Paints and their allied products like varnishes, enamels, pigments, printing inks and synthetic resins protect assets from corrosion. These are increasingly being used in automotive, engineering and consumer durable sectors. Paint testing can be done in a number of different ways. The fact of the matter is that many industries use several different paint testing methods in order to ensure accurate results. Paint should be tested in a wet form for particular properties but also in the dry form. Testing of paints generally falls into three categories: testing of the raw materials, testing of the finished product and performance testing using accelerated weathering and other simulation type methods of evaluation. Coatings technologists deal with interfaces of all classes gas liquid as in an aerosol spray liquid liquid, as in an emulsion gas solid, as in a dry pigment before its immersion in a vehicle liquid solid, as in a pigment dispersion and solid solid, as when the crystal faces of two different pigment particles are in tight contact. Paint scientists are particularly interested in the formation of liquid solid interfaces that are stable in the package, that is, in the permanent replacement of the air at the air solid interface of the pigment by the vehicle to give the liquid solid interface of the dispersion. In coatings and similar products, the criteria for best performance particulate ingredients; inorganic, organic, extender and metallic flake pigments and dispersed phase of latexes depends on the size and shape of particles composing the particulate materials. The purpose of paint testing is to help and ensure that the minimum requirements for ingredients and material characterization are met by the manufacturer on a batch basis, and to help ensure that the formulated product will provide satisfactory performance in the environment.
Handbook on Paint Testing Methods explains about aspect of gloss, specular glass, sheen, contrast gloss, absence of bloom gloss, distinctness of image gloss, specular gloss evaluation, specular reflectance, geometric considerations, instrumentation, goniophotometers, specular glossmeters, basic factors producing hiding power, refractive indexes of white pigments, refractive indexes of organic pigments, films for testing preparation of films for test, pigments and extenders, metallic flake pigments, latexes, methods for determining particle, treatment of data, particle size with light microscope etc.
This handbook elaborates the different testing methods of paints with an understanding of the various tests that can be performed on product performance. This handbook will be very helpful to its readers who are related to this field and will also find useful for upcoming entrepreneurs, existing industries, technical institution, etc.
1. OPTICAL PROPERTIES COLOUR AND LIGHT
Introduction, Light source, Standard Illuminants, Color Temperature, Color Matching Booth, Metamerism, Non-visible Radiation, The observer, Color Deficiency, Reflectance and Transmittance, Color mixing, Addition of Lights, Subtractive colorimetry, Color order systems, Munsell system, Ostwald color system, ISCC-NBS System, Din – color system, Atlas de los colores, Federal color standard, Specialized color order systems, Gardner Liquid color standards, Loviond Tintometer, Parlin color standards, Gardner – Delta color comparator, ASTM color Scale, ASTM Method D 1500, Intermental color measurement, Spectrophotometers, Abridged Spectrophotometers, Tristimulus colorimeters
Aspect of gloss, Specular Glass, Sheen, Contrast Gloss, Absence of Bloom Gloss, Distinctness – of- Image Gloss, Specular Gloss Evaluation, Specular Reflectance, Geometric Considerations, Instrumen-tation, Goniophotometers, Specular Glossmeters, Distinctness – of – Image Glossmeters, Specular Gloss Methods, Two–Parameter Methods, Distinctness – of Image – Methods, Gloss standards, Material for standards, Calibration of standards, Use of standards
3. HIDING POWER
Definition of Hiding Power, Basic Factors Producing Hiding Power, Refractive Indexes of White Pigments, Refractive Indexes of Organic Pigmenfs, Practical Determination of Hiding Power, Checkerboard Brush-Out Method, Haslam Method, Early Hiding-Power Methods, Krebs Method, ASTM Relative Hiding Power, Pfund Cryptometer, Black and White Cryptomet, Rotary Cryptometer, Pfund Precision Cryptometer, Assessment of Cryptometers, Hallett Hidimeter, Hanstock Method, Bruce Hiding-Power Tests, Gordon-Gildon Method, Some Hiding-Power Findings-I, Pigment Concentration Versas Hiding Power, Contrast Design and Visual Sensitivity, Fell Equation, Hiding Power of Colored Pigments, Kubelka-Munk Two-Constant Theory, Importance and Applicability of Kubelka-Munk Tlreory, Equation, Judd Groph, Schmutz-Gallagher Melhod, New York Club Method, Van Eyken-Anderson Method, Federal Test for Dry Opacity, ASTM Method, First Method-Uses Cardboard , Procedure, Computation, Precision, DIN Method, Universally Applicable Technique, Bruehlman-Ross Method, Day Method, Some Hiding-Power Findings-II, Hiding Power VersusConcenlration for Titanium Pigments, Hiding Power Versus Concentration for Zinc Sulfide Pigments, Reflectance and Hiding Power of Tinted Paints-l, Reflectance and Hiding Power of Tinted Paints-II, Some Applications of Kubelka-Munk S. and K.Values, Unification of Paint Phenomeno-I, Unificattion of Paint Phenomena-II, Influence of Particle Size of Extender on S-Value, Influence of Particle Size of Titanium Pigment on S-Value Versus PVC, Formulation of Paints from Predetermined S-Values, Instrumental Color Malching Using Both S- and K.Values, Relation Detween Tinting Strength and Hiding Power, Hallell Equation, Scallering Cofficient and Tinting Strength, Calculation of While Hiding Power from Tinting Strength
4. MASS COLOR AND TINTING STRENGTH
Definition, Mass color, Tinting Strength, Back Factors Producing MC and TS, Mixing Pigment and Vehicle, Spatula and Muller Methods, Hoover Automatic Muller, Laboratory Ruller Mill, Pall Glass Mill, Pigment concentration, Application, Dispersion Time, Visual Mass – color Methods, ASTM Method, Other Methods, Mass color of white Pigments, Visual Tinting –strength Methods, ASTM Method for colored pigments, NPIRI Method for Colored Pigments, TAPPI Method of Colored Pigments, Tintograph, ASTM Method for White Pigments, NPIRI Method for White Pigments, NJZ Method for Zine Oxide and Titanium Dioxide, duPont Method for Titanium Dioxide, Reynolds Constant Volume Method, Instrumental Mass Color, Maxwell Color Triangle, MC, atul S- and K-Values , Mass Color of While Pigments, Visual Versus Instrumental White, Instrumental Timing-Strength Methods, Early Methods, DIN Method, Japanese Method, Mttnk Theory, ASTM Method for White Pigments, Some Tinting-Strength Findings, Pigment Concentration, Lightness Versus PVC, Tone Versus PVC, Tone of Colored Pigments, Calculation of Instrumental Color Matches, History, Simple Case—One Constant, General Case—One Constant, More Than Three Wavelengths, General Case—Two Constants, Pigment Standards for Federal Specifications, Artist’s Oil Paints Commercial Standard CS98-42, Permanent Palettes
5. PHYSICAL PROPERTICS
Density, Specific Gravity, Density of Liquids with Pycnometer, Procedure, Weight Per Gallon, Specific Gravity of Liquids with the Specific Gravity Balance, Specific Gravity of Liquids with the Hydrometer, Specific Gravity of Pigments, Vacuum Method, Method B-Accurate Testing of Single Specimens, Method C-Rapid and Accurate Testing of Single Specimens, Centrifuge Methods for Specific Gravity of Pigmeots, Zieglemann Method, Baker-Martin Method, Dunn Method for Specific Gravity of Pigments, Calculating Specific Gravity of Mixed Pigments, Apparent Density of Pigments, Primitive Method, Becker Method, Displacement Method for Specific Gravity, Flotation Method for Specific Gravity, Settling of Pigments in Paints, ASTM Evalutation, New Jersey Zinc Company (NJZ) Test , Hancock-Brown Test, Arnold Test, New Jersey Zinc Company (NIZ) Accelerated Test, Eagle-Picher Accelerated Test, Hancock-Brown Accelerated Settling Test, ASTM Accelerated Settling Test, New York State Accelerated Settling Test, Paint Formula Yield
6. VISCOSITY AND CONSISTENCY
Introduction, Definitions, Rheology, Flow, Viscosity, Absolute Dynamic, Newtonian Liquid, Consistency, Non-Newtonian Liquid, Plastic Flow, Plastic Viscosity, Pseudoplastic flow, Dilatant Flow, Thixotropy, False-Body, Instrument Types, Capillary Viscometers, Standard Capillary Viscometers, Hercules Capillary Viscometer, Bingham-Green Plustometer, Vacuum Plastometer, Caster Severs Viscometer, Gardner Pressurized Flow Cup, Eflux Type Viscometers, Saybolt Viscometer, Ford Cup, Shell Cup, Zahn Cup, ASTM Consistency Cup, Parlin Cups, Prall and Lambert Cup, Gottsch Consistency Cone, Scott Viscometer, Westinghouse Cup, Demmler Cup, Viscosity Cup Correlation Duta, Rotational Viscometers, Brookfield Viscometer, MacMichael Viscometer, Krebs-Stormer Viscometer, Brabender Recording Viscometer, Kämpf Viscometer, The Wolffe-
Hoepke Turboviscometer, High-Shear Rotational Viscometer, Brushometer, Interchemical Rotational Viscometer, Devilbiss Electro-Viscometer, Rotovisco Viscometer, ICI Rotothinner, ICI Cone and Plate Viscometer, Ferranti-Shirley Cone and Plate Viscometer, Ferranti Portable Viscometer, Wells-Brookfield Micro Cone and Plate Viscometer, Falling Ball Viscometers, Hercules Falling Ball Method, Astom Method for Cellulose Derivatives, Hoeppler Viscometer, Band Viscometer, Bubble Viscometer, Gardner-Holdt Bubble Viscometer, Other Instruments, Gardner Vertical Viscometer, Interchemical Inclined Tube Viscometer, Collins Bubble Viscometer, Steiner Bubble Viscometer, Gardner Mobilometer, SIL Mobilometer, Laray Viscometer, Clarvoe Consistometer, Influx Viscometer, Flowmeters, Gardner Flowmeters, Flowmeters, Inclined Plane Type, Thixotrometers, Brushability, Brushability from Stormer Data, Brushability by High-Shear Method, Sagging, Sagging Measurements Using Modified Stormer, Sagging Measurements using the Rotovisco, Sag Test Instruments, Leveling, Tensiometer for Leveling , Recent Leveling Investigations, Practical Evaluations of LevelingComb Tests, Leveling by Drawdown Method, Leveling by Shell Flow Comparator
7. SURFACE ENERGETICS
Free Interfacial Energy, Wetting, Surface Tension, Surface Tension Measurements, Capillary Rise Method, Maximum Bubble Pressure Method, Drop-Weight Method, Ring Method, Other Methods, Contact Angle, Shadow Method, Tilting Plate Method, Displacement Cell Method
8. PARTICLE SIZE MEASUREMENT
Pigments and Extenders, Metallic Flake pigments, Latexes, Methods for Determining Particle, Treatment of Data, Particle Size with Light Microscope, Direct Measurement Method, Reticle Method, Dark Field Technique, Particle Size with Electron Microscope, Particle Size by Sieving, Hand Sieving, Machine Sieving, Particle size by Sedimentation, Gravity Sedimentation, Centrifugal Sedimentation, M-S-A Particle Size Analyzer, Sedimentation by Ultracentrifuge, Particle
Size by Photometry, Transmission Methods, Spectrophotometric Techniques, Angular-Dependence Techniques, X-ray Scattering, Particle Size by Elutriation, Thompson Classifier, Roller Particle Size Analyzer, Felvartion, Particle Size from Surface Area, Adsorption of Gas, Adsorption of Solutes, Soap Titration Method, Permeation Method, Electronic Size Analyzer, Particle Size and Thickness of Metallic Flake Pigments, Coarse Particles, Sieve Method, Gallie-Parritt Apparatus, Dunn Test, Thin-Film Drawdown for Oversize Particles, Dunn Texture Test for Dry Pigments, North Standards, Fineness-of-Dispersion Gages, X-ray Microradiograply Technique
9. OIL ABSORPTION OF PIGMENTS
Introduction, Nature of Oil Adsorption, Methods for Determining Oil Absorption, ASTM Rubout Method, British Standards Institution Method, Azam Method, Hoffman Method, Smith Stead Method, National Lead Company Method, Density End Point Method, Bessey-Lammiman Method, Gardner–Coleman Method, Free Binder, Liquid Absorption by Pigments, Critical Pigment Volume, Critical Pigment Volume Concentration Cell, Pigment Packing Factor, Cole Method for CPVC, Pierce-Holsworth Method for CPVC, Procedure, CPVC AND OA, CPVC, OA, and Viscosity, Calcula-ting OA of Pigment Mixtures, Characterization of Dispersions, Dispersant Demand of Extender Pigments
10. FILMS FOR TESTING PREPARATION OF FILMS FOR TEST
Preparation of Films by Spray, Bell Laboratories Method, Battelle Automatic Sprayer, Preparation of Films with the Doctor Blade, Gardner Adjustable Film Casting Knife, DiCostanzo Adjustable Doctor Blade, Gardner Ultra Applicator, Parks Film-O-Graph, Dow Film Caster for Latex, Bird Film Applicator, Boston-Bradley Adjustable Doctor Blade, Parks Rapid Coater, Brier-Wagner Spreader, Grooved Rod Applicators, Baker Film Applicator, Automatic Doctor Blade, Motor Drive for Doctor Blades, Magnetic Chuck, Wedge-Shape Films, Tape Method, Howard Suction Plate, Preparation of Films by Flowing, Preparation of Films by Dipping, Bruins Method, Payne Dip Coater, Hot Rolling Method (Asphalt Trimmer), Hydraulic Press Method, Preparation of Films by Spinning, Preparation of Free Films, Sized Paper Substrate for Free Films, Mercury Substrate for Free Films, Aluminum Substrate for Free Films, Polyethylene Substrate for Free Films, Silvered-Glass Substrate for Free Films.
11. MEASUREMENT OF FILM THICKNESS
Wet Film Thickness, Inmont Wet Film Gage, Pfund Wet Film Gage, Tooth Gages, Needle Micrometer, Dry Film Thickness, Machinists’ Micrometer, Gardner Needle Thickness Gage, Gardner Carboloy Drill Thickness Gage, Gardner Gage Stand, Gardner Micro-Depth Gage, Microscope for Film Thickness, Magnetic Thickness Gages, Inductance Thickness Gage, Eddy-Current Thickness Gage, General Electric Gage, Type B, Elcometer, Minitector, Gardner Scratch Thickness Gage, Profile Measurement, Keane-Tator Surface Profile Comparator, Elcometer Surface Profile Gage
12. DRYING TIME
Effects of Environment, Set-to-Touch Time, Dust-Free Time, Cotton Fiber Method, Powder Method, Glass Bead Method, Tack-Free Time, Tack-Free Time with Paper, Zapon Tester, Blom Drying Time Tester, Gardner Magnetic Tack Tester, General Electric Tackmeter, Siccometer, Final Drying Times, Dry, Dry Hard, Dry-Through, Dry-To-Recoat, Touch Controller, Gardner Drying Time Meter, Parks Dry-O-Graph, Gardner Drying Time Recorder, Sanderson Drying Time Meter, Paraffin Companies Drying Time Machine, Gardner Circular Drying Time Recorder, RCI Drying Time Recorder, Erichsen Universal Drying Time Recorder, Rolling Ball Testers, Drying Time with Hardness Rocker
13. MECHANICAL PROPERTIES OF FILMS
HARDNESS AND RELATED PROPERTIES
Concept and Definition, Scratch Hardness, Laurie-Baily Hardness Tester, Graham-Linton Hardness Tester, Clemen Hardness Tester, duPont Scratch Testing Machine, Hoffman Scratch Tester, Taber Shear/Scratch Tester, DEF Scratch Resistance, Bierbaum Microcharacter, Schopper Hardness Tester, Parker-Siddle Scratch Tester, Simmons Scratch Tester, Dantuma Scratch Tester, Rondeau Scratch Tester, Sheppard-Schmitt Scratch Dynamometer, Arco Microknife, Pencil Method, Mechanical Pencil Method, Pendulum-Rocker (damping) Hardness, Walker-Steele Swinging Beam, Persoz Pendulum, Konig Pendulum, The Sward Rocker, Identation Hardness, Methods and Devices, Identation Rheology, Indentation Hardness Miscellaneous, Theory of Indentation Hardness and Rheology, Comparative Results, Mar Resistance, Concept and Definition, Single Scratch Methods, Impinging Abrasive Method, Scuffing Methods, Miscellaneous Methods
14. ABRASION RESISTANCE
Introduction, Definition, Relation to Other Physical Properties, Mar Resistance, Hardness, Modulus of Elasticity and Tensile Strength, Correlation with Service Performance, Mechanism of Abrasion, Classification of Test Methods, Methods Using Loose or Falling, Falling Sand Abrasion Test, Pebble Abrasion Test, Olsen Wearometer, Gloss Reduction Methods, Abrasive Blast Methods, Bell Laboratory Abrasiometer, Roberts Jet Abrader, Gravelometer, Methods Using Rotating Disks, Bell Laboratory Rotating Disk Abrasion Test, Wolf Abrasion Method, Camp Abrasion Machine, FDC Wear Test, Schiefer Abrasion Testing Machine, Methods Using Rotating Wheels, Taber Abraser, Methods Employing Rectilinear Motion, Armstrong Abrader, Gardner Heavy-Duty Wear Tester, Parlin Abrasion Testing Machine, Rain or Water Erosion, Wet Abrasion Methods, Gardner Wet-Abrasion (washability) Machine, PEl Abrasion Tester, Peters Abrasion Block, Traffic Paint Tests, Miscellaneous Methods
Concept and Definition, Classification of Test Methods, Method of Removal, Knife Removal Methods, Penknife, Rossmann Chisel Adhesion Test, König Knife-Wedge Device and Method, New York Club Chisel Adhesion Test, Koole Chisel Adhesion Test, Arco Microknife, Adherometer, Wolf Adhesion Chisel, Adherometer-Integrometer, Graham-Linton Edge Adhesion Test, Meredith and Guminski Chisel Test, duPont Sharp Tool Adhesion Test, Hesiometer, Scraping and Scratching Methods, Crosscut Adhesion Test, Window Adhesion Test, Balanced-Beom Scrape-Adhesion, Automatic Scrape-Adhesion, Pocket Scrape-Adhesion Tester, Scratchmaster, ASTM Pre-cut Scrape Adhesion, Hoffman Scratch Tester, Rondeau Scratch Tester, van Laar Scratch Test, Angular Scribe-Stripping Technique, Pass Test, Pencil Test, Princeton Adhesion and Scratch Tester, Adhesive Joint Methods, Tensile Shear Methods (lap joint), Torque Shear Methods, Cleavage Tests, Peel Tests, Gardner-van Heuckeroth Adhesion Test, Courtney-Wakefield Adhesion Test, Russian Method, Adhesive Tape Tests, Weyerhaeuser Paint Adhesion Tester, Procedure, Method of DIN 53 151, Brown and Garnish Crosshatch-Metal Strip Tape Test, Ford Motor Company Crosshatch Tape Test, Liquid Jet Test, Liquid Wedge Test, Dannenberg Blister, Hoffman Air Pressure Method, Inertia Tests, Ultrasonic Vibration Test, Ultracentrifuge Adhesion Test, ICI Bullet Method Adhesion Test, Impact and Bending Methods, Other Methods, Hydrophil Balance
Definition, Interpretation, External Factors Affecting Flexibility, Humidity, Temperature, Strain Rate, Determination of Flexibility, Mandrels, T-Bend, Cupping Tests, Forming Tests, Impact Tests, Cold Crack, Exposures
17. TENSILE STRENGTH AND ELONGATION
Definition, Interpretation, Determination, Specimen Preparation, Tension Testing Machines, Film Mounting, Controlled Conditions Cabinets, Reproducibility, Predicting Durability
18. CHEMICAL PROPERTIES OF FILMS
Resistance to Water Vapour and Liquid in the Atmosphere, Introduction, Water Vapor Transmission, General Method for Materials in Sheet Form, Resistance to Rain and Condensation, ASTM Method D 1735, Water Fog Testing of
Organic Coatings, JAN-H-792 Humidity Cabinet, ASTM Method D 2247, Testing Coated Metal Specimens at 100 Percent Relative Humidity, Early Condensation Tests, Cleveland Condensation Tester (ASTM Method D 2247, Appendix II), Resistance to Water from Within a Structure, Levin-Christian Blister Box, Forest Products Laboratory Blister Box, Veer Blister Box, ASTM Method D 2366, Accelerated Testing of Moisture Blister Resistance of Exterior House Paints on Wood, Blister Houses, Moisture Content of Substrates, Electric Moisture Meters , Electric Hygrometers, Hair Hygrometer, Salt Color-Change Hygrometer
19. CHEMICAL RESISTANCE
Introduction, Spot Tests, Staining from Household Chemicals, Staining in the Transportation Industry, Immersion Tests, Resistance to Water, Resistance to Alkali, Resistance to Detergents, Battelle Chemical Resistance Cell, Bratt Conductivity Cell for Chemical Resistance, Gearhart-Ball Solvent Resistance Tests, Perspiration Resistance, Salt Fog Test
20. FIRE RETARDANCE AND HEAT RESISTANCE
Introduction, Cypress Shingle Tests, Schulz Firl-Retardant Tester, New Jersey Zinc Company Box Test, British Box Test, ASTM Cabinet Test, Stick and Wick Test, Westgate Vertial Match Test, Crib Test, Fire-Tube Test, Roof Corner Test, Sidewall Test, Corner-Wall Test, SS-A-118 Test, Schlyter Method, Radiant Panel Test, Twenty-Five-Foot Tunnel Test, Eight-Foot Tunnel Test, SURD 16-Foot Tunnel Test, Two-Foot Tunnel Test, Heat Resistance, 400 F Test, 1200 F Test (on aluminun paint), 1400 F Test, ASTM Heat/Service Test, Melting Point Bars for Testing Heat-Resistant Paint, Houston Heat Resistant Tester, New Jersey Zinc Company HeatResistant Tester, Spontaneous Combustion, Mackey Apparatus for Spontaneous Combustion, Sawdust Method, Louisville Methods
OPTICAL PROPERTIES COLOUR AND LIGHT
Man ability to discriminate colors increases both his
enjoyment and appreciation of the world around him. Because color is
such a significant factor in the appearance of an object, it is an
important characteristic of any paint. Appearance is one quality of a
product that every customer can judge for himself. No matter how good
the physical properties of a paint, if the color does not meet the
expectation of the customer, he will think the finished product
unsatisfactory. Color is often thought to be a property of the paint
itself, and to a degree, it is. Actually it is more complex than that
for color of a paint depends on three things (1) the spectral
reflectance of the paint, (2) the spectral composition of the light in
which the paint is viewed, and (3) the spectral sensitivity of the eye
of the observer for these reasons matching the color of a paint or
other material requires considering all three and not merely the
spectral characteristic of the material.
This chapter deals with ways in which the color analyst
can obtain consistent measurements of the colors of his specimens. It
will also touch briefly on the mechanism of vision where this helpful
to understand the requirements for evaluation of appearance. The
physics, physiology, and psychology of color, however, are broad
subjects, and only enough disground for understanding the development
of test methods. Readers desiring to purpose these subjects in detail
should consult an appropriate text.
Light is electromagnetic radiation weighted by the
response of the normal human eye. It involves the portion of the
electromagnetic spectrum between approximately 380 and 780 nanometers
(nm) (fig 1). Note that visible radiation occupies a very narrow band
in the electromagnetic spectrum between ultraviolet and infrared
In the past the most important sources were daylight and
candlelight, and in spite of the construction of many windowless
buildings, daylight is still an important source since most objects at
some time or other are viewed outdoors. The composition of daylight
however is quite variable, depending upon the hour of day, the season
of year, and very importantly, the amount of cloud cover. Other sources
must replace daylight at the end of the day or when there are no
windows to illuminate dwellings and offices. Flame sources were first
used but of course, have now been supplanted by electric lamps. But
incandescent lamplight is generally preferred of use indoors because it
imparts the same soft, mellow effect as that of candlelight. In offices
and in places of business where high level of illumination
are needed, duplication of daylight is usually preferred, and for this
purpose fluorescent lamps can simulate daylight to a degree, but the
spectral character is not identical. It should be noted that while
fluorescent lamps can be used for visual color evaluation they are
generally unsuitable for use in instrumental color measurement
fluorescent lamps are made the most common of which is known as cool
white. Even this color has deluxe and superdeluxe versions the distinct
improvement being an increase if radiation in the red portion of the
spectrum light sources are described most accurately by curves are
described most accurately by curves or tables of their relative
spectral power distributions.
In order to standardize color evaluation work, the CIE
(commission International de Eclairage international commission on
illumination) in 1931 established three standard Illuminants designated
A,B, and C (fig2). Illuminants A is intended to represent ordinary
tungsten lamplight, Illuminants B and illuminants C are derived by
using liquid filters specified by the CIE with Illuminant A illuminant
C does not have provided data for what has been called reconstituted
daylight of various correlated color temperature. D65 is suggested for
most color measurement, but D 75 can be used when a bluer illuminant is
desired, and d55 can be used when a more yellow illuminant is desired.
Other distributions of D illuminant can be also used note that color
evaluation for D illuminants is easy when spectral data are available.
On the other hand, it is difficult to produce a D illuminants for use
with a colorimeter, and most measurements with colorimeters continue to
be made for CIE illuminant C.
The term color temperature has been often used to identify
radiation from incandescent lamps and other sources. As a body is
heated it begins to radiate, first in the infrared, then in the
visible. As heating continues the color shifts from red, to yellow, to
white, to blue white at the highest temperature. Figure 3 shows the
blackbody locus of color temperatures potted on the CIE chromaticity
diagram. The chromaticity is a function of temperature. The temperature
is expressed in kelvins and abbreviated K. A radiator whose emissivity
is less than that of a blackbody, such as a tungsten lamp, called a
gray body. Since its emissivity is a function of wavelength it will not
have exactly the same chromaticity as a blackbody. However, when its
chromaticity most nearly matches that if the black body, it is said to
have a correlated color temperature of the same numerical value as that
of the blackbody. The color temperature of the sun is about 5000k that
of a cloudless summer sky is between 20,000 and 25,000 k. The color
temperature of a household type 100 W tungsten lamp is about 2850k (CIE
Illuminant A). Tungsten melts at about 3600k, so the color temperature
of an incandescent tungsten lamp cannot exceed this value and
practically is limited to about 3200k.
Because of the varied chromaticities of different natural
and artificial sources, it is essential that visual color matching
boot. This device allows the color analyst to make a visual match at
any hour of the day or night with the confidence that the match will be
equally satisfactory in a plant thousands of miles away.
A paint of specified color should be matched with a paint
having the same combination of pigments. Frequently a different combination
of pigments will be used. Matches of this type of illumination such as
incandescent lamplight, but may not match under another type of
illumination such as daylight. This phenomenon is known as metamerism,
and the colors are said to be metameric. Such paints are found to have
different spectral reflectance curves (fig 4). Only when colors have
identical spectral characteristics can they be expected to match under
all types of light this is why the
same pigment formulation should be used when remarking the color.
Whenever pigment used for the match have different spectral
characteristics from the sample, the resultant color match should be
verified under at least two different types of illumination, for
example, illuminant A and illuminant C. if the match is not
satisfactory under each type, spectrophotometric analysis of the two
formulation should be adjusted to minimize these differences.
The sensation of color is evoked by visible radiation, but
ultraviolet and infrared radiation can have important effects in paint.
Ultraviolet radiation promotes de composition of binder, is the
principal stimulus of fluorescence of certain pigments, and is an aid
to identification and analytical determination of certain ingredients
of paint. Infrared radiation is a factor in heat transfer and also an
aid in the identification and analysis if chemical compounds.
materials encountered in nature have the unique property of fluorescing
when irradiated by ultraviolet energy (invisible to the eye) contained
in the source. In other words, the object will emit light in the
visible range even though the radiation used to energize it is not
itself visible. The effect of fluorescence is to increase the apparent
reflectance. Most colorimeters and spectrophotometers have not been
designed to properly evaluate the colors of fluorescent materials, and
most test methods are applicable only to nonfluorescing materials. It
is usually sufficient in most applications to expose the paint or other
coating to ultraviolet radiation to determine that it does not
There are recently developed pigments that use
fluorescence to convert ultraviolet radiation, as well as visible blue
light, to green orange, or red light, thus increasing the luminance in
these regions to levels much higher than would otherwise be obtained in
daylight fluorescent they have low hiding power and should be applied
over a white background for good results. Table 1 lists the
fluorescence of some paint materials.
Infrared Radiation Every hot object emits infrared
radiation, and while this radiation is not visible to the eye, it is
reflected by certain materials including paint. Object painted to
conceal them from observation need to reflect infrared radiation to the
same extent as their surroundings in order to escape detection. For
this reason evaluation of infrared reflectance is important, and
devices for detecting infrared radiation have been developed,
especially for military operations.
Absorbed radiant energy, invisible as well as visible, is
converted into heat. Hence, to the extent that painted surfaces absorb,
reflect fluence the temperature of a building, increase heat transfer
from a radiator, reduce heat transfer into a refrigerator, or reduce
evaporation from a gasoline storage tank.
The perception if objects and their colors depend on the
reaction of both the eye and the brain. Vision is thus psychophysical
phenomenon physical in that light
must reach the eye, psychological in that the brain must interrupt the
nerve signals to form an impression of the light in the mind. The
psychological factor determines whether a given color combination is
pleasing or displeasing. The mechanism of seeing is physical the
impression is psychological. Objective color measurement is confined to
Psychological factors cannot be evaluated objectively. For
example, the apparent color of a specimen may be changed by the color
of the adjacent area. The effect can be observed by placing one half of
a sheet of pastel colored paper in the middle of a large area of a high
reflectance color, such as white, and the other half on a low
reflectance color, such as black. The specimen on the white background
will appear stronger and darker than the identical specimen on the
black background. Reversing the specimen reverses the direction of
their apparent color difference. The kind of color evaluation cannot be
Another example of a psychological color factor involves a
phenomen called chromatic adaptation or color constancy. When one goes
outdoors from a windowless office, he requires a few minutes to adapt
to a large increase in light level. Also there probably will be a
change in the spectral composition of the light reflected from his
friend s clothing, hair, and face will be different. But one has
learned to expect this kind of change and, therefore, thinks little
about it, except under spectral composition of the light incident on
them. By the same token the eye brain combination cannot be trusted to
pick date. Most persons will select a color considerably stronger than
the original they think they remember.
The human eye is still an excellent device for comparing
colors, but only for direct side by side
comparisons. Even here we find that result depend on the observe in
that some observes report large differences where others claim that
there is hardly any difference at all. What the reason for such
differences is among observes? The explanation lies in the fact that
the color sensitivity of the human eye differs among individuals. Some
individuals can be identified or the by tests for color vision such as
those devised by Ishihara2, Hardy et al or
In order to evaluate colors consistently, a standard
observe was defined by the CIE in 1931. One might assume that to define
a standard observe representative of entire human population might
require evaluation of the spectral sensitivities of thousands of
individuals. Actually the difficulty of obtaining such data precluded
using many observe, and the standard observer is based on data from
only17 individuals (all trained color matchers). The spectral responses
of the CIE 1931 standard observer are defined by the tri stimulus
values of the spectrum plotted in fig. 5. At any wavelength the values
of are the amount of primaries, x (red), y (green), and z (blue)
required to produce the color of the spectrum at that wavelength. Any
part of the spectrum at that be produced by the positive amounts of
these primaries, which are imaginary because no additive mixture of
real lights will duplicate all of the spectrum colors. In the actual
experiment with spectrum lights negative amounts of some standard
lights were required to match other spectrum lights. That is in the
attempt to match a spectrum color with an additive mixture of the three
primary lights, it was found necessary to add some light from the
primaries too the test color. In effect, this subtracted light from the
primaries, producing negative amount however, the CIE decided not to
use any negative quantities, and the data were transformed so that all
quantities would be positive. Another restriction was imposed, namely,
that all of the luminosity should be contained in one of the standard
lights, which these restrictions on the experimental data, the CIE 1931
standard observer was born and it has performed very successfully, if
not perfectly, for 40 years.
The data for the 1931 observer were obtained with commonly
used visual photometer of that era in which the field of view subtended
an angle of only 2 deg at the eye of the observer. Actually the usual
field of view in commerce is more nearly 10 deg, and data showed that
the spectral sensitivity of the eye is different for these two viewing
fields. For this reason additional data were obtained with a 10 deg,
field and the CIE established the 1964 Supplementary Observer. However,
it should be pointed up that, for many colors, differences computed for
the two fields are small and that most filter colorimeters are still
based on the 1931 Standard Observer.
No individual is average and very few have color vision
identical with that of the Standard Observer. This circumstance does
not make the Standard Observer less useful, but it must be kept in mind
when making visual examinations of colored objects. Color blindness is
a term used to denote color vision grossly different from that of the
average individual. About three people in 1000 are completely
colorblind. There are other types of color defectives who see color
slightly, though significantly, different than the average observer
although about 95 percent of all people see color approximately the
same as the Standard Observer. Since, in many instances, Color matches
are made by visual observation, it is important to know just how well a
particular observer compares with be average. As mentioned before,
color aptitude tests have been devised to test the color matching
ability of individuals required to make such judgements.
What happens to light when it strikes an object Some
may be reflected, some may be absorbed, and if the object is
transparent, some may be transmitted. The reflected light may be
concentrated at an angle numerically equal to the angle of the incident
ray, or scattered uniformly in all directions, or distributed between
these two extremes which are known as specular (minor) reflection and
diffuse reflection. A highly polished white metal, such as silver,
reflects as much as 99 percent of the incident light in a specular
direction. A white powder, such as magnesium oxide or barium sulfate,
scatters light in all direction, and it, too, reflects as much as 99
percent of the incident light. Specular reflection is related to gloss diffuse
reflection is related to lightness and color. Transmission can also be
regular (specular) or diffuse, depending on whether or not light is
scattered in passing through a material. The reflectance or
transmittance of light by an object is usually wavelength dependent. A
spectro photometer is used to provide information on the spectrally
selective character of a material. Figure 6 shows several examples of
spectral curves of colored paints. This type of chart provides a
profile of the spectral reflectance or transmittance of an object.
Trained analysts can look at a spectral curve and obtain valuable
information, but spectral data alone are unsatisfactory means for color
Gloss is often described as the attribute responsible for
the shiny or lustrous appearance of an object. A more formal definition
is given by the CIE.
Gloss (of a surface) Directionally selective reflecting
properties responsible for the degree to which reflected highlights or
the images of object may be seen as superimposed on the surface.
Gloss is second only to color in importance as an
appearance characteristic of an object. Like color, gloss can be
subdivided into several aspects depending on the particular
characteristic that is being considered. Hunter identified five aspects
of gloss and function of reflectance by which they can be evaluated
1). Experience has shown that no single objective measurement
requirement of an object be determined and the most useful aspect
selected. It must be pointed up, however, that the measurement
techniques for the various aspects are not developed equally well.
Aspect of gloss
Each aspect of gloss is associated with the shininess of
the object, but depending on other appearance characteristics or the
use of the object, one aspect of gloss may predominate, or the
distribution of reflected flux may have to be investigated.
Goniophotometry is the technique used for investigating the geometric
distribution of reflected light. Goniophotometry is required in any
research type investigation in order to determine the nature of the
geometric distribution of reflected light.
Specular gloss or specular reflectance is simply the
fraction of the incident light reflected from a surface in the mirror
direction within a specified angular tolerance. This is the aspect of
gloss that is measured most frequently because it is the one for which
an instrument is constructed most easily. In practice the angle of
incidence is specified as well as the divergence angle or aperture of
source and receptor (fig 1). For simplicity the geometry is identified
frequently by the giving only the incidence angle,
but the associated apertures play a vital role in instrumental readings.
Sheen is often described as specular gloss at a near
grazing angle of incidence. However, sheen is defined in ASTM
Designation E 284, standard Definitions of terms relating to Appearance
of Materials, as specular gloss at a large angle of incidence or an
otherwise mat specimen, that is one whose specular gloss is very low at
low angle of incidence such as a mat wall paint. The usual angle for
sheen measurement is 85 deg from the perpendicular to the specimen.
This is about the maximum angle that can be used without excessive
difficulty in positioning the optics to illuminate and view the
Contrast gloss is defined as the ratio of specular to
diffuse reflectance. It is the aspect that takes into account the
relation between specular gloss, reflectance factor, and glossy
appearance. It can be used to explain why a black object appears to be
more glossy than a white object having the same specular reflectance.
In the case of the black object the reflected highlight is not diluted
by high, diffuse reflectance from the surround, as is the case with the
white object. Contrast gloss can be used as a means to make this
differentiation, but is seldom required to compare gloss
readings with visual estimated of glossiness for both black
and white specimens.
This type of gloss is associated with high gloss paints
and similar materials that on occasion exhibit a hazy or milky
appearance adjacent to the reflected highlight. It is observed most
easily in a darkened room by viewing the image of the filament of an
unfrosted lamp (operated at reduced voltage) reflected in the surface
of the specimen. Some people try to differentiate between haze and
bloom. Bloom is said to appear on the surface as efflorescence and can
be rubbed off. Haze on the other hand is attributed to scatting by
pigment particles just beneath the surface. for either
haze or bloom the measurement technique is the same. basically
it requires a specular gloss aperture to differentiate between the
small differences in near forward
scattering associated with bloom or haze.
A very sensitive measure of haziness can be obtained by
providing a glossmeter with a specular stop, so that only the reflected
rays that are, scattered beyond the usual specular aperture are
accepted for measurement.
In general high gloss surfaces are best measured for
specular gloss with an incidence angle near the perpendicular, and
conversely low gloss surfaces are best differentiated by using an
incidence angle close to grazing. This axiomatic truth is found in a
paper on gloss measurement by fund. High gloss measurement also
requires that the divergence of rays accepted for measurement be as
small as possible consistent with ease of specimen preparation and
positioning on the instrument. With small apertures warped of curved
specimens are likely to give erroneous readings.
The light flux entering a glossmeter receiver after
reflection from a perfectly diffusing surface is a direct function of
the size of the receptor entrance window. Readings for low gloss
specimens not perfectly polished will be a function of the size of the
source image formed at the receptor entrance window. If the source
image formed by reflection from a polished specimen nearly fills the
window, instrument readings. The effect on gloss readings of variations
in 60 deg glossmeter geometry is discussed by Hammond and Nimeroff.
Another consideration that influences the choice of the
receiver source aperture ratio is the planarity or flatness of
specimens. For specimens having a wavy surface, the receiver source
aperture ratio should not be too small otherwise
specimens ranked by eye as having high gloss will be rated too low by
the instrument. The gloss of a warped specimen is always difficult to
measure accurately because the flux reflected from the specimens ranked
by eye as having high gloss will be rated too low by the instrument.
The gloss of a warped specimen is always difficult to measure
accurately because the flux reflected from the specimen is not likely
to be centered on the receptor entrance window. A warped specimen
distorts the reflected flux distribution so that the glossmeter is
likely to indicate an erroneous gloss value.
Standardization of specular gloss
measurements necessitates specification of the angles of illumination
and receptors field angles or apertures. The aperture sizes must be
specified with tolerance. Specification for these apertures was often
omitted from the appartures requirements of early test methods. In some
cases, instrument type was specified, but this was found to be
Early instruments for gloss measurement were visual
instruments, and, as a result objective gloss evaluation was not
carried out with great frequency until the production of the
photovoltaic cell in the 1930 s. Since then a large number of
photoelectric glossmeters have been designed and built, and several
designs of photoelectric goniophotometers have been marketed.
Distinctness of image evaluation. However, has yet to be accomplished
to complete satisfaction by a means other than visual.
Gonio means angle, and a photometer is an instrument that
measures light so a goniophotometer
is an instrument for measuring the angular distribution of reflected or
transmitted light. This type of instrument is used regularly in the
research laboratory to investigate the geometric distribution of light
flux (fig 3). This instrument can be also used to provide gloss data
for a wide variety of angles and apertures. When goniophotometric
measurement are desired, reference should be made to ASTM Recommended
Practice E 166, Goniophotometry of Transmitting objects and Materials,
and Recommended Practice E 167, Goniophotometry of reflecting objects
and Materials. Analysis of goniophotometric curves was treated by
Goniophotometry is not new. A goniophotometer was devised
by Bouguer in an attempt to corroborate Lambert s (consine) law for
uniform diffusers by experimental evidence. A number of
goniophotometers have been described since this earliest known
instrument. Only a few will be mentioned here. McNicholas described a
sophisticated visual goniophotometer with provision for diffuse or
unidirectional illumination and Multiplan view. Wet laufer and Scott
described a photoelectric goniophotometer with rectangular source and
receiver apertures however they
receiver apertures, whereas for replication of data the separate
dimensions are required.
Two types of goniophotometers are currently available
commercially. In the Gardner goniophotometer the receptor direction is
fixed, and the source and the specimen angles are set manually as
desired. In the Hunterlab instrument the geometry is selected speeds,
and the receptor signal is plotted on rectangular coordinates as a
function of angle.
All current specular glossmeters are photoelectric
instruments. They embody the geometric requirements set forth in the
various test methods. In the paint industry the geometeries of 20, 60,
and 85 deg are used exclusively. The geometeries are designated by the
angle of incidence, but there are also receptor aperture changes for
each geometry. Some materials are measured regularly for gloss with
geometeries different from those used for paint, such as 45 deg for
ceramic material, 75 deg for paper. A good way to evaluate an
instrument is to use it in a particular application.
Instruments for gloss measurement are made in various
degrees of complexity and configuration. One class of instruments is
known as portable, by which is meant that the instrument in this class
is battery operated so that they can be used where the usual electric
power line is not available. Some instruments have a self contained
photometer and readout device (microammeter), notably the portable
Other instruments are constructed with the optical unit
separate from the photometric unit. This type of construction permits
use of a common photometric unit to read out the signals from a number
of optical units that can be either plugged into the photometric unit
is provided with a switch so that the operator can change easily from
one optical unit to another. Some instruments have been constructed
with several optical units built into a single housing, thereby
permitting a change from one geometry to another by the flick of a
switch. This multiangle design also has the advantage of eliminating
the connecting cables that are required when separate optical units are
used with a common photometric unit.
Instrument of different manufactures vary in their
complexity, precision of measurement, and cost. schreckendgust
and Gowing described a high speed, direct reading. Digital readout is
convenient, rapid and less likely to be recorded incorrectly, but it is
also more costly. However, the saving in operator time may offset the
increase in instrument cost.
Pellegrini patented a visual instrument, which was later
improved by Harrison. This instrument has been used widely in the
porcelain enamel industry and is also useful for evaluating the image
quality of paint specimens.
Special specular gloss methods have been developed for a
number of different materials such as ceramics, paper, plastics and the
like however. The discussion here will be confined to paint methods.
A method for classifying paints according to gloss was
developed originally by Hunter and Judd and this same year the first
edition of ASTM Method D 523, Test for specular Gloss, was published.
This method originally contained only the 60 deg geometry for general
classification, subsequently it was expanded to include the 20 deg
geometry for better differentiation of high gloss specimens and the 85
deg geometry for sheen or low gloss evaluation.
The ASTM method has been adopted by the America National
standards Institute (ANSI) and the basic requirements of apertures and
angles have been incorporated into Federal Methods and into an
Australian method. They are being currently considered for use by the
International standards organization (ISO). Other geometries have been
adopted by some other countries,
notably Great Britain, where a 45 deg geometry designated as British
standard (BS) 3900 has been used for high gloss evaluation.
It has long been recognized that a measurement of specular
gloss at one angle of incidence and for one receptor was insufficient
to characterize the appearance of a surface. Yet the recording of a
goniophotometric curve takes considerable time and equipment and may
provide more data than are needed. The logical compromise is to make
gloss measurements at two angles of incidence or for two different
apertures or a combination of both and combine the result in some form
to provide a two parameter index. A procedure of this type was
developed by Nimeroff originally to provide a more satisfactory
evaluation of the gloss of clear finishes over wood.
Images gloss measurement by objective means has not
provided sufficient reproducibility to warrant publication of a method.
The only published method is ASTM C 540 67, Test for Image Gloss of
porcelain Enamel surfaces, a visual method. Although the method was
developed for porcelain it can be applied to organic enamels.
The general requirements for gloss standards are similar
to those for color standards namely, the standard should be uniform
over its surface, should have permanence, and should not be damaged
easily in use. In addition gloss standards should be quite planar and
should have surfaces that provide reflected
light flux distributions closely duplicating those of paint specimens.
Vitreous or gloss like
materials is preferred for use as standards because of their
permanence. Polished black gloss is used as the primary gloss standard
because its specular reflectance can be computed from its index of
refraction. Liquid surfaces of different indexes of refraction were
proposed by Moore and Hunter. ceramic
tiles of the 4 ½ in. (11cm) square type commonly used for bathroom
walls have been used in large quantities. The main difficulty is to
obtain tiles that are reasonable flat. For bathroom walls a convex
surface is preferred. Specimen of ground sand blasted and acid etched
glass have been used as standard. The first two frequently have flux
distribution that depart markedly from paint specimen having the same
glossmeter readings an acid etched glass surface is particularly good
for providing high gloss standard of high planarity, but it is
difficult to obtain with high uniformity, in fact it is difficult to
obtain with high all because the glass must be etched with a hot
combination of concentrated hydrofluoric and sulfuric acids.
Porcelain enamel on steel can be also made in a wide range
of gloss. Such standards are less fragile than ceramic tile, and there
seems to be little difficulty in obtaining uniform and relatively plane
The upper portion of each gloss scale is set by using
polished black glass standards whose specular gloss has been computed
from its index of refraction by using the Fresnnel snell equation. A
factor may be introduced to provide the desired scale value. All other
gloss standards are calibrated relative to a polished black glass
primary standard by measuring them on a glossmeter whose apertures and
angles conform accurately to the requirements of the published method.
MASS COLOR AND TINTING STRENGTH
The color, when viewed by reflected light, of a pigment
vehicle mixture of such thickness as to obscure completely the
background is called the mass color (MC) of the pigment. Sometimes this
is called mass tone, over tone, or self colors. MC encompasses
lightness, hue, and saturation. Although generally applied to white
The power of a pigment to color a standard paint or
pigment is called the tinting strength (TS) of a pigment. When applied
to white pigments, Haslam suggested a better term would be lighting
power or brightening power, but this is not ordinarily done. Sometimes,
for white pigments, the definition for TS is given as the ability to
resist discoloration by colorants.
Factors Producing MC and TS
The TS of pigments is often used as a guide for estimating
hiding power. Consequently, it is not surprising to find that the same
basic factors given for hiding power apply for MC and TS. Thus the
refractive Index of the pigment is the primary factor followed by its
particle size and its degree of dispersion in the vehicle. For colored
pigments, the inherent light absorbing ability is also a primary factor.
Pigment and Vehicle
TS determinations involve through mixing of definite
quantities of white pigment and colored pigment. The preliminary mixing
is sometimes done dry, after which vehicle is added, but it is probably
better practice to first prepare separate pastes in the vehicle, and
then to mix the pastes. Thus, for colored pigments, MC is determined
first after which tinting strength is determined with the paste
prepared for the MC test. For white pigment lampblack or ultramarine
blue is the usual tinting pigments, white for colored pigment, it is
The mixing of pigment
and vehicle may be carried out by a number of methods as will be
described for all methods great
care must be exercised to avoid contamination of one color with another.
and Muller Methods
Ayers fond that a muller is more reliable than a spatula
and that the texture of the rubbing surface may vary considerable
without affecting the accuracy of the test.
Stutz in a study of the TS of white pigments also found a
muller to be better than a spatula but that a weighted muller is no
better than an unweighted one, as shown in table 1. In the rubbing
operation, the muller should travel over areas about 3 in. wide and 12
in. long, being pushed up one side and pulled down the other side of
the strip so that all color particles receive the same amount of
rubbing. By one rub is meant one complete up and down circuit.
The number of rubs depends upon the type of pigment and
actual factory grinding operation. For example, Ayers found that
excessive mulling changes the MC and TS of many iron oxide pigments and
some chemical pigments. Table 2 shows relations between different methods
of paste preparation obtained by stutz on TS of zinc oxide and basic
carbonate of lead.
In the Hoover Automatic Muller the mixture of pigment and
oil is rubbed between two glass disks, one stationary the other rotated
by a motor. Since the rotation is about the centers of the disks, paste
at the center receives less mulling than paste near the edge. To
minimize this it is helpful to spread the paste in a ring halfway
between center and edge. This method of paste preparations is becoming
favored over either type of hand mulling.
small three roller mill that has been found useful for grinding small
laboratory batches of paint. The rolls of this mill are 4 in. in
diameter by 8 in. in length. Batches as small as 5 g have been prepared.
The pall glass Mill was designed for preparing small
quantities of pigment pastes without contamination by metal spatulas or
metal rolls. It consists of a ground glass stopper and the female
section of a heavy glass joint.
The mixed but unground ink is placed in the female section
and the plunger inserted. The male section is rotated by a small motor
at about 150 rpm. The grinding pressure ranges from 20 to 30 psi.
Quantities of 1 to 8 g can be handled.
The mill is said to be superior to hand milling by saving
time and by greater development of TS it
is superior to roller mill grinding in freedom from contamination
although not as efficient in dispersing very hard grinding
pigments. No comparison with automatic mullers is available.
Consistency of the paste influences the efficiency of the
rubbing procedure and consequently the degree to which the color is
developed. In a study of the relationship between plant grinding and
laboratory testing on the color of iron oxides, Ayers showed that the
higher the consistency of the paste, the more rapid is the development
of color. Above a certain point, increase of consistency had no
appreciable effect. At the point of maximum reflectance, 700nm (fig 1),
the low consistency paste had a reflectance of 26 percent, while the
high consistency paste had a reflectance of 23 percent.
Stutz gives the relationship found in table 3 between the
amount of oil used during rubbing and the tinting strength of a sample
of zinc sulfide. Note that the final amount of oil is the same in all
cases, and that the thickest paste is 4 percent greater in strength
than the thinnest one.
To test at equal consistency, preliminary tests would have
to be made. However, for convenience in routine work, definite weights
of pigment and vehicle may be used for each type of pigment. The
weights vary from, for example, 0.3 g for carbon and lampblack to 2.0 g
for chromium oxide and white pigments. The proper amount of vehicle
depends upon the oil absorption of the pigment and the type of vehicle.
Refined linseed oil is generally acceptable. Experience has shown,
however that for best correlation with factory findings, better results
are obtained if the dispersion is carried out in the same vehicle or at
least one more comparable to factory practice. For example, polymerized
linseed oil or solvent free alkyds have been used.
After rubbing is done, apply liberal portions of each
paste (standard and sample) side by side, on glass, tinplate or
lacquered chart. Draw a wide spatula, French scraper, or doctor blade
over both daubs at least about 1 by 2 in. edges should
touch but not overlap. Judge colors immediately. If panel is glass,
view colors from above, not through the panel.
Venuto designed a scraper (fig 2) for leveling surfaces of
rubouts. A spring steel blade is attached to a rigid steel plate which
in turn is fastened to a table. Adjustable sides are attached to the
plate to guide the panel along the flexible blade whose tension is
regulated by an adjusting screw S. The panel with the rubouts facing
down ward is placed against the upper portion of the guides and then
drawn over the spring steel wiping blade.
One of the properties of oil color that affects its
suitability is the speed with which it may be dispersed into white
paints. A method for measuring this speed has been directed by Paul and
Diehlmen. It consists in mechanically rotating a bottle containing a
white paint, the oil color, and No. 11 lead shot. The bottle is changed
with 550g of shot, 2 ml of the oil color, and 75 ml of white paint. It
is then closed with a cork (concave in the inner end to match glass end
of the bottle) and placed in the holder of the rotating machine.
Immediately after charging the test is run. The tube is
first tilted and rotated slightly by hand to wet the walls with the
white paint. The mechanical rotation is continued until no
streaking is noted. The time in seconds for mixing is taken with a stop
watch. Results on a number of samples by one operator are shown in
The first methods developed for mass color measurement,
and those requiring the least equipment involve subjective, visual
comparisons instrumental methods
will be considered later. Thus, for routine examination, it is
generally sufficient to determine, visually, if a sample is lighter or
darker, redder or bluer, etc., than the standard.
ASTM Method D 387, Mass color and Tinting Strength of
color pigments, is based on the experience reported in the foregoing.
Two methods of rubbing are provided (1) slab and Muller and (2) Hoover
Muller. Appropriate quantities of pigment and oil are given in Table 5.
otherwise specified, the oil is No. 2 transparent lithographic varnish.
Other methods for MC of colored pigments are in general
agreement with the ASTM method. The National printing Ink Research
Institute (NPIRI). Method E 1 specifies No 1 lithographic oil for most
pigment to oil for a few. Ratios of pigment to oil are about
the same as those in Table 5. The Technical Association of the pulp and
paper Industry (TAPPI) method is limited to inorganic pigments and
pastes it includes procedures for
using a casein vehicle for water dispersible pigments and also for
extending the color pigment with clay.
color of white Pigments
The criteria of quality for the MC of white pigments are
the lightness (that is Y value in terms of colorimetery sometimes
called brightness) and tone (hue) of the pigment paste. All assessment
are generally given as lighter or darker, and bluer or yellower than
standard. Although the color dry powder has some finance in the paper
trade, it does provide a realistic evaluation of the color obtained by
dispersing the pigment into a vehicle. Like other optical properties, Mc is
influenced by the color of the medium pigment concentration, and degree
of the pigment dispersion
One MC color test for all grades of titanium dioxide
pigments uses a 50 50 mixture of
refined linseed oil and soybean oil. Four grams of standard pigment are
transferred to the bottom plate of a hoover
muller, and 2.4 ml of color oil is added from a syringe. After a
preliminary mix with a spatula the paste is mulled 25 revolutions and
transferred to a glass panel or a cardboard panel. The pastes are then
drawn down in juxtaposition with a 0.015 in. applicator blade. The
sample is rated against the standard in north daylight or a color
Tinting strength Methods
The sample is visually rated as lighter as or darker than
the standard. Identify the standard should have the same tone (under
tone, hue) as the sample or else the visual estimate of lightness is
affected, if a numerical estimate of TS is required, the relative
proportions of white and colored pastes are varied for one of the
pastes (either standard or sample, depending upon the particular method) until the lightness of
the two pastes match.
Method for colored pigments
The ASTM method for TS of colored pigments is combined
with the method for MC. In addition to pastes prepared for
determination of MC, it requires a reduction paste. If prepared in
advance, the paste may be kept in collapsible tubes. There are three
alternative pastes the most frequently used one is prepared by grinding
100 lb green or white seal zinc oxide in 25 lb of No.2 transparent
Weigh small amounts of sample paste and standard paste on
counterbalanced watch glass. Add an appropriate amount of reduction
paste as given in Table 5. transfer
pastes to separate sheets of glass or other impervious substrates and
mix thoroughly with spatulas. Apply sample and standard in side by side
position as directed for MC. If the colors are not the same repeat
using smaller amounts of the stronger paste (sample or standard) until
reduced pastes match. The amount of reduction paste is held constant.
VISCOSITY AND CONSISTENCY
Viscosity or consistency is an extremely significant
property that is included routinely in specifications and is an
important factor in the purchase and use of not only formulated paint
products but also the countless raw materials used in the industry.
The successful formulation of a surface coating or paint
requires that it be able to be satisfactorily applied to the object to
be protected or decorated in a prescribed or predetermined manner or
both. It is necessary, therefore, that the paint be formulated to have
a proper level of a property that may be designated by any of several
terms consistency, body, flowability, viscosity that will allow it to
be handled and applied as intended. Measurement of this property is
carried out usually with anyone of a variety of different instruments
developed for the purpose of measuring it either accurately,
approximately, or practically. Also since this property, which is
basically its viscosity or its ability to flow, influences certain
performance properties such as leveling or flowout, resistance to
sagging, brushability, etc., some instruments have been devised with
the intent of measuring these particular performance properties
As the paint industry has matured over the past three or
four decades and its technology is gradually becoming less of an art
and more of a science, researchers have come to realize that a single
viscosity determination on a paint is not sufficient. A series of
viscosity determinations, a viscosity profile, is required to give a
complete picture of rheological properties of the material. Instruments
capable of making these more sophisticated measurements have been
developed and have become available. As a result many of the
subjectively evaluated properties, the attainment of which was not
always accompli shed and hence considered part of the art
are now attained quickly, measured
precisely, explained more easily, and then altered at will.
The subject of rheology, which includes viscosity and
consistency measurement, is broad, and the reader is urged to consult
any of the many more recent references cited throughout this section
for a more comprehensive treatment of the subject.
A subjective term descriptive of the flow properties of a
liquid usually measured in an empirical manner and in arbitrary units.
Often used as a synonym for viscosity.
Any liquid that does not satisfy the requirements for a
Newtonain liquid. A liquid that displays plastic, pseudoplastic, or
dilatant flow characteristics. Most paints are non Newtonian liquids.
A material displaying pseudoplastic flow has no yield
value, and the curve of the plot of shear stress versus shear rate is
nonlinear with the shear rate increasing faster than the shear stress.
A type of flow characterized by an increase in viscosity
as shear stress is increased. The curve of the plot of shear stress
versus shear rate is nonlinear with the shear stress increasing faster
than the shear rate.
Thixotropic materials are those whose consistency depends
on the duration of shear as well as on the rate of shear. Thixotropy is
a reversible process, and after resting the structure of the material
builds up again gradually. The thixotropy or thixotropic material meant
when the terms are used in the paint industry is generally a
thixotropic Newtonian or a thixotropic pseudoplastic material.
A term used to distinguish types of thixotropic behavior
in materials exhibiting plastic now (Bingham bodies).
Countless instruments are available for measurement of the
rheological properties of liquids. Many of these instruments have been
accepted by and found considerable use in the paint industry. A few of
these are described or noted in this chapter.
Broadly one may consider viscometers to fall into two
categories those that are capable of a fundamental measurement of the
property and those that are approximate to varying degrees, or
arbitrary. In the fundamental group, Patton includes capillary (tube
and orifice), concentric cylinders (rotary motion and axial motion),
falling sphere, band, and cone and plate viscometers. In the
approximate or arbitrary group he includes bubble, rotating disk,
orifice (single and multiple), and rotating paddle. This classification
and grouping is somewhat similar to classification into the following
groups by Weltman those that express consistency in arbitrary terms,
those that can represent flow conditions mathematically for Newtonian
and liquids that can be tested at low shear and are not applicable to
non Newtonian materials, and those that can measure consistency under
controllable flow conditions and establish accurate relationship
between shearing force and rate of shear. Myers groups viscometers
according to the method of imposing shear such as efflux, viscous drag
(falling weight), rotational, telescopic flow, and transverse flow. Le
Sota classifies instruments as capillary, efflux, bubble, rotational,
and perforated disk. Rotational is further divided into cup, bob,
paddle, disk, and cone and plate. Westgate also categories instruments
in essentially the same manner. Karam lists viscometers as capillary,
falling sphere, rotational, vibrational, and compression or extension
In the following paragraphs, instruments are grouped by
simple physical type capillary, rotational, falling ball, band, efflux,
bubble, and orifice. The overall listing goes from fundamental to
approximate however, within anyone class of instrument there may be
also a range between fundamental and approximate.
This is an early capillary viscometer (Fig. 1) developed
for the purpose of determining the viscosity of the low viscosity types
of nitrocellulose (that is, 18 25 centipoise and 30 35 centipoise
types) where the standard solution viscosity was too low to be
determined by the Hercules Falling Ball method. The instrument is
calibrated by the user with oil having a viscosity in the range in
which the instrument will be used. For nitrocellulose solutions, castor
oil having an absolute viscosity of about 665 centipoise was used as
the calibrating fluid. To use the instrument, tilt it so that the
reservoir is vertical. Fill to the etched line with the sample being
tested, allowing time for the material to reach an equilibrium level in
the capillary. Then, holding a finger over the other capillary end,
lower the instrument to the original horizontal position. Remove finger
and determine the time for the material to travel from the first mark
on the capillary to a second mark suitably located further down the capillary. The
second mark (or marks) is determined in calibrating the viscometer.
The instruments rely on gravity to move the liquid through
the capillary. Instruments were also developed that used either
pressure or vacuum to move the liquid through the capillary. One such
instrument is the Bingham Green Plastometer (Fig. 2). Many early
studies of consistency were made with this apparatus. Because it
combines air under pressure and capillary tubes of various sizes,
almost any substance that exhibits now may be run in it. This apparatus
was used primarily in research work, being too costly and time
consuming for general routine work. It consists of a container for the
paint, at the bottom of which a carefully calibrated glass capillary
tube may be attached, Air pressure is used to force the paint through
the capillary into a receiver. Rotational devices have largely
displaced it today.
This instrument was devised for measuring the rheological
properties of plastisols. Basically, the instrument utilizes air or gas
pressure to force the test material through an orifice. The time
required for a given weight or volume in poises is calculated from
basic equations. Apparent Viscosity of Plastisols at High Shear Rates
contains a recommended procedure for determinations using this
Pressurized Flow Cup
While this device may be looked upon as a simplified
plastometer, the dimensions of its orifice placed it more in the
category of a pressurized simple efflux cup rather than in the class of
the capillary instrument. It comprised a cylinder, inside diameter
about 2 in., length 1.7 in., with a conical bottom terminating in a
detachable orifice, and a cover with fittings to admit air under
pressure. Three different sizes of orifice were provided.
The capillary viscometers are efflux type instruments.
However, the term efflux instrument or efflux viscometer has come to
mean a cup type instrument containing an orifice. Whatever precision
and accuracy an efflux instrument may have is essentially dependent
upon the dimensions of the orifice. The closer an orifice resembles a
capillary, the more accurate is the instrument. Theoretically, the
behavior of the Newtonian liquid in an efflux viscometer can be
expressed by the last equation.
Normally, however, the viscosities obtained with an efflux
viscometer or viscosity cup are expressed in time units such as
seconds, No.4 Ford Cup, have a limited useful viscosity range, and are
not precise. However, they are practical instruments, easy to use, and
easy to clean, features that account for their widespread popularity.
One of the more accurate cup type instruments, the Saybolt
viscometer utilizes two capillary type orifices to measure viscosity in
terms of Universal seconds or Furol seconds. The time of measurement
for this instrument is that required for 60 cm3 of
liquid to pass through the orifice. The Saybolt viscometer instrument
has been adopted by organizations, such as the National Bureau of
Standards, American Petroleum Institute, and American Society for
Testing and Materials. The procedure may be summarized as follows. Pour
some of the sample through the outlet tube and insert cork stopper into
air chamber below tube, sufficiently tight to prevent escape of air,
but not to touch tube. Heat material in separate container to slightly
above 25 C (or specified temperature), stirring with thermometer. Pour
material into viscometer until its fills gutter. Maintain temperature
for one minute, remove thermo meter, pipet material from gutter until
level in gutter is below level in viscometer proper. Place receiver (60
below viscometer so that stream of material from outlet tube will
strike neck of receiver, thus avoiding foaming. Snap cork stopper from
its position and simultaneously start a stopwatch. Stir bath liquid
continually during test and maintain temperature. Stop the watch when
bottom of meniscus of material reaches mark on neck of receiver. Time
in seconds is Saybolt Universal Viscosity.
The creation of more surfaces, as in the atomization of a
liquid, requires an investment of energy. Part of this energy goes into
the surface as free energy and is supplied by the mechanical agent
creating the surface. The rest is contributed by the environment,
generally as the heat influx required to maintain the experiment at its
initial temperature, since surface formation causes a temperature drop.
If neither of the two phases is a gas or vapor, the
boundary between them is an interface. However, the terms surface and
interface are used interchangeably.
The superficial similarity between blowing a soap bubble
and inflating a balloon, which suggests the existence of a skin under
tension, led early investigators to the assumption of a surface
tension. But surface effects can be
correlated more readily in terms of the thermodynamics of interfacial
energetics. It is easily shown that the numerical value of the
hypothetical surface tension of a liquid is identical with its very
real free surface energy. Surface tensions are expressed in dynes per
centimeter and free surface energy in ergs per square centimeter.
Coatings technologists deal with interfaces of all classes
gas liquid as in an aerosol spray liquid liquid, as in an emulsion gas
solid, as in a dry pigment before its immersion in a vehicle liquid
solid, as in a pigment dispersion and solid solid, as when the crystal
faces of two different pigment particles are in tight contact. Paint
scientists are particularly interested in the formation of liquid solid
interfaces that are stable in the package, that is, in the permanent
replacement of the air at the air solid interface of the pigment by the
vehicle to give the liquid solid interface of the dispersion.
Wetting is not merely the submersion of the pigment in the
vehicle. If it were, any pigment forcibly submerged in any liquid would
be considered as wetted by the liquid, and wetting would lose all
discriminatory meaning since it would be universal. Wetting is the
unforced, spontaneous spreading of a liquid over a solid, or a second
liquid, and is synonymous with a decrease in free interfacial energy.
For each 1 cm2 of
the solid surface that becomes wet, 1 cm2 of
air liquid surface and 1 cm2 of
liquid solid interface is formed and 1 cm2 of
air solid surface disappears. The change in the free interfacial energy
balance sheet is called the spreading coefficient, S
air solid and liquid solid interfaces are of the utmost importance to
coatings technologists, the corresponding free energies cannot be
measured directly because existing methods require that the interface
be mobile, as between two liquids, or a liquid in a gas, or a liquid in
a vacuum saturated with its own vapor, and not rigid, as when one of
the two phases is a solid. Hence, the scope of surface and interfacial
tension measurements is limited. Nevertheless, much can be deduced from
related determinations for example, observation whether a liquid
spreads over a solid in air or in a second liquid and, if not, what the
magnitude of the resulting finite contact angle is.
ranges of common organic liquids are not broad. At room temperatures,
the saturated aliphatic hydro carbons extend from 18 to 28 dynes per cm
the saturated cyclic hydrocarbons, 22 to 28 the aromatic hydrocarbons,
28 to 30 the alcohols, ketones, and esters, 21 to 28 the fatty acids,
25 to 38 other oxygenated aliphatic liquids, 17 to 28 the aromatic
amines, 33 to 43 and practically all the halogenated liquids, 21 to 35.
The cause of this narrowness of range is not far to seek. Molecules
orient themselves in the surface of a liquid in the general direction
that leaves the least possible free surface energy under the
circumstances. For organic compounds, this means that the non polar
ends of the molecules at the surface point toward the air phase, while
the polar ends, if any, point into the liquid. Thus, organic liquids
expose a hydrocarbon exterior, and their surface tensions are those of
hydrocarbons somewhat elevated by the influence of any polar groups
Water can expose
only polar groups therefore, it shows a higher surface tension, around
70 dynes per cm at laboratory temperatures. However in an aqueous
solution of an organic compound, the solute molecules are able to
decrease the free surface energy by collecting in the surface, oriented
with their nonpolar groups out, even if this means depleting the
solution of much of the solute in the case of a very dilute solution.
Aqueous solutions of organics have surface tensions intermediate
between that of water and those of the organics. The slightest
contamination drops the surface tension of the water. The surface
tension of the aqueous phase of paints is about half of that of pure
On the other hand,
molten salts show very high surface tensions. Aqueous solutions of
strong electrolytes reveal surface tensions somewhat higher than that
of pure water. In these solutions, the solute leaves the surface since
this desorption drops the free surface energy. But there is some
electrolyte sufficiently dose to the surface to give a net effect above
that of pure water.
Thus, the surface
and interfacial tensions of pure liquids have limited significance in
the coatings industry. The liquids in which the paint chemist is
interested are, first, the liquids as supplied and, second, the liquids
with their interfacial characteristics modified after they have become
saturated by the other ingredients of a coating.
The extent to
which the dass of otganic derivatives known as surface active agents
lowers the surface tension of water and, to an even greater degree,
depresses the interfacial tension of a mineral oil emulsion is
summarized graphically in Figs. 1 and 2.
tension of organic liquids against water, mutually saturated, ranges
from below 10 to over 50 dynes per cm.
methods used to measure surface and interfacial tensions may be classed
either as absolute or empirical. The readings made by an absolute
method may be converted directly into surface tension values. An
example is the capillary rise method. An empirical method must first be
standardized by a series of measurements with liquids of currently
known surface tension, after which the previously unknown surface
tension of additional liquids may be determined. Examples are the ring
method and the drop weight method. Many empirical methods have been
developed. As more becomes under stood about the fundamental nature of
matter, theoretical developments will convert empirical into absolute
methods by supplying rational duplications of the correction curves
used with empirical approaches.
representation of the arrangement is shown in Fig. 3. This method is
obviously direct since one need only to determine r, d, and h and know
g to calculate. However, to obtain the accuracy inherent in this
method, several factors, in addition to those common to other physical
measurements, must be considered. As with all surface tension
determinations, contamination must be scrupulously avoided as the
slightest impurities may concentrate in the surface and greatly affect
the results. The vessel in which the bulk liquid is held must be
wide or else a correction must be introduced for the minor capillary
effect caused by the vessel the wider the vessel in proportion to the
capillary, the smaller the correction. The meniscus in the narrow tube
is a part of the capillary rise, and the true value of h is the rise to
the bottom of the meniscus plus an allowance for the liquid in the
meniscus calculated with sufficient accuracy by assuming the meniscus
to be hemispherical. The density, d, is actually not quite that of the
liquid but the difference in densities between the liquid and the gas
or vapor above it. Most important, the liquid must wet the capillary
that is spread spontaneously to its very top, so that the column of
liquid is suspended not from the dry material of which the capillary is
made but from a very thin layer of the liquid spread as a so called
duplex film, not as a monolayer, over it. If the liquid does not wet
the capillary wall, it shows a contact angle q at
the junction which will diminish h because only the vertical component
of the surface tension is effective in raising the column of liquid.
The capillary rise
method is not confined to surfaces between a liquid and a gas or vapor.
two phases involved may both be liquids, with the liquid of lower
density replacing the gas as the supernatant phase. The experimental
ingenuity required has been recorded in the literature.
While overtly a
simple procedure, capillary rise, except for meticulous research
investigations, does not rank as a practical method for the
determination of interfacial energies in the coatings industry.
Bubble Pressure Method
a static bubble held at the tip of a capillary, the dimensions of the
bubble, the pressure within it, and the surface tension of the liquid are
interrelated. Properly contrived measurements, therefore, may permit
the calculation of the surface tension in an absolute manner. However,
the procedures may be simplified by slowly increasing the size of the
bubble until it breaks away from the tip and measuring only the
pressure maximum required to achieve this rupture.
drop weight method (Fig. 4) centers around a vertical tube conveniently
about 7 mm in diameter with a narrow capillary along its axis. The
bottom of the tube is ground flat, perpendicular to the axis. The
liquid is permitted to flow very slowly down the capillary from an
upper reservoir and to spread along the circular tip of the tube, from
which it hangs. As the drop is allowed to enlarge, it ultimately
becomes infinitesimally too heavy to continue to be supported by the
tip and falls. For greater accuracy, a number of drops are collected
under the same carefully controlled conditions,
weighed, and the average weight of a drop is calculated.
factor has been painstakingly determined empirically and relates to
the fact that, once the drop has just grown to the size where it is
destined to fall, only a specific fraction of it is severed, that
fraction being governed only by the dimensionless ratio of the radius
of the tip r to the cube root of the drop volume.
The drop weight method is thus not an absolute method, since it must be
standardized by establishing the values of through Eq 5 for many
liquids and solutions of known r with tips
covering a range of r values. The simple apparatus
involved is easy to construct. Obviously, the method is adaptable to
interfacial tension measurements, since the drops may be allowed to
fall from an immersed tip through a second, lighter, immiscible liquid
or rise up through a second heavier liquid from an inverted tip. In
such interfacial studies, m of Eq 5 is not the weight of the drop in
air or vapor but the differential weight which takes into account thc
buoyancy of the second liquid. Particularly in interfacial experiments
it is the drop volume rather than the weight that is measured and which
can be converted into the weight by knowledge of the difference in
densities between the two fluid phases. Simple as the method is, it has
been over simplified with a sacrifice in accuracy by adaptions which,
for example, count drops from relatively primitive tips for a
predetermined volume of the liquid under study.
the most widely employed method for measuring surface tension is the
ring method. It is based on the ideal concept that if a submerged thin
horizontal wire 1 cm long is to be pulled up through a surface, a force
of 2 r dynes must be
on it against the surface forces, the factor 2 appearing because there
is an interface on each of the two sides of the thin film. To avoid
corrections for the effect of the wire ends, the wire may be bent into
a flat ring, in which case the force to pull it up through the surface
is ideally 4 Rr where R
is the radius of the ring in centimeters. For this method to apply the
liquid must wet and spread over the wire. However, the geometry of the
attachment of the film to the ring is not ideal. Consequently, a
correction factor p
enters the consideration as a measure of the extent to which the system
departs from the ideal.
method may be applied to the measurement of interfacial tension, in
which case the composition of the ring must be such that it is
preferentially wet by one of the liquids and not by the other. If the
wetting liquid is the denser, the ring is pulled up from it into the
other. If the wetting liquid is the lighter, the ring is pulled down
from it into the lower liquid. Instruments are available from several
commercial sources for the practice of the ringless method.
interfaces occur in a structure to an extent
great enough to influence its physical behavior, interfacial energy or
its equivalent, interfacial tension, appears as a variable in the
equations governing some aspects of the system. Each such equation,
therefore, may be adapted into a method for determining interfacial
tension provided the other variables are readily controlled and are
measurable with sufficient accuracy. A number of other methods have
been proposed on this basis for measuring interfacial tension in
addition to those described above. For example, the shape parameters of
a hanging drop have been so used, and also those of a jet of liquid
passing through an elliptical orifice, which imparts an oscillation to
the stream. However, except for special applications, such methods are
has been pointed up that since the paint industry deals with stable
mixtures of powders and liquids as articles of commerce, liquids that
wet the surfaces of the powders involved are of paramount interest.
Nevertheless, situations arise in coatings technology where wetting
does not occur. Examples are the specific pairing of a liquid and a
powder, and using a particular liquid over an intended substrate. In
such instances of nonspreading, a finite contact angle is involved, and
occasions arise where it is desirable to measure it. As in all
measurements concerned with interfacial energy, the greatest precaution
must be taken to ensure that the surface used is actually the one of
interest, or that all contaminants are avoided. Thus, the magnitude of
the angle of contact shown by a small drop of a highly pure non
spreading liquid, resting on the clean horizontal facet of a large
crystal of rutile has nothing to do with the practical realities in a
sample of paint in which the surface of commercial titanium dioxide pigment is
anything but pure titanium dioxide and the liquids used in coatings are
anything but pure single chemical species.
PARTICLE SIZE MEASUREMENT
coatings and similar products, the criteria for best performance of
particulate ingredients inorganic, organic, extender, and metallic
flake pigments, and the dispersed phase of latexes depends in large
measure on the size and shape of the particles composing the
particulate materials. Examples of these relationships appear in other
parts of this book. The importance of particle size measurement
including the amount of coarse oversize particles is evident. In this
book it is practical to cover in detail the many procedures employed in
the measurements, but the brief descriptions are supported by
references to the literature. Also cited are textbooks for the reader
who desires to acquaint himself with general theory. The recent book by
Allen can be particularly helpful.
size distribution, and shape of pigment particles greatly influence the
optical and other physical properties of paints. Relatively large
particles may protrude through the surface of the film and detract from
its gloss. But extreme fineness is not always desirable. There is an
optimum size below and above which hiding power decreases.
Eide showed that coarse acicular zinc oxide imparted
greater durability to a paint than did round zinc oxide. Morris and
Nelson found evidence that acicular asbestine of a wide distribution of
sizes favorably influenced the durability of paints. A review by
Jacobsen cites many examples of the significance of particle size and
shape of pigments and extenders on the optical and physico chemical
properties of coating systems.
good generalization of the relationship between particle dimensions and
film properties is shown in Fig. 2
paint, Varnish, and Lacquer Association has published a Pigment Index
that gives available information of the particle size of pigments on
the American market.
size and shape of metallic flake pigments are also important. The
different tone effects in polychrome finishes are obtained by using
different sizes of non leafing aluminium flakes. The covering area of
leafing pigments is a function of size and thickness. For nonleafing
flakes, microscopical and surface area measurements are used for
leafing pigments, a method based on the Langmuir balance principle is
particle size of the dispersed phase of emulsions (latexes) influences
the performance of paints made with these materials. Martens show with
graphs how the many parameters of paint performance are influenced by
the particle size of the dispersed polymeric materials. Brown considers
the particle size of the polymer as one parameter involved in the
coalescence of the particles, and Kreider connects rheological
properties with particle size. Extensive investigations into light
scattering reflect the great interest in influence of particle size.
Methods for the determination of the particle size of
emulsions are numerous. Bradford and Vanderhoft cite references
concerned with the following techniques light scattering,
ultracentrifuge, soap titration, low angle X ray scattering and
microscopy. Except for soap titration, these methods also apply to
are commonly expressed in terms of frequency against size, or weight
percent against size, that is, the number of particles in each of the
various size fractions, or the weight percent of the fractions that
compose the specimen. Another way of representing the data is by
average diameter, of which there are several kinds. The arithmetic mean
diameter is commonly used. Others, with their symbols and mathematical
definitions, are given in Table.1
particle size ctiaracteristics, Reporting Particle Size characteristics
of Pigments, offers three parameters size, coarseness, and dispersion.
These were selected because it appeared that they could be derived from
data obtainable by most analytical methods.
The size parameter
(SSD) is the specific surface diameter the diameter in micrometers that
the particle would have if they were particles of uniform size. It is
also known as the surface mean diameter.
Size with Light Microscope
Above about 0.2 m, the microscope
is a basic tool for determining particle size. The lower limit is set
by its resolving power and the upper limit by its depth of focus. The
microscope is especially useful for measurement of platelike and needle
shape particles that do not obey Stokes law on which the sedimentation
methods are based. Disadvantages of the method are that it is slow and
laborious. Hence, it is used chiefly for the calibration of the more
rapid relative methods. Microscopy and in ASTM Recommended Practice E
20, Analysis by Microscopical Methods far Particle Size Distribution of
Particulate Substances of Subsieve Sizes, and to a paper by Loveland.
Following are same general comments.
Green was the first investigator to systematize the
microscopical method. He dispersed the pigment in a medium on a
microscope slide, photographed the dispersion at a known magnification,
and projected the image on a screen so as to increase the magnification
for measurement. Dunn bypassed the photomicrograph by projecting the
image of the particles directly from the slide on the screen.
Ideally, the pigment should be dispersed in the medium in
which it is used, but this is rarely done. Green used turpentine, Allen
recommended a viscous vehicle, and Eide used fused resins. Gehman and
Morris milled the pigment in rubber, dissolved the mix in a solvent,
and applied the suspension to the slide.
For some purposes, comparison under the microscope with
linear scales, or with circles or ellipses in graduated sizes, may be
advantageous. The comparison scale may be a micrometer eyepiece, an
eyepiece reticle, or a scale engraved on the micro scope slide. For an
approximate comparison of coarse particles, Gardner used the grooves of
The technique of dark field illumination uses the
detecting power of the microscope rather than the resolving power,
which is a limiting factor. Therefore it is used essentially for sizes
below the resolving power of the light microscope.
The slit ultramicroscope, as well as the Cardioid
condenser dark field microscope, is also used for estimating particle
Size by Sieving
Sieving appears to be a simple means for separating
pigments into fractions according to size. A significant addition to
the physical laws governing the process has been made by Whitby who
investigated variables such as size distribution, mesh size, sieve
loading, sieve motion, sieve material, and relative humidity. A
comprehensive manual on sieving methods has been published by the ASTM.
For routine testing, sieves conforming to ASTM
Specification E II, Sieves for Testing Purposes, are recommended, Table
3. These sieves are made of woven wire cloth, supported in frames 8 in.
in diameter and 2 in. high, having below the sieve a skirt that nests
into the frame of another sieve. Frames for sieves 149 m (No. 100) and
lower may be 3 by 1 in.
The openings in successive sieves progress from a base of
1.00 mm in the ratio when selecting a range of sieves from the series,
it is recommended that each sieve, each alternate sieve, or each fourth
sieve be taken. In this way, the basic ratio between successive sieves
The U.S. Series of sieves, Table 3, is patterned after the
Tyler Series that was introduced in 1910. Infact, the two are now
interchangeable, the only difference between them being the
designations of the individual sieves. Those in the U.S. Series are
identified preferably by the sizes of the openings
in millimeters or micrometers. An alternate means is by a number
approximately equal to the mesh. Tyler sieves are identified by mesh.
Equivalent sieves are available in both series.
The sieves proposed as standard by the International
Standards Organization (ISO) correspond too many of the sieves in the
A recent development, especially useful for sizes under
100 m is a sieve made
by a photoengraving and electroplating process. Such sieves are
intended to serve mainly as primary reference standards.
If the sieves are used singly, the following directions
that appears in several ASTM methods of test. Sieve Analysis of Mineral
Filler, may be used.
After transferring the specimen to the sieve, Hold
the sieve, with pan and cover attached, in one hand in a slightly
inclined position so that the specimen will be well distributed over
the sieve, and at the same time gently strike the side about 150 times
per minute against the palm of the hand on the upstroke. Turn the sieve
every 25 strokes about one sixth of a revolution in the same direction.
Continue the operation until not more than 0.05 g passes through the
sieve in 1 min of continuous sieving.
Sources of error in hand sieving are fatigue and mere
casual attention to directions. Machine sieving eliminates these
errors, but some specifications require hand sieving unless it can be
shown that machine sieving gives the same results.
Machine sieving has the advantages of uniformity of
treatment and saving time since the operator is free to perform other
tasks while the machine is working. Several types of machine are
available. The conventional ones impart to the sieve an oscillating or
rotating motion or both, with regular tapping. None appear to reproduce
the motions of hand sieving specified in the preceding section. But we
might ask what is sacred about hand sieving? Perhaps the restriction
should be reversed permit hand sieving provided the results agree with
Ro Tap Sieve Shaker This machine
manipulates a series of sieves, graduated with respect to mesh size, so
as to permit separation of a specimen into sizes. As the name implies,
the sieves are given a special rotary motion accompanied at regular
intervals by a tap or jar. The nature of the specimen dictates the size
of the sieve openings and the timing cycle. This type of machine is
recommended in ASTM Test E 276, Particle Size or Screen Analysis at
No.4 Sieve and Finer for Metal Bearing Ores and Related Materials.
Besides the machines that agitate the specimen by rotary
and tapping actions, those that use air are also available. Two of
these are described next.
OIL ABSORPTION OF PIGMENTS
A common preliminary test applied to a pigment is mixing
it into a paste with linseed oil. The test gives a rough idea of color
and texture and, if done quantitatively, tells how much oil is needed
to make a stiff paste. According to some authorities the oil absorption
(OA) test predicts consistency of ready mixed paint. Mills writes when
equal volumes of paste (oil absorption paste) are thinned with equal
volumes of thinners, the resulting paints have equal consistencies. Stieg
shows that OA by the rubout method, expressed on a volume basis, is
proportional to critical pigment volume, judged by uniformity of flat
alkyd paints made with varying percentages of pigment, or determined by
the Asbeck Van Loo method.
of Oil Adsorption
The most reasonable explanation of what happens during an
OA test appears to be
The surface of each particle of pigment is wetted with
oil, that is, each particle becomes enclosed in a shell the thickness
of which has been estimated to be as much as eight molecules. The
amount of oil required depends on the specific area of the pigment,
that is, particle size, roughness of the surface of the particle, and
presence of cracks and pores in the particles. Other things being
equal, it depends on the vigor and duration of rubbing and on the
wetting power of the oil for the pigment. With the addition of more
oil, the interstices between the particles (with their shells of oil)
now become filled with oil. The amount of oil required for this stage
depends on the type of packing assumed by the pigment particles, which
may range from rhombe hedral to cubic. It is also influenced by the
presence of aggregates (clusters of pigment not broken up by the
procedure) and agglomerates (clusters formed after particles have been
wetted). Agglomeration is influenced by the nature of the oil. Figure l
is a stylized representation of conditions existing in an oil
for Determining Oil Absorption
This is the classical method, and the following directions
are essentially those in ASTM Method D 281, Oil Absorption of Pigments
The apparatus required for the test includes a glass plate
or marble slab, about 10 by 20 in. in size a dropping bottle or buret,
Fig. 1, graduated in 0.1 ml a stiff spatula and raw linseed oil, having
an acid value of I to 3.
One to 0.05 g, or multiple thereof, of pigment is
transferred to the rubbing plate. Oil is added, drop by drop, the mix
being thoroughly rubbed with the spatula between additions until a very
stiff putty like paste is obtained. This paste should not fall apart
when separated from the rubbing plate with the spatula. Results are
expressed as pounds of oil per 100 lb of pigment,
or grams of oil per 100 g of pigment. The amount of pigment should be
regulated to require at least 1 g of oil, for example, 20 g of white
lead or 1 g of carbon black.
A, B, and C are
each 1 g of zinc oxide with increasing amounts of oil. A is still
granular and crumbles when separated from the plate the end point has
not been reached. B may be rolled without crumbling the end point has
just been reached. C may not be rolled the end
point has been passed. Table 1 gives some typical results.
Unless extreme care is exercised, precision is not
particularly good. Results by a single operator are often relatively
satisfactory, within ± 5 percent. On the other hand, results among
different operators may differ by as much as ± 25 percent. Contributing
to the low order of reproducibility among different operators are
several uncontrolled or poorly controlled conditions, such as spatula,
acid value of oil, rate of addition of oil or pigment, sequence of
additions, energy of rubbing, duration of rubbing, end point, and use
of standard specimen.
The following variations of the method show how several
authorities have tried to surmount the difficulties.
This method differs from Method D 281 mainly in the acid
value of the oil (7.5 to 8.5) and in a requirement that the rubbing
shall occupy from 20 to 25 min. Also, the spatula is a palette knife
with a tapered blade, 140 mm long, 20 to 25 mm wide at its widest
point, and not less than 12.5 mm at its narrowest point, but this
difference would seem to be minor. The method allows the addition of
several drops of oil to the pigment between rubbings until agglomerates
are formed but does end the test with single drop increments. The final
paste should be just spreadable without cracking.
This method is practically the same as the ASTM method,
except that the end point is defined more rigorously as the point where
the paste just adheres to the spatula. Rubbing is continued for about
In the course of many determinations of OA, Azam noted
that sticking of the paste to the spatula seemed to coincide with a
definite transition point. Investigation indicated that a complete
paste absorbs no more oil when immersed in oil but that an incomplete
paste absorbs oil in an amount equal to the deficiency. This check of
the end point was made by immersing the rubout in a known amount of oil
in a graduated cylinder and, after 2 or 3 days, noting any decrease in
the volume (Table 2).
This method introduced two changes. Pigment is added to
the oil, and much smaller quantities are used in order to reduce the
labor. Some operators get better results while others find no practical
The Smith stead method, below, also adds the pigment to
the oil and works with larger quantities, but makes the rubout with a
This method eliminates the variable and poorly defined
rubbing effort of the rubout method by the use of a mechanical muller.
It follows the Hoffman practice of adding the pigment to the oil on the
hypothesis that this procedure more readily displaces occluded air.
Exactly 1 g of oil. is transferred to the lower plate of
the muller. A watchglass or dish containing an excess of pigment is
weighed. About half of the required quantity of pigment is transferred
to the oil and mixed with a spatula until no dry pigment remains. The
muller is closed and run for 50 revolutions. Another portion of pigment
is transferred to the muller and mixed with the spatula, and the muller
is closed again and run for 25 revolutions. This procedure is repeated
with smaller quantities of pigment until the end point is reached. At
the end point, the paste may be lifted without distortion when the
spatula is drawn across the plate at an angle of about 20 deg. The
leftover pigment is weighed, and the OA is computed in the usual way.
The test should be scheduled to occupy from 30 to 35 min and requires
from 8 to 12 additions of pigment.
Lead Company Method
This method is similar to the ASTM and Azam methods but
differs in a few respects. It attempts to regulate the pressure of
rubbing and the area of contact of the spatula with the rubbing plate.
The spatula is calibrated by holding it at an angle of 45 deg against
the pan of a torsion balance, under a load of about 1500 g, under which
conditions it should flex sufficiently to make contact with the balance
pan for a distance of about 1 in. The blade of a 4 in. spatula may need
to be shortened to about 3 in. to meet this requirement. Dutch 37 AA
linseed oil, or equivalent, as the liquid, and 5 g of pigment are used.
A slight excess of oil is first added to a portion (about two thirds)
of the pigment to make a thin paste, increments then being added until
a stiff paste is formed. Oil and pigment are then alternately worked
into the paste until a dry, crumbly mass is formed. Finally, oil is
added until the crumbs are converted to a coherent paste that adheres
slightly to the glass plate. The pressure during the rubbing should
flex the spatula to the same degree as in the calibration, and the
rubbing should normally occupy from 2 to 5 min, depending on the type
Finally, a correction factor should be applied. This is
obtained from a determination on a pigment whose OA has been
established by experienced operators.
Standard deviation for 72 degree of freedom has been found
to be 0.66 for a single determination.
End Point Method
In this method in the absence of a chemical reaction
between oil and pigment, it is postulated that the density of a mixture
of oil and pigment is an additive function of the densities of the
components (Curve AB of Fig. 2). Bulk (apparent) densities of mixtures
having oil contents lower than those required to produce the putty like
pastes corresponding to oil absorption end points lie along a curve C
similar to Curve CD. The volume of oil corresponding to Curve D is the
true OA. With greater proportions of oil, bulk density falls under
Curve AB, probably because all air is not removed from the sticky mass.
If mixing is inadequate, the entire Curve CD is displaced downward, and
a lower oil absorption is found.
This method is an indirect one for determining OA. A known
volume weight of the pigment is compressed into a wafer, and the volume
of the wafer is determined. From this value is subtracted the actual
volume computed from the density of the pigment. The difference is the
volume of the pores.
The apparatus consists of a 10 ton hand operated hydraulic
press, cylindrical mold, rams, and upper and lower plates. The mold is
3 in. long, with an inside diameter of 1 in. and an outside diameter of
2 in. The lower plate fits into a hole in the bottom plate. The upper
plate is bolted to the press.
With the lower ram sandwiched between the bottom plate and
the mold 10 g of pigment is transferred to the mold and distributed
uniformly by tapping the assembly against the table top a few times.
The upper ram is inserted in the mold and pressed against the pigment
under hand pressure with a slight twist. The assembly is placed in the
press, and the pressure is applied. The thickness of the compacted
wafer is obtained from a dial micrometer gage mounted in the upper
The pressure is released, and the bottom plate is lowered
to make room for a small beaker (tared) inside a metal ring, slightly
taller than the beaker, under the mold. Pressure is applied to eject
the pigment wafer into the beaker. The void volume of the specimen is
The time required for a test, including cleanup, is about
For whitings, the void volume at 12,800 psi is numerically
equal to true oil absorption, which is the OA obtained by the density
end point given by the density end point. True OA is substantially
under the values obtained by the rubout method, as specified in British
For other pigments, different pressures may be required.
In this method oil is stirred into the pigment until a
soft paste is formed. The labor of the rubout method and its
accompanying errors are avoided. The following directions are
essentially those of ASTM Method D 1483, Oil Absorption of Pigments by
the Gardner Coleman Method.
Apparatus for the test includes a 250 ml round bottom
glass container, buret graduated in 0.1 ml, spatula with a blade
approximately 18 by 100 ml, and raw linseed oil with acid number of 3
Variations in the method have been proposed but none
appears to have received much support, for example, stirring with a ½
in. steel rod in a metal dish. The end point is reached when a Vicat
needle, fitted with a disk, 1.875 by 0.125 in. under a load of 280 g,
penetrates the paste to distance of 0.5 in. within 1 min. Another
investigator preferred a glass rod as a stirrer, and still another used
a mortar and pestle. In the last named test, the end point is reached
when a single drop of oil changes the mixture from a mealy mass to a
sticky paste. At the end point, the paste, lightly touched with the
finger, will adhere to it.
Another concept related to OA is that of testing the
performance of exterior house paint at equal free binder content. The
property is strictly comparative and is measured by consistency,
usually at a low rate of shear in an apparatus, such as the Krebs
Stormer viscometer. Paints are considered to have equal free binder
when they are made with identical vehicle and have equal consistency.
The method requires a paint of known performance. This is
represented by Paint 1 in Fig. 3. It is desired to replace the
magnesium by calcium carbonate. The first step is to replace it with an
equal volume of the calcium carbonate. This gives Paint 2. Next, Paint
3 is prepared with an extra volume of calcium carbonate replacing an
equal volume of complete vehicle. Step 2 gives a paint of lower
consistency than that of Paint 1. While Step 3 gives one of higher
consistency. Finally, Paints 2 and 3 are blended to give Paint 4,
having tho consistency of Paint 1. It has the same content of titanium
oxide and zinc oxide as paint 1.
Absorption by Pigments
Oil absorption concepts may be extended to liquids other
than oil. For example, pigments for cellulosic lacquers may be
dispersed in plasticizer and the paste so formed be stirred into the
other ingredients of the lacquer. Some typical data are given in Table
The application of scientific principles in the paint
industry upsets the long held fallacy that oil is the life of the paint
and with the almost universal practice of formulating paints on a
In 1926, as a result of a statistical study of the North
Dakota house paint tests, Calbeck concluded that the pigment/binder
system should contain at least 28 volume percent of pigment. A short
time later, Wolff in Germany, related optimum performance of exterior
paints to their critical oil contents. Wolff and Zeidler devised a
simple graphical method that is illustrated in Fig. 4. Elm also studied
this relationship and, in general confirmed Wolff s conclusions, Table
4, but reversed the point of reference to express the critical
composition in terms of pigment content, as Calbeck had done.
In 1929, and again in 1961, Vannoy used consistency,
determined with the Krebs Stormer viscometer, to monitor the effects of
replacing, on a volume basis, certain pigments by others in
Measurement of FILM THICKNESS
Control of film thickness is an important part of paint
testing. Film thickness measurements are a check on other associated
tests in the laboratory, on the skill and will of the journeyman
painter, and on the performance of automatic painting machinery.
A number of methods for measuring film thickness are
available. The choice depends on where the film is located (laboratory
or field), the substrate (wood, plaster, or metal, and the electrical
or magnetic properties of the metal), of the surface texture of the
substrate (smooth or rough, or flat or wavy), and whether the film is
wet or dry.
The principal reason for determining the thickness of a
wet film is to check on the spreading rate at which the paint is being
applied. Two devices for producing this information have been
standardized in ASTM Method D 1212, Measurement of Wet Film Thickness
of Organic Coatings, the Inmont gage (formerly known as the Inter
chemical), and an alternate, the Pfund gage. These are described in the
sections following. Two nonstandardized methods are also described.
Measurements on wet films must be made without delay after
the film has been prepared in order to avoid shrinkage due to
evaporation of volatile thinner.
Wet Film Gage
This gage is essentially a pair of wheels and a ring of
shorter radius mounted eccentrically between them so that on one radius
the wheels and the ring are tangent. Thus, when placed on a plane
surface at the tangent radius, the clearance of the ring is zero. At
the opposite radius, the clearance is maximum. Two symmetrical scales
on one face of the gage show the clearance of the ring at any point on
the circumference. With the maximum clearance at the bottom, the gage
is placed on the specimen and rolled. The point on the circumference of
the ring where the paint and the ring first meet denotes the thickness
of the film. Best practice is to start from the maximum clearance.
Rolling from zero clearance may pile up paint ahead of the ring which
would indicate thickness greater than actual.
Reproducibility is stated to be within one half of the
smallest graduation of a gage.
It has been observed that a substantial proportion of
paints do not obey the 1 to 1 relationship. The actual
thickness, obtained by independent methods, may be several
times, or only a fraction of, the thickness calculated by the equation.
A small amount of thinner added to a paint may increase the diameter of
the spot on the lens and give a corresponding increase in the
calculated thickness. This phenomenon has been ascribed to the effects
of surface tension. Hence, for best results, a correction factor should
be established for each type of paint based on the known thickness of a
freshly prepared film measured by the Inmont gage.
Reproducibility is within about 2 percent for films 2 mils
thick, decreases to about 10 percent for films about 5 mils thick, then
becoming better as thickness increases.
This gage is a small square or rectangle of metal or
plastic with teeth of graduated clearances cut in the edges. It is a
simple, low cost gage, useful when approximate values are satisfactory.
The gage is simply dipped vertically in the film until it rests firmly
on the substrate. The thickness of the film lies between the highest
tooth wet by the coating and the next highest tooth. The gages are
available in many ranges.
This method was used to study the relationship between the
clearance of a doctor blade and the thickness of the wet film left by
the blade. A needle is attached vertically to the objective holder of a
microscope. The barrel is lowered until the needle just touches the
film that has been spread on a plain metal panel. The contact is
observed through a horizontal microscope. When contact is made the
needle and its image reflected by the film just meet. The needle is
then lowered into the film until it just touches the metal panel. This
contact is noted by the deflection of a galvanometer in series with the
panel, a dry cell, and the needle. The thickness of the film is
calculated from the number of turns made by the focusing screw of the
vertical microscope between the two points of contact.
Many methods are in use for measuring dry film thickness,
but only a few are suitable for all types of specimens and
circumstances. Some require removal of the film from the substrate and
thereby destroy or damage the film others
make small holes in the film but, in some instances, may be considered
nondestructive a large number
depend on magnetic or electrical functions and are nondestructive.
Naturally, these types are limited to metallic substrates, specifically
ferromagnetic metals for the magnetic methods.
This tool may be used in the conventional manner to obtain
the thickness of a free chip of paint by direct measurement, or of a
film on a substrate by measuring the total thickness, removing the film
by solvent or scraper, and measuring the thickness of the substrate at
the spot where the film was removed. The thickness of the film is the
difference between the two measurements. ASTM Method D 374, Thickness
of Solid Electrical Insulation, contains instruction for calibrating
and using machinists micrometers, with and without ratchet, and with
dead weight loading.
A preferred procedure is, set forth in ASTM Method D 1005,
Measurement of Dry Film Thickness of Organic Coatings. This method
directs the use of a dial micrometer rigidly mounted on a support and
the use of clamps to hold the specimen firmly during the measurement,
and sets forth approved manipulation and adjustment of the micrometer.
The method is recommended only for thicknesses over 0.5 mil and is
accurate within about 0.1 mil.
Needle Thickness Gage
This instrument is designed to measure the thickness of
electrically nonconducting films on metal (conducting) substrates. It
is small enough to be used in the field where the substrate can be made
part of the electric circuit. The needle makes only a minute puncture
in the film. In many instances, particularly in a go no go
determination, the damage is so slight that the method may be
considered nondestructive for many end
cast aluminium housing contains the needle, screws for forcing the
needle through the film, and a lamp to signal when the needle contacts
the substrate. For use in the field and for occasional use in the
laboratory, the electric circuit comprises the needle, the substrate, a
dry cell, the lamp, and a cord that connects with the substrate. In the
laboratory, if many measurements are to be made, it is advisable to use
a step down transformer and to connect to a 110 V source.
The zero setting of the needle is obtained by retracting
the needle within the housing, placing the gage on a plane metal block,
and lowering the needle until the lamp signals contact. The block is
replaced by the specimen, and the needle is lowered until the lamp
again signals contact. The difference between the two readings is the
thickness of the film.
Carboloy Drill Thickness Gage
In many instance films are so hard that they successfully
resist penetration by the needle penetrometer described above. This
difficulty has been overcome by substituting a Carboloy drill for the
needle. The drill is a needle terminating in a pyramid having three
faces. The drill is secured in a chuck that can be rotated and advanced
independently. The rotation is controlled by finger action on a knob at
the upper end of the chuck shaft. In all other respects the operation
is the same as that of the needle gage.
This method has heen standardized by ASTM as Procedure B
in Method D 1400. Measurement of Dry Film Thickness of Nonmetallic
Coatings of Paint, Varnish, Lacquer and Related Products Applied on a
Although the Gardner Needle Gage and the Carboloy Drill
Gage may be operated by manually holding the gage against the specimen,
using the Gardner Gage Stand is less tiring and more accurate,
especially when many measurements are to be made. The stand provides
constant known pressure (up to 10 lb) on the specimen and ensures that
the needle or drill is always perpendicular to the specimen.
Micro Depth Gage
Measurement is not restricted to nonmetallic films on
metal any type of film on any type of substrate may be measured, and
the film is always damaged.
In this gage, a chisel replaces the needle of the gage.
The zero setting having been established, the chisel is advanced by an
amount estimated to be less than the thickness of the film. The gage is
placed on the specimen and drawn toward the operator through a distance
of a few millimeters. If the scratch made by the chisel does not
penetrate the film, the chisel is advanced by a small increment, and
another scratch is made. The procedure is repeated until the substrate
is reached and exposed. Inspection is best made with the aid of a low
power magnifier. The range is 0 to 40 mils. Repeatability depends on
the magnitudes of the increments and the compressibility of the film
and substrate. For example, specimens with poor adhesion may be torn
off and expose the base even if the chisel does not penetrate the film.
for Film Thickness
Classical Method The classical method of using the
microscope for determining film thickness is to measure the width with
a micrometer eyepiece. For approximate results a chip of paint, which
may or may not include the substrate, is used. For more accurate
results, the specimen should be mounted in a block of wax, and the face
of the mount is cut or ground to a smooth surface. This is a common
method of preparing many kinds of material for microscopical
ASTM Method D 2691, Microscopic Measurement of Dry Film
Thickness of Coatings on Wood Products, is based on the above
procedure. It states that the specimen may be sliced with a microtome
or a razor blade.
Whitehead points up that brittle paint films are usually
shattered when cut with a microtome. Also, the microtome blade is
easily damage by hard paint films. He recommends grinding and
polishing, somewhat as metallurgical specimens are prepared. Molten wax
is flowed around the specimen and the face is leveled with a dead
smooth file and finally polished with a fine hone lubricated with
dilute soap solution and alcohol. Whitehead does not recommend abrasive
paper as the particles of abrasive become lodged in the film and cannot
On the other hand, others have had success with abrasive
paper. The face of the specimen is rubbed on a sheet of No. 80 wet or
dry paper lubricated by a stream of cold water. When a suitable cross
section has been exposed, it is polished successively with finer grades
Various waxes have been tried, such as a mixture of equal
parts of beeswax and carnauba (carnauba is too brittle, beeswax is too
soft), bayberry, montan, and others. A synthetic wax (Acrawax B) is
very good. It does not fill the pores of the abrasive paper excessively.
Brightwell Method This method does not
require removal of a chip and elaborate mounting and preparation. A
tiny furrow is made in the film or a small chip is removed. A prism or
ribbon of light is projected on the selected area at an angle of 45
deg. The distortion of the beam is examined with a microscope equipped
with a micrometer eyepiece. Apparatus for this is available in the
Schmaltz Optical Surface Analyzer.
Another type somewhat easier to
manipulate. A balanced beam is attached to the shaft of a light duty
motor spring. At the end of one arm is the magnet, which is
counterbalanced by a rider on the other arm. The free end of the spring
is attached to a ring. Rotation of the ring raises or lowers the magnet.
In the determination the gage is placed on the specimen
and is held firmly with the thumb of one hand on the housing near the
magnet with the thumb or finger of the free hand, the ring is rotated
counterclockwise to bring the magnet into contact with the specimen
the ring is then rotated clockwise until the magnet breaks loose. The
thickness is read on the scale on the rim of the ring.
Probably no test on coatings has given as much trouble as
drying time, unless it is hardness. One reason is the subjective nature
of the tests. Another is the unawareness of the influence of
environment and of the mechanical details of instrumental aids.
Subjective methods have been used since time immemorial.
Says Werner It
s been a long time since our illustrative and artistic ancestor touched
the first paint film on the wall of his cave. Little did he know that
he was the initiator of the world s longest established test method.
Authorities may not agree that all of the stages actually
exist and, if they do, that they appear in the order named.
For many purposes, the early stages may be identified by
that universal tool, the finger, especially if experienced. One
exception is the dust free stage, which obviously needs dust of some
kind. However, for specification purposes, instruments or tools are
Widely used directions for determining drying time by
finger methods appear in ASTM Method D 1640, Drying, Curing, or Film
Formation of Organic Coatings at Room Temperature, and in Method
4061.1, Drying Time of Coatings, of Federal Test Method Standard No.
141 (FTMS). Except in a few minor respects, the two versions agree in
the technical requirements. Two methods dust free and tack free times
call for simple accessories dust, or lint, and Kraft paper,
The directions include expressions such as touch lightly, feel
sticky, and firm pressure, which mean different things to different
people. Mr. Brobdingnad has a bigger finger than has Miss Lilliput, and
it would be amazing if they agreed in their subjective observations.
Lately, there has been added the International Standards
Organization (ISO) Method CHM 02 64, Determination of the Degrees of
Drying of Paint Films. Three degrees dust dry, tack free, and hard dry
require only simple accessories ballotini (fine glass beads), paper,
and wire gauze and do not depend on the finger.
As already mentioned, drying rates are influenced by the
environment, mainly, temperature, relative humidity, circulation of
air, and light. Frequently these factors exert more influence than the
method of test.
Among the first investigators to examine the effects of
environment were Schmutz and Palmer who studied the drying rates of
exterior oil base house paint in environments consisting of various
combinations of temperature, 32 and 100 F, relative humidity, under 10
and over 90 percent, and light, with and without, Table 1. In general,
low temperature retards drying more than does excessive humidity or
absence of light. Light accelerates drying, but, under very humid
conditions at high temperatures, the effect is slight.
Sheetz studied the drying time and hardness of alkyd
finishes over a much smaller range of temperature, 70 to 77 F, and
relative humidity, 50 to 95 percent. In these ranges he found that
relative humidity was a greater factor than the temperature.
In a study of factors influencing the drying time of
paint, Algeo observed a difference of 4 h for a paint dried at 73 and
77 F, both at 50 percent relative humidity. The two temperatures were
within the limits specified in ASTM Method D 1640.
For films that dry by oxidation the rate of drying should
be a function of the concentration of oxygen at the interface. Since
oxygen can reach the surface only by diffusion, the rate of drying is a
function of the thickness of the stationary air film.
For films that dry by solvent evaporation, in whole or in
part, the continuous removal of solvent laden air hastens drying. It is
well established that the thickness of such stationary films is related
to the velocity at which the main body of air is moving. Hence, the
reaction rate (drying time) is profoundly influenced by local air
currents. Since it is almost impossible to provide absolutely
stationary air, Scofield provided a uniform current by mounting the
panels on a turntable that carried them through the air at
approximately 6 ft/min, a rate higher than that of local air currents,
so that the latter may be disregarded.
Algeo went a step further. They regulated the flow of
fresh air through the drying area to that of the environment that they
wished to reproduce. Uniform composition of the air was achieved by
Typical standard conditions are given in Table 2. If such
space is not available, a control specimen should be tested at the same
Besides controlling the environment, it is usually
desirable to control other parameters, such as type of substrate,
method of preparing the film, and the thickness of the film.
The panels for the test may be glass, metal, or other
material agreed upon. The surface should be smooth. Glass, for example,
should be ground and polished plate. Ordinary window glass may deviate
as much as 1 mil from flatness.
Ordinarily no restriction is placed on the method of
preparing the film, but doctor blade preparation is often specified.
The base of the doctor blade bearing on the panel should be lubricated
with a drop of high flash naphtha.
Unless otherwise specified, the film thicknesses given in
Table 3 are suggested.
to Touch Time
According to ASTM Method D 640 and FTMS 4061.1, a film is
set to touch when it clings weakly to the finger under gentle pressure,
but none of the film transfers to the finger. To confirm, press the
finger tip on clean glass. There should be no transfer to the glass.
A test for the efficiency of liquid drier is somewhat more
According to ASTM Method D 1640 and FTMS 4061.1, prepared
dusts are used for this test cotton fibers and a powder, usually
calcium carbonate pigment for good visibility on white opaque coatings,
the dust may be darkened by mixing with it a trace of carbon black.
Other colors or clears should give no trouble.
Individual fibers of absorbent cotton are separated with
tweezers. At regular intervals several fibers are dropped onto the
specimen from a height of 1 in. The film is considered to be dust free
when a gentle current of air removes the fibers.
Rossmann proposed a similar method whereby a thread is
unwound automatically from a reel or otherwise placed on the wet film.
A machine for dispensing the thread is available.
The powder (Standard Multifex, Diamond Alkali Co.) is
deposited on the film
at regular intervals with the aid of the New York Dust Free Tester.
Charge the tester with the powder to a depth of about ½
in., place tester over the film, and operate the vibrator for 2 s.
immediately blow air under about 5 psi on the deposit and gently rub
the spot with a soft cloth. The film is at the dust free stage when it
is clean after the rubbing. If not clean repeat at regular time
intervals on different spots.
According to ISO Method CHM 02 64, ballotini (glass
beads), 0.2 mm in diameter, are sprinkled on the film. After 10 s, the
specimen is tilted 20 deg from the horizontal, and the beads are
brushed gently with a soft hair brush. The film is dust dry or surface
dry if no trace of beads is left. Machines that automatically dispense
the beads have been made, but none provides for removal of the beads in
the manner mentioned. The nearest is the Paraffin machine in which the
specimen is oriented at 45 deg to the stream of beads (sand), and the
excess rolls off immediately.
During a study of artists, colors. The Federal Art Project
of the Works Progress Administration for Massachusetts devised a
metering device for sand used to measure surface dry time. It was
essentially a small funnel having a valve in the stem. Sand from a
reservoir is admitted through a similar valve and then discharged onto
the specimen oriented at an angle of 45 deg below the funnel. All
operations are manual.
Tack is the tenacity with which an accessory, such as a
piece of paper that has been pressed into contact with the film, clings
to the film. The accessories and the weights used to impress them on
films hardly merit being classed as instruments.
This tester is made of sheet metal, 0.015 to 0.018 in.
thick,. I in, wide, and about 3 in. long, bent as shown in Fig. 3. The
base is 1 in. square and the angle between the two arms is
approximately 135 deg, such that a 5 g weight placed on the geometric
center of the base just balances the side arm. Aluminum foil is wrapped
around the base cushioned on the underside with felt, blotting paper,
or smooth white paper. The New York Production Club recommends 2 SO
plain coiled aluminum, 0.5 mil thick, dull side out. ASTM Method D
1640, however, specifies the shiny side out.
To make a test, place a weight (300 g for most films) on
the base of the tester and set the tester on the film. Remove the
weight after 5 s. The film is tack free if the tester tips over
immediately upon removal of the weight.
Table 5 compares tack free times obtained with the Zapon
tester, the bare finger, and the finger wrapped in aluminum foil. The
shorter times of the bare finger method may have been caused by
moisture on the finger interfering with good contact.
Drying Time Tester
A 1 cm2 disk
of chrome tanned leather attached to one end of a balance beam is
impressed on the film under a 50 g load for 30 s. A counterbalancing
rider is drawn along the beam by a motor until it pulls the
leather disk away from the film. A pen attached to the counterbalance
makes a time load record on a chart.
Magnetic Tack Tester
This consists of a
leather disk to be impressed on the specimen and an arrangement for
pulling the disk from the specimen with magnetic force.
The disk, cross section 5 cm2, is
located at the bottom of a rod hanging from the left arm of a balanced
beam and counter balanced with a rider on the right arm, the right arm
terminates in a fixed magnet. Below the fixed magnet is another magnet
vertically adjustable with a worm and gear. A dial calibrated in
millimeters indicates the extent of the vertical movement.
The specimen is placed under the disk, which is lowered
into contact with the film and further impressed with a 75 g load. The
dial is manipulated to raise the movable magnet until its pull on the
other magnet separates the disk from the film. When the distance
between the two magnets is 20 mm or more the film is tack free. When
more than about 31 mm, the film may be considered dry.
MECHANICAL PROPERTIES OF FILMS
Before it is possible to arrive at a definition of
hardness, one must realize that hardness is conceptual. It means
different things to different people and is measured in different ways.
Dealing with the hardness of relatively thin organic finishes over
various substrates presents other complications not experienced when
one deals with materials in bulk. That is, the apparent or measured
value of hardness is dependent on factors outside of the organic finish
itself. Included among these factors are the thickness of the organic
finish and the nature of the substrate. Another way of expressing this
is to say that hardness is not an absolute or intrinsic material
property of organic finishes. It is not an absolutely definable value
such as mass, length, velocity, etc. It is descriptive of many things
and is variable. Trying to limit the hardness of organic finishes to a
particular, intrinsic material property, is like trying to confine
mercury in one s fist.
Over the years many methods and devices have been employed
to measure the hardness of organic finishes. In 1946 ASTM conducted a
survey to determine what methods were being used by the paint industry
to measure hardness. They found that some forn of scratching or
abrasion was used 21 times, pendulum or damping hardness (also referred
to as entropy hardness) 14 times, and indentation hardness 5 times. The
modem trend has been towards an increasing use of indentation methods.
Indentation hardness has been universally accepted throughout the
scientific and engineering communities. In addition to those methods
just mentioned, some work has been done using impact and rebound.
However, these latter methods have not been seriously considered for
measuring paint hardness.
All of this leads us to a definition of hardness based on
practical concepts. A material is considered to be hard if it resists
indentation or is not easily scratched. Therefore, hardness is
resistance hardness is a resistance to indentation or scratching.
However, the latter part of this definition presents more problems than
Resistance to strictly surface scratching is within the
province of mar resistance, which is covered later on Organic finishes
are often doped with silicones or waxy additives to increase this
surface scratch resistance, while maintaining the same resistance to
indentation. Scratching through to the substrate has been used to
evaluate adhesion. Perhaps the distinguishing difference here is that
cutting through with a relatively wide instrument or tool, wherein
removal of the finish from the substrate occurs, is more in the realm
of adhesion while resistance to scratching through with a relatively
narrow or pointed tool is considered indicative of hardness. The use of
scratch tests for assessing the hardness of organic finishes was
undoubtedly influenced by the use of scratch tests for minerals and
However, at the expense of being redundant, we may note
that the modem trend in all areas has been towards the increasing use
and adaptation of indentation hardness tests. These tests are far more
objective and precise than the scratch tests. If resistance to
indentation is used as a measure of hardness, then such hardness should
be related to the rigidity of the material or more precisely, the
elastic modulus. With certain Qualifications this is true.
Baily Hardness Tester
This apparatus was among the first to be developed for
measuring the scratch hardness of varnish films and was invented by A.
P. Laurie and F. G. Baily of Heriot Watt College, Edinburg. The
apparatus consists essentially of a hardened, blunt steel point upon
which pressure is exerted by a vertical coil spring. The spring tension
is controlled by an adjusting screw and can vary from 0 to 2000 g. The
finish to be tested is placed under the point and slowly moved
horizontally by hand as the pressure is increased until a scratch is
made. Then the reading on the scale is recorded.
Linton Hardness Tester
This device might also be considered an adhesion tester.
As shown in the figure, it is essentially a small, circular blade upon
which pressure is exerted by a coil spring. A scale, graduated in 100 g
increments from 0 to 2000 g, indicates the load on the blade.
consists of a
balanced beam, on one arm of which is a short knife and a post upon
which weights are placed. The specimen rests on a tray that is drawn
under the knife during the test.
Scratch Testing Machine
This type of instrument was once used at the duPont
lacquer plant at Parlin, N.J., for determining the hardness of lacquers
and their resistance to scratching. It was one of the first to
electrically signal the end point of the test.
The device consists of a wooden base on which are mounted (1)
a fulcrum holding a graduated lever equipped with weights and a needle
point, (2) a transformer, (3) a 6 V lamp, and (4) a metal plate. These
parts are connected in series, the transformer being used to step down
ordinary light voltage to that of the small lamp.
The metal panel, coated with the lacquer, is placed coated
side up on the metal plate under the needle. The weight is adjusted,
and the panel is drawn along the plate by the operator in the direction
of the long axis of the instrument. This operation is repeated, each
time using an increased weight, until the needle penetrates the film.
When this happens, the electric circuit is closed, and the lamp lights.
this to be a low
carriage with a weighted lever on one end. The scratching tool is a
sharp edged, hardened steel cylinder with its axis at an angle of 45
deg to the plane of the film. This cylinder is attached to the lever
arm, and the load is varied by varying the position of the weight on
the lever. This has also been used for adhesion and mar resistance tests
Shear, Scratch Tester
In this tester, the tool is fixed to the underside of a
beam, pivoted on ball bearings. Riders provide for adjusting the load
on the tool between zero and 1000 g. The test films for this tester are
prepared on panels containing a hole in the middle for locating on a
turntable. In making a test, the panel is rotated counterclockwise.
Three tools are provided thumb
nail contour shear tool (S 20)
lapped to a 25 mm radius with a 30 deg clearance, a diamond cut to the
shape (diamond pyramid) of a corner of a cube, and a diamond cut to the
shape of a cone.
This device uses the corner of a diamond cube (diamond
pyramid) as a scratching tool. It is a rather elaborate device
consisting of a microscope, stage, and diamond tool on a balanced arm.
Bierbaum scratch hardness is the ratio of the load on the diamond, in
kilograms, to the square of the scratch width, in millimeters. Although
it was designed for testing metals, it had been used on plastics for a
while. It never gained much acceptance for use on organic finishes.
This device was one of the first to provide for
automatically increasing the load on the scratching tool while the
scratch is being made. Arms extending upward from the panel carrier end
in slots above the beam carrying the scratching tool A roller resting
on the beam is guided by the slots. As the panel carrier is drawn
along, the roller travels with it, thereby increasing the load on the
scratching tool. Provision is made for automatically lifting the load
from the specimen at the end of each trip and also for a sidewise
displacement of the specimen in order to provide a new path for repeat
tests. The interpretation of results is the same as with other types of
scratching devices, that is, according to the character of the mark
under a particular load or tool, or the load at which a particular tool
makes a mark.
Siddle Scratch Tester
This tester is a very simple form of the Schopper type in
which the load on a needle is increased as the scratch is made. The
point of the needle is a hemisphere 0.2 mm in diameter. The panel
carrier is moved by hand, the speed recommended by Parker and Siddle
being 30 cm per min. appreciably greater speeds give inconsistent
This is another tester of the increasing load type and is
suitable only for films on metal. When the stylus breaks through the
film, a relay stops the machine. Hardness is reported as the weight
necessary to penetrate the film.
This device employs a novel means of increasing the load
during the travel of the scratching tool across the film. as arm B,
with tool G resting on panel H,
is lowered, arm A follows. The scratching tool
travels from H to L, a distance
of 5 cm. The load varies from 0 to 5000 g. Operation may be by hand
crank or by motor.
Four types of hardened steel scratching tools are provided
a ball 1 mm in diameter, a simulated finger nail, and two wedges.
This device shown in Fig. 1, also belongs to the type
where the load on the scratching tool automatically increases as the
test is being made. The tool, parabolic in shape, is mounted on the
free end of a cantilever spring, one end being movable in a slot in the
frame. At the start of a test, the tool rests on the test surface under
zero load. The finish end of the slot is 0.100 in. nearer to the test
surface than it is at the start. At the finish end, the load is the
rated value of the spring. At intermediate distances, the load is
proportional to the distance. Three springs are provided, giving loads
of 300, 600, and 1200 at the finish end.
Schmitt Scratch Dynamometer
The principle of this device is similar to that of the
Schopper tester. The scratching tool is a hardened steel, 45 deg
Measurement of scratch resistance is expressed either as
the threshold load producing a scratch, or by a curve expressing the
relation between the load and size (width) of the scratch.
this device in
which the scratching tool, a 60 deg diamond point, rests nearly
vertically on the test panel which is clamped to a movable stage. In
making a scratch, the assembly containing the diamond point is moved by
means of an electric motor. The load is adjusted until two strokes in
the same place just cut through the coating and a third stroke cuts
into the supporting panel. This load (in grams) is reported as, the
hardness of the film. By means of a micrometer screw, the stage may be
displaced for successive determinations. The Arco Microknife used in a
different manner has been standardized as an adhesion tester. This is
covered in the section on adhesion.
Rating the hardness of an organic finish according to the
hardness of a lead (graphite) pencil that will just scratch it was
probably an old custom when described by Wilkinson. Gardner studied the
method using pencils sharpened to different shapes sharp cones, rounded
cones, and chisels. He found that the principal source of error lay in
the character of the point, because it was difficult to reproduce
points. Other sources of error were the pressure on the pencil and the
angle at which the pencil was held while it was moved over the organic
finish. Gardner built a device to hold eight pencils at one time, at an
angle of 45 deg to the panel, but found that it was impossible to align
all pencils uniformly.
Hold the pencil in a writing position, that is, at
approximately 45 deg, and push forward against the film. Use pressure
short of breaking the lead. By turning the pencil after a test a new
edge is available for use, and three or four trials may be made with
one dressing of the lead.
Clean the marks with a soap or artgum eraser. Any marring
of the surface, visible at an oblique angle in strong light, indicates
that the pencil is harder than the film. The hardness is expressed as
the grade of the next softer pencil.
In developing this method, Smith determined the pencil
hardness of 14 different organic finishes that varied widely in
hardness. Five different brands of pencils were used, and the results
are shown in Table 1. The results indicated that there were variations
in the hardness between different brands of pencils and that it was
necessary to use only one brand in order to obtain reproducibility.
The Erichsen Company in Germany has marketed a mechanical
Hardness Test Pencil (DBGM), Model 318, which somewhat resembles a
mechanical pencil in appearance, but whose point is a spring loaded
tungsten carbide hemisphere or ball point of a 0.75 mm diameter. Two
interchangeable helical springs are supplied with each Hardness Test
Pencil to give ranges of 0 to 400 or 0 to 1000 g load on the ball
point. The hardness is defined as the minimum load or force, in grams,
on the ball point, that leaves a mark in the surface just visible to
the unaided eye.
Rocker (damping) Hardness
As indicated by the title, this hardness is measured by
some sort of pendulum or rocker, the mechanism being the damping of the
oscillations due to the hysteresis losses of the material, rolling
friction, shear modulus, and in the case of relatively thin organic
coatings, the hardness of the substrate. While damping of the
oscillations due to rolling friction and hysteresis losses are self
explanatory, one can readily see that, all other things being equal, a
lower modulus (stiffness) will result in deeper indentation of the ball
or rocker into the material, resulting in a greater rate of decrease of
the oscillations. It logically follows, therefore, that this type of
hardness should be related to the shear modulus of a material. It is,
as will be shown in the section on the Sward Rocker.
The principle of this beam is that of the Herbert Pendulum
Hardness Tester, a device for testing metals. This is a metal form
weighing from 2 to 4 kg, supported by a 1 mm ball of ruby or steel.
Normally the center of gravity is adjusted to a fraction of a
millimeter below the center of the ball. With the ball resting on the
test surface, the pendulum is set into oscillation through a small
angle, and the time required for ten oscillations is determined. This
time depends on the nature of the surface. The pendulum may also be
tilted from the horizontal through a selected angle and then released.
The angle reached at the swing in the other direction is called the
scale hardness number. A curved
spirit level and scale on the pendulum allow angles to be read.
The harder the film, the longer it will require for the
beam to come to rest, after having been set to oscillating through any
specific angle. However, instead of the dead stop scheme of
measurement, it is easier to determine the time for the amplitude to
decrease by a specified fraction, say 50 percent, or what amounts to
the same thing, to count the number of oscillations. Amplitude may be
measured on a circular scale behind the end of one of the arms. The
scale may be divided in degrees or in arbitrary divisions.
It is necessary to have a reference standard of known
hardness. Since, to date, no organic composition can be relied upon,
polished plate glass is used.
The problem of resistance to abrasion exists all around us
on floors, walls, furniture, automobiles, highways, signs, military
gear, spacecraft, etc. Because abrasion or wear resistance is a basic
factor in the durability of a coating, its measurement is of practical
importance to both producer and consumer.
Abrasion resistance is not a unique or isolated property
of a material but is related to other physical properties, such as
hardness and mar resistance cohesive and tensile strength modulus of
elasticity, and toughness. From the standpoint of retaining its
protective or decorative function, the thickness of a coating is an
important factor in addition to its intrinsic abrasion resistance.
When we seek to measure abrasion resistance, we are
measuring a complex composite of many different but inter related
properties. It is not surprising, therefore, that a wide variety of
testing devices and machines have been developed in the quest for a
practical test method that correlates with actual service performance.
Abrasion resistance may be defined generally as the
ability of a material to withstand mechanical action, such as rubbing,
scraping, or erosion that tends progressively to remove material from
its surface. This definition applies also to organic coatings or paint
films. Implicit in the definition is the concept of a gradual or
progressive wearing away of the entire thickness of the coating. In
this respect, it is distinguished from mar resistance, which pertains
only to surface effects manifest as visual changes in appearance as a
result of mechanical action.
to Other Physical Properties
Let us consider briefly the relationship of abrasion
resistance to the other physical so properties mentioned in our
introductory paragraph. This may help us to understand the mechanism of
abrasion and the problems involved in its measurement.
While this property is sometimes confused with abrasion
resistance, those concerned with the problem in the American Society
for Testing and Materials (ASTM) have found it practical to consider
mar resistance as a surface property, whereas abrasion resistance
involves the body of the material as well. For example, in an
automobile finish, where good appearance is a prime requirement, mar
resistance would be a very important property. On an army tank,
however, where surface appearance is of secondary importance, it is the
abrasion resistance of the coating throughout its thickness that
determines how long it will remain in place to perform its primary
function of protecting the underlying steel from corrosion.
Abrasion resistance obviously is related to hardness, yet
the relationship is not a simple one. It might seem, at first thought,
that the harder a coating or other material, the better would be the
abrasion resistance. This is not necessarily true. Steel is much harder
than rubber, but stell tires on automobiles, besides giving a very
rough ride, would not last long on concrete roads compared to the
service life of a good rubber tire. The ability of a material, like a
rubber tire, to undergo elastic deformation or to ride with the blow is
associated with good abrasion resistance.
transferred to an elastic material by an impacting object or particle
is returned largely to the particle, though redirected, instead of
being expended in the destruction of (separation and removal of
material from) the impacted surface. From a fundamental point of view,
this is a consequence of the smaller deceleration and hence smaller
force generated when the impact is with a material that will deform or
give, if the deformation is not elastic, of the material will yield and
flow. Hence, soft materials of low tensile strength are not abrasion
The fact that elastic materials often are abrasion
resistant does not mean that hard materials are not abrasion resistant.
Theoretically, however, given two materials of equal tensile strength,
the material of lower modulus should have the better abrasion
resistance. The deceptive factor here is that a hard material usually
has a much higher tensile strength than a soft one. Thus, when we
compare rubber with steel, we are comparing different orders of
magnitude in tensile strength. The fact that rubber is abrasion
resistant does emphasize the value of a low modulus of elasticity,
coupled with adequate tensile strength, as factors in good abrasion
In theory, a very hard material, of hardness and cohesive
strength adequate to completely resist any impact force it might
encounter, would not be dependent upon rubber like elasticity to reduce
impact stresses, and would be more abrasion resistant than the best
elastic but weaker material. In practice, the writer feels that
hardness bears a closer correspondence to mar resistance than to
abrasion resistance, while abrasion resistance is related more closely
to toughness than to hardness.
of Elasticity and Tensile Strength
Since hardness and modulus are closely interrelated, what
has been stated about the relation of abrasion resistance to hardness
holds quite well for modulus of elasticity also but we must not forget
the role of tensile strength. Other factors being equal, the higher the
tensile strength (or cohesive strength) of a material, the more
abrasion resistant it is likely to be.
with Service Performance
From the foregoing, it is evident that the measurement of
abrasion resistance involves measuring a complex composite of many
interrelated properties among which there is no 1 to 1 correspondence.
The task of devising a test methodology that will correlate with
service performance is therefore complex and difficult but not
impossible. If the test method we use, whether by chance or by design,
subjects the material under test to a composite of destructive forces
similar to those encountered in service, then the test method will
correlate with or predict the service performance of the material, at
least with respect to the ranking of materials.
With all accelerated tests, even those that rank materials
in the same sequence as actual service tests, a quantitative
correspondence with actual service is seldom expected or obtained.
Actual service tests, while the most reliable in providing an
indication of the probable long term durability of a material, suffer
from the difficulties of ensuring equivalent usage and measurement,
especially when the inter comparison of different materials is
attempted. Because of such difficulties and because service tests are
usually very time consuming, a wide variety of test machines have been
developed to provide an accelerated indication of the abrasion
resistance of coatings and related materials such as plastic tile
flooring, linoleum, and wall coverings. An investigation by the
International Study Committee for Wear Tests of Flooring Materials in
which seven commercial, organic flooring materials were systematically
tested on 21 abrasion machines of 17 different types, indicated that
very few of these machines were capable of providing a reliable inter
comparison of the abrasion resistance of widely different materials
that could be correlated with service performance nor did the different
machines correlate well with each other.
On the basis of a round robin conducted by ASTM on six
different clear floor coatings evaluated by six different abrasion test
methods only two of the methods were found to correlate with actual
service performance and to have the reproducibility necessary for
acceptance as ASTM standards. These were the falling sand method and
the air blast abrasion tester. The jet abrader which has since become
available appears to correlate well with the two aforementioned methods
and with various types of service performance while offering greater
speed and precision of measurements. All three of these methods are
discussed in the accompanying text.
The success of the particle impingement types of abrasion
testers in correlating with service performance is perhaps not
surprising if one considers the abrading mechanism. Although a
comprehensive discussion of the mechanism of abrasion is beyond the
scope of this article, some useful observations can be made.
Whether or not a particular type of abrasion test
correlates with service performance depends not only on a similarity of
abrading mechanisms in the two cases but also on the extent to which
that mechanism is maintained during the course of the abrasion test. It
is on the latter point that many of the available methods fail.
Rubbing (friction) and scraping methods obviously wear
away the test surface in a different way than methods in which abrasive
particles are blasted at the surface. One aspect of the mechanistic
difference lies in the angle of attack upon the surface. Abrasive
particles striking the surface of a coating at near normal incidence
would tend to compress, scar, and cut into the coating, so that minute
portions of the coating eventually would be crosscut and displaced. On
the other hand, the rubbing and scraping types of abrasion that take
uplace at near grazing incidence would tend to undercut and to shear
through minutely thin layers of the coating in successive slices that
would ultimately wear it away. Different machines might incorporate
various degrees of these basic processes, depending on the angle and
force of attack. Apart from the nature of the mechanisms considered
before, it should be apparent that whatever the mechanism, it is not
maintained uniformly in friction methods. The latter inevitably suffer
from change in the abrading conditions as the testing proceeds, either
because of heating of the specimen or clogging of the abradant or both.
If, to avoid undesirable friction effects, we choose an
abrasion test employing the direct impingement of free particles, we
might have some doubt about whether the method would correlate well
with a type of service in which the coating, for example, is walked
upon. Yet, the previously mentioned ASTM round robin of abrasion tests
on floor coatings clearly established the validity of particle
impingement type tests for evaluating this type of service.
It must be concluded that until the mechanism of abrasion
is more fully the studied and understood, we must continue to rely to a
large extent on experimentation, intuition, or genius in devising
abrasion test methods and we must depend upon properly designed test
programs to establish the extent to which a given method correlates
with actual service performance.
of Test Methods
Despite the failure of most abrasion testing machines to
rank a widely differing range of materials consistently or to correlate
with actual service performance, many of the machines and methods give
useful results when comparisons are limited to similar types of
materials or it when the results are performed and interpreted on a
single machine in a single laboratory experienced in its use. A wide a
variety of such useful abrasion test methods and machines is described
in the following sections.
Abrasion testing devices for coatings may be classified
either by the type of abrading action they exert (for example, abrasive
blast, rotating wheels, etc.) or according to the nature or function of
the material they are designed to test (for example, traffic paints,
automotive finishes, etc.). In this chapter, we have elected to
classify the test methods according to type of abrading action, except
for traffic paints and a miscellaneous category. Traffic paints are
mentioned here only briefly.
The following sections present a description and
discussion of a selected group of abrasive testing devices that have
found acceptance and use in the coatings field.
Before one can evaluate various instruments and methods
for measuring the adhesion of organic coatings, it is important to have
a fundamental understanding of what measuring adhesion means in the
light of modern science, since so many significant discoveries have
been made within the past decade. Only after we have this understanding
can we appreciate which methods are useful and which have little or no
practical value. ASTM Designation D 907, Definition of Terms Relating
to Adhesion, defines adhesion as the state in which two surfaces are
held together by interfacial forces which may consist of valence forces
or interlocking action, or both. If
we speak of adhesion as these interfacial forces, then we must abandon
hopes of trying to measure adhesion. That is, we cannot measure these
interfacial forces by means of mechanical tests. However, even if we
could, knowledge of the magnitude of these forces would not necessarily
mean anything in a practical sense, since adhesive failures have
invariably proven to be cohesive.
To review briefly the aspects involved in trying to
measure adhesion as defined in ASTM Designation D 907, assume for the
moment that interfacial failure is possible and that the organic
coating material has freely wet the substrate. Also assume that the
substrate is flat and planar (on an atomic scale) and that we can apply
a force to the coating in such a way that it is normal to the substrate
surface, and that no energy is expended in hysteresis or deforming the
coating or substrate.
With regard to what is wetting the substrate, we know that
during application organic finishes are liquid mixtures (even dry resin
powders must achieve a fluid state in order for them to flow and
adhere) of resins, pigments, solvents, diluents, and various additives,
any of which can result in weak boundary layers at the coating
substrate interface. Even with the resins themselves, there often exist
low molecular weight fractions which can form weak boundary layers. By
a weak boundary layer we mean inherently weak substance at or near the
coating substrate interface, which will substantially reduce the
apparent adhesion. Some examples are stearates, heat and ultraviolet
stabilizers, plasticizers, mar reducing additives (such as silicones,
waxes, and polyethylene), low molecular weight fractions of polymeric
resins, and others mentioned in the following paragraph. Even though
surface chemistry tells us that organic materials in the liquid state
should freely wet and therefore adhere to comparatively high free
energy surfaces such as metals and metal oxides, one of the weaker
substances in our organic coating mixture may preferentially wet the
substrate. Zisman has shown that preferential wetting can and does
occur with mixtures of organic liquids. Hence, there is a problem in
ascertaining just which component of the mixture is wetting the
Identification of the substrate is also a problem. As an
example, one rarely has a pure iron, aluminum, zinc, etc., surface.
Instead, there exist layers of water, mixed oxides, carbonates,
carbides, water soluble salts, and various other materials referred to
as dirt. Vapor
degreasing applies a layer of organic material to the surface and
perhaps metal carbonyls. Any of these can result in weak boundary
layers. As an illustration, our laboratory experienced difficulty in
obtaining strong adhesive joints with brass. Investigation revealed
that a zinc compound was causing the weak boundary layer. The brass,
therefore, was treated to etch the zinc away from the surface, leaving
a copper surface which was then treated to form a strong layer of
copper oxide. This resulted in adhesive joints so strong that the brass
broke instead of the joint. Zinc by itself is usually treated to form a
phosphate layer at the surface. Aluminum and iron can be treated to
form strong oxide layers on the surface. However, even if the surface
is treated to form a relatively strong surface layer materials in the
atmosphere impinge, fall, and are adsorbed onto the treated surface,
thereby contaminating it. In some cases where an organic coating is
applied to a thermoplastic substrate, an interface, as such, may not
even exist. Instead, there may be a zone of mutually inter diffused
material between the coating and substrate, due to solution of the
substrate by solvents in the liquid coating material.
In light of the above, we cannot really answer the two
questions previously posed. To carry the argument further, it is
obvious that we can never have an absolutely flat, planar substrate
surface. This consideration precludes the possibility of ever obtaining
a force that is normal to the interface. Also, the existence of true
interfacial failure has never been proven. Studies employing electron
microscopy and other techniques have shown that there is usually a
layer of polymer left on the substrate surface, although this may not
be apparent under an optical microscope.
In practice, we can never be sure of the substrate area
that has been wet or covered by the coating. Even a highly polished
surface looks like a rough mountain range on a microscopic scale. As an
example, fire polished glass has an average peak to valley distance of
about 400 A. If we have a non wetting liquid, interfacial contact area
is low. If we have a wetting liquid, the interfacial contact area is
usually high. Even then, high viscosity may limit contact to the peaks.
Capillary pressure may prevent the liquid polymer from flowing into the
pits and similar irregularities.
The state or configuration of the polymer chains in
solution can also influence the interfacial contact area. The solvent
system determines whether the dissolved polymer chains are in coil form
or relatively flat and open. Adsorption studies of polystyrene on
graphited, nonporous carbon black illustrated the effect of solvent on
configuration and surface orientation. In poor solvents wherein the
polymer forms a loop or coil structure, only part of the polymer
segments was attached directly to the carbon surface. In good solvents,
the polymer attached itself to the carbon surface as a flat, oriented
However, even if all of our initial assumptions were
correct and we knew the answer to the two questions, we still could not
measure the interfacial forces by means of mechanical tests, because no
test has ever been, devised that can eliminate the influence of the
rheological and other physical properties of both coating (or adhesive)
and substrate, joint geometry, stress concentrations, etc., nor can any
test method ensure the same manner or mode of failure for all
materials. For instance, a direct tension test of adhesive joints may
have many of the features of a rapid stripping or peeling. Further, in
peeling, an increase in the elastic modulus of the coating or adhesive
will result in a sharper angle of peel and apparent lower adhesion due
to the higher stress concentrations at the root of the peel apex. This
can also change the mode of failure from ductile shear to brittle
fracture. One can cause an increase in the elastic modulus of the
coating merely by increasing the speed of peeling or by lowering the
Therefore, if we cannot measure the interfacial forces of
adhesion as defined by, ASTM Designation D 907, what can we measure?
The answer is that we measure a force of removal,
which is probably totally unrelated to the interfacial forces. Let us
therefore define adhesion in a practical sense, as
the force required to remove an organic coating, break an adhesive
joint, peel a tape, etc., the nature of the test being dictated by the
expected mode of stress or removal in service. To repeat, when we
measure what we refer to as adhesion, what we are really measuring is a
force of removal.
If we were testing adhesive tapes, then some sort of
or peel test would be used to measure the peel strength. If we
were using adhesives to bond metals together to form cabinets, we would
at least want to perform lap shear tests to obtain the joint strength.
We would not normally use a peel test for organic coatings
unless this is the expected mode of failure and either the coating or
substrate is flexible and strong enough to be peeled. Similarly,
organic coatings are not used to make adhesive joints and we would not
normally use tests that measure joint strength. However, there are
exceptions. One exception would be in the case of relatively thick,
hard, strong, tile like coatings that may defy other commonly used
methods. To summarize then, we could say that the type of test used
should be dictated by the mode of failure expected in service, which
may require more than one type of test.
of Test Methods
For the most part organic coatings are removed in service
by abrasion, chipping, cutting, or scraping off through the use of
knives, coins, and other instruments, picking away at exposed edges,
corrosion of the substrate, impact or impingement by stones, etc. Most
failures start at the edges or corners of objects and not in the middle
of a flat section.
At one time the automotive industry made extensive use of
the Gravelometer, because they believed that it best duplicated the
kind of impact and chipping failure (chipping at door edges, etc.)
encountered in service. Over the past decade they have placed less
reliance on it, and this writer believes that most people who own a
late model automobile will agree that they should place more reliance
on it. However, most technicians judge how well an organic coating
adheres to a substrate by how difficult it is to remove with a
penknife. A penknife is used almost universally because, among other
things, its mode of removing coatings is fairly representative of the
type of removal failure encountered in service. Many devices developed
for testing the adhesion force of removal of organic coatings have
been, literally speaking, mechanized knives. Of course there have also
been other types of cutting devices, scratching devices and a wide
variety of exotic and gimmick types of devices. We must bear in mind
that the usefulness of any device or method should not be judged by its
complexity, impressiveness, or novelty. Indeed, in this writer s
opinion, some of the latter are the most useless.
Chisel Adhesion Test
The first record of a chisel adhesion test appears to be
that of Rossmann. A chisel under a definite load is caused to bite into
the film at a fixed angle. Meanwhile the panel supporting the film is
drawn against the edge of the chisel by means of a spring. The tension
of the spring diminishes during the test until it is nolonger
sufficient to separate the film from its support. The principle of the
device is shown in Fig. 1, but in practice the chisel is a safety razor
blade, a new one being used for each test. It is turned to such an
angle that it cuts a path 1 cm wide. This allows convenient loads and
forces to be used. The razor blade is inclined at an angle of 45 deg to
the panel, the load on it is usually 10 kg, and the starting tension on
the spring varies from 2 to 10 kg. A time tension curve is recorded on
a rotating drum. From this curve, various characteristics of the
adhesive forces may be read or calculated.