1. ADHESIVE PROPERTIES AND GENERAL CHARACTERISTICS
Epoxies
Phenolic Adhesives
Nitrile Adhesives
Vinyl Adhesives
Neoprene
Polyurethanes
Silicones
Polyesters
Acrylics
Rosin (Sometimes Called Colophony)
Polysulfide Rubber Adhesives
Ceramic Adhesives
Cyanoacrylate Adhesives
Polyaromatic Adhesives
Vinyl Phenolic Adhesives
Neoprene Phenolic Adhesives
Epoxy-Silicone Adhesives
Epoxy-Polysulfide Adhesives
Epoxy-Nylon Adhesives
Epoxy-Phenolic Adhesives
Nitrile-Phenolic Adhesives
Modified Epoxy Intermediate Curing Films
2. ADHESIVE MATERIALS AND PROPERTIES
The Components of An Adhesive
Adhesives Types
Thermoplastic Adhesives
Thermosetting Adhesives
Rubber-Resin Blends
Properties of Basic Adhesives Types
Acrylics
Acrylic Acid Diesters
Allyl Diglycol Carbonate
Animal Glues
Blood Albumen
Butadiene-styrene Rubbers
Butyl Rubber and Polyisobutylene
Casein
Cellulose Derivatives
Cellulose Acetate
Cellulose Acetate-butyrate
Cellulose Caprate
Cellulose Nitrate (Nitrocellulose or Pyroxylin)
Ethyl Cellulose
Hydroxy Ethyl Cellulose
Methyl Cellulose and Sodium Carboxy Methyl Cellulose
Ceramic or Refractory Inorganic Adhesives
Cyanoacrylates
Epoxy Adhesives
Epoxy-Nylon
Epoxy-Polyamide
Epoxy-Polysulphide
Epoxy-Polyurethane
Fish Glue
Furanes
Hot-Melt Adhesives
Inorganic Adhesives and Cements
Sodium Silicate
Phosphate Cements
Basic Salts (Sorel Cements)
Litharge Cements
Sulphur Cements
Hydraulic Cements
Inorganic Polymers
Ionomer Resins
Isocyanates
Isocyanate Adhesives
Isocyanate—Modified Adhesives
Isocyanate—Polyester Methane Adhesives
Melamine Formaldehyde
Natural Rubber
Nitrile Rubbers
Permanence
Nylon Adhesives
Solution Adhesives
Hot-melts
Phenolic-nylon
Phenolic-epoxy
Phenol Formaldehyde (Acid Catalysed)
Phenolic Formaldehyde (Hot Setting)
Phenolic-Neoprene
Phenolic-Nitrile
Phenolic-Polyamide
Phenolic-Vinyl Butyral
Phenolic-Vinyl Formal
Phenoxy
Polyamides
Polyaromatics
Polyimides (PI)
Polybenzimidazoles (PBI)
Polybenzothiazoles (PBT)
Polyphenylenes (PP)
Polychloroprene (Neoprene) Rubbers
Polyesters
Allyls
Alkyds (or Glyptals)
Polyesters (Unsaturated)
Polystyrene
Polysulphide (Thiokol)
Polyurethanes
Polyvinyl Acetals
Polyvinyl Acetate
Polyvinyl Alkyl Ethers
Polyvinyl Alcohol
Polyvinyl Chloride
Reclaim Rubber
Resorcinol Formaldehyde and Phenol
Resorcinol Formaldehyde
Rubber Derivatives
Chlorinated Rubber
Cyclised Rubber
Rubber Hydrochloride
Silicones
Silicone Rubber
Epoxy-silicone
Soy(a)bean and Vegetable Proteins
Starch
Thermoplastic Resins (Miscellaneous)
Coumarone-indene
Shellac
Rosin (Colophony)
Oleo-Resins (Vegetable Oils + Rosin, Phenolic or Alkyd Resins)
Bitumen (Including Asphalt)
Urea Formaldehyde
Water and Solvent Based Adhesives
Waxes
3. PHYSICAL TESTING OF ADHESIVES
Introduction
Strength Properties
Assessment of Durability and Strength
Parameters
Fatigue
Creep
Flexural Strength
Peel Strength
Durability
Non-Destructive Testing
Standard Test Methods
4. POLYVINYL ACETATE WOOD ADHESIVES
Introduction
Background
Chemistry of Polyvinyl Acetate
A. Production of Vinyl Acetate Monomer
B. Polymerization of Vinyl Acetate
Formulating A Pva-Based Adhesive
A. General Considerations
B. Formulating and Compounding
C. Guide Formulations
Aspects of Application
A. Joint Design
B. Surface Preparation
C. Adhesive Preparation
D. Application
E. Assembly Conditions
F. Influence of Temperature
Performance of Pva Adhesives
A. Factors Affecting Durability
B. Specifications
C. Testing
Conclusion
5. AMINORESIN WOOD ADHESIVES
Introduction
Chemistry of Aminoresins
A. Urea-Formaldehyde Condensation
B. Melamine-Formaldehyde Condensation
C. Aniline-Formaldehyde Condensation
D. Reaction Kinetics: Urea-Formaldehyde
E. Reaction Kinetics : Melamine-Formaldehyde
F. Reaction of Methylolureas in the Presence of Cellulose
G. Reaction Mechanisms: Urea-Formaldehyde
H. Reaction Mechanisms: Melamine-Formaldehyde
I. Hardening
J. Analysis
Chemistry and Technology of Application of
Aminoresin Adhesives for Wood
A. General Principles of Manufacture and Application
B. Formulaire
C. Plywood and Particleboard Adhesives
D. Melamine Laminates
E. Glulam, Finger Jointing and Joinery Adhesives
F. Toxicity
6. PHENOLIC RESIN WOOD ADHESIVE
Introduction
Chemistry of Phenol-Formaldehyde Condensations
A. Reaction Mechanisms
B. Nature of Mechanism : Methylene and Methylene-Ether Bridges
C. Acid Catalysis
D. Alkaline Catalysis
E. Metallic Ions Catalysis and Orientation of the Reaction
F. Reaction Kinetics
G. Hardening
H. Resorcinol and Meta-Aminophenol Condensates
Chemistry and Technology of Application of
Phenolic Resin Adhesives for Wood
A. General Principles of Manufacture
B. Plywood and Particleboard Adhesives an
the Factors Regulating Their Application
C. Properties of Phenolic Adhesives for Plywood
D. Formulation of Plywood Glue Mixes
E. Plywood Manufacturing Variables
F. Wood-Related Factors
G. General Observations on Particleboard Manufacture
H. Dry-Out Resistance
I. Wood Laminating and Finger Jointing Adhesives
J. Fast Setting Adhesives for Finger Jointing
7. TANNIN-BASED WOOD ADHESIVES
Introduction
Chemistry of Condensed Tannins
A. General
B. Monoflavonoids
C. Biflavonoids
D. Triflavonoids and Tetraflavonoids Condensed Tannins
E. Methods for the Analysis of Phenolic Materials Content in Tanning Extract
Reactivity of Tannins as Macromolecules
A. Reactivity and Orientation of Electrophilic Substitutions of Flavonoids.
B. A- and B-Ring Reactions with Aldehydes and Their Kinetics
C. Metal Ions Catalysis
D. Hydrolysis and Acid and Alkaline Autocondensation
E. Sulfitation
Chemistry and Technology of Industrial Tannin Adhesive Formulations
A. General
B. Standardization of Industrial Tanning Extracts
C. Exterior-Grade Plywood Adhesives
D. Cold-Setting, Fast-Setting and Radio-Frequency Laminating Adhesives
E. Exterior-Grade Particleboard Adhesives
F. Corrugated Cardboard Adhesives
G. Generation of Resorcinol
H. Infrared Analysis of Resorcinol Content in Tannin-Based Adhesives
8. URETHANE STRUCTURAL ADHESIVE SYSTEMS
Introduction
A. Historical
B. Advantages and Limitations
Chemistry
A. Basic Concepts
Application—Meter-Mix Equipment
Curing, Testing and Durability
A. Curing
B. Testing and Durability
Health, Safety and Environmental Considerations
Quality Control of Urethane Adhesives
9. MODIFIED ACRYLIC STRUCTURAL ADHESIVES
Introduction
History
Performance Properties
A. Advantages
B. Disadvantages
C. General Performance
Curing Properties
Technology
Handling Properties
A. Accelerator Lacquer Method
B. Two-Component Mix Method
C. Two-Component, No-Mix Method
Representative Case Histories
A. Solar Heating Panels
B. Ceramic Magnets
C. Shipbuilding
D. Sporting Goods
E. Aircraft
Meter, Mix, Dispense Equipment
Present Limitations and Future Directions of
Modified Acrylic Structural Adhesives
10. PHENOLIC ADHESIVES AND MODIFIERS
Introduction
Chemistry of Phenolic Resins
Analytical Test Methods
Phenolic Adhesives
Phenolic Modifiers
Phenolic Modifiers as Tackifiers
Solvent-Based Contact Adhesives
A. Neoprene-Phenolic Contact Adhesives
B. Adhesive Compounding
C. Adhesive Testing and Performance
D. Solvent Blend
E. Nitrile-Phenolic Contact Adhesives
Phenolic Dispersions
Other Uses for Phenolic Tackifiers
Structural Adhesives
A. Vinyl-Phenolic Structural Adhesives
B. Nitrile-Phenolic Structural Adhesives
C. Epoxy-Phenolic Structural Adhesives
Summary
Suppliers of Trade-Name Material
11. CYANOACRYLATE ADHESIVES
Introduction
Types of Cyanoacrylate Adhesives
Mechanism of Bond Formation
Advantages
Limitations
Bonding Characteristics on Various Substrates
A. Metals
B. Plastics
C. Rubber
D. Glass
E. Wood and Porous Materials
Dispensing Cyanoacrylates
Requirements for Successful Use of Cyanoacrylate Adhesives
Commercial Applications in Product Assembly
Toxicity and Handling Precautions
A. Toxicity
B. Handling Precautions
Cleaning Up Excess Adhesive
How to Release Bonds
Shelf Life of Cyanoacrylates
12. HOT-MELT ADHESIVES
Introduction and Definition of Hot-Melt Adhesives
Advantages and Limitations of Hot-Melt Adhesives
A. Advantages
B. Limitations
Types of Hot Melts Based on the Backbone Polymer
Elementary Principles of Joint Design
Hot-Melt Adhesive Usage by Industry
Where Hot-Melt Adhesives are Used
Summary of Adhesives by Base Polymer or Use
What to do when Problems Occur while using Hot-Melt Adhesives
Safety Suggestions for using Hot-Melt Adhesives
Hot-Melt Adhesives—Forms and Shapes
Hot-Melt Adhesives—Anticipated Future Developments
Thermoplastic-Thermoset
Foamable Hot Melts
Exotic Polymers
13. PRESSURE-SENSITIVE ADHESIVES
Introduction
Theory
Surface Tack
Peel Adhesion
Shear Resistance
The Influence of Polymer Structure on Performance Properties
Market and Trends
A. Introduction
End Uses
Solvent-Based Pressure-Sensitive Adhesives
Water-Based Systems
Hot-Melt Pressure-Sensitive Adhesives
Radiation Curing
Coating Methods
Test Methods
14. WATER-BASED ADHESIVES
Introduction
Types of Water-Based Adhesives
Chemistry and Formulating of Water-Dispersed Adhesives
A. Natural-Rubber Latices
B. Synthetic-Rubber (Polymer) Latices
Postformed-Rubber (Polymer) Latices
Film Formation of Water-dispersed Adhesives
Bonding Techniques
A. Wet Bonding
B. Open-Time Bonding
C. Contact Bonding
D. Solvent Reactivation
E. Heat Reactivation
Forced Drying of Latex Adhesives
Properties of Latex Adhesives Versus Solvent-Based Adhesives
Applications for Various Types of Latex Adhesives
Characterization of Latex Adhesives
A. Physical Properties
B. Application Properties
C. Performance Properties
Adhesive Selection
15. THE BONDING PROCESS
Storage
Preparation of the Adhesive
Methods of Adhesive Application
Brushing
Flowing
Spraying
Roll Coating
Knife Coating
Silk Screening
Melting
Methods of Adhesive Bonding
Wet Bonding
Reactivation Bonding
Pressure-Sensitive Bonding
Curing
Other Methods of Bonding
Inadequate Bonding
Methods of Bond Curing
Direct Heat Curing
Radiation Curing
Electric Heaters
High Frequency (Radio Frequency) Dielectric Heating
Induction Heating
Low-Voltage Electric Heating (L.V.H.)
Ultrasonic Activation
Bonding Pressure
    Equipment for Processing Adhesives

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Adhesive Properties and General Characteristics

An adhesive or formulation is generally a mixture of several materials. The extent of mixture and the ratio usually depend upon the properties desired in the final bonded joint. The basic materials may be defined as those substances, which provide the necessary adhesive and binding properties.

Solvents are employed in many systems to provide vehicle and viscosity control. In some cases, low-molecular-weight resins of high fluidity are added to the basic resin to help control viscosity.

Fillers such as metallic oxides, mineral powders and various fibers are sometimes used to control reinforcement, decrease shrinkage, lower the coefficient of thermal expansion, control temperature operating ranges, and in some instances provide a more satisfactory system for a special environmental condition. Fillers are also used to control viscosity, especially if a thixotropic paste is desired. The most common of these are the ultrafine mesh silicas such as Cab-O-Sil. Most fillers also lower the cost of the system. They also prevent waste by virtue of improving the handling properties. They are often referred to as “extenders.”

Catalysts and hardeners are employed to activate the resin systems, especially where thermosetting resins are concerned, in order to speed up hardening and make the adhesive system practical. Acids, bases, salts, alcohols, sulfur compounds, and peroxides are a few of the basic catalyst materials. The selection must be based upon a knowledge of the mechanics of polymerization reactions which account for the curing or hardening of adhesives. Catalysts are very important in forming the final joint. The amount of catalysis is critical. Overcatalying may result in a poor joint, and the same holds true for undercatalyzing.

There are several classes, types, and groups of adhesives. These have been classified as to use, chemical composition, mode of application, setting factors, vehicle, etc. The first general classifications to be considered are structural and nonstructural adhesives. These classifications are sometimes difficult to clarify. A structural adhesive would normally be defined as one, which can be employed where, joints or load-carrying members associated with primary design are required. This type of adhesive will be subjected to large stress loads. The term “structural bonded joints” equates “structural” with the importance of its mission. In this concept, a further definition may be required where “primary structural” means loss of the aircraft or vehicle through joint failure and “secondary structural” means severe damage and impairment of the mission. The criteria in many cases have been defined on the basis of bond strength using the arbitrary top value strength of 1000 psi.

This is considered by many as a very poor definition, and since many people have disagreed on these terms, this discussion is included only to raise the question and allow the individual concerned to draw his own conclusion. It must be considered because a large portion of companies have specifications placed in these general categories. The major problem is that they all differ in context.

Nonstructural adhesives are not capable of supporting appreciable loads and are generally required to locate parts in an assembly. They will be employed many times where only a temporary bond is required. Their failure would not usually result in the loss of a vehicle. Adhesives, sealants, and coatings usually fall into this category, but could be responsible for the full accomplishment of the mission.

The type of adhesive material is easier to define and usually falls into three categories:

          1.          Thermosetting resins are synthetic organic substances, which can be converted by chemical reaction into a permanently hard, practically infusible, and insoluble solid. These resins are high-molecular-weight polymers, which react by polymerization to form hard substances, usually rigid and possessing high strength properties. Thermosetting resins usually have a high modulus of elasticity, do not support combustion, and resist the action of most chemicals. When reacted, the thermosetting system will not be liquified by heat but will deteriorate or decompose under heat ranges beyond its limitations. We might compare this to the baking of bread. Once it is catalyzed (baking powder), further baking will only burn it.

          2. The thermoplastic resins are often employed in metal and plastic bonding and usually adhere well to both. They do not lend themselves to use as good load-bearing adhesives, especially if they would be subjected to elevated temperatures. They will soften when heated and harden when cooled. An example here would be butter, placed in a molten liquid state by heating and becoming solid upon cooling.

                    The more common thermoplastic resins include the polyvinyls, acrylics, polystyrenes, celluloses, and polyamides. They are sometimes used effectively with thermosetting resins for specific formulations.

          3. Elastomeric resins are used widely for modification of the thermosetting systems. They generally fall into a distinct class; e.g., natural and synthetic rubber. A true elastomer is usually defined as a material that will stretch twice its original length without inherent loss of elastic properties. When used as a modifying agent for other resins, they usually induce flexibility and increase peel strength of the systems. They are often used alone or in slightly modified form for sealants, but lack the strength to be used alone for structural applications. Examples of this class are the butyls, nitriles, polysulfides, and neoprenes.

Epoxies

A thermosetting system, 100 percent reactive when in a pure state, the epoxies are very desirable and more widely used than any other chemical type. Epoxy is one of the newer types and has penetrated more fields of manufacturing operations in a shorter space of time than any of its predecessors. The epoxies have been formulated from more materials than any other class. They are very versatile and can be formulated to do any job, limited only by heat. Some formulations will withstand 800°F for short periods. The heat ranges of the epoxies are usually determined by the catalysts utilized to harden the system. The many catalysts used with epoxies produce systems of variable properties. The most common are the aromatic amines and cyclic anhydrides. The amines produce the low-temperature cure cycles and limited heat range, while the anhydrides usually require higher cure cycles and withstand higher operational temperatures. Table 1 shows the general properties of a basic epoxy resin hardened by various catalysts with 301 stainless steel adherends.

Epoxies are available in liquid, paste, and film forms (supported and unsupported). The two-component systems are more widely used because of the extended shelf life. They may be stored for long periods and, naturally will not activate until mixed. A few of the early epoxies were one part, in a stick form that was heated prior to application, which proved to be impractical from an application standpoint. Epoxy adhesives are not widely used in the film form unless they are modified, that is, alloyed with another adhesive system.

The epoxies are not affected by bond-line thickness as compared to many other structural adhesives. This is important for application and processing, because the epoxies require very little pressure, and become very fluid when heated prior to the gel or “B” stage. The thin bond line is preferred, but is sometimes difficult to control with a paste or liquid. If bond-line control is essential, it may be accomplished by utilizing glass beads of the desired size in the resin, which do not adversely affect the mechanical strength unless used excessively. The bond-line control is one of the prime advantages of the film-type adhesives, especially if a carrier is utilized. When utilizing the epoxy with a carrier, care must be exercised in pressure application, because bond-line starvation will occur, due to the very fluid state of the resin under heat and pressure.

The epoxies have low peel and impact strength as compared to many other structural adhesives because of their brittle nature after cure. To improve the undesirable properties, they are alloyed with various other adhesive systems to produce a system to meet the demand of design requirements.

In summary, these factors should be remembered pertinen to resins:

          1.          Adhesion. The expoxies have high specific adhesion to metals, glass, plastics, ceramics, paper, concrete, wood, and various other substrates. Because of their brittle nature, epoxies are not recommended for bonding the rubbers and elastomeric adherends, although they will adhere to these types of materials. The epoxies can be formulated to create mixtures of low viscosity and improved wetting, spreading, and penetrating action. If the substrate to be joined is cleaned and processed properly, adhesion presents very few problems.

          2.          Cohesion. When properly cured, the cohesive properties are considered very good, but are usually the limiting strength factor. The adhesive properties are superior to the cohesive properties in most formulations, thus cohesive failures will be experienced during testing from room temperature to the maximum operating limits of the system.

          3.          100 percent solids. The epoxies in the unmodified state cure without releasing water or other condensation byproducts. This makes them desirable where contact pressures are necessary for manufacturing. They are also convenient for bonding such materials as glass or thermoplastics, where high heat and pressures would be unsatisfactory. This characteristic also makes them desirable as potting compounds, since the possibility of air bubbles or inclusions is reduced. The addition of silver, carbon, or other conductors has proven very successful in varying the electrical properties of epoxies without the problems of discontinuities in the bond line and also without adversely affecting the mechanical properties of the system.

          4.          Low shrinkage. The epoxies cure with only a fraction of the shrinkage of vinyl-type adhesives such as polyesters and acrylics; consequently less strain is built into the glue line, and the bond is stronger. The shrinkage can be reduced to a fraction of 1 percent by incorporation of silica, aluminum oxide, or other organic fillers. A shrinkage factor of 3 percent would be considered extremely high for epoxies.

          5.          Low creep. They maintain their shape under prolonged stress better than thermoplastics and many thermosetting systems. This is an important asset in favor of the use of epoxies, because creep is considered a major problem in structural adhesive bonding, and an area of prime concern by designers. Creep, in all probability, has hampered the use of adhesives and plastics in the building industry more than any other single factor.

          6.          Resistance to moisture and solvents. The epoxies are resistant to moisture. Moisture does not effect an epoxy in the least but will migrate through the joint and deteriorate the substrate. When epoxy bonded joints are subjected to moisture or water immersion, the failures usually occur at the interface. This indicates the importance of proper surface preparation of the adherends. Their resistance to solvents is considered outstanding and accounts for their rapid advancement in the coating field. Because fluids do migrate through an epoxy with little or no effect to the system the substrate problem does exist, Which makes other systems more desirable for use in long-term exposure to such fluids as fuels, although when modified with an elastomeric system, for example, they may possess very desirable properties in these areas.

          7.          Versatility applicable to modification. The properties of the epoxy may be changed by:

          •          Varying of the base resin and curing agents.

          •          Varying cure cycles, both temperature and cure time.

          •          Alloying the compound with another resin.

          •          Compounding the various fillers. This may affect the cost factor, but the economics of epoxies are governed more by the type of catalyst utilized.

They are effective barriers to heat and electric current, yet at the same time may be modified easily for conduction of electricity. They are versatile in applying due to their wide range of modification, and may be applied manually, semiautomatically, or automatically.

Phenolic adhesives

The phenolics or phenol-formaldehyde resins are formed by the condensation reaction of phenol and formaldehyde. This material was discovered in 1872. The phenolics are very rigid, strong, and have excellent resistance to fungi. They have moderate to good resistance to moisture, and very good high-temperature properties. The phenolic resins have been used extensively in the lamination of plywood and in filament-wound structures. They enjoy a wider range in the structural-adhesive category when alloyed with other materials.

There are two basic classes of phenolic resins: resoles and novalacs, and both begin as phenol alcohols. They are catalyzed with either an acid or an alkali. Regardless of the formulation of phenolic resins, they are considered to have high resistance to deteriorating influences encountered in service. They would not be considered excellent in resistance to stresses caused by thermal expansion, and extenders should not be used in attempts to correct this weakness. When combined or alloyed with other adhesive systems, they become excellent structural adhesives and are widely used in this manner throughout the aerospace industry.

Nitrile adhesives

The nitrile rubbers are elastomers and copolymers of unsaturted nitriles and dienes. The nitriles are not used as structural adhesives in this form, but yield many one-part adhesives that are used for bonding small nonstructural parts, especially in the electronics and plastics The nitrile rubbers, when prepared for use as a cement, are milled on tight cold-mill rolls, broken down, and rendered soluble in some type of solvent. The most widely used nitrile rubber adhesives are cured by the solvent escape drying method, but they may be catalyzed by the utilization of sulfur compounds and cured at room or elevated temperatures. The nitriles are available from the manufacturers in a variety of formulations, but the important role of this rubber system for structural adhesives comes as a result of being alloyed or mixed with another resin. The nitriles, like the phenolic resins, do not have the desired properties for structural bonding when used alone, but, for example, if the nitriles and phenolics are combined their mechanical properties change to a system with excellent properties for structural use. The nitriles give the rigid resins flexibility that produces high peel strengths and better than average shear strengths.

Vinyl adhesives

The vinyl polymers do not stand alone as a structural adhesive, but hundreds of adhesives are formulated by the use of this class of polymer. Vinyl is the univalent radical CH2:CH-, derived from ethylene, a compound which undergoes polymerization to form high-molecular-weight resins. More generally, the term “vinyl polymer” has been used to include a variety of resins, plastic films, and elastomers obtained by polymerizing monomers having one or more unsaturated double or triple bonds, including diolefins, such as butadiene, vinyldienes such as vinyldiene chloride or methyl methacaylate, and unsaturated compounds such as maleic anhydride.

The vinyls are important to adhesive bonding not only from the adhesive standpoint, but because the films derived from these substances are widely used as vacuum bags, slip sheets, etc. The more widely used ones are polyvinyl chloride, polyvinyl alcohol, and polyvinyl fluoride.

Neoprene

Neoprene was the first synthetic elastomer developed that possessed properties comparable to natural rubber. It is defined as an oil-resistant synthetic rubber obtained by polymerizing chloroprene. The neoprenes were limited in use due to the cost factor until the shortage of natural rubber in World War II. At that time, neoprene was the only synthetic rubber available for use in adhesives and as a result, formulators began to experiment with it. They found that neoprene adhesives were just as good, if not better in many cases, than those based on natural rubber. The neoprenes are used in three general capacities in the adhesive industry. They are used structurally when alloyed with another resin, as a rubber cement and as a noncuring “tacking” paste. The neoprene cements are usually dispersed in solvents such as toluene, which is one of the morewidely used. It may be dissolved in mixtures of aromatic and aliphatic hydrocarbons.

The neoprene cements may be cured at room temperature or by the use of heat, depending upon the accelerator used. Magnesium and zinc oxides are two of the more common accelerators, which effect a slow, room-temperature cure.

The maximum operating temperature does not usually exceed 170°F and would show signs of degradation if used at that temperature for long periods of time. As neoprene ages, traces of hydrochloric acid are formed by decomposition of the chlorine-containing molecules; this acid tends to deteriorate most fabrics such as cotton, rayon, linen, etc.

Polyurethanes

A wide variety of polyurethanes can be formed by cross linking highly reactive isocyanates with various polyols. This elastomeric material provides a bond which resists not only the shear and tensile stresses satisfactorily but has very high impact resistance and excellent cryogenic properties. This has brought them into widespread use in space applications, especially for insulation problems. They are also widely used as sprayable coatings for aircraft wing assemblies, and for bonding solid propellants. They are utilized for bonding metal to metal, elastom-eters, foam, plastics, nylon, glass, ceramic, and the fluorocarbons. Due to the flow characteristics they are not considered a good material for honeycomb construction.

The cohesive strength is usually better than the adhesive strength, but good cohesive failures are obtained by careful processing with a majority of formulations. They yield from 3000 to 5000 psi in shear at room temperature, but shear strength varies with cure conditions and pressure. There is a correlation between the bond-line thickness and shear strength, the ideal bond-line thickness being in the range of 2 to 6 mils. They will yield up to 8000 psi in shear at -423°F, but are limited to approximately 250°F at elevated temperatures.

The polyurethanes are not considered ideal. They pose processing problems due to their reaction with water and their gaseous nature. The systems that are MOCA catalyzed require hot mixing to diffuse the catalyst into the resin. The ratio of MOCA to resin has varying effects on the final joint and should be carefully controlled. They may be degassed before application, but the amount of degassing affects the pot life. When applied before gelling starts, they are very fluid and sometimes tend to cause starved bond lines. This has been controlled in special applications by the addition of 6 to 12 percent of nylon fibers.

Another problem associated with the polyurethane system applies to storage. Storage must be maintained that will inhibit fractional crystallization; in fact, it should inhibit any crystallization and water accrual in the raw material.

In summary, the polyurethanes are excellent cryogenic materials, exhibit excellent shock properties, are more difficult to process than many systems, suffer from excessive creep at room temperature, and show changes in properties on aging, some of which are undesirable. They still hold the answer to cryogenic application, but have poor elevated-temperature strength.

Silicones

Silicones are semi-inorganic polymers made up of a skeleton structure of alternate silicone and oxygen atoms with various organic groups attached, and are thermosetting-type resins. A large variety of the RTV (room-temperature vulcanizing) compounds are formulated utilizing the silicone resins. They do not possess the mechanical properties to be used as structural adhesives, but are widely used as sealants and potting compounds. The unit was designed to pass heat from the electronic equipment to the outside radiation system.

The silicones vary in curing temperatures from room temperature to 250°F, depending on the formulation and vulcanizing agent. The majority of these systems require only contact pressures during cure.

The silicones have many very desirable characteristics as listed:

1. Good high-temperature properties. They have good thermal and oxidative stability at temperatures up to 600°F and will withstand short exposures up to 800°F.

2.          Silicones are good thermal insulators, which accounts for their utilization as thermal insulation and heat sinks.

3.       They have good low-temperature properties when compared to many other systems. The methyl silicones have brittlepoints at -100°F. but the methyl-phenyl silicones may be used to -175°F.

4.       They maintain good electrical properties over a wide temperature range.

5.       They have adequate resistance to aging and weathering and remain, stable when exposed to ozone, corona, and sunlight.

6.       They have fair resistance to water and moisture.

7.          Generally, all silicones will withstand radiation; however, the most effective group is the silicone resins, followed closely by the silicons rubbers. In all probability, the most outstanding characteristic is their ability to resist combined heat and radiation.

8.       The ease of handling and low-temperature cures brand the silicones for future growth.

The silicones are handicapped by low shear strength and many do not possess the adhesion or tack quality level desired. Adhesion may promoted by the use of primers. They deteriorate under constant contact with fuels, which limits their usage in fuel areas.

Polyesters

The reaction of organic acids and alcohols produces a class of materials called “esters.” When the acids are polybasic and the alcohols are polyhydric, they can react to form very complex esters. They are usually called “alkyds” and have long been useful as surface coatings and glass-reinforced plastics. This same principle, utilized with various modifications, brings the polyesters into the adhesive field. The polyester adhesive systems cure rigid, and have a temperature operating range up to 500°F. They reveal poor adhesion to metals, especially aluminum.

The polyesters are attacked by most solvents and have a high shrinkage rate when compared to other adhesives. Shrinkage rates may run as high as 4 percent. Attempts are made to combat the high shrink rate by the utilization of fillers such as calcium carbonate and aluminum silicate.

Recently a new polyester has been developed that is flexible. The evaluation of this system is incomplete but indications are that it adheres better to metals than many of the earlier polyester systems.

Acrylics

The acrylics are a group of thermoplastic resins formed by polymerizing the esters or amides of acrylic acid. They are usually transparent, low viscosity, polymerizable liquids and were developed primarily for use as liquid locknuts. They are now used as adhesives, but are more important pertinent to structures as transparent sheets (plexiglass and Lucite).

The acrylic adhesives have indefinite shelf life when stored at ambient temperature with access to oxygen. When oxygen is excluded by applying the material in a thin film between two mating surfaces, gelation occurs at room temperature in a matter of minutes. To prevent gelation before application, the liquid is packaged in a low-density polyethylene container permeable to oxygen. Curing may be accelerated by elevating the temperature using an oven, heat lamp, or press. The acrylics have been cured successfully in a vapor degreaser when small details are being joined. The heat causes gelation to occur before the solvent extracts the adhesive from the joint. Perchloroethylene, with a boiling point of 250°F results in a more rapid cure with less leaching than trichloroethylene with a 190°F boiling point. Also, the vapor degreaser removes the thin film of liquid that is kept from curing by contact with the air.

If the liquids are applied to sensitive electromechanical devices, be sure the uncured surface liquid is removed. Outgassing and condensation of volatiles in sealed systems may cause problems in service or storage.

Certain metals, such as zinc, cadmium, and gold, do not promote cure of these materials. For these metals an organometallic activator is supplied in solution in a chlorinated solvent, which is applied directly to the metal and allowed to dry. Zinc and cadmium plated surfaces can also be activated by a chromic rinse prior to sealing the surface.

Strength is far too low for the acrylics to be used as structural adhesives in lap joints, but resistance to torque shear is outstanding for joining cams, sleeves, pulleys, and gears to shafts in lieu of conventional fasteners.

Rosin (sometimes called colophony)

Spirit-soluble thermoplastic materials are available in two forms: gum rosin, the more popular form of which is obtained by distillation of the exudation fom pine trees, and wood rosin, which is prepared from pine trees.

Rosin adhesives are used for metal-container labeling either as hot melts or in solvent solution, often with added plasticizers. It is also used in the modification of other resins. Rosin is used in the powder form for bonding wood components in aircraft, but is not usually termed a structural adhesive. It has very good strength, and good water and moisture resistance, but poor resistance to fuels and solvents.

Polysulfide rubber adhesives

The polysulfide adhesives are synthetic polymers obtained by the reaction of sodium polysulfide with organic dichlorides such as dichloro-diethyl formal, alone or mixed with ethylene dichloride.

The polysulfides are used as adhesives where high strengths are not required, but are used more often as sealants. They are sometimes used as binders for solid propellants. This system offers good resistance to light, oxygen, oils, and solvents, and impermeability to various gases. They adhere well to almost any adherend, but have poor tensile properties. They exhibit poor properties when subjected to high humidity conditions and have an operating temperature range of –67 to +250°F.

The polysulfides are usually procured in two-component paste or liquid systems, have good shelf life, and require no special storage facilities. They may be catalyzed, mixed, and frozen for several days to eliminate production handling problems. The polysulfides are considered a wise choice (if the service requirements do not exceed their capabilities) because of the economy involved.

Ceramic adhesives

A typical formulation of ceramic adhesives may contain silica, sodium nitrate, boric acid, and ferric oxide. These materials are heated above 2000°F, blended, and then crystallized. A hard frit is formed, dried, milled, and then passed through a screen of the desired size to ensure uniform grain size. Oxides and water are then added and the results are ceramic adhesives. The viscosity may be controlled by the amount of water added to the mixture.

In recent years, investigations have been carried out to adapt the ceramic adhesives to structural bonding. They possess high shear strengths up to 1500°F (1800 psi in shear) and may reach 5000 psi in shear at room temperature.

The prime disadvantages are low-peel and flexural strengths coupled with very-high-temperature cure cycles. Much attention has been diverted from the ceramic systems since the newer polyaromatics became a reality. At present, the ceramics are not practical but research is currently in progress to improve the unfavorable properties they possess. The ceramics must be improved to become a sound structural adhesive but show great promise as an encapsulation material for high-remperaturc rocket nozzles and nose cones.

Cyanoacrylate adhesives

A special-purpose proprietary cyanoacrylatc adhesive (Eastman 910) is a one-part, clear, watery liquid. It is free of solvents and cures at room temperature in contact with many surfaces without the addition of a catalyst or hardener. The system sets by an anionic polymerization mechanism, which is catalyzed by weak bases such as traces of moisture on most surfaces in contact with the atmosphere. Surfaces such as phenolic, polyester, polyethylene, and polystyrene plastics tend to inhibit the curing rate, and may be pretreated with a diluted solution of an activator, phenylethylethanolamine (910 surface activator). However, most common metals, glass, wood, and rubber surfaces bond very rapidly and cure in a matter of minutes.

Shear strengths up to 4000 psi can be obtained, but peel and impact strengths are poor. When exposed to elevatrd temperatures the mechanical properties are poor, and aging as low as 160°F reflects degradation. When exposed to temperatures above this, The adhesives turn yellow and decompose. The prime advantage is the fast cure, and relatively little effort its required for well-mated joints. It is widely used for bonding small electrical components and as a tacking adhesive; however, it is expensive.

Problems have been encountered on production lines, as the adhesive presents an operator hazard because of its strong and rapid adhesion to the skin. Curing may be slow when the humidity is low, but this problem may easily be solved by placing an open container of water near the parts being bonded, but this material should never be placed in an oven for cure.

Adhesive Materials and Properties

THE COMPONENTS OF AN ADHESIVE

The components of the adhesive mixture are usually determined by the need to satisfy certain fabrication properties of the adhesive, or properties required in the final joint. The basic component is the binding substance which provides the adhesive and cohesive strength in the bond; it is usually an organic resin but can be a rubber, an inorganic compound or a natural product. Other constituents of the adhesive fulfil other functions.

Diluent

This is employed as a solvent vehicle for other adhesive components and also to provide the viscosity control which makes a uniformly thin adhesive coaling possible. Occasionally, liquid resins are added to control viscosity.

Catalysts and hardeners

These are curing agents for adhesive systems. Hardeners effect curing by chemically combining with the binder material and are based on a variety of materials (monomeric, polymeric, or mixed compounds). The ratio of hardener to binder determines the physical properties of the adhesive and can usually be varied within a small range. Thus, polyamides combine with epoxy resins to produce a cured adhesive. Catalysts, which themselves remain unchanged, are also employed as curing agents for thermosetting resins to reduce cure time and increase the crosslinking of the synthetic polymer. Acids, bases, salts, sulphur compounds and peroxides are commonly used and, unlike hardeners, only small quantities are required to effect curing. The amount of catalyst is critical and poor bond strengths result where resins are over or under catalysed.

Accelerators, inhibitors and retarders

These substances control the curing rate. An accelerator is a substance that speeds up curing caused by a catalyst by combining with the binder (a catalyst may have the same effect but will not lose its chemical identity during the process). An inhibitor arrests the curing reaction entirely whereas a retarder slows it down and prolongs the storage and/or the working life of the adhesive.

Modifiers

There are many chemically inert ingredients which are added to adhesive compositions to alter their end-use or fabrication properties. Modifiers include fillers, extenders, thinners, plasticisers, stabilisers, or wetting agents, and each material is used for a special purpose.

Fillers are non-adhesive materials which improve the working properties, permanence, strength, or other qualities of the adhesive bond and those commonly used are; wood flour, silica, alumina, titanium oxide, metal powders, china clay and earths, slate dust, asbestos and glass fibres. Some fillers may act as extenders.

Extenders are substances which usually have some adhesive properties and are added as diluents to reduce the concentration of other adhesive components and thereby the cost of the adhesive. Extenders often have positive value in modifying the physical properties of the glue-line by providing reinforcement to resins which would otherwise craze. Common extenders are flours, soluble lignin and pulverised partly-cured synthetic resins. Thinners are generally volatile liquids which are added to an adhesive to modify the consistency of other properties. Plasticisers are incorporated in a formulation to provide the adhesive bond with flexibility or distensibility, Plasticisers may reduce the melt-viscosity of hot-melt adhesives or lower the elastic modulus of a solidified adhesive. Stabilisers are added to an adhesive to increase its resistance to adverse service conditions such as, light, heat, radiation, etc. Wetting agents promote interfacial contact between adhesive and adherends by improving the wetting and spreading qualities of the adhesive.

ADHESIVES TYPES

One objective of this handbook is to indicate the basic properties of the different adhesives types. To a large extent the mechanical properties depend on the thermosetting or thermoplastic nature of the bond and the following general discussion of these differences provides background information to the detailed listing of the adhesives types (based on the major chemical ingredient) which follows.

Thermoplastic Adhesives

The thermoplastic adhesives are classified under the general categories of thermoplastic resin and thermoplastic rubber adhesives. As a class, thermoplastic adhesives are fusible, soluble, soften when heated and are subject to creep under stress. Unlike the thermosetting resins, they do not change chemically in establishing a bond. The thermoplastic nature of these materials confines their application as adhesives to low load assemblies formed from metals, ceramics, glass, plastics and porous materials based on paper, wood, leather and fabrics and which are not subject to severe service conditions. Hot-melt adhesives, which fall into this class, are being increasingly employed for fast assembly of packaging materials and plastic film laminates.

Thermoplastic resin adhesives are based on various synthetic materials (typified by the polyamide, vinyl and acrylic polymers and cellulose derivatives) or on natural products such as rosin, shellac, oleoresins and the mineral waxes. The important hot-melt adhesives are invariably compounded from polyethylene, vinyl polymers and co-polymers, polystyrene, polycarbonates, polyamides and other polymers. Additives, including plasticisers, fillers, and reinforcing materials, are frequently compounded with the resins to confer particular properties on the adhesive. With the exception of pastes, these adhesives are available in the same forms as the thermosetting adhesives, i.e. liquid forms can be solutions, dispersions, or emulsions of the polymer and other modifying components in a volatile medium. Solid forms are also available as films (supported and unsupported), pellets, sticks, or extruded cord lengths, suitable for machine application. Other solvent-free liquid forms (‘100% solids systems’) contain the thermoplastic material as a monomer or as a pre-polymer which requires a catalyst to bring about polymerisation to a high molecular-weight solid.

Thermoplastic rubber adhesives are some of the most versatile industrial adhesives currently used. The rubber based adhesives discussed in the following pages include; natural and reclaim rubbers and synthetic elastomers such as polychloroprene (neo-prene), butyl, styrene-butadiene and acrylonitrile-butadiene (nitrile). Most of the elastomers are available in solvent and latex forms or as water dispersions and other types are supplied with vulcanising agents. The thermoplastic rubber adhesives are generally modified with fillers, plasticisers and compounding ingredients. The types of rubber and solvent vehicle used partly determines the physical and chemical properties of the adhesives and the many compounding techniques employed result in widespread variations in strengths, tack ranges, drying rates, environmental resistance and other properties.

Heat or solvent activation is used to convert film adhesives into the fluid state prior to bonding. Solvent activation is applicable only to situations where an adherend is porous enough to permit solvent release by absorption and diffusion and heat activation is employed where adherends are impermeable and able to withstand the temperatures involved. Heating also has the effect of curing any thermosetting component which may be present in the adhesive. Both techniques are also used prior to bonding to activate substrates which have previously been coated with a solvent-base adhesive and dried to a tack-free state. Bonding is usually carried out under heat and pressure after joint assembly. Solid thermoplastic adhesives of the hot-melt type rely on heat to render them fluid and on cooling to bring about the setting action.

An unusual cure mechanism is displayed by the cyanoacrylates which are an example of chemically blocked materials. When confined between close-fitting parts, these one-component liquid monomers undergo polymerisation in a very short period (often 15 s). The thin moisture film which is usually present on exposed surfaces is sufficient to harden these materials if the glue-line is thin enough.

Thermosetting Adhesives

As a group, the thermosetting adhesives form bonds which are essentially infusible and insoluble through the action of heat, catalysts or combinations of these. In contrast to thermoplastics, the thermosetting resins display good creep resistance and provide the basis for many structural adhesives intended for high-load applications and exposure to severe environmental conditions such as heat, cold, radiation, humidity and chemical atmospheres. Thermosetting adhesives include materials of natural origin such as animal glues, soybean and vegetable proteins, casein and miscellaneous water-based adhesives as well as synthetic products based on epoxy, phenolic, polyester, polyaromatic and other thermosetting polymers.

Water-based adhesives prepared from low strength materials of animal or vegetable origin were the earliest adhesives used and are still, important for furniture and plywood manufacture, paper and packaging materials, and similar applications where low strength and a limited durability to outdoor conditions are acceptable. In addition there are thermosetting rubber-resin adhesives and other blends referred to as thermosetting-thermoplastic resin adhesives. These adhesives have increased toughness and strength while their improved resilience enhances stress-distribution properties. Examples of the latter class are the phenolic resins modified with nylon or various vinyl resins. The characteristics of these materials are, in general similar to those of thermosetting adhesives and hence many of these products are employed as structural bonding agents for metal to metal. Epoxy resins are modified with poly-sulphides to improve their flexibility and are thermosetting materials. However, polysulphide adhesives often function as sealing materials and may be thermoplastic or thermosetting according to formulation and cure.

Thermosetting adhesives are supplied as liquids, pastes and solids. Liquid types are generally one- or two-component systems which are already non-solvent, containing 100% solids materials, or react to become so by catalytic action. Some liquid adhesives contain a volatile solvent which is non-reactive and which acts as a dispersant or improves the handling and processing properties of the system. The curing agent for a liquid system may be a powder which requires to be melted before mixing the components. As a result of added modifying agents, pastes are usually thixotropic and may be applied to vertical joints as non-sag adhesives which will not flow out during assembly and cure of a bonded structure. Film forms may be supported or unsupported and of various thicknesses. They have the advantages of easy, clean handling and can be cut to conform to the shape of the joint. The shelf-life of film and one-component types is increased by refrigeration but in the case of some film adhesives cold storage is essential to prevent room temperature curing.

Natural product thermosets, like animal-glue, set by loss of solvent. Many two-component liquid types, such as epoxy resins, cure by catalytic action with or without the aid of heat. Other two-part thermosetting rubber-resin adhesives can be vulcanised at room temperature but otherwise curing with heat and pressure is necessary. Some rubber-resin adhesives which are used to bond unvulcanised rubber to metal cure during subsequent vulcanisation of the rubber while other adhesives, employed to bond already vulcanised rubber to metal, are cured separately. Structural film adhesives invariably require heat and pressure to realise the maximum mechanical properties and curing temperatures ranging from 150-250°C with bonding pressures up to 100 N/cm2 are not uncommon. Post-cures are often an additional processing requirement for structural adhesives where optimum strength is sought.

Rubber-Resin Blends

There are innumerable adhesives in which rubbers and resins, both natural and synthetic, are blended to obtain combinations of desired properties of both types of material. Blended adhesives may be employed for structural or general purpose bonding according to the type of resin and rubber used and their ratio in a formulation. Those consisting mainly of thermosetting resins modified with synthetic rubber are used for the structural bonding of metal and other rigid materials. Phenolic-nitrile and phenolic-neoprene adhesives are examples of this type, in which the rubber component serves to improve the flexibility of the cured bond and promote its resistance to impact or shock loading. Thermosetting resins alone lend to be brittle. Adhesives based on rubber, with a certain amount of natural or synthetic resin as a modifying component, represent the other end of the scale. In practice, the various types of rubber are rarely used alone as adhesives but are invariably modified with resins to improve such properties, as tack, cohesive strength, specific adhesion to surfaces and heat resistance. Within these extremes are numerous formulations in which various ratios of resin to rubber are used. These adhesives have a wide range of applications which include: bonding of textiles; bonding of synthetic fabrics to wood and metal; affixing wallboards and tiles; lamination of paper, metal foil and plastic films; laying of flooring materials; and various other industrial or domestic applications.

The structural rubber-resin adhesives are available as films or tapes (supported or unsupported on fabric carrier cloths) and occasionally as solvent solutions. The films are cured at elevated temperatures up to 200°C and under bonding pressures ranging from 30-100 N/cm2. Post-curing is often included to ensure optimum mechanical properties for the cured adhesive. Liquid types are dried to remove solvent and then processed as film ad-hesives. The non-structural rubber-resin adhesives are generally supplied as solutions in organic solvent mixtures and can be applied by brush, spray, dio or roller coater, spatula, or flow techniques. Because these adhesives rely on a loss of solvent before adhesive action can take place the shelf-life and working life are usually indefinitely long provided the solvent content is maintained. With porous adherends the assembly can be made with wet adhesive and time allowed for solvents to escape by diffusion through the material. Where solvents have a high volatility assembly times may be as short as 15 min by which time the substrates have lost the tackiness required for contact bonding. Impermeable materials are coated with adhesive and bonded together only after the bulk of the solvent has been dried off to leave the adhesive in a tacky state. Light assemblies can frequently be handled after a few hours but heavier assemblies require a setting period of at least 24 h. Maximum joint strength is not realised until after a few days following the removal of residual solvent traces. Wet-bonding generally produces joints having good strength and durability but poor solvent resistance. Optimum performance is given by heat curing (according to manufacturer’s instructions) which has the effect of removing the trace solvents otherwise retained by these adhesives and which act as plasticiscrs and increase the thermoplasticity of the system. Heat also promotes cross-linking of the adhesive constituents and thereby increases the creep resistance of the joint. Processing conditions depend on the adhesive with bonding pressures ranging from 10 300 N/cm2 according to joint factors such as rigidity, dimensions, closeness of fit and glue-line thickness. Low bonding pressures are more satisfactory for glue-lines exceeding 0.2 mm. Curing schedules range from 1 h at 80°C to 20-30 min at 140°C, with optimum properties resulting from the longer curing periods.

PROPERTIES OF BASIC ADHESIVES TYPES

This section has been prepared almost entirely from published material appearing in technical books and journals. The length of an entry is not indicative of the importance of the adhesive type under consideration since the amount of information available was found to vary considerably. Due regard has been paid to technical information in the trade literature received from the various adhesives manufacturers. Manufacturers’ literature was found particularly useful for confirmation of such adhesive properties as colour, available form, processing factors, and applications. These data necessary to supplement material from published sources and effort has been made to keep the section free from any trade bias.

It has already been noted earlier that adhesives based on the same material may show considerable variation in their properties where modifying materials have been added to the formulation. Properties are dependent, not only on the adhesive composition, but also on the conditions under which it is prepared and used. Because of these possible variations any values given in this section should be regarded as representative of the probable behaviour of a basic type of material used under certain conditions. This is further complicated by the fact that a large number of commercial adhesives are blends of two or more basic adhesive types; in particular both natural and synthetic rubbers and resins are often used together and form the basis of the numerous ‘resin-rubber’ or ‘rubber-resin’ adhesives which are available. Consultation with the manufacturers concerned is strongly recommended where detailed information is required on the behaviour of specific commercial adhesives under various service conditions.

Acrylics

Type

Thermoplastic resins based on acrylates (properties of polymethyl methacrylate types are discussed below) or derivatives (amides and esters).

Physical form

Available as emulsions, solvent solutions, and monomer-polymer mixtures (one or two components) with catalysts (liquid or powder). One component liquids which polymerise under ultra-violet radiation are available.

Setting action

Emulsion-solvent types set by evaporation and-absorption of solvent. Polymer mixtures set through polymerisation by heat, ultra-violent radiation and/ or chemical catalysts.

Processing conditions

Solvent types set over a period of 20 days at 20°C or 6 h at 80°C. Polymer mixture setting times depend on polymerising method used: chemical action, 14 d at 20°C or 4h at 80°C; ultra-violet action, 5 h exposure; heal action, 2 h at 5.5° C followed by 8 h at 80°C. Bonding pressures range from contact to 17 N/cm2.

Permanence

Resistance to weathering and moisture varies from poor (solvent types) to excellent (polymer mixtures). Change from transparent to yellow colour may occur with time (over 1 yr period). Not affected by alkalies, non-oxidising acids, salt spray, petroleum fuels but attacked by alcohols, strong solvents and hydrocarbons (aromatic and chlorinated). Highly resistant to ultra-violet exposure. Service temperature range of acrylic resin adhesives is — 60°C to 52°C.

Applications

Light structural assemblies based on acrylic plastics to themselves, wood, glass, metals, rubber, leather and fabrics; colourless jointing of decorative plastic laminates; production line assembly of components (ultra-violet effective here); outdoor applications such as plastic name-plates; aluminium foil work; windshields, instrument panels, lenses and optical components in aircraft, marine, and automotive industries. One-type, n-butyl methacrylate (alone or modified with Canada Balsam) is used as an optical cement. Ross-24 (modified type) is a heat-setting cement which is transparent ( = 1.485) and has good thermal shock resistance.

Physical Testing of Adhesives

INTRODUCTION

A detailed description of the various test methods that have been developed for adhesive bonds is beyond the scope of this handbook. A short outline, in chart form, of commonly used lest specimens follows the remarks on the evaluation of adhesive strength. Non-destructive test methods and the effects of adverse service conditions on bond strength are dealt with in subsequent paragraphs and the section concludes with a list, of titles of widely accepted standard test methods.

STRENGTH PROPERTIES

Specialised testing methods are required for the evaluation of the strength properties of adhesives. In addition to joint strength determination these methods provide a means for checking the efficacy of the processes used to make the bonds. The joint strength is invariably dependent on bonding technique factors such as adhesive application, adherend pretreatment and the adhesive curing conditions. Bonding conditions also determine the repro-ducibility of test results and complete information on a number of variables is, therefore, necessary before undertaking an adhesive evaluation. The following particulars are essential for the fabrication of reliable test specimens.

—          Instructions for the preparation of the adhesive

—          Adherend surface pretreatment procedures recommended for the adhesive under consideration. (Special treatments may be involved where certain environmental tests are envisaged)

          —          Adhesive application and processing before bonding. (Attention to coating thicknesses and their control, or drying conditions is often important).

—          Manufacturers’ specified conditions for joint assembly (temperatures, humidities or times)

Adhesive curing conditions relating to bonding temperatures, pressures and times.

Test specimens required to give reproducible failing strengths need to be carefully designed and prepared; unsatisfactory bonds will result from the faulty execution of any stage in the assembly process. Most of the standard test methods employ test specimens of definite shape and size, which have to be machined to specified tolerances. Most methods specify the number of specimens to be tested in order to obtain a reliable result because testing factors and slight differences between adhesive batches prepared under identical conditions lead to joint strength variations. Ten or more specimens may be required to give meaningful data. The equipment used for testing is important and will influence the reliability of strength values obtained. Some variations in the performance between machines of the same type are to be expected. Often, test machine accuracy is greatest over a limited working range of the loading capacity (usually 10-90%), and test specimens should fail at loads within this span. The rate at which test specimens are stressed is another factor influencing the strength values obtained for adhesive bonds. Standard test methods generally specify the testing rate although it may be better to adopt a rate, which more closely resembles the stressing rate that an actual assembly is likely to experience.

The data obtained from test methods is useful for comparing the performance of several adhesives prior to the selection of one for a particular assembly job. It must be emphasised that the test specimen rarely simulates the actual configuration of an assembly and that the test data cannot therefore, be relied upon to predict the performance of the assembly in service. The same limitation applies to test specimens removed from an assembly; these are unlikely to represent the behaviour of the whole structure. Short of testing an assembly under service conditions it is necessary to adopt a test specimen and method, which simulate the assembly and its working environment as closely as is practicable. The testing procedure finally employed must produce results that are likely to show a good correlation with the results that would he obtained in tests on assemblies. In this respect, selected standard test procedures are frequently employable without modification.

ASSESSMENT OF DURABILITY AND STRENGTH PARAMETERS

Fatigue

Fatigue testing refers to the repeated application of a specified load or deformation on a bonded specimen. Tests may be conducted under static or dynamic conditions (or both separately if necessary) according to the data required to evaluate an adhesive under service conditions.

Static fatigue properties are determined by measuring the maximum loading sustained by an adhesive over a given time. Various weight loadings applied to shear or tensile specimens provide a measure of the time required for bond failure.

Dynamic fatigue properties are measured by cycling test specimens with specified minimum to maximum stress loading for a given period or number of cycles or until failure. Cycle frequencies usually vary within the range 5000 to 107 Hertz. In addition to frequency, fatigue life is determined by amplitude, temperature and mode of stressing; these variables must be specified along with the extent of loading. These tests do not determine the damping properties or elastic moduli of adhesives.

Creep

There is no standard test for measuring the distortion or dimensional change in a bonded specimen under sustained loading (Creep). Deformation of the adhesive is generally measured by noting the dimensional change occurring when a bonded specimen is subjected to a constant load for a specified time and temperature. Room temperature creep is known as ‘cold flow’. Higher temperatures usually increase the rate of creep significantly.

Creep tests are often carried out to determine joint deformation when stressed below the failing load required to break the bond. Joints may be loaded by springs (ASTM D2294-64T) or dead weights (MIL-A-5090E) to maintain constant loading in a specified environment. Optical measurement of the shift in scribed reference lines on a lap joint edge is a useful method of creep assessment. Alternatively, the relaxation, or the ability of an adhesive to restore to its former state, may be optically determined on removal of stress. Rigid thermosetting adhesives display little or no creep under stress in contrast to thermoplastic or plasticised adhesives. Prolonged stressing of thermoplastic adhesives always reduces bond strength.

Flexural Strength

The shear strength of beams composed of adhesive laminated strips may be determined by flexural loading. The load is applied to the mid-span to develop maximum shear stress and delamination in the centre layer of adhesive. The method gives higher shear strengths than are obtained with tensile or compressive shear specimens because the resistance of the adhesive to shear failure is increased by the compressive loading normal to the glue-line.

Peel Strength

Peel tests involve complex stress distributions. Peel strengths vary with the speed of testing (particularly with low modulus adhesives) and the forces needed to start and sustain peeling action are determined by the physical properties of the adherends, test specimen geometry (adherend thickness and width), and the adhesive strength characteristics. Peel strength increases with adherend thickness and adhesive thickness but decreases with adhesive modulus of elasticity. Steel adherends give higher peel strengths than aluminium adherends of similar thickness. Low peel strengths are usually a feature of brittle adhesives with high tensile strengths. Peeling rates of 15.2 cm/min for adherend widths of 2.54 cm are commonly specified.

Durability

The test specimens described previously may be used to determine the effects of adverse environments on an adhesive bond although no single test (or series) exists which will enable the user to predict its service life. A suitable test will provide information on the permanence of the bond when it is exposed to deteriorating circumstances such as temperature changes leading to oxidation, thermal degradation or softening of the adhesive. Other destructive hazards include low temperatures, sunlight and radiation, water, chemical reagents, oils and biodeterioration. Test specimens and procedures should be selected to simulate the type of service conditions envisaged for the bonded assembly. Consideration must be given to the adherend material for certain tests, e.g. for the evaluation of the acid resistance of an adhesive, certain metals would be unsuitable as adherends. Some of the environments which often provide the basis for unfavourable long-term conditions of exposure for adhesive bonds are discussed here.

Temperature

Adhesives and adherends are affected by high, low and varying temperatures. Elevated temperatures may decompose adhesive materials by oxidation or thermal degradation. Long exposure to moderate temperatures often leads to polymerisation changes in adhesives. Displacement of bonded surfaces occurs where high or low temperatures accentuate differences between the thermal coefficients of expansion for adherends and adhesives; stresses are set up at the interface which influence bond strength. Low temperatures embrittle many adhesives causing a reduction in their peel and cleavage strengths.

Test chambers with heating or cooling units can be employed for environment simulation. Adhesive durability is best determined at the service temperature; higher test temperatures should be regarded for their comparative value only. Destructive temperature effects may become apparent in hours or years.

Weathering

The long term ageing or weathering properties of bonded structures are difficult to predict since there are no standard short term permanence tests. Actual long time weathering is usually a reliable guide to adhesive durability although variations in exposure conditions over long test periods can make data difficult to interpret. Rainfall, humidity and temperature vary widely with locality. Accelerated weathering tests designed to reduce the long exposure periods are useful if the results can be correlated with actual weathering. Several tests have been adopted in the U.K. for providing a uniform testing procedure for military equipment. Referred to as I.S.A.T. (Intensified Standard Automating Trials), these tests are claimed to be equivalent to prolonged storage under existent weather conditions.

Chemical

The permanence of a bond may be affected by exposure to external chemical agents or by the latent chemical reactivity of the adhesive for an adherend.

Several tests have been specified to evaluate adhesive bond strength on exposure to reagents such as acids, alkalies, water, sea water, petrol, organic solvents and lubricating oils. The deterioration in adhesion is sometimes dependent on reagent concentration. Temperature and exposure period should also be considered as test factors. Other tests are concerned with the effects of atmospheric constituents, which are known to cause adhesive deterioration, e.g., salt spray or ozone.

Chemical constituents in the adherends, such as plasticisers, can migrate into the adhesive and destroy adhesion. Additionally, the by-products of an adhesive curing reaction may attack the adherend at the interface and cause loss of adhesion.

Biological

Certain adhesive types based on natural products such as casein, cellulose, dextrine or protein, etc., are subject to attack by bacteria, fungi, insects and rodents. Tests are available to check the effectiveness of preservative agents for adhesive formulations otherwise subject to biodeterioration.

Radiation

The effects of light, either artificial or natural, on bonded glass or optical assemblies, involving transparent or translucent materials, may be important. Adverse effects include the loss of adhesive strength and the discoloration of the glue-lines following photochemical changes in the adhesive. Light is unlikely to present a hazard for impervious adherend structures but is often an important factor with glass and transparent or translucent plastics.

Nuclear radiation is known to effect structural changes in high molecular weight polymers but has been scarcely studied for adhesive systems. The advent of nuclear technology and space research can be expected to produce test methods for this type of environment soon. An extensive literature on radiation induced changes in polymers provides a basis for studying adhesive performance. Some selected references appear at the end of the section.

Polyvinyl Acetate Wood Adhesives

Introduction

Polyvinyl acetate is a thermoplastic polymer that has gained wide acceptance over the years as a raw material for the adhesives industry. Modified or unmodified, in solution or emulsion form, and as homopolymer or copolymer, it exhibits a versatility that makes it suitable for bonding a wide variety of substrates. In particular, it is capable of producing strong and durable bonds on wood and wood-derived products, and this has been a major contributing factor to the tremendous growth of polyvinyl acetate-based adhesives in recent years, from almost nothing in the early 1930s to an estimated worldwide production of all types of 2 million tonnes in 1977. It is familiar to people all over the world as the binder for interior and exterior emulsion paints, as the so-called “cold glue” that replaced the heated pot of animal glue for carpentry, and as the “white glue” used in millions of households as a general-purpose adhesive.

Background

Vinyl acetate is a colorless flammable liquid with a viscosity of 0.4 cP at 20°C, a solubility in water of about 2% at 25°C, a boiling point of 72.7°C, and a characteristic odor. This is the starting point for the production of polyvinyl acetate (PVA), and, in conjunction with other vinyl monomers, acrylic esters, dialkyl maleates and fumarates, ethylene, and certain other monomers, of a range of speciality copolymers and terpolymers, while graft polymers can be produced with monomers such as styrene that will not copolymerize with vinyl acetate.

It is not certain when the first polymerization of vinyl acetate to polyvinyl acetate was performed. During the period between 1915 and 1925 the free radical initiation of polymerization of various vinyl monomers was widely studied and by 1930 polyvinyl acetate was commercially available. It was, however, only in the years following World War II that polymers of vinyl acetate began to be used in significant quantities, particularly in the paint and adhesive industries, and since then, PVA has shown the same rapid expansion as most other well-known thermoplastic materials. Today some 50 major manufacturers and countless small suppliers around the world make material available in solid form, dissolved in solvent, or dispersed in water, as homopolymer, copolymer, or terpolymer, for a variety of applications that include paint manufacture, production of general-purpose and specialized adhesives, textile coating, sizing and sealing of paper and related products, concrete additives, and production of sealants. Formulated products span the entire range of viscosities, from thin liquid to heavy paste, and may be fast or slow drying. Dried films may be clear or opaque, pigmented or unpigmented, flexible or brittle, hard or soft, as required. The product may be internally or externally plasticized or unplasticized, and may dry hard and tack-free or pressure sensitive. With this versatility it is hardly surprising that polyvinyl acetate is now available almost anywhere in the world, with factories in most developed countries. At this point, the United States and Germany are the major producers, with companies such as Air Products, Borden, Monsanto, and Union Carbide in the United States, and Hoechst in Germany.

Chemistry of Polyvinyl Acetate

The steps involved in the manufacture of polyvinyl acetate are shown schematically in Figure 1.

A. Production of Vinyl Acetate Monomer

Originally, vinyl acetate monomer was produced by reacting acetylene and acetic acid together with suitable catalysts.

H º CH + CH3COOH H2C= CH OOC CH3

The earlier technique was to carry out this reaction in liquid phase but this method was replaced in the early 1930s by more efficient gaseous-phase processes performed at high temperature. The acetylene and acetic acid were both derived from coal, acetylene directly by the action of water on calcium carbide

CaC2 + H2O  HC º CH + Ca0

and acetic acid by reacting the acetylene so produced with water to yield acetaldehyde, and subsequently oxidizing this acetaldehyde to acetic acid.

HCº CH + H2O  CH3CHO

CH3CHO + [O]  CH3COOH

This process remained viable for as long as calcium carbide was readily and cheaply available. With the increasing cost of energy, however calcium carbide has become steadily more expensive and less readily available, while an ever-increasing range of chemicals has been made available by the rapid expansion of the petrochemical industry. These two factors have combined to make the modern method of production increasingly attractive. Here ethylene is used as the starting point for the production of both acetylene and acetic acid. Acetylene is made by removing hydrogen from ethylene, while acetic acid is obtained by oxidizing the ethylene to acetaldehyde, which is oxidized further to acetic acid.

B. Polymerization of Vinyl Acetate

Vinyl acetate monomer may be polymerized by many of the conventional polymerization techniques, including mass polymerization, solvent polymerization, and emulsion polymerization. The reaction is usually initiated and controlled by the use of free radical or ionic catalysts, although experimental methods of catalysis, including redox catalysis or activation by light, may be used for specialized products. The polymerization is characterized by three successive stages of reaction: initiation, the growth of the polymer, and termination.

1. Initiation

Initiation occurs when a free radical or ion attaches itself to a vinyl acetate molecule. This leads to a rearrangement of the electrons in the double bond, transferring the reactive site to the vinyl acetate monomer.

The initiator is usually a free radical derived from a peroxide such as benzoyl, lauroyl, or even hydrogen peroxide, although other initiators, such as persulfates, may also be used.

2. Polymerization

This highly reactive initiated molecule reacts with further monomer molecules by the same transfer mechanism, retaining the terminal reactive site for further growth.

3. Termination

Growth of the macromolecule is terminated when the reactive site is removed, either by combination with the reactive site of some other molecule, or by transfer of the reactive site to some other molecule.

Selection of the initiating catalyst, the ratio of catalyst to monomer, and the reaction conditions allows control over the average molecular weight of the polymer formed, and also of the degree of branching, if any, in the macromolecule.

In the case of polvinyl acetate the polymerization may be carried out by a wide range of techniques, including mass polymerization, solution polymerization, and emulsion polymerization. Most of the poly vinyl acetate produced is made using emulsion polymerization techniques, and this is particularly true of those grades used in the production of wood adhesives. Vinyl acetate is particularly suited to emulsion polymerization, owing to the relatively high solubility of the monomer in water, and the complete solubility of the polymer in the monomer. In this process the vinyl acetate monomer is dispersed by means of relatively high speed stirring in water that contains suitable emulsifiers or protective colloids. A more or less stable suspension of monomer particles will be formed in the water. To this suspension the initiator is added, and typically the mixture will be heated to a temperature, which will substantially speed up the rate of reaction, while allowing it to remain controllable. As polyvinyl acetate is soluble in the monomer, the reaction will take place within the individual droplets of the suspension, producing a stable emulsion of polyvinyl acetate. Typical commercial polymer emulsions will contain between 40 and 60 parts by weight of the polymer, a very low residual level of monomer, and a viscosity between 0.1 and 20 Pa-sec. In addition, emulsions intended for use as the basis of formulated wood adhesives will also have a relatively large particle size, usually within the range 0.3-5 × 10-6 m.

Other characteristics of the polymer emulsion that will goven its suitability for use in a specific application will include the molecular weight; degree of copolymerization or plasticizing, if any; film strength and film-forming properties at low temperature; and ability of the emulsion to withstand both the mechanical effects of mixing and changes in temperature, especially where freezing may be involved. Manufacturers of PVA emulsions will supply most or all of this information for their products to assist in the selection of the most suitable grade.

Formulating a Pva-Based Adhesive

A. General Considerations

Formulating a PVA-based wood adhesive, a number of factors, sometimes conflicting, must be borne in mind. It follows that the final product will often be a compromise in which the conflicting factors have been carefully considered in order to give most weight to those, which seem to be the most important in the specific application. Factors to consider will include the following:

1. Substrates

Where the adhesive is to be used for bonding wood to wood, consideration must be given to whether the wood will be hardwood or softwood. For the hardwoods an adhesive of high solid content is usually advantageous, while with softwoods, adhesives of lower solid content may be used. In addition, if the species to be glued is known to be oily, incorporation of a wetting agent or solvent will assist in adhesive penetration.

In many cases, however, wood will be only one of the substrates. The other substrate can vary from concrete in the case of adhesives for parquet or mosaic wood blocks to decorative laminates which may be cellulosic or plastic. Although, in general, PVA adhesives are used principally on cellulosic materials because of their exceptionally good adhesion to such surfaces, special applications may call for wood to be bonded to rubber, to foams, both flexible and rigid, to synthetic or natural fibers, or even to metal or other nonporous surfaces. Each of these will impose restrictions, sometimes severe, on the freedom of the formulator, and corresponding limitations on the applications for which the formulated adhesive may be suitable.

2. Surface Preparation

The preparation of the surfaces to be glued will also influence the formulation to a certain extent. In the case of adhesives for gluing wood to wood, inaccurately machined surfaces may necessitate the formulation of gap-filling adhesives in order to produce a satisfactory bond over the entire surface. Adhesives may need to be formulated to give good adhesion to greasy, loose, or dusty surfaces or to seal a very porous substrate. In certain applications the adhesive must be capable of bonding surfaces that have been coated or lacquered,

3. Application

The viscosity of the formulated adhesive will largely be governed by the method of application of the adhesive. Application methods will include manual application by brush, roller, smooth or notched trowel or spray, machine application from smooth or embossed rollers, with or without a doctor blade, or by cascade coaters, by nozzle or jet, or even mechanical extruders or sprays. Consideration should always be given to making the adhesive as easy to use as possible, especially where it will be handled by unskilled people unfamiliar with the proper handling of adhesives. This applies particularly to adhesives intended for household use.

4. Assembly Conditions

Intricate or multicomponent assemblies will demand an adhesive with a long open assembly period in order to enable all the components to be brought together and placed under pressure before the adhesive has started to dry. At the other extreme, applications such as core composing, the pressure station is usually very short, require an adhesive that can develop high bond strength very quickly. High production rates will similarly demand a quick-setting adhesive. Application conditions involving high or low temperatures or humidities will also influence the formulation. The formulator must also consider whether or not the glued article requires machining. If this is the case, fillers must be chosen carefully or eliminated in order to minimize damage to cutters. Adhesives containing solvent should only be used in well-ventilated locations.

5. Service Conditions

Although PVA adhesives are not, in general, used for joints that are under continuous load or subjected to high temperatures or high humidity, these adhesives can be formulated to give better performance under such conditions. The conditions under which the completed assembly will be expected to operate should always be taken into account when designing the adhesive.

6. Appearance

Again, the use to which the completed article is to be put may influence the formulation. The appearance may be marred by unsightly glue lines, in which case it may be necessary to design the adhesive to dry completely transparent, or even to tint the adhesive so that the glue line will be less obtrusive. Tinting or pigmenting the adhesive may be essential the finished article is to be stained, as squeezed-out adhesive may seal the surface in the vicinity of the glue line and prevent subsequent penetration of stain in that area.

7. Storage Conditions

If the adhesive is likely to be stored under adverse conditions, attention must be given to ensuring that it has adequate freeze-thaw stability. The maximum storage life of the adhesive.

8. Price

Very often the most serious limitation will be that of formulating to a particular price. In this regard, ease of application, reliability, and spread rate are factors to take into account, as it may be the case that a relatively expensive adhesive will prove more economical in the specific application than an inferior but cheaper product. When comparing prices it is important to take the density of the products into account, especially if they are sold in units of mass.

B. Formulating and Compounding

A number of different components will normally be incorporated into a PVA wood adhesive. Each of these has a specific function in the finished product. To formulate successfully, it is necessary to understand not only the performance criteria, but also the function of these components.

1. The Base Polymer

Since the PVA emulsion provides the major proportion of the adhesive strength, and will in many cases be the only binder in the formulation, it is worth considering its function in some detail. While the mechanism of adhesion is not fully understood, adhesion probably occurs as a result of secondary forces, principally Debye forces and London dispersion forces operating at close range, with some hydrogen bonding with the cellulosic fibers especially where polyvinyl alcohol has been used as the protective colloid. In addition, mechanical bonding occurs as a result of adhesive penetration into the open-cell structure of the wood. All these combine to produce an excellent bond, which in a properly formulated adhesive will be stronger than the wood itself.

Since the PVA is in emulsion form, coalescing of the particles must occur in order to produce a continuous film during the drying process, this will happen only if the drying takes place at a temperature above the solidification temperature of the polymer particles. If this condition is not met, the particles will not coalesce properly, leading to a loss of mechanical properties in the dried film. If the temperature at which evaporation of the water takes place is significantly below this minimum film-forming temperature or white point, no coalescing will take place, and a white, chalky film will result with no mechanical strength whatsoever. This white point is thus an important aspect of the base PVA to consider when formulating a wood adhesive.

Viscosity of the emulsion will be determined by the solid content of the emulsion, the particle size distribution, and the emulsifier or protective colloid system used, and may vary between very wide limits. For production of wood adhesives it is most common practice to use grades with a coarse to medium particle size (in the range 0.3-10 × 10-6 m) and a solid content between 40 and 60% by weight of the total emulsion. Even with these restrictions viscosity may still be anywhere between 0.05 and 50 Pa-sec, however, and it will usually be necessary to modify this viscosity in the finished product.

The resistance of the dry film from a PVA emulsion to water is mainly dependant on the type and quantity of protective colloid used. Where this is polyvinyl aocohol, the water resistance of the dried film is generally poor. It is possible, however, either by using emulsions protected by cellulose-based colloids, or by Incorporating certain additives, to produce PVA adhesives that have a fair resistance to water.

Because polyvinyl acetate is a thermoplastic polymer, it loses cohesive strength as the temperature increases. In general, higher molecular weight polymers lose less strength at elevated temperatures than those of lower molecular weight, but the differences are not great. In addition, thermoplastic polymers are subject to cold creep, which is the tendency for a fully dried film to flow slowly under a sustained load. Plasticized grades are much more subject to cold creep than are unplasticized grades. To a certain limited extent, the tendency to cold creep can be reduced, but in general PVA wood adhesives are not suitable for applications in which the glue line is highly stressed or subject to temperatures above about 50°C, and a combination of these two factors will completely rule out the use of PVA wood adhesives.

Different grades of PVA emulsions will also have different drying times and will therefore offer the possibility of formulating adhesives with long or short open assembly times. All the larger manufacturers issue comprehensive data sheets for their various grades from which it is possible to select, with a good deal of precision, the best grade to use as a starting point.

2. Other Binders

In addition to the PVA emulsion it is common practice to incorporate other binders into the formulation of various purposes. Probably the most widely used cobinder is polyvinyl alcohol. Incorporation of polyvinyl alcohol into a formulation increases the cold creep resistance of the dried film, but reduces its water resistance, especially where the polyvinyl alcohol has a low degree of hydrolysis. In addition, polyvinyl alcohol will increase the open assembly time of the adhesive substantially. Use of high molecular weight polyvinyl alcohols is common in low-cost formulations, as these produce relatively high viscosity solutions allowing incorporation of extra water into the formulation. Adhesives containing polyvinyl alcohol will exhibit good machine stability and running properties and faster initial bond strength development.

The use of starch as an additive is also common practice. Here the major advantage is the cost reduction, which is again achieved at the expense of the water resistance.

Because of their affinity for water, starches will also extend significantly the open assembly time of the adhesive. As starches are particularly susceptible to microbial attack, care must be taken to ensure that the formulated adhesive is adequately protected. A wide variety of starch types may be incorporated, including pre-gelatinized, water-soluble, and oxidized starches. Where borax is used to solubilize the starch, the compatibility of the starch solution with the base emulsion must be checked. Borax has the effect of insolubilizing poly vinyl alcohol, and may therefore destroy the protective colloid of the PVA emulsion, thereby destabilizing the emulsion.

While cellulose derivatives such as carboxymethyl cellulose are often added, their function is usually that of viscosity modifier rather than additional binder, and the proportion is invariably small. Thermosetting resins such as phenol, resorcinol, or urea formaldehyde resins are occasionally added to PVA wood adhesives to improve their water resistance. Their use will be more fully discussed subsequently. For specialized application, a range of other binders may be incorprated, especially where wood is to be bonded to some other substrate. Thus all or part of the PVA homopolymer may be replaced by an ethylene vinyl acetate copolymer for the lamination of polyvinyl chloride film to wood, while vinyl acrylate polymers or copolymers may be added to improve adhesion to nonporous substrates. While dextrins may be added to PVA emulsions used in packaging applications, they are not normally added to PVA wood adhesives.

3. Plasticizers

Plasticizers may be regarded as high-boiling solvents with very low vapor pressures at the operating temperature of the adhesive. They will thus remain permanently in the dried film. Plasticizers form a film around the particles of the dispersion, increasing the distance between them and thus lowering the forces between them. In addition to increasing the flexibility of the dried film, they also lower the minimum film-forming temperature of the adhesive. However, they also increase the tendency of the film to creep under load and should thus be used with caution in wood-to-wood adhesives. They should be avoided completely in adhesives for critical applications, especially highly stressed structures, where the use of a solvent to promote film-forming characteristics is preferred.

One of the substrates is flexible, they may be used to improve adhesion and match the characteristics of the adhesive film more closely to those of the substrate, especially where this is flexible. Only plasticizers that are compatible with PVA should be used. Those in common use include esters, particularly alkyl phthalates such as dibutyl phthalate, and aromatic phosphates such as tricresyl phosphate, which are chiefly used where flame retardancy is a consideration. Speciality Plasticizers are not normally used in wood adhesives. Some physical properties of plasticizers in common use are shown in Table 1. The commonly used plasticizers are more or less immiscible with water, the addition of plasticizer to the base emulsion does not present undue difficulty. It is good practice to add the plasticizer slowly while stirring the emulsion vigorously. Once added, the stirring rate may be decreased, but stirring should be continued for at least 30 min to ensure thorough dispersion and to allow the plasticeser to solvate the emulsion particles. Plasticiser will seldom be added at a level above 10% based on polymer solids in formulations for wood adhesives.

Aminoresin Wood Adhesives

Introduction

Aminoresins are polymeric products of aldehyde reaction with compounds carrying – NH2 or –NH groups. Such groups are mainly amide groups, such those in urea and melamine. They constitute the most important members of this class of compounds, more so than the amine groups as in the case of aniline. Formaldehyde is the main aldehyde used. Other aldehydes, such furfural, are generally not used for wood adhesives. The advantage of aminoresin adhesives (or amino-plastic adhesives as they are often called) are their (1) initial water solubility (this renders them eminently suitable for bulk and relatively inexpensive production), (2) hardness, (3) nonflammability, (4) good thermal properties, (5) absence of color in cured polymers, and (6) easy adaptability to a variety of curing conditions.

Although many amidic and aminic compounds have been investigated for use in production of aminoresins, only urea and melamine and, in rare cases aniline, are extensively used. Thermosetting aminoplastic resins produced from urea and melamine are built up by condensation polymerization. Urea and melamine are reacted with formaldehyde, which results in the formation of additional products, such methylol compounds. Further reaction, and the concurrent elimination of water, leads to the formation of low molecular weight condensates which are still soluble. Higher molecular weight products, which are insoluble and infusible, are obtained by further condensing the low molecular weight condensates.

Urea- and melamine-formaldehyde (UF and MF) resins have a great deal in common as regards the chemical and physical characteristics of both the cured and uncured resins. MF is superior to UF because of its superior water and heat resistance, hardness, and shorter curing time under less drastic conditions. The greatest disadvantage of these aminoplastic resins is their bond deterioration, caused by water and moisture. This is due to the hydrolysis of the aminoplastic or amino-methylenic bond, which is the same for both UF and MF resins.

The higher resistance of MF resins to water attack is due to the considerably lower solubility of melamine in water. (Melamine dissolves in hot water only, whereas urea dissolves in cold water as well.) Therefore, UF adhesives are used for interior application only; MF or melamineurea-formaldehyde (MUF) resins can be employed successfully even for rather severe outdoor conditions. If full exterior-grade quality is needed, it is safer to use phenolic-type resins rather than aminoplastic resins.

Chemistry of Aminoresins

A. Urea-Formaldehyde Condensation

The reaction between urea and formaldehyde is very complex. The combination of these two chemical compounds results in both linear and branched polymers, as well as tridimensional networks, in the cured resin. This is due to a functionality of 4 in urea (due to the presence of four replaceable hydrogen atoms), and a functionality of 2 in formaldehyde. The most important factors determining the properties of the reaction products are (1) the relative molar proportion of urea and formaldehyde, (2) the reaction temperature, and (3) the various pH values at which the condensation takes place. These factors influence the rate of increase of the molecular weight of the resin. Therefore, the characteristics of the reaction products differ considerably when lower and higher condensation stages are compared, especially solubility, viscosity, water retention, and rate of curing of the adhesive. These all depend to a large extent on molecular weights.

The reaction between urea and formaldehyde is divided into two stages. The first is the alkaline condensation to form mono-, di-, and trimethylolureas. (Tetramethylolurea has never been isolated.) The second stage is the acid condensation of the methylolureas, first to soluble and then to insoluble cross-linked resins. On the alkaline side, the reaction of urea and formaldehyde at room temperature leads to the formation of methylolureas. When condensed, they form methylene-ether links between the urea molecules. The products from urea and formaldehyde, and from mono- and dimethylolureas, are as follows:

The reaction also produces cyclic derivatives: uron, monomethyloluron, and dimethyloluron.

In weak alkaline solutions, the first product of the reaction is a complex (II), which is capable of rearranging itself exothermically into monomethylolurea. On acidification, the complex (II) eliminates water, resulting in unstable trimethylene urea hydrate (IV). The hydrated azomethin (III), which was identified by Fahrenhorst in urea-formaldehyde resins, is regarded as characteristic of the intermediate stage of the reaction.

The electron theory provides a possible bonding mechanism between azomethin groups and either the solvent or other resin molecules. In this bonding, theelectrons of the C=N bonds and the free electron pair on the nitrogen atoms are involved. The higher pH stabilizes the degree of polymerization or association by permitting the formation of ionic complexes with water or the solvent. The lower pH causes the loosening of the water from the hydrated azomethin groups, allowing association. The resin eventually proceeds to the liophobic stage.

Indirect evidence strongly points to the existence of this mechanism as well as of the mechanism proposed by the classic theory of UF resin formation. However, no conclusive evidence of the participation of structure IV in the UF resinification process has yet been obtained. The association through azomethine-type intermediates has been mentioned to explain resin formation and to interpret the mechanism of etherification of methylol groups under acid, neutral, and alkaline conditions. This theory opposes the classic theory.

In the first reaction, monomeric methyleneurea is formed as a result of the intramolecular loss of water. An unsaturated azomethine group is formed, followed by rapid polymerization. This gives the insoluble end product. The other reactions are condensation polymerizations in which the methylolureas are merely the building blocks of the polymers and of the insoluble end product. The polymers formed in both cases are mainly linear polymers obtained by the intermolecular splitting off of water. Under certain conditions water may also be split off intra- molecularly. Cyclic compounds called urones are then formed. In both cases, further splitting off of water and formaldehyde leads to the formation of hardened or cured resins.

B. Melamine-Formaldehyde Condensation

The condensation reaction of melamine (V) with formaldehyde is similar to the reaction of formaldehyde with urea. Formaldehyde first attacks the amino groups of melamine, forming methylol compounds.

Formaldehyde addition to melamine occurs more easily and completely than to urea. The amino group in melamine accepts easily up to two molecules of formaldehyde. Thus up to six molecules of formaldehyde are attached to a molecule of melamine. The methylolation step leads to a series of methylol compounds with two to six methylol groups.

Because melamine is less soluble in water than urea, the hydrophilic stage proceeds more rapidly in MF resin formation than in UF condensations. Therefore, hydrophobic intermediates of the MF condensation appear early in the reaction. Another important difference between MF and UF is that the MF condensation and curing occurs not only under acid conditions, but also under neutral or even slightly alkaline conditions.

The mechanism of the further reaction of methylolmelamines to form hydrophobic intermediates is the same as for UF resins, with splitting off of water and formaldehyde. Methylene and ether bridges are formed and the molecular size of the resin rapidly increases. These intermediate condensation products constitute the large bulk of the commercial MF resins. The final curing process transforms the intermediates to the desired MF insoluble and infusible resins through the reaction of amino and methylol groups which are still available for reaction.

A simplified schematic formula of cured MF resin has been given by Koehler and Fry . They emphasize the presence of many ether bridges besides unreacted methylol groups, and the methylene bridges. This is because in curing MF resins at temperatures of up to 100°C, no substantial amounts of formaldehyde are liberated. Only small quantities are liberated during curing up to 150°C. However, UF resins curing under the same conditions liberate a great deal of formaldehyde.

Wohnsieldler, Updegraff, and Hunt have tr