NIIR

Contents

  	1.	Synthesis and Characteristics of Epoxy Resin	

		Introduction

		Structure of Epoxides

		Epoxipation of Unsaturated Hydrocarbons

		Catalytic Oxidation of Ethylene and Higher Olefins

		Epoxidation by Peroxy Acids and Their Esters

		Preparation of Peroxy Acids

		In Situ Epoxidation

		The Epoxidation Mechanism

		Unsaturated Materials

		Epoxidation by Inorganic Peroxy Acids

		Epoxidation with Aliphatic and Aromatic Hydrocarbon Hydroperoxides

		Epoxidation with Chromic Acid and Chromyl Compounds

		Biological Epoxidation

		Dehydrohalogenation of Substituted Hydroxyl Compounds

		The Epoxidation Mechanism

		Halohydrin Formation

		Epoxides from Epichlorohydrin

		Glycidyl Ethers

		Glycidyl Esters

		Nitrogen-Containing Epoxides

		Thioglycidyl Epoxides

		Silicon-Containing Epoxides

		Organophosphorus Epoxides

		Halogen-Containing Epoxides

		Epoxides from Hydroxy Sulfonates or Halogenated Acetates

		Epoxides from Glycols

		Epoxidation by Condensation

		Darzens Glycidic Ester Condensations

		Epoxides from Ylids

		Epoxides from Halogenated Ketones and Nickel Carbonyl

		Epoxides from the Reaction of Diazomethane with Aldehydes or Ketones

		Epoxides Containing Unsaturation

		Conclusions



	2.	Manufacture of Epoxy Resins	

		Raw Materials

		Manufacture

		Plant Location

		Machinery Needed

		Profit



	3.	Epoxide-Curing Reactions	

		The Effect of Epoxide Structure on Reactivity with Curing Agents

		The Mechanism of the Curing Reaction

		Polyaddition Reactions

		Polyamines

		Polyamides

		Polyureas

		Polyurethanes

		Polyisocyanates

		Polymercaptans

		Polyhydric Alcohols

		Polyphenols

		Polycarboxylic Acids

		Polybasic Acid Anhydrides

		Silanes and Silanols

		Others

		Polymerization

		Anionic Catalysts

		Cationic Catalysts



	4.	The Dynamic Mechanical Properties of Epoxy Resins	

		Basic Parameters

		The Glassy Transition and Dynamic Mechanical Dispersion

		Temperature and Frequency Interdependence

		Experimental

		Results and Discussion

		Standard Measurements

		Dynamic Measurements

		Comparison of Results

		Treatment by Reduced Variables

		Conclusions



	5.	Physical and Chemical Properties of Epoxy Resins	

		Solubility and Surface Properties

		Network Structure and Physical Properties

		Aging and Chemorheology

		Bisphenol a Epoxy Homopolymers and Copolymers

		Thermal Transition Effects

		Dynamic Mechanical Response

		Relaxation and Fracture Properties

		Properties Compared with Elastomers and Thermoplastics



	6.	Epoxy Resin Adhesives	

		Introduction

		Theories of Adhesion and Aohesive-joint Strength

		Wetting and Spreading Phenomena

		Boundary-Layer Theory

		Surface-Attachment Theory of Adhesive-Joint Strengths

		Stress Distribution in Adhesive Joints

		Rheological Aspects of Adhesives

		Unified Interpretation of Adhesive-Joint Strengths

		Physical and Mechanical Aspects of Epoxy-Resin Adhesives

		Dynamic Mechanical Techniques

		Mechanical Behavior of Epoxy Adhesives During Joint Formation

		Strength of Adhesive Materials

		Chemical Aspects of Epoxy-based Adhesives

		Curing Agents for Bisphenol A Epoxy Adhesives

		Modifiers for Bisphenol A Epoxy Adhesives

		Adhesives Based on Other Epoxy Materials

		Technological Properties of Epoxy-adhesive Systems

		Cure and Thermal Softening Behavior of Epoxy Adhesives

		Stress and Environmental Durability of Adhesive Joints

		Applications of Epoxy Adhesives

		Future Prospects



	7.	Epoxy Resin Coatings	

		Classification of Epoxy-Resin Coatings

		Epoxy Resins Commonly Used in Coatings

		Epoxy-Resin Esters 

		Esters Produced from Solid Epoxy Resins

		General Remarks

		Formulation Latitude

		Esters Produced from Liquid Epoxy Resins

		Precatalyzed Liquid Epoxy Resin for the Production of Solid Epoxy Resins and Epoxy-Resin Esters

		Cooking Procedure

		“Two-Step” Liquid-Epoxy-Resin Route to Epoxy-Resin Esters

		Cooking Procedure

		Solid-Epoxy-Resin Solution Coatings

		Cold-Cured Epoxy-Resin Systems

		Polyamine Curing Agents

		Polyamine-Adduct Curing Agents

		Polyamide-Resin Curing Agents

		Polyamide-Adduct Curing Agents

		Tertiary Amine Curing Agents

		Industrial Maintenance Coatings Based on Cold-Cured Epoxy-Resin Systems

		High-Film-Build Cold-Cured Epoxy-Resin Coatings

		Application Instructions

		Manufacturing Instructions

		Epoxy Baking Finishes

		Epoxy-Phenolic Coating Systems

		Epoxy-Urea-Formaldehyde Resin Coating Systems

		Epoxy-Thermosetting Acrylic Coating Systems

		Liquid Epoxy Resins in Solventless and Super-High-Solids Systems

		Special Application Equipment and Formulation for Solventless Systems

		Manufacturing Instructions 

		Application

		Ketimine Curing Agents

		Manufacturing Instructions 

		Application

		Curing Characteristics

		Powder Coatings

		Application Equipment

		Epoxy-Resin Powder-Coating Formulations

		Fusion-Produced Epoxy-Resin Powders

		Manufacturing Instructions

		Applications Instructions

		Dry-blended Epoxy-Resin Powders

		Manufacturing Instructions

		Application Instructions

		Properties and Applications

		Thermoplastic Epoxy Resins

		Zinc-Rich and General Purpose Shop Primers

		Manufacturing Instructions

		Application Instructions

		Manufacturing Instructions

		Application Instructions

		Thermoplastic-Epoxy-Resin Crosslinked Systems

		Water-Reducible Epoxy Resin Coatings

		Water-Reducible Epoxy-Ester Baking Finishes

		Manufacturing Instructions

		Application Instructions 

		Water-Reducible Polyamide-Cured Epoxy-Resin Coatings

		Manufacturing Instructions

		Manufacturing Instructions

		Water-Reducible Epoxy-Resin Coatings for Electrodeposition

		General Remarks

		Maleinization Step After Complete Esterification of the Epoxy Resin with Organic Acids

		Cooking Procedure

		Application Instructions



	8.	Epoxy Coating Give into Water	



	9.	Electrical and Electronic Applications : Sealants and Foams	

		Electronic and Electrical Applications

		Introduction

		Casting

		Potting

		Encapsulation

		Coatings

		Sealing

		Molding

		Formulation of the Resin System

		Internal Stresses

		Rapid Cures

		Flexibilizing Epoxy Resins

		Fillers

		Reactive Diluents

		Cycloaliphatic Epoxides

		High-Temperature Epoxy-Resin Systems

		Flame-Retardant Epoxy Resins

		Colorless Epoxy Resins

		Epoxy Formulations

		Molding

		Molding Compounds

		Molding Technology

		Liquid-Injection Molding

		Pellets and Preforms

		Epoxy Sealants

		Epoxy Foams

		Gas-Blown Foams

		Syntactic Foams

		One-Package Foams

		Epoxy-Foam Applications

		Epoxy Strippers

		Handling of Epoxy Casting Systems



	10.	 Analysis of Epoxides and Epoxy Resins	

		Uncured Epoxy Resins

		Qualitative Tests

		Detection of Free Epoxy Groups

		Determination of Epoxy Group—Lithium-Chloride Test

		Reagents

		Procedure

		Determination of Epoxy Group—Periodic Acid Test

		Reagents

		Procedure

		Determination of Epoxy Group—Pyrolysis Test

		Reagents

		Procedure

		Determination of Epoxy Group—Lepidine Test

		Reagents

		Procedure

		Detection of the Bisphenol A Skeleton

		Determination of Bisphenol A Epoxy Resins—Mercuric Oxide and Nitric Acid Tests

		Reagents

		Procedure

		Determination of Bisphenol A Epoxy Resins in Coatings—Nitric Acid Test Reagents

		Reagent

		Procedure

		Determination of Bisphenol A Epoxy Resins—Filter-Paper Test

		Reagents

		Procedure

		Determination of Bisphenol A Epoxy Resin—Formaldehyde Test

		Reagents

		Procedure

		Determination of Bisphenol A Epoxy Resins—Phenylenediamine Test

		Reagent

		Procedure

		Detection of Epoxy Resins Based on 4,4-’-Diamino-diphenylmethane

		Determination of Epoxy Resins Based on 4,4'-Diaminodiphenylmethane

		Reagents

		Procedure

		Detection of Other Epoxy Resins

		Quantitative Tests of the Epoxy Group

		Hydrohalogenation Methods

		Estimation of Epoxy Group—Hydrochloric Acid in Dioxane, Methyl Ethyl Ketone, or Dimethylformamide

		Reagents

		Procedure

		Calculations

		Estimation of the Epoxy Group—Pyridinium Chloride in Pyridine

		Reagents

		Procedure

		Hydrohalogenation by Direct Titration

		Estimation of Epoxy Group

		Reagents

		Procedure

		Calculations

		Other Chemical Methods

		Estimation of Other Functional Groups

		Hydroxyl Group

		-Glycol Group

		Estimation of a-Glycol Group

		Reagents

		Procedure

		Calculation

		Chlorine

		Esterification Equivalent Weight

		Estimation of Esterification Equivalent Weight

		Reagents

		Procedure

		Calculation

		Infrared Spectroscopy

		Technique

		Epoxide Absorption Bands

		Epoxy Resins

		Quantitative Estimation

		Following the Degree of Cure

		Other Physical Methods

		Ultraviolet Spectroscopy

		Electron Spin and Nuclear Magnetic Resonance Methods

		Gas Chromatography

		Paper Chromatography

		Thin-Layer and Gel-Permeation Chromatography

		Handling Properties

		Molecular Weight

		Softening Point

		Viscosity

		Color

		Blends and Compounds

		Hardeners and Accelerators

		Organic Acid Anhydrides

		Determination of Acid and Anhydride Content

		Reagents

		Procedure

		Calculations

		Amines

		Determination of Amine Number

		Reagents

		Procedure

		Calculation

		The Curing Process

		Curing Characteristics of Epoxy Resin-Hardener Systems

		Determining the Degree of Cure

		Analysis of Cured Epoxy Resins



	11.	The Toxicology of Epoxy Resins	

		Introduction

		Experimental Method

		Materials

		Acute Toxicity

		Chronic Toxicity

		Irritation

		Sensitization

		Results

		Acute Toxicity

		Chronic Toxicity

		Irritation

		Sensitization

		Medical Experience with Epoxy Resins

		Comment



	12.	Photographs of Machinery with Suppliers 

		Contact Details				

Sample Chapters

SYNTHESIS AND CHARACTERISTICS OF EPOXY RESIN

Introduction

      The term “epoxide” is a prefix referring to a bridge consisting of an oxygen atom bonded to two ther atoms already united in some way. This chapter deals only with the synthesis and characteristics of the a- or 1,2-epoxides wherein the other two atoms are carbon. The capability of this group to undergo a large variety of addition and polymerization reactions leads to the numerous thermoplastic and thermosetting forms of epoxy resins. We define an epoxy resin as any molecule containing one or more a-epoxide groups.

The nomenclature of epoxides is confusing. In addition to the 1,2-epoxide, there exist a large variety of heterocyclic compounds with similar sounding names. The reader can thus easily go astray in searching the literature. To avoid this pitfall we briefly point out some of the more misleading facets.

Monocyclic compounds containing more than one type of atom in a heterocyclic ring are named by combining a prefix that describes the additional element with a stem that describes the ring size and degree of unsaturation. Shown in Table 1 are the prefixes for the various elements in a heteroring system (the “a” is elided where necessary). Table 2 lists the stems for the various oxygen and nitrogen saturated and unsaturated heterocycles in 3- to 10-membered rings. Note the similarity between ethylene oxide, trimethylene oxide, and tetrahydrofuran, which are called oxirane, oxetane, and oxolane, respectively. A further point of confusion arises from the numerous epoxy prefixes. In this system ethylene oxide is called epoxyethane.

 Table 1. Prefixes of Monocyclic Compounds with-One or More Hetero Atoms in a 3- to 10-Membered Ring

Element         Valence        Prefix              Element          Valence        prefix

Oxygen            II            Oxa           Antimony          III          Stibab

Sulfur             II           Thia           Bismuth           III          Bisma

Selenium          II          Selena           Silicon            IV           Sila

Tellurium         II         Tellura       Germanium         IV         Germa

Nitrogen          III           Aza               Tin             IV         Stanna

Phosphorus     III       Phosphab           Lead             IV        Plumba

Arsenic           III          Arsab           Mercury           II        Mercura

bWhen immediately followed by “-in” or “-ine”, “phospha-” should be re­placed by “phosphor-,” “arsa-” should be replaced by “arsen-,” and “stiba-should be replaced by “antimon-.”

Table 2. Stems for Monocyclic Compounds with One or More Hetero Atoms in a 3- to 10-Membered Ring

Number of                   Rings containing                        Rings containing Members                             nitrogen                                    no nitrogen

in ring             Unsaturatedb       Saturated       Unsaturatedb      Saturated

3                      -irine             -iridine             -irene            -iranec

4                       -ete             -etidiine             -ete             -etane

5                       -ole              -olidine              -ole              -olane

6                      -ined                 (e)                 -ind               -anef

7                     -epine               (e)                -epin            -epane

8                     -ocine                (e)                -ocin             -ocane

9                     -onine               (e)                -ocin             -ocane

10                    -ecine                (e)                -ecin             -ecane

bCorresponding to the maximum number of noncumulative double bonds, the hetero elements having the normal valences shown in Table 1.

cThe syllables denoting the size of rings containing 3, 4, or 7 to 10 members are derived as follows: “ir” from tri, “et” from tetra, “ep” from hepta, “oc” from octa, “on” from nona, and “ec” from deca.

dFor phosphorus, arsenic and antimony, see footnote b of Table 1.

eExpressed by prefixing “perhydro” to the name of the corresponding unsaturated compound.

fNot applicable to silicon, germanium, tin, and lead, for which “perhydro” is prefixed to the name of the corresponding unsaturated compound.

Table 3. Some Epoxy Prefixes

Prefix                                Structural                    Prefix                  Structural                                               group                                                         group

Epoxy                            -O-               Epoxyethano        -O-CH2CH2-

Epoxyethenothio       -O-CH=CH-S-         Epoxyimino            -O-NH-

Epoxymethano             -O-CH2-          Epoxymethanoxy      -O-CH2-O-

Epoxymetheno             -O-CH=             Epoxynitrilo            -O-N=

Epoxythio                      -o-s-               Epoxythioxy            -o-s-o-

Epidioxy                        -o-o-                 Epidithio               -s-s-

Epithio          -s-           Epithioximino    -S-O-NH-

There are additional names for 1,2-epoxides that add further to the confusion. Cyclohexene oxide is variously referred to as 1,2-epoxycyclo-hexane, 1,2-oxidocyclohexane, and 7-oxabicyclo [4.1.0] heptane. When epoxides are regarded as derivatives of a-glycols, the prefix “anhydro” is used. This term is prevalent in the naming of sugar epoxides. Further­more, several of the simple epoxides have common names that tell little about their structure. l-Chloro-2,3-epoxypropane (1) is called epichloro-hydrin, l-hydroxy-2,3-epoxypropane (2) is called glycidol, and 2,3-epoxypropanoic acid (3) is called glycidic acid.

    OCH2CHCH2CI            OCH2CHCH2OH          OCH2CHCOOH

             (1)                               (2)                             (3)

The terms “epoxy” and “epoxide” are most commonly used to describe the oxirane ring. The latter is doubtless more correct. The terms “epoxide” and “oxide” are used throughout the text in conjunction with the useful “glycidyl.” The term “glycidyl” is used to describe the terminal epoxide group, OCH2CHCH2-. The prefix is modified by “ether,”’ “ester,” “amine,” etc., according to the group attached to the third carbon atom.

Since this chapter is concerned with recent developments in the synthesis of epoxides, reference is made only to investigations that appear to throw light on the epoxidation reactions involved. This is sometimes arbitrary, but we have tried to present all of the important evidence. In addition, a section on structure is included, since it is desirable to have this clearly in mind in considering the characteristics of epoxides.

Structure of Epoxides

The bond lengths and angles in ethylene oxide have been determined by electron diffraction and by microwave spectroscopy. The bond angles of C-O-C, O-C-C, and H-C-H are 61°24', 59°18', and 116°15', respectively. The bond lengths are C-C = 1.47, O-C = 1.44, and C-H = 1.08 Å, respectively. Each CH2 group is in a plane at right angles to the plane of the ring, and the angle between each CH2 plane and the C-C bond is 159°25'.

The dipole moment of ethylene oxide is 1.82 D in benzene solution and 1.91 D in the gas phase. The heat of mixing with chloroform, the frequency shift of the OD band in the infrared specteum of CH3OD solutions and the chemical shifts in proton magnetic resonance show that the electron density on the oxygen atom is unusually low compared with that in acyclic and larger ring cyclic ethers. The calculated charge distributions for a series of epoxides are shown in Table 4. The electronic states of other epoxides are shown in Table 5. The strain energy, taken as the difference between the experimental and the calculated heats of formation, has been found to be 13 kcal/mole for ethylene oxide.

The Dynamic Mechanical Properties of Epoxy Resins

Basic Parameters

      The viscous and elastic response of a polymer to a cyclic stress, or strain (varying sinusoidally with time) is termed the dynamic mechanical response. The gross apparent stiffness of a polymeric material at a particular temperature and frequency is measured by the dynamic (Young’s) modulus (ER). This gross stiffness factor may be resolved in terms of an elastic and a viscous contribution. The elastic or springlike contribution to total stiffness is termed the storage modulus (E1) and is a measure of the mechanical energy stored and recovered each dynamic cycle. The viscous contribution to total stiffness is called the loss modulus (E2) and is direct measure of the energy dissipated each dynamic cycle and lost primarily as heat.

It is well known that the gross stiffness of a polymer changes with both temperature and cyclic frequency. As the gross stiffness changes so does the elastic (storage) and viscous (loss) contributions to this total stiffness. Characterization of the total stiffness, in terms of these energy storage and loss contributions, and as a function of frequency and temperature, provides information useful to both the research chemist and product engineer.

Figure 1 indicates the manner in which the stress (Curve A) and strain (Curve C) varies with time. In the case of the Maxwell device both stress and strain are tensional-compressional. The dynamic (Young’s) modulus (ER) is just equal to the ratio of maximum stress (So) divided by maximum strain (g0)

\

Fig. 1. Curves show how stress and strain vary with time.

In general the strain wave (Curve C) lags the stress wave (Curve A) by some phase angle (d). Under these conditions we can divide the stress wave into two components (Curves B) an elastic stress (SE) which is in phase with the strain (g), and a viscous stress which is 90 degrees or p/2 radians out of phase with the strain (g). The ratio of the maximum elastic stress (oSE) to maximum strain (go) is termed the storage modulus (E1).

In a similar manner the ratio of the maximum viscous stress (oSv) to maximum strain (g0) is termed the loss modulus (E2).

A characteristic of the decomposition of the stress (S) into its components (SE) and (Sv) is that the ratio (oSE/oSv) and the corresponding modulus just equals the tangent of the stress (S)—strain (g) phase angle (d). This ratio of moduli is termed the loss tangent (tan d)

A further property of the division of the stress (Curve A) into its components (Curve B) is that the maximum stress is given by the following expression:

So = (oSE2+oSv2)1/2.

From this expression and the preceding relations it may be shown that:

ER = (E12+E22)1/2

where |ER| is the absolute value of the dynamic modulus.

The Glassy Transition and Dynamic Mechanical Dispersion

The glassy transition temperature is dependent on the time scale or frequency of the measurement. This time dependency results from the fact that a diffusion rate controlled mechanism is normally associated with the “freezing in” of molecular mobilities in the transition region. A standard procedure for determining the glassy transition temperature is to measure some function of density with change in temperature. In phenol-formaldehyde resins the glassy transition temperature has been associated witii the heat distortion temperature and dynamic mechanical dispersion.

One of the physical phenomena with which dynamic mechanical measurements are concerned is that of dispersion. Optical and dielectric dispersion are well known phenomena. The features of a mechanical dispersion are:

Fig. 2. Curves are illustrative of dynamic mechanical dispersion.

    (a)   over a narrow frequency region the dynamic moduli (E1) and (ER) drop from higher to lower values as the frequency decreases or temperature increases.

    (b)   the loss tangent (tan d) goes through a maximum at the frequency at which the dynamic modulus (ER) curve has its inflection point, and E2 goes through a maximum.

    (c)   the dispersion region shifts to lower frequencies as the temperature is lowered.

In the glassy state it has been shown experimentally that each dispersion region is associated with a specific molecular group. As the frequency is increased or temperature lowered this group is “frozen in” and thus contributes to an increase in stress and resulting modulus.

In the region of the glassy transition a number of molecular mobilities are gradually frozen in with increased frequency and lowered temperature. This heterogeneous effect generally leads to a much broadened dispersion region and is described in terms of a continuous spectrum of processes. Here also the change in

complex dynamic modulus (ER) through the dispersion region and maximum tan d value will be much larger than for a glassy state dispersion region. These properties of dynamic mechanical dispersion are illustrated in Figure 2.

Temperature and Frequency Interdependence

The now standard “method of reduced variables” as applied to mechanical response of amorphous polymers involves two assumptions. The first is that the moduli ER, E1 and E2 are proportional to absolute temperature (T) and density (P), hence:

where E0, Po and E, P are respectively associated with temperatures To and T. Secondly that all molecular mobilities have the same frequency temperature dependence as expressed by the following equation:

where A is the activation energy of flow, R the gas constant and log aT the logarithmic frequency shift relating temperatures T and To.

The “reduced variables treatment” checks well for polymers having weak intermolecular binding forces in the region of the glass transition. In the glassy state it would appear that each mobility may have a discrete activation energy and that the shape of the dispersion curves depend on temperature.

Experimental

The diglycidyl ether of bisphenol-A was chosen as the common epoxy base for the several cure compositions. This epoxy was crosslinked catalytically with a boron trifluoride amine complex to produce the homopolymer and coreacted stoichiometrically with four diamine curatives, of varying structure and molecular weight. The chemical reactants and formulation and curing conditions are indicated in Table 1 and Table 2 respectively. Specimens for mechanical properties testing were cast in rectangular molds and machined to final shape. Specimens for dynamic property testing were either cast in glass tubes or machined from rectangular shape.

Table 1. Chemical Reactants

Number           Compound                                                              Equivalent                                                                                                               Weight

1                 Diglycidyl ether of Bisphenol A                        200

2                 Boron trifluoride amine complex                       —

3                 Diethylene triamine                                       20.6

4                 Metaphenylene diamine                                 27.0

5                 Methylene bis(orthochloroaniline)                     66.5

6                 Aliphatic diamine                                          158

Table 2. Formulation and Curing Conditions

Cure Recipe                       Parts by Wt.                              Cure Conditions

No.                               Resin              Curative               Time                 Temp.                                                                                            (Hr.)                    (ºC)

1-2                        100                 2                 12                132

                                                                     24                178

                                                                      1                 204

1-3                        100              10.3               24                R.T.

                                                                      2                 120

1-4                        100              13.5               24                R.T.

                                                                     16                120

1-5                        100              32.7               24                160

1-6                        100                79                 24                R.T.

                                                               16              120  

The test fixture and specimen for the modified Vicat heat penetration test, as used in our laboratories, is iliustrated. The test is adapted for use with a standard neat distortion tester. The penetrator head, having a flat penetrator needle of square cross section and 1.0 mm2 area, attaches to the load shaft. The compression weight is adjusted to a total of 1.0 Kg. The rate of bath temperature rise remains at 2.0°C/min. The test permits semimicro testing of samples of simple geometry and provides a penetration depth versus temperature curve as shown in Figure 3. By convention, the intersection of the extrapolated linear portion of the penetration curve to zero penetration is termed the heat penetration temperature.

The glassy transition temperature is obtained from linear expansivity curves as shown in Figure 4. The curves are plotted with adjusted base values of DL to provide for convenient vertical displacement.

Epoxy Resin Adhesives

Introduction

      The outstanding adhesive-bonding properties of epoxy resins    were first recognized by Preiswerk and Gams in 1944. At that time epoxy-resin adhesives were recognized as the first cast-in-place adhesives featuring a versatile chemical functionality and a remarkably low shrinkage on curing. This led to adhesive joints with low internal stress. Since the transition to the cured state takes place by a polyaddition reaction, no low-molecular-weight substances are evolved. These properties were then of unusual significance in that, for the first time, it was possible to obtain reliable adhesive joints with excellent cohesion, structural integrity, and outstanding adhesion to all kinds of substrates. For example, bonds could be made to metals and glass without resorting to the application of pressure during the bonding process and without any problem in bonding irregular surfaces. It was of technological significance that the manner in which these adhesives were used was reminiscent of well-known metal-soldering techniques. Therefore the discovery of the bonding function of epoxy resins introduced a new concept in adhesive materials and inaugurated the modern approach to the technology of adhesive bonding.

Since the commercial introduction of epoxy resins in 1946, adhesives have been one of the most valuable uses for such resins. Thus of the 161.7 million lb of epoxy resins (unmodified basis) sold in the United States in 1969, 19.4 million lb, or 12%, were used for bonding and adhesive purposes. Modern adhesives technology has led to the development of many types of epoxy-based adhesive systems. Adhesives have been formulated to meet various specifications and use criteria. The principal applications are in the field of structural metal bonding, particularly in the aerospace industry and in military equipment, as well as in miscellaneous small-part assembly of plastics and metals. To demonstrate the scope and versatility of epoxy-resin-based adhesives technology, Table 1 illustrates the storage, application conditions, service temperature, and joint-strength behavior of various formulated epoxy adhesives. Most of the information in this table was abstracted from an article by Carson on the specifications of structural adhesives. The variation of properties with formulation indicates the wide range of structural applications for which epoxy-resin adhesives can be designed. For example, phenolic-modified epoxy resins produce the best high-temperature adhesives, whereas nylon-epoxy combinations produce adhesives with the highest joint strengths.

The formulation of epoxy-based adhesives into serviceable adhesive-bonding systems is a broad field of technology. Epoxy-resin-based adhesives can be produced in various forms: one- and two-part liquids, film, or solvent based. This wide variety of formulations is indicative of the advanced state of the art of epoxy adhesives.

Generally the adhesion function and mechanism of adhesive-joint strength is poorly understood. However, a more systematic understanding of adhesion phenomena and adhesive-joint strengths is beginning to take form. This chapter treats the adhesive properties of epoxy resins from a scientific viewpoint. As a background, the most recent theories of adhesion and adhesive-joint strength will be presented, followed by an analysis of epoxy adhesive materials from a strength-of-materials viewpoint.

The chemical aspects of epoxy resins specific to the field of adhesion and adhesive applications will be presented. In particular the chemical background pertinent to the development of epoxy-resin-based adhesives of the type listed in Table 1 will be reviewed. This will be followed by a discussion of the technological aspects of adhesive joints concerned with the stress, environmental aging, and curing behavior of various practical epoxy-based adhesives.

Table 1. Types and Properties of Epoxy-Resin-Based Adhesives

Adhesive                         Shelf or                        Cure                       Service                            Lap shear strength (psi)b type                                  pot life                    conditions              temperature                                                                                                                                     (ºC)                       21ºC                 93ºC                 260ºC

General epoxy           40 sec to            24 to 204°C        –240 to +177         2000 to          300 to           500 to                               indefinite         5 days to 1 min,                                 4300            4000             1300                                                       0 to 100 psi

Polyamide-epoxy       30 min to            21 to 93°C,        –129 to +127         1800 to          500 to              —                                 1 year           7 days to 1.3 h,                                  4000            3900                                                       0 to 100 psi

Nelon-epoxy            2 months to         24 to 204ºC,       –251 to +204         2500 to         3000 to              —                               indefinite          8 h to 20 min,                                   6300            4000                                                      10 to 100 psi

Phenolic-epoxy          3 days to           107 to 204ºC        –251 to 260          2400 to         2400 to          2000 to                               indefinite              2 to 1 h,                                       3700            3300             2400                                                       10 to 50 psi

Anhydride-epoxy       20 min to           24 to 316ºC,        –196 to 371          1700 to         1600 to           500 to                               4 months          5 h to 12 min,                                   3600            3400             2200                                                        0 to 75 psi

Polysulfide-epoxy      15 min to            24 to 66ºC,         –21 to +121          1700 to         1200 to              —                                   5 h                 48 to 1 h,                                      3600            3000                                                        0 to 10 psi

Silicone-epoxy            90 days             177ºC, 1 h,          –21 to 371            2200            1500             1000                                                           15 psi

Polyurethanepoxy      30 sec to           24 to 121ºC,        –21 to +121           5200            4200                                                   2 h            7 days to 30 min,                                                        0 to 70 psi

bAluminum-to-aluminum lap joints tested acording to ASTM D1002.

Theories of Adhesion and ADhesive-joint Strength

Adhesion is a scientifically and technically diverse subject. In essence adhesion is an interfacial phenomenon. Physical and chemical driving forces are always operative when matter comes together to form an interface. Therefore, in its strictest sense, the adhesion strength of an interface is a measure of the degree to which the two surfaces are attracted. In practice the molecular basis of adhesion is rarely studied, and one must resort to secondary interpretations to determine the strength of adhesion. Hence the status of adhesion at an interface between two materials is presently reflected only by the test procedure used. Nevertheless, the process of adhesion can be systematically analyzed.

     The mechanism of adhesion is manifested by two stages: wetting and setting. However, from a practical standpoint, the entire process of adhesion must include the performance behavior of the joint since in many ways, the type of wetting or setting mechanism markedly influences the environmental resistance of an adhesive. For example, adhesive joints may appear to be satisfactory at first, but they may fail drastically under common use conditions, such as a moist environment or mechanical impact. The key to this problem lies in the state of the interfacial contiguous layer that exists between the polymeric adhesive material and the substrate.       Specifically, it will depend on what types of interaction prevail at the interface, whether the absorbed or bonded species are sensitive to moisture, and the presence or absence of voids due to improper wetting. A scientific approach to this problem is now beginning to take form, and it is believed that the answers should come from a deeper understanding of the absorption theory of adhesion; more specifically by probing the roles that chemisorption and diffusion play in the manifestation of adhesion effects.

Epoxy Resin Coatings

      A brief look into the background of the surface-coatings industry gives an understanding of the position epoxy resins hold in this field today. At the outset it should be noted that the formulation and manufacture of coatings are going through a period of transition from a craft type of endeavor to a scientifically regulated industry. For many years natural sources were the basis for the polymers and film-forming materials used in paints and varnishes. Paintmaking at that time was a typical craft, with artists and painters mixing their own paints from pigment and oil. The properties of the finished product depended to a great degree on their judgment and experience.

This period was followed by a transition that saw the establishment of paint and varnish factories whose products were controlled by the crafts­manship of the paint and varnish maker.

The development, in more recent years, of synthetic polymers, such as alkyd- and varnish-type phenolic resins, represents a significant step in the transition of the industry from an art to a science. Since this transition a steady flow of new synthetic polymers has become available. Procedures used in their production, control, evaluation, and application have formalized a technology based to a large degree on scientific principles and methods. This is substantiated by the increase in scientifically trained personnel and scientific technique being used in the laboratories of the coatings manufacturers. The transition is continuing, and the balance is shifting more and more from art to science.

The technical literature is abundant with references to the evaluation of the new synthetic polymers in coatings applications. Comparisons are shown of the new materials used both alone and in combination with established products. In many instances at least some advantage is claimed for the new materials. The number of new products finding practical applications indicates that many never leave the laboratory for commercialization. Quite a few of those that do leave the laboratory are found only in a limited number of low-volume, specialty applications.

On the other hand, epoxy resins based on epichlorohydrin and bisphenol A, introduced to industry a little over 20 years ago, are well-established coatings materials used in a wide variety of applications. Nearly 50% of the total epoxy-resin production finds its way into coatings formulations. The importance of epoxy resins to the coatings industry is shown by the consumption of 69 million lb in 1970 in coating applications.

Classification of Epoxy-Resin Coatings

Few coatings formulations are based on vehicles containing epoxy resins as the sole vehicle. The commercial success of these materials has been realized in coatings resulting from the reaction of other materials with them through the secondary hydroxy1, terminal epoxide, or both types of groups in the molecular structure. This has resolved the technology into four main types of system:

     1.   Epoxy-resin esters.

     2.   Cold- or room-temperature-cured systems based on curing agents containing amine groups.

     3.   Baking systems, particularly those in which the epoxy resins are reacted with phenolic resins, amino resins, or thermosetting acrylic resins.

     4.   Thermoplastic epoxy-resin systems based on extremely-high-molecular-weight epoxy resins.

These types of system, with a further subdivision into solvent-borne and liquid epoxy-resin solventless and super-high- solids systems, powder coatings, epoxy-resin-ester emulsions, water-reducible epoxy-resin poly-amide-cured coatings, and water reducible epoxy-resin coating systems for electrodeposition, are covered in this chapter. Emphasis is placed on the more recent developments in epoxy-resin coatings technology.

Epoxy Resins Commonly Used in Coatings

The earliest and even the current epoxy-resin coatings technology revolves mainly around solid epoxy resins or their solutions. The commercially available types can be classified, as in Table 1, according to end use, average molecular weight, WPE range, viscosity, hydroxyl functionality, and weight per gallon.

Because of the decided trend in the coatings industry to utilize liquid epoxy resins, Table 2 is included to show the differences in the materials finding their way into coatings formulations.

Epoxy-Resin Esters

Esters Produced from Solid Epoxy Resins

General Remarks

The solid types of epoxy resin can be reacted with vegetable-oil fatty acids to form epoxy-resin esters. The most commonly used resin type for these products has an average molecular weight of about 1400. In this reaction the epoxy resin is regarded as a resinous polyol containing epoxide and hydroxyl groups, both esterifiable. The epoxide groups have a hydroxyl functionality of 2 since two ester groups can be formed with each epoxide. Normally esterification proceeds under typical alkyd- or polyester-processing conditions. Temperatures ranging from 425 to 550°F and typical esterification catalysts (sodium carbonate, lithium naphthenate, or calcium acetate—0.05 wt % based on total solids) can be used. Kettles used are of the fusion or azeotropic type, as in the production of alkyd resins. Better color and lower viscosity characteristics result from the use of azeotropic equipment.

Typical surface-coating            Average                WPE               Viscosityb                mpc                 Hydroxyl              Weight end use                                        mol wt                rangea                                               (ºC)             functionalityd          (lb/gal)

Thermosetting acrylic              710             290-335            A1-B              40-45e               5+                 9.9

Polyamine curing (solventborne), thermosetting acrylic              900             450-550             D-G              65-75                6                  9.9

Polyamine curing (solventborne), powder coatings                    1060            600-700             G-K              75-85               6+                 9.9

Esterification, powder coatings                    1400           875-1025            Q-U              95-105              7+                 9.6

Phenolic or amino resin curing, esterification, powder coatings                    2900          2000-2500           Y-Z1             125-135              13                 9.6

Phenolic or amino resin curing                         3750          2500-4000          Z2-Z5            145-155             17+                9.9

aGrams of resin containing 1 gram equivalent of epoxide.

bGardner-Holdt viscosity of 40 wt% solids in butyl carbitol.

cDurran’s mercury method.

dIncludes hydroxyl functionality of two hydroxyl groups for each epoxy group.

eReadily pourable only when heated slightly above room temperature.

Table 2. Commercially Available Liquid Epoxy Resins for Surface Coatings

Typical surface-             Average           WPE            Viscosity           Weight coating end use               mol wt            range         at 25°C (P)a         (lb/gal)

Polyamine curing (solventless)                 330b        175-195          5-7               9.5

Polyamine curing (solventless), emulstion paints liquid epoxy resin for ester production      380         185-192       100-160           9.7

Coal-tar epoxy paint, thermosetting acrylic     470         230-280       4.1-9.7            9.7

aKinematic viscosity, ASTM D445-53T.

bContains a reactive diluent.

Formulation Latitude

Esters produced from the higher-molecular-weight (2900 and 3750) resins have higher viscosity, greater impact and chemical resistance, and flexibility. Drying rate and application solids are lowered.

The choice and amount of fatty acids related to the amount of epoxy resin affect final properties. A variety of epoxy-resin esters, similar in scope to the various types of alkyd resins, can be made by use of these two variables. Long-, medium-, and short-oil-length esters are formulated in this manner.

Long-oil linseed or soya epoxy-resin esters are soluble in mineral spirits, and, by adding driers, an air-drying system for industrial maintenance applications can be formulated.

Short-drying-oil epoxy esters are not soluble in aliphatic solvents but are soluble in aromatic hydrocarbons. These products can be used by themselves. Specific improvements in properties can be obtained by cross-linking them with urea-formaldehyde resin (Beckamine 21-511, Reichhold Chemicals, Inc.; Beetle 227-8, American Cyanamid Co.; Uformite F-240, Rohm and Haas), or melamine-formaldehyde resins (Resimene 875, Monsanto Chemical Co.; Uformite MM-55, Rohm and Haas).

Overprint varnishes (short-oil dehydrated castor epoxy-resin esters) for metal containers, bottle caps, and screw-cap enclosures constitute a large-volume usage. A highly detergent-resistant appliance primer can be formulated by crosslinking a short-oil soya ester containing an epoxy resin (molecular weight 2900) with a melamine resin.

Epoxy-resin esters based on low-rosin-content tall oil crosslinked with an amino resin have attained significant importance in automotive primers.

Esters Produced from Liquid Epoxy Resins

Precatalyzed Liquid Epoxy Resin for the Production of Solid Epoxy Resins and Epoxy-Resin Esters

There is an alternative procedure for producing epoxy-resin esters wherein a selectively precatalyzed liquid epoxy resin (Epon Resin 829, Shell Chemical Co.) with a narrow range of properties is reacted with varying amounts of bisphenol A to prepare a broad group of solid epoxy resins or their fatty-acid esters. Higher-molecular-weight-resin esters at shorter oil lengths than are feasible with commercial solid resins can be prepared. The composition listed in Table 3 is proving quite interesting in coating evaluations, as it shows noticeably improved through- drying, flexibility, impact strength, and chemical resistance properties.

Table 3. High-Molecular-Weight Ester via Precatalyzed Liquid Epoxy Resin

Material                                                                                  Quantity (wt %)

Precatalyzed liquid epoxy resina                                      44.0

Bisphenol A                                                                23.0

Linseed fatty acidsb                                                      33.0

Sodium carbonate (anhydrous)c                                        —

Xylened                                                                        —

                                                                                100.0

aEpon Resin 829, Shell Chemical Co.

bWoburn’s Supra, Woburn Chemical Co.

cEsterification catalyst; 0.1 wt % based on resin components.

dApproximately 3 wt % as an azeotropic solvent.

Epoxy Coating Give into Water

      The pollution elimination-motivated desire to limit solvent  usage is the basis for a large amount of research on water-borne coatings. If the vehicle involved is hydrophylic or “water-loving” or if emulsifiers can be included which are complementary to whatever hydrophylic character the polymer itself may have, successful coatings result. The vehicle which started the “water revolution” in the coatings industry was, as is well-known, a styrene-butadiene copolymer. This material obviously is not very hydrophylic. On the other hand, it is compatible with emulsifying agents and the preparation of an emulsion is relatively simple. At least it is simple once you know how to do it. The technique for emulsifying styrene-butadiene copolymers was a major part of the rubber program during World War II and, accordingly, a great body of knowledge existed for preparing such emulsions.

Poly (vinyl acetate) was also readily emulsified. The acetate groups being polar are eager to be solvated by water molecules and dispersion can be helped along with the proper emulsifiers and stabilizers such as poly (vinyl alcohol). Similar comments apply to acrylic polymers and copolymers.

Protective coatings chemists soon found, however, that there are other polymers which do not emulsify readily. Heading the list are the epoxy resins. An epoxy resin had ether and hydroxyl groups in the chain and, of course, epoxy end groups. All of these groups are polar and one might think that they would assist in the emulsification of the epoxy resin. Not so, for a complex reason about which theoreticians like to debate. An epoxy resin has a very rigid structure because of the bisphenol A component. It is this rigidity, perhaps, which prevents the hydroxyl and ether groups from exhibiting polar character. On the other hand, epoxy resin compositions have relatively poor water resistance because of the polarity presented by the ether and hydroxyl groups. Thus there is an inconsistency that requires complex theoretical rationalization. The practical point, however, is that epoxy resins are difficult to emulsify.

The difficulty varies with the type of epoxy resin composition involved. An epoxy resin ester can be emulsified readily by esterifying some of the hydroxyl groups with one carboxyl of a dibasic acid or anhydride such as maleic anhydride. The unreacted carboxyl can be neutralized with sodium hydroxide or an amine to provide an intrinsic emulsifier. Correspondingly, the unsaturated fatty acid groups of the ester can be maleinized by heating with maleic anhydride and, again, carboxyl groups are present which, after neutralization, make emulsification possible. This type of composition is used for electrocoating.

On the other hand, a two-component epoxy composition comprising an epoxy resin and a coreactant such as an amine or a polyamide resin is much harder to convert from a solvent-based to a water-borne system. Persistent work on the part of coatings chemists, however, has produced such water-borne systems and these are in use today. Much of the success in emulsifying epoxy resins depends on the coreactant. Amino-containing polyamide resins are coreactants which are readily emulsifed by neutralizing some of the free amine groups with acids. Intrinsic emulsifiers result which make possible the emulsification or dispersion of the material in water. The amine-containing polyamide with its intrinsic emulsifying groups will emulsify not only itself but will also disperse the epoxy resin. To be sure, the dispersion must be assisted by extrinsic emulsifiers and when these are carefully chosen a water-borne vehicle results which can be pigmented and which has many of the properties of the corresponding solvent-based system.

Perusal of the patent literature provides interesting insight into the type of work underway to make epoxy compositions water-dispersible. If a baked coating is contemplated the problem is somewhat easier because the baking contributes to good properties. On the other hand, if a room temperature-curing system is desired the problem is complicated if only because the emulsifiers present tend to detract from properties such as water resistance.

Typical of the patents which have issued recently on water-borne epoxy coatings are the following.

German patent 2,627,697 issued to Ciba-Geigy AG describes technology in which an epoxy resin with a molecular weight varying from 340 to 5,000 is reacted with an aminoalcohol such as diethanolamine. Presumably the amine group of the diethanolamine reacts with the epoxy linkages to give a composition with primary hydroxyl groups. These are in turn ethoxylated to increase the hydrophylic properties of the polymer. When the ethoxylated composition is combined with a methylated urea-formaldehyde resin, a water-dispersible baking finish results.

Another baking finish which is intended primarily as a cathodic electrocoating vehicle is described in Dutch patent 11,958 assigned to BASF. Here the epoxy resin is made more hydrophylic by reacting it with an equivalent amount of mercaptan to provide a thiol ether. This in turn is quaternized with an epoxide and acid to provide a sulfonium compound. The free hydroxyl groups which were initially present in a the epoxy resin are partially masked by reaction with a polyisocyanate, although insufficient polyisocyanate is used so that gel formation is avoided. In a typical example, a bisphenol A-based epoxy resin with a molecular weight of 1,000 and an epoxy value of about 500 is reacted with mercaptoethanol in ethylene glycol. The thiol ether that results is in turn reacted with ethylene oxide and acetic acid. The water-dispersible composition that results is combined with a melamine resin.

Another patent for a baking coating which makes use of diethanolamine is British patent 1,469,495 assigned to M&T Chemicals. Here an epoxy resin derived from bisphenol A is reacted with mono- or diethanolamine in a ratio such that all of the epoxy functionality is consumed. An aqueous solution of this material is applied to a substrate which has been heated to 200 to 300°C. A coating is said to result which is durable, solvent- resistant, and flexible. It is not clear, however, from the description what kind of crosslinking, if any, takes place.

A heat-curable water-borne epoxy resin composition is described by Mobil Oil Corporation in U.S. patent 4,029,620. The coating comprises two parts, one of which is an emulsified epoxy resin in which the stabilizing agent is a nonionic surfactant. The second portion which cures the epoxy resin on heating is a solvent-soluble copolymer containing carboxyl functionality. This is dispersed in water by forming a salt of some of the carboxyl groups with a volatile amine or ammonia. If ammonia is used, up to 90% of the carboxyl groups may be neutralized. If an amine is used, there is a 30% maximum for neutralization. Also, when the two components are mixed there must be a 25% excess of epoxy functionality over carboxyl functionality.

A related baking coating is described in U.S. patent 4,021,396 assigned to duPont de Nemours Co. This, however, is a one-component system in which both the epoxy resin and the carboxyl-containing coreactant are dispersed in aqueous medium at a level of 25 to 50 weight percent. Five to forty percent of epoxy resin may be used with 60 to 95% of an acrylic polymer whose number average molecular weight is above 10,000. The copolymer is based on styrene, methylstyrene, methyl methacrylate, or acrylonitrile with an alkyl acrylate or methacrylate and up to 10% of an unsaturated carboxylic acid.

A number of patents describe epoxy resin emulsions. Thus one in which the epoxy resin is emulsified with a nonionic surfactant is claimed in Japanese patent 013530 assigned to the Dai-Nippon Toryo Co. The dispersed epoxy resin is combined with a suitable hardener and the mixture is said to be stable on storage which means that the coating must be baked in order to achieve curing.

Analysis of Epoxides and Epoxy Resins

The problems that confront the epoxy analyst are the following:

     1.   Determination of the structure and physical properties of the cured and uncured resins.

     2.   Control of resin charges, analysis of hardeners, flexibilizers, fillers, etc.

     3.   Following the progress of the curing reactions and determining the extent of final cure.

     4.   Identification of unknown samples that are supposed to be epoxy resins.

Recently physical methods have become valuable tools in epoxy-resin analysis; however, the older, more classical chemical methods still remain important and accordingly cannot be disregarded.

Uncured Epoxy Resins

Qualitative Tests

Several classification tests for epoxy resins depend on the detection of free epoxy groups, but a series of reactions that are specific for the bisphenol A ether skeleton are used predominantly. The latter tests do not require the existence of free epoxy groups and therefore can also be used to detect cured or esterified bisphenol A epoxy resins. In all cases the solubility of the epoxy resin in the reaction medium has to be considered.

Qualitative identification of epoxides and epoxy resins by means of infrared spectroscopy will be discussed in Section I.D.

Detection of Free Epoxy Groups

a. Addition of Chloride Ions. The addition of chloride ions to epoxides proceeds even in neutral solution, as was stated by BrOnstedt et al. [1].

According to Wurtz the hydroxyl ions formed can be detected by the precipitation of metallic hydroxides from the concentrated solutions of the corresponding chlorides (e.g., aluminum or magnesium chlorides). Lenher recommends a saturated neutral solution of manganous chloride as the best test; however, the halides of aluminum and the divalent halides of magnesium, iron, tin, and zinc are also useful reagents. For the determination of the hydroxyl ions formed Deckert used the color change of an acid-base indicator. Water-soluble epoxides can be treated with an aqueous solution of sodium chloride or potassium thiocyanate. A suitable indicator is phenolphthalein, used in a procedure described by Fisch to detect epoxy groups of substances that are insoluble in aqueous salt solutions.

Determination of Epoxy Group—Lithium-Chloride Test

Reagents

Lithium chloride solution, 10%. Mix 85 ml of methyl Cellosolve and 5 ml of water and dissolve 10 g of lithium chloride.

Phenolphthalein indicator solution, 1% in ethanol.

Methyl Cellosolve.

Procedure

Dissolve 0.5 to 1 g of the substance to be tested in 2 ml of methyl Cellosolve, add a few drops of phenolphthalein indicator,

and, if necessary neutralize the solution. After mixing with 2 ml of lithium chloride solution, heat the sample for 20 sec to boiling. An intense red color indicates free epoxy groups.

The various quantitative hydrochlorination analytical methods are also suitable classification reactions and can be used for this purpose with somewhat simplified procedures.

b. Selective Oxidation. A further test for the detection of epoxides and epoxy resins has been given by Fuchs, Waters, and Vanderwerf. The method employs hydration of the epoxide, followed by periodic acid oxidation of the corresponding glycol. The test is interfered with by a-glycols, a-oxyaldehydes, a-oxyketones, a-diketones, and a-dicarboxylic acids. Alcohols, aldehydes, and ketones do not interfere.

Determination of Epoxy Group—Periodic Acid Test

Reagents

Periodic acid solution, 0.5% in water.

Silver nitrate solution, 1% in water.

Nitric acid, 60%.

Dioxane or glacial acetic acid.

Procedure

Dissolve 1 to 2 drops of the substance to be tested in 2 ml of dioxane or glacial acetic acid, add 2 ml of the periodic acid solution, and shake for several minutes. Add 1 to 2 drops of silver nitrate solution. Precipitation of white silver iodate indicates that free epoxy groups were present.

c. Pyrolysis. A test described by Feigl employs the pyrolytic splitting off of acetaldehyde from epoxides when heated at 240 to 250°C. The acetaldehyde thus formed can be detected by the color reaction with sodium nitroprusside and piperidine. Other materials (e.g., cellulose) evolve acetaldehyde only when heated to much higher temperatures. The test is also positive for cured epoxy resins.

Determination of Epoxy Group—Pyrolysis Test

Reagents

Sodium nitroprusside solution, 5% in water.

Piperidine or morpholine solution, S% in water.

Procedure

Heat the substance to be tested in a tube immersed in a metal bath at 240 to 250°C. Place a filter paper moistened with a few drops of sodium nitroprusside and piperidine solutions in the tube. A blue color indicates free epoxy groups.

d. Color Reactions. Several epoxides, with tertiary amines like pyridine, â-picoline, quinoline, etc., give specific color-producing reactions, which sometimes allow a quantitative estimation, as in the case of ethylene oxide. An intensive study of these reactions was published by Lohmann. Gunther employed the reaction with lepidine (4-methylquinoline) for a microdetermination of ethylene oxide. Swann suggested the following procedure for a qualitative test, which is also suitable for epoxy resins.

Determination of Epoxy Group—Lepidine Test

Reagents

Lepidine (4-methylquinoline).

Cellosolve.

Procedure

Dissolve 1 drop of the sample in 3 ml of Cellosolve, add 5 drops of lepidine, and heat to 125°C in an oil bath. Development of a blue color indicates that free epoxy groups are present.

e. Preparation of Derivatives. Pure alkylene oxides can be characterized by the preparation of the readily crystallizing sulfides, which are formed by the addition of dinitrothiophenols.

According to Parker pure epoxides can be converted to the carbonyl compound by acid catalysis. The resulting aldeyhyde or ketone is then determined as the 2,4-dinitrophenyl hydrazone,

or semicarbazone; however, quite often two isomeric compounds are obtained.

Ulbrich reported that bis (1-naphthyl-urethanes) are suitable derivatives for the identification of glycidyl ethers.

Detection of the Bisphenol A Skeleton

Besides the described procedures for the determination of epoxy groups there are a series of reactions that are characteristic tests for epoxy resins based on bisphenol A. These tests do not require the existence of free epoxy groups; that is, they are also positive for cured and esterified epoxy resins. Compounds of the phenyl glycidyl ether type give only a very weak positive reaction.

a. Mercuric Oxide and Nitric Acid Tests. These two reactions, which were first described by Foucry, are used to a large extent. Rudd and Zonsveld recognized the fact that these tests are specific reactions of epoxy resins based on bisphenol A. Other epoxides, lacquer solvents, phenol formaldehyde resins, phenol, bisphenol A, melamine resins, etc., do not interfere. It is customary to apply both tests and to carry out a blank test on an epoxy resin of the bisphenol A type.

Determination of Bisphenol A Epoxy Resins—Mercuric Oxide and Nitric Acid Tests

Reagents

Sulfuric acid, 98%.

Denigè’s reagent. Mix 10 ml of concentrated sulfuric acid and 50 ml of water, dissolve in the hot mixture 2.5 g of mercuric oxide, and filter the solution.

Nitric acid, 60%.

Sodium hydroxide solution, 5% in water.

Procedure

Dissolve with shaking but without heating 0.25 g of the powdered or liquid resin to be tested in 25 ml of concentrated sulfuric acid. Fillers or pigments must be previously separated by centrifuging or by extracting the resin with a suitable solvent. (An ether-benzene-methanol-acetone mixture, 10:6:4:1, has been recommended.)

     1.   Mix 1 ml of the sulfuric acid-resin solution with 5 ml of Denigè’s reagent and let stand for 30 min. An orange precipitate indicates an epoxy resin of the bisphenol A type.

     2.   Shake 1 ml of the sulfuric acid-resin solution with 1 ml of concentrated nitric acid. After 5 min pour the solution with stirring into 100 ml of sodium hydroxide solution. Epoxy resins of the bisphenol A type are indicated by an orange color.

The latter test has been studied by Swann who stated that the reaction also permits an estimation of the epoxy-resin portion of simple mixtures. To determine epoxy resins in cured coatings they recommend the following procedure: