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Epoxy Resins for Adhesives and Sealants

Epoxy resin-based adhesives are probably the most versatile family of adhesives used today! Since they bond well with many substrates and can be easily modified to achieve widely varying properties, they found wide applications in applications such as automotive, industrial, aerospace etc.

Get comprehensive information about fundamentals of epoxy resins used in adhesives, main types of chemical classes and selection tips to find the right product for your adhesive or sealant formulation.

Epoxy Resin Fundamentals


TAGS:  Epoxy Adhesives    

Epoxy Resins in Adhesives and Sealants Epoxies are probably the most versatile family of adhesives because they bond well to many substrates and can be easily modified to achieve widely varying properties. This modification usually takes the form of:

  1. Selection of the appropriate epoxy resin or combination of resins of which many are available,
  2. Selection of curing agent and associated reaction mechanism,
  3. Simple additions of organic or inorganic fillers and components.

Epoxy resins represent only one major ingredient in the formulation. They gain their ultimate performance characteristics when reacting with curing agents and additives or modifiers.


What is Epoxy Resin?


The term "epoxy","epoxy resin", or "epoxide" (Europe) refers to a broad group of reactive compounds that are characterized by the presence of an oxirane or epoxy ring as shown in figure below:

Epoxy resin structure

Epoxy resin structure

Epoxy resin Structure & Properties


This is represented by a three-member ring containing an oxygen atom that is bonded with two carbon atoms already united in some other way.

Structure of Epoxy Resin


  • The epoxy groups at both terminals of the molecule and the hydroxyl groups at the midpoint of the molecule are highly reactive.
  • The outstanding adhesion of epoxy resins is largely due to the secondary hydroxyl groups located along the molecular chain; the epoxy groups are generally consumed during cure.
  • The large part of the epoxy resin backbone contains aromatic rings, which provide a high degree of heat and chemical resistance.
  • The aliphatic sequences between ether linkages confer chemical resistance and flexibility.
  • The epoxy molecule can be of different molecular weight and chemistry. Resins can be low viscosity liquids or hard solids. Low viscosity can be obtained at 100% solids, which results in good penetration and wetting.
  • A large variety of polymeric structures can be obtained depending on the polymerization reaction and the curing agents involved. This can lead to versatile resins that can cure slowly or very quickly at room or at elevated temperatures.
  • No small molecules such as water are liberated during the curing process. Thus, epoxies exhibit low shrinkage, providing a very low degree of internal stress when cured.

Epoxy resins that are commercially produced are not necessarily completely linear or terminated with epoxy groups. Some degree of branching occurs, with the end groups being either epoxy or hydroxyl. The amount and degree of branching varies from resin to resin and from supplier to supplier.

Epoxy resins are not completely difunctional. Tri-, tetra-, and polyfunctionality are possible. Various end groups can be introduced as a consequence of the manufacturing process.


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Epoxy Resins Curing


The epoxy resins are capable of reacting with various curing agents or with themselves (via a catalyst) to form solid, crosslinked materials with considerable strength and adhesion. This transformation is generally referred to as curing or hardening. This ability to be transformed from a low viscosity liquid (or thermoplastic state) into a tough, hard thermoset is the most valuable single properties of epoxy resins.

This transformation or conversion is accomplished by the addition of a chemically active compound known as a curing agent or catalyst. Depending on the particular details of the epoxy formulation, curing may be accomplished at room temperature, with the application of external heat, or with the application of an external source of energy other than heat such as ultraviolet (UV) or electron beam (EB) energy.

Epoxy resins curing is initiated once the resin is mixed with a curative. The cure of epoxy resins is an exothermic process where heat is generated as a natural result of the chemical reaction. Success in using most epoxies is dependent on handling the product in the correct way in order to avoid premature cure and unwanted side reactions.


Major Epoxy Resin Types


A wide range of epoxy resins are produced and the main chemical categories of epoxy resins used in adhesive and sealant formulations are:

  1. Glycidyl epoxies- are prepared via a condensation reaction of an appropriate dihydroxyl compound, dibasic acid or a diamine and epichlorohydrin. These can be further classified as:
  2. Non-glycidyl epoxy

There are several epoxy resin types that can be used in adhesive formulations. The most commonly used epoxy resin type is based on diglycidyl ether of bisphenol A (DGEBA). Epoxy novolac, flexible epoxy, high functionality, and film forming epoxy resins are also used in specialty applications. Check out the table below to know more about various epoxy resins types:

Epoxy Resin
Chemical Family
Characteristics
Primary Cure*
General purpose use
RT, ET
Glycidyl ether
Flame resistance
RT, ET
Glycidyl ether
Very low viscosity
RT, ET
Glycidyl ether
High heat and chemical resistance
ET
High functionality resins (e.g., tetraglycidyl ether of tetraphenolethane, diglycidyl of resorcinol)
Glycidyl and non-glycidyl
High heat and chemical resistance
ET
Flexibilized epoxy resins (e.g., glycidyl ether of aliphatic polyol)
Glycidyl and non-glycidyl
Flexibility and high elongation
RT, ET
Glycidyl amine
High modulus and Tg
ET
Aliphatic epoxy
Non-glycidyl
High reactivity and low viscosity
RT, ET
Cycloaliphatic epoxy
Non-glycidyl
Good electrical characteristics, chemical resistance, low viscosity
ET
Glycidyl and non-glycidyl
Very fast cure, chemical resistance
RT
Primary Epoxy Resin Types Used in Adhesives and Sealants


Epoxy resins are commercially available as either liquids or solids. The liquids are available as:

  • Solvent-free resins, ranging in viscosity from water-like liquids to crystalline solids
  • Water borne emulsions; and
  • Solvent borne solutions

The higher the molecular weight of the epoxy resin molecule, the higher the viscosity or melting point

Let's understand them in detail.

#1. DGEBA Epoxy Resins


Diglycidyl ether of bisphenol A also known as DGEBA epoxy resins are not only the oldest type of epoxy resins but also the most valuable in adhesive formulations. DGEBA epoxy resins can be divided into:

  1. Low molecular weight liquids
  2. High molecular weight semi-solids and solids, and
  3. Brominated resins

The primary reasons for their popularity have been the relatively low cost of raw materials used to synthesize these resins, and the ability to be cured by a large number of different crosslinking agents at both room and elevated temperature.

Brominated epoxy resins are the reaction product of epichlorohydrin and brominated bisphenol A. They are primarily used in applications where ignition resistance is a requirement such as printed circuit boards and other products that need to be flame retardant.

The resulting resins are available primarily as semisolids or solids in solvent solutions. They have properties similar to other DGEBA epoxy resins except that the high bromine content (18-21%) in the finished resins provides outstanding flame ignition resistance. Brominated epoxy resins are usually blended with other epoxy resins to provide the concentration of bromine required to provide ignition temperature resistance. They have also been used as flame retardant additives in thermoplastic compounds.

#2. Epoxy Novolac Resin


  • Epoxy novolac resin is prepared by reacting epichlorohydrin with a novolac resin.
  • The most common epoxy novolac resins are based on medium molecular weight molecules with phenol and o-cresol novolacs.
  • Epoxy novolac resins differ from standard DGEBA based epoxy resins primarily in their multifunctionality, which is about 2.5 to 6.0.
  • The multiplicity of epoxy groups allows these resins to achieve increased crosslink density.
  • The commercial epoxy novolac resins are semi-solid to solid resins with EEW in the range of 170 to 230.
  • Recently low viscosity epoxy novolac resins have been produced (18000-28000 cps) to provide easy processing; however, these generally have lower epoxy content.

When cured with any of the conventional epoxy curing agents, epoxy novolac resins produce a product with better elevated temperature performance, chemical resistance, and adhesion than the bisphenol A based resins.

In order to develop the properties of epoxy novolac resins to their fullest extent, a high temperature cure is necessary. With room temperatures cures, the properties of the final product are similar to conventional DGEBA systems. The thermal stability of most epoxy novolac resins is affected markedly by the length of the cure cycle.

#3. Flexible Epoxy Resins


Flexibility can be provided through the epoxy resin constituent by incorporating large groups in the molecular chain, which increase the distance between crosslinks. Long chain aliphatic epoxy resins provide flexible molecules with high elongation but little toughness. Because of their lack of hydrolytic stability and lack of strength, flexible epoxy resins are generally not used alone but are blended as modifiers with other epoxy resins. When used in a concentration range of 10-30%, flexible epoxy resins improve flexibility without greatly impairing other properties.

Flexible Epoxy Resins Applications:

  • Adhesives to laminate safety glass,
  • Adhesives and sealants to dampen vibration and sound in addition to providing joining, and
  • Encapsulants for electrical components and other delicate components where thermal cycling is expected.

Glycidyl ethers of aliphatic polyols based on polyglycol, glycerin, and other polyols are flexible epoxy resins. They are used as reactive diluents and flexibilizers for solvent-free epoxy resin formulation.

Commercial products are available where "n" varies from 2 to 7.

#4. Waterborne Epoxy Resins


Epoxy resins are hydrophobic in nature and consequently are not, by themselves, dispersible in water. However, water dispersability can be conveyed to epoxy resins by two general methods:

  • "Chemical modification" of the epoxy resin, or
  • Through the process of emulsification

Chemical modification of the epoxy resin generally includes either attaching hydrophilic groups to the epoxy resin or attaching the epoxy resin to hydrophilic polymers.  The emulsification method is primarily used for waterborne epoxy resin adhesive systems. The epoxy resin is made water dispersible by partitioning the epoxy resin within a micelle, effectively separating the resin from the water. Emulsification can be achieved by a suitable surfactant. The choice of surfactant and processing parameters determines the long-term mechanical and chemical stability of the dispersion.

Epoxy resin emulsions are commercially available from several sources. As a group, typical particle size of the dispersion is in the 0.5-3.0 micron range. Solids typically range from 50-65% and viscosity from 10,000-12,000 cps. Characteristics of these epoxy dispersions are summarized in the table below.

Property

Bisphenol – A

Polyfunctional

Modified

WPE (Weight per epoxy based on resin solids)

195 – 2200

Viscosity, cps

500 – 20,000

Functionality, epoxy groups per molecule

2

3-8

Depends on base resins

Other characteristics

Higher MW provides greater flexibility

Lower MW provides greater crosslinking density

Higher Tg and crosslinking density than bis-A systems

Modifications are generally for greater toughness, flexibility, and adhesion to substrates such as vinyl


Characteristics of Major Classes of Epoxy Dispersions Used in Adhesive Applications


#5. Epoxy Acrylate Resins


There are basically two types of epoxy acrylate resins used in formulating adhesive systems. One is a vinyl ester resin that is used in two component adhesive formulations much like a DGEBA epoxy resin. The other is a special type of resin that is used in radiation cure processes. This latter type of epoxy acrylate does not have any free epoxy groups, but reacts through its unsaturation.

Epoxy Acrylate Resins Production

Epoxy acrylate resins or "vinyl esters" are made from the esterification of epoxy resin. The resultant polymer is typically dissolved in a reactive monomer such as styrene. These epoxy acrylate resins act more like polyester resins than they do epoxy resins. They are easily processed, have fast cure rates at room temperature, and can be cured with peroxides. Once crosslinked vinyl esters have excellent chemical resistance and mechanical properties at both room and elevated temperatures. In adhesive systems epoxy acrylate resins impart low viscosity, flexibility, and superior wetting characteristics compared to DGEBA type epoxy resins. However, their shrinkage is greater than any conventional epoxy resins and often the formulator will have to counteract this.

Epoxy acrylate resins are also commonly used as oligomers in radiation curing adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. Epoxy acrylate resins are formulated with photoinitiators to cure via ultraviolet (UV) or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. Epoxy acrylate oligomers that are used in UV/EB curing are very low viscosity systems with high vapor pressures. Within this group of oligomers, there are several major sub-classifications.

#6. Other Epoxy Resins


Bisphenol F

Bisphenol F epoxy resins are produced by condensing phenol with formaldehyde. Bisphenol F epoxy resins have lower viscosity than DGEBA for the same molecular weight. Cured bisphenol F epoxy resins also have increased solvent resistance. Bisphenol F resins are often mixed with conventional DGEBA epoxy resins because of the relatively high cost of the bisphenol F product. When mixed with bisphenol A resins, the two form crystallization-free resins of moderate viscosity.

Tetraglycidyl ether of tetraphenolethane

Tetraglycidyl ether of tetraphenolethane is an epoxy resin noted for high temperature and high humidity resistance. It has a functionality of 3.5 and, thus, exhibits a very dense crosslink structure. The resin is commercially available as a solid. It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur.

Diglycidyl ether of resorcinol

Diglycidyl ether of resorcinol based epoxy resins provide the highest functionality in an aromatic diepoxide. It is one of the most fluid of epoxy resins, with a viscosity of 300-500 cps at 25°C. Because of its high functionality, it is a very reactive resin and cures more rapidly than DGEBA epoxies with most conventional curing agents.

Cycloaliphatic epoxy resins
Cycloaliphatic epoxy resins have better weather resistance and fewer tendencies to yellow and chalk than aromatic epoxy resins. These resins possess excellent electrical properties and are often used in electrical / electronic applications. Their use in adhesive systems is limited because they are relatively brittle and higher in cost than aromatic resins. However cycloaliphatic epoxy resins are used in cationically cured (UV and EB) epoxy resin adhesive formulations.

Glycidyl amine

Glycidyl amine epoxy resins are reaction products of aromatic amines and epichlorohydrin. They have high modulus and high glass transition temperature. These resins find use in specialty aerospace composites and high temperature adhesive formulations.

Biobased epoxy resins Biobased epoxy resins are based on agricultural sources such as plant-derived glycol used in the manufacture of epichlorohydrin. Otherwise the characteristics are similar to petroleum synthesized epoxy resins.


Selection Criteria for Epoxy Resin


The selection of an epoxy resin for an adhesive or sealant formulation must consider these stages:


The epoxy adhesives with the best combination of properties are often blends of two or more resins. One must also remember that the selection of a curative and additives or modifiers is also critically important.

Formulation Properties


Most of the uncured properties of an epoxy resin are indicated by the number of repeating units, "n", or its molecular weight. The most important are:
  • viscosity
  • epoxy equivalent weight, and
  • vapor pressure

These properties are shown in the table below for the most common types of epoxy resins used in adhesives and sealants.

Number of Repeating Units, n

Molecular Weight

Epoxy Equivalent Weight, EEW

Viscosity

Vapor Pressure

Poise at 25°C

Melting Point, °C

0

380

175-210

50-225

High

Low

2

900

450-525

Solid

65-75

3.7

1400

870-1025

Solid

95-105

8.8

2900

1650-2050

Solid

125-135

12.0

3750

2400-4000

Solid

145-155

Comparison of Formulating Properties for Epoxy Resins


Viscosity

Controlling flow or viscosity is an important part of the adhesive formulation process. If the adhesive has a propensity to flow easily, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface.

The viscosity of an epoxy resin is dependent primarily on its molecular weight. Low molecular weight resins typically have a viscosity in excess of 6,000 cps, and conventional DGEBA epoxy resins (EEW = 190) have a viscosity around 12,000 cps. General relationships between structure and viscosity are provided below:
  • For a given resin, viscosity increases in proportion to the molecular weight. Epoxy molecules having "n" greater than 2 are semisolid or solid.
  • Linear resins of the same molecular weight often give higher viscosity than do branched resins.
  • Aromatic epoxies with three or more groups per molecule are semisolid or solid at room temperature.

Epoxy Equivalent Weight


An important term that is used in formulating epoxy adhesive compositions is epoxy equivalent weight or EEW. This is defined as the weight of resin in grams that contains one equivalent of epoxy. As the resin's molecular weight increases, the EEW will also increase. If the resin chains are assumed to be linear with no side branching, and it is further assumed that an epoxy group terminates each end of the molecule, then the epoxide equivalent weight is one-half the average molecular weight of the epoxy resin.

The EEW is useful in determining curing agent concentrations. Generally, the higher the EEW, the less curing agent is required because there are less functional epoxy groups per unit weight of resin. Figure below illustrates how EEW affects other properties of an epoxy resin. Most of the epoxy resins that are used in the formulation of adhesives have EEW's in the range of 180 to 3200, corresponding to molecular weight range from 250 to 3750.

Comparative physical and curing properties for DGEBA epoxy resins
Comparative Physical and Curing Properties for DGEBA Epoxy Resins


Vapor Pressure


Vapor pressure is largely determined by the molecular weight of the epoxy resin. High vapor pressure materials are more likely to be present in vaporous form in the surroundings. Thus, health and safety issues are a concern if these materials are toxic or cause skin irritation. Vapor pressures of epoxy curing agents and solvents are more important considerations because they are generally higher than epoxy resins.

Vapor pressure must also be taken into account if vacuum is used to remove entrapped air from the formulation after mixing. High vapor pressure components can be evaporated and lost during the vacuum processing operation. The result can be a mixed formulation having different component proportions than originally intended.

Curing Properties


Reactivity is determined primarily by the epoxy resin and curing agent or catalyst that is used. The structure of the epoxy resin molecule and the number and type of functional groups greatly influence reactivity.

Reactive groups other than epoxy rings may be present in the molecule. Hydroxyl and olefinic groups are the most common. These can affect reactivity as well as the course and sequence of the polymerization reactions. Catalytic sites (e.g., tertiary nitrogen) on the epoxy molecule can also influence the reactivity and favor certain reactions. Steric factors influence reactivity by blocking possible reaction sites. Bulky side chains that are developed during the course of polymerization may also inhibit reaction.

Reactivity can be increased by externally heating the epoxy formulation to a preselected curing temperature. Epoxy resin reactions roughly obey Arrhenius' law that for every 10°C rise in temperature, the reaction rate doubles. Certain epoxy resin systems must be heated for any reaction to take place at all. This is beneficial in that these "latent" adhesive formulations are one-component products that do not require metering or mixing yet have long, practical shelf lives.

Service Properties


In addition to good adhesion, an adhesive must have satisfactory cohesive strength and durability once it is cured. The physical and chemical properties of the cured product will be determined by:
  • The structure of the resin, particularly the number, location, and reactivity of the epoxy and other groups present, and
  • The curing agent, stoichiometry, and cure conditions.
These parameters will establish the rigidity, thermal stability, chemical resistance, and overall usefulness of the adhesive joint. Factors that can affect cohesive strength include (1) the molecular weight and the nature of the molecules between crosslinks and (2) the degree of crosslinking.

The structure of a crosslinked epoxy resin can be represented by a "ladder-type" model (Figure below) in which the dimensional network is formed by tying together a relatively high molecular weight backbone polymer with random and relatively short crosslinking segments. The properties of the network are then determined by the nature of the backbone polymer chain segment, crosslink segments, and secondary force attraction.

Relationship between cured epoxy molecule and physical properties
Relationship between cured epoxy molecule and physical properties

Change in Parameter and results


Crosslink Density


This may be defined as the number of effective crosslinks per unit volume. The crosslink density is a key parameter in determining the properties of epoxy resin after cure. It is dependent on the number of reactive sites (functionality), the molecular distance and chain mobility between functional sites, and the percentage of these sites that enter into reaction.

Polymers that have a high crosslink density are thermosets and are infusible, insoluble, and dimensionally stable under load. Perhaps the most significant property that is controlled by the degree of crosslinking is the glass transition temperature, Tg. Polymers that have a low crosslink density are more flexible and show greater resistance to stress concentration, impact, and cold.

Crosslink density is inversely related to the molecular weight between crosslinks, Mc. Figure below 5 shows the general physical relationship between Mc and the physical state of epoxy resins.

Effect of molecular weight between crosslinks on the physical state of epoxy resins
Effect of Molecular Weight between Crosslinks on the Physical State of Epoxy Resins


Flexibility


Much of the development in epoxy adhesive technology has been toward attempts to combine the advantages of rigid and flexible epoxy systems and to minimize their disadvantages. Generally, the rigid structural adhesive, which displays excellent tensile or shear strength do not perform well when tested in peel, impact, or fatigue. The reason for this is that the flexibility distributes stresses over a much larger area (reducing stress concentration points).

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength.  In general flexibility can be controlled by the parameters shown in the table below.

Increasing Parameter
Effect on Flexibility
Epoxy molecular weight
Generally increases
Epoxy crosslink density or reactivity
Decreases
Epoxy chain branching
Generally increases
Crystallinity
Decreases
Glass transition temperature
Decreases
Filler concentration
Decreases
Plasticizer concentration
Increases
Flexibilizers
Increases
Effect of Various Formulation Parameters on the Flexibility of an Epoxy


Adhesion


The hydroxyl equivalent weight is the weight of the resin containing one equivalent of hydroxyl group, and it may also be expressed as equivalents per 100 gms. Several studies have demonstrated that the hydroxyl content greatly influences the adhesion of the epoxy resin. These results are best explained by the polar character of the hydroxyl groups. Figure below shows the relation between adhesive strength and hydroxyl content for a series of epoxy resins bonding stainless steel substrates.

Role of Adhesion in Epoxy Resin Selection
Relation between Adhesive Strength of Epoxy Resins and their Hydroxyl Content


Commercially Available Epoxy Resins Grades for Adhesives






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1 Comments on "Epoxy Resins for Adhesives and Sealants"
Lenka M Feb 28, 2019
Dears, can you kindly recommend us adhaesivum with thermal stability about 200-220 C? Thank you in advance. Best regards, Lenka Martinov√°

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