Epoxy Resins Selection for Adhesives and Sealants: Complete Guide

Fundamentals of Epoxy Resins

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:

Selection of the appropriate epoxy resin or combination of resins of which many are available,
Selection of curing agent and associated reaction mechanism,
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.

Epoxy adhesives are relatively inexpensive materials that are available as:

Solventless resins
Solvent solutions
Water based dispersions, and
Solids (e.g., film and powder)

They adhere well to a wide range of substrates, offer resistance to many environments, and can be formulated to provide pot lives and cure rates that can be adjusted to meet practical production requirements.

The advantages and disadvantages of epoxy adhesives are compared generally to other structural adhesives in the table below.

Advantages	Disadvantages
Low shrinkage on cure Low level of creep under sustained load High strength and stiffness Moderately good thermal and chemical resistance Good gap filling properties Low or no VOCs present in formulation Great formulation capability due to low viscosity Many types of base resins and curing agents are available Ability to cure under a wide range of conditions	Rigid and brittle Low impact strength Low peel strength Requires metering and mixing Requires time and/or temperature to cure Irritant and certain chemicals can be a health hazard

Advantages & Disadvantages of Epoxy Adhesives Over Other Structural Adhesives

However, unmodified epoxy resins cause certain problems for both the adhesive formulator and the end-user. Unmodified epoxies are often rigid and brittle; hence, impact resistance and peel strength are poor. Since they are thermosetting systems that cure by chemical reaction, metering and mixing of the unreacted components and time and temperature are required to polymerize the epoxy system. Although epoxy adhesives are environmentally friendly and often formulated without solvents, clean-up or dilution may require the addition of an organic solvent that contributes to environmental, health, and safety concerns.

Formulators have been vigorously working to minimize the major shortcomings of these adhesives. Fortunately, there are a wide variety of starting epoxy raw materials, modifiers, and additives that can be used in this pursuit. As a result, significant achievements have been made in epoxy adhesive technology concerning:

Improvements in flexibility and toughness,
Advancement of waterborne systems and solid (powder, film, etc.) systems,
Development of formulations for fast, simple application ("performance-on-demand"), and
Incorporation of performance-enhancing additives.

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 the figure below:

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 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 vary 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.

Effect of Adding Curing Agent to Epoxy Resins

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 property 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.

Types of Curing Agents for Epoxy Resins

Once the epoxy resin and the curing agent are mixed, the crosslinking reaction begins immediately, and the resulting working life is limited. The cure rate and working life will be dependent on the specific formulation as well as the ambient temperature. Although the adhesive will harden to handling strength in usually a short period of time, it will continue to develop strength for a much longer period.

Depending on the type of curing agent chosen, epoxy adhesive systems can reach full strength in minutes to several days.

The most common types of epoxy adhesives will reach handling strength in several hours and full cure in five to seven days.
The faster-curing agents can reach handling strength in as little as five minutes at room temperature. These fast cure formulations also provide practical cure times at low temperatures and they are well suited for outdoor application in winter.

There are six primary classifications of curing agents that are generally used as shown in the table below.

Low or Room Temperature Curing	Elevated Temperature Curing
Aliphatic amines and modified aliphatic amines Polyamides and polyamide amines Polysulfides and mercaptans	Aromatic amines and modified aromatic amines Anhydrides Lewis acids Catalytic and latent hardeners

Main Types of Epoxy Curing Agents

The choice of a particular curing agent or catalyst depends on the:

Processing requirements (e.g., viscosity, pot life, application method, curing temperature, rate of reactivity, mix ratio) and
End-use requirements (e.g., thermal and chemical resistance, shear strength, toughness) of the cured adhesive

The curing agents along with the epoxy resin will determine the type of chemical bonds and the degree of crosslinking that will occur.

Curing agents can also be used to improve the flexibility of inherently rigid epoxy resins. Certain common epoxy formulations can be flexibilized by altering the stoichiometric mix ratio of resin to curing agent or by changing the type of curing agent to one that has a more flexible molecule. For example, by changing from hexahydrophthalic anhydride to hexamethylenediamine, one can double the impact resistance of a resin system and increase its tensile elongation at break.¹

In our exclusive guide, learn more about the importance of adding curing agents to base resins in order to enhance the performance properties of the system.

Room Temperature Curing Agents for Epoxy Resins

Room temperature curing epoxy adhesives are "two-component" adhesives indicating that the curing agent portion and the epoxy resin portion are packaged separately. These are often referred to as "2K" adhesives, the "K" derived from the German spelling of the word "komponent".

One of the two components is commonly referred to as a base and it contains the epoxy resin including additives.
The second component is commonly referred to as the curing agent, hardener, catalyst, activator, or accelerator, and it contains the triggering agent for the crosslinking reaction.

The two components are also sometimes referred to as Part A (base) and Part B (curing agent).

The two parts are mixed just prior to application. Metering either occurs before the mixing operation at the time of assembly or the two components are packaged in premixed containers of various types and sizes.

There are also one-component epoxy systems that do not require mixing and will cure at room temperature. These, however, need an energy source such as ultra-violet light or electron beam for the crosslinking reaction to proceed.
There are also two-component epoxy formulations that are premixed by the supplier, immediately frozen, and shipped to the user in the frozen state. These, of course, must be kept in the frozen state prior to use, and even in this situation the shelf life is relatively limited. However, to the user, this type of adhesive has many of the application advantages of a one-component adhesive.

Once mixed, the two-part room temperature curing epoxy adhesives are designed to react at ambient conditions and temperatures near room temperature (20°-25°C) or lower. In many cases, the cure time may be accelerated with heat, but this is not a requirement.

The crosslinking reaction begins immediately, and the resulting working life is limited. The cure rate and working life will be dependent on the specific formulation as well as the ambient temperature. Although the adhesive will harden to handling strength in usually a short period of time, it will continue to develop strength for a much longer period.

With room temperature curing epoxy adhesives, very reactive curing agents are required. An exothermic temperature increase will be generated during the cure reaction, the degree of which will depend on the specific epoxy adhesive system and the mass of adhesive mixed together. This exotherm may limit the working time of the adhesive before it is applied to the substrate. However, any heat that is produced by the exothermic reaction is quickly dissipated due to the thin film of the adhesive and thermal conductivity of the adjoining substrates.

The major advantages and disadvantages of room temperature curing epoxy adhesives are shown in the table below.

Advantages

Disadvantages

Very long shelf life of stored components at room temperature
No thermal energy is required to either heat the adhesive or the parts being bonded
Can be accelerated by exposure to elevated temperatures
Adhesives can be cured at room temperature to a handling strength and then exposed to elevated temperatures for final cure
Less shrinkage and internal stress resulting from thermal expansion differences
Moderate strength as well as heat and chemical resistance
Good toughness and moderate peel and impact strength
Lower cost due to less energy and no heating equipment
Less hazardous (less vapors, no high-temperature equipment)

Short working life of mixed adhesive can result in waste, difficult application
Components must be metered accurately and mixed thoroughly
Tensile strength, heat and chemical resistance are not as high as when cured at elevated temperatures
Long cure times may limit production and require fixturing

Advantages and Disadvantages of Room Temperature Curing Epoxy Adhesives

Examples of Room Temperature Curing Agents

Many curing agents can be used in two-part, room temperature curable epoxy adhesives. However, the most useful curing agent families are polyamides, amidoamines, and aliphatic amines.

These curing agents react directly with the epoxy and become part of the crosslinked epoxy structure once cured. The advantages, disadvantages, and applications for the major types of epoxy curing agents are summarized in the table below.

Curing Agent	Advantages	Disadvantages	Applications
Polyamides	Convenience Room temperature cure Low toxicity Good bond strength and flexibility Moderately high peel and impact strength	High formulation cost Long cure times at room temperature High viscosity Low heat and chemical resistance	General-purpose adhesives Casting and encapsulation
Polysulfides and mercaptans	Moisture Resistance Quick set time Flexible	Odor Poor elevated temperature performance Poor tensile strength	Adhesives and sealants Civil engineering Casting and encapsulation Coatings
Aliphatic amines	Convenience Room temperature cure, fast elevated temperature cure Low viscosity Low formulation cost Moderate chemical resistance	Critical mix ratios Strong skin irritant High vapor pressure Short working life, exothermic Poor bond strength above 80°C Rigid, poor peel and impact properties	Adhesives and sealants Casting and encapsulation Coatings
Amidoamines	Reduced volatility Convenient mix ratios Good toughness	Poor elevated temperature performance Some incompatibility with certain epoxy resins	General-purpose adhesives Construction adhesives Concrete bonding Toweling compounds

Advantages, Disadvantages, and Applications of Common Epoxy Curing Agents

The required mix ratios, curing temperatures, and the resulting heat distortion temperatures of the cured products are provided in the table below.

Curing Agent	Physical Form	Amount Required (a)	Cure Temperature, °C	Pot Life at 23°C (b)	Complete Cure Conditions	Max Use Temperature, °C
Triethlyenetetramine	Liquid	11-13	20-135	30 min	7 days at 25°C	70
Diethylenetriamine	Liquid	10-12	20-95	30 min	7 days at 25°C	70
Diethylaminopropylamine	Liquid	6-8	27-150	5 hrs		85
Metaphenylenediamine	Solid	12-14	66-200	8 hrs	1 hr at 85°C 2 hrs at 163°C	150
BF3MEA Complex	Solid	1-4	135-200	6 months	3 hrs at 163°C	163
Nadic Methyl Anhydride	Liquid	80-100	120-200	5 days	3 hrs at 160°C	163
Triethlyamine	Liquid	11-13	20-95	30 min	7 days at 25°C	83
Polyamides
Amine value 80-90	Semisolid	30-70	20-150	5 hrs	5 days at 25°C	-
Amine value 210-230	Liquid	30-70	20-150	5 hrs	5 days at 25°C	-
Amine value 290-320	Liquid	30-70	20-150	5 hrs	5 days at 25°C	-
Notes: (a) - Per 100 parts by weight for an epoxy resin with an EEW of 180-200 (b) - Five hundred gms per batch; with a DGEBA epoxy with an EEW of 180-200 (c) - Highly dependent on curing agent concentration

Characteristics of Curing Agents Used with Epoxy Resins in Adhesive Formulations

The polyamides and amidoamine curing agents provide flexibility and a degree of toughness to the cured epoxy. The aliphatic amines are somewhat more reactive and provide moderate temperature performance and good chemical resistance. However, they are generally more brittle than polyamides or amidoamines. The table below shows the effect of each of these curing agents on the selected properties of an epoxy adhesive.

Property	Polyamine	Amine Adduct	Polyamide	Amidoamine
Tensile strength	+	+	-	+
Peel, impact strength			+	+
Heat resistance	+	+	-	-
Chemical resistance	+	+	-	-
Water resistance	+	+	-	+
Safety, hazardous vapors	-	+	+	+
+ Relatively Good, - Relatively Poor

Curing Agent Selection for General Purpose Room Temperature Curing Epoxy Adhesives

Polyamides and Amidoamines

Polyamides and amidoamines are perhaps the most popular type of curing agents used in general-purpose epoxy adhesives. They are used either as the sole curing agent or blended with other curing agents or accelerators in consumer epoxy adhesive products, such as two-tube ready-to-mix systems. They provide:

Low toxicity
Excellent flexibility
Good toughness, an
Good water resistance

The polyamide and amidoamine curing agents react directly with the epoxy resin and become an integral part of the cured product. Their flexibility helps to distribute the stress and impact loads throughout the adhesive joint. A typical liquid DGEBA cured with a polyamide will give peel strengths of about 16 lb/in on metal. Peel strengths of only 4 to 5 lb/in are common for the more rigid epoxy adhesives.

Higher percentages of polyamide or amidoamine curing agents in the epoxy adhesive will improve peel strength; lower percentages will provide higher temperature and chemical resistance at the cost of lower peel and impact properties. The epoxy:curing agent ratio for these systems can range from 1:2 to 2:1.

Pot life of polyamide or amidoamine cured epoxy adhesives are generally on the order of hours at room temperature. Full cure is achieved in 5-7 days at room temperature, and handling strength is achieved in about 16-24 hrs. A faster cure can be achieved in 20 mins to 4 hours by heating to 60-150°C. When room temperature cures are required, an accelerator such as an amine is often added to the formulation.

The table below shows a typical formulation for an accelerated and unaccelerated epoxy–amidoamine adhesive system. The accelerated formulation provides a faster curing adhesive, but it is not as flexible. Common fillers such as alumina, talc, and silica can be incorporated up to a level of 100 pph before any deleterious effect on properties is observed.

Component	Parts by Weight
Epoxy resin (EEW: 180-200)	100	100
Amidoamine (e.g., Versamid® 115 or equivalent)	70	35
Tertiary amine (e.g., DMP-30)	-	5
Filler or reinforcement	As desired	As desired

General Purpose Epoxy Adhesive with Amidoamine Curing Agents²

As a family of curing agents for epoxy resins, the amidoamines are lower in viscosity than the polyamides. They exhibit very good adhesive properties due to their chemical structure and easy penetration. Amidoamine cured epoxy adhesives have shown very good properties on concrete and other porous substrates. They cure extremely well under humid conditions. In fact, amidoamine cured epoxy formulations have been used to cure underwater in certain applications.

Aliphatic Amines

Aliphatic amines typically provide two-component room temperature curing epoxy formulations with fast reactivity. They are often used as accelerators for polyamide and amidoamine curing agents. When compared to polyamide and amidoamine curing agents, aliphatic amines provide high shear strength, moisture insensitivity, good chemical resistance particularly to solvents, and somewhat higher operating temperatures, especially when cured at elevated temperatures.

Typical of the aliphatic amines are diethylene tetramine (DETA), triethylenetetramine (TETA), and catalytic amines such as DMP-30.

A typical room temperature cured aliphatic amine cured epoxy adhesive for general purpose use is also shown below. This table also shows the difference that is achieved in shear strength by curing at elevated temperatures versus room temperature. As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile-shear strength that is dependent on the type of filler and concentration.

Component	Part by Weight
Part A
Liquid epoxy resin (EEW: 190)	70
Aliphatic epoxy resin (e.g., DER 732, Dow)	30
Alumina T-60	50
Part B
Triethylenetetramine (TETA)	13
Cure conditions	7 days at 25°C	2 hrs at 95°C
Tensile shear strength, psi
on Aluminium, 16 gauge	1150	1600
on Stainless steel, 16 gauge	1400	1730

Room Temperature Cure General Purpose Epoxy Adhesive Cured with Triethylenetetramine

Other Common Unmodified Aliphatic Amines

Cycloaliphatic amines provide good adhesion and very good chemical resistance. They are also noted for excellent low color and color stability. Cycloaliphatic amines cure well at low temperatures, even under damp conditions. A range of working lives and cure schedules is possible depending on the type of cycloaliphatic amine that is used in the epoxy adhesive formulation. When cured at elevated temperatures, cycloaliphatic amines provide:

High glass transition temperatures
Excellent chemical resistance, and
Strong mechanical properties

However, they are relatively brittle and exhibit low peel strength and poor impact characteristics in unmodified systems.

Aminoethyl piperazine (AEP) is a cycloaliphatic amine possessing primary, secondary, and tertiary amine groups. AEP is a clear, high boiling liquid with low vapor pressure. It is often used instead of DETA or TETA when improved toughness is required.

Optimum cures with AEP catalyzed DGEBA are obtained using 20-22 pph. However, the crosslink density is not great and the cured product does not attain a high degree of thermal or chemical resistance. The pot life and exotherm are similar to DETA and TETA, but a post-cure (2 hrs or more at 100°C) is required to develop properties fully.

Other cycloaliphatic diamines such as isophorone diamine, bis-p-amino-cyclohexyl methane, and 1, 2-diaminocyclohexane are used as epoxy resin curing agents for both ambient and heat-cured epoxy resin systems. While they have advantages, such as light color and good chemical resistance, they provide rather sluggish cure rates at low temperatures.

Diethylaminopropylamine (DEAPA) is an aliphatic polyamine that is used for curing epoxy adhesives where extended pot life and low exotherm are required. The stochiometry of DEAPA cured systems is less critical than with DETA or TETA since DEAPOA acts both as a crosslinking agent and a catalyst. Dimethylaminopropylamine (DMAPA) provides similar cure characteristics and properties to DEAPA, but it has a slightly shorter pot life.

Secondary amines can be represented by piperidine and diethanolamine. Secondary amines when used alone may be considered a special class of tertiary amine in that after the secondary hydrogens have reacted, the resulting crosslinking is believed to occur via the tertiary amine mechanism. These curing agents generally have limited temperature resistance, poorer chemical resistance, and equivalent mechanical properties to the primary amines. In adhesive formulations, they are used as blends with primary amines for providing specialized properties.

Tertiary Amine Catalysts

Tertiary amines are a type of Lewis Base catalyst and are, perhaps, the most widely used catalyst. Two of the most widely used tertiary amines are:

DMP-10: tris-(dimethylaminomethyl) phenol, and
DMP-30: o-(dimethylaminomethyl) phenol

The "DMP" designation comes from the original manufacturer of these chemicals, Rohm and Haas, and continues today with their manufacturer, Resolution Performance Polymers LLC.

Generally, when used as a sole catalyst, tertiary amines are used only in specialty applications where:

Short pot life can be tolerated, and
Maximum physical or chemical properties are not required

DMP-10 and DMP-30 are used at concentrations of 4-10 pph with liquid DGEBA epoxy resins. They achieve fairly fast cures overnight, even at room temperatures since the hydroxyl groups present in the epoxy molecule enhance the catalytic activity of the tertiary amine groups.

Tertiary amine salts of DMP-30 provide extended room temperature pot life (6-10 hrs at 20°C) when used at concentrations of 10-14 pph in liquid DGEBA epoxy resins. It cures at moderately elevated temperatures (4-8 hrs at 60°C), or even at room temperature with a heat bump. The tertiary amine salts are claimed to provide epoxy resin formulations with very good adhesion to metal.

The cured resins also show a hydrophobic effect when in contact with water or at high humidities. The strength, toughness, and elongation (4.7%) of the cured epoxy resin are very good. However, heat distortion temperature is only in the range of 70°-80°C, and chemical resistance is relatively poor for an epoxy. The physical properties fall off rapidly with any rise in temperature.

Elevated Temperature Curing Agents for Epoxy Resins

Room temperature curing cannot achieve the same degree of crosslinking as is obtained by curing at elevated temperatures. At elevated temperatures the epoxy resin and curing agent molecules are mobile, and there is a greater potential for reaction than at room temperature. The figure below shows the glass transition temperatures of common epoxy formulations cured at room and elevated temperatures.

Glass transition temperatures of several epoxy formulations cured at room and elevated temperatures

Another reason for higher Tg with elevated temperature curing systems is that the hydroxyls along the epoxy chain can react. At ambient temperature, reaction between an epoxy group and a hydroxyl group proceeds very slowly. Hence, the diepoxy group, which is formed when the epoxy group reacts with an amine, generally cannot enter into further reaction with a second epoxy ring at room temperature. As a result, heat cured diepoxies can be regarded as being potentially tetrafunctional, rather than difunctional.

Post curing at elevated temperatures after a room temperature cure is a common process in epoxy technology, and this can moderately increase the Tg in some systems. Such effects could be due to secondary reactions (irreversible) or to free volume effects (reversible). These effects could also be realized during the normal aging of the epoxy system in service. One should be careful, however, in assuming that a low temperature cure followed by an elevated temperature post cure will provide properties equivalent to only a high temperature cure.

It is also not generally desirable to rely on the service environment for completion of the cure reaction and establishing optimum adhesive strength. Incomplete reaction is undesirable since the presence of reactive, polar groups increases the susceptibility to uptake of moisture and other small molecules. This could be detrimental to long-term durability of the adhesive. Under-cure should not be used as a potential source of flexibility for these reasons. Rather another adhesive formulation should be chosen which produces the desired level of flexibility in its fully reacted state.

The major advantages and disadvantages of elevated temperature curing epoxy adhesives are shown in the table below. One-component (or "1K"), elevated temperature curing epoxy adhesives have the catalyst or curing agent incorporated directly with the epoxy resin. Thus, no metering or mixing is required. However, since the curative is integrated with the resin, these systems have limited shelf life, and special storage conditions such as refrigeration may be required.

Elevated temperature curing epoxy adhesives can be desirable when the short working life of a room temperature curing adhesive cannot be tolerated. One-component epoxy adhesives are advantageous when it is necessary to eliminate measuring or mixing errors or when dispensing equipment cannot handle multi-component systems.

Advantages

Disadvantages

Fast cure time results in higher production speed and less tie-up of fixturing equipment
Long working life
High-temperature resistance
Good chemical resistance
One-component systems possible (no need to meter and mix)
Long shelf life for two-component systems
Solid systems (film, preforms, powders) are possible
Viscosity decreases at elevated temperatures and provides more efficient wetting of the substrate

Greater internal stresses than room temperature curing adhesive because of
higher shrinkage on polymerization and
thermal expansion coefficient differences
More brittle than room temperature curing adhesives (poorer peel and impact properties)
Safety and hazardous nature of high temperatures
One-component systems a have shorter shelf life; may require refrigeration
Solid systems are easier to apply, are less of a health hazard than liquids or pastes
Energy consumption
Viscosity decrease at elevated temperature could result in starved joint

Advantages and Disadvantages of Elevated Temperature Curing Epoxy Adhesives

Elevated temperature curing epoxy formulations provide widely varying application and performance properties depending on the formulation employed. The following sections highlight certain formulations and commercial products that fall under this popular classification of epoxy adhesives.

Examples of Elevated Temperature Curing Agents

Most elevated temperature curing epoxy adhesives are cured with aromatic amines, acid anhydrides, dicyandiamide, imidazoles, and a host of other curatives. Latent curing agents, such as dicyandiamide, are typically used in one-component epoxy adhesives systems.

The advantages, disadvantages, and applications for the major types of epoxy curing agents are summarized in the table below.

Curing Agent	Advantages	Disadvantages	Applications
Aromatic amines	Moderate heat and chemical resistance	Critical mix ratios Solid at room temperature (except for certain eutectics) Strong skin irritant Rigid Long elevated temperature cures required Relatively short pot life	Generally used as a 2K adhesive Composites Electrical components Components in the chemical processing industry
Dicyandiamide	Latent cure Long shelf and pot life Good elevated temperature properties Good chemical resistance Relatively good combination of tensile and peel strength Impact and toughness can be increased with additives	Relatively long elevated temperature cure Cure temperatures >150°C generally required Insoluble before elevated temperature cure Rigid, poor peel and impact properties unless modified	Generally used as a1K paste adhesive Solid, film, or paste adhesives Composite and metal bonding
Anhydrides	Good heat and chemical resistance Excellent electrical insulation properties Very low viscosity liquids	Critical mix ratios Strong skin irritant Rigid, poor peel and impact properties unless modified Relativley short pot life	Used as a 2K adhesive Casting and encapsulation Bonding electrical components Construction adhesive
Catalytic curing agents	Long shelf and pot life High heat resistance Can be used as an accelerator or as the sole curative	Long elevated temperature cures required Relatively poor moisture resistance Rigid, poor peel and impact properties unless modified	Used as either a 1K or 2K adhesive Electrical encapsulation Composite and metal bonding Solid, film, or paste adhesives

Advantages, Disadvantages, and Applications of Elevated Temperature Curing Agents

The required mix ratios, curing temperatures, and the resulting heat distortion temperatures of the cured products are provided in the table below.

Curing Agent (Type)	Physical Form	Amount Required (a)	Cure Temperature, °C	Pot Life at 23°C (b)	Complete Cure Conditions	Max Use Temperature, °C
Metaphenylene diamine (aromatic amine)	Solid	12-14	66-200	8 hours	1 hrs @ 85°C plus 2 hrs @ 163°C	150
BF3-MEA complex (catalytic)	Solid	1-4	135-200	6 months	3 hrs @ 163°C	163
Nadic methyl anhydride (anhydride)	Liquid	80-100	120-200	5 days	3 hrs @ 163°C	163
Dicyandiamide	Solid	5-7	150	6 months	1 hr @ 150°C	150
2-ethyl-4-methyl-imidazole (catalytic)	Liquid	10	60-170	8-10 hours	6-8 hrs @ 60°C	125

Characteristics of Curing Agents Used with Epoxy Resins in Adhesive Formulations

Aromatic Amines

Aromatic amines are widely used as curing agents for epoxy resins. However, they are not used as widely in adhesive formulations are they are in composites, molding compounds, and castings. They offer cured epoxy structures with good heat and acid resistance.

The primary advantage of aromatic amines over aliphatic amines for curing epoxy resins are the longer pot life as well as the development of higher heat resistance and greater chemical resistance. The major disadvantages are that they are solids at room temperature and generally require heat for processing as well as elevated temperature for cure. The added heat required for mixing and cure increases the dermatitis and toxicity potential by releasing irritating vapors.

The lower reactivity of the aromatic amines in adhesive formulations is an advantage in that epoxy resin mixtures can be "B-staged" at room temperature (react to a glassy but fusible and thermoplastic intermediate structure) and will not fully cure for months. In this way, dry films, solid powders, and molding compounds can be formulated as elevated temperature curing, one-component adhesives with long shelf life.

When compared to aliphatic amines, aromatic amines generally have a reduced exotherm and reactivity. Elevated temperatures are required to achieve optimum properties. In certain cases aromatic amines can be cured at room temperature with catalysts such as phenols, BF3 complexes, and anhydrides.

Metaphenylene diamine (MPDA) is one of the most common of the aromatic amines used to cure epoxies. This product is amber to very dark in color. It is a solid that melts at 65°C and is generally mixed with the epoxy resin at that temperature. The molten liquid or vapors from MPDA can stain the skin and near-by structures rather badly.

Once the pot life is exceeded at room temperature, the MPDA – epoxy mixture will "B-stage". This technique is used to produce dry filament winding materials (prepreg) and solid molding compounds. In adhesive compounding this technique can be used to produce one-component, dry adhesives in the forms of solid stick, powder, or film. To cure the B-stage, the product is exposed to temperatures in the range of 150°-175°C, which causes the B-staged material to flow and then cure. The adhesive is then post cured at 175° for optimal property formation. The B-stage can also be dissolved in solvent and used to impregnate reinforcement of a carrier.

Methylenedianiline (MDA) is also a solid diaromatic amine. Similarly to MPDA, MDA is not often used in adhesive formulations because of the difficulty in compounding and curing practical epoxy formulations and the resulting brittleness of cured structures. MDA does not stain skin or equipment as badly as MPDA. For this reason along with its lower cost, MDA is often preferred to MPDA in epoxy formulations. MDA does not provide the high temperature strength or chemical resistance of MPDA for equivalent cure conditions. However, these properties are significantly superior to those of epoxy resins cured with primary amines.

There are several curing agents available that consist of eutectics of various aromatic amines. These perform very much like MPDA and MDA. However, the eutectics are liquids with viscosity of approximately 2000 cps at room temperature. They are readily miscible with liquid epoxy resins at room temperature.

Diaminodiphenylsulfone (DADS) is another solid aromatic amine that is primarily used in elevated temperature applications. DADS provides the best strength retention after prolonged exposure to elevated temperatures than any other amine curing agent. The curing agent can be used with DGEBA epoxy resins of various molecular weights. It is used with higher functionality solid epoxy resins (e.g., tetrafunctional resins of the epoxy novolac type, for maximum crosslink density, thermal resistance and heat distortion temperatures. It has been reported that a mixture of 100 parts EPON 1031 (tetrafunctional bisphenol A) and 30 parts DADS, cured for 30 mins at 180°C will provide bond strength in excess of 1000 psi at 260°C.

DADS melts at 135°C and is employed stoichiometrically with DGEBA at 33.5 pph. Fortunately, it is relatively unreactive so it can be mixed with solid epoxy resin at elevated temperatures. It can also be used in epoxy solutions to provide an adhesive formulation for manufacturing supported or unsupported film with long shelf life. Because of the low reactivity of the system, the DADS is generally (1) employed at a concentration that is about 10% greater than stoichiometric or (2) an accelerator, such as BF₃MEA, which is employed at about 0.5-2 pph. When DADS is mixed with liquid DGEBA resin, it provides a pot life of 3 hrs at 100°C and requires a rather extended high temperature cure to achieve optimal physical properties.

Anhydrides

Acid anhydrides are an important class of epoxy curing agents, although they are not as often used in adhesive systems as they are in casting compounds, encapsulants, and molding compounds. The most common types of anhydrides are:

Hexahydrophthalic anhydride (HHPA)
Phthalic anhydride (PA)
Nadic methyl anhydride (NMA)
Pyromellitic dianhydride (PMDA), and several others.

These compounds do not readily react with epoxy resins except in the presence of water, alcohol, or some other base, called an accelerator. Tertiary amines, metallic salts, and imidazoles often act as accelerators for anhydride cured epoxy systems. The reaction between acid anhydride and epoxy resins is illustrated in the figure below.

Anhydride epoxy reaction

Anhydrides are sometimes used in epoxy adhesives to provide specific performance properties or to provide improvements in formulations (e.g., low viscosity). The most important anhydride in epoxy adhesive formulations is pyromellitic dianhydride (PMDA), which provides very high temperature properties.

The mix ratio of anhydride to epoxy resin is less critical than with amines and can vary from 0.5 to 0.9 equivalents of epoxy. The specific ratio is generally determined experimentally to achieve desired properties. Compared to aliphatic amine cures, the pot life and exotherm generated by anhydride cured epoxies are low. Elevated temperature cures up to 200°C and post cures are required to develop optimal properties.

The reactivity of the epoxy-anhydride reaction is slow, therefore, an accelerator is often used at a 0.5-3% to speed gel time and cure. Most often the accelerator is a tertiary amine, and the optimum concentration is dependent on the anhydride, the resin used, and the cure conditions. The accelerator concentration, like the anhydride concentration, is usually determined experimentally based on a specific set of end-properties.

Catalysts and Other Latent Curing Agents

Catalytic curing agents achieve crosslinking by initially opening the epoxy ring and causing homopolymerization of the resin. The resin molecules react directly with each other, and the cured polymer has essentially a polyether structure. The catalysts do not themselves participate in the epoxy polymerization reactions as do the curing agents described above which provide a polyaddition reaction mechanism.

Therefore, catalysts merely act as an initiator and promoter of epoxy resins curing reactions. The amounts of catalysts used with epoxy resins are usually determined empirically and are chosen to give the optimum balance of properties under the required processing conditions. Generally, only several pph of catalyst is used with an epoxy resin. Excess amounts of catalyst can result in poor physical properties and degraded resin.

The most popular catalysts for epoxy resins are boron imidazoles, boron trifluoride complexes, and dicyandiamide. Many of these catalysts provide very long pot lives (months) at room temperatures and require elevated temperatures for reaction with the epoxy groups. These catalysts are often referred to as latent hardeners.

Imidazoles

2-Ethyl-4-Methyl-Imidazole (EMI) is used as a single catalyst or as an accelerator. EMI is a substituted imidazole that is a liquid at room temperature (4000-8000 cps at 25°C) with a high boiling point. EMI cured epoxy adhesive formulations are claimed to have outstanding adhesion to metals and for this reason it is added as a co-curing agent in many compositions.

When used as a single catalyst in concentrations of about 10 pph, the mixed epoxy formulation shows a very low viscosity, which is ideal for the incorporation of high filler contents. The catalyzed resin has a pot life of 8-10 hrs at 25°C and a normal cure of 6-8 hrs at 60°C. EMI cures to a densely crosslinked structure with liquid DGEBA epoxy resins. These mixtures can cure at relatively low temperatures (60°C) or at higher temperatures (170°C) in very short times.

Compared with other catalysts which homopolymerize epoxies, the imidazole offers improved thermal properties and retention of mechanical properties at more elevated temperatures. The cured resin has a heat distortion temperature of between 85°-130°C, which can be further increased by a post cure to about 160°C.

Latent imidazole catalysts have also been developed to provide cure rates considerably faster than dicyandiamide cured epoxy resins. They also exhibit excellent adhesive characteristics and heat and chemical resistance. A unique feature of these imidazole catalysts is that they do not have the high exotherm that dicyandiamide produces when cured in epoxy resins. Thus, they do not char or burn when exposed to high cure temperatures for fast cure. This is an important factor for adhesives that are cured via induction or dielectric heating. These adhesive systems are also much safer to ship via airfreight than conventional dicyandiamide catalyzed epoxy formulations due to their low exotherm.

Dicyandiamide

Dicyandiamide (DICY) is a solid latent catalyst that reacts with both the epoxy terminal groups and the secondary hydroxyl groups. DICY has the advantage that it only reacts with the epoxy resins on heating to an activation temperature, and once the heat is removed, the reaction stops. It is widely used with epoxy resins where long shelf life (up to 6 months) is required prior to curing. Significantly longer shelf lives can be obtained by storage under refrigeration until use.

As a result of the latency and excellent properties produced by DICY cured epoxies, DICY is used in many "B-Staged" supported film adhesives. DICY is also the leading catalyst used in one-component, elevated temperature curing epoxy adhesives.

DICY is considered a catalyst and polymerizes epoxy resin through homopolymerization mechanism. But DICY has also shown behavior with epoxies that indicates some breakdown at cure temperatures to produce a curing agent that contributes to the polyaddition reaction mechanism.

DICY is used at about 5-7 pph of liquid epoxy resins and 3-4 pph for solid epoxy resins in adhesive formulation. It is generally ball milled into the epoxy resin. DICY forms very stable mixtures with epoxy resins at room temperature because the catalyst is not soluble at low temperatures. However, on being exposed to temperatures greater than 140°C the DICY becomes soluble in the epoxy resin, and cure progresses rapidly.

When formulated into one component adhesive systems, the product is stable for 6 months to a year at room temperature. It will then cure when exposed to 145°-160°C for about 30-60 mins. Since the reaction rate is relatively slow at lower temperatures, the addition of 0.2-1% benzyldimethylamine (BDMA) or other tertiary amine accelerators is common to reduce cure times or cure temperatures. Other common accelerators are imidazoles, substituted urea, and modified aromatic amine.

Substituted DICY derivatives have been developed to increase solubility and lower the required activation temperatures. These techniques can reduce the activation temperatures for DICY - epoxy resin mixtures to as low as 125°C.

Epoxy resins cured with DICY exhibit a good balance of physical properties with heat and chemical resistance. The glass transition temperature of a DGEBA liquid epoxy resin cured with 6 pph of DICY is on the order of 120°C whereas, an elevated temperature curing aliphatic amine would provide a glass transition temperature of no greater than 85°C. Tougheners can be added to the adhesive formulation to achieve relatively high levels of peel and impact strength.

BF3-MEA Complex

Boron trifluoride monoethylamine (BF3-MEA) is Lewis acid type catalyst. Lewis acids are electron pair acceptors that function as curing agents by coordinating with the epoxy oxygen, facilitating transfer of the proton (See figure below). BF₃MEA is the only Lewis acid that has achieved broad commercial use in epoxy resin systems. BF₃MEA are effective catalysts for the polymerization of linear and cycloaliphatic epoxies as well as for the glycidyl ethers.

Reactions of BF₃MEA with an epoxy resin

The BF₃MEA must be melted and dissolved in liquid epoxy resins. For small batches, the procedure is to heat the resin to 85°C and stir in the curing agent. For larger batches, about three parts by weight of the catalyst is stirred into about five parts by weight of the resin preheated to 50°C. This produces a smooth paste, which then may be added to the remainder of the resin heated to 85°C. Alternatively, the BF₃MEA can be dissolved in a solvent such as furfural alcohol, which will also dissolve the epoxy resin.

The monoethylamine component blocks the reaction sufficiently so that BF3-MEA can be considered to be a latent catalyst. It provides a pot life of 6-12 months at room temperature. It does not show significant curing activity until temperatures of 100°-125°C have been reached.

In most formulations the concentration of BF₃MEA in DGEBA epoxy resins is on the order of 2-4 pph. Normal curing temperatures are usually 2 hrs at 105°C followed by a post cure at 150°-200°C for 4 hrs for optimum properties. The rate of cure is very sensitive to temperature; below 100°C the rate is negligible, and at 120°C it is very rapid and accompanied by a significant exotherm.

The epoxy product cured with BF₃MEA is densely crosslinked and has excellent physical properties at high temperatures (150°-175°C). When reacted with an unmodified epoxy resin, the resulting product is very hard and brittle. The chemical resistance, however, is only fair and somewhat less than epoxies that are cured with aliphatic amines.

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:

Glycidyl epoxies are prepared via a condensation reaction of an appropriate dihydroxyl compound, dibasic acid, or diamine and epichlorohydrin. These can be further classified as:
- Glycidyl ether
- Glycidyl ester, and
- Glycidyl amine
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*
Diglycidyl ether of bisphenol A (DGEBA)	Glycidyl ether	General-purpose use	RT, ET
Brominated DGEBA	Glycidyl ether	Flame resistance	RT, ET
Diglycidyl ether of Bisphenol F (DGEBF)	Glycidyl ether	Very low viscosity	RT, ET
Epoxy novolac	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 epoxy	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
Epoxy acrylate	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:

Low molecular weight liquids
High molecular weight semi-solids and solids, and
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 temperatures.

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 increases 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 dispersibility 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, the 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 a 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 provides 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.

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Selection Criteria for Epoxy Resins

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

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
Number of Repeating Units, n	Molecular Weight	Epoxy Equivalent Weight, EEW	Poise at 25°C	Melting Point, °C	Vapor Pressure
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 of over of 6,000 cps, and conventional DGEBA epoxy resins (EEW = 190) have a viscosity of 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. The 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 a molecular weight range from 250 to 3750.

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 a 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 the 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 the 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:

The molecular weight and the nature of the molecules between crosslinks and
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

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)
Molecular distance and chain mobility between functional sites, and
Percentage of these sites that enter into the 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, M_c. The figure below 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

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 does 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

Get inspired: Learn How to Improve the Toughness of Structural Epoxy Adhesives

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. The figure below shows the relation between adhesive strength and hydroxyl content for a series of epoxy resin bonding stainless steel substrates.

Relation between Adhesive Strength of Epoxy Resins and their Hydroxyl Content

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.

Related Read: A Complete Guide on Adhesion Myths & Reality

Find Suitable Epoxy Resin for Your Formulation

View all the commercially available epoxy grades for adhesives, analyze technical data of each product, get technical assistance or request samples.

References

Epon Resin Structural Resin Manual - Additive Selection, Resolution Performance Polymers LLC (Now Momentive), 2001, p. 8.
Meath, A. R., "Epoxy Resin Adhesives", Chapter 19 in Handbook of Adhesives, 3nd ed., I. Skeist, ed., van Nostrand Reinhold, New York, 1990, p. 356.

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