**Abstract**

Epoxy adhesives are primarily composed of epoxy resin and curing agent. Epoxy adhesives are supplied in both one-component package and two-component package depending on curing agent used and curing method applied. Two-component epoxy adhesives are prepared by packing epoxy composition and curing agent composition separately. They cure soon after mixing the two components together. Almost all room temperature cure epoxy adhesives are supplied in two-component package. One-component epoxy adhesives are prepared and supplied by mixing all formulated components in advance including epoxy resin and curing agent. One-component epoxy adhesives usually need cure at elevated temperature and store at low temperature in a refrigerator or even freezer. Epoxy adhesives have been widely used as typical reactive adhesives for various applications ranging from general industry, construction, electronics assembly, automobile production to aerospace market. Typical room temperature cure epoxy adhesives, thermal cure epoxy adhesives and UV cure epoxy adhesives are introduced in detail.

**Keywords:** epoxy adhesive, one-component, two-component, room temperature cure, thermal cure, UV cure, latent curing agent

## **1. Introduction**

Epoxy adhesive was first invented in 1936 by Dr. Pierre Castan for dental application via curing bisphenol A epoxy resin with phthalic anhydride. Commercial supply of epoxy adhesives was started in late 1940s in Europe and USA. Various epoxy adhesives have been developed and commercialized since then and widely used as typical reactive adhesives for various structural bonding applications ranging from general industry, construction, electronics assembly, automobile production to aerospace market [1–11]. Their typical application area and examples as well as their supply type, curing method are summarized in **Table 1**. Major global suppliers for epoxy adhesives are Henkel AG & Co. KGaA, H.B. Fuller Company, 3M, Huntsman Corporation, Sika Corporation, Arkema Corporation, Cemendine Co., Ltd., Three-Bond Co., ltd., Huitian Adhesive, etc.

Epoxy adhesives show good adhesion on various substrates and are suitable to bond metals, glass, concrete, ceramics, wood and many plastics. Curing shrinkage is very low. Cured epoxy resin possesses strong and rigid cross-linked chemical structure suitable for structural bonding applications. By combination of various epoxy resins and different curing agents, a number of epoxy adhesives have been commercialized for different applications. On the other hand, room temperature and thermal cure epoxy adhesives need relatively long cure time. Most cured epoxy adhesives are very rigid and are not suitable for bonding flexible substrates.


#### **Table 1.**

*Typical applications of epoxy adhesives.*

In selection and use of epoxy adhesives, cautions need to be paid on their pot life, cure condition, cure method, physical properties of un-cure and cured resin as well as adhesion performance.

#### **2. Epoxy adhesive chemistry**

Epoxy adhesives are primarily composed of epoxy resin and curing agent. Filler, toughener, plasticizer and other additives such as silane coupling agent, deformer and colorant, etc., can be formulated as needed. Common compositions and their main role of epoxy adhesives are illustrated in **Table 2**.

Epoxy resins are mainly synthesized from reaction of active hydrogen in phenols, alcohols, amines and acids with epichorohydrin, abbreviated normally as ECH at certain well controlled conditions. Epoxy resin can be also prepared by oxidation of olefin with peroxide as in the case of preparation of cycloaliphatic epoxy resins. Main commercial epoxy resins, their preparation and key features are shown in **Table 3**. Bisphenol A diglycidyl ether, often called as bisphenol A type epoxy resin, is the first commercialized and still most widely used epoxy resin. Synthesis of DGEBA is illustrated in **Figure 1** [12]. In volume base, it is estimated that over 75% of epoxy resin used in industry is this type. **Figure 2** illustrates chemical structure and key features of various functional groups for bisphenol A diglycidyl ether [13], the most common epoxy resin used in epoxy adhesives.

Epoxide group is chemically very active. Epoxy resin can react with active hydrogen almost equivalently via polyaddition mechanism with polyamines, mercaptan compounds, phenols and anhydrates to become cross-linked strong thermoset polymers. Epoxy resin can also polymerize homogeneously via anionic polymerization mechanism by initiating of Lewis bases such as tertiary amines or imidazole compounds. It can also polymerize via cationic polymerization mechanism via initiating of Lewis acid such as Boron trifluoride amine complex or strong acid such as onium salts, iodonium salts. **Table 4** lists typical curing agent, initiator used in epoxy adhesives. By combination of suitable epoxy resin with curing agent, epoxy adhesive is designed for various substrate bonding in different applications. It is supplied in both two-component and

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one-component package depending on the curing agent and curing method used. Two-component epoxy adhesive is prepared by packing epoxy composition and curing agent composition separately before use. It will cure soon after mixing based on designed mixing ratio. Almost all room temperature cure epoxy adhesives

*Epoxy Adhesives*

**Table 2.**

**Table 3.**

**Figure 1.**

**Figure 2.**

*Epoxy adhesive compositions.*

Glycidyl ether of Bisphenol A Bisphenol F Novalac

*Commercial epoxy resins.*

*Synthesis of DGEBA (diglycidyl ether of bisphenol A).*

*Chemical structure and key features of DGEBA.*

*DOI: http://dx.doi.org/10.5772/intechopen.86387*

**Constituent Ingredient Main role** Primary Epoxy resin, reactive diluent Adhesive base

Additive Coupling agent Adhesion

**Epoxy resin type Preparation from Key features**

Bisphenol A and ECH Bisphenol F and ECH Novalac with ECH

Curing agent/catalyst, accelerator Curability Modifying Filler Property modification Toughener Toughening Plasticizer Flexibility

Colorant Color

Glycidyl ester Carboxylic acids and ECH Mainly for anhydrate cure

Glycidyl amine Amines and ECH Multi-functional Cycloaliphatic Oxidation of olefin by peroxide Cationic cure

Standard epoxy resin Low viscosity Multi-functional


#### **Table 2.**

*Adhesives and Adhesive Joints in Industry Applications*

Construction Concrete repairing

Automotive Structural bonding

Electronics Electrically conductive

well as adhesion performance.

*Typical applications of epoxy adhesives.*

**Table 1.**

Others Sports tools

**2. Epoxy adhesive chemistry**

main role of epoxy adhesives are illustrated in **Table 2**.

the most common epoxy resin used in epoxy adhesives.

In selection and use of epoxy adhesives, cautions need to be paid on their pot life, cure condition, cure method, physical properties of un-cure and cured resin as

**Application area Application examples Package type Cure method**

Two components

One component; Two components

One component; Two components

One component; Two components

Two components R.T. cure

Two components R.T. cure

R.T. cure Thermal cure

Thermal cure

Thermal cure

Thermal cure UV cure R.T. cure

Industrial Structural bonding One component;

Anchor bolt fixture

Hemming adhesion

Display assembly Image sensor assembly

Consumer applications

Aerospace Metal, honeycomb & composite bonding, repairing

> Underfills Medical bonding

Epoxy adhesives are primarily composed of epoxy resin and curing agent. Filler, toughener, plasticizer and other additives such as silane coupling agent, deformer and colorant, etc., can be formulated as needed. Common compositions and their

Epoxy resins are mainly synthesized from reaction of active hydrogen in phenols, alcohols, amines and acids with epichorohydrin, abbreviated normally as ECH at certain well controlled conditions. Epoxy resin can be also prepared by oxidation of olefin with peroxide as in the case of preparation of cycloaliphatic epoxy resins. Main commercial epoxy resins, their preparation and key features are shown in **Table 3**. Bisphenol A diglycidyl ether, often called as bisphenol A type epoxy resin, is the first commercialized and still most widely used epoxy resin. Synthesis of DGEBA is illustrated in **Figure 1** [12]. In volume base, it is estimated that over 75% of epoxy resin used in industry is this type. **Figure 2** illustrates chemical structure and key features of various functional groups for bisphenol A diglycidyl ether [13],

Epoxide group is chemically very active. Epoxy resin can react with active hydrogen almost equivalently via polyaddition mechanism with polyamines, mercaptan compounds, phenols and anhydrates to become cross-linked strong thermoset polymers. Epoxy resin can also polymerize homogeneously via anionic polymerization mechanism by initiating of Lewis bases such as tertiary amines or imidazole compounds. It can also polymerize via cationic polymerization mechanism via initiating of Lewis acid such as Boron trifluoride amine complex or strong acid such as onium salts, iodonium salts. **Table 4** lists typical curing agent, initiator used in epoxy adhesives. By combination of suitable epoxy resin with curing agent, epoxy adhesive is designed for various substrate bonding in different applications. It is supplied in both two-component and

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*Epoxy adhesive compositions.*


#### **Table 3.**

*Commercial epoxy resins.*

#### **Figure 1.**

*Synthesis of DGEBA (diglycidyl ether of bisphenol A).*

#### **Figure 2.**

*Chemical structure and key features of DGEBA.*

one-component package depending on the curing agent and curing method used. Two-component epoxy adhesive is prepared by packing epoxy composition and curing agent composition separately before use. It will cure soon after mixing based on designed mixing ratio. Almost all room temperature cure epoxy adhesives


**Table 4.**

*Epoxy curing agents.*

are supplied in two-component package. Epoxy adhesives can be formulated in one-component package where all components including epoxy resin and curing agent has been mixed in advance. One-component epoxy adhesives usually use elevated temperature cure and need to be stored at low temperature conditions in a refrigerator or even freezer for long shelf life.

#### **3. Room temperature cure epoxy adhesives**

Room temperature cure epoxy adhesives are normally prepared and supplied in two-component package with epoxy resin component parked in one resin part and curing agent packed as the other hardener part. By mixing these two parts together, epoxy resin will react with curing agent quickly at room temperature conditions to become cross-linked strong thermoset structure that can bond adhesion substrates tightly. By use of different type of curing agents, pot life and cure time can be designed as needed.

#### **3.1 Fast room temperature cure epoxy adhesives**

Mercaptan compounds are usually selected as curing agent for fast room temperature curable epoxy adhesive because its reaction with epoxy resin is very fast in the existence of small amount of basic chemicals such as tertiary amine or imidazole as accelerator. As shown in **Figure 3**, epoxy resin reacts with mercaptan group equivalently via polyaddition reaction mechanism [14]. Fixture time can be <30 minutes or even 15 minutes at room temperature. Full cure time will need 24 hours. Precautions need to be paid on its very short work life, <10 minutes or even 5 minutes. Commercial fast cure epoxy adhesives supplied by Henkel AG & Co. KGaA and their typical properties are illustrated in **Table 5** [15].

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*Epoxy Adhesives*

**Table 5.**

**Figure 4.**

**Table 6.**

*DOI: http://dx.doi.org/10.5772/intechopen.86387*

*Fast room temperature cure epoxy adhesives.*

**Product LOCTITE EA E-05MR LOCTITE EA E-00NS** Color clear translucent Viscosity, mPas/25°C 25,000 100,000 Mix ratio 1:1 1:1 Work life, minutes 5 3 Fixture time, minutes 15 10 Room temperature cure time, hours 24 24 Shear strength, psi on steel 3360 1600

**3.2 Room temperature cure epoxy adhesives**

*Room temperature cure epoxy adhesives.*

*Polyaddition reaction between epoxy resin and amine.*

Aliphatic polyamines are most commonly used curing agents in epoxy resin technology. A number of modified polyamine type curing agents with adjustment on curability, handling or other physical properties for easy use have been commercialized in the market by curing agent suppliers. As shown in **Figure 4**, active hydrogen of primary and secondary amine reacts equivalently with epoxide via polyaddition mechanism [16]. Fixture time and work life can be adjusted by combination with suitable curing agent. **Table 6** shows commercial room temperature epoxy adhesives supplied by Henkel AG & Co. KGaA and their properties [17].

**Product LOCTITE EA E-20HP LOCTITE EA E-120HP** Color Off-White Amber Viscosity, mPas/25°C 30,000 30,000 Mix ratio 2:1 2:1 Work life, minutes 20 120 Fixture time, minutes 120 >180 Room temperature cure time, hours 24 24 Shear strength, psi on steel 3270 4300

**Figure 3.** *Polyaddition reaction of epoxy resin with mercaptan.*


#### **Table 5.**

*Adhesives and Adhesive Joints in Industry Applications*

Polyaddition Polyamines

Anionic Tertiary amines

Cationic Boron trifluoride monoethylamine

**Polymerization mechanism Curing agent, initiator**

a refrigerator or even freezer for long shelf life.

**3. Room temperature cure epoxy adhesives**

**3.1 Fast room temperature cure epoxy adhesives**

*Polyaddition reaction of epoxy resin with mercaptan.*

designed as needed.

**Table 4.**

*Epoxy curing agents.*

in **Table 5** [15].

are supplied in two-component package. Epoxy adhesives can be formulated in one-component package where all components including epoxy resin and curing agent has been mixed in advance. One-component epoxy adhesives usually use elevated temperature cure and need to be stored at low temperature conditions in

Onium salts Iodonium salts

Modified polyamines Mercaptans Phenols Anhydrates Dicyandiamide

Imidazole compounds

Room temperature cure epoxy adhesives are normally prepared and supplied in two-component package with epoxy resin component parked in one resin part and curing agent packed as the other hardener part. By mixing these two parts together, epoxy resin will react with curing agent quickly at room temperature conditions to become cross-linked strong thermoset structure that can bond adhesion substrates tightly. By use of different type of curing agents, pot life and cure time can be

Mercaptan compounds are usually selected as curing agent for fast room temperature curable epoxy adhesive because its reaction with epoxy resin is very fast in the existence of small amount of basic chemicals such as tertiary amine or imidazole as accelerator. As shown in **Figure 3**, epoxy resin reacts with mercaptan group equivalently via polyaddition reaction mechanism [14]. Fixture time can be <30 minutes or even 15 minutes at room temperature. Full cure time will need 24 hours. Precautions need to be paid on its very short work life, <10 minutes or even 5 minutes. Commercial fast cure epoxy adhesives supplied by Henkel AG & Co. KGaA and their typical properties are illustrated

**40**

**Figure 3.**

*Fast room temperature cure epoxy adhesives.*

#### **Figure 4.**

*Polyaddition reaction between epoxy resin and amine.*


**Table 6.**

*Room temperature cure epoxy adhesives.*

#### **3.2 Room temperature cure epoxy adhesives**

Aliphatic polyamines are most commonly used curing agents in epoxy resin technology. A number of modified polyamine type curing agents with adjustment on curability, handling or other physical properties for easy use have been commercialized in the market by curing agent suppliers. As shown in **Figure 4**, active hydrogen of primary and secondary amine reacts equivalently with epoxide via polyaddition mechanism [16]. Fixture time and work life can be adjusted by combination with suitable curing agent. **Table 6** shows commercial room temperature epoxy adhesives supplied by Henkel AG & Co. KGaA and their properties [17].
