**4. Thermal cure epoxy adhesives**

Thermal cure epoxy adhesives are prepared and supplied in both onecomponent and two-component packages depending mainly on curing agent type used. Compared to room temperature cure type, thermal cure two-component epoxy adhesives usually have higher glass transition temperature that is suitable for high temperature resistance applications. One-component epoxy adhesives do not need pre-mixing in use and thus can be handled much easily. Many new onecomponent epoxy adhesives have been commercialized and become more and more important in recent years.

#### **4.1 Two-component thermal cure epoxy adhesives**

When use cycloaliphatic amine or aromatic amine as curing agent, post thermal cure process is usually required to achieve full cure as their reactivity, especially aromatic amines and secondary amine in cycloaliphatic amine, with epoxide is much lower with compared to aliphatic amines applicable for room temperature cure. Chemical structure of commonly used cycloaliphatic amine IPDA (isophorone diamine) and aromatic amine DDM (methylene dianiline) is shown in **Figure 5** [18, 19]. Thermal cure epoxy adhesives have much stronger and rigid structure and normally possess higher glass transition temperature with compared to room temperature cure epoxy adhesives mainly based on aliphatic amines or mercaptans. Two-component thermal cure epoxy adhesives are mainly used for higher temperature resistance required applications such as automobile production and aerospace market.

#### **4.2 One-component thermal cure epoxy adhesives**

One-component epoxy adhesives do not require pre-mix before use since all components have been mixed together and there is no concern on insufficient mixing problem as often occurred in two-component use. Pot life of one-component epoxy adhesives is usually long and one-component adhesives are thus suitable for automatic dispensing systems. Compared to two-component type, one-component epoxy adhesives can be handled much easily. On the other hand, one-component epoxy adhesives usually need cure at higher temperature because of long enough room temperature stability needed for adhesive preparation and storage. Most one-component epoxy adhesives require storage condition at lower temperatures in a refrigerator or even freezer.

Recently one-component thermal cure epoxy adhesives have become more and more important especially in electronics assembly and automotive production where high production efficiency is required. With selection of suitable latent curing agents, a number of one-component epoxy adhesives have been developed and commercialized by epoxy adhesive suppliers for various applications. Typical commercial latent curing agents are summarized in **Table 7**.

**43**

*Epoxy Adhesives*

separation

**Table 7.**

**Figure 6.**

*Chemical structure of DICY.*

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

Chemical block and physical

*Typical commercial latent curing agents.*

**Latency mechanism Latent curing agent Curing** 

Physical separation Modified imidazoles Fine powder ≥80

Chemical block Onium salts Solid ≥80

be further lowered to 120°C [21].

**5. UV cure epoxy adhesives**

DICY (dicyandiamide), chemical structure shown in **Figure 6** [20], is the oldest and widely used latent curing agent for epoxy resin technology. It is a solid chemical with a melting point at 208°C. DICY formulated epoxy composition is very stable, up to 6 months at room temperature. Latency mechanism is a combination of physically separation and chemically blocking with epoxide group. DICY cured epoxy resin shows high adhesion and possesses high glass transition temperature especially suitable for high performance required applications such as vehicle parts bonding in automobile production. Cure temperature of DICY alone with epoxy resin normally needs at least 150°C to achieve full cure. By addition small amount of accelerator such as modified urea compounds and imidazole compound, cure temperature can

**agent state**

DICY Solid ≥150 Dihydrazines ≥120

Modified polyamine ≥80

Amine-BF3 complex Liquid ≥130

**Typical curing temperature °C**

In recent years, new type latent curing agents have been developed and commercialized by several curing agent suppliers [22–26]. These latent curing agents are supplied as fine powder with average particle size well controlled in a few microns or premix of fine powder latent curing agent in epoxy resin. They are manufactured by grinding specially synthesized modified polyamine or imidazole solid with a softening point from 80 to 150°C. Latency mechanism is mainly physically separation between curing agent and epoxide. Curing temperature has been be lowered to as low as 80°C and its formulated epoxy composition can be still quite stable at room temperature. Many one-component epoxy adhesives commercialized recently are based on these new type latent curing agents because of their lower temperature curability suitable for use in bonding heat sensitive substrates such as plastics. By combination of small amount of liquid phenol compound with latent curing agent, it has been found that cure time of one-component epoxy adhesives can be shortened significantly [27, 28].

Ultra-violet light (UV) curable epoxy adhesives can be cured quickly and have been very successfully used in several new electronics assembly and general bonding

**Figure 5.** *Chemical structure of IPDA and DDM.*


#### **Table 7.**

*Adhesives and Adhesive Joints in Industry Applications*

**4.1 Two-component thermal cure epoxy adhesives**

**4.2 One-component thermal cure epoxy adhesives**

commercial latent curing agents are summarized in **Table 7**.

Thermal cure epoxy adhesives are prepared and supplied in both onecomponent and two-component packages depending mainly on curing agent type used. Compared to room temperature cure type, thermal cure two-component epoxy adhesives usually have higher glass transition temperature that is suitable for high temperature resistance applications. One-component epoxy adhesives do not need pre-mixing in use and thus can be handled much easily. Many new onecomponent epoxy adhesives have been commercialized and become more and more

When use cycloaliphatic amine or aromatic amine as curing agent, post thermal cure process is usually required to achieve full cure as their reactivity, especially aromatic amines and secondary amine in cycloaliphatic amine, with epoxide is much lower with compared to aliphatic amines applicable for room temperature cure. Chemical structure of commonly used cycloaliphatic amine IPDA (isophorone diamine) and aromatic amine DDM (methylene dianiline) is shown in **Figure 5** [18, 19]. Thermal cure epoxy adhesives have much stronger and rigid structure and normally possess higher glass transition temperature with compared to room temperature cure epoxy adhesives mainly based on aliphatic amines or mercaptans. Two-component thermal cure epoxy adhesives are mainly used for higher temperature resistance required applications such as automobile

One-component epoxy adhesives do not require pre-mix before use since all components have been mixed together and there is no concern on insufficient mixing problem as often occurred in two-component use. Pot life of one-component epoxy adhesives is usually long and one-component adhesives are thus suitable for automatic dispensing systems. Compared to two-component type, one-component epoxy adhesives can be handled much easily. On the other hand, one-component epoxy adhesives usually need cure at higher temperature because of long enough room temperature stability needed for adhesive preparation and storage. Most one-component epoxy adhesives require storage condition at lower temperatures

Recently one-component thermal cure epoxy adhesives have become more and more important especially in electronics assembly and automotive production where high production efficiency is required. With selection of suitable latent curing agents, a number of one-component epoxy adhesives have been developed and commercialized by epoxy adhesive suppliers for various applications. Typical

**4. Thermal cure epoxy adhesives**

important in recent years.

production and aerospace market.

in a refrigerator or even freezer.

*Chemical structure of IPDA and DDM.*

**42**

**Figure 5.**

*Typical commercial latent curing agents.*

**Figure 6.** *Chemical structure of DICY.*

DICY (dicyandiamide), chemical structure shown in **Figure 6** [20], is the oldest and widely used latent curing agent for epoxy resin technology. It is a solid chemical with a melting point at 208°C. DICY formulated epoxy composition is very stable, up to 6 months at room temperature. Latency mechanism is a combination of physically separation and chemically blocking with epoxide group. DICY cured epoxy resin shows high adhesion and possesses high glass transition temperature especially suitable for high performance required applications such as vehicle parts bonding in automobile production. Cure temperature of DICY alone with epoxy resin normally needs at least 150°C to achieve full cure. By addition small amount of accelerator such as modified urea compounds and imidazole compound, cure temperature can be further lowered to 120°C [21].

In recent years, new type latent curing agents have been developed and commercialized by several curing agent suppliers [22–26]. These latent curing agents are supplied as fine powder with average particle size well controlled in a few microns or premix of fine powder latent curing agent in epoxy resin. They are manufactured by grinding specially synthesized modified polyamine or imidazole solid with a softening point from 80 to 150°C. Latency mechanism is mainly physically separation between curing agent and epoxide. Curing temperature has been be lowered to as low as 80°C and its formulated epoxy composition can be still quite stable at room temperature. Many one-component epoxy adhesives commercialized recently are based on these new type latent curing agents because of their lower temperature curability suitable for use in bonding heat sensitive substrates such as plastics. By combination of small amount of liquid phenol compound with latent curing agent, it has been found that cure time of one-component epoxy adhesives can be shortened significantly [27, 28].

## **5. UV cure epoxy adhesives**

Ultra-violet light (UV) curable epoxy adhesives can be cured quickly and have been very successfully used in several new electronics assembly and general bonding


#### **Table 8.**

*Comparison of UV acrylate, cationic epoxy and hybrid epoxy adhesives.*

applications such as image sensor module assembly, display panel and module assembly where fast production speed and high adhesion performance are required. Various UV cationic epoxy adhesive and UV acrylate hybrid thermal cure epoxy adhesives have been commercialized in recent years. As compared in **Table 8**, UV cure epoxy adhesives have no oxygen inhibition issue, low curing shrinkage and show better adhesion with compared to common UV acrylate adhesives.

#### **5.1 UV cationic epoxy adhesives**

UV cationic epoxy adhesives are primarily composed of epoxy resin and cationic photo-initiator [29–31]. Cycloaliphatic type epoxy resins are usually selected for UV cationic epoxy adhesives because of faster cationic polymerization rate than that of normal bisphenol A diglycidyl ether type epoxy resin. As illustrated in **Figure 7** [32], cationic photoinitiator formulated in UV epoxy adhesives absorbs UV energy to generate strong acid that will react with epoxy to produce cationic which can initiate homo-polymerization of epoxy resin. Compared to common acrylate based UV adhesives, UV cationic epoxy adhesives have lower cure shrinkage because of epoxy structure and have no surface cure issue resulted from oxygen inhibition to free radical polymerization since they cure via cationic polymerization. On the other hand, UV cationic epoxy adhesives are not suitable for bonding basic substrates which terminate cationic polymerization. UV cationic epoxy adhesives will need some longer cure time. In real use, a post thermal cure of UV cationic epoxy adhesives after the UV radiation is commonly used for full cure to assure satisfactory adhesion performance.

UV cationic epoxy adhesives have been commercialized and used in optical parts bonding, camera module sensor packaging and OLED panel assembly applications [33–37]. The authors have found that adhesion reliability performance of UV cationic epoxy adhesives can be much improved by combination use of cationic photo initiator with thermal cationic initiator [38].

**45**

*Epoxy Adhesives*

**Figure 7.**

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

**5.2 UV hybrid epoxy adhesives**

*UV cationic polymerization of epoxy adhesives.*

where UV light cannot reach.

Most widely used UV cure adhesives are acrylate compositions [39–41]. Acrylate based UV cure adhesives are mainly composed of acrylate monomer, acrylate oligomer and radical photo-initiator. Acrylate based UV cure adhesives can be cured instantly, within seconds. Limitations of UV cure acrylate adhesives are surface cure issue, shadow cure problem, high cure shrinkage and poor humidity reliability. By combination of UV acrylate composition with thermal cure epoxy composition, UV and thermal cure hybrid epoxy adhesives have been developed and commercialized for over two decades [42–44]. Acrylate monomer, epoxy resin, photo-initiator and epoxy curing agent are primarily formulated in the UV and thermal cure hybrid adhesives. UV hybrid epoxy adhesives combine advantages from both UV acrylate proportion and thermal cure epoxy part. Adhesion reliability performance could be much improved by introduction of epoxy composition with compared to normal UV acrylate adhesives. In the meantime, production efficiency could be much improved by shortening the fixture time to seconds via UV radiation with compared to at least dozens of minutes needed for thermal cure epoxy adhesives. Surface cure issue, shadow cure issue and cure shrinkage problem of acrylate UV adhesives could also be improved to certain degree because of lower contents of free radical curable acrylate compositions. In some cases, thermal initiator such as peroxide is also formulated in the hybrid adhesive to assure curing remained acrylate compositions after UV radiation or those at shadow areas

Successful development and industrialization of so called ODF (One Drop Fill) process for large size LCD (liquid crystal panel) panel production was one important technology revolution in early 2000s that have made a big impact on our modern life. Development and commercialization of LCD ODF main sealant, an UV hybrid epoxy adhesive, played a key role in its mass production success [45–47]. LCD ODF main sealant is an adhesive material that is used to bond two glass substrates and seal liquid crystal material between them. It is a UV hybrid epoxy adhesive, typically composed of acrylate monomer, photo-initiator, partially acrylate epoxy resin and latent curing agent. Main steps for the adhesive use in this process are: (1) dispensing

LCD main sealant; (2) dropping off liquid crystal materials into each cell;

liquid crystal material and high performance [48–50].

(3) alignment and assembly; (4) UV cure the sealant; and (5) thermal cure the sealant. The author has invented initiator free UV hybrid thermal cure epoxy adhesives by combination with bismaleimides that shows much better compatibility with

*Adhesives and Adhesive Joints in Industry Applications*

Polymerization

Key features

**Table 8.**

applications such as image sensor module assembly, display panel and module assembly where fast production speed and high adhesion performance are required. Various UV cationic epoxy adhesive and UV acrylate hybrid thermal cure epoxy adhesives have been commercialized in recent years. As compared in **Table 8**, UV cure epoxy adhesives have no oxygen inhibition issue, low curing shrinkage and

**Adhesive type UV acrylate UV cationic epoxy Hybrid thermal cure** 

Thermal cure N.A. Cationic Polyaddition, anionic

Photoinitiator Cationic photoinitiator Photoinitiator

Main compositions Acrylate Epoxy resin Acrylate

UV cure Radical Cationic Radical

Oxygen inhibition Yes Yes Partially Alkali inhibition No Medium No UV curability High Preferred High Post thermal cure No need Partially Need Shadow cure No Low Yes Cure shrinkage High Good Low Adhesion Moderate Good

**epoxy**

Epoxy resin Curing agent

UV cationic epoxy adhesives are primarily composed of epoxy resin and cationic photo-initiator [29–31]. Cycloaliphatic type epoxy resins are usually selected for UV cationic epoxy adhesives because of faster cationic polymerization rate than that of normal bisphenol A diglycidyl ether type epoxy resin. As illustrated in **Figure 7** [32], cationic photoinitiator formulated in UV epoxy adhesives absorbs UV energy to generate strong acid that will react with epoxy to produce cationic which can initiate homo-polymerization of epoxy resin. Compared to common acrylate based UV adhesives, UV cationic epoxy adhesives have lower cure shrinkage because of epoxy structure and have no surface cure issue resulted from oxygen inhibition to free radical polymerization since they cure via cationic polymerization. On the other hand, UV cationic epoxy adhesives are not suitable for bonding basic substrates which terminate cationic polymerization. UV cationic epoxy adhesives will need some longer cure time. In real use, a post thermal cure of UV cationic epoxy adhesives after the UV radiation

show better adhesion with compared to common UV acrylate adhesives.

*Comparison of UV acrylate, cationic epoxy and hybrid epoxy adhesives.*

is commonly used for full cure to assure satisfactory adhesion performance.

UV cationic epoxy adhesives have been commercialized and used in optical parts bonding, camera module sensor packaging and OLED panel assembly applications [33–37]. The authors have found that adhesion reliability performance of UV cationic epoxy adhesives can be much improved by combination use of cationic photo

**5.1 UV cationic epoxy adhesives**

initiator with thermal cationic initiator [38].

**44**

**Figure 7.** *UV cationic polymerization of epoxy adhesives.*
