3.3.2 Cyanoacrylate adhesive characteristics

Characteristics of cyanoacrylate are the following: single-component, very fast fixture time (fixturing in seconds) and curing speed, almost all materials may be bonded, wide range of viscosities, very high shear and tensile strength, good aging resistance, and easy automation with dispensing equipment as a single component.

## 3.3.3 Cyanoacrylate by the monomer chemistry

The monomer chemistry of cyanoacrylate consists of methyl grade, ethyl grade, and alkoxy grade.

## 3.3.3.1 Methyl grade

This is the first-generation cyanoacrylate. Just like the chemistry, the molecular weight of "methyl" is the smallest, so the number of molecules that can be attached to the bonding surface is the most which means it has a high bonding strength but as the total weight is the lightest therefore it is easy to evaporate and stimulate people's nose as a strong smell and cause a "blooming" phenomenon, too (see Figure 7 and Table 2) [12].

### 3.3.3.2 Ethyl grade

This is the second-generation cyanoacrylate. Just like the chemistry, the molecular weight of "ethyl" is medium, it means the number of molecules that can be attached to the bonding surface is medium, consequently it has a medium bonding strength but as the total molecular weight is medium therefore it is still not light and not easy to evaporate and stimulate people's nose as a less strong smell and cause less "blooming" phenomenon (see Figure 8 and Table 2) [12].

3.3.4 Recent trend of CA adhesive properties

Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

3.3.4.1 Surface insensitive grade

grade [12].

3.3.4.2 Toughened grade

3.3.4.3 Low bloom/low odor grade

3.3.5 Typical applications

81

metal, plastic and elastomer substrates.

and more clean appearance of bonded assembly.

property (fast fixture), too. This is ethyl grade [12].

sitive property (fast fixture), too. This is alkoxy grade [12].

3.4 Adhesives cured with activator systems (modified acrylics)

Recent bonding performance improvements achieved are for fast fixture time for porous and acid surface, good impact resistance, high temperature resistance,

Typical CA products are surface sensitive which means no effective bonding to

porous substrates and acidic surface. It wicks into the porous area and leaves nothing on the surface, so there is no good bonding and the curing time is slow when applied to acidic surface. This surface insensitive grade when applied to porous substrate stays on the porous surface to bond well and, due to some special

chemistry, bond fast to the acidic surface. They have a higher temperature

resistance and more surface insensitive property (faster fixture time), this is ethyl

Typical CA products are not tough which means there is no effective resistance against impact and vibration. It breaks very easily when the assembly is exposed to impact. This toughened grade contains some elastomer inside, so when bonded the bond line absorbs the external impact and maintains a very good bonding. They have a higher temperature and moisture resistance and have surface insensitive

Typical CA products have a strong odor and "blooming" which means there are no good working environment and no good assembly appearance quality. This low bloom/low odor grade due to a heavy molecular weight has no pungent smell and almost no blooming. They have a higher temperature resistance and surface insen-

Typical applications of cyanoacrylates are used broadly in the various assembly industries and can cover various applications and their requirements such as bonding of porous substrates, acidic surfaces, difficult to bond plastics, dissimilar materials, flexible materials with very fast fixturing, high impact and vibration resistance, high temperature resistance (120C), high humidity resistance, high heat aging resistance, high optical appearance as well as bonding of the most general

These adhesives are regarded as structural adhesives. Structural adhesive is different from others essentially in the bond of the adhesive, in the monomer type, and in the cure chemistry. The compositional differences result in differences in cure characteristics and in surface affinity and in basic cured properties such as durability of adhesive. Structural adhesives are strong and tough as they possess a high cohesive and adhesive strength. The cohesive strength represents the ability of the material to tolerate stress without failure. Adhesive strength is the ability of the

#### 3.3.3.3 Alkoxy grade

This is the third-generation cyanoacrylate. Just like the chemistry, the molecular weight of "methyl" is the biggest, so the number of molecules that can be attached to the bonding surface is the smallest which means it has a low bonding strength, but as the total weight is the highest, therefore it hardly evaporates, has no smell to people's nose, and causes almost no "blooming" phenomenon (see Figure 9 and Table 2) [12].

Figure 7. Methyl grade molecule.


Table 2.

Property difference among CA grades.

Figure 8. Ethyl grade molecule.

Figure 9. Alkoxy grade molecule.

Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

#### 3.3.4 Recent trend of CA adhesive properties

Recent bonding performance improvements achieved are for fast fixture time for porous and acid surface, good impact resistance, high temperature resistance, and more clean appearance of bonded assembly.

#### 3.3.4.1 Surface insensitive grade

not easy to evaporate and stimulate people's nose as a less strong smell and cause

This is the third-generation cyanoacrylate. Just like the chemistry, the molecular weight of "methyl" is the biggest, so the number of molecules that can be attached to the bonding surface is the smallest which means it has a low bonding strength, but as the total weight is the highest, therefore it hardly evaporates, has no smell to people's nose, and causes almost no "blooming" phenomenon (see Figure 9 and

R (alkyl) Odor Adhesion Temp. resistance

Methyl High High High Ethyl High High High Alkoxy Moderate Moderate Moderate

less "blooming" phenomenon (see Figure 8 and Table 2) [12].

Adhesives and Adhesive Joints in Industry Applications

3.3.3.3 Alkoxy grade

Table 2) [12].

Figure 7.

Table 2.

Methyl grade molecule.

Property difference among CA grades.

Figure 8.

Figure 9.

80

Alkoxy grade molecule.

Ethyl grade molecule.

Typical CA products are surface sensitive which means no effective bonding to porous substrates and acidic surface. It wicks into the porous area and leaves nothing on the surface, so there is no good bonding and the curing time is slow when applied to acidic surface. This surface insensitive grade when applied to porous substrate stays on the porous surface to bond well and, due to some special chemistry, bond fast to the acidic surface. They have a higher temperature resistance and more surface insensitive property (faster fixture time), this is ethyl grade [12].

#### 3.3.4.2 Toughened grade

Typical CA products are not tough which means there is no effective resistance against impact and vibration. It breaks very easily when the assembly is exposed to impact. This toughened grade contains some elastomer inside, so when bonded the bond line absorbs the external impact and maintains a very good bonding. They have a higher temperature and moisture resistance and have surface insensitive property (fast fixture), too. This is ethyl grade [12].

#### 3.3.4.3 Low bloom/low odor grade

Typical CA products have a strong odor and "blooming" which means there are no good working environment and no good assembly appearance quality. This low bloom/low odor grade due to a heavy molecular weight has no pungent smell and almost no blooming. They have a higher temperature resistance and surface insensitive property (fast fixture), too. This is alkoxy grade [12].

#### 3.3.5 Typical applications

Typical applications of cyanoacrylates are used broadly in the various assembly industries and can cover various applications and their requirements such as bonding of porous substrates, acidic surfaces, difficult to bond plastics, dissimilar materials, flexible materials with very fast fixturing, high impact and vibration resistance, high temperature resistance (120C), high humidity resistance, high heat aging resistance, high optical appearance as well as bonding of the most general metal, plastic and elastomer substrates.

#### 3.4 Adhesives cured with activator systems (modified acrylics)

These adhesives are regarded as structural adhesives. Structural adhesive is different from others essentially in the bond of the adhesive, in the monomer type, and in the cure chemistry. The compositional differences result in differences in cure characteristics and in surface affinity and in basic cured properties such as durability of adhesive. Structural adhesives are strong and tough as they possess a high cohesive and adhesive strength. The cohesive strength represents the ability of the material to tolerate stress without failure. Adhesive strength is the ability of the material to stick to the surface. Cohesive failure will leave adhesive on both substrates. Adhesive failure occurs at the interface of the adhesive and the substrate and after debonding, one side substrate will have no cured adhesive left bonded while all the cured adhesive will be left bonded to the other side substrate. These adhesives cure at room temperature when used with activators. Depending on the adhesive, for two-part no-mix type, both the adhesive and activator can be applied separately to the bonding surfaces. It does not react as long as they are not mated allowing some flexibility of application condition. After mating both surfaces with a pressure, reaction starts and adhesive cures to a tough structural adhesive. For twopart premix type, product is dispensed through a double cartridge by a double plunger, and it is mixed and dispensed through static mixer. This way, separate mixing procedure is not required. A precise amount is mixed by volume. For twopart separate dispensing type, adhesive parts A and B are dispensed through a separate nozzle on to the substrate bead by bead or bead beside bead. This way the product is not cured after dispensing, and when the two substrates are mated, the adhesive will be mixed by pressing movement and cures to a tough acrylic adhesive. This is useful when the high-speed production and wide mixing ratio tolerance are required. Three types can be selected properly according to application requirements.

Adhesives with multifunctional oligomers cure faster than monofunctional oligomer. Multifunctional oligomers have faster cure speed, better surface cure,

adhesives result in a high viscosity, and many different types of elastomers are

Elastomers are responsible for flexibility and toughness; therefore the structural

The following requirements have to be considered when selecting a right adhesive such as fixture time, ultimate bond strength, on-part life, toughness, tempera-

Typical applications of structural adhesive are magnet bonding for different types of electric motors, magnet bonding for loudspeakers, security alarm sensor bonding, metal structure bonding, flat coil bonding, ferrite core bonding, and voice

Methyl methacrylate (MMA) adhesives are structural adhesives which are toughened by special elastomers which are "super tough" because they result in a graft polymer that has both excellent peel and impact strength. They cure at room temperature. Their two-component cure is based on redox system. They have a good performance on most substrates especially plastics. They have a good performance on dissimilar substrates. They have a good adhesion to fiber-reinforced plastic (FRP) and gel coat with little or no surface preparation. They have a high modulus and toughness. They have an off-ratio tolerant within 20% during mixing. However, cure rate is temperature dependent, such as in summer and winter, and heat of cure and shrinkage can cause a distortion to read-through at FRP panel. They will cure when adhesive (part A) and activator (part B) are statically mixed. They will fillet cure and cure speed is dependent on mass. Plastics that are solvated by MMA will fixture fast. The type of mixing ratio is ranging from 1:1 to 10:1. Some products which contain glass or polymer beads that can control bond

For some special applications, if MMA is used with "0" gap, then the assembly cannot take the advantage of "toughness" of MMA. By using beads, the cohesive strength and impact strength become very high as the bond line absorbs all the external impact. Beads also ensure a controlled exotherm reaction in the bond line, too. Figure 11 shows a certain bond line gap achieved with spacer. This design

better durability, better hardness, and much low odor and toxicity than

monofunctional oligomer.

Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

3.4.2.1 Product selection criterion

ture resistance, and carrier solvent.

coil magnet plate bonding for hard disk drive.

3.4.3 Structural adhesive (two-component premix: MMA)

line thickness are ranging from 0.1 to 0.8 mm diameter.

3.4.3.1 Glass (or polymer) beads

Figure 11.

83

Structural bonding with spacer.

3.4.2.2 Typical applications

used in this adhesive.

## 3.4.1 Structural adhesive characteristics

Structural adhesive characteristics are very high shear and tensile strengths, good impact resistance, wide operating temperature range (55°C to +120°C), almost all materials can be bonded, good gap-filling capacity (especially premixed acrylics), and good environmental resistance.

#### 3.4.2 Structural adhesive (two-component no-mix)

Some grades have special oligomers to provide more toughness. Other grades have multifunctional acrylate and elastomeric oligomer. They are minimally crosslinked. They have a big portion of elastomeric toughening agent, and they generate free radical through redox reaction. The following is the typical oligomeric structure of structural acrylic adhesives (see Figure 10) [13].

$$\mathsf{H}\_{\mathsf{H}}\mathsf{C}=\mathsf{C}\mathsf{H}\mathsf{C}\mathsf{O}(\mathsf{C}\mathsf{H}\_{2})^{\mathsf{L}}\cdot\left[\bigoplus\_{\mathsf{O}}^{\mathsf{C}}(\mathsf{C}\mathsf{H}\_{2})\_{\mathsf{H}}\mathsf{C}\mathsf{O}(\mathsf{C}\mathsf{H}\_{2})\_{\mathsf{K}}\right]\_{\mathsf{D}}\cdot\mathsf{O}\mathsf{C}^{\mathsf{L}}\mathsf{C}\mathsf{H}=\mathsf{C}\mathsf{H}\_{2}\mathsf{D}$$

Figure 10. Different acrylate molecular structures. Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

Adhesives with multifunctional oligomers cure faster than monofunctional oligomer. Multifunctional oligomers have faster cure speed, better surface cure, better durability, better hardness, and much low odor and toxicity than monofunctional oligomer.

Elastomers are responsible for flexibility and toughness; therefore the structural adhesives result in a high viscosity, and many different types of elastomers are used in this adhesive.

#### 3.4.2.1 Product selection criterion

material to stick to the surface. Cohesive failure will leave adhesive on both substrates. Adhesive failure occurs at the interface of the adhesive and the substrate and after debonding, one side substrate will have no cured adhesive left bonded while all the cured adhesive will be left bonded to the other side substrate. These adhesives cure at room temperature when used with activators. Depending on the adhesive, for two-part no-mix type, both the adhesive and activator can be applied separately to the bonding surfaces. It does not react as long as they are not mated allowing some flexibility of application condition. After mating both surfaces with a pressure, reaction starts and adhesive cures to a tough structural adhesive. For twopart premix type, product is dispensed through a double cartridge by a double plunger, and it is mixed and dispensed through static mixer. This way, separate mixing procedure is not required. A precise amount is mixed by volume. For twopart separate dispensing type, adhesive parts A and B are dispensed through a separate nozzle on to the substrate bead by bead or bead beside bead. This way the product is not cured after dispensing, and when the two substrates are mated, the adhesive will be mixed by pressing movement and cures to a tough acrylic adhesive. This is useful when the high-speed production and wide mixing ratio tolerance are required. Three types can be selected properly according to application requirements.

Structural adhesive characteristics are very high shear and tensile strengths, good impact resistance, wide operating temperature range (55°C to +120°C), almost all materials can be bonded, good gap-filling capacity (especially premixed

Some grades have special oligomers to provide more toughness. Other grades

have multifunctional acrylate and elastomeric oligomer. They are minimally crosslinked. They have a big portion of elastomeric toughening agent, and they generate free radical through redox reaction. The following is the typical oligomeric

structure of structural acrylic adhesives (see Figure 10) [13].

3.4.1 Structural adhesive characteristics

Figure 10.

82

Different acrylate molecular structures.

acrylics), and good environmental resistance.

Adhesives and Adhesive Joints in Industry Applications

3.4.2 Structural adhesive (two-component no-mix)

The following requirements have to be considered when selecting a right adhesive such as fixture time, ultimate bond strength, on-part life, toughness, temperature resistance, and carrier solvent.

#### 3.4.2.2 Typical applications

Typical applications of structural adhesive are magnet bonding for different types of electric motors, magnet bonding for loudspeakers, security alarm sensor bonding, metal structure bonding, flat coil bonding, ferrite core bonding, and voice coil magnet plate bonding for hard disk drive.

#### 3.4.3 Structural adhesive (two-component premix: MMA)

Methyl methacrylate (MMA) adhesives are structural adhesives which are toughened by special elastomers which are "super tough" because they result in a graft polymer that has both excellent peel and impact strength. They cure at room temperature. Their two-component cure is based on redox system. They have a good performance on most substrates especially plastics. They have a good performance on dissimilar substrates. They have a good adhesion to fiber-reinforced plastic (FRP) and gel coat with little or no surface preparation. They have a high modulus and toughness. They have an off-ratio tolerant within 20% during mixing. However, cure rate is temperature dependent, such as in summer and winter, and heat of cure and shrinkage can cause a distortion to read-through at FRP panel.

They will cure when adhesive (part A) and activator (part B) are statically mixed. They will fillet cure and cure speed is dependent on mass. Plastics that are solvated by MMA will fixture fast. The type of mixing ratio is ranging from 1:1 to 10:1. Some products which contain glass or polymer beads that can control bond line thickness are ranging from 0.1 to 0.8 mm diameter.

#### 3.4.3.1 Glass (or polymer) beads

For some special applications, if MMA is used with "0" gap, then the assembly cannot take the advantage of "toughness" of MMA. By using beads, the cohesive strength and impact strength become very high as the bond line absorbs all the external impact. Beads also ensure a controlled exotherm reaction in the bond line, too. Figure 11 shows a certain bond line gap achieved with spacer. This design

Figure 11. Structural bonding with spacer.

provides a very high toughness to the assembly as a very high bond line stress is eliminated with this design (see Figure 11) [14].

the site of crosslinking, once surface is cured, moisture becomes more difficult to penetrate the already cured layer of silicone to react with internal uncured silicone; therefore the whole curing takes relatively a long time. The depth of cure is limited

One-component silicone characteristics are the following: excellent thermal resistance ("up to 270°C"), flexible, tough, high elongation, low to medium modu-

One-component silicone applications are gasketing and sealing in automotive industry, sealing in high temperature applications, sealing and bonding for small

When part A and part B are mixed in a proper mixing ratio, a chemical reaction occurs initiated by the catalyst reacting with water. This chemical reaction occurs not only at the surface but also inside the mixed product. Therefore, it has a very fast fixturing and curing time for faster fixture and assembly, and it allows a structural bonding assembly in more compact assembly line and automated assembly with less manpower. This technology can replace various assemblies that have used double-sided tape or RTV silicone that requires a manual and lengthy assembly line and time with much manpower. During the chemical reaction, moisture ("H2O") comes in and alcohol ("OH") that was generated during chemical reaction

Two-component silicone characteristics are such as fast tack free time, fast fixture time and fast full curing time. Its curing speed is not affected by moisture and bond line thickness. It is flexible, tough, high modulus and UL 746C listed for high temperature applications. It also has an excellent gap filling capability and high temperature resistance up to 270C. Three colors such as black, gray and almond are

Typical applications of two-component silicone are glass bonding for refrigera-

tor, washing machine, microwave, cook top, dish washer, air conditioner and

lus, effective sealants for a variety of fluid types, and excellent gap filling.

to 10–15 mm.

3.5.1.1.1 Silicone characteristics

Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

3.5.1.1.2 Typical applications

3.5.1.2 Two-component silicone

3.5.1.2.1 Silicone characteristics

3.5.1.2.2 Typical applications

bridge structure bonding.

Two-component silicone curing mechanism.

available.

Figure 13.

85

parts, sealing and potting, and coating of PCBs.

as a by-product comes out and evaporates (see Figure 13) [15].

#### 3.4.3.2 Typical applications

Typical applications are galvanized steel bonding for construction to prevent corrosion bonding failure; school bus structure bonding for fire wall, side wall, and roof; marine yacht bonding for deck, stringer, hull, and gel coat; boom bonding for special vehicles; structure bonding with spot welding; office cubicle bonding; composite structure bonding; and case bonding of hand-held display device.

#### 3.5 Adhesives cured by ambient moisture

These adhesives and sealants are polymerized by (in most cases) a condensation reaction which involves reaction with ambient moisture. Two general chemistry types fall into two categories which are silicone and polyurethane.

#### 3.5.1 Silicone

Depending on the curing mechanism, it consists of one-component RTV silicone and two-component silicone. For one-component silicone, it vulcanizes at room temperature either by reaction with moisture in the air and substrates. It starts to cure from the surface and cures into depth. Depending on the type of formulation, it generates a by-product and evaporates. For two-component silicone, it vulcanizes at room temperature by part A and B reactions. Its curing starts from both surface and inside of mixed product. Therefore, its curing is not affected by the lack of moisture in the air.

#### 3.5.1.1 One-component RTV silicone

This is widely used in various industries. Depending on the type of formulation, a by-product is generated such as vinegar smell and non-vinegar smell. Acetic acid type has a pungent odor and is mainly used in the construction area and some machinery repair applications. Oxime type is the most popular in the assembly industry as it has no pungent odor and is regarded as non-acetic acid silicone. The cure speed of silicone primarily depends on the relative humidity which means it is fast in summer time and slow in winter time (see Figure 12) [15].

Because of the nature of the curing mechanism, silicones vulcanize from the outside to the inside of the bond line. Due to the necessary moisture migration to

Figure 12. RTV silicone curing mechanism.

#### Functional Adhesive Trend for Assembly Industry DOI: http://dx.doi.org/10.5772/intechopen.84880

the site of crosslinking, once surface is cured, moisture becomes more difficult to penetrate the already cured layer of silicone to react with internal uncured silicone; therefore the whole curing takes relatively a long time. The depth of cure is limited to 10–15 mm.
