3.1.5.2.1 Conventional thread sealing method

Those conventional methods used in the assembly industry are polytetrafluoroethylene (PTFE) tape, O-ring, pipe dope, etc.

They have shortcomings such as PTFE tape shards generated during pipe assembly contaminate internal hydraulic pipe system, and PTFE tape can have a leak once readjusted after pipe location setting and requires a manual wrapping of time and cost. Another conventional method O-ring has no sealing effect at dynamic environment and requires an additional machining or molding. Pipe dope has a low pressure sealing [5].

### 3.1.5.2.2 Liquid thread sealing adhesive

Those liquid thread sealing adhesives have the following benefits: they provide a good sealing, they prevent a self-loosening, they cause no contamination inside hydraulic piping system, they cure slowly to allow pipe location adjustment after initial tightening yet provide an instant low pressure sealing, they protect pipe threads against corrosion and galling, and they show a high pressure resistance after cure [6].

#### 3.1.5.3 Retaining

#### 3.1.5.3.1 Conventional retaining methods

Those conventional methods used in the assembly industry are positive drive (pins, keys and splines), friction drive (press fit, shrink fit), and welding/soldering.

They have shortcomings such as the following: they have a notorious back lash that causes an extremely high stress, deformation, and consequent failure of assembly; they have a fretting corrosion that causes a lower torque transmitting capability, and their machining is costly and lengthy to meet the right assembly tolerance; they need additional equipment required for press fit and shrink fit; and in overall they are more complicated, time-consuming, and less reliable [7].

prevents a curing of the adhesive as the power of light energy is so high to overcome all the other hindrance. The free radicals formed in turn start the polymerization by reacting with a monomer nearby, and it becomes unstable, too, and then reacts with the next monomer and finally polymerized in a very fast speed (see Figure 2). It is critically important to use a right light curing lamp equipment that emits a proper wavelength of radiation spectrum ideally matched to the curing properties of light

There are three types of light cure adhesives, namely, UV cure adhesive, UV+

UV light is in the range of 40–400 nm wavelength. The whole UV wavelength does not necessarily cure adhesive. A specific wavelength of UV light affects the adhesive. UVC is a short wavelength, and it cures the surface of adhesive as it is a short wavelength and not easy to penetrate the adhesive inside; therefore it is used for surface cure of adhesive. UVA is a long wavelength, and it cures the volume of adhesive as it is a long wavelength and easy to penetrate the adhesive depth; therefore it is used for a depth cure. Care should be taken when selecting a clear plastic for light cure. Some plastics such as UV stabilizer filled PC looks like a clear plastic to the eyes of people, but UV light cannot go through the plastic as the UV stabilizer inside the PC can absorb the UV light and cannot reach the adhesive bond line. It is also critical to know that the temperature inside UV curing chamber or conveyor should be kept less than 60–70°C depending on the thermal sensitivity of parts exposed to UV light as the UV light bulb can irradiate not only UV light but also emit infrared (IR) and heat. Otherwise temperature can reach above the limit of plastic parts. UV adhesive was the first-generation light cure adhesive introduced

Ultraviolet (UV) + visible light is in the range of 40–405 nm wavelength. UV photo initiator had a limited cure through volume (CTV) capability, and some visible light initiators were added together to achieve better CTV and faster curing with less UV dependence that was harmful to operators. The photo initiator for visible light at the time had a limited (partial) visible light wavelength coverage. This is a second-generation light cure adhesive introduced in the assembly industry.

Visible light cure is in the range of 400–1000 nm wavelength. This is only for visible light cure adhesive, and it can cover wider visible light wavelength coverage; therefore it can cure through different colored clear plastics which previous light cure adhesive was not able to cure as it cannot go through the colored plastics. It also can cure CTV 12 mm deep as it has a longer wavelength. The visible light cure

cure adhesives [11].

Light curing mechanism.

Figure 2.

in the assembly industry.

3.2.3 Visible light cure and adhesive

75

visible cure adhesive, and visible cure adhesive.

3.2.2 Ultraviolet (UV) light + visible light cure adhesive

3.2.1 Ultraviolet (UV) light cure adhesive

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

#### 3.1.5.3.2 Liquid retaining compound adhesive

Those liquid retaining compound adhesives have following benefits such as they are less costly in machining as they don't need a tight tolerance; they cause no back lash, and therefore it allows a high durability and reliability of the assembly; they allow the assembly assembled simpler and easier; and they can be used to conventional designs if they can't change the design and it can increase the durability of the assembly, too [8].

#### 3.1.5.4 Gasketing

#### 3.1.5.4.1 Conventional gasketing methods

Those conventional methods used in the assembly industry are various compression gaskets (pre-cut gaskets, O-ring seals and profile packings).

They have shortcomings such as the following: they achieve a sealing by compression for a long time which is not realistic; they have a shrinkage, creep, and compression set under high flange pressure; they require to store multiple shapes to seal different flange shapes; they are costly with an intricate flange shape; they are susceptible to external or internal impact or pressure; and they require additional machining or smooth machining to achieve a sealing with a cut gasket [9].

#### 3.1.5.4.2 Liquid gasketing sealant

Those liquid gasketing sealants have the following benefits: they have a metal-to-metal contact, thus becoming a very stable assembly, they can be used to all different shapes of flange with one product, they can provide a unitized assembly with "keyway effect" between flanges, they can eliminate all the mistakes that occur with the manual operation, they can be used for automatic dispensing and assembly, and in overall they can increase the durability, reliability, as well as the cost reduction of the assembly [10].

#### 3.2 Adhesives cured by light (ultraviolet and visible)

The cure time of these adhesives depends on the intensity and on the wavelength of the light. Polymerization initiated by the light thus always requires a proper curing property of the adhesive and correct exposure to a light radiation. When a light cure adhesive is applied and goes through the area of light irradiation, the photo initiators in the adhesive are split to become free radicals which are very unstable. In this situation, unlike to anaerobic cure, there is no condition that

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

#### Figure 2.

3.1.5.3 Retaining

the assembly, too [8].

3.1.5.4 Gasketing

3.1.5.3.1 Conventional retaining methods

Adhesives and Adhesive Joints in Industry Applications

3.1.5.3.2 Liquid retaining compound adhesive

3.1.5.4.1 Conventional gasketing methods

3.1.5.4.2 Liquid gasketing sealant

cost reduction of the assembly [10].

74

3.2 Adhesives cured by light (ultraviolet and visible)

Those conventional methods used in the assembly industry are positive drive (pins, keys and splines), friction drive (press fit, shrink fit), and welding/soldering. They have shortcomings such as the following: they have a notorious back lash

Those liquid retaining compound adhesives have following benefits such as they are less costly in machining as they don't need a tight tolerance; they cause no back lash, and therefore it allows a high durability and reliability of the assembly; they allow the assembly assembled simpler and easier; and they can be used to conventional designs if they can't change the design and it can increase the durability of

Those conventional methods used in the assembly industry are various com-

They have shortcomings such as the following: they achieve a sealing by compression for a long time which is not realistic; they have a shrinkage, creep, and compression set under high flange pressure; they require to store multiple shapes to seal different flange shapes; they are costly with an intricate flange shape; they are susceptible to external or internal impact or pressure; and they require additional machining or smooth machining to achieve a sealing with a cut gasket [9].

Those liquid gasketing sealants have the following benefits: they have a metal-to-metal contact, thus becoming a very stable assembly, they can be used to all different shapes of flange with one product, they can provide a unitized assembly with "keyway effect" between flanges, they can eliminate all the mistakes that occur with the manual operation, they can be used for automatic dispensing and assembly, and in overall they can increase the durability, reliability, as well as the

The cure time of these adhesives depends on the intensity and on the wavelength

of the light. Polymerization initiated by the light thus always requires a proper curing property of the adhesive and correct exposure to a light radiation. When a light cure adhesive is applied and goes through the area of light irradiation, the photo initiators in the adhesive are split to become free radicals which are very unstable. In this situation, unlike to anaerobic cure, there is no condition that

pression gaskets (pre-cut gaskets, O-ring seals and profile packings).

that causes an extremely high stress, deformation, and consequent failure of assembly; they have a fretting corrosion that causes a lower torque transmitting capability, and their machining is costly and lengthy to meet the right assembly tolerance; they need additional equipment required for press fit and shrink fit; and in overall they are more complicated, time-consuming, and less reliable [7].

Light curing mechanism.

prevents a curing of the adhesive as the power of light energy is so high to overcome all the other hindrance. The free radicals formed in turn start the polymerization by reacting with a monomer nearby, and it becomes unstable, too, and then reacts with the next monomer and finally polymerized in a very fast speed (see Figure 2). It is critically important to use a right light curing lamp equipment that emits a proper wavelength of radiation spectrum ideally matched to the curing properties of light cure adhesives [11].

There are three types of light cure adhesives, namely, UV cure adhesive, UV+ visible cure adhesive, and visible cure adhesive.

#### 3.2.1 Ultraviolet (UV) light cure adhesive

UV light is in the range of 40–400 nm wavelength. The whole UV wavelength does not necessarily cure adhesive. A specific wavelength of UV light affects the adhesive. UVC is a short wavelength, and it cures the surface of adhesive as it is a short wavelength and not easy to penetrate the adhesive inside; therefore it is used for surface cure of adhesive. UVA is a long wavelength, and it cures the volume of adhesive as it is a long wavelength and easy to penetrate the adhesive depth; therefore it is used for a depth cure. Care should be taken when selecting a clear plastic for light cure. Some plastics such as UV stabilizer filled PC looks like a clear plastic to the eyes of people, but UV light cannot go through the plastic as the UV stabilizer inside the PC can absorb the UV light and cannot reach the adhesive bond line. It is also critical to know that the temperature inside UV curing chamber or conveyor should be kept less than 60–70°C depending on the thermal sensitivity of parts exposed to UV light as the UV light bulb can irradiate not only UV light but also emit infrared (IR) and heat. Otherwise temperature can reach above the limit of plastic parts. UV adhesive was the first-generation light cure adhesive introduced in the assembly industry.

#### 3.2.2 Ultraviolet (UV) light + visible light cure adhesive

Ultraviolet (UV) + visible light is in the range of 40–405 nm wavelength. UV photo initiator had a limited cure through volume (CTV) capability, and some visible light initiators were added together to achieve better CTV and faster curing with less UV dependence that was harmful to operators. The photo initiator for visible light at the time had a limited (partial) visible light wavelength coverage. This is a second-generation light cure adhesive introduced in the assembly industry.

#### 3.2.3 Visible light cure and adhesive

Visible light cure is in the range of 400–1000 nm wavelength. This is only for visible light cure adhesive, and it can cover wider visible light wavelength coverage; therefore it can cure through different colored clear plastics which previous light cure adhesive was not able to cure as it cannot go through the colored plastics. It also can cure CTV 12 mm deep as it has a longer wavelength. The visible light cure

Figure 3. Light wavelength.

lamp is safer than UV lamp as it emits no UV light. Therefore, there is also no need to wear safety UV block glasses and skin protection wear as well as it has lower cost than UV lamp (see Figure 3) [11].

Also as shown in Figure 4, care should be taken to select a right light curing lamp to some special clear plastics. One may think that UV stabilized clear plastics

stabilizer that blocks the UV light that cannot reach out to the adhesive. It is all absorbed by the UV stabilizer in the plastic. In that case, visible light cure adhesive

But our eyes use a visible light to see its clearness. Such clear plastics contain UV

In numerous applications the light does not reach all areas of adhesive bond line due to opaque substrates. Therefore, other functional light cure adhesives have been developed with a secondary cure system for areas not exposed to the light such as

Also there are various dual cure adhesive technologies, namely, light cure anaerobic, light cure CA, light cure acrylic, light cure silicone, and light cure epoxy.

Light cure adhesives have the following characteristics: on demand cure, high strength, high gap filling capability, very short curing time in handling strength, good to very good environmental resistance, and good dispensing capacity with

Typical applications of light cure adhesives such as bonding glass to itself or to metal, bonding transparent plastics, sealing electrical components (e.g., relays), bonding electrical components to printed circuit boards (PCBs), conformal

coating of PCBs, bonding/sealing in high temperature applications, retaining metal parts + cure excess adhesive by light for fast fixturing, bonding metal and plastic parts + cure excess adhesive by light for fast fixturing, and magnet bonding with

Single-component cyanoacrylate adhesives polymerize on contact with slightly weak base (alkaline) surfaces. In general, ambient humidity in the air and on the substrate surface is sufficient to initiate curing within a few seconds. This is called

can be cured by UV light curing lamp because it looks clear to our eyes.

can be used to cure adhesive through the UV stabilized clear plastics [11].

anaerobic curing, heat, ambient moisture, and activators.

automatic application systems as a single-component adhesive.

3.3 Adhesives cured by anionic reaction (cyanoacrylates)

"anionic" polymerization (see Figure 5) [12].

3.2.8 Curing by secondary mechanisms

Light penetration to different substrate.

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

Figure 4.

3.2.9 Light cure adhesive characteristics

3.2.10 Typical applications

a fillet cure.

77

#### 3.2.4 Light curing lamp equipment by the application type

Light curing lamps by the application type consist of benchtop lamp that can be used for relatively big irradiation area of small batch production or testing at development lab, conveyor type that can be used for relatively big irradiation area of volume production, and fiber optic/pencil type that can be used for specific small irradiation area of development lab and small and volume productions that require a localized area curing.

#### 3.2.5 Light curing lamp equipment by the lamp type

Types of light curing lamp consist of medium pressure mercury arc lamp, high pressure mercury arc lamp, metal halide lamp, fusion lamp, and LED lamp.

Also, light curing process demands are divided into three types, namely, depth curing by light radiation, surface curing by light radiation, and secondary cure systems.

#### 3.2.6 Depth curing

To cure adhesive to maximum depth, light curing systems which emit high intensity light in the wavelength in the band from 300 to 400 nm (longer UV wavelength, UVA light) should be specified. This is useful for applications of potting or bigger gap and big fillet cure bonding [11].

#### 3.2.7 Surface curing

Surface curing is especially important when potting or bonding with light cure adhesives are done. If adhesive surface becomes tacky after light cure, it can contaminate the adjacent area and sensitive components by collecting dust and outgassing of uncured adhesive. Wavelength at 254 nm is the one that can achieve a proper surface cure. A selection of proper light cure lamps that emit a proper wavelength is crucial for successful curing application. It is worthwhile to reiterate that this is very important for the application of surface coating or fillet cure to prevent contamination from tackiness on the cured adhesive surface (see Figure 4) [11].

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

#### Figure 4.

lamp is safer than UV lamp as it emits no UV light. Therefore, there is also no need to wear safety UV block glasses and skin protection wear as well as it has lower cost

Light curing lamps by the application type consist of benchtop lamp that can be

Types of light curing lamp consist of medium pressure mercury arc lamp, high

Also, light curing process demands are divided into three types, namely, depth curing by light radiation, surface curing by light radiation, and secondary cure

To cure adhesive to maximum depth, light curing systems which emit high intensity light in the wavelength in the band from 300 to 400 nm (longer UV wavelength, UVA light) should be specified. This is useful for applications of pot-

Surface curing is especially important when potting or bonding with light cure adhesives are done. If adhesive surface becomes tacky after light cure, it can contaminate the adjacent area and sensitive components by collecting dust and

outgassing of uncured adhesive. Wavelength at 254 nm is the one that can achieve a proper surface cure. A selection of proper light cure lamps that emit a proper wavelength is crucial for successful curing application. It is worthwhile to reiterate that this is very important for the application of surface coating or fillet cure to prevent contamination from tackiness on the cured adhesive surface

pressure mercury arc lamp, metal halide lamp, fusion lamp, and LED lamp.

used for relatively big irradiation area of small batch production or testing at development lab, conveyor type that can be used for relatively big irradiation area of volume production, and fiber optic/pencil type that can be used for specific small irradiation area of development lab and small and volume productions that require

than UV lamp (see Figure 3) [11].

a localized area curing.

systems.

Figure 3. Light wavelength.

3.2.6 Depth curing

3.2.7 Surface curing

(see Figure 4) [11].

76

3.2.4 Light curing lamp equipment by the application type

Adhesives and Adhesive Joints in Industry Applications

3.2.5 Light curing lamp equipment by the lamp type

ting or bigger gap and big fillet cure bonding [11].

Light penetration to different substrate.

Also as shown in Figure 4, care should be taken to select a right light curing lamp to some special clear plastics. One may think that UV stabilized clear plastics can be cured by UV light curing lamp because it looks clear to our eyes.

But our eyes use a visible light to see its clearness. Such clear plastics contain UV stabilizer that blocks the UV light that cannot reach out to the adhesive. It is all absorbed by the UV stabilizer in the plastic. In that case, visible light cure adhesive can be used to cure adhesive through the UV stabilized clear plastics [11].

#### 3.2.8 Curing by secondary mechanisms

In numerous applications the light does not reach all areas of adhesive bond line due to opaque substrates. Therefore, other functional light cure adhesives have been developed with a secondary cure system for areas not exposed to the light such as anaerobic curing, heat, ambient moisture, and activators.

Also there are various dual cure adhesive technologies, namely, light cure anaerobic, light cure CA, light cure acrylic, light cure silicone, and light cure epoxy.

#### 3.2.9 Light cure adhesive characteristics

Light cure adhesives have the following characteristics: on demand cure, high strength, high gap filling capability, very short curing time in handling strength, good to very good environmental resistance, and good dispensing capacity with automatic application systems as a single-component adhesive.

#### 3.2.10 Typical applications

Typical applications of light cure adhesives such as bonding glass to itself or to metal, bonding transparent plastics, sealing electrical components (e.g., relays), bonding electrical components to printed circuit boards (PCBs), conformal coating of PCBs, bonding/sealing in high temperature applications, retaining metal parts + cure excess adhesive by light for fast fixturing, bonding metal and plastic parts + cure excess adhesive by light for fast fixturing, and magnet bonding with a fillet cure.

#### 3.3 Adhesives cured by anionic reaction (cyanoacrylates)

Single-component cyanoacrylate adhesives polymerize on contact with slightly weak base (alkaline) surfaces. In general, ambient humidity in the air and on the substrate surface is sufficient to initiate curing within a few seconds. This is called "anionic" polymerization (see Figure 5) [12].

Adhesives and Adhesive Joints in Industry Applications

3.3.1.5 Cure versus open time

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

3.3.1.6 Cure versus activator

3.3.1.7 Storage

and alkoxy grade.

3.3.3.1 Methyl grade

3.3.3.2 Ethyl grade

79

(see Figure 7 and Table 2) [12].

Even though cyanoacrylate fixtures and cures are relatively fast, there are various types with different fixture times depending on application requirements because some applications require a very fast fixture and others require some time to adjust the parts location then fix it. It is not recommended to apply too much adhesive as it needs more time to cure and will slow down the fixture time.

As cyanoacrylate cures by moisture, the lack of moisture causes a slow cure of adhesive. In this situation, accelerator can be used to speed up the cure even in dry environment. Accelerator can be applied to substrate before adhesive application or can be applied after adhesive application. Both ways can speed up the cure. The latter is often used when some part is secured by adhesive in an open area without

When bonding a difficult to bond plastic or rubber, polyolefin primer can be

The proper storage of cyanoacrylate adhesive is crucial for the best product shelf life. Refrigeration (4C) in an unopened state is recommended. If adhesive bottle opened already, it is recommended to close the bottle tightly and keep it at room

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.

The monomer chemistry of cyanoacrylate consists of methyl grade, ethyl grade,

This is the first-generation cyanoacrylate. Just like the chemistry, the molecular

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

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

temperature and continue to use it. It is not necessary to be refrigerated.

two-part bonding such as wire tacking on a PCB board, etc.

used to enhance the bonding reliability.

3.3.2 Cyanoacrylate adhesive characteristics

3.3.3 Cyanoacrylate by the monomer chemistry

Figure 5. CA anionic reaction 1.

Figure 6. CA anionic reaction 2.

The adhesive contains acidic stabilizer, and when adhesive was applied to a substrate surface, the acidic stabilizer that keeps the adhesive stable reacts with moisture on the surface. As the acidic stabilizer is consumed by moisture on the surface, the adhesive becomes unstable, and monomer reacts to each other, and the negative partial load is then passed to the next monomer in a chain reaction as seen in Figure 6 [12].

The polymerization continues until no monomer is available or a molecule with positive partial load (e.g., stabilizer) is reached.

#### 3.3.1 General characteristics

#### 3.3.1.1 Rule of thumb

The rule of thumb for CA adhesives such as the lower the viscosity, the faster the curing speed; the higher the viscosity, the slower the curing speed, and it generates while curing(exothermic reaction); the thinner the bond line, the faster the curing time, and the bigger the package size, the longer the shelf life, and not recommended for glass or glazed ceramics, and not ideal for a large area bonding, and liquid monomer can cause a stress crack on some sensitive plastics, and some product may cause a blooming.

#### 3.3.1.2 Cure versus bond line

To achieve the fastest cure to handling strength, a "zero gap" condition is desirable.

#### 3.3.1.3 Cure versus moisture

The best results are achieved when the relative humidity in the working environment is 40–60% at room temperature. Lower humidity leads to slower curing; higher humidity accelerates it but may impair the final strength of the bond. Longer cure times, however, slow down production. With the help of a humidity control system, favorable humidity levels can be kept constant in the bonding workplace.

#### 3.3.1.4 Cure versu surface pH

Acidic surfaces (pH value <7) may delay or even prevent curing, whereas alkaline surfaces (pH value >7) accelerate curing.

#### 3.3.1.5 Cure versus open time

Even though cyanoacrylate fixtures and cures are relatively fast, there are various types with different fixture times depending on application requirements because some applications require a very fast fixture and others require some time to adjust the parts location then fix it. It is not recommended to apply too much adhesive as it needs more time to cure and will slow down the fixture time.
