Functional Adhesive Trend for Assembly Industry

ChulSoo Woo

#### Abstract

Recent applications in the industry require more and more cost-saving, more effective, and more reliable assembly of various substrate parts that are used for end-use product for manufacturers. This chapter can provide an insight on the solutions of different functional bonding and sealing technologies available to manufacturers who have used conventional methods for a long time which help them open their eyes to new solutions that can provide a faster, lighter, lower cost yet achieve more reliable assemblies resulting in more competitive assemblies in the market place. Various technologies provide an optimum solution to different industries and market segments for OEM industry such as appliance, HVAC, filter, medical, construction/agricultural equipment, recreational vehicles, railway, marine, electric motor, loudspeaker, elevator, small engine, valves, pumps, hydraulic system, etc. and for repair industry such as power plant, steel mill, mining, and car repair markets, too.

Keywords: anaerobic, cyanoacrylate, light cure (ultraviolet and visible), acrylic structural (two-part and no-mix), acrylic structural (two-part and pre- and post-mix), silicone, polyurethane

#### 1. Introduction

Adhesives have been a long history ever since the presence of mankind on this earth. It was found out that the jewelry box unearthed from the tomb of ancient Egyptian Pharaoh Tutankhamen had used a glue to assemble it with a beautiful appearance. When the Genghis Khan of Mongolian Empire had conquered the most of Asia and some Europe, the most powerful weapon they had used a bow made of a buffalo bone with laminations bonded with a glue which had become much more powerful than that of enemies. Globally famous musical instrument Stradivarius violin had been found out that a flue was used for laminations that help generate a beautiful sound for generations to come. Through Industrial Revolution, the key trend was how many and how fast they can manufacture within a short amount of time. Nowadays assembly industries have been looking for ways to help how they can manufacture faster yet increase the reliability of their products more. Those functional reactive adhesive and sealants featuring their benefits and typical applications will be introduced in this article in comparison with conventional assembly methods.

They are mainly machinery adhesives (anaerobic), light cure adhesives (LCA), cyanoacrylate adhesives (CA), acrylic structural adhesives, silicone and polyurethane, etc.

### 2. Overview of adhesive solution

Bonding solution has provided a faster assembly, lighter assembly, and more reliable assembly than various conventional mechanical assemblies. The following section will help you understand the features and benefits of reactive adhesive and sealant technology with the target of explaining the adhesive engineering terminology in order to understand a common basis for the following chapters.

according to the curing property. They are anaerobic reaction, exposure to light (ultraviolet and visible—also secondary curing option), anionic reaction (cyanoacrylate), activation system (modified acrylics), and moisture curing (silicones, poly-

When adhesive is applied onto one side of a metal substrate, adhesives contact with metal ion (Fe2+ and Cu2+) for some anaerobic initiator to be broken into free radicals which are unstable, but the presence of air to contact adhesive is stronger; therefore it remains as a liquid. As a next step, when the other mating part comes in contact with the first part with adhesive in between, more anaerobic initiators contact more metal ions to be broken into more free radicals, and at the same time, oxygen contact is blocked by both substrates; then those unstable free radicals react with a monomer nearby which becomes unstable, too, and then it reacts with

In case of inactive metal substrates that contain low metal ions such as Cu2+ or Fe2+ and have a slow reaction with photo initiator and slow cure, an acceleration method such as primer and heating can be used to speed up the curing. Anaerobic

This cured thermoset plastic property provides an excellent resistance to various chemical environment, excellent resistance against external impact, and vibrational force due to its mechanical "keyway" effect and high temperature environment up to 180°C and some special formulation is up to 350°C. It also has a controlled viscosity for optimum dispensing and controlled cure speed depending on the requirement of the application. This allows a normal machining by saving a machining cost, and it also allows an automatic dispensing of adhesive for more

Curing, especially the reaction speed of anaerobic products, is mainly influenced by the following: substrates, as some are active substrates that contain a rich metal ion, whereas inactive substrates contain less metal ion; the bond line gap, as the reaction takes a short time with a thin bond line, while a thick bond line takes a long time to polymerize; temperature, as elevated temperature activates the reaction of adhesive, while low temperature slows down the reaction of adhesive; and activa-

tor, as it will activate the inactive surface as if an active substrate is used.

other monomers nearby, and gradually all the monomers react and become

adhesives become a very reliable thermoset plastic after cure [2].

urethane).

3.1 Adhesives cured by anaerobic cure

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

polymerized (cure) (see Figure 1) [1].

3.1.1 Anaerobic adhesive features

effective manufacturing.

Figure 1.

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Anaerobic cure mechanism.

#### 2.1 Universal truth on bonding

Wood is not a proper substrate for welding as it will burn, not melt, plastic is not a proper substrate for soldering or brazing as it will melt away, and steel is not a proper substrate for nail as it will not bond and cause a localized high stress peak. However, adhesive can bond wood, plastic, and steel without burning, without melting and without having a high localized stress peak.

#### 2.2 The adhesive joint

Adhesives are connecting between two substrate surfaces, where they are of the same or different materials. The bonding mechanism depends on the adhesion and cohesion.

#### 2.3 Adhesion

"Adhesion" is an interaction between adhesive and substrate surfaces. There are two main parameters for achieving a good bonding. One is molecular interactions called "van der Waals" force, and another one is a mechanical interaction called "mechanical keyway" in the assembly. Adhesive was applied on the surface flows, due to its rheology, into the valley of surface roughness to fill and grip the surface. This way the mechanical keyway assembly can exhibit a high resistance against external vibration or impact. In case of low surface energy substrates such as Teflon, polypropylene, and polyethylene, adhesive wetting the surface flow into the valley is not easy resulting in improper mechanical keyway effect. In order to overcome this, a surface treatment is required to increase the surface roughness and mechanical keyway effect such as primer, plasma and flame treatment, etc.

#### 2.4 Cohesion

"Cohesion" is an interaction between the molecules of adhesive monomer. This is a combination of "van der Waals" force and molecular reaction of adhesive monomers. This helps to have a toughness of the adhesive. This property is one of the important parameters especially when a reliable structural bonding against impact and vibration is required and multifunctionality of oligomer is used together with elastomers.

#### 3. Various functional adhesives

Most functional adhesives are reactive polymers; in most cases they change from liquid to solid through various chemical polymerization reactions. Numerous functional adhesives have been developed with special curing properties for specific application situations. Those adhesives can be classified into the following groups

2. Overview of adhesive solution

Adhesives and Adhesive Joints in Industry Applications

2.1 Universal truth on bonding

2.2 The adhesive joint

and cohesion.

2.3 Adhesion

2.4 Cohesion

with elastomers.

70

3. Various functional adhesives

Bonding solution has provided a faster assembly, lighter assembly, and more reliable assembly than various conventional mechanical assemblies. The following section will help you understand the features and benefits of reactive adhesive and sealant technology with the target of explaining the adhesive engineering terminol-

Wood is not a proper substrate for welding as it will burn, not melt, plastic is not a proper substrate for soldering or brazing as it will melt away, and steel is not a proper substrate for nail as it will not bond and cause a localized high stress peak. However, adhesive can bond wood, plastic, and steel without burning, without

Adhesives are connecting between two substrate surfaces, where they are of the

"Adhesion" is an interaction between adhesive and substrate surfaces. There are two main parameters for achieving a good bonding. One is molecular interactions called "van der Waals" force, and another one is a mechanical interaction called "mechanical keyway" in the assembly. Adhesive was applied on the surface flows, due to its rheology, into the valley of surface roughness to fill and grip the surface. This way the mechanical keyway assembly can exhibit a high resistance against external vibration or impact. In case of low surface energy substrates such as Teflon, polypropylene, and polyethylene, adhesive wetting the surface flow into the valley is not easy resulting in improper mechanical keyway effect. In order to overcome this, a surface treatment is required to increase the surface roughness and

same or different materials. The bonding mechanism depends on the adhesion

mechanical keyway effect such as primer, plasma and flame treatment, etc.

is a combination of "van der Waals" force and molecular reaction of adhesive monomers. This helps to have a toughness of the adhesive. This property is one of the important parameters especially when a reliable structural bonding against impact and vibration is required and multifunctionality of oligomer is used together

"Cohesion" is an interaction between the molecules of adhesive monomer. This

Most functional adhesives are reactive polymers; in most cases they change from liquid to solid through various chemical polymerization reactions. Numerous functional adhesives have been developed with special curing properties for specific application situations. Those adhesives can be classified into the following groups

ogy in order to understand a common basis for the following chapters.

melting and without having a high localized stress peak.

according to the curing property. They are anaerobic reaction, exposure to light (ultraviolet and visible—also secondary curing option), anionic reaction (cyanoacrylate), activation system (modified acrylics), and moisture curing (silicones, polyurethane).

### 3.1 Adhesives cured by anaerobic cure

When adhesive is applied onto one side of a metal substrate, adhesives contact with metal ion (Fe2+ and Cu2+) for some anaerobic initiator to be broken into free radicals which are unstable, but the presence of air to contact adhesive is stronger; therefore it remains as a liquid. As a next step, when the other mating part comes in contact with the first part with adhesive in between, more anaerobic initiators contact more metal ions to be broken into more free radicals, and at the same time, oxygen contact is blocked by both substrates; then those unstable free radicals react with a monomer nearby which becomes unstable, too, and then it reacts with other monomers nearby, and gradually all the monomers react and become polymerized (cure) (see Figure 1) [1].

In case of inactive metal substrates that contain low metal ions such as Cu2+ or Fe2+ and have a slow reaction with photo initiator and slow cure, an acceleration method such as primer and heating can be used to speed up the curing. Anaerobic adhesives become a very reliable thermoset plastic after cure [2].

### 3.1.1 Anaerobic adhesive features

This cured thermoset plastic property provides an excellent resistance to various chemical environment, excellent resistance against external impact, and vibrational force due to its mechanical "keyway" effect and high temperature environment up to 180°C and some special formulation is up to 350°C. It also has a controlled viscosity for optimum dispensing and controlled cure speed depending on the requirement of the application. This allows a normal machining by saving a machining cost, and it also allows an automatic dispensing of adhesive for more effective manufacturing.

Curing, especially the reaction speed of anaerobic products, is mainly influenced by the following: substrates, as some are active substrates that contain a rich metal ion, whereas inactive substrates contain less metal ion; the bond line gap, as the reaction takes a short time with a thin bond line, while a thick bond line takes a long time to polymerize; temperature, as elevated temperature activates the reaction of adhesive, while low temperature slows down the reaction of adhesive; and activator, as it will activate the inactive surface as if an active substrate is used.

Figure 1. Anaerobic cure mechanism.

#### 3.1.2 Active and inactive metal substrates

Anaerobic products react fast to active metal substrates and react slow to inactive substrates. Therefore, it is important to understand what type of substrates they have for bonding and sealing application. Active substrates contain more Cu2+ and Fe2+ ions, while inactive substrates have less Cu2+ and Fe2+ ions (see Table 1) [1].

3.1.5 Typical applications

3.1.5.1 Thread locking

machined metal flanges against fluid leakage.

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

spring washer, nylon lined nut, serrated bolt, etc.

3.1.5.1.1 Conventional fastening methods

3.1.5.1.2 Liquid thread locking adhesive

3.1.5.2.1 Conventional thread sealing method

oethylene (PTFE) tape, O-ring, pipe dope, etc.

3.1.5.2 Thread sealing

has a low pressure sealing [5].

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3.1.5.2.2 Liquid thread sealing adhesive

There are four major applications, namely, thread locking application for locking bolt and nut assembly, or stud and tap assembly, in place against loosening; thread sealing application for sealing various fittings and hydraulic and pneumatic thread connections against fluid leakage; retaining application for shaft and gear, bearing, and housing against slippage or free spinning; and gasketing application for

Those conventional methods used in the assembly industry are flat washer,

They have shortcomings such as the following: they loosen under vibration, thermal expansion, and/or plastic deformation, they do not seal thread as the space between bolt and nut is all empty, they require extensive inventory of several fastener shapes and sizes, they are prone to rust and damaged surface due to empty space and poor handling, they have a wide bolt tension scatter as each bolt assembled with the same tightening torque shows a different bolt tension on a flange, they show lower break loose torque than tightening torque resulting in unstable assem-

Those liquid thread locking adhesives have the following benefits: they lock bolt and nut in place, they seal against leakage and internal pressure, they prevent thread corrosion, they provide a controlled lubricity for consistent bolt tension on a flange, they provide a torque augmentation with higher break loose torque than tightening torque, they provide a controlled strength for easy disassembly for repair, they have a

Those conventional methods used in the assembly industry are polytetrafluor-

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

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].

bly, and they are more costly than liquid thread locking adhesive [3].

low inventory with less storage space, and they have a cost saving [4].

#### 3.1.3 Anaerobic shelf life

Anaerobic liquid products need a constant oxygen contact to stay as a liquid and to become stable. When a new bottle was opened and the adhesive product level is only half of the container, it is important to know that it is to ensure that adhesive is in constant contact with oxygen in the container. Therefore, if the product's net amount is "250 ml," then the total volume of packing is "500 ml"; likewise in case of a "50 ml" product, the total volume of packaging is "100 ml."

Anaerobic adhesives have a different shelf life depending on the size of the packaging. The smaller the packaging, the longer the shelf life; in other words, the bigger the packaging, the shorter the shelf life. The whole mechanism behind this is that for smaller packaging, the distance oxygen penetrated into the center of adhesive product is shorter, so the whole amount is under the influence of oxygen; however for bigger packaging, the distance of oxygen to penetrate into the center of adhesive product is longer, so the monomers in the center of adhesive products have less amount of oxygen contact and become unstable resulting in increased viscosity and premature reaction of monomers inside the packaging.

#### 3.1.4 Recent technological trend

Recent technological trend of anaerobic adhesive and sealants is to bring the performance of core products to a next level so that those few core products can cover various application requirements; in the past, there were separate products to meet fast cure, high temp resistance, oily surface cutting, active reaction to inactive substrates, etc. However currently, those few core products can meet most of the above requirements at the same time such as this adhesive has a high temperature resistance up to 180°C (constant condition) so it can be used for those applications that could not be used due to high temp requirement; this adhesive has fast reaction to inactive substrates such as stainless steel, aluminum and zinc plated steel, and this adhesives cuts through a slight oil coating on the substrate and it adheres well to the substrate; therefore no special cleaning is required.


#### 3.1.5 Typical applications

3.1.2 Active and inactive metal substrates

Adhesives and Adhesive Joints in Industry Applications

(see Table 1) [1].

3.1.3 Anaerobic shelf life

3.1.4 Recent technological trend

Brass Bronze Copper Iron Steel

Table 1.

72

Active and inactive substrates.

Anaerobic products react fast to active metal substrates and react slow to inactive substrates. Therefore, it is important to understand what type of substrates they have for bonding and sealing application. Active substrates contain more Cu2+ and Fe2+ ions, while inactive substrates have less Cu2+ and Fe2+ ions

Anaerobic liquid products need a constant oxygen contact to stay as a liquid and to become stable. When a new bottle was opened and the adhesive product level is only half of the container, it is important to know that it is to ensure that adhesive is in constant contact with oxygen in the container. Therefore, if the product's net amount is "250 ml," then the total volume of packing is "500 ml"; likewise in case of a "50 ml" product, the total volume of packaging is "100 ml." Anaerobic adhesives have a different shelf life depending on the size of the packaging. The smaller the packaging, the longer the shelf life; in other words, the bigger the packaging, the shorter the shelf life. The whole mechanism behind this is that for smaller packaging, the distance oxygen penetrated into the center of adhesive product is shorter, so the whole amount is under the influence of oxygen; however for bigger packaging, the distance of oxygen to penetrate into the center of adhesive product is longer, so the monomers in the center of adhesive products have less amount of oxygen contact and become unstable resulting in increased

viscosity and premature reaction of monomers inside the packaging.

the substrate; therefore no special cleaning is required.

Recent technological trend of anaerobic adhesive and sealants is to bring the performance of core products to a next level so that those few core products can cover various application requirements; in the past, there were separate products to meet fast cure, high temp resistance, oily surface cutting, active reaction to inactive substrates, etc. However currently, those few core products can meet most of the above requirements at the same time such as this adhesive has a high temperature resistance up to 180°C (constant condition) so it can be used for those applications that could not be used due to high temp requirement; this adhesive has fast reaction to inactive substrates such as stainless steel, aluminum and zinc plated steel, and this adhesives cuts through a slight oil coating on the substrate and it adheres well to

Active materials (speed cure) Passive materials (slow cure)

Aluminum Ceramics Chromate films Glass High-alloy steel Nickel Oxide films Plastics Stainless steel Zinc

There are four major applications, namely, thread locking application for locking bolt and nut assembly, or stud and tap assembly, in place against loosening; thread sealing application for sealing various fittings and hydraulic and pneumatic thread connections against fluid leakage; retaining application for shaft and gear, bearing, and housing against slippage or free spinning; and gasketing application for machined metal flanges against fluid leakage.

### 3.1.5.1 Thread locking
