**3. Assembly solutions**

In this section, we are going to list up and review major solutions that have been used in the industry and new solutions that can be used in this industry. Conventional methods consist of mainly double-sided tape, 1 component silicone, and mechanical fasteners, and new special adhesive solutions are polyurethane hot melt technology and 2 component silicone technology. We will review characteristics of each solution.

#### **3.1 Double-sided tape**

This technology has been used widely in the assembly industry. It has had a wide range of awareness in the market place. It can fix the parts within 1 min and no

#### *Global Trend of Glass Bonding for Appliance Industry Assemblies DOI: http://dx.doi.org/10.5772/intechopen.90515*

chemical reaction is required. But at the same time, it requires a high laborintensive pre-work such as priming the surface with a primer to improve a bonding durability. It also requires a peel-off of release paper on one side and place the sticky part on the application area mostly manually. It is very difficult to re-adjust the location once it is placed on the substrate, and very high labor cost is required if it has to be re-adjusted. It has to be pressed for a certain amount of time; otherwise, there is no good affinity achieved between the sticky surface of tape and the substrate. The de-bonding happens when bonding a surface not completely flat. Tape solution also generates a bulky waste of release paper. Tape solution tends to release the bonding property under aging condition; therefore, a gap is generated in the glass bonding area as time goes causing a repair. The adhesion of tape is the surface temperature dependent; therefore, if the temperature of assembly line, tape, and substrate was not properly controlled, especially in the winter time due to the effect of cold temperature on the bonding durability of tape, it generates a poor adhesion to substrate and consequently a bonding failure, scrap, and liability issue [6].

### **3.2 Mechanical fastener**

**2.5 Air conditioner**

*Advanced Functional Materials*

**2.6 Dish washer**

**2.8 Styler**

**2.7 Kitchen ventilation hood**

glass bonding in front of styler.

**3. Assembly solutions**

**3.1 Double-sided tape**

**296**

Air conditioner has a different type of design that can adopt glass plate bonding in terms of shape and function such as window type, wall type, standalone vertical type, and ceiling type. The most popular application is for bonding of a glass plate to plastic frame for wall type air conditioner. This type of application needs a reliable bonding solution as the glass might fall off from air conditioner and cause a safety problem for people if an improper solution is used. The below **Figure 5** shows a

Dish washer has a different type in terms of shape and function that can use a glass bonding application such as table top, built in stand-alone type, and drawer type that can adopt glass bonding application in the front side. The most popular application is for bonding of glass plate to plastic front door for stand-alone type. The below

Kitchen ventilation hood can have three glass plate bonding application for its function of easy cleaning of contamination from cooking and high-end image and appearance. This type of design has the glass plate in the middle which is in a closed state. When it is turned on, the glass plate in the middle opens up and the fan inside sucks those fumes, smokes, and microparticles generated during cooking. For a situation where the glass plate is contaminated with oil due to splash of boiling oil during cooking, it can be wiped off easily for clean surface. The most popular application is for bonding of glass plate to moving chassis. Other applications are for bonding of glass to control panel chassis and bonding of glass to other chassis. The below **Figure 7** shows an overall glass bonding area of kitchen ventilation hood.

Styler has a glass plate bonding application such as bonding of glass plate to plastic frame on the front side of it. It is a stand-alone type and has a function of eliminating dust by vibrating the clothe, deodorization by vacuum fan, sterilization by steam heating, and ironing of the clothe by heat pressing overnight inside it and next day they can take it out of the styler and put it on. The below **Figure 8** shows a

In this section, we are going to list up and review major solutions that have been used in the industry and new solutions that can be used in this industry. Conventional methods consist of mainly double-sided tape, 1 component silicone, and mechanical fasteners, and new special adhesive solutions are polyurethane hot melt technology and 2 component silicone technology. We will review characteristics of each solution.

This technology has been used widely in the assembly industry. It has had a wide range of awareness in the market place. It can fix the parts within 1 min and no

**Figure 6** shows a glass bonding for stand-alone type washing machine [5].

glass bonding to plastic frame of wall type air conditioner [4].

This technology has been available with a history of mankind for a very long time. This can provide a very fast mechanical fixture and consequent assembly, but at the same time, this requires mechanical fasteners and drilled thread holes on both parts, and operator has to assemble them by using fastening tools manually. The assembly leaves a screw mark that is not good in appearance and it is also difficult to make an automation assembly line, too [7].

#### **3.3 1 component RTV silicone**

This technology is a room temperature vulcanized (RTV) silicone that reacts with moisture in the air and substrate, and generates some by-products such as oxime, alcohol, acetic acid, and acetone. This technology in general has a high viscosity with a slow curing property that takes a few days for functional bonding and more than 1 week for a complete curing. Normally this technology requires mechanical clips or double-sided tapes for initial fixture time and a stacking space and lengthy curing time before it goes to the next process. Some process adopts 1 component RTV silicone or double-sided tape which requires more manual process and manpower cost [8].

#### **3.4 Polyurethane hot melt**

This technology is at first a thermoplastic (remeltable) material that contains some free isocyanate groups, and then, it is similar with 1 component polyurethane technology that reacts with moisture to form a tough thermoset (not meltable) plastic material. Its initial thermoplastic property can melt easily and provide a very excellent high position tack capability that can hold the two substrates in place by losing an energy from molten liquid state, and also its thermoset property provides a high temperature heat and creep resistance with a good durability. Polyurethane hot melt is available with a wide range of open time. This technology has a good adhesion and is compatible with a wide variety of substrates. It has better bonding strength performance and environment resistance compared to normal hot melt due its cross-linked chemical reaction property. It has a relatively lower operation temperature around 120°C; therefore, a dispensing at lower energy is possible. It also has longer open time 1–4 min; therefore, it has a wide bonding window for

### *Advanced Functional Materials*

assembly. This technology is just like other 1 component and 2 component polyurethane adhesives, and during the reaction, generates some carbon dioxide (CO2) to evaporate.

Additional characteristics are excellent toughness, flexibility, high elongation, excellent gap filling, paintable when cured, and excellent chemical resistance.

As shown in **Figure 9**, isocyanate NCO reacts with moisture in air and substrate and becomes an oligomer with by-product carbon dioxide evaporating; then, these oligomers react each other to form a cured polyurethane [9].

As shown in **Figure 10**, polyurethane hot melt is in a solid state in the sealed aluminum foil packaging to prevent the ingress of moisture in the solid product that can react to become a thermoset plastic. After the product becomes a molten state at elevated temperature by melting machine, it is dispensed onto the substrate, and substrates are mated at "a: joining time", as it loses the energy, and it solidifies just

like normal hot melt and starts to build an initial bonding strength "d" at "b: holding time." Once it solidifies by losing energy, it starts to react with moisture and cures chemically to form a much higher bonding strength at "curing time c" and finally comes to a final bonding strength "e." Therefore, a very fast fixturing and reliable bonding can be achieved with this technology. Polyurethane hot melt is dispensed by melting machine and robot or by roll coating machine; in this chapter, for glass

When part A and part B are mixed in a proper mixing ratio, a chemical reaction occurs which is 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 fast tack free, fixture and curing time, and cure speed not affected by moisture, and cure speed less affected by bond line thickness, and excellent thermal resistance ("up to 270°C"), and flexible, tough, high modulus, and agency UL 746°C listed for high temperature applications for appliance, and excellent gap filling, and black, gray and almond colors are

First of all, in this section, we are going to compare polyurethane hot melt adhesive solution vs. tape solution based on parameters such as assembly process, environment reliability, tensile shear strength against open time, cross pull strength against fixture time, design benefit, air pockets on bond line, bond line thickness, creep resistance, dispensing equipment, and overall value analysis. The application

Tape solution is to apply primer to plastic frame by 1 worker (5 s) ! apply tape to plastic frame and cut manually by 2 workers (20–40 s) ! peel off release paper manually by 2 workers (5 s) ! place glass plate onto the plastic frame manually by 1 worker (10 s) ! compress the glass plate by pressing (5–10 s) ! next process. This

is for bonding of glass to plastic frame for various refrigerator doors [1].

bonding application, melting machine and robot are used mostly [9].

*Global Trend of Glass Bonding for Appliance Industry Assemblies*

*DOI: http://dx.doi.org/10.5772/intechopen.90515*

as a by-product comes out and evaporates (see **Figure 11**) [8].

**3.5 2 component silicone**

*2C silicone curing mechanism.*

**Figure 11.**

available [8].

**4. Refrigerator**

**4.1 Assembly process**

**299**

solution takes a total of 4 workers and 70 s.

**Figure 9.** *PUR HM curing mechanism with moisture.*

**Figure 10.** *PUR HM curing strength vs. curing time.*

*Global Trend of Glass Bonding for Appliance Industry Assemblies DOI: http://dx.doi.org/10.5772/intechopen.90515*

**Figure 11.** *2C silicone curing mechanism.*

assembly. This technology is just like other 1 component and 2 component polyurethane adhesives, and during the reaction, generates some carbon dioxide (CO2) to

Additional characteristics are excellent toughness, flexibility, high elongation, excellent gap filling, paintable when cured, and excellent chemical resistance.

As shown in **Figure 10**, polyurethane hot melt is in a solid state in the sealed aluminum foil packaging to prevent the ingress of moisture in the solid product that can react to become a thermoset plastic. After the product becomes a molten state at elevated temperature by melting machine, it is dispensed onto the substrate, and substrates are mated at "a: joining time", as it loses the energy, and it solidifies just

oligomers react each other to form a cured polyurethane [9].

As shown in **Figure 9**, isocyanate NCO reacts with moisture in air and substrate and becomes an oligomer with by-product carbon dioxide evaporating; then, these

evaporate.

*Advanced Functional Materials*

**Figure 9.**

**Figure 10.**

**298**

*PUR HM curing strength vs. curing time.*

*PUR HM curing mechanism with moisture.*

like normal hot melt and starts to build an initial bonding strength "d" at "b: holding time." Once it solidifies by losing energy, it starts to react with moisture and cures chemically to form a much higher bonding strength at "curing time c" and finally comes to a final bonding strength "e." Therefore, a very fast fixturing and reliable bonding can be achieved with this technology. Polyurethane hot melt is dispensed by melting machine and robot or by roll coating machine; in this chapter, for glass bonding application, melting machine and robot are used mostly [9].

#### **3.5 2 component silicone**

When part A and part B are mixed in a proper mixing ratio, a chemical reaction occurs which is 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 as a by-product comes out and evaporates (see **Figure 11**) [8].

Two-component silicone characteristics are fast tack free, fixture and curing time, and cure speed not affected by moisture, and cure speed less affected by bond line thickness, and excellent thermal resistance ("up to 270°C"), and flexible, tough, high modulus, and agency UL 746°C listed for high temperature applications for appliance, and excellent gap filling, and black, gray and almond colors are available [8].

#### **4. Refrigerator**

First of all, in this section, we are going to compare polyurethane hot melt adhesive solution vs. tape solution based on parameters such as assembly process, environment reliability, tensile shear strength against open time, cross pull strength against fixture time, design benefit, air pockets on bond line, bond line thickness, creep resistance, dispensing equipment, and overall value analysis. The application is for bonding of glass to plastic frame for various refrigerator doors [1].

#### **4.1 Assembly process**

Tape solution is to apply primer to plastic frame by 1 worker (5 s) ! apply tape to plastic frame and cut manually by 2 workers (20–40 s) ! peel off release paper manually by 2 workers (5 s) ! place glass plate onto the plastic frame manually by 1 worker (10 s) ! compress the glass plate by pressing (5–10 s) ! next process. This solution takes a total of 4 workers and 70 s.

Polyurethane hot melt adhesive solution is to apply adhesive to plastic frame by machine (10 s) ! place glass plate onto the plastic frame manually by 1 worker or by machine (0–10 s) ! compress glass plate by pressing (5–10 s) ! next process.

substrate. A variation of tensile shear strength between adhesive and tape was checked and compared in **Figure 13**. As it is shown below, adhesive solution has 10 times of higher bonding strength than the tape solution from open time of 15–30 s to 2 min; and the adhesive solution still maintains 7–8 times of higher tensile shear strength than the tape solution from open time of 4 min to max 15 min. It explains that adhesive solution has a lot of assembly process freedom as same as tape solution, yet it achieves a much higher and reliable bonding strength. The realistic handling time that manufacturer allows is within 2 min after adhesive dispensing as the assemblies need to move to the next station as early as possible. Therefore, the quality of bonding strength with adhesive solution can be achieved in most appli-

*Global Trend of Glass Bonding for Appliance Industry Assemblies*

*DOI: http://dx.doi.org/10.5772/intechopen.90515*

Tape solution maintains the same level of tensile shear strength but in a much

Testing has been carried out to measure an initial fixture strength per holding time between adhesive and tape solution. An initial fixture time means a bonding

strength developed after dispensing and assembly. As shown in the below **Figure 14**, tape solution shows a higher initial fixture strength at 1 min holding time; however, as time goes by adhesive solution achieves higher strength than tape, and as time goes further, adhesive solution reacts with moisture. The holding time in **Figure 14** is the one adhesive solution that still maintains a solidified hot melt status which is not chemically reacted, yet shows higher strength after 2 min of holding time. As time goes further, the adhesive reacts with moisture and achieves a chemically reacted bonding strength which is at least 10 times higher than the tape solution as seen in **Figure 12**. The holding time of less than 2 min is not realistic condition as most of manufacturers use more than 2 min of holding time due to handling other parts until the next process. The substrate tested was glass bonded to

ance assembly process conditions.

**4.4 Cross pull strength vs. fixture time**

lower figure.

combined PC and ABS.

*Tensile shear strength vs. open time.*

*Cross pull strength vs. fixture time.*

**Figure 13.**

**Figure 14.**

**301**

This polyurethane hot melt adhesive solution takes only a total of 1 or no workers and 30 s, which means it requires almost no worker and a half process time compared to tape solution. This adhesive bonding solution can save 5 or 6 workers and can use them in other more important work places, can save overall assembly time in half, and can reduce assembly line space more compact and eliminate waste coming from release paper from tape solution. Tape solution requires a primer process for more reliable bonding as its adhesion decreases when the ambient temperature was relatively low; then, the complete wetting of tape onto the substrate was not fully established resulting in bonding reliability issue down the road, while adhesive bonding solution wets all the mating surfaces and penetrates all the surface imperfections and mechanically grip the surface after cure contributing more reliable bonding, and also much reduced process steps allow more reduction of quality check points, too [1].

## **4.2 Tensile shear strength vs. aging conditions**

Testing has been carried out to measure a tensile shear strength to compare the bonding reliability before and after aging condition between adhesive solution and tape solution. Substrate was a high temperature ink printed glass and a plastic combined with PC and ABS. Testing conditions were at room temperature (23°C), low temperature (�40°C for 7 days), high temperature (80°C for 7 days), high temperature, and high humidity (50°C, 95%RH for 8 h, 5°C 95%RH for 8 h as 1 cycle for 1000 h) and thermal cycle tests (60°C for 4 h to �30°C for 4 h as 1 cycle for 1000 h). As shown in **Figure 12**, adhesive solution has at least 10 times higher bonding strength for all aging conditions. It was shown that both adhesive and tape have increased their bonding strength a bit, but adhesive solution still has maintained at least 10 times higher bonding strength than the tape solution. If we review the thermal cycle test, strength has increased for adhesive solution, but it has decreased for tape; consequently, adhesive solution has 13 times of higher bonding strength than the tape solution. Overall, it is very clear that as far as environmental durability is concerned, adhesive solution provides a far more reliable bonding than the tape solution.

#### **4.3 Tensile shear strength vs. open time**

Testing has been conducted to measure a tensile shear strength per different open time which means the time after adhesive dispensing until mating the other

**Figure 12.** *Tensile shear strength vs. different aging conditions.*

#### *Global Trend of Glass Bonding for Appliance Industry Assemblies DOI: http://dx.doi.org/10.5772/intechopen.90515*

substrate. A variation of tensile shear strength between adhesive and tape was checked and compared in **Figure 13**. As it is shown below, adhesive solution has 10 times of higher bonding strength than the tape solution from open time of 15–30 s to 2 min; and the adhesive solution still maintains 7–8 times of higher tensile shear strength than the tape solution from open time of 4 min to max 15 min. It explains that adhesive solution has a lot of assembly process freedom as same as tape solution, yet it achieves a much higher and reliable bonding strength. The realistic handling time that manufacturer allows is within 2 min after adhesive dispensing as the assemblies need to move to the next station as early as possible. Therefore, the quality of bonding strength with adhesive solution can be achieved in most appliance assembly process conditions.

Tape solution maintains the same level of tensile shear strength but in a much lower figure.

#### **4.4 Cross pull strength vs. fixture time**

Polyurethane hot melt adhesive solution is to apply adhesive to plastic frame by machine (10 s) ! place glass plate onto the plastic frame manually by 1 worker or by machine (0–10 s) ! compress glass plate by pressing (5–10 s) ! next process. This polyurethane hot melt adhesive solution takes only a total of 1 or no workers and 30 s, which means it requires almost no worker and a half process time compared to tape solution. This adhesive bonding solution can save 5 or 6 workers and can use them in other more important work places, can save overall assembly time in half, and can reduce assembly line space more compact and eliminate waste coming from release paper from tape solution. Tape solution requires a primer process for more reliable bonding as its adhesion decreases when the ambient temperature was relatively low; then, the complete wetting of tape onto the substrate was not fully established resulting in bonding reliability issue down the road, while adhesive bonding solution wets all the mating surfaces and penetrates all the surface imperfections and mechanically grip the surface after cure contributing more reliable bonding, and also much reduced process steps allow more reduction

Testing has been carried out to measure a tensile shear strength to compare the bonding reliability before and after aging condition between adhesive solution and tape solution. Substrate was a high temperature ink printed glass and a plastic combined with PC and ABS. Testing conditions were at room temperature (23°C), low temperature (�40°C for 7 days), high temperature (80°C for 7 days), high temperature, and high humidity (50°C, 95%RH for 8 h, 5°C 95%RH for 8 h as 1 cycle for 1000 h) and thermal cycle tests (60°C for 4 h to �30°C for 4 h as 1 cycle for 1000 h). As shown in **Figure 12**, adhesive solution has at least 10 times higher bonding strength for all aging conditions. It was shown that both adhesive and tape

have increased their bonding strength a bit, but adhesive solution still has

maintained at least 10 times higher bonding strength than the tape solution. If we review the thermal cycle test, strength has increased for adhesive solution, but it has decreased for tape; consequently, adhesive solution has 13 times of higher bonding strength than the tape solution. Overall, it is very clear that as far as environmental durability is concerned, adhesive solution provides a far more reliable bonding than

Testing has been conducted to measure a tensile shear strength per different open time which means the time after adhesive dispensing until mating the other

of quality check points, too [1].

*Advanced Functional Materials*

the tape solution.

**Figure 12.**

**300**

**4.3 Tensile shear strength vs. open time**

*Tensile shear strength vs. different aging conditions.*

**4.2 Tensile shear strength vs. aging conditions**

Testing has been carried out to measure an initial fixture strength per holding time between adhesive and tape solution. An initial fixture time means a bonding strength developed after dispensing and assembly. As shown in the below **Figure 14**, tape solution shows a higher initial fixture strength at 1 min holding time; however, as time goes by adhesive solution achieves higher strength than tape, and as time goes further, adhesive solution reacts with moisture. The holding time in **Figure 14** is the one adhesive solution that still maintains a solidified hot melt status which is not chemically reacted, yet shows higher strength after 2 min of holding time. As time goes further, the adhesive reacts with moisture and achieves a chemically reacted bonding strength which is at least 10 times higher than the tape solution as seen in **Figure 12**. The holding time of less than 2 min is not realistic condition as most of manufacturers use more than 2 min of holding time due to handling other parts until the next process. The substrate tested was glass bonded to combined PC and ABS.

**Figure 13.** *Tensile shear strength vs. open time.*

**Figure 14.** *Cross pull strength vs. fixture time.*
