Gluing

Gluing for assembly of plastic parts is an effective method of making permanent connections. This method produces aesthetically clean looking joints with low weight and sufficiently strong connections. This is a very effective joining method for heat sensitive plastics which would normally deform if welded.

The most common types are

1. Solvent bonding 

2. Adhesive bonding 

1. Solvent bonding: Solvent bonding or solvent welding is a process in which the surfaces of parts to be joined are treated with a solvent. 

1. This swells and softens the surfaces and by applying pressure on the joint and with the evaporation of the solvent, the two surfaces bond.

2. The matching surfaces must be clean and free of grease before bonding. 

3. Parts having a single joining surface are simply pressed against a sponge or felt pad that has been impregnated with solvent. 

4. The quantity of solvent used should be kept to a minimum to avoid drips and crazing. 

5. It may be necessary to allow a few seconds to ensure sufficient swelling. 

6. The parts are then clamped together with a moderate pressure. 

7. The parts are removed from the clamping equipment and must not be used for a period of 24 to 48 hours to ensure that full strength has been achieved. 

8. Heat can be used to accelerate the overall rate of evaporation and cycle time.

9. The process is commonly used with amorphous thermoplastics.

Advantages 

1. Homogeneous distribution of mechanical loads

2. Good aesthetics 

3. Economic assembly

4. Low weight, no heavy screws, bolts and nuts

5. Heat sensitive constructions or materials, which welding would distort or destroy, can be joined

6. Good sealing and insulating properties.

Disadvantage

1. Entrapment of solvent in the joint

2. Stress cracking or crazing

3. Dissimilar materials can only be joined if both are soluble in a common solvent or in a mixture of solvents.

4. Differences in thermal expansion of components are not compensated if dissimilar materials are bonded.

5. Reproducibility/process control

6. High solvent evaporation time due to its entrapment in the polymer matrix

7. No disassembly possible

8. Assembly hazards such as fire or toxicity.

2. Adhesive Bonding:

Adhesive bonding is based on the following five theories:

1. Mechanical Locking

2. Electronic Theory

3. Theory of boundary layers and interfaces  

4. Adsorption Theory

5. Diffusion Theory

6. Chemical Bonding Theory

1. Mechanical Locking:

The mechanical interlocking model conceives mechanical keying or interlocking of the adhesives into the cavities, pores and asperities of the solid surface to be the major factor in determining the adhesive strength. 

A high level of adhesion should be achieved by improving both the surface morphology and physico-chemical surface properties of substrate and adhesive. In most cases the enhancement of adhesion by Mechanical Keying can be attributed simply to the increase in interfacial area due to surface roughness in so far as the wetting conditions are fulfilled to permit penetration of the adhesive in to pores and cavities. 

2. Electronic Theory:

The electronic theory of adhesion suggests that an electron transfer mechanism between the substrate and the adhesive having different electronic bond structures. This phenomenon could induce the formation of a double electrical layer at the interface. It is consider that adhesive strength results from the attractive electro-static forces across the electrical double layer. 

3. Theory of boundary layers and interfaces:

It is known that alterations and modifications of the adhesive / or adherend can be found in the vicinity of the interface leading to the formation of an interfacial zone exhibiting properties that differ from those of the bulk materials. 

The first approach to this problem is due to cohesive strength of a weak boundary layer can always be considered as the main factor in determining the level of adhesion even when failures appears to be interfacial. According to this assumption the adhesion energy is always equal to the cohesive energy of the weaker interfacial layer.

4. Adsorption Theory:

According to this theory, adhesive will adhere to the substrate because of interatomic and intermolecular forces established at the interface. The most common interfacial forces results from Van der Wall’s and Lewisacid-base interactions. Generally the formation of an assembly goes through a liquid-solid contact step and therefore criteria of good adhesion become essentially criteria of good wetting. 

5. Diffusion Theory: 

The diffusion theory of adhesion is based on the assumption that the adhesion strength of polymers to themselves or to each other is due to mutual diffusion of macro molecules across the interface thus creating an interface. 

6. Chemical Bonding Theory:

Chemical bonds formed across the adhesive substrate interface can greatly participate to the level of adhesion between both materials. These bonds are generally considered as primary bonds in comparison with physical interactions such as Van der Wall’s force of attraction which are called secondary force interactions. 

The main criteria for achieving good adhesive bonding are surface wetting and curing of the adhesive. Important variables for applying and distributing the adhesive on a substrate are the surface contact angle, adhesive viscosity and the chemical resistance of the substrate to the adhesive. 

Advantages 

1. Application on various substrates like thermoplastics, thermosets, elastomers and metals

2. Homogenous distribution of mechanical loads

3. Differences in thermal expansion of components can be compensated for by using a thick adhesive layer

4. Good aesthetics / no special requirements to hide the bond.

5. Economic assembly

6. Low weight, no heavy screws, bolts and nuts

7. Heat sensitive constructions or materials, which welding would distort, can be joined

8. No thermal stresses introduced.

9. Good sealing and insulating properties. 

Limitations 

1. Long term behavior may not be very good

2. Stress cracking or crazing of the plastic may take place.

3. Dissimilar materials can only be joined if both are compatible with the adhesive.

4. Reproducibility/process control.

5. Curing time can be high depending on the adhesive.

6. No disassembly possible.

7. Assembly hazards such as fire or toxicity.