ADHESIVES

An adhesive is a substance which is used to join two or more parts so as to form a single unit. Adhesives are largely used in the manufacture of veneers, plywoods and other laminated wooden products and for fixing wall and ceiling linings, tiles and other floor coverings. The application of adhesives has many advantages over the conventional methods of bolting, riveting and welding. Glue is a general term which is used to indicate an adhesive substance. Following are the various types of adhesives.

1. Albumin glue: It is a glue of better quality. It is not attacked by water. It is used for making furniture.
2. Animal protein glue: It is obtained by boiling waste pieces of skins, bones, etc. of animals in hot water. Animal glue develops strong and tough joints and it is easy to apply. But it is affected by damp and moist conditions.
3. Glue from natural resins: It is prepared from natural resins. It is used for labelling, building paper, etc.
4. Glue from synthetic resins: This glue is based on synthetic resins. It may be either thermo-setting glue or thermo-plastic glue. The thermo-setting glue becomes permanent once it is set. The thermo plastic glue can be made plastic again by reheating.
5. Nitrocellulose glue: It is prepared from pyroxylin, which is a nitrated cellulose. It is derived by treating cellulose with nitric acid. It produces films, which strongly adhere to glass.
6. Rubber glue: It is prepared by dissolving rubber in benzene. It is used for joining rubber, plastics, glass, etc.
7. Special glue: It is specially prepared to join metals. Cycle weld is a modified form of rubber and it is used to join aluminium sheets. Araldite is another variety of special glue. It is used to join light metals.
8. Starch glue: It is prepared from vegetable starch. It has good strength in dry conditions. But it is not moisture resistant. It is cheap and is used from interior quality of plywood.
9. Vegetable glue: It is prepared from natural gums and starches. It is used for preparing paper board articles, labelling, etc.

11.6.1 Advantages of adhesives

1. A wide variety of combinations in joining is possible.
2. It can be used for bonding the surface of glass, metal, plastics and wood.
3. It creates a massive effort.
4. It is possible to prevent corrosion between different metals joined by adhesives.
5. It produces adequate strength.
6. The process of applying adhesives is easy, economical and speedy.

11.6.2 Disadvantages of adhesives

1. It is not possible to adopt any adhesive for all substances. Depending upon the properties of substances to be joined, a suitable adhesive has to be selected.
2. The adhesive substance does not become strong immediately after its application.
3. The adhesive substance generally does not remain stable at high temperatures.