Adhesive

Any substance which is capable of attaching material together by surface action is an adhesive. The material attached by an adhesive are called adherents and the process are called adherents and the process by which they attached is called bonding.

ADHESIVE ACTION
There are three types of adhesive action.
1. Specific adhesive action: The attachment of surfaces by an adhesive is mostly by an adhesive is mostly due to chemical or primary bonds and physical or intermolecular force of attraction. This type of surface attachment is called specific adhesion.
2. Mechanical adhesion: This adhesive may enter the minute pore and voids of the porous or rough surface and attachment of surfaces occur due to the interlocking action of adhesive molecules. This type of surface attachment is called mechanical adhesion.
3. Fusion adhesion: The surface of attaching body may partially dissolve in an adhesion solution and attachment takes place on solidification. This type of adhesion is called fusion adhesion.

DEVELPOMENT OF ADHESION STRENGTH
Adhesive develop bond strength in different ways depending on the physical and chemical nature of the adhesives. Adhesive solution is applied to the bonding surface and brought in contact with each other. Here the adhesive develop the bond strength due to the loss of solvent by evaporation or by entering in to the minute pores or voids of the attaching surfaces. Adhesive of this type of thermoplastic polymer are applied to the bonding surfaces in hot liquid conditions and brought and brought in contact with each other. On cooling, the hot molten mass solidifies and acquires bond strength. Adhesive of this type of thermosetting polymers develop bond strength by the application of pressure on the attaching surfaces.

PHYSICAL AND CHEMICAL FACTORS AFFECTING ADHESIVE ACTION

PHYSICAL FACTOR
1. Surface tension and viscosity: the wetting tendency of a liquid adhesive depends on properties like surface tension and viscosity. The bonding between the attaching surfaces by an adhesive is effective only when the interfacial tension between the adhesive and adherent is minimum.
2. Roughness and smoothness of the contacting surfaces: For better quality bonding, a smooth and non-porous surfaces is necessary instead of rough and porous surfaces, since the porous surfaces have minute pores, voids or capillaries which absorbs mobile solvent portion of the adhesive solutions which disturb the equilibrium between adhesive and solvent. Therefore the bond formed will be less strength.
3. Thickness of the adhesive film: A thick adhesive film imparts poorer strength then a thin adhesive film. Solvents can be used to reduce the thickness of the film so that strength can be improved.
4. Elasticity and tensile strength: If the tensile strength and elasticity are higher for the adhesive film produced, the bond strength will be more.

CHEMICAL FACTORS
1. Presence of polar functional groups in adhesive molecules: The adhesive molecules containing polar functional groups such as –OH, -COOH, -Cl, can attach to surface effectively than adhesives containing non-porous functional group.
2. Degree of polymerization: The degree of polymerization of a polymeric adhesive must be in the range corresponding to the oligomers or polymers having slightly higher molecular weight than them. These polymer molecules will have more free valencies to attach to the surfaces more effectively than the other high molecules.
3. Complexity of the adhesive molecules: Adhesive molecules with lengthy side chains have best adhesive action. The chemical nature, length and complexity of the side chains in the adhesive molecule increase the bond strength.
4. Effect of pH: Presence of strong acids and alkalis in the adhesive adversely affect the bond strength.

CLASSIFICATION OF ADHESIVES

I.NATURAL ADHESIVE
a) Starch adhesives: Starch is mixed with water or dilutes acids to get a pasty mass. They can be applied in hot as well as in cold conditions. The bonds formed by them are weak and they are less water resistance. They are used in manufacture of envelopes, stamps, notebooks, binding books etc.
b) Vegetable glues or protein glues: Soya bean glue can be crushed and processed and then mixed with calcium hydroxide and sodium hydroxide to get a pasty mass, which is used as an adhesive paste.
c) Casein glue: Skimmed milk is treated with dilute acid to obtain protein casein. It is then mixed with calcium hydroxide and other presertives and used as adhesive paste.
d) Animal glue: the bones and other waste of animals are chemically processed to remove the unwanted fat and calcium content and mixed with preservatives to get an adhesive paste

II. SYNTHETIC ADHESIVES
a) Thermosetting polymeric adhesives: They form a three dimensional cross linked structure during the bond formation. Hence they have better bond strength. They form insoluble and infusible adhesive having good resistance towards heat, moisture and other biological microorganisms. They are used for the manufacture of water proof plywood laminates and ship and aviation industries. Phenol formaldehyde resins, urea formaldehyde, polyesters etc., are some of the examples of thermosetting polymeric adhesives.
b) Thermoplastic polymeric adhesives: These types of polymeric adhesives should be applied while in hot condition to the surface to be attached. On cooling they acquire bond strength. They form fusible and soluble adhesives having poor water resistance. Cellulose nitrate, acetate, polyvinyl acetate, polyvinyl chloride etc. are some of the adhesives which are thermoplastic in nature. They are mainly used in garments and foot wear industries.