Introduction:

Adhesive is the substance employed for sticking two unlike bodies altogether because of molecular forces existing in the area of contact. All the adhesives either have polymers or polymers are made within the adhesive bond. The growth of adhesives has continued over the centuries to meet up the needs of different civilizations however it was not until the industrial revolution that demands were made for main advances in the adhesive technology.

Adhesives:

The adhesive is any substance applied to the surfaces of materials which binds them altogether and resists separation. The word 'adhesive' might be employed interchangeably by 'glue'. Adhesives might be found naturally or prepared synthetically. 

The four major requirements for a material to carry out as an adhesive comprise:

a) It should 'wet' the surfaces. This signifies the material should flow over the surfaces being bonded.

b) It should adhere to the surfaces. This signifies the material should stay in position.

c) It should develop strength. This signifies that the material should become strong.

d) It should remain stable. This signifies that the material should remain influenced via external conditions as long as the bond is needed.

Types of adhesives:

The two main kinds of adhesives are natural and synthetic adhesives. 

1) Natural adhesives:

Natural adhesives come from a diversity of animal and vegetable sources. The main kind of natural adhesive is animal glue. The animal glues are prepared from collagen, a protein found in bone, skin and sinew. Natural adhesives have been in use from the ancient times, specifically casein glue, animal glue and adhesives prepared from plant resins. Beeswax and pitch were employed as adhesives for centuries. Some 3,300 years ago, the ancient Egyptians employed animal glue to build the furniture, covering it by fine ornamental wood veneers and ivory. 

2) Synthetic adhesives:

The synthetic adhesives are compounded from simple chemicals. The major classes of synthetic adhesives are:

• Water-based adhesives
• Reactive adhesives
• Solvent-based adhesives
• Sealants 

Production of adhesives:

The production process of adhesives comprises the combination of different substances in a homogeneous mixture to form final products which are then packaged in the variety of containers.

1) Water-based adhesives:

The raw materials employed in the manufacture of water-based adhesives comprise polymer emulsions, plasticizers, polymers, preservatives and fillers. Fine polymer dispersion is made by high intensity mixing of the polymer in water. The polymer emulsions are blended by other components in stirred mixers. Some of the operations will comprise chemical reactions. 

2) Hot-melt adhesives:

Th hot-melt adhesives include solid material needing high temperature and application. The raw materials employed comprise polymers, fillers and anti-oxidants.

3) Vegetable-based adhesives:

Cellulose is reacted by methyl chloride at high pressures and temperatures. The resulting product is generally a flaky solid. Biocidal agents are added to the product prior to packing.

4) Solvent-based adhesives:

The natural or synthetic rubber or polymer is mixed by organic solvent and additives in a closed mixing vessel to prepare a homogeneous product.

5) Reactive adhesives:           

Reactive adhesives are prepared as two-part systems, resin and hardener, which are packed separately and mixed prior to use. The Epoxy resins are prepared via the reaction of bisphenol-A and epichlorohydrin in a stirred reaction vessel. The vessel is heated and sodium hydroxide is added. The epoxy resin made by the reaction is filtered off, further purified via solvent distillation and then mixed by the needed additives and packaged. 

6) Sealants:

The requisite raw materials are weighed and then added to the mixing vessel where the ingredients are batch mixed. The product is then discharged for packaging, which might be automatic or semi-automatic.

Common adhesives and their uses:

Fig: Common adhesives and their uses

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