Classification of Adhesives

Adhesives are substances that are used to bind or stick things together. These have been used by our ancestors from all historic civilizations, for many purposes like building, wood cutting, for making boats, etc. The ones used by them are now being replaced by modern ones.

There are various types that are made from materials available in nature or artificially made from chemicals.

(i) Soybean Flour: Soybean flour when mixed with a strong alkali like sodium hydroxide or trisodium phosphate acts as a good binding material for softwood plywood.

(ii) Animal Blood Glues: The proteins available in the animal blood such as serum albumin, globulin and red cell hemoglobin are used to make glue. Mostly blood of cows, hogs, sheep, horses, etc., are used for making binding materials in fixing plywood.

(iii) Casein Glues: This is made from dried curd casein, lime and certain chemicals. This is used as a binding material for furniture woods, paints and sizing agents in wooden canvas.

(i) Cellulose: Cellulose from wood pulp and cotton linters which is converted into cellulose esters and ethers by the process of esterification and etherification, is dissolved in organic solvents to form glue, which is used as plastic binders.

(ii) Starch: Starch from plants are used as binding material in the manufacture of paper bags.
(iii) Natural Gum: Natural gums are used as pressure-sensitive tapes, denture glues, medicinal tablet binders, etc.

(i) Latex: These are made from the latex of natural rubber. These are used in envelopes, leather, tire cords, carpet and textile industries, etc.
(ii) Solution Adhesive: These are prepared using solid natural rubber, zinc oxide, antioxidant, sulfur and solvents such as toluene, naphtha, etc. They are mostly used as sealants in industries.

(iii) Elastomers: Elastomers are materials that are made from natural rubber, which are highly flexible, cross-linkable, high molecular weight polymers. They are used in belt and shoe manufacture. Butyl rubber is an elastomer used for auto glass repair, rubber roof installation and various other applications in the marine industry.

(iv) Pressure-Sensitive Adhesives: These are used in surface protection films, covering the tapes, papers, promotional graphic materials, and skin contact materials such as anesthetic patches, wound dressings, etc.

(i) Lignin: Lignin is a complex polymer that is produced from paper mills. Binding materials made from lignin are basically used as binders in roads (non-tarred) and wood. Premethylolated lignin is becoming very popular in North America.
(ii) Tannin: Hydrolyzable tannin and condensed tannins are two different types of tannins, which are used for making plywood panels.

(i) Urea-Formaldehyde: These are hard, inflammable amino resin adhesives. The bonds of UF can be affected by water and moisture, so it has interior applications only.
(ii) Melamine-Formaldehyde: Melamine-Formaldehyde resins are used in exterior plywood designs and impregnation of papers.

(i) Ethylene Vinyl Acetate Hot-Melts: Ethylene-vinyl acetate (EVA) hot-melt is made up of EVA polymer, resins such as hydrocarbon, terpene, rosin esters, etc., fillers such as barium sulfate, calcium carbonate and antioxidants.

They are used in edge veneering and edge banding of wooden furniture.
(ii) Polyamide Hot-Melts: They are higher in cost as compared to EVA hot-melts and have good heat-resistive property. Polyamide hot-melts are used in veneer splicing process of linking the veneer edges of a decorative wooden surface.

(i) Resorcinol: Resorcinol is a phenol which is produced from resins. It is used as a binding material in the manufacture of structural fingerjoints and timbers. Its strength is not affected even during harsh climatic conditions.
(ii) Furan: It is used in wood, cement and glass productions. It is also used in grinding wheel manufacture.

(iii) Unsaturated Polyester Resins: Unsaturated polyester resins are used as binding materials in casting plastic materials, glass fiber lamination, etc.
(iv) Polyurethane: These are used in glass, plastics, ceramics, wood bindings and also in binding leather and textile fabrics.

(v) Anaerobic: These are made from a combination of acrylic esters, which is made by polymerization of acrylonitrile. It is used for sealing metal castings, pipes and flange joints.
(vi) Ultraviolet Curing: Their bonding strength is very strong and it can withstand extreme temperatures. They are used in electronics and telecommunication, medicine, optical glasses, etc.
(vii) Heat Curing: Epoxies, urethane and polyimides are thermosetting materials and are used as heat curing adhesives in plastics and for various coatings.

All these binding materials are applied on the surfaces using brushes, rollers, guns, etc. The best property of these materials should be high tensile strength, high thermal resistance, high cohesive strength, high damping capacity, resistivity to environmental and chemical fluctuations.