Engineering Chemistry S Datta

ADHESIVES 335
3. Natural resin adhesives:
(a) Shellac–Hot shellac sol is an oldest known adhesive, which is hard.
Use. For making conveyer belts.
(b) Asphalt–Used for bonding metal, paper etc.
4. Starch and dextrin adhesives. The raw materials used for various types of starch and
dextrin based adhesives are potato starch, corn starch etc. A suspension of starch is heated for
sometime to obtain the adhesive which is cheap, can be applied in cold and hot states but bond
strength is low and susceptible to moisture.
Uses. For making envelopes, stamps, books etc.
5. Vegetable glue. These protein glues are obtained from soyabean, corn, casein, albumin
etc. The oily or the fatty matter is extracted or precipitated from soyabean or milk and then
the protein is treated with lime and the water solution form a jelly-like adhesive. They are
very cheap but of poor bond strength.
Uses. Similar to starch adhesive.
6. Animal glue. Animal wastes obtained from slaughterhouses etc. and hide scraps from
leather industries are degreased with solvent, treated with lime. The product obtained is
hydrolysed with hot water to form liquid glue. This is bleached and mixed with preservatives
and concentrated to form solid glue. The solid cake or powder is boiled with water to form the
adhesive solution. The bonding strength of the adhesive is high but susceptible to heat and
moisture.
Uses. For manufacturing cardboard boxes and furnitures.
7. Inorganic adhesives. Sodium silicate alongwith litharge (lead monoxide) and MnO 2
etc. forms gel when suspended in water. This gel has adhesive properties. The bond is moisture
and high temperature sensitive but fungus resistant.
Uses. For cardboard boxes making.
Testing of Adhesives
Testing of adhesives mainly encompasses:
• Solid content
• Rheologic properties
• Shelf life
• Flammability and inflammability
• Risks involved in transport and application
• Measurement of the mechanical properties like tensile strength, breaking elongation
of the films or test specimens made with the adhesives.
Growth Rates of Different Adhesives
The highest growth rate is expected in automotive industry, transport and electronics.
Continued growth is expected in construction adhesives, solvent free adhesives whereas
reduction in demand is expected for solvent containing adhesives and animal and vegetable
glues.