Dr. Dmitri Kopeliovich
Adhesives are classified by different classification systems:
Thermosets molecules are cross-linked by strong covalent intermolecular bonds, forming one giant molecule. Cross-linking is irreversible therefore thermosets can not be reprocessed (re-melt). Cross-linking is achieved in curing process initiated by heat, chemical agents, radiation or evaporation of Solvents. Curing results in sharp increase of strength, elasticity and stability of thermosets.
Most of thermosetting adhesives are based on epoxies, polyesters, polyimides and phenolics.
Thermoplastics are Polymers, which soften (become pliable and plastic) and melt when heated. No new cross-links form (no chemical curing) when a thermoplastic cools and harden. Thermoplastics may be reprocessed many times by heating or applying a solvent. Molecules of most of thermoplastics combine long polymer chains alternating with monomer units.
Polyamides, cyanoacrylates, polyacrylates, polyvinyl acetate (PVA) are typical thermoplastic adhesives.
Elastomers are polymers possessing high elasticity - may be reversibly stretched at high degree. Elastomers consists of long lightly cross-linked molecules. Elastomers are set (strengthened) by thermal curing or solvent evaporation. Curing results in increase of cross-linking of the molecules.
Typical elastomeric adhesives are based on natural rubbers, silicones, acrylonitride butadiene (nitrile), neoprene, Butyl, polyurethane, styrene-butadiene.