Лако-красочные материалы — производство

5.2.1

Composition of Contact Adhesives

Typical base polymers of contact adhesives are polyvinyl acetates, chloroprene rubbers, polyethylene copolymers (with polar components), or poly (vinyl chloride) (PVC) derivatives. In general, the solvents used have different vapor pressures to prevent cobwebbing during the application process or skinning during the drying process. For the bonding of thermoplastics, for example PVC, specific solvents are added to obtain swelling or dissolution ofthe adherents and, in addition to adhesion, molecular diffusion between the adhesive and the solid object in terms of a welding effect. However, this may lead to microcracking (crazes) in adherents that are sensitive to stress cracking, such as polymethyl methacrylate (PMMA), polycarbonate or polystyrene. Microcracking starts in a localized polymer zone where the molecules are highly stretched, and extraneous molecules penetrate into the free volume, thus inducing the formation of cracks.

Besides solvents, 10-30% of a solvent-containing contact adhesive is composed of solids which may contain added materials such as wax or dissolved rosin to generate tack in the open dried bond-line, or anti-aging agents (e. g. metal oxides) to degrade any hydrochloric acid contamination from damaged base resins containing chlorine, or fungicides and UV stabilizers. Furthermore, slowly and weakly crosslinking prepolymers (e. g. phenolic resins in low concentrations) or, as a second component added prior to application, isocyanates, are added which improve the long-term creeping properties [8].