The use of terpene resins has expanded rapidly during the last decade with the rapid growth of hot-melt adhesives. The hot-melt adhesive is defined as a material containing 100% active components applied in a molten state which will form a bond between substrates upon cooling below its melting point. Hot-melt adhesives generally offer versatility not obtained with conventional aqueous adhesive systems in bonding to a wide range of materials under varied conditions, the following advantages are realized by the use of hot melt adhesives: 100% active components, fast set, cleaner operation, less sensitive to change in surrounding conditions, reduction of fire hazard easier storage, and better resistance of bonds to moisture. There are many variables which affect bonding with hot melt adhesives. The most important are: temperature and viscosity of the adhesive melt, porosity of the substrate, adhesion of the substrate, cohesive strength, condition of the substrate, compression during bond formation, and storage conditions.
Pressure Sensitive Adhesives
PSA is one of permanently tacky, requires no activation by heat, solvent, or moisture, and which will adhere strongly to most surfaces upon applications with a minimum of pressure. Tack is defined as instant low order adhesion developed by mere contact with a variety of dissimilar surfaces. The major components of a pressure sensitive formulation are elastomer and tackifier. The latter can be polyterpene, hydrocarbon or rosin ester-based. The elastomers, generally rubbers, have molecular weights between 60,000 and 350,000 which correspond to degrees of polymerization of 1,000 to 5,000. Because of their extreme chain length, they provide latent tackiness. There high molecular weights allow modification with large amounts of other substances withour serious loos of cohesive strength. The terpene resins, because of their chemistry and pysical characteristics, combine with the elastomer to produce formulations with the characteristics of tack, adhesion and cohesion required of a successful pressure sensitive adhesive.