polychloroprene (Neoprene) are commonly used contact adhesives. Both of these elastomers undergo strain crystallization. Contact adhesives must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together. Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held together. Once the surfaces are pushed together, the bond forms very quickly. It is usually not necessary to apply pressure for a long time, so there is less need for clamps. Hot Hot adhesives, also known as hot melt adhesives, are thermoplastics applied in molten form (in the 65–180 °C range) which solidify on cooling to form strong bonds between a wide range of materials. Ethylene-vinyl acetate-based hot-melts are particularly popular for crafts because of their ease of use and the wide range of common materials they can join. A glue gun (shown at right) is one method of applying hot adhesives. The glue gun melts the solid adhesive, then allows the liquid to pass through its barrel onto the material, where it solidifies. Thermoplastic glue may have been invented around 1940 by Procter & Gamble as a solution to the problem that water-based adhesives, commonly used in packaging at that time, failed in humid climates, causing packages to open. Reactive Anaerobic Anaerobic adhesives cure when in contact with metal, in the absence of oxygen. They work well in a close-fitting space, as when used as a Thread-locking fluid. Multi-part Multi-component adhesives harden by mixing two or more components which chemically react. This reaction causes polymers to cross-link into acrylates, urethanes, and epoxies . There are several commercial combinations of multi-component adhesives in use in industry. Some of these combinations are: Polyester resin & polyurethane resin Polyols & polyurethane resin Acrylic polymers & polyurethane resins The individual components of a multi-component adhesive are not adhesive by nature. The individual components react with each other after being mixed and show full adhesion only on curing. The multi-component resins can be either solvent-based or solvent-less. The solvents present in the adhesives are a medium for the polyester or the polyurethane resin. The solvent is dried during the curing process. Pre-mixed and frozen adhesives Pre-mixed and frozen adhesives (PMFs) are adhesives that are mixed, deaerated, packaged, and frozen. As it is necessary for PMFs to remain frozen before use, once they are frozen at −80 °C they are shipped with