Most modern proprietary adhesives are based on synthetic rubber/resin formulations. Synthetic adhesives are generally called synthetic resins because the natural adhesives they most resemble are the resin types. There are many variations, including one not found in natural adhesives: the two-part adhesive, where the adhesive is mixed with a separate hardener or catalyst to make it set. Synthetic resins are normally classed as thermoplastic (melting when heated) and thermosetting (heat speeds hardening).
Thermoplastic adhesives include the vinyl resins, a versatile group that stick well to glass and metal, but are also used in many other applications. Polyvinyl acetate (PVA) based adhesive is water-soluble and used for woodwork, ceramic tiling, flooring and general purpose bonding agents.
Other types of adhesive have organic solvents or are hot-melt types, such as the resin that is sandwiched between two thin layers of glass to make laminated safety glass for automobile windshields, making them stronger and safer.
There are several types of acrylic resin adhesives, one- and two-part, both cured by adding chemicals. They can develop very strong bonds, and are more transparent than other types of adhesive. Objects are often embedded in clear acrylic resin for protection or display. One unusual type is cyanoacrylate adhesive, the popular Superglue that cures to a high strength in a few seconds. This quality makes it useful for production lines.
Cellulose adhesives consist of chemicals derived from cellulose (such as cellulose acetate) in an air drying organic solvent, and are not the same as the water-based natural cellulose pastes mentioned above. They are quick drying and water resistant.
Other thermosetting resins include the phenolics, which are available both as chemical solvent types and in thin, solid, pressure-sensitive sheets which are used by the plywood industry for glueing layers of wood together.
Thermosetting adhesives include epoxide resins, among the strongest of all adhesives. Some types will withstand a shearing stress of up to 7000 lb/sq in. (500 kg/cm2) in correctly designed joints.
Polyester resins are cheaper than epoxy resins, and are therefore suitable for use in bulk. Their commonest use is with glass fiber to make glass reinforced plastics.
Synthetic rubber is used with organic solvents and with water to make many types of adhesives. The pressure-sensitive adhesives which are used on adhesive tape can also be of this type. Synthetic rubber adhesives are widely used in automobiles for attaching interior trim panels.
Two products that do not fit into any of these categories, but which are nonetheless adhesives, are solder and hydraulic cement.