Synthetic resins are materials with a property of interest that is similar to natural plant resins: they are viscous liquids that are capable of hardening permanently. Otherwise, chemically they are very different from the various resinous compounds secreted by plants (see resin for discussion of the natural products).
The synthetics are of several classes. Some are manufactured by esterification or soaping of organic compounds. Some are thermosetting plastics in which the term "resin" is loosely applied to the reactant or product, or both. "Resin" may be applied to one of two monomers in a copolymer (the other being called a "hardener", as in epoxy resins). For those thermosetting plastics which require only one monomer, the monomer compound is the "resin." For example, liquid methyl methacrylate is often called the "resin" or "casting resin" while it is in the liquid state, before it polymerizes and "sets." After setting, the resulting PMMA is often renamed acrylic glass, or "acrylic." (This is the same material called Plexiglas and Lucite).
Types of synthetic resins
The classic variety is epoxy resin, manufactured through polymerization-polyaddition or polycondensation reactions, used as a thermoset polymer for adhesives and composites. Epoxy resin is two times stronger than concrete, seamless and waterproof. Accordingly, it has been mainly in use for industrial flooring purposes since the 1960s. Since 2000, however, epoxy and polyurethane resins are used in interiors as well, mainly in Western Europe.
Synthetic casting "resin" for embedding display objects in Plexiglass/Lucite (PMMA) is simply methyl methacrylate liquid, into which a polymerization catalyst is added and mixed, causing it to "set" (polymerize). The polymerization creates a block of PMMA plastic ("acrylic glass") which holds the display object in a transparent block.
Another synthetic polymer sometimes called by the same general category, is acetal resin. By contrast with the other synthetics, however, it has a simple chain structure with the repeat unit of form -[CH2O]-.
Ion exchange resins are used in water purification and catalysis of organic reactions. See also AT-10 resin, melamine resin. Certain ion exchange resins are also used pharmaceutically as bile acid sequestrants, mainly as hypolipidemic agents, although they may be used for purposes other than lowering cholesterol.
Solvent Impregnated Resins (SIRs) are porous resin particles, which contain an additional liquid extractant inside the porous matrix. The contained extractant is supposed to enhance the capacity of the resin particles.
A large category of resins, which constitutes 75% of resins used, is the unsaturated polyester resins.
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