Rigid Polymers

Polymers:
5.3.1 Rigid Polymers
Two types of chemical polymerization reaction, Condensation and Addition, are used to make dental polymers. Examples of polymers derived from condensation reaction include polycarbonates used for provisional crowns. Examples of addition reaction include acrylic acids and methyl methacrylate monomers, which respectively form poly(acrylic acid) and poly(methyl methacrylate) or acrylic. The latter (acrylic) is the most widely used rigid polymeric material. Its applications include the fabrication of denture bases, provisional crowns, and bridges.
Acrylic materials can be heat- or self-cured. The composition of self-cured materials is similar to heat-cured materials except that activators, such as dimethyl-p-toluidine, are present in the liquid component. Self-cured acrylics are also called autopolymerizing or cold-curing materials. They have lower molecular weights, higher residual monomer contents, poorer rheological properties, poorer color stability, and greater porosity, and are weaker than heat-cured acrylics. Although acrylic prostheses (e.g., dentures and dental splints) can theoretically be processed by a number of techniques including compression and injection molding, the most commonly used one is the dough technique. In this technique, a dough is formed from a mixture of the monomer (liquid) and polymer (powder). This is subsequently packed into a mold and polymerized under the appropriate conditions to give a solid prosthesis.