Dental polymers.

Dental polymers are primarily used in restorative dentistry and must serve both cosmetic and functional purposes. They must be adaptable for easy forming and final cure by simple techniques and equipment available in a dental laboratory or dentist’s office. The final prosthetic device must fit exactly in spite of shrinkage due to polymerization and high coefficients of expansion. The plastic must withstand the occasional high shock loads of severe biting as well as the constant low loads that prevail in the mouth. Dental polymer materials must be free from toxicity, esthetically pleasing, and stable in the fluid environment of the mouth. The first practical dental polymer application took place in 1839 when Goodyear invented the vulcanization of rubber and this material was used to make a denture base. It was not long afterwards, in 1868, that Hyatt’s invention of celluloid was likewise adapted for use in the production of denture-base resins. Almost every subsequent polymer material of any commercial significance has received at least research consideration by dental investigators. Some resins such as glyptals, phenol-formaldehydes, vinyl chloride copolymers, polystyrene, and epoxies have had varying periods of popularity. It was not until 1937, when acrylic resins were introduced, that a truly universal dental polymer appeared on the scene. Acceptance was immediate, and now acrylic resins and their modifications constitute the largest volume dental resin used. They have low water absorption and are dimensionally stable. The ease of fabrication and inherent attractiveness of acrylics made it possible for the dentist to provide his patient with plastic dentures which are very difficult to distinguish from natural teeth. Acrylics still have deficiencies. The severe shrinkage which they undergo on polymerization can cause high internal stress of as much as 3,000 psi in an upper denture.l The abrasion resistance of plastic teeth is not as good as that of porcelain teeth. Acrylic resins used in restoration show poor adhesion to wet tooth substance and they discolor when used with amine-peroxide room-temperature initiators. New polymer materials must overcome these deficiencies without any sacrifice of the many advantages of acrylics. The market for dental materials is small when measured by current industrial standards of pounds consumed. The total resin market amounts to somewhat more than 600,000 pounds of finished products per year. Of this, almost 95% are acrylic and related resins since they provide the best combination of physical, chemical, and cosmetic properties for applications to denture-base and tooth materials. The actual purchase requirements for dental resins in 1964 were closer to 1,000,000 pounds since the mo!ding operations and fabrication techniques require a considerable amount of over-mixture. Many other polymers are currently used in smaller amounts in order to achieve special effects. These include polystyrenes, polyesters, vinyls, nylons, natural polymers, silicones, polycarbonates, epoxides, and a variety of very special copolymers. Several surveys based on American Dental Association figures have been evaluated in order to arrive at the figures given in TABLE 1 for the resin consumption in 1964.?r3 Specifications which cover the use of plastic materials in dentistry are set forth by the American Dental Ass~ciation.~ This group is assisted by the National Bureau of Standards where the Dental Association maintains a number of re-