1. Field of the Invention
This invention relates to the field of artificial dentures, and more particularly, to such dentures prepared from polyurethane elastomers.
2. Description of the Prior Art
It has been proposed to provide dentures with a soft layer in contact with the gums and other mouth parts to provide tissue relief. Such soft layers have been composed of acrylics, silicones, and like rubber-like materials. But on aging, such soft layers tend to harden and give off undesirable odors. In addition, some decomposition of the polymer may also occur presumably due to an oxidation process as well as to pH fluctuations within the mouth. By way of overcoming these disadvantages, U.S. Pat. Nos. 4,024,636 and 4,080,412, both to Colpitts et al., and both incorporated by reference herein, describe dentures in which teeth are anchored in a gum member comprising a tooth-holding portion fabricated from a hard nonhydrophilic polyurethane elastomer having a hardness of not less than about Shore D40, and a mouth-engaging portion fabricated from a soft non-hydrophilic polyurethane elastomer having a hardness of not greater than Shore A65 integrally and chemically bonded into a unitary mass. U.S. Pat. No. 4,024,637 to Colpitts which is also incorporated by reference herein describes a denture in which hard non-hydrophilic polyurethane elastomer teeth are imbedded in and chemically bonded to a soft non-hydrophilic polyurethane elastomer. Preferred non-hydrophilic elastomers are those formed by isocyanate-terminated prepolymers which are cross-linked or cured by mixing with a cross-linking agent and heating as required to effect curing. Isocyanate-terminated prepolymers suitable for preparing the hard non-hydrophilic polyurethane elastomers are prepared by the reaction of polyether diols or triols with aliphatic or cycloaliphatic or aralkyl di- or polyisocyanates in proportion to give free NCO groups. The prepolymers are then cured or cross-linked with a diol, polyol, an alkanolamine, a diamine or a tertiary amine containing polyol, or blends thereof. Advantageously, the diol or polyol is a polyether diol or polyol or a hydroxyl-terminated prepolymer. While dentures prepared from the foregoing polyurethane resins represent a significant advance over dentures fabricated from such materials as acrylic resins, silicones, and the like, it has been observed that the polyurethanes are susceptible to distortion under the conditions prevailing in the mouth. Such distortion, accompanied by a loss in dimensional stability of the dentures, interferes with the ability of the dentures to retain a good fit with the mouth and remain in place without slippage.