Лако-красочные материалы — производство

Closely related to the zinc phosphate luting cements, the silicophosphate materials are two-part bonding systems, which are mixed and applied essentially as described in the preceding section. Whereas the liquid component is a buffered aqueous phosphoric acid solution as before, the solid is a powdered mixture of a fluoride-containing, ion-leachable aluminosilicate glass and zinc oxide, and the hardening process yields a matrix of phos­phates of zinc and aluminum, embedding zinc oxide, and glass particles. The fully set cement is less soluble than zinc phosphate in aqueous media, yet still prone to erosion, especially under acidic oral conditions. The fluoride content is beneficial in retarding secondary cavity formation, as fluoride ion gradually leaches from the lute. Because of their superior translucency, the silicophosphates are preferentially used for luting porce­lain crowns, although their bonding mode is one of micromechanical interlocking, requir­ing specialized surface treatment of the porcelain restoration. The cements are weak under tensile and flexural loads but adequate in compression (Table 4).

unreacted ZnO and eugenol. The products are stabilized through coordinative bonding of the metal center to two ether oxygen atoms with generation of five-membered chelate rings. The commercial products are two-part systems supplied either as powder-liquid or paste-paste combinations. In the former case, the powder is composed of zinc oxide as the primary ingredient, usually containing 1 to 5% zinc acetate added to accelerate the reaction, and the liquid component is made up of eugenol mixed with small proportions (5 to 15%) of cottonseed or olive oils, added for viscosity control. The paste-paste combina­tions, supplied for greater ease of mixing, generally contain the ZnO-ZnOAc solids admixed to a plant oil, on the one hand, and the eugenol admixed to an inert mineral filler, on the other. In the absence of water, the two parts mixed together by the clinician can be handled conveniently without premature setting; once applied, the mixture, now exposed to the moist and slightly warmer oral environment, sets rapidly. The cement is weak, however, having a compressive strength of no more than 10 to 20 MPa (somewhat higher upon additional resin reinforcement). In addition, eugenol leaching from the lute and subsequent hydrolysis may lead to significant deterioration of the material. For this reason, the ZOE cements should be used only for temporary luting.