Лако-красочные материалы — производство

Although prevalently used as luting and cavity-lining cements, the glass ionomers play a moderate part as cavity-filling materials, largely on the strength of their adhesion to the enamel and dentin of the tooth structure, the polyacid components participating in ionic bond formation with calcium cations of the hydroxyapatite in addition to undergoing weak ionic and/or covalent bonding with basic or nucleophilic sites in the dentinal col­lagen. The structural features and bonding mechanisms were discussed in Sections II. B.6 and IV. B.5. The compositions and properties of the GI filling materials are quite similar to those of the luting and lining varieties, the main difference being a more viscous consis­tency of the filling material, brought about by increased filler/liquid ratios and/or varied types and sizes of the glass-particulate fillers. As pointed out earlier, the weak link in GI — enamel bonding frequently is not so much the interface but the cement itself, which is quite brittle and possesses low flexural (15 to 20 MPa) and diametral tensile (8 to 12 MPa) strengths. It is largely for this reason that the GI cements are not routinely employed for restoration of permanent teeth, where premature failure would be expected under the load of masticating forces.

Metal-containing GI materials, known as cermets, are the latest in specialty devel­opment in the field of dental ionomer cements. The cermets contain a filler phase obtained by fusing silver and other metals or alloys together with aluminosilicate glass and pulverizing the molten mass. This is then combined with poly(acrylic acid) in one — or two-part fashion as described in Section II. B.6. The cermets display setting and bonding characteristics resembling those of the metal-free parent cements while displaying better fatigue limits, and in properly poly(acrylic acid)-conditioned cavities, cause significantly less marginal leakage. However, there appears to be no clear super­iority with respect to other strength characteristics, both tensile and compressive strength values being in the same ranges as observed for representative GI cements, although for a silver-tin-zinc alloy as the metal component, encouraging compressive and diametral tensile strength data (187 and 18 MPa, respectively) have been reported [24]. An inter­esting potential application for reinforced-glass ionomers in restorative dentistry suggests itself for building up cores in severely destructed teeth prior to the placement of crowns. Perfect dimensional stability is required for a core to support a superimposed crown efficaciously. Conventional composites exposed to moisture are not sufficiently stable dimensionally for this kind of application as a consequence of unduly high water sorp­tion, and there are indications that reinforced GI cements, on account of better dimen­sional stability, may more adequately fulfill that requirement [25].