Лако-красочные материалы — производство

Developed some 35 years ago, the polycarboxylate materials are based on polycarboxylic acids, such as poly(acrylic acid), poly(maleic acid) and various acrylic acid copolymers, and their principal setting reaction involves carboxylate salt and chelate formation with polyvalent cations, mainly Zn2+. Because of the polyvalent nature of the cations, the reaction leads to three-dimensional cross-linking. The polycarboxylates are generally

supplied as two-part, solid-liquid systems. Finely powdered zinc oxide, sometimes admixed with magnesium oxide and other oxides, represents the solid component, whereas the liquid is a solution of poly(acrylic acid) (ca. 40%) and other polyacids in water. Rapid reaction occurs on mixing of the components; within 15min some 75% of ultimate strength (compressive; approximately 80 MPa) is attained. Other commercial products are supplied as powders, which require mixing with water for cementation. These solids are composed of zinc oxide and anhydrous poly(acrylic acid) in the proper proportions, and reaction sets in upon admixture of water, which provides the vehicle for the ionic reaction sequence. Some materials contain fluoridation agents such as tin(II) fluoride in the powder component, thus providing protection of adjacent tooth structure against secondary caries without affecting the bonding characteristics. The polycarboxylate cements are more readily soluble than the phosphates in aqueous media. On the other hand, they offer an additional contribution to retention insofar as they form ionic bonds through salt formation of free carboxyl groups with the cationic calcium present in the hydroxyapatite tooth structure. The micromechanical bonding contribution in polycarboxylate cementation nevertheless is a major one, rendered highly efficacious as a result of surface porosity and wettability of the enamel adherend brought about by etching with the poly(acrylic acid). Bonding to dentinal tooth structure (invariably much weaker than to enamel) may also involve carboxyl group interaction with reactive groups (OH, NH2) in the collagen constituent, which makes up a substantial proportion (Table 1). The polycarboxylates bond strongly to stainless steel, whereas there is little or no chemical adhesive bonding to noble metal alloys, porcelain, and resin restorations.