Лако-красочные материалы — производство

Luting cements based on zinc phosphate have been known for more than a century and are still in major use today. The fundamental process leading to cementation is the

formation of hydrated zinc phosphate from zinc oxide and phosphoric acid:

3ZnO + 2H3PO4 + H2O! Zn3(PO4)2 • 4H2O

Accordingly, the material is supplied as a two-part system: a powder component consisting predominantly of zinc oxide, and an aqueous solution of phosphoric acid (50 to 60wt%) and other minor ingredients. The two components are mixed by the clinician in a con­trolled fashion immediately prior to use. The reaction is vigorous and strongly exothermic. For clinical application some retardation is called for; in most commercial products this is achieved by high-temperature (>1300° C) sintering of the zinc oxide reactant with some 10% of other, less reactive oxides, such as MgO, resulting in partial deactivation. To buffer the reaction further, up to 10% of aluminum and zinc phosphates are added to the phosphoric acid solution. With a powder/liquid ratio of 1.4g/mL the mixture possesses adequate fluidity and working time to permit thin-film (ideally, 35 to 40 pm) application and allow for proper seating of the restoration. The lute so prepared sets within 4 to 7 min and undergoes further hardening thereafter; ultimate compressive strength after more than 24 h is typically in the vicinity of 80 MPa.[34]

To achieve satisfactory mechanical characteristics of the cement, the lute margin requires protection from moisture during the setting period (e. g., by varnish application); otherwise, phosphoric acid leaches out from the fresh cement, and the latter turns chalky and porous. The retention effects of the zinc phosphate cement rest on mechanical inter­locking rather than chemical bond formation to the adherends. Surface roughness there­fore contributes decisively to lute retention, although a limit is set by the inherent strength of the cured cement. The shear bond strengths of the phosphate and other classical luting cements to the tooth structure are in a very low range, generally not exceeding 2 MPa. Cement solubility in the aqueous oral fluids is slight but noticeable and leads to slow lute erosion and loss of retention. The erosion is affected by lute margin width, zinc oxide load, and particle size. Taken together, such erosion effects render the zinc phosphates inferior in this respect to most resin and composite cements.