Лако-красочные материалы — производство

In the process of hardening, the overlying filling material may exert considerable pressure on the liner. Additional forces, mostly compressive in nature, will be transmitted to the liner through the hardened restorative as the completed restoration is subjected to the stresses of mastication. To prevent liner deformation and flow under the packing load in the process of cavity filling, it is important for the lining cement to have undergone sufficient hardening before the filling step is initiated. Application of the cement requires a pastelike consistency, and this contrasts with the low-viscosity materials used for luting. As a rule, depending on type of cavity, type of restorative, filling technique, and other variables, a lining cement must attain a compressive strength of up to 26 MPa prior to filling, and a sufficiently long time interval between placement of the liner and that of the restorative is therefore indi­cated. Ideally, a liner should display good micromechanical and/or chemical bonding characteristics vis-a-vis both the tooth structure and the restorative so as to minimize microleakage. In terms of strength and mechanism, the bonding effects depend on the materials’ composition; in practice, they are found to be rather weak.

The base or liner should provide a chemical barrier protecting the underlying dentin from attack by acids or acrylic monomers and other irritants that may diffuse out of the restoration. Needless to say, cements emitting irritants themselves may be restricted in use to the lining of shallow cavities where pulp irritation represents a less severe problem. Another important function of the cement base or lining, especially with amalgam fillings in deep cavities, is thermal and electrical insulation, so as to minimize heat transfer or transmission of electrical currents to the sensitive pulp area. Although electrical conduc­tion is not always prevented by the common water-based ionic or metal-chelating cements, their thermally insulating properties generally are more than adequate for the purpose.

Not unexpectedly in view of the related functions of luting and lining materials, most of the materials discussed in Section II. B as luting agents are more or less equally useful as cavity base and liner cements. Cements classified as cavity liners include the calcium hydroxide materials, the zinc phosphates, zinc chelating agents, polycarboxylates, and glass ionomers.