In dentistry the term form, “composite” initially referred to a restoration material, usually in paste form, consisting of an organic binder containing at least 60 per cent inorganic filler by weight incorporated into a system that would induce polymerization. Likewise, the filler particles are coated with a “coupling” agent to bond them the resin matrix. Thus, the essentials of a composite are a resin binder, a filler and a coupling agent.
A number of parameters have a marked influence upon the properties that can be obtained by the addition of fillers to a resin matrix. The characterization of the dispersed phase in terms of its shape, size, hardness, concentration and distribution is very important. The composition of the composition of the continuous phase, i.e., the resin matrix, is equally significant. Thus the term composite distinguishes this class of materials from the unreinforced direct acrylic filling resins, including those materials to which small amounts of a filler have been added.
Resin Matrix
The dimethacrylate monomer (BIS-GMA) is the chief composition of resin matrix. It can be synthesized by the reaction between bisphenol-A and glycidyl methacrylate. It also can be obtained by the reaction of glycidyl ether of bisphenol-A and methacrylic acid. This hybrid molecule is classified as a thermo-setting methacrylate resin. It has proved to be suitable for use as a binder for reinforcing fillers. It has a somewhat lower polymerization shrinkage than does methyl methacrylate and hardens rapidly under oral conditions.
The resin for the matrix of commercial composite resins must be modified in order to improve certain properties. The dimethacrylate resin is too viscous for convenient use at ambient temperatures, so it is diluted by the addition of other methacrylate monomers of low viscosity. These monomers may be difunctional so as to form a cross-; inked polymer. Stabilizers are added to improve shelf life. Since polymerization is generally accomplished by means of the peroxide-amine system, ultraviolet light absorbing compounds must be added to minimize color change in the material when exposed to sunlight.
Most of the conventional composites are chemically activated via the peroxide-amine initiation system. The two pastes are identical except that one contains the benzoyl peroxide the other the tertiary amine activator or accelerator. When the two pastes are mixed together the material polynierizes. Light cured systems are employed in the currently popular composites.
Fillers
The filler in a composite must be in high concentration if the dispersed particles are to inhibit deformation of the matrix. Also, another function of the filler is to reduce the coefficient of thermal expansion of the resin matrix. Although the filler concentration varies from one commercial product to another, ，英语论文范文，英语论文题目 |
