A composite is defined as the combination of two or more chemically different materials that have properties better than when the compounds act alone. Resin composites, which are used in dentistry, are compounds composed of three separate agents that act together to form a rigid polymer.
The three agents that act together to form the polymer include, an organic resin matrix, inorganic filler, and a coupling agent. The most important agent, the organic resin matrix, is typically composed of a compound referred as bis-GMA, or bisphenol A glycidyl methacrylate. This aromatic methacrylate is considered to be a rigid polymer because of the benzene rings near its center, and also provide specific sites for free radical polymerization from its terminal methacrylate groups. Since this structure has hydroxyl groups near the benzene ring, this polymer is considered to have high viscosity. Since high viscosity will affect the rigid structure after polymerization, a compound, typically triethyleneglycol dimethacrylate, or TEGDMA, is added to reduce the viscosity.
Another disadvantage of using bis-GMA is that it absorbs light, which could possibly lead to color changes. Knowing this, the compound, 2-hydroxy-4-methoxy benzophenone, is added to the matrix to keep light from being absorbed and prevent the discoloration.
Since bis-GMA is capable of polymerizing spontaneously under normal storage conditions, polymerization inhibitors are added to the matrix. The most common inhibitor is the monomethyl ether of hydroquinone. Another inhibitor that prevents the dimethylcrylate groups on the bis-GMA from polymerizing spontaneously is the use of butylated hydroxytoluene.
In order for the matrix to be condensed, inorganic filler molecules are added to improve the properties of this organic matrix. The properties of the matrix are affected by the percentage of filler because there will be less room within the matrix for the polymer to shift or crack. With the addition of...