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.
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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 the filler, the coefficient of thermal expansion, water absorption, and polymerization shrinkage decreases, while elasticity, tensile strength, and fracture toughness increases. This molecule usually consists of colloidal silica, barium silicate, zinc silicate, and other particles that contain some silica backbone. This silica structure is important for bonding purposes between the organic matrix and the coupling agent.
The coupling agent, which main purpose is to bind the filler molecule to the organic matrix, is connected to the filler molecule through the silanating agent. The silane agent also improves the physical properties of the resin by preventing hydrolytic breakdown along the filler/matrix interface, which could result in cracking within the resin matrix. The most common silanating agents used for bonding filler molecules to the matrix are an organosilane referred to as gamma-methacryloxypropyltrimethoxy silane. This silanating agent has a silane group on one end that bond to the hydroxyl group on the filler particles through a condensation reaction that produces the siloxane bond. The methacrylate group on the agent undergoes polymerization with the resin under the light or chemically activation.
Chemically Activated Resins:
Other molecules that are necessary in carrying out this reaction are polymerization initiators. For chemical activated resin composites, the use of benzoyl peroxide and tertiary amines is used as a source for free radicals. After breaking the O-O bond on the benzoyl peroxide, the radical on the oxygen interacts with the methacrylate groups on the composite, allowing composite to then interact with other compounds as seen in the light activated resins.
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Visible Light Activated Resins:
This process of light activated reactions is similar to chemically activated resins in that the free radicals need to be produced in order for the compound to react with the resin matrix. Light activated resin composites use a diketone photoactivator, one such as camphoroquinone, which reacts with the tertiary amine under light conditions and extracts electrons from this coinitiator. Camphoroquinone, once exposed to light, is converted to an excited triplet state. Within this state, it collides with a tertiary amine, converting itself and the amine into free radicals. These radical components can then initiate the polymerization process.
Ultraviolet Light Activated Resins:
Along with light-activated and chemically composite resins, a third way to activate the compounds is through the process of using an ultraviolet light activated resin, which also produces free radicals by using the compound benzoin methyl ether. This UV photoinitiator interacts with the tertiary amine, once it is excited, and extracts electrons from the amine and produces radicals on each. When the UV light produces free radicals on the benzoin methyl ether compound, it then interacts with the methacrylate groups on the composite. Since these groups provided for site of free radical polymerization, the compound can now interact with other materials such as coupling agents and polymerization inhibitors
Practical problems existed with each method. For UV light activated reactions, the light had a limited depth of cure, due to the UV light not penetrating deeply enough into the resin. Visible light activated resins, when compared to chemically activated resins, show nonuniform polymerization as well as depth penetration by the curing light.
When the resin composites polymerize, the reaction is very quick. This in fact happens so quickly that some polymer molecules have unreacted carbon double bonds at the terminal ends. Using a degree of conversion, they are able to determine the amount of material that reacted within the polymer matrix. This degree of conversion is directly affected by the amount of time exposed to the curing light. The longer exposed to the duration of exposure, the greater conversion rates will be within the matrix.
