Composition and Reaction
The organic polymer matrix in currently available composites is most commonly an aromatic or urethane diacrylate oligomer such as bisphenol A glycidyl methacrylate (bis-GMA) or urethane dimethacrylate, represented by the simplified formula:
where R may be any of a number of organic groups, such as methyl-, hydroxyl-, phenyl-, carboxyl-, and amide-.
Oligomers have reactive double bonds at each end of the molecule that are able to undergo addition polymerization in the presence of free radicals. The oligomer molecules are highly viscous and require the addition of low-molecular-weight diluent monomers, usually triethylene glycol dimethacrylate, so a clinically workable consistency may be maintained when the filler is incorporated.
A bond between filler particle and matrix in the set composite is achieved by use of an organic silicon compound, or silane coupling agent. The silane molecule has reactive groups at both ends and is coated on the filler particle surface by the manufacturer before mixing with the oligomer. During polymerization, double bonds on the silane molecule also react with the polymer matrix. A bond between filler and matrix allows the distribution of stresses generated under function. The net result is a material with strength properties greater than those of the particulate filler or the matrix separately or the filler combined with the unsilanated matrix. Bonding also enhances the retention of the filler particle during abrasive action at the composite surface, greatly improving the wear resistance of the material. Effective silane coupling of each filler particle to the organic resin matrix also reduces the absorption of water from the oral environment. Decreased water sorption will result in better dimensional and color stability of the material over time.
Initiators and accelerators
Polymerization of composites may be initiated by chemical means (self cure) or by visible-light activation. Dual cure is a combination of light and chemical curing. In chemically activated systems, an organic peroxide initiator (or catalyst) reacts with a tertiary amine accelerator, producing free radicals that attack the double bonds of oligomer molecules and initiate addition polymerization.
Initiation of polymerization in light-activated systems depends on generation of free radicals with a diketone (camphorquinone) and aliphatic amine when irradiated with blue light. For both systems, the following general reaction occurs:
Because dimethacrylate oligomers as well as dimethacrylate diluent monomers have reactive double bonds at each end of the molecules, polymerization results in a highly cross-linked polymer.
Inorganic oxide pigments are added to composites in small amounts to provide a range of standard shades. Most often, four shades, ranging from yellow to gray, are supplied. In response to consumer interest, manufacturers now offer an extended range of 16 to 25 shades, matched to the Vita ceramic shade guide. Most manufacturers offer modifiers such as highly pigmented tints for characterizing standard shades and creating opaque layers to block out tooth discolorations. There are also highly translucent incisal shades, extra-white bleach shades, and surface glazes to customize esthetic procedures. Polymerization inhibitors and stabilizers are added to the composite to lengthen shelf life.
Filler particles are of inorganic composition. In addition to quartz, fine-sized particles may be composed of barium or lithium aluminum silicate glasses; borosilicate glass; or barium, strontium, or zinc glasses. Colloidal silica particles make up microfilled composites, and radiopaque composites are made by incorporating elements of high atomic weight, such as barium, strontium, zirconium, or ytterbium, into the glass filler particles. Resin composites are often classified according to the size of the ceramic filler particle (Fig 8-1 and Tables 8-1 and 8-2).
Fine-particle resin composites contain ground glass or quartz particles 0.5 to 3.0 µm in diameter, which occupy 60% to 77% of the composite by volume. Since the filler has a density greater than that of the polymer matrix, the fraction of the filler by weight is higher, about 70% to 90%. Particles may be of uniform diameter or have a distribution of diameters, in which case smaller particles fit in the spaces between larger particles, and packing is more efficient.
Microfilled resin composites contain spheric colloidal silica particles 0.01 to 0.12 µm in diameter. Colloidal silica is produced by vapor-phase hydrolysis of silicon compounds, resulting in an average surface area of 200 m2/g, which greatly increases the viscosity of the polymer matrix on incorporation. Filler loading in these composites is therefore limited to about 30% to 55% by volume or 35% to 60% by weight, and low-molecular-weight organic diluents of low viscosity are often added to give the composite a workable clinical consistency. Filler content may be increased and properties improved by grinding a polymerized microfilled composite into particles 10 to 20 µm in diameter and subsequently using these reinforced particles as filler along with colloidal silica. Heavily filled microfilled composites have a filler content of 32% to 66% by volume or about 40% to 80% by weight (Fig 8-2).
|Inorganic filler content (vol %)||30–55||60–70|
|Coefficient of thermal expansion (/°C × 10–6)||50–68||20–40|
|Water sorption (mg/cm2)||1.2–2.2||0.5–0.7|
|Compressive strength (MPa)||225–300||200–350|
|Tensile strength (MPa)||25–35||35–60|
|Young modulus (GPa)||3–5||7–14|
|Polymerization shrinkage (%)||2–3||1–1.7|
|Filler size||0.01–0.12 µm||0.01–3.0 µm|
|Appearance||Optical properties similar to enamel||Good gloss, luster, and smoothness|
|Usage||Non-stress-bearing esthetic restorations*||Anterior and posterior restorations|
* Only heavily filled microfilled materials may be used for posterior restorations.
Hybrid resin composites have a combination of colloidal and fine particles as filler. The colloidal particles fill the matrix between fine particles, resulting in a filler content of around 60% to 65% by volume. Hybrids (Fig 8-3a) have dominated the market over the past decade.
Microhybrid resin composites contain a combination of microfillers and ultrafine glass particles and are so called because of their reduced filler particle size range (0.04 to 1.0 µm). They are marketed as all-purpose “universal” composites, offering both esthetics and enhanced wear resistance for use in both anterior and posterior applications (Fig 8-3b).
Nanofilled composites are microhybrid composites with even smaller sized particles prepared using nanotechnology. Nanometer-sized particles range from 20 nm to 75 nm, depending on the desired shade and translucency. They can be partially fused or sintered into small nanoclusters that act as silanated fillers. These materials tend to have a smoother surface texture after finishing and also after clinical wear. They blend shades better with natural tooth structure and create more esthetic restorations in critical anterior areas. With these characteristics comes no decrease in the physical properties. Nanofilled composites are recommended for both anterior and posterior restorations (Fig 8-3c).
Composite Product Systems
Chemically cured composites supplied as two pastes are typically used as resin cements or for core applications. Each jar contains dimethacrylate and filler; one jar also contains the peroxide initiator, and the other contains the amine accelerator. The initiator and the accelerator are kept separate until mixing. A few composites are offered as two-paste, dual-cure systems. Setting begins after the catalyst and base are mixed and can be accelerated by light curing.
Composites designed for restorative applications are supplied as single pastes in opaque, disposable syringes or in color-coded compules to be used with a syringe. Light-activated composites are currently the most widely used systems available.
Commercially available curing units transmit light from a halogen lamp to the tooth surface by way of a curved quartz rod, a liquid-filled transmission tube, or a bundle of flexible quartz fibers attached to a fiber-optic handpiece. Ultraviolet light is generally filtered out at the light source.
The initiator present in most photocuring monomers is camphorquinone, which is sensitive to light with the spectrum shown in Fig 8-4. To initiate polymerization, curing lights must emit light within this spectrum, which is in the blue range. Filtered halogen lights produce a broad range of wavelengths within the camphorquinone spectrum and are the standard.
Other lights that have higher intensities for faster polymerization have been introduced. These include plasma arc lamps (PAC) and argon laser lights (see Fig 8-4). Although more intense, not all PAC and laser lights have the broad spectrum of the halogen lamps. It is important to match the spectrum of any light to the absorption characteristic of the initiator in the product being used. Lasers are expensive, and the quality and dimensional change of the composite material cured at such fast speeds is still in question.
Halogen curing lights are available with continuous operation and programmed cycles. One program is called a stepping function, which raises the light intensity from about 25% to 100% in specified steps over the curing period. Another program is the ramped function, which has a similar action but works on a continuously increasing light intensity over the set curing period. These curing variables reduce polymerization shrinkage during the curing process by having some of the initial shrinkage take place while the material is still in a plastic stage. Figure 8-5a shows a halogen curing light for composite materials.
The latest advancement in light technology to enter the dental marketplace is the light-emitting diode (LED) curing lights. They are usually portable and rechargeable instruments that are very conducive to clinical applications (Fig 8-5b). However, the light source is emitted from a stimulated blue chip or an array of chips that can produce high-power densities but over more restricted spectral ranges. The first and second generations of these lights were somewhat underpowered because the higher intensities generated internal heat sufficient to melt the chip and external heat that could affect the dental pulp in the prepared tooth. In a newer generation of the LED lights, a pulsed intensity (micropulsing) and an array of smaller chips with variations in frequencies produce a more controlled heat.
In selecting a light source, every effort must be made to have the spectral distribution of the light match the absorption of the initiator in the material. In general, it is most desirable to use a light with the highest power density level that is consistent with minimal biologic response from the heat generated. From a material standpoint, the most important factor is to obtain sufficient depth of cure. The external surface will cure with most available light sources, but the internal aspects of the material may be insufficiently activated and result in a restoration with inferior physical properties. The best measure of the effectiveness of a light/resin composite combination is the depth of cure that can be obtained within the specific material.
As with ultraviolet light used in early curing units, blue light has the potential to cause retinal damage if observed directly. Protective eyewear during operation of curing units is highly recommended; however, the glasses or shields used should be wavelength specific to absorb between 450 and 500 nm. Normal sunglasses will not provide adequate protection.
Composite systems that are chemically activated have setting times ranging from 3 to 5 minutes from the start of mixing. The setting time is determined at the time of manufacture by control of the concentrations of initiator and accelerator. However, studies show that even after a curing time of 24 hours, polymerization is incomplete, and 25% to 45% of double bonds remain unreacted.
The setting time and the depth of cure of light-initiated materials depend on the intensity and penetration of the light beam. Polymerization is approximately 75% complete at 10 minutes after exposure to blue light, and curing continues for a period of at least 24 hours. At 24 hours, up to 30% of double bonds still remain unreacted.
The occurrence of shrinkage during polymerization creates stresses (~18 MPa) at the tooth-composite interface that may exceed the strength of any bond between composite and enamel or dentin. Bond failure at the interface allows an influx of oral fluids and greatly contributes to the possibility of postoperative sensitivity, marginal staining, and secondary caries. In addition, stresses at the tooth-composite interface may exceed the tensile strength of enamel perpendicular to the enamel rods, resulting in fractures through the enamel along the interface.
Shrinkage is a direct function of the volume fraction of polymer matrix in the composite, and thus occurs to a larger degree in microfilled composites than in fine-particle composites or hybrids. Microfilled composites typically show setting contractions of 2% to 4% as compared with 1.0% to 1.7% for fine-particle composites (see Table 8-1).
The shrinkage problem can be partially overcome in three ways. First, incremental addition and polymerization of thin layers of a light-initiated material will result in decreased total setting contraction as opposed to bulk curing a single thick layer. However, although this method does result in lower stresses at the tooth-composite interface, studies show that marginal gaps may still occur.
A second method is to use a graduated curing process by varying the light intensity during the curing exposure. The initial polymerization is done at a low intensity and then the final aspect is cured at full light intensity. This stepped sequence allows some of the initial shrinkage that takes place when the reaction kinetics are at the highest rate to be accommodated for while the material is still in the flow state. As a result, the absolute shrinkage is reduced and the stresses on the adhesive bonds are reduced.
The third approach involves the preparation of a composite inlay either directly in the mouth or indirectly as a laboratory procedure. In the latter procedure the inlay is heat processed (Fig 8-6), allowing the degree of polymerization to approach 100%, and then cemented in the mouth with a thin layer of resin cement. The bulk of the resin composite cement layer needed is small, producing a very small amount of shrinkage and low interfacial stresses. Composite inlays produced in this way are expected to show improved durability and increased wear resistance due to enhanced physical properties. The major drawback of this procedure is that a more extensive cavity preparation is required, and the procedure is significantly more expensive.
The organic polymer matrix has low thermal conductivity, and composites therefore provide good thermal insulation for the dental pulp. The thermal conductivity of all composites closely matches those of enamel and dentin and is much lower than that of dental amalgam.
As a consequence of the weak physical bonds by which individual polymer molecules are held together, polymers have a marked tendency to expand and contract in response to temperature changes. In contrast, the highly inorganic content of tooth structure is affected to a much smaller degree. Dimensional changes in the resin composite resulting from thermal cycling in the mouth (approximately between 5°C and 55°C) produce further strain on the bond at the tooth-composite interface, increasing the possibility of marginal percolation. This effect occurs to a larger extent with resin-rich microfilled composites than with the more highly filled hybrid materials. The normal thermal cycling of resin materials over extended periods of time also produces a thermal fatigue effect within the material that can be manifested in surface, bulk, or adhesive failure.
Water sorption and solubility
The polymer matrix is able to absorb water; thus, there is some swelling of the composite but not enough to counteract polymerization shrinkage. The uptake of water by composites has been correlated with decreases in surface hardness and wear resistance. As a result of their larger volume fraction of matrix, microfilled composites have higher water sorption values and therefore a greater potential for discoloration by water-soluble stains.
The solubility of resin composites ranges from 1.5% to 2.0% of the original material weight. The major component found to leach in water is residual oligomer or monomer, revealing that incomplete polymerization of the composite results in markedly increased solubility. Additional leachable molecules include degradation products of various composite components and may include formaldehyde, benzoic acid, and methacrylic acid. The largest part of the dissolution occurs within the first few hours of placement.
Elements from filler particles dissolve in water to varying degrees and are detected in quantities as high as 180 µmol/g. Boron and silicon are the main elements, but barium, strontium, and lead—other additives to glass particles—also leach out. The presence of silicon in solution may indicate degradation of the surface treatment of the filler.
Alcohol, a solvent of bis-GMA, and acidulated fluoride gels increase the rate of dissolution of filler particles. Therefore, alcohol-free rinses and neutral fluoride products should be used.
Darkening and a color shift to yellow or gray has often been noted in self-curing systems, and has been attributed to the tertiary amine accelerator, which produces color on oxidation. Photo-initiated systems do not contain a tertiary amine and have shown considerable improvement in color stability over long periods of time.
|COMPRESSIVE STRENGTH (MPa)||TENSILE STRENGTH (MPa)||YOUNG MODULUS (GPa)||HARDNESS NUMBER KNOOP (VICKERS)|
Under accelerated aging conditions in a weathering chamber, erosion of the resin matrix and exposure of filler particles of microfilled composites resulted in lightening of color. The color stability of microfilled composites, however, was affected by erosion only to a small degree.
A degree of radiopacity slightly exceeding that of enamel may be useful in diagnosis. Radiopacity may be conferred by incorporating elements of high atomic number, such as barium, strontium, and zirconium, into the filler. The relative number of these atoms is still small, however, and the materials are much less radiopaque than is amalgam. Almost all composites currently available have some degree of radiopacity. The international standard1 for the radiopacity of resin composites used in posterior restorations requires it to be equal or greater than that of enamel.2 This property is judged by comparing the radiograph of the material with that of an aluminum stepped wedge with graded radiopacities.
The higher compressive and tensile strengths of fine-particle and hybrid composites, as compared with microfilled composites, reflect the higher volume fraction of the high-strength filler component. Note that for all materials, compressive strengths are several times higher than tensile strengths, reflecting the somewhat brittle behavior of composites. More highly filled composites have tensile strengths near that of dentin and compressive strengths similar to or higher than that of dentin (Table 8-3). Some highly filled composites have compressive strengths greater than that of enamel.
The Young modulus, also called the elastic modulus