important to place these composites in increments due to limited depth of cure. In addition, special adhesives are needed.
Although the composite restoration appears hard and fully cured after exposure to the curing light source, the setting reaction continues for a period of 24 hours. All of the available unsaturated carbon double bonds of methacrylate-based composites do not react; studies report that about 25% remain unreacted in the bulk of the restoration. A thin layer of air-inhibited, unpolymerized material remains on the surface of the polymerized layer, which is advantageous for subsequent incremental place- ment during layering. It is useful to protect the sur- face of the contoured restoration with a transparent matrix, to minimize the amount of unpolymerized resin in the final restoration. Although for some composites the final physical properties may not be reached until about 24 hours after the reaction is initiated, enough of the mechanical strength is attained immediately after curing, so the restoration can be finished and polished with abrasives and is functional.
For most composites that are initiated by visible light, bright operatory lights can initiate cure prema- turely if the composite is left unprotected on a mix- ing pad. Within 60 to 90 seconds after exposure to ambient light, the surface of the composite may lose its capability to flow readily against the tooth, and further work with the material becomes difficult. The dispensed paste can be covered with an opaque or orange cover to present premature exposure to light.
The setting times for chemically activated com- posites range from 3 to 5 minutes. These short set- ting times have been accomplished by controlling the concentration of initiator and accelerator.
Polymerization Shrinkage and Stress
As explained in the section Polymerization Reactions, all composites undergo volumetric shrinkage upon setting. Typical shrinkage values are listed in Table 9.3. Volumetric shrinkage results in the development of contraction stresses as high as 13 MPa between the composite and the tooth structure. These stresses severely strain the interfacial bond between the com- posite and the tooth, leading to a very small gap that can allow marginal leakage of saliva and microor- ganisms. Recurrent caries and marginal staining may result. This stress can exceed the tensile strength of enamel and result in stress cracking and enamel frac- tures along the interfaces. Because the development of shrinkage stress depends on the volumetric shrink- age strain and the stiffness of the composite at the time of shrinkage, low-shrinkage composites might exhibit high stress if the composite has a high elastic modulus. Adding the composite in 2-mm increments and polymerizing each increment independently can reduce the net effect of polymerization shrinkage.
Net shrinkage stress is less because a smaller volume of composite is allowed to shrink before successive additions. A recent paper has reviewed the clinical and laboratory properties of several low-shrink/
low-stress composites. Most of these products are universal composites but two products are described as flowable composites. The shrinkage values are dependent on the method used. Volumetric polym- erization shrinkage for low-shrink universal com- posites using pycnometer varies from 0.9% to 1.8%, whereas that of low-shrink flowables is 2.4% to 2.5%.
When the ACTA linometer was used, the values were 1.0% to 1.4% and 2.6% to 2.9%, respectively. The polymerization stress was measured from 1.2 to 1.6 MPa. In comparison, traditional universal compos- ites have been reported to have polymerization stress of 0.8 to 2.4 MPa, whereas flowables were reported to have stress values ranging from 1.3% to 3.2%.
Thermal Properties
The linear coefficient of thermal expansion (α) of composites ranges from 25 to 38 × 10−6/°C for com- posites with fine particles to 55 to 68 × 10−6/°C for composites with microfine particles. The α values for composites are less than the mean of the constituents added together; however, the values are higher than those for dentin (8.3 × 10−6/°C) and enamel (11.4 × 10−6/°C). The higher values for the microfilled com- posites are related mostly to the greater amount of polymer present. Certain glasses may be more FIG. 9.10 Dual-curing composite for core build-ups.
(Courtesy DMG Chemisch-Pharmazeutische Fabrik GmbH, Hamburg, Germany.)
1499. REsToRATiVE MATERiAls: REsin CoMPosiTEs And PolyMERs
Property Nanocompositea
Multipurpose Composite
Microfill Composite
Packable Composite
Flowable Composite
Laboratory Composite
Core Composite
Conventional Glass Ionomer
Resin- Modified Glass Ionomer
Flexural strength (MPa) 180 80–160 60–120 85–110 70–120 90–150b ― 7–15 50–60
Flexural modulus (GPa) ― 8.8–13 4.0–6.9 9.0–12 2.6–5.6 4.7–15b ― ― ―
Flexural fatigue limit
(MPa) ― 60–110 ― ― ― ― ― ― ―
Compressive strength
(MPa) 460 240–290 240–300 220–300 210–300 210–280 210–250 10–15 200–250
Compressive modulus
(GPa) ― 5.5–8.3 2.6–4.8 5.8–9.0 2.6–5.9 ― 7.5–22 7.2–10.3 3.2–6.9
Diametral tensile
strength (MPa) 81 30–55 25–40 ― 33–48 ― 40–50 7–15 30–40
Linear polymerization
shrinkage (%) ― 0.7–1.4 2–3 0.6–0.9 ― ― ― ― ―
Color stability, accelerated aging: 450 kJ/m2 (ΔE*)c
― 1.5 ― ― 15 1.1–2.3 ― ― ―
Color stability, stained
by juice/tea (ΔE*)c ― 4.3 ― ― ― 1.7–3.9 ― ― ―
aFrom Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. J Am Dent Assoc. 2003;134(10):1382–1390.
bWithout fiber reinforcement.
cΔE < 3.3 is considered not clinically perceptible.
effective in reducing the effect of thermal change than are others, and some resins have more than one type of filler to compensate for differential rates.
Thermal stresses place an additional strain on the bond to tooth structure, which adds to the detrimen- tal effect of the polymerization shrinkage. Thermal changes are also cyclic in nature, and although the entire restoration may never reach thermal equi- librium during the application of either hot or cold stimuli, the cyclic effect can lead to material fatigue and early bond failure. If a gap forms, the differ- ence between the thermal coefficient of expansion of composites and teeth could permit percolation of oral fluids.
The thermal conductivity of composites with fine particles [25 to 30 × 10−4 cal/s/cm2 (°C/cm)] is greater than that of composites with microfine par- ticles [12 to 15 × 10−4 cal/s/cm2 (°C/cm)] because of the higher conductivity of the inorganic fill- ers compared with the polymer matrix. However, for highly transient temperatures, the composites do not change temperature as fast as tooth struc- ture and this difference does not present a clinical problem.
Water Sorption
The water sorption of composites with hybrid particles (5 to 17 μg/mm3) is lower than that of composites with microfine particles (26 to 30 μg/
mm3) because of the lower volume fraction of polymer in the composites with fine particles.
The quality and stability of the silane coupling
agent are important in minimizing the deteriora- tion of the bond between the filler and polymer and the amount of water sorption. Expansion associated with the uptake of water from oral fluids could relieve some polymerization stress, but water sorption is a slow process when com- pared to polymerization shrinkage and stress development. In the measurement of hygroscopic expansion starting 15 minutes after the initial polymerization, most resins required 7 days to reach equilibrium and about 4 days to show the majority of expansion. Because composites with fine particles have lower values of water sorp- tion than composites with microfine particles, they exhibit less expansion when exposed to water.
Solubility
The water solubility of composites varies from 0.25 to 2.5 mg/mm3. Inadequate light intensity and dura- tion can result in insufficient polymerization, particu- larly at greater depths from the surface. Inadequately polymerized composites have greater water sorption and solubility, possibly manifested clinically with early color instability.
During the storage of microhybrid compos- ites in water, the leaching of inorganic ions can be detected; such ions are associated with a break- down in interfacial bonding. Silicon leaches in the greatest quantity (15 to 17 μg/mL) during the first 30 days of storage in water and decreases with time of exposure. Microfilled composites leach silicon more slowly and show a 100% increase in amount during the second 30-day period (14.2 μg/mL).
Boron, barium, and strontium, which are present in glass fillers, are leached to various degrees (6 to 19 μg/mL) from the various resin-filler systems.
Breakdown and leakage can be a contributing fac- tor to the reduced resistance to wear and abrasion of composites.
Color and Color Stability
The color and blending of shades for the clinical match of esthetic restorations are important. The characteristics of color are discussed in Chapter 4, and these principles can be applied specifically to composites for determining appropriate shades for clinical use. Universal shades vary in color among currently marketed products. Modern-day compos- ites are often supplied by the manufacturer in multi- ple opacities. This allows for better esthetic outcome using multiple shades of different opacities to con- struct the restoration.
Change of color and loss of shade match with surrounding tooth structure are reasons for replac- ing restorations. Stress cracks within the polymer matrix and partial debonding of the filler to the TABLE 9.4 Requirements for Polymer-Based Filling and
Restorative Materials Based on iso 4049
Property Class 1 Class 2 Class 3
Working time (min,
seconds) 90 — 90
Setting time
(max, min) 5 — 10
DEPTH OF CURE (MIN, MM)
Opaque shades — 1.0 —
Other shades — 1.5 —
Water sorption (max,
μg/mm3) 40 40 40
Solubility (max, μg/mm3) 7.5 7.5 7.5 FLEXURAL STRENGTH (MPA)
Type 1 80 80a
100b 80
Type 2 50 50a 50
aGroup 1: cured intraorally.
bGroup 2: cured extraorally.
resin as a result of hydrolysis tend to increase opac- ity and alter appearance. Discoloration can also occur by oxidation and result from water exchange within the polymer matrix and its interaction with unreacted polymer sites and unused initiator or accelerator.
Color stability of current composites has been studied by artificial aging in a weathering chamber (exposure to UV light and elevated temperatures of 70 °C) and by immersion in various stains (coffee/
tea, cranberry/grape juice, red wine, and sesame oil). As shown in Table 9.3, composites are resistant to color changes caused by oxidation but are suscep- tible to staining.