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The extent of polymerization of light-cured composite resins cured in relatively large class II cavities was estimated by measuring Knoop hardness on sectioned surfaces of resin restorations. The influences of the incremental placement technique, exposure time, and post-curing of resin inlay on the extent of polymerization of two different resin materials were examined. For both MFR type (HE) and hybrid type (P50) materials, the 3-step incremental placement technique was superior when the cavity was restored by the direct filling method. For resin inlays, the post-heating improved the conversion of resin restorations. However, because the final degree of conversion of resin attained by heating was influenced by the initial conversion of the resin at the stage of light-curing, the 2-step incremental placement technique was recommended at the light-curing stage for even resin inlays. As the exposure time is influenced by the transmittance of material and the intensity of activator light, it should be optimized in accordance with manufacturers' directions.

In an effort to improve the light transmittance of light-cured composite resins while avoiding degradation of their physical properties, 4 dimethacrylates having low refractive indices and a bulky backbones such as alicyclic or fluorine-substituted bisphenol groups were synthesized. The depths of cure and physical properties of 6 experimental composite resins containing these new monomers were, then, examined and compared to those of 3 control composite resins containing UDMA, BisMEPP, or BisGMA. The depths of cure of the experimental group containing the synthesized monomers were greater than those of the control group with the exception of the composite containing UDMA. The depth of cure increased as the difference between the refractive indices of the matrix monomer and silica filler decreased. The physical properties of 2 composites in the experimental group were comparable to those of the control group. The matrix monomers of these 2 composites were a mixture of 4,8-dimethacryloxymethylene tricyclo [5.2.1.0] decane or 2,2-bis (4-methacryloxyethoxy phenyl) hexafluoropropane and TEGDMA in a molar ratio of 1/1.

To improve the transmittance of light-cured composite resins, the matching of the refractive indices of the matrix monomer and filler were examined. Eight fluorinated aromatic dimethacrylates and diacrylates with a low refractive index were synthesized. Three of these monomers had a poor solubility to TEGDMA. The depths of cure and the other physical properties of eight experimental composites prepared from one of five fluorinated monomers, TEGDMA and UDMA were determined compared with those of the control composites prepared from Bis-GMA or Bis-MEPP/ and TEGDMA in the same ratio. These composites contained 70 wt% of silanized silica filler. The depths of cure of the experimental composites except for one were significantly greater than those of the control composites. The mechanical properties of the experimental composites were comparable to those of the control composites with a few exceptions, but the difference in the mechanical properties between these exceptional composites and the control was small. The water sorption of the experimental composites was markedly lower than that of the composite containing Bis-GMA and comparable to that of the composite containing Bis-MEPP. These five fluorinated monomers are considered useful as the matrix monomer of light-cured composite resins.

The durability of four commercially available light-cured composite resins was investigated by thermal cycling, GR containing inorganic fillers treated with the graft polymerization of acryl ester, LF inorganic fillers treated with a silane coupling agent, PC silanized inorganic fillers and organic composite fillers, and the MFR-type SI containing the organic composite fillers. These materials were given 10,000, 30,000 and 50,000 thermal cycles (4 degrees C-60 degrees C) and the deterioration of materials by thermal cycling was evaluated by the measurement of the mechanical properties and the SEM observations of the surface of the thermocycled materials. Compressive strength and bending elastic moduli for all materials did not change greatly by thermal cycling. However, bending strength, toothbrush abrasion resistance and surface hardness decreased with increasing number of thermal cycles between 0 and 30,000, and changed little after 30,000 cycles. The percentage of bending strength after 50,000 thermal cycles to that of the non-thermocycled sample was 75% for GR, 60% for LF, 50% for PC and 65% for SI, respectively. Deterioration of materials was observed as cracks on the surface, which generated at the interface of the filler and matrix. The cracks generated relatively earlier during thermal cycling for SI and PC which contained the organic composite filler, later for LF which contained the silanized inorganic fillers, and the number of cracks on LF were fewer than SI and PC. On the other hand, for GR, no cracks were observed even after 50,000 thermal cycles. From these results, it can be presumed that the pre-treatment of filler by the graft polymerization is more effective to improve the durability of composite resin.

The various mechanical properties of seven microfilled composite resins were evaluated, comparing with two conventional composite resins. The following results were obtained. 1) Though microfilled composite resins were more excellent than conventional ones on surface roughness after polishing or brushing, but they were inferior to conventional ones on the resistance to brushing abrasion. 2) Tensile, compressive and bending strength were dependent on inorganic filler content and properties of matrix resin. Particularly, their mechanical properties in the wet condition were strongly influenced by whether matrix resin was hydrophilic or hydrophobic. Compared with conventional composite resins, microfilled ones contained 30 to 40 wt% of filler had lower values of tensile and bending strength, and near values of compressive strength. While, microfilled ones contained 50 to 60 wt% of filler had higher values of compressive strength, and near values of tensile and bending strength, compared to conventional ones. 3) Modulus of elasticity and hardness were mainly dependent on inorganic filler content. Moduli of elasticity of microfilled composite resins were about 25 to 33% of them of conventional ones, and hardnesses of microfilled ones were approximately 50% of them of conventional ones except some materials. Hardnesses of microfilled composite resins with 50 to 60 wt% of filler were similar to them of conventional ones.

Method of impact test for composite resins was investigated according to JIS K 7110, "Method of Izod impact test for rigid plastics". The following views were suggested from the results of the experiments. 1) For the heterogeneous paste-type materials such as composite resins, the test specimens should be prepared with a metal mold that can add the V-notch to a test piece on polymerization process, rather than mechanically done with frise cutter. 2) On the testing according to JIS K 7110, sample could not be saved in large amount against our expectation. Therefore, it may be necessary to develop the testing machine that has a smaller capacity.

Linear coefficients of thermal expansion (beta) of twelve restorative composite resins, one Bis-GMA based resin and one self-curing acrylic resin were determined in the temperature range of 4 to 60 degrees C under the dry and the wet conditions on a thermomechanical analyzer. The results were as follows. 1) The thermal expansion curves obtained were nonlinear, and the transition appeared at about 40 degrees C. 2) beta reduced with the elapse of stored time and the number of times of measuring run. It appears that these phenomena were attributable to decrease of monomer remained in each case. 3) beta under the wet condition was larger than one under the dry condition, when specimens were stored for 20 days in each condition at 37 degrees C. Values of beta for composites ranged from 16.1 to 35.3 x 10(-6)/degrees C under the dry condition and from 18.1 to 55.4 x 10(-6)/degrees C under the wet condition. 4) beta of composites varied with filler content and kinds of filler and resin used. Value of beta reduced with increase of filler content, and was lower for Bis-GMA based composite resin than for MMA based composite resin.

Thermal expansion, expansion by water sorption, water sorption and solubility to water, which will influence to the marginal closure of cavities restored, were evaluated in seven microfilled composite resins, comparing with two conventional composite resins. The following results were obtained. 1) Coefficients of linear thermal expansion of microfilled composite resins were 41 to 84 x 10(-6)/degrees C in the dry condition and 57 to 93 x 10(-6)/degrees C in the wet condition. Those values were about 2 to 3 times those of conventional composite resin, and about 6 to 8 times that of teeth. 2) Linear expansion by water sorption was highly correlated to water sorption. Microfilled composite resins had higher values of liner expansion by water sorption compared to conventional ones. 3) Generally, water sorption of microfilled composite resins were higher than those of conventional ones. After 7 days, they were about 0.4 to 1.9 mg/cm2. Values of microfilled ones which contained larger amount of filler or was composed of more hydrophobic matrix resin, however, were lower than others. 4) Solubilities to water of microfilled composite resins also were higher than those of conventional ones. Particularly, microfilled ones which were used organic composite filler and UDMA monomer as base material and photopolymerized had relatively higher values.