A slow contraction may result in reduced gap formation when a restorative resin polymerizes in a dental cavity. It was the aim in the present work to investigate the rate of contraction in relation to composition of experimental light-curing resin composites. The monomer of the resin composites consisted of mixtures of BisGMA, TEGDMA, and in one series HEMA. The resins contained varying amounts of initiators, co-initiators, and inhibitor, and were made composite by adding a silanized glass filler to a content of 74% by weight of the composite paste. The polymerization contraction up to 120 sec was determined by means of the bonded-disk method. Within the ranges studied, the concentration of initiator and co-initiator in the monomer mixture had only an insignificant influence on rate of polymerization. In comparison to camphorquinone, the initiators 1-phenyl-1,2-propanedione and benzil reduced the rate of polymerization without affecting the final contraction. In comparison to N,N-dimethyl-p-aminobenzoic acid ethyl ester, N,N-cyanoethyl methylaniline was as effective, while N,N-diethanol-p-toluidine was less effective as co-initiator. A relatively high content of the inhibitor methoxyhydroquinone reduced the initial rate but not the final polymerization contraction. The rate of polymerization increased with the level of HEMA and TEGDMA in the monomer mixture. It was concluded that intrinsic slow cure may be obtained with certain compositions of resin composites without impairing the final extent of polymerization.
