The nature of the bis-GMA diluent present in composite formulations greatly affects such properties of the cured resin as setting time, strength, water sorption, and coefficient of thermal expansion. J Dent Res 60(6):983-989, June 1981 Introduction.Composite resins comprise the majority of anterior dental restorations. These materials have many desirable properties; however, their overall performance (in particular, their finishing, wear resistance, adaptation to cavity walls, color and storage stability, adhesion to tooth surface, and biocompatibility) indicates a need for improvement.Most commercial materials use bis-GMA as the major monomeric ingredient. This substance has a molasses-like viscosity. To obtain pastes or liquids that can be mixed readily in the dental operatory, monomeric diluents are incorporated into the composite formulations. The role of the composition of the organic (polymerizable) phase in determining the wall-to-wall polymerization contraction of composite restorative resins has been demonstrated by Asmussen.1 Thus, the diluent monomer employed may influence the physical properties, as well as the marginal adaptation of bis-GMA-based composites. Different diluent monomers vary in chain length, viscosity, surface tension, storage stability, and solubility compatibility with bis-GMA. These properties affect the mixing characteristics and the polymerization of the material. Cured resins with different molecular weight, crosswhich must be minimized for a restoration possessing optimal marginal adaptation.The objective of this study was to determine the effect of the diluent monomer on the physical properties of both the monomer solution and the cured resin in the bis-GMA-based powder-liquid system. In subsequent work we will determine the influence of diluent on the marginal adaptation of dental composite restorations. Thus, the results of this study will allow the selection of composites with desirable characteristics for the subsequent measurements of marginal leakage.Materials and methods.* Preparation of liquid. -Liquids were prepared containing 0.2% bis-GMA and 0.23% butylated hydroxytoluene (BHT) as inhibitor.N,N-dimethyl-p-toluidine (DMPT) and various diluents of two main types were used: 1) RO(CH2)nOR where R = methacryl (H2C = 9-) and n = CH3 2, 4, 6, 10 (hydrocarbon diol series) and 2) RO(CH2CH20)nR, where n = 2, 3,^4 (glycol ether series). These and other miscellaneous diluents used in this study are given in Table 1. All diluents were used as received. To better compare the effect of the diluent component of the liquid on physical properties of the cured composites, various diluent concentrations were incorporated into the formulations.

The overall characteristics of the composites cured with a number of newly synthesized, tertiary aromatic amines compare favorably to those of resins polymerized with commonly used accelerators. Maximum compressive and tensile strength for the composites are obtained only over a narrow concentration range of accelerator used.

The characteristics of newly synthesized tertiary aromatic amines as accelerators for restorative resins have been evaluated. Comparison of the composites prepared with these and presently used accelerators indicate that resins formulated with 4-N,N-dimethylaminophenylacetic acid, its methyl ester or N,N-dimethylaminoglutethimide have properties generally better than comparable resins prepared with commercially used amines.

The aim of this study was to determine how otherwise acceptable diluent monomers affect the marginal adaptation of BIS-GMA-based composites. Based on the results of the investigation, the following conclusions can be drawn: 1. Addition to dimethacrylate diluents containing (CH2) recurring units generally yields composites having better marginal adaptation than do those containing (CH2 CH2 O) groups. Best marginal adaptation for a single diluent is obtained for compositions using 1, 4 and 1, 10-polymethylene glycol dimethacrylate as diluent. 2. Marginal adaptation is improved on lowering the diluent concentration. Optimum adaptation will be obtained for a formulation containing a minimum percentage of diluent with clinically acceptable working properties. 3. Volume changes on temperature cycling resulting from differences in thermal expansion coefficients of composites do not effect the marginal integrity as much as does curing shrinkage.