EFFECTS OF CURING TIME AND FILLER CONCENTRATION ON CURING AND POSTCURING OF URETHANE DIMETHACRYLATE COMPOSITES - A MICROCALORIMETRIC STUDY

The isothermal enthalpy changes with time of a dental composite were e xamined by microcalorimetry to isolate the effects of different filler concentrations and curing times on chemical aging of these composites . Urethane dimethacrylate (UDMA) monomer, zirconia-silica (ZS) powder, and 3-methacryloxypropyltrimethoxysilane (MAPM) were used as organic and inorganic matrices, and a coupling agent, respectively. The compos ite was mixed in different ratios and cured by visible light. The enth alpy changes with time for 0, 15, 45, 75% ZS-filled UDMA and 75% MAPM- silanated ZS-filled UDMA cured for 13, 30, 90, 150, and 300 s were mea sured at 37.0 degrees, 57.0 degrees, and 65.5 degrees C until equilibr ium. Increased curing time and filler concentration caused the excesss enthalpy changes (dH) and their rate of change (dH/dt) to increase wi th annealing time and apparent equilibrium was reached faster. In addi tion, dH showed non-linear dependence with the increase in filler conc entration by showing a maxima for samples containing 25 wt % filler. F urther, filler silanation caused dH/dt to increase and required shorte r times to reach apparent equilibrium. dH also reached a minimum when samples contained silanated filler, compared to composites containing unsilanated filler. It was concluded that the shorter curing time caus ed the occurrence of spontaneous densification, which facilitated cont inual resin curing; and longer curing time caused higher crosslinking of the organic phase. Moderate concentration of inorganic phase restri cts the molecular motion of the surface layer of polymer onto filler p articles, and the polymer is regarded as highly crosslinked, while a h igher filler concentration forms aggregates that are covered by the po lymer which causes a decrease in the molecular packing of the resin, a nd is reflected as low enthalpy values. Finally, silanation of the fil ler showed a highly endothermic reaction that is probably due to break ing and forming of bonds at the interface between the organic and the inorganic phases in the composites. (C) 1998 John Wiley & Sons, Inc.