Dimensional change in dental polymeric composite materials

Linear coefficients of thermal expansion and dimensional changes during setting of some dental polymeric composite materials were determined using thermomechanical analysis. Composite specimens were subjected to five successive heating and cooling cycles (25–160°). Three distinct regions were obtained in the first heating run but not in successive runs. A “hysteresis” behaviour was noted in the first thermal cycle as a result of the materials entering a transitional phase of further curing as well as of the release of all the internal stresses. The linear pattern of behaviour obtained in the successive runs, with lower expansion coefficient values, was attributed to both completed polymerization reaction and release of stresses. Immediately after the end of mixing, composite specimens were subjected to isothermal heating at 37° for 24 hr. A polymerization contraction ranging from 0.07 to 0.17% was observed. Such a contraction was pronounced in the first 2–4 hr of setting owing to the marked conversion occurring immediately after mixing.