Polymerization shrinkage versus Conversion in Commercial Composites

Composite polymerization shrinkage is a significant problem and is directly proportional to conversion. Objectives: The goal of this investigation is to determine the polymerization shrinkage and the extent of conversion achieved under consistent polymerization conditions. Methods: 32 commercial dental restorative and flowable composites were tested. Polymerization shrinkage was followed for 10 minutes on a linometer, with 40s exposure to 500 mW/cm². The specimen was transferred immediately to the FTIR and the near-IR absorption peak area at 6170 cm-1 was measured. Another specimen of the same diameter was created in a mold in which both the monomer and polymer absorption peak area could be measured. The monomer absorption peak was used (with thickness normalization) to determine the conversion at 10 minutes for each of the shrinkage specimens. In addition one composite was examined at a series of total energy exposures from 0.4 – 20 J by varying exposure time (2-40s) and light radiance (200-500 mW/cm²). The shrinkages and conversions were determined for these specimens. Results: The polymerization shrinkage and conversion versus total energy exposure was a direct relationship approaching a plateau above 5 J (shrinkage ~ 2.5%, conversion ~ 0.55). There are still significant increases in conversion and shrinkage that occurs up to at least 20 J. The restorative composites varied between 0.5% to 3.0% shrinkage at 40s; the flowables, 4.0 to 5.5%. The conversions for restorative materials varied between 0.4 and 0.6 with some flowables somewhat higher at the same total energy of 20 J produced by 500 mW/cm² at 40s. Conclusions: The extent of polymerization shrinkage increases with increased conversion, but that is one of several factors contributing to shrinkage. The volume percent filler needs to be evaluated also. (Gift supporting student research from Septodont/Confi-Dental)