Varying Hydrophilic Resin Composite Matrices Affects on Strength versus Time

Composite resin matrices can be made from dimethacrylates with varying potentials to absorb water. A new comonomer system using dimer-acid dimethacrylate (DAD) polymerizes into a phase separating structure which could have unique water sorption potential. Water sorption can affect flexural strength and flexural modulus over time varying with hydrophilicity of the resin. Also the composite continues to have a slow increase in conversion which can increase strength and modulus.

Objective: Determine the effects of water sorption and continued conversion on flexural strength of different comonomer composites exposed to varied intervals of either wet or dry storage conditions.

Methods: Composites were formulated with the following comonomer systems: BisGMA/TEGDMA, UDMA/TEGDMA, EthoxyBisGMA/HDDMA, BisGMA/HDDMA, and UDMA/EthoxyBisGMA/DAD. Each resin was filled 75wt% with conventional glass filler. Flexural strength beams 2mmx2mmx25mm were prepared, and conversion determined initially with FTIR spectra. The specimens were stored in either water or dry at 37°C, for 24 hours, 48 hours, and 1 month. At time of testing the composites were allowed to equilibrate to ambient temperature, the conversion determined again and the specimens tested in 3-point bending.

Results: The highest initial strength was observed in the dimer-acid dimethacrylate composite. By 2 days the wet storage conditions produced lower flexural strengths than the dry storage for all composite formulations. The differences increased by one month. The flexural modulus remained constant or increased when stored dry. The wet storage conditions decreased the modulus in most composite formulations by one month. Flexural strengths varied from 80 to 150 MPa for the various comonomer composites.

Conclusion: Generally the wet storage conditions caused a lower flexural modulus and flexural strength than that produced by dry storage conditions for the same composite stored dry. The highest initial strength was exhibited by the dimer-acid based composite.

Supported by NIH/NIDCR 2R01DE14227 and a gift from Septodont