Polymerization development of “low-shrink” resin composites: Reaction kinetics,polymerization stress and quality of network

Objectives

To evaluate “low-shrink” composites in terms of polymerization kinetics, stress development and mechanical properties.

Methods

“Low-shrink” materials (Kalore/KAL, N’Durance/NDUR, and Filtek P90/P90) and one control (Esthet X HD/EHD) were tested. Polymerization stress (PS) was measured using the Instron 5565 tensometer. Volumetric shrinkage (VS) was determined by the ACTA linometer. Elastic modulus (E) and flexural strength (FS) were obtained by a three-point bending test. Degree of conversion (DC) and polymerization rate (Rp) were determined by NIR spectroscopy (6165 cm⁻¹ for dimethacrylates; 4156 and 4071 cm⁻¹ for P90). Photopolymerization was performed at 740 mW/cm² × 27 s. Glass transition temperature (T_g), degree of heterogeneity and crosslink density were obtained in a DMA for the fully cured specimens. Analysis of extracts was done by ¹H NMR. Data were analyzed with one-way ANOVA/Tukey's test (α = 0.05).

Results

The control presented the highest shrinkage and T_g. P90 showed the highest modulus, and NDUR demonstrated the highest conversion. The polymerization rates were comparable for all materials. NDUR and KAL had the highest and the lowest network homogeneity, respectively. The multifunctional P90 had the highest crosslink density, with no difference between other composites. The control had the greatest stress development, similar to NDUR. Crosslinking density and polymer network homogeneity were influenced by degree of conversion and monomer structure.

Significance

Not all “low-shrink” composites reduced polymerization stress. P90 and NDUR had no leachable monomers, which was also a function of high crosslinking (P90) and high conversion (NDUR).