DC-normalised composite-shrinkage versus volume-fractions of spherical-filler

Objectives: Conventional wisdom accepts that resin-composite shrinkage decreases with increasing filler-loading (V_f). However, degree-of-conversion (DC) may also decrease with V_f. Hence the aim was to investigate the null-hypothesis that shrinkage/DC ratios may be invariant over specified ranges of V_f.

Methods: Three series of model spherical-particle hybrid composites, C, P and D, were investigated, each comprising four filler-fractions spanning the range: 55-75 % w/w or 37-52 % v/v. The matrix chemistry: bisGMA/TEGDMA (60:40); CQ/DMBE was common to all series. C series had SiO₂-TiO₂ fillers (80 nm); P series had SiO₂-ZrO₂ fillers (150 nm); D series had SiO₂-ZrO₂ fillers (450 nm). Degrees-of-Conversion (DC) of all composites were measured at 23 ^oC by FTIR on thin films, (10 min after 60 s irradiance: 200, 400 and 600 mW/cm², by QTH). The 600 mW/cm² sub-groups were re-measured after 1 h and 1 & 7 d. Shrinkage-strain measurements were made on all composites at 23 ^oC (after 60s irradiance 200-400-600 mW/cm²) with the bonded-disk method, continuously for 1 h. DC, final shrinkage-strain (S_max) and strain-rate data were analysed by ANOVA, post hoc tests and linear regression.

Results:  For each composite, DC increased moderately with irradiance and with post-irradiation time (t_pi). Globally, DC ranged from 53 – 71 %. For each series, DC diminished progressively with V_f by 3-6 percentage-points. Qualitatively similar trends were observed in S_max, with a global range from 3.1 to 5.3 %. For each series, again S_max diminished progressively with V_f. Nevertheless, when appropriate normalised ratios of (S_max/fractional DC) were plotted against V_f, these still diminished, but moderately, from circa 8 to 6 over the V_f  range 42-52 %. NH rejected.

Conclusions: DC-reductions with filler-volume-fraction are attributable to viscosity increases. For these spherical-particle composites, the attenuated (DC-normalised) shrinkage-strain reductions with V_f are of practical importance and theoretical-design interest.