Effect of Heating Delay on Conversion and Strength of a Post-cured Resin Composite

Abstract

Physical property enhancement in light-cured resin composites from post-cure heating is attributed to free radicals created during initial photocuring, the number of which decreases following initial light-curing. Clinically, it is important to know when the number of remaining free radicals is too low to provide for additional conversion of monomer in post-cure-heated specimens. The hypothesis tested is that the potential for additional conversion in post-cure-heated resin composite restorations is dependent upon the time after initial light-curing at which the specimen is exposed to heat treatment. This research examined the effect of delay in post-cure heating after initial photo-activation on strength and monomer conversion of a commercial resin composite material. Discs (10 x 1 mm) of Herculite XRV (Kerr/Sybron, Orange, CA) were photocured at standardized conditions. One group was left unheated, and another was subjected to post-cure heating (Brilliant DI-500, Coltène AG, Altstätten, Switzerland) at the following times after being light-cured: 5 and 30 min, and 6, 24, 48, 72, 96, and 120 hrs. After the appropriate delay time, unheated and heated specimens (n = 10) were tested for biaxial flexural strength at a constant stressing rate. Recovered, fractured strength specimens (n = 10) were analyzed for cure by means of IR spectroscopy. Post-cure heating increased strength over that of the unheated specimens only for the shortest delay times: 5 or 30 min. Thereafter, strength values were statistically equivalent (p < 0.05). Delay in heating did not significantly enhance strength of post-cure-heated specimens, but delay in time did improve strength of the unheated groups. The greatest monomer conversion was obtained when post-cure heating was applied within 6 hrs following light-curing. The difference in cure between unheated and heated specimens remained significant up to 96 hrs of delay. Flexural strength of post-cure-heated specimens remained unchanged with time delay for heating specimens. Maximal monomer conversion of post-cured specimens is obtained only within 6 hrs of light-curing. The potential for additional conversion arising from post-cure heat treatment is dependent upon the time following initial curing at which heat is applied following initial light-curing. However, delay in heat application has no influence on flexural strength.

Pre-heated dual-cured resin cements: analysis of the degree of conversion and ultimate tensile strength

“…Although the increased temperature resulted in significantly higher DC in both RCs, the 8% to 10% increase in the DC values had no influence on the UTS of cements light-activated at 50º C. According to Loza-Herrero et al, 20 a parabolic relationship can be observed between DC and mechanical properties. In that study, as conversion increased, strength values tended to plateau or even decline.…”

“…As a consequence, the material develops high shrinkage strain and internal stress during polymerization. 20,21 Moreover, the required slow homogenization of the internal stress created in the polymers during polymerization may be compromised by such a fast cure rate as well. 20 The SC groups of VII cured at 50º C exhibited higher UTS than at 25º C. It has been shown that the degradation rate of the self-curing component benzoyl peroxide to radicals increases with increased temperatures.…”

“…20,21 Moreover, the required slow homogenization of the internal stress created in the polymers during polymerization may be compromised by such a fast cure rate as well. 20 The SC groups of VII cured at 50º C exhibited higher UTS than at 25º C. It has been shown that the degradation rate of the self-curing component benzoyl peroxide to radicals increases with increased temperatures. 22 For this reason, more radicals are created more quickly, so the decrease in resin viscosity and increased radical mobility promoted by higher resin temperatures during polymerization most likely were not the only factors contributing to the higher DC and UTS values in the heated RCs when polymerization was initiated via this curing condition.…”

Heat treatment of a direct composite resin: influence on flexural strength

“…According to Loza-Herrero et al 6 (1998), heat application, aiming at an increase in the degree of conversion, must be used, ideally, immediately after photoactivation. Higher monomer conversion took place up to 6 h after photoactivation.…”

“…These can be used as an option to improve the mechanical properties of direct composite resins. 3,6,7 An advantage would be the low cost. Besides that, this treatment can provide an increase in the degree of conversion, also named as secondary cure or post cure.…”