Quantifying the Optimal Two-Step Photocuring Protocol for Maximally Reduced Shrinkage Stress during Photopolymerization

Background

Shrinkage stress accumulated during photopolymerization significantly impairs the quality and lifespan of photopolymerized materials. Soft-start photocuring protocols like ramp, two-step, and pulse-delay have been proposed to mitigate this issue, among which the two-step protocol has been proved to be the most effective. However, the accuracy and underlying mechanisms of the previously proposed strategy for quantifying the optimal two-step protocol have not been validated.

Objective

In this study, the universality of the strategy for quantifying the optimal two-step photocuring protocol was validated experimentally under varying working conditions, and the mechanism was systematically investigated to correct previous conjectures.

Methods

Shrinkage stress and reaction temperature of typical methacrylate resins during photopolymerization were measured under various working conditions (irradiation intensity, stiffness of constraint, and particle filling content of the testing material) using a standardized cantilever beam instrument.

Results

The optimal first irradiation time and delay time were identified based on the initiation of the stress evolution and the inflection point where shrinkage stress stabilized under the standard protocol, respectively. This optimal protocol resulted in a 20%-40% reduction in the shrinkage stress across all the working conditions tested with the shortest total curing duration. The stress reduction can be attributed to delayed gelation and a lower peak temperature change after gelation.

Conclusions

The present study not only validates the broad applicability of the optimal two-step curing protocol for maximally-reduced shrinkage stress, it also uncovers the underlying mechanism that should guide the manufacturing and application of photopolymers for improved service quality and longevity.