Force application and decay characteristics of untreated and treated polyurethane elastomeric chains.

In this study the effects of prestressing, environmental acidity, oxygen content, and temperature on mechanical properties were measured for three polyurethane-based orthodontic elastomeric chains. Specimens of each chain were treated for 10 and 100 days, and their mechanical properties were compared with those of the untreated specimens via stress-relaxation tests. Conditioning treatments were determined to affect the magnitude of residual load after relaxation, with the largest effect due to prestressing. Among the other variables studied, an increase in temperature of the environment appeared to significantly influence the degradation mechanism responsible for the deterioration of the mechanical properties of polyurethane elastomers (p < 0.001); acidity and oxygen content had no significant effects. Force decay profiles from the specimens were derived using a Maxwell-Weichert model consisting of springs and dashpots. This model allows the orthodontist to predict load magnitude supplied by the chain at activation and at any time during treatment.