Studies of orthodontic elastomeric modules. Part 1: glass transition temperatures for representative pigmented products in the as-received condition and after orthodontic use.

The purpose of this study was to investigate the glass transition temperatures (T(g)) of representative elastomeric chain products (plastic modules) in the as-received condition and after orthodontic use to determine differences between brands and pigments. Values of T(g) were determined by differential scanning calorimetry. Products were obtained from 3 manufacturers: Rocky Mountain Orthodontics (RMO, Denver, Colo), Ormco (Glendora, Calif), and G&H (Greenwood, Ind). Three colors (gray, red, and purple) were selected for each brand-pigment combination to evaluate the as-received products, and test specimens for each brand-pigment combination from the same batches were placed in patients' mouths for 4 weeks to evaluate the products after clinical use (sample size of 7 specimens for both groups of experiments). Results were analyzed statistically by multiple nonparametric Mann-Whitney tests. A post hoc step-down Bonferroni analysis followed, to examine differences in T(g) due to pigmentation within brands and differences due to brands within each pigmentation category. For the as-received products, the RMO modules had mean T(g) ranging from -24 degrees C to -21 degrees C, whereas the Ormco and G&H modules had significantly lower mean T(g), ranging from -46 degrees C to -39 degrees C, indicating substantial compositional or polymer structural differences compared with the RMO modules. After clinical use, the RMO products had mean T(g) ranging from -31 degrees C to -25 degrees C, whereas the Ormco and G&H products had mean T(g) ranging from -46 degrees C to -30 degrees C. The in vivo specimens also exhibited a second, higher-temperature glass transition of unknown origin. Results suggest that the Ormco and G&H products should have greater flexibility than the RMO products and that there should be significant differences in clinical force-degradation behavior for the Ormco and G&H products compared with the RMO products.