Objectives : Denture soft liner materials are known to harden during clinical use. This study evaluated hardness changes in a variety of commercial soft liner products during long-term water storage.

Materials and methods : Three disc-shaped specimens (6 mm thick) were made of polysiloxane-based products (Sofreliner S, Sofreliner Tough M, GC Reline Extra Soft, GC Reline Soft, Ufi Gel SC, SoftSil 25, Molloplast B), and a plasticized acrylic-based material (EverSoft) following manufacturer’s instructions. Cured specimens were stored in distilled water (grade 3) at 37 °C up to 111 days. Shore A surface hardness measurements (Model S1 Handheld, Digital Durometer, Instron Corp.) were made using a 5-s dwell time. Five measurements were taken on each specimen after 1, 2, 7, 9, 13, 20, 28, 40, 55, and 111 days of water storage. Data were analyzed using two-way, repeated measures ANOVA and pair-wise comparisons were done using Tukey’s HSD test at a pre-set alpha of 0.05.

Results : SofrelinerS showed the lowest initial hardness value (6.8 ± 0.7) whereas Molloplast B showed the highest (42.7 ± 0.9). After 111 days, hardness values of all materials increased significantly ( p < 0.05), except for Molloplast B ( p > 0.05). The largest hardness changes were observed during the first 13 days storage. SofrelinerS showed the highest overall change (357%) and Molloplast B showed the lowest change (2%) during the immersion period. The plasticized acrylic (Eversoft) demonstrated a 12% increase in hardness and was among the materials that exhibited minimal change.

Conclusions : Hardness of the polysiloxane-based soft liner Molloplast B did not change during storage, but gradual hardening of all other soft liner products was found over the immersion period, with some products demonstrating only small increases, and others exhibiting very great ones. Clinicians need to be aware of the variation in hardness change with time among commercial soft liner products to best select the material for a specific clinical treatment.