Mechanical Test for Assessing Early Changes in UV-Radiated Facial Elastomers

Tensile and tear tests outlined in ASTM D412 & D624 for rubber cannot detect mechanical property changes occurring during early damage of weathered facial elastomers, particularly along thin edges of a prosthesis. Objective: This study assessed a fatigue protocol for identifying early physical changes occurring within polydimethyl siloxane (PDMS) following short-term exposure to ultraviolet-B (UVB) radiation. Methods:  PDMS dumbbells were constructed with 13 wt % silica, no pigment, thickness either 2 mm or 0.3 mm and placed into one of three groups: (I) immediate mechanical testing, (L) 600 h UVB exposure (0.72 J/cm²/h), (D) 600 h storage in darkness (n=6). A modified Stepwise Increasing Strain Test (El Fray et al., Polymer, 2004) was followed to produce 4 stress-strain (s-s) curves for each dumbbell:  (1) baseline, (2) after 1000 cycles 25% tensile strain, (3) additional 1000 cycles 50% strain, (4) additional 1000 cycles 100% strain.  Triangular waveforms and 1 Hz cycling were followed.  Three parameters were measured for each s-s curve:  E modulus at 80-100% strain, area within load-unload hysteresis (strain energy lost), area below unload curve + ½ hysteresis area (strain energy stored). 3-way ANOVA/Tukey assessed effects of dumbbell thickness, weathering condition and no. cycles on each measured s-s parameter. Results: Thickness: Thin dumbbells exhibited higher modulus (662±15 vs 426±15 kPa), energy loss (24.6±1.5 vs 19.0±1.5 kPa-mm/mm) and energy stored (510±16 vs 337±16 kPa-mm/mm) than thick dumbbells (all p≤0.05). Weathering:  Modulus values for thin dumbbells were L>D>I (p≤0.05); n.s.differences were observed for thick dumbbells.  Energy lost: thin dumbbells L=D>I; n.s.differences for thick. Differences in energy stored were n.s. for thick and thin specimens.  Cycles: Cycle no. rendered few effects on measured properties. Conclusion: Elastic modulus measurements of thin dumbbells provided the highest sensitivity for this testing regimen. Supported by UNMC COD SSRF and VA Rehabilitation R & D grant A4290I.