Photoelastically observed effects of heat treatment on polyethylene vinyl acetate

Objectives: During the course of work carried out on polyethylene vinyl acetate (pEVA) the effect of heat during processing on its’ physical properties was felt to be worth further investigation. Methods: For the heat treatment of pEVA a furnace was used to heat the test material to near its’ glass transition temperature (Tg) of 84°C ± 3 °C. The material was brought up to Tg and held at that point for 10 minutes. The specimens were then removed from the furnace and allowed to cool to room temperature. Heat treated and non-heat treated samples were placed in a polariscope to observe stress within the material. Dropweight impact tests were then conducted on all samples using an instrumented impact testing rig. All samples were circularly clamped and force-time and displacement-time plots obtained. The samples were placed again in the polariscope and any changes in stress were noted. To observe the processing effects of the manufacturing procedure five mouthguards were made on the same cast and were brought to various stages of completion. Results: The Peak Impact Force (PIF) of heat treated pEVA was lower (PIF<140N) than that of untreated pEVA (PIF=160N). The displacement of the heat treated sample during impact increased by 66%, (untreated pEVA>18mm centre displacement, heat treated pEVA >30mm centre displacement). Photographic results from the polariscope show that the heat treatment of pEVA virtually eliminates stress and following impact the amount of stress, seen photoelastically, was also reduced. Digital photographic images from the polariscope also indicate that the finishing techniques employed during the manufacturing process have a direct effect on the stress distribution within the mouthguard. Conclusions: The mouthguard forming process has a direct effect on the internal stresses of pEVA and therefore its’ physical response during an impact event will be different from that of unprocessed pEVA.