Laminated structures for sports mouthguards
The aims and objectives of this study are to examine the role of mouthguards and the materials that are used for their manufacture, the heating process they undergo during manufacture and how the lamination of different materials into a multi-layered system to form a composite structure may affect the impact absorbing capabilities. The effect of heat on pEVA during the manufacturing process was investigated using an instrumented dropweight impact testing rig and a polariscope to observe internal stress as it was felt that the physical properties of the material could be adversely affected by this part of the process. Laminated structures, using several different materials, (pEVA, PMMA, silicone rubber, synthetic wax, modelling clay, soft denture lining material and a semi-solid synthetic rubber) were tested as it was felt that the lamination of different materials with a range of physical properties would exhibit less deformation and transmit less of the impact energy. To ascertain how a mouthguard may react during an impact event by simulation tests in the impact test rig. 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 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. Different 'lay-ups' of pEVA along with laminations and sandwiches of pEVA, PMMA, silicone rubber, synthetic wax, modelling clay, semi-solid synthetic rubber and denture soft lining were also tested using the dropweight impact tester. For the impact simulation tests samples of 50mm. diameter were placed on top of a PMMA substrate, that was clamped in the impact rig, to see how the test sample would protect the substrate during impact. 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). Digital photographic images 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. Images of material in the polariscope also indicate that the finishing techniques employed during the manufacturing process have a direct effect on the stress distribution within the mouthguard. A 5mm laminated structure of pEVA, PMMA and silicone rubber was able to absorb more impact energy (PIF = 275N) and exhibited less deformation (1.4mm) than that of a monolithic structure of 5mrn heat treated pEVA (PIF <140N, displacement >30mm). Simulation tests showed that the 5mm. thick pEVA protected the PMMA better (PIF = 325, displacement 6.8mm) than the Imm pEVA (PIF = 340, displacement 7.7mm). Conclusions: The mouthguard forming process has a direct effect on the internal stresses of pEVA and therefore its physical response. When pEVA is laminated with PMMA and silicone rubber the impact absorbing capabilities are better than a monolithic structure of pEVA. Mouthguards for use in contact sports, therefore, should incorporate a laminated section of pEVA, PMMA and silicone rubber. Simulation tests show that 5mm thick samples protect a substrate more effectively than I- 4mm test samples.