Potential Drawbacks of Using Ethylene-Vinyl Acetate (EVA) in Fabricating Athletic Mouthguards

Abstract

Although the most commonly used material in athletic mouthguards, ethylene-vinyl acetate (EVA) may not be the most ideal material for the absorption of shock forces. Non-laminate form EVA absorbs approximately 40-65% of the energy of impact leaving a significant amount of momentum transferred to the substrate (teeth). EVA is a thermoplastic material that can be softened by the application of heat and is subject to the development of a permanent deformation set upon continued impact. PURPOSE: This investigation tested the mechanical properties of a popular non-laminate EVA material used in fabricating athletic mouthguards to determine: 1.) the effect of permanent deformation on EVA's energy absorption capability, and 2.) the effect of temperature on energy absorption. METHODS: We measured the capacity of the material to dissipate energy upon impact using a Charpy tup impact head mounted on a Tinius Olsen Model 92T pendulum (ASTMD6110-02). The device was set to impart either 1.13 J or 3.0J of energy to the specimen. To determine permanent deformation set, the EVA material was impacted in the same location over a series of 25 drops. To determine the effect of temperature on EVA energy absorption, the specimen was cooled to −50°C and impacted every 5°C as it warmed to room temperature. The specimen was then heated to +50°C in a convection oven and impacted every 5°C as it cooled to room temperature. RESULTS: Energy absorbed, when impacted with 1.13J of energy, decreased 2.3% by the 4th impact and a total of 3.4% by the 25th impact. Energy absorbed, when impacted with 3.00J of energy, decreased 5.7% by the 2nd impact and a total of 7.0% by the 25th impact. When the specimen was impacted with 1.13J between ±50°C, energy absorption varied from a high of∼75% of the impact energy at −10°C to a low of ∼36% of the impact energy at −50°C. Energy absorption continued to decrease as its temperature continued to increase from approximate room temperature (∼22°C) until the EVA material started to become more viscous at ∼40°C. CONCLUSION: The mechanical performance characteristics of commercially available EVA-based mouthguard materials decreased in their ability to dissipate energy at the extremes of temperature and as frequency of focal impact increased thereby indicating a strong need for a new material platform resistant to these effects.