Youna Kim1,Jinkee Hong1
Yonsei University1
Every year 2 million people have experienced sports injuries. Especially, in ball games such as basketball, baseball, and golf, serious injury could be caused by a high-speed ball. To prevent sports injuries, players should wear sports protective equipment. For example, in a baseball game, catchers wear protective equipment such as chest protectors, protective helmets, and shin guards. However, as the total weight of the protective equipment approaches 5 kg, the catchers suffer from extreme physical fatigue during the game. Therefore, it is necessary to develop materials for protective equipment that are lightweight and block impact.<br/>As shock-preventing materials, there are representative viscoelastic hydrogels and organogels. These materials are effective in blocking impact due to their viscosity properties that absorb impact and elastic properties that dissipate impact. However, hydrogels have limited ability to withstand the impact energy of spherical projectiles due to their weak mechanical properties. In addition, it is difficult to apply to sports games due to shrinkage and weakening of physical properties due to water evaporation. Organogels overcome the limitations of hydrogels but use harmful solvents. It is necessary to develop viscoelastic protective materials that are lightweight, non-harmful, and can block impact.<br/>In this study, a shock-preventing viscoelastomer (SPV) is designed by using non-toxic deep eutectic solvents (DES) and polyvinyl alcohol (PVA). Shock-preventing materials should absorb all impact energy and convert it into kinetic energy, preventing the energy from being transferred to the wearer. SPV could regulate the dissipation and absorption of impact energy through hydrophobic vapor treatment.<br/>The entangled polymer chains within the amorphous phase could dissipate impact energy. PVA is a hydrophilic semi-crystalline polymer. DES is an azeotropic mixture composed of hydrogen bond donors and hydrogen bond acceptors, which hinder hydrogen bonds between PVA chains. For the increment in the amorphous phase, DES is used as an additive. As the amorphous phase increases in SPV, SPV shows viscoelasticity and could dissipate impact energy. Moreover, SPV is tougher than conventional hydrogels.<br/>Hydrophilic SPV has a different chemical affinity with hydrophobic methyl salicylate (MeS), an analgesic and mint flavoring agent. Due to the different affinity, the solvation effect of MeS within SPV increases the chain entanglement density by H-bonding PVA chains. However, as the MeS treatment does not affect the crystallinity of SPV, MeS-treated SPV still maintains the rubbery phase at a high amplitude sweep. That is, MeS-treated SPV could absorb the impact energy to break the H-bonding and dissipate impact energy.<br/>The MeS-treated SPV is restored to its original state after 50 min. It can be used as protective equipment for baseball and hockey within 1 h per inning. In addition, SPV can be applied to sports protective equipment in various forms such as thin film, fiber, and spray coating. We expect that SPV is suitable as an impact protection material to prevent sports injuries.