Expanded Polystyrene (EPS) < back
Expanded polystyrene (EPS) is a crushable foam, widely used in helmet liners (and other energy management applications like automobile bumpers). EPS is sometimes called “Styrofoam”, though that particular name actually refers to a single trademarked brand of EPS owned by Dow Chemical.
The EPS used in helmet liners is of a higher quality than the EPS used in disposable cups, coolers and packing popcorn. But the process of creating it is fairly similar. EPS is created by placing small (.5 to 1.5-millimeter) polystyrene beads into a mold and applying steam and a blowing agent called pentane. The polystyrene beads then expand up to 40 times their original size, fusing together into a solid mass that assumes the shape of the mold.
EPS is lightweight, effective across a wide range of temperatures and conditions and highly effective at reducing the amount of energy transferred to your head during an impact. EPS accomplishes that energy reduction by collapsing during the impact, converting some of the energy into heat and slowing the transmission of energy. A key advantage of EPS is the reliability of the molded part to deal with impact energy over a wide range of conditions. Another advantage is that EPS is not damaged by some of the common chemicals that can affect other materials (although it can be damaged by some chemicals). Additionally, EPS maintains its protective characteristics over time, (within limits, of course.)
Once those expanded polystyrene beads collapse, however, they do not regain their shape (or ability to absorb energy); for that reason, EPS is strictly suited to single impacts. This is why helmets with EPS liners should be replaced after a crash. Similarly, helmet owners should be vigilant about not dropping or knocking their helmet during daily use. You’ll find EPS foam liners employed in the vast majority of cycling, snow sports and motorcycle helmets. Athletes who freqently fall down or experience multiple impacts (snowboarders, football players, etc.) may prefer helmets equipped with more resilient liner materials.
Manufacturers can precisely “tune” their EPS foams by adjusting foam density to provide just the right amount of impact attenuation needed for a given application. Harder EPS foams work well to reduce energy transfer during high-speed impacts. Softer EPS foams are better suited to slower-speed, lower-energy impacts. Several helmet brands have created dual-density EPS foam liners that combine hard and soft EPS foams, which may help protect riders in some accidents.< back