Vinyl nitrile (VN) is a closed-cell foam derived from synthetic rubber. This material is soft, flexible and offers good impact attenuation. Vinyl nitride is also resilient—meaning that it crushes on impact and reduces energy transfer to the head, before regaining its shape and protective qualities. For that reason, vinyl nitrile is used in football, hockey, snowsports and kayaking helmets.
Helmet manufacturers can also fine tune vinyl nitrile helmet liners to mitigate both high and low-energy impacts by utilizing multiple layers of varying-density foam. Paired with a flexible shell, vinyl nitrile is frequently featured in the latest generation of “soft shell” snowsport helmets.
Reading through the benefits above, you might be wondering why cycling helmets aren’t using vinyl nitrile. The primary reason is that VN’s ability to manage energy is diminished in warmer temperatures. That attribute alone currently rules out its use in cycling helmets. The material is also heavier than EPS and EPP.
EPS is very attractive to helmet makers—the crushable foam is lightweight, yet offers excellent impact protection. One downside, however, is that EPS is petroleum based and will not decompose after it’s been retired. Its environmental footprint, so to speak, is heavy.
Expanded polyactic acid (E-PLA) is a relatively new addition (2016) to the world of helmet liners. This single-impact, crushable foam functions very similarly to EPS and can perform to the same safety standards, and it can be much friendlier to the environment. E-PLA is plant based (it’s derived from corn, at this time) and biodegradable.
E-PLA has been used for some time in packing materials. In order to be utilized in helmets, it has been reformulated to provide better impact attenuation and to degrade very slowly—helmet owners expect several years of safe use from their helmet. E-PLA is currently being used in a small number of cycling helmets and could potentially replace EPS in more types of helmet liners in the near future.
Though it looks identical to crushable EPS foam, expanded polypropylene (EPP) is a more resilient crushable foam, which is to say that it slowly regains its shape following an impact and retains some of its protective quality. For that reason, EPP can be integrated into helmets designed to help protect users from multiple impacts. It has seen use, for example, in skateboarding helmets. EPP is most commonly used in the automobile industry, where it can be found into products such as bumper bars, sun visors, roof pillars, armrests and headrests.
Though EPP could be an attractive liner material for certain cycling enthusiasts, this particular type of foam generally isn’t utilized for cycling helmets. The reason? EPP offers slightly less impact attenuation for a given thickness than EPS foam, which means that more EPP foam must be employed to provide the same level of energy management. The end result would be a heavier and thicker helmet—both traits are “hard sells” in the cycling, snow sports and motorcycle markets.
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.
Acrylonitrile butadiene styrene (ABS) may be the most common helmet outer shell material in the world—thanks to its affordability, ease of manufacturing, its strength and reliability. As a thermoplastic, ABS can be heated to 221 degrees Fahrenheit (105 °C) melted and easily injection molded or 3D printed into finished shapes. This is why ABS debuted in helmet shells during the 1960s and is still popular with helmet manufacturers today.
Once cooled, there is usually little required in the way of hand finishing the ABS product, which reduces labor costs. That said, ABS is also easily machined, sanded, glued and painted, which makes it a popular material for both prototype and finished products.
From a performance standpoint, ABS is attractive in that it has high impact resistance, functions well in cold weather and is, simply put, quite tough.
For all these reasons (and more) ABS products are omnipresent in our daily lives—the light switch cover on the wall, the keys on your computer’s keyboard, the interior in your car—ABS is everywhere, including, of course, in helmet shells.
Polycarbonate is a thermoplastic, meaning it is easily melted and injection molded into final products, such as helmet shells, visors and face shields. Polycarbonate is highly impact resistant, which makes it the material of choice for products such as “bullet proof glass” and safety glasses. It’s rugged nature has also led to its use in smartphone cases, compact discs and a wide range of other consumer products.
Polycarbonate is stronger than ABS, the other popular thermoplastic frequently used to construct helmet shells, but also more expensive and therefore tends to be featured on higher price point helmets. Helmets using polycarbonate typically feature slightly thinner shells, creating a more streamlined look with the same degree of impact resistance and performance.
Technically speaking, any product that combines multiple materials is a composite, but when people refer to composites in the helmet world, they are generally referring to helmet shells that are constructed from sheets of glass, carbon or Kevlar fibers that have been fixed within a matrix of polymer resin. You can think of it as a high-tech laminate.
While glass, carbon and Kevlar fibers all possess different qualities (carbon fibers are, for instance, generally stronger and stiffer than fiberglass), composite shells and components can be “tuned” to specific needs – say lighter or stiffer – than ABS or polycarbonate thermoplastic helmet shells. This makes them particularly attractive to consumers, whether those consumers happen to be helicopter pilots, baseball batters, football players, motorcyclists…
Composites, however, also require a great deal of labor and the source materials can be expensive. Whereas ABS and polycarbonate can be inexpensively injection molded and demand little in the way of hand finishing, each layer of a composite must be precisely laid up, by hand, before being molded. Once the composite shell emerges from the mold, it often requires sanding and other detail finishing. For that reason, helmets featuring composite shells are often considerably more expensive than their polycarbonate counterparts.