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Foams Used in Bicycle Helmets
10/6/2010 10:31:17 AM

Foams Used in Bicycle Helmets

Summary: Foam is used for energy management in a helmet. There are many types, but EPS is the choice for most bike helmets. The ideal foam would be stiffer in hard impacts, softer in lighter impacts, light, cheap, reliable to manufacture and easy to ventilate.

Crash Energy Management
In lab tests to US standards, helmets are dropped with headforms inside and are expected to keep the g forces registered inside the headform below 300 g. (We have a page on "What is a g" if you are not familiar with the term.)
To do that a helmet does two things: through the crushing or deformation of foam it tries to convert a small part of the crash energy to heat, and it slows the stopping process so that the head stops in about six milliseconds rather than one millisecond or less. Most of what the foam in a helmet does for you is in that second function, slowing head more gradually in the stop. The helmet spreads the energy of the crash out over that very short six milliseconds, reducing the peak spike of energy to the head and brain.
In a lab test graphs of the energy traces look like this, with a smooth curve extending over 6ms for the good helmet (on the left below), and a huge spike for the bare head (right). 
 
Any physics text will tell you that the Law of Conservation of Energy means that the energy of the crash cannot be "absorbed" but can only be converted to some other form of energy. So we refer to what a helmet does in a crash as "energy management" rather than "absorbing" energy. To confirm that, hit a piece of styrofoam with a hammer. The indentation will be warm to the touch. So some energy was converted to heat. But the blow was very hard and concentrated on a very small area the size of the hammer head, and the warmth you feel is not extreme. Even so, the blow of the hammer is blunted and the sound is deadened.

Current helmets mostly perform energy management with foam of some type. There are many different types and grades of foam. There are two major categories of helmet foam: some foams are stiff and crushable, while other foams are rubbery or squishy. Their characteristics make it possible to design a helmet for one very hard impact, a number of hard impacts, or a very large number of softer impacts. But each foam has its limitations.

All foams have some common characteristics. Generally they are all cheap to manufacture. Because they have jillions of air cells incorporated in them, they are all good insulators. That means that most helmets will have to have some air circulation inside the helmet to prevent heat buildup unless the weather is very cold. And all foams will immediately or eventually deteriorate under impact, even though some can take multiple impacts before deterioration sets in.

The crush or deformation of foams can be varied by changing their density. Denser foams resist very hard impacts better before compressing or crushing to their ultimate limit and "bottoming out," passing all the rest of the impact energy on to the head. Softer foams compress, deform or crush more easily in lesser impacts, giving better protection against the milder injuries we call concussions. A helmet can have both types of foam in layers, but that usually increases the thickness.

The slowing of the head is dependent on the foam characteristics and on the foam thickness. The best foam for your crash provides an optimal rate of crush or deformation for the particular impact you are experiencing. If that foam is one inch thick it gives you longer to stop in a very hard impact than foam that is one half inch thick. If it is a less dense foam, it can give you a softer landing in a lesser impact, and if it is thicker it can do that without bottoming out in a hard one. But when helmets get too thick, they look like a mushroom on the rider's head, and consumer acceptance drops like a stone. In addition, thicker helmets that stick out farther from the head might possibly add to "rotational" injuries by jerking the head around more in a skidding impact.
In short, the ideal foam is just thick enough for your crash impact, and just firm enough to minimize your g's without bottoming out. Since you don't know exactly what your impact will be, the ideal foam would adapt for each blow. That's a "rate-sensitive" foam that stiffens in major impacts but cushions more in lesser impacts.

Crushable Foams
Crushable foams are ideal for helmets designed for one hard impact. The cell walls crush on impact and slow the head gradually. A small portion of the energy is converted to heat as noted above. When foam crushes, it does not bounce back at the bottom like a spring to make the impact worse. But when you reach its crush limit, it will pass the rest of the impact energy straight through to your head.

One of the major design parameters of every helmet is the specification of foam density. That is what "tunes" the helmet for a specific range of impacts. High density for harder impacts, lower density for a softer landing but with the possibility of bottoming out in a hard blow. With experience you can make a guess at the density of a foam by squeezing it with your thumb enough to make a small impression. (Don't do this with a helmet somebody will be wearing!)

Crushing the cell walls destroys the impact management ability for most stiff foams, so the helmet has to be replaced after a single impact. The crushing is not always visible and can be hidden by the outer shell. The foam can also recover some of its thickness over a period of hours, but not its ability to manage impact. Crushed and partially recovered one-use foam will feel rubbery and soft. Experts measure the foam thickness carefully for crush, but for consumers the recommendation has to be "replace after every impact." Often the damage is not apparent in a casual inspection, even when there are cracks in the foam. And the rider often underestimates the impact because crushing of the foam dampened the shock.

Some crushable foams:
EPS
Expanded PolyStyrene is one of the most widespread foams used in our society. It is the white picnic cooler foam that you see eggs and stereo gear packed in. It is the peanuts in your mail order package. It is the white food carton or drink cup you get at a carry-out. It is cheap to manufacture, light, and has almost ideal crush characteristics with no bounce-back to make the impact more severe. It can be reliably manufactured with reasonable quality control procedures.

EPS is formed by placing polystyrene beads (granules) about the size of table salt in a pressure mold shaped like the helmet liner and expanding the bead from 2 to 50 times with a blowing agent like pentane under pressure and heat. The beads expand to form the cells and fill the mold. The cells are tightly bonded--under ideal conditions. Under poor conditions the steam/pentane temperature is not just right or the pressure is off a little and the foaming may not be uniform, or there may be hidden recesses where the granules did not expand correctly. (A helmet liner with such a recess "rattles" with unexpanded beads inside when shaken.) The foaming is often done by a "foam shop" outside the manufacturer's plant, and the challenge for helmet quality control programs is to design testing that will catch any problem liners. Foam density is measured by weighing the liner, then placing it in water and weighing the amount of water displaced, and comparing the two weights.

The version of EPS you see in a helmet is several quality grades above what normally is used for picnic coolers. It can be tuned to produce optimal crush for a given impact level by varying the density of the foam cells. Additives can provide increased cell adhesion, cutting down the splitting of helmets in very hard impacts. (GE's GeCet foam is an example of a product that adds a resin to make the EPS more resistant to cracking.) Additives can also be used to color the foam, although they may change the impact characteristics. Manufacturers can add internal reinforcing using nylon, carbon fiber or various types of plastics to reduce cracking as well, enabling designers to open up wider vents and still pass the lab impact tests.

Molding techniques for EPS have evolved over the half century that it has been used for helmets, enabling manufacturers to push the envelope by producing a helmet liner with harder and softer foam in layers (variable density foam). That lets the softer inner layer of foam crush in a lesser impact, where harder foam would just resist and pass the energy on to the head. The harder outer layer is still there when the soft foam bottoms out to keep managing the energy in a hard impact. Over the years there have been several helmets that used this technique. At present the most advanced one we know of is from Kali, a manufacturer who entered the US market in late 2008.
Kali is using two different approaches to dual-density foam. Their first use of it was to mix the two densities in different areas of the same layer, as in these photos. 
 


 
The black layer is the harder, denser foam. The result is a lighter helmet with a stiffer liner in the black areas. Kali is the only manufacturer using this technique.
In 2010 Kali began making helmets with a technique invented by Australian Don Morgan originally called "Conehead" foam. It uses a layer of harder foam next to the head, with cones or pryamids sticking up into the layer of softer foam in the outer layer. The photo below shows a cone section, while the second photo shows a cross-section with both layers in place. The slice of the cross-section makes the cone look flat.
 


 
Note that the conical shape of the cones means that more of the dense foam is involved as the crush continues down the cone. It is easy to crush the tip of the cone, but as the crush zone moves downward the volume of dense foam involved increases rapidly, in effect stiffening the liner. This tunes the foam to give a softer landing in the first stages of crush, while stiffening up as the crush continues to prevent bottoming out in the hardest impacts. The technique should offer better management of lower level impacts that could otherwise cause a concussion.

The cone technology is licensed through Strategic Sports of Hong Kong, who call it Contego foam, the same term Kali uses. Strategic makes helmets for hundreds of different brands worldwide, and some of the models they make may eventually have the technology incorporated in them. At present Kali is the only manufacturer we know of using the technique.
There are other dual-density helmets where the foam in one section is a different density, notably one of Bell's skate helmets and the Specialized S-Works. By using higher-density foam in one section of a skate-style helmet, Bell was able to avoid thickening the helmet. Specialized was able to make the S-Works lighter by using less dense foam.

The lab tests for helmet standards are pass/fail tests, and are not designed to reveal the "softer landing" helmets. Legal worries prevent companies from advertising anything about impact performance beyond meeting the standard, a point that can be defended in court even if the user was injured. Consumers don't understand the advantage of a softer landing, and really don't ever expect to crash anyway. The injury prevention community is now focusing on the problem of mild traumatic brain injuries from concussions. As the dialogue advances you might look for innovation in foam densities in coming years. The thinner helmets and the ones with bigger vents have to use denser foam to pass the lab tests to meet a standard. We had thought that was an indication that they may not be best for soft landings. Our project to test cheap and expensive helmets showed otherwise, since the two types performed almost exactly the same.

In 2005 the Italian company Shain made new claims, supported by data published in their catalog, that their helmets with EPS foam and inner shells can perform with two hits in the same spot. Inner shells are not a new idea, but Shain was the first to claim that they can meet standards with two hits at the same spot due to the inner shell. We do not see their helmets in the US market now.

You can learn more about EPS, including information on recycling it, a the Alliance of Foam Packaging Recyclers. EPS is not generally recycled in helmets, since the quality control problems would be multiplied. In fact, it is difficult to get your old helmet to a foam recycler without wasting more fossil fuel resources than recycling the helmet can save.
EPP Expanded PolyPropylene is very similar in appearance to EPS, with just a touch of rubbery feel on the surface by comparison and a little bit of give if you squeeze it with your thumb. EPP is a multi-impact foam, recovering its shape and most of its impact protection slowly after a crash. It can be trickier to work with than EPS, costs a little more, and has a modest amount of rebound (in technical terms a less favorable coefficient of restitution) that usually requires a little bit thicker helmet than one using EPS. Most of the rebound takes place after test rigs have stopped measuring the impact severity, so that characteristic is not well documented. EPP looks identical to EPS, and only the label can tell you if your helmet has this multi-impact foam or the one-use-only EPS. There are some, but not many, EPP helmets on the market, mostly for multi-impact sports like skateboarding. In 2004 Pro Tec introduced a modified EPP that they are calling SXP. They say that it permits them to meet multi-impact standards without thickening their helmets.

EPU Expanded PolyUrethane, also abbreviated PU or EPU, is another crushable foam similar to EPS. It has a dense, fine and very uniform cell structure. It skins over in the mold to form a surface shell that is adequate to protect the bottom part of the helmet from some dings and adds to the esthetic appeal. It is heavier than EPS and has a very solid feel. Most of the EPU we have seen is manufactured in Taiwan, apparently because the manufacturing process produces toxic byproducts that are tolerated there but not in other countries.

Tau Multi Impact Technology or Re-Up Foam was introduced by Pro Tec and Shain 2004. It is an EPS formulation with the granules suspended in EPP. Shain has published test results in their catalog that show their helmet handling four hard impacts in the same spot before registering over 300g. That is not true multi-impact performance, but a lot closer to it than any standard EPS helmet can manage.

Other Beaded Foams
Brock Foam Brock USA has a proprietary multi-impact foam formulation using expanded polypropylene or polyethylene beads held together by an elastic adhesive that produces a closed-cell foam that is still porous. Brock Foam is made by fusing the round foam beads together just touching at their tangent points. The result is a resilient foam that allows moisture and air to pass through it. Depending on the size, roughness and pre-compression of the beads, they will compress under the force of a blow in various ways. Brock Foam can be made in cross-linked polyethylene for durability and softness, or in polypropylene for strength. The foam is used for many different products besides helmets, and until late 2005 we had not actually seen a helmet made with it, although we knew that some manufacturers have experimented with it. For the 2006 model year, Bern Unlimited introduced several new helmets made with Brock foam. They have the hard shells characteristic of Brock foam helmets. Some of them meet the US CPSC bike helmet standard. There are many more interesting details in the patent, including a lot of information on how the beads behave in an impact. Brock Foam is manufactured in Shenzen, China and Butler, Pennsylvania.

Rubbery Foams
Most bicycle helmet foam is the crushable type made with beads, and we know a lot less about the squishy foam side of things. Football, hockey and skateboard helmets are made with rubbery foam to provide the multiple impact protection needed in those sports. For a given thickness the rubbery foams are less protective in a very hard impact, but more protective in a lesser impact, where they deform while stiffer EPS is still resisting. The liners in rubbery foam helmets can deteriorate with many impacts, however, and football helmets must be reconditioned on a regular schedule by replacing the liners. That's not a big problem for football teams, who can send all their helmets back to the manufacturer for relining during off-season.

Zorbium We can only report on one specific rubbery foam for bicycle helmets. A company called W Helmets is producing ski and BMX bike helmets from a foam they call Zorbium. It is a "rate-sensitive" foam that deforms easily in a lesser impact to prevent milder injuries, while stiffening up in a harder impact to prevent bottoming out. It may be a real advance, but we have not seen lab test data confirming to what degree the rate sensitivity is benficial in a helmet, so we are still cautious about this one. The W Helmet models we have seen so far are heavy and not well enough vented for cycling, but this is early on the curve for this new foam and improvements may come along later. We did notice a tendency for the foam to absorb a lot of sweat. Meantime, check our writeup on Dual-Certified Helmets for a review of the helmet.

SALi (Shock Absorbing Liquid) is another concept altogether, where the foam beads are encased in plastic and floating in a liquid. Impact pressure on the liquid works on the beads from all directions, compressing them to manage the peak force. This concept has a long way to go before you will see it in a bike helmet, if ever. The weight constraint alone could make it impractical. But it is at least a new technology that is less than fifty years old. Check out the details and the latest on development efforts on the Cheshire Innovation site.

And for Something Completely Different . . .
Cascade Helmets introduced in May, 2007, a new impact management system that uses no foam at all. It is multi-impact, and probably flexible enough to be tailored for most helmets. It was first introduced in hockey and lacrosse helmets. It uses small cylinders of plastic bunched together with the open ends toward the head and the helmet. We are excited about this new technology and hope Cascade will bring it to the bicycle world soon. You can see an ad on their Web site, but without a lot of detail on the technology.

Air is again a different concept that eliminates foam (or could be used in conjunction with it). Israeli industrial designer Amos Wagon has this air bladder design up on the Web. The concept has been tried before and found lacking. More Foam might be another way to go. A company called Blitzshield has a product that puts a second helmet layer over a large football helmet. We don't know what it would look like, or how it might perform. It increases the outside diameter of the helmet considerably, so if you snag it or in an oblique impact it would tend to move the head more. That might more than cancel the value of the added impact foam. If the video clip on their Web site stalls, try this one on Youtube. The sledgehammer demo is not very impressive compared to real lab test results.

The bottom line
Although there are many types of foams available, EPS remains the choice for most bike helmets because it performs well in hard impacts and it is light, cheap, durable in use, reliable to manufacture and easy to ventilate. A rate-sensitive foam would probably permit better protection against the mild concussions that today's bike helmets just have to accept to provide protection against catastrophic brain injury in very hard impacts, but has other drawbacks. Research continues, and new foams appear from time to time, but for the present they all have disadvantages that keep them from replacing EPS.

Reference:
Bicycle Helmet Safety Institute
4611 Seventh Street South
Arlington, VA 22204-1419 USA

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