Interesting descriptions of EPS vs EPP foam

southpawboston

New member
i've been obsessing lately about bike safety, as i've been riding my bike to work every day this fall and also biking DD1 to preschool in the morning (both of us wear new helmets of course!).

anyway, i've found an amazing encyclopedia of helmet knowledge, the bicycle helmet safety institute. without going into detail about all the fun-facts i've learned about helmets, i'll just mention that they have a very nice section on impact-managing foams (they don't like the term "impact-absorbing", because energy is not really absorbed, but redistributed in a managed way).

i'm quoting the entries on EPS and EPP, both of which fall under the category of "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.

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 take up the slack in a hard impact. Over the years there have been several helmets that used this technique, but we do not know of any currently on the market.

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 just beginning to understand the problem of mild brain injuries. As the dialogue advances you might look for innovation in foam densities in coming years. In the meantime, the foam in most bike helmets ranges in density from about 4 to 6 pounds per cubic yard. The thinner helmets and the ones with bigger vents have to use denser foam to pass the lab tests to meet a standard.

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.
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.
bolded by me: what's interesting is that EPS might be better in an impact because it has no rebound characteristics, whereas EPP can have rebound which may not be picked up by the sensors which measure initial impact loads.

:twocents:
 
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safeinthecar

Moderator - CPS Technician
Very interesting indeed. So, what helmets are considered the best then? I had to throw all of ours away last week because they were hanging on the bikes and they melted in the sun.

Kimberly
 

southpawboston

New member
reading from the helmet safety institute, it appears to be very similar to carseats-- buying an expensive one may get you features and "style" but not necessarily greater safety, and in fact some very inexpensive helmets have tested better than ones costing 10 times as much. all new helmets have to pass US standards just as carseats do, and there are even additional non-required standards that some helmets comform to. typically the more expensive helmets only get you better ventilation (larger air fins) and lighter weight. but there is speculation that the more expensive ones with better ventilation, while passing the head drop test, may cause more severe localized forces, which aren't measured in the head drop test.

personally, i bought the whole family new helmets made by troxel. they are not a name-brand, but they conform to not only the required ASTM standard but also the "optional" CPSC standard. they cost about $9 through my health insurance benefit.

it's also interesting to learn about changes in helmet designs and standards over the past decade. just like carseats, new standards and design elements get introduced over time, making old helmets obsolete. my old helmet that i threw away had none of the current design features that all new helmets have, yet to the untrained eye, it looked like a "perfectly good" helmet, lol...

if you browse the weblink in the original thread, there are sections for finding safe helmets and what to look for.
 

skaterbabs

New member
Good points. Bike helmets also have a shelf life, form what I understand. I tossed mine after three or four years, which is what the manufacturer recommended. I need a new one; I'd like to actually USE my bike rather than have it take up room in the garage. :rolleyes:
 

singingpond

New member
> anyway, i've found an amazing encyclopedia of helmet knowledge, the bicycle helmet safety institute. without going into detail about all the fun-facts i've learned about helmets, i'll just mention that they have a very nice section on impact-managing foams (they don't like the term "impact-absorbing", because energy is not really absorbed, but redistributed in a managed way).<

Thanks for posting the link. I've been using a very antique bicycle helmet, and have been wondering about expiration (especially after getting more informed about carseats in the last year or so). My helmet is so old, I'm embarassed to say how old it is :eek:, so I think I'll go to your referenced website so I can get myself worried enough to finally get a new one. And probably replace DH's not-quite-so-antique expired helmet at the same time. I know we also have relatively cheap helmets available through our health insurance's website -- probably made by the same company you mentioned.

The discussion about EPS vs. EPP is interesting.

Katrin
 

southpawboston

New member
Thanks for posting the link. I've been using a very antique bicycle helmet, and have been wondering about expiration (especially after getting more informed about carseats in the last year or so). My helmet is so old, I'm embarassed to say how old it is :eek:, so I think I'll go to your referenced website so I can get myself worried enough to finally get a new one. And probably replace DH's not-quite-so-antique expired helmet at the same time. I know we also have relatively cheap helmets available through our health insurance's website -- probably made by the same company you mentioned.
most of the new features that are common on even the cheapest new $10 helmets that didn't exist years ago on the most expensive helmets have to do with the way the helmet is held onto the head (strap, positioner and buckle design), the molding techniques, and the basic shape of the helmet. plus, the new voluntary CPSC standard only became implemented in 1999, so any helmet older than that is automatically obsolete, regardless of its design or cost.

i found it interesting that the helmet institute holds consumer reports in high regard when it comes to helmet testing. perhaps CR's helmet testing department is run by more competent scientists and technicians than their carseat testing dept. :rolleyes: the institute actually recommends the cheaper helmets found at discount stores like target and wal-mart as opposed to the more expensive ones found at specialty bike shops, provided you look for the right certification stickers and the right design elements.
 

joolsplus3

Admin - CPS Technician
Phew, I've been feeling guilty about my kids' Walmart helmets when I hear how nice the name brands one are (Gyro?... fancy strap adjuster on that!). We use the all purpose skate/bike ones, they are easier to adjust and I guess it's good they don't have any big fancy air vents ;)
 

southpawboston

New member
Phew, I've been feeling guilty about my kids' Walmart helmets when I hear how nice the name brands one are (Gyro?... fancy strap adjuster on that!). We use the all purpose skate/bike ones, they are easier to adjust and I guess it's good they don't have any big fancy air vents ;)
right... just look for a CPSC sticker inside the helmets... that's all you really need to look for to know that the helmet is safe. that, and the fit must be good. just like carseats. :)
 

safeinthecar

Moderator - CPS Technician
OK so how should I store them to keep them in good condition, We usually just hang them on the handle bars, but the sun melted them because I have no shade since I moved. Is the garage ok since it gets so hot? I don't want to keep them in the house since I have so little room. I generally have a helmet each for the 4 of us, and 3-4 extras for spare children.

Kimberly
 

southpawboston

New member
OK so how should I store them to keep them in good condition, We usually just hang them on the handle bars, but the sun melted them because I have no shade since I moved. Is the garage ok since it gets so hot? I don't want to keep them in the house since I have so little room. I generally have a helmet each for the 4 of us, and 3-4 extras for spare children.

Kimberly
i had no idea they could melt! did the foam melt or did the top shell melt? i would guess the sun basically cooked them. i would think they'd be a lot cooler in a hot garage than out in the southwest sun. i didn't find anything on storage, and the only thing i found on replacing is, "if it's been in an accident".
 

tentoes

New member
I know this is an ooold thread, but correct me if I'm wrong- aren't helmets bearing the Snell Foundation logo rated higher (impact-wise)? I was under the impression that Snell (a not-for-profit) tests to much higher standards than other ones do- and in turn [motorcycle I know for sure] helmets are more expensive.
 

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