Here's a little foam info that I found from Tom at shred ready:

There are many types of foam for helmet liners here is run down. These are the major ones.

Single Impact: Expanded Polystyrene: amy variations and densities. Foam of choice for Single impacts. Usually must be molded in the densities good for bike helmets. Has no memory

Vinyl Nitrile: VN used in football helmet, hockey helmets and some kayaking helmets. Multiple impact. better than EPP at 2 3 hits and equal at 6 impacts. Sorta heavy compare to EPP and EPS. We use this in the Shaggy, Vixen, Sherlock, Shensu, FMJ, TDUB

EPP: Expanded Polyprolyene: by far the choice for most helmets needing mulitple impacts. Light. Must be molded in densities for meeting bike, snow standards. Hard to find. Some really cool new EPP's coming out and we are using it in the Phly and Lektor protektor

Zorbium Foam: a break through in foam. Soft at low impacts where EPS would hurt in low impact velocities, but hardens up in higher velocity impacts. It is a speical varation of PU foam. so it soaks up water. You have to use a coating to stop the water from being soaked up. Not used in many sports helmets right now. Was a bit heavy when they put it out on the market, but have since reduced the weight considerably. Being used mainly in helicopter pilot helmets.

Polyethene, cross linked. This is mini cell. Not as stiff as VN and rebounds quicker than VN.

I am sure there are lots more foams. This is where most of the research is in helmets. Figuring out new foams that allow a thinner liner that will still manage impacts at the same or better than other foams.

What you want in a mulitple impact foam is one that is hard to push on with your fingers and rebounds slowly.

Also, manageing the energy of impacts is also dependent on the thickness of the foam. The idea of a helmet is to stop the head before the foam bottoms out and your head hits the inside of the shell. The thicker the foam the more distance you have to slow down the head before it hits the inside of the helmet. You want as thick as foam as possible .

tom
shred ready