Originally Posted by FastEddie
Nope. It could have been with older, polycarbonates helmets. High-end helmets - but low-end too, now - aren't made by PC. Arai trademarked its CLC & S-CLC, most manufacturers use strong Kevlar percentages, and even cheap helmets are mainly made from fiberglass.
I think you are missing something here. The vast majority of new helmets sold today are made with an injection molded polycarbonate shell. They have absolutely no Kevlar in them, because they are injection molded, not laminated.
In particular, "cheap" helmets are almost ALWAYS injection molded from polycarbonate, since this is a FAR cheaper process than laminating one from fiberglass or kevlar cloth.
It is impossible to injection mold Kevlar. Kevlar is a fiber, not a resin, and therefore cannot be mixed with other plastics, as (say) ABS and polycarbonate are often mixed.
Helmets that have kevlar in them are made by a lamination process just like fiberglass helmets. Glass or kevlar cloth is applied layer by layer and saturated with polyester of epoxy resin.
By the way, polycarbonate does not harden with age. I know something about this; plastic design is what I do for a living. Clear polycarbonate that is not a UV-inhibited type, can yellow a bit with exposure to UV, but helmets are completely opaque. The pigment blocks all light (including UV) from penetrating. Since UV cannot penetrate, UV damage is impossible. In other regards, polycarbonate is extremely resistant to chemical attack, and enviroinmental damage, with one exception: It is sensitive to some solvents.