The first class of fibers are "aromatic polyamides," the two terms merge into the portemanteau of aramid (See Wikipedia).
The other type of fabric is polyethylene (or simply PE). Bullet resistant polyethylenes have high molecular weight. Low molecular weight PE is used in all sorts of "wicking" fabrics.
PE is generally sensitive to sunlight and heat. Bullet resistant panels are typically housed in "carriers" made of nylon and cotton that help protect the panels from UV and heat.
One new class of PE fabric is Dyneema. Dyneema is "ultra high molecular weight polyethylene" or UHMWPE (See Wikipedia). Dyneema is a very lightweight and flexible bulletproof fabric that floats, so it is good for naval operations.
Aramids, on the other hand, have excellent heat resistance. Kevlar and Twaron are chemically similar to Nomex (the material used to protect NASCAR drivers from flame).
There are 2 different ways of manufacturing aramids, however: woven and laminated. One example of a laminated aramid is Goldflex. The fibers are essentially sealed in a matrix, which creates a more comfortable (and less bulky) fabric.
According to this forum post, laminate aramids are not as strong as woven aramids:
Vests with Goldflex or any other hybrid laminate materials have a record of deterioration which begins almost immediately after manufacture. This deterioration is accelerated by water/humidity/perspiration (vests get hot!) and has been responsible for issued vests failing to stop ballistic threats on many documented occasions. Contact shots have also been known to burn through them (brand new vests) like butter. Stick with 100% Kevlar or Twaron which have a proven record of working after decades of wear under most environments. Kevlar and Twaron are a little heavier and are more stiff but you may as well wear something that works. Another advantage is that Kevlar/Twaron can often be found at a lower price since it is considered "older" technology.
A good resource for comparing the two types of fabrics is the Handbook of Composites avaialble thru Google Books.
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