The final class of materials for use in rifle plates is a familiar face- UHMWPE. Despite its drawbacks in soft armor format, in hard/rigid applications, this material does not show as many weaknesses. For reasons that are still not fully understood, the heat tolerance of PE hard armors is much better than soft armors (showing a danger zone of 195-200 degrees F rather than 180 F). This may be due to the typically thicker profile, thus providing a larger thermal mass to heat (taking longer and requiring more ambient heat to achieve irreversible denaturing). Also, contact shots are not as likely to have such a high risk of penetration due to the physical properties of a rigid defense compared to a flexible.
In addition to finding wide application as the backing material in many (if not most) ceramic plates (and a VERY effective steel/UHMWPE hybrid by Armored Mobility, the TAC3S), it is also used as the sole material in a significant number of plates by various manufacturers. In a hard armor format, UHMWPE offers some notable advantages: it is positively buoyant (it floats), is immune to acids, zero spallation/splash, and makes for the lightest rifle plate available. The drawbacks are moderate-high cost, and the thickest profile of any rifle plate (some have likened it to wearing a Wheaties box on your chest). In addition, UHMWPE plates typically do not stop M855 Green Tip ammunition, while having no difficulty with M193 (the opposite of most steel plates).
UHMWPE plates stop rounds by means of frictive braking. The fibers of UHMWPE squeeze and apply compressive braking force to rounds that strike. Generally the projectile is embedded about halfway into the armor when it is stopped. This leads to no splash or spallation, since the round remains mostly intact. M855 is thought to penetrate due to the incompressibility of the steel penetrator. Regardless of the mechanism, it is important to take into account when considering potential threats.
Due to their properties, some applications (maritime, swimmers) tend to benefit from their use. If the thicker profile is not a hindrance, they can be quite effective. As always, assess your needs and potential threats before making a decision.
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