Posts Tagged ‘UHMWPE dangers’

In several of my posts, I mention that while UHMWPE UD armor is an excellent choice for certain applications, and has material advantages over woven or laminate Aramid ballistic fabrics (higher potential V50, positive buoyancy, UV resistant, waterproof), it suffers from several glaring weaknesses (degrades to complete ineffectiveness above 170F, no breathability, delaminates/curls, and is WEAK AGAINST CONTACT SHOTS).

It is important to reiterate that last weakness: a large number (if not the majority) of self-defense and duty scenarios take place at 0-5 feet, where contact shots are a high likelihood. Woven Kevlar soft armor has shown to provide EXTREMELY good protection against contact shots (defined as the muzzle of the weapon being in physical contact with the vest or armor panel). The point at which Kevlar chars is around 500F, and it will retain its strength below this temperature.

The failure mechanism for UHMWPE in contact shots is the high temperature propellant gases that exit the muzzle microseconds after the bullet. These gases heat the area surrounding the muzzle and bullet path, and cause the laminate to melt/denature. This allows the bullet to penetrate much further than would normally be possible. In the case of large caliber revolvers (with a large muzzle blast footprint), this can allow the round to completely defeat the vest.

Test Round

Test Round

For the sake of the test, the round chosen was the .357 Magnum, rather than a .44 Magnum, as I wanted to see if the (relatively!) more modest caliber would still defeat the level II ballistic panel. The panel consisted of 15 layers of Dyneema SB-38. Round chosen was Hornady Custom 158 gr. XTP @ 1250 fps muzzle velocity, from 6″ barrel. The level II panel is specced to stop an equivalent round.

Test Panel

Test Panel

Backing

Backing

The panel was placed against a backing material consisting of 8 layers of bubble wrap, covered in a dish towel. In retrospect, this was probably a bit too “springy,” giving the panel an advantage by permitting it to move away from the hot muzzle blast faster than if the armor was being worn.

Test Panel Ready To Shoot

Test Panel Ready To Shoot

The test panel and backing were placed upon the ground, and the muzzle pressed firmly (but not forcefully) against the surface. The round was discharged into the center of the panel.

First shot

First shot

First shot, through the backing

First shot, through the backing

First shot, rear of panel...

First shot, rear of panel…

The panel was defeated, showing that the muzzle blast had melted a moderately large area around the point of contact. The round penetrated the backing and buried itself into the dirt (as shown).

To verify, a second round was fired into the lower left area of the panel (away from the heat affected zone of the first round). The second round performed identically, burying itself into the dirt beneath the panel.

Second Contact Shot

Second Contact Shot

Second Round, showing penetration

Second Round, showing penetration

Second shot, layers peeled back

Second shot, layers peeled back

Second shot, showing exit into backing...

Second shot, showing exit into backing…

...And into the dirt

…And into the dirt

Conclusions:

The results of the test shows that UD UHMWPE laminates are at risk vs. contact shots. Heat from the muzzle gases (especially medium to large caliber revolvers) “blazes a trail” so to speak, for the round to penetrate further than it normally would.

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It is no big mystery that I am at best a reluctant advocate of UHMWPE in soft armor. While the material itself *does* have incredible properties, these properties come at a steep price if the end user is not aware of the limitations and weaknesses inherent in the material.

These limitations and weaknesses are exacerbated by (in MY OPINION, ahhh, I promised you would see that word hurled around here!) a tendency to “softball” the armor test protocols. Even the current “best practices” protocols (The FBI and DEA tests for soft armor), have this same inherent kid glove treatment when it comes to UHMWPE containing vests.

“How can this be?” you may ask. Well, let’s review:

UHMWPE (“Ultra High Molecular Weight Poly Ethylene”) is an exceedingly strong material made up of long chains of ethylene molecules. The tensile strength is astounding, exceeding para-aramid (Kevlar/Twaron) and steel easily. It is positively buoyant, waterproof and does not degrade with exposure to UV light (three of the Achillies heels of aramids). However, as has been mentioned before, UHMWPE (regardless of brand- both Spectra and Dyneema are at their root the same basic molecule) will denature when exposed to temperatures exceeding ~168 F.

Think of it as exposing hardened steel to it’s normalization (annealing) temperature. The hardness disappears, and it becomes soft again. Unlike steel, it is impossible to change the UHMWPE back to its “super” state. In its denatured state, the material is identical to the stuff used to make milk jugs.

This denaturation temperature is well-known.

In the real world, temperatures often climb to well above this temperature, in both storage and incidental use (especially hotter regions of the world where .Mil users often find themselves). Why then are the test protocols seemingly designed to AVOID this issue?

Take the current NIJ 06 protocol. It incorporates many new rigors that an armor must pass in order to be certified (a GOOD thing, no doubt), including environmental conditioning. However, the temperature does not exceed the KNOWN denaturation/transition temp of the UHMWPE (highest temp in the conditioning phase is 149 F):

http://www.ncjrs.gov/pdffiles1/nij/223054.pdf

Even the FBI protocol, which excels the NIJ 06 standard in many ways, still only exposes the armor to a MAX temp of 140 F:

http://www.bidsync.com/DPXViewer/FBI_TEST_2006.pdf?ac=auction&auc=108034&rndid=338514&docid=748537

Quite frankly, this is a ludicrous state of affairs. Since temperatures can *regularly* reach 200 F in a car trunk or APC on hot days in CONUS or OCONUS, to not expose armor to these realistic circumstances could be perceived as softballing.

Even recent tests done by DSM which supposedly showed that their UHMWPE material did OK in elevated temperatures STILL tiptoed around the transition threshold temp:

http://www.dyneema.com/~/media/Downloads/Research%20papers/PASS%202010%20-%20Meulman%20-%20DSM%20Dyneema%20-%20Ballistic%20performance%20of%20Dyneema%20at%20elevated%20temperatures.ashx

In the paper, “temperatures up to 90 C (194 F)” were used. However, these temps were *only* applied to hard/rigid armor, which does not experience the same sensitivity as soft armor, most likely due to an insulative effect of the material in thicker section. The soft armor samples were still only exposed to 70 C (which is 158 F, 10 degrees BELOW the known transition temp). The test should have applied the same max temp to ALL samples, regardless of whether they were hard or soft.

In conclusion, test protocols should be designed to apply REAL WORLD rigors to life saving equipment. The current protocols SEEM TO incorporate temperature threshold requirements that allow the limitations and weaknesses of a specific material (UHMWPE) to pass. Designing tests with less rigorous standards so as to avoid excluding a material does not live up to the purpose of testing in the first place. Providing the absolute best lifesaving gear, regardless of any other considerations, should always be the goal.