Posts Tagged ‘laminates’

From time to time, I will post information about the cutting edge, or recently developed armor technology. The first installment will focus on carbon nanotube armor.

Carbon nanotubes are a fairly new material, a dividend from the growing field of nanotechnology (the technology which focuses on nano-scale structures and engineering). Based off of the C60, or “Buckyball” molecule, nanotubes consist of sheets of carbon atoms rolled into tubular structures. This morphology gives them unheard of tensile strength, currently the highest of any known material. So strong are carbon nanotubes, that they are projected as being the only current material suitable for use in the tether ribbon for the upcoming Space Elevator (using unidirectional sheets of nanotubes in a ribbon 8 feet wide).

The major stumbling block to widespread use of this material in body armor is COST. Producing even a few ounces of nanotubes is still prohibitively expensive. And producing a continuous fiber that can then be woven into a cloth is still at least a decade away.

Even so, several companies have started marketing “Nanotube Armor,” which is, unfortunately, not 100% true. My assessment of these armors has shown them to consist of a ballistic package made up of UHMWPE laminates, with a single sheet of nanotube-doped UHMWPE on the strike face. While this enhanced material does have higher ballistic effectiveness, the armor is hampered by the known flaws and weaknesses inherent in the UHMWPE laminates.

There is one area that nanotubes can and do have a significant affect on armor, and that is hard-face armors. Several companies are currently producing ceramics (including Boron Carbide and Silicon Carbide) containing both nanotubes and other nano-scale high-strength whiskers. This acts, on a much smaller scale, as rebar in reinforced concrete, allow for an increase in fracture toughness of the ceramics, which in turn improves the protective qualities of the armor. This subject will be touched upon again, when hard armors are discussed.

Until next time…

Advertisements

In the late 80’s and early 90’s a relatively new material was making an appearance in concealable body armor. Based on the Ultra High Molecular Weight Polyethylene molecule, this material offered tensile strength 8-15 times that of steel on a weight-to-weight basis. This was up to 40% higher than Aramid fiber. Developed by DSM, this material became known by two different trade names, Dyneema (DSM) and Spectra (Honeywell). Initially, this material was utilized as both a woven panel (which had immediate problems, as will be discussed below), and later in a laminate form (called Shield technology, similar to Gold Shield aramid laminate).

In the same way that aramid laminates utilized a poly-film matrix, so too did Dyneema and Spectra laminates. The UHMWPE fibers in laminate armor materials are unidirectional (all running in the same direction) and offset by 90 degrees in each successive layer. While this material, which is still widely used in soft armor, has impressive performance (much lower AD than aramid based armors, no UV susceptibility, positive buoyancy), there are fatal flaws that an end-user must be made aware of.

In addition to having the drawbacks of aramid laminates (de-lamination/peeling, extremely poor breateability), UHMWPE laminates also suffer from heat sensitivity. The UHMWPE molecule is chemically similar to garden-variety Polyethylene (the same material used in plastic milk jugs). When Spectra or Dyneema is exposed to temperatures above 170 Degrees F, it permanently and irreversibly denatures/reverts to the same milk-jug plastic (which has absolutely NO ballistic properties at all).

Because armor is often exposed to a wide range of temperatures (for example, in many parts of the country, a car trunk/interior can easily reach 180-190 F), this is a major concern. Furthermore, since there is no visible change to the material, there is no way for the end user to know if their armor is still viable, or merely layers of coffee can lid. Originally, woven UHMWPE armors were produced (called Spectraflex), but since the higher surface area of the woven fibers made the armor even more prone to heat degradation (a hot cup of coffee, for instance, would have a greater effect on a woven UHMWPE vest compared to a laminate, due to the vapor/moisture barrier properties of the laminate), they were quickly withdrawn from the market.

In addition, Spectra/Dyneema based armors fare poorly in situations where they may be subjected to contact shots- the hot muzzle blast gasses can melt the armor around the impact area, allowing the bullet to penetrate more layers (sometimes even the entire vest) than would have been possible with a woven aramid based vest. All laminates suffer poor contact shot resistance, but UHMWPE is especially susceptible. I will be dedicating a post on contact shots in the coming months.

What does all this mean for you, the end user? First of all, it is vital to identify armor containing UHMWPE, and to a similar extent, first and second generation laminates (I will be posting a tutorial on armor material identification in the coming months). If you are able to assess your needs prior to purchasing armor, ask yourself if you will be operating in environments that expose the weaknesses of UHMWPE or laminates (potential for elevated temperatures, likelihood of contact shots, requirement for high exertion/perspiration). If none of these circumstances are likely to be encountered, the dangers of UHMWPE/Laminates will be minimized. But if one or more apply, it is strongly recommended you find an armor system consisting of 100% WOVEN ARAMID.

It is important to note that this applies only to soft armor. UHMWPE/Spectra/Dyneema/Aramid Laminates find extensive use in hard/rigid armors (both as a pure defense and as backing material for the strike face. This will be discussed in a later post, but for now, please note that evidence strongly suggests in a rigid configuration, UHMWPE/Laminates do not exhibit the same dangers/weaknesses as when utilized in soft armor.

Next: The most dangerous (to the wearer) soft armor material. Stay tuned.

And so it was that a great need was upon the land. With projectiles achieving higher velocities, and greater penetration, Nylon just was not cutting the mustard. Even though it excelled silk for use in soft armor, it still lacked the requisite tensile strength to stop modern copper jacketed handgun rounds in anything approaching wearable ADs.

In 1965, a Dupont chemist named Stephanie Kwolek stumbled upon a new material while searching for alternatives to steel in tire reinforcements. This new material had a tensile strength 5 times that of steel on an equal weight basis. The structure resembled natural silk, but what made Kevlar outstanding was the propensity for the fibers to form cross-linked hydrogen bonds at 90 degrees to the polymer chain. This gave the new fiber exceptional tenacity, making it ideally suited for use in ballistic armor.

This, combined with excellent heat and flame resistance (Aramid fibers do not burn, they char at around 700 F), lead to a resurgence in concealable personal body armor. Richard Davies, founder of Second Chance, immediately saw the potential of this fiber, and the modern “bulletproof vest” was born.

Kevlar is the trade name for aramid fiber developed by Dupont, but there are several different brands of aramid fiber, including Teijin’s Twaron. Though originally discovered by Dupont, Teijin, a Netherlands based company, perfected and patented an aramid fiber processing method that Dupont later licensed to use themselves. Whether we are talking about Kevlar aramid or Twaron aramid, the properties are very similar.

To this day, aramid is widely used in armor applications. During the 70’s and 80’s, the only form used was woven fabric, cut and layered up to 35 plies deep. In the 90’s, new iterations of bullet resistant composites were brought to market, including laminates.

Laminates were introduced in search of the ever moving goal post of thinner and lighter armor. Of course, as has been the case throughout history, heavy and cumbersome armor is not fun to wear. To get folks to wear their armor, thin and light make sense. However, as will be seen, laminates were not necessarily better, and could even be seen as a step backwards (at least the first and second generation iterations).

Laminates such as Goldflex and Gold Shield utilize polypropylene films (chemically similar to food preservative film) to sandwich unidirectional aramid fibers in alternating 0 degree and 90 degree layers. Admittedly, this results in a very good material for stopping bullets, including hits near the edge of a panel, and at acute angles to the panel.

Unfortunately, several drawbacks rear their ugly heads with aramid laminates. First off, they have the breatheability of plastic wrap. Which is zero, since similar materials are used as vapor barriers. Secondly, the plastic film has a nasty tendency to melt when the panel is subjected to the hot muzzle blast of contact shots (an event all-to common in the course of law enforcement). In contrast, woven aramid is extremely effective at resisting contact shots. Finally, armor made with first and second generation laminates experience accelerated wear, since the adhesion of the film is degraded by repeated exposure to flexing, heat, cold and moisture.

While an armor built with woven aramid could reasonably be expected to survive (and remain fully effective!) for over 25 years (and I have personally verified that they HAVE), a laminate constructed armor is usually toast after only two years of normal wear. The edges curl, the layers peel apart, and the ballistic effectiveness drops to unsafe levels. In much the same way that a chain is only as strong as its weakest link, first and second generation laminates are hobbled by their use of, essentially, plastic wrap in their construction.

Next episode, we will look at another laminate, one that has great numbers, but hidden dangers…