April 07, 2022 5 min read
Part IV: The Future
UHMWPE fibers have come a long way from humble beginnings. 20 years ago, it was far from clear that they’d enjoy the dominance that they have now come to hold. Back then, the military still utilized Kevlar in just about all of its body armor systems, Zylon was heralded as the “fiber of the future,” and up-and-comers like M5 Fiber were capturing imaginations. Today, given improvements in UHMWPE fiber processing,it’s stronger than Zylon ever was. It’s also very difficult to see how it can be surpassed by M5 Fiber -- for although M5 is stronger, it’s also much more dense, and its performance would likely prove inferior to UHMWPE on an equal weight basis.
The list ofpotential near future competitors to UHMWPE is very short. It consists primarily of nanomaterials such as carbon nanotubes and graphene, upgraded derivatives of aramid with altered chemical structures, stabilized derivatives of Zylon, and a few less likely fibers such as spider silk derived from genetically engineered silkworms.
Armor derived from carbon nanotubes and graphene are unlikely to emerge any time soon, so I don’t think that they’ll compete with UHMWPE in the “near future.” For ballistic applications, they’d need to bevery large -- carbon nanotubes, for instance, would need to be many inches long -- and they’d also need to be substantially free of defects. (They would therefore be, in effect, carbon fibers of the very finest and highest-quality type.) Nanomaterial production technologies just aren’t there yet. In fact, they’re nowhere close. So while armor of woven nanotubes is very interesting in theory -- with new academic papers written on the subject with some regularity, e.g.this one -- it will surely be a very long time before that theory is reduced to practice.
Aramid derivatives are sometimes discussed as a potential high-performance alternative to UHMWPE.
Up until very recently, the Russian producer Kamenskvolokno was in the business of producing extremely high grade “aramid” -- actually a modified compound which resembled, but was not identical to, the material that the rest of the world calls “aramid” -- which exhibited much better performance than DuPont’s KM2 and Teijin’s Twaron in ballistic experiments conducted by the US Army’s PEO Soldier. This Russian aramid apparently performsalmost as well as mid-grade UHMWPE. But “almost” isn’t “superior,” and there’s little chance that any new grade of aramid will come close to beating the best grades of UHMWPE on performance. (Rumor has it that DuPont is hard at work on a new grade of a Russian-style aramid derivative, so this may soon be put to the test.)
Zylon is an interesting case. Unlike carbon nanomaterials, Zylon fibers are fairly easy to make. Unlike aramid, they can potentially beat UHMWPE on performance. It’s theoretically possible to stabilize it – either by altering its chemistry, or by applying a barrier coating during the manufacturing process. This could, potentially, result in a top-tier fiber material for armor composites.
But stabilized zylon is unlikely to ever come about, for it’s explicitly banned in most armor products. If you’re an armor company and you’d like to bid on a Military project, you’ll quickly find that most of the solicitations ban Zylon by name. The last one I saw required a “written declaration by the manufacturer that the Zylon anti-ballistic material (IUPAC name: poly(p-phenylene-2,6-benzobisoxazole)) or any other material, based on the same category of materials, has not been used.” The NIJ, for its part, refuses to certify any product that contains Zylon. So even if a Zylon derivativecan be stabilized, and even if it performs better than UHMWPE, such a material would face almost insurmountable challenges in the market.
M5 Fiber, which is structurally similar to Zylon in certain respects, has an outside chance of potentially competing with UHMWPE. But it seems that serious development work on this polymer material is not ongoing, perhaps due to production difficulties, or perhaps because the non-military market for M5 fiber is limited. (As a high-stiffness fiber, it competes with much cheaper carbon fiber in non-military applications.) And it’s unlikely, in any case, that it can out-perform UHMWPE; for it is considerably more dense – in fact, at 1.7 gm/cc, it’s nearly twice as dense – and, by all projections, even the best grades of M5 would have a substantially lower specific tensile strength than the best modern grades of UHMWPE fiber. Those “best” grades of M5 are mere speculation, besides, and in reality the material might not perform that well.
So for now, and until the carbon nanomaterial guys get their act together, UHMWPE’s status as the high-end armor fiber of choice seems entrenched. Where does it go from here, and how can it cement its position?
Perhaps most obviously, improved fiber production techniques are going to bring UHMWPE even closer to its theoretical strength. This simply represents the continuation of an ongoing process which began decades ago. Unlike aramid -- where Kevlar-29, Kevlar KM2, and Kevlar KM2+ are separated by decades -- new and meaningfully improved grades of Dyneema and Spectra-Shield are released every few years. There are real fiber quality differences between 2017’s Dyneema HB212 and last year’s HB311, and there are no indications that this process of refinement and optimization is slowing down.
As all of the UHMWPE used in armor is used in composite form, there’s another potential avenue for improvement in resin chemistry. Performance will improve whenever polyurea, polyolefin, and polystyrene resins get tougher or stronger. But, more interestingly, other resin types may also see some use. Polysulfide -- which is extremely tough, flexible, and ductile -- might perhaps see use in ballistic UHMWPE composites where deformation resistance is not a design priority, such as shields and barriers. Epoxy is generally considered too stiff and brittle for ballistic applications, but new semi-ductile grades of epoxy may challenge that assumption and find use in UHMWPE composites where rigidity and deformation control are important, such as helmets.
And it’s not just about the fibers and the resin systems considered separately – there’s plenty of work to be done on how they interface with each other. UHMWPE is a famously “slick” material, and resins don’t tend to bond well to it. This is unfortunate, for, all else being equal, an increased resin-fiber bond strength improvesevery functional property of a fiber composite system -- from the composite’s strength, to its kinetic energy absorption capability, to its rigidity and deformation resistance. Bonding, however, is strongly influenced by fiber surface roughness and other surface characteristics, and surface treatments which involve roughening UHMWPE fibers with femtosecond lasers or plasma-based techniques have shown promise in strengthening UHMWPE composite materials. The small reduction in fiber quality is more than offset by a much improved bond strength between UHMWPE and its resin matrix. These techniques are still being researched, and have not yet been commercialized, but the time seems very near.
Ultimately, there isn’t going to be a revolutionary shift in the performance or properties of UHMWPE ballistic composites, but a process of incremental evolution as fibers get better, resins get stronger, and tighter bonds are formed at the resin-fiber interface.