April 04, 2022 8 min read
UHMWPE and the modern helmet
This is the first part of a planned four-part series covering some of the basics of ultra-high molecular weight polyethylene (UHMWPE) composites in helmet design. We are going to discuss why UHMWPE is the best material for lightweight combat helmets, its historical development in armor systems, and its outlook in near-future helmets and armor systems. We may draw certain parallels to the development of body armor systems.
Part I: UHMWPE Basics
Polyethylene is the simplest hydrocarbon polymer. It consists solely of repeat units of CH2.
Contrast with aramid, which has a much more complicated chemical structure:
Aramid’s structural complexity is more typical. That is to say, most synthetic polymers, such as most polyesters, are closer to aramid in their structural complexity, whereas polyethylene is anomalous in its simplicity.
Another point of difference between polyethylene and aramid is that the former is a thermoplastic polymer, whereas the latter is a thermoset polymer. What this means is that polyethylene softens when it’s exposed to heat, and can be melted down and re-formed. Aramid, and all other thermoset polymers, cannot be melted down and reprocessed -- they irreversibly degrade when exposed to sufficient heat. Aramid’s heat tolerance is, however, much greater than polyethylene’s -- aramid decomposes at temperatures above 930°F, whereas polyethylene will melt at approximately 230°F.
Because ballistic impacts generate heat, it was assumed through the 1950s-1970s that (a) aramid’s high heat tolerance contributes to its ballistic performance, and (b) thermoplastic polymer fibers are generally unsuitable for use in ballistic systems, for they would soften when struck. Both assumptions seemed reasonable at the time, for, among other reasons, the only “ballistic fibers” known were very heat resistant: Nylon, fiberglass (doron), aramid, and even silkworm silk. Nevertheless, those assumptions turned out to be false.
You might note that we’ve been using the word “polyethylene” rather than “UHMWPE.” This is because the two substances are chemically identical. Polyethylene, used in grocery bags and milk jugs, is chemically indistinguishable from UHMWPE, and they share the same thermoplastic characteristics. There are many different variants of polyethylene, and they have different functional characteristics, but generally the relationship can be broken down along two parameters:
- Molecular weight. For ballistic applications, the higher, the better.
- Size. For ballistic applications, the smaller the better, and very fine fibers are best of all.
“Molecular weight” simply refers to the mass, and therefore length, of those CH2-CH2 chains. In those chains, each CH2 link weighs 14.01 daltons. In low molecular weight or low-density polyethylene, which has a molecular weight of 2000-5000 daltons on average, those chains are just hundreds of repeat units long, and generally are entangled with each other. This results in a material that is soft and weak, but highly ductile and elastic, because those bent and entangled chains of molecules can straighten out when tensile stresses are applied. LDPE is primarily used in plastic bags, plastic wraps, and containers such as shampoo bottles.
Increasing polyethylene’s molecular weight to 2 to 7.5 million daltons -- where its constituent molecular chains can be more than 500,000 units long -- results in a material that’s stronger, stiffer, denser, but somewhat more brittle due to reduced elasticity. HDPE and bulk sheets or blocks of UHMWPE, which are usually produced via compression molding or extrusion from a melt, are used in seals, bearings, biomaterials, and structures built for friction or impact resistance, like marine fenders and dump truck bed liner.
(Dump truck liner made of non-composite bulk UHMWPE plates)
Though stronger than LDPE, HDPE and UHMWPE aren’t strong materials; the average tensile strength of an extruded UHMWPE plate is just 30MPa. Fibers derived from UHMWPE, however, are extremely strong, tough, and damage tolerant; the tensile strength of UHMWPE fibers has been measured at as much as 7000 MPa. This vast, and seemingly puzzling, performance gap between bulk polyethylene (of any type) and UHMWPE fibers is precisely analogous to the difference between plate glass and fiberglass.
So, to shine a light on why UHMWPE fibers are strong whereas bulk plastic UHMWPE is weak, let’s take a closer look at glass.
Though fiberglass is very strong, glass itself is synonymous with weakness. For instance, if you have a “glass jaw,” you’re unfortunately prone to getting knocked out in fights. So how is it that the same exact material, when drawn into fine fibers, becomes every bit as strong as steel on an equal weight basis?
The short answer is that glass is anintrinsically strong material -- it’s made up primarily of strongly-bonded silicon and oxygen atoms -- but it’s tremendously sensitive to flaws and defects such as scratches and microcracks. Water, also, degrades it -- breaking the Si-O bond and replacing it with a hydrogen-bonded bridge that’s far weaker. This hydrolysis reaction can occur at highly stressed microcrack tips, which results in a “weak zone” of material right where it’s most likely to fail. And, unlike metals, amorphous and brittle materials such as glasses can’t absorb mechanical stresses via crystal dislocation motion -- so that, ultimately, it takes very little force to set off a chain-reaction that results in catastrophic material failure, most commonly in shattered glassware or window panes.
(Schematic illustration of the hydrolytic degradation of glass by water.)
When glass is drawn into very fine fibers, something interesting happens: Flaws and defects are reduced by orders of magnitude, and sometimes, in the very finest pristine fibers, are eliminated entirely. This allows the highinnate strength of glass to shine through; the strength of very fine glass fibers then begins to approach the strength of the Si-O atomic bond.
(Glass gets much stronger as it gets smaller. Reduce a glass fiber’s diameter from 50µm to 5µm, and you’ve roughly quintupled its strength.)
The same mechanism accounts for the high strength of both UHMWPE fibers and carbon fibers. In bulk materials, microstructural imperfections, atomic inclusions, microcracks, voids, inhomogeneities, and other faults areinvariably present. In most bulk materials failure will initiate at, and spread from, these faults. But in ultra-thin fibers or nanomaterials like graphene, all of those imperfections are minimized, and in some cases they are entirely absent.
Nobody has figured out how to make perfect plates of glass, and such a thing may be practically impossible. (Interestingly, there are indications that you’d need to begin by making glass in a totally water-free environment -- likely in space, utilizing anhydrous raw materials from asteroids or the moon.) But very fine glass fibers are edging closer and closer to perfection.
And in this respect -- in the attainment of perfection -- fibers derived from UHMWPE are way ahead of fibers derived from glass. The theoretical strength of polyethylene fiber has been estimated at roughly 30,000-40,000 MPa -- based largely on the strength of the C-C bond that forms the backbone of all polyethylene plastics. As mentioned previously, the strength of isolated UHMWPE fibers has been measured at as much as 7,000 MPa in laboratory experiments, so it’s at roughly 25% of its theoretical strength, whereas other fibers are, for the most part, stuck at around 10%. UHMWPE fibers are much closer to their theoretical strength than any other type of fiber.
But strength itself is not always enough. Fibers are highly anisotropic -- which is to say that they don’t have equal properties in all three directions. As bulk materials, on macroscopic scales, they are at their strongest when they are aligned in parallel and laid-up in a laminate or spun into yarn. Theory predicts that a “structureless” fibrous material made up of fibers randomly arranged in three dimensions would be, at best, 1/6th as strong as that sort of parallel arrangement. Such a thing would not be worth making, even if it could be made in the first place.
UHMWPE holds another advantage here: Its fibers are highly crystalline and highly rigid, they don’t branch very much, and they naturally orient themselves into well-aligned sheets and structures made of very long molecular chains. This allows it to make the most of its high strength.
Combine the extremely high strength-to-weight ratio of UHMWPE fibers with their tendency towards linearity and high directional orientation, and you have what has become the armor material of choice in recent years.
And UHMWPE fibers aren’t only for armor. They’re also used in high-end ropes, cords, tethers, fishing line, sailcloth, and many other applications that require a high tensile strength and low density. In such cases, UHMWPE fibers are usually used “neat,” that is, in pure form.
In armor applications, UHMWPE fibers are always used in “composite” form -- or, in other words, the fibers are combined with a resin binder or glue. This is because holding the fibers in place with glue improves their stiffness, shear resistance, damage tolerance, heat resistance, abrasion resistance, and frictional properties, forcing them to stick together even upon impact. The last part is especially important, for UHMWPE fibers have such a low coefficient of friction that, without a resin binder, they are liable to part under ballistic impact conditions, clearing space for a projectile to travel through; this has been observed in various depreciated soft armor products, released a couple of decades ago and long since discontinued, which used woven UHMWPE without a resin binder, and which performed very poorly in comparison with Kevlar. Today, even flexible sheets of “soft” UHMWPE, which are generally used in handgun-rated “soft” armor panels, are composites made up of 2-6 layers of UHMWPE fibers, laid-up in a 0/90° orientation, held in place with resin.
Resin selection can profoundly impact the performance of a UHMWPE fiber composite part. Hard UHMWPE fiber-composite laminates made with stiff polyurethane resins will exhibit lower backface deformation upon impact, whereas if that same laminate plate was made with a more ductile polystyrene (“Kraton”) matrix, it would exhibit more backface deformation, but better absolute performance, for deformation is a kinetic energy absorption mechanism. In armor systems, the choice of resin ultimately comes down to the intended use of the armor part. Deformation-insensitive applications favor ductile resins, whereas body armor and especially helmets typically favor stiff resins, even if their use results in somewhat reduced performance, i.e. results in a lower V50.
Having said all of that, here’s the short version:
- UHMWPE is made of, and is chemically identical to, the extremely simple and very common polymer polyethylene.
- In the production of UHMWPE fibers, it’s drawn into very long and fine fibers of high (ultra high) molecular weight, chain alignment, linearity, and crystallinity.
- UHMWPE fibers boast an extremely high specific tensile strength because those strands of UHMWPE are substantially free of defects and are very highly oriented, so their strength begins to approach the strength of polyethylene’s C-C molecular bond.
- In armor systems, UHMWPE fibers are almost always encased in resin glues, which bind them together and improve all of their functional properties.
- Resin chemistry can strongly affect the ballistic performance of the finished armor part. Flexible resins allow for better ballistic performance at a penalty to backface deformation performance.
In part 2, we’ll discuss the development of UHMWPE armor and how it was first developed for use in armor systems.
"Jake Ganor is the chemist and ballistics researcher behindAdept Armor, a developer of leading-edge body armor products and materials. If you enjoyed this article, you might also enjoy his book, "Body Armor and Light Armor Materials and Systems."