April 04, 2022 8 min read
Understanding the role of ultra-high molecular weight polyethylene (UHMWPE) in combat helmets and other ballistic protection first requires understanding its chemical makeup and properties. Polyethylene is the simplest “hydrocarbon polymer.” Breaking down that term:
Polyethylene is a relatively simple hydrocarbon that consists solely of repeat methylene (CH2) units:
It’s relevant to contrast UHMWPE with aramid, another polymer widely used in ballistic protection and popularly known by the trademarked Kevlar®. Aramids have a much more complicated chemical structure:
Note the “repeating units containing large phenyl rings” (C6H5) … “linked together by amide groups” (CO-NH). In terms of both atom types and their configuration, aramids are way more complicated.
Most synthetic polymers, such as most polyesters, are closer to aramid in their structural complexity, whereas polyethylene stands out for its simplicity.
Another critical difference between polyethylene and aramid is that the former is a thermoplastic polymer, whereas the latter is a thermoset polymer.
This means that polyethylene softens when exposed to heat and can be melted down and reformed. In contrast, aramid, and all other thermoset polymers, cannot be melted down and reprocessed—they irreversibly degrade when exposed to sufficient heat.
However, aramid’s heat tolerance is much greater than polyethylene’s—it decomposes at temperatures above 930°F, whereas polyethylene will melt at approximately 230°F.
Because ballistic impacts generate heat, scientists assumed throughout the 1950s to 1970s that:
Both assumptions seemed reasonable at the time because, among other reasons, the only “ballistic fibers” known then were very heat resistant: nylon, fiberglass (Doron), aramid, and even silkworm silk. Nevertheless, these early ideas about heat turned out to be false.
This plastic shopping bag has more chemically in common with modern body armor and combat helmets than you might think.
You’ve likely noticed that we’ve been using the words “polyethylene” and “UHMWPE” interchangeably. 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. But many different variantsof polyethylene have different functional aspects.
Generally, these main variables and how they impact ballistic protection can be broken down into two parameters:
"Molecular weight" simply refers to the mass and, therefore, length of those CH2-CH2 chains. Each CH2 link in a chain weighs 14.01 daltons (Da).
In low-molecular-weight polyethylene, aka low-density polyethylene (LDPE), which has a molecular weight of about 2,000–5,000 Da, those chains are just hundreds of repeat units long and generally entangled with each other. This results in a material that is soft and weak but highly ductile and elastic because the bent and entangled molecule chains can straighten out when tensile stresses are applied.
Thus, LDPE is primarily used in plastic bags, wraps, and containers such as shampoo bottles.
Increasing polyethylene’s molecular weight to 2 million–7.5 million daltons—where its constituent molecular chains can be more than 500,000 units long—results in high-density polyethylene (HDPE). This material is much stronger, stiffer, and denser but somewhat more brittle due to reduced elasticity.
HDPE and bulk sheets or blocks of UHMWPE, 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 liners.
A dump truck liner made of non-composite bulk UHMWPE plates.
But though HDPE and UHMWPE in isolation are stronger than LDPE, they aren’t particularly strong materials overall; the average tensile strength of an extruded UHMWPE plate is just 30 MPa (megapascals). For comparison, standard glass has an ultimate tensile strength of 33 MPa, A36 steel has 400–550 MPa, and aramid fibers are 3,757 MPa.
Fibers derived fromUHMWPE, however, are extremely strong, tough, and damage tolerant. Their tensile strength has been measured at as much as 7,000 MPa. So, why the difference between fibers and other forms of the same substance?
The vast and seemingly puzzling performance gap between bulk polyethylene (of any type) and fine UHMWPE fibers is analogous to the difference between plate glass and fiberglass.
Though fiberglass is very strong, regular glass 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 an intrinsically strong material primarily made of strongly bonded silicon and oxygen atoms. But it's tremendously sensitive to flaws and defects such as scratches and microcracks.
Water also degrades glass, breaking the Si-O (silicon-oxygen) bonds and replacing them with a hydrogen-bonded bridge that’s far weaker. This “hydrolysis” reaction can occur at highly stressed microcrack tips, resulting in a "weak zone" of material where it's most likely to fail.
A schematic illustration of the hydrolytic degradation of glass by water.
And, unlike metals, amorphous and brittle materials such as glass can't absorb mechanical stresses via crystal dislocation motion. So, ultimately, it takes very little force to set off a chain reaction that results in catastrophic material failure, commonly shattered glassware or windowpanes.
All of that said, something interesting happens when glass is drawn into very fine fibers. Flaws and defects are reduced by orders of magnitude and sometimes eliminated entirely in the finest, pristine fibers. This process allows the high innate toughness of glass to shine through; the strength of very fine glass fibers begins to approach that of its robust 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 reduction mechanism accounts for the high strength of both UHMWPE and carbon fibers.
Microstructural imperfections, atomic inclusions, microcracks, voids, inhomogeneities, and other faults are invariably present in bulk materials. And most failures 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, totally absent.
Nobody has yet figured out how to make perfect glass plates—and it may be practically impossible. (Interestingly, there are indications 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 still edging closer and closer to the ideal.
And in this respect—the attainment of perfection—fibers derived from UHMWPE are way ahead of those from glass. The theoretical strength of polyethylene fiber has been estimated at roughly 30,000–40,000 MPa, mainly based 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 mostly stuck at around 10%. UHMWPE fibers are much closer to their theoretical strength than any other type of fiber.
UHMWPE, glass, and other types of fibers are strong, but strength itself is not always enough. Fibers are highly “anisotropic,” which means they don’t have equal properties in all three directions.
As bulk materials, on macroscopic scales, fibers are strongest when 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.
So, UHMWPE holds another advantage here: Its fibers are highly crystalline and rigid, don't branch very much, and naturally orient themselves into well-aligned sheets and structures of very long molecular chains. These properties allow UHMWPE 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 lines, sailcloth, and many other applications requiring high tensile strength and low density. In such cases, UHMWPE fibers are usually used "neat"—that is, in their pure form.
In armor applications, UHMWPE fibers are always used in a "composite" form; 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 when hit by a projectile.
The last part is essential. UHMWPE fibers have such a low friction coefficient that they are liable to part under ballistic impact conditions without a resin binder, clearing space for a projectile to travel through. This effect has been observed in various depreciated soft armor products released a couple of decades ago and long since discontinued. They used woven UHMWPE without a resin binder, performing very poorly compared to Kevlar.
Today, even flexible sheets of UHMWPE, generally employed in handgun-rated “soft” armor panels, are composites made up of two to six layers of UHMWPE fibers, laid up in a 0/90° orientation and held in place with resin.
Resin selection can profoundly impact the performance of a part. For example, hard UHMWPE fiber-composite laminates made with stiff polyurethane resins will exhibit lower “backface deformation”—the bulge a projectile makes, causing blunt trauma—upon impact.
In contrast, that same laminate plate made with a more ductile polystyrene (“Kraton”) matrix would exhibit more backface deformation but better absolute performance, since deformation is a kinetic energy absorption mechanism.
So, 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 (e.g., a lower “ballistic limit” (V50: the point where half the shots get through).
Now that you know the chemistry, let’s take a look at the history. Read part two of this series to learn about UHMWPE combat helmets and armor and how this substance was first developed for use in ballistic protection.
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.
Hard Head Veterans stays on top of helmet research to ensure we provide the best protection possible. Readmore blog posts, and be sure tocheck out our gear, including a selection ofthe best tactical helmets andessential helmet accessories.