The Effect of Resin Selection on UHMWPE armor performance

(Working on an expanded second edition of the book, which will probably be released sometime next year, and figured I’d drop some notes here in advance.)

If an armor plate or soft armor panel is made entirely from ultra-high molecular weight polyethylene (UHMWPE,) it will nevertheless always be a composite armor plate.  A composite, in a technical sense, is an indivisible material made up of two or more constituent materials.  UHMWPE fibers, like many fiberglass and carbon fiber parts, are always encased within a polymer resin.  Technically, these materials are fiber-reinforced plastics, where the resin is the plastic, and serves an important structural role.  

In ballistic systems derived from UHMWPE, the resin (1) imparts stiffness, (2) improves heat resistance and damage tolerance, (3) improves abrasion resistance and mechanical durability, and (4) holds UHMWPE fibers in place so that they don’t slide past each other upon impact.  The last point is especially important, because, unlike aramid fibers, UHMWPE fibers have an exceptionally low coefficient of friction, and can slip past each other if they’re impacted or compressed at a very high strain rate.  This lesson was learned at a cost:  Woven UHMWPE fibers, without a resin binder, were once used in soft armor panels — but they performed very poorly and unreliably, precisely because the fibers would stretch when impacted and bullets would push through the gaps.  This resulted in embarrassing product failures.  A resin binder would have prevented this sort of slip.

Resins are important in general — but what separates different grades of UHMWPE, guides their selection, and dictates their performance characteristics is the selection of particular resins.  A recent paper from the US Army Research Laboratory at Aberdeen [1] illustrates this.  

The ARL tested two different types of UHMWPE composite panel from DSM — one of which was the commercial grade “HB-210,” the other was the commercial grade “HB-212.”  HB-210 and HB-212 are high-end UHMWPE composite materials, each made of the same “SK-99” fiber type, with the ratio of fiber to resin the same in both, at around 17% resin by volume.  If only fiber characteristics determined ballistic performance, both grades would exhibit indistinguishable performance characteristics, after making allowances for data scatter.  That’s not what the researchers found.

HB-210 utilizes a relatively stiff polyurethane resin matrix, whereas HB-212 utilizes a more ductile  polystyrene-copolymer (“Kraton”) elastomer matrix.  Upon impact, to simplify things a little bit and summarize the main point, the softer HB-212 panels were able to spread impact forces out over a larger area, and were also able to absorb more kinetic energy via plastic deformation.  Ultimately, for these reasons, the HB-212 panel’s ballistic performance was 5% better than the HB-210 panel’s performance, in terms of V50; in other words, against the same projectile and under the same experimental conditions, a projectile that can reliably penetrate a HB-210 panel at 1200 fps would need to move at 1260 fps to reliably penetrate a HB-212 panel.

Which isn’t to say that HB-210 is simply worse than HB-212.  Panel backface deformation was also measured, and here the situation was reversed.  The HB-210 panel exhibited backface deformation roughly 25% lower than the HB-212 panel.  So, all else being equal, if a HB-210 armor panel exhibits 30mm BFD upon impact, an equivalent HB-212 armor panel would exhibit 37.5mm, against the exact same threat at the exact same velocity.  

So it appears a UHMWPE composite’s resin matrix significantly influences ballistic performance; by single-digit percentage points where kinetic energy absorption and absolute performance characteristics are concerned, but in a hugely significant way where deformation characteristics are concerned.  On final analysis, this seems to come down to shear stress, which was very high in the HB-212 panel, for the ductile polymer matrix 

To validate these results, the researchers then turned to DuPont’s Tensylon 30A, which is not a UHMWPE fiber composite, but instead is made of ribbons (or “tape”) of extruded UHMWPE.  Tensylon 30A is a “self-reinforced composite.”  Self-reinforced composites are an unusual class of composite material, built from fibers or tape that are bound with a lower melting-point and generally disoriented polymer with the same chemical composition.  In Tensylon’s case UHMWPE ribbons are held in place with a UHMWPE polymer resin.

Tensylon 30A has an in-plane shear stiffness 20 times higher than HB-210, but it’s much less strong and its ribbons have a much lower intrinsic modulus than the fibers of HB-210.  In other words, it’s stiffer upon impact than the fiber composites, but less capable of kinetic energy absorption.  Continuing the trend — and sufficient to validate the experiment — the Tensylon 30A panel exhibited much worse ballistic performance in comparison with both fiber composite panels, yet, simultaneously, exhibited much lower BFD than both fiber composite panels.

The performance differential between HB-210 and HB-212 is threat and system specific.  It can’t be generalized.  In some experiments, the two grades exhibit equal V50 ballistic performance values.  In others [2] it appears to depend on how the panels were processed: