Fiber Composite Armor III: UHMWPE
Consisting of highly-oriented fibers drawn from the simple hydrocarbon polymer polyethelene (C2H4), this class of material offers an outstanding performance-to-weight ratio on account of the high strength and stiffness of the bond between carbon atoms. There are a number of grades available, ranging from low-cost grades with a weight efficacy roughly equal to aramid, to very highly-optimized grades, such as Dyneema SB117 and Spectra Shield 5143, which are much more effective than aramid on a weight basis.
UHMWPE has a low melting-point that leads to performance degradation at high temperatures. This becomes a serious consideration when the material is exposed to temperatures over 70°C for appreciable lengths of time, or to temperatures over 100°C for even very short lengths of time. Unlike aramid, however, UHMWPE does not degrade following exposure to UV radiation or water.
UHMWPE composites are frequently made with rubber, polyurea, polyurethane, or epoxy resins. Unlike aramid, UHMWPE is never supplied “neat.” Even the thin sheets used in soft armor panels consist of two or more layers of unidirectional fibers held in place with flexible resins.
UHMWPE – Spectra 1000
Ultimate Tensile Strength: 2570 MPa
Modulus of elasticity: 120 GPa
Elongation at break: 3.5%
Uses in armor systems:
Soft body armor.
Plate backer in ceramic body armor systems.
Stand-alone rifle plates.
Aerospace and naval armor.
Many other non-structural applications where a very high strength-to-weight ratio is required.
Scientists at DSM in the Netherlands discovered ultra-high molecular weight polyethylene (UHMWPE) fibers by accident in 1963, during experiments in fractionating out different polyethylene lengths from a solution. It was immediately noted that these very long-chained polyethylene molecules were incredibly strong, but, initially, only small amounts were produced, and it seemed impossible to produce meaningful quantities. This is on account of the fact that the UHMWPE fibers were nearly impossible to orient in solution, so they’d typically form intractable aggregates and clumps, and only a small fraction of each production batch would result in usable UHMWPE fibers.
(Spaghetti is the usual analogy. On a molecular level, strong UHMWPE fibers resemble bundles of stiff, uncooked spaghetti. UHMWPE in solution resembles tangled clumps of soft, overcooked spaghetti. The art of drawing strong Dyneema fibers — and it is an art — resembles turning cooked and tangled spaghetti into uncooked spaghetti!)
In 1978, 15 years after the original discovery, DSM engineers discovered the ingenious gel-spinning method, which enabled the production of commercial quantities. Dr. Piet Lemstra, who worked on the project, described as follows:
“It’s actually very simple. The more you dissolve the long chain molecules in a solvent, the more they become separated. Then you cool it down to a gel state and the molecules are more or less disentangled, so it is easy to stretch them. You can then remove the solvent, or later, during the stretching/drawing process – either way you end up with nearly perfectly aligned molecules.”
Although the development of this clever gel-spinning process enabled the industrial production of UHMWPE fibers, DSM was not an industrial fiber company. It was — and to a large extent remains — a food and agricultural supply company that discovered UHMWPE fiber by accident. The UHMWPE team at DSM was essentially a “skunkworks” crew that did their experiments in spare time and on weekends, with little corporate oversight. In the years since its discovery and the development of a commercial production process, the decision-makers at DSM showed no interest in commercializing UHMWPE fiber, and this was not helped by the fact that the consulting companies they asked to evaluate their potential product did not think very highly of it.
“This [consulting] company said the fiber was just candlewax unable to resist high temperatures and it lacked any good properties. They saw no future in it,” says Lemstra. “And the management at DSM believed them.”
Around that time, in the very early 80s, Allied Signal (now Honeywell) obtained a patent on a very similar UHMWPE fiber product, though they claimed that the melting point of their material was 20-30 degrees higher than that of DSM’s fiber. As it turned out, the two UHMWPE fibers were largely identical, the melting point differences were due to an methodological error on Allied Signal’s part, and, as the DSM patent had priority, Allied was compelled to purchase a license for the original DSM patent.
With the license in hand, and no potential intellectual property conflicts on the horizon, Allied started marketing their own fiber, Spectra, as a competitor to Dupont’s Kevlar. Dupont countered with effective marketing campaigns which showed how Spectra would weaken at moderately high temperatures — from 60-80°C. The military, aided by institutional inertia, essentially said “if it ain’t broke, don’t fix it,” and stuck with aramid armors.
It could be argued, in hindsight, that Allied and DSM didn’t do enough to promote their material. Yet, at the same time, the first grades of UHMWPE fiber were quite poor. We have seen continual improvement since those early days, and today’s grades of UHMWPE fiber are more than 44% superior to the best grades of Kevlar on an equal-weight basis.
Dyneema and Spectra are presently used in high-end body armors, the very newest models of military helmet, and lightweight ballistic shields. Unlike aramid, moderately thick panels of UHMWPE (2cm+) are capable of stopping lead-cored rifle rounds at high velocity, but invariably fail against steel-cored rifle rounds. In fact, they can fail against steel-core pistol rounds; that the SS190 AP 5.7x28mm pistol round can punch through a UHMWPE Level III plate has been well documented.
In any case, there’s significant room for improvement in UHMWPE fiber materials, and it’s likely that further gains will be made within the next 10-15 years.