Fiber Composites – Other Fiber-Composite Materials
Carbon fiber is a fibrous material utilized primarily in the manufacture of composite panels for structural applications. It is comprised of very fine strands of graphitic (sp2) carbon, and is rarely used as a ballistic material on account of its brittleness, relatively high density, and lack of ductility. It boasts an extremely high yield strength and flexural modulus, however, and its strength and stiffness make it attractive for certain niche applications in armor. It is also highly resistant to UV light, chemical exposure, and temperature extremes.
The very best grades of carbon fiber — Toray’s T1100G (ultra-high strength) and M60J (ultra-high stiffness) — represent the current state of the art, and are uncommon, whereas the grades T300 and T800 represent the fibers most frequently used in aerospace and automotive applications. T300, in particular, dominates the carbon fiber industry — 95% of sales are of the “standard modulus” T300 grade.
Carbon Fiber – Toray T300 (Standard modulus)
Ultimate Tensile Strength: 3800 MPa
Modulus of elasticity: 230 GPa
Elongation at break: 1.5 – 1.76%
Carbon Fiber – Toray M60J (Ultra-high modulus)
Ultimate Tensile Strength: 3950 MPa
Modulus of elasticity: 588 GPa
Elongation at break: 0.7%
Uses in armor systems:
Negligible in body armor and vehicular armor.
Structural use in aerospace armor.
M5 fiber (polyhydroquinone-diimidazopyridine), developed just over 20 years ago by scientists at the Dutch chemical firm AkzoNobel, is an extremely strong and rigid polymer fiber. Small-batch samples have been produced for evaluation by the US Military, but those samples were rough, which is to say that they contained more defects than strong fibers usually do. They nevertheless performed well in ballistic experiments — and if M5 were reasonably free of defects, as UHMWPE and aramid fibers produced via mature manufacturing processes tend to be, it would exhibit vastly improved mechanical properties and still better performance. For this reason there are “measured” and “goal” values below.
It is unknown why M5 was (seemingly) not evaluated or developed further. AzkoNobel sold the rights to M5 to a company called Magellan Systems International, who went on to sell to DuPont. All indications are that M5 is not in development at this time. The most plausible hypothesis is that DuPont and Magellan attempted to develop high quality M5 fiber on bulk commercial scales, but either failed to produce it to a high enough quality standard, or failed to produce it at a commercially feasible price. Indeed, the production of M5 Fiber requires complex “aging” and “tempering” processes that are unusual for a polymer, and these extra production steps may have added to the difficulty or cost of its manufacture.
It is generally assumed that M5 fiber should meet or slightly exceed the performance of today’s best UHMWPE materials, on a weight basis. At the same time, it has thermal resistance comparable to aramid, and chemical resistance better than both.
M5 now seems to be off-patent, as of early 2021. If it can be made, somebody may eventually get around to it.
Ultimate Tensile Strength: 3960 MPa
Modulus of elasticity: 271 GPa
Elongation at break: 1.4%
Ultimate Tensile Strength: 9500 MPa
Modulus of elasticity: 450 GPa
Elongation at break: 2 – 2.5%
Use in armor systems:
Not known at this time, but presumably none.
Zylon is a para-phenylene benzobisoxazole (PBO) fiber produced by the Toyobo Company of Osaka, Japan. It became widely available in 1998, and rapidly became popular in high-end armor vests, for its mechanical properties were far superior to the aramid and UHMWPE materials available at that time — and, indeed, its mechanical properties are substantially superior to the grades of aramid and UHMWPE available today.
Shortly after its introduction in 1998, it was noticed that some Zylon armor panels exhibited anomalously poor performance, and this culminated in a 2003 shooting where a police officer wearing a Zylon vest was attacked by an assailant wielding a .40 caliber handgun. The bullet passed through the officer’s vest, and he died of his injuries.
It turns out that although highly-pure Zylon is chemically and thermally stable to a substantial extent, commercial grades contained defects that rendered it unstable in the face of ambient heat and humidity — and, therefore, the older a Zylon armor panel, the poorer its performance and reliability. Some of those defects were intramolecular — apparently not all of the oxazole rings in the PBO molecule fully close, leaving some “weak links” in the PBO chain. Others defects were attributed to porosity within fibers, or micro-porosity between PBO chains. Still other defects were attributed to excess phosphoric acid in the Zylon fibers.
There was some 2003-2005 research into Zylon stabilization for ballistic applications, and it seems as though it can be stabilized and made useful — the PBO molecule itself is extremely stable, so it’s just a matter of reducing particularly deleterious defects — but the NIJ has a hard ban on Zylon vests and refuses to certify them.
Ultimate Tensile Strength: 5200 – 5800 MPa
Modulus of elasticity: 169 GPa
Elongation at break: 3.1%
Use in armor systems:
None at this time.