Yarns used for stab/slash resistance
There are a common selection of fibres used extensively for armour products. Fibres such as
- DuPont’s para-aramid (Kevlar®);
- Honeywell’s ultra-high molecular weight polyethylene
- (UHMWPE) Spectra®;
- DSM’s UHMWPE Dyneema®;and
- Toyobo’s polyphenylene polybenzobisoxazole (PBO) Zylon®.
Here we seek to explain courtesy of Bolton University some of the virtues of each of the fibre types.
Aramid
Aramids are aromatic polyamides (for instance, Kevlar from DuPont and Twaron from Teijin) that are defined as having at least 85% of the amide groups linked to two aromatic rings. They are produced by the polymerization of long stiff molecules such as para-phenylene terepthalamide, achieving molecular weights averaging around 20 000. The stiff aromatic rings and hydrogen bonding cross-linkages combine the best features of both the polyamides and polyester in an extended chain configuration.
In para-aramids, links are formed on the opposite corners of the rings. Para-aramids do not melt, but decompose above 430°C. They absorb a small amount of waterii.
Ultra-high molecular weight polyethylene
UHMWPE is a type of polyolefin that is commercially produced in the fibre form as Dyneema by DSM and as Spectra by Honeywell. It is produced by gel-spinning, a drawing technique that uses dilute solution of ultra-high molecular weight polymer,such as polyethylene,to unfold chains further and so increases both tensile strength and fibre modulus.
These fibres derive their strength from the extremely long chains of polyethylene that can attain parallel orientation of greater than 95%. The weak Van der Waals bonds between the molecules in UHMWPE give it poor heat resistance. The fibres melt at 150°C and properties deteriorate as temperature increases above room temperature. Under high stress the fibres tend to creep extensively and can break after short times under load. A secondary, slow heating, under tension, when approaching the melting point increases modulus and reduces creep. It is extremely resistant to chemical and biological attack and has better abrasion and fatigue resistance than aramid fibresiii.
Polyphenylene polybenzobisoxazole
Toyobo produces a commercial version of PBO fibre, called Zylon, by dry-jet wet spinning from solution in phosphoric acid. The five membered rings on either side of the benzene ring result in a stiffer chain moleculeiv.
The modulus and strength of the fibre are twice as high as for aramid fibres, with other properties remaining similar. The fibre degrades by hydrolysis, which makes it unsuitable for applications that expose the material to warm and moist environmentsv.
Fabrics used for stab/slash resistance
Various fabric structures are currently used to provide resistance against stabs and slashes. They include tightly woven structures, woven and non-woven composites, and warp- and weft-knitted fabrics. Honeywell’s Gold Flex®, for instance, is a roll product consisting of four plies of unidirectional aramid fibre, cross-plied at 0˚/90˚/0˚/90˚ and sandwiched in a thermoplastic film. The composite produces ultra-thin vests that are light and comfortable to wear.
Honeywell claims they are about 25% lighter than Kevlar or Zylon vests, but offer high trauma protection. The tight structure of this composite protects from cuts and slashes and, to a degree, from stabs. This material also offers flame and heat protection and is able to withstand temperatures of up to 500°C.
Another example is Dyneema UD, a unidirectional laminate made of two layers of extended chain polyethylene filament tows, cross plied and sandwiched in a thermoplastic film. DSM claims it is the world’s strongest fibre laminate designed for ballistic protection. It is thin and one of the lightest ballistic protection materials available with a relative density lower than that of water. It offers good protection from cuts and slashes and will also help to protect against stabs. It will withstand temperatures of 150°C and maintains its protective properties down to –150°C.
As a final example, there is Twaron SRM, which exploits particles of silicone carbide to form a protective coating deposited on a special matrix composition that coats the Twaron aramid fabric substrate. The major function of the carbide particles is to blunt sharp blades and points, adding to the energy dissipation and penetration resistance of the fabricvi.
A major issue with the above-mentioned slash or stab resistant fabrics is that they are very stiffvii, which makes it impractical for them to be used on a regular basis over long periods of time. This is not a problem for the wearers of Sark Gloves and Clothing.