Regularly faced with life-threatening situations, fire-fighters naturally depend on a strong outer-shell fabric in their turnout gear.
High tensile and tear strength usually translates into good wear life and consequent better whole life value, with garments requiring fewer repairs and lower replacement levels as a result of the mechanical failure of the outer-shell. Does it, therefore, follow that the highest fabric strength possible should be welcomed across the board?
Not necessarily!
Of course, acceptance depends on varying end user requirements that have to be determined by a PPE Assessment as required under the UK PPE at Work Regulations 1992. High strength normally comes with a higher percentage Kevlar® content in the outer-shell which can lead to a variety of other problems. Kevlar® is very strong (five times stronger than steel on a filament to filament basis) but it is very rigid and, therefore, extremely fragile and will shatter when flexed – a process called fibrillation. This gives the characteristic frosted appearance to Nomex® Tough blends (23% Kevlar®) and Pbi Gold (60% Kevlar®).
This fragile nature of Kevlar® also leads to poor abrasion resistance as, rather than moving and flexing with the wearer, the yarn fibres simply break. This is especially apparent on areas of high stress such as elbows, knees and the interface with the BA set – on the shoulders in particular. Kevlar® also transmits heat more quickly than Nomex®. Thus, fabrics with a high Kevlar® content will transmit heat through to the wearer faster, especially at lower working temperatures, when compared to fabrics with a lower Kevlar® content e.g. Nomex® Antistatic or Nomex® Comfort.
Fabric strength is not static throughout the life of the garment and can be detrimentally influenced by external factors such as abrasion (through everyday wear and laundering) and UV degradation. A fabric that is initially very strong may not stay that way, so any perceived benefits of this higher strength may be lost over time.
This is one of the reasons why Hainsworth® TITAN removes the majority of Kevlar® from the face of the fabric and positions it on the back. Here, it is protected from the effects of UV degradation and abrasion, but still offers strength and integrity to the outer shell fabric.
When identifying the benefits of higher strength, we should first define strength parameters. EN469:2005 sets the tensile strength requirement of an outer-shell fabric at 450N. This translates into a fabric that cannot be torn or ripped by hand without the use of a tool e.g. knife or scissors. Most outer-shell fabrics on the market exceed this level by a considerable factor coming in at 1000N or higher, prompting the question: “Why?”
A significantly higher tensile strength will not offer better thermal performance, nor will it reduce heat stress or increase comfort. In fact, it could quite possibly have the opposite effect. If we draw an analogy with driving a car, and the current speed limit is 450mph, what real benefit is a car that can do 1500mph or 2000mph compared to one that can only do 1000mph or 1200mph?
Being faster doesn’t mean that your brakes are automatically better, or your seat belt has been upgraded, in the same way that high tensile strength doesn’t mean your abrasion resistance or thermal performance are increased. Claims for increased tensile strength can end up being about performing against a specific, intensive lab test and may not bear any relation to the requirements of real life or increased protection for the wearer.
Focusing on strength is one small element of a garment assembly as a whole and adding a matrix of ‘go faster stripes’ doesn’t automatically make it a better performing fabric in the long run!
Source: Fire Times
