Thermal protection - a double edged sword
The issue of thermal protection has traditionally been a double edged sword; if a protective garment insulated the wearer from outside heat and flame, it also insulated in the reverse direction. It could trap body heat and moisture inside the garment, leading to fatigue, discomfort, moisture build-up and loss of the body’s ability to maintain proper body temperature (heat stress). This resulted in the so called “plastic wrap effect”.
Synthetic flame resistant fibres (aramids, polyesters, modacrylics, vinyls, etc) can all be used to produce functionally flame resistant fabrics, but fabrics made from these synthetic fibres are often unsatisfactory with respect to heat and moisture management (comfort of the wearer).
Much research has been done with various topical treatments to make synthetics more comfortable, but the improvements are marginal at best, and are progressively degraded by laundering.
“topically treated fabrics can expose the wearer to toxic pyrolysis gases”
An alternative approach was to start with a natural, comfortable fibre such as cotton, and treat it with topical flame retardant chemicals to make it flame resistant. Unfortunately, this approach resulted in fabrics which lost most of the comfort characteristics of cotton. And the flame resistance is not permanent; with each laundering the flame resistance is reduced. Eventually the garment produced from topically treated cotton no longer meets flammability standards.
Furthermore, if involved in flame or high thermal conditions, topically treated fabrics can expose the wearer to toxic pyrolysis gases generated by the fabric treatment. Highly toxic heavy metals (for example antimony) and halogen compounds (for example fluorine, chlorine, bromine and iodine) can typically be evolved and inhaled by the wearer.
So a novel approach had to be found. The ideal solution to building a state-of-the-art flame resistant garment would be to start with fabric based on a natural fibre that is inherently comfortable as well as inherently flame resistant.
Viscose - a pure, natural, wood based cellulosic fibre (originally called man made silk) was chosen for research in this area. An ambitious development program resulted in a revolutionary, inherently flame resistant, inherently comfortable, breathable, anti-static, non-irritating fibre. Such a fibre would make possible the current advance in thermal protection fabrics/apparel. This was the birth of the inherently flame resistant viscose fibre.
A new generation of thermal fabrics for specific end users
Using the inherently flame resistant viscose fibre Lenzing FR®, a whole new generation of thermal fabrics has now been developed. These fabrics effectively combine the historically mutually exclusive properties of permanent flame resistance and wearer comfort.
In addition to providing “comfortable thermal protection” the new generation fabrics also have some ancillary properties which are of interest to specific end users.
For utility workers, innovative fabrics have been commercialised which are blends of viscose and aramids. These fabrics are comfortable yet offer excellent protection against flame and high voltage electrical arcs. Utility workers appreciate the natural comfort as well as superior UV stability of the new generation garments.
For hot metal splash protection, (for example in aluminum smelters and foundries), fabrics composed of Lenzing FR® and wool are not only comfortable, but have superior metal shed and cryolite resistance properties compared to previous generation fabrics. Because of the hot environment where these garments are worn, the excellent wicking properties contributed by viscose add to wearer comfort. As with flash hoods, perspiration is wicked away from the skin and to the surface of the fabric. The subsequent evaporation helps to cool the wearer because of the latent heat of the vaporisation phenomenon.
“perspiration is wicked away from the skin and to the surface of the fabric”
Fabrics made with wool and Lenzing FR® fulfil Norm EN 531 with the following results:
EN 532 – Limited flame spread Pass
EN 366 – Radiant heat Level 2
EN 367 – Convective heat Level 1
EN 373 – Molten metal splash (aluminium) Level D-2
Molten metal splash (iron) Level E-2
Several companies throughout the world in the aluminium, steel and iron sector have clothed their workers in garments made of wool/Lenzing FR® blends and the results have been highly satisfactory.
For welding industry applications, new generation fabrics composed of Lenzing FR® and para-aramids have been developed. They are proving to be the lightweight, comfortable solution for protecting the welder. There is also an added benefit; the wear life of these garments is up to 10 times that of traditional treated cotton products, providing excellent cost effectiveness, despite the higher initial cost.
For petrochemical and general industrial applications where protection against flash fire is required, Lenzing FR®/aramid blends are replacing heavier, less effective, chemically treated cotton shirts, trousers and coveralls. These new blends have resistance to a wide variety of solvents, acids, alkalis and other industrial substances. Yet, these new generation fabrics are comfortable, inherently flame resistant and cost effective.
Fabrics made with a blend of wool and Lenzing FR® offer natural protection. Highlights include:
Permanent flame resistance regardless of the amount of laundering
Good protection against convective heat
Good protection against radiant heat
Excellent resistance against molten metal splash
Excellent wear comfort
Easily dyed – produces brilliant colours
Good colour fastness to light and laundering
Breathable and non-irritating to the skin
Features of aramid/Lenzing FR® blends include:
Lightweight, comfortable fabrics with higher moisture regain than 100% aramid
Soft, smooth and comfortable against the skin
Long lasting durability versus FR treated cotton and cotton blends
Natural ‘chambray’ appearance
Better light fastness than piece-dyed aramids
Excellent pilling resistance
Easy care; no touch up ironing required
Finally, testing has concluded that the toxicity of pyrolysis gases for Lenzing FR® is no different than that of regular viscose. Additional Gas Chromatographic analysis of pyrolysis gases confirmed that there were no phosphorous compounds or sulphur dioxide (SO2) detectable. Formation of hydrogen cyanide or hydrogen chloride in the smoke gas is not possible because the flame retardant agent used does not contain nitrogen or chlorine.
The balance between protection and comfort
So, for numerous applications, Lenzing FR® flame resistant viscose fibres would seem to be “the perfect solution”. But we all realise that in an imperfect world, there are seldom perfect solutions...
By way of analogy, in the consumer market, polyester was introduced years ago as a solution to enhance the care properties of pure cotton garments. But simply replacing the cotton with polyester would solve one problem and simultaneously create another. Pure polyester was not very comfortable. Eventually, the consumer determined that a blend of cotton and polyester gave the best of both worlds. Cotton for comfort, polyester for easy care. Ultimately, the optimum blend ratio was determined by the balance that the consumer wanted between comfort and easy care. Similarly with thermal protection fabrics; there are balances to be determined. Let us consider the properties of two widely used fibres with inherently flame resistant properties.
At one end of the spectrum are the synthetic fibres, such as aramids, which are inherently flame resistant and extremely durable. At the other end of the spectrum are the viscose fibres which are inherently flame resistant and extremely comfortable. Here, as in the cotton/polyester example above, a balance has to be achieved between two competing properties - durability and comfort. Or, to be more precise; advantage can be taken of the synergy between flame resistant viscose fibres and aramid fibres. The balance is determined by the end use application of the fabric/garment.
“the optimum blend ratio was determined by the balance that the consumer wanted between comfort and easy care”
For example, for firefighter’s bunker gear, physical strength and durability is of the essence, so 100% aramid construction makes sense.
Alternatively, for children’s nightwear, comfort is of the essence, so 100% inherently flame resistant Lenzing FR® makes sense.
For most industrial applications, however, the best balance between protection and comfort lies somewhere in between. Research in this area shows that Lenzing FR®/aramid blend levels of 50/50 are a good starting point. But these levels can range from 20/80 to 80/20 depending on the balance of properties to be achieved. Lenzing AG, in cooperation with other fibre producers, developed yarns and fabrics based on varying blends. The results obtained from several tests clearly showed that this new generation of fabrics was a great improvement in many ways. Compared to 100% aramid fabrics, these new blends offered equal or better protection against heat and flame and at the same time brought about a whole new level of comfort.
The three C’s
The protective apparel industry speaks of the three C’s that it needs to address on an ongoing basis; Compliance, Cost and Comfort.
Compliance is a given; today there is an array of fabrics available which are compliant with current government, military and industry requirements and standards.
Cost of compliant fabrics is determined by the competitive nature of the protective apparel industry. Effectively this has resulted in increasingly better protection at lower prices.
Comfort is an ongoing and increasingly more important issue. Comfort can now be incorporated as a permanent feature in thermal protective apparel with, in most cases, simultaneous cost savings to the end user.
Published: 10th Oct 2002 in Health and Safety International