Too much para-aramid may reduce protection performance of firefighting clothing
Too much para-aramid may reduce protection performance of firefighting clothing
Brief characteristics of flame-resistant fibres such as Nomex®
The heat-resistant meta-aramid fibre Nomex® can withstand exposure to extreme heat for over eight seconds and does not melt or drip even at high temperatures. It offers excellent heat insulation, by absorbing the heat energy when exposed to a heat source. It also boasts high UV resistance, abrasion resistance and durability and is soft to the touch. Fabrics made of Nomex® are extremely easy to care for and maintain their appearance even after many washes.
Property profile of tear-resistant fibres such as Kevlar®
The high-strength para-aramid fibre Kevlar® has an extremely high level of tear and cut-resistance as well as good heat resistance. However, unlike meta-aramid fibres, para-aramid fibres have a few properties that can be classified as restrictive for fire protection clothing. When exposed to flames, para-aramid fibres absorb hardly any heat energy, so a large proportion of the energy can penetrate the fabric. Fabrics with a high content of para-aramid fibres are generally relatively heavy, feel rigid to touch and are uncomfortable to wear in view of their high tensile strength. Furthermore, a fabric made of para-aramid is less UV stable and abrasion-resistant than a meta-aramid fabric and has a tendency to pilling, which must always be taken into account especially after washing.
How does para-aramid impact on the fibre ?
In view of these differing property profiles, hybrid blend fabrics made of flame-resistant and tear-resistant fibres are expected to exhibit differences depending on the specific content of the two different components. As part of the DuPont quality assessment project, tests were carried out on high performance fabrics containing different contents of para-aramid which are used in firefighting clothing. Fabrics with a para-aramid content of 5%, 12%, 23% and 60% were tested : Nomex® Comfort with intrinsic weight 230 g/m2 (5% para-aramid), Nomex® III antistatic 265g/m2 (5%), a special fabric Nomex® (Hainsworth® TITAN) 220 g/m2 (12%), Nomex® Tough 195g/m2 (23%) and a fabric comprising 60% para-aramid fibres and a heat-resistant fibre 210 g/m2 (60%). DuPont analysed the performance values that are particularly important for protective firefighting clothing, such as behaviour when in contact with heat and intense flames, tear and abrasion resistance, air permeability and impact of UV radiation and washing. The focus below is primarily on the properties of the fabric Nomex® (Hainsworth® TITAN) with 12% para-aramid content and the fabric with 60% para-aramid content, as the four types of Nomex® fabric do not significantly differ from each other and Nomex® (Hainsworth® TITAN) can be considered as a mean value.
Contact heat test according to ISO 12127-1:2007
The behaviour of the fabric when exposed to contact heat was tested in accordance with standard ISO 12127-1:2007. The value for the contact heat was determined for temperatures of 100, 250 and 350°C. In all temperature ranges, the pain threshold was reached later with the Nomex® fabric containing 12% para-aramid than with the fabric containing 60% para-aramid. At a contact heat of 100°C, a time difference of over six seconds was recorded until the pain threshold was reached, which is a difference of around 50%.
Fig : Contact heat ISO 12127-1:2007
Heat transmission behaviour according to ISO 17492
To ascertain the heat transmission through the fabric, a Thermal Protection Performance (TPP) test was carried out, in which the fabrics were exposed to flames with arc rating 2 cal/cm2 (84kW/m2) for 8 seconds and the extent to which heat transmission to a PTFE membrane located beneath the fabric is detected was determined. While only moderate thermal breakdown was revealed with the fabric with low para-aramid content, clear changes in the membrane were detected with the fabric with 60% para-aramid content.
Fig : TPP results ISO 17492
Tear resistance according to ISO 13937-2:2000
The test provided for in ISO 13937-2:2000 (tear growth test) indicates fabric durability and endurance. Tear strength is tested in two ways : after 25 washes and after exposure to UV radiation, setting an upper limit value of 25 Newtons according to EN 496. In their original condition, all fabrics far exceeded the value set in the standard. However, the fabric with 60% para-aramid content tear strength decreased by 48% after 25 washes and by 12% after exposure to UV radiation compared with its original condition. The values of the fabric with clearly lower para-aramid content only revealed a decrease of 16% in tear strength after 25 washes and 1% after exposure to UV radiation.
Fig : Tear strength ISO 13937-2:2000
Abrasion resistance according to ISO 13937-2:2000
The abrasion characteristics of a fabric can be used to draw conclusions about the expected life of clothing. The test was carried out in accordance with ISO 13937-2:2000 (Martindale abrasion test) with a load of 12kPa and produced a clear result : The lower the content of para-aramid, the higher the number of abrasion cycles achieved. The Nomex® fabric blends with 5%, 12% and 23% para-aramid scored a value in the region of 100,000 abrasion cycles, while blends with 60% para-aramid only withstood approximately 25,000 abrasion cycles.
Air permeability according to ISO 9237
The higher the permeability of the outer shell of the protective clothing to air, the better the performance of the properties of the membranes processed in the protective clothing. The measurements were taken in accordance with ISO 9237 on the original fabric and after 25 washes. The fabric with a lower content of para-aramid exhibited five times greater air permeability in its original condition and eight times after 25 washes than the fabric containing 60% aramid. Although with both fabrics, permeability to air decreased with washing, the decrease was clearly lower with lower para-aramid content. In practice, this means significantly greater comfort and lower heat stress for firefighters.
The fabric tests revealed differences in the criteria that are important for protective clothing for firefighters. The fabrics with a para-aramid content of between 5 and 23% achieved better values in terms of thermal protection (heat penetration resistance), abrasion resistance and UV resistance as well as washing behaviour than the fabric with a 60% content of para-aramid. The reason for greater heat insulation is in the way in which the Nomex® fibres work, as they absorb a large part of the heat energy when exposed to heat – unlike with para-aramid fibres. Nomex® works in the same way as an airbag, in that the fabric carbonises, causing it to thicken and form pockets of air, explains Andreas Fries, Sales Manager at DuPont. And every kilojoule absorbed means less heat transfer to the skin.
The fabric containing 60% para-aramid exhibited a significantly higher level of tear resistance in its original condition than the fabrics containing less para-aramid. This is due to the very high tensile strength of the para-aramid fibres. However, the tear resistance of the fabric with very high para-aramid content clearly deteriorates more significantly in the course of its life than with the fibres containing far lower amounts of para-aramid. The test results show that the appearance of the fabric worsens the longer it is stored and worn. According to findings based on tests, this is because of the lower UV resistance and poorer abrasion and washing characteristics of the para-aramid component.
These findings are backed up by the experience gained by DuPont around the world with its various fibre blends. It is therefore shown that the tensile strength and cut resistance of Kevlar® para-aramid fibre are key additional components for increasing the performance of flame-resistant fabrics. It is nevertheless assumed that too high a content of para-aramid fibres in heat- and flame-resistant clothing only increases the properties required for use in firefighting to a limited extent and is questionable from the point of view of ensuring a service life which is as long as possible.