Fire Fighters face many life threatening situations and conditions when they are at the Fire Face. The most critical of these would be the most obvious one, that of burning, and normally this can arise from either Flame or Radiant heat source.
In addition, they also face a huge risk of exhaustion and heat fatigue, which in the most severe cases can lead to death, either by passing out from de-hydration and heat exhaustion, or by making poor decisions due to their altered state as a result of this heat fatigue, which can have fatal consequences.
The contributing factors to this heat fatigue are numerous and would include:
- The stress of the environment in which they find themselves
- The amount of weight they have to carry into the fire
- The comfort and confidence the wearer has in the protective garment they are wearing
- Other physical factors such as hydration and maintenance of body temperature
Whilst Protective clothing cannot influence the wearer’s state of mind and stress, it can go a long way towards optimising all of the other factors mentioned above.
The protective garment
First and foremost, it is designed to insulate the wearer from the heat sources mentioned in the introduction, as this has always been considered the major risk factor for a Fire Fighter.
To achieve this the garment was traditionally made of a number of layers of inherently non-flammable materials to maximise the insulation from the risk of burning, however this comes with its own factors contributing to the risk of heat fatigue.
Garments achieved the required standards of insulating the wearer from the risk of burning, but with heavy garments, that did not breathe well due to the multiple layers that were required to achieve these standards. Insulation from heat from the outside also meant keeping the wearer’s heat and perspiration in as insulation is not a one way process.
So, the challenge appears to be to manufacture the lightest possible fabric assembly that will conform to the requirements of heat protection and minimise the adverse effects of weight, body temperature regulation, hydration and moisture management (perspiration) and breathability, contributing to the overall wearer comfort and ergonomics.
The most effective way of achieving this is by means of “trapped air.” This minimises weight contribution, but the bulk achieved by this trapped air imparts excellent insulation without contributing additional mass to the garment. Traditionally this was done using multi-layers of non-woven materials, or quilted wadding of non flammable materials, but these solutions always lead to new issues, namely poor breathability and a high degree of absorbency, which meant heavy when wet.
More recent innovations have created space to trap air by “printing” bubbles of heat resistant rubber on to one of the layers, thus creating a dead air space between that layer and the adjacent layer. This works very well, with respect to insulation, but does have a significant reduction in insulation when compression is applied ( typical of knee or elbow bending), because the insulation trapped air is squeezed out. In addition, the rubber bubbles do not have exceptional abrasion resistance and so reduce in size with ageing due to rubbing between the layers, with a corresponding reduction in insulation due to reduced volume of trapped air.
The most popular thinking currently is to have a hydrophilic membrane, which wicks away perspiration moisture, by transporting the water across a pressure gradient. This means that even if this membrane is saturated on one side it will still transport moisture away and the evaporation on the other side will also have a cooling effect. This type of membrane is the most ergonomic and the best against heat fatigue as it leaves the wearer feeling dry and slightly cooled and thus comfortable in his garment.
These membranes are typically made of high heat tolerant material like PTFE and are given a coating of PU for added elasticity.
The closer the membrane is to the wearer’s skin, the better the effect of moisture transport and the better the breathability.
Using the above principles Alpex designed a new concept of product, which utilises the trapped air insulation solution, but minimises the negative aspects mentioned above. In addition, because some of the product is based on upcycled aramid it is also sustainable in these days of owning one’s waste.
The product is based on a non-woven fabric made of aramid fibres, with large diameter weft inserts “woven” across the fabric to create channels of trapped air between the large diameter yarns. These large diameter yarns are of recycled aramid fibres ( See Diag 1.)
This resultant fabric is then laminated to an E-PTFE hydrophilic membrane as mentioned in the Breathability paragraph above.
This results in a fabric which has:
- Excellent insulation due to the channels of trapped air, without being heavy or bulky,
- Improved compression insulation, due to the thick yarns being incompressible
- Durability of effect with ageing, due to good abrasion resistance of the insert yarns
- Good breathability, due to E-PTFE and positioning in the garment complex.
As the target market for this study was Australia and New Zealand it was decided to test (where possible) in local accredited laboratories to AS/NZS 4967 : 2009 requirements, and to concentrate on the critical aspects mentioned, namely Protection against Flame Heat, Radiant Heat and Breathability.
Tests were carried out on 3 different complexes of typical Fire Fighter garment assemblies, based on blends of FR fibres, notably Nomex/Kevlar, Nomex/PBI/Kevlar and Kermel/Para-aramid, constructed outer shells, with the lining fabric the same in 2 of the 3 cases, and the FR FABRIC EOLINER 260 always oriented with the membrane skin side to the wearer. The Assembly weights varied from 500g/m² – 540g/m² total complex mass. (See Diag 2. For details)
Heat Transfer Flame (Protection convective heat)
The heat source is a standardised gas flame. A sensor is placed above and in contact with the horizontally positioned sample.
The rise in temperature of the sensor is recorded and the time necessary to reach an elevation of 24°C (parameter HTI24) is also noted.
The rise to 12°C is taken as the first perception of heat by the wearer and the HTI 24 as the theoretical 2nd degree burn time. The difference between the 2 is a measure of escape time
Protection radiant heat
The heat source is an electric radiator. A sensor is placed behind and in contact with the specimen which is set vertically, opposite the radiator.
The rise in temperature of the sensor is recorded and it is compared to the theoretical skin tolerance curves. As above the t1 is an indication of the first perception of heat by the wearer and the t2 as the theoretical 2nd degree burn time.
The difference between the 2 is again a measure of escape time
The RET value is the measurement of water-vapour resistance under steady-state conditions.
The test fabric is placed on the surface of a porous metal plate.
The plate is heated, and water is channeled into the metal plate, simulating perspiration. The plate is then kept at a constant temperature.
As water vapor passes through the plate and the fabric, it causes Evaporative Heat Loss and therefore more energy is needed to keep the plate at a constant temperature.
Ret is the measurement of the resistance to evaporative heat loss. The lower the Ret value, the less resistance to moisture transfer and therefore higher breathability.
In reality, RET value is a measure of wearer comfort in the garment, because it is a measure of the amount of energy required to keep the wearer dry and at a constant body temperature.
EN 31092 Ret <25m2Pa/W
The Results can be seen in the Table below. We find that as long as the outershell and lining fabrics are of modern relatively lightweight breathable constructions of reputed inherently FR fibres, we obtain similar results, and these are:
- A general total reduction of complex weight of around 20%
- A general improvement of RHTI of around 30%
- A general improvement of Flame HTI of around 20%
- A general improvement of RET of up to 40%
For more information, go to www.Alpex.fr/gb