Leather is a combustible material and must be treated with appropriate flame retardant products in order to give passive protection in fire prevention.
Fireproof leather offers the obvious advantage of always having an element available that can reduce the speed of fire propagation so that the flames do not touch other combustible materials. This increases the time allowed for evacuation before the fire spreads.
Slowing the spread of a fire means having a low participation in combustion but also a rapid cooling time so as not to contribute to the overheating of the environment. The level of passive safety can also include minimal formation of dense and toxic fumes to promote visibility and breathability.
Fire Regulation and Reaction Tests
Each country has fire regulations and tests which must be respected.
In order for the leather to meet the regulation requirements, flame retardants must be selected that are suitable for the preparation of the leather for the test and for the dynamics of combustion during the test.
A good example is the UNI EN 91/75 standard for the 1IM classification and the BS 5852 – source 1- standard which are very similar in carrying out the test, however only the UNI EN 91/75 provides an initial wash of the specimen which aims to remove any deposited treatments and anything not fixed on the support. It follows that the use of flame retardants that can be washed out prejudices the result of the test according to Italian law.
How Fire Reacts to Leather
Let us now try to highlight what contributes to the behavior of fire on leather in order to meet the correct performance criteria. Depending on the application sectors and the type of environment in which the leather is used, the tolerance of measured parameters can vary greatly (for example, in both aircraft and trains, the development of smoke can be taken into account but in the first case tolerance is much lower).
As a premise, we have fragmented the course of combustion in four phases:
Let’s now consider which parameters to keep under observation during leather combustion and the factors that influence them (considering a generic finished leather):
Fatliquors provide the greatest amount of fuel vapor, and particularly those with mineral oils. In order for the fireproof leather system to work, it must be balanced: the flame retardants used must be available in the immediate vicinity of the flame and must degrade around the temperature in which combustion takes place. An example of an unbalanced system where the flame is far from the fireproofing agents is when there is a simultaneous high presence of mineral oils and a very thick finishing layer. Overheating generates gases on the flesh side, which is more permeable than the finishing side. In the case of a vertical test conducted on leather, the flame acquires a large volume that remains distant from the flame retardants and as a result they offer a limited contrast. In this case the criteria of reduced post-combustion time and reduced destruction of the material are at risk. On the one hand the flame mainly feeds on the back of the specimen with gases expelled, and on the other, being durable and in light contact with the leather, it manages to carbonize it at a temperature without giving rise to a real combustion that degrades the flame retardants.
An example of an unbalanced system in which the combustion temperature is too low to degrade the flame retardants is observed when a small flame that brings little destruction but is very persistent is fed over the oil that oozes from the surface of the leather. It is a candle effect by which the wick burns with slow destruction fed by the vapors of the overheated wax. In both cases flame retardants fail to intervene correctly and risk not complying with the standard performance requirements.
We report another experiment in which the dynamics of combustion must be adequate for the test method. In the case of applications for furnishings, according to UNI EN 91/75 and BS 5852, the use of padding is foreseen. This adds a variable which complicates things. The filling foam, even if it is fire-retardant, releases combustible gases. If the fire-retardant leather is destroyed only a little it helps to keep the gases imprisoned and this condition maintains the flame inside the pillow. The latter is consumed more quickly and, once its thickness is consumed, it does not satisfy one of the requirements required by law. The leather must be destroyed to the extent that it allows the outflow and dilution of the gases on the outside to avoid excessive consumption of the padding.
The persistence of embers in the leather as a result of combustion is a parameter that is not considered in the aeronautical sector but is instead very important in the public spaces sector.
It is well known that embers are a problem to consider if fire-retardant leathers are produced from mineral tanned hides, while organic tanning agents do not present this phenomenon. This is due to the absence of metals for which the stabilization of collagen is different as well as the chemistry of combustion. With the presence of metals, combustion results in more solid residue which is compacted by absorbing more heat and offering more resistance to destruction. In the absence of metals, organic substances have better combustion and are destroyed more quickly with the advantage of dispersing heat more quickly. However, it must be taken into account that a wet-white leather must be more lubricated than a wet-blue leather. For now, we just say that where the fatliquors are deposited in the dermis and their type are factors that contribute to combustion.
Deciding to face a test for fireproof leather is certainly helpful to understand the dynamics of combustion. We have reported some experiments that will be implemented in the future in order to get a more complete overview and to choose the type of processing best adapted to the purpose.
We will soon examine:
- Density of fumes emitted
- Toxicity of the fumes emitted
- Heat developed by combustion
- Influence of mechanical operations
- Combination of multiple materials