Leather is used in a number of public transport situations, for example for upholstery in aeroplanes, trains and ships, as well as for furniture in public buildings. For these uses there is normally a requirement that the material meet a range of fire-resistance specifications.
Leather in itself is not a readily burnable substance. However, to provide an added margin of safety to give time for evacuation of vehicles or buildings, an additional fire-resistant protection for leather is necessary. The protection normally comes in the form of a chemical treatment to the leather so that flame development is inhibited and the release of smoke and toxic gases is minimised.
Protecting leather with flame retardants A fire requires a number of factors to occur. These are the combination of a combustible material, a source of ignition and an oxidising agent such as oxygen. Normal practice is to treat combustible materials with a flame retardant to meet the strict safety regulations which apply to public transport and buildings.
These retardants prevent fires from starting or spreading by making the material more difficult to ignite and significantly reducing the spread of the fire. Importantly this delay in the spread of the flame allows more time for passengers and public to escape.
Fires often reach more than 500°C after the flash-over, which causes all combustible materials to simultaneously ignite and the fire to spread quickly. Hot gases emitted can mix with oxygen causing the fire to spread more rapidly. Fumes of toxic gases, such as carbon monoxide, cyanide and nitrous oxides, can very quickly overcome people, leading to asphyxia.
With the increasing use of combustible synthetic materials for furnishings, as well as wall and floor coverings, there has been a noticeable reduction in the escape time. Hence, any increase in the escape time by using non-combustible materials is a major advantage.
Leather offers a major advantage in that it is not readily combustible. It does not melt or form flaming drips during fires. Therefore, when used for furniture covering, it noticeably improves the thermal protection of the underlying foam or cushion material. However, the strict safety regulations for public transport and buildings, where the safety and protection of human lives are the key issue, mean that additional flame retarding properties are required.
Test methods For aircraft and ships, the regulations covering fire-resistant protection are governed to a large extent by international agreements, although some aircraft manufacturers apply their own additional requirements. This makes the overview of the regulatory aspects reasonably manageable. However, in the case of trains and commercial
furniture in public buildings, the regulations are at a national level, meaning that they vary considerably from country to country.
For aircraft, the CS/JAR/FAR Part 25 tests are used. These are comprised of several aspects:
• flammability tests
• smoke density test
• heat release test
• oil burner test for seat cushions
Aircraft manufacturers Airbus, require an additional test to measure the build-up of six defined toxic gases (HCN, CO, NO/NO2, SO2/H2S, HF and HCl).
For shipping, the International Maritime Organisation (IMO) sets the FTP-Code Annex 1 fire tests. The relevant parts for typical upholstered furniture leather are:
• smoke and toxicity
• surface flammability
It is worth noting that the toxicity classification requires the measurement of the emission of eight different toxic gases.
For public transport such as rail, there is an increasing use of leather upholstered seats in the 1st Class sections of the new fast European trains. However, the regulations for rail differ from country to country and have not been unified. In
addition there can be variations in the requirements for each type of use within a railcar.
For example, in Germany, the DIN 5510-2 Standard and the German Railways (Deutsche Bahn) set the technical specifications for seating materials.
In public buildings, again national regulations vary from country to country. Germany and Austria usually apply the B1 and B2 test in the DIN 4102-1 Standard. In the UK, there is the BS 5852 Standard and in addition wooden crib tests to assess the ignitability of upholstery furniture in commercial use. The fire safety legislation has developed differently in each country according to the types of building structures in use.
Important factors for fire-resistant leather The propensity for leather to burn is dependent on a number of factors. Some of them are dependent on the physical parameters of the leather itself and others dependent on the wet-end and finishing treatment processes. The leather substance has a strong influence on the flammability behaviour of leather. Ideally the thicker the leather the better its natural flame resistant properties. However, the aircraft industry has strict limitations in terms of weight.
Leather competes with textile materials that have a density of 400g/m2 compared with 800g/m2 for a 1.2mm leather. So in order to reduce the weight of the leather, the first thought is to reduce the thickness, leading to a negative influence on the flammability properties. Consequently the thinner the leather the more important the fire-resistance treatments are.
An open leather structure contains air pockets that can assist ignition. The leather does not extinguish as readily and keeps burning after the removal of the flame. This will result in a longer burn length.
To avoid this, it is necessary that leather with a tighter and denser fibre structure is prepared in the wet-end. Therefore, the selection of the retannage and fatliquoring chemicals has an influence on the resultant fire-resistance properties.
In the development phase of trials to produce fire-resistant leather, the aircraft vertical flame test method was used to evaluate and optimise the performance. A full test series for each public transport sector was completed afterwards.
In Figure 1, the TFL Protect Line sample has immediately extinguished and only a small charred section remains.
In Figure 2 the leather sample continues to burn slowly after the flame is removed indicating it has been inadequately treated.
TFL Protect Line – where safety meets comfort At TFL, the whole leather manufacturing process was studied and each step evaluated so that the optimum flame resistance effect could be obtained.
In the wet-end, specially formulated syntans and fatliquors were selected to improve the evenness of the distribution in the leather cross-section.
It was found that some flame retardants currently used commercially to protect leather are simply physically absorbed into the leather cross-section and not fixed. This not only limits the amount that can be added to the leather but also allows them to migrate during mechanical and coating operations. TFL offers additional benefits by providing products that are chemically fixed to the leather fibres.
The grain layer in particular is very difficult to protect using normal flame retardants. A thorough penetration of the fire-resistance treatment is essential to avoid having a layer directly under the finish coat that is not protected.
This untreated layer was found to be effectively feeding the burning. It was recognised by the jet-like flame, which often appears as the gases under the finish coat layer are emitted perpendicular to the leather surface (see Figure 3).
With the correct selection of fire-resistant chemicals, which are fixed in the leather cross-section, this build-up of gas under the finish coat is eliminated and leads to a much improved fire-resistance performance.
By selecting the appropriate individual wet-end components, the leather treatment can be varied so that the fire-resistant specifications for each public transport and building requirement could be reached.
An additional challenge was to extend the protection to also include the finish layer. The negative influence of an untreated finish coat is immediately visible in the vertical flame test. Specially formulated polyurethane and acrylic base coat compounds were developed so that the treated finish layer also complied with the public transport requirements for fire-resistance. Products are now available to treat the crust leather and also to make the finish coat fire-resistant. Using the vertical flame test for aircraft, the improved fire-resistant properties of the TFL Protect Line leather sample, on the left in Figure 4, can be clearly shown.
By combining all these developments into one system, called the TFL Protect Line, it is now easier to reach the stringent requirements for fire-resistance of leather.

Products in the TFL
Protect Line:
Sella tec SAFE
Sella tec FILL
Sella tec SOFT
Roda tec B 02
Roda tec B 03