Introduction
The most essential feature of leather is its fibrous structure. Leather is characterised by a high permeability for water vapour and other gases and, at the same time, very low water permeability. The characteristic structure of leather can explain many properties of tanned leather such as tear resistance, flexibility, permeability for air, thermal insulation, resistance to water and shape maintenance. Leather shows these properties at different temperatures and different humidity levels. And in the case of leather upholstered seats, the behaviour of leather in fire conditions is of utmost importance.
In this study, results of flammability measurements of natural leather, applied to the production of upholstered furniture and upholstered seats in the automotive industry, are presented and discussed.
Experimental
The evaluation of flammability was performed on natural leather with a surface mass of 620g/m2, applied to the manufacture of luxury upholstered seats. Flammability measurements were carried out using three tests for the evaluation of flammability of upholstered materials applied to furniture and means of transport.
* Road vehicles ISO 3795:1989(E)
Samples are placed in an U-shaped frame in a horizontal position and subjected to a 15-second exposure to a low-energy flame
* Textile fabrics – Burning behaviour EN ISO 6940:2004
Samples are placed in a rectangular frame in a vertical position. They are exposed for 1-20s contact with the flame situated perpendicularly to the sample surface or directly below the sample edge
* Cone calorimeter 2 ISO-5660 – 1:2002 (E)
This is the standard device for measuring the rate of heat release from a burning material under a controlled radiant heat source (radiant heat is the major cause of fire spreading). The apparatus consists of a conical electric heater (typically 10-100kW/m2) delivering uniform radiation to the sample situated horizontally, parallel to the source of radiation. Moreover, leather samples were subject to thermogravimetric analysis (TG) and differential thermal analysis (DTG) on a Setsys 12 instrument (made by Setaram). The analyses were carried out in the temperature range from room temperature to 700°C in air flow (30cm3/min) at linear temperature growth of 10°C/min, using corundum crucibles. Results of the measurements were compared with those obtained for artificial polyurethane (pu) leather with surface mass of 630g/m2 that is applied to standard upholstered seats
Results and discussion
Flammability of tanned leather is resistant to a short-time exposure to temperatures up to 200°C (higher temperatures result in pyrolysis). Exposure to temperatures in the range of 130-170°C for several dozen minutes does not cause structural changes1. Resistance to higher temperatures requires appropriate finishing. To determine flammability of natural and artificial leather, measurements were carried out using ISO 3795:1989(E) and EN ISO 6940:2004 methods. Results of these measurements are presented in Tables 1 and 2.
Results obtained by using the ISO 3795:1989(E) method allow the classification of the test leather into non-ignitable materials (combustion time: 0mm/min). On the other hand, artificial pu leather ignites and the flame spreads at a rate of 66mm/min. The time when the sample is not ignited and does not glow during surface and edge exposure to flame was determined by the EN ISO 6940:2004 method.
In the case of natural leather, the time of surface exposure to the flame is 18s, whereas for artificial leather, it is 3s (Table 2). When the flame is situated at the edge, natural leather ignites after 5s and artificial leather ignites immediately after contact with the flame, burns slowly and then glows until the sample undergoes total ashing. Natural leather, when ignited, smoulders slowly, glows for a long time and shrinks. Results of flammability measurements performed on a cone calorimeter are in Table 3.
In the case of natural leather, heat release rate [HRR] is 190kW/m2 (on average) and is clearly lower than that observed for artificial leather (Table 3). Time to sustained ignition [TI] in the case of natural leather is 2.1 times longer than that of artificial leather and mass loss rate [MLR] is 2.1 times lower for natural leather, which points to a slow burning.
Smoke emission, expressed by specific extinction area [SEA], in the case of natural leather is almost twice as low as that for artificial leather. Natural leather is also safer because the emission of carbon monoxide is one and a half times lower. All three tests have proved that natural leather is relatively resistant to ignition.
However, it is also characterised by the ability to glow slowly which, in the case of its application to upholstered furniture, can cause fire after several hours of latent smouldering. In some cases, combustion can remain at the stage of smouldering, while in others the material can suddenly burst into flames.
When smouldering combustion occurs, the damage is usually limited to the place where it began and losses are relatively low. However, when the upholstery bursts into flames, conditions hazardous to life are generated quickly and such fires can spread rapidly, causing serious material losses and casualties. That is why materials of diversified extent of flame retardation should be used for the manufacture of upholstered seats.
Flame retardant treatment of natural leather
It is well-known that making leather fully resistant to charring and decomposition caused by contact with fire or high temperature is impossible. There is, however, a possibility of providing an increased level of fireproofing by applying appropriate flame retardants. On the basis of published information about the application of flame retardants used for textiles to natural leather, the authors decided to perform this study based on the experience of the Institute of Natural Fibres in the field of flame retardation of wool and natural fibres5.
In the first experiments on the protection of leather, they chose flame retardants applied to non-water durable and permanent flame retardation of textile raw materials such as boric acid, orthophosphoric acid and three commercial flame-retardants for the treatment of natural raw materials based on polyurea – phosphates and borates. The use of impermanent flame-retardants was justified by the fact that leather in upholstered products is not subjected to laundering with water during its use.
The advantages of these compounds are their low cost and flame-retarding effectiveness. Different formulae of flame retardants used in the experiments for the impregnation of leather (intended for upholstered furniture) of surface mass of 620g/m2 are listed in Table 4.
Flame-retarded leather samples were first air-dried at about 20°C and then drying continued in a forced air circulation dryer at 35°C. Leather samples, after their treatment and drying, were evaluated from the point of view of changes in their external appearance. On the basis of the evaluation, sample No 12 was eliminated from further studies due to considerable stiffening as a result of the flame retardant protection.
The effectiveness of the protection was evaluated on the basis of results of flammability tests as well as DTA and TG analyses. Differential thermal analysis curves (Figure 1) show one endothermic and two exothermic peaks. The first of them at 100°C should be ascribed to the removal of water; the second one, at about 350°C, may originate from the oxidation of trivalent chromium (present in chrome tanned leather) to hexavalent chromium3 and the third peak reflects the combustion of leather.
Flame retardation according to formula variant No 7 shifts most of the third peak from 475°C (the case of untreated leather) to 491°C (sample No 7). An analogous shift in the third peak towards higher temperatures is observed for sample No 11. In the case of the latter sample, an additional exothermic effect appears at 533°C, which suggests that some components of the material subjected to flame retardation according to formula variant No 11 respond even more effectively to the flame retardant treatment.
In Figure 2, results of thermogravimetric analysis are shown as the first derivative (DTG) of TG curves. On the DTG curves, counterparts of all effects observed on DTA curves appear at slightly lower temperatures which results from a difference in the rate of response of detecting devices of differential thermal analysis and thermogravimetric analysis.
Despite the mentioned difference in temperatures recorded on DTA and DTG curves, the direction of changes shown by both curves is the same – the employed flame retardants shift the temperature associated with leather combustion to higher values compared with that of untreated leather. Results of flammability measurements of effective different flame retardants are presented in Tables 5 and 6. For the sake of comparison, results obtained for untreated leather are shown as well.
Analysis of flammability characteristics of fire-retarded samples carried out by comparing them with untreated leather has shown that:
* for sample No 7 a small extension of time to sustained ignition [TI] (by 3s) occurs, whereas HRR remains on a similar level. The smoke emission, as expressed by specific extinction area [SEA], increases by 125% (Table 5) and time increases by 4s during edge exposure to flame. Moreover, glowing is eliminated (Table 6).
* for sample No 11 time to sustained ignition increased by 27%, heat release rate [HRR] was reduced by 20%, a two-fold growth of mass loss rate was observed, smoke emission increased by 50% (Table 5), time was extended by 3s during edge exposure to flame and glowing was eliminated (Table 6). Unfortunately, flame retardants increase the release of carbon monoxide roughly by a factor of two, although it reduces the emission of carbon dioxide (Table 5).
Flammability of difficult-to-ignite upholstery composites based on natural leather and barrier non-woven [Lin FR]300
In the case of upholstery composites, the application of protective barriers situated between covering and filling materials can be an efficient way of flame retardancy of seats2. Such a barrier reduces the susceptibility of filling material present in a piece of furniture to the development and spreading of fire.
As a barrier material, a non-woven made of flame-retarded flax fibres, which was developed at the Institute of Natural Fibres6,7 was employed. The barrier flax non-woven [LIN FR]300 that is situated in an upholstery composite directly under a covering material, considerably increases moisture adsorption and upgrades softness of upholstery system4.
Results of flammability measurements of upholstery composites, consisting of natural leather or artificial pu leather and barrier flax non-woven [LIN FR]300 are presented in Table 7. Time to sustained ignition of the composites and covering material is shown in Figure 3.
Results from Table 7 show that a more effective composite is that containing natural leather because in the case of this composite, time to sustained ignition is about 60% longer than that of an artificial leather-containing composite. This fact is clearly seen in Figure 3. Moreover, the heat release rate is lower for the composite with natural leather which is, of course, the advantage of the latter composite.
Summary
Flammability studies of natural and artificial leather have shown that natural leather is safer in case of fire because of longer time to ignition (as evaluated by applying three different tests) and a two-fold lower emission of carbon monoxide compared with artificial leather.
A drawback of natural leather is its tendency to glow which can cause a long smouldering phase. Hence, there is the necessity of applying flame retardants or flame-retarding barriers to the upholstery.
Among the fire retardants investigated, the most efficient seemed to be a commercial product based on ammonium polyphosphate and boric acid (formula variant No 11), extending time to ignition, reducing total heat released and eliminating glowing of natural leather. Unfortunately, the presence of fire retardants increases optical density of smoke and carbon monoxide emission during combustion.
In the case of applying a flame-retarding barrier, optical density of smoke observed during combustion of composites is lower and time to ignition longer. Can a flame-retarding barrier effectively stop the development of glowing of upholstery systems? This will be the subject of further studies.
