Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Improving the fire behaviour of synthetic leather

Due to its aesthetic appearance and feel, together with its good to very good performance characteristics such as cleaning and resistance, synthetic leather is primarily utilised in public places, for example in hotels, meeting locations and restaurants as well as in modes of transport, e.g. in buses, trains and aeroplanes. The applications range from floor coverings to high-quality items of seating furniture. Due to its high polymer content, however, synthetic leather is problematic as regards fire behaviour and is becoming increasingly powerless against the stricter fire protection demands from the core markets and the requirements of the processing industry. Current technology, however, does not enable synthetic leather to fulfil these relatively high requirements. For this reason, investigations were carried out into whether the incorporation of intumescent aggregate mixtures in PU and PVC bulk can result in improved fire behaviour.

The regulations stipulate that only fire protection Class B1 building materials may be used in escape routes – these include foyers. Similarly strict requirements already apply to other areas (e.g. meeting locations) and it is already foreseeable that these requirements will apply to other areas in the future, in particular in the transport sector.

Our investigations were carried out on seven commercially-available and 19 modified aqueous PU binders. In addition, a commercial PVC paste was also evaluated during the preliminary tests. The modifications undertaken were organic modifications (phosphor-based) to the binder system, inorganic modifications with silicates and silicon dioxide nanoparticles, and hybrid modifications using a sol-gel process in combination with inorganic constituents.

The behaviour of the various binders in reaction to the intumescence process was investigated using a commercial intumescent mixture and six intumescent aggregate mixtures. The intumescent mixtures were incorporated in 30 to 50 percent by mass in relation to the overall recipe. The production of the recipes was carried out with the help of a laboratory disperser. The compatibility and the curing were tested by applying the mixture to a glass plate. The fire behaviour was investigated under laboratory conditions in accordance with WKI methods and in a small-furnace test in accordance with the standard DIN 4102.

The commercial intumescent mixture could not be incorporated into any of the binders – each attempt resulted in phase separation. The incorporation of the aggregate mixtures 1,2 and 3 in the PVC paste resulted in homogeneous coating compounds and the intumescent reaction during fire-testing was not impaired. At an aggregate concentration of 50%, the majority of the PU coatings showed large cracks. In order to determine the binder proportion which would not result in a significant deterioration during fire-testing, a series of test were carried out in which the proportion of the organically-modified PU binder was varied. The most favourable intumescent aggregate concentration was determined thereby to be 30%. This also led to an improved layer. However, this result is not applicable for all PU types. For all the aggregate mixtures in combination with the PU, a significantly lower amount of swelling was determined compared to the PVC paste. A large majority of the commercial PU binders formed only a slight foam layer. The insulation effect during fire-testing resulted mainly from cavity formulation with a foam surface layer of varying thickness. With the organically-modified binders, the swollen layers also contained cavities. The results for the inorganically-modified and hybrid-modified binders fluctuated most strongly. The best fire protection was provided by two hybrid modifications and an organic modification.

© Fraunhofer WKI
Small-furnace test of a) a hybrid PU dispersion b) a commercial PU dispersion.


The illustration shows the course of the small-furnace test on the recipe with a) a hybrid PU dispersion and b) a commercial PU dispersion. Release paper was used for the substrate. In the case of the hybrid PU binder, the foam developed regularly under the influence of the flame. The coating compound with the PU dispersion, which is used industrially in the manufacture of synthetic leather, also formed a foam layer. This, however, burned.

It was possible to incorporate intumescent aggregate mixtures into PU and PVC compounds. Furthermore, it was possible to integrate the effect mechanisms of the high-performance fire protection coatings into synthetic leather. The application of phosphor-containing PU dispersion increased the fire resistance. A higher proportion of intumescent aggregate mixtures led to a cracked film. The cracking problem was solved by varying the binder and/or aggregate concentration, without deterioration of the insulation effect. Inorganically-modified PU dispersions burn less quickly than conventional standard polyurethanes. Furthermore, the glass-hard surface provides the opportunity to utilise various forms of flame-retardant materials, without significant deterioration of the film hardness. The insulation effect of the voluminous carbon layer can be reinforced through the application of inorganic, glass-like materials.