Highest flame resistance of lightweight composite materials based on basalt fibers

June 2022 edition: Technical Textiles – Innovation, Technology, Application

Partition walls for flexible spatial concepts or large tents for events, exhibitions and fairs or emergency tents, which must meet the requirements of construction material class A2, at present are made of rigid composite panels. Currently this is the only solution, which can meet the strict fire test criteria stipulated in the aforementioned class. Though the currently used materials are safe, they do have many disadvantages. These rigid composites are heavy and occupy more space. Solid rigid elements, which are used to partition the space into compartments in industrial buildings, typically have a wall thickness of 100 mm. High-quality products for the partitioning of empty space into rooms needs circular steel tubes, crossbars and chipboards to be planked on both sides. Thinking of flexible materials at use now, one would point to PVC coated textiles. But, due to the high percentage of organic matter they do not meet the fire test criteria of construction material class A. At the maximum, they could meet the criteria of construction material class B1 only. Moreover, there is danger of toxic gases being released during fire. Flexible partition walls which could also serve as firewalls are in huge demand for instance, at airports, in shipbuilding, rail traffic, for fire services, hall buildings or public buildings.

Textile

Basalt is a high-performance material, which has some similarity to carbon and glass fibres and finds use in various fields. Predominantly, the continuous filaments of basalt offer an enormous potential for technical use. The fibres are produced in a spinning process from a hot melt and wound on coils. They have a cross section between 8 to 14 µm posing no health hazard, an extremely high tensile strength of 4,800 MPa (compared to E glass fibre: around 3,000 MPa) and a high temperature resistance between -200 °C to +700 °C, a range which is suitable for several applications (compared to glass fibre: -60 °C to +460 °C). The reason for such a high thermal resistance is attributed to the presence of inorganic matter in the basalt fibres, principal components being SiO2, Al2O3, CaO, MgO, Fe2O3 and Na2O. The fibres are non-combustible and melt at a very high temperature of about 1,450 °C. Both the properties – the high strength and the thermal stability – qualify the basalt fibres to be used in a flame-resistant composite. Moreover, Basalt is a naturally abundant mineral and can be recycled to 100 per cent at the production plant. To be compatible with process of composite production and utilize the outstanding properties of basalt, the basalt filaments were processed to a woven fabric. Subsequently, to make the textile surfaces suitable for customary coating processes, the surface of the fibres as well as the woven fabric were adjusted to meet the requirements of the silicone coating. This is the only way to achieve an adhesive composite attractive in terms of mechanical sturdiness as well as visual appearance.

Beschichtung

The application of conventional polymers for textile coating is typically limited to a temperature range between -30 °C and 130 °C as they are composed of organic compounds. Carbon-carbon bonds in the polymers cleave at higher temperatures and undergo oxidative degradation. On one hand, this results in rapid ageing if there is a permanent exposure of a higher temperature (> 100 °C), on the other hand, this necessitates the use of special additives to equip the materials in a way that they are flame-retardant in case of exposure to extreme temperatures and thus to meet the fire test criteria. The extreme fire test criteria of construction material class A2, cannot be met with conventional polymers (e.g. PVC or PU). For this reason, a polymer layer made of silicone was used to produce the new composite materials. Silicones have inorganic Si-O-bonds, which have a 100 kJ/mol higher bond energy compared to that of the C-C-bond existing in conventional polymers, which renders it with a high temperature resistance and inherent flame resistance. Furthermore, in case of fire, no toxic gases are released due to the presence of major proportion of inorganic elements in the chemical composition of the silicones (Si, O, C, H). Standard silicones can endure permanent exposure to a temperature of 200 °C and certain special types of silicones can endure up to a temperature of 250 °C. Correspondingly, standard silicones and special types of silicones can withstand short-term temperature peaks up to 250 °C and 300 °C, respectively. Another advantage of using silicone is that they are hydrophobic due to the shielding of the inorganic main chain using organic methyl groups, hence resulting in a stain-resistant surface. For the described lightweight composite material of the project a one or two-sided coating which can also contain foamed layers was realized (see Fig.1).

Regulations

Until now no flexible composite material is available which can be manufactured and processed like tarpaulin materials in a roll-to-roll process, which meet the strict fire test criteria of construction material class A2, according to the standard DIN EN ISO 13501-1. This standard classifies material class A into A1 (without combustible parts) and A2 (with combustible parts to a small extent). For a material to be categorised into construction material class A2, it should survive a series of tests as specified in the test standard EN 13501-1. These include determination of the gross heat of combustion according to DIN EN ISO 1716 or DIN EN ISO 1182, thermal stress by single burning item (SBI) according to EN 13823 and burning behaviour according to EN ISO 9239, which is meant for floorings. Compared to the national classification standard (DIN 4102-1), the European classification standard of construction materials (DIN EN 13501-1) comprises a larger spectrum of classes, as the side effects of fires such as smoke emission and dripping of burning droplets are also considered. On grounds of higher safety, it is important for the new high-performance composites to comply with the class A2 – s1 d0 in DIN EN 13501-1 relating to burning behaviour. This class corresponds to DIN 4102-1 A2, which means that there is no or hardly any emission of smoke (s1) and no dripping of burning droplets (d0).

Properties of composite material

The combination of the inorganic basalt fabric and the special inorganic silicone-based coating resulted in a tarpaulin-like composite material, which shows high strength even at a low mass per unit area, extreme high temperature and flame resistance. In addition, the material exhibits excellent heat insulation properties, which can be further optimized by a foamed silicone layer. The coating ensures a sealed surface and homogeneous heat distribution. The maximum temperature resistance is about 1,200 °C and the long-term heat resistance about 280 °C. The exact values of the individual tests for three different types of material are compiled in table 1.
Until now, two products have been certified according to construction material class A2 s1 – d0 (DIN 13501) and one for FAR/JAR/CS-25.853 App. F Part I-IV for Cargo Compartment Liners all listed in the table 1 for market launch by the company Fulcoline USA.

Field of applications

There is an enormous potential for the non-combustible tarpaulin like material in the following fields of application:

textile construction for partition walls and fire protection
lightweight construction as a part of multi-layered lightweight construction composites especially for housings and coverings for ships, aircrafts, railway vehicles and road vehicles and related containers.

Fig.: 2 Temperature profile at the back of a composite plate during the flammability test with a temperature load of 1,000 °C at the other side of the material. SMC = Sheet Molding Compound

First analyses concerning the integration of the basalt composite material into lightweight construction structures, in addition to flame resistance, also show a high insulation potential to thermal load. Fig. 2 shows the temperature profile at the back of an SMC composite material with basalt hybrid material with a thickness of 3 mm compared to the standard SMC plate previously used. With a temperature load of 1,000 °C at the surface of both the materials, the temperature profile at the back shows a considerably slower increase for the composite material with basalt hybrid material. After five minutes the standard material reaches a temperature of about 500 °C, whereas the temperature of the basalt hybrid SMC structure rises to a temperature of about 250 °C only.

For further information regarding the described new flame-resistant materials please contact Tobias Mader from Fulcoline USA via (t.mader@fulcoline.de)

V. Gatterdam *, T. Mader*, K. Trommer**
* Fulcoline USA, Hofbieber
**FILK Freiberg Institute gGmbH, Freiberg