Basalt stones in their original form are not flexible or elastic. Under room temperature basalt ston
Basalt stones in their original form are not flexible or elastic. Under room temperature basalt stone breaks before gaining plastic properties. In contrast, the basalt fibers exhibit flexible properties to a certain degree. This flexibility is related to a small fiber diameter, the presence of amorphous areas, and the application of sizing agent, acting like a polymer matrix gluing the single basalt filaments together. However, basalt fibers break easily under mechanical stress and have to be treated carefully during textile production processes like weaving or knitting [28].
The term heat resistance summarizes the stability of all fiber properties under the influence of heat, meaning increasing temperature. In fact, most synthetic fibers from organic polymers melt, burn, and decompose at temperatures up to 300°C. Compared to these synthetic fibers, basalt fibers exhibit high thermal stability. Basalt fibers are inorganic fibers, they do not burn, and the melting point is around 1350–1450°C [14]. For this reason, the thermal stability of basalt fibers can be supposed to be excellent. However, if the view is set to a technical property like the fiber strength, even at lower temperatures a change in fiber properties is reported.
Besides the melting temperature, other temperatures are also mentioned in the literature, which are claimed to be the thermal limitation for the use of basalt fibers. An overview of various temperatures found in literature is shown in Fig. 9.8 [15,41,42]. It can be clearly seen that the thermal limitation is significantly below the melting temperature. The mentioned working temperature as reported by King et al. is at 700°C which is only the half of the melting temperature [15]. The strong differences in the different thermal limitation temperatures are probably caused by two issues. First, the strong variation in different types of basalt fiber materials. Second, the variation in application and the parameter important for this applications.
Fig. 9.8. Overview of different temperatures for basalt fibers as thermal limitation for use. The temperatures given are from different sources: softening temperature from Ref. [41], working temperature from Ref. [15], and other temperatures presented in the figure from Ref. [42].
However, even the exposition to lower temperatures can influence the properties of basalt fibers. Even the temperature impact of 400°C applied for only 2 h can decrease the strength of basalt fibers significantly [39,43]. Militiky et al. reported even a significant decrease in the strength of basalt fibers heated up to temperatures of 300°C [32] (Fig. 9.9). In these experiments the fiber strength was determined at the heating temperature and after cooling at room temperature, as reported by Overkamp et al. [28].
Fig. 9.9. Influence of the heating temperature on the tenancy of basalt fibers [32].
Mainly two factors are responsible for the decrease in the strength of basalt fibers. First, the decomposition of applied sizing agents as discussed above. Second, crystallization processes in the fiber [44]. The spinning process of basalt fiber is driven to form basalt fibers with high amounts of amorphous area to gain best mechanical properties. In the case of crystallization, the amorphous areas are removed and the fiber strength is decreased.
The crystallization behavior of basalt fibers is mainly determined by the iron oxide content of the fibers. It is supposed that under the influence of heat the crystallization of amorphous areas starts in the presence of iron oxide. As a result, crystallization process covers the whole fiber starting from fiber surface and being progressed into the interior of the fiber [45].
The influence of iron oxide is related to oxidation processes occurring at higher temperatures. Iron (II) oxide (FeO) is oxidized to iron (III) oxide (Fe2O3). This is probably the reason why crystallization of basalt fibers starts from the surface of basalt fiber, where oxygen from air is present as oxidizing agent [26]. Besides the oxidation to Fe2O3, the formation of magnetite (Fe3O4) is also suggested to be part of the crystallization process in basalt fibers [43].
One of the main applications of basalt fibers is their use in fiber-reinforced composite materials. Thus, it is logical to discuss the thermal stability of basalt fibers in such composites. A related study comparing different glass fibers and basalt fibers is given by Cerny et al. [46]. They claimed that in a thermally stable matrix basalt fiber can serve up to 550–600°C. However, even at lower temperature of 400°C a significant decrease in tensile modulus can occur. This change in tensile performance is explained by crystallization processes but the fiber matrix interface should also be taken into account [46].