Boris Mahltig, in Inorganic and Composite Fibers, 2018
9.3 Spin Process and Fiber Properties
The basalt fibers can be produced from the melt of basalt stones . In principle two different kinds of basalt fibers are distinguished—staple fibers and filaments . For both types different production methods have been reported. The production of staple fibers is possible directly from small and molten basalt stones. However, these staple fibers possess asymmetrical properties and only a low mechanical performance in mentioned. For industrial production of basalt staple fibers two methods are mentioned: the “Junkers type” and the “centrifugal-multirole system” [14,30]. For advanced applications basalt fibers are produced as filaments. These filaments are produced by a spinneret process. The product of this process consists usually of several hundred monofilaments building up the rovings. This process is quite similar to the production of glass fibers . An example for such basalt monofilament fibers is presented in Fig. 9.3.
For the preparation of fibers from basalt stones, a content of silica of 46% or more is necessary. Only under this precondition it is possible to melt the stone completely without residues, to reach an adequate viscosity for fiber formation, and to gain after freezing a homogenous amorphous phase without crystalline areas . In general, the preparation of basalt fibers can be outlined in following steps: preparation of raw materials, melting of the stones, homogenization of the melt, the spinning of the fibers, and finally the application of the size . Compared to the preparation of melts for glass fiber production, the melting of stones for the basalt fiber production is more challenging. The reason for this is the low thermal conductivity and low transparency for infrared (IR) radiation of basalt fibers. The IR radiation is also named as heat radiation and a material which has a good transparency to heat radiation is warmed homogeneously and liquefy more easily. For this reason, the melting of transparent glass is easier compared to the IR-intransparent basalt. To reach a melt of basalt stones a preheating at 1450°C is described . Another challenge during the preparation of the basalt melt is posed by possible inhomogenities of the natural basalt stones . A sufficient temperature for the spinning of basalt fibers is reported to be in the range of 1350–1420°C .
After realizing a homogeneous melt as starting material for the spinning process, the next step is the spinning including the filament formation accompanied by the cooling and solidification of the melt. During this step a problematic crystallization can occur, which can be avoided by thermo-isolation and controlled cooling procedures [26,32]. A fast cooling process leads to a high amorphous basalt fiber, while a slow cooling process increases the crystallization rate of the basalt fiber . If the cooling process is done step by step and not continuously, different types of crystalline phases such as plagioclase, magnetite, and pyroxene can occur . Altogether it should be clear that the exact controlling of temperature of molten basalt and temperature of the cooling is absolutely necessary to obtain basalt fibers of excellent and reproducible properties.
After filament formation and cooling a size is applied to the basalt filaments. This chemical size is of high importance, because it significantly influences the mechanical properties of basalt fibers . The size can be described generally as aqueous solution of various chemicals, which is applied during the spinning process after the filament formation. The first task of the size is to keep the filaments together and improve the mechanical properties. The second task of the size is to improve the attraction of fiber and matrix in fiber-reinforced composite materials . For inorganic fibers such as glass or basalt fibers often sizes containing silane compounds are used. Silane compounds are metalorganic compounds, in which the metal part can bond to the surface of the inorganic fiber, while the organic part has greater attraction to the organic matrix of the fiber-reinforced material . A schematic overview of reaction of silane compounds on basalt fiber surfaces is displayed in Fig. 9.4, while Fig. 9.5 shows some examples of those silane coupling compounds in detail .
Besides attaining the above-mentioned properties by the size, other properties are also often aimed such as an improved corrosion stability, antistatic properties, and an improved abrasion stability . A special development is the combination of the size with new materials such as carbon nanotubes (CNTs). A silane treatment of basalt fibers can as well be used to apply CNTs onto the fibers. In this application the silane is used to fix and arrange the CNTs on the basalt fiber surface. In this way modified basalt fibers are used for the preparation of fiber-reinforced materials, which are described as CNT/epoxy/basalt composites and exhibit significant improved fracture toughness [34,35]. Other innovative sizing agents are reported by Wei et al. [36,37]. They described a surface modification of basalt fibers with the so-called hybrid sizings containing nanosilica and epoxy functions.
Such systems can be realized by sol-gel process using tetraethoxysilane (TEOS) and epoxy modified silane compounds, for example, GLYMO shown in Fig. 9.5. The silica particles exhibit diameters of only few nanometers and the epoxy function introduces an enhanced adhesion to the polymer matrix in the final fiber-reinforced material. The main idea is here to realize a compound at the interface of basalt fiber surface to the polymer matrix, which contains an inorganic silica component and an organic epoxy function. The final goal is to improve the adhesion of polymer matrix to the basalt fibers [36,37]. Another aspect of using the size during basalt fiber production is avoiding micro-cracks on the fiber surface by the sizing. By the application of the size the growth of these micro-cracks can be avoided and the tenancy of the fibers can be stabilized . The mechanical stability guaranteed by the size is reported to be absolutely necessary for the production steps like the production of hybrid yarns, weaving, knitting, and the finishing processes. The mechanical forces acting on the fibers during these processes are quite high, so a size giving the fiber a sufficient elasticity and flexibility is necessary .
One point to be kept in mind is that if the sizing compounds are made from organic material, then they have a higher thermal sensitivity as the inorganic basalt fibers. It was observed that rovings made from basalt fibers have already lost significant strength after a heat treatment at 300°C [28,39]. For these materials, it was determined that by heat treatment the amount of carbon on the basalt fiber surface can be deleted . Before the heat treatment a significant amount of carbon (15%) was detected on the basalt fiber surface, probably related to an organic sizing agent. By heating process in air, this size is probably burned away and the positive influence of the size on the strength of the rovings is also eliminated .
One conclusion from this investigation is that it is necessary to develop sizing agents of high thermal stability especially for use in inorganic fiber with high thermal stability. Only with a thermally stable size, it is possible to take the full benefit of the thermal stability of the inorganic basalt fiber.
Various heat-resistant sizing agents and their applications were investigated by Shayed et al. . They have investigated basalt fiber roving supplied from Asamer Basaltic Fibers GmbH (Austria). These rovings already contain a silane-containing size. Further modification is done by using different heat-resistant polymers applied as sizing agent by dip coating. Two types of sizing agent are applied—a polysilazan (KiON HTT 1800) and a polysiloxane (Silikophen P80/MPA). For testing the rovings are heated with increasing temperature and the testing is carried out according to ISO 3341 standard on the heated fibers . Some results of these mechanical testings are presented in Figs. 9.6 and 9.7.
These investigations lead to the following results. First, the supplied basalt roving already exhibited a mechanical stability at 400°C. Second, by the application of the polysiloxane size the mechanical stability of the basalt roving is significantly improved, probably because the sizing agent glues the basalt fibers together strongly. Third, both additional sizing agents (polysilazane and polysiloxane) lead to improved mechanical properties after heat treatment at 500°C compared to the original basalt roving. However, a thermal treatment at 600°C eliminates mostly the mechanical stability for all the samples .
It is concluded that the sizing agents that form a metalorganic polymer film onto the basalt fiber surface act as a protective barrier layer against heat. By this crystallization processes introduced by heat are suppressed and the fiber strength is retained . Further, this polymer film could also act as a barrier layer against oxygen from air. The oxidation of FeO present in the basalt fiber is avoided and the following crystallization is suppressed. The heating up to higher temperatures of 600°C probably also destruct the metalorganic polymer film, so its protective properties for the basalt fibers are diminished.
Altogether, it can be concluded that the sizing agent is an elemental component of basalt fibers, which influences the properties of the basalt fibers significantly. The type of sizing agent used has to be selected according to the demand and the type of application of basalt fibers.