Basalt Fibers: Alternative To Glass?
High-temperature performance and superior strength properties may make this late-comer a better choice in some applications.
By Anne Ross, Assistant Editor | August 2006
ROCK TO FIBER
As crushed basalt enters the furnace, the material is liquefied at a temperature of 1500°C/2732°F (glass melt point varies between 1400°C and 1600°C). Unlike glass, which is transparent, the opaque basalt absorbs rather than transmits infrared energy. Therefore it is more difficult for the overhead gas burners used in conventional glass furnaces to uniformly heat the entire basalt mix. With overhead gas, the melting basalt must be held in the reservoir for extended periods of time — up to several hours — to ensure a homogenous temperature. Basalt producers have employed several strategies to promote uniform heating, including the immersion of electrodes in the bath. But Ihor Markuts, sales and marketing director for Technobasalt, notes that his company prefers gas heating to electric, for quality reasons, despite increased manufacturing costs. Finally, a two-stage heating scheme is employed, featuring separate zones equipped with independently controlled heating systems. Only the temperature control system in the furnace outlet zone, which feeds the extrusion bushings, requires great precision, so a less sophisticated control system may be used in the initial heating zone.
Source: Basaltex
A simplified diagram of a basalt fiberization processing line: 1) crushed stone silo; 2) loading station; 3) transport system, 4) batch charging station, 5) initial melt zone, 6) secondary heat zone with precise temperature control, 7) filament forming bushings, 8) sizing applicator, 9) strand formation station, 10) fiber tensioning station, 11) automated winding station.
Like glass filaments, basalt filaments are formed by platinum-rhodium bushings. As they cool, a sizing agent is applied and the filaments are moved to speed-controlled fiber stretching equipment and then on winding equipment, where the fiber is spooled.
Because the basalt filament is more abrasive than glass, the expensive bushings once needed more frequent refurbishing. As bushings wear, their cylindrical holes wear unevenly, degrading process control. Without timely maintenance, the out-of-round apertures form filaments with an unacceptably wide diameter range, producing a roving with unpredictable breaking loads, explains Nolf. While glass fiber bushings last six months or more before they need to be melted, reformed and redrilled, a bushing used for basalt fiber production previously lasted anywhere from three to five months. Kamenny Vek, however, reports that process control efforts have extended bushing life to a similar six-month cycle.
FIBER VS. FIBER
On balance, these differences in processing and maintenance lead to overall operating costs that exceed those for processing E-glass fiber, but basalt fiber proponents say that their product clearly outperforms E-glass in composites. In chopped mat, roving and unidirectional fabric forms, basalt fibers exhibit a higher breaking load and higher Young's modulus (a measure of the stiffness of a given material) than E-glass. In a study of basalt fibers and E-glass fibers, conducted by Professor Ignaas Verpoest at the Composites Dept. of the University of Leuven in Belgium, unidirectional prepregs were produced by impregnating E-glass and basalt roving with epoxy and winding each on a mandrel, and then compacting the laminate until complete cure was achieved. Samples of 135-mm by 15-mm (5.3-inches by 0.6-inch) were cut and measured for thickness. The pieces were then subjected to a three-point bending test (ISO 178) and the ILSS test (ISO 14130) to test strength and stiffness. Verpoest reports that each sample had a fiber volume fraction of 40 percent, but the basalt/epoxy sample's strength tested 13.7 percent higher than that of the E-glass sample and exhibited 17.5 percent greater stiffness, although the basalt sample was 3.6 percent heavier than the E-glass sample.
Additionally, basalt fibers are naturally resistant to ultraviolet (UV) and high-energy electromagnetic radiation, maintain their properties in cold temperatures, and provides better acid resistance. Reportedly, basalt also is superior in the realm of worker safety and air quality as well. Markuts points out that since basalt is the product of volcanic activity, the fiberization process is more environmentally safe than that of glass fiber. The "greenhouse" gases that might otherwise be released during fiber processing, he says, were vented millions of years ago during the magma eruption. Further, basalt is 100 percent inert, that is, it has no toxic reaction with air or water, and is noncombustible and explosion proof.
FIBER TO FABRIC
Once producers mastered fiber manufacture, they faced additional challenges as the product was converted to useful reinforcement forms. Basaltex, for example, found early on that woven basalt fabrics straight from a weaver's loom were fragile and easily damaged when handled, exhibiting broken fibers when sharply folded or bent, and were irritating to the skin. In order to make the product more stable, Basaltex developed a proprietary silane-based sizing that facilitates the post-manufacture processing. The coating doesn't generate toxic smoke when heated and does not degrade the fiber's fire-resistance properties. Mislavsky observes that a significant factor in initially poor fabric performance was fiber damage that occurred during the fiberization process. He maintains that, today, a combination of sizing and refined production techniques minimizes damage and enables basalt fiber manufacturers to produce strong fibers that can be braided and woven without inhibiting desired performance.
While basalt fiber is still not widely used, it is slowly making its way into the hand of consumers. At price points that vary between S-glass ($5/lb to $7/lb) and E-glass ($0.75/lb to $1.25/lb), basalt fibers have properties akin to S-glass. A common use is in the fire protection sector because of its high melt-point. Fire-blocking tests performed by Basaltex placed its basalt fabric in front of a Bunsen burner, placing the yellow tip of the flame in direct contact with the fabric. The yellow tip reaches temperatures of 1100°C to 1200°C (2012°F to 2192°F) and causes the fabric to become red hot, similar to a metal fabric. When exposed to the flame, basalt fiber maintains its physical integrity for extended periods of times, but the company found that a fabric made of E-glass with the same density can be pierced by the flame in a matter of seconds.
