Basalt fiber: an alternative to glass?

Basalt is a hard, dense volcanic rock that can be found in most countries around the world. It is an igneous rock, which means it starts in a molten state. Basalt has been used in casting processes for many years to make tiles and slabs for architectural applications. In addition, cast basalt linings for steel pipes have shown very high wear resistance in industrial applications. Basalt is also used as an aggregate in concrete when crushed.

Recently, continuous fibers extruded from natural refractory basalt have been investigated as an alternative to asbestos fibers in almost all of its applications. Over the past decade, basalt has emerged as a strong contender in the field of fiber reinforcement for composites. Proponents of this latecomer claim that their product offers similar properties to S-2 glass fibers at a price point between S-2 glass and E-glass, and may offer manufacturers a cheaper alternative to carbon fibers for producing the latter products representing over-design.

Basalt fiber composition

Basalt fibers are produced in a continuous process similar in many respects to the process used to make glass fibers. The mined basalt is first crushed, then cleaned and loaded into a box connected to a feeder that moves the material into a molten pool in a gas-fired heating furnace. Here, the process is actually simpler than glass fiber processing because the composition of basalt fiber is less complex. Glass is typically made from 50% silica sand mixed with oxides of boron, aluminum and/or several other minerals - materials that must be fed independently into the metering system before entering the furnace. Unlike glass, basalt fiber has no secondary materials. The process requires only a single feed line to transport the crushed basalt to the furnace. On the other hand, basalt fiber manufacturers have less direct control over the purity and consistency of the virgin basalt. Although basalt and glass are both silicates, molten glass forms an amorphous solid when it cools. However, the crystal structure of basalt will vary depending on the specific conditions during each geographically located lava flow. Basalt combines three silicate minerals - plagioclase, pyroxene, and olivine. Plagioclase describes a number of triclinic feldspars composed of sodium silicate and calcium silicate. Pyroxene is a group of crystalline silicates containing any two of the three metal oxides of magnesium, iron, or calcium. Olivine is a silicate that combines magnesium and iron - (Mg, Fe) However, its crystal structure varies depending on the specific conditions during each geographic lava flow. Basalt combines three silicate minerals - plagioclase, pyroxene and olivine. Plagioclase describes a number of triclinic feldspars composed of sodium and calcium silicates. Pyroxene is a group of crystalline silicates containing any two of the three metal oxides of magnesium, iron, or calcium. Olivine is a silicate that combines magnesium and iron - (Mg, Fe) However, its crystal structure varies depending on the specific conditions during each geographic lava flow. Basalt combines three silicate minerals - plagioclase, pyroxene and olivine. Plagioclase describes a number of triclinic feldspars composed of sodium and calcium silicates. Pyroxene is a group of crystalline silicates containing any two of the three metal oxides of magnesium, iron, or calcium. Olivine is a silicate combining magnesium and iron - (Mg, Fe) Pyroxene is a group of crystalline silicates containing any two of the three metal oxides of magnesium, iron, or calcium. Olivine is a silicate combining magnesium and iron - (Mg, Fe) Pyroxene is a group of crystalline silicates containing any two of the three metal oxides of magnesium, iron or calcium. Olivine is a silicate that combines Mg and Fe - (Mg, Fe)₂ SiO ₄. This potential for compositional diversity means that the mineral content and chemical composition of basaltic formations may vary from site to site. In addition, the rate of cooling as the primary flow reaches the Earth's surface can affect the crystal structure. Although this basalt can be obtained from mines and open quarries around the world, only a few dozen sites contain basalt that has been analyzed and qualified as suitable for the manufacture of continuous filaments.

From rock to fiber

When crushed basalt enters the furnace, the material liquefies at a temperature of 1500°C/2732°F (the glass melting point varies between 1400°C and 1600°C). Unlike clear glass, opaque basalt absorbs rather than transmits infrared energy. As a result, it is more difficult to uniformly heat the entire basalt mixture with the elevated gas burners used in conventional glass melting furnaces. For tower-top gases, the molten basalt must remain in the reservoir for a longer period of time - several hours at most - to ensure uniform temperatures. Basalt producers employ a variety of strategies to promote uniform heating, including immersing electrodes in the bath. Despite the increased manufacturing costs, we prefer gas heating over electric heating for quality reasons. Finally, a two-stage heating solution was used, featuring separate zones equipped with independently controlled heating systems. Only the temperature control system in the exit zone of the furnace that feeds the extrusion bushing requires a high degree of precision, so that a less complex control system can be used in the initial heating zone.

As with glass filaments, basalt filaments are formed from platinum-rhodium sleeves. As they cool, a sizing agent is applied and the filaments are moved to a speed-controlled fiber stretching apparatus and then to a winding apparatus where the fibers are wound.

Because basalt filaments are more wear-resistant than glass, the expensive bushings used to require more frequent refurbishment. As the bushings wore, their cylindrical bores wore unevenly, reducing process control. If not maintained in a timely manner, the unrounded holes could form filaments with unacceptably large diameter ranges, resulting in rovings with unpredictable breaking loads. While glass fiber casing can last six months or more before needing to be melted, reformed and re-drilled, casing previously used for basalt fiber production could last three to five months. However, recent reports say that process control efforts have extended casing life to a similar six-month cycle.

Basalt fiber vs. glass fiber

Overall, these differences in processing and maintenance result in overall operating costs that exceed the cost of processing alkali-free glass fiber, but proponents of basalt fiber say that their product performs significantly better than alkali-free glass in composites. In short-cut mat, roving and unidirectional fabric forms, basalt fibers exhibit higher fracture loads and higher Young's modulus (a measure of stiffness for a given material) than E glass. In a study conducted on basalt and alkali-free glass fibers, unidirectional prepregs were produced by impregnating alkali-free glass and basalt roving with epoxy resin and winding them each around a mandrel, then compacting the laminate until fully cured. Samples of 135 mm x 15 mm (5.3 in. x 0.6 in.) were cut and their thickness measured. The parts were then subjected to a three-point bending test (ISO 178) and an ILSS test (ISO 14130) to test strength and stiffness. The basalt/epoxy samples were tested to be 13.7% stronger and 17.5% stiffer than the E-glass samples, even though the basalt samples were 3.6% heavier than the E-glass samples.

In addition, basalt fibers are naturally resistant to ultraviolet (UV) and high-energy electromagnetic radiation, retain their properties at low temperatures, and provide better acid resistance. Basalt is also reported to be superior in terms of worker safety and air quality. Because basalt is a product of volcanic activity, the fibrillation process is more environmentally friendly than fiberglass. The "greenhouse" gases that may be released during the fiber process were released during magma eruptions millions of years ago. In addition, basalt is 100% inert, meaning that it does not react toxically with air or water, and is non-flammable and non-explosive.

Although basalt fiber is still not widely available, it is slowly making its way into the hands of consumers. At a price point between S glass ($5/lb. to $7/lb.) and E glass ($0.75/lb. to $1.25/lb.), basalt fiber has similar properties to S glass. Because of its high melting point, its common use is in the fire protection sector. We place its basalt fabric in front of a Bunsen burner so that the yellow tip of the flame is in direct contact with the fabric. The yellow tip reaches temperatures of 1100°C to 1200°C (2012°F to 2192°F) and reddens the fabric, similar to a metallic fabric. When exposed to flame, basalt fibers retain their physical integrity for long periods of time, but we found that fabrics made from E glass with the same density could be pierced by flame within seconds.

The flame retardant nature of basalt fiber makes basalt fiber an asbestos substitute in friction applications such as composite brake pads because it does not soften at high temperatures or deposit on its counterpart (disc or drum) system during braking. Continuous basalt fibers are also used as reinforcement for other traditional composite structures. Basalt fibers are easily wetted and can therefore be quickly impregnated with resin, making them suitable for resin transfer molding, infusion molding and pultrusion molding. All products made from glass can be made from basalt.