A number of companies are trialling the use of basalt fibres in the filament winding of CNG storage vessels.
A number of companies are trialling the use of basalt fibres in the filament winding of CNG storage vessels.
Ring specimens of basalt/epoxy composite.
Ring specimens of basalt/epoxy composite.
Kamenny Vek offers a wide range of basalt rovings with different linear densities and various sizings for filament winding CNG cylinders.
Kamenny Vek offers a wide range of basalt rovings with different linear densities and various sizings for filament winding CNG cylinders.

The manufacture of compressed natural gas (CNG) cylinders is growing rapidly worldwide. This is in line with growing demand from the automotive industry for cheaper and ecologically ‘cleaner’ fuel, and in particular compressed natural gas.

However, the use of CNG creates the problem of gas storage and safe transport-ation. Vessels for CNG storage (CNG cylinders) must be strong, lightweight, and resistant to impact and temperature. At present, cylinders can be divided into the following three groups, depending on the materials used in their manufacture:

  • metallic;
  • metal-composite (metal liner strengthened by polymer composite material); and
  • composite (polymer liner strengthened by polymer composite material).

Today, all three groups are widely available on the market and each has its own advantages and disadvantages. For example, the weight of metal cylinders is a serious disadvantage. The weight of a cylinder can be reduced by partially replacing the metal with a lower weight polymer composite material as well as replacing the metal liner with a polymer liner. In this case the cylinder maintains its durability because of the high strength, lightweight epoxy impregnated carbon fibres used. But the extremely high price and current shortage of supply of carbon fibre may force manufacturers of CNG cylinders to look for an alternative material. Basalt fibre, which has better mechanical properties than E-glass fibre and is more widely available and cheaper than carbon fibre, could be a good alternative in this application.

Basalt fibre

Russian company Kamenny Vek produces continuous basalt fibres. These are being used in a variety of applications, including the manufacture of metal-composite and composite CNG storage cylinders.

Basalt is an inert, naturally occurring, volcanic rock that is found worldwide. The first attempts to produce basalt filaments from the melt were made in the USA in 1923. During World War II, and continuing into the 1950s, research in several countries advanced the science and technology of basalt fibre manufacture, but no commercial products were produced. In the past 30–40 years most of the research and the commercialisation of basalt fibre products has occurred in Russia and in the former republics of the Soviet Union.

Basalt-based materials are environmentally friendly and are non-hazardous. The current technology for producing continuous basalt fibres is very similar to that used for E-glass manufacturing. The main difference is that E-glass is made from a complex batch of materials whereas basalt filament is made from melting basalt rock with no other additives. Basalt fibre reinforcement is sized during manufacture (in exactly the same way as E-glass) to protect the fibre and to impart the resin compatibility needed for optimum performance.

Table 1: Kamenny Vek basalt roving properties (impregnated strand, 13 micron filament diameter,
1200 tex.)

Characteristic (in epoxy resin) ASTM Values
Tensile strength D2343 2900-3200 MPa
Tensile modulus D2343 85-95 GPa
Elongation D2343 3.2%
'Dry fibre' tensile strength D3822 700-750 mN/tex
Basalt fibre specific gravity   2.66 g/cm3


Once the continuous basalt fibres have been produced, they are converted into a suitable form for each particular application. For these CNG evaluations the continuous strands were assembled into rovings for filament winding. Some typical basalt roving properties are shown in Table 1.

Filament winding

To investigate the filament winding of a CNG cylinder using basalt fibre, to evaluate the advantages of basalt fibre and determine potential problems, Kamenny Vek and US company Spencer Composites Corp (SCC) performed some tests. These included the optimisation of the basalt/epoxy filament winding process, manufacture of material with specified properties, and production of a high pressure cylinder with optimal physical and mechanical properties.

Basalt roving BCF 13.2400.KV12 with 13 micron monofilament diameter and 2400 tex linear density manufactured by Kamenny Vek for use with epoxy resin, was used for testing. The epoxy resin system used in the test was Epon 862 with an amine curing agent. In the first stage of the work, unidirectional cylindrical samples of basalt/epoxy composite were produced. The basalt fibre content in all samples was 65% by volume. The winding process was conducted on SCC's proprietary test equipment with the following optimal parameters: winding speed 30 m/min; fibre tension 3–5 lbs; resin temperature – ambient 25°C; resin curing conditions – 315°F (157°C) for 4 hours.

The cylinders were then cut to produce ring test specimens and tested for tensile properties using SCC's proprietary ring burst test method.

The basalt fibre was found to process just like glass fibre and no fabrication problems were encountered in the winding of the rings. The basalt fibre also exhibited excellent wet-out during fabrication.

Table 2: Fibre property comparison.
Material Failure stress,
MPa (ksi)
Modulus,
MPa (msi)
Cost, US$/kg (lb)
BCF 13.2400.KV12 2937 (426) 109 (15.8) 3.2 (1.45)
Grafil 34-700 12K* 4881 (708) 231 (33.5) 28 (12.7)
OCF 450 Yield Type 30 E-glass** 2586 (375) 74.4 (10.8) 2 (0.91)
* www.mrc.co.jp ** www.owens-corning.com    

Ring specimens made with carbon and glass fibre were wound using similar process parameters. All the specimens were tested using SCC's proprietary test method (test report for SAMPE conference, May 2006). Table 2 compares the results obtained from the basalt rings with those from the carbon fibre and E-glass fibre specimens. The table also lists the current price for each. It can be seen from these results that basalt fibre is at least 15% higher in tensile strength, and around 35% higher in modulus, than E-glass. At the same time, the price of basalt is definitely comparable to E-glass and nine times lower than carbon. This gives filament winders the opportunity to reduce their expenses and get quality products at effective cost.

All of the data can be confirmed by calculations made using the composite-oracle engineering tools (www.composite-oracle.com). According to these calculations, the weight of a basalt high pressure vessel is 15% lower than similar E-glass vessels of the same strength. Since the amount of resin and fibre used to manufacture one cylinder is around 15% less, and manufacturing time and labour costs are also reduced, the cost of the basalt/epoxy cylinder is approximately 5% less than that of a cylinder made using glass.

Several Russian and international companies are interested in introducing basalt fibres into CNG cylinder manufacturing. One Russian company has manufactured metal-composite cylinders of different volumes using basalt/epoxy composite. These cylinders successfully passed all testing and were certificated according to Russian norms.

Some European companies have also produced composite cylinders with basalt and tested them. After cyclic testing, such vessels, which are designed for 200 bars of working pressure, exhibited high fatigue stress (17 000 cycles of loading and unloading at 260 bar pressure) with the burst pressure of 625 bar after that.

These theoretical and practical results allow us to say that basalt is a promising material for this application. All the advantages of basalt fibres – higher tensile strength, modulus, high chemical resistance, the possibility of recycling, and an extended range of working temperatures – can be realised in filament wound components.