A composite breakthrough that has initiated the commercial manufacture of plastic gas cylinders has been uncovered by Tom Shelley during a visit to the Czech Republic

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Commercially produced plastic cylinders for compressed propane and butane are both lighter and much safer than their steel equivalents while costing about the same. The breakthrough comes from using thin glass-- reinforced epoxy shells in combination with an impervious liner and a protective outer shell.

This is not a particularly new idea but, to our knowledge, the cylinders, developed for the Russian market, are the first to be commercially manufactured on any scale. They could soon see use in the UK and may act as a precursor to other low-cost, high- performance composite developments.

The cylinders, their materials and the methods behind their manufacture are the brainchild of Prague-based Kompozit-Praha. Its essential skill lies in its ability to fabricate continuous glass filament-- wound composite products for the military and aerospace markets. One of the skills, crucial to the Czech development, is its ability to produce very thin composite constructions which are able to keep weight and cost to a minimum. In the case of the smallest sized gas bottle the glass-epoxy is only 1.2mm thick (even the largest bottle has a wall thickness of just 3.5mm). The gas is contained within a PET (Polyethyleneterephthalate) liner with a glass-epoxy over wrap providing the strength. One of the features of this combination of materials is transparency, allowing users to see how much liquefied gas remains within the bottle. The outer HDPE (High Density Polyethylene) casing provides additional protection and allows the cylinders to be styled to supplier's needs or coloured to indicate hazards or content.

The biggest advantage of the new method of construction, however, is neither its style nor its low weight - being half that of steel - but its fire resistance. One might imagine that steel is the ultimate fireproof material. It is, until it gets very hot or internal pressure rises to a degree sufficient to rupture it. Bear in mind, the most powerful non-- nuclear bombs are those that produce a sudden mixture of fuel and air that subsequently ignites. Conventional steel gas cylinders have the potential to perform in just such a manner.

Flexible ceramic coatings

Also encountered in the Czech Republic is CeRamKote, a US- developed, flexible ceramic composite coating. Unlike most ceramic coatings, a piece of metal coated with this material can be bent back and forth in the fingers without cracking.

According to Czech agent Zdenek Vyroubal of Ceramed this behaviour arises because the ceramic particles are so close together they interact with each other directly by physical means. The matrix is epoxy resin with the solid content being 80% and the gradation of particle sizes ranges from 40(mu)m down to nm.

The material, developed and patented some 20 years ago by Freecom in Texas, is used in the oil industry. In Europe, it was, for a long time, only used for coating cylinders for pulp and paper manufacture. Now manufactured by CeRam-Kote Europe in Germany, take- up has been described as .very promising. In the Czech Republic, it has been used to coat 280 tanks for Libya's 'Great Man-Made River project which extracts underground water from beneath the desert and delivers it to agricultural areas on the coast. Vyroubal says the coating can be supplied in any of 10,000 colours and costs euro 10/ litre in large quantities or euro 30/litre in small quantities. Application is best undertaken by spraying but it can also be rollered on.

The new cylinders, on the other hand, have a failure mode that is non-explosive. If an 85% full, 5litre cylinder is subjected to a 'bonfire test', failure begins four minutes from the start of the test when the inner liner melts. The gas then leaks evenly through the composite over-wrap, with all gas leaking out in 25 minutes.

The cylinders fulfil all current European legislation and the company is looking for additional customers in the European Union which the Czech Republic is expected to join in 2004. One of the companies it is actively talking to is BP.

Editor's note: This story was brought to our attention at the 44th International Engineering Fair in Brno in the Czech Republic by another Czech company, Zalesi, based in Luhacovice. Zalesi supplied both tools and resin for the project. (www.zalesi.cz)

We must mention at this point that Advanced Lightweight Engineering, based in The Netherlands, also has expertise in this type of construction. This company has for some time been promoting composite LPG for cars (More information at www.lightweight.nl) but the seem still to be at the promotion and testing phase.

Take a shine to natural composites

Polished tiles of re-crystallised basalt, a ceramic-- ceramic composite material, are used to make skid pans for all the major automotive companies. The material is very hard and abrasion resistant, and can be given a high polish or made non-slip. Its abrasion resistance also makes it very suitable for lining pipes used for the pneumatic and hydraulic conveying of bulk materials.

To our knowledge, one company stands above all others in its manufacture - the Czech company Eutit (www.eutit.cz and kompozit@cbox.cz). The firm started out as a glass works, but has spent the last 50 years perfecting the black art of selecting natural basalts, and then melting, casting and heat treating them to produce optimum mechanical properties. Although basalt is a natural volcanic rock, found all over the world in large quantities, turning it into useful engineering product requires considerable technical expertise. The present process is the result of much research undertaken in various institutes in the Czech Republic and in the Technical University in Prague. For tasks too arduous even for basalt, the firm also produces another ceramic, which it calls 'Eucor, made by melting corundum (aluminium oxide) and baddeleyite (zirconium oxide). However, basalt is cheaper, and while Eutit has a regular customer base, it is eager to find new markets. Hardness on the Mohs scale is 8 (Diamond is 10), compression strength is 309 to 483MPa and bend strength, 48 to 64MPa. The UK agent is Greenbank.

* Composite gas cylinders now in commercial production are explosion proof and half the weight of steel cylinders
* Ceramic powder in epoxy coatings use nano sized particles to achieve flexibility on metal substrates
* Natural ceramic-- ceramic composites can be made by melting and recrystallising natural basalt to produce a material with exceptional hardness and abrasion resistance