From Trees to Trousers :

A century of man-made cellulosic fibres .

A public lecture given at Kew Gardens for the Centenary of the invention of the Viscose process by Cross and Bevan in the Kew Laboratories

By Calvin R Woodings, Research Fellow, Courtaulds, Coventry

Introduction

From the dawn of civilisation up to the early years of this century, man's textiles had been based directly on natural products. However a chain of events commencing in 1890's have led to a global textile industry based increasingly on fibres manufactured by industrial processes.

These processes can be subdivided into those which use the purified cell walls of plants ("Cellulose") as their starting point, and those plastic fibres which start from non-renewable fossil reserves. Plastic or synthetic fibres nevertheless do come from trees, its just that the current production route is rather lengthy: millions of years being needed to convert the dead vegetable matter into the coal and oil from which they are made.

In this article, we start with the early attempts to make fibres and then go on to look at the so-called viscose process which has been the 20th century's dominant method for converting plant matter into fibres. We will finish up by introducing a new method for making fibres from trees and consider how , "when the oil runs out", plant material will have to be used directly to make plastic as well as cellulosic fibres.

Early man-made fibres

The inspiration for the man-made fibre industry is generally credited to Robert Hooke who in 1664 proposed "wire drawing with some transparent artificial glutinous composition to make silk as the worm does." In those days, as now, silk was the luxury fibre, but far too expensive for general use in clothing. There were some abortive attempts to use the silk worm gum and extrude it artificially, but the first real indication that Hooke was on the right track came in 1842 when Louis Scwabe extruded molten glass into fibres.

The first successful attempts to use plant cellulose to make fibres can be traced to George Audemars who dissolved the nitrated form of cellulose in ether and alcohol and discovered that fibres formed by solvent evaporation as the dope was drawn into the air. These soft strong fibres could be woven into fabrics but had, however a very serious drawback: they were explosive, nitrated cellulose being the basis of gun-cotton.

It was Sir Joseph Swan who, as a result of his quest for carbon fibre for lamp filaments, learnt how to convert the nitrocellulose back into harmless cellulose. In 1885 he exhibited the first textiles made from the new "artificial silk", but with carbon fibre being his main theme he failed to follow up on the textile possibilities.

Meanwhile Count Hilaire de Chardonnet was following the same route and exhibited his first textiles at the Paris Exhibition in 1889. There he got the necessary financial backing for the first "Chardonnet Silk" factory in Besancon in 1890. Although the route was simple in concept, it proved slow in operation and difficult to scale up safely. The fibre was produced sporadically until 1949, when the last factory in Brazil closed down after a major fire.

The second cellulosic fibre process to be commercialised was invented in 1890 and involved the direct dissolution of cotton fibre in ammoniacal copper oxide liquor. This solvent had been developed by M E Schweizer in 1857. The solution of cellulose was spun into water, with dilute sulphuric acid being used to neutralise the ammonia. The process is still used to day, most notably in Japan where artificial silk, and medical disposable fabrics still provide a lucrative market. However its relatively high cost, associated with the cotton fibre starting point, prevented it from reaching the large scale manufacture that the third process did.

The Viscose Process

It was in fact at Kew, in 1891 that Charles Cross and Ernest Bevan discovered that cellulose, from either cotton or wood, could be dissolved as cellulose xanthate following treatment with alkali and carbon disulphide. The treacle-like yellow solution could be coagulated in an ammonium sulphate bath and then converted back to pure white cellulose using dilute sulphuric acid. They patented their process in 1892 (without considering the fibre making potential of their solution), and in 1896 formed the British Viscoid Co Ltd to exploit the process as a route to moulded plastic materials. In 1902 they were taken over by the Viscose Development Co. who, I believe, still use the process for non-fibrous products.

More important from our viewpoint was the establishment of another laboratory at Kew to develop silk like filaments. Here, C H Stearn, and C F Topham, who had worked for Sir Joseph Swan on lamp filaments, developed the continuous filament spinning process and the machinery needed to wash and collect them (1898).

In 1904, the Kew laboratories were visited by representatives of Courtaulds, who were at the time silk weavers looking for new raw materials and new opportunities to grow. The Samuel Courtauld Company was about to "go public", and its success and profitability had been built on the 19th century fashion for black silk mourning crepe. By 1904 however, the good times were over, and the company was stagnating.

The visitors to Kew knew that the Chardonnet filament process was making money, and that a lucrative market for artificial silk was being created. They believed that Cross and Bevan's viscose route could make a similar fibre much more economically. They liked what they saw, and, later in 1904, at the second attempt, managed to persuade a reluctant Board into acquiring the viscose process rights. The first commercial production was set up in Coventry in 1905, and while that original plant was closed in the 1960's we have carried out R&D on the site continuously since then.

Commercial Success...

As is often the case in major new ventures, the market used to justify Courtaulds acquisition of the viscose process - artificial silk - was not the most successful. Viscose rayon was no match for silk in luxury fabrics, but it could be used in blend with cotton or silk in lower priced apparel fabrics. Used alone, it made an excellent lining material for jackets and skirts, and its silk-like texture still makes it one of the most desirable fibres for this market. Chemically identical to cotton, it provided absorbency, comfort, lustre and a soft silky handle.

By 1908 sales were rocketing, and a series of expansions of output commenced with setting up the American Viscose Corporation in 1910 and the first Courtaulds US factory in 1911. By 1939, Courtaulds had 6 US factories, 7 UK factories, 1 in France 1 in Canada, and joint ventures in Germany and Italy. Silk weaving was all but forgotten, the small British company having been transformed into the world's leading producer of man-made fibre. New developments to make very strong yarns allowed the viscose rayon to become the fibre of choice for longer-life tyres, and the simple expedient of chopping the yarn into staple fibre gave it many new outlets through those Lancashire and Yorkshire mills used to processing cotton and wool into spun yarns.

These new outlets spawned a series of developments aimed at matching the characteristics of wool and cotton more closely. Viscose rayon was after all silk-like, and compared with wool it lacked bulk, resilience and abrasion resistance. Compared with cotton it was less strong, especially when wet, tended to shrink and crease more easily, and had a rather lean, limp "hand".

Coarser crimped staple fibres were therefore developed and by the late 60's these were widely used in the new tufted carpets which for a period became the single largest market for the cellulosic fibre. Stronger, finer more lustrous staples ("Polynosic rayon") were introduced for use in blend with cotton, and staple analogues of the tougher tyre-yarn fibre ("Modal rayon") were introduced for use in industrial textiles, and for blending with the now rapidly growing synthetics.

In the 1970's, attempts to make viscose rayon feel more like cotton led to the development of a range of hollow fibres. Two of these were commercialised, one, the uncollapsed tube version found worthwhile new business in thermal underwear, and the other, the collapsed tube version had a very high absorbency which allowed it to enjoy large sales in sanitary protection products.

These hollow fibres were replaced in the late 80's by easier-to-make "I" and "Y" shaped solid fibres which performed as well as the hollow versions in the main applications.

... and then slow decline?

In 1940 a new man-made fibre appeared in the statistics for the first time. This was of course nylon, and it heralded the development of a whole series of synthetic fibres, the most popular being polyester. These fibres were tough, non-absorbent, crease resistant, rot and moth-proof and as such have fuelled the expansion of fibres and textiles ever since.

Garments made entirely from these fibres have superb durability and easy care properties, but have in general proved rather uncomfortable because they hinder moisture loss from the skin. As a consequence, the absorbent cellulosic fibres still have to be used in blend so that even today, despite their relatively high cost, and inferior strength characteristics, they are still important in garments worn close to the skin.

Nevertheless, when the overall trends in consumption are considered it is clear that the man-made cellulosics share of world fibre usage is declining, rapidly in terms of share of market, and gradually in terms of volume. The old favourite, cotton, maintains its position leaving the growth to the synthetics. So, are the wood-based man-made from wood going to disappear, leaving the oil-based synthetics and cotton dominating the market.?

Processes and the Environment

To deal with this we must consider the fibre making process in a little more detail and examine some of the environmental issues which are rapidly becoming the most important factors in long range planning.

Cellulose is the natural polymer which makes up the living cells of all vegetation. It is the material at the centre of the carbon cycle, and the most abundant and renewable biopolymer on the planet. As outlined above, viscose rayon fibre producers have converted it from the fine short fibres which come from trees into the fine long fibres used by textiles for almost a century. Viscose rayon nevertheless remains unique among the mass produced man-made fibres because it is the only one to use the natural polymer directly.

Polyesters, nylons, polyolefins, and acrylics all come indirectly from vegetation. They come from the polymerisation of monomers obtained from fossil fuels, which in turn are formed by the incomplete biodegradation of vegetation which grew millions of years ago.

The industrial grade of cellulose used to make our rayon comes from tree-farms, where specially chosen species are grown from sapling to maturity in 7-10 years. New trees grow from the stumps of the cut trees, and this happens on marginal land, generally unsuitable for food crops and without the intensive use of fertilisers or pesticides. The best farms yield in excess of 2.5 tonnes of pure cellulose per acre per year. For comparison, cotton growing at its most intensive yields about 0.35 tonnes/acre and needs good soil.

Cutting down trees is popularly regarded as an unfriendly activity, and it is therefore quite important to put the usage of trees as a raw material in the correct perspective. The first point to emphasise is that the type of cellulose which is best for fibre making cannot easily be obtained from rain-forests. If we wanted to expand our industry dramatically, new tree-farms would have to be planted to provide the right sort of cellulose. The second point is that the fibre makers use of cellulose is tiny compared with its present availability:

* 100 billion tonnes of vegetation grow and decay annually on land. This represents about 12% of the planet's total production of vegetation, the majority being produced in the oceans.

* 12% of this land-based vegetation is in the form of wood (trees).

* Of this 12 billion tonnes of wood, a maximum of 3 billion tonnes is removed by man. Half of this is burnt, either as fuel or to clear land for agriculture. The other half is converted into wood products. (Compare this with 6 billion tonnes of fossil reserves "mined" each year.)

* Of the 1.5 billion tonnes of wood used to make products, a fifth gets made into woodpulp for paper and board.

* About 4.5 million tonnes of this woodpulp output are a high quality dissolving grade, or "industrial cellulose" for the world's chemical industry.

* Viscose rayon manufacture consumes 2,600,000 tonnes of this cellulose, with about 2 million tonnes going into the staple fibre process.

From the above figures, it can be seen that the 2.6 million tonnes of dissolving grade pulp currently manufactured to feed the rayon fibre industry represents one ten-thousandth of the annual production of cellulose on land in nature.

In other words, the equivalent of the annual polymer requirements for the cellulosic man made fibre industry are produced in nature every 50 minutes.

Converting the woodpulp into fibres by the viscose route involves soaking the pulp in strong caustic soda, reacting this "Alkaline Cellulose" with carbon disulphide to form the yellow xanthate "crumbs" and then dissolving these crumbs in dilute caustic soda. The solution has to be well filtered and deaerated before extruding it through a very small holed spinneret into a bath of dilute acid. The fibres are then washed to remove acid and suphurous by-products, dried and baled ready for use in, for example, yarn spinning.

Whilst the basic viscose process has changed little in the last 50 years, the process details and equipment have required continuous upgrading both to minimise the manufacturing costs and to meet the increasingly stringent environmental regulations. The viscose process pioneers were not unlike other industrialists at the turn of the century: they concentrated their efforts on making the best product and had little regard for waste treatment or recycling within the process.

Today we can foresee ways of operating the viscose process in a "closed-box", and in recent years have made enormous progress towards that ideal. Such technology is however complex and costly, and is, in accounting terms non-productive.

In fact it is now cheaper to make synthetic fibres from trees after they have "matured in the ground" for millions of years rather than directly. (It is perhaps worth noting that they are only cheap because our economic system currently attaches no intrinsic value to the irreplaceable fossil reserves on which they, and in fact our entire energy-intensive lifestyle, are based.)

A new process

At the same time as the viscose process was evolving to cope with the combined pressures of cheap synthetics and expensive process modernisation, completely new routes from trees to trousers were being sought.

The simple solvent for cellulose had been a research goal for over a century, and in the seventies, numerous new chemicals and combinations of chemicals were being tested in laboratories around the world, including our own. To our scientists a relatively new chemical - an amine oxide -appeared to have the greatest potential for making fibres. During the last 12 years we have used this chemical and developed a fibre-making process based on it. The overall R&D project was code-named "Genesis", and the new fibre is now called "Tencel". In the last few years we have taken the process through scale up and market testing and are now building the world's first full-scale plant in Mobile Alabama. This is due to start up this summer.

The amine oxide is the only major chemical used in the new process, and from the outset we planned to recycle it to a very high degree. In our Grimsby plant, which has been operating semi-commercially to produce the "Tencel" fibre we have progressively developed the techniques which allow us to recycle virtually all of the solvent used to dissolve the pulp.

Furthermore, extensive health and safety testing has shown that the solvent is harmless over the range of concentrations used in the plant, and especially so in the tiny amounts likely to be released into the effluent.

Energy factors

Energy is a major component of the environmental impact of most complex industrial processes.

Not surprisingly, most of the published work on energy use by fibre processes was carried out during the energy crises in the 1973-81 period. They assess the total energy required to make baled staple fibre from naturally occuring raw materials, wood in the case of cellulosics and oil in the case of synthetics. In general they break the fibre production sequence into monomer making, polymer making and fibre production, and while a variety of fibres are covered, only viscose rayon and polyester are mentioned in all of them. The overall picture that emerges from these early studies was that whilst the wet-spun cellulosic fibres required more energy than melt spun polyester for the fibre making step, they had no monomer energy requirement, and the polymer-making requirement was minimal.

In fact for an integrated pulp and viscose plant situated close to its tree-farms the viscose route only requires about half the "fuel oil equivalent" of polyester. This is because the monomer (sugar) and polymer (cellulose) are derived using direct solar power (photosynthesis). When the tree is pulped, the pulp mills energy needs can be obtained by burning the parts of the tree which are not needed in the final product. Fossil fuel is thus eliminated from a major part of the cellulosic fibres route while for polyester it is the very raw material from which the polymer is made.

Finally, in the integrated arrangement, wet pulp can be fed directly into the viscose process without incurring any transport or drying cost. This provides an additional energy bonus in that never-dried pulp is easier to react and dissolve than the dried form.

The new solvent route to cellulosic fibres is identical to the viscose route up to the point where the cellulose enters the solvent. The energy requirements for the non-cellulosic raw material is significantly lower in the case of the solvent route, but the solvent route will require similar energy levels in dope handling, spinning, washing and recycling. The lower water retention of the solvent fibre (65% versus 95%) will yield savings in fibre drying and of course in any subsequent washing and drying operations.

Summarising the energy situation, the solvent route will show a useful economy in this important resource when compared with viscose production on the same scale, and viscose already has a significant advantage over polyester.

Pollution

In the viscose process, gaseous effluent control and treatment is a fundamentally important part of the overall process and is continuously improving as the technology of the "closed-box" process evolves. The air handling and cleaning systems employed are costly and most of the emissions to atmosphere are collected and discharged through tall stacks.

The solvent process produces very little atmospheric emission. There are traces of volatile organic compounds associated with the solvent and the soft finish which will leave the plant in the normal course of ventilation. There is no need for any central air handling or emissions stack.

The fibre spinning and washing liquors from the viscose process are recycled to allow re-use of the chemical components wherever this is feasible. Nevertheless, in common with most industrial washing and bleaching systems, large volumes of process water have to be cleaned on-site before discharge. As is the case with gaseous effluents, most rayon producers stay well ahead of the regulatory requirements and this means continuously working towards improved plant designs.

The solvent route uses much less water overall, and the process effluent needs significantly less treatment because so little solvent escapes the recovery system.

Disposability/Recycling

Man made cellulosic fibres are simply a tiny subset of the most abundant bio-polymer on the planet.

Like natural vegetation, they can become food for micro-organisms and higher life forms (they biodegrade) and they will burn with a rather greater yield of energy than natural vegetation.

In complete biodegradation or incineration, the final breakdown products are carbon dioxide and water, and so in the overall sense these disposal methods simply recycle the cellulose to the atmospheric components from which it was made.

It is also possible to liberate and use some of the "free" solar energy which powered the polymerisation step. In the case of incineration this is straightforward in that the cellulose burns freely. In the case of landfill disposal, it is now well known that slow anaerobic biodegradation occurs in all landfill sites dealing with municipal solid waste. This process generates methane from cellulose, which can, and increasingly is, being used to drive gas-turbines directly. Admittedly, this process makes only a small contribution to reducing the volume of waste in the landfill, but as fuel costs rise, and tips are designed to digest the rubbish rather than store it, this "free" and renewable energy source will become more important.

In fact, in the more distant future when fossil reserves are less abundant and more costly, it will be necessary to derive the monomers for plastics production from the biodegradation of plant matter. Fermentation of sugars from cellulose into ethanol, followed by dehydration to ethylene, already provides a route to one of the key building blocks for plastics. The lignin component of wood could be used to derive the other key ingredients, the aromatic compounds, benzene, toluene and ethylbenzene. The processes could even be driven by natural gas derived from the anaerobic digesters used to treat the nation's rubbish!

In Conclusion

Viscose rayon fibres, made for a century by the direct conversion of abundant vegetable matter, have always had much to recommend them in textiles compared with synthetics made from fossil fuels. The renewability of their main raw material, their overall energy efficiency, their lack of dependence on fossil fuels, their long history of safe use in skin contact applications, and their easy disposal and natural recyclability make them strong contenders for tomorrow's textile industry also.

Our new solvent route to rayon reinforces these inherent strengths by using a high-technology fibre production system which, being physical rather than chemical, reduces the environmental impact of the production route to a minimum. Our "Tencel" investment, coupled with the continuous improvement of the old viscose route, gives us what we believe is a winning approach to textile industry fibre supply for some time to come.

C R Woodings

18/2/92

"Tencel" is the Courtaulds trade mark for the solvent-spun cellulosic Lyocel fibre which will be produced commercially in the USA from mid-1992.