Advanced Cellulosic Nonwovens

By Calvin Woodings

Introduction

Prior to 1960, regenerated cellulosic fibres enjoyed 50 years of rapid expansion. Since then, synthetics have grown to dominate the market. Cotton, for centuries the most important of all fibres is taking second place to the combined weight of synthetics and viscose rayon. Rayon now appears relegated to little more than a niche in a global fibre market driven by the ready availability of cheap fossil fuels and the demand for commodity textiles and nonwovens.

Nonwoven production was founded on the ready availability of low-cost viscose rayon fibres and these continued to dominate the industry until the mid seventies. Since then the reducing cost of synthetics coupled with their easy conversion into binder-free spun-laid and melt-blown fabrics caused a steady decline in rayon's nonwoven market share.

Is the relative decline in use of biopolymers such as cellulose in nonwovens and textiles just another, if long-enduring example, of the last stage of the inevitable growth/maturity/decline life-cycle of most markets? Or is there any suggestion that, on a longer time scale, biopolymers will prove to be a serious rival for the synthetics?

This paper reviews how rayon arrived at its current position in the nonwovens market, records the technology-based attempts to reverse the decline, and examines possible futures.

The Current Positioning of Cellulosic Fibres

Graph 1 illustrates how fibres based on fossil fuels have replaced fibres based on biopolymers over the last century according to the CIRFS statistics on World fibre usage in all markets.

Roughly half of the 45 million tonnes of fibre consumed annually in the world are now made from synthetic polymers. The only perturbations in an impressively smooth growth curve appear in the 73-74 period, the 78-84 period and the 90-91 period and are explained later.

On a more local level, fibre usage in the European Nonwovens industry is illustrated in Graphs 2 and 3.

While the tonnage of viscose rayon sold has held remarkably constant for 30 or more years, rayon has not participated at all in the massive growth of the industry and its market share is now a tenth of the 1970 figure.

The enormous expansion of the synthetics in the 60's and 70's put the viscose rayon industry on the defensive. Despite having taken almost 25% of a 14 million ton cellulosic fibre market without ever promoting rayon strongly against "King Cotton", and despite having the potential to double this share, the viscose producers felt the future of man-made fibres to be synthetic.

With hindsight, the viscose industry entered an end-game strategy at this time. Profits, substantial during the upside of the textile cycle, were spent on synthetic fibres or in diversification ventures rather than in marketing rayon against cotton or in building efficient new plants. A progressive tightening of the regulations governing the release of gaseous and liquid effluents from the viscose process compounded the problem. Effluent control projects and the 'modernisation' of existing plants took whatever funds were made available. During the downside of the textile cycle, the higher cost viscose capacity was simply closed down. By the end of the seventies nobody expected to see a new viscose plant being built anywhere.

In the absence of substantial reinvestment in new plants, the repositioning of viscose rayon became inevitable, and from 1985, the fibre was successfully transmogrified from a commodity to a premium-priced speciality fibre for the more lucrative niches in apparel and nonwoven markets.

The reasons for the decline are obviously related to relative fibre price and performance, but a more detailed analysis is needed to decide if a simple extrapolation of the graphs correctly identifies the most likely future.

Price

Graph 4 brings together fibre, oil and pulp prices from a variety of sources to throw some light on the competitive positioning of the key fibres in the period from 1970. They are not all on the same basis and the more readily available US figures are used in preference to less complete and more volatile European data.

They nevertheless illustrate the following:

• A doubling of oil price due to the oil embargo during the 1973 Yom Kippur War.
• The oil price controls during the Iranian Revolution/Iran-Iraq War and aftermath (1978-86) cause a massive disturbance in oil-price.
• Return to "normal" oil pricing interrupted by the Gulf War in 1990.
• Cotton increased from the 20c/lb to the 60 c/lb region during the 1970's, opening the door to polyester.
• Ignoring the political perturbations in oil-price, the pulp/oil price ratio is increasing.
• A quadrupling of oil-price (1972-82) hardly affected the relative price of polyester.
• Polyester and cotton prices appear to be linked.
• Rayon prices remain broadly in line with polyester and cotton until 1986 when they move rapidly ahead of both the competitive fibres and of reasonable price expectations based on pulp prices.
• By 1990 the repositioning of rayon at prices 50-100% higher than cotton was complete.

Performance

The aesthetic, absorbency and comfort advantages still enjoyed by the biopolymer fibres has slowed synthetic penetration of the apparel sector, and has extended their life at premium prices in the hygiene sectors of the nonwovens industry. Synthetics are used for their low-cost, thermal bondability, resilience, dryness, and durability characteristics. In the absence of fibres combining all these properties, synthetic/cellulosic blends have been a most popular combination. Ratios varying between 35% cellulosic and 35% synthetic depending on the market positioning of the fabric and the relative prices of the fibres have been typical.

Within the cellulosic part of a nonwoven blend, rayon now has to compete with woodpulp and to a much lesser extent, cotton.

While the versatility of rayon has ensured its continued use in a wide range of absorbent disposables, its fortunes appear to have been dominated by a few key markets. Technical developments aimed at responding to the needs of these markets extended the boundaries of rayon technology and identified ways of significantly altering its performance. While these are worth reviewing, it goes without saying that none were capable of reversing the market share decline.

Market Oriented Developments

The first major market share loss occurred in a market where the skin-friendly, absorbent nature of cellulose was felt to be a major advantage, coverstock.

During the late '60's when disposable diapers came in two pieces (reusable plastic pant with rectangular absorbent pad), latex bonded rayon was the cover of choice. At this time, 'flushability' was becoming a key development issue. The rectangular inserts with their heat-sealed latex-bonded rayon covers were too stable to be disposed of in the toilet even after tearing in half. New wet-laid nonwovens made from the specially developed self-bonding collapsed-tube rayon fibres had no wet-strength at all and dispersed easily in flowing water. However when treated with the standard wet-strength agents used in the paper industry it became strong enough in use and remained disintegratable in toilet turbulence. Rayon producers in Europe, Japan and the USA developed such fibres and a small market developed in the USA. The introduction of the more convenient one-piece diaper pushed mothers concerns about flushability into the background.

Latex-bonded 100% rayon continued as the leading cover on one-piece diapers, but in 1974 coverstocks containing 50% polyester were market-tested for the first time. Consumers found they could not really spot the difference from the 100% rayon fibre versions so in a second test the latex bonded rayon/polyester blend was put through a point embossing calender to give it a textured surface. This time the mothers could express a preference for the patterned over the plain and a "unique" new product was born. This first use of synthetic fibre in what had been regarded as an absorbent fibre 'fortress' appeared to be driven by nothing more than the concern over the escalating price of rayon. Technically however, the success of polyester was put down to its easy embossability.

In the course of the introduction of polyester to coverstock, the diaper industry discovered a major new marketing angle to support a move up to 100% polyester. Coverstocks containing polyester were found to be drier to the touch than the rayon versions. This was initially explained as a consequence of the fact that the synthetics did not absorb water, and so the rayon industry was asked to develop hydrophobic embossable fibres to stay in the game.

Embossability, achievable through alloying rayon with polyethylene emulsions, proved difficult to scale up, but hydrophobic rayons made simply by using hydrophobic finishes, were nevertheless commercialised in both the US and Europe.

One other feature of the new synthetic coverstocks was proving to be at least as important to surface dryness as their hydrophobicity. Resilience when wet, coupled with much greater dry-bulk associated with their low-collapse in latex bonding allowed them to provide a greater mechanical barrier to urine wetback than a water-plasticised viscose fibre ever could.

Solutions to the wet-collapse problem have been many and various. Dry cross-linking technology had been used on-line in rayon plants in the 1960's to improve the resiliency of fibres used in carpets. Wet cross-linking, using technology not dissimilar to that now used to make 'curly pulp' (for AD layers) had been possible since the early 70's but was rejected as compromising the chemical simplicity of rayon. Hollow and multi-limbed fibres gave benefits that allowed them to become major products, but only in markets where the premium prices allowed them to be considered.

Despite their advantages, 100% polyester latex bonded coverstocks had a short life span. Concerns over latex chemistry (e.g. formaldehyde) led to reformulation of the binders, but the progress in making thermal bonded polypropylene nonwovens allowed the diaper producers to move swiftly on to this even cheaper, even cleaner technology.

Attempts to develop a thermally bonded rayon nonwovens looked at several approaches.

Acetylation of the surface of rayon and the use of a solvent bonding process perfected for stabilising acetate in cigarette filters was one obvious approach This, like the surface grafting of thermoplastic materials, was felt to add more cost than value. Alloying with polyethylene, developed to improve embossability, gave inadequately bonded nonwovens in calendering.

Through-air bonding of 70/30 rayon/bicomponent fibres to lock the rayon into a high volume configuration allowed the manufacture of coverstocks with attractive softness and a good balance of strikethrough and surface dryness. The former was achieved without recourse to the high levels of surface finish necessary on polypropylene. However the rapid expansion of point-thermal bonding on wide high-speed calender lines allowed 100% PP coverstock to be made at prices that could not be matched on any less dedicated hardware using more costly fibres. The rayon industry quickly decided that this was a wave of nonwoven expansion that held no opportunity.

The rayon tonnage lost in the coverstock market was largely regained in new latex-bonded in-drier fabric softeners. These were lightweight latex bonded rayon nonwovens produced from rather coarser fibres than had been possible in coverstock. They were made on purpose-built wide lines at speeds well in excess of those possible on coverstock lines a few years earlier. Rayon gave better thermal stability than polypropylene and more strength than latex-bonded polyester. As this market matured, the fact that rayon absorbed too much softener was identified as a technical disadvantage because it allowed the sheets to be used in more than one drying cycle. Technical solutions were of course possible, and one involved harnessing the technical advantages of the new lyocell fibre (high strength, less absorbent rayon which could be converted into very light nonwovens - see later). However spun-laid polyester emerged as a lower cost alternative.

With fabric softeners as with coverstock, the absorbency of rayon appeared to have changed from a fundamental advantage at the outset, to a fundamental disadvantage as the converters experience in the marketplace grew.

Rayon's absorbency advantage has been less transient in the wiping and hygiene markets, and it is here that ingress of cheaper synthetics has been resisted best. This has been aided by the rapid growth of the hydroentanglement (HE) bonding method that allows the true character of pure cellulose to be realised in major nonwovens markets for the first time. The silky softness of rayon had, up until the development of HE always been masked by the need to use external bonding agents. Here again though, the poor wet resilience of cellulose has been disadvantageous, and many wipe producers now use up to 50% of synthetic fibres to prevent wet-collapse.

The growth of HE bonding has led to a turn around in rayon's fortunes and for the last couple of years tonnages consumed in nonwovens have reached their highest-ever levels in Europe. World HE capacity now exceeds 350,000 tonnes and an estimated 40% of this remains to be utilised. One of the major new products made possible by HE bonding of rayon is the ultra-soft baby wipe as exemplified by the European "Pampers" product from P&G. Many other companies are now imitating this and it may be just a matter of time before similar products appear in the US market.

Spunbonded Cellulose Nonwovens

Several cellulosic fibre producers attempted to improve the performance/cost ratio by making nonwovens themselves.

Viscose Process Based

• Courtaulds (now Acordis), Rhone-Poulenc and Enka Glanstoff AG (now Acordis) developed spun-laid rayon nonwovens on a pilot scale in the late 60's and early seventies.
• Kanebo and Daiwabo researched similar techniques.
• Asahi worked with viscose and cuprammonium pilot lines.
• Mitsubishi Rayon developed a process based on hydroxymethyl cellulose xanthate in which the webs were point-bonded in a thermal calender before regeneration.
• Kosabura Miura spun viscose vertically downwards from oscillating spinnerettes onto a conveyor, and oversprayed the liquid filaments with acid.
• The Tachikawa Research Institute developed a polynosic viscose spun-laid process.

Courtaulds introduced the product first at the first IDEA show in 1971, but closed the development 2 years later for reasons associated with the undesirability of competing with its rayon customers and the small size of the available market at that time.

This early cellulosic filament nonwoven was not a true spunbond. It was a wet-laid continuous filament web made from a fully washed viscose tow, laid on-line on an inclined wire former, dried and print-bonded with latex. It nevertheless made an excellent wiping cloth, demonstrably outperforming the market leader (at that time "J-Cloth" from Johnson and Johnson's Chicopee Division.) However also at that time the European market for nonwoven wipes of all types was at best 10,000 tonnes, and the minumum economic process scale-up would have produced 25% of this.

In 1973, Asahi introduced "Bemliese" the cuprammonium version of a similar on-line wet-laid cellulosic tow process. Here the continuous filaments could be laid in the incompletely regenerated state to give a true spunbond of pure cellulose. Because the cupro-based process was significantly more costly to operate than the Courtaulds viscose-based approach, they targeted higher value medical nonwovens and made much capital in Japan of the fact that they dissolved cotton-linters rather than wood-pulp. This allowed them to describe the resulting nonwoven as made from cotton, just like the woven products they planned to replace.

In 1976, the Mitsubishi Rayon Company introduced nonwovens from their methyol cellulose process as "TCF" (Textiles Continuously Formed). This was made in a viscose plant, and while it looked and felt similar to the "Bemliese" fabrics it was in fact "thermal-bonded" using a calender. Today, both of these Japanese processes use HE bonding to arrive at softer materials than spunbonding allows. The TCF fabrics are now made by Futamura and are now made from short-cut fibres, wet-laid on-line, rather than from tows.

Also in 1976, Courtaulds and their in-house nonwoven producer Bonded Fibre Fabrics (now the BFF Nonwovens division of Lamont Holdings) decided to reopen the development of spun-laid viscose. A second large pilot-plant was built in the Coventry Research Labs., this time along lines more similar to the Asahi approach, with tow-laying preceding washing. This gave the benefit of higher web uniformity from the perfectly parallel filaments laid straight from the special spinning heads, and the ability to use different bonding methods, before or after washing, or both.

Unlike Asahi and Mitsubishi, the Courtaulds route involved either spinning regular fibres and viscose bonding them immediately after laying, or aperturing and hydroentangling the acid tow-web using a very early in-house version of the HE technology which began to grow so rapidly 10 years later. Surgical swabs and binder-free, lint-free wipes were among the main target markets. The development met its technical objectives and plans to install a large commercial line in a UK viscose plant were drawn up. However in 1982 the company decided to spend all available capital automating the back end of the viscose plant rather than adding cost (and value) to the front end. The viscose pilot operation could not be sustained further, but the machinery did not go to waste.

At the start of the project calcium alginate nonwovens had been spun-laid, partly to investigate medical applications, and partly because they were an easy spinning proposition which allowed the design of the laying section to be finalised without the complications of the viscose process. By the time viscose spun-laid was shelved, these alginate spun-laid nonwovens were in demand as advanced wound dressings. The spun-laid pilot plant was converted to alginate and became the foundation of the current Speciality Fibres business of Acordis.

A further reason for mentioning alginate nonwovens at this stage is that their development led to an understanding of gel-dressings for advanced wound care. This in turn led to the development of another advanced cellulosic nonwoven made by carboxymethylation of viscose and more recently, lyocell. However one of the first commercially produced carboxymethyl cellulose nonwovens was made by Asahi, and the product, based on carboxymethylation of their "Bemliese" cuprammonium rayon spunbond was marketed as "Super AB" superabsorbent sheet in the 1980's.

Other processes

Of the numerous other routes to cellulosic fibres researched in the last 3 decades, within the context of this paper, two are worth mentioning and one of these is worth discussing in more detail.

The Carbamate route, developed by Neste Oy, reacted cellulose with urea to form a stable derivatised "pulp" which could be stored indefinitely and was easily dissolved in sodium hydroxide. The resulting solution, which could be prepared on viscose process hardware was also spinnable into sulphuric acid on viscose spinning machinery, or into dilute alcohol. The process was operated on a pilot scale by Kemira and small amounts of fibre were evaluated in nonwovens. The investment needed to prepare the carbamate pulp on an economic scale was never obtained, partly because the second process dealt with below was showing more promise in a wider range of markets.

The NMMO route employs the cyclic amine n-methyl morpholine n-oxide to dissolve cellulose prior to spinning the dope into water. This lyocell process was developed and scaled up by Enka and Courtaulds (now together in Acordis) over the last 20 years.

Lyocell Fibres

The lyocell process offered an environmentally acceptable way of converting natural cellulose into a premium quality rayon fibre with tensile properties approaching those of polyester. Furthermore the technology offered the potential for converting pulp to fibre on a scale and at a cost that would give polyester and cotton some serious competition.

The high capital requirement of the first plant necessitated launching the fibre (in 1990) at a premium price into the quality fashion-apparel market. This proved highly successful and led to rapid expansion up to the current capacity of around 100,000 tonnes. However this represents only about one half of one percent of the current cellulosic fibres market, and lyocell fibre is still only available commercially from two sources, Acordis and Lenzing.

At the time of writing, demand for lyocell has yet to increase to absorb the latest expansions. Some repositioning of the fibre in the market may well be necessary. The general softening of the world fibre market as indicated by the steep declines in cotton and polyester prices was a key element in the lower than anticipated demand for lyocell.

Lyocell Nonwovens

Lyocell makes excellent nonwovens, especially in those processes that allow it's superior aesthetics to shine through, like needle-punching and HE. Its high strength is of little intrinsic value in disposables, but it enables the nonwoven producer to reduce basis weight while meeting strength targets. It's freedom from shrinkage and high wet stability allows higher area yields in HE processes, and its high modulus prevents it from collapsing in the wet to the same extent as viscose rayon. Fibrillation, the development of surface microfibres on wet abrasion or in high-pressure entanglement, adds an additional dimension for the nonwoven developer. Unfortunately, while it has established itself in several profitable niches, its premium positioning has so far prevented it's use in mainstream disposables

Spun-Laid Lyocell?

Most fibre-forming polymers or polymer solutions can also be converted into continuous yarns, films, sponges or indeed nonwoven fabrics. Lyocell dope is no exception and many of the characteristics of the lyocell process make it a better basis for spun-laid nonwovens than the viscose process ever was. Technically speaking, the challenges are not great. Economically and commercially speaking, they are however enormous.

In the nonwovens industry the leading products are nearly always those with the lowest cost, and justification of spun-laid cellulosics on added-value alone has failed several times already. The ultimate in economy arises from inherently low cost raw materials converted on state of the art machinery at the largest possible scale. The nonwoven industry enjoys the economies of (say) polypropylene because PP is a by-product of the world's largest industry, energy. Viscose rayon, a premium product of the timber industry, requires the most costly grades of woodpulp. Lyocell is currently similar, but its simple production process has the so far unexplored potential to use cheaper pulps. It also has the potential to achieve very high levels of sales in textiles, and hence the economies of scale that may ultimately interest the major nonwoven converters.

Possible Futures

A recent study of world fibre demand argued that continued population growth and increased per capita fibre use would result in a demand for a further 70 million tonnes of fibres by 2050. With the potential for further cotton yield or hectarage increases now limited, and with the synthetics still looking unlikely to provide the comfort element of future textiles, a "Comfort Gap", or shortage of biopolymer fibres of up to 20 million tonnes, is postulated.

For 60 years, cotton production has grown almost entirely due to increased fibre yield per hectare. Land area under cotton cultivation has been constant for that period, and pressure for the same top quality agricultural land will increase due to the need to feed increased population. Better irrigation, higher pesticide use, higher chemical fertiliser use and genetic improvements achieved the cotton yield increase. In fact cotton cultivation in Russia became so intensive that despite its benign environmental image it has been the origin of environmental degradation on a massive scale. In the 1980's, a tonne of Central Asian cotton required approximately 800 kgs of fertilisers, 100 kgs of pesticides and an aggregate of 1.5 metres depth of water per square metre of growing area. In recent years cotton yield per acre has peaked and now appears to be unlikely to increase dramatically in the foreseeable future.

It is of course taken for granted that the implied need for an additional 50 million tonnes of synthetics will be readily provided. What is less easy to assume is that these synthetics will include varieties as comfortable, absorbent and as readily newable as cellulose. Will they also be cheaper, despite the inevitable oil shortage that will bite by the middle of the next century? Crop-based polymer production certainly holds promise, and polylactic acid could well prove to be a contender to be taken seriously. However it seems unlikely that agriculture could yield thermoplastics at the same price as oil currently valued at no more than the costs of extraction. If this proves to be the case, the real cellulosic gap could be higher.

The Role of Man Made Cellulosics

In order to realise the opportunity offered by the hypothetical cellulosic gap, or even a gap of a tenth of that size, man-made cellulosics must rapidly close that other gap, the price differential with cotton and polyester. They must do it in such a way that the financial community can conclude that investment in man-made cellulosics is at least as good as an investment in more synthetics. The technology to achieve this has existed for some time, but the justification for the necessary long-term investment is hard to sell.

The following points are relevant to the concluding argument:

• Cotton is the real competition for rayon.
• Cotton is still regarded as the invulnerable giant against which rayon cannot possibly be marketed.
• Polyester is clearly the main competitor for "Cellulosics" (rayon and cotton taken together) and having hitched its price to cotton, has achieved massive market share and impressive economies of scale.
• Polyester prices are now likely to increase to the long-term trend line from the unusually depressed levels of the last few years.
• Rayon (viscose or lyocell type) has enormous potential for growth at the expense of cotton, if available at cleaned cotton prices.
• Existing rayon plants are unlikely to be viable at such prices if dissolving pulp remains at current price levels. Over the longer term however, the pulp/oil price ratio must change in pulps favour.
• Rayon plants could now be built on the same scale as recent polyester staple plants (350,000 tonnes/year). On this scale their economics would be challengingly close to the required level.
• Lyocell, more than viscose, has the potential to break out of rayon's long-standing need for high quality dissolving pulp.
• Lyocell, more than rayon, has the potential to replace cotton.

Conclusions

• Wood-based cellulose is a "crop-based polymer" that is well positioned to take share from oil-based polymers and cotton as these materials become scarcer in the first half of the next century.
• A 350,000 tonne lyocell plant linked to a source of low-cost dissolving pulp could even now make a "viscose replacement" fibre at price close to polyester.
• A spunlaid nonwoven line in the same plant could make a wide range of 100% cellulose webs.
• These webs could be bonded in a variety of ways including hydroentangling on-line.
• Technology now available could surface the web with synthetics or add in high-bulk pulps for further economy before bonding.
• Technology now in use to make carboxymethylated lyocell for gel-dressings could be used to make spun-laid cellulosic SAP's in-situ.
• Technology now appearing in the patent literature could be used to make microfibre webs and nonwovens.
• Advanced cellulosic nonwovens based on these technologies are feasible now.

"Fibres made by Wet Spinning of Cellulose Carbamate Solutions", Turenen et al, Neste Oy. Presented at the TAPPI International Dissolving and Speciality Pulp Conference 1983.

Alceru (Schwarza), FCFC ( Taiwan), Hanil Synthetic Fibre Co. (Cocel), and the Birla Group ( India) are all working with lyocell pilot plants.

"World Fibre Demand 1890-2050 by Main Fibre Type." T F N Johnson, Courtaulds Fibres UK, presented to the Chinese National Textile Council, 1997.

"Poisoned Waters", New Scientist 21 October 1995.