Opportunities arising from the Fibrillation of LyocellBy Calvin Woodings, CWC Ltd, Eathorpe, Warwickshire, UK IntroductionThe uncontrolled fibrillation that occurred when textiles made from early lyocell fibres were wet processed (dyeing/finishing) or washed led to unacceptable fabric appearance and texture. The fibrils splitting away from the fibre surface became apparent as a frosty-looking surface which prevented the true dyed colour from emerging. In extreme cases, and especially on knitted fabrics where the fibres were less tightly bound into the structure, the fibrils caused these looser fibres to entangle and form pills. Thus early lyocell had restricted potential in textiles, being confined to woven products dyed and finished by gentle processes to lighter shades for dry-clean only garments. This unacceptable situation was addressed by two courses of action:
In the event, Action 1 succeeded more rapidly than 2 resulting in a full-scale process which could not produce fibre to match the expectations of those nonwoven producers who had tried the easy fibrillating pilot-plant fibres which had been introduced as part of Action 2. This situation led to the “interim” development of a way of increasing the fibrillation rate of the production fibre for those customers who needed it. Here the new hard-to-fibrillate fibre was degraded during washing until it matched the original fibre. This was to be marketed only for sufficient time to prove (or otherwise) that there was an acceptably large market for such a product. Given a large market, a production line would then be dedicated to the easy-fibrillating product, allowing its manufacture by a more cost-effective route. As we will see later, reducing fibrillation adds cost, and this interim wash-machine degradation route to easy fibrillation added further cost, making it unattractive to operate even for short periods. Inherently highly-fibrillating lyocell has therefore never been properly tested in the market place, and the current case for doing this is the subject of this paper. Structural ConsiderationsNatural cellulosic fibres grow with a microfibrillar structure (Fig 1), and when these fibres are broken down by hammering or abrasion, especially when wet, fibrils and microfibrils can be generated. These fibrils have properties similar to those of the original fibre, but have a great tendency to bond to one another due to their very high surface area and increased flexibility. Regenerated cellulose generally abrades into short lengths rather than fibrils, and while these particles can hydrogen bond in the same way, their lack of length prevents any useful bond strength from developing. As the modulus of regenerated cellulose fibres is increased, the molecular structure becomes more fibrillar, and versions with the highest modulus, the polynosic rayons, fibrillate highly. One of the best examples of such a fibre was Toyobo’s “Tufcel”. Dry-jet/wet-spinning of lyocell fibre has been described as being similar to the extrusion of lyotropic rigid-rods, giving rise to a highly microfibrillar structure which is consistent across the entire cross-section of the fibre – i.e. differing from the skin-core structure observed in viscose rayon. Wide-angle X-ray diffraction data shows lyocell to have a crystallinity of 60% (c.f. 40% for viscose), the celluose crystals having a a lateral dimension of about 4 nanometers. However, microfocus small-angle X-ray scattering has recently shown a definite skin-core structure with the thin skin having better oriented voids than the core. This observation is more in-keeping with the macro observations of fibrillation in papermaking: the surface appearing to fibrillate easily, whereas breaking up the core region of current production fibre proves very difficult. Factors Affecting Lyocell FibrillationFibrillation, or the tendency of a fibre to develop fibrillar and microfibrillar “hairs” on the surface when it is abraded can be assessed in the obvious microscopic ways. However to quantify it, it is necessary to standardise the method of creating the fibrils and the method of assessing the extent of fibrillation. Papermaking science provides one of the best methods, the “disintegration” of a slurry of fibres followed by measuring it’s rate of drainage as it forms a web on a standardised sieve. Quicker, but less precise is to subject a slurry of fibres to ultrasonic agitation for a fixed time and then to assess the extent of fibrillation microscopically with the aid of an image analyser. Given a standard method for quantifying fibrillation, factors affecting the ease and extent of the phenomenum can be established. Compared with the viscose process the direct dissolution of cellulose in N-Methyl Morpholine N-Oxide has very few variables, but most of them have now been shown to affect the fibre’s ease of fibrillation:
A more recent study at Hanyang University also showed that spinning of a 15% (c.f. 12%) solution of cellulose (DP 940) in NMMO with a hydration number of 0.72 (c.f. 1.0) produced very finely-fibrillated structures after ultrasound treatment. Other factors of importance are the pulp type and cellulose DP, the presence of particulate additives such as titanium dioxide in the fibre, and any after-treatment that would reduce the cellulose DP such as hypochlorite bleaching. (The early matt bleached versions of lyocell fibrillated more rapidly than the equivalent bright unbleached versions.) Effects of Fibrillation on Nonwoven PropertiesIn nonwovens, lyocell could be seen as a source of cellulosic microfibres, and several markets, filtration and synthetic leather being but two, paid relatively highly for sources of microfibres. Early lyocell was in fact a cellulosic analogue of the “islands in a sea” bicomponent synthetic fibres used to make the ultrasuede type of artificial leather (e.g Alcantara). The “sea” was a loose array of amorphous cellulose holding together (but not as strongly as required for textile uses) a parallel array of very high modulus cellulose crystallites of great length but incredibly small diameter. In the dry, unswollen state, this amorphous cellulose conferred more than adequate lateral strength to the composite, but as it became hydrated and swollen in the presence of high humidity, its ability to hydrogen-bond the whole structure together became catastrophically weak. In wet-laid processes and in papermaking, this swollen amorphous region could be torn apart by beating or refining a fibre slurry, and in hydroentanglement, it could be penetrated by high pressure water jets, fibrils splitting away from the fibre surface in the hydraulic turmoil. If synthetic leather was the objective, then the after-treatments to which the bonded nonwoven was subjected could further develop the microfibre content. Fibrillation in PapermakingThe main conclusions from several years work on fibrillating lyocell for use in special papers were published in 1996 and 1997. Both papers refer to an experimental lyocell which fibrillated faster and more completely than the standard fibre. This experimental or “interim fibre” mentioned in the introduction was briefly marketed as “HF 100”, and could be fibrillated to 200 CSF in 100 minutes, half the time taken by standard fibre . The process involved subjecting the fiber to severe bleaching, for example by application of an aqueous liquor containing 0.1 to 10 percent by weight sodium hypochlorite (as available chlorine) to the fiber during tow-washing. A reduction of the degree of polymerisation by about 200 units was sufficient to boost the fibrillation rate by the desired amount. The resulting fibres could be refined more rapidly and therefore with less energy input than the standard fibre. They could in fact be refined through the point of zero Canadian Standard Freeness into a region where the CSF started to rise again due to the extremely fine fibres being lost through the sieve in the test equipment. At zero CSF or finer, the refined product could be used as a filter aid, small percentages being added to filtration papers to boost their performance. Fibrillation in Wet-laid NonwovensAt Index 90 in Geneva the properties of wet-laid nonwovens made from “Tencel” lyocell were described. Here the high wet modulus of the fibre was shown to allow wet-laying at 12mm and 15mm staple length, comparable with polyester, and significantly longer than possible with the more flexible viscose rayon fibres of the same denier. However in this work, the lyocell was wet-laid in the unfibrillated state, and strength comparisons with the polyester control were made after hydroentanglement, where it gave strengths ten-times higher than the same length polyester entangled at the same water pressure. If the lyocell was fibrillated before laying, the reduction in average denier meant that the longer staple lengths could no longer be formed into an even sheet. In fact fibrillation emerged as a problem in wet-laid processes attempting to operate close to the critical fibre diameter/length/concentration limits, because some fibrils form even on gentle agitation and these act as nuclei for unwanted entanglement before the web can be formed. Wet-laying of pre-fibrillated lyocell is only possible with short fibres and this is better described as papermaking. The main wet-lay applications for fibrillated fibre relied on its ability to form lighter-weight sheets with the same or lower porosity than unfibrillated fibres, properties which could be expected to be of interest to makers of tea-bag nonwovens. In practice the first and possibly most potentially significant application to develop was the improved efficiency cigarette filter, see below. Fibrillation in HydroentanglementThe effect of high pressure water jets on lyocell fibre was first described publicly in 1989 . This paper also mentions the experimental version of lyocell that gave fabric properties out of line with those expected from a knowledge of the tensile properties of the fibre. In addition to the higher fabric strengths, partly due to the fact that at 140 bar water pressure, extensive fibrillation had occurred, the fabrics were stiffer (bonding between the fibrils), and the fibre took on a chalky white appearance as bonding proceeded. (light scattering from the fibrils). Absorbent capacity and wicking rates were higher than for the higher water imbibition viscose rayon controls: this too being an effect of the microfibre content. However the most significant alteration in properties caused by fibrillation was in the dry and wet texture of the fabrics. Dried conventionally after hydroentanglement, hydrogen-bonding between the fibrils of the highly fibrillated fibres stiffened up the fabrics giving them an unattractive papery handle. Once rewetted however the material took on a suede or chamois-leather-like texture which proved interesting to makers of durable wipes for car and window cleaning and for artificial leather, especially for shoe construction where the inherent absorbency of the fibre could improve the comfort of leather substitutes made from, for example, splittable synthetic/lyocell blends. One unusual and perhaps unexpected application for fibrillated hydroentangled fabrics was cigarette filters, made from staple fibre or spun-laid from a fibrillated tow. Fibrillated lyocell wet-laid from refined fibre had been convertible into cigarette filters with considerably higher tar and nicotine retention than either cellulose acetate tow or the conventional semi-crepe tissue filters. While such filters were commercialised and are still made, they are expensive compared with the market leading products made from acetate tow and hence restricted to speciality cigarettes. Cigarette producers needed fibrillated lyocell tows in the same deniers as acetate cigarette-tow, but tows of this size could not be made on the lyocell production lines and there were obvious problems of pre-fibrillating the tow and converting it into filter rods at high speed. An intermediate solution was to fibrillate a parallel-laid lyocell staple web in hydroentanglement and slit this into widths to give the required “tow” denier. This proved potentially more economical and made higher quality tips than the paper route, but could not match the costs of the tow route. A solution to the pre-fibrillation problem which could be applied during fibre production involved high pressure hydroentanglement, either radially on round tows of the right denier, or conventionally on spun-laid webs which could then be slit to the right size. Clearly neither of these configurations of lyocell is producable on the production plants now installed to make conventional staple fibre. What if… It is interesting to speculate what the lyocell process and product range would look like if fibrillation could be regarded as an advantage to be built on and optimised rather than a problem to be solved at any cost:
Concluding RemarksCurrent lyocell, like the almost identical polynosic viscose rayons developed half a century ago, has so far proved to have similarly limited and variable potential in the fashion textile markets for which it, and the process for making it, was designed. The low-fibrillation fibre now produced is too expensive ever to reach the 2 million tonne scale confidently predicted in the project justifications of the early 1990’s. It is, like the polynosics, a fibre with unique properties restricted to niche - compared with polyester or cotton - applications where a return can be made despite its high price. Whether a cheaper, high-fibrillation version could improve lyocell’s prospects is still open to debate, but it would be a shame if it was allowed to go the way of the polynosic rayons without this version ever being tested in the hard light of commercial reality. Calvin Woodings N.B. The author is not affiliated to any company now producing lyocell fibre. Drainage rate is proportional to the mean pore size in the forming web, which in turn is related to fiber surface area. H A Coulsey and S B Smith, “The formation and structure of a new cellulosic fibre”, Dornbirn Conference, Austria 1995. CE Moss et al J App Poly Sci , 83, 13, 2002, 2799-2816 S A Mortimer and A A Peguy, J App Poly. Sci, 60, 3, 1996, 305-316. Dong Bok Kim et al, J App Poly Sci, 83,5,2002, 981-989 Dr C D Potter. “Wet-Laid Applications for Solvent Spun Cellulose Fibres”, INDEX 96 Congress, Geneva, February 1996 P.A. Johnson. “Courtaulds Lyocell for Papermaking Applications”, TAPPI Nonwovens Conference, Memphis, March 1997 WO 95/35399 Gannon et al (28/12/1995) to Courtaulds Fibres and USP 6,042,769 (28/3/20000) to Acordis Fibres. D. Cole and C.R.Woodings, “Solvent Spun Cellulosic Fibres in Nonwovens”, INDEX 90 Congress, Geneva, April 1990. USP 5,671,757 Woodings to Courtaulds Fibres 30/9/1997 C.R. Woodings, “The hydroentanglement of a range of staple fibres”, Impact Conference, Florida, March 1989. USP 5,738,119 Woodings et al to Courtaulds Fibres 14/4/1998 USP 5,839,448 Woodings to Courtaulds Fibres 24/11/98 “Regenerated Cellulose Fibres”; ISBN 1 85573 459 1; Woodings et al, published by Woodhead Publishing Ltd, Cambridge England (www.woodhead-publishing.com.) |
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