Thursday, 16 September 2004

Alternative Cellulose Conference – TITK Rudolstadt 1-3rd September 2004


 TITK 2004 Audience

Dr Bauer, MD of the Thüringische Institute of Textile Science (TITK) opened the conference observing that world lyocell capacity stood at 120,000 tonnes/year and the fibre was at a crucial point in its history. Would it remain a niche product produced only by Lenzing (who now own Tencel) or would it grow into a mainstream fibre? To grow it needed investment, but the barriers to entry were high and the markets for the fibre currently appeared saturated. TITK will continue to work on high value specialities using their version of the lyocell technology. He hopes to continue to increase volume and market penetration in speciality areas.

25 years of Tencel development

Pat White, Tencel Ltd's Technical Director reviewed 25 years spent taking the lyocell process from curiosity to reality. Charged in 1979 with developing a low cost, environmentally-friendly route to rayon which would allow the venerable viscose process to retire gracefully, he rapidly identified NMMO as the most promising solvent. Fibres made from NMMO were all but indistinguishable from the premium Japanese polynosic rayon in character, so the key developments were to devise a highly efficient recovery system for the expensive solvent, and to optimise the process to deliver acceptable throughputs despite the solvent's tendency to degrade cellulose explosively if pushed too hard. The success of the engineering development coupled with a strong fashion-led Japanese demand for fibre led to rapid expansion and Lenzing's entry into the technology. The result: excess capacity because the projected long-term increase in fashion-sales proved unsustainable. Fibrillation and its control has dominated process and market development from the start:
  • The inherent fibrillatability of the first fibres led to the nonwovens market being targeted while solutions to the fibrillation “problem” were developed.
  • Japanese success at harnessing fibrillation to develop peach-touch fabrics by enzymatic selection of fibril sizes led to the first major sales, and to the optimism for rapid plant expansion.
  • New techniques were developed (by Tencel Ltd and “pioneering” mills) to cope with the new fibre in dyeing and finishing.
  • Low fibrillation achieved by process optimisation increased process costs but did not make the fibre universally acceptable in textiles.
  • Low fibrillation using additional on-line cross-linking increased the range of textiles possible.
  • High fibrillation routes were developed to suit special papers and wet nonwoven processes.
Currently, lyocell sales of ~90,000 tonnes/year are equally split between nonwovens, standard fibre in textiles and cross-linked fibre in textiles, with disposable wipes being the biggest single market. For the future, Mr White foresaw:
  • The Lenzing takeover of Tencel will achieve the “critical mass” to allow further development of the technology and markets.
  • Variable-cost reductions by moves to lower cost pulps and improved operating efficiency.
  • Fixed-cost reductions by integration, maximising factory output, and higher sales.
  • Lower cost processing to garments in Asia allowing lower costs to the consumer.
  • Further rapid expansion of nonwovens markets.
  • New products in the pipeline will further reduce costs and expand markets.

    Pat White MBE

Filtration Products from Lyocell

Dr Kolbe of TITK presented results for tetrachloromethane (TCM) and toluene adsorption by lyocell fibres and nonwovens containing 100% active carbon on cellulose i.e. 50/50 mixtures of carbon and cellulose. The carbon from coconut shells had been ground to <25 micron diameter and the final fibre had an apparent diameter of 60 microns, with 10 micron particles clearly visible at the surface. The efficiency of adsorption for carbon in fibre over carbon alone was 60% with TCM and 55% with toluene. 50/50 zeolite/lyocell fibres had also been made, but here the efficiencies were much lower, <10%, presumably due to the effects of the alkaline dope on the zeolite structure. The carbon-containing nonwovens had been tested against specifications for cigarette filters and water filters but no results were given. One of the problems with this technology relates to the inability to use the best adsorbing carbons in the lyocell process, because these do not give adequate spinning stability, maybe due to particle size distribution after grinding.

Edible Food Casings from Lyocell

TITK have collaborated with Kalle GmbH on replacing the collagen sausage skins with an edible version based on their lyocell tubular film “wurst” casing. Dr Meister of TITK pointed out that most collagen now comes from intestines processed “abroad” and suggested that the risks of biological contamination could be high. Attempts to replace collagen with alginate or casein-based substitutes have both failed. The problem with lyocell film relates to its “chewability” or rather lack of it, and so TITK sought to improve this by adding globular proteins such as casein, gluten or soyabean protein. Whilst giving the desired improvement in chewability (i.e. reduction of wet strength) these unfortunately caused an unacceptable increase in wet extension. They deduced that powdery fillers which were insoluble in NMMO should correct this and the resulting “Nalobite”, a three-component film comprising 33% cellulose, 9% corn protein and 58% bran appears to provide an acceptable balance of wet and dry properties. The process now needs scaling up to allow extensive application trials. Asked if the product met FDA regulations, Dr Meister indicated that all materials were FDA approved. What about NMMO residues? These had been measured at about 15 ppb in the finished product. Could they do blown films? Not yet, but they have a project awaiting food industry requirements.

Innovative Dyeing and Finishing of Lyocell

Jim Taylor of Tencel Ltd examined the various commercial lyocell fabric processing methods that have evolved over the last decade, ranging from those required for fully-fibrillated highly casual jeanswear to those for classic formal garments:
  • Full garment wash : abrasion and fibrillation of garment seams occurs giving a very casual appearance and texture. Long treatment times and high abrasion meant sewing threads and garment trim choices are restricted to the more robust products.
  • Reduced garment wash . The fabric is degraded by magnesium chloride treatment prior to make up. Garment processing can then be more gentle and hence a wider range of trims can be used.
  • Softner-only garment wash : The fabric is protected by resin finishing before make-up so that a semi-formal “smart-casual” unfibrillated appearance and soft texture arises from the garment wash.
  • Fibrillated Mill Finish : a more formal look is created by fibrillation/defibrillation using enzymes in air-jet processing. Again the magnesium chloride pre-degradation can be used to shorten the process.
  • Non-Fibrillated Mill Finish : a classic look is obtained by avoiding fibrillation altogether, either in open-width cold pad batch or fully continuous jig or beam dyeing. Resin finishing is essential to prevent fibrillation in use, so soft-touch cannot be achieved. Alternatively the non-fibillating A100 Tencel or Lyocell LF (cross-linked during fibre production) can be used on conventional cotton finishing systems. A further possibility is to resin treat a fabric after the Fibrillated Mill Finish route.

  • Jim Taylor

Finishing Lyocell Fabrics with Enzymes

Alina Popescu of the Rumanian R&D Institute for Textiles and Leather has experimented with the enzymatic finishing of woven lyocell fabrics. The sequence of experiments used in air-jet processing were:
• Washing and/or causticisation to create primary fibrils.
• Enzymatic defibrillation
• Dyeing which creates secondary fibrils
• Bio-polishing with enzymes to create a more uniform distribution of secondary fibrils
• Tumbler processing
• Further enzymatic defibrillation
• Drying, which creates a few fibrils but the final fabric looks unfibrillated.

“Then Airflow” or “Thies LuftRoto” open-width jet machines gave the best results. Rope processing was possible but creased fabrics above 200gsm.

Conductive Lyocell for plastic reinforcement

Dr Niemz of TITK showed that loading lyocell with carbon and compounding the resulting fibres with plastic increased the plastic's conductivity significantly more than adding the same amount of carbon directly to the plastic. The effect was dependent on fibre length and fineness: the longer and finer the fibres the higher the conductivity for the same total carbon load. Clearly the fibres were overlapping in the final structure forming conductive pathways which would not exist in carbon particle dispersions. Lyocell loaded with 30-40% of carbon black has a resistance around 1 ohm/cm compared with 10 12 ohms/cm for pure lyocell. At this loading it's mechanical properties are similar to viscose. Scaling up the compounding process indicated that fibre breakage was a problem. TITK's pultrusion rig allowing the preparation of master batches of fibre reinforced plastic have allowed the inclusion of longer fibres and helped to overcome this problem.

Lyocell, viscose and polyester spin-finishes compared

Bernhard Goosens of BGB Stockhausen reported results from an experimental programme to measure the effects of 21 different spin-finish formulations on the frictional characteristics of continous filament yarn.
  • Sulphonated paraffins and ethoxylated fatty acid esters reduced dynamic fibre/metal friction on lyocell and viscose more than polyester.
  • For best dynamic fibre/fibre friction the three fibres require different finishes, the ethoxylated fatty acid esters and sulphonated paraffins again being best on lyocell.
  • For static fibre/fibre friction lyocell is also unique, but again the ethoxylated fatty acid esters appear best.
Mr Goosens was surprised that the smooth-surfaced lyocell fibre appeared so different to viscose or polyester. He felt there was something special about its surface which needs further investigation.

Silver-loaded SeaCell®

Christina Hipler of Friedrich-Schiller University, Jena has, with funding from Alceru-Schwarza, investigated the antibacterial and antifungal effects of the Alceru-Schwarza fibre “SeaCell Active”, a silver-containing version of the lyocell/alginate alloy fibre. The fibre proved effective against Candida albicans, C parapsilosis, C glabrata, C tropicalis, C krusei, Staph. aureus, and E coli in in-vitro testing. A 100 person patch test proved that it caused no allergic reactions or sensitization of skin. Ms Hipler suggested it could be used as a component of bed-sheets, shirts, socks, shoes, underwear, sportswear, protective clothing, hygiene products and home textiles. It would be especially useful for people with atopic dermatitis, transpiration problems, diabetes or obesity. The fibre contains 5,000-10,000 ppm of silver and should be usable in 5-20% blends with conventional fibres in textiles and nonwovens.

Bioactive Cellulosics

Nina Kotelnikova of the Russian Academy of Sciences reported the first reactions of modified microcrystalline cellulose (MMC) with biologically active substances such as N-dimethyl benzyl alkyl ammonium chloride (DMBAA), polyvinylpyrrolidone, and its derivatives Sovican and Catapol. She had also reduced MMC with silver nitrate and showed that metallic silver forms as nano-particles (35nm) in the cellulose. All the new compounds were shown to be antimicrobial.

Lyocell Fibre Structure

Dr Mohammad Rous of Lenzing , winner of this years “Most Promising Young Scientist” award, described the use of Raman spectroscopy on fibres dyed with fluorescent additives to elucidate further details of the fine-structure of lyocell. Calcoflour (fluorescent dye- MWt 960) could be seen to penetrate to the centre of never-dried fibres which collapsed irreversibly on first drying. (Never-dried modal and viscose fibres did not allow the dye to penetrate at all.) On dried fibres the dye penetrated part-way into the fibre, this depth of penetration increasing with alkalinity. Thus, after 24 hours immersion, the Calcofluor could be detected to a depth of 2.5 or 3.7 microns (neutral or caustic-dyed respectively). The observations fit the hypothesis that lyocell has a thin semi-permeable membrane on the outside of an accessible skin region covering a denser, less-accessible core.

Another Solvent for Cellulose

Dr Christopher Michels of TITK compared 5 routes to cellulosic solutions, 4 of these familiar (acetate, viscose, cupro, lyocell) and one unfamiliar: 1-butyl-3-methylimidazolium chloride or BMIMCl. BMIMCl melts at 60-70 0 C to give a yellow liquid stable up to 250 0 C with no measurable vapour pressure. It dissolves cellulose under conditions similar to NMMO with the difference that BMIMCl has to be totally non-aqueous. The solution reprecipitates in water to give fibrils comparable to those from NMMO or cellulose xanthate. Interestingly, the BMIMCl/cellulose dope (12% cellulose of 530 DP) has an endothermic melting peak 45 0 C below the melting point of the solvent and is thus stable at room temperature. Furthermore it only begins to oxidise the cellulose at 213 0 C, and is therefore dramatically more stable than an NMMO solution. Fibre properties from air-gap spinning into water show BMIMCl to be higher in tenacity and modulus but lower in extension than conventional lyocell fibre.

Dissolving Cellulose in Molten Salts

Dr Thomas Heinze of the Friedrich Schiller University, Jena classified cellulose solvents into derivatizing and non-derivatising, the latter being further subdivided into aqueous and non-aqueous. Comprehensive lists of each type were presented. Of particular novelty were the molten inorganic salt hydrates of the general formula LiX.nH 2 0 where X = iodide, nitrate, acetate or chlorate. Of these, the LiClO 4 .3H 2 0 salt in molten form would dissolve 1500 DP cellulose to form a clear yellow solution within minutes. NMR spectra could be obtained from these and cellulose II could be regenerated.
Dr Heinze was also interested in fully homogeneous processes for making cellulose derivatives by etherification or esterification. Acetylation to a Degree of Substitution (DS) of 2.4 could be achieved in 3 hours by adding an excess of acetic anhydride to a molten salt solution of cellulose at 130 0 C. In this case the molten salt was a mixture of sodium, potassium and lithium thiocyanates, but LiClO 4 .3H 2 0 also worked at 110 0 C. Carboxymethyl celluloses with a DS of 2.2 and unusual structure could be obtained from dimethylamine/lithium chloride solutions using monochoracetic acid and non-aqueous NaOH powder in a one step procedure. The same molten salt solution could also be used to prepare the silyl derivative of cellulose.
In a second paper on a similar theme, Dr Steffan Fischer of the Fraunhofer Institute, Potsdam discussed the use of the above mentioned molten salts in acetylation and carboxymethylation reactions. He reported that LiClO 4 .2H 2 0 while only swelling cellulose, did so sufficiently for it to convert from the I to the II form. LiClO 4 .4H 2 0, and LiClO 4 .5H 2 0 also swell cellulose but not as much as the dihydric form. He too found molten salt hydrates effective and efficient routes to cellulose solutions and derivatives of cellulose with high DS. Perhaps of most interest was the slide of apparently homogeneous blends of cellulose and polyacrylonitrile dissolved in, and reprecipitated from, a molten salt, showing a clear glass transition temperature.

Melt Spinning of Silyl Cellulose

Thomas Karstens of Rhodia Acetow AG reviewed their joint work with ITCF Denkendorf on the melt spinning of cellulosic fibres via the silyl derivative. Bis-(trimethyl)-silylcarbamate (BSC) penetrates cellulose easily and swells it sufficiently to allow silylation to occur salt-free and without catalyst. The resulting silyl cellulose could be melt-spun without additives at temperatures below 250 0 C and at speeds up to 1000 m/min. Hydrolysis of the silyl groups (1N HCl for 1 minute at 40 0 C) yields pure cellulose fibres and hexamethyldisiloxane, the latter being recyclable into BSC via the catalysed insertion of isocyanuric acid obtained from the thermal decomposition of urea. Lab. Scale operation has been demonstrated at high efficiency and the process is felt to have great potential for scale up. The fibres from the laboratory operation are 0.8dtex after de-silation, with a tenacity of 34 cN/tex and an extension of 4.5%. Wet properties are slightly better than the dry. Fibre Crystallinity = 55%.
The BSC is a costly analytical reagent so the team have synthesized it from the more readily available hexamethyldisilazane (HMDZ) and ammonium carbamate with a yield of 97%.

LIST dissolution of cellulose in NMMO

Two papers covered the progress made by LIST with their horizontal “Discotherm” dissolvers since Courtaulds rejected them as unsuitable for Tencel and chose “Filmtruders” in the early 1990's. Dr W Zhao of LIST Singapore mentioned 6 years of development with TITK resulting in the single-step dissolution process suitable for speciality products up to 1000 tpa. This appears to be the basis of the Alceru-Schwarza range of special fibres such as Sea Cell – see later. The first semi-commerical unit has now been ordered and will be installed in China for start up in mid 2005. Exactly where was not revealed, but the assumption is that it will be on a pilot line for the production of standard lyocell fibres.

Andreas Diener also of LIST AG pointed out that since their rejection by Courtaulds they had developed to achieve:
  • Highest quality fibre spinning
  • Safer use of NMMO
  • Flexibility of throughput
  • Ability to use cheaper pulps than a Filmtruder
  • Lower operational temperatures, (<100 0 C) these being safer, with less NMMO decomposition and higher recovery rates
  • Operation without stabilisers is possible (patent reasons?)
  • Water quenching in the rare event of an approaching exotherm
  • Better for blends with fillers, polymers, dyes etc.
Asked how big the new Discotherm system could be built, Mr Diener thought 10,000 tpa of cellulose thoughput would be possible, i.e. below Filmtruder capabilities.

Lyocell Fibre Spinning

The abstract promised a review of the methods used to increase the efficiency of fibre formation in commercial Tencel spinning. Malcolm Hayhurst of Tencel Ltd Spondon described a computer simulation of fluid flow in the air-gap illustrated with unit-free graphs to show how well the model had been validated against real measurements taken in spinning. The air-gap model was said to have been central to understanding the fundamental parameters which must be controlled to enable viable lyocell production. Tencel Ltd has also modelled the dope flow in the spinneret and the spin-bath flow around the filaments, but Mr Hayhurst could not expand on this.

Characterisation of Exotherms

Frank Wendler of TITK described the use of thermal analysis and UV/Visible spectroscopy to study the autocatalytic degradation of cellulose in NMMO solution. Extinction-time graphs at various temperatures and pressures, with and without stabilisers and other additives were measured using both a mini-autoclave and a Radex security calorimeter. Exotherm onset conditions could be established with some accuracy and these showed the importance of propyl gallate stabiliser in raising the onset temperature by 20 0 C. However two of the additives favoured by TITK to convert lyocell into speciality fibres, active carbon and ion exchange resins, both destabilised the solution and exothermed after 3-4 hours contact at temperatures around 130 0 C. This observation presumably led to the topic of the next paper – late injection of additives.

Lyocell with Additives

Mr Jürgen Melle of TITK compared static and dynamic mixers for mixing additives into lyocell dope just before extrusion. He chose the dynamic flow mixer “DLM HY HO” from Indag Maschinenbau as the most successful with the simplest design, easy maintenance and high reliability. This mixer was used to blend additives pre-dispersed in an 80/20 NMMO/water solution with the main dope stream to give 50/50 alloys of cellulose. Pigments and dyes could also be added to create spun-dyed fibres.
Superabsorbents , carbon and ion-exchange resins could now be safely incorporated by the new method, and clean-down time between runs was dramatically reduced. Injection of microspheres showed that the late injection process could be used to increase additive levels up to 160% on cellulose, at which point the fibre tenacity had only fallen to 17 cN/tex.

Confocal Raman Spectroscopy (CRS) on Cellulosics

Thomas Röder of Lenzing Pulp illustrated how CRS could allow the analysis of microscopic areas of fibres or their cross-sections to identify chemicals and elucidate the structure of the cellulose. CRS on sections through wood cells differentiated lignin and cellulose, and if the wood had been impregnated with melamine for composite use, the distribution of the melamine could be visualised and quantified. For instance while the lumen contained 100% melamine, the cellulose in the cell walls could be seen to have absorbed 11% melamine.
Illumination of the samples with polarised lasers, first parallel to and then across the fibre axis allowed crystal orientation to be determined with accuracy similar to wide angle x-ray techniques. The technique takes 6-7 minutes per fibre and Mr Röder suggested using three fibres and taking 10 measurements per fibre. Total chain orientation (as well as crystal orientation) would be a possible output when the data analysis techniques had been refined further.
Additive distribution could be visualised by the technique if the additives were good Raman scatterers. Quantification of additive levels at different points in the fibre was not yet possible.

Chromophores in Cellulosics

Thomas Rosenau of the University of Natural Resources and Applied Life Sciences at Vienna described a procedure to allow the identification of ppm-ppb traces of coloured matter in viscose or lyocell fibres. The fibres were extracted with a boron trifluoride-acetic acid complex, said to be more laborious but more effective than straightforward methanol extraction. Sodium sulphite was also needed to prevent oxidative phenolic coupling which would give spurious results. Cotton linters, being free of chromophores, were used as a control to check that the extraction method did not introduce any colours. Comparisons of lyocell pulp and fibre showed that the chromophores came from the process not the pulp, a fact which may not be true if cheaper pulps were used. The lyocell process created 6 primary chromophores, i.e. those arising from the degradation of cellulose, hemi, and lignin, and 5 secondary chromophores i.e. nitrogenous compounds arising from the solvent and its reactions with the primary chromophores. Structures for each were presented. The secondary chromophores have high extinction coefficients and can be seen as coloured even in nano-molar solutions. They are destroyed by peroxide or hypochlorite. Total chromophore concentration in lyocell was put at <10ppm.

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