Rayon Lyocell

Environmentally Friendly High Wet Strength Rayon—Lyocell... 

Whereas cotton represents the purest form of cellulose, wood contributes the vast majority of it. Depending on end use, paper, board or chemical grade, wood conversion to some type of cellulose-rich fiber amounts to about 200 x 10 t/a a worldwide [29]. This compares to about 15 X 10 t/a cotton [13]. Only a minor amount, ca. 7 x 10 t/a, of high-purity cellulose is used for chemical purposes, mainly regenerated fibers (viscose rayon, lyocell) and derivatives (esters, ethers). Smaller amounts are also used for hydrocolloids [microcrystalline cellulose (MCC)]. 

Regenerated fibres from cellulose - R on and viscose are regenerated cellulosic fibres. The issue that is often brought up about regenerated fibres is that the processing uses harsh chemicals and is environmentally damaging. Tencel is considered a more environment-friendly regenerated fibre, and it has different properties from rayon. Lyocell/ Tencel is a cellulose-based fabric from farmed trees. 

N-McLhylmorpholine-N-oxidc monohydrate, a tertiary, aliphatic amine N-oxide, is able to dissolve cellulose directly, i.e. without chemical derivatization, which is used on an industrial scale as the basis of the Lyocell process [ 1, 2], This technology only requires a comparatively low number of process steps compared for instance to traditional viscose production. Cellulose material - mainly fibers - are directly obtained from the cellulose solution in NMMO no chemical derivatization, such as alkalization and xanthation for rayon fibers, is required [3]. The main advantage of the Lyocell process lies in its environmental compatibility very few process chemicals are applied, and in the idealized case NMMO and water are completely recycled, which is also an important economic factor. Even in industrial production systems NMMO recovery is greater than 99%. Thus, compared with cotton and viscose the Lyocell process pertains a significantly lower specific environmental challenge [4]. Today, Lyocell fibers are produced on an industrial scale, and other cellulosic products, such as films, beads, membranes and filaments, are also currently being developed or are already produced commercially. 

The process of cellulose regeneration in the form of lyocell fibers is significantly simpler than that of the viscose rayon process. It is illustrated in O Fig. 16. A solution containing 14% cellulose, 10% water, and 76% NMMO plus stabilizers is extruded at a temperature slightly above 100 °C into an aqueous NMMO-bath from which cellulose is precipitated [74,75]. The extrusion has been described as a melt-spinning process that has recently made it possible to manufacture cellulosic self-bonded meltblown nonwovens as well [76]. 

The use of acid cellulases are recommended for fast treatments and neutral cel-lulases for more severe treatments when marked effects are required [56,91 ]. Endo-enriched acid cellulase is found to be best for easily weekened fabrics such as 1 inen and viscose rayon. Standard whole acid cellulases are best for sturdy fabrics sucli as lyocell, modal rayon and heavy weight cotton [92]. 

Chang Lang et al. [CHA 05] Viscose (Rayon and Lyocell) Cellulase produced by Trichoderma viride... 

CHA 05] Chang Lang Y., Chung Hee P., Yun-kyung K., et al, BiodegradabiUty of viscose Rayon and Lyocell fibers , Journal of the Korean Society of Clothing and Textiles,... 

As Rosenau et al. [49] indicated that paper grade pulp, unbleached chemical pulp, cotton and rayon fiber wastes, or even paper wastes can be used as raw materials for Lyocell fiber production, even though problems with spinnability may be encountered in some cases. In preparation for spinning dope, a 50-60% aqueous NMMO is used with the addition of 0.01 0.10% antioxidant to prevent cellulose degradation. A typical antioxidant is PG [50]. In a typical Lyocell industrial process, the slurry is produced from cellulose pulp and an aqueous NMMO solution. Typical compositions are 50-60% NMMO, 20 30% water, and 10-15% pulp [48]. Subsequently excess water is efficiently evaporated at temperatures lower than 150°C and... 

As for viscose fibres, these are mostly produced as staple fibres for textile and nonwoven applications. In 2011, world production was 3.246 million tons [49] while filament yam for textile and technical applications reached 332 000 tons in 2011 [49] with a share of technical yarns of 56 000 tons. Technical viscose fibres, also called rayon or viscose rayon, are used mainly as carcass reinforcing fibres in fast-running and run-flat tyres. Lyocell fibres are produced only as staple and virtually exclusively by Lenzing AG, Austria, with a production capacity of 140 000 tons in 2011 [50]. 

Fig. 18.1 Cross sections of rayon tire cord (/eft), Lyocell (middle), and cellulose carbamate fibers as obtained by transmission electron microscopy...

As shown in Fig. 18.1, fiber cross sections clearly differ between viscose (including rayon tire cord as shown) on the one hand and Lyocell (NMMO) and Carbamate fibers on the other, as demonstrated in the transmission electron micrographs of ultra thin cuts (60 nm). 

Rayon (left) has a lobed cross section which is typical for fibers spun industrially by the viscose process. In contrast, Lyocell (middle) displays a round shape similar to the cellulose carbamate example (right). (In all the micrographs, the dark streaks are overlaps of sample material caused by the cutting movement of the ultra microtome knife.) Cross sectional shape and pore structure are strongly dependent on the precipitation conditions. For NMMO fibers [11] it was shown that harsh precipitation with water gave dense, round structures while softer precipitation into alcohols lead to a skin-core stmcture with dense skin and a porous inner region. StiU, cross sections are round in contrast to viscose-ty fibers. 

With 25% pure rayon, a moderate increase to 80°C can be obtained, as shown in Table 18.3. In this table, the HDT-A values for pure PP and cellulose man-made fiber-based composites are given together with other important mechanical characteristics. In order to increase the HDT to more than 100°C, three routes were pursued (1) change the cellulose fiber type from rayrai to Lyocell, (2) use fiber mixtures with natural fibers or LyoceU, and (3) add organic fillers (like talcum) to the matrix. 

Mixtures of rayon with jute or Lyocell show the desired balance of high HDT and a high level of the other properties. HDT values are shifted to above 100°C, stiffness is increased slightly and the losses in impact strength remain below 10%. As compared to Lyocell, rayon again proves to be an excellent impact modifier. 

A relatively recent addition to the rayon family is Lyocell, trade named Tencel. The fiber is claimed to have high wet integrity and dry and wet strengths comparable to those of cotton, and is therefore suitable for use in regular apparel and other durable products. Being also a manufactured fiber, which means that its size, shape and properties can be varied as needed, the new rayon fiber can be engineered for use in more intimate or demanding absorbent applications. 

The polynosic rayons enjoyed a revival of fortimes in the 1990s when the new lyocell fiber produced by Courtaulds (Tencel )—a solvent spim fiber with polynosic properties— became popular in ladies apparel and denim. By 2001 however the Toyobo Tufcel plant in Japan had been closed and at the time of writing, it is likely to be transferred to China where lower labor costs would allow it to compete more effectively with lyocell (see below). 

Current lyocell, like the almost identical polsmosic viscose rayons developed half a century ago (33), has so far proved to have similarly hmited and variable potential in the fashion textile markets for which it, and the process for making it, was designed. Whether a cheaper, high-fibrillation version would improve lyocell s prospects in nonwovens is still open to debate. 

Lenzing of Austria commenced production of their lyocell fibre in 1997. This rapid expansion led to a temporary overcapacity and no further production occurred until Lenzing expanded their capacity by 20,000 tonnes per year in 2004. Development of a continuous filament spinning process by AKZO/ Tencel continued throughout the 1990s but has not yet been commercialised because of the decline in the market for filament rayon and the high investment cost required. 

Wood pulp is the principal raw material of the lyocell process in terms of cost and volume. The grade used is similar to the dissolving pulp used for viscose rayon but has a slightly lower degree of polymerisation (DP) Tencel fibres have a DP of 500 to 550. The pulp is pulled from the reels into a shredder, which cuts the pulp into small pieces for mixing with the amine oxide solvent. The amount of pulp fed to the mixer has to be accurately measured so that the cellulose content in solution is closely controlled. 

PET, polyolefin (PP/PE), nylon, cotton, rayon, wool, lyocell, modacrylic. 

The conversion of cellulosic wood pulp into a textile fiber dates to the earliest periods of synthetic fiber manufacture viscose rayon and lyocell processes are well known. However, it is stiU conceivable to improve upon... 

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