Correlation of regenerated fibres morphology and surface ... - Lenzing

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Lenzinger Berichte, 82 (2003) 83-95 p 2 γ m (13) s = d 2 γ b (14) s = Results and discussion X-Ray analysis Small angle X-ray scattering SAXS. All types of regenerated cellulose fibres exhibit a welldefined maximum on the meridional SAXS diagram (Figure 3) indicating periodical structures due to alternation of crystalline and amorphous regions along the fibre axis. The long period values were calculated (Table 3) 2θ 2 .I(2θ) 2θ 2 .I(2θ) 2θ 2 .I(2θ) 88 from the peak position and are almost similar for all untreated fibres (approx. 13.5 nm). This This This finding is in good agreement with the values reported [8,17]. Fibre’s treatment in alkaline solution or a bleaching medium causes distinct changes. In the case of viscose fibres an increase of the long spacing for about 15% occurs regardless of the treatment conditions applied. Modal fibres show a significant change of long spacing (by about 15%) only in case of alkaline treatment. The opposite effect was observed with lyocell fibres showing a decrease of long spacing of about 10% (bleaching) and 5% (mercerisation). 2θ 2θ 2θ Figure 3. Plot of 2θ 2 . I(2θ) vs. 2θ of meridional SAXS curve for the untreated viscose, modal and lyocell fibres, respectively. Viscose fibres are of low crystallinity but highly accessible to different media due to their mainly amorphous molecular arrangement and an extensive inner surface. This morphology enables a better sorption of water and low molecular species, compared to the two other fibres [11,28] causing major structural reorganization shown by the changes in long spacing. Cellulose molecules in CMD fibres are well oriented, have a higher degree of polymerisation and are densely packed. A nonextensive void system is formed because of the spinning process at high drawing ratios [9]. This structural parameter is the reason for the lower hydrophilic capacity of modal fibres in comparison to viscose or lyocell fibres. Mercerisation is the only treatment process that causes significantly increased long spacing and promotes the swelling process. Lyocell fibres have a more complicated structure. They show high hydrophilicity caused by the fibres larger void system (comparable with viscose) and not by the fibre’s superstructure. It was shown that water swollen solvent spun fibres contain more separated elementary fibrils and less nonswelling clusters than fibres spun from cellulose derivates [17]. The interstices that separate single elementary fibrils, the interfibrillar and intrafibrillar voids are accessible to water, sodium hydroxide and other treating media. The dense fibre inner structure prevents major structural reorganization if the lyocell fibres are bleached or mercerised without tension. Swelling does not create new voids nor do existing voids disappear; only the voids diameter is increased [16]. In the case of viscose and modal fibres the increase of long spacing is accompanied by a more pronounced crystallinity increase that could additionally influence changes in the periodical structure.
Lenzinger Berichte, 82 (2003) 83-95 Fibre Treatment Long period nm Crystalli-nity index xfc Orientation function 89 fc *Voids volume cm 3 /g *Voids diameter nm *Inner surface m²/g Tenacity cN/tex Viscose untreated 13.5 0.25 0.580 0.68 3.1 439 22.0 bleaching 15.5 0.25 0.494 20.0 mercerisation 15.6 0.38 0.722 19.5 Modal untreated 13,5 0.37 0.706 0.49 2.4 409 34.0 bleaching 13.7 0.44 0.583 32.0 mercerisation 15.5 0.57 0.561 30.5 Lyocell untreated 13.8 0.44 0.664 0.62 3.0 432 33.0 bleaching 12.3 0.46 0.677 34.5 mercerisation 13.0 0.50 0.768 27.0 Table 3. Long periods, crystallinity index, Herman’s orientation function, *voids volume, diameter and inner surface of voids (determined using Size Exclusion Chromatography) and tenacity of untreated and treated fibres Wide angle X-ray scattering WAXS. All analysed fibres show intensive scattering with negligible differences between the solvent spun and the two other fibre types. Fibre treatment always causes separation of the scattering function’s peaks. The obtained results are presented in Table 3, one example of the randomised scattering curves of viscose is shown in Figure 5. Crystallinity index. The crystallinity index (Figure 6) of viscose and modal fibres differ as expected, viscose has a lower crystallinity (0.25) than modal (0.37), lyocell fibres have the highest crystallinity index of 0.44. Any kind of treatment significantly increases the crystallinity index, which agrees very well with those results given in the literature [15]. Lyocell fibres are more stable than viscose or modal fibres in alkaline medium and therefore crystallinity increase is more pronounced for derivate spun cellulose fibres in comparison to solvent spun fibres. Viscose fibres crystallinity index is strongly dependent on the treatment conditions with an almost negligible increase caused by bleaching and a large increase caused by mercerisation (52%). The bleaching of viscose fibres may cause some oxidative damage and therefore a limited increase of crystallinity, nevertheless, it causes a significant increase of long spacing. A more pronounced crystallinity increase is observed after the bleaching of CMD (19%) and a strong one (53%) after alkaline treatment. Longer cellulose molecules in CMD fibres are less sensitive and more resistant to oxidative damages and therefore the influence of the alkaline environment predominates and an increase of crystallinity is observed. The large degree of polymerisation of about 640 and the more complex structure are the reasons why lyocell fibres are more stable under any of the treatment conditions. A comparable small increase of crystallinity index occurs, however, and a correlation between the pH value of the treating medium and the crystallinity increase can be assumed. The pH value at bleaching is 10.7 and at mercerisation 12.8, the increases of crystallinity 5% and 14%, respectively. Iodine sorption data confirms the X-ray results of crystallinity changes (Figure 7) especially when chemical changes caused by the oxidative bleaching process are taken into account. The lower the ISV the higher is the degree of crystallinity. This data also show that there are only very small differences in lyocells crystallinity caused by both treatments. However, the degree of cellulose products oxidation influences the iodine sorption. Crystalline orientation. Fibres reactivity and accessibility as well as tensile properties are mainly influenced by crystalline orientation. This parameter shows such different trends for each fibre type, that no unique model for its explanation could be established. Warwicker
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Correlation of regenerated fibres morphology and surface ... - Lenzing

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