Cotton internal stresses

In this section, we examine the internal stress within the cotton fibers treated with SMPU and DMDHEU. Eichhom and Young (Eichhom et al, 2001 Eichhom and Yoimg, 2001,2003 Eichhom eta/., 2003) showed that the internal stress of cellulose fibers can be measured by shifts of C-O-C vibrations in the Raman spectrum an increase in the wave intensity indicates a reduction of the internal stress, and vice versa. Using the strain-induced shift in the special Raman peak, incorporated with micro-Raman spectroscopy, is a technique unique to micro-mechanics. A Raman spectrum of imtreated cotton is shown in Fig. 10.15. The C-O-C peaks are 1096 and 960cm , depending on the internal stress of the cellulose chain. 

Change of internal stress of cotton after SMPU/DMDHEU treatment (Figs 10.16 to 10.19) show the Raman spectrams of the C-O-C peak at nearly 1095 cm for PU, DMDHEU and PU mixed DMDHEU treated cotton. 

The shifts of internal stress obtained above could be explained by the differences in the finishing process and the reaction between the SMPU and DMDHEU on the cotton fabrics. 

The shape memory effects of SMPU as a finishing agent of cotton have been reviewed at macroscopic and microscopic levels. At the macroscopic level, the shape memory effects enhance the surface appearance of cotton fabrics. The fabrics form a permanent shape after the curing process, which are deformed in normal use and then returned to a permanent shape after thermal stimulation. At the microscopic level, the SMPU layer deforms and recovers in the dry-cure process, resulting in changes of internal stress within the cellulose chains. 

As mentioned in the introduction to this paper, scientific study has concentrated on the tensile mode. Except for two forms of break in cotton, all the tensile failures discussed in this paper consist of breaks that run transversely aeross the fibre. However, the fibres arc fairly highly oriented, so that the bonding across the fibre is much weaker than along the fibre. Transversely, there are weak intermolecular bonds plus a small component of the covalent bonding. In use, failure is rarely due to a direct tensile overload, unless this is on fibres weakened by chemical degradation. The common forms of wear in use are due to weakness in the transverse direction, related either to shear stresses or to axial compression. There is no detailed structural prediction of the response to shear stresses or axial compression at a molecular or fine-structure level. All that one can say is that at a certain level of shear stress cracks will form and that at a certain level of axial compressive stress the structure will buckle internally. What can be described is how these stresses occur. 

Within this area, nonwovens find application as internal product components, such as support and cover materials for mattresses as well foam replacements. As discussed, bonded polyester wadding (used in low stress applications), thermally bonded nonwovens, and nonwovens laminated with woven or knitted fabrics to provide covers with high dimensional stability are used in the production of foam-backed mattresses for upholstery as support and cover materials. Needled waddings and paddings are incorporated into furniture as insulation and comfort layers. Fibres used for such applications include recycled natural and synthetic fibres obtained from waste clothing, bast fibres, cotton, and virgin synthetic fibres, such as PET, PP, and acrylic (Anand et al., 2007, p. 253). 

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