Yam crimp

The other factors that indirectiy influence yam crimp and o-FVF are nesting, ply waviness, side interaction of yams and yam overlap. If they are not in a model, it may mean that either these pecuharities have not been observed in the materials studied or they have not been taken into account. The overview aims at giving an overall picture rather than demanding all the models to have the same level of geometric refinement. 

In addition, the crimp ratio (i.e., the difference between the length of the yarn and the length of the fabric) can be determined for woven fabrics that feature yam undulations. The combination of the yam crimp ratio and the yarn modulus is needed to describe the tensile mechanical behavior of a composite fabric (Fig. 6.17). 

Even when crimp is fully developed it is easy to pull out (low energy) and difficult to translate into noticeably bulkier woven and knitted fabrics. It does however improve the absorbency and the cohesion of the staple (important in spun-yarn and nonwoven making) and gives a subtly different texture to woven fabrics. Coarse crimped rayon was the leading synthetic carpet fiber in Europe in the 1960s, but has since been replaced by the highly durable bulked continuous filament nylon yams. Crimp is most important in rayon used for hygienic absorbent products. 

It should be appreciated that lyocell fibres are stable in length when wetted and dried (i.e. they do not shrink). Fabric shrinkage arises only from the fact that the fibres and yams swell in diameter swelling in diameter forces yam crimp to increase so that the fabric will contract (shrink) and become thicker. Thickness is due to an increase in the yam crimp amplitude. 

Fabric shrinkage and contraction effects occur as a consequence of swelling in fibre diameter and the resulting increases in yam crimp. There are three particular factors that affect the amount of yam crimp development ... 

Processing tensions. For example, greater warp tensions will give either (a) more weft yam crimp or (b) greater residual warp shrinkage (or both ). 

Frequency and diameter of yarns yarn density. Where yams are more numerous and/or larger in diameter then yarn crimp will develop only to the point at which adjacent yams just touch. When such yams touch one another the fabric becomes jammed or locked so that no more yam crimp can develop without dismpting the alignment of yams in that plane. Jamming prevents further contraction in that plane but can readily force increased contraction (shrinkage) or fabric movement (creasing) elsewhere. 

Fibre and yarn swelling in diameter. Swelling adds to the jamming effect by thickening the yams. Fabrics that are apparently quite loosely woven when dry can become stiff and firm when wet. Again, this hmits the amount of yam crimp that can develop within the plane of the fabric (i.e. without buckling). 

This is an extension of the open width processing principles in combination with causticisation. The fabric construction should be devised so that there is space to allow yam crimp to develop. Causticisation will cause the crimp development and impart a memory which together with the fibre s high modulus and resilience will allow stretch with high degrees of recovery, equal to that achievable with elastane containing fabrics. It should be borne in mind though, that this is a comfort stretch rather than the power stretch associated with elastomerics. 

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