Cold sizing of non-conventional worsted warp yarns

Sizing is essential to improve weavability. However conventional sizing is not suitable for wool because of its heat sensitivity. Cold sizing can only solve this purpose, say Rajesh Mishra & B K Behera

On the weaving machine, the warp yarns are subjected to several types of actions i.e. cyclic strain, flexing, abrasion at various loom parts and inter yarn friction. To ensure less number of breaks, the warp yarn must withstand these complex actions. At the same time, weaving technologists have to understand the behavior of warp, when they are subjected to these complex stresses. In order to know effect of these action, it is essential to study the breakage mechanism of warp yarns.

Weavablility of Sized Single Worsted Yarns

Performance of worsted yarns during weaving

It is well known that the breakage of spun yarn normally occurs either due to inter-fiber slippage or due to their breakage. Unlike the short-staple spun yarns, worsted yarns show a different behavior with regards to weavability and breakage phenomenon. This is because, the staple length of wool fibres may be as high as 7 cms in contrast to a length of maximum 4 cms in case of cotton. Failure or worsted yarns therefore is not because of fibre breakage but, due to slippage only. Longer staple length facilities for a high elongation at break.

Weavability of unsized worsted yarns

It is a common practice in worsted weaving, to use 2-ply warp yarns. Thus an experiment was carried out to compare the weavability of 2-ply worsted warp yarns with equivalent single yarns of the relevant linear densities that are used for worsted suitings. A wide variety of worsted suitings. A wide variety of worsted yarns were taken ranging from 100 per cent wool to blends like wool-tussah silk with three different blend proportions, wool-mulberry silk, wool-linen, wool-cotton etc. 100 per cent silk yarns were also tested for comparison purpose. The weavability tester developed by Sulzer-Ruti used for measuring the average cycles for breakage of the yarns.

It is normally expected that the 2-ply yarns should have better weavability as compared to equivalent single yarns as is the common prejudice and practice in industry.

But, contrary to this, most of the single yarns of 100 per cent wool, blends of wool with tussah and mulberry silk gave better results than their corresponding 2-ply yarns when tested on the weavability tester. Only in case of 60:40 wool-tussah blend and 70:30 wool:linen blend the 2-ply yarns break after a higher number of cycles as compared to the single yarns. These exceptions in wool:silk blends may be due to the adhesivity of silk with wool fibres. Even in the case of 60:40 wool:tussah silk blend, the difference in weavability between the single and 2-ply yarns is not significant.

In 100 per cent wool yarn also the single 40 Nm yarn breaks after more number of cycles than 2/78 Nm yarn. It can be inferred that there is loss of twist in component yarns due to doubling operation, which leads to lower weavability of plied yarns. Moreover breakage of any one component yarn results in yarn failure and it is more relevant in case of extra fine worsted yarns of 78-100 Nm range. Wool fibres have better cohesiveness among themselves due to the fibre crimp and and when the number of fibres in the cross-section are higher in 40 Nm yarn, this results in higher weaving abrasion resistance on the weavability tester. Mulberry silk shows better adhesivity with wool as compared to tussah silk due to more finer fibres and more crimp in fibres. The weavability is dominated by the wool component of the yarns as it is observed that higher the percentage of wool, more is the number of cycles before breakage/collapse of yarn.

Wool:linen yarns show better weavability because of more number of fibres in cross section of coarser yarns of 24-48 Nm range. It is no wonder that 2/48 Nm yarn breaks after more number of cycles as compared to 1/24 Nm yarn.

In silk:line blend the poor weavability is the result of poor binding between the two dissimilar fibres. However the 100 per cent silk yarns don’t give satisfactory weavability because they are manufactured on false-twisting principle unlike the other ring spun yarns.

The 1/20 Nm wool:cotton yarn though not comparable to other yarns, has got good potential weavablility. There are sufficient reasons behind its behavior, as because it is spun only from the comber noils and is manufactured on the rotor spinning machine without any positive twisting mechanism like he ring traveller arrangement on the ringframe.

Looking to the results of single yarns on weavability tester, it is apparent that they can successfully withstand the stresses on the weaving machines and they will perform still better when treated with suitable sizing recipe.

Improving weavability of single worsted yarns

Single spun warp yarns essentially need sizing to make them weavable. During sizing, size-fiber interaction improves structural stability, constituent fiber integrity and therefore, weaving stress bearing capacity of yarn. Degree of improvement in this stress bearing capacity mainly depends on nature of size-fiber interaction, which in turn depends on adhesion force between fiber and size, size penetration as well as encapsulation of yarn. Changing size add on and some process parameters like squeeze pressure, roller hardness and chemical nature of size material can vary the size penetration and encapsulation. Besides all these parameters, size fibre interaction largely depends on yarn structure and properties.

Single worsted yarns have a lower tensile strength as compared to equivalent 2-ply yarns. These can be weavable only through a sizing process. The technique of conventional sizing is not suitable for wool based yarns due to its sensitivity towards high temperature and moisture. Thus a cold sizing operation involving non-aqueous based size recipe can only be suitable for worsted yarns.

Sizing of worsted material: Evaluation of size recipes

The adhesively of some commercially available cold size recipes was tested for cotton polyester cotton and worsted materials (rovings) and evaluated in comparison to conventional size materials like modified starch and PVA. PVA proves to be the best adhesive material for all kinds of fibres followed by Filawax-HV. Polyester-cotton in general shows better adhesivity in comparison to 100 per cent or worsted material, when sized with almost all kinds of size recipes.

For worsted material, PVA and Filawax-HV can be used as sizing adhesives. The adhesion force of Softex-RX with worsted material though higher, it has other disadvantages like stickiness and it does not dry very quickly. Whereas Filawax-HV is a liquid solution at room temperature, and dries very fast, leaving only the solid material on the fibre substrate. Thus Filawax-HV should be used as a cold sizing recipe for worsted yarns. PVA however can be used for normal sizing of worsted single yarns.

Weavability of conventional sized single worsted yarns

Sizing of single worsted yarns was carried out with modified starch and PVA solutions at boil, adopting conventional technique of dip and squeeze on a laboratory model Zell sizing machine. The weavability of these yarns was measured using the same Sulzer-Ruti weavability tester maintaining identical conditions.

Conclusions

As is apparent from the results of conventional sizing, the yarns rather lose weavability after application of the size solution. It is because, the properties of wool deteriorate at higher temperatures. Thus cold sizing is the only option for worsted yarns.

(The authors are with Department of Textile Technology, Indian Institute of Technology, Delhi)