M. Senthilkumar, C. V. Koushik, B. B. Jambagi*, and V. Subramaniam**
Department of Apparel and Fashion Technology - Sona College of Technology-Salem
Email: cmsenthilkumar@yahoo.com

ABSTRACT

Knitting technology has advanced considerably during the past two decades with the introduction of various knitted structures, the use of new and modified yarns and the versatility of modern knitting equipments. The successful use of yarns in knitting depends upon the yarn characteristics and the selection of the knitting process parameters.

The yarn quality requirements for knitting extend beyond those expected of yarns for weaving. For example, yarns would do well in knitting if they possess better evenness, good elasticity and elongation, low hairiness and low coefficients of friction, soft feel and lower variation in count. Further, good fabric performance can be expected with yarn properties such as high work of rupture, loop strength and knot strength, yarn twist, reasonable tenacity, low flexural rigidity and adequate torsional rigidity.

This paper deals with problems encountered in silk knitting. Silk material has exceptional lustre and good tenacity among the natural fibres. Silk knitted fabric is regarded as a special product that can hold its own and survive in the competitive and diversified global textile market.

With regard to silk filament yarn knitting, it has been observed that the number of plies in the yarn and the yarn twist are interrelated and their effect on fabric properties is significant. If the number of plies increases, the result would be a coarser yarn suitable for producing heavy knitted fabric. But, with increase in the number of plies, a greater level of twist would be required to bind the plies and the filaments together. Additionally, as twist increases, the tendency for the yarn to snarl will be high, and this in turn would lead to needle breakage during knitting or yarn breakage because of yarn entanglement.

A major finding of this work is that the selection of silk filament denier, number of plies, twist level, machine gauge and fabric structure are important factors in the commercial knitting of silk filaments.

Chemical processing of silk filament yarn is one way to enhance the feasibility of silk knitting. Silk knitted fabrics made from the salt shrunk, fibre dispersed and resin treated silk yarns are reportedly more flexible and elastic, have a higher resilience, and are soft and extensible. This area holds great promise in investigations aimed at making silk more amenable to knitting.

Problems associated with knitting of silk filament yarn involves silk filament yarn treatment, supply package feed, yarn tension and knitting of silk yarn are also discussed.




* Professor, DKTE, Ichalkaranji ** Director, Jaya College of Engineering, Chennai

INTRODUCTION

A silk knitted fabric has a smooth surface and provides a pleasant feel when worn next to the body. Such a fabric would be especially suitable for ladies garments because it is wrinkle-free, soft, light weight and has aesthetic feel. Generally, knitted fabrics tend to mould easily to body shapes and adapt themselves easily to body movements. [1]

Knitted fabrics possess a high order of elasticity and recovery, unlike woven fabrics that posses a low degree of elongation. The unique property of a knitted fabric is that of stretch. This is because of its inherent intermeshed loop arrangement. A knitted fabric is essentially a highly elastic material.

Effect of fibre and yarn quality on knitted fabric properties

Fibre properties such as length, work of rupture, elongation, frictional characteristics and fibre quality index are the major factors that influence the properties of knitted fabrics. [2]

Yarn characteristics like count, twist, frictional behaviour, bending rigidity, presence of knots, yarn defects and yarn strength play a critical role in deciding the knitting performance of a yarn. . [2-5] Other than these yarn characteristics, the short fibres present in a spun yarn are also important.

Various yarns for knitting

Today, the various yarns used for knitting are ring, rotor, air jet, siro-spun yarns, etc. These yarns fulfil some of the properties mentioned above from the engineering point of view; however, they also cause some undesirable properties and lead to insufficient requirements in the finished fabric especially in properties such as pilling resistance, comfort, handle, compression properties, abrasion resistance and dimensional properties.[6]

Development of silk yarn for knitted fabrics

Research work done by Hiroshi Kato and Tamako Hata, shows that processes like salt shrinkage treatment, fibre dispersion and resin treatment are used to enhance the knitting performance of silk. Salt shrinkage is brought about by soaking degummed twisted yarns and treating them for about 30-60 seconds at 80"aC in a concentrated solution of calcium nitrate of specific gravity 1.415; a shrinkage scatter in the range of 30 - 50% can be obtained. The yarn samples are then treated twice in degumming baths of 10-g/l concentration at 95o C. Thereafter, they are subjected to fibre dispersion, then surface-treated with resin and finally given a treatment with an antistatic agent.

The yarn treated as described above is found to be slightly thicker and possesses greater crimp and bulk. It was also observed that the twisting and bending of the yarn become easier with increasing salt shrinkage. This behaviour may be attributed to a partial destruction of the crystalline regions in the silk fibre and a consequent increase in the amorphous content. The degree of molecular orientation in the amorphous part of the fibre, which is usually high in silk compared to other fibres, is also lowered by the salt shrinkage treatment.

The ease of twisting increases with increasing salt shrinkage, and results in a yarn with hard feel and lacking in fluffiness and bulk; an increased resistance to bending deformation and increased bending strength has also been observed. However, the treatment and the number of twists do not seem to affect fabric handle.[7]

SILK KNITTING

There is a ready availability of silk yarn that facilitates the development of diversified products of silk.[10]. Silk knitting is hardly a new technology. However, it is still not as popular as might be expected because of the problems associated with the knitting process as such and the cost of raw silk yarn. The potential for development of the silk knitted garment market is tremendous. But it is expensive when compared to cotton knitted fabric.

Diversification of products in knitting is one way to overcome the competition in the open market. India can boast of the second position in the world silk market. India can utilise its available raw material resources to develop an increasing range of diversified silk products and thus enhance its silk export from the present share of about two per cent. Such an initiative will also add immensely to the export earnings of our country.

Problems associated with silk knitting

Yarn count

One of the biggest drawbacks of a knitting machine as compared to a weaving machine is that on a particular knitting machine only a certain range of yarn counts can be knitted; in a weaving machine, on the other hand, yarn of any count can be woven just be changing the reed and healds. The gauge of a knitting machine cannot be changed (as the distance between two needles is fixed). Hence only a particular yarn can be knitted on a machine of specific gauge.

If the selection of yarn denier is improper, problems will arise every time the needle moves to form a new loop. This is because fresh incoming yarn has to pass through the space in between two sinkers. If the space between two sinkers is not sufficient due to a large yarn diameter, it will give rise to the problems of yarn or needle breakage.

Yarn count is a key factor in getting a predetermined fabric density. Finer denier silk yarn (say less than 20 denier) is not suitable for circular knitting because the yarn would tend to escape from the path or hook part in the needle and lead to missed loops in fabric.

Yarn twist

A low level of twist is usually used in the production of yarn for knitting. This is because a yarn with high twist will create a lot of problems during knitting; it will also affect fabric feel and create a fabric defect known as spirality.

With reference to silk filament yarn, the number of plies in the yarn and the yarn twist are both interrelated. If the number of the plies increases, the count will increase correspondingly and the resulting yarn will be suitable for producing heavy or dense fabric. But, if the number of plies is increased, a greater degree of twist would also be required to bind the filaments and the plies together. Additionally, as twist increases, the tendency for snarling of the yarn will also be high and this could lead to needle breakage or yarn breakage (because of yarn entanglement) or both.

The problem of high twist in the yarn can be reduced by water or steam treatment. But it is better to produce a yarn with lower twist level as compared to yarn used for woven fabric so that it can perform better during the process of knitting as well as to produce fabric with minimum defects.

Yarn frictional property

The frictional behaviour of a yarn is very important with regard to its performance on any knitting machine. This behaviour is dependent upon yarn surface characteristics, which determine the frictional resistance developed when yarn moves in contact with different machine parts.

During the process of knitting, silk yarn has to pass in contact with a number of different surfaces. As it wraps around the needle and sinker during its passage through the machine, its tension increases. This in turn will hamper smooth knitting and lead to increased end breakages and holes in the fabric.

Yarn bending rigidity

Bending rigidity is defined as the force required to bend a material through unit curvature. In knitting, the needle causes straight yarn to bend into a loop. So the rigidity of the yarn will determine the force required to bend the yarn in order to form a loop. During knitting, the same force will also act on the needle hook and the needle, but mainly as a force of reaction.

The bending rigidity of silk filament yarn is normally higher than that of a cotton yarn. As a result there will be a higher reaction force on the needle, and therefore a greater chance for the needle hook or butt to break. In order to eliminate this problem it is wiser to select a flexible yarn for knitting (particularly for finer gauge machines).

Type of feed package

In circular knitting, the feed package is usually in cone form. Silk yarn wound into cone or cheese form generally tends to slough off during unwinding. To obviate this a double-flanged bobbin is used to prevent the slough-off and give easy unwinding. The only drawback with double-flanged bobbins is the low material content in the package and the variation in unwinding tension during knitting.

Type of machine

Both circular weft knitting machines and flat rib knitting machines are suitable for knitting silk filament yarn. Positive feeders in knitting play a vital role in silk knitting by the tension variation control they provide during the inter-looping. Knitting machines without the positive feeder devices could easily cause yarn tension variations during knitting that would result in the occurrence of missed loops. [8, 9]

Materials, Machines and Methods

Preliminary work done for selection of the right yarn for knitting

The types of filament silk yarn available commercially and the denier range normally found are listed below.

Filament silk yarn: 20 V 22 denier 40 V 44 denier 100 V 120 denier

Dupion silk: 200 denier

The linear density range of filament silk yarn for initial trials was selected as 40V44 denier as it is in the middle order of the available linear densities and when used as doubled yarn they would still be fine enough for knitting.

The 100V120 denier range of yarns are fine enough for knitting but as single yarns they would tend to slip out of the needle and cause the formation of missed loops; doubling them would result in rather coarse a yarn for knitting.

The 20V22 denier range of yarns are so fine as to require a large number of plies and a very high level of twist to bind the filaments together, besides the problem of needle breakage that they could cause due to the high twist.

Initial knitting trials

Knitting trials were undertaken to determine the optimum number of plies and the twist level in the 40f{44 denier yarn that would give easy, trouble-free knitting. Three different combinations of these parameters were tried.

Case 1:

The first trial was taken with a folded yarn consisting of 8 plies each of 40-denier filament silk yarn, the folding twist being 15 TPI. The knitting was tried out on a hand-knitting machine of machine gauge 8. It was observed that the yarn got caught frequently in the feeding section due to the snarling tendency of the filament silk yarn.

Case 2:

The second trial was taken with filament yarn of the same number of plies [Eight plies] but the twist level was reduced from 15 to 4 TPI. The same 8-machine gauge hand-knitting machine was used for knitting. It was noticed that frequent end breakages occurred and missed loops showed up in the fabric due to filamentation of the material in the needle.

Case 3:

In this case, the number of plies was reduced from 8 to 3. The twist was also further reduced to 3 TPI. This combination of parameters gave smooth knitting without any needle or end breakages.

Knitting on industry-scale machines

On the basis of the preliminary trials described above, the following yarn and machine specifications were used for knitting of silk on industry-grade machinery. Cotton yarn of equivalent count was also used for the sake of comparison.

Yarn details



Properties

Low-stress mechanical properties

The following table presents the low-stress mechanical properties of the fabrics in this study.

In actual practice, yarn wastage is less in the case of silk filament yarn as it gives lower breakage during knitting. The reduction of raw material cost is the one big way to succeed in silk knitting, as it would help to reduce the fabric cost. The cost of Chinese silk yarn is half that of Indian silk. That is why silk knitting is so popular in China.

CONCLUSION

The technical feasibility of filament silk yarns for knitting is investigated. Filament-silk yarns are proved to be immensely suitable for knitting. This is a great benefit to the knitting industry in view of its survival in the international fashion market. The silk knitted fabrics are special quality fabrics that can be suitable for all seasons. Selective use of silk knitted fabrics is one way to compensate for the economical constrains.

Filament-silk Knitted Fabrics - Photographs



REFERENCES

1. Siva Kumar. M, "Role of silk in knitted fabrics production", Textile Magazine, August 2000, p- 45.
2. Sivakumar. V.R, " A study of the fiber and yarn quality requirements for knitting ", 22 nd technological conference, p 7.1.
3. Parate. D.M. " Evaluation of yarn properties for knitting (part 1 & part 2) ", Man Made Textiles in India, October 1995.
4. Shankara narayanan. K.S - " Effect of yarn quality on the properties of the knitted fabrics and performance in knitting ", 22 nd technological conference, p 159.
5. Krishna varma, T.M, " Influence of yarn faults on the quality of knitted fabric ", ITJ, January 1992, p 88.
6. Vasavada. D.A , " Various yarns for knitting ", ITJ, Dec 1999, p 144.

7. Hiroshi Kato and Tamako Hata, "Development of Silk Yarns for Knitted Fabrics", Published at Department of Insect Processing Engineering, National Institute of Sericultural and Entomological Science, Japan
8. Subrata ghosh - " Effect of yarn characteristics on knitting performance" Textile Trends, Feb 1997, p 32.
9. Patil. U.J " Factors affecting yarn behavior " ITJ, OCT 2002, P 67.
10. Rajiv Kumar , "Feasibility of Spinning Silk / Silk Blends on Cotton System", Textile Asia, February 2001, Part I, II and III, p 27
11. Knapton, J. J. F., "The Dimensional Properties of Knitted Wool Fabrics - Part 1 - 5", TRJ, 1963, p 158
12. Knapton, J. J. F., "The Geometry and Dimensional Properties and Stabilization of the Cotton Plain Jersey Structure", JTI, 1975, No. 12
13. Munden, D. L., "The Geometry and Dimensional Properties of Plain Knit Fabrics", JTI, 1959, p 448
14. Ajgaonkar, D. B., "Performance of Open-end, Twist-less and Ring Yarns in Weft Knitted Fabric", TRJ, June 1974, p 405
15. Sharma, I. C., "Dimensional and Physical Characteristics of Single Jersey Fabrics", TRJ, March 1985, p 149
16. Doyle, P. J., "Fundamental Aspects of the Design of Knitted Fabrics", JTI, 1953, p 561




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