It is a well-known fact that knitted fabric is characterised by its superior elastic properties over others such as woven and non-woven fabrics. Fabrics of different categories like sportswear, casual wear etc. which require extra elastic property, knitted fabric are found to be very useful. The inclusion of yarn containing elastane, further elasticises the fabric, resulting in immediate and complete construction of garments. In this way, fabric becomes perfect for some special garments such as aerobic exercise wear, cycling shorts, swimwear etc. Blending of Elastane which is also known as Lycra (Trade name of Du-Point) with cotton is a common practice in knitting so that the fabric will have the comfort of cotton and the superb elasticity of spandex. Due to this marvellous application in knitting, it is often known as "Miracle Fibre" and the knitwear made out of this yarn called "Magic Knit". Spandex is a polyurethane polymer (having more than 85% of segmented polyurethane) produced by the reaction of polyester or polyether glycol with di-isocyanate. Polyester-based spandex is more easily hydrolysed than the polyether-based. The yarn are ordinarily white, with a dull lustre and are supplied as zero twist, in which the individual filaments are joined together to form a monofilament like yarn.

THEORY OF HIGH ELASTICITY

Compared with other synthetic fibres, elastofibres are distinguished by their high elasticity or stretch properties. As opposed to permanent extension, elasticity is a temporary extension; once the tension is relaxed, the fibre and thus the textile fabric made from it, recovers its original shape and position. According to the textile designation law, elastofibres are designated as being fibres which, when subjected to a tensile force, are capable of being stretched to three times their original length and immediately return to their original state when the tensile force is released. Segmented polyurethane, being the chemical basis of the elastane fibre is crystalline hard segments with a length-wise orientation and amorphic soft segment as cross-links from a macromolecular forces. The soft segments are in the form of a disorderly cluster. When the soft segments are stretched to an orderly configuration, they constantly strive to regain their original cluster form. This is the principle of high elasticity and fast recovery from the stretched state.

COMPARISON OF LYCRA YARN WITH OTHER YARN

Comparing lycra with some other yarn, (specially rubber) we found that lycra has some superb edge over its counterpart. Following are some special properties of Lycra:

1. Thickness: Lycra yarn is presently available in 40 denier while the finest rubber thread available in about 125 gauge, which is equivalent to 290 spandex denier.
2. Specific gravity: The specific gravity of spandex fibre is 1.0, which makes the spandex yarn lighter. The specific gravity of different fibres are given in Table-2.
3. Exposure to Heat and Ultraviolet Rays: Rubber is badly affected by heat and ultraviolet rays, while spandex remains unaffected. For single jersey, the  following counts of yarns are popular for particular fabrics:


Yarn Count End Product

22.33 and 44 dtex Underwear

44 and 78 dtex Outwear

156 dtex Fabric with higher retractive force

4. Stretchability: Spandex can be stretched 6 to 7 time to its original length. The speciality of this yarn is that this stretched length, it does not recover vigorously after released, but recovers slowly, this is the reason why spandex based knitwear fit to the body, without exerting much pressure.
5. Washability: The fabric made with spandex yarn is easily washable in home. The precaution during washing should be that, the detergent must be chlorine free.
6. Pulling Tension: Spandex yarn has much higher pulling strength than rubber yarn.
7. Flexing: Durability of Lycra is significantly greater than that of conventional rubber thread. A comparison was made Lycra vs Rubber in representative foundation fabric after 50 cycles of machine washing at 60^oC and tumble drying at 82^oC.

For various end products, three types of spandex yarns are used. These are:

i) Bare yarn; ii) Covered yarn; iii) Core-spun yarn

1. Bare Yarn: This is 100 per cent spandex yarn. The following factors should be taken into consideration before using bare Lycra:
1. Special feeders and tension devices are required to unwind the spool of Lycra or spandex on a multifeed circular knitting machine. These can either be adopted to the existing machines or installed by the manufacturers on new machines. Positive feed devices are used in order to control yarn delivery and ensure that yarn tension and elongation is the same in all the feeders of spandex. Such a device, especially developed GmbH under the name of MER (Memminger Elastane Roller).
2. Special care is required to prevent Lycra slippage from the seam of garments made with fabrics containing Lycra. Knitting construction should be carefully selected to minimize this risk.
2. Covered Yarn: To make spandex yarn knittable, covered yarns with different fibre such as cotton, nylon, wool, rayon, polyester, acrylic etc. are used as the spandex covering. This type of covered spandex yarn has less stretchability than bare yarn, and can be easily knitted by doing some modification on knitting machine. These types of yarns do not require special tension devices on the knitting machine. In this category three types of yarns are available. These are single covered, double covered and core-mingled yarns.
1. Single covered yarn: If only one layer is wrapped around the spandex as it covering, it is called single covered yarn. The characteristics of single covered Lycra compared to bare Lycra are:

• Improved abrasion resistance
• Greater strength
• Uniform stretch in subsequent processing
• Torque similar to false twist textured nylon

1. Double covered yarn: If two layers are wrapped around the spandex as its covering, it is known as double covered yarn. The characteristics of double covered Lycra compared to bare Lycra are:

• Controlled elongation and power
• Conventional "hard" fibre hand
• Improved abrasion resistance
• Greater strength
• Uniform stretch in subsequent processing
• No torque, no snarling.

1. Core mingled yarn: This type of yarn is produced by entangling the filaments of a multifilament yarn (specially synthetic) around the extended Lycra core in an air-intermingling jet. The applications of core mingled or air mingled yarn are limited. The characteristics of core-mingled Lycra compared to bare Lycra are:

• Improve abrasion resistance
• Greater strength
• Easier stretch control in subsequent processing

Following are the few important points which are necessary to run Lycra on knitting machine.

• Tape system feeders are preferable for knitting covered yarns containing Lycra on multiple feeder machines, in order to ensure equivalence and consistency of the extensions. A slower feeding tape is required for covered Lycra, because its throughout is lower.

• The layout of the knitting construction for bare-Lycra should be such as to clear the stitch containing the Lycra at the next feeder. This procedure minimizes tight courses and Lycra breaks which become apparent when the fabric is knitted.

• Bare Lycra not covered with a non-elastic yarn should never be knitted alone. As the stitch extension is not limited by a companion yarn, the resulting extension would be excessive. In addition there would also be knitting difficulties such as laddering of stitches, poor appearance etc. As an exception it may be possible to use bare Lycra alone, if it is tucked at each alternate needle.

• Compared to bare Lycra, covered Lycra and core-spun yarns are relatively easy to knit. They do not need a special unwinding system and conventional tension devices are suitable. It is recommended to knit the covered Lycra at full extension, independently of the Lycra dtex. Consequently, the covering thread is also fully stretches.

• Plaiting: Plaiting bare Lycra or Lycra covered with a non-elastic yarn is relatively easier than plaiting two non-elastic yarn because:

• The non-elastic yarn is always coarser or of equal count.

• The stitches are larger.

• The knitting tension are different and keep the yarn separate during introduction to the needles.

The yarn size of Lycra is expressed in denier, which is the weight in grams of 9000 meters of yarn (decitex = g/10,000 m). The size of both cut and extruded rubber threads, however, is expressed in Gage. The relationship between Gage and denier is given below:

1. Cut Rubber (Square Cross Section):


Denier = 5.81

1000

(dtex) = (6.46) x (-------------------------)²

Gage of cut rubber

2. Extruded Rubber (Round Cross Section)

The normal range of tension per Lycra thread is:

• 22 dtex Lycra – 2 to 4 CN
• 33 dtex Lycra – 3 to 5 CN
• 44 dtex Lycra – 4 to 6 CN
• 78 dtex Lycra – 7 to 12 CN
• 156 dtex Lycra – 10 to 20 CN
• 310 dtex Lycra – 20 to 35 CN

The introduction of a fine dtex Lycra at frequent intervals gives a better appearance than the use of a coarse dtex introduced at greater intervals. In spite of the need for more unwinders, finer Lycra is preferable, in particular for flat surface fabric (1/1 and 2/2 ribs or jersey). Fine Lycra also gives more flexibility for other fabric constructions.

A ridgy fabric results, of the Lycra feeds are spaced too far apart. Fabric containing Lycra in alternate feed will appear less triped than fabric produced using Lycra in every 3^rd or 4^th feed.

WAY OF INTRODUCTION OF LYCRA AND PRECAUTIONS

Lycra, bare or covered, should always be introduced on the back surface of the fabric by knitting on only one of the needle beds, usually at the rear of a flat knitting machine or on the dial of a circular knitting machine. The main reasons are:

1. Reduced "grin through" of any contrasting colour.
2. Maximum initial power of the fabric obtained when the Lycra lies as straight as possible.

• Protection of the knitting machine against unit from other areas of the workshop by plastic shields which enclose the creel.

• The use of pulleys with protected bearings, so that lint does not accumulate and is gradually eliminated.

• The machine should be cleaned as soon as lint deposits become critical. Eyelets should be checked with particular care.

Fold mark on fabric roll edges. The long the folds are subjected to pressure, the more difficult it is to remove these marks.

Fabric cold setting. Long storage alters fabric snap-back and can lead to a partial loss of fabric contractive power.

It is imperative to unwind the fabric roll after removal from the knitting machine. As soon as possible, the roll should be flet and relaxed before any further treatment including storage. If convenient, it is not necessary to wind the fabric at all, it may be plaited directly on the machine.

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