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(xi) Anti Pill Fibres: Pilling has been a problem with polyester fabrics particularly with spinners preferring high tenacity fibres in 1.2 / 1.4 D X 44 or 51 mm cut lengths to spin 30 s / 40 s / 50 s yarns at high spindle speeds pf 20,000 rpm plus. The yarns so produced are hairy. It is these protruding fibres which form pills after rubbing. Pills do get formed with cotton or viscose fibres, but the pills fall off during use as the fibre breaks because of lower strength. The tenacity of polyester being high - in the range of 6.4 grams / denier, the pills formed do not fall off and so give a bad appearance to the garment.
Therefore today anti pill fibres with a tenacity of 3.5 gpd are available for 100 % polyester and polyester / viscose fabrics spun on cotton system and super anti pill fibre with even lower tenacity of 2.8 gpd are recommended for polyester / wool blends to be spun or worsted spinning system.
These fibres are produced by reducing the intrinsic viscosity of the polymer from the normal 0.63 to as low as 0.53 for anti pill and 0.48 for super anti pill fibres.
These fibres are available only in 2 and 3 D in 44 / 51 mm for use on cotton system and 66 to 110 mm cut lengths in 2 mm steps for worsted system.
One advantage of particularly super anti pill fibre is its natural fibre feel. It is very 'wool' like which could be its bigger advantage.
These fibres are recommended for premium quality suiting in polyester / viscose and polyester / wool for sophisticated markets in US, Europe and Japan.
(xii) Sewing Thread Fibre: This fibre is also called Super High Tenacity Fibre. It is normally made only in 1.2 D in 44 mm cut length and in bright lustre. (May be because cotton sewing threads are mercerized and so are brighter). Its tenacity at break is around 7.5 gpd, elongation at break is around 16% and its T 10 is around 6.0 gpd. Its dry heat shrinkage at 180 deg. C. is around 3.5% (against 5 to 8 with normal apparel fibres). This fibre has to have the high tenacity since it is used solely for stitching polyester fabrics. A general rule is the sewing thread has to be stronger than the fabric it is stitching; otherwise sewing threads could break during the use of the stitched garment and may cause embarrassment to the wearer.
Its low hot air shrinkage ensures puckering free stitching.
The end use is 100% polyester sewing threads. Rkm of the single yarn should be around 40 - 41. The fibre becomes almost 100% crystalline in the effort to get 7.5 gpd; and a true sewing thread fibre does not easily dissolve in the solvent for polyester at room temperature. Also, because of high crystallinity, the dyeability of this fibre is lower by 15-20% compared to that of normal 1.2 Den fibre.
As this fibre is heat set at very high temperatures - 225 deg. C, or so and for a longer duration, its crimping is poor - crimps / inch are lower than the normal value of 12 - 14 per 25 cm and both crimp stability and crimp take up are also inferior. This fibre cannot, therefore, be processed at high speeds in preparatory.
(xiii) High Shrink Fibre: This fibre shrinks upto 59% in boiling water in 30 seconds in one step. One condition is - this fibre - even in bale form - is stored below 27 deg. C. before use i.e. during transportation, at the ports and in mill godown. At higher room temperature the fibre will start shrinking. This fibre is available in 1.4, 2.0 and 3.0 D in all usual cut lengths. It is recommended to use in apparel to get crinkle effect, for non-woven felts and for making artificial leather. One possible use is to replace high bulk acrylic yarn by doubling 100 % high shrink fibre with normal fibre yarn with a light twist; knit a garment and then process it to allow the high shrink fibre yarn to shrink.
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