A fabric experiences the effects of both indoor and outdoor environments under mild to severe conditions throughout its life. Weaving and use parameters can clearly be seen to influence the durability and serviceability of a fabric, whilst further less noticeable parameters, such as UV radiation, may also affect its tensile strength (Gezer and Merdan, 2010).
2.5.1 Selecting uniform yarn characteristics
Both longitudinal and transverse yarns in the fabric must have consistency in their constructional features, as any point along their length could be the first to register unexpected instances of damage or shock. Yarns possessing uniform diameter throughout, identical twist, and homogeneity in blending should be used to produce fabric.
2.5.2 Optimising fabric geometry
Selection of the right weave and fibre combination enhances the end-use perform- ance, and efficient load sharing is key to fabric geometry. Each end product is required to meet certain criteria in order to perform most effectively, so changes to either the weave or fibre in the fabric can deteriorate the overall performance. The performance requirement of any product must be considered before its manufac- ture begins, so that extensive research into the most suitable combination of fibre and weave can be carried out.
Unevenness in tension over the warp affects the breaking strength of the woven fabric across the warp width, and a decrease in warp tension or an increase in weft density or weft yarn thickness can cause a fall in the warp breaking strength. A densely woven fabric with a lower warp tension will show less variation in the warp-directional breaking strength (Süle et al., 2011).
Anti-ballistic textiles are preferably woven using a multilayered basket or satin
Strength properties of fabrics 51
weave, with an end and pick density of around 6.7 and 18.9 cm–1 respectively, a yarn count of 840–2000 denier, and gsm around 231–672. In contrast, parachute canopies used for gliding and flight control are made of ripstop nylon fabric woven in a specialised manner with extra thick thread. This ripstop weave is a plain weave with heavier threads woven into the material, resulting in a boxlike pattern of small squares, which, combined with the heavier thread, inhibits the spread of tears.
A further example of a situation in which higher breaking and tearing strengths and breaking elongation are requirements is in the design of advanced automobile seat covers. Studies reveal that flat woven and woven velour seat cover fabrics provide the best breaking and tearing strength performances, whilst circular knitted automobile seat cover fabrics produce the greatest elongation measures (Pamuk and Çeken, 2009).
2.5.3 Reducing damage during chemical processing
Reducing the damage caused during chemical processing ensures superior strength and serviceability. A combination of processes reduces wastage of energy, waste- water load, handling and cost, whilst ensuring the quality of product. Desizing, scouring and bleaching could be combined, and prolonged dyeing and redyeing could be avoided. Due attention must be paid while dyeing with vat and sulphur dyes, due to the strong reducing agents involved in their use. Selective catalysts can be used during functional finishing, and the use of softening agents including silicones (polysiloxanes), for example, can improve handling and strength remark- ably. Because of their characteristically pleasant handle, drape and lustre, viscose fibres are invariably blended with cotton and polyester to manufacture apparel.
However, viscose is highly sensitive to water; washing, contact and convection drying all reduce the dimensional stability, whiteness, crease resistance and breaking strength of the fabric, necessitating optimal processing conditions (Ismal, 2008).
2.5.4 Reducing yarn and seam slippage
Yarn slippage can be reduced by working along the stitch line. Seams sewn parallel to the warp, in comparison to seams sewn perpendicularly, result in greater yarn slippage, suggesting that alternative stitch and seam constructions may be a better option. Seams sewn with a single needle lockstitch machine may be changed to a safety-stitch with two rows of stitches (Fig. 2.4). Similarly, a two needle over- edged superimposed seam could be changed to an LSq* Seam, in which an additional row of stitches is used to cord or topstitch the seam. Stitch density is an important issue in achieving optimum seam performance, and may be 12–14 inch–1 in general.
Seam slippage can be reduced in many ways. Selection of a wider seam margin, from a margin of 1/4 inch to a margin of 1/2 inch for example, may help. Changing
52 Understanding and improving the durability of textiles
2.4 Reducing yarn slippage by double-row stitching.
2.5 Types of seams to reduce seam slippage.
the seam and stitch combination, or stitching with either french, single lapped, double lapped or flat felled seams in place of plain seams are also effective ways of reducing seam slippage (Fig. 2.5).
Plain seam
Flat felled or lapped seam
Double lapped seam Single lapped
or raised seam
French seam
Strength properties of fabrics 53
2.5.5 Selection of sewing thread and needle
Sewing thread must possess adequate strength. Any force working to separate the pieces causes elongation from the line of joining until the sewing thread is able to retract the force, or the fabric is ruptured. The necessary thread strength is dependent on the characteristics of the fabric to be joined, and the average forces to which the garment will be subjected in the end-use, in combination with the actual seam strength (Carr and Latham, 1994). Sewing needle design involves modifications to reduce over-heating during high-speed sewing. This is to avoid the scorching of cotton fabrics and fusing of fabrics such as rayon or nylon. Needle cooler, thread lubrication or needle lubrication are other methods for the avoidance of damage to the fabric (Scott, 1951; Sondhelm, 1953). Commercial fabrics must be sewn with the specific prescribed thread. Sewing thread used to stitch parachute canopies, for example, must be a 100% nylon thread with a diameter of 0.33 mm, breaking strength 40 N and denier 250/2, whilst the seam binding used to join the canopy panels must possess a breaking strength of 271 N (longitudinal) and 81 N (transverse).
2.5.6 Minimising pilling
Pilling causes progressive loss of fibre from the fabric, and arises from inadequate binding of fibres due to a less cohesive force along the main strand. The projection of fibre tips on the surface form pills under abrasive forces, with the severity of the problem dependent upon such factors as the nature of the fibre, fibre length, twist level, length of projection and nature of abrader. One hundred percent polyester fabrics produced through carbonisation of polyester–cotton fabric have a reduced yarn diameter, due to removal of the cotton, reducing the effective twist level and enhancing pilling. Fabric appearance and serviceability are both drastically re- duced by pilling, with the progressive loss of fibre further reducing the life of the garment. Imparting optimum twist to the yarn prior to weaving and minimising loss during processing may reduce the problem. Open structures, such as knits, increase the problem, whilst fabrics with closely interlaced woven structures are more resistant. Alternatively, an anti-static, anti-pilling or silicone finish may be imparted to improve pill rating (Mittal and Trivedi, 1980).