Dimensional changes of knitwear depend on fibre, garment construction and finishing. Appearance changes depend on garment construction (Chow, 2006).
Shrinkage of a knitted fabric can be determined by various factors, such as fibre type, stitch length, machine gauge, yarn twist, knitting tension and washing and drying conditions. According to Collins (1939) and Suh (1967), the most significant factor in knit fabric shrinkage is the swelling of yarn and the relaxation of internal stress that is produced during the knitting process. The knit fabric is formed by yarn loop formation under high tension so the knit fabric on the machine is distorted compared with its relaxed state.
Herath and Kang (2008) investigated the dimensional stability of core spun cotton/spandex single jersey structures with high, medium and low tightness factors under dry, wet and full relaxation conditions. Higher dimensional constants (K-values) were reported for the cotton/spandex single jersey structures than for the 100% cotton, and the cotton/spandex showed a more stable structure under full relaxation. Tightness factor (TF) can be calculated by Equation 3.1, which refers to the area of a knitted fabric covered by the yarn for defining the relative looseness or tightness of the knitted fabric (Horrocks and Anand, 2004).
66 Understanding and improving the durability of textiles
√—–tex
Tightness factor (TF) = ————— (tex1/2cm–1) [3.1]
stitch length
Yarns with elastomeric components show an increase in tightness factor, which has a significant effect on dimensional behaviour and gives a better dimensional stability in single jersey fabrics.
de Souza et al. (2010) stated that there are variables that directly influence the behaviour of a knit fabric when it is processed or when it attains the complete relaxation state. These variables can be classified into variables of the manufac- tured fabric and processing variables. Variables of the manufactured fabric include fibre type, yarn, yarn count, type of circular knitting machine and stitch length.
Processing variables include factors such as knit density (number of courses and wales), dyeing and finishing process employed.
Knitting conditions and the state of fabric relaxation will also have a significant influence on the dimensions of knitted fabrics. Quaynor et al. (1999) studied different fabric relaxation states of cotton and silk fibres including: (a) the cast off of knit fabric from the knitting machine after a few minutes; (b) dry relaxation of the knit fabric after 24 hours; (c) dry state after wet relaxation in water; and (d) dry state after one laundering cycle. During the knitting process, the fabric is formed by the yarn under continuous stress, so that the fabric is highly distorted when compared with the relaxation state. The phenomenon of (a) is that both cotton and silk yarns were almost straight after cast-off from the knitting machine. This indicates that there was not enough time for the stress to relax. The phenomenon of (b) shows the tendency to retain the yarn crimp after 24-hour dry relaxation, which indicates the importance of time in dry relaxation. The phenom- enon of (c) shows there was further retention of knitting crimp after the fabric was immersed in water, which means that stage (b) was not enough for full relaxation.
The phenomenon of (d) shows that the silk yarns retained more loops per unit than cotton yarns after one laundering cycle. Silk fabric seems to require at least one laundering cycle to attain minimum energy of stress.
Munden (1959) showed that hydrophobic fabrics may return to almost full relaxation in the dry relaxation state if they are given enough time to relax. On the other hand, when knitted fabrics with hydrophilic yarns are treated with water, there is a significant effect on the dimensional stability. For hydrophilic fabrics, fabric relaxation cannot be achieved by dry relaxation. Full relaxation must be completed by water, which acts as a lubricant on the hydrophilic yarn in the loop structure of knitted fabrics. As mentioned above, the different knit fabric structures will have an effect on the dimensional stability. Plain knit cotton shrinks more than silk, while 1 × 1 rib knitted cotton can be considered to have good dimensional stability.
Since the dimensional changes of knit fabrics are directly related to relaxation of the tension acquired during the yarn manufacturing processes, shrinkage may be predicted through mathematical models correlating the relevant variables of the
Dimensional stability of fabrics 67 process. When the complete relaxation state of cotton knit fabric is known, a dimensional constant or K factor can be established. A knowledge database of the relaxation process of cotton knit fabrics, ranging from production to the final product, has been developed by de Souza et al. (2010). This system simulates all of the process variables (e.g. the characteristics of raw materials, machines and processes) to obtain a dimensional stability for cotton knit fabrics so that the quality of the fabric can be predicted before production in order to meet the quality requirements of the client. The proposed program is seen as a useful tool for cotton manufacturers producing circular knitted cotton fabrics and will allow the final quality of the product to be determined in advance without generating costs and wastage due to experimental testing. The computer program may also assist textile companies involved in all stages of the industry – from fabric manufacturing to the garment production stages – in resolving many problems related to the knit fabric specifications, saving time and money while developing or improving the quality of knit fabrics for their clients (de Souza et al., 2010).
In response to consumer demand for wool garments that show excellent dimensional stability following washing and tumble drying, the manufacture of easy-care wool knitwear was investigated by a long-term collaboration effort between CSIRO and The Woolmark Company in Australia. They developed the term ‘Total Easy Care’ (TEC) to describe garments that retain their appearance after repeated machine washing and tumble drying. The garments can be worn immediately without having to spend a considerable amount of time restoring the garment to a pristine or ‘just pressed’ appearance. Woolmark accreditation schemes for Total Easy Care yarns indicate that they should be colour-fast, free from felting shrinkage and from spirality, with good regularity, good strength, low friction and good afterwash appearance, while TEC knitwear is said to have minimum relaxa- tion shrinkage and good afterwash appearance (Chow, 2006). Australian Wool Innovation Ltd (AWI) have been involved in the promotion of Total Easy Care technologies worldwide. Dr Roy Kettlewell from AWI described the steps in- volved in producing easy-care wool knitwear as follows (Chow, 2006):
(i) Select a suitable wool yarn; for instance, yarn count should be within 1.0 Nm counts with assured quality;
(ii) define the state of equilibrium of structures;
(iii) calculate knitting program/statements;
(iv) produce prototype;
(v) test prototype according to AATCC 150 standards (Chow, 2006).
Dr Kettlewell has also provided some rules that should be adhered to in the manufacture of easy-care wool knitwear:
(i) do not re-wind the yarn;
(ii) do not use de-knit yarn;
(iii) ensure that sewing needles are in good condition;
68 Understanding and improving the durability of textiles (iv) do not overload the tumble drier;
(v) dry wet garments as soon as possible;
(vi) make thin knots;
(vii) ensure knots have tails long enough to prevent failure;
(viii) ensure knots are placed at panel edges;
(ix) size downwards not upwards.