CHAPTER 8 EFFECT OF HEATSETTING AND COMPACTING TEMPERATURES ON DYNAMIC ELASTIC BEHAVIOUR OF COTTON / SPANDEX KNITTED FABRICS

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1 92 CHAPTER 8 EFFECT OF HEATSETTING AND COMPACTING TEMPERATURES ON DYNAMIC ELASTIC BEHAVIOUR OF COTTON / SPANDEX KNITTED FABRICS 8.1 INTRODUCTION Spandex back plated cotton knitted fabrics are more dimensionally stable which is used for sportswear construction. Normally, these fabrics undergo by heat setting and dyeing, followed by compacting process. Heat setting process is to stabilise the desired fabric properties of width, weight, stretch and power (Reginald 1971). Heat setting sets the spandex in an elongated form. This is also known as redeniering, wherein a spandex of higher denier is drafted, to a lower denier, and then heated to a sufficiently high temperature, for specific duration. It is to stabilize the spandex at the modified denier. The spandex permanently changes at a molecular level and remains at the denier permanently. So that, recovery tension in the stretched spandex is mostly relieved and the spandex becomes stable at new and lower denier. Heat setting temperature for spandex, is generally in the range of C to 200 C (Lay cock 2006). If heat-setting is not used to set the spandex, after the fabric is knitted and released from the circular knitting machine, the stretched spandex in the fabric will start relaxing and compresses the fabric loops so that, the

2 93 fabric will shrink and dimensions will become lower as compared to what those dimensions would be if the spandex was not present. Compression of the loops in the knitted fabric exerts three major effects that are directly related to elastic fabric properties. First, loop compression reduces fabric dimensions and increases fabric weight beyond desired ranges for single jersey knit fabrics for use in elastic garments. As a result, the finishing process for elastic knitted fabric includes a fabric stretching and heat setting treatment. Heat setting is carried out at sufficiently high temperatures and sufficiently long residence time, so that the spandex yarn in the fabric will get set at desired stretched dimensions. After heat setting, the spandex yarn will either not retract, or will retract only modestly below its heat-set dimension. Thus, the heat-set spandex yarn will not significantly compress the knit loops from the heat-set dimensions. Stretching and heat setting parameters are chosen to yield the desired fabric basic weight and elongation, within relatively rigid limits. For typical cotton single jersey fabric, the desired elongation is at least 60%, and the basic weight ranges from about 140 to about 240 grams per square meter (Lay cock 2006). Secondly, higher the yarn loop lateral compression, the higher will be the fabric elongation, thus far exceeding minimum standards and practical needs. When a plated knitted fabric with elastic yarn is compared with a fabric which was knitted without elastic yarn, it is common for the plated elastic knit fabric to be 50% more compressed than the fabric without elastic yarn. The plated knitted fabric will be able to stretch 150% in lengthwise direction or more from its compressed state, and such excessive elongation is generally undesirable in single jersey fabrics for cut and sews application. This extension is in the course wise direction of the fabric. Fabrics with high elongation in length (stretch) are more likely to be cut irregularly, and are also

3 94 more likely to shrink excessively upon washing. Similarly, loops are compressed by spandex in the widthwise direction, so that fabric width is reduced by about 50% as well, far beyond the 15 to 20% as fabric width reduction normally meets with non-elastic fabrics (Lay cock 2006). Thirdly, the compressed loops in the finished fabric are at equilibrium condition with spandex recovery forces and resistance to loop compression by the accompanying companion yarn. Washing and drying of the fabric can reduce the companion yarn resistance, probably in part because of agitation of the fabric. Thus, washing and drying may permit the spandex recovery forces to further compression of the knit loops, which can result in unacceptable levels of fabric shrinkage. Heat-setting the knitted fabric serves to set and fix the spandex and reduces the spandex recovery force. The heat setting process therefore, improves the dimensional stability of the fabric, and reduces the amount of fabric shrinkage during repeated washing (Lay cock 2006). Heat setting is preferably carried out at the early stage of the textile processing rather than at the end, in order to avoid yellowing in drying. Under-setting results in eventual loss of fabric dimensions; over heat-setting lowers power and can discolor the spandex and companion fibers.relaxation treatment is used to reduce potential distortion or deformation of the fabric from residual uneven tension. It develops the power and recovery of the fabric. The fabric should be relaxed prior to heat-setting to avoid rope marks and puckering during dyeing to ensure good dimensional stability of the final garment (69 and 71). Fabric set can be achieved by heat setting the spandex, which is not cross-linked, in fabric form to stabilize the structure at specific dimensions and to eliminate the remaining force that would lead to contraction or

4 95 deformation. This is accomplished by heating the fabric at C. At that condition, some of the spandex network tie points are broken and reformed, and the polymer chains can rearrange. If the spandex in the fabric is under stretch during heat-setting, the chains get disoriented and the retractive forces drop to almost zero. Spandex size is reduced at that extension while heat setting. And the elastic power upon stretching beyond this point is reduced correspondingly. A similar process with steam can be used to reduce the fibre size in core spun yarns (Lay cock 2006). The fabric has to be compacted after heat setting and dyeing, in order to stabilize the fabric dimensions. Compacting is a mechanical process by compactors that physically rearrange the geometrical relationship of yarns in a fabric. In the knitted fabrics, the loops can be rearranged to overcome the distortion in the length versus width caused by stretching tensions. Knit compactors balance the length to width loop ratio, thereby stabilizing the residual shrinkage due to laundering. (Hassan 2005) In a knitted fabrics, the intermeshed loops are organized in such a way that they are free to move towards each other orthogonally along their related loop row so as to reduce volume of the fabric. Effective compressive shrink-proofing of knitted textile fabrics of natural fibers depends up on partly expansion of steam heat and / or moistened yarn caused by partial unraveling of their fibers. Steam puffing gives lubricating effects on natural yarn loops of knitted fabrics (Lay cock 2006). Though the contribution of spandex in elastic fabric is very small, it is responsible to the fabric elastic recovery. Spandex has high stretch and its elastic recovery characteristics are due to combination of high melting (hard) and low melting (soft) polymer segments linked together in the same molecular chain. The lateral bonding forces, physical size and molecular

5 96 shape of the hard segments contribute to the modulus and thermal stability. The glass transition temperature of the soft segment of spandex fibers decreases after steaming. Soft segments within the chain molecules are responsible for elasticity (Reginald 1971). The spandex in the fabric will lose its residual elastic energy during heat treatment. Once its elastic energy is reduced, the elastic properties of the fabric will also reduce. Therefore, it is necessary to study the effect of heat setting and compacting temperatures on dynamic elastic properties of spandex plated cotton knitted fabric. Though studies have been carried out on dimensional properties of plated fabric, studies reported on elastic properties of elastic knitted fabrics are only limited. The present work is to study the effect of heat setting and compacting temperatures on dynamic elastic properties of spandex plated cotton knitted fabrics. The effect of heat setting temperature has been studied by varying heat setting temperature as C (Low), C (Medium) and C (High) for 36 seconds. Similarly, the effect of compacting temperature has been studied by varying compacting temperature as 70 0 C (Low), 85 0 C (Medium) and C (High) for 22 seconds. The range of temperatures for both heat setting and compacting has been selected based on trial studies. 8.2 MATERIALS AND METHODS Commercially available tex cotton yarn and 40 denier spandex have been selected for the study. The yarn has been converted into fabric in the knitting machine, and the specifications are listed in Table 8.1.

6 97 Table 8.1 Knitting machine specifications Model MV4 Mayer and Cie (2001) Machine gauge (Needles per Inch) 28 Machine diameter (Inches) 26 Number of feeders used 84 Type of spandex feed % 5 Machine speed (rpm) 25 The fabric production methods for studying the effect of heat setting and compacting temperatures are given below; Effect of Heat setting Relaxation Effect of Compacting Relaxation Heat setting (Three levels) Heat setting Dyeing Dyeing Compacting Compacting (Three levels) The cotton / spandex single jersey knitted fabric was heat set, dyed and compacted as mentioned in the section , and respectively. Geometrical characteristics of cotton / spandex knitted fabrics were tested as per standards mentioned in the section and the results are given in Table 8.2.

7 RESULTS AND DISCUSSION The effect of heat setting and compacting temperatures on dynamic elastic behaviour of spandex plated cotton knitted fabrics at different extension levels have been analysed and the results are tabulated in Table Geometrical Characteristics Geometrical characteristics such as wales per centimeter, course per centimeter and loop density are found to decrease with the increase in heat setting temperatures from C to C. This is due to the stretched form of the fabric, when it is treated for heat setting. After this, the fabric was relaxed. The spandex gets elongated when the temperature increases. Table 8.2 Geometrical characteristics of cotton / spandex fabrics treated with different heat setting and compacting temperatures Specification Wales per centimeter Courses per centimeter Loop length (mm) Loop density (loops/ cm 2 ) Thickness (mm) Heat setting Temperature Low Medium High Compacting Temperature Low Medium High

8 99 Generally, the stretched spandex in the fabric will retract to compress the fabric loops. This is may be because the spandex was not set with cotton yarn (inadequate heating) or due to loss of its residual energy (excessive heating). When the heat setting temperature increased to C, further spandex retraction was observed. Fabric thickness drastically reduces due to lack of yarn loop lateral compression. Not much of changes observed in loop length, when the heat setting temperature increases. When the compacting temperature increases from 70 0 C to C, loop density and thickness of the fabric first increases from 70 0 C to 85 0 C and then decreases up to C. No change was found in loop length with respect to increase in compacting temperature. In the case of medium compacted fabric, yarn loops have to move much more close to each other orthogonally. That is, the fabric was treated for mechanical pushing of the inter-meshed loops so as to reduce the volume of the fabric. When the volume of the fabric reduces, loop density and thickness increases. No reason was found for lower loop density and lower thickness at the compacting temperature of C Dynamic Work Recovery Effect of heat setting temperature on DWR of cotton / spandex fabrics Effect of heat setting temperature on dynamic work recovery of the cotton/ spandex fabric has been given in Figure 8.1. When the heat setting temperature was increased from C to C, the DWR of the fabric at walewise direction first increases from C to C and then starts decreasing from C to C. In the case of coursewise direction, DWR of the fabric decreases with the increasing heat setting temperature for all extension levels.

9 100 Table 8.3 DWR of cotton/ spandex fabrics treated with different heat setting and compacting temperatures Effect of Heat setting Temperature Walewise direction Coursewise direction 20% 30% 40% 50% 20% 30% 40% 50% Low Medium High Effect of Compacting Temperature Walewise direction Coursewise direction Low Medium High The dynamic work recovery of the fabric increases with increasing fabric extension from 20 % to 30 % and then decreases for 40 % and 50 % extensions in all the cases at both walewise and coursewise directions. It is observed that the fabric which was heat set at C has shown apparently higher DWR value (2-10%) in both walewise and coursewise directions. The DWR of the fabric at walewise direction is higher than that of the DWR at coursewise direction in most of the cases. The possibility of yarn loop lateral compression is more in the case of wale wise direction than that of coursewise direction, as there is a restriction in the yarn movement from one needle to another due to shorter clearance between the needles. Higher the machine gauge, the lower the yarn loop lateral compression, which is the reason for higher DWR value in walewise direction.

10 101 DWR (%) Low Medium High Heat setting temperature 20% 30% 40% 50% (a) Walewise direction DWR (%) Low Medium High Heat setting temperature 20% 30% 40% 50% (b) Coursewise direction Figure 8.1 Effect of heat setting temperature on DWR of cotton / spandex fabrics

11 102 Generally, as the heat setting is carried out on the elastic fabric in elongated form, the spandex molecular structure rearranges itself in the fabric and reduces the recovery tension. For the normal spandex yarn, the softening temperature is C to C, which will vary depending upon its structural arrangement and its fineness. When the heat setting temperature increases above its softening temperature, the spandex loses its residual energy. As the spandex soft segments became harder, it shrinks to compress the knit loops. Thus, higher the heat setting temperature, the lower will be the DWR of the fabrics. Effect of heat setting temperature and extensions level on dynamic work recovery of knitted fabric has a significant effect on both walewise direction [F observed > F critical at F (2, 6) = (for Heat setting temperature) and 6.45 (for extension level) P <0.05] and coursewise direction [F observed > F critical at F (2, 6) = (for Heat setting temperature) and (for extension level) P <0.05] Effect of compacting temperature on DWR of cotton / spandex fabrics Effect of compacting temperature on dynamic work recovery of the cotton/ spandex fabric has been given in Figure 8.2. When the compacting temperature increases, DWR of the fabric first increases from 70 0 C to 85 0 C and then decreases from 85 0 C to C in most of the cases at walewise and coursewise directions. For medium compacting temperature (85 0 C), dynamic work recovery of the fabric is higher (2 10 %) than that of fabrics treated at low and high temperatures in walewise direction. In the case of coursewise direction, the fabrics compacted at 85 0 C and at C have higher DWR than that of fabric compacted at 70 0 C. When the fabric extension increases from 20% to 50%, the DWR of the fabrics first increases from 20% to 30% and then decreases for 40 % and 50% extension in both walewise and coursewise directions.

12 103 DWR (%) Low Medium High 20% 30% 40% 50% Compacting temperature (a) Walewise direction 78 DWR (%) Low Medium High Compacting temperature 20% 30% 40% 50% (b) Coursewise direction Figure 8.2 Effect of compacting temperature on DWR of cotton / spandex fabrics

13 104 Compacting is the process of mechanically pushing the loops preferably towards each other to make the loops as close as possible to control the elastic loop deformation. Medium compacted fabric has given good compaction with respect to geometrical characteristics. Effect of compacting temperature and extension levels on dynamic work recovery of the fabric has significant effect in walewise direction. F observed > F critical at F (2, 6) = (for Compacting temperature) and 8.77 (for extension level) P <0.05. Compacting temperature has no significant effect on dynamic work recovery of the fabric and different levels of extension have significant effect at coursewise direction. F observed < F critical at F (2, 6) = 4.90 (for compacting temperature) P >0.05 and F observed > F critical at F (2, 6) = (for extension level) P < Stress at Specific Extension Effect of heat setting and compacting temperatures on the fabric stress at different extensions were observed and tabulated in Table 8.4. Stress values of the fabrics at different heat setting temperatures are given in Figure 8.3. The fabric extension increases with increasing stress value of the fabric in both walewise and coursewise directions. Low and medium heat set fabrics have shown almost same kind of trend in both the directions. The heat setting temperatures in the range of C to C is preferable to reduce the stress value at both the directions, as most of the stress values are lesser than 0.1 N /mm 2. Heat setting temperature up to C may not raise the problem of skin irritation or friction when the fabric is converted into garments.

14 105 Table 8.4 Stress values of cotton / spandex fabrics treated with different heat setting and compacting temperatures Effect of heat setting temperature Walewise Direction Coursewise Direction 20% 30% 40% 50% 20% 30% 40% 50% Low Medium High Effect of compacting temperature Walewise Direction Coursewise Direction Low Medium High Heat setting temperature has no significant effect on the fabric stress and different levels of extension have significant effect at walewise direction. F observed < F critical at F (2, 6) = 2.27 (for heat setting temperature) P >0.05 and F observed > F critical at F (2, 6) = (for extension level) P <0.05. Effect of heat setting temperature and different levels of extension on stress of the fabric have significant effect at coursewise direction. F observed > F critical at F (2, 6) = (for heat setting temperature) P >0.05 and F observed > F critical at F (2, 6) = (for extension level) P <0.05.

15 Stress Low Medium High % 30% 40% 50% Extension levels (a) Walewise direction Stress Low Medium High % 30% 40% 50% Extension levels (b) Coursewise direction Figure 8.3 Effect of heat setting temperature on stress values of cotton / spandex fabrics

16 107 Stress % 30% 40% 50% Extension levels Low Medium High (a) Walewise direction Stress % 30% 40% 50% Extension levels Low Medium High (b) Coursewise direction Figure 8.4 Effect of compacting temperature on stress values of cotton / spandex fabrics

17 108 Stress values of the fabrics at different compacting temperatures are given in Figure 8.4. The fabric extension increases with the increasing stress values of the fabric at both the directions. Not much change in the stress value up to 30 % extension level is observed and after that the high compacted fabric shows higher stress value in walewise direction. Medium compacted fabric has shown higher stress value in the case of coursewise direction. The compacting temperature has significant effect on fabric stress values. Most of the stress values are lesser than 0.1 N / mm 2. Effect of compacting temperature and extension level on stress value of knitted fabric has significant effect in walewise and coursewise directions. F observed > F critical at F (2, 6) = 7.65 (for compacting temperature) and (for extension level) P <0.05 in walewise direction. In the case of coursewise direction, F observed > F critical at F (2, 6) = (for compacting temperature) and (for extension level) P < CONCLUSION The study analyses the effect of heat setting and compacting temperature on dynamic elastic behaviour such as DWR and stress at specific extensions of cotton / spandex knitted fabric. The heat setting temperatures have significant influence on DWR of the fabric in both walewise and coursewise direction. But, the effect of compacting temperature on DWR of the fabric influences only in walewise direction. The stress at specific extension is significantly affected by both the heat setting and compacting temperatures in most of the cases. It is necessary to optimise the heat setting temperature for different construction of elastic knitted fabrics, in order to achieve the better dynamic work recovery of the fabric for designing a good elastic garment for the right end use application.