Contribution of fabric characteristics and laundering to shrinkage of weft knitted fabrics

Contribution of fabric characteristics and laundering to shrinkage of weft knitted fabrics

Textile Research Journal, Mar 2003 by Onal, Levent, Candan, CevzaABSTRACT

Cotton and cotton/polyester blended weft knitted fabrics are prone to shrinkage during finishing processes and customer usage. The effect of various fabric characteristics on the shrinkage behavior of weft knits is as important as that of the fiber characteristics. In this work, three different single jersey knits with varying fabric tightnesses, yarn types, and fiber blends are selected for their characteristics. The contribution of each characteristic to shrinkage behavior is presented statistically using an analysis of variance. The fabrics are subjected to several laundering cycles followed by tumble drying. Changes in the dimensions are measured right after each process and cycle. Double pique knits shrink less widthwise but more lengthwise than the other knit types. Yam type and fiber blend have a relatively more significant contribution to fabric shrinkage lengthwise than widthwise. Knit loop formation is defined in each relaxation stage from dry relaxed to fully relaxed. The effect of dyeing on fabric relaxation is also studied.

Fabric shrinkage is a serious problem for knitwear, originating from dimensional changes in the fabric, particularly stitches. It has become even more prevalent in recent years due to the popularity of casual wear such as tights, pants, blouses, and sportswear. Shrinkage is a result of the combined effect of numerous factors such as relaxation, finishing, dyeing, and effects of machinery. The significance of this problem has been investigated by several researchers, who focused mainly on the geometry and dimensional relations of knitted structures [1, 2, 7, 8, 10-12, 14-19, 21, 22]. Suh [21] discussed the relationship of relaxation shrinkage in cotton knitwear and yarn.

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Due to the influence of fashion, the production of knitted goods has been expanded, with new fabric designs created with different fiber blends and knit structures. The effect of the former on fabric shrinkage was extensively investigated in various combinations, but so far as the latter is concerned, there is still a lack of information in the literature. Most work has focused particularly on double jersey knits [1, 8, 10, 11, 14, 18, 22], with some on plain single jersey knits [15-17, 19].

Using the data presented in our work, we have an opportunity not only to re-evaluate some of the previous studies, but in particular to examine the extent of the effects of different single jersey knits (double pique, plain jersey, and fleecy), fiber content, and yam types on the shrinkage of knitted fabrics laundered several times. We also explain the role of the tuck stitch and fabric dyeing on fabric relaxation to determine their significance on fabric and loop relaxation. In doing so, we present statistically the contribution of each factor to the shrinkage behavior of weft knitted fabric using an analysis of variance (ANOVA).

Experimental

For the work, three different fabrics-double pique, plain jersey, and fleece-were knitted on a 28-cut Monarch circular knitting machine in 30" in diameter samples. Detailed information about the yams used to produce the fabric samples with the relevant structures (Table I, reference 6) was given in our previous study [6].

Each fabric type was produced with three different stitch lengths, i.e., fabric tightnesses, and the samples were classified in accordance with the following general form: S, L, and F were designated plain jersey, double pique, and fleecy, respectively; R or O where R/O stand for yarn type (ring spun and open-end spun), and C/M where C represents 100% cotton open-end and M represents 50/50 PET/cotton blend for fiber blend classification. For instance, SRC is a plain knit, ring spun, 100% cotton fabric. In the case of the fleecy knits, FOMM defines both sides of the fabric and the inlay yam made from 50/50 blended open-end yam. All fleecy fabrics were produced only from open-end yarn with 100% cotton and 50/50 cotton/polyester fiber contents.

In the case of the fleecy fabrics, the inlay stitch length was held (more or less) constant. Except for the fabrics knitted from the blend yarns (see Table I in reference 6), some of the yardage of the gray goods was set aside, and the remainder was scoured, bleached, and dyed under the same controlled conditions. The fabrics made from blend yams were only rinsed. All the fabric samples were then subjected to the relaxation treatments, which consisted of dry relaxation and three cycles of laundering at 60 deg C with 1% Lissapol-N (wetting agent), followed by tumble drying at 70 deg C for 60 minutes after each cycle. The details of these processes can be found in our previous work [6].

where 1 represents the distance between the data lines marked on the fabric, and subscripts 0 and I represent the initial condition and following laundering, respectively. Shrinkage was measured in both the lengthwise and widthwise directions. Measurements were repeated five times in different sections of the sample, and the average was reported.

Only 100% cotton fabrics were dyed. The process started with NaOH liquor to keep the bath caustic, and peroxide, a peroxide stabilizator, a stain remover, and a wetting agent were also added. The dyebath and final washing (to remove excess dye from the fabric and to normalize the pH) finalized the dyeing process at 60 deg C. Dyed fabrics were then calendared to dry them.

Results and Discussion

Loop STRUCTURE AND DYEING

The loop structure of a knit fabric changes as relaxation treatments progress. The contribution of the dry relaxation process to the dimensional stability of the fabrics is quite slow when compared to the laundering and tumble-drying treatments. Coban [7] stated that 34 days of open-width relaxation is necessary for a fully relaxed fabric, which is nearly impossible in commercial applications. It appears that agitation, together with heated water with a wetting agent, helps to remove the stress on the loops much more effectively in washing and forced drying cycles. The dimensions of the loops change semi-permanently and retain the new shape. Fabric dyeing is also a kind of wet treatment, whose effect on loop shape is particularly apparent widthwise, as in the case of wet relaxation.

The yarns retain water and dyestuff when fabric is immersed in the dyebath. Press-drying in the dyeing process, which leads the different loop shapes from the dry relaxed loops, follows the dyebath. Despite very few course/cm changes, wale count (wales/cm) changes considerably (Tables II-IV in reference 6). Hence, haspel dyeing can be accepted as somehow modified wet relaxation with an additional press-drying stage, in that length shrinkage decreases due to calendaring in the stenter.

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The effect of relaxation steps on loop shape variation can be inferred from the ratio of courses/cm to wales/cm, which is defined as the loop shape factor [15]. Figure 1 is a schematic of the loop shape factor versus relaxation stages at the same tightness factor for each sample group. Unlike the irregular pattern observed from the dry relaxed to initial laundry stages, loop distortions are pretty much in line with laundry stages. The variation is different for each knit structure. The loop shape factor shows a wavy trend for pique fabrics, especially those made from ring spun yarn. The cause of the lower shape factor in the dyeing stage might be attributed to relatively higher width shrinkage compared to length shrinkage. The origin of the variation may be due to the double tuck stitches in the fabric unit cell. Nearly constant values for fleecy and plain knits maintained throughout all the relaxation steps can be attributed to the equal change of wales/cm and courses/cm.

Repeated laundering followed by tumble-drying helps the loops to approach their relaxed shape, which could be accepted as the minimum energy state. After loops reach their fully relaxed shape, the fabric becomes more dimensionally stable with fewer tendencies to shrink.

EFFECT OF KNIT STRUCTURE

For the three different single jersey knits, width and length shrinkage is different. This is not very surprising because various stitch formations (knit, tuck, or miss) create individual knit structures, causing the relevant fabric structures to have different dimensional behaviors.

The shrinkage amount of each knit structure has a reverse proportion with shrinkage direction. With pique knits, length shrinkage is apparently greater than width shrinkage, while the case is reversed for plain and fleecy knits, shown in Figures 2a and b. Our results agree with those of Quaynor et al. [17] for plain knits. The existence of double tuck stitches in pique knit is believed to be responsible in this reverse relation. The shorter loop length of the tuck stitch possibly decreases the amount of width shrinkage.

The nature of shrinkage for fleecy knits is different from other single jersey knit structures. Yam counts on the face and back of the fabric are Ne30 and Ne20, respectively. Since the surface yarn counts of plain jersey and fleecy knits are the same, the existence of the inlay yam is expected to be the important factor affecting the shrinkage dissimilarities of these two knit structures. For the same yarn type and fiber percentage, the shrinkage amounts get closer to each other from the first to the third laundry cycle (Figure 2). The shrinkage difference not only depends on the knit type but also the fiber percentage. The ANOVA reveals the significance of that interaction where the relation is stronger for length shrinkage. For the same yarn type, 100% cotton fleecy fabric has nearly the same width shrinkage as plain knit, whereas polyester/cotton blend fleecy fabric shrinks less than plain jersey fabric. Despite tuck stitches in fleecy knits, this unexpectedly lower widthwise shrinkage of 100% cotton plain knits may be attributed to the higher shrinkage potential of the inlay yarn, which has a lower linear density (Ne20). Heap [9] and Sharma et al. [19] agreed with the effect of linear density on shrinkage. Sharma et al. [19] implied that shrinkage increases for the same tightness factor in plain knit fabrics with increasing linear yarn count.

The effect of each relaxation step on shrinkage is almost the same for all knit structures. There is a big jump from dry relaxed fabric to the initial laundry step. Nevertheless, consecutive laundering does not cause higher shrinkage increments as does the first laundering. The results of former researchers [7, 14, 17] are the same as our findings.

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EFFECT oF FABRIC TIGHTNESS

The fabric cover factor increases with further relaxation steps. During the relaxation period, the knitted structure tends to reach the minimum energy level, resulting in fabric shrinkage.

As length shrinkage decreases with repeated laundering, the tightness factor decreases as well. Lengthwise shrinkage with laundering cycles decreases with the decreased tightness factor, perhaps because fabrics with a lower cover factor are able to recover much more after removal from the machine. Thus, the slacker the fabric, the lower the length shrinkage during laundering. The case is reversed for widthwise shrinkage. We saw this behavior in each of the three knit types. The literature survey shows that some of the former studies [17, 20] disagree with our work, whereas others do not [1, 11]. We believe the methodologies and materials selected for all these works cause such differences.

EFFECT OF YARN TYPE AND FIBER CONTENT

Even with the same knit type, shrinkage changes with both yarn type and fiber blend. Fiber distribution and physical properties of ring and open-end spun cotton yarns affect the amount of fabric shrinkage. Since of spun yam possesses better extensibility, evenness, and bulk [4, 13], knitted fabrics from these yarns are expected to be more dimensionally stable than ring spun yarn. Therefore, open-end fabrics shrink somewhat less widthwise than ring spun samples for both plain and pique knits (Figure 2). However, the shrinkage amount is nearly the reverse lengthwise.

A comparative study of fabrics knitted from 100% cotton and blended yarns revealed that blended fabrics shrink less. This might be attributed to the bulkier characteristics of blended yarns as well as the polyester fibers, which are less prone to shrinkage. Fiber blends of inlay yarn cause slight shrinkage variations in fleecy knits. The effect of inlay yarn type so little changes shrinkage that it is insignificant compared to other variants based on the ANOVA for a 5% significance interval.

ANOVA RESULTS

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In order to demonstrate the importance of each variable on which the study was based, we performed an ANOVA for both width and length shrinkage behavior of the fabrics under discussion. We used the statistical software package JMP(R) to interpret the experimental data at the 5% significance level. We determined the contribution of main variables such as knit structure, fiber blend ratio, yarn type, fabric tightness, laundering processes, and their interactions using all experimental data. We evaluated the results based on the F-ratio and the probability of the F-ratio (prob>F). The lower the probability of the F-ratio, the stronger the contribution of the variation and the more significant the variable. Thus, contributions of tightness, knit structure, and laundering, given in the order starting from the strongest, were highly significant in terms of the shrinkage behavior of each fabric type in both directions. The contribution of fiber blend was only important in lengthwise shrinkage. Even though yarn type was insignificant in widthwise shrinkage, it was nearly as important as other variables for lengthwise shrinkage. This may result from the fact that width shrinkage measurements of ring and open-end fabrics after dyeing and laundering do remain close to each other (Figure 2a).

Analysis results indicate that the interaction of two and three variables also has a large effect on fabric shrinkage. Knit type X fiber blend X fabric tightness and knit type X yarn type X fabric tightness interactions are particularly as significant as the main variables for length shrinkage. The significance of even three variants proves the complexity of the fabric shrinkage mechanism in weft knitting.

The effect of inlay yarns and their fiber blends on fleecy fabric shrinkage were statistically analyzed separately. Inlay yarns were made from either 100% cotton or 50/50 cotton/polyester. As we expected, the ANOVA revealed that inlay yarns in fleecy fabric partially contribute to width shrinkage, but are accepted as insignificant when compared with other variables such as fabric tightness.

It is clear from the statistical and experimental evaluations that fabric shrinkage strongly affects knit characteristics and wet treatments. Loops gradually relax from dry relaxation to further washing cycles, but the relaxation amount in each shrinkage direction changes according to knit type. There is a big jump between dyed fabric and the first laundering cycle. The effect of dyeing on fabric relaxation appears to be between dry relaxation and full relaxation, or so-called modified wet relaxation.

Knit type and fabric tightness greatly influence fabric shrinkage. While length shrinkage is more than width shrinkage for pique fabric, the tendency is reversed for plain and fleecy knits. For fleecy knits, we have investigated the contribution of inlay yarn count and fiber blend to fabric shrinkage, and the significance level is not as strong as the other variables.

There is a reverse proportion between fabric tightness and the direction of fabric shrinkage. Width shrinkage increases with the increased tightness factor as the laundering process progresses. Hence, the tighter the fabric, the higher the width shrinkage during laundering. The relation is reversed for length shrinkage.

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Yarn type and fiber percentage contribute significantly to knit fabric shrinkage: 50/50% cotton/polyester blends shrink less than 100% cotton samples. However, the contribution of yarn type is not as strong as that of fiber blend in width shrinkage.

To clarify the significance level of each variant on fabric shrinkage, we performed an analysis of variance, concluding that tightness, knit type, and laundering, which are given in the order starting from the strongest in both length and width shrinkage, are highly significant. However, yarn type and fiber percentage are particularly important for length shrinkage.

AcKNOWLEDGMENTS

We are thankful to Hakan Karakaya and the other plant managers of Bilkont's spinning, dyeing, and knitting divisions for permission to produce the fabrics used for the work.

Literature Cited

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3. Booth, J. E., "Principles of Textile Testing," Butterworths, U.K., 1968, pp. 344-355.

4. Burnip, M. S., and Saha, M. N., The Dimensional Properties of Plain Knitted Cotton Fabrics Made from Open-end Spun Yam, J. Textile Inst. 64, (3), 153-169 (1973).

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