Structural Analysis of Cotton/Lycra Corespun Single Jersey Knitted Fabrics

Lycra/Cotton garments find numerous applications in knitwear industry and in this study the corespun yarn is used with Lycra in the core and Cotton forming the sheath. The advantage of using corespun (core cover) instead of plaiting technique where cotton and lycra are fed separately is that the loop formation of corespun yarn is better than plaited yarn [1]. The ability of the corespun knitted fabric to maintain the dimensional stability and loop shape factor is studied for three different structures with 3 different loop length and the results are discussed in this study [2-5]. In accordance to Munden [6] the dimensions of the relaxed fabrics in most of the simple weft knitted fabrics are solely determined by the length of yarn in the knitted loop. Munden recognized two relaxed states, namely a Dry relaxed state, taken up after some day’s exposure to air and a wet relaxed state, taken up after soaking in water. Nutting [7] has shown that the wet relaxation process is irreversible, but the fabric dimensions depend upon water temperature used for relaxation and the regain of the fabrics when they are measured. The use of single dry or wet relaxed state is not only convenient but a necessary simplification.


Introduction
Lycra/Cotton garments find numerous applications in knitwear industry and in this study the corespun yarn is used with Lycra in the core and Cotton forming the sheath. The advantage of using corespun (core cover) instead of plaiting technique where cotton and lycra are fed separately is that the loop formation of corespun yarn is better than plaited yarn [1]. The ability of the corespun knitted fabric to maintain the dimensional stability and loop shape factor is studied for three different structures with 3 different loop length and the results are discussed in this study [2][3][4][5]. In accordance to Munden [6] the dimensions of the relaxed fabrics in most of the simple weft knitted fabrics are solely determined by the length of yarn in the knitted loop. Munden recognized two relaxed states, namely a Dry relaxed state, taken up after some day's exposure to air and a wet relaxed state, taken up after soaking in water. Nutting [7] has shown that the wet relaxation process is irreversible, but the fabric dimensions depend upon water temperature used for relaxation and the regain of the fabrics when they are measured. The use of single dry or wet relaxed state is not only convenient but a necessary simplification.
As the determination of loop length and dimensional parameters are of prime importance in defining the end use of the knitted fabrics, the present study is the continuation of the earlier studies in this field. So the objective of this research work is to establish the dimensional constants of three samples and to investigate the effect of loop length on non-dimensional parameters in different relaxed states.

Materials and Methods
The experimental part of this study involves corespun yarn having lycra in the core and cotton in the sheath of 30's Ne was purchased from M/s.Arunachala Gounder Textiles, Pallipalayam, Erode district. The yarn count was measured in indirect system and the test results are shown in table 1.
The yarn samples are tested to find unevenness and imperfections on an Uster-UT3 evenness tester using testing speed of 100m/min and time 3 min. The results are shown in

Dry Relaxed State (DRS):
The fabric is relaxed in grey state under standard test conditions at 65 ± 2% RH and 27 ± 2°C for two days.

Wet Relaxed State (WRS):
This is achieved by subjecting the fabric to static soak with 0.7% wetting agent for one day and the fabric is allowed to relax for at least 3 days.

Heat Setting (HS):
The fabric is stretched to the required level and set permanently by passing through the heating and curing chamber at 180°C for 40 seconds.

Bleaching:
The fabric is treated with 0.7% wetting agent and to avoid creasing, 0.5% lubricant is added. Stabilizer of 0.3%, Caustic Soda of 3% and Peroxide of 2.5% is added to the sample and the machine is run for one hour at 95°C. After one hour the bath is drained and the machine is run with killer of 0.7% and acetic acid of 1.0% at pH of 6.5 and a temperature of 80°C for half an hour. Killer is used to remove peroxide and acetic acid is used for neutralization.

Dyeing (DY):
The recipe for dyeing is Blue EDG of 0.90%, Salt of 1.35 g/l, Soda ash of 20 gms, M:L ratio is 1:8 and the temperature is 65°C. The salt is added to obtain evenness of color throughout the fabric and soda ash is added to fix the color in the fabric.

Compacting (COM):
It is done to stabilize the cotton yarn. The samples are compacted at 100°C for 30 seconds. The fabric is subjected to compressive shrinkage in the width direction.
The non directional parameters like K c (Course constant), K w (Wales constant), S (Stitch density), K s (Stitch density constant) and K c / K w (Loop shape factor) are calculated as per the formula given below. Loop shape factor=K c /K w .

Relationship between loop length and loop shape factor
It is established from figures 2-6 the loop shape factor (K c /K w ) for            of the samples at different relaxed states reduces with increase in loop length which shows that loop shape is not maintained with increase in loop length. This shows that the fabric undergoes progressive length wise shrinkage, resulting in gradual decrease in course spacing. The analysis also shows that the loop shape factor for Honey comb structure is higher in all relaxed states, which is due to its stabilized structure.

Relationship between loop length and aerial density
From the test results it is observed that the aerial density increases after a sample is subjected to relaxation. This is due to the tendency of the yarn in a loop to try to regain its original shape after each relaxation stages. This is irrespective of the loop lengths and the value is higher in compacting stage because the fabric is subjected to compressive shrinkage in the lengthwise direction.

Relationship between loop length and bursting strength
From the observations it is known that the bursting strength increases after the fabric has completely relaxed because the course and wale density of the sample has increased from dry relaxed stage to compacted stage. But on comparison of the bursting strength in the same state, but with increase in loop length, it showed a reduction because of less stitch density. This trend is same on all samples.

Relationship between loop length and thickness
From the test results it is established that the thickness of the fabric reduces after different relaxation states for a particular loop length, which is due to the yarn attaining the maximum relaxed state of minimum residual energy. But as the loop length is increased the thickness is reduced which is obvious in a grey fabric. But after subsequent relaxation process the thickness is reduced.

Regression analysis
The regression analysis for the relationship between loop shape factor and loop length at different stages of relaxation are made and this is shown in each of the figures 2-6.
Statistical analysis of loop shape factor between loop length and different relaxed states using 2 WAY ANOVA (within structure) at 95% confidence level Popcorn Hypothesis 1: There is no significant difference in the loop shape factor between relaxed states.

Hypothesis 2:
There is no significant difference in the loop shape factor between loop lengths. F ratio between relaxed states is 6.3 and F ratio between loop lengths is 21.2 The calculated F value for both the conditions was more than the table value and hence both the hypothesis is rejected ( Table 3).
The analysis showed that there is significant difference in the loop shape factor between relaxed states and loop lengths which imply that the loop length of the fabric sample is not able maintain its shape regardless of the different relaxation stages which is confirmed by the earlier work of Nutting [8].

Double airtex
Hypothesis 1: There is no significant difference in the loop shape factor between relaxed states.   The calculated F value between relaxed state was less than the table value and hence hypothesis 1 is accepted and the calculated F value between loop length was more than the table value and hence hypothesis 2 is rejected. The possibility of significant difference between loop shape factor and relaxed states is due to its complexity of the structure of the sample which is reflected in the increase in loop length against loop shape factor (Table 4).

Honeycomb
Hypothesis 1: There is no significant difference in the loop shape factor between relaxed states.

Hypothesis 2:
There is no significant difference in the loop shape factor between loop lengths. F ratio between relaxed states is 0.19 and F ratio between loop lengths is 1.02 The calculated F value between relaxed state and between loop lengths was less than the table value and hence both the Hypothesis is accepted. This result shows that there is no significant difference in the loop shape factor between loop lengths and different relaxed states (Table 5). This is due to torsional resistance of the yarn and more balanced structure of the sample.