Silk is a natural protein fibre with a soothing lustre and elegance. India produces four varieties of silk, namely: mulberry, eri, tussar and muga. Mulberry silk is used for weaving Kancheepuram sari. The procedure for making a Kancheepuram sari starts with preparing the silk threads. The unique features of this sari is its strength, which is achieved by the twisted silk yarns running in both warp and weft direction (Bhasin, 2011).
Silk yarns are dyed in a variety of colours, dipped in rice water and dried in sun before they are set in the handloom for waving. The twisted yarns used for weaving these saris are strong enough to last for 30 to 40 years (Bhasin, 2011). The silk remnants left after weaving the sari are collected from the handloom and processed to develop the recycled yarn.
107 4.5.1.1 Yarn twist
Yarn twisting is an essential process in the production of both staple and filament yarn. The yarn twist has two-fold effects; the twist increases the cohesion between the fibres by increasing the lateral pressure in the yarn, thus giving strength to the yarn. Secondly, twist affects the softness of the yarn and fabric (Lorenz, 1987). In the case of Kancheepuram saris the silk yarns are sized in rice starch.
In order to desize the silk remnants collected from the handloom before carding, the following desizing experiments were conducted:
4.5.1.2 Desizing experiments 1. Hydrolytic desizing
Rod steeping
This is the oldest and simplest desizing method to remove the starch size. The grey fabric is saturated with water (90-100% pick up) by a padding mangle, and allowed to dwell in the rope form for 24h. at 40-45°c. The starch size is degraded by natural microorganisms in the water and can be easily removed by hot water washing. It is a normal practice to treat the batch with some of the process liquor from the previous batch processed, so that the appropriate microorganisms start to degrade the starch present in the yarn. The time and temperature which are usually inversely related, are the only process factors controlled in this method. This procedure does not involve chemical usage, also it is the cheapest method of desizing. The disadvantages are: it can be time consuming and requires large floor space. Defects like: uneven removal size and damage to the fabric are common. For the above reasons rod steeping is uncommon. (Au and Holme, 1999).
Desizing of silk remnants before carding help the silk fibres open and improve the mixing of silk fibres with lamb’s wool while carding. Machine carding method is adopted in order to improve the handle and reduce the hairiness on the surface of the hand-spun yarn. The researcher
108 experimented with rod steeping, alkali steeping and acid steeping methods to desize the yarns. The result for alkali desizing was not satisfactory. as it was noticed in the experiment that the original dyes in the silk remnants were bleached out and had lost the required smooth handle, texture and strength.
Experiment 1
Rod steeping method done in Roaches Pyrotec Dyeing machine Yarn weight - 25g before desizing
L/R= 1:30
98°c 10 min
2°c min 2°c min 40°c
The liquor was boiled to 98°c at a rate of 2°c per min. Once it reaches 98°c it was kept for 10 min and then gradually cooled at 2°c per min till it reaches
40°c. The weight of the yarn reduced to 22g and a large amount of colour bleeding from the silk remnants was noticed.
2. Experiment 2
Acid desizing using sulphuric acid
Weight of yarn before desizing 9 g and L/R =1:200 0.2% of H2SO4 at room temperature and dwell time 7 h.
The desized yarns were dried at room temperature.
The dyed silk yarn had less colour bleeding during the desizing process and the opening of silk fibres in the carding process was comparatively better than the rod-steeped yarn. The results showed that acid steeping is more
109 efficient and it can obtain a more uniform desized fibre. The weight of the yarn had dropped to 6.5g after desizing.
Experiment 3
Acid desizing using acetic acid
Weight of the yarn before desizing 150g
The yarn was conditioned for 24h. before desizing to improve the physical properties.
L/R =1:200
0.2% of acetic acid at room temperature and dwell time 7h.
The desized yarns were dried at room temperature and conditioned for 24h. before carding
The dye bleeding was minimal.
The experiments undertaken involved treatment of dyed silk fibres with diluted sulphuric acid and diluted acetic acid. The difference of desized yarn weight was similar in the case of sulphuric acid and acetic acid desizing. 0.2% acetic acid was chosen for desizing silk fibres as acetic acid is a moderately weak acid and the effluent was rinsed in cold water. 4.5.1.3 Results of desizing experiments
Table 4.1 Comparison of desizing results
Chemical Dwell time Temperature
(°c) Yarn weight difference Rod steeping Distilled water 10min 98 0.88% Acid steeping H2SO4 7-12h 25 0.72% Acid steeping CH3COOH 7-12h 25 0.71%
110 From the table 4.1 acetic acid steeping was finalised for desizing silk yarn in a liquor ratio 1:200 (1g of fibre mixed in 200g of water) with 0.2% of acetic acid in grams is added. The desized yarn was then dried at the room temperature and conditioned for 24h. before carding. The portion of acetic acid used for desizing was minimal and more eco-friendly compared to the rod steeping and sulphuric acid steeping methods.