Novel trends in continuous pretreatments

Dept. Of fiber and TextileProcessing, Udct, Matunga

Textiles have been subjected to chemical pretreatment processes since time immemorial. Human ingenuity and imagination, craftsmanship andresourcefulness are evident in textile products throughout the ages; we are tothis day awed by beauty and sophistication of textiles sometimes found inarcheological excavations. This paper mainly describes the need of continuous pretreatments its recipe. It also gives the update of new machinery design for continuous pretreatments.

INTRODUCTION

After January 2005 the quota restriction ended and theentire textile sector is now open for foreign direct investment (FDI) withultimate aim to be par with the new economy information technology (IT) sector.The textile sector Foreign exchange earning amounts to about US$11b a year isabout 35% of total foreign exchange earned by the nation. This amounts to approximately Rs. 51700 crore per year (@ Rs47 per dollar). This figure is to be raised 5 fold atUS$ 50b per year (Rs 2, 35,000 crore per annum) by the year 2010, a level projected for the Indian IT sector by 2008¹.

To achieve the above shown statistics in todays era ofglobal competition its crucial to produce goods in shortest possible timewhile consuming least possible amount of energy, materials and water andconsequently at lower processing costs. Well a logical approach is to shortenthe sequence by combining the three operations-desizing, scouring bleachingwhich is popularly known as the D-S-B process. Thus efforts have, been made toimprove the efficiency of these processes and also to combine these processes to reduce the time consumed and also the energy and water consumption along withincurring lesser cost of production and increased productivity. This alsoreduces the effect of chemical pre-treatments of textiles on the ecology byreducing substantially the amount of effluent released from the process house.

This paper discusses the single stage preparatory process which combines the desizing, scouring and bleaching operations into a single stage thatare carried out in individual unit operations otherwise².

COMBINED PREPARATORY PROCESSES

In a combined process, hydrogen peroxide or sodiumhypochlorite can be used as a desizing and bleaching agent and sodium hydroxidecan be used as a conventional scouring agent along with a wetting agent at pHof 10.5 11.5 (using soda ash). Various combined processes have been developedin the past using the above chemicals and also using new scourant formulations³.

Some attempts have, also been made to review the variousmethods of single stage preparatory processes earlier. Preoxygen boosters wereconsidered to be the key compound in developing a DSB process, which can, also,interact with starch and PVA sizes and to act as desize-scour boosters.Processes without peroxygen boosters were, also, developed incorporating higherstrength of peroxide bur were not widely accepted. Role of peroxide activatorsnamely tri-sodium phosphate, tetra sodium pyrophosphate, sodium hexametaphosphate, magnesium sulphate and sodium salts of EDTA were analyzed incombination of sodium silicate in the single stage preparatory process³.

PEROXIDE BASED SINGLE STAGE PROCESS

Single stage preparatory process using hydrogen peroxide hasbeen developed successfully in past for starch and acrylic-base sized textilematerials, and, also has been used for processing various fabrics. In acombined process, hydrogen peroxide can be used as a conventional scouringagent along with a wetting agent. The treatments can be carried out in thestainless steel containers. Caustic soda provides required alkalinity forscouring and activation of hydrogen peroxide. When activated, hydrogenperoxide degrades the sizing materials at lower temperatures and at highertemperature, bleaching occur along with completion of desizing.

Silicate is used as a stabilizer and to obviate the catalytic activation of heavy metals as well as acts as a buffer. Colouring materials are decomposed into soluble products and acidic impurities in cotton fibers. Buffered systems showed improved whiteness index compared to the un-buffered system since pH is maintained throughout the bleaching operation. Un-buffered system the loss in tensile strength was observed after 55-60% of peroxide decomposition, even though there is no improvement in whiteness⁴.

Table-1 Recommended Recipes for Single Stage Preparatory Process²

In another novel approach steam procedure has been use to combine all the preparation processes into a single step helps to complete the preparation within 2-4 minutes, applying the solution, consisting alkali, hydrogen peroxide and auxiliary to the fabric, it is steam with saturated steam followed by wash.

EFFECTIVENESS OF THE PROCESSES

Peroxide-Alkali Process

Absence of either sodium hydroxide or hydrogen peroxide in the peroxide based process, results in very low weight loss, indicating very low efficiency, Increase in weight loss due to scouring action and desizing occurs with increase in concentrations of peroxide and sodium hydroxide and higher weight loss is achieved at a temperature of 95^oC. The whiteness index is found to be directly proportional to the concentration of sodium hydroxide, which is mainly due to the increased formation of per hydroxyl ion at higher pH, which acts as the bleaching agent and also with the processing time⁵.

Peroxide-Solvent Assisted

In peroxide-solvent assisted scouring agent based process substantial weight loss, strength loss a lowering of wetting time occurs up to a concentration of 4% of scouring agent. Also a very small change in these parameters was observed at higher concentration levels. For a given temperature and scouring agent concentration a rapid increase in weight loss, a steep decrease tensile strength and wetting time are observed with respect to an increase in peroxide concentration upto2%.

Apparent linear relation exists between a) weight loss and tensile strength. B) Wetting time and weight and loss and c) strength and wetting time.

Whiteness index show a linear relationship with the concentration of hydrogen peroxide and other factors like scouring agent concentrations, time and temperature had a little effect. Under identical conditions, acrylic, sized fabrics showed superior results than scratch sized fabrics in terms of whiteness and wettability. The poor results in the case of starch sized samples can be attributed to higher add-on levels of size and incomplete removal of tallow from the fabric during desizing.

COMBINED DESIZING SCOURING-BLEACHING MERCERISATION PROCESS

Cotton fabrics after impregnation with a mixed solution containing sodium hydroxide hydrogen peroxide and trichloroethylene for 3 minute at different temperatures and fabrics are taken for curing at 120^oC, followed by neutralization and washing. In this case, the strength retention, whiteness index, dye ability are mainly controlled by concentration of sodium hydroxide, dipping time, temperature and curing duration. But no further literature is available in this regard.

COMBINED DESIZING SCOURING AND BLEACHING OF COTTON USING OZONE

In the present work, an attempt has been made to combine all the three grey preparatory processes, such as desizing, scouring and bleaching. Here, ozone is used to desize, dewax and decolour the grey cotton fabric. To improve the fabric properties two stage bleaching is suggested wherein the fabric is treated with ozone followed by hydrogen peroxide⁶.

APPARATUS

The equipment used for the bleaching with ozone has three components: the ozone generator, the applicator and the ozone destroyer. The ozone generator of 8g/h capacity was supplied by Ozone Tek Ltd, India. The input for the generator is oxygen from a pressurized cylinder. The generator supplies the required concentration of ozone-oxygen mixture to the applicator. The applicator is a glass cylindrical tube with a diffuser at the bottom.

PROCEDURE

Wet grey cotton fabrics having 24% moisture content (wet pickup) were placed in the application chamber and exposed to 100 g/m concentration of ozone oxygen gas mixture at pH 5 using acetic acid for a specified time (1-7 min) at room temperature (about 30 C) for the combined desizing, scouring and bleaching. The gas mixture flow was maintained at a constant rate of 0.5 liter/min for the all experiments.

Inference

The above study shows that grey preparation with ozone can be completed in one or two minute. This process has additional advantages such as savings in thermal energy, water and chemicals. The quality of the fabrics processed with ozone is comparable with that of the fabrics processed by conventional method of desizing scouring and hydrogen peroxide bleaching.

ENVIRONMENTALLY FRIENDLY PROCESSES

Alkaline scouring is a commercially well established and efficient process. The drawback of this process, however is the use of large amounts of caustic (up to 80 kg/ton fiber material) as well as large quantities of rinse water once the scouring process is completed while the rinse water could be reused, the scouring effluent with its high BOD, high COD, and alkalinity presents a major problem. Increasingly stringent effluent regulations have thus generated interest in procedures that are nontoxic and environmentally caring⁷.

Pectinase

Pectinic components as found in raw cotton mainly consist of neutral and acidic hetero polysaccharides with different molecular weights and degrees of esterification. Pectinases capable of hydrolyzing pectinic substances are generally enzyme complexes containing esterases and depolymerases with random or terminal activities. Their activity optimum usually lies in the slightly acidic region at temperatures around 50C. With the conventional caustic scouring procedure it is not possible to completely remove pectins especially if they are associated with calcium^7.

Cellulase

Cellulases enhance the effect of pectinase to a certain extent and add softness to the cotton fabric. They often accompany pectinases in small amounts. If used or scouring cellulases hydrolyze cotton cellulose lifting off non-cellulosic impurities in the course of the reaction. Typical fiber damage caused by cellulase cannot be excluded⁷

Lipase

Cotton waxes consist of various hydrocarbons, fatty alcohols and acids, and their respective esters which are the major reason for the hydrophobic nature of the unscoured cotton fiber⁷.

Another option is scouring with organic solvents during which these hydrophobic components are extracted. Scouring in presence of a non-specific lipase can ease the removal of fats and waxes. Many lipase enzymes are capable of non specifically cleaving various natural lipids and oils, generating glycerides, glycerol, and short-chained fatty acids⁷.

Synergism Effect of Cellulase and Protease:

Cellulase and protease mixtures have good synergistic effect for the scouring of cotton fabric. Cellulose breaks the linkage of cellulose chain in primary wall and proteases hydrolyse the protein linkage from the surface of the fiber. The reaction of protease exhibits more sites in the primary wall for cellulase reaction. These synergistic enzymes combination removes effectively the protein based impurities with partial hydrolysis of cellulose. Pections and other impurities which are present in cotton fiber to about 1.5-2.5% cannot be effectively removed. Pectinase and other specific enzymes along with above synergistic system can produce scouring efficiency just equivalent to conventional caustic ecological balance⁷.

MACHINERY USED

MACHINERY FOR CONTINUOUS PROCESSING

Dornier has developed open width machinery for pretreatment of knitted fabric called EcoCombine. The EcoCombine mercerizing and bleaching range consists of an impregnation unit and washing sections. The mercerizing unit is improved by heating and dwelling zone for the peroxide bleach. This not only improved quality but also significant cost advantages both on wet-on-wet and dry-on-dry applications⁸.

The parameters and function of machine are:

Fabric speeds in individual sections are accurately mentioned as per the indication of fabric type and conditions.

It has washing water comparison with set point for washing unit.

Dancer roll position can be change as per the fabric type.

Swastik Group of company has developed continuous bleaching plant with Combi Steamer for all type of textile substrate. Continuous pretreatment machine consists of impregnators which are equipped with wiper Rollers of suitable rubber to obtain 100% or more liquor pick up (upto 130%). A vacuum slot is located at the entry of fabrics in impregnator from preceding Hi squeeze Nip passes through the trough with minimum liquor for light interchange dip before being subjected to a spray of chemicals with designed concentration and quality with the help of a well designed, fully automatic dosing system. A set of wiper Rollers are provided in this machine for smooth operation.

Main features of machine:

Well equipped with various automatic controls such as level controller in impregnator, micro Processor controller for dosing of chemicals.

Swastik offers pretreatment plant in single of multiple stages and in combination with various units to cater to small production of 30,000 mts/day to full size range for 100000 mts/day production⁹.

RACO YET SYSTEM

Ramisch-Kleinwefers Raco Yet System is latest technology which combines D-S-B with reaction time of 1-3 minutes. This technology is independent of type of size and even blends. A new multifunctional auxiliary has been developed for this technique which involves forming a aerosol of steam and chemical liquor which has high specific surface. The recipe contains soft water, silicate, hydrogen carbonate, additives such as cattoclarin AS and H₂O₂. Recipe changes are rapid and optimal for environmental protection¹⁰.

CONTINUOUS DYEING AND FINISHING MACHINE

Prashant Texmach Pvt. Ltd. developed continuous dyeing and finishing machine for open width nylon, acrylic and polyester tape.¹¹

Machine specification:

It is equipped with disentangling unit with positive feeder at entry side.

A set of padding mangles (Two rollers having 350 mm width) pre dryer (10 Mts tape content), steaming chamber (40 Mts tape content), five washing device (with provision of hot water in two tank) and drying and finishing chamber.

COMPARISON BETWEEN CONTINUOUS VS BATCH WISE PRETREATMENT

Conclusion

In todays era textile companies are increasing there market share internationally by leaps and bounds. With this every processor is forced to make maximum utilization of resources available thereby increasing the pollution load. With stringent government laws for environment protection many companies should find substitute for conventional preparatory process. In developing countries like India were electricity, water, and other resources are limited. Substitute like combined preparatory process would hold key position to abate pollution problem and saves enormous amount energy.

Reference

Deo H.T.,Indian Journal of Fiber and Textile Research, Vol. 26 (3), 61(2001).

Sarvanan D., Journal of Textile Association, March-April (2005), pp277

Gulrajani M. L., Sukumar N., Journal of Society of Dyers and Colourists, 100(1), (1984),pp21.

Gulrajani M. L., Colourage, 36(2), (1990) 19.

Ammayappan L., Muthukrishanan G., Sarvanan Prabhakar C., Man Made Textiles in India (1), 46, (2003), pp23.

Prabaharan M., Venkatarao J., Indian Journal of Fibre and Textile Research, 28, December(2003), pp437-443.

Moussa K., Traope, Gisela Buschle, Textile Chemist and Colourists and American Dyestuff Reporter, 32, 12 (2000).

www.lindauer-dornier.com

www.swastiktextile.com

Int. Dyer, 184(2), 15(1999).

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