Rayon has the distinction of being the first man-made fibre developed in the wake of industrialisation of textiles, says Dr Ashok Athalye.
Though rayon was conceptualised in 1855 before synthetic dyestuffs were discovered, considering the complexity of the manufacturing process, it took over 50 years for its commercial production.
Until about a hundred years ago, textiles consisted of only the fibres available in nature. Clothing was made up of fibres of plant origin like cotton, jute, linen or wool and silk derived from animals. However, with growing population and increasing per capita consumption, technical advancements and industrial revolution, the need for newer textile fibres was felt.
Count Chardonnet of France patented the first process of rayon manufacturing in 1884 and is known as the 'Father of Rayon'. In 1905, he started bulk scale fibre production in the United Kingdom. This new fibre soon became popular and owing to its characteristic lustre, texture and feel, initially it was called artificial or imitation silk. In 1925, the US Federal Trade Commission (FTC) officially recognized and confirmed the term rayon as the regenerated cellulosic fibre.
Cotton is the most widely used natural cellulosic fibre but its consumption has almost reached a level of saturation owing to the limited area of cultivation, varying yield per hectare and growing need for other agricultural food crops. The global cotton production in 2017-18 is about 2.6 million MT with an average yield of approximately 800 kg/hectare.
Like cotton, rayon is also made of cellulose but it is regenerated from wood pulp through a chemical manufacturing process. At present, the global production of rayon is estimated to be about 6 million MT and India is the leading supplier having close to 24 per cent market share.
The chemical characteristics of rayon are similar to cotton and it provides soft, smooth and comfortable handle, while its drape and fall simulate properties similar to nylon. Further, the higher moisture absorption and the non-body clinging nature make it ideal for use in hot and humid weather.
Types and generations
Rayon manufacturing has evolved noticeably during its journey over the past century. Based on various modifications and changes, starting from the source of the basic raw material and usage of chemicals solvents to the incorporation of advanced mechanical technologies, it has transformed considerably. However, rayon fibres from each of these generations have unique properties and specific end use applications and thus are still being produced universally.
Viscose rayon is the most widely produced and used regenerated manmade fibre. In 1894, English chemists Charles Frederick Cross, Edward John Bevan and Clayton Beadle patented a safe and practical process of rayon manufacturing and named it Viscose. It is termed as the first-generation rayon.
It is made from cotton linter or wood pulp generally obtained from pine and spruce trees and passes through various processing stages involving alkaline soaking, xanthation and ageing. Recently, bamboo-based viscose was developed and gained popularity.The resultant material is supplied in both the staple fibre and in continuous filament forms.
Fibres are spun using different methods, namely pot spinning, spool spinning and continuous spinning, which characterise yarn size, diameter and strength. Different varieties of yarns, such as mono-filament yarns, multi-filament yarns and spun yarns, permit the manufacturing of a wide variety of fabrics suitable for apparel and home furnishings.
Two other manufacturing routes and processes were developed during the initial phase. One, discovered by French chemist Paul Schutzenberger and modified further by Swiss chemist brothers Camille Dreyfuss and Henri Dreyfuss in 1865, is called acetate rayon. Being hydrophobic, it found usage mainly in non-textile industrial applications. The other variety known as cuprammonium rayon was developed in 1899 and later commercially launched by German businessman Johann Peter Bemberg, was initially used as linings in suits and dresses, but became obsolete due to environmental concerns regarding its manufacturing process.
Modal, developed in Japan in 1951, is considered to be the second-generation rayon. Considering the wet strength weakness of viscose, demand for high-strength rayon variety was growing, which led to the development of the high wet modulus fibre variety. Modal is strong when wet and extremely soft. Its higher durability and appearance retention make it popular for use in intimate garments and sleepwear.
Polynosic is another term used for modal. Like cotton, it can be further mercerised for increased strength and lustre.It is used alone or in blend with other fibres for the intended usage in apparel clothing and home furnishings. Modal fabrics are machine washable and have less pilling tendency due to lower surface friction. High-tenacity rayon is another modified version of viscose that has almost twice the strength of modal. This type of rayon is typically used for industrial purposes like tire cord.
Lyocell, considered the third-generation rayon, was originally developed by US company American Enka in 1972 and later commercialised in 1990 by Courtaulds in United Kingdom. It uses the eucalyptus tree as its source of cellulose and is made using a specialty solvent recovery process and involves dry jet-wet spinning. It is promoted as the most environment-friendly rayon and is also widely known as tencel by one of its commercial name.
A new variety of rayon has been developed recently. It is claimed to have the lowest environmental impact in terms of extent of water consumption and carbon footprint during its entire manufacturing value chain. It is based on sustainable wood resource, ecological production and supply chain transparency. This could be termed as the fourth-generation rayon.
Properties and processing
The building block of rayon is cellulose as in cotton, but during the process of regeneration from wood, the cellulosic chain length gets reduced to almost 10-15 per cent of that present in cotton. Also, it has comparatively more amorphous region than crystallinity, which accounts for its higher moisture regain as well as dye absorption capacity. However, unlike cotton it has a distinctive skin-core structure. Generally, the skin is crystalline while the core is amorphous. Depending on the extent of skin and core composition of the fibre, the textile wet processing is determined. Usually a hygroscopic chemical like urea is preferred for swelling of fibre skin during colouration, alternately causticisation with sodium hydroxide under specific time, temperature and wash-off process is advised for better results. Also, comparatively higher temperature and time during the dyeing and printing is considered to help in effective dye diffusion. Practically, the high energy, high exhaust type mono chlorotriasine-based reactive dyes like Tulactiv XLEP are highly effective.
Generally, the textile wet processing of rayon is considered to be more environment-friendly compared to cotton. The extent of water and energy consumption and the effluent generation during the wet processing is much less as it hardly requires any pre-treatment process.
Among all type of rayons, viscose is the highest-produced fibre having over 80 per cent market by volume, basically owing to the availability of raw material and economical manufacturing. It is used in staple as well as in filament form with varying densities between 1.5 to 4.5 and yarn counts from 30s to 80s.
As it has a distinct characteristic of being weak when wet, it tends to shrink or stretches up to 10 per cent when wet. Therefore, care must be taken during wet processing, like soft package winding, applying less pull during soft flow dyeing. Though rayon manufacturers take utmost care in ensuring removal of heavy metal ion impurity like zinc, which is usually used during viscose manufacturing, the residual traces can affect dyeability of the metal ion sensitive dyes. Hence, it is usually advisable to incorporate a suitable chelating agent during colouration. Also, the presence of residual sulphide content, based on its extent of removal during fibre manufacturing stage, may cause reduction of dyestuffs. Therefore, it is advisable to incorporate a mild oxidising agent like Rucorit RS-PD during colouration.
It absorbs about 13 per cent moisture, which is almost double than cotton and hence, exhibits superior comfort wearing properties. Microorganisms (moulds, mildew, fungus, bacteria) affect its colour, strength and lustre. It is ecologically more biodegradable than cotton.
Modal and lyocell are generally supplied in micro-fibre form and have a characteristic surface hairiness. In case of lyocell, the extent of such micro-fibrils on the yarn surface is much higher and it tends to affect uniformity levelness during colouration and hand feel after finishing. Therefore, it is recommended to remove such micro-fibrils through a process of de-fibrillation before the dyeing step. Normally such de-fibrillation is done by cellulase enzyme based bio-polishing process. Finally, after finishing, a re-fibrillation process is carried out to achieve the characteristics peach skin effect.
Blends and uses
Rayon is commonly used in blends with different natural as well as synthetic fibres and has the distinction of being the most widely used and the largest consumed component in composite blends. Owing to its versatile nature, having chemical properties of cotton and physical appearance of synthetic material, it blends well with any other fibre in any proportion.
Rayon is used in almost all type of textile and clothing material. Right from the conventional suiting, shirting, stocking and dress material to the trendier fashionwear covering intimates, sleepwear and sportswear. Rayon blends are also popular inhome furnishings-terry towels, upholstery and bed linen. It is also preferred in various non-textile end use application like the wipes for cosmetic cleansing, infant care, wound pads, surgical swabs and components of surgical gowns.
About the author: Ashok Athalye is from the technical service of Atul Ltd, Colors Business, based in Valsad, Gujarat.