The Manufacturing Process of Rayon
For a very long period of human history, man depended on natural elements to make fibers for clothing. These were natural fibers. However, with the technological advances many man made synthetic fibers were developed. Man-made fibers have two main categories: One that are made from natural products (cellulosic fibers) and the other that are synthesized from chemical compounds (noncellulosic polymer fibers). Rayon fiber is a natural-based material made from the cellulose of wood pulp or cotton fiber. In fact, they are made from reformed or regenerated cellulose. As such, they are identified as regenerated cellulose fibers.
History of Rayons
The invention of rayon can be related to the invention of an apparatus in 1840 that drew synthetic filaments through small holes. In 1855, Georges Audemars, a Swiss chemist, discovered how to make cellulose nitrate. Later in 1884, Count Hilaire de Chardonnet made the first man made fibers from nitrocellulose. He came to be known as the 'father of rayon'. He got the original French patent and also won financial support through which, he established the world's first rayon factory. Subsequently, other scientists developed more cost-effective ways of making many other types of rayon.
The Federal Trade Commission (FTC) officially recognized man made textile filaments in 1925 when it allowed the use of the name, "rayon" for yarns obtained from cellulose or its derivatives. Till now, it was known as artificial or imitation silk. With the production and types of man made fibers increasing and given various trademarks, the FTC again made a rule in 1937 that any fiber or yarn produced chemically from cellulose must be designated as rayon. However, confusion prevailed among the masses because at least four different types of rayon with some similar and some different properties came into being. It was because of the fact that there were basically two groups of rayon- one consisting of regenerated pure cellulose and the other of a cellulose compound. These different compositions gave different properties. Therefore, in 1952, the FTC ruled that there would be two categories of cellulose fibers- rayon fiber ( pure cellulose) and acetate fiber (cellulose compound).
Production of Rayon Filament- the Basic Method
The basic method of producing rayon filament is similar to the natural silk manufacturing process followed by the silk worms who transform the cellulose of mulberry trees into two fine filaments. A liquid substance of cellulose is forced through a metal cap or nozzle called spinneret. The spinneret is perforated with small holes through each of which, a filament is extruded. The extruded filament gets solidified by a liquid bath as it comes from the spinneret. The number of holes in the spinneret vary from 1 to 20,000 and filaments of equal size are produced simultaneously. Subsequently, these filaments are combined by twisting to make rayon yarn of required diameter.
Types of Rayon
There are basically two types of rayons- viscose rayon and high-wet-modulus (HWM) rayon- categorized on the basis of two principal methods of making rayon. As the two methods differ in specific features of manufacture so the two fibers differ in important characteristics.. However, both, the viscose fiber and high-wet-modulus fiber are classified as regenerated rayons because the original raw material (cellulose) is changed chemically into another form, which is then changed (regenerated) into cellulose again. Purified cellulose in fiber form is produced due to these changes.
Viscose Rayon- The Viscose Process for Manufacturing Rayon
- Viscose rayon is made from cotton linter or wood pulp generally obtained from pine, spruce, or hemlock trees. Cotton linter is the residue cotton fiber which cling to cotton seed after the ginning process.
- Wood chips or cotton fibers are treated to make sheets of purified cellulose, resembling white blotters. The cellulose sheets are then soaked in sodium hydroxide (caustic soda), producing sheets of alkali cellulose. The resultant substance is broken up into fluffy white flakes or grains known as cellulose crumbs. These crumbs are aged in metal containers for 2 to 3 days under controlled temperature and humidity.
- Liquid carbon disulfide is added to the aged crumbs which turns the cellulose into cellulose xanthate, light orange-colored crumbs. The cellulose xanthate is dissolved in a weak solution of caustic soda, resulting into a thick viscose solution that appears like honey in color and consistency. The viscose is aged, filtered and vacuum treated to remove air bubbles which cause the filament to break.
- The viscose solution is then forced through a spinneret into sulfuric acid which coagulates the cellulose of the soluble cellulose xanthate to produce pure regenerated cellulose filaments. If staple fiber is to be produced, a large spinneret with large holes is used. If filament fiber is being produced, then a spinneret with smaller holes is used.
- After extrusion from spinneret, the viscose rayon fibers are processed into filament or spun staple yarns by one of several methods. There are basically three methods of such processing- Pot, or Box Spinning; Spool Spinning; and Continuous Spinning.
- In Pot Spinning, the filaments are removed from the coagulating bath by passing over a series of godet wheels and are stretched thereon under controlled tension. This stretching reduces the diameter of the filaments and makes them more uniform in size. It also make the filaments more strong. The filaments are then put into a rapidly spinning cylinder called Topham Box, resulting in a cake-like strings that stick to the sides of the Topham Box. After removing the cake, it is thoroughly washed, treated to remove any residual chemical substances, bleached, rinsed, dried, and wound on cones or spools.
- In Spool Spinning, the filaments may be drawn from the coagulating bath over guides and rollers and wound on perforated spools. A washing solution is forced through the spools' holes to cleanse filaments which are then bleached, rinsed, dried, and wound on cones or spools or put in skeins.
- In Continuous Spinning, upon extrusion from the spinneret, the filaments may be carried in one continuous process over the godet wheels through a series of reels as they are washed, purified, bleached, dried, twisted, finished and wound onto desired package.
- After treatment with textile chemicals, various weaving processes are carried out to produce rayon fabrics. Different varieties of yarns such as monofilament yarns, multifilament yarns, spun yarns etc. permit the manufacturing of a wide variety of fabrics. Spun rayon yarns can be used for making fabrics similar to cotton fabrics, linen fabrics or wool fabrics. Rayon filament yarns can make fabrics resembling silk fabrics.
- Finally, various finishing processes are carried out to enhance the appearance of these fabrics and to improve their serviceability. Most common finishes include calendaring for smoothness, embossing for decorative effects, flame resistance, napping (spun rayons only) for softness & warmth, preshrinking for greater dimensional stability, stiffening, water resistance, wrinkle resistance etc.
High-Wet-Modulus (HWM) Rayon – The Differences in Manufacturing Process
Viscose rayon has its own limitations. It loses up to 70% of its strength when wet. In fact, in 1950s, when first developed, rayon was used in industrial products and home furnishings only and not for clothing. This was due to the fact that viscose rayon fibers were too weak as compared to other fibers intended to be used in apparel making. In 1960, commercial production was started for a rayon that had similar strength as of cotton fabrics and retained most of the strength, even when wet. It was high-wet-modulus (HWM) rayon. It is also known as 'modified rayon'. The process for manufacturing high-wet-modulus rayon is almost similar to that for making viscose rayon but with a few exceptions.
- Initially, when the purified cellulose sheets are bathed in a caustic soda solution, a weaker caustic soda is used while making HWM rayon.
- Alkali crumbs or the viscose solution, neither of the two are aged in the HWM process of making rayon.
- When making HWM rayon, the filaments are stretched to a greater degree than when making viscose rayon.