Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.
Rayon is a synthetic fibre produced from cellulose (wood pulp) that has been chemically treated. It is used alone or in blends with other synthetic or natural fibres to make fabrics that are strong, highly absorbent and soft, and which can be dyed in brilliant, long-lasting colours.
The manufacture of rayon had its origins in the quest for an artificial silk. In 1664, Robert Hooke, a British scientist noted for his observations of plant cells, predicted the possibility of duplicating silk by artificial means; almost two centuries later, in 1855, fibres were made from a mixture of mulberry twigs and nitric acid. The first successful commercial process was developed in 1884 by the French inventor Hilaire de Chardonnet, and in 1891, the British scientists Cross and Bevan perfected the viscose process. By 1895, rayon was being produced commercially on a rather small scale, and its use grew rapidly.
Rayon is made by a number of processes, depending on its intended use.
In the viscose process, cellulose derived from wood pulp is steeped in a sodium hydroxide solution, and the excess liquid is squeezed out by compression to form alkali cellulose. Impurities are removed and, after being torn into shreds similar to white crumbs that are allowed to age for several days at a controlled temperature, the shredded alkali cellulose is transferred to another tank where it is treated with carbon disulphide to form golden-orange crumbs of cellulose xanthate. These are dissolved in dilute sodium hydroxide to form a viscous orange liquid called viscose. Different batches of viscose are blended to obtain uniform quality. The mixture is filtered and ripened by several days of storage at rigidly controlled temperature and humidity. It is then extruded through metal nozzles with fine holes (spinnerets) into a bath of about 10% sulphuric acid. It can be wound as a continuous filament (cakes) or cut into the required lengths and spun like cotton or wool. Viscose rayon is used to make wearing apparel and heavy fabrics.
In the cuprammonium process, used to make silk-like fabrics and sheer hosiery, the cellulose pulp dissolved in the sodium hydroxide solution is treated with copper oxide and ammonia. The filaments come out of the spinnerets into a spinning funnel and are then stretched to the required fineness by the action of a jet stream of water.
In the viscose and cuprammonium processes, the cellulose is reconstituted, but acetate and triacetate are esters of the cellulose and are considered by some to be a separate class of fibre. Acetate fabrics are known for their ability to take brilliant colours and to drape well, features that make them particularly desirable for apparel. Short fibres of acetate are used as fillers in pillows, mattress pads and quilts. Triacetate yarns have many of the same properties as acetate but are particularly favoured for their ability to retain creases and pleats in garments.
Hazards and Their Prevention
The principal hazards in the viscose process are the exposures to carbon disulphide and hydrogen sulphide. Both have a variety of toxic effects depending on the intensity and duration of the exposure and the organ(s) affected; they range from fatigue and giddiness, respiratory irritation and gastrointestinal symptoms to profound neuropsychiatric disturbances, auditory and visual disorders, deep unconsciousness and death.
Moreover, with a flashpoint below –30 °C and explosive limits between 1.0 and 50%, carbon disulphide has a high risk of fire and explosion.
The acids and alkalis used in the process are fairly dilute, but there is always danger from the preparing of the proper dilutions and splashes into the eyes. The alkaline crumbs produced during the shredding process may irritate workers’ hands and eyes, while the acid fumes and hydrogen sulphide gas emanating from the spinning bath may cause a kerato-conjunctivitis characterized by excessive lachrimation, photophobia and severe ocular pain.
Keeping the concentrations of carbon disulphide and hydrogen sulphide below the safe exposure limits requires diligent monitoring such as may be provided by an automatic continuous recording apparatus. Complete enclosure of the machinery with efficient LEV (with intakes at floor levels since these gases are heavier than air) is advisable. Workers must be trained in emergency responses in the event of leaks, and, in addition to being provided with proper personal protective equipment, maintenance and repair workers must be carefully schooled and supervised to avoid unnecessary levels of exposure.
Rest rooms and washing up facilities are necessities rather than mere amenities. Medical surveillance through preplacement and periodic medical examinations is desirable.