Rayon is a manufactured fiber that begins with natural materials, placing it in a unique category of textiles known as regenerated cellulose fibers. It is not a purely synthetic material like polyester, nor is it a simple natural fiber like cotton, as it requires extensive chemical processing to transform its structure. This versatile fabric, sometimes called “artificial silk,” was first commercially produced in the late 19th century, marking the beginning of the man-made fiber industry. The fiber is prized for its ability to mimic the soft drape and luxurious feel of silk, cotton, and linen at a more affordable cost.
Sourcing the Cellulose Raw Material
The foundation of all rayon production begins with purified cellulose pulp, which is overwhelmingly sourced from wood. Trees such as pine, spruce, beech, and eucalyptus are harvested, then chipped into small pieces before undergoing chemical pulping. This process is designed to remove lignin and hemicellulose.
The result of this purification is known as “dissolving pulp,” a bleached, high-purity cellulose product, typically containing 90–98% alpha-cellulose. This purified pulp is pressed into large, blotter-like sheets, which serve as the standardized raw material for the subsequent chemical transformations.
The Traditional Viscose Method
The most common method for producing rayon is the Viscose process. The process begins with steeping the cellulose pulp sheets in a solution of 17.5% caustic soda (sodium hydroxide) for about an hour, which converts the cellulose into alkali cellulose. After pressing out the excess liquid, the sheets are shredded into fluffy, white crumbs to increase the material’s surface area.
These alkali cellulose crumbs are then subjected to an aging process for up to three days, where controlled exposure to oxygen causes the long cellulose polymer chains to break down into shorter, more manageable lengths. This step is precisely controlled to achieve the desired molecular weight necessary for the subsequent spinning process. Following aging, the alkali cellulose crumbs are placed in vats and reacted with carbon disulfide (CS2).
This reaction chemically alters the cellulose, forming a compound called sodium cellulose xanthate. The xanthate is then dissolved in a weak caustic soda solution, resulting in a thick, honey-like fluid known as “viscose.” This viscose solution is filtered and degassed to remove any trapped air bubbles that could cause defects in the final fiber.
The final stage involves extruding the viscose through a spinneret directly into a spin bath containing dilute sulfuric acid and various salts. The acid immediately neutralizes the caustic soda and hydrolyzes the cellulose xanthate, causing the cellulose to precipitate back into a solid filament form. The continuous streams of liquid viscose are solidified into the final rayon fibers, which are then washed, bleached, and cut into staple or filament lengths. The traditional Viscose method, while highly effective, generates significant chemical byproducts, including hydrogen sulfide and carbon disulfide, which are challenging to manage environmentally.
Cleaner and Specialized Production Processes
Alternative, more efficient manufacturing techniques were developed due to environmental concerns regarding the traditional Viscose method, most notably the Lyocell process. The Lyocell process employs a direct-dissolution method using N-methylmorpholine N-oxide (NMMO) to dissolve the cellulose pulp.
In the Lyocell process, the cellulose is dissolved directly in the NMMO solvent and water mixture to form a clear, viscous spinning solution. This solution is then extruded through a spinneret into a dilute aqueous bath, where the cellulose fiber precipitates, and the solvent is recovered. A major advantage of this method is the closed-loop system, which allows for the recovery and reuse of up to 99% of the NMMO solvent.
Another specialized rayon type is Modal, which is produced using a modification of the traditional Viscose process. Modal fiber is typically derived exclusively from beechwood pulp and is often classified as a High Wet Modulus (HWM) rayon. The manufacturing adjustments, such as a modified coagulation bath or stretching the fibers during spinning, result in a fiber with significantly improved strength, particularly when wet. These process modifications contribute to Modal’s characteristic softness and durability, making it a higher-performing alternative to regular Viscose rayon.
Final Fiber Properties and Applications
Standard Viscose rayon is known for its excellent drape, high absorbency, and silk-like luster, making it a popular choice for linings, blouses, and dresses. However, a major drawback of regular Viscose is its low wet strength, meaning it loses a significant portion of its tensile strength when saturated, often leading to shrinkage or shape loss during washing.
The modified processes result in a fiber that maintains high strength and stability even when wet, allowing for machine washing and greater resilience in items like towels, underwear, and casual wear. Lyocell fibers, due to their distinct and uniform internal structure resulting from the NMMO process, exhibit exceptional dry and wet strength, superior dimensional stability, and a smooth, luxurious hand. Lyocell is frequently used in high-quality apparel, denim, and specialized non-woven products like medical wipes. High Tenacity Rayon is specifically engineered for industrial applications requiring extreme strength, such as tire cords and conveyor belts.