Viscose is a semi-synthetic textile fiber derived primarily from wood pulp. It is not considered purely natural, like cotton or linen, because it undergoes significant chemical processing to transform the raw material. Conversely, it is not fully synthetic, like polyester, which is derived from petroleum-based compounds. This classification stems from the chemical alteration of cellulose, the natural polymer that forms the foundational component of the fiber.
Understanding Regenerated Cellulose
Viscose is a type of rayon, a generic term for any fiber made from regenerated cellulose. The term “regenerated” signifies that the cellulose polymer is dissolved and then chemically reformed, or spun, into a usable fiber structure. The resulting fiber is known for its excellent drape, soft feel, and lustrous appearance, making it an early substitute for silk. Viscose is also highly absorbent and breathable, qualities it shares with natural fibers like cotton.
Sourcing the Natural Pulp
The starting point for viscose production is high-purity cellulose, typically sourced from wood pulp known as “dissolving pulp.” This pulp is commonly derived from fast-growing trees such as beech, pine, or eucalyptus. Other sources, including bamboo and cotton linters, can also be used. The cellulose component must be isolated from other wood materials, such as lignin and hemicellulose, to ensure the purity required for the chemical process. The industry faces an ongoing challenge to source this pulp from responsibly managed forests to mitigate risks of deforestation.
The Standard Viscose Manufacturing Process
The transformation of wood pulp into viscose fiber involves a multi-step chemical procedure known as the xanthate method. The process begins with steeping, where sheets of purified wood pulp are immersed in a concentrated solution of sodium hydroxide (\(\text{NaOH}\)), also called caustic soda. This alkalization step converts the cellulose into alkali cellulose, which makes the long polymer chains more reactive.
The alkali cellulose is then broken down, or shredded, into small, fluffy pieces, often referred to as “white crumb,” to increase the surface area. These crumbs are allowed to age under controlled temperature and humidity, which intentionally reduces the length of the cellulose polymer chains. This reduction in chain length is necessary to achieve the specific viscosity required for the next stages of spinning.
Next, the aged alkali cellulose crumbs are placed in airtight vats and treated with liquid carbon disulfide (\(\text{CS}_2\)) in a process called xanthation. This reaction forms sodium cellulose xanthate, changing the crumb color to a yellowish-orange and making the cellulose soluble. The cellulose xanthate is then dissolved in a weak sodium hydroxide solution, creating a thick, honey-like liquid known as viscose, from which the fiber gets its name.
The final stage is spinning, or regeneration, which forms the filament. The viscous solution is filtered and forced through a spinneret into a coagulation bath. This bath contains sulfuric acid (\(\text{H}_2\text{SO}_4\)), sodium sulfate, and sometimes zinc sulfate (\(\text{ZnSO}_4\)). The acid instantly neutralizes the alkali and regenerates the cellulose, causing the xanthate to decompose and the cellulose to solidify into continuous filaments.
Environmental Variations and Processing Innovations
The traditional viscose process, while effective, utilizes hazardous chemicals like carbon disulfide, which poses environmental and occupational health risks if not managed carefully. This has led to the development of alternative manufacturing methods that are more environmentally contained.
One prominent alternative is the Lyocell process, which is used to produce fibers like Tencel. Lyocell production does not use carbon disulfide and instead employs an organic solvent, N-Methylmorpholine N-oxide (NMMO), to dissolve the cellulose directly. This solvent is non-toxic and can be recovered and reused in a highly efficient, closed-loop system with a recovery rate of over 99%.
Another variation is Modal, a type of high wet modulus rayon often made from beechwood. Modal fibers are spun using a slightly modified viscose process that results in a fiber with greater strength and stability, especially when wet.