Viscose Rayon — The First Artificial Fiber from Wood Cellulose

Begin with purified wood pulp — cellulose extracted from spruce, beech, or eucalyptus wood by chemical pulping processes that remove lignin and hemicellulose. The purified cellulose arrives as thick white sheets containing over 90% alpha-cellulose. This is the same molecule as cotton — (C₆H₁₀O₅)ₙ — but sourced from wood instead of cotton bolls. The entire viscose process is about dissolving this solid cellulose and reconstituting it as fiber.

Immerse the cellulose sheets in a 17–20% sodium hydroxide (NaOH) solution at 18–20°C for 1–2 hours. The caustic soda penetrates the cellulose, breaking hydrogen bonds between chains and converting it to alkali cellulose (sodium cellulose). The sheets swell to roughly twice their original thickness. This step is chemically identical to mercerization — the same reaction Mercer discovered in 1844, now applied as a precursor to dissolution rather than as a finishing treatment.

Remove the swollen sheets from the caustic bath and press them between rollers to squeeze out excess NaOH (recovering it for reuse). Then shred the pressed sheets into fine crumbs using a rotating shredder. The crumbled alkali cellulose has a vastly greater surface area than intact sheets, which is essential for the next reaction step — carbon disulfide must contact every cellulose particle evenly.

Spread the alkali cellulose crumbs on trays and store them in a controlled-temperature room (18–22°C) for 2–3 days. During aging, oxygen from the air slowly breaks cellulose chains into shorter lengths through oxidative depolymerization. This reduces the viscosity of the final solution to a spinnable range. Without aging, the viscose would be too thick to force through spinneret holes. Too much aging makes it too thin and produces weak fiber.

Place the aged alkali cellulose crumbs in a sealed rotating drum (called a baratte or churn) and add carbon disulfide (CS₂) — approximately 30–35% by weight of the cellulose. The CS₂ reacts with the alkali cellulose over 2–3 hours at 25–30°C, forming cellulose xanthate — an orange, crumb-like solid. Carbon disulfide is extremely toxic (damages the nervous system) and highly flammable (autoignition at 100°C). The reaction must be conducted in sealed, ventilated equipment.

Transfer the orange cellulose xanthate crumbs into a dilute sodium hydroxide solution (5–7%) and stir vigorously for several hours. The xanthate dissolves to form a thick, orange, honey-like liquid — this is 'viscose' (from Latin viscosus, meaning sticky). The viscose solution contains approximately 7–10% cellulose by weight. It must be perfectly homogeneous, free of undissolved particles, and free of air bubbles — all of which would cause defects in the final fiber.

Store the viscose at 10–18°C for 4–5 days (ripening). During this period, chemical changes occur that improve the fiber's properties when it is later regenerated. Filter the ripened viscose through fine mesh screens and deaerate it under vacuum to remove dissolved gas. Any air bubble trapped in the viscose creates a void in the fiber — a weak point that breaks during spinning or weaving.

Force the viscose through a spinneret — a small metal disc perforated with thousands of tiny holes, each 50–80 micrometers in diameter — directly into a coagulation bath of dilute sulfuric acid (10–15%) containing sodium sulfate and zinc sulfate. As the viscose stream meets the acid, the xanthate groups are stripped away and the cellulose regenerates as a solid filament. Each spinneret hole produces one continuous filament; a spinneret with 1,500 holes produces 1,500 filaments simultaneously.

The freshly regenerated filaments are soft and weak. Pull them through the acid bath under tension — stretching them by 50–100% of their original length. This mechanical stretching aligns the cellulose molecules parallel to the fiber axis, dramatically increasing tensile strength. The degree of stretch determines the fiber's final strength: more stretch produces stronger, less extensible fiber. This principle — drawing to orient molecules — later became fundamental to all synthetic fiber production.

Pass the stretched filaments through a series of washing baths to remove residual acid, sulfur compounds, and salts. The characteristic smell of carbon disulfide clings to unwashed viscose. Bleach lightly with sodium hypochlorite if a white fiber is needed. Apply a soft finish (a light oil or emulsion) to reduce static and improve handling. The finished filaments are smooth, lustrous, and drape beautifully — properties that earned rayon the nickname 'artificial silk.'

For filament yarn (used in linings, lingerie, and dress fabrics), wind the continuous filaments onto bobbins or cones. For staple fiber (blended with cotton or wool), cut the filament tow into short lengths (38–50 mm for cotton-type blends, 60–150 mm for wool-type) and crimp them to give the straight filaments the waviness needed for spinning on conventional cotton or wool machinery.

The viscose process uses carbon disulfide — a volatile, toxic liquid that attacks the central nervous system, causing blindness, psychosis, and death at high exposures. Early viscose workers suffered severe CS₂ poisoning. Sulfuric acid in the spin bath is corrosive; hydrogen sulfide gas (toxic, smells of rotten eggs) is released during coagulation. Modern viscose plants have enclosed systems and vapor recovery, but the process remains inherently hazardous. Newer alternatives — lyocell (Tencel), which uses a non-toxic solvent — address these concerns.

Viscose rayon proved that useful textile fibers could be manufactured from molecular components rather than harvested from nature. This was a conceptual revolution. Once chemists understood that fiber properties came from molecular structure — chain length, orientation, crystallinity — they could design fibers to specification. Wallace Carothers at DuPont synthesized nylon in 1935 using this principle. John Rex Whinfield created polyester in 1941. Today, synthetic and artificial fibers account for over 60% of global fiber production. Every one of them traces its lineage to the moment Cross, Bevan, and Beadle dissolved a tree and spun it into silk.