Rayon (often Viscose) is the oldest commercially produced man-made fiber, classed as a Regenerated Cellulosic Fiber. It is considered semi-synthetic because it uses a natural polymer (cellulose, typically from wood pulp) which is then dissolved and chemically treated to regenerate it into a fiber form.
1. Preparation: The Viscose Process (Chemical Regeneration)
The Viscose process involves dissolving pure cellulose and regenerating it using a chemical pathway. This process relies on key reagents to temporarily solubilize the cellulose.
1.1. Chemical Steps
The process is designed to break down the natural hydrogen bonding of cellulose so it can be extruded, and then rebuild the cellulose structure in a filament form.
- Steeping: Cellulose pulp is soaked in a strong Sodium Hydroxide (NaOH) solution to swell the fibers and convert cellulose into alkali cellulose.
- Shredding: The alkali cellulose is broken into smaller crumbs to increase the surface area.
- Xanthation: The crumbs are treated with Carbon Disulfide (CS2). This forms a highly soluble intermediate called Cellulose Xanthate.
- Dissolving: The Cellulose Xanthate is dissolved in a dilute NaOH solution, creating a thick, syrupy solution known as Viscose.
- Spinning (Regeneration): The Viscose is extruded through tiny holes (a spinneret) into a coagulation bath containing dilute Sulfuric Acid (H2SO4) and salts. The acid reverses the xanthation reaction, regenerating the pure, non-soluble cellulose fiber (Rayon filament).
[C6H10O5]n + n NaOH → [C6H9O4·ONa]n + n H2O
Cellulose → Alkali Cellulose (swelling and mercerization)
[C6H9O4·ONa]n + n CS2 → [C6H9O4·OCS2Na]n
Alkali Cellulose + Carbon Disulfide → Cellulose Xanthate (orange, viscous)
[C6H9O4·OCS2Na]n + n H2SO4 → [C6H10O5]n + n CS2 + n Na2SO4
In acid bath: Xanthate → Regenerated Cellulose + byproducts
2. Types of Rayon
| Type | Process | Key Features | Applications |
|---|---|---|---|
| Viscose Rayon | Xanthate process | Soft, absorbent, drapable | Apparel, home textiles |
| Modal | High-wet-modulus viscose | Higher strength when wet | Towels, activewear |
| Lyocell (Tencel™) | NMMO solvent spinning | Eco-friendly, strong, smooth | Denim, intimates, bedding |
| Cuprammonium Rayon | Cuprammonium hydroxide | Very fine, silk-like | High-end fabrics (less common) |
3. Chemical Structure and Fiber Properties
3.1. Structural Chemistry
Chemically, Rayon is pure cellulose, structurally identical to cotton, consisting of repeated glucose units linked by beta;-1,4-glycosidic bonds.
However, because the regeneration process uses a solution and not a growing plant, the resulting fiber:
- Has a lower Degree of Polymerization (DP) compared to natural cotton.
- Is less crystalline and has a more random internal structure, which affects its strength.
3.2. Engineering Properties Table
| Property | Relevance to Chemistry/Structure | Engineering Consequence |
|---|---|---|
| Absorbency | High concentration of accessible hydroxyl (-OH) groups. | Excellent moisture absorption; high comfort in warm weather. |
| Dry Strength | Moderate tensile strength, suitable for most apparel. | Relatively weaker than synthetics like Nylon or PET. |
| Wet Strength | Significant loss of strength (up to 50-70%) when wet. | The OH groups weaken inter-chain H-bonds upon water absorption. Requires careful washing/handling. |
| Drape and Softness | Low crystallinity and smooth, continuous filament structure. | Highly prized for its silk-like texture and flow in clothing. |
4. Applications
- Apparel: Blouses, dresses, jackets, and linings, valued for its comfort and drape.
- Home Furnishings: Bedspreads, blankets, upholstery.
- Non-Wovens: Absorbent products like wipes, towels, and medical bandages (due to high absorbency).
- High-Tenacity Rayon: Chemically modified or highly stretched during spinning to improve strength, used in tire cords and industrial belting.
5. Environmental Impact: Viscose vs. Lyocell (Modern Solution)
From an engineering chemistry perspective, the environmental footprint is defined by the process solvent used.
5.1. Traditional Viscose Process Challenges
The reliance on the chemical reagents creates significant environmental and safety concerns:
- Carbon Disulfide (CS2) Toxicity: CS2 is highly volatile, toxic to humans (especially the nervous system), and flammable. Emissions must be rigorously controlled.
- Water Pollution: The coagulation bath releases zinc and large amounts of sulfates into the effluent, requiring extensive water treatment.
5.2. Modern, Green Engineering Solution: The Lyocell Process
Lyocell (e.g., Tencel) is a type of rayon developed specifically to address the pollution of the Viscose process. It represents a successful shift toward Green Chemistry in the textile industry.
The key innovation is the solvent: N-methylmorpholine N-oxide (NMMO).
- Solvent: NMMO is an organic solvent that is non-toxic and easily separates from cellulose via simple evaporation.
- Closed-Loop System: Over 99% of the NMMO solvent can be recovered, purified, and reused, making the process highly sustainable with minimal waste discharge.
Understanding the transition from the CS2-based Viscose process to the NMMO-based Lyocell process is a perfect example of applying green engineering principles to industrial chemistry.