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Industrial Chemistry
March 09, 2025 | UR Gate

Preparation of Pure Cellulose "Pure Silk"



Introduction

Cellulose is a natural polymer composed of glucose sugar units linked together by glycosidic bonds. The number of glucose units in a cellulose molecule ranges between 6,000 and 12,000. It is the primary component of plant cell walls, with cotton being one of its most important sources.


Types of Cellulose

Cellulose is classified into three main types based on its solubility in 18% sodium hydroxide (NaOH) solution:

α-Cellulose:

  • This is the insoluble fraction in NaOH (18%) solution.
  • Considered pure cellulose due to its high molecular weight.
  • Used in textile fibers, paper, and bioplastics industries.

β-Cellulose:

  • Dissolves in NaOH (18%) solution but precipitates upon acidification.
  • Has a lower molecular weight compared to α-cellulose.
  • Used in some chemical applications.

γ-Cellulose:

  • Dissolves in NaOH (18%) solution and cannot be precipitated by acidification.
  • Has the lowest molecular weight among the three types.
  • Considered the least chemically stable.


Viscose Method for Producing Pure Cellulose

The viscose method is one of the most important industrial techniques for producing cellulose and is widely used in the production of artificial silk (rayon). This method consists of three main processes:

1. Steeping Process

  • Paper or cotton is treated with 18% sodium hydroxide (NaOH) solution.
  • Alpha-cellulose is converted into sodium cellulose, while beta-cellulose and gamma-cellulose are separated.
(C6​H10​O5​)n ​+ NaOH → (C6​H9​O5​Na)n ​+ H2​O
  • The mixture is left in the air for three days in a process known as aging.

2. Xanthation Process

  • Sodium cellulose reacts with carbon disulfide (CS₂) at a ratio of 30% by weight.
  • This reaction produces sodium cellulose xanthate, a compound soluble in 5% diluted NaOH.
  • The mixture turns into a viscous yellow solution called viscose.
(C6​H9​O5​Na)n​ + CS2 ​→ (C6​H9​O5​SC-SNa)n​


3. Fiber Liberation (Spinning Process)

  • The viscose solution is forced through tiny holes in a spinneret.
  • The fibers are immersed in a spinning bath containing sulfuric acid (H₂SO₄), zinc sulfate (ZnSO₄), and activating agents.
  • During this process, the xanthate reverts to pure cellulose, causing the fibers to solidify and form rayon fibers.
(C6​H9​O5​SC-SNa)n​ + H2​SO4 ​→ (C6​H10​O5​)n ​+ CS2 ​+ Na2​SO4​



Conclusion

Cellulose is one of the most important natural polymers used in textile, paper, and plastic industries. The viscose method is among the most widely used processes for producing rayon, due to its high quality and cost-effectiveness. Through the steeping, xanthation, and spinning processes, pure cellulose is extracted and applied in various industrial fields.

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