March 09, 2025 | UR Gate

Preparation of Pure Cellulose (Rayon)

Rayon is a type of cellulose where its molecular structure is reduced to smaller molecules. Its solution is dissolved and aged, then pushed through fine holes into a precipitating liquid, where it comes out as fibers and is transformed into threads of various sizes and strengths. It can also be cut into yarns or formed into sheets, and so on. Cellulose consists of three different types:

Alpha Cellulose: This is the part of cellulose that does not dissolve in an 18% sodium hydroxide solution at 20°C for one minute.
Beta Cellulose: This is the part of cellulose that dissolves in an 18% sodium hydroxide solution, and can be precipitated by acidification.
Gamma Cellulose: This is the part of cellulose that dissolves in an 18% sodium hydroxide solution but does not precipitate when acidified.

Alpha cellulose is the original cellulose and is known in the industry as rayon. It is obtained by performing the following steps on lignin-free cellulose:

1. Impregnation Process: In this process, cellulose is converted to alkaline cellulose by reacting with an 18% NaOH solution. This process dissolves the beta and gamma cellulose, leaving only the alpha cellulose.

(C₆H₉O₄O⁻Na⁺)_x + xH₂O → (C₆H₉O₄OH)_x + xNaOH

2. Aging: In this process, cellulose sheets are cut, conditioned, and the fibers separate, mix, and open up (oxidation occurs here). Cutting accelerates the reactions and homogenizes the material. The aging process takes place under certain thermal conditions for 1-2 days, during which oxidation occurs. During this process, the polymer chain breaks into smaller polymer molecules, and the length of the cellulose polymer chain is reduced by about a third.

3. Xanthation: This is the process of converting aged alkaline cellulose into a compound that dissolves in caustic soda by reacting with carbon disulfide. The resulting product is cellulose xanthate. CS2 is added slowly, where it is absorbed by the cellulose and changes color from yellow to orange.

(C₆H₉O₄O⁻CS-S⁻Na⁺)_x + xCS₂ → (C₆H₉O₄O⁻Na⁺)_x

4. Dissolving: Cellulose xanthate is dissolved in a dilute sodium hydroxide solution, forming what is known as viscose solution. This is a viscous liquid with a golden-greenish color, containing 7-8% xanthate and 6-7% sodium hydroxide.

5. Maturation: In this phase, the air is removed from the solution and filtered at 16°C. Sulfur content in the molecule decreases during this stage.

(C₁₈H₂₇O₁₂(OH)₂−OCS−S⁻Na⁺)_x + 2xH₂O → 3(C₆H₉O₄O⁻CS−S⁻Na⁺)x + 2xNaOH + 2xCS₂

The maturation period lasts between 20-36 hours.

6. Spinning: Spinning is the process of regenerating cellulose in the form of fibers from the basic xanthate solution by using an acidic bath. The bath consists of sulfuric acid (H2SO4) at 8-10% concentration, sodium sulfate at 13-20%, zinc sulfate at 1%, and glucose at 4%. The viscose solution loses its alkalinity when the caustic soda reacts with the acid, and the xanthate is broken down into carbon disulfide, hydrogen disulfide, and sulfur dioxide. The xanthate fibers are coagulated by zinc sulfate and sodium sulfate, recovering the cellulose. The glucose prevents crystallization of salts in the fibers.

(C₆H₉O₄OH)₃x + xCS₂ + x/2 Na₂SO₄ + x/2 H₂SO₄ → (C₁₈H₂₇O₁₂(OH)₂−OCS−S⁻Na⁺)_x

Procedure:

Place about 1 gram of filter paper, cut into very small pieces, into a conical flask, and add 25 ml of 18% sodium hydroxide solution. Mix the mixture with a glass rod, then leave it aside for an hour.
Decant the excess sodium hydroxide solution, then dry the filter paper by placing it between two glass plates and squeezing it as dry as possible. Place the small paper pieces back into the conical flask, seal it tightly with a rubber stopper, and keep it for the following week at room temperature.
After a week, take the conical flask containing the filter paper, and add 1.5 ml of carbon disulfide in a fume hood. Seal the flask with a suitable rubber stopper and shake it intermittently for an hour until the paper pieces turn yellowish-red. Decant the excess CS2, add as little NaOH solution as possible to obtain a viscous solution, and store this solution in the refrigerator for the next week.
Prepare the spinning bath by mixing the following materials in a 250 ml beaker: 15 ml H2SO4, 15.5 g Na2SO4, 0.25 g glucose, 15 g ZnSO4, and 86 ml water. This bath should be at room temperature.
Prepare rayon fibers as follows: Take a 20 ml syringe, fill it with the prepared paste, and push the plunger to extrude the paste through the needle into the spinning bath, where the rayon fibers will form. Allow the fibers to remain in the bath for only 5 seconds, then remove them with tweezers and air-dry them by placing them on a balance watch glass. Weigh the watch glass with the rayon fibers.

Discussion of the Procedure:

Why is the spinning bath solution used?

The viscose solution fibers lose their alkalinity when caustic soda reacts with the acid, and the xanthates decompose into carbon disulfide, hydrogen disulfide, and sulfur dioxide. The xanthate fibers coagulate with zinc sulfate and sodium sulfate, recovering cellulose. The glucose prevents the crystallization of salts in the fibers.