Describe the process of manufacture of rayon from wood.

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Rayon begins at a processing plant, just like the mill in Thurso, and actually has to go through numerous processes to be transformed from wood into fiber.

From Wood to Pulp

Derived from German for “strength” or “power,” turning wood chips into pulp is called the kraft process. “Cooked” in a machine called a digester, wood chips are boiled for several hours at a temperature of 130 to 180 °C (266 to 356 °F) producing a kind of sludge-like substance.

This substance is then pressurized and put through a screening process where large shives, knots, dirt and other debris is removed. Once the rejected materials are removed, what remains is pulp.

The pulp is washed to remove any further debris, bleached to remove its colour and then steam-dried, packaged in the form of sheets and sold to buyers.

What makes regular pulp different from dissolving pulp (the substance needed to make rayon) is an extra chemical process that removes hemicellulose – a weaker polysaccharide inside the biological composition of the individual pulp cells. By removing these weaker elements, this process ensures that the pulp has higher cellulose content – usually around 92 per cent – so much so that it is regarded in the pulp and paper industry as “specialty cellulose.”

Its microbiological strength is what makes it perfect for creating rayon.

From Pulp To Rayon

To create the fiber, the cellulose has to be put through a series of chemical and physical procedures.

First, the cellulose is dissolved in sodium hydroxide (also know as caustic soda) then the solution is pressed between rollers to remove excess liquid. The pressed sheets are crumbled or shredded to produce what is known as “white crumb.”

The white crumb is aged by exposing it to oxygen, then mixed with carbon disulfide in vats under a controlled temperature – usually around 20 to 30°C. This changes the chemical makeup of the cellulose mixture and results in a product called cellulose xanathate, or “yellow crumb.”

The yellow crumb is dissolved in a caustic solution and forms yet another product called “viscose” because of its very high resistance to force – or – viscosity.

The viscose is set to stand for a period of time to “ripen,” allowing the cellulose to regenerate when it’s finally formed into a filament.

After it is ripened, the viscose is filtered to remove any un-dissolved particles, degassed to remove any bubbles of air and put through a spinneret – a multi-pored device that forms numerous individual filaments.

As the viscose exits the spinneret, it lands in a bath of sulfuric acid resulting in the formation of rayon filaments. The rayon filaments are then stretched to straighten out the fibers, washed to remove any residue chemicals and cut into spools and – depending on the client’s wishes – dyed.

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