With the recent acquisition of a near-bankrupt mill in Thurso, Quebec, Vancouver’s Fortress Paper announced its plans to convert the one time pulp mill into a world-class producer of specialty cellulose.
Specialty cellulose (also known as dissolving pulp) is a product used in the textile industry to manufacture a silk-like fiber called rayon typically used to make clothing.
So how exactly do you get from wood to wearable?
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.
From Rayon to You
Ready to be packaged, the spools of rayon are shipped to textile manufacturers around the world who use rayon for many forms of clothing such as blouses, dresses, jackets, lingerie, scarves, neckties and more.
Rayon isn’t only used in clothing however. It can also be used in medical surgery products, tire cords, and is even used as the filling in Zippo lighters.