Editing Rayon (section)

=== Viscose method ===
[[File:Viscose Rayon spinning machine.JPG|thumb|A device for spinning viscose rayon dating from 1901]]
[[File:Xanthation.png|thumb|Simplified view of the xanthation of cellulose<ref name="UllCell" />]]

The viscose process builds on the reaction of [[cellulose]] with a strong base, followed by treatment of that solution with [[carbon disulfide]] to give a [[xanthate]] derivative. The xanthate is then converted back to a cellulose fiber in a subsequent step.

The viscose method can use [[wood]] as a source of cellulose, whereas other routes to rayon require [[lignin]]-free cellulose as a starting material. The use of woody sources of cellulose makes viscose cheaper, so it was traditionally used on a larger scale than the other methods. On the other hand, the original viscose process generates large amounts of contaminated wastewater. Newer technologies use less water and have improved the quality of the wastewater.

[[File:Viscose Dope 1.jpg|thumb|alt=A beaker being filled with a viscous reddish orange liquid|A viscous solution of cellulose xanthate (with some excess carbon disulfide), referred to as "viscose dope", prior to regeneration of the cellulose fibers<ref>{{Cite web |date=2025-04-02 |title=Cellulose Xanthate Image |url=https://www.epichem.com/pages/gallery |publisher=Epichem |access-date=2025-05-02 }}</ref>]]

The raw material for viscose is primarily [[wood pulp]] (sometimes [[Bamboo textile|bamboo pulp]]), which is chemically converted into a soluble compound. It is then dissolved and forced through a [[spinneret (polymers)|spinneret]] to produce filaments, which are chemically solidified, resulting in fibers of nearly pure cellulose.<ref name="fibersource">{{cite web |url=http://www.afma.org/f-tutor/rayon.htm |title=Rayon Fiber (Viscose) |website=AFMA.org |archive-url=https://web.archive.org/web/20080406101953/http://www.afma.org/f-tutor/rayon.htm |archive-date=April 6, 2008 }}</ref> Unless the chemicals are handled carefully, workers can be seriously harmed by the [[carbon disulfide]] used to manufacture most rayon.<ref name="blancbook">{{cite book |title=Fake silk: the lethal history of viscose rayon |last=Blanc |first=Paul D. |publisher=Yale University Press |date=2017 |isbn=978-0-300-20466-7 |oclc=961828769 }}</ref>{{Page needed|date=May 2025}}<ref name="Monosson 2016">{{cite journal |last=Monosson |first=Emily |date=25 November 2016 |title=Toxic Textiles |url=https://www.science.org/doi/pdf/10.1126/science.aak9834 |journal=Science |volume=354 |issue=6315 |page=977 |bibcode=2016Sci...354..977M |doi=10.1126/science.aak9834 |pmid=27884997 }}</ref>

To prepare viscose, pulp is treated with aqueous sodium hydroxide (typically 16–19% [[Mass fraction (chemistry)|by mass]]) to form [[Mercerisation|alkali cellulose]], which has the approximate formula [C<sub>6</sub>H<sub>9</sub>O<sub>4</sub>−ONa]<sub>{{mvar|n}}</sub>. This material is allowed to [[Polymer|depolymerize]] to an extent. The rate of depolymerization (ripening or maturing) depends on temperature and is affected by the presence of various inorganic additives, such as metal oxides and hydroxides. Air also affects the ripening process, since oxygen causes depolymerization. The alkali cellulose is then treated with carbon disulfide to form sodium cellulose [[xanthate]]:<ref name="UllCell" />

{{block indent|[C<sub>6</sub>H<sub>5</sub>(OH)<sub>4</sub>−ONa]<sub>{{mvar|n}}</sub> + {{mvar|n}}CS<sub>2</sub> &nbsp;→&nbsp; [C<sub>6</sub>H<sub>5</sub>(OH)<sub>4</sub>−OCS<sub>2</sub>Na]<sub>{{mvar|n}}</sub>}}

Rayon fiber is produced from the ripened solutions by treatment with a mineral acid, such as [[sulfuric acid]]. In this step, the xanthate groups are hydrolyzed to regenerate cellulose and carbon disulfide:

{{block indent|[C<sub>6</sub>H<sub>5</sub>(OH)<sub>4</sub>−OCS<sub>2</sub>Na]<sub>2{{mvar|n}}</sub> + {{mvar|n}}H<sub>2</sub>SO<sub>4</sub> &nbsp;→&nbsp; [C<sub>6</sub>H<sub>5</sub>(OH)<sub>4</sub>−OH]<sub>2{{mvar|n}}</sub> + 2{{mvar|n}}CS<sub>2</sub> + {{mvar|n}}Na<sub>2</sub>SO<sub>4</sub>}}

Aside from regenerated cellulose, acidification gives [[hydrogen sulfide]] (H<sub>2</sub>S), sulfur, and carbon disulfide. The thread made from the regenerated cellulose is washed to remove residual acid. The sulfur is then removed by the addition of [[sodium sulfide]] solution, and impurities are oxidized by bleaching with [[sodium hypochlorite]] solution or [[hydrogen peroxide]] solution.<ref name="Wheeler 1928" />{{Page needed|date=May 2025}}

Production begins with processed cellulose obtained from wood pulp and plant fibers. The cellulose content in the pulp should be around 87–97%.

The steps:<ref name="fibersource" />

# Immersion: The cellulose is treated with [[caustic soda]].
# Pressing. The treated cellulose is then 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 through exposure to [[oxygen]]. This is a depolymerization step and is avoided in the case of polynosics.
# The aged "white crumb" is mixed in vats with carbon disulfide to form the xanthate. This step produces "orange-yellow crumb".
# The "yellow crumb" is dissolved in a caustic solution to form viscose. The viscose is set to stand for a period of time, allowing it to "ripen". During this stage the molecular weight of the polymer changes.
# After ripening, the viscose is filtered, degassed, and then extruded through a spinneret into a bath of [[sulfuric acid]], resulting in the formation of rayon filaments. The acid is used as a regenerating agent. It converts cellulose xanthate back to cellulose. The regeneration step is rapid, which does not allow proper orientation of cellulose molecules. So to delay the process of regeneration, [[zinc sulfate]] is used in the bath, which converts cellulose xanthate to zinc cellulose xanthate, thus providing time for proper orientation to take place before regeneration.
# ''Spinning.'' The spinning of viscose rayon fiber is done using a wet-spinning process. The filaments are allowed to pass through a coagulation bath after extrusion from the spinneret holes. The two-way mass transfer takes place.
# ''Drawing.'' The rayon filaments are stretched, in a procedure known as drawing, to straighten out the fibers.
# ''Washing.'' The fibers are then washed to remove any residual chemicals from them.
# ''Cutting.'' If filament fibers are desired, then the process ends here. The filaments are cut down when producing [[Staple (textiles)|staple fibers]].