SOLVENTS FOR COTTON BASIC INFORMATION AND TUTORIALS

Cotton Solvents
What can be used as solvents for cotton?

Cellulose is s oluble only in unusual and com plex s olvent systems. Solvents for cellulose are c entral to the rayon and cellulose film industries, but are also necessary for s olubilizi ng cotton for the determination of molecular weight and degree of polymerization ( DP) by chromatographic m ethods.

These solvents fall into several categories. The solvents discussed do not include processes w here cellulose is converted t o a derivative that i s subsequently dissolved in a nother m edium. For e xample, cellulose acetate i s solubl e in acetone, but this is not a solut ion of cellulose. H ow ever, t he viscose process that f orm s a cell ulose xanthate deri v at ive, f rom which c el lul ose is readil y regenerated, is gene rally c onsidered to use a cellulose solution because s olvation and derivatization oc cur simultaneously.

The viscose process is the most important method for m aking cellul ose solutions for industrial use. Alkali cellulose ( pulp s wollen in NaOH) i s pressed a nd aged to reduce m olecula r weight. Xanthation (a reaction with CS2 ) takes place in a vessel t hat c ontains an inert atmosphere (CS2 –air mixtures ar e explosive).

The orange xanthate i s s ubsequently dissolved in aque ous alkali to make the spinni ng dope. The dope is pumped t hrough spinnerets in which t here are f rom 14 t o 40,000 holes. The spun dope is converted back to c ellulose by the sulfuric acid i n t he coagul ating bath. Another s ystem with simultaneous derivitization and dissolution uses dimethyl sulfoxide and f ormaldehyde [ 133].

Several solvent s, such as cuprieth ylenedia mine (C UEN) hy droxide, de pend on the formation of metal–i on c omplex es with cellulose. Wh ile not as wi despread in use as the viscos e process , CUEN and its relative s with diff erent metals and ammoni um hyd roxide find substantial indust rial use [131]. The cad mium comp lex, cadoxen , is now the so lvent of choice in laborat ory work [134].

Aqueous salt solut ions such as satur ated zinc chloride or calci um thiocy anate can dissol ve limit ed amounts of cellulose [131] . Two nona queo us salt solut ions wi th a lengthy hist ory are ammoni um thiocy ana te=ammon ia and dimethylac etam ide =lithium chlori de (DMAc =LiCl). Soluti ons up to abou t 15% can be prepared with these solvent s. DMA c–LiC l has be en used for mo lecular weight determinat ions of cotton.

Trifluoroacet ic acid–m ethyl ene chlorid e and N -methyl mo rpholin e N -oxide mon ohydrate (NMMO) [136–138] are two other solvent systems that have been studied [139]. The new generic class of regenerated cellulose fibers, lyocell (e.g., Tencel [Courtaulds Fibres Limited, London, England]), is spun from aqueous solutions of NMMO [140].

Lyocell is an alternative to the generic name ‘‘rayon’’ for a subcategory of rayon fibers where the fiber is composed of cellulose precipitated from an organic solution in which no substitution of the hydroxyl group takes place and no chemical intermediates are formed.

Lyocell may have a different crystalline structure (a mixture of cellulose II and cellulose III [141]) than other rayons and cotton cellulose. No information has been published on cotton molecular weight determinations using NMMO as the solvent.