Cellulose derivatives

1. Cellulose derivatives Ian Suckling APPI 2012

2. Overview • Reactivity and accessibility of cellulose • Swelling and dissolution of cellulose • Esters of inorganic acids • Cellulose xanthates • Esters of organic acids • Cellulose ethers

3. Reactivity and accessibility of cellulose • Etherification requires ionisation of hydroxyl • i.e. C-OH → C-O- • Ease of ionisation: HO-2 > HO-3 > HO-6 • Hence HO-2 most easily etherified • Esterification HO-6 most reactive (primary alcohol) • HO-6 has highest reactivity to bulky substituents

4. Accessibility • Reactivity will depend on accessibility to hydroxyl groups, i.e. can be reached by reactants • Initial reaction heterogeneous at cellulose surfaces • Reactivity amorphous >> crystalline • Pre-swelling of cellulose needed prior to both etherification (by alkali) and esterification (by acid) Acetylation of cotton fibres. original fibres crystallinity destroyed, but dried so H-bonds re-form crystallinity destroyed , never dried (i.e. no H-bonding)

5. Swelling of cellulose • Cellulose swells in different solvents • Depends on the solvent and cellulosic sample, e.g. cellulose crystallinity, hemicellulose & lignin contents • Intercrystalline swelling – agent enters only disordered regions and between microfibrils • Intracrystalline swelling – also penetrates crystalline regions • Swelling of dry cellulose fibres on exposure to humidity

6. Cellulose solvents • Non-derivatising solvents – dissolve cellulose only by intermolecular interactions • Derivatising solvents dissolution accompanied by covalent derivitisation to ester, ether or acetal • Decomposed to regenerated cellulose by changing medium or pH • Examples • Phosphoric acid/water (Cell-O-PO3H2) • Formic acid/ZnCl2 (Cell-O-(O)CH) • Carbon disulfide/NaOH/water (Cell-O-C(S)SNa)

7. Non-derivatising solvents

8. Alkali celluloses • NaOH most important swelling solvent • Actually hydrates of cellulose alkoxides • X-ray diffractogram changed • Some other inorganic and organic bases also effective • Preparation of alkali celluloses called mercerisation • Swollen structure exhibits markedly enhanced accessibility to reagents and hence reactivity • Mercerisation in 12-18% cold NaOH transforms Cellulose I → Cellulose II (changes unit cell dimensions) • concentration required depends on cellulose source

9. Sources of cellulose • Purified cellulose cotton linters • Generally higher purity than dissolving pulps • Dissolving pulps (5-8 M t/yr): • Acid sulfite • Multi-stage sulfite • Prehydrolysis-kraft • Applications: • Regenerated celluloses (~two-thirds of cellulose) • Cellulose derivatives

10. Esters of inorganic acids

11. Cellulose nitrates I • Cellulose nitrates first commercial cellulose derivatives • Usually prepared by nitrating using mixtures of nitric + sulfuric acid • Competes with ordinary dissociation • Amount of water determines NO2+ concentration and hence extent of nitration (i.e. DS) • Then

12. Cellulose nitrates - Ctd • Properties determined by type of nitrating agent, as well as reaction time and temperature • Ratio HNO3:H2O:H2SO4 chosen to yield desired DS and maintain fibrous structure

13. Cellulose nitrates - Ctd • Physical properties and applications depend on DS • Rather hydrophobic & soluble in organic solvents • Non-toxic to humans • Flammable or explosive Commercial grades of cellulose nitrate.

14. Cellulose xanthates • Cellulose xanthate as Na salt soluble in water or alkali • Resulting aqueous solution (viscose) passed through slit (cellophane) or spinneret (rayon) into an acid bath for regenerating almost pure cellulose

15. Viscose process Alkali cellulose Cellulose xanthate Viscose Regenerated cellulose

16. Regenerated celluloses • Viscose process dominant route to regenerated celluloses • Issues • consumption of CS2 (~30% consumed) • dealing with sulfur-containing discharges • Rayon fibres have great variety of uses, e.g. textiles, non-wovens, tyre cords • Properties can be adjusted in manufacturing process • Recent new direct solvent spinning process using NMMO monohydrate “Tencel”

17. Esters of organic acids • Dominant product is cellulose acetate (~80,000 t/yr) • properties depend on DS and DP (viscosity) • Acetate rayon prepared by passing solution of cellulose acetate through fine holes of a spinnaret • solvent evaporation → solid filaments Commercial grades of cellulose acetate.

18. Cellulose acetate – raw materials • Cellulose raw material mainly wood pulps (sulfite or prehydrolysis kraft), but cotton linters also suitable • Quality important in final product Typical specifications for acetylation grade pulp.

19. Commercial grades of cellulose acetate

20. Chemistry of acetylation

21. Cellulose acetate process Ac2O acetic anhydride CTA cellulose triacetate

22. Other esters • Can tailor properties using other esters • Longer alkyl chains, e.g. propionate, butyrate, etc • Mixed esters, e.g. cellulose acetate propionate • used in lacquers, sheetings, plastics and films • Mixed esters, e.g. propionate-valerate • used to provide grease- and water-repelling properties • Aromatic esters, e.g. cinnamates and salcylates • Nitrogen-containing esters, e.g. dialkyl diaminoacetate

23. Cellulose ethers • Products of considerable commercial importance • formed by treating alkali cellulose with variety of reagents • Most ethers water-soluble, used as thickeners in foods, cosmetics, pharmaceuticals, drilling muds, paints etc • Critical properties: • solubility • water-binding capacity • non-toxicity • chemical stability Types of commercial cellulose ethers.

24. Alkylcelluloses • Methyl & ethyl celluloses (total ~100,000 t/yr) • Formed by reaction of alkali celluloses with alkyl chlorides: • Side reaction consuming alkyl halide then

25. Methyl cellulose • DS determines properties • <0.6 soluble in cold water and alkali • 1.5-2.0 soluble in water • >2.5 soluble in organic solvents • Chemically stable on storage • Non-toxic • Form gels at higher temperatures • Main applications as thickener and emulsifier in: • construction materials • paints • agricultural & food products • cosmetics, pharmaceuticals • detergent aids

26. Ethyl cellulose • DS again determines properties • 0.7-1.7 soluble in water • >2.5 soluble in organic solvents • Main applications: • Stable and tough films (solvent-soluble EC products) • In lacquers • Binder and granulation aid for tablets

27. Hydroxyalkylcelluloses • Hydroxyethyl cellulose (HEC) R = H • Hydroxypropyl cellulose (HEC) R = CH3 • Side reactions: • Consumes the ethylene oxide • DS [of cellulose] < molar substitution (MS)

28. Degree of substitution vs molar substitution • Degree of substitution (DS) = average number of –OHs substituted on anhydroglucose unit • Molar substitution (MS) = average of total number of alkylene oxide groups per anhydroglucose unit

29. Hydroxyalkylcelluloses – ctd • HEC • MS 0.05-0.5 soluble only in alkali • MS > 1.5 water soluble • HEC applications (~60,000 t/yr, MS 1.8-3.5): • Thickener for latex paints • For emulsion polymerisation of polyvinyl acetate • Paper sizing • Wet strength agent for paper • HPC more hydrophobic than HEC • Only water-soluble at MS approaching 4 • similar commercial applications but volumes lower • can be extruded without softener at 160ºC, c.f. HEC not thermoplastic

30. Carboxymethylcellulose (CMC) • Commercially most important cellulose ether • DS 0.4 – 1.4, DP 200 – 1,000 • DS > 0.6 – 0.8 gives good water solubility • Main applications (>300,000 t/yr): • Detergents – anti-deposition agent • Thickener, stabiliser and water binder in foods & cosmetics • Drilling mud dispersion stabiliser • Emulsion paints • Ceramics • Pharmaceuticals

31. Cyanoethyl cellulose • Highly cyanoethylated products (DS 2.5) soluble in polar organic solvents • Can be used as insulating materials because of high dielectric constant and low dissipation factors • Cyanoethylated papers used for insulation papers for transformers • Also have good thermal & dimensional stability