Chemical analysis of wood and pulps

1. Chemical analysis of wood and pulps Ian Suckling APPI May 2012

2. Wood analysis methods overview Extractives Extractives Klason + acid-soluble lignin Lignin Uronic acids Hemicelluloses Acetyl content Holocellulose Neutral carbohydrates Cellulose crystallinity Celluloses -cellulose Ash Silica, salts, etc

3. General issues • Representative sampling critical! • Sample storage may be an issue • Contamination, oxidation, microbiological activity, etc • Solid samples often need drying and grinding • Results for solid samples normally reported on oven-dry basis even though sample often analysed air-dried • Requires parallel determination of moisture content • Liquid samples may be analysed as-is or dried prior to analysis • Filtration may be an issue • Method must be appropriate to the sample analysed • Standard methods exist, e.g. Tappi, ASTM, SCAN, ISO, ASNZS B. Holmbom & P. Stenius – Analytical Methods in Forest Products Chemistry Vol. 3 Papermaking Science and Technology, FapetOy (2000)

4. Summative analysis Wood/pulp Extraction Extractive-free wood/pulp Acid hydrolysis (i) 72% H2SO4 at 30ºC 4% H2SO4 at 121ºC Filtration Insoluble Soluble Acid-insoluble (Klason) lignin Acid-soluble lignin Sugar monomers Uronic acids Total lignin

5. Extractives • Extractives can interfere with subsequent analyses • Solvent(s) depend on substrate, e.g. • Dichloromethane or ethanol/benzene in Soxhlet extractor for softwoods • Kraft pulps generally do not require extraction • Sometimes multi-stage extraction is required • Level of extractives then determined gravimetrically by removing the solvent and weighing (e.g. Tappi T204 cm-97) • Sometimes better to determine losses in weight of material extracted • Extractives may then be further analysed to determine detailed composition, e.g. by GC, GPC etc • Dr Karen Stack to discuss in later lectures

6. Klason + acid-soluble lignin • Most common way to quantify lignin in woods, e.g. Tappi T222 om-88 and variants on this • Procedure not difficult, but requires careful attention to details • Gravimetric, so not very accurate at low lignin contents • Potential interferences: ash, protein, extractives, acid degradation products of carbohydrates • Hydrolysis is carried out in two stages: • 72% H2SO4 for 1 h at 30ºC – impregnation, swelling, preliminary hydrolysis of carbohydrates • 4% H2SO4 for 1h at 121ºC – to complete hydrolysis • For some substrates, e.g. hardwoods, sulfonated pulps, some lignin is soluble in dilute acid, so quantified via the absorbance of the solution at 205 nm (e.g. Tappi UM 250)

7. Neutral carbohydrates • Principle: Hydrolysis of insoluble carbohydrates to monomeric sugars, e.g. glucose, under conditions to minimise their subsequent degradation, and then quantification of the resulting monomeric sugars • Hydrolysis • Commonly use soluble material following Klason lignin determination • Alternative hydrolysis methods exist if material soluble or not interested in determining lignin content, e.g. using trifluroacetic acid • Sugar analysis • Mostly commonly now by liquid chromatography (HPLC) using anion exchange columns as avoids need to derivatise sugars • e.g. proprietary Dionex PA10 anion exchange column + pulsed amperometric detector • By gas chromatography after derivitisation, e.g. as aliditol acetates (Tappi T249 cm-85) • Glucose analysers

8. Monomeric sugar separation – Dionex PA10 column • fafsd Courtesy of Katrina Martin, Scion

9. Summative analysis results • For radiata pine wood: • Cellulose = Glucose – 0.27 Mannose • Hemicelluloses = 1.27 Mannose + Galactose + 1.18 Xylose + Arabinose • Galactoglucomannan = 1.31 Mannose

10. Uronic acid groups • 4-O-methyl-glucuronic acid groups difficult to quantify • Conditions to completely hydrolyse glycosidic bond to monosaccharide cause severe decomposition of uronic acid units • Options: • Colourimetric assay for uronic acids in hydrolysates from Klason lignin detn (e.g. using m-hydroxydiphenyl) • Not very good for low levels of uronic acids, e.g. in woods • Hydrolyse in acidic methanol, then quantify fairly stable methyl glucuronosides • Measure monomeric glucouronic acids and correct back

11. Acetyl content • Hemicelluloses in wood contain acetyl substituents • Liberated on treatment with either acid or alkali • Determined by hydrolysis with oxalic acid and determination of released acetic acid by gas chromatography • Alternatively, instrumental methods can be used • diffuse reflectance infrared spectroscopy • solid state nuclear magnetic resonance spectroscopy • Common product in mill process streams, effluents and air discharges, so range of methods to quantify

12. Other lignin analysis methods • Kappa number widely used to determine lignin in chemical & semi-chemical pulps (e.g. Tappi T236 cm-85) • Defined as the volume in mL of 0.1 N potassium permanganate solution consumed by 1 g moisture-free pulp under specified conditions • Applicable to pulps with yields <60% (or <70% with care) • Hexeneuronic acids (HexA) formed during kraft pulping & bleaching consume permanganate, esp. for hardwood pulps. Methods to correct for HexA content are available • Approx. Klason lignin = 0.13 x Kappa # • Acetyl bromide lignin • Dissolve wood/pulp in acetyl bromide & quantify using UV absorbance of solution at 280 nm • Benefit is small sample size

13. Holocellulose • Extractive-free wood/pulp can be delignified with certain oxidative reagents → extractive- and lignin-free carbohydrate fraction (holocellulose) • Traditionally used as one step in summative analysis of wood, most commonly using chlorite • Now mainly used as a research method • Other conditions and oxidative reagents possible, .e.g Cl2/monoethanolamine, peroxyacetic acid, nitric/acetic acid • Some carbohydrate oxidation and dissolution of hemicelluloses can occur

14. Solubility of pulps in alkali • Traditionally used for determining cellulose and degraded celluloses in fully bleached pulps (and holocelluloses) • Multiple variants • Now replaced to large extent by summative analyses • Still used to characterise pulps for certain applications, e.g. dissolving pulps • Tappi T203 om-93 • -cellulose – insoluble in 17.5% and 9.45% NaOH at 25ºC – undegraded cellulose • -cellulose – soluble fraction precipitated on acidifcation of the solution – degraded cellulose • -cellulose – that fraction remaining in the solution – mainly hemis • Tappi T212 om-93: One percent NaOH solubility of wood & pulp • Insoluble residue after extraction with 1% NaOH at reflux for 1 h. • Low-molecular-mass carbohydrates in woods and pulps • Can be used as an indicator of cellulose degradation

15. Pulp viscosity • Cellulose is soluble in cupriethylenediamine (CED) • Viscosity of solution of cellulose or pulp in CED under standard conditions used as an indicator of the molecular weight of the pulp • Relative viscosity, e.g. Tappi T230 om-94 • Limiting viscosity, e.g. SCAN • Often used as a predictor of pulp strength potential & to monitor fibre degradation during pulping & bleaching • Empirical correlations between Tappi and limiting viscosity • int (mL/g) = 954 x log[Tappi viscosity] – 325 (MPa.s) • Empirical correlations to cellulose DP • DPv0.90 = 1.65 x [int ] • int must first be corrected for cellulose content of pulp • Only applicable to pulps that fully dissolve in CED, e.g. chemical pulps up to ~4% lignin

16. Cellulose crystallinity • X-ray diffraction one of most common methods • Get maxima from specific reflections of crystalline regions Typical X-ray diffractogram for cellulose I.

17. Changes in cellulose crystallinity during kraft pulping Evans, R et al. Holzforschung 49(5): 498 (1995)

18. Cellulose accessibility • Wide range of methods used to measure cellulose accessibility using physical, chemical and sorption methods • Wide range of accessibilities for a given substrate, good agreement on order of increasing accessibility: • Cotton < wood pulps < mercerised cotton < regenerated celluloses

19. Inorganics • Ash after combustion used as measure of total inorganics • Typically done in muffle oven at 525ºC or at 900ºC • At 900ºC CaCO3 decomposes (to CaO and CO2), so ashing at both temperatures useful for estimating CaCO3 of papers etc • Ashing often used to concentrate inorganics prior to analysis • Methods for analysis of inorganics • Atomic absorption, e.g. for Na, Ca, Cu, Fe • Plasma techniques (ICP-AES, ICP-MS) – replacing AAS as multi-element and more sensitive • Ion chromatography – sulfide, sulfate, thiosulfate, chloride, carbonate • X-ray fluorescence spectroscopy – pigments, major & minor inorganics in woods, bark, furnace smelt, dust etc

20. Sulfonic and carboxylic acid groups • Conductometric titration • Pulp pre-treated to acid form • Titrate with NaOH and monitor conductivity • Initially neutralisation of strong sulfonic acid groups • The conductivity constant as weaker carboxylic groups neutralised • Conductivity then rises once all acids neutralised