Structural changes to Pinus radiata wood lignin during kraft pulping and bleaching
The aim of this thesis was to improve the quality of the information available concerning the structure of native (within wood) and residual (within pulps) lignins. A better understanding of the structure of lignin will assist the development of improved pulping and bleaching sequences. However, before this can be done, it is necessary to develop approaches to lignin isolation which provide more representative lignin samples without resorting to what are often very tedious extraction procedures. A key requirement for such isolation methods would be to isolate lignins in high yield with a structure as unmodified as possible. This would allow meaningful comparisons to be drawn with respect to structural changes occurring during the pulping and bleaching processes. An improved method for the isolation of lignin from Pinus radiata wood was developed. The method involves a three day enzymatic hydrolysis of ball milled wood, followed by a second ball milling stage. A combination of extraction and purification steps enables isolated yields of approximately 75% of the lignin present in the wood to be obtained. The key step is the second ball milling treatment, which allows essentially all the lignin in the enzyme digest residue to be extracted. The resulting cellulolytic enzyme lignins (CEL) contain approximately 5-8% carbohydrates and less than 2% protein. The CEL lignins, which were soluble in common lignin solvents, were characterised chemically and spectroscopically. They were shown to be remarkably similar to milled wood lignin (MWL) in terms of the total phenolic hydroxyl content, uncondensed β-O-4 ether content, and the distribution of interunit linkages, although there was some indication that the CEL lignins had a higher molecular weight than MWL. This shows that MWL, as far as a soluble lignin is concerned, is a reasonable representation of native lignin. However, the CEL method developed in this study is superior to the MWL method in that much higher yields of lignin can be obtained, thus less effort is required to obtain the same quantities of lignin from wood by the CEL method as compared to the standard MWL method. Unfortunately, when this CEL method (and modifications of) were applied to Pinus radiata kraft pulps no significant improvement, in terms of yield or protein contamination, were achieved compared to other reported CEL methods. However, as part of this work it was shown that an enzyme hydrolysis treatment to enrich the residual lignin content in kraft and kraft-oxygen pulps had the potential to make the analysis of in situ residual lignin by degradation studies easier. In order to study the structure of the residual lignin in pulps bleached with elemental chlorine free sequences residual lignins were isolated by dioxane acidolysis instead. The approach taken in this thesis was to compare residual lignin from pulps bleached to the same kappa number. Such a comparison should remove the variables associated with the different oxidising powers of each reagent and yield a more comparable residual lignin, thus a kraft-oxygen pulp was bleached to approximately kappa number 10 with cyanamide activated hydrogen peroxide (KOQPn), ozone (KOQZE), and chlorine dioxide (KODE). For comparison a hydrogen peroxide bleached pulp (KOQP) was generated using the same peroxide charge as KOQPn; and unextracted ozone and chlorine dioxide bleached pulps (KOQZ and KOD, respectively) were also studied. The residual lignins from the bleached pulps were very similar in structure and resembled the kraft pulp residual lignin in many aspects except in having slightly elevated levels of oxidised groups. Additionally, many of the structural elements expected to be highly reactive during bleaching (for example phenolic and catechol type structures) were detected in the residual lignins. These results support the view that the removal of lignin during bleaching is controlled to a significant extent by the accessibility of the lignin and penetration of the bleaching reagents rather than just by the nature of the functional groups present in the lignin.
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