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UBC Theses and Dissertations

Topochemistry of delignification and its effect on fiber properties of spruce organosolv pulp Behera, Nikhil Chandra


The catalysed organosolv process is a novel method of pulping that has many advantages over other chemical pulping processes. One of the most important advantages is its 6-12 percentage point higher yield of pulp in comparison to other chemical pulping processes. Short cooking time, low disintegration and refining energy requirements, ease of pulp washing and simplified method of by-product recovery are some of the other advantages. However, due to differences in the chemical nature of the cooking liquor, the basic properties of the fibers differ considerably. In this thesis a detailed study has been carried out on some of the unique phenomena i.e., fiber liberation at a high yield, topochemical preference of delignification and their manifestation on morphology and strength properties of fibers. Pulping results show that softwoods can be pulped easily to a high pulp yield (60%) with a high viscosity of cellulose of 50 mPas. The observed delignification pattern indicates two distinct stages both having first order kinetics. By this process, fast delignification occurs in the bulk delignification stage within which about 70% of the lignin is removed. Loss of residual lignin occurs at a slower rate in the residual delignification stage. The ease of penetration of the cooking liquor and preferential removal of lignin from the middle lamella result in complete fiber liberation at a pulp yield of 57.3% and a Kappa number of 72 (7% residual lignin). The loss of lignin to carbohydrate ratio at 57.3%pulp yield is 1.21:1. The topochemistry of delignification in organosolv pulping is limited to a preferential removal of lignin from the cell corner and middle lamella regions rather than from the secondary wall. In the initial stages of pulping, lignin removal is mostly from the cell corner and middle lamella region. Secondary wall lignin was removed quite slowly and a substantial amount of lignin remained in the secondary wall even after extended delignification. This can be accounted for by the slow hemicellulose removal (loss) from the secondary wall. The relatively high residual lignin retained in the cell corner in comparison to the complete delignification in the middle lamella raises questions about chemical differences and solubility characteristics of the cell corner lignin. The fibers of high-yield pulps are found to be stiffer and form a low density paper with high tear, and average burst and tensile strength. These factors can be correlated with the higher amount of residual lignin material in the fiber secondary wall and the low bonding properties of the fibers. High residual lignin content decreases the internal fibrillation and ability of the fibers to conform with each other during sheet formation. On the other hand, the low-yield fibers (49.8%) were found to be quite flexible and showed higher strength than obtainable with high yield pulps. Organosolv handsheets contain 20 to 30% fewer fibers than kraft papers of the same basis weight. However, the apparent difference in strength properties between organosolv and kraft papers is not disproportionately large. Organosolv lignins isolated from the spent liquor have low molecular weight (1400-2400) and low polydispersity (1.95) when recovered from extended pulping liquors. This indicates that most of the lignin is degraded to a fairly uniform low molecular weight polymer without substantially affecting the reactivity of the natural lignin as it occurs in the native fibers. The simplicity of the pulping process together with the comparable strength properties of the fibers even at higher yields, reveals large potentials of this method as a new pulping process. With some refinements and closer optimization, pulp fully acceptable commercially could be produced by this process.

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