Compressive stress relaxation of wood pulps with particular reference to pulp chemistry
Creators: name: Kirbach, Eberhard
Publisher: name: University of British Columbia
Year: 2011

description: Stress relaxation responses in compression following steeping either in distilled water or 18.6% NaOH were examined on wood pulp mats with the objective to relate quantitative, physical, as well as structural features of pulp polymers to mat rheology. The study included 32 commercial and and laboratory prepared pulps (groundwood, holocellulose, paper grade, viscose, acetate and alpha-cellulose pulps) which provided wide variation in chemical characteristics. Rate of stress relaxation was found to differ considerably between individual pulps, indicating that each pulp possesses a different ability to dissipate stress. The stress relaxation traces, following quasi steploading (1.0 - 1.5 sec), suggest that pulp mat relaxation is governed by two mechanisms (M₁ and M₂). M₁, dominating between 0.0 to 1.0 min, is believed to involve primarily inter-fibre processes, while M₂, controlling the response thereafter, comprises intra-fibre, essentially molecular processes. Swelling media were found to influence pulp mat rheology. Considerably higher rates of stress dissipation were observed with caustic (18.6% NaOH) than with water swollen mats. This indicates that intra-crystalline swelling, in addition to inter-crystalline, enhances time dependent responses. The effect of any swelling medium on pulp mat rheology appears to be highly dependent on length of swelling treatment, since it was observed that the capacity of pulps to dissipate stress changes inversely with length of steeping time. This phenomenon suggests inter-changeability of chemical and physical stress systems. Evidence presented in the study shows that pulp chemistry controls time dependent: behavior of pulp mats. Quantitative and structural characteristics of hemicelluloses appeared to be most important for rheological processes. In the caustic swollen state, decrease in hemicellulose caused proportional changes in stress relaxation. The same observation was made for water swollen groundwood, holocellulose and paper pulps. In water saturated low yield pulps, however, the viscoelastic effect of hemicelluloses was reversed, due to degradation and redeposition phenomena. Here, residual hemicelluloses were found to retard stress dissipation. From these observations, it was deduced that this group of wood polymers functions as an important linkage in stress distribution and dissipation systems of the undegraded or only slightly degraded lignin-carbohydrate system. Low DP and high degree of branching make bemicelluloses highly suited to dissipate stress in the wood of living trees, as well as ground-woods and holocellulose pulps and to a lesser extent in other pulp types. Cellulose was found to account for most of the dissipated stress in pulp polymeric systems. Its quantitative contribution to pulp rheology appeared to vary little between pulps of no or limited cellulose degradation. Severe degradation, e.g. in low yield pulping, however, enhanced the time dependent response. This was demonstrated further by relaxation tests on viscose pulps degraded by radiation. Lignin appeared to be of only subordinate importance for pulp mat stress relaxation in compression, primarily attributable to orientation or layer effects and to the dominant role of hemicelluloses. Relaxation measurements on wood pulp products are suggested as useful tools for predicting and estimating pulp and paper properties, such as pressing behavior and alkali solubility of viscose pulps, pulp beating response, runnability and printability of paper.