Wolfgang J. Fischer

How xylan effects the breaking load of individual fiber–fiber joints and the single fiber tensile strength

Bleached and unrefined softwood kraft fibers were treated with aqueous solutions of xylans and potassium hydroxide (KOH) to vary the amount of xylan present on the fiber surface and within the fiber wall. This was done in order to directly measure the influence of xylan on the joint strength of individual fiber–fiber joints as well as the tensile strength of individual fibers to determine the elastic modulus. The results were compared with two unbleached and unrefined softwood kraft pulps (Kappa 42 and 54). Additional xylan had a statistically significant effect on the mean values of the breaking load of individual fiber crossings. Moreover, the breaking load was strongly affected by the cooking time during pulping. However, no correlation between the optical bonded area and the joint strength was observed. The modulus of elasticity of individual fibers was not influenced by additional xylan or alkaline extraction. The elastic modulus was influenced more by chemical pulping and bleaching. The results indicate that additional xylan on the fiber surface has a strength-enhancing effect on the joint strength individual fiber crossings which also depends on the distribution of xylan. On the other hand, the elasticity of a fiber seems to be more influenced by the morphology and the composition of the fiber walls, especially in the secondary S2 wall.

Nonspecific protein adsorption on cationically modified Lyocell fibers monitored by zeta potential measurements

Nonspecific protein deposition on Lyocell fibers via a cationization step was explored by adsorption of two different N,N,N-trimethyl chitosan chlorides (TMCs). Both, the cationization and the subsequent protein deposition steps were performed and monitored in situ by evaluating the zeta potential using the streaming potential method. Both employed TMCs (degree of substitution with N+Me3Cl groups: 0.27 and 0.64) irreversibly adsorb on the fibers as proven by charge reversal (-12 to +7mV for both derivatives) after the final rinsing step. Onto these cationized fibers, BSA was deposited at different pH values (4, 5, and 7). Charge titrations revealed that close to the isoelectric point of BSA (4.7), BSA deposition was particularly favored, while at lower pH values (pH 4), hardly any adsorption took place due to electrostatic repulsion of the cationic fibers and the positively charged BSA. This work sets the foundation for further investigations to use zeta potential measurements for protein adsorption studies on fibrous materials.

Influence of relative humidity on the strength of hardwood and softwood pulp fibres and fibre to fibre joints

Cellulosic materials are highly sensitive towards environmental changes such as temperature and especially towards humidity. Besides morphological changes like swelling and/or shrinking, the mechanical properties of pulp fibres and fibre to fibre joints change as well. The current study sets to elucidate the changes and the extent to which elevated or decreased relative humidity (RH) influences the load bearing capacity of individual hardwood and softwood fibres and joints. The device used for the measurements is a micro bond tester with a custom-built humidity chamber in which the humidity can be controlled in a range from 25 to 95% RH. Individual fibres and joints have been tested at 30, 50 and 80% RH after equilibration time of 120 min. The breaking load of individual softwood fibres and joints displayed a maximum breaking load at 50% RH, with the values showing a decreasing trend towards higher or lower RH. In case of hardwoods, no change in the breaking load of either individual fibres or fibre to fibre joints has been observed at different RH.

The effect of Dean Flow in a tube flow fractionation device

Fractionation in laboratory scale is a common way to evaluate size distributions of pulp samples. The method of interest in the present study, is the Tube Flow Fractionation (TFF) device. This device separates pulp samples according to their particle size based on the flow regime inside the tube. Longer particles such as fibres are dragged with the faster middle flow and concentrate at the front end of the flowing column. Smaller particles and fines are concentrated at the end of the flow. In the current work, a TFF similar to those described in the literature was set up to investigate the effect of secondary flow (Dean Flow) in the flow regime of a coiled tube on the fractionation performance. Centrifugal forces acting on the fluid generate a pressure difference between the inner and outer bend of the tube. This pressure difference in turn leads to the formation of a secondary flow. Based on a simple design of the tube flow fractionator, settings such as flow velocity or the curvature can be easily adjusted. Trials based on different pulp samples as well as fractions of these pulps show the influence of secondary flow on the fractionation performance in the tube flow.