A Model for the Prediction of Fiber Elasticity

A model is presented that enables the elastic properties of wood fibers to be estimated from the properties of its polymeric constituents, cellulose, hemicellulose, and lignin. The influence of the value of the axial stiffness of the cellulose crystal is demonstrated, its proper value being discussed in comparison with experimental data on fibers. The effects on fiber stiffness of the S2 fibril angle, the fibril angles of other layers, the crystallinity, and layer thicknesses are analyzed. The manner in which the effect of a variation in yield can be simulated by a change in shape factor of the reinforcing cellulose crystals is demonstrated, the cell wall thus being considered to be a discontinuous reinforced composite

Experimental micromechanical characterisation of wood cell walls

International audienceThe properties of wood and wood based materials are strongly dependent on the properties of its fibres; i.e. the cell wall properties. The ability to characterize these in order to increase our understanding of structure-property relationships is thus highly important. This article gives a brief overview of the state of the art in experimental techniques to characterize the mechanical properties of wood at both the level of the single cell and that of the cell-wall. Challenges, opportunities, drawbacks and limitations of single fibre tensile tests and nanoindentation are discussed with respect to the wood material properties

Chemical pulping: Localization of xylan (native and added) during cooking

The aim of this study was to investigate how the xylan content is changed within a wood chip and within the fiber cell walls at different positions of the chip during cooking. It was hoped that such information would give an understanding regarding the critical factors for xylan deposition within the cell wall and possible ways of affecting strength delivery of the pulp. Two sets of cooking trials were performed; one Ref-pulp and one with beech xylan addition in the impregnation stage (10g/L). The cooks were interrupted at specific times and the chips taken out and analyzed for chemical composition and distribution of wood polymers within the chips as well as within the fiber cell walls. The pulps produced with beech-xylan addition had about 2 % higher xylan content at the end of cooking compared to the Ref-pulp. Results indicated that the chips already contained an increased amount of xylan after 10 minutes of impregnation when beech xylan was added to the cooking liquor. However, only chips at the end of the cook showed a substantial increase in xylan sorption. During cooking, a large gradient in lignin content was noted between the surface and the inside of the chips which always showed higher lignin content even at the end of the cook. A progressive increase in xylan content was also noted in the chips as cooking proceeded. From the studies made it was not possible however, to determine if the addition of xylan led to any increased xylan content within the fibre walls. In cooks with beech xylan added during early stages, an increase in xylan was noted in the fibre lumen areas. In later stages and with increased delignification, increased xylan content was also noted between fibres in degraded middle lamellae regions

CRUW Mechanical Pulping sub-project 1: Effect of different refining pressures and energy using spruce TMP pulps from Braviken

The mechanical pulping industry faces continued rising energy costs and increasing competition for raw material. In order to produce improved products based on mechanical pulp at lower energy consumption it is necessary to have a better understanding of the development of fundamental fibre properties during the processes. In particular, changes in fibre collapsibility, fibre fibrillation and fibre and surface development are of great interest. The overall goal of the CRUW Mechanical Pulping project is “Support development of more energy efficient mechanical pulping processes by increasing the knowledge on ultrastructural phenomena in mechanical pulping”. This project is working closely together with the Industrial Research College for Mechanical Pulping Technology bringing in the ultrastructural competence to more clearly understand and explain phenomena observed in these projects thus making it easier to develop new and improved processes to reduce energy consumption. This report presents results from CRUW Mechanical Pulping sub-project 1: ”Effect of different refining pressures and energy using spruce TMP pulps from Braviken”. The influence of temperature on the softening of lignin and hence improved (easier) fibre separation and treatment was noted earlier (Becker et al. 1977; Salmén 1984). Based on this knowledge, different process alternatives have been suggested to reduce energy demand for the refining process. One of the earliest publications on a technical system utilizing higher temperature and pressure was by Höglund et al. 1997 (Thermopulp). These results have been reproduced in many studies and are today considered general knowledge. There are however many practical problems with such a system. For example, the resulting very small refining gaps are difficult to control and it has therefore taken time to establish this technology in the industry. In the new TMP line at Braviken, the refiners are equipped to run at higher temperature/pressure than normal and it has therefore been interesting to study these pulps in order to explain the effects on pulp/fibres at an ultrastructural level. It should be noted that in a fibre-water-steam system, temperature and pressure are not independent variables and higher pressure means higher temperature and vice versa

Structural characterisation and orientation of cell wall polymers in Arabidopsis thaliana stem

Plant cell walls are composed of a framework of cellulose microfibrils that are interconnected with heteropolysaccharides (lignin, hemicelluloses) in a specific manner. Plant cell walls form a large part of the plant body and define its characteristics. Structural organisation of the cell wall and related polymers is important for both mechanical properties of plants and chemical reactions occurring in the wall space, especially in the response to stress.By using imaging FTIR microscopy, run in transmission mode and at different polarisation modes (from 0° to 90°), it is possible to follow the chemical variability and the orientation of cell wall polymers (cellulose, hemicelluloses and lignin) of the Arabidopsis thaliana stem. The polarised FTIR measurements indicated that both xylan and lignin have parallel orientation with regard to the orientation of cellulose. It is believed that this structuring of lignin in the S2 layer of the cell wall might be a result of the spatial constraints within the cell wall, occuring due to the previous deposition of cellulose/hemicellulose in a strongly oriented assembly

Imaging FTIR microscopy – technique for rapid screening of plant cell walls

It has been shown that xylan is oriented in parallel to the cellulose and more or less parallel to the axis of a cell wall, in isolated CW fragments from maize leaves. There was also a clear indication of lignin orientation parallel to the longitudinal CW axis. This means that all of these components show strong anisotropic behaviour and organisation

STRUCTURAL CHARACTERISATION AND ORIENTATION OF CELL WALL POLYMERS IN MAIZE LEAVES

Cell wall can be considered as a nano-composite in which cellulose, lignin and hemicelluloses are interconnected in a specific manner. Mechanical and physical propreties of plant fibres are dependent on the orientation of constituent polymers (cellulose, hemicellulose, lignin). Fourier transform infrared (FTIR) microscopy was used to examine the orientation of the main plant polymers in transversal and longitudinal direction of the isolated cell wall of the maize leaves. Polarised FTIR measurements indicated an anisotropy, i.e. orientation of the cellulose microfibrils that was more or less parallel to the longitudinal axis of the cell wall. Xylan has parallel orientation with regard to the orientation of cellulose, as well as lignin

Polarized FT-IR study of cell wall of a hardwood (maple branch)

Mechanical and physical propreties of wood fibres are dependent on the orientation of constituent polymers (cellulose, hemicellulose, lignin). Fourier transform infrared (FTIR) microscopy was used to examine the orientation of the main wood polymers in transversal and longitudinal direction of the isolated cell wall of the maple branch. The polarised FTIR measurements indicated that glucomannan and xylan appear to have parallel orientation with regard to the orientation of cellulose. Lignin has also parallel orientation

Chemical pulping: the influence of the molecular weight of added xylan on pulp properties

The aim of this study was to investigate if added beech xylan with low molecule weight (Mw) could penetrate into the fiber wall to a greater extent compared to xylan with high Mw. The high Mw (ca 11000) xylan was degraded using enzymes to obtain xylan with low Mw (ca 1800). The influence from the added xylan on the strength properties was evaluated. The cooks in this study were performed without mechanical damage introduced during the cooking process, thus any conclusions of the impact from adding low Mw xylan on the sensitivity towards mechanical damage was not possible. The different microscopy analyses performed could not show any evidence for a higher penetration of xylan into the fiber wall when producing the pulp with addition of low molecular weight (low Mw) xylan. The low Mw xylan did not contribute to any improved pulp properties, rather the opposite. Addition of low Mw-xylan did not result in straighter fibers compared to the Ref-pulp, which was the case for the high Mw-xylan-pulp. The tensile indexdevelopment (tensile vs PFI-beating and tensile vs density) for the low Mw-pulp was even worse compared to the Ref-pulp. The pulp produced with addition of the high Mw-xylan showed, as earlier seen, an improved tensile index development compared to the Ref-pul