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Cross-sectional structure of the secondary wall of wood fibers as affected by processing
STFI, Swedish Pulp and Paper Res. Inst..
STFI, Swedish Pulp and Paper Res. Inst..
2003 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 38, no 1, 119-126 p.Article in journal (Refereed) Published
Abstract [en]

Understanding the arrangement of wood polymers within the fiber wall is important for understanding the mechanical properties of the fibers themselves. Due to their high load bearing ability, the arrangement of cellulose fibrils within the cell wall are of special interest. In this work AFM-Atomic Force Microscopy-in combination with image processing has been used to obtain more information about the arrangement of cellulose aggregates (fibrils) in the secondary cell wall layer of spruce wood. The effects of chemical processing on the arrangement of these cellulose aggregates were also studied. Enlargement of cellulose aggregates was found in the initial phase of the kraft cook. This increase in cellulose aggregate dimensions depended mostly on temperature for treatment temperatures above 140degreesC, regardless of the amount of alkali present. Although hemicelluloses are lost to various degrees under alkaline conditions, the increase in cellulose aggregate size was mainly related to thermally induced rearrangement of the cellulose molecules. The mean side length of cellulose aggregates was found to be around 18 nm in unprocessed wood and 23 nm in processed wood. The cellulose aggregates were assumed to be square shaped in cross section in both cases.

Place, publisher, year, edition, pages
2003. Vol. 38, no 1, 119-126 p.
Keyword [en]
Atomic force microscopy, Cellulose, Image processing, Mechanical properties, Molecular structure, Natural fibers, Thermal effects, Cellulose aggregate, Cross-sectional structure
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4934DOI: 10.1023/A:1021174118468ISI: 000179246400016OAI: oai:DiVA.org:kth-4934DiVA: diva2:7106
Note
QC 20101012Available from: 2005-02-14 Created: 2005-02-14 Last updated: 2017-12-05Bibliographically approved
In thesis
1. The cell wall ultrastructure of wood fibres: effects of the chemical pulp fibre line
Open this publication in new window or tab >>The cell wall ultrastructure of wood fibres: effects of the chemical pulp fibre line
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Knowledge of the ultrastructural arrangement within wood fibres is important for understanding the mechanical properties of the fibres themselves, as well as for understanding and controlling the

ultrastructural changes that occur during pulp processing.

The object of this work was to explore the use of atomic force microscopy (AFM) in studies of the cell wall ultrastructure and to see how this structure is affected in the kraft pulp fibre line. This is done in order to eventually improve fibre properties for use in paper and other applications, such as composites. On the ultrastructural level of native spruce fibres (tracheids), it was found that cellulose fibril aggregates exist as agglomerates of individual cellulose microfibrils (with a width

of 4 nm). Using AFM in combination with image processing, the average side length (assuming a square cross-section) for a cellulose fibril aggregate was found to be 15–16 nm although with a broad distribution. A concentric lamella structure (following the fibre curvature) within the

secondary cell wall layer of native spruce fibres was confirmed. These concentric lamellae were formed of aligned cellulose fibril aggregates with a width of about 15 nm, i.e. of the order of a single cellulose fibril aggregate. It was further found that the cellulose fibril aggregates had a

uniform size distribution across the fibre wall in the transverse direction.

During the chemical processing of wood chips into kraft pulp fibres, a 25 % increase in cellulose fibril aggregate dimension was found, but no such cellulose fibril aggregate enlargement occurred during the low temperature delignification of wood into holocellulose fibres. The high temperature in the pulping process, over 100 ºC, was the most important factor for the cellulose fibril aggregate enlargement. Neither refining nor drying of kraft or holocellulose pulp changed the cellulose fibril aggregate dimensions.

During kraft pulping, when lignin is removed, pores are formed in the fibre cell wall. These pores were uniformly distributed throughout the transverse direction of the wood cell wall. The lamellae consisting of both pores and matrix material (“pore and matrix lamella”) became wider and their numeral decreased after chemical pulping. In holocellulose pulp, no such changes were seen.

Refining of kraft pulp increased the width of the pore and matrix lamellae in the outer parts of the fibre wall, but this was not seen in holocellulose.

Upon drying of holocellulose, a small decrease in the width of the pore and matrix lamellae was seen, reflecting a probable hornification of the pulp. Refining of holocellulose pulp led to pore closure probably due to the enhanced mobility within the fibre wall. Enzymatic treatment using

hemicellulases on xylan and glucomannan revealed that, during the hydrolysis of one type of hemicellulose, some of the other type was also dissolved, indicating that the two hemicelluloses were to some extent linked to each other in the structure. The enzymatic treatment also decreased the pore volume throughout the fibre wall in the transverse direction, indicating enzymatic accessibility to the entire fibre wall.

The results presented in this thesis show that several changes in the fibre cell wall ultrastructure occur in the kraft pulp fibre line, although the effects of these ultrastructural changes on the fibre properties are not completely understood.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 70 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2005:2
Keyword
Chemical engineering, atomic force microscopy, cellulose, cell wall, drying, fiber, Kemiteknik
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-129 (URN)
Public defence
2005-02-18, STFI-salen, Drottning Kristinas väg 61, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101012Available from: 2005-02-14 Created: 2005-02-14 Last updated: 2010-10-12Bibliographically approved

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