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Effects of Cell Wall Structure on Tensile Properties of Hardwood: Effect of down-regulation of lignin on mechanical performance of transgenic hybrid aspen. Effect of chemical degradation on mechanical performance of archaeological oak from the Vasa ship.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wood is a complex material and the mechanical properties are influencedby a number of structural parameters. The objective of this study has been toinvestigate the relationship between the structure and the mechanical propertiesof hardwood. Two levels were of special interest, viz. the cellular structureand morphology of the wood, and the ultra-structure of the cell wall. In thenext step, it was of interest to examine how the mechanical properties ofhardwood change with spontaneous/induced changes in morphology and/orchemical composition beyond the natural variation found in nature.

Together, this constituted the framework and basis for two larger projects,one on European aspen (Populus tremula) and hybrid aspen (Populus tremulax Populus tremuloides), and one on European oak (Quercus robur). Amethodology was developed where the concept of relative density and compositemechanics rules served as two useful tools to assess the properties ofthe cell wall. Tensile testing in the longitudinal direction was combined withchemical examination of the material. This approach made it possible to revealthe mechanical role of the lignin in the cell wall of transgenic aspen trees,and investigate the consequences of holocellulose degradation in archaeologicaloak from the Vasa ship.

The study on transgenic aspen showed that a major reduction in lignin inPopulus leads to a small but significant reduction in the longitudinal stiffness.The longitudinal tensile strength was not reduced. The results are explainableby the fact that the load-bearing cellulose in the transgenic aspen retained itscrystallinity, aggregate size, microfibril angle, and absolute content per unitvolume. The results can contribute to the ongoing task of investigating andpinpointing the precise function of lignin in the cell wall of trees.

The mechanical property study on Vasa oak showed that the longitudinaltensile strength is severely reduced in several regions of the ship, andthat the reduction correlates with reduced average molecular weight of theholocellulose. This could not have been foreseen without a thorough mechanicaland chemical investigation, since the Vasa wood (with exception fromthe bacterially degraded surface regions) is morphologically intact and witha micro-structure comparable to that of recent oak. The results can be usedin the ongoing task of mapping the condition of the Vasa wood.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , viii, 67 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:14
Keyword [en]
tensile strength, transgenic hybrid aspen, lignin down-regulation, the Vasa ship, chemical degradation
Keyword [sv]
longitudinell draghållfasthet, transgen hybridasp, ligninnedreglering, Vasaskeppet, kemisk nedbrytning
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-32190ISBN: 978-91-7415-914-1OAI: oai:DiVA.org:kth-32190DiVA: diva2:409533
Public defence
2011-04-29, F3, Lindstedtsvägen 26, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20110420Available from: 2011-04-20 Created: 2011-04-08 Last updated: 2011-04-26Bibliographically approved
List of papers
1. Mechanical characterization of juvenile European aspen (Populus tremula) and hybrid aspen (Populus tremula × Populus tremuloides) using full-field strain measurements
Open this publication in new window or tab >>Mechanical characterization of juvenile European aspen (Populus tremula) and hybrid aspen (Populus tremula × Populus tremuloides) using full-field strain measurements
2008 (English)In: Journal of Wood Science, ISSN 1435-0211, E-ISSN 1611-4663, Vol. 54, no 5, 349-355 p.Article in journal (Refereed) Published
Abstract [en]

Functional analysis of genes and proteins involved in wood formation and fiber properties often involves phenotyping saplings of transgenic trees. The objective of the present study was to develop a tensile test method for small green samples from saplings, and to compare mechanical properties of juvenile European aspen (Populus tremula) and hybrid aspen (Populus tremula × tremuloides). Small microtomed sections were manufactured and successfully tested in tension parallel to fiber orientation. Strain was determined by digital speckle photography. Results showed significantly lower values for juvenile hybrid aspen in both Young's modulus and tensile strength parallel to the grain. Average Young's moduli spanned the ranges of 5.9-6.6 and 4.8-6.0 GPa for European aspen and hybrid aspen, respectively. Tensile strength was in the range of 45-49 MPa for European aspen and 32-45 MPa for hybrid aspen. The average density (oven-dry) was 284 kg/m3 for European aspen and 221 kg/m3 for hybrid aspen. Differences in mechanical properties correlated with differences in density.

Keyword
Hybrid aspen, Juvenile, Populus tremula × tremuloides, Tensile strength, Young's modulus
National Category
Wood Science
Identifiers
urn:nbn:se:kth:diva-8760 (URN)10.1007/s10086-008-0960-x (DOI)000260150000002 ()2-s2.0-54249086657 (ScopusID)
Note
QC 20100922. Uppdaterad från Accepted till Published (20100922).Available from: 2008-06-17 Created: 2008-06-17 Last updated: 2011-04-20Bibliographically approved
2. Ultrastructure and Mechanical Properties of Populus Wood with Reduced Lignin Content Caused by Transgenic Down-Regulation of Cinnamate 4-Hydroxylase
Open this publication in new window or tab >>Ultrastructure and Mechanical Properties of Populus Wood with Reduced Lignin Content Caused by Transgenic Down-Regulation of Cinnamate 4-Hydroxylase
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2010 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 11, no 9, 2359-2365 p.Article in journal (Refereed) Published
Abstract [en]

Several key enzymes in lignin biosynthesis of Populus have been down-regulated by transgenie approaches to investigate their role in wood lignification and to explore their potential for lignin modification. Cinnamate 4-hydroxylase is an enzyme in the early phenylpropanoid pathway that has not yet been functionally analyzed in Populus. This study shows that down-regulation of cinnamate 4-hydroxylase reduced Klason lignin content by 30% with no significant change in syringyl to guaiacyl ratio. The lignin reduction resulted in ultrastructural differences of the wood and a 10% decrease in wood density. Mechanical properties investigated by tensile tests and dynamic mechanical analysis showed a decrease in stiffness, which could be explained by the lower density. The study demonstrates that a large modification in lignin content only has minor influences on tensile properties of wood in its axial direction and highlights the usefulness of wood modified beyond its natural variation by transgene technology in exploring the impact of wood biopolymer composition and ultrastructure on its material properties.

Keyword
Axial direction, Cinnamate 4-hydroxylase, Down-regulation, Guaiacyl, Key enzymes, Klason lignin, Lignin biosynthesis, Lignin contents, Lignin modifications, Lower density, Material property, Natural variation, Phenylpropanoid pathways, Populus, Tensile tests, Transgene, Transgenic approaches, Transgenics, Wood density, Biochemistry, Density (specific gravity), Dynamic analysis, Dynamic mechanical analysis, Enzymes, Lignin, Plants (botany), Stiffness, Tensile testing, Wood
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-26700 (URN)10.1021/bm100487e (DOI)000281629600023 ()2-s2.0-77956536493 (ScopusID)
Note
QC 20101129Available from: 2010-11-29 Created: 2010-11-26 Last updated: 2011-04-20Bibliographically approved
3. Towards improved understanding of PEG-impregnated waterlogged archaeological wood: A model study on recent oak
Open this publication in new window or tab >>Towards improved understanding of PEG-impregnated waterlogged archaeological wood: A model study on recent oak
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2010 (English)In: Holzforschung, ISSN 0018-3830, Vol. 64, no 2, 243-250 p.Article in journal (Refereed) Published
Abstract [en]

To prevent deformation and cracking of waterlogged archaeological wood, polyethylene glycol (PEG) as a bulk impregnation agent is commonly applied. PEG maintains the wood in a swollen state during drying. However, swelling of wood can reduce its mechanical properties. In this study, the cellular structure of oak and cell wall swelling was characterized by scanning electron microscopy (SEM) of transverse cross-sections, and the microfibril angle of oak fibers was determined by wide angle X-ray scattering (WAXS). Samples of recent European oak (Quercus robur L) impregnated with PEG (molecular weight of 600) were tested in axial tension and radial compression. Mechanical tests showed that axial tensile modulus and strength were only slightly affected by PEG, whereas radial compressive modulus and yield strength were reduced by up to 50%. This behavior can be explained by the microstructure and deformation mechanisms of the material. Microfibril angles in tensile test samples were close to zero. This implies tensile loading of cellulose microfibrils within the fiber cell walls without almost any shear in the adjacent amorphous matrix. These results are important because they can help separate the impact of PEG on mechanical properties from that of chemical degradation in archaeological artifacts, which display only small to moderate biological degradation.

Keyword
axial tension, dynamic vapor sorption (DVS), European oak, impregnation, polyethylene glycol (PEG), radial compression, wide angle X-ray scattering (WAXS), Young's modulus
National Category
Materials Engineering Forest Science
Identifiers
urn:nbn:se:kth:diva-28815 (URN)10.1515/HF.2010.024 (DOI)000274423900015 ()2-s2.0-76849084829 (ScopusID)
Funder
Knut and Alice Wallenberg Foundation
Note
QC 20110126Available from: 2011-01-26 Created: 2011-01-21 Last updated: 2011-04-20Bibliographically approved
4. Significant loss of mechanical strength in archeological oak from the 17th century Vasa ship: correlation with cellulose degradation
Open this publication in new window or tab >>Significant loss of mechanical strength in archeological oak from the 17th century Vasa ship: correlation with cellulose degradation
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(English)Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:kth:diva-32819 (URN)
Note
QC 20110420Available from: 2011-04-20 Created: 2011-04-20 Last updated: 2011-11-09Bibliographically approved

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