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.
INGELA BJURHAGER
Doctoral Thesis
Stockholm, Sweden 2011
TRITA-CHE Report 2011:14
ISSN 1654-1081
ISBN 978-91-7415-914-1
KTH School of Chemistry
SE-100 44 Stockholm
SWEDEN
Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges
till offentlig granskning för avläggande av Teknologie doktorsexamen i polymerteknologi fredagen den 29 april 2011 klockan 13.00 i F3, Lindstedtsvägen 26, Kungl
Tekniska högskolan, Stockholm.
© Ingela Bjurhager, May 2011
Tryck: E-print AB, Stockholm
iii
Abstract
Wood is a complex material and the mechanical properties are influenced
by a number of structural parameters. The objective of this study has been to
investigate the relationship between the structure and the mechanical properties of hardwood. Two levels were of special interest, viz. the cellular structure
and morphology of the wood, and the ultra-structure of the cell wall. In the
next step, it was of interest to examine how the mechanical properties of
hardwood change with spontaneous/induced changes in morphology and/or
chemical 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 tremula x Populus tremuloides), and one on European oak (Quercus robur). A
methodology was developed where the concept of relative density and composite mechanics rules served as two useful tools to assess the properties of
the cell wall. Tensile testing in the longitudinal direction was combined with
chemical examination of the material. This approach made it possible to reveal the mechanical role of the lignin in the cell wall of transgenic aspen trees,
and investigate the consequences of holocellulose degradation in archaeological oak from the Vasa ship.
The study on transgenic aspen showed that a major reduction in lignin in
Populus leads to a small but significant reduction in the longitudinal stiffness.
The longitudinal tensile strength was not reduced. The results are explainable
by the fact that the load-bearing cellulose in the transgenic aspen retained its
crystallinity, aggregate size, microfibril angle, and absolute content per unit
volume. The results can contribute to the ongoing task of investigating and
pinpointing the precise function of lignin in the cell wall of trees.
The mechanical property study on Vasa oak showed that the longitudinal tensile strength is severely reduced in several regions of the ship, and
that the reduction correlates with reduced average molecular weight of the
holocellulose. This could not have been foreseen without a thorough mechanical and chemical investigation, since the Vasa wood (with exception from
the bacterially degraded surface regions) is morphologically intact and with
a micro-structure comparable to that of recent oak. The results can be used
in the ongoing task of mapping the condition of the Vasa wood.
Keywords: tensile strength, transgenic hybrid aspen, lignin down-regulation,
the Vasa ship, chemical degradation
iv
Sammanfattning
Trä är ett komplext material vars mekaniska egenskaper påverkas av en
rad olika strukturparametrar. Målet med det här arbetet var att undersöka
kopplingen mellan struktur och mekaniska egenskaper i lövved. Två nivåer var
av speciellt intresse: morfologi och struktur på cellnivå, och ultrastrukturen
på cellväggsnivå. I nästa steg undersöktes hur de mekaniska egenskaperna hos
lövved förändras med spontana/inducerade förändringar i morfologi och/eller
kemisk sammansättning utöver den naturliga variation man finner i naturen.
Detta utgjorde grunden för två större projekt, det första på Europeisk asp
(Populus tremula) och hybridasp (Populus tremula x Populus tremuloides),
och det andra på ek (Quercus robur). För att erhålla information om cellväggsegenskaperna i materialet utvecklades en metodik som kombinerar relativ
densitet med kompositmekanik. Draghållfasthetstester i axiell riktning och
kemiska undersökningar gjorde det möjligt att undersöka konsekvenserna av
ligninnedreglering i transgen asp och nedbrytning av holocellulosa i arkeologisk ek från Vasaskeppet.
Den mekaniska studien på transgen asp visade att en stor reduktion i
lignininnehåll hos veden leder till en liten men signifikant nedgång i axiell
dragstyvhet. Den axiella draghållfastheten var däremot helt opåverkad. Resultaten kunde förklaras utifrån att den lastbärande cellulosan i materialet
var oförändrad, både i fråga om kristallinitet, aggregatstorlek och orientering
hos mikrofibrillerna (mikrofibrillvinkel) och absolut volymsinnehåll. Resultaten från studien kan bidra till en ökad förståelse av ligninets roll i cellväggen.
Studien på mekaniska egenskaper hos Vasaek visade på en stor nedgång
i axiell draghållfasthet på flera platser runtom i skeppet. Försämringen korrelerade mot en nedgång i medelmolekylvikt hos holocellulosan. Utan en kombination av mekaniska och kemiska analyser skulle detta ha varit svårt att
förutsäga, eftersom eken i Vasa är morfologiskt intakt (med undantag för
de bakteriellt nedbrutna ytregionerna) och uppvisar en mikrostruktur som
är fullt jämförbar med den hos nutida ek. Resultaten kan användas i det
pågående arbetet med att fastställa statusen hos träet i Vasaskeppet.
Nyckelord: longitudinell draghållfasthet, transgen hybridasp, ligninnedreglering, Vasaskeppet, kemisk nedbrytning
List of Publications
I.
Mechanical characterization of juvenile European aspen (Populus tremula) and hybrid aspen (Populus tremula x Populus tremuloides) using full-field strain measurements
I. Bjurhager, L. A. Berglund, S. L. Bardage, B. Sundberg (2008)
Journal of Wood Science 54(5):349-355
II.
Ultrastructure and mechanical properties of Populus wood with reduced lignin content caused by transgenic down-regulation of cinnamate 4-hydroxylase
I. Bjurhager, A-M Olsson, B. Zhang, L. Gerber, M. Kumar, L. A. Berglund, I.
Burgert, B. Sundberg, L. Salmén (2010)
Biomacromolecules 11(9):2359-2365
III.
Towards improved understanding of PEG-impregnated waterlogged archaeological
wood: A model study on recent oak
I. Bjurhager, J. Ljungdahl, L. Wallström, E. K. Gamstedt, L. A. Berglund (2010)
Holzforschung 64(2):243-250
IV.
Significant loss of mechanical strength in archeological oak from the 17th century
Vasa ship – correlation with cellulose degradation
I. Bjurhager, H. Nilsson, E-L Lindfors, T. Iversen, G. Almkvist, K. E. Gamstedt,
L. A. Berglund
(manuscript)
v
vi
The author’s contributions to the appended papers are as
follows:
I. Performed all the experimental work. Did most of the preparation of the manuscript.
II. Performed the dynamical mechanical analysis including evaluation of the results. Did most of the preparation of the manuscript.
III. Performed the mechanical testing parallel to grain; evaluated the results from
all the mechanical tests; planned and evaluated the results from the DVS study;
planned the SEM-study; performed parts of the X-ray microtomography study;
planned, participated in, and evaluated the results from the WAXS study. Did
most of the preparation of the manuscript.
IV. Performed and evaluated the results of all the mechanical tests; planned and
evaluated the results from the SilviScan study. Did most of the preparation of the
manuscript.
vii
Other relevant publications not included in the thesis
Large-area, lightweight and thick biomimetic composites with superior material
properties via fast, economic, and green pathways
A. Walther, I. Bjurhager, J-M Malho, J. Pere, J. Ruokolainen, L. A. Berglund, O.
Ikkala (2010)
Nano Letters 10(8):2742-2748
Supramolecular control of stiffness and strength in lightweight high-performance
nacre-mimetic paper with fire-shielding properties
A. Walther, I. Bjurhager, J-M Malho, J. Ruokolainen, L. A. Berglund, O. Ikkala
(2010)
Angewandte Chemie - International Edition 49(36):6448-6453
Ultra-structural organisation of cell wall polymers in normal and tension wood
of aspen revealed by polarisation FT-IR microspectroscopy
A-M Olsson, I. Bjurhager, L. Gerber, B. Sundberg, L. Salmén (2011)
Planta xx:xx-xx
Contents
Contents
viii
1 Introduction
1
2 Morphology of hardwoods
2.1 From macroscale to nanoscale . . . . . . . . . . . . . . . . . . . . . .
2.2 Different types of wood . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 The species studied . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Mechanical testing of wood
3.1 Basic solid mechanics . . . . . .
3.2 Quasi-static mechanical testing
3.3 Dynamic mechanical testing . .
3.4 Strain measurements . . . . . .
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4 Mechanical properties of wood
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4.1 Impact from density . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2 Impact from microfibril angle . . . . . . . . . . . . . . . . . . . . . . 18
4.3 Impact from moisture content . . . . . . . . . . . . . . . . . . . . . . 20
5 Effect of genetic modification on mechanical properties of hybrid
aspen
23
5.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3 Mechanical properties of wild-type and transgenic hybrid aspen . . . 30
5.4 Conclusions - hybrid aspen . . . . . . . . . . . . . . . . . . . . . . . 34
6 Effect of chemical degradation on mechanical properties of archaeological oak
35
6.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.3 Mechanical properties of recent oak and archaeological Vasa oak . . 45
6.4 Conclusions - archaeological oak . . . . . . . . . . . . . . . . . . . . 52
viii
CONTENTS
ix
7 Acknowledgements
55
Bibliography
59
8 Appendix I
69
Chapter 1
Introduction
Wood has played an important role throughout the history of mankind. Until
the beginning of the 19th century the material had been primarily a source of
energy. As construction materials, hardwoods and softwoods were valued equally,
mainly outgoing from their mechanical performance. This was however soon to
be changed with the development of the modern pulp and paper industry, which
favored softwoods rather than hardwoods as raw material. One reason was the
abundance of softwood in the surroundings close to the paper mills and industries,
while the morphology of the species has been claimed as another. In softwoods, 90%
or more of the volume consists of softwood fibers which are often preferred from a
paper mechanics point of view, since these long and slender elements contribute to
the strength of the paper. The percentage of fibers in hardwood is much lower, and
hardwoods have traditionally also been used to lesser extent in paper production.
However, the need for new natural resources have increased, and along with this
the research on these resources. The number of chemical and mechanical studies
on hardwoods has also increased steadily during the last decades (not least due to
the transgenic tree technology which is focusing mainly on fast growing hardwood
species).
Objectives
The objective of this study has been to investigate the relationship between the
structure and the mechanical properties of hardwood. Two structural levels were
of special interest, viz.:
a) The cellular structure and morphology of the wood
b) The ultra-structure of the cell wall
1
2
CHAPTER 1. INTRODUCTION
In the next step, it was of interest to examine how the mechanical properties of
hardwood change with spontaneous/induced changes in morphology and/or chemical composition beyond the natural variation found in nature. It therefore became
necessary to develop a methodology for excluding the influences of natural parameters (such as density and microfibril angle) which are known to greatly affect the
mechanical performance of wood.
Together, this constituted the framework and basis for two larger projects, one
on European aspen (Populus tremula) and hybrid aspen (Populus tremula x Populus tremuloides) and one on European oak (Quercus robur). In the case of the
aspen species, the biological material included samples with induced tension wood
and transgenic samples. The goal was to assess effects from down-regulation in
lignin content on mechanical properties. In the case of the oak, the material included both archaeological wood from the Vasa ship and samples of recent oak
which had undergone chemical treatment to mimic the wood from the ship. The
goal was to assess chemical degradation effects on mechanical properties.
Chapter 2
Morphology of hardwoods
The structure of wood is highly sophisticated, with several hierarchical levels. This
section aims at describing these levels. In this section a number of important terms
used later are also introduced.
2.1
From macroscale to nanoscale
The three main directions in a piece of timber are denoted the radial (R), the
tangential (T), and the longitudinal (L) direction (see Figure 2.1). The transverse
cross section of a tree trunk can be divided into different concentric layers: the
outer bark, inner bark, vascular cambium and xylem, where the innermost part of
the xylem (normally located at the center of the cross section of the trunk) is called
the pith.
Cell division takes place in the thin vascular cambium layer between the inner
bark and the xylem. The growth of the vascular cambium is visible as more or
less concentrically oriented rings referred to as annual growth rings. The tissue
produced in one of these growth rings early in the growth season is referred to as
earlywood, while tissue produced later is called latewood.
In both hardwood and softwood, the xylem tissue is composed of different types
of cells which are oriented mainly in two directions, the axial and the radial (i.e.
in the pith-to-bark direction). Hardwoods are considered to have a much more
advanced structure with more speciated cells than softwoods. The four main cell
types in hardwoods are fibers (for mechanical support), vessels (for conduction),
tracheids (for conduction), and parenchyma cells (for storage of nutrients). From a
mechanical perspective, the fibers are of greatest interest since these are the main
contributors to the mechanical strength of the material [1, 2]. The diversity of
the hardwood cells is demonstrated in Table 2.1, where the volume fraction and
dimensions of the different cell types span over a wide range, contributing to the
heterogeneity not only among different hardwood species but also within a single
species, and even within individuals of a species. Hardwood cells are large enough
3
4
CHAPTER 2. MORPHOLOGY OF HARDWOODS
Figure 2.1: Schematic representation of a sector of hardwood showing the outer and
inner bark, xylem including the vascular cambium, annual rings, (vessels seen as
pores in the transverse section of the trunk), and pith. The radial (R), tangential
(T), and longitudinal (L) directions are indicated by arrows.
to be seen by the naked eye, and are usually said to have a length roughly 100 times
their width.
A simple division of the hardwood species into sub-groups is based on the arrangement and size of the vessels, which are seen as small pores in the transverse
section of the trunk. In ring-porous species (such as oak), the earlywood pores are
larger and arranged in a ring around the pith (as in Figure 2.1), while in diffuseporous species (such as aspen) the pores are fairly uniform in size and scattered
(see also Figure 2.5 and Figure 2.6).
Table 2.1: Volume fractions and dimensions of hardwood cells
Volume fraction [%]
Radial dimension [µm]
Tangential dimension [µm]
Axial dimension [mm]
Cell-wall thickness [µm]
After Gibson
Fibre
Vessel
Ray cells
37-70
6-55
10-32
10-30
20-350
10-30
20-500
0.6-2.3 0.2-1.3
1-11
LJ and Ashby MF [3]
The wood polymers
The three major chemical components of wood are cellulose, hemicelluloses, and
lignin, where cellulose and hemicelluloses sometimes go under the common name
2.1. FROM MACROSCALE TO NANOSCALE
5
of holocellulose.
Cellulose ((C6 H10 O5 )n ) is a non-branched polysaccharide synthesized by the
polymerization of monosaccharide glucose (C6 H12 O6 ) units into linear chains with
a degree of polymerization (DP) of approximately n = 8000-15000.
Figure 2.2: Chemical structure of cellulose including the repeating unit
Hemicelluloses are also polysaccharides, but unlike cellulose they have a much
lower DP (n = 150), a more branched structure, and are composed of a number of
different monosaccharides.
Lignin is a polymer with a complex and heterogeneous structure and a DP of several thousands (the actual DP is difficult to determine since the three-dimensional
lignin network is impossible to extract without some decomposition). Wood lignin
is composed mainly of three different monolignols with an aromatic structure: pcoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (see Figure 2.3). Hardwood
lignin consists mostly of coniferyl and sinapyl alcohols. In the lignin network, these
monolignols are found as guaiacyl and syringyl residues.
The wood cell wall structure
Wood can be looked upon as a composite material, in which the structural arrangement and the proportions of the different polymers (cellulose, hemicelluloses, lignin)
in the cell wall have a large effect on the wood on a macro-scale. The mechanical
behavior of wood can to a large extent be ascribed to the cell-wall micro-structure
and chemical composition.
Cellulose constitutes the backbone of the cell wall, and the structure of the
polymer is hierarchical. The polymer contributes stability, stiffness, and strength.
On the nano-scale, cellulose chains are bonded together through hydrogen bonds,
forming flat sheets. These sheets are in turn stacked on top of each other (and
held together by van der Waals forces), forming three dimensional bundles called
(micro)fibrils. These fibrils are oriented differently in different parts of the cell wall,
and differences in the orientation can be used to distinguish the different cell wall
layers (see Figure 2.4), depending on which part of the cell wall is being studied. In
the thin primary wall, the microfibrils are deposited in a random fashion, while in
the thicker secondary wall the fibrils are well organized in a parallel fashion forming
fibrillar bundles (fibril aggregates) oriented at a certain angle towards the fibre axis.
This angle is called the microfibril angle (MFA) and it is highly important for the
6
CHAPTER 2. MORPHOLOGY OF HARDWOODS
Figure 2.3: The monolignols (upper row) and their respective residues as found
in the lignin polymer (lower row). R1 , R2 are either hydrogen or lignin. After
Holmgren A [4]
mechanical properties of the wood. The MFA is large in the S1 and S3 layer, but
usually varying between 5° and 15° and rarely exceeding 30° in the S2 layer. (The
effect of variations in MFA is discussed further in Chapter 4.)
Due to imperfections, chain overlapping etc., the degree of organization in the
cellulose structure decreases from the highly ordered molecule chains to the less
ordered fibril aggregates, and amorphous regions are believed to exist in the surface
layers of the microfibrils and probably also in the axial direction of the cellulose
chains (see for instance the review by Samir et al. [5] and Neagu et al. [6]). However,
due to the linearity and hydrogen bonding ability of the cellulose molecule, most
of the cellulose in wood is in a crystalline state. This is also the reason why Xray techniques can be used to determine the MFA orientation in the cell wall (see
further Papers II, III, and IV).
Hemicelluloses do not form aggregates, but they contribute to the mechanical
properties of the material through bonding to both cellulose and lignin. During cell-wall biosynthesis, the lignin is polymerized last to form a complex amorphous network enclosing both the cellulose and hemicelluloses. Thus, in a crosssection of the mature cell wall alternating layers of fibrillar cellulose aggregates
and lignin/hemicellulose lamellae can be identified. (See further Paper II, where
the aggregate and lamellar size of transgenic hybrid aspen were measured). The
lignin network is less hygroscopic and difficult to penetrate, and lignin therefore
contributes both to the water conduction properties and the stiffness of the wood,
and it also helps to protect the tree from microbial attack. The syringyl-to-guaiacyl
2.2. DIFFERENT TYPES OF WOOD
7
(S/G) ratio is a good measure of the cross-linking density in the lignin network.
This can be understood by studying Figure 2.3, where the chemical structures of
guaiacyl and syringyl are shown. There are two possible positions at which the
former can bond to another molecule (either at position 4 or 5), but the latter has
only one bonding possibility (position 4). A high S/G ratio implies that there are a
greater number of syringyl units and thus less probability of cross-linking. This is of
interest, especially in pulping, since the S/G ratio indirectly indicates how difficult
the delignification will be and thus how high the energy and/or chemical consumption will be. (The S/G ratio of transgenic hybrid aspens with down-regulated lignin
content was measured in Paper II.)
Figure 2.4: Schematic representation of the cell-wall layers in a fiber with respective
orientation of microfibrils. Detail of the cell wall is displaying the fibril aggregates
embedded in the hemicellulose-lignin matrix. ML - middle lamella, P - primary
wall, S1, S2, S3 - layers of the secondary wall.
2.2
Different types of wood
Sapwood and heartwood
From a biological activity perspective, the xylem in a mature tree can be divided
into sapwood and heartwood. While the sapwood consists of both living and dead
cells and plays a biological role by conducting sap from the roots to the leaves,
the heartwood is composed only of dead cells and is non-conductive. This is an
optimization strategy since conduction and metabolic activity in the mature tree
is not necessary throughout the whole stem.
The heartwood contains leveled amounts of extractives, which in many cases
give a darker color to the wood. This characteristic feature is therefore broadly
used to identify heartwood, although the precise distinction between heartwood
and sapwood lies in their biological activity. In hardwood species, a type of outgrowth called tyloses are also formed in the vessels from adjoining rays or vertical
8
CHAPTER 2. MORPHOLOGY OF HARDWOODS
parenchyma cells during the transformation of sapwood into heartwood. These tyloses plug the vessels and make penetration more difficult [7]. The extractives and
the tyloses provide natural durability to the wood, and heartwood is therefore preferred in wood constructions where such properties are important. A good example
is the archaeological Vasa ship, which was mainly constructed of heartwood from
oak (see Paper IV).
Juvenile wood
From an age perspective, the xylem can instead be divided into juvenile wood and
mature wood. The juvenile wood is considered to be the tissue produced in the
tree during its first 10-30 years, while mature wood is the tissue produced thereafter. The difference between juvenile and mature wood is both morphological
and chemical. Juvenile wood generally contain more earlywood than mature wood
and therefore has a lower density. Juvenile cells are shorter, contain less cellulose
and have a larger microfibril angle. All this contributes to the weaker mechanical
properties of the juvenile wood, which makes it less desirable for constructional
purposes. (The mechanical behavior of juvenile wood is further discussed in Paper
I and Paper II.)
Tension wood
Hardwood trees subjected to stress (e.g. intentional leaning of the wood plant or
mechanical forces from wind and snow) develop tension wood on the upper side of
the subjected organ, in order to prevent further deformation and/or displacement.
The cell walls of the fibers of such wood are abnormally thick. A gelatinous layer
(called the G-layer) is formed in the cell lumen and can be present as an additional
layer on top of the S1, S2, and S3 layers, or it may replace the S3 and in some
cases also the S2 layer. The microfibril angle in the G-layer is close to zero, and the
layer is almost entirely composed of cellulose which means that tension wood has a
higher density than normal wood. Tension wood also displays a greater shrinkage.
The mechanical properties of tension wood are not necessary lower than those of
normal wood, but the abnormal deformation during drying and the fact that the
machining of tension wood is more difficult makes it less desirable for constructional
purposes. (The mechanical behaviour of tension wood is further discussed in Paper
II.)
2.3
The species studied
Aspen (Populus sp.)
Aspen belongs to the Populus genus within the Salicaceae family. Characteristic
of all members of the family are their vegetative propagation. The Aspen species,
2.3. THE SPECIES STUDIED
9
including a large number of cross-breeds between different species, are one of the
most abundant tree species in the world [8]. Poplars were the first forest trees to be
stably genetically transformed [9], and the species within the Populus genus have
come to play an important role within the field of genetics [10].
Hybrid aspen (Populus tremula x Populus tremuloides) is a fast growing crossbreed of the European aspen (Populus tremula) and the American aspen (Populus
tremuloides); two species with similar features, mechanical properties, and densities [11, 12, 13, 14, 15]. The hybrid aspen is used in a large number of projects
using transgene technology with the purpose of studying, for instance, tension and
reaction wood formation, and lignin content in stems [16].
Aspen wood is considered to be a low-performance material and is not used in
load-carrying elements in large constructions. The reason for its low mechanical
performance lies in the low density of the material. Aspen is however used for production of chop-sticks, lollipop sticks and wooden cutlery, since the wood is almost
odour-free and tasteless. Aspen pulp is also used to a certain extent within the
pulp and paper industry. The wood structure is homogeneous and diffuse porous,
and no transition from earlywood to latewood can be distinguished (see Figure 2.5).
Figure 2.5: Photomicrographs displaying cross-sections of the innermost annual
ring(s) of a) European aspen, and b) hybrid aspen.
European oak (Quercus robur)
Oak species belong to the Quercus genus within the Fagaceae family. The European
oak (Quercus robur L.) is native to Europe, but the species can also be found in
the northern and western parts of Africa and Asia.
Oak wood has been highly valued as a construction material for thousands
of years. The wood has high density and is used in many high-performance applications intended for long-term use such as floors, stairs, furniture, barrels and
spokes [17]. Oak wood has also been a popular material in larger architectural
constructions such as churches and cathedrals, boats, and ships [18]. The fact that
oak (heart)wood, stored under the right conditions, is highly resistant to abrasive
forces, fungi and bacterial attack, has resulted in many well-preserved archaeologi-
10
CHAPTER 2. MORPHOLOGY OF HARDWOODS
cal oak findings.
Oak is a ring-porous hardwood species. The material is characterized by its
porous earlywood containing large vessels easily identified with the naked eye, while
the latewood is much denser with vessels visible only in a microscope. The transition
from early- to latewood is abrupt, and the density of the latewood is 2-3 times higher
than that of the earlywood. One of the morphological characteristics is the large
and small rays in the radial direction which constitute approximately 20% of the
wood volume (see Figure 2.6) [19]. Another feature is the high density which, as in
all other ring-porous hardwood species, is maintained or even increased with annual
ring width [7, 20].
Figure 2.6: Photograph of a cross-section of European oak (heartwood). The wood
is yellow in color from extractives and tyloses have developed and are filling the
vessels at some places. Rays are running in the radial direction and are seen as
horizontal lines (marked by arrows).
Chapter 3
Mechanical testing of wood
Investigation of wood from a materials point of view includes (quasi-static and dynamic) mechanical testing. The principle of mechanical testing is to examine the
behavior of a material exposed to an external force. Basic laws of solid mechanics
are related to loads, deformations, and specimen dimensions. Accurate measurement of these parameters is essential in order to produce reliable results. The
following sections include a brief description of the mechanical methods used in
Paper I - Paper IV.
3.1
Basic solid mechanics
Solid mechanics includes the response of a solid body subjected to external forces.
Naturally, a larger body will be able to withstand higher loads than a smaller one
made from the same type of material, but in order to obtain information about the
material’s ability to resist forces we introduce the concept of stress σ, where stress
is defined as the (average) force F divided by the (projected) area A of the body
on which the force is acting;
σ=
F
A
(3.1)
We are further interested in the relative deformation of the body as a result of
the load, and the strain is defined as the dimensional change δ compared to the
initial dimension L of the body in the direction of the applied stress;
=
δ
L
(3.2)
When choosing a material for a given construction application, it is important to
know how much stress the material can withstand and also how much the material
will deform under a given stress. A simple yet useful empirical formula is Hooke’s
law, which states that the stress σ is linearly related to strain according to;
11
12
CHAPTER 3. MECHANICAL TESTING OF WOOD
σ =E·
(3.3)
Many materials including wood obey Hooke’s law (as long as the material’s
elastic limit is not exceeded). The constant E in Equation 3.3 is called the Young’s
modulus, and it is a material parameter describing the stiffness of the material.
3.2
Quasi-static mechanical testing
Quasi-static mechanical testing requires the application of an external force at a
moderately low speed, so that the effect of speed does not need to be considered.
Due to its heterogeneous structure, the mechanical behavior of wood is expected
to depend on the direction (longitudinally, tangentially, or radially) and manner
(tension, compression, shear, bending etc.) in which a load is applied. In the
following, the behavior of wood under axial tension will be discussed in more detail,
since this was the main loading direction examined in these studies.
A wooden material loaded in tension in the longitudinal direction (parallel to
the grain) will display a stress-strain curve with a characteristic appearance, as
shown in Figure 3.1. The curve can be divided into two regimes; the linear-elastic
regime at low stresses and the plastic regime at higher stresses, followed by fracture.
Figure 3.1: Characteristic curve of stress versus strain for wood loaded in tension
parallel to grain
At low stresses the material is stretched elastically and the relation between
stress and strain can be considered as linear. Hooke’s law (Equation 3.3) is valid
within this region, and the slope of the stress-strain curve corresponds to the
Young’s modulus E of the material. Above a certain critical strain the material
enters the plastic regime where the strain is no longer proportional to the stress.
The sample experiences irreversible (plastic) deformation and if unloaded again it
displays a permanent deformation. (Quasi-static mechanical testing was used in
Papers I-IV.)
3.3. DYNAMIC MECHANICAL TESTING
3.3
13
Dynamic mechanical testing
Dynamic mechanical testing involves the application of an oscillating force at a
frequency f to a sample and analyzing the material’s response to that force [22, 23].
Loading and unloading of the sample is usually sinusoidal, and the stress σ varies
with time as;
σ = σ0 · sin(ωt)
(3.4)
where σ0 is the stress amplitude, ω the oscillation frequency, and t time. For a
material with a completely elastic behavior, the strain response is immediate, i.e.
in phase with the stress, and the strain can therefore be expressed as;
= 0 · sin(ωt)
(3.5)
where 0 is the strain amplitude. In reality, there is no such thing as an ideally
elastic material. Instead, all materials are more or less viscoelastic and inclined to
flow (i.e. to deform as a fluid under an applied stress). In a viscoelastic material
the strain is out of phase with the stress, and the phase shift δ determines the time
lag;
= 0 · sin(ωt + δ)
(3.6)
The dynamic modulus E for the viscoelastic material can be considered to consist of one elastic part and one viscous part according to E = E 0 + iE 00 , where <(E)
is the storage modulus E 0 , and =(E) is the loss modulus E 00 (see Figure 3.2 for a
schematic illustration):
E0 =
σ0
· cos(δ)
0
(3.7)
E 00 =
σ0
· i sin(δ)
0
(3.8)
The practical interpretation of this is the following: When a sample is loaded
dynamically, a certain amount of energy is stored in the material during each stress
cycle. The storage modulus E 0 describes the elastic response of the material, and
can be seen as a measure of how much energy is released when the sample is
unloaded again. The loss modulus E 00 , on the other hand, represents the energy
loss due to internal motion in the material.
Dynamic mechanical tests are often used to characterize the wood under the
influence of changes in temperature or moisture. An important parameter in this
context is the glass transition temperature (Tg ) which is the temperature region
in which the material softens and goes from a solid into a more rubbery state
with a decrease in dynamic modulus. Cellulose, hemicelluloses, and lignin each has
its own glass transition temperature, and wood therefore softens gradually over a
temperature interval, rather than at a certain temperature [24, 25]. The same line
14
CHAPTER 3. MECHANICAL TESTING OF WOOD
Figure 3.2: The dynamic modulus E illustrated schematically, where the storage
modulus E 0 and the loss modulus E 00 are the projection of the dynamic modulus
vector E on the real (Re) and imaginary (Im) axes, respectively.
of reasoning can be applied to moisture, and wood softens gradually with increasing
moisture content (see Figure 3.3). (Dynamic mechanical testing was used in Paper
II.)
Figure 3.3: Characteristic softening behavior (seen as decrease in dynamic modulus)
for wood loaded dynamically in tension parallel to the grain, and subjected to an
increase in the surrounding relative humidity.
3.4
Strain measurements
Modern universal testing equipment usually provides internal logging of load F and
deformation δ. However, additional measurement of the displacement using external equipment is commonly used for a number of reasons:
3.4. STRAIN MEASUREMENTS
15
a) The testing of smaller samples requires high accuracy in the deformation
measurements, and this cannot always be provided by a universal testing equipment.
b) Universal testing equipment can usually only estimate the total deformation.
In theory, the total deformation is considered to be homogeneous throughout a
loaded sample, but in practice heterogeneous deformation patterns are frequently
observed due to the inhomogeneous structure of the material. These deformation
patterns may be of special interest, since they can provide information concerning
microstructure and fracture behavior etc.
c) During the clamping of the sample in the testing device, external loads are
necessarily applied on the sample before the test has even begun, and this may
in many cases lead to deformation artifacts influencing the final strain data in a
negative way.
One way to deal with these problems is fix of strain gauges onto the sample,
while optical methods provide another alternative. The advantage of the latter are
that no device has to be mounted directly on the sample which is an advantage
with small and/or delicate samples.
Digital Speckle Photography and Video extensometry
The digital speckle photography (DSP) technique is a widely used non-destructive
method for measuring two- (2D) or three-dimensional (3D) surface deformations
in materials such as wood. The procedure involves the identification of a natural
or artificially applied pattern on the sample surface. Images of the surface are
captured before and during displacement, and strain calculations can be made by
relating the surface pattern in the strained to that in the unstrained state [21].
The DSP technique was in this work used to determine the strain (in 2D) on small
aspen samples loaded in tension parallel to the grain (Paper I).
Video extensometry is another optical method for external measurement of deformation. As with the DSP technique, marks (natural or applied) on the specimen surface are identified and captured before and during displacement. Strain is
thereafter calculated by relating the surface pattern in the strained to that in the
unstrained stage. The video extensometry technique was used in Papers III and IV
to determine the strain on oak samples loaded in tension parallel to the grain.
Chapter 4
Mechanical properties of wood
The mechanical properties of wood depend on a number of parameters of which
density, microfibril angle (MFA) and moisture content are the three most important. Density and moisture content can be determined by simple methods, while
more advanced equipment and techniques are required for measuring of the MFA.
In the following sections, the effects of density and MFA are discussed together with
two simple approaches for compensating for variations in these parameters during
mechanical testing. The influence of moisture content is also briefly mentioned together with softening with the wood impregnation agent polyethylene glycol (PEG),
which is similar to that with water.
4.1
Impact from density
Density has a large impact on the mechanical properties of both soft- and hardwood, the mechanical performance of wood with low density being inferior to that
of wood with high density [7]. This is simply because density is a measure of the
fraction of solid material in the wood material able to withstand loads. The relationship between density and the mechanical properties in the axial direction is
approximated as linear in most cases1 .
Over the years, several attempts have been made to create models for prediction of mechanical properties of wood and wood fibers. A model presented by
Gibson and Ashby is based on an idealized two-dimensional honeycomb structure,
where wood is represented as an aggregate of hexagonal cells (Figure 4.1) [3]. This
coarse simplification of the cell wall structure has proven to be surprisingly useful
when modeling wood, and not only for loading in the longitudinal but also in the
tangential and radial directions.
Based on experimental results, it is shown that stiffness (EL ) of wood during
loading in the longitudinal direction is linearly proportional to the volume fraction
of solid material in the structure, which is given by the relative density ρ/ρS (ρ
1 The
exception being fracture toughness.
17
18
CHAPTER 4. MECHANICAL PROPERTIES OF WOOD
Figure 4.1: The idealized honeycomb structure and illustration of the influence of
density
is the density of wood, ρS the density of the cell wall, and ES the longitudinal
cell-wall stiffness);
EL
ρ
= C1 ·
ES
ρS
(4.1)
This can be applied by analogy for the longitudinal tensile strength (σL ) in
wood, which also has been shown to be linearly related to the relative density
(although the relationship is not as strong as for longitudinal stiffness) [7];
σL
ρ
= C2 ·
σS
ρS
(4.2)
C1 and C2 are empirical constants to compensate for deviations in the real wood
material (usually set to C1 =C2 =1) and σS is the longitudinal cell-wall strength.
The density of the solid material ρS is almost the same for all wooden species and
has been estimated to be ρS =1500 kg/m3 [3]. This can be used in combination
with experimental values of tensile stiffness and strength to estimate the cell wall
stiffness ES and strength σS using equations 4.1 and 4.2.
The honeycomb model can thus serve as a useful tool to make predictions of the
cell wall structure in materials exhibiting different densities. A direct comparison
of a high and a low density material confirms that mechanical properties increase
with increase in density, while calculations of cell wall stiffness/strength can show
whether or not there is actually a difference in the cell wall structure. (The concept
of relative density was used in Papers I and IV for calculation of cell wall properties.)
4.2
Impact from microfibril angle
The orientation of the microfibrils in the cell wall in relation to the fiber axis, i.e.
the microfibril angle (MFA), is another parameter which has been shown to have
a considerable impact on both the longitudinal tensile stiffness and the strength.
4.2. IMPACT FROM MICROFIBRIL ANGLE
19
A small MFA indicates that the microfibrils are more aligned along the fiber axis.
Longitudinal loading of a material with low MFA gives a more direct loading of
the cellulose chains, reflected on the meso- and macro-level as higher stiffness and
strength and less plastic deformation before failure. The opposite is also true, and
the influence of variations in MFA has been shown, both for solid wood [26] and
single fibers [27, 28].
A simple composite mechanics rule which can be used to take into account the
MFA is the maximum stress criterion [29] (which, in contrast to e.g. Hankinson’s
formula [30, 31] does not require any information concerning the perpendicular
tensile strength). Here, the cell wall is modeled as a flat fiber mat with fibers
(analogous to the microfibrils) oriented at an angle θ (analogous to the MFA) to
the fiber axis. Failure of the mat happens either at a critical (local) stress value
σ1 ≥ σ1u parallel to the fibers, σ2 ≥ σ2u perpendicular to the fibers, or at a shear
stress τ12 ≥ τ12u along the fibers.
The (local) in-plane stresses working parallel and perpendicular to the fibers
(σ1 , σ2 , τ12 ) can then be expressed as the (global) stresses applied in the x- and
y-directions of the fiber mat (σx , σy , τxy ) according to;
 
 
σx
σ1
 σ2  = T ·  σy 
(4.3)
τxy
τ12
where the transformation matrix is given by:

cos2 θ
sin2 θ
2
cos2 θ
T =  sin θ
−cosθ · sinθ cosθ · sinθ

2cosθ · sinθ
−2cosθ · sinθ 
cos2 θ − sin2 θ
(4.4)
If uniaxial tension (σ2 = τ12 = 0) is considered, the stress σxu to cause failure
in the material can be expressed for each or the three failure modes as;
σ1u
cos2 θ
σ2u
σxu =
sin2 θ
τ12u
σxu =
cosθ · sinθ
σxu =
(4.5)
(4.6)
(4.7)
Under the assumption of independent modes of failure (i.e. no interaction between the different failure modes) and using experimental data for σ1u , σ2u , and
τ12u , the equations 4.5-4.7 can be used to calculate the maximum tensile strength
of a fiber material with fibers (microfibrils) oriented at a given angle θ (see also
Figure 4.2). Also, in reverse, data for the maximum (global) tensile strength can
be used together with information on θ to calculate the local (cell wall) properties
σ1u , σ2u , and τ12u for a material.
20
CHAPTER 4. MECHANICAL PROPERTIES OF WOOD
In practice, the local failure mode in wood loaded in the longitudinal direction
is considered to be either failure parallel to the microfibrils or shear along the fibrils
(depending on if the MFA is small or large). The maximum stress criterion was
used in Paper IV for calculation of local tensile stress at failure parallel to the
microfibrils σ1u .
Figure 4.2: Predicted dependence on fiber angle θ of the applied stress for the onset
of different failure modes. After Hull D and Clyne TW [29]
4.3
Impact from moisture content
The moisture content in wood is an important parameter which needs to be monitored during mechanical testing since the swelling induced in the material has a
large effect on the mechanical properties. The hygroscopic nature of wood originates from polar hydroxyl groups which are more abundant in the cellulose and
hemicellulose. When moisture is introduced into the cell wall, attraction forces will
lead to the formation of new hydrogen bonds between the hydroxyl groups in the
non-crystalline regions and water molecules, while the number of hydrogen bonds
in the wood constituents will decrease. This increases the distance between the
wood elements leading to swelling and allowing more water molecules to enter into
the cell wall. Swelling continues until the cell walls are fully saturated (i.e. the fiber
saturation point), and water molecules may thereafter enter the cavities and voids
of the material without affecting the dimensions further. A higher moisture content
in wood will reduce not only the stiffness but also the strength of the material.
4.3. IMPACT FROM MOISTURE CONTENT
21
The impact on wood of polyethylene glycol impregnation
Water is not the only substance to have a swelling effect on wood; any polar molecule
small enough to enter the cell wall will initiate the same behavior. Although swollen
wood is mechanically weaker, artificial bulking of the cell wall is in some cases necessary. In archaeological waterlogged wood, for instance, drying induces strong capillary forces in the often biologically degraded and thereby weakened material, which
therefore cracks and warps severely during drying. For this reason, the water in archaeological waterlogged wood is replaced by polyethylene glycol (PEG) molecules
which, due to the difference in concentration outside and inside the wood, diffuses
into the material. The PEG molecule (chemical formula H(OCH2 CH2 )n OH) has
a non-branched structure, where both the highly polar hydroxyl end groups and
the moderately polar ether-bonded carbons in the repeating unit contribute to the
overall polarity of the molecule (Figure 4.3) [32].
Figure 4.3: Chemical structure of polyethylene glycol
The molecular weight (Mw) plays an important role during PEG impregnation.
The polarity of the molecule decreases with increasing Mw, and this reduces the
affinity between the PEG and the wood. A higher Mw also means a larger molecule,
and PEG molecules large enough will not enter the cell wall at all but instead deposit
in cavities, voids and on the surface of the wood. (The effect of PEG impregnation
on wood was examined in Paper III.)
Chapter 5
Effect of genetic modification on
mechanical properties of hybrid
aspen
5.1
Background
The general principle of transgenic technology is to add new genetic material into an
organism’s genome. The reason has often been to alter (and improve) the properties
of the organism (this applies particularly to crops, for instance, where resistance to
pests and an increase of certain nutritionals are desirable traits). In other cases,
studies of a certain gene and its function have been the purpose.
Genetic modification is done by inserting the extra DNA into the cell nucleus of
the organism. One method is to exploit the natural ability of microbes to transfer
genetic material to plants. Here, the Agrobacterium genus (especially Agrobacterium tumefaciens) is widely used [33]. Besides the chromosomal DNA, these
bacteria contain a separate DNA molecule, a plasmid, which can replicate independently of the chromosomal DNA. The strategy of the bacteria is to infect a wounded
host by establishing a connection with a host cell. A DNA segment, transfer-DNA,
is then transferred from the plasmid to the cell and incorporated in the cell DNA.
Expression of the natural transfer-DNA will disturb the biosynthesis of the cell
and cause a tumor in the host. However, the tumor causing transfer-DNA can be
exchanged for genes of interest, which when expressed will result in, for instance,
resistance to herbicides, a change in growth traits, or a change in chemical composition of the host. In this context, the use of molecular markers is a very useful tool
widely applied within the transgenic field [34]. These markers are specific fragments
of DNA in the genome which are flagging the position of a particular gene (in this
case the transfer-DNA), and can be used to select the plants in which incorporation of the new DNA sequence was successful. Gene function can be investigated in
23
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
24
PROPERTIES OF HYBRID ASPEN
greater detail by down- or up-regulating mRNA1 availability, and thereby enhance
or suppress the traits associated with this particular gene.
During the last few decades, the rapid process in identification and functional
analysis of genes involved in wood formation has opened the way to novel approaches in tree breeding. The greatest effort in transgene technology of trees has
been devoted to the reduction and/or modification of lignin, since lignin constitutes
an obstacle during pulping, requiring large amounts of chemicals and/or energy for
its removal [34]. In the transgenic research on trees, Populus is used as a major
model tree for a number of reasons. The species has a relatively small genome
and is easily cross-bred and genetically transformed, the trees are fast growing and
common almost all over the world, and they can provide almost infinite access to
cloned material (due to vegetative propagation) [10].
Lignin modification in these species has resulted in improved pulping properties
such as a lower Kappa number and lower consumption of chemicals during delignification [35, 36, 37]. However, only a few studies have been done on the mechanical
properties of wood from transgenic trees (see e.g. [23, 38, 39, 40, 41]). The results
indicate that both the lignin content and the chemical composition (in terms of the
syringyl to guaiacyl (S/G) ratio) are of importance.
Objectives and strategies
Transgenic trees with significant alterations in lignin composition provide good
material for exploring the importance of lignin content and structure on wood mechanics. Therefore, the study focused on juvenile transgenic hybrid aspen (Populus
tremula x Populus tremuloides) with down-regulated lignin content, together with
wild-type hybrid aspen (i.e. non-transgenic plants grown under controlled conditions). Measurement on small saplings only a few weeks/months old was desired in
order to keep the project time to a minimum.
The first part of the study included the development of a test procedure for
small samples, and evaluation of the mechanical properties of material from the
innermost annual ring of wild-type hybrid aspen. In the next step, the manner in
which down-regulation of lignin content influenced the mechanical properties was
studied. Axial tensile strength and stiffness were of great interest, since mechanical properties in the fiber direction can provide information on microstructural
changes on the cell-wall level (provided that any changes in density and microfibril
angle are compensated for). Moreover, the effect on storage modulus of changes
in temperature and relative humidity was examined. Information about the softening behavior with elevated temperature or RH can serve as a good complement
to chemical methods and it provides an indirect indication of changes in chemical
composition of the samples.
1 mRNA - messenger ribonucleic acid; a molecule of RNA carrying coding information from a
DNA template to the ribosomes which are responsible for the protein synthesis
5.2. MATERIALS AND METHODS
5.2
25
Materials and methods
A brief description of the experimental work on hybrid aspen is given here. For
details, see Papers I and II.
Wood material
The material consisted of wild-type (clone T89) and transgenic hybrid aspen with
down-regulation of cinnamate-4-hydroxylase (C4H), a key enzyme in the early
phenylpropanoid pathway through which monolignols are synthesized. By downregulation of this enzyme, a lower lignin content in the transgene samples was
expected, which in turn was expected not only to influence the (quasi-static and
dynamic) mechanical behavior but also to result in a change in the micro-structure
of the material. In addition to the normal wild-type hybrid aspen, wild-type hybrid
aspen with induced tension wood was used as a reference for the transgenic aspen,
since tension wood is known to have a naturally low lignin content [42]. European
aspen (Populus tremula) was also used as a reference to the wild-type hybrid aspen
for comparison of species.
Mechanical testing of the samples
(Quasi-static) tensile behavior
Initial quasi-static mechanical tests focused on measuring and comparing the longitudinal Young’s modulus (EL ) and tensile strength (σL ) of wild-type hybrid aspen
and European aspen. For the mechanical tests stem sections were microtome cut
[43] and thereafter cut to dog-bone shape using a scalpel (specimen dimensions
0.2 (R) x 4 (T) x 40 (L) mm3 ). Axial tensile tests were performed using a mini
materials tester MiniMat 2000 (Rheometric Scientific) with a 20N load cell and a
distance of 30 mm between the clamps. The rate of displacement was 0.5 mm/min
(or strain rate 1.7% per min). Sections were tested in the green state.
Because of the small sample size, the digital speckle photography (DSP) technique [21] was used to measure the strain (). A DSP system with a CCD camera
and ARAMIS software (Gom, Germany) was used. The camera recorded the visible
area of the specimen onto which a black high-contrast dot pattern had been applied.
The specimen was automatically meshed into facets. An area (size approximately
1 x 8 mm2 ) of the reduced cross-sectional area of the dog-bone was selected for the
calculations, and the strain was obtained by averaging the facet strain data parallel
to the fibers in this area. From the recorded data for force (from the MiniMat
system) and strain (from the DSP system) the tensile strength and stiffness were
obtained.
Next, quasi-static mechanical tests were performed (at the Max Planck Institute
of Colloids and Interfaces, Potsdam) on transgenic and wild-type hybrid aspen, also
in order to determine and compare the longitudinal Young’s modulus and tensile
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
26
PROPERTIES OF HYBRID ASPEN
strength. The procedure in this case differed from that previously described, since
the two test series were parts of different projects. Specimens were embedded in
polyethylene glycol (PEG, Mw 2000) and microtome cut into specimens with dimensions of 1.5 (R) x 0.08 (T) x 20 (L) mm3 and thereafter kept in a wet state
before and during testing. Axial tensile tests were performed using a mechanical
tester (Owis, Germany) with a 50N load cell (Honeywell, USA) and a distance of
10 mm between the clamps. The rate of displacement was 0.15 mm/min (or strain
rate 1.5% per min). From the recorded data for force and displacement (both from
the mechanical tester) the tensile strength and stiffness were obtained.
Dynamic and softening tensile behavior
In order to study the softening of the samples under the influence of relative humidity (RH) and temperature (t), dynamic mechanical tests were performed on
microtome sections of transgenic and wild-type hybrid aspen (with and without
induced tension wood). Specimen dimensions were 0.07 (R) x 2 (T) x 20 (L) mm3 .
The tests were performed parallel to the grain using a Perkin Elmer dynamic mechanical DMA analyzer with a 7N load cell and a distance of 12 mm between the
clamps. Because of the experimental setup the strain had to be calculated from
recorded data of the (average) displacement.
The sample was first mounted and subjected to a small load (10 mN) for one
hour (t 30 ℃, RH 5%). The humidity was raised from 5% to 95% RH at a rate of
1% per minute and then lowered in one step down to 5% RH, where the sample was
kept for four hours. The purpose of this initial procedure was to prevent buckling
and thereby get a uniform load distribution in the sample, and to subject the sample
to a well-defined humidity cycling history.
Thereafter followed the dynamic test. A static (Fstat ) and a dynamic (Fdyn )
load were applied to the sample at an amplitude of 10 µm and a frequency (f ) of
1 Hz. A humidity scan was performed where the RH was raised from 5% to 95%
at a rate of 0.33% per minute, followed by four hours at RH 5%. (The humidity
scan was repeated to assess the repeatability of the measurement.) Thereafter,
the sample was soaked in water and subjected to a temperature scan where the
temperature was raised from 30 ℃ to 90 ℃ at a rate of 0.5 ℃ per minute. The
storage modulus (E 0 ) obtained from the temperature scan was used to calculate the
glass transition temperature (Tg ). (The dynamic mechanical tests were performed
at Innventia AB.)
Chemical composition of the samples
Differences in chemical composition
The Fourier transform infra-red (FTIR) technique makes measurements in the infrared region of the electromagnetic spectrum, and utilizes the fact that different types
of molecules absorb radiation at different wavelengths, depending on their molecular
5.2. MATERIALS AND METHODS
27
structure. An IR beam is passed through the material and the absorbance recorded
in an instrument-dependent dimension is then translated through the Fourier transform into absorbance at different wavenumbers displayed in an absorbance spectrum. This is used in combination with the absorbance bands associated with
cellulose, hemicelluloses, and lignin to examine the chemical composition of wood
(see e.g. [44]) and also to elucidate the orientation, structure and interaction between different types of molecules in the cell-wall (see e.g. [45, 46]).
Since the DMA analysis in some of the specimens displayed a different softening
behavior than the others, FTIR was used on all the specimens from the DMA studies
to examine the chemical composition. Transmission spectra were recorded over a
wavenumber interval from 700 to 4000/cm (Perkin Elmer, Spectrum 100 FTIR
equipped with a microscope, Spectrum Spotlight 400 FTIR imaging system). (The
FTIR tests were performed at Innventia AB.)
Determination of cellulose, hemicelluloses, and lignin content
Lignin content was expected to be lower in the transgenic samples, and wet chemistry analysis (performed at SLU) was therefore adopted including the determination of (Klason) lignin, α-cellulose, and hemicellulose content according to Ona
et al. [47], who developed a method to determine the wood composition in small
samples.
Determination of the syringyl to guaiacyl ratio
Pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) is a technique for
the analysis of samples which can be vaporized. In the first pyrolysis step, heat is
used to degrade large molecules in an oxygen-free environment into fragments which
are small enough to be carried by a gas stream. The (inert) gas stream containing
the vapourised fragments is then passed through a gas chromatography column, a
narrow tube coated internally with different stationary phases. Depending on the
chemical composition, the gaseous compounds interact differently with the phases
and are therefore eluted after different retention times. In order to determine the
elemental composition of the compounds, mass spectrometry is used as a third step.
Here the eluted sample is first ionized, forming charged molecule fragments, whose
mass-to-charge (m/z) ratio is measured by acceleration in an electric field. Both
the retention time and m/z provide a finger print of a certain molecule fragment,
which is displayed as a peak in a pyrogram.
To further investigate the lignin structure of the wood samples, Py-GC/MS was
applied to determine the of S/G ratio in wood fractions using a pyrolysis equipment
(Pyrola 2000, Pyrol AB, Lund, Sweden) mounted on a GC/MS (7890A/5975C,
Agilent Technologies AB Sweden, Kista, Sweden). The sample was first subjected
to pyrolysis (2 s at 450 ℃) followed by separation of the vaporized molecules in
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
28
PROPERTIES OF HYBRID ASPEN
a column (J&W DB-5, Agilent Techologies Sweden AB, Kista, Sweden) where the
temperature was raised from 40 ℃ to 320 ℃ (most of the peaks from phenolic
products derived from lignin appear in the temperature range 118.75 ℃ to 250 ℃
[48]). Full-scan spectra were recorded in the range of 40-500 m/z, and m/z channels
for calculation of the S/G ratio were chosen according to Faix et al. [48]. (The
Py-GC/MS tests were performed at SLU.)
Micro-morphology of the samples
Determination of cellulose crystallinity and microfibril angle
X-ray diffraction (XRD) is a non-destructive method for the determination of a
number of different wood parameters, such as the cellulose crystallinity and the microfibril angle (see e.g. [49, 50]). The theory behind XRD measurements on wood
is that X-rays striking a wood sample will deflect from their path at a number of
deflection angles, which are directly related to the planes and dimensions of the
cellulose structure [49, 50]. In practice, a beam of rays from an X-ray source is
emitted and passed through a monochromator to yield X-rays of a certain wavelength λ (since a well-defined wavelength is fundamental for the accuracy of the
measurements). When the beam strikes the sample, it will scatter and the scattering pattern will be recorded by a detector as differences in X-ray intensity. The
data is displayed as an intensity curve (1D), as an intensity pattern (2D), or (in
rare cases) as an intensity profile (3D) (see e.g. Appendix 1, Figure 8.1) [51, 52].
When XRD is used on wood, diffraction patterns characteristic of the crystalline
regions of cellulose will dominate, but, since wood also contains amorphous substances the image is a result of the reflections from the cellulose molecular structure
superimposed on an amorphous background [53]. Therefore, the reflection pattern
from the amorphous compounds (e.g. lignin) is subtracted from the intensity data
for the measuring of e.g. crystallinity.
In order to determine the crystallinity of the cellulose in the wood samples, a
wide angle XRD equipment (D8 advance, Bruker AXS, Germany) was used in the
symmetric reflection mode. Intensity was measured as a function of the scattering
angle in a 2θ by θ-2θ scan with a scanning angle of 10-50◦ , and the signal from
organosolv lignin (Sigma, USA) used as an amorphous standard was subtracted
from the results. The peak detected at 2θ ≈ 22◦ (the 200-reflection of cellulose)
and the baseline intensity at 2θ ≈ 18◦ (representing the scatter from the amorphous
fraction of the sample) were used to calculate the crystallinity [54].
To determine the microfibril angle, a wide angle XRD equipment (Nanostar,
Bruker AXS, Germany) was used in the symmetrical transmission mode. The peak
at 2θ = 22.5◦ (corresponding to the 200 reflection of cellulose) was used as the
sample was rotated, and the intensity was plotted against the azimuthal angle Φ.
5.2. MATERIALS AND METHODS
29
(The experiments were performed at the Max Planck Institute of Colloids and
Interfaces, Potsdam.)
Determination of cell wall nanostructure
Atomic force microscopy (AFM) is a technique which has been successfully used
to examine the nano-structure of wood fibers, and has made it possible to image
for instance the arrangement of cellulose fibril aggregates in the cell wall [55]. The
principle of the technique is to use a cantilever with a very small (nano-scale) tip
to scan the sample surface. When brought close to the surface, forces between the
tip and the surface lead to a cantilever deflection, which it is possible to detect
and monitor. The tip is operated in either a static or a dynamic mode. In the
first case, the tip is dragged over the sample surface, while in the second case the
tip is instead oscillated up and down. When run in the tapping mode (which is a
dynamic mode), the amplitude is large enough to cause the tip to touch the surface,
but without the drawbacks associated with the static mode (such as dragging of the
tip and therefore potential damage of the surface). An image of the sample surface
can be obtained by monitoring the force required to keep the cantilever amplitude
at a constant level.
Wood-cross sections were examined (NanoScope© IIIa Multimode AFM, Digital
Instruments, USA) in the tapping mode (tip radius 4.18-4.29, cantilever length 125
µm, spring constant 40 N/m, resonance frequency 300 kHz). Image processing
software was used to evaluate the images with regard to cellulose aggregate size,
lamellar size, and number of lamellas per µm. (The AFM measurements were
performed at Innventia AB.)
Additional measurements
Density
Since density is known to influence the mechanical behavior of wood, both dry
(oven-dry weight/oven-dry volume) density (ρdry ) and basic (oven-dry weight/wet
volume) density (ρbasic ) were determined for the samples by measuring of specimen
weight and dimensions.
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
30
PROPERTIES OF HYBRID ASPEN
5.3
Mechanical properties of wild-type and transgenic
hybrid aspen
In the first part of the study mechanical properties of non-transgenic hybrid aspen
and European aspen (both tested in green condition) were compared. The tests
showed that both longitudinal stiffness (EL ) and tensile strength (σL ) were significantly higher in the European aspen (EL 6.2±1.3 GPa, σL 47±9 MPa) than in
the hybrid aspen (EL 5.5±0.9 GPa, σL 38±8 MPa). These values correlated well
with the oven-dry density (ρdry ) which was higher for the European aspen (284±10
kg/m3 ) than for the hybrid aspen (221±15 kg/m3 ). The stiffness and strength
values were lower than those presented in a number of previous studies (see Table
5.1), and this may be due to both the juvenile character and the green condition of
the samples used in the study. The results confirm the strong relationship between
mechanical properties and density shown previously for European aspen alone [56],
and European aspen in comparison to hybrid aspen [57].
Besides density, the microfibril angle is a parameter known to influence the mechanical performance of wood. Instead of measuring the MFA in the first part of
the study, the cell wall stiffness ES was estimated using equation 4.1 (Chapter 4).
The calculations resulted in similar values of ES for the European aspen and hybrid
aspen (24 and 27 GPa, respectively). This suggests that the microstructure of the
species, including MFA, was similar and that the difference in longitudinal mechanical properties could probably be explained by the difference in density alone.
The second part of the study included wild-type and transgenic hybrid aspen
with down-regulated cinnamate 4-hydroxylate (C4H) activity. Here, no difference
in tensile strength σL between the two groups could be found, although the basic density was significantly lower for the transgenic aspen (ρbasic 260±30 kg/m3 )
compared to the wild-type aspen (ρbasic 296±6 kg/m3 ). A lower density usually
means less wood material capable of withstanding forces. However, this is only
true if the lignin-to-cellulose-to-hemicellulose ratio is unaltered. Here, the chemical analysis showed that the Klason lignin content was reduced from 22% in the
wild-type to 15% in the transgenic material (see Table 5.2). This was confirmed by
the Py-GC/MS and FTIR-measurements, and manifested in the latter as a lower
lignin absorption at the wave-numbers 1500 cm−1 and 1591cm−1 . The relative
cellulose content increased correspondingly from 33% to 38%. More importantly,
the absolute cellulose content (calculated as a fraction of the basic density) in the
transgenic samples remained unchanged (see Table 5.2). Since the quantity of loadbearing component was unchanged, this fully explains why the tensile strength was
preserved in the transgenic samples.
With regard to stiffness, the quasi-mechanical tests showed a slight but significant lower value for the transgenic aspen than for the control. In contrast to tensile
strength, a relatively good correlation was found between stiffness and basic density
in both the transgenic and the wild-type aspen (see Figure 5.1). The stiffness of
5.3. MECHANICAL PROPERTIES OF WILD-TYPE AND TRANSGENIC
HYBRID ASPEN
31
transgenic aspen with a lower lignin content has been examined previously. Horvath
et al. [39] showed that the modulus of elasticity and compression strength parallel
to the grain were much lower in samples with a reduced lignin content, and the
stiffness results were confirmed in a later study [40]. Since the down-regulation of
lignin content in the present study affected the stiffness but not the tensile strength,
the experiments clearly showed the role of lignin in the cell wall as a stiffening and
reinforcing component.
(The tensile stiffness and strength of the wild-type hybrid aspen were higher
in the first than in the second part of the study, probably due to differences in
the methodology. In the first part of the study, strain was measured using DSP,
whereas in the second part, strain was based on machine path basis which can lead
to underestimated stiffness values.)
Previous studies on poplars down-regulated in enzyme activity other than that
of C4H have reported morphological changes [23] and a disorganization in the cellwall manifested as e.g. loss of compactness, the formation of sublayers [60], a
change in size and shape of the cells, and even cell collapse [61]. Therefore, it was
of interest in this study to examine how the micro- and nanostructure of the cell
wall was affected by the C4H down-regulation.
The results from of AFM study showed that the lamellar size was significantly
lower in the transgenic aspen samples (13±6 nm) than in the wild-type hybrid
(6±2 nm), indicating a more loose arrangement of the cellulose aggregates, as observed in previous studies on Populus with a reduced lignin content [60]. On the
other hand, no significant difference in cellulose aggregate size was found, and the
FTIR and X-ray measurements determining cellulose crystallinity and microfibril
angle, respectively, indicated that neither of these parameters had changed. This
strengthened the impression that the cellulose ultrastructure and arrangement had
been preserved in the transgenic samples, and that the impact of the C4H downregulation was restricted to affecting the lignin.
The softening behavior of the solid wood was also of interest in this study. The
humidity and temperature scans showed consistent property trends. The humidity
scans revealed a softening with increasing RH for both transgenic and wild-type
samples (see Figure 5.2). A major transition, probably due to hemicellulose softening [25, 62], was observed at about 80% RH. Similarly, the temperature scans
indicated a softening region at about 70 ℃ for both the transgenic and the wild-type
samples due to lignin softening (see e.g. [25, 62]). The results of the DMA measurements were in agreement with those of the quasi-static measurements, showing that
the absolute (dynamic average) moduli were considerably lower in the transgenic
wood than in the wild-type.
The softening behavior of the wild-type and the transgenic groups did not resemble that of the hybrid aspen samples with induced tension wood, except for one
of the replicate trees from each group which, on the contrary, showed a behavior
very similar to that of the tension wood material. When examined by FTIR spec-
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
32
PROPERTIES OF HYBRID ASPEN
Table 5.1: Mechanical properties and density of American aspen, European aspen,
and hybrid aspen listed in the literature
Species
Young’s modulus
EL [GPa]
8.9[30]
8.3, 8.7[11]
Tensile strength
σL [MPa]
102[58] , 110[59]
122[56]
Density
[kg/m3 ]
American aspen
380(ad)[7]
European aspen
376(bd)[13] , 420(ad)[58] ,
423(ad)[15] , 440(ad)[15] ,
460(ad)[7]
hybrid aspen
93[58]
363(bd)[13] , 380(bd)[23] ,
423(ad)[58]
References: [7] Tsoumis G (1991), [11] Kufner M (1978), [13] Heräjärvi H and Junkkonen
R (2006), [15] Säll H et al. (2007), [23] Horvath B (2009), [30] Bodig J and Jayne BA
(1993), [56] Nagoda L (1981), [58] Heräjärvi H (2009), [59] Karlmats U and Olsson
Tegelmark D (2004); (ad) - air-dry density; (bd) - basic density
Figure 5.1: Correlation between basic density and stiffness in wild-type and transgenic trees. The figure shows technical replicates from 5 wild-type and 5 transgenic
trees. r2 = 0.65.
Table 5.2: Wet chemical analysis and calculated absolute values of wood components from the transgenic and the wild-type trees based on basic density.
Transgenic
Wild-type
Transgenic
Wild-type
Klason lignin
[% dry weight]
15.1±1.2
22.4±3.2
Klason lignin
[kg/m3 ]
∼40
∼68
α-cellulose
[% dry weight]
37.6±3.2
32.8±2.0
α-cellulose
[kg/m3 ]
∼100
∼99
5.3. MECHANICAL PROPERTIES OF WILD-TYPE AND TRANSGENIC
HYBRID ASPEN
33
Figure 5.2: Absolute dynamic modulus of wild-type (thick line) and transgenic (thin
line) wood as a function of a) relative humidity, and b) temperature. The dotted
lines indicate the onset of (a) hemicellulose and (b) lignin softening, respectively,
as the point where the modulus deviates from this line (outliers excluded).
troscopy, the outliers were also found to contain tension wood, evident as lower
absorption peaks at certain characteristic wavenumbers. The outliers were therefore excluded from the analysis.
From a papermaking perspective, the glass transition temperature Tg especially
of lignin is of interest. Pulping is carried out under water-saturated conditions
and at a high temperature where the hemicelluloses and amorphous region of the
cellulose are already in their rubbery state and therefore offer little resistance to
fiber separation, fiber swelling and/or defibration. The energy or the amount of
chemicals required thus depend on the glass transition temperature of the lignin,
and even a small decrease in lignin Tg can result in large savings.
The Tg of wood has been shown to be related to the chemical composition,
and more specifically associated with the cross-linking of the lignin [62]. A higher
S/G ratio is expected to lower the Tg since it is associated with a lower crosslinking density in the lignin structure. In the present work no significant difference
in Tg was found between the transgenic and wild-type samples, as seen from the
temperature scans in Figure 5.2b). This was in agreement with the results of the
chemical analysis, which showed no major difference in the lignin S/G ratio.
In general, manipulating the number of syringyl and guaiacyl groups in the
lignin structure has been found to have only a minor effect on the bending stiffness
in transgenic aspen samples, although a small decrease in modulus of elasticity can
be seen with increasing S/G ratio [39, 40]. This indicates that the lignin structure
plays a peripheral role, and that lignin content is much more important when it
comes to the mechanical rigidity of wood.
CHAPTER 5. EFFECT OF GENETIC MODIFICATION ON MECHANICAL
34
PROPERTIES OF HYBRID ASPEN
5.4
Conclusions - hybrid aspen
In a wider perspective, the study of transgenic hybrid aspen, wild-type hybrid aspen and European aspen show the importance of investigating and determining the
parameters in wood (density and microfibril angle) which usually cause the largest
variations in mechanical properties. Not until these are either compensated for or
shown to be unimportant in a specific case, can one proceed to examine the effect of
manipulating certain parameters beyond the natural range of variation, as in this
case the low lignin content in the transgenic samples. The study on hybrid aspen
showed that simple rules can be applied on hardwood, to compensate for variations
in a certain parameter but also in reverse to predict cell wall properties as in this
case the microfibril angle.
In greater detail, the study showed the value of transgenic technology in producing biological material with an altered microstructure and a variation in chemical
composition beyond the natural range to enable the importance of components
crucial for the mechanical behavior of wood to be evaluated.
A major reduction in lignin in Populus led to a small but significant reduction
in the longitudinal stiffness, but the longitudinal tensile strength was not reduced.
The results were fully explainable by the fact that the load-bearing cellulose in the
transgenic aspen retained its crystallinity, aggregate size, microfibril angle, and absolute content per unit volume. The glass transition temperature of the transgenic
wood in the wet state was unaffected since S/G ratio of the lignin did not change
with the down-regulation.
The results presented here can contribute to the ongoing task of investigating
and pinpointing the precise function of lignin in the cell wall of trees.
Chapter 6
Effect of chemical degradation on
mechanical properties of
archaeological oak
6.1
Background
Wood is extremely durable and can, under favorable conditions, be preserved in a
more or less unaltered state for thousands of years, both structurally [63] and mechanically [64, 65]. The age of a wooden object is not necessarily a good indication
of its condition, and the degradation rate is to a very high degree dependent on
the storage conditions in combination with the species-dependent resistance of the
material.
The best way of natural preservation of wood is by protection from decay by
wood-degrading fungi. This is done by keeping the wood in an environment which
is either very dry or poor in oxygen. The latter is the easier to achieve, simply by
submersion in water, and the number of archaeological wooden objects recovered
from a long-term storage in a very moist environment vastly exceed the number of
objects recovered from a dry environment. Based on this, archaeological artifacts
are sometimes divided into dry and waterlogged wood, where waterlogged wood
can be defined as wood which has been stored under wet conditions, e.g. at the
bottom of a river or a lake, or in a marine environment.
The 17th century man-of-war Vasa is probably one of the world’s most famous
archaeological artifacts, and it is also a magnificent example of successful (although
unintentional) wood preservation in underwater conditions. Setting sails on her
maiden voyage in August 1628, she was undoubtedly one of the most splendid ships
of her time, with extravagant ornamentation and heavy armament. However, the
64 cannons on board and the high stern castle had a negative influence on the
stability and maneuverability of the ship and, instead of becoming the pride of the
35
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
36
PROPERTIES OF ARCHAEOLOGICAL OAK
Swedish navy, Vasa sank at a depth of 32 m in the Stockholm harbor after travelling
only a very short distance in open water. The exact location of the ship gradually
sank into oblivion, until she was located in 1956 and raised in 1961. An extensive
conservation process was immediately initiated, and after nearly three decades, in
1990, the ship was finally put on permanent display in the Vasa museum (see Figure
6.1).
Figure 6.1: The Vasa ship on display
The brackish waters where Vasa sank are hostile to shipworm (Teredo navalis)
which contributed to the preservation of the ship. Therefore, although the surface
(0-15 mm) of the wood is microbiologically degraded, the inner parts are morphologically intact including even such delicate wood elements as the tyloses.
The complex chemistry of the Vasa ship is a result of the prevailing conditions on the sea bed in combination with the preservation treatment and chemical
degradation of the ship. The wood displays a content gradient of iron, sulfur, and
polyethylene glycol (PEG) which generally decreases with distance from the surface. Both the iron and the sulfur in the wood originate from the period during
which the Vasa was under water. The iron comes mainly from the thousands of
iron bolts which originally held the ship together, but which rusted away during
the centuries on the sea bed. The sulfur is a remnant from the slow biodegradation of sulfur-reducing and erosion bacteria [66]. These organisms, which use the
naturally occurring sulfate ions from the sea water in their metabolic cycle, were
favored by the oxygen-poor underwater conditions created by the heavy pollution
by the Stockholm sewage at the end of the 19th century. As a result of the bacterial breakdown of organic matter, hydrogen sulfide was formed, and the small gas
molecules penetrated and were deposited in the Vasa wood.
6.1. BACKGROUND
37
The PEG was intentionally applied during the conservation process by spraying
the hull with an aqueous solution of PEG and boric acid/borax (acting as a fungicide) for 17 years. The main molecular weights (Mw) of PEG were 600, 1500, and
4000 [67, 68]. Substituting the water in the cell walls with PEG during the drying
of the ship was a necessary measure. Waterlogged archaeological wood is more
or less degraded, and degradation by erosion bacteria is characterized by gradual
disintegration of the S3 and S2 cell-wall layers, which usually provide strength and
integrity to the cell-wall structure [69]. As water evaporates from the drying wood,
strong gradients resulting in large strains are liable to occur and cause severe deformation of the fragile material. This was avoided by bulking the Vasa wood with
PEG.
In 2000, heavy rains caused large fluctuations in the RH in the museum, and
considerable precipitation, identified as different sulfur salts and elemental sulfur,
was found in several positions on the ship. At the same time, pH values below 2 were
monitored on the outermost surfaces [70]. The most immediate threat was believed
to be further sulfur oxidation into sulfuric acid followed by acid-catalyzed hydrolysis
of the load-bearing cellulose. Later, it was shown that extensive degradation of the
polysaccharides and the PEG had already taken place in the timbers of the ship
[71, 72]. Depolymerization seemed to occur in the context of a high iron and
low sulfur content [71, 72, 73]. Surprisingly, only a relatively low degradation of
polysaccharides was detected in sulfur-infested surface samples. It has therefore
been suggested that iron in combination with oxygen could degrade the wood in
Fenton-type reactions [71].
Although the chemistry of the Vasa is not yet fully understood, it is extremely
important to determine the condition of the wood in the ship. The Vasa is a galleon
with a displacement of 1210 tons, and, except from the supporting stanchions keeping the hull in an upright position, the ship is almost completely self-supporting.
This is putting high demands on the mechanical performance of the material.
Objectives and strategies
An examination of the mechanical properties in relation to the chemical degradation
in the Vasa wood can provide useful information which may be applicable also to
other archaeological artifacts, since the Vasa ship is far from being the only wooden
object suffering from chemical degradation. A knowledge of the present mechanical
condition of the ship is important and can be used in the ongoing work of preserving
the ship.
Cellulose is a tensile material which means that its most important mechanical
function in the wood fiber is to carry tensile loads [74]. Since the cellulose microfibrils are oriented mainly in the longitudinal direction and an eventual degradation of
these would be most likely to influence the mechanical performance of the material
in this direction, longitudinal tensile tests were of primary interest.
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
38
PROPERTIES OF ARCHAEOLOGICAL OAK
Polyethylene glycol was believed to have a certain negative influence on the
mechanical properties. This influence had to be investigated in order to be able to
separate the influence of PEG impregnation from that of chemical degradation in
the Vasa material. Access to archaeological wood is for natural reasons restricted.
Therefore, the first part of the study included an evaluation of the mechanical
properties of recent oak impregnated with PEG.
The next step focused on a mechanical examination of wood from the Vasa ship
itself. In this step a strategy to compensate for variation in density and microfibril
angle was developed in order to make comparisons between material with different
morphology possible. This part of the study also included the development of a
method for micro-testing, to both optimize the use of archaeological material while
still obtaining reliable mechanical data.
6.2
Materials and methods
A brief description of the experimental work on recent oak and archaeological Vasa
oak is given here. For details, see Papers III and IV.
Wood material
In the first part of the study investigating the influence of PEG on the mechanical
properties, heartwood from a stem of recent oak (Quercus robur) was used. Pieces
were selected containing only mature wood.
In the second part of the study, a large block of Vasa oak (ID 65542) was taken
from the orlop deck of the ship (starboard side, position 1, see Figure 6.2). The
block had served to fill a gap, and had therefore probably not been subjected to any
previous loading. The wood in the block was visibly sound, with evenly thick annual
rings, and the characteristic ring porous structure of oak was clearly discernible. In
addition, a number of drilling cores taken from three different positions on the ship
were used for mechanical evaluation (position 2: ID 65695, 65696; position 3: ID
65698, 65699; position 4: ID 65700) and chemical evaluation (position 2: ID 65694;
position 3: ID 65697; position 4; 65702; see Figure 6.2) together with reference
samples of recent oak.
Chemical pre-treatment with PEG
The first part of the study was performed in order to elucidate the influences on
mechanical properties of the impregnation agent polyethylene glycol (PEG) with
which the Vasa had been treated during the conservation process. Dog-bone shaped
samples of recent oak (for preparation, see the Mechanical testing section below)
were pre-swollen in deionized water and immersed in a PEG (Mw 600)/water solution at two different concentrations (23% and 38% w/w) in order to achieve a PEG
content similar to that in the oak of the Vasa ship. Treatment concentrations and
treatment time (3 weeks) were based in part on the work of Ralph and Edwards
6.2. MATERIALS AND METHODS
39
Figure 6.2: Sampling site (1) for the block and the drilling cores (2-4) on the Vasa
ship.
[75]. The PEG content was determined as the increase in dry weight (vacuum
drying at 50 ℃ for 5 days) after impregnation.
Mechanical testing
Quasi-static mechanical tests were carried out on PEG-treated samples, samples
from the Vasa block and the Vasa drilling cores, and untreated reference samples
of recent oak (Figure 6.3a).
Large dog-bone shaped specimens (4 (R) x 2 (T) x 170 (L) mm3 ) were milled,
cut, conditioned (RH 55%, t 23℃) and tested in axial tension using an Instron
universal testing machine model 5566 (10kN load cell) with a span length of 85
mm (cross-head speed 1 mm/min, or a strain rate of ∼1.1% per min). Strain was
measured by a video extensometer, and the longitudinal Young’s modulus (EL ) and
tensile strength (σL ) were calculated from the test curves.
The second series of small samples (0.3 (R) x 3 (T) x 20 (L) mm3 ) from the
drilling cores was prepared by cutting slices parallel to the grain using a slit saw
and a dog-bone shaped punch. Axial tensile tests were performed using a Deben
Microtester (50N load-cell) with cross-head speed of 1 mm/min or a strain rate of
approximately 10% per min. Tensile strength (σL ) was calculated from the results.
Chemical composition of the samples
Determination of iron and sulfur content
Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is a method
widely used to determine the total content of elements in dissolved samples. The
technique uses inductively coupled plasma (i.e. ionized gas in which the energy is
supplied by electrical currents produced by electromagnetic induction) to produce
excited atoms and ions. When the sample is introduced into the plasma, it is degraded into atoms which, in turn, lose and collect electrons repeatedly in the plasma
stream. This gives rise to the emission of radiation at wavelengths characteristic of
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
40
PROPERTIES OF ARCHAEOLOGICAL OAK
Figure 6.3: Schedule for the micro- and macrotests on Vasa oak: Preparation of a)
small samples from the drilling cores, and b) large samples from the block.
the elements involved [76].
To determine the total iron and sulfur content, samples were milled and dissolved
in nitric acid and hydrogen peroxide, followed by ICP-AES. (The experiments were
performed at Innventia AB.)
Preparation of holocellulose
Preparation of holocellulose was a necessary step preceding the size exclusion chromatography step in which the molar mass distribution of the holocellulose was
determined. From the different sub-steps in the holocellulose preparation, aliquots
were used to determine low molecular weight organic acids, sulphate, and PEG
content.
Wood from the Vasa and reference samples were milled, extracted and treated
in a number of steps for removal of extractives, PEG, waxes, metals, lignin, and
tannins (see Figure 6.4). More information on the holocellulose preparation can
be found in [71, 77]. (The preparation of holocellulose with the following chemical
sub-steps was performed at Innventia AB.)
6.2. MATERIALS AND METHODS
41
Figure 6.4: Schematic protocol for the preparation of holocellulose from Vasa samples.
Determination of PEG content
Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF MS) is an ionization technique suitable for the analysis of large molecules
which can be ionized. In practice, the sample is embedded in a carrier material
(known as the matrix) and put on a plate which is hit by a laser beam. The matrix
material, which separates the analyte molecules from each other, ensures energy
absorption from the laser pulse, and facilitates ionization of the analyte molecules,
is desorbed from the plate together with the sample molecules. These are simultaneously ionized and accelerated in an electric field. Since ions of different masses
attain different velocities, it takes a longer time (’time-of-flight’) for molecules with
high mass to reach the detector. The intensity of the detected signal from the test
sample is used together with that of a reference sample with known concentration
to determine the amount of molecules of a certain size in the test sample [78, 79].
PEG content in oak from the Vasa was determined by MALDI-TOF MS, where
aliquots from all the Vasa samples were prepared together with aqueous reference
solutions of PEG 600 (at a number of concentrations), and analyzed using a Bruker
Ultraflex with a Scout-MTB ion source. An aqueous reference solution with known
concentration of γ-cyclodextrin was added to each sample. The amount of PEG in
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
42
PROPERTIES OF ARCHAEOLOGICAL OAK
the samples could then be estimated by comparing the areas from the PEG signal
and the signal from the Na+ -ions (originating from the γ-cyclodextrin).
Determination of molecular mass distribution of holocellulose
Size exclusion chromatography (SEC) is a technique for determining the size of
small (i.e. lower mass) molecules which may be insufficiently ionized or even unable to form ions, and therefore not suitable for evaluation by MALDI-TOF. The
principle of SEC is separation of molecules depending on their size. A dissolved
sample is filtered through a column containing an inert and porous packing material with a certain pore size distribution. Molecules are retained inside the column
for different times, depending on their size and ability to penetrate the pores of
the packing material. The molecules are eluted from large to small as a result of
which the refractive index (RI) of the eluted solution changes with time. The RI
can be monitored and the molar mass distribution (MMD) determined based on
light scattering theory [80, 78]. A reference with known MMD is used to calibrate
the experimental curves. Average molecular weight (AMW) is commonly calculated
from the MMD.
In order to determine whether there had been any chemical degradation of
the Vasa holocellulose, SEC was used. A sample of holocellulose from the earlier holocellulose preparation (see previous section) was evaluated according to the
method developed by Lindfors et al. [71], and based mainly on the previous work
by Berthold et al. [81]. Lithium chloride/N,N -dimethyleacetamide was used as solvent, together with a chromatographic system consisting of four separation columns
(Mini Mixed-A 20 mm, 4.6x250 mm, Polymer Laboratories) and a refractive index
detector (Waters 2410).
Determination of acids and sulphate
The ion chromatography (IC) technique allows for the separation and detection of
different ions. A liquid sample is introduced together with an ionic eluent into a
separation column which contains a porous packing material with charged functional surface groups. The analyte ions are initially retained in the column but can
be eluted by exposure to the eluent which flows continuously through the column
and which gradually will replace the sample ions. Since different ions have different affinities to the surface groups, they are eluted at different times and can be
detected as they leave the column by measuring a physical or a chemical property
of the eluent (for example electrical conductivity or refractive index) [82].
The presence and concentration of acids and sulphate can give an indirect indication of the chemical degradation of the Vasa wood. To determine low molecular
weight organic acids (acetic acid, formic acid, glycolic acid), oxalic acid, and sulphate, ion chromatography was used. An aliquot from the holocellulose (see pre-
6.2. MATERIALS AND METHODS
43
vious section) was analyzed using a Dionex ICS-2000 Ion chromatography system
and 35mM KOH as eluent.
Micro-morphology of the samples
Examination of structural features
X-ray microtomography is based on the tomography technique, where X-rays are
used to generate a three-dimensional image of an object. An X-ray beam is focused
onto a sample and the signal from the beam is collected by a detector, which creates a two-dimensional contrast image of the sample cross section. The sample is
thereafter moved slightly into a new position and another image is produced. In
this way a large number of 2D images are produced without destructive examination of the sample. The images can be put together to generate a 3D image of the
material, including its internal structure. In the case of microtomography, features
at a micro-scale are revealed.
The internal structure of recent oak was documented by using X-ray microtomography. The resolution of a volume element (voxel) was 1.439 (R) x 1.439 (T)
x 1.439 (L) µm3 . Experiments were performed at beamline 2 at HASYLAB in
Hamburg, Germany.
Examination of the structural impact of swelling by PEG
The scanning electron microscopy (SEM) technique is a widely used method for
the examination of surfaces and the determination of surface structures. In principal, a high energy beam of electrons is focused onto a sample, and a variety of
signals from the interaction of the incident electrons and the sample surface can be
detected. The signal collected from the beam-specimen interaction varies from one
location to another, and the intensity is detected by an electron detector, displaying
differences in signal intensity as a contrast image [83].
The swelling of cell walls and the deposition of PEG in impregnated (dry)
material of recent oak were examined using scanning electron microscopy (SEM).
Small specimens were cut using the laser cutting technique [84] in order to produce
specimen surfaces with a minimum of structural damage due to sample cutting.
Thereafter, the cell wall thickness in the specimens before and after impregnation
with PEG 600 was determined (using an SEM model JEOL-6460LV operated at 15
kV). (The SEM study was performed at LTU.)
Determination of microfibril angle
For a description of the X-ray diffraction technique, see Chapter 5.
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
44
PROPERTIES OF ARCHAEOLOGICAL OAK
It was considered important to measure the microfibril angle (MFA), since MFA
is one of the main parameters influencing the mechanical properties of wood in the
axial direction. Two different X-ray techniques were used.
In the initial part of the study (performed at the University of Helsinki in
Finland) of PEG-impregnated recent oak, the wide angle X-ray scattering (WAXS)
technique was used in the symmetrical transmission geometry mode (measuring in
one dimension). The sample was hit by an X-ray beam perpendicular to the fibers
and the intensity (of the 004 reflection of cellulose at 2θ = 34.6◦ ) was detected as
a function of the azimuthal angle Φ as the sample was stepwise rotated around its
tangential axis [49].
In the second part of the study (performed at Innventia AB), the SilviScan
technique which is analogous to WAXS in the perpendicular transmission geometry
mode [52, 49] was applied (measuring in two dimensions) on a strip from the Vasa
block. The PEG in the strip had previously been extracted with water at 50 ℃. A
CCD area detector recorded the two-dimensional diffraction pattern and an image
analyzing software was used to calculate the reflection intensity of the 200 reflection
of cellulose at 2θ = 22.5◦ and its variation with the azimuthal angle Φ [52].
Additional measurements
Sorption behavior
Dynamic vapor sorption (DVS) involves a gravimetrical measurement of the uptake/loss of mass of samples with relative humidity at specific temperatures. The
results are presented as sorption isotherms which provide information on the adsorption/desorption behavior of the material.
Moisture uptake of wood is important, since it leads to swelling and softening
of the material which influence the mechanical properties. PEG is more or less
hygroscopic and therefore affects the moisture uptake of PEG-impregnated wood.
In order to monitor this, DVS was applied on samples of recent oak containing
different amounts of PEG 600 and on samples from the Vasa block (taken at different different depth from the surface) using a Surface Measurement Systems Plus
II Oven DVS equipment. (The experiments were performed at Innventia AB.)
Density
Density, which is one of the main parameters influencing the mechanical properties
of wood, was determined by two different methods, Archimedes’s principle applied
on PEG-treated samples of recent oak and SilviScan applied on the Vasa block.
In the latter case the X-ray transmission technique was used. The sample was lit
through by an X-ray, and the variation in transmission intensity with radial position
of the sample was recorded. This information was then used together with data on
average density (from weight and dimensions of the whole sample) to estimate the
local density (see e. g. [85]).
6.3. MECHANICAL PROPERTIES OF RECENT OAK AND
ARCHAEOLOGICAL VASA OAK
6.3
45
Mechanical properties of recent oak and archaeological
Vasa oak
When studying Vasa oak, it must be borne in mind that the behavior of the material is dependent on a number of additional parameters, besides those usually
influencing the mechanical properties of wood. As PEG was believed to be one of
these additional parameters, it was necessary to examine the impact from PEG on
the axial mechanical properties. Therefore, model experiments on recent oak were
performed. The mechanical results showed that longitudinal tensile strength σL
was completely unaffected by the PEG (Mw 600) impregnation, and only a minor
decrease in stiffness EL was recorded (see Table 6.1). The uptake of PEG in the
two groups of PEG treated samples was 15% and 24% which is comparable to the
uptake in Vasa oak [67].
Table 6.1: Mechanical properties of samples tested in tension parallel to grain:
Young’s modulus and tensile strength
PEG content
Reference 1 (0%)
PEG 600, 23%
Reference 2 (0%)
PEG 600, 38%
Young’s modulus
EL [GPa]
10.8 ±1.6
9.5 ±1.4
10.9 ±1.5
9.4 ±1.7
Tensile strength
σL [MPa]
75.3 ±15.5
76.1 ±12.9
70.9 ±12.2
69.3 ±9.4
A literature study revealed that the impact on the mechanical properties of
impregnation with PEG differs greatly between species, and that the decrease in
mechanical performance of the material seems to depend both on the molecular
weight of the PEG and on the loading direction (see Table 6.2). The only previous
work found on longitudinal properties of PEG-impregnated wood reports a 20%
decrease in tensile strength in pine with a PEG content of 15% [86]. The discrepancy
between these results and the results of the present study can be explained by the
microstructure of the oak samples. The WAXS measurements revealed that the
MFA in the samples ranged from 0.4◦ to 1.8◦ [87], whereas pine displays a MFA of
approximately 10-15◦ [49]. During impregnation, the PEG molecules interact with
the amorphous parts of the wood, leading to a reduction in stiffness of the cell-wall
matrix [88, 89]. The influence is however smaller in fibers with small MFA, as this
means that there is a more direct tensile loading of the cellulose microfibrils with
less shear in the surrounding cell wall matrix.
As seen in Table 6.2, the decrease in mechanical performance with increasing
PEG content is initially high, but that the effect diminishes at a content of approximately 25-30%. In the present study, no significant difference in either longitudinal
tensile strength σL or stiffness EL was found between the groups with lower (15%)
and higher (24%) PEG contents, see Table 6.1. This agrees with the results on radial and tangential shrinkage, which was only slightly lower in the specimens with
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
46
PROPERTIES OF ARCHAEOLOGICAL OAK
Table 6.2: Reduction in mechanical properties of different PEG-impregnated species
in the literature
Species
Pine[86]
PEG
Mw
1000
Content
[%]
15.4
Spruce[90]
Pine[91]
Pine[86]
Beech[91]
Spruce[92]
Spruce[90]
Pine[91]
Beech[91]
Spruce[92]
1000
1500
1000
1500
1000
1000
1500
1500
1000
41.8, 53.2
∼10, ∼20, ∼30
22.2
∼10, ∼20, ∼30
15.3, 30.5, 45.5
40.7, 49.7
∼10, ∼20, ∼30
∼10, ∼20, ∼30
15.3, 30.5, 45.5
Property
Tensile strength
(longitudinal)
Compression
yield strength
(longitudinal)
Bending
strength MOR
Property quotient
treated/untreated [ - ]
0.81
0.65, 0.76
∼0.92, ∼0.88, ∼0.85
0.84
∼0.91, ∼0.82, ∼0.76
0.87, 0.67, 0.62
0.86, 0.82
∼0.95, ∼0.95, ∼0.95
∼0.91, ∼0.87, ∼0.86
0.99, 0.90, 0.85
Bending
stiffness MOE
References: [92] Stamm AJ (1959), [91] Cronström Ö and Forssander T (1968),
[90] Schneider A (1969), [86] Kreicuma V and Svalbe S (1972)
the higher PEG content (results not shown). Hence, cell-wall bulking followed by a
decrease in mechanical properties of wood is restricted, and the PEG molecules at
higher concentrations must therefore be deposited elsewhere in the material than in
the cell wall. This assumption was confirmed during the SEM study where a sample containing approximately 25% PEG displayed cell-wall swelling but where PEG
was also deposited in vessels and voids (see Figure 8.2, Appendix I). A comparison
between the SEM images and X-ray microtomography images confirmed that the
morphology of the samples was preserved throughout the laser cutting procedure,
and this strengthened the SEM results (see Figure 8.3, Appendix I).
The moisture content in the samples was of relevance for the reliability of the
mechanical results, since moisture is known to have a great influence on wood stiffness and strength [93]. Based on both measurements including the weight of PEGimpregnated samples before and after drying, and the results of dynamic vapor
sorption experiments on wood fragments, it was evident that the moisture content
in both the PEG-impregnated and non-impregnated material was very similar under the mechanical test conditions (23◦ , RH 55%), see Figure 6.5. The results from
the diagram also confirm the interaction (in terms of hydrogen bonding) between
PEG and the wood constituents [94, 32] which reduces the ability for both wood
and PEG to adsorb water at low RH.
In the next step the mechanical properties of Vasa oak was investigated. The
measurements revealed some interesting results. In the first sample series, which
included large specimens from the block (Figure 6.6a), the longitudinal tensile
strength σL was determined to 48±13 MPa. Compared to the reference samples
6.3. MECHANICAL PROPERTIES OF RECENT OAK AND
ARCHAEOLOGICAL VASA OAK
47
Figure 6.5: Equilibrium moisture content of oak wood plotted as a function of
relative humidity. Legend indicates the PEG content of the samples (zero PEG
content = reference) and pure PEG 600. The inset figure shows enlargement of the
region RH ≤ 60%.
Figure 6.6: The (previously interior) area of the Vasa sample 65542 (the sample
was originally a corner of a larger piece of oak). Positions of large tensile test
samples are indicated by dots in the image. Mechanical series A and B are indicated
by the large and small square, respectively. Shadowed areas A and B indicate
the positions at which chemical evaluation were performed. The sample used for
SilviScan was positioned at the same X- and Y-coordinates as the chemical series A,
but at different depths in the fiber direction (Z). The dotted line indicates surfaces
previously internal.
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
48
PROPERTIES OF ARCHAEOLOGICAL OAK
Figure 6.7: a) Variation in tensile strength σL from mechanical series A and average
molecular weight of holocellulose from the chemical series A with distance from the
surface(Figure 6.6a). Average molecular weight of the holocellulose is from the
chemical series A (Figure 6.6). b) Variation in microfibril angle and density with
distance from the surface (from the SilviScan measurements).
(tensile strength 112±27 MPa) this is approximately 60% lower. A strong gradient in tensile strength was observed, with the highest σL values located to the
near-surface regions of the block (10-30 mm) and gradually declining towards the
interior (see Figure 6.6b). The decrease in mechanical strength was accompanied
by a decrease in average molecular weight (AMW) of the holocellulose as measured
by SEC (see Figure 6.7a).
The same trend in tensile strength was found also after compensation for variation in MFA and density was made using equation 4.2 and 4.7. The local tensile
stress at failure parallel to the microfibrils σ1u was found to be lower in the interior
region, while the surface region displayed higher values (see Table 6.3). The difference in σ1u between the surface and interior regions is probably even higher than
calculated as density was abnormally high in the surface due to water-insoluble
compounds which influenced the SilviScan measurements (see Figure 6.7b).
Table 6.3: Values of local tensile stress at failure parallel to the microfibrils σ1u
at different depth calculated from MFA and density data in combination with the
tensile strength values from mechanical series A in the Vasa block 65542.
Y-position [mm]
Tensile strength σL (series A, average) [MPa]
Density ρ [kg/m3 ]
MFA [ ◦ ]
Local tensile stress at failure parallel
to the microfibrils σ1u [MPa]
10
68
709
15
133
30
46
614
16
103
50
42
602
18
94
70
40
616
17
88
90
45
568
17
110
110
43
515
15
116
Instead of compensating for variation in the density and MFA, one can use
another approach and chose samples from an area where density and MFA can be
6.3. MECHANICAL PROPERTIES OF RECENT OAK AND
ARCHAEOLOGICAL VASA OAK
49
considered to be constant and therefore need not to be compensated for. This was
done for a group of samples which were located to the same set of annual rings
(sample series B oriented diagonally in the block, see Figure 6.6a). Once again, a
drop in both tensile strength (from ∼65 MPa to ∼35 MPa) and average molecular
weight of the holocellulose (from ∼600 kDa to ∼400 kDa) was observed from the
surface and inwards (see Figure 6.8).
The mechanical results from the micro-tensile testing on the drilling cores were
in line with the results from the block. The Vasa samples displayed significantly
lower tensile strength (position 2: 38±16 MPa; position 3: 60±25 MPa; position 4:
35±20 MPa) compared to the reference (75±40 MPa).
The DVS results on Vasa oak showed, as with the measurements on recent oak,
that the moisture content of the samples were very similar at RH 55%, and was
therefore not taken into account when evaluating the mechanical results.
Very few mechanical investigations on archaeological wood are found in the
literature. These show on a high diversity, with mechanical properties varying
from those which are completely maintained (or even increased) to those which are
severely reduced in comparison to recent wood [64, 95, 65]. Only a small number
of studies have been performed on the Vasa ship. Reduction in bending properties
have been confirmed (on a plank from the Vasa) [96], and negative correlations
between PEG content and compression properties have been found in the radial
and tangential directions [97] (see Figure 6.9 for a summary of the mechanical
results on radial stiffness and yield strength).
Figure 6.8: Depth profile for sample series B located to the same set of annual rings
in the block: tensile strength and average molecular weight.
As seen from the results in this study, the longitudinal tensile strength was
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
50
PROPERTIES OF ARCHAEOLOGICAL OAK
greatly reduced in the block and the drilling cores, and the tensile strength in the
block was closely related to the degradation of the holocellulose. The only other
Vasa study performed in 1958 in the longitudinal direction (compression testing on
samples from a non-impregnated block of water-logged oak) showed lower density,
lower stiffness, and lower yield strength than in recent water-logged oak, but no difference in mechanical properties was found between the surface and internal Vasa
material [100]. One possible explanation is that a large part of the degradation in
the Vasa timbers is believed to have taken place after the salvage [71], and it had
therefore not yet occurred at the time of the axial compression tests.
Figure 6.9: Mechanical properties in the radial direction of wood from the Vasa and
PEG impregnated recent oak: a) Compression stiffness and b) compression yield
strength. Data have been compiled from Ljungdahl J and Berglund LA [97], and
Ljungdahl J [98, 99]
Extensive investigations on the chemistry of the Vasa ship have been performed
since the salt outbreaks were first discovered in 2000 [66, 78, 101]. The composition
of the salts was first investigated. The precipitates were found to be different iron
sulphates (e.g. natrojarosite and melanterite), white gypsum and elemental sulfur.
Different iron sulphates were also found in the interior of the wood. Evidently, the
sulfur (originating from the bacterial degradation) had to a great extent reacted
with the iron (from the rusting bolts and iron objects). When the speciation of the
sulfur from the surface to the interior of the wood was examined, the relative amount
of oxidized sulfur was found to increase with increasing depth from the surface
(although the total sulfur amount was found to decrease). The possible oxidation
of the sulfur and sulfur compounds (especially reactive iron sulfides) caused most
concern as it was argued that this could lead to the formation of sulfuric acid and
hydrolysis of the cellulose [102]. Low pH values were taken as proof that this had
already occurred at some sites both on the surface and inside the wood.
However, when the pH trend was investigated further the low pH values in the
6.3. MECHANICAL PROPERTIES OF RECENT OAK AND
ARCHAEOLOGICAL VASA OAK
51
Figure 6.10: Chemical composition with distance from the surface in sample series
B from the Vasa block 65542: content of acids (from IC) and pH.
Figure 6.11: a) Molar mass distributions from the SEC measurements: samples
from chemical series A positioned at different depth from the surface in the Vasa
block and a recent oak reference. b) Average molecular weight as a function of total
acids content in the drilling cores.
interior were found in a context of low total sulfur content, and correlated with a
high iron-to-sulfur ratio. The degradation of PEG, hemi-, and cellulose had also
taken place in areas rich in iron but poor in sulfur (usually corresponding to wood
segments taken at a greater depth from the surface), whereas only relatively low
polysaccharide degradation was detected in salt-infested surface samples [71, 73,
CHAPTER 6. EFFECT OF CHEMICAL DEGRADATION ON MECHANICAL
52
PROPERTIES OF ARCHAEOLOGICAL OAK
72]. This subdued the importance of the sulfuric acid as a general threat, and
oxidative processes including the formation of radicals1 with iron working as a
catalyst in Fenton-type reactions was suggested as a second degradation hypothesis
[71, 78]. Reduced mechanical properties of recent wood in contact with iron is a
well-documented phenomena in the literature (see e.g. [103, 104, 105, 106]) although
the results might not be directly applicable to archaeological wood. It has also been
suggested that reduced sulfur species can function as a radical-quenching agent [78],
presenting a possible explanation to the lower degradation found in the (sulfur-rich)
surface regions of the wood.
The chemical experiments in this study clearly showed that the average molecular weight of the Vasa samples is lower compared to recent oak, and also that the
AMW in Vasa oak is reduced with increase in depth from the surface. Significantly
higher concentrations of acids were found in the interior compared with the surface
region (see Figure 6.10), and the acidity is increasing towards the interior of the
wood with a strong negative correlation to the average molecular weight, see Figure
6.11a) and b). From this study together with previous studies it is evident that the
degradation is not isolated to a few local regions or elements, but is more likely a
typical state for the interior wood in the whole Vasa ship [71, 72].
The origin of the holocellulose degradation in the Vasa remains partly unknown.
The most important question whether degradation is still taking place in the ship
also remains unanswered. However, a new piece of the puzzle has been added with
the results from this study, which clearly show that mechanical performance (in
terms of longitudinal tensile strength) is greatly reduced in the Vasa oak compared
to recent oak.
6.4
Conclusions - archaeological oak
The study on archaeological and recent oak tested in tension parallel to grain shows
how rules, developed for and applied to man-made composite materials, can be applied also to wood. Here, density and microfibril angle were compensated for in
a first attempt to reveal the effect of chemical degradation on archaeological oak
from the Vasa ship.
The review of PEG-impregnated wood together with the experimental results
from this study showed that the impregnation agent can have a large negative
influence on the mechanical properties of wood, but also that the impact of PEG
when testing oak in the longitudinal direction is small and can therefore be ignored
provided that the microfibril angle in the material is small.
The mechanical results on Vasa oak show that the tensile strength is severely
reduced, and that the reduction correlates with reduced average molecular weight of
the holocellulose. This could not have been foreseen without a thorough mechanical
1 Atoms,
molecules, or ions which have unpaired electrons, and are therefore highly reactive.
6.4. CONCLUSIONS - ARCHAEOLOGICAL OAK
53
and chemical investigation, since the Vasa wood (with exception from the bacterially
degraded surface regions) are morphologically intact and with an micro-structure
comparable to that of recent oak.
The results presented here can be used in the ongoing task of mapping the
condition of the Vasa wood.
Chapter 7
Acknowledgements
This thesis is based on research performed at the Department of Fibre and Polymer
Technology, the Department of Solid Mechanics, and the Department of Aeronautical and Vehicle Engineering at the Royal Institute of Technology (KTH), at Innventia AB (previously STFI-Packforsk) and SP Trätek in Stockholm, at the Luleå
University of Technology (LTU) in Luleå, at the University of Agricultural Sciences (SLU) in Umeå, at the Division of X-ray Physics at the University of Helsinki
in Finland, at the Max Planck Institute of Colloids and Interfaces in Potsdam,
Germany, and at the Hamburg Synchrotron Radiation Laboratory (HASYLAB) in
Hamburg, Germany.
The work was funded by the Swedish Center for Biomimetric Fiber Engineering (BioMime), the Knut and Alice Wallenberg Foundation, The Swedish Research
Council (VR), The Swedish Foundation for Strategic Research (SSF), The Swedish
Research Council for Environment, Agricultural Sciences and Spatial Planning
(FORMAS), and the Swedish Agency for Innovation Systems (VINNOVA). This
support is gratefully acknowledged.
First of all, I wish to express my gratitude to my supervisor Professor Lars Berglund,
who has given me the opportunity to work in a stimulating environment where students are encouraged to develop their initiative and self-confidence as scientists.
I would also like to thank my co-supervisor Professor Kristofer Gamstedt for his
great support and interest in the projects, and for always taking time for interesting
discussions.
Former and present members of the Biocomposite group and Wallenberg Wood Science Center (WWSC) - in particular Aihua, Anders, Andong, Anna, Carl, Houssine,
Joby, Jonas, Julia, Ngeza, Malin, Marielle, Michaela and Sylvain - deserve to be
mentioned also, not only for their generous and kind help during my research but
also for their friendship.
Colleagues and friends at the Department of Fibre and Polymer Technology - Dina
55
56
CHAPTER 7. ACKNOWLEDGEMENTS
Dedic, Yvonne Hed, Inga Persson, Brita Gidlund, and Mona Johansson in particular - are thanked for their invaluable help and encouragement.
Anders Beckman, Bo Magnusson, Bertil Dolk, Hans Öberg, and Martin Öberg are
thanked for assistance with the laboratory equipment.
Finally, I wish to thank my colleagues outside KTH:
Tommy Iversen, Lennart Salmén, Anne-Mari Olsson, Eva-Lisa Lindfors, Helena
Nilsson, and Åke Hansson at Innventia AB (previously STFI-Packforsk) in Stockholm,
Anders Ahlgren, Emma Hocker, Ove Olsen, and Magnus Olofsson at the Vasa Museum in Stockholm,
Joakim Seltman at SP Trätek in Stockholm,
Gunnar Almkvist, Charles Johansson, and Stig Bardage at the University of Agricultural Sciences (SLU) in Uppsala,
Lennart Wallström at the Luleå University of Technology (LTU) in Luleå,
Jonathan Love, Björn Sundberg, Lorenz Gerber, and Manoj Kumar at the University of Agricultural Sciences (SLU) in Umeå,
Kirsi Leppänen, Seppo Andersson, Marko Peura, and Ritva Serimaa at the University of Helsinki in Finland,
Andreas Walther at the Aalto University in Helsinki in Finland, and
Ingo Burgert and Bo Zhang at the Max Planck Institute of Colloids and Interfaces
in Potsdam in Germany.
I have really enjoyed our collaborations and look forward to the coming ones!
◦◦◦◦
57
Förutom kollegor och arbetskamrater finns det förstås ett par personer som jag
skulle vilja rikta ett speciellt tack till, trots att de inte har haft en direkt koppling
till min forskning.
Jag vill börja med att nämna mina mentorer, Jaana Zaino och Sverker Blomberg.
Ni har hjälpt mig att utveckla mitt tålamod och min förmåga till eftertanke och
strukturerat tänkande; verktyg som är lika användbara inom forskningen som i
övriga livet.
Sedan vill jag tacka mina före detta befäl Johan Lindqvist, Håkan Friman, Joacim
Karlsson och Martin Kretz. Ni visade vad verkligt bra föregångsmannaskap och
ledarskap kan innebära, och fick mig att inse min inneboende envishet och styrka.
Min morbror Ulf förtjänar också ett stort tack, eftersom jag utan hans hjälp hade
haft det mycket svårare att genomföra mina KTH-studier.
Mina vänner - speciellt Anna-Mona, Linda och Tamira - er vill jag tacka för er
trogna vänskap och ert stöd genom alla år.
Jag vill också nämna mina syskon Emilia och Gustav som betyder mycket för mig,
och mina föräldrar Christina och Tommy: Tack, mamma och pappa, för att ni
trodde på mig och uppmuntrade mig att studera vidare efter gymnasiet, och för
att ni gav mig en uppväxt ute på landet där ett barn hade nära till naturen och
skogen. Där väcktes mitt intresse för träd.
Till sist vill jag tacka min bättre hälft och älskade Anders som alltid finns där och
som får mig att skratta så ofta. Du är det bästa som har hänt mig. Du fyller mitt
liv med mening och jag är verkligen lycklig över att få dela det med dig!
◦◦◦◦
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Chapter 8
Appendix I
69
70
CHAPTER 8. APPENDIX I
Figure 8.1: 1D, 2D, and 3D representations of diffraction intensity from wood. The
azimuthal intensity profile for the 200 crystal plane of cellulose is indicated by the
thick solid line in all three images. After Evans et al.[51]
71
Figure 8.2: Microstructure of oak before (a, c, e) and after (b, d, f) treatment with
38% PEG/water resulting in an approximate PEG content of 25%. a), b) Latewood
cells, c), d) earlywood cells. (e) and (f) show enlargements of the areas marked by
squares in (c) and (d) respectively.
72
CHAPTER 8. APPENDIX I
Figure 8.3: Micro-structure in recent oak examined by X-ray microtomography.
Earlywood (a) and latewood (b).