MASTER'S THESIS
Dimensional stability of chemical modified
aspen by using oxidized glucose
Qian Yang
Master of Science (120 credits)
Wood Technology
Luleå University of Technology
Department of Engineering Science and Mathematics
Abstract
Wood modification is an attractive study topic in the wood industry to improve the
properties of wood for outdoor structure applications. Development of environmentally
friendly and renewable materials to improve dimensional stability of wood is of high interest.
In this project, Fenton´s reagent was used to oxidize glucose and then small cubes of solid
aspen wood was impregnated with the aqueous solution using water vacuum followed by
drying/curing by heating in oven. A series of experiment was conducted by DOE (design of
experiment) for multivariate statistic analysis on the influence of various processing
parameters. Curing temperature, impregnate pH and their interaction effects was found to be
important in improving dimensional stability of impregnated wood. A second experiment was
performed at high curing temperature of 140oC aiming to achieve highest Anti Swelling
Efficiency (ASE). This was performed with larger dimensions of solid wood. Water repellent
effectiveness (WRE) and ASE results indicated that the dominate mechanism for reducing the
swelling property of wood was more like conditions prevailing under thermal modification of
wood such as hemicelluloses degradation.
A third series of experiment was performed aiming at reduce thermal degradation reactions
and prove the effect of cell wall bulking and cross-linking effect on ASE. The research results
showed that under low temperature heating conditions at 103oC and fairly strong acid
conditions (pH 2.4), higher ASE of around 45% was achieved by the wood impregnated with
oxidized glucose after one day water soaking rather than when impregnated with oxidized
sucrose (ASE was around 35%). An even lower ASE of 10% was obtained for wood treated
with Fenton’s reagent. The WPG for oxidized sucrose treated sample was twice as high as the
sample with oxidized glucose while the cell wall bulking of oxidized sucrose treated sample
was lower than oxidized glucose treated sample. Though large amount of impregnate had
been leached out, an ASE of 20% for the wood samples which had been impregnated with
oxidized glucose was obtained after 7 days water soaking test. It suggested that under our
impregnation and curing treatment, there might be small amount of reactive compounds
generated from the glucose oxidization that form possible cross-linking with wood
components inside cell wall. From scanning electron microscopy (SEM) pictures, it can be
seen that the cell wall thickness of all the impregnated samples were smaller than control
aspen samples. Furfural can be detected from the first day of soaking in water of the samples
impregnated with Fenton Reagent. The degradation of wood cell wall compounds might occur
and could influence the dimensional stability of treated wood under the conditions used in this
study. WPG played an important role in inhibiting water uptake, oxidized sucrose
impregnated wood with higher WPG showed better WRE than oxidized glucose impregnated
samples with a lower WPG.
Keywords: Fenton Reagent, Glucose, Curing temperature, pH of impregnate, cross linking,
water repellency, anti swelling efficiency (ASE), water repellent effectiveness(WRE) .
2
Preface
This work has been carried out at Luleåuniversity of Technology from Feburary 2012 to May
2012.
Many people have been important to my project work.
First, I would like to thank my supervisor Olov Karlsson for his patient guiding and teaching
during the whole thesis work. His suggestions help and inspire me get through the difficult
stage of experiment.
Also I would also like to thank the course supervisor Micael Öhman letting me understanding
how to plan and execute a thesis project. Special thanks to Sheikh Ali Ahmed for his kind help
during the different stages in my work.
It is a novel treatment, a lot of problems have been shown in the experiment, I hope further on
this project can be continued to study.
In my master degree study, all the faculty, classmates, friends I met in LTU skellefteågave me
guidance, inspiration and help. It is a great experience for me. Here I want to thank all of them
and give my best wishes to everyone.
Finally, I would like to express my sincere thanks to my parents, for their support and love.
Without them, I cannot be who I am now. Thank you Xiaodong Wang, Ke Jiang, Victoria
Krasnoshlyk, Niclas Björngrim for accompanying and chocolates.
Skelleftea, May 2012
Qian Yang
3
List of contents
Abstract ........................................................................................................................................ 2
Preface ......................................................................................................................................... 3
List of contents ............................................................................................................................ 4
1.Introduction .............................................................................................................................. 6
1.1 Background ....................................................................................................................... 6
1.2 Theory ............................................................................................................................. 11
1.3 Mission & Vision ........................................................................................................... 14
2.Materials and Methods .......................................................................................................... 16
2.1 Materials.......................................................................................................................... 16
2.2 The importance of different impregnation processing parameters .............................. 16
2.2.1 Experiment design (screening study) ................................................................... 16
2.2.2 Impregnation and heat treatment .......................................................................... 17
2.2.3 Swelling properties after repeated leaching with water ...................................... 18
2.3 Interaction effects of pH and temperature on dimensional stability of impregnated
wood .................................................................................................................................... 18
2.3.1 Impregnation and heat treatment .......................................................................... 18
2.3.2 Swelling properties after repeated leaching with water ...................................... 18
2.4 Effect of oxidized carbohydrates on dimension stability of impregnated wood ........ 19
2.4.1 Impregnation and heat treatment .......................................................................... 19
2.4.2 Swelling properties after repeated leaching with water ...................................... 20
2.5 Analysis ......................................................................................................................... 20
2.5.1 HPLC analysis ..................................................................................................... 20
2.5.2 Dimensional stability and Weight Percentage Gain ........................................... 22
2.5.3 Scanning electron microscope(SEM) .................................................................. 22
3.Results and Discussion .......................................................................................................... 24
3.1 Screening experiment ..................................................................................................... 24
3.1.1 Studies on importance of process parameters ..................................................... 24
3.1.2 Influence of oxidation of glucose ......................................................................... 25
3.1.3 Impregnate pH effect ............................................................................................ 27
3.1.4 Interaction effect between impregnate pH and curing temperature ................... 27
3.1.5 Weight Percentage Gain(WPG) and Bulking effect(BC) related to ASE ......... 29
3.2 pH, temperature and their interaction with dimenisonal stability of impregnated wood
................................................................................................................................................ 30
3.2.1 Water Repellency .................................................................................................. 30
3.2.2 Dimensional stability ............................................................................................ 32
3.2.3 HPLC analysis ....................................................................................................... 35
4
3.3 The effect of oxidized carbohydrates on dimension stability of impregnated wood . 37
3.3.1 Water Repellency .................................................................................................. 37
3.3.2 Dimensional stability ............................................................................................ 38
3.3.3 HPLC analysis ....................................................................................................... 39
3.3.4 Swelling properties of impregnated wood after 7 days non-leachable water
soaking................................................................................................................................... 40
4.Conclusion .............................................................................................................................. 43
5.Hypothesis and suggestion to future work ........................................................................... 44
6.References .............................................................................................................................. 46
Appendix I ................................................................................................................................. 49
Appendix II................................................................................................................................ 50
Appendix III .............................................................................................................................. 51
Appendix IV .............................................................................................................................. 52
5
1. INTRODUCTION
1.1 Background
Nowadays, with the pollution and exhaustible consumption problem generated from
nonrenewable material, the awareness of using green materials in a sustainable way is
increasing worldwide. Wood is one of the green materials which is light, have high strength
and is environmentally friendly. On the other hand, it also exist some properties of wood that
can be critical from an end-user point of view like relatively poor dimensional stability
compared with other nonrenewable composites due to the hydrophilic property of wood and
more severe degradation if suffered from biological attack, susceptibility appearance change
when exposed to weathering condition.
Improvement of wood hydrophobic and dimensional stability properties is very
important for outdoor applications such as decking boards. Dimensional stability of wood will
influence its performance like in surface coated products and adhesion with other materials.
As wood is sensitive to dimensional change responding to atmospheric conditions,
movements in wood may generate small cracks in coating layer, generating openings were
water can enter. Wood is liable to biological attack when sufficient water, oxygen and
nutrients have been provided. In order to prevent wood from rot fungi degradation, one of the
best ways is to keep wood component from reaching high moisture content (over 20%).
Wood is a hygroscopic material mainly constituted of cellulose, hemicelluloses, lignin,
some other carbohydrates and extractives. The cell wall is where the absorption and
desorption of moisture take place and the change of cell wall dimension will be the main
contribution to the change of the wood shape during swelling and shrinking. When comes to
the ultra structure of cell wall, briefly speaking that the cellulose is the skeleton surrounded by
hemicelluloses and lignin [1]. The hemicelluloses have strong affinity to water due to large
amount of accessible hydroxyl group. Accessible hydroxyl group is the main reason causing
the swelling and shrinkage.
Impregnation with preservatives or modifying chemicals is a way to improve the properties
and prolong the life cycle of wood product. When looked at the structure of wood cell wall
with the aim to achieve good dimensional stabilization it requires that the modifying agent
resides within the cell wall micropore structure rather than lumen fill. There has been
proposed primarily two mechanisms of the impregnate [2] reacting in the cell wall and
blocking the micropores in the cell wall. The micropores are open when the cell wall is fully
swollen. Normally a maximum size for cell wall micropores is in the region of 2-4nm[2].
When green wood is dried, the water will be removed from cell wall and the micropores
might collapse but re-open to a large extent when exposed to moisture again. So when
chemicals molecule diameters are smaller than micropores diameter, they can access with
interior cell wall part. One mechanism is due to cell wall bulking, keeping cell wall in swollen
6
state and where chemical and physical factors can both influence the degree of cell wall
bulking. Dimensional improvements occur due to bulking of the cell wall by impregnating
chemicals such as in treatments with anhydrides which can react with accessible hydroxyl
groups and become fixed to the wood (See Figure 1). Dimensional stability of wood is
improved at a high WPG (weight percent gain) in many cases of chemical modification due to
less water uptake in wood cell wall. WPG is the weight gain due to chemical addition
comparing with untreated sample in oven dry state. It indicates how well the impregnated
chemicals stayed inside wood after chemical modification. WPG might help reducing the rate
of water uptake by physically blocking the lumen. In Papadopoulos study [3], they suggested
that cell wall bulking is caused by the volume of adduct deposit rather than hydroxyl
substitution by anhydride modification.
Figure 1. Anhydride modification of hydroxyl groups in wood, where R=CH 3 (acetic
anhydride).
Strong bonding between cell walls which is formed by chemicals cross-linking with the
cell wall polymers is another mechanism for improving dimensional stability. The formation
of such bonds with cell wall polymeric constituent prevents the micropores from opening
when the wood is exposed to moisture
[2]
(Figure 2). For example in formaldehyde
modification, it is reported that with low WPGs high values of ASE can be achieved, pointing
to a large extent of cross linking [4].
Figure 2. Formaldehyde modification of wood cell wall components.
Impregnating chemicals like CCA (copper- chrome- arsenic) has its own problem although
it has been one of the most effective and used preservatives. In contrast to what is described
above treatment with CCA or other copper-containg preservatives do not lead to improvement
7
of dimensional stability and the improved durability is due to the its toxic properties to rot
fungi and other microorganisms. CCA is widely forbidden to use in Europe as a wood
preservative for residential or domestic constructions use because of the probability of
releasing arsenic and chromium to the ecosphere and as there is no green techniques to
recycle it. When developing new preservative methods the modified wood should itself be
nontoxic under usage conditions and furthermore, there should be no release of any toxic
substances during service, or at end of life following disposal or recycling of the modified
wood [2].So nowadays, environmental friendly chemicals with developed green techniques are
desired in the wood modification industry.
A lot of studies
[2-6]
have been focusing on using acetic anhydride, carboxylic acids,
isocyanates, epoxides, aldehydes, furfuryl alcohol to chemically modify surface or impregnate
the wood and reacting with accessible hydroxyl group. Acetylation has been broadly studied
as a wood modification reaction[2]. Acetylated wood is non toxic and has high durability. The
modification is accompanied by formation of acyl adduct by effectively change hydroxyl
group into acetyl groups (Figure 1) that keeps wood in swollen state. Studies have shown that
the dimensional stabilization during acetylation has increased with increased WPG or larger
extent of bulking of cell wall. The contribution from the cross-linking effect is very minor
practically[2]. Compounds containing carboxylic acid, acid chloride, isocyanate can also
react and reduce the amount of hydroxyl groups in the cell wall. Especially, isocyanate is
reactive and forms chemical bonds with itself and wood components that are involved in
wood cell wall bulking and cross linking mechanisms. Such bonds are quite hydrolytically
stable especially as no reactive by-product (such as HCl) is generated during the curing
process which can influence the modification process or stability in end-products. But some
low molecular weight isocyanate monomers are volatile and toxic which can be harmful
during manufacturing and maybe also in in-door products if they are not fully reacted. How to
control the isocyanate reagent reaction comes to a question for environment issues [5]. Epoxide
modification could form an ether-linkage with hydroxyl groups in the cell wall. At the same
time another new hydroxyl group is formed for possible reaction with another epoxide reagent
group, so polymerization can occur increasing chemical cell wall bulking (Figure 3). Also
difunctional epoxides can cross link cell wall s [7]. Rowell and Gutzemer [8]pointed out that the
polymerization of epoxide contributed to a bulking effect inside of cell wall, but above a
weight percent gain (WPG) of 30%, the anti-swelling efficiency (ASE) was reduced when
using propylene oxide, butylene oxide. Pandey [9]also reported that in propylene epoxide (PO)
treatment at WPG>30 percent, a reduction of ASE in the PO modified specimen occurred
which may be caused by oligomers that destroy the cell wall microstructure, leading to that
inaccessible hydroxyl sites being exposed. Rowell et al. observed that ASE of epoxide
modified wood reduced a lot from water-soaking/ oven drying cycles which indicated that the
chemicals inside of cell wall were non-bonded [2].
8
Figure 3. Reaction of wood with an epoxide
PEG (polyethylenglycol) is considered as a non-bonded leachable impregnate. Though
it can perform quite well after treatment, the ASE after weathering and other properties are
reduced a lot due to leaching of the impregnate [10]. Resin treatments like PF resin, UF resin
and other monomers like furfuraldehyde and dimethylol dihydroxy ethyleneurea (DMDHEU)
have been reported for some time
[2]
. It was concluded that the resin should be less
polymerized so that the molecular size of the chemical is small enough to penetrate into the
cell wall. These treatments can make cell wall bulking and form cross-links inside of cell
wall.
As mentioned above, formaldehyde increased the dimensional stability of wood at quite
low WPG, but with the concern of the health risk from formaldehyde vapor, the focus is shift
to non-formaldehyde cross-linking agent like glyoxal, glutaraldehyde [11]. Xiao et al.[12] used
the glutaraldehyde (GA) as a cross linking agent in presence of magnesium chloride
hexahydrate (MgCl2 ﹒6 H2O ) as catalyst to react with the cell wall of Scots pine (Pinus
sylvestris L.) sapwood. After 10 days of leaching with daily change of tap water and drying at
103°C, GA treated wood achieved about 70% ASE compared with untreated wood at a WPG
of 22%. After 10 cycles of leaching experiment, the untreated and treated specimen had equal
weight loss which suggested that only water-soluble extractives inside of wood had been
washed out as GA probably formed a strong bonding (with itself or with wood) inside of cell
wall during impregnation. It shows the potential of replacing formaldehyde.
Sefc.B et.al[13]discussed the dimensional stabilization of fir and beech wood modified by
citric acid with NaH2PO2 or NaH2PO4 as a catalyst under three different heating temperatures.
Data showed that when treated with water solution of 6.9% citric acid and catalyst under
140 °C for 5 hours, the ASE was around 40% in beech wood and 55% in fir wood. They also
observed a particularly color change under treatment at 180 °C. It was claimed that citric acid
has the potential to be a non-formaldehyde cross-linking reagent, but the extent of
cross-linking is still unknown.
Though some green material or modification methods have been studied as mentioned
above, experiment conditions and the treatment cost did not allow a large development of
wood modification, so the need of less expensive environmental safety material and methods
has been raised as an issue.
Another popular way to improve the dimension stability of wood is thermal modification.
Thermal modification is normally performed between the temperature of 160°C and 260°C [2].
However, treatment temperature could be significantly reduced by treatment in saturated
9
steam than in superheated steam conditions and still give similar brownish colour of thermally
modified wood [14]. Colour could be used as an indicator for other properties of modified
wood[14] due to the chemical change of wood components. Hemicelluloses degradation has
been regarded as the most thermally labile structural wood components and loss of it is the
main reason for the increase of dimension stability. When the temperature was lower than
140 °C, there was only slightly changes in material properties[2]. Ramiah showed that the
initial temperature of active pyrolysis of a glucomannan derived from spruce was 140 oC while
some weight loss from this hemicelluloses was noted at temperature as low as 100oC from its
dynamic thermal gravimetric analysis[15]. We found evidence that the presence of monosugars
(arabinose and galactose) released from side chain groups or polysaccharides (arabinan and
arabinogalactan) was low at high temperature drying of spruce boards in oven at 130 °C but
higher under more extreme conditions (saw-dust from spruce was heated in water at 110 °C)
[16]
. Degradation of monosugars in wood surfaces of spruce was high after 24 hours of drying
in oven at 130 °C as well as by gentle heat pressing for 2.5 min. at press plate temperature of
200 °C[16]. The presence of acetyl groups which are thermally labile and lead to the formation
of acetic acid when wood is heated can cause acid-catalysed degradation of the
polysaccharides[17]
(See
Figure
4).
The
removal
of acetyl
group
from
larch
galatoglucomannan had been shown to increase the thermal stability of the deacetylated
hemicelluloses[2]. Dehydration reactions began to occur above about 140°C which leads to a
decrease in hydroxyl content[2]. Fengel and Wegener[2] proposed that hemicelluloses polymers
were degraded via free-radical intermediates which were dehydrated to form furfural and
hydroxymethyl furfural under thermal degradation (Figure 4). Hardwood was more thermally
labile than softwood due to having more pentosans than hexosans in hemicelluloses
constituent [2]. In 1996, Abad et al.[18] used 95% acetic acid solutions containing 0.2-0.4% HCl
as catalyst to treated Eucalypt at 120-130°C. The HCl-catalyzed acetic acid generated furfural
during the fractionation of eucalyptus wood.
10
Figure 4. Probable thermal degradation pathways for hemicelluloses, according to Fengel and
Wegener (1989) [2] .
1.2 Theory on reactions of oxidized sugars with wood
Fenton reagent is a solution of hydrogen peroxide and ferrous/ferric iron catalyst. It is an
11
effective oxidant and applied mostly to treat the organic waste and pollutant. The probable
pathway of Fenton reagent reactions are shown below:
(1) Fe2+ + H2O2 → Fe3+ + OH·+ OH −
(2) Fe3+ + H2O2 → Fe2+ + OOH·+ H +
2 H 2O2 → H2O +
O2
The generated hydroxyl radical is a very active oxidant, capable of attacking most organic
compounds including those containing hydroxyls and ether linkages. When it reacts with
wood, it involves complex reactions which are still not fully understood. The radicals can
react with both lignin and carbohydrates. Such oxidation of lignin produces new phenols and
free radicals. It was reported that one of the important reactions is the generation of phenoxy
radicals by attack of hydroxyl radicals[19]. Both lignin and carbohydrates can be further
oxidised to carboxyl and keto-groups [20]. Cellulose oxidized by Fenton reagent tends to form
carbonyl groups under acid condition while under alkaline condition more carboxyl group
forms [21]. In alkaline condition, the oxidized cellulose chain are more likely to fissure[21]. But
a mild one electron oxidation of wood under neutral or acidic conditions has been shown to
give self-bonding properties to the wood[22].
In previous studies, Westermark
[22]
used hydrogen peroxide and catalyst to oxidize the
wood particles and then hot press to resin-free particleboard. Small spring back has been
achieved and the internal bonding (IB) was reported to be excellent. Lower swelling
coefficient was reached when the amount of oxidant was higher. Also in that study higher
moisture content of wood particles before oxidation may lead to a weakening of the final
board and an increase in swelling. It may indicate that water plays a role preventing Fenton
reaction to occur by cooling the wood during oxidation of wood particles.
The effect of pH on the auto-adhesive bonding of wood is needed to be considered. A pH
value higher than 3.5 is preferred and the experiments suggest that the autoadhesive bonding
reaction can be performed up to pH 5.5 without negative effect on swelling and strength. But
when pH is above 6, the swelling was increasing a lot
[24]
In 2003 Widsten
[23]
.
used the Fenton’s reagent to activate spruce and beech fibers. It is
proposed that the adhesion strength improves a lot highly due to interfiber bonds formed by
radical coupling of radicals (phenoxy radicals see above) formed in wood components such as
lignin during high temperature pressing, but the thickness swelling was quite high which
limits the application for outdoor conditions.
As discussed above, Fenton reagent can oxidize both the lignin and carbohydrates which
contributes to the bonding ability. Interestingly, Westermark et.al
[20]
suggested that the
oxidized carbohydrates of the dialdehyde type played an important role in the bonding with
the lignocellulosic material. They investigated the bonding ability of oxidized wood particles
12
in binder-less particle boards by extracting the particles with water and then re-add the water
extracts into the wood particles, evaporate the water followed by hot pressing. Analysis of the
bonding activity (measured as internal bonding strength) showed that a high portion of the
gluing capability can survive dilution in water and evaporation which means that neither
soluble nor stable lignin radicals contribute to the majority of the bonding activity. Then they
tested Fenton chemistry to improve the wet-strength of lignin-free paper by oxidizing
different types of sugars followed by spraying a water solution of them on paper surface and
drying the treated papers under heating. The result also showed that oxidized carbohydrates
can give high wet-strength especially when the molar ratio of monosaccharide and Fe2+/H 2O2
is 1:2. The monosaccharides as glucose, xylose, galactose, manonose were reported to
perform well in wet-strength test while sorbitol and gluconic acid on the other hand gave
lower strength. They also tested the hydroxyl acids: 3-hydroxy butyric acid, 2-hydroxy
butyric acid and 2-keto-butyric acid without any oxidation and addition of 2-keto-butyric acid
showed an increase in wet-strength but an increase that was smaller than for the oxidized
monosaccharides.
After oxidation of glucose, analysis using the GC-MS spectrum shows that it can have
products with two extra functional groups (aldehyde and/or keto groups) [20]. They proposed
that glucose with one-extra aldehyde and a keto group or two keto groups are the most active
products in the system[20].
Glyoxal, gluconic acid, ribose, glucitol, glycolate, glyceraldehydes and formic acid [19-27]
have been reported to be generated from oxidation of carbohydrates with hydrogen peroxide
(Figure 5). Some of these products could function as cross linker, for example glyoxal has
bifunctional group (dialdehyde), between cell walls while other oxidized products can be
reacte with free hydroxyl groups on wood by one functional group to form bulking inside the
cell wall.
Glyoxal
Gluconic acid
Glyceraldehyde
Figure 5.
β-D-ribofuranose
Formic acid
Glucitol
Glucose
Glucose and oxidation products by using Fentons’ reagent.
13
Thus, the oxidation of glucose by Fenton chemistry is not selective and many products are
formed, therefore it is difficult to say which product is the reason of wet strength
improvement of paper. Morelli et al.[25] tried to confirm the relationship between carbohydrate
molecular structure and its reactivity against free radical activity by studies of presence of
degradation products such as formic acid. It was reported that maltose and sucrose were very
active in the Fenton conditions used at room temperature, also if the temperature was
increased to 80°C, it produced higher amounts of formic acid than other assayed sugars.
1.3 Mission and Vision
As far as author know, use of oxidized sugars have hitherto not been used to try to
improve dimensional stability of solid wood, so in this study glucose oxidized with Fenton´s
reagent will be firstly used in treating solid wood. In mild conditions ester and acetal may be
formed by the reaction of accessible hydroxyl group on wood with carboxylic group[28] and
keto group.[12] See the figure below (Figure 6). Acetal (or ketal) formation takes place in two
steps where the intermediate and unstable hemiacetal reacts further with hydroxyl groups in
wood (Figure 6). Formation of reactive functional groups, carboxylic and keto-groups, is
described in section 1.2.
Though the composition of impregnate (oxidized glucose) is complex (section 1.2), it is
hoped that some bifunctional group formed during oxidation can be active to make
cross-linking inside the cell wall to improve the dimensional stability of the wood.
Esterification
Sugar-COOH
+
HO-WOOD

Sugar-CO-O-WOOD
+
H2 O
Acetal formation
Sugar-CHO + HO-WOOD  Sugar-C(OH)-O-WOOD (hemiacetal)
Sugar-C(OH)-O-WOOD+HO-WOOD  WOOD-O-CH(sugar)-O-WOOD + H2O
Figure 6.
Probable pathway of reactions scheme between oxidized sugar and wood
In an earlier experiment of author’s pre-study trial, oxidized glucose was used to improve
the dimensional stability of solid aspen [29]. Aspen had been chosen as it is widely spread over
the globe and coefficient of shrinking anisotrophy is relatively high [30]. Water retention was
over 100% when small cubic aspen sample was soaked in water under reduced pressure,
which indicated a successful impregnation of impregnate into small cubic aspen sample.
Results indicated that Fenton oxidized glucose might improve the dimensional stability of
solid aspen. But a large amount of impregnate was observed soaking out when soaking the
treated aspen in water. We concluded that processing parameters which was adopted needed to
be studied more in detail. It is also of interest to find evidence whether cross-linking effect
may be formed by the reaction between wood and oxidized glucose leading to high
14
anti-swelling efficiency (ASE) after long time water soaking test.
In this study, the focus is to optimize the dimensional stability of solid aspen by adjusting
the processing parameters like glucose and H 2O 2 molar ratio, curing temperature, curing time,
and pH of impregnate solution. We wanted also to testify our hypothesis that by using glucose
oxidized by Fenton´s reagent it is possible to reduce accessibility of hydroxyl groups in
hemicelluloses by achieving permanent bulking in the cell wall by performing repeated
soaking treatments without intermittent drying. By using modified ASE also study extent of
formation cross linking between the wood components in the cell wall. Furthermore also
study whether stabilization was resistant to water soaking
Due to the limitation of time, the mechanical properties and weathering experiment will not
be studied in this project but may be further studied in other project.
15
2. MATERIALS AND METHODS
2.1. Materials
Aspen (Populus tremuloides) sapwood. Anala R@ glucose(C 6H12O6) from VMR. 99% purity,
magnesium chloride hexahydrate( MgCl2*6H2O) from Merck KGaA, Germany, 99% purity,
ferrous sulphate heptahydrate（ FeSO4 x 7 H2O）from VWR, 50% stabilized hydrogen peroxide
(H2O2) from AKZO NOBEL were used.
2.2 The importance of different impregnation processing parameters
2.2.1 Experiment design (screening study)
The aim of this study was to investigate the effect of various processing parameters on how
they influenced the dimensional stability of aspen. The process parameters investigated were
molar ratio of glucose and hydrogen peroxide, pH of the impregnate, curing temperature,
curing time, air drying or not. To get an initial indication of the effect of the chosen parameter,
it was decided to use statistical design and to perform a screening study on the system to study
both the main effects and the interaction effects. A D-optimal design was used with 5
controlled variables. The 3-Quantitative controlled variables and the value of each level are
shown in table 1, whereas 2-Qualitative controlled variables and constant variables are shown
in table 2. The confidence level is 0.95 with the explained variance R 2 and predicted variance
Q2. The experiment requirements were imported to MODDE software (Umetric AB, Sweden,
2008)[31] to investigate the inner-relation of the experimental factor and each parameter.
Table 1. Quantitative factors and their design level
Level
Molar ratio
Curing
Curing time
between
temperature
(h.)
o
H2O2 and
( C)
glucose
(-)
2:1
100
48
0
4:1
120
96
(+)
6:1
140
144
Table 2. Qualitative and constant factors
Factors
Concentration of
Constant factors
1 mol/L (M)
impregnate solution
16
Qualitative
Qualitative
factors(No)
factors(Yes)
/
/
Moisture content
0%
/
/
Air drying before
/
Without air drying
With air drying
/
Without addition of
With addition of
diluted NaOH
diluted NaOH
curing
pH adjustment
The design above included 19 experiments. 3 experiments of samples (See No.20-22, Table
3) treated only by Fenton reagent without glucose reactant in the impregnate solution were
also added into experiment design to investigate the effect of oxidized glucose (comparing
with 3 center points experiments (See No.16-19, Table 3)).
Table 3. Sheet of designed experiments
2.2.2 Impregnation and heat treatment
Reagent: The reactant solution contains 2.4g glucose, 200mg MgCl 2 x6H 2O and 100mg
FeSO4 x 7 H2O per 100ml water solution. Three different impregnates were prepared which
molar ratio between glucose and hydrogen peroxide was controlled to 1:2, 1:4, 1:6. 50 %
H2O2 was added into solution slowly due to that Fenton reaction is known to be a vigorous
reaction. The color gradually changed from green to reddish brown. The solutions were put
into hood for water evaporation at room-temperature for several days until the impregnate
solutions reached 1M concentration of the reagent. Diluted water-solution of NaOH was
added to half volume of the impregnate solution to adjust the pH to 3.5. Totally six different
types of 100 ml impregnate solutions were prepared. The solutions were put into capped
plastic flasks and stored in refrigerator until used.
Wood specimen, 10*20*10mm3 (L*T*R), were dried in the oven at 103 °C for 48 h. before
impregnation and totally 22 wood samples were used in this pre-study. The wood specimen
17
were submersed into the solution under vacuum (50 mbar, 120 min.), and then stayed in the
solution for 2 days until the samples weight was stable.
After impregnation treatment, samples were taken from impregnate solution for drying and
curing. Two different curing conditions were used, one was directly oven drying without air
drying and one was air-drying to slowly remove excess of water for 4 days before oven heat
curing was performed. The samples were put into oven for curing according to the experiment
design at 103 °C, 120 °C and 140 °C for 48 h., 96 h., 144 h., separately.
2.2.3 Swelling properties after repeated leaching with water
In order to check the leachability of the chemicals and the durability of dimensional
stability, a nonleachable test was applied. The test consisted of determining the oven dry
volume of the test specimen, then submerging the specimen in tap water and removing
entrapped air in wood using vacuum desiccator. Vacuum was continued for 30 minutes,
released and left for 1 hour, reapplied for 30 minutes and then released and sample was left
for 24 hours at room temperature. The volumetric swelling was calculated and Anti swelling
efficiency (ASE) (see section 2.5.2) was studied after soaking in 100ml water. pH of the
soaking water was measured and then tap water was added to change the soaking liquid every
day for totally 7 days. ASE was measured before each change of water.
2.3 Interaction effects of pH and temperature on dimensional stability of
impregnated wood
2.3.1 Impregnation and heat treatments
After the multivariate analysis of the design experiment (section 2.2) the processing
parameters which gave the highest ASE for treated wood were chosen for studies on
interaction of pH and temperature on dimensional stability by impregnation of large size of
wood samples (Appendix I). A molar ratio between glucose and H 2O2 of 1:6, air drying
before curing and curing at 140 °C without addition of NaOH were chosen. Fenton reagent
without addition of glucose was also used as impregnate solution to study the influence of
oxidized glucose. In this experiment, the pH of the oxidized glucose solution was varied to 4
different levels, pH 1.4, 2.1, 3.0 and 4.0, by addition of 5 M NaOH solution to investigate the
importance of pH during the treatment. The Fenton reagent was also controlled to pH 2.1, pH
3.0 and pH 4.0 in a similar way. In all, 7 different impregnate solutions were used to
impregnate the wood samples.
Impregnation specimen size: 10*20*190mm3 (R*T*L) and the wood specimen were dried
in the oven at 103 °C for 48 h. before impregnation. Totally 27 pieces of sample were divided
into 9 groups. Each group consisted of 3 samples. The scheme of experiments is shown in
Table 4.
18
Table 4. Experiment groups with different impregnate solutions, pH and with or without
heat treatment.
Sample Groups
Impregnate
Impregnate pH
Heat treatment
st
None
None
None
nd
2 group
None
None
140oC
3rd group
Oxidized glucose
1.4
140oC
4th group
Fenton reagent
2.1
140oC
5th group
Oxidized glucose
2.1
140oC
6th group
Fenton reagent
3.0
140oC
7th group
Oxidized glucose
3.0
140oC
8th group
Fenton reagent
4.0
140oC
9th group
Oxidized glucose
4.0
140oC
1 group
The wood specimens were submersed into the respective chemical reagent solution under
vacuum (50 mbar, 120 min.), vacuum was released and specimens were left staying in
solution for several days until the samples weight became stable.
Curing conditions: Samples were air-dried to remove excess of water for 4 days before
heating and curing in oven at 140 °C for 48 h.
The samples after treatment were cut into different sizes for soaking test. Dimensions of
one sample was 10*10*180mm3(R*T*L) and of another it was 10*20*10mm3 (R*T*L) which
was the same dimensions as used in section.2.2 and was cut from one end of longer test
sample.
2.3.2 Swelling properties after repeated leaching with water
The nonleachable test was done as in the screening test (section 2.2.3). The pH of soaking
water and swelling volume were recorded every day till 7 days. The first day soaking solution
was collected and analyzed with HPLC to investigate the possible existence of furfural
(section 2.5.1). ASE was calculated to characterize the dimensional stability of wood sample
(section 2.5.2).
The dimensions of the specimens were determined by immersing the wood samples in
water and adopt the Archimedean principle.
2.4 Effect of oxidized carbohydrates on dimensional stability of
impregnated wood
2.4.1 Impregnation and heat treatment
Sample size were 10*20*10mm3 (R*T*L). Totally 20 pieces of sample were divided into 4
19
groups with 5 specimens in each group (control group, treated with reagent I, reagent II,
reagent III). The wood specimens were dried in the oven at 103 °C, 48 h. before
impregnation.
Solutions for impregnation:
There were 3 types of reagent solutions in this trial as shown in table 5.
Table 5. Chemicals and proportion of chemicals used in impregnate solutions.
Reagent
Sugar
and MgCl2 x6H2O (g)
amounts
FeSO4 x 7 H2O 50%
(g)
(ml)
I
/
1.5
0.75
12
II
18g Glucose
1.5
0.75
12
III
34.4g Sucrose
1.5
0.75
12
H 2O 2
For preparation of reagent, 12 ml hydrogen peroxide (50%) was diluted with water to 36 ml
before adding this solution into a water solution where all other chemicals had been
previously dissolved (Table 5). H2O2 was added dropwise with magnetic stirring using a
burette to get a more smooth reaction.
The wood specimens were submersed into prepared chemical reagent solution as described
above (section 2.2.2).
Curing conditions: Samples were air-dried for 4 days, before curing in oven, to remove
excess of water without severe migration of impregnating chemicals to wood surface. The
samples were heated at 103 °C for 48 h.
2.4.2 Swelling properties after repeated leaching with water
The non-leachable test presented above (section 2.3.2) was also performed after 7 days in
this experiment. pH of soaking water and volume of swelling sample was measured every day.
The solution from first day soaking was collected to investigate the existence of furfural in
wood sample treated under different conditions using HPLC (section 2.5.1). After 7 days
soaking in water, the samples was air dried for 4 days, then put into oven drying at 60 oC for
several days until the MC is close to 0%, the bulking effect (BC) and WPG were calculated by
comparing the volume and weight before impregnation and after 7 days of water soaking. The
non-leachable test was repeated again for 1 day to measure the volumetric swelling of sample
for calculating the Anti-Swelling efficiency (ASE) after 7 days soaking.
2.5 Analysis
2.5.1 HPLC analysis
High performance liquid chromatography (HPLC) was used to investigate the extent of
20
oxidization of glucose and the generation of furfural. The principle of the HPLC is shown in
figure 7.
Figure 7. HPLC working principle
Figure 8. Different carbohydrates retention time and peak height in High performance
liquid chromatography.
A sample is injected into a loop with specified volume which is then suddenly filled with a
continuous flow of mobile phase or eluent under certain pressure created by a HPLC pump.
The stream of mobile phase will then bring the sample to go through a densely packed column.
Due to different interactions between compounds and the stationary phases in the column,
compounds weakly bonded to the phase will be eluted first out of the column and others that
are bonded more strongly to the stationary phase will come out later. Detection of individual
compounds coming out from the column are performed through different type of detectors
that will be related to physical properties of compounds such as refractive index or
UV-absorption at various wave lengths. The absorption will be transferred into electric signal
and show up as peaks at different retention time in chromatography (Figure 8). The extent of
absorption is related to the concentration of the compound so the area under the peak of
21
absorption indicates the proportional relation with compound concentration (Figure 8). By
comparison of the peak area of for example glucose in the spectrum with a reference solution
consisting of pure glucose with known concentration, the concentration can be recognized and
calculated (Figure 8).
In this experiment, HPLC was used to estimate the extent of oxidization of glucose
during oxidation treatments as well as formation of furfurals after curing. They were analyzed
according to the affinity of glucose and furfural compounds towards stationary and mobile
phase in the column. A Water Hi-plex Pb-column (8μm and 250*7.7mm) was used for this
purpose. The column was heated in an oven temperature at 60 °C to reduce viscosity of eluent
and keep the pressure under limitation of column (50bar) to protect the separating gel inside
of column from degradation. For this reason a higher eluent flow than 0.3 mL/min could not
be used. Before analysis the solutions of treated wood from first day of soaking test were
collected and filtered (50μm). The analyzing time was 35 min. 5-hydroxymethylfurfural
(HMF) and furfural were detected with UV detector operating at 280nm around, and glucose
was detected with a RI-detector eluting around 6.5 min in chromatogram as shown below
(Figure 9). Sugars do not have UV absorption, so RI detector had been chosen as detector.
2.5.2 Dimensional stability and weight percentage gain
The weight percent gain (WPG) after treatment was measured by comparison of oven-dry
weights. The weight and volume of the sample before impregnation and after curing were
recorded.
WPG(%)=(m1-m0)/m0
(2.1)
m1 is the oven dry weight of the sample after curing
m0 is the oven dry weight of the sample before impregnation
The cell wall bulking (BC) is often taken place in chemical modification of wood if
penetration of wood cell wall occurs. It is defined by the equation below where the increase of
volume of oven dried test piece after modification is measured.
BC(%)=100*(Vt-V u)/V u
(2.2)
Vt is oven-dried volume of sample after curing treatment
Vu is oven-dried volume of sample before impregnate treatment
The swelling coefficient (S) is described as below excluding the cell wall bulking of treated
samples after impregnation
22
S(%)=100*(V w-Vd)/Vd
(2.3)
Vw is the volume of the sample swelled in water
Vd is the volume of the sample when it is oven dried after impregnation (excluding BC)
The stabilization effect is characterized as anti swelling efficiency (ASE) with the equation
below[2]:
ASE(%)=100*(SR-ST)/SR
(2.4)
SR is the swelling of the control sample submerse into water before impregnation treatment and
curing treatment.
ST is the swelling of the impregnated sample after impregnation treatment and curing treatment.
Water repellent effectiveness(WRE) is estimated based on water absorption(WA) under water soaking test:
WA(%)= 100*( m2-m1)/ m1
(2.5)
m1 is the oven dry weight of the sample after curing
m2 is the wet weight of the sample after 7 days soaking in water
WRE(%)=100*（Wc-Wt）/ Wc
(2.6)
Wc is the water absorption of the control sample
Wt is the water absorption of the impregnated sample.
The dimensions of the specimens were determined by immersing the wood samples in
water and measure the weight when the samples were immersed and suspended in water. The
principle is the Archimedean principle, the volume of wood sample is the same as the sample
mass because the water density is 1.00 g/cm3 [26].
2.5.3 Scanning electron microscope(SEM)
A Jeol, JSM-5200 was used for the SEM examination .The specimens were oven dried,
sputter coated with a gold layer. The accelerating voltage was 30kV.
The treated samples (See methods in section 2.4) were scanned with the microscope to
check the cell wall thickness and cross-section morphology. 10 pictures of each sample were
then used to determine if cell wall bulking had existed after 7 days water soaking test. In each
picture 6 different positions were chosen to measure the two cell wall thickness in tangential
direction due to the irregular shape of the cells.
23
3. RESULTS AND DISCUSSION
3.1 Screening Experiment
3.1.1 Studies on importance of process parameters
In an earlier report[29], which was mentioned in Section 1.3, the influence of impregnate
concentration for improving dimensional stability of wood was studied. The experiments were
performed by impregnation with various concentrations of oxidized glucose solution, dried
and heat treated. The results indicated that oxidized glucose could achieve fairly stable and
high extent of dimensional stability when solid aspen was treated with oxidized glucose
impregnate at a concentration that was larger than 0.5M. However, a lot of impregnate had
been leached out during water soaking and oven drying cycles which means that an extensive
cross-linking of the impregnate did not take place during the curing step. Due to the small
scale of sample amount and the many processing parameters the modification process could
involve, we lacked the understanding of which processing parameters were the most
important factors contributing to high dimensional stability. So screening test was performed
to study influence of individual parameters including curing temperature, time, molar ratio
between glucose and H2 O2, pH of the impregnate and air drying or not (Table 3) in the
impregnation process. In the screening study impregnate with 1M concentration was adopted
(see above).
The results, after importing all the data into MODDE software (Appendix I), are presented
in Table 6. By auto fitting the model, R2 which showed the explanation of this model was
about 0.88 while the Q2 value which showed the prediction of model was 0.26. The Q 2 value
was very low which suggested that it was a poor model, so it was necessary to refine the
model. Low reproducibility, which indicates high noise level, could be one reason causing
low Q2. As wood is a biomaterial, its properties differs from each other by a lot of factors like
the growth and even the position on the boards. So the variability of sample properties might
cause poor control of experiment set up that resulted high noise level. Insignificant terms in
the model could also lead to low Q 2.
Table 6. Summary of the fit of model.
X-axis
R2
Q2
ASE 0,881185 0,255534
Model Validity Reproducibility
0,878769
0,510355
Coefficient plot (Appendix II) is displayed below to check the insignificant terms and
remove the noises. The plot is based on the Factorial and Plackett Burmann designs, where the
coefficients are half the size of the effects[31]. So in coefficient plot, it expresses the change in
the response when the factors vary from the low to the high level. It can be observed that two
24
main factors pH, curing temperature seemed to be important variables for the model while other
controlled main factors such as air-drying and the molar ratio between glucose and H 2O2 which
was initially thought to be important seemed to have not so strong influence on the model
(Appendix II).
After we excluded other effects in the model and only kept pH of impregnate as well as
curing temperature as the two factors, the model Q 2 value was increased to 0.47 (Table 7). In
MODDE when R2 value is above 0.5, it can be considered to be good at explanation of the
model. R2 value was 0.60 in this case which indicated that new model was still good in
explaining the dimensional stability property. Q 2 is an underestimated measure of the
goodness of fit of the model
[31]
. The model is not as weak as before as Q2 increased from
0.26 to 0.51.
Table 7.
Summary of the fit of refined model with two main factors: curing temperature and
pH of impregnate
X-axis
R2
Q2
Model Validity Reproducibility
ASE 0,595559 0,506717
0,879458
0,510355
The study on the relationship between these two effects (pH of impregnate and curing
temperature) and ASE is showed below (Table 8). The effect plot displays the change in the
response when a factor varies from its low level to its high level when all other factors are
kept at their averages. Impregnate solution pH at low pH without adding diluted
water-solution of NaOH and curing temperature played very important role contributing to
dimensional stability of impregnated wood since the effect values were fairly high (Table 8).
Table 8.
The significance of effects (pH and curing temperature) according to the effects
plot.
ASE
Effect Conf. int(±)
pH(Without NaOH) 31,2749
14,1218
Temp 19,3937
15,9316
3.1.2 Influence of oxidation of glucose
Glucose was found at ca 6.5 min in the chromatogram from analysis with HPLC (Figure
9). By comparison of area under glucose peak with the glucose calibration curve where
concentration of pure glucose is plotted against the corresponding area under the peak the
concentration of glucose in the water solutions from leaching test was estimated. As could be
seen absorption from other compounds than glucose was large in the range from 6 to 9 min
25
and overlapped with the glucose peak (Figure 9). This means that only a rough estimation of
glucose content in the soaking water could be done.
Figure 9. The HPLC chromatogram of oxidized glucose impregnate, here the molar ratio of
glucose and H2O2 reagent was 1:2.
However, results from HPLC analysis still displayed that the extent of glucose oxidation
were high and that glucose had been well oxidized into other products (Table 9). In spite of
overlap with other compounds, with a higher ratio between hydrogen peroxide and glucose
the extent of oxidation seemed to have been increased (Table 9).
Table 9. HPLC analysis of glucose oxidation extent and pH of impregnate
Molar ratio
Extent of
pH of impregnate
glucose
without addition of
oxidation
diluted NaOH
glucose/H2O2=1:2
93.4%
1.51
glucose/H2O2=1:4
98.7%
1.28
glucose/H2O2=1:6
99.5%
1.43
-
2.71
0%
-
Fenton reagent Without glucose
Glucose
In the screening test, it was interesting to find that the molar ratio between glucose and
H2O2 was not a key factor (section 3.1.1). Furthermore, at a curing temperature of 120 oC and
curing time of 96 h. without preceding air drying including pH conditioning, a fairly good
ASE which was nearly 44 11% (See Appendix I, No.20-22) after 7 days of water soaking
was obtained for the impregnate solution, containing Fenton reagent without glucose. While
26
wood treated with oxidized glucose impregnation with the same processing conditions above
had ASE around 42% 19 (See Appendix I, No.17-19). The effect of oxidized glucose under
those conditions was questioned by similar ASE that was obtained without glucose. Under
those conditions the effect of impregnate on ASE might mostly be due to the acid catalyzed
degradation of wood components and not bulking of oxidized glucose. So the impregnate pH
effect was studied further below.
3.1.3 Impregnate pH effect
It could also be seen that a lower pH of impregnate solution was obtained in presence of
glucose (Table 9). This could be due to possible oxidized products for example gluconic acid
and formic acid from glucose reaction with Fenton reagent which could give more protons to
impregnate solution.
pH of impregnate was found to be the most important factor (see section 3.1.1) affecting
the dimensional stability of treated wood (Table 8). Treatment of wood with impregnate
solution at lower pH gave a higher ASE compared to treatment with impregnate adjusted to a
higher pH (in which NaOH has been added) (Table 10). More acidic environment was
preferred in improving the ASE. It might be due to some possible products from oxidized
glucose that reacted with hemicelluloses and acceleration of the degradation of hemicelluloses
rather than creating cross-links with wood cell wall.
Table 10. Influence of pH effect on anti-swelling efficiency (ASE) from effect plot.
Label (X)
ASE Confidence interval
WithNaOH
17,8601
11,2653
Without NaOH 49,1349
8,51578
3.1.4 Interaction effect between impregnate pH and curing temperature
Results above indicate that conditions during curing could be of high importance. Since the
complexity of the curing temperature, the interaction effect between impregnate pH and
curing temperature was added in the model expected for better prediction of ASE. But Q2
value decreased from 0.51 (Table 7) to 0.45 (Table 11) with adding the new interaction effect.
Table 11. Summary of the fit of refined model with three factors including curing
temperature, pH of impregnate and their interaction effect
X-axis
R2
Q2
ASE 0,600134 0,445124
Model Validity Reproducibility
0,867813
0,510355
Results from the effects plot (Table 12) showed that pH of impregnate and curing temperature
27
were still the most important effects and the interaction effect between them was also a
significant factor since the effect coefficient value is over 1.
Table 12.
The effects overview of refined model with three factors including curing
temperature, pH of impregnate and their interaction
ASE
Effect Conf. int(±)
pH(Without NaOH) 31,5364
14,4808
Temp 17,5995
16,3365
Temp*pH(Without NaOH) -4,12712
16,3365
From the interaction effect plot (Figure 10a), it was obvious that the model had highest
ASE value when higher temperature and treatment without pH conditioning were used. But
also data showed that when the condition was adjusted to low temperature (103oC) without
addition of diluted NaOH, higher ASE than the situation using higher curing temperature
(140oC) and addition of diluted NaOH (to pH 3.5) was obtained (Figure 10b). It could be
concluded that, the interaction effect between pH of impregnate and temperature cannot be
neglected in further ASE improvement. This might suggest different mechanisms were taken
place for improving the dimensional stability of wood during impregnation process. The ASE
achieved in higher temperature and pH condition might be more likely due to the heat
degradation of wood component while lower temperature and pH of impregnate might
provide more suitable condition for the activity of oxidized glucose, contributing to possible
chemical bonding reaction between oxidized glucose and cell walls inside of wood.
Investigation: ase (PLS, comp.=2)
Interaction Plot for Temp*PH, resp. ASE
PH (WithNaOH)
PH (Without NaOH)
PH (Without NaOH)
50
ASE
40
PH (Without NaOH)
30
PH (WithNaOH)
20
10
PH (WithNaOH)
98
100 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 134 136 138 140 142
Temperature
N=22
DF=18
R2=0,600
Q2=0,445
RSD=15,55
MODDE 9 - 2012-05-09 14:33:04 (UTC+1)
(a)
28
Investigation: ase (PLS, comp.=2)
Interaction Plot for Temp*PH, resp. ASE
Temp (low )
Temp (high)
Temp (high)
50
Temp (low )
ASE
40
30
Temp (high)
20
10
Temp (low )
WithNaOH
Without NaOH
PH of Impregnation
N=22
DF=18
R2=0,600
Q2=0,445
RSD=15,55
MODDE 9 - 2012-05-09 14:33:26 (UTC+1)
(b)
Figure 10. The impregnate pH and curing temperature interaction effect plot
3.1.5 Weight percentage gain (WPG) and bulking effect (BC) related to ASE
It was difficult to find correlation between WPG, BC and ASE in the screening experiments
after water soaking for one day (Figure 11). It indicated that WPG and BC were not the
suitable parameters to describe the wood dimensional stability which contradicts to our
previous study that with increasing WPG, a larger ASE was obtained (section 1.3). It might
also suggest that more than one mechanism contributing to dimensional stability of wood
existed in the modification process.
29
Figure 11. WPG (upper part) and BC (lower part) after impregnation against ASE after first water soaking
day.
3.2 pH, temperature and their interaction with dimensional stability of
impregnated wood
As shown above it was uncertain whether it could be concluded that oxidized glucose
contributed to strong bonding inside of wood causing high ASE. This together with the
complexity of many processing parameters in screening test lead to another series of tests.
The study emphasized on the mechanism of Fenton reagent effect, pH and curing temperature
as well as their interaction effects as described in section 2.3.1. Wood samples which had
larger dimensions (section 2.3.1) compared with previous test (section 3.1) were used for
impregnation.
3.2.1 Water Repellency
In Figure 12 the water repellent effectiveness (WREs) of impregnated wood samples could
be seen (see section 2.5.2). Comparing with untreated samples, all of these impregnated wood
samples have improved water repellent property especially sample impregnated with oxidized
glucose without addition of diluted NaOH had the most significant WREs. The increments of
average WREs in oxidized glucose impregnated sample were observed higher than Fenton’s
reagent treated one when the impregnate pH were in the same level (Figure 12). The extent of
cell wall bulking no matter whether impregnate was bonded or not bonded inside of cell wall
could be a reason causing different water uptake among oxidized glucose and Fenton’s
reagent impregnated sample before 7 days water soaking. Also another possible reason might
30
be that hydroxyl groups in the cell walls react with reactive compounds from oxidized glucose
and form cross link between cellulosic fibers while the improvement of hydrophobic property
in Fenton reagent impregnated wood might be only due to its degradation of carbohydrates
and hemicelluloses inside of wood compounds.
Figure 12. Water repellence effectiveness(WRE) of small size imprengated wood in water for 7 days.
Control 140 °C was the 2nd group in table 4, oxidized glucose was the 3rd group in table 4, Fenton’s
reagent was the 4th group in table 4, oxidized glucose at pH 2.1 was the 5th group in table 4, Fenton’s
reagent at pH 3 was the 6th group in table 4, oxidized glucose pH 3 was the 7th group in table 4, Fenton’s
reagent 4 was the 8th group in table 4 and oxidized gluocse at pH 4 was the 9th group in table4, see section
2.3.1.
The figure data indicated that the WRE seemed to be the highest at pH 1.4 (3 rd group in
table 4) in impregnate treated wood. Due to the large variance of every group, whether WRE
from oxidized glucose treated wood were sensitive to pH of impregnate solution or not is
uncertain. T-test was used to describe the significance of the difference between different
group of oxidized glucose impregnated wood. The probability of its WRE was higher than
other samples with higher pH of oxidized glucose was shown below (Table 13). They did not
reach the statistically significant level at 95% probability.
Table 13. P-value from T-test comparing the ASE of wood samples impregnated with pH
1.4 of oxidized glucose and pH2.1, pH3, pH4 oxidized glucose, respectively. (The criteria of
significant is that p-value is less than significance level 0.05)
pH of oxidized
Impregnate pH
glucose
31
p-value
1.4
2.1
0.207918
1.4
3.0
0.301628
1.4
3.0
0.288381
3.2.2 Dimensional stability
As it observed from Figure 13, data from measurements of pH of the soaking water was
collected from every day and was increasing with the days soaking into water till it reached
neutral state, except for impregnate with no alkali addition which was still mildly acidic
(Figure 13). pH changes of soaking water and the HPLC analysis of the first day soaking
water (Section 3.2.3) suggested some impregnate or wood components had been extracted out
by water soaking. Large amount of impregnate were still not bonded inside of cell wall. Due
to lack of consideration about the volume change caused by removal non-bonded material
during soaking, the ASE values except the first day shown(Appendix III)could not describe
the real performance of the impregnated wood dimensional stability. The increasing pH of
soaking water until neutral state suggested all the non-bonded impregnate had been washed
out. So ASE might decrease with the increasing of soaking water pH until. So we only took
the first day ASE into consideration later. However, a larger ASE observed for modification
with oxidized glucose in the first day soaking than with only Fenton’s reagent indicated that
some chemical reaction had been taken place inside of wood and contributed to different level
of dimensional stability of wood rather than simple physical bulking.
(a)
32
(b)
Figure 13. (a) pH of soaking water from small size impregnated wood during the nonleachable soaking
test (b) pH of soaking water from large size impregnated wood during the nonleachable soaking test.
Control 140 °C was the 2nd group in table 4, oxidized glucose was the 3rd group in table 4, Fenton’s
reagent was the 4th group in table 4, oxidized glucose at pH 2.1 was the 5th group in table 4, Fenton’s
reagent at pH 3 was the 6th group in table 4, oxidized glucose pH 3 was the 7th group in table 4, Fenton’s
reagent 4 was the 8th group in table 4 and oxidized gluocse at pH 4 was the 9th group in table4, see section
2.3.1.
The result of dimensional stability of wood after the first days soaking test was showed in
Table 14 in terms of Anti Swelling Efficiency (ASE) and Volumetric swelling coefficient(S).
Larger dimensions of treated wood had higher ASE compared to ones with smaller
dimensions (Table 14). It might be due to that the non-bonded impregnate inside of smaller
dimensions of treated wood was easier to wash away. It was shown that the wood samples
without impregnate but treated at the same temperature as performed with other modified
wood samples at 140 oC gave around 10% ASE. Sugars and hemicelluloses might start to
degrade probably involving dehydration which reduces the hydroxyl groups that coordinates
water molecules resulting in an increase of ASE. From table 14, it is clear that oxidized
glucose impregnate with lower pH gave better ASE with high curing temperature (140 oC)
than other impregnate. Furthermore, with milder acid impregnate (to which diluted NaOH
was added), the ASE of small dimensions of samples were lowered and even below 0 at pH
2.1 and pH 3. The results suggested that these impregnation processes when the curing
temperature was 140oC could not improve the dimensional stability of wood if the impregnate
pH was above 2.1.
33
Table 14. Dimensional stability of wood samples treated with different impregnation
process after first day water soaking.
Sample Impregnate Impregnate Curing
Group
pH
Tempe
Sample
size
rature
1st
group
/
/
103oC
Small
Large
Volumetric
Anti Swelling
Swelling
efficiency
coefficient(
(ASE%)
S%)
17.71
5.73
16.25
3.82
2nd
/
/
140 oC
Small
group
16.01
0.52
Large
14.79
0.13
3rd
group
4th
group
Oxidized
glucose
140 oC
reagent
140 oC
glucose
9.93
0.14
43.91
Large
8.06
3.17
50.42
Small
15.38
1.67
Large
11.38
Small
18.63
2.83
Large
6th
140 oC
Fenton
reagent
11.98
17.84
Large
10.27
2.42
Small
17.70
2.56
3.0
Large
18.37
4.69
8th
Fenton
group
reagent
29.97
-5.24
26.31
-0.78
3.0
140 oC
Oxidized
glucose
Small
2.58
7th
group
13.16
2.1
1.07
group
8.93
2.1
5th
Oxidized
9.57
Small
2.26
group
/
1.4
140 oC
Fenton
/
4.0
140 oC
Small
14.57
2.03
34
36.77
-13.07
-3.61
17.74
9th
group
140 oC
Large
13.48
0.22
Small
15.70
0.82
Oxidized
4.0
glucose
Large
20.40
1.89
17.07
17.70
-25.53
Comparing dimensional stability of small dimensions wood impregnated with Fenton’s
reagent and oxidized glucose and cured at 140 oC, at same levels pH of impregnate
(pH2.1,pH3,pH4), showed that they gave a similar low ASE (Table 14). pH seemed to play a
dominant effect contributing to the swelling properties for impregnated wood rather than
impregnate including oxidized glucose or not. Since heating at high temperature (140 oC)
without impregnate could lead to ASE around 10% (Table 13), temperature and it’s interaction
with pH could also be significant effects rather than oxidized glucose chemical bonding
effect.
It was shown that small size wood, which was impregnated with oxidized glucose without
addition of diluted NaOH, resulted in ASE of around 50% (Table 14). However due to high
temperature and too acidic conditions, it is difficult to estimate how significant the influence
of thermal modification on ASE of treated wood was. The effect of glucose, which we
assumed would lead to chemical bonding with wood components such as hemicelluloses
generated in the process and leading to lower swelling, might not be possible to interpret from
table 13 and it might not be the mechanism causing high ASE in this test.
3.2.3 HPLC analysis
Furfural could be formed via the dehydration of pentose like xylose whereas
5-(hydroxymethyl)furfural (HMF) is formed from hexoses as glucose (Fig. 4 in section 1.1).
The treated glucose impregnate, in which glucose and H 2O2 molar ratio was 1:6, had oxidized
to a high extent and no more glucose was left to degrade and dehydrate (Fig. 4). The HPLC
chromatogram
showed
that
a
considerable
amount
of
furfural
compared
with
5-Hydroxymethylfurfural (HMF) existed in first day of soaking with water both for wood
impregnated with Fenton’s reagent and oxidized glucose (Table 15). As xylan is dominating
hemicellulose in aspen, this implies that hemicelluloses were degraded severely when pH was
low during heat treatment at 140 oC. In table 15, we calculated the degradation of
hemicelluloses based on the assumptions that for every xylose unit one furfural was formed
and that content of xylan In Aspen was 16% of the total mass. The extent of hemicelluloses
degradation values of the sample impregnated with Fenton reagent (Table 15) indicated that
strong acid impregnate (pH2.1) induced more degradation of hemicelluloses at 140oC than
mild acid impregnate(pH3 and pH4). However, different degree of hemicelluloses degradation
35
looked like not correlated with the WPG of the samples impregnated by Fenton reagent
(Table 15). It was difficult to find the relation between estimated hemicelluloses degradation
and sample mass loss because the amount of impregnate stayed inside of wood samples is
unknown.
Thus, the degradation of wood component, which remove large amount of active hydroxyl
group leaded to high ASE rather than formation of chemical bond between the hydroxyl
groups in wood components and products from glucose oxidation. The mechanism of
improving dimensional stability of wood seemed to be more similar to thermal modification
of wood by degradation of hemicelluloses. Also by the fact that furfural can solidify into
thermosetting resin in the presence of acid, there were still other possible effects influencing
wood dimensional stability.
Table 15 5-Hydroxymethylfurfural (HMF) and furfural in 1 st day soaking liquid. The
soaking liquid for one sample group were 100 ml; each group has 3 pieces of treated sample.
Sample
Group
Impregnate Impregnate
HMF
Furfural
Extent of WPG(%)
pH
amount per amount per hemicellulo of wood
No.
1ml
1ml
soaking
soaking
ses
degradatio impregnate
solution(m solution(m n in Fenton
g)
1
Oxidized
glucose
g
sample’s
d with
reagent
Fenton
treated
wood(%)
reagent
2.1
0.038
5.205
/
/
2.1
1.849
8.176
32.63
-4.69
3
0.024
2.915
/
/
3
1.899
6.170
17.15
-5.14
4
0.763
6.612
17.90
-4.35
Fenton
2
Reagent
Oxidized
3
glucose
Fenton
4
Reagent
Fenton
5
Reagent
36
3.3 The effect of oxidized carbohydrates on dimension stability of
impregnated wood
Due to high influence of pH and little effect of oxidized glucose contributing to the
dimension stability in the previous experiment, an additional study was added to prove
whether there were some chemical bonding formed to reduce wood swelling from
carbohydrates oxidation compounds. It was suggested from screening test (section 3.1.4) that
different mechanisms were taken place that improved the dimensional stability of wood when
curing temperature and pH was varied. Temperature was reconsidered and seemed to be too
high in the previous study.
In tests performed in this section (3.3), all samples were
controlled to milder temperature condition (see section 1.4.1).
3.3.1 Water Repellency
The water repellent effectiveness was measured for samples treated with or without
saccharides (glucose and sucrose) (Figure 14). Oxidized sucrose impregnated sample gave
highest WRE which was over 50%. Oxidized glucose impregnated sample also gave
considerable increase in WRE to inhibit water uptake. But treatment with Fenton’s reagent
here only gave about 5% WRE. This is much smaller when comparing with treatment at
140°C (Figure 12). Temperature (103oC) might not induce hemicelluloses degradation to
considerable extent. Thus, it indicated that lower temperature give mild condition to let
impregnate form cross-linking inside of wood to dominate.
Figure 14. Water repellence effectiveness (WRE) of imprengated wood in water for 7 days.
Table 16 indicated that the higher Weight Percentage Gain (WPG) of impregnated sample
after impregnation, the higher WRE were the impregnated sample before water soaking. The
WPG seems to influence the water repellency of wood impregnated with oxidized
carbohydrates a lot.
The reason of impregnate that leads to WRE is complex, physical bulking, hemicelluloses
37
degradation and cross-linking formed by oxidized glucose impregnate reaction inside of wood
could be mechanisms for improving its water repellent that exist under different impregnation
process conditions from the result in section 3.2.1(Figure 12) and the discussion
above(Figure 14) .
Table 16. Dimensional stability of wood samples treated with different impregnate under
103oC curing temperature in the first day of water soaking
Sample
Impregnate Impregnate Impregna
(control)
2nd group
3rd group
Cell Wall
Anti
te pH
Percentage bulking(BC Volumetric
Swelling
after pH Gain(WPG
%)
Swelling
efficienc
adjustment adjustme %) after
coefficient(
y
nt
impregnati
S%)
(ASE%)
on
Group
1st group
Weight
pH
pH
before
13.60
/
/
/
/
/
Oxidized
glucose
1.81
2.41
24.03
0.39
10.85
1.95
1.76
2.41
2.41
2.41
Oxidized
sucrose
57.71
1.99
8.56
0.51
2.72 7.81
Fenton
th
4 group
reagent
0.36
0.17 7.17
6.41
1.40
3.34
/
45.13
1.27
13.60
2.59
34.28
10.31
3.3.2 Dimensional stability
The same situation happened as in section 3.2.2 that the impregnate were also leaching out
from wood samples during the soaking test. Because of lack the consideration about the
volume change of impregnated wood which non-bonded impregnate had come out, the ASE
value might decrease along with the soaking days due to the change of bulking cell wall. The
actual ASE might not followed the trend seen in Appendix IV in which ASE seemed stable
during the 7 soaking days. Only the first day of soaking ASE can describe the real
performance of the impregnated wood dimensional stability after impregnation Removal of
loose bulking chemicals by water soaking might reduce ASE.
The swelling coefficient and ASE of impregnated wood were shown in table 15. When all
the impregnate were controlled to pH 2.41 and temperature was 103 oC, wood impregnated
with oxidized carbohydrates (both glucose and sucrose) displayed good dimensional stability
38
in the first soaking day. The oxidized glucose impregnated wood have around 45% of ASE
and oxidized sucrose impregnated one have nearly 35% of ASE in the first soaking day
(Table 16). Meanwhile, improvement of dimensional stability for the Fenton’s reagent
impregnated wood was smaller and the ASE was around 10% (Table 16).
Table 16 indicated that Fenton reagent could not lead to high ASE by itself with low curing
temperature, thus pH and its interaction effect with low curing temperature was not clear.
However obviously when the temperature was set to lower temperature (103°C), oxidized
products from carbohydrates oxidation brought positive effect on wood dimensional stability.
Wood modified with different oxidized carbohydrates impregnate showed different ASE
(Table 16). Oxidized glucose performed better dimensional stability than oxidized sucrose
impregnate while WPG of oxidized sucrose impregnated wood were much higher than
oxidized glucose impregnated one (Table 16). Also the relation between cell wall bulking and
ASE among wood modified in presence of two different oxidized carbohydrates compared
with control samples seemed to be almost linear (Figure 15) in the beginning of water
soaking test. It suggested that bulking effect was the most important factor that caused high
ASE in low curing temperature treatment in the beginning of non leachable test. But since
large amount of impregnate or wood components had been washed away by water, it indicated
that these impregnate cannot form chemical bonding with hydroxyl groups in hemicelluloses
in wood,
the influence of non-bonded impregnate on BC and ASE should be taken into
consideration. Whether cross-linking between impregnate compounds had occurred or not
was still unknown.
Anti Swelling coefficient(ASE%)
The relation between BC and ASE
50
45
40
35
30
25
20
15
10
5
0
-5 0
y = 4.1171x - 0.1677
R² = 0.9991
The relation between
BC and ASE
线性 (The relation
between BC and ASE)
5
10
Cell Wall Bulking(BC%)
15
Figure 15. The relationship between cell wall bulking effect (BC) and Anti swelling efficiency(ASE) of
oxidized carbohydrates impregnated wood and control sample(origin point)
3.3 HPLC analysis
39
When soaking solution from first day from wood modified with Fenton’s reagent was
analyzed in HPLC, even though the curing temperature was only 103 oC, still considerable
amount of furfural was detected (Table 17). Residual carbohydrates or xylose inside of
hemicelluloses can be dehydrated to furfural in the presence of Fenton reagent inside of wood.
However, due to the mass of modified wood and water extraction was different from this two
conditions, quantitative comparison between this two conditions could not be done.
Table 17 5-Hydroxymethylfurfural (HMF) and furfural in 1 st day soaking liquid from
treatments with Fenton’s reagent.
Sample
No.
Impregnate
Impregnate Curing
HMF
Furfural
pH
Temperatu amount per amount per
re
1ml
1ml
soaking
soaking
solution(m solution(m
Fenton
1
Reagent(3.2.3)
Reagent
g/ml
2.1
140oC
1.849
8.176
2.4
103 oC
1.761
27.197
Fenton
2
g/ml)
3.3.4 Swelling properties of impregnated wood after 7 days non-leachable water soaking
After 7 soaking days in water most of impregnate had been leached out and the WPG and
cell wall thickness of impregnated wood were investigated. The table below (Table 18)
showed that WPG of oxidized carbohydrates impregnated wood (See Methods in section 2.4.2)
had decreased a lot compared with the values after impregnation (Table 15). Together with
pH change of soaking water and disappearance of color when it is close to more neutral pH
indicated that a lot of impregnated material was still not bonded inside of cell wall. All the
samples volume (at 0%MC) was smaller than the samples before water soaking. It can be
assumed that the actual swelling coefficients of impregnated wood were changing every
soaking day during the non-leachable test (section 3.3.2).
Table 18. Dimensional stability of wood samples treated with different impregnate under
103oC curing temperature after 7 days water soaking
Sample
Group
Impregnate
Weight
Cell wall
Percentage bulking(BC
Two cell
wall
Volumetric
Swelling
Gain(WPG
thickness
coefficient(S efficiency(AS
) after 7
%) after 7 days water after 7 days %) after 7
40
Anti
Swelling
E%) after 7
days water
soaking
soaking
1st group
(control)
2nd group
3rd group
/
-0.65 0.06 -4.26 2.52
water
days water
days water
soaking
soaking
soaking
6.05 0.54
17.58
0.86
/
Oxidized
glucose
2.83 1.88
5.66 0.91
14.54 0.86
22.45
10.94 1.44 0.01 2.36
5.53 0.94
15.64 0.98
16.58
0.36 0.17
4.59 0.89
18.75 1.48
4.34
5.02 0.30
Oxidized
sucrose
Fenton
4th group
reagent
3.58 3.54
Cell wall thickness which also describes the extent of cell wall bulking was measured with
Image J tool with the cross-section pictures scanned by scanning electron microscope picture
(Methods in section 2.5.3) (Figure 16). The impregnated wood cell wall thickness (at 0% MC)
after 7 days soaking test was smaller than controlled samples. Fenton’s reagent impregnated
wood had the smallest average cell wall thickness among the samples treated with different
impregnate. Cell wall structure might be destroyed by the impregnate since the pH of
impregnate was very low (pH 2.1). No impregnate can be seen from the SEM pictures that
filled inside of lumen after 7 days soaking, however, there are still some WPG existed inside
of oxidized carbohydrates treated wood (Table 18). It suggested that some of the impregnate
had stayed inside of wood cell wall and might be bonded with cell wall. Since oxidized
glucose and sucrose had smaller reduction of cell wall thickness compared with the wood
treated with Fenton’s reagent (Table 16), the left and stable oxidized carbohydrates inside of
cell wall may be compensated the loss of cell wall structure from acid and Fenton’s reagent
attack.
(a)
(b)
41
(c)
(d)
Figure 16.(a) Measurement of two cell wall thickness of aspen control sample(1000X) (b)
Measurement of two cell wall thickness of aspen sample treated with oxidized glucose (1000X)
(c) Measurement of two cell wall thickness of aspen sample treated with oxidized sucrose
(1000X)
(d) Measurement of two cell wall thickness of aspen sample treated with Fenton’s reagent (1000X).
In order to exclude the effect of non-bonded impregnate on ASE, one water soaking-oven
drying cycle had been added (Methods see in section 2.4.2), the dimensional stability test
showed that the ASE after 7 days soaking test was 22% of oxidized glucose impregnated
sample while the Bulking coefficient of cell wall was only less than 3% and WPG was only
around 5% while Fenton Reagent impregnated wood had almost no positive effect that
contributing to wood dimensional stabilization (Table 18). It seems that despite the cell wall
bulking caused by non-bonded impregnate, there was still some dimensional stability effect
from oxidized carbohydrates impregnated wood. Thus, cross-linking between oxidized
carbohydrates reactive compounds and cell wall might have occurred inside of wood to some
extent.
42
4. Conclusion
The aim of this study was to investigate impregnation processing parameters and
mechanism that influence the dimensional stability of solid aspen after impregnating with
oxidized glucose followed by heating. Results presented in this paper showed that it is
possible to significantly reduce the wood swelling by impregnating with glucose which was
activated by Fenton’s reagent followed by heating. Most of the impregnate was, however, not
bonded to the wood under our impregnation and curing processes.
Mechanism of dimensional stabilization when impregnated with oxidized glucose seems to
be related to curing temperature and pH. No matter the impregnate contained activated
oxidized glucose or not, high curing temperature (140 oC) and low pH in impregnation process
generated an extreme condition which seemed to result in degradation of hemicelluloses and
damage the structure of wood component. In fact, existence of furfural in the soaking water
from Fenton’s reagent treated wood suggested the significant degradation of hemicelluloses.
Under these conditions, the effect of possible cross linking inside of cell wall which may be
formed by bifunctional groups in oxidized glucose is not clear at higher pH level than 2. But
since wood modified with oxidized glucose without addition of diluted NaOH, gave much
larger ASE compared with Fenton reagent (which were without pH adjustment), the effect of
oxidized glucose itself may also influence the dimensional stability at a low pH level of 1.4
under high temperature heating.
On the other hand, when the curing temperature was low (103 oC), wood modified with
oxidized carbohydrates, controlled to the same pH as pH of Fenton reagent, gave much better
dimensional stability than untreated one and Fenton’s reagent impregnated one. At this low
temperature, temperature as a parameter influencing the modification was considered to be
less important factors that influenced the reaction. Cell wall bulking effect looks like
prevailing over other effects such as cross-linking or hemicelluloses degradation and was
closely linerary correlated with ASE after the first day of soaking test. However, after 7 days
of soaking, the cell wall thicknesses of impregnated wood were smaller than control samples
and samples oven dry volume were also smaller than themselves before soaking into water.
The cell wall bulking from non-bonded impregnate was lost after soaking in water and its
effect on ASE was no longer the significant factor. Still around 20% of ASE was found after
non leachable test. From our point of view, with low temperature (103°C), the effect of
possible cross-linking formed by oxidized carbohydrates became more obvious after non
leachable test excluding the non-bonded impregnate cell wall bulking effect. ASE results
suggests that cross-linking occurs but it is difficult to estimate the extent since a lot of
impregnate was not bonded to cell wall, the amount of reactive compounds and their activity
that contributed to cross-linking inside of cell wall from oxidized glucose impregnate were
unknown.
WRE results showed that oxidized glucose impregnated wood had higher WRE than
43
Fenton’s reagent treated sample at each pH level, respectively. Influence of pH of impregnate
solution on WRE, when using oxidized glucose treated wood, were not significant. With
higher temperature (140°C), Fenton’s reagent impregnated wood had better performance in
inhibiting water uptake than at 103°C. At low curing temperature (103°C), larger WPG of
oxidized carbohydrates had less water absorption when we compared the WRE from the
sample impregnated by oxidized glucose and sucrose. Samples treated with oxidized sucrose
gave a WRE above 50% which is the largest among all the treated samples. Physical bulking,
hemicelluloses degradation and cross-linking formed by oxidized glucose impregnate reaction
inside of wood all helped to improve its water repellent under different impregnation process
conditions.
5. Hypothesis and suggestions to future work
Results obtained on increasing dimensional stability of wood by impregnation and curing
of soluble oxidized sugars are interesting but needs to be further studied. We observed that
when the impregnated sample was heated at 60°C there is no color change. Maybe low
temperature can be a better option when the study objective is aiming at investigating the
chemical cross-linking and bulking formed from oxidized glucose reaction inside of wood.
It was possible to impregnate small cubes of aspen with only water vacuum; however,
larger samples need probably also a pressure step to reach full penetration of the voids in the
wood. Larger samples gave larger variation in dimensional stability than small samples. It is
better to start with small sample size to study the correlation between activated oxidized
carbohydrates and dimensional stability. More studies can be focused on how to optimize the
impregnation process to achieve even higher ASE.
A problem was shown up that large amount of the impregnate was leaching out with
exposure to water. A large part of this is due to leaching of oxidized glucose from lumen
which means that still not all of the impregnate was chemically reacted with wood.
Water-soaking and oven-drying cycles test might be more suitable to describe more accurately
ASE of impregnated wood due to the decrease of cell wall bulking caused by leaching of
impregnating materials. However, the temperature for oven drying needs to be carefully
chosen to avoid the impregnate from further curing and hemicelluloses degradation during the
leaching test.
The dimensional stability of sample impregnated with oxidized glucose after water soaking
was not good in our study as we were expected, the yield of reactive compounds during
preparation of impregnate products and their free radical activity were unknown.
Identification of active compounds in our impregnate solutions that lead to increased
dimensional stability of modified wood seems to be important to really find the most
appropriate conditions. Another way is to try to make oxidation more specific as Fenton
oxidation is quite vigorous. It can be interesting to use experimental design to investigate the
44
reaction conditions like pH (buffer solution) control during oxidation, the mode of addition
H2O2 (the concentration of H2O 2 and the speed of addition), the catalyst type and amount and
so on.
Another interesting thing is related to the reaction of Fenton reagent with wood, the reason
for large amount of furfural can be investigated further. Mechanical properties and durability
should also be paid attention. Higher pH level of impregnate solution than used successfully
in this report needs to be studied at low temperature conditions. This is needed to find mild
condition for retaining good mechanical properties.
45
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[37] Soulounganga, P., Loubinoux, B., Wozniak, E., Lemor, A., Gerardin, P. (2004) Improvement of wood
47
properties by impregnation with polyglycerol methacrylate. Holz. Roh. Werkst. 62:281-285.
[38] Ohmae, K., Minato, K., Norimoto, M. (2002) The analysis of dimensional changes due to chemical
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48
Appendix I
The ASE results from study of the importance of different impregnation parameters. For
conditions during treatments see Methods in section 2.2.
WPG(%)
BC(%)
ASE1(%)
ASE2(%) ASE3(%)
ASE4(%)
ASE5(%) ASE6(%) ASE7(%)
N1
15,40877 5,247071
33,99606
31,65964 31,46494
31,07553
37,30599
37,8901
41,00533
N2
10,27699 2,155806
29,32842
27,41838 26,94087
32,43224
44,13125 42,22121
35,2973
N3
14,08899 5,106168
58,06542
58,74178 53,10542
56,71269
59,86906 58,06542 60,54542
N4
6,389442 0,051361
38,82452
31,28529 30,20826
34,30099
29,56204
N5
17,64648 8,218504
61,55386
55,48341 59,75521
66,27531
56,15791 62,00352 49,18814
N6
9,096873 1,450054
33,43279
18,7112
21,27148
26,60539
27,45881 27,67217 14,01736
N7
7,372865 1,082677
33,12203
29,33649 26,30805
29,33649
26,56042 34,88862
22,5225
N8
3,188736 1,416287
70,67958
64,16393 63,94674
65,24987
61,77486 63,51236
59,1686
N9
18,98524 3,36008
46,36965
42,93731 48,72939
48,08582
45,29704 51,08912 41,43566
N10
8,766859 -1,71933
16,98173
14,66925 9,350527
10,96927
3,56931
N11
10,33511 3,941909
43,56655
46,06544 41,90061
44,19127
36,48633 45,64896 36,06985
N12
5,63792 5,429162
74,5005
67,93245 69,86423
70,83012
67,73927 73,92096 66,77338
N13
14,82437 -4,74096
-31,3615
-35,8525
-23,5022
-43,9924
-27,4319 -29,6774
N14
8,942318 5,246753
52,81578
48,50671 55,18576
43,9822
57,98665 52,60032 57,98665
N15
10,20192 2,466598
55,50285
52,49338 62,38164
55,07293
60,44698 57,43751 60,44698
N16
4,271087 -0,14395
29,7234
24,67226
31,2607
25,33111
32,13915 32,57838 32,57838
-0,87674
39,12513
37,40429 33,10218
47,08404
31,16623 31,38133
N18
9,880869 3,476581
65,55169
71,5628
66,47648
66,70767
65,78289 65,55169 63,23972
N19
9,365079 -2,74164
29,93081
33,73892 31,45405
33,48504
28,66144 30,69243 31,70793
N20
-6,66805 -2,68219
59,46555
66,95281 58,69101
58,43283
57,91647 66,17827 57,91647
N21
-7,24608 -6,02056
39,57836
40,38042 43,32129
56,68891
36,90484 51,34187 40,64777
N22
-7,05194 -6,34146
37,07639
34,51389 25,97222
35,65278
32,80556 31,95139 33,65972
N17
9,5083
31,5007
26,33094
8,656781 -2,21191
-27,4319
29,8756
(WPG is the abbreviation of weight percentage gain, BC is the abbreviation cell wall bulking,
ASE is the abbreviation Anti swelling efficiency.)
49
Appendix II
It is the coefficient plot of the model with all main and interaction factors in MODDE. For the design of
model see Methods in 2.2.1 and the multivariate analysis results in section 3.1.1
20
15
10
5
0
-5
-10
-15
-20
Mol
Temp
Air(no air drying)
Air(air drying)
PH(WithNaOH)
PH(Without NaOH)
time
Mol*Temp
Mol*Air(air drying)
Mol*PH(WithNaOH)
R2=0,881
Q2=0,256
Mol*PH(Without NaOH)
Mol*time
Temp*Air(no air drying)
RSD=14,68
Conf. lev.=0,95
Temp*Air(air drying)
Temp*PH(WithNaOH)
Temp*PH(Without NaOH)
Temp*time
MODDE 9 - 2012-05-09 14:11:52 (UTC+1)
Air(no air drying)*PH(WithNaOH)
Air(no air drying)*PH(Without NaOH)
Air(air drying)*PH(WithNaOH)
Air(air drying)*PH(Without NaOH)
Air(no air drying)*time
Air(air drying)*time
PH(WithNaOH)*time
PH(Without NaOH)*time
50
Investigation: ase (PLS, comp.=2)
Scaled & Centered Coefficients for ASE (Extended)
N=22
DF=6
Mol*Air(no air drying)
Appendix III
According to the method shown in section 2.3.2, with the soaking days increased, the ASE
values from different impregnation process groups heated at 140 °C were shown on the figure
below.
ASE of small size impregnated samples
80%
60%
Control 140oC
Fenton Reagent
ASE(%)
40%
Fenton Reagent 3
Fenton Reagent 4
20%
Oxidized glucose
0%
Oxidized glucose 2.1
0
2
4
6
8
Oxidized glucose 3
-20%
Oxidized glucose 4
-40%
Soaking days
(a)
ASE of Larg size impregnated samples
60%
40%
Control 140oC
Fenton Reagent
ASE(%)
20%
Fenton Reagent 3
Fenton Reagent 4
0%
0
2
4
6
8
Oxidized glucose
-20%
Oxidized glucose 2.1
Oxidized glucose 3
-40%
Oxidized glucose 4
-60%
Soaking days
(b)
(a) Anti Swelling efficiency(ASE) of impregnated wood (10*20*10mm)(R*T*L) during the
nonleachable
soaking
test
(b)
Anti
Swelling
51
efficiency
(ASE)
of
impregnated
wood
(10*10*180mm)(R*T*L) during the nonleachable soaking test. Control 140 °C was the 2nd group in table
4, oxidized glucose was the 3rd group in table 4, Fenton’s reagent was the 4th group in table 4, oxidized
glucose at pH 2.1 was the 5th group in table 4, Fenton’s reagent at pH 3 was the 6th group in table 4,
oxidized glucose pH 3 was the 7th group in table 4, Fenton’s reagent 4 was the 8th group in table 4 and
oxidized gluocse at pH 4 was the 9th group in table4, see section 2.3.1.
Appendix IV
According to the method shown in section 2.4.2, with the soaking days increased, the ASE
values from different impregnate were shown on the figure below.
ASE(%)
ASE of different imprengted sample
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
Fenton Reagent
Oxidized glucose
Oxidized sucrose
0
2
4
6
8
Soaking days(d)
Anti Swelling efficiency (ASE) of impregnated wood (10*20*10mm) (R*T*L) during the nonleachable
soaking test. Fenton’s reagent was the reagent I in table 5, oxidized glucose was the reagent II in table 5,
oxidized sucrose was the reagent III in table 5.
52