CHARACTERISTICS OF WOOD PLASTIC COMPOSITES BASED
ON MODIFIED WOOD
- MOISTURE PROPERTIES, BIOLOGICAL RESISTANCE AND
MICROMORPHOLOGY
CHARACTERISTICS OF
WOOD PLASTIC COMPOSITES
BASED ON MODIFIED WOOD
- MOISTURE PROPERTIES, BIOLOGICAL
RESISTANCE AND MICROMORPHOLOGY
Kristoffer Segerholm
Doctoral Thesis
KTH Building Materials Technology, Stockholm,
Sweden 2012
KTH Royal Institute of Technology
School of Architecture and the Built Environment
Dept. of Civil and Architectural Engineering
Div. of Building Materials
SE-100 44 Stockholm
SWEDEN
TRITA-BYMA 2012:2
ISSN 0349-5752
ISBN 978-91-7501-554-5
© 2012 Kristoffer Segerholm
All aboard!
Ozzy
ABSTRACT
Biobased materials made from renewable resources, such as wood, play an
important role in the sustainable development of society. One main
challenge of biobased building materials is their inherent moisture
sensitivity, a major cause for fungal decay, mold growth and dimensional
instability, resulting in decreased service life as well as costly maintenance.
A new building material known as wood-plastic composites (WPCs) has
emerged. WPCs are a combination of a thermoplastic matrix and a wood
component, the former is usually recycled polyethylene or polypropylene,
and the latter a wood processing residual, e.g. sawdust and wood shavings.
The objective of this thesis was to gain more insight about characteristics
of WPCs containing a modified wood component. The hypothesis was
that a modified wood component in WPCs would increase the moisture
resistance and durability in outdoor applications. The study comprises
both injection molded and extruded WPC samples made with an
unmodified, acetylated, thermally modified or furfurylated wood
component in a polypropylene (PP), high density polyethylene (HDPE),
cellulose ester (CAP, a cellulose ester containing both acetate and
propionate substituents) or polylactate (PLA) matrix. The WPCs were
prepared with 50-70 weight-% wood. The emphasis was on studying the
moisture sorption, fungal resistance and micromorphological features of
these new types of composites. Water sorption in both liquid and vapor
phases was studied, and the biological performance was studied both in
laboratory and in long term outdoor field tests. Micromorphological
features were assessed by analyzing of the wood component prior to and
after processing, and by studying the composite microstructure by means
of a new sample preparation technique based on UV excimer laser ablation
combined with scanning electron microscopy (SEM).
Results showed that the WPCs with a modified wood component had a
distinctly lower hygroscopicity than the WPCs with unmodified wood,
which resulted in less wood-plastic interfacial cracks when subjected to a
moisture soaking-drying cycle. Durability assessments in field and marine
tests showed that WPCs with PP or CAP as a matrix and 70 weight-%
v
unmodified wood degraded severely within a few years, whereas the
corresponding WPCs with a modified wood component were sound after
7 years in field tests and 6 years in marine tests. Accelerated durability tests
of WPCs with PLA as a matrix showed only low mass losses due to decay.
However, strength losses due to moisture sorption suggest that the
compatibility between the PLA and the different wood components must
be improved. The micromorphological studies showed that WPC
processing distinctly reduces the size and changes the shape of the wood
component. The change was most pronounced in the thermally modified
wood component which became significantly reduced in size. The
disintegration of the modified wood components during processing also
creates a more homogeneous micromorphology of the WPCs, which may
be beneficial from a mechanical performance perspective. Future studies
are suggested to include analyses of the surface composition, the surface
energy and the surface energy heterogeneity of both wood and polymer
components in order to tailor new compatible wood-polymer
combinations in WPCs and biocomposites.
Keywords: Wood plastic composites, WPC, acetylation, thermal
modification, furfurylation, moisture sorption, biological durability, UV
excimer laser, micromorphology.
vi
SAMMANFATTNING (IN SWEDISH)
Biobaserade material tillverkade av förnybara råvaror spelar en betydande
roll för en hållbar samhällsutveckling. En utmaning för biobaserade
byggnadsmaterial är deras inneboende fuktkänslighet, som ökar risken för
biologiskt angrepp, mögelpåväxt och dimensionsförändringar, vilket i sin
tur kan leda till en nedsatt livslängd och kostsamt underhåll. En ny typ av
byggnadsmaterial, trä-plastkompositer (eng. wood plastic composites,
WPCs), finns nu på byggnadsmaterialmarknaden i Sverige. WPCs är en
kombination av en träkomponent, till exempel i form av träbearbetningsrester såsom sågspån eller hyvelspån, och en termoplastisk matris ofta i
form av återvunnen polyeten eller polypropen.
Syftet med denna avhandling har varit att få en djupare insikt om några
egenskaper hos WPCs med en modifierad träkomponent. Hypotesen var
att en modifierad träkomponent i WPCs skulle minska fuktkänsligheten
och samtidigt öka den biologiska beständigheten för tillämpningar
utomhus. Studien innefattar både formsprutade och extruderade WPCmaterial med en träandel mellan 50 och 70 vikts-%. Träkomponenterna
bestod av omodifierade, acetylerade, termiskt modifierade eller
furfurylerade träspån eller fibrer. Matrismaterialen bestod av polypropen
(PP), högdensitetspolyeten (HDPE), cellulosaester (CAP, en cellulosaester
innehållande både acetat och propionat substituenter) eller polylaktat
(PLA). Tonvikten har lagts på att studera fuktegenskaper, resistens mot
rötangrepp och mikromorfologi hos dessa nya typer av kompositer.
Mikromorfologin studerades genom analys av träkomponenten före och
efter bearbetning samt genom studier i svepelektronmikroskop (SEM) på
ytor beredda med en ny provberedningsmetod baserad på ablatering med
UV-excimerlaser.
Resultaten visade att WPCs med en modifierad träkomponent hade en
betydligt lägre hygroskopicitet än WPCs med en omodifierad träkomponent, vilket minimerade gränskiktsprickor mellan trä och plast då
kompositerna utsattes för en uppfuktning-uttorkningscykel. Beständighetsutvärdering i fältförsök ovan och i mark, samt i marin miljö, visade att
WPCs med 70 vikts-% modifierad träandel har klarat sig utan biologiska
vii
angrepp efter 7 år i fältförsöken och efter 6 år i de marina försöken, medan
motsvarande WPCs med en omodifierad träkomponent uppvisade kraftig
nedbrytning redan efter några år. Accelererad beständighetsutvärdering av
WPCs med PLA som matris påvisade inga rötangrepp, emellertid så
sänktes hållfastheten på grund av fuktsorption vilket tyder på att
kompatibiliteten mellan PLA och de olika träkomponenterna bör
förbättras. De mikromorfologiska studierna visade att tillverkningsprocessen för WPC-materialen drastiskt minskar storleken och ändrar
formen hos träkomponenten. Förändringen var störst för den termiskt
modifierade träkomponenten som kraftigt reducerades i storlek under
processen. Förändringen av de modifierade träkomponenterna under
processen resulterade i en mer homogen mikromorfologi, vilket kan ge
förbättrade mekaniska egenskaper hos WPC-materialen. Framtida studier
föreslås omfatta analyser av ytkemisk sammansättning, ytenergi och ytors
heterogenitet hos både trä- och polymerkomponenter för att kunna
skräddarsy nya kompatibla trä-polymerkombinationer i WPCs och
biokompositer.
viii
PREFACE
This work has been carried out at Kungliga Tekniska Högskolan,
avdelningen för Byggnadsmaterial (KTH Royal Institute of Technology,
Division of Building Materials), Stockholm, Sweden. The work has been a
part of the Institute Excellence Centre EcoBuild, hosted by SP Wood
Technology in Stockholm. The over-all objective of this centre is to
engineer and develop innovative, eco-efficient and durable wood based
materials and products for building, furniture and textile applications, see
also www.ecobuild.se for further information. Types of materials of
primary interest are: modified solid wood such as thermally modified,
furfurylated and acetylated wood, biobased textile fibres, biobased binders
and coatings, solvent-spun cellulose fibres and biocomposites. A top
priority effort is to engineer fully biobased alternative products entirely
based on renewable resources. EcoBuild is mainly financed by VINNOVA
(Swedish Governmental Agency for Innovation Systems), SSF (Swedish
Foundation for Strategic Research) and the Knowledge Foundation and
together with contributions from the participating companies.
An acknowledgement is addressed to FORMAS for their financial support
(Dnr 24.3/2003-0690) regarding the development of the UV laser
technique for sample preparation used in this work and their financial
support (Dnr 243-2007-1166) regarding acquisition of the scanning
electron microscope used in this work, as well as the Knut and Alice
Wallenberg foundation (Dnr KAW 1998.0130) for funding the UV laser
laboratory.
I wish to express my sincere gratitude to my supervising group at KTH
and SP Wood Technology consisting of main supervisor Prof. Magnus
Wålinder, Dr. Pia Larsson Brelid, Dr. Mats Westin and Prof. em. Ove
Söderström for their enthusiasm and support.
I am grateful to Joachim Seltman for the assistance in preparing specimens
by UV laser. I would also like to express my thanks to the staff at SP
Wood Technology for fruitful discussions and guidance when needed. Dr.
Rebecca Ibach and colleagues at the Forest Products Laboratory are
ix
greatly acknowledged for the collaborations and help during and after my
stay in Madison. A large recognition is sent to Dr. Harald Brelid and Dr.
Dennis Jones for their involvement in reading and commenting on the
thesis. A special thank is expressed to Prof. em. Roger M Rowell for his
encouraging thoughts and comments about the thesis and manuscripts.
Finally, I would like to express my profound gratitude to my other half,
Johanna for her support and understanding and to my children, Pontus
and Klara for reminding me that there also are other important things in
life.
Stockholm, November 2012
Kristoffer Segerholm
x
CONTENTS
ABSTRACT .......................................................................................................... v
SAMMANFATTNING (IN SWEDISH) ...................................................... vii
PREFACE ............................................................................................................ ix
CONTENTS ....................................................................................................... xi
LIST OF PAPERS ............................................................................................ xiii
OUTLINE OF THE THESIS ...................................................................... xvii
1. INTRODUCTION ......................................................................................... 1
1.1
1.2
Wood plastic composites ................................................................................. 1
Statement of objectives ................................................................................... 7
2. MATERIALS PREPARATION ................................................................... 9
2.1
2.2
2.3
2.4
Modified wood................................................................................................. 9
Preparation of wood components ................................................................. 11
Polymer matrices ........................................................................................... 12
Manufacturing of WPC samples .................................................................... 13
3. MOISTURE CHARACTERISTICS .......................................................... 15
3.1
3.2
Water vapor sorption..................................................................................... 15
Water soaking ................................................................................................ 18
4. BIOLOGICAL DURABILITY .................................................................. 19
4.1
4.2
Laboratory testing .......................................................................................... 19
Field testing.................................................................................................... 22
5. MICROMORPHOLOGICAL FEATURES ............................................ 27
5.1 Changes in size and shape of the wood component due to the extrusion
processing ................................................................................................................ 27
5.2 Specimen preparation technique by means of UV excimer laser ablation .... 29
5.3 Micromorphology of the WPC samples ......................................................... 30
6. CONCLUSION AND FUTURE WORK ................................................ 35
6.1
6.2
Conclusions .................................................................................................... 35
Future work.................................................................................................... 36
7. REFERENCES.............................................................................................. 37
APPENDED PAPERS (I-VII) ....................................................................... 45
xi
LIST OF PAPERS
This doctoral thesis is based on the following papers, referred to in the
text by their roman numerals:
I.
Wålinder, M.E.P., Omidvar, A., Seltman, J. and Segerholm, B.K. (2009).
Micromorphological studies of modified wood using a surface
preparation technique based on ultraviolet laser ablation. Wood Material
Science and Engineering, 4(1), 46-51.
II.
Segerholm, B.K., Vellekoop, S. and Wålinder, M.E.P. (2012). Processrelated mechanical degradation of the wood component in high-woodcontent wood-plastic composites. Wood and Fiber Science, 44(2), 1-10.
III.
Segerholm, B.K., Walkenström, P., Nyström, B., Wålinder, M.E.P. and
Larsson Brelid, P. (2007). Micromorphology, moisture sorption and
mechanical properties of a biocomposite based on acetylated wood
particles and cellulose ester. Wood Material Science and Engineering, 2(3),
106-117.
IV.
Segerholm, B.K., Ibach, R.E. and Wålinder, M.E.P. (2012). Moisture
sorption in artificially aged wood-plastic composites. BioResources, 7(1),
1283-1293.
V.
Segerholm, B.K., Rowell, R.M., Larsson Brelid, P., Wålinder, M.E.P.,
Westin, M. and Alfredsen, G. (2007). Improved Durability and Moisture
Sorption Characteristics of Extruded WPCs made from Chemically
Modified Wood. In: Proceedings of the 9th International Conference on Wood &
Biofiber Plastic Composites, Madison, WI, USA, pp. 253-258 (Received the
“Best Student Formal Presentation” award).
VI.
Segerholm, B.K., Ibach, R.E. and Westin, M. (2012). Moisture sorption,
biological durability, and mechanical performance of WPC containing
modified wood and polylactates. BioResources, 7(4), 4575-4585.
VII.
Segerholm, B.K., Larsson Brelid, P., Wålinder, M.E.P., Westin, M. and
Frisk, O. (2012). Biological outdoor durability of WPC with chemically
modified wood. In: Proceedings of the 6th European conference on wood
modification, Jones, D., Militz, H., Petrič, M., Pohleven, F., Humar, M. and
Pavlič, M. eds. Ljubljana, Slovenia, pp. 47-54.
xiii
Other relevant publications not included in this thesis:
Clemons, C.M., Rowell, R.M., Plackett, D and Segerholm, B.K. (2012).
Chapter 13. Wood/nonwood thermoplastic composites. In: Handbook of
wood chemistry and wood composites, second edition. Rowell, R.M. ed. CRC Press,
Boca Raton, Florida.
Jones, D., Englund, F., Henriksson, M., Segerholm, B.K., Trey, S., Sterley,
M., Ziethen, R., Gonzales, S. and Segui, L. (2012). Development of a novel
wood based panel for use in internal door manufacture. In: 5th International
Conference on Environmentally-Compatible Forest Products, Porto, Portugal.
Jermer, J., Wong, A.H.H., Segerholm, B.K. and Nilsson, T. (2011).
Durability testing of coconut shell according to ENV 807. In: 42nd Annual
Meeting of the International Research Group on Wood Protection, Queenstown,
New Zealand. 8 pp. IRG/WP 11-10761.
Larsson Brelid, P., Segerholm, B.K., Alfredsen, G., Westin, M. and
Wålinder, M.E.P. (2006). Wood Plastic Composites with Improved
Dimensional Stability and Biological Resistance. In: 2nd International
Conference on Environmentally-Compatible Forest Products. Porto, Portugal, 6 pp.
Larsson Brelid, P., Segerholm, B.K., Westin, M. and Wålinder, M.E.P.
(2006). Wood Plastic Composites from Modified Wood; Part 1 Conceptual idea, mechanical and physical properties. In: The 37th Annual
Meeting of the International Research Group on Wood Preservation. Tromsø,
Norway, 13 pp. IRG/WP 06-40338.
Olsson, S., Östmark, E., Ibach, R.E., Clemons, C.M., Segerholm, B.K. and
Englund, F. (2011). The use of esterified lignin for synthesis of durable
composites. In: Proceedings of the 7th meeting of the Nordic-Baltic Network in
Wood Material Science and Engineering (WSE). Larnøy, E. and Alfredsen, G.
eds. Oslo, Norway, pp. 173-178.
xiv
Segerholm, B.K. (2007). Wood plastic composites made from modified
wood – Aspects on moisture sorption, micromorphology and durability.
Licentiate thesis, KTH Royal Institute of Technology, Stockholm, Sweden.
Segerholm, B.K. and Wålinder, M.E.P. (2010). Dimensional changes due
to water sorption in high wood-content WPCs prepared with modified
wood. In Proceedings of the 5th European Conference on Wood Modification,
ECWM5. Hill, C.A.S., Militz, H. and Andersons, B. eds. Riga, Latvia. pp.
201-204.
Segerholm, B.K. and Wålinder, M.E.P. (2012). Inverse gas
chromatography characterization of wood composite components. In:
Proceedings of the 8th meeting of the Northern European Network for Wood Science
and Engineering (WSE). Baltrušatits, A. and Ukvalbergienė, K. eds. Kaunas,
Lithuania, pp. 58-63.
Segerholm, B.K., Ibach, R.E. and Westin, M. (2011). Durability of PLA Modified Wood Composites. In: Proceedings of the 11th International Conference
on Wood & Biofiber Plastic Composites. Madison, WI, USA. 3 pp.
Segerholm, B.K:, Omidvar, A. and Wålinder, M.E.P. (2009). Acetylation to
Minimize Water Uptake and Deformation of High Wood Content WPC.
In: Proceedings of The 4th European Conference on Wood Modification, ECWM4.
Stockholm, Sweden, pp. 239-242.
Segerholm, B.K., Rowell, R.M., Larsson Brelid, P. Wålinder, M.E.P.
Westin, M. and Söderström, O. (2007). Micromorphology and Durability
of WPCs made from Chemically Modified Wood. In: Proceedings of the 3rd
meeting of the Nordic Baltic Network in Wood Material Science and Engineering
(WSE). Rikala, J. and Sipi, M. eds. Helsinki, Finland, pp. 57-63.
Segerholm, B.K., Westin, M., Larsson Brelid, P. and Wålinder, M.E.P.
(2009). Wood Plastic Composites from Modified Wood and CAP. In:
Proceedings of the 4th Wood Fibre Polymer Composites International Symposium.
Bordeaux, France. 10 pp.
xv
Segerholm, B.K., Wålinder M.E.P. and Bardage, S.L. (2011). Mould
growth resistance of fungicide-containing WPC. In: Proceedings of the 7th
meeting of the Nordic-Baltic Network in Wood Material Science & Engineering
(WSE). Larnøy E. and Alfredsen G. eds. Oslo, Norway. pp. 25-30.
Segerholm, B.K., Wålinder, M.E.P. and Holmberg, D. (2010). Adhesion
studies of scots pine-polypropylene bond using ABES. In: Proceedings of the
6th meeting of the Nordic-Baltic Network in Wood Material Science and Engineering,
(WSE), Meier, P. ed. Tallinn, Estonia. pp. 142-146.
Segerholm, B.K., Wålinder, M.E.P., Larsson-Brelid, P., Walkenström, P.
and Westin, M. (2005). Wood Plastic Composites made from Acetylated
Wood - Effects on Water Vapour Sorption Behaviour and Durability. In:
Proceedings of the 9th European Panel Products Symposium. Llandudno, UK, pp.
233-242.
Westin, M., Larsson Brelid, P., Segerholm, B.K. and Van den Oever, M.
(2008). Wood Plastic Composites from Modified Wood; Part 3. Durability
of WPCs with bioderived matrix. The 39th Annual Meeting of the International
Research Group on Wood Preservation. Istanbul, Turkey. 17 pp. IRG/WP 0840423.
Wålinder, M.E.P., Larsson Brelid, P., Segerholm B.K., Long, C.J. and
Dickerson, J. (2012). Wetting characteristics and coatings penetrability in
acetylated Southern yellow pine. In: Proceedings of the 6th European conference
on wood modification. Jones, D., Militz, H., Petrič, M., Pohleven, F., Humar,
M. and Pavlič, M. eds. Ljubljana, Slovenia. pp. 39-46.
Wålinder, M.E.P., Segerholm, B.K. and Söderström, O. (2009). Water
sorption properties and dimensional changes of high wood-content WPC.
In: Proceedings of the 5th meeting of the Nordic Baltic Network in Wood Material
Science and Engineering. Bergstedt, A. ed. Copenhagen, Denmark. pp. 153160.
xvi
Wålinder, M.E.P., Segerholm, B.K. and Westin, M. (2010). Durability of
high wood content WPCs. In: Proceedings of the 53rd International Convention
Society of Wood Science and Technology/UNECE-TC. Palais des Nations,
Geneva, Switzerland. Paper WS-89, 10 pp.
Wålinder, M.E.P., Segerholm, B.K., Larsson Brelid, P. and Westin, M.
(2010). Liquids and coatings wettability and penetrability of acetylated
scots pine sapwood. In: Proceedings of the 5th European Conference on Wood
Modification, ECWM5. Hill, C.A.S., Militz, H. and Andersons, B. eds. Riga,
Latvia. pp. 381-388.
OUTLINE OF THE THESIS
Chapter 1 presents the general context of the thesis, a short background of
wood plastic composites and the chemical modification of wood using
acetylation, thermal modification and furfurylation, and finally the
statement of objectives.
Chapter 2 describes the materials manufactured and evaluated in this
thesis.
Chapters 3, 4 and 5 summarize and discuss the experiments and results of
the moisture sorption, the biological durability and the micromorphology
studies.
Finally, Chapter 6 presents conclusions that can be drawn from the work
in this thesis as well as some suggested future activities related to this
work.
xvii
1. INTRODUCTION
1.1 WOOD PLASTIC COMPOSITES
New types of biobased building materials made from renewable resources,
such as wood, are becoming more interesting. The main reason for this
can be related to global environmental challenges and the need for a
sustainable development in society. There is a long tradition of using
thermoplastic materials, such as polyethylene and polypropylene, and an
even a longer tradition of using wood-based materials, such as
particleboards and fiberboards. A new material has emerged, which is a
combination of a thermoplastic component and a wood based component,
known as wood-plastic composites (WPCs). The term wood-plastic
composites is historically a broad term that includes all wood containing
composites in both a thermoset or a thermoplastic matrix. In this thesis,
only wood-thermoplastic composites are considered, and the term WPCs
will be used throughout this thesis for this type of material.
To produce WPCs, wood residuals, e.g. sawdust, wood shavings or wood
flour, are mixed with a thermoplastic polymer and melt processed (or
thermoformed) into its final shape, either as a continuous profile forming
through extrusion or as a three dimensional form through injection
molding. During recent decades, WPCs have rapidly increased their market
share as a building material (Carus and Gahle 2008). Despite the recent
economic downturn, which has leveled off this growth, a continued global
market advance is predicted (Anonymous 2011). This success may partly
be attributed to the fact that WPCs are a competitive alternative to tropical
hardwoods and that they are considered to require less maintenance than
conventional wood products. Using WPCs it is also possible to
manufacture more complex shapes than with solid wood products, with a
raw material yield close to 100%.
1
Figure 1. WPC cable channel lid manufactured by OFK Plast AB, Karlskoga,
Sweden.
The initial growth of WPCs as a building material was in the decking
market (Clemons 2002), other profile products, for example sidings,
fencing, and piling also exists to a lesser extent. Three dimensional forming
of WPCs by injection molding allows for more freedom when designing
the product, and the product can be finalized in one processing step.
Figures 1 and 2 demonstrate two innovative products made out of WPC
material. Figure 1 shows a cable channel lid made of WPC used to cover
cable channels along the railroads, and Figure 2 demonstrates a new type
of injection molded pallet that meets the mechanical requirements of EU
pallets. The pallet is designed with a fit function that enables stacking at a
height that is less than the sum of the height of the individual pallets, and
consequently, lower than the stacking height of conventional pallets.
Some advantages of WPCs compared with glass fiber reinforced or
mineral filled thermoplastics are less environmental impact (e.g. lower
embedded energy, smaller carbon footprint, and better recyclability), a less
abrasive processing, lower price, increased cooling rate (leading to a
decrease in product cycle time in injection molding) (Youngquist 1995),
2
and in addition, lower thermally induced movements than unfilled plastics.
From the same perspective, some disadvantages of WPCs are that the
wood component adds the risk of moisture sorption and biological
degradation, and the necessity of maintaining a lower temperature during
processing due to the risk for severe thermal degradation of the wood
polymers. Another main challenge of WPCs is their more brittle behavior,
both in comparison with solid wood and glass fiber reinforced plastics.
Compared with conventional unmodified and preservative treated solid
wood, WPCs are more expensive to produce and have a lower strength to
weight ratio.
Figure 2. Injection molded WPC pallet, design IKEA of Sweden. Construction
currently not in use.
The wood component used in conventional WPCs often originates from
planer shavings or sawdust. The producers of WPCs normally use
commercial wood flour, which has a broad size distribution, and
consequently makes it more difficult to predict the properties of the WPC
products. Typical particle sizes used in WPCs are 10-80 mesh (Clemons
2002). In a comparative study on the effects of particle Stark and
Rowlands (2003) have concluded that it is the particle shape, not the size
that has the greatest influence on strength and stiffness. A more slender
particle will redistribute and transfer stresses better between the particles
3
and the matrix. Therefore, the process for wood particle preparation
should be designed to give particles with high length to width ratio.
However, the size distribution may also be of great importance. For indoor
products, the challenge is mainly to achieve sufficient mechanical
properties of the material for the intended product, since better
mechanical properties give more freedom for the design of a product.
Outdoor durability of WPCs
In the case of outdoor use of WPCs, the challenges are related to the
requirements of being a long lasting and minimum maintenance material.
Exposure to the outdoor environment implies moisture and temperature
fluctuations, the risk of attack by micro-organisms and degradation by UVradiation. The moisture properties, dimensional stability and biological
durability of WPCs are highly dependent on the constituents and the
processing of the material. If the wood component is allowed to sorb
moisture, the risk of irreversible dimensional changes arises, i.e. woodpolymer delamination, and the risk of degradation by fungi and other
micro-organisms also arises.
WPC manufacturers often promote their products as maintenance-free
and highly durable with a lack of cracking and splintering, often offering
10-year warranties (Clemons 2002). However, the first generation of WPCs
has shown to lack in long time durability and failures have led to class
action law suits (Morris and Cooper 1998). Studies on WPCs and moisture
transport have shown that the initial moisture content (MC) is very low
and the rate of sorption is very slow. However, the moisture in the
composites is not uniformly distributed and the outermost part can reach
very high moisture levels, high enough to support fungal degradation
(Gnatowski 2009, Wang and Morrell 2004). It is also important to stress
that even if moisture uptake is slow in WPCs, even when immersed in
water, the uptake may continue over a long period of time. The rate and
extent of moisture sorption increase when the wood content exceeds
approximately 50% of the total weight of the composite (Clemons et al.
2012). In addition, a moist environment will swell the wood particles close
to the surface, and the particles will shrink upon drying. This will cause
stresses within the material and create microcracks, which will expose
4
more particles deeper in the material. This swelling and shrinkage will also
cause cracks at the interfaces between the wood particles and the matrix.
Laboratory soil block tests have shown mass losses of 3-8% due to the
fungal degradation of WPCs after 12 weeks of exposure (Ibach et al. 2003,
Clemons and Ibach 2002). The mechanical durability of WPC products has
been evaluated for WPC products for use in marine waterfront
applications, and it was concluded that moisture ingress into the composite
can reduce strength up to 50% (Smith and Pooler 2000). Improvements
are needed to minimize these problems, and moisture in WPCs with a high
loading of the wood component is definitely one of the key reasons for
their shortcoming in both mechanical durability and resistance to fungal
decay.
WPCs with modified wood
A chemically modified wood component can be introduced into the
composite which may increase both dimensional stability and resistance to
biological attack (see e.g. Segerholm 2007, Ibach and Clemons 2006). This
would improve durability and resistance to decay.
Several environmentally friendly methods for the chemical modification of
wood have been developed, most of them are not new, but have
previously failed to gain commercial interest. Three methods that have
recently have been commercialized are acetylation, furfurylation and
thermal modification (see e.g. Jones et al. 2012, Hill et al. 2010, Englund et
al. 2009, Hill et al. 2007). Acetylation of wood was mentioned as early as in
1928, where acetylation was used in a procedure for isolating lignin (Fuchs
1928). Early work on the acetylation of solid wood in order to improve its
dimensional stability was performed by Stamm and Tarkow in 1947 and in
1950 (Stamm and Tarkow 1947, Tarkow et al. 1946). Research pertaining
to furfurylation was initiated by Stamm in the early 1950s. Most of the
early work was performed by his student Goldstein (cf. e.g. Goldstein and
Dreher 1960, Goldstein 1955). Some of the scientific reports on thermal
modification of wood goes as far back as to Tiemann in 1915 (Hill 2006),
who discovered that thermally modified wood showed reduced moisture
sorption with only small reductions in strength. The reduced moisture
5
sorption of thermally modified wood depends on the initial thermal
degradation of the hemicelluloses (Stamm 1964), which are the most
hygroscopic constituents in the wood. These modification methods have
been extensively studied in the literature, and several improvements have
been made since the work was initiated. These and other modification
methods are thoroughly described in the recent book by Hill (2006).
Figure 3 illustrates two demonstrator products made from modified wood.
Abdul Khalil et al. (2002) have shown that thickness swelling and moisture
levels of WPCs can be reduced by using acetylated Acacia magnum as a
wood component. The amount of wood in that study was between 20 and
50% by weight. Resistance to biological decay has been studied in
laboratory tests, both in single fungi test jars and in soil boxes with a
variety of different fungi and micro-organisms (Westin et al. 2006, Ibach et
al. 2003, Clemons and Ibach 2002), as well as in field and marine tests.
Current standards for fungal resistance have been developed for testing
solid wood (see e.g. ENV 807, EN 252, EN 113), however, the slow
moisture uptake of WPCs makes these standard test methods
inappropriate for use directly, and modifications of such test procedures
are needed. Pre-treatment to increase the initial moisture content has been
used, but remains to be further refined (Defoirdt et al. 2010, Van Acker
2006, Ibach et al. 2003, Clemons and Ibach 2002).
In the present investigation, the conceptual idea was to use residuals from
the production of modified wood or fibers, such as sawdust, shavings or
boards rejected because of cracks or discoloration. This will mean that no
additional wood resources were used and the waste products were turned
into value added products. An increase in the resistance of the wood
component to moisture and fungal decay could enable a significantly
higher weight-% wood in WPCs for outdoor use, which could result in a
lower overall cost of the composite because of less use of the generally
more expensive thermoplastic matrix.
6
Figure 3. Examples of new products with modified wood. Left image: Boat deck
made of furfurylated wood (Ziethén et al. 2009), right image: Row boat made of
acetylated wood.
1.2 STATEMENT OF OBJECTIVES
The objective of this thesis was to study some characteristics of wood
plastic composites that incorporate an acetylated, a thermally modified or a
furfurylated wood component in a polypropylene, a polyethylene, a
cellulose ester or a polylactate matrix to gain knowledge about the
behavior of such materials in a long term perspective. The emphasis has
been on studying:
•
•
•
Moisture properties of these composites both in liquid and vapor
phases.
Biological performance both in laboratory tests and long term
outdoor field tests.
Micromorphological features of the WPCs and mechanical damage
done to the wood component during processing.
7
2. MATERIALS PREPARATION
All the WPCs prepared in this work contained at least 50 weight-% (wt.%) wood, and the extruded WPC materials contained 60-70 wt.-% wood.
Such a high wood content entails challenges both to dispersing the wood
components during processing and also to protecting the WPCs against
moisture sorption, moisture induced movements and biological
degradation. The primary idea in this thesis was to develop a new type of
durable high-wood-content WPCs containing a chemically modified wood
component, that exhibit superior moisture resistance, dimensional stability
and resistance to biological degradation.
2.1 MODIFIED WOOD
A brief description of the modified wood used in this thesis is given in this
section. The unmodified wood used in the thesis was prepared from Scots
pine (Pinus silvestris) sapwood boards.
Acetylation with acetic anhydride
Modification of wood by acetylation is a single-site reaction where one
acetyl group replaces one hydroxyl group in the cell wall polymers. In the
modification procedure, the wood material is impregnated with acetic
anhydride and then reacted at an elevated temperature (Rowell 2006a). The
only by-product produced is acetic acid. The resulting modified wood
material exhibits increased dimensional stability, maintained strength,
decreased equilibrium moisture content (EMC), and superior resistance to
biological degradation (Larsson Brelid 1998).
In this thesis, acetylated Scots pine (Pinus silvestris) sapwood boards were
prepared by A-Cell Acetyl Cellulosics AB in their pilot plant (now located
at SP, Borås, Sweden) with a 0.66 m3 reactor (Figure 4), according to a
simplified procedure without the use of any catalyst or co-solvent in the
reaction (Larsson Brelid 1998, Rowell et al. 1986). The acetylation level of
the wood material used in this study was approximately 18-23% expressed
as wood acetyl content. The acetylation of fibers was conducted in the
9
liquid phase by BP Chemicals in 100 kg batches, and the degree of
acetylation was about 20% expressed as wood acetyl content.
Figure 4. Microwave heated wood acetylation pilot plant reactor situated in the
EcoBuild laboratory at SP, Borås, Sweden. Left: overview, Right: inside the cylinder.
Thermal modification
Thermal modification of wood results in a change in color and a partial
degradation and rearrangement of the cell wall polymers. The
hemicelluloses are the most affected constituents (Rowell et al. 2009). It is
important to note that the thermal treatment of resinous wood species also
results in modification and a spatial redistribution of the wood extractives
(Nuopponen et al. 2003). The resulting material exhibits a higher
dimensional stability, a reduced hygroscopicity and improved resistance to
microbial decay, but has reduced mechanical properties. There are four
major heat treatment methods used in Europe today; ThermoWood, Oil
Heat Treatment, Plato Wood and Retification. These four are similar in
that solid wood is subjected to a temperature of around 200 °C for several
hours in a low oxygen atmosphere (Rapp 2001). In this thesis, thermally
modified Norway spruce (Picea abies) boards were prepared according to
10
the ThermoWood D (Anonymous 2003) procedure. The process has a
peak temperature of 212 °C.
Furfurylation
So-called furfurylation of wood involves treatment with furfuryl alcohol,
which is pressure impregnated into the wood and then in-situ polymerized
within the cell wall using elevated temperature and catalysts (Lande 2008).
There are also strong indications that the furfuryl alcohol initially reacts
with the cell wall lignin and that the formed furan polymer is thereby
chemically bonded to the cell wall (Nordstierna et al 2008). The resulting
material exhibits high dimensional stability, improved mechanical behavior,
except for impact resistance, and improved resistance to fungal decay
(Lande et al. 2004). In this thesis, furfurylated radiata pine (Pinus radiata)
boards were prepared according to Lande et al. (2004) in an industrial pilot
plant of former Wood Polymer Technologies ASA (WPT), now Kebony
ASA, Skien, Norway. Results related to WPCs with a furfurylated wood
component are included in Paper VII.
2.2 PREPARATION OF WOOD COMPONENTS
In Papers II-VII ground wood was prepared from the modified and
unmodified solid wood samples using a two-step grinding process
involving first a disk flaker to produce thin veneers, followed by a knife
ring mill to produce the finer wood components (Figure 5). The size and
shape of the wood component vary depending on the modification of the
solid wood prior to grinding; thermally modified wood disintegrates into
very fine particles with minor variation in size, whereas the acetylated and
unmodified wood disintegrates into larger particles with a greater variation
in size. Paper VI also involved liberated softwood fibers, medium density
fiberboard (MDF) type fibers, which were used as they were or modified
through acetylation.
11
1 mm
Figure 5. Left: disk flaker for preparation of thin flakes below. Right: knife ring mill
for preparation of the wood component for WPCs.
2.3 POLYMER MATRICES
Thermoplastic matrices for the use in WPCs need to be processable at
temperatures below 200 °C, since wood constituents start to thermally
degrade at approximately 150 °C, and around 200 °C the degradation may
be substantial for many wood species (Fengel and Wegener 1983).
Thermal degradation is also dependent on the wood-plastic residence time
at higher temperatures, i.e. if the processing cycle is kept short it is possible
to use a higher peak temperature. Thermoplastic matrices currently used in
commercial WPCs are normally high density polyethylene (HDPE),
polypropylene (PP) or polyvinyl chloride (PVC). Other matrices that have
gained increased interest in recent years are so-called bioderived plastics,
e.g. cellulose esters and polylactates, which are made partly or fully from
renewable resources.
This thesis comprises work with both traditional olefin thermoplastics
such as polypropylene (PP) and high density polyethylene (HDPE), and
12
bioderived matrices, such as cellulose acetate propionate (CAP) and
polylactate (PLA). Both CAP and PLA are able to sorb moisture, and
therefore, need to be pre-dried before processing.
2.4 MANUFACTURING OF WPC SAMPLES
The two main techniques for manufacturing WPCs are injection molding
and extrusion. Normally, a physical mixture of wood components, plastic
and additives is dry blended and compounded on an extruder to achieve
WPC granules prior to injection molding or extrusion. In injection
molding, the granules are melted and injected into a mold to give the detail
its final shape. In extrusion the granules are melted and pushed through a
die, which will gives the profile its shape.
In this thesis, WPC materials with wood and matrix components as
described in Sections 2.2 and 2.3 were prepared by either injection molding
or extrusion. In some cases processing additives were added to the
formulation. Figure 6 illustrates the different WPC materials used in
Papers II–VII. Papers III and VI involve injection molded WPC samples
with a cross section of 4 x 10 mm2 (Figures 6d and e), Paper IV involves
extruded WPC profiles with a cross section of 3 x 13 mm2 (Figure 6c),
Papers II, V and VII involve extruded hollow WPC profiles with a cross
section measuring 60 x 40 mm2 with a wall thickness of 8 mm prepared
using a conical extruder (Conex®) (Figure 6a), and Paper VII also involves
extruded decking profiles with a cross section measuring 140 x 25 mm2
(Figure 6b). As mentioned in Section 2.1, WPCs with a furfurylated wood
component were only included in Paper VII. This is due to extrusion
processing difficulties, which meant that only a very limited amount of the
composites were considered acceptable for inclusion in the further
research studies.
13
a
c
b
d
e
Figure 6. WPC materials used in the thesis: a) Extruded hollow profile, b)
Extruded solid decking profile, c) Extruded 3 x 13 mm2 profile, d) Injection molded
tensile test bar, e) Injection molded bend test bar (dark color due to thermally modified
wood component).
14
3. MOISTURE CHARACTERISTICS
Moisture intrusion in WPCs directly affects the dimensions of the material
due to the swelling forces of the wood component. Indirectly, if the
material is able to sorb moisture, it will be more susceptible to decay by
fungi and other micro-organisms. In WPCs with PP and HDPE, the
polymer matrix constitutes a hydrophobic phase and the wood component
a hydrophilic phase. Because of this, the moisture transport into the
material mainly takes place through the wood component and internal
void spaces in the material. When CAP and PLA are used, the matrices are
able to sorb moisture and, thus, also contribute to the moisture transport
into the material. The surfaces of WPCs are usually smooth, and a thin
surface layer generally consists of a high proportion of the polymer matrix.
In the initial stage of moisture exposure, this polymer-rich surface phase
retards the rate of moisture sorption. Therefore, the WPCs will appear to
be very moisture resistant. However, when the composite is subjected to
UV radiation and/or water, this surface layer may be degraded, resulting in
a decreased moisture resistance, and a critical intrusion of water into the
bulk of the material may occur more easily.
3.1 WATER VAPOR SORPTION
In Papers III-VI the water vapor sorption properties of the WPC materials
have been evaluated. Thin specimens were used in these studies; a
thickness of 2 mm in Papers III and V and 1 mm in Paper IV. All surfaces
were sealed except for the original outer surface of the WPCs. In Paper VI,
the inlet piece for the injection molded bending test bars was used for
equilibrium moisture content measurements. The moisture sorption rate
was very slow for all the WPC materials, especially slow for the injection
molded WPCs due to the lower amount of wood component in those
composites compared to the extruded WPCs. In Paper III for WPCs with
PP the test was ended after 18 months, and in Paper VI for WPCs with
PLA as matrices the test was ended after 21 months. In both tests the
specimens had still not reached their equilibrium moisture content (EMC)
at the end of the tests. However, in Paper III, another set of the WPCs
with PP as the matrix was subjected to artificial weathering prior to the
15
water vapor sorption test, and in that case the specimens reached EMC in
much shorter times (approximately 120 days). In addition, composites with
CAP as the matrix were evaluated in Paper III, and due to the ability of
CAP to sorb and transport moisture, the times to reach EMC were much
shorter and no difference was observed between non-aged and pre-aged
specimens.
In Paper IV, WPC samples subjected to artificial ageing procedures
involving weathering, and white- and brown- rot decay (Table 1) were
evaluated in dynamic vapor sorption experiments. Figure 7 shows the
moisture uptake of WPC samples with an unmodified wood component
when the climate was changed from a conditioned state at 65% relative
humidity (RH) to a climate with 90% RH at a constant temperature of
27 °C. As can be seen, the artificial weathering of the specimens changed
the rate of moisture sorption substantially compared to the non-weathered
samples. The WPCs with an acetylated wood component were also
affected by the artificial weathering giving higher sorption rates than the
non-weathered samples However, the rate and level of moisture gain was
much lower for the WPCs with an acetylated wood component than it was
for the WPCs with an unmodified wood component (Figures 7 and 8).
Table 1. Artificial ageing procedures used for the WPC samples with 50 wt.-% wood
component in Paper IV
Material
notation
Artificial ageing procedure
In
Figure
U1
A1
Room temperature (22°C) water soaking for 2 weeks
7
8
U2
A2
Artificial weathering for 1000 hours, followed by room
temperature (22°C) water soaking for 2 weeks
7
8
U3
A3
U4
A4
Artificial weathering for 1000 hours and room temperature (22°C)
water soaking for 2 weeks, followed by a modified soil block test
procedure based on ASTM D1413 with Trametes versicolor, a whiterot fungus
Artificial weathering for 1000 hours and room temperature (22°C)
water soaking for 2 weeks, followed by a modified soil block test
procedure based on ASTM D1413 with Gloeophyllum trabeum, a
brown-rot fungus
16
7
8
7
8
U1
moisture uptake (%)
4
U2
U3
U4
3
2
1
0
0
100
200
300
0.5
√time (s )
400
500
Figure 7. Moisture uptake versus square root of time when relative humidity is raised
from 65% to 90% for extruded WPC specimens with 50 wt.-% unmodified wood
component.
A1
moisture uptake (%)
4
A2
A3
A4
3
2
1
0
0
100
200
300
0.5
√time (s )
400
500
Figure 8. Moisture uptake versus square root of time when relative humidity is raised
from 65% to 90% for extruded WPC specimens with 50 wt.-% acetylated wood
component.
17
The rate of sorption is, to a large extent, governed by the polymer matrix
and the wood content in the WPCs. However, the modifications by
acetylation, furfurylation or thermal treatment of the wood component
have a substantial effect on the amount of moisture that can be sorbed by
the material. By using an acetylated wood component in the WPCs, the
levels of moisture sorbed can be reduced to at least 50% of that for the
WPCs with unmodified wood. Such a reduction in moisture sorption in
the material also has an effect on the dimensional stability of the material.
3.2 WATER SOAKING
Water soaking experiments were performed, both to precondition the
specimens to be evaluated regarding biological durability and to study
micromorphological changes due to moisture sorption.
In Paper V, specimens for biological testing were preconditioned by water
soaking. Specimens measuring 5 x 10 x 100 mm3 were cut from the
extruded hollow profiles with approximately 70 wt.-% wood. The water
soaking resulted in very high moisture contents (MC), for the extruded
WPC samples with unmodified, acetylated and thermally modified wood
components after a total soaking time of 4 weeks (Table 2). On the other
hand, in Paper VI the injection molded WPC samples with 50 wt.-% wood
and PLA matrix subjected to 2 weeks of soaking only reached 3.7% MC
for the sample with unmodified wood. The MC for the WPC samples with
a modified wood component was even lower.
Table 2. Moisture content (MC) before and after soaking in water for extruded WPC
specimens with 70 wt.-% wood component.
WPC material
(30/70 w/w)
Initial MC
in WPCs
Final MC
in WPCs
Final MC based on
wood component
PP/unmodified pine
3.2
24.1
34.4
PP/acetylated pine
1.4
8.1
11.6
PP/thermally modified spruce
2.5
15.2
21.7
18
4. BIOLOGICAL DURABILITY
There are three main types of wood degrading fungi, white-, brown-, and
soft-rot. White rot preferentially degrades lignin, brown rot mainly attacks
hemicelluloses and cellulose, and soft rot degrades all cell wall polymers
(Eaton and Hale 1993). A criterion for these fungi to attack wood is that
moisture within the material reaches a certain level. As described in
Chapter 3, the moisture transport in WPCs is very slow. Consequently, the
time for gaining sufficient moisture suitable for attack by decaying fungi is
much longer for WPCs than for solid wood. Another factor influencing
the biological durability of WPCs is the amount of wood in the
composites. At higher levels (above about 50%), the wood component
may form an interconnecting network in the matrix which accelerates the
moisture transport into the material. This both affects the biological
durability and the dimensional stability of the composite.
4.1 LABORATORY TESTING
The laboratory testing included in this thesis was performed using a
terrestrial microcosm (TMC) test according to an extended version of
ENV 807. Prior to the TMC test the specimens were subjected to
preconditioning involving a 2 week leaching procedure in de-ionized
water, according to EN 84, followed by a 14-day water soaking in deionized water and then direct insertion into the test soils. Scots pine (Pinus
silvestris) control specimens were used to follow the fungal activity in the
soils. Three different soils were used in the TMC tests, TMC1 was a
compost soil with a high activity of soft rot, TMC2 was a soil from the
Simlångsdalen test field with predominately brown-rot decay, and TMC3
was a forest soil with high activity of white-rot decay. The specimens were
buried three quarters of their lengths in the soil, Figure 9 shows the
experimental setup and a schematic drawing of the specimens inserted in
the soil. After the test, the specimens were cleaned and dried, and after
weighing the mass losses were calculated. The corrected mass loss, solely
caused by decay was calculated by subtracting mass loss due to leaching
from specimens from sterile soils.
19
Specimen size
100 x 10 x 4 mm3
25 mm
Soil
75 mm
Figure 9. TMC soil box with inserted specimens and right: schematic drawing of the
specimen inserted in the soil.
Extruded profiles with 70 wt.-% modified wood are evaluated in the TMC
test in Paper V. Specimens 5 x 10 x 100 mm3 were cut out from the
extruded profiles in such a way that the outer polypropylene (PP)-rich
surface was removed. The findings showed that the WPC control sample
with unmodified wood had mass losses due to decay. The WPCs with
thermally modified wood also had slight mass losses, but decay could not
be verified by microscopy. Mass losses for WPCs with acetylated wood
were very low and no decay could be detected by microscope. Figure 10
shows the corrected mass losses after 32 weeks of exposure to the test
soils.
20
60
Mass loss (%)
50
40
55
Compost soil
Simlångsdalen soil
42
Forest soil
30
20
18
7
10
0
0 0 0
Pine control PP control
5 3
1 1 1
0 0 0
WPC
WPC
WPC
Unmodified Thermally Acetylated
wood
modified
wood
wood
Figure 10. Corrected mass loss after 32 weeks exposure to three different TMCs for
extruded WPCs with 70 wt.% of unmodified, thermally modified or acetylated wood
component as well as for pine solid wood and PP control specimens.
Injection molded WPCs are evaluated in Paper VI. The wood components
used were both in the shape of fibers and also in the shape of particles.
The thermoplastic used in this study was a polylactate (Nature Works R
4042D, NatureWorks LLC, Minnesota, USA) which originates from
renewable resources and is possible to compost at the end of life. Tensile
test bars with 50 wt.-% were prepared, the specimens measured 4 x 10 x
80 mm3, no further processing of the WPCs before the biological testing
were needed. The biological test resulted in very low mass losses due to
decay, however, the mechanical evaluation before and after the biological
test showed major differences. The mechanical evaluation showed that the
PLA unmodified wood fiber composites lost great amounts in strength
and stiffness, this was mainly due to moisture-induced movements of the
wood component and not due to biological decay.
21
4.2 FIELD TESTING
In field tests, materials are subjected to an environment which, in
comparison to laboratory experiments, represents the real world.
However, field tests are very time-consuming and several years may pass
before any detectable decay occurs, and an even longer time may pass
before there is substantial decay in the tested materials. The correlation
between results from laboratory experiments and field tests is often
difficult to do, especially for new types of materials. This is true for WPCs
for which a limited number of field test studies of biological durability
have been conducted.
The field and marine tests are evaluated in Paper VII. Extruded profiles
with PP and CAP have been in a horizontal double layer test (Figure 11)
and in ground tests (EN 252, Figure 12) for 7 years and in a marine test
(EN 275) for 6 years. The samples in each test have been evaluated
annually and remain in the field and marine sites.
Figure 11. Double layer test set up.
22
Figure 12. In ground test set up.
Horizontal double layer
In the horizontal double layer test specimens have been placed
horizontally in two layers on top of each other with a half width shift of
the upper layer to the lower layer, the specimen size is 50 x 25 x 500 mm3
except for the extruded hollow profiles which were only cut in length to
500 mm in length. The rig has been built so that the specimens can be
placed 200 mm above ground level. The samples with an unmodified
wood component showed major dimensional changes due to moisture
pick-up at an early stage, and all stakes had a substantial index of decay
after 7 years of testing in the horizontal double layer. The samples with a
modified wood component still perform very well and are still considered
to be sound. Figure 13 shows three extruded WPC stakes from the double
layer test after 7 years of exposure. The bottom stake has been made with
an unmodified wood component and shows severe growth of fungal
mycelium.
23
Figure 13. Three extruded WPC stakes with PP matrix and 70 wt.-% wood from
the double layer test after 7 years of exposure. From top to bottom; thermally modified,
acetylated and unmodified wood component.
In ground test
In the in ground test, stakes have been buried to half their length in soil.
The specimen size is 50 x 25 x 500 mm3 except for the extruded hollow
profiles which were only cut in length to 500 mm in length. The findings
from the test sites in Borås and Simlångsdalen are presented in Paper VII.
In the in ground test, the degradation of the materials has been at a higher
rate than in the horizontal double layer. All the stakes with unmodified
wood component show decay. Some of the stakes with modified wood
components show signs of decay, but are still considered to be sound.
Figure 14 shows two extruded WPC profiles with an unmodified and an
acetylated wood component. The right part of the stakes has been above
ground and the left part in the ground. The white appearance is due to
UV-degradation and the swollen and twisted appearance of the upper
stake is due to moisture induced movements of the unmodified wood
component.
24
Figure 14. Two extruded WPC stakes with PP matrix and 70 wt.-% wood from the
in ground test after 7 years of exposure, top stake with unmodified wood component and
bottom stake with acetylated wood component.
Marine test
Marine testing of WPC specimens is being conducted according to EN
275. The test specimens measuring 50 x 25 x 200 mm3 with a hole in the
center were hung on ladder like rigs, as shown in Figure 15. The test site is
in the bay outside the Sven Lovén Centre for Marine Sciences in
Kristineberg, 100 km north of Gothenburg, Sweden. The rigs are removed
annually from the water, specimens are cleaned of fouling (overgrowth)
organisms, visually rated, and X-rayed. After this, the specimens are
immediately placed at the sea bottom again. In this test, specimens with
CAP as the matrix with an unmodified, an acetylated or a thermally
modified wood component were included. Results show that the CAP
unmodified wood WPCs were destroyed by shipworm attack after two
years. Reference solid unmodified pine wood is often destroyed after one
year. The CAP WPCs with modified wood are still sound after 6 years and
the test is still in progress. Figure 16 shows an X-ray image of a sound
board of CAP thermally modified wood WPCs after 6 years of exposure
and one solid wood control board which was completely destroyed by
shipworm after one year of exposure.
25
Figure 15. Test rig used for the marine test, left image: just after removal from the sea
bottom at the annual evaluation, right image: after annual evaluation, before
replacement at sea bottom.
Figure 16. X-ray image of an extruded CAP thermally modified wood WPC after 6
years exposure(left) and a solid wood control board after one year exposure (right).
26
5. MICROMORPHOLOGICAL FEATURES
5.1 CHANGES IN SIZE AND SHAPE OF THE WOOD
COMPONENT DUE TO THE EXTRUSION PROCESSING
The thermoforming processing of WPCs requires both high temperatures
(in general at least 170–180 ºC) as well as intense shearing action in order
to ensure sufficient mixing and dispersion of the wood component in the
matrix. It is evident that such processing conditions may lead to severe
degradation of the wood component, i.e. apart from the obvious thermal
degradation, also a mechanical damage, and combinations thereof, with
resulting change in size and shape (Rowell 2007). Studies by Glasser et al.
(1999), Nitz et al. (2000) and Rowell (2006b) indicate that the extrusion of
WPCs causes damage to the wood component, mainly as a reduction in
size of the wood component. Gacitua et al. (2008) have applied a
nanoindentation technique to investigate the damage to the wood cell wall
caused by WPC processing. The processing in this case resulted in a
reduction of the Young’s modulus of the S2 layer of the wood cell wall by
40-70%. Furthermore, latewood cell walls were found to suffer less
collapse and damage than earlywood cell walls.
The damage done to the wood component varies depending on many
parameters e.g. the inherent wood properties, the original size and shape
of the wood component, the melt properties of the matrix, the effects of
additives used, screw design and the size of the extruder. The effects of
using different types of modified wood components on such extrusionrelated mechanical degradation are investigated in Paper II. The wood
components originated from solid acetylated, thermally modified and
unmodified wood and were ground by means of a newly developed twostep process. This process enables a greater aspect ratio (fiber direction
length vs. width) than conventional wood meal or sawdust raw material.
The WPCs were extruded into hollow profiles on a conical extruder. Paper
III involves unmodified and acetylated wood components which were
injection molded into WPC tensile test bars. In both Papers II and III it
was observed that the wood component became significantly reduced in
size in all samples due to the thermoforming processing. The most
27
extensive changes could be observed for the thermally modified wood
component. By isolating the wood component from the composites
through an extraction process in boiling xylene, it was possible to analyze
the wood component size and shape after processing (Paper II). Figure 17
shows the extracted wood components from the conical extruded WPCs
with 70 wt.-% wood. The difference between the wood components can
clearly be seen, i.e. the estimated average size of the thermally modified
wood component is several times smaller than the acetylated and
unmodified wood components. The shape of the acetylated wood
component appeared to have a more splinter like shape than the
unmodified wood component which had a more rounded shape.
a
b
c
Figure 17. Wood components extracted from the extruded WPCs with (a) thermally
modified spruce, (b) acetylated pine, and (c) unmodified pine.
Micromorphological examination of the fracture surfaces revealed a higher
amount of larger sized wood particles in the fracture zone for the WPCs
with unmodified and acetylated wood components than in the WPCs with
a thermally modified wood component. These larger wood components
could possibly have less ability to redistribute stresses in the material than
smaller ones, which could also explain why the WPCs with thermally
modified wood had the highest tensile strength according to the
micromechanical evaluation presented in Paper II.
28
5.2 SPECIMEN PREPARATION TECHNIQUE BY MEANS OF UV
EXCIMER LASER ABLATION
There are several methods used for examining the inner micromorphology
(or microstructure) of materials such as wood and composites. One way to
study the microstructure of composites is to examine fracture surfaces
after, e.g. tensile strength testing, from which the internal microstructure
may be estimated. However, it must be emphasized that the microstructure
in the fracture zone after such tests has most likely been deformed, and
subsequently represents the weakest part of the material and not the
average structure of the composite. Nevertheless, examinations of the
fracture surfaces may provide important information, such as fiber-matrix
compatibility, fiber embedment and fiber pullout.
A key challenge for the evaluation of the micromorphology of WPCs
relates to the specimen preparation technique for preparing mechanically
undamaged surfaces for scanning electron microscopy (SEM). Traditional
techniques for preparing surfaces for microscopy of soft materials such as
wood often involve an initial moistening followed by mechanical treatment
of the surface, such as microtoming, razor blade cutting or polishing. Such
techniques may lead to uncontrolled damage such as microcracks and
distortion of the surface morphology as well as contamination and
redistribution effects. It is especially difficult to prepare surfaces of
material with two distinctively different phases such as in wood plastic
composites.
A preparation technique based on UV excimer laser ablation is
demonstrated in Paper I. This technique for ablating wood surfaces was
developed at KTH (Stockholm, Sweden), in the early nineties (Stehr et al.
1998, Seltman 1995). The laser used is a pulsing UV excimer laser
(LAMBDA PHYSIK LTD 210 ICC) that applies the wavelength 248 nm.
Each pulse corresponds to 1 J/cm2 energy and the pulse frequency was
varied between 1 and 100 Hz in this study. Figure 18 shows the UV laser
laboratory set-up and an SEM micrograph of a sawn wood cross section
before and after ablation.
29
5
2
1
3
4
6
0.5 mm
Figure 18. Left: UV excimer laser apparatus, 1) operating panel, 2) UV laser,
3) focus lenses, 4) specimen holder. Right: SEM micrograph of 5) sawn wood surface,
6) UV laser ablated wood surface.
The technique has shown to be well suited for precision micromachining
of wood and other lignocellulosics, as well as certain polymers and plastics.
The prepared samples can reveal the micromorphology features of the
composites, for example, the orientation and size of the wood component,
cell wall damage, the collapse and compression of the cells, the polymer
filling of the lumens, porosity and the interface delamination between the
components.
5.3 MICROMORPHOLOGY OF THE WPC SAMPLES
Figures 19 to 22 in this section show SEM micrograph examples of the
micromorphology of some WPC samples, where all surfaces have been
prepared using the micromachining technique with UV excimer laser
ablation. Figure 19 shows an injection molded WPC with 50 wt.-%
unmodified wood and PP as the matrix. In this figure the following
important micromorphological features can be seen:
30
e
b
d
a
c
Figure 19. Micromorphology of an injection molded WPC with 50 wt.-% unmodified
wood (lighter colored phase) and PP as matrix (darker colored phase);
a) cell wall fragments, b) earlywood tissue, c) compressed earlywood tissue,
d) latewood tissue, e) interface delamination.
1) A pronounced variation in the size and shape of the wood component
(lighter phase), i.e. from fine cell wall fragments (a) to larger portions of
wood tissue or bundles of wood fibers (d). 2) The lumens of the wood
tissue are filled with the thermoplastic matrix (darker colored phase),
leading to a low porosity, normally in the range of a few percent, in the
composites. 3) Wood tissue consisting of mostly latewood cells often
shows no cell wall compression or collapse (d). 4) Wood tissue consisting
of mostly earlywood cells often shows compressed or collapsed cells (b) or
is separated into cell wall fragments (a). 5) Wood-polymer interfacial
delamination or microcracks (e). The disintegration into cell wall fragments
was mostly pronounced for the WPCs with a thermally modified wood
component, which resulted in a more homogeneous WPC
micromorphology than the WPCs with unmodified and acetylated wood
31
components (Paper II). This was also reflected in the micromechanical
evaluation where WPCs with a thermally modified wood component
showed the greatest strength.
Figure 20 shows a micrograph of an injection molded WPC with a 50 wt.% acetylated wood component. In the figure it can be seen that many of
the wood particles have been split into single fibers and fiber fragments.
No such extensive fragmentation could be observed in the WPCs with an
unmodified wood component (Figure 19). This micromorphological
observation is in good agreement with the light microscopy analysis of
extracted wood component from the composites (cf. Figure 17).
Figure 20. Micromorphology of an injection molded WPC with 50 wt.-% acetylated
wood and PP as matrix.
The local interfacial delamination as indicated in Figure 19, see the
denotation (e) indicates a poor adhesion or compatibility between the
hydrophilic wood and the hydrophobic PP matrix, and that such adhesion
32
properties may vary greatly within the composites. In a parallel
investigation to this thesis (Bryne and Wålinder 2010 and Bryne et al.
2010), surface energy and surface chemical composition characteristics of
modified wood and so-called wood-thermoplastic interaction or adhesion
parameters have been studied; and the results indicate that acetylated wood
has distinctly different adhesion abilities with certain thermoplastics.
Another possibility to study such wood-polymer adhesion and to explore
this area further would be by using a technique called inverse gas
chromatography (IGC), see e.g. Tze et al. (2006). This technique is
suggested as a possible future area of research related to this thesis.
When WPCs are processed, moisture is ventilated out from the composite
so the final product contains very little moisture, less than the equilibrium
moisture content of the wood component in most environments. This
implies that the WPC gains moisture with time and the wood component
swells due to the moisture uptake. Depending on the level and rate of
uptake as well as cyclic moisture gain and loss, irreversible deformations of
the WPC may occur along with interfacial delamination between the wood
and thermoplastic. The micromorphology of extruded WPCs with 70 wt.% wood and PP is evaluated before and after moisture soaking and drying
in Paper V. Figure 21 shows a micrograph of a WPC with unmodified
wood subjected to one cycle of water soaking followed by drying. There
large cracks are visible between most of the wood components and the
thermoplastic matrix. The WPCs with acetylated and thermally modified
wood components were also subjected to the same soaking procedure, but
crack formation at wood-polymer interfaces was greatly reduced due to the
more dimensionally stable wood component (Figure 22).
33
Figure 21. Extruded PP-unmodified wood WPC with 70 wt.-% wood after one
soaking drying cycle.
Figure 22. Extruded PP-acetylated wood WPC with 70 wt.-% wood after one
soaking drying cycle.
34
6. CONCLUSION AND FUTURE WORK
6.1 CONCLUSIONS
The work presented in this thesis demonstrates the feasibility of using a
modified wood component in wood plastic composites. The incorporation
of a modified wood component into WPCs allows for a higher wood
content without compromising on the long term durability of this type of
material. The following conclusions can be drawn from this thesis:
•
•
•
•
•
•
The use of a modified wood component in WPCs decreases moisture
movements in the material. Accelerated weathering or the use of a
hygroscopic matrix material will increase the rate of moisture sorption
into the composite, however the final moisture levels are unchanged.
WPCs with an acetylated wood component showed greatly reduced
crack formation caused by moisture induced movements than
compared to WPCs with an unmodified wood component.
Laboratory decay tests show low mass losses due to decay for WPC
materials. However, a mechanical evaluation prior to and after testing
may reveal losses in strength due to decay at an earlier stage than mass
loss.
High wood content WPCs with a modified wood component perform
well in field testing. After a 7year in ground test all composites, except
the high wood content WPCs with an unmodified wood component,
are considered sound.
Mechanical action during processing severely damages the wood
component. The damage was most pronounced for the thermally
modified wood component, which after processing showed
significantly shorter and more damaged wood tissues than before the
extrusion process.
Surface preparation by means of an UV-excimer laser ablation
technique was shown to be well suited for analysis of the
micromorphology of these types of wood-plastic combinations.
35
6.2 FUTURE WORK
Future challenges for WPCs are, among others, to try to tailor interface
properties in order to achieve a composite that performs well even after it
has been subjected to moisture and temperature fluctuations. By analyzing
the surface properties of the different wood components as pertains to
surface energy, wetting properties and surface heterogeneity, and
comparing these to different thermoplastics properties, theoretical
predictions of interaction parameters can be obtained for different wood
thermoplastic combinations. Inverse gas chromatography (IGC) will be
used to study surface energetics (gas-solid interactions) and surface
heterogeneity of the components. This will give a basis for the theoretic
determination of the properties for different wood-thermoplastic
combinations. Such measurements, in combination with practical adhesion
studies, may contribute to the knowledge about adhesion phenomena in
wood plastic composites (WPCs).
36
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43
APPENDED PAPERS (I-VII)
Division of the work in the appended papers:
I.
Wålinder, M.E.P., Omidvar, A., Seltman, J. and Segerholm, B.K.
“Micromorphological studies of modified wood using a surface
preparation technique based on ultraviolet laser ablation”
Wålinder initiated the work. Segerholm, Omidvar and Seltman
performed the experiments. Segerholm and Wålinder interpreted the
results and jointly wrote the paper.
II.
Segerholm, B.K., Vellekoop, S. and Wålinder, M.E.P.
“Process-related mechanical degradation of the wood component in
high-wood-content wood-plastic composites”
Segerholm and Wålinder initiated the work. Segerholm and
Vellekoop performed the experiments and all authors jointly
interpreted the results. Segerholm wrote the paper.
III.
Segerholm, B.K., Walkenström, P., Nyström, B., Wålinder, M.E.P.
and Larsson Brelid, P.
“Micromorphology, moisture sorption and mechanical properties of
a biocomposite based on acetylated wood particles and cellulose
ester”
The consortium of Ecocomp initiated the work. Segerholm
performed the sorption and micromorphological experiments,
Walkenström performed the mechanical characterisation, Nyström
performed the particle characterisation. The authors jointly
interpreted the results. Segerholm wrote most of the paper.
45
IV.
Segerholm, B.K., Ibach, R.E. and Wålinder, M.E.P.
“Moisture sorption in artificially aged wood-plastic composites“
Segerholm and Ibach initiated the work, performed the experiments
and all authors jointly interpreted the results. Segerholm wrote the
paper.
V.
Segerholm, B.K., R.M. Rowell, P. Larsson Brelid, M.E.P. Wålinder,
M. Westin and G. Alfredsen.
“Improved Durability and Moisture Sorption Characteristics of
Extruded WPCs made from Chemically Modified Wood”
Segerholm initiated the work. Segerholm performed the sorption
experiments. Westin and Alfredsen performed the durability tests
and all authors jointly interpreted the results. Segerholm wrote the
paper.
VI.
Segerholm, B. K., Ibach, R. E., and Westin, M.
“Moisture sorption, biological durability, and mechanical
performance of WPC containing modified wood and polylactates”
Segerholm initiated the work. Segerholm, Ibach and Westin
performed the experiments. All authors jointly interpreted the
results. Segerholm wrote the paper.
VII. Segerholm, B. K., Larsson Brelid, P., Wålinder, M. E. P., Westin, M.,
and Frisk, O.
“Biological outdoor durability of WPC with chemically modified
wood”
Westin initiated the work. Westin and Larsson Brelid performed the
durability assessment. All authors jointly interpreted the results.
Segerholm and Larsson Brelid jointly wrote the paper.
46