Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
Recent Progress in the Industrial Implementation of ThermoHydro (TH) and Thermo-Hydro-Mechanical (THM) Processes
Dick Sandberg1 and Andreja Kutnar2
1
Luleå University of Technology, Wood Technology, SE-931 87 SKELLEFTEÅ, Sweden
[email: [email protected]]
2
University of Primorska, Andrej Marušič Institute, Muzejski trg 2, SI-6000 Koper, Slovenia
[email: [email protected]]
Keywords: ageing, densification, heat treatment, laser treatment, moulding, welding
ABSTRACT
The implementation of thermo-hydro (TH) and thermo-hydro-mechanical (THM)
techniques on an industrial scale dates back to 1850, when Michael Thonét introduced
his invention for the bending of solid wood. The development and commercialization of
processes for the softening of wood and for the kiln drying of sawn wood followed.
More recently, several thermal wood treatments have reached an industrial scale and the
markets, and new thermal processing techniques are continuously being developed and
introduced onto the market. As a result, the volumes of thermal-treated wood for interior
and exterior use are continuously growing. This paper presents the latest developments
in the field of TH and THM processing with a focus on processes that are in an
implementation phase for industrial production or have the potential for such
development. Challenges faced during the commercialization of TH and THM processes
are also discussed. In this paper, the following TH/THM treatments are presented:
•
•
•
•
•
Densified engineered wood products (EWP)
Moulding for two-directional or three-directional shaping of veneer
Ageing of wood
Frictional wood welding and wood surface fusion
Laser treatment for the TH processing of wood surfaces
INTRODUCTION
One of the emerging treatments involves the combined use of temperature and moisture,
and force can be added, so that one speaks of thermo-hydro (TH) and thermo-hydromechanical (THM) methods, Figure 1. TH/THM processing is implemented to improve
the intrinsic properties of wood, to produce new materials, and to acquire the form and
functionality desired by engineers without changing the eco-friendly nature of the
material.
Researchers have discussed whether the term “hydro” or “hygro” should be used to
describe the influence of water during the processing of wood. Hydro comes from Greek
hudōr and is a prefix that means water. Hygro comes from Greek hugrós and is a prefix
that means wet or moist. Depending on which processes are in focus, one of the two
prefixes is more suitable than the other. For simplicity, in this article, we use only the
prefix hydro.
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
Figure 1: Classification of thermo-hydro (TH) and thermo-hydro-mechanical (THM) processes
TH/THM processing has been used by humans for thousands of years. A 5200-year-old
find of a pair of skis shows that the front tips of the skis were bent with the help of THM
action. The ancient Egyptians also used this technique for the manufacture of tools,
weapons, and utility equipment (Navi and Sandberg 2012). Nevertheless, it was not
before 1849, when the inventor of the Vienna Chair, Michael Thonét, started a
workshop for the production of bent solid wood furniture, that the first step was taken
towards the industrialization of a TH/THM process intended for mass production. In the
1850s, the Thonét method was developed for bending thick, solid pieces of beech wood
on a large scale. This method involved wood plasticization by treatment in hot steam,
bending in a special mould and drying in a fixed position. Before 1900, the Thonét Bros
factory had already produced over 50 million of their famous café chair No. 14 and the
chair was being delivered in six parts as the world's first "knock-down” piece of
furniture, Figure 2. At the time of Michael Thonét's death in 1871, the company had
4000 employees and an annual production of 450 000 items of furniture. Twenty years
after Michael Thonét's death, there were 60 factories, mostly within the AustroHungarian Empire.
Figure 2: The famous Thonét café chair No. 14; left - ready for delivery in parts from a factory (36
chairs in 1 m3), and right - finally assembled
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
In the beginning of the twentieth century, the use of heat and moisture into wood
processing came in focus. It had been observed that wood dried at a high temperature
changes colour, and has a greater dimensional stability and a lower hygroscopicity
(Tiemann 1915, Koehler and Pillow 1925). After the First World War, comprehensive
studies were made on the effect of the kiln drying temperature on the strength of wood
for the aviation industry in the United States (Wilsson 1920). In 1946, Stamm et al.
reported the first systematic experiments of wood heat treatment, illustrating an increase
in dimensional stability and in the decay resistance of wood treated at a temperature
between 120 and 320°C, but their results did not come into industrialization before the
mid of 1990s when several thermal-treatment processes were commercialized; the
Finish ThermoWood process one of the best known.
In recent decades, scientists have raised concerns about urgent and complex problems
impacting our environment. As threats to the Earth's ozone layer mount, the planet
warms, agricultural lands turn to deserts, and the threat of carbon dioxide emissions
increase, our very survival may be at stake. One way of reducing the emission of carbon
dioxide is to use more wood products and to increase the life of these products so that
the carbon is bound over a longer period of time. Another possibility is to replace
energy-intensive materials with wood and wood-based products. However, for timber as
a material to be competitive against other materials, its environmental advantages alone
are not enough. Wood must also be competitive for its technical qualities, show a high
material utilisation during further processing and, not least, a competitive economic
yield during usage. The TH/THM technology may be one way to strengthen the
competiveness of wood. In this context, there has since the 1990s been renewed interest
in developing TH/THM products.
The purpose of this paper is to present the latest developments in the field of TH and
THM processing with a focus on wood processes that are close to the implementation
stage for industrial production or have the potential for such development.
RECENT DEVELOPMENTS IN TH/THM PROCESSING
Engineered densified wood products, moulded veneer products, and wood products
modified through ageing, frictional welding or laser treatment are areas in which the
development of TH/THM-technology is especially concentrated. This is especially true
when we are looking at the modification of solid wood and veneer.
Densified engineered wood products (EWP)
Densification is an old modification method to improve wood properties such as
hardness and resistance to abrasion. A major problem with densified wood is, however,
its ability to retain its original dimensions under the influence of moisture. Recent
developments in the compression of wood in the transverse direction have focused on
limiting the shape memory of compressed wood with the help of a TH action or by
mechanically “locking” the compressed wood within the structure (Inoue et al. 1993,
Dwianto et al. 1997, Nilsson et al. 2011). Applications of “densified and shape memory
fixed wood” in different EWPs include panels, beams, columns, and other 3D-shaped
components used mainly for interiors and furniture. The developments are aiming either
(1) to increase the strength and usability of wood for structural purposes, or (2) to
increase the potential for using wood in free 3D-shapes for non-structural purposes.
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
With the aim of providing engineered wood products for structural applications, tubes or
columns of densified glued laminated timber boards of spruce have been developed by
Haller et al. (2004). The tube has a much larger load-bearing capacity than solid wood.
The THM-formed tubes can be optionally reinforced with technical fibres and/or textiles
laminated to the outer wood surface to strengthen and protect the construction against
decay, Figure 3. Besides structural applications in civil engineering, the tubes can also
be used as water pipes (Putzger et al. 2011). This invention is now productized and is
close to being ready for introduction onto the market.
Increasing the density of the surface only, to improve surface hardness rather than
modifying the whole material, could be an advantage in applications where it is
desirable to maintain the low bulk density of wood but improve the surface properties
(Rautkari 2010). In surface densification, only the first few millimetres beneath the
wood surface are compressed, in effect just the first few cell layers. Inoue et al. (1990)
have developed the THM technique for surface densification to improve abrasion
resistance and hardness. The surface was saturated with water through narrow grooves
(2 mm wide and 5 mm deep) before the surface was heated irradiated by microwaves,
followed by compressing in the radial direction and drying under restraint to fix the
compressive deformation. Densified wood from different densifications method has
been introduced in commercial wood flooring products, but these products have had
difficulty in competing with high-density hardwoods.
The free 3D-shaping of solid wood for non-structural purposes has not achieved any
significant commercial success, except in the traditional solid wood bending technique
and the forming of veneers (see below). Densification of wood in the transverse and/or
longitudinal direction makes the wood more flexible in these directions (see e.g. Navi
and Sandberg 2012, pp. 290, 320). Shigematsu et al. (1998), Kyomori et al. (2000), and
Tanahashi et al. (2000) estimate that more fundamental knowledge on wood THM
behaviour is required before the issues that currently stand in the way of developing a
new manufacturing system for the compressive moulding of solid wood can be
overcome. Recently, the Olympus Corporation presented wood products, such as the
outer casings of electronic products, manufactured in a three-dimensional compression
moulding process for wooden materials (Tatsuya, 2007). In the USA, commercial
interest in low density species has driven the wood-research community to investigate
the possibility of employing densified wood products in the structure of composite
materials (Kamke 2004, 2006, Kutnar et al. 2009). To densify small low-density hybrid
poplar specimens, Kamke and Sizemore (2008) have developed the continuous
viscoelastic thermal compression (VTC) process.
Only few wood THM-densification applications have been industrialized to some extent
and there are several reasons for this relatively low transfer of research results to a full
up-scaled industrial production. Initially the reason was a lack of adequate consideration
of the plasticization or stability of the products. The latter issue was solved by
development of resin-impregnated laminated THM products during the first half of the
20th century and such products are today commercially produced, e.g. Panzerholz and
Dehonit (Germany), Permawood (France), Ranprex (Italy), Permali (USA), Insulcul
(Australia), Surendra (India), and MyWood (Japan).
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
Figure 3: Engineered wood products from densified wood: THM-modified columns with and without
fibre reinforcement (left), and VTC wood-based composites (laminated composites with the VTC wood
laminae placed in the two outer layers, and a piece of undensified wood in the core layer)
Moulding for two-directional or three-directional shaping of veneer
Veneers have been used for a long time for the production of complex forms in wood,
i.e. laminated veneer products. Although formability is significantly better when using
veneer than when using solid wood of thicker dimensions, there is still a problem with
the lower strength in the transverse than in the longitudinal direction. A major problem
in laminated moulding is the stretching of the veneers and the risk that the veneers can
crack if they are subjected to large three-dimensional deformations. The bending
direction of the construction is therefore normally in the longitudinal direction of the
external veneers.
To overcome the problem with low formability of the veneers in the transverse
direction, the so-called “3D-veneer” has been developed. This veneer can be formed
extremely three-dimensionally. This special veneer was developed by the company
Reholz GmbH and has now been successfully introduced onto the market (Müller,
2006). During the production of 3D veneers, narrow grooves spaced 0.1 to 1 mm apart
and through the thickness of the veneer are introduced into an ordinary veneer along the
fibre direction. To keep the wooden strips together, lines of glue are spread on the rear
of the veneer, Figure 4. Other type of 3D-veneer also occurs in various products.
Glue
Strips
Figure 4: 3D veneer showing the strips and the glue to keep the strips together (upper). 3D-shaped
form in five layers manufactured from (left) ordinary beech veneers and (right) 3D veneers (lower)
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
Ageing of wood
Wood ageing is a further development of classical thermal-treatment processes currently
used industrially at temperatures between 150 and 260°C. Compared to these thermaltreatment processes, wood is aged at lower temperatures (100-150°C). The negative
effects that a classical thermal treatment normally has on the strength and brittleness of
wood are therefore reduced. The main purpose of this development is to increase the use
of thermally treated wood in construction, enabling its use for load-bearing construction
elements. Obataya et al. (2000) have studied the effect of artificial ageing on the
mechanical properties of wood for use in the repair of ancient Japanese wooden
constructions and in musical instruments. During the last year, this work has been
continued in Europe by Froidevaux et al. (2012). No industrial process for ageing wood
is however known today, but with all the knowledge built up during the development of
the thermal treatment, it should not take long to reach an industrial implementation.
Frictional wood welding and wood surface fusion
For several decades, friction welding has found broad applications in many metal and
plastic industries. Suthoff et al. (1996) made the first attempts to join wood by means of
pressure and frictional movement. It is stated that two pieces of wood can be welded by
means of an oscillating or linear frictional movement. The time required for the welding
process is short, taking less than a few minutes, and the mechanical performance of the
pieces joined by friction welding can be as good as that achieved by gluing with
conventional adhesives, but it is highly dependent on the process parameters and on the
wood species. Frictional welding techniques have also been shown to be viable for
increasing the hardness of wood surfaces in the presence of oils (Pizzi et al. 2005).
Friction welded products still present several challenges which need to be overcome
before the process can be applied on an industrial level. One weakness is that the
absorption of water lowers the strength of friction-welded joints and limits their use in
construction, but there is ongoing research to overcome this problem (Vasiri, 2011).
Laser treatment for the TH processing of wood surfaces
The phenomena that arise in wood at the micro-level during friction welding have an
interesting relation to the laser treatment of wood. Laser treatment, depending on irradiation parameters, is a form of thermal treatment which is concentrated in space and time
and exhibits effects such as a molten surface. Studies on the effect of laser irradiation on
wood with respect to changes in structural, chemical and physical properties have led to
the identification of laser parameters which guarantee melting wood without pyrolysis.
This type of molten surface was observed by e.g. Sandberg (1999) during the
preparation of weathered samples of pine and spruce with the help of UV-laser ablation
(Seltman 1995). Haller et al. (2001) have investigated the effect of laser irradiation on
wood with regard to changes in structural, chemical and physical properties, and they
have identified the laser parameters which guarantee the melting of wood without
pyrolysis. These laser treatments can be used to modify wood surfaces and to improve
their mechanical or water-related properties. The technique is not well developed and
further research is needed before the commercial potential can be evaluated.
Laser has also been used for engraving of wood, which is one of the most promising
technologies for use in wood carver operations (Leone et al. 2009). In this method, a
laser beam is used to ablate a solid piece of wood, following predetermined patterns.
The sculpture is obtained by repeating this process on each successive thin layer.
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
CONCLUSIONS
The TH/THM processing techniques discussed in this presentation can be utilised to
enhance wood properties, to produce new materials and to develop new products with a
low environmental impact. These processes are at various stages of development, and
the challenges that must be overcome in scaling up to industrial applications differ
among them. The benefit of THM treatments are that several process parameters may be
manipulated to create intermediate or final products with a broad range of applications
and the treatments should be used for high-value product applications. Understanding
these treatments requires, however, the appropriate scientific knowledge of the wood.
At the present time, the technology of several TH/THM wood processes has been
industrialized and their products are being commercialized, and ongoing research in the
field of wood THM moulding and wood densification as well as the post-treatment of
compressed wood by THM actions aims to widen this field to large-sized wood
elements for applications in the building industry. Moreover, many other TH/THM
treatments are advancing at the laboratory level, e.g. wood welding and the ageing of
wood. However, one challenge that all TH/THM developments need to consider is the
economic viability of the industrial scale production.
ACKNOWLEDGEMENTS
The authors wish to acknowledge COST Actions FP0904 and FP1303, and Andreja
Kutnar wishes to acknowledge the Slovenian Research Agency for financial support
within the frame of the project Z4-5520.
REFERENCES
Dwianto, W., Inoue, M. and Norimoto, M. (1997). Fixation of compressive deformation
of wood by heat treatment. Journal of the Japan Wood Research Society (Nippon
Mokuzai Gakki), 43, 303−309.
Froidevaux, J., Volkmer, T., Ganne-Chédeville, C., Gril, J. and Navi, P. (2012).
Viscoelastic behaviour of aged and non-aged spruce wood in the radial direction. Wood
Material Science and Engineering, 7(1), 1-12.
Haller, P., Wehsener, J. and Ziegler, S. (2004). Wood profile and method for the
production of the same. U.S. Patent No. 10/479439.
Inoue, M., Norimoto, M., Otsuka, Y. and Yamada, T. (1990). Surface compression of
coniferous lumber, I. A new technique to compress the surface layer. Journal of the
Japan Wood Research Society (Nippon Mokuzai Gakki), 36(11), 969-975.
Inoue, M., Norimoto, M., Tanahashi M. and Rowell R.M. (1993). Steam or heat fixation
of compressed wood. Wood and Fiber Science, 25, 224-235.
Kamke, F.A. (2004). A novel structural composite from low density wood. In:
Proceedings of the 7th Pacific Rim Bio-Based Composites Symposium. Nanjing, China,
Oct. 31-Nov. 2, pp. 176-185.
Kamke, F.A. (2006). Densified radiata pine for structural composites. Maderas. Ciencia
y Tecnología, 8, 83–92.
Nunes, L., Jones, D., Hill, C. & Militz, H. (Eds.) The seventh European Conference on Wood Modification (ECWM7),
(abstract pp. 91-92), March 10-11, Lisbon, Portugal, 2014
Kamke, F.A. and Sizemore, H. (2008). Viscoelastic thermal compression of wood. U.S.
Patent No. 7404422.
Koehler, A. and Pillow, M.Y. (1925). Effect of high temperatures on the mode of
fracture of a softwood. Southern Lumberman, 121, 219−221.
Kutnar, A., Kamke, F.A. and Sernek, M. (2009). Density profile and morphology of
viscoelastic thermal compressed wood. Wood Science and Technology, 43, 57–68.
Leone, C., Lopresto, V. and de Iorio, I. (2009). Wood engraving by Q-switched diodepumped frequency-doubled Nd:YAG green laser. Optics and Lasers in Engineering,
47(1), 161–168.
Müller, A. (2006). Method for the production of a three-dimensional, flexibly
deformable surface element. U.S. Patent No. 7131472B2.
Navi, P. and Sandberg, D. (2012). Thermo-hydro-mechanical processing of wood.
Lausanne, Switzerland: EPFL Press.
Nilsson, J., Johansson, J., Kifetew, G. and Sandberg, D. (2011). Shape stability of
modified engineering wood product subjected to moisture variation. Wood Material
Science and Engineering, 6, 132−139.
Obataya, E., Norimoto, M. and Tomita, B. (2000). Moisture dependence of vibrational
properties of heat-treated wood. Journal of the Japan Wood Research Society, 46:88-94.
Pizzi, A, Leban, J.-M., Zanetti, M., Pichelin, F., Wieland, S. and Properzi, M. (2005).
Surface finishes by mechanically induced wood surface fusion. Holz als Roh- und
Werkstoff, 63(4):251-255.
Putzger, R., Eckhard, R., Eichhorn, S., Nendel, K. and Haller, P. (2011). Investigation
on fibre reinforced moulded wood pipes for the conduction of brine at high temperature
and pressure. In: Current and Future Trends of Thermo-Hydro-Mechanical
Modification of Wood. Opportunities for new markets? Cost Action FP0904 Workshop,
March 26-28, Nancy, France, pp. 174-176.
Rautkari, L. (2010). Surface modification of solid wood using different techniques. PhD
Thesis, Aalto University, Helsinki, Finland.
Sandberg, D. (1999). Weathering of radial and tangential wood surfaces of pine and
spruce. Holzforschung, 53, 355−364.
Seltman, J. (1995). Abtragen von Holzoberflächen durch UV-bestrahlung. (Removal of
wood surfaces by UV irradiation.) Holz als Roh- und Werkstoff, 53, 100.
Stamm, A.J., Burr, H.K. and Kline, A.A. (1946). Staybwood. Heat-stabilized wood.
Industrial and Engineering Chemistry, 38, 630−634.
Suthoff, B., Schaaf, A., Hentschel, H. and Franz, U. (1996). Verfahren zum reibschweiβartigen Fügen von Holz. (Method for joining wood.) Offenlegungungsschrift
DE 196 20 273 A1. Deutsches Patent und Markenamt.
Tiemann, H.D. (1915). The effect of different methods of drying on the strength of
wood. Lumber World Review, 28, 19−20.
Vaziri, M. (2011). Water resistance of Scots pine joints produced by linear friction
welding. PhD Thesis, Luleå University Technology, Skellefteå, Sweden.