Enhancing the Functionality of Wood Products Puu-0.4310 Thermal modification Friday 27th November 2015 Marc Borrega
Learning outcomes To explain the concept of thermal modification of wood To describe the changes in wood structure and properties To identify the main applications of thermally treated wood
Content Principles of thermal modification Degradation of wood polymers Effects of process variables Changes in wood properties Applications of thermally modified wood Commercial thermal modification processes
Introduction Thermal modification of wood dates back centuries, e.g. burning the ends of wood poles in ground contact. Research work in the 30s and 40s led to the introduction of several processes such as Lignostone and Lignifol in Germany and Staypak and Staybwood in the US. Increased interest in environmentally-friendly methods for wood modification led to development of commercial processes in the 90s. Thermal modification of wood is the most commercially advanced modification technology. 4
What is thermal modification? Thermal modification is the application of heat to bring about desired property changes, largely resulting from the controlled degradation of the wood polymers. Thermal modification is usually carried out at temperatures between 150 o C and 250 o C. A variety of process variables need to be considered: Temperature and time Treatment atmosphere Type of system, i.e., open or closed Wood species Moisture conditions (wet or dry) Sample dimensions 5
Thermal degradation of wood components Thermal degradation in wood occurs through complex chemical reactions that are not fully understood but are dependent upon the process variables. Esteves and Pereira. 2009. BioResources 4:370 6
Chemical changes - Hemicelluloses Hemicelluloses are the most heat sensitive polymers in the cell wall. Their depolymerization starts at temperatures over 100 o C and accelerates as temperature rises. Degradation products include acetic acid formed from the hydrolysis of acetate groups as well as furanic (furfural and HMF) compounds formed by dehydration of C5 and C6 monosugars. Acetate Acetic acid Acid production can further degrade the remaining hemicelluloses and amorphous cellulose. Tjeerdsma and Militz. 2005. Hols Roh Werkst 63:102 Rowell et al. 2009. Wood Mater Sci Eng 1-2:14 7
Chemical changes - Cellulose Cellulose is more stable than hemicelluloses due to the presence of crystalline regions, a linear configuration and high degree of polymerization. Cellulose crystals start to degrade at temperatures above 320 o C. Degradation of amorphous regions and re-conformation of cellulose chains lead to greater crystallinity in heat treated wood. R.t. 300 C 340 C Krässig, 1993 X-ray diffraction patterns and profiles after heating Populus wood for 1 hour Kim et al. 2001. Holzforschung 55:521 8
Chemical changes - Lignin Lignin degrades over a wide range of temperatures owing to its complex structure with different types of chemical bonds. Lignin is generally considered to be the most thermally stable component of wood, but some changes occur below 200 o C. Typical reactions in lignin include depolymerization by cleavage of aryl-ether bonds and condensation of lignin fragments. Lignin may also condense with degradation products from carbohydrates, which leads to an apparent increase in lignin content. Li et al. 2007. Bioresour Technol 98:3061 9 Brosse et al. 2007. Polym Degrad Stabil 95:1721
Anatomical changes The degradation of structural components in wood during thermal treatment damages the tissues and increases the wood pore sizes. Damage in tracheids and pit deaspiration in red cedar wood after 2 hours heat treatment at 220 o C Awoyemi and Jones. 2011. Wood Sci Technol 45:261 10
Thermal degradation and mass loss At temperatures above 100 o C, some of the wood polymers start to degrade. Water vapor, carbon dioxide and traces of organic compounds are released, often resulting in a loss of wooden mass. Beyond 250 o C, significant thermal degradation of wood takes place, followed by extensive mass loss. Because mass loss in heat treated wood depends on the extent of thermal degradation, it is generally used as a quality indicator. The extent of mass loss depends on a number of process variables. 11
Temperature and duration of treatment Increasing temperature and treatment time increases the extent of mass loss. Increasing time is analogous to increasing temperature. Pine wood Esteves and Pereira. 2009. BioResources 4:370 12
Treatment atmosphere Thermal modification can be performed in: Air Vacuum Inert atmosphere (e.g. steam / nitrogen) Under a blanket, such as oil or molten metal In air, the presence of oxygen leads to oxidative processes, so most commercial processes exclude air either by using a shield gas or by immersing in oil. 13
Closed and open systems In a closed system (i.e. the wood is heated in a sealed container), the gases given-off by the wood are not allowed to escape, whereas in an open system, the gases are removed. If it is a closed system, the degradation products which include acidic vapours can lead to a more rapid degradation of the polysaccharides. Source: www.silvapro.eu 14
Wood species softwoods vs hardwoods Some species effects have been noted, but the main difference is between softwoods and hardwoods. Hardwoods generally experience greater weight loss than softwoods following thermal modification processes, due to their higher content in hemicelluloses. Esteves et al. 2007. Wood Science and Technology 41:193 15
Moisture conditions The presence of water/steam can act as a barrier or shield gas, preventing oxidative reactions. Water present in wood also affects the heat transfer. The presence of water in the atmosphere (relative humidity) increases the rate of thermal degradation of wood components. Rate of mass loss in spruce wood 170 C 150 C Borrega and Kärenlampi. 2008. Journal of Wood Science 54:323 16
Sample dimensions Wood is a good insulator. Larger pieces, especially when dry, are more difficult to heat evenly throughout. Temperature at the center of the wooden piece Temperature, C Time, h Trcala and Cermak. 2014. Holzforschung 17
Summary - factors affecting mass loss Lower rate of mass loss Decrease in temperature/time Inert atmosphere (no oxygen) Open systems Dry conditions Softwoods Large wood dimensions Higher rate of mass loss Increase in temperature/time Presence of air (oxygen) Closed systems Wet conditions Hardwoods Small wood dimensions 18
Changes in wood properties Following thermal treatment, a number of changes occur in wood as a result. These include: Lower density due to mass loss Color changes Lower hygroscopicity and permeability Increased dimensional stability Increased stiffness and hardness, but loss of toughness and strength Reduced susceptibility to biodeterioration Improved weathering resistance 19
Color changes Wood darkens with increasing treatment temperature. Pine wood Because of the darker (brownish) color, heat treated wood is often used to replace tropical woods for aesthetic purposes. ThermoWood Handbook. 2003 20
Hygroscopicity Degradation of hemicelluloses by thermal modification leads to reduced hygroscopicity. EMC of spruce wood at 19 C and 65% RH Equilibrium moisture content (EMC) of wood decreases with mass loss. At a given mass loss, wood heated at intermediate RH conditions (50-60%) shows lower EMC than wood heated in dry or water-saturated conditions. Borrega and Kärenlampi. 2010. Eur J Wood Prod 68:233 21
Sorption behavior In heat treated wood, adsorption in the first cycle differs from consecutive cycles. Hysteresis is much higher in heat treated wood than in untreated wood for the first sorption cycle, but is similar in subsequent cycles. Hill et al. 2012. Journal of Materials Science 47:3191 22
Water absorption Heat treated wood absorbs less liquid water than untreated wood in flotability tests. The decrease in water absorption is clearly more marked in wood treated at higher temperatures. Heartwood has lower water absorption than sapwood. Metsä-Kortelainen et al. 2006. Hols Roh Wekst 64:192 23
Dimensional stability Because of lower EMC, heat treated wood has better dimensional stability, as determined by anti-shrinkage efficiency (ASE) tests. Welzbacher et al. 2007. Wood Mater Sci Eng 2:66 Esteves et al. 2007. Wood Science and Technology 41:193 24
Bending strength and stiffness Heat-treated wood shows a general decrease in mechanical properties, but bending stiffness may be somewhat higher if mass losses are below 4-5%. Borrega and Kärenlampi. 2008. Holz Roh Werkst 66:63 Esteves et al. 2007. Wood Science and Technology 41:193 25
Toughness and compressive strength Heat treated wood exhibits lower toughness, that is, the wood becomes more brittle.the compressive strength also decreases. Kubojima et al. 2000. Journal of Wood Science 46:8 Yildiz et al. 2006. Building and Environment 41:1762 26
Hardness Hardness in heat-treated wood increases slightly due to the thermal treatment, which is a benefit for flooring applications. Poncsák et al. 2006. Wood Sci Technol 40:647 ThermoWood Handbook. 2003 27
Fungal degradation Thermal treatment generally improves the wood resistance to fungal decay, but temperatures above 200 o C are needed for effective resistance. Reduction in mass loss due to fungal decay may be somewhat offset by mass loss during thermal treatment Metsä-Kortelainen et al. 2009. Wood Mater Sci Eng 3-4:105 Welzbacher et al. 2007. Wood Mater Sci Eng 2:66 28
Weathering resistance Heat treated wood losses its brownish color when exposed to UV light, turning grey and cracking. However, the color changes in heat treated wood are less significant than in untreated wood. Jämsä et al. 2000. Pigment Resin Technol 29:68 Ayadi et al. 2003. Holz Roh Werkst 61:221 29
Summary of wood properties Color Hygroscopicity Water absorption Dimensional stability Mechanical properties Fungal degradation Weathering resistance Based on these property changes, write down 3 posible applications for thermally treated wood (5 min)
Applications for heat treated wood Loss of mechanical properties not suitable for structural applications Lower moisture/water uptake suitable for moist ambients Improved dimensional stability suitable for widely varying moisture conditions or where accurate dimensions are essential Improved resistance to decay and to weathering suitable for outdoor applications Change in color aesthetic purposes (i.e. furniture) 31
Main applications Façade and exterior cladding Decking and flooring Windows and door frames Fences Outdoors (urban and garden) furniture Interior furniture Saunas Source: www.lunawood.fi 32
Thermal modification processes Several commercial processes are now in operation, principally in Europe. The five main processes are: Plato Wood (The Netherlands) Bois Perdure (France) Retification (France) OHT- Oil Heat Treatment (Germany) ThermoWood (Finland) Finland is the largest producer of thermally treated wood, with nearly 128 000 cubic meters of ThermoWood sold in 2013. Each of the processes differs in detail, such as the use of different shield gas, modification temperature, etc. 33
Plato Wood BV (Proving Lasting Advanced Timber Option) A five stage process: Pre-drying in conventional kiln to 14-18% MC Hydro-thermolysis in a stainless steel pressure vessel under saturated steam: Heating at 150-180 o C for 4-5 hours Drying in a convential kiln to 8-9% MC (3-5 days) Curing in a special stainless steel reactor under dry atmospheric conditions Heating at 150 180 o C for 12-16 hours Wood MC decreases to about 1% Conditioning in a conventional kiln with steam Increasing the wood MC to 4-5% over a 3 days period Drying and conditioning Hydro-thermolysis Source: www.platowood.com Curing 34
Le Bois Perdure Green wood is rapidly dried and then heattreated at temperatures between 200 and 240 o C in steam. Retification Source: www.perdure.com Wood dried to about 12% MC is heated in a nitrogen atmosphere, which contains less than 2% oxygen (i.e. a shield gas) to temperatures between 180 and 250 o C. Source: www.emse.fr 35
OHT Oil Heat Treatment Menz Holz uses vegetable oil (rapeseed, sunflower or linseed oils). Oil acts as a heat transfer mechanism and to shield the wood from oxygen. Three stage treatment, with warming up, heating and cooling. Treatment at 180 and 220 o C for 20 to 40 hours. Oil is absorbed by the wood in the process. Source: www.menzholz.de 36
ThermoWood The ThermoWood process was developed and patented at VTT. It s the most commercially developed thermal modification process. The wood is heated in a steam atmosphere at temperatures between 185 and 215 o C. The duration of the treatment is up to 36 hours. Three stages, including drying, heating and cooling and conditioning. Two classes of ThermoWood :Thermo-S ( S for stability ) and Thermo-D ( D for durability ). Dry raw material Modified wood 37 ThermoWood Handbook. 2003
Comparison among wood heat treatment processes Media Initial MC (%) Steps Temperature o C Plato Wood BV Steam/Air Green 5 150 180 o C Le Bois Perdure Steam Green 2 200 240 o C Retification Nitrogen Air-dried 1 180 250 o C OHT Wood Oil Dried 3 180 220 o C ThermoWood Steam Green or dried 3 185 215 o C 38
Further reading Borrega, M. 2011. Mechanisms affecting the structure and properties of heat-treated and hightemperature dried Norway spruce (Picea abies) wood. Dissertationes Forestales 134. http://www.metla.fi/dissertationes/df134.pdf Esteves, B.M. and Pereira, H.M. 2009. Heat treatment of wood: a review. BioResources 4:370-404. Hill, C. 2006. Wood Modification. Chemical, thermal and other processes. John Wiley & Sons Ltd. 239 p. Militz, M. 2002. Heat treatment technologies in Europe: scientific background and technological state-of-the-art. In: Proceedings of Conference on Enhancing the durability of lumber and engineered wood products, 11-13 TH February, Kissimmee, Orlando, USA. http://www.thermotreatedwood.com/researches/heattreatmenteurope.pdf Rapp, A.O. 2001. Review on heat treatments of wood. COST ACTION E22: Environmental optimisation of wood protection. http://projects.bre.co.uk/ecotan/pdf/heat_treatment_processes_andreas_rapp%20.pdf ThermoWood Handbook. 2003. Finnish ThermoWood Association. http://files.kotisivukone.com/en.thermowood.kotisivukone.com/tiedostot/tw_handbook_080813.pdf 39