THERMALLY MODIFIED TIMBER: RECENT DEVELOPMENTS IN EUROPE AND NORTH AMERICA Dick Sandberg*{ Professor Wood Science and Engineering Lulea ˚ University of Technology SE-931 87 Skelleftea ˚, Sweden E-mail: [email protected]Andreja Kutnar{ Associate Professor University of Primorska SI-6000 Koper, Slovenia E-mail: [email protected](Received July 2015) Abstract. This state-of-the-art report presents the basic concepts in manufacture of thermally modified timber (TMT) to achieve increased dimensional stability and resistance to biological degradation. The reasons for the growing interest in thermo–hydro and thermo–hydro–mechanical techniques in Europe and the United States are discussed, and the physical and chemical changes that occur in wood during processing according to the latest research are also presented. Finally, the role of thermal wood processing in a sustainability of resource utilization context is discussed, along with future need in TMT research and development to contribute to the low-carbon economy. The results clearly show a knowledge gap in data supporting the environmental benefits of TMT compared with unmodified timber or other modification methods for timber. Keywords: Environmental product declarations, low-carbon bioeconomy, product category rules, sustainability. INTRODUCTION Forest-based industries are continually develop- ing advanced processes, materials, and wood- based solutions to meet evolving demands and increase competitiveness. One emerging treat- ment involves the combined use of temperature and moisture, in which force can be applied. This is referred to as the thermo–hydro (TH) and thermo–hydro–mechanical (THM) processes. Figure 1 shows a simplified synoptic diagram of the most common TH and THM processes based on what is achieved through the process. Ther- mal wood processing (thermal treatment) involves temperatures of 100-300 C and can have two dis- tinctly different purposes: 1) softening the wood in steam or water to release internal stresses and make the wood easier to further process or 2) controlled degradation of the wood involv- ing temperatures between 150 C and 260 C with the purpose of improving shape stability and decay resistance. Heat treatment of wood at greater than 300 C is of limited practical value because of the severe degradation of the wood material (Sandberg et al 2013). Wood aging is a further development of the classic thermal treatment processes currently used industrially. Wood aging operates in a temperature range between wood drying and thermal treatment (100-150 C), and the negative effects that a classic thermal treatment normally has on strength and brittleness of wood are therefore decreased. TH processing is also applied in many other processes that can be attributed to reconstituted wood products. A thermal treatment process can also be followed by compression in the axial or transverse direction of the wood. THM * Corresponding author { SWST member Wood and Fiber Science, 48(2015 Convention, Special Issue), 2016, pp. 28-39 # 2016 by the Society of Wood Science and Technology
Transcript
THERMALLY MODIFIED TIMBER: RECENT DEVELOPMENTSIN EUROPE AND NORTH AMERICA
Forest-based industries are continually develop-ing advanced processes, materials, and wood-based solutions to meet evolving demands andincrease competitiveness. One emerging treat-ment involves the combined use of temperatureand moisture, in which force can be applied.This is referred to as the thermo–hydro (TH)and thermo–hydro–mechanical (THM) processes.Figure 1 shows a simplified synoptic diagram ofthe most common TH and THM processes basedon what is achieved through the process. Ther-mal wood processing (thermal treatment) involvestemperatures of 100-300�C and can have two dis-tinctly different purposes: 1) softening the woodin steam or water to release internal stresses
and make the wood easier to further process or2) controlled degradation of the wood involv-ing temperatures between 150�C and 260�Cwith the purpose of improving shape stabilityand decay resistance. Heat treatment of wood atgreater than 300�C is of limited practical valuebecause of the severe degradation of the woodmaterial (Sandberg et al 2013). Wood aging isa further development of the classic thermaltreatment processes currently used industrially.Wood aging operates in a temperature rangebetween wood drying and thermal treatment(100-150�C), and the negative effects that aclassic thermal treatment normally has on strengthand brittleness of wood are therefore decreased.TH processing is also applied in many otherprocesses that can be attributed to reconstitutedwood products. A thermal treatment processcan also be followed by compression in theaxial or transverse direction of the wood. THM
* Corresponding author{ SWST member
Wood and Fiber Science, 48(2015 Convention, Special Issue), 2016, pp. 28-39# 2016 by the Society of Wood Science and Technology
processes include three main areas of modifica-tion, namely joining, densification, and shapingof wood.
Thermally modified timber (TMT) is, accordingto CEN (2007), wood at which the compositionof the cell wall material and its physical prop-erties are modified by exposure to temperaturegreater than 160�C and conditions of decreasedoxygen availability. The wood is altered in sucha way that at least some of the wood propertiesare permanently affected through the cross sec-tion of the timber. This product is related toheat-treated wood, but to distinguish it from heatsterilization at lower temperature (�55�C) withthe purpose of killing pests in solid wood mate-rials and preventing their transfer between con-tinents and regions, we use the terms thermaltreatment/processing and TMT.
In recent decades, developments in the area ofthermal treatment have accelerated considerably.During the 1980s, French and Japanese indus-tries began to modify wood with the help ofheat to increase resistance to microbial attack.
Since then, interest in thermal treatment hasincreased and several thermal treatment pro-cesses for treatment of sawn timber were devel-oped in Europe.
Thermal treatment is wood modification in astrict sense because the material undergoes chemi-cal changes. The process essentially involves acontrolled degradation of the wood primarilyresulting in the destruction of hemicelluloses.The thermal treatment process is in most casesperformed in a vacuum, in air, or with an inertgas such as nitrogen. Preheated oil can also beused; in which case, the oil acts as a heat trans-fer medium and also excludes oxygen from thewood. The underlying reason for applying ther-mal treatment is the increasing demand for envi-ronmentally friendly high-durability wood, ie toincrease the service life of wood materials with-out the use of toxic chemicals.
Kutnar et al (2015) gave an overview of recentdevelopments of THM-treated wood rangingfrom surface densification, compressed wood,and shaped wood. In this study, the purpose was
Figure 1. Classification of TH and THM processes (Navi and Sandberg 2012).
Sandberg and Kutnar—THERMALLY MODIFIED TIMBER RECENT DEVELOPMENTS 29
to present the role of thermal wood processingin a sustainability of resource utilization contextand to document what thermal wood processingshould achieve to contribute to a low-carboneconomy and beyond.
BIOECONOMY
For many years to come, the world’s politicaland economic decisions will be determined byresource and energy scarcity and by climatechange as a topic related to consumption offossil energy. In these circumstances, a balancehas to be achieved between economics, ecology,and social welfare that can be summed up assustainability, which was put forward at the endof the 20th century and has been inseparablylinked to forestry ever since.
The forest sector and wood-based industries arechallenged by this change and tend to participatein the sustainability debate. This is for obviousreasons, because the concept of sustainability thatis now related to the global economy first camefrom these industries. Indeed, nobody questionsthe value of the forest for mankind and the envi-ronment, and nobody questions the value of themultitude of products made of wood. Wood,especially sawn timber, has during the last 50 yrto a large extent, disappeared from technologicalapplications (Radkau 2007). Therefore, its con-tribution to sustainability fails to appear in theone area in which it would be most significant—as a substitute for energy-intensive materials. Newmodification methods for wood, such as thermalmodification, are attempts to reintroduce woodto technological applications without changingthe eco-friendly characteristics of the material.
Wood is in volume the most important renewablematerial resource (Rowell 2002). In all aspectsof human existence, wood use appears to be themost effective way to optimize resources and todecrease the environmental impact associatedwith mankind’s activities. However, becausetimber possesses good but not outstanding prop-erties, this is not an easy thing to achieve, and inview of the new materials emerging, it becomesnoticeably more difficult. The only properties
wood has that reign supreme are ecological fit-ness and, possibly, low cost.
In the European Union (EU), measures are beingdiscussed in economics and science that seekto improve the sustainability of resource utiliza-tion. European policy is affecting and, indeed,directing current research, development, and mar-keting in the EU. This has been especially truefor the introduction of industrial processes formanufacture of TMT. The main policies havinga direct impact on the forest-based sector are theEU Sustainable Development Strategy (EuropeanCommission 2009), which was published in 2006,and revised in 2009, the EU Roadmap 2050(European Commission 2011), and the recy-cling society directive, Directive 2008/98/EC(European Parliament Council 2008). The forest-based sector can considerably contribute tothe European Commission’s ambitious CO2
emissions reduction goal of 80% by 2050, ieRoadmap 2050, with innovative productiontechnologies, decreased energy consumption,increased wood products recycling, and the reuseand refining of side-streams.
The need to decrease the whole-life energy con-sumption of buildings has highlighted the rolethat wood can play in construction. When build-ings have net-zero energy consumption, a majorpart of their overall environmental burden con-sists of their embodied energy and the associatedgreenhouse gas emissions. Compared with otherconstruction materials, the energy needed toconvert a tree into the final product is signifi-cantly lower, resulting in wood products havinga low embodied energy.
It is vital that wood can be used effectivelythrough the whole value chain, from forest man-agement and multiple uses of forest resourcesthrough new wood and fiber-based materials andprocessing technologies to new end-use concepts.Fossil fuel consumption, potential contributionsto the greenhouse effect, and quantities of solidwaste tend to be minor for wood products com-pared with competing products (Werner andRichter 2007). Preservative-impregnated woodproducts tend to be more critical than comparative
30 WOOD AND FIBER SCIENCE, MARCH 2016, V. 48(2015 CONVENTION, SPECIAL ISSUE)
products with respect to toxicological effectsand/or photo-generated smog, depending on thetype of preservative. Unfortunately, the numberof life cycle assessment (LCA) studies of wood-based composites and modified wood is relativelylimited, are geographically specific, and use ofa variety of databases and impact assessmentprotocols. The aim of the “carbon economy” isto mitigate climate change and promote sus-tainable development by decreasing energyconsumption, pollution, and emissions whileincreasing performance.
As a consequence of increased competition fromtraditional and new industries based on renew-able resources, forest resources must be consid-ered limited. There are forecasts showing that,already in 2020; European consumption of woodmight be as large as the total European com-bined forest growth increment (Jonsson et al2011). According to the Communication “Inno-vating for Sustainable Growth: A Bio-economyfor Europe,” Europe needs to radically change itsproduction, consumption, processing, storage,recycling, and disposal of resources (EuropeanCommission 2012). Thus, bioeconomy is con-sidered one of the key elements for smart andgreen growth in Europe. The “Strategic Researchand Innovation Agenda for 2020 of the Forest-based Sector and the Horizons-Vision 2030”view forest-based sector as a key actor andenabler of the bio-based society.
THERMAL PROCESSING OF WOOD
Thermal treatments of sawn timber have beeninvestigated for many years and are now com-mercialized. In the beginning of the 20th century,the use of heat and moisture in wood process-ing came into focus, and it was observed thatwood dried at a high temperature changed colorand had greater dimensional stability and lowerhygroscopicity than untreated wood (Tiemann1915; Koehler and Pillow 1925). After WorldWar I, comprehensive studies were made of theeffect of kiln-drying temperature on the strengthof wood for the aviation industry in the UnitedStates (Wilson 1920). Systematic research on
how to improve wood properties by thermal treat-ment was made by the US-based group led byAlfred Stamm in the 1940s (Stamm 1964) andin Germany by Burmester (1973). Burmesterstudied the effects of temperature, pressure, andmoisture on wood properties in a closed system,and the process was named Feuchte-Warme-Druck (FWD).
The exact method of treatment can have a sig-nificant effect on the properties of the modifiedwood, and the most important process variablesare the treatment atmosphere, the choice of closedor open system, the choice of wet or dry systems,the use of a catalyst, the wood species, the timeand temperature of treatment, and the dimensionsof the processed material.
One of the first commercial thermal treatmentunits in Europe was based on Burmester’s workand was started in Germany about 1980 but wasnever industrialized on a great scale (Giebeler1983). During the two decades preceding theturn of the century, the development of TMT inEurope was intensive and four new methodsentered the market. The Plato process (ProvingLasting Advanced Timber Option) was devel-oped in the 1980s by Royal Dutch Shell in TheNetherlands and is now used by the Plato Com-pany in the Netherlands. In France, the firststudies related to thermal treatment were relatedto renewable energy from biomass in middle1970s by Ecoles des Mines de Paris and SaintEtienne (Candelier 2013). In the late 1980s,torrefied wood was developed and advanta-geous properties of the thermal-treated woodsuch as increased dimensional stability and dura-bility from fungal attacks were highlighted. Theretification process for thermal treatment wasdeveloped from that research. It was not until1993, when VTT Technical Research Center ofFinland together with industry developed theThermoWood process that got established asan industrial process for improving wood proper-ties. This process is licensed to members of theInternational ThermoWood Association (foundedin 2000), and the ThermoWood process is domi-nating the market for manufacture of thermal-treated wood in Europe. Because the boiling
Sandberg and Kutnar—THERMALLY MODIFIED TIMBER RECENT DEVELOPMENTS 31
points of many natural oils and resins are greaterthan the temperature required for the thermaltreatment of wood, the thermal treatment in a hotoil bath is a feasible option. The oil heat treat-ment (OHT) process was developed in Germany,and the process is performed in a closed processvessel. Table 1 shows the processing conditionsfor the previously mentioned thermal processes.
On the basis of the development of these pro-cesses, new thermal treatment processes havealso been emerging in other countries, mainlyin Europe. Several wood species are used, withdifferent process conditions depending on spe-cies and the final use of the product. Commonfor the different processes is the use of sawntimber and treatment temperatures in the rangeof 160-260�C, but differences exist in termsof process conditions such as the use of mediafor the process (steam, nitrogen, or differentvegetable oils).
Finland (ThermoWood) is the largest producerof TMT in the world and by far the biggest of allthe commercial wood modification businesses.The production volume has been progressivelyincreasing since 2001, and in 2014, the produc-tion of ThermoWood was 150,000 m3. Produc-tion is mainly from scots pine and norway spruce,and 80% of the production is sold within Europe.Today, the total annual production of TMT inEurope is about 300,000 m3.
Outside Europe, research has not been as focusedon development of industrial processes for ther-mal treatment for improved wood properties.North America has recently showed a growinginterest in both the product and the process.In 2004, the United States restricted the useof chromated copper arsenate–treated softwoodtimber for children’s playgrounds, finishingmaterials for waterfront homes, and in palletsused to transport food. Similar restriction hasbeen in action in the EU and Canada since 2004and 2015, respectively. This opens opportuni-ties for the increased use of wood not treatedwith toxins. In North America, annual produc-tion of softwood timber is roughly 90 millionm3, of which �20 million m3 are treated for T
able
1.
Exam
plesofthermal
treatm
entprocesses
andtheirprocessingconditions.
Process
Approxim
ate
year
Tradem
arks
InitialMC
(%)
Tem
perature
a(�C)
Process
duration(h)
Pressure
(MPa)
Atm
osphere/heat
transportationmedia
Comments
FWD
1970
10-30
120-180
�15
0.5-0.6
Steam
Closedsystem
Plato
1980
PlatoWood
14-18
150-180/170-190
4-5/70-120up
to2wk
Super
atmospheric
pressure
(partly)
Saturatedsteam/
heatedair
Afour-stageprocess
ThermoWood
1990
ThermoWood
10to green
130/185-215/80-90
30-70
Atm
ospheric
Steam
Continuoussteam
flowthrough
thewoodunder
processing
thatremoves
volatile
degradationproducts
LeBois
Perdure
1990
Perdure
green
200-230
12-36
Atm
ospheric
Steam
Theprocessinvolves
drying
andheatingthewoodin
steam.
Retification
1997
Retiwood,Bois
Retifie,Reti,
Retibois,Retitech,
Retifier
�12
160-240
8-24
Nitrogen
or
other
gas
Thenitrogen
atmosphere
guaranteeamaxim
um
oxygen
contentof2%
OHT
2000
OHT(Company
nam
eþ
OHT,
egMenzOHT)
10to green
180-220
24-36
Vegetableoils
Closedsystem
aTreatmenttemperature
fordifferentstages
oftheprocess
isseparated
by/.
32 WOOD AND FIBER SCIENCE, MARCH 2016, V. 48(2015 CONVENTION, SPECIAL ISSUE)
outdoor decks, sill plates, interior framing intermite-prone locations, and other uses. Treatingsoftwood lumber with preservatives can makethe difference between complete disintegrationwithin months in the worst conditions and thesame wood lasting decades.
Production units and research were establishedin Canada slightly after the breakthrough of TMTin Europe. The Perdure process (Le Bois Perdure)is a French thermal treatment process that was thefirst process established in Canada (by PCI Indus-tries in 2003). Today, there are several industrialplants in the Quebec region (Esteves and Pereira2009). In addition to the Perdure process, theThermoWood process as well as “own built”thermal treatment units are in production inCanada. In 2012, there were 7 manufacturers ofTMT in Canada and 10 in the United States. In2004, Westwood Corp. was the first companyin the United States exhibiting thermal-treatedwood at the International Woodworking Fair inAtlanta, and two years later at the same fair, theFinish companies Jartek Inc. and Stellac Inc.were also exhibitors. The Westwood process isa further-developed ThermoWood process spe-cially adapted for hardwood species. The com-pany markets both TMT and their patentedprocess. The companies Jartek and Stellac werelicensees of the ThermoWood process.
In 2008, Jartek installed their first equipment ina plant located in Minnesota, and Stellac startedtheir plant in Indiana for manufacturing thermal-treated decks made of softwood under the brandname PureWood. The market for thermal-treatedsoftwood in the United States is different fromthat in the EU. Softwoods in the United Statesare typically used for construction purposes,which demand completely different moisture stan-dards than those of Europe. In the United States,the MC for building materials is 20-25%, com-pared with the European standards of 8-14%. In2008, the Perdure process was also introduced inthe US market with a production plant locatedin New Hampshire. Mostly softwood is processed,and the products can be found in the UnitedStates under the brand name Cambia. Inspiredby the activities in Europe, the Natural Resources
Research Institute at the University of Minnesota,Duluth, took the initiative to develop an AmericanSection of the International Association for Test-ing and Materials standard similar to the EuropeanTechnical Specification for TMT (CEN 2007),but still there is no such standard.
Although thermal-treated wood can now be usedin many common applications, the market is stilllimited. Heat-treated wood is suitable for varioususes, mainly in which it is exposed to weatherand humidity variations above ground eg for out-side use as cladding, terraces, garden furniture,saunas, and windows but also for interior usesuch as kitchen furniture, flooring, decorativepanels, and stairs. However, its properties andlow physical strength do not allow it to be usedin wooden structures. TMT was first developed toimprove the performance and durability of soft-woods, but it has more recently been extended toboost the performance of hardwoods, allowingcertain low-durability hardwoods to be used out-doors with no additional protection. Examples ofthermally modified hardwood species are birch,aspen, ash, soft maple, tulipwood, and red oak(with the best results from quartersawn timber).
PHYSICAL CHANGES IN WOOD DUE
TO THERMAL PROCESSING
Thermal treatment significantly influences theproperties of wood, eg hygroscopicity and dimen-sional stability, resistance against fungi andinsects, mechanical properties, and also proper-ties such as color, odor, gluability, and coatingperformance (Table 2). Loss of mass of thetimber during thermal treatment is, however, atypical effect of the process. A decrease in massup to 20% can occur depending on type of pro-cess. Most properties of TMT are, in addition toproperties of the raw material, affected by theintensity of the thermal treatment process, ie bythe temperature and duration of the process.
In general, most thermal treatments, even atmild temperatures, decrease the hygroscopicityof wood, ie its capacity for reabsorption of mois-ture from the air, but the decreased hygroscopicitycan in some cases be recovered by moistening
Sandberg and Kutnar—THERMALLY MODIFIED TIMBER RECENT DEVELOPMENTS 33
(Maejima et al 2015). Because of the loss ofhygroscopic hemicellulose polymers during thermaltreatment, the EMC is decreased. Consequently,the swelling and shrinking of thermal-treatedwood are drastically decreased. On average,the EMC is decreased to about half the valueof the untreated wood. The hygroscopicity ofthermal-treated wood can vary considerably withvarying process parameters. Table 3 shows asummary of the effect of thermal treatment onthe EMC according to various researchers.
Welzbacher and Rapp (2007) compared differ-ent types of industrially thermal-treated prod-
ucts using several different fungi in laboratorytests and in different field and compost condi-tions. Table 4 shows the weight loss during aCEN (1996) test using three different fungi.The thermal treatment in oil was the most effec-tive, but the effect of the oil in the decay test isnot known. Schwarze and Spycher (2005) reportedthat densified wood posts treated at 180�C weremore resistant to colonization and degradationby brown rot fungi. In contrast, results obtainedby Welzbacher et al (2008) showed that a verydurable-to-durable THM-densified wood is pro-duced onlywhen thermo–mechanical densificationis used in combination with OHT. In addition,
Table 2. Main changes of properties for thermal-treated wood compared with untreated wood.
Wood species Temperature (�C) Time (h) Decrease in EMC (%) Reference
Fagus silvatica 180 4 40 Teichgraber (1966)
190 2.5 60 Giebeler (1983)
Pinus pinaster 190 8-24 50 Esteves et al (2006)
Pinus sylvestris 220 1-3 50 Anon (2003)
Eucalyptus globulus 190 2-24 50 Esteves et al (2006)
P. sylvestris Plato 50-60 Tjeerdsma (2006)
Table 4. Weight loss (%) of different species of wood after different thermal treatment processes (Welzbacher and
Rapp 2007).
MaterialPostiaplacenta
Coriolusversicolor
Coniophoraputeana
Control species
P. sylvestris L.a 31.0 5.1 47.5
P. sylvestris L.b 26.2 35.7 60.3
Pseudotsuga menziesii F. 14.0 2.6 27.4
Quercus petrea Liebl. 0.8 14.3 3.9
Thermal-treated wood Process
Pices abies (L.) Karst. Plato 10.0 6.8 3.7
P. sylvestris L. ThermoWood 16.0 9.0 1.9
Pinus maritima Mill. Retification 13.3 7.8 12.2
P. sylvestris L. OHT 7.4 5.6 3.4a Including both sap and heartwood.b Including only sapwood.
34 WOOD AND FIBER SCIENCE, MARCH 2016, V. 48(2015 CONVENTION, SPECIAL ISSUE)
Skyba et al (2008) found that THM treatmentincreased the resistance of spruce but not ofbeech wood to degradation by soft rot fungi.
It is well known that changes in the cell wallchemistry, such as changes in the hemicellulosesand lignin structures, cellulose depolymerization,and increased crystallinity, affect the strengthproperties of thermal-treated wood (Schneider1971). Bending strength, which is a combinationof tensile stress, compressive stress, and shearstress, is commonly used to compare mechanicalproperties of different processes. Table 5 presentsa summary of data on bending strength changes(modulus of rupture [MOR]) in thermal-treatedtimber from different processes and of differentspecies. In general, there is only a small changein modulus of elasticity (MOE) but a majordecrease in MOR independent of process or spe-cies. In some cases, even an increase in MOEis reported. Boonstra et al (2007) tested scotspine timber from the Plato process and got anincrease in MOE of 10%, whereas MOR showeda decrease of 3% compared with untreatedtimber. For structural timber of scots pine andnorway spruce with cross-sectional dimensionsof 45 � 145 mm and treated in the ThermoWoodprocess, Bengtsson et al (2002) found decreasedbending strength of up to 50% but only minorMOE changes.
Impact bending strength is decreased consider-ably for all thermal treatment processes and is aconsequence of the increased brittleness. Impactbending strength can be decreased up to 50% com-pared with untreated wood, but an 80% decreasehas been reported by Hanger et al (2002).
There is no major difference in health and safetyconsiderations for TMT compared with untreated
wood. A typical detectable difference is the smellof the material and the dust generated in theprocessing. TMT has a smoke-like smell, whichcomes from chemical compounds, mainly furfu-rals, which develop during processing. TMT wastecan be handled similar to any other untreated woodwaste, eg burned or pelletized and briquetted ifa mixture with normal sawdust is used.
Gluability is not changed to any great extentbecause of thermal treatment, and TMT can beglued with many industrial adhesives. The hydro-phobic wood surface causes lower penetration ofthe solvents from the adhesive into the woodmaterial, which may cause a need of increasedpressing times.
Decreased MC of TMT improves its stability,which in turn decreases cracking and flakingof the surface coating in changing weather con-ditions. To prevent color changes and surfaceshakes, a surface treatment against UV radiationis recommended. Normal painting processes arein general not problematic, but when electro-static painting is used, moisturizing is required.To prevent color changes, the treatment sub-stance should contain pigment, which usuallyresults in a slightly darker appearance.
ENVIRONMENTAL IMPACTS OF THERMALLY
MODIFIED WOOD
Awareness of climate change and its potentiallydisastrous consequences are stimulating a trans-formation toward sustainable development, withincreasing economic efficiency, protection andrestoration of ecological systems, and improve-ment of human welfare. Wood as a renewablebiological raw material in numerous applica-tions is therefore gaining in importance. This
Table 5. Decrease in MOR of thermal-treated sawn timber in relation to untreated wood.
Species Treatment ProcessDecrease inMOR (%) Reference
Birch (Betula pendula) Vapor, 200�C, 3 h ThermoWood 43 Johansson and Moren (2006)
Scots pine (P. sylvestris) Oil, 220�C, 4.5 h OHT 30 Rapp and Sailer (2001)
Eucalyptus (Eucalyptus globulus) Vapor, 200�C, 10 h Closed system 50 Esteves et al (2006)
Norway spruce (Picea abies) Aqueous environment
8-10 bar, 165�C, 30 min
Plato 31 Boonstra et al (2007)
Sandberg and Kutnar—THERMALLY MODIFIED TIMBER RECENT DEVELOPMENTS 35
presents an opportunity for the forest-basedsector to become a leader in achieving the globaltarget of decreased CO2 emissions with innova-tive production technologies, decreased energyconsumption, increased wood products recycling,and the reuse and refining of side-streams (eg byutilizing by-products). The systematic evaluationof material databases shows that, in spite of themajor achievements made in material science,timber as a structural material can hardly beoutperformed by any other materials, that it evenremains the first choice as far as plate bendingis concerned, even outperforming carbon fibercomposites, and that it is undisputedly unrivalledin terms of cost and environmental perfor-mance. These results show that timber is a high-performance material for structures, playing anessential role in construction and lightweightdesign. Several of the species used by industryhave deficiencies related to poor resistance tobiological degradation and low shape stability,which previously could be decreased by pres-ervation with more or less toxic substances,which today are forbidden to use in many coun-tries and regions. Consequently, there has beenrenewed interest in developing thermal processesin recent years.
Thermal processing will be implemented toimprove the intrinsic properties of wood and toobtain the form and functionality desired byarchitects, designers, and engineers. Great per-formance at low weight and low price createsa considerable market potential for thermal-treated timber products that can replace energy-intensive materials and methods of constructionand reveals a great potential in construction,architecture, light-weight construction, and fur-niture manufacture. Different modification pro-cesses and parameters yield modified woodwith different properties suitable for a varietyof product lines. However, they also have dif-ferent environmental impacts, which are con-sequently transferred into the materials andfinal products. Interactive assessment of processparameters, product properties, and environ-mental impacts should be used to aid develop-ment of innovative modification processes and
manufacturing technologies. Recycling, upcycling,the cradle-to-cradle paradigm, and end-of-lifedisposal options need to be integrated in a fullydeveloped industrial ecology for modified woodprocesses. New advances in wood-based mate-rial processing should support and promote effi-cient product reuse, recycling and end-of-lifeuse, and a low-carbon economy.
The low-carbon economy aims to mitigate cli-mate change and promote sustainable devel-opment by decreasing energy consumption,pollution, and emissions while increasing per-formance. This goal is especially stated withinthe EU. Therefore, research into thermally basedtimber processing and the resultant products mustplace more emphasis on the interactive assess-ment of process parameters, developed productproperties, and environmental impacts. Energyconsumption contributes considerably to the envi-ronmental impact of thermally treated wood.However, the improved properties during theuse phase might decrease the environmentalimpact of the thermally based timber process-ing. To achieve sustainable development, cer-tain criteria within a framework of economic,environmental, and social systems must also befollowed. The effective use of wood throughoutits whole value chain from forest management,through multiple use cycles, and end-of-life dis-posal can lead to truly sustainable development.This is especially true for the cascading useof wood—sequential use of a certain resourcefor different purposes (Hoglemeier et al 2013).Therefore, to contribute to the low-carbon econ-omy, thermal wood processing should imple-ment the following:
1. Establish a baseline of environmental impacts.Identify and quantify the environmental loadsinvolved, ie the energy and raw materials usedand the emissions and waste released. Thenevaluate the potential environmental impactsof these loads, which should be followed byan assessment of the opportunities availableto bring about environmental improvement.
2. Decrease emissions by redesigning existingtechnologies.
36 WOOD AND FIBER SCIENCE, MARCH 2016, V. 48(2015 CONVENTION, SPECIAL ISSUE)
3. Demonstrate a manufacturer’s commitmentto sustainability and showcase the manufac-turer’s willingness to go above and beyond.Therefore, product category rules, whichinclude the requirements for environmentalproducts declarations (EPD) for thermallyprocessed wood, should be defined in aninternationally accepted manner based onan open, transparent, and participatory pro-cess. Furthermore, EPD in which relevant,verified, and comparable information aboutthe environmental impact of products result-ing from thermal processing of wood shouldbe acquired. The EPD can then be used asproof of environmental claims in the publicprocurement arena.
4. Develop an “upgrading” concept for recov-ered products resulting from the thermal pro-cessing of wood as a source of clean andreliable secondary wooden products for theindustry. This will further strengthen TMTmarket competitiveness and sustainability andmitigate climate change by longer storage ofcaptured carbon in wooden materials.
Development of new building materials shouldconsider human well-being and should go beyondachieving minimal environmental impacts. Thesustainable design principles that emphasizedecreasing environmental impact of buildingconstruction, location, and utilization do so bytheir material choice, site choice, and energyuse across all phases of the building’s lifetime.These principles should be performed togetherwith the restorative environmental design (RED)paradigm, which brings together the ideas ofsustainable design and biophilic design (Kellert2008; Derr and Kellert 2013). RED attempts topromote a stronger connection between buildingoccupants and nature. Derr and Kellert (2013)believe RED is the next evolution of “green”design, which offers an opportunity for increasedwood use. Wood that is harvested from healthy,well-managed forests is a renewable material thatprovides carbon storage and satisfies both gen-eral tenets of the RED paradigm, sustainabilityand a connection to nature, making it an idealmaterial for RED. It is also possible to emphasize
the aspects of wood that people recognize asnatural, such as grain patterns and color pro-viding building occupants with a connectionto nature (Nyrud and Bringlimark 2010). Anymodifications to wood should minimize changesthat decrease its apparent naturalness (Burnardand Kutnar 2014, 2015). To successfully inte-grate TMT wood into RED practices, it musthave minimal environmental impacts (or posi-tively affect the environment) and be a recog-nizable element of nature.
CONCLUSION
Thermal treatment of wood is an innovative pro-cess currently being implemented in industrialapplications. Although many technical aspectsof thermal treatment are well known, the fun-damental influence of the process on productperformance, the environment, and end-of-lifescenarios remains unknown. Several studies havedealt with LCA of forests and primary woodproducts, but most of these tend to be cradleto gate and there is still a lack of data for thewhole value chain. The number of LCA studiesin the wood sector is relatively limited, andthese studies are geographically distributed anduse a variety of databases and impact assessmentprotocols. Comparison among different produc-tion processes is not possible given the lack ofavailable information, different system bound-aries, and different assessment criteria. A com-parison of different production methods usingcommon calculation rules is clearly required.This requires an integrated approach. It is essen-tial to integrate interactive assessment of pro-cess parameters, developed product properties,and environmental impacts. To optimize modi-fication processing to minimize environmentalimpacts, much more information must be gath-ered about all process-related factors affectingthe environment (volatile organic compounds,energy use, end-of-life use, etc.). This studyclearly shows that such data are missing or atleast are not documented in a systematic andtransparent way. Research in the future shouldinvestigate and characterize the relationshipsamong thermal modification processing, product
Sandberg and Kutnar—THERMALLY MODIFIED TIMBER RECENT DEVELOPMENTS 37
properties, and the associated environmentalimpacts. This will require analysis of the wholevalue chain, from forest through processing,installation, in-service use, end of life, secondand third life (cascading), and ultimately incin-eration with energy recovery.
ACKNOWLEDGMENTS
The authors acknowledge COST Action FP1407.Furthermore, Andreja Kutnar is pleased toacknowledge the support of WoodWisdom-Net þ and the Slovenian Ministry of Education,Science, and Sport of the Republic of Slovenia fortheir support of the What We Wood Believe andCascading Recovered Wood projects; EuropeanCommission for funding the project InnoRenewCoE (#Grant Agreement 664331) under theHorizon2020 Widespread-2015 program, andinfrastructure program IP-0035.
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