Trends and reasons for development of heat treated wood

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2009:001

M A S T E R ' S T H E S I S

Trends and reasons for development of heat treated wood

Ekaterina Sidorova

Luleå University of Technology Master Thesis, Continuation Courses

Wood Technology Department of Skellefteå Campus

Division of Wood Physics

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Sammanfattning

Projektet omfattar 5 delar:

1. Värmebehandlat trä. Egenskaper i relation till tillämpningar.

2. Storälgenprojektet

3. Konferens artikel: Värmebehandling av trä i olja 4. Utomhusexponering av trä

5. Värmebehandlat trä i extrema klimat

Den första delen omfattar allmänna studier av värmebehandlat trä och rådande samt framtida trender vad gäller tillämpningar.

Andra delen behandlar projekt inom forskning inom kvalitetsökning för utomhusexponerat värmebehandlat trä. Ett syftet med projektet är att ta fram underlag för att kunna bygga Storälgen, där en tanke är att dess exteriör skulle kunna utgöras av värmebehandlat trä. Del tre – fem behandlar forskning inom detta område.

Den tredje delen omfattar inte enbart studier gjorda i ovanstående projekt, utan även mitt deltagande i en internationell konferens: The 4th meeting of the Nordic Baltic network in wood material science and engineering (WSE).

Fjärde delen presenterar vissa resultat från färgmätningar.

Femte delen behandlar studier av hur värmebehandlat trä reagerar på ett extremt och varierat klimat.

Summary

This project consists of 5 parts:

1. Heat treated wood. Properties in relation to applications 2. Stoorn project

3. Conference paper: Oil heat treatment of wood 4. Outside exposure of wood

5. Extreme climate changes and heat treated wood

The first part includes the general studies of heat treated wood applications and trends for the present and future. The second part is telling about the project which I am involved in order to do research in the field of improvement the quality of heat treated timber for outside exposure. The project is aiming to build a huge entertaining complex in the shape of big moose; the cladding for the moose should be performed from heat treated timber. Parts 3 – 5 are the research for the Stoorn project. The third part was not only studies of one of the heat treatment ways which can be developed in future, but also participation in the conference. The fourth part displays the results from my colleague’s studies. The fifth part aimed to test the heat treated timber into relation to changeable climate.

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Introduction

Heat treatment (HT) is the process which changes the appearance and properties of wood.

Timber becomes darker, achieves consistent colour through the piece. The colour darkness depends on the HT temperature: the higher is the temperature, the darker is the colour. Thus HT timber can be an alternative to the expensive tropical species. HT makes wood more durable against decay and water resistant, so the wood can be used for outdoor applications. It is easier to design the items from HT timber, than from the untreated because of improved dimensional stability of HT wood. Reduced thermal conductivity of HT timber is an advantage for sauna applications. There are drawbacks of HT wood such as reduced bending and splitting strength; so this timber is not recommended for structural constructions. The strength properties are inversely proportional to the increase of heat treatment temperature.

Also the HT wood becomes grey like the untreated one during outside exposure. Thus special surface coatings are needed for the outdoor applications of HT timber. At the same time HT removes the resin from wood and makes the wood a good substrate for dressing treatments; so there is no reason for appearance of yellow spots coming through the surface coatings in later years^4,12.

It is always important to enhance the properties of the material, because the material technologies are developing faster and the competition is increasing. The researches dealing with heat treated wood are doing many efforts in order to improve the material, so it will have fewer drawbacks in the future.

The future development of the HT wood depends on its actual applications and future projects which require certain properties of the material. Those properties will be enhanced which are have the biggest interest. It will certainly take much work of the researchers in contact with industries, but it should result in the development of new HT wood meeting the demands of the customers.

The aim of this project is to study the Scandinavian market of HT wood products, finding out applications and trends for HT wood and to carry out the practical part of the project by making contribution into the development of heat treatment technologies. Perhaps the practical part will become a good effort for the future developments of heat treated timber.

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Part 1 Heat Treated Wood. Properties in Relation to Applications

In recent years the heat treatment industrial process has been developed successfully on Scandinavian market. The Scandinavian companies working with heat treatment include the members of ThermoWood Association, Finnforest, Stora Enso, Stellac, SWM-Wood, Bitus, Oy Esse Möbel Ab and others.

Classes

Thermally treated wood has a number of applications for both internal and external uses.

Different properties of the material are prioritized depending on the final application of the product. Thus there are various ways to run the heat treatment processes in order to achieve required properties. Several classes of HT timber are known in Scandinavian market.

The most commonly used classes are ThermoWood classes. ThermoWood classifies the HT timber in two types: ThermoD and ThermoS (table 1-1)⁴.

ThermoD class has durability as the key property in the end use applications of products. The wood of this class is suitable for the applications where resistance against decay is required;

also ThermoD can be used in the case when the darker colour is desired⁴.

ThermoS class recognizes stability as the key property. The class is used where the strength properties are prioritized⁴.

The treatment temperature for ThermoD softwoods is 212°C, hardwoods is 200°C and for ThermoS softwoods is 190°C, hardwoods is 185°C (fig. 1-1)⁴.

Figure 1-1 – ThermoWood classes

Other classes are five Stellac classes: D1, D2, D3 and T4, T5. HT according to D-classes makes timber durable; T-classes are made for the applications where durability importance is not critical and the strength properties are prioritized. The thermal modification levels of these classes are within the limits of 190-250°C (fig. 1-2, table 1-1, Appendix 1). The exact temperature depends on the species and treatment class level; the temperature settings are Stellac’s best know-how⁹. Thus the information about exact temperature for each Stellac class is not available.

Treatment level D1 is required when the excellent decay resistance is critical as in the case of extremely hard moisture conditions. But strength properties are slightly reduced for this class,

ThermoS (softwood) ThermoS

(hardwood)

ThermoD (hardwood)

185ºC 190 ºC 200ºC 212ºC

ThermoD (softwood)

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therefore HT wood class D1 is not recommended where a high quality wood working finish is required⁹.

Treatment level D2 can be used for conditions requiring good decay resistance. Thus resistance to decay is considered to be sufficient for the most outdoor applications⁹.

Treatment level D3 is intended for the applications demanding fair decay resistance as in light exposure of weather stress. This level provides the best possible decay resistance without significantly reducing the strength properties; it also ensures the good quality of surface finishing⁹.

Treatment level T4 is performed to make HT timber requiring excellent strength properties (such as surface hardness). This treatment is intended for applications where low water absorption, reduced moisture deformation and precise colour shade are essential⁹.

Treatment level T5 is suitable for interior applications where desirable colour shade is needed.

The physical properties are not changed significantly. The wood of this class is recommended to similar uses as untreated wood⁹.

190ºC... 250 ºC

Natural T5 T4 D3 D2 D1

Figure 1-2 – Stellac classes (the information about the exact temperature for each class is not available)

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Table 1-1: Summary of recommended applications of HT wood treated according to several classes^4,9

Softwood Hardwood Stellac D1

Extremely hard moisture conditions Sink tops

Stellac D2

Wall claddings Terrace floors Playground furniture

Garden fences Windows

Doors Noise barriers

Garden furniture Parquet Furniture Wall panels Kitchen cabinets

Sink tops

Stellac D3

Wall claddings Terrace floors Playground furniture

Garden fences Windows

Doors Mouldings Kitchen cabinets

Furniture

Garden furniture Parquet Furniture Wall panels Kitchen cabinets

Carpentry work

ThermoD

Cladding Outer doors

Shutters

Environmental constructions Sauna and bathroom furnishing

Flooring Garden furniture

Furnishing Fixtures Furniture

Flooring Sauna structures

ThermoS

Building components Furnishing in dry conditions

Fixtures in dry conditions Furniture Garden furniture

Sauna benches Door and window components

Furnishing Fixtures Furniture

Flooring Sauna structures

Stellac T4

Interior floors Wall panels Prefabricated wall elements

Mouldings Kitchen cabinets

Furniture Rail upright floors

Parquet Furniture Wall panels Kitchen cabinets

Carpentry work Interior floors Cutting boards Sauna benches and panels

Stellac T5

Interior floors Wall panels Prefabricated wall elements

Mouldings Kitchen cabinets

Wall panels Furniture Carpentry work Sauna benches and panels

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Applications: external and internal

The key property for the external uses of the HT wood is high durability in the changing climate conditions. External applications of heat treated wood include: outdoor cladding;

façades; landscaping and park structures (decks and patios, garden planks and furniture, fences and fence posts, flower boxes, sand boxes, pergolas); shingles; outside doors, windows and shutters; balconies; pedestrian bridges.

Outdoor claddings, façades, outer doors, windows and shutters require high decay and weather resistance (fig. 1-3). Commonly the coniferous wood species such as pine and spruce treated according to Thermo-D^®, Stellac^®Wood D2, D3 categories or Stellac^®Wood D1 (for extremely hard moisture conditions) are used for the external claddings¹².

www.swm-wood.com www.swm-wood.com

Figure 1-3: Outdoor applications (claddings, façades, doors, windows)

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Landscaping structures, park and playground constructs, pedestrian bridges require not only high durability, but also stability (fig. 1-4). Heat treated wood of classes standing for durability without critical strength reduction can be used in these applications. Stellac^®Wood D2, D3 categories could be used in these cases.

Figure 1-4: Outdoor applications (yard and garden products)

The indoor applications include the production of internal claddings, inner doors, inner shutters and frames, panelling, furniture, flooring, stairs and banisters, decorative details (fig.

1-5). The durability of the material is not critical in the case of indoor applications. Material has to meet the esthetical requirements. In many cases the strength properties are important especially for the furniture applications. So the treatment level is chosen to meet the colour and strength requirements. Stellac^®Wood D1 is not recommended, since this class meets the poorest strength properties.

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www.swm-wood.com www.essemobel.fi

Figure 1-5: Indoor applications

www.thermowood.fi www.swm-wood.com www.suomenlampopuu.com

www.suomenlampopuu.com www.suomenlampopuu.com

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There is other type of indoor applications such as sauna benches and panels, sauna and bathroom furniture (fig. 1-6). They demand good moisture resistance and reduced thermal conductivity. Heat treated timber meets their requirements. ThermoS, Stellac T4 and T5 suit for these applications.

Figure 1-6: Sauna applications

The application of HT wood for kitchens is also wide spread (fig.1-7). All classes except Stellac D1 can be used. As mentioned, this class has the worst strength properties and is recommended only for extremely hard moisture conditions.

www.swm-wood.com www.thermo-timber-team.com

Figure 1-7: Kitchens made of HT wood

Species

Different wood species can be heat treated, but one of the ideas of heat treatment is to use the cheaper wood for the treatment in order to enhance its properties and make it more valuable.

The commonly used coniferous wood is pine and spruce. The treatment of hardwoods commonly includes: aspen, ash, birch, oak, beech. The use of softwoods is more common in

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outdoor applications; the use of hardwoods is more common for indoor applications and also garden furniture.

Surface coatings

To prevent the colour changes of HT wood different surface coatings are applied. Water- or oil-based paints are used. To prevent colour changes of the material in future, treatment substance should contain pigment; and the more pigment is used the longer maintenance interval is⁴.

Tendencies and Future Development

Heat treatment is a remarkable process because it enhances the wood properties without adding any chemicals into the material. HT wood can be utilized like normal wood after its service life is complete⁴. So the HT timber can be a substitute for CCA impregnated timber since HT itself is chemical free process. The price of HT wood is higher than CCA timber, but it is difficult to make the customers pay too much for the durable material.

The reason is that the customers focus on the visual appearance of the material first of all when they buy it. The material colour is the actual sales argument. So, HT wood became an alternative to expensive tropical wood species or Western red cedar (which is popular in the North America). At the same time HT wood degrades in its colour, becoming grey in the outdoor applications, so there is a needing in the surface coatings of the material. In the case of visual appearance argument, this drawback makes some customers think that the heat treatment is useless process.

In cases of building and construction the thermal isolation value of HT wood is and will be considered important, especially for window components. Later it can result as the important property for façade material. The reduced thermal conductivity of HT wood is important in sauna applications, where the material can find a good future.

HT wood is appreciated for its good dimensional stability. It makes much easier the work of architects to design the items from HT timber than from ordinary wood, because the material does not swell in the humid conditions. Also it is important in the case of using the surface coatings for HT wood: the coated surface does not expand with the time and the coating serves longer than for the untreated wood.

The reduced strength properties deprive the HT wood possibility to be used in load-bearing constructions. One of the ways to improve the strength of the material is making the laminated beams.

The HT timber has a good future, but much research work must be held in the area of heat treatment. As it was mentioned, the actual sales argument of HT wood is the colour, so it is important to perform a treatment process which can make the colour permanent. The HT has to meet much competition with other technologies, such as wood modification (for example, acetylation). Thus HT process has to be improved and combined with other technologies. One of the ways is to make an impregnated HT wood. In future there should be a demand of the material which is coloured not only superficially but throughout the whole piece.

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There are different ways to heat treat the wood besides the air heat treatment, for example, oil heat treatment. Oil heat treatment is appreciated by the producers due to the technology advantages, because oil serves as a perfect separation of oxygen from wood from during the HT process and oil is a great heat transfer to the wood. That is why it is possible to heat treat the samples thicker than during the ThermoWood Process. Bitus AB has been using oil heat treatment for the Thermex product, but now they have discontinued this production, because they are not ready to finance the research. Big scale companies such as the members of ThermoWood Association, Stora Enso and Finnforest have a great advantage in this case;

they have higher capacity and are able to invest much into the research of their product². It seems like for the nearest future the biggest companies, such as members of ThermoWood Association, Stora Enso and Finnforest will develop their products, because they are leading and have enough finances to develop their production further^3,4,11. Probably they will meet much competition in the far future with other treatment technologies. In this case they will have to bring the new technologies into their companies and develop them more.

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Part 2 The Stoorn Project

www.membrana.ru

Figure 2-1: Storälgen

Stoorn or Storälgen is a project aiming to build a big entertaining complex in the shape of large moose in the North of Sweden (fig. 2-1). The Big Moose having enormous sizes: height of 45 metres, length of 47 metres and width of 12 metres, is supposed to become an innovative tourist attraction (fig. 2-2)¹⁰.

http://www.ohgizmo.com/

Figure 2-2: The large moose construction

Stoorn’s facilities are set to become a commercial tourist and adventure centre in the middle of the forest and wildlife. Top-class restaurant, exhibition venues, a concert hall and modern conference facilities will be inside the edifice. Stoorn will be on the top of the 510-metre

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Vithatten Mountain. The location of Stoorn will give visitors the opportunity to admire the expansive view of mountain forest landscape and the sea (fig. 2-3). The Stoorn’s antlers will be an alfresco dining area and lookout point (fig. 2-4)¹⁰.

www.stoorn.se

Figure 2-3: Big Moose on Vithatten Mountain

www.membrana.ru

Figure 2-4: View from the antlers of Stoorn

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Stoorn, having a foundation of concrete and legs and a steel framework, is going to be made of glulam. The cladding is supposed to be made of heat treated wood. The Big Moose will bite into an artificial pine. The lift up to the moose head will be in the trunk of the pine. The visitors will enter the moose through moose’s muzzle (fig. 2-5)¹⁰.

www.membrana.ru

Figure 2-5: The entrance into the facility

This ambitious project was inspired by Thorbjörn Holmlund, tourism entrepreneur, founder and owner of Svansele Vildmarkscenter AB. Holmlund’s business started in 1996 and has grown with the time, having 25000 visitors per year for the present. Thorbjörn Holmlund, having been an organizer of many moose safaris, got an idea of building Big Moose which should become not only tourist facility, but have a large national and international attraction and become a high symbolic value of Sweden. Stoorn will be a symbol not only in the means of popularity of elks in Sweden. Big Moose construction will become a symbol of innovation in Swedish forestry, Swedish wood material production, technology and construction. It will become a centre to visitors to the forests of the far north of Sweden. The location in the living environment will play a significant role, because besides the Stoorn’s conference venues, restaurant and concert hall, the facility will offer different activities in the area of adventure tourism within forest and wildlife. The activities such as snowmobile safaris, sled-dog tours, hunting, fishing and guided nature walks will be offered¹⁰.

www.stoorn.se

Figure 2-6: Big Moose in the middle of the forest and wildlife

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On the official web page of Stoorn it is said:

“The originality of the project makes it a valuable point of reference for modern and advanced Swedish timber construction. The reference value lies in:

Showing that advanced construction solutions can be achieved today using entirely wood- based construction solutions, created using advanced glulam techniques.

Heat-treated wood creating aesthetically stimulating, living and maintenance-free surface finishes.

The ability to create interiors which bring out wood’s aesthetic qualities – an aim which has been realised in this project as a result of modern research.

Using an exciting and aesthetically appealing mix of other materials in indoor surfaces.

Stimulating interest in and respect for forest, lakes, wetlands, flora and fauna on the part of both visitors and residents of the region.”¹⁰

Thus this project gives a huge opportunity for the present research in the area of material science and technology (especially wood) and environment. The Stoorn project was granted an environmental licence in 2007 and building permit was granted in 2008. The aim is to finish the project and open Stoorn’s doors to the public in 2010¹⁰.

For the present the project organisation consists of:

- Håkan Widmark Chairman;

- Sven-Olof Holmström, SSC Skellefteå Snickericentral;

- Peter Lugnegård, Thule In AB;

- Thorbjörn Holmlund, Svansele Vildmarkscenter AB;

- Börje Hörnlund, former Minister of Labour, County Governor;

- Lorentz Andersson, former County Governor¹⁰.

Stoorn is a pilot project within the National Wooden Building Programme, headed by County Governor Lorentz Andersson, Umeå. The project work is performed in very close cooperation with Luleå Tekniska Universitet (LTU), Skellefteå¹⁰.

The Stoorn project and the research at Luleå University of Technology

I have been working at the department of Wood Physics at LTU. The task for our department is the research in the area of Stoorn’s cladding which is going to be made of heat treated wood as it was mentioned. We are to find the best way to heat treat wood, so it will be durable and save its colour in the long terms¹⁰.

We have been testing the heat treated wood ThermoWood which has been the leader in Scandinavian heat treatment production. We are ready to try different ways to heat treat wood, combine heat treatment with wood impregnation and compare to ThermoWood. We would like to make a material which will have all benefits from the heat treatment process and will have reduced drawbacks caused by heat treatment. Our goal is to improve heat treatment,

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because this project requires high technology since it is going to bear a significant symbolic value of Sweden.

One of the heat treatment ways that we would like to study more is oil heat treatment (Part 3).

I have been doing research on the oil absorption during the process of oil heat treatment. The knowledge about oil absorption is important because the next step for the research is to use some chemical additives during the process. The additives are supposed to penetrate into the wood together with oil. The purpose of adding chemicals is to increase durability and improve the colour resistance, since the material is supposed to be for the outside exposure.

Also I have been studying the relation of wood samples heat treated in different ways to extremely changeable climate conditions: soaking in water, direct freezing and warming (Part 5). I would like to find out the best material which quality will degrade less in the North Swedish climate.

Åke Olofsson, the owner of Woodline AB, Skellefteå, has been doing the outside exposure test and colour measurements of the material (Part 4).

The project is not finished. The studies presented in my Master Thesis are the start for the further research into Stoorn project. Stoorn is a good example of future application of heat treated timber. Heat treated timber has a reason to be developed further, since there is an interest in it and the material properties should be improved.

www.stoorn.se

Figure 2-7: The way to the future Stoorn…

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Part 3 The Paper “Oil Heat Treatment of Wood” and Conference

As it was mentioned we would like to investigate the oil heat treatment of wood, because it is possible to add chemicals into the oil which is absorbed by timber during the process. The impregnation can make the material more durable and colour resistant. The combination of heat treatment and impregnation is rather new trend and can find many applications in the future.

The aim of my work has been to study the oil absorption of wood during the process of oil heat treatment. This work has included the heat treatment of spruce, pine (heartwood and sapwood) and aspen in rape seed oil. The heat treatment was performed in deep-fryers and a vessel heated by magnetic stirrer (fig. 3-1) at 180, 210 and 240°C during 30, 60 and 120 minutes. Two sets of samples were run, the first one was heat treated and left to cool in the air, the second one was heat treated and directly cooled in the oil bath for 1 hour. At 180°C there was a trend of increasing oil absorption during the heat treatment with the increasing the treatment time for all species. At 210°C the percentage of mass growth was reducing with increasing the time of the process. Pine heartwood started to lose its weight already at 210°C.

At 240°C all species lost the weight and the percentage of weight loss was increasing proportionally with increasing heat treatment time. During the heat treatment aspen had the highest mass increase and pine heartwood had the lowest. Results showed that wood absorbed significantly more oil in the stage of cooling in oil than during heat treatment. All samples gained the weight during storing in the cool oil. The wood was sinking in the oil bath after cooling in it. All species had a tendency to have approximately the same oil pick up during storing in the oil bath after the heat treatment at one temperature, so the heat treatment time did not have an effect on the oil absorption in the stage of cooling. The oil pick up during the stage of cooling had the lowest values for all species treated at 240°, because all the species lost the mass during the heat treatment at 240°. The colour changed during heat treatment and depended mainly on the heat treatment temperature (fig. 3-2). The darkness was increasing proportionally with increasing the thermal treatment temperature. The colour varied among the species. After heat treatment pine (especially heartwood) was darker than spruce, aspen was the darkest and gained a sombre brown colour during treatment at 240°C.

a b c

Figure 3-1: Equipment for the oil heat treatment: a – Deep-fryer Co-line; b – Deep-fryer Frifri Basic 411; c – magnetic stirrer with heating VMS-C10 and Contact Thermometer VT-5 from VWR International

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Figure 3-2: Samples before and after heat treatment

Untreated Treated at 180° Treated at 210° Treated at 240°

Spruce

Pine heartwood

Pine sapwood

Aspen

I wrote a paper “Oil Heat Treatment of Wood”, including the description and results of my research (Appendix 2). On the 13-14 November, 2008, I participated in the conference at the 4th meeting of the Nordic Baltic network in wood material science and engineering (WSE), held in Riga, Latvia, and presented my work.

Nordic Baltic network was established by SNS-Nordic Forest Research Cooperation Committee in 2004. The purpose of the network is exchange of know-how and experience, establishment of contacts and possibilities for further co-operation projects among the researches dealing with wood science and engineering^1,13.

The first meeting for Nordic Baltic network was arranged by Skogforsk and held in Honne, Norway, in August 2005. Second meeting was arranged by the Royal Institute of Technology, KTH, and the Swedish National Testing and Research Institute, SP, and held in Stockholm, Sweden, in October 2006. The third meeting was arranged by the University of Helsinki, Department of Forest Resource Management and held in Helsinki, Finland, in October 2007.

The fourth meeting in Riga was arranged by the Latvian Institute of Wood Chemistry¹³. The paper presentations of the meeting in Riga covered the following areas: wood resources, wood biodeterioration and protection, wood modification and new products, surface treatment and properties, moisture content, drying, physical and mechanical properties, sawing.

My paper “Oil Heat Treatment of Wood” was published in the ”Proceedings of the 4th meeting of the Nordic Baltic network in wood material science and engineering (WSE), 13-14 November, 2008, Riga, Latvia” (edited by Bruno Andersons and Henn Tuherm). I got WSE Student Award for the best paper and presentation.

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Part 4 Outside Exposure of ThermoWood

Åke Olofsson has been doing the outside exposure test of heat treated ThermoD and untreated spruce, pine and aspen since July 2008. The test samples had sizes 10x120x300 mm. These samples were placed on the roof of Luleå University of Technology (Skellefteå) (fig. 4-1) in three different positions: horizontally, 45° to the South, vertically to the North side (fig. 4-2).

Colour measurements (lightness, colour saturation, hue) of samples have been performed⁶. The results of the outside exposure test after four weeks are summarized in this part.

Picture: Åke Olofsson

Figure 4-1: The constructions for the outside exposure test on the roof of Luleå University of Technology

b

c

a

Figure 4-2: The constructions for the outside exposure test on the roof of Luleå University of Technology and three different positions of the samples: a – horizontally, b - 45º to the South, c – vertically to the North

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Picture: Åke Olofsson Picture: Åke Olofsson

Figure 4-3: The test samples on the roof

Lightness

Untreated aspen samples decreased the lightness less than spruce and pine. Untreated pine had higher lightness decrease in lightness than aspen and spruce. The highest lightness increase for the untreated samples was in the case of position 45° to the South. The samples placed vertically to the North had lowest lightness decrease. It can be the result of the UV radiation;

the Sun is the most active from the South side⁶.

Treated samples increased the lightness. Aspen had the highest increase in lightness. Pine had lower lightness increase than spruce. The highest lightness increase was for the samples placed 45° to the South⁶.

Colour saturation

The untreated aspen placed horizontally and 45° to the Southhad weak decrease of colour saturation. Other untreated wood samples showed weak increasing of colour saturation⁶. All heat treated aspen samples and heat treated pine placed horizontally showed weak decrease of colour saturation. Other treated samples had stable colour saturation or a bit increasing⁶.

Hue

Untreated wood became redder, treated wood became more yellow⁶.

Heat treated and untreated aspen samples places horizontally and 45° to the South lost the hue difference after four weeks⁶.

Heat treated and untreated pine and spruce samples lowered hue difference after test⁶. Conclusions

It is important to protect the colour of heat treated and untreated timber against UV radiation by using special coatings or perform the impregnation of timber in order to use the material for outside exposure in the future.

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Part 5 Extreme Climate Changes and Heat Treated Wood:

Soaking in water, Freezing and Warming

ABSTRACT

Untreated, ThermoD and oil heat treated at 180°C wooden samples were tested for the extreme climate changes. The species used in the experimental were pine, spruce and aspen.

The experimental contained of five cycles of soaking in water, freezing and warming. After experimental all samples degraded in colour. Aspen had lowest colour degradation. Pine heat treated according to ThermoD and untreated pine got the cracks on the surface. Oil was coming out of oil heat treated samples during soaking. Oil appeared on the surface of oil heat treated pine during warming.

INTRODUCTION

The importance of performing this experimental is evident, because it is necessary to choose the best heat treated material for the external cladding of Stoorn. The Swedish climate is characterized by changes in the temperature and humidity especially in the winter period. It is important that the cladding material will save its colour during many years. So to make the experimental shorter the very extreme climate changing conditions were made for the tested material.

MATERIALS AND METHODS Materials

The heat treated samples of pine, spruce and aspen with the dimensions 10x120x300 mm were tested. The samples were heat treated in two ways: ThermoD and oil heat treatment during one hour with cooling in oil bath for one hour. 8 samples of each species and each treatment variant were tested for the extreme climate changes. 8 reference untreated samples of pine, spruce and aspen were also tested.

Methods

The material was tested for 5 cycles. Each cycle contained: soaking in water for 24 hours, direct freezing for 6 days and direct warming for 30 minutes.

Soaking in water was performed in a big vessel for 24 hours at the room temperature. The samples were under water and pressed by the weight. In the first cycle the water absorption (W) of samples was measured and calculated in percentage (Eq. 5-1)

W = (Mw – Md)/Md x 100% (5-1) Md – mass before soaking,

Mw – mass after soaking.

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Freezing was performed in the Freezer Cylinda (Denmark) for 6 days at -28ºC (fig. 5-1).

Figure 5-1: Freezer Cylinda and the samples

Warming was performed in front of the patio infrared lamp Ningbo Nationstar Electrical Appliance Co., Ltd (China) (fig. 5-2). The warming was performed until the sample surface reached +40°C; it took 30 minutes for all the samples.

Figure 5-2: Infrared lamp and the samples

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RESULTS AND DISCUSSION Soaking and water absorption

Results of the water absorption during soaking in the first cycle are displayed in the table 5-1.

Water absorption of untreated samples had higher values than water absorption of treated samples, because heat treatment reduces absorption of water⁴.

Regarding untreated wood pine had the highest water absorption compare to spruce and aspen. It is difficult to compare figures for water absorption of untreated spruce and aspen, because the standard deviation had high values, but the average water absorption of spruce was higher than the one of aspen.

Pine has higher mass increase than spruce, because of the difference in the connections between longitudinal tracheids and horizontal parenchyma cells in rays of species⁸. Looking at the radial view it can be seen that pine has a large pit (fenestriform pit) and thin membrane between adjacent cells. Spruce has a number of small piceoid or cupressoid pits in the membrane (fig. 5-3)⁸.

Figure 5-3: Interconnecting pits between ray parenchyma and vertical tracheids in pine and spruce, earlywood and latewood ⁸

Aspen did not have so high water absorption like pine in the case of ThermoD and untreated timber, even though aspen is hardwood which is known to have much wider cells (vessel elements) than softwood⁷. But aspen is diffuse-porous and its pores are very small⁵, probably it can explain the lower water absorption.

ThermoD timber had strongly reduced water absorption for spruce and aspen as it was expected. Pine reduced absorption compare to untreated pine, but still pine had higher water absorption values than spruce and aspen. Spruce had a bit lower absorption than aspen, but it is not significant, because of high standard deviations.

Interesting results were achieved for oil heat treated wood. The water absorption of pine was much lower than the absorption of spruce and aspen; water absorption of oil heat treated pine was also lower than pine treated according to ThermoD. It can be explained by the fact that pine absorbed much of oil in the stage of cooling after heat treatment. Water absorption of oil heat treated spruce was higher than of ThermoD spruce, because oil heat treatment was performed at 180°C (not like ThermoD which has treatment temperature 212°C for

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softwoods). The higher treatment temperature results in the lower water absorption of the timber.

Table 5-1: Water absorption of samples (W – water absorption, St D – standard deviation) ThermoD Oil heat treatment at

180° C

Untreated

W, % St D, % W, % St D, % W, % St D, %

Spruce 16,3 3,1 28,7 5,9 30,8 3,1

Pine 28,2 7,7 12,0 2,4 36,8 1,7

Aspen 18,1 2,8 19,7 3,2 27,7 4,1

The oil heat treated samples had a tendency to lose the oil during soaking, because the water always contained oil spots on the surface and had a disagreeable odour after soaking of oil heat treated wood for 24 hours. Water did not have a disagreeable odour after soaking of ThermoD and untreated wood.

Results after 5 cycles of testing

ThermoD pine practically did not change colour after cycling (fig. 5-4). Some cracks appeared on the pine samples (fig. 5-5). This can be explained by high water absorption during soaking.

Figure 5-4: ThermoD pine (up – before cycling; down – after cycling)

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Figure 5-5: Cracks on ThermoD pine after cycling

ThermoD spruce became darker and greyish, but there was no cracking on the samples (fig. 5- 6).

ThermoD aspen became a bit greyish but the colour change was not significant (fig 5-7).

Figure 5-6: ThermoD spruce (up – before cycling; down – after cycling)

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Figure 5-7: ThermoD aspen (up – before cycling; down – after cycling)

Oil heat treated pine became a bit darker, but there were no cracks like in pine ThermoD (fig.

5-8). It can be explained by the fact that oil heat treated pine had low water absorption, since pine absorbed much oil during heat treatment process. At the same time oil started to come out on the surface of pine during warming of the samples (fig. 5-9).

Figure 5-8: Oil heat treated pine (up – before cycling; down – after cycling)

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Figure 5-9: Oil on the surface of oil heat treated pine after warming

Oil heat treated spruce degraded in colour, but there was no oil on the surface of the samples during warming (fig. 5-10).

Aspen also became a bit darker, but the change for aspen was less than for pine and spruce (fig. 5-11).

It is possible to see that aspen degraded less of all three species and spruce had the highest degradation of colour.

Figure 5-10: Oil heat treated spruce(up – before cycling; down – after cycling)

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Figure 5-11: Oil heat treated aspen(up – before cycling; down – after cycling)

Untreated pine, spruce and aspen degraded in colour.

Pine and spruce got big bluish spots on the surface (fig. 5-12, 5-14). Pine also got some cracks (fig. 5-13). Aspen became darker, but no spots were visible (fig. 5-15).

Figure 4-12: Untreated pine(up – before cycling; down – after cycling)

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Figure 4-13: Cracks on the untreated pine after cycling

Figure 4-14: Untreated spruce(up – before cycling; down – after cycling)

Figure 4-15: Untreated aspen(up – before cycling; down – after cycling)

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CONCLUSIONS

The water absorption of untreated wood had the highest values as it was expected. Pine had higher water absorption than spruce and aspen in the case of untreated wood and wood treated according to ThermoD. Oil heat treated pine had lowest water absorption compare to oil heat treated spruce and aspen and even ThermoD wood because pine absorbed much of oil during treatment process.

Oil heat treated samples were losing oil during soaking in water; pine had oil on the surface during warming.

The low water absorption is an advantage for oil heat treated pine, but unfortunately the oil which is leaving the pine during soaking and warming will increase the water absorption with the time. Thus it is important to develop coating or impregnation which will prevent the loss of oil from the timber.

All the samples decreased in colour. Aspen had the lowest colour degradation. Spruce had highest colour degradation. Thus it is important to protect colour not only from UV radiation but also from changing climate conditions.

Untreated and ThermoD pine had cracks after cycling. The absence of cracks on the oil heat treated pine can be explained by low water absorption. Cracking of the material showed that it is very weak to resist the changeable climate conditions.

The weak colour resistance is the drawback for all the heat treated wood. Necessary coatings or impregnations should be used for the HT timber.

Comparing all three species, aspen showed the best results, because it did not cracked and its colour degradation was lowest.

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Future Work

The future work will include the improvement of the process of oil heat treatment, continuation of the outside exposure test and extreme climate change test for samples heat treated in different ways.

Regarding oil heat treatment, the research will be carried out by adding some chemicals into the oil in order to enhance the product durability and colour resistance.

Outside exposure test will be performed on oil heat treated samples.

Extreme climate change test will be performed on the samples oil heat treated at 210 and 240ºC.

Outside exposure and extreme climate change tests will be also performed on the new heat treated samples, when the new ways of heat treatment will be determined.

The fungi and durability tests of the oil heat treated wood should be performed.

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Conclusions

During this project the certain knowledge about HT wood and its applications was gained.

The practical work was carried out in order to do the research for the Stoorn project.

The first step in the research of oil heat treatment of wood was done by studying oil absorption during the heat treatment process. The high oil absorption can be achieved during cooling in the oil directly after treatment and the type of specie is the main factor influencing on the absorption. The conference success of the paper and presentation on the topic “Oil heat treatment of wood” showed that this topic is urgent for the modern research and development.

The HT wood made according to ThermoD and oil heat treatment degrades in colour after outside exposure and climate changes. ThermoD pine can not resist extreme changes of the climate and starts to crack. The colour degradation and cracking make the material lose the aesthetic look with the time. Cracking of cladding can result in the bad protection of the building.

It is necessary to think of a new heat treatment way in order to improve the quality of heat treated timber. The idea is to investigate different heat treatment ways and combine them with chemical timber impregnation.

The research is not finished. HT wood material should be more improved in order to be the cladding of Stoorn. The HT wood that would be made for Stoorn should be high durable and colour resistant material. This material should represent the future HT wood for outdoor applications. Much research should be performed to develop a new technology for the outside cladding in the changeable climate.

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References

1. Andersons B, Tuherm H (2008) Proceedings of the 4^th meeting of the Nordic Baltic network in wood material science and engineering (WSE), 13-14 November, 2008, Riga, Latvia.

2. Bitus AB (2008) Homepage, http://www.bitus.se/, 1/11 2008

3. Finnforest (2008) Homepage, http://www.finnforest.com/, 23/12 2008

4. Finnish Thermowood Association (2003) ThermoWood® Handbook. ThermoWood:

Homepage, http://www.thermowood.fi/, 1/3 2008.

5. Mackes KH, Lynch DL (2001) Aspen forest products. USDA Forest Service Proceedings RMRS-P-18

6. Olofsson Å (2008) Färgmätning av utomhusexponerade paneler av obehandlad och värmebehandlad asp, furu och gran. Luleå Tekniska Universitet, Instutionen i Skellefteå, Sverige

7. Rowell R (1984) The chemistry of solid wood. American Chemical Society, Washington, USA.

8 Sehlstedt-Persson M, Johansson D, Morén T (2006) Effect of Heat Treatment on the Microstructure of Pine, Spruce and Birch and the Influence on Capillary Absorption. In:

Proceedings of The 5th International IUFRO Symposium Wood Structure and Properties. 3-4 September, Sliač - Sielnica, Slovakia

9. Stellac® (2008) Homepage, http://www.stellac.fi/, 9/11 2008.

10. Stoorn (2008) Homepage, http://www.stoorn.se/, 15/11 2008 11. StoraEnso (2008) Homepage, http://www.storaenso.com/, 1/5 2008 12. SWM-Wood (2008) Homepage, http://www.swm-wood.com/, 1/3 2008

13. Wood science and engineering (WSE) (2008) Nordic Baltic network, Homepage, http://www.wse.no/, 1/12 2008

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Appendix 1. Stellac Treatment Levels

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Appendix 2. Oil Heat Treatment of Wood

(from Proceedings of the 4

^th

meeting of the Nordic Baltic network in wood material science and engineering (WSE), Edited by B.

Andersons and H. Tuherm)

OIL HEAT TREATMENT OF WOOD

Sidorova, K.

ABSTRACT

The investigation of thermal treatment of wood has led to the improvement of heat treatment with vegetable oils. The aim of this work has been to study the oil absorption of wood during the process of oil heat treatment. This work has included the heat treatment of spruce, pine (heartwood and sapwood) and aspen in rape seed oil. The heat treatment was performed in the deep fryer at 180, 210 and 240°C during 30, 60 and 120 minutes. Two sets of samples were run, the first one was heat treated and left to cool in the air, the second one was heat treated and directly cooled in the oil bath for 1 hour. At 180°C there was a trend of increasing oil absorption during the heat treatment with the increasing the treatment time for all species. At 210°C the percentage of mass growth was reducing with increasing the time of the process. At 240°C all species lost the weight and the percentage of weight loss was increasing proportionally with increasing heat treatment time. During the heat treatment aspen had the highest mass increase and pine heartwood had the lowest. Results showed that wood absorbed significantly more oil in the stage of cooling in oil than during heat treatment. All species had a tendency to have approximately the same oil pick up during storing in the oil bath after the heat treatment at one temperature, so the heat treatment time did not have an effect on the oil absorption in the stage of cooling. The oil pick up during the stage of cooling had the lowest values for all species treated at 240°, because all the species lost the mass during the heat treatment at 240°. The colour changed during heat treatment and depended mainly on the heat treatment temperature. The darkness was increasing proportionally with increasing the thermal treatment temperature. The colour varied among the species.

Key words: oil heat treatment, pine, spruce, aspen, oil absorption

INTRODUCTION

The thermal modification of wood has been known as a process enhancing wood properties by reducing moisture absorption, improving dimensional stability and biological durability [12,11,3,5]. In recent years the heat treatment industrial processes have been developed successfully in Europe. These processes use air steam, nitrogen or oil as the heat transfer and include the Finnish ThermoWood [13] and Dutch PlatoWood [2] using steam, French Retification using nitrogen [4], German OHT-Process using oil [8].

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The treatments using air or nitrogen demand accurate control of high temperatures for the planned process time to enhance wood properties [14]. Hot oil provides fast and equal heat transfer to the wood and the same conditions all over the whole vessel; also oil serves as a perfect separation of oxygen from wood [8]. It was found that the hydrophobic properties and resistance against biological attack of oil-heat treated wood not only benefit from the heat treatment, but also from the shell formed by water-repellent oil during the process [14].

At the same time oil heat treatment reduces nail holding resistance significantly as well as the heat treatment in gaseous atmosphere. The colour resistance to weathering is not improved in the case of oil-heat treatment. So, special surface treatments and coating systems should be developed for oil heat treated wood for the outdoor exposure. Also oil thermally modified wood is not recommended in the structural uses where the strength properties are critical [14].

The purpose of this paper is to investigate the oil absorption of three wood species, pine, spruce and aspen, heat treated at different temperatures, 180, 210 and 240°C, and different time intervals, 30, 60 and 120 min, during the process of oil heat treatment. The importance of the knowledge about the oil penetration into wood during the process is evident, because the next step for this work is to enhance the properties of oil heat treated wood by adding chemical compounds into the oil.

MATERIALS AND METHODS

Materials

Rape seed oil was used for the wood treatment. For each time interval and heat treatment temperature five samples of spruce, pine heartwood, pine sapwood and aspen were prepared. The test samples of the sizes 150x20x20 mm were predried before heat treatment.

Treatment process

Deep-fryer Frifri Basic 411 (Switzerland) was used for the oil heat treatment. The wood was treated at 180, 210 and 240°C during 30, 60 and 120 minutes. Two sets of samples were run, the first one was heat treated and left to cool in the air, the second one was heat treated and directly cooled in the oil bath for 1 hour. The mass increase during heat treatment and during heat treatment with cooling in the oil bath was calculated (Eq. 1).

M = (m2 – m1)/m1 x 100% (1) m1 – mass before the heat treatment, m2 – mass after the process.

RESULTS AND DISCUSSION

The results for the mass changes during heat treatment are displayed in the Table 1. At 180° there was a trend of increasing oil absorption with the increasing the heat treatment time for all species (see also Fig. 2:1). At 210°C the percentage of mass growth was reducing with increasing the time of the process (see also Fig. 2:2); pine heartwood lost its weight already at 210°C because heartwood contains a lot of extractives which are evaporated during heat treatment process. The decrease in mass growth for the samples treated at 210°C can be explained by the fact that the wood having reached 200°C changes its properties rapidly because of the degradation of its components; after 200°C lignin starts to decrease [1,7]. At 240° all species lost the weight and the percentage of weight loss was increasing proportionally with increasing heat treatment time (see

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also Fig. 2:3). After 240°C the cellulose can start to degrade as well, because the decomposition temperature for cellulose is about 240–350°C [7].

Table 1: Mass increase during heat treatment (M – mass increase, St D – standard deviation)

Spruce Pine (heartwood)

Pine (sapwood) Aspen

Tem- pera- ture,

°C

Heat treat-

ment time, min

M, % St D, % M, % St D, % M, % St D, % M, % St D, % 30 3,19 1,12 1,39 0,87 3,61 0,23 10,84 3,1 60 3,47 0,25 1,98 0,99 5,72 1,28 12,8 5,33 180

120 6,18 0,65 2,99 0,48 6,99 1,28 19,79 11,22 30 3,52 0,87 -4,03 1,31 3,76 1,21 10,27 3,49 60 1,66 0,66 -4,45 1,74 2,58 0,93 7,07 0,81 210

120 0,09 1,13 -4,6 2,28 2,34 0,41 6,83 0,4 30 -1,52 0,35 -8,72 2,94 -0,76 0,33 -3,92 0,93 60 -3,88 0,39 -15,04 4,99 -2,89 0,74 -5,5 1,79 240

120 -5,42 0,39 -16,17 2,17 -3,43 0,76 -6,23 7,51

According to the results from Table 1 the species have different changes in mass after treatment.

It can be explained by the difference in the anatomical structure. Aspen had the highest mass increase. Aspen is hardwood which is known to have much wider cells (vessel elements) than softwood [9]. Thus it makes the aspen easier to penetrate.

Pine sapwood has higher mass increase than spruce, because of the difference in the connections between longitudinal tracheids and horizontal parenchyma cells in rays [10]. Looking at the radial view it can be seen that pine has a large pit (fenestriform pit) and thin membrane between adjacent cells. Spruce has a number of small piceoid or cupressoid pits in the membrane (Fig. 1) [10].

Fig. 1: Interconnecting pits between ray parenchyma and vertical tracheids in pine and spruce, earlywood and latewood [10]

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The results for the mass increase during heat treatment and direct cooling in the oil are displayed in the Table 2. Oil pick up occurred for all species, because the wood absorbed tremendously more oil in the stage of cooling (Fig. 2) and started to sink after laying in the cool oil bath. The reason for the higher oil absorption in the stage of cooling is that there is an internal pressure in the wood during heating and low pressure during soaking [6] since the pressure and volume of the gases which are present in the wood cells are directly proportional to the temperature. All species had a tendency to have approximately the same oil pick up during the heat treatment at one temperature and storing in the oil bath, so the heat treatment time did not have an effect on the oil absorption in the stage of cooling (see also Fig. 2). Pine heartwood had the lowest oil pick up (Fig. 2) because it had the lowest values for mass increase in the stage of heating. Pine sapwood had the highest oil pick up in comparison with other species (Fig. 2). The oil pick up during the stage of cooling had the lowest values for all species treated at 240°, because all the species lost the mass during the heat treatment at 240°C (Table 1, Fig. 2:3).

Analyzing the oil pick in the stage of cooling (Table 2), it is possible to see the trend for every species: the oil pick up is higher during the treatment at 210°C than at 180°C; and at 240°C the oil pick up is lower than at 210°C.

Table 2: Mass increase during heat treatment and cooling in the oil bath Spruce Pine

(heartwood)

Pine (sapwood) Aspen

Tem- pera- ture,

°C

Heat treat- ment time, min

M, % St D, % M, % St D, % M, % St D, % M, % St D, % 30 14,99 1 10,87 6,72 75,15 22,8 78,53 7,61 60 13,98 4,8 9,65 3,25 77,95 8,67 69,21 7,71 180

120 13,8 1,14 7,89 1,72 77,99 13,61 74,98 7,34 30 20,61 4,64 17,43 12,95 98,54 10,09 84,58 8,49 60 15,27 3,65 11,55 5,45 84,33 7,57 88,55 3,89 210

120 17,24 2,35 14,18 7,49 91,93 17,3 75,25 25,66 30 10,14 3,87 5,67 5,12 94,19 13,31 48,64 29,71 60 7,81 0,8 3,82 5,15 77,1 8,37 58,09 19,06 240

120 11,69 0,88 3,99 3,37 77,29 21,53 52,63 25,09

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1 2

3

Fig. 2: Mass increase during heat treatment and direct cooling in the oil bath (1 - Heat treatment at 180°C, 2 - 210°C, 3 - 240°C. Spruce A, Pine A, Aspen A – samples cooled in the air after heat treatment; Spruce B, Pine B, Aspen B – samples cooled in the oil bath after heat treatment).

The colour changed during heat treatment and depended mainly on the heat treatment temperature. The darkness was increasing proportionally with increasing the thermal treatment temperature (Fig. 3). The colour varied among the species. After heat treatment pine (especially heartwood) was darker than spruce, aspen was the darkest and gained a sombre brown colour during treatment at 240°C.

Fig. 3: Samples before and after heat treatment

Untreated Treated at 180° Treated at 210° Treated at 240°

Spruce

Pine heart- wood

Pine sap- wood

Aspen

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CONCLUSIONS

According to the results obtained during oil heat treatment process the following conclusions were made:

The degradation processes within the wood during oil heat treatment are similar to the heat treatment in the gaseous atmosphere. The variations in colour are significant between samples of different species and samples treated at different temperatures.

The percentage of oil pick up during heat treatment is not major. The wood absorbs the highest percentage of oil during the stage of cooling. The heat treatment time did not have an effect on the oil absorption in the stage of cooling.

The species have different oil pick up for the reason of the differences on the anatomical level, variations in porosity and permeability, heartwood content

REFERENCES

1. Beall FC, Eickner HW (1970) Thermal degradation of wood components: a review of literature. USDA Forest service research paper. US Department of Agriculture. Forest Products Laboratory. Madison, USA.

2. Boonstra MJ, Tjeerdsma BF, Groeneveld HAC (1998) Thermal modification of non-durable wood species. 1.

The Plato technology: Thermal modification of wood. IRG/WP 98-40123, IRG Secretariat, Stockholm, Sweden.

3. Burmester A (1973) Effect of heat-pressure-treatment of semi-dry wood on its dimensional stability. Holz als Roh und Werkstoff, Germany.

4. Dirol D, Guyonnet R (1993) The improvement of wood durability by retification process. The international research group on wood preservation, IRG/WP 93–40015, IRG Secretariat, Stockholm, Sweden.

5. Giebeler E (1983) Dimensional stabilization of wood by moisture-heat-pressure-treatment. Holz als Roh und Werkstoff, Germany.

6. Grenier D, Baillères H, Méot JM, Langbour P, Lanvin JD (2003) Oil absorption during oleothermic treatment of wood. In: The First European Conference on Wood Modification ECWM 2003, Ghent, Belgium, 3-4 April 2003.

7. Finnish Thermowood Association (2003) ThermoWood® Handbook. ThermoWood: Homepage, http://www.thermowood.fi/, 1/10 2008.

8. Rapp AO, Sailer M (2001) Oil heat treatment of wood in Germany – State of the art. In: Review on heat treatments of wood. COST Action E22, Environmental optimisation of wood protection. Proceedings of the special seminar held in Antibes, France, on 9 February 2001, Forestry and Forestry Products, France.

9. Rowell R (1984) The chemistry of solid wood. American Chemical Society, Washington, USA.

10. Sehlstedt-Persson M, Johansson D, Morén T (2006) Effect of Heat Treatment on the Microstructure of Pine, Spruce and Birch and the Influence on Capillary Absorption. In: Proceedings of The 5th International IUFRO Symposium Wood Structure and Properties. 3-4 September, Sliač - Sielnica, Slovakia.

11. Stamm AJ (1964) Wood and Cellulose Science. Ronald Press, New York, USA.

12. Stamm AJ, Hansen LA (1937) Minimizing wood shrinkage and swelling: Effect of heating in various gases.

In: Industrial and Engineering Chemistry, US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, USA.

13. Syrjänen T, Kangas E (2000) Heat treated timber in Finland. The international research group on wood preservation, IRG/WP 00–40158, IRG Secretariat, Stockholm, Sweden.

14. Wang J (2007) Initiating evaluation of thermal-oil treatment for post-MPB lodgepole pine. Forintek Canada Corp., Vancouver BC, Canada.

Trends and reasons for development of heat treated wood

M A S T E R ' S T H E S I S