CHARACTERIZATION OF MODIFIED WOOD IN RELATION TO WOOD BONDING AND
COATING PERFORMANCE
Workshop Proceedings
Edited by
Dr. Sergej Medved & Dr. Andreja Kutnar
COST Action FP0904 “Thermo–Hydro–Mechanical Wood Behaviour and Processing”
and
COST Action FP1006 “Bringing new functions to wood through surface modification”
Department of Wood Science and Technology
Biotechnical Faculty, University of Ljubljana
and
University of Primorska, Andrej Marušič Institute
Rogla, Slovenia, October 16th to 18th, 2013
modified wood in relation to wood bonding and coating performance, Rogla, Slovenia, October 16th to 18th 2013
Editor: Sergej Medved, Andreja Kutnar
Publisher: University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Rožna dolina, Cesta VIII/34, 1000 Ljubljana, Slovenia. Phone: +386 1 320 30 00, Fax: +386 1 257 22 97
University of Primorska, Andrej Marušič Institute, Muzejski trg 2, 6000 Koper, Slovenija. Phone +386 5 611 75 00, Fax: +386 5 611 75 30
Edition: 100 copies Ljubljana, 2013
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Narodna in univerzitetna knjižnjica, Ljubljana 674.028.9(082)
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COST Action FP0904 Thermo-‐Hydro-‐Mechanical Wood Behaviour and Processing (2013 ; Rogla)
Characterization of modified wood in relation to wood bonding and coating performance : workshop proceedings / COST Action FP0904 Thermo-‐Hydro-‐Mechanical Wood Behaviour and Processing and COST Action FP1006 Bringing New Functions to Wood through Surface Modification, Rogla, Slovenia, October 16th to 18th, 2013 ; edited by Sergej Medved & Andreja Kutnar ; [organizers] Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana and University of Primorska, Andrej Marušič Institute. -‐ Ljubljana : Biotechnical Faculty, Department of Wood Science and Technology ; Koper : University of Primorska, Andrej Marušič Institute, 2013
ISBN 978-‐961-‐6144-‐37-‐7 (Biotechnical Faculty, Department of Wood Science and Technology)
1. Gl. stv. nasl. 2. Medved, Sergej 3. COST Action FP1006 Bringing New Functions to Wood through Surface Modification (2013 ; Rogla) 4. Biotehniška fakulteta (Ljubljana). Oddelek za lesarstvo 5. Univerza na Primorskem (Koper). Inštitut Andrej Marušič
269303808
All rights reserved. No parts of these Proceedings may be reproduced or transmitted in any form or by any means, including photocopy, recording, or any information storage and retrieval system, without permission in writing from publisher.
Technical editor: Sergej Medved
Printed by: Somaru, d.o.o. Rožna dolina, Cesta XV/26, 1000 Ljubljana, Slovenia
The polymeric components of wood and its porous structure allow its properties to be modified under the combined effects of temperature, moisture and mechanical action – so-‐called Thermo-‐Hydro-‐Mechanical (THM) treatments. Various types of processing techniques, including high temperature steam with or without an applied mechanical force, can be utilized to enhance wood properties, to produce eco-‐friendly new materials and to develop new products. During these THM treatments, wood undergoes mechano-‐
chemical transformations, which depend upon the processing parameters and material properties. An investigation of these phenomena requires collaboration between groups from different wood disciplines; however, to date research has been rather fragmented.
This COST Action aims to apply promising techniques in the fields of wood mechanics, wood chemistry and material sciences through an interdisciplinary approach to improve knowledge about the chemical degradation and mechanical behavior of wood during THM processing. This will help overcome the challenges being faced in scaling-‐up research findings, as well to improving full industrial production, process improvement and the enhancement of product properties and the development of new products.
About COST Action FP1006
Many applications of products are determined by their special surface properties, and based on the physical, chemical and biological interchange of various molecules with the materials surface. This is especially true for the use of wood and wood based products due to the special wood characteristics like anisotropy, UV-‐degradation. Thus, bringing new functions to wood through surface modification is needed in order to enhance the quality of the existing wood products and to open the way to new applications, products or markets.
This COST Action aims to provide the scientific-‐based framework and knowledge required for enhanced surface modification of wood and wood based products towards higher functionalization and towards fulfillment of higher technical, economic and
interaction and Process and Service life modelling.
The aim of this event is to present materials, technologies, and characterization techniques in relation to wood bonding and coating performance of modified wood:
• Modification techniques (new and/or improved)
• Characterization of modified wood surface
• Formation and properties of the bond line/coating system
• Performance of coated modified wood
• Performance of bonded modified wood
• Performance of surface wood – based panels made from/or in combination with modified wood
The Workshop has been organized by the Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana and Andrej Marušič Institute, University of Primorska. Support and help was also provided by the Scientific Committee, reviewers and by the COST FP0904 and FP1006 Management Committee. The organisers and the editors would like to thank to all that help at organizing this Workshop, reviewers, speakers and also session moderators.
We hope that you enjoyed the Workshop and that you will find these papers useful in your future work.
Sergej Medved Andreja Kutnar
Parviz Navi (FP0904 Chair)
Lone Ross Gobakken (FP1006 Vice Chair) Dennis Jones (FP0904 Vice Chair)
Mark Hughes (FP0904 WG1 Leader) Lennert Salmen (FP0904 WG2 Leader) Peer Haller (FP0904 WG3 Leader) Gerhard Gruell (FP1006 WG1 Leader) Holger Militz (FP1006 WG1 Vice-‐Leader) Electra Papadopoulou (FP1006 WG2 Leader) Graham Ormondroyd (FP1006 WG2 Vice-‐Leader) Sergej Medved (FP1006 WG3 Leader)
Jakub Sandak (FP1006 WG3 Vice-‐Leader) Andreja Kutnar (FP0904 member)
Frederick A. Kamke ... 8 Characterization of laser modified wood surfaces for resin-‐free adhesion
Scott Renneckar, W. Travis Church, Jeffrey Dolan, Zhiyuan Lin, Charles E. Frazier ... 16 Emissions of thermally modified timber products
Lothar Clauder, Maria Rådemar, Lars Rosell, Marcus Vestergren, Alexander Pfriem .... 23 Application of FT-‐NIR for recognition of substances used for conservation of wooden parquets of 19th century manor houses located in South-‐Eastern Poland
Anna Rozanska, Anna Sandak ... 32 Gluability of thermally modified ash wood with EPI adhesives
Krystofiak Tomasz, Lis Barbara, Muszyńska Monika, Sobota Karolina ... 43 Bondability of phenol formaldehyde modified beech wood glued with phenol
resorcinol formaldehyde and polyvinyl acetate adhesives
Alireza Bastani, Holger Militz ... 52 Bonding properties of wood modified with various siloxanes and silanes
Marcus Müller, Markus Hauptmann, Christian Hansmann ... 61 Viscoelastic thermal compressed wood as a component in green building composites
Milan Sernek, Aleš Ugovšek, Andreja Kutnar, Frederick A. Kamke ... 67 Effect of heat treatment of spruce on adhesive bond performance after soaking in water
Mirko Kariz, Manja Kitek Kuzman, Milan Sernek ... 74 Effect of treatment medium on the moisture uptake rate and colour change during natural weathering of heat treated wood
Miklós Bak, Róbert Németh, Diána Csordós, László Tolvaj ... 80 The Effect of Surface Weathering on the Water Sorption Properties of Wood
Callum Hill ... 87 Coated Surface Densified Wood: Water Vapour Absorption and Desorption and
Related Dimensional Changes
Marko Petrič, Mark Hughes, Borut Kričej, Andreja Kutnar, Kristiina Laine, Sergej
Medved, Matjaž Pavlič, Lauri Rautkari ... 94
Wood moisture analysis under THM-‐conditions by employing scaling properties of room temperature moisture isotherms
Wim Willems ... 116 A structural study of the white rot biodegraded lime wood coated with poly(hydroxy urethane acrylate)
Carmen-‐Mihaela Popescu, Maria-‐Cristina Popescu ... 123 Wax impregnation slows down photodegradation processes of wood
Boštjan Lesar, Matjaž Pavlič, Marko Petrič, Miha Humar ... 130 Weathering performance of coatings on acetylated, furfurylated and heat treated wood at two exposure sites in Europe
Laurence Podgorski, Gerhard Grüll, Michael Truskaller, Jean-‐Denis Lanvin, Véronique Georges, Susanne Bollmus ... 140 Surface performance of thermally modified wood during weathering
Michael Altgen, Jukka Ala-‐Viikari, Antti Hukka, Timo Tetri, Holger Militz ... 149 Surface qualification of weathered wood
Jean Strautmann, Marion Noël, Thomas Volkmer ... 157 The Influence of the Sodium Carbonate Treatment of Narrow-‐leaved Ash on the Lap Shear Strength
Jasmina Popović, Milanka Djiporović-‐Momčilović, Ivana Gavrilović-‐Grmuša, Mladjan Popović, Sergej Medved ... 167 Combined treatment using boron impregnation and thermo-‐modification to improve properties of heat treated wood -‐ Effects of additives on boron leachability
Solafa Salman, Anélie Petrissans, Marie France Thevenon, Stéphane Dumarcay, Benoît Pollier, Philippe Gerardin ... 175 Superb wood surface finishing – SWORFISH project approach
Jakub Sandak, Anna Sandak, Mariapaola Riggio, Ilaria Santoni ... 191 Contact angle measurement as a method for quantitative analysis of wettability of plasma treated thermal modified timber
Judith Sinic, Uwe Müller ... 198 Spectral study of hydro-‐thermal treated lime wood
Maria-‐Cristina Popescu, Carmen-‐Mihaela Popescu ... 206
Exploratory Thermal-‐Hydro-‐Mechanical Modification (THM) of Moso Bamboo (Phyllostachys pubescens Mazel)
K.E. Semple, F.A. Kamke, A. Kutnar, G.D. Smith ... 220 The sorption properties of some thermally treated hardwoods analysed by
thermodynamics, surface fractality and FT-‐NIR spectroscopy
Aleš Straže, Željko Gorišek, Stjepan Pervan, Anna Sandak, Jakub Sandak ... 228 Modification of wood acoustic, hygroscopic and colorimetric properties due to
thermally accelerated ageing
Elham Karami, Miyuki Matsuo, Iris Bremaud, Sandrine Bardet, Julien Froidevaux,
Joseph Gril ... 238 Changes in technological properties of thermally treated Gympie messmate wood
Pedro Henrique Gonzalez de Cademartori, Patrícia Soares Bilhalva dos Santos, Darci Alberto Gatto, Jalel Labidi ... 246 Changes in chemical composition occurring in wood during the hydrothermal
treatment process
René Herrera, Xabier Erdocia, Jalel Labidi ... 254 Colour changes in coated hydrothermally modified wood after artificial and outdoor exposure
Sansonetti E., Cirule D., Grinins J., Andersone I., Andersons B., ... 261 Characterization of wood surface degradation using activation spectra approach
Vjekoslav Živković, Martin Arnold, Klaus Richter, Hrvoje Turkulin ... 268 Advantage of vacuum versus nitrogen to achieve inert atmosphere during wood
thermal modification
K. Candelier, S. Dumarçay, A. Pétrissans, P. Gérardin, M. Pétrissans ... 279 A Rapid Method for Assessing Check Development in Veneer Overlays
Michael Burnard, Lech Muszyński, Scott Leavengood, Lisa Ganio ... 287 The grindability of heat treated biomass: effect of treatment intensity on the
production of particles suitable for the 2nd generation of BtL chain
F. Pierre, P. Lu, G. Almeida, P. Perre ... 296 Stresses in the plans of bond lines in reconstituted solid wood under moisture
variation: a numerical approach
Sung-‐Lam Nguyen, Rostand Moutou Pitti, Jean-‐François Destrebecq ... 303
Analysis of the effects of the European oak natural variability on the modification of the density distribution and chemical composition during the heat treatment
Joël Hamada, Anélie Petrissans, Frédéric Mothe, Mathieu Petrissans, Philippe Gerardin ... 317
Keynote paper/presentation
THM – a Technology Platform or Novelty Product?
Frederick A. Kamke
Dept. Wood Science & Engineering, Oregon State University, Corvallis, Oregon USA 97331, fred.kamke@oregonstate.edu
ABSTRACT
Thermal-‐Hydro-‐Mechanical (THM) processing is an old idea that has arisen with a new life. THM wood has impressive mechanical and physical properties, but this exciting technology has some serious challenges for commercialization. This paper defines the concept and scope of THM technology and provides some examples of commercial application. Recent research in Europe, Asia, and North America has clearly demonstrated that THM processing of wood improves strength, stiffness, hardness, and moisture resistance; and this implies that the value of wood is also enhanced. The broad array of process parameters and unique conditions clearly differentiates THM as a technology platform. However, THM adds cost to processing and reduces wood volume. THM wood, depending on the specific process conditions, may have large potential for swelling when exposed to water. Technical challenges and process cost may limit THM processing to novelty products. Clever scientists and engineers can address most of the technical disadvantages of THM processing. However, the challenge for an entrepreneur, who has visions for commercialization, is to create THM value that exceeds THM cost.
Keywords: wood modification, compression, densification, thermo-‐hydro-‐mechanical.
1 INTRODUCTION
Thermo-‐Hydro-‐Mechanical (THM) processing is an old idea that has been given new life via research efforts around the world. It’s a very interesting concept – take some wood, soften it with heat and steam, compress, and viola!, the result is a high density material
with improved strength, stiffness, and hardness. The process is simple, it requires no chemicals, and the properties of the wood are dramatically improved. So why hasn’t THM processing been readily adopted? This paper will provide some historical background, discuss challenges of commercialization, and present some personal observations.
1.1 BRIEF THM HISTORY
THM processing implies the strategic application of high temperature, moisture, and mechanical compression toward the goal of reducing the void space in wood, and thus increasing density. Prior to 1960, researchers and practitioners quickly recognized that temperatures above 100°C, along with some moisture, sufficiently softens wood such that it may be compressed without catastrophic failure. The moisture content was usually not controlled during the densification process, and microscopic fractures in the cell wall were often ignored. Compression was performed in hydraulic pressing systems that were open to atmospheric conditions (open systems). Many wood densification equipment designs and processing conditions were reported. While the details of why this process worked were perhaps not clearly understood, the efforts produced wood products with interesting characteristics. For the purpose of the present discussion, the use of high temperature and mechanical compression, in the presence of significant moisture, and with the intent to increase density, will be called THM processing.
Previous reviews of THM wood, also called “compressed wood” or “densified wood”, reveal that a significant amount of research and some commercialization has occurred in Europe and the United States (Kollman 1936, Morsing 2000, Sandberg et al. 2012) prior to 1960. Kollman (1936) described the state-‐of-‐the-‐art for compressed wood, and even mentioned some investigations in Germany in the late 19th century. Seborg and Stamm (1941) reported results from some early investigations of compressed wood that was performed at the U.S. Forest Products Laboratory in Madison, Wisconsin. Readers are referred to these previous reviews for more information about early THM processing.
Research in the U.S. led to very limited commercial application. In 1943 the Formica Insulation Company (Cincinnati, Ohio) marketed Pregwood, which was a phenol-‐
formaldehyde impregnated, laminated, veneer product that was processed with 50%
degree of compression. Pregwood was designed for the hub of aircraft propeller blades with length up to 5m (Weick 1939). Pregwood was produced from parallel-‐laminated maple veneer, with density of 1300 kg/m3, MOE of 24 GPa, and MOR of 330 MPa. Resin impregnation was needed to overcome the greatest technical challenge for THM wood, namely moisture-‐induced dimensional instability. Other early applications of THM wood for commercial products were bobbins, picker sticks, and shuttles used in the textile industry; as well as machine dyes, antenna masts and knife handles. Most of these products were resin-‐impregnated (presumed for dimensional stabilization).
Since 1990 there has been renewed interest in developing THM products. Research in Japan explored surface densification of lumber (Inoue et al. 1990), shape-‐forming of round wood into prismatic shapes (Ito et al. 1998), and fixation of set recovery by hydro-‐
thermal and chemical treatment (Inoue et al. 1993a, Inoue et al. 1993b). In Europe, one critical area of focus was the problem of shape recovery upon exposure to water (Navi et al. 1997, Tomme et al. 1998). For more details refer to Sandberg et al. (2012). The current COST Action FP0904 is further evidence of renewed research interest in THM processing, however, commercial application is still very limited.
1.2 COMMERCIAL PRODUCTS
Solid THM wood products are rare to find on the commercial market. Calignum (Gothenburg, Sweden), who developed a densified solid wood via an isostatic membrane press, liquidated its assets in 2012. The Calignum technology was acquired by the Tarkett Company (Nanterre Cedex, France), who also announced their intension to produce a densified eucalyptus flooring product in 2011. Apparently, this has not occurred. There are some commercial operations in Japan. However, information has been difficult to obtain. MyWood2 Corporation (Iwakura, Aichi, Japan) manufactures densified solid cedar wood products. Their primary market is flooring in Japan and China, and products are sold for use in furniture. The MyWood2 product is impregnated with a proprietary polymer to provide resistance to water, and compression is approximately 50 percent.
Electric transmission support components made from THM wood are typically resin-‐
impregnated, laminated veneer. Low molecular weight resins (typically phenol-‐
formaldehyde) are used to impregnate veneer, which is then partially cured in an oven. A billet is then formed from the laminas, with orientation of the veneer dependent on the intended application. The billets are compressed in a heated press (open system) to density of approximately 1300 kg/m3. This line of products is desired for high electrical resistivity, high dimensional stability, and high strength to weight ratio. Another application or resin-‐impregnated veneer THM is liquid natural gas (LNG) storage containers and associated support structures. A laminated design permits components with wide dimensions that could not be achieved with THM lumber. For this application, low thermal conductivity, high dimensional stability, and high strength to weight ratio are important. Other applications for resin-‐impregnated veneer THM include wear plates for machinery and transportation vehicles, machine pattern molds, bulletproof barriers, and some structural building components. There are several products in this market, including, but not limited to, Insulam® by CK-‐Composites (Mount Pleasant, Pennsylvania USA), Lignostone® by Röchling (Harren, Gemany) and Lignostone® (Ter Apel, Netherlands), dehonit® by Deutsche Holzveredelung Schmeing GmbH & Co. KG (Kirchhundem/Würdinghausen, Germany), and Ranprex® by Rancan Srl (Vincenza, Italy).
The PureTimber company (Gig Harbor, Washington USA) produces a cold bendable wood product that was patented by the Danish Technical Institute (Hansen et al. 1993). Other companies have licensed the technology (e.g. Compwood Products, Hungary). The method employs THM techniques to compress wood elements (approx. 2.5 m) in the longitudinal direction. Length is reduced approximately 20% in process, but recovers to approximately 90% of original dimension when complete. The wood moisture content must be above 25%. Side restraint prevents buckling, however, the cell walls buckle, and partial shear failure between adjacent cell wall layers probably occurs. While the wood is still wet, it may be bent in one or two axes with little mechanical force. Once dried, the wood is no longer bendable. Applications include architectural woodwork, furniture,
musical instruments, and boat hulls. Retail cost is approximately $19,000/m3, so this is an example of very high value and low production volume THM wood.
2 CHALLENGES FOR COMMERCIALIZATION
The greatest challenges for expanded commercialization of THM technology are: 1) scale-‐
up from laboratory processes, 2) loss of volume yield, 3) swelling potential, and 4) profitability. All of these challenges are related in some way, and any one of them may be overcome with clever engineering or the right product application.
The challenge of scale depends on the industrial application. If the application is high value, a simple batch process, with moderate capital investment, may be adequate. One must also consider the physics of heat and mass transfer, as well as the compression forces required for a large THM device. The time required for a specific temperature or moisture content change via unsteady-‐state heat and mass transfer is approximately proportional to the second power of the principal thickness of the material being processed. If one doubles the thickness, the processing time required to achieve the desired change in temperature or moisture content increases by a factor of four. For example, a THM process step that requires 10 minutes in a small laboratory device may require 100 minutes to produce a larger commercial product. The impact on production capacity is devastating. Compression force (e.g. N, not N/mm2) on a small laboratory test sample increases in proportion to the area normal to the direction of applied force.
Consequently, hydraulic pumps and press frames in a commercial THM device must be sized accordingly, with significant impact on capital investment.
Most wood processing companies closely monitor the volume of wood that enters the factory and the volume of production that leaves the factory. Productivity is often expressed as percentage of volume yield. Most THM technologies dramatically reduce volume, perhaps by 50% or more. Traditional wood processing mentality resists any change that reduces volumetric productivity, even if the process is profitable.
The moisture-‐induced swelling potential of wood is proportional to its density. Swelling of untreated wood is reversible, but most swelling of THM wood is not reversible. Additional
process steps may be used to reduce THM swelling, such as hydro-‐thermal treatment or some chemical treatment. However, additional treatment adds cost to the final product, and may cause undesirable characteristics, such as darker color and embrittlement.
Processing cost is not the critical factor for THM wood. Profitability determines commercial success. All of the technical challenges may be overcome, and indeed have been achieved as demonstrated by numerous research reports. If the value of the final product significantly exceeds the cost of production, then the commercial enterprise will be viable. As researchers, we provide technical solutions to problems. However, sometimes we must define the problem within the limitations of commercial reality.
3 PROPOSED APPLICATIONS AND OBSERVATIONS
THM processing is a technology platform. There are several process parameters that may be manipulated to create intermediate or final products with a broad range of application. With a robust menu of products, a manufacturer may readily adapt to changing markets and price volatility. The long-‐term success of the manufacturers of resin-‐impregnated veneer THM products is due to the many high value product applications. The same capital equipment is used to produce products for the tool and dye industry, electrical power distribution, and cryogenic fluid storage and transport industries. The manufacturer manipulates density, resin content, and veneer orientation to effectively support each of these industries. Solid THM wood flooring is the target market for MyWood2 and Tarkett. I believe an engineered composite would be more cost effective and a more efficient use of raw materials in flooring applications. My own research has examined the use of THM wood veneer in laminated veneer lumber (LVL) for building construction. Profitability for commodity building products depends on low cost.
THM-‐LVL has been demonstrated to have superior mechanical properties than conventional LVL, but processing cost is higher. Therefore, THM-‐LVL can’t compete against current LVL products. Either higher value LVL applications are needed or raw material cost must be lower. The key to commercial success is to establish a technology
platform, where one facility has the capability to manufacture a broad range of THM products as the market evolves.
4 REFERENCES
Hansen O., Ljorring J., Thomassen T. 1993. Method and apparatus for compressing a wood sample, US Patent No. 5190088A.
Inoue, M., Norimoto, M., Otsuka, Y., Yamada, T. 1990. Surface compression of coniferous lumber, I. A new technique to compress the surface layer. Mok. Gak. 36(11):969-‐975.
Inoue, M., Norimoto, M., Tanahashi, M. 1993a. Steam or heat stabilization of compressed wood. Wood and Fiber Sci. 25(3): 224-‐235.
Inoue, M., Norimoto, M., Tanahashi, M., Rowell, M. 1993b. Steam or heat fixation of compressed wood. Wood and Fiber Sci., 25(3), 224-‐235.
Inoue, M., Norimoto, M., Tanahashi, M., Rowell, M. 1993c. Fixation of compressed wood using melamine-‐formaldehyde resin. Wood and Fiber Sci. 25 (4): 404-‐410.
Ito, Y., Tanahashi, M., Shigematsu, M., Shinoda, Y. and Otha, C. 1998. Compressive-‐
molding of wood by high-‐pressure steam-‐treatment: Part 1. Development of compressively molded squares from thinnings. Holzforschung 52: 211-‐216.
Kollmann, F. 1936. Technologie des Holzes. Springer-‐Verlag, Berlin.
Morsing, N. 2000. Densification of wood: The influence of hygrothermal treatment on compression of beech perpendicular to the grain. Dept. Structural Engineering and Materials, Technical University of Denmark, Series R, No. 79., Lyngby, Denmark.
Navi, P., Huguenin, P., Girardet, F. 1997. Development of synthetic-‐free plastified wood by thermohygromechanical treatment. In: Proc. The Use of Recycled Wood and Paper in Building Applications. For. Prod. Soc. Proc. No. 7286, Madison, Wisc. P. 168-‐171.
Seborg, R.M., Stamm, A.J. 1941. The compression of wood. US For. Prod. Lab. Rep. No.
R1258, Madison, Wisc. USA.
Tomme, F-‐P., Girardet, F., Gfeller, B., Navi, P. 1998. Densified wood: an innovative product with highly enhanced characteristics. In: Proc. World Conf. on Timber Engineering, Eds. Natterer, J., Sandoz, J-‐L., Swiss Fed. Inst. Tech., August 17-‐20, 1998.
Keynote paper/presentation
Characterization of laser modified wood surfaces for resin- free adhesion
Scott Renneckar1, W. Travis Church2, Jeffrey Dolan1, Zhiyuan Lin1, Charles E. Frazier1
1 Department of Sustainable Biomaterials, 230 Cheatham Hall, Virginia Tech, Blacksburg, VA 24060, USA, srenneck@vt.edu
2 5919 New Albany Rd W, New Albany, OH 43054, USA ABSTRACT
Laser irradiation of wood is a new method of bonding two wood substrates. Irradiating the surface of wood with laser light, within an optimal set of parameters, causes the wood surface to change and subsequently undergo bonding when hot-‐pressed. Light microscopy and scanning electron microscopy were utilized for surface topology analysis.
Dependent upon the amount of energy density, laser modification created a grooved surface or a flat surface. Chemical analysis of the residue after laser-‐modification was conducted and the polysaccharide and Klason lignin content of the extracted products were evaluated using ion chromatography. Additionally, chemical analysis of the wood surface was performed using FTIR spectroscopy. The surface of wood after laser light exposure was decorated with a “glass-‐like” layer, which consists of modified lignin with some polysaccharide degradation products, and evidence of cellulose melting.
Subsequently, wood samples with modified surfaces were hot-‐pressed together creating a wood composite. Screening of multiple factors that would contribute to surface modification and adhesion was performed utilizing mechanical testing. Laser light modified wood composites were tested in shear for mechanical strength, using a design of an experiment (DOE) approach to optimize hot-‐pressing parameters. It was found via initial screening and DOE experiments that laser power density as well as and hot press pressure were significant factors to optimize bonding. Laser-‐modified 3-‐ply veneer
samples had values that were comparable to control samples created using phenol formaldehyde resins. The data suggests that laser-‐activated bonding of wood can yield a wood composite requiring no liquid adhesives as the wood itself serves the dual role of adhesive and substrate.
Keywords: CO2 laser, wood surface chemistry, plywood, auto-‐adhesion
1 INTRODUCTION
Wood surfaces are complex arising from the destruction of the cell wall material, migration of water and extractive components to the surface, and the contamination of the surface with dust and other air-‐borne materials. Additionally, heat is generated at the surface during cutting operations and subsequent elevated temperatures during drying can alter the type and quantity of functional groups at the surface (Sernek et al. 2005).
Moreover, wood surfaces can be purposefully modified through exposure to different forms of energy. One of the oldest and classic examples is the flame treatment of wood for storage of food and drink. Due to the high quantity of energy transferred, in a short period of time, lasers are able to modify wood differently than other modification methods with slower heat rates. Laser light interaction with wood results in charring, ablation, and melting, depending on a multitude of variables that are related to wood properties and laser parameters. Laser light affects the irradiated area and the resulting laser modification is described in 3 levels, which are cumulatively known as the heat affected zone, or HAZ. This zone can be up to 100 microns thick, depending on wood variables, laser variables, and their interaction. Parameswaran (1982) described the first level as a black, smooth laser cut surface that is approximately 25 microns thick. Softening and melting as a result of laser light and wood interaction suggests the process is such that kinetics of softening and melting can surpass the kinetics of thermal degradation (Schroeter and Felix 2005).
Past research indicated that the laser modification of wood affects the lignin and hemicellulose components to the greatest extent, while effecting cellulose to a lesser degree (Kubovsky 2009). It was found that laser modification primarily caused
degradation of hemicellulose and lignin (Kubovsky 2009). Specifically the hemicellulose underwent deacetylation, while bond cleavage occurred in lignin, specifically the aryl-‐
alkyl ether bonds in lignin were broken. This bond cleavage induced further condensation reactions. Other research indicated that the modification of components was preferential towards reducing the methoxy side groups of lignin (Walinder et al. 2009). Additional studies have investigated laser modification specific to cellulose. The studies were based on the concept of applying enough energy, induced by a combination of frictional heat and via laser, in order to chemically and/or physically change viscose grade wood pulp, which is noted to have a high α-‐cellulose content, into clear films. A calculation was made for the amount of energy required to “weaken and unlock” the intermolecular hydrogen bonds in cellulose. It was found that this energy would need to be 20kJ/mol, or 3.3*10-‐20 J per bond, which is equivalent to 1 photon with a wavelength of 6 microns (Schroeter and Felix 2005). Although the IR spectra indicated little change, qualitatively it was evident that the material changed from a fibrous opaque structure to a transparent smooth structure. With these results the researchers concluded they were able to melt cellulose.
Previous researchers have observed the “melting” of lignocellulosic materials with laser modification, some suggesting lasers melting only related to a specific component, with other suggesting that all wood components can undergo thermal softening and melting.
In the current research, we examine the surface of laser modified wood with a variety of chemical analysis techniques to understand chemical changes induced by the laser modification as well as investigate the parameters that lead to strong bondlines when two laser modified surfaces are hot pressed together.
2 MATERIALS AND METHODS
2.1 MATERIALS
3.2 mm thick “Grade A” Yellow-‐poplar (Liriodendron tulipifera) and southern yellow pine (Pinus spp.) rotary-‐peeled veneer were obtained from a southeastern US laminated veneer mill. The as-‐received 60x60cm samples were conditioned to 12% moisture content in a walk-‐in environmental humidity controlled room.
2.2 METHODS
Wood veneer samples were modified utilizing a ULS-‐V460 60W carbon dioxide laser with high power density focusing optics, resulting a circular spot size with a minimum diameter of approximately 50 μm. Laser wattages of 3 to 60W were utilized, with the laser moving at a speed of 0.1 m/s to 0.5 m/s, at 40,000 pulses per meter. Specific parameters utilized in the ULS print driver included a maximum image density of 6, tuning of zero, while utilizing vector mode. The trajectory of the laser was designed in AutoCAD, using the smallest resolution usable by the laser between lines, 0.002 in (50.4 μm) to irradiate the surface. Laser modified samples were placed in contact matching their irradiated surfaces and subsequently hot pressed together utilizing a MP2000 mini hot press at 200 °C (Fig.
1). The time between laser modification and hot pressing ranged from 5 min to weeks;
with no detectable difference in bond strength. Pulse Amperometric Detection Ionic chromatography (IC) was used for sugar analysis of the laser modified poplar versus remaining bulk wood.
Figure 1: Process segments for laser modified wood bonding; (left) Autocad file for read/write laser modification of wood surface, (center) laser modification of southern yellow pine veneer, (right) hotpress system used to make test specimens.
3 RESULTS AND DISCUSSION
In Fig. 2, a 3-‐D light microscopy image indicates a distinct difference of the laser-‐modified wood surface. The samples appear to have a glossy, reflective surface with dark spots that speckle the surface. The sides of the sample reveal that laser modification is limited
show earlywood tracheids are modified on average of 19.7 micrometers in depth, while latewood tracheids are modified on average of 8.1 micrometers. Fiber tracheids and ray parenchyma have approximately 11 micrometers of modification. Others have reported that the density variations in the wood greatly impact depth of treatment, as cell wall thickness directly impacts the laser ablation process.
Figure 2: 3-‐D light microscopy image of laser modified yellow-‐poplar.
The spacing of the line and the focus of the laser spot size were controlled to manipulate surface geometries from a flat surface to a highly grooved surface. As the in-‐focus laser spot size was smaller than the resolution of the laser's motion system, ridges develop between laser lines during laser modification (Figure 3a). In addition the Gaussian shape of the power of the laser beam creates a spot size with additional energy in the center.
As mechanical interlock of ridges may promote adhesion, the presence of the ridges was thought to have a substantial positive effect on the bonding (Fig. 3b). However, for laser bonding to occur, mechanical interlock “micro-‐finger joints”, was not required (Fig. 3c,d).
Surprisingly, smooth surfaces showed higher compressive shear strengths than the grooved samples (6.2 MPa vs. 3.5 MPa, respectively). Highest shear strengths were found for samples with the most pressure during bonding (2 MPa). This data suggests that the laser-‐modified surfaces are limited by their inability to bridge differences in surface roughness. When appropriate conditions were used for bonding, 3-‐ply specimens had a bending modulus between 7 and 10 GPa and bending strength 60 to 80 MPa.
Figure 3: (a) SEM image of laser modified wood with high concentrated energy causing grooves in surface; (b) image of bondline of two specimens from (a); (c) SEM image of laser modified wood with low concentrated energy resulting in relative flat surface topography; and (d) image of bondline of two specimens from (b)
Surface material of laser treated wood was isolated through extraction. A number of solvents were evaluated for their ability to refresh the surface, removing all residues from the surface. Solvents tested were the following: alcohols like methanol and ethanol;
acetone; selective lignin solvents like aqueous dioxane; dimethylsulfoxide (DMSO), 0.1 M NaOH, and water. Dimethylsulfoxide was successful in removing the primary heat affected zone of the surface. The DMSO extract was evaluated by precipitating the polymeric materials with acidic ethanol, followed by a two-‐step acid hydrolysis for composition analysis of these materials. This data provided the monomeric carbohydrate component percentage related to the wood polysaccharides extracted as well as the Klason lignin content. It was found that the cellulose content increased from 47.7 to 61.9% and the lignin content of the surface increased from 21.1 to 27.9%, and the xylan content was greatly reduced to less than 4%. Analysis of the water extracted surface material revealed a large amount of xylose, as well as monomeric degradation products such as hydroxymethyl-‐furfural, levoglucosan, xylitol and sorbitol. While a number of possible compounds were found to be present that could be reactive, no specific reactive chemistry was detected suggesting bonding occurred because of intimate contact of surfaces during hot-‐pressing.
a b
c d
4 CONCLUSIONS
CO2 laser modification of wood ablates the surface of wood leaving a residue on the wood surface. Dependent upon the laser energy density, the surface topology changes from a grooved surface to a relatively flat surface. The residue remaining on the wood surface is composed of the native wood polymers and degradation products. This residue alone provides adequate adhesion to form composite materials when two specimens are bonded together under pressure and heat.
5 REFERENCES
Buschbeck L., Kehr E., Jensen U. 1961a. Untersuchungen über die Eignung verschiedener Holzarten und Sortimente zur Herstellung von Spanplatten – 1. Mitteilung: Rotbuche und Kiefer. Holztechnologie, 2, 2: 99–110
Kubovsky, I., 2009. FT-‐IR Study of Maple Wood Changes due to CO2 Laser Irradiation.
Cellulose Chemistry and Technology, 43(7-‐8): p. 235-‐40.
Parameswaran, N., 1982. Feinstrukturelle Veränderungen an durch laserstrahl getrennten Schnittflächen von Holz und Holzwerkstoffen. European Journal of Wood and Wood Products, 40(11): p. 421-‐428.
Schroeter, J. and F. Felix, 2005. Melting cellulose. Cellulose, 12(2): p. 159-‐165.
Sernek, M., Kamke, F.A. and Glasser, W.G., 2004. Comparative analysis of inactivated wood surfaces. Holzforschung, 58:22-‐31
Walinder, M., et al., 2009.Micromorphological studies of modified wood using a surface preparation technique based on ultraviolet laser ablation. Wood Material Science and Engineering, 4(1): p. 46 -‐ 51.
Emissions of thermally modified timber products
Lothar Clauder1, Maria Rådemar2, Lars Rosell2, Marcus Vestergren2, Alexander Pfriem1
1 Eberswalde University for Sustainable Development, Friedrich-‐Ebert-‐Straße 28, 16225 Eberswalde, Germany, lothar.clauder@hnee.de
2 SP Technical Research Institute of Sweden, Box 857, SE-‐501 15 Borås, Sweden, marcus.vestergren@sp.se
ABSTRACT
In this study the applicability of wood in the museum environment was investigated. The applied method focused on an appropriate selection of materials and adequate control of their noxious compounds as keys to achieve compatibility between display materials and artworks. Therefore specimens of fresh-‐sawn Fir (Abies alba, Mill.) and Alder (Alnus glutinosa, (L.) Gaertn.) were pre-‐treated with a buffer-‐solution and heat-‐treated at low temperatures. The Field and Laboratory Emission Cell (FLEC) were applied for measuring the volatile organic compounds (VOC) and the formaldehyde (FA) emissions from wood.
The emissions were characterised by using gas chromatography (GC) in combination with mass-‐spectra (MS) and flame ionization detection (FID), ion chromatography (IC) and high-‐performance liquid chromatography (HPLC). Compared to samples of green Fir, the formaldehyde emissions increased in the kiln-‐dried samples. However these emissions were decreased in the impregnated and thermally modified samples. Thermally treated and dried variants of Alder samples showed low amounts of VOC, in particular due to aldehydes (>C2). The low amount of acidity and decreased formaldehyde formation in the Alder samples increased the positive trend. Concerning the detection limits for substances with high contamination potential for individual display case construction materials, this study gives a first hint on how the VOC emissions of thermally modified timber could be minimized by using a buffer solution before the heat treatment.
Keywords: thermally modified timber, gas chromatography, high-‐performance liquid
1 INTRODUCTION
Wood emits volatile organic compounds (VOC). Thus, the applicability of wood in the museum environment, e.g. as material for the construction of display cabinets, is almost entirely restricted due to the required controlled climate and air purity, which is very different from normal indoor air, e.g. in dwellings, offices, schools etc. The desire to preserve exhibits, constituted by all imaginable materials, from deterioration allows in principle only low levels of air pollutants with possible detrimental effects (Englund 2010).
The purpose of this study was to develop and test a suitable method to minimize the emissions. Schäfer and Roffael (2000) proposed reaction mechanisms of FA formation from wood and demonstrated an increase of FA emission at elevated temperatures and prolonged heating times during panel production. Especially during the pressing step at elevated temperatures increased FA and VOC emissions were detected in the absence of any resin (Carlson et al. 1995). Even at temperatures below 100°C, as during the kiln drying of the wood, the hydrolysis of cell wall components cellulose, polyose (hemicellulose) and lignin leads to formation of furfural, formaldehyde and very volatile acids (VVOC, e.g. formic acid). The approach was to reduce the emissions of thermally modified timber products, based on impregnation with a sodium-‐boric-‐buffer-‐solution.
2 EXPERIMENTAL
Alder (Alnus glutinosa, (L.) Gaertn.), which is low emitting, and Fir (Abies alba, Mill.) were selected. Sample preparation was performed at the Eberswalde University for Sustainable Development. The fresh-‐sawn specimens (210×210×20 mm³) were taken out of two stems, each approximately 60 years old, harvested in Northeast Germany. The experimental test set-‐up consisted of 2 samples for each variant of treatment (Table 1).
Table 1: Experimental test set-‐up
Specimen [n] Treatment
2 untreated
2 kiln dried
2 modified
2 impregnated kiln dried
2 impregnated modified
3 METHODS
3.1 SET UP FOR pH VALUE MEASUREMENTS
In general, wood species range in pH from 3.0 to 5.5 (Stamm 1964). A pH range of 4.00 to 5.86 for hardwoods, e.g. 5.52 for Alder and 4.02 to 5.82 for softwoods, e.g. 4.02 for Fir was found (Johns 1980). The impregnation with the buffer solution was performed in a Pressure Impregnation plant. After impregnation, small samples were dissolved in distilled water then pH-‐value measurements were carried out with a WTW pH meter, Model inoLab by using an electrode to measure the extracts. To determine the potential of the buffer solution with a pH-‐value of 9.4, the following equilibrium equation (Eq.1) was used, e.g. to calculate the amount of protonated acetic acid molecules inside the pre-‐
treated wood.
pH = pKa + lg [b]/ [a] (Eq. 1)
[b] = base (e.g. sodium acetate); [a] = acid (e.g. acetic acid); pKa = negative logarithm of the equilibrium constant (e.g. acetic acid & sodium acetate)
3.2 SET UP FOR FIELD AND LABORATORY EMISSION CELL
A variety of test methods for determining FA emissions from wood and wood products have evolved over time. As reference methods the American National Standards (ANSI), e.g. for particleboard (A.208.1 2009), as the emission standards of the California Air Resources Board (CARB 2008) and the chamber method according to DIN EN 717-‐1 (2004) specify large chamber tests. The large chamber test is expensive, time consuming and needs a large amount of samples. Therefore it is impractical for quality assurance in
commercial production (Birkeland et al. 2010). Due to reliable correlation to these reference methods, derived and approved secondary methods, e.g. the perforator method (EN 120 1993), gas analysis method (EN 717-‐2 1994) and desiccator-‐test, as described in ASTM D 5582-‐00 (2006) have been established. For this reason in this study the measurements of the emissions from wood were performed according to ISO 16000-‐
10 (2006) with a Field and Laboratory Emission Cell (FLEC), which provides a simulation of realistic indoor air conditions with respect to temperature and relative humidity (Fig. 1 and 2). In contrast to real air conditions, the air exchange rate in the FLEC is higher (171 times/hour). This emission cell is designed to measure area specific emission rates of general VOCs and separately the lowest aldehydes, formaldehydes and acetaldehyde, as well as the lowest carboxylic acids, formic acid and acetic acid.
1 air inlet 2 air outlet 3 channel
4 sealing material 5 slit
1 Specimen is located in the subunit (stainless steel cylinder) 2 Sorbent tubes (e.g. stainless steel tubes filled with Tenax TA®)
Figure 1:Schematic of Field and Laboratory Emission Cell (FELC) (EN ISO 16000-‐10 Test cell method)
Figure 2: Application of the Field and Laboratory Emission Cell (FELC) combined with a subunit containing the specimen
Before each measurement on the specimen, a background air sample of the test chamber was performed, to quantify any contribution of organic compounds from the clean air system and the empty cell. The samples were prepared according to EN ISO 16000-‐11, formatted (Ø14.8 cm) and stored in a conditioning room (23 ± 2°C and 50 ± 5%). Prior to the tests fresh surfaces were planed and the edges were sealed with an alloy tape. The stainless steel cell and subunit allowed a controlled climate at 23 ± 1°C and 50 ± 3% RH.
1 2
