Introduction to Thermo-hydro-mechanical (THM) Wood Processing Dick Sandberg and Parviz Navi Price 175 SEK, Available from School of Technology and Design Växjö University SE-351 95 Växjö Sweden www.vxu.se ISSN: 1652-8433 School of Technology and Design ISBN: 978-91-7636-561-8 Reports, No. 30 Växjö University Växjö, Sweden 2007 Introduction to Thermo-hydro-mechanical (THM) Wood Processing Dick Sandberg and Parviz Navi Report No. 30 ISSN: 1652-8433 ISBN: 978-91-7636-561-8 Copyright © 2007 Dick Sandberg and Parviz Navi 1 Preface Wood is the ultimate renewable material. It possesses qualities that have made it a material of choice for millennia, these qualities further enhanced by its recognised carbon sequestration. However, as a biological material it suffers the same fate as any natural material, namely degradation. There are ways in which wood can be enhanced, including eco-friendly methods. One of the emerging eco-friendly methods is the combined use of temperature, moisture and mechanical action – so-called Thermo-Hydro- Mechanical (THM) treatments. THM processing can improve the intrinsic properties of wood, to produce new materials and to acquire a form and functionality desired by engineers without changing its eco-friendly characteristics. There are numerous THM processing techniques and the number of these processes is growing continuously. THM processing can be divided into two major categories; Thermo- Hydral treatments (TH) and Thermo-Hydro-Mechanical treatments (THM). TH is usually used to enhance wood properties, and is of importance in increasing stress relaxation during drying under high temperature, in the formation of wood-based composites or veneer products and in the artificial ageing of wood. THM on the other hand, is employed in the producing of new materials by densification, shaping by moulding, welding of wood by friction, embossment, bending of wood, wood fusion, and chip-less manufacturing. During THM treatment, wood undergoes large deformation, stress relaxation and chemical degradation, but with strong shape memory depending on parameters like temperature, moisture content, applied forces, processing time and the type of wood. Several TH and THM processes have recently been developed in Europe, Japan, the USA and Canada, but only some of them have been scaled-up industrially. However, to overcome the problems associated with TH/THM wood during processing at the laboratory scale, during scale up and controlling the end use properties, a detailed knowledge of these phenomena becomes important. This requires a close collaboration between experts in wood chemistry, wood mechanics and material science from both academia and from industry. I With this report we would like to give a brief state-of-the-art of the important field of THM-processing. The aim of this work has not been to cover the complete field of THM-processing, just give an introduction. Dick Sandberg and Parviz Navi II Contents 1 WOOD MODIFICATION...........................................................................................1 1.1 INTRODUCTION .........................................................................................................1 1.2 MODIFICATION OF WOOD ..........................................................................................2 1.2.1 Chemical modification .....................................................................................3 1.2.2 Thermo-hydro-mechanical treatments, THM...................................................5 1.3 REFERENCES ...........................................................................................................14 2 ANCIENT USES OF THERMO-HYDRO-MECHANICAL PROCESSES.........17 2.1 INTRODUCTION .......................................................................................................17 2.2 WOODLAND CRAFTS .................................................................................................22 2.2.1 Bending of solid wood in woodland craft.......................................................22 2.3 THM-PROCESSING IN THE CONSTRUCTION OF WOODEN VESSELS............................24 2.4 THE MANUFACTURE OF WOODEN CASKS .................................................................35 2.4.1 traditional coopering......................................................................................35 2.4.2 Machine coopering.........................................................................................40 2.5 THE INDUSTRIALIZATION OF SOLID WOOD BENDING ...............................................42 2.5.1 The Thonét process ........................................................................................43 2.5.2 The Vienna Chair which conquered the world...............................................45 2.5.3 What was it that made the Thonét method so successful?..............................47 2.6 THE USE OF VENEER FOR CURVED SHAPES ..............................................................49 2.7 REFERENCES ...........................................................................................................58 3 ELASTO-VISCOPLASTICITY OF WOOD UNDER THERMO-HYDRO- MECHANICAL ACTIONS...........................................................................................65 3.1 COMPRESSIVE FORMING OF MASSIVE WOOD BY THM ACTIONS ..............................65 3.1.1 Historical background....................................................................................66 3.2 INTRODUCTION TO THE ELASTIC, PLASTIC AND VISCOELASTIC BEHAVIOR OF WOOD UNDER CONSTANT AND VARIABLE CLIMATIC CONDITIONS .................................70 3.2.1 Behavior of wood under mechanical loading.................................................70 3.2.2 Instantaneous elastic response.......................................................................71 3.2.3 Mechanisms of wood cellar deformations under longitudinal compression ..74 3.2.4 Deformation of wood under compression in the radial and tangential directions.................................................................................................................77 3.2.5 Constitutive equations of an elastic orthotropic material ..............................79 3.2.6 Constitutive equations for the large nonlinear deformation of materials ......82 3.3 ELASTO-VISCOPLASTIC BEHAVIOUR OF WOOD CONSTITUENTS UNDER THERMO- HYDRO-MECHANICAL (THM) ACTIONS AT TEMPERATURES UNTIL 200°C.....................85 3.3.1 Introduction....................................................................................................85 3.3.2 Glass transition temperature of amorphous and semi-crystalline polymers..87 Glass transition temperature (Tg) of wood components..........................................88 3.3.3 Influence of temperature and humidity on the hygro-plasticity of wood........92 3.3.4 Influence of temperature on the thermal degradation of wood ......................95 III 3.3.5 Deformation of the cellular structure of spruce during transverse compression ............................................................................................................98 3.4 CHEMICAL DEGRADATION OF WOOD CONSTITUENTS UNDER THERMO-HYDRO- MECHANICAL (THM) ACTIONS AT TEMPERATURES UNTIL 200°C ...............................105 3.4.1 Reactivity of the wood components in acid conditions.................................105 Effects of temperature and moisture content.........................................................105 3.4.2. Reactivity of polysaccharides......................................................................106 Reducing terminal.................................................................................................106 Glucosidic links.....................................................................................................106 The hydroxyl groups..............................................................................................106 3.4.3 The hydrolysis of polysaccharides ...............................................................107 Factors influencing the hydrolysis ........................................................................109 3.4.4 Dehydration and condensation reactions.....................................................110 3.4.5 Reactivity of the cellulose microfibrils.........................................................111 Characteristics of the cellulose microfibrils .........................................................111 3.4.6 Reactivity of hemicelluloses .........................................................................113 Xylans....................................................................................................................114 Glucomannans ......................................................................................................114 3.4.7 Reactivity of lignin .......................................................................................115 Reactivity of the bonds ether .................................................................................115 Reactivity of the hydroxyl groups..........................................................................115 Reactivity of the lignin-carbohydrate complex......................................................116 3.4.8 Physicochemical modifications of wood components during THM post- processing .............................................................................................................116 Quantitative analysis of hemicelluloses by GPC...................................................116