Technology of Adhesives and Wood-Based Panels

Manfred Dunky has over 40 years of experience in the chemical and wood-based panels industry as a researcher and application manager, continuing now as a consultant for this industry. He earned his doctoral degree in 1980 from the University of Leoben, Austria. He is the author of two comprehensive books, (co-)editor of several conference proceedings, and of about 50 original and extensive review papers on synthetic and natural adhesives and wood-based panel technology. During his career, he worked with synthetic wood adhesives as well as with adhesives based on natural resources and their application in panel production. Besides his industry career, he has lectured at universities for 30 years and in 2000 received his habilitation (post-doctoral lecturing qualification) for "Wood Science with special consideration of wood-based panels." He speaks regularly at international wood science conferences.

• Preface xiAcknowledgments xv1 Introduction 11.1 Wood-Based Panels 31.1.1 Plywood and Solid Wood-Type Panels 91.1.2 Panels Based on Strands and Particles 121.1.3 Panels Based on Fibers 171.1.4 Composites 201.1.5 Taylor-Made Wooden Products (Functional Boards) 201.1.6 Wood–Plastic Composites (WPC) 221.2 Production of Wood-Based Panels 231.3 Factors for Selecting an Adhesive 231.4 Interdisciplinary Approach to Wood-Based Panels 241.4.1 Forestry 241.4.2 Wood Anatomy (Structure of Wood as Raw Material) 251.4.3 Wood Physics and Mechanics 261.4.4 Wood Technology 271.4.5 Wood Chemistry 271.4.6 Biotechnology 281.4.7 Organic Chemistry and Type and Composition of Adhesives 281.4.8 Process Technology 311.4.9 Ecological Sciences and Circular Economy in Forestry and Wood Industry 311.4.10 Economic Sciences 351.5 Summary of Actual Developments with Wood-Based Panel 352 Components 392.1 Wood 392.1.1 Characteristics and Structure of Wood and Wooden Raw Materials 392.1.1.1 Wood Species 402.1.1.2 Recycled and Waste Wood, Cascade Use of Wood 422.1.1.3 Density and Porosity 482.1.1.4 Wood Surface and Mechanical Weak Boundary Layer (MWBL) 492.1.2 Size and Size Distribution of Particles, Flakes, and Strands 552.1.3 Refining Process and Fiber Quality 622.1.4 Moisture Content 672.1.5 Chemical Behavior of Wood 682.1.5.1 Analysis and Chemical Composition of Wood and Wood Surfaces 682.1.5.2 Wood-Inherent Chemicals (Extractives) 692.1.5.3 Acid Content and Buffer Capacity, pH of the Wood Surface 702.1.5.4 Age of Wood Surfaces, Inactivation 732.1.5.5 Formaldehyde in Wood 752.1.5.6 Seasonal Variations of the Wood Quality 792.1.6 Chemical and Physical Activation, Modification, and Pre-Treatment of Wood and Wood Surfaces 802.1.6.1 Modification and Activation of the Wood Surface by Chemical and Thermochemical Treatment 812.1.6.2 Citric Acid 882.1.6.3 Thermal and Physical Pre-Treatments of Wood 922.1.6.4 Hydrothermal Treatment (Steam Pre-Treatment) 972.1.6.5 Enzymatic Pre-Treatment of the Wood Surface 1002.1.6.6 Binderless Bonding (Bonding Without Use of an External Adhesive, Adhesive-Free Bonding) 1032.1.7 Biofibers and Plant Raw Materials 1102.1.8 Modified Wood 1122.2 Adhesives 1142.2.1 Formaldehyde Condensation Resins 1242.2.1.1 Aminoplastic Resins 1282.2.1.2 Phenol- and Resorcinol-Based Resins 1462.2.1.3 Molar Mass Distribution 1512.2.1.4 Reactivity and Hardening Reactions 1522.2.2 Isocyanate and Polyurethane Adhesives 1582.2.2.1 Chemistry of Isocyanate Adhesives 1582.2.2.2 Polyurethane Adhesives 1632.2.2.3 Emulsion Polymer Isocyanate (EPI) 1642.2.3 Adhesives Based on Natural Resources 1662.2.3.1 Protein-Based Adhesives 1712.2.3.2 Carbohydrate-Based Adhesives 1842.2.3.3 Tannin-Based Adhesives 1892.2.3.4 Lignin-Based Adhesives 1942.2.3.5 Unsaturated Oils 1992.2.3.6 Liquefied Wood 2002.2.3.7 Pyrolysis Products 2012.2.3.8 Mimicking Nature and Bio-Inspiration 2022.2.3.9 Detachable Bonding (Debonding) and Self-Healing Polymers (see also Section 5.5) 2052.2.3.10 Mycelium Biocomposites 2062.2.3.11 Nanomaterials 2072.2.3.12 Replacement of Synthetic Raw Materials by Natural Components from Bio-Refineries 2122.2.3.13 Crosslinkers 2132.2.4 Thermoplastic Adhesives 2162.2.4.1 PVAc 2162.2.4.2 Hot Melts 2192.2.4.3 Polylactic Acid (PLA) 2212.3 Additives and Adhesive Mix Formulations 2222.3.1 Fillers and Extenders 2222.3.2 Hydrophobing Agents (Paraffins, Wax) 2242.3.3 Flame-Retardant Agents, Fire-Retardant Treatment of Wood 2252.4 Analysis and Test Methods (Composition and Properties) for Adhesives 2262.4.1 Molar Mass Distribution (MWD) and Molar Mass Averages 2282.4.2 Spectroscopic Methods and Determination of the Molecular Structure 2302.4.3 Crystallinity of Aminoplastic Condensation Resins 2313 The Behavior of Components During and After the Application of the Adhesive 2333.1 Wetting Behavior 2333.1.1 Wetting Behavior of Solid Wood and Aging of Wood Surfaces 2343.1.2 Inactivation of Wood Surfaces, Aging of Wood Surfaces, Chemical Weak Boundary Layer (CWBL) 2423.1.3 Wetting and Penetration Studies on Wood-Based Panels 2433.2 Penetration Behavior 2443.2.1 Penetration into Wood Tissue 2443.2.2 Diffusion of an Adhesive into Cell Walls (Infiltration) 2843.3 Application Technology of the Adhesive 2883.3.1 Adhesive Resin Mixes 2883.3.2 Technique of Application of the Resin on Solid Wood and Veneers 2893.3.3 Techniques of Application of the Adhesive on Particles and Distribution of the Adhesive 2903.3.4 Adhesive Spread on Strands in the OSB Production 2983.3.5 Fiber Blending 3003.3.6 Determination of the Resination Factor 3063.4 Water Balance Before Pressing 3073.4.1 Wood Moisture Content (MC) Before and After the Application of the Adhesive 3103.4.2 Open and Closed Assembly Time 3133.4.3 Wet (Green) Gluing 3143.5 Forming Process 3173.5.1 Modeling of Mats for Wood-Based Panels 3183.5.2 Orientation of Particles and Strands 3193.5.3 Ratio of Densification (Compaction Ratio, Compression Ratio) 3203.5.4 Reinforcement of Mats and Boards 3214 The Behavior of Components During the Press Cycle 3254.1 Press Technology 3264.1.1 Hot-Press Strategy 3264.1.2 Heating Up a Particle or Fiber Mat, Gas Permeability 3354.1.3 Water Balance and Steam Pressure 3474.2 Behavior of Wood During the Press Cycle 3544.2.1 Wood Strength Depending on Temperature, Moisture Content, and Densification of the Wood Structure 3544.2.2 Chemical and Thermal Impact on Wood, Thermal Degradation of Wood Components 3614.3 Behavior of the Adhesive During the Press Cycle 3614.3.1 Chemical Curing of Formaldehyde-Based Condensation Resins 3724.3.1.1 pH Drop and Buffering Behavior of Aminoplastic Resins (See also Sections 2.2.1.1 and 2.2.1.4) 3724.3.1.2 Addition of Chemicals During the Hot-Press Cycle 3744.3.1.3 Gel Times and Pot Life of Formaldehyde-Based Adhesives Resins 3754.3.1.4 Differential Thermoanalysis (DTA) and Differential Scanning Calorimetry (DSC) 3764.3.1.5 Dielectric Thermal Analysis (DETA), Dielectric Analysis (DEA) 3854.3.1.6 Arrhenius Plots and Apparent Activation Energy E a for the Gelation Process and the Formation of Bond Strength 3864.3.2 Mechanical Hardening and Monitoring the Development of Bond Strength 3894.3.2.1 Rheological Characterization, Thermal Scanning Rheometry (tsr) 3894.3.2.2 Dynamic Mechanical (Thermal) Analysis (DMA, DMTA) and Thermomechanical Analysis (TMA) 3924.3.2.3 Torsional Braid Analysis (TBA) 3954.3.2.4 Development of Bond Strength and Automatic Bonding Evaluation System (ABES) 3964.3.3 Correlations Between Chemical Curing, Mechanical Curing, and the Development of Bond Strength 4014.3.4 Emissions During the Press Cycle 4084.3.4.1 Formaldehyde Emission During the Production of Wood-Based Panels 4084.3.4.2 VOC Emissions 4104.4 Generating Bondlines and Panels During the Press Cycle 4104.4.1 Formation of Bondlines 4124.4.2 Compression of Particle and Fiber Mats and Formation of the Density Profile 4134.4.3 Simulation Models for the Production Process of Wood-Based Panels 4224.5 Theories of Bonding 4374.5.1 Basics of Adhesive Bonding 4374.5.1.1 Cohesion 4394.5.1.2 Adhesion 4404.5.2 Bonding Theories (“How Does Wood Bonding Really Work?”) 4404.5.2.1 Secondary Forces and Physical Bonding 4414.5.2.2 Covalent Chemical Bonding Between Wood Surface and Adhesive 4414.5.2.3 Mechanical Anchoring (Interlocking) of the Adhesive in Wood (Mechanical Adhesion) 4434.5.2.4 Autoadhesion (Diffusion Theory) 4444.6 Wood Welding 4445 Bonded Products 4495.1 Post-Press Cycle Technology 4495.1.1 Cooling, Hot Stacking, and Maturing Conditions (Temperature and Duration) 4495.1.2 Post-Treatment Procedures 4505.1.2.1 Steam Post-Treatment 4505.1.2.2 Heat Post-Treatment 4515.2 Consolidated Bondline 4515.2.1 Test Methods for Hardened Adhesives 4515.2.1.1 Chemical, Physical, and Physico-Chemical Tests, and Content of Residual Monomers 4515.2.1.2 X-Ray Scattering Methods 4555.2.1.3 Mechanical Tests on Solidified/Hardened Adhesive Films 4565.2.2 Characterization of a Bondline 4615.2.2.1 Microstructure and Morphology of a Bondline 4615.2.2.2 Mechanical Properties of a Bondline 4685.2.3 Bondlines Under Strain, Stress, and Fracture 4775.2.3.1 Fracture Behavior of a Bondline and of Wood-Based Panels 4775.2.3.2 Strain and Stress Distribution in Adhesive Joints and Wood-Based Panels 4845.3 Testing and Properties of Wood-Based Panels 4855.3.1 Density and Microstructure of Wood-Based Panels 4905.3.1.1 Density and Density Distribution 4905.3.1.2 Wood-Based Panels with Low Density 4925.3.1.3 Microstructure of Wood-Based Panels 4945.3.2 Mechanical Properties 5065.3.2.1 Mechanics of Wood-Based Panels 5065.3.2.2 Fracture Toughness of Boards (See Also Section 5.2.3.1) 5075.3.3 Hygroscopic Properties 5085.3.3.1 Equilibrium Moisture Content 5085.3.3.2 Thickness Swelling and Water Absorption 5095.3.4 Spectroscopic Analysis of Wood-Based Panels 5105.3.5 Environmental Impact on Bondlines and Wood-Based Panels 5115.3.5.1 Durability (Permanence) of Bondlines and Wood-Based Panels 5115.3.5.2 Influence of Temperature and Moisture Content on Wood Joints and Wood-Based Panels 5175.3.6 Mechanical Modeling and Prediction of Board Properties 5235.4 Emissions 5245.4.1 Subsequent Formaldehyde Emission from Wood-Based Panels 5245.4.1.1 Test Methods for the Subsequent Formaldehyde Emission and Correlations Between the Various Test Methods 5255.4.1.2 Regulations Concerning the Subsequent Formaldehyde Emission 5295.4.2 Volatile Organic Compounds (VOC, SVOC, and VVOC) 5365.4.3 Acids 5405.5 Recycling of Wood and Wood-Based Panels and Furniture 5405.5.1 Recycling of Particleboards and Furniture 5435.5.2 Recycling of Fiberboards and MDF 544List of Abbreviations 549References 555Index 613