Preface xi
Acknowledgments xv
1 Introduction 1
1.1 Wood-Based Panels 3
1.1.1 Plywood and Solid Wood-Type Panels 9
1.1.2 Panels Based on Strands and Particles 12
1.1.3 Panels Based on Fibers 17
1.1.4 Composites 20
1.1.5 Taylor-Made Wooden Products (Functional Boards) 20
1.1.6 Wood-Plastic Composites (WPC) 22
1.2 Production of Wood-Based Panels 23
1.3 Factors for Selecting an Adhesive 23
1.4 Interdisciplinary Approach to Wood-Based Panels 24
1.4.1 Forestry 24
1.4.2 Wood Anatomy (Structure of Wood as Raw Material) 25
1.4.3 Wood Physics and Mechanics 26
1.4.4 Wood Technology 27
1.4.5 Wood Chemistry 27
1.4.6 Biotechnology 28
1.4.7 Organic Chemistry and Type and Composition of Adhesives 28
1.4.8 Process Technology 31
1.4.9 Ecological Sciences and Circular Economy in Forestry and Wood Industry 31
1.4.10 Economic Sciences 35
1.5 Summary of Actual Developments with Wood-Based Panel 35
2 Components 39
2.1 Wood 39
2.1.1 Characteristics and Structure of Wood and Wooden Raw Materials 39
2.1.1.1 Wood Species 40
2.1.1.2 Recycled and Waste Wood, Cascade Use of Wood 42
2.1.1.3 Density and Porosity 48
2.1.1.4 Wood Surface and Mechanical Weak Boundary Layer (MWBL) 49
2.1.2 Size and Size Distribution of Particles, Flakes, and Strands 55
2.1.3 Refining Process and Fiber Quality 62
2.1.4 Moisture Content 67
2.1.5 Chemical Behavior of Wood 68
2.1.5.1 Analysis and Chemical Composition of Wood and Wood Surfaces 68
2.1.5.2 Wood-Inherent Chemicals (Extractives) 69
2.1.5.3 Acid Content and Buffer Capacity, pH of the Wood Surface 70
2.1.5.4 Age of Wood Surfaces, Inactivation 73
2.1.5.5 Formaldehyde in Wood 75
2.1.5.6 Seasonal Variations of the Wood Quality 79
2.1.6 Chemical and Physical Activation, Modification, and Pre-Treatment of Wood and Wood Surfaces 80
2.1.6.1 Modification and Activation of the Wood Surface by Chemical and Thermochemical Treatment 81
2.1.6.2 Citric Acid 88
2.1.6.3 Thermal and Physical Pre-Treatments of Wood 92
2.1.6.4 Hydrothermal Treatment (Steam Pre-Treatment) 97
2.1.6.5 Enzymatic Pre-Treatment of the Wood Surface 100
2.1.6.6 Binderless Bonding (Bonding Without Use of an External Adhesive, Adhesive-Free Bonding) 103
2.1.7 Biofibers and Plant Raw Materials 110
2.1.8 Modified Wood 112
2.2 Adhesives 114
2.2.1 Formaldehyde Condensation Resins 124
2.2.1.1 Aminoplastic Resins 128
2.2.1.2 Phenol- and Resorcinol-Based Resins 146
2.2.1.3 Molar Mass Distribution 151
2.2.1.4 Reactivity and Hardening Reactions 152
2.2.2 Isocyanate and Polyurethane Adhesives 158
2.2.2.1 Chemistry of Isocyanate Adhesives 158
2.2.2.2 Polyurethane Adhesives 163
2.2.2.3 Emulsion Polymer Isocyanate (EPI) 164
2.2.3 Adhesives Based on Natural Resources 166
2.2.3.1 Protein-Based Adhesives 171
2.2.3.2 Carbohydrate-Based Adhesives 184
2.2.3.3 Tannin-Based Adhesives 189
2.2.3.4 Lignin-Based Adhesives 194
2.2.3.5 Unsaturated Oils 199
2.2.3.6 Liquefied Wood 200
2.2.3.7 Pyrolysis Products 201
2.2.3.8 Mimicking Nature and Bio-Inspiration 202
2.2.3.9 Detachable Bonding (Debonding) and Self-Healing Polymers (see also Section 5.5) 205
2.2.3.10 Mycelium Biocomposites 206
2.2.3.11 Nanomaterials 207
2.2.3.12 Replacement of Synthetic Raw Materials by Natural Components from Bio-Refineries 212
2.2.3.13 Crosslinkers 213
2.2.4 Thermoplastic Adhesives 216
2.2.4.1 PVAc 216
2.2.4.2 Hot Melts 219
2.2.4.3 Polylactic Acid (PLA) 221
2.3 Additives and Adhesive Mix Formulations 222
2.3.1 Fillers and Extenders 222
2.3.2 Hydrophobing Agents (Paraffins, Wax) 224
2.3.3 Flame-Retardant Agents, Fire-Retardant Treatment of Wood 225
2.4 Analysis and Test Methods (Composition and Properties) for Adhesives 226
2.4.1 Molar Mass Distribution (MWD) and Molar Mass Averages 228
2.4.2 Spectroscopic Methods and Determination of the Molecular Structure 230
2.4.3 Crystallinity of Aminoplastic Condensation Resins 231
3 The Behavior of Components During and After the Application of the Adhesive 233
3.1 Wetting Behavior 233
3.1.1 Wetting Behavior of Solid Wood and Aging of Wood Surfaces 234
3.1.2 Inactivation of Wood Surfaces, Aging of Wood Surfaces, Chemical Weak Boundary Layer (CWBL) 242
3.1.3 Wetting and Penetration Studies on Wood-Based Panels 243
3.2 Penetration Behavior 244
3.2.1 Penetration into Wood Tissue 244
3.2.2 Diffusion of an Adhesive into Cell Walls (Infiltration) 284
3.3 Application Technology of the Adhesive 288
3.3.1 Adhesive Resin Mixes 288
3.3.2 Technique of Application of the Resin on Solid Wood and Veneers 289
3.3.3 Techniques of Application of the Adhesive on Particles and Distribution of the Adhesive 290
3.3.4 Adhesive Spread on Strands in the OSB Production 298
3.3.5 Fiber Blending 300
3.3.6 Determination of the Resination Factor 306
3.4 Water Balance Before Pressing 307
3.4.1 Wood Moisture Content (MC) Before and After the Application of the Adhesive 310
3.4.2 Open and Closed Assembly Time 313
3.4.3 Wet (Green) Gluing 314
3.5 Forming Process 317
3.5.1 Modeling of Mats for Wood-Based Panels 318
3.5.2 Orientation of Particles and Strands 319
3.5.3 Ratio of Densification (Compaction Ratio, Compression Ratio) 320
3.5.4 Reinforcement of Mats and Boards 321
4 The Behavior of Components During the Press Cycle 325
4.1 Press Technology 326
4.1.1 Hot-Press Strategy 326
4.1.2 Heating Up a Particle or Fiber Mat, Gas Permeability 335
4.1.3 Water Balance and Steam Pressure 347
4.2 Behavior of Wood During the Press Cycle 354
4.2.1 Wood Strength Depending on Temperature, Moisture Content, and Densification of the Wood Structure 354
4.2.2 Chemical and Thermal Impact on Wood, Thermal Degradation of Wood Components 361
4.3 Behavior of the Adhesive During the Press Cycle 361
4.3.1 Chemical Curing of Formaldehyde-Based Condensation Resins 372
4.3.1.1 pH Drop and Buffering Behavior of Aminoplastic Resins (See also Sections 2.2.1.1 and 2.2.1.4) 372
4.3.1.2 Addition of Chemicals During the Hot-Press Cycle 374
4.3.1.3 Gel Times and Pot Life of Formaldehyde-Based Adhesives Resins 375
4.3.1.4 Differential Thermoanalysis (DTA) and Differential Scanning Calorimetry (DSC) 376
4.3.1.5 Dielectric Thermal Analysis (DETA), Dielectric Analysis (DEA) 385
4.3.1.6 Arrhenius Plots and Apparent Activation Energy E a for the Gelation Process and the Formation of Bond Strength 386
4.3.2 Mechanical Hardening and Monitoring the Development of Bond Strength 389
4.3.2.1 Rheological Characterization, Thermal Scanning Rheometry (tsr) 389
4.3.2.2 Dynamic Mechanical (Thermal) Analysis (DMA, DMTA) and Thermomechanical Analysis (TMA) 392
4.3.2.3 Torsional Braid Analysis (TBA) 395
4.3.2.4 Development of Bond Strength and Automatic Bonding Evaluation System (ABES) 396
4.3.3 Correlations Between Chemical Curing, Mechanical Curing, and the Development of Bond Strength 401
4.3.4 Emissions During the Press Cycle 408
4.3.4.1 Formaldehyde Emission During the Production of Wood-Based Panels 408
4.3.4.2 VOC Emissions 410
4.4 Generating Bondlines and Panels During the Press Cycle 410
4.4.1 Formation of Bondlines 412
4.4.2 Compression of Particle and Fiber Mats and Formation of the Density Profile 413
4.4.3 Simulation Models for the Production Process of Wood-Based Panels 422
4.5 Theories of Bonding 437
4.5.1 Basics of Adhesive Bonding 437
4.5.1.1 Cohesion 439
4.5.1.2 Adhesion 440
4.5.2 Bonding Theories ("How Does Wood Bonding Really Work?") 440
4.5.2.1 Secondary Forces and Physical Bonding 441
4.5.2.2 Covalent Chemical Bonding Between Wood Surface and Adhesive 441
4.5.2.3 Mechanical Anchoring (Interlocking) of the Adhesive in Wood (Mechanical Adhesion) 443
4.5.2.4 Autoadhesion (Diffusion Theory) 444
4.6 Wood Welding 444
5 Bonded Products 449
5.1 Post-Press Cycle Technology 449
5.1.1 Cooling, Hot Stacking, and Maturing Conditions (Temperature and Duration) 449
5.1.2 Post-Treatment Procedures 450
5.1.2.1 Steam Post-Treatment 450
5.1.2.2 Heat Post-Treatment 451
5.2 Consolidated Bondline 451
5.2.1 Test Methods for Hardened Adhesives 451
5.2.1.1 Chemical, Physical, and Physico-Chemical Tests, and Content of Residual Monomers 451
5.2.1.2 X-Ray Scattering Methods 455
5.2.1.3 Mechanical Tests on Solidified/Hardened Adhesive Films 456
5.2.2 Characterization of a Bondline 461
5.2.2.1 Microstructure and Morphology of a Bondline 461
5.2.2.2 Mechanical Properties of a Bondline 468
5.2.3 Bondlines Under Strain, Stress, and Fracture 477
5.2.3.1 Fracture Behavior of a Bondline and of Wood-Based Panels 477
5.2.3.2 Strain and Stress Distribution in Adhesive Joints and Wood-Based Panels 484
5.3 Testing and Properties of Wood-Based Panels 485
5.3.1 Density and Microstructure of Wood-Based Panels 490
5.3.1.1 Density and Density Distribution 490
5.3.1.2 Wood-Based Panels with Low Density 492
5.3.1.3 Microstructure of Wood-Based Panels 494
5.3.2 Mechanical Properties 506
5.3.2.1 Mechanics of Wood-Based Panels 506
5.3.2.2 Fracture Toughness of Boards (See Also Section 5.2.3.1) 507
5.3.3 Hygroscopic Properties 508
5.3.3.1 Equilibrium Moisture Content 508
5.3.3.2 Thickness Swelling and Water Absorption 509
5.3.4 Spectroscopic Analysis of Wood-Based Panels 510
5.3.5 Environmental Impact on Bondlines and Wood-Based Panels 511
5.3.5.1 Durability (Permanence) of Bondlines and Wood-Based Panels 511
5.3.5.2 Influence of Temperature and Moisture Content on Wood Joints and Wood-Based Panels 517
5.3.6 Mechanical Modeling and Prediction of Board Properties 523
5.4 Emissions 524
5.4.1 Subsequent Formaldehyde Emission from Wood-Based Panels 524
5.4.1.1 Test Methods for the Subsequent Formaldehyde Emission and Correlations Between the Various Test Methods 525
5.4.1.2 Regulations Concerning the Subsequent Formaldehyde Emission 529
5.4.2 Volatile Organic Compounds (VOC, SVOC, and VVOC) 536
5.4.3 Acids 540
5.5 Recycling of Wood and Wood-Based Panels and Furniture 540
5.5.1 Recycling of Particleboards and Furniture 543
5.5.2 Recycling of Fiberboards and MDF 544
List of Abbreviations 549
References 555
Index 613
