Structural Adhesives
Description
Uniquely provides up-to-date and comprehensive information on the topic in an easily-accessible form.
A structural adhesive can be described as a high-strength adhesive material that is isotropic in nature and bonds two or more parts together in a load-bearing structure. A structural adhesive material must be capable of transmitting the stress/load without loss of structural integrity within design limits. There are many types of established structural adhesives, including epoxy, urethane, acrylic, silicone, etc.
Structural Adhesives comprises nine chapters and is divided into two parts: Part 1, Preparation, Properties, and Characterization; Part 2, Applications.
The topics covered include: structural epoxy adhesives; biological reinforcement of epoxies as structural adhesives; marble dust reinforced epoxy structural adhesive composites; characterization of various structural adhesive materials; effects of shear and peel stress distributions on the behavior of structural adhesives; the inelastic response of structural aerospace adhesives; structural reactive acrylic adhesives: their preparation, characterization, properties, and applications; application of structural adhesives in composite connections; and naval applications of structural adhesives.
Audience
This book should be of much use and interest to adhesionists, materials scientists, adhesive technologists, polymer scientists, and those working in the construction, railway, automotive, aviation, bridge, and shipbuilding industries.
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Persons
Kashmiri Lal Mittal was employed by the IBM Corporation from 1972 through 1993. Currently, he is teaching and consulting worldwide in the broad areas of adhesion as well as surface cleaning. He has received numerous awards and honors including the title of doctor honoris causa from Maria Curie-Sklodowska University, Lublin, Poland. He is the editor of more than 160 books dealing with adhesion measurement, adhesion of polymeric coatings, polymer surfaces, adhesive joints, adhesion promoters, thin films, polyimides, surface modification surface cleaning, and surfactants.
S. K. Panigrahi, PhD, is a professor in the Department of Mechanical Engineering of the Defence Institute of Advanced Technology (DIAT), Pune, India. He has worked as an international visiting academic with the University of New South Wales at the Australian Defence Force Academy. With over 27 years of extensive teaching, research, training, and administrative experience, Dr. Panigrahi is currently focused on advanced finite element methods, nonlinear finite element analyses, and modeling engineering structures with functionally graded/monolithic adhesively-bonded joints. His publications include over 190 research articles and peer-reviewed scholarly papers, four books, a monograph, and many conference proceedings, including a series of lecture materials.
Content
Preface xiii
Part 1: Preparation, Properties and Characterization 1
1 Structural Epoxy Adhesives 3 Chunfu Chen
1.1 Introduction 4
1.2 Epoxy Adhesive Chemistry 4
1.2.1 Epoxy Resins 4
1.2.2 Curing Agents and Catalysts 7
1.2.3 Formulating Epoxy Adhesives 10
1.3 Properties, Testing and Characterization 11
1.4 Typical Epoxy Adhesives 13
1.4.1 Room Temperature Cure Epoxy Adhesives 13
1.4.2 Thermal Cure Epoxy Adhesives 14
1.4.3 UV Cure Epoxy Adhesives 16
1.5 Recent Developments and New Trends 18
1.5.1 High Performance Toughened Epoxy Adhesives 18
1.5.2 Low Temperature Cure One-Component Epoxy Adhesives 19
1.5.3 Instant Bonding Epoxy Adhesives 20
1.5.4 Sustainable Epoxy Adhesive Development 22
1.6 Summary 22
References 23
2 Biological Reinforcement of Epoxies as Structural Adhesives 31 Anna Rudawska, Jakub Szabelski, Izabela Miturska-Baranska and Elzbieta Doluk
2.1 Introduction 31
2.2 Epoxy Resins and Curing Agents 33
2.2.1 Epoxy Resins 33
2.2.2 Curing Agents 34
2.2.3 Curing Methods 38
2.2.4 Epoxy Structural Adhesives 40
2.3 Modification of Epoxies, and Modifying Agents 41
2.3.1 Epoxy Modification Methods 41
2.3.2 Fillers Properties 44
2.3.3 Fillers Types 45
2.3.3.1 Filler Classification Criterion: Type of Material 46
2.3.3.2 Filler Classification Criterion: Shape of the Filler Particles 47
2.3.3.3 Filler Classification Criterion: Filler Particle Size 48
2.3.3.4 Filler Classification Criterion: Origin 49
2.3.3.5 Filler Classification Criterion: Activity 50
2.4 Biological Reinforcement of Epoxy Adhesives 51
2.4.1 Introduction 51
2.4.2 Types of Biological Reinforcements 51
2.4.2.1 Natural Fibers 53
2.4.2.2 Wood 53
2.4.2.3 Vegetable Oils 58
2.4.2.4 Fungi 59
2.4.2.5 Extracted Plant Ingredients 60
2.4.2.6 Nut Shells 64
2.4.2.7 Straw 65
2.4.3 Natural Fibers 66
2.4.4 Plant Fibers 68
2.4.4.1 Cotton Fibers 68
2.4.4.2 Hemp Fibers 68
2.4.4.3 Linen (Flax) Fibers 69
2.4.4.4 Jute Fibers 71
2.4.4.5 Sisal Fibers 72
2.4.4.6 Coconut (Coir) Fibers 73
2.4.4.7 Cellulose Fibers 74
2.4.4.8 Bamboo Fibers 76
2.4.4.9 Kenaf Fibers 77
2.4.4.10 Other Fibers 78
2.5 Fungi-Modified Adhesives 80
2.6 Prospects 83
2.7 Summary 84
References 85
3 Marble Dust Reinforced Epoxy Structural Adhesive Composites 105 Amar Patnaik, Pankaj Agarwal, Ankush Sharma, Deepika Shekhawat and Tapan Kumar Patnaik
3.1 Introduction 106
3.2 Materials and Methods 110
3.2.1 Procurement of Raw Materials 110
3.2.2 Fabrication of Composites 111
3.2.3 Physical and Mechanical Characterization 113
3.2.3.1 Density and Void Content 113
3.2.3.2 Water Absorption 114
3.2.3.3 Vickers Hardness 115
3.2.3.4 Tensile Test 115
3.2.3.5 Flexure Test 117
3.2.3.6 Impact Test 117
3.2.3.7 Thermal Conductivity 117
3.2.3.8 Specific Wear Rate 117
3.2.3.9 TOPSIS Approach 118
3.3 Results and Discussion 119
3.3.1 Density and Void Content 119
3.3.2 Water Absorption 120
3.3.3 Hardness 121
3.3.4 Tensile Strength and Tensile Modulus 121
3.3.5 Flexural Strength and Flexural Modulus 122
3.3.6 Impact Energy 123
3.3.7 Thermal Conductivity 123
3.3.8 Specific Wear Rate 125
3.3.9 Ranking of Epoxy Adhesive Composites 126
3.4 Summary and Conclusions 131
References 132
4 Characterization of Various Structural Adhesive Materials 135 Srujan Sapkal, Pooja Maske, S. K. Panigrahi and Himanshu S. Panda
List of Abbreviations 136
List of Symbols 137
4.1 Introduction 138
4.2 Various Structural Adhesives and their Properties 139
4.2.1 Phenolic Structural Adhesives 139
4.2.2 Epoxy Structural Adhesives 140
4.2.3 Polyurethane (PU) Structural Adhesives 141
4.2.4 Acrylic Structural Adhesives 142
4.2.5 Cyanoacrylate Structural Adhesives 143
4.2.6 Silicone Structural Adhesives 143
4.3 Characterization Techniques for Structural Adhesives 144
4.3.1 Chemical Characterization 144
4.3.1.1 Energy Dispersive X-ray (EDX) 144
4.3.1.2 X-ray Photoelectron Spectroscopy (XPS) 145
4.3.1.3 Fourier Transform Infrared Spectroscopy (FTIR) 147
4.3.1.4 Gas-Liquid Chromatography (GLC) 151
4.3.1.5 Nuclear Magnetic Resonance 153
4.3.1.6 Raman Spectroscopy 156
4.3.2 Physical Characterization 161
4.3.2.1 Contact Angle Measurement 161
4.3.2.2 Scanning Electron Microscopy (SEM) 164
4.3.2.3 Gelation Time 165
4.3.2.4 Small Angle X-ray Scattering (SAXS) 166
4.3.2.5 Atomic Force Microscopy (AFM) 167
4.3.3 Thermal Characterization 168
4.3.3.1 Thermogravimetric Analysis (TGA) 170
4.3.3.2 Differential Thermal Analysis (DTA) 171
4.3.3.3 Differential Scanning Calorimetry (DSC) 172
4.3.4 Mechanical Characterization 176
4.3.4.1 Tensile Test 177
4.3.4.2 Lap Shear Test 178
4.3.4.3 Dynamic Mechanical Analysis (DMA) 179
4.4 Summary 185
Acknowledgements 186
References 186
5 The Effects of Shear and Peel Stress Distributions on the Behavior of Structural Adhesives in Tubular Composite Joints 193 Mohammad Shishesaz
5.1 Introduction 194
5.1.1 A Brief Review of Loads (Stresses) and Failure of Adhesively Bonded Tubular Composite Joints 194
5.1.2 Major Factors Affecting the Peel and Shear Stresses in the Adhesive Layer and its Performance (Failure) 199
5.2 Governing Equations Based on Linear Elasticity 200
5.2.1 Typical Assumptions in a Tubular Lap Joint under Torsion 200
5.3 Factors Influencing the Adhesive Behavior and Stresses 209
5.3.1 The Effects of Geometric and Mechanical Properties of the Adhesive and Adherends 209
5.3.2 The Effects of Load Type on the Adhesive Stresses and Behavior 217
5.3.3 The Effects of Damages due to Voids, Debonds, or Delaminations 221
5.3.4 Additional Factors Influencing the Adhesive Behavior and Its Performance 230
5.3.5 The Effect of Nonlinear Behavior of the Adhesive on Its Performance 236
5.3.6 Factors Influencing the Failure Behavior of the Adhesive Layer 238
5.4 Design Aspects Regarding the Selection of Adhesive Layer 239
5.5 Summary 244
Acknowledgement 245
Nomenclature 245
References 249
6 Inelastic Response of Structural Aerospace Adhesives 255 Yi Chen and Lloyd Smith
List of Symbols 255
6.1 Introduction 257
6.2 Time-Independent Plasticity 258
6.2.1 Yield Stress 258
6.2.2 Elasto-Plastic Models 262
6.3 Time-Dependent Inelasticity 263
6.3.1 Creep Loading 263
6.3.2 Cyclic Loading 267
6.3.3 Time-Dependent Models 270
6.3.3.1 Modeling of Creep 270
6.3.3.2 Modeling of Ratcheting 274
6.4 Environmental Factors 276
6.4.1 Temperature 276
6.4.2 Moisture 277
6.4.3 Modeling 278
6.5 Summary 280
References 281
Part 2: Applications 291
7 Structural Reactive Acrylic Adhesives: Preparation, Characterization, Properties and Applications 293 D.A. Aronovich and L.B. Boinovich
7.1 Introduction 293
7.2 ¿ompositions and Chemistries 295
7.2.1 Base Monomer 296
7.2.2 Thickeners and Elastomeric Components 299
7.2.3 Adhesive Additives 308
7.2.4 Initiators 310
7.2.5 Aerobically Curable Systems 319
7.2.6 Fillers 319
7.3 Physico-Mechanical Properties of SAAs 323
7.4 Adhesives for Low Surface Energy Materials 329
7.4.1 Initiators Based on Trialkylboranes 329
7.4.2 Comparison of the Initiation System Containing Trialkylborane with the Redox System Benzoyl Peroxide (BPO) - Tertiary Aromatic Amine 340
7.4.3 Alternative Types of Trialkylborane Initiators 342
7.4.4 Additives Modifying the Curing Stage 344
7.4.5 Other Components of SAAs 346
7.4.6 Hybrid SAAs 348
7.5 Comparison of the Properties of SAAs and Other Reactive Adhesives 354
7.6 Summary and Outlook 358
References 359
8 Application of Structural Adhesives in Composite Connections 375 M. D. Banea and H.F.M. de Queiroz
8.1 Introduction 375
8.2 Factors Affecting the Performance of Composite Adhesive Joints 376
8.2.1 Effect of Surface Preparation 377
8.2.2 Effect of Joint Configuration and Failure Mode 378
8.2.3 Effect of Mechanical Properties of Adhesive and Adherend Materials 383
8.2.4 Effect of the Environmental Conditions 386
8.3 Recent Developments and Trends 388
8.4 Summary 389
References 390
9 Naval Applications of Structural Adhesives 397 Bikash Chandra Chakraborty
List of Abbreviations 398
List of Symbols with Units 399
9.1 Introduction 400
9.2 Type of Marine Adhesives 401
9.2.1 Essential Characteristics 402
9.2.2 Flexible Adhesives 403
9.2.2.1 Bonding Multilayer Rubber Tiles 406
9.2.2.2 Bonding Silicone Rubber Gaskets 407
9.2.3 Thermoset-Based Marine Adhesives 408
9.3 Application on Naval Platform 415
9.3.1 Vibrodamping Arrangements 415
9.3.2 Underwater Application 416
9.3.3 Acid-Resistant Rubber Bonding 421
9.3.3.1 Example 422
9.4 Diffusion of Water in Adhesive Matrix 423
9.4.1 Fickian Diffusion 423
9.4.1.1 Example 427
9.4.2 Dual-Fickian Prediction 430
9.4.3 Effect on Flexural Strength 431
9.4.3.1 Example 432
9.5 Summary 436
References 437
Index 445
1
Structural Epoxy Adhesives
Chunfu Chen
Henkel Technology Center - Asia Pacific, Henkel Japan Ltd., Isogo-ku, Yokohama, Kanagawa, Japan
Abstract
Epoxy adhesives form very strong and durable bonds with most materials and are widely used in various structural bonding applications. Epoxy adhesives are formulated in the form of a compound containing epoxy resin and curing agent, catalyst with modifiers and additives. Typical epoxy resins include glycidyl ether epoxy resins such as Bisphenol A, Bisphenol F and novolac type, and glycidyl amine epoxy resin, glycidyl ester epoxy resin as well as cycloaliphatic epoxy resin. Epoxy adhesives are supplied in both one-component and two-component packages depending on curing agent used and curing method applied. Two-component epoxy adhesives are prepared by packaging epoxy composition and curing agent composition separately. Almost all room temperature cure epoxy adhesives are supplied in two-component packages. One-component epoxy adhesives are prepared and supplied by mixing all formulated components in advance including epoxy resin and curing agent. One-component epoxy adhesives usually need cure at elevated temperature and storage at low temperature conditions. Typical room temperature cure epoxy adhesives, thermal cure epoxy adhesives and UV cure epoxy adhesives are described. Toughened high performance epoxy adhesives, low temperature cure one-component epoxy adhesives, instant bonding epoxy adhesives and sustainable epoxy adhesives are recent progressions and new trend in epoxy adhesive technology developments.
Keywords: Epoxy adhesive, structural bonding, two-component, one-component, toughened, instant bonding, sustainable epoxy
1.1 Introduction
Epoxy resins are one of polymer materials containing at least one carbon-oxygen-carbon three ring known as the epoxy group, epoxide or oxirane. Epoxy resin was initially discovered in late 1890's. It was first synthesized by N. Prileschajew in 1909 via oxidation of olefin with benzoic acid peroxide. The first epoxy adhesive was invented in 1936 by Dr. Pierre Castan for dental application via curing bisphenol A epoxy resin with phthalic anhydride. Commercialization of epoxy adhesives was started around late 1940's in Europe and USA [1, 2]. Various epoxy adhesives have been developed and commercialized since then. Epoxy adhesives are widely used as typical reactive adhesives for various structural bonding applications ranging from general industry, construction, electronics assembly, automobile production to aerospace and defense markets [3-20]. Major global suppliers for epoxy adhesives are Henkel AG & Co. KGaA, H.B. Fuller Company, 3M, Huntsman Corporation, Sika Corporation, Arkema Corporation, Cemendine Co., Ltd., Three-Bond Co., Ltd., Huitian Adhesive, etc.
Epoxy adhesives show good adhesion to a wide range of materials such as metals, glass, concrete, ceramics, wood and many plastics. Curing shrinkage is very low. Cured epoxy resin possesses strong and rigid cross-linked chemical structure especially well-suited for structural bonding applications. By combination of various epoxy resins with different curing agents, a number of epoxy adhesives have been commercialized for various applications. On the other hand, room temperature and thermal cure epoxy adhesives need relatively long cure time. Most cured epoxy adhesives are rigid and thus are not suitable for bonding flexible substrates. In selection and use of epoxy adhesives, their pot-life, cure condition, cure method, physical properties of un-cured and cured resin as well as adhesion and reliability performance should be taken into consideration.
1.2 Epoxy Adhesive Chemistry
1.2.1 Epoxy Resins
Epoxy resins are usually synthesized from the reaction of active hydrogen in phenols, alcohols, amines or acids with epichlorohydrin at certain well-controlled conditions. Epoxy resins can also be prepared by oxidation of olefin with peroxide as in the case of cycloaliphatic epoxy resins. Typical epoxy resins are glycidyl ether resins such as Bisphenol A, Bisphenol F and novolac type, glycidyl amine epoxy resin, glycidyl ester epoxy resin and cycloaliphatic epoxy resin.
Bisphenol A glycidyl ether, often called DGEBA, was the first commercialized epoxy resin. It is still the most standard and widely used epoxy resin, constituting the majority, estimated over 75% in sales volume, of all epoxy resins used today. Bisphenol A epoxy resin is normally synthesized by the reaction of bisphenol A and epichlorohydrin at 70 - 80°C in alkaline condition as illustrated in Figure 1.1 [21]. Various grades of bisphenol A epoxy resins in either liquid or solid state with different viscosity or melting point, EEW (abbreviated for epoxy equivalent weight) and purity have been commercialized and supplied by all major epoxy resin manufacturers. Chemical structure and key features of functional groups of bisphenol A epoxy resin are illustrated in Figure 1.2 [22], indicating potential balanced properties of good reactivity, good chemical and thermal resistance as well as high adhesion.
Bisphenol F glycidyl ether, often called Bisphenol F epoxy resin, is synthesized by the reaction of bisphenol F and epichlorohydrin. Figure 1.3 shows its typical chemical structure. Bisphenol F epoxy resin has lower viscosity, better solvent and chemical resistance as compared to standard bisphenol A epoxy resin.
Novolac glycidyl ether, often called novalac epoxy resin, is synthesized by the reaction of novalac phenol and epichlorohydrin. Figure 1.4 shows its typical chemical structure. Novolac epoxy resin has multi-functional epoxy groups and possesses especially high thermal resistance. Novolac epoxy resin is usually used in combination with bisphenol A or bisphenol F epoxy resin due to its high viscosity.
Figure 1.1 Synthesis of DGEBA from bisphenol A and epichlorohydrin.
Figure 1.2 Chemical structure and key features of DGEBA.
Figure 1.3 Chemical structure of bisphenol F epoxy resin.
Figure 1.4 Chemical structures of cresol (left) and phenol (right) novolac epoxy resins.
Glycidyl ether epoxy resins, synthesized from alcohols and epichlorohydrin, usually have much lower viscosity and are used mainly as reactive diluent to lower viscosity for better handling property adjustment.
Glycidyl amine epoxy resin is synthesized by the reaction of amino compound with epichlorohydrin. Figure 1.5 shows chemical structures of tri-functional and tetra-functional glycidyl amine epoxy resins. Glycidyl amine epoxy resins with less than tri-functional epoxy groups have low viscosity and are often used as epoxy diluent especially for applications requiring high thermal resistance. Tetra-functional glycidyl amine epoxy resin possesses extremely high glass transition temperature and excellent mechanical properties after full cure, suitable for use as base resin in applications requiring high thermal resistance and high performance.
Glycidyl ester epoxy resin is prepared by the reaction of carboxylic acid with epichlorohydrin. Figure 1.6 (left) illustrates chemical structure of typical glycidyl ester epoxy resin. It is often cured by anhydride, offering good insulation, high thermal resistance and good UV resistant performance. Cycloaliphatic epoxy resin is usually prepared by the oxidation of olefin with peroxide. Figure 1.6 (right) illustrates chemical structure of typical cycloaliphatic epoxy resin. Cycloaliphatic epoxy resin is principally cured with anhydrides or cationic initiators, offering good weathering and thermal resistant performance.
Figure 1.5 Chemical structures of tri- (left) and tetra-functional (right) glycidyl amine epoxy resins.
Figure 1.6 Chemical structures of glycidyl ester (left) and cycloaliphatic (right) epoxy resins.
1.2.2 Curing Agents and Catalysts
Epoxide group is chemically very active. Epoxy resin can react, almost equivalently, with active hydrogen in polyamines, mercaptan compounds, phenols and anhydrates via polyaddition mechanism at certain conditions to become cross-linked strong thermoset polymers. Epoxy resin can polymerize homogeneously via anionic polymerization mechanism by initiating it with Lewis bases such as tertiary amines or imidazole compounds. It can also polymerize via cationic polymerization by initiating it with Lewis acids such as onium salts, iodonium salts. Table 1.1 lists typical curing agents, catalysts and initiators used for epoxy adhesives.
Table 1.1 Typical epoxy resin curing agents, catalysts and initiators.
Polymerization mechanism Curing agent, catalyst Polyaddition PolyaminesModified polyamines
Mercaptans
Phenols
Anhydrides Anionic Tertiary amines
Imidazole compounds Cationic Onium salts
Iodonium salts
Figure 1.7 Chemical structures of typical aliphatic polyamine curing agents.
Polyamines including aliphatic, cycloaliphatic and aromatic types, are the most widely used curing agents for epoxy adhesives. Active hydrogen in aliphatic polyamines can rapidly react with epoxy resins at room temperature. Figure 1.7 shows chemical...
