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Cataloging- in- Publication Data
Names: Pizzi, A. (Antonio), 1946- editor. | Mittal, K. L., 1945- editor.
Title: Handbook of adhesive technology / editors, A. Pizzi & K.L. Mittal.
Description: Third edition. | Boca Raton : CRC Press, 2018. | Includes bibliographical references.
Identifiers: LCCN 2017026609| ISBN 9781498736442 (hardback : alk. paper) | ISBN 9781351647267 (epub) | ISBN 9781498736473 (web pdf) | ISBN 9781351637763 (mobi/kindle)
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Part 1 Fundamental aspects
Chapter 1 Theories and Mechanisms of Adhesion
Douglas J. Gardner
Chapter 2 Surface Mechanical (Physical) Treatments Prior to Bonding ....................................
Janette Brezinová, Anna Guzanová, and Dagmar Draganovská
Chapter
Nitu Bhatnagar
Chapter
Farid Taheri
Part 2 adhesive Classes
Chapter
Charles R. Frihart and Linda F. Lorenz
Chapter
A.A. Shybi, Siby Varghese, Hanna J. Maria, and Sabu Thomas
Chapter 7
Johann Klein and Christina Despotopoulou
Chapter
Antonio Pizzi
Chapter 9
Antonio Pizzi
Part 3 applications of adhesives
Chapter 22 Adhesives in Dentistry .............................................................................................
Erdem Özdemir
Chapter 23 New Adhesive Technologies in the Footwear Industry ............................................
Elena Orgilés-Calpena, Francisca Arán-Aís, Ana M. Torró-Palau, and César Orgilés-Barceló
Chapter 24 Adhesives in the Automotive Industry 619
Klaus Dilger and Michael Frauenhofer
Index 635
Preface
This volume constitutes the Third Edition of the popular book Handbook of Adhesive Technology which made its debut in 1994 and the second edition appeared in 2003.
Adhesives are used in a myriad of applications to bond similar or dissimilar materials. The applications of adhesives range from mundane DIY (Do-It-Yourself) projects to bonding composite materials to fabricate structural elements. The use of adhesives is ubiquitous and adhesives find their utility in a legion of varied and diverse industries, e.g., construction, aerospace, automotive, packaging, microelectronics, dentistry, and medical (surgical procedures).
As a matter of fact, adhesive bonding is a branch of the general discipline called “zygology” (the science of joining). The popularity of the use of adhesives for bonding is owing to the fact that adhesive bonding offers many advantages vis-à-vis other methods of joining, e.g., riveting, welding, soldering, nailing which, inter alia, can be enumerated as: uniform distribution of stress, lightweight assemblies, cost effectiveness, and no danger from corrosion as is the case with nails.
Recently, there has been a flurry of R&D activity in ameliorating the existing adhesive products and to come up with new and improved adhesives with desirable new functionalities and performance characteristics. The holy grail in the field of adhesive bonding is the durability of adhesively bonded joints against the deleterious effect of water. Water has been called the “God’s cruelest liquid” or the worst enemy of adhesion. Also, there is presently an accelerated pace of research to understand and control the factors affecting the performance of adhesively bonded joints.
This book (Third Edition) containing 24 chapters written by internationally renowned authors is profusely illustrated and copiously referenced. It differs from its predecessor volumes in many respects and its particular and important characteristics are: some completely new chapters are included; some topics are the same but written by other subject matter experts with different perspective; and some topics are the same but these have been revised and updated. These days the mantras are: nano, green, renewable, sustainable, biobased and biomimetic, and some new as well as certain chapters on established adhesives address these catchwords.
The book is divided into three parts as follows: Part 1: Fundamental Aspects; Part 2: Adhesive Classes; and Part 3: Applications of Adhesives. The topics covered include: theories and mechanisms of adhesion; surface mechanical (physical) treatments prior to bonding; plasma surface treatment to enhance adhesive bonding; applications of nanoparticles in adhesives; protein adhesives, rubber-based adhesives; elastic adhesives; phenolic resin adhesives; natural phenolic adhesives derived from tannins and lignin; urea and melamine aminoresin adhesives; polyurethane adhesives; reactive acrylic adhesives; anaerobic adhesives; aerobic acrylic adhesives; biobased acrylic adhesives; silicone adhesives; epoxy adhesives; biosourced epoxy resins; pressure-sensitive adhesives and products; adhesives in the wood industry; bioadhesives in drug delivery; adhesives in dentistry; new adhesive technologies in the footwear industry; and adhesives in the automotive industry.
This compendium contains a wealth of information in the arena of adhesives and adhesive bonding and represents a commentary on the current state of the knowledge and R&D activity in the domain of adhesives—a fascinating class of materials. Anyone interested centrally or tangentially in adhesives should find this volume of much interest and value. The field of adhesive bonding is veritably inter-, multi- and transdisciplinary, so researchers in seemingly different disciplines should find the information compiled here useful, particularly it should appeal to polymer scientists, surface chemists, materials scientists, adhesionists and those who need to use adhesives.
The editors sincerely hope that this Third Edition will receive the same warm welcome from the scientific and technological community as its predecessors. Here we have attempted to take due cognizance of the many important aspects of adhesives and adhesive bonding.
Now comes the important but fun part of writing the Preface. First and foremost, we would like to profusely thank all the authors for their sustained interest, enthusiasm, unwavering cooperation and contribution which were sine qua non for materializing this book. Also we very much appreciate the steadfast interest and support of Barbara Knott and other staff members of Taylor & Francis to give this book a body form.
A. Pizzi and K. L. Mittal
Editors
Antonio Pizzi is full professor of industrial chemistry at the ENSTIB, University of Lorraine. Prof. Pizzi, who holds a Dr. Chem. (Polymers, Rome, Italy), a Ph.D. (Organic Chemistry, South Africa) and a D.Sc. (Wood Chemistry, South Africa), is the author of more than 700 research and technical articles, patents, contract reports and international conference papers as well as 7 books on adhesion and adhesives published in New York. He is the recipient of numerous prestigious international prizes for new industrial developments in his fields of specialization, such as, among others, the Descartes Prize of the European Commission and the Schweighofer Prize for Wood Research Innovation. His best-known area of specialization is on wood and fiber glueing and wood adhesives chemistry, formulation and application, in particular in bioadhesives and their application to composite products based on natural materials.
Kashmiri Lal (Kash) Mittal received his Ph.D. from the University of Southern California in 1970 and was associated with the IBM Corp. from 1972 to 1994. He is currently teaching and consulting worldwide in the areas of adhesion and surface cleaning. He is the editor of more than 125 published books, as well as others that are in the process of publication within the realms of surface and colloid science, and adhesion. He has received many awards and honors and is listed in many biographical reference works. Dr. Mittal was a founding editor of the Journal of Adhesion Science and Technology in 1987 and was its editor-in-chief until April 2012. He has served on the editorial boards of a number of scientific and technical journals. He was recognized for his contributions and accomplishments by the international adhesion community with the First International Congress on Adhesion Science and Technology in Amsterdam organized in his honor on the occasion of his 50th birthday in 1995 (235 papers from 38 countries were presented). In 2002, he was honored by the global surfactant community, which instituted the Kash Mittal Award in the surfactant field in his honor. In 2003, he was honored by the Maria Curie-Sklodowska University, Lublin, Poland, which awarded him the title of doctor honoris causa. In 2010, he was honored by both adhesion and surfactant communities on the occasion of publication of his l00th edited book. In 2013, he initiated a new journal titled Reviews of Adhesion and Adhesives. In 2014, 2 books entitled Recent Advances in Adhesion Science and Technology, and Surfactant Science and Technology: Retrospects and Prospects were published in his honor.
List of Contributors
Francisca Ará n-Aí s
INESCOP
Spanish Footwear Technology Institute Alicante, Spain
Istvá n Benedek
Pressure-Sensitive Consulting Wuppertal, Germany
Nitu Bhatnagar Department of Chemistry
Manipal University Jaipur, Jaipur, Rajasthan, India
David Birkett
Henkel Adhesive Technologies Technology Centre Europe Dublin, Ireland
Bernard Boutevin
Institut Charles Gerhardt Université de Montpellier Montpellier, France
Janette Brezinová Faculty of Mechanical Engineering Technical University of Koš ice Koš ice, Slovakia
Sylvain Caillol
Institut Charles Gerhardt Université de Montpellier Montpellier, France
David Condron
Henkel Adhesive Technologies Technology Centre Europe Dublin, Ireland
Paul Cranley
The Dow Chemical Company Freeport, Texas
Christina Despotopoulou Henkel AG & Co D ü sseldorf, Germany
Klaus Dilger
Institute of Welding and Joining
Technical University of Brunswick Braunschweig, Germany
Dagmar Draganovská
Faculty of Mechanical Engineering
Technical University of Koš ice Koš ice, Slovakia
Manfred Dunky Institute for Wood Technology and Renewable Resources
University of Natural Resources and Life Sciences
Vienna, Austria
Michael Frauenhofer Audi AG Ingolstadt, Germany
Charles R. Frihart USDA Forest Service, Forest Products Laboratory Madison, Wisconsin
Douglas J. Gardner University of Maine Advanced Structures and Composites Center Orono, Maine
Anna Guzanová
Faculty of Mechanical Engineering Technical University of Koš ice Koš ice, Slovakia
John Hill
Lord Corporation Cary, North Carolina
Johann Klein
Henkel AG & Co D ü sseldorf, Germany
Jerome M. Klosowski Klosowski Scientific Inc Bay City, Michigan
Dennis G. Lay
The Dow Chemical Company
Freeport, Texas
Linda F. Lorenz
USDA Forest Service, Forest Products Laboratory
Madison, Wisconsin
Hanna J. Maria
International and Inter University Centre for Nanoscience and Technology
Mahatma Gandhi University
Kottayam, Kerala, India
Cé sar Orgilé s-Barceló INESCOP
Spanish Footwear Technology Institute Alicante, Spain
Elena Orgilé s-Calpena INESCOP
Spanish Footwear Technology Institute
Alicante, Spain
Erdem Ö zdemir
Metin Kasapoğ lu Caddesi
Antalya, Turkey
Paramjot
Department of Pharmaceutics
Chitkara College of Pharmacy
Chitkara University
Raypura (Patiala), Punjab, India
Jean-Pierre Pascault INSA-Lyon
Villeurbanne, France and Université de Lyon Lyon, France
Emmanuel Pitia
Lord Corporation Erie, Pennsylvania
Antonio Pizzi LERMAB
University of Lorraine
Epinal, France
Anna Rudawska
Lublin University of Technology
Mechanical Engineering
Department of Production Engineering
Lublin, Poland
A. A. Shybi
Rubber Research Institute of India
Kottayam, Kerala, India
Inderbir Singh
Department of Pharmaceutics
Chitkara College of Pharmacy
Chitkara University
Raypura (Patiala), Punjab, India
Nigel Sweeney Henkel Ireland Operations and Research Ltd
Dublin, Ireland
Farid Taheri
Department of Mechanical Engineering
Dalhousie University Halifax, Canada
Sabu Thomas
International and Inter University Centre for Nanoscience and Technology
Mahatma Gandhi University Kottayam, Kerala, India and
School of Chemical Sciences
Mahatma Gandhi University
Kottayam, Kerala, India
Ana M. Torró -Palau INESCOP
Spanish Footwear Technology Institute Alicante, Spain
Siby Varghese
Rubber Research Institute of India
Kottayam, Kerala, India
Part 1 Fundamental Aspects
1 Theories and Mechanisms of Adhesion
Douglas J. Gardner
CONTENTS
1.2.3
1.1 INTRODUCTION
The concept of joining things together through the use of sticky or glue-like substances has been around for thousands of years [1]. Early humans were quite adept at utilizing products found in nature that are sticky, such as pitches and bitumen, and that could contribute to the manufacture of useful bonded articles such as tools and building materials as well as artisanal objects. It is only more recently, within the past century or so, that man has tried to classify adhesion based on the fundamental behavior of materials. As such, the study of adhesion has gained importance in the fields of materials science, engineering, and biomedical science. It is the goal of this chapter to provide an overview of the current theories and mechanisms of adhesion.
1.1.1
Adherend M AteriAl ProPerties relevAnt to Adhesion
In the adhesion science and technology community, most materials to be adhesively bonded or glued are referred to as adherends. Adherends being bonded are usually in a solid form, while adhesives can be in either solid or liquid form (Table 1.1). There are a wide variety of adherend and adhesive types, as well as different processes to bond materials, such that many adhesion scientists will specialize in a specific area of adhesion/adhesives. A list of common adherend materials is found in Table 1.2. Examples of adherend materials include
wood, tapes, coated abrasives, building materials, and materials in the automotive and aerospace industries. The processes of joining materials through adhesive bonding to form a bonded
TABLE 1.1
Examples of Adherend and Adhesive Types
Adherend Type Examples Adhesive Type
Dense solid Metals, polymers
Brittle solid Glass
Porous solid Wood, foams
Soft solid Elastomers
Biological solid Teeth
Highly viscous elastomeric liquid
Medium-viscosity liquid
Low-viscosity liquid
Solids
Low- and medium-viscosity liquid
Examples
Sealants, caulks
Thermosetting or cold setting catalyzed polymer solutions
Adhesion promoters or highly reactive low molecular weight polymer adhesives such as Superglue
Hot-melt adhesives, powdered adhesives typically require heat to achieve liquid state to facilitate adhesion and curing
Acrylate adhesives
TABLE 1.2
List of Common Adherend Materials with Product Examples
Adherend Materials
Plastics
Textiles
Product Examples
Consumer goods, composites
Waterproof clothing
Wood Furniture
Laminates
Coated abrasives
Building materials
Automotive composites
Aerospace composites
Tapes, labels
Sandpaper
Tiles, flooring
Vehicle bumpers
Fuselage assembly
durability requirements of the resulting adhesive bond. Because of the variability in adhesive bonding processes, there is no single adhesive bonding mechanism that describes all adhesive bond types. To better understand adhesive bonding processes, adhesion scientists have categorized adhesion mechanisms or theories based on the fundamental behavior of materials being bonded (adherends) as well as the adhesives used to bond the materials. Understanding adhesion requires an intimate knowledge of the bulk and surface material properties of the adherend, as well as the material property behavior of the adhesive. A list of general material property characteristics to be considered in studying or assessing adhesion is shown in Table 1.3. Surface properties of interest related to adhesion include topography, surface thermodynamics, chemical functionality, and hardness. Adhesive characteristics to be considered include molecular weight, rheology, curing characteristics, thermal transition of polymers, and viscoelasticity. For the bonded assembly, the ultimate mechanical properties and durability characteristics are of prime importance.
1.1.2 length scAle of Adherend –Adhesive inter Actions
The prevailing adhesion theories can be grouped into two types of interactions: (1) those that rely on interlocking or entanglement, and (2) those that rely on charge interactions. Furthermore, it is useful to know the length scale(s) over which the adhesion interactions occur. The comparison of adhesion interactions relative to length scale is listed in Table 1.4.
TABLE 1.3
General Materials Related to Adhesion and Their Assessment Methods
Material Assessment Methods
Adherend
Adhesive
Bonded assembly
Topography, wettability, chemical functionality, hardness
Comparison of Adhesion Interactions Relative to Length Scale
Category of Adhesion Mechanism
Mechanical
Diffusion
Type of Interaction
Interlocking or entanglement
0.01–1000 μm
Interlocking or entanglement 10 nm–2 µm
Electrostatic Charge 0.1–1 μm
Covalent bonding Charge 0.1–0.2 nm
Acid–base interaction Charge 0.1–0.4 nm
Hydrogen bonding Charge 0.235–0.27 nm
Lifshitz–van der Waals Charge 0.5–1 nm
TABLE 1.5
Orders of Scale for Adherend–Adhesive Interactions
Scale
1 m, 100 cm
10−1 m, 10 cm
10−3 m, 1 mm
10−4 m, 100 µm
10−6 m, 1 µm
10−7 m, 100 nm
10−8–10–9 m, 1–100 nm
Test Specimen or Material Characteristics for Determining Adherend–Adhesive Interactions
Glulam beam laminates
Furniture bondlines
Polymer microdroplet on a glass fiber
Microscopic evaluation of adherend–adhesive bondline
Small paint droplets on automobile panels
Scale of cellulose nanofibrils
Scale of adhesive polymer chains
Source: Adapted from Gardner, D. J. et al., Rev. Adhesion Adhesives, 2, 127–172, 2014.
It is apparent that the adhesion interactions relying on interlocking or entanglement (mechanical and diffusion) can occur over greater length scales than the adhesion interactions relying on charge interactions. Most charge interactions involve interactions on the molecular level or nanolength scale.
The length scale of adherend–adhesive interactions is also of importance in understanding adhesion mechanisms, because although many practical aspects of adhesion occur on the macroscopic length scale (millimeter to meter), many of the basic adhesion interactions occur on a much smaller length scale (nanometer to micrometer) (Table 1.5). Evaluations of laminate adhesion failure in wood glulam beams are determined on the meter length scale, whereas many gluelines in furniture occur on the centimeter length scale. Interactions between polymer droplets on individual glass
fibers occur on the millimeter length scale, and microscopic evaluation of the adherend–adhesive bondline is carried out on the 100 µ m length scale. The smallest paint droplets on automobile panels are of the order of 1–10 µ m in diameter. Cellulose nanofibrils are on the scale of 100 nm in length and 10–20 nm in diameter. The smaller molecular weight fraction(s) of many thermosetting adhesive polymers range from 1 to 100 nm in length.
1.2 THEORIES OF ADHESION
There are seven accepted theories of adhesion [3–5]. These are:
1. Mechanical interlocking or hooking
2. Electronic, electrostatic, or electrical double layer
3. Adsorption (thermodynamic) or wetting
4. Diffusion
5. Chemical (covalent) bonding
6. Acid–base
7. Weak boundary layers
It should be noted that these mechanisms are not self-excluding, and several can occur simultaneously in an adhesive bond depending on the specific bonding situation. An additional adhesive mechanism for pressure-sensitive or elastomeric adhesives should be included in this list given the nature of that particular bonding mechanism, although some adhesion scientists have attempted to explain the bonding behavior of pressure-sensitive adhesives using surface energetics and the concept of tack [6]. We will discuss the issue regarding elastomeric adhesives in greater detail later.
1.2.1 MechAnicAl interlocking theory
Conceptually, the ubiquity of mechanical interlocking has long been a topic of interest in nature, art, and society [7]. In the field of adhesion, mechanical interlocking was first proposed in the early part of the last century [8,9]. There have been changing perceptions on the importance of mechanical interlocking in adhesion as analytical methods to study adhesion and our fundamental understanding have improved [10]. Essentially, mechanical interlocking can be divided into two groups: locking by friction and locking by dovetailing (Figure 1.1). For mechanically interlocked adherends, there are irregularities, pores, or crevices where adhesives penetrate or absorb into, and thus the mechanical properties of the adherends are involved [11]. In addition to geometry factors, surface roughness has a considerable influence on adhesion. Rougher adherend surfaces produce better adhesion than smooth surfaces. High-level adhesion can be attained by
FIGURE 1.1 Schematic diagram of mechanical interlocking mechanisms.
improving the adherend surface properties, and mechanical keying can be enhanced by increasing the surface area [12].
Absorption is an important factor in mechanical interlocking, because it affects penetration of adhesives into pores or irregularities on adherend surfaces. Greater absorption produces better adhesion in mechanical interlocking systems [13]. The length scale, which changes according to the type of interaction, is another factor that affects adhesion. Mechanical interlocking is strongly dependent on the surface properties. When studying mechanical interlocking, the adherend surface properties, including the presence of crevices, pores, roughness, and irregularities, should be well characterized. Optimizing the surface properties—for instance, increasing the roughness of the surface—will produce stronger or enhanced mechanical interlocking. A primary limitation of the mechanical interlocking theory is that it does not inherently take into account charge interactions that may also occur in the creation of an adhesive bond.
Over the past several decades, the focus of mechanical interlocking in the adhesion field has been in the area of micro- and nanomaterials [14,15]. There are two popular research areas in polymer materials that address the mechanical interlocking theory: mechanically interlocked molecules (MIMs) [16] including dendrimers [17], and surface microstructuring to enhance adhesion in polymer composites [14]. In evaluating the effect of mechanical interlocking on adhesion strength of polymer–metal interfaces, micropatterned topographies were introduced on metal surfaces via a machining process. It was found that the molecular dissipation of the polymer in the vicinity of the interface is the major cause of the practical energy of separation during mechanical testing [13]. Mechanical interlocking also provides a simple and effective means of enhancing adhesion between dissimilar materials in microelectromechanical systems (MEMS) [18,19].
The morphological properties of nanoparticles are also germane to the understanding of mechanical interlocking on the nanoscale. Nanoporous gold particles added to silicone in film applications show excellent adhesion to the silicone attributable to mechanical interlocking with the elastomer substrate [20]. In polymer nanocomposites with low nanofiller content, graphene platelets perform better than carbon nanotubes in terms of enhancing mechanical properties, and this is partially attributed to improved mechanical interlocking/adhesion at the nanofiller–matrix interface [21]. Indeed, the role of wrinkles in thermally exfoliated chemically modified graphene may possibly contribute to nanoscale surface roughness that could enhance mechanical interlocking in polymer nanocomposite applications [22]. Nanomechanical interlocking has been observed at the nanotube–polymer interface, and this contributes to improved mechanical properties in polymer nanocomposites [15].
1.2.2 electrostAtic theory
The electrostatic mechanism of adhesion was proposed in 1948 [23]. The primary tenet of the electrostatic mechanism is that the two adhering materials are viewed as akin to the plates of an electrical condenser across which charge transfer takes place and adhesion strength is attributed to electrostatic forces (Figure 1.2) [4]. The concepts and quantities important in electrostatic adhesion are listed in Table 1.6.
FIGURE 1.2 Schematic of the formation of an adhesion bond attributed to transfer of charge from an electropositive material to an electronegative material.
TABLE 1.6
Concepts and Quantities Important in Electrostatic Adhesion
Concept Definition
Electric field Generated by electrically charged particles.
Coulomb’s law Electrostatic interaction between electrically charged particles.
Capacitor Consists of two conductors separated by a nonconductive area.
Charge density Measure of electric charge per unit volume of space, in one, two, or three dimensions.
Van der Waals force Close-range force between two molecules attributed to their dipole moments.
Hamaker constant Augmentation factor for van der Waals force when many molecules are involved, as in the case of nanoparticles.
DLVO theory Named after Derjaguin, Landau, Verwey, and Overbeek. Theory explains the aggregation of particles in aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counterions.
Zeta potential The potential difference between the dispersion medium and the stationary layer of liquid attached to the dispersed particle.
Smoluchowsky approximation Used to calculate the zeta potentials of dispersed spherical nanoparticles.
Source: Adapted and augmented from Horenstein, M.N., J. Electrostatics, 67, 384–393, 2009.
FIGURE 1.3 Interaction between electrically charged particles. F1 and F 2 are the forces of interaction between two point charges (q1 and q2) and the distance (r) between them.
Coulomb’s law describes the electrostatic interaction between electrically charged particles (Figure 1.3) as:
where:
F is the force ke is Coulomb’s constant q1 and q2 are the charges r is the distance between the charges
Capacitance C is defined as the ratio of charge Q on each conductor to the voltage V between them:
Derjaguin conveyed the force F(h) acting between two charges separated from one another to the strength of an adhesion bond where:
where W(h) is the interaction energy per unit area between the two planar walls and Reff the effective radius.
In considering electrostatic interactions in liquids, the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory defines the interactions between charged surfaces where the total adhesion force FA is equal to the sum of the van der Waals force FvdW and the electric double layer force F EDL:
The van der Waals force is a function of the system Hamaker constant, particle diameter, contact radius, and particle–surface separation distance. The electric double layer force is a function of the liquid medium dielectric constant, zeta potential, reciprocal double layer thickness, particle diameter, and particle–surface separation distance.
The electrostatic theory is often used to describe adhesion behavior of powders to solid surfaces [23–26]. Electrostatic adhesion that occurs in the liquid phase through colloidal interactions has received much greater emphasis in the scientific literature, and practical applications are plentiful in various fields. Electrostatic self-assembly in liquids is an important area in nanoscience applications [24,27]. A primary limitation of the electrostatic theory is that charge neutralization through grounding or a similar mechanism can potentially disrupt bonding.
Recent research in electrostatic adhesion has focused on the biomimetic aspects of gecko lizard toe adhesion using synthetic materials. Dry adhesives inspired by biomimetic gecko lizard toe pad adhesion using Teflon amorphous fluoropolymer nanopillar sheets was attributed to electrostatic adhesion [28]. Improved controllable adhesion on both rough and smooth surfaces can be achieved with a hybrid/gecko-like adhesive [29]. Other research has addressed comparisons of adhesion forces between electrostatic and Coulombic attraction [30], electrostatic adhesion of nanosized particles, and the cohesive role of water [31]. Electrostatic forces greatly impact adhesion interactions from the micro- to the nanoscale [32] including micromanipulation of micrometer-scale objects [33].
1.2.3 WettAbility, surfAce free energy, ther ModynAMic Adhesion theory
Thermodynamic adhesion or wetting refers to the atomic and molecular interactions between adhesives and adherends. Surface tension or surface free energy are manifestations of these forces and are regarded as fundamental material properties to understand adhesion, because they are associated with adhesive bond formation [3]. Bond formation arises from the highly localized intermolecular interaction forces between materials. Therefore, good wetting is beneficial to strong adhesive bonding. It is well known that the dominant surface chemical and energetic factor influencing joint strength is interfacial tension between the adhesive and the adherend (γsl): the joint strength increases as γsl decreases [34]. The atomic and molecular forces involved in wetting include: (1) acid–base interactions, (2) weak hydrogen bonding, or (3) van der Waals forces (dipole–dipole and dispersion forces) [3]. The condition necessary for spontaneous wetting is given as:
where γsg, γsl, and γlg are, respectively, the interfacial free energies for solid–gas, solid–liquid, and liquid–gas interfaces. If γsl is insignificant, the criterion can be simplified to:
which means that the adhesive will wet the surface of the adherend when the surface free energy of the substrate is greater.
The surface free energies of solids can be determined by measuring the contact angles of appropriate probe liquids on a solid surface. Different contact angle analysis techniques are applied in the measurements of various forms of substrates. One is the sessile drop method, which is also referred to as the static contact angle technique. Another method is the Wilhelmy plate technique, which is suitable for making contact angle measurements on thin plates and single fibers. The contact angle can be calculated using the Wilhelmy equation (Equation 1.7) [35].
where:
F is the advancing or receding force on the sample in liquid
γL is the surface tension of the liquid
P is the perimeter of the wetted cross-section
M is the mass of the specimen
g is the acceleration due to gravity
ρL is the liquid density
A is the cross-sectional area of the specimen
h is the depth of immersion
For particles (also fibers), by recording the process of liquid going through a column attributed to capillary forces where particles of interest are packed inside, the contact angle can be calculated from the Washburn equation (Equation 1.8) [36], which governs the capillary wicking process:
where:
h is the height to which liquid has risen as a function of time t
R is the effective interstitial pore radius between the packed particles
γL is the surface tension of the liquid
η is the viscosity of the liquid
The methods of determining surface free energy of solids based on contact angles are various; for example, the Zisman approach [37], the equation of state [38], the Chibowski approach, the harmonic mean approach, the Owens and Wendt approach (the geometric mean), and the acid–base approach, which are described in a recent review [39]. Although satisfactory wetting or intrinsic adhesion is desirable in the creation of an adhesive bond, it does not necessarily ensure that the final mechanical bond strength will be optimal for a given bonding situation.
1.2.4 diffusion theory
The diffusion theory is based on the concept that two materials are soluble in one another, that is, compatible, and if they are brought into close contact, they dissolve in one another and form an interphase, which is a solution of both materials in one another and therefore does not form a discontinuity of physical properties between the two materials (Figure 1.4) [6]. The diffusion theory was first mentioned by Voyutskii and Vakula, and considered the role of polymer–polymer interactions in the creation of an adhesive bond based on the diffusion phenomenon [40].
For the diffusion mechanism of adhesion to occur, there must be similar solubility parameters for the adhesive and adherend [41]. This phenomenon is well illustrated by solvent welding in thermoplastic systems. The adhesive is typically a low molecular weight polymer solution in a compatible solvent that is applied to the adherend, and the solvent–polymer solution will diffuse into the
FIGURE 1.4 Schematic of diffusion theory of adhesion: (a) two compatible materials are brought into close contact (b) and an interphase (c) is formed where both materials mix and/or entangled with one another.
adherend to create molecular entanglement characterizing a diffusion bond. Thermal welding of thermoplastic polymers by various heating techniques is an adhesion bonding subject area in itself [42]. Thermal welding offers a way to create an adhesive bond between two adherends without the addition of a separate adhesive, because the adherends themselves essentially contribute to the adhesive bond. Polymer–polymer adhesion of plastic parts made by the additive manufacturing process of fused layer or fused deposition modeling is also dependent on diffusion bonding (welding) interactions [43]. Diffusion bonding is not applicable in situations where an adherend is not capable of absorbing a polymer adhesive, as in the case of bonding glass.
1.2.4.1
Interpenetrating Polymer Network (IPN)
There is a class of polymer interactions where two different polymer types will overlap in the same three-dimensional space on a molecular length scale. These overlapping polymers comprise a class of materials known as interpenetrating polymer networks (IPNs). The International Union of Pure and Applied Chemistry (IUPAC) defines an IPN as “A polymer comprising two or more networks which are at least partially interlaced on a molecular scale but not covalently bonded to each other and cannot be separated unless chemical bonds are broken.” A more detailed description of IPNs can be found in Lipatov [44] and Sperling [45]. In many instances, the formation of IPNs requires interdiffusion among polymer types, so it is worth mentioning them here. In addition, the concept of IPNs has been explored extensively in adhesive bonding of different materials such as wood bonding [46], fiber-reinforced polymer (FRP) composites in dental applications [47], and combined thermoset/thermoplastic FRP composites [48].
1.2.5 cheMicAl (covAlent) bonding theory
A covalent bond is a bond where two atoms share an electron pair, and is believed to improve the bond durability between the adherend and an adhesive. The bond strength of covalent bonds is tantamount to its importance in adhesion and adhesive bond strength. In a given material, the bond energy of a covalent bond (cohesive bond strength) is approximately 1000 times greater than the surface free energy of the same material. Therefore, creating a covalent bond between adhesive and adherend should provide a high-strength adhesive bond.
In composite material systems where two dissimilar materials are being joined, the use of coupling agents that bridge the chemical interaction between two substances has been an important area of adhesion technology development [49–51]. An example of a silane coupling agent undergoing (1) hydrolysis and (2) reaction with a hydroxyl functional substrate (glass) is depicted in Figure 1.5.
Coupling agents enable the creation of strong adhesive bonds between materials that are chemically dissimilar, such as glass fibers and polyester, epoxy and aluminum, and polypropylene and talc.
1.2.5.1 Hydrogen Bonding
The role of hydrogen bonding in adhesion is well recognized, but the historical interpretation of hydrogen bond strength typically placed it in the range of Lifshitz–van der Waals or acid–base
FIGURE 1.5 Hydrolysis of organofunctional silane and reaction of hydrolyzed organosilane with hydroxyl functional substrate.
TABLE 1.7
Bond Strength of Various Types of Chemical Bonds and Intermolecular Forces
Chemical Bond or Intermolecular Force
Source: Adapted from Gardner, D. J. et al., Rev. Adhesion Adhesives, 2, 127–172, 2014.
a Gilli, G. and Gilli, P. The Nature of the Hydrogen Bond. Outline of a Comprehensive Hydrogen Bond Theory, Oxford University Press, New York, 2009.
interaction bond strengths (8–25 kJ/mol) (Table 1.7). Recent evidence suggests that hydrogen bond strengths (4–188 kJ/mol) approach the range of covalent bond strength (147–628 kJ/mol) [52]. Many common synthetic and biobased adhesives such as epoxies, polyurethanes, proteins, and formaldehyde-based resins have strong hydrogen bonding functionalities. The new bond strength data elevate the importance of hydrogen bonding in regard to the chemical bonding theory of adhesion.
1.2.6 Acid – bAse theory
Based on the correlation of acid–base interactions by Drago et al. [53], Fowkes and Mostafa [54] proposed a new method to interpret the interactions during polymer adsorption where the polar interaction is referred to as an acid–base interaction. In this interaction, an acid (electron-acceptor) is bonded to a base (electron-donor) by sharing the electron pair offered by the latter, which forms a coordinate bond.
The following briefly summarizes the Lewis acid–base concept in wetting-related phenomena. According to Fowkes [55] and van Oss et al. [56], the total work of adhesion in interfacial interaction
Mechanisms
between solids and liquids can be expressed as the sum of the Lifshitz–van der Waals (LW) and the Lewis acid–base (AB) interactions, namely,
The separation of the work of adhesion into LW and AB components is also applicable to the surface free energies according to:
An advance in the understanding of wetting phenomena was the Good–Girifalco–Fowkes “geometric mean” combination rule for the LW interactions between two compounds i and j, which can be expressed as [57, 58]:
Hence, if the contact angle (θ) is determined for both a nonpolar and a polar liquid, with known γLW parameters on the same surface, then Wa LW and Wa AB can be determined using Equations 1.9 through 1.11.
The acid–base theory plays a critical role in surface chemistry and adhesion, and it has been exploited broadly on different materials [59–61]. Several models of calculating the surface free energy of solids were proposed where acid–base theory was applied, including the Fowkes method, the Good method, the van Oss method, and the Chang-Chen method [39]. On the nanoscale, LW forces are important in the bonding of silver nanoparticles to polyimide in printed electronic applications [62]. The importance of acid–base interactions in the adhesion field can be assessed by comparing the adhesive bond strength of nonpolar versus polar polymer substrates.
1.2.7 WeAk boundAry l Ayers concePt
Bikerman [63] first introduced the concept of a weak boundary layer (WBL) in adhesion science. Three different classes of WBLs were specified: air bubbles, impurities at the interface, and reactions between components and the medium. Good [64] further implied a WBL on the surface of adherends to be responsible for lower mechanical strength. The interface is the location of adhesion failure of a bonded assembly when a WBL is present. If the tenets of proper adherend preparation are followed in the creation of an adhesive bond, especially the bonding of a freshly prepared surface, then the concept of WBLs is not an issue. However, in many bonding situations, a freshly prepared, clean adherend surface may not be possible. It simplifies our understanding of WBLs to categorize them as being mechanical or chemical in nature (Figure 1.6).
Mechanical WBLs can arise from improper machining and lack of cleaning of an adherend surface prior to bonding, while chemical WBLs can be attributed to processing aids or lubricants used to prepare a surface. Examples of mechanical WBLs are common in wood adhesion [2,65], while chemical WBLs are common in preparing metal surfaces (oils) and extruded plastic surfaces (lubricants) for bonding. In addition, “aged” surfaces are often chemically altered because of environmental influences such as exposure to moisture, ultraviolet light, oxygen, or heat. Aged surfaces tend to have lower surface free energies and are thus more difficult to be wetted by adhesives.
Weak boundary layer
Mechanical WBL
• Trapped air bubbles
• Machining sur face damage
• Dir ty surface
Chemical WBL
• Lubricant contamination
• Plasticizer and other additives
• Aged or inactivated sur face
• Weathered sur face
Adhesives can be formulated to accommodate WBLs in certain bonding situations, but it is recommended to try to remove WBLs prior to bonding if at all possible. A great example of an adhesive group that can tolerate moisture in a “wet” WBL is based on isocyanate functionality. Isocyanates can chemically react with water (hydroxyl groups) to form urea linkages that contribute to the adhesive bond. Adhesives that are catalyzed by strong acids or bases for the curing process can also impact the adherend surface and help “activate” an aged surface.
1.2.8 sPeciAl MechAnisM of el AstoMeric-bAsed Adhesives
An important class of adhesives that exhibit characteristics of both a solid and liquid are the elastomeric-based adhesives, which include pressure-sensitive and contact bond adhesives. Many elastomeric-based adhesives are in the form of highly viscous liquids that are combined with flexible substrates in the form of tapes that can be bonded to a variety of material substrates in an instantaneous manner using low bonding pressure (pressure-sensitive adhesives). Contact bond adhesives are represented by the extrudable construction-based adhesives and caulks and sealants that are highly viscous and also form relatively instantaneous semistructural bonds. The major differences between the pressure-sensitive and contact bond adhesives are the bond strength of the adhesive application and the length of time required to hold a bond [4].
The elastomeric-based adhesives have a characteristic adhesion behavior described as tackiness or stickiness that aids in the creation of an almost instantaneous adhesive bond. Tackiness is generated by adding low molecular weight, resinous tackifiers to elastomeric polymers used in the formulation of elastomeric-based adhesives [4,6]. The glass transition and softening temperatures of tackifiers are often much above room temperature. There are several definitions for tack, including one promulgated by the Pressure-Sensitive Tape Council, “the condition of the adhesive when it feels sticky or highly adhesive” and the ASTM definition, “the property of an adhesive that enables it to form a bond of measurable strength immediately after the adherend and the adhesive are brought into contact under low pressure.” A visual example of tackiness is shown in Figure 1.7.
An interesting characteristic of elastomeric-based adhesives is that the magnitude of stickiness or tackiness that is formulated to occur in a particular adhesive is greatest at the application or use temperature and that tackiness will decrease both below and above the formulated application temperature.
Elastomeric-based adhesives—and any adhesive that exhibits tackiness, for that matter— will also need to consider other adhesion characteristics, including surface tension, wettability, mechanical interlocking, and so forth in creating proper adhesion with a substrate. However, in this author’s opinion, the concept of stickiness or tackiness deserves to be considered among adhesion mechanisms.
FIGURE 1.6 Characteristics of mechanical and chemical WBLs.
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