Adhesion in Pharmaceutical, Biomedical, and Dental Fields

By K.L. Mittal

The phenomenon of adhesion is of cardinal importance in the pharmaceutical, biomedical and dental fields. A few eclectic examples will suffice  to underscore the importance/relevance of adhesion in these three areas. For example, the adhesion between powdered solids is of crucial importance in tablet manufacture. The interaction between biodevices (e.g., stents, bio-implants) and body environment dictates the performance of such devices, and there is burgeoning research activity in modifying the surfaces of such implements to render them compatible with bodily components. In the field of dentistry, the modern trend is to shift from retaining of restorative materials by mechanical interlocking to adhesive bonding.

This unique book addresses all these three areas in an easily accessible single source. The book contains 15 chapters written by leading experts and is divided into four parts: General Topics; Adhesion in Pharmaceutical Field; Adhesion in Biomedical Field; and Adhesion in Dental Field. The topics covered include:

- Theories or mechanisms of adhesion.
- Wettability of powders.
- Role of surface free energy in tablet strength and powder flow behavior.
- Mucoadhesive polymers for drug delivery systems.
- Transdermal patches.
- Skin adhesion in long-wear cosmetics. 
- Factors affecting microbial adhesion. 
- Biofouling and ways to mitigate it. 
- Adhesion of coatings on surgical tools and bio-implants. 
- Adhesion in fabrication of microarrays in clinical diagnostics. 
- Antibacterial polymers for dental adhesives and composites. 
- Evolution of dental adhesives. 
- Testing of dental adhesives joints.

• Language: English
• Publisher: Wiley
• Release date: Jun 15, 2017
• ISBN: 9781119323792

Book preview

Preface

The phenomenon of adhesion is of cardinal importance in the pharmaceutical, biomedical, and dental fields. A few eclectic examples will suffice to underscore the importance/relevance of adhesion in these three areas. For example, the adhesion between powdered solids is of crucial importance in tablet manufacture. A mundane example is the bandage where the role of adhesion in its performance (sticking and peeling) is all too familiar. The interaction between biodevices (e.g., stents, bio-implants) and body environment governs the performance of such devices, and there is burgeoning research activity in modifying the surfaces of such implements to render them compatible with bodily components. Essentially, there are two distinct approaches which are exploited in this vein: one is to modify the surface chemistry of biodevices by a host of techniques ranging from simple to very sophisticated (e.g., laser treatment) and the second is to deposit suitable coatings. Irrespective of the intended function of the coating, it must adhere to the substrate; so many schemes have been devised to obtain adequate adhesion. The topic of microbial adhesion and biofouling is of grave concern with wide-ranging implications. In the field of dentistry, there has been paradigm shift from retaining of restorative materials by mechanical interlocking to adhesive bonding; so the mantra adhesive bonding dentistry has gained much popularity. Those who wear dentures very well know the importance of adhesion. If dentures come out in a social setting, it can be very embarrassing. In the recent past, there has been much activity in ameliorating the existing adhesives or in formulating stronger and better adhesives.

This unique book addresses all these three areas in an easily accessible single source. The impetus for bringing out this compilation stemmed from the lack of a book dealing primarily and specifically with the adhesion aspects in these three areas. So this book was conceived with the express intention to fill this lacuna in the literature.

The book contains 15 chapters written by internationally-renowned subject matter experts and is divided into four parts: Part 1: General Topics; Part 2: Adhesion in Pharmaceutical Field; Part 3: Adhesion in Biomedical Field; and Part 4: Adhesion in Dental Field. The topics covered include: theories or mechanisms of adhesion; wettability of powders; role of surface free energy in tablet strength and powder flow behavior; mucoadhesive polymers for drug delivery systems; transdermal patches; skin adhesion in long-wear cosmetics; factors affecting microbial adhesion; biofouling and ways to mitigate it; adhesion of coatings on surgical tools and bio-implants; adhesion in fabrication of microarrays in clinical diagnostics; antibacterial polymers for dental adhesives and composites; evolution of dental adhesives; and testing of dental adhesive joints.

This book covering many subtopics highlighting the importance/relevance of adhesion should be of great interest and considerable importance to R&D personnel in pharmaceutical, biomedical and dental industries as well as to researchers in academia and other research labs. Also, advanced graduate students carrying out research in these three disciplines will find it very instructive and beneficial. We sincerely hope this book will spur cross-pollination of ideas in these three seemingly different fields and thus new research vistas will emerge.

Now it is our great pleasure to express our thanks to those who were instrumental in materializing this book. First and foremost, we would like to profusely thank the authors of these chapters for sharing their knowledge and experience and for their interest, enthusiasm and cooperation, without which this book would not have seen the light of day. We would like to extend our appreciation to Martin Scrivener (publisher) for his unwavering interest in and steadfast support for this book project.

Kash Mittal

P.O. Box 1280

Hopewell Jct., NY 12533

E-mail: [e-mail]

Frank M. Etzler

LECOM

Erie, PA 16509

E-mail: [e-mail]

April 2017

Part 1

GENERAL TOPICS

Chapter 1

Theories and Mechanisms of Adhesion in the Pharmaceutical, Biomedical and Dental Fields

Douglas J. Gardner

University of Maine, Advanced Structures and Composites Center, Orono, ME., U.S.A

Corresponding author: [e-mail]

Abstract

Adhesion is an important attribute of material behavior in the pharmaceutical, biomedical, and dental fields that influences the interactions among different substances in the human body, and it is also important as it plays an important role in various processes, including, but not limited to, the manufacture of drugs, medical devices and dental care. Adhesive bonding is an important area focusing on the creation of joined substrates and composite materials. Based on the wide variety of adhesive bonding situations, the concept of adhesion can be broadly applied across different material types and interactions. Mechanisms of adhesion fall into two broad areas: those that rely on mechanical interlocking or entanglement and those that rely on charge interactions. There are seven accepted theories of adhesion. These are: mechanical interlocking; electrostatic theory; adsorption (thermodynamic) or wetting theory; diffusion theory; chemical bonding theory; acid-base theory; and theory of weak boundary layers. In addition, elastomeric-based adhesives exhibit a characteristic adhesion behavior described as tackiness or stickiness that aids in the creation of an almost instantaneous adhesive bond. This chapter provides an overview of adhesion theories and mechanisms relative to applications in the pharmaceutical, biomedical and dental fields.

Keywords: Adhesion, mechanisms, theories, adhesives, bonding, mechanical interlocking, electrostatic, adsorption, wetting, diffusion, chemical, acid-base, weak boundary layers, tackiness

1.1 Introduction

Adhesion mechanisms in the pharmaceutical, biomedical, and dental fields are similar to those encountered in other fields of materials science. However, the biggest challenge is that the adhesion mechanisms will typically occur in or will be influenced by the environment of the human body. The primary challenges facing adhesion in the environment of the human body include: creation of an adhesive bond in contact with various bodily fluids, blood, saliva, etc.; durability of an adhesive bond when exposed to various bodily fluids; the biochemical onslaught related to the body’s immune response and cellular regeneration; and exposure to inherent bodily microorganisms such as bacteria and fungi. Common examples of adhesion in the pharmaceutical, biomedical, and dental fields include the manufacture of respiratory inhalants such as albuterol; the application of medical bandages such as Band-aids® used to cover wounds; and the use of denture adhesives to secure false teeth. It is the goal of this Chapter to provide an overview of the current theories and mechanisms of adhesion with reference to applications in the pharmaceutical, biomedical, and dental fields.

1.1.1 Adherend Material 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 in the human body being bonded are usually in a solid form while adhesives are typically in the liquid form (Table 1.1).

Table 1.1 Examples of adherend and adhesive types in the human body.

The processes of joining materials through adhesive bonding to form a bonded assembly in the pharmaceutical, biomedical, and dental fields are quite variable in terms of adherend types and bonding processes including the strength and durability requirements of the resulting adhesive bond. To better understand adhesive bonding processes, adhesion scientists have characterized 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 a close familiarity with the bulk and surface material properties of the adherend and the material property characteristics of the adhesive being used. A list of general material property features to be considered in studying or assessing adhesion is shown in Table 1.2. Surface properties of interest related to adhesion include topography, surface thermodynamics, chemical functionality, hardness, and surface charge. Adhesive features to be considered include: molecular weight, rheology, curing characteristics, thermal transition of polymers, and viscoelasticity. For the bonded assembly, the ultimate mechanical properties, durability, and biological compatibility characteristics are of major importance. In addition, when considering adhesion in the pharmaceutical, biomedical, and dental fields, one also needs to consider cell adhesion. Cellular adhesion is involved with the bonding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules [1]. Cell adhesion continues to receive considerable attention in the adhesion field.

Table 1.2 General materials related to adhesion and their assessment methods.

1.1.2 Length Scale of Adherend-Adhesive Interactions

The prevailing adhesion theories can be assembled into two types of interactions: 1) those that rely on interlocking or entanglement; and 2) those that rely on charge interactions. Furthermore, it is beneficial to know the length scale(s) over which the adhesion interactions occur. The comparisons of adhesion interactions relative to length scale are listed in Table 1.3. It is obvious that the adhesion interactions relying on interlocking or entanglement, mechanical and diffusion, can occur over larger length scales than the adhesion interactions relying on charge interactions. Most charge interactions involve interactions on the molecular level or nano length scale.

Table 1.3 Comparison of adhesion interactions relative to length 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 centimeter), many of the basic adhesion interactions occur on a much smaller length scale (nanometer to micrometer) (Table 1.4). Wound protection using a Band-Aid® typically occurs on the cm length scale. Interactions between inhaler droplets in the lung occur on the millimeter length scale, and typical microscopic evaluation of the adherend-adhesive bondlines is performed at the 100 μm length scale. Bacteria are on the order of 0.5 to 5 μm in diameter. Nanoparticles are generally in the scale of 10 to 100 nm in diameter.

Table 1.4 Orders of scale for adherend-adhesive interactions in the pharmaceutical, biomedical and dental fields.*

*Adapted from Gardner et al. [2].

1.2 Mechanisms of Adhesion

There are seven mechanisms or theories of adhesion [3–5]. These are:

Mechanical interlocking or hooking

Electronic, electrostatic or electrical double layer

Adsorption (thermodynamic) or wetting

Diffusion

Chemical (covalent) bonding

Acid-base

Weak boundary layers

It should be mentioned that these adhesion mechanisms are not self-excluding, and several may occur simultaneously in a specific adhesive bonding situation. The concept of stickiness or tack that occurs in rubber-based or elastomeric adhesives will be discussed in more detail later [6].

1.2.1 Mechanical Interlocking Theory

In the field of adhesion, mechanical interlocking was proposed early in the last century [7, 8]. There have been changing views on the importance of mechanical interlocking in adhesion as analytical methods to study adhesion and our fundamental understanding have improved [9]. Mechanical interlocking of adhesives occurs in porous materials like bone through anchoring within the cellular substrate (Figure 1.1). For mechanically interlocked adherends, there are irregularities, pores, or crevices where adhesives penetrate or absorb into and the mechanical properties of the adherends are involved [10]. Using adhesives in an attempt to repair damaged joints in hip or knee replacement surgery is a good example of bonding a porous structure. 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 improving the adherend surface properties and mechanical keying can be enhanced by increasing the surface area [11].

Graphic

Figure 1.1 Micrograph of the porous structure of bone (Courtesy of Michael Mason, University of Maine).

Absorption is an important factor in mechanical interlocking, because absorption affects penetration of adhesives into pores or irregularities on adherend surfaces. Greater absorption produces better adhesion in mechanical interlocking systems [12]. The length scale, which changes according to 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.

1.2.2 Electrostatic Theory

The electrostatic mechanism of adhesion was proposed in 1948 [13]. The primary tenet of the electrostatic mechanism is that the two adhering materials are viewed as analogous 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 1.5.

Graphic

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 1.5 Concepts and quantities important in electrostatic adhesion.*

*Adapted and augmented from Horenstein [14].

Coulomb’s Law describes the electrostatic interaction between electrically charged particles (Figure 1.3) as:

(1.1)

Graphic

where: F is force, ke is Coulomb’s constant, q1 and q2 are the charges and 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

(1.2)

Graphic

Derjaguin expressed the force F(h) acting between two charges separated from one another to the strength of an adhesion bond where:

(1.3)

GraphicGraphic

Figure 1.3 Interaction between electrically charged particles. F1 and F2 are the forces of interaction between two point charges (q1 and q2) and the distance (r) between them.

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 FEDL

(1.4)

Graphic

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 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 [15–17]. 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 [18, 19]. A primary limitation of the electrostatic theory is that charge neutralization through grounding or a similar mechanism can potentially disrupt bonding. Electrostatic interactions are commonly encountered in pharmaceutical solid systems [20].

1.2.3 Wettability, Surface Free Energy, Thermodynamic Adhesion Theory

Thermodynamic adhesion or wetting refers to the atomic and molecular interactions between adhesives and adherends. Surface tension or surface free energy represent 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 [21]. The atomic and molecular forces involved in wetting include: (a) acid-base interactions, (b) weak hydrogen bonding, or (c) van der Waals forces (dipole-dipole and dispersion forces) [3]. The condition necessary for spontaneous wetting is given below:

(1.5)

Graphic

where: γsg, γsl and γlg are respectively the interfacial free energies for solidgas, solid-liquid and liquid-gas interfaces. If γsl is insignificant, the criterion can be simplified to:

(1.6)

Graphic

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 on various forms of substrates. One is the sessile drop method which is also referred to as static contact angle technique. Another method is the Wilhelmy Plate technique that is suitable for making contact angle measurements on thin plates and single fibers [22]. 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 (θ) can be calculated from the Washburn equation (Equation 1.7) [23] that governs the capillary wicking process:

(1.7)

Graphic

where:

h = height to which liquid has risen as a function of time t

R = effective interstitial pore radius between the packed particles

γL = surface tension of the liquid

η = viscosity of the liquid.

The methods of determining surface free energy of solids based on contact angles are various, for example the Zisman approach [24], the equation of state [25], the Chibowski approach, the harmonic mean approach, Owens and Wendt approach (the geometric mean)and the acid-base approach, which are described in a recent review [26]. 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 optimum 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, i.e. 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 diffusion phenomenon [27].

Graphic

Figure 1.4 Schematic of diffusion mechanism of adhesion: (a) two compatible materials are brought into close contact (b) and an interphase (c) is formed where both materials mix and/or entangle with one another.

For the diffusion mechanism of adhesion to occur, there must be similar solubility parameters for the adhesive and adherend [28]. 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 adherend to create molecular entanglement characterizing a diffusion bond. Thermal welding of thermoplastic polymers by various heating techniques is an important practical adhesion bonding process [29]. 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. Adhesion of plastic parts made by the additive manufacturing process of fused deposition modeling is also dependent on diffusion bonding (welding) interactions [30]. The manufacture of tissue scaffolds and organs is also being explored in three-dimensional printing [31]. 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.

1.2.5 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 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 which bridges the chemical interaction between two substances has been an important area of adhesion technology development [32–35]. 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.

Graphic

Figure 1.5 Hydrolysis of an organofunctional silane and reaction of a hydrolyzed organosilane with a hydroxyl functional substrate (Adapted from [4]).

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 Theory

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 interaction bond strengths (8 to 25 kJ/mol) (Table 1.6). Recent evidence suggests that hydrogen bond strengths (4 to 188 kJ/mol) approach the range of covalent bond strength (147 to 628 kJ/mol) [36]. Many common synthetic and bio-based adhesives such as epoxies, polyurethanes, acrylates, proteins, and starch-based resins have strong hydrogen bonding functionalities. The new bond strength data elevate the importance of hydrogen bonding in regards to the chemical bonding theory of adhesion.

Table 1.6 Bond strengths of various types of chemical bonds and intermolecular forces.*

*Aadapted from Gardner et al. [2];

**Gilli and Gilli [36].

1.2.6 Acid-Base Theory

Based on the correlation of acid-base interactions by Drago et al. [37], Fowkes and Mostafa [38] 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 summarize the Lewis acid-base concept in wetting-related phenomena. According to Fowkes [39] and van Oss et al. [40], the total work of adhesion (Wa) in interfacial interaction between solids and liquids can be expressed as the sum of the Lifshitz-van der Waals (LW) and the Lewis acid-base (AB) interactions, viz.

(1.8)

Graphic

The separation of the work of adhesion into LW and AB components is also applicable to the surface free energies according to:

(1.9)

Graphic

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 [41, 42]:

(1.10)

Graphic

Hence, if the θ is determined for both a non-polar and a polar liquid, with known γLW parameters, on the same surface, then WaLW and WaAB can be determined using equations (8–10).

The acid-base theory plays a critical role in surface chemistry and adhesion and it has been exploited broadly on different materials [43–45].

1.2.7 Weak Boundary Layers Concept

Bikerman [46] first introduced the concept of a weak boundary layer (WBL) in adhesion science. Three different classes of WBLs were specified, i.e., air bubbles, impurities at the interface, and reactions between components and the medium. Good [47] further implicated a WBL on the surface of adherends for the lower mechanical strength. The interface is the location of adhesion failure of a bonded assembly when a weak boundary layer 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 weak boundary layers is not an issue. However, in many bonding situations, a freshly prepared, clean adherend surface may not be possible, and this is especially relevant in the use of adhesives in the human body. It simplifies our understanding of weak boundary layers to categorize them as being mechanical or chemical in nature (Figure 1.6).

Graphic

Figure 1.6 Characteristics of mechanical and chemical weak boundary layers.

Mechanical weak boundary layers can arise from improper machining and lack of cleaning of an adherend surface prior to bonding, while chemical weak boundary layers can be attributed to processing aids or lubricants used to prepare a surface. Examples of mechanical weak boundary layers have received considerable attention in the wood adhesion field [3, 48] while chemical weak boundary layers 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 thus are more difficult to be wetted by adhesives.

Adhesives can be formulated to accommodate weak boundary layers in certain bonding situations, but it is recommended to try and remove weak boundary layers prior to bonding if at all possible. A great example of an adhesive group that can tolerate moisture in a wet weak boundary layer 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. In the dental field, implant surface preparation is important in addressing the issue of weak boundary layers [49].

1.2.8 Special Mechanism of Elastomeric-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) as previously discussed regarding the use of Band-aids® in wound dressing. Contact bond adhesives are represented by the extrudable construction adhesives, caulks and sealants that are highly viscous and also form relatively instantaneous semi-structural bonds. The major differences between the pressure-sensitive and contact bond adhesives are the bond strength of the adhesive and the length of time required to hold a bond [5].

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 [5, 7]. 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.

Graphic

Figure 1.7 Behavior of a tacky (sticky) elastomeric adhesive used in bonding wood.

(Photo courtesy Justin Crouse, University of Maine).

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 also will need to consider other adhesion characteristics including surface tension, wettability, mechanical interlocking, etc., in creating proper adhesion with a substrate. It is suggested that the concept of stickiness or tackiness deserves to be considered among adhesion mechanisms.

1.3 Summary

At present no practical unifying theory describing all adhesive bonds exists, although a unified adhesion theory was proposed twenty-five years ago [50]. However, adhesion phenomena are too complex in terms of the materials to be bonded and the diversity of bonding conditions encountered to be simplified into a single theory [6]. Understanding adhesion requires an intimate knowledge of the bulk and surface material properties of the particular adherend to be bonded as well as the material property behavior of the particular adhesive used in the bonding process. The length scale over which practical adhesion occurs also impacts the evaluation and study of adhesive bonding. Adhesion mechanisms relying on entanglement occur over a wider length scale than those relying only on charge interactions. The study of adhesion theories has and will continue to be an important topic for researchers, and practitioners of adhesive bonding.

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Chapter 2

Wettability of Powders

Emil Chibowski*, Lucyna Holysz and Aleksandra Szczes

Department of Physical Chemistry-Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, Lublin, Poland

*Corresponding author: [e-mail]

Abstract

Wettability of solid surfaces plays a crucial role in many industrial processes and everyday life. Water is the liquid which most often is involved in surface wetting. The surfaces which are well wetted by water are termed hydrophilic in contrast to those poorly wetted called hydrophobic. Wetting contact angle of a droplet is an important parameter to determine hydrophobic/hydrophilic character of the surface. Then using the measured contact angles the solid surface free energy can be calculated which can better characterize the interactions present between the surface and the liquid. While in case of a flat solid surface measurement of contact angle does not present a great problem, but in the case of powdered solids it does and as a consequence determination of surface free energy of such solids is a problem too. In this chapter first the fundamentals of wetting processes are described and then the most common methods for contact angle and surface free energy determination on powdered solids are discussed.

Keywords: Wettability, powders, capillary rise, contact angle, surface free energy

2.1 Introduction

In a general sense, wetting is a physical phenomenon that relies on displacing a fluid (liquid or gas) from a solid or immiscible liquid surface by other liquid during its spreading. In most practical cases wetting deals with solid/liquid systems. Whether a given liquid wets or not a solid surface depends on the force (energy) balance between the liquid and the solid, i.e. the balance between cohesion forces in the liquid and the solid/liquid adhesion forces. If the cohesion forces are larger than the adhesion ones, the liquid does not spread over the solid surface but forms a droplet with a defined wetting contact angle. It is the angle between the solid surface and the tangent to the liquid droplet at the three-phase solid/liquid/gas contact line and is measured through the liquid phase. If the contact angle is less than 90 degrees