The determination of solid and solid-liquid surface tensions is of importance in a wide range of problems in pure and applied science. There exist, at present, many indirect approaches for obtaining these values because it is not possible to measure directly surface tensions involving a solid phase. These various methods are often in considerable disagreement, both quantitatively and from a theoretical standpoint. The problem persists since most of these approaches have not been tested objectively through the prediction of physical phenomena which could be independently observed and thus used to validate the various theories.

The accurate measurement of contact angles is essential in many areas of applied surface thermodynamics. As was seen in Chapters 3 and 5, the contact angle provides a unique means of determining solid-vapor and solid-liquid surface tensions. The range of applications of this measurement is remarkable, both as a simple tool to assess, for example, the cleanliness of surfaces, and as a highly sensitive scientific measurement aimed at providing information on the solid surface tension and the physical state of the surface. When first encountered, the measurement of contact angles appears to be quite straightforward. This apparent simplicity is, however, very misleading, and experience has shown that the acquisition of thermodynamically significant contact angles requires painstaking effort. This chapter addresses the many practical issues pertaining to the measurement of contact angles and liquid surface tensions, including the preparation of suitable solid surfaces and measuring liquids.

In the vast majority of contact angle studies in the literature, the method used is direct measurement of sessile drops. Recent developments in image analysis and processing have increased the accuracy and reduced the subjectivity considerably (see Chapter 10). Nevertheless, there are certain limitations which leave room for other techniques. One of the limitations of the classical sessile-drop method is that the camera or imaging device will be focused on the largest meridian section, and hence reflect only the contact angles at the point in which the meridian plane intersects the three-phase line. Surface heterogeneity and/or roughness could well cause variations of the contact angle along the three-phase line. An alternate approach is to infer the contact angle from the drop contact diameter (see Chapter 10). But even on very good solid surfaces, on which such effects are absent, there is a systematic problem with this method: contact angles will change as a function of drop size for drops up to approximately 1 cm diameter. While this dependence can indeed be used to determine line tension (see Chapter 4), this and other similar effects interfere with the interpretation of contact angles in terms of surface energetics (see Chapter 3).

Numerous methodologies have been developed for the measurement of contact angles and surface tensions as outlined in Chapter 8 and Refs. 1-4. Liquid surface tension measurements commonly involve the determination of the height of a meniscus in a capillary, or on a fiber or a plate. Contact angles are most commonly measured by aligning a tangent with the profile of a sessile drop at the point of contact with the solid surface. Other notable methods are the Wilhelmy slide (Chapter 8) and the capillary rise technique (Chapter 9). An overview of such techniques reveals that in most instances a balance must be struck between the simplicity, the accuracy, and the flexibility of the methodology.

The interfacial energetics and wettability of small particles are of technological interest in many areas of applied science. Areas where such phenomena are important include the preparation of stable suspensions of particles (e.g., colour pigments in paints), the adhesion of particles to solid surfaces in various scenarios (e.g., lubrication), the dispersion of particles into a liquid or melt of a polymer, and the modification of particle surface properties through the adsorption of polymeric macromolecules or surfactants. The successful manipulation of the process being considered is largely determined by the physicochemical surface properties of the interacting surface components, and particularly the wettability and the surface (or interfacial) tension of the particles. The complexities of contact angle phenomena and surface tensions were discussed in Chapter 3.

The reversible work of adhesion (W) is the free energy change per unit area in creating an interface between two bodies (Fig. 1). The work W is related to the intermolecular forces that operate at the interface between two materials, eg, an adhesive and an adherend. However, in practice, the reversible work of adhesion may be obscured by other factors (eg, mechanical interlocking, interdiffusion) because it is always a few orders of magnitude lower than the measured adhesive joint strength [1, 2]. One important contribution to practical joint strength is the energy loss due to irreversible deformation processes within the adhesive. Nevertheless, Gent and Schultz [3] showed using peel strength measurements that viscoelastic losses were proportional to the reversible work of adhesion. For this reason, one of the important tasks is to determine the nature of interfacial chemical and physical forces and to understand how they control the reversible work of adhesion.

In recent years, we have witnessed a remarkable growth in the use of the synthetic organic polymers in technology, both for high-technology and for consumerproduct applications (see Fig. 1 [1]). Polymers have been able to replace more traditional engineering materials such as metals, because of their many desirable physical and chemical characteristics (high strength-to-weight ratio, resistance to corrosion, etc.) and their relatively low cost. However, fundamental differences between polymers and other engineering solids have also created numerous important technical challenges, which manufacturing operations must overcome. An important example is the characteristic low surface energy of polymers and their resulting intrinsically poor adhesion [2–6]; the term “adhesion,” as it is used here and elsewhere in this text, may be briefly defined as the mechanical resistance to separation of a system of bonded materials [7]. Because adhesion is largely a surface property, often governed by a layer of molecular dimensions, it is possible to modify this near-surface region without affecting the desirable bulk properties of the material.

Good adhesion to polymers is required in a number of important technologies including adhesive bonding, printing, and painting. To achieve a satisfactory level of adhesion it is often necessary to pretreat the polymer by one of a wide range of methods. Two books are of particular interest [1, 2]. In the case of polar polymers such as nylon 66 and epoxide thermosets, a treatment may not be necessary, or if the surfaces are contaminated, a physical method such as solvent degreasing or grit blasting to remove the contaminants may be all that is required. On the other hand, if a polymer lacks suitable functionality, it will be necessary to modify its surface chemically. Polymers with no active functionality include low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). A wide variety of methods for introducing new groups is available, including the use of low-pressure plasmas, corona discharges, flames, etchants, and active gases. Flame treatment to enhance adhesion to polymers has been used since the early 1950s, one of the first applications being to enhance print adhesion to lowdensity polyethylene. Since that time, flame treatment has been used with many other polymers in a variety of applications. Flame treatment has a number of advantages over the other main method of treating large areas of polymers, ie, the corona treatment. These include no reverse-side treatments, no creation of pinholes, no Ozone production, and better aging characteristics.

The three states of matter are solid, liquid, and gas. A plasma state exists as its fourth state. A plasma consists of positively charged particles and negatively charged electrons existing at almost the same electrical density, it is overall electrically neutral, and it was named plasma by Langmuir in 1928. The easiest way to obtain a plasma state is to induce an electrical discharge in a gas. A corona discharge treatment is a kind of plasma treatment. Plasmas are classified roughly into two categories: equilibrium plasmas and nonequilibrium plasmas. In equilibrium plasmas, the temperatures of electrons and of the gas are the same. Mainly equilibrium plasmas have been studied, and temperatures of approximately 10,000 C have been reported. In nonequilibrium plasmas the gas is at ambient temperature, but the temperature of electrons is very high (about 10,000 C). These nonequilibrium plasmas are used in chemical applications and are called low-temperature plasmas or cold plasmas. The low-temperature plasmas are classified roughly into two categories:(1) ordinary low-temperature plasmas at low pressure and (2) corona discharges at atmospheric pressure. Ordinary low-temperature plasmas are widely used in chemical modification of the surfaces of materials, especially in semiconductor industries [1] as well as for polymers [2].

By now, the effect of microorganisms on polymer materials has been well studied. However, most of the investigations were aimed at polymer protection against biocorrosion or, on the contrary, biodegradation of polymer wastes. Using microbial treatment for polymer adhesion improvement was initiated only in the past decade. Nevertheless, such treatment, being a variant of chemical surface modification, has a number of advantages in comparison with other known treatment techniques: It needs no expensive chemicals and solvents. It is conducted at moderate temperatures and needs no energy expenditure. It is ecologically clean. Because of the great variety of existing microorganisms, it can offer the desired degree of treatment for different polymer materials.

Most printing inks used in major printing processes such as lithography, gravure, and flexography contain organic solvents. Some of these solvents, e.g., toluene, are toxic and can be harmful to humans. Both environmental and workplace safety considerations exert growing pressure on the printing industry to limit the use of toxic organic solvents. Due to this pressure, the share of water-based liquid printing inks in packaging printing (including corrugated) has achieved a respectable level of about 50%. In newspaper printing the share is estimated at about 10 to 15% of the total. To prepare for a possible ban on the use of toxic organic solvents, the printing industry is exploring the viability of water-based technology.

Adhesion phenomena are relevant to many scientific and technological areas and in recent years have become a very important field of study. The main application of adhesion is bonding by adhesives, which is replacing, at least partially, more classical mechanical attachment techniques such as bolting or riveting. It is considered to be competitive primarily because it saves weight, ensures better stress distribution, and offers better aesthetics because the glue line is practically invisible. Applications of bonding by adhesives can be found in many industries, particularly in advanced technological domains such as the aeronautical and space industries, automobile manufacture, and electronics. Adhesives have also been introduced in areas such as dentistry and surgery. However, adhesive joints are not the only application of adhesion. Adhesion is concerned whenever solids are brought into contact, for instance, in coatings, paints, and varnishes; multilayered sandwiches; polymer blends; filled polymers; and composite materials. Because the final performance or use properties of these multicomponent materials depend significantly on the quality of the interface that is formed between the solids, it is understandable that a better knowledge of adhesion phenomena is required for practical applications. The field of adhesion began to create real interest in scientific circles only about 50 years ago. Thus, adhesion became a scientific subject in its own right, but it is still a subject in which empiricism and technology are slightly ahead

Adhesion, whether the bonding of polymers or the adhesion of coatings to polymer surfaces, is a recurring and difficult problem for all industries that use these materials as key components in their products. Designers must often select specially formulated and expensive polymeric materials to ensure satisfactory adhesion (albeit even these materials often require surface preparation). In some cases, entire design concepts must be abandoned due to the prohibitive cost of the required polymer or the failure of crucial bonds. Historically, surface treatments to improve adhesion of coatings to plastics consisted of mechanical abrasion, solvent wiping, solvent swell that was followed by acid or caustic etching, flame treatment, or corona surface treatment. Each of these treatments has limitations, thus providing a strong driving force for the development of alternative surface preparation methods. Many of the common methods mentioned are accompanied by safety and environmental risks, increased risk of part damage, and expensive pollution and disposal problems.

A study on metal-polymer interface formation following an in situ plasma treatment is presented. The plasma treatment is performed in a dual frequency ECR plasma. This enables to control some of the main plasma parameters. The study is focused on a model system consisting of a polypropylene substrate and a magnesium metal overlayer. Due to large variations in the interface properties depending on the surface treatment, this system allows deeper insight in the interface formation.

Knowing that the oriented positive ion bombardment plays an important role in the plasma treatments of polymers, some investigations using a positive ion beam-plasma system were carried out. Preliminary results concerning the surface modifications of poly (ethylene terephthalate) films induced by the action of oxygen ion beam are presented. Ion energies (50-500 eV) and doses (3.0 x1015 1.5 x1016 ions/cm²) are those used in a reactive ion etching device. Techniques such as: determination of the surface free energy components by the contact angle method, thermal methods (DTA, DSC, etc.), IR spectroscopy, SEM, XPS, were used to characterize the surface modifications. The relation between chemical and physical modifications is discussed.

Ion bombardment of polymer surfaces is a method used in promoting metal/polymer adhesion. The adhesion of these multicomponent interfaces can be attributed to chemical bonding, physical bonding, or a combination of both. By evaluating the resistivity of thin films of Cr, TaSiz, Pd and Au deposited on polymer, the interfacial reactivity was determined, and the contribution due to chemical bonding identified. The adhesion strength of these interfaces, determined by peel strength measurements, increases with interfacial reactivity. Interfacial reactivity increases with the total energy of all the ions bombarding the polymer surface (dosage). Cr and TaSi₂ show extensive interfacial reactivity than noble Au and Pd.

A basic understanding of rheology and surface chemistry, two primary sciences of liquid flow and solid-liquid interaction is necessary for understanding coating and printing processes and materials. A generally qualitative treatment of these subjects will suffice to provide the insight needed to use and apply coatings and inks and to help solve the problems associated with their use. Rheology, in the broadest sense, is the study of the physical behavior of all materials when placed under stress. Four general categories are recognized: elasticity, plasticity, rigidity, and viscosity. Our concern here is with liquids and pastes. The scope of rheology of fluids encompasses the changes in the shape of a liquid as physical force is applied and removed. Viscosity is a key rheological property of coatings and inks. Viscosity is simply the resistance of the ink to flow-the ratio of shear stress to shear rate. Throughout coating and printing processes, mechanical forces of various types and quantities are exerted. The amount of shear force directly affects the viscosity value for non-Newtonian fluids. Most coatings undergo some degree of" shear thinning" phenomenon when worked by mixing or running on a coater. Heavy inks are especially prone to shear thinning. As shear rate is increased, the viscosity drops, in some cases, dramatically. This seems simple enough except for two other effects. One is called the yield point. This is the shear rate required to cause flow. Ketchup often refuses to flow until a little extra shear force is applied. Then it often flows too freely. Once the yield point has been exceeded the solidlike behavior vanishes. The loose network structure is broken up. Inks also display this yield point property, but to a lesser degree. Yield point is one of the most important ink properties.

The idea of the treatment of textiles with plasma is a few decennia old. There is no consensus about who was “the first,” but it is clear that the treatment of textiles is historically linked to the plasma treatment of polymers in general. As one of the most promising alternatives in many fields, the importance of plasma treatments results from the exceptional advantages it offers. It does have specific action only at the surface, keeping the bulk properties unaffected. The future of plasma is closely linked to the fact that this technique gives the treated surface some properties that cannot be obtained by conventional techniques, and this is without the need to use water as a reaction medium. At the level of textiles, this means changing an almost inert surface into a reactive one, and in this way, it becomes a surface engineering tool. The transfer of research results into the technological field would lead to nonpolluting and very promising operating conditions. In the prospect of chemical finishing using plasma, two main methods can be considered: grafting of a compound on the fiber or surface modification by means of discharges.

Any review on the shape of a liquid droplet on top of a solid surface has to start with the pioneering work by P.S.Laplace and Sir Thomas Young almost two centuries ago [1, 2]. Young and Laplace set out to describe the phenomenon of “capillary action” in which the liquid inside a small capillary tube may rise several centimeters above the liquid outside the tube [3]. To understand this effect, two fundamental equations were derived by Young and Laplace. The first equation, known as the Laplace or Young-Laplace equation [1], relates the curvature at a certain point of the liquid surface to the pressure difference between both sides of the surface, and we consider it next in more detail. The second equation is Young’s equation [2], which relates the contact angle to the surface tensions involved.