The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch^® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.

One of technologically imminent problems related to the use of pressure sensitive adhesives (PSAs) in the LCD industry is how to properly control the surface properties of various polymeric films used in devices to obtain sufficient bond strength with PSAs. To provide practical solutions to this issue, we used two types of surface treatments, NaOH and corona, to control the surface properties of polymeric films that are widely used in the LCD industry. Here we report a significant increase in surface tension in triacetyl cellulose (TAC) and discotic liquid crystal (D-LC) films along with a remarkable enhancement of bond strength in TAC/PSA and D-LC/PSA systems. The major portion of surface tension increase, in both types of films, was found to be due to an increase of polar component. The continuous increase of OH functionality in TAC with NaOH treatment time supported this observation. Furthermore, we established a map of surface treatment by studying the sequential effects of the two treatments, and based on this, we clearly demonstrated that each treatment had its own limiting value that could not be altered regardless of the sequence of surface treatment.

This study focused on the behavior of the paper-polypropylene-paper (PPP) laminate while aging in hot oil in the absence of voltage stress. The results provide an understanding of both the quality of the interfacial bond and the performance of this bond during service. X-ray photoelectron spectroscopy performed on two different peeled laminates suggest that the bond failed primarily adhesively. Weibull statistical analysis of the peel strength data obtained on unaged laminates and those aged in polybutene oil at 90°C for 120 hours showed that the strength loss is consistent with one failure mechanism and the failure rate increases with applied stress. For the aged sample, Weibull analysis results are consistent with the prior loss of peel strength due to the aging. Experiments on the solubility of the oil show that lamination reduces the amount of absorption in comparison to the unlaminated composite. Swelling experiments on the individual components show differential swelling between the paper and polypropylene to be the source of the strength loss. The polypropylene swells, and the paper shrinks. Measurements on the laminate show that both paper and polypropylene shrink, indicating that the paper governs the laminate swelling process. During aging, the differential swelling generates internal stresses on the interface. In addition to yielding the magnitudes of these stresses, finite element analysis also predicts plastic deformation and creeping of the polypropylene as well as tensile stresses between the paper and polypropylene at a free edge. Very likely these processes damage the bond and contribute to the loss of bond strength.

Contact angle measurement has wide application in studying the wettability of a surface. This paper presents a contact angle measurement system developed using simple apparatus. The system consists of a bespoke measurement software, USB microscope, motorized linear position slider and a sample holder with back lighting system. The advantages of this system include user friendly, compact size, allow manual and automatic measurements and cost effective. This system is established with the contact angle and surface tension measurement experiment which is based on Fox-Zisman theory. Different probe liquids were suggested and the critical surface tension of polydimethylsiloxane (PDMS) and polyimide were determined using both the software and the hardware system developed.

An RTV silicone and high density polyethylene are exposed in an activated gas plasma for varying times and varying conditions. Both oxygen and argon are used. Changes in critical surface tension of wetting as determined by contact angle measurements are reported. Bondability of the treated surfaces is evaluated with both the aged bonds and aged surfaces prior to bonding being evaluated. In contradiction to some of the recent work reported in the literature on the effect of activated inert gas on surface characteristics, contact angles always decreased on the materials studied indicating an increase in surface energy. The significance of the results on present adhesion theories is discussed.

In atmospheric pressure corona systems, the densities of electrons and ions determine the level of treatments. Here, the electron and ion densities in a corona plasma are evaluated for a DC positive-polarity wire discharge in dry air at atmospheric pressure, in the coaxial wire-cylinder geometry. We use a new numerical iterative approach to solve the coupled equations for the electric field and charge densities. The role of electron diffusivity is discussed and the influence of the charge distribution between electrodes on the electric field strength and on the plasma region is analyzed.

Many approaches have been used to infer the surface tension of solids from liquid contact angles. In most cases the different methods have not been verified by independent means because of the inherent difficulty in directly measuring a solid surface tension. This paper examines a range of diverse experiments which, together with appropriate thermodynamic models, permit such an independent verification to be made.

As part of an ongoing study, the focus has been on two methods of interpreting contact angles which often yield conflicting results; namely, the equation of state approach and the theory of surface tension components. The previous work has led to the conclusion that the latter approach is incorrect. In this paper the accuracy of these two methods is examined in a strictly empirical way through the interpretation of a wide body of experimental results. It is seen that the predictions of the equation of state approach are in much closer agreement with the various experiments than are those derived from the Fowkes equation.

Striking a balance between applied and theoretical research, this work details many of the uses of wettability and interprets experimental data from a variety of viewpoints, including the ‘separation of forces’ and the ‘equation of state approaches.’

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.

The behavior of a silicone sealant during its application, from its extrusion from the carнtridge to the completion of the joint, has been identified as a key feature for professional applicators. This feature called “ease of use’* is very complex and includes many different criteria such as the ease of extrusion and joint smoothing, the aesthetic of surface finish, the stringing, and the action of a tooling aid (aqueous solution of surfactant) during the smoothing operation. Several of these criteнria seem directly linked to surface properties of the uncured sealant. In an attempt to translate these subjective properties into quantitative measurements in the laboratory and to understand the underнlying parameters that can be used to control these features, the surface energy of uncured sealants was measured using the solid-liquid contact angle technique. The surface energy data were further correlated with ratings collected from professional applicators with regards to ease-of-use criteria. A correlation was also built between the contact angle values obtained with various tooling aid soluнtions against the surface of the sealant and the ease of smoothing obtained by using these tooling aids at the application stage. The evolution of the contact angle of a water droplet at the surface of uncured sealant with time provided some insights in the understanding of the migration and/or reнorientation of polar entities from the sealant bulk to the sealant surface.

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.

Gravity-induced sagging can amplify variations in goniometric measurements of the contact angles of sessile drops on super-liquid-repellent surfaces. The very large value of the effective contact angle leads to increased optical noise in the drop profile near the solid–liquid free surface and the progressive failure of simple geometric approximations. We demonstrate a systematic approach to determining the effective contact angle of drops on super-repellent surfaces. We use a perturbation solution of the Bashforth–Adams equation to estimate the contact angles of sessile drops of water, ethylene glycol, and diiodomethane on an omniphobic surface using direct measurements of the maximum drop width and height. The results and analysis can be represented in terms of a dimensionless Bond number that depends on the maximum drop width and the capillary length of the liquid to quantify the extent of gravity-induced sagging. Finally, we illustrate the inherent sensitivity of goniometric contact angle measurement techniques to drop dimensions as the apparent contact angle approaches 180°.

Electret charging methods on the basis of gas discharge in air offer many advantages, including a very simple arrangement, no direct contact to the electret surface, and no restrictions on charging temperature. For discharge in air two arrangements are in use: (1)discharge in a parallel air gap and (2) corona discharge. A comparison of both methods, showing significant similarities, is given.

Starting with investigations of charging in a parallel air gap, the practical knowledge is applied to the more complex corona charging. The characteristics of equilibrum electret voltage and its dependence on applied voltage are measured and interpreted theoretically. The influence of deviations in gap spacing on electret voltage is discussed. Electrical breakdowns of the electret foil affect the results. Therefore the role of breakdown is investigated in more detail.

The spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid–vapor interfacial tension and hydrophilize the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations.