The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γ_s, is reviewed. The basis for the three components γ^LW_s, γ⊕_s, and γ⊖_s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γ^p') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, π_e. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ^⊖ and γ^⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γ^LW at the hydrophobic corner, and γ^⊕ and γ^⊖ at the two hydrophillic corners, is suggested.

Wenzel’s modification of Young’s equation for contact angles, equation (1), may be derived from considerations of free surface energy, though not from the assumption that surface “tensions” may be represented by vectors. A theory is presented for the hysteresis of contact angles. The “driving force” toward the attainment of an equilibrium contact angle is found to be equal to the surface tension of the liquid times the deviation of the cosine of the contact angle from its equilibrium value. It is shown that this may be equated to the “contortional energy” Fc that the drop must have in order for its edge to surmount a ridge. The result is in the same form as the equation of Adam and Jessop: í-(-yi— 72)= 72 cos 6,, r± Fc, but with a new and physically meaningful interpretation of Fc.

A RECENT communication by Gray¹ illustrates a possible pitfall in the use of the theories of Fowkes^2–5 and Good and Girifalco^6,7 to estimate surface energies, and the various components of surface energy, from contact angles. This source of error is the incorrect identification of the surface tension terms, and the equating of the contact angle in a contaminated, experimental system to that in a system composed of properly pure components. Thus, Gray wrote Fowkes's equation in the form

We wish to propose that the surface entropy of a liquid may be taken as a criterion of surface orientation. Orientation in the surface will lead to a lower entropy than that in the condition where the surface molecules are disordered. The question is, first, how much lower is the surface entropy of polar substances than that of non-polar substances? And second, can we set up a simple model which will account for the lower entropy of polar liquids, as resulting from surface orientation?

Ramsay and Shields4 reached the conclusion empirically that there was a “normal” value (2.1) for the Eötvös constant, which is directly related to the molar surface entropy.^6-7 From various theo-retical studies,^6-9 particularly that of Born and Courant, it might be expected that there should be a “normal” value for nearly spherical non-polar molecules. (The extension of this concept to non-spherical molecules cannot be made very simply, because the number of molecules “in the surface” per unit area depends on the degree of orientation as well as the ratio of length to thickness.⁷) The hypothesis of Ramsay and Shields, that the degree of association could be calculated from the ratio of the observed Eötvös constant to the “normal” value, 2.1, has of course long sincebeen discredited;⁷ but it persists in textbooks and the literature, probably because of the lack of a plausible alternative. We will show that surface orientation furnishes a much more reasonable explanation.

This chapter, and the following one by Neumann and Good, deal with the measurement of contact angles in three-phase systems. The contact angle is, intrinsically, a macroscopic property, and one that should be amenable to a phenomenological treatment, e.g., to measurement without regard to its thermodynamic or microscopic interpretation. But inevitably, the desire for a thermodynamic and molecular interpretation arises. In addition, some fundamental questions about solid surfaces come up, and the problem of hysteresis must be given explicit consideration. So there is a need to go beyond phenomenology.

It has been found that if a film of a polymer such as polypropylene or Teflon FEP is oriented by stretching, the contact angle of liquid becomes anisotropic, being higher for liquid front advancing or retreating perpendicular to the direction of stretch than for advance or retreat in the parallel direction. Electroscanning microscopy has revealed a small degree of anisotropy of the surface roughness. Comparison with samples that have been abraded with a single stroke of abrasive paper of various grit sizes showed that a far greater degree of anisotropic roughness would be required, to produce the observed contact angle anisotropy and hysteresis, than is actually observed on the stretched samples. It is concluded that the observed contact angle anisotropy is probably due to the anisotropic force field of the oriented polymer molecules.

The dependence of cell or bacteria adhesion and growth on the polarity of a polymer substrate, as controlled by the composition of a HEMA-EMA copolymer, has been studied by contact angle measurements. These have been analyzed by the acid/base hydrogen bonding methodology of van Oss, et al. It was found that adhesion and growth of mouse 3T3 cells occurred on surfaces for which the acidic parameter, γ^⊕_S, was negligibly small. This was the case above 50% EMA, for which γ^p was zero, and both attachment and growth occurred. The γ^⊕ parameter was appreciable, but approximately constant, independent of composition of the copolymer. The acid/base theory thus supplants the simple polar-nonpolar (γ^pandγ^d) hypothesisin regard to cell adhesion. A new 3-dimensional representation of hydrophilic/hydrophobic behavior is suggested, to implement the acid/base description.

Contact angle hysteresis on rough surfaces is caused by the contortion of the liquid surface that must occur as the liquid front passes from one metastable configuration to another. We have combined the Wenzel equation for the effect of roughness on the contact angle, 0, with the well-known equation relating contact angles to the surface free energy of the solid and of the liquid, and with Good’s hypothesis of a free energy barrier to liquid front motion. The method that is developed calls for measuring 0 for a series of liquids and plotting cos0a vs./yi and extrapolating to the limit of (\/yi) 0. On a perfectly smooth, homogeneous surface, the intercept is—1 and the Wenzel ratio for a rough surface is given, approximately, by the negative of the value of the intercept. A shift of the yc value for the solid, due to roughness, is also predicted. Experimental data are presented for measurements with Teflon FEP.

The theory of the apolar components of interfacial forces was examined in the previous chapter of this volume.(1) It has been possible to develop that theory of apolar components at this time owing to the existence of quantitative, mathematically formulated theories of forces between molecules (e.g., the London theory) together with the Lifshitz electromagnetic theory of the interaction of macroscopic bodies. (See the previous chapter for references.)

The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γ^LW, γ^⊕ and γ^⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed.

The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ^⊕ and γ^⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ^⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m². The acidic component, γ^⊕, rose from zero to as high as 1.9 mJ/m². Its decay with time was only qualitatively the same as that of γ^⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data.

The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.

A method has been devised to determine the acid/base parameters of reference liquids as absolute numbers, and not as values relative to a conventional set of parameters for water. Contact angle measurements are employed, using three liquids on three solids. The theory calls for the solution of nine simultaneous, nonlinear equations in nine variables–and unreasonably formidable task.

A preliminary set of solutions has been computed, for one set of polar liquids on five solids. These results must be rejected on grounds of physical reasonableness. They also fail the test of predicting liquid-liquid interfacial tension, which for miscible liquids must be negative or zero.

We owe a great debt to W. A. Zisman and his colleagues at the Naval Research Laboratory for their extensive, pioneering work that opened up the field of contact angles and made possible the development of the modern theory of wetting and adhesion. Their data on the wetting of solids by apolar liquids and by hydrogen bonding liquids pointed the way to the recent introduction of a theory of hydrogen bond interactions across interfaces. We will devote this chapter to a review of this new theory.

The contact angle of CH₂I₂, α-bromonaphthalene and aniline on crosslinked polystyrene has been measured. The polymer was swollen for as long as 60 days in the liquid whose contact angle was to be measured. The surface free energy, γ_s, of unswelled polystyrene was estimated from the contact angles and the swelling results, using an equation which had previously been proposed by Good. The value of γ_s is estimated to be 42±2 ergs/cm².

1. It is to be expected that the hysteresis of contact angles on polystyrene would be the effect of a number of simultaneous causes. Whatever the interpretation, the differences in the observed angles found with surfaces that had been formed in different ways point to structural differences among the samples; and the existence of structural differences would indicate that all the surfaces depart appreciably from ideality, e.g., from ideal flatness, rigidity, and homogeneity.

2. We have shown that effects due to film pressure, πe, would be in the opposite direction from those observed in hysteresis. Hence film pressure is not responsible for contact angle hysteresis.

A theory is proposed by which the surface free energy of certain solids can be estimated from the contact angles ofliquids on them. The method is verified using contact angle and surface tension data from the literature, for benzene and a-methylnaphthalene on liquid and solid fractions of a fluorinated lubricating oil. The method is then applied to data on the contact angles of various liquids on Teflon and on an octadecylamine monolayer. The surface tensions of these solids are estimated to be, respectively, about 28 and 30 ergs/cm…

The theory of the apolar components of interfacial forces was examined in the previous chapter of this volume.(1) It has been possible to develop that theory of apolar components at this time owing to the existence of quantitative, mathematically formulated theories of forces between molecules (e.g., the London theory) together with the Lifshitz electromagnetic theory of the interaction of macroscopic bodies. (See the previous chapter for references.)

We have established experimentally that, for certain liquid-solid systems, the contact angle formed by a drop of liquid varies with drop size, below a critical diameter which is probably a function of the nonuniformity of the solid surface. This effect has been observed with water on Teflon FEP and on polymethylmethacrylate (PMMA), in regard to both advancing and receding contact angles. The decrease was about 8° in θ_a and 16° in θ_r for water on Teflon FEP, between diameters of 4 mm and about 1 mm; and for other systems, a comparable or even greater effect was observed. For water on PMMA, θ_r decreased from 51° to 26°, between diameters of 8 and 3 mm. With ethylene glycol on Teflon FEP, the decrease in θ with drop size was observed in the retreating angle only. With n-decane on Teflon FEP, the contact angle was independent of drop size, between diameters of 1 and 12 mm. For all liquid-solid systems studied, the limiting contact angles for large drops were in good to excellent agreement with the values obtained by the vertical plate method. Qualitatively, this effect could be explained by hypothesizing the existence of a large, negative line tension. It was found that there was quantitative disagreement between observed results and predictions based on this hypothesis. A theory is proposed to explain the experimental results, based on the known heterogeneity of many polymer surfaces and a previously discussed theory of hysteresis. A negative pseudo-line tension can be used in a phenomenological description of the motion and contortion of the three-phase line.

Three methods of interpretation of contact angles in terms of surface tension of liquid and solid are correlated. It is shown that the critical surface tension for wetting γ_c is dependant in a specific way on the nature of the liquid system used regardless of the nature of the solid. Equations are derived which express the limiting values γ_c may assume for a liquid/solid system. γ_c is also correlated with the Good-Girifalco treatment of contact angles and experimentally determined values of the parameter φ are compared with theoretical calculations from molar volumes of normal alcohol-paraffin wax interfaces.

The Fowkes method of contact angle interpretation is used to derive values for the polar and dispersion force components of liquid surface tensions for three alcohol/water solution series and two organic acid/water solutions.

The effects of exposure to [72 nm ultraviolet (UV) excimer light in ambient air on the wettability and surface free energy of polymer films were investigated from contact angle measurements. The polymer films used were polyethylene (PE), polypropylene (PP), poly (ethylene terephthalate)(PET), nylon 6 (Ny6) and polyimide (Pl). As a measure of the wettability, the water contact angle was determined by the sessile drop and the Wilhelmy methods. For all films, considerable increase in wettability was accomplished by UV exposure within a few tens of seconds. After the UV exposure, a decrease in the wettability, the hydrophobic recovery, was observed over a time period of several days. Even after the recovery, the wettability was sufficiently higher compared to that before the UV exposure. The Lifshitz-van der Waals component and Lewis acid-base parame-ters of the surface free energy of the films were determined by contact angle measurements using certain probe liquids. The base parameter was found to increase considerably by the UV exposure. XPS analysis and AFM observation of the film surfaces showed that such increases in the wettability and the surface free energy were due to the increased atomic oxygen concentration at the film surfaces. The effect of the UV exposure on particle deposition onto PP and PET in water was also examined using spherical polyethylene and nylon 12 particles. The apparent equilibrium number of particles deposited on the polymer substrate decreased drastically after UV exposure. The particle deposition behavior was explained well in terms of the free energy change due to deposition, which was calculated from various surface free energies.

The poly(ethylene terephthalate), PET, film was exposed to atmospheric pressure plasma under various plasma processing parameters. The wettability of the PET film immediately after the exposure and after storage in air, which was determined by the sessile drop method, was strongly dependent on the plasma processing parameters. The contact angle hysteresis on the plasma-exposed PET film was examined by the Wilhelmy method. It was found that the hydrophobic recovery of the PET surface on storage after the plasma exposure was observed only for the advancing contact angle and that the receding angle remained almost the same. These experimental findings were explained on the basis of the calculation by Johnson and Dettre for the advancing and receding contact angles on model heterogeneous surfaces.

Wettability of polyimide (PI) films modified by atomic oxygen (AO) was investigated by contact angle measurements. The PI films with/without being covered by a metal mesh were exposed to the AO beam with fluences from 1.4x 1016 to 9x 1018 atoms/cm2. The atomic force microscopy (AFM) and the X-ray photoelectron spectroscopy (XPS) were used to characterize the PI film surfaces. Both the roughness and the oxygen concentration at the PI surface increased by the AO exposure. The advancing and the receding contact angles of water on the PI films were measured both by the sessile drop method and by the Wilhelmy method. The contact angles measured by these two methods were identical for the PI samples both with/without AO exposures. In the case of the AO-exposed PI films being covered by the metal mesh, the contact angles were evaluated by the Wilhelmy method. A difference in contact angles on the exposed and the covered areas was clearly observed. It was also found that the wettability of the AO-exposed PI films was related to the amount of oxygen detected by the XPS.

Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films were treated with an atmospheric-pressure plasma (APP) jet and a 172-nm ultraviolet (UV) excimer light in air. The advancing and receding water contact angles on both films decreased after the treatments, especially after APP treatment. After the treatments, the hydrophobic recovery was observed and almost diminished within a week. The dispersive component of the surface free energy of the two polyester films did not change due to the APP and UV exposure, whereas the acid–base component drastically increased after the treatments. The X-ray photoelectron spectroscopy results showed that the polyester film surfaces were oxidized by the treatments. From the AFM images, the topographical change on the film surfaces due to the treatments was clearly observed. It was found that the APP treatment of the PET film prevented the deposition of particulate soils in air due to the decrease in the contact area between the film and the soil particle. Furthermore, the soil release in the aqueous solutions was promoted as a result of the hydrophilization of the polyester films due to the APP treatment.

Effects of the exposure of ultraviolet (UV) excimer light on the physicochemical surface properties of polymer films were investigated by contact angle measurements and X-ray photoelectron spectroscopy (XPS). The UV light at wavelength of 172 nm was exposed to polyethylene (PE), polyimide (PI), and polytetrafluoroethylene (PTFE) films in ambient air. The advancing and receding contact angles of water on the unexposed and UV-exposed films were determined by the sessile drop and the Wilhelmy methods as a measure of the wettability. For the PE and PI films, remarkable decrease in the water contact angle was accomplished by the UV exposure of several or several 10 s. The XPS data showed that such increase in the wettability was attributed to the increased atomic oxygen concentration at the film surfaces. The wettability of the PTFE film did not change due to the UV exposure. When the UV-exposed PE and PI films were stored in ambient air, the increase in the water contact angle, i.e. the hydrophobic recovery, was observed over a time scale of several days. It was suggested that the gasification of the low-molecular weight oxidized materials as well as the reorientation and the migration of polymer chains in the oxidized surface layer was responsible for the hydrophobic recovery in air. The UV exposure was also attempted to the PI film being covered with a metal mesh to prepare the film having both non-exposed and UV-exposed surface regions. The differences in the advancing and receding contact angles between the both regions were observed on the continuous weight recording at constant interfacial moving velocity by the Wilhelmy method. The Wilhelmy method in combination with the UV lithography technique enabled the simultaneous evaluation of the wettabilities of the treated and untreated surfaces.

The present invention relates to the treating of plastic film materials and more particularly to a method and apparatus for improving the exposed surface adhesion qualities of films of plast'c such as polyethylene which have been applied to electrically conductive substrates such as metal foils.

Plasma web treatment is a common practice for promoting adhesion, wettability and other surface or interfacial properties in the conversion industry. While the objective of creating new surface functional groups is conceptually simple, it can be difficult to choose the most appropriate kind and configuration of plasma source, the most appropriate feed gas composition and the most appropriate operating pressure for a given application. Such difficulties arise from the variety of species that can be formed in the plasma and the variety of possible plasma-surface interactions that can occur. A brief review of the importance of various plasma parameters (eg, specific energy, species concentrations, and energy distributions) and an example relating nitrogen uptake in poly (ethylene-2, 6-naphthalate) to plasma diagnostic data in a low-radiofrequency capacitivelycoupled nitrogen plasma are presented. The importance of driving frequency and treatment configuration is discussed in detail. Uptake kinetics for samples treated at floating potential at low radiofrequency is compared with that for samples treated in the cathode sheath. Analysis of the treatment kinetics is based on a simple model of surface saturation. This approach can be used not only to compare practical treatment results as a function of process conditions, but also to compare different treatment techniques in a practical manner.

The present invention is a polyester film base which has a surface approximately 5 nm thick. The surface of the film base has been altered to include about 6 to 15 atomic percent nitrogen in the form of imines, secondary amines and primary amines in the ratio of about 1:1:2. The invention also includes a film base whose surface includes oxygen in the form of hydroxyl, ether, epoxy, carbonyl or carboxyl groups wherein the oxygen is about 4 to 10 atomic percent above the original surface content of the base. The polyester film base can be either polyethylene terephthalate or polyethylene naphthalate.

The present invention is a polyester film base which has a surface approximately 5 nm thick. The surface of the film base has been altered to include about 6 to 15 atomic percent nitrogen in the form of imines, secondary amines and primary amines in the ratio of about 1:1:2. The invention also includes a film base whose surface includes oxygen in the form of hydroxyl, ether, epoxy, carbonyl or carboxyl groups wherein the oxygen is about 4 to 10 atomic percent above the original surface content of the base. The polyester film base can be either polyethylene terephthalate or polyethylene naphthalate.

A method and apparatus are taught for treating paper webs for obtaining the proper surface characteristics to promote adhesion of nonphotosensitive coating materials and/or layers typically coated thereon. The web is passed through a high-voltage sheath region or dark space of the plasma generated by a powered electrode residing in a discharge zone. The frequency of the driving voltage must be above a lower bound dictated by the properties of the paper support and the plasma, and it must be below an upper bound beyond which the sheath voltages drop significantly and it is observed that the benefits of this approach diminish. The dark space is generated by a treatment electrode in a treatment zone. There is a counter electrode having a surface area in said treatment zone which is at least as great as the surface area of the treatment electrode. A power supply is included for driving the treatment electrode with an oscillating high voltage at a frequency less than about 2 MHz and greater than 1/t_c where t_c is the charging time of a web surface exposed to a rms ion current in the plasma.

A method and apparatus are taught for treating polyolefin containing or polyolefin-coated webs or laminates for obtaining the proper surface characteristics to promote adhesion of photosensitive coating materials and/or layers typically coated thereon. The web is passed through a high-voltage sheath region or dark space of the plasma generated by a powered electrode residing in a discharge zone. The frequency of the driving voltage must be above a lower bound dictated by the properties of the paper support and the plasma, and it must be below an upper bound beyond which the sheath voltages drop significantly and it is observed that the benefits of this approach diminish. The dark space is generated by a treatment electrode in a treatment zone. There is a counter electrode having a surface area in said treatment zone which is at least as great as the surface area of the treatment electrode. A power supply is included for driving the treatment electrode with an oscillating high voltage at a frequency less than about 2 MHz and greater than 1/tc where tc is the charging time of a web surface exposed to a rms ion current in the plasma.

Plasma treatment of polymers encompasses a variety of plasma technologies and polymeric materials for a wide range of applications and dates back to at least the 1960s. In this article we provide a brief review of the United States patent literature on plasma surface modification technologies and a brief review of the scientific literature on investigations of the effects of plasma treatment, the nature of the plasma environment, and the mechanisms that drive the plasma–surface interaction. We then discuss low‐radio‐frequency capacitively coupled nitrogen plasmas and their characteristics, suggesting that they provide significant plasma densities and populations of reactive species for effective plasma treatments on a variety of materials, particularly when placing the sample surface in the cathode sheath region. We further discuss surface chemical characterization of treated polymers, including some results on polyesters treated in capacitively coupled nitrogen plasmas driven at 40 kHz. Finally, we connect plasma characterization with surface chemical analysis by applying a surface sites model to nitrogen uptake of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) treated in a 40 kHz nitrogen plasma. This example serves to suggest an interesting practical approach to comparisons of plasma treatments. In addition, it suggests an approach to defining the investigations required to conclusively identify the underlying treatment mechanisms.