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 wettability of oxidized polyethylene films was studied with pure liquids (water and methylene iodide) and practically nonpolar mixtures of Decalin and methylene iodide. A linear variation was found of the wettability of these films with the chemical composition of their surfaces (determined by adsorption of radioactive ⁴⁵Ca ions).

A value of γ_s^d for the polyethylene was found with the nonpolar mixtures of methylene iodide and Decalin and the values of the solid-liquid polar interactions (I_sl^P) for oxidized polyethylene were deduced.

Dipole-dipole and induced dipole-dipole interactions between the pure liquids and the oxidized and unoxidized polyethylene were calculated for two possible orientations of the hydrocarbon chains to the surface and compared with the experimental results. Generally a poor agreement was obtained, mainly due to the difficulty in estimating the correct values for the distances between molecules or groups. However, a better agreement was obtained assuming that the chains were perpendicular to the surface.

The surface free energy and polarity values of a number of organic pigments are obtained from contact angle measurements and the interfacial tension equation of Wu(1). The pigment types studied are phthalocyanine, quinacridone, toluidine red, isoindolinone, indanthrone, β-oxynaphthoic acid derivative, and thioindigoid red. The surface free energies obtained agree reasonably well with those predicted from parachor and density values.

Static and dynamic contact angles of aqueous solutions of three surfactants--anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C(₁₂)E(₅))-were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid-vapor interface. Obviously, another process involving adsorption at the expanding solid-liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.

Advancing and receding contact angles of four organic liquids and water were measured on a variety of polymer surfaces and silicon wafers using an inclinable plane. Contact angles varied widely from liquid to liquid and from surface to surface. Surface roughness was relatively unimportant. Instead, the contact angles seemed to be more closely tied to the chemical nature of the surfaces. In general, contact angles increased with the liquid surface tension and decreased with the surface tension of the solid. Several definitions were used to calculate contact angle hysteresis from the experimental data. Although hysteresis is usually considered an extensive property, we found that on a given surface a wide range of liquids gave a unique value of reduced hysteresis. Apparently, reduced hysteresis represents an intrinsic parameter describing liquid–solid interactions.

This paper describes the photochemical surface modification of polystyrene (PS) substrates using vacuum ultraviolet (VUV) light 172 nm in wavelength. We have particularly focused on the effects of atmospheric pressure during VUV irradiation on the obtained surface's wettability and the stability of the wettability, in addition to its chemical structure, morphology, and photooxidation rate. Samples were photoirradiated with VUV light under pressures of 10, 10(3), or 10(5) Pa. Although, in each case, the originally hydrophobic PS surface became highly hydrophilic, the final water-contact angle and photooxidation rate depended on the atmospheric pressure. The samples treated at 10 Pa were less wettable than those prepared at 10(3) and 10(5) Pa due to the shortage of oxygen molecules in the atmosphere. The minimum water-contact angles of the samples treated at 10, 10(3), and 10(5) Pa were about 8 degrees, 0 degrees, and 0 degrees, respectively. With the samples prepared at 10 and 10(3) Pa, photooxidation reactions proceeded in the topmost region closest to the surface, while at 10(5) Pa photooxidation was found to be greatly enhanced in the deeper regions, as evidenced by angle-resolved X-ray photoelectron spectroscopy. Photoetching rates were determined through atomic force microscope observation of microstructured PS samples prepared by a simple mesh-contact method. As estimated from AFM images of the latticed microstructures obtained, the rates of samples prepared at 10(3) and 10(5) Pa were about 1.5 and 1.3 nm/min, respectively. However, no photoetched features were observable on the sample surface prepared at 10 Pa. Hydrophilic stability also varied greatly depending on atmospheric pressure. The hydrophilicity of samples treated at 10 and 10(3) Pa gradually decreased as they were exposed to air. On the other hand, the sample surface prepared at 10(5) Pa showed excellent hydrophilicity even after being left in air for 30 days.

Contact angle (θ) measurements on poly(tetrafluoroethylene) (PTFE) and polymethyl methacrylate (PMMA) surface were carried out for the systems containing ternary mixtures of surfactants composed of: p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols), Triton X-100 (TX100), Triton X-165 (TX165) and Triton X-114 (TX114), and fluorocarbon surfactants, Zonyl FSN100 (FSN100) and Zonyl FSO100 (FSO100). The aqueous solutions of ternary surfactant mixtures were prepared by adding TX114, FSN100 or FSO100 to binary mixtures of TX100+TX165, where the synergistic effect in the reduction of the surface tension of water (γ(LV)) was determined. From the obtained contact angle values, the relationships between cosθ, the adhesion tension and surface tension of solutions, cosθ and the reciprocal of the surface tension were determined. On the basis of these relationships, the correlation between the critical surface tension of PTFE and PMMA wetting and the surface tension of these polymers as well as the work of adhesion of aqueous solutions of ternary surfactant mixtures to PTFE and PMMA surface were discussed. The critical surface tension of PTFE and PMMA wetting, γ(C), determined from the contact angle measurements of aqueous solutions of surfactants including FSN100 or FSO100 was also discussed in the light of the surface tension changes of PTFE and PMMA under the influence of film formation by fluorocarbon surfactants on the surface of these polymers. The γ(C) values of the studied polymeric solids were found to be different for the mixtures composed of hydrocarbon surfactants in comparison with those of hydrocarbon and fluorocarbon surfactants. In the solutions containing fluorocarbon surfactants, the γ(C) values were different taking into account the contact angle in the range of FSN100 and FSO100 concentration corresponding to their unsaturated monolayer at water-air interface or to that saturated.

Polyethylene films were prepared with phase separation at lower temperatures. The wettability of such films varied from hydrophobicity to superhydrophobicity as the processing temperature decreased owing to the increase of surface roughness. Storing the as-prepared films at subzero temperature (−15 °C), it was found that the water contact angle of the film decreased obviously, and the decrease depended on the corresponding roughness. Further keeping the as-prepared films at room temperature for 30 min, the water contact angle would return to the normal value, which indicated that the reversible switching of surface wettability can be controlled by the environmental temperature.

A theory, based on the presence of an adsorbed film in the vicinity of the triple contact line, provides a molecular interpretation of intrinsic hysteresis during the measurement of static contact angles. Static contact angles are measured by placing a sessile drop on top of a flat solid surface. If the solid surface has not been previously in contact with a vapor phase saturated with the molecules of the liquid phase, the solid surface is free of adsorbed liquid molecules. In the absence of an adsorbed film, molecular forces configure an advancing contact angle larger than the static contact angle. After some time, due to an evaporation/adsorption process, the interface of the drop coexists with an adsorbed film of liquid molecules as part of the equilibrium configuration, denoted as the static contact angle. This equilibrium configuration is metastable because the droplet has a larger vapor pressure than the surrounding flat film. As the drop evaporates, the vapor/liquid interface contracts and the apparent contact line moves towards the center of the drop. During this process, the film left behind is thicker than the adsorbed film and molecular attraction results in a receding contact angle, smaller than the equilibrium contact angle.

Plasma treatment of polyimide surfaces not only causes structural modification during the plasma exposure, but also leaves active sites on the surfaces that are subject to post-reaction. In this work, the effects of atmospheric fluorine plasma treatment on the surface properties and dielectric properties of polyimide thin film were investigated by using X-ray photoelectron spectroscopy (XPS), Fourier transform-IR (FT-IR) spectroscopy, and contact angle measurement. The results indicated that plasma treatment successfully introduced fluorine functional groups on the polyimide surfaces. The polyimides also exhibited good thermal stability and a lower dielectric constant. It appears that the replacement of fluorine led to the decrease of the local electronic polarizability of polyimide. Consequently, it was found that the atmospheric fluorine plasma-treated polyimides possessed lower dielectric characteristics than the untreated polyimides.

The surface chemical and physical character of offset paper was studied before and after application of model fountain solutions based on isopropyl alcohol and an alcohol-free surfactant solution. The paper surface features were characterised with atomic force microscopy and the surface energies were determined by contact angle measurements. Changes in the surface chemical properties induced by the fountain solutions were investigated with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. Coated papers wetted with the surfactant solution revealed a slight increase in the root mean square roughness, but the isopropyl alcohol solution led to no observable changes. The change in sub-micro roughness is ascribed not only to substrate swelling or migration of coating constituents but also to the presence of surfactant on the surface. A change in the surface energy and particularly the polar contribution was observed after application of the surfactant solution. The X-ray photoelectron spectroscopy showed an increase in the oxygen-to-carbon ratio, which confirms the presence of surfactant on the surface. Time-of-flight secondary ion mass spectroscopy showed that the isopropyl alcohol solution did not change the elemental composition of the surface whereas the surfactant solution clearly did so. The distribution of surfactant on the surface was confirmed by mapping the characteristic fragments of the molecule.

In a previous paper it was shown that negative interfacial tensions between predominantly monopolar surfaces (i.e., surfaces with mainly H-acceptor properties) and polar liquids are real phenomena. Such negative interfacial tensions do however decay rapidly. For miscible liquids, the decay of the interface is, in general, so rapid that it practically excludes measurement of interfacial tension. However, if one liquid is present in the form of a gel, and if the other liquid is placed as a drop upon the gel, there is often enough time to measure contact angles. This may be done at various concentrations of the liquid encased in the gel, and an extrapolation made to zero concentration of the gelling agent. With this method we found the existence of negative interfacial tensions at liquid/liquid interfaces.

This paper considers the correctness of the application of Good–van Oss theory (vOGT) to the calculation of acid–base components of solid surface free energies. Theory equations are written in a matrix form, and their application is analyzed particularly from a mathematical point of view. A calculation procedure similar to the approach used for other scales of acid–base strength is suggested to obtain in a straightforward manner the necessary material coefficients. The chemical consequences of these considerations are also discussed. The acceptability of current experimental literature data and the validity of the calculations obtained from them by the proposed method are considered, showing the origin of some inconsistencies in current results. Some general considerations regarding the difficulties commonly encountered in the application of vOGT are also discussed, and it is shown that they can be rationalized or eliminated with more acid solvents being included in the solvent set and the properties of the reference solvent being correctly chosen. The difficulties encountered in correctly expressing the acid properties of some polymers are closely examined. Finally, some indications are given of the future possible developments of vOGT.

The surface characteristics of ethylene-vinyl acetate (EVA) were modified by argon, air, and oxygen plasma at atmospheric pressure. The surface energies of the EVA were evaluated by contact angles according to a sessile-drop method and adhesion energy (G(IC)) was estimated by a 180 degrees peel test with polyurethane (PU). After the plasma treatments, the surface free energies (or specific polar component) of the EVA increased about five times compared to that of virgin EVA. The adhesion between the EVA and the PU is significantly improved by the plasma treatment. Especially, Ar/air/O(2) plasma treatment increases G(IC) of EVA/PU up to about 600% compared to that of the sample using virgin EVA.

In this work, the effect of atmospheric-pressure plasma treatments on surface properties of polyimide film are investigated in terms of X-ray photoelectron spectroscopy (XPS), contact angles, and atomic force microscopy (AFM). The adhesion characteristics of the film are also studied in the peel strengths of polyimide/copper film. As experimental results, the polyimide surfaces treated by plasma lead to an increase of oxygen-containing functional groups or the polar component of the surface free energy, resulting in improving the adhesion characteristics of the polyimide/copper foil. Also, the roughness of the film surfaces, confirmed by AFM observation, is largely increased. These results can be explained by the fact that the atmospheric-pressure plasma treatment of polyimide surface yields several oxygen complexes in hydrophobic surfaces, which can play an important role in increasing the surface polarity, wettability, and the adhesion characteristics of the polyimide/copper system.

In an attempt to modify the hydrophobic surface properties of polypropylene (PP) films, this study examined the optimum process parameters of atmospheric pressure plasma (APP) using Ar gas. Under optimized conditions, the effects of a mixed gas (Ar/O2) plasma treatment on the surface-free energy of a PP film were investigated as a function of the O2 content. The polar contribution of the surface-free energy of the PP film increased with increasing O2 content in the gas mixture. However, slightly more oxygen-containing polar functional groups such as CO, CO, and COO were introduced on the PP film surface by the Ar gas only rather than by the Ar/O2 gas mixture. In addition, AFM analysis showed that the Ar plasma treatment of the PP film produced the smoothest surface as a result of the relatively homogeneous etching process.

The hydrophobic recovery of polydimethylsiloxane elastomers was studied after exposure to partial electrical discharge. Silicone elastomers that were thoroughly extracted of free oligomeric impurities as well as those deliberately contaminated with low molecular weight (LMW) silicone fluids were used for these studies. Contact angle and X-ray photoelectron spectroscopy revealed that the recovery rates of the oxidized extracted samples are strongly influenced by the applied voltage, humidity, and aging condition. The recovery rates increase considerably as the applied voltage and the humidity during discharge increase. Remarkably, the oxidized samples stored under high vacuum (10⁻⁷ Torr) exhibit lower recovery rates than those aged in air. Free silicone fluid, when added to the elastomer, affects the recovery rate as well; however, significant recovery is seen even without any added fluid. These results imply that the LMW species that are formed in situ during electrical discharge are sufficient to cause the hydrophobic recovery of oxidized PDMS elastomers.

Two different experimental approaches based on ellipsometry and zeta potential measurements have been employed to determine the dispersion and polar surface free energy components of glass. From ellipsometry the adsorption isotherms of n-octane and water have been determined, yielding values for the film pressures of n-octane and water and the dispersion and polar surface free energy components of glass. Similarly, zeta potentials in water of glass covered with various amounts of n-octane and n-hexanol have been determined. Next, the film pressures of these liquids and surface free energy components of glass were also calculated. Thus determined values are 32 and 80 mJ/m² (from ellipsometry) and 25 and 80 mJ/m² (from zeta potentials) for the dispersion and polar components, respectively. The correspondence between the surface free energies obtained by two completely independent methods gives confidence to the approaches employed.

A technique has been developed for rapid and extremely accurate measurements of contract angles formed by liquids on the surface of small-diameter filaments. The light beam reflection technique first de- scribed by Langmuir and Sehaeffer (1) and recently by Fort and Patterson (2) for liquid drops on flat plates has been refined for use with filaments and microscope equipment.

A new method for the measurement of apparent contact angles at the global energy minimum on real surfaces has been developed. The method consists of vibrating the surface, taking top-view pictures of the drop, monitoring the drop roundness, and calculating the contact angle from the drop diameter and weight. The use of the new method has been demonstrated for various rough surfaces, all having the same surface chemistry. In order to establish the optimal vibration conditions, the proper ranges for the system parameters (i.e., drop volume, vibration time, frequency of vibration, and amplitude of vibration) were determined. The reliability of the method has been demonstrated by the fact that the ideal contact angles of all surfaces, as calculated from the Wenzel equation using the measured apparent contact angles, came out to be practically identical. This ideal contact angle has been compared with three methods of calculation from values of advancing and receding contact angles.

The critical surface tension γ_c of polyvinyl butyral has been measured with polyhydric alcohols and halogenated hydrocarbons. Despite variations in polymer composition (residual OH content) and modes of preparation, γ_c is found to be 24–25 dynes/cm. with the former class of liquids. The —CH₃ groups appears to predominate over —CH₂, ether oxygen, and OH groups present. Steric effect may account for this biasing of the γ_c values toward the lowest surface energy group present. Fowkes' relation based on dispersion force interactions only is found to fit the data reasonably well. Comparative data on polyethylene are also presented.

Sessile-drop and captive-bubble techniques were used for contact angle measurements. The advancing and receding contact angles were measured for water and ethylene glycol at self-assembled monolayer surfaces of dodecanethiol, for water at methylated quartz surfaces, and for water at roughened polyethylene and polytetrafluoroethylene surfaces. It was found that for each technique used, sessile-drop and captive-bubble, different advancing contact angles and different receding contact angles were frequently obtained for nonideal systems with rough and heterogeneous solid surfaces. The disagreement between contact angles, as measured with the two different techniques, increased with increasing imperfection of the solid surface. Also, it was observed that solid surface roughness and heterogeneity affected a variation of the advancing and receding contact angles with drop (bubble) size. No contact angle change with respect to drop (bubble) size (in the range 1–7 mm base diameter) was observed when smooth and homogeneous solid surfaces were well prepared. It is possible that metastable states, which are responsible for the contact angle hysteresis, also affect the contact angle/drop (bubble) size relationship. These three-phase systems with sessile drop and captive bubble at heterogeneous and/or rough solid surfaces are complex because solid surface heterogeneity and roughness cause contortions in the shape of the three-phase contact line and the drop (bubble) surface in the vicinity of the three-phase contact line. These contortions may affect a variation of the internal free energy of the liquid drop (gas bubble). It is shown that a slight variation in the advancing contact angle value over a few millimeters change in drop (bubble) diameter does not guarantee a high-quality surface state. Measurements of the receding contact angles provide more information on the quality of the solid surface and they should always be included with the measurements of advancing contact angles.

The acid-base approach to the calculation of solid surface free energy and liquid-liquid interfacial tensions is a practical example of application of correlation analysis, and thus it is an approximate approach. In these limits, and provided that wide and well-obtained sets of contact angles or interfacial tension data are used for their computation, surface tension components can be considered as material properties. Although their numerical value depends on the characteristics of the chosen reference material, their chemical meaning is independent on the selected scale. Contact angles contain accessible information about intermolecular forces; using surface tension component (STC) acid-base theory, one can extract this information only making very careful use of the mathematical apparatus of correlation analysis. The specific mathematical methods used to obtain these results are illustrated by using as an example a base of data obtained by the supporters of the equation-of-state theory (EQS). The achievements are appreciably good and the agreement between STC and EQS is discussed.

This paper follows the “defense” of the Good-van Oss-Chaudhury (GvOC) acid-base approach made in Part I and carries out a detailed analysis of the Zisman critical surface energy and, mainly, of the Neumann equation-of-state (EQS) theory. The analysis is made on both a “practical” and a theoretical basis, trying to highlight the acceptable fitting results of axisymmetric drop shape analysis (ADSA) methods and their independence of the assumed thermodynamic foundations of EQS. Some new and original criticisms of the EQS approach are raised and it is shown that other purely semiempirical models, represented by different fitting equations with the same number of parameters, can represent the data measured by ADSA method with the same goodness as EQS. The equation of state appears as one of many semiempirical approaches for the evaluation of surface free energy of solids. Independent of the previous analysis, the criteria used in ADSA measurements are evaluated and some comments made on them.

Hypothesis: Both the surface free energy (γ) and solubility (δ) parameters of substances are related to their cohesive energies which are decided by intermolecular interactions, and there should be some intrinsic relationships between the two parameters. Understanding of the γ-δ correlations is of great fundamental and practical importance. Several empirical γ-δ equations have been proposed so far, but their application to solids is limited. This is because the molar volume (V~) as a parameter exists in these equations while the V~ of solids is commonly hard to be obtained. Hence, the development of γ-δ equations without the parameter V~ is essential for solids.

Method: The γ and δ data of 21 solids including polymers and layered solid materials were chosen, and possible γ-δ relationships were systematically explored using the parameter data of solids by a trial and error fitting method.

Finding: Six γ-δ equations without the parameter V~ are proposed. The γ parameters include total (γ_t), dispersive (γ_d), and polar (γ_p) ones, and the δ parameters include the Hildebrand parameter (δ_t) and the Hansen dispersive (δ_d), polar (δ_p), and hydrogen-bonding (δ_h) ones. Interestingly, the so-obtained V~-free γ-δ equations are also valid for most liquids including nonpolar and polar ones. These γ-δ equations can provide a way to estimate non-measurable parameters from measurable parameters for solid materials, which is beneficial to the application of the characteristic parameters (γ and δ) for solid material engineering.

Contact angle data for sets of probe liquids allow the determination of components of solid surface energies which in turn can be used to calculate the work of adhesion of other materials to the solid surface. There is much debate currently about the correct choice of the acid–base components for the probe liquids. For many systems, the strength of adhesion measured independently correlates well with the calculated work of adhesion. Recent trends in this area include adhesion under water and the adhesion of bacterial and other cells to immersed solids.

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.

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.

By measuring contact angles with water, glycerol and formamide on a number of polar surfaces, an estimate could be made of the electron-acceptor (γ⁺ ) and the electron-donor (γ⁻ ) parameters of glycerol (G) and formamide (F), relative to the parameters of. water (W), for which a reference value of γ⁺ _W = γ⁻ _W = 25.5 mJ/m² has been assumed. The values thus found are: γ⁺ _G ≈ 3.92 mJ/m² (which yields γ⁻ _G ≈ 57.4 mJ/m²) and γ⁺ _F ≈2.28 mJ/m² (which yields γ⁻ _F ≈ 39.6 mJ/m²).

The effect of plasma treatment on surface characteristics of polyethylene terephthalate films was investigated using helium and oxygenated-helium atmospheric plasmas. Sample exposure to plasma was conducted in a closed ventilation test cell inside the main plasma chamber with variable exposure times. The percent weigh loss of the samples showed an initial increase followed by decrease with extended exposure time, indicating a combined mechanism of etching and redeposition. The wettability as measured by the contact angle showed a sharp initial increase followed by a steady-state trend with increased exposure time, suggesting a change in surface functionality. Atomic force microscopy analysis revealed increase in surface roughness, as well as evidence of redeposition of etched volatiles. Functionality changes were measured using X-ray photoelectron spectroscopy and these changes were correlated to the new plasma-induced properties. © 2004 The Electrochemical Society. All rights reserved.

The surface of polyimide (PI) films before/after plasma surface treatment using a remote-type modified dielectric barrier discharge was investigated to improve the adhesion between the PI substrate and the metal thin film. Among the plasma treatments of the PI substrate surface using various gas mixtures, the surface treated with the N-2/He/SF6/O-2 plasma showed the lowest contact angle value due to the high C=O bondings formed on the PI surface, while that treated with N-2/He/SF6 showed the highest contact angle value due to the high C-F-x chemical bondings on the PI surface. Specifically, when the O-2 gas flow was varied from 0 to 2.0 slm in the N-2(40 slm)/He(1 slm)/SF6(1.2 slm)/O-2 (x slm) gas composition, the lowest contact angle value of about 9.3 degrees was obtained at an O-2 gas flow of 0.9 slm. And it was due to the high content of oxygen radicals in the plasma, which leads to the formation of the highest C=O bondings on the PI surface. When the interfacial adhesion strength between the Ag film and PI substrate was measured after the treatment with N-2(40 slm)/He(1 slm)/SF6(1.2 slm)/O-2(0.9 slm) followed by the deposition of Ag, a peel strength of 111 gf/mm was observed, which is close to the adhesion strength between a metal and the PI treated by a low pressure plasma.

In a number of automotive applications of polypropylene (PP) good paint adhesion is essential. For industrial applications, the initially poor adhesion properties of PP compounds are often improved by flame treatment, resulting in good paint adhesion on the PP parts. For control purposes, intensive and time consuming paint tests are required. To relate adhesion behaviour with the parameters used in the flame treatment modification, several methods are applied such as simple surface tension tests with inks (wetting tests), contact angle measurements and standard XPS(ESCA) measurements. Unfortunately a good correlation is rarely obtained between these tests and the desired adhesion behaviour under rigid test conditions. Good wetting, for example, is necessary, but is no guarantee for good adhesion. In this paper results are presented on the characterisation of flame treated PP compounds by XPS, as well as the predictive information obtained from the XPS study concerning the paint adhesion behaviour of the modified PP surfaces.

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.

In this paper, the corona charge stability in electret polytetrafluoroethylene Teflon film is investigated after the film is treated by radio-frequency plasma. It is found that the charge stability depends strongly on the plasma composition and the film exposure to plasma, especially for negative charge. When a non-metalized film is held horizontally on the ground holder, i.e. with one side facing the plasma, oxygen plasma treatment achieves a superior negative charge retention on the front side, while its rear side retention decreases significantly. Under the same conditions in oxygen/helium and helium plasmas, the charge stability also increases but the potentials are lower compared with pure oxygen plasma after annealing. In a hydrogen plasma, the stability only slightly enhances. If the film is held vertically on the holder, so that both sides contact the plasma, the surface potential on both sides decreases dramatically and arrives at a few volts within 2 min, after annealing at 170°C. By Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), we conclude that oxidation formed on the front side is responsible for increasing the stability of negative charge. The positive carriers, generated in the film during plasma treatment, recombine with charge from corona charging and causes the surface potential in the rear side of the horizontal non-metalized film, or in both sides of the vertical non-metalized film, to discharge after heating.

Currents during corona charging and surface potential decay after corona charging have been studied on polymeric films. As has been reported before, surface potential is a useful tool for investigating the electrical properties of an insulating material, making it possible to discriminate charge injection from polarization processes, when data are correctly analysed, and it has also been shown that, on thin polymeric films, slow polarization processes leading to heterocharge formation dominate at low fields, while charge injection occurs above a given field threshold. We present here a combined study of the surface potential after charge deposit and current flowing on the back electrode during the corona charge; we show that current measurements during the charge confirm the interpretation of potential measurements after corona charge. The outbreak of “hollows” in the potential distribution on the surface is clearly linked to the predominance of injected charge on the polarization charge. However, even at high fields, polarization phenomena will dominate again a given time after corona discharge stopping.

We examined the influence of the geometric parameters of the system on the modification power as determined by the contact angle on the surface of the treated low-density polyethylene (LDPE). We have found that (1) with a constant electric energy to generate a corona discharge, the modification power decreases as the distance from the center of surface (the point on the film immediately below the point electrode) increases and that the corona discharge in a point-to-grid system can modify the film surface over a wider area than in a point-to-LDPE system without grid; (2) with a constant discharge current, the modification power on the center of surface decreases when the point-to-grid gap in negative corona treatment increases, but increases in positive corona treatment; (3) the modification power compared to the electric energy used to generate a corona discharge (the yield) is inversely proportional to the point-to-grid gap. However, in a positive corona discharge, the yield did not reach zero when the point-to-grid gap was extrapolated to infinity, possibly because the streamer reduces the effective point-to-grid gap and produces neutral activated species along the streamer; and (4) in a negative corona, the modification power as measured by the temperature increases at the plain electrode (anode) and varies with the energy dissipated by neutral activated species.

Surface potential decay after negative corona charge deposition has been studied for polypropylene films 50μm thick, over a wide range of decay times and charging voltage (500–2800V). At low initial potential (500–800V), the surface potentials were stable and did not decay. At high initial potential (1200–2800V), the surface potential decay has been clearly observed with differences in the decay rate. The observed decay behavior has been computer simulated to elucidate the transport mechanism in polypropylene films. A good agreement between measured and calculated decay curves was obtained with a model in which charges are transported by hopping of injected carriers between localized trapping centers. The model is characterized by parameters such as the free carrier mobility, the mean free time of carrier between traps and the mean capture time in trap. The transport parameters have been determined by a fitting procedure.

A viscous-liquid drop spreads on a smooth horizontal surface, which is uniformly heated or cooled. Lubrication theory is used to study thin drops subject to capillary, thermocapillary and gravity forces, and a variety of contact-angle-versus-speed conditions. It is found for isothermal drops that gravity is very important at large times and determines the power law for unlimited spreading. Predictions compare well with the experimental data on isothermal spreading for both two-dimensional and axisymmetric configurations. It is found that heating (cooling) retards (augments) the spreading process by creating flows that counteract (reinforce) those associated with isothermal spreading. For zero advancing contact angle, heating will prevent the drop from spreading to infinity. Thus, the heat transfer serves as a sensitive control on the spreading.