PTFE foils were plasma-treated in order to enhance their adhesion to thin films. The effect of plasma treatment using argon and oxygen discharge gases on the surface energy of PTFE foils was examined by measuring the contact angles of water droplets placed on the foil surface. Exposure to the plasma for only about 10-20 s was very effective in enhancing the surface energy. By depositing gold films onto the PTFE substrates, it was found that this enhancement in surface energy was directly related to an increase in the film adhesion. It was also found that Ar plasma treatment of a few tens of seconds followed by O₂ plasma treatment for 10 s was even more effective for adhesion enhancement.

Surface graft polymerization of a hydrophobic monomer, 2,2,3,3,3-pentafluoropropyl methacrylate (5FMA), onto hydrophilic poly(vinyl alcohol) (PVA) and cellulose films was studied after corona discharge of the films. It was found that grafting strongly depended on the reaction medium; especially, addition of alcohol to the monomer greatly accelerated graft polymerization. For instance, when an ethanol/ water /5FMA mixture (65/25/10, by volume) was used as the polymerization medium. the PVA and cellulose films corona-discharged for a few minutes exhibited a high contact angle up to 100° after 30 min polymerization, the graft density being approximately 170 μg/cm² for cellulose and 80 μg/cm² for PVA. © 1993 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1993.070481016

A remote plasma reactor, with air as the plasma gas, has been used for in-line surface modification of linear low-density polyethylene tape (LLDPE) passing 10 cm below the main plasma zone. Line speeds of up to 0.70 m/s were tested, allowing the study of 0.014 s exposure times to the plasma. Oxygen to carbon (O/C) ratios averaging 0.11 were observed on a reproducible basis. The reactor was also used for static plasma treatment under similar experiment onditions. This allowed a comparative study of short-term (milliseconds) vs. long-term (several seconds) plasma treatment. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the treated polymer surface suggested the formation of hydroxyl (COH), carbonyl (CO) and carboxyl (OCO) groups, even after short plasma treatment. The intensities of these components were seen to increase in approximately equal quantities with increasing O/C ratio. Water washing of polyethylene surfaces with high O/C ratios showed a loss of oxygen, apparent as a decrease in OCO groups in the C 1s spectra. A smaller loss in oxygen was observed when washing samples that had been plasma-treated for milliseconds. A surface ageing study revealed that polyethylene surfaces that had been plasma-treated for short time periods showed only a negligible loss of oxygen on prolonged exposure to air. Surfaces treated for longer time periods showed a loss of up to 50% of the total oxygen on the surface within a few days of treatment. Static secondary ion mass spectrometry has provided some supporting evidence for surface damage of the treated films.

The interfacial region of coated plastics is an example of a multicomponent polymer system. Practical adhesion, as determined by the peel test, has been found to be strongly dependent on the composition of the system and the degree of interaction between its components. Several interactions are possible during the coating process of polypropylene (PP)/ethylenepropylene-diene-monomer (EPDM) blends with chlorinated polyolefin (primer) and polyurethane (PUR) paint. Wettability, a necessary but not sufficient condition alone for molecular interdiffusion, was found to be good in all cases. The lack of interfacial adhesion between PP and PUR and between EPDM and PUR was explained by high interfacial tensions calculated from surface energetics, which, in turn, were determined by contact angle and inverse gas chromatography (IGC) measurements. The improvement of interfacial adhesion between PUR and PP by chlorinated polyolefin was explained by acid-base interactions detected by IGC. The creation of surface topography by extraction of low molecular weight fractions during the coating process does not influence the adhesion. Molecular interdiffusion was shown to be facilitated by solvents.

To enhance polyimide-to-polyimide adhesion, we have investigated the effect of surface modification in water vapor plasma. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface before subsequent layers of polyimide are applied results in dramatically enhanced interfacial adhesion. The polyimide-to-polyimide interfacial adhesion strength attained following water vapor plasma treatment exceeds the cohesive strength of the applied polyimide layer. The effect of surface modification in water vapor plasma on metal-to-polyimide adhesion has also been investigated. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface prior to metallization results in increased metal-to-polymer interfacial adhesion. A study of both electroless and vacuum-deposited metal was conducted. The use of contact-angle measurements, peel tests, Fourier transform infrared spectroscopy, optical emission spectroscopy, nuclear forward scattering, and X-ray photoelectron spectroscopy has led us to a preliminary understanding of the resulting surface modification and the subsequent effect of adhesion promotion. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1992.070461216

The bonding properties of polyolefin films in composites are improved by a treatment with a low pressure plasma.

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 surface free energy of solids is a characteristic parameter that determines most of the surface properties such as adsorption, wetting, adhesion, etc. The surface energetics of solids may be characterized by measurement of contact angles of different liquids. Nevertheless, the calculation of surface free energy from contact angle measurements has been the subject of much controversy. Indeed, this characteristic of a solid cannot be measured directly because of elastic and viscous restraints of the bulk phase, which necessitate indirect methods.

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.

Low dynamic surface tension is an important factor in achieving superior film formation in water-borne coatings. Dynamic coating application methods require surfactants with low dynamic surface tensions in order to prevent defects such as retraction, crawling and cratering. Comparative basic and empirical data are presented that will demonstrate the ability of acetylenic diols to lower the dynamic surface tension of water-borne coatings and hence improve the quality of the cured film.

This paper reports on a three-part study: (1) to determine the effect of surface pretreatment of BDS, a siloxane/polyimide copolymer, on adhesion; (2) to determine the extent of segregation of components of BDS against metal substrates; and (3) to determine the properties of ultrathin polymer films against metal substrates.

Surface pretreatment of bds films with aqueous NaOH etched away the top siloxane layer, roughening the polymer surface and producing surface functional groups. These changes resulted in increased wettability and peel strength. Siloxane segregation when bds films were formed against metal surfaces was in the order: Al > Ti > Zn. The relative acidity of the metal oxides as measured by polyvinyl chloride adsorption was in the same order. Reflection-absorption measurements using Fourier transform infra-red spectroscopy were found to be useful in studying the crystallinity of thin polyphenylene sulphide (pps) films. X-ray photoelectron spectroscopy was used to show that failure occurred through a thin layer of residual pps polymer close to the copper oxide substrate.

According to this invention, there is provided an atmospheric pressure plasma surface treatment process comprising the steps of introducing a gas in a plasma reaction apparatus having a pair of dielectric-covered electrodes having opposing surfaces on at least one of which is provided with a solid dielectric; performing plasma excitation under atmospheric pressure; and surface treating an object placed between the opposing electrodes, wherein the gas introduced is a gaseous composition consisting essentially of argon, helium and/or ketone. This process makes it possible to quickly imparting hydrophilic nature to surfaces of an object made of a plastic to be treated. The hydrophilic nature given lasts long.

The present invention is a process for treating plastic substrates to improve their surface properties. The process involves exposing the plastic substrate to UV radiation at wavelengths of 185 and 254 nm in the presence of atomic oxygen for about 5 to 60 minutes. The treated plastic substrates have improved wettability. The process is particularly well suited for the treatment of plastic intraocular lenses and plastic packaging materials such as PVC film.

Methods for the surface free energy determination of solids based on wetting by liquids are reviewed. Some critical remarks and new ideas are included.

Self-assembled monolayers (SAMs) of alkylsiloxanes on elastomeric PDMS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies (γsv) of these chemically modified surfaces were estimated by measuring the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles.

The present work examined the susceptibility of contact angle data to specific interactions taking place between solids and contacting liquids. The polymers involved were polystyrene, polyvinyl chloride and polyethylene, representing respectively basic, acidic and neutral substrates. Contacting fluids also were chosen to represent acid and base interaction categories.

Significant time-dependent changes in contact angles were observed when acid/base pairs were involved in the experimental sequence. In specific cases it was possible to identify initial (zero contact time) contact angles, as well as equilibrium values, attained after prolongued contact times. Local solvation, or plasticization, of the polymer by the wetting fluid was postulated as the operative mechanism. The differences between initial and final values of the contact angles were correlated with parameters of specific interaction, calculated from the acceptor/donor numbers for the pertinent materials as measured by inverse gas chromatography. In contrast, when acid/acid or base/base combinations of polymer and wetting fluid were studied, equilibrium values of the contact angle were established rapidly. Since accurate information on acid/base properties of polymers and wetting fluids is not always available, it seems prudent to record contact angles as a function of contact time, and by extrapolation to determine the initial (true) value for further use in surface characterizations of polymers.

Pretreatments of polyolefins and fluoropolymers are usually necessary to achieve satisfactory adhesion for bonding and related technologies. In this paper results for various pretreatments of these polymers are presented. These are the treatment of polyolefins with aqueous reagents, dilute fluorine and a natural gas flame, the treatment of PTFE with sodium naphthalenide and the treatment of ECTFE with sodium naphthalenide and a flame. X-ray photoelectron spectroscopy was used to investigate the chemical changes caused by the treatment and the adhesion levels were discussed in relation to wetting, interactions across interfaces and weak boundary layers.

The wettability and adhesion of the coating and printing films on the polymer substrates depend on the surface properties of the formulation ingredients and polymer surface. In addition, the transfer of ink from flexographic printing plates to substrates depends on the surface properties of the printing plate, water-based ink and polymer substrate. Among several surface properties such as surface composition, surface roughness, surface tension/surface energies and surface defects, the surface energies: polar, nonpolar and total energies of the dried coating films, flexographic printing plates and polymer substrates were determined by measuring contact angles of water and methylene iodide. These results were used to understand the ink transfer from printing plates to substrates during flexographic printing process, and ink spreading, wetting as well as ink adhesion behavior of the coatings and inks on the polymer substrates. The data indicates that for good ink transfer and adhesion to occur, the surface energy of the water-based ink should be lower than that of the printing plates and substrates.

A detail discussion of the theoretical aspects of surface tension measurements by ring method is provided with special emphasis on the sources of error. The accuracy of the measured data is mainly limited by the correction factor “f”, which compensates for the non-symmetrical shape of the surface. Based on the experimental findings, it is suggested to include the correction factor during the evaluation of the surface tension, especially if an accuracy of less than 0.1mN/m is required. The effect of meniscus shape and size on the surface tension is discussed. In addition to the surface tension measurements, several other physical properties of the coating systems such as settling behavior and hardness of the settlement can be measured by using the dynometer from BYK-Gardner as a measuring device. The results on different coating systems are presented to study the settling and hardness of the settled material.

Dynamic Contact Angle Technique offers a unique, non-optical alternative to solid surface energy analysis. The technique provides advancing and receding hysteresis profile scans of a surface recorded in real time as the liquid meniscus traverses the solid surface. Changes in the wetting hysteresis scan can be used to characterize the qualitative effects of surface roughness, surface homogeneity, and surface polarity, as well as measure the quantitative surface energy of the solid. Applications in flexography in which wettability plays a critical role are numerous, and the switch from solvent-based inks to water-based inks gives impetus for future study.

A cell theory for the prediction of the surface tension of polymer liquids is modified to include an entropic effect due to molecular asymmetry. Also considered is the extent of the effect of the preservation of connectivity in the vicinity of the surface upon the potential energy zero term due to missing nearest neighbors of orders greater than one. Theory and experiment are in good agreement without an adjustable surface parameter.

The surfaces of five EUROBIOMAT standard biomaterials are characterized by contact angle measurements applying the sessile drop method using a goniometer. The advancing angles of ten liquids composed of formamide and ethyl CellosolveR according to ASTM D 2578-84 and aqua bidest. are measured. From these data, surface characteristics such as critical surface tension, surface free energy and dispersive surface tension are determined.