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.

The contact angles θ of dispersion (D), polar (P) and hydrogen bonding (H) liquids on poly(vinylidene fluoride-co-hexafluoroacetone) (P(VDF-HFA); HFA content 6.5, 8.3 and 10.4 mol%) were measured. The critical surface tensions γ_c of P(VDF-HFA) were evaluated by the Zisman plot (cos θ versusγ_L), Young-Dupre-Good-Girifalco plot $\text{(1 +}\text{cos}\text{θ versus}\text{1}\text{γ}{}^{0.5}{}_{\mathrm{<mtext>L</mtext>}}\text{)}$ and the log(1 + cos θ) versus log (γ_L) plot. The following results were obtained: the γ_c values of P(VDF-HFA) evaluated for the P liquids were larger than those for the D and H liquids; the γ_c values estimated by the Zisman plot were smaller than those obtained by the other plots; the surface tension γ_s values of P(VDF-HFA) revealed a minimum at the HFA content of 8.3 mol%. It was expected that P(VDF-HFA) with HFA = 8.3 mol% induced surface segregation most easily.

Treatment of fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE) by plasmas established in water vapour or ammonia gas enabled the rapid and facile modification of their surface chemistries. Under comparable plasma conditions, ammonia plasma exposures produced considerably lower air/water contact angles than water vapour plasmas. On storage of samples in air at ambient temperature, contact angles increased markedly within a few days on ammonia plasma-treated samples but remained constant over many weeks on water plasma-treated surfaces. Angle-dependent X-ray photoelectron spectroscopy (XPS) demonstrated a very low depth of modification in the case of ammonia plasma exposure, whereas the oxygen content of water plasma-treated samples was invariant with depth within the XPS analysis region. The long-term stability of water plasma-treated fluorocarbon polymer surfaces is believed to be due to this deep modification which prevents polymer chain reorientation, whereas the shallow modification in ammonia plasmas allows the rapid partial burial of the newly attached chemical groups inside the polymer. When ammonia plasma-treated samples stored in air were immersed in water, the contact angles remained constant, suggesting that the buried groups could not resurface. Contact angle measurements provided a simple and sensitive method for studying the time-dependent reduction in plasma treatment effects and the segmental mobility of modified fluorocarbon polymer surfaces; very shallow reorientation movements can be detected.

The thermodynamic energies associated with conventional wetting, spreading, adhesion, cohesion, and disjoining pressure, as defined in classical equations, are re-examined for their significance in a force field. They are then converted into dimensionless form such that the equilibrium properties of both wetting and spreading all fall on the same line when the dimcnsionless spreading coefficient is plotted as a function of the dimensionless work of adhesion. The effects of a force field such as gravity are examined and it is further shown that spreading is always thickness-dependent, whether in a force field or in a gravity-free field. Non-equilibrium processes such as autophobicity are shown on the same dimensionless plot and indicate clearly that the speed with which the process approaches equilibrium depends on the difference between the initial and equilibrium spreading coefficients. All these processes are expressed in terms of a dimensionless group P_n, the reduced wetting energy, which, when lying between the values of + 1 and -1, equals the cosine of the contact angle, . The implication of this approach to non-equilibrium processes is discussed.

To improve the adhesion between poly(p-phenylene terephthalamide), PPTA, fiber and silicone rubber, the surface modification of PPTA was investigated. Combining plasma treatment and coupling agent treatment with the silicone adhesive was found to be effective in improving adhesion. The combination process made the pull-out force of the PPTA yarn/silicone rubber composite 2.5 times higher, compared with the plasma treatment or the coupling agent treatment alone. The plasma treatment led to the elimination of carbonized layer from the PPTA yarn surface and the formation of oxygen functionalities including CO and CO groups. The elimination of the carbonaceous deposits from the PPTA surface and the interaction between the silicone adhesive and the oxygen functionalities created by the plasma treatment contribute to the improvement of adhesion with silicone rubber.

The wettability of aluminum foil is an important concern in many industrial converting processes. X-ray photoelectron spectroscopy (XPS or ESCA) and water contact angle results indicated that relatively mild (i.e. 250 W, 15 s) oxygen plasma treatments efficiently removed residual carbon contamination from cold-rolled foil surfaces. This resulted in a significant improvement in the foil wettability. It was also found that the wettability of plasma-treated foils degraded with time, apparently due to the adsorption of hydrophobic, airborne carbon species and other contaminants. Furthermore, oxygen plasma treatments caused additional aluminum oxide to grow on the metal surface. The composition of this additional oxide was found to be similar to that of the native passivation oxide. The thickness of the aluminum oxide layer increased with both the plasma RF power and the plasma exposure time.

Contact angles of binary liquid mixtures on Teflon FEP were measured. It was found that the equation of state for interfacial tensions, γ_SL = f (γ_LV, γ_sv), cannot be used to determine solid surface tensions from these contact angles of binary liquid mixtures. These findings are explained in terms of the thermodynamic phase rule.

The effect of surface heterogeneity on contact angle hysteresis is studied by using the model of Neumann and Good of a vertical plate with horizontal heterogeneous strips. The results of this study explain well known, but not understood patterns of contact angle behaviour: On the one hand, the advancing contact angle on a carefully prepared solid surface is generally reproducible; on the other hand, even a very small amount of surface heterogeneity may cause the receding contact angle to be less reproducible and to depend on several non-thermodynamic factors.

Contact angles of 17 liquids on 3 hydrophobic solid surfaces, FC721, fluorinated ethylene propylene, and polyethylene terephthalate, were measured by using the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) technique. Details of the surface preparation and the experiments are presented. The accuracy of these contact angle data is better than 0.2° in most cases. These data were used to calibrate an equation of state for interfacial tensions of solid—liquid systems. The end results of the analysis is an equation of state for interfacial tensions with a single parameter β = 0.0001247 (m²/mJ)², cf., Eqs. [22]–[24]. Within the experimental limitations, there is no evidence for the notion that β might change from system to system.

The inconsistencies in contact angle data presented in the literature can be attributed to a number of factors. The awareness of these factors would allow novice researchers to make meaningful contact angle measurements and interpretations. In this survey the effects of surface roughness and heterogeneity, surface preparation and the presence of contaminants, the vapor environment, pressure and temperature, drop size, electrical charge, and heat transfer on the wettability of polymer surfaces were examined.

The effect of a cold remote N₂ plasma (CRNP) or N₂ + O₂ plasma (CRNOP) on poly(ether ether ketone) (PEEK) is studied. The amount of nitrogen or oxygen uptake and functionalities are determined by x-ray photoelectron spectroscopy (XPS). After CRNP treatment, the N/C and O/C atomic ratios are 0.301 and 0.333, respectively. Nitrogen functional groups are not detected by CRNOP treatment, and the O/C atomic ratio is then 0.785. The ageing process of the treated PEEK surface in the open air is investigated in both cases. For CRNOP treatment the O/C atomic ratio decreases by carbonate function departure, whereas for CRNP treatment the total amount of nitrogen and oxygen graft atoms goes through a maximum after 1 h of air exposure.

An investigation of the surface by XPS photoelectron spectroscopy has shown that the process of production of cast contact lenses based on poly(2-hydroxyethyl methacrylateco-diethyleneglycol methacrylate) is accompanied by mass transfer at the lens-mold boundary. This phenomenon, which impairs the compatibility of the lens during its application, can be considerably suppressed by employing a suitable surface modification of polypropylene molds. The surface treatment consisting in the oxidation of the mold surface by an AC corona discharge in the oxygen atmosphere increased hydrophilicity of the material, thus facilitating separation of the lens from the mold. The results of the XPS study were also confirmed microscopically by employing the SEM method. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jab.770030406

The effects of oxygen plasma treatment on ultra-high-modulus polyethylene monofilaments have been investigated with particular reference to their adhesion to epoxy resins. The adhesion strength was monitored by pull-out tests, and the effects on the monofilaments was also studied by contact angle measurements, determination of gel content and scanning electron microscopy. The results of this investigation suggest that there are three contributions to the improvements in adhesion obtained from the plasma treatment. First, at comparatively short treatment times there is a general oxidation of the surface. Secondly, at intermediate treatment times cross-linking of the surface occurs, which increases the cohesive strength of the fibre surface. Finally, at long exposure times there is a pitting of the surface, which could give rise to a mechanical keying effect but may also reduce the fibre strength.

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.

Contact-angle measurements in air and water environments and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface properties of air-corona-treated polypropylene (PP) and poly(ethylene terephthalate) (PET) films. Surface properties were examined as a function of the storage time at various temperatures. Corona treatment forms water-soluble, low-molecular-weight oxidized materials on both polymer films. Corona-treated PP and corona-treated PET films have markedly different responses to aging. For corona-treated PP stored at ambient temperatures, only a slight decrease in wettability was observed. This decrease was attributed to the reorientation of oxidized functionalities within the surface region. At elevated storage temperatures, migration of oxidized species out of the surface region occurred under some conditions. For corona-treated PET, extensive migration and reorientation of oxidized groups occurred even at ambient temperatures, leading to significant decreases in wettability and a loss of surface oxidation. The contrasts in the responses of PP and PET to corona treatment are primarily due to differences in the properties of the base polymer resins.