In this study, atmospheric plasma treatment has been used to modify the wetting properties of ethylene-methacrylic acid sodium ionomer. The effects of the plasma treatment on surface properties of this ionomer have been followed by contact angle measurements, Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). With the use of these techniques, the overall effects on activation–functionalization and surface topography changes have been determined in terms of the processing parameters of the atmospheric plasma treatment (rate and distance). The obtained results show a remarkable increase of the wetting properties and optimum balanced behavior is obtained for atmospheric plasma treatment with a rate of 100 mm/s and a distance of 6 mm; in this case, surface free energy is increased from 33 mJ/m² (untreated ionomer) up to 62 mJ/m², maintaining good transparency. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

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.

The polyethylene (PE) coatings could be very promising for various branches of industry due to their chemical stability and impact resistance. Plasma modification of powder has recently attracted much interest because of new prospects to control the interfacial properties. Plasma modification also significantly enhanced the adhesion of the polymer to the substrate. Powders find wide application in various branches of industry like paintings, biotechnology, filling for composite materials etc., but the plasma modification of powder surface has not found such application as plasma modification of flat solid materials. This is due to problems connected with the three dimensional geometry, necessity of solid mixing (due to the aggregation phenomenon) and the large surface area of powders which should be treated. We investigated plasma modification of PE powder, its adhesion properties on steel surface and mechanism influencing this adhesion. PE powder was modified using various working gases and chemicals. It was found that adhesion properties were strongly influenced by concentration of oxygen containing groups and also by PE crosslinking after modification. The value of crosslinking depends on used working gas and chemicals. The ternary mixture of O₂/H₂O/methanol was found to be an appropriate working gas for plasma treatment of PE for adhesion purposes. The treated PE had good wettability, low crosslinking and very high adhesion to the steel substrate.

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.

The effects of corona discharge treatment (CDT) on ink drop impact and spreading on a coated polypropylene film substrate were investigated. Substrate surface energies were determined from static contact angles with water and ethylene glycol. The polar component increased with increasing CDT. Drops 39 um in diameter of an acrylate-based UV-curable ink were printed on to the substrate, and the spreading process studied by high-speed photography. No changes occurred during the initial stages, but the wetting phase was shorter for higher doses of CDT. Drops spread further on substrates with low doses of CDT than with higher doses. White light interferometry was used to determine the final heights of drops after UV-curing. The height was significantly affected by CDT, with minimum height at low doses. The relationship was investigated between the static contact angle for large sessile drops and the equilibrium contact angle for printed drops after spreading. Contact angle measurements with millimeter-sized sessile drops of ink provide a reliable method to determine the effects of corona treatment on wetting by ink jet printed drops.

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

Polymers are commonly used in industry for packaging applications and as protective coatings but are sometimes unsuitable to use due to their low surface energies. For these latter applications, surface modification is usually necessary to improve wettability, printability and adhesive properties. In the past decades, plasma surface treatment of polymers has been extensively studied and different treatment conditions have been investigated. However, the influence of polymer crystallinity on plasma treatment effects is not well-known and therefore this study focuses on plasma treatment of PET foils with different degrees of crystallinity. The different PET foils are treated with a DBD discharge operating in air at medium pressure and the effect of polymer crystallinity on the treatment efficiency is studied in detail using contact angle measurements and atomic force microscopy (AFM). Also the ageing behavior of the different types of plasma-treated samples is investigated in this work. Results clearly show that the crystallinity of the PET foil influences both the plasma treatment effect as well as the ageing process of the samples. AFM analysis indicated that the DBD plasma homogeneously etches the surface of the amorphous PET foil resulting in smoother surfaces after plasma treatment, while for the more crystalline PET foil the etching effect is more randomly leading to rougher surfaces. As a result, amorphous polymer regions are most likely more prone to plasma etching than crystalline regions.

Drop-on-demand inkjet printing, familiar to most of us from small home and office printers, is taking an increasing role in printing for the broader commercial and industrial market. Inkjet printing has made serious inroads into the market for printing banners and signs of all sorts. Wide-format and super wide-format printing is now the norm and has, to an increasing degree, superseded analog printing as the method of choice for printing large format and point-of-purchase signage. Overall, inkjet printing has now taken over 30 percent of the general sign and banner market. One area of printing that holds promise for future growth is that of packaging and labels. Many forms of commercial printing, although a huge market today, are threatened on multiple fronts from various forms of electronic media. Printing and decoration for packaging, on the other hand, is expected to increase in volume in the foreseeable future. In spite of this great promise, penetration of digital printing, in general, and inkjet printing, in particular, into packaging and label printing is still in the low single digits. This article will focus on the label market as an example of printing for packaging. Printing for packaging is a much broader and diverse subject than just labels, but many of the conclusions that follow can be extrapolated to the broader packaging market. Toner-based methods, both wet and dry, have been at the vanguard of penetrating the label market. Today, inkjet is slowly gaining market share. Inkjet has great potential because there is more flexibility in the type and characteristics of fluids that can be applied from an inkjet head. While there are many possible explanations for the relatively low penetration of digital printing into this market, this article will concentrate on the technical challenges involved in reliably printing labels of acceptable quality with inkjet printing. Only now is the inkjet printing industry overcoming these challenges.

In this study, the surfaces of ultrahigh molecular weight polyethylene (UHMWPE), poly(ethylene terephthalate) (PET), and polytetrafluoroethylene (PTFE) films were treated with a helium-water vapor plasma at atmospheric pressure and room temperature. Surface changes related to hydrophilicity, chemical funtionalization, surface energy, and adhesive strength after plasma treatment were investigated using water contact angle (WCA) measurements, X-ray photoelectron spectroscopy (XPS), and mechanical T-peel tests. Results indicate increased surface energy accompanied with enhanced hydrophilicity. WCA decreased by 36, 50, and 16% for UHMWPE, PET, and PTFE, respectively, after only 0.4 s treatment. For UHMWPE, it is shown that the surface functionalization can be tailored depending on the plasma exposure time. Aging studies performed for these three polymers show the stability of the surface groups as indicated by a small increase in WCA values of plasma treated samples which can be attributed to cross-linking of surface and subsurface polymer chains. XPS analysis of the surfaces show increased oxygen content via the formation of polar, hydroxyl-based functional groups. Furthermore, major changes in the polymer structure of PET are observed, possibly due to the opening of the aromatic rings caused by the plasma energetic species. T-peel test results show an 8, 7.5, and 400-fold increase in peel strength for UHMWPE, PET, and PTFE, respectively. Most importantly, it is shown that water-vapor based plasmas can be a promising, “green,” inexpensive route to promote the surface activation of polymers.

The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (α) and advancing (θ_A) and receding (θ_R) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to solution coated polymer films to chemical vapor deposition polymerized polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and two natural surfaces derived from the front and back side of the rose petal. Static water contact angle data suggest that these surfaces vary from hydrophilic with θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces versus α, (cos θ_R – cos θ_A), and the contact angle hysteresis (θ_A – θ_R) all yield scattered plots, indicating that there is little correlation between θ and α, (cos θ_R – cos θ_A) and (θ_A – θ_R). Since the later three parameters have been mentioned to relate to adhesion semiempirically between a liquid droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. In this work, we study both the wetting and adhesion forces between water and these 20 surfaces on a microelectromechanical balance (tensiometer). When the water drop first touches the surface, the attractive force during this wetting step was measured as the “snap-in” force. The adhesion force between the water drop and the surface was measured as the “pull-off” force when the water drop separates (retracts) from the surface. The snap-in force is shown to decrease monotonously as θ_A decreases and becomes zero when θ_A is >150°. The very good correlation is not unexpected due to the similarity between the wetting and the “snap-in” process. The analysis of the pull-off force data is slightly more complicated, and we found that the quality of the water–surface separation depends on the surface “adhesion”. For surfaces that show strong adhesion with water, there is always a small drop of water left behind after the water droplet is pulled off from the surface. Despite this complication, we plot the pull-off force versus α, (cos θ_R – cos θ_A) and (θ_A – θ_R), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle θ_R. Specifically, pull-off force decreases monotonically as θ_R increases, suggesting that θ_R is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between θ_R and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when θ_R is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design are discussed.

After the impact of the great review of M. Strobel and C. S. Lyons on contact angle measurements, we discuss some claims of the authors. The Wilhelmy method is not generally “the best technique for measuring the contact angle hysteresis” as the authors claimed. Otherwise, we think that, even though equilibrium contact angle is an “unattainable” angle, the most-stable contact angle obtained from the system relaxation is experimentally accessible. The most-stable contact angle is energetically significant for evaluating quantitatively the surface energy value of rough, chemically homogeneous surfaces from the Wenzel equation, and the average surface energy of smooth, chemically heterogeneous surfaces from the Cassie equation. The most-stable contact angle, the advancing contact angle and the receding contact angles enable the thermodynamic description of the range of contact angle hysteresis and the distribution of metastable system configurations.

Improving wettability of the polycarbonate (PC) surface to triple distilled water has been carried out by Ar+ ion irradiation with blowing oxygen gas. The amount of Ar+ was changed from 1014 to 5 × 1016 ions/cm2 at 1 keV energy by a Kaufman-type ion source. Contact angle of the water to PC has been reduced from 78° to 50° with Ar+ irradiation, and to 12° with Ar+ irradiation in various vacuum pressures adjusted by oxygen gas flow rate (0-4 sccm). Strong O-H stretching vibration peaks at about 3370 cm−1 on FT-IR spectra of the polymer appeared after the surface treatments, and the wetting angle of the treated PC was returned to its value (78°) when the PC was exposed in air environment. The minimum contact angles were maintained with the same value when the irradiated polymers were kept in dilute HCl solution. The improved wettability and surface chemical reaction by Ar+ ion irradiation with oxygen was explained by the formation of a hydrophilic functional group. Enhanced adhesion between aluminum and PC was confirmed by the scotch tape test, and was discussed with relation between the hydrophilic group on the polymer surface and the deposited metal.

An attempt was made to study the effect of plasma surface activation on the adhesion of UV-curable sol-gel coatings on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrates. The sol was synthesized by the hydrolysis and condensation of a UV-curable silane in combination with Zr-n-propoxide. Coatings deposited by dip coating were cured using UV-radiation followed by thermal curing between 80 °C and 130 °C. The effect of plasma surface treatment on the wettability of the polymer surface prior to coating deposition was followed up by measuring the water contact angle. The water contact angle on the surface of as-cleaned substrates was 80° ± 2° and that after plasma treatment was 43° ± 1° and 50° ± 2° for PC and PMMA respectively. Adhesion as well as mechanical properties like scratch resistance and taber abrasion resistance were evaluated for coatings deposited over plasma treated and untreated surfaces.

A uniform and smooth transfer of stresses across the polymer matrix/fiber interface is enhanced when adhesion between the matrix and fiber surface is optimized. In the absence of covalent bonds matching the Hansen solubility (cohesion) parameters (HSP) of the fiber surface with the HSP of a matrix polymer assures maximum physical adhesion to transfer loads uniformly. Plasma treatment of ultra-high-molecular-weight (UHMWPE) fibers is shown to significantly increase the amount of oxygen in the surface. There are two distinct types of surfaces in both the plasma treated and the untreated UHMWPE fibers. One type is typical of polyethylene (PE) polymers while the other is characteristic of the oxygenated surface at much higher values of HSP. The oxygenated surface of the plasma treated fibers has the HSP δ_D, δ_P, and δ_H equal to 16.5, 15.3, and 8.2, compared to the pure PE surface with HSP at 18.0, 1.2, and 1.4, all in MPa^½. The dispersion parameter has been lowered somewhat by the plasma treatment, while the polar and parameters are much higher. The HSP methodology predicts enhanced adhesion is possible by skillful use of anhydride and nitrile functional groups in matrix or tie polymers to promote compatibility in the system.

Semi-crystalline polyetheretherketone (PEEK) is of interest for providing hermetic sealing of implantable medical devices. Self bonding or autohesion is achieved by mild temperature and pressure treatment and is potentially useful to form joints in the encapsulation of active medical implants. The surfaces of PEEK films were treated in a radio-frequency (RF) plasma containing one of the gases Ar, N₂ and O₂, to achieve surface activation. The bond strength developed over the interface of PEEK films was evaluated by lap-shear testing. The effects of plasma conditions on the surface morphology, composition, and properties were determined using the profilometer, contact angle measurements, X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). Results obtained show that plasma treatment of the PEEK films enhances their bonding strength, with the Ar treated films exhibiting the highest bond strength and nitrogen the lowest. Bond strength was shown to correlate positively with total oxygen content, with C–O group concentration and with the polar component of surface energy. Bond strength correlated negatively with CO group concentration.

Contact angles are used to solve research and manufacturing problems in an industrial environment. Contact angle measurements are scientific, readily acquired using relatively low-cost instruments and simple procedures, are agreeable for use in environments from academic research laboratories to industrial manufacturing facilities, and are an extremely powerful method for characterizing surfaces. The measurement of dynamic contact angles is rate-dependent at high capillary numbers. Water is a preferred probe liquid for contact angle measurements not only because of the importance of aqueous systems in science and industry, but also because water has the highest surface tension of any commonly available probe liquid and therefore has measurable contact angles on most polymeric materials. Most theories of solid surface energy have a basis in Young's equation, which employs the equilibrium contact angle. If surface energy or surface energy component calculations are made, both the advancing and the receding contact angle data should be used in those calculations.

In this commentary we discuss the assay by M. Strobel and C. S. Lyons on contact angle measurements, critical and popular topic in surface/plasma science community. We agree with stressing the importance of dynamic contact angle measurements (i.e. the evaluation of both advancing and receding). However, we make some remarks about the meaning of angle hysteresis with particular regard to the concepts of roughness and chemical heterogeneity, on the basis of our experience in hydrophobic and super-hydrophobic surfaces. Further, we describe our different point of view in the dispute between Wilhelmy balance and sessile drop methods.

The potential of contact angle measurements (CAM) as an analytical tool to characterize surface treatments or modifications is often not fully exploited. Agreeing with Strobel and Lyons, comparing contact angles is often much more reasonable than comparing deduced data like surface energies, because the latter are based on models, in turn involving the influence and knowledge of intermolecular forces at the respective interfaces. For a comprehensive picture, the measurement of contact angles itself has to be considered together with the appropriate model and the available techniques to carry out CAM. An appropriate measurement procedure will be given and a brief discussion of some models to derive free surface energy from CAM.