The measurement and interpretation of contact angles deceptively appear to be simple. This paper attempts to summarize the pitfalls in the field, and how to avoid them. First, the fundamental underlying theory that is necessary in order to properly measure and interpret contact angles is discussed, emphasizing recent developments. Then, the practical implications of these theoretical aspects are presented. In addition, the discussion highlights the missing pieces of the picture that need to be completed through future research.

This article attempts to give an overview of atmospheric plasma sources and their applications. The aim is to introduce, in a first part, the main scientific background concerning plasmas as well as the different atmospheric plasma sources (description, working principle). The second part focuses on the various applications of the atmospheric plasma technologies, mainly in the field of surface treatments.Thus this paper is meant for a broad audience: non-plasma-specialized readers will find basic information for an introduction to plasmas whereas plasma spectroscopists who are familiar with analytical plasmas may be interested in the synthesis of the different applications of the atmospheric pressure plasma sources.

Polyolefins such as polyethylene, polypropylene and their copolymers have excellent bulk physical/chemical properties, are inexpensive and easy to process. Yet they have not gained considerable importance as speciality materials due to their inert surface. Polyethylene in particular holds a unique status due to its excellent manufacturer- and user-friendly properties. Thus, special surface properties, which polyethylene does not possess, such as printability, hydrophilicity, roughness, lubricity, selective permeability and adhesion of micro-organisms, underscore the need for tailoring the surface of this valuable commodity polymer. The present article reviews some of the existing and emerging techniques of surface modification and characterisation of polyethylene. Surface modification of polymers, polyethylene in particular, has been extensively studied for decades using conventional tools. Although some of these techniques are still in use, they suffer from distinct shortcomings. During the last two decades, different means of surface modification have been thoroughly explored. The increasing expectancy for smart materials in daily life has, of late, sharply influenced research in the area of surface modification. Technologies that involve surface engineering to convert inexpensive materials into valuable finished goods have become even more important in the present scenario. In this review article we have attempted to broadly address almost all conventional and modern techniques for the surface modification of different physical forms and chemical compositions of polyethylene. This article will hopefully stimulate further research in this area and result in the development of polyolefins with multi-functional and responsive surfaces, which would ultimately lead to the commodities of polyolefins with smart surfaces.

The hydrophobic characteristic of polymers is considered a limiting property for its applications. To some extent, this has been overcome by techniques such as non-thermal plasma, which, even with a few seconds of application, can increase the surface energy and hydrophilic character of polymers. However, this technique is associated with advantages and disadvantages. Surface degradation related to oxidation and crosslinking are considered irreversible changes, in most cases, while the hydrophobic character is quickly restored, presenting a challenge to researchers all over the world. As a reversible behavior, efforts have been made to understand this particular characteristic of the hydrophobic recovery (or the aging effect) of polymers. The application of non-thermal plasma on polymeric surfaces has also been used in biomedicine as a sterilization device to control the growth of biofilms, as well as to increase the biocompatibility of prosthetic surfaces. This chapter discusses some particular characteristics of polyolefins exposed to plasma.

Polyolefins are well-known and the most commonly used polymers worldwide. Advantages like outstanding mechanical properties, chemical resistance, low cost, and processability are neighboring with some drawbacks like relatively high gas and vapor permeability, low surface energy. This chapter introduces surface plasma modification as an environmentally friendly, fast, and versatile technique. Details regarding different plasma reactor designs, generation methods, working parameters suitable for treating polyolefins are presented. Furthermore, plasma activation, grafting, and etching are described as the most commonly used techniques for surface energy modification to enhance polyolefins' biocompatibility, printability, adhesion to materials, and other parameters. For instance, plasma activation cross-linking of the polymer chains can be achieved, which leads to gas and vapor permeability improvement. Choice of working conditions allows controlling the degree of cross-linking, the type, and the concentration of the incorporated functional groups on the surface. Plasma polymerization is introduced as a technique for coating deposition with different properties and functionality depending on the operating parameters and monomer selection. Improvement of barrier layer performance and modification of the surface energy are the main applications of plasma polymerization of polyolefins.

Capillarity is the study of the interfaces between two immiscible liquids, or between a liquid and air. The interfaces are deformable: they are free to change their shape in order to minimize their surface energy. The field was created in the early part of the 19th century by Pierre Simon de Laplace (1749–1827) and Thomas Young (1773–1829). Henri Bouasse wrote a wonderful account of developments in capillarity in a book he published in 1924.1 This discipline enables us to understand the games water can play to break the monotony of a rainy day or the tricks it performs while washing dishes. On a more serious note, capillarity plays a major role in numerous scientific endeavors (soil science, climate, plant biology, surface physics, and more), as well as in the chemical industry (product formulation in pharmacology and domestics, the glass industry, automobile manufacturing, textile production, etc.).

When we place a liquid drop on a clean, planar, solid surface, we can observe a contact angle θ _E , which is precisely the angle contained in Young’s formula. Quite often, though, the surface is marred by defects that are

• either chemical (stains, blotches, blemishes)

• or physical (surface irregularities).

The effect of air dielectric barrier discharge plasma treatment on the chemical structure and morphology of polypropylene (PP)film was studied using UV-VIS (ultraviolet-visible),FT-IR,(Fourier transform infrared),SEM (scanning electron microscopy)and AFM (atomic force microscopy).Polypropylene samples were printed using solvent-based gravure ink.An evaluation of the print quality criteria of the treated PP films included measurement of print density and print gloss.SEM investigated the ink laydown on the modified PP film.The results showed that after a few seconds of plasma treatment,both the surface energy and the surface roughness of the treated PP film increased.There was an increase in the absorbance at the almost-visible range,and C=C and C=O bands were found after the air discharge plasma treatment.A short plasma treatment of 15 seconds was found to bring about a dramatic increase in the print density readings,but a decrease in print gloss.The time of the air discharge plasma treatment was found to have no effect on the print density or print gloss at a high ink film thickness.The results showed that air dielectric barrier discharge plasma treatment,for a few seconds,is effective in printing and is economical for industrial use (this will be studied in detail in future work).

Through the controlled use of selected pretreatments, significant improvements to adhesion levels can be realised. Pretreatment options include chemical activation, corona discharge treatment, plasma-induced modifications and grafting. Using such methods, adhesion levels that render substrates fit for the intended purpose can be achieved. Such improvements can be realised without compromising the inherent properties of the materials being treated. Various approaches are considered as is the nature of the adhesion process. Several reasonably recent examples of the use of surface activation are presented.

The activation of polymer surfaces in glow discharges and the deposition of metals from organo-metallic vapours, both at low pressure, are standard laboratory processes. The upscaling to industrial mass product applications is, however, hampered by cost and the time consumption needed for establishing a sufficient vacuum. Atmospheric pressure processes based on the same physical surface interactions show great promise as replacements of some steps in galvanic plating. Successful metallizations are reported after treatment of polymers in barrier discharges at atmospheric pressure. These may be applied directly to the surface of the workpiece or indirectly from within large-area monochromatic excimer UV lamps. Comparisons with excimer UV laser treatment are made.

The current broad interest in wetting characterization of solid surfaces is driven by recent advances in the formulation of surfaces and coatings that are superhydrophobic, superhydrophilic, oleophobic, oleophilic and so on. Unfortunately, the contact angle data presented in many publications raise some concerns among the surface chemists and physicists who work with contact angle measurement techniques on a regular basis. In those articles, best practices are often ignored, and the data presented are limited to the static contact angles measured for small droplets, a few times smaller than typically recommended. The reported contact angles are neither advancing nor receding, and their reproducibility in different laboratories is therefore questionable. In this note, guidelines to measurements of reproducible and reliable advancing and receding contact angles are summarized.

The surface characteristics of polymers are important factors determining their interfacial properties and their technological performance. Changes in physical and chemical properties of a polymer film may be induced by subjecting the material to a variety of surface modification techniques, one of which is ion-beam modification. In order to understand the underlying mechanisms X-ray photoelectron spectroscopy (XPS) was used to study the alterations of the polystyrene (PS) surface after Ar-ion treatment under well controlled conditions with low ion doses from 10¹² to 10¹⁶ cm^-2. The ion bombardment leads to surface functionalization, loss of aromaticity, and free radical formation. Induced surface cross-linking and the formation of polar groups raised the surface glass transition temperature of PS film.

Polymers have been applied successfully in fields such as adhesion, biomaterials, protective coatings, friction and wear, composites, microelectronic devices, and thin-film technology. In general, special surface properties with regard to chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity, and cross-linking density are required for the success of these applications. Polymers very often do not possess the surface properties needed for these applications. However, they have excellent bulk physical and chemical properties, are inexpensive, and are easy to process. For these reasons, surface modification techniques which can transform these inexpensive materials into highly valuable finished products have become an important part of the plastics and many other industries. In recent years, many advances have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces without affecting bulk properties. Common surface modification techniques include treatments by flame, corona, plasmas, photons, electron beams, ion beams, X-rays, and γ-rays.

Plasma treatment is probably the most versatile surface treatment technique. Different types of gases such as argon, oxygen, nitrogen, fluorine, carbon dioxide, and water can produce the unique surface properties required by various applications. For example, oxygen-plasma treatment can increase the surface energy of polymers, whereas fluorine-plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking at a polymer surface can be introduced by an inert-gas plasma. Modification by plasma treatment is usually confined to the top several hundred ångströms and does not affect the bulk properties. The main disadvantage of this technique is that it requires a vacuum system, which increases the cost of operation.

Thin polymer films with unique chemical and physical properties are produced by plasma polymerization. This technology is still in its infancy, and the plasma chemical process is not fully understood. The films are prepared by vapor phase deposition and can be formed on practically any substrate with good adhesion between the film and the substrate. These films, which are usually highly cross-linked and pinhole-free, have very good barrier properties. Such films find great potential in biomaterial applications and in the microelectronics industry.

Very high-power microwave-driven mercury lamps are available, and they are used in UV-hardening of photoresist patterns for image stabilization at high temperatures. Other applications of UV irradiation include surface photo-oxidation, increase of hydrophilicity, and photocuring of paintings.

Pulsed UV-lasers are used in surface modification in many areas. Pulsed UV-laser irradiation can produce submicron periodic linear and dot patterns on polymer surfaces without photomask. These interference patterns can be used to increase surface roughness of inert polymers for improved adhesion. These images can also be transferred to silicon surfaces by reactive ion etching. Pulsed laser beams can be applied to inert polymer surfaces for increased hydrophilicity and wettability. Polymer surfaces treated by pulsed UV-laser irradiation can be positively or negatively charged to enhance chemical reactivity and processability. Pulsed UV-laser exposures with high fluence give rise to photoablation with a clean wall profile. There are many other practical applications of laser photoablation, including via-hole fabrication, and diamond-film deposition. The present review discusses all these current applications, especially in the biomedical and microelectronics areas.

Polymers have been applied successfully in fields such as adhesion, biomaterials, protective coatings, friction and wear, composites, microelectronic devices, and thin-film technology. In general, special surface properties with regard to chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity, and cross-linking density are required for the success of these applications. Polymers very often do not possess the surface properties needed for these applications. However, they have excellent bulk physical and chemical properties, are inexpensive, and are easy to process. For these reasons, surface modification techniques which can transform these inexpensive materials into highly valuable finished products have become an important part of the plastics and many other industries. In recent years, many advances have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces without affecting bulk properties. Common surface modification techniques include treatments by flame, corona, plasmas, photons, electron beams, ion beams, X-rays, and γ-rays.

Plasma treatment is probably the most versatile surface treatment technique. Different types of gases such as argon, oxygen, nitrogen, fluorine, carbon dioxide, and water can produce the unique surface properties required by various applications. For example, oxygen-plasma treatment can increase the surface energy of polymers, whereas fluorine-plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking at a polymer surface can be introduced by an inert-gas plasma. Modification by plasma treatment is usually confined to the top several hundred ångströms and does not affect the bulk properties. The main disadvantage of this technique is that it requires a vacuum system, which increases the cost of operation.

Thin polymer films with unique chemical and physical properties are produced by plasma polymerization. This technology is still in its infancy, and the plasma chemical process is not fully understood. The films are prepared by vapor phase deposition and can be formed on practically any substrate with good adhesion between the film and the substrate. These films, which are usually highly cross-linked and pinhole-free, have very good barrier properties. Such films find great potential in biomaterial applications and in the microelectronics industry.

Very high-power microwave-driven mercury lamps are available, and they are used in UV-hardening of photoresist patterns for image stabilization at high temperatures. Other applications of UV irradiation include surface photo-oxidation, increase of hydrophilicity, and photocuring of paintings.

Pulsed UV-lasers are used in surface modification in many areas. Pulsed UV-laser irradiation can produce submicron periodic linear and dot patterns on polymer surfaces without photomask. These interference patterns can be used to increase surface roughness of inert polymers for improved adhesion. These images can also be transferred to silicon surfaces by reactive ion etching. Pulsed laser beams can be applied to inert polymer surfaces for increased hydrophilicity and wettability. Polymer surfaces treated by pulsed UV-laser irradiation can be positively or negatively charged to enhance chemical reactivity and processability. Pulsed UV-laser exposures with high fluence give rise to photoablation with a clean wall profile. There are many other practical applications of laser photoablation, including via-hole fabrication, and diamond-film deposition. The present review discusses all these current applications, especially in the biomedical and microelectronics areas.

Two engineering thermoplastic polymers (polycarbonate, PC, and acrylonitrile butadiene styrene copolymer, ABS) were treated with atmospheric plasma torch using different treatment rates (1, 5 and 10 m/min). The modifications produced by the treatment were analysed by contact angle measurements, XPS, SEM and ATR-IR spectroscopy. Particular emphasiswas placed on the ageing (up to 30 days) after atmospheric plasma treatment on both polymers. The slower the atmospheric plasma treatment, the greater the wettability of the treated polymers. The decrease in water contact angle was mainly ascribed to a significant increase in oxygen content due to the formation of carboxylic and hydroxyl groups and a decrease in the carbon content on the polymer surfaces. After natural ageing, there was an increase in the water contact angle, although the values of the untreated polymer surface were never reached.

This work concerns the ageing effect of the atmospheric plasma and corona treatments when used to treat paper substrates. Pigment coated and surface sized papers were modified using two types of atmospheric plasma equipment; one at the pilot scale and one at the laboratory scale. In addition, the plasma treatments were compared to conventional corona treatment. Surface energy was estimated by contact angle measurements and surface chemistry by X-ray photoelectron spectroscopy (XPS) as a function of the time during three months. The treatments increased surface energy and oxidation level of surface for both papers. The ageing effect could be detected only in the surface energy, whereas the oxidation level remained stable during the twelve weeks. The decay in surface energy was faster during the first weeks of storage and subsequently leveled off leading to a permanent change. The permanent change was explained as a contribution of oxygen containing polar molecular groups, which were detected by XPS. The ageing effect was suggested to originate from already existing polar molecular groups, which have rotated on the surface by plasma-related process and then rotate back into the material in time. A part of the decay was also explained by the plasma cleaning model, in which the ageing effect occurred through re-contamination. Paper is a multicomponent system, where the constituents that have the lowest surface energy were suggested to migrate to paper surfaces.

Different types of plasma, irradiative and chemical activation were compared in terms of surface functionalization. Corona and spark jet plasmas are characterized by low gas temperatures and high rates in surface modification. UV irradiation in the presence of ozone does not involve any particle bombardment and acts only by enhanced photooxidative processes. Although ion implantation can be avoided, this method is not free of radiative damage in both the surface-near region and the bulk of polymers. Furthermore, its functionalization rate is low. In relation to low-pressure O₂ plasma modification, all treatments mentioned here have a low efficiency in adhesion promotion due to oxidative degradation of macromolecules and formation of molecular debris known as the “weak boundary layer”.

The treatment of surfaces by corona discharges is a well-established method to improve surface properties. The surface to be treated is moved continuously and is exposed to transient gas discharges, known as microdischarges, in air at atmospheric pressure between electrodes, where at least one electrode is covered with a dielectric barrier. Because of the short duration, only some 10 ns, the current through the microdischarges is predominantly carried by electrons. The ion temperature remains close to room temperature. Owing to these properties such discharges are qualified to treat surfaces which are sensitive to higher temperatures. For a large number of applications this treatment is adequate, but the adhesion of aqueous glues and inks to some plastic materials is insufficient if the surfaces are treated in this way. Furthermore, it is difficult to meet the requirements of surface properties of, for instance, polyolefine film (e.g. surface tension, adhesion). This material is not based on monomers containing chlorine or fluorine and is preferred for ecological reasons. This paper presents the results of experiments which demonstrate that in comparison to a common corona treatment significant improvements in surface properties of plastic materials can be achieved if repetitively generated pulse trains and reactive gases are used instead of air. If, for instance, the microdischarges are established in acetylene, thin films with a thickness of several namometres are formed on surfaces, which increase and stabilize the surface tension up to a level of 72 mN m⁻¹. The state of the art of this new technology is discussed.

Surface modifications of polyetheretherketone (PEEK) made by chemical etching or oxygen plasma treatment were examined in this study. Chemical etching caused surface topography to become irregular with higher roughness values R_a and R_q. Oxygen plasma treatment also affected surface topography, unveiling the spherulitic structure of PEEK. R_a, R_q and surface area significantly increased after plasma treatment; topographical modifications were, nonetheless, moderate. Wetting angle measurements and surface energy calculations revealed an increase of wettability and surface polarity due to both treatments. XPS measurements showed an increase of surface oxygen concentration after both treatments. An O:C ratio of 3.10 for the plasma-treated PEEK surface and 4.41 for the chemically-etched surface were determined. The results indicate that surface activation by oxygen plasma treatment for subsequent coating processes in supersaturated physiological solutions to manufacture PEEK for biomedical appiications is preferable over the chemical etching treatment.

For vacuum web coating for permeation barrier coatings in flexible packaging, the final functionality of the packaging media is extremely dependent on the whole chain of processing steps up to the final laminated packaging film. The most sensitive sector appears to be, on the one hand the hand-shake between substrate film pretreatment–substrate surface properties and the coating process with its characteristics on the other. The influence of different surface pretreatment processes (Corona activation, oxygen and ammonia plasma treatment) on active surface groups of BOPP (biaxial-orientated polypropylene) substrates is shown together with: (1) specifities of the thermal deposition (electron beam source); (2) the reactive deposition–microwave plasma process (plasma species, excitation characteristics, kinetic energies, obtained by in situ process monitoring); and (3) structural properties (chemical composition, adhesion and oxygen permeation) of the thin barrier films (Al₂O₃ and SiO_x), in correlation with the achieved functional properties of the barrier coated films.

Electroless plating of non-conducting materials needs, prior to the metal deposition itself, to make the sample surface catalytically active. The route involving the chemical reduction of a thin solid metal–organic coating has, for this purpose, a significant potential in reducing the number of steps which are required today in conventional wet chemical metallization processes. In this work, a novel activation process using a dielectric barrier discharge (DBD) is described for the first time. This process is based on the plasma-induced chemical reduction at atmospheric pressure in air of palladium acetate (PdAc) layers resulting in the formation of palladium (Pd) on non-active surfaces. Fast surface activation of polymers like polyimide (PI) was found to occur in only a few seconds using a simple DBD device instead of expensive excimer UV lamps or complicated laser systems. The DBD-induced Pd layers on PI exhibit high activity with regard to initiation of the electroless copper plating. Indeed, copper deposition starts immediately after dipping the activated PI samples in the electroless solution without any inhibition time. Homogeneous copper coatings on PI were achieved under optimal plasma treatment conditions. The results are compared to those achieved with excimer UV lamps and excimer UV lasers.

The means by which plasma treatment enhances the adhesion of polymer materials, remains obscure. Thus far, two possible mechanisms have been proposed: an increase in surface energy, and the anchor effects imparted by plasma etching. Independently from these mechanisms, reactions between free radicals, generated by plasma irradiation and adhesives are also likely to affect the adhesive properties of polymer materials. Free radicals generated on polyethylene (PE) by glow-discharge plasma were exposed to air and converted to peroxide. The peroxides were converted back to free radicals with the application of heat, and then graft polymerization was initiated, by adding a hydrophilic monomer such as acrylic acid. The peroxides formed by the reaction between free radicals and the oxygen in air was detected by chemiluminescence (CL). In this work, plasma-treated PE surfaces were bonded to aluminum boards, using epoxy resin as an intermediate adhesive and then subjected to a series of peeling tests. The sample with the highest peeling strength also had the highest level of CL-detected peroxides. These findings suggest that the free radicals generated by plasma treatment influence the adhesive properties of the polymer materials.

The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with Langmuir–Blodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.

The hydrophobic recovery of oxygen-plasma-treated hydrophilic surfaces of polyethylene and polypropylene films was investigated by measuring the dependence of the contact angle with the water on the vacuum storage time. It could be shown that the rehydrophobation of the polyethylene surface may be retarded by the previous controlled surface cross-linking in a hydrogen plasma and/or by repeated plasma treatment. However, the loss in hydrophilicity cannot be suppressed for ever. After certain individual periods all treatments lead to the same final state. Nevertheless, in particular, controlled cross-linking seems to be a suitable way for improving the long-term stability of plasma-functionalized polymer surfaces for polymers not tending to chain scission during plasma treatment.

o optimize the effects of some discharge parameters on the surface wettability of polypropylene (PP) in atmospheric pressure dielectric barrier discharge, a surface modification model is created based on statistical theory and orthogonal experimental design method. Contact angle measurements, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) are used to study the changes in the surface wettability, surface topology and chemical compositions of the samples. The results show that surface wettability has been improved due to roughness increasing and the introduction of oxygen-containing functional groups. High-resolution XPS of C1s peak deconvolution indicates that the types and contents of oxidized functional groups are different in different discharge conditions or plasma energy. Moreover, the model analysis reveals that it has better predictive ability, and different discharge parameters has selective influence on water contact angle and surface O atom percentage.

Air plasma treatment at low pressure was applied to modify the surface of a cellulose film with the aim to improve its wettability, dyeability and adhesion properties. The contact angles of different polar liquids on the treated films show an exponential decay with treatment time at a given power; the power–time reciprocity is followed. The calculated surface tension values exponentially rise to the same maximum value with a decrease of the polar fraction. ATR-FTIR analyses suggest that a cellulose dehydration takes place rather than a surface oxidation. The plasma treatment improves also the cellophane dyeability with typical dyes for cellulose fibers: the results of dye uptake follow the same trend as the surface energy. The bond strength of lap joints of cellophane with LLDPE film shows a strong improvement of the adhesion depending on the duration and the power of treatment. The whole results are consistent with ablation effects like those observed with air corona treatment rather than oxygen plasma.