A summary of recent studies on the corona treatment of films is presented. Chemical functional groups generated by the corona discharge on these films are identified and their effect on ink film wettability and adhesion discussed.

The wetting property of polymers is very important in different applications such as biomaterials, textiles, aerospace (fluid management and materials processing in microgravity), and thin film adhesion. Therefore, there is a strong interest in the development of a new technology for the modification at will of this property. The use of low energy ion beams allows the modification of the first surface atomic layers. Nitrogen ions of 500 eV/at. were used to bombard the surface of polycarbonate (PC) samples to a fluence of 5×1016 at/cm2. Five different environments (oxygen, nitrogen, argon, dry air and vacuum) were used to store the samples for some hours (1 to 24 hours) after the implantation. Aging studies of the contact angle (advancing and receding) have shown that the environment gas influences the long term value of the contact angle and helps to maintain the stability of the treated surfaces with the passage of time. XPS and ToF-SIMS have been used to study the chemical effects of both N2 + ion irradiation and storage gas surrounding the samples. The results show faster aging in the case of the samples stored in vacuum, a harmful effect of nitrogen gas on the treatment and the formation of new chemical species for all treatments.

The surface modification of isotactic polypropylene (PP) in a microwave plasma of CO2 is described. The modified PP is characterized in bulk and also at its surface. The mechanism of plasma modification is discussed in terms of degradation and oxidation. The degradation leads to volatile products and to the formation of a layer of oxidized oligomers of PP. The oxidation leads to ketone, acid or ester groups. The degradation and oxidation rates depend on plasma parameters (duration, discharge power, gas flow, pressure, discharge or post-discharge treatment). The oxidation rates vs the various plasma parameters show a maximum. The crosslinking of PP (Crosslinking Activated Species of INert Gases) seems to be negligible.

Cold plasma treatments of polymers, dry processes, allow either the elaboration of hydrophilic or hydrophobic surfaces. For example, a poly(propylene) film treated in nitrogen plasma shows a surface having a hydrophilic and basic character since amino groups are attached onto the surface during the treatment. The treatment induces an increase of the surface tension of the polymeric material, which may be sometimes destroyed by an aging effect. For the treatment of poly(propylene) in nitrogen plasma, the aging is mostly due to a motion of attached groups from the surface to the bulk of the material and some oxidation of radicals formed during plasma treatment. The surface radicals formed and used for a post-reaction such as grafting are characterized in terms of chemical nature, density and reactivity.

The interactions between CO₂ plasma, less degrading than O₂ plasma, and polymeric surfaces are studied. CO₂ discharge and the relationships between the density of plasma reactive species are analyzed by optical emission spectroscopy and mass spectrometry. The optical emission spectrum was identified and five principal systems of carbon monoxide were assigned: the 4th and 3rd positive systems, Angstrom and 3A systems. Other systems dealing with ionized species CO⁺ ₂ and CO⁺ were also found. Mass spectrometry showed that the carbon monoxide and atomic oxygen were created through CO₂ dissociation by electronic impact. The detected molecular oxygen coming from the atomic oxygen recombination was associated with the power. The study of plasma/polymer interface showed the consumption of ionized species, the appearance of atomic hydrogen due to methyl groups transformation into exomethylene groups onto the polypropylene surface, and a degradation mechanism dependent on atomic oxygen density in the plasma phase.

The surface energies of poly (methyl methacrylate), polycarbonate and poly (tetrafluoroethylene) which have been exposed to UV radiation in an ambient ozone-air atmosphere have been elucidated from surface tension and contact angle data using these test liquids: ethylene glycol, formamide, glycerol, methylene iodide and water. Comparisons of surface energy values obtained using Kaelble’s two-liquid method, Good’s three-liquid method and Neumann’s macroscopic apнproach are reported. It is tentatively suggested that atmospheric moisture may play a role in producнing discordant values since the test liquids ethylene glycol, formamide and glycerol are highly hyнgroscopic in nature. It has been demonstrated that UV/ozone irradiation produces changes in surface roughness. Poly (tetrafluoroethylene) shows three distinct regions: first, where at low irradiation times the surface roughness is enhanced and following this, the roughness decreases before increasнing finally to a terminal value. The behavior is somewhat similar for polycarbonate although the dramatic increase in roughness exhibited by poly (tetrafluoroethylene) is absent. The roughness characteristics are quite different for poly (methyl methacrylate) where a large change in roughness is observed at only one specific irradiation time. Thus presently it is not possible to predict surface roughness changes for a particular polymer and more studies on the morphological changes occurнring at different surfaces are being carried out.

It is demonstrated that extraneous electric charge can produce a large variation in contact angle for water drops on polytetrafluoroethylene surfaces.

Polyethylene (PE) is often used in several industrial applications including the building, packaging and transport industries. Aluminum (Al) is widely used in different applications in the automotive, railway, aeronautic, and naval industries because of its excellent mechanical and chemical properties. Laminates prepared from Al and PE lead to an enhancement in physical and mechanical properties. These materials play a main role in the packaging and building sectors, such as in TetraPak containers and aluminum composite panels. The main problem observed is associated with the adhesion between polymers and metals. This research focused on investigating the enhancement in the adhesion of the PE/Al laminate using the corona discharge. The corona treatment of the surfaces led to a significant increase in the adhesion of the PE/Al laminate as a result of improved surface properties confirmed by peel test measurements. Moreover, the positive effect of the corona treatment in combination with a primer on the improvement of adhesion characteristics was observed too. Different analytical techniques were employed to characterize the effect of the corona treatment on the improvement in adhesion of PE/Al. A significant increase in wettability was confirmed by the measurement of contact angles. Changes in the surface morphology of the PE and Al surface, after the corona treatments at different operating conditions, were observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). In addition, x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to analyze changes in chemical composition after the corona discharge effect on PE and Al surfaces.

Polymers/metal laminates are often used to improve physical and mechanical properties, especially those required in building applications. A flat aluminum composite panel (ACP) consisted mainly of two thin metal sheets usually made of aluminum (Al) and a non-metal core, such as polyethylene (PE). The lack of adhesion associated with the low wettability of PE is a serious problem. An eco-friendly, dry, non-destructive corona treatment technique can be applied to solve this problem. In this work, the use of a corona treatment to enhance the adhesion properties of linear low-density polyethylene (LLDPE) was studied. The changes in surface and adhesion properties were thoroughly analyzed using various analytical techniques and methods to obtain the optimal parameters for corona discharge using contact angle measurements, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). AFM force adhesion measurements were used to analyze the effect of the corona treatment on the adhesion enhancement of LLDPE, and the peel tests confirmed a significant increase in peel resistance in the LLDPE/Al laminate. A synergy effect from using the corona treatment in combination with an ethylene acrylic acid dispersion primer was observed.

A method of treating plastic film to improve the adhesion of ink impressions imprinted thereon comprises subjecting the opposite surfaces of the plastic film simultaneously to the action of high voltage stress accompanied by corona discharge. As shown, a web 10 is unwound from a supply reel 12 and after passing over rollers 16, 18 passes between electrodes 24, 26 one of which is earthed and the other of which has a high voltage applied thereto by means of a high-voltage transformer 60. The web then passes over rollers 20, 22 on to a take-up roll 14. Alternatively the web may be fed directly to a printing machine. The ground electrode 24 is carried on posts 30 and is provided with notches, Fig. 3 (not shown), on each longitudinal edge so that a glass cord may be threaded therethrough and held in position, thus acting as a spacing member for the web. The high voltage electrode 26 is secured in a recess formed in the inner periphery of a frame 40 formed of insulating material and a solid dielectric 42 is disposed on the upper surface of the electrode. The electrode 26 is supported on posts 50, and a glass cord 46, for spacing purposes, is wound on pins .44 extending from the frame 40. In alternative arrangements the glass cords may be wound spirally around the electrodes, Fig. 4 (not shown), or a fabric may be disposed over each electrode, Fig. 5 (not shown), or both methods employed. The voltage on the high voltage electrode 26 is preferably between 10,000 and 20,000 v., and the electrode may be made of stainless steel.

The surface treatment of plastics as well as metals or ceramics includes a thorough surface cleaning as an essential step prior to adhesive bonding and coating processes. Besides this, surface activation of polymers is often needed because their surface free energy is too low for durable adhesion of a coating or adhesive. In this chapter various types of UV/Ozone sources with different light spectra as well as the influences of spectra and ozone concentration are investigated and compared. Also the surface wetting and adhesive bond strength as a result of UV/Ozone, atmospheric plasma, or corona treatments on thermoset, thermoplastic, and rubber materials are presented. UV/Ozone treatment was found to show an excellent cleaning performance on all kinds of materials, and especially as a very useful technique for surface functionalisation of polymers, resulting in durable adhesion both for adhesives as well as coatings. This chapter is a condensed overview of over 30 years of experiments done with UV/Ozone treatments at The Delft University of Technology.

Physical and chemical surface modifications of polyethyleneterephthalate (PET) films due to treatment with excimer UV lamps (222 nm) have been studied. Interpretations of the reactions and products were made in comparison to known PET irradiations with excimer UV lasers and broad-band UV sources. In this context the advantages of the excimer UV lamps as a light source, i.e., a quasi monochromatic radiation source with a power density which is sufficient for initiating surface reactions without changing the topography of the substrate, have been shown. Analytical data on treated PET to characterize the surface modifications were obtained by contact-angle measurements, dyeing with cationic dyestuff, scanning electron microscopy (SEM), photoacoustic Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) studies. With regard to possible industrial applications, the relevance for textile finishing and some perspectives for future developments are pointed out.

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.

The relationships of the optical characteristics of monomers (refractive index and specific refraction) and the cohesion parameters of polymers (effective cohesional energy and molar volume of the repeating unit) have been analyzed. New relationships are proposed that allow the calculation of the surface energies of homo- and copolymers using refractometric data of monomers. The relationships were tested with good consistency for a number of polymers of various chemical nature.

The apparatus for the electrical pretreatment of foils to increase the surface tension (surface energy) comprising a roller electrode forming a counter-electrode and over which is passed the foil to be treated, and at least three knife electrodes, which are arranged parallel to one another and at right angles to the direction of movement of the foil. The discharge edges of the knife electrodes have a constant spacing from the roller electrode surface, which leads to a much better treatment result.

A method to modify the surface or fluoropolymer by a reactive gas plasma so that the surfaces have improved fluoropolymer to other substrate adhesive bond strength.