The surface modifications produced by treatment of a synthetic sulfur vulcanized styrene-butadiene rubber with oxidizing (oxygen, air, carbon dioxide) and non oxidizing (nitrogen, argon) RF low pressure plasmas, and by treatment with atmospheric plasma torch have been assessed by ATR-IR and XPS spectroscopy, SEM, and contact angle measurements. The effectiveness of the low pressure plasma treatment depended on the gas atmosphere used to generate the plasma. A lack of relationship between surface polarity and wettability, and peel strength values was obtained, likely due to the cohesive failure in the rubber obtained in the adhesive joints. In general, acceptable adhesion values of plasma treated rubber were obtained for all plasmas, except for nitrogen plasma treatment during 15 minutes due to the creation of low molecular weight moieties on the outermost rubber layer. A toluene wiping of the N{2 } plasma treated rubber surface for 15 min removed those moieties and increased adhesion was obtained. On the other hand, the treatment of the rubber with atmospheric pressure by means of a plasma torch was proposed. The wettability of the rubber was improved by decreasing the rubber-plasma torch distance and by increasing the duration because a partial removal of paraffin wax from the rubber surface was produced. The rubber surface was oxidized by the plasma torch treatment, and the longer the duration of the plasma torch treatment, the higher the degree of surface oxidation (mainly creation of C O moieties). However, although the rubber surface was effectively modified by the plasma torch treatment, the adhesion was not greatly improved, due to the migration of paraffin wax to the treated rubber-polyurethane adhesive interface once the adhesive joint was produced. On the other hand, the extended treatment with plasma torch facilitated the migration of zinc stearate to the rubber-adhesive interface, also contributing to deteriorate the adhesion in greater extent. Finally, it has been found that cleaning of SBS rubber in an ultrasonic bath prior to plasma torch treatment produced a partial removal of paraffin waxes from the surface, and thus improved adhesion was obtained.

Interest has grown over the past few years in applying atmospheric pressure plasmas to plasma processing for the benefits this can offer to existing and potential new processes, because they do not require expensive vacuum systems and batch processing. There have been considerable efforts to efficiently generate large volumes of homogeneous atmospheric pressure non-thermal plasmas to develop environmentally friendly alternatives for surface treatment, thin film coating, sterilization, decontamination, etc.

Many interesting questions have arisen that are related to both fundamental and applied research in this field. Many concern the generation of a large volume discharge which remains stable and uniform at atmospheric pressure. At this pressure, depending on the experimental conditions, either streamer or Townsend breakdown may occur. They respectively lead to micro-discharges or to one large radius discharge, Townsend or glow. However, the complexity arises from the formation of large radius streamers due to avalanche coupling and from the constriction of the glow discharge due to too low a current. Another difficulty is to visually distinguish many micro-discharges from one large radius discharge. Other questions relate to key chemical reactions in the plasma and at the surface. Experimental characterization and modelling also need to be developed to answer these questions.

This cluster collects up-to-date research results related to the understanding of different discharges working at atmospheric pressure and the application to polymer surface activation and thin film coating. It presents different solutions for generating and sustaining diffuse discharges at atmospheric pressure. DC, low-frequency and radio-frequency excitations are considered in noble gases, nitrogen or air. Two specific methods developed to understand the transition from Townsend to streamer breakdown are also presented. They are based on the cross-correlation spectroscopy and an electrical model.

Dielectric barrier discharge (DBD) is the discharge involved in corona treatment, widely used in industry to increase the wettability or the adhesion of polymer films or fibers. Usually DBD's are filamentary discharges but recently a homogeneous glow DBD has been obtained. The aim of this paper is to compare polypropylene surface transformations realized with filamentary and glow DBD in different atmospheres (He, N₂, N₂ + O₂ mixtures) and to determine the relative influence of both the discharge regime and the gas nature, on the polypropylene surface transformations. From wettability and XPS results it is shown that the discharge regime can have a significant effect on the surface transformations, because it changes both the ratio of electrons to gas metastables, and the space distribution of the plasma active species. This last parameter is important at atmospheric pressure because the mean free paths are short (∼μm). These two points explain why in He, polypropylene wettability increase is greater by a glow DBD than by a filamentary DBD. In N₂, no significant effect of the discharge regime is observed because electrons and metastables lead to the same active species throughout the gas bulk. The specificity of a DBD in N₂ atmosphere compared to an atmosphere containing oxygen is that it allows very extensive surface transformations and a greater increase of the polypropylene surface wettability. Indeed, even in low concentration and independently of the discharge regime, when O₂ is present in the plasma gas, it controls the surface chemistry and degradation occurs.

Recently, a glow like dielectric controlled barrier discharge (GDBD) working at atmospheric pressure has been observed. Such a discharge could replace a filamentary dielectric controlled barrier discharge (FDBD) used in corona treatment systems to improve the wettability or the adhesion of polymers. So it is of interest to compare these two types of discharges and their respective effect on a polymer surface. This is the aim of an extensive study we have undertaken. The first step presented here is the comparison of a filamentary discharge in air with a glow discharge in helium. Helium is the most appropriate gas to realize a glow discharge at atmospheric pressure. Air is the usual atmosphere for a corona treatment. The plasma was characterized by emission spectroscopy and current measurements. The surface transformations were indicated by the water contact angle, the leakage current measurement and the X-ray photoelectron spectroscopy. Results show that the helium GDBD is better than air FDBD to increase polypropylene wettability without decreasing the bulk electrical properties below a certain level. Contact angle scattering as well as leakage current measurements confirm that the GDBD clearly results in more reproducible and homogeneous treatment than the FDBD.

Three different dielectric barrier-controlled discharge regimes in helium at atmospheric pressure under sinusoidal excitation have been obtained by varying the excitation frequency or the gas chemical composition: the filamentary discharge, which is the discharge that is usually obtained; the glow discharge, which is controlled by cathode secondary emission; and the homogeneous discharge, which is of a nature in between those of the filamentary and the glow discharges. All the characteristics that have been studied, such as the discharge current, the emission spectrum, the wettability and the chemical transformations of a polypropylene film, are related to the discharge-regime variation. The glow discharge is clearly more efficient than the others as a means of increasing the polypropylene-surface energy. Values as high as 62 mJm^-2 are obtained with this discharge whereas the maximum value after interaction with the filamentary one is 45 mJm^-2. This improvement in wettability is due to there being more O atoms implanted at the surface as well as to the addition of N atoms. The differences among in surface transformations have been correlated to the characteristics of these different discharges and more specifically to the localization of the electrical energy transfer into the gas and to the nature of the ions created during the discharge.

A novel plasma chemical process PPCP (pulse corona induced plasma chemical process) can produce copious active radicals in air under NTP (normal temperature and pressure) by using an extremely fast rising narrow high voltage pulse between corona electrodes and grounded counter electrodes so that intense streamer coronas are generated. This provides an effective means of surface treatment to a plastic material placed between the two electrodes through generation of free bonds on the surface directed to the corona electrodes. Special features of this method are that it can cope with a complex shape of the material to be treated, and that it does not spark even at the periphery near the grounded counter electrode. This method is suitable for the surface treatment of polypropylene bumpers so as to provide a strong adhesion of color paint to it. The adhesion strength of a paint film is raised from zero to ca. 1000 g/cm/sup 2/ by PPCP treatment for 60 seconds.

Combined surface treatments using plasma fluorination and surface roughening were applied to investigate whether they could increase the peel property of PET beyond the value needed for use as a release coating of pressure-sensitive adhesive tapes. The peel strength of PET treated with CF₄/He APG plasmas decreased to approximately 100 N · m⁻¹, but not quite to the ideal value of PTFE, 20 N · m⁻¹. We also prepared PET with a rough surface (matte PET) to examine the effect of surface roughening. The matte PET peel strengths were decreased by plasma fluorination; the roughest matte PET showed even lower peel strength than PTFE. We conclude that the combined treatments could be effective in the formation of a surface with high peel property on PET.

The lack of data concerning all the species and the microscopic phenomena involved in low pressure plasmas has always been the major obstacle for the complete understanding of the process mechanisms. As a matter of fact, even today, there is no gas-discharge system for which we can have, by whatever diagnostic tools, a complete picture of the concentration profiles of the species, either charged or neutral, involved in the various gas phase, gasfield, and gas-surface interactions (figure 1). This is more pronounced as we go from simpler noble or molecular gas plasmas to the more and more complex" chemical" plasmas used for the deposition of thin films. The main difference between classical chemical reactors and these plasma reactors comes from the presence of the electromagnetic field in interaction with various charged particles and surfaces. This makes the different plasma processes not easily predictable, controllable and comparable with each other. The rf power used in all these processes implies special reactor design and operation regimes which are different from the idealized plug-flow (PFR) or continuous stirred tank (CSTR) chemical reactors. Therefore, what one measures outside the reactor has no straightforward relation with what is happening inside, and there is no universal way of translating this information since, the" reacting gas volume" is not known, isotropic or homogeneous, while all the microscopic plasma quantities are also functions of space, in the specific reactor. On the other hand, a variation of one of the process parameters, like power or pressure, is not only associated with a change in the value of other macroscopic and microscopic quantities, but also with the way they interact with each other, with the electric field, and the surrounding surfaces. Two basic requirements arise from the discussion above: the need for more efficient, yet simple, non-intrusive diagnostics, and the need for more accurate process control. In fact, both requirements end up to the need for more accurate measurements. This is essential if the characterization of the discharge in a universal way is to be pursued, and it is also the main prerequisite for understanding the basic mechanisms governing the process, for building realistic mathematical models and in general for the development of the discharge theory.

In the present study the mechanisms and effectiveness of various pretreatments for fluoropolymers were studied. The pretreatments were “Tetra-Etch,” various plasmas, flame and potassium hydroxide. “Tetra-Etch” was found to be much more reactive than potassium hydroxide (KOH) towards fluoropolymers. The plasma treatment of PTFE showed that it was possible to get substantial increases in adhesion with little or no chemical change to the polymer. However, to obtain large increases in adhesion it may be necessary to modify PTFE chemically as with “Tetra-Etch.” Consideration of the bonding of these fluoropolymers shows that sharp interfaces between these substrates and adhesives do not exist.

An ultraviolet-ozone oxidation process is shown to be an effective adhesion pretreatment for polyethylene (PE) and polyetheretherketone (PEEK). The data obtained indicate that the treatment gives considerable oxidation and improved wettability for PE and PEEK surface types. Treated surfaces were analysed using X-ray photoelectron spectroscopy (XPS) and water contact angles. XPS was also used to follow the chemistry and mechanism of the oxidation. Adhesion with a two-part epoxy was measured for PE and PEEK and was observed to improve significantly after pretreatment.

Surface composition, fluorine distribution, and morphology were determined for polyimide films modified downstream from microwave plasmas containing CF₄/O₂. Complementary analytical techniques including x‐ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, and scanning electron microscopy yielded a more complete understanding of polyimide fluorination and subsequent etching of the modified film. Depth of fluorination increased nonlinearly with treatment time for films exposed downstream from a CF₄‐rich plasma. Exposure downstream from an O₂‐rich plasma resulted in a reduction of thickness in both the fluorinated layer and the unmodified polyimide during etching. Finally, a model for fluorination of polyimide and subsequent removal is proposed.

Microbial colonisation of synthetic plastic films is normally slow, which affects the total period of biodegradation. Correlation between the modified surface condition and the ability for microorganisms to colonise low-density polyethylene (LDPE) film was studied. Corona discharge treatment was applied to obtain enriched and activated surface condition of LDPE film. It was found from water contact angle and FTIR spectrum evaluations that surface energy was significantly increased due to production of free radicals. Stabilised oxidised LDPE surface was also obtained by further exposure to the corona which gave more suitable condition for subsequent colonisation. Results were compared with UV irradiated (photo-oxidised) LDPE films. Colonisation of corona discharged and UV treated LDPE films were tested in the laboratory environment using known fungal isolates and in a natural compost environment. More active microbial colonisation was observed in all cases for corona discharged and UV treated LDPE films. Far longer UV exposure was required to have the same physicochemical and biological effect as the corona discharge treatment.

The dispersion force component of surface free energy, γ, and the nondispersive interaction free energy between solid and water, I, were determined by the two-liquid contact-angle method, i.e., by the measurement of contact angles of water drops on plain solids in hydrocarbon, for commercialy available organic polymers such as nylons, halogenated vinyl polymers, polyesters, etc. A method to estimate the I values from the knowledge of the polymer composition is also proposed, on the basis of the assumption of the spherical monomer unit and the sum of interactions between functional groups and water molecules at the surface.

The London dispersive component of surface free energy γ_s^d and the nondispersive interactions with polar liquids W I_swⁿ were determined for hydrophilic polymers S, that is, cellulose, poly(vinyl alcohol) (PVA), and poly(methylmethacrylate) (PMMA). On applying the geometric-mean relation $\text{2(γ}{}_{\mathrm{s}}{}^{\mathrm{d}}\text{γ}{}_{\mathrm{w}}{}^{\mathrm{d}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ to the dispersive interaction with W I_sw^d, the γ_s^d values were found to be 30, 29, and 37 erg·cm⁻² for cellulose, PVA, and PMMA, respectively. Each of them is completely independent of the nature of the testing liquids W, indicating that the geometric-mean equation is appropriate for representing the dispersive interaction. On the contrary, such a geometricmean expression is shown to be inapplicable to the nondispersive interactions. It is suggested that Fowkes' approach, in which intermolecular forces are regarded to be dominated by dispersion force interactions and electron donor-acceptor interactions, is more reasonable than the popular approach.

A semicontinuous, if capacitively coupled plasma polymerization apparatus was designed and constructed to coat the internal surface of a small-diameter plastic tubing. The glow zone was restricted to a small area to obtain a uniform coating of plasma polymer over the entire length of tubing (13 m long). It was found that a uniform coating can be achieved by maintaining the glow discharge parameters and velocity of moving substrate. In such a reactor, it was found that the deposition rates obtained for plasma polymers of tetrafluoroethylene, hexafluoroethane, and hexafluoroethane/hydrogen were very high compared with those polymerized in a conventional plasma polymerization apparatus. Special attention was needed to avoid deposition of an excessively thick coating, which was found to damage the barrier characteristics of the coating

The effect of time and surface contact on corona treated material can be measured and evaluated. Both components cause degradation of dyne level readings of wettability. However, the most significant cause, though it can be isolated, cannot be completely eliminated. A solution to the problem is possible but requires cooperation between material producers and converters.

Surface free energy (SFE) is a property which depends on the chemical state and roughness of the surface and it is necessary to develop a reliable method to evaluate SFE value on a small area, taking into account these two different contributions. Today contact angle methods are the most used and they allow to evaluate the global mean value of SFE on areas of mm² size. With these methods, it is not possible to evaluate the effects of roughness, surface defects, chemical contamination on SFE value. In addition, it is difficult to determine the surface free energy value on small components which have dimensions smaller than drop diameter. Nanoindentation and atomic force microscopy techniques provide alternative direct measurement methods to evaluate the SFE on small areas (on the order of μm² or nm²) through a contact mechanism triggered by the contact of two bodies. In order to evaluate the adhesion properties, currently three models, Johnson– Kendall–Roberts, Maugis–Dugdale and Derjaguin–Muller–Toporov, use the value of pull-off force (force required to separate the indenter tip from the sample). All influences of surface morphology on SFE values are lost using these methods. In fact the adhesion value obtained refers to the energy balance between two conformal surfaces, which depends mainly on the morphology of the harder material (i.e., diamond tip). In this work we describe a new methodology for the SFE determination consisting in the modeling and quantitative evaluation of the interaction between the tip and sample surface during the approach phase in a nanoindentation test. During the test, the nanoindenter tip is attracted to the sample surface until the sample reaction forces become significant (in this case physical contact between two bodies is achieved). The SFE value is evaluated using experimental force of attraction and displacement of the nanoindenter spherical tip when it approaches the sample surface. In this method the sample surface is not altered by the tip, therefore unlike pull-off force method, it could be very useful to evaluate the actual SFE considering the effect of sample morphology (controlled roughness or pattern).

The Lewis acid-Lewis base properties of various polymers have been determined by measuring the contributions γ_s ⁺ and γ_S ^- to the solid surface free energy using the contact angle approach of van Oss, Chaudhury, and Good. A new linear method to solve for γ_S ⁺ and γ_S ^- is employed in addition to the usual approach which uses three simultaneous equations. The set of liquid surface tension parameters developed by van Oss, Chaudhury, and Good, and the recent set of values developed by Della Volpe and Siboni are both useful in distinguishing between acidic and basic polymers. The adhesion (peel force) of an acidic pressure-sensitive adhesive is greatest on a basic oxide film. In addition, the adhesion (pull-off force) of the basic polymer poly(methyl methacrylate) is greatest for acidic oxide films. Thus, direct experimental evidence is provided as to the importance of Lewis acid-Lewis base effects in the adhesion of polymers on oxide-covered metals.

A droplet of liquid placed on a flat high-energy solid surface spreads to give a thin film so that no macroscopic droplet shape exists. On a chemically identical solid surface with only the geometry changed to a cylinder, the same droplet can have an equilibrium conformation. When the equilibrium conformation is of a barrel type, the profile of the droplet changes rapidly in curvature as the three-phase contact line is approached and the direct measurement of the contact angle is difficult. This work considers the theoretical profile for barrel-type droplets on cylinders and discusses how the inflection angle in the profile depends on droplet parameters. Experimental results are reported for poly(dimethylsiloxane) oils on a range of fiber surfaces and these are used to estimate the equilibrium contact angle from the inflection angle. The drop radius and volume dependence of the inflection angle is confirmed.