Corona Treatment Research Articles

Surface modification of polymer materials by transient gas discharges at atmospheric pressure

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.

Characterization of surfaces of various polymers by electron spectroscopy

X-ray induced photo and Auger electron spectroscopic techniques have been used to characterize the surface composition of biaxially oriented polypropylene sheets with and without corona discharge treatment. The surfaces of polypropylene sheets without corona treatment contain very small amounts of O and the Cls peak is narrow, consisting mainly of CH groups and less than 5% CO groups. After corona treatment the intensity of the Ols peak increases by a factor of 8, the Cls peak broadens and CO intensity increases by a factor of 3. The surfaces of aluminized polypropylene contains both aluminium oxide and metallic aluminium. From the measured intensity ratio of the Al 3+ Al 0 peaks the thickness of the oxide layer is estimated to be smaller than 4 nm. Aging increases the carbon content of the surface without changing the aluminium oxide/metal composition.

Adhesion phenomena in (co)extrusion coating of paper and paperboard

In extrusion coating, the inadequate adhesion between the polymer coating and the fiber-based paper substrate (paper and paperboard) is both a common and a constant problem. The lack of adhesion between the printing ink, or glue, and the polymer coating is another area where adhesion improvement is needed. The common means of improving adhesion are flame, corona, and ozone treatments. A modem extrusion coating line is equipped with both a pretreatment and a post-treatment unit. From the work presented here, the following observations were made. The higher the applied corona power and the thicker the coating, the higher the surface energy and polarity of the low density polyethylene (PE-LD) surface. When a high corona power was applied to the coating, only the polar component of the surface energy was increased. The surface energy decreased sharply as a function of aging, but remained more or less constant after about 2 weeks' storage time. The contact angles of water on paper correlated well with the oxygen contents (determined by ESCA) and with the applied corona power. The polarities of both paper and paperboard increased as a function of the applied corona power. Corona pretreatment of paper and paperboard improved their adhesion to PE-LD remarkably. The adhesion of the polypropylene (PP) homopolymer is based more on mechanical interlocking than on interfacial bonding. On the other hand, the oxidizing pretreatments of the paper substrates significantly promoted the adhesion of the PP copolymer.

Air corona removal of phenols

Exposure to the neutral activated species of an a.c. point-to-plane type corona discharge in air of aqueous alkaline solutions of phenol and substituted phenols (pentachlorophenol, triiodo 2,4,6 phenol and picric acid) leads to the oxidative destruction of the solutes up to the CO2 state in the case of phenol. The influence of the discharge parameters, such as the current intensity, is considered. The destruction of phenates follows overall pseudo first-order kinetics, and the kinetic constants are linearly dependent on the current. A scheme for the degradation kinetics is proposed involving a multistep mechanism, and the occurrence of dihydroxybenzenes as reaction intermediates is suggested. Applications of corona treatment to waste waters may be considered as a versatile method for the elimination of toxic chemicals and the purification of industrial waters.

Attachment of Vero cells to corona-treated, plastic surfaces

Corona treatment of polystyrene (PS) and polyethylene resulted in an increase in the interfacial energy ( γ sv) of these surfaces, primarily due to an increase in the polar forces. The number of Vero cells which attached and the strength of attachment were higher on corona-treated surfaces, increasing as γ sv increased. Inoculum concentration did not affect the percentage of cells that attached. Experiments were performed in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum hence cellular attachment was most likely preceeded by adsorption of serum proteins. A minimum of 1 h of incubation under static conditions resulted in the maximum number of cells which attached and which remained attached after shear stress (33.0 dynes/cm 2 for 5 min) was applied. Experimental results with corona treated surfaces agree with the theoretical predictions for the free energy of adhesion of Vero cells in the medium used, indicating that interfacial energy plays an important role in attachment. The model predicted that substrates with a γ sv>55.6 ergs/cm 2 ( i.e. Θ H 2 O <39°) would have a positive energy of adhesion. Such substrates did have > 60% cell attachment. Since the model is valid of only van der Waals and Lewis acid-base interactions are dominant, these forces probably played a major role in the adsorption of serum proteins. Polylysine-treated PS and tissue culture PS did not follow the model. In the first of these two cases, electrostatic interactions probably played a more important role since the surface was covered with a positively charged protein.

Surface Characterization of Polysaccharides, Lignins, Printing Ink Pigments, and Ink Fillers by Inverse Gas Chromatography

Inverse gas chromatography (IGC) was used to determine the dispersive contribution to surface energy of three families of fibrous or powder-like materials: (i) polysaccharides; (ii) lignins from different sources; and (iii) printing ink pigments and ink fillers. The dispersive component of unpurified alpha hardwood cellulose was found to be close to 32 mJ·m−2. This parameter increased substantially (up to 50%) when fibers were purified by acetone extraction or by corona treatment using a current discharge of 40 mA. The surface properties of other polysaccharides, namely starch, chitin, and two chitosans with different degrees of acetylated groups, were also investigated. Except for the soda–anthraquinone lignins, which showed the highest dispersive component of surface energy (115.6 mJ·m−2), other lignins, namely kraft, steam-explosion, and organosolv materials, showed very similar dispersive properties (45 to 49 mJ·m−2). As expected, lignosulfonates displayed a higher dispersive energy of about 67 mJ·m−2. The study of printing ink pigments and ink fillers suggested that carbon black and calcium carbonate used for that purpose must have been subjected to a specific treatment in order to increase their surface energy. The dispersive component of the surface energy of other pigments, viz., red, yellow, blue, and white powders, was close to 40 mJ·m−2.

Surface Morphology of Polymer Films Imaged by Atomic Force Microscopy

Abstract The surface morphology of commercial polymer films has been studied by contact mode and tapping mode atomic force microscopy. Flame-treated isotropic HDPE obtained by cast-film extrusion exhibited randomly oriented, 20–50-nm thick lamellar features of 200–400 nm in length, while uniaxially oriented films showed a shish-kebab-like morphology. The surface features of extrusion-blown films of blends of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) appeared as stacks of lamellae arranged in a cauliflower-like pattern. Images of isotactic polypropylene (PP) films obtained by cast-film extrusion consisted of branched fibrillar features. Corona treatment of the PP films resulted in the formation of 400–500 nm large ‚droplet-like‘ features at the surface. During metallization, 20–40-nm diameter A1 particles were deposited. The metal layer formed followed the topography of the surface. The surface of ethylene-vinyl alcohol (EVOH) copolymer skins on PP consisted of radiating lamellae of 9–11 nm thickness. Contact mode AFM imaging of solvent-cast atactic polystyrene (PS) and polyvinylchloride (PVC) films induced surface erosion. During the scanning of PS and PVC films, wavy features formed with a predominantly perpendicular orientation with respect to the scan direction. The height of these features increased with plasticizer content from ca. 10 nm (unplasticized films) to over 100 nm. The height did not show any measurable relaxation over 24 h. This plastic deformation of the surface of the glassy polymer films studied is discussed. It is assumed that the glass-transition temperature of the outermost thin layer at the surface of glassy polymers is significantly lower than the bulk value.

Hydrogen Bonding and the Interfacial Component of Adhesion: Acid/Base Interactions of Corona-Treated Polypropylene

Abstract The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γLW, γ⊕ and γ⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed. The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ⊕ and γ⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m2. The acidic component, γ⊕, rose from zero to as high as 1.9 mJ/m2. Its decay with time was only qualitatively the same as that of γ⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data. The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.

Energy efficiency of NO removal by pulsed corona discharges

Pulsed positive corona discharges are used to remove NO from the flue gas of a methane burner. At low power input this leads to an increase in NO2, which shows that the process is oxidative. Removal efficiency is greatest when discharges are produced with high-voltage pulses, which are shorter in duration than the time required by the primary streamers to cross the discharge gap, in combination with a dc bias. Other important parameters are input power density and residence time. The best result obtained so far is an energy consumption of 20 eV per NO molecule removed, at 50% deNOx i.e., a removal of 150 ppm NOx, using a residence time of 15 s and an input power density, of 3.5 Wh/Nm3. [Wh/Nm3 stands for watt-hour per normal cubic meter, i.e., at normal conditions (273 K and 1 bar). This implies that 1 Nm3 contains 2.505 1025 molecules.] There appears to be room for improvement by the addition of gaseous and particulate chemicals or the use of multiple corona treatment. It is argued front comparison between results from models and experiments that the direct production of OH by the discharge is only the initiation of the cleaning process.

Chemistry and reactivity of carboxylic acid-containing surfaces

Polymer surface modification techniques, such as corona, plasma, and ultraviolet treatments, promote adhesion to polymer surfaces through introduction of specific functional groups that interact with subsequent coating layers. Carboxylic acid groups formed during corona and plasma treatments may play a significant role in adhesion promotion, so such groups have been chosen for model surface chemistry studies. Thin films (<30 nm) of the potassium salt of poly(acrylic acid) (PAA) and poly(vinyl methyl ether-co-maleic acid) (VME-MA) copolymer were analyzed before and after surface protonation by dilute acid solution. The reactive nature of the acid groups was probed using infrared reflection absorbance spectroscopy (IRRAS) to analyze the entire thin film and high-resolution electron energy loss spectroscopy (HREELS) to analyze the top 1-2 nm. The HREELS results show that it is possible to obtain surfaces enriched in carboxylic acid groups by protonation of the potassium salts of PAA and VME-MA. Anhydride formation, followed as a function of temperature, went to completion quickly (5-10 min) at the surface, whereas complete conversion did not occur in the bulk thin film even after 4 h. PAA formed anhydride at 160°C, as compared to 100°C for VME-MA. The large difference between the temperature for anhydride formation in PAA and VME-MA may be a result of the adjacent acid groups available for forming anhydride in VME-MA, or the high Tg of PAA (105°C) relative to that of VME-MA (80°C). IRRAS and HREELS show that PAA and VME-MA form imide functionalities following butylamine exposure and heating at 125°C. The reactive nature of PAA and VME-MA at elevated temperatures suggests that acid-containing surfaces are good candidates for reaction with organic materials containing primary amine groups, an example being gelatin, which is the binder used in photographic emulsions.

Surface modification of polypropylene in an impulse corona discharge

The impulse corona discharge was applied to modify surface of polypropylene films. The modified surface was analyzed by contact angle measurements and ESCA. The results were compared to those treated with AC corona discharge. The impulse corona treatment provided a similar contact angle and a similar [O]/[C] ratio on the polymer surface to those in AC corona treatment. On the other hand, [N]/[C] in impulse discharge was much lower than that in AC pulse discharge. The total power input in the impulse corona discharge, however, was lower than that in AC corona discharge to achieve a similar level of surface modification. It was also observed that the lower repetition frequency, the lower contact angle, and the higher [O]/[C].

Improved bonding strength of polyethylene/polymethylmetacrylate bone cement – a preliminary study

The adhesion of polymethylmetacrylate (PMMA) bone cement and low density polyethylene (LDPE) substrate by means of corona discharge treatment are presented. The surfaces were characterized by water contact angle goniometer, Fourier-transform infrared spectroscopy in the attenuated total reflectance mode (FTIR-ATR), electron spectroscopy for chemical analysis (ESCA), and scanning electron microscopy (SEM) for surface morphology. The bonding strength between bone cement and LDPE with varying intensity of corona treatment was evaluated by a 180 degrees peel test. The water contact angles decreased with increasing treatment intensity from 95.0 +/- 1.8 to around 43.5 degrees. It was observed that the oxygen containing groups as hydroxyl, ether, carboxylic acid, ester, ketone or aldehyde groups were introduced on the LDPE surfaces by corona treatment. The peel strength steeply increased from 0 to 1.75 kgf/cm with increasing the intensity of corona treatment. The improved adhesion of bone cement onto LDPE substrate may be due to only secondary force such as hydrogen bonding between PMMA bone cement and hydrophilized LDPE substrate.

Surface characterization of cellulose fibres by XPS and inverse gas chromatography

Inverse gas chromatography (IGC) was used to determine the dispersive component of the free energy as well as the acid-base properties of cellulose fibre surfaces, before and after modification by corona treatment. It was found that the corona treatment increases both the dispersive contribution to surface energy and its acidic character, whereas only a slight increase in its basicity was observed. It was also found that some chemical degradation of the surface occurs at high corona currents. The extent of modification of the surface properties, as revealed by IGC, was correlated to the surface chemical composition deduced from XPS analysis as well as with the electrical conductance and the pH of the water suspensions of the cellulose fibres.

Comparing the effect of corona treatment and block copolymer addition on rheological properties of polystyrene/polyethylene blends

The effect of corona treatment and copolymer modification on linear viscoelastic properties of PS/PE blends in the molten state has been investigated. The results show that corona treatment has little effect on rheological properties of the blends. On the other hand, SEBS addition results in an appreciable decrease of the elastic modulus in the low‐frequency region and in a more evident secondary plateau. The data have been analyzed quantitatively by Palierne’s emulsion model, which takes into account the presence of an emulsifier agent. The model has been used to estimate the interfacial tension in PS/PE blends as modified by corona treatment and by SEBS copolymer modification. Addition of as little as 1 wt % copolymer resulted in a fivefold decrease of interfacial tension, whereas only a little change was observed in the case of corona modified blends. Further, corona treatment did not affect the size of the dispersed particles, whereas 1 wt % SEBS addition reduced the particle size of the dispersed pha...

Solvent-Induced Changes on Corona-Discharge-Treated Polyolefin Surfaces Probed by Contact Angle Measurement

Contact angle measurement was employed to study the effect of water, acetone, and 2-propanol on the surface properties of air corona-discharge-treated polyolefins. The results show that these solvents can significantly change the surface energy and contact angle hysteresis of corona-treated polyolefins. A model based on the corona treatment effects is proposed to depict the treated surfaces and is found to reasonably explain the results obtained.

A comparison of gas-phase methods of modifying polymer surfaces

Oxidation is the most common surface modification of polymers. This paper presents a comparison of five gas-phase surface oxidation processes: corona discharge, flame, remote air plasma, ozone, and combined UV/ozone treatments. Well-characterized biaxially oriented films of polypropylene and poly(ethylene terephthalate) were treated by each of the five techniques. The surface-treated films were then analyzed by X-ray photoelectron spectroscopy (XPS or ESCA), contact-angle measurements, and Fourier-transform IR (FTIR) spectroscopy. Corona, flame, and remote-plasma processes rapidly oxidize polymer surfaces, attaining XPS O/C atomic ratios on polypropylene of greater than 0.10 in less than 0.5 s. In contrast, the various UV/ozone treatments require orders of magnitude greater exposure time to reach the same levels of surface oxidation. While corona treatment and flame treatment are well known as efficient means of oxidizing polymer surfaces, the ability of plasma treatments to rapidly oxidize polymers is not as widely appreciated. Of the treatments studied, flame treatment appears to be the 'shallowest'; that is, the oxygen incorporated by the treatment is most concentrated near the outer surface of the film. Corona and plasma treatments appear to penetrate somewhat deeper into the polymers. At the other extreme, the UV/ozone treatments reach farther into the bulk of the polymers.

Surface pretreatment of plastics for adhesive bonding

Abstraet-Many plastics have a poor tendency to bond to other materials because of their inherent inert chemical structure and thus require a pretreatment. Wet chemical methods are expensive because of the disposal of the waste liquids. In this study, the corona treatment (Ional process), the low-pressure plasma process, and the fluorination process were tested and compared with each other. The following plastics were tested: PP (polypropylene), PBT (polybutyleneterephthalate), PBT blends, and a high-temperature thermoplastic, PEEK (polyetheretherketone). In particular, the low-temperature plasma process results in excellent adhesion strength. In addition, we have shown that the stability of freshly plasma-treated surfaces could be maintained for time periods of at least several days.

Direct Measurement of the Surface Energy of Corona-Treated Polyethylene Using the Surface Forces Apparatus

In this study we report the direct mechanical measurement of enhancement of surface energy of polyethylene films due to corona treatment. Surface energy was measured using the surface forces apparatus (SFA). Unlike practical adhesion tests, the SFA measurements are free from dissipative effects. The results of direct surface energy measurements using the SFA were compared to the contact angle measurements. The extent of modification was controlled by varying the corona energy density. The surface composition is measured using X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SSIMS). The surface energy of polyethylene increases from 33 to 55 mJ/m 2 due to corona treatment. The surface energy estimated from contact angle measurements varied depending on the probe liquids used and the model used to analyze the data

The effect of corona treatments on the hygral expansion of wool worsted fabrics

The effect of dry corona and aqueous treatments on the hygral expansion of wool gabardine has been investigated. Increasing the severity of the corona treatment produced a concomitant reduction in the hygral expansion of the fabric. X‐ray photoelectron spectroscopy has been used to study the chemical modification of the wool fibre surface.

Effect of corona modification on the mechanical properties of polypropylene/cellulose composites

AbstractThe effect of various corona treatment conditions on the mechanical properties of cellulose fibers/polypropylene composites was studied. The cellulose fibers and polypropylene were modified using a wide range of corona treatment levels and concentrations of oxygen. The treatment level of the fibers was evaluated using the electrical conductance of their aqueous suspensions. The mechanical properties of composites obtained from different combinations of treated or untreated cellulose fibers and polypropylene were characterized by tensile stress–strain measurements; they improved substantially when either the cellulose fibers alone or both components were treated, although composites made from untreated cellulose fibers and treated polypropylene showed a relatively small improvement. The results obtained indicate that dispersive forces are mostly responsible for the enhanced adhesion. The relationship between the electrical conductance of the fibers, the mechanical properties, and the mechanism of improved adhesion is discussed. © 1994 John Wiley &amp; Sons, Inc.