We present a study on ageing of polymethyl methacrylate (PMMA) polymer treated with oxygen plasma. Oxygen plasma was created with an RF generator operating at a frequency of 27.12 MHz and a power of 200 W. The oxygen pressure was 75 Pa. The samples were treated for different time from 5 s to 60 s. The chemical modifications of the surface after plasma treatment were monitored by XPS (X-ray photoelectron spectroscopy), while the wettability and ageing effects were studied by WCA (water contact angle measurements). The samples were aged in dry air or in water. In the case of dry air, the least pronounced ageing was observed for the sample treated for 60 s. For samples aged in water, however, the lowest ageing rate was observed for the sample treated for 5 s. The samples were ageing slightly faster in water than in air. We also investigated the temperature effect on ageing of plasma treated samples. A set of samples was stored in a refrigerator at 5 °C and the other set was placed into an oven at 50 °C. The ageing rate of the samples stored at 5 °C was significantly lower than for the samples stored at 50 °C, so cooling the samples help keeping the required surface properties.An atmospheric pressure plasma syste

A method to corona treat a polymeric surface with a fixed and pre-set treatment energy level is proposed. In order to reduce the effect of humidity, the Corona Discharge Energy – CDE value was controlled during the corona treatment by an in-line monitoring apparatus, which indicates when the energy level is attained. The method was tested applying a negative corona treatment discharge on a polypropylene copolymer film. Chemical and morphological surface changes were examined by IR spectra, contact angle and morphology. Treated samples show the presence of carbonyl, hydroxyl and unsaturations, the first being less concentrated. The formation of polar groups on the film surface led to a reduction of the contact angle and an increase in the surface tension. Film surface morphology was also modified, showing a second phase granular structure which grows with the increase of the CDE level.

An atmospheric pressure plasma system has been used to treat amorphous polyethylene terephthalate (APET) to enhance its healseal properties to a polyethylene terephthalate (PET) film. The plasma treated APET sheet material was thermoformed into trays for use in the food packaging industry and heatsealed to a PET film. The heatsealing properties of the resulting package were assessed using the burst test technique. It was found that the plasma treatment significantly enhanced the adhesive properties and an increase in burst pressure from 18 to 35 kPa was observed for plasma treated food trays. The APET surface chemistry was assessed after plasma treatment where it was found that the plasma treatment had affected an increase in oxygen and an addition of nitrogen species to the polymer surface. The surface roughness (R_a) of the plasma treated samples was also observed to increase from 0.4 to 0.9 nm after plasma treatment.

Adhesion is a surface phenomenon occurring in many processes, e.g., bonding, painting or varnishing. Knowing the adhesion properties is critical for evaluating the usability or behaviour of materials during these processes. Good adhesion properties favour the processes of bonding, resulting in high strength of adhesive joints. Adhesive bonded joints are used in many industries, and the subject of this study was 7075 aluminium alloy sheet bonded joints as typically used in the aviation or construction industry. Surface free energy (SFE) can be used to determine the adhesion properties of the materials. The SFE of the tested sheets was determined with the Owens–Wendt method, which consists in determining the dispersion and polar components of SFE. The purpose of this work was to correlate the bonded joint strength of selected aluminium alloy sheets to the surface free energy of the sheets that had been subjected to degreasing only and no other prior treatment was used. Single-lap bonded joints of 7075 aluminium alloy sheets were tested. Higher joint strength was measured for the thinner sheets, while the lowest strength was measured for the thickest sheets. This suggests that the thickness of the joined parts is an important factor in the strength of bonded joints. The comparison of adhesion properties to the strength of adhesive joints of tested materials shows that there is no direct relation between good adhesion properties (i.e., high SFE) and joint strength. As for degreasing, the highest joint strength was observed for aluminium alloy sheets with the lowest SFE; the sheets which were not degreased gave the highest SFE and highest joint strength.

In atmospheric pressure plasma treatments water molecules in the substrate material may disrupt the molecular arrangement in the substrate and thus greatly influence the outcome of the plasma treatment. This paper summarizes the results of our recent studies on how moisture influences the etching, surface chemical modification, crystallinity and aging of aramid, ultrahigh molecular weight polyethylene (UHMWPE), polyamide fibers, and poly(vinyl alcohol) (PVA) films. Overall, a higher moisture regain often results in a greatly enhanced etch rate, less surface chemical composition change, increased near-surface crystallinity, which could lead to a higher surface wettability, higher interfacial shear strength between the fibers and resin, decreased water solubility for PVA films, and delayed hydrophobic recovery of plasma treated fibers. Therefore, it is important to control the moisture contained in the substrate in atmospheric pressure plasma treatments.

In this work, the effect of medium pressure plasma treatment on thin poly-ϵ-caprolactone (PCL) layers on glass plates is investigated. PCL is a biocompatible and biodegradable polymer which potentially can be used for bone repair, tissue engineering and other biomedical applications. However, cell adhesion and proliferation are inadequate due to its low surface energy and a surface modification is required in most applications. To enhance the surface properties of thin PCL layers spin coated on glass plates, a dielectric barrier discharge (DBD) at medium pressure operating in different atmospheres (dry air, argon, helium) was used. After plasma treatment, water contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to examine the PCL samples. These measurements show that the medium pressure plasma treatment is able to increase the hydrophilic character of the samples, due to an incorporation of oxygen groups at the surface and that the surface roughness is significantly decreased after plasma treatment.

Droplet dynamics analysis concerns the measurements of droplet volume, cap and base areas and contact angles, as they change in time to study evaporation, wettability, adhesion and other surface phenomena and properties. In a typical procedure, the two-dimensional measurements are based on a series of images recorded at successive stages of the experiment from a single view. Only a few basic dimensions of sessile droplets are commonly measured from such images, while many other quantities of interest are derived utilizing geometrical relationships. The reliability of these calculations is limited by the necessary assumption that the droplet shape can be approximated as a spherical cap. In reality, the sessile droplet shapes are influenced by gravity, liquid surface tension, local surface anisotropy and microstructure, which often produce non-spherical cap shapes.

This paper describes an experimental methodology for determination of key parameters, such as volume and contact angle for dynamic sessile droplets that can be approximated either by spherical or ellipsoidal cap geometries. In this method, images collected simultaneously from three cameras positioned orthogonally to each other are used to record the dynamic behavior of non-spherical droplets. Droplet shape is approximated as an ellipsoid of arbitrary orientation with respect to the cameras, which allows determination of volume and contact angle along the base perimeter. A major advantage of this method is that the dynamic parameters of droplets on anisotropic surfaces can be determined even when the orientation of the axes changes throughout the droplet lifetime. The method is illustrated with experimental results for a spherical and an ellipsoidal droplet.

A critical review of published studies investigating the dielectric barrier discharge (DBD) treatment of four polymers widely employed in the packaging sector, namely: polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET) and polystyrene (PS) is presented. The DBD treatment process operates at atmospheric pressure in air, and thereby offers a low cost method of enhancing the surface properties of polymers. The method is suitable for high volume in-line applications such as packaging. It has been reported that treatment doses as low as 0.01 J/cm² result in significant increases in surface energy and wettability, leading to enhanced adhesive bonding and printing performance. Two critical issues limit the improvements obtained via the DBD processing of polymers. Firstly, DBD processing can produce a poorly adhered surface layer of low molecular weight material, which can then interfere with bonding and printing processes. Secondly, the properties of DBD treated polymers tend to revert towards that of the untreated state during storage.

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 vast majority of polymers and composites have low surface energy; this is due to the low presence of functional groups on their surface which results in low adhesive properties. In order to modify this intrinsic property chemical and physical processes are commonly used. These processes present disadvantages, such as the use of products harmful to the environment. An alternative to these processes is the use of plasma technology. The main objective of this study is the improvement of the adhesive properties of the low density polyethylene (LDPE). In order to achieve the target, atmospheric plasma pretreatment has been optimized in order to promote subsequent adhesion processes, as the ones needed in the toy industry or the application of dyes or printing on surfaces. Plasma surface treatment is a clean process from the environmental viewpoint. This process does not emit any residue and it is easy to implement in an industrial process. Moreover the atmospheric plasma treatment is suitable to be applied in a large variety of materials even at high speeds when the treatment lasts less than a few seconds. In the present study it is examined the physical and chemical processes that occur in the LDPE surface as function of speed rate and distance of treatment. An increase both of the polar groups on the surface and the roughness after the treatment may increase its adhesive properties. It has been analyzed the influence of the speed rate and the nozzle distance on the final results. The adhesive properties have been evaluated using the T-peel test. The results show that at low speeds rates and low nozzle/substrate distance there is a greater inclusion of polar molecules at the surface. Consequently the adhesion properties of LDPE are improved.

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

Contact angle (θ) measurements on poly(tetrafluoroethylene) (PTFE) and polymethyl methacrylate (PMMA) surface were carried out for the systems containing ternary mixtures of surfactants composed of: p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols), Triton X-100 (TX100), Triton X-165 (TX165) and Triton X-114 (TX114), and fluorocarbon surfactants, Zonyl FSN100 (FSN100) and Zonyl FSO100 (FSO100). The aqueous solutions of ternary surfactant mixtures were prepared by adding TX114, FSN100 or FSO100 to binary mixtures of TX100+TX165, where the synergistic effect in the reduction of the surface tension of water (γ(LV)) was determined. From the obtained contact angle values, the relationships between cosθ, the adhesion tension and surface tension of solutions, cosθ and the reciprocal of the surface tension were determined. On the basis of these relationships, the correlation between the critical surface tension of PTFE and PMMA wetting and the surface tension of these polymers as well as the work of adhesion of aqueous solutions of ternary surfactant mixtures to PTFE and PMMA surface were discussed. The critical surface tension of PTFE and PMMA wetting, γ(C), determined from the contact angle measurements of aqueous solutions of surfactants including FSN100 or FSO100 was also discussed in the light of the surface tension changes of PTFE and PMMA under the influence of film formation by fluorocarbon surfactants on the surface of these polymers. The γ(C) values of the studied polymeric solids were found to be different for the mixtures composed of hydrocarbon surfactants in comparison with those of hydrocarbon and fluorocarbon surfactants. In the solutions containing fluorocarbon surfactants, the γ(C) values were different taking into account the contact angle in the range of FSN100 and FSO100 concentration corresponding to their unsaturated monolayer at water-air interface or to that saturated.

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

Polyethylene films were prepared with phase separation at lower temperatures. The wettability of such films varied from hydrophobicity to superhydrophobicity as the processing temperature decreased owing to the increase of surface roughness. Storing the as-prepared films at subzero temperature (−15 °C), it was found that the water contact angle of the film decreased obviously, and the decrease depended on the corresponding roughness. Further keeping the as-prepared films at room temperature for 30 min, the water contact angle would return to the normal value, which indicated that the reversible switching of surface wettability can be controlled by the environmental temperature.

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

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

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

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