In order to functionalize the surface of blown low-density polyethylene (LDPE) and cast polypropylene (CPP) films, and ultimately to maximize the attachment of active molecules onto them, the optimum treatment parameters of capacitively-coupled radio-frequency (13.56 MHz) oxygen plasma were investigated by using contact angle, toluidine blue dye assay, X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Contact angle values of LDPE and CPP samples decreased significantly after oxygen plasma treatment. They further decreased as the plasma power level increased. The treatment time had no substantial effect on contact angle value. The optimum treatment conditions for LDPE and CPP films for maximizing carboxyl functionality without causing observable surface changes were found to be 200 W/200 mTorr and 250 W/50 mTorr, respectively, when treated for 3 min. The maximum carboxyl group concentration obtained with LDPE and CPP films were 0.46 and 0.56 nmol/cm², respectively. The percent of oxygen atoms on the surface of plasma-treated LDPE and CPP films was determined by XPS analysis to be 22.6 and 28.7%, respectively. The ATR-FTIR absorption bands at 1725–1700 cm⁻¹ confirmed the presence of carboxylic acids on LDPE and CPP films. By exposing the plasma-treated sample to air rather than water and treating films repeatedly with oxygen plasma, a higher carboxyl group concentration could be obtained. Copyright © 2008 John Wiley & Sons, Ltd. https://onlinelibrary.wiley.com/doi/abs/10.1002/pts.829

Long-term durability of a structural adhesive joint is an important requirement, because it has to be able to support the required design loads, under service conditions, for the planed lifetime of the structure. One way of improving bond durability is through the use of surface treatments prior to bonding, which will activate the adherends’ surface, making it more receptive to the adhesive. In this study, the effects of two surface pre-treatments (corona discharge and flame ionization) on three timbers (maritime pine, iroko, and European oak) were evaluated quantitatively through contact angle measurements. These measurements allowed the determination of the changes in the timber surface thermodynamic characteristics, thus indicating which pre-treatment performed better. The results showed that both techniques increased each timber's surface free energy, which could translate into a durability enhancement of bonded joints. Overall, the corona-discharge treatment looks more promising, since this treatment leads to a bigger increase in the timber's surface energy, especially in its polar component, whilst also tended to be less species specific, less susceptible to variation, and the treatment effects lasted longer for this type of treatment.

The article throws light on the physical methods to modify natural fibers to be used in composites. Physical methods in natural fiber processing are used to separate natural fiber bundles into individual filaments and to modify the surface structure of the fibers so as to improve the use of natural fibers in composites. Steam explosion and thermomechanical processes fall in the first category while plasma, dielectric barrier techniques and corona fall in the second. The physical treatments have also been used to modify the thermoplastic polymeric films like polyethylene and polypropylene in a bid to impart reactivity. Reviewing such developments, the areas for further research are suggested.

Polyester and polyamide fabrics were treated with plasma under atmospheric pressure for different durations, 3, 5 and 7 s. The wettability of polyester and polyamide fabrics, measured in terms of contact angle and longitudinal wicking, was improved after plasma treatment. The oxygen content of the fabrics was increased indicating that hydrophilic groups had been introduced into the fabric leading to the improved wettability. However, there was no obvious improvement in dryability because bulk properties of the fibres did not change. Moreover, with the help of plasma treatment, water repellency of the fabrics was greatly improved when water repellency finishing agent was added.

The surface modification of polyethylene powder using a plasma reactor based on a dielectric barrier discharge in air at atmospheric pressure and ambient temperature is investigated. The process is inexpensive, and the necessity of any vacuum equipment and technical gases is alleviated. The efficiency of the modification process was successfully demonstrated by ESCA measurements that proved formation of new functional groups at the modified powder surface. The modification effect was also evaluated by means of dynamic capillarity rising measurements. Powder capillarity tests proved significant powder capillarity changes. The reduction of the modification effect was also limited (max. reduction of about 20% during 1 100 d after the modification date).

The surface chemical and physical character of offset paper was studied before and after application of model fountain solutions based on isopropyl alcohol and an alcohol-free surfactant solution. The paper surface features were characterised with atomic force microscopy and the surface energies were determined by contact angle measurements. Changes in the surface chemical properties induced by the fountain solutions were investigated with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. Coated papers wetted with the surfactant solution revealed a slight increase in the root mean square roughness, but the isopropyl alcohol solution led to no observable changes. The change in sub-micro roughness is ascribed not only to substrate swelling or migration of coating constituents but also to the presence of surfactant on the surface. A change in the surface energy and particularly the polar contribution was observed after application of the surfactant solution. The X-ray photoelectron spectroscopy showed an increase in the oxygen-to-carbon ratio, which confirms the presence of surfactant on the surface. Time-of-flight secondary ion mass spectroscopy showed that the isopropyl alcohol solution did not change the elemental composition of the surface whereas the surfactant solution clearly did so. The distribution of surfactant on the surface was confirmed by mapping the characteristic fragments of the molecule.

The industrial use of polypropylene (PP) films is limited because of undesirable properties such as poor adhesion and printability. In the present study, a DC glow discharge plasma has been used to improve the surface properties of PP films and make it useful for technical applications. The change in hydrophilicity of modified PP film surface was investigated by contact angle (CA) and surface energy measurements as a function of exposure time. In addition, plasma-treated PP films have been subjected to an ageing process to determine the durability of the plasma treatment. Changes in morphological and chemical composition of PP films were analyzed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The improvement in adhesion was studied by measuring T-peel and lap shear strength. The results show that the surface hydrophilicity has been improved due to the increase in the roughness and the introduction of oxygen-containing polar groups. The AFM observation on PP film shows that the roughness of the surface increased due to plasma treatment. Analysis of chemical binding states and surface chemical composition by XPS showed an increase in the formation of polar functional groups and the concentration of oxygen content on the plasma-processed PP film surfaces. T-peel and lap shear test for adhesion strength measurement showed that the adhesion strength of the plasma-modified PP films increased compared with untreated films surface.

BACKGROUND: Polytetrafluoroethylene (PTFE) is utilized in many engineering applications, but its poor wettability and adhesion properties with other materials have limited its use. The study reported was aimed at achieving surface modification of PTFE films by radiofrequency NH₃ and N₂ plasma treatment, followed by graft copolymerization, in order to improve the interfacial adhesion of PTFE and bismaleimide.

RESULTS: X-ray photoelectron spectroscopy results showed that a short-time plasma treatment had a distinct defluorination effect and led to nitrogen functional group formation. The nitrogen chemical bonding form was different for NH₃ and N₂ plasma treatments. Under the same experimental conditions, the NH₃ plasma exhibited a better etching effect than did the N₂ plasma. Contact angle measurement showed an improvement in both surface energy and wettabliity by short-time plasma treatment. The concentration of the surface-grafted bismaleimide on PTFE increased after the plasma pretreatment. The lap shear strength between PTFE and bismaleimide increased significantly after surface modification.

CONCLUSION: This study found that plasma treatment caused changes in surface chemistry, thus leading to an increase of the wettability of PTFE surfaces. Hence, the adhesion properties of PTFE with bismaleimide were significantly improved. Copyright © 2008 Society of Chemical Industry

The peel characteristics of sealed low-density polyethylene/isotactic polybutene-1 (PE-LD/iPB-1) films, with different contents of iPB-1 up to 20 m.-% (mass percentage), were evaluated and simulated in dependence on the iPB-1 content, and in dependence on the peel rate. Sealing involves close contact and localized melting of two films for a few seconds. The required force, to separate the local adhered films, is the peel force, which is influenced, among others, by the content of iPB-1. The peel force decreases exponentially with increasing iPB-1 content. Transmission electron microscopy studies reveal a favorable dispersion of the iPB-1 particles within the seal area, for iPB-1 concentrations ≥6 m.-%. Here, the iPB-1 particles form continuous belt-like structures, which lead to a stable and reproducible peel process. The investigation of the peel rate-dependency on the peel characteristics is of important interest for practical applications. The peel force increases with increasing peel rate by an exponential law. A numerical simulation of the present material system proves to be useful to comprehend the peel process, and to understand the peel behavior in further detail. Peel tests of different peel samples were simulated, using a two-dimensional finite element model, including cohesive zone elements. The established finite element model of the peel process was used to simulate the influence of the modulus of elasticity on the peel behavior. The peel force is independent of the modulus of elasticity, however, the peel initiation value increases with increasing modulus of elasticity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 https://onlinelibrary.wiley.com/doi/10.1002/app.28999

In this work we modified the surface of polyethylene in order to coat it with highly-fluorinated UV-curable coatings and assure good adhesion between the two polymers. Different methods were investigated, and a successful treatment was the grafting of a monomer containing a group able to dissociate under UV light. The surface modification was assessed by XPS, ATR–FT-IR and AFM analyses. The modified substrate was easily coated with the photocurable fluorinated formulation, and the highly hydrophobic and oleophobic layer formed after irradiation adhered well to the polyethylene substrate.

The surface of polymer materials has been successfully modified by ion implantation. The plasma-source ion implantation (PSII) technique was applied as a surface modification method for poly (ethylene terephthalate)(PET) films. A mass-separated ion accelerator was used for low energy implantation in poly (vinyl chloride)(PVC) and polyamide (PA). The surface electrical conductivity of these polymers was measured. Using our computer program TRIM-MV for simulation of the accelerated ion transport through the polymers, the penetration depths of bombarding ions in the studied plastic films were calculated. The experimentally observed changes in physical properties cannot be explained by the calculated ion ranges and implanted particle energy distributions. For the experimental conditions used, the chemical structure modification of the polymer surface, polymer material erosion, and gas creation and its diffusion through the surface layer are more important reasons for the modified material characteristics. The kind of bombarding ions and the composition of polymer material are found to be of prime importance.

Surface energy (1) of a substrate plays a vital role in determination of surface characteristics in terms of interactions with liquid phases, such as inks. The polar and dispersive components of surface energy are employed so as to estimate the interaction of a substrate with polar and non-polar liquids. This study involves the determination of dependence of surface energy of coated paper on various calendering conditions and effect of post-calendering environmental conditions, such as relative humidity, on surface energy, which is of high importance in estimation of ink-substrate interactions and so as to interpret various print attributes. A laboratory-scale cylindrical coater has been used to coat paper sheets with different coating formulations and a laboratory-scale soft-nip calender has been used with different pressure and temperature combinations, so as to generate a series of coated paper samples, which were further conditioned at different environmental conditions by varying the levels of relative humidity. The surface energy determinations were made on these samples by measuring the contact angle of liquids with different polar and dispersive natures.

Understanding, measuring, and interpreting equilibrium contact angles appear to be simple, but may actually be quite confusing. This paper is an attempt at a guide to the perplexed. First, a comprehensive, clearly defined terminology is suggested. Then, the theory of equilibrium contact angles on smooth, rough, or chemically heterogeneous surfaces is briefly discussed. Finally, the practical implications of the theory to contact angle measurement and interpretation are indicated and explained.

We examined the effect of hydrogen content in various polymers in a N₂/H₂ discharge for surface amine functionalization. Three polymers (polyethylene (PE), polyvinylidene fluoride (PVDF), and poly(tetrafluoroethylene) (PTFE)) containing various amounts of hydrogen and fluorine were treated with an atmospheric pressure dielectric barrier discharge (DBD). While surface modification was observed on the PE and the PVDF in a pure N₂ discharge, adding H₂ in a N₂ discharge was necessary to observe the fluorine etching on the surface of the PVDF and PTFE polymers. The presence of a slight amount of hydrogen in the gas mixture was also a prerequisite to the formation of amino groups on the surface of all three polymers (max NH₂/C ∼ 5%). Aging revealed that the modified polymer surfaces treated in a N₂−H₂ discharge were less prone to hydrophobic recovery than were surfaces treated in pure N₂, due primarily to the presence of a higher density of polar groups on the surfaces. We demonstrated that H atoms in the discharge are necessary for the surface amine functionalization of polymers in a N₂ atmospheric pressure DBD, regardless of polymer chemical composition. It is therefore possible to control the plasma functionalization process and to optimize the concentration and specificity of NH₂ grafted onto polymer surfaces by varying the H₂ concentration in a N₂ atmospheric pressure DBD.

In modern technology, there is a constant need to solve very complex problems and to fine-tune existing solutions. This is definitely the case in modern medicine with emerging fields such as regenerative medicine and tissue engineering. The problems, which are studied in these fields, set very high demands on the applied materials. In most cases, it is impossible to find a single material that meets all demands such as biocompatibility, mechanical strength, biodegradability (if required), and promotion of cell-adhesion, proliferation, and differentiation. A common strategy to circumvent this problem is the application of composite materials, which combine the properties of the different constituents. Another possible strategy is to selectively modify the surface of a material using different modification techniques. In the past decade, the use of nonthermal plasmas for selective surface modification has been a rapidly growing research field. This will be the highlight of this review. In a first part of this paper, a general introduction in the field of surface engineering will be given. Thereafter, we will focus on plasma-based strategies for surface modification. The purpose of the present review is twofold. First, we wish to provide a tutorial-type review that allows a fast introduction for researchers into the field. Second, we aim to give a comprehensive overview of recent work on surface modification of polymeric biomaterials, with a focus on plasma-based strategies. Some recent trends will be exemplified. On the basis of this literature study, we will conclude with some future trends for research.

Low pressure plasma treatment using radiofrequency (rf) discharge of argon gas was employed to improve the hydrophilicity of polypropylene. The effects of argon plasma on the wettability, surface chemistry and surface morphology of polypropylene were studied using static contact angle measurements, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Increase in surface energy of polypropylene was observed as a result of argon plasma treatment. SEM and AFM images revealed the increased surface roughness. A set of identified process variables (rf power, pressure, argon flow rate and time) were used in this study and were optimized using central composite design (CCD) of response surface methodology (RSM). A statistical model was developed to represent the surface energy in terms of the process variables mentioned above. Accuracy of the model was verified and found to be high.

In this investigation, the surface modification of poly vinylidene fluoride (PVDF) and poly methyl methacrylate (PMMA) film induced by air plasma has been investigated using contact angle measurement, electron spectroscopy for chemical analysis (ESCA), and ATR-FTIR spectroscopy. Plasma treatment affects the polymer surfaces to an extent of several hundreds to several thousand angstroms deep, and the bulk properties of the polymer substrate are never modified because of its low penetration range. Plasma surface treatment also offers the advantage of greater chemical flexibility. The plasma exposure leads to weight loss and changes in the chemical composition of the polymer film surfaces. The contact angle of water shows decrease in surface wettability of PVDF and PMMA as the treatment time increases. The improvement in adhesion was studied by measuring T-peel strength. In addition, printability of plasma treated PVDF and PMMA was studied by cross test method. It was found that printability increases considerably for plasma treatment of short duration. Surface energy and surface roughness can be directly correlated to the improvement in the aforementioned surface related properties. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

We review our 2006−2009 publications on wetting and superhydrophobicity in a manner designed to serve as a useful primer for those who would like to use the concepts of this field. We demonstrate that the 1D (three-phase, solid/liquid/vapor) contact line perspective is simpler, more intuitive, more useful, and more consistent with facts than the disproved but widely held-to-be-correct 2D view. We give an explanation of what we believe to be the reason that the existing theoretical understanding is wrong and argue that the teaching of surface science over the last century has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. We review our analyses of the phenomena of contact angle hysteresis, the lotus effect, and perfect hydrophobicity and suggest that needlessly complex theoretical understandings, incorrect models, and ill-defined terminology are not useful and can be destructive.

We respond to a recent report in this journal that criticizes our experiments, which disproved the Wenzel and Cassie theories. The criticism is that we measured contact angles “with drops that were too small, ignoring the indications of existing theoretical understanding.” We take a step back to give an explanation of what we believe to be the reason that the “existing theoretical understanding” is wrong. We explain that the teaching of surface science has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. This continues as evidenced by this recent criticism and numerous recent papers. We describe several demonstrations that were designed to help teachers, students, and scientists overcome the widespread learning disability that is rooted in their faulty intuition and to help them regard wetting from the perspective of lines and not areas.

The van Oss–Chaudhury–Good theory (vOCGT) was checked for a large artificial set of work of adhesion input data calculated for 15 solids and 300 liquids. Numerical values of LW component and acid (A) and base (B) parameters were assigned to 15 solids. These 15 solids were grouped in 5 sets of 3 solids in each. Also numerical values of LW component and A and B parameters were assigned to 300 liquids (three sets of 100 liquids in each). Data for these solids and liquids were especially selected to represent real types of materials encountered in practice. For all 15 solids and 300 liquids the work of adhesion values were calculated and these values were assumed to be error-free. Next, new values of the work of adhesion were obtained by adding a random homoscedastic error (A vector of random variables is homoscedastic if it has the same finite variance.) of the normal distribution (Also called the Gaussian distribution — it is continuous probability distribution defined by two parameters: the mean and variance (standard deviation squared, σ²).), belonging to 8 distributions of a mean value equal to the error-free work of adhesion value and standard deviations of 0.5, 1, 2, 5, 7, 10, 15 and 20 mJ/m². The LW components and A and B parameters for these solids were back-calculated for each error level. Two different methods for the solution of a 3-equation set were used and they gave practically the same results irrespective of the error level and liquids and solids used. It was found that there existed a linear correlation between the RMSE (root mean square error) of the solution and the standard deviation of the work of adhesion data. This correlation was highly significant (with a correlation coefficient higher than 0.999) and was true separately for LW component, A and B parameters as well as for the total solution vector (i.e., combinedly for the LW component, A and B parameters). The RMSE values of the total solution vector (having as elements values of the LW component, A and B parameters) as well as separately for LW component and A and B parameters were correlated with the condition number of a given 3-equation set. A very good correlation was found only for the total solution, much worse for A or B parameters, and practically there was a lack of correlation for the LW component. Based on the correlation between the RMSE and the standard deviation of the work of adhesion it was possible to determine what should have been the maximal standard deviation of the work of adhesion if the calculated value of a given LW component or A or B parameter did not differ by more than 1 mJ/m² from an error-free (true) value.

Flock coating is a widely used process to create a textile-like texture on substrates of various shapes and materials. In the process, flock fibers—short fibers typically 1–3 mm long—are oriented and accelerated towards the substrate by means of an electric field. Impacting fibers are stuck to the substrate surface by an appropriate adhesive. Primary quality criteria are adhesion of the flock fibers to the adhesive, and also the so-called flock density, ie number of fibers per unit area, and evenness. The influential physical and chemical factors refer to interfacial adhesion, but also charging effects by the impacting fibers. The system presently under investigation is based on aliphatic polyamides as material for a molded car component, hot-melt adhesive, and flock fibers. Experiments reported here refer to the application of an air plasma pretreatment of the polyamide (PA) substrate, mainly in order to increase the adhesion of the hot-melt layer. It was found that the plasma treatment affects the polar energy of the PA surface with a related increase in wettability due to a reduction of C–C and C–H bonds and an increase of carboxylic groups. Surface carbonization occurred at higher plasma doses. The effect on hot-melt adhesion was rather small, however two types of failures were observed in these experiments, either due to insufficient adhesion of the hot-melt or due to a break of one of the PA plates with the bond still intact. The characterization of flock coatings on these samples showed no effect on flock fiber adhesion in pull-out as well as on abrasion resistance, but an increased flock density was observed. This is assumed to be due to enhanced dissipation of charges by the conductive water layer adsorbed on the substrate surface.