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.

Four types of contact angles (receding, most stable, advancing, and “static”) were measured by two independent laboratories for a large number of solid surfaces, spanning a large range of surface tensions. It is shown that the most stable contact angle, which is theoretically required for calculating the Young contact angle, is a practical, useful tool for wettability characterization of solid surfaces. In addition, it is shown that the experimentally measured most stable contact angle may not always be approximated by an average angle calculated from the advancing and receding contact angles. The “static"” CA is shown in many cases to be very different from the most stable one. The measured contact angles were used for calculating the surface tensions of the solid samples by five methods. Meaningful differences exist among the surface tensions calculated using four previously known methods (Owens-Wendt, Wu, acid-base, and equation of state). A recently developed, Gibbsian-based correlation between interfacial tensions and individual surface tensions was used to calculate the surface tensions of the solid surfaces from the most stable contact angle of water. This calculation yielded in most cases higher values than calculated with the other four methods. On the basis of some low surface energy samples, the higher values appear to be justified.

The influence of the pulsed CO₂ laser irradiation on the surface structure of the LDPE film was investigated. Significant changes were observed on the surface of laser treated films as it was verified by the attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy, scanning electron microscopy and contact angle-measurement. Formation of polar functional groups onto the LDPE surfaces exhibited by the ATR–FTIR spectra was shown to be strongly dependent on the number of the CO2 laser pulses. The intensity of the polar groups increased with increasing the number of pulses up to two and then slightly decreased at three laser pulses. This was also confirmed with the contact angle measurements in which the sample subjected to two laser pulses showed the highest wettability i.e. the lowest water drop contact angle. The concentration of peroxide groups formed on the surface of the laser treated films was determined quantitatively by UV spectroscopic method using iodide procedure. The latter results showed a similar trend with the results obtained using FTIR spectroscopy.

Corona and chemical treatment worked cooperatively for increasing and stabilizing the polyethylene film surface energy. Gentle and varied corona discharge treatment conditions were applied for each polyethylene film to reach 40 dynes/cm. A rather low blending amount of additive could stabilize the film surface energy obviously. Compared with neat PE film, of which the surface energy decreased to 36 dynes/cm at the 12th day, films blended with 1000 ppm A7-OH or PE-PEG 4k -PE showed stable surface energy (36–38 dynes/cm) over 150 days. The influence of industrial applied slipping agent was investigated as well. Morphological and chemical changes were studied by X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscope (AFM). The surface energy was determined by the dyne pens. Mechanism investigation of hydrophilization and hydrophobic recovery processes showed that proper crystallization behavior and enough C[dbnd]O groups on the film surface guarantee satisfactory stability of the surface energy.

To analyze various approaches for the determination of surface free energies of solids from liquid-solid contact angles, comb-like polymers with controlled grafting rates and macromolecular structures have been synthesized. The surface free energy parameters were calculated from the contact angles of standard liquids on the solid surfaces. A mathematical approach of the so-called acid-base theory of adhesion was used to characterize the nucleophilic and/or electrophilic behavior of the polymeric solid surfaces. Thus, correlations were established between the macromolecular structures and the dispersive component of the surface free energy, on the one hand, and the acid and base components, on the other. The main conclusion is that the surface free energy components are relevant for the characterization of functional comb-like polymeric materials: the dispersive and base components increase with the number of grafted electron-donating groups, whereas the acid component decreases.

Interactions of cellulose fiber surfaces with water and other liquids depend on surface morphology as well as intrinsic material properties. Smooth nanocrystalline cellulose (I) films can be used as models to study surface phenomena, where the effects of surface morphology and roughness are minimized. Contact angle measurements are particularly sensitive to surface roughness. In this work, we measured the advancing and receding contact angles for water on thin model cellulose (I) and regenerated cellulose (II) films. The advancing and receding contact angles on model cellulose (I) surfaces were lower than on cellulose (II) surfaces, and the contact angle hysteresis was also lower for the smooth model cellulose (I) surfaces prepared from nanocrystal suspensions. The surface free energy was evaluated for the various cellulose surfaces from contact angle measurements.

Critical surface tensions γ_e of nine representative polymer surfaces with four series of polar liquids differed considerably from commonly accepted values. The Good-Girafalco-Fowkes-Young equation is used to explain the results, and it is shown that if certain precautions are observed, the equation may be used to predict γ_c of solid polymers for “standardized” series of liquids. The theoretical concepts of Fowkes and Good are shown to be compatible with Zisman's approach to the determination of γ_c. Serious errors may result, however, in the evaluation of contact angle data from misuse of the theoretical concepts of Fowkes or from misinterpretation of critical surface tension values as determined by the Zisman technique. Curve of cos θ vs. γ_L are straight lines only for one particular series of liquids and normal curves are of power form. It is suggested that many of the experimental contact angle data in the literature may be reinterpreted, including those for poly-(styrene), human skin, nylon 11, poly(ethylene), and monolayers of perfluorolauric acid.

A modification of the Good-Girafalco-Fowkes-Young equation is used to calculate nondispersion interactions I_SL^P at the interface for nine polymeric solids and four polar series of liquids. The relationship of I_SL^P to work of adhesion W_A and the spreading coefficient S_e is shown. A linear relationship is found to exist between I_SL^P and γ_L^P, the nondispersion energy component of the liquids, for the series of polar liquids and the solids studied. The slopes of the I_SL^P vs. γ_L^P curves vary depending upon the polymer surface. Intercepts of the curves may be a measure of π_s, the reduction in the surface energy of the solid resulting from adsorption of vapor from the liquid.

Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces were exposed to vacuum UV (VUV) photo-oxidation downstream from Ar microwave plasma. The modified surfaces showed the following: (1) an improvement in wettability as observed by water contact angle measurements; (2) surface roughening; (3) defluorination of the surface; and (4) incorporation of oxygen as CF—O—CF₂, CF₂—O—CF₂ and CF—O—CF₃ moieties. With long treatment times, a cohesive failure of copper sputter-coated onto the modified surface occurred within the modified FEP and not at the Cu–FEP interface.

The use of plasma treatment for the modification of polystyrene microtiter wells has been evaluated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The contact angle data suggests that the effect of plasma treatment is first to clean the surface of volatile contamination, increasing the hydrophobicity, and then to introduce oxygen functionality into the surface, decreasing the hydrophobicity. The cleaning effect appears to occur in the first few seconds of treatment while the oxygenation effect increases with increased exposure to the plasma. The XPS and ToF-SIMS measurements show increasing surface oxygen concentration with plasma treatment time, with a concomitant reduction in aromaticity. Scanning tunnelling microscopy (STM) imaging reveals that plasma treatment significantly affects the adsorption of bovine serum albumin (BSA). Untreated surfaces exhibited areas where no BSA adsorption occurred. These regions ranged in size between 20 and 60 nm in diameter. Plasma treated surfaces, however, exhibited no such areas, with BSA adsorption appearing to be more uniform across the surface. The regions on the untreated surfaces where no BSA adsorption occurred are thought to be hydrocarbon (volatile) in nature possibly from the moulding process, which is removed in the first few seconds of plasma treatment.

A new method has been developed for calculating surface free energies of polymeric materials using a simplified solution to the Fowler equation and polarizabilities and diamagnetic susceptibilities for polymer constituent groups. Comparison of this new method with estimates from group parachors, contact angle measurements, or extrapolation of data for melts indicates generally good agreement among the different methods. Discussions are also included on the effect of limited rotation on dipolar interactions and on the proper application of Good and Girifalco's method for estimating surface free energies of solids.

The Young's equation is the well-known relationship used to describe a sessile drop at equilibrium on top of a solid surface. This relationship has been discussed thermodynamically and microscopically for purely flat surfaces in the literature. To characterize the non-flatness of a surface, one may introduce the Wenzel's roughness r defined as the area of the wall surface devided by the area of its projection onto the horizontal plane. Obviously, r is equal to 1 once the surface is flat. For r>1, it is known that Young's equation has to be modified to take into account the increase of surface. The generalization of Young's relation is the so-called Wenzel's law. In this presentation, we will study this relation within microscopic models. We will in particular show that the roughness may enhance the wetting of the substrate even at the microscopic scale.

In this paper, polyester (PET) and polypropylene (PP) films are modified by a dielectric barrier discharge in air, helium and argon at medium pressure (5.0 kPa). The plasma-modified surfaces are characterized by contact angle measurements and X-ray photoelectron spectroscopy (XPS) as a function of energy density. The polymer films, modified in air, helium and argon, show a remarkable increase in hydrophilicity due to the implantation of oxygen-containing groups, such as C–O, O–CO and CO. Atomic oxygen, OH radicals, UV photons and ions, present in the discharge, create radicals at the polymer surfaces, which are able to react with oxygen species, resulting in the formation of oxygen-containing functionalities on the polymer surfaces. It is shown that an air plasma is more efficient in implanting oxygen functionalities than an argon plasma, which is more efficient than a helium plasma. In an air plasma, most of the created radicals at the polymer surface will quickly react with an oxygen particle, resulting in an efficient implantation of oxygen functionalities. However, in an argon and helium plasma, the created radicals can react with an oxygen particle, but can also recombine with each other resulting in the formation of an oxidized cross-linked structure. This cross-linking process will inhibit the implantation of oxygen, resulting in a lower efficiency. In argon plasma, more ions are present to create radicals, therefore, more radicals are able to react with oxygen species. This can explain the higher efficiency of an argon plasma compared to a helium plasma.

Poly(ethylene terephthalate) (PET) Mylar^® samples were treated by corona discharge in order to improve their adhesive properties. The corona treatments were performed in different atmospheres including nitrogen, ammonia and air.

X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical modifications induced at the PET surface by these corona treatments. XPS results show that nitrogen incorporation takes place in the form of non-oxygenated nitrogen functionalities, like amine or cyano groups. These are present at the surface of all the corona-treated samples but in different concentrations depending on the gases used in the corona discharge. Furthermore, XPS analyses performed after heating of the treated samples show a higher thermal stability of the corona-induced surface modifications in the case of nitrogen and ammonia.

Ion scattering spectroscopy (ISS) and static secondary ion mass spectroscopy (SIMS) analyses were also performed because of their higher surface sensitivity compared with XPS: ISS reveals that nitrogen is not present at the topmost surface layer of the treated samples but is incorporated just beneath. The outermost surface layer presents a composition rich in oxygen. Finally, static SIMS spectra show that corona treatment induces more surface degradation when performed in air compared with nitrogen or ammonia.

These results are discussed in relation to adhesive properties of PET.

Contact angles can be of great value; however, making meaningful contact angle measurements and interpreting those measurements is complex. For years, researchers have addressed a wide variety of issues concerning contact angles. Some questions have been qualitatively answered; others remain open. In this paper, we focus on three issues which are particularly important for the measurement and use of contact angles: the appropriate definitions and use of macroscopic and microscopic contact angles, a brief survey of the length scales relevant to phenomena controlling contact angles, and the role of vibrations in determining contact angles. We emphasize contact angle issues relevant to heterogeneous surfaces, specifically, ambient surfaces prevalent in nature and industry.

Wetting on ambient, heterogeneous surfaces is characterized by contact angle hysteresis. Quantitative models of contact angle hysteresis are essential in order to design surfaces with specific wetting behavior or to interpret experiments seeking to characterize a surface through its wetting properties. We focus on the successes and failures of theoretical models as well as experiments on model surfaces in describing contact angle hysteresis on ambient surfaces. We describe experimental observations of contact line structure and dynamics as well as contact angle hysteresis on laboratory surfaces. We discuss three general classes of models treating one-dimensional periodic heterogeneity, two-dimensional periodic heterogeneity, and random heterogeneity. We show where these models succeed and where they fail to agree quantitatively and qualitatively with experimental observations. New models treating strong, dense heterogeneity as well as temporal relaxation of contact angles in experimental environments need to be developed to provide quantitative descriptions of contact angle hysteresis on ambient surfaces.

Surface activation of polymer films to modify surface properties is widely practiced to enhance wettability, printability and adhesion properties of these films. Sigma Technologies has been actively pursuing the area of longlasting activation of polymer films using various techniques including plasma technology. Atmospheric glow discharge plasma technology has been a recent addition towardsthese efforts. Plasma activation using a variety of precursors has been investigated for long lasting functionilization of various substrates. Latest developments in the area of surface activation of polymer films will be presented.

Plasma treatment is a common process for cleaning, etching and chemically functionalizing surfaces of polymer films. High speed plasma treatment is performed at atmospheric and subatmospheric pressure with a treatment device combining magnetic and hollow cathode effects, which enhance the gas ionization and focus and direct the plasma energy onto the surface of the moving web. Plasma treated polymer films show highly increased micro roughness. Polymer films metallized after plasma treatment exhibit highly improved adhesion between polymer and metal and better oxygen and water vapor barrier behaviour. Fabrics can be treated to attain hydrophilic or hydrophobic properties.

A brief review of the surface tension of polymer liquids is presented. A strong emphasis is placed on recent measurements of surface tensions of homologous liquid series up to high-molecular-weight polymers, and the thermodynamic liquid properties of these same homologous series obtained from sources such as pressure-volume-temperature (PVT) data. The accuracy and limitations of the thermodynamic information which are used as input to many of the theories applied to the surface properties of polymer molecules are discussed. By scaling the surface tension data using a true measure of the cohesive energy density of the liquid state, we can clearly observe the entropic contribution to the surface tension caused by the conformational restriction of a large molecule at the liquid-vapor interface. The scaling implies the existence of a corresponding states principle for both polymer liquids and for low-molecular-weight liquids. The ramifications of the existence of a corresponding states principle for the surface tension of polymer melts are discussed. One consequence of the corresponding states principle is that it allows us to use surface tension measurements to compute the cohesive energy density of polymer melts using PVT data.

The biocompatibility of Si⁺ implanted medical polyurethane was studied. Si ion implantation was performed at energies of 40, 60, 80, and 100 keV with doses ranging from 2 × 10¹³ to 2 × 10¹⁶ cm⁻² at room temperature. The results show that the wettability, blood adsorption, anticoagulability and anticalcific bahaviours of the surface were changed significantly by ion bombardment. The results of SEM and XPS analyses indicate that some of the original chemical bonds in the surface region were broken and the degree of destruction was increased after implantation, which was probably the main reason for the surface modification. ESR shows that the number of radicals is not beyond the range 10¹² to 10¹⁴ cm⁻³, which is advantageous for the clinical utilization of polyurethane.

The acid-base approach to the calculation of solid surface free energy and liquid-liquid interfacial tensions is a practical example of application of correlation analysis, and thus it is an approximate approach. In these limits, and provided that wide and well-obtained sets of contact angles or interfacial tension data are used for their computation, surface tension components can be considered as material properties. Although their numerical value depends on the characteristics of the chosen reference material, their chemical meaning is independent on the selected scale. Contact angles contain accessible information about intermolecular forces; using surface tension component (STC) acid-base theory, one can extract this information only making very careful use of the mathematical apparatus of correlation analysis. The specific mathematical methods used to obtain these results are illustrated by using as an example a base of data obtained by the supporters of the equation-of-state theory (EQS). The achievements are appreciably good and the agreement between STC and EQS is discussed.