Wetting involves the interaction of a liquid with a solid. It can be the spreading of a liquid over a surface, the penetration of a liquid into a porous medium, or the displacement of one liquid by another. It can help to characterize surfaces and to determine solid/liquid interactions. Wettability is most often described by a sessile or resting drop. A schematic diagram is shown in Fig. 1. The contact angle (6) is a measure of wettability. A low contact angle means high wettability and a high contact angle means poor wettability. Zero contact angles are possible but they are always less than 180.(The highest commonly observed angle, mercury on glass, has been reported to be as high as 148 [1].) Systems having more than one stable contact angle are said to show contact-angle hysteresis.

The fiber/epoxy resin adhesion increases after plasma treatment on ultrahigh molecular weight polyethylene (UHMW-PE) fibers. The surface modification of UHMW-PE monofilaments was studied using a combination of techniques: contact-angle measurements, SEM, and pullout tests. The results may be summarized as follows: Infiuenced by different plasma parameters and draw ratios of the monofilaments, the adhesion increases by at least four times by plasma treatment. Failure in the pullout tests involve rupture within a treated monofilament and the skin of it was peeled off; the degree of peeling-off is affected by different plasma treatment conditions and draw ratios of the monofilaments. There is only a slight decrease in the surface energy of the treated monofilaments with aging time. Ways of combining plasma etching with other chemical treatments to further improve the fiber/resin adhesion have also been studied. © 1993 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1993.070471116

This article reviews theoretical and practical aspects of electrical discharge plasma treatment for automotive parts at atmospheric pressure. Paints and bonding compounds adhere poorly to polyolefins because of their intrinsic non-polar chemical structure. Therefore, these materials require pretreatment before bonding and finishing to improve their adhesive properties. The electrical discharge plasma treatment at atmospheric pressure offers several advantages to automotive suppliers, such as the high treatment level, its repeatability and cost effectiveness, versatility of in-line processing, and the environmentally-safe nature of the process. Despite its increasing use, industry standards for surface treatment of plastic pa* have not been developed.

Untreated aluminium and aluminium hydrated for 60 s in boiling water have been extrusion-coated with low-density polyethylene (LDPE). The hydration transforms the oxide surface into a porous oxyhydroxide, known as pseudoboehmite. LDPE samples with different melt indices (4.5, 7.5, and 15) were used, which influence the ability to penetrate into the pores. Compared with untreated aluminium, a superior peel strength was obtained for the laminates with hydrated aluminium. In almost all cases, the peel strength for the laminates with hydrated aluminium could not be measured, due to rupture in the polymer film. This improvement is suggested to be due to stronger acid-base interactions, increased contact surface, and mechanical keying into the porous surface. The obtained peel strength and analysis by means of scanning electron microscopy indicated that the polymer with the highest melt index or lowest melt viscosity had the greatest ability to penetrate into the formed pores. After ageing up to 12 weeks in solutions with 1% and 3% acetic acid, the peel strength dropped rapidly for the untreated Al laminates, but remained constant for the hydrated Al laminates. This is explained by the fact that, besides the improved adhesion, the hydrated oxide prevents corrosive attack.

Asymmetric poly(vinyl alcohol) (PVA) membranes were prepared by the phase inversion technique, and crosslinked with glutaraldehyde. The degree of crosslinking of the membrane was controlled by varying the crosslinking conditions. The effects of the degree of crosslinking on the swelling characteristics, contact angles, critical surface tensions, and pervaporation characteristics were examined. A method for the evaluation of the degree of crosslinking, which needs only the gluaraldehyde concentration of the crosslinking solution to be measured after the crosslinking reaction, is proposed, and was found useful. The degree of swelling of PVA membrane for water decreases abruptly as the degree of crosslinking increases. However, the degree of swelling for ethanol is nearly independent of the degree of crosslinking. The critical surface tension of the membrane increases more or less within the range of 37.0–40.0 dyn cm⁻¹ with increasing degree of crosslinking below 30%. But, it is nearly constant at 40.5 dyn cm⁻¹ above 30%. The wetting behavior of the membrane may not be greatly affected by the degree of crosslinking. The selectivity factor and permeate flux of the membrane in the pervaporation of the ethanol-water mixture of 95 wt% ethanol concentration decrease similarly with increasing degree of crosslinking. The pervaporation characteristics seem to be closely related to the swelling behavior. The degree of crosslinking is an important variable for swelling behavior and pervaporation characteristics.

The virtues of chemical inertness and low surface energy which make polytetrafluoroethylene (PTFE) a valuable engineering polymer also account for the difficulty in achieving structural adhesive bonds. While plasma surface treatment has proven to be the most effective means of maximizing strength and permanence of adhesive bonds with the most inert of engineering polymers, a simple plasma treatment has proven elusive for PTFE. The following studies evaluate two very different plasma processes, activation and deposition, as a means to achieve reliable and high-strength structural adhesive bonds. Sodium naphthalene-etched PTFE is used as a control. Presented are ESCA data which support a theory that improvement is limited by a weakened boundary layer of the PTFE.

The effects of corona discharge treatment on polyethylene and polypropylene homopolymers have been studied. X-ray photoelectron spectroscopy was used to determine surface compositions which were related to surface free energy estimates from contact angle measurements. Changes in composition and surface free energy were measured as a function of treatment level. The work of adhesion was seen to increase with oxygen incorporation. The increase was not linear and this is attributed to an increase in the degree of sub-surface to near-surface oxidation at intense treatment levels. Aging of samples followed by XPS and contact angle measurement showed that surface wettability is reduced whereas a slight increase in surface oxygen was found. This phenomenon was attributed to the reorientation/migration of functional groups. Morphological examination by scanning electron microscopy indicated no surface roughening at any power level.

The extent of the surface crosslinking of high-density polyethylene (HDPE) under various plasma treatment conditions was investigated. The plasma modification efficiency was studied by surface energy and adhesive bond strength measurements. The results show that the surface crosslinking of HDPE takes place as soon as the HDPE is exposed to the plasma and that the crosslinking rate is a function of the plasma conditions. The surface energy and the adhesion of HDPE are greatly increased by the plasma treatment and these improvements are independent of the depth of surface crosslinking. Based on these results and our previous studies on the surface chemical composition and free radical density on the surface of HDPE after plasma treatment, the relationships among various surface changes and the surface modification efficiency are discussed.

The dispersive component of the surface free energy, the nondispersive interaction, with polar liquids were determined for cellulose, cellulose acetate and cellulose grafted with alkyl ketene dimer (AKD). and were calculated in the dry state as well as the fully hydrated state by the two liquid contact angle method. was found to be independent of AKD coverage. Iⁿ_sw was found to be highly dependent on AKD coverage and differed significantly between the dry and fully hydrated states. Using the work of adhesion as a criterion, it was postulated that in the dry state, the AKD molecule renders the cellulose hydrophobic, and undergoes surface restructuring in the hydrated state leading to a hydrophilic surface.

The ability of polypropylene (PP) to adhere to mild steel depends to a large extent on the surface characteristics of both PP and steel. The adhesion of PP was improved by treatment in a cold plasma from oxidizing gases (O₂, H₂O, etc.). This surface functionalization was followed ex situ by means of contact angle measurements and XPS (X-ray photelectron spectroscopy) analysis. The polymer/steel assembly was fabricated by hot-pressing in vacuum, or after exposure to ambient air. Adhesion to steel, as determined by the lap-shear test, increased when the PP was treated with Ar-containing plasma gas and joined to steel after exposure to room atmosphere. Correlations between the polarity, the atomic (O/C, N/C) ratio, the dispersive component of the surface energy, and the degree of PP/steel adhesion are discussed.

Polyethylene and polystyrene film surfaces have been plasma-oxidized and subsequently characterized by X-ray core level and valence band spectroscopies. The extent of polyethylene surface oxidation was found to be dependent on the power of the oxygen glow discharge employed and the length of time that the treated sample was left exposed to air prior to analysis. In marked contrast to these observations, plasma-oxidized polystyrene surfaces were much less dependent on the oxygen glow discharge power and were also found to retain their oxygenated character over much longer periods of ageing. These differences in oxidative behaviour are explained in terms of the molecular structures of the respective polymers.

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.

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 surface energetic properties of different areas of the offset printing plate are the key factors of this printing process, since they control the ink transfer during printing. The importance of these factors is discussed for both waterless offset and conventional offset. The printing process is highly dynamic. New surfaces are created and their lifetimes are short. From recent theories of dynamic wetting, it has been concluded that spontaneous removal of ink films from nonimage areas is a very slow due to the high ink viscosity and the low dynamic contact angle. Thus it is of less importance.

Low density polyethylene (PE) and polyethylene terephthalate (PET) films were treated in air plasma of a low pressure (500 mTorr) large area microwave (2.45 GHz) discharge, or in a corona discharge at atmospheric pressure. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) for their oxygen content [O] and surface chemical structure, which were related to the corresponding peel strength of PE/PE and PE/PET laminates. Although the oxygen concentration at the surface increased monotonically with the degree of treatment, the peel force reached a maximum and then sharply decreased. Regardless of the treatment type, the maximum peel force occurred for [O] values between 10-15 at.%. The highest peel force has been found to occur when the concentration of CO (hydroxyl, ether, epoxy,…) groups is highest and that of OCO (carboxyl) groups is lowest, which corresponds to the situation when the effect of a weak boundary layer, due to low molecular weight materials, is minimal (low OCO concentration).

The hydrophobic recovery of oxygen-plasma-treated hydrophilic surfaces of polyethylene and polypropylene films was investigated by measuring the dependence of the contact angle with the water on the vacuum storage time. It could be shown that the rehydrophobation of the polyethylene surface may be retarded by the previous controlled surface cross-linking in a hydrogen plasma and/or by repeated plasma treatment. However, the loss in hydrophilicity cannot be suppressed for ever. After certain individual periods all treatments lead to the same final state. Nevertheless, in particular, controlled cross-linking seems to be a suitable way for improving the long-term stability of plasma-functionalized polymer surfaces for polymers not tending to chain scission during plasma treatment.