A plot of the initial spreading pressures F sub ba or initial spreading coefficients S sub ba against the surface tensions of a homologous series of organic liquids b can be used to determine the critical surface tension for spreading on a second substrate liquid phase a. Straight-line relations are found for various homologous series. The intercept of that line with the axis of abscissas F sub ba 0, or S sub ba 0 defines a value of spreading for that series. This method is advantageous because it eliminates the need for measuring or calculating the contact angle of lens b floating on liquid a, it can be applied to any liquid substrate, and it is applicable even when spreading does not lie within the range of surface tensions of the members of the homologous series of liquids b. The value of spreading for the waterair interface was determined in this way using several homologous series of pure hydrocarbon liquids. The lowest value found was 21.7 dynescm at 20 deg C for the n-alkane series. Higher spreading values were obtained using olefins or aromatic hydrocarbons as the result of interaction between the unsaturated bond and the water surface. Since the results are analogous to those reported earlier for solid surfaces, it is concluded that the clean surface of water behaves as a low-energy surface with respect to low-polarity liquids. This result is to be expected if only dispersion forces are operative between each alkane liquid and water.

Many wetting processes involve interpretation using Young’s equation to describe contact angle equilibrium on a solid surface. By assuming the solid to be rigid, no account is made of the component of the liquid surface tension perpendicular to the solid surface. It is shown that a wetting ridge must be formed and, although negligible for hard solids, this mesoscopic disturbance of the solid near the wetting front can have significant consequences on a soft solid. The theory of triple line displacement, taking into account viscoelastic dissipation in the wetting ridge, is developed both for wetting and dewetting processes. Experimental studies using tricresyl phosphate and two types of model solid—a rigid polymer and silicone elastomers—have been carried out. Both for wetting and dewetting, triple line motion is markedly slowed down on the soft solids as a result of viscoelastic dissipation near the triple line. Theoretical predictions and experimental findings are found to be in good agreement.

Wetting hysteresis due to isolated surface heterogeneities is now fairly well understood but when the solid presents a population of defects, complex cooperative effects between neighbours may exist. One such effect is that of ‘shadowing’, in which a proportion of the flaws near the triple line, and which would otherwise contribute to hysteresis, are masked by already existing deformations to the wetting front caused by neighbouring heterogeneities. This renders them inactive and, as a result, the hysteretic wetting force is only expected to be a linear function of density for sparse populations. Theoretical predictions are compared with experimental results obtained with model heterogeneous surfaces consisting of overhead projector transparencies bestrewn with circular ink spots - the defects. Agreement is found to be satisfactory when intrinsic angles on both the homogeneous solid and the flaws are finite, whereas the concordance is less satisfactory when the contact angle of the liquid on the homogeneous solid is zero.

In investigating the effect of the surface energetics of substrate materials on the adhesion characteristics of poly(p-xylylene) and poly(chloro-p-xylylene) by the “Scotch Tape” method, it was found that if the substrates had not been preconditioned (treated with argon or a methane plasma), the adhesion was poor. The characteristics of water resistant adhesion that were observed when coated substrates were boiled in 0.9% sodium chloride solution were found to vary from excellent (when the polymer did not peel from the substrate after three cycles of 8 hours of boiling and 16 hours at room temperature) to poor (when the polymer peeled off almost immediately). It was noticed that water resistant adhesion depends on the hydrophobicity of the substrate material (the greater the hydrophobicity, the greater the adhesion) and is not related to the dry adhesive strength of poly(p-xylylene). The oxygen glow discharge treatment of the substrates decreased both the dry and wet adhesive strength of the polymer. The effect of the argon glow discharge treatment depended on the surface energetics of the substrate, and the methane glow discharge treatment increased both the dry and wet adhesive strength of the polymer. These preconditioning processes are discussed in terms of the sputtering of the material from the wall of the reactor in contact with the plasma and the deposition of the plasma polymer of the sputtered material on the substrate surface.

This paper describes various aspects of water-based coatings and printing processes with special emphasis on the surface characteristics of coating/printing films. The film formation depends significantly on the surface properties of formulated coating/ink, and their interactions with substrates. Several surface parameters in relation to coating defects are briefly described. The mechanisms of printing processes and coating/ink film formation by water-based systems are presented. It has been shown that the formation of surface tension gradient during film curing determines the quality of the coating and printing films. Results demonstrate that the incorporation of suitable additives in the formulation can considerably minimize the crater formation. The hydrophilic-lipophilic balance (HLB) concept and the effect of surfactant concentration on pigment dispersion in an aqueous medium are discussed.

The magnitude of the hydrophobic effect, as measured from the surface area dependence of the solubilities of hydrocarbons in water, is generally thought to be about 25 calories per mole per square angstrom (cal mol^-1 Å^-2). However, the surface tension at a hydrocarbon-water interface, which is a "macroscopic" measure of the hydrophobic effect, is ≈72 cal mol^-1 Å^-2. In an attempt to reconcile these values, alkane solubility data have been reevaluated to account for solute-solvent size differences, leading to a revised "microscopic" hydrophobic effect of 47 cal mol^-1 Å^-2. This value, when used in a simple geometric model for the curvature dependence of the hydrophobic effect, predicts a macroscopic alkane-water surface tension that is close to the macroscopic value.

Certain aspects of the adsorption theory of adhesion are developed more fully than has been done previously. The consequences of nonreciprocity of spreading are pointed out, and are used to develop a more general practical point of view with respect to the adhesive bonding of materials of low-surface free energy. The system epoxy adhesive-(nonsurface-treated) polyethylene, normally considered nonadherent, is investigated experimentally in some detail. It is shown how this system, without material modification, can be made adherent. An area of study for possible adhesives for materials of lowsurface free energy is suggested.

In conventional offset lithographic printing, it has been well established that the existence of a continuous layer of fountain solution (FS) on the surface of the non-image area is an essential condition to ensure correct operation of lithography. However, the mechanistic function of FS in preventing the ink from being transferred onto the non-image area has not been fully understood. Several major mechanistic interpretations can be found in the literature, which are based either on comparing of static works of adhesion and cohesion of ink and FS, or on the splitting of the 'weaker' FS layer. Although the latter becomes more accepted, direct experimental evidence is difficult to find in the literature. On the other hand, confusing information found in the literature showed that the ink-transfer (or non-transfer) observations reported in many case studies correlate well with simple comparisons of works of adhesion, cohesion and spreading data of ink/FS, ink/plate and FS/plate obtained under the static condition. These results, therefore, imply that, in explaining the function of FS in preventing ink transfer to the non-image area, the ink/FS interfacial adhesion failure would be the dominant mechanism. The work presented in this study covered two specific areas in order to address and better understand the responses of ink and FS layers and their interface to forces encountered during ink transfer. Firstly, an analysis of lithographic plates contaminated with a cationic polymer revealed that the violation of the ink non-transfer condition of the plate non-image area due to contamination could be predicted by traditional criteria of plate wetting and works of adhesion and cohesion. However, these traditional criteria cannot reliably predict the non-transfer condition of the ink on the clean non-image area that was covered by FS. Secondly, in some novel experiments conducted in this study using ice or Teflon as a substrate, the works of adhesion and cohesion were not able to predict ink transfer in most cases. Direct experimental evidence from this work revealed that splitting of the FS layer was involved in the prevention of ink transfer to the non-image areas, and that the thickness of the FS layer was critical in allowing the splitting to occur.

The effects of flame treatment on the surfaces of a propylene-ethylene copolymer have been studied using XPS, contact angle measurement, vapour-phase derivatization and an adhesion test. The results obtained were compared to those from the homopolymer. An optimum air-to-gas ratios of ∼11:1 has been found. Close correspondence between water contact angle and oxygen concentration was found, with the exception of high oxygen concentrations. The orientation or migration of functional groups away from the surface has been proposed to cause the non-correspondence between water contact angle and oxygen concentration. Diiodomethane advancing contact angle was found to remain constant, independent of flame conditions. XPS analysis in conjunction with vapourphase derivatization with trifluroacetic anhydride (TFAA) suggests that up to 20% and 30% of the oxygen introduced in the surfaces is present as hydroxyl groups for propylene homopolymer and the copolymer, respectively. High adhesion levels of the flame-treated copolymer with a polyurethane-based paint were found. In most cases, the adhesion failure was complex, but involved the cohesive failure of the copolymer.

X-ray photoelectron spectroscopy (XPS) has been used to study the effects of flame treatment on three propylene polymers, i.e. a homopolymer, an ethylene-propylene copolymer and a rubber-modified polypropylene. Angle-resolved XPS has shown an enrichment in oxygen concentration at the near surface for all three propylene polymers when treated with a mild flame. A depletion in oxygen has been shown at the near surface of the rubber-modified polypropylene treated with an intense flame. The use of simple surface composition models shows that the oxidation depth induced by a mild flame treatment is around 50 Å, and that oxygen-containing functional groups may have reoriented or migrated a few ångströms away from the near surface of the rubber-modified polypropylene during the treatment with an intense flame.

We demonstrate that stable microwave-coupled atmospheric pressure nonequilibrium plasmas (APNEPs) can be formed under a wide variety of gas and flow-rate conditions. Furthermore, these plasmas can be effectively used to remove surface contamination and chemically modify polymer surfaces. These chemical changes, generally oxidation and crosslinking, enhance the surface properties of the materials such as surface energy. Comparisons between vacuum plasma and atmospheric plasma treatment strongly indicate that much of the vacuum-plasma literature is pertinent to APNEP, thereby providing assistance with understanding the nature of APNEP-induced reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 95–109, 2002
https://onlinelibrary.wiley.com/doi/abs/10.1002/pola.10056

An atmospheric pressure non-equilibrium plasma (APNEP) developed in the UK by EA Technology Ltd is currently being investigated in collaboration with the University of Surrey. Of the many applications of surface modification that can be induced using plasmas, adhesion enhancement is one of the most commercially important. In this paper, we illustrate the use of an atmospheric plasma to enhance the adhesion characteristics of low-density polyethylene (LDPE) and poly(ethylene terephthalate) (PET). The polymers were treated in the remote afterglow region of an atmospheric pressure plasma to avoid the thermal effects that can cause degradation for thermally sensitive materials when placed in direct contact with the plasma. Reactive (oxygen containing) and inert (oxygen free) atmospheric plasmas rapidly impart adhesion enhancement by a factor of two to ten as measured by 180° peel tests. However, extended exposure to the atmospheric plasma does not impart additional adhesion enhancement as the surface is ablated revealing the underlying polymer with poor adhesive characteristics. In contrast, vacuum plasma treated LDPE and PET show increased adhesion with extended plasma treatment. An adhesion enhancement in excess of two to three orders of magnitude was found to be achievable for vacuum plasma treatment times greater than 10 min.

Atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in a joint project with the University of Surrey. APNEP has been used to induce surface modification changes on commodity polymers such as high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA).

A stable atmospheric pressure glow discharge can be formed with a variety of gases, (e.g., nitrogen, air, argon and helium). In all cases, the plasmas are capable of inducing surface modification of commodity polymers in the near-field and remote afterglow regions. However, as APNEP can have a significant thermal component, care must be taken to avoid thermal decomposition of the polymers.

This study has used differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to investigate the thermally induced transitions and thermal decomposition behaviour of commercial polymers. The DSC measurements give melting points, heats of fusion and crystallinities. TGA has been used to measure the onset of thermal degradation in both air and nitrogen atmospheres. In parallel with these experiments, temperature profiles of the downstream region of APNEP have been recorded. As a result, positioning of samples and residence times to avoid thermal damage to the substrates can now be achieved.

An atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in collaboration with the University of Surrey. The main focus is the use of atmospheric pressure plasmas to modify the surfaces of commercially important polymers including polyolefins, poly(ethylene terephthalate) and poly(methyl methacrylate). These surface modifications include surface cleaning and degreasing, oxidation, reduction, grafting, cross-linking (carbonization), etching and deposition. When trying to achieve targeted surface engineering, it is vital to gain an understanding of the mechanisms that cause these effects, for example, surface functionalization, adhesion promotion or multi-layer deposition. Hence comparisons between vacuum plasma treated surfaces have also been sought with a view to using the extensive vacuum plasma literature to gain further insight. In this paper, we will introduce the APNEP and compare the key characteristics of the plasma with those of traditional vacuum plasma systems before highlighting some of the surface modifications that can be achieved by using atmospheric plasma. Data from the analysis of treated polymers (by spectroscopy, microscopy and surface energy studies) and from direct measurements of the plasma and afterglow will be presented. Finally, our current understanding of the processes involved will be given, particularly those that are important in downstream surface treatments which take place remote from the plasma source.