Extensive study has been made of the effects of various types of glow discharge plasmas on the changes of the surface properties of some fluorine polymers. The properties were investigated as a function of such factors as the exposing period, aging after exposure, type of plasma, and so on.

It was found that the wettability and the critical surface tensions were changed considerably with plasma exposure and that periods of several tens of seconds are long enough to cause changes. The extents of change were not so prominent for fluorine polymers as for polyethylene, and this fact may show the important role of the fluorine atom in the surface properties even after the plasma treatments.

An apparent link between the surface properties of polar group-containing polymers, such as PMMA and Styrene/Acrylic copolymers, and the thermodynamic quality of solvents used in solutions from which the polymers were cast, was described in earlier papers.^1,2 In these polymers, significant variations have been observed in critical surface tensions(γ_c), and in the thermodynamic interaction parameters for selected vapor-polymer pairs, when the configuration of the polymer in solution was varied through the suitable selection of solvents of differing thermodynamic quality. The “solvent history” effect on surface properties of solid film was not detected however for non-polar polymers such as polystyrene (PS).^1,2 Apparently the distinct chain configurations adopted in solution by PMMA are carried over into the solid and result in different proportions of non-polar (backbone) and polar (side chain) moieties being located in the surface layer of the solid. Since only one surface state can correspond to a thermodynamic equilibrium, it may be expected that the film surface properties will change with time, as the thermodynamically preferred state is attained. As a consequence, use properties of these films should also display (initially) the “solvent history” effect, and should vary similarly with time. The present communication is concerned with these points.

Corona treatment of films, mainly polypropylene (PP)-copolymers, was studied at commercial levels in a 2.7 kVA treater. The films were produced on a flat-film extruder with chill rolls. Degree of treatment was characterized by power of the generator divided by web speed and width of film (m Ws/cm²).

The effectiveness of the treatment was measured in terms of the polar and dispersion components of surface-energy, the peel adhesion of pressure sensitive tape (similar to ASTM Adhesion Ratio) and the peel adhesion of polyurethane adhesives.

The polar component of surface energy is a measure of the effectiveness of corona pretreatment. For a given degree of treatment, the polar surface energy component becomes greater as the film cooling rate increases (and the degree of crystallization falls).

A comparison of homopolymers and copolymers does, however, reveal that even where these have the same density or the same degree of crystallization one cannot count on them having equally-sized polar components.

Peel strengths of pressure-sensitive tapes and polyurethane-bonded patches confirm the influence of cooling conditions on wetting properties.

The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch^® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.

Plasma-treated poly-p-xylylene films have been characterized by neutron activation oxygen analysis, internal reflection (IRS) and transmission infrared spectroscopy, transmission electron microscopy (TEM), and surface contact angle measurements. The results indicate that an oxygen plasma roughens the surface and that oxygen is incorporated into the surface. Oxygen is not detected in the bulk of the sample. The infrared transmission spectra exhibited no carbonyl band, but the relative band intensities changed, indicating a change in ring substitution by a loss of chlorine in the chlorinated poly-p-xylylenes. The IRS spectra of the surface of films treated with oxygen plasma did contain carbonyl bands at 1730 and 1640 cm⁻¹. Argon and helium plasmas generally decreased the water contact angle measured on plasma-treated poly-p-xylylene surfaces more than oxygen or nitrogen plasma treatments. Regardless of the plasma utilized, the water contact angles increased with time after the treatment but did not recover to the original level. IRS spectra of the surface of films treated with argon plasma contained carbonyl bands at 1730 and 1695 cm⁻¹. The adhesion of a polyurethane thermosetting material to a poly-p-xylylene surface is greatly improved if a plasma treatment is used prior to the application of the polyurethane. The degree of improvement in adhesion was dependent on the type of plasma and the treatment time.

Coating of polyethylene (PE), poly(vinyl fluoride) (PVF), poly(tetrafluoroethylene) (PTFE), and poly(vinyl chloride) (PVC) films with flow discharge-polymerized acetylene was investigated. The influence of glow discharge experimental conditions on the nature of the coated layer, the films' wettability, surface roughness, and adhesive joint strength was determined. It was found that coating of these films with plasma-polymerized acetylene led to the formation of a rough, crosslinked, irremovable layer with an improved wettability. The presence of the plasma-polymerized acetylene on the film surface lead to a large improvement in the adhesive joint strength of these films with epoxy adhesive. Best results were obtained with films coated at a low acetylene flow rate. Increase in glow discharge power and treatment time lead to a further improvement.

Poly(ethylene terephthalate) (PET) film has been discharge-treated under controlled conditions and the resulting surface modifications analysed via X.p.s. (ESCA), contact angle and surface energy measurements. Changes in surface properties have been followed as a function of ageing time. These measurements have been correlated with the adhesive properties of the treated surfaces using autoadhesion (treated-treated seals) as the probe. Discharge treatment introduces phenolic -OH and carboxylic acid -COOH groups into the surface resulting in increased wetting and much enhanced autoadhesion via hydrogen bonding of phenol groups to carbonyl groups. Much chain scission also occurs; the low molecular weight material is easily removed by washing and migrates into the film on ageing. The new functionalities in relatively immobile chains slowly reorientate and internally H-bond. The former process is largely responsible for the wettability change on aging, the latter for the loss of adhesive properties.

The contact angle (θ) of a sessile liquid drop on a horizontal solid surface can be calculated from the drop volume and the radius of the contact circle at the liquid/solid interface. A simple apparatus which allows simultaneous estimation of these two parameters is described, and tests of the method for two systems are reported. The first system is a 3.25 mole liter⁻¹ solution of 1-propanol in water on paraffin wax. The advancing (θ_a) and receding (θ_r) contact angles at 20°C are found to be (59.5 ± 1.0)° and (54.3 ± 0.3)°, respectively, in good agreement with the literature values and those found by direct measurement. The second system chosen is cyclohexane (a volatile liquid) on cleaved mica at 20°C. Two mica sheets were used. Mean contact angles of cyclohexane on the first mica sheet are θ_a = (7.45 ± 0.10)°, θ_r = (6.99 ± 0.12)°. For cyclohexane on the second sheet the mean contact angles are θ_a = (6.48 ± 0.31)°, θ_r = (5.56 ± 0.06)°. The difference between advancing and receding contact angles is statistically significant (P < 0.01) for both sheets. Other methods of comparable accuracy exist for θ ⪆ 30°, but the accuracy of most of these methods diminishes rapidly if θ ⪅ 30°. If calculation of the contact angle from the spacing between interference fringes is not appropriate, then estimation from drop volume and contact circle radius becomes the method of choice if θ <~ 30°.

The oxidation of polyethylene, polypropylene, and polystyrene by exposure to plasmas excited in pure oxygen and helium–oxygen mixtures at low power levels has been studied. A detailed curve resolution procedure is outlined, and the rate of oxidation is shown to be a strong function of the polymer structure for pure oxygen plasmas, as is the composition of the oxidized layer; this is not the case, however, for oxidation effected by helium–oxygen mixtures. It seems likely, from a consideration of the available data, that the oxidation is confined to the outermost monolayer and is initiated by a crosslinking mechanism that involves oxygen-containing functionalities.

Previous work established the importance of the fibrous substrate topography in obtaining good adhesion of polyethylene to matt black oxide films formed on copper in alkaline solution. In this paper the effect of the very rough surface topography is shown to be general. Anodising treatments for copper and zinc and a high temperature oxidation for steel are described which give a very rough surface consisting (respectively) of fibrous, dendritic and blade-like growths. The peel strength of polyethylene to these substrates is high even under circumstances, for example when the polymer is stabilised with anti-oxidant, where adhesion to a chemically similar smooth surface is low. The high peel strength is associated with large amounts of energy being dissipated during peeling in plastic deformation of the polymer near the interface. It is suggested that this is caused by the development of high shear stress concentration at the fibre ends causing yielding in a large volume of polymer.

We have established experimentally that, for certain liquid-solid systems, the contact angle formed by a drop of liquid varies with drop size, below a critical diameter which is probably a function of the nonuniformity of the solid surface. This effect has been observed with water on Teflon FEP and on polymethylmethacrylate (PMMA), in regard to both advancing and receding contact angles. The decrease was about 8° in θ_a and 16° in θ_r for water on Teflon FEP, between diameters of 4 mm and about 1 mm; and for other systems, a comparable or even greater effect was observed. For water on PMMA, θ_r decreased from 51° to 26°, between diameters of 8 and 3 mm. With ethylene glycol on Teflon FEP, the decrease in θ with drop size was observed in the retreating angle only. With n-decane on Teflon FEP, the contact angle was independent of drop size, between diameters of 1 and 12 mm. For all liquid-solid systems studied, the limiting contact angles for large drops were in good to excellent agreement with the values obtained by the vertical plate method. Qualitatively, this effect could be explained by hypothesizing the existence of a large, negative line tension. It was found that there was quantitative disagreement between observed results and predictions based on this hypothesis. A theory is proposed to explain the experimental results, based on the known heterogeneity of many polymer surfaces and a previously discussed theory of hysteresis. A negative pseudo-line tension can be used in a phenomenological description of the motion and contortion of the three-phase line.

This chapter, and the following one by Neumann and Good, deal with the measurement of contact angles in three-phase systems. The contact angle is, intrinsically, a macroscopic property, and one that should be amenable to a phenomenological treatment, e.g., to measurement without regard to its thermodynamic or microscopic interpretation. But inevitably, the desire for a thermodynamic and molecular interpretation arises. In addition, some fundamental questions about solid surfaces come up, and the problem of hysteresis must be given explicit consideration. So there is a need to go beyond phenomenology.

The contact angle of CH₂I₂, α-bromonaphthalene and aniline on crosslinked polystyrene has been measured. The polymer was swollen for as long as 60 days in the liquid whose contact angle was to be measured. The surface free energy, γ_s, of unswelled polystyrene was estimated from the contact angles and the swelling results, using an equation which had previously been proposed by Good. The value of γ_s is estimated to be 42±2 ergs/cm².

The previous chapter was largely theoretical, in that it dealt with the interpretation of contact angle results in terms of solid surface energies. It also delved into the question of how the structure of a solid surface affects the contact angle that a liquid forms on the solid. The level of structure considered there included features that are not macroscopically observed, such as microheterogeneities, or minute peaks, pits, hills, and grooves in various geometries. Their existence may be inferred from certain observations, such as contact angle hysteresis, and sometimes they can be observed directly, e.g., with the optical or electron microscope.

The curent state of problems connected with the definition and experimental determination of surface free energy and surface tension of polymers is discussed. An analysis of the application of some equations based on classical and modern thermodynamics of polymer solutions shows that present theories need an essential improvement to fit experimental data. The Zisman concept of critical surface tension and Fowkers' hypothesis of additivity in the contribution of polar and dispersion forces to surface tension are criticized and a new approach to the problem is proposed.

Ni, Fe, Ti, Al, Au, and Cu were each evaporated and deposited onto both sides of polyethylene and poly(tetrafluoroethylene) (PTFE) films. Adhesive joint strengths of the different metal–polymer–metal composites were compared and subsequent surface modifications due to metalization were investigated. Studies show no change in wettability of polyethylene or PTFE after a metal layer was deposited onto their surfaces and subsequently removed. There was also no evidence of oxidation or unsaturation of the surface. Gel fractions of polyethylene show a definite correlation between joint strength and crosslink at the surfaces of the different metal–polymer composites. Metals forming the strongest joints with polyethylene yield the greatest amount of crosslinking. Conversely, metals forming the weakest joints result in the least amount of crosslinking.

Contact angles of various liquids and surfactant solutions on polytef and paraffin were measured. Critical surface tension values were obtained by extrapolation of plots of cosine of the contact angles versus corresponding surface tension values. Contact angles measured using polyoxyethylene octylphenols produced linear Zisman plots and yielded critical surface tensions that agreed with accepted values. This liquid series provides a reasonable approach to the measurement of critical surface tension for solid drugs that are soluble in organic liquids but relatively insoluble in water.

X-ray photoelectron spectroscopy showed that a normal flame treatment caused a high level of oxidation in low-density polyethylene. 0.02% of the antioxidant 2,6-ditertbuty-p-cresol did not reduce the degree of oxidation or the level of adhesion in contrast to the extrusion of low-density polyethylene. It is estimated that the depth of oxidation is between 40 and 90 Å which is much less than for a moderate chromic acid treatment or with extrusion. There were no significant changes in the XP-spectra or adhesion levels of flame treated samples after 12 months.

The surface energy of a polymer can be increased by compression moulding against a metal substrate. After removal of the substrate relaxation to the equilibrium value sets in. We determined the rate of polymer surface energy relaxation as a function of temperature. For a vinyl chloride-vinyl acetate copolymer we determined an activation energy of the relaxation process that could be correlated to segmental motions in this polymer. For a plasticized polyvinylchloride we found a lower activation energy and a larger rate of relaxation, which is the result of the action of plasticizers on segmental motions. In the case of polyethylene the results indicate segmental motions in amorphous regions in the polymer. With polyethylene the activation energy drops when nearing the melt temperature. The movements of molecular segments correspond to a desorption process at the polymer surface, after removal of the substrate. This agrees with the adsorption during compression moulding, as repotted in earlier work.

The wetting (or dewetting) of a solid by a liquid is an integral part of many important processes such as coating, petroleum recovery, distillation and the handling of liquid fuels in low gravity conditions. Several experiments^1–4 have shown that wetting lines (where liquid, air and solid phases meet) which are straight at slow rates of movement over the solid have a sawtooth shape at sufficiently high speeds. We now offer a quantitative explanation for this phenomenon based on the postulate that, for a given system, there is a maximum rate at which wetting can proceed. The consequences of this interpretation are likely to be important, since, in many practical situations, the aim is to maximise the speed of wetting without entraining the displaced phase.

The study of polymer—water interfaces by contact angle methods can be accomplished directly at the polymer—water interface. Using two water-immiscible liquids or a liquid and a vapor, one can deduce the dispersion and polar components of the hydrated solid surface free energy and the solid—water interfacial free energy. The theory is presented and a numerical analysis procedure is developed to solve the equations in the general case. The special case of n-octane and air is also presented. Data and results are given for poly(hydroxyethyl methacrylate-methoxyethyl methacrylate) copolymers of varying composition and equilibrium water contents. The results show that the hydrophilic component dominates the polymer—water interfacial properties, even at relatively low hydrophilic component compositions. The method presented should be useful for the study of polymer—water interfaces, particularly for hydratable or mobile polymers which can reorient to equilibrate differently with a water environment than with the air or vapor environment commonly used in contact angle studies.

A reexamination of previous studies concerning the electrical (‘corona’) discharge treatment of polyethylene and the resulting enhancement of autoadhesion has been carried out. X-ray photoelectron spectroscopic data provide new insight into the phenomenon by showing surface oxidation to result from treatment in ‘inert’ gases. Treatment in hydrogen is an exception and results in no autoadhesion enhancement even though energy input into the film is more efficient than in air. Autoadhesion theories based on electret formation are rejected; those based on hydrogen bonding are largely up-held and shown to be more generally applicable than at first imagined.

1. It is to be expected that the hysteresis of contact angles on polystyrene would be the effect of a number of simultaneous causes. Whatever the interpretation, the differences in the observed angles found with surfaces that had been formed in different ways point to structural differences among the samples; and the existence of structural differences would indicate that all the surfaces depart appreciably from ideality, e.g., from ideal flatness, rigidity, and homogeneity.

2. We have shown that effects due to film pressure, πe, would be in the opposite direction from those observed in hysteresis. Hence film pressure is not responsible for contact angle hysteresis.

A remarkable agreement between Weber's linear analysis and experiment makes it possible to determine the dynamic surface tension of viscous liquids from the growth rate of axisymmetric disturbances on excited capillary jets. The method is very accurate and can be used to determine the surface tension at as short as 10⁻⁴ sec surface age. Aqueous glycerol solution and inks developed for inkjet printing were used as test liquids in the experiments. While a dye base ink showed time-dependent surface tension, the surface tension of inks which were colloid suspensions of small pigment particles and contained surfactant micelles equalled their equilibrium value at 10⁻⁴ sec surface age. In the tentative explanation of this phenomenon, the dynamic equilibrium between surfactant molecules in solution and in micelles was substituted for long-range surfactant transport by diffusion. A result of this assumption is that surface tension in nonequilibrium states depends only on the composition of the surface layer.

Corona treatment of films, mainly polyethylene, was studied at commercial levels in a small continuous treater. Degree of treatment was characterized by measuring polar and dispersion components of surface energy, ASTM Wipe and ASTM Adhesion Ratio (“peel adhesion”). The chief factors studied were corona current, applied frequency, web speed, dielectric thickness and air-gap thickness between electrode and film. Other factors less intensively investigated were type of film, film additives, aging time after treatment, humidity and corona atmosphere. The polar component of surface energy, γ, is the key to understanding the changes in adhesive behavior of the films during treatment. We found that, for the equipment used, γ is accurately given by the equation