A new method to detect surface oxidation of an otherwise untreated, cross-linked and filled silicone rubber is described. Our method is established on the principle that surface wettability increases during the progress of oxidation. Surface wettability is determined in terms of critical surface tension.

Abhesive polymers, of which silicone rubber is a typical example, are characterized by low surface energy, low friction coefficient and low release value. The problem associated with silicone rubber is its poor adhesion to other polymers. Its adhesional ability, however, can be improved by surface modification, e.g. oxidation, treatment with corona discharge, or ionic bombardment with inert gases.

By our method we found that the oxidation of silicone rubber is comparatively mild below 260°C, but is intensified at 287°C. Excessive oxidation at 316°C results in the formation of low molecular weight siloxanes which lower the wettability of the oxidized surface. Mechanisms of thermal oxidation are discussed.

We have explored the possibility of acquiring information on the molecular nature of some novel polymer films by following Zisman's approach of determining the critical surface tension γ_C which is considered to reflect the molecular composition of the solid surface. This led us to obtain equilibrium contact angle data, using both a series of pure liquids and various alcohol solutions, not only with the polymer films but also with solid surfaces of polystyrene and polymethylmethacrylate.

Lower values of γ_C were obtained with the solutions than with the pure liquids; these lower values are attributed to the preferential adsorption of the alcohol molecules at both the polymer-liquid and the polymer-vapour interfaces. The value of γ_C depends on the alcohol used, and is relatively independent of the solid : it is inferred that the alcohol is adsorbed with the hydroxyl group towards the polymer surface.

It is concluded that in certain cases, the value of γ_C obtained using solutions cannot be used as being characteristic of the solid (as has been suggested by Zisman), and that changes at the solid-vapour interface cannot be neglected when interpreting contact angle data. Several sets of data reported in the literature are discussed from this viewpoint. The Gibbs adsorption isotherms is applied to the contact angle data and the results add further weight to the conclusions regarding the occurrence of adsorption at both interfaces.

Plasma polymer interactions are actively being studied because of their unique ability to surface modify polymers without affecting their bulk properties. Plasma polymer interactions are characterized by those which result from the addition of molecules to the polymer surface and by those which result from polymer bond rearrangement. Surface crosslinking is a bond rearrangement reaction, and gives rise to the following surface property improvements: improved adhesion (l), improved absorption (2), and improved resistance from environmental attack (heat, ultraviolet radiation). Plasma induced surface crosslinking has been studied for a period in excess of ten years. Previous experiments have measured the existence of surface crosslinking as a function of the gas used for the plasma and the technique by which the plasma has been produced (3-6). These studies have completely neglected the plasma properties which were responsible for the surface crosslinking. This approach has failed to provide a comprehensive picture which explains the experimental data and has resulted in the requirement that the polymer be in physical contact with the plasma. The present experiment investigates the coupling mechanism which exists between the plasma and the polymer. The results of the experiment demonstrates that ultraviolet radiation (uv), produced by a hydrogen glow discharge, is sufficient to account for plasma induced surface crosslinking of high density commercial polyethylene. Physical contact between the plasma and the polymer is not required.

A theoretical treatment of the effect of surface heterogeneity on contact angles is given, by means of a model employing the capillary rise of a liquid in contact with a stripwise heterogeneous surface. Local contortions of the liquid-vapor surface are postulated to conform to an assumed periodic shape of the three-phase line. A minimum of free energy is found in a configuration in which Young's equation is obeyed locally. When the width of the strips is below some value of the order of 0.1 μ, the amplitude of the periodic contortion of the three-phase line is less than about 10 A, which is operationally indistinguishable from a straight line. Extension of this model is made to a patchwise heterogeneous surface, and a mechanism for hysteresis is developed. For patches smaller than about 0.1 μ, it is shown that heterogeneity should make a negligible contribution to hysteresis.

Progress of research on wetting, spreading, and liquid-to-solid adhesion is reviewed with emphasis on advances since 1963. Carefully controlled experiments with pure materials, clean solids, special atmospheres, and thermodynamically acceptable physical conditions have resulted in new knowledge of surface science, polymer properties, and the role of interfering properties and chemicals. New uses and relationships have already been produced in many fields of technology and science. Recent advances shed much light on molecular conformational differences, polymer tacticity, and hydrogen bonding. Several current research problems are identified.

According to Bikerman, who attributes failure in adhints to a weak boundary layer, it is almost impossible and meaningless to correlate adhesive strength to surface-chemical properties of adhints. Though his assertion seems to be confirmed by the recent studies of Schonhorn and his coworkers on the methods of CASING and TCR, not a few results have yet been accumulated, which show a close relation between them. In this paper surface-chemical criteria for the optimum adhesion are investigated and the minimum interfacial tension or the maximum wetting pressure is deduced from the published data and our own as a first approximation. It is emphasized that, when critical surface tension γ _c would be used as a measure of surface-chemical properties of solid, its variability according to liquid series (nonpolar, polar and hydrogen bonding liquids) should be carefully taken into consideration. The importance is shown for polyethylene and its fluorine substituted polymers, using newly measured contact angle data and Zisman's data. Results of Levine et al. and Schonhorn et al. on adhesive shear strength with epoxy adhesives are replotted against available values of γ _c obtained by the use of hydrogen bonding liquid (γ _c ^c ), which are thought to reflect wetting behaviors of epoxy adhesives quite well. Each curve shows a maximum around γ _c ^c = 40 dyne/cm with few points falling off the curves.

A known relationship between heat of vaporisation, surface tension and molar volume applicable to spherical non-polar molecules is modified to apply also to linear molecules; the treatment involves calculation of molar surface areas corresponding to the appropriate “fully-packing” molecular shapes.

A linear relationship between the ratio cohesive energy density/surface tension and the reciprocal of molar volume is predicted for members of homologous series and demonstrated, with data for the n-paraffins.

In forced spreading systems, three different modes of θ_d-V behavior have been found, each of which predominates in a different velocity range. In the lowest velocity range, with systems involving the low viscosity, low boiling, nonpolar liquid hexane, θ_d was found equal to θ_eq (the Elliott-Riddiford or Hansen-Miotto mode). In the next higher velocity range, which extends to very low velocities for all other systems studied, the behavior described by Eq. 6 (the Blake-Haynes mode) predominates. At still higher velocities, the behavior described by Eq. [9] (the Friz mode) becomes superposed on the Blake-Haynes mode, and eventually predominates up to the range where θ_d approaches 90° and Eq. [9] becomes inapplicable. In the Blake-Haynes mode the major force opposing advance of the liquid front is the solid-liquid interfacial viscosity. In the Friz mode it is the bulk viscosity of the liquid.

The roughness of solid surfaces has no appreciable effect on the θ_d-V relationship, provided the physicochemical character of the surfaces is the same and the roughness is random. If the process of roughening alters the physicochemical character the θ_d-V behavior of the roughened surface may differ from that of the smooth one.

There is no qualitative difference between the θ_d-V behavior of systems in which θ_eq is zero and systems in which θ_eq is positive.

The effect of velocity upon the advancing and receding contact angles of water, glycerol, formamide and methylene iodide on glass coated with a dimethyl siloxane layer has been investigated. The cosines of the dynamic contact angles of water and glycerol vary linearly with the interfacial velocity, as predicted by hydrodynamic theory. However, this treatment cannot account for the magnitudes of the velocity effects observed with water and glycerol. Qualitative molecular considerations explain certain features of the velocity dependence of advancing contact angles. Initial rapid relaxation of the contact angles occurs on removal of the drive because of molecular reorientation of liquid molecules at the solid interface. There are further slow changes of water and methylene iodide advancing contact angles with time because of penetration of liquid changing the solid-liquid interfacial tensions. No penetration occurs with glycerol and formamide because of their greater effective molar volumes and with the former liquid the advancing and receding angles relax to a common value of about 90° which is considered to be the equilibrium value.

An expression is derived, from simple statistical thermodynamical considerations, to relate the cohesive energy density (C.E.D.) to intermolecular interaction parameters. From analogous theoretical relations in the literature for the surface tension and the index of refraction together with the derived expression for the C.E.D., we can obtain relations between the C.E.D. on the one hand and the surface tension and the index of refraction, respectively, on the other. These relations are compared with empirical relations in the literature. The exponents of the surface tension and the index of refraction in the empirical relations are different from those in the relations obtained here. The derived relation between C.E.D. and surface tension is shown to be applicable to experimental data with excellent agreement for both organic liquids and polymers. As surface tension measurements are very simple to perform, another main point of interest of the derived relation is knowledge of the solubility parameters of polymers (the square root of the C.E.D.) from critical surface tension measurements without the necessity of solubility and swelling experiments which are often awkward. The relation between the C.E.D. and the index of refraction derived in this paper implies grave theoretical objections to applications in practice.

The surface free energy and polarity values of a number of organic pigments are obtained from contact angle measurements and the interfacial tension equation of Wu(1). The pigment types studied are phthalocyanine, quinacridone, toluidine red, isoindolinone, indanthrone, β-oxynaphthoic acid derivative, and thioindigoid red. The surface free energies obtained agree reasonably well with those predicted from parachor and density values.

By applying a simple device comprising a power supply for arc welding and a plasmajet torch, a new method for plastic surface treatment to improve adhesion of the plastic was developed. The method enables such surface treatment instantly in the air atmosphere by applying the plasmajet to test pieces and is effective to various k-inds of plastics, especially to crystalline plastics as polyethylene.

When several pieces of polyethylene were treated under the following conditions in our experiment, the contact angle of water on the surface was improved from 80" to 20" and the adhesive strength by the tensile test was also remarkably improved from a few kg/cm2 to 120kg/cm2.

The purpose of this research was to classify fluoropolymer surfaces with regard to their wettability by various liquids. Thin solid blades, coated with various fluoropolymers, were suspended from a force transducer balance. The blades penetrated various hydrocarbon/air and hydrocarbon/water interfaces, and the curves of force acting on the blade versus displacement were recorded. The contact angles hydrocarbon/solid/air and hydrocarbon/solid/water were calculated using: $\text{COS}\text{θ =}\text{t}\text{γL}$ and $\text{COS}{}_{\mathrm{i}}\text{θ =}\text{T}{}_{\mathrm{i}}\text{γi}$ where θ, θ_i: contact and interfacial contact angle; γL, γi: surface and interfacial tension; and τ, τ_i: adhesion and interfacial adhesion tension. Cos θ versus γ_L for various γ_L were plotted and γ_c (for cos θ = 1) was determined. It was found that there are two critical surface (γ_c) and critical interfacial (γ_cⁱ) tensions due to contact angle hysteresis. Three different fluoropolymer surfaces were investigated. When the surface can be used in the form of a well-defined blade and when enough liquid to be tested is available, the method was found to be useful, rapid, and reproducible. Under these circumstances it can be used in place of Zisman's well-known sessile drop method.

The surface tension and surface entropies of different molecular weight polyethylene glycols and polypropylene glycols have been measured. The surface entropy of a mixture of polyethylene glycol and polypropylene glycol and that of block copolymers have also been determined. In the case of homopolymers, there is no effect of molecular weight on surface free energy and the increase in free energy on passing from the interior to the surface is due mainly to the heat content with the entropic contribution being very small.

In the case of a mixture of homopolymers and block copolymers, a minimum is observed when surface entropy is plotted against composition. At any particular composition, the surface entropy of a mixture is higher than that of a block copolymer of the same composition.

By means of an equation containing two adjustable coefficients it is possible to relate the surface free energy to the energy of vaporization, using the Hansen parameters from London force energy, polar energy, and hydrogen-bonding energy. The technique is applicable to simple organic liquids, mixtures of simple liquids, and most liquid metals. Hydroxy compounds, acidic and basic organic liquids, certain hexagonal and irregular metals, and most fused halides require special versions of the basic equation.

In this investigation the fracture surface between bovine dentine and bovine enamel and a dental cement was observed using the scanning electron microscope at magnifications up to 10,000 ×. The results indicated that the topography of the adherend plays an important role in the formation of an adhesive bond and in the fracture pattern of an adhesive joint, even when cohesive failure is involved.

We propose an equation, based on “reciprocal” mean and force additivity, for calculating the interfacial tension between polymers or between a polymer and an ordinary liquid:

If a low-density polyethylene sheet is treated in a corona discharge and subsequently pressed to a similarly treated sheet at 45°C, the bond formed is much stronger than that between similarly pressed but untreated sheets. Several series of observations have indicated that this enhanced autohesion is not due to surface oxidation or to surface crosslinking (CASING). Evidence is presented that the effect may be related to some type of electret formation induced in the polymer sheet by the corona discharge.

Corona treatment of low-density polyethylene in oxygen or oxygen-containing gases produced bumps on the surface, while treatment in nitrogen, hydrogen, argon, or helium caused no detectable surface change. Bumps made by an oxygen corona increased in size with time and temperature of the treatment. The bumps were removed when a treated polymer sheet was dipped into solvents such as CCl₄, ethanol, or 0.2% aqueous NaOH. Infrared analysis indicated that most of the oxidized layer was eliminated from the polymer surface by solvent dipping and that the degraded products contained a substantial proportion of CH₂ groups. It is suggested that the bumps are caused by the migration of low molecular weight degradation products to charged areas of the polymer surface.

Average values for dispersion γ_s^d and polar γ_s^d contributions of the solid surface tension γ_s γ_s^d + γ_s^p for poly(methylene oxide) (PMO) and Na-treated polytetrafluoroethylene (PTFE) are determined by a new computational analysis of wettability data. PMO displays γ_s^d equals; 21.8 ± 0.9 and γ_s^p = 11.5 ± 1.5 dyn/cm while Na-treated PTFE displays γ_s^d = 36.1 ± 3.0 and γ_s^p = 14.5 ± 2.9 dyne/cm. These surfaces present the highest fractional surface polarity p_s = γ_s^p/γ_s = 0.29-0.35 yet encountered for organic polymers or oriented monolayers. These unusual surface tension properties are correlated with surface chemistry and adhesion phenomena.

It has been found that if a film of a polymer such as polypropylene or Teflon FEP is oriented by stretching, the contact angle of liquid becomes anisotropic, being higher for liquid front advancing or retreating perpendicular to the direction of stretch than for advance or retreat in the parallel direction. Electroscanning microscopy has revealed a small degree of anisotropy of the surface roughness. Comparison with samples that have been abraded with a single stroke of abrasive paper of various grit sizes showed that a far greater degree of anisotropic roughness would be required, to produce the observed contact angle anisotropy and hysteresis, than is actually observed on the stretched samples. It is concluded that the observed contact angle anisotropy is probably due to the anisotropic force field of the oriented polymer molecules.

Polypropylene–nylon 6 10 composites were prepared by the in situ polymerization of the nylon monomers on polypropylene films. The adhesion between the nylon and the polypropylene was markedly improved by a brief corona discharge treatment of the films in nitrogen prior to coating. This improvement was demonstrated by an increase in the peel strength of the nylon coating and a decrease in brittleness of photo-oxidized compesites when corona treatment was used. Adhesive bonding between the nylon and substrate was sufficiently strong to cause cohesive failure in the corona-treated polypropylene. Only interfacial failure was observed at untreated surfaces. These effects were demonstrated by electron microscopy of the surfaces produced in peel tests. The effects of corona treatment on adhesive bonding characteristics of surfaces are discussed in terms of the chemical and physical changes observed in treated surfaces.

The CASING (crosslinking by activated species of inert gases) treatment of polypropylene film in both oxygen and nitrous oxide is shown to be an effective surface treatment for conventional adhesive bonding. A crosslinked surface extending to a depth of about 300 Å, apparently independent of exposure time, is produced in both excited oxygen and nitrous oxide.

Corona treatment improved bonding between sheets of cellulose and synthetic polymers. The bond strength increased at higher temperatures of pressing. Physical changes in the surface were detected microscopically after corona treatment in air. Sheets treated in pure nitrogen made strong bonds although the surface treated in nitrogen was indistinguishable from the untreated surface.

A new definition for work of adhesion W_a is applied to computationally define the dispersion γ_s^d and polar γ_s^d components of the solid surface tension γ_s = γ_s^d + γ_s^d for twenty-five low energy substrates. These calculated surface properties are correlated with surface composition and structure. Surface dipole orientation and electron induction effects are respectively distinguished for chlorinated and partially fluorinated hydrocarbons. Published values for critical surface tension of wetting γ_c are correlated with both γ_s^d and γ_s.

The solubility parameters of a broad spectrum of solvents and chemicals are calculated from vapor pressure data using an expression derived from the relationship of Haggenmacher. In the case of high boiling liquids, the available vapor pressure data are found to be unreliable when extrapolated to room temperature and an alternate method of calculation is proposed. A structure correlation is made using the method of Small and new values of the molar cohesion constants are developed. The problem of associations of certain molecular species is discussed and the concept of chameleonic character introduced as a qualitative explanation.

The reaction of a radiofrequency-excited oxygen plasma with the surfaces of cured and uncured polymethylsiloxane produces intense hydroxylation of the surface region as followed by FMIR spectroscopy. Characteristic infrared features indicative of intraor intermolecular hydrogen bonding are evident. Plasma oxidation differs markedly from thermal oxidation processes. Reactions of polymethylsiloxane with nitrogen and air plasmas were also investigated and compared to corona reactions of oxygen, nitrogen, and air. In air corona, nitrogen moieties appear to be introduced. The behavior of polymethylsiloxane surfaces in oxidizing acids is also described.