Ellipsometrically determined adsorption isotherms are reported for water on two types of pyrolytic carbon, on polyethylene, and on stearic acid-coated copper, for relative pressures up to close to the saturation pressure, P⁰, and for various temperatures. Contact angle data for bulk water on the same solids are included; advancing angles of 60°–90° were found. The adsorbed film thickness reaches 40–80 Å in the first two systems, but only a few angstroms in the second two; correspondingly, the surface pressures of P⁰, π⁰, are large in the first two cases and small in the second two. Large contact angle thus does not necessarily imply low π⁰. The data are fitted to a previously published potential-distortion model, which allows adsorption and contact angle behavior to be related.

The wettability of stereospecific poly(methacrylates) was studied. In the wettability of poly(methacrylates) having bulky substituents such as phenyl and chloroethyl groups, it was found that the critical surface tensions for isotactic polymers were low compared to those for attactic polymers. The steric effect of the bulky substituent on wetting was also discussed.

The wettability of a number of low energy solid surfaces, including hoof keratin and human skin, has been examined using two liquids, water and methylene iodide, and employing Wu's empirical approach to obtain γ_s^d and γ_s^P, the dispersion and polar components of the solid “surface tension.” The sum of these parameters, (γ_s^d + γ_s^p) was found to be in good agreement with reported values of γ_c, the critical surface tension, based on Zisman plots. Using the latter method, γ_c values of solids selected from the above group were determined using aqueous ethanol solutions. The values were lower than those obtained using nonpolar liquids, thus confirming earlier findings. A compilation of our own data and data from the literature reveals that the derived values of γ_c show little or no dependence on the type of solid surface, the type of alcohol or its chain length. The results can be explained in terms of adsorption of alcohol at the surface of the solid.

The dispersion force contributions to the surface free energies of octane and water are equal—21.8 dyn/cm. Octane's surface free energy has no polar component, whereas water has a polar contribution of 50.2 dyn/cm. Therefore, the increase in the contact angle of octane on various polar polymer surfaces underwater is a quantitative measure of the interfacial stabilization energy from polar forces. Octane contact angles were measured underwater on polyethylene, polytetrafluoroethylene, and polyethyleneglycolterephthalate surfaces before and after surface oxidation in a low temperature asher. The octane contact angles increased in each case as the surfaces became oxidized. When simple lap joints were prepared from these polymers and then broken in an Instron Tester, the measured breaking forces correlated well with the octane contact angles. Breaking strength increases of 1.1, 1.2, and 1.8 psi were realized with the polyethylene, polytetrafluoroethylene, and polyethyleneglycolterephthalate, respectively, when the polar forces were increased by 1 erg/cm².

Amorphous and graphitic carbon fibers and film surfaces are characterized by wettability measurements and surface energy analysis which isolate the (London-d) dispersion γ^d _svand (Keesom-p) polar γ^p _sv contribution to solid-vapor surface tension γ_sv =γ^d _sv + γ^p _sv Graphitized carbon fibers which are surface treated to provide strong bonding to polar matrix resins show consistent strong polar contributions to total surface tension with γ^d _sv|γ_sv ≃ γ^p _sv|γ_sv ≃ 0.50. Amorphous carbon films prepared for biological implant applications display dominant dispersion character in surface energy with γ^d _sv|γ_sv ≃ 0-74 to 0.95 and γ^p _sv|γ_sv ≃ 0.05 to 0.24.

It is shown that the best fit of experimental data to the correlation equation as used by Fowkes¹ cannot be considered as a criterion of correctness with which the mathematical formula expresses the way of polar interaction at interfaces. Examples of other evidence are given that the polar part of work of adhesion may be well represented by the geometrical mean of polar components of surface energies.

Wettability measurements and surface energy analysis are applied to isolate the (London-d) and (Keesom-p) polar contributions to solid-vapor surface tension γ_sv=γ^d _sv + γ^p _sv of surface treated graphite fibers. Surface treatments include metal coatings with Al, Cu, and Ni, chemically reducing heat treatments in H₂ and vacuum, and films of highly chlorinated polymers such as polyhexachlorobutadiene and polychloral. This study shows that the highly polar surface properties γ^p _sv|γ_sv ≃ γ^d _sv|γ_sv ≃ 0.50 of commercial graphite fibers can be modified by surface treatment to display dominant dispersion character with γ^d _sv|γ_sv ≃ 0.79 to 0.92 without substantial reduction in total surface energy γ_sv. For adsorption bonded fiber/matrix interfaces a new method of mapping the surface energy effects of an immersion phase upon the Griffith fracture energy γ_G is applied to define criteria for strong interfacial bonding under both air and water immersion.

Interfacial and surface tensions of polymers are important in the technology of plastics, coatings, textiles, films, and adhesives through their roles in the processes of wetting, adsorption, and adhesion. Because of experimental difficulty due to high viscosity, however, reliable measurements of these quantities were not reported until 1965 for surface tensions [l, 2] and 1969 for interfacial tensions [3, 4]. A body of scattered data has been accumulated in the literature. This review will evaluate, compile, and interpret these results.

Low-temperature radiofrequency excited gas plasma was applied to the surfaces of a number of polymers. Polymers that are known to crosslink as well as those that only degrade under irradiation were included in the investigation. Surface changes were studied by viscosity, gel content, and contact-angle measurements. Changes in adhesive bond strength were used as a measure of overall practical effects of plasma treatment. In each case the response of the polymer surface to an oxidizing (oxygen) and a nonoxidizing (helium) plasma environment is discussed. Further indications of the nature of the surface changes were suggested by statistical treatment of the bond-strength data.

Exposure of polyethylene film to UV radiation at wavelengths of ≤2537 Å is sufficient to induce surface crosslinking and to facilitate the formation of strong adhesive joints to these surfaces with conventional adhesives. Reduction of the vapor pressure in the reaction vessel to about 1 torr apparently maximizes the efficiency of the crosslinking process. Examination of the treated films which have been exposed for times necessary to form strong adhesive joints has revealed an absence of surface oxidation. It appears that crosslinking to improve the mechanical strength of the surface region of the polyethylene is sufficient to allow the formation of strong adhesive joints.

An equation of state is developed which allows the surface tension of a low-energy solid to be determined from a single contact angle formed by a liquid which is chemically inert with respect to the solid and whose liquid surface tension is known. The equation of state is obtained using two independent methods. In the first one, similar arguments to those in previous papers are used; however, the qualitative argument, based on the general appearance of plots, is replaced by computer curve fitting and statistical analysis. The second method, which has not been employed heretofore, treats the solid surface tension as an adjustable parameter. Molecular arguments in conjunction with the interaction parameter Φ are used to eliminate poor choices of the solid surface tension. The results are in excellent agreement with the first method.

The range of validity of the equation of state and practical points in its application are discussed.

By combining four techniques—X-ray photoelectron spectroscopy (ESCA), soft X-ray spectroscopy, contact-angle hysteresis, and electron microscopy—a powerful method to elucidate the nature of solid surfaces is created. ESCA provides semiquantitative elemental analysis of the uppermost 5–100 Å of the sample. Soft X-ray spectroscopy extends the elemental analysis to a depth of about a micron. Contact-angle measurements can be interpreted in terms of the distribution of surface energy and roughness, and a view of the microtopography is obtained with the electron microscope. This method of surface characterization has been applied to several problems in fluoropolymer surface chemistry. For example, certain sodium complex solutions are shown to react with fluoropolymer surfaces, removing most of the fluorine and leaving a sponge-like surface with characteristics of an unsaturated, oxidized hydrocarbon. Also analyzed are surface changes that occur upon exposure of these sodium-etched films to environmental conditions. In another application, films of poly(tetrafluorethylene/hexafluoropropylene) melted and recrystallized against a gold substrate were analyzed. The unusual wettability of such films has been attributed to the presence of a “transcrystalline” surface region, but our analysis indicates the presence at the surface of a very thin layer of materials with the characteristics of an oxidized hydrocarbon. The increased wettability is evidently due to the presence of this layer.

Chromic acid solutions were used to oxidatively etch linear (high density) and branched (low density) polyethylene and isotactic polypropylene. Etched surfaces were characterized by surface IR spectroscopy, wettability, electron microscopy and aqueous adhesive bonding (peel test). Polypropylene was found to etch rapidly, but showed little residual chemical or topographical change. The polyethylenes etched more slowly, especially in the case of the linear polymer, but showed large changes in surface chemistry as a result of oxidative attack. Adhesion onto polyolefin films and fabric increased very rapidly during the first few seconds of attack. This increase is interpreted in terms of wettability, topography and cohesive strength of the surface layer. The differences in etch behavior between the polyolefins is interpreted in terms of ease of oxidative attack at branch points, and surface accessibility to the acid.

Mild plasma cleaning of metal surfaces was shown to be effective in removing organic contaminants. Auger electron spectroscopy and surface wettability measurements were used to evaluate the plasma cleaning procedure and to provide a comparison with conventional solvent cleaning methods.

The relationship between wetting and pressure-sensitive adhesion was studied using an adhesive composed of poly(butyl acrylate) and various adherends of different surface tension. The amount of adhesive deposit was determined quantitatively by tracer technique although the unbonding process was apparently observed as interface failure. The adhesive force and amount of deposit were both dependent on the critical surface tension of the adherends. Maximum tack value and contamination were observed with adherends whose critical surface tension was close to that but a little higher than that of the adhesive. The adhesive force obtained was lower than cohesive strength of adhesive. From this evidence, a mechanism for pressure-sensitive adhesion was discussed: the bond breaks in the addesive mass around the very minute spots where interaction is at work between adhesive and adherend. Inasmuch as the density of the minute spots per unit area depends on the surface tension, the adhesive force also depends on the surface tension.

Three methods of interpretation of contact angles in terms of surface tension of liquid and solid are correlated. It is shown that the critical surface tension for wetting γ_c is dependant in a specific way on the nature of the liquid system used regardless of the nature of the solid. Equations are derived which express the limiting values γ_c may assume for a liquid/solid system. γ_c is also correlated with the Good-Girifalco treatment of contact angles and experimentally determined values of the parameter φ are compared with theoretical calculations from molar volumes of normal alcohol-paraffin wax interfaces.

The Fowkes method of contact angle interpretation is used to derive values for the polar and dispersion force components of liquid surface tensions for three alcohol/water solution series and two organic acid/water solutions.

The wettability of oxidized polyethylene films was studied with pure liquids (water and methylene iodide) and practically nonpolar mixtures of Decalin and methylene iodide. A linear variation was found of the wettability of these films with the chemical composition of their surfaces (determined by adsorption of radioactive ⁴⁵Ca ions).

A value of γ_s^d for the polyethylene was found with the nonpolar mixtures of methylene iodide and Decalin and the values of the solid-liquid polar interactions (I_sl^P) for oxidized polyethylene were deduced.

Dipole-dipole and induced dipole-dipole interactions between the pure liquids and the oxidized and unoxidized polyethylene were calculated for two possible orientations of the hydrocarbon chains to the surface and compared with the experimental results. Generally a poor agreement was obtained, mainly due to the difficulty in estimating the correct values for the distances between molecules or groups. However, a better agreement was obtained assuming that the chains were perpendicular to the surface.

Experiment has shown that the nitrogen corona-induced autohesion of polyethylene and the nitrogen-corona induced sorption of iodine by polyethylene both follow similar mechanisms. The controlling factor is postulated to be the formation of short-lived electrets within the polymer surface.

The resultant surface activation of polymers by corona discharges has been found to be markedly influenced by the type and purity of gases used in the corona. In this work it is shown that for the nitrogen gas corona treatment (15 KV, 15 mins) of polyethylene and polypropylene, traces of oxygen, >0.5% and <0.15% respectively, are sufficient to produce chemical changes in the polymer surface.

The effect of a variety of wetting liquids on the resistance to peeling separation for a lightly crosslinked rubbery adhesive in contact with a Mylar substrate has been studied over a wide range of peeling rates and at two temperatures. Although the magnitude of the peel strength is much greater than the thermodynamic work of detachment, it is reduced by alcohols and alcohol/water mixtures in good agreement with calculated reduction factors. It is concluded that the measured strength is a product of two terms: the thermodynamic work, and a numerical factor, generally large, denoting inefficiency. The latter term is strongly dependent on peel rate and temperature for viscoelastic adhesives. Two anomalies are pointed out: particularly low adhesion is observed at low rates of peel for certain liquids, attributed to swelling of the adhesive, and smaller effects are found for some other liquids than predicted.

Equations for polar and nonpolar interactions across the interface are developed by using energy additivity concept in a semi-continuum model. Interfacial and surface tensions of molten polymers are measured directly and used to test the resulting equations: The first expression may be called the harmonic-mean equation preferred for low energy systems such as organic liquids, water, polymers, and organic pigments. The second may be called the geometric-harmonic-mean equation preferred for high energy systems such as mercury, glass, metal oxides and graphite. The third may be called the geometric mean equation which is found unsatisfactory. The harmonic-mean equation is used to obtain the “optimum” wettability condition for adhesion. The importance of polar interactions and matching of the polarity are analyzed and emphasized.

The mechanics of adhesion have been investigated both theoretically and experimentally, using model adhesive joints consisting of a crosslinked amorphous rubber bonded to a variety of rigid polymeric substrates.

An adhesive failure energy, θ, is defined which is characteristic of the bond but independent of test-piece geometry. Both theory and experiment show that θ has the form, θ=θ₀f(R) where θ₀ is the “intrinsic adhesive failure energy” which depends only on the physical and chemical nature of the adhesive-substrate interface, and f is a function of R, the “reduced” rate of failure propagation obtained from rate and temperature data using the WLF equation.

θ₀ is the work of bond fracture across the interface and, for clean interfacial failure, is equal to the thermodynamic work of adhesion w_A. Where failure is not purely interfacial, θ₀ can be expressed as θ₀=iI+r𝒯₀+sF where i, r, and s are respectively the area fractions of interfacial, cohesive-in-rubber and cohesive-in-substrate failure, and I, θ₀, and F are the intrinsic failure energies for the interface, rubber, and substrate, respectively.

It is believed that this work is the first to demonstrate explicitly and quantitatively the separate contributions of interfacial properties and bulk rheological behavior to the strength of adhesive joints.

A technique is described for performing temperature scanning measurements of the surface tension of polymer solutions. Measurements on solutions of a high molecular weight and a low molecular weight monodisperse polystyrene in tetralin, decalin, and n-hexadecane are reported. Whereas previous investigations of other physical properties of polystyrene solutions had revealed only one anomaly, at about 50°C in some cases and at about 70–80°C in others, the curves presented here show two anomalies, near 45°C and 70°C, respectively. These anomalies are tentatively attributed to conformational changes of the polymer chains.

The strength of joints between Teflon FEP (Type A) and 500- to 1000-Å gold layers deposited by evaporation can be greatly increased if the Teflon surface is subjected to electron-beam bombardment prior to the evaporation process. Typically, joint strengths of about 60 kg/cm², approaching the bulk strength of Teflon, are obtained for treatments with electron-beam energies in the range of 5 to 20 keV and intercepted charge densities of about 5 X 10⁻⁶ C/cm². This compares with gold–Teflon joint strengths of about 10 kg/cm² for untreated material. The increase in joint strength is believed to be primarily due to crosslinking caused by the electron bombardment. Compared to the other known treatments to improve gold–Teflon joints, the present method has the advantage that the charge-storage properties of the Teflon are not irreversibly degraded. It is possible, for example, to store charge densities up to 3 X 10⁻⁸ C/cm², on 25-μm films treated with this method, with the same favorable charge-retention properties and thermally stimulated current characteristics as obtained for untreated Teflon.

Equations for polar and nonpolar interactions across the interface are developed by using energy additivity concept in a semi-continuum model. Interfacial and surface tensions of molten polymers are measured directly and used to test the resulting equations:

The first expression may be called the harmonic-mean equation preferred for low energy systems such as organic liquids, water, polymers, and organic pigments. The second may be called the geometric-harmonic-mean equation preferred for high energy systems such as mercury, glass, metal oxides and graphite. The third may be called the geometric mean equation which is found unsatisfactory. The harmonic-mean equation is used to obtain the “optimum” wettability condition for adhesion. The importance of polar interactions and matching of the polarity are analyzed and emphasized.

Three methods of manipulating wetting data appear satisfactory in providing estimates of the components of solid surface free energy due to dispersion, Keesom, and hydrogen-bonding forces. These include: an extension of the Hansen method with contact angle data, which had been applied to solubility parameters, to surface free energy calculations; Hansen plots of absorption volume data; and Zisman plots using the components of surface tension rather than total surface tension. The Fowkes equation for the dispersion component of surface free energy agrees well with the results from the empirical methods. Extensions and modifications of the Fowkes equation to provide the polar components of solid surface free energy have not worked well when evaluated with a wide range of reference liquids.

A detailed analysis is reported of the chemical and physical modifications which occur to PTFE surfaces exposed to glow discharges in ammonia gas and in air. The analytical methods used were infra-red attenuated total reflectance and differential attenuated total reflectance spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements and scanning electron microscopy. The suitability for bonding with adhesives and the stability of the modified surfaces to attack by oxidizing acids are also reported.

Contact angles of polar liquids on various nonpolar solids immersed in a nonpolar liquid have been measured. They agreed satisfactorily with values calculated by combining Young's equation either with measured contact angle data of the liquids on the substrates in air, or with the Fowkes relation valid at each of the interfaces. The latter method also applies when one of the liquids spreads on the substrate in air. Polyethene, polystyrene, polytetrafluorethene and glyceryltristearate were used as nonpolar solids; paraffin oil and heptane as nonpolar liquids; water and glycerol as polar liquids.

The finding that the dispersion force contributions to the surface free energies of octane and water are equal enabled a simple method to be developed to characterize the hydrophilic nature of solid surfaces. This technique involves measuring octane contact angles on solid surfaces under water. Nonhydrophilic solids unable to interact by polar forces exhibit a predicted 50° contact angle, whereas those able to interact by polar forces give values greater than 50°. The greater the contact angle, the stronger are the polar interactions. The deviation of the contact angle from 50° can be used to evaluate, Isw, defined as the interfacial stabilization energy from the nondispersion (polar) forces.

The bondability of the following polymers as a function of length of exposure to excited helium or oxygen was investigated: low-density polyethylene, high-density polyethylene (two types), poly(4-methyl-1-pentene), poly(vinyl fluoride), poly(vinylidene fluoride), FEP Teflon, poly(oxymethylene) copolymer, nylon 6, nylon 66, poly(ethylene terephthalate), and polystyrene. Generally, the bond strength increase rapidly initially and then remains nearly constant, perhaps decreasing in some cases at long exposure times. A method is presented for calculating bond strength-versus-exposure time curves. The calculated curves generally fit the data reasonably well. Polypropylene showed a rapid increase in bondability with exposure to excited oxygen. Helium was ineffective toward this polymer under normal conditions, but could produce good bond strength at higher temperatures.

An RTV silicone and high density polyethylene are exposed in an activated gas plasma for varying times and varying conditions. Both oxygen and argon are used. Changes in critical surface tension of wetting as determined by contact angle measurements are reported. Bondability of the treated surfaces is evaluated with both the aged bonds and aged surfaces prior to bonding being evaluated. In contradiction to some of the recent work reported in the literature on the effect of activated inert gas on surface characteristics, contact angles always decreased on the materials studied indicating an increase in surface energy. The significance of the results on present adhesion theories is discussed.