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.

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 critical surface tension γ_c of polyvinyl butyral has been measured with polyhydric alcohols and halogenated hydrocarbons. Despite variations in polymer composition (residual OH content) and modes of preparation, γ_c is found to be 24–25 dynes/cm. with the former class of liquids. The —CH₃ groups appears to predominate over —CH₂, ether oxygen, and OH groups present. Steric effect may account for this biasing of the γ_c values toward the lowest surface energy group present. Fowkes' relation based on dispersion force interactions only is found to fit the data reasonably well. Comparative data on polyethylene are also presented.

The critical surface tension γ_c of polyvinyl butyral has been measured with polyhydric alcohols and halogenated hydrocarbons. Despite variations in polymer composition (residual OH content) and modes of preparation, γ_c is found to be 24–25 dynes/cm. with the former class of liquids. The —CH₃ groups appears to predominate over —CH₂, ether oxygen, and OH groups present. Steric effect may account for this biasing of the γ_c values toward the lowest surface energy group present. Fowkes' relation based on dispersion force interactions only is found to fit the data reasonably well. Comparative data on polyethylene are also presented.

Copper films evaporated on argon-oxygen plasma-treated poly(phenylene-vinylene) films have been studied by scratch test, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The adhesion of the metallic film to the polymer substrate was greatly enhanced after treatment and found to increase with the treatment time. SEM observation of the treated samples revealed that the morphology of the polymer surface was gradually changed with the treatment time as compared with that of the bare polymer film. On the other hand, XPS analysis of the polymer-metal interface showed that the bonding between carbon, oxygen and copper were subsequently modified as compared with those obtained in untreated samples. The high adhesion strength observed on these substrates was related to the modification in the surface morphology on the one hand and to the formation of new compounds at the polymer-metal interface on the other. The nature of the interfacial layer and its influence on the adhesion of the copper layer was discussed by comparing the results with those obtained in poly(phenylene-vinylene) (PPV)-Al systems.

The exposure of a polymer to ozone in the presence of ultraviolet light (UV/ozone) is a simple and effective way to improve the wettability of the surface. Using atomic force microscopy (AFM) we were able to examine the changes in morphology and the increase in the adhesion force at the surface of biaxially oriented polypropylene (PP) films after treatment with UV/ozone. It is clearly shown by atomic force microscopy (AFM) that UV/ozone treatment modified the original, fine, fiber-like structure to one displaying the formation of mounds or droplets. These droplets are most likely comprised of short chains of oxidized polymer or low-molecular weight oxidized materials (LMWOM). The size of the mounds increased with increasing treatment time. More interestingly, lateral force imaging AFM were capable of distinguishing these mounds from the surrounding surface, indicating that the mounds were formed on aggregation of the loose LMWOM during the UV/ozone treatment, while the surrounding surface was covered by bound moderately oxidized materials. The adhesion force was estimated from measurements made on the amount of force required to retract the tip from the surface after the two had made contact. A clear increase in adhesion force was observed on the modified PP film surface, which indicates an increase in the surface energy. We have demonstrated that mechanical scratching can alter the surface morphology and increase the surface energy of a polymer on a micrometer scale. The mechanically-scratched areas are more susceptible to modification than the surrounding unscratched surface when exposed to UV/ozone.

The degree of roughness and the linear direction of the abrasion process operated over the adherend surface are two important design factors for the adhesive joint. Thus, in the first part of this study, the surface roughness was varied by means of different grades of abrasive paper and its effect on the joint strength was studied. An investigation involving changing the linear direction with respect to the loading direction was also carried out. These experiments were done to determine the effectiveness of the abrasion process for the pretreatment of the adherend. A significant increase in joint strength was found for the abrasion treatment. However, it was shown that different linear directions did not have any significant effect on the joint strength. In the second part of this study, thermodynamic analysis of the bonding of dissimilar polymeric materials using different adhesives in terms of their surface tension, critical surface tension, and joint strength was carried out. The aim of the study was to determine the thermodynamic criteria for maximum joint strength in bonding dissimilar materials. The results showed that the joint strength was dictated by the adherend with the lower critical surface tension. Maximum joint strength for bonding dissimilar materials is attained when the surface tension of the adhesive used is close to that of the adherend with the lower critical surface tension.

Injection-moulded plates of four commercial thermoplastic polyolefins (TPOs) were subjected to oxygen plasma treatment. The modified surfaces were analyzed by water contact angle measurements and X-ray photoelectron spectroscopy (XPS), and the adhesion properties of the plates were evaluated by a 90 peel test after being lacquered with a two-component polyurethane lacquer. The study included treatments in two different plasma reactors operating at different frequencies. The influence of certain processing parameters, such as discharge power, flow rate and gas pressure, was investigated, as was that of frequency (using the same reactor). While the results revealed that oxygen plasma treatment indeed led to improved wettability, the degree of surface modification was not highly affected by changes in the processing conditions. In contrast, there was a great effect on the lacquer adhesion, in particular by changes in discharge power and gas pressure. The results also showed that the TPOs were sensitive in different ways towards changes in the processing conditions. It was also found that, regardless of the absolute peel force, the failures occurred in the substrate at some distance below the oxidized layer. These observations were attributed to a VUV-induced formation of radicals which, in the case of polypropylenebased materials, predominantly lead to^-scissions. As secondary radicals have a higher tendency to form crosslinks that can compensate for chain scission reactions, the difference in the sensitivity of the TPOs was proposed to be related to the amount and distribution of ethylene in the materials.

The adhesion properties achieved after oxygen plasma treatments of ten polypropylene (PP) and thermoplastic polyolefin (TPO) materials of different compositions were studied. It is shown that the adhesion between a polyurethane (PUR) lacquer and plasma-treated materials was strongly influenced by the plasma treatment conditions and the chemical composition of the materials. Generally, a low power-to-gas pressure (P/G) ratio during the plasma treatment and a high ethylene content, preferably in the form of blocks, and/or the presence of double bonds in the matrix, are favourable for adhesion properties. Moreover, the TPOs were less sensitive towards the plasma treatment conditions than the corresponding PPs. The properties and the type of rubber may also be important for the adhesion properties. Furthermore, it was shown by X-ray photoelectron spectroscopy (X.p.s.) and Fourier transform infrared (FTi.r.) spectroscopy (using the attenuated total reflectance (ATR) technique) that all failures—even the apparently interfacial failures—were located in the substrate, below the oxidized surface layer, the only difference being the depth of failure. The fracture surfaces of samples showing low peel forces generally had a more PP-like composition than fracture surfaces that were clearly cohesive in the substrate. This observation offers evidence that the lacquer adhesion is determined by the extent to which chain scission reactions occur in the near-surface region of the substrate during the plasma treatment. © 1997 Elsevier Science Ltd.

Injection-moulded plates of four polypropylene-based copolymers with ethylene or an unconjugated diene as the comonomer were subjected to oxygen plasma treatments. The main objective was to investigate how the degree of wettability and the adhesion properties were influenced by the type and amount of comonomer and by selected plasma parameters. The change in wettability was monitored by static water contact angle measurements and the adhesion between plasma-treated polypropylene plates and a two-component polyurethane lacquer was evaluated by a 90° peel test. No significant difference in the degree of wettability depending on material composition or treatment conditions could be observed. However, the lacquer adhesion was shown to be a function of both material composition and discharge power, while the influence of gas pressure was less clear. For all procssing conditions used, the lacquer adhesion was distinctly improved as the diene content was increased. An increasing extent of crosslinking reactions combined with a reduction in the number of main chain scissions are proposed to account for the observed results.

Injection-moulded plates of ten polypropylene (PP) and thermoplastic polyolefin (TPO) materials with varying material composition (different type of rubber, varying degree of ethylene etc.) were characterized before and after oxygen plasma treatments. Untreated materials were studied by means of differential scanning calorimetry (d.s.c.), size exclusion chromatography (s.e.c.), Fourier-transform infrared spectroscopy (FTi.r.), attenuated total reflectance (ATR) and transmission measurements, and the effect of plasma treatment conditions was followed by X-ray photoelectron spectroscopy (X.p.s.) and contact angle measurements. S.e.c. analysis revealed only minor variations among the materials, while the d.s.c. and FTi.r. experiments confirmed that the differences were to be expected as a result of the variation in material composition. The FTi.r.-ATR results showed that all samples had a gradient in material composition. The materials were generally more rich in PP in the topmost ∼ 200 nm than in the first ∼800 nm, and a lesser extent of ethylene modification and/or rubber was observed in the topmost ∼ 200 nm. It was also shown that the degree of surface crystallinity was normally greater at ∼ 800 nm than at ∼ 200 nm, and that a higher mould temperature led to a higher degree of surface crystallinity. The water contact angles and the atomic composition showed that the materials were more oxidized after plasma treatment at high power-to-gas pressure (P/G) ratios than at low ratios. Moreover, the dependence on material composition was weak for samples that were plasma-treated at low P/G ratios whereas the materials that were least ethylene-modified were less oxidized than the others at high P/G ratios. © 1997 Elsevier Science Ltd.

The polymers PET, PA6, PVDF, HD-PE, and PP are activated by a commercially available plasma jet system at atmospheric pressure to improve adhesive bondability. The adhesion properties of the activated surfaces are evaluated by lap shear tests. The results are correlated with the surface properties that are investigated by XPS, AFM, and contact angle measurements. In addition the influence of operational parameters of the plasma treatment is studied. The activated samples exhibit a substantially increased bonding strength. The improvement can be related to an increase of oxygen concentration, and to changes of the topology of the substrate surface induced by the thermal component of the plasma. The most influential parameters in the plasma treatment are the distance between substrate and nozzle exit and the treatment time.

Polyolefin surfaces, namely isotactic poly(propylene) (iPP) and low-density polyethylene (LDPE), were modified by corona discharge plasma. The chemical changes on the modified surfaces were observed, deeply affecting the surface and the adhesive properties of the studied materials. The hydrophobic recovery in the case of iPP is considerably dependent on the polymer crystallinity. The presence of the processing agents in the LDPE has a significant influence on the surface hydrophobization dynamics.

The surface of isotactic poly(propylene) foils was oxidized by corona discharge plasma in order to improve the adhesive characteristics. The dependence of the degree of surface oxidation on either the current density or the time of exposure was determined. Rapid increase of the free surface energy was observed at current densities ranging from 0.4 to 0.6 mA. A reduction of the exposure time of discharge at the foil surface has an effect similar to the reduction of current density. The change of free surface energy of extruded poly(propylene) was rapid, especially during the first 24 h, while for modified biaxially oriented poly(propylene) the decrease of free surface energy was substantially slower.

Surface modification of iPP in vapors of phosphoryl chloride under UV irradiation is an effective method for the increase of adhesive properties. Phosphoryl chloride acts as an sensitizer that decomposes under the effect of UV irradiation.

Hydrophilicity of polyethylene modified by electric discharge in the course of aging was investigated. The experiments were carried out with the foils of low-density polyethylene (PE) containing additives as well as with additive-free foils. Antiblocking or sliding agents, antioxidants and antistatic agents were used as additives. The results showed that the absence of additives in PE was responsible for the higher degree of modification of PE foils by corona discharge when compared with the polymer containing additives. The value of surface free energy (SFE) found after 30 days of aging of the modified PE foils was lower than the value recommended for inking the printing foils.

The adhesive properties of isotactic polypropylene with different degrees of crystallinity, surface-modified by corona discharge plasma have been studied, during the process of ageing. Considerable decrease in the surface free energy and its polar component was observed. A significant correlation was found between the mechanical work of adhesion to polyvinyl acetate and the polar fraction during ageing. The influence of the crystallinity of the polymer on the resulting adhesion parameters of aged polypropylene foil was confirmed. © 2001 Society of Chemical Industry

The change in adhesive properties of discharge plasma-modified biaxially oriented polypropylene was not very strong. Very low changes were detected in free surface energy (FSE) values; more intensive was the decrease in the polar contribution to FSE, polar fraction and the mechanical work of adhesion to polyvinyl acetate. After 30 days of aging of the biaxially oriented polypropylene foils, the measured value of FSE was suitable for printing. It has been determined that the correlation between mechanical adhesion work to polyvinyl acetate and polar fraction of polypropylene modified by discharge plasma is linear.

The adhesion of polypropylene and printing with various dyestuffs represents a serious problem which cannot be solved in satisfactory manner without modification. Because of practical usability, simple manipulation, suitability to continuous modification processes and efficiency the modification by plasma produced by electric discharge at atmospheric pressure in the medium of air oxygen was used. The free surface energy value of discharge-plasma pretreated biaxially oriented polypropylene in the course of short-time aging was determined. The free surface energy of modified polypropylene two weeks after modification exceeds the empirically established value 38 mJ.m^-2, that is regarded as a condition of acceptable surface modification of discharge plasma modified polypropylene foils.

This study concerns the surface and adhesive properties of isotactic polypropylene (iPP) modified by an electric discharge plasma and affected by long-term hydrophobic recovery of the polymer surface after modification. The investigations were focused on the change in polarity of the modified polymer expressed by the polar fraction as well as on the decrease in the surface free energy, its polar component and mechanical work of adhesion (A _m) to polyvinyl acetate. A _m of modified iPP to polyvinyl acetate as a function of polar fraction can be described by a mathematical formula. It has been confirmed that the most intensive decrease in the surface and adhesive properties investigated is produced by the long-term hydrophobic recovery of the polymer appears in the course of the first 30 days after its modification. During subsequent aging the process of polymer hydrophobic recovery proceeds more slowly. It has been found that the value of surface and adhesive properties of iPP as well as the dynamics of the decrease in these properties during hydrophobic recovery of the surface after modification is, in the main, dependent on the iPP crystallinity.