This invention relates to the surface treatment of hollow plastic articles to provide thereon a surface receptive to inks, coatings, adhesives and the like and especially to the interior treatment of hollow containers to insure that interior coatings will adhere reliably and uniformly thereto.

Polymers offer a wide range of bulk physical, chemical and mechanical properties, are inexpensive and are relatively easy to synthesize. However, very often these polymer surfaces do not possess properties needed for application in certain external environments which could be one of the limiting factors in their usage. Polymer surface characterization and modification is of fundamental importance for the good functioning of a composite material and plays a key role in determining successful implementation of the polymer. Tailoring of polymer surface properties such as adhesion, wettability, surface roughness and chemical reliability (including heat-soak) is critical for several applications like decorative coatings, protective films, thin film technologies and biomaterials, to name a few. For these reasons, surface modification techniques which can transform these inexpensive materials into highly valuable finished products have become an important part of the polymer and nanocomposite industries. This chapter reviews the importance and applications of polymer surface modification using corona treatment to enhance adhesion by reporting most recent advancements in the field. Novel corona treatment (type of plasma, pressure, and power) and its potential in several applications is briefly discussed in latter part of this chapter.

Dynamic Contact Angle Technique offers a unique, non-optical alternative to solid surface energy analysis. The technique provides advancing and receding hysteresis profile scans of a surface recorded in real time as the liquid meniscus traverses the solid surface. Changes in the wetting hysteresis scan can be used to characterize the qualitative effects of surface roughness, surface homogeneity, and surface polarity, as well as measure the quantitative surface energy of the solid. Applications in flexography in which wettability plays a critical role are numerous, and the switch from solvent-based inks to water-based inks gives impetus for future study.

Atmospheric pressure plasmas are commonly used to improve the wetting and adhesion properties of polymers. In spite of their use, the mechanisms for achieving these properties are unclear. In this regard, we report on a computational investigation of the gas phase and surface kinetics during humid-air corona treatment of polypropylene (PP) and the resulting modification of its surface properties while varying energy deposition, relative humidity (RH), web speed, and gas temperature. Using results from a global plasma chemistry model validated against experiments, we found that increasing energy deposition increased the densities of alcohol, carbonyl, acid, and peroxy radicals on the PP surface. In doing so, significant amounts of gas phase O₃ and N_xO_y are produced. Increasing the RH increased the production of peroxy and acid groups, while decreasing those of alcohol and carbonyl groups. Production of O₃ decreased while that of HNO₃ increased. Increasing the temperature decreased the concentrations of alcohol, carbonyl, and acid groups on PP while those of the peroxy radicals increased. For a given energy deposition, higher web speeds resulted in decreased concentrations of alcohols, peroxy radicals, carbonyl, and acid groups on PP.

An atmospheric pressure plasma system has been used to treat amorphous polyethylene terephthalate (APET) to enhance its healseal properties to a polyethylene terephthalate (PET) film. The plasma treated APET sheet material was thermoformed into trays for use in the food packaging industry and heatsealed to a PET film. The heatsealing properties of the resulting package were assessed using the burst test technique. It was found that the plasma treatment significantly enhanced the adhesive properties and an increase in burst pressure from 18 to 35 kPa was observed for plasma treated food trays. The APET surface chemistry was assessed after plasma treatment where it was found that the plasma treatment had affected an increase in oxygen and an addition of nitrogen species to the polymer surface. The surface roughness (R_a) of the plasma treated samples was also observed to increase from 0.4 to 0.9 nm after plasma treatment.

The interfacial forces between a polyethylene particle and a toner substrate in alkaline aqueous solutions were studied using an atomic force microscope colloidal probe technique. Measurements were taken at pH 9 in water and solutions of 5 × 10^-4 M CaCl₂, 1 × 10^-4 M Na oleate, and 1 × 10^-4 M Na oleate plus 5 × 10^-4 M CaCl₂ in order to mimic the conditions present during de-inking flotation. A polyethylene particle was used to represent the air bubble. The observed interaction forces were described by the extended DLVO theory. An energetic barrier caused by electrical double-layer repulsion was observed in water and Na oleate solutions but was greatly diminished in CaCl₂ solution. A long-range attractive force was found to be present in these systems and was described using a simple exponential function. The long-range attractive force was virtually the same in water and CaCl₂ solution but decreased significantly in Na oleate solution because of the reduced hydrophobicity of the interacting surfaces caused by the adsorbed carboxylate layer. However, in the presence of oleate and calcium ions the observed attraction was even stronger and of longer range than in water and CaCl₂ solutions. Moreover, no energetic barrier was observed. These results can be attributed to the presence of precipitated calcium oleates on the interacting surfaces.

Sessile-drop and captive-bubble techniques were used for contact angle measurements. The advancing and receding contact angles were measured for water and ethylene glycol at self-assembled monolayer surfaces of dodecanethiol, for water at methylated quartz surfaces, and for water at roughened polyethylene and polytetrafluoroethylene surfaces. It was found that for each technique used, sessile-drop and captive-bubble, different advancing contact angles and different receding contact angles were frequently obtained for nonideal systems with rough and heterogeneous solid surfaces. The disagreement between contact angles, as measured with the two different techniques, increased with increasing imperfection of the solid surface. Also, it was observed that solid surface roughness and heterogeneity affected a variation of the advancing and receding contact angles with drop (bubble) size. No contact angle change with respect to drop (bubble) size (in the range 1–7 mm base diameter) was observed when smooth and homogeneous solid surfaces were well prepared. It is possible that metastable states, which are responsible for the contact angle hysteresis, also affect the contact angle/drop (bubble) size relationship. These three-phase systems with sessile drop and captive bubble at heterogeneous and/or rough solid surfaces are complex because solid surface heterogeneity and roughness cause contortions in the shape of the three-phase contact line and the drop (bubble) surface in the vicinity of the three-phase contact line. These contortions may affect a variation of the internal free energy of the liquid drop (gas bubble). It is shown that a slight variation in the advancing contact angle value over a few millimeters change in drop (bubble) diameter does not guarantee a high-quality surface state. Measurements of the receding contact angles provide more information on the quality of the solid surface and they should always be included with the measurements of advancing contact angles.

The current broad interest in wetting characterization of solid surfaces is driven by recent advances in the formulation of surfaces and coatings that are superhydrophobic, superhydrophilic, oleophobic, oleophilic and so on. Unfortunately, the contact angle data presented in many publications raise some concerns among the surface chemists and physicists who work with contact angle measurement techniques on a regular basis. In those articles, best practices are often ignored, and the data presented are limited to the static contact angles measured for small droplets, a few times smaller than typically recommended. The reported contact angles are neither advancing nor receding, and their reproducibility in different laboratories is therefore questionable. In this note, guidelines to measurements of reproducible and reliable advancing and receding contact angles are summarized.

Polyamide 12 (PA12) films have been modified by CF₄ and CF₄+H₂ (50/50 v/v) microwave plasma with different treatment times. The surface modifications have been followed versus plasma exposure duration by water contact angle measurements and atomic force microscopy (AFM) Pervaporation measurements were carried out to characterize the effects of these plasma treatments on water transport through PA12 films. From these measurements, water permeability was determined for each duration time of treatment. The efficiency of these plasma treatments in reduced permeability is compared. For both plasma treatments (CF₄ and CF₄+H₂), the analysis of the experimental data shows an increase and then a decrease of the permeability coefficient P with treatment durations. These observations are related with the evolution of the surface versus treatment time. From all these experimental results, it is clearly shown that the barrier effect to water in plasma-treated layers of PA12 is improved significantly, especially with CF₄.

The effect of oxygen and ammonia plasma treatments on changes of the surface properties of linear high-density polyethylene (HDPE) was studied. Surface energies of the polymer substrates were evaluated by contact angle measurements using Lifshitz-van der Waals acid-base approach. The surface energy of untreated HDPE is mainly contributed by Lifshitz-van der Waals interactions. After 5 min of plasma treatment, hydrogen bonds are formed on the surface, which is reflected in predominant acid-base interactions. The SEM results obtained demonstrate considerable changes of the surface roughness due to different types of the plasma gas used. Evolution of oxygen- or amino-containing moieties was detected by XPS and ATR FT IR. The prepared polyethylene surfaces were used as a basic support for further fabrication of novel hybrid biocomposite sandwich structures.

Surface free energies have been calculated for solid fluorocarbon materials by employing a method that utilizes dielectric data and theoretical predictions of van der Waals (dispersion) interactions. Excellent agreement between the results of direct force measurements and those of the theory for retarded van der Waals interactions supports the methodology. Two relatively new fluorocarbon polymers have been identified as having the lowest known surface free energies of all bulk homogeneous polymeric solids. This study provides confirmation that estimates of solid surface free energies based on contact angle measurements with dispersive organic liquids depend on the dielectric properties of both the liquids and the solid.

Parallel plates dielectric barrier discharge (DBD) at atmospheric pressure has been investigated to modify and functionalize the surface of different polymer substrates, e.g. polyolefins, poly(ethylene terephtalate), polyamide, in order to enhance their hydrophilic properties. Surface properties have been altered to meet the requirements of specific applications by introducing the appropriate functionalities through the use of either acetic acid or ethyl acetate. The coatings have been characterized through wettability measurements, labeling coupled with X-Ray photoelectron spectroscopy, and IR spectroscopy.

Recent progress in the correlation of contact angles with solid surface tensions are summarized. The measurements of meaningful contact angles in terms of surface energetics are also discussed. It is shown that the apparent controversy with respect to measurement and interpretation of contact angles are due to the fact that some (or all) of the assumptions made in all energetic approaches [7–14] are violated when contact angles are measured and processed. For a large number of polar and non-polar liquids on different solid surfaces, the values of γ_1v cos θ are shown to depend only on γ_1v and γ_sv when the appropriate experimental techniques and procedures are used. An equation which follows these experimental patterns and which allows the determination of solid surface tensions from contact angles is discussed.

This chapter discusses the physico-chemical hydrodynamics of rising bubble. At small Reynolds numbers effective approximate analytical methods allow to characterize different states of dynamic adsorption layers quantitatively: weak retardation of the motion of bubble surfaces, almost complete retardation of bubble surface motion, transient state at a bubble surface between an almost completely retarded and an almost completely free bubble surface. The measurement of bubble terminal velocity in water cannot be used for the experimental verification of these theories because uncontrolled impurities in water immobilize a small bubble surface almost completely without any addition of surfactant. The rising bubble velocity relaxation caused by the dynamic adsorption layer (DAL) formation can be measured. The DAL study is more realistic for large bubbles and large Reynolds numbers (Re) because trace concentrations of surface active impurities cannot retard the bubble surface movement completely.

The thin-layer wicking technique was used to determine the surface free energy components and the surface character of three celluloses (Sigmaccll 101, Sigmacell 20, and Avicel 101), using an appropriate form of the Washburn equation. For this purpose, the penetration rates of probe liquids into thin porous layers of the celluloses deposited onto horizontal glass plates were measured. It was found that the wicking was a reproducible process and that the thin-layer wicking technique could be used for the determination of the celluloses' surface free energy components. The size of the cellulose particles was characterized with the Galai CIS-100 system and their crystallinity was measured by X-ray diffraction. The three celluloses have high apolar (y^LWS = 50-56 mJ/m²) and electron donor (γs = 42-45 mJ/m²) components, while the electron acceptor component (γS⁺ ) is practically zero. The free energy interactions of cellulose/water/cellulose calculated from the components are positive, regardless of the cellulose crystallinity. This would mean that the cellulose surfaces have a hydrophilic character. However, the work of spreading of water has a small negative value (3-9 mJ/m²), indicating that the surfaces are slightly hydrophobic. It is believed that the work of spreading characterizes better the hydrophobicity of the surface than the free energy of particle/water/particle interaction, because in the latter case, no electrostatic repulsion is taken into account in the calculations.

The surface of oxygen-plasma-treated polystyrene (PSox) was investigated using X-ray photoelectron spectroscopy (XPS), streaming potential measurements and a dynamic study of the wetting properties at different pH (Wilhelmy plate method). The PSox surface is functionalized with various oxygen-containing groups, including carboxyl functions, and must be viewed as covered by a polyelectrolyte which swells depending on pH. The wetting hysteresis, its evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upon emersion; the retained water may evaporate progressively and allow macromolecule compaction and/or reorientation. Modification of the PSox surface upon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissolution of PSox macromolecular chains, as indicated by adsorption of released fragments on a check PS sample placed nearby. However, the concentration of functionalized molecules at the surface of water-aged PSox was still sufficient to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faster in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plasticizing effect of water. Hydrophobicity recovery upon aging in air was reversed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swelling.

Cleanliness can be characterized in industrial applications via "simple" wettability measurements, and has been successfully done so for at least two centuries. A problem in much of general manufacturing and maintenance industries is not that more sophisticated measurement and evaluation technology is necessary to provide value, but that technology developed at least several generations ago has not been more widely and profitably used. This paper describes and references that technology, and identifies published case histories where it has been both successfully and unsuccessfully used.

Static and dynamic contact angles of aqueous solutions of three surfactants--anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C(₁₂)E(₅))-were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid-vapor interface. Obviously, another process involving adsorption at the expanding solid-liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.

Detailed physical and chemical surface characterization of fractured adhesive joints, guided by qualitative fracture mechanics theory, constitutes a semiempirical method to elucidate adhesive bonding phenomena. Inherent flaws, interfacial separation, viscoelastic and plastic responses, and crazing and crack propagation are the main factors governing overall bond strength. Surface analyses (primarily by SEM/EDAX² and ESCA³) provide an estimate of the nature and extent of each mechanism. Results from various fluoropolymer joints are presented and rationalized in terms of the elastic modulus and fracture work in the failure zone. Bond strength on untreated fluoropolymers is negligible, but ESCA shows a small amount of fluorocarbon transfer to the adhesive. Surface treatments increase surface energy via a hydrocarbon layer ∼20 to >500 Å thick, and useful peel strength results. SEM shows fracture relatively deep in the fluoropolymer with pronounced microdeformation. When the surface treatment is depleted by heat or light, bond strength varies with surface composition. Also, copolymers with perfluoropropylvinyl ether side chains in place of perfluoromethyl groups are superior hot melt adhesives. The combination of SEM and ESCA shows cohesive failure in both instances, but the latter separates closer to the interface and with relatively little deformation.

Acid–base interactions across interfaces are shown to have predictable influences on adhesion. The history of this development, and methods to assay the acid–base character of solvents, polymers and a variety of powders and fibers are reviewed briefly. Recent studies are described that demonstrate directly how acid–base interactions influence both ‘fundamental’ and ‘practical’ adhesion.

The role of Lewis acid-base interactions at the fiber-matrix interface in composites is studied with both glass and Teflon fibers. In the glass fiber case, surface chemistry is modified with amino-, methacryloxy- and glycidoxy-silane coupling agents (A-1100, A-174 and A-187, respectively). Silane adsorption mechanisms as well as the properties of filament-wound, unidirectional epoxy and polyester composites are explained by a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and flow microcalorimetry. The heats of adsorption of pyridine and phenol prove that the coupling agents add acidic sites to the glass fiber surface as well as stronger basic sites. The subsequent adhesion of the matrix polymers and the short beam shear strengths of composites are explained on this basis. The Teflon fibers are first etched with sodium naphthalene solutions, and then sequentially hydroborated and acetylated, producing approximately monofunctional hydroxyl (acidic) and ester (basic) groups on the surfaces, as determined by XPS, FTIR, and electrophoretic mobility analyses. Composites prepared with the acetylated fibers and a chlorinated polyvinyl chloride (acidic) matrix are superior in tensile properties, and SEM fractography shows PTFE fibrillation, indicative of good fiber-matrix adhesion and stress transfer, in this case only.

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.

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.

With increasingly stringent EPA guidelines for controlling emissions of volatile organic compounds on the horizon, the desirability to move to water-based printing inks is evident This paper examines the effects of corona discharge treatments which are commonly used to improve ink adhesion to polvethylene. Electron spectroscopy for chemical analysis (ESCA) was used to determine the surface chemical changes induced by corona treatments in pure polyethylene extruded films and in formulated resin systems This data was correlated with surface tension and ink adhesion measurements to show the effects of treatment and additives on the final printability of the films with particular emphasis on water-based inks. In addition, the effects of stonng treated film prior to printing and of retreating these films were also examined The results of these tests have shown that formulated linear low density polyethylene (LLDPE) films treat and print at least as easily as high-pressure low-density polyethylene (HP-LDPE) counterparts.

With increasingly stringent EPA guidelines for controlling emissions of volatile organic compounds on the horizon, the desirability to move to water-based printing inks is evident This paper examines the effects of corona discharge treat ments which are commonly used to improve ink adhesion to polvethylene. Electron spectroscopy for chemical analysis (ESCA) was used to determine the surface chemi cal changes induced by corona treatments in pure polyethylene extruded films and in formulated resin systems This data was correlated with surface tension and ink adhesion measurements to show the effects of treatment and additives on the final printability of the films with particular emphasis on water-based inks. In addition, the effects of stonng treated film prior to printing and of retreating these films were also examined The results of these tests have shown that formulated linear low den sity polyethylene (LLDPE) films treat and print at least as easily as high-pressure low-density polyethylene (HP-LDPE) counterparts.