This chapter summarizes our present understanding of surface preparations for structural adhesive bonding of aluminum, titanium, and steel adherends. Both the initial bond strength and the subsequent bond durability depend critically on the interaction of the adhesive (and/or primer) with a pretreated adherend. There are two mechanisms of adhesion that are prominent in structural adhesive bonding: mechanical interlocking of the polymer adhesive with a microscopically rough adherend surface, and chemical bonding (with either covalent bonds or weaker van der Waals bonds) of adhesive molecules to the (intentional) adherend oxide. The magnitude and relative importance of both of these interactions depend greatly on the nature of the adherend surface prior to bonding and on the rheology and chemistry of the adhesive.

The surface characteristics of a two-part polyurethane adhesive formulation, based on controlled amounts of polyol, isocyanate, and catalyst, have been studied by methods including contact angle analysis, 1R spectroscopy, and inverse gas chromatography (IGC). The response of surface properties to various cure regimes and to exposure to water has been established. IGC analyses show that the adhesive surface is mildly basic, and as first evaluated by contact angle methods, has a surface energy close to 40 mJ/m². This is largely accounted for by dispersion forces. Following immersion in water at 60°C, however, the surface energies change, the most important effect being an increase in the non-dispersive component. FTIR spectra show that immersion in water also produces chemical changes in the surface region, likely related to enolization effects. On subsequent immersion of the adhesive surface in non-polar n-heptane, the non-dispersive component of the surface energy is again reduced, showing that surface restructuring of polyurethane chains contributes significantly to the observed surface dynamics. The magnitude of the restructuring effects was shown to vary with, but to persist for, all cure regimes applied to the formulation. The documented surface dynamics of the polymer are fully analogous to earlier results obtained for a series of two-part (soft-segment) polyurethanes. As expected, the surface dynamics in this family of polymers affect the bond strength of joints using the polyurethanes as adhesives.

The process of modifying the surface of polytetrafluoroethylene in a glow discharge plasma in vapors of organic compounds of various classes was investigated. It was established that the greatest increase of wettability is seen when modification is done in acrylic acid vapor. Multiple attenuated total internal reflection infrared spectroscopy was used to study the spectra of the coatings that formed and to demonstrate their difference in the case of acrylic acid.

A novel plasma chemical process PPCP (pulse corona induced plasma chemical process) can produce copious active radicals in air under NTP (normal temperature and pressure) by using an extremely fast rising narrow high voltage pulse between corona electrodes and grounded counter electrodes so that intense streamer coronas are generated. This provides an effective means of surface treatment to a plastic material placed between the two electrodes through generation of free bonds on the surface directed to the corona electrodes. Special features of this method are that it can cope with a complex shape of the material to be treated, and that it does not spark even at the periphery near the grounded counter electrode. This method is suitable for the surface treatment of polypropylene bumpers so as to provide a strong adhesion of color paint to it. The adhesion strength of a paint film is raised from zero to ca. 1000 g/cm/sup 2/ by PPCP treatment for 60 seconds.

X-Ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS) were used to study the chemical modification of polyphenylene sulfide (PPS) thin films on exposure to both oxygen and ammonia plasmas. The XPS results for the oxygen plasma treatment indicated a large oxygen increase with the incorporation of various oxidized carbon species as well as oxidized sulfur. For the ammonia plasma, both nitrogen and oxygen were incorporated. IRRAS proved to complement the XPS results, showing a wide range of C O and CO functionalities incorporated on oxygen plasma exposure. For the ammonia plasma treatment, an increase in hydrocarbon, alkene-type fragments, and possibly amine groups was detected. Both the XPS and IRRAS results indicated that exposure of plasma-treated surfaces to epoxy with subsequent carbon tetrachloride washes removed most of the modification originally present after plasma treatment. IRRAS analysis showed that a thin layer of epoxy remained after repeated solvent washes and that the film seemed to be cured. For untreated PPS films, a non-cured epoxy film adsorbed. This work suggests that the plasma-modified layer plays a role in the formation of a covalent interphase region between PPS and epoxy.

The study of the ionization of carboxylic acid groups at the interface between organic solids and water demonstrates broad similarities to the ionizations of these groups in homogeneous aqueous solution, but with important systematic differences. Creation of a charged group from a neutral one by protonation or deprotonation (whether -NH₃⁺ from -NH₂ or -CO₂- from -CO₂H) at the interface between surface-functionalized polyethylene and water is more difficult than that in homogeneous aqueous solution. This difference is probably related to the low effective dielectric constant of the interface (ε≃9) relative to water (ε≃80). It is not known to what extent this difference in ε (and in other properties of the interphase, considered as a thin solvent phase) is reflected in the stability of the organic ions relative to their neutral forms in the interphase and in solution, and to what extent in differences in the concentration of H⁺ and OH- in the interphase and in solution. Self-assembled monolayers (SAMs)-especially of terminally functionalized alkanethiols (HS(CH₂)_nX) adsorbed on gold-provide model systems with relatively well-ordered structures that are useful in establishing the fundamentals of ionization of protic acids and bases at the interface between organic solids and water. These systems, coupled with new analytical methods such as photoacoustic calorimetry (PAC) and contact angle titration, may make it possible to disentangle some of the complex puzzles presented by proton-transfer reactions in the environment of the organic solid-water interphase.

The effects of acid oxidation on the surface properties of gel-spun ultra-high modulus and molecular weight polyethylene (UHMWPE) fibers were investigated. Three acid-assisted reactions with CrO₃ (I), K₂Cr₂O, (II), and one base-catalyzed reaction with K₂Cr₂O₇ (III) were studied. In reaction II, two levels of sulfuric acid were used for IIa and IIb, with reaction IIa containing the higher concentration. Under the reaction conditions chosen, i.e. 1 min at 23°C, the effects of these oxidations were restricted to the fiber surfaces. All oxidation reactions either significantly reduced or eliminated the axially oriented macrofibril striations and changed the lamellae perpendicular to the fiber axis to irregular hairline surface structures. The oxidative attacks on the fiber surfaces appeared to have occurred in the fibrillar structure and likely at the disorder regions along the fibrils. The epoxy resin wettability and the interfacial adhesion to the epoxy resin were both improved with reactions I and IIa, whereas reaction III did not affect either of these properties. A positive relationship between surface wettability and interfacial adhesion on single fibers was observed on the untreated and acid oxidized gel-spun UHMWPE fibers.

The ageing of thin PET films in an oxygen plasma was investigated. After several hours exposure a large decrease in mechanical strength was observed. Plasma particle bombardment, chemical reactions and the plasma vacuum UV radiation cause extensive chemical and structural changes. The chemical reactions leading to the ageing process were identified.

Poly(tetrafluoroethylene) (PTFE) samples were surface modified in gas plasma atmospheres of air, oxygen, argon and water vapour in order to increase the surface energy. Its dispersive and polar components were determined by contact angle measurements after various treatment times. Plasma treatment times of only 15s were sufficient in all gases studied for substantial surface modification of PTFE. The chemical composition of the surfaces was studied by X-ray photoelectron spectroscopy (X.p.s.). The main results of all the plasma treatments were the abstraction of fluorine and the production of surface crosslinks, whereas only a low level of oxygen-containing groups was attached into the surface layer.

A modified axisymmetric drop shape analysis approach, ADSA-MD (maximum diameter) was developed to measure the contact angles of non-wetting drops front top-view images of the drop. The approach numerically solves the Laplace equation of capillarity given the following input parameters: maximum diameter and volume of the drop, liquid surface tension, density difference between the two fluid phases and the gravity constant. The computed contact angles are in good agreement with those from ADSA-P, an approach which uses the profile of the drop to determine the contact angle. This new technique is particularly suited for systems where the quality of the solid substrate is poor, such as in the case of biological systems. For these situations contact angle determination from the profile is difficult, if not impossible, due to the difficulty in locating the three-phase contact line. The ADSA-MD approach was used to determine the contact angle of water sessile drops on colon sections of New zealand white rabbits.

The effects of a thin liquid film on contact angles are studied using a simplified thermodynamic model. (in this model, the small transition zone between the liquid-vapour interface and the fiat thin liquid film is neglected). A set of mechanical equilibrium conditions have been derived for contact angle systems with a flat thin liquid film. The equilibrium condition at the three-phase intersection explicitly predicts the effects of the film tension, the disjoining pressure and the film thickness, on contact angles.

The number of degrees of freedom for a two-component solid-liquid-vapour surface system with a flat thin liquid film is shown to be three, implying the existence of an equation-of-state-type relationship among the solid-liquid interfacial tension, γ_sl, liquid surface tension, γ_lv, the disjoining pressure, Π, and the film tension, γ_f. An approximate, explicit form of such an equation of state has been derived. The combination of this equation of state with the equilibrium condition of the the three-phase intersection can be used to estimate the film tension, γ_f, and the solid-liquid interfacial tension, γ_sl, from the measured data for the vapour pressure, P_v, the film thickness, h, the curvature of the liquid-vapour meniscus, J, the liquid surface tension, γ_lv, and the contact angle, θ.

The effect of the thin film on the drop-size dependence of contact angles is also investigated and found to be negligible.

Contact angle measurements with three different liquids were performed on: (i) butyl rubber PB 101-3 (Polysar Ltd.) and (ii) Dow Corning 236 dispersion. Contact angles were measured at different temperatures within the range from 23°C (room temperature) to 120°C. The surface tensions, γsv, of the polymeric coatings at each temperature were calculated from the contact angles. The temperature coefficients of the surface tensions, d_γsv/dT, i.e., the surface entropies, were established for the temperature range covered.

No single step in the coating process has more impact on film adhesion than surface preparation. Film adhesion to a plastic is primarily a surface phenomenon and requires intimate contact between the substrate surface and the coating. However, intimate contact of that plastic surface is not possible without appropriate conditioning and cleansing. Plastic surfaces present a number of unique problems for the coater. Many plastics, such as polyethylene or the fluorinated polymers, have a low surface energy. Low surface energy often means that few materials will readily adhere to the surface. Plastic materials often are blends of one or more polymer types or have various quantities of inorganic fillers added to achieve specific properties. The coefficient of thermal expansion is usually quite high for plastic compounds, but it can vary widely depending on polymer blend, filler content, and filler type. Finally, the flexibility of plastic materials puts more stress on the coating, and significant problems can develop if film adhesion is low due to poor surface preparation.

The surface chemical modification of polypropylene by CF₃Cl plasma treatment was studied by ESCA, wettability measurements, and pressure-sensitive-adhesive performance tests. Improved adhesion was observed on polypropylene treated under CF₃Cl plasma conditions that maximized Cl and minimized F and O incorporation. Polypropylene treated using CF₃Cl plasmas had a high dispersive component of surface energy, as indicated by low diiodomethane contact angles. High dispersive energy is characteristic of chlorinated surfaces, and may contribute to the improved adhesion.

A basic understanding of rheology and surface chemistry, two primary sciences of liquid flow and solid-liquid interaction is necessary for understanding coating and printing processes and materials. A generally qualitative treatment of these subjects will suffice to provide the insight needed to use and apply coatings and inks and to help solve the problems associated with their use. Rheology, in the broadest sense, is the study of the physical behavior of all materials when placed under stress. Four general categories are recognized: elasticity, plasticity, rigidity, and viscosity. Our concern here is with liquids and pastes. The scope of rheology of fluids encompasses the changes in the shape of a liquid as physical force is applied and removed. Viscosity is a key rheological property of coatings and inks. Viscosity is simply the resistance of the ink to flow-the ratio of shear stress to shear rate. Throughout coating and printing processes, mechanical forces of various types and quantities are exerted. The amount of shear force directly affects the viscosity value for non-Newtonian fluids. Most coatings undergo some degree of" shear thinning" phenomenon when worked by mixing or running on a coater. Heavy inks are especially prone to shear thinning. As shear rate is increased, the viscosity drops, in some cases, dramatically. This seems simple enough except for two other effects. One is called the yield point. This is the shear rate required to cause flow. Ketchup often refuses to flow until a little extra shear force is applied. Then it often flows too freely. Once the yield point has been exceeded the solidlike behavior vanishes. The loose network structure is broken up. Inks also display this yield point property, but to a lesser degree. Yield point is one of the most important ink properties.

A review of studies of polymer-paper adhesion illustrates the thermodynamic nature of the bondability of polymers to plain, uncoated paper surfaces. The bond strength depends strongly on the chemical nature of the polymer surface and on that of the fibrous paper surface. Adhesion to paper may be characterized indirectly through thermodynamic analysis of the paper substrate, or directly through paper laminate or adhesion tape peel testing. The need for adequate paper adhesion is emphasized, particularly for some of the newer printing processes (electrophotographic and thermal imaging). It is concluded that some of the indirect methods of adhesion characterization (surface energetics analysis via contact angle measurements or the inverse gas chromatography technique) may serve to characterize paper adhesion in these processes.

An oriented, heat sealable polypropylene film is provided having a metallizable surface. The film includes a core layer derived from isotactic polypropylene containing an effective amount of adhesion promoting agent. A copolyester layer is bonded to the core layer, the adhesion promoting agent protecting against the delamination thereof. A heat sealable layer formed from an ethylene-propylene random copolymer is bonded to the opposite side of the core layer. The film is formed as a coextrudate and is biaxially oriented.

X-ray photoelectron spectroscopy (ESCA), wettability measurements, and an ink adhesion test were used to characterize changes in the surface properties of air-corona-treated polypropylene (PP) films upon aging under a variety of storage conditions. No changes in ESCA O/C atomic ratios as a function of aging were observed for corona-treated PP films. The wettability data indicated a slight decrease in wettability upon aging. Aging did not affect ink adhesion for the particular PP and ink studied. The responses obtained were independent of the various film storage conditions employed. The slight decrease in wettability observed upon aging was attributed to reorientation of oxidized functionalities within the surface region.

The effects of ion irradiation on polyimide surfaces have been studied using x‐ray photoemission techniques. Ion bombardment with energies in the range 0.5–2.0 keV and doses between 8×10¹³ and 1×10¹⁵ ions/cm² were carried out in situ in the x‐ray photoelectron spectrometer and the chemistry of the modified surface was monitored using core level spectral changes. At low doses and energies, carbonyl groups were preferentially sputtered keeping the rest of the monomer intact. Loss of nitrogen was insignificant compared to losses of carbon and oxygen. At higher energies and doses, the polymer undergoes extensive bond scission, restructuring of various functional groups and species, together with radical and anion formation. High resolution spectra indicated a binding energy scale shift to a lower value, which increased with ion energy and dose, and which was related to the creation of a surface negative charge. The effects of exposure to moisture in the ambient on the surface charge, on the surface structure, and on the surface chemistry was studied.