In the last 25 years, surface sensitive analytical techniques have made a major contribution to our understanding of adhesion phenomena and problems. There are several areas where these techniques have provided important information including the identification of failure modes, the chemistry of a substrate before and after pretreatments, the stability of surfaces and interfaces, the identification of surface contaminants, the interaction across an interface and the nature of interphases. X-ray photoelectron spectroscopy (XPS or ESCA), Auger electron spectroscopy (AES) and static secondary ion mass spectrometry (SSIMS) have proved to be especially useful. Many examples of the usefulness of these techniques are given.

The most effective commercial methods for pretreating fully fluorinated polymers at present are sodium in liquid ammonia and sodium naphthalenide in a solvent such as tetrahydrofuran. A number of other methods can produce large increases in bondability but are relatively inefficient. Plasma treatments of fully fluorinated polymers such as PTFE often introduce functional groups and/or increase wettability but result in moderate improvements at best in bondability. However, recent work with ammonia plasmas has resulted in big improvements in bondability. Some batches of partially fluorinated polymers may be effectively bonded without a pretreatment. However, other batches possess surface regions of low cohesive strength. Partially fluorinated polymers such as poly (vinyl fluoride) may be rapidly and effectively treated with a conventional flame treatment. Where a flame treatment is unsuitable, as with complex shapes, potassium hydroxide solutions provide effective treatments for partially fluorinated polymers such as poly (vinyl fluoride) and poly (vinylidene fluoride).

Good adhesion to polymers is required in a number of important technologies including adhesive bonding, printing, and painting. To achieve a satisfactory level of adhesion it is often necessary to pretreat the polymer by one of a wide range of methods. Two books are of particular interest [1, 2]. In the case of polar polymers such as nylon 66 and epoxide thermosets, a treatment may not be necessary, or if the surfaces are contaminated, a physical method such as solvent degreasing or grit blasting to remove the contaminants may be all that is required. On the other hand, if a polymer lacks suitable functionality, it will be necessary to modify its surface chemically. Polymers with no active functionality include low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). A wide variety of methods for introducing new groups is available, including the use of low-pressure plasmas, corona discharges, flames, etchants, and active gases. Flame treatment to enhance adhesion to polymers has been used since the early 1950s, one of the first applications being to enhance print adhesion to lowdensity polyethylene. Since that time, flame treatment has been used with many other polymers in a variety of applications. Flame treatment has a number of advantages over the other main method of treating large areas of polymers, ie, the corona treatment. These include no reverse-side treatments, no creation of pinholes, no Ozone production, and better aging characteristics.

Surface and interface characterization of polymeric materials has not enjoyed the multi-technique approach which typifies other types of materials. X-ray photoelectron spectroscopy (XPS) has dominated, despite several major disadvantages. New approaches are discussed which either improve XPS (particularly derivatization techniques) or utilize ‘static’ secondary ion mass spectrometry (SIMS) to overcome these limitations, with examples of their application in materials problem solving.

XPS has been used to elucidate the mechanisms of surface modification of low density polyethylene by electrical (“corona”) discharge treatment and by chromic acid treatment. The use of derivatisation techniques for improving the precision of functional group analysis is described. These techniques also allow the role of specific interactions in adhesion to discharge treated surfaces to be investigated. The role of residual Cr on the adhesion of deposited metal to polymer surfaces is discussed.

Poly(ethylene terephthalate) (PET) film has been discharge-treated under controlled conditions and the resulting surface modifications analysed via X.p.s. (ESCA), contact angle and surface energy measurements. Changes in surface properties have been followed as a function of ageing time. These measurements have been correlated with the adhesive properties of the treated surfaces using autoadhesion (treated-treated seals) as the probe. Discharge treatment introduces phenolic -OH and carboxylic acid -COOH groups into the surface resulting in increased wetting and much enhanced autoadhesion via hydrogen bonding of phenol groups to carbonyl groups. Much chain scission also occurs; the low molecular weight material is easily removed by washing and migrates into the film on ageing. The new functionalities in relatively immobile chains slowly reorientate and internally H-bond. The former process is largely responsible for the wettability change on aging, the latter for the loss of adhesive properties.

A series of low-density polyethylene (LDPE) surfaces, chemically modified using a number of oxidative techniques employed for adhesion enhancement (pretreatments), have been studied by time-of-flight (ToF) SIMS and XPS. The methods consisted of corona discharge, flame, electrochemical, chromic acid, acid dichromate and acid permanganate treatment. All except flame treatment were performed under mild and fairly severe conditions to yield a range of surface chemistries. The XPS analysis, using high energy resolution and a refined approach to C 1s curve-fitting, provided some new insights into the quantitative assessment of the type and concentration of functional groups. Both positive and negative ion ToF-SIMS spectra were obtained at high mass resolution. The oxygen-containing fragments were identified by accurate mass analysis and subjected to a detailed comparison with the XPS results. No convincing relative intensity correlations could be identified that would allow particular secondary ion fragments to be associated strongly with particular functional groups (in this multi-functional surface situation). Inorganic residues resulting from wet chemical treatments were also investigated and here the two techniques were found to be more complementary. Copyright © 2003 John Wiley & Sons, Ltd.
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/sia.1515

X-ray photoelectron spectroscopy showed that a normal flame treatment caused a high level of oxidation in low-density polyethylene. 0.02% of the antioxidant 2,6-ditertbuty-p-cresol did not reduce the degree of oxidation or the level of adhesion in contrast to the extrusion of low-density polyethylene. It is estimated that the depth of oxidation is between 40 and 90 Å which is much less than for a moderate chromic acid treatment or with extrusion. There were no significant changes in the XP-spectra or adhesion levels of flame treated samples after 12 months.

The previously observed, but unexplained, deleterious effect of high relative humidity on the efficiency of electrical (‘corona’) discharge treatment for rendering polypropylene film printable has been re-examined. The effect of film temperature during treatment has also been studied. A consistent explanation of both effects based on the degree of surface coverage by physically adsorbed water is put forward, supported by X-ray photoelectron spectroscopy analysis of treated film surfaces.

Films of poly(methyl methacrylate) (PMMA) of both medium and high molecular weight have been prepared by casting onto clean glass. The difference in water contact angle of the surface originally in contact with glass. and air and the variation over time of this parameter have been studied. By use of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and, particularly, static secondary ion mass. spectroscopy (SSIMS), it has been shown that migration of low molecular weight impurities from the bulk of the film to the film/air interface is responsible for the contact angle behavior.

Specific reactions for the derivatization of oxygen-containing functional groups in polymer surfaces have been developed in order to improve the precision of analysis by X-ray photoelectron spectroscopy (xps). These have been used to probe the chemical composition of low density polyethylene (ldpe) surface-modified by electrical discharge treatment. Simultaneously the effect of derivatizing particular groups on the auto-adhesive behaviour of these surfaces has been examined. Two independent specific interaction mechanisms have been identified.

A method is described for measuring the level of pretreatment of polyethylene films in terms of critical surface tensions. Drops of water-dioxan mixtures of various surface tensions are placed upon the pretreated films and their critical surface tensions assessed from the spreading behaviour of the liquids.

A suggestion is made for using this method as a process control test.

A review is presented of the adhesion between polymers with particular emphasis on the processes that occur during failure at the level of polymer chains and how these processes relate to the macroscopic interface toughness. The same processes at the chain level, pull-out and scission, occur in both glassy polymers and elastomers, but the two classes of material are considered separately because their deformation processes around a crack tip are so different. Emphasis is placed on the work in which the author has participated and so the review makes no attempt to be an unbiased survey of the field.

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus. Scanning electron and optical microscopic observations have been used to examine the microscopic basis for these results, which are compared with results previously obtained for aramid/epoxy and extended-chain polyethylene/epoxy composites. It is concluded that the increased interlaminar shear and flexural properties of vinylester matrix composites are due to improved wetting of the surface-treated fibres by the vinylester resin, rather than covalent chemical bonding.

A process for flame treating and heat sealing an uncoated biaxially oriented heat shrinkable film formed from a homogeneous blend of low density and high density ethylene polymers and copolymers comprising passing the film through a flame fed by a fuel of paraflinic and olefinic hydrocarbons, said fuel having a fuel equivalency when mixed with oxygen-enriched air of between 0.95 and 1.05, with said film being supported as it traverses the flame on a surface maintained at a temperature from about room temperature to 40 C., said film being exposed to the flame for a time between .0005 and .1 second; and heat sealing the flame treated film to itself or to another such film similarly treated.

This invention pertains to the surface treatment of film, and more particularly to the flame treatment of the surface of polyvinyl fluoride film whereby to enhance the ad herability characteristics of said surface.

Polyvinyl fluoride is noted for its attractive properties, and in film form possesses an unusual combination of excellent resistance to outdoor weathering exposures, a high degree of physical toughness, chemical inertness, abrasion resistance, resistance to soiling and the action of solvents as well as an amazing retention of these properties at both low and elevated temperatures. The above combination of properties not only suggests many areas of use for polyvinyl fluoride in the form of self-supporting films, but also the use of such films as the outer layers of a wide variety of laminar structures destined chiefly for outdoor use wherein the polyvinyl fluoride films serve to upgrade less functional substrates, imparting to the final structure a degree of utility not to be found solely in either film or substrate.

A method and device of treating an irregularly shaped article to prepare the article for painting is provided. The device includes a burner which can produce an adjustable flame tongue which can fit into crevices, openings and other irregular topographical features of an item to be painted or otherwise coated. The burner device further provides means to apply a grafting chemical on a freshly oxidized surface. Further, the invention provides means to colorize treated objects so that they may be recognized as having been treated. In another embodiment, the grafting chemicals may be enhanced with electrolytic solutions such that electrostatic methods of painting may be subsequently employed on the item. In an alternate embodiment, the burner is adapted to spray a powder inside of a generally enclosed flame, and is used in conjunction with chop guns to manufacture glass or carbon fiber preforms.

A method and device of treating an irregularly shaped article to prepare the article for painting is provided. The device includes a burner which can produce an adjustable flame tongue which can fit into crevices, openings and other irregular topographical features of an item to be painted or otherwise coated. The burner device further provides means to apply a grafting chemical on a freshly oxidized surface. Further, the invention provides means to colorize treated objects so that they may be recognized as having been treated. In another embodiment, the grafting chemicals may be enhanced with electrolytic solutions such that electrostatic methods of painting may be subsequently employed on the item. In an alternate embodiment, the burner is adapted to spray a powder inside of a generally enclosed flame, and is used in conjunction with chop guns to manufacture glass or carbon fiber preforms. In a preferred embodiment all of the main systems of the present invention are provided with redundancies that allow the continuous operation of the device. Further, the device provides means to run and modify the process either locally or by remote communication means.

Joining of thermoplastic composite primary structures is an area of great importance in the aerospace industry. In order to achieve a strong and durable joint, an effective surface treatment is needed. Preadhesion surface treatment of thermoplastic composites is limited due to their chemical inertness to aggressive chemicals or mechanical treatments. The feasibility of using a new technique of ArF UV (193 nm) excimer laser irradiation as a preadhesion treatment of PEEK (polyaryletheretherketone) reinforced with carbon fibers is demonstrated. This method presents an alternative to other limited and polluting conventional surface treatment methods, such as sand blasting, etching or welding. Experimental results indicated that laser preadhesion treatment significantly improved the shear and tensile adhesion strength of various structural epoxy adhesives FM 300 2K and AF 163-2 bonded to PEEK composite adherends compared with untreated and SiC blasted substrates. Best results were obtained with laser pulse fluences of 0.18 or 1 J/Pcm2. Shear strength of laser treated PEEK composite joints improved by 450% compared with that of untreated PEEK composite and by 200% compared to SiC blasted adherends, at ambient and at—30 C and 120 C temperatures. An order of magnitude improvement was found in the tensile strength of laser treated PEEK composite in a sandwich structure compared to non-treated or abraded sandwich joints. The mode of failure changed from interfacial to cohesive as the number of pulses or laser energy increased during treatment. A similar improvement was achieved in fracture toughness (Mode I and II) performance of laser treated compared to abraded or non-treated PEEK composite adherends. Surface analysis of laser treated adherends and of the fractured joints revealed surface cleaning (XPS), morphology changes (Scanning Electron Microscopy), chemical modification (FTIR spectroscopy and XPS), changes in crystallinity (X-ray) and in wetting property (contact angle), all correlated with the laser irradiation improving the joint’s performance. The bulk properties of the PEEK composite adherend did not deteriorate by the laser irradiation during treatment, as indicated by the identical flexural strength before and after laser treatment. Durability tests showed no change in performance of joints produced from laser treated adherends compared to untreated and abraded ones, even after exposure of 60 days at 60 C/95% RH. It can be concluded that the excimer laser has a potential as a precise, clean and simple preadhesion surface treatment for PEEK composite.

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.

Polar and dispersion surface free energy (SFE) can be determined with the Owens-Wendt method. Thereby, contact angles (CAs) of at least two liquids with known surface tension (ST) components are measured. The ST components can either be determined through experiment or drawn from literature. However, it is important to know how big the difference is between SFE component values that have been calculated with experimentally-determined ST values or values derived from literature. In this study, STs of different test liquids were analyzed by Pendant Drop method and the components by CA measurement on a non-polar surface. CAs on different polymer surfaces were measured to calculate SFE components with the Owens-Wendt method. The calculations conducted were either based on experimentally-determined ST parts or different sets of values found in the literature. The findings of the survey show that, depending on the set of literature values used, the SFE results deviate significantly from the values obtained from experiment. Expressing this deviation in figures, in extreme cases the polar part differs for some polymers by -100% to +100%, with the dispersion component spanning -50% to +43%. In comparison, the expected relative uncertainties exhibited by the experimentally-determined ST values are about 15% for the polar and approximately 5% for the dispersion SFE part. Hence, the results show that the SFE uncertainty can be reduced significantly by means of analyzing the ST parts experimentally.

The interfaces formed by evaporating copper, nickel, and chromium layers on polystyrene, polyvinyl alcohol, polyethylene oxide, polyvinyl methyl ether, polyvinyl acetate, and polymethyl methacrylate have been studied with x‐ray photoemission spectroscopy (XPS). The adhesion strengths of the metal films to the polymers were measured by a tensile‐pull test. At submonolayer coverages of the metals, the peak positions and widths of the metallic electron core levels measured with XPS vary significantly from one polymer substrate to another. Most of these variations can be accounted for in terms of changes in the atomic and extra‐atomic relaxation energies during the photoemission process. Much of this change is brought about when the metal atom deposited on an oxygen‐containing polymer interacts with the substrate oxygen and forms a metal‐oxygen‐polymer complex. The presence of this complex is verified by changes in the photoemission lineshapes of the substrate carbon and oxygen atoms. The XPS signatures of these various complexes are quite similar and suggest that they are chelate‐like complexes. The adhesion strength of any metal on an oxygen‐containing polymer is greater than on the oxygen‐free polystyrene. In general, the increased adhesion strength correlates with the presence of the metal‐oxygen chelate complexes.