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.

Surface pretreatment methods to enhance adhesion to polymers with low surface energies generally either remove a region of low strength from the surface or introduce new surface functional groups. The relative importance of these two mechanisms is examined in the present paper for various combinations of pretreatment and polymer.

Metallization techniques based on electroless plating are widely used to coat polymer materials in a large variety of technological applications. Traditionally, dilute tin chloride (SnCl₂) and palladium chloride (PdCl₂) solutions in HCl are used to render the surface of such non-conductive substrates catalytically active towards metal deposition in the electroless plating solution. In the present work, it is shown how X-ray photoelectron spectroscopy has allowed to monitor the chemical and compositional surface modifications of polymer substrates (polypropylene, polycarbonate) subjected to plasma and UV or VUV irradiation (use of ArF^* excimer laser and Xe₂^* incoherent excimer lamp, respectively) in oxygenated (O₂, air) or nitrogenated (N₂, NH₃) atmospheres, as well as to understand the mechanisms of the catalyst (palladium species) chemisorption on the so-grafted surfaces through the use of a simple dilute palladium chloride solution in HCl. In addition, this work has allowed to bring into light the precise role that the reducer plays (sodium hypophosphite) present in the electroless nickel bath. In short, this research has been successful in allowing the development of new approaches for the electroless metallization of polymer surfaces.

The effects of 13 pre-treatments were examined to determine their effect on the surface region of homopolymer polypropylene. Five of the pre-treatments were examined in detail due to excellent joint strengths. They were: corona discharge, flame, fluorination, low-pressure vacuum plasma and atmospheric plasma. The pre-treatments were examined using X-ray photoelectron spectroscopy (XPS), angle resolved XPS (AR-XPS) and atomic force microscopy (AFM), to determine surface chemistry and topography. Of the 13 pre-treatments examined, it was found that the first five all showed the highest surface chemical modification of the pre-treatments studied. It was identified that the surface chemistry, concentration depth and topography varied widely across the five pre-treatments. However, all have been shown to have similar bond strengths with polyurethane adhesives, indicating that a number of significant factors were responsible for bond strength. It is surmised that the depth of the functional group concentration is the determinant joint strength parameter and not the O : C ratio or surface roughness.

Polyetheretherketone (PEEK) has been treated with oxygen-, air-, argon- and ammonia-plasmas, which greatly improve adhesion to an epoxide film adhesive. Treated surfaces can be stored under laboratory conditions for up to 90 days without significant loss of the improved adhesion properties. Contact angle measurements show that the surface energy of PEEK is much increased by plasma-treatment. X-ray photoelectron spectroscopy shows that the plasmas increase the amounts of oxygen and in some cases the amounts of nitrogen, and that new surface groups include -OH and -CO-. Wiping treated surfaces with acetone can reverse the effects of plasma-treatment.

A styrene-butadiene-styrene (S6) rubber was treated with corona discharge to increase its surface energy and adhesion to polyurethane (PU) adhesive. The influence of the length of treatment (the speed of the upper plate was varied from 80 to 900cm/min) during corona discharge was analyzed. The corona energy applied to S6 rubber surface ranged from 0.4 to 4.6J/cm ² . The surface modifications produced as a consequence of the corona discharge were characterized immediately after treatment was carried out and were monitored by means of different surface analysis techniques, mainly contact angle measurements (ethylene glycol, 25 ^o C), ATR-IR spectroscopy, XPS and Scanning Electron Microscopy (SEM). T-peel tests of corona-discharge-treated S6-rubber/polyurethane (PU) adhesive/leather joints (72h after joint formation) were carried out to evaluate the influence of the surface modifications produced by the corona discharge on the adhesion properties of the treated S6 rubber.The corona discharge improved the wettability of the S6 rubber due to the formation of polar moieties, mainly C-O, C=O and COO ^- groups. These chemical modifications were not detected by ATR-IR spectroscopy (depth of analysis about 5μm), indicating that a nanometer-range oxidized layer was created on the S6 rubber surface by treatment with corona discharge. Besides, surface cleaning and removal of rubber contaminants (mainly silicon moieties) were produced but roughness was not created as a consequence of the treatment. These modifications were enhanced when a low speed treatment (long treatment and high corona energy) was carried out. Peel strength values of corona-discharge-treated S6 rubber/PU adhesive/leather joints only moderately increased (mainly for long length of the corona discharge). Although corona treatment chemically modified the surface of the S6 rubber, the absence of surface roughness might likely be responsible for the slight improvement in its adhesion properties.

Poly(tetrafluoroethylene-co-perfluoro [alkyl vinyl ether]) (PFA) and polytetrafluoroethylene (PTFE) films were treated by three kinds of atmospheric pressure glow plasmas: an untreated sample was treated by He plasma or trimethoxyborane (TMB)/H₂/He plasma, and a TMB-absorbed sample was treated by H₂/He plasma. TMB was a new reactant for the treatment, to increase the films’ adhesive strength with an epoxy glue. These films were also treated by a wet method using a sodium solution (Tetra-Etch compound) and such films were used as the control samples. The peel strength values of the controls of PFA and PTFE were 3.5 and 9.5 N cm⁻¹, respectively. The adhesive strengths of all plasma-treated PFAs were stronger than those of untreated one. Especially, the peel strength of the TMB/H₂/He plasma-treated PFA showed the maximum value of 4.5 N cm⁻¹, which was bigger than that of the control one. The adhesive strength of the TMB/H₂/He plasma-treated PTFE films also showed the maximum peel strength, 7.9 N cm⁻¹, but this value did not exceed that of the control PTFE. Such results suggested that the TMB/H₂/He plasma had the advantage of providing better adhesive improvement of those polymers, especially PFA than the wet method could provide. The results of XPS and SEM indicated that TMB actively removed fluorine atoms from the polymer surface. Therefore, boron compounds are effective for the improvement of the adhesive strength between the fluorinated polymer and the epoxy glue.

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.

The aim of this work was to improve adhesion to tefzel using plasma surface treatment. The plasmas used were O₂, and NH₃. Joints ,ade from the adherends using several commercially available epoxy adhesives were tested using a double lap shear configuration. Measured bond strenghts for the treated adherends were as much as 30 times greater than those for the untreated materials. Examination of the O₂ plasma-treated Tefzel by electron spectroscopy for chemical analysis indicated a surface oxidation increase of about 7–8% over the untreated material, with the oxide being primarily in the form of an ester.

Adhesion, whether the bonding of polymers or the adhesion of coatings to polymer surfaces, is a recurring and difficult problem throughout the plastics industry. This paper introduces a proven, yet relatively unknown technology which provides an efficient, economic and versatile solution to adhesion problems: cold gas plasma surface treatment. Through plasma processing, it is possible to re-engineer the furface chemistry of any polymer to maximize its adhesive qualities. The result is optimum performance, even from inexpensive materials, and maximum flexibility in design decision-making.

In this paper are discussed some of the fundamental principles which are relevant to an understanding of the influence that interfacial roughness may have on adhesion. The surface energies of the adhesive, substrate and of the interface between them determine the extent of wetting or spreading at equilibrium. Numerical values for surface energies may be obtained either from contact angle measurements or from analysing force–displacement curves obtained from the surface forces apparatus. The extent to which the relationships, appropriate for plane surfaces, may be modified to take into account interfacial roughness are discussed. For modest extents of roughness, the application of a simple roughness factor may be satisfactory, but this is unrealistic for many of the practical surfaces of relevance to adhesive technology which are very rough, and is ultimately meaningless, if the surface is fractal in nature. Some examples are discussed of published work involving polymer–metal and polymer–polymer adhesion, where the roughness of the interface exerts a significant influence on the adhesion obtained. Roughness over a range of scales from microns to nanometres may strengthen an interface, increasing fracture energy by allowing bulk energy dissipating processes to be activated when the bond is stressed.

Polymeric hard/soft combinations consisting of a rigid, thermoplastic substrate and an elastomeric component offer many advantages for plastic parts in industry. Manufactured in one step by multi-component injection moulding, the strength of the thermoplastics can be combined with sealing, damping or haptic properties of an elastomer. Bonds of self-adhesive liquid silicone rubber (LSR) on high performance thermoplastics such as polyetheretherketone (PEEK) or polyphenylene sulphide (PPS) are especially interesting e.g. for medical applications due to their outstanding resistance properties. To ensure good adhesion between the two components, surface treatments from an atmospheric pressure plasma jet (APPJ) and a Pyrosil^® flame are applied. Chemical changes on the thermoplastic surfaces are verified by water contact angle measurement (CA) and X-ray photoelectron spectroscopy (XPS). Plasma treatment causes a decline in water contact angle, indicating the formation of functional groups, especially –OH, on the surface. XPS measurements confirm the increase of oxygen on the surface. Thus, the number of functional groups on the thermoplastic surface is enlarged by plasma treatment, leading to stronger bonding to the organofunctional silanes of the self-adhesive silicone rubber. A thin layer of silanol groups is created by the Pyrosil^® flame on the thermoplastic substrates, which could be verified by XPS. A hydrophilic behaviour of the coated surface is noticed. Both surface modification methods lead to enhanced adhesion properties of self-adhesive LSR on thermoplastic surfaces. This is confirmed by 90°- peel tests of the injection-moulded composites leading to an increase in peel force by the applied surface modification techniques.

Improvement of paint adhesion to aluminium surfaces is one of the main challenges in many industrial applications. In this paper, we introduce the atmospheric pressure air plasma jet as an appropriate candidate for preparation of 5052 aluminium surface alloy to improve paint adhesion in the industrial level. The employed plasma jet can promote paint adhesion to aluminium surface at the treatment velocity of 2 m/min and plasma size of 10 mm. Based on the cross-cut test, adhesion of polyurethane paint to the surface greatly increases from 1B to 5B level due to the plasma treatment. According to the results, the surface wettability increases under the influence of the plasma treatment so that water droplet contact angle reduces from 79.0°±2.0°–27.5°±2.0° after the treatment. Dyne test ink also denotes the increment of surface energy to the greater than 72 mN/m. Besides, we employ various analytical methods to investigate the physical and chemical changes arise from the plasma processing to the surface. Atomic force microscopy (AFM) results show a twofold increase in the roughness parameters of plasma treated surface which can result in a stronger paint and surface interlocking. Chemical analysis of the surface reveals that plasma treatment of the aluminium surface leads to the surface cleaning and formation of hydrophilic functional groups that attract much more water towards the surface and improves the paint adhesion.

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.

Effect of the electron radiation generated by a high-voltage linear accelerator on the wettability, contact angle and surface free energy (SFE) of a biaxially oriented polypropylene (BOPP) film was studied. Radiation doses of 25, 50, 100, 250, and 500kGy were used. Water, glycerol, formamide, diiodomethane, and α-bromonaphthalene were applied as measuring liquids. The calculations of SFE were made with the methods of Owens–Wendt and van Oss–Chaudhury–Good, using the results of measurements of the contact angle with various sets of the measuring liquids. Wettability tests were also performed. It was found that the contact angle decreased with the rising radiation dose for all the measuring liquids and the shapes of these dependences were similar to one another. However, significant quantitative differences were observed. The largest changes in the contact angle were detected in the dose range of up to 100kGy. The SFE values when measured with different methods and various measuring liquids differed generally in the whole range of the applied doses. Therefore, the surface free energy cannot be accepted as an absolute measure of the thermodynamic state of the surface layer of a radiation-modified BOPP film. Its values can be compared with one another only when they were determined with the same method and same measuring or standard liquids.

The surface free energy parameters of ethylcellulose (EC) films were determined using the Lifshitz-van der Waals/acid-base approach and the influence of plasticizers on their surface energetics was assessed. Films were prepared by dip-coating glass slides in organic solvents containing EC and the advancing angles of drops of pure liquids on the EC films were measured with a contact angle goniometer using the captive drop technique. EC has lower surface free energy than cellulose. The acid-base (AB) term made only a slight contribution to the total surface free energy and the surfaces exhibited predominantly monopolar electron-donicity. The addition of plasticizer (dibutyl sebacate or dibutyl phthalate) resulted in a small decrease in the total surface free energy. The effects of film forming variables, including solvent system, concentration and post-formation treatment (annealing), on the surface free energy parameters of EC films were also investigated. These data were then used to analyze how the surface energetics affect the interaction of the EC films with other surfaces based on interfacial tension, work of adhesion and spreading coefficient calculations. Lifshitz-van der Waals (LW) interactions provided the major contribution to the work of adhesion for EC with all of the solid substrates analyzed. However, the AB interactions contributed significantly to the work of adhesion for EC with 'bipolar' substrates and to the spreading coefficients of EC over substrates. The consideration of work of adhesion and spreading coefficient based on surface free energy parameters may have potential use in evaluating factors affecting film adhesion and, furthermore, in optimizing pharmaceutical film coating processes.

The effect of CO₂-laser treatment on the wettability of mild steel is presented. In addition, data are presented on the initial joint strengths and durability of joints formed between a single-part epoxide and both mild steel and aluminium. A large increase in stressed durability performance was observed with the laser-treated aluminium compared with degreased-only controls. The laser treatment was shown to efficiently remove the organic contamination from the metallic substrates. Auger analysis showed that the laser interacts more with the mild steel than the aluminium adherends, to produce a relatively thick surface oxide. The changes to the mild steel surface introduced by the CO₂-laser treatment facilitate an durability trials was greater with the laser-treated adherends than with degreased-only controls.

A corona discharge treatment of low-density polyethylene film (LDPE) was carried out in preparation for flexographic printing. Such treatment of the PE film is necessary if maximum adhesion of ink is to be achieved. This project involved three different treating machines for which the current had to be manipulated in all the machines so that a standard treatment could be accomplished. Using a mathematical relation, current requirements for each machine were calculated and used to standardize treatment level of PE films. Standardization was achieved by controlling input current in all the three machines so as to attain a treatment level of 38 dynes/cm. This level of treatment showed the best results in adhesion of ink to the PE film during printing. The exercise also confirmed that printing must be carried out within 24 h of treatment since the level of treatment deteriorates with time.

In the gas flame treatment of low surface free energy (SE) substrates, such as high-density polyethylene (HDPE), problems might arise from under or over flaming, oxygen concentration differences in and around of the flame, etc. Consequently, in printing applications, the possible variation of induced SE existing on the surface, could cause distortion on printed letters. In this research, a new method based on the wetting and spreading phenomena was developed to display and study details of the SE variation on HDPE flame treated substrates. It was an easy and quick method. Results showed good agreements with previous works done on the flame treatment characteristics. The optimal flaming was achieved, while the substrate surface had been positioned about 10 to 12 mm below the tip of the flame's blue part. Also when the flaming speed had been controlled about 80 mm/s. Results from the adhesion strength test supported the optimum situations found previously by others. It was hoped that this new method could also be capable of estimating the critical SE of solid surfaces in future works.

The properties of a polymer surface can be decisive for the function of the polymer. Both in the assessment of existing polymer systems and the development of new ones the possiblity of characterizing the chemical composition and structure of the polymer surface becomes important. Various instruments and chemical methods used to characterize polymer surfaces and interfaces are reviewed. The pros and cons of electron spectroscopy for chemical analysis and derivatization schemes to enhance the detectability of functional groups, Fourier transform infrared spectroscopic methods (ATR, RIFT, PAS, micro), Raman spectroscopy, static secondary ion mass spectrometry, high resolution solid state nuclear magnetic resonance, microscopy and contact angle measurements are presented. The importance of the fact that the polymer surface can undergo comparatively rapid reorientations leading to a changed surface chemistry is discussed and exemplified.

Polyethylene (PE) and polypropylene (PP) were oxygen plasma treated and aged in carefully reproducible conditions. The effect of aging on the surface chemistry, wettability and adhesion were studied using a combination of techniques: contact angle measurements, XPS, SSIMS, adhesion tests (shear and pull).

PE was found to be relatively insensitive to aging both in terms of wettability and adhesion, due to crosslinking during plasma treatment, which is likely to reduce macromolecular mobility within the surface layer.

In the case of PP, dramatic decreases of wettability occur with time, due to macromolecular motions leading to minimization of oxygen-containing functions at the surface. This behavior was shown to affect the adhesion performance of treated PP.

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.

A “blister test” technique has been used to determine the fracture surface energy of a range of adhesive joints formed using a polyurethane adhesive and a range of solid substrates. For each adhesive pair examined the work of adhesion was calculated from the contact angles formed by liquids for which the polar and dispersion force components of the surface tension are known. For each adhesive pair, the solubility parameter of adhesive and substrate were determined by swelling measurements in a range of liquids. Although cohesive failure of the joints was observed for some of the pairs for which the solubility parameters were matched, this was not true for all such pairs and an explanation of this behaviour has been sought in a new calculation of the volume interaction component of the molecular interaction parameters.

Examples of surface characterization using cohesive energy parameters and surface energy parameters are given. In general the two approaches yield essentially equivalent results. The predictive ability of the cohesive energy approach suggests its use where directed modification of surface properties is desired.

A known relationship between heat of vaporisation, surface tension and molar volume applicable to spherical non-polar molecules is modified to apply also to linear molecules; the treatment involves calculation of molar surface areas corresponding to the appropriate “fully-packing” molecular shapes.

A linear relationship between the ratio cohesive energy density/surface tension and the reciprocal of molar volume is predicted for members of homologous series and demonstrated, with data for the n-paraffins.

Surface energies of solids can be estimated using contact angles of liquids of known surface tension and susceptibilities for polar or acid-base interactions. Interfacial tensions and work of adhesion can be calculated using these estimated energies. There are three circumstances in which performance or bond strengths are related directly to surface energies: when separation occurs interfacially, when interfaces are not completely wetted, and when third phases are present at the interface.

A new definition for work of adhesion W_a is applied to computationally define the dispersion γ_s^d and polar γ_s^d components of the solid surface tension γ_s = γ_s^d + γ_s^d for twenty-five low energy substrates. These calculated surface properties are correlated with surface composition and structure. Surface dipole orientation and electron induction effects are respectively distinguished for chlorinated and partially fluorinated hydrocarbons. Published values for critical surface tension of wetting γ_c are correlated with both γ_s^d and γ_s.

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.

According to Bikerman, who attributes failure in adhints to a weak boundary layer, it is almost impossible and meaningless to correlate adhesive strength to surface-chemical properties of adhints. Though his assertion seems to be confirmed by the recent studies of Schonhorn and his coworkers on the methods of CASING and TCR, not a few results have yet been accumulated, which show a close relation between them. In this paper surface-chemical criteria for the optimum adhesion are investigated and the minimum interfacial tension or the maximum wetting pressure is deduced from the published data and our own as a first approximation. It is emphasized that, when critical surface tension γ _c would be used as a measure of surface-chemical properties of solid, its variability according to liquid series (nonpolar, polar and hydrogen bonding liquids) should be carefully taken into consideration. The importance is shown for polyethylene and its fluorine substituted polymers, using newly measured contact angle data and Zisman's data. Results of Levine et al. and Schonhorn et al. on adhesive shear strength with epoxy adhesives are replotted against available values of γ _c obtained by the use of hydrogen bonding liquid (γ _c ^c ), which are thought to reflect wetting behaviors of epoxy adhesives quite well. Each curve shows a maximum around γ _c ^c = 40 dyne/cm with few points falling off the curves.

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.

A thermal gradient bar has been used for convenient measurements of γ_c and dγ_c/dT in complex polymers used as film-formers. The technique yields both γ_c and its temperature variation in one experimental sequence well suited for rapid, routine applications. Surface tension data have been obtained for a styrene-acrylic terpolymer, and these have also been used to characterize the compatibility of external plasticizers for the polymer. The surface tension approach has shown that glyceryl dibenzoate, though compatible with the polymer at temperatures above ∼70°C becomes incompatible at use temperatures, and exudes to the polymer film surface. Measurements of moisture sensitivity in plasticized polymer samples have confirmed the incompatibility and illustrated one of the applications to which the gradient bar and its data generation potential may be put.

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 CASING (crosslinking by activated species of inert gases) treatment of polypropylene film in both oxygen and nitrous oxide is shown to be an effective surface treatment for conventional adhesive bonding. A crosslinked surface extending to a depth of about 300 Å, apparently independent of exposure time, is produced in both excited oxygen and nitrous oxide.