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1882. Wells, R.K., J.P.S. Badyal, I.W. Drummond, K.S. Robinson, and F.J. Street, “Plasma oxidation of polystyrene vs. polyethylene,” J. Adhesion Science and Technology, 7, 1129-1137, (1993) (also in Plasma Surface Modification of Polymers: Relevance to Adhesion, M. Strobel, C.S. Lyons, and K.L. Mittal, eds., p. 113-122, VSP, Oct 1994).

Polyethylene and polystyrene film surfaces have been plasma-oxidized and subsequently characterized by X-ray core level and valence band spectroscopies. The extent of polyethylene surface oxidation was found to be dependent on the power of the oxygen glow discharge employed and the length of time that the treated sample was left exposed to air prior to analysis. In marked contrast to these observations, plasma-oxidized polystyrene surfaces were much less dependent on the oxygen glow discharge power and were also found to retain their oxygenated character over much longer periods of ageing. These differences in oxidative behaviour are explained in terms of the molecular structures of the respective polymers.

1883. Guezenoc, H., Y. Segui, S. Thery, and K. Asfardjani, “Adhesion characteristics of plasma-treated polypropylene to mild steel,” J. Adhesion Science and Technology, 7, 953-965, (1993).

The ability of polypropylene (PP) to adhere to mild steel depends to a large extent on the surface characteristics of both PP and steel. The adhesion of PP was improved by treatment in a cold plasma from oxidizing gases (O2, H2O, etc.). This surface functionalization was followed ex situ by means of contact angle measurements and XPS (X-ray photelectron spectroscopy) analysis. The polymer/steel assembly was fabricated by hot-pressing in vacuum, or after exposure to ambient air. Adhesion to steel, as determined by the lap-shear test, increased when the PP was treated with Ar-containing plasma gas and joined to steel after exposure to room atmosphere. Correlations between the polarity, the atomic (O/C, N/C) ratio, the dispersive component of the surface energy, and the degree of PP/steel adhesion are discussed.

1884. Toussaint, A.F., and P. Luner, “The wetting properties of grafted cellulose films,” J. Adhesion Science and Technology, 7, 635-548, (1993).

The dispersive component of the surface free energy, the nondispersive interaction, with polar liquids were determined for cellulose, cellulose acetate and cellulose grafted with alkyl ketene dimer (AKD). and were calculated in the dry state as well as the fully hydrated state by the two liquid contact angle method. was found to be independent of AKD coverage. Insw was found to be highly dependent on AKD coverage and differed significantly between the dry and fully hydrated states. Using the work of adhesion as a criterion, it was postulated that in the dry state, the AKD molecule renders the cellulose hydrophobic, and undergoes surface restructuring in the hydrated state leading to a hydrophilic surface.

1885. Yao, Y., X. Liu, and Y. Zhu, “Surface modification of high-density polyethylene by plasma treatment,” J. Adhesion Science and Technology, 7, 63-75, (1993).

The extent of the surface crosslinking of high-density polyethylene (HDPE) under various plasma treatment conditions was investigated. The plasma modification efficiency was studied by surface energy and adhesive bond strength measurements. The results show that the surface crosslinking of HDPE takes place as soon as the HDPE is exposed to the plasma and that the crosslinking rate is a function of the plasma conditions. The surface energy and the adhesion of HDPE are greatly increased by the plasma treatment and these improvements are independent of the depth of surface crosslinking. Based on these results and our previous studies on the surface chemical composition and free radical density on the surface of HDPE after plasma treatment, the relationships among various surface changes and the surface modification efficiency are discussed.

1886. Strohmeier, B.R., “Improving the wettability of aluminum foil with oxygen plasma treatments,” J. Adhesion Science and Technology, 6, 703-718, (1992) (also in Contact Angle, Wettability and Adhesion: Festschrift in Honor of Professor Robert J. Good, K.L. Mittal, ed., p. 453-468, VSP, Nov 1993).

The wettability of aluminum foil is an important concern in many industrial converting processes. X-ray photoelectron spectroscopy (XPS or ESCA) and water contact angle results indicated that relatively mild (i.e. 250 W, 15 s) oxygen plasma treatments efficiently removed residual carbon contamination from cold-rolled foil surfaces. This resulted in a significant improvement in the foil wettability. It was also found that the wettability of plasma-treated foils degraded with time, apparently due to the adsorption of hydrophobic, airborne carbon species and other contaminants. Furthermore, oxygen plasma treatments caused additional aluminum oxide to grow on the metal surface. The composition of this additional oxide was found to be similar to that of the native passivation oxide. The thickness of the aluminum oxide layer increased with both the plasma RF power and the plasma exposure time.

1887. Inagaki, N., S. Tasaka, and H. Kawai, “Surface modification of Kevlar fiber by a combination of plasma treatment and coupling agent treatment for silicone rubber composite,” J. Adhesion Science and Technology, 6, 279-291, (1992).

To improve the adhesion between poly(p-phenylene terephthalamide), PPTA, fiber and silicone rubber, the surface modification of PPTA was investigated. Combining plasma treatment and coupling agent treatment with the silicone adhesive was found to be effective in improving adhesion. The combination process made the pull-out force of the PPTA yarn/silicone rubber composite 2.5 times higher, compared with the plasma treatment or the coupling agent treatment alone. The plasma treatment led to the elimination of carbonized layer from the PPTA yarn surface and the formation of oxygen functionalities including CSingle BondO and CDouble BondO groups. The elimination of the carbonaceous deposits from the PPTA surface and the interaction between the silicone adhesive and the oxygen functionalities created by the plasma treatment contribute to the improvement of adhesion with silicone rubber.

1889. Gatenholm, P., C. Bonnerup, and E. Wallstrom, “Wetting and adhesion of water-borne printing inks on surface-modified polyolefins,” J. Adhesion Science and Technology, 4, 817-827, (1990).

Polyolefin films were surface-modified by different methods to improve the wetting and adhesion of water-borne printing inks. Polyethylene (PE) films were treated with corona at various energy levels. Surface-modified PE films were characterized by contact angle measurements and electron spectroscopy for chemical analysis (ESCA). Good wetting was already achieved with treatment at a lower energy level. Various degrees of adhesion were obtained at various degrees of treatment. A hydrophilic monomer, 2-hydroxyethylmethacrylate (HEMA), was polymerized onto the surfaces of polypropylene (PP) with radiation-induced grafting, which was carried out at two different radiation doses. In both cases, a thick, visible layer of polyHEMA was formed on the surface of PP, and satisfactory wetting was already achieved at lower radiation doses. Scanning electron microscopy (SEM) showed that different degrees of roughness were achieved at various radiation doses. Like the case of corona-treated PE, different degrees of adhesion were obtained at different degrees of surface treatment. This study shows that improved wetting alone is not satisfactory for good practical adhesion', regardless of the surface modification method used.

1891. Friedrich, J.F., L. Wigant, W. Unger, et al, “Barrier properties of plasma-modified polypropylene and polyethylene terephthalate,” J. Adhesion Science and Technology, 9, 1165-1180, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 121-136, VSP, May 1996).

Plasma treatment changes the solvent absorption and permeation as well as the swelling properties of polymers. Enchanced solvent absorption and swelling are effects of an improved solvent compatibility. The plasma introduces a large number of different groups at the polymer surface depending on the nature of the plasma. Fluorine-containing plasmas can replace hydrogen atoms of the polymer molecule with fluorine atoms. Moreover, fluorine-containing plasma polymer layers can be formed. All these processes reduce the resulting surface free energy, reduce the diffusion length of solvent molecules, and produce a barrier layer. We have studied the formation of solvent barriers by plasma fluorination and by crosslinking by ultraviolet (UV) radiation. Thin foils of polypropylene (PP) and polyethyleneterephthalate (PET) were used as substrates. CF4, SF6, and SOF2 were applied as sources of fluorine atoms. Hexafluoropropene, tetrafluorethylene, and perfluorohexylethylene form plasma polymer layers on the polymer substrates. Test solvents were n-pentane, tetrachloroethylene, dimethylsulfoxide, and mixtures of n-pentane and methanol.Plasma treatment changes the solve The permeation rate of solvents through plasma-modified polymers was measured gravimetrically. Mass spectrometry was applied to analyze the permeating components of the solvent mixtures. Fluorination of surface layers by plasma-chemical (CF4, SF6) means considerably reduces the permeation rate of PP (95% barrier effect) and PET (100%). The preferred permeation of one component of the pentane/methanol mixture is influenced by the polarity of plasma-introduced groups at the polymer surface.

1892. Friedrich, J.F., W. Unger, A. Lippitz, et al, “The improvement in adhesion of polyurethane-polypropylene composites by short-time exposure of polypropylene to low and atmospheric pressure plasmas,” J. Adhesion Science and Technology, 9, 575-598, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 49-72, VSP, May 1996).

The surfaces of polymers, namely polypropylene, copolymers and blends, were exposed to low pressure oxygen and atmospheric pressure air plasmas to improve their adhesion to polyurethane adhesives. A correlation is attempted between lap shear strengths of polypropylene-polyurethane composites and the relevant XPS, AFM and NEXAFS data. It was found that plasma functionalization improved the adhesion to maximum values even when the time of exposure was low: 1 to 10 seconds for low pressure plasmas, and 0.1 to 1 seconds in case of atmospheric plasma jet treatments. Thus, high lap shear strengths were obtained at relatively small oxygen contents. The improvement in shear strength at short time plasma exposures seems to be correlated to the complete smoothening of the supermolecular structure of stretched polypropylene foils as shown by AFM. Valence band XPS and derivatization techniques revealed more details of the oxygen functionalization on polypropylene. NEXAFS experiments confirmed a re-orientation of bonds and segments of the macromolecules by plasma exposure which are assumed to be responsible for adhesion improvement.

1893. Kruse, A., G. Krueger, A. Baalman, and O.-D. Henneman, “Surface pretreatment of plastics for adhesive bonding,” J. Adhesion Science and Technology, 9, 1611-1621, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 291-302, VSP, May 1996).

Many plastics have a poor tendency to bond to other materials because of their inherent inert chemical structure and thus require a pretreatment. Wet chemical methods are expensive because of the disposal of the waste liquids. In this study, the corona treatment (Ional process), the low-pressure plasma process, and the fluorination process were tested and compared with each other. The following plastics were tested: PP (polypropylene), PBT (polybutyleneterephthalate), PBT blends, and a high-temperature thermoplastic, PEEK (polyetheretherketone). In particular, the low-temperature plasma process results in excellent adhesion strength. In addition, we have shown that the stability of freshly plasma-treated surfaces could be maintained for time periods of at least several days.

1894. Murahara, M., and M. Okoshi, “Photochemical surface modification of polypropylene for adhesion enhancement by using an excimer laser,” J. Adhesion Science and Technology, 9, 1593-1599, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 223-232, VSP, May 1996).

Polypropylene (PP) surface in water was photochemically modified to render it hydrophilic using ArF excimer laser radiation. The chemical stability of PP is attributed to the CSingle BondH and CSingle BondH3 bonds present. Thus, it is considered that H atoms are selectively pulled out from the area irradiated with ArF excimer laser light and are replaced with OH functional groups in the presence of water. In this treatment, the irradiated sample becomes hydrophilic with enhanced adhesion properties. The experimental conditions for this surface treatment were ArF laser fluence of 12.5 mJ/cm2 and a shot number of 10000. The treated PP and stainless steel were bonded with epoxy adhesive and the tensile shear strength was 46 kg/cm2.

1895. Murahara, M., and K. Toyoda, “Excimer laser-induced photochemical modification and adhesion improvement of a fluororesin surface,” J. Adhesion Science and Technology, 9, 1601-1609, (1995).

Modification of a selective area of a fluororesin surface was accomplished by using ArF excimer laser radiation and a boron complex with oleophilic or hydrophilic functional groups. The chemical stability of fluororesin is attributed to the presence of C-F bonds. The F atoms were abstracted by B atoms selectively from the area irradiated with excimer laser radiation and were replaced with the desired functional groups. In this modification, B(CH3)3 and B(OH)3 were used: a boron compound with methyl groups to generate an oleophilic surface, and one with hydroxyl groups to generate a hydrophilic surface. As a result, the resin surface exposed to ArF laser radiation becomes oleophilic or hydrophilic. Both samples were bonded to stainless steel plates with an epoxy bonding agent and the tensile shear strength was 1.2 x 107 Pa in both cases.

1896. Hill, J.M., E. Karbashewski, A. Lin, M. Strobel, and M.J. Walzak, “Effects of aging and washing on UV- and ozone-treated poly(ethylene terephthalate) and polypropylene,” J. Adhesion Science and Technology, 9, 1575-1591, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 273-290, VSP, May 1996).

In this study we investigated the stability of poly(ethylene terephthalate) (PET) and polypropylene (PP) surfaces modified using three combinations of UV light and ozone: ozone only, UV light in air (producing ozone), and UV light in air supplemented by additional ozone in the incoming air. Analysis was done using X-ray photoelectron spectroscopy and dynamic contact angle measurements. Our results showed that PET film is oxidized using these treatment conditions and it changes significantly within the first week of aging and after washing with water. These changes are reflected in the decrease in the Δ(O : C) ratio and the increase in the contact angle. Conversely, PP changes very little on aging or washing. Low-molecular-weight oxidized material (LMWOM), produced on the polymer surfaces treated with UV/air or UV/air + ozone, is easily removed with water washing. On aging PET, a number of the oxidized groups at the surface disappear, seeming to migrate into the bulk. The PP, however, does not favour migration as a path to reduce the overall free energy of the system, so the oxidized groups remain at the surface. Treatment with ozone only, in the absence of UV light, is a much different modification process in terms of the mechanism and the functional groups formed on the surface. This is reflected in the aging and washing behaviour of both the PET and the PP treated with ozone only.

1897. Walzak, M.J., S. Flynn, R. Foerch, et al, “UV and ozone treatment of polypropylene and poly(ethylene terephthalate),” J. Adhesion Science and Technology, 9, 1229-1248, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 253-272, VSP, May 1996).

The effects of exposure to ultraviolet (UV) light and ozone, separately and in combination, were investigated with respect to polypropylene (PP) and poly(ethylene terephthalate) (PET) surfaces. Three combinations of UV light and ozone were studied: ozone only, UV light in air (producing ozone), and UV light in air (producing ozone) supplemented by additional ozone in the incoming air. The effect of the exposure time of the PP and PET to each treatment was studied. The samples were analyzed by X-ray photoelectron spectroscopy (XPS) to determine the surface composition, and by dynamic contact angle to determine the water wettability. The results showed that the effect of the treament was dependent on the properties of the exposed polymer, with PET being more sensitive to the UV light and PP being more sensitive to the reactive species in the gas. The exposure times studied ranged from 1 to 90 min. By monitoring the oxygen uptake levels, we were able to determine that surface modification occurred within minutes. The possible reactive species and mechanisms are discussed.

1898. Grundke, K., H.-J. Jacobasch, F. Simon, and S.T. Schneider, “Physico-chemical properties of surface-modified polymers,” J. Adhesion Science and Technology, 9, 327-350, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 431-454, VSP, May 1996).

Contact angle, electrokinetic, and X-ray photoelectron spectroscopic (XPS) measurements have been used to study the surface properties of flame- and oxygen plasma-pretreated polypropylene/ ethylenepropylene-diene monomer rubber (PP-EPDM) blends and of ethylene vinyl acetate (EVA) copolymers grafted with carboxyl group-containing monomers. The contact angles of pure test liquids (water, methylene iodide, and ethylene glycol) were used to calculate the dispersive and polar components of the surface free energy according to Owens and Wendt, and the acid-base parameters according to Van Oss and co-workers. In addition, the acid-base properties of the differently pretreated polymers could be evaluated quantitatively by measuring the zeta potential vs. the pH in a 10-3 mol/l KCI solution. The zeta potential measurements show that oxygen plasma-treated PP-EPDM and grafted EVA indicate an acidic surface character, whereas the flame-treated PP-EPDM blends possess both acidic and basic surface groups. The basic surface character of flame-treated PP-EPDM injection-moulded sheets could be enhanced by the presence of sterically hindered amine light stabilizers in the blend. This increase in the basic surface character was not only proved by zeta potential measurements, but also by the contact angle method according to Van Oss and co-workers. These results correlate with an increase of the oxygen content in the surface region and the occurrence of nitrogen-containing functional groups detected by XPS. The plasma-treated surface region of PP-EPDM blends contained an increased amount of carboxyl group-containing species (O=C-O). Flame-treated surfaces with additional light stabilizers in the blend indicated an increased concentration C-OH groups together with protonated nitrogen in the surface region. It was found that the adhesion strength of water-based primers was higher at these surfaces. A general interrelation between the acidic and basic parameters determined by zeta potential measurements, on the one hand, and the acidic and basic parameters determined by contact angle measurements, on the other hand, could not be found. A direct correlation was found between the increasing acidic character of EVA grafted with different amounts of carboxyl group-containing monomers and the decrease in the receding contact angle.

1899. Greenwood, O.D., R.D. Boyd, J. Hopkins, and J.P.S. Badyal, “Atmospheric silent discharge versus low-pressure plasma treatment of polyethylene, polypropylene, polyisobutylene, and polystyrene,” J. Adhesion Science and Technology, 9, 311-326, (1995) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 17-32, VSP, May 1996).

Polyethylene, polypropylene, polyisobutylene, and polystyrene films have been exposed to high- and low-pressure non-equilibrium electrical air discharges. The modified surfaces have been characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Atmospheric silent discharge treatment causes a greater level of topographical disruption, whereas surface oxygenation is dependent on the chemical nature of the polymer substrate and its reactivity towards the electrical discharge medium. Oxygen incorporation occurs much more readily for the unsaturated polystyrene surface than for the saturated polyethylene, polypropylene, and polyisobutylene substrates.

1900. Lunkwitz, K., W. Burger, U. Lappan, H.-J. Brink, and A. Ferse, “Surface modification of fluoropolymers,” J. Adhesion Science and Technology, 9, 297-310, (1995).

COF groups are formed by electron irradiation of PTFE [poly(tetrafluoroethylene)] powders in air, especially at the surface and in near-surface regions which can be easily hydrolysed to carboxyl groups by air humidity. The application of special additives during irradiation leads to modified micropowders. Fourier transform infrared (FTIR) spectroscopy enables the detection of carboxyl and COF groups. γ-Irradiation of PTFE mainly causes degradation of the polymer; the concentration of carboxyl groups is much lower. Carboxylated micropowders created via radiation treatment retain the essential properties of PTFE. With increasing radiation dose, the increasing concentration of functional groups in the micropowders causes an increase in the surface free energy. This diminishes the strong water and oil repellency of PTFE in such a way that homogeneous incorporation into aqueous and organic liquids or other polymers is possible. So, the special properties of PTFE can be made effective in these media. Modified PTFE micropowders have been successfully tested in many application areas. The aim of our present work was to increase the concentration and vary the nature of functional groups by radiation-chemical methods or chemical conversion of COF groups (polymer-analogous reactions). A highly modified PTFE powder was used to reduce the repellent properties of PTFE diaphragms for application in brine electrolysis. The COF groups of the micropowders were modified by γ-aminopropyltriethoxysilane. The irradiation of FEP [poly(tetrafluoroethylene-co-hexafluoropropylene)] and PFA [poly(tetrafluoroethylene-co-perfluoroalkylvinylether)] yields products which contain a higher content of carboxyl groups than PTFE.

1901. Shanahan, M.E.R., and J.M. Di Meglio, “Wetting hysteresis: Effects due to shadowing,” J. Adhesion Science and Technology, 8, 1371-1380, (1994) (also in Fundamentals of Adhesion and Interfaces, D.S. Rimai, L.P. DeMejo, and K.L. Mittal, eds., p. 225-234, VSP, Dec 1995).

Wetting hysteresis due to isolated surface heterogeneities is now fairly well understood but when the solid presents a population of defects, complex cooperative effects between neighbours may exist. One such effect is that of ‘shadowing’, in which a proportion of the flaws near the triple line, and which would otherwise contribute to hysteresis, are masked by already existing deformations to the wetting front caused by neighbouring heterogeneities. This renders them inactive and, as a result, the hysteretic wetting force is only expected to be a linear function of density for sparse populations. Theoretical predictions are compared with experimental results obtained with model heterogeneous surfaces consisting of overhead projector transparencies bestrewn with circular ink spots - the defects. Agreement is found to be satisfactory when intrinsic angles on both the homogeneous solid and the flaws are finite, whereas the concordance is less satisfactory when the contact angle of the liquid on the homogeneous solid is zero.

1903. Zhang, J.-Y., H. Esrom, U. Kogelschatz, and G. Emig, “Modification of polymers with UV excimer radiation from lasers,” J. Adhesion Science and Technology, 8, 1179-1210, (1994).

Photochemical dry etching and surface modification of various polymers, e.g. polymethylmethacrylate (PMMA), polyimide (PI), polyethyleneterephthalate (PET) and polytetrafluoroethylene (PTFE) were investigated with coherent and incoherent excimer UV sources. Ablation rates of PMMA were measured as a function of laser fluence and laser pulse at the wavelength λ = 248 nm (KrF*). Decomposition and etch rates of PMMA and PI were determined as a function of UV intensity and exposure time at three different wavelengths λ = 172 nm (Xe*2), λ = 222 nm (KrCl*) and λ = 308 nm (XeCl*). The transmittance of the polymeric films was determined with a UV-spectrophotometer after different exposure times. The morphology of the exposed polymers was investigated with scanning electron microscopy (SEM). The gaseous products occurring during UV exposure were measured using mass spectrometry (MS). Chemical surface changes of the photoetched PMMA were determined by X-ray photoelectron spectroscopy (XPS). The mechanism of the photo-oxidation process of PMMA is discussed. The etching of PMMA can be explained as a result of extensive photo-oxidation. The results are compared with those obtained from mercury lamp and excimer laser experiments. Good adhesion of electrolessly deposited metal layers was achieved by irradiation of the polymeric surfaces from incoherent UV source before depositing the metal layer.

1904. Collaud, M., P. Groening, S. Nowak, and L. Schlapbach, “Plasma treatment of polymers: The effect of the plasma parameters on the chemical, physical, and morphological states of the polymer surface and on the metal-polymer surface interface,” J. Adhesion Science and Technology, 8, 1115-1127, (1994) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 87-100, VSP, May 1996).

1905. Owen, M.J., and P.J. Smith, “Plasma treatment of polydimethylsiloxane,” J. Adhesion Science and Technology, 8, 1063-1075, (1994) (also in Polymer Surface Modification: Relevance to Adhesion, K.L. Mittal, ed., p. 3-16, VSP, May 1996).

1906. Poncin-Epaillard, F., B. Chevet, and J.-C. Brosse, “Reactivity of a polypropylene surface modified in a nitrogen plasma,” J. Adhesion Science and Technology, 8, 455-468, (1994) (also in Plasma Surface Modification of Polymers: Relevance to Adhesion, M. Strobel, C.S. Lyons, and K.L. Mittal, eds., p. 167-182, VSP, Oct 1994).

1908. Inagaki, N., S. Tasaka, and K. Hibi, “Improved adhesion between plasma-treated polyimide film and evaporated copper,” J. Adhesion Science and Technology, 8, 395-410, (1994) (also in Plasma Surface Modification of Polymers: Relevance to Adhesion, M. Strobel, C.S. Lyons, and K.L. Mittal, eds., p. 275-290, VSP, Oct 1994).

1909. Khairallah, Y., F. Arefi, J. Amouroux, D. Leonard, and P. Bertrand, “Surface fluorination of polyethylene films by different glow discharges. Effects of frequency and electrode configuration,” J. Adhesion Science and Technology, 8, 363-381, (1994) (also in Plasma Surface Modification of Polymers: Relevance to Adhesion, M. Strobel, C.S. Lyons, and K.L. Mittal, eds., p. 147-166, VSP, Oct 1994).

1910. Oh, T.S., L.P. Buchwalter, and J. Kim, “Adhesion of polyimides to ceramic substrates: Role of acid-base interactions,” J. Adhesion Science and Technology, 4, 303-317, (1990) (also in Acid-Base Interactions: Relevance to Adhesion Science and Technology, K.L. Mittal and H.R. Anderson Jr., eds., p. 287-302, VSP, Nov 1991).

Adhesion of polyimides to ceramic substrates such as SiO2, Al2O3' and MgO, and interfacial interactions were studied using XPS, SEM, and the peel test. The peel strength of polyimides on SiO2 and Al2O3 is almost identical and higher than that on MgO at the same polyimide thickness. Contrary to the failure within the polyimides on SiO2 and Al2O3' Mg was found on the peeled PMDA-ODA acid-derived polyimide surface, implying weakening of MgO by interfacial reactions with polyamic acid. With the neutral polyamic ethyl ester, the locus of failure on MgO was changed to the apparent weak boundary layer of the ester-derived polyimide. On SiO2 and Al2O3 the peel crack propagated with a discontinuous stick-slip process. The constant interspacing between transverse stick-slip striations on the peeled polyimide surfaces has confirmed that plastic bending is the major energy dissipation process with a minimal contribution from tensile loading.

1911. Whitesides, G.M., H.A. Biebuyck, J.P. Folkers, and K.L. Prime, “Acid-base interactions in wetting,” J. Adhesion Science and Technology, 5, 57-69, (1991) (also in Acid-Base Interactions: Relevance to Adhesion Science and Technology, K.L. Mittal and H.R. Anderson Jr., eds., p. 229-242, VSP, Nov 1991).

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.

1912. Webster, H.F., and J.P. Wightman, “Effects of oxygen and ammonia plasma treatment on polypropylene sulfide thin films and their interaction with epoxy adhesive,” J. Adhesion Science and Technology, 5, 93-106, (1991) (also in Acid-Base Interactions: Relevance to Adhesion Science and Technology, K.L. Mittal and H.R. Anderson Jr., eds., p. 329-342, VSP, Nov 1991).

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 CSingle Bond O and CDouble BondO 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.

1913. Padday, J.F., “Spreading, wetting, and contact angles,” J. Adhesion Science and Technology, 6, 1347-1358, (1992) (also in Contact Angle, Wettability and Adhesion: Festschrift in Honor of Professor Robert J. Good, K.L. Mittal, ed., p. 97-108, VSP, Nov 1993).

The thermodynamic energies associated with conventional wetting, spreading, adhesion, cohesion, and disjoining pressure, as defined in classical equations, are re-examined for their significance in a force field. They are then converted into dimensionless form such that the equilibrium properties of both wetting and spreading all fall on the same line when the dimcnsionless spreading coefficient is plotted as a function of the dimensionless work of adhesion. The effects of a force field such as gravity are examined and it is further shown that spreading is always thickness-dependent, whether in a force field or in a gravity-free field. Non-equilibrium processes such as autophobicity are shown on the same dimensionless plot and indicate clearly that the speed with which the process approaches equilibrium depends on the difference between the initial and equilibrium spreading coefficients. All these processes are expressed in terms of a dimensionless group Pn, the reduced wetting energy, which, when lying between the values of + 1 and -1, equals the cosine of the contact angle, . The implication of this approach to non-equilibrium processes is discussed.

1914. Xie, X., T.R. Gengenbach, and H.J. Griesser, “Changes in wettability with time of plasma-modified perfluorinated polymers,” J. Adhesion Science and Technology, 6, 1411-1431, (1992) (also in Contact Angle, Wettability and Adhesion: Festschrift in Honor of Professor Robert J. Good, K.L. Mittal, ed., p. 509-529, VSP, Nov 1993).

Treatment of fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE) by plasmas established in water vapour or ammonia gas enabled the rapid and facile modification of their surface chemistries. Under comparable plasma conditions, ammonia plasma exposures produced considerably lower air/water contact angles than water vapour plasmas. On storage of samples in air at ambient temperature, contact angles increased markedly within a few days on ammonia plasma-treated samples but remained constant over many weeks on water plasma-treated surfaces. Angle-dependent X-ray photoelectron spectroscopy (XPS) demonstrated a very low depth of modification in the case of ammonia plasma exposure, whereas the oxygen content of water plasma-treated samples was invariant with depth within the XPS analysis region. The long-term stability of water plasma-treated fluorocarbon polymer surfaces is believed to be due to this deep modification which prevents polymer chain reorientation, whereas the shallow modification in ammonia plasmas allows the rapid partial burial of the newly attached chemical groups inside the polymer. When ammonia plasma-treated samples stored in air were immersed in water, the contact angles remained constant, suggesting that the buried groups could not resurface. Contact angle measurements provided a simple and sensitive method for studying the time-dependent reduction in plasma treatment effects and the segmental mobility of modified fluorocarbon polymer surfaces; very shallow reorientation movements can be detected.

2165. Wolkenhauer, A., G. Avramidis, E. Hausweld, H. Militz, and W. Viol, “Plasma treatment of wood-plastic composites to enhance their adhesion properties,” J. Adhesion Science and Technology, 22, 2025-2037, (2008).

In this study, the adhesion properties of adhesives and paints on wood–plastic composites (WPCs) after plasma treatment at atmospheric pressure and ambient air were investigated. Surface energy determination by means of contact angle measurements according to the Owens–Wendt approach and atomic force microscopy to detect changes in surface topography were carried out. An increase in the polar component of surface energy and an increase in surface roughness after plasma treatment were detected, indicating enhanced bond strength. To confirm these results, bond strength tests were conducted. By tensile bond strength tests, increased adhesion of waterborne, solventborne and oil-based paints on plasma treated surfaces was found. Furthermore, by shear bond strength tests, an increase in bond strength of plasma treated WPCs bonded with poly(vinyl acetate) and polyurethane adhesives was ascertained.

2166. Bialopiotrowicz, T., “Influence of erroneous data on the results of calculations from acid-base surface free energy theories, III: Solution of a three-equation set in the case of homoscedastic error,” J. Adhesion Science and Technology, 23, 799-813, (2009).

The van Oss–Chaudhury–Good theory (vOCGT) was checked for a large artificial set of work of adhesion input data calculated for 15 solids and 300 liquids. Numerical values of LW component and acid (A) and base (B) parameters were assigned to 15 solids. These 15 solids were grouped in 5 sets of 3 solids in each. Also numerical values of LW component and A and B parameters were assigned to 300 liquids (three sets of 100 liquids in each). Data for these solids and liquids were especially selected to represent real types of materials encountered in practice. For all 15 solids and 300 liquids the work of adhesion values were calculated and these values were assumed to be error-free. Next, new values of the work of adhesion were obtained by adding a random homoscedastic error (A vector of random variables is homoscedastic if it has the same finite variance.) of the normal distribution (Also called the Gaussian distribution — it is continuous probability distribution defined by two parameters: the mean and variance (standard deviation squared, σ2).), belonging to 8 distributions of a mean value equal to the error-free work of adhesion value and standard deviations of 0.5, 1, 2, 5, 7, 10, 15 and 20 mJ/m2. The LW components and A and B parameters for these solids were back-calculated for each error level. Two different methods for the solution of a 3-equation set were used and they gave practically the same results irrespective of the error level and liquids and solids used. It was found that there existed a linear correlation between the RMSE (root mean square error) of the solution and the standard deviation of the work of adhesion data. This correlation was highly significant (with a correlation coefficient higher than 0.999) and was true separately for LW component, A and B parameters as well as for the total solution vector (i.e., combinedly for the LW component, A and B parameters). The RMSE values of the total solution vector (having as elements values of the LW component, A and B parameters) as well as separately for LW component and A and B parameters were correlated with the condition number of a given 3-equation set. A very good correlation was found only for the total solution, much worse for A or B parameters, and practically there was a lack of correlation for the LW component. Based on the correlation between the RMSE and the standard deviation of the work of adhesion it was possible to determine what should have been the maximal standard deviation of the work of adhesion if the calculated value of a given LW component or A or B parameter did not differ by more than 1 mJ/m2 from an error-free (true) value.

2167. Kurihara, Y., H. Ohata, M. Kawaguchi, S. Yamazaki, and K. Kimura, “Improvement of adhesion between liquid crystalline polyester films by plasma treatment,” J. Adhesion Science and Technology, 22, 1985-2002, (2008).

Surface modification of thermotropic liquid crystalline aromatic polyester (LCP) films was carried out by low-pressure plasma treatment to improve the initial adhesion as well as the long-term adhesion reliability, a measure of durability between the LCP films used as substrates for printed circuit boards. Plasma irradiation was carried out in various plasma gases with different plasma modes such as reactive-ion-etching, and direct-plasma (DP) with pressures ranging from 6.7 Pa to 26.6 Pa. The introduction of polar groups on the film surface such as phenolic hydroxyl groups and carboxyl groups enhanced the initial adhesion by increased chemical interaction. However, if the concentration of polar groups became too high, the longterm adhesion reliability estimated by the pressure cooker test was degraded due to the acceleration of the penetration of water molecules into the interface. A large surface roughness was also effective in preventing the decrease in the long-term adhesion reliability. However, too much increase in surface roughness decreases the long-term adhesion reliability. The DP-treatment in the O2 atmosphere at a gas pressure of 6.7 Pa was found to be the best plasma condition for both the initial adhesion as well as the long-term adhesion reliability between the LCP films.

2250. Simor, M., Y. Creyghton, A. Wypkema, and J. Zemek, “The influence of surface DBD plasma treatment on the adhesion of coatings to high-tech textiles,” J. Adhesion Science and Technology, 24, 77-97, (2010).

The surface of high-performance poly(ethylene terephthalate) (PET) fibers is difficult to wet and impossible to chemically bond to different matrices. Sizing applied on the fiber surface usually improves fiber wetting, but prevents good adhesion between a matrix and the fiber surface. The present study demonstrates that the plasma treatment performed by Surface dielectric barrier discharge (Surface DBD) can lead to improved adhesion between sized PET fabric and polyurethane (PU) or poly(vinyl chloride) (PVC) coatings. Moreover, it points out that this plasma treatment can outperform current state-of-the-art adhesion-promoting treatment. Plasma treatment of sized fabric was carried out in various gaseous atmospheres, namely N2, N2 + H2O, N2 + AAc (acrylic acid) and CO2. The adhesion was assessed by a peel test, while wettability was evaluated using strike-through time and wicking rate tests. Changes in fiber surface morphology and chemical composition were determined using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Only the CO2 plasma treatment resulted in improved adhesion. As indicated by the analyses, increased surface roughness and the incorporation of specific oxygen-containing groups were responsible for enhanced adhesion. The results presented were obtained using a plasma reactor suitable only for batch-wise treatment. As continuous treatment is expected to provide higher homogeneity and, therefore, even better adhesion, a scaled-up Surface DBD plasma system allowing continuous treatment is presented as well.

2251. Tuominen, M., J. Lahti, J. Lavonen, T. Penttinen, J.P. Rasanen, J. Kuusipalo, “The influence of flame, corona, and atmospheric plasma treatments on surface properties and digital print quality of extrusion coated paper,” J. Adhesion Science and Technology, 24, 471-492, (2010).

Polymer and paper structures have been successfully utilized in several fields, especially in the packaging industry. Together with barrier properties, printability is an important property in packaging applications. From the point of view of printing, the dense and impervious structure of extrusion coatings is challenging. Flame, corona and atmospheric plasma treatments were used to modify the surface of low density polyethylene (LDPE) and polypropylene (PP) and the influence of these surface modifications on print quality, i.e., toner adhesion and visual quality was studied. The traditional surface treatment methods, i.e., flame and corona treatments, increased the surface energy by introducing oxygen containing functional groups on the surfaces of LDPE and PP more than helium and argon plasma treatments. Only in the case of flame treatment, the higher surface energy and oxidation level led to better print quality, i.e., toner adhesion and visual quality, than the plasma treatments. The morphological changes observed on LDPE surface after flame treatment are partly responsible for the improved print quality. Atmospheric plasma treatments improved the print quality of LDPE and PP surfaces more than corona treatment. The electret phenomenon observed on LDPE and PP surfaces only after corona treatment is the most likely reason for the high print mottling and low visual quality of corona treated surface.

2252. Klages, C.-P., A. Hinze. P. Willich, and M. Thomas, “Atmospheric-pressure plasma amination of polymer surfaces,” J. Adhesion Science and Technology, 24, 1167-1180, (2010).

Using dielectric barrier discharges (DBDs) in suitable gas atmospheres, appreciable densities of amino groups can be generated on polymer surfaces. After the introduction and a few remarks on analytical methods for the determination of functional groups densities, this paper presents a short summary of recent studies on the mechanism of the polymer surface amination from nitrogen and nitrogen/hydrogen mixtures, and possible relevant precursor species. Combination of chemical derivatization with quantitative FT-IR spectroscopy was employed for the determination of primary amino groups densities introduced on polyolefin surfaces in DBD afterglows in N2 and N2 + H2 mixtures. Owing to the possibility to generate atmospheric-pressure plasmas in sub-mm3 volumes, DBD plasmas can be used to modify polymer surfaces area selectively: a new process termed 'plasma printing' can be applied for the achievement of micropatterned surface modifications, such as hydrophilization/hydrophobization or chemical functionalization. Direct-patterning polymer surface modification processes are of interest for biochemical/biomedical applications as well as for polymer electronics. Two examples are presented in more detail: • the area-selective plasma amination of carbon-filled polypropylene minidiscs to manufacture microarrays with peptide libraries utilizing parallel combinatorial chemical synthesis, and •the continuous treatment of polymer foils by means of reel-to-reel patterned plasma amination for the subsequent electroless copper metallization, leading to a fast and highly efficient process for the manufacture of structured metallizations for flexible printed circuits or RFID antennas.

2253. Encinas, N., M. Pantoja, J. Abenojar, and M.A. Martinez, “Control of wettability of polymers by surface roughness modification,” J. Adhesion Science and Technology, 24, 1869-1883, (2010).

Most polymeric materials, particularly polyolefins and their derivatives, present a low surface energy which is the cause of their poor wettability and limits processes such as adhesive bonding, painting, or metalizing. Many methods have been developed and used to modify polymer surfaces for improved wetting, including mechanical treatments, wet-chemical treatments with strong acids or bases, and exposure to flames or corona discharge. In this paper the improvement of wetting properties of several polymeric materials widely used in the automotive industry, such as high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and silicone, is studied by means of surface mechanical abrasion using sandpapers of different grain sizes (1000, 180 and 80). Measurements of the surface roughness are performed using a Hommel Tester T8000 device equipped with a diamond stylus, which provides data on the arithmetic average roughness Ra parameter and Abbott–Firestone curve. Variations in the polymers surface energy (SE) are estimated through contact angle measurements using five test liquids of different polarities. Both components of the SE, dispersion (σD) and polar (σP), as well as total (σT) at different conditions of treatment are analyzed using the Owens–Wendt–Rabel–Kaelble (OWRK) method. Morphological changes induced in the surface are analyzed by Scanning Electron Microscopy (SEM). Additionally, measurements of the static friction coefficient (μs) are carried out by the standard method ASTM D 1894-08. A slight enhancement in surface wettability is found with the mechanical abrasion pre-treatment from the SE increase. Finally, a higher value of μs is achieved for the abraded specimens as the normal force acting onto the system is increased.

2254. Gotoh, K., A. Yasukawa, and K. Taniguchi, “Water contact angles on poly(ethylene terephthalate) film exposed to atmospheric pressure plasma,” J. Adhesion Science and Technology, 25, 307-322, (2011).

The poly(ethylene terephthalate), PET, film was exposed to atmospheric pressure plasma under various plasma processing parameters. The wettability of the PET film immediately after the exposure and after storage in air, which was determined by the sessile drop method, was strongly dependent on the plasma processing parameters. The contact angle hysteresis on the plasma-exposed PET film was examined by the Wilhelmy method. It was found that the hydrophobic recovery of the PET surface on storage after the plasma exposure was observed only for the advancing contact angle and that the receding angle remained almost the same. These experimental findings were explained on the basis of the calculation by Johnson and Dettre for the advancing and receding contact angles on model heterogeneous surfaces.

2255. Langowskia, H.-C., “Surface modification of polymer films for improved adhesion of deposited metal layers,” J. Adhesion Science and Technology, 25, 223-243, (2011).

Plasma treatment and vacuum Al deposition on films from biaxially oriented polypropylene is a multistep large scale industrial process, mainly ending up in packaging film laminates. As atmospheric plasma treatment processes suffer from lack of reproducibility, low pressure plasma treatment processes that can be operated in-line with the metal deposition are being developed. Process development is difficult, because the final packaging film laminate has to deliver optimum properties of adhesion as well as of the barrier against oxygen and water vapor permeation. As a typical production run involves tens of thousands to up to one hundred thousand square meters of film, experiments on an industrial scale are expensive, so smaller scale experimental processes are needed, which so far do not match well enough with industrial process characteristics. Moreover, bonding mechanisms between the treated substrate film and the deposited Al layer are not sufficiently understood. This paper describes the sequence in development and optimization of substrate films and plasma treatment that has been performed on an experimental as well as on an industrial scale. A sufficient correlation between experimental and industrial scales was achieved, which helps to perform development and optimization on a small scale before scaling up to industrial processes. However, improvement is still needed both in fundamental understanding of the aluminum–polypropylene interface as well as in experimental equipment and methodology.

2256. Dankovich, T.A., and D.G. Gray, “Contact angle measurements on smooth nanocrystalline cellulose (I) thin films,” J. Adhesion Science and Technology, 25, 699-708, (2011).

Interactions of cellulose fiber surfaces with water and other liquids depend on surface morphology as well as intrinsic material properties. Smooth nanocrystalline cellulose (I) films can be used as models to study surface phenomena, where the effects of surface morphology and roughness are minimized. Contact angle measurements are particularly sensitive to surface roughness. In this work, we measured the advancing and receding contact angles for water on thin model cellulose (I) and regenerated cellulose (II) films. The advancing and receding contact angles on model cellulose (I) surfaces were lower than on cellulose (II) surfaces, and the contact angle hysteresis was also lower for the smooth model cellulose (I) surfaces prepared from nanocrystal suspensions. The surface free energy was evaluated for the various cellulose surfaces from contact angle measurements.

2257. Ruiz-Cabello, F.J.M., M.A. Rodriguez-Valverde, and M.A. Cabrerizo-Vilchez, “Contact angle hysteresis on polymer surfaces: An experimental study,” J. Adhesion Science and Technology, 25, 2039-2049, (2011).

In order to characterize a solid surface, the commonly used approach is to measure the advancing and receding contact angles, i.e., the contact angle hysteresis. However, often an estimate of the average wettability of the solid–liquid system is required, which involves both the dry and wetted states of the surface. In this work, we measured advancing and receding contact angles on six polymer surfaces (polystyrene, poly(ethylene terephthalate), poly(methyl methacrylate), polycarbonate, unplasticized poly(vinyl chloride), and poly(tetrafluoroethylene)) with water, ethylene glycol and formamide using the sessile drop and captive bubble methods. We observed a general disagreement between these two methods in the advancing and receding contact angles values and the average contact angle determined separately by each method, although the contact angle hysteresis range mostly agreed. Surface mobility, swelling or liquid penetration might explain this behaviour. However, we found that the 'cross' averages of the advancing and receding angles coincided. This finding suggests that the cross-averaged angle might be a meaningful contact angle for polymer–liquid systems. Hence, we recommend using both the sessile drop and captive bubble methods.


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