XPS Studies of Polymer Surface Modifications and Adhesion Mechanisms

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.     

Investigation of the locus and mechanism of adhesion failure during peel tests on copper-chromium coated polyimide systems

Auger depth profiling and X-ray photoelectron spectroscopy (XPS) techniques were used to elucidate the adhesion process in copper-chromium coated polyimide. Metal layers were sputter-deposited on polyimide, which was modified by glow discharge under various conditions. The results showed that glow discharge could increase the chromium coverage on polyimide, improve or deteriorate adhesion at various interfaces, and change the locus of adhesion failure. Auger depth profiles could explain the various failure modes by revealing the chemistry at each interface. Additional information was provided by XPS results obtained from surfaces generated by failure. The effects of accelerated aging on this metal-polymer system were also studied.     

Improved adhesion of low-density polyethylene/EVA foams using different surface treatments

The adhesion between a polyurethane (PU) adhesive and four foams containing different low-density polyethylene (LDPE)/ethylene vinyl acetate (EVA) blends was improved by using different surface treatments. UV-ozone, corona discharge, and low-pressure oxygen plasma treatments for different times were used to increase the surface energy of the foams. The low-pressure oxygen plasma was the most successful surface treatment to promote the adhesion of the foams. A reduced length of treatment was needed to improve the adhesion of the foams containing higher EVA content. The surface treatments produced a noticeable decrease in contact angle values due mainly to the creation of different carbon–oxygen moieties and to the formation of cracks/heterogeneities on the foams surfaces. After oxygen plasma, removal of non-polar material from EVA surfaces allowed to expose acetate groups which are likely to be important for increasing the adhesion of the foams.     

Determination of the degree of adhesion in plasma-treated polyethylene/paper laminates

It is shown that an oxygen or ammonia plasma treatment significantly improves the adhesion between low density polyethylene (LDPE) and cellulose. Plasma treatment of the polymer was more effective than treatment of the paper, and ammonia plasma seemed somewhat more effective than the oxygen treatment. The adhesion between polyethylene and cellulose was evaluated at room temperature using a non-linear double cantilever beam test. The effect of the discharge treatment on the surface composition of LDPE and cellulose was characterized using electron spectroscopy for chemical analysis. The improved adhesion may be due to an improved penetration of the porous paper surface by the LDPE-melt and to an increase in the interfacial attraction forces as a result of the introduction of polar groups in the surfaces.     

Immobilization of polyethylene oxide surfactants for non-fouling biomaterial surfaces using an argon glow discharge treatment

A non-fouling (protein-resistant) polymer surface is achieved by the covalent immobilization of polyethylene oxide (PEO) surfactants using an inert gas discharge treatment. Treated surfaces have been characterized using electron spectroscopy for chemical analysis (ESCA), static secondary ion mass spectrometry (SSIMS), water contact angle measurement, fibrinogen adsorption, and platelet adhesion. This paper is intended to review our recent work in using this simple surface modification process to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in particular.     

Role of helium plasma pretreatment in the stability of the wettability,adhesion, and mechanical properties of ammonia plasma-treated polymers. Application to the Al-polypropylene system

The stability of a plasma-treated polymer surface is an important issue, but very often a surface rendered wettable by the treatment is found to revert to a less wettable state with time. The purpose of this work was to minimize the ageing phenomenon by stabilizing the surface layer via crosslinking using an inert gas discharge. The stability of the wettability, adhesion, and mechanical properties of treated polypropylene (PP) has been investigated by a comparative study of two different plasma treatments (i.e. an NH₃ plasma and a He plasma pretreatment carried out before the NH₃ plasma; He plasma is well known to crosslink polymer surfaces). The aluminium-polypropylene (Al-PP) interfaces present very different features depending on the gas treatment. The role of the treatment time has been pointed out and under our experimental conditions, a treatment time of the order of a few seconds seems to be an overtreatment leading to degradation of the adhesion and mechanical properties. A broad interphase was obtained for an NH₃-overtreated PP, in contrast to the abrupt one formed when pretreated with the helium gas followed by NH₃ treatment. Good correlations between wettability and mechanical properties with adhesion measurements were established.     

Effects of corona treatment on composite formation. Adhesion between incompatible polymers

Polypropylene–nylon 6 10 composites were prepared by the in situ polymerization of the nylon monomers on polypropylene films. The adhesion between the nylon and the polypropylene was markedly improved by a brief corona discharge treatment of the films in nitrogen prior to coating. This improvement was demonstrated by an increase in the peel strength of the nylon coating and a decrease in brittleness of photo-oxidized compesites when corona treatment was used. Adhesive bonding between the nylon and substrate was sufficiently strong to cause cohesive failure in the corona-treated polypropylene. Only interfacial failure was observed at untreated surfaces. These effects were demonstrated by electron microscopy of the surfaces produced in peel tests. The effects of corona treatment on adhesive bonding characteristics of surfaces are discussed in terms of the chemical and physical changes observed in treated surfaces.     

Thin Film Adhesion: Effect of Glow Discharge on Substrate

Glow discharge treatment of soda lime glass and silica substrates prior to depositing copper films in oil-free ultra high vacuum as well as in conventional vacuum has been found to increase film adhesion suggesting that gross contamination removal is not the only effect of importance in this method of improving film adhesion. There is evidence from Auger electron emission spectroscopy for sorption of gas from the discharge into the substrate surface layers. Desorption of gases from the substrate also appears to take place. A difference in electron emission from a variety of insulating surfaces following glow discharge was observed in a scanning electron microscope, the sign of the change depending on the nature of the discharge gas. No sputtered material from system components could be detected on the substrate and there was no detectable micro-roughening nor net surface electric charge.     

Thin Film Adhesion: Effect of Glow Discharge on Substrate

Glow discharge treatment of soda lime glass and silica substrates prior to depositing copper films in oil-free ultra high vacuum as well as in conventional vacuum has been found to increase film adhesion suggesting that gross contamination removal is not the only effect of importance in this method of improving film adhesion. There is evidence from Auger electron emission spectroscopy for sorption of gas from the discharge into the substrate surface layers. Desorption of gases from the substrate also appears to take place. A difference in electron emission from a variety of insulating surfaces following glow discharge was observed in a scanning electron microscope, the sign of the change depending on the nature of the discharge gas. No sputtered material from system components could be detected on the substrate and there was no detectable micro-roughening nor net surface electric charge.     

Investigation on the effectiveness of silane-based field level surface treatments of aluminum substrates for on-aircraft bonded repairs

The aim of this study was to assess the role of silane-based field level surface treatment processes on aluminum substrate with a film-type epoxy adhesive. Two silane-based surface treatment compositions based on a dilute aqueous solution of GPS (3-glycidoxypropyltrimethoxy silane) and a hybrid sol-gel solution of TPOZ (zirconium n-propaxine) and GPS were used. The surface morphology of the treated aluminum substrates was characterized by profilometry. Contact angle measurement and X-ray photoelectron spectroscopy were carried out to analyze surface wettability, which in turn is related to the surface chemistry and cleaning efficiency for bond performances. Quantitative evaluation of the joint strength and environmental durability presented that two GPS- and TPOZ-GPS based sol-gel coatings improved the initial adhesion and environmental durability, and hence can be considered promising alternative surface treatment techniques to the existing on-aircraft anodizing process for bonded repairs. Finally, observation of the fracture surfaces revealed that a loss of interfacial integrity between the adhesive and aluminum substrate was the dominant mechanism behind the permanent loss of adhesion; the loss of interfacial integrity induced the low-strength interfacial adhesion failure mode.     

Proliferation rate of fibroblast cells on polyethylene surfaces with wettability gradient

Surface wettability on anchorage‐dependent cells has an important role in cell growth rate. In our previous studies, we prepared a wettability gradient on polyethylene (PE) surfaces using corona discharge treatment from a knife‐type electrode whose power increased gradually along the sample length. The PE surfaces were oxidized gradually with increasing power and characterized by Fourier transform infrared spectroscopy, contact angle goniometry, and electron spectroscopy for chemical analysis. The purpose of this study is to determine the rate of proliferation on polymer surfaces with different wettability. The behavior of cell growth for NIH/3T3 fibroblast cells attached on the polymer surfaces with different hydrophilicity was investigated using wettability gradient PE surfaces prepared by corona discharge treatment. They were investigated for the number of grown cells from 24 to 60 h in terms of surface wettability. From the slope of cell number on PE gradient surface versus culture time, the proliferation rates (number of cell/cm² · h) were calculated. It was observed that the proliferation rate was increased more on positions with moderate hydrophilicity of the wettability gradient surface than on the more hydrophobic or hydrophilic positions, i.e., 1111 (number of cell/cm² · h) of 57° of water contact angle at the 2.5‐cm position (P < 0.05). This result seems closely related to the serum protein adsorption on the surface: the serum proteins were also adsorbed more on the moderately hydrophilic surface. In conclusion, surface wettability plays an important role in cell adhesion, spreading, and proliferation on the polymer surfaces. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 599–606, 2004     

Mechanism of corona treatment on polyolefin films

This paper reviews recent studies on the mechanism of corona treatment of polyolefin films, specifically the chemical and physical changes of this process and the self-adhesion mechanism. Corona discharge of polymeric films introduces polar groups into the surfaces, which increases the surface energy and, as a consequence, improves substrate wettability and adhesion. The main chemical mechanism of corona treatment is oxidation. In addition, corona treatment can crosslink surface regions and increase the film cohesive strength.     

Sulfonation of polymer surfaces - I. Improving adhesion of polypropylene and polystyrene to epoxy adhesives via gas phase sulfonation

A new method for surface treating polymers and polymer composites based on gas phase sulfonation has been shown to quickly and effectively increase wettability and adhesion to epoxy. A gas mixture containing a low concentration of sulfur trioxide in nitrogen (～ 1% v/v) was used to treat the surfaces of polypropylene and polystyrene films. The sulfonated surfaces were then neutralized with ammonium hydroxide. The effectiveness of sulfonation on the adhesion of these polymers to an epoxy adhesive was investigated using mechanical testing of sandwich lap-shear specimens. The lap-shear adhesive joint strength of epoxy to sulfonated polypropylene was compared with polypropylene treated with currently accepted surface treatments including chromic acid etching and flame treatment. Sulfonation greatly improves the adhesion of polypropylene to epoxy compared with other surface treatment techniques as measured by lap-shear strength. An optimum sulfonation treatment level was shown to exist for polypropylene. For polystyrene surfaces, it was shown that while sulfonation immediately increased wettability, it did not greatly improve its adhesion to epoxy; it did, however, significantly reduce the scatter in the ultimate strength values. Excess sulfonation treatment reduced the lap-shear strength for both polymers. X-ray photoelectron spectroscopic examination of the locus of failure of tested joints has shown that failure occurs in a weak boundary layer for these surface-treated polymers.     

Sanding vs. plasma treatment of aged wood: A comparison with respect to surface energy

To compare sanding and plasma treatment by dielectric barrier discharge (DBD) with respect to their effects on wood surface characteristics, beech, oak, spruce, and Oregon pine were investigated. For this purpose, the surface energy of aged, freshly sanded or plasma-treated surfaces was examined by contact angle measurement and calculation of work of adhesion. For both methods, sanding and plasma treatment, an increase in surface energy caused by a heavily increased polar part was found. Plasma treatment turned out to be superior to sanding. To see whether a combined treatment amplified this effect, a combination of sanding and plasma treatment was also investigated.     

Ultra-Thin Polymer Layer Deposition by Aerosol–Dielectric Barrier Discharge (DBD) and Electrospray Ionization (ESI) at Atmospheric Pressure

Polyolefins are chemically inert and do not adhere well to metals, polymers or inorganics. To overcome this problem, polyolefin surfaces were modified thermally, plasmachemically, or by flame treatment with different oxygen-containing groups, however, unfortunately, such treatments were accompanied by undesired, adhesion lowering polymer degradation. To solve this dilemma, solutions of synthetic polymers and copolymers were prepared, sprayed into the barrier discharge or electrosprayed without discharge and deposited as thin adhesion-promoting layers. The deposited polymer layers from poly(vinylamine), poly(ethylene glycol)–poly(vinyl alcohol) copolymers and poly(acrylic acid) were endowed with monotype functional groups. Using the aerosol — dielectric barrier discharge only a fraction of functional groups survived the deposition process in contrast to the electrospray in which all functional groups were retained.     

Modification of titanium surface with collagen and doxycycline as a new approach in dental implants

In this work, we investigate for the first time several issues involved in bio-adhesion process for a new type of chemically modified titanium surfaces (in their initial form and after collagen deposition), in order to assess their potential in dental implant surface modification. For this purpose, we studied the following: collagen adhesion, cytotoxicity, osteoblast cytomorphology, cell adhesion and proliferation, doxycycline embedding and modifications in the collagen film deposed on the metal surfaces, drug release from the collagen films. The improvement of adhesion between collagen film and titanium substrate, when hydroxyl and amino functional groups are assisting the surfaces was presented, all materials showing no cytotoxic effects as revealed by lactate dehydrogenase-based assay. The drug release from titanium–coll–doxy systems offers a dual mechanism of the delivery profile (burst release followed by moderate discharge of the antibiotic), with perspectives in soft tissue recovery postoperative stage.     

Investigation of the Electroadhesion Properties of Modified Polymer Surfaces

The effect of modification of the PTFE film surface on its adhesion and on mechanoelectron emission has been studied. If previously treated in glow discharge, PTFE gains significant adhesion strength up to the value 200 kg. per sq. sm. The increase of the adhesion strength is due to free peroxy radicals of the activated PTFE surface. The disjunction of the contact (treated PETF-polyepoxide) gives rise to emission with electron energies of 40 kev and initial intensity of 10⁴ imp/sec. It becomes possible to vary the electroadhesion properties of treated surfaces through reactions of free peroxy radicals of activated surface with diverse reagents. The adhesion strength and emission intensity depend on the nature of functional groups on the surface. There is a correlation between the adhesion strength and emission intensity for different modified surfaces. These results are in a good agreement with the electronic theory of adhesion, the strength of adhesive joint depending on the charge density of the electric double layer, produced by collective donor-acceptor interactions at the interface. The acceleration of electrons in high electric fields in gaps formed on breakdown of adhesion contact accounts for phenomena of high energy electron emission in vacuum. Intensity of the post-emission from freshly-formed breakdown surfaces is a function of time. Mechanoemission is considered to be a type of autoelectron emission, caused by the residual charge field of freshly formed surface.     

Adhesion Improvement of Glass-Fibre-Reinforced Polyester Composites by Gliding Arc Discharge Treatment

A gliding arc is a plasma that can be operated at atmospheric pressure and applied for plasma surface treatment for adhesion improvement. In the present work, glass-fibre-reinforced polyester plates were treated using an atmospheric pressure gliding arc discharge with an air flow to improve adhesion with a vinylester adhesive. The treatment improved wettability and increased the polar component of the surface energy and the density of oxygen-containing polar functional groups at the surfaces. Double cantilever beam specimens were prepared for fracture mechanics characterisation (fracture resistance as a function of nominal mode mixity) of the laminate adhesive interface. It was found that gliding arc treatment significantly increases the interfacial fracture energy and fracture resistance in comparison with a standard peel ply treatment, although the mixed mode fracture energy of the gliding arc treated specimen was not as high as that of the laminate itself.     

Synthetic paper from plastic waste: Influence of a surface treatment with corona discharge

Polymers generally have a chemically inert surface, are nonporous, and have low surface energy, which is characterized by their relatively weak adhesion to other materials. Numerous methods have been developed to modify polymer surfaces chemically. This study was an investigation of how the surface energy of synthetic paper (composite film) could be altered after treatment by corona discharge and of the stability of the surface energy of a film produced from postconsumer polypropylene (PP). A PP/filler (70/30 w/w) composite film was processed in a single‐screw extruder with a flat‐film dye. In this study, the film stretching rate and electric charge potential of the film surface treatment were the process parameters analyzed. The results showed that the conditions of the surface treatment by corona discharge and the presence of a filler (calcium carbonate) influenced some surface properties: the contact angle, surface energy, and ink adhesion. The surface energy of the composite film increased from 38.7 × 10^?3 N/m² without treatment to 52.3 × 10^?3 N/m² with treatment. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Plasma surface modification of carbon fibers: a review

The properties of the fiber/matrix interface in carbon fiber-reinforced composites play a dominant role in governing the overall performance of the composite materials. Understanding the surface characteristics of carbon fibers is a requirement for optimizing the fiber-matrix interfacial bond and for modifying fiber surfaces properly. Therefore, a variety of techniques for the surface treatment of carbon fibers have been developed to improve fiber-matrix adhesion as well as to enhance the processability and handling of these fibers. Many research groups have studied the effects of plasma treatments, correlating changes in surface chemistry with the interfacial shear strength. This article reviews the recent developments relative to the plasma surface modification of carbon fibers.