Multitechnique study of hexatriacontane surfaces modified by argon and oxygen RF plasmas: effect of treatment time and functionalization,and comparison with HDPE

Hexatriacontane (C₃₆H₇₄) has been used as a model molecule for the study of the surface modifications of high-density polyethylene (HDPE) in argon and oxygen radio-frequency (RF) plasmas. The combination of static secondary ion mass spectrometry (SIMS), ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements has constituted a powerful method for the investigation of the surface modifications induced by the plasma treatments. The surface degradation and functionalization are shown to depend on both the nature of the treated material and the nature of the plasma atmosphere. The SSIMS results obtained on plasma-modified hexatriacontane and HDPE are compared in order to identify the nature of the functionalities present at the plasma-treated surfaces. Finally, plasma treatment ¹⁸O atmosphere was performed on HDPE, C₃₆H₇₄, and polystyrene (PS). In that case, the isotopic specificity of both ISS and SIMS allowed the determination of the relative concentrations of ¹⁶O and ¹⁸O in relation to the probed depth and plasma atmosphere.     

Corona discharge treatment of EVAs with different vinyl acetate contents

Four ethylene vinyl acetate (EVA) co-polymers with different vinyl acetate (VA) contents (9–20 wt%) were treated with corona discharge to improve their adhesion to polychloroprene (PCP) adhesive. The thermal properties of the EVAs decreased as their VA content increased, caused by a decrease in crystallinity. The elastic and viscous moduli of the EVAs decreased and the temperature and modulus at the cross-over between these moduli decreased with increasing VA content. Contact-angle measurements (water), infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyse the surface modifications produced in the corona-discharge-treated EVAs. The corona discharge treatment produced improved wettability and created roughness and oxygen moieties on the EVA surfaces. The higher the VA content and the higher the corona energy, the more significant modifications were produced on the EVA surface. The VA content also affected the T-peel strength values of treated EVA/polychloroprene + isocyanate adhesive joints, as the values increased with increasing VA content. Mixed failure modes (interfacial + cohesive failure in the EVA) were obtained in the adhesive joints produced with corona discharge treated EVAs containing more than 9 wt% VA. The accelerated ageing of the joints did not affect the T-peel strength values, but the locus of failure in most cases became fully cohesive in the EVA, likely due to the higher extent of curing of the adhesive.     

Hydrophobic recovery of repeatedly plasma-treated silicone rubber. Part 2. A comparison of the hydrophobic recovery in air,water, or liquid nitrogen

Surfaces of medical grade silicone rubber (Q7-4750, Dow Corning) were modified by repeated (six times) RF plasma treatments using various discharge gases: oxygen, argon, carbon dioxide, and ammonia. The treated samples were stored for a period of 3 months in ambient air, water, or liquid nitrogen. Subsequently, the temporal behavior of the effects of the plasma treatment on the physicochemical surface properties of the silicone rubber was investigated using water contact angle measurements and X-ray photoelectron spectroscopy (XPS). Hydrophobic recovery during 3 months storage in ambient air was considerable and nearly complete for all four plasmas used. Hydrophobic recovery was almost completely suppressed during storage in liquid nitrogen, and only a minor increase of around 10° in advancing water contact angle was observed for all four plasma treatments. Also during storage of treated samples in water, hydrophobic recovery was minimal and initiated again by returning the treated samples to ambient air. XPS analyses showed that argon, carbon dioxide, and ammonia plasma-treated silicone rubber all had increased carbon percentages at the expense of oxygen and silicon after storage in water, or in liquid nitrogen, compared with after storage in ambient air. Interestingly, the carbon content of oxygen plasma-treated silicone rubber decreased during storage in water, or in liquid nitrogen, compared with storage in ambient air, while its oxygen and silicon percentages increased.     

Wood flour: New filler for the rubber processing industry. IV. Cure characteristics and mechanical properties of natural rubber compounds filled by non‐modified or corona treated wood flour

It is presented here how some cure characteristics and mechanical properties of rubber compounds based on nonpolar rubber, natural rubber (NR), are affected by filling with conifer wood flour, both nonmodified or corona treated in air or in ammonia. The corona treatment was performed at room temperature and varied both: treatment duration and voltage to find out the maximal reinforcing effect of the modified wood flour. The discharge voltage was varied at 10, 12, and 15 kV to include both (1) the range of relative low voltages (lower than 10 ÷ 12 kV in which the major active species arise from ionized and activated air oxygen and (2) the range of relative high voltages (12 kV and higher) in which the active species arise from simultaneously ionized and activated air oxygen and nitrogen. The chemical composition of the nonmodified and corona‐treated wood surface was controlled by X‐ray photoelectron spectroscopy (XPS) analysis. The activity of the modified wood flour was evaluated by comparative measurement of some basic mechanical parameters of rubber compounds, filled at equal level by nonmodified or plasma‐treated wood flour. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 651–658, 2006     

Surface degradation and hydrophobic recovery of polyolefins treated by air corona and nitrogen atmospheric pressure glow discharge

The surface degradation and production of low molecular weight oxidized materials (LMWOM) on biaxially oriented polypropylene (BOPP) and low‐density polyethylene (LDPE) films was investigated and compared for two different dielectric barrier discharge (DBD) treatment types, namely air corona and nitrogen atmospheric pressure glow discharge (N₂ APGD). Contact angle measurements, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses were performed in conjunction with rinsing the treated films in water. It is shown that N₂ APGD treatments of both polyolefins result in much less surface degradation, therefore, allowing for a significantly higher degree of functionalization and better wettability. Hydrophobic recovery of the treated films has also been studied by monitoring their surface energy (γ_s) over a period of time extending up to several months after treatment. Following both surface modification techniques, the treated polyolefin films were both found to undergo hydrophobic recovery; however, for N₂ APGD modified surfaces, γ_s ceases to decrease after a few days and attains a higher stable value than in the case of air corona treated films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1291–1303, 2004     

A comparison of gas-phase methods of modifying polymer surfaces

Oxidation is the most common surface modification of polymers. This paper presents a comparison of five gas-phase surface oxidation processes: corona discharge, flame, remote air plasma, ozone, and combined UV/ozone treatments. Well-characterized biaxially oriented films of polypropylene and poly(ethylene terephthalate) were treated by each of the five techniques. The surface-treated films were then analyzed by X-ray photoelectron spectroscopy (XPS or ESCA), contact-angle measurements, and Fourier-transform IR (FTIR) spectroscopy. Corona, flame, and remote-plasma processes rapidly oxidize polymer surfaces, attaining XPS O/C atomic ratios on polypropylene of greater than 0.10 in less than 0.5 s. In contrast, the various UV/ozone treatments require orders of magnitude greater exposure time to reach the same levels of surface oxidation. While corona treatment and flame treatment are well known as efficient means of oxidizing polymer surfaces, the ability of plasma treatments to rapidly oxidize polymers is not as widely appreciated. Of the treatments studied, flame treatment appears to be the 'shallowest'; that is, the oxygen incorporated by the treatment is most concentrated near the outer surface of the film. Corona and plasma treatments appear to penetrate somewhat deeper into the polymers. At the other extreme, the UV/ozone treatments reach farther into the bulk of the polymers.     

射流等离子体放电气氛对木塑复合材料表面性质的影响

利用空气、氮气、氧气3种不同气氛的射流等离子体放电对聚乙烯木塑复合材料（PE-WPC）表面进行处理以改善其胶接性能，其中空气、氮气、氧气气氛处理后的试样分别记为PE-WPC-A、PE-WPC-N和PE-WPC-O。通过对剪切强度、表面接触角、表面形貌、表面官能团以及表面元素含量的测试与表征，研究了不同气氛射流等离子体处理对PE-WPC表面物理化学性质的影响。研究结果表明：射流等离子体处理可以通过改变PE-WPC的表面性质，进而大幅度提高材料的胶接剪切强度，由未处理样品的0.62 MPa提高到处理后试样的11.32~13.79 MPa。对于胶接性能来说，不同气氛的射流等离子体处理效果差别不大；而对于处理后材料表面的微观结构，不同气氛射流等离子体的处理效果存在差别，氮气气氛处理以表面化学改性为主，在材料表面引入更多的含氮基团；氧气气氛处理以表面氧化刻蚀为主，在材料表面引入更多含氧基团；空气气氛处理则是以上2种作用的综合体现。     

PET薄膜表面性能及其与紫外光固化涂层粘接性能相关性研究

通过粘结性能实验研究了表面处理方法对PET薄膜/紫外光固化涂层体系粘接性能的影响,结果表明:表面处理提高了PET薄膜/涂层体系粘接强度。研究了PET表面处理对PET表面性能如表面形貌、光泽度(粗糙度)、表面张力、表面官能团含量的影响,结果表明:PET薄膜经电晕处理后,其表面粗糙度有增大的倾向,而涂层处理对PET薄膜表面粗糙度无影响;经电晕处理和底涂处理后,PET薄膜表面张力均有增加,底涂处理提高PET薄膜表面张力的幅度更大。研究了PET薄膜表面性能与剪切强度的关系,发现PET薄膜/涂层粘接体系剪切强度τ与PET薄膜表面张力γ具有线性关系。     

Surface and adhesion changes of atmospheric barrier discharge-treated polypropylene in air and nitrogen

Oriented polypropylene treated by atmospheric barrier discharges in air and nitrogen was investigated using several techniques: contact angle measurements, ATR-FT-IR spectroscopy and two adhesion tests based on the stripping of an applied ink layer. The activation in an air discharge was found to be much weaker compared to the activation in industrial grade nitrogen, particularly with respect to adhesion. The adhesion was found to be much better in nitrogen in spite of the common use of air in industrial 'corona discharges'. A new 'abrasive shear-stripping' (AS) test for ink coating adhesion was designed and performed. It was shown that the AS test was much more sensitive than the classical adhesive tape test and was sensitive enough to monitor ageing and overtreatment. The contact angle measurements did not correlate completely with the adhesion properties and could not monitor the overtreatment, while the ATR-FT-IR technique indicated changes just for overtreated foils.     

Comprehensive study of polymer fiber surface modifications Part 1: high‐fluence UV‐excimer‐laser‐induced structures

Recently, there has been great interest in physico‐chemical surface treatments for modifying polymer surfaces. Ultraviolet (UV)‐excimer‐laser irradiation of polymers is of particular interest. In this study, polyamide was irradiated by a 193 nm excimer laser with a fluence above its ablation threshold (high‐fluence). Morphological changes of the resulting samples were characterized by scanning electron microscopy (SEM) and tapping mode atomic force microscopy (TM‐AFM). Chemical modifications by laser treatment were studied by X‐ray photoelectron spectroscopy (XPS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and chemical force microscopy (CFM). Topographical results indicated that ‘ripple‐like’ structures of micrometer size were formed after laser irradiation. XPS and Tof‐SIMS results showed that bond scission occurred on the polymer surface under the action of high‐fluence. Changes in surface chemical properties of the laser‐irradiated polyamide were supported by CFM experiments. Copyright © 2004 Society of Chemical Industry     

An Investigation of the Effect of Plasma Treatment on the Surface Properties and Adhesion in Sheet Molded Composite (SMC) Material

Sheet molded composite was treated with different plasmas (oxygen, dry air, nitrogen, and argon). Plasma treatment of SMC alters the surface properties in a manner dependent on the type of plasma used and the time of treatment. The surface properties were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Fourier transform infrared spectros-copy (FTIR). A two-part urethane adhesive was used to prepare lap shear specimens. Untreated SMC, plasma-treated SMC, and primer-treated SMC were prepared, bonded and tested. The surface properties of the failed specimens were measured. The adhesion characteristics of SMC and the surface properties of the failed specimens were correlated with the type of treatment and the surface properties of treated SMC. Comparison of the surface and adhesive properties of plasma-treated samples with those for untreated samples indicates a) an increase in roughness, b) an increase in the level of SMC surface oxidation, and c) an increase in the failure force for lap shear tests.     

An Investigation of the Effect of Plasma Treatment on the Surface Properties and Adhesion in Sheet Molded Composite (SMC) Material

Sheet molded composite was treated with different plasmas (oxygen, dry air, nitrogen, and argon). Plasma treatment of SMC alters the surface properties in a manner dependent on the type of plasma used and the time of treatment. The surface properties were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Fourier transform infrared spectros-copy (FTIR). A two-part urethane adhesive was used to prepare lap shear specimens. Untreated SMC, plasma-treated SMC, and primer-treated SMC were prepared, bonded and tested. The surface properties of the failed specimens were measured. The adhesion characteristics of SMC and the surface properties of the failed specimens were correlated with the type of treatment and the surface properties of treated SMC. Comparison of the surface and adhesive properties of plasma-treated samples with those for untreated samples indicates a) an increase in roughness, b) an increase in the level of SMC surface oxidation, and c) an increase in the failure force for lap shear tests.     

Plasma‐induced surface modification and adhesion enhancement of polypropylene surface

Unoriented (UPP) and biaxially oriented (BOPP) polypropylene films were treated under radio frequency plasma of air, nitrogen, oxygen, and ammonia. Surface modification of polypropylene films was investigated by using surface energy measurement and attenuated total reflection (ATR)‐FTIR spectroscopy. Surface energy of air and nitrogen plasma‐treated polypropylene film increased for shorter treatment time and then decreased and attained an equilibrium value. Such changes in surface energy were not observed for oxygen and ammonia plasma‐treated polypropylene film, which increased to an equilibrium value. ATR‐FTIR studies revealed characteristic differences in the absorption spectra for short‐duration and long‐duration treatments. From the relative intensity change in the C—H stretching vibration, the mechanism of surface chemical reaction could be inferred. Studies regarding the durability of surface modification due to plasma treatment were evaluated by investigating surface energy of samples aged for 2 months. Treated films subjected to peel strength measurement showed improvement in bondability for UPP and BOPP film by hydrophilic surface modification accompanied by surface crosslinking. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 925–936, 2002     

Adhesion to Plasma-Modified LaRC-TPI I. Surface Characterization

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry. Both XPS and IR-RAS results indicated the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surface. Oxygen and argon plasmas were highly ablative, whereas ammonia plasma was only moderately so. Oxygen and argon plasmas appear to react with the LaRC-TPI via a fragmentation/oxidation mechanism; the effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups.     

Treatment of PET nonwoven with a water vapor or carbon dioxide plasma

Gas plasma treatment of poly(ethylene terephthalate) nonwoven (NW–PET) was used to increase the hydrophilicity of single‐ and multilayer NW–PET. NW–PET was treated with a pulsatile CO₂ or with a pulsatile H₂O glow discharge. X‐ray photoelectron spectroscopy (XPS) showed significantly more oxygen with CO₂ glow‐discharge‐treated NW–PET than with H₂O glow‐discharge‐treated‐NW–PET surfaces. Moreover, the introduction rate of oxygen at a single layer of NW–PET was higher for a CO₂ than for a H₂O glow‐discharge treatment. Titration data revealed significantly higher surface concentrations of carboxylic groups for CO₂ glow‐discharge NW–PET than for H₂O glow‐discharge‐treated NW–PET. Mass spectrometry analysis revealed that the entire internal surface of a single layer of NW–PET was modified. XPS and contact measurements confirmed the modification of the internal surface of multilayers of NW–PET. H₂O and CO₂ glow‐discharge‐treated substrates consisting of six layers of NW–PET had a nonuniform surface concentration of carboxylic acid groups as determined with titration experiments. The outside layers of the substrate contained a higher surface concentration of carboxylic acid groups than did the inside layers. XPS analysis and titration data showed that the rinsing of H₂O and CO₂ glow‐discharge‐treated NW–PET with water changed the surface composition considerably. Part of the carboxylic acid group‐containing species were washed off. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 480–494, 2000     

Surface modification of poly(ethylene) in an rf downstream remote plasma reactor

Polyethylene (PE) was treated in a remote downstream reactor with oxygen, nitrogen, hydrogen, and mixed gas plasma. The effects of these treatments were investigated by contact angle goniometry and x-ray photoelectron spectroscopy (XPS). The oxygen plasma treatment allowed a rapid and efficient hydrophilation of the PE, although the samples were placed far outside the main discharge region. In hydrogen plasma treatment, only a small amount of polar functional groups are formed, while the nitrogen plasma results in a surface similar to that in an oxygen plasma treatment. Thirteen percent of oxygen was found by XPS in these samples, while only 3% of nitrogen was present. The mixed oxygen/hydrogen plasma treatment revealed that 0.1% oxygen in the process gas produced a detectable degree of oxidation. By experiments with quenched atomic oxygen, the singlet molecular oxygen (O½Δ) was found to be the major reactive species in the surface reaction in our system. The role and the source of radiation in this reaction is discussed. © 1993 John Wiley & Sons, Inc.     

XPS study of the PET film surface modified by CO2 plasma: effects of the plasma parameters and ageing

Chemical modification of the PET surface by carbon dioxide plasma treatment has been studied using X-ray photoelectron spectroscopy (XPS). The plasma process results mainly in the formation of carbonyl, carboxyl, and carbonate groups at the PET surface. Under rather mild treatment conditions (low plasma power combined with a short treatment time), the formation of C-O bonds was found to be dominant, whereas the formation of highly oxidized carbon or double-bonded oxygen-containing groups required a high plasma power or a relatively long treatment time. The treatments performed under excessive conditions frequently led to degradation at the polymer surface. Angle-resolved XPS analyses performed on a freshly modified PET film showed a slight decrease in the O/C atomic ratio when the take-off angle (TOA) increased, indicating a relatively uniform distribution of oxygen within the sampling depth (estimated to be about 8 nm at 80° TOA). The chemical composition of the plasma-modified surface was found to be relatively stable on extended storage in air under ambient conditions. The decrease in oxygen-containing groups at the carbon dioxide-plasma-treated PET surface upon ageing is mainly ascribed to the surface rearrangement of macromolecular segments, the loss of oxygen-containing moieties introduced by the plasma treatment, and the possible migration of non-affected PET chains from the bulk to the surface region.     

Improvement of the adsorption properties of carbon black granules by means of plasma enhanced-chemical vapour deposition with acrylic acid vapours

Carbon black granules were coated by thin polymers in radio frequency glow discharges fed with acrylic acid/argon mixtures at different input powers, in order to improve the adsorption ability of basic compounds in the liquid and gas phases.The treated surfaces were characterized by X-ray photoelectron spectroscopy analyses and water contact angle measurements, performed immediately after the plasma treatment, after ageing in water and in air, and after heating up to 150 °C.The adsorption ability of basic compounds in the liquid and gas phases was evaluated with Boehm titrations and ammonia adsorption, respectively.Scanning electron microscopy observations and Brunauer, Emmett, Teller specific surface area measurements were performed to evaluate the morphological changes induced by the plasma treatments on the carbon black granule surface.The results show that the plasma treatments significantly improve the absorption ability of carbon black mainly for grafting of carboxylic groups and not for surface morphological modifications.     

Plasma Surface Treatment of Aerospace Materials for Enhanced Adhesive Bonding

The increased use of polyphenylene sulphide (PPS) and polyetheretherketone based composites for aircraft structures has highlighted the need for reliable methods of bonding these materials to metallic components such as titanium. Both composite and titanium adhesive bonds exhibit poor long-term durability when exposed to hot/wet conditions, aerospace fluids and solvents. As a result, surface treatments are employed to enhance surface energy, surface roughness and alter surface chemistry to provide better long-term durability. In this initial study the adhesive bonding of glass fibre reinforced GFR-PPS and commercially pure titanium was investigated. Prior to bonding, both materials were plasma treated using argon and oxygen gases in a RF discharge. Surface characterisation was carried out to optimise these treatments. Surface energy and wettability were examined using contact angle analysis, surface roughness was examined using scanning electron microscopy and atomic force microscopy, while X-ray photo-electron spectroscopy (XPS) was employed to study the surface chemistry. Bond strengths were determined using lap shear tests. Initial results reveal that these optimum plasma treatments produce a significant increase in bond strength.     

Surface properties of an anhydride-epoxy resin cured against different mould surfaces

An epoxy resin consisting of diglycidylether of bisphenol A (DGEBA) and methyltetrahydrophthalic anhydride (MTHPA) was cured against moulds with different surface characteristics: poly(ethylene terephthalate) (PET), perfluorinated ethylene propylene copolymer (FEP), and air. The epoxy surfaces were analysed using contact angle measurements and X-ray photoelectron spectroscopy (XPS). The results presented are interpreted in terms of differences in surface energy between the surface of the mould and the epoxy resin. With PET as the mould surface, the surface content of ester groups resulting from the anhydride increased as compared to the average bulk content. With the non-polar FEP mould, the amount of ester groups decreased instead. Shear tests on overlap joints obtained by adhesive bonding with polyurethane and epoxy adhesives showed, however, a high adhesive joint strength, both for epoxy surfaces obtained with FEP as mould, and for ground surfaces with a bulk composition. The surfaces generated in PET moulds yielded only poor adhesive joint strength. These differences in joint strength could be related to the concentration of reactive functional groups (-OH, -COOH) in the outermost surface of the cured epoxy resin.