Surface Characterization of Plasma Treated Carbon Fibers and Adhesion to a Thermoplastic Polymer

The surface chemistry of IM7 carbon fibers was characterized by x-ray photoelectron spectroscopy (XPS). The fiber surface energetics were determined from a two-liquid tensiometric method. The adhesion between as-received and plasma-treated carbon fibers and polyethersulfone (PES) was measured by the microbond pull-out test.

The surface characterization techniques showed that the effect of any plasma treatment is attained within less than 15 seconds. It was found that both argon and air plasmas increased the oxidation state of the fiber surface and that they reduced the dispersive component (γ_s^d) of the fiber surface free energy considerably. The ammonia plasma treatment resulted in a cleaning of the surface. This plasma treatment was also effective in improving the fiber/matrix adhesion of quenched samples. A similar adhesion enhancement between as-received fibers and PES is obtained by annealing the samples above the Tg of the polymer. The air plasma treatment did not have any significant effect on the fiber/matrix adhesion.     

Measurement of Adhesion Between Carbon Fibers and Bismaleimide Resins

The adhesion between carbon fibers and bismaleimide resins was evaluated using the microbond single fiber pull-out test. A commercially-available, methylene dianiline-based bismaleimide resin and a novel phosphorus-containing bismaleimide were tested with as-received and plasma-treated polyacrylonitrile-based carbon fibers. The surface chemical composition, topography, tensile strength, and surface free energy of the carbon fibers were studied using x-ray photoelectron spectroscopy, scanning electron microscopy, single fiber tensile tests, and dynamic contact angle analysis. The carbon fiber-bismaleimide adhesion improved when the carbon fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding.     

Measurement of Adhesion Between Carbon Fibers and Bismaleimide Resins

The adhesion between carbon fibers and bismaleimide resins was evaluated using the microbond single fiber pull-out test. A commercially-available, methylene dianiline-based bismaleimide resin and a novel phosphorus-containing bismaleimide were tested with as-received and plasma-treated polyacrylonitrile-based carbon fibers. The surface chemical composition, topography, tensile strength, and surface free energy of the carbon fibers were studied using x-ray photoelectron spectroscopy, scanning electron microscopy, single fiber tensile tests, and dynamic contact angle analysis. The carbon fiber-bismaleimide adhesion improved when the carbon fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding.     

Effect of Organic Gas Plasmas on the Adhesion of Matrix Resins to Carbon Fibers

IM7 carbon fibers were surface treated in methane, ethylene, trifluoromethane and tetrafluoromethane plasmas. The surface chemical composition of the fibers was determined by X-ray photoelectron spectroscopy (XPS). The adhesion between as-received and plasma-treated carbon fibers and polyethersulfone (PES) and an epoxy resin was measured by the microbond pull-out test. XPS showed that the methane and ethylene plasmas deposited a thin layer of hydrocarbon on the fiber surface. The trifluoromethane plasma deposited a layer of fluorocarbon on the surface of the fibers. The tetrafluoromethane plasma etched the fibers and introduced a significant amount of fluorine on the surface. The microbond pull-out test results indicated that an etching plasma, such as the tetrafluoromethane plasma, improved the adhesion between carbon fibers and PES. These results are consistent with earlier work performed with ammonia plasma. The adhesion is believed to be due primarily to the differential thermal shrinkage between the fiber and the matrix. It was shown that in the case of a reactive matrix such as an epoxy resin, the fiber chemical composition plays a role in the fiber-matrix adhesion. However, this chemical effect is secondary to the cleaning effect of the surface treatment.     

The Adhesion of Carbon Fibers to Thermoset and Thermoplastic Polymers

The adhesion of three carbon fibers, AS1, AS4, and XAS to thermosetting and thermoplastic polymers has been investigated using the single, embedded filament test. All three fiber types exhibited strong adhesion to the thermosets (epoxies) whereas only the XAS bonded strongly to the thermoplastics. Common explanations for low adhesion, such as weak boundary layers and surface roughness, were investigated and shown not to be responsible for the differences in adhesion. Different levels of fiber surface treatment and various organic sizings also had no effect. Surface analysis of the fibers using XPS and retention time chromatography indicate a subtle difference in the surface chemical constitution of the three fibers but the exact nature of these differences was not determined.     

Effect of Plasma Treatment on the Adhesion of Carbon Fibers to Thermoplastic Polymers

A study has been made of the effect of RF plasmas on the adhesion of carbon fibers to polycarbonate and polysulfone. Treatment in oxygen plasma significantly increased the adhesion to both polymers. The effect is lost if the treated fiber is stored in air for a week. Surface analysis using XPS indicated an increase in atom percent oxygen but the spectra were unchanged for the stored fibers even though there had been a significant loss in adhesion. It is suggested that oxygen surface functionality is responsible for the improved adhesion but that this surface activation is lost on storage. Due to a sampling depth of 5-10 nm, XPS would not be expected to detect this small change in surface functionality.     

Effect of Plasma Treatment on the Adhesion of Carbon Fibers to Thermoplastic Polymers

A study has been made of the effect of RF plasmas on the adhesion of carbon fibers to polycarbonate and polysulfone. Treatment in oxygen plasma significantly increased the adhesion to both polymers. The effect is lost if the treated fiber is stored in air for a week. Surface analysis using XPS indicated an increase in atom percent oxygen but the spectra were unchanged for the stored fibers even though there had been a significant loss in adhesion. It is suggested that oxygen surface functionality is responsible for the improved adhesion but that this surface activation is lost on storage. Due to a sampling depth of 5-10 nm, XPS would not be expected to detect this small change in surface functionality.     

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.     

Adsorption of Proteins and Cell Adhesion to Plasma Treated Polymer Substrates

Plasma treatment is often used to alter cell interaction with polymer surfaces used in biomedical application. The influence of surface hydrophilicity/hydrophobicity on human mammary epithelial cell (HMEC) proliferation and adhesion of protein albumin to plasma treated polystyrene (PS) was studied. The PS surface was made hydrophilic or hydrophobic by treatment either in O₂ or CF₄ plasma. The rate of protein adhesion was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) after incubation of PS in albumin solution for different periods, while cell viability and morphology was studied by MTT assay and scanning electron microscope (SEM). XPS measurements have shown that the quantity of adsorbed protein was higher for both plasma treated samples than for the untreated one. No significant difference regarding protein adhesion on hydrophilic or hydrophobic plasma treated surface was found by XPS. Contrary, the results for cell proliferation showed much better proliferation on hydrophilic surface.     

The Adhesion Properties of a Thermoplastic Olefin Elastomer Grafted with Maleic Anhydride Under Nitrogen Plasma

The attenuated total reflection infrared spectroscopy (ATR), the lap shear test, and the contact angle measurement are used to analyze adhesion properties of the thermoplastic olefin elastomer (TPO) after plasma exposure. TPO is dip-coated with maleic anhydride (MA) that mixed with benzophenone (BP), benzoyl peroxide, and azobisisobutyronitrile initiators before exposed to nitrogen plasma. The amounts of grafted MA, the lap shear strength (LSS), and the surface energy depend on the plasma exposure time and the type of initiator applied. An increase in plasma exposure results in the increase of the total surface energy and the polar groups, but a decrease in the LSS of the grafted TPO. The MA grafted greatly improves the LSS of TPO and the specimen prepared with BP initiator has the greatest LSS after plasma exposure. It is also confirmed that the nature of the grafted layer could have a significant effect on the adhesion strength.     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

激光和等离子处理对复合材料表面涂层附着力的影响研究

为了提高环氧涂料在纤维增强聚丙烯复合材料上的附着力,采用激光和等离子体表面前处理方法,应用超景深显微镜、粗糙度测定仪、接触角测试仪以及附着力测试仪,研究了激光和等离子体表面处理对纤维增强聚丙烯复合材料表面形貌、表面粗糙度和表面水接触角的影响,并且探究了这 2种表面处理方式对环氧涂层在复合材料上附着力的影响。结果表明： 2种处理方式均可明显提高环氧涂层在基材上的附着力,附着力均可由不到 1 MPa提高至 8 MPa以上。     

Surface Properties for Blended Films of Poly(methylmethacrylate) and Terpolymers Composed of Methylmethacrylate, Methoxypoly(ethyleneglycol-methacrylate) and Poly(dimethylsiloxanemethacrylate)

Terpolymers composed of methylmethacrylate (MMA), poly(dimethylsiloxanemethacrylate) (PDMSMA) and methoxypoly(ethyleneglycolmethacrylate) (MPEGMA), which have blood compatibility, were blended with poly(methylmethacrylate) (PMMA) in order to improve their mechanical properties. It was expected that low surface free energy components such as the poly(dimethylsiloxane) (PDMS) and methoxy groups of terpolymer would predominate at the blend surface. The adsorptions of PDMS to the blended surfaces were confirmed via X-ray photoelectron spectroscopy (XPS). A large contact angle hysteresis was observed for the blended films via a dynamic contact angle. Advancing contact angles for blended films showed the same values as that of the silicone. The receding contact angles for those blends incorporating PDMSMA-rich terpolymer showed high values and decreased with hot water treatment, while MPEGMA-rich terpolymer blended films exhibited low values and maintained those values after hot water treatment. Adhesion tension relaxations for these blended films were also observed. These phenomena were interpreted to be caused by the reorganization of a hydrophobic segment to the polymer surface or hydrophilic segment to the water/polymer interface so as to decrease the surface or interfacial tension, respectively. Although the mechanical properties slightly decreased with blending of these terpolymers, the blended films could be applied for various practical uses.     

Plasma treatment effect on the surface energy of carbon and carbon fibers

The surface energy dispersive (γ^D_S) and polar (γ^P_S) components of carbon model surfaces (bal planes, prismatic surfaces, vitreous carbon) and of carbon fibers (high strength and high modulus, respectively) were determined systematically before and after plasma treatment. The method used is essentially based on the wetting contact angle measurements within two liquids. In all cases (γ^P_S) is markedly increased by the plasma treatment. For carbon fibers, with increasing plasma treatment duration (γ^P_S) is increasing toward a limiting value (-30 mJ/m²) while (γ^D_S) is depressed toward low values (-10 mJ/m²). The parallel evolution of surface topography is followed by SEM observations. The changes in surface energy of carbon model surfaces is also discussed.     

Surface composition of carbon fibers subjected to oxidation in nitric acid followed by oxygen plasma

Type II, PAN-based carbon fibers (unsized and commercially treated) have been exposed to nitric acid and oxygen plasma individually and also to combined nitric acid/oxygen plasma treatments and the surface compositions have been determined using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS). Most of the oxygen on the as-received carbon fibers resides within the outermost 10-15 Å of the surface. Fiber exposure to nitric acid at 115°C for 20-90 min enhances the oxygen surface concentration to a point of saturation and the oxygen depth distribution is increased and becomes more uniform within the maximum XPS sampling depth (~60-100 Å). In addition, the fiber surface area is believed to be increased. After treating fibers to various degrees in nitric acid, subsequent exposure to oxygen plasma yields an additional increase in the surface oxygen content, particularly in the outermost fiber layers (10-15 Å). Under the conditions of the investigation, the maximum amount of surface oxidation occurs after sequential fiber exposure to nitric acid at 25°C for 30 s and oxygen plasma. As the extent of initial nitric acid treatment is increased, the synergism with subsequent plasma oxidation decreases, and the oxygen concentration becomes more uniform within the outer layers of the oxidized fibers. Overall, the data are consistent with a proposed oxidation mechanism in which oxygen plasma acts to enhance the surface density of oxygen on roughened and pitted nitric acid-oxidized fiber surfaces. As the duration of nitric acid exposure is increased, it is hypothesized that subsequent exposure to oxygen plasma smoothes the fiber surfaces but the surface density of oxygen remains essentially constant.     

Surface Characterization of Vulcanized Rubber Treated with Sulfuric Acid and its Adhesion to Polyurethane Adhesive

Modifications produced on a vulcanized styrene -butadiene rubber surface by treatment with sulfuric acid were studied and several experimental variables were considered.

The treatment of R1 rubber with sulfuric acid produced a noticeable decrease in contact angle which was mainly ascribed to an increase in surface energy due to the formation of sulfonic acid moieties and C=O bonds, and the removal of zinc stearate. The rubber surface swelled and became brittle as a result of the treatment, and when flexed microcracks were created. A rubber surface layer modification was produced with a consequent decrease in tensile strength and elongation-at-break values. The treatment enhanced the T-peel strength of R1 rubber/polyurethane adhesive joints and the locus of failure was cohesive in the rubber.

The optimum immersion time in H₂SO₄ solution was less than 1 min., and the reaction time in air was not found to be critical; the neutralization with ammonium hydroxide and the high concentration of the sulfuric acid (95 wt%) were essential to produce adequate effectiveness of the treatment.     

Surface Characterization of Vulcanized Rubber Treated with Sulfuric Acid and its Adhesion to Polyurethane Adhesive

Modifications produced on a vulcanized styrene –butadiene rubber surface by treatment with sulfuric acid were studied and several experimental variables were considered.

The treatment of R1 rubber with sulfuric acid produced a noticeable decrease in contact angle which was mainly ascribed to an increase in surface energy due to the formation of sulfonic acid moieties and C?O bonds, and the removal of zinc stearate. The rubber surface swelled and became brittle as a result of the treatment, and when flexed microcracks were created. A rubber surface layer modification was produced with a consequent decrease in tensile strength and elongation-at-break values. The treatment enhanced the T-peel strength of R1 rubber/polyurethane adhesive joints and the locus of failure was cohesive in the rubber.

The optimum immersion time in H₂SO₄ solution was less than 1 min., and the reaction time in air was not found to be critical; the neutralization with ammonium hydroxide and the high concentration of the sulfuric acid (95 wt%) were essential to produce adequate effectiveness of the treatment.     

Plasma Modifications to the Interface in Carbon Fiber Reinforced Thermoplastic Matrices

Radio frequency glow discharge oxygen plasma was used to modify the surfaces of PAN-based and mesophase pitch-based carbon fibers. Surface chemical changes to the fibers were monitored by X-ray photoelectron spectroscopy and by fiber wetting studies evaluated in terms of dispersive-polar components of surface energy and acid-base contribution to the work of adhesion. Physical changes to these fibers were monitored by scanning electron microscopy. Stress transferability of these fibers was evaluated by the embedded single fiber test in poly(methyl methacrylate), poly(ethyl methacrylate), poly(methacrylonitrile) and poly(vinyl chloride) as these matrices offered varying degrees of dispersive-polar and acid-base character. Experimentally determined critical aspect ratios were compared to the theoretical work of adhesion determined by dispersive-polar interactions and with the Lewis acid-base nature of the matrices.     

Plasma Modifications to the Interface in Carbon Fiber Reinforced Thermoplastic Matrices

Radio frequency glow discharge oxygen plasma was used to modify the surfaces of PAN-based and mesophase pitch-based carbon fibers. Surface chemical changes to the fibers were monitored by X-ray photoelectron spectroscopy and by fiber wetting studies evaluated in terms of dispersive-polar components of surface energy and acid-base contribution to the work of adhesion. Physical changes to these fibers were monitored by scanning electron microscopy. Stress transferability of these fibers was evaluated by the embedded single fiber test in poly(methyl methacrylate), poly(ethyl methacrylate), poly(methacrylonitrile) and poly(vinyl chloride) as these matrices offered varying degrees of dispersive-polar and acid-base character. Experimentally determined critical aspect ratios were compared to the theoretical work of adhesion determined by dispersive-polar interactions and with the Lewis acid-base nature of the matrices.