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.     

Chemistry of the Interface Between Aluminum and Polyester Films

It has been observed that the adhesion between vacuum-evaporated aluminum and poly(ethylene isophthalate-co-ethylene sodium sulfoisophthalate) copolymer is approximately five times greater than the adhesion between vacuum-evaporated aluminum and biaxially-oriented poly(ethylene terephthalate) film. To describe the interface between the aluminum and these polymeric substrates, thermoanalytical, spectroscopic and microscopic techniques have been applied. Definite changes in surface elemental composition and chemical functionality occur upon metallization of the polymer films. Aluminized samples contained two new oxygen functionalities; one due to the aluminum oxide and the other due to an organoaluminum species. Thermal degradation, as may occur during vacuum evaporation, would be expected to yield a carboxylic acid endgroup and a vinyl endgroup for each chain scission reaction that occurred. Reaction of aluminum with these carboxylic acid endgroups is thought to be responsible for the organoaluminum oxygen peak that was observed. Based on the XPS data, however, the level of this new functionality was comparable for both types of polyester film. Thus, this new functionality may be involved in promoting aluminum/polyester adhesion, but by itself cannot explain the differences in the level of adhesion that are attained. It appears, based on the transmission electron micrographs, that the aluminum deposit penetrates the copolymer coating to a greater depth than it does the PET. The greater level of penetration could be responsible for the greater adhesion obtained between vacuum-evaporated aluminum and the copolymer film compared with the level of adhesion obtained with the PET film. Based on this work, it appears that the adhesion of the vacuum-evaporated aluminum to both polyesters has a similar chemical component (type and amount) but a different extent of the mechanical component.     

Chemistry of the Interface Between Aluminum and Polyester Films

It has been observed that the adhesion between vacuum-evaporated aluminum and poly(ethylene isophthalate-co-ethylene sodium sulfoisophthalate) copolymer is approximately five times greater than the adhesion between vacuum-evaporated aluminum and biaxially-oriented poly(ethylene terephthalate) film. To describe the interface between the aluminum and these polymeric substrates, thermoanalytical, spectroscopic and microscopic techniques have been applied. Definite changes in surface elemental composition and chemical functionality occur upon metallization of the polymer films. Aluminized samples contained two new oxygen functionalities; one due to the aluminum oxide and the other due to an organoaluminum species. Thermal degradation, as may occur during vacuum evaporation, would be expected to yield a carboxylic acid endgroup and a vinyl endgroup for each chain scission reaction that occurred. Reaction of aluminum with these carboxylic acid endgroups is thought to be responsible for the organoaluminum oxygen peak that was observed. Based on the XPS data, however, the level of this new functionality was comparable for both types of polyester film. Thus, this new functionality may be involved in promoting aluminum/polyester adhesion, but by itself cannot explain the differences in the level of adhesion that are attained. It appears, based on the transmission electron micrographs, that the aluminum deposit penetrates the copolymer coating to a greater depth than it does the PET. The greater level of penetration could be responsible for the greater adhesion obtained between vacuum-evaporated aluminum and the copolymer film compared with the level of adhesion obtained with the PET film. Based on this work, it appears that the adhesion of the vacuum-evaporated aluminum to both polyesters has a similar chemical component (type and amount) but a different extent of the mechanical component.     

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.     

Chemical oxidation of high-density polyethylene: Surface energy,functionality, and adhesion to liquid epoxy

The application of polyolefins has increased significantly over the past few decades. However, their chemical inertness and low surface energy limits their application in many industries where high adhesion to polar materials is required, such as for composites and protective coatings. Herein, six different acids are used to create polar functional groups on High-Density Polyethylene's (HDPE) surface and to increase its adhesion to liquid epoxy (LE). Contact angle measurements, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and pull-off strength measurements are used to analyze the surface energy and functionality of HDPE and to measure its adhesion to LE. The results show that each acid increases both the polar and disperse surface energies of HDPE to a different extent, but that this is not necessarily a function of acid strength. Chlorosulfonic acid and chromic acid increase the oxygen to carbon ratio by a factor of 8 and increase HDPE's adhesion to LE by more than 400%. Furthermore, a comparison between predicted work of adhesion values from the OWRK model and experimental results shows that the latter are significantly higher than what is predicted, especially with increasing surface polarity.     

Surface modification of aromatic polyamide film by aminoethanethiol solution for silicon rubber composites

The surface modification of poly(p-phenylene terephthalamide) (PPTA) film with 2-aminoethanethiol (AET) to adhere to silicon rubber was investigated. The combination of the AET treatment and the silane coupling treatment is an effective surface modification of the PPTA film for this adhesion. The x-ray photoelectron spectroscopy (XPS) analyses show that the AET treatment does not generate sulfur functionalities at the surface of the PPTA film but does generate oxygen functionalities. In the AET treatment process, a part of the amide groups near the surface of the PPTA film is hydrolyzed to form carboxylic acid groups and amino groups. The oxygen functionalities are condensed at the film surface, and nitrogen functionality is diluted at the film surface. The C(O)O moiety at the PPTA film surface may be a key factor for the adhesion with silicon rubber. The C(O)O moiety is mobile from the bulk of the PPTA film to the film surface. Hot water treatment of the original PPTA film makes the impossible adhesion with the silicon rubber possible. The hot water treatment, however, is not as powerful a surface modification as the AET treatment. © 1995 John Wiley & Sons, Inc.     

Effect of surface oxygen content and roughness on interfacial adhesion in carbon fiber–polycarbonate composites

The effect of surface chemistry and rugosity on the interfacial adhesion between Bisphenol-A Polycarbonate and a carbon fiber surface subjected to surface treatment to add surface oxygen groups was investigated. The surface oxygen content of PAN based intermediate modulus IM7 carbon fibers was varied by an oxidative surface treatment. The oxygen content of the carbon fiber surface increased from 4 to 22% by changing the degree of surface treatment from 0 to 400% of nominal commercial surface treatment levels. The oxidative surface treatment also causes an increase in surface roughness by creating pores and fissures in the surface by removing carbon from the regions between the graphite crystallites. To decouple the effects of surface roughness and the surface oxides on the interfacial adhesion, the oxidized fiber surface was passivated via hydrogenation at elevated temperature. Thermal hydrogenation removes the oxides on the surface without significantly altering the surface topography. The results of interfacial adhesion tests indicate that an increase in the oxygen content of the fiber does not increase the fiber-matrix interfacial adhesion significantly. Comparing adhesion results between oxidized and hydrogen passivated fibers shows that the effect of the surface roughness on the interfacial adhesion is also insignificant. Overall, dispersive interactions alone appear to be the primary factor in adhesion of carbon fibers to thermoplastic matrices in composites.     

Comparison Between FT-IR and XPS Characterization of Carbon Fiber Surfaces

Fourier transform infrared internal reflection spectroscopy and X-ray photoelectron spectroscopy XPS have been applied to investigate the surface of polyacrylonitrile-based carbon fibers treated by chemical oxidation and electrochemical oxidation. We have found that infrared spectroscopy has comparable sensitivity to XPS and that the amount of the functionality introduced at the fiber surface depends on the oxidation time in the case of chemical oxidation and on the electrolyte used in the case of electrochemical oxidation.     

Effects of atmospheric pressure helium/air plasma treatment on adhesion and mechanical properties of aramid fibers

In order to investigate the effect of atmospheric pressure plasmas on adhesion between aramid fibers and epoxy, aramid fibers were treated with atmospheric pressure helium/air for 15, 30 and 60 s on a capacitively-coupled device at a frequency of 5.0 kHz and He outlet pressure of 3.43 kPa. SEM analysis at 10 000× magnification showed no significant surface morphological change resulted from the plasma treatments. XPS analysis showed a decrease in carbon content and an increase in oxygen content. Deconvolution analysis of C1s, N1s and O1s peaks showed an increase in surface hydroxyl groups that can interact with epoxy resin. The microbond test showed that the plasma treatment for 60 s increased interfacial shear strength by 109% over that of the control (untreated). The atmospheric pressure plasma increased single fiber tensile strength by 16-26%.     

Improvement of carbon fiber/PEEK hybrid fabric composites using plasma treatment

A carbon fiber (CF)/polyetheretherketone (PEEK) composite was manufactured using hybrid fabrics composed of CF and PEEK fiber. The fiber/matrix interface was modified by low temperature oxygen plasma treatment. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform attenuated total reflection infrared spectroscopy (FTIR-ATR) were used to relate the roughness and the functionality of the CF surface with the interfacial adhesion strength of the CF/PEEK composite. Scanning electron micrographs showed that plasma treatment increased the roughness of the CF surface up to 3 min of plasma treatment time; and prolonged treatment resulted in overall smoothing. XPS results confirmed that increasing treatment time marginally increased surface functionality: treatment for more than 5 min decreased the surface functionality by removing the active site of the CF surface. In addition, flexural strength and interlaminarshear strength (ILSS) of the CF/PEEK composite were measured. Their maximum values were observed at 3 min of plasma treatment time as a result of surface roughening by plasma etching. The SEM results were correlated with mechanical properties of the CF/PEEK composite.     

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.     

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.     

X-ray photoelectron spectroscopic studies of carbon fiber surfaces VIII—A comparison of type I and type II fibers and their interaction with thin resin films

Type I (high-modulus HM) and Type II (high-strength HT) carbon fibers electrochemically treated in a variety of electrolytes have been analyzed using X-ray photoelectron spectroscopy. A comparison of the differences in surface functionality and the possible interaction of treated fibers with epoxy resin is reported. The amount of carbon/oxygen functionality is greater for type II for the untreated and electrochemically treated fibers. Carboxylic/ester groups are produced at edge sites in the fiber surface whereas keto-enol groups are produced on the basal planes. Conclusive evidence for a chemical reaction between the fiber surface and 828-resin for fibers polarized in acidic electrolyte is given. It is not possible to conclude whether chemical bonding is responsible for the increased interlaminar shear strength of composites produced from treated fibers.     

The Tribological Properties of Oxidation-Treated Carbon Fiber-Reinforced PTFE Composite under Oil-Lubricated Conditions

The effect of air-oxidation and ozone surface treatment of carbon fibers (CF) on tribological properties of CF reinforced Polytetrafluoroethylene (PTFE) composites under oil-lubricated condition was investigated. Experimental results revealed that ozone treated CF reinforced PTFE (CF/PTFE) composite had the lowest friction coefficient and wear. X-ray photoelectron spectroscopy (XPS) study of carbon fiber surface showed that the increase in the amount of oxygen-containing groups enhanced interfacial adhesion between CF and PTFE matrix. With strong interfacial adhesion of the composite, stress could be effectively transmitted to carbon fibers; carbon fibers were strongly bonded with PTFE matrix.     

Energy filtered low voltage “in lens detector” SEM and XPS of natural fiber surfaces

Most analyses of natural fibers give the average composition of the fiber and not the nature and distribution of surface species present. The nature of the fiber surface is important since it governs interfacial adhesion between fiber and matrix and the transfer of stress to the fiber in composite materials. The surface of caustic treated flax fibers is analyzed using X‐ray photoelectron spectroscopy (XPS) and a low voltage scanning electron microscopy (SEM) technique that uses a filtered in‐lens electron detector. XPS shows that the fiber surface is not composed of a single polymer but is a mixture of materials, probably degraded lignin and hemicellulose and extractives. The SEM technique shows patches of material on the surface with different contrast and this contrast is shown to result from different average atomic number (Z). The variation in surface composition has obvious implication in variable interfacial properties in composites made using natural fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39572.     

Influence of plasma surface treatments on kink band formation in PBO fibers during compression

The effect of nitrogen and oxygen plasma surface treatments on the compressive strength of PBO fibers has been studied. To this end, the nucleation and propagation of compression‐induced kink bands was carefully monitored by means of in situ bending tests inside a scanning electron microscope. The micromechanisms of deformation were identical irrespective of fiber surface condition (either as‐received or modified by plasma) but the critical stress necessary to induce irreversible damage in compression in the nitrogen‐plasma treated fibers was 40% higher than in the as‐received fibers. This improvement occurred without any reduction in the fiber tensile properties. The source of this behavior is discussed in the light of the morphological and chemical changes induced by the plasma treatments on the fiber surface, as studied by AFM and XPS. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Surface modification of ultra high modulus polyethylene fibers by an atmospheric pressure plasma jet

To improve their adhesion properties, ultra high modulus polyethylene (UHMPE) fibers were treated by an atmospheric pressure helium plasma jet (APPJ), which was operated at radio frequency (13.56 MHz). The surface properties of the fibers were investigated by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and contact angle measurement. The surface dyeability improvement after plasma treatments was investigated using laser scanning confocal microscopy (LSCM). The adhesion strengths of the fibers with epoxy were evaluated by microbond tests. In addition, the influence of operational parameters of the plasma treatment including power input and treatment temperature was studied. XPS analysis showed a significant increase in the surface oxygen content. LSCM results showed that the plasma treatments greatly increased fluorescence dye concentrations on the surface and higher diffusion rate to the fiber center. The tensile strength of UHMPE fiber either remained unchanged or decreased by 10–13.6% after plasma treatment. The contact angle exhibited a characteristic increase in wettability, due to the polar groups introduced by plasma treatment. The microbond test showed that the interfacial shear strengths (IFSS) increase significantly (57–139%) after plasma treatment for all groups and the optimum activation is obtained at 100°C and 5 W power input. SEM analysis showed roughened surfaces after the plasma treatments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Effect of fuming nitric acid surface treatment of ultra-high modulus polyethylene fibers on the mechanical properties of their composites

The effect of hot fuming nitric acid (FNA) treatment on the adhesion of ultra-high modulus polyethylene fabrics to an epoxy resin has been investigated. Mechanical and molecular characterization of the interface has been attempted. Fourier transform infrared diffuse transmittance spectroscopy has been used to monitor the chemical changes introduced by the FNA treatment as well as the nature of the interface between the fibers and the epoxy resin on the molecular level. Scanning electron microscopy has been used to examine the morphological consequences of the FNA treatment. Flexural and interlaminar shear properties of the composites have been measured as a function of the extent of surface treatment. Esterification of the FNA treated polyethylene fibers is used to examine the role of surface functionality to the mechanical performance.     

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.