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.     

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.     

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.     

Development and evaluation of surface treatments to enhance the fiber-matrix adhesion in PAN-based carbon fiber/liquid crystal polymer composites. Part I: Coupling agent and amine surface treatments

Several surface treatments, using both commercially available coupling agents and reagents containing multiple amines, were applied to commingled continuous as-received AS4 carbon reinforcing fiber/liquid crystal polymer (LCP) matrix fibers. Unidirectional composites (normally 60 vol% carbon fiber) were prepared from as-received and treated commingled fibers and characterized. To estimate the effect the effect of the treatments on fiber-matrix adhesion, short beam shear (SBS) tests were conducted, the failure surfaces were examined, and spectroscopic studies wee performed. The mean SBS strength of the as-received unidirectional AS4 carbon fiber/LCP matrix composite system was 49 MPa. The best coupling agent and amine treatments yielded increases in composite shear strength of ∼ 10 to 20%, relative to the as-received AS4/LCP system. For the amine treatments, ESCA and FTIR analyses suggested of both the carbon and LCP fibers may have caused the increased adhesion. Moreover, SEM analysis of the failure surfaces of SBS specimens from composites prepared with the treated fibers may have caused the increased adhesion. Moreover, SEM analysis of the failure surfaces of SBS specimens from composites prepared with the treated fibers (both with coupling agents and amines) showed that strong fiber-matrix adhesion was present. That is, failure occurred in the LCP matrix material.     

Evaluation of tribological performance of surface‐treated carbon fiber‐reinforced thermoplastic polyimide composite

The effect of surface treatment [rare earth solution (RES) and air oxidation] of carbon fibers (CFs) on the mechanical and tribological properties of carbon fiber‐reinforced polyimide (CF/PI) composites was comparatively investigated. Experimental results revealed that surface treatment can effectively improve the interfacial adhesion between carbon fiber and PI matrix. Thus, the flexural strength and wear resistance were significantly improved. The RES surface treatment is superior to air oxidation treatment in promoting interfacial adhesion between carbon fiber and PI matrix. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Interfacial adhesion of plasma‐treated carbon fiber/poly(phthalazinone ether sulfone ketone) composite

Interfacial adhesion between fiber and matrix has a strong influence on composite mechanical performance. To exploit the reinforcement potential of the fibers in advance composite, it is necessary to reach a deeper understanding on the relation between fiber surface treatment and interfacial adhesion. In this study, air plasma was applied to modify carbon fiber (CF) surface, and the capability of plasma grafting for improving the interfacial adhesion in CF/thermoplastic composite was discussed and also the mechanism for composite interfacial adhesion was analyzed. Results indicated that air plasma treatment was capable of increasing surface roughness as well as introducing surface polar groups onto CF; both chemical bonding and mechanical interaction were efficient in enhancements of interlaminate shear strength of CF/PPESK composite, while mechanical interaction has a dominant effect on composite interfacial adhesion than chemical bonding interaction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Interfacial adhesion between ionic copolymers and high-modulus surface-treated carbon fibers

High-modulus carbon-fiber-reinforced thermoplastic composites typically fail at the interface due to poor adhesion between fiber and matrix. To increase interfacial strength, the research described herein focuses on modifying the fiber surface (via high-temperature acid treatment or zinc electrolysis) to facilitate chemical functional groups on the fiber that might increase fiber-matrix inter-actions. The thermoplastic matrix materials used in this study were random copolymers of ethylene and methacrylic acid in which the carboxyl groups in the methacrylic acid segments were neutralized with either sodium or zinc counterions. Mechanical tests were performed to determine the macroscopic effects of fiber pretreatment on the ultimate mechanical properties of the composites. Fabrication was designed such that fiber-matrix separation provides the dominant contribution to mechanical gracture. Composites containing fibers treated with nitric acid, or a mixture of nitric and sulfuric acids exhibit a 20 to 25 percent increase in transverse (tensile) fracture stress relative to composites fabricated with as-received fibers. Scanning electron microscopy of the fiber-matrix interface at fracture allows one to “zoom-in” and obtain qualitative details related to adhesion. Fracture surface micrographs of the above-mentioned acid-treated fiber-reinforced composites reveal an increase in the amount of matrix material that adhered to the fiber surface relative to the appearance of the fracture surface of composites fabricated with as-received fibers. The presence of acid functionality in the matrix, rather than the divalent nature of the zinc counterions, produces the largest relative enhancement of transverse (tensile) fracture stress in the above-mentioned composites containing surface-treated carbon fibers.     

CO2 plasma modification of high-modulus carbon fibers and their adhesion to epoxy resins

Interfacial bond strength is often a performance-limiting factor of carbon-fiber-reinforced composites. This limitation is most prevalent when higher-modulus fibers or relatively unreactive matrix resins, such as engineering thermoplastics or high-temperature thermoset resin systems, are used. Radio-frequency (RF) glow discharge plasmas are an effective means of modifying carbon-fiber surface chemical characteristics to promote adhesion. It has been previously shown that oxidizing plasmas are especially effective compared with electro-oxidative treatments for treating carbon fiber surfaces as revealed by titrations, electron spectroscopy, wetting, and inverse gas chromatography measurements. This study evaluated the effectiveness of CO₂ plasmas on two experimental high-modulus carbon/graphite fibers and correlated the plasma surface modification with interfacial adhesion in an epoxy matrix composite system. The results show that CO₂ plasma treatment increased the surface oxygen content by nearly a factor of 2 over typical electro-oxidation treatments. The increased oxygen is mainly in the form of hydroxyl, ketone, and carboxyl-like moieties. Unidirectional composites were prepared from as-received and plasma-modified versions of each type of experimental fiber. The composites containing plasma-modified filaments exhibited 1.5-3.0 times the strength of composites fabricated with untreated or electro-oxidized filaments in transverse-flexural tests. Short-beam shear strength increased by two times over those with as-produced filaments and is equivalent to that of composites containing electro-oxidized filaments.     

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.     

INFLUENCE OF PROCESSING CONDITIONS ON ADHESION BETWEEN CARBON FIBERS AND ELECTRON-BEAM-CURED CATIONIC MATRICES

Adhesion between an electron-beam-cured Diglycidyl Ether of Bisphenol A (DGEBA) epoxy matrix and AS4 carbon fibers has been evaluated with the microindentation test method and compared with similar thermally cured materials. The results indicate that the absence of amine compounds and of high temperature treatment associated with thermally cured epoxy matrices are detrimental to fiber-matrix adhesion in electron-beam-cured epoxy matrices when measured by the microindentation test. Electron beam processing was not found responsible for any adsorption and/or deactivation of the irradiated carbon fiber surface as determined by surface analysis with X-ray Photoelectron Spectroscopy (XPS). Moreover, the relationship between electron-beam processing conditions (namely, dose and dose increment) with the resulting matrix properties and the adhesion to carbon fiber have revealed a strong dependency of fiber-matrix adhesion on the bulk matrix properties independent of the electron beam processing history. Undercured electron-beam-processed matrices exhibit higher adhesion to carbon fibers that can be explained by a higher matrix shear modulus.     

Influence of processing conditions on adhesion between carbon fibers and electron-beam-cured cationic matrices

Adhesion between an electron-beam-cured Diglycidyl Ether of Bisphenol A (DGEBA) epoxy matrix and AS4 carbon fibers has been evaluated with the microindentation test method and compared with similar thermally cured materials. The results indicate that the absence of amine compounds and of high temperature treatment associated with thermally cured epoxy matrices are detrimental to fiber-matrix adhesion in electron-beam-cured epoxy matrices when measured by the microindentation test. Electron beam processing was not found responsible for any adsorption and/or deactivation of the irradiated carbon fiber surface as determined by surface analysis with X-ray Photoelectron Spectroscopy (XPS). Moreover, the relationship between electron-beam processing conditions (namely, dose and dose increment) with the resulting matrix properties and the adhesion to carbon fiber have revealed a strong dependency of fiber-matrix adhesion on the bulk matrix properties independent of the electron beam processing history. Undercured electron-beam-processed matrices exhibit higher adhesion to carbon fibers that can be explained by a higher matrix shear modulus.     

Wetting and adhesion behavior of armos fibers after dielectric barrier discharge plasma treatment

To investigate the influence of atmospheric plasma treatment on aramid fiber wetting and adhesion behavior, an air dielectric barrier discharge (DBD) was applied to the Armos aramid fiber surface at different discharge power densities. Dynamic contact angle analysis indicated that the total surface free energy was increased from 49.6 to 68.3 mJ/m 2 , an increment of 37.7%, whereas the single-fiber tensile strength testing showed that the mechanical properties of the Armos fibers were almost unaffected. With the enhancement of fiber surface wettability, the interlaminar shear strength, which was used to determine the interfacial adhesion in Armos-fiber-reinforced thermoplastic poly(phthalazinone ether sulfone ketone) composites, increased by 17.2% to 71.4 MPa. Scanning electron microscopy photos showed that the predominant failure mode of the composites changed from interface failure to matrix and/or fiber failure after the plasma treatment. Taken together, these results suggest that the air DBD plasma was an effective technique for improving the surface and interfacial performance of the Armos fibers without damaging their bulk properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Enhanced interfacial adhesion of carbon fibers to vinyl ester resin using poly(arylene ether phosphine oxide) coatings as adhesion promoters

Poly(arylene ether phosphine oxide)s (PEPO) were prepared and utilized to coat carbon fibers to enhance the interfacial adhesion with vinyl ester resins. For comparison, poly(arylene ether sulfone) (PES), Udel^® P-1700, and Ultem^® 1000 were also used. The interfacial shear strength (IFSS) of thermoplastic polymer-coated fibers was measured via microbond pull-out tests. The interfacial adhesion between thermoplastics and as-received carbon fibers was also measured in order to investigate the adhesion mechanism. Thermoplastic polymer-coated fibers exhibited a higher IFSS than the as-received fibers with vinyl ester resin, and with thermoplastic polymers. PEPO-coated fibers showed the highest IFSS, followed by Udel^®, PES, and Ultem^®-coated fibers. The high IFSS obtained with PEPO coating could be attributed to the phosphine oxide moiety, which provided a strong interaction with functional groups in the vinyl ester resin and also on carbon fibers. A diffusion study revealed the formation of a clear interphase not only between PEPO and the vinyl ester resin, but also between Udel^® (PES or Ultem^®) and the vinyl ester resin, although the morphology of the two interphases differed greatly.     

空气等离子体处理芳纶纤维及其与天然橡胶/乳聚丁苯橡胶的黏合性能

考察了空气等离子体处理对芳纶纤维表面结构形态的影响,研究了空气等离子体和间苯二酚-甲醛-胶乳(RFL)浸胶处理芳纶纤维与天然橡胶(NR)/乳聚丁苯橡胶(ESBR)的黏合性能,并对经处理的芳纶纤维与NR/ESBR体系的界面层作了动态力学分析。结果表明,芳纶纤维经空气等离子体处理后,表面粗糙度增大,表面积增加,结晶度减小,但处理功率过大、处理时间过长时,芳纶纤维的表面又变得比较光滑、结晶度又呈增大趋势。随着等离子体处理时间的延长,芳纶纤维与NR/ESBR的黏合性能增强,但处理时间过长时,芳纶纤维与NR/ESBR的黏合性能下降;等离子体处理芳纶纤维经RFL进一步浸胶处理后,芳纶纤维与NR/ESBR的黏合性能大幅度提高。芳纶纤维与NR/ESBR的界面存在介于高模量芳纶纤维和低模量橡胶之间的过渡层。     

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.     

Three‐dimensionally braided carbon fiber–epoxy composites,a new type of materials for osteosynthesis devices. II. Influence of fiber surface treatment

Interfacial adhesion between carbon fiber and epoxy resin plays an important role in determining performance of carbon–epoxy composites. The objective of this research is to determine the effect of fiber surface treatment (oxidization in air) on the mechanical properties (flexural strength and modulus, shear and impact strengths) of three‐dimensionally (3D) braided carbon‐fiber‐reinforced epoxy (C_3D/EP) composites. Carbon fibers were air‐treated under various conditions to improve fiber–matrix adhesion. It is found that excessive oxidation will cause formation of micropits. These micropits are preferably formed in crevices of fiber surfaces. The micropits formed on fiber surfaces produce strengthened fiber–matrix bond, but cause great loss of fiber strength and is probably harmful to the overall performance of the corresponding composites. A trade‐off between the fiber–matrix bond and fiber strength loss should be considered. The effectiveness of fiber surface treatment on performance improvement of the C_3D/EP composites was compared with that of the unidirectional carbon fiber–epoxy composites. In addition, the effects of fiber volume fraction (V_f) and braiding angle on relative performance improvements were determined. Results reveal obvious effects of V_f and braiding angle. A mechanism was proposed to explain the experimental phenomena. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1040–1046, 2002     

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.     

Production and characterisation of vapour grown carbon fiber/polypropylene composites

Polymeric composites are widely used in the aircraft and automotive industries. Their high strength-to-weight ratio makes significant weight reduction possible. Beside this advantage, the polymer materials also offer a good corrosion resistance but the mechanical and electrical properties are not satisfactory. In order to increase these properties, vapour grown carbon fibers (VGCF) with high strength and metal-like electrical conductivity can be embedded in the polymeric matrix. To ensure a good adhesion between the fibers and the polymer matrix a functionalization of the chemically inert surface of the fibers is necessary.In the present research work oxygen-containing functional groups were introduced on the fiber surface through cold plasma treatment. Measurements of the fiber surface energy after plasma functionalization showed an enhancement of at least 50% of the initial value. The VGCF/PP composites with different amounts of VGCF were made through extrusion and injection molding. The results show that the degree of fiber surface functionalization and the fiber distribution and orientation in the polypropylene (PP) matrix may strongly influence the mechanical properties of the composite.     

Enhancing hygrothermal resistance of aeronautical carbon fabric/epoxy composites via air plasma processing

Air plasma processing is introduced as a surface modification technique for carbon fibers to enhance the hygrothermal resistance of carbon fabric/epoxy composites. On carbon fiber surface subjected to 4-min plasma processing, there are 37.6% of carbon species present as –C–O–C groups and 9.3% of carbon species as –COOH groups. The moisture adsorption behavior of composites can be described by Fick’s law. Increase in temperature accelerates the initial moisture adsorption rate and results in a higher diffusion coefficient. The decreasing interlaminar shear strength (ILSS) of composites is mainly ascribed to the loss of adhesion at the fiber/matrix interface. A lower equilibrium moisture content of composites is caused by air plasma processing, which leads to the improved interfacial bonding strength and the higher retention rate of ILSS of carbon fabric/epoxy composites. The interface sensitivity and temperature dependence of moisture adsorption for carbon fabric/epoxy composites are discussed. The results presented herein demonstrate an effective strategy for enhancing hygrothermal resistance of carbon fiber-reinforced composites.