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.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber–polymer matrix composites can be obtained from this study.     

Nanoscale toughening of carbon fiber‐reinforced epoxy composites through different surface treatments

Interests in improving poor interfacial adhesion in carbon fiber‐reinforced polymer (CFRP) composites has always been a hotspot. In this work, four physicochemical surface treatments for enhancing fiber/matrix adhesion are conducted on carbon fibers (CFs) including acid oxidation, sizing coating, silane coupling, and graphene oxide (GO) deposition. The surface characteristics of CFs are investigated by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, interfacial shear strength, and interlaminar shear strength. The results showed that GO deposition can remarkably promote fiber/matrix bonding due to improved surface reactivity and irregularity. In comparison, epoxy sizing and acid oxidation afford enhancement of IFSS owing to effective molecular chemical contact and interlocking forces between the fiber and the matrix. Besides, limited covalent bonds between silane coupling and epoxy matrix cannot make up for the negative effects of excessive smoothness of modified CFs, endowing them inferior mechanical properties. Based on these results, three micro‐strengthening mechanisms are proposed to broadly categorize the interphase micro‐configuration of CFRP composite, namely, “Etching” “Coating”, and “Grafting” modifications, demonstrating that proper treatments should be chosen for combining optimum interfacial properties in CFRP composites. POLYM. ENG. SCI., 59:625–632, 2019. © 2018 Society of Plastics Engineers     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber-polymer matrix composites can be obtained from this study.     

The Study of Reactive Functional Groups in Adhesive Bonding at the Aramid-Epoxy Interface

In polymeric composites, reactive functional groups on the fiber surface are assumed to enhance the mechanical strength of the fiber-matrix interface greatly by forming covalent bonds with the matrix. To test this assumption, we sought to promote covalent bonding at the aramid fiber-epoxy matrix interface by attaching flexible reactive pendent groups to the fiber surface. Other factors that could affect interfacial adhesion were kept constant, i.e., surface energy and surface topography. Quantitative analysis showed a pendent group attachment level of 1.5 to 4.5 groups per 100 Ų of fiber surface, a level that agrees well with the theoretical amount. Surprisingly, in adhesive performance tests, the presence of these reactive pendent groups did not improve the fiber-matrix interface strength. Specific chemical tests for covalent bond formation between the terminal amine of the pendent group and the epoxy molecule showed that covalent bonding did not occur, thus explaining the unexpected lack of improvement in adhesive performance.     

The effects of surface treatments of fibers on the interfacial properties in single-fiber composites

The interfacial properties between fibers and the matrix contribute to the overall properties in high performance composites. Plasma treatments (Ar, O₂, CF₄/O₂, N₂/H₂) have been performed on carbon fibers to improve the fiber-matrix interaction. The treatment efficiency was checked by the single-fiber technique, while the surface chemistry and morphology were characterized by X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectroscopy (SSIMS), and scanning electron microscopy (SEM). The O₂- and N₂/H₂-plasma treatments proved most effective both for introducing oxygen-containing functionalities at the fiber surface and for improving the interfacial shear strength of carbon fiber/epoxy composites. A relationship between the oxygen concentration at the fiber surface and the interfacial shear strength is demonstrated.     

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.     

Improvement of the interfacial shear strength of poly(p‐phenylene benzobisoxazole) fiber/epoxy resin composite via a novel surface coating agent

Poly(p‐phenylene benzobisoxazole) (PBO) fiber with a smooth surface exhibits limited interfacial interaction with resin matrix. One of the effective strategies to improve the adhesion between the fiber and resin matrix is through surface modification of the fiber. In this study, we have proposed a novel surface treatment agent based on phosphoester cross‐linked castor oil (PCCO) for effective surface treatment of PBO fibers. The surface treatment agent was prepared by a simple cross‐linking reaction between hydroxy phosphorylated castor oil (PCO) and epoxy resin, with alcohol as the solvent at 65°C. Once the PBO fiber was treated with this agent, the interfacial adhesion between the PBO fiber and the epoxy resin could then be improved. Systematic analyses suggest that the surface treatment with (PCO + epoxy)/alcohol solution improves the interaction of the PBO fiber with the epoxy resin matrix. The PCCO coated onto the surface of PBO fiber acts as a coupling agent, improving the interfacial shear strength (IFSS) of the PBO fiber/epoxy resin composite. Results indicate a 156% increase in IFSS without compromising the mechanical properties of the fiber. POLYM. COMPOS., 37:1198–1205, 2016. © 2014 Society of Plastics Engineers     

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.     

Effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites

Interfacial adhesion between graphite fibers and epoxy matrices is one of the important parameters in controlling composite performance. Experiments were performed to verify the idea that polymerizable and nonpolymerizable nonionic surfactants can promote wetting and increase interfacial shear strength. Using a single-fiber-composite (SFC) technique with a Poisson/Weibull model of fiber strength and flaws, the effective interfacial shear strength (ISS) was determined. The effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites was also compared with the effects of treatment with deionized water, absolute alcohol, and acetone. The experimental results showed that the fibers treated with the nonpolymerizable surfactant gained 37% in interfacial shear strength relative to those untreated or those treated with deionized water, absolute alcohol, or acetone. The polymerizable surfactants showed a 10% increase in interfacial shear strength. The lognormal distribution provided the best fit for critical aspect ratios obtained from the SFC tests.     

The Effect of Nitric Acid Oxidization Treatment on the Interface of Carbon Fiber-Reinforced Thermoplastic Polystyrene Composite

The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibers and the composition of the matrix. The composition of polystyrene was modified by the addition of maleic anhydride (MAH) grafted polystyrene. The surface properties of the various matrix formulations were characterised by contact angle. Carbon fibers were modified by oxidation in nitric acid. The surface composition of the carbon fibers was characterized. The interaction between modified polystyrene and the carbon fibers was studied by single fiber pull-out tests. The best adhesion behavior was achieved between polystyrene containing grafted MAH and nitric acid oxidation carbon fibers. The addition of MAH-grafted polystyrene to the unmodified polystyrene caused the interfacial shear strength to increase. The apparent interfacial shear strength of this fiber-matrix combination allowed for the utilisation of 100% of the yield tensile strength of polystyrene.     

Adhesion of Graphite Fibers to Epoxy Matrices: II. The Effect of Fiber Finish

Reinforcing fibers are available from various manufacturers with matrix compatible “finishes” applied to them. Usually these finishes or coatings are 100-200 nm thick resin layers applied after surface treatment. Their function has been hypothesized as being to enhance adhesion through either protecting the fiber from handling damage, protecting the fiber surface reactivity, or improving fiber wettability. This study of finished and unfinished graphite fibers concludes that the mechanism by which an epoxy compatible finish operates is different from what has been hypothesized to date. The finish layer creates a brittle interphase layer between the fiber and matrix which increases the interfacial shear strength but at the expense of changing the failure mode from interfacial to matrix.     

The influence of the interphase on composite properties: Poly(ethylene-co-acrylic acid) and poly(methyl vinyl ether-co-maleic anhydride) electrodeposited on graphite fibers

The electrodeposition of saturated copolymers onto carbon fibers is investigated, focusing particular attention on improvement of shear and impact properties of the corresponding composites. Carbon fibers are electrocoated with poly(ethylene-co-acrylic acid) and poly(methyl vinyl ether-co-maleic anhydride) from aqueous media, and fabricated into epoxy composites. The results of interlaminar shear strength (ILSS) tests, initially employed to assess fibermatrix adhesion, are vitiated by the occurrence of mixed-mode failure. Interfacial shear strength (IFSS) is hence evaluated by stressing single-fiber composite specimens to obtain ultimate aspect ratios of the fiber fragments. The data are combined with fiber strengths by a recently developed statistical theory (1) to yield a distribution for IFSS. Both copolymer interphases improve fiber-matrix bonding to an extent greater even than that obtained with commercial fiber surface treatment. Good fiber-matrix adhesion is further apparent from SEM studies of fractured ILSS test specimens. A key to this improved adhesion is the interpenetration of matrix resin and interphase polymer, revealed by electron microprobe analysis (2). Notched Izod impact strength is also increased over uncoated-fiber composites. These copolymer interphases behave as deformable interlayers, absorbing impact energy and blunting the growing crack tip. Further energy is absorbed in deflecting the crack through a more tortuous path. Simultaneous improvements in impact and shear strengths are thus obtained, which may be further enhanced by optimizing the electrodeposition parameters and the coating thickness. The influence of the interphase on composite properties is better understood from this study, paving the way for refinement in interphase design.     

Evaluation of fiber surfaces treatment and sizing on the shear and transverse tensile strengths of carbon fiber-reinforced thermoset and thermoplastic matrix composites

The mechanical performance of advanced composite materials depends to a large extent on the adhesion between the fiber and matrix. This is especially true for maximizing the strength of unidirectional composites in off-axis directions. The materials of interest in this study were PAN-based carbon fibers (XA and A4) used in combination with a thermoset (EPON 828 epoxy) and a thermoplastic (liquid crystal poymer) matrix. The effect of surface treatment and sizing were evaluated by measuring the short-beam shear (SBS) and transverse flexural (TF) tensile strengths of unidirectional composites. Results indicated that fiber surface treatment improves the shear and trasverse tensile strengths for both thermosetting and thermoplastic matrix/carbon fiber-reinforced unidirectional composites. A small additional improvement in strengths was observed as the result of sizing treated fibers for the epoxy composites. Scanning electron microscope photomicrographs were used to determine the location of composite failure, relative to the fiber-matrix interface. Finally, the epoxy composites SBS and TF strengths appear to be limited to the maximum transeverse tensile strength of the epoxy matrix, while the thermoplastic composite SBS and TF strengths are limited by the LCP matrix shear and transverse tensile strengths, respectively.     

Interfacial interaction in sisal/epoxy composites and its influence on impact performance

To obtain comprehensive knowledge of the interfacial effect on the impact performance of sisal fiber reinforced epoxy composites, the fiber surface was modified in different ways prior to compounding. By using a surface tensiometer and dynamic mechanical analyzer, interfacial interactions in the composites were characterized. The results indicated that the chemical treatments brought about strong bonding between sisal bundles and the epoxy matrix. The subsequent impact tests revealed that the microfailure mechanism involved is a function of interfacial adhesion and fiber length continuity (i.e., continuous or discontinuous fiber). In the case of unidirectional laminates, an optimum fiber treatment should be able to result in an increased affinity between fiber bundles and matrix and a decreased intercellular adhesion. In this way, extension and uncoiling of the spirally arranged microfibrils, a main energy consumption process of plant fibers, can impart significant toughness to the composites. For short fiber composites, the interfacial strength should be properly tailored so as to increase energy dissipation through debonding and pullout of fiber bundles.     

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.     

Surface modification of UHSPE fibers through allylamine plasma deposition. II. Effect on fiber and fiber/epoxy interface

The surface of ultra-high strength polyethylene (UHSPE) fibers was modified using allylamine plasma deposition to improve their adhesion to epoxy resins. Allylamine plasma polymerization was investigated at different power inputs and polymerization times. The adhesion of treated fibers to epoxy resin was studied by single-fiber, pull-out tests. A special silicon rubber mold was developed to embed the single fiber in epoxy resin. The results show that the interfacial shear strength (IFSS) increased by a factor of 2 to 3 after allylamine plasma treatments. The greatest improvement, by a factor of 3.25, was obtained at 30 W for 10 min. Scanning electron microscopy (SEM) was also used to study the surface topography of fibers pulled from the epoxy resin. In most cases, it was observed that pull-out failure occurred at the interface, as evidenced from clean fiber surfaces. In a few cases, however, fibrils were peeled from fibers. The fiber strength decreased, but initial modulus increased after the plasma treatments. The decrease in fiber strength was insignificant for treatments at a lower power input, but was significant at higher power inputs. Treatment time, however, had no significant effect on fiber strength.     

环氧基体与竹节状有机纤维之间的界面性能研究

本文采用单丝拔出试验和动态力学分析研究了环氧树脂基复合材料中基体与竹节状有机短纤维之间的界面特性.有关的试验结果表明:在弱界面结合的条件下,由于在竹节状有机短纤维中凸节的存在,可以提高纤维与基体之间的界面结合强度,也有利于纤维末端界面剪切应力的传递.     

The dependence of interfacial shear strength on matrix and interphase properties

Experiments were conducted to determine the dependence of the interfacial shear strength on the bulk material matrix properties using model compounds based on epoxy/amine chemistry. AS4, carbon fibers were used as the subject for these measurements with both a difunctional epoxy (DGEBA) system as well as a tetrafunctional epoxy (MY720) system. Amine curing agents were carefully chosen to produce matrices which produced a range of matrix properties from brittle, elastic to ductile, plastic. The fiber-matrix interfacial chemistry was constant throughout this study by always using a stoichiometric amount of curing agent. The results indicate that, for both the difunctional as well as the tetrafunctional epoxy system, the interfacial shear strength (as determined by the fragmentation test) decreases nonlinearly with decreasing modulus of the matrix. Linear elastic analysis yields a nearly linear relationship, for both systems, between the interfacial shear strength and the product of strain to final break and the square root of the matrix shear modulus. A linear relationship is also found between the difference in test temperature and glass transition temperature of the cured matrix and the interfacial shear strength. Additionally, the failure mode is seen to remain interfacial as the ductility of the matrix changes.