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.     

The Effect of Silane Surface Treatment of Carbon Fiber on the Tribological Properties of Bismaleimide(BMI) Composite

Silane surface modification method was used for the surface treatment of carbon fiber to improve the interfacial adhesion of the carbon fiber reinforced bismaleimide(BMI) composite. The surface characteristics of untreated and treated carbon fiber were characterized by Fourier transform infrared (FT-IR) spectroscope. The friction and wear properties of the BMI composites filled with differently surface treated carbon fibers(20 vol%), were investigated on a ring-on-block tribometer. Experimental results revealed that silane treatment largely reduced the friction and wear of CF/BMI composites. Scanning electron microscope (SEM) of worn surfaces of BMI composites showed that surface treated CF/BMI composite had the strongest interfacial adhesion.     

Formation of a carbon fiber/polyhedral oligomeric silsesquioxane/carbon nanotube hybrid reinforcement and its effect on the interfacial properties of carbon fiber/epoxy composites

A carbon fiber/polyhedral oligomeric silsesquioxane/carbon nanotube (CF–POSS–CNT) hybrid reinforcement was prepared by grafting CNTs onto the carbon fiber surface using octaglycidyldimethylsilyl POSS as the linkage in an attempt to improve the interfacial properties between carbon fibers and an epoxy matrix. X-ray photoelectron spectroscopy, scanning electron microscopy, dynamic contact angle analysis and single fiber tensile testing were performed to characterize the hybrid reinforcements. Interlaminar shear strength (ILSS), impact toughness, dynamic mechanical analysis and force modulation atomic force microscopy were carried out to investigate the interfacial properties of the composites. Experimental results show that POSS and CNTs are grafted uniformly on the fiber surface and significantly increase the fiber surface roughness. The polar functional groups and surface energy of carbon fibers are obviously increased after the modification. Single fiber tensile testing results demonstrate that the functionalization does not lead to any discernable decrease in the fiber tensile strength. Mechanical property test results indicate the ILSS and impact toughness are enhanced. The storage modulus and service temperature increase by 11 GPa and 17 °C, respectively. POSS and CNTs effectively enhance the interfacial adhesion of the composites by improving resin wettability, increasing chemical bonding and mechanical interlocking.     

Improvement of interfacial properties in bismaleimide composites using functionalized graphene oxide grafted carbon fiber

A hierarchical reinforcement, which was used to improve the interfacial properties of bismaleimide (BMI) composites, was prepared by grafting functionalized graphene oxide (GO) onto a carbon fiber surface. The GO and carbon fibers were first functionalized separately to create interactional functional groups on their surfaces. The grafting process was then realized by an amidation reaction of the amine and acyl chloride function groups formed on GO and carbon fibers, respectively. The surface groups of functionalized GO and carbon fibers were identified by an X‐ray photoelectron spectroscopy (XPS). The resulting reinforcement was further characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and dynamic contact angle analysis. Experimental results showed that the functionalized GO were successfully grafted onto the carbon fibers surfaces and significantly increased the surface energy of carbon fibers. The study also indicated that the prepared hierarchical reinforcement could significantly improve the interfacial adhesion of resulting BMI composite. POLYM. ENG. SCI., 58:886–893, 2018. © 2017 Society of Plastics Engineers     

常压等离子体改善高性能纤维粘结性的研究

以氦气为载气,氧气为反应气体,对高强度聚乙烯和Twaron 1000芳纶两种高性能纤维进行常压等离子体处理,来改善纤维的粘结性能;采用单纤维抽拔实验测定等离子体处理前后纤维与环氧树脂之间的界面剪切力;利用原子力显微镜和X射线光电子能谱仪分析等离子体处理前后纤维表面形态和化学成分的变化。结果表明:高强度聚乙烯纤维和芳纶经常压等离子体处理后,纤维表面粗糙度增加,纤维表面碳元素含量下降,羟基、羧基等含氧或氮的极性基团增加,纤维粘结性能得到提高,但其强度无明显变化。     

The Effect of Surface Activation in Polymer Matrix-Carbon Fiber Interactions

Various carbon fibers (CF) were surface modified by chemical and electrochemical treatments for the purpose of establishing organic functional groups on the fiber surface. A series of fibers, surface oxidized for various periods of time, was prepared. The amounts of surface functionalities formed were assessed by means of contact angle measurements on single fibers. A suitable set of probe liquids was used to determine the LW (Lifshitz-van der Waals) and acid-base components of carbon fiber surfaces. Similar tests were made on commercial, sized carbon fibers, polystyrene (PS) and polymethyl methacrylate (PMMA), and their surface energies determined in terms of LW, surface acidity and surface basicity components. Work of adhesion values were calculated of all combinations of CF and polymer matrix couples by using these surface energies of both constituents. The calculated work of adhesion values were correlated to the ILSS values obtained from single fiber pull out tests with PS and PMMA as matrices.     

Influence of oxygen plasma treatment parameters on the properties of carbon fiber

This study is focused on the impact of oxygen plasma treatment on properties of carbon fibers and interfacial adhesion behavior between the carbon fibers and epoxy resin. The influences of the main parameters of plasma treatment process, including duration, power, and flow rate of oxygen gas were studied in detail using interlaminar shear strength (ILSS) of carbon fiber composites. The ILSS of composites made of carbon fibers treated by oxygen plasma for 1 min, at power of 125 W, and oxygen flow rate of 100 sccm presented a maximum increase of 28% compared to composites made of untreated carbon fibers. Furthermore, carbon fibers were characterized by scanning electron microscopy (SEM), tensile strength test, attenuated total reflectance Fourier transform infrared (ATR-FTIR), and Raman spectroscopy analyses. It was found that the concentration of reactive functional groups on the fiber surface was increased after the plasma modification, as well the surface roughness, which finally improved the interfacial adhesion between carbon fibers and epoxy resin. However, high power and long exposure times could partly damage the surface of carbon fibers and decrease the tensile strength of filaments and ILSS of treated fiber composites.     

Acid-base character of carbon fiber surfaces

The acid-base character of the surfaces of commercially available carbon fibers used in advanced composites is determined using both pulsed and continuous flow microcalorimetry techniques. The carbon fibers investigated include unsized versions of AS4, IM6, IM7X, T300, and several Apollo fibers with different levels of surface treatment. All of these carbon fiber surfaces are amphoteric and energetically heterogeneous. In general, the heats of preferential adsorption in the pulsed flow mode of bases dissolved in n-heptane are larger than for acids. While most of the adsorbates used are reversibly and physically adsorbed onto the carbon fibers, some primary and secondary amines exhibit irreversible binding to a portion of the surface. In continuous flow experiments the adsorption heat isotherms for several bases on T300 display a sharp jump at low probe concentrations, reflecting the energetically heterogeneous nature of these surfaces. Comparisons between the flow microcalorimetry data and other measures of the surface chemistry are made. Pulsed flow adsorption heats correlate with the amount of oxidized carbon species on the fiber surfaces as detected using ESCA, and recently reported results of inverse gas chromatography and programmed thermal desorption (S. Wesson, Textile Research Institute). Calorimetry results are also compared with fracture mechanical measures of fiber-resin adhesion in manufactured composites. Adsorption heats of both acids and bases on selected carbon fibers correlate with edge delamination and 90° flexural strengths of composites composed of these fibers in both epoxy and bismaleimide resins. This supports a causal connection between carbon fiber surface adsorption heats and a practical measure of fiber-resin adhesion.     

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.     

Recycling carbon fiber from composite waste and its reinforcing effect on polyvinylidene fluoride composite: Mechanical,morphology, and interface properties

Recycled carbon fiber (RCF) was reclaimed from thermoset composite waste and employed as reinforcement from 0 to 30 wt% to prepare polyvinylidene fluoride (PVDF)/RCF composite. Commercial virgin carbon fiber (VCF) was used as comparison. The surface morphology, chemistry, and tensile properties of carbon fibers were investigated by Scanning Electron Microscopy (SEM), X‐Ray Photoelectron Spectroscopy (XPS), and tensile test. Results showed that the roughness, O/C ratio and –COO content of RCF surface were significantly improved after recycling. In addition, the single fiber tensile strength and modulus of RCF was lower than that of VCF. The interfacial adhesion between RCF and PVDF was much stronger due to the high chemical activity and roughness over the RCF surface. Mechanical properties of composites were investigated by flexural test, impact test, and Dynamic Mechanical Analysis (DMA). It is found that the PVDF/RCF composite showed higher flexural properties, storage modulus, and lower impact strength, which indicated the strong interfacial adhesion, played an important role in reinforcing. The morphology of fracture further demonstrated the strong interface in PVDF/RCF composite. The fiber length distribution and crystallinity of composites were also evaluated to characterize the composites. The work develops potential for recycling and reuse of carbon fiber, and also expands the application of PVDF based composite. POLYM. COMPOS., 38:2544–2552, 2017. © 2015 Society of Plastics Engineers     

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.     

Plasma etching and plasma polymerization coating of carbon fibers. Part 2. Characterization of plasma polymer coated carbon fibers

Unsized AS-4 carbon fibers were subjected to RF plasma etching and/or plasma polymerization coating in order to enhance their adhesion to vinyl ester resin. Ar, N₂ and O₂ were utilized for plasma etching, and acetylene, butadiene and acrylonitrile were used for plasma polymerization coating. Etching and coating conditions were optimized in terms of plasma power, treatment time, and gas (or monomer) pressure by measuring the interfacial adhesion strength. Interfacial adhesion was evaluated using micro-droplet specimens prepared with vinyl ester resin and plasma etched and/or plasma polymer coated carbon fibers. Surface modified fibers were characterized by SEM, XPS, FT-IR, α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. Interfacial adhesion between plasma etched and/or plasma polymer coated carbon fibers and vinyl ester resin was reported previously (Part 1), and characterization results are discussed is this paper (Part 2). Gas plasma etching resulted in preferential etching of the fiber surface along the draw direction and decreased the tensile strength, while plasma polymer coatings altered neither the surface topography of fibers nor the tensile strength. Water contact angle decreased with plasma etching, as well as with acrylonitrile and acetylene plasma polymer coatings, but did not change with butadiene plasma polymer coating. FT-IR and XPS analyses revealed the presence of functional groups in plasma polymer coatings.     

Surface modification of Kevlar by grafting carbon nanotubes

A novel and efficient method was developed for surface‐modification of Kevlar fibers by multi‐wall carbon nanotubes (MWCNTs). Kevlar fibers were immersed in a solution mixed with Hexamethylene diisocyanate, 1,4‐diazabi‐cyclo [2,2,2] octane (DABCO), and toluene to introduce pendant amine groups before the COCl‐functionalized carbon nanotubes were chemically grafted onto the surface of modified fibers under ultrasonic condition. The characterization of resulting fiber involved in SEM, infrared spectroscopy, and tensile measurement. Results indicated over 20% of the fiber surface were coated by MWCNTs even after washing, which indicated a good adhesion. Furthermore, the mean value of tensile strength of Kevlar fiber was improved by 12% compared with original one. And the interlaminar shear strength (ILSS) of the fiber‐reinforced bismaleimides composite was increased by 30%. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

低温等离子体改性UHMWPE纤维的表面性能

采用空气低温等离子体改善超高相对分子质量聚乙烯(UHMWPE)短纤维的黏着性,设计正交试验对改性后纤维的黏着性进行测试与分析,确定出较优试验方案;然后对未处理和经较优方案改性后的UHMWPE短纤维的表面形貌、表面化学成分、表面润湿性和强伸性进行测试分析。结果表明:空气低温等离子体改性UHMWPE短纤维黏着性的较优处理条件为功率50 W、压强15 Pa、反应时间120 s,此时,纤维的剥离功是未处理的4.14倍,黏着性得到了大幅度的提升,且单纤维强力损失率仅为3.29%;经较优方案处理后,纤维表面的粗糙程度有所增加,表面润湿性有明显改善,纤维表面的C元素含量明显减少,O、N元素含量有所增加,且出现了相对含量为22.2%的C=O官能团,有利于UHMWPE短纤维黏着性的改善。     

Modification of polyacrylonitrile (PAN) carbon fiber precursor via post-spinning plasticization and stretching in dimethyl formamide (DMF)

This study investigates the possibility of using a post-spinning plasticization and stretching process to eliminate suspected property-limiting factors in polyacrylonitrile-based carbon fibers. This process was performed with the intention of removing surface defects (to improve tensile strength), attenuating fiber diameter (to promote more uniform heat treatment), and reducing molecular dipole interactions (to facilitate further molecular orientation). Among the various organic and inorganic solutions tested, treatment using aqueous dimethyl formamide (DMF) offered far and away the best properties and was therefore selected for further testing. Tested individually (as single filaments), fibers exposed to 80% DMF for 10 s gave the highest precursor values of elastic modulus (9.07 GPa) and tensile strength (675 MPa). While fibers treated in 80% DMF gave a 73% improvement in elastic modulus and a 53% improvement in tensile strength over as-received PAN, limitations in sample preparation and carbonization necessitated a reduction in DMF concentration (to 30%) to allow extraction of individual carbon fibers for tensile testing. Despite this compromise, results for fibers carbonized at 1000°C ultimately showed a 32% improvement in carbon fiber elastic modulus and a 14% improvement in carbon fiber tensile strength over regularly prepared carbon fibers. These results show that, to a certain extent, improvements in PAN precursor properties can translate to corresponding improvements in subsequently produced carbon fibers. Additional characterization using wide angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) suggests that these improvements are due in part to improved lateral order as well as the successful elimination of surface defects and prevention of skin-core formation.     

Processing,structure, and properties of gel spun PAN and PAN/CNT fibers and gel spun PAN based carbon fibers

Polyacrylonitrile (PAN) and PAN/carbon nanotube (PAN/CNT) fibers were manufactured through dry‐jet wet spinning and gel spinning. Fiber coagulation occurred in a solvent‐free or solvent/nonsolvent coagulation bath mixture with temperatures ranging from ?50 to 25°C. The effect of fiber processing conditions was studied to understand their effect on the as‐spun fiber cross‐sectional shape, as well as the as‐spun fiber morphology. Increased coagulation bath temperature and a higher concentration of solvent in the coagulation bath medium resulted in more circular fibers and smoother fiber surface. as‐spun fibers were then drawn to investigate the relationship between as‐spun fiber processing conditions and the drawn precursor fiber structure and mechanical properties. PAN precursor fiber tows were then stabilized and carbonized in a continuous process for the manufacture of PAN based carbon fibers. Carbon fibers with tensile strengths as high as 5.8 GPa and tensile modulus as high as 375 GPa were produced. The highest strength PAN based carbon fibers were manufactured from as‐spun fibers with an irregular cross‐sectional shape produced using a ?50°C methanol coagulation bath, and exhibited a 61% increase in carbon fiber tensile strength as compared to the carbon fibers manufactured with a circular cross‐section. POLYM. ENG. SCI., 55:2603–2614, 2015. © 2015 Society of Plastics Engineers     

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.     

Surface functionalization of unsized carbon fiber using nitrenes derived from organic azides

The surface of both oxidized and unoxidized unsized carbon fiber was functionalized using an aziridine linking group derived from reactive nitrenes. The aziridine functionality arose from the cyclization of a reactive nitrene species onto the highly electron rich graphitic surface of the carbon fibers; the nitrene species evolved from thermal N₂ elimination from the corresponding (room temperature stable) azide. Surface functionalization using the nitrene approach was supported by X-ray Photoelectron Spectroscopy, in both oxidized and unoxidized carbon fiber. Attempts were also made to functionalize using amide chemistry, the two-step acid chloride coupling being successful for oxidized fibers by utilizing the carboxylic acid rich defect sites on the carbon fiber. None of the chemical treatment pathways had a significant impact on the tensile strength of the individual fibers, and atomic force microscopy revealed that fibers undergoing these treatment methodologies remained intact, without creating additional surface defects.     

Study on surface- and mechanical fiber characteristics and their effect on epoxy composite properties tuned by continuous anodic carbon fiber oxidation

Continuous anodic oxidation was employed to alter the surface chemical properties of carbon fibers. As expected, the wetting behavior by water improved and that of non-polar liquid diiodomethane deteriorated. The calculated surface tensions mirror the changes in the physicochemical surface properties. The zeta (ζ)-potential measurements performed also reflect changes in the surface chemistry of the investigated carbon fibers. A correlation between the measured ζ-potentials and the wetting behavior of water on anodically oxidized carbon fibers was found. The influence of anodic carbon fiber oxidation on the epoxy composite properties was studied by a modified axial tensile test, which allows additionally the measurement of the so-called 'notching force' as a measure of the interfacial composite properties. Common model-composite samples were used to check the reliability of this test. The determined 'notching force' as a measure of adhesion correlates with the increased polar component of the fiber surface tension.