Effect of carbon fiber surface functionality on the moisture absorption behavior of carbon fiber/epoxy resin composites

Polyacrylonitrile (PAN)‐based carbon fibers were electrochemically oxidized in aqueous ammonium bicarbonate with increasing current density. The electrochemical treatment led to significant changes of surface physical properties and chemical structures. The oxidized fibers showed much cleaner surfaces and increased levels of oxygen functionalities. However, it was found that there was no correlation between surface roughness and the fiber/resin bond strength, i.e. mechanical interlocking did not play a major role in fiber/resin adhesion. Increases in surface chemical functionality resulted in improved fiber/resin bonding and increased interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composites. The relationship between fiber surface functionality and the hydrothermal aging behavior of carbon fiber/epoxy composites was investigated. The existence of free volume resulted from poor wetting of carbon fibers by the epoxy matrix and the interfacial chemical structure were the governing factors in the moisture absorption process of carbon fiber/epoxy composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of low current density electrochemical oxidation on the properties of carbon fiber‐reinforced epoxy resin composites

Changes in surface physicochemical structures of polyacrylonitrile‐based carbon fibers resulted from low current density electrochemical oxidation were monitored by scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS). The relationship between the interlaminar shear strength (ILSS) values of carbon fiber‐reinforced polymers (CFRPs) and carbon fiber surface chemistry including elemental ratios and the relative content of oxygen‐containing functional groups were researched. SEM results revealed that the electrochemical oxidation got rid of surface contaminants generated during the production process. XPS analysis showed that the relative contents of oxygen and nitrogen increased by 446% and 202%, respectively, after the electrochemical oxidation. Carbon fiber surface chemistry was of paramount importance to the interfacial properties of CFRPs. The higher the carbon fiber surface activity, the better the interfacial bonding was, and an increase in the acidic‐group contents was responsible for a higher ILSS value. However, when the current density increased to 1.0 A/m², the interfacial bonding between carbon fiber and the epoxy resin became weak which led to the decline in ILSS values. Copyright © 2012 John Wiley & Sons, Ltd.     

Wettability of carbon fibers modified by acrylic acid and interface properties of carbon fiber/epoxy

The oxidation-reduction and pre-irradiation induced methods were employed to study the effect of acrylic acid modification on the wetting and adsorption ability of carbon fiber (CF) in epoxy solution and the interfacial properties of CF/epoxy. Systematic experimental work was conducted to determine the surface topography, surface energy, surface chemical composition, absorbability and tensile strength of carbon fibers and interfacial adhesion of CF/epoxy before and after modification. The roughness, surface energy, amount of containing-oxygen functional groups and wetting ability were all found to increase significantly after modifications. The tensile strength of carbon fibers was improved marginally by γ-ray pre-irradiation while was decreased little by oxidation-reduction modification. Consequently, the surface modifications of carbon fibers via both oxidation-reduction and pre-irradiation led to an improvement (more than 15%) of the interlaminar shear strength of CF/epoxy composites. The mechanisms of interfacial improvement of modified CF/epoxy composites are proposed.     

An improved microbond test method for determination of the interfacial shear strength between carbon fibers and epoxy resin

An improved microbond method, with a corresponding testing device, was developed to measure the interfacial shear strength (IFSS) between carbon fibers and epoxy resin. Compared to other methods, this proposed approach is both highly efficient and easy to operate. As a case study for this new method, we measured the IFSS between carbon fibers and epoxy resin. Although the average IFSS obtained was only 7.08 MPa, which is much lower than values documented in several previous studies, the displacement-load curves demonstrate the strong reliability of this method. The lower IFSS could be explained by the highly inert surface of the carbon fibers, which was highly graphitized and had no sizing treatment. Therefore, this method has high potential in applications for screening the sizing agents of carbon fibers or optimizing the surface sizing processes.     

Increased interfacial adhesion between carbon fiber and poly(vinylidene fluoride) by an aqueous sizing agent

The adhesion behavior of poly(vinylidene fluoride) (PVDF) to carbon fiber (CF) has always been a huge challenge, on account of the inertness nature of PVDF and the lack of reactive functional groups. In this work, a novel maleic anhydride grafted PVDF (MPVDF) aqueous sizing agent was prepared to modify the interface between CF and PVDF matrix. The surface properties of desized, MPVDF‐sized, and PVDF‐sized carbon fibers were characterized by the scanning electron microscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, and dynamic contact angle analysis test. The results demonstrated that the surface roughness increased from 39 to 55 nm, and surface energy increased from 40 to 74 mN m^?1 after MPVDF sizing treatment. The content of activated carbon atoms increased from 31.0% to 48.4%. Subsequently, the interlaminar shear strength was examined, for which was a critical indicator of the interfacial adhesion between CF and matrix. Compared with the desized CF, the value of interlaminar shear strength increased from 14.8 MPa to 25.5 MPa improved by 72% because of the improved H‐bonding formation, surface roughness, and wettability for MPVDF‐sized CF. In addition, the flexural strength and modulus were also improved by 47% and 74%, respectively. Copyright © 2016 John Wiley & Sons, Ltd.     

Improved mechanical properties of epoxy composites reinforced with surface‐treated UHMWPE fibers

The surface treatment of ultra‐high molecular weight polyethylene fiber using potassium permanganate and the mechanical properties of its epoxy composites were studied. After treatment, many changes were happened in the fiber surface: more O‐containing groups (―OH, ―C═O, and ―C―O groups), drastically decreased contact angles with water and ethylene glycol, slightly increased melting point and crystallinity, and formed cracks. Different contents (0.1–0.5 wt%) ultra‐high molecular weight polyethylene fibers/epoxy composites were prepared. The results indicated that the surface treatment decreased the tensile strength of epoxy composites, but increased the bending strength. When the fiber content was 0.3 wt%, the above properties reached the maximum. At the same fiber content, the interlaminar shear strength of the composites was increased by 26.6% up to the as‐received fiber composites. Dynamic mechanical analysis of the composites suggested the storage modulus and tanδ were decreased due to the surface treatment. Fractured surface analysis confirmed that the potassium permanganate treatment was effective in improving the interface interaction.     

Effects of noncovalently functionalized multiwalled carbon nanotube with hyperbranched polyesters on mechanical properties of epoxy composites

In order to improve the dispersibility and interface properties of multi-walled carbon nanotubes (MWCNTs) in epoxy resin (EP), aromatic hyperbranched polyesters with terminal carboxyl (HBP) and aromatic hyperbranched polyesters with terminal amino groups (HBPN) were used for noncovalent functionalization of MWCNTs. Epoxy composites reinforced by different types of MWCNT were prepared. The effects of noncovalent functionalization of MWCNTs on the dispersibility, wettability, interface properties and mechanical properties of epoxy composites were investigated. The results show that the dispersibility and wettability of MWCNTs are significantly improved after noncovalent functionalization. A large number of terminal primary amines (NH₂) on noncovalently functionalized MWCNT with HBPN (HBPN-MWCNT) form covalent bonds with EP matrix, and thus the interfacial adhesion is enhanced significantly, resulting in high load transfer efficiency and substantial increase in mechanical properties. The interface with covalent bonding formed between the flexible hyperbranched polyester layer on the surface of HBPN-MWCNT and the EP matrix promotes plastic deformation of the surrounding EP matrix. The toughening mechanisms of HBPN-MWCNT are MWCNT pull-out and a large amount of plastic deformation of the surrounding EP matrix.     

The effect of coupling agents on the interfacial properties of wood‐fiber–reinforced polyimide composites

Wood‐fiber–reinforced polyimide (PI) has been widely used in many engineering fields because of its high specific strength and stiffness. However, PI does not adhere well with wood fibers because it has a low free surface energy. In addition, high viscosity in the melted phase causes poor impregnation. In this study, surface treatment methods, ie, coupling agents with plasma treatment on wood fibers, were applied to increase the interfacial strength between the wood fibers and the PI matrix. The modified wood fiber surfaces were analyzed by X‐ray photoelectron spectroscopy and scanning electron microscopy. To analyze the effectiveness of the surface treatment method, the interlaminar shear strength (ILSS) was measured using the 3‐point bending test. From the test results, the ILSS of the specimens treated with the silane coupling agent after the plasma treatment increased by 48.7% compared with those of the untreated specimens.     

Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar fibers/epoxy composites using micromechanical techniques and surface wettability

Comparison of interfacial properties and microfailure mechanisms of oxygen-plasma treated poly(p-phenylene-2,6-benzobisoxazole (PBO, Zylon) and poly(p-phenylene terephthalamide) (PPTA, Kevlar) fibers/epoxy composites were investigated using a micromechanical technique and nondestructive acoustic emission (AE). The interfacial shear strength (IFSS) and work of adhesion, Wa, of PBO or Kevlar fiber/epoxy composites increased with oxygen-plasma treatment, due to induced hydrogen and covalent bondings at their interface. Plasma-treated Kevlar fiber showed the maximum critical surface tension and polar term, whereas the untreated PBO fiber showed the minimum values. The work of adhesion and the polar term were proportional to the IFSS directly for both PBO and Kevlar fibers. The microfibril fracture pattern of two plasma-treated fibers appeared obviously. Unlike in slow cooling, in rapid cooling, case kink band and kicking in PBO fiber appeared, whereas buckling in the Kevlar fiber was observed mainly due to compressive and residual stresses. Based on the propagation of microfibril failure toward the core region, the number of AE events for plasma-treated PBO and Kevlar fibers increased significantly compared to the untreated case. The results of nondestructive AE were consistent with microfailure modes.     

The interfacial feature of thermoplastic polystyrene composite filled with nitric acid oxidized carbon fiber

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 characterized 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 (IFSS) to increase. The IFSS of this fiber‐matrix combination allowed for the full utilization of the tensile strength of polystyrene. Copyright © 2009 John Wiley & Sons, Ltd.     

Interfacial adhesion and microfailure modes of electrodeposited carbon fiber/epoxy-PEI composites by microdroplet and surface wettability tests

Interfacial properties and microfailure modes of electrodeposition (ED)-treated carbon fiber-reinforced polyetherimide (PEI) toughened epoxy composite were investigated using microdroplet test and the measurement of surface wettability. ED was performed to improve the interfacial shear strength (IFSS). As PEI content increased, IFSS increased due to enhanced toughness and plastic deformation of PEI. In the untreated case, IFSS increased with adding PEI content, and the IFSS of the pure PEI matrix showed the highest. On the other hand, for the ED-treated case IFSS increased with PEI content with rather low improvement rate. In the untreated case, neat epoxy resin appeared brittle microfailure mode, whereas the pure PEI matrix exhibited a more likely ductile microfailure mode. In the ED-treated case, neat epoxy exhibited a more ductile fracture than that of the untreated case. Critical surface tension and polar surface free energy of ED-treated carbon fiber was higher than those of the untreated fiber. The work of adhesion between fiber and matrix was not directly proportional to IFSS for both the untreated and ED-treated cases. The matrix toughness might contribute to IFSS more likely than the surface wettability. Interfacial properties of the epoxy-PEI composite can be affected efficiently by both the control of matrix toughness and ED treatment.     

Orientation and mechanical properties of laser‐induced photothermally drawn fibers composed of multiwalled carbon nanotubes and poly(ethylene terephthalate)

This study presents a novel photothermal drawing of poly(ethylene terephthalate) (PET)/multiwalled carbon nanotube (MWCNT) fibers. The photothermal drawing was carried out using the near infrared laser‐induced photothermal properties of MWCNTs. An uniform fiber surface was obtained from a continuous necking deformation of the undrawn fibers, particularly at a draw ratio of 4 and higher. The breaking stress and modulus of the photothermally drawn PET/MWCNT fibers were significantly enhanced, in comparison to those of hot drawn fibers at the same draw ratio. The enhanced mechanical properties were ascribed to the increased orientation of PET chains and MWCNTs as well as PET crystallinity due to photothermal drawing. In particular, a significantly higher degree of orientation of the MWCNTs along the fiber axis was obtained from photothermal drawing, as shown in polarized Raman spectra measurements. The photothermal drawing in this study has the potential to enhance the mechanical properties of fibers containing MWCNTs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 603–609     

Effects of surface synergy for the dispersion of short carbon fibers sized by adipic acid modified epoxy resin potassium

The carbon fiber (CF) surface plays a critical role in the performance of CF composite materials. Adipic acid modified epoxy resin potassium (AAEK) prepared with epoxy resin and adipic acid, and KOH was employed as the CF sizing agent. Then, series of surface properties of AAEK‐treated carbon fiber (CF‐AAEK) including surface charge, morphology, and groups were characterized by using Faraday cup, friction coefficient gauge, atomic force microscopy, X‐ray photoelectron spectroscopy, and thermogravimetry. The results indicated that the dispersion coefficient of CF‐AAEK was increased by 1.72 times and there were synergistic effects for the dispersion of short CFs during the sizing treatment process with AAEK. In addition, the flexural strength of treated short CF composite proved to increase by 168%, which evaluated that the better CF dispersion in the matrix was a critical factor for the mechanical property improvement of short CF‐AAEK/epoxy resin composites.     

Effect of scale and surface chemistry on the mechanical properties of carbon nanotubes‐based composites

In this article, Multi‐Walled Carbon Nanotubes (MWCNTs) of varying diameters, both untreated and polycarboxylated, were dispersed at constant weight percentage in an epoxy matrix, and resulting fracture toughnesses (K_Ic) were measured in each case. We show that changing the MWCNT diameter has two effects on the composite fracture toughness: (i) a small MWCNT diameter enables larger interfacial surface for adhesion maximization, which increases toughness; (ii) at the same time, it limits the available pull‐out energy and reduces the MWCNT ability to homogeneously disperse in the matrix due to this same large active surface: this decreases toughness. Most commercially available MWCNTs have a length range of several μm, thus an optimal diameter exists which depends on MWCNT wall thickness and surface treatment. Such optimal diameter maximizes pull‐out energy and thus composite fracture toughness. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

碳纳米管改性聚苯硫醚熔纺纤维的结构与性能研究

将多壁碳纳米管(MWCNTs)和聚苯硫醚(PPS)经过熔融挤出后制备成复合材料切片,并采用熔融纺丝法制得碳纳米管改性聚苯硫醚复合纤维.采用扫描电镜(SEM)、拉曼光谱、示差扫描量热分析(DSC)、动态机械分析(DMA)以及力学性能测试等表征手段研究了复合纤维中碳管的分散状态,与基体的界面作用,复合纤维的结晶性能以及力学性能,从而探讨了聚苯硫醚/碳纳米管复合纤维体系的微观结构与宏观性能之间的关系.研究表明,聚苯硫醚分子结构与碳纳米管之间具有的π-π共轭作用使碳管较为均匀的分散在基体中,界面结合较为紧密.同时熔融纺丝过程中的拉伸作用使碳管进一步解缠并使碳管沿纤维拉伸方向取向.另一方面,拉曼光谱显示拉伸作用有效地增强了界面作用,有利于外界应力的传递.碳管的良好分散以及强的界面作用使复合纤维力学性能得到大幅度的提高,当碳管含量达到5 wt%时,复合纤维的模量有了明显的提高,拉伸强度较纯PPS纤维提高了近220%.     

Enhanced mechanical properties of carbon fiber composites by grafting different structural poly(amido amine) onto fiber surface

The mechanical properties of carbon fiber composites depend on the interfacial strength between fiber and epoxy matrix. Different poly (amido amine) (PAMAM) dendrimers were grafted onto carbon fiber to improve the interfacial strength of the resulting composites. Functional groups on the carbon fiber surface were examined by X-ray photoelectron spectroscopy. The surface morphology of the resulting materials was characterized by scanning electron microscopy and atomic force microscope. The characterization results revealed that PAMAM dendrimers were chemically grafted onto the surface of carbon fiber. More importantly, the mechanical properties of the resulting composites were enhanced owing to the presence of sufficient functional groups on the carbon fiber surface. In addition, after PAMAM containing chair conformations were grafted, the interlaminar shear strength had the highest increase of 53.13%, higher than that of the fiber grafted with PAMAM containing terminated linear amine. This work provides an alternative approach to enhance the mechanical properties of fiber composites by controlling the interface between fiber and epoxy matrix.     

The effect of mixing methods on the dispersion of carbon nanotubes during the solvent-free processing of multiwalled carbon nanotube/epoxy composites

Several solvent-free processing methods to disperse multiwalled carbon nanotubes (MWCNTs) in bisphenol F-based epoxy resin were investigated, including the use of a microfluidizer (MF), planetary shear mixer (PSM), ultrasonication (US) and combinations. The processed mixture was cured with diethyl toluene diamine. Three complimentary techniques were used to characterize the dispersion of the MWCNTs in cured composite samples: optical microscopy, micro Raman spectroscopy, and scanning electron microscopy (SEM). For sample MF + PSM, optical micrographs and Raman images showed reduced agglomeration and a homogeneous distribution of MWCNTs in the epoxy matrix. SEM analysis of fractured specimen after tensile testing revealed breakage of nanotubes along the fracture surface of the composite. A comparison of the MWCNT dispersion in the epoxy samples processed using different methods showed that a combination of MF and PSM processing yields a more homogeneous sample than the PSM or US + PSM processed samples. Mechanical testing of the composites showed about 15% improvement in the tensile strength of samples processed by the MF + PSM method over other methods. Thermogravimetric analysis (TGA) results showed a small decrease in the onset degradation temperature for poorly dispersed samples produced by PSM compared with the well-mixed samples (MF + PSM). These results strongly suggest that the MF + PSM processing method yield better-dispersed and stronger MWCNT/epoxy composites. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

原位合成纳米SiO2增强阴离子聚酯型上浆剂及其对碳纤维复合材料层间剪切强度的影响

采用溶胶-凝胶法, 在侧链带有羧基的线性不饱和聚酯中加入正硅酸乙酯(TEOS), 使TEOS在酸性条件下发生水解反应, 原位合成纳米SiO₂增强阴离子型聚酯乳液(SEAPE). 利用傅里叶变换红外光谱(FTIR)仪、 激光粒度分析仪和冷冻扫描电子显微镜(Cryo-SEM)对SEAPE进行分析与表征. 将SEAPE与聚乙二醇单油酸酯润滑剂、 非离子型表面活性剂FC-4430及抗氧剂1010进行复配, 原位制备纳米SiO₂增强阴离子型聚酯乳液上浆剂(SEAPEs), 用扫描电子显微镜(SEM)、 视频动态接触角测量仪、 X射线能谱(EDS)仪和纤维强力仪对SEAPEs上浆后碳纤维的表面形貌、 表面能、 碳纤维(CF)表面元素及碳纤维增强不饱和聚酯(UPR)复合材料(CF/UPR)的层间剪切强度(ILSS)进行测试与表征. 结果表明, 当TEOS添加质量分数为5%时, SEAPEs上浆后的碳纤维有效增强了其与UPR的结合强度, CF/UPR复合材料的ILSS达到40.03 MPa, 与市售环氧树脂型上浆剂上浆后碳纤维增强UPR复合材料相比, ILSS提高90.1%. SEAPEs中原位生成的纳米SiO₂分散均匀, 乳液储存稳定, 上浆后SiO₂均匀吸附在碳纤维表面, 增加碳纤维表面能, 改善碳纤维与树脂间的浸润性, 可有效提高碳纤维增强不饱和聚酯树脂复合材料的ILSS.     

Acid treatment for functionalizing polyethylene fiber surfaces for enhanced adhesion to epoxy resins

The utility of high-strength, high-modulus polyethylene fibers in fiber-reinforced composites is limited due to its poor interfacial adhesion to various polymeric matrices. One way to overcome this limitation is to introduce reactive functionalities on the fiber surface capable of covalently bonding to matrix resins. Ultra high-strength polyethylene (UHSPE) fibers were treated with chlorosulfonic acid. The surface acid groups were found to considerably improve the interfacial adhesion between polyethylene fibers and epoxy resins as shown by the microbond test. These surface functionalities were found to improve the fiber wettability, as shown by contact angle measurements using the Wilhelmy balance method. Colorimetric measurements of methylene blue absorption were used to quantify the surface concentrations of the acid groups. It was possible to functionalize the UHSPE fiber surfaces using this method to obtain fibers that formed a stronger adhesive bond with epoxy resins; this was achievable without sacrificing other fiber mechanical properties.