Effects of sizing materials on the properties of carbon fiber‐reinforced polyamide 6,6 composites

This article aims to study the effect of the sizing materials type on the mechanical, thermal, and morphological properties of carbon fiber (CF)‐reinforced polyamide 6,6 (PA 6,6) composites. For this purpose, unsized CF and sized CFs were used. Thermogravimetric analysis was performed, and it has been found that certain amounts of polyurethane (PU) and PA sizing agents decompose during processing. The effects of sizing agent type on the mechanical and thermomechanical properties of all the composites were investigated using tensile, Izod impact strength test, and dynamic mechanical analysis. Tensile strength values of sized CF‐reinforced composites were higher than that of unsized CF‐reinforced composites. PA and polyurethane sized CF‐reinforced composites exhibited the highest impact strength values among the other sized CF‐reinforced composites. PU and PA sized CF‐reinforced composites denoted higher storage modulus and better interfacial adhesion values among the other sizing materials. Scanning electron microscope studies indicated that CFs which were sized with PU and PA have better interfacial bonding with PA 6,6 matrix among the sized CFs. All the results confirmed that PA and PU were suitable for CF's sizing materials to be used for PA 6,6 matrix. POLYM. COMPOS., 34:1583–1590, 2013. © 2013 Society of Plastics Engineers     

Properties of carbon fiber sized with poly(phthalazinone ether ketone) resin

We studied thermoplastic poly(phthalazinone ether ketone) (PPEK) resin as a sizing agent on carbon fiber, with emphasis on its thermal stability, surface energy, wetting performance, and interfacial shear strength (IFSS). X‐ray photoelectron spectroscopy characterization was carried out to study the chemical structure of sized/unsized carbon fibers. Scanning electron microscopy and atomic force microscopy were used to characterize surface topography. TGA was used to analyze the thermal stability. Meanwhile, contact angle measurement was applied to analyze the compatibility between the carbon fibers and PPEK and the surface energy of carbon fibers. IFSS of carbon fiber/PPEK composite was examined by microbond testing. It is found that carbon fibers uniformly coated with PPEK resin had better thermal stability and compatibility with PPEK resin than the uncoated fiber. The contact angle is 57.01° for sized fibers, corresponding to a surface energy of 49.96 mJ m^?2, much smaller than that for unsized ones with contact angle value of 97.05°. The value of IFSS for sized fibers is 51.49 MPa, which is higher than the unsized fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Interface improvement of carbon fiber/methylphenylsilicone resin composites by fiber surface coating of polyhedral oligomeric silsesquioxanes

To enhance interfacial properties of carbon fibers (CFs)-reinforced methylphenylsilicone resin (MPSR) composites, we introduced an appropriate interface reinforced by trisilanolphenyl-polyhedral oligomeric silsesquioxanes (trisilanolphenyl-POSS) between CFs and MPSR with a liquid phase deposition strategy. Chemical bonds among silanol groups of trisilanolphenyl-POSS, hydroxyl-functionalized CF (CF–OH), and silanol end groups of MPSR in the coating were expected to be formed through condensation reaction during the prepared process. CFs with and without sizing treatment-reinforced MPSR composites were prepared by a compression molding method. X-ray photoelectron spectroscopy revealed that trisilanolphenyl-POSS particles enhanced the contents of fiber surface oxygen-containing groups and silicon-containing functional groups. Scanning electron microscopy and atomic force microscopy images showed that trisilanolphenyl-POSS nanoparticles have been introduced onto the fiber surface obviously and the surface roughness increased sharply. Dynamic contact angle analysis indicated that trisilanolphenyl-POSS-modified sizing agent could improve the fiber wettability and surface energy significantly. Short-beam bending test and impact toughness test results showed that the interlaminar shear strength and impact resistance of the sized CFs composites were enhanced greatly with increasing amplitudes of more than 35 and 27% in comparison with those of untreated CF composites, respectively. Cryo-fractured surface topographies of composites confirmed that interfacial adhesion between CFs and MPSR has been improved after sizing treatment. Meanwhile, the sizing treatment does not decrease single fiber tensile strength.     

Improved interfacial adhesion in carbon fiber/epoxy composites through a waterborne epoxy resin sizing agent

A series of self‐emulsified waterborne epoxy resin (WEP) emulsions were used as surface sizing for carbon fibers (CFs) to improve the interfacial adhesion between the CF and epoxy matrix. In this work, the hydrogenated bisphenol‐A epoxy resin (HBPAE) was modified by polyethylene glycol (PEG) with molecular weights of 400, 800, 1000, 1500, 2000, 4000, and 6000 g/mol. The properties of the WEP emulsion were examined by Fourier transform infrared spectroscopy, dynamic light scattering, and transmission electron microscopy. The surface characteristics of sized CFs were evaluated using scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy. Afterwards, CF/EP composites were prepared and their fracture surface and interlaminar shear strength (ILSS) were examined. The results indicated that PEG2000 modified HBPAE sizing had the optimum emulsion stability and film‐forming ability. Meanwhile, the results also demonstrated that a continuous and uniform sizing layer was formed on the surface of CFs and the surface sizing was excellent in improving the chemical activity of CFs. Compared with unsized CFs, the O1s/C1s composition ratio was observed to increase from 11.51% to 33.17% and the ILSS of CF/EP composites increased from 81.2 to 89.7 MPa, exhibiting better mechanical property than that of commercial Takemoto S64 sized CFs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44757.     

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     

Development of multiwalled‐carbon‐nanotube‐welded carbon fibers and evaluation of the interfacial strength in epoxy composites

Multiwalled carbon nanotube (MWCNT)‐welded carbon fibers (CFs) were prepared by a three‐step process, which included polyacrylonitrile (PAN) coating, MWCNT absorption, and heat treatment. The structure of these materials was characterized by scanning electron microscopy, Fourier‐transform infrared spectroscopy, and Raman spectroscopy. The MWCNTs were uniformly assembled on the surface of the PAN‐coated CFs and welded by a PAN‐based carbon layer after heat treatment. The contact angle of the MWCNT‐welded CFs in the epoxy resins was 41.70°; this was 22.35% smaller than that of the unsized CFs. The interfacial shear strength (IFSS) of the MWCNT‐welded CF–epoxy composite was 83.15 MPa; this was 28.89% higher than that of the unsized CF–epoxy composite. The increase in the IFSS was attributed to the enhancement of adhesions between the CFs and polymer matrix through the welding of the MWCNTs on the CFs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45027.     

Effects of electrophoretically deposited carbon nanofibers on the interface of single carbon fibers embedded in epoxy matrix

Electrophoretic deposition (EPD) was used to deposit carboxylic acid-functionalized carbon nanofibers (O-CNFs) on the surface of single carbon fibers. Using the single fiber fragmentation technique and Weibull analysis, interfacial shear strength (IFSS) was estimated for different fiber surface treatments. Samples for sized, unsized, O-CNF deposited sized, and O-CNF deposited unsized carbon fibers were tested. Additionally, the effects of EPD were investigated by testing sized and unsized carbon fiber samples exposed to an electric field in water. Removal of the fiber sizing decreased IFSS by approximately 27%, but addition of O-CNFs to the unsized fiber surface led to an increase of 15% compared to the sized base fiber. The O-CNF deposited sized fibers provided IFSS increases of 207.6% and 66.9% for 1 and 5 min deposition durations, respectively. The surface morphology of all samples was characterized, and those containing homogeneous deposition of closely bound O-CNFs provided the highest IFSS values. Exposing sized fibers to the electric field for 1 min led to an IFSS increase of 79%, while unsized fibers undergoing the same treatment provided increases of 7.7% and 46% compared to the base sized fiber and unsized fiber samples, respectively.     

上浆剂对CF/EP界面粘结的影响

本文通过单纤维碎裂法，分别对未上浆及不同上浆剂处理的碳纤维与基体的界面粘结性进行表征。结果表明，不同上浆剂对界面粘结的影响不同。含活性基团较多的上浆剂可改善界面粘结，而含活性基团较少的上浆剂反而会使其减弱。     

Influence of new thermoplastic sizing agents on the mechanical behavior of poly(ether ketone ketone)/carbon fiber composites

In this study, unidirectional poly(ether ether ketone)/carbon fiber (CF) composite sheets were elaborated with unsized, epoxy‐sized, and thermoplastic‐sized CFs by hot‐press molding. The thermoplastic sizings that we used were poly(ether imide) (PEI) and poly(ether ketone ketone) oligomer aqueous dispersions. Scanning electron microscopy observation of the composites freeze fractures showed that unlike unsized or epoxy‐sized CFs, the thermoplastic sizings improved the interaction between the fibers and the matrix. A comparative study of the mechanical relaxations by dynamic mechanical analysis was carried out on the different composites before and after immersion in kerosene. At low temperature, the PEI sizing had a significant influence on the β relaxation, particularly after kerosene immersion. The thermoplastic sizings did not modify the glass‐transition temperature but improved the kerosene resistance on the composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42550.     

Prospect of nanoscale interphase evaluation to predict composite properties

For meeting the requirements of lightweight and improved mechanical properties, composites could be tailor-made for specific applications if the adhesion strength which plays a key role for improved properties can be predicted. The relationship between wettability and adhesion strength has been discussed. The microstructure of interphases and adhesion strength can be significantly altered by different surface modifications of the reinforcing fibers, since the specific properties of the interphase result from nucleation, thermal and/or intrinsic stresses, sizing used, interdiffusion, and roughness. The experimental results could not confirm a simple and direct correlation between wettability and adhesion strength for different model systems. The main objective of the work was to identify the interphases for different fiber/polymer matrix systems. By using phase imaging and nanoindentation tests based on atomic force microscopy (AFM), a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites was conducted. As model sizings for PP composites, γ-aminopropyltriethoxysilane (APS) and either polyurethane (PU) or polypropylene (PP) film former on glass fibers were investigated. The EP-matrix was combined with either unsized glass fibers or glass fibers treated with APS/PU sizing. It was found that phase imaging AFM was a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficiently small indentation force was found to be sufficient for measuring actual interphase properties within a 100-nm region close to the fiber surface. Subsequently, it also indicated a different gradient in the modulus across the interphase region due to different sizings. The possibilities of controlling bond strength between fiber surface and polymer matrix are discussed in terms of elastic moduli of the interphases compared with surface stiffness of sized glass fibers, micromechanical results, and the mechanical properties of real composites.     

Enhanced interfacial strength and mechanical properties of carbon fiber composites by introducing functionalized silica nanoparticles into the interface

Introducing nanoparticles onto the surface of carbon fibers (CFs) is a useful method for enhancing the quality of fiber-matrix interface. In this work, a liquid sizing agent containing functionalized silica nanoparticles (SiO₂) was well prepared to improve interfacial strength and mechanical properties of composites. In order to enhance the dispersion of SiO₂ nanoparticles in sizing agent, SiO₂ nanoparticles were chemically grafted with 3-aminopropyltriethoxysilane (APS), and then silanized silica (SiO₂-APS) was introduced into the interphase by a conventional sizing process as well. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) confirmed the successful preparation of SiO₂-APS. Scanning electron microscopy (SEM) showed that a uniform distribution of SiO₂-APS on the fiber surface and the increased surface roughness. The sized fibers (CF/SiO₂-APS) exhibited a high surface free energy and good wettability based on a dynamic contact angle testing. Interfacial microstructure and mechanical properties of untreated and sized CFs composites were investigated. Simultaneous enhancements of interlaminar shear strength (ILSS) and impact toughness of CF/SiO₂-APS composites were achieved, increasing 44.79% in ILSS and 31.53% in impact toughness compared to those of untreated composites. Moreover, flexural strength and modulus of composites increased by 32.22 and 50.0% according to flexural test. In addition, the hydrothermal aging resistance of CF/SiO₂-APS composites has been improved significantly owing to the introduced Si-O-Si bonds at the interface.     

芳纶纤维增强树脂基复合材料的界面粘结性能研究

为了改善芳纶纤维增强树脂基复合材料的界面粘结性能,从树脂基体入手,依据相似相容原理和芳纶的结构特点,合成出新型热固性树脂(AFR–T)用作芳纶复合材料的基体,以未经表面处理的芳纶作增强材料,采用热压成型法制备了AFR–T/芳纶纤维复合材料,并通过测定溶度参数、接触角、线膨胀系数、层间剪切强度(ILSS)和横向拉伸强度等方法研究了复合材料的界面粘结性能。结果表明,AFR–T树脂浇注体与芳纶的溶度参数相近,AFR–T树脂溶液在芳纶纸表面的接触角为36.9°,小于环氧树脂(EP)溶液与芳纶纸的接触角(53.2°),说明AFR–T树脂对芳纶的浸润性优于EP;AFR–T/芳纶纤维复合材料的ILSS和横向拉伸强度为73.0 MPa和25.3 MPa,分别比EP/芳纶纤维复合材料提高了25.9%和32.5%,这表明AFR–T树脂与芳纶纤维之间的浸润性和界面粘结性能较好。     

Interfacial stress transfer in nylon‐6/E‐Glass microcomposites: Effect of temperature and strain rate

In this study, the interfacial properties between E‐glass fibers with different commercial sizings have been investigated on model composites with a nylon‐6 matrix. In particular, the fiber critical length was measured by means of the single‐fiber fragmentation test over a wide range of temperatures (from 25 to 175°C) and strain rates (from 0.0008 to 4 min⁻¹). The general trend observed is that the fiber critical aspect ratio increases as the temperature increases and it decreases as strain rate is increased. The fiber critical aspect ratio for unsized fibers resulted to be reasonably well linearly related to the square root of the fiber to matrix modulus ratio. This results is in accordance with the Cox's shear‐lag theoretical model and the Termonia's numerical simulations. Sized fibers display an higher deviation from the theoretical prevision probably because of the presence of interphases whose properties are different from the bulk matrix. As a consequence, the interfacial shear strength values resulted to be dependent on the fiber sizing. In particular, the fibers coated with an epoxy sizing showed a superior thermal stability of the fiber matrix‐interface with respect to the unsized or nylon compatible sized fibers.     

Studies on surface energetics of glass fabrics in an unsaturated polyester matrix system: Effect of sizing treatment on glass fabrics

The surfaces of glass fibers were sized by polyvinyl alcohol (PVA), polyester, and epoxy resin types in order to improve the mechanical interfacial properties of fibers in the unsaturated polyester matrix. The surface energetics of the glass fibers sized were investigated in terms of contact angle measurements using the wicking method based on the Washburn equation, with deionized water and diiodomethane as the wetting liquids. In addition, the mechanical behaviors of the composites were studied in the context of the interlaminar shear strength (ILSS), critical stress intensity factor (K_IC), and flexural measurements. Different evolutions of the London dispersive and specific (or polar) components of the surface free energy of glass fibers were observed after different sizing treatments. The experimental result of the total surface free energies calculated from the sum of their two components showed the highest value in the epoxy‐sized glass fibers. From the measurements of mechanical properties of composites, it was observed that the sizing treatment on fibers could improve the fiber–matrix interfacial adhesion, resulting in improved final mechanical behaviors, a result of the effect of the enhanced total surface free energy of glass fibers in a composite system. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1439–1445, 2001     

Interfacial microstructure and properties of carbon fiber-reinforced unsaturated polyester composites modified with carbon nanotubes

To improve the interfacial properties in carbon fiber (CF)-reinforced unsaturated polyester (UP) composites, we directly introduced functionalized carbon nanotubes dispersed in the fiber sizing onto the fiber surface. For comparing the influence of polymer type on sizing effect, two different polymers (UP MR13006 and water-soluble epoxy (EP)) were used to prepare sizing agent. Morphology and surface energy of CFs were examined by scanning electron microscopy and dynamic contact angle analysis test. Tensile strength was investigated in accordance with ASTM standards. Mechanical properties of the composites were investigated by interlaminar shear strength (ILSS) and impact toughness. Test results indicate that TS, ILSS, and impact toughness were enhanced simultaneously. For UP matrix, the sizing agent containing UP has better reinforcing and toughening effect than the sizing agent containing water-soluble EP.     

Effect of hybrid polysphosphazene coating treatment on carbon fibers on the interfacial properties of CF/PA6 composites

Carbon fiber (CF) reinforced polyamide 6 (PA6) composite has an extensive application. However, the performances of CF/PA6 composite are constrained by the poor interfacial adhesion between CF and PA6 matrix. In this article, in order to strengthen the interfacial adhesion of CF/PA6 composite, a layer of poly(cyclotriphosphazene-co-4,4′-sulfonyldiphonel) (PZS) hybrid coating with plenty of PZS microspheres (PZSMS) was successfully introduced onto CF surface through facile in situ polymerization. After surface modification, the surface morphologies and the surface chemical structures of fibers changed distinctly. On one hand, the PZSMS provided more contact points and increased mechanical interlocking between CF and PA6 matrix. On the other hand, numerous hydrogen bonds between CF and PA6 were formed due to a great amount of unique polar groups on modified CF surface. Consequently, in comparison with untreated CF, the interfacial shear strength of CF-PZSMS/PA6 composites was improved from 37.68 ± 3.16 to 53.79 ± 3.38 MPa, by 42.75 ± 3.02%. The results indicated that PZS hybrid coating on fiber surface effectively improved the interfacial adhesion of CF/PA6 composites, and the stronger hydrogen bonding and the enhanced mechanical interlocking synergistically played a major role in such significant improvements.     

芳纶纤维/AFR树脂复合材料界面粘结性能的研究

为了改善芳纶纤维复合材料的界面粘结性能,合成了一种新型树脂(AFR)作为基体,以未经任何表面处理的芳纶纤维作增强材料,制备了芳纶纤维/AFR复合材料。采用测定表面能、接触角、层间剪切强度、横向拉伸性能和扫描电镜观察形貌等方法,从宏观和微观等方面研究了芳纶纤维/AFR复合材料的界面粘结性能。结果表明,AFR树脂与芳纶纤维有相近的表面能,AFR树脂溶液与芳纶纤维的接触角为42.8°,而环氧树脂(EP)与芳纶纤维的接触角为68°,说明AFR树脂对芳纶纤维的润湿性优于EP树脂;芳纶/AFR复合材料的层间剪切强度、横向拉伸强度和纵向拉伸强度分别为74.64MPa、25.34MPa和2256MPa,比芳纶/EP复合材料的相应强度分别提高了28.7%、32.5%和13.4%,其复合材料破坏面的形貌也说明芳纶纤维与AFR树脂之间的界面粘结性能较好。     

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     

Improving the interfacial and flexural properties of carbon fiber–epoxy composites via the grafting of a hyperbranched aromatic polyamide onto a carbon fiber surface on the basis of solution polymerization

Hyperbranched aromatic polyamide (HBP) was grafted successfully onto carbon fibers (CFs) on the basis of solution polymerization to enhance the interfacial adhesion strength of CF-reinforced epoxy resin composites. The microstructure and interfacial properties of the CFs before and after decoration were researched. The results indicate that HBP was deposited uniformly onto the CFs with γ-aminopropyl triethoxysilane as the bridging agent. The active groups, roughness, and surface energy of the modified fiber [hyperbranched aromatic polyamide grafted carbon fiber (CF–HBP)] increased visibly in comparison with those of the untreated CFs. The CF–HBP composites revealed simultaneous remarkable enhancements (65.3, 34.3, and 84.8%) in their interfacial shear strength, flexural strength, and modulus, respectively; this was attributed to the improvement in the fiber–epoxy interface through enhanced chemical interactions, mechanical interlocking, and wettability. These agreed with the scanning electron microscopy observations from the fracture surface morphologies of the composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47232.     

Improving the interfacial properties of carbon fiber–epoxy resin composites with a graphene-modified sizing agent

We successfully prepared a graphene-modified carbon fiber (CF) sizing agent with good dispersity and stability by dispersing reduced graphene oxide (RGO) into an emulsion-type sizing agent. RGO was obtained by the reduction of graphene oxide (GO) with the help of gallic acid. The influence of the graphene-modified sizing agent on the interfacial properties of the CF–epoxy resin composites was investigated with microbond testing and the three-point bending method. The results show that optimized interfacial properties were achieved when the size of the modified graphene was less than 1 μm, the content of RGO was 20 ppm, and the pH value of the sizing agent was 10.5. The interfacial shear strength of the composites reached 92.3 MPa, which was 29.6% higher than that of the composites with unmodified CFs. Compared with commercial-CF-fabric-reinforced composites, the interlaminar shear strength of the composites treated with the RGO-modified sizing agent increased by 21.5%. Both the interfacial and interlaminar failure morphologies of the composites were examined with scanning electron microscopy (SEM). The results show that a large amount of residual resin adhered to the surfaces of the CFs treated with the RGO-modified sizing agent; this indicated good interfacial properties between the CFs and the resin matrix. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47122.