Increasing the interfacial strength in carbon fiber/epoxy composites by controlling the orientation and length of carbon nanotubes grown on the fibers

Multi-walled carbon nanotubes (MWCNTs) were grafted onto carbon fibers (CFs) using an injection chemical vapor deposition method. The orientation and length (16.6–108.6 μm) of the MWCNTs were controlled by the surface treatment of the CFs and the growth time, respectively. The interface between the MWCNTs and the CFs indicated the grafted CNTs were immobilized by embedding catalyst on CFs. Two orders of magnitude increase in the specific surface areas of CFs was obtained by grafting the MWCNT. MWCNT–CF hybrids exhibited good wettability with the epoxy resin due to the surface roughness and capillary action. Single-fiber composite fragmentation tests revealed an remarkable improvement of interfacial shear strength (IFSS) controlled by the orientation and length of MWCNTs. MWCNTs with an perpendicular alignment and long length showed a high IFSS in epoxy composites due to better wettability and a large contact interface between the hybrids and the resin. Hybrids with an optimum length (47.2 μm) of aligned MWCNTs showed a dramatic improvement of IFSS up to 175% compared to that of pristine CFs.     

Effects of surface treatment of carbon fiber: Tensile property,surface characteristics,and bonding to epoxy

In this article, effects of electrochemical oxidation and sizing treatment of PAN‐based carbon fibers (CFs) on the tensile properties, surface characteristics, and bonding to epoxy were investigated. As found, the electrochemical oxidation improves the tensile strength of single CF by 16.0%, due to weakening the surface stress concentration and smoothing the surface structure. Further sizing treatment shows a negligible effect on the tensile strength. Both oxidation and sizing treatments significantly improve the wettability and surface energies of CFs by introducing oxygen‐containing functional groups. Microbond test was conducted to characterize the interfacial shear strength (IFSS) between a single fiber and an epoxy droplet. The oxidation treatment increases IFSS slightly, which is due to the contradictory effects of the formation of chemical bonds between the resin and CFs, and the reduced mechanical interlocking. Further sizing treatment significantly enhances IFSS from 73.6 to 81.0 MPa, due to the formation of vast chemical bonds. Furthermore, the oxidation and sizing treatment can effectively reduce the degradation of IFSS to the hygrothermal ageing for the CF/epoxy system. POLYM. COMPOS., 37:2921–2932, 2016. © 2015 Society of Plastics Engineers     

Improving thermal conductivity and shear strength of carbon nanotubes/epoxy composites via thiol‐ene click reaction

This study investigates the effect of the thiol‐ene click reaction on thermal conductivity and shear strength of the epoxy composites reinforced by various silane‐functionalized hybrids of sulfhydryl‐grafted multi‐walled carbon nanotubes (SH‐MWCNTs) and vinyl‐grafted MWCNTs (CC‐MWCNTs). The results of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermal gravimetric analysis (TGA), and transmission electron microscopy (TEM) show that the sulfhydryl groups and vinyl groups are successfully grafted onto the surface of MWCNTs, after treatment of MWCNT with triethoxyvinylsilane and 3‐mercaptopropyltrimethoxysilane, respectively. Scanning electron microscopy (SEM), HotDisk thermal constant analyzer (HotDisk), optical microscope, and differential scanning calorimetry (DSC) are used to characterize the resultant composites. It is demonstrated that the hybrid of 75 wt % SH‐MWCNTs and 25 wt % CC‐MWCNTs has better dispersion and stability in epoxy matrix, and shows a stronger synergistic effect in improving the thermal conductivity of epoxy composite via the thiol‐ene click reaction with 2,2′‐azobis(2‐methylpropionitrile) as thermal initiator. Furthermore, the tensile shear strength results of MWCNT/epoxy composites and the optical microscopy photographs of shear failure section indicate that the composite with the hybrid MWCNTs has higher shear strength than that with raw MWCNTs. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44579.     

Interfacial shear strength in carbon fiber‐reinforced poly(phthalazinone ether ketone) composites

We studied interfacial shear strength (IFSS) in carbon fiber (CF)‐reinforced poly (phthalazinone ether ketone) (PPEK) composites system, with emphasis on the influence of forming temperature of composite and sizing agent on CFs. To obtain apparent IFSS of CF‐reinforced PPEK composites shaped at various forming temperatures ranged from 20 up to 370°C, microbond test was carried out at single‐fiber composites. Results of microbond test showed that apparent IFSS was directly proportional to the difference between the matrix solidification temperature (forming temperature) and the test temperature and approximately 80% of the apparent IFSS in CF/PPEK composite system was attributed to residual radial compressive stress at the fiber/matrix interface. By sizing CF with sizing agent, the wettability of the fiber by the matrix was improved and the final apparent IFSS was also improved. POLYM. COMPOS., 34:1921–1926, 2013. © 2013 Society of Plastics Engineers     

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.     

Rheological properties and fracture toughness of epoxy resin/multi‐walled carbon nanotube composites

Multi‐walled carbon nanotubes (MWCNTs), surface‐treated via chemical functionalization, i.e., oxidation and amidation, were used to reinforce diglycidylether of bisphenol F (DGEBF) epoxy resin. The effects of the functionalization on the dispersion stability, rheological properties, and fracture toughness of DGEBF/MWCNT composites were investigated. The dispersion homogeneity of the MWCNTs in the epoxy matrix improved after functionalization. In addition, isothermal rheology measurements revealed that the DGEBF/dodecyl amine‐functionalized MWCNT (D‐MWCNT) composite had a longer gel time and higher activation energy of cross‐linking than the DGEBF/acid‐treated MWCNT (A‐MWCNT) composite. The fracture toughness of the former was also significantly higher than that of the latter; this resulted from the relatively high dispersion stability of the D‐MWCNTs in the epoxy matrix, owing to the presence of alkyl groups on the D‐MWCNT surface. POLYM. ENG. SCI., 55:2676–2682, 2015. © 2015 Society of Plastics Engineers     

Synthesis and characterization of LiFePO4–carbon nanofiber–carbon nanotube composites prepared by electrospinning and thermal treatment as a cathode material for lithium‐ion batteries

Binder‐free LiFePO₄–carbon nanofiber (CNF)–multiwalled carbon nanotube (MWCNT) composites were prepared by electrospinning and thermal treatment to form a freestanding conductive web that could be used directly as a battery cathode without addition of a conductive material and polymer binder. The thermal decomposition behavior of the electrospun LiFePO₄ precursor–polyacrylonitrile (PAN) and LiFePO₄ precursor–PAN–MWCNT composites before and after stabilization were studied with thermogravimetric analysis (TGA)/differential scanning calorimetry and TGA/differential thermal analysis, respectively. The structure, morphology, and carbon content of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composites were determined by X‐ray diffraction, high‐resolution transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, and elemental analysis. The electrochemical properties of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composite cathodes were measured by charge–discharge tests and electrochemical impedance spectroscopy. The synthesized composites with MWCNTs exhibited better rate performances and more stable cycle performances than the LiFePO₄–CNF composites; this was due to the increase in electron transfer and lithium‐ion diffusion within the composites loaded with MWCNTs. The composites containing 0.15 wt % MWCNTs delivered a proper initial discharge capacity of 156.7 mA h g^?1 at 0.5 C rate and a stable cycle ability on the basis of the weight of the active material, LiFePO₄. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43001.     

Dispersion of multiwalled carbon nanotubes in polyacrylonitrile‐co‐starch copolymer matrix for enhancement of electrical,thermal, and gas barrier properties

Nanomaterials gained great importance on account of their wide range of applications in many areas. Carbon nanotubes (CNTs) exhibit exceptional electrical, thermal, gas barrier, and tensile properties and can therefore be used for the development of a new generation of composite materials. Functionalized multiwalled carbon nanotubes (MWCNTs) reinforced Polyacrylonitrile‐co‐starch nanocomposites were prepared by in situ polymerization technique. The structural property of PAN‐co‐starch/MWCNT nanocomposites was studied by X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. The conductivity, tensile strength, and thermal properties of nanocomposites were measured as a function of MWCNT concentrations. The thermal stability, conductivity, and tensile strength of PAN‐co‐starch/MWCNT nanocomposites were improved with increasing concentration of MWCNTs. Oxygen barrier property of PAN‐co‐starch/MWCNT nanocomposites was calculated and it was found that, the property was reduced substantially with increase of MWCNTs proportion. The synthesized PAN‐co‐starch/MWCNT nanocomposites may used for electrostatically dissipative materials, aerospace or sporting goods, and electronic materials. © 2013 Society of Plastics Engineers     

Modifying the carbon fiber–epoxy matrix interphase with graphite nanoplatelets

In this study, a novel method consisting of coating carbon fibers (CF) with graphite nanoplatelets (GnP) is investigated for its ability to modify the mechanical properties in the interphase region. Coating the CF was achieved by immersing CF in a solution of GnP dispersed in an epoxy‐based solution for a few seconds. The influence of the processing conditions on the properties of the coating (thickness, homogeneity, quality of the GnP dispersion) is reported. Interfacial adhesion and the associated failure modes were evaluated by the single fiber fragmentation test. The maximum value of interfacial shear strength (IFSS) was achieved when a relative GnP concentration of 7.9 wt% on CFs, which led to 45 and 34% improvements in IFSS in comparison with the non‐coated CF and epoxy coated CF, respectively. POLYM. COMPOS., 37:1549–1556, 2016. © 2014 Society of Plastics Engineers     

Effect of polyurethane sizing on carbon fibers surface and interfacial adhesion of fiber/polyamide 6 composites

Commercial epoxy sized carbon fibers (CFs) or unsized CFs have poor interfacial adhesion with polyamide 6 (PA6). Here, CFs are coated with polyurethane (PU) and their surface properties in terms of surface chemistry, contact angle, roughness, and morphology, are investigated. The results of Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy demonstrate PU sizing evidently increases the quantity of polar functional groups on the CFs surface. The surface energy of the PU sized fiber is calculated according to the Owens–Wendt method. Compared with unsized fibers, the contact angle of PU sized fibers is decreased while their total surface energy is increased, indicating superior wettability. Moreover, transverse fiber bundle tests are performed to determine the interfacial adhesion between the CFs and PA6 matrix. The transverse fiber bundle strength of unsized CF is measured to be 12.57 MPa. For PU sized CFs processed with sizing concentration of 1.2%, this value is increased to 24.35 MPa, showing an increase of more than 90%. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46111.     

Temperature effect on graphene‐filled interface between glass–carbon hybrid fibers and epoxy resin characterized by fiber‐bundle pull‐out test

The overall mechanical performance of glass–carbon hybrid fibers reinforced epoxy composites depends heavily upon fiber–matrix interfacial properties and the service temperatures. Fiber‐bundle pull‐out tests of glass (GF) and/or carbon fiber (CF) reinforced epoxy composites were carried out at room and elevated temperatures. Graphene nanoplatelets were added in the interfacial region to investigate their influence on the interfacial shear strength (IFSS). Results show that IFSS of specimens with fiber‐bundle number ratio of GF:CF = 1:2 is the largest among the hybrid composites, and a positive hybridization effect is found at elevated temperatures. IFSS of all the specimens decreases with the increasing of test temperatures, while the toughness shows a contrary tendency. As verified by scanning electron microscopy observations, graphene nanoplatelets on fiber surface could enhance the IFSS of pure glass/carbon and hybrid fibers reinforced epoxy composites at higher temperatures significantly. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46263.     

Preparation and properties of multiwalled carbon nanotube/epoxy‐amine composites

Different amounts of multiwalled carbon tubes (MWCNTs) were incorporated into an epoxy resin based on diglycidyl ether of bisphenol A and both epoxy precursor and composite were cured with 4,4′‐diamino diphenyl sulfone. Transmission and scanning electron microscopy demonstrated that the carbon nanotubes are dispersed well in the epoxy matrix. Differential scanning calorimetry measurements confirmed the decrease in overall cure by the addition of MWCNTs. A decrease in volume shrinkage of the epoxy matrix caused by the addition of MWCNTs was observed by pressure–volume–temperature measurements. Thermomechanical and dynamic mechanical analysis were performed for the MWCNT/epoxy composites, showing that the T_g was slightly affected, whereas the dimensional stability and stiffness are improved by the addition of MWCNTs. Electrical conductivity measurements of the composite samples showed that an insulator to conductor transition takes place between 0.019 and 0.037 wt % MWCNTs. The addition of MWCNTs induces an increase in both impact strength (18%) and fracture toughness (38%) of the epoxy matrix with very low filler content. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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     

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     

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.     

Thermal conductivity and dynamic mechanical property of glycidyl methacrylate‐grafted multiwalled carbon nanotube/epoxy composites

The well dispersed multiwalled carbon nanotube (MWCNT)/epoxy composites were prepared by functionalization of the MWCNT surfaces with glycidyl methacrylate (GMA). The morphology and thermal properties of the epoxy nanocomposites were investigated and compared with the surface characteristics of MWCNTs. GMA‐grafted MWCNTs improved the dispersion and interfacial adhesion in epoxy resin, and enhanced the network structure. The storage modulus of 3 phr GMA‐MWCNTs/epoxy composites at 50°C increased from 0.32 GPa to 2.87 GPa (enhanced by 799%) and the increased tanδ from 50.5°C to 61.7°C (increased by 11.2°C) comparing with neat epoxy resin, respectively. Furthermore, the thermal conductivity of 3 phr GMA‐MWCNTs/epoxy composite is increased by 183%, from 0.2042 W/mK (neat epoxy) to 0.5781 W/mK. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of amino‐functionalization of multi‐walled carbon nanotubes on the dispersion with epoxy resin matrix

Amino‐functionalization of multiwalled carbon nanotubes (MWCNTs) was carried out by grafting triethylenetetramine (TETA) on the surfaces of MWCNTs through the acid–thionyl chloride way. The amino‐functionalized MWCNTs show improved compatibility with epoxy resin and, as a result, more homogenous dispersion in the matrix. The mechanical, optical, and thermal properties of the amino‐functionalized MWCNT/epoxy composites were also investigated. It was found that introducing the amino‐functionalized MWCNTs into epoxy resin greatly increased the charpy impact strength, glass transition temperature, and initial decomposing temperature of cured epoxy resin. In addition, introducing unfunctionalized MWCNTs into epoxy resin was found greatly depressing the light transmission properties, which would affirmatively confine the application of the MWCNTs/epoxy composites in the future, while much higher light transmittance than that of unfunctionalized MWCNTs/epoxy composites was found for amino‐functionalized MWCNTs/epoxy composites. SEM of the impact cross section and TEM of ultrathin film of the amino‐functionalized MWCNTs/epoxy composites showed that the amino‐functionalized MWCNTs were wetted well by epoxy matrix. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 97–104, 2006     

Electrical conductivity and oxygen permeability of polyacrylonitrile/multiwalled carbon nanotubes composites

Polyacrylonitrile (PAN)/Multiwalled carbon nanotube (MWCNT) nanocomposites were prepared by nonconventional ultrasonic‐assisted emulsifier free emulsion polymerization technique with variable percentage of functionalized carbon nanotube. PAN/MWCNT nanocomposites were characterized by ultraviolet‐visible (UV‐visible) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The result from UV‐visible suggested that the functionalized MWCNT had interfacial interaction with PAN matrices. The surface morphology of functionalized MWCNT and PAN/MWCNT nanocomposites were studied by scanning electron microscopy (SEM). Electrical properties of PAN/MWCNT nanocomposites were measured and the result indicated that the conductivity increased with increasing concentration of MWCNTs. The oxygen permeability of PAN/MWCNT nanocomposites gradually increased with increase of MWCNT concentration, the result which was in agreement with the vertical alignment ofMWCNT in SEM. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Influence of aminosilane‐functionalized carbon nanotubes on the rheometric,mechanical, electrical and thermal degradation properties of epoxidized natural rubber nanocomposites

We describe the preparation, characterization and physical properties of multiwalled carbon nanotube (MWCNT)‐filled epoxidized natural rubber (ENR) composites. To ensure better dispersion in the elastomer matrix, the MWCNTs were initially subjected to aminopropyltriethoxysilane (APS) treatment to bind amine functional groups (?NH₂) on the nanotube surface. Successful grafting of APS on the MWCNT surface through Si–O–C linkages was confirmed using Fourier transform infrared spectroscopy. Grafting of APS on the MWCNT surface was further corroborated using elemental analysis. ENR nanocomposites with various filler loadings were prepared by melt compounding to generate pristine and APS‐modified MWCNT‐filled elastomeric systems. Furthermore, we determined the effects of various filler loadings on the rheometric, mechanical, electrical and thermal degradation properties of the resultant composite materials. Rheometric cure characterization revealed that the torque difference increased with pristine MWCNT loading compared to the gum system, and this effect was more pronounced when silane‐functionalized MWCNTs were loaded, indicating that this effect was due to an increase in polymer–carbon nanotube interactions in the MWCNT‐loaded materials. Loading of silane‐functionalized MWCNTs in the ENR matrix resulted in a significant improvement in the mechanical, electrical and thermal degradation properties of the composite materials, when compared to gum or pristine MWCNT‐loaded materials.© 2013 Society of Chemical Industry     

Preparation and properties of multi‐walled carbon nanotubes/carbon fiber/epoxy composites

Multi‐walled carbon nanotubes/carbon fiber (MWCNTs/CF) hybrid fillers are employed to prepare MWCNTs/CF/epoxy composites. Results reveal that a great improvement of the thermal conductivities of the epoxy composites with the addition of MWCNTs/CF hybrid fillers, and the thermal conductivity of the MWCNTs/CF/epoxy composites is 1.426 W/mK with 8 vol% treated MWCNTs/CF hybrid fillers (5 vol% MWCNTs + 3 vol% CF). Both the flexural and impact strength of the MWCNTs/CF/epoxy composites are increased firstly, but decreased with the excessive addition of MWCNTs. The flexural and impact strength of the MWCNTs/epoxy composites are optimal with 2 vol% MWCNTs. For a given MWCNTs/CF hybrid fillers loading, the surface treatment of MWCNTs/CF hybrid fillers can further increase the thermal conductivities and mechanical properties of the MWCNTs/CF/epoxy composites. POLYM. COMPOS., 35:2150–2153, 2014. © 2014 Society of Plastics Engineers