Effects of surface modification on rheological and mechanical properties of CNT/epoxy composites

Despite superior properties of carbon nanotubes (CNTs), physical properties of the CNT/epoxy composites are not improved significantly because interfacial bonding between the CNTs and the polymer matrix is weak. CNTs were treated by an acidic solution to remove impurities and modified subsequently by amine treatment or plasma oxidation to improve interfacial bonding and dispersion of nanotubes in the epoxy matrix. The functional groups on the surface of treated CNTs were investigated by X-ray photoelectron spectroscopy. The surface modified CNTs were embedded in the epoxy resin by ultra-sonication and the cured nanotube containing composites were characterized by field emission scanning electron microscopy. Rheological properties of nanotube containing epoxy resin and mechanical properties of the modified CNT/epoxy composites were improved because the modification of CNTs improved dispersion and interaction between the CNT and the epoxy resin.     

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.     

Interfacial improvement of carbon fiber/epoxy composites using a simple process for depositing commercially functionalized carbon nanotubes on the fibers

An aqueous suspension deposition method was used to coat the sized carbon fibers T700SC and T300B with commercially carboxylic acid-functionalized and hydroxyl-functionalized carbon nanotubes (CNTs). The CNTs on the fiber surfaces were expected to improve the interfacial strength between the fibers and the epoxy. The factors affecting the deposition, especially the fiber sizing, were studied. According to single fiber-composite fragmentation tests, the deposition process results in improved fiber/matrix interfacial adhesion. Using carboxylic acid-functionalized CNTs, the interfacial shear strength was increased 43% for the T700SC composite and 12% for the T300B composite. The relationship between surface functional groups of the CNTs and the interfacial improvement was discussed. The interfacial reinforcing mechanism was explored by analyzing the surface morphology of the carbon fibers, the wettability between the carbon fibers and the epoxy resin, the chemical bonding between the fiber sizing and the CNTs, and fractographic observation of cross-sections of the composites. Results indicate that interfacial friction, chemical bonding and resin toughening are responsible for the interfacial improvement of nanostructured carbon fiber/epoxy composites. The mechanical properties of the CNT-deposited composite laminate were further measured to confirm the effectiveness of this strategy.     

Quantification of carbon nanotube dispersion and its correlation with mechanical and thermal properties of epoxy nanocomposites

An experimental study is carried out to quantitatively assess the dispersion quality of carbon nanotubes (CNTs) in epoxy matrix as a function of CNT variant and weight fraction. To this end, two weight fractions (0.05% and 0.25%) of as-grown, oxidized, and functionalized CNTs are used to process CNT/epoxy nanocomposites. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared analysis of different variants of CNTs are used to establish the efficiency of purification route. While the relative change in mechanical properties is investigated through tensile and micro-hardness testing, thermal conductivity of different nanocomposites is measured to characterize the effect of CNT addition on the average thermal properties of epoxy. Later on, a quantitative analysis is carried out to establish the relationship between the observed improvements in average composite properties with the dispersion quality of CNTs in epoxy. It is shown that carboxylic (-COOH) functionalization reduces the average CNT agglomerate size and thus ensures better dispersion of CNTs in epoxy even at higher CNT weight fraction. The improved dispersion leads to enhanced interfacial interaction at the CNT/epoxy interface and hence provides higher relative improvement in nanocomposite properties compared to the samples prepared using as-grown and oxidized CNTs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48879.     

Multiscale fiber‐reinforced composites prepared by vacuum‐assisted resin transfer molding

Carbon nanotube (CNT)/aramid fiber epoxy composites were produced using a new manufacturing method proposed in this study. The rheological and morphological experiments of the CNT/PEO nanocomposites indicates that the PEO nanocomposites have a good dispersion state of the CNTs. The flexural mechanical properties of the aramid fiber/CNT epoxy composites were measured. The CNTs dispersed in the epoxy resin between the aramid fibers were observed using field emission scanning electron miscroscope (FESEM). It was found that the flexural properties of the multiscale fiber‐reinforced composites were higher than those of aramid fiber/epoxy composites. POLYM. COMPOS., 28:458–461, 2007. © 2007 Society of Plastics Engineers.     

Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials

Carbon nanotubes (CNTs) with superior mechanical properties have been of interest as reinforcement for polymer composites. However, the length of individual CNTs is limited. As a solution, yarns spun by twisting together multi-walled carbon nanotubes (MWCNTs) have been reported. In this study, untwisted CNT yarns were prepared by a non-conventional method drawing CNTs through a die. The MWCNTs in these yarns are held together by strong van der Waals forces that arise due to the interactions on the long and smooth surfaces of the MWCNTs. Here, mechanical properties of untwisted CNT yarn were studied by tensile tests. The strength of the CNT yarn was increased by increasing the apparent density of the yarn. The CNT yarns showed high tensile strength of 1 GPa and elastic modulus of 79 GPa at a yarn diameter of 35 μm. The interfacial shear strength between the CNT yarn and epoxy resin was studied by the microdroplet method, and it was very low. The wettability of the CNT yarn was affected by a type of curing agent. A unidirectional composite of epoxy resin and CNT yarn was prepared by the pultrusion molding method. Mechanical properties of the unidirectional composite were affected by the type of curing agent.     

Improvement of dispersion of carbon nanotubes in epoxy resin through pyrogallol functionalization

Multiwalled carbon nanotubes (MWNTs) were functionalized with pyrogallol. The functionalized MWNTs were well‐dispersed in the epoxy/curing agent/ethanol solution, as demonstrated by UV‐vis spectra and optical micrographs. Epoxy resin/MWNTs composites were prepared via solution mixing method. The cure behavior was characterized using differential scanning calorimetry. Pyrogallol‐functionalized carbon nanotubes (CNTs) reacted with the epoxy through the mediation reaction of pyrogallol with the curing agent, leading to the interfacial bonding between the functionalized carbon nanotubes (CNTs) and the resin matrix. Due to the excellent dispersion and interfacial bonding, the mechanical strength and electrical conductivity of the epoxy resin/CNTs composites have been improved. POLYM. ENG. SCI. 56:1079–1085, 2016. © 2016 Society of Plastics Engineers     

Structure and properties of aeronautical composites using carbon nanotubes/epoxy dispersion as nanocomposite matrix

The structure and properties of hybrid multiscale composites containing carbon nanotubes (CNTs) was reported. CNTs were dispersed in epoxy by using high energy ultrasonication, followed by the fabrication of CNT hybrid composites via resin transfer molding (RTM) processing. The processability of CNTs/epoxy systems was explored by a capillary experiment. The dependences of mechanical and electrical properties of the hybrid composites on CNT content were investigated. Microscopic observation confirms the formation of CNTs percolation network. The different roles of CNT networks in mechanical reinforcement and electrical amelioration were analyzed. One explanation based on the dispersion and distribution of CNTs is proposed. It is found that the variations of the hybrid composites with respect to mechanical and electrical properties are attributed to the hierarchical structure in the hybrid composites. As far as the hybrid multiscale composites produced via RTM process is concerned, the formation of CNT percolation network, subjected to dynamic impregnation, is hindered by the presence of continuous fibrous reinforcement. The hierarchical structure influenced by several competing factors reveals great potential in being able to tailor the structural and functional performance of the CNT hybrid composites. The effects of CNTs on the dimensional stability of polymer based composites are also assessed. POLYM. COMPOS., 34:1690–1697, 2013. © 2013 Society of Plastics Engineers     

The reduction of carbon nanotube (CNT) length during the manufacture of CNT/polymer composites and a method to simultaneously determine the resulting CNT and interfacial strengths

Carbon nanotube/epoxy composites with an excellent dispersion of carbon nanotubes (CNTs) were prepared using a three-roll calandering technique. CNT length after processing of composites is measured and then characterized using a two-parameter Weibull distribution function. Significant reduction of the CNT length is observed as a result of the processing and it is thus suggested that great attention should be paid to the retention of CNT length after processing in order to obtain good mechanical properties. Because of the difficulties in manipulating nanometer sized CNTs during measurement of CNT strength and CNT-polymer interfacial strength, CNT strength and CNT-polymer interfacial strength have previously been determined using complicated methods with expensive or specially designed equipments. In this work a simple methodology based on the modified rule of mixtures is proposed to simultaneously determine the CNT strength and CNT-polymer interfacial strength.     

Carbon nanotube/epoxy composites fabricated by resin transfer molding

Carbon nanotube (CNT)/epoxy composites with controllable alignment of CNTs were fabricated by a resin transfer molding process. CNTs with loading up to 16.5 wt.% were homogenously dispersed and highly aligned in the epoxy matrix. Both mechanical and electrical properties of the CNT/epoxy composites were dramatically improved with the addition of the CNTs. The Young’s modulus and tensile strength of the composites reach 20.4 GPa and 231.5 MPa, corresponding to 716% and 160% improvement compared to pure epoxy. The electrical conductivity of the composites along the direction of the CNT alignment reaches over 1 × 10⁴ S/m.     

Mapping local microstructure and mechanical performance around carbon nanotube grafted silica fibres: methodologies for hierarchical composites

The introduction of carbon nanotubes (CNTs) modifies bulk polymer properties, depending on intrinsic quality, dispersion, alignment, interfacial chemistry and mechanical properties of the nanofiller. These effects can be exploited to enhance the matrices of conventional microscale fibre-reinforced polymer composites, by using primary reinforcing fibres grafted with CNTs. This paper presents a methodology that combines atomic force microscopy, polarised Raman spectroscopy, and nanoindentation techniques, to study the distribution, alignment and orientation of CNTs in the vicinity of epoxy-embedded micrometre-scale silica fibres, as well as, the resulting local mechanical properties of the matrix. Raman maps of key features in the CNT spectra clearly show the CNT distribution and orientation, including a 'parted' morphology associated with long grafted CNTs. The hardness and indentation modulus of the epoxy matrix were improved locally by 28% and 24%, respectively, due to the reinforcing effects of CNTs. Moreover, a slower stress relaxation was observed in the epoxy region containing CNTs, which may be due to restricted molecular mobility of the matrix. The proposed methodology is likely to be relevant to further studies of nanocomposites and hierarchical composites.     

Effect of various methods of pre-treatment of carbon fibers on the mechanical properties of PIP-based ceramics/carbon composites

The purpose of the work was to determine the conditions of CF preparation to obtain carbide composites with favorable mechanical response. The relationships between the interfacial properties of fiber/polymethylsiloxane composite, and mechanical properties of the resulting fiber/carbide composites were investigated. The CF/resin interfacial strength was modified by oxidation of CF surface with nitric acid, silanization, and depositing CNT or a pyrolytic carbon layer (PyC). The study of composite interphases (ILSS and SEM) and surface tests of the modified CF (XPS, FT-IR, wettability measurements) showed different nature of the bonding occurring at the fiber/resin and fiber/ceramics boundary. The CF silanization significantly improved the ILSS between CFs and resin by 38.5%, while reduced flexural properties of carbide composites. The most promising treatment method of CF for PIP-based ceramic composites was modification with PyC, which provided 2 times higher ILSS, 1.5 times higher flexural strength and improved work to fracture (WF) as compared to unmodified CF.     

热固性树脂/碳纳米管复合材料的研究进展

介绍了碳纳米管(CNTs)在改性环氧树脂(EP)、酚醛树脂(PF)、双马来酰亚胺树脂(BMI)以及氰酸酯树脂(CE)等热固性树脂方面的应用,综述了EP/CNTs、PF/CNTs、BMI/CNTs、CE/CNTs复合材料的近期研究进展,详细叙述了CNTs对热固性树脂的力学性能、热性能、摩擦性能的改善情况,结合扫描电镜分析结果研究了CNTs在EP和BMI中的分散情况,并指出了热固性树脂/CNTs复合材料存在的问题和应用前景。     

Effect of ultrasound on wettability between aramid fibers and epoxy resin

Good wetting of reinforced fiber by resin was a main factor in the improvement of the interface adhesion of their composites. Ultrasound with a frequency of 20 kHz was used to improve the wettability between aramid fibers and epoxy resin during the winding process of the composites. The effects of ultrasound on the viscosity and surface tension of epoxy resin and on the surface characteristics of aramid fibers were investigated. The wettability of aramid fibers and treated epoxy resin under different conditions and of aramid fibers and epoxy resin under ultrasonic online treatment were compared. The results indicated that the main action of ultrasound was to force epoxy resin to impregnate aramid fibers, in addition to the influence of ultrasound on the properties of epoxy resin and aramid fibers. The results of microdebond testing showed that the interfacial shear strength (IFSS) of aramid/epoxy composites could be 26% higher than that of untreated composites because of the improved wettability between aramid fibers and epoxy resin subjected to ultrasonic online treatment. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Incorporation of hyperbranched polyamide‐functionalized graphene oxide into epoxy for improving interfacial and mechanical properties

The incorporation of hyperbranched polyamide‐functionalized graphene oxide (HPA‐GO) into epoxy was proposed to improve the interfacial and mechanical properties. Benefiting from improved dispersion and strengthened interfacial interaction, epoxy composites with HPA‐GO showed significant improvements in mechanical and thermomechanical properties at low GO loading. The interaction at the HPA‐GO/epoxy interface was investigated to confirm the occurrence of chemical bonding. Strong interfacial bonding improved the stress transfer and distribution of HPA‐GO/epoxy interface. Accordingly, the overall strength of epoxy composites was effectively improved on account of the uniform dispersion of HPA‐GO and interfacial chemical interaction between HPA‐GO and epoxy. Compared with neat epoxy resin, the inclusion of 0.10 wt% HPA‐GO led to 310.5 and 37.2% increase in impact strength and tensile strength, respectively. © 2019 Society of Chemical Industry     

The effect of surface functionalization of carbon nanotubes on properties of natural rubber/carbon nanotube composites

The objective of this study was to prepare natural rubber composites filled with carbon nanotubes (CNTs) that show an electrical percolation threshold at very low CNT concentrations. Therefore, two methods of surface functionalization of CNTs were investigated to enable an improved dispersion of CNTs and chemical interaction between CNTs and rubber matrix. On one hand, the CNTs have been functionalized ex situ by acid treatment and silanization reaction with bis(triethoxysilylpropyl) tetrasulfide before mixing with the rubber and otherwise in situ functionalization was directly carried out during the processing of the composites in the internal mixer. The grafting of silane molecules onto CNT surface was established by Fourier transform infrared spectroscopy and scanning electron microscopy. Tensile tests revealed the outstanding properties of composites prepared by in situ silanization method. The in situ silanization led to a better dispersion of the CNTs and the formation of chemical linkages between CNT surface and rubber and this became manifest in higher reinforcement of the rubber, higher crosslink densities, and a lower electrical percolation threshold. It was also shown that the in situ silanization is retarding the vulcanization reaction. POLYM. COMPOS., 36:2113–2122, 2015. © 2014 Society of Plastics Engineer     

Surface functionalization of carbon nanotubes by biological adhesive polymers carbopol for developing high‐permittivity polymer composites

Surface functionalization of carbon nanotubes (CNTs) by biological adhesive polymers carbopol (CP) was developed by simply mixing CNT suspension and an aqueous solution of CP without any toxic solvents. CP can be easily coated onto CNTs through hydrogen bonds O?C? OH?NH₂? C?O and electrostatic interaction between ? COO? on CP and ? NH₃⁺ on CNTs. After modification, the surface of the CNT is endowed with a large number of carboxyl groups, which can effectively prevent the reaggregation of CNT by electrostatic repulsion between the ionized carboxyl groups. Hence, highly dispersed functionally modifying CNT by CP (CP‐CNT) filler in polydimethylsiloxane (PDMS) matrix can be obtained. More important, with the help of adhesive properties of CP, the interfacial compatibility between fillers and matrix can also be improved. Thus, the CP‐CNT/PDMS composites exhibited higher dielectric permittivity comparing with CNT/PDMS composites at the same filler content. We present a potential and green approach of surface functionalization of CNT for preparing high‐permittivity polymer composites. J. VINYL ADDIT. TECHNOL., 26:165–172, 2020. © 2019 Society of Plastics Engineers     

Electrophoretic deposition of carbon nanotubes onto carbon-fiber fabric for production of carbon/epoxy composites with improved mechanical properties

Carbon nanotubes (CNTs) have been deposited onto carbon-fiber fabric using electrophoretic deposition (EPD) prior to the infusion of epoxy resin for the production of carbon/epoxy composites. The carbon-fiber fabric employed for EPD was used in the as-received condition, in which the proprietary epoxy sizing-agent was present. CNTs were functionalized prior to EPD using ozone treatment for oxidation, followed by chemical reaction with polyethyleneimine. The CNT oxidation used a novel recirculating system which enabled ozonolysis to be conducted on large-volume solutions of CNTs in the presence of high-powered sonication, facilitating preparation of stable dispersions suitable for EPD. Significant increases in the shear strength and fracture toughness of the carbon/epoxy composites with the CNT treatment have been measured relative to composites without the CNT treatment. Analysis of fracture surfaces revealed interlaminar regions with high levels of CNTs and evidence of good adhesion between the carbon nanotubes and sized carbon-fiber, which is believed to have contributed to the measured improvement in mechanical properties.     

Raman microscopy of residual strains in carbon nanotube/epoxy composites

Carbon nanotube (CNT) strain variations with temperature, as measured with Raman microscopy, are reported for pristine and functionalized CNT/epoxy composites. The CNT strain is derived from the difference in frequencies of the CNT vibrational G⁺-mode in the composite and that of a relaxed CNT, and shown to serve as a measure of the local residual strains in the composites. The magnitudes of these strains vary with both CNT functionalization and CNT concentration. At room temperature and with the same local concentration of CNTs in the composite, the strains of oxidized and polyamidoamine-functionalized CNTs are found to be 2.5 times higher than that of the composite containing pristine CNTs. The higher residual strain of the composites loaded with functionalized CNTs reflects their better adhesion and integration in the polymer matrix. These findings are in accordance with the improved tensile properties measured for the functionalized CNT composites.     

Carbon nanotube film/epoxy composites with high strength and toughness

A strong and tough carbon nanotube (CNT)/epoxy composite was fabricated by resin solution impregnation process based on floating catalyst chemical vapor deposition (CVD)–growth CNT films, which had a tensile strength and toughness of 405 MPa and 122 J/g, respectively, and good damping properties as well. Evolution of the composite structure revealed that the CNTs aligned along the tension direction with decreasing orientation angle, and the CNT bundle size enlarged during the tensile test process, which contributed to efficient load transfer among the composite network. Results showed that the proper resin content could bring benefit for strong connections and dense packing of CNTs/bundles, but excessive resin content was unfavorable for improving mechanical properties and conductivities of the nanocomposite. In addition, the resin in CNT film/epoxy composites had a lower crosslink density than that in a neat epoxy system, which endowed the CNT composites with large deformation capability. POLYM. COMPOS., 38:588–596, 2017. © 2015 Society of Plastics Engineers