Processing and properties of syntactic foams reinforced with carbon nanotubes

This article presents synthesis and mechanical characterization of carbon nanotube (CNT)‐reinforced syntactic foams. Following a dispersion approach (comprising ultrasonic, calendering, and vacuum centrifugal mixing), single‐ and multi‐walled functionalized CNTs (FCNTs) were incorporated into two foam composites containing various commercially available microballoon grades (S38HS, S60HS, and H50 from 3M). The FCNT‐reinforced composites were tested for compressive strength and apparent shear strength before and after hot/wet conditioning. The results showed that the FCNT‐reinforced composites' mechanical properties depended on the vacuum pressure used during processing. Compared with pristine and commercially available syntactic foam (EC‐3500 from 3M), the FCNT‐reinforced composites processed at high vacuum (0.2 kPa) showed significant increase in compressive strength and apparent shear strength before and after hot/wet conditioning. Dynamic mechanical analysis showed an increase of about 22°C in glass transition temperature for composites processed at high vacuum with 0.5 wt % FCNT and 45 wt % S38HS–5 wt % S60HS microballoons. Thermogravimetric analysis indicated water absorption and lower decomposition temperature for the FCNT‐reinforced composite mixed at atmospheric pressure, whereas no significant change was observed for the compound processed at high vacuum. Fracture analysis showed matrix failure for the composite processed at high vacuum and microballoon crushing for the composite mixed at atmospheric pressure. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Multifunctional polymer foams with carbon nanoparticles

Increasingly demanding industry requirements in terms of developing polymer-based components with higher specific properties have recently aroused a great interest around the possibility of combining density reduction through foaming with the addition of small amounts of functional nanosized particles. Particular interest has been given to the creation of lightweight conductive polymers by incorporating conductive carbon-based nanoparticles, related to processing improvements in attaining homogeneous nanoparticle dispersion and distribution throughout the polymer as well as new processes that enable a higher control and throughput of highly pure carbon nanoparticles, which could overcome some of the common problems of conductive polymers, such as high cost and poor mechanical properties. This review article considers the use of carbon nanoparticles in polymer foams, initially focusing on the important aspects of foam preparation, the main results found in the literature about conductive polymer composites containing carbon nanoparticles, as well as the main polymer foaming processes and types of foams. The main section is dedicated to the properties of multifunctional polymer foams with carbon nanoparticles, with special focus being given to the electrical and transport properties of these materials.     

High-shear processing induced homogenous dispersion of pristine multiwalled carbon nanotubes in a thermoplastic elastomer

We report a high-shear processing technology that allows the homogenous dispersion of unmodified multiwalled carbon nanotubes (UMWNTs) in a thermoplastic elastomer, poly(styrene-b-butadiene-co-butylene-b-styrene) (SBBS). We demonstrated that the dispersion of UMWNTs in a polymer matrix depends greatly on the shear stress exerted during melt processing. Mechanical tests showed that the tensile modulus, tensile strength and elasticity of the composites with fine nanotube dispersion processed at a high-shear rate are much higher than those of the composites processed at a low shear rate. The results indicated that high-shear processing is an effective method of improving the dispersion of unmodified carbon nanotubes in a polymer matrix.     

Processing of yttria stabilized zirconia reinforced with multi-walled carbon nanotubes with attractive mechanical properties

The improvement of the mechanical properties of carbon nanotube reinforced polycrystalline yttria-stabilized zirconia (CNT-YSZ) was questionable in earlier investigations due to several difficulties for processing of these composites. In the present article, the authors are proposing a successful technique for mixing pre-dispersed CNTs within YSZ particles followed by a fast spark plasma sintering at relatively low temperature, resulting in near full-dense structure with well-distributed CNTs. Composites with CNT quantities ranging within 0.5-5 wt% have been analyzed and a significant improvement in mechanical properties, i.e. Young's modulus, indentation hardness and fracture toughness with respect to monolithic YSZ could be observed. To support these interesting mechanical properties, high-resolution electron microscopy and Raman spectroscopy measurements have been carried out. The analysis of densification shows that the lower densification rate of CNT reinforced composites with respect to the pure YSZ could be attributed to a slower grain boundary sliding or migration during sintering.     

Poly(vinyl alcohol) foams reinforced with carbon nanotubes for stapedial annular ligament applications

This study aims to develop carbon nanotubes (CNTs) reinforced poly(vinyl alcohol) (PVA) foams as a possible material for stapedial annular ligament (SAL) application. As-grown (AG) and purified CNTs are used as reinforcing fillers for PVA foams. Uniaxial and cyclic compression tests reveal that specific modulus and energy dissipation behavior improve after reinforcing foam with CNTs. A relatively higher improvement in specific modulus is recorded for purified CNTs as they tend to produce stiffer cell walls. Thermogravimetric analysis shows thermal stability improves after addition of CNTs in PVA foams. The 50 wt % degradation temperature is higher for PVA_AG foam in comparison to neat PVA foam. Under dynamic loading storage, modulus is found to be higher for CNT doped foams with higher relative improvement with purified CNTs than AG CNTs. It is shown that reinforcing PVA foams with purified CNTs is a feasible strategy to improve their average mechanical properties and microstructure for SAL application. While the specific elastic modulus of neat PVA foam found to be in range of 0.05–0.06 MPa gcc⁻¹ with almost zero porosity. The addition of CNTs provides a wide range of specific elastic modulus 0.1–1.3 MPa gcc⁻¹ with an average pores size of about 300 μm. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48736.     

Thermo-oxidative aging of high-density polyethylene reinforced with multiwalled carbon nanotubes

The purpose of this study was to investigate the influence of aging on the properties of high-density polyethylene (HDPE) reinforced with multi-wall carbon nanotubes (MWCNTs). Nanocomposites were prepared with nanotubes at 0, 1, 3, and 5 wt%. The long-term durability of the prepared materials was evaluated by thermo-oxidative aging test. Test bodies were aged at 110°C for up to 10 weeks. The nanocomposites were characterized by differential scanning calometry, thermogravimetric analysis (TGA), ¹³C-NMR, elongation at break, and transmission electron microscopy. The aging mainly occurred on the surface of the samples and the neat HDPE showed a strong yellowing after the aging. A strong reduction in elongation at break was seen. For neat HDPE, the elongation at break was reduced from roughly 1400–25%. When HDPE was reinforced with the nanotubes, the reduction was less dramatic.     

Improvement in mechanical and thermal properties of phenolic foam reinforced with multiwalled carbon nanotubes

Phenolic foams reinforced with pristine and functionalized multiwalled carbon nanotubes (MWCNTs) were fabricated to develop fire‐resistant materials with improved mechanical properties. The influences of the contents of carboxyl multi‐walled carbon nanotubes (MWCNTs‐COOH) and of MWCNTs types on the compressive properties of the composite foams were investigated. The microstructure and detailed failure behavior of MWCNTs/phenolic composite foams were studied using scanning electron microscopy (SEM) and in situ quasistatic compression inside SEM, respectively. In addition, thermal performances were evaluated by thermogravimetric analysis (TGA) and vertical burning method. It is found that as heterogeneous nucleation agents, MWCNTs increase cell density and decrease cell size of the produced foams, and that as reinforcements located in cell walls, MWCNTs impart high strength and stiffness to brittle foams. Moreover, MWCNTs reinforced foams have higher thermal stability than raw foams and exhibit similar excellent resistance to flame, confirming the effectiveness of MWCNTs as stabilizers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1479–1488, 2013     

Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

An experimental and theoretical investigation of the compressive properties of multi-walled carbon nanotube/poly(methyl methacrylate) nanocomposite foams

Polymer nanocomposite foams are promising low density substitutes for nanocomposites. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Good control of foam properties is of great importance in the application of such materials. In the current study, multi-walled carbon nanotubes (MWNTs) with controlled aspect ratio were used to alter the foam morphology in MWNT/poly(methyl methacrylate) (PMMA) nanocomposite foams produced by a supercritical carbon dioxide (CO₂) foaming process. It was found that with the addition of one weight percent of MWNTs, the Young’s modulus of polymer foams increased by as much as 82%, and the collapse strength increased by as much as 104%. The influence of MWNT aspect ratio on the compressive properties of nanocomposite foams was investigated. The addition of MWNTs influenced the foam properties in two ways: improving the compressive properties of the solid matrix, and reducing the bubble size of the nanocomposite foams. A modified constitutive model for predicting the compressive properties of high density closed-cell polymer foams was developed. The influence of the bubble size on the mechanical properties of polymer foams was discussed based on the new model.     

An effective surfactant-free isolation procedure for single-wall carbon nanotubes

An optimised isolation procedure of single-wall carbon nanotubes (SWNTs) from a SWNT soot without using any surfactant is reported. Amorphous carbon and small graphitic particles were washed away with N-methyl-2-pyrrolidone (NMP) and acetone. A large amount of graphite-coated metal particles were removed with the oxidation of the SWNT material with HNO₃ (6.5 and 4 M) and by washing the oxidised SWNT material with a mixture of methanol (MeOH) and deionised water. The isolated material was investigated with transmission electron microscopy (TEM) and Raman scattering (647.1 and 532 nm). An elemental analysis of the content of Co and Ni in the SWNT samples isolated at different steps of the isolation procedure was performed. On the basis of the TEM images and elemental analysis it was estimated that the purified material contains more than 75 wt.% of SWNTs.     

Mechanical properties of carbon fiber reinforced bisphenol A dicyanate ester composites modified with multiwalled carbon nanotubes

A novel electrophoretic deposition (EPD) method was employed for grafting multiwalled carbon nanotubes (MWCNTs) on carbon fibers, which, after impregnation with bisphenol A dicyanate ester (BADCy), synergistically reinforced BADCy matrix composites (CNT‐C/BADCy). The effect of MWCNT presence on the mechanical properties of the composites was investigated. Composite tensile strength increased by 45.2% for an EPD duration of 2 min, while flexural strength exhibited a decreasing trend with EPD duration. Optical microscopy revealed that the existence of MWCNTs enhanced the fiber‐matrix interface while a large number of CNTs were observed to have pulled‐out from the matrix, a finding which explained the observed tensile strength increase in terms of energy dissipation by the specific toughening mechanism. The flexural strength decrease of the composites with CNTs as compared to specimens without nanotubes was found linked to the increased stress concentration in the BADCy matrix due to tube presence which weakens the adhesion between carbon fabrics. In a word, carbon nanotubes will enhance the micro interface and weaken the macro interface of the composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45100.     

Mechanical behavior of phenolic-based composites reinforced with multi-walled carbon nanotubes

Two types of carbon nanotubes (CNTs), the network multi-walled nanotubes (MWNTs) and the dispersed MWNTs, were used for fabricating MWNTs/phenolic composites. The MWNTs were synthesized using the floating catalyst method through the chemical vapor deposition process. The effects of the MWNT content on the mechanical properties of the composites were investigated. Modified Halpin-Tsai equation was proposed to evaluate the Young’s modulus and tensile strength of the MWNTs/phenolic composites by adopting an orientation factor and an exponential shape factor in the equation. It is found that the results obtained from the modified Halpin-Tsai equation on tensile strengths and Young’s moduli fit successfully the experimental ones. The tensile fracture surfaces of MWNTs/phenolic composites were examined using field emission scanning electron microscope to study the failure morphologies of the MWNTs/phenolic composites.     

A multi-step treatment to effective purification of single-walled carbon nanotubes

An efficient procedure for the purification of single-walled carbon nanotubos (SWCNTs) based on chemical treatment and physical separation is proposed and studied step-by-step by high-resolution transmission electron microscopy, thermogravimetric analyses, and nitrogen adsorption techniques. Impure raw samples prepared by the arc-discharge method containing SWCNTs, amorphous carbon, graphitic particles, carbon shells, metallic catalysts, and fullerenes were employed in this study. The procedure reported herein was used to remove each impurity of the sample in a step-by-step way. Gas-phase oxidation and the popular oxidation by heating in nitric acid were avoided to minimize damages to structure of the SWCNTs and to keep eventual chemical modifications of sample to a minimum. This new procedure is efficient and appropriate for the obtention of highly pure SWCNTs with minimal damage to the walls and minimal modification in the length of the tubes. The amount of each kind of impurity present in the raw material yielded in an arc-reactor was quantitatively estimated. Results of specific surface area and t-plots are also discussed.     

Microstructure and mechanical properties of a metakaolinite-based geopolymer nanocomposite reinforced with carbon nanotubes

The preparation and mechanical behavior of metakaolin-based geopolymer nanocomposite reinforced with multi-walled carbon nanotubes are presented in this study. In this work, Multiwall carbon nanotubes (MWCNTs) were added to the metakaolin-based geopolymer paste at 0, 0.5, or 1 wt% concentration. For each specimen, the mechanical properties were tested at the age of 7, 14 and 28 days. TEM and FESEM were employed to evaluate the dispersion quality of MWCNTs within the metakaolin geopolymer matrix and determine their strengthening mechanism. The test results showed that the addition of about 0.5 wt% MWCNTs increased the compressive and flexural strength by as much as 32% and 28%, respectively. Based on these results, the MWCNTs can act as effective bridges to minimize and limit the propagation of micro cracks through the metakaolin-based geopolymer nanocomposite under the conditions of homogenous dispersion and good bonding between the MWCNTs and the surrounding metakaolin-based geopolymer paste.     

Tribological behaviors of hybrid PTFE/nomex fabric/phenolic composite reinforced with multiwalled carbon nanotubes

Modification of composites was a general method to improve their tribological behaviors. On the way to explore composites with enhanced tribological behaviors, we have successfully prepared hybrid PTFE/Nomex fabric/phenolic composite filled with multiwalled carbon nanotubes (MWCNTs) or MWCNTs modified by polystyrene (PS) with a grafting to method. The results of pin‐on‐disc type wear tests indicated tribological behaviors were improved both for hybrid PTFE/Nomex fabric/phenolic composite filled with MWCNTs and MWCNTs‐PS, especially for that of filled with MWCNTs‐PS. And the probable reason was also discussed based on the characterization results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Processing and assessment of poly(butylene terephthalate) nanocomposites reinforced with oxidized single wall carbon nanotubes

Thermoplastic composites with carbon nanotubes (CNT) have a great potential as structural material because of their superior mechanical properties and ease of processing. The objective of this report is to evaluate the effect of oxidized single walled carbon nanotubes (oSWCNT) on the properties of poly(butylene terephthalate) (PBT) thermoplastic polymers, as a function of their weight content. The nanocomposites are obtained by introducing the oSWCNT into the reaction mixture whilst the synthesis of PBT. The polymers without and with carbon nanotubes were synthesised using an in situ polycondensation reaction process. Weight percentages ranging from 0.01 to 0.2 wt% of single walled nanotubes were dispersed in 1,4-butanediol (BD) by ultrasonication and ultrahigh speed stirring. After polycondensation the nanocomposites were extruded followed by injection moulding. The samples were characterised by thermal analysis, electron microscopy, dynamic-mechanical analysis, and tensile testing.The addition of only a small amount of oSWCNT was enough to improve the thermo-mechanical properties of the nanocomposites. The Young's modulus, tensile strength, and strain to failure increased with increasing amount from 0.01 to 0.1 wt% of CNT in the PBT matrix. However, when the content of CNT was increased from 0.1 to 0.2 wt%, the strength and the strain of the nanocomposites decreased slightly.     

KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation

New information was obtained on the mechanism of porosity development during chemical activation by KOH and NaOH using various multiwalled nanotubes (MWNTs) of different structural organization. The high purity MWNTs were prepared by acetylene decomposition on a cobalt-based catalyst at different temperatures. The obtained samples ranged from MWNTs with well organised graphitic walls to nanotubes with disorganised layers mixed with some pyrolytic carbon when decreasing synthesis temperature. The results of transmission electron microscopy (TEM) observations were linked with gas adsorption measurements and X-ray diffraction data. They show that NaOH is only effective with disordered materials whereas KOH is effective whatever the structural order. After reaction of the poorly ordered precursor with KOH, the nanotubular morphology is completely destroyed, whereas it is preserved when NaOH is used. However for the more ordered materials, the morphology remains unchanged with both reactants. Effects of activation are only seen with KOH, which generated a large concentration of defects in the nanotubes walls. The differences found between KOH and NaOH during activation are related with an additional intercalation step of metallic K or Na produced during the redox reactions. It is shown that metallic K has the ability to be intercalated in all materials in contrast with Na which can only intercalate in the very disorganised ones. The conclusions obtained from the study on ordered nanotubes were confirmed with an ordered carbon black, demonstrating that the structural organization of the carbon precursor is an important parameter which must be taken into account when alkali reactants are used for the activation.     

Fabrication of composites with excellent mechanical properties based on cubic boron nitride reinforced with carbon nanotubes

Composites consisting of cubic boron nitride (cBN) as a matrix and carbon nanotubes (CNTs) as reinforcing additives were fabricated by high-temperature and high-pressure sintering (HTHP). Microstructures, mechanical properties, fracture modes and toughening mechanisms of these composites were investigated. Composites exhibited excellent bending strength, wear resistance, and fracture toughness. Fracture toughness of composites reached 7.02 MPa·m^1/2. Comparing to pure cBN matrix, bending strength improved from 475.27 to 600.15 MPa, and wear resistance increased by 43.23%. Such improvements of mechanical properties were mainly attributed to pullout and bridging reinforcements by CNTs. CNTs incorporation also changed fracture mode from inter-to trans-granular.