Polypropylene Carbon Nanotubes Research Articles

Effects of Orientation and Dispersion on Electrical Conductivity and Mechanical Properties of Carbon Nanotube/Polypropylene Composite.

The orientation and dispersion of nanoparticles can greatly influence the conductivity and mechanical properties of nanocomposites. In this study, the Polypropylene/ Carbon Nanotubes (PP/CNTs) nanocomposites were produced using three different molding methods, i.e., compression molding (CM), conventional injection molding (IM), and interval injection molding (IntM). Various CNTs content and shear conditions give CNTs different dispersion and orientation states. Then, three electrical percolation thresholds (4 wt.% CM, 6 wt.% IM, and 9 wt.% IntM) were obtained by various CNTs dispersion and orientations. Agglomerate dispersion (Adis), agglomerate orientation (Aori), and molecular orientation (Mori) are used to quantify the CNTs dispersion and orientation degree. IntM uses high shear to break the agglomerates and promote the Aori, Mori, and Adis. Large Aori and Mori can create a path along the flow direction, which lead to an electrical anisotropy of nearly six orders of magnitude in the flow and transverse direction. On the other hand, when CM and IM samples already build the conductive network, IntM can triple the Adis and destroy the network. Moreover, mechanical properties are also been discussed, such as the increase in tensile strength with Aori and Mori but showing independence with Adis. This paper proves that the high dispersion of CNTs agglomerate goes against forming a conductivity network. At the same time, the increased orientation of CNTs causes the electric current to flow only in the orientation direction. It helps to prepare PP/CNTs nanocomposites on demand by understanding the influence of CNTs dispersion and orientation on mechanical and electrical properties.

Unified modeling and experimental realization of electrical and thermal percolation in polymer composites

Correlations between electrical and thermal conduction in polymer composites are blurred due to the complex contribution of charge and heat carriers at the nanoscale junctions of filler particles. Conflicting reports on the lack or existence of thermal percolation in polymer composites have made it the subject of great controversy for decades. Here, we develop a generalized percolation framework that describes both electrical and thermal conductivity within a remarkably wide range of filler-to-matrix conductivity ratios (Yf/Ym), covering 20 orders of magnitude. Our unified theory provides a genuine classification of electrical conductivity with typical Yf/Ym≥1010 as insulator–conductor percolation with the standard power-law behavior and of thermal conductivity with 102≤Yf/Ym≤104 as poor–good conductor percolation characterized by two universal critical exponents. Experimental verification of the universal and unified features of our theoretical framework is conducted by constructing a 3D segregated and well-extended network of multiwalled carbon nanotubes in polypropylene as a model polymer matrix under a carefully designed fabrication method. We study the evolution of the electrical and thermal conductivity in our fabricated composites at different loading levels up to 5 vol. %. Significantly, we find an ultralow electrical percolation threshold at 0.02 vol. % and a record-low thermal percolation threshold at 1.5 vol. %. We also apply our theoretical model to a number of 23 independent experimental and numerical datasets reported in the literature, including more than 350 data points, for systems with different microscopic details, and show that all collapse onto our proposed universal scaling function, which depends only on dimensionality.

Influence on the thermophysical properties of nanocomposites of the duration of mixing of components in the polymer melt

A set of experimental studies has been carried out to establish the effect of the mixing time of components of nanocomposite materials on their thermal conductivity, specific heat, and density. The physical properties of polypropylene-carbon nanotube composites were to be studied. During the experiments, the duration of mixing of the components in the melt of the polymer varied from 5 to 52 minutes, the mass fraction of the filler ‒ in the range of 0.3...10 %, and nanocomposite temperature – from 290 K to 475 K. It was found that an increase in the mixing time of components of nanocomposite materials could lead to a significant (more than 70 times) increase in their thermal conductivity. It is also shown that the influence of the specified time is limited to its value equal to 27 minutes, above which the change in the thermal conductivity of nanocomposites can be neglected. It was found that the sensitivity of the thermal conductivity of nanocomposites to the time of mixing of their components decreases with a decrease in the mass fraction of the filler. Temperature dependences of the specific heat capacity of the studied composites were obtained by varying the mixing time of their components and the mass fraction of the filler. It was found that with an increase in the specified time, there is a decrease in the heat capacity of nanocomposites, which is significantly manifested only in the region of temperatures close to the melting point of the composite matrix. It is shown that the dependence of the density of nanocomposites on the mixing time of their components in qualitative terms is similar to the corresponding dependence for their thermal conductivity. The obtained data can be used to choose the mixing time of components of nanocomposite materials in the development of appropriate technology for their production

Basic Study of Extensional Flow Mixing for the Dispersion of Carbon Nanotubes in Polypropylene by Using Capillary Extrusion

Abstract This study evaluated the mixing effect of simple uniaxial extensional flow for the dispersion of multiwalled carbon nanotubes (MWCNTs) into polypropylene (PP) as a nonpolar matrix. An only converging flow allowed for a high strain rate and was suitable for the compounding process. The extensional flow was characterized from the entrance pressure drop (ΔP0) at the converging section. Thus, in this study, capillary extrusion was employed to generate uniaxial extensional flow. Based on the hypothesis that the dispersion of nanofillers depends on the magnitude of flow-induced stress, ΔP0, which related to extensional stress, was measured directly during capillary extrusion by using an orifice die. The influences of the mass flow rate and the hole diameter in the orifice die, which affected ΔP0, on the extrusion of PP nanocomposites with an MWCNT loading of 1.0 wt.% were studied. The extruded samples were collected, and the dispersion state was evaluated based on the melt viscoelastic properties, volume resistivity, and morphological observations by optical microscopy (OM) and transmission electron microscopy (TEM). The agglomeration area of the MWCNTs decreased with higher ΔP0 (higher mass flow rate and smaller hole diameter), which increased the uniformity of the dispersion. Moreover, the influence of the length-to-diameter (L/D) ratio of the hole in the capillary die on the dispersion state of the MWCNTs was investigated. A higher L/D ratio of the capillary die did not improve the dispersion state, although shear and extensional stresses were provided.

Injection molding of segregated carbon nanotube/polypropylene composite with enhanced electromagnetic interference shielding and mechanical performance

The formation of segregated structure in conductive polymer composites reveals great superiority in gaining improved electromagnetic interference (EMI) shielding performance. Nevertheless, the restricted processing method and seriously deteriorated mechanical properties hamper the practical applications of such segregated composites. Herein, we demonstrate simultaneously improved EMI shielding and mechanical performance in a segregated carbon nanotube/polypropylene (CNT/PP) composite, via a facile and eco-friendly method composed by pre-coating, melt mixing, and injection molding. The segregated CNT/PP composite containing only 3.5 wt% CNT exhibits an average EMI shielding effectiveness (EMI SE) of 32 dB, which shows 130% and 30% improvements in comparison to 14 dB for the CNT/PP composite prepared by conventionally injection molding and 25 dB for the CNT/PP composite prepared by compression molding. Our segregated CNT/PP composite also attains a high elongation at break (EB) of 78%, about 4 and 17 times higher than 16% for the conventionally injection molded one and 4% for the compression molded one, with comparable tensile strength and Young's modulus. This work provides a feasible approach to guide the scaled preparation of segregated composites with satisfactory EMI shielding and mechanical performance.

Highest recorded electrical conductivity and microstructure in polypropylene–carbon nanotubes composites and the effect of carbon nanofibers addition

In the last decade, numerous investigations have been devoted to the preparation of polypropylene–multiwalled carbon nanotubes (PP/MWCNT) nanocomposites having enhanced properties, and in particular, high electrical conductivities (> 1 S cm−1). The present work establishes that the highest electrical conductivity in PP/MWCNT nanocomposites is limited by the amount of nanofiller content which can be incorporated in the polymer matrix, namely, about 20 wt%. This concentration of MWCNT in PP leads to a maximum electrical conductivity slightly lower than 8 S cm−1, but only by assuring an adequate combination of dispersion and spatial distribution of the carbon nanotubes. The realization of such an optimal microstructure depends on the characteristics of the production process of the PP/MWCNT nanocomposites; in our experiments, involving composite fabrication by melt mixing and hot pressing, a second re-processing cycle is shown to increase the electrical conductivity values by up to two orders of magnitude, depending on the MWCNT content of the nanocomposite. A modest increase of the highest electrical conductivity obtained in nanocomposites with 21.5 wt% MWCNT content has been produced by the combined use of carbon nanofibers (CNF) and MWCNT, so that the total nanofiller content was increased to 30 wt% in the nanocomposite with PP—15 wt% MWCNT—15 wt%CNF.

Nucleation efficiency of fillers in polymer crystallization studied by fast scanning calorimetry: Carbon nanotubes in polypropylene

The influence of multi-walled carbon nanotubes (0–5 wt%) on the crystallization process in isotactic polypropylene has been investigated by DSC and fast scanning calorimetry. In the non-isothermal experiments the crystallization temperature shifts to higher temperatures with increasing nanotube content, αCNT, whereas the polymer crystallinity was not significantly influenced. The critical cooling rate, βc, at which the PP does not crystallize, increases with increasing of αCNT. From βc and αCNT a nucleation efficiency parameter was derived, which is independent of crystallization temperature and filler content. Isothermal crystallization experiments allow differentiating between α-phase and mesophase crystallization. Nanotubes accelerate only the α-phase formation. To describe the efficiency of a nucleation agent an acceleration factor, ε, was introduced, which is the ratio of the characteristic crystallization time of unfilled and filled polymer. For the α-phase formation the acceleration factor ε is related to the number of nuclei per nanotube and the specific effect of the filler on the growth rate.

Polypropylene Carbon Nanotubes Nanocomposites: Combined Influence of Block Copolymer Compatibilizer and Melt Annealing on Electrical Properties

We study the influence of melt annealing and the presence of a block copolymer compatibilizer on the electrical properties of polypropylene carbon nanotubes (CNT) nanocomposites from the DC limit to microwave frequencies and link it to the morphological details. We show that the compatibilizer concentration controls three types of morphologies: separate CNT agglomerates, a network of well dispersed but interconnected CNT, and individualized but separate nanotubes. This explains why conductivity reaches an optimum over the whole frequency range at a low compatibilizer concentration. We model the corresponding structures by a semiquantitative schematic equivalent electrical circuit. A key outcome of the work is the understanding and control of dispersion mechanisms in order to optimize the electrical performances for efficient EMI shielding depending on the targeted frequency range.

Surface durability of injection-moulded carbon nanotube–polypropylene spur gears

Short fibre reinforced thermoplastics are being considered for light- and medium-duty engineering applications because of their improved mechanical strength combined with cost-effective advantages. In recent years, the carbon nanotube reinforced thermoplastics are being preferred over the short fibre reinforced thermoplastics because of the absence of directional shrinkage characteristics, directional mechanical and tribological properties. In this work, 1 wt% carbon nanotube–polypropylene material was injection-moulded to spur gears and evaluated for the durability using in-house developed power absorption gear test rig. During testing, the net surface temperature of the test gears and in-line torque available at the driver and driven shafts were continuously measured. The measured torque was used to evaluate the transmission efficiency of the test-steel gear pair. The measured net surface temperature was correlated with the gear failure mode. Test gears were run up to failure or up to 8.6 × 105 cycles, whichever occurred first. Worn-out gear tooth surfaces were observed using an optical and scanning electron microscope to understand the wear mechanism. In the initial stage of service, test gears exhibited less wear near the pitch region compared to the tip and root regions. This behaviour is due to the maximum sliding velocities at the tip and root regions compared to the pitch region. The carbon nanotube–polypropylene gears exhibited lower surface temperature (5−10 ℃), improved service life (30%−80%) and higher transmission efficiency (1−1.5%) compared to the polypropylene gear.

Mechanical and tribological properties of carbon nanotube reinforced polypropylene composites

In this work, carbon nanotube (CNT) reinforced polypropylene (PP) composites (0.5, 1.0, 3.0, and 5.0 wt%) were developed using the melt compounding process. The developed composites were injection-molded into tensile specimens and pins to evaluate the mechanical and tribological properties of the composites. As the CNT content increased, the tensile strength and Young’s modulus of the PP composites increased. The addition of the CNTs to the PP matrix beyond 1 wt% demonstrated agglomeration, and fractured tensile specimens confirmed this behavior. Developed materials demonstrated enhanced crystallinity up to 1 wt% CNT and, subsequently, decreased crystallinity beyond 1 wt% CNT, and an X-ray diffraction investigation confirmed this behavior. The measured coefficient of friction, online wear, and weight loss from the sliding wear test confirmed the least frictional resistance and maximum wear resistance for the 1 wt% CNT–PP composite. As the CNT content increased, the hardness of the CNT–PP composite increased up to 1 wt% CNT and decreased beyond this threshold. The worn-out surfaces of the CNT–PP composite observed using a scanning electron microscope and noncontact three-dimensional profiler confirmed the superior wear resistance of the 1 wt% CNT–PP composite. The CNT–PP composites considered in this study exhibited increased surface temperatures in the sliding wear condition because of the addition of the CNTs. The addition of the CNTs to the PP material increased the thermal conductivity of the composite.

3D RVE Models Able to Capture and Quantify the Dispersion, Agglomeration, and Orientation State of CNT in CNT/PP Nanocomposites

The focus of this study is to investigate the capabilities of 3D RVE models in predicting the tensile modulus of carbon nanotube polypropylene (CNT/PP) composites which differ slightly in the dispersion, agglomeration and orientation states of CNT within the PP matrix. The composites are made using melt mixing followed by either injection molding or melt spinning of fibers. The dispersion, agglomeration and orientation of CNT within the PP are experimentally altered by using a surfactant and by forcing the molten material to flow through a narrow orifice (melt spinning) that promotes alignment of CNT along the flow/drawing direction. An elaborate image analysis technique is used to quantify the CNT characteristics in terms of probability distribution functions (PDF). The PDF are then introduced to the 3D RVE models which also account for the CNT-PP interfacial interactions. It is concluded that the 3D RVE models can accurately distinguish among the different cases (dispersion, distribution, geometry and alignment of CNT) as the predicted tensile modulus is in good agreement with the experimentally determined one.

Properties of PP/MWCNT-COOH /PP composites made by melt mixing versus solution cast /melt mixing methods

An approach on improvement of the properties of polypropylene / carbon nanotube (PP/CNT) composites is reported. PP blend compositions with carboxylic acid functionalized multi-walled carbon nanotubes (MWCNT-COOH) at filler content 1.0 wt.% were researched. One part of the composites was manufactured by direct thermoplastic mixing PP with the filler, but the other one was made from solution casted masterbatch with the following thermoplastic mixing. An increase of mechanical properties (Young's modulus, storage modulus and tensile strength), compared to an increase of glass transition temperature indicated a reinforcement effect of CNTs on PP matrix, determined from the tensile tests and differential mechanical analysis (DMA), while the elongation was reduced, compared to PP matrix. By differential scanning calorimetry (DSC) analysis, the effect of nanofiller on the reorganization of PP crystallites was observed. A noticeable enhanced effect on increase of the crystallization temperature was indicated for masterbatch manufactured composite. An increase of thermal stability was also observed, compared to pristine PP and the composite made by direct thermoplastic mixing PP with the filler.

Experimental Investigation of Static and Dynamic Characteristics of Multiwall Carbon Nanotubes Reinforced Polypropylene

The change in static and dynamic characteristics of polypropylene (PP) of grade H110MA by adding 2% multi walled carbon nanotubes (MWCNT) was investigated experimentally in this paper. The nanocomposite material is prepared by mixing neat PP and MWCNT in an extruder in a specified proportion. The extruded material is then used in an injection molding machine to make tensile test and impact test specimen. Tensile test specimen is tested on universal testing machine (UTM) to check the Young’s modulus of nanocomposite material. The impact test specimen is used to find the dynamic characteristics of this material by using the technique of experimental modal analysis using Single point Laser Doppler Vibrometer (SLDV), modal hammer and FFT analyzer. The scanning electron microscopic (SEM) images were taken to check the orientation and dispersion of carbon nanotubes in polypropylene.

Fabrication of Multiwalled Carbon Nanotube/Polypropylene Conductive Fibrous Membranes by Melt Electrospinning

A novel multiwalled carbon nanotube/polypropylene (CNT/PP) conductive fibrous membrane with fiber diameter of 1–3 μm was fabricated by melt electrospinning. To improve the dispersibility of CNT and enhance the spinnability of PP fibers, CNTs were first mixed with small amounts of paraffin liquid (PL) and then melt-blended with PP for melt electrospinning. The morphology of fibrous membranes and the orientation of CNTs in PP fibers were observed via scanning and transmission electron microscopy (SEM and TEM). The effect of PL and CNTs on crystallization behavior of PP was studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Tensile test and impedance analysis were performed to investigate the mechanical and electrical properties of the fibrous membranes. The results indicated that CNT has a distinct nucleating effect on PP, and the addition of PL can improve the spinnability of the CNT/PP compound remarkably. This novel conductive fibrous membrane fabricated by melt electrospinni...

Dispersion of carbon nanotubes in polypropylene via multilayer coextrusion: Influence on the mechanical properties

Multilayer coextrusion was used to disperse Carbon Nanotubes (CNT) in polypropylene (PP). The dilution of commercially available masterbatches using a twin-screw extruder was first applied to produce several formulations, which were then mixed with PP using a multilayer coextrusion device to obtain films or pellets with CNT concentrations between 0.1 and 1%wt. The influence of the specific mechanical energy (SME) during the dilution step, of the addition of a compatibilizer, and of the multilayer tool on the CNT dispersion within the matrix was highlighted. The effect of the dispersion on the thermomechanical properties of the resulting materials was studied. We showed notably that films containing 0.2%wt CNT, 1%wt of PPgAm, prepared at high SME presented a Young's modulus increase of 25–30% without significant decrease in the elongation at break. These results, using low amounts of CNT and industrially available devices, may show a new path for producing nanocomposites.

Fabrication of a magnet‐assisted alignment device for the amperometric detection of capillary electrophoresis using a carbon nanotube/polypropylene composite electrode

A magnet-assisted alignment device was designed and fabricated for the amperometric detection of CE. It mainly consisted of a magnet-containing electrode holder, a capillary-based microdisc detection electrode, a detection cell, and a micrometer adjuster. To demonstrate the feasibility and performance of the alignment device, it was used in combination with a carbon nanotube/polypropylene (CNT/PP) composite electrode for the determination of p-phenylenediamine, m-aminophenol, and m-dihydroxybenzene in commercial hair dye by CE. The CNT-based electrode was fabricated by packing a melt mixture of CNTs and PP in a piece of fused silica capillary under heat, offering significantly lower operating potentials, substantially enhanced signal-to-noise characteristics, and high resistance to surface fouling. Because magnetic force was employed to move the detection electrode, the alignment system was significantly simplified. It is characterized by simple design and fabrication, high alignment reproducibility, reduced alignment time, and low cost. Both the alignment device and the CNT/PP composite electrode should find a wide range of applications in microchip CE, flowing injection analysis, and other microfluidic analysis systems.

Carbon nanotube induced structure and phase evolution in polypropylene crystallised at elevated pressures: effect on physical properties

The dispersion of carbon nanotubes (CNTs) in polypropylene (PP) introduces a significant change in the structure and phase evolution during crystallisation at ambient and elevated pressures. The differences between the behaviours of PP and PP containing CNTs are attributed to CNT/polymer interface driven nucleation, which is also responsible for the significant reduction in spherulite size and is associated with the change in equilibrium melting point and percentage crystallinity. This opens the gateway for the use of polymer CNT composites for functional devices, including biomedical and electronic devices. A study of the impact of 1 wt-%CNT additions on the microstructure evolution of PP during pressure induced crystallisation is reported. It is demonstrated that the dispersion of CNTs produces a significant difference in the macromolecular structure and phase evolution during crystallisation at ambient and elevated pressures. The change in equilibrium melting point with pressure is higher for PP containing CNT, which is indicative of the chemical interaction between polymer and CNT. It is found that CNT additions promote the formation of γ-phase at low crystallisation pressure of 0·1–25 MPa, in contrast to the neat PP where α-crystals nucleate at low to moderate pressures (about 0·1–60 MPa) and the γ-phase nucleates at crystallisation pressures exceeding ∼60 MPa. The dependence of equilibrium melting temperature on crystallisation pressure and spherulite size delineates two distinct regimes, characterised predominantly by α- and γ-phases respectively, for both PP and PP–CNT systems.

Effect of Nanoscale Species on the Behavior of Polymer Nanocomposites Subjected to Laser Pulse Heating

ABSTRACTPolymers and polymer nanocomposites have been studied under conditions of extremely high heating rates. Traditionally, these materials have been examined by the flammability research community using methods which have heating rates on the order of 10 degrees C/min. In this study, we have examined how polypropylene-nanoclay (montmorillonite) and polypropylene-carbon nanotube nanocomposites behave subjected to heating rates on the order of one million degrees C/min when irradiated with a 1064 nm Nd-YAG variable pulse millisecond laser. Time-resolved temperature data and mass loss data was collected for each sample as well as post-mortem surface characterization using spectroscopy and electron microscopy. The analysis shows that the nanospecies are effective in providing a protective barrier that decreases the amount of degradation and mass loss to the underlying polymer material. The effect is clearly seen after irradiating with a single pulse and multiple pulses. A comparison between the performance of the nanoclay and carbon nanotube composites is given.

Influence of maleic anhydride compatibiliser on properties of polpropylene/multiwalled carbon nanotube composites

The effect of maleic anhydride compatibilser on the mechanical, rheological and thermal properties of polypropylene–carbon nanotube composites is reported. A commercial grade of polypropylene copolymer was melt compounded with acid purified multiwalled carbon nanotubes at loadings up to 12 wt-%, with and without the addition of a maleic anhydride grafted polypropylene. Tensile and flexural moduli were found to increase with filler content at all nanotube loadings and these properties were enhanced by addition of maleic anhydride. Ultimate tensile strength increased with nanotube loadings up to 8 wt-% and then decreased; maleic anhydride had a negative effect on tensile strength most likely resulting from nanotube agglomerations. Rheological and morphological characterisations confirmed that dispersion was relatively poor in both systems suggesting that maleic anhydride improved interfacial bonding without significantly improving dispersion, for the materials and compounding conditions examined in this work.

Effect of processing technique on the dispersion of carbon nanotubes within polypropylene carbon nanotube‐composites and its effect on their mechanical properties

AbstractCarbon nanotube‐reinforced polymer composites are being investigated as promising new materials having enhanced physical and mechanical properties. With regards to mechanical behavior, the enhancements reported thus far by researchers are lower than the theoretical predictions. One of the key requirements to attaining enhanced behavior is a uniform dispersion of the nanotubes within the polymer matrix. Although solvent mixing has been used extensively, there are concerns that any remaining solvent within the composite may degrade its mechanical properties. In this work, a comparison is carried out between solvent and “solvent‐free” dry mixing for dispersing multiwall carbon nanotubes in polypropylene before further melt mixing by extrusion. Various weight fractions of carbon nanotubes (CNTs) are added to the polymer and their effect on the mechanical properties of the resulting composites is investigated. Enhancements in yield strength, hardness, and Young's modulus when compared with the neat polymer, processed under similar conditions, are observed. Differences in mechanical properties and strain as a function of the processing technique (solvent or dry) are also clearly noted. In addition, different trends of enhancement of mechanical properties for the solvent and dry‐mixed extrudates are observed. Dry mixing produces composites with the highest yield strength, hardness, and modulus at 0.5 wt% CNT, whereas solvent mixing produces the highest mechanical properties at CNT contents of 1 wt%. It is believed that this difference is primarily dependent on the dispersion of CNTs within the polymer matrix which is influenced by the processing technique. Field emission scanning electron microscopy analysis shows the presence of clusters in large wt% CNT samples produced by dry mixing. Samples produced by solvent mixing are found to contain homogeneously distributed CNTs at all CNT wt fractions. CNT pull‐out is observed and may explain the limited enhancement in mechanical properties. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers