Melt flow behavior of polypropylene composites filled with multi-walled carbon nanotubes during extrusion

Polypropylene (PP) composites filled with multi-walled carbon nanotubes (MWCNTs) were prepared using a twin-screw extruder. The melt flow properties of the composites were measured with a capillary rheometer in a temperature range from 180 to 230 °C and at various apparent shear rates varying from 100 to 4000 s⁻¹. The results showed that the melt shear stress increased almost linearly while the melt shear viscosity decreased almost linearly with increasing shear rates in a bi-logarithmic coordinate system. The melt shear flow followed the power law relationship and the dependence of the melt shear viscosity on temperature obeyed the Arrhenius equation. The relationship between the melt shear viscosity and the MWCNT weight fraction was roughly linear under the investigated range of temperature or shear rate.     

Electroactive shape memory cyanate/polybutadiene epoxy composites filled with carbon black

Electroactive shape memory composites were synthesized using polybutadiene epoxy(PBEP) and bisphenol A type cyanate ester(BACE) filled with different contents of carbon black(CB). Dynamic mechanical analysis(DMA), scanning electron microscopy(SEM), electrical performance and electroactive shape memory behavior were systematically investigated. It is found that the volume resistivity decreased due to excellent electrical conductivity of CB, in turn resulting in good electroactive shape memory properties. The content of CB and applied voltage had significant influence on electroactive shape memory effect of developed BACE/PBEP/CB composites. Shape recovery can be observed within a few seconds with the CB content of 5 wt% and voltage of 60 V. Shape recovery time decreased with increasing content of CB and voltage. The infrared thermometer revealed that the temperature rises above the glass transition temperature faster with the increase of voltage and the decrease of resistance.     

Nonlinear current-voltage characteristics of conductive polyethylene composites with carbon black filled pet microfibrils

Current-voltage electrical behavior of in situ microfibrillar carbon black (CB)/poly(ethylene terephthalate) (PET)/polyethylene (PE) (m-CB/PET/PE) composites with various CB concentrations at ambient temperatures was studied under a direct-current electric field. The current-voltage (I-V) curves exhibited nonlinearity beyond a critical value of voltage. The dynamic random resistor network (DRRN) model was adopted to semi-qualitatively explain the nonlinear conduction behavior of m-CB/PET/PE composites. Macroscopic nonlinearity originated from the interfacial interactions between CB/PET micro fibrils and additional conduction channels. Combined with the special conductive networks, an illustration was proposed to interpret the nonlinear I-V characteristics by a field emission or tunneling mechanism between CB particles in the CB/PET microfibers intersections.     

Polymer blends filled with carbon black: structure and electrical properties

Morphology and electrical conductivity of polymer blends based on polypropylene, polyethylene, polyoxymethylene, polyamide 12, co‐polymer ABS filled with carbon black (CB) were studied. When filler is introduced in polymer blend, three cases of filler spatial distribution can be obtained. CB can be distributed randomly within polymer matrix, can be situated in one of the polymer components or can be localized on the boundary of the polymer components. The existence of different cases of filled blends structure is defined by following reasons: thermodynamic factors (relationship between interface surface tensions of polymer‐filler γ_pf and polymer‐polymer surface tension γ_pp); kinetic factors (relationship between viscosities of polymer components); processing factors (methods of the filler introduction in the complex polymer matrix, which can enhance or depress the influence of thermodynamic and kinetic factors).     

Electrical properties and conductive mechanisms of immiscible polypropylene/Novolac blends filled with carbon black

Carbon black (CB)-filled immisicible thermoplastic/thermosetting polymer blends consisting of polypropylene (PP) and Novolac resin were reported in this paper. The PP/Novolac/CB blends with varied compositions and different processing sequences were prepared by melt-mixing method. The CB distribution, conductive mechanism and the relationship between morphology and electrical properties of the PP/Novolac/CB blends were investigated. Scanning electron microscopy (SEM), optical microscopy and extraction experiment results showed that in PP/Novolac blends CB particles preferentially localized in the Novolac phase, indicating CB has a good affinity with Novolac resin. The incorporation of CB changed the spherical particles of the dispersed Novolac phase into elongated structure. With increasing Novolac content, the elongation deformation of Novolac phase became more obvious and eventually the blends developed into co-continuous structure, which form double percolation and decrease the percolation threshold. When CB was initially blended with PP and followed by the addition of Novolac resin, the partial migration of CB from PP to the Novolac phase was possibly occurred. The addition of Novolac to PP evidently increases the storage modulus G′, loss modulus G″ and complex viscosity η^∗. The addition of CB to PP/Novolac blends further increase η^∗, and it increases with increasing CB loading, which was related to the change of composite morphology.     

Electroactive shape memory effect of radiation cross-linked SBS/LLDPE composites filled with carbon black

The poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer and linear low-density polyethylene (LLDPE) were blended and irradiated by γ rays to prepare shape memory polymer (SMP). Different weight fractions of conductive carbon black (CB) were filled into SMP to form a novel electroactive shape memory CB/SBS/LLDPE composite. The CB reinforced radiation cross-linked SBS/LLDPE blends for the improvement of the mechanical weakness and conductivity of SBS/LLDPE bulk and for wide practical engineering uses. The electroactive shape memory CB/SBS/LLDPE composites were investigated by electrical properties, mechanical, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electroactive shape memory effects. It is found that the tensile strength, storage modulus, and resistance against mechanical and thermal mechanical cycle loading in the developed composites increased due to the role of reinforcement of CB. The melting temperatures and volume resistance of the composites decreased with the increment of CB for excellent electrical conductivity of CB. The electroactive shape memory effects of developed CB/SBS/LLDPE composites were affected by CB weight fractions and applied voltage, while good shape recovery could be obtained in the shape recovery test. When the CB fraction is more than 5 wt%, full recovery can be observed after tens of seconds and shape recovery speed increased with CB fractions and voltage increasing. However, the shape recovery rate decreases slightly with increment of cycle times.     

Impact behaviour of polypropylene filled with multi-walled carbon nanotubes

Multi-walled carbon nanotubes/polypropylene composites were compounded using a twin-screw extruder. Here, nanotubes with different lengths, i.e. 1-2 μm and 5-15 μm, respectively, were applied at a constant volume content of 1%. Notched Charpy impact tests showed that toughening effects of nanotubes depended highly on testing temperatures. The impact resistance was notably enhanced at a temperature above the glass transition temperature of matrix. Longer nanotubes performed more effective in toughening compared to the shorter ones. The increment of impact resistance of nanotube-filled polypropylene was considered due to enhanced load-carrying capability and much-increased deformation of matrix. SEM fractography further revealed the toughening mechanisms in a micro-scale. The impact energy was improved via nanotube breakage and pullout, which likely led to a series of energy consuming actions. In addition, the smaller spherulite size induced by nanotubes would be favourable to the impact resistance partially.     

Nucleation, structure and lamellar morphology of isotactic polypropylene filled with polypropylene-grafted multiwalled carbon nanotubes

Polypropylene-based nanocomposites filled with polypropylene-grafted multiwalled carbon nanotubes (PP-g-MWNT) were compared to PP samples filled with pristine MWNT. The effect of such additives on the structure and morphology of the polymer matrix was studied by small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD), polarized light optical microscopy (PLOM) and differential scanning calorimetry (DSC). PP-g-MWNT allowed a more efficient and unhindered crystallization at a lamellar level, while MWNT disrupted the order of lamellar stacks, probably because of their tendency to aggregate. A common trend of tensile properties and lamellar morphology as a function of filler content was noted in the series filled with functionalized carbon nanotubes.     

Crystallization properties and thermal stability of polypropylene composites filled with wollastonite

The influence of wollastonite (CaSiO₃) content on the crystallization properties and thermal stability of polypropylene (PP) composites was investigated. The results showed that the crystallization temperature, crystallization end temperature and crystallization temperature interval, as well as the degree of crystallinity of the composites, were higher than those of the unfilled PP resin, while the crystallization onset temperature was little changed from that of the unfilled PP resin. The increase of degree of crystallinity for the composites could be attributed to the heterogeneous nucleation of the CaSiO₃ in the PP matrix. The thermal stability increased with increasing filler weight fraction (ϕ_f); the thermal decomposition rate decreased nonlinearly with increasingϕ_f. Finally, the dispersion of the filler particles in the matrix was observed, and the mechanisms of thermal stability and crystallizing behavior were discussed.     

The strain-sensing behaviors of carbon black/polypropylene and carbon nanotubes/polypropylene conductive composites prepared by the vacuum-assisted hot compression

The strain-sensing behaviors of carbon black (CB)/polypropylene (PP) and carbon nanotubes (CNTs)/PP conductive composites prepared by the vacuum-assisted hot compression were studied and compared. When ten extension-retraction cycles were applied, it was found for CB/PP, the value of the maximum responsivity (ΔR/R ₀, ΔR—the instantaneous variation of the resistance during the test, R ₀—the original resistance) decreased gradually with increasing the cycle number, but it began to rise from the seventh cycle. The value of the min ΔR/R ₀ increased during the whole test. While for CNTs/PP, both the values of the max and min ΔR/R ₀ decreased rapidly. It is suggested that the different behaviors mainly depend on the distinction in the dimension of the conductive fillers and the preparation technique.     

Enhancing the conductivity of isotactic polypropylene/polyethylene/carbon black composites by oscillatory shear

The influence of oscillatory shear on the conductivity of the isotactic polypropylene/polyethylene/carbon black composites is studied. It is found that the oscillatory shear under high strain amplitude can enhance the conductivity of the ternary composites with a HDPE/CB concentration in the percolation region. This is related to the fact that the high-strain oscillatory shear can improve the continuity of the conductive HDPE/CB phase in the composites. This finding has not been previously reported, and it may be used in industry to improve the conductivity of the ternary conductive composites with a low filler loading.     

Mechanical properties and morphology for polypropylene composites filled with microencapsulated red phosphorus

The effects of the microencapsulated red phosphorus (MRP) content and specimen thickness on the mechanical properties of the filled polypropylene (PP) composites were studied at room temperature. The results showed that the influence of the MRP on the tensile and impact properties of the composites was significant, and the Young's modulus and impact strength of the composites increased nonlinearly while the tensile strength decreased slightly with increasing the MRP weight fraction; the tensile elongation at break increased outstandingly when the MRP weight fraction was less than 2% and then decreased with increasing the MRP weight fraction. Furthermore, the impact fracture surface was examined by scanning electronic microscope. The results suggested that the toughening effect was attributed to the absorption of impact fracture energy by the microencapsulated layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Percolation in carbon black filled epoxy resin

The conditions to form a conductive network in carbon black filled epoxy resin were examined. It was found that the formation is controlled by particle-particle interaction which can be influenced by shear forces or by the ionic concentration of the resin. Therefore the network morphology and thus the percolation threshold depends strongly upon the processing route. On best conditions the percolation threshold was reduced to 0.06 vol.-%. That a continuous network can form with such a low volume fraction can be explained by the fractal dimension of agglomerates the network is made of.     

Crystallisation of polypropylene matrix in composites filled with wooden parts of rapeseed straw

Nucleation ability of native and modified rapeseed straw during the polypropylene crystallization from the melt was investigated by the DSC method. Composites were made from isotactic polypropylene and lignocellulosic material using extrusion and injection moulding techniques. They were obtained using polypropylenes differing with respect to melt flow rates and different varieties of rapeseed straw. Chemical modification was carried out in two stages: through mercerisation and treatment with acetic acid anhydride. In the course of investigations, it was found that both native and modified rapeseed straw acted as an active nucleant of polypropylene crystallisation characterised by low values of MFR indices. It was found that for polypropylenes with high MFR values, the values of crystallization temperatures and crystallization half-time in composites were identical when compared with non-filled polymers. The investigations demonstrated that there were insignificant differences among composites containing straw from different varieties of rapeseed. The analysis of crystallization temperatures confirmed that rapeseed straw modification failed to change this parameter of the crystallization process. A similar tendency was observed in the case of changes of the half-time crystallization process. Moreover, the analysis of the crystallization temperature and crystallization half-time showed that the presence in composites of lignified rapeseed straw particles played an important role in the crystallization conditions.     

Effects of thermal volume expansion on positive temperature coefficient effect for carbon black filled polymer composites

Polymeric positive temperature coefficient (PTC) materials have been prepared by incorporating carbon black (CB) into two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS). The effects of thermal volume expansion on the electrical properties of conductive polymer composites were studied. The volume fraction of conductive particles behaves like a switch from insulator to conductor in the polymeric PTC composite. Our mathematical model and experimental model have proved that the abrupt resistivity increase at PTC transition range and at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of the key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment. Furthermore, the model predicts PTC curves of CB/PS composite more successfully than it does for the CB/HDPE composite, and the reasons for this are also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3078–3083, 2007     

Nanostructured carbon black filled polypropylene/polystyrene blends containing styrene-butadiene-styrene copolymer: Influence of morphology on electrical resistivity

This paper is part of a comprehensive study on using selective localization of carbon black (CB) at the interface of immiscible polymer blends in order to reduce the percolation threshold concentration and enhance the conductivity of the blends. CB was successfully localized at the interface of polypropylene/polystyrene (PP/PS) blend by introducing styrene-butadiene-styrene (SBS) tri-block copolymer to the blend. In CB-PP/PS/SBS blends, CB has higher affinity for the polybutadiene (PBD) section of the SBS copolymer, whereas in CB-PP/PS blends, CB prefers the PS phase. PP/PS interface is one of the preferred locations for the SBS copolymer in the (PP/PS) blend; at which the PBD section of the SBS copolymer forms a few nanometers thick layer able to accommodate the CB nano-particles. The influence of SBS addition on the morphology and electrical properties of various PP/PS blends filled with 1 vol% CB were studied. SBS influence on the conductivity of PP/PS blends was found to be a function of the PP/PS volume ratio and SBS loading. The most dramatic increase in conductivity was found in the (60/40) and (70/30) PP/PS blends upon the addition of 5 vol% SBS. 5 vol% SBS was found to be the optimum loading for most blends. Using 10 vol% of SBS was reported to deteriorate electrical conductivity of the conductive co-continuous PP/PS blends. For all blends studied, SBS addition was found to compatibilize the blends. Finer morphologies were obtained by increasing SBS loading.     

Degradability of composites of low density polyethylene/polypropylene blends filled with rape straw

Composites of LDPE/PP blends and comminuted rape straw as a filler were obtained and characterized. The aging performance of these composites has been investigated. The composites were aged using various types of wet soil and selected species of mould fungi as aging environment as well as using Atlas UV 2000 apparatus as simulation of abiotic conditions. The changes between non-aged and partially degraded composites were evaluated by measuring their mechanical properties and mass change and using FT-IR spectroscopy, FT-IR/PA spectroscopy and scanning electron microscopy. The highest extent of degradation was observed after aging in compost. The band corresponding to cellulosic fibres is no longer recorded in the FT-IR spectra.     

Mechanical properties and morphology of intumescent flame retardant filled polypropylene composites

The intumescent flame retardant (IFR) filled polypropylene (PP) composites were prepared using a twin‐screw extruder. The tensile and impact fracture behavior of the composites were measured at room temperature. It was found that the Young's modulus increased roughly, while the tensile strength decreased slightly with increasing the IFR weight fraction; the toughening effect of the filler on the PP resin was significant. Both the V‐notched Izod impact strength and the V‐notched Charpy impact strength of the PP/IFR composites showed a nonlinear increase with increasing the filler weight fraction (φ_f) as φ_f was less than 20%, then it decreased. The limited oxygen index of the composites increases nonlinearly with increasing φ_f. The relationship between them obeyed a quadratic equation. The impact fracture surface was observed by means of a scanning electronic microscope to understand the toughening mechanisms for the composite systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Polymorphism and transcrystallization of syndiotactic polystyrene composites filled with carbon nanotubes

Syndiotactic polystyrene (sPS) composites filled with well-dispersed multi-walled carbon nanotubes (CNTs) were readily prepared through a coagulation method. Fourier-transform infrared spectroscopy and wide-angle X-ray diffraction revealed the effect of CNTs on the polymorphism of sPS. When crystallized from the melted state, the formation of the β-form was always favored after CNT addition regardless of crystallization conditions (isothermal or non-isothermal). In the case where liquid nitrogen was used to quench the melt, the uncrystallized material that was not able to crystallize in the extremely short crystallization time crystallized in the α form upon subsequent cold crystallization. Regardless of the CNT content, the glass transition and equilibrium melting temperature of the sPS matrix were unchanged at ∼96 and 290 °C, respectively. With a gradual increase in CNT loading, the sPS crystallization rate initially increased but then reached a plateau value at high CNT concentrations because of the reduction in chain mobility. Moreover, the Avrami exponent was changed from 2.8 for samples at low CNT contents to 2.0 for samples with a CNT concentration above 0.1 wt.%, at which the rheological threshold was approached and a polymer-CNT hybrid network was formed. The enhanced crystallization kinetics was attributed to the high nucleating ability of CNTs to induce a transcrystalline layer (TCL) at its surface, as revealed by transmission electron microscopy. For composites with low levels of CNT, the growth of sPS spherulites in the bulk between CNTs prevailed. Provided that the CNT-related networks were developed, the two-dimensional growth of cylindrical TCL at the CNT surface became dominant and led to the expected Avrami exponent.     

Mechanical and conducting properties of polypropylene fibers filled with carbon nanotubes with functionalized surface

A procedure for functionalization of the surface of multiwall carbon nanotubes, based on treatment of the nanofiller with a strong oxidant and 1-hexadecylamine and aimed to improve the compatibility of nanotubes with a polypropylene matrix, was examined. Variation of the mechanical and electrical properties of the composite fiber based on polypropylene in relation to the concentration of the modified nanotubes was studied.