A two-step process was used to prepare PP/MAPP/C15A/MWNT ternary nanocomposites system. The effect of the addition of MWNT on the delamination of clay layers in the polymer matrix has been studied through XRD. The similar crystallinity level was noticed after the addition of clay and MWNT together in the PP matrix through XRD. Higher mechanical properties of the ternary nanocomposites system were noticed than neat PP and its binary nanocomposites systems. Differential scanning calorimeter (DSC) technique was utilized to investigate the effect of both nanofillers on crystallization temperature (
Polymers, reinforced with nanoscaled fillers, developed an advanced multifunctional material with improved properties which can be used in many fields ranging from microelectronics to aerospace. And because of good balance between properties and cost, low density, and easy moldability, polypropylene is the chosen material among all polymers [
Nanoscale fillers are also important in improving mechanical and other properties along with changes in crystallization behaviour [
Studies also have been reporting that MWNT, because of their exceptional mechanical and thermal properties, has become attractive class of inclusion [
Combination of these two fillers nanoclay and MWNT has attracted much of the current interest owing to their extended structure allowing for clay nanolayer and carbon nanotubes network assemblies. Earlier various studies were primarily focused on dispersion of single nanofiller (clay or MWNT) into the polymer matrix, wherein various short falls have been reported. However very few literatures have reported the combined effect of MWNT and clay on single polymer matrix. It has been noticed preciously that MWNT and nanoclay exhibit synergism in improving the flame retardancy of ABS [
The aim of this work was to investigate the effect of incorporation of MWNT and layered silicates (MMT) based on nanoclay on the polypropylene (PP) matrix. The process parameters of the preparation of ternary nanocomposites were optimized in order to achieve the improved mechanical properties. The morphological conditions (intercalation/exfoliation) were also investigated through XRD and TEM. The dynamic mechanical analysis of the PP and its binary and ternary nanocomposites has been performed in order to evaluate the molecular mobility transition such as glass transition. This transition represents the ability of heat resistance [
Polypropylene (M110) was procured from M/s Haldia Petrochemicals, Kolkata, India, having a density of 0.94 g/cm3 and the MFI of 11 g/10 min. The clay minerals used were commercially available Cloisite 15A (cation exchange capacity (CEC) of 125 meq/100 g clay,
The masterbatch route was employed to prepare the nanocomposites using microinjection molding technique. First the PP/MAPP/C15A hybrid was synthesized with a 5 Wt% loading of MAPP and 3 Wt% loading of C15A using microcompounder, M/s DSM explore Netherlands, (Micro 15 at a temperature of 180, 185, and 185°C in front, middle, and rear zone resp. for 20 minutes as obtained from a previous study [
In this step variable mixing time and screw speed of 60 rpm for 5 min, 30 rpm for 15 min, and 30 rpm for 22 min were used in order to optimize the process parameters as well as improved dispersion of MWNT within PP matrix. The mixing parameters have been optimized as 180°C with 30 rpm for 22 minutes, on the basis of mechanical properties.
Wide angle X-ray diffraction (WAXD) analysis was used to analyze the interlayer gallery spacing of nanoclays in the nanocomposites, using Philips X’Pert MPD (Japan), with graphite monochromator and a Cu K
The tensile properties of the virgin matrix as well as the nanocomposites were determined using Universal Tensile Machine (3382 Instron, UK) as per ASTM D 638. Microinjection moulded sample having dimension of
Differential scanning calorimetry (DSC) measurement was carried out by scanning the sample about 10 mg, from −50 to 200°C with heating rate of 10°C/min under nitrogen atmosphere using Q 20 series of TA instrument.
The thermal degradation temperatures and thermal stability of the PP and its nanocomposites were studied using a thermogravimetric analyzer (Q50 M/s TA instrument, USA). Samples of ≤10 mg were heated from 50 to 680°C at a heating rate of 10°C/min. corresponding initial, maximum, and final degradation temperature were calculated.
The DMA analysis was conducted in dual cantilever mode at variable temperature using dynamic mechanical analyzer (DMA Q-800; TA instruments) technique. The experiment performed on the microinjection moulded samples having a dimension
TEM analysis was of the samples carried using the transmission electron microscope (JEOL 1200EX, Japan). The thin sections have been taken from injection moulded bar, using microtome, Lieca EM UC6 microtome (M/s Leica, Germany), under cryo conditions, for analysis. The sections were collected from water on 300-mesh carbon-coated copper grids. TEM imaging was carried out at an accelerating voltage of 100 kV. Images were captured using a charged couple detector (CCD) camera for further analysis using Gatan Digital Micrograph analysis software.
In order to achieve high performance nanocomposites, good dispersion of the nanofiller and strong interfacial adhesion between nanofiller and the matrix are the major requirements. The microstructural configuration was examined using XRD. Figure
X-ray diffraction pattern (a) from
Table
Mechanical properties of the PP and its nanocomposites.
Properties | VPP | PP/MAPP/C15A | PP/MAPP/C15A/MWNT | ||
---|---|---|---|---|---|
5 min |
15 min |
22 min | |||
Impact strength (j/m) | 25 | 22 | 20 | 22 | 31 |
Tensile stress (MPa) | 30 | 33 | 32 | 37 | 41 |
Tensile strain (%) | 8 | 6 | 6 | 5 | 8 |
Modulus (MPa) | 1136 | 1454 | 1408 | 1443 | 1521 |
Analysis revealed the higher tensile properties of PP/MAPP/C15A nanocomposites as compared to neat PP was noticed. This was due to uniform nanodispersion and intercalation of nanoclays. It is assumed that the strong hydrophilic interaction between the MA group and the polar clay surfaces has been achieved, which might be driving force of the intercalation. The tensile properties of PP/MAPP/MWNT nanocomposite were also higher than that of neat PP. This was also ascribed to the nanodispersion of clay particles inside the PP matrix leading to the even stress distribution.
Further, in case of ternary nanocomposites, increase of 23.6%, 4.65, and 19.5% was noticed in tensile strength, tensile modulus, and impact strength, respectively, as compared to PP/MAPP/C15A. Similarly as compared to PP/MAPP/MWNT also, ternary nanocomposites had 15%, 3%, and 11% higher tensile strength, tensile modulus, and impact strength, respectively. This might be ascribed to the more intact 3D network achieved by clay-clay, clay-MWNT, and clay-polymer-MWNT interaction, resulting in disruption of MWNT network by the layered silicates forming islands isolated by clay platelets [
Further, the elongation at break was found to be lower for nanocomposites. This supports the fact that increase in modulus leads to decreased mechanical ductile nature. However, it is not disadvantageous, since several applications require high stiffness of material and low deformation in order to ensure higher-dimensional stability [
Further, in order to strengthen the finding of the tensile data the Izod impact strength was analyzed. The analysis revealed the higher impact strength of the ternary nanocomposites. It is expected that the nanofillers create tie points between crystallites, bridging the amorphous phase, due to different possible interactions as mentioned earlier in this section.
The mechanical properties of the polymer matrix are very much dependent on the crystallinity and microstructure, since failure of material takes place at the microscopic level. The microstructural properties are directly reflected by exothermic or endothermic transition in DSC thermogram. The result obtained from DSC analysis is represented in Table
Differential scanning calorimeter results of PP and its nanocomposites.
Material |
|
|
|
|
---|---|---|---|---|
PP | 166 | 119 | 110 | 109 |
PP/MAPP/MWNT | 167 | 131 | 109 | 123 |
PP/MAPP/C15A | 169 | 129 | 107 | 118 |
PP/MAPP/C15A/MWNT | 168 | 139 | 110 | 127 |
The thermal stability is a unique ability to promote flame retardancy in polymer nanocomposite. The degradation temperature at different stages has been observed and reported in Table
Thermogravimetric analysis of PP and its nanocomposites.
Composition |
|
|
|
|
|
|
---|---|---|---|---|---|---|
PP | 384 | 430 | 450 | 319 | 440 | 440 |
PP/MAPP/C15A | 387 | 431 | 433 | 360 | 441 | 429 |
PP/MAPP/MWNT | 391 | 438 | 445 | 326 | 476 | 445 |
PP/MAPP/C15A/MWNT | 421 | 445 | 452 | 401 | 491 | 458 |
Further, the
The onset degradation temperature (
However, more delayed degradation in case of ternary nanocomposites was noticed. An increase of 82°C in the onset degradation temperature was evident. This was relatively higher as compared to its binary counterparts (i.e., PP/MAPP/C15A and PP/MAPP/MWNT). This suggests the presence of porous char layer with microcracks in PP/MAPP/C15A, whereas the PP/MAPP/MWNT is supposed to have continuous char layer with less density. The highly delayed degradation of ternary nanocomposites is ascribed to relatively tighter and denser char. This can be attributed to two types of mechanism. Some MWNTs act as the bridge and overlap the pores between clay layers, and the remaining MWNTs get inserted between clay layers and form a sandwich structure leading to intercalation of clay layers [
Also the decomposition of nanocomposites was found sharper than PP, pertaining to a reduced range of decomposition for nanocomposites as compared with virgin PP. This supports the fact that the addition of nanofiller improves the uniformity of the crystalline structure of the polymer matrix. Hence, in ternary nanocomposites more uniformity of crystalline structure was evident from sharper degradation peak. This may also be attributed to the restricted thermal motion of polymer and MWNT in the intergallery of clay platelets. These results were in accordance with the findings reported earlier [
Therefore, it is evident that, in order to increase the overall thermal stability of polymer matrix, a high degree of exfoliation accompanied with fine dispersion is required. In this study this exfoliation was achieved with the incorporation of MWNT in PP/MAPP/C15A hybrids, which in turn resulted in improved thermal stability.
Hence it may be concluded that plate like particles of high aspect ratio are not the only filler suitable for thermal improvement. Independent of aspect ratio nanoparticles with high specific area are also suitable entity for improving thermal stability, by absorbing radicals and high polar groups. This mechanism involves the physical/chemical adsorption of volatile degradation product on particle surface. This imparts more thermal stabilization in the polymeric system [
In order to interpret the unique property variation of PP and its nanocomposites, including melt fluidity, crystallization habit, and thermal stability as well as storage modulus, the role of the addition of nanofillers on the mobility of PP chains needs to be identified. The storage modulus is related to the stiffness of the material and measures the elastic response of the polymer. The loss modulus denotes the energy dissipated by the system in the form of heat and measures the viscous response of the polymer material. The damping factor (tan
The dynamic storage modulus is shown in Figure
Dynamic mechanical properties of PP and its nanocomposites: (a) storage modulus and (b) loss modulus.
Furthermore, it was noticed that the storage modulus of nanoclay and MWNT reinforced binary nanocomposites was higher as compared to the neat PP. This confirmed the uniformly distributed stress over the nanoclay and MWNT in their individual nanocomposites. Subsequently, higher storage modulus of ternary (PP/MAPP/C15A/MWNT) nanocomposites as compared to the its binary counterparts exhibited more intact percolated network in PP/MAPP/C15A/MWNT than in PP/MAPP/C15A and PP/MAPP/MWNT. Hence it is evident that coexistence of clay and MWNT in the polymer matrix creates more effective confined space and enhances the networked structure, leading to the relatively more even distribution of the stress.
It was also observed that the curves tend to converge to that of pure PP when approaching the melting temperature of PP. This convergence at higher temperature reveals the successful exploitation of nanofillers as reinforcement for PP.
The loss modulus curve of PP, as represented in Figure
The damping in polymeric materials is sensitive of segmental mobility of the polymer chain and in nanocomposites it is indicative of the interfacial interaction between the polymer and the filler. Strong interfacial interaction between the polymer and the filler tends to restrict the polymer mobility thereby reducing the damping. The damping factor of PP/MAPP/C15A was found lower than that of neat PP indicating the appropriate interaction between macromolecular polymeric chains and fillers (clay or MWNT). However it could be noticed that damping of PP/MAPPMWNT nanocomposites was higher as compared to PP/MAPP/C15A. This is because the low loading of MWNT could be dispersed uniformly in matrix but the formation of MWNT network could not take place [
Subsequently, a significant decrease in the damping of the ternary nanocomposites was noticed, revealing the much enhanced interaction between matrix and both the fillers together, as compared to their binary counterparts. These interactions may be in various forms as clay-clay network, clay-MWNT network, MWNT-polymer-clay bridging, and polymer-polymer network. In the clay-MWNT network clay platelets impede the motion of MWNT in polymer matrix during deformation, leading to the enhanced interaction between polymer chain and fillers, which might have played a major role in reducing the damping of ternary nanocomposites. Hence the synergistic effect of the clay and MWNT was evident.
From Figure
Tan
The TEM micrographs of PP/MAPP/C15A, PP/MAPP/MWNT, and PP/MAPP/C15A/MWNT are depicted in Figure
TEM micrographs of (a) PP/MAPP/C15A, (b) PP/MAPP/MWNT, and (c) PP/MAPP/C15A/MWNT.
On the contrary, incorporation of MWNT as secondary filler in, PP/MAPP/C15A nanocomposite revealed uniform dispersion of the MWNTs and clay layers within the PP matrix as compared to the PP/MAPP/C15A nanocomposites. Higher degree of exfoliation was observed in PP/MAPP/C15A/MWNT nanocomposites. This might be due to the insertion of MWNT within the clay layers which might have resulted in the delamination of the clay layers.
The entanglement of MWNT and clay layers is evident from the micrographs, which is marked in Figure
The present study revealed that the melt intercalated nanocomposite hybrids from PP clay and MWNT in the presence of compatibilizer can be obtained at relatively low screw speed and high residence time. Morphological investigation through XRD revealed much enhanced d-spacing of clay layers in ternary nanocomposites, revealing the exfoliated morphology. Incorporation of nanoclay within the PP matrix in the presence of compatibilizer results in improvement of tensile properties with no considerable improvement in the impact properties of the matrix polymer. On the contrary the addition of MWNT within the PP/MAPP/C15A nanocomposite hybrid resulted in significantly improved mechanical performance. The thermal stability of the nanocomposites increased phenomenally with the addition of MWNT, thus revealing the synergistic effect of MWNT. The dynamic mechanical analysis revealed the higher modulus in binary nanocomposites which was further higher in case of ternary nanocomposites. TEM micrographs revealed higher degree of exfoliation in the presence of both types of nanofillers (C15A and MWNT) as compared with that of C15A individually.
The authors declare that there is no conflict of interests regarding the publication of this paper.