Since the 1940s, synthetic polymers has been experiencing fast development and growth due to their incomparable properties and low cost. Polypropylene (PP), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polyethylene terephthalate (PET), polyethylene (PE), and polystyrene (PS) are some of the most common synthetic polymers. They are increasingly utilized in the developing world due to their low production costs and diverse uses in applications such as mobile phones, various parts of vehicles and aircrafts, medical-related products, construction, toys, packaging and protection . PP is a widely adopted thermoplastic polymer, noted for its widespread use in industrial applications. However, PP has two negative properties: insufficient resistance at low temperatures and high physical effects. To overcome these problems, PP is often blended with various other polymers . Exfoliated graphite nanoplatelets (xGnPs) are newly introduced additives that show promising outcomes toward superior properties when mixed with PP [3, 4, 5, 6, 7, 8]. xGnPs are usually composed of a maximum of ten graphene layers, leading to a total thickness of a few nanometres . Notably, xGnPs are extraordinary reinforcing materials and have been proven to ameliorate mechanical, thermal, and electrical polymeric matrix properties [10, 11]. MgO nanoparticles are among the most diversely utilized metal oxides due to their simple synthesis, which adopts cheap mineral raw materials such as bearing minerals, magnesium salts, and brines . In addition to the low raw material costs, MgOs have exceptional mechanical properties of superior strength, modulus, and hardness, plus a remarkably high melting point and exceptional thermodynamic stability . Consequently, MgO is utilized as an appropriate filler in this work.
Thermal stability is one of the most studied characteristics of polymers because organic-based polymers have low thermal degradation resistance. Recent studies have shown that the addition of nanomaterials such as carbon nanotubes, nanoclays, and graphene to the polymers improves their thermal stability. The incorporation of inorganic fillers such as GNPs has been shown to further enhance many of these properties, including thermal properties [14, 15]. Furthermore, studies discussing the adoption of MgO as a nanofiller for PP based on xGnP composites are very limited in the literature. Consequently, this work investigates the thermal properties of PP based on xGnP/n-MgO composites.
2 Experimental procedures and methods
xGnP and n-MgO, having purities of 99.9%, were utilized as the filler materials in this work. They were supplied by Tritrust Industrial Co, China. The xGnP has a diameter of 10 μm and a thickness of 5 nm, whereas the MgO nanoparticles have a particle size of 10 nm. The PP powders were provided by SABIC, Kingdom of Saudi Arabia.
2.2 Fabrication of composites
Different gravimetric proportions of xGnP and n-MgOs were used to reinforce the PP matrix to obtain different PP/xGnP/n-MgO composites, as presented in Table 1.
A mechanical disperser was used to mix the weighed powders for some time, aiming for a homogeneous state. Afterwards, the powder mix was added to a Battenfeld HM 1000/750 injection moulding machine to produce the samples. Injection moulding was carried out using a 22 L/D ratio, 45 mm screw diameter, 10 tons of clamping force, 230∘C barrel and nozzle temperatures, ~ 20.68 MPa pressure, and 20∘C mould temperature. Five different sample compositions, presented in Table 1, were tested to investigate the effect on mechanical properties of adding n-MgOs to PP-xGnP composites.
3 Characterization methods
3.1 X-ray diffraction analysis
X-ray diffraction (XRD) was utilized to confirm composition of the nanocomposites and to determine the composites existing phases. Wide-angle X-ray diffraction (WAXD) patterns were obtained with an X-ray diffractometer equipped with a CuKα radiation source. These patterns aim at supplying the characteristics of nanoparticles. The survey scan range was chosen to be from 10∘ to 90∘, with a scanning speed of 2∘ every sixty seconds.
3.2 Scanning electron microscopy
The samples were prepared through fracture (cryo- fracture) to one-centimetre-long pieces from the neck portion of tensile samples. These pieces were then attached to Al stubs with a diameter of 12.5 mm. Afterwards, the samples were sputter-coated with Au-Pd for one to two minutes at an Ar partial pressure of ~0.1 Torr and a deposition current of 25 mA. The samples were then examined through 20 Kv FEI Quanta 200 scanning electron microscopy (SEM). The working depth ranged from 15-25 mm, with an image resolution of 1024x784.
3.3 Fourier transform infrared (FTIR)
The spectra of the composite surfaces were acquired using a FTIR 783 Perkin Elmer spectrometer. Each spectrum was obtained from an average of 24 scans between 4000 and 400 cm−1, at intervals of 1 cm−1, with a resolution of 4 cm−1. An FTIR spectroscopic study was performed to assess the structural degradation of the immersed samples or the change in the chemical structure.
3.4 Thermo-gravimetric analysis (TGA)
TGA was performed using a TGA Q500. The samples were placed in a platinum crucible and heated in a nitrogen filled environment at a heating rate of 20∘C/min from room temperature to 600∘C. The initial weights of the samples were approximately 22 mg. The data from the test is displayed as TG, weight loss as a function of temperature, and as DTG (derivative thermal gravimetry), weight loss rate as a function of temperature.
4 Results and discussion
4.1 X-ray diffraction analysis
Wide-angle X-ray diffraction (WAXD) is a widely used technique for studying intercalation and exfoliation in order to characterize composites. X-ray diffraction is typically utilized to elucidate intercalation or exfoliation structures through calculations of inter-gallery spacings, which allow recognition of the composite structures . This technique is used in this section for identifying and presenting the inter-gallery d-spacings of different particle loading composites, as well as for the neat polymer.
Figure 1 shows the XRD patterns of the neat PP and PP/xGnP composites with various compositions of fillers. In the XRD patterns obtained, the neat PP shows four strong peaks, as shown in Figure 1a These peaks are characteristic of neat PP and are consistent with previous work . xGnP and n-MgO were found in samples 2 to 5, shown in Figure 1b-1e xGnP peaks are clearly detected at a two theta value of 26.6∘ in all composite samples, which is consistent with the previously published report . The n-MgO peak was observed at a two theta value of 42.9, which is clearly indicative of a face-centred cubic crystalline phase (JCPDS No 45-0496) . In addition, the intensity of the xGnP peaks became stronger with increasing concentration of xGnP added into the PP matrix, whereas the intensity of n-MgO was relatively consistent, as seen in Figure 1. Table 2 represents the change in the basal spacing (d001 spacing) of the composites, which was calculated by Bragg’s Law using the values extracted from the XRD patterns according to .
4.2 Microstructural investigation
The SEM micrographs of PP along with various composites of xGnP and n-MgO are shown in Figure 2. The microstructure of the neat PP sample showed a spherulite structure, which is a known PP microstructure characteristic (Figure 2a). Moreover, Figure 2b-2e present the microstructures of the composites with different amounts of filler, which are essential for the justification of filler dispersion within the PP matrix. The microstructures of the composites showed good dispersion of filler particles, which were almost embedded into the matrix and well dispersed. Moreover, their interfaces are well bonded, which is expected to improve the mechanical properties of the composites. However, small agglomerations were found in the 5 and 10 wt.% xGnP composite samples, as shown in Figure 2d and 2e respectively. Although xGnP is a potent filler, achieving homogeneous dispersion remains the key challenge for effectively reinforcing the polymer, particularly for non-polar polymers such as polypropylene (PP) [18, 19, 20, 21].
4.3 Fourier transform infrared (FTIR)
The FTIR spectra of the PP composites are shown in Figure 3 The peaks of PP were somewhat altered with the addition of graphite and a constant amount of magnesia oxide nanoparticles. The high concentration of graphite addition has slightly decreased its absorbance level; however, it was not too significant since the molecular structure is relatively constant. In other words, there are no significant differences among peaks produced at composites, which means the molecular structure of the polymer matrix was relatively stable.
4.4 Thermal properties
Figure 4 presents the TGA and DTG analyses, of the neat PP matrix and corresponding nanocomposites during heating rates varying from 20 to 600∘C/min. The addition of graphite and magnesium oxide nanoparticles into the polymer matrix resulted in enhancement of thermal stability. As seen in Figure 4, the neat polymer reached the onset temperature at 425∘C, whereas it was reached for the PPG1M2 and PPG2.5M2 composites at 440∘C and 442∘C, respectively. The incorporation of 5 wt.% graphite and 2 wt.% magnesium oxide into the PP matrix improved the onset temperature to 448∘C. Moreover, an amelioration of the onset temperature up to 450∘C was reached when adding 10 wt.% graphite and 2 wt.% magnesium oxide. In addition, the char yield of the nanocomposites has significantly increased, whichwas observed to be proportional to the graphite content. The thermal stability enhancement of the nanocomposites, shown in Figure 4, could be attributed to the network structure formed by the interfacial interaction between the PP matrix and graphite, which led to an increase in the activation energy for the degradation process . A possible interpretation is that the magnesium oxide nanoparticles and graphite act as insulators, which could inhibit the diffusion of volatile products from the bulk of the polymer matrix into the gas phase during the degradation process .
On the other hand, the crystallinity tended to be reduced by the increase of nanocomposite loading, as seen in Table 3, which is in accordance with the literature explaining that nanocomposites with plate-like particles correspond to a reduced crystallinity degree . As neat PP already exhibits relatively high crystallinity, the incorporation of graphite increases the possibility of reducing free volume and hindering polymer chain mobility, thus discouraging the formation of larger, more ordered crystals . These findings are in close agreement with those of the study performed by Wegrzyn et al. .
PP-based composites reinforced with xGnP and n-MgO have been successfully fabricated using an injection moulding machine. The presented results demonstrated that the incorporation of combinations of xGnP with n-MgO fillers led to an improvement in the thermal properties of the PP composites. In general, all composite samples showed better thermal stability than the neat PP. This confirms that the addition of different types of fillers improves the thermal stability of the composites. From this investigation, it was found that the PP/10xGnP/2n-MgO composites had the best performance in terms of thermal properties. Gradual incorporation of xGnPs led to an excellent combination with improved thermal properties. The improvement of thermal stability could be attributed to the act of fillers as obstacles, which may inhibit the diffusion of volatile products from the bulk of the polymer matrix into the gas phase during the degradation process. The network structure formed by the interfacial interaction between the PP matrix and graphite played an important role in increasing the activation energy needed for the degradation process to start. In addition, the reduction of crystallinity was proportional to the fillers’ contents, which could be explained by indications that the plate-like particles reduce the degree of crystallinity. Additionally, the neat PP already exhibits relatively high crystallinity; therefore, the incorporation of graphite increases the possibility to reduce free volume and hinder polymer chain mobility, thus discouraging the formation of larger, more ordered crystals.
The authors would like to thank and appreciate the efforts of SABIC for their assistance, tremendous help, and contribution in the preparation and characterization of composites.
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About the article
Published Online: 2018-11-20
Conflict of interestConflict of Interests: The authors have no conflicts of interest to declare.
Citation Information: Open Engineering, Volume 8, Issue 1, Pages 432–439, ISSN (Online) 2391-5439, DOI: https://doi.org/10.1515/eng-2018-0052.
© 2018 A. I. Alateyah, published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0