n-MgO and xGnP with a purity of 99.9% were used in this study as filler materials. The n-MgO and xGnP were purchased from 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 HDPE matrix particles were supplied by SABIC, Kingdom of Saudi Arabia.

In this study, the different weight percentages of xGnP and n-MgO reinforcement used are shown in Table 1.

The weighed powders were mixed using a mechanical disperser to obtain a homogeneous distribution of the filler materials in the HDPE matrix. Subsequently, the mixed powders were put into a Battenfeld HM 1000/750 injection moulding machine to produce the samples. Injection moulding was carried out using an L/D ratio of 22, screw diameter of 45 mm, a clamping force of 10 tons, barrel and nozzle temperatures of 230˚C, pressure of 3000 psi, and mould temperature of 20˚C. The different HDPE-xGnP/n-MgO composites shown in Table 1 were examined to investigate the effect on the thermal properties of HDPE-xGnP composites of the addition of n-MgO.

XRD (Schimadzu 7000, Japan) was used to confirm the existing phases of the composites. Wide-angle X-ray diffraction (WAXD) patterns, which provide the characteristics of the nanoparticles, were obtained with an X-ray diffractometer equipped with CuKα radiation. The survey scan was run between 10˚ and 90˚ with a scanning speed of 2˚ per minute.

The samples were prepared by fracturing (cryo-fracturing) the samples into 1 cm long pieces at the neck portion of tensile samples and attaching each to a 12.5 mm diameter Al stub with sticky 12 mm diameter C tabs. The samples were Au-Pd sputter-coated for 1 - 2 minutes at a deposition current of 25 mA and a partial pressure of ~0.1 TorrAr. The samples were then transferred to be examined by SEM.

A spectrogram of the composite surface was collected using an FTIR 783 Perkin Elmer spectrometer. Each spectrum was obtained for 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 changes in the chemical structure.

TGA was performed using a TGA Q500 instrument. The samples were placed in

a platinum crucible and heated in a nitrogen-filled environment with 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 extracted from the test were used to plot both the weight loss as a function of temperature and the thermogravimetric analysis/derivative thermal gravimetry (TGA/DTG) relations.

Wide angle X-ray diffraction (WAXD) is a widely applied technique in the study of intercalation or exfoliation and has an advantage for composite characterization. X-ray diffraction (XRD) is used to explain the intercalation or exfoliation structures by utilizing methods for the inter-gallery spacing calculations, which are responsible for identification of the composite structures [2] . Using this technique, it is useful to identify and present the amount of inter-planner spacing (d-spacing) of the inter-gallery for different particle loading composites as well as for the neat polymer.

Figure 1 shows the XRD patterns of the neat HDPE and HDPE/xGnP/n-MgO composites with different amounts of fillers. From the XRD patterns obtained, xGnP and n-MgO were found in samples b to d, as shown in Figure 1. It is clear that the xGnP peaks were detected ata two theta of 26.6˚ in all composite samples, which matches the previous study [23] . In contrast, n-MgO was observed at a two theta of 42.9˚, which suggests the face-centred cubic crystalline phase (JCPDS No 45-0496) [24] . In addition, the intensity of the xGnP peak became stronger with increasing amounts of xGnP added into the HDPE matrix, whereas the intensity of the n-MgO peak was relatively consistent, as shown in Figure 1. Table 2 represents the change of 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 literature [25] . The d-spacing can be described as

the distance between planes of atoms that gives rise to diffraction peaks. Each peak in a diffractogram results from a corresponding d-spacing. The planes of atoms can be referred to as a 3D coordinate system and can be described as a direction within the crystal. Therefore, the d-spacing, in addition to having a dimension, which is usually given in Ångstroms, can be labelled with a plane direction, hkl. With the development of nanotechnology, an increasing number of materials with d-spacings in the nanometre range have been made.

Figure 2 shows the FTIR spectra of neat HDPE and its composites. The peak of HDPE was somewhat altered with the addition of graphite and a constant amount of magnesium oxide nanoparticles. The absorbance level was slightly decreased by the incorporation of high amounts of graphite. The reduction was not significant as the molecular structure was relatively constant. In summary, differences in the peaks of the composites were not clearly observed, which indicates the stability of the polymer molecular structure. Of note in this paper, the FTIR spectra are presented as the absorbance percentage as a function of wavelength, not as the transmittance percentage as a function of wavelength. Therefore, if we carefully look at the FTIR spectra, the spectra show a decrease in the absorbance percentage with increasing filler content. Furthermore, this decrease in intensity can be attributed to a decrease in the number of free polyethylene chains, meaning a number of the polymer chains were coordinated to the carbon particles, and hence, the flexibility and molecular motion of the polymer chain are restricted.

The TGA/DTG analysis of monolithic HDPE and HDPE/xGnP/n-MgO composites during heating from 20˚C to 600˚C/min are shown in Figure 3. From Figure 3, it is clear that the addition of graphite and magnesium oxide nanoparticles into the HDPE matrix resulted in a slight increase in the thermal stability. Additionally, the onset temperature of the neat polymer was 465˚C, whereas for HDPE/1xGnP/2n-MgO and HDPE/2.5xGnP/2n-MgO, the onset temperatures were 464˚C and 464˚C, respectively. By incorporating 5 wt.% graphite and 2 wt.% magnesium oxide into the HDPE matrix, the onset temperature was improved to 466˚C. The char yield of the composites was significantly increased in a manner proportional to the graphite content. Crystallinity tended to be reduced

by increasing the composite loading, as shown in Table 3, which agreed with the literature that composites with plate-like particles have a reduced degree of crystallinity [26] . There is a robust relationship between Figure 3 and Table 3. The melting temperature (Tm), crystallization temperature (Tc) and degree of crystallinity are observed and calculated from the TGA curves.

These findings are in close agreement with those in the study by Wegrzyn et al. [27] .

To investigate the surface morphology of the composites, SEM micrographs were taken for the monolithic HDPE and HDPE-xGnP/n-MgO composites, as shown in Figure 4. Figure 4(a) reveals some white strand regions as a characteristic of polymer materials. The composites with 1 and 2.5 wt.% xGnP exhibited good dispersion/distribution of fillers in the matrix, which shows no evidence

of aggregation. This is due to the high affinity between the particles and polymer during fabrication at the optimal injection conditions. Furthermore, less intercalation was obtained in the polymers with the addition of 5 wt.% xGnP, which could be associated with eitherparticle agglomeration or the possibility of losing the platelet morphology of xGnP, leading to the development of a rolled-up structure or folds during preparation and dispersion differences [3] . It is worth mentioning here that achieving a homogeneous dispersion is the most challenging obstacle in obtaining an efficient reinforced polymer, especially in the case of non-polar polymers such as polyethylene (PE) [2] [28] .