e 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

Figure 2. FTIR images of HDPE and its composites: (a) 100 wt.% HDPE, (b) HDPE/1xGnP/2n-MgO, (c) HDPE/2.5xGnP/2n-MgO, and (d) HDPE/10xGnP/2n-MgO (too bad resolution-not clear).

(a) (b) (c) (d)

Figure 3. TGA results of (a) 100 wt.% HDPE, (b) HDPE/1xGnP/2n-MgO, (c) HDPE/2.5xGnP/2n-MgO, and (d) HDPE/5xGnP/ 2n-MgO.

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] .

3.4. Microstructure

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

Table 3. Thermal properties of HDPE and its corresponding composites.

Figure 4. SEM images of HDPE and its composites: (a) 100 wt.% HDPE (2 mm), (b) HDPE/1xGnP/2n-MgO (2 mm), (c) HDPE/2.5xGnP/2n-MgO (400 µm), and (d) HDPE/ 5xGnP/2n-MgO (400 µm).

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] .

4. Conclusion

HDPE/xGnP/n-MgO composites were fabricated using an injection moulding machine. The present results show that the combination of xGnP and n-MgO filler provides varying degrees of reduction in the thermal properties of composites. Generally, all of the composite samples revealed lower thermal properties compared with the monolithic HDPE. This reduction can be attributed to the agglomeration of particles within the matrix as a consequence of inadequate matrix-reinforcement adhesion. Other preparation method such as in situ polymerization, can be utilised for investigating the intercalation level.

Acknowledgements

The author would like to thank SABIC for their assistantship and tremendous help and contribution in the preparation of the composites and their characterization.

Cite this paper
Alateyah, A. (2019) High-Density Polyethylene Based on Exfoliated Graphite Nanoplatelets/Nano-Magnesium Oxide: An Investigation of Thermal Properties and Morphology. Materials Sciences and Applications, 10, 159-169. doi: 10.4236/msa.2019.103013.
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