materials | Performance PP
composites. The crystalline microstructure of the composites has been characterised by X-ray diffraction (XRD), with scanning electron microscopy (SEM) used to study the interface between the matrix and the fillers and the orientation of the glass fibres, and polarised Raman spectroscopy to determinate the orientation of the GNPs in the composite samples. The Young’s (elastic) modulus of the hybrid material
A schematic image of a single layer of graphene. Nanoplatelets of 10 or more layers are being investi- gated for PP modification
In a paper published in Materials Science and Engineering, Young and his colleagues say that it was realised soon after the discovery of graphene in 2004 that its addition to a polymer matrix could result in a significant improvements in mechanical properties, particularly stiffness and strength. It was also thought that the addition of graphene could influence crystal nucleation and the microstructure of the nanocomposite. Graphene nanoplatelets can be a cost-effective filler
for nanocomposites, the researchers report. “Strictly speaking, such materials should be termed ‘graphite nanoplatelets’ since they contain more than 10 gra- phene layers,” they say. However, they point out that the terminology “graphene nanoplatelets” is now commonly used.
“It is clear that the addition of graphene nanoplate-
lets to PP to form PP/GNP nanocomposites leads to a major modification of both the microstructure and mechanical properties of the polymer,” the researchers say. “The thermal stability of the PP is improved significantly and the melting temperature and degree of crystallinity are both increased through enhanced crystal nucleation.” The Manchester researchers say the nanocomposite
system investigated in their study is far from optimised. “A nucleated PP homopolymer matrix has been employed and it would be of interest to investigate the effect of using other forms of PP,” they say. “Only one size of unfunctionalized GNPs has been used and better performance may be obtained with different particle sizes and functionalised GNPs. Again, only one set of mixing processing conditions has been employed and different conditions may lead to better particle dispersions. These issues all need to be investigated in future studies.”
Graphene hybrids Another project led by Young is looking at hybrid multifunctional graphene/glass-fibre polypropylene
28 COMPOUNDING WORLD | August 2017
Figure 1: Graph showing variation of PP modulus with a range of GNP (graphene nanoplatelet) and glass fibre contents Image courtesy of Composites Science and T
echnology
www.compoundingworld.com
was found to be higher than that of the materials containing the fillers individually, showing an additive effect, the researchers say. There was also good interfacial stress transfer between the PP matrix and the GNPs while the thermal conductivity of the materi- als was found to be significantly higher for the compos- ites containing GNPs alone. The researchers found that the addition of the glass
fibres decreases both the degree of crystallinity and the glass transition temperature of the PP, while both of these parameters are increased significantly upon the addition of the graphite nanoplatelets, improving the thermal properties of the material. “The addition of both of these fillers to PP increases the Young’s modulus of the polymer to a similar extent for a given volume fraction and there is some evidence from Raman spectroscopy and from the use of a 3-phase modified rule of mixtures that, when added together, the two fillers may give better reinforcement by acting additively,” the researchers conclude (see Figure 1). “The addition of the glass fibres is found to increase
the fracture stress but reduces the failure strain of the polymer whereas the addition of the graphite nano- platelets causes less reduction in the failure strain. Finally, the presence of glass fibres in the PP is found to have little effect upon the thermal conductivity of the
PHOTO: SHUTTERSTOCK
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