In nanocomposites, different surface states of nanoparticles can potentially provide different interactions with the base polymer and in turn change the bulk properties. Aluminium nitride (AlN) nanoparticles were surface functionalised with three different silane coupling agents (SCAs) with varying organofunctional and hydrolysable groups. The effects of the filler surface chemistry on the resulting AlN/polypropylene (PP) nanocomposites were examined and compared with an unfilled reference system. It is observed that different organofunctional groups can provide different nucleating effects and the dispersion states of nanoparticles while the hydrolysable group is not the dominant factor. The dielectric spectroscopy results show the hydrolysable group of SCA will also result in a difference of the interphase since the trimethoxy silane treated systems show much higher imaginary permittivity than the triethoxy silane treated systems when the frequency is below 1 Hz. The grafted organofunctional layer on the particle surface can provide a significant improvement of the thermal conductivity of the composite materials, e.g. 15 % improvement in thermal conductivity was observed when adding 10 wt% methacrylate silanes treated nano-AlN into PP, while the untreated counterpart only has 5 % improvement.

The influence of water immersion and silane treatment on the AC breakdown and the complex dielectric response of polypropylene/nano-aluminium nitride (PP/nano-AlN) composites has been investigated. The as-received filler was examined to have a nanoscale particle distribution with a hexagonal shape and slight hydrolysation. Grafting the aluminium nitride with an octyl silane reduces the weight increase in samples filled with 10 wt% of aluminium nitride during water immersion by 3, from 0.29 to 0.09%. The results suggest that the AC breakdown strength and complex permittivity of ''wet'' composite samples are related to the silane treatment of the nanofiller. The AC breakdown strength of octyl silane-treated samples after 9 days of water immersion shows comparable results to the dry samples, while a reduction can be seen on non-treated samples. Although silane-treated samples still show an increased dielectric loss at low frequency after water immersion, a significant reduction in low-frequency dispersion of real and imaginary permittivity can be seen when compared to the non-treated composites. This indicates that significant gains can be obtained for PP/nano-AlN composites by suitable silane treatments.

Polypropylene (PP) is considered a competitive candidate to replace widely used cross-linked polyethene (XLPE) in cable material application, as it can potentially withstand higher operating temperature and has excellent electrical properties. Besides, it is a more environment-friendly material, since PP does not require crosslinking and, therefore, can be easily recycled at the end of its life. This paper investigated the influence on the tensile strength, space charge and breakdown behaviours of the PP by adding polyolefin elastomer (POE) and Magnesium Oxide (MgO) nanoparticles. 10 or 20 wt. % of POE and 5 or 10 wt. % of nano-MgO were introduced into PP. Results show that the PP with 10 wt. % of PE02 and 5 wt. % of surface treated-MgO has the highest breakdown strength among all systems. Also, the introduction of POE and nano-MgO can significantly improve the mechanical flexibility. In combination with the observed suppression of space charge accumulation, which shows that PP/POE/MgO nanocomposites are a viable option for high voltage (HV) cable insulation material in the future.

Polymer nanocomposites can potentially provide many advantages and the interaction region between polymer and nanoparticles-the so-called interphase is usually considered to be responsible for the change of properties. In this work, nanoaluminium nitride powders with various surface states obtained by three different silane coupling agents (SCA) and an untreated powder, were blended with isotactic polypropylene. AC breakdown strength and dielectric spectroscopy were used to study the effect of different particle surfaces and polymerparticle interphases. The nano-aluminium nitride powder grafted with different organofunctional groups can provide a number of potential interactions with the matrix polymer. The results show that it can not only affect the dispersion state of nanoparticles but will provide different impacts on the dielectric properties of the bulk material. The hydrolysable groups of SCA, notably, can also influence the dielectric properties through altering the surface chemistry of nanoparticles.

In this work, four loading amounts of trimethoxy (octyl) silane (C8-M) were used in preparing C8-M treated AlN/polypropylene nanocomposites. Although the thermal gravimetric analysis shows a significant difference in the amount of C8-M grafted on the AlN surface, and a saturation point can be reached as the amount of C8-M added increases, the AC breakdown strengths of composites present virtually identical results. Similar behaviours can be observed in the real and imaginary permittivity from 0.01 Hz to 100 kHz, although the upturns of the imaginary permittivity at low frequency are observed increasing with the amount of C8-M used. Seemingly contradictory to a previously reported work on epoxy with nanosilica, AlN/polypropylene is largely unaffected by the amount of the silane coupling agent applied during the surface treatment.