Dielectric properties of isotactic polypropylene and montmorillonite nanocomposites

Polymer nanocomposites typically consist of inorganic particles dispersed in a polymer matrix. Layered-silicate-based polymer nanocomposites have great potential as low-cost, high-performance materials with various engineering applications. They exhibit enhanced mechanical, thermal, and electrical properties compared with their macro- and micro- counterparts. Polypropylene (PP) exhibits good hardness, thermal, and electrical properties and provides outstanding resistance to moisture and chemicals such as grease, detergent, and oils. In addition to this, PP provides good resistance to fatigue, environmental stress, and cracking. It is commonly used as an insulating material in power capacitors and cable wraps. Natural montmorillonite (MMT) is a commonly-used layered silicate filler. Nanocomposites formed from isotactic polypropylene (iPP) filled with MMT are candidates for power cable insulation materials with enhanced properties. In this work, the a.c. dielectric permittivity and losses of iPP-based nanocomposites, filled with 2% and 6% by weight of Cloisite 20A nanoclay with natural MMT, have been investigated. Measurements were made using a Novocontrol dielectric spectrometer with sample cell BDS1200 housed in a temperature-controlled chamber, in the frequency range 10^-2 to 10⁶ Hz at temperatures 25, 50, 75, 100 and 125°C. The nanocomposites show increased permittivity and loss factor compared with unfilled iPP, attributed to the appearance of two thermally-activated relaxation processes in this frequency range. The mechanisms underlying these relaxation processes will be discussed, along with observations of other parameters such as glass transition temperature and melting temperature, and their relation to the percentage of nanoclay filler.