Thermal and mechanical properties of polypropylene-iron-diamond composites

Polypropylene-iron and polypropylene-iron-diamond composites with various amounts of iron and natural or synthetic diamond particles were prepared in a co-rotating laboratory kneader. Test samples were made by injection molding. Heat flow measurements with a differential scanning calorimeter and dynamic mechanical properties in the forced vibration mode were measured from 225 K to 475 K, and thermal diffusivity was measured from 225 K up to 425 K by the laser flash method. Specific heat capacity of the polypropylene-iron-diamond composite can be significantly reduced by the addition of iron particles, while the addition of diamond particles show only a minor influence. Conventional, reversal and non-reversal heat flow measurements are showing two different monoclinic crystal structures (α1, α2) and crystallinities around 50% in the polypropylene. In a heating cycle a conversion between both crystalline structures can appear. Due to different crystallinities and crystal structures the activation energies for the main relaxations at glass transition temperature vary between 275−400 kJ/(mol⋅K) in the amorphous and between 65−190 kJ/(mol⋅K) in the crystalline part of the polypropylene. The storage modulus of polypropylene is increased by the iron and diamond particles from 4000 MPa up to 12500 MPa. The thermal diffusivity can be increased from 0.13 mm2/s up to 1.05 mm2/s with iron and up to 1.45 mm2/s with the secondary addition of diamond powder. With increasing temperature the thermal diffusivity decreases. The particle network shows a dominant contribution of the polymer matrix for mechanical properties whereas the particles show a dominant contribution to thermal properties. The mean free path of phonons in polypropylene decreases with increasing temperature, while it decreases up to 300 K and increases then up to 15 times the value of neat polypropylene again for temperatures above 375 K in the composites.