Composites of polyamide 6 and silicate nanotubes of the mineral halloysite: Influence of molecular weight on thermal, mechanical and rheological properties

In this work, the potential of silicate nanotubes of the naturally occurring mineral halloysite as filler for polyamide 6 (PA 6) nanocomposites is evaluated. Several PA 6/halloysite composites with 0 wt% to 30 wt% filler loading using two different grades of PA 6 were prepared. In order to elucidate the influence of molecular weight on the properties of the nanocomposites, mechanical resp. rheological experiments (i) below the glass transition temperature T_g of PA 6, (ii) between T_g and the melting temperature T_m of PA 6 and (iii) above T_m were performed. Our investigations reveal that the addition of halloysite nanotubes favours the formation of the γ-modification for the low molar mass PA 6. Furthermore, the storage modulus, the tensile modulus and the yield stress of the composites increase with concentration of halloysite, an effect which is strongly pronounced at very low filler fractions for the low molar mass PA 6 composites. The increase of the storage modulus which was measured in dynamic-mechanical experiments is mostly dominant in the temperature interval from 55 °C to 100 °C, i.e. above the glass transition temperature of PA 6. Rheological investigations showed that the shear viscosity is only moderately increased by the addition of a low fraction of halloysite to PA 6, and nanocomposites with 30 wt% halloysite can be still processed. In summary, halloysite nanotubes are promising and inexpensive candidates for increasing the stiffness of PA 6 while maintaining very good flow properties.