Processing and barrier performance of syndiotactic poly(propylene) nanocomposites
Dirk Kaempfer, Rolf Muelhaupt, Mariarosaria Tortora, Vittoria Vittoria
University of Freiburg
Germany
Keywords: syndiotactic poly(propylene), nanocomposite, barrier performance
Polypropylene exhibits an attractive combination of low cost, low weight and the extraordinary versatility in terms of properties, applications, and recycling. In order to improve poly(propylene)'s competitiveness in engineering resin applications, it is an important objective to simultaneously increase dimensional stability, stiffness, strength and the impact resistance. According to Okada and coworkers this can be achieved by melt intercalation of organophilic layered montmorillonite in the presence of an compatibilizer e.g. maleic anhydride-grafted poly(propylene).
This work is aimed of evaluating the syndiotactic polypropylene (sPP) nanocomposites containing organophilic layered silicates prepared by means of melt extrusion. Synthetic sodium fluoromica was used as water swellable layered silicate, which was rendered organophilic by means of cation exchange with protonated octadecylamine. Only compounding of the modified silicate in conjunction with maleic-anhydride-grafted isotactic polypropylene afforded exfoliation and dispersion of individual silicate layers, encapsulated in an iPP-g-MA shell, within the polypropylene matrix. Interlayer distance increased with increasing content and increasing molecular weight of the compatibilizer. The Young's modulus of the nanocomposite increased five-fold from 490 MPa up to 2640 MPa due to nanoreinforcement and nucleation of sPP crystallization via the iPP-g-MA shell of the dispersed organophilic silicate layers. The yield stress was increased to 28.7 MPa with respect to 16.1 MPa of the bulk sPP. Morphology, measured by means of transmission electron microscopy and mechanical properties were examined as a function of the silicate - and compatibilizer- content. The transport properties, were measured with respect to two organic vapors, dichloromethane and n-pentane. The zero-concentration diffusion parameter strongly decreased on increasing filler content. Therefore the permeability decreased for both vapors, as a consequence of the decreased value of the diffusion parameter.