The objectives of this paper are to present approaches to build up melt strength in linear polypropylene with dispersed nanoclay (montmorillonite) and to examine the strain hardening in extensional flow of polypropylene clay nanocomposite melts with varying loading and surface treatment.

Two different surfactants have been incorporated in the different grades of montmorillonite used here. The primary onium ion surfactant with a C-18 tail is hydrogen bonded more tightly to the clay and tends to orient perpendicular to the clay surface. The quaternary onium ion surfactant with two C-18 tails is more mobile in the interlayer galleries; it also packs to a greater extent in the interlayer galleries, cf. [1].

The matrix material is maleated polypropylene contains 0.5 wt% maleic anhydride, has a molecular weight of 70,000 and melts at 145 C. The compounding was done at 165 C and 150 rpm in a Banbury mixer with 5 and 8 wt% clay. The nanocomposites were found to melt at 146 C. The dispersion of clay in the nanocomposite was checked by X-Ray diffraction and found to be better for the primary onium ion treated clay. However, the strain hardening in extensional flow was found to be much much greater for the nanocomposite with the quaternary ammonium ion treatment. Uniaxial extensional flow tests were run at melt temperatures of 155C and 160C with the help of a rotary extensional viscosity fixture (EVF) on an ARES rheometer.

The results show that nanocomposites can be prepared that have much greater melt strength than the matrix of linear polymer chains. The state of dispersion is less important for this effect than the coupling between the matrix and clay. This can be accomplished with much less when adding clay than when adding branched polymer.

1. Heinz, H., Vaia,R.A. Krishnamoorti, R. and Farmer, B.L., Chem. Mater. 19, 59-68 (2007)

2. Stange, J., Uhl, C. and Munstedt, H., J. Rheol. 49 (5), 1059-1079 (2005)

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