Branching of polypropylene through reactive extrusion

Abstract

Long chain branched polupropylenes (PPs) have been produced by reactive extrusion (REX) and the thermal, rheological and molecular properties of the modified PPs have been fully characterized. For the long chain branched samples, it is found that there is good agreement among the results from GPC, thermal properties, apparent elongational viscosity, flow activation energy, and linear viscoelastic properties.

The work consists of two parts: a) the reactive extrusion of an acrylic acid grafted PP with hexadecylamine, and b) the reactive extrusion of PP with a combination of Lupersol 101 and pentaerythritol triacrylate (PETA). In the first route, model branched polypropylenes were produced by the reaction of acryclic acid grafted PP with hexadecylamine in solution and REX. It was found that the attachment of the alkyl chains lowers the glass transition, melting and crystallization temperatures of PP, and increases the moduli and shear viscosity.

Two stages were involved in the REX experiments of the second route. Stage 1 involved preliminary experiments to search for the possibility of producing branches on PP and suitable concentrations of Lupersol 101 and PETA for further experiments. Based on the results of stage 1, REX experiments were carried out in the second stage, using stabilized linear PP with very low concentrations of peroxide (50 - 200 ppm) and PETA (0 - 3 wt%). The products have very low levels of macrogels and the majority of macrogel formed during REX is PETA homopolymer. The sols of these modified PPs have higher thermal stabilities than linear PPs, investigating the coexistence of crosslinking structures. Generally, the sols from the PETA/peroxide modified PPs have higher melting and crystallization temperatures than those of linear PPs.

GPC-viscometry analysis shows that there is virtually no long chain branching in linear PPs. For the long chain branched PP produced, the average LCB frequency for the whole samples is estimated to be from 0.01 to 0.05 LCB/1000C, which corresponds to about 0.2 to 0.8 branches per weight average molecule. The change in the molecular weight averages with PETA and peroxide concentration in the PETA/peroxide reaction system seems nonmonotonic and it depends on PETA/peroxide relative concentrations. It is found that all the PETA/peroxide modified PPs have smaller Mn and Mu than those of virgin PP but some samples have M-s similar to that of the virgin PP. There is some broadening in the high molecular weight end in some long chain branched samples. Experimental results also suggest that there is a certain relationship between the inverse MFI and Mu for linear polypropylenes. However, this does not apply for their long chain branched counterparts.