EFFECT OF CHROMIUM RESIDUES ON THE STABILITY OF GAS-PHASE HIGH-DENSITY POLYETHYLENE PRODUCED BY SUPPORTED CATALYSTS

The thermo-oxidative stability of high-density polyethylene (HDPE) prepared by silica supported Ziegler-Natta, bis(triphenylsilyl)chromate and bis(cyclopentadienyl)-chromium(II) catalyst systems was examined by chemiluminescence (CL) techniques and thermogravimetric (TG) analysis. In particular, the technique of dynamic CL in nitrogen allows the concentration of polymer hydroperoxides and the relative stability of these to be simultaneously determined. Polymer hydroperoxides in Ziegler-Natta catalyzed HDPE are relatively stable because titanium and aluminum residues are poor pro-oxidants compared with chromium residues. HDPE produced by bis (cyclopentadienyl) chromium(II) has a low intrinsic thermo-oxidative stability due to the chromium-catalyzed conversion of polymer hydroperoxides into degradation products during thermal aging. Concentrations of residual chromium as low as 1 ppm can profoundly affect the oxidative stability of the polymer and exert a much greater influence than either the extent of branching or the degree of unsaturation. The appearance of the silica catalyst support before and after gas-phase polymerization was studied by scanning electron microscopy. During polymerization the silica support shatters and the resulting submicron fragments are dispersed throughout the polymer particles. On exposure of the nascent polymer to the atmosphere, bis(cyclopentadienyl)chromium(II) is converted to a trivalent chromium species which remains associated with the silica substrate. The presence of trivalent chromium in the poisoned chromium catalyst was confirmed by UV/visible spectrophotometry and the nature of the Cr(III) species was investigated by diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The high surface area of the catalyst residue renders it a powerful pro-oxidant despite its low concentration.