Millimeter-size polyethylene hollow spheres synthesized with MgCl2-supported Ziegler-Natta catalyst

Polyethylene hollow spheres with diameters of 0.4-2 mm were synthesized by a two-step slurry polymerization in a single reactor with a spherical MgCl2-supported Ziegler-Natta catalyst activated by triethylaluminum, in which the first step was prepolymerization with 0.1 MPa propylene and the second step was ethylene polymerization under 0.6 MPa. The prepolymerization step was found necessary for the formation of hollow spherical particles with regular shape (perfectly spherical shape). The effects of adding small amount of propylene (propylene/ethylene < 0.1 mol/mol) in the reactor after the prepolymerization step were investigated. Average size of the polymer particles was increased, and the polymerization rate was markedly enhanced by the added propylene. Development of the particle morphology with polymerization time was also studied. The polymer particles formed by less than 20 min of ethylene polymerization showed hollow spherical morphology with thin shell layer. Most of the particles had ratio of shell thickness/particle radius smaller than 0.5. By prolonging the ethylene polymerization, the shell thickness/particle radius ratio gradually approached 1, and the central void tended to disappear. Central void in polymer particles formed from smaller catalyst particles disappeared after shorter time of polymerization than those formed from bigger catalyst particles. The shell layer of the hollow particles contained large number of macro-, meso-and micro-pores. The mesopore size distributions of four typical samples were analyzed by nitrogen adsorption-desorption experiments. A simplified multigrain model was proposed to explain the morphogenesis of the hollow spherical particles. (C) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43207.