Dramatic changes occur during the initial stages of olefin polymerization over heterogenous catalysts. As polymer accumulates, the catalyst fragments and the void space within the growing particle becomes filled with polymer. The changing monomer transport rate to the active sites, dissipation of heat and stress with the particle, and eventually, dispersion of catalyst fragments within the growing particle can control the polymerization. We focus on the changes in, and influence of, polymer, catalyst and void morphology during the nascent polymerization for silica supported chromium oxide catalysts. We find that the course of the polymerization depends on the initial stages yet is seldom considered in polymerization models. We review several years of morphological characterization that identify the important physical phenomena which occur during nascent heterogeneous olefin polymerization. ~~ ~~ ~ ~~~Des changements importants surviennent durant les stades initiaux de la polymerisation de I'olkfine sur des catalyseurs hkttrogknes. Lorsque le polymkre s'accumule, les fragments de catalyseurs et I'espace intersticiel B I'intCrieur de la particule qui grossit se remplit de polymkre. La variation de vitesse de transport de monomkre vers les sites actifs, la dissipation de la chaleur et des contraintes avec la particule, et finalement, la dispersion des fragments de catalyseurs B I'interieur des particules qui croissent contrblent la polymtrisation. Nous nous interessons en particulier aux changements dans le polymkre, le catalyseur et la morphologie du vide et B leur influence durant la polymkrisation naissante pour de la d i c e supportke sur des catalyseurs d'oxyde de chromium. Nous avons trouvt que le cours de la polymerisation dkpendait des stades initiaux bien que cela soit rarement considCrC dans les modkles de polymerisation. Nous avons examine sur plusieurs annkes la caractirisation morphologique qui difinit les phknomknes physiques qui surviennent durant la polymkrisation naissante de l'olkfine hiterogkne.

Transport influences during nascent ethylene polymerization over Phillips chromium oxide-silica catalyst were evaluated by using pulse solid-gas chromatography. Measures of effective intraparticle diffusivity and tracer-accessible intraparticle void space were made using helium as a tracer. Measures of surface reaction rate and equilibrium adsorption were made using ethylene monomer as the tracer. Quantitative site poisoning using oxygen was performed. The Kubin-Kucera model was used for the transport measurements; the Suzuki-Smith model was used for the kinetic analysis. The Thiele modulus for ethylene polymerization at 50 °C, 1 atm of monomer pressure, and yields less than 0.1 g of polymer/g of catalyst is estimated to be 1.5. This value increases as polymer is formed inside the catalyst pores. Significant intraparticle concentration gradients and transport control may exist even at low yields. Without particle fracture, the rate of polymerization, which would occur mainly at the particle surface, would be inhibited. The rate of adsorption of monomer over active chromium oxide is very fast relative to the surface reaction. The apparent first-order polymerization rate constant at these conditions is roughly 0.06 s-1. The estimated activation energy for the surface insertion step is 14 kcal/mol. The equilibrium adsorption constant is 6; the free energy for reversible adsorption is -3 kcal/mol. The effective intraparticle monomer diffusivity prior to particle fragmentation and accumulation of polymer is 2.5 X 10"4 cm2/s, a very low value when compared to that estimated by using effective medium transport theory.

The realtime evaluation of normal, tangential and perpendicular cutting forces and spindle vibration provides a simple measure of blade friction, wear performance, diamond-bond wear rate differences, blade stiffness and machine support. These observations can be used to evaluate and diagnose problems in diamond sawing that might inhibit use of high-grade diamond crystals.

No abstract

No abstract