SynopsisThermally activated ethylene polymerization catalysts which contain chromium and titanium on silica differ from those catalysts which contain only chromium on silica. The characteristics of chromium-titanium catalysts vary with the method of incorporating the titanium and the thermal activation procedure. Titanated catalysts of the kind examined in this article have faster initiation and a higher polymerization rate per unit catalyst weight than corresponding catalysts without titanium. High-density polyethylene produced by this type of titanium-chromium catalyst tends to have a higher melt index and a broader molecular weight distribution than polyethylene made with chromium on silica catalysts. Iodometric titration showed that reduction from the initial hexavalent chromium to trivalent occurs when the dry catalyst starting material is treated with titanium tetraisopropoxide. A study of the reaction between chromium trioxide and titanium tetraisopropoxide in carbon tetrachloride revealed that (1) it is not necessary to have a reaction between surface silanols and titanium tetraisopropoxide for the reduction to occur, and (2) the reaction product has an absorption near 660 nm in the visible range. Comparison of spectra showed that chromium trioxide on silica reduced by isopropyl alcohol has a shifted absorption, i.e., 600 nm. These findings are interpreted to mean that titanium atoms come sufficiently close to chromium atoms to change their electron density in the starting material and remain close neighbors in the activated catalysts.

It was shown that three material functions are required to relate various phenomena in the capillary flow of polyethylene. The phenomena examined are the entrance pressure loss, laminar flow through a capillary, memory decay in the flow, and extrudate swelling. A unified picture was constructed for the capillary flow and the energy balance was calculated.

A method is given for the analysis of long‐chain branching in polymers by using combined GPC and intrinsic viscosity measurements. A computer program was written to evaluate branching indices by a tabular, iterative method. The method was applied to the evaluation of long‐chain branching in low‐density polyethylene.

In order to predict the processing behavior of a high density polyethylene resin one must know the resin flow behavior over a wide range of shear rates. Low shear properties are important in applications where melt strength, sagging, etc. are critical. On the other hand, high shear flow properties are a determining factor in applications where melt instability, melt fracture and heat generation are important. The flow behavior of a resin can be established by measuring the zero shear viscosity, η0, the maximum relaxation time, τ0, and the shape of the flow curve. We have measured these basic rheological parameters on a large number of high density polyethylene resins. A shear sensitivity parameter which is independent of molecular weight was derived from a correlation between η0 and τ0. This parameter, together with η0, provide the vital information needed in order to predict the processing behavior of the resin. This method is applicable to other polymer systems provided that the rheological parameters η0 and τ0 can be experimentally obtained.

The apparent uniaxial extensional viscosity or elongational viscosity, ηE, of several polymer melts in a wide range of temperatures was determined as a function of strain rate, ε̇, using Cogswell's analysis of converging flow at the die entry. In particular, ηE was derived from knowledge of the dependence of the steady-flow viscosity as well as the entrance pressure drop, δP0, on shear rate. Quantitative agreement was found in ηE−ε̇ dependence derived from the convergent flow analysis (CFA) along with that measured from isothermal melt-spinning experiments for melts of high-density and low-density polyethylene, polypropylene, and polystyrene. Further support of CFA comes from the finding here that the normalized entrance pressure drop, δP0/τ, is uniquely related to the vortex angle in a manner similar to that determined from flow visualization studies by Ballenger and White. Here τ is the shear stress at the die wall. Additionally, the flow activation energy in extension is found to be equal to that in shear for melts of low-density polyethylene, polypropylene, and polystyrene. Such observations are in line with the earlier results of Munsted on polystyrene melts. An anomaly exists for high-density polyethylenes since ηE increased with increases in temperature above a critical temperature. Finally, the shapes of extensional flow curves are found to be similar to the published data, although an exception such as low-melt-index, low-density polyethylene, where ηE decreases with ε̇, is cited.