Patent GB-925753-A (GB)
Title: Removal of impurities from polyolefines
IPC: C08F10/00
Filed: 1961-01-10
Abstract: Solid impurities are removed from polyolefines by filtering the polyolefine in a liquefied state at a temperature at which the impurities are solid and under pressure of a gas which is soluble in the liquefied polyolefine.  The polyolefine may be in a molten state or in a solution and specified olefines are ethylene and propylene whilst the gases specified are ethylene, propylene and ethane. Preferably the filtration is carried out at a temperature of 150 DEG -250 DEG C. with a discharge pressure from the filter unit higher than the partial pressure of the dissolved gas.  In the examples, polyethylene is prepared by pressurizing with ethylene an autoclave containing mineral spirits and molybdenum oxide of reduced valency of 4,42 as catalyst an g -alumina with sodium dispersed thereon.  After polymerization, the polyethylene is filtered directly under the pressure of the reaction (1,000 p.s.i.g.) through a heated filter press.  After filtration, the polymer solution is flushed to a vessel at atmospheric pressure and any remaining solvent removed.  In Example 4, polypropylene solution, prepared by using a catalyst complex of lithium aluminium hydride and titanium trichloride, is discharged from the reaction vessel, pressurized with ethane (2,000 p.s.i.g.), filtered through a heated filter press and flushed off to remove the solvent.ALSO:Solid impurities are removed from polyolefines by filtering the polyolefine in a liquefied state, i.e. in solution or a molten state, at a temperature at which the impurities are solid and under pressure of a gas which is soluble in the liquid polyolefine.  Specified defines are ethylene and propylene and specified gases are ethylene, propylene and ethane.  Preferably the filtration is carried out at a temperature of 150-250 DEG C. with a discharge pressure from the filter unit equal to or higher than the partial pressure of the dissolved gas.  The desired discharge pressure may be obtained either by (i) feeding the molten polymer or polymer solution to the suction side of the pump and pumping it to the filter at a pressure sufficient to overcome the pressure drop across the filter, or by (ii) adding solvent to the polymer solution so so that the concentration of dissolved gas is below saturation level.  In the examples, ethylene at 1000 p.s.i.g. was used to force polyethylene solution through two stages of filtration in a heated plate-and-frame filter press with seven plates being used in the first section and three plates being used in the second section.  The filter plates were hollow frames containing a 4 mesh back-up screen, 1/8 inch diameter wire, covered with a 10 mesh screen 1/16 inch diameter wire.  Each of the filter leaves in the first section was covered with a double layer of filter paper and each filter leaf in the second section with a triple layer.  Before starting the filtration, the filter was heated to 200 DEG  to 220 DEG C. with hot oil and flushed with mineral spirits.  During filtration, back pressure on the filter was maintained by an air-operated valve. Alternatively the filter plates may be of stainless steel, each plate being covered with a sandwich composed of glass paper between two pieces of glass cloth.