Robust spray-dried ziegler-natta procatalyst and polymerization process employing same
A Ziegler-Natta procatalyst composition in the form of solid particles and comprising magnesium, halide and transition metal moieties, said particles having an average size (D50) of from 10 to 70 μm, characterized in that at least 5 percent of the particles have internal void volume substantially or fully enclosed by a monolithic surface layer (shell), said layer being characterized by an average shell thickness/particle size ratio (Thickness Ratio) determined by SEM techniques for particles having particle size greater than 30 μm of greater than 0.2.
1. A method for making a Ziegler-Natta procatalyst composition in the form of solid particles and comprising magnesium chloride and transition metal moieties, said particles having an average particle size (D50) of from 10 to 70 μm and comprising at least 5 percent of the particles having internal void volume substantially or fully enclosed by a monolithic surface layer (shell), said layer having an average shell thickness/particle size ratio (Thickness Ratio) determined by Scanning Electron Micrograph (SEM) techniques for particles having particle size greater than 30 μm of greater than 0.2, the steps of the method comprising:
a) providing a liquid composition comprising reduced titanium tetrachloride with magnesium metal in the presence of a Lewis base electron donor free of substituents containing active hydrogen as the sole electron donor in a solvent or diluent, and optionally a filler;
b) spray-drying the composition to form a spray-dried particle; and
c) collecting the resulting solid particles,
characterized in that the concentration of a magnesium chloride in the spray dried liquid composition is greater than 90 percent of the saturation concentration of the magnesium chloride at the temperature of the atomization.
2. The method for making a Ziegler-Natta procatalyst composition of claim 1 , wherein the step of providing a liquid composition comprising reduced titanium tetrachloride with magnesium metal in the presence of a Lewis base electron donor free of substituents containing active hydrogen as the sole electron donor comprises providing the reduced titanium tetrachloride with the magnesium metal in the presence of tetrahydrofuran.
3. The method for making a Ziegler-Natta procatalyst composition of claim 2 , wherein the step of providing a liquid composition comprising reduced titanium tetrachloride with magnesium metal in the presence of tetrahydrofuran in a solvent or diluent, and optionally a filler, further comprises adding a filler to the liquid composition.
4. The method for making a Ziegler-Natta procatalyst composition of claim 3 , wherein the filler is silica particles having an average aspect ratio from 1.0 to 2.0, where aspect ratio is the ratio of the largest linear dimension of a particle to the smallest linear dimension thereof as determined from SEM images, and an average particle size ranging from 0.01 μm to 12 μm.
5. The method for making a Ziegler-Natta procatalyst composition of claim 1 , wherein the magnesium metal is in the form of particles having an average particle size of from 0.25 mm to 10 mm.
6. The method for making a Ziegler-Natta procatalyst composition of claim 1 comprising mixing one mole of the magnesium metal for every two moles of the titanium tetrachloride.
7. The method for making a Ziegler-Natta procatalyst composition of claim 1 , comprising mixing 5 moles to 400 moles of the electron donor compound per mol of the titanium tetrachloride, and removing most of the excess prior to or during spray-drying.
8. The method for making a Ziegler-Natta procatalyst composition of claim 1 wherein the reduction of the titanium tetrachloride with the magnesium metal comprises forming a solution containing one mole of magnesium dichloride for every two moles of titanium trichloride, and which is substantially free of undesirable by-products.
9. The method for making a Ziegler-Natta procatalyst composition of claim 8 comprising adding additional magnesium dichloride to the solution to increase a Mg/Ti ratio.
10. The method for making a Ziegler-Natta procatalyst composition of claim 8 comprising adding sufficient magnesium dichloride to the solution to result in a Mg/Ti molar ratio ranging from 1.5:1 to 15:1.
11. The method for making a Ziegler-Natta procatalyst composition of claim 1 wherein the average particle size (D50) of the procatalyst composition is from 20 to 35 μm.
12. The method for making a Ziegler-Natta procatalyst composition of claim 1 wherein at least 25 percent of the particles of the procatalyst composition have a Thickness Ratio of greater than 0.2.
13. The method for making a Ziegler-Natta procatalyst composition of claim 12 wherein at least 25 percent of the particles of the procatalyst composition have a Thickness Ratio of greater than 0.25.
14. A process for making a polymer comprising contacting at least one olefin monomer with a procatalyst made by the method of claim 1 , and a cocatalyst under olefin polymerization conditions to form a polymer product.
15. A process according to claim 14 wherein the at least one olefin monomer is ethylene, and homopolymerizing the ethylene or copolymerizing the ethylene with one or more α-olefins.