Polyethylene Molding Composition for Producing Blown Films Having Improved Mechanical Properties

Abstract

The invention relates to a polyethylene molding composition having a multimodal molar mass distribution particularly suitable for blow molding films having a thickness in the range from 8 to 200 μm. The molding composition has a density at a temperature of 23° C. in the range from 0.940 to 0.948 g/cm 3 and an MFR 190/5 of the final product after extrusion in the range from 0.10 to 0.50 dg/min. The composition comprises from 40 to 60% by weight of a first ethylene polymer fraction made of a homopolymer A having a first molecular weight, from 25 to 45% by weight of a second ethylene polymer fraction made of a first copolymer B of ethylene and at least one first comonomer from the group of olefins having from 4 to 8 carbon atoms, the first copolymer B having a second molecular weight higher than the first molecular weight, and from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C having a third molecular weight higher than the second molecular weight. The molding composition of the invention allows to produce thin films having improved mechanical properties, in particular in terms of dart drop impact strength, at a high take-off speed without impairing the film bubble stability.

1 . A polyethylene molding composition having a multimodal molar mass distribution; a density at a temperature of 23° C., measured in accordance with ISO 1183, in the range from 0.940 to 0.948 g/cm 3 ; and an MFR 190/5 , measured in accordance with ISO 1133, of the final product after extrusion in the range from 0.10 to 0.50 dg/min, said polyethylene molding composition comprising:
 from 40 to 60% by weight of a first ethylene polymer fraction made of an ethylene homopolymer A having a first molecular weight;   from 25 to 45% by weight of a second ethylene polymer fraction made of a first copolymer B of ethylene and at least one first comonomer selected from the group of olefins having from 4 to 8 carbon atoms, said first copolymer B having a second molecular weight higher than said first molecular weight of the homopolymer A; and   from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C of ethylene and at least one second comonomer, said second copolymer C having a third molecular weight higher than said second molecular weight of the copolymer B,   all percentages being based on the total weight of the molding composition.   
     
     
         2 . The polyethylene molding composition according to  claim 1 , comprising:
 from 45 to 55% by weight of the first ethylene polymer fraction;   from 30 to 40% by weight of the second ethylene polymer fraction, the first copolymer B containing from 0.4 to 3.0% by weight, based on the weight of copolymer B, of said at least one first comonomer; and   from 15 to 25% by weight of the third ethylene polymer fraction, the second copolymer C containing from 3 to 15% by weight, based on the weight of the second copolymer C, of said at least one second comonomer.   
     
     
         3 . The polyethylene molding composition according to  claim 1 , wherein said first comonomer and said second comonomer are independently selected from 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene and mixtures thereof. 
     
     
         4 . The polyethylene molding composition according to  claim 1 , wherein the density is from 0.944 to 0.948 g/cm 3  and the MFR 190/5  of the final product after extrusion is in the range from 0.15 to 0.23 g/10 min. 
     
     
         5 . The polyethylene molding composition according to  claim 4 , further comprising a viscosity number VN 3 , measured in accordance with ISO/R 1191 in decalin at a temperature of 135° C., in the range from 300 to 480 cm 3 /g. 
     
     
         6 . A process for preparing a polyethylene molding composition comprising
 from 40 to 60% by weight of a first ethylene polymer fraction made of an ethylene homopolymer A having a first molecular weight;   from 25 to 45% by weight of a second ethylene polymer fraction made of a first copolymer B of ethylene and at least one first comonomer selected from the group of olefins having from 4 to 8 carbon atoms, said first copolymer B having a second molecular weight higher than said first molecular weight of the homopolymer A; and   from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C of ethylene and at least one second comonomer, said second copolymer C having a third molecular weight higher than said second molecular weight of the copolymer B,   all percentages being based on the total weight of the molding composition,
 wherein the polyethylene molding composition has a multimodal molar mass distribution; a density at a temperature of 23° C., measured in accordance with ISO 1183, in the range from 0.940 to 0.948 g/cm 3 ; and an MFR 190/5 , measured in accordance with ISO 1133, of the final product after extrusion in the range from 0.10 to 0.50 dg/min, 
 the process comprising the step of polymerizing ethylene, said at least one first comonomer and said at least one second comonomer in suspension at temperatures in the range from 20 to 120° C., at a pressure in the range from 2 to 10 bar and in the presence of a Ziegler catalyst comprising a transition metal compound and an organo-aluminum compound. 
   
     
     
         7 . The process according to  claim 6 , wherein said polymerization step is carried out in multiple successive polymerization stages comprising a first stage, a second stage, and a third stage performed in corresponding multiple reactors comprising a first reactor, a second reactor and a third reactor arranged in series, wherein the first, second and third stages each comprise a molar mass of a polyethylene composition and a hydrogen concentration and the molar mass of the polyethylene composition prepared in each stage is adjusted in each case by means of hydrogen. 
     
     
         8 . The process according to  claim 7 , wherein the hydrogen concentration in the first polymerization stage is adjusted to obtain in the homopolymer A a viscosity number VN 1 , measured in accordance with ISO/R 1191, in the range from 60 to 110 cm 3 /g. 
     
     
         9 . The process according to  claim 7 , wherein the hydrogen concentration in the second polymerization stage is adjusted to obtain in a mixture of homopolymer A plus copolymer B a viscosity number VN 2 , measured in accordance with ISO/R 1191, in the range from 300 to 400 cm 3 /g. 
     
     
         10 . The process according to  claim 7 , wherein the hydrogen concentration in the third polymerization stage is adjusted to obtain in a mixture of homopolymer A plus first copolymer B plus second copolymer C a viscosity number VN 3 , measured in accordance with ISO/R 1191, in the range from 300 to 480 cm 3 /g. 
     
     
         11 . A process comprising forming a blown film having a thickness in the range from 8 to 200 μm, the film comprising a polyethylene molding composition having a multimodal molar mass distribution; a density at a temperature of 23° C., measured in accordance with ISO 1183, in the range from 0.940 to 0.948 g/cm 3 ; and an MFR 190/5 , measured in accordance with ISO 1133, of the final product after extrusion in the range from 0.10 to 0.50 dg/min, said polyethylene molding composition comprising:
 from 40 to 60% by weight of a first ethylene polymer fraction made of an ethylene homopolymer A having a first molecular weight;   from 25 to 45% by weight of a second ethylene polymer fraction made of a first copolymer B of ethylene and at least one first comonomer selected from the group of olefins having from 4 to 8 carbon atoms, said first copolymer B having a second molecular weight higher than said first molecular weight of the homopolymer A; and   from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C of ethylene and at least one second comonomer, said second copolymer C having a third molecular weight higher than said second molecular weight of the copolymer B,   all percentages being based on the total weight of the molding composition.   
     
     
         12 . The process according to  claim 11 , wherein the blown film is formed via a blown film process comprising the step of melting the polyethylene molding composition to obtain a polyethylene melt, extruding the polyethylene melt by forcing the polyethylene melt through an annular die to form a bubble having a frost line oscillating at a maximum of ±2 cm in axial direction during the shock test at a maximal take-off speed. 
     
     
         13 . A blown film having a thickness in the range from 8 to 200 μm; a dart drop impact DDI, measured in accordance with ASTM D1709 method A, of more than 400 g, measured on a film having a thickness of 20 μm, comprising a polyethylene molding composition having a multimodal molar mass distribution; a density at a temperature of 23° C., measured in accordance with ISO 1183, in the range from 0.940 to 0.948 g/cm 3 ; an MFR 190/5 , measured in accordance with ISO 1133, of the final product after extrusion in the range from 0.10 to 0.50 dg/min; said polyethylene molding composition comprising:
 from 40 to 60% by weight of a first ethylene polymer fraction made of an ethylene homopolymer A having a first molecular weight;   from 25 to 45% by weight of a second ethylene polymer fraction made of a first copolymer B of ethylene and at least one first comonomer selected from the group of olefins having from 4 to 8 carbon atoms, said first copolymer B having a second molecular weight higher than said first molecular weight of the homopolymer A; and   from 10 to 30% by weight of a third ethylene polymer fraction made of a second copolymer C of ethylene and at least one second comonomer, said second copolymer C having a third molecular weight higher than said second molecular weight of the copolymer B,   all percentages being based on the total weight of the molding composition.