Process for the manufacture of a composition comprising ethylene polymers

Abstract

Process for the manufacture of a composition comprising ethylene polymers, in at least two polymerization reactors connected in series, according to which, in a first reactor, from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A) having a melt flow index MI 2 of 5 to 1000 g/10 min is formed and, in a subsequent reactor, from 30 to 70% by weight with respect to the total weight of the composition of a copolymer of ethylene and of hexene (B) having a melt flow index MI 5 of 0.01 to 2 g/10 min is formed. The compositions obtained by this process exhibit a good compromise between the processing properties and the mechanical properties, which renders them capable of being used in the manufacture of articles shaped by extrusion and extrusion blow-molding, such as films and pipes.

1 . Process for the manufacture of a composition comprising ethylene polymers, in at least two polymerization reactors connected in series, according to which: 
 in a first reactor, ethylene is polymerized in suspension in a mixture comprising a diluent, hydrogen, a catalyst based on a transition metal and a cocatalyst, so as to form from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A) having a melt flow index MI 2  of 5 to 1000 g/10 min,    the said mixture, additionally comprising the homopolymer (A), is withdrawn from the said reactor and is subjected to a reduction in pressure, so as to degas at least a portion of the hydrogen, then    the said at least partially degassed mixture comprising the homopolymer (A), as well as ethylene and 1-hexene and, optionally, at least one other α-olefin are introduced into a subsequent reactor and the suspension polymerization is carried out therein in order to form from 30 to 70% by weight, with respect to the total weight of the composition, of a copolymer of ethylene and of hexene (B) having a melt flow index MI 5  of 0.01 to 2 g/10 min.    
     
     
         2 . Process according to    claim 1   , characterized in that the hexene content in the copolymer (B) is at least 0.4% and at most 10% by weight.  
     
     
         3 . Process according to    claim 1   , characterized in that the copolymer (B) is composed essentially of monomer units derived from ethylene and from 1-hexene.  
     
     
         4 . Process according to    claim 1   , characterized in that the homopolymer (A) exhibits an MI 2  of at least 50 and not exceeding 700 g/10 min and that the copolymer (B) exhibits an MI 5  of at least 0.015 and not exceeding 0.1 g/10 min.  
     
     
         5 . Process according to    claim 1   , characterized in that the diluent is isobutane.  
     
     
         6 . Process according to    claim 1   , characterized in that the amount of hydrogen introduced into the first reactor is adjusted so as to obtain, in the diluent, a molar ratio of hydrogen to ethylene of 0.05 to 1.  
     
     
         7 . Process according to    claim 1   , characterized in that the ratio of the concentration of hydrogen in the first reactor to the concentration in the subsequent polymerization reactor is at least 20.  
     
     
         8 . Process according to    claim 1   , characterized in that the amount of 1-hexene introduced into the subsequent polymerization reactor is such that, in this reactor, the hexene/ethylene molar ratio in the diluent is from 0.05 to 3.  
     
     
         9 . Process according to    claim 1   , characterized in that the catalyst comprises from 10 to 30% by weight of transition metal, from 0.5 to 20% by weight of magnesium, from 20 to 60% by weight of a halogen and from 0.1 to 10% by weight of aluminium.  
     
     
         10 . Composition comprising ethylene polymers comprising from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A) having a melt flow index MI 2  of 5 to 1000 g/10 min and from 30 to 70% by weight with respect to the total weight of the composition of a copolymer of ethylene and of hexene (B) having a melt flow index MI 5  of 0.01 to 2 g/10 min capable of being obtained by the process according to    claim 1   .  
     
     
         11 . Composition according to    claim 10   , characterized in that it exhibits an MI 5  of at least 0.07 and not exceeding 10 g/10 min and an HLMI/MI 5  ratio of greater than 10.  
     
     
         12 . Use of a composition according to    claim 10    for the manufacture of films or of pipes.  
     
     
         13 . Pipes obtained by extrusion of a composition according to    claim 10   .