Polymer compositions and method of making pipes

Abstract

A polymer composition comprises a low-molecular-weight (LMW) ethylene polymer component and a high-molecular-weight (HMW) ethylene polymer component. Preferably, the LMW polyethylene component and the HMW polyethylene component co-crystallize in the composition such that it exhibits a single or substantially single peak in a lamella thickness distribution (“LTD”) curve. The ethylene polymer for the LMW and the HMW polyethylene components can be either homopolyethylene or ethylene copolymer. Preferably, both components are an ethylene copolymer of the same or different composition (i.e., with the same or different comonomers). A method of making a pipe that includes selecting a polymer composition having a substantially single peak in the LTD curve is described.

What is claimed is:  
     
         1 . A polymer composition, comprising: 
 a) a LMW polyethylene component; and    b) a HMW polyethylene component, 
 wherein the composition has a substantially single peak in an LTD curve and a PENT value of greater than about 1000 hours at about 80° C. and about 2.4 MPa.  
   
     
     
         2 . The polymer composition of  claim 1 , wherein the composition has a PENT value of greater than about 6000 hours at about 80° C. and about 3 MPa.  
     
     
         3 . The composition of  claim 1 , wherein the polymer composition has a PENT value of greater than about 6500 hours at about 80° C. and about 3 MaP.  
     
     
         4 . The composition of  claim 1 , wherein the polymer composition has a density greater than about 0.940 g/cm 3 , an average molecular weight ranging from about 200,000 to about 350,000 and a melt flow ratio (I 21 /I 5 ) of from about from 15 to about 40.  
     
     
         5 . The composition of  claim 1 , wherein the HMW polyethylene component includes a comonomer selected from the group consisting of C 4  to C 10  olefins.  
     
     
         6 . The composition of  claim 5 , wherein the comonomer content ranges from greater than 0 to about 40%.  
     
     
         7 . The composition of  claim 1 , wherein the LMW polyethylene component includes a comonomer selected from the group consisting of C 4  to C 10  olefins.  
     
     
         8 . The composition of  claim 7 , wherein comonomer content ranges from greater than 0 to about 30%.  
     
     
         9 . The composition of  claim 1 , wherein the polymer composition is bimodal.  
     
     
         10 . The composition of  claim 1 , wherein the HMW polyethylene component comprises from about 48 to about 67 percent by weight of the combined weight of the HMW component and the LMW polyethylene component in the polymer composition.  
     
     
         11 . The composition of  claim 1 , wherein the LWM component comprises from about 33 to about 52 percent by weight of the combined weight of the HWM component and the LMW polyethylene component in the polymer composition.  
     
     
         12 . The composition of  claim 1 , wherein the composition has 
 1) a density of at least about 0.940 g/cm 3  as measured by ASTM Method D-1505;    2) a melt flow index (I 5 ) of from about 0.2 to about 1.5 g/10m;    3) a melt flow index ratio (I 21 /I 5 ) of from about 20 to about 50; and    4) a molecular weight distribution, M w/M   n , of from about 15 to about 40; and    wherein the HMW polyethylene component comprises from about 30 to about 70 wt. percent of the composition; has a density of at least about 0.890 g/cm 3  as measured by ASTM D-1505; has a melt flow index (I 2 ) of from about 0.01 to about 0.2 g/10 min; and a melt flow ratio (I 21 /I 2 ) of from about 20 to about 65; and    wherein the LMW polyethylene component comprises from about 30 to about 70 wt. percent of the composition; has a density of at least about 0.940 g/cm 3  as measured by ASTM D-1505; has a melt index (I 2 ) of from about 40 to about 2000 g/10 min; and has a melt flow ratio (I 21 /I 2 ) of from about 10 to about 65.    
     
     
         13 . The composition of  claim 1 , wherein the pipe has a rapid crack propagation (RCP) S4-value of −5 degrees C. or lower at 10MPa.  
     
     
         14 . A polymerization process, comprising: 
 contacting at least one catalyst composition with at least one ethylene alpha-olefin mixture under polymerization conditions in a reactor system to form a blend comprising a higher molecular weight ethylene/alpha olefin (HMW) polymer component and a lower molecular weight ethylene/alpha olefin (LMW) polymer component in the reactor system,    wherein the blend has a substantially single peak in a lamellar thickness distribution (LTD) curve.    
     
     
         15 . The process of  claim 14 , wherein contacting the at least one catalyst composition with the at least one ethylene alpha-olefin mixture includes contacting a catalyst precursor, a cocatalyst, and a first gaseous ethylene/alpha-olefin composition in a first reactor to form the HMW polymer component and contacting a second gaseous ethylene/alpha-olefin composition with the HMW polyethylene component in a second reactor.  
     
     
         16 . The process of  claim 15 , wherein the HMW polyethylene component is formed in the first reactor and the LMW polyethylene component is formed in the second reactor.  
     
     
         17 . The process of  claim 14  wherein; the catalyst composition comprises a Ziegler-Natta catalyst and a cocatalyst.  
     
     
         18 . The process of  claim 14  wherein; the catalyst composition includes a titanium/magnesium catalyst precursor and a hydrocarbyl aluminum cocatalyst.  
     
     
         19 . The process of  claim 15 , wherein the first gaseous composition comprises: 
 i) a mole ratio of the alpha-olefin to ethylene of from about 0.02:1 to about 0.35:1; and    ii) a mole ratio of hydrogen to ethylene of from about 0:1 to about 0.2:1, and wherein the second gaseous composition comprises:    i) a mole ratio of alpha-olefin to ethylene of from about 0:1 to about 0.42:1; and    ii) a mole ratio of hydrogen to ethylene of from about 0:1 to about 2.2:1; and    wherein the ratio of the weight of HMW polymer to the weight of the LMW polymer is in the range of about 30:70 to about 70:30.    
     
     
         20 . The process of  claim 19  wherein the ratio of the weight of the HMW polymer to the weight of LMW polymer is in the range of about 40:60 to about 60:40.  
     
     
         21 . The process of  claim 15 , wherein contacting the catalyst composition and the first gaseous ethylene/alpha-olefin composition is conducted at a temperature of from about 70° C. to about 110° C.; and wherein contacting the HMW polymer with the second gaseous composition is conducted at a temperature of from about 70° C. to about 110° C.  
     
     
         22 . The process of  claim 15 , further comprising providing additional cocatalyst to the second reactor.  
     
     
         23 . The process of  claim 14  wherein the at least one catalyst composition comprises at least two titanium/magnesium precatalysts and a cocatalyst or at least one multimodal titanium/magnesium precatalyst and cocatalyst.  
     
     
         24 . A method of making a pipe, comprising: 
 a) selecting a polymer composition having a substantially single peak in a LTD curve;    b) extruding the polymer composition to form the pipe.    
     
     
         25 . The pipe of  claim 24 , wherein the pipe is a water pipe, gas pipe, or oil pipe