Melt blended high density polyethylene compositions with enhanced properties and method for producing the same

Abstract

Melt blended HDPE for pipe and fitting material having a density in the range of 0.945 to 0.955 grams per cubic centimeter, values of melt flow index according to ASTM D1238 less than 0.4 with enhanced physical properties and process characteristics and blend methods are described in which virgin, reprocessed, wide specification, flake or regrind HMW homopolymer and copolymer HDPE (5 to <50 %) are included as a minor component and HMW HDPE (>50 to 95%) is the major component. The method of selecting and blending the HMW HDPE copolymer, LMW HDPE homopolymer and LMW HDPE copolymer provides the corrugated HDPE pipe manufacturer with enhanced HDPE compositions and the means to independently design physical properties and enhance processability while exceeding AASHTO's standard for ESCR.

1 . A melt blended high density polyethylene (HDPE) composition comprising a principal weight fraction component of a high molecular weight (HMW) copolymer or homopolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE, blended together with a remainder weight fraction component of one or more than one of a low molecular weight (LMW) HDPE homopolymer and a LMW HDPE copolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE, in a ratio in which the logarithm of the melt index (MI) of the blend equals the sum of the products of 1) the weight fraction of the HMW HDPE and the logarithm of the MI of the HMW HDPE, 2) the weight fraction of the LMW HDPE homopolymer and the logarithm of the MI of the LMW HDPE homopolymer, and 3) the weight fraction of the LMW copolymer and the logarithm of the MI of the LMW HDPE copolymer, and in which the density of the blend equals the sum of 1) the product of the weight fraction and density of each HMW HDPE, 2) the product of the weight fraction and density of each LMW HDPE homopolymer, and 3) the product of the weight fraction and density of each LMW HDPE copolymer in the blend.  
     
     
         2 . The blend of  claim 1  formed into a shape.  
     
     
         3 . The melt blend of  claim 1  in which the HMW HDPE component is about 65% to about 80% by weight fraction.  
     
     
         4 . A melt blend of  claim 1  having an ESCR equal to or exceeding about 24 hours.  
     
     
         5 . A corrugated polyethylene pipe formed from the melt blend of  claim 1 .  
     
     
         6 . The melt blend of  claim 1  in which the HMW HDPE component is about 50% to about 95% by weight fraction.  
     
     
         7 . A method of blending grades of high molecular weight (HMW) and low molecular weight (LMW) high density polyethylene (HDPE) copolymers or homopolymers selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE comprising the steps of: 
 selecting a density and a melt index (MI) desired for a melt blend;    selecting from the group of a high molecular weight (HMW) copolymer or homopolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE, a HMW HDPE as a first component of about 50% to about 95% by weight fraction of a mixture of components to be blended;    selecting from the group of a high molecular weight (HMW) copolymer or homopolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE, a low molecular weight (LMW) HDPE copolymer as a second component in the mixture;    selecting as a third component in the mixture, in the instance that the density selected for the blended composition is greater than the density of the HMW HDPE first component, a LMW HDPE homopolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE;    determining a weight fraction ratio of LMW HDPE homopolymer to HMW HDPE in the melt blend, to attain the selected density of the melt blend, according to the formula: 
 the density of a mixture of the LMW HDPE homopolymer and the HMW HDPE component equals the sum of 1) the product of the weight fraction of the LMW HDPE homopolymer in the mixture and the density of LMW HDPE homopolymer and 2) the product the weight fraction of the HMW HDPE in the mixture and the density of HMW HDPE;  
   determining the MI of the mixture according to the formula; 
 the logarithm of the MI of the mixture equals the sum of 1) the product of the weight fraction of the LMW HDPE homopolymer in the mixture and the logarithm of MI of the LMW HDPE homopolymer and 2) the product the weight fraction of the HMW HDPE in the mixture and the logarithm of the MI of the HMW HDPE;  
   determining the ratio of the weight fraction of a LMW HDPE copolymer selected from the group of virgin pellets, recycled, reprocessed, off specification, and wide specification grades of HDPE to be added to the mixture to attain the selected MI for the blended composition according to the formula: 
 the logarithm of the MI of the melt blend equals the sum of the products of 1) the weight fraction of the HMW HDPE and the logarithm of the MI of the HMW HDPE, 2) the weight fraction of the LMW HDPE homopolymer and the logarithm of the MI of the LMW HDPE homopolymer, and 3) the weight fraction of the LMW HDPE copolymer; and 
 melt blending the HMW HDPE and the LMW HDPE copolymer, and, if the selected density for the blended composition is greater than the density of the HMW HDPE, the LMW HDPE homopolymer, in the amounts determined.  
 
   
     
     
         8 . The method of  claim 7  including the step of forming a shape from the melt blend composition.  
     
     
         9 . The method of  claim 7  in which the step of selecting a density and MI desired for the melt blend comprises selecting a density in the range of from about 0.945 to about 0.955 grams per cubic centimeter and the step of selecting a MI comprises selecting a MI of less than about 0.4.  
     
     
         10 . The method of  claim 7  in which the step of selecting a HMW HDPE comprises selecting a HMW HDPE copolymer.  
     
     
         11 . A corrugated polyethylene pipe formed from the melt blend of  claim 1   
     
     
         12 . A method for manufacturing a corrugated polyethylene pipe in accordance with  claim 7  in which, after the step of melt blending the HMW HDPE and the LMW HDPE copolymer, and, if the selected density for the blended composition is greater than the density of the HMW HDPE, the LMW HDPE homopolymer, in the amounts determined, the materials blended are formed into a corrugated polyethylene pipe.