Polyethylene foams and methods of their production

Abstract

High-density polyethylene (HDPE) foams that are essentially free of residual chemical blowing agents and reaction-by-products of chemical blowing agent are provided. The HDPE foams can be either microcellular foams or conventional foams. The foams can be produced in extrusion, injection molding, and blow molding processes that utilize a physical blowing agent. Specific die designs useful for making high quality foams are described.

What is claimed is:  
     
         1 . A foam article comprising: 
 a matrix of polymeric material including a plurality of cells, the polymeric material consisting essentially of high-density polyethylene and being essentially free of residual chemical blowing agent and reaction-by-products of chemical blowing agent and having a shape essentially identical to that of a continuous extrudate.    
     
     
         2 . A foam article comprising: 
 a matrix of polymeric material including a plurality of cells, the polymeric material consisting essentially of high-density polyethylene and being essentially free of residual chemical blowing agent and reaction-by-products of chemical blowing agent and having a shape essentially identical to that of the interior of a mold.    
     
     
         3 . The foam article of  claim 1  having an average cell size of less than 100 microns.  
     
     
         4 . The foam article of  claim 1  having an average cell size of less than 50 microns.  
     
     
         5 . The foam article of  claim 1  having an average cell size of less than 20 microns.  
     
     
         6 .. The foam article of  claim 1  having a void volume of greater than 10%.  
     
     
         7 . The foam article of  claim 1  having a void volume of greater than 20%.  
     
     
         8 . The foam article of  claim 1  having a void volume of greater than 50%.  
     
     
         9 . The foam article of  claim 1  having a void volume of greater than 10% and less than 50%.  
     
     
         10 . The foam article of  claim 1  having an essentially closed-cell structure.  
     
     
         11 . The foam article of  claim 1 , including at least about 1% by weight nucleating agent.  
     
     
         12 . The foam article of  claim 1 , wherein the nucleating agent is talc.  
     
     
         13 . A system for extruding foam material, comprising: 
 a polymer forming die including an inlet at an upstream end thereof constructed and arranged to receive a foam extrudate precursor, an outlet at a downstream end thereof, defining a die gap, for releasing foamed polymeric material, and a fluid pathway connecting the inlet with the outlet, the fluid pathway including a nucleating pathway that decreases in cross section in a downstream direction with an included angle of greater than 4°.    
     
     
         14 . The system of  claim 13  further comprising: 
 an extruder constructed and arranged to provide microcellular extrudate precursor comprising a single-phase, non-nucleated solution of polymeric material and blowing agent to the polymer foaming die including an accumulator positionable to receive microcellular extrudate precursor from the extruder and to accumulate a charge of foam extrudate precursor.  
 
     
     
         15 . The system of  claim 13 , wherein the polymer forming die includes an accumulator constructed and arranged upstream of the die gap to periodically accumulate the microcellular extrudate precursor prior to extruding through the die gap.  
     
     
         16 . The system of  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 6°.  
     
     
         17 . The system of  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 7°.  
     
     
         18 . The system  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 8°.  
     
     
         19 . The system of  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 4° and less than 18°.  
     
     
         20 . The system of  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 4° and less than 80°.  
     
     
         21 . The system of  claim 13 , wherein the nucleating pathway decreases in cross section in a downstream direction with an included angle of greater than 4°.  
     
     
         22 . A system for extruding microcellular polymeric material, comprising: 
 a polymer forming die including an inlet at an upstream end thereof constructed and arranged to receive microcellular extrudate precursor, an outlet at a downstream end thereof, defining a die gap, for releasing foamed polymeric material, and a fluid pathway connecting the inlet with the outlet, the fluid pathway including a nucleating pathway constructed such that when a single-phase, non-nucleated solution of polymeric material and blowing agent is introduced into the die and conveyed through the die at a flow rate of about 100 lbs/hr, nucleation of the solution occurs to form a nucleated polymeric stream that is released from the die in a period of time of no more than about 0.002 second after nucleation.    
     
     
         23 . A method of a foam article comprising: 
 conveying polymeric material in a downstream direction in a polymer processing apparatus, the polymeric material consisting essentially of high-density polyethylene;    introducing a physical blowing agent into the polymeric material in the polymer processing apparatus; and    forming a foam article from the polymeric material.    
     
     
         24 . The method of  claim 23 , wherein the blowing agent comprises carbon dioxide.  
     
     
         25 . The method of  claim 23 , wherein the blowing agent comprises nitrogen.  
     
     
         26 . The method of  claim 23 , wherein forming the foam article comprises extruding the polymeric material through a die connected to a downstream end of the polymer processing apparatus.  
     
     
         27 . The method of  claim 23 , wherein forming the foam article comprises molding the polymeric material in an injection mold connected to a downstream end of the polymer processing space.  
     
     
         28 . The method of  claim 23 , wherein forming the foam article comprises blow molding the article.  
     
     
         29 . The method of  claim 23 , wherein the foam article has an average cell size of less than 100 microns.  
     
     
         30 . The method of  claim 23 , wherein the foam article has an average cell size of less than 50 microns.  
     
     
         31 . The method of  claim 23 , wherein the foam article has an average cell size of less than 20 microns.  
     
     
         32 . The method of  claim 23 , wherein the polymeric material is essentially free of residual chemical blowing agents or by-product of chemical blowing agent.  
     
     
         33 . The method of  claim 23 , wherein the foam article has a void volume of greater than 10%.  
     
     
         34 . The method of  claim 23 , wherein the foam article has a void volume of greater than 20%.  
     
     
         35 . The method of  claim 23 , wherein the foam article has a void volume of greater than 50%.  
     
     
         36 . The method of  claim 23 , wherein the foam article has a void volume of greater than 10% and less than 50%.  
     
     
         37 . A method comprising: 
 introducing a single-phase solution of polymeric material and blowing agent into an inlet of a polymer forming die;    nucleating the single-phase solution in a nucleating pathway that decreases in cross-section in a downstream direction with an included angle of greater than 4° to form a nucleated polymeric stream; and    releasing the nucleated stream as a polymeric foam extrudate from an outlet of the die.    
     
     
         38 . The method of  claim 37 , wherein the die includes a nucleating pathway that decreases in cross section in a downstream direction with an included angle of greater than 6°.  
     
     
         39 . The method of  claim 37 , wherein the die includes a nucleating pathway that decreases in cross section in a downstream direction with an included angle of greater than 7°.  
     
     
         40 . The method of  claim 37 , wherein the die includes a nucleating pathway that decreases in cross section in a downstream direction with an included angle of greater than 8°.  
     
     
         41 . The method of  claim 37 , wherein the polymeric material comprises high-density polyethylene.  
     
     
         42 . The method of  claim 37 , wherein nucleating the single-phase solution comprises subjecting the single-phase solution to a pressure drop rate of at least about 0.3 GPa/sec.  
     
     
         43 . The method of  claim 37 , wherein nucleating the single-phase solution comprises subjecting the single-phase solution to a pressure drop rate of at least about 1.0 GPa/sec.  
     
     
         44 . The method of claims  37 , wherein the single-phase solution includes blowing agent at a level of no more than about 3% by weight based on the weight of the solution.  
     
     
         45 . The method of  claim 37 , wherein releasing the nucleated stream as a polymeric microcellular extrudate from the outlet of the die, in a period of time of no more than about 0.001 second after nucleation.  
     
     
         46 . The method of  claim 37 , wherein releasing the nucleated stream as a polymeric microcellular extrudate from the outlet of the die, in a period of time of no more than about 0.002 second after nucleation.  
     
     
         47 . The method of  claim 37 , further comprising varying the width of a die gap defined by the die outlet, while releasing polymeric microcellular extrudate from the outlet, while maintaining a constant nucleating pathway gap within the die by which the solution is nucleated.  
     
     
         48 . The method of  claim 37 , wherein the die comprises a blow molding forming die and the polymeric material extrudate comprises a microcellular parison.  
     
     
         49 . The method of  claim 48 , further comprising blow molding the parison into a blow-molded microcellular polymeric article.  
     
     
         50 . A method comprising: 
 introducing a single-phase solution of polymeric material and blowing agent into an inlet of a polymer forming die;    nucleating the single-phase solution to form a nucleated polymeric stream; and    releasing the nucleated stream as a polymeric foam extrudate from an outlet of the die in a period of time no more than about 0.002 second after nucleation.