Foam and method of making

Abstract

The present invention is directed to a process for producing a polypropylene foam by mixing a high melt strength polypropylene and a blowing agent to form a melt mixture, reducing the temperature of the melt mixture at the exit of the apparatus to an exit temperature that is no more than 30° C. above the melt temperature of the neat polypropylene while maintaining the melt mixture at a pressure sufficient to prevent foaming; passing the mixture through said exit shaping orifice and exposing the mixture to atmospheric pressure, whereby the blowing agent expands causing cell formation resulting in foam formation, and orienting said foam. The foam is useful as tape backings, thermal and acoustical insulation and as a diffuse reflector for use in optical applications such as computer displays.

1 . A process for making a foamed article comprising: 
 (1) mixing at least one high melt strength polypropylene and at least one blowing agent in an apparatus having an exit shaping orifice at a temperature and pressure sufficient to form a melt mixture wherein the blowing agent is uniformly distributed throughout the polypropylene;    (2) reducing the temperature of the melt mixture at the exit of the apparatus to an exit temperature that is no more than 30° C. above the melt temperature of the neat polypropylene while maintaining the melt mixture at a pressure sufficient to prevent foaming;    (3) passing the mixture through said exit shaping orifice and exposing the mixture to atmospheric pressure, whereby the blowing agent expands causing cell formation resulting in foam formation, and    (4) orienting said foam.    
   
   
       2 . The process of  claim 1  wherein said foam is oriented under conditions such that the density of the extruded foam is decreased.  
   
   
       3 . The process of  claim 2  wherein said orientation is at or above the alpha transition temperature and below the melt temperature of the polypropylene.  
   
   
       4 . The process of  claim 1  wherein said orientation is uniaxial.  
   
   
       5 . The process of  claim 1  wherein said orientation is biaxial.  
   
   
       6 . The process of  claim 5  wherein said orientation is simultaneous biaxial.  
   
   
       7 . The process of  claim 1  wherein said high melt-strength polypropylene comprises homo- and copolymers containing 50 weight percent or more propylene monomer units, and having a melt strength in the range of 25 to 60 cN at 190° C.  
   
   
       8 . The process of  claim 7  wherein said polypropylene copolymers are selected from random, block, and grafted copolymers of propylene and an α-olefin selected from the group consisting of C3-C8 α-olefins and C4-C10 dienes.  
   
   
       9 . The process of  claim 1  wherein said mixture comprises a blowing agent and a blend of a major amount of a high melt strength polypropylene and a minor amount of a semicrystalline or amorphous polymer.  
   
   
       10 . The process of  claim 1  wherein said extruding step comprises extruding said mixture at a pressure ≧2500 psi (17.2 Mpa).  
   
   
       11 . The process of  claim 4  wherein said orientation is at least 3×.  
   
   
       12 . The process of  claim 5  wherein said orientation is 3 to 50 total draw ratio.  
   
   
       13 . The process of  claim 1  wherein said blowing agent is selected from physical blowing agents and chemical blowing agents.  
   
   
       14 . The process of  claim 13  further comprising the step of elevating the temperature of the melt mixture to a temperature sufficient to activate said chemical blowing agent prior to step (2).  
   
   
       15 . The process of  claim 1  wherein said foam comprises 70% or greater closed cells prior to orientation.  
   
   
       16 . The process of  claim 1  wherein said foam, prior to orientation, has an average cell dimension of 50 micrometers or less.  
   
   
       17 . A foamed article prepared by the process of  claim 1.