Microcellular foam of thermoplastic resin prepared with die having improved cooling property and method for preparing the same

Abstract

The present invention relates to a microcellular foam of a thermoplastic resin and a method for preparing the same, and more particularly to a microcellular foam comprising a skin layer having a porosity of below 5% and a core layer having a porosity of at least 5%, wherein the thickness of the skin layer accounts for 5 to 50% of the entire foam, and a method for preparing the same. The microcellular foam of the present invention is advantageous in that it has a thicker skin layer and smaller and uniform micropores in the core layer, compared with conventional microcellular foams, while having mechanical properties comparable to those of conventional non-foamed sheets.

1 . Microcellular foam comprising a skin layer having porosity, which is defined in Equation 1 below, of below 5%, and a core layer having porosity of at least 5%, wherein the thickness of the skin layer accounts for 5 to 50% of the entire thickness of the foam:  
       Porosity(%)=(ρ N −ρ F )/ρ N ×100  [Equation 1] where ρ N  is the density of a non-foamed portion and ρ F  is the density of a foamed portion.    
   
   
       2 . The microcellular foam of  claim 1 , wherein the foam is a sheet, a “ ”-shaped cross-sectional body, or a chassis having a chamber inside thereof.  
   
   
       3 . The microcellular foam of  claim 2 , wherein the cross-sectional thickness of the foam is from 0.5 to 5 mm.  
   
   
       4 . The microcellular foam of  claim 1 , wherein the skin layer has an average thickness of 50 to 500 μm.  
   
   
       5 . The microcellular foam of  claim 1 , wherein the overall porosity of the foam is from 5 to 80%.  
   
   
       6 . The microcellular foam of  claim 1 , wherein the overall porosity of the foam is from 15 to 30%, and the impact absorption energy of the foam measured by a rheometric drop test in accordance with ASTM D4226 is at least 70% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       7 . The microcellular foam of  claim 6 , wherein the overall porosity of the foam is from 15 to 30%, and the impact absorption energy of the foam measured by a rheometric drop test in accordance with ASTM D4226 is 90 to 150% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       8 . The microcellular foam of  claim 1 , wherein the core layer has pores having an average diameter of 0.1 to 50 μm.  
   
   
       9 . The microcellular foam of  claim 1 , wherein the overall porosity of the foam is from 15 to 30%, and the elongation of the foam measured in accordance with ASTM D638 is at least 70% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       10 . The microcellular foam of  claim 9 , wherein the overall porosity of the foam is from 15 to 30%, and the elongation of the foam measured in accordance with ASTM D638 is 90 to 150% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       11 . The microcellular foam of  claim 1 , wherein the overall porosity of the foam is from 15 to 30%, and the tensile strength of the foam measured in accordance with ASTM D638 is at least 70% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       12 . The microcellular foam of  claim 11 , wherein the overall porosity of the foam is from 15 to 30%, and the tensile strength of the foam measured in accordance with ASTM D638 is from 90 to 150% of that of a non-foamed counterpart prepared under comparable conditions.  
   
   
       13 . The microcellular foam of  claim 1 , which comprises at least one polymer selected from the group consisting of an acrylonitrile-butadiene-styrene (ABS) copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyester, polypropylene (PP), and nylon.  
   
   
       14 . The microcellular foam of  claim 1 , which is an interior/exterior construction material.  
   
   
       15 . The microcellular foam of  claim 1 , which is prepared by a process comprising the steps of: 
 a) mixing a plasticized thermoplastic polymer resin with a foaming agent using an extruder;    b) forming micropores by passing the plasticized mixture through a pressure drop region; and    c) cooling the melted mixture in which the micropores have been formed by passing it through a cooling region,    by using an extrusion die comprising both the pressure drop region and the cooling region, wherein a temperature difference at the end of the pressure drop region and the beginning of the cooling region is 30 to 200° C.    
   
   
       16 . The microcellular foam of  claim 15 , wherein the extrusion die further comprises a temperature change region between the pressure drop region and the cooling region, wherein a temperature change rate in the temperature change region defined by Equation 2 below is 2 to 40° C./mm:  
         T   L =( T   h   −T   c )/ L   [Equation 2] where T L  is the temperature change rate, T h  is the temperature at the end of the pressure drop region, T c  is the temperature at the beginning of the cooling region, and L is the length of the temperature change region.    
   
   
       17 . A preparation method of a microcellular foam comprising the steps of: 
 a) mixing a plasticized thermoplastic polymer resin with a foaming agent using an extruder;    b) forming micropores by passing the plasticized mixture through a pressure drop region; and    c) cooling the melted mixture in which the micropores have been formed by passing it through a cooling region,    wherein a temperature difference at the end of the pressure drop region and the beginning of the cooling region is 30 to 200° C.    
   
   
       18 . The preparation method of  claim 17 , wherein the extrusion die comprises a heating means at the end of the pressure drop region for preventing a temperature decrease.  
   
   
       19 . The preparation method of  claim 17 , wherein the extrusion die comprises a cooling means at the beginning of the cooling region for preventing a temperature increase.  
   
   
       20 . The preparation method of  claim 17 , wherein the temperature at the end of the pressure drop region is 150 to 250° C.  
   
   
       21 . The preparation method of  claim 17 , wherein the temperature at the beginning of the cooling region is 40 to 150° C.  
   
   
       22 . The preparation method of  claim 17 , wherein the temperature change of the pressure drop region and the cooling region is maintained to be within ±5° C.  
   
   
       23 . The preparation method of  claim 17 , wherein the transfer rate of the thermoplastic polymer resin is 0.5 to 20 m/min.  
   
   
       24 . The preparation method of  claim 17 , wherein a temperature change region is present between the pressure drop region and the cooling region, and the temperature change rate defined by Equation 2 below at the temperature change region is 2 to 40° C/mm:  
         T   L =( T   h   −T   c )/ L   [Equation 2] where T L  is the temperature change rate, T h  is the temperature at the end of the pressure drop region, T c  is the temperature at the beginning of the cooling region, and L is the length of the temperature change region.    
   
   
       25 . The preparation method of  claim 24 , wherein the length of the temperature change region is 1 to 150 mm.  
   
   
       26 . The preparation method of  claim 17 , wherein the thermoplastic polymer resin comprises at least one polymer selected from the group consisting of an acrylonitrile-butadiene-styrene (ABS) copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyester, polypropylene, and nylon.