Foamed sheet of polylactic acid resin, foam molding of polylactic acid resin and method of preparing foam molding

Abstract

A foamed sheet of a base resin containing 50 to 100% by weight of a polylactic acid resin and having an apparent density of 63 to 630 kg/m 3 , a thickness of 0.5 to 7 mm and endothermic and exothermic calorific values of ΔH endo:2 and ΔH exo:2 , respectively, as measured by heat flux differential scanning calorimetry at a heating rate of 2° C./min. The endothermic calorific value ΔH endo:2 is at least 10 J/g and the difference (ΔH endo:2 -ΔH exo:2 ) between the endothermic calorific value ΔH endo:2 and the exothermic calorific value ΔH exo:2 is more than 20 J/g and less than 40 J/g. A foam molding such as a receptacle is prepared by thermoforming the foamed sheet.

1 . An extruded foamed sheet of a base resin comprising at least 50% by weight of a polylactic acid resin, said foamed sheet having an apparent density of 63 to 630 kg/m 3 , a thickness of 0.5 to 7 mm and endothermic and exothermic calorific values of ΔH endo:2  and ΔH exo:2 , respectively, as measured by heat flux differential scanning calorimetry at a heating rate of 2° C./min, wherein the endothermic calorific value ΔH endo:2  is at least 10 J/g and the difference (ΔH endo:2 −ΔH exo:2 ) between the endothermic calorific value ΔH endo:2  and the exothermic calorific value ΔH exo:2  is more than 20 J/g and less than 40 J/g. 
   
   
       2 . An extruded foamed sheet as claimed in  claim 1 , wherein the extruded foamed sheet has an exothermic calorific value ΔH exo:10 , as measured by heat flux differential scanning calorimetry at a cooling rate of 10° C./min, of at least 20 J/g. 
   
   
       3 . An extruded foamed sheet as claimed in  claim 1 , wherein the extruded foamed sheet has a melt tension of at least 2 cN at 190° C. 
   
   
       4 . An extruded foamed sheet as claimed in  claim 1 , wherein the extruded foamed sheet has cells having an average cell diameter in the extrusion direction of X mm, an average cell diameter in the transverse direction of Y mm and an average cell diameter in the thickness direction of Z mm, and wherein X, Y and Z satisfy the following conditions:
 0.05 mm<Z<0.8 mm   0.2<Z/X<0.8   0.2<Z/Y<0.65.   
   
   
       5 . A method of producing an open-topped foam receptacle having a draw ratio of S 2 /S 1  where S 1  represents an area of the top opening thereof and S 2  represents an inside surface area thereof, comprising thermoforming in a mold an extruded foamed sheet of a base resin containing at least 50% by weight of a polylactic acid resin, said extruded foamed sheet having an apparent density of 63 to 630 kg/m 3 , a thickness of 0.5 to 7 mm and endothermic and exothermic calorific values of ΔH endo:2  and ΔH exo:2 , respectively, as measured by heat flux differential scanning calorimetry at a heating rate of 2° C./min, wherein the endothermic calorific value ΔH endo:2  is at least 10 J/g and wherein the draw ratio S 2 /S 1  and the difference ΔH x  (ΔH x =ΔH endo:2  −ΔH exo:2 ) between the endothermic calorific value ΔH endo:2  and the exothermic calorific value ΔH exo:2  satisfy the following equation:
   S 2 /S 1 ≦−0.08ΔH x +4.2.   
   
   
       6 . An extruded foamed sheet as claimed in  claim 1 , wherein the endothermic and calorific value ΔH endo:2  is at least 20 J/g. 
   
   
       7 . An extruded foamed sheet as claimed in  claim 1  wherein the polylactic acid resin has been modified by reaction with an organic peroxide, an isocyanate, an epoxy compound, a metal complex, a polyvalent carboxylic acid or a mixture thereof. 
   
   
       8 . An extruded foamed sheet as claimed in  claim 7  wherein the polylactic acid resin has a melt tension of at least 3 cN. 
   
   
       9 . An extruded foamed sheet as claimed in  claim 1  wherein the polylactic acid resin has been reacted with an organic peroxide. 
   
   
       10 . An extruded foamed sheet as claimed in  claim 1  wherein the polylactic acid resin has been crosslinked by reaction with an organic peroxide to raise its melt tension to at least 3 cN. 
   
   
       11 . An extruded foamed sheet as claimed in  claim 1  wherein the polylactic acid resin has an endothermic calorific value ΔH endo:Material  of at least 10 J/g as measured by heat flux differential scanning calorimetry at a heating rate of 2° C./min. 
   
   
       12 . An extruded foamed sheet as claimed in  claim 9  wherein the organic peroxide is dicumyl peroxide. 
   
   
       13 . A method as claimed in  claim 5  wherein the polylactic acid resin has been modified by reaction with an organic peroxide, an isocyanate, an epoxy compound, a metal complex, a polyvalent carboxylic acid or a mixture thereof. 
   
   
       14 . A method as claimed in  claim 5  wherein the polylactic acid resin has been reacted with an organic peroxide.