Method for producing semicrystalline polylactic acid articles

Abstract

Amorphous sheets of PLA resins are thermoformed by heating the sheets until they become semicrystalline, and then forming the sheets on a relatively cold mold. Semicystalline formed articles having improved heat resistance are made by the process.

What is claimed is:  
     
         1 . A method for making formed heat-resistant PLA articles, comprising heating a sheet of an amorphous, crystallizable PLA resin until it has obtained a surface temperature from about 80 to about 155° C., then thermoforming the heated sheet on a mold that is at a temperature below 80° C.  
     
     
         2 . The method of  claim 1 , wherein the mold is at a temperature below the T g  of the PLA resin.  
     
     
         3 . The method of  claim 2 , wherein the sheet is heated under conditions such that the PLA resin attains a crystallinity of at least 15 Joules/gram.  
     
     
         4 . The method of  claim 3 , wherein the PLA resin is a homopolymer of L-lactic acid or D-lactic acid, a random copolymer of L-lactic acid and D-lactic acid, a block copolymer of L-lactic acid and D-lactic acid or a mixture of two or more of these.  
     
     
         5 . The method of  claim 4  wherein the PLA resin contains a nucleating agent, a plasticizer, or both.  
     
     
         6 . The method of  claim 5  wherein the PLA resin contains from about 1 to about 40 percent of a finely divided talc, based on the combined weight of the PLA resin and the talc.  
     
     
         7 . The method of  claim 4  wherein the PLA resin has a crystallization half-time at the temperature to which it is heated during the heating step of less than 3 minutes.  
     
     
         8 . The method of  claim 7  wherein the sheet is heated under conditions such that the PLA resin attains a crystallinity of at least 20 Joules/gram.  
     
     
         9 . The method of  claim 8  wherein the sheet is heated under conditions such that the PLA resin attains a crystallinity of at least 24 Joules/gram.  
     
     
         10 . The method of  claim 1  wherein the PLA sheet contains one or more additives selected from the group consisting of nucleants, other inorganic fillers, plasticizers, reinforcing agents, slip agents, lubricants, UV-stabilizers, thermal stabilizers, flame retardants, foaming agents, antistatic agents, antioxidants and colorants.  
     
     
         11 . The method of  claim 1  wherein the amorphous PLA sheet is cellular.  
     
     
         12 . The method of  claim 1  wherein the amorphous PLA sheet contains a foaming agent, and expands during the heating step to become cellular.  
     
     
         13 . The method of  claim 1  wherein the PLA resin contains a plasticizer.  
     
     
         14 . The method of  claim 1  wherein the sheet of amorphous PLA resin forms a layer of a multilayer structure having at least one other layer which is not a crystallizable PLA resin.  
     
     
         15 . A thermoformed article made according to the process of  claim 1 .  
     
     
         16 . A method for making formed heat-resistant PLA articles, comprising heating a sheet of a semicrystalline PLA resin having a crystallinity of at least 10 Joules/gram to a temperature at which the sheet can be thermoformed but at which the crystallinity of the sheet is not reduced below 10 Joules/gram, and then thermoforming the sheet on a mold that is at a temperature below 80° C. to produce a formed article of PLA resin having a crystallinity of at least 10 Joules/gram.  
     
     
         17 . A method for making formed heat-resistant PLA articles, comprising heating a sheet of an amorphous, crystallizable PLA resin to a temperature at which the PLA resin forms crystallites and for a time sufficient to impart a crystallinity of at least 10 Joules/gram to the PLA resin, and then thermoforming the heated sheet on a mold that is at a temperature below 80° C. to produce a formed article of PLA resin with a crystalinity of at least 10 Joules/gram.