High Modulus Crosslinked Polyethylene with Reduced Residual Free Radical Concentration Prepared Below the Melt

Abstract

The present invention provides an irradiated crosslinked polyethylene containing reduced free radicals, preferably containing substantially no residual free radical. Disclosed is a process of making irradiated crosslinked polyethylene by irradiating the polyethylene in contact with a sensitizing environment at an elevated temperature that is below the melting point, in order to reduce the concentration of residual free radicals to an undetectable level. A process of making irradiated crosslinked polyethylene composition having reduced free radical content, preferably containing substantially no residual free radicals, by mechanically deforming the polyethylene at a temperature that is below the melting point of the polyethylene, optionally in a sensitizing environment, is also disclosed herein.

1 . An irradiated crosslinked composition made by a process comprising:
 a) irradiating a composition comprising polyethylene at a temperature that is below the melting point of the composition; and   b) mechanically deforming the composition at a temperature below the melting point of the irradiated composition in order to reduce the concentration of residual free radicals, and   c) annealing the mechanically deformed composition at a temperature below the melting point of the deformed composition in order to permit shape recovery.   
     
     
         2 . The composition of  claim 1 , wherein the deformed composition is crystallized at the deformed state. 
     
     
         3 . The composition of  claim 1 , wherein the composition has substantially no trapped residual free radical detectable by electron spin resonance. 
     
     
         4 . The composition of  claim 1 , wherein crystallinity of the composition is about equal to or higher than that of the starting unirradiated composition. 
     
     
         5 . The composition of  claim 1 , wherein crystallinity of the composition is at least about 51%. 
     
     
         6 . The composition of  claim 1 , wherein elastic modulus of the composition is about the same as or higher than that of the starting unirradiated composition. 
     
     
         7 . The composition of  claim 1 , wherein starting composition is in the form of a consolidated stock. 
     
     
         8 . The composition of  claim 1 , wherein starting composition is a finished product. 
     
     
         9 . The composition of  claim 8 , wherein the finished product is a medical prosthesis. 
     
     
         10 . The composition of  claim 1 , wherein the polyethylene is selected from the group consisting of a low-density polyethylene, high-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, and mixtures thereof. 
     
     
         11 . The composition of  claim 1 , wherein the composition is in intimate contact with a metal piece. 
     
     
         12 . The composition of  claim 1 , wherein the composition is in functional relation with another polyethylene or a metal piece, thereby forming an interface. 
     
     
         13 . The composition of  claim 1 , wherein the mechanical deformation is one of uniaxial, channel flow, uniaxial compression, biaxial compression, oscillatory compression, tension, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), or a combination thereof. 
     
     
         14 . The composition of  claim 1 , wherein the deforming temperature is less than about 140° C. 
     
     
         15 . A method of making an irradiated crosslinked composition, the method comprising:
 a) irradiating a composition comprising polyethylene at a temperature that is below the melting point of the composition; and   b) mechanically deforming the composition at a temperature that is below the melting point of the irradiated composition in order to reduce the concentration of residual free radicals, and   c) annealing the mechanically deformed composition at a temperature below the melting point of the deformed composition in order to permit shape recovery.   
     
     
         16 . The method of  claim 15 , wherein the deformed composition is crystallized at the deformed state. 
     
     
         17 . The method according to  claim 15 , wherein the annealing temperature is less than about 145° C. 
     
     
         18 . The method according to  claim 15 , wherein irradiation is carried out using gamma radiation or electron beam radiation. 
     
     
         19 . The method according to  claim 15 , wherein irradiation is carried out at an elevated temperature that is below the melting point of the composition. 
     
     
         20 . The method according to  claim 15  wherein radiation dose level is between about 1 and about 10,000 kGy. 
     
     
         21 . The method according to  claim 15 , wherein the mechanical deformation is performed in presence of a sensitizing environment. 
     
     
         22 . The method according to  claim 15 , wherein the mechanical deformation is performed at a temperature that is below the melting point of the irradiated composition and is above room temperature. 
     
     
         23 . The method according to  claim 15 , wherein mechanical deformation is performed in presence of a sensitizing gas at a temperature that is below the melting point of the irradiated composition and is above room temperature. 
     
     
         24 . The method according to  claim 15 , wherein the mechanical deformation is one of uniaxial, channel flow, uniaxial compression, biaxial compression, oscillatory compression, tension, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), or a combination thereof. 
     
     
         25 . The method according to  claim 15 , wherein the mechanical deformation is performed at a temperature that is less than about 135° C.