Shaped part of an ultra high molecular weight polyethylene

Abstract

Shaped parts of ultra high molecular weight polyethylene (UHMWPE) are made by a process comprising melt processing, wherein a) the UHMWPE has a weight average molecular weight (Mw) of at least 1*106 g/mol, b) during the shaping the storage plateau modulus of the UHMWPE (G*) is kept at a value of at most 1.5 MPa, c) whereafter, before the cooling, the G* is raised to its final value. The shaped parts may advantageously be used in medical applications, e.g., as an element of a hip or knee prosthesis.

1 . An essentially grain boundary free shaped part of an ultrahigh molecular weight polyethylene (UHMWPE) made by a process comprising heating the UHMWPE to a temperature above the melting temperature, shaping the resulting melt, and cooling the melt to a temperature below the melting temperature, wherein
 a) the UHMWPE has a weight average molecular weight (Mw) of at least 1*10 6  g/mol,   b) during the shaping the storage plateau modulus (G*) of the UHMWPE is kept at a value of at most 1.5 MPa,   c) whereafter, before the cooling, the G* is raised to its final value.   
   
   
       2 . A shaped part according to  claim 1 , wherein the UHMWPE is heated at a heating rate (Θ) which is at most 1 K/minute, as of a temperature of 350K. 
   
   
       3 . A shaped part according to  claim 1 , wherein the UHMWPE is heated at a heating rate (Θ) which is at most 5 K/minute. 
   
   
       4 . A shaped part according to  claim 1 , wherein the UHMWPE has a MWD of between 1.2-3.0, inclusive. 
   
   
       5 . A shaped part according to  claim 1 , wherein the initial value of G* is at most 0.75 MPa. 
   
   
       6 . A shaped part according to  claim 1 , wherein G* builds up to a value of 1.5 MPa at a speed (Ψ) less than 3 MPa/hour. 
   
   
       7 . A shaped part according to  claim 6 , wherein Ψ is less than 0.5 MPa/hour. 
   
   
       8 . A shaped part according to  claim 1 , wherein the UHMWPE is obtained through a solution or suspension polymerization at a temperature of between 225 and 325 K, using an unsupported catalyst in a concentration of less than 1×10 −4  mol/L. 
   
   
       9 . A shaped part according to  claim 1 , wherein the UHMWPE is either a homopolymer of ethylene, or a copolymer of ethylene with another α-olefin or cyclic olefin. 
   
   
       10 . A shaped part according to  claim 8 , wherein the polymerisation takes place at a temperature between and inclusive 260 and 305 K. 
   
   
       11 . A shaped part according to  claim 1 , wherein the UHMWPE is annealed during the heating, at a temperature of not less than 398 K and not more than 410 K. 
   
   
       12 . A medical device which comprises a shaped part according to  claim 1 . 
   
   
       13 . A medical device according to  claim 12 , wherein the shaped part is an element of a hip or knee prosthesis. 
   
   
       14 . A shaped part comprised of an ultrahigh molecular weight polyethylene (UHMWPE) made by a process comprising the steps of:
 (a) heating at a heating rate (O) which is at most 1 K/minute an ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of at least 1*10 6  g/mol and having an initial storage plateau modulus (G*) value of at most 0.75 MPa to a temperature above the melting temperature of the UHMWPE to form a processable melt thereof;   (b) allowing the initial G* value of the UHMWPE melt to build at a speed (Ψ) of less than 3 MPa/hour to a processing G* value of at most 1.5 MPa;   (c) shaping the melt of the UHMWPE while maintaining the processing G* value of at most 1.5 MPa to form a shaped part thereof; and thereafter   (d) prior to cooling, raising the G* value of the shaped part formed according to step (c) from a G* value of at most 1.5 MPa to a final G* value of about 2.0 MPa; and then   (e) cooling the shaped part.   
   
   
       15 . A shaped part according to  claim 14 , wherein step (a) is practiced by heating the UHMWPE at a heating rate (Θ) which is at most 5 K/minute. 
   
   
       16 . A shaped part according to  claim 14 , wherein the UHMWPE has a molecular weight distribution (MWD) of between 1.2-3.0, inclusive. 
   
   
       17 . A shaped part according to  claim 14 , wherein step (b) is practiced by allowing the initial G* value of the UHMWPE to build at a at a speed (T) of less than 0.5 MPa/hour. 
   
   
       18 . A shaped part according to  claim 14 , wherein the UHMWPE is the solution or suspension polymerization product obtained at a polymerization temperature of between 225 and 325 K, using an unsupported catalyst in a concentration of less than 1×10 −4  mol/L. 
   
   
       19 . A shaped part according to  claim 18 , wherein the polymerisation takes place at a temperature between and inclusive 260 and 305 K. 
   
   
       20 . A shaped part according to  claim 14 , wherein the UHMWPE is either a homopolymer of ethylene, or a copolymer of ethylene with another α-olefin or cyclic olefin.