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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a divisional of U.S. application Ser. No. 10 / 561,290 filed on Jan. 25, 2006, which in turn is the US national phase of international application PCT / NL2003 / 000473 filed Jun. 26, 2003 which designated the U.S., the entire content of each being hereby incorporated by reference.FIELD OF INVENTION[0002]The present invention relates to a process for the preparation of a shaped part of an ultrahigh molecular weight polyethylene (UHMWPE) by 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. The invention further relates to a shaped part obtainable with this process, and to the use thereof, especially in medical applications.BACKGROUND AND SUMMARY OF INVENTION[0003]Such a process is known from WO 03 / 037590. In this publication a shaped part of UHMWPE is prepared wherein the UHMWPE is annealed for at least one hour a...

AI Technical Summary

Benefits of technology

[0006]The motion of chains within a highly entangled melt is described by the reptation model introduced by De Gennes in J. Chem. Phys. 55, p. 572 (1971). In this model a chain within a melt moves in worm-like fashion through a virtual tube, which is delineated by entanglements formed by neighbouring chains. The time needed for a chain to renew its tube (reptation time), i.e. to change its position within the melt is also highly dependent on molecular mass (τ0˜MW3). These fundamental restrictions make high molecular mass polymers rather intractable via conventional processing routes. On the other hand, final properties like tenacity, strength and wear improve with increasing molecular mass. Superior properties are necessary to meet the requirements of demanding applications.

[0007]The discrepancy between intrinsic properties related to high values of molecular mass and insufficient product performance due to difficulties in processing is encountered in UHMWPE as well as in other polymers of very high molecular mass. UHMWPE is a linear grade polyethylene, as is high-density polyethylene (HDPE), but possesses a weight average molecular mass (Mw) of at least 7.5*105 g / mol (according to ASTM D4020). Preferably the UHMWPE has a weight average molecular mass of at least 3*106 g / mol, because of excellent mechanical properties.

Problems solved by technology

This difference in viscosity of the two molecular mass regimes is due to the ability of long chains to entangle, which imposes a restriction on the flowability of a melt.

These fundamental restrictions make high molecular mass polymers rather intractable via conventional processing routes.

The discrepancy between intrinsic properties related to high values of molecular mass and insufficient product performance due to difficulties in processing is encountered in UHMWPE as well as in other polymers of very high molecular mass.

In the case of melt crystallised UHMWPE, entanglements are trapped upon crystallisation and limit the extent to which the chains can be drawn.

Experimental results however showed that highly disentangled solution crystallised films of UHMWPE, which are drawable below the melting temperature lose their drawability immediately upon melting.

Consequently the chains entangle immediately upon melting, which causes the sudden loss in processability and drawability once the sample has been molten.

These results showed that the fundamental restrictions resulting from the strong dependence of the zero-shear viscosity on molecular mass cannot be easily overcome.

Simple disentanglement of the chains prior to melting will not lead to a less entangled melt and accordingly it cannot be used to improve the melt processability of UHMWPE.

Images