Solid State Processing
ďA method of solid state deformation processing of crosslinked polymers includes deforming a polymer bulk material by compressing it in a direction orthogonal to a main axis of the bulk material and optionally cooling the bulk material while maintaining the deformation force. When the polymeric material is made of UHMWPE and the crosslinking is by irradiation such as .gamma.-irradiation, products of the method are particularly suitable for use in bearing components and implants for total hip replacement and the like.
In one aspect, the invention involves solid state extrusion of an elongate bulk material through a reducing die while the material is at a compression deformable temperature, preferably below the melting point. The extruded bulk material is then cooled, preferably while held in the deformed state. After cooling, the bulk material is stress relieved by reheating to an annealing temperature to below the melting point, this time without applying pressure.
An oriented UHMWPE molded article can be obtained according to methods of the invention by crosslinking a UHMWPE raw article with a high energy ray such as gamma-irradiation, heating the crosslinked UHMWPE to a compression deformable temperature, and compression deforming the UHMWPE, followed by cooling and solidifying. The material has a detectable level of free radicals and yet is resistant to oxidative degradation evidenced by a very low, preferably undetectable, increase in infrared absorption bands of the UHMWPE material that correspond to formation of carbonyl groups during accelerated aging.
By compression deforming in a direction orthogonal to the main axis of a bulk material, an anisotropic material is formed wherein mechanical properties in the direction of the main axis differ from mechanical properties in the orthogonal or transverse direction. After stress relieving, mechanical properties can differ by 20% or more in the axial direction as opposed to the orthogonal directions.
Polymers treated by the methods exhibit a desirable combination of high tensile strength and resistance to oxidative degradation. Transverse deformation of UHMWPE, for example, leads to material having a tensile strength at break greater than 50 Mpa and preferably greater than 60 Mpa, measured in the axis orthogonal to the deformation. At the same time, the material is resistant to oxidative degradation, showing in preferred embodiments essentially no change in oxidation index on accelerated aging."
[Schroeder et al, US Patent 8,398,913 (3/19/2013)]
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Roger D. Corneliussen
Maro Polymer Links
Tel: 610 363 9920
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Copyright 2013 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 4/12/2013.