Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-01-15
2004-01-13
Lechert, Jr., Stephen J. (Department: 1732)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C264S331140, C264S331170, C526S351000, C526S352000, C623S018110, C623S023580
Reexamination Certificate
active
06677415
ABSTRACT:
The present invention relates to compression moulding of high molecular weight polymers, and components made by these moulding processes.
The processing of ultra-high molecular weight polymers into load-bearing components by techniques conventional for thermoplastics is not feasible because of the exceptionally high viscosity of such materials. The polymer manufacturer produces a polymerised powder which then has to be compacted into a continuous solid. One method of compaction is to produce large slabs or rods by melting the powder and compressing or extruding the melt followed by cooling to form the solid. Individual components are then manufactured from this bulk stock by machining.
An alternative is direct net-shape compression moulding of the powder in heated moulds of an appropriate shape. A weighed charge of the powder is poured into the fixed part of the mould and pressure applied through the moving part of the mould. Components of the same dimensions in plan, but of different thickness, can be made by varying the weight of the charge.
In order to ensure the mechanical integrity of components and to maximise their strength and wear resistance, it is essential for the powder to be fully compacted. In addition, for there to be no residual planes of weakness at the original particle boundaries (which give rise to preferential sites for fatigue crack growth), the moulding process must achieve complete homogenisation of the polymer. The process of homogenisation of the polymer may also be described by the terms welding, diffusion, self-diffusion, consolidation, and/or fusion. Complete homogenisation requires two steps to be completed, aided by elevated pressure and temperature. Firstly, there must be complete compaction of the powder, with the particles being deformed and pressed into intimate contact (fully wetted) at the molecular level, with all voids removed. Secondly, the polymer chains must interdiffuse across the particle interfaces, until the entanglements on either side of the interfaces are fully knitted together, and no memory of the original boundaries remains.
For these two processes to be achieved, sufficient time is required for thermal conduction from the heated surfaces of the mould to raise the temperature in the centre of the mould to that required for homogenisation; in addition, this temperature must be maintained for a sufficiently long period of time for homogenisation to take place. Manufacturers do not reveal what pressure, time and temperature cycles are used in the production of their components and these doubtless vary from manufacturer to manufacturer.
Compression moulding techniques find particular application in the manufacture of joint replacement prostheses, such as artificial hip and knee joints.
Over 1 million joint replacements are implanted annually world-wide. Virtually all of them include ultra-high molecular weight polyethylene (UHMWPE) elements which provide a low-friction arthroplasty when articulating with polished metal surfaces.
Direct moulding has certain advantages, when compared to moulding of large slabs or rods, followed by machining. The only external machining marks are those of the mould and it is possible to achieve a highly smooth and glossy surface finish. Moreover, the polymer may be moulded around metallic inserts to produce composite components.
UHMWPE has been the most widely used material for bearing surfaces in total knee and hip joint replacement prostheses since the 1970s, because of its suitable properties of biocompatibility, high impact strength, low friction and high wear resistance (Li, S. and Burstein, A. H.,
The Journal of Bone and Joint Surgery, Incorporated
, 76-A, 1080 (1994)). However, recent research into the microstructure of the material has shown that incomplete consolidation of the UHMWPE powder, resulting in “fusion defects”, is implicated in the failure of the material due to fatigue (Mayor , M. D., Wrona, M., Collier, J. P. and Jensen, R. E.,
Clinical Orthopaedics and Related Research
, 299, 92 (1994)). Cracking and delamination, specifically associated with fatigue failure, have been found in retrieved knee components. A large literature has been produced characterising the morphology of the wear particles, describing the cellular reactions to the particles, defining the effects of oxidisation and irradiation on the mechanical properties of the polymer, describing the results of wear tests in hip and knee simulators in the laboratory and retrieval studies of components removed from patients at revision. To minimise the occurrence of wear, cracking and deformation, there is an urgent need to improve understanding of the factors governing homogenisation and removal of fusion defects, and to develop reliable means of ensuring their absence, by proper engineering of the manufacturing process. In practice, there is a range of prosthesis designs in use, and each design of component is produced in a variety of sizes, to suit the needs of different patients. It is necessary to develop ideal moulding conditions for each size of component.
Therefore, there is a need for a flexible computer-aided engineering methodology, that can be used at the design stage for products to optimise the moulding process for each individual component. There has been virtually no discussion in the scientific community about optimising the moulding/manufacturing process, other than the demonstration that components produced by different manufacturers exhibit differing wear rates and have different molecular weights.
In the compression moulding of powders of high molecular weight such as UHMWPE, once the powder particle surfaces are in intimate contact, homogenisation requires interdiffusion of the polymer chains across the interfaces. This occurs by the motion of entangled polymer chains along tortuous paths defined by neighbouring molecules: a process known as reptation. When molecules of given molecular weight have reptated to the extent that their new shapes have no correlation with their original shapes, they are said to have reptated. The time taken for polymer chains to reptate increases strongly with molecular weight and a typical sample of UHMWPE is normally believed to contain a wide spectrum of molecular weights. Thus, to maximise homogenisation, it is necessary to mould articles where as much as possible of the polymer has reptated, and hence the maximum reptated molecular weight is as high as possible.
The practical problem for UHMWPE and the moulding of prostheses from UHMWPE is that homogenisation, wetting and molecular diffusion take place exceptionally slowly in UHMWPE even in the molten state. Moreover, they are highly sensitive to temperature time history, and therefore in a typical moulding will occur to differing extents in different parts of a moulding. The problem is exacerbated by the need for supplying moulded components in different sizes, according to the needs of the patient.
The present invention provides a method by which the bonding of already compacted particles of a polymer throughout a moulding of arbitrary size and shape can be controlled. A finite-element model of the component is formed, the continuous solid being represented as a mesh of tetrahedral, brick-shape or other elements, joined together at their apices. The temperature at the surface of the model component is taken through a cycle of elevated temperature followed by fall in temperature. Conventional theory of heat conduction is used to calculate numerically the time/temperature history throughout the component. Standard finite-element packages can be used for these calculations.
The maximum reptated molecular weight may be calculated as a function of position and time within the moulding, according to the equations below. The controlled cycle of time/temperature at the surface and/or the component design is modified until the calculation of reptated molecular weight shows that a satisfactory maximum reptated molecular weight is achieved throughout the moulding.
According to the present invention there is provided a process
Buckley Christopher Paul
O'Connor John Joseph
Wu Junjie
Bacon & Thomas PLLC
O'Connor John Joseph
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