Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-10-27
2002-08-13
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C528S50200C, C528S503000
Reexamination Certificate
active
06433120
ABSTRACT:
The present invention relates to a method for processing polyethylene having an ultra-high molecular weight (UHMWPE) of at least 400,000 by heating under an elevated pressure, followed by cooling, to UHMWIPE thus obtained and to a biomedical application of such UHMWPE.
Such a method for processing polyethylene having an ultra-high molecular weight (UHMWPE) of at least 400,000 is known from U.S. Pat. No. 4,587,163. Said method comprises the heating of powder UHMWPE or a similar partially fused, melt-crystallized material to a temperature of 220-320° C. in the absence of oxygen at a compression load of about 1500-2000 pounds (1 pound=0.454 kg) for about 10 minutes. Train the material is subjected to a compression load of about 12,000-14,000 pounds (1 pound =0.454 kg) at a temperature of about 150° C. for 10 minutes, after which the resulting material is cooled down to ambient temperature under a compression load of about 1500 pounds (1 pound=0.454 kg), in order to obtain a semi-crystalline morphology, whereby the memory of the granular nature of the original starting material is no longer present. Although the compression value is precisely indicated, it is not clear which area unit applies when the pressure is indicated in force/area. Circumstances such as these cause the starting material to melt completely so as to form a homogeneous melt, in which the previous granular structure is no longer preserve. According to said US patent complete fusion of the powder particles of ultra-high molecular weight polyethylene takes place, as a result of which a uniform, melt-crystallized morphology is obtained upon heating to a temperature higher than 220° C. In addition to that it is known from said reference that only partial fusion of the powder particles takes place at temperatures lower than 220° C., which is caused by the high melting viscosity (in the temperature range of 145-210° C.), which hinders the formation of a molecular network between the powder particles. Furthermore, the high degree of physical entanglements hinders chain mobility during the compression moulding process; according to said US patent an effective chain interpenetration between powder particles can occur upon heating of the raw powder material or a partially fused melt-crystallized material to a temperature above 220° C., at which temperature the melt viscosity is significantly lower.
Experiments have shown that UHMWPE thus prepared, when used in artificial hip-joints, wherein a metal pin placed into the leg is provided with a layer of UHMWPE, which pin is present in the hollow space of the hip, exhibits so much wear after a period of use of about 7 years, that the metal pin must be surgically removed from the body in order to be replaced by a new pin. Such an operation after a relatively short period of use is undesirable. Moreover, in the future the number of artificial hip-joints being used will increase, which makes a longer period of use desirable.
The object of the present invention is, therefore, to provide polyethylene having an ultra-high molecular weight of at least 400,000, which material exhibits greater resistance to wear than the UHMWPE that is known from prior art.
Another object of the present invention is to provide a method for processing polyethylene having an ultra-high molecular weight of at least 400,000, which method is carried out under significantly lower temperature and pressure conditions than is known from prior art.
The method according to the present invention as referred to in the introduction is characterized in that UHMWPE having a lamellar thickness <12 nm and a melting temperature at atmospheric pressure >141° C. is used, wherein said heating at an elevated pressure takes place via a transient hexagonal phase, which phase is represented by the hatched area in the Figure, which lies below the equilibrium line of the phase transition orthorhombic phase-liquid phase, which area is enclosed by paints (P
1
, T
1
′), (P
2
, T
1
′), (P
2
, T
2
) and (P
1
, T
1
), wherein T
1
and T
2
represent the equilibrium temperatures associated with pressures P
1
and P
2
respectively, followed by heating to melting and cooling to ambient temperature.
The phase diagram of UHMWPE is generally subdivided into three separate phase areas, viz. an orthorhombic phase, a hexagonal phase and a liquid phase, and it is described in the article titled “Equilibrium triple point pressure and pressure-temperature phase diagram of polyethylene”, Polymer, 1992, volume 33, issue 12, by Hikosaka, M., et al. The thermodynamically critical point Q
U
is associated with a pressure Q
P
of 3.5 kbar and a temperature Q
T
of 230° C., and for UHMWPE having a molecular weight of at least 400,000 it does not depend on the magnitude of the molecular weight. The transient hexagonal phase lies below the liquid phase-orthorhombic phase equilibrium line, as represented by the hatched part in the appended diagrammatic FIG. 1.
Such a method for processing UHMWPE is also known from European patent application No. 0 614 347, wherein polyethylene having an ultra-high molecular weight of at least 400,000, a crystalline melting point of greater than 144° C., and a crystal morphology comprising a bimodal distribution of the molecular chain fold spacings, wherein one group is dimensioned 200-800 nm, and another group is dimensioned 5-50 nm, is processed by subjecting polyethylene having an ultra-high molecular weight of at least 400,000 to a fluid under pressure of at least 2 kbar and a temperature of 190-300° C. for at least 0.5 hour, subsequently reducing the temperature to about 160-170° C. or lower, wherein the pressure is maintained at at least 2 kbar, and finally cooling to a temperature of about 130° C. or lower, and reducing the pressure to about 1 bar.
The method according to the present invention differs significantly from the method disclosed in European patent application No. 0 614 347, since heating at an elevated pressure is according to the present invention carried out via a transient hexagonal phase, which phase is achieved under lower pressure and temperature conditions than is the case with the hexagonal phase described in said European patent application. The processing of the starting material via the transient hexagonal phase in order to obtain a material which possesses satisfactory properties as regards resistance to wear is not known from said publication. In addition to this, the starting material of European patent application No. 0 614 347 differs significantly from that of the present application, since the present inventors use a special polyethylene having an ultra-high molecular weight, which polymer has a lamellar thickness of less than 12 nm and a melting temperature at atmospherical pressure >141° C., preferably a lamellar thickness of 5-12 nm and a melting temperature at atmospherical pressure of 141-148° C. Experiments have shown that if the lamellar thickness is more than 12 nm, there will be no complete fusion of the starting material. A corresponding effect is observed if the melting temperature is lower than 141° C., for example 140° C. The upper limit of the suitable melting temperature is determined by the UHMWPE that is used.
The experiments according to the present invention have shown the relation between the melting temperature at atmospheric pressure, the lamellar thickness and the method to be carried out for processing UHMWPE via the transient hexagonal phase so as to obtain UHMWPE which possesses better wear resistance properties than UHMWPE that has been processed according to methods which employ temperature and pressure conditions which are significantly higher than those employed in the present invention. The presence of such a transient hexagonal phase in the orthorhombic phase area constitutes an essential aspect of the method according to the present invention.
The present inventors explain this as follows, though it should be understood that they are by no means obliged to give such an explanation. It is assumed that the melting
Koets Peter Paul
Lemstra Pieter Jan
Rastogi Sanjay
Cheung William K
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Technische Universiteit Eindhoven
Wu David W.
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