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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a novel ultrahigh molecular weight linear
polyethylene (UHMWLPE). This novel UHMWLPE, in the form of a shaped
article, exhibits in various embodiments a unique combination of
properties making the material useful as a bearing surface, in general,
but particularly useful as a prosthetic hip joint cup and as other
prosthetic shapes for replacement of other joints of the human body.
2. Description of the Prior Art
In U.S. Pat. No. 3,944,536 (March 1976), Lupton et al. describe UHMWPE in
the form of a fabricated article exhibiting an elastic modulus of 340,000
to 500,000 psi, a tensile impact strength of 140 to 600 ft lb/in.sup.2, a
density of 0.95 to 0.98 g/cc at 25.degree. C., a crystalline melting point
of 142.degree. to 148.degree. C. (as measured by differential thermal
analysis) and a unique crystalline form characterized by the absence of
fold spacings of 50-2000 Angstrom units (.ANG.) and the presence of
crystal spacings of about 10,000 .ANG.. The critical feature of the
process of producing this UHMWPE is disclosed to involve inducing
crystallization of the molten polymer above 150.degree. C. by rapidly
increasing the applied pressure from an initial level of 1 to 1000
atmospheres to a second level of 2000 to 7000 atmospheres and then cooling
rapidly while maintaining a pressure sufficient to maintain the
polyethylene in the solid phase until the temperature is below the
crystalline melting point of the polyethylene at atmospheric pressure.
In Kunstuffe German Plastics 77 (1987) pp. 617-622, in an article entitled
"Ultrahigh Molecular Polyethylene for Replacement Joints", Eyrer et al.
point out that the service life of joint replacements made of UHMWPE is
limited. Analysis of the damage to over 250 explanted hip cups and tibial
plateaus revealed a changed property profile which they explained by
post-crystallization resulting from oxidative chain decomposition. They
suggested optimizing the processing of polyethylene under higher pressure
and higher temperature to increase the degree of crystallinity. The Eyrer
et al. product displays a creep of above 5% at a compression of 1000 psi
(6.9 N/mm.sup.2) for 24 hours at 37.degree. C.
One of the most remarkable advances in the medical field in recent years is
the development of prosthetic joints, particularly the load bearing hip.
The crippled and sometimes bed ridden elderly can walk again. The key to
this development is UHMWPE because, not only does it have the necessary
impact strength, but it initiates no adverse blood reactions. But at
present, these prosthetic joints are limited to the older, less active
segment of the population because the polymer tends to creep under the
pressure that a younger more active person might develop while involved in
recreation or employment. The creep would cause the loss of the close
tolerance required between the plastic socket and the polished metal ball
attached to the femur. These changes in dimensions disturb the
distribution of walking forces which in turn accelerates more creep and
wear. Eventually the increased pain requires a traumatic revision
operation. One objective of this invention is to provide UHMWPE prosthetic
joints with improved creep resistance hence removing some of the age
restriction existing on the present polyethylene joints. This invention
can also function in other UHMWPE-based prosthetic devices, for example,
non-conforming joint assemblies such as knees which require a special
balance of properties, especially with respect to tensile modulus, creep
resistance, and long term dimensional stability.
SUMMARY OF THE INVENTION
This invention provides tough ultrahigh molecular weight linear
polyethylene (UHMWLPE-1), and shaped articles therefrom, having a
homoeomerous morphology, unusually low creep, and excellent tensile
flexural properties, said polyethylene and articles being substantially
free from internal stresses and having unusually long-term dimensional
stability.
The invention also provides ultrahigh molecular weight linear polyethylene
(UHMWLPE-2), and shaped articles therefrom, having a folded chain
morphology and unusually high elongation and impact resistance.
Both polyethylenes of the invention have a molecular weight of at least
800,000, preferably at least 4,000,000, most preferably at least
6,000,000.
The homoeomerous polyethylene of the invention, UHMWLPE-1, exhibits two
crystalline DSC melting points, the higher of which is greater than
144.degree. C., said higher melting point decreasing by at least
11.degree. C. when the polyethylene is remelted; an infrared crystallinity
index of at least about 0.35, preferably at least 0.45.
UHMWLPE-1 is prepared in a novel process (Process 1) consisting essentially
of the following steps:
(a) forming, by milling, casting or cold pressing and sintering or the
like, an article from UHMWLPE having a molecular weight of at least
800,000, preferably at least 4,000,000, most preferably at least
6,000,000;
(b) placing the article in a pressure vessel substantially filled with a
liquid that is inert to the polymer under process conditions, preferably
water, heating the vessel to a temperature of at least 190.degree. C.,
preferably 200.degree.-300.degree. C. and, after the article is molten,
raising the pressure in the vessel, usually by adding more liquid, to at
least 230 MPa, preferably at least 280 MPa;
(c) thereafter, cooling by reducing the temperature to about
160.degree.-170.degree. C. or below, preferably to 160.degree. C. or
below, most preferably to below 140.degree. C., while maintaining a
pressure of at least 230 MPa, the rate of cooling being such that
temperature gradients producing internal stresses in the article are
substantially avoided; and
(d) cooling to a temperature below about 130.degree. C., preferably below
100.degree. C., and reducing the pressure to about 100 kPa, either
sequentially or simultaneously, in a manner such that remelting of the
article is prevented.
UHMWLPE-2 is a preferred starting polyethylene for use in Process 1 for
preparing UHMWPE-1.
UHMWLPE-2 is prepared in a novel process (Process 2) consisting essentially
of the following steps:
(a) forming, by milling or casting or the like, an article from UHMWLPE
having a molecular weight of at least 800,000, preferably at least
4,000,000, most preferably at least 6,000,000;
(b) subjecting said article to a temperature of 280.degree.-355.degree. C.,
preferably 320.degree.-355.degree. C., for at least 0.5 hour, preferably
at least 3 hours, in an inert atmosphere; and
(c) cooling the article non-precipitously to a temperature of about
130.degree. C. or below, the rate of cooling being such that temperature
gradients producing internal stresses in the article are substantially
avoided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the apparatus for preparation of the polyethylene of the
present invention.
FIG. 2 shows a comparison of dimensional stability of the polyethylene of
the present invention and that of prior art.
DESCRIPTION OF THE INVENTION
UHMWLPE-1 is characterized by a homoeomerous morphology comprising a
bimodal distribution of crystalline molecular chain fold spacings, one
group of said spacings being greater than 200 nm reflecting a population
of very highly extended molecular chains, the other group being less than
50 nm. By "homoeomerous" is meant UHMW linear polyethylene which exhibits
two distinct crystalline melting points corresponding to two distinct
groups of crystalline molecular chain fold spacings yet which is
essentially homogeneous in appearance and properties. Non-homogeneous UHMW
polyethylenes of the art which may exhibit bimodal distributions of chain
fold spacings and dual melting points are not homoeomerous, but rather a
mixture of folded chain and partly or fully extended chain polyethylenes
whose properties and appearance vary throughout the polymeric body.
UHMWLPE-1 exhibits a flexural modulus of 200-600 kpsi, a tensile stress at
yield of 3.5-5.4 kpsi, a tensile stress at break of 4-9 kpsi, a tensile
modulus of 250-700 kpsi, an elongation at break of 200-600%, a notched
Izod impact resistance of 12-25 ft lb per in. of notch, a creep at a
compression of 1 kpsi of less than 1.4% after 24 hours at a temperature of
23.degree. C. and relative humidity of 50%.
A preferred embodiment of UHMWLPE-1, prepared by keeping the pressure in
steps (b) and (c) of Process 1 over 280 MPa, exhibits compressive creep of
less than 1% and a tensile modulus of at least 350 kpsi.
For certain prosthetic devices, particularly knee joints, UHMWLPE-1 having
a tensile modulus in the range of 250 to about 430 kpsi is preferred.
These lower modulus products are conveniently prepared by limiting the
maximum pressure in steps (b) and (c) of Process 1 to between 230 MPa and
280 MPa. It is recommended, in preparing said lower modulus products at
pressures between 230 MPa and less than 280 MPa, especially at pressures
between 230 MPa and about 250 MPa, to use in step (a) linear polyethylene
having a molecular weight of at least 4,000,000.
It has been found during the manufacture of UHMWLPE-1 products by Process 1
that use of a liquid, preferably water, in step (b) results in products
having properties that are superior to those similarly processed but
contained in a rigid mold made of metal or other rigid material, as
describer in U.S. Pat. No. 3,944,536, even when exposed to an inert gas
such a argon, as described in U.S. Pat. No. 5,037,928. Containment in a
rigid mold during pressure recrystallization, in contrast to the
hydrostatic method of the present invention process, results in built-in,
internal stresses which adversely affect final product properties,
including markedly reduced long-term dimensional stability (Example 19,
Comparative Example 1) .
By "internal stresses" is meant unrelieved compressive or tensile forces
locked within a pressure-recrystallized UHMWLPE product, said forces
resulting from uneven application of pressure to an UHMWLPE article and/or
significant temperature gradients within said article during its
crystallization from the molten state under pressure. Such internal
stresses are believed to be anisotropic and usually lead to a
heterogeneous product having markedly reduced long-term dimensional
stability.
A remarkable difference is found between UHMW linear polyethylene articles
pressure-recrystallized by the present hydrostatic method compared to
those pressure-recrystallized as in U.S. Pat. No. 5,037,928 after
treatment in water a 100.degree. C. for 4 hours; the latter expand about
10 times more than the former. Dramatic differences are also seen between
hip cups of UHMWLPE processed by the present method and that of '928 after
heating in air at 170.degree. C. for 12 hours; the latter emerge yellow
and grossly misshapen while the former remain white and dimensionally
unchanged. Severe stress-related defects are also found in products of the
art which are pressure-recrystallized under conditions wherein the
material is not fully surrounded, such as in a platen press as described
in U.S. Pat. No. 4,587,163; articles so processed are significantly
deformed and of very limited commercial value.
It is also preferred, in step (b) of Process 1, to preheat the starting
ultrahigh molecular weight polyethylene (UHMWLPE) to the required
temperature of at least 190.degree. C. outside the pressure vessel because
heating UHMWLPE is very time consuming due to the high heat of fusion and
low thermal conductivity of the polymer. The UHMWLPE may by heated in a
dry environment, e.g., an oven, preferably in an inert atmosphere to
prevent oxidation, and then transferred to the preheated pressure vessel.
Because molten UHMWLPE is too viscous to flow, the article will not deform
during transfer from, for instance an oven to the pressure vessel. It has
also been found that dry preheating of the polymer leads to a lower final
water content in the UHMWLPE-1 product when water is used as the
heat/pressure transfer medium. For example, the water content of the
UHMWLPE-1 product, while dependent on the surface:volume ratio of the
article formed in step (a), is usually in the range of about 0.04-0.2% by
weight when the starting polymer is preheated in an oven, but in the range
of about 0.7 to 2% by weight when the starting polymer is heated in water
in the pressure vessel.
Further, in step (b) of Process 1, it is important to allow the starting
UHMWLPE to melt completely before raising the pressure. The required
heating time will depend on the thickness of the article; means of
determining temperature at the center of an article, e.g., a thermocouple,
is desirable for insuring that the article is molten, In repetitive
operations (runs) of Process 1, it is usually sufficient to measure the
article's central temperature in a first, calibration run to determine the
heating time for subsequent runs.
In step (c) of Process 1, the polymer should be cooled under full process
pressure until it has completely crystallized and is below the melting
point of the polymer as measured at one atmosphere. Cooling rate should be
sufficiently slow to avoid significant temperature gradients which produce
internal stresses in the article. Preferably the cooling rate is such that
a minimal difference in temperature between the pressure vessel and the
polymer therein is maintained until crystallization is complete,
particularly if the pressure vessel construction does not permit means for
measuring the temperature of the polymer itself. For example, a cooling
rate of about 10.degree. C. per hour is desirable for a 1 inch.times.6
inch rod. However, although cooling rates of about 10.degree. C. per hour
are preferred, cooling rates of up to about 60.degree. C. per hour
(Example 6) have been used to provide products of this invention. Rapid
cooling, as taught in the prior art, will not provide the products of this
invention.
Fortunately, in the practice of steps (b)-(d) of Process 1 wherein the
UHMWLPE article is immersed in a liquid within a pressure vessel, the
temperature differential between the interior wall of said vessel and the
polymer usually remains satisfactorily small even when the exterior of
said pressure vessel is quenched with coolant. Thus, the rate of cooling
of the vessel exterior is usually not critical in the practice of Process
1 for maintaining a polymer cooling rate that is sufficiently low to avoid
significant temperature gradients leading to internal stresses within the
polymer.
Control of cooling rate, as described above, is a particularly important
feature of both Process 1 and 2 of this invention in the manufacture of
larger UHMWLPE shaped articles having a smallest dimension of at least 0.2
inch, especially cross sectional dimensions of 1 inch.times.1-2 inches
wherein avoidance of temperature gradients leading to internal stress
development during cooling is more difficult yet critical to the long-term
dimensional stability of the article.
In step (d) of Process 1, cooling the polymer to a temperature below its
melting point at any particular pressure is necessary to ensure that none
of the polymer melts as the pressure is reduced, since lowering the
pressure lowers the melting point.
After step (d) of Process 1, it is optional but advisable to shave the
surface of the article, i.e., remove approximately the outer 2 millimeters
that might contain any liquid-affected polymer.
Products of the aforementioned process possess superior strength
properties, resistance to creep under load and long-term dimensional
stability, and are excellent material for orthopedic replacement parts.
In addition to utility in the field of orthopedic replacement, the products
prove useful in other applications also requiring the special properties
of the products. Not only shaped articles are of interest, but also films,
including oriented films, and fibers as well as other "downstream" forms
and unshaped granular forms of the product will prove useful. Film formed
of the product of Example 4 is exemplified in Example 12. These examples
are illustrative only, and other forms, shaped and unshaped, of the
present products are contemplated within the scope of the invention.
Therefore, "article" shall include both shaped articles and unshaped
articles.
It will be understood that the articles produced by Process 1 may be
fabricated "downstream" by any of several means into other articles, care
being taken to avoid remelting of the polymer.
UHMWLPE-2, a further product of the invention having a fully folded chain
morphology, exhibits a flexural modulus of 150-300 kpsi; a tensile stress
at yield of 3.5-4.3 kpsi; a tensile stress at break of 4-6 kpsi; a tensile
modulus of 150-300 kpsi; a notched Izod impact resistance of 15-25 ft lb
per inch of notch; an elongation at break of 200-1400%; a creep at
compression of 1 kpsi of less than 2% after 24 hours at a temperature of
23.degree. C. and a relative humidity of 50%; and an infrared
crystallinity index of at least about 0.35. UHMWLPE-2 is prepared by heat
treatment without application of pressure in Process 2 described
hereinabove. In step (b) of Process 2, the polymer should be heated as
close as possible to, without reaching, its decomposition temperature. In
step (c) Process 2 the hot polymer should be cooled slowly because very
rapid cooling, such as immersion in cold water, causes internal voids to
form. Voids result from a combination of large volume change (about 30%)
on melting and poor heat conductivity in polyethylene. It is convenient to
allow the polymer to cool wrapped in insulation.
The thermally treated, folded chain product, UHMWLPE-2, has improved
elongation, impact resistance and crystallinity over the starting UHMW
polyethylene. Preferred embodiments exhibit elongation at break of up to
about 1400% (Example 10). However, UHMWLPE-2 is not equivalent in overall
tensile properties and creep resistance to the pressure-recrystallized
UHMWLPE-1.
As indicated hereinabove, a very important property of the UHMWLPE-1
products of this invention is creep resistance. For prosthetic devices,
e.g., knee, hip, elbow joints, etc., any substantial creep can be
devastating in the loss of the benefits of extremely expensive surgery. In
such applications, very low creep together with high stiffness, high
elongation, and high tensile strengths at yield are required. It has been
found that products having these superior properties can be obtained by
either using UHMWLPE-2 as the starting UHMW linear polyethylene in Process
1 or, alternatively, by inserting between steps (a) and (b) in Process 1
the atmospheric pressure heating step (b) of Process 2. Accordingly,
UHMWLPE-2 is a preferred starting UHMW polyethylene for preparing
UHMWLPE-1 by Process 1 for use in demanding applications such as
prosthetic devices. Certain preferred embodiments of UHMWLPE-1 prepared in
Process 1 wherein the maximum pressure in step (b) is at least 280 MPa and
UHMWLPE-2 is used as the starting polyethylene, exhibit compressive creep
of less than 1%, preferably less than 0.6%.
When UHMWLPE-2 is used as the starting polymer in Process 1, it may be
introduced in either step (a) or as an article in step (b) and handled as
described hereinabove for conventional starting UHMWLPE.
It is envisaged that the additional preliminary step of heating the
starting UHMWLPE to 280.degree.-355.degree. C. will also provide superior
characteristics to the product described in U.S. Pat. No. 5,037,928.
By inert atmosphere in the processes of this invention is meant a gaseous,
vaporous or liquid environment that is stable and inert to process
conditions. Suitable gases, vapors or liquids include water, nitrogen and
the noble gases, and nonflammable, chemically inert and thermally stable
liquids such as the perfluoroalkylpolyethers (Example 9). Vacuum may also
be employed but is not preferred.
For purposes of this invention, ultrahigh molecular weight linear
polyethylene (UHMWLPE) is defined as a linear polyethylene having an
estimated weight-average molecular weight greater than about 800,000,
usually 4,000,000 to 10,000,000 as defined by a melt index (ASTMD-1238) of
essentially zero and a reduced specific viscosity (RSV) greater than 8,
preferably 25-30. The relationships of RSV to intrinsic viscosity and to
molecular weight are those developed by R. Chaing as presented by P. S.
Francis et al. in J. Polymer Science, 31, 453 (1958).
Another characteristic property of the products of this invention is their
infrared crystallinity index (IRCI). This property, which fairly
accurately reflects product crystallinity, is higher than in conventional
UHMW polyethylene. To determine this index, samples are first obtained by
microforming thin sections. Heat should be avoided during preparation of
the samples. ICRI is the ratio of the band at 1894 reciprocal centimeters
(cm.sup.-1) to the band at 1305 reciprocal centimeters (cm.sup.-1) . Since
the band at 1894 cm.sup.-1 is attributed to the crystal line nature of the
material and the band at 1305 cm.sup.-1 is attributed to its amorphous
nature, ICRI increases as the crystallinity increases. The product of this
invention displays an IRCI of at least about 0.35, preferably at least
0.45. In fact, values of 0.73 and higher have been obtained. On the other
hand, IRCI values for prior known UHMWLPE's seldom reach above 0.3.
The invention will be more clearly understood by referring to the drawings
and examples, which follow. In the drawings, FIG. 1 is a schematic diagram
of the equipment used in the process for forming the product of the
invention using the hydrostatic process. FIG. 2, shows photographically
the results of a comparison of material made by U.S. Pat. No. 3,944,536,
Example 2 (article "15"), and the claimed material as embodied in Example
24 (article "103").
In the examples, most of the properties are measured using standard ASTM
tests.
All of the physical measurements were carried out under constant humidity
(50% relative humidity) and temperature (23.degree. C.) conditions.
Tensile modulus, ultimate tensile strength, yield strength and elongation
were measured according to ASTM D-638 with the following modifications:
______________________________________
samples machined into shape without lubricating liquid
type I tensile bar
cross head speed = 0.2"/min for tensile modulus
2.0"/min for tensile stress and elongation.
______________________________________
Resistance to deformation (creep) was measured in accordance with ASTM
D-621, where noted ASTM F-648, with the following modifications:
samples machined into cylinders or cubes without the use of lubricating
liquids
samples measured 0.5".times.0.5".times.0.5"
(ASTM F-648 only) 90 minute recovery period before measurement
Flexural properties were measured according to ASTM D-790 with the
following modifications:
samples machined into shape without the use of lubricating liquids
typical flex bar measures 0.125" thick.times.0.5" width.times.5" length
span or gage is 2.0". (This was determined by a span/depth ratio of 16/1.)
cross head speed=0.05"/min (calculated based on span) .
Impact resistance was measured using the notched Izod test given in ASTM
D-256 with the following modifications:
samples machined into shape without the use of lubricating liquid
type A or notched IZOD
specimen size is 0.5".times.2.5"
0.4" from bottom of vertex to opposite side
1.25" impacted end (from end of bar to vertex of notch)
the notch should be the specified angle of 22.5 degrees.
The following non-limiting examples, including the improved and superior
embodiments, illustrate the basic principles and unique advantages of the
present invention. Various changes and modifications may be made without
departing from the spirit and scope of the present invention.
EXAMPLE 1
The material used in this example is an ultrahigh molecular weight
polyethylene obtained from Jet Plastics, Inc.
With reference to FIG. 1, a rod 21 measuring 6".times.11/8" was placed in
the cavity 22 of a stainless steel, seamless, cylindrical pressure reactor
23. The cavity 22 had a diameter of 1.35" and was about 9" long.
Water was fed into the cavity 22 at the entry port 24 through the use of a
high pressure water pump 25 powered by compressed air. Simultaneously, the
reactor was heated by electrical heaters 26 surrounding the reactor.
In the first step, the rod 21 was heated to a temperature of 220.degree. C.
under a hydrostatic pressure of 200 MPa. The pressure was raised to 300
MPa while the temperature was maintained at 220.degree. C. for 2 hours.
The temperature was permitted to fall to 209.degree. C. over another 2
hour period, and then to about 182.degree. C. in 4 hours. Finally, the rod
was cooled to 49.degree. C. by subjecting the reactor 23 to compressed air
from the blower 27 over a period of one hour and the pressure released.
The rod was removed from the reactor and the surface was shaved. The
product, a sample taken substantially from the center of the rod,
displayed a DSC melting point of 154.5.degree. C., and, on reheating, a
DSC melting point of 140.degree. C.
The material, when subjected to a compression pressure of 1000 psi for 24
hours at a temperature of 23.degree. C. and a relative humidity of 50% in
accordance with ASTM D-621, deformed only 0.4%.
The other properties of the product were:
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flexural modulus over 250 kpsi
tensile modulus over 300 kpsi
tensile stress (yield)
over 3500 psi
tensile stress (break)
over 4000 psi
elongation (break) less than 500%.
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Its infrared crystalinity index was over 0.5.
The hydrostatic process described in this example is the best mode for
preparing the product of this invention. This process has important
advantages. The pressure transfer liquid, water, is non-toxic and
inexpensive. The hydraulic pressure is applied equally in all directions
resulting in a substantially homogeneous product. This compares to
processes shown in the prior art Where hydraulic pressure is applied by a
piston or a ram. In these latter cases, the high shrinkage polymer tends
to solidify along the heat-escaping walls making it difficult for the
pistons to advance and still apply the pressure uniformly. The result is a
heterogeneous product.
It should be understood that although water is the preferred liquid to use
in the process, other liquids provided they are chemically inert and
thermally stable under process conditions are also useful. Thus, methanol,
ethanol, glycerin, glycols, etc. in addition to various aqueous solutions
may be used.
The salt selected for an aqueous solution may be one that imparts a
desirable property to the surface of the shaped article.
EXAMPLE 2
This experiment was carried out in a manner similar to Example 1 except
that the pressure in the first step was 300 MPa. The material was
maintained at 220.degree. C. under 300 MPa for 4 hours. The temperature
was allowed to fall to 190.degree. C. over an 8-hour period. After which,
it was cooled to 100.degree. C. in 1 hour.
Samples were taken from 1/8" inside both ends of the rod and had melting
points of 150.8.degree. C. and 153.2.degree. C. When reheated, the melting
points were 135.5.degree. C. and 138.degree. C., respectively.
The infrared crystallinity index was 0.791; and the creep, when measured in
accordance with ASTM D-621, was less than 1%. These measurements were
obtained on a sample taken from the center of the rod.
EXAMPLE 3
The experiment was also carried out in a manner similar to Example 1 except
for the following changes in the heating/cooling cycle:
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Heat at 211.degree. C. and 300 MPa and maintain for 1 hour;
Cool to 200.degree. C. in 1 hour at 300 MPa;
Cool to 180.degree. C. over 5 hours at 300 MPa
(cooling rate 200 .fwdarw. 180.degree. C., 4.degree./hour); and
Cool to 33.degree. C. in 1 hour and 3 minutes.
______________________________________
The product from inside both ends melted at 150.degree. C. and on
reheating, at 135.5.degree. C. The product, when tested in accordance with
ASTM D-621 displayed a creep of less than 1%. Its infrared crystalinity
index was 0.652.
EXAMPLE 4
A reactor with the general configuration shown in FIG. 1 having an internal
diameter of 4" and being 22" long, was charged with a 31/8".times.181/16"
rod of UHMWPE (made from polymer from American Hoechst, Hostalen GUR 415).
The closed vessel was evacuated, filled with water, and heated to
232.degree. C. at which point the pressure was increased to 300 MPa with
the water pump. This pressure was maintained until the end of the
experiment. The reactor was held between 210.degree. and 230.degree. C.
for 3 hours, cooled over 1 hour to 200.degree. C. cooled to 175.degree. C.
in 5 hours (5.degree./hour) and then cooled to 80.degree. C. in 71/2
hours.
The resulting product rod was still in a cylindrical form with very little
warpage. It measured 31/8".times.1715/16". End pieces, 1/2" thick, were
cut off each end of the rod revealing a uniform white color. Samples taken
from the center of the rod on these cuts gave melting points of
152.9.degree. C. (201 J/g) and 152.1.degree. C. (207 J/g) when heated at
20.degree. C./minute. When reheated, the melting points were 137.5.degree.
C.
A six inch section of the rod was sawed into 3/16" thick shapes for
physical tests, then carefully milled to remove saw marks to 1/8"
thickness. The resulting polymer had the following properties:
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IZOD 18.7 ft.lb./in. of notch
Flexural Modulus 298.9 kpsi
Tensile Properties
Stress at yield 4190 psi
Stress (at break) 5430 psi
Elongation (at break)
280%
Modulus 378.3 kpsi
Creep (at 1000 psi load)
0.6%
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All tests at room temperature.
The crystallinity index (IRCI) was 0.528.
Additional evidence of the products' distinctiveness is found in data
produced by small angle X-ray testing. A truly characteristic small-angle
X-ray scattering plot of desmeared intensity (by the method of P. W.
Schmidt, Acta Cryst., 13, 480 (1960) and Acta Cryst., 19, 938 (1965))
(I.times.(2 theta) squared) versus scattering angle (2 itheta) for the
material of the invention exhibits two distinct scattering peaks
associated with crystal long-spacings in the range of 480 angstroms (at 2
theta=0.184 degrees) and 4610 angstroms (at 2 theta=0.0192 degrees). The
presence of the sharp diffraction peak at the lower angle is indicative of
an extended polymer chain conformation (with a lamellar thickness greater
than 2000 angstroms ) whereas the more diffuse higher-angle peak
corresponds to a lamellar thickness characteristic of conventional folded
chain PE. This provides clear evidence for the presence of two scattering
peaks in the subject invention material which correspond to lamellar
thicknesses both above and below 2000 angstroms. By comparison, the
previously patented extended chain polyethylene of Lupton et al., U.S.
Pat. No. 3,944,536, was reported to exhibit a complete absence of any
detectable small angle X-ray scattering in the range of 50 to 2000
angstroms. Consequently this work demonstrates that the subject invention
material is morphologically distinguishable from Lupton et al.
EXAMPLE 5
In this example, the product was prepared with an exceedingly smooth
surface.
A polished brass disk, about 11/2 diameter, 1/4" thick was pressed at
160.degree. C. against a UHMW polyethylene plug. The combination was
cooled under pressure and sealed in a heat shrinkable Teflon FEP.TM. tube.
The polyethylene was converted in a hydrostatic system by this procedure
in the vessel used in Example 4.
The heating, cooling cycle was as follows:
300 MPa and 210.degree. C. in 1 hour;
300 MPa 210.degree. C. to 200.degree. C. in 1 hour;
300 MPa 20020 C. to 178.degree. C. in 6 hours, 45 minutes; and
300 MPa 178.degree. C. to 23.degree. C. in 2 hours, 20 minutes.
The polyethylene did shrink so that it had a smaller diameter than the
disk, but the polymer stuck to the surface. When forced apart, the surface
was extremely smooth.
This technique is important in preparing complicated surfaces where
smoothness is extremely important, such as on bearing surfaces such as
medical prosthesis for knee and hip joints, or bearings for motor shafts,
etc. Machine cutting polymers always leaves very small ridges.
EXAMPLE 6
The reactor of FIG. 1, internal diameter 4" by 22" long was charged with a
3".times.18" rod fabricated from American Hoescht, Hostalen GUR 415
ultrahigh molecular weight polyethylene, water, and a nominal pressure of
100 psi (690 kPa). The system was heated to 170.degree. C. to 176.degree.
C. and held there for 1 hour, then the pressure was raised to 300 MPa. The
temperature was maintained at 179.degree. C.-174.degree. C. for 3 hours,
during which time the polyethylene crystallized. The reactor was cooled to
79.degree. C. in 1.7 hours (approximately 60.degree. C. per hour).
Two-samples were taken; one from the center of the rod and another 1/2 inch
from the outer surface of the rod. The melting points, as measured by DSC,
were 150.9.degree. C. and 150.4.degree. C., respectively, and upon
reheating, 136.6.degree. C. and 137.3.degree. C. Thus, the increases in
melting points were 14.3.degree. C. and 12.7.degree. C., respectively. The
infrared crystallinity index was 0.5.
EXAMPLE 7
This example shows that the polymer can be cooled at a rate as high as
34.5.degree. C. per hour in the critical cooling step (step 5) if proper
precautions are taken to limit temperature gradients.
A one inch rod of UHMWLPE from Jet Plastics, Inc. was used. It was placed
in the pressure vessel with water and subjected to the following
treatments:
300 MPa and 220.degree. C. for 2 hours;
300 MPa, cool to 200.degree. C. in 50 minutes;
300 MPa, cool to 177.degree. C. in 40 minutes;
300 MPa, cool to 35.degree. C. in one hour.
A test sample taken one-half inch from the end of the rod and in the center
displayed a DSC melting point of 153.8.degree. C. and on reheating a DSC
melting point of 139.7.degree. C.
The material, when subjected to a compression pressure of 1000 psi for 24
hours at 23.degree. C. and a relative humidity of 50% in accordance with
ASTM D-621 deformed 0.5%.
EXAMPLE 8
Superior Enhanced UHMW Polyethylene prepared by Preheating Polymer to
325.degree. C.
A 31/16".times.15" rod of UHMW polyethylene (Hoechst GUR415, fabricated by
PolyHi) was heated to 325.degree. C. in an atmosphere of N.sub.2 for six
hours. The hot rod was quickly placed in a pressure vessel preheated to
212.degree. C. The vessel was sealed immediately and pressured with water
to 300 MPa. The cooling schedule was as follows:
212.degree. to 191.degree. C. 65 minutes
191.degree. to 181.degree. C. 63 minutes
181.degree. to 175.degree. C. 2 hours
175.degree. to 174.degree. C. 6 hours, 26 minutes
174.degree. to 45.degree. C. 3 hours, 15 minutes
The rod was cut into test samples and analyzed with the following results:
______________________________________
DSC Center 1 cm from
(Differential Scanning Calorimetry)
of Bar Bar Edge
______________________________________
m.p., .degree.C.
1st heat 150.5 152.4
2nd heat 137.9 139.0
.DELTA.T 12.6 13.4
Heat of Fusion
1st heat 198.8 J/g
2nd heat 134.4 J/g
Infrared Crystallinity Index
(Samples cut from within
5 mm of bar edge)
In Bar Direction 0.613
Perpendicular to Bar Direction
0.633
Flex Modulus, (kpsi) 424.0
386.1
Deformation (Creep)
(% at 1000 psi load)
In Bar Direction 0.4
Perpendicular to Bar Direction
0.6
Density g/ml
Gradient column 0.9595
Infra Red 0.957, 0.958
______________________________________
Perpendicular
In Bar to Bar
Direction
Direction
(6" Test (21/2" Test
Bars) Bars)
Tensile Properties: (Type I) (Type V)
______________________________________
Stress, psi
Yield: 4743 4516
4758 4526
Max: 4743 5614
4758 5005
Break: 4396 5004
3695 5040
Modulus, kpsi 611.1 520.3
613.0 513.9
Elongation, % at break
355 433
315 400
______________________________________
Perpendicular
Bar to Bar
Direction
Direction
______________________________________
IZOD IMPACT, ft.lb./in. of notch
24.8 26.1
22.0 25.0
______________________________________
EXAMPLE 9
Effect of Sequence of Heat-treatment, Cooling, Reheating to a Lower
Temperature, and Pressure Recrystallization on UHMWPE.
A UHMW PE bar (3".times.15") was heated for five hours at 325.degree. C.
under N.sub.2, then slowly cooled to room temperature. It was reheated to
225.degree. C., and pressure recrystallized as described in Example 8
according to the following schedule:
______________________________________
241.degree. to 191.degree. C.
300 MPa 2 hours, 15 minutes
191.degree. to 181.degree. C.
300 MPa 2 hours
181.degree. to 171.degree. C.
300 MPa 6 hours
______________________________________
The resulting product was machined into test pieces and analyzed with the
following results:
______________________________________
1 cm in from
Center of Bar
Bar Edge
______________________________________
DSC
m.p., .degree.C.
1st heat 149.3 149.1
2nd heat 134.3 135.2
.DELTA.T 15 13.9
Heat of Fusion
1st heat 223.6 J/g 229.6 J/g
2nd heat 156.1 J/g 162.3 J/g
Infrared Crystallinity Index
In Bar Direction 0.745
Perpendicular to Bar Direction
0.759
Tensile Properties
Stress, psi
At Yield 4706 4463
At Break 5362 5326
Modulus, kpsi 649.7 404.2
Elongation, %
At Yield 4.7 4.5
At Break 330 335
Deformation (Creep)
0.4
(% at 1000 psi load)
0.3
______________________________________
Effect of Preheating
The preliminary heating of this example may be achieved in an atmosphere of
refluxing vapors instead of N.sub.2, as described below.
A 3".times.18" rod of UHMWLPE (American Hoechst, Hostalen GUR 415) was
heated in refluxing vapors of Krytox.RTM.-143AZ (E. I. du Pont de Nemours
and Company, Wilmington, Del.) (at 333.degree.-335.degree. C.) for 2
hours, 40 minutes. Krytox.RTM.-143AZ is a perfluoroalkylpolyether that is
a nonflammable, chemically inert liquid having unusually high thermal and
oxidative stability. Vapors of other liquids demonstrating these
characteristics may also be suitable. The overall system was wrapped with
glass insulation to facilitate slow cooling and protected by a nitrogen
atmosphere. As compared to the starting material, the resulting product
has improved crystallinity (IRCI 0.47 versus 0.27), a tensile modulus (300
kpsi versus 210), and tensile strength at yield (3850 psi versus 3400).
Most significantly, the product displays a large increase in elongation at
break (893% versus 315%).
When the above described material was recrystallized from 220.degree. C.
under 300 MPa, a new polyethylene resulted possessing extremely high
elongation at break (667%) along with the high tensile strength at yield
(4900 psi) and the tensile modulus (574 kpsi) expected of the superior,
enhanced UHMWLPE materials.
______________________________________
Flex Modulus, kpsi 436.4
431.2
433.80 (av)
Density .9684
IZOD IMPACT, 17.1
(ft.lb./in. of notch) 15.9
16.5 (av)
______________________________________
EXAMPLE 10
Effect of Heating Temperature on UHMW PE
Cubes (3/4) i of UHMW polyethylene (Hoechst Hostalen GUR 415, m.w. 4-6
million, fabricated by Westlake) were wrapped in Teflon.RTM. film and
placed in a large test tube protected from air with N.sub.2. A small
thermocouple was inserted into the center of one of the cubes to determine
the time necessary for the samples to reach test temperature. A plug of
glass wool was placed above the sample to control convection currents. The
tube was heated with a Wood's | | |
