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SUMMARY OF THE INVENTION
This invention relates to thermal treatment of ultra-high molecular weight
polyethylene and to the novel polyehtylene compositions thus formed. More
particularly, it relates to a method of treating powders of ultra-high
molecular weight polyethylene, i.e., polyethylene having molecular weight
of above about one million, at elevated temperatures above its crystalline
melting point for a short period of time. This process affords
polyethylene powders having improved properties which facilitate
compression molding. Molded pieces made from heat-treated powder have
better properties with respect to transparency, surface gloss and
low-temperature mechanical properties.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1: Scanning electron microscope (SEM) picture (.times.5,000) of a
commercial ultra-high molecular weight polyethylene powder marketed by
American Hoechst Corporation under the trademark .RTM.HOSTALEN GUR 413,
before heat treatment. This material had a molecular weight of about
5.times.10.sup.6, and more than 99 percent of the powder passed 60 mesh
screen. The surface area as measured by B.E.T. method using nitrogen was
0.45 m.sup.2 /g. FIGS. 2-6 were also obtained at the.times.5,000
magnification whereas FIGS. 7 and 8 were obtained at.times.10,000.
FIGS. 2: SEM picture of the same material as FIG. 1 after heat treatment at
250.degree. C. for two minutes.
FIG. 3: SEM picture of the same material as FIG. 1 after heat treatment at
160.degree. C. for twelve minutes.
FIG. 4: SEM picture of the same material as FIG. 1 after heat treatment at
120.degree. C. for sixteen hours.
FIG. 5: SEM picture of a commercial ultra-high molecular weight
polyethylene powder marketed by Hercules Incorporated under the trademark
.RTM.HIFAX 1900, before heat treatment. This material had a molecular
weight of about 7.times.10.sup.6, and more than 99% of the powder passed
60 mesh screen.
FIG. 6: SEM picture of the same material as FIG. 5 after heat treatment at
250.degree. C. for two minutes.
FIG. 7: SEM picture of the freeze-fractured surface of a sheet molded from
untreated .RTM.HOSTALEN GUR 413 powder.
FIG. 8: SEM picture of the freeze-fractured surface of a sheet molded from
the same material as FIG. 7 after heat treatment at 250.degree. C. for two
minutes.
FIG. 9: Nomarski interference photograph (for procedure, see "Polarizing
Microscope" 3rd edition, A. F. Halland, published by Vickers Intruments,
York, England, 1970) of the surface of a plaque compression molded from
.RTM.HOSTALEN GUR 413 untreated powder. Magnification was at.times.200.
FIG. 10: Nomarski interference photograph (.times.200) of the surface of a
plaque compression-molded from .RTM.HOSTALEN GUR 413 powder heat-treated
at 250.degree. C. for two minutes.
The scales inside the FIGS. 1-6 correspond to 2.mu., and those of FIGS. 7-8
correspond to 1.mu..
DETAILED DESCRIPTION OF THE INVENTION
Although ultra-high molecular weight polyethylene (UHMW-PE) possesses
various outstanding properties such as very high energy absorption
characteristics, it presents certain melt processing problems, which has
hampered the broadening of its application. The process of the present
invention has unexpectedly been found to produce UHMW-PE having improved
properties.
Typically such powders are used for making various articles by compression
molding. Presently available ultra-high molecular weight polyethylenes are
generally fine powders, the predominant portion of which comprises
particles smaller than 1 mm size. Observation of these particles with a
microscope even at a moderate magnification of 400, shows that the
particles are not round and smooth, but have complex and irregular shapes.
Use of scanning electron microscope at.times.5000 magnification is a
convenient method for observing detailed structure of the powder surface.
FIG. 1, for instance, shows that the single particle of .RTM.HOSTALEN GUR
413 before heat treatment has a very complex irregular surface structure
comprising nodules or spherules of about 1.mu. or less, crevices and
fibrous structures. We will refer to these details of the powder surface
as observed by SEM at.times.5000 as "fine structure." FIG. 5 shows that
.RTM.HIFAX 1900 has less detailed surface structure than .RTM.HOSTALEN GUR
413 but still has a substantial extent of fine structure in comparison to
the powders heat-treated by the method of this invention as exemplified by
FIGS. 2 and 6.
The ultra-high molecular weight polyethylene powders which are
advantageously treated by the process of the present invention are
characterized by a complex, irregular surface appearance when observed
under SEM at .times.5000 or by the "fine structure" referred to above and
by a molecular weight of above about 1.times.10.sup.6, preferably between
about 1.times.10.sup.6 and about 12.times.10.sup.6. Particular
polyethylenes which have been found useful in this invention have
molecular weight in the range of about 5.times.10.sup.6 -7.times.10.sup.6
as well as the irregular surface appearance or "fine structure" of the
type described above.
Examples of specific ultra-high molecular weight polyethylenes of the type
described above useful in this invention are .RTM.HOSTALEN GUR (molecular
weight is about 5 million and the compression molded sheets have the
following typical physical properties; density 0.94, tensile strength
3,100 psi, ultimate tensile strength 6,300 psi and Shore-hardness D
64-67), and .RTM.HIFAX 1900 (molecular weight is about 7 million and the
compression molded sheets have the following physical properties; density
0.94 tensile strength 3,400 psi, ultimate tensile strength 6,300 psi and
Shore--hardness D 67). Two samples of .RTM.HOSTALEN GUR, namely, GUR 413
and 403 were used in the examples given below. More than 99% of the powder
passed 60 mesh screen in either sample.
I have discovered that heat treatment of the UHMW-PE particles of the type
described above for a short period of time at a temperature between the
crystalline melting point and about 275.degree. C. results in powders
having properties which facilitate the compression molding of these
powders and which form compression molded articles having improved
properties such as high transparency, high surface gloss and low
freeze-fracture temperature.
With respect to molding conditions, it has been found that, in order to
obtain the same level of product quality with respect to transparency and
surface gloss, pre-heat treated material of this invention requires less
strenuous molding conditions such as time, temperature and pressure than
the untreated material, and when compared at a given molding condition the
heat treated material generally gives better transparency and gloss than
the untreated material. Thus in many instances more economical molding can
be achieved by the process of this invention.
Below the crystalline melting point, even prolonged heating does not cause
any appreciable improvements in the particles with respect to these
properties. There is no practical advantage in using higher temperatures
than about 275.degree. C. because of the risk of chemical changes of the
particles such as oxidation, degradation, etc., and increased cost. The
suitable length of time for the heat treatment naturally depends upon the
temperature but typically it is a few minutes at 250.degree. C. and more
than thirty minutes at 140.degree. C. The particular temperature-time
combination necessary to effectuate the advantages of this invention
depends to some extent, on the particle size of the resin and
effectiveness of heat-transfer in the heating device employed. Typical
time-temperature condition useful in this invention is about 140.degree.
to 270.degree. C. for about one to thirty minutes. Preferred
time-temperature condition useful in this invention is about 160.degree.
to 250.degree. C. for about 2 to 16 minutes.
In a preferred embodiment of the process of the present invention it has
been found desirable to heat treat the UHMW-PE powders useful in this
invention at a temperature between about the crystalline melting point of
the polyethylene powder and about 275.degree. C. for a period of time
effective to substantially destory the "fine structure" of the particles
as observed by SEM at.times.5000. FIGS. 2 and 6 exemplify powders in which
the fine structure has substantially been destroyed. As will be noted from
FIGS. 2 and 6, the powders thus treated have a smooth, regular surface
appearance when viewed by SEM at.times.5000. There is no practical
advantage in continuing the heating more than is necessary to effectuate
the substantial destruction of fine structures, and particularly when the
temperature is relatively high within this range, prolonged heating will
lead to the yellowing of the molded product. Typical time temperature
conditions useful to achieve the substantial destruction of "fine
structures" are about 140.degree. to 270.degree. C. for about one to about
thirty minutes, preferably about 160.degree.-250.degree. C. for about two
to sixteen minutes.
Specific UHMW-PE powders were treated in accordance with the process of
this invention to achieve substantial destruction of "fine structures"
under the following conditions; .RTM.HOSTALEN GUR 413 powder was treated
at about 250.degree. C. for about three minutes in order to achieve
substantial descruction of fine structure. .RTM.HOSTALEN GUR 403 powder
was treated at about 250.degree. C. for about three minutes in order to
achieve substantial destruction of fine structure. .RTM.HIFAX 1900 was
treated at about 250.degree. C. for about three minutes to achieve
substantial destruction of "fine structure."
Under the temperature-time conditions of this invention, the heat treatment
does not require inert atmosphere such as nitrogen, but can be done in air
without accompanying any appreciable oxidation or degradation of the
material. Although a conventional oven is conveniently employed in the
process of this invention, the particular device or mechanism of
heat-transfer used is not critical, and other known methods of heating
powders may be used. For instance, the resin powder may be placed in a
column as a fluidized bed and hot gas such as air or nitrogen may be blown
through the column, although the optimum temperature-time condition found
for this device may be somewhat different from the one found for the
oven-heating.
Another aspect of the present invention is the novel UHMW-PE compositions
having improved molding properties, which novel compositions are prepared
by the process of the instant invention.
Such novel UHMW-PE powder composition of this invention has a molecular
weight of above about 1.times.10.sup.6 as measured by the method described
hereinafter, and a substantially smooth surface appearance free of fine
structures such as nodules of less than about one micron in size, crevices
and fibrous structures, as viewed under scanning electron microscope at
5000 magnification.
As compared to conventional polyethylene molding powders, the ultra-high
molecular weight polyethylene powders subjected to the process of the
present invention gives molded products with better properties,
particularly with respect to the transparency, gloss and low-temperature
mechanical properties such as brittle temperature.
A typical commercial use of the improved powders of this invention is the
manufacture of compression-molded sheets of various sizes and thickness
which in turn are used for a wide variety of end applications such as
linings for chutes.
Molecular weights of the materials used here were determined from the
viscosity measurements of decalin solutions at 135.degree. C. by the
formula.
Molecular weight =5.37.times.10.sup.4 (intrinsic viscosity).sup.1.49.
The term crystalline melting point as used here is determined by placing
the powder sample in Mettler Hot Stage FP-5 equipped with a polarized
microscope and by detecting the loss of birefringence through the use of
polarized light while the temperature is continuously raised at the rate
of 10.degree. C./min. A typical value of the crystalline melting point of
commercial ultra-high molecular weight polyethylene is 140.degree. C. as
measured by this technique, but there are some variations among the
individual commercial materials.
Following examples are given for illustrative purposes and should not be
construed as a limitation on the invention.
EXAMPLE 1
A powder sample of a commercial ultra-high molecular weight polyethylene,
.RTM.HOSTALEN GUR 403 (molecular weight was 5.times.10.sup.6 and more than
99% of the powder passed 60 mesh screen) was placed in a porcelainized
metal pan as a layer of 1/4 inch thickness and the pan was introduced to
an oven maintained at 250.degree. C. After three minutes of heating the
pan was taken out. In view of the time necessary for the oven to return to
the original temperature and the initial warm-up period of the particles,
this condition was effectively two minutes at 250.degree. C. (This
"effective time" was ascertained by comparing the extent of change of the
fine structure caused by the heating in this oven with the one caused by
the heating in Mettler Hot Stage which had very little time lag of heat
transfer, the temperature being the same in both cases). A similar heating
experiment was carried out using the same apparatus and material except
the heating condition was 250.degree. C. for five minutes (the effective
condition was 250.degree. C. for four minutes). In both experiments, when
the powder cooled, a skin was covering most of the surface. The materials
were crushed by hand and passed through a 20 mesh screen prior to molding.
Comparison between FIG. 1 and FIG. 2 exemplifies a typical difference of
morphology of the .RTM.HOSTALEN GUR particles before and after the heat
treatment at 250.degree. C. for two minutes. (These two figures were
obtained on a .RTM.HOSTALEN GUR 413 sample. See Example 3 below for a more
detailed explanation).
A prosthetic cup of about one inch radius hemisphere having 3/16 to 1/4
inch wall thickness was compression molded from each (a) untreated
.RTM.HOSTALEN GUR 403, (b) .RTM.HOSTALEN GUR 403 pre-heat treated at
250.degree. C. for three minutes, and (c) .RTM.HOSTALEN GUR 403 pre-heat
treated at 250.degree. C. for five minutes. These three samples were
designated Sample A, B and C respectively. The processing cycle in all
three cases was the following:
1. The resin powder was placed in the mold and pressed at 32,640 psi for
seven minutes.
2. The temperature was brought up to 400.degree. F. and the pressure
reduced to 4,080 psi. These conditions were maintained for 25 minutes.
3. While maintaining the temperature at 400.degree. F., the pressure was
raised to 20,400 psi and kept there for five minutes.
4. While maintaining the pressure at 20,400 psi, the material was gradually
cooled to 25.degree. C. in about thirty minutes.
Comparison among the Samples A, B and C led to the following conclusions:
Samples B and C processed more easily than Sample A; Sample B gave a
clearer, glossier and smoother finished product than Sample A; and Sample
C had a yellow color indicating an excessive pre-heat treatment.
The .RTM.HOSTALEN GUR 403 sample used here had a crystalline melting point
of 139.5.degree. C. as measured by the aforementioned technique. There was
no substantial change in the crystalline melting point of the sample after
the heat treatment.
EXAMPLE 2
An untreated powder sample of .RTM.HOSTALEN GUR 413 and one heat-treated at
250.degree. C. for two minutes (effective time) were used to compression
mold plaques of 12 inch.times.12 inch.times.1.5 mm size at the following
conditions, (a) heat at 390.degree. F. (199.degree. C.) for 60 minutes at
2 tons of force and (b) cool down to room temperature in 30 minutes at 60
tons of force.
Transparencies of the molded sheets were measured in terms of the thickness
of the layers of the sheet through which one can read black letters of 4.5
mm.times.4.5 mm size printed on a white paper. In the case of the sheet
molded from the untreated powder, the letters were legible through 3.0 mm
thickness but not through 4.5 mm thickness, whereas in the case of the
sheet molded from heat-treated powder, the letters were legible through
4.5 mm thickness but not through 6.0 mm thickness, indicating a higher
transparency for the latter sheet.
Surface gloss of the two sheets mentioned above was measured by using an
ABS plaque as a reference standard. By using a Nomarski Interference
Contrast System the surface to be examined was put in focus and the
intensity of light reflected from the surface was measured on a Vickers
J-35 camera system. A "Gardner Gloss Meter" was adjusted at 100 when the
subject was the ABS plaque. The readings of the reflected light
intensities were 40-45 for the sheet made from the untreated powder and
75-80 for the sheet made from the heat-treated powder. Normalski
interference pictures of the surfaces of the two sheets were taken
at.times.200 magnification and they are shown at FIG. 9 (untreated) and
FIG. 10 (heat-treated). It is noticed that the sheet molded from the
heat-treated material has a smoother surface than the one from the
untreated material when they are examined under this condition, explaining
the difference in the gloss as measured by the above technique.
EXAMPLE 3
The crystalline melting point of a .RTM.HOSTALEN GUR 413 sample was
measured by the procedure described earlier, and it gave 141.4.degree. C.
After a heat treatment at 250.degree. C. for two minutes, the crystalline
melting point was substantially the same as before.
The surface area measurement of the untreated and the heat-treated powder
sample by the B.E.T. (Brunauer-Emmett-Teller) method using nitrogen gave
0.45 m.sup.2 /g and 0.38 m.sup.2 /g, respectively.
EXAMPLE 4
A comparative experiment similar to Example 1 was carried out using a
powder sample of another commercial ultra-high molecular weight
polyethylene, .RTM.HIFAX 1900 (molecular weight is 7.times.10.sup.6).
Untreated material (Sample D), material treated at 250.degree. C. for
three minutes (Sample E), and material treated at 250.degree. C. for five
minutes (Sample F) were subjected to the same molding cycle as described
in Example 1 to form the prosthetic cups described above.
Comparison among the molding runs based on Samples D, E and F led to the
same conclusion as recited in Example 1, namely that the molded piece made
from the material pre-heat treated at 250.degree. C. for two minutes had a
better transparency, gloss and smoothness than the one made from the
untreated material, that the heat treatment at 250.degree. C. for three to
five minutes facilitated the compression molding and that some yellowing
of the material resulted from the treatment at 250.degree. C. for five
minutes.
EXAMPLE 5
A commercial ultra-high molecular weight polyethylene powder, .RTM.HOSTALEN
GUR 413, was studied by scanning electron microscopy both before heat
treatment and after heat treatment under varying conditions. FIG. 1 is
before heat treatment and shows a complex structure of the particle. FIG.
2 is a picture of the same material after a heat treatment in an oven at
250.degree. C. for two minutes showing a complete destruction of the fine
structures resulting in the particle having a much smoother surface than
before. FIG. 3 is a picture of the same material after a heat treatment in
the oven at 160.degree. C. for twelve minutes showing a substantial loss
of the fine structures. FIG. 4 corresponds to a heat treatment at
120.degree. C. for 16 hours showing that at 120.degree. C. even an
extended period of heating (16 hours) does not cause any appreciable
change of morphology of the particle.
The molecular weight of the material before heat treatment was
5.2.times.10.sup.6, whereas it was 5.3.times.10.sup.6 after the heat
treatment at 250.degree. C. for two minutes, indicating that there was
little chemical change of the material under this condition of heat
treatment.
EXAMPLE 6
A powder sample of .RTM.HIFAX 1900 was studied in a similar manner as
Example 5.
FIG. 5 is a SEM picture of the material before heat treatment. FIG. 6 is a
SEM picture of the same material after a heat treatment at 250.degree. C.
for two minutes. Again, it is noticed that the heat treatment of
250.degree. C. for two minutes causes a considerable change in the
morphology of the particles; the fine structures of the surface of the
particles are lost to a large extent and the particles become smoother.
EXAMPLE 7
A plaque of 12".times.12".times.1/4" size was compression molded using
.RTM.HOSTALEN GUR 413 after it had been heat treated at 250.degree. C. for
two minutes (effective heating time). The molding condition was (a) heat
at 390.degree. F. (199.degree. C.) for sixty minutes at two tons of force
and (b) cool down to room temperature in thirty minutes at 60 tons of
force. A test piece about three inches long, 1/2 inch wide and 1/4 inch
thick was band-sawed out of this plaque. Another test piece of
2".times.1/2".times.1/8" size was carved out of a sheet which had been
molded under the same condition as above from untreated .RTM.HOSTALEN GUR
413 powder, and these two test pieces were compared with regard to the
freeze fracture characteristics.
The two test pieces were placed in liquid nitrogen for a sufficient length
of time for temperature equilibration and were snapped by hand instantly
after they were taken out of the liquid nitrogen bath. In the case of
untreated material, the sheet broke brittlely and the picture of the
fractured surface, FIG. 7, showed a distinct morphology similar to the one
possessed by the original untreated powder as shown in FIG. 1.
In the case of pre-treated material, however, it did not brittle fracture
at liquid nitrogen temperature; it needed a notching in order to be
broken. The scanning electron microscopic picture of the fractured
surface, FIG. 8, shows that the characteristic morphology of the original
untreated powder is absent, and that the fractured surface is completely
different from characteristic morphology of brittle structure, indicating
that a plastic flow has occurred.
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