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STATE OF THE ART
Ultra high molecular weight polyethylene (UHMWPE) is taken to mean linear
polyethylenes produced by a low pressure process and having
viscometrically measured average molecular weights of at least
1.times.10.sup.6 g/mol, preferably from 2.5.times.10.sup.6 g/mol to more
than 1.times.10.sup.7 g/mol. The procedure for determining the
abovementioned molecular weights is described, for example, in
CZ-Chemische Technik, Vol. 4 (1974), 129 ff.
UHMWPE has a special position among polyethylenes as it has a number of
physical properties which make many applications possible. Worthy of
particular mention are its high abrasion resistance, its low coefficient
of friction and its exceptional toughness. It is furthermore markedly
resistant to numerous chemicals. Because of its favorable mechanical,
thermal and chemical properties, UHMWPE has found use in various fields as
a high-value special material. Examples are the textile industry,
mechanical engineering, the chemical industry and mining.
Yet the scope for using ultra high molecular weight polyethylene is limited
by the difficulties in its processing. This is because, unlike the low
molecular weight types of polyethylene, the high molecular weight products
can only be processed using presses and screw or ram extruders because of
their extremely high melt viscosity. Pressing and extrusion often give
only partly formed pieces from which parts of the desired shape must be
produced by machining off material.
A process for producing solid objects from UHMWPE is described in DE-A-2A
25 396 which starts from pulverulent material, the particles of which have
an average particle size of less than 100 .mu.m with a distribution
function (which can be determined from the particle count for a number of
particle size intervals) of less than 0.8. The powder is pressed to give a
solid preform at a pressure of at least 140 kg/cm.sup.2 at a temperature
below the crystalline melting point of the polyethylene. After releasing
the pressure, the preform is sintered at a temperature above the
crystalline melting point. Since the as-synthesized ultra high molecular
weight polyethylene usually contains particles larger than 100 .mu.m, this
procedure requires comminution of the starting material by appropriate
means, for example by milling. An additional disadvantage of the known
procedure is that the moldings are not homogeneously plasticated
throughout. They can therefore expand during the sintering process as a
function of the temperature applied. This phenomenon can be attributed to
the preform being sintered outside the pressing mold. As a consequence,
the physical properties are not reproducible and vary greatly. Strict
dimensional accuracy of the moldings is furthermore not ensured.
Another process for producing moldings from pulverulent polyolefins having
molecular weights of at least 1 million is taught in DE-C-26 34 537
wherein the pulverulent polymers are converted into the rubber-elastic
state at from 150.degree. to 250.degree. C., preferably from 190.degree.
to 210.degree. C., in a mold loosely closed by a punch. The material is
then immediately densified in the mold at pressures of from 2.5 to 25
N/mm.sup.2 in a first stage of at least 32 minutes, then at pressures of
from 40 to 100 N/mm.sup.2 in a second stage of at least 1 minute and the
molding is then allowed to cool in the mold without pressure being
applied. This process has proved successful in practice, but only makes
possible the production of moldings having a simple geometry.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a process for the molding of
complex-shaped moldings from ultra high molecular weight polyethylene
powder free of the disadvantages of the prior art.
It is another object of the invention to provide novel moldings which are
completely homogeneous and have uniform physical properties throughout the
molding.
These and other objects and advantages of the invention will become obvious
from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The novel process of the invention for producing moldings from pulverulent
ultra high molecular weight polyethylene comprises heating said polymer
powder to 160.degree. to 280.degree. C. in a mold loosely closed by a
punch in a press without applying pressure or at a pressure of up to 0.5
MPa, and then cooling it to room temperature at a pressure of from 4 to 20
MPa.
The novel process allows the production from UHMWPE of moldings having
varying and even demanding geometries having high homogeneity and
dimensional accuracy.
The starting material can be any type of pulverulent ultra high molecular
weight polyethylene, regardless of how they were produced. It is possible
to use polymers obtained by the Ziegler process by polymerization of
ethylene in the presence of transition metals of the 4th to 6th group of
the Periodic Table of the elements together with organometallic compounds
of the elements of the 1st to 3rd group of the Periodic Table of the
elements. It is, however, also possible to use ultra high molecular weight
polyethylenes which have been produced from anhydrous and oxygen-free
ethylene in the gas phase in the presence of supported catalysts
containing chromium oxide and metal alkyl. It is particularly remarkable
that the polymers can be used with the particle size obtained in the
synthesis. The average particle size is, depending on the polymerization
process, about from 100 to 400 .mu.m, but may also be smaller or larger.
The process of the invention can be carried out in a simple manner. The
pulverulent polymer is introduced into a mold of any desired shape which
is loosely closed by a fitting but not airtight punch lying on top. In
this process stage, the punch applies only a slight pressure (contact
pressure) of up to 0.5 MPa to the powder. The powder in the mold is then
heated to a temperature of 160.degree. to 280.degree. C., preferably
220.degree. to 250.degree. C., which procedure can be carried out directly
in the press, heat being conducted from the upper and lower platens. A
heatable mold can, however, also be used. Finally, it is possible to carry
out the heating in a special facility, for example in an electric oven.
Owing to the poor thermal conduction, a temperature gradient is established
in the powder bed with the result that the temperature decreases from the
outside layers into the middle of the mold. To avoid inhomogeneities in
the final pressed product, care must be taken that temperatures in all
parts of the powder bed are within the abovementioned ranges. Overheating,
even in limited regions, must be avoided so that the material is not
thermally damaged.
Oxidative damage to the heated UHMWPE can be combated by maintenance of an
inert gas atmosphere, for example nitrogen. The duration of heating is
dependent on the powder volume and on the geometry of the molding. During
heating, the ultra high molecular weight polyethylene which possesses not
an actual melting point but a crystalline melting range (about 130.degree.
to 135.degree. C.) becomes viscoelastic. The weight of the loosely
overlying punch leads to a minor densification of the powder, and to
substantial degassing of the material.
As soon as the processing temperature of 160.degree. to 280.degree. C.,
preferably 220.degree. to 250.degree. C., has been reached, the polymer in
the mold is placed under a pressure of from 4 to 20 MPa, preferably 8 to
14 MPa. The pressure to be used depends particularly on the amount of
polymer to be processed and on the geometry of the molding. Higher
pressures should be used for large amounts of polymer and for shapes which
are composed of regions of widely differing wall thicknesses.
The processing temperature is maintained until the punch reaches its final
position in the mold at constant pressure which stage is generally
attained within from 1 to 5 minutes. The molding is then cooled to room
temperature while maintaining the pressure. Cooling may occur by heat
exchange with the surrounding atmosphere and can be accelerated by use of
coolants in the upper and lower platens or in the mold. The cooling time
depends on the size of the molding and on the type of the heat exchange.
In each phase of the process prior to the cooling under pressure, it is
possible to add more pulverulent UHMWPE to the mold, if the amount of
material needed for production of the desired molding has to be
supplemented during the process. Account must here be taken of the fact
that the bulk volume of the polyethylene powder is from about 2 to 2.5
times as great as the volume of the molding manufactured from it. The
newly added powder must, of course, be subjected to the same thermal and
pressure treatment as that originally used. It is, however, more
advantageous to provide the mold with an upstream chamber from which
further polymer powder can be continuously fed in such quantity as is
necessary to fill the mold, and to calculate the stroke so that the powder
can be densified in one operation. The cooled body is completely
plasticated and free of voids and can easily be removed from the mold.
The process of the invention makes possible the production of dimensionally
accurate moldings from ultra high molecular weight polyethylene by
pressing. No wasteful machining to remove material is necessary. The
pressurization of the plasticated bodies takes only a short time so that
press capacity can be effectively used. The pressurization can furthermore
be carried out at room temperature after the heating stage, so that way it
consumes little energy.
The novel procedure ensures a high production rate by use of multi-die
presses or of automatic processing methods and is suitable for
manufacturing moldings of any desired shape. Complex-shaped machine parts
such as pump housings, pump rotors and valve components, for example for
chemical apparatus, can be produced in high quality. It is, however, also
possible to manufacture sheets of UHMWPE having a thickness of about 10 mm
and less, which could previously only be obtained by splitting polyolefin
blocks.
Various modifications of the process and products of the invention may be
made without departing from the spirit or scope thereof and it should be
understood that the invention is intended to be limited only as defined in
the appended claims.
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