 |
Description  |
|
|
BACKGROUND OF THE INVENTION
Ultra-high molecular weight, linear polyethylene, having molecular weight
of at least about 1.times.10.sup.6 and density of between 0.92 to 0.99
has been known for some time. On account of its outstanding physical
properties including toughness, impact strength, abrasion resistance, low
coefficient of friction, combined with excellent resistance to attack by
solvent and corrosive chemicals, it has been found useful in demanding
applications including vibration dampener pads, hydraulic cyclinders,
mallet heads, flexible drive couplings, gears, belt and chain guides, door
stops, bumpers, machinery carriages, conveyor equipment components,
bearings, and especially in textile machine parts such as loom pickers,
drop box pickers, lock straps, sweep sticks, lock connectors, pick arm
stop bolts, and the like. Its use, however, has been limited because of
fabrication difficulties. Due to its high molecular weight it has very
high melt viscosity, which makes it difficult or impractical to fabricate
it by conventional thermoplastic processing techniques, such as injection
molding. Not only that, it is also sensitive to melt shear, which causes
melt fracture and physical degradation.
Previously available methods for fabricating ultra-high molecular weight
polyethylene include compression molding, flow molding, transfer molding
and ram extrusion, all of which are cumbersome and slow as compared to
melt extrusion usually employed for thermoplastic materials. Compression
molding of such ultrahigh molecular weight polyethylene requires
maintenance of the molded part under elevated temperature and pressure in
the mold for extended periods of time to melt the polyethylene and to
allow it to melt flow into a solid article free of voids, and then cooling
it under pressure while confined in the mold. Equipment required for such
operation is complex and expensive and production rates are exceedingly
slow. A plurality of identical molds are usually required in order to
permit acceptable production rates. Therefore, there is a need for a
simplified molding process for forming articles of ultra-high molecular
weight, linear polyethylene.
It is an object of the present invention to provide a new method for
forming solid articles of ultra-high molecular weight, linear
polyethylene.
DESCRIPTION OF THE INVENTION
It has now been discovered that ultra-high molecular weight, linear
polyethylene of very specific finely divided powder form can be fabricated
into solid articles having excellent physical properties by (1)
compressing the polyethylene powder at a temperature below its crystalline
melting point under pressure of at least about 2,000 p.s.i. to form a
preform, (2) releasing the pressure, and (3) free-sintering the preform by
subjecting it to elevated temperature above its crystalline melting point
usually in the range of 275.degree. to 350.degree.F. for time sufficient
to permit the powder particles to sinter into a solid article having the
shape of the preform.
The present invention provides a method for making solid articles of
ultra-high molecular weight, linear polyethylene by free-sintering
procedure which comprises (1) compressing molding powder of ultra-high
molecular weight, linear polyethylene having molecular weight of at least
1.times.10.sup.6 and density of from 0.92 to 0.99 comprising particles of
less than 100 micron mean particle size having distribution function of
less than 0.80 under pressure of at least 2,000 p.s.i. at a temperature
below its crystalline melting point to form a solid preform, (2) releasing
the pressure, and (3) free-sintering the preform at temperature above the
crystalline melting point of the polyethylene, usually at a temperature in
the range of 275.degree. to 350.degree.F.
DETAILED DESCRIPTION OF THE INVENTION
Ultra-high molecular weight polyethylene suitable for use in the present
invention may be prepared by known procedures. It is commercially
available. It has been prepared by the Phillips low pressure ethylene
polymerization process using a chromium oxide catalyst on a silica or
silica-alumina support in paraffinic or cycloparaffinic solvent to form
the polymer in solution or as discrete particles in a hydrocarbon slurry.
It has also been prepared by the Ziegler process using active metal alkyl
catalyst, or by such processes as described in U.S. Pat. No. 3,050,514,
and especially by the process outlined in U.S. Pat. No. 3,051,993. The
latter process involves at least intermittently contacting anhydrous
oxygen-free ethylene in gaseous phase with an inorganic, porous,
frangible, solid contact catalyst prepared from an inorganic compound of
chromium and oxygen and an active metal alkyl.
Ultra-high molecular weight, linear polyethylene suitable for use in the
present invention has molecular weight, calculated from viscosities in
decalin solution at 135.degree.C. by the P. Francis et al. formula [N] =
6.77 .times. 10.sup.-.sup.4 (M.sup.0.67) (c.f. P. Francis et al. J. Pol.
Sci. 31, 453 (1958)) in the range between about 1.times.10.sup.6 and about
5.times.10.sup.6 and above. Since abrasion resistance and other
advantageous properties increase with increasing molecular weight,
ultra-high molecular weight polyethylene having molecular weight of at
least about 2.times.10.sup.6 is preferred. Ultra-high molecular weight
polyethylene suitable for use in the present invention has densities in
the range from 0.92 to about 0.99, usually from about 0.935 to about 0.960
at 23.degree.C., determined by ASTM Method D792. Its crystalline melting
point is in the order of about 275.degree.F.
The term "polyethylene" as used in the specification and claims refers to
homopolymer of ethylene as well as to copolymers thereof with minor
amounts of .alpha.-olefins copolymerizable therewith, such as, for
example, 1-alkenes having 3 to 8 carbon atoms such as propylene, butene-1,
2-methylpropene-1, 4-methylpentene-1, and 2,4,4-trimethylpentene-1,
generally containing not less than 85 weight percent, and preferably not
less than 96 weight percent of polymer units derived from ethylene. Such
copolymers have essentially the same characteristics as the ethylene
homopolymer of the same ultra-high molecular weight, and they have the
same preforming and sintering characteristics.
Typical properties of some commercial types of ultrahigh molecular weight,
linear polyethylenes suitable for use in the present invention are shown
in Table I below.
TABLE I
__________________________________________________________________________
TYPICAL PROPERTIES* OF ULTRA-HIGH MOLECULAR WEIGHT, LINEAR
__________________________________________________________________________
POLYETHYLENE
ASTM Test
Property Units Method Value
__________________________________________________________________________
Molecular Weight 1.5 .times. 10.sup.6
2.8 .times. 10.sup.6
>5.0 .times.
10.sup.6
Crystalline Melting Point
.degree.F. 275 275 275
Specific Gravity gm/cc D792-66 0.940 0.936 0.930
Melt Index (21.6 kg, 190.degree.C.)
gm/10 min.
D1238-65 0.00 0.00 0.00
Tensile Strength at 73.degree.F., 2"/Min.
p.s.i. D638-68 5,600 5,600 6,000
Yield Strength at 73.degree.F., 2"/Min.
p.s.i. D638-68 3,100 3,100 3,000
Ultimate Elongation at 73.degree.F., 2"/Min.
% D638-68 525 450 200
Izod Impact (Notched)
Ft-lb/in..sub. 2 notch
D256-56 >20** >20** >20**
Tensile Impact (Type L)
Ft-lb/in. D1822-68 >1,000 850
Stiffness, Cantilever Beam D747-63 80,000 80,000 80,000
Compression Modulus p.s.i. D695-69 110,000
110,000
Rockwell Hardness R. Scale D785-65 50 50 50
Tear Strength gm/Mil D1004-66 550 550
Heat Deflection Temperature, 264 psi
.degree.F.
D648-56 113 113 115
Coefficient of Thermal Linear
Expansion (-22.degree.F. to 86.degree.F.)
in/in.degree.F. 7.2 .times. 10.sup.-.sup.5
7.2 .times. 10.sup.-.sup.5
Brittleness Temperature
.degree.F.
D746-64 <-94 <-94 <-94
Coefficient of Friction, Static 0.11 0.11
Environmental Stress Crack Resistance
(0.75", 100% Igepal, 100.degree.C.)
Hrs. D1693-70, P-34
>2,000 >2,000
Abrasion Resistance (Taber)
gm/10,000 cycles
D1044-56 0.013 0.008
Dielectric Constant
100.sub.6 Hz D150-68 2.34 2.34 2.34
10.sup.6 Hz 2.30 2.30 2.30
Dissipation Factor
100.sub.6 Hz D150-68 0.0003 0.0003 0.0003
10.sup.6 Hz 0.0002 0.0002 0.0002
Dielectric Strength
Short Time Volts/Mil D149-64 710 710 710
Stepwise 680 680 680
Volume Resistivity ohm/cm D257-66 >10.sup.16
>10.sup.16
>10.sup.16
Water Absorption % 24 hrs., 1/8"
D570-63 Nil Nil <0.01
Burning Rate D635-68 Very Slow
Very Slow
Very
__________________________________________________________________________
Slow
*Determined at 73.degree.F. and 50% Relative Humidity
**Deflects but does not break
In order to be free-sinterable in accordance with the method of the present
invention, ultra-high molecular weight, linear polyethylene must have mean
particle size of less than 100 microns, and it must have a distribution
function of less than 0.80.
Mean particle size as used herein is the particle size determined by use of
the Coulter Counter. The Coulter Counter provides a method for determining
particle size in the 1 to 100 micron range which is based on the principle
of changes in the electrical conductance of an electrolyte solution
containing suspended therein the particles the size of which is to be
determined as the solution and suspended particles pass through a small
orifice. Coulter Counters are commercially available instruments.
Distribution function may be determined from particle counts at a number of
particle size intervals as determined on a Coulter Counter. Average
particle counts at a number of predetermined particle size intervals are
determined, the weight percent and cumulative weight percent are
calculated for each particle size interval, and the cumulative percentages
of particle size in microns within the particle size intervals are plotted
on probability paper. The best fit line is drawn through the plot. The
particle size at the 50 and 84 percent probability level is determined and
the distribution function is calculated as average particle size in
microns at the 50 percent probability level minus average particle size in
microns at the 84 percent probability level, divided by the average
particle size in microns at the 50 percent probability level.
Ultra-high molecular weight linear polyethylene as obtained from the
polymerization process is usually a granular powder of particle size well
above 100 microns. Molding powder of the present invention is obtained by
comminuting such coarse, granular powder until it has been reduced to mean
particle size of 100 microns at distribution function of less than 0.80.
Any commerically available milling equipment capable of achieving such
combination of particle size and particle size distribution is suitable
for making the molding powder of the present invention. Suitable equipment
includes fluid energy mills wherein milling is principally effected by
interparticulate collision of the polymer particles, mechanical mills
wherein particle size reduction is principally effected by collision of
polymer particles with rotating mill parts, or mills combining the
principles of fluid energy milling and mechanical milling.
In fluid energy mills the granular polymer powder to be reduced in particle
size is subjected to action of expanding grinding fluid such as air or an
inert gas, e.g., nitrogen, argon or fluorocarbons. Suitable equipment
includes air mills such as the "Jet-O-Mizer" manufactured by the Fluid
Energy Processing and Equipment Company. A high degree of interparticulate
collision is accomplished by utilizing this type of air mill which has the
shape of a hollow elongated toroid. The mill stands vertically with one
curved end at the top and the other curved end at the bottom so that the
straight elongated sides which are substantially parallel are in vertical
position. The coarse, granular material is fed into the lower curved end
of the mill (grinding chamber). The grinding fluid is fed into the same
end of the mill under high pressure to effect grinding of the feed
particles by interparticulate collision. Temperature is preferably held
within the range of 0.degree. to 200.degree.F. It is not permitted to rise
above about 250.degree.F. Low grinding temperatures tend to improve mill
performance due to increased brittleness of the resin. Starting material
feed to the grinding chamber is accomplished through a venturi feeder. The
air or other grinding fluids are forced under high pressure through
nozzles into the grinding chamber wherein the expanding grinding fluid
causes repeated impact of the resin particles resulting in rapid size
reduction. The material is forced from the grinding chamber through an
upstack, which is one of the straight elongated portions of the mill, into
the upper curved section wherein classification takes place. In the
classification zone smaller particles are removed and the larger particles
are thrown to the outside by centrifugal force and remain in the mill
returning to the grinding chamber by means of a "downstack" which is the
counterpart of the upstack and is substantially parallel thereto.
Another type of fluid energy mill suitable for effecting size reduction to
make molding powder of ultra-high molecular weight, linear polyethylene in
accordance with the present invention in the circular air mill of the
"Micronizer" type which is a hollow horizontal toroid consisting of a
circular grinding chamber having peripherally spaced orifices for
introducing the grinding fluid tangentially into the grinding chamber, and
having a centrally located exhaust opening. The coarse, granular material
is introduced into the grinding chamber through a venturi feed jet.
Tangential positioning of the orifices through which grinding fluid is
introduced into the mill under high pressure insures rotation of mill
content in one direction. Expansion of grinding fluid within the mill
causes high speed rotation of the feed material to be pulverized and
interparticulate collision thereof, resulting in size reduction.
Centrifugal force causes larger size particles to remain near the
periphery of the grinding chamber, while material of fine particle size is
drawn with the exhausting grinding fluid towards the inside of the
grinding chamber and is removed from the mill through the centrally
located exhaust opening. In this mill grinding and classification occur
concurrently.
A different type of fluid energy mill suitable for making the molding
powder of the present invention is disclosed in U.S. Pat. No. 2,704,635
which has opposing jets and a circulatory classification system.
A further type of mill suitable for making the molding powder of the
present invention is the "Cyclo-Jet" mill of the type described in U.S.
Pat. Nos. 3,348,779 and 3,468,489 supplied by the Fluid Energy and
Processing Equipment Company. "Cyclo-Jet" mills combine the comminuting
action of expanding high velocity grinding fluids, resulting in
interparticulate collision, with that of rotating anvils, resulting in
mechanical impact of particles with rotating mill parts.
Molding powder in accordance with the present invention may also be made by
comminuting coarse, granular ultra-high molecular weight, linear
polyethylene as obtained from the polymerization process by means of an
enclosed bladed rotor rotating at high peripheral speeds in the order of
about 10,000 feet per minute in a vortex of air or other gaseous medium,
such as provided by the "Hurricane Mill" supplied by Bauer Bros. of
Dayton, Ohio.
Other means, though generally less efficient for making molding powder of
the present invention, include the purely mechanical-mills, such as hammer
mills. Such mills generally require repeated passage of the material
through the mill in order to reduce particle size to required degree of
fineness and to obtain narrow particle size distribution corresponding to
distribution function of less than 0.80. Similarly, molding powder
obtained by grinding in an air mill having required mean particle size of
less than 100 microns, but having distribution function in excess of 0.80
can be subjected to repeated passages through such mill in order to reduce
the distribution function to a value below 0.80.
As previously stated, the molding process of the present invention involves
compressing the molding powder under pressure of at least 2,000 p.s.i. at
a temperature below its crystalline melting point into a cavity having the
size and shape of the desired article to obtain a solid preform having
strength sufficient for normal handling. The requirement that molding of
the preform is accomplished at a temperature below the crystalline melting
point of the molding powder means that the molding powder is present in
the mold substantially in solid form, not in the melt, although if
desired, the mold itself may have higher temperatures, and even though
some of the molding powder may have such higher temperatures. This
requirement, however, makes it clear that the present process does not
rely on melt flow under pressure in order to form desired solid articles,
but that the preform obtains its solidity substantially solely from
cohesion of the molding powder particles, and consolidation into final
product occurs in the following free-sintering operation in the absence of
external pressure. The term "free-sintering" means subjecting the solid
preform to elevated temperature in the absence of externally applied
pressure without being confirmed in a mold. Green strength of preforms so
obtained will generally be sufficiently high so that these preforms can be
handled conveniently without need for onerous handling precautions. They
are strong enough to be transported and stored prior to the free-sintering
step, and to withstand the presintering step without deformation. The
preforms are then subjected to temperatures above the crystalline melting
point of the polyethylene, usually at least about 275.degree.F. for time
sufficient to sinter them into solid articles having the form and shape of
the preform and having tensile strength of at least about 4,000 p.s.i. and
elongation of at least about 200 percent. Tensile strength and elongation
are determined by ASTM Method D638-68. Suitable free-sintering
temperatures usually range from about 275.degree. to about 350.degree.F.,
preferably about 300.degree. to 320.degree.F. Temperatures above about
350.degree.F. are preferably avoided since they may cause discoloration
and/or sagging of the preform. In the free-sintering step it is essential
that the preform is brought to minimum sintering temperature (above the
crystalline melting temperature of the polyethylene, say at least about
1.degree.F. above that crystalline melting point) throughout, and is then
maintained at that temperature for time sufficient to allow sintering into
an article having the required tensile strength and elongation. In order
to bring the center of the preform to sintering temperature, it will
ordinarily be necessary to heat the preform for a period of 2 to 4 hours
per inch of thickness. In order to obtain uniform heating of the preform
throughout it will often be desirable, though it is not required, to heat
it slowly to desired final sintering temperature for example at a rate of
temperature increase of 20.degree.F./hr. to 150.degree.F./hr. particularly
if the preform embodies portions of substantial thickness. Similarly, it
will generally be of advantage to allow gradual cooling of the sintered
article from sintering temperature at similar rates, thereby minimizing
thermal stresses within the article. In order to prevent yellowing of the
polyethylene in the sintering operation, it is often preferred to conduct
that operation in a inert medium, e.g., in a nitrogen atmosphere.
Preform pressures should be at least about 2,000 p.s.i. Although it is
possible to obtain solid preforms at lower preform pressures, preforms so
obtained have high void content and upon sintering yield only articles
having inferior tensile and elongation properties. Preform pressures of
about 6,000 p.s.i. usually will result in articles having good tensile and
elongation properties, although preform pressures of 8,000 p.s.i. or
higher will yield article having slightly better properties yet. At
pressures above about 10,000 p.s.i. any improvement in properties will
generally be too small to justify the expense of using the type of
equipment required for obtainment of such pressures. Of course, there is
no upper limit to the preform pressure that may be employed, other than
that of economies in equipment design.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The Example below illustrates preferred embodiments and sets forth the best
mode for practice of the invention presently contemplated. It is not be be
construed as a limitation on the invention.
EXAMPLE
Ultra-high molecular weight, linear polyethylene having molecular weight of
2.8.times.10.sup.6 and density of 0.94, having mean particle size of about
320.mu. was milled to fine particle size in a Model 0202 Jet-O-Mizer air
mill manufactured by the Fluid Energy Processing and Equipment Company.
Air was supplied to the mill at a pressure of 95 p.s.i.g., a temperature
of 72.degree.F. and a rate of about 100 S.C.F.M. Polyethylene feed rates
of individual batches to the mill varied from 0.5 to 50 pounds per hour.
Feed rates were varied in order to obtain molding powder of different
particle size, higher feed rates resulting in larger mean particle size
and numerically greater distribution function. Molding powders so obtained
were formed into 2 inch diameter billets by subjecting them in a mold to
pressure of 6,000 p.s.i., holding the pressure for three minutes,
releasing the pressure, removing the billet preforms from the mold and
free-sintering them in an oven by subjecting them to programmed heating,
first from ambient temperature (about 70.degree.F.) to 225.degree.F. at a
rate of 150.degree.F./hr, followed by heating from 225.degree. to
325.degree.F. at a rate of 75.degree.F./hr, holding at 325.degree.F. for 4
hours and then cooling at approximately the same rates as employed for
heating. In each case, molding at 6,000 p.s.i. pressure yielded preforms
which could easily be handled without danger of breakage. Properties of
the free-sintered billets were determined on tape skived from the billets.
Table II below summarizes these experiments, showing polymer feed rate to
the mill, particle size and distribution function of the resultant molding
powder, as well as tensile strength and elongation of the free-sintered
billet. 1
TABLE II
__________________________________________________________________________
Polymer Particle
Distribution
% Retained on 100
Tensile Percent
Run No.
Feed Rate
Size Function
U.S. Mesh Screen
Strength
Elongation
__________________________________________________________________________
1 .5 pph 64.mu.
.04 -- 7400 psi
371
2 1 62 .06 1 7800 386
3 2 66 .10 4 6400 328
4 4 88 6000 220
5 25 150 39
6 2 44 .43 4 5600 311
7 4 75 .68 20 5200 274
8 8 66 .62 16 4500 256
9 25 123 .56 31 2900 117
10 50 150 .57 39 2700 103
11 .5 54 .17 1 8300 430
12 1 54 .15 2 8000 480
13 2 62 .10 11 6600 386
14 4 56 .53 5800 352
15 8
16 25 60 .58 10 3800 227
17 35 67 .60 13
18 50 116 .64 27 2650 104
19 1 58 .09 1 5900 362
20 2 8 6600 382
21 4 67 9 6000 368
22 8 64 .67 14 4300 271
23 25 87 .62 24 3600 223
24 50 106 .50 28 2900 34
__________________________________________________________________________
In Table II, runs designated 5, 9, 10, 17 and 22 resulting in particle size
of more than 100 microns represent comparative experiments.
The data in Table II indicate that, although there is some variation in
individual runs, molding powder in accordance with the present invention
having particle size of less than 100 microns can be free-sintered at a
temperature above its crystalline melting point (here at a sintering
temperature of 320.degree.F.) into solid objects having tensile strength
of at least 4,000 p.s.i. and having elongation of at least 200 percent.
Molding powder having particle size of more than 100 microns can be
free-sintered only to form inferior product having substantially lower
tensile strength and elongation. The data in Table II clearly demonstrate
the criticality of particle size required to obtain molding powder
free-sinterable into solid objects having acceptable tensile and
elongation properties.
It has further been found that, molding powders free-sinterable to form
solid objects having tensile strength of at least 4,000 p.s.i. and
elongation of at least 200 percent, must have distribution function of
less than 0.80 in combination with particle size of less than 100 microns.
Molding powder of ultra-high molecular weight polyethylene having particle
size of less than 100 microns, but distribution function in excess of 0.80
are not free-sinterable to obtain solid objects having tensile strength of
at least 4,000 p.s.i. and elongation of at least 200 percent, but yield
only products having lesser tensile and elongation properties which are
not commercially acceptable for uses in which ultra-high molecular weight
polyethylene articles are commonly employed.
In general, in the molding powders of the present invention tensile
strength and elongation of free-sintered articles made therefrom increases
with decreasing particle size. For that reason, preferred molding powders
in accordance with the present invention have mean particle size of less
than about 75 microns and more preferred yet, particle size of less than
50 microns. Size reduction to substantially below 50 micron mean average
particle size results only in relatively small increases of tensile
strength and elongation. Molding powders in accordance with the present
invention preferably have distribution function of less than 0.70 or, more
preferably yet, of less than about 0.50.
Fabrication of the molding powders of the present invention, and properties
of articles made therefrom may, if desired, be improved by incorporating
therein additives such as stabilizers or inert fillers.
To improve thermal stability during free-sintering operation, it is often
desirable to add one or more stabilizers to the molding powders, of the
type and in the amounts usually employed to improve thermal stability,
oxidation resistance and/or color stability of hydrocarbon polymers, such
as butylated hydroxy toluene, sterically hindered phenols, e.g.,
3,4,4'-thiobis-(2-tert.butyl-5-methyl phenol) and the like. Suitable
stabilizers are well known to those skilled in the art. Preferred molding
powders of the present invention contain added stabilizer. Stabilizer may
be incorporated into the molding powder by any method conventionally
employed for that purpose.
Suitable fillers include asbestos, pigments, glass, metal powders, abrasive
powders, graphite, polytetrafluoroethylene powder, especially if partially
degraded, and the like. Such fillers generally may be incorporated for the
purpose of improving such properties as resistance to creeping under load,
lubricity, wear resistance, stiffness, thermal conductivity, electrical
insulating properties and hardness. In general, fillers may be
incorporated in any amount up to about 40 percent, preferably up to about
30 percent by volume. In order to optimize properties of the molded
product, the filler should be of fine particle size and of narrow particle
size distribution, similar to those of the polymer component of the
molding powder. In preferred operation filled molding powders of the
present invention are made by co-milling polymer and filler.
It will be apparent that many modifications and variations can be effected
without departing from the scope of the novel concepts of the present
invention and the illustrative details disclosed are not to be construed
as imposing undue limitations on the invention.
* * * * *
|
|
 |
|
 |
Description |
|
