Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a process for
granulating homo- or copolymerizates of olefins having
2 to 5 carbon atoms, thus rendering the polymers
suitable for preparing foils and strips.
Stretched polyolefin foils and strips are widely
used in many technical fields. Foils are increasingly
important in the field of pac~aging for the building
industry and agriculture, while strips in woven form
are especially desirable for the manufacture of bags.
Foils are formed by extrusion and strips are prepared
by straight line slitting of the foils. They are of
rectangular cross section and low thickness. The foils
and strips possess high strength at low weight due to
the stretching procedure employed.
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The foils and strips are prepared from granulated
starting materials by extrusion of blown tubing or with
the aid of a slot die, the resulting foils being
subsequently stretched to enhance their strength.
Strips are prepared by stretching and subsequentiy
slitting the foils to strips of desired band width
or by slitting the foils to strips and subsequently
stretching the latter. Stretching of ~ ns is performed
by means of roller stretching machines,while strips are
stretched by means of godets in hot-air channels.
The temperature~prevailing during the above-
mentioned working steps depend upon the material to be
processed and lie just below the c,rystall,ite melting
range. Temperatures of 100 to 125C are for instance
stated for polyethylene and temperatures of 135 to 165C
for polypropylene. The amount of stretching of polyethylene
is rom about 1 : 7 to 1: 8, and for polypropylene from
about 1 : 8 to 1 : 15. The stretching operation causes
some of the chain molecules to be oriented in the
longitudinal direction, increasing the tensile strength
in this direction to a multiple of the original value
but correspondingly decreasing the elongation at break.
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The stretched foils and strips may be reeled off
and further used without thermal after-treatment. However,
if the materials are to be exposed to temperatures above about 70C,
a further f~ionheat treating step is necessary in order
to prevent shrinkage of the foils and strips. The
fixation is effected by means of devices used for
stretching. It may however be carried out at temperatures
ranging between 10 to 30C below the stretching temperatures.
After stretching and fixation the foils and
strips are cooled. While the foils may be reeled off
immediately, the strips must be aligned by combing
in order to prevent crossing.
But in processes such as the foregoing a universal
problem exists in that the technical design
commercially available extrusion devices allows the
processing of polyolefins only with low through-put rates.
At increased throughput rates inferior products are
obtained, as for instance foils with rough surfaces.
In contrast, low through-put
of the plastic material in stretching devices considerably
increases the tear sensitiveness of the strips.
As a consequence,difficulties arise during the
continuous production of foils and strips by extrusion
and subsequent stretching, causing material losses and
precluding economical performance of the process.
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Accordingly, an object of this invention is
the provision of homo- and copolymerizatés of olefins
having 2 to 5 carbon atoms which do not exhibit the
foregoing drawbacks and which can be continuously
extruded with high through-put rates and stretched
in corresponding devices.
- Surprisingly, it has now been found that desired
working properties can be imparted to the plastic
material by defined measures during the granulation
of raw plastic materials obtained by the corresponding
synthesis processes.
- According to the intended use of the foils and
strips, the starting materials are-homo- and copolymerizates
of ethylene, propylene and higher olefins having about
2 to 5 carbon atoms, obtained by high-pressure or low-
pressure polymerization processes.
~ w~ng to the conditions of the polymerization process
by which it was prepared, the starting material polyolefin
is obtained as a powder made up of small particles or
agglomerates of small particles. Easily dosable compact
part~cles,being free of occluded air, are desirable for
the processing by means of extrusion devices. Accordingly,
the polymerizates are generally made up asa granulate
having high apparent density by melting it in continuously
working screw extruders, pressing the viscous mass
through orifices and cutting the strands.
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The screw presses essentially comprise a horizontally
arranged steel cylinder, surrounding one or several
screw conveyors. The steel cylinder has a charging
op~ening and a discharge opening, the latter being
connected with a mold through which the extrudate flows.
The raw material is conveyed through the screw press
to the discharge end and extruded through the mold
under the pressure built up in the screw press.
Generally, the cylinders are divided into three
separately heated and differently designed zones; a
charging zone, a plastification zone and a discharge zone.
The charging zone takes up, compresses, conveys and
often preheats the plastic material to be extruded.
In the plastification zone it is subsequently me~ted,
fEeed of occluded gas, homogenized and uniformly
heated. The discharge zone takes up the material
exiting from the plastification zone and extrudes
it under the necessary pressure in uniform flow through
the connected mold.
The moae of operation of screw presses is
essentially determined by the geometrical design
of the screw, the screw speed and the temperature
profile over the length of the screw. These values
can be varied according to the individual conditions.
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The single screw extruder conveys the mass
to the discharge zone owing to its friction at the
cylinder wall. This forward motion, the so-called
'volumetric drag flow" is counteracted by the volumetric
- "pressure flow" along the channels and the volumetric
"leakage flow" across the flights over the space of
the screw. Although this diminishes the effective
conveying capacity of the screw press, it greatly
enhances the homogenization efficiency of the screw.
The pressure flow is a function of the geometrical dimensions
of the screw threads, the pressure and the viscosity of
the melt. The leakage flow depends upon the geometrical
dimensions of the gap between the land of the screw
and the-cylinder, the pressure conditions and the viscosity
of the melt. ..
The multiple screw extruder on the other ~and: -
operates with mating screws, i.e. screws being engaged
with each other causing forced conveyance. The effective
conveying capacity is comprised of the partial volume
of the conveyed material, and is limited by the threads
and the generated surfaces of the screw threads, the
generated surfaces of the screw roots and theinterior
walls of the cylinder, as well as the leakage flow.
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It has now been found, that the leakage flow, hereinbefore consider-
ed as substantially uncontrollable and undesirable, is of essential signifi-
cance for the realization of distinct properties of the product.
Accordingly, the invention comprises a process for preparing a C2 to
C5 olefin homo- or copolymer, especially low pressure ethylene homo- or copoly- - -
merizates, suitable for granulation and extrusion into foil and strip, which
comprises charging the polymer into a screw extruder press having a charging
zone, a plastification zone, and a discharge zone, converting the polymer into
a melt at 150 to 350C and homogenising the melt in the plastification zone,
10 and adjusting the viscosity of the outer layer of that portion of the polymer ~-
melt which undergoes backward leakage flow across the flights of the screw in -
the discharge zone, by cooling means provided in the discharge zone, so that
the leakage flow in the discharge zone is maintained constant at a desired -~
value determined experimentally. Preferred melt conversion temperatures are
fro~ 180 to 220C.
Since the adjustment of the leakage flow to a determined constant
value is indispensable for the performance of the process according to this
invention, the novel procedure requires apparatuses which allow control of
parameters influencing the leakage flow constant during the entire working
time, thereby enabling accurate leakage flow control.
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As stated above, the leakage flow is
a function of the geometrical design of the device, the
pressure conditions and the viscosity of the melt,
the latter being dependent On the character of the
polymerizate and the temperature~Having the benefit
of the present disclosure, it is-merely a matter of
determining which parameters best suit the particular
polymer. Such determination is easily within the
ambit of expertise of a skilled chemist.
In conventional extruders, the geometrical design
and the pressure conditions are considered as being
constant, while the temperature conditions may be variable
in certain ranges according to the construction of the
particular device. Owing to the temperature control within -
a certain range, continuous fluctuations
in temperature at the interior surface of the cylinder,
and consequently in the outer layer of the material
sub~ect to the leakage flow,arise during the process.
It is an essential characteristic of the process
according to this invention, that the temperature in the
discharge zone is not regulated by means of a heating-
cooling- system, but that a sufficient amount of heat
is added to the material in the upstream plaatification
and homogenization zones, so that the melt need only
be cooled,if desired. Since constant temperatures may
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easily be obtained by simple cooling, large temperature fluctuations at the
interior wall of the cylinder created by heating-cooling-systems can be avoid-
ed. - -
With adequate cooling of the discharge zone it is possible by means
of continuous adjustment of the cooling conditions, to attain a constant temp-
erature at the interior surface of the cylinder. Consequently, the surface
layer of the plastic material adjacent to the cylinder wall remains substanti-
ally constant in temperature. The novel process requires devices in which
the material is delivered to the discharge zone in the fully plasticized state,
being constructed in such manner, that constant temperature conditions can be
maintained at the entire interior surface of the cylindrical discharge zone.
The cooling means for the discharge zone advantageously comprises a water-
cooling system around the cylinder, consisting of cooling channels, parallel-
ly arranged with respect to the cylinder axis and uniformly jacketing the cy-
linder.
Since an intensive heat exchange takes place between the cylinder
surface and the thin outer layer of the plastic material subject to the leak-
age flow, a control of the leakage flow is possible within wide limits.
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The amount of the leakage flow in the discharge
zone is a characteristic value for the respective
polymerizate or even for a certain fraction of a poly-
merizate. It cannot be evaluated by technical considerations,
but must be determined by experimental tests well within
reasonable skills. It is therefore necessary to test
the'workability of the respective polymerizate by
means of a testing device dependent upon the volumetric
leakage flow in order to determine the optimal value
for any polymerizate to be processed.
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The treatment of the olefin polymerizates according,to
the invention results in a decrease in molecular orientation
so that the proper orientation of the molecules occurs
predominantly during the stretching process.
With the novel process any olefin polymerizate
or copolymerizte having 2-5 carbon atoms suited for
the preparation of foils and strips can be worked,
especially polyethylenes, polypropylenes, copolymerizates
of ethylene and propylene with each other and/or with
and C5 olefins. The olefin polymerizates are generally
used in pulverulent form? i.e. in the form of powder
directly obtained by the polymerization, but it is also
possible, to subject conventionally granulated starting
material to the treatment according to this invention.
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Pulverulent low-pressure polyethylene was converted
to a granulate in a granulation device having charging,
plastification and discharge zones. The amount of
volumetric leakage flow was varied by removing
heat at differing rates from the discharge zone.
The results are tabulated below as Tests 1-4.
In -the tests, 2.5 t/h pulverulent polyethylene
respectively were used.
T A B L E
Test No.Removal of heat in the Polyethylene throughput
discharge zone in the extrusion device
~, k cal/~ kg/h
1 about 100 000 about 60
2 70 000 7Q- 75
3 about 40 000 80- 85
4 about 10 000 55- 60
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From the tests it can readily be seen that the workability of the
plastic material in the extrusion device depends upon the amount of heat re-
moved from the discharge zone. This amount is in direct relation to the magni-
tude of the volumetric leakage flow. It can be seen from the above Table
that, as heat is removed, the polyethylene throughput reaches a maximum value.
The most favourable value is determined by testing the granulated material in
an experimental plant. This value can then be transmitted to the proper pTO-
duction plant. The optimal conditions for processing different olefin poly-
merizates and copolymerizates are determined in similar manner.
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