CA1047721A - Adjusting outer polymer layer viscosity by cooling in screw extruder - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Granular homo- and co-polymerized olefins especially suitable for foil extrusion processing are provided by melting the homo- or copolymers, homogenizing the melt and extruding the polymer while adjusting the volumetric leakage flow in the discharge zone by means of regulating the viscosity of that portion of the outer layer by cooling of the polymerizate which is subject to leakage flow, to a constant value.

Description

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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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1~4772.i 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.

. : . . . .: ~ : . -~1477Zl 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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)477;~ -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 '_g_ .

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1q~477Zl 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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1(~477Zl 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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)47721 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

3 about 40 000 80- 85

4 about 10 000 55- 60 , :

l''J477Zl 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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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a C2 to C5 olefin homopolymer or copolymer suitable for granulation and extrusion into film, which comprises charging the polymer into a screw extruder press having a charging zone, a plastifica-tion zone, and a discharge zone, converting the polymer into a melt at 150 to 350°C and homogenising the melt in the plastification zone, and adjusting the viscosity of the outer layer of that portion of the polymer melt which under-goes 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.

2. The process of claim 1 in which the polymer is converted to a melt at about 180 to 220°C.

3. The process of claim 1 or 2, in which the cooling means comprises cooling channels positioned uniformly about the discharge zone which is substan-tially cylindrical in shape, the channels being aligned parallel to the axis of the discharge zone and being supplied by liquid coolant.