CH620635A5 - Process and device for the continuous extrusion and blowing of thin films of plastic material, especially of rigid PVC - Google Patents

Abstract

The process and the device in question differ from the known ones in that they permit the working of the PVC in powder form at very low temperatures, while achieving a high degree of plasticity even with a limited percentage of stabilisers and slip agents. Thin films having a thickness of 10 mu to 60 mu are obtained by continuously introducing the raw material in the form of powder, after intense mixing with stabilisers and slip agents, into the feed aperture of a single-screw extruder. Said single-screw extruder exhibits 7 different zones: an inlet zone (E) preferably without heating; a compression zone (K1) with slight heating; a heated homogenisation zone (H), a heated decompression zone (D); a heated degassing zone (G); a heated compression zone (K2) and a heated expulsion zone (A). <IMAGE>

Description

The present invention relates to a process and a device for the continuous extrusion and blowing of thin films, in plastic material, particularly of rigid polyvinyl chloride (PVC), having a thickness from 10 p, up to 60 p. by continuously introducing the raw material in powder form, after intense mixing with stabilizers and glides, into the introduction opening of a single-screw extruder.

The process in question differs from the known ones in that it allows the powder material to be processed at very low temperatures, while achieving a high degree of plasticity even with a limited percentage of stabilizers and glides.

The production of PVC films is currently in continuous development. The goal to be reached is the increase in the passage through extruders operating continuously, and the safety of the processing, guaranteeing a high level of quality with lower costs.

PVC (polyvinyl chloride) is thermally sensitive from 100 ° C. Above this temperature it is increasingly damaged by the separation of hydrochloric acid.

For granulation, mass temperatures of around 175-190 ° C are required, and for processing (extrusion) temperatures of around 185-220 ° C. Granulation and extrusion are possible only by adding stabilizers and glides to the mass, which are partially consumed during thermal stress.

Said stabilizers and glides if in excessive quantities can adversely affect the quality of the product. With the elimination of granulation, i.e. using powdered material, it undergoes a high thermal stress only once, that is, during extrusion alone. Therefore discolorations, crosslinks and other defects can be reduced or avoided by resorting to powders.

The quantity of additives, that is, of stabilizers and glides, can also be decreased if granulation is avoided, so that in addition to a better final product, there is also an economic advantage, since these additives are very expensive.

However, the heating of the rigid PVC powder in the extrusion machine presents difficulties, since the permitted temperature range is very limited, both for the resistance opposed to the flow of the material to be laminated, and for the limited thermal stability.

Local overheating may occur particularly in those parts of the extrusion machine which have low thermal conductivity, which lead to thermal damage to the already pre-stressed mass.

The processing of rigid PVC in extruders is also very limited by the low thermal stability of the mass. The request for the lowest possible thermal stress is opposed to obtaining products that are qualitatively flawless through high processing temperatures and therefore low flow resistance. High mass temperatures are only possible for short periods of time so that the mass is not damaged.

The thermal decomposition of PVC during processing is characterized as mentioned, by the separation of hydrochloric acid, by self-oxidation and cross-linking. Low thermal damage that does not yet affect the extrusion process can instead cause discoloration, as well as through the decrease in mechanical qualities, the unusability of the product. Strong thermal damage can be seen in the form of strips which are first of a light brown color and then turn dark brown before the extrusion of completely charred pieces is achieved. Once the outbreak is born, part of the damaged mass remains attached to the metal surface of the extruder and causes an extension of the so-called "burns" over the whole mass.

When the aforementioned strips appear, production must generally be stopped. After dismantling the die and screw, all parts that come into contact with the mass must be cleaned carefully. After restarting the extruder, it takes hours, depending on the size of the machine, to arrive at the normal production state, so that said assembly and cleaning works cause a considerable waste of time and therefore of production.

The process and the device according to the present invention allow to overcome the difficulties mentioned, making possible the optimal processing of rigid PVC powder with single screw extruders and the production of non-toxic and thin-walled films of uniform thickness.

The process in question is, as mentioned, characterized by the fact that the raw material, in the form of powder, is continuously pre-compressed, in the cold state, into the extrusion screw and further compressed (Kx) through said extrusion screw and after this it is plasticized under heating and after a homogenization phase (H) it is conducted through throttling openings which determine the pressure increase in said homogenization phase (H), after which it is decompressed (D) and degassed in a phase of elimination of the pressure (G), then increased again the pressure (K2) and passed to the extrusion, adjusting the thickness

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of the film, by varying the pulling speed and the diameter of the tubular.

The advantages of the process in question consist in the fact that too long heating of the material and therefore its carbonization is avoided. This is due not only to the 5 subdivision in a plasticizing zone and a homogenizing zone but also in the relatively short stress of the material in the plasticizing zone.

Preferably the temperature in the extruder remains below about 190 ° C. 10

The invention will now be better explained below in an example of embodiment of the device, i.e. of the extruder, which allows to carry out the process in question.

The drawings represent:

in fig. It is an extruder according to the invention which is strongly semi-automated in longitudinal section;

in fig. 2 a section along line II-II of fig. 1 on an enlarged scale;

in fig. 3 the longitudinal section of the central area of the extruder according to fig. 1 on an enlarged scale, at 20 of the degassing zone adjacent to it;

in fig. 4 the section along line IV-IV of fig. 3 through the degassing pump on an enlarged scale;

in fig. 5 the partial section V-V of fig. 5 rotated 90 ° through the piston of the degassing pump, on an enlarged scale;

in fig. 6 the diagram of the stroke of the degassing pump rod, as a function of time.

The extruder generally indicated in fig. 1 with the reference number 10 has the extrusion screw 12 housed in the extruder cylinder 14. From a feed hopper 16, the material to be processed, i.e. the resin powder mixed with stabilizers and glides, is fed through the feed screw 20 (fig. 2) arranged in the channel 18, to the extrusion screw 12 shown in fig. 1, passing through the inlet opening 24 (fig. 2).

The extrusion screw 12 has 7 different areas (compare figs. 1 and 3).

In the area of the material feeding hopper 16 there is the introduction area E. In this area the extrusion screw 12 has an almost constant external, internal diameter and pitch and is preferably without heating. The introduction zone E is followed by a first compression zone K, which is slightly heated, in which 15 screws have a constant external diameter, an increasing internal diameter (conical core) and a constant pitch. The first compression zone Kt is followed by a heated homogenization zone H in which the screw has a constant internal and internal diameter and pitch. The homogenization zone H is followed by a heated decompression zone D, in which the screw has a constant external diameter, a constant internal diameter, but reduced compared to the previous diameters and a constant pitch.

The degassing zone G is followed by a second heated compression zone K2 in which the screw has a constant external diameter, an increasing internal diameter (conical core) and a constant pitch.

The second compression zone K2 is followed by a heated expulsion zone A, in which the screw has a constant external diameter but limited to a certain length, an internal constant diameter for the same length and tapered at the end near the extrusion head, from which the perfectly plasticized mass comes out in tubular form and is thinned by blowing the tubular in a radial direction and stretching in an axial direction.

In the diagram in fig. 6, as already mentioned, the stroke in millimeters of the degassing pump rod as a function of time in seconds is shown.

As can be seen, the piston stroke occurs very quickly at the time tj - it remains constant in the time between tx and t2 and abruptly returns to the starting point at time t2 -. The same is true for times t3

and t4.

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Claims (2)

620 635 2 CLAIMS 1. Process for the continuous extrusion and blowing of thin films in plastic material, particularly of rigid polyvinyl chloride, having a thickness of 10 [x up to 60 (i by continuously introducing the raw material after mixing with stabilizers and glides, in the feed opening of a single-screw extruder, characterized in that the raw material in the form of powder is fed continuously in the cold state, pre-compressed in the extrusion screw and further compressed (Kt) through said extrusion screw, and after which it is plasticized under heating and after a homogenization phase (H) it is conducted through throttling openings which determine the pressure increase in said homogenization phase (H), after which it is decompressed (D) and degassed in a phase of elimination of the pressure (G), then again increased the pressure (K2) and passed to the extrusion, adjusting the desired thickness of the film, by varying ion of the towing speed and the diameter of the tubular tire. 2. Device for carrying out the process according to claim 1, characterized in that it comprises a single-screw extruder continuously fed by a feeding screw (18, fig. 2) which supplies cold powder under pressure, mixed with stabilizers and glides; said single-screw extruder having 7 different zones: an introduction zone (E) in which the extrusion screw has an almost constant internal diameter and pitch; a heated compression zone (K ^), in which the screw has a constant external diameter, an increasing internal diameter and a constant pitch; a heated homogenization zone (H) in which the screw has a constant external, internal diameter and pitch; a heated decompression zone (D) in which the screw has a constant external diameter, a decreasing internal diameter and a constant pitch; a heated degassing area (G) in which the screw has a constant external diameter, a constant internal diameter but reduced compared to the previous diameters and a constant pitch; a heated compression zone (K2) in which the screw has a constant external diameter, an increasing internal diameter and a constant pitch; a heated expulsion zone (A) in which the screw has a constant external diameter but limited to a certain length, a constant internal diameter for the same length and tapered at the end near the extrusion head.