PROCESS AND DEVICE FOR THE EXTRUSION OF POLYMERS WITH HIGH PERCENTAGE OF INERT MATERIALS

Abstract: Process for the extrusion of polymers with a high percentage of inert materials; being meant with such an expression percentages from 35-40 % by weight to 70-80 % by weight on the total, including the stages of: feeding of precisely measured amounts of the components and the additives in axially distanced points of a screw mixer; intimate mixing of said components and their extrusion through the extrusion head of said mixer. Said process is carried out through a device including an extruder (1) for the transformation of a mixture consisting of at least one polymer, an inert material and one or more additives such as lubricants, stabilizers or else, means (2, 3, 4) (2, 3, 4) for the ensiling of each of said mixture components, devices (21, 31, 41) for the continuous weighing of each of said components and means (6, 8, 9) for the feeding of said components. Such feeding means (6, 8, 9) are positioned in prearranged points along the extruder (1) according to the composition of the mixture to be transformed.

Description: TITLE : Process and device for the extrusion of polymers with high percentage of inert materials. DESCRIPTION: The present invention refers to a process and a device for the extrusion of polymers, such as for instance PVC, having high percentage of inert materials, such as for instance carbonates, talc, silica, wood dust, glass, etc., being such extrudates obtained without using intermediate processes and then with considerable energy saving. Actually, to obtain polymeric resins, with high percentage of inert materials, they have to be subjected to several transformation treatments before they can be extruded to obtain the desired polymeric profiles or manufactured products. First of all it is necessary a vigorous mixing of the polymers resins with additives such as stabilizers, lubricants, shockproofers , as well as with inert materials (carbonates, silica...) in order to improve the mechanical and/or physical characteristics and, additionally or alternatively, to control the costs. The mixing or turbomixing process, requires expensive plants and remarkable amount of energy (0.3 to 0.5 k pro Kg of product to be mixed) . Moreover, the overheating of the mixture, caused by the friction but essential for this type of known mixing, is detrimental to the further transformation and it is necessary to keep it down through a slow mixer, provided with a powerful cooling system (entailing other energy costs besides the ones necessary to heat it) . The percentage of inert materials on the mixture is very important for the further processing of the mixture. In fact if such percentage does not exceed approximately the 20% on the total of the mixture components it is possible, by means of special two screws extruders, to directly transform the mixture, in a very difficult way, into finished product. If the inert materials percentage exceeds this limit but remains within approximately the 40% of the total it is necessary an intermediate processing called "granulation" entailing further energy costs. The mixtures having percentages of inert materials higher than about 40% of the total cannot be used anymore since they cannot be managed by the extruders, which, afterwards, should gelate said mixture to transform it into granules which in turn will be transformed into finished product or, in the even less easy to realize hypothesis, gelate said mixture and directly transform it into finished product . Another problem connected with the mixing of a resin with inert materials is the uncertainty that the elements forming the mixture are equally and homogeneously distributed in it because of the granulometries and the completely different specific weights, and such uncertainty is increased by a de-mixing due to the necessary, mechanical, pneumatic or some other moving ways, to the production units . The aim of the present invention is then to overcome the drawbacks of the known technique previously mentioned by means of a device for the forming and the extrusion of polymers in just one working stage with high percentages of inert materials, being meant with such an expression percentages in the range to 70 - 80% of the total of the mixture, including an extruder for the transformation of a mixture consisting of at least one polymer, an inert material and one or more additives such as lubricants, stabilizers or else, means for the ensiling of each of said mixture components, devices for the continuous weighing of each of said components and means for the feeding of said components, being such feeding means positioned in provided points along the extruder according to the composition of the mixture to be transformed. According to one aspect of the invention the extruder used for the transformation of the mixture into profiles or manufactured products is a co- rotating double screw extruder, that is to say with the two screws rotating in the same direction, characterized by length and geometry of the screws, as well as number and position of the feeding points of the mixture components, adapted according to the mixture to be extruded. The manufactured products obtained through the device of the present invention are characterized as said before by high percentages of inert materials, for instance up to 80% of the total of the starting mixture. With the treatment process according to the present invention two stages are in fact avoided: the turbomixing and the granulation, as said before very expensive for plant engineering and energy consumption and at the same time it is guaranteed a precise scattering of the mixture components, giving the profiles or manufactured products obtained by the extruder such mechanical char- acteristics, under the same ratio polymers/inert materials, which cannot be reached with the known present techniques, since such techniques don't guarantee a suitably precise and intimate mixing. The present invention will be better understood by the following description of an embodiment form, to be considered as a not limitative illustration and with reference to the one enclosed drawing, where : Fig. 1 is a side view of the device for the extrusion of the polymers with high percentage of inert materials according to the present invention. In Fig. 1 it is shown a double screw extruder 1 fed with a polymeric mixture composed by a polymer, the PVC for example, an inert material, as calcium carbonate CaC0 3 , and additives, it is to say lubricants, stabilizers or else. These mixture components are contained in their storage bins to keep them separated one from the other before they are introduced into the extruder 1, and precisely: the bin 2 for the storage of the calcium carbonate, the bin 3 for the storage of the PVC and the bin 4 for the storage of the additives. All these bins are supported by a suitable supporting structure 5 and feed devices for the continuous weighing of the mixture components; these devices are: the device 21 for the continuous weighing of the CaC0 3 , the device 31 for the continuous weighing of the PVC and the device 41 for the continuous weighing of the additives; the supporting structures (not shown) of said weighing devices 21, 31, 41 are preferably isolated from the supporting structure 5 of the bins 2, 3, 4, so that said weighing devices don't feel the effects of the vibrations which the bins 2, 3, 4 are subjected to. Moreover it is necessary that during the feeding, the weighing devices 21, 31, 41 don't come into contact with their bins because their precision on the weighing of the different components of the mixture could be disad- vantageously changed. The CaC0 3 continuous weighing device 21 feeds a hopper 6 positioned in provided point of the extruder 1 and, optionally, can feed through a conveyor 7 a further hopper 8 for the feeding of the extruder, providing for an introduction of calcium carbonate into the extruder 1 in two different points and so ensuring an intimate and uniform mixing of the PVC and the calcium carbonate. The PVC and the additives are premixed and introduced into the extruder 1 through the hopper 9, positioned upstream of the hoppers 6 and 8. The extruder 1, as mentioned before, has a co-rotating double screw, that is to say that the two screws rotates in the same direction and can be characterized by different pitches between the teeth and different lengths in order to get a treatment of the polymeric mixture as uniform as possible. The shaped or manufactured product with high percentage of inert materials, for example up to 80%, coming out of the mouth 10 of the extruder 1, can be further treated for example by other two extruders to obtain for example plates formed by a central part in PVC with inert materials, having a thickness of, for example, 2 to 4 mm, and by surface protective and ennobling layers, such as an upper layer in PMMA having a thickness of, for example, 0.2 mm and a lower layer in shock-proof PVC having a thickness of, for example, 0.4 mm. As a not limitative illustration it is here reported a composition referred to 100 parts of polymeric mixture to be extruded: Calcium Carbonate (CaC0 3 ) 65 parts Polyvinyl Chloride (PVC) 30 parts Primary Stabilisers 3 parts Inner Lubricants 1 part Outer Lubricants 1 part The polymeric manufactured products or profiles with high percentage of inert materials obtained through the device of the present invention has excellent mechanical properties, such as a remarkable stiffness with a very low bending modulus and excellent physical properties, such as a remarkable dimensional stability and a restrained linear expansion due to the temperature ranges. Said manufactured products can have a considerable length and don't require special adjustments to compensate the dilations which, as well-known, represent one of the greatest problems in the realization of extruded with average or high quantity of inert materials and which in many cases restrict their use. Besides these mechanical and physical characteristics the manufactured products of the present invention show remarkable economical advantages since a good inert material like the calcium carbonate has a cost definitively lower, even of a tenth, than the cost of the polymer. The use of a device for the extrusion of polymers with high percentage of inert materials according to the present invention, finally, allows to realize manufactured products which can be util- ized for different uses such as covering plates, roof gutters etc., moreover the scraps of the polymers coming from other processings or from the invention process itself, can be put back in the cycle in the device and valued for their percentage of polymers included and not of inert materials, so revaluing the scraps themselves .

Claims: CLAIMS 1. A process for the extrusion of polymers with a high percentage of inert materials, being meant, with such an expression, percentages from 35-40% by weight to 70-80% by weight on the total, including the stages of: feeding of precisely measured amounts of the components and the additives in axially distanced points of a screw mixer; intimate mixing of said components and their extrusion through the extrusion head of said mixer. 2. A device for the extrusion of polymers with a high percentage of inert materials for the realization of the process according to claim 1, including an extruder (1) for the transformation of a mixture consisting of at least one polymer, an inert material and one or more additives such as lubricants, stabilizers or else, means (2, 3, 4) for the ensiling of each of said mixture components, devices (21, 31, 41) for the continuous weighing of each of said components and means (6, 8, 9) for the feeding of said components, being such feeding means (6, 8, 9) positioned in prearranged points along the extruder (1) according to the composition of the mixture to be transformed. 3. A device according to claim 2, character- ized in that the polymer and the additives materials, taken from their bins (3, 4) and weighed by means of their continuous weighing devices (31, 41) , are premixed and introduced into the extruder (1) through the same feeding hopper (9) . 4. A device according to claim 2, characterized in that it includes two hoppers (6, 8) for the feeding of the inert material positioned along the extruder (1) at a prearranged distance one from the other, being the first (6) of said hoppers directly fed by the continuous weighing device (21) of the inert material and the second one (8) of said hoppers fed by said device (21) by a conveyor (7) . 5. A device according to any preceding claim, characterized in that the extruder (1) is of the double screw type. 6. A device according to claim 5, characterized in that the two screws of the extruder (1) are co-rotating. 7. A device according to claim 5, characterized in that the geometry of the screws of the extruder (1) is differentiated to obtain a treatment of the mixture as uniform as possible. 8. A device according to any preceding claim, characterized in that said ensiling means (2, 3, 4) are fastened to a supporting structure (5) isolated from the supporting structure of said continuous weighing devices (21, 31, 41) . 9. A device according to any preceding claim, characterized in that the manufactured product coming out of the mouth (10) of the extruder (1) is further subjected to other extrusion or co- extrusion treatments to obtain finished products such as multi-layer plates or else. 10. A device according to any preceding claim, characterized in that the percentage of inert materials introduced into the mixture takes up values to 80% of the mixture total. 11. A device according to any preceding claim, characterized in that the polymers scraps coming from other treatments or the device itself can be recycled in said device.

Inventors: ZAIDI HAJER BENT-MAHFOUDH, ZAIDI, HAJER, BENT-MAHFOUDH
Applicants: TESSA TRADING LTD, ZAIDI HAJER BENT MAHFOUDH, TESSA TRADING LTD., ZAIDI, HAJER, BENT-MAHFOUDH
Application year: 1999
Main classification: B29C47
IPC codes: B29C47