Process and apparatus for recycling of synthetic plastics materi

Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Measuring a weight or volume

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Details

264102, 264916, 425143, 425145, 425289, B29C 4792

Patent

active

058825584

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for recycling of synthetic plastics material containing gas, for example foamed polystyrene, in a plant in which synthetic plastics material is molten, filtered, degased, granulated and mixed with gas again, this gas re-introduction being performed in the same plant as the granulation. Further, the invention relates to an apparatus for performing such a process.
The usual process steps when re-introducing gas into foamed synthetic plastics material, in particular polystyrene, consist in that the synthetic plastics material is comminuted in a tearing and densifying apparatus and is then plasticized in an extruder and de-gased. The so produced plasticized, de-gased synthetic plastics material is worked up to granules in a granulating apparatus and is solidified thereby. The granulate forms a pourable homogeneous, de-gased mass which then is introduced again into an extruder and is molten therein. During the extrusion which can be performed by means of extruders comprising one or more extruder screw, a gas is fed in a controlled manner to the extruder wherein gas is introduced within the extruder into the synthetic plastics melt and is homogenized therewith, considering the necessary holding time in the extruder. When doing this, the use of a pourable homogeneous granulate is compulsory necessary because only in such a manner a continuous flow of the melt in the extruder and therefore a continuous filling degree of the foaming gas in the melt can be ensured.
A similar process has become known from the document "Plastver-arbeiter", volume 42, 1991, number 6, pages 124, 125. This document suggests to mill packages of polystyrene and subsequently to heat this material in a roller extruder so that the material is de-gased and plasticized. The thus obtained solidified final product, namely cristallized polystyrene, is then foamed again in an oxtruder.
These known processes have the disadvantage that two melting steps are necessary which, as a rule, have to be carried out at different places, because the homogeneous introduction of the gas is a critical thing and, therefore, cannot be made everywhere. Therefore, as a rule, the synthetic plastics material to be recycled is granulated there where an apparatus for comminuting and densifying the material is at disposal. The granulate is then sent to a plant where the material is provided with a gas content again. This involves costs for transport and transformation which lead to a high price of the recycled and gas-containing material so that the thus obtained recycling product can hardly be sold. The consequence is an environmental load by foamed synthetic plastics material scrap or by improper processed synthetic plastics material. In this connection it is of detrimental influence that in the past frequently such gases have been used as foaming gases (for example fluorocarbons) which are critical for environmental reasons and, therefore, should not get into the atmosphere.
It is also known to melt and degas scraps of foamed thermoplastic synthetic plastics material in an extruder comprising several worms. In parallel to this extruder a further extruder is provided in which thermoplastic particles, hence new material, are molten and mixed with gas. The outlets of the two extruders are combined to a mixing and cooling station to which a further extruder as well as a mould are connected, in which the material is foamed. Finally, the foamed material is cut to the desired shape or is granulated. For performing this process, a considerable effort in apparatus and space is required and the above described disadvantages can not fully be avoided by this process, because the granulate of new material must at first be produced and then be stored so that the enthalpy is lost which was in the new material until its granulation.
The invention has as its object to improve a process of the last described kind so that the recycling process of the gas-containing synthetic plastics material can be effected quic

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