PROCEDURE FOR THE PRODUCTION OF RECYCLAT ABSTENTIONS, EXPAND-CASH STYRENE POLYMERS

Expandable styrène polymers comprising recycled material The present invention relates to expandable styrene polymers comprising recycled material which can be converted into foams having an improved foam structure. Plastics based on styrene and styrene-containing copolymers are produced in large quantities and are used in many areas of industry. Considerable importance is also attached to foamed products. The production and use of these products have been known for some time and have been described in a variety of publications. A problem which has become particularly important in recent years is the recycling of used plastics. In the case of polystyrene and polystyrene foams, various recycling methods are known. One possibility comprises dissolving used polystyrene in monomeric styrene and re-polymerizing this solution by known methods. Processes of this type are described, for example, in us_A-5,269,948 and JP-A-56-60096. Another possibility comprises using polystyrene foam or unfoamed polystyrene waste for the extrusion of polystyrene foams. This recycling material can be used either alone or as a mixture with fresh polystyrene. The polystyrene foams to be recycled should be compacted or ground before processing. This can be carried out mechanically, for example by pressing or grinding, but preferably by extrusion. The production of foams from the recycled materials can then be carried out directly, as usual, by steam treatment of the polymer in the extruder with subsequent foaming to give boards or by extrusion to give expandable minigranules. It is also possible to produce minigranules which are impregnated with blowing agent in a second step. All the known methods have the disadvantage that the foam structure of the foamed styrene polymers produced using recycled material is unsatisfactory. Cells are usually large and irregular, and voids are formed. These disadvantageous effects are caused, for example, by dirt particles {or additives) in the recycled material which cannot be removed completely even by melt filtration. A further problem which adversely affects the use of styrene-containing copolymers is the effect of the other polymer components. There has been no lack of attempts in the past to suppress these adverse effects and to prepare expandable styrene polymers containing recycled material whose foam structure and mechanical property level are just as good as those made from fresh polymer. Thus, nucleating agents, such as finely divided water or talc, have been added to the styrene polymers. Although this resulted in a more homogeneous foam structure, metering and homogeneous distribution of these inorganic components are difficult. In addition, in particular in the case of water, corrosion problems during extrusion cannot be excluded. It is an object of the present invention to prepare polystyrene comprising recycled material which can be converted into foams having a uniform foam structure and good mechanical properties and which is simple to prepare. We have found that, surprisingly, this object is achieved by adding finely divided polyethylene wax to the expandable styrene polymers comprising recycled material. The present invention accordingly provides expandable styrene polymers comprising recycled material to which homogeneously distributed, finely divided polyethylene wax has been added. The content of recycled polystyrene in the novel products is from 10 to 100% by weight, based on the total weight of the polystyrene. The finely divided polyethylene wax is added to the polystyrene comprising recycled material in an amount of from 0.01 to 5%, preferably from 0.05 to 0.5%, by weight, based on the total amount of polystyrene. If smaller amounts of polyethylene wax are added, the action is too weak, and if larger amounts are added disadvantages can occur in the mechanical properties of resultant foams. The fact that the object of the present invention can be achieved by the addition of polyethylene wax was unforeseeable for the person skilled in the art. Although the use of polyethylene wax in the production of polystyrene foams is known, these products are used, as described, for example, in GB—A—997 356, with the aim of shortening the cooling time in particle foams. The polyethylene wax used in accordance with the invention generally has a mean molecular weight (number average) of from 2000 to 6000, preferably from 2000 to 4000, particularly preferably from 2800 to 3500. An example of a suitable polyethylene wax is Luwax® supplied by BASF AG, which has a mean molecular weight (number average) of 3000. The polyethylene wax preferably has a mean particle size of from 5 to 50 [im. If the particles are too large, problems can occur with distribution in the polymer, and if the products are too fine, dust nuisance can arise. The polyethylene wax is expediently added to the molten polystyrene, preferably during extrusion. In the production of extruder foam, the polyethylene wax can be added during the extrusion step before foaming, and in the production of polystyrene particle foam, it can be added during the extrusion step to give minigranules. Polyethylene wax can expediently be fed into the extruder together with the polystyrene, but can also be introduced at any other point of the extruder. The polystyrene foam used for the production of the recycled polystyrene must generally be compacted. This can be carried out mechanically by pressing or grinding the recycled polystyrene foam, but it is also possible to compact the recycled polystyrene foam by extrusion. In order to remove mechanical impurities which are insoluble in the melt, the molten recycled polystyrene can advantageously be subjected to melt filtration. The novel expandable styrene polymers comprising recycled material are expediently mixed with fresh polystyrene in order to improve the properties of the resultant foams. This mixing is usually carried out in the melt in an extruder. However, it is in principle also possible to re-foam the recycled material alone. The foaming is carried out by processes known per se. Thus, it is possible to treat the molten polystyrene with steam in the extruder and to foam the product directly thereafter to give boards (extruder foam technology). However, it is also possible to granulate polystyrene after extrusion, to impregnate these granules with blowing agent and subsequently to foam the granules by known methods (particle foam technology). The impregnation with the blowing agent can be carried out while the granules are still in the extruder, in which case the melt must be cooled rapidly in order to prevent expansion of the particles (quenching). However, the particles are usually impregnated in suspension. In this case, the granules are suspended in a liquid, usually water, in the presence of conventional auxiliaries and additives in a pressure container, and the latter is rendered inert and heated to a temperature above the softening point, but below the melting point of the polymer. The blowing agent is injected at this temperature. After cooling and decompression, the impregnated granules are separated off, purified and dried, the latter preferably at < 50°C in a stream of air. This process is described, for example, in EP—A—343 473. For the production of foams, the expandable styrene polymers are expanded in a known manner by heating to temperatures above their softening point, for example by means of hot air or preferably by means of steam. After cooling and, if desired, interim storage, the foam beads can be re-expanded by further heating. Finally, they can be welded in a known manner in non-gas-tight molds to give moldings. Further details on conventional impregnation, extrusion and foaming processes are given, for example, in Polystyrene, edited by R. Vieweg and G. Daumiller, Carl—Hanser—Verlag, Munich, 1969. The novel polystyrene foams are distinguished from other polystyrene foams comprising recycled material through a more uniform foam structure. Even a small amount of contamination by other polymers and additives in the recycled material, which cannot be removed by conventional purification methods, such as melt filtration, do not have any disadvantageous effect on the foam structure of the novel products. This allows even foamed and unfoamed polystyrene waste produced outside the actual production process, which varies in material characteristics and in some cases also has different additives, to be re-converted into high-quality foams by a very simple process• The polyethylene wax used in accordance with the invention is readily available. It has no disadvantageous effects on the properties and the processing behavior of the foams. In addition, molecular weight degradation of the recycled polystyrene, measured as the viscosity number, is significantly reduced, ie. the polymer suffers less damage during extrusion. The invention is described in greater detail by means of the examples below: Examples 1-3 (comparison) Granulated recycled polystyrene (produced using an EREMA extruder, insoluble content 0.12% by weight, viscosity number 65 ml/g), if desired blended with pure polystyrene having a viscosity number of 75 ml/g (polystyrene VPT from BASF AG), was extruded in a Werner und Pfleiderer ZSK 53 extruder with introduction of 5% by weight of pentane and subsequently granulated under water to a size of about 1.4 x 1.5 mm. The granules were pre-foamed by means of a pressureless foaming box (Rauscher system) and converted into board-shaped moldings after 12 hours. Examples 4—12 The procedure was as in Examples 1-3, but a polyethylene wax having a molecular weight of about 3000 and a particle size of about 10 fim (Luwax® AF 31 from BASF AG) was added during the extrusion. Examples 13-15 ( comparison) The procedure was as in Examples 1-3, but the blowing agent was not added in the extruder, but instead the granules were suspended in water with a Pickering stabilizer based on magnesium pyrophosphate, transferred into a nitrogen-flushed pressure reactor, impregnated with 7% by weight of pentane and foamed as described in Examples 1-3. Examples 16 — 18 The procedure was as in Examples 13 - 15, but a polyethylene wax having a molecular weight of about 3000 and a particle size of about 10 (im (Luwax® AF 31 from BASF AG) was added during the extrusion. The precise mixing ratios of the starting materials and the properties of the foams are shown in Table 1. The viscosity number (0.5% in toluene) was measured in accordance with DIN 53 726, and the foam structure and cell count were assessed visually. 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