EP0000513B1 - Process for the reutilisation of decantation material resulting from the treatment of waste water from the production of polystyrene - Google Patents

Description

Polystyrene, in particular expandable polystyrene, is produced on an industrial scale by polymerizing styrene in an aqueous suspension. After the polymerization has ended, the majority of the polystyrene particles are separated from the aqueous phase by centrifuging. Part of the polymer, generally about 2 to 4% by weight, based on the styrene used, remains in the form of very fine beads in the waste water. If the suspension polymerization was controlled in such a way that fine-particle polymer particles are preferably obtained, the amount of polymer remaining in the waste water increases to 5% or more. These finest particles are recovered from the wastewater by decanting, primarily for environmental reasons. The so-called decanter material obtained consists largely of very fine-particle polystyrene, which may contain blowing agent, and which contains relatively large amounts of emulsion polymer and other hydrophilic impurities. It is not directly for further processing, e.g. suitable for the production of foams.

There are a number of ways to use the decanter material economically, e.g. by reworking with the help of extruders, which leads to colored or specially shaped granules. The main reason why these methods are costly is that before they are processed on an extruder, they have to be dried, which is associated with emissions in the form of exhaust air.

It would therefore be of great advantage if you could reuse the decanter material as it is separated from the decanter. It would be obvious to dissolve the decanter material in monomeric styrene and then to polymerize the styrene in aqueous suspension by a process as described in FR-B 1 368 544. However, it has been shown that the risk of coagulation is very great and that at best a bead polymer is obtained which is cloudy, has very high internal water contents and leads to foams with an irregular foam structure.

GB-A 1 416 405 describes a process for the preparation of polystyrene particles with a particle size in which pre-formed polystyrene particles with a diameter of greater than 0.4 mm are suspended in water, monomeric styrene is added to the suspension and this is present in the presence is polymerized by polymerization catalysts and retarders. The patent also states that attempts have been made to dissolve smaller polystyrene particles in styrene and to polymerize them. However, no useful results were obtained.

The object of the invention was to economically process the decanter material and to reuse it.

It has been found that this object is achieved if the decanter material is dissolved in styrene and the styrene is polymerized, but the decanter material is treated with water before the styrene is polymerized. It can be assumed that in this cleaning operation a large part of the substances interfering with the polymerization are removed with the aqueous phase.

The invention accordingly relates to a process for the reuse of decanter material, which is obtained in the form of fine polystyrene beads with an average particle diameter of at least 0.05 mm and at most 0.3 mm, if the processing of the waste water from the production of polystyrene by suspension polymerization Solids separated, by dissolving the particles in monomeric styrene in the weight ratio of styrene to decanter material from 100: 1 to 1: 1, and polymerizing the styrene in aqueous suspension in the presence of radical initiators and optionally other polymerization aids at elevated temperature. It is characterized in that, before the polymerization of the styrene, the decanter material was treated with water in a weight ratio of 1:50 to 1: 1 at temperatures between 10 ° C. and 100 ° C., but in the event that the treatment in the presence of Styrene and radical initiators is made, the temperature must not exceed 80 ° C.

The decanter material is obtained by separating the solids from the wastewater from the production of expandable polystyrene. This separation is conveniently done by decanting, e.g. by centrifuging with a solid bowl centrifuge. This produces fine polystyrene beads with an average particle diameter of at least 0.05 and at most 0.3 mm.

The decanter material is suspended in water, the water: decanter material weight ratio being able to vary within wide limits between 50: 1 and 1: 1 and preferably between 20: 1 and 2: 1. Dispersing aids, e.g. Polyvinyl alcohol or polyvinyl pyrrolidone are added, furthermore buffer substances which may be required in a later suspension polymerization.

The decanter material can be cleaned with water in various ways: In a preferred procedure, the aqueous suspension of the decanter material is at temperatures between 10 ° and 100 °, preferably between 20 ° and 50 ° C, for 5 to 90 minutes, preferably 15 to 45 minutes touched. The cleaned material is then separated from the water again by decanting. This may be followed by another washing operation. The moist powder obtained is then dissolved in monomeric styrene, the weight ratio of monomer: decanter material here preferably being between 50: 1 and 5: 1. This organic phase is then suspended in water together with polymerization aids, such as, for example, polymerization initiators and suspending aids, and polymerized in a conventional manner at elevated temperature. But you can also add the cleaned wet powder with stirring a suspension of styrene in water, whereby it is detached from the styrene, and then carry out the suspension polymerization.

In another preferred procedure, monomeric styrene is added to the aqueous suspension of the decanter material, the weight ratio of monomer: decanter material here preferably being between 10: 1 and 2: 1.

The aqueous phase and the organic phase are then stirred together at temperatures between 10 and 100 ° C., preferably between 50 and 90 ° C. The stirring time depends on the temperature and the ratio of monomer; Decanter material. The higher the temperature and the higher the monomer: decanter material ratio, the shorter the stirring times required. In any case, stirring should be carried out here for at least one hour in order to extract the polystyrene in the decanter material sufficiently by the monomer. The stirring time is generally between 2 hours and 2 days. With a styrene: decanter material ratio of 1: 1, one has to stir at 20 ° C. for about 60 hours to extract essentially all of the styrene; after 18 hours, only 20% are released. A particular advantage of this method is that. there is no need to separate organic and aqueous phases after the extraction, but rather that the organic phase can be polymerized directly in the presence of the aqueous phase, giving uniformly round, clear pearls with a low internal water content. It is advantageous if the extraction is already carried out in the polymerization kettle. After the extraction has ended, all that is required is to add to the styrene phase the auxiliaries and free radical initiators required for the preparation of a particular bead polymer, and the polymerization can then be carried out at elevated temperature without further intermediate operations.

It is also possible to add the radical initiators already dissolved in the styrene during the extraction. The temperature during the extraction must then not exceed 80 ° C., since otherwise there is a risk that the polymerization will start even before the extraction has ended.

In principle, instead of styrene alone, a mixture of styrene with up to 50% by weight of other copolymerizable monomers, such as e.g. l1'-methylstyrene, nuclear halogenated styrenes, divinylbenzene, acrylonitrile, esters of acrylic or methacrylic acid, N-vinyl compounds or 1,3-diolefins can be used.

The suspension polymerization of the styrene or of the monomer mixture is carried out under customary conditions. The decanter material dissolved in the monomer does not interfere. The polymerization is generally carried out at temperatures between 80 and 130 ° C., in particular between 100 and 120 ° C. The usual polymerization initiators, such as peroxides or azo compounds, are used to start the polymerization. Either organic protective colloids, such as polymers of vinylpyrrolidone, polyvinyl alcohol or cellulose ether, or sparingly soluble inorganic salts, such as calcium phosphate, are used as suspension stabilizers. They can be added before or during the polymerization. The bead spectrum can be regulated by varying the time at which the suspension stabilizers are added. During or after the suspension polymerization, volatile hydrocarbons or halogenated hydrocarbons are added as blowing agents in a known manner. To produce flame-retardant styrene polymers, organic halogen compounds can be added to the suspension as flame retardants.

The blowing agent-containing styrene polymers obtained in the suspension polymerization are suitable for the production of foams. The process according to the invention is particularly suitable for the production of fine-pearled polymers which are used in the porosity of bricks.

The parts and percentages given in the examples relate to the weight.

example 1 A. Detach the decanter material

The starting point was the waste water from a suspension polymerization of styrene, from which polymer beads had been removed by centrifuging. This wastewater contained 2% by weight of fine polystyrene beads with an average diameter of less than 0.3 mm. It was fed to a decanter (Westfalia SDA 230) and a polymer material with a moisture content of 3 to 8% was separated off by centrifuging.

B. Cleaning the decanter material

• a) The decanter material from A. was slurried in a ratio of 1: 9 in water and at 20 ° C for 20 min. long stirred. Then it was decanted again. The cleaning effect can be recognized from the fact that the decantate only contains particles with a diameter of less than 0.05 mm. The solids content of the De cantats was 0.4%. The yield, based on the material used, was 95%.
• b) The execution was carried out analogously to a). However, 1% polyvinylpyrrolidone, based on organic material, was added to the mixture before the separation. The solids content of the decant was 0.6%, consisting of particles with a diameter of less than 0.05 mm. The yield was 94%.

C. Reinstalling the cleaned decanter material

• 100 Teilen Wasser
• 0,1 Teilen Natriumpyrophosphat
• 95 Teilen Styrol mit
• 5 Teilen Dekantermaterial,
• 0,155 Teilen Dibenzoylperoxid,
• 0,266 Teilen tert.-Butylperbenzoat und
• 0,04 Teilen Hexabromcyclododecan.

a) 5 parts of the material treated according to B. a) were dissolved in 95 parts of styrene. A mixture was poured into a pressure-resistant stirred tank equipped with a stirrer

• 100 parts water
• 0.1 part sodium pyrophosphate
• 95 parts of styrene
• 5 parts decanter material,
• 0.155 parts of dibenzoyl peroxide,
• 0.266 parts of tert-butyl perbenzoate and
• 0.04 parts hexabromocyclododecane.

The mixture was polymerized with stirring in a closed vessel for 4 hours at 90 ° C., 2 hours at 105 ° C. and then for 3 hours at 120 ° C. 3.5 parts of a 10% strength polyvinylpyrroldione solution in water were metered in as suspension stabilizer after 130 minutes, so that an average size of 1.2 to 1.6 mm was formed. 3 hours after reaching 90 ° C., 7 parts of pentane were added over the course of 10 to 15 minutes.

To determine the internal water content, a sample (pearl fraction from 1.25 to 2.0 mm) was treated with methanol for 2 minutes to remove the external water, then suctioned off on a suction filter and blown dry with air at 20 ° C. in countercurrent for 5 minutes. The internal water content was determined using the Karl Fischer method. It was 0.61%.

• a); es wurden jedoch als Auflösematerial 5 Teile des nach B. b) behandelten Materials eingesetzt. Das Perlpolymerisat hatte einen mittleren Durchmesser von 1,32 mm; der Innenwassergehalt betrug 0,50 %.

The bead polymerization was carried out analogously to C.

• a); however, 5 parts of the material treated according to B. b) were used as the dissolving material. The bead polymer had an average diameter of 1.32 mm; the internal water content was 0.50%.

• a), wobei aber die 5 Teile des nach B. a) behandelten Materials nicht in Styrol gelöst, sondern direkt der wäßrigen Phase hinzugefügt wurden. Das Perlpolymerisat hatte einen mittleren Durchmesser von 1,35 mm. Der Innenwassergehalt betrug 0,51 %.

The bead polymerization was carried out analogously to C.

• a), but the 5 parts of the material treated according to B. a) were not dissolved in styrene, but were added directly to the aqueous phase. The bead polymer had an average diameter of 1.35 mm. The internal water content was 0.51%.

Comparison test

The execution was analogous to C. a); however, 5 parts of the material which had been separated off but not further processed were used. The bead polymer obtained had an average diameter of 1.38 mm. The determination of the internal water content gave 3.5% water.

Example 2

• a) In a 1 m ³ pressure vessel equipped with a stirrer, thermometer and heating, 0.414 parts of sodium pyrophosphate were dissolved in 521 parts of water with stirring. 136 parts of moist decanter material which contained 10% of water and had an average particle diameter of approximately 0.15 mm were suspended in this solution. 371 parts of styrene were added to this suspension and, after sealing and purging the kettle with nitrogen, the mixture was stirred at 25 ° C. for 16 hours. Then 0.488 parts of dibenzoyl peroxide, 0.879 parts of tert-butyl perbenzoate and 0.082 parts of hexabromocyclodecane were added, nitrogen was injected and the mixture was heated to 120 ° C. in the course of 8 hours and kept at this temperature for 4 hours. In the heating phase, 60 minutes after reaching 80 ° C., 17.6 parts of a 10% strength aqueous solution of polyvinylpyrrolidone were added, and 46 parts of pentane were metered in 1 hour later. After the polymerization, cooling, separating the aqueous phase and drying, 482 parts of clear beads were obtained (92% yield). The pearls had a water content of 0.8% and an average particle size of about 0.7 mm.
• b) The procedure is as in Example 2a, but the suspension is stirred together with the styrene at 98 ° C. for 1 hour. The mixture is then cooled to 40 ° C., the peroxides are added and the mixture is polymerized as described in Example 2a. 490 parts of clear pearls are obtained with an internal water content of 0.8% and an average particle size of 1.1 mm.
• c) Comparative example
  296 parts of styrene were placed in a stirred kettle and 218 parts of moist decanter material (with 10% water) were dissolved therein by stirring at room temperature. After 16 hours, 0.075 parts of hexabromocyclododecane, 0.472 parts of dibenzoyl peroxide and 0.703 parts of tert-butyl perbenzoate were added and the mixture was stirred for 30 minutes. Then 509 parts of water were run in, 0.424 parts of nadrium pyrophosphate and 17.6 parts of 10% strength aqueous polyvinylpyrrolidone solution were added and the mixture was heated to 120 ° C. in 8 hours with stirring and under a nitrogen pressure of 1 bar. The mixture was kept at a temperature for 4 hours and then cooled. 48 parts of pentane were added at 80 ° C. during the heating. After the practice After processing, 485 parts (92.5% yield) of pelletized granules with an average particle diameter of about 1.5 mm were obtained. The pearls were very cloudy and about every third contained cavities. The water content was 7.6%.

Example 3

In a 1 m ³ pressure vessel equipped with a stirrer, thermometer and heating, 0.411 parts of sodium pyrophosphate are dissolved in 517 parts of water with stirring. 181 parts of moist decanter material (approx. 10% water, average particle size 0.15 mm) are suspended in this solution. 330 parts of styrene, 0.392 parts of dibenzoyl peroxide and 0.781 parts of tert-butyl perbenzoate are added with stirring. The mixture is pressed to 1 bar with nitrogen and heated to 60 ° C. The reaction is carried out at this temperature with stirring for four hours, then heating is continued to 120 ° C., whereupon the polymerization begins, and the mixture is kept at this temperature for a further four hours. In the heating phase, 17.6 parts of a 10% strength aqueous solution of polyvinylpyrrolidone are added 30 minutes after reaching 80 ° C .; 35 parts of pentane are added 1 hour later.

After cooling, separating off the aqueous phase and drying, 466 parts of clear beads are obtained (88% yield); they have an internal water value of 0.6% and an average particle size of 0.95 mm. 7% solids are analyzed in the separated aqueous phase.

Claims (4)

1. A process for reusing decanter material obtained in the form of fine polystyrene beads having a mean particle diameter of at least 0.05 mm and at most 0.3 mm when separating the solids in processing the waste water from the production of polystyrene by suspension polymerization, by dissolving the particles in monomeric styrene in a weight ratio of styrene to decanter material of from 100 : 1 to 1 : 1 and polymerizing the styrene in aqueous suspension in the presence of free-radical initiators and, if desired, further polymerization aids at elevated temperature, characterized in that prior to polymerization of the styrene the decanter material is treated with water in a weight ratio of from 1 : 50 to 1 : 1 at a temperature between 10° and 100°C, provided, however, that when carrying out the treatment in the presence of styrene and free-radical initiators the temperature of 80°C must not be exceeded.

2. A process for reusing decanter material according to claim 1, characterized in that the particles are suspended in water, stirred at temperatures between 20° and 50°C for from 5 to 90 minutes and, after separation of the water, dissolved in styrene.

3. A process for reusing decanter material according to claim 1, characterized in that the particles are suspended in water, the styrene is added to this suspension and the aqueous and organic phases are stirred together for at least 1 hour.

4. A process for reusing decanter material according to claim 3, characterized in that stirring is carried out at temperatures between 50 and 90°C.