Method for recycling polystyrene

The invention relates to a method for bringing polystyrene, particularly foamed polystyrene regarded as problem waste, to recycling. The recycling method comprises collecting, processing and converting the polystyrene waste to a reusable form.

Foamed polystyrene (FPS), i.e. cellular polystyrene, is used in large quantities e.g. as building insulation and as packing material to protect products in cardboard packages by enclosing them in whole or in part by FPS shaped pieces conforming to the product to be protected. Cellular polystyrene is also used as a floating material. In this context, FPS refers to any polystyrene material whose structure is characterized by that the volume of the material has been increased from the normal volume of solid polystyrene polymer by including gas in the polystyrene.

A problem in the recycling and re-use of cellular polystyrene is its structure: when used as such, it is poorly suited for working. The differ- ent shapes of pieces used for protecting packages, and particularly the low density of the material, lead to high transportation costs, and the transportation of the material to collection points is not economically viable. Consequently, most of the FPS waste ends up in dumping areas. In Finland, FPS waste is used to some extent in mechanically ground form in cellular concrete elements to improve the thermal insulation properties of the material. This is an example of recycling where the original (cellular) macrostructure of the FPS material is maintained; in other words, the FPS waste is worked mechanically for recycling.

The present invention is based on a method in which polystyrene is mixed into a solvent, from which the polystyrene is recovered later. Prior art patent publications disclose a variety of methods for compacting foamed polystyrene to reduce the transportation costs by dis- solving FPS waste in an organic solvent.

In the dissolving methods of prior art, the FPS waste is recycled by first dissolving the cellular polystyrene in a suitable solvent and then recovering the polystyrene by a suitable separation method so that the resulting polystyrene is denser than the starting material and can be re- used in technical plastics manufacturing processes. Because the dissolving simultaneously removes the air contained in cellular polystyrene, the final volume of the solution becomes smaller than the volume of the starting material, which facilitates the processing. Dissolving methods are described, among others in publications US 5,629,352; US 4,031 ,039; US 5,223,543; US 5,891 ,403; and US 4,517,312. Publication WO 02/38659 discloses a particular solvent system which collapses the foamed polystyrene without actually dissolving it, wherein the polystyrene can be separated by filtering from the solvent system.

Japanese application publications 2002-069230 and 11-181145 disclose the use of acetone as a solvent. In the publications, the mixture of acetone and FPS waste is called a paste or a gel, from which the acetone is evaporated because of its easy volatility, wherein the polystyrene is recovered. The mixing ratio is not mentioned.

A common problem in evaporation methods is that large quantities of solvent must be evaporated from polystyrene. The evaporation of all the solvent to leave no residues requires energy and is a relatively slow method for recovering polystyrene. The introduced heat may damage the polystyrene, particularly at the end of the process.

The aim of the invention is to eliminate the above-mentioned drawbacks and to present a method for recycling polystyrene without the disadvantages of prior art. To achieve this aim, the method according to the invention is primarily characterized in that the polystyrene is dissociated physically into an organic aqueous solvent to form a gel, after which the polystyrene is separated either by bringing the gel into contact with water or by separating the gel portion from the solvent and evaporating the solvent residue from it. In this case, the separation refers to the separation of polystyrene from the solvent in such a way that it is converted physically from the gel state to the solid state by the effect of a large quantity of water (regeneration), or the separation of

the gel phase from an excess of the solvent, after which it can be converted to the solid state in contact with water or by evaporating the solvent residue in the gel, which is present in a significantly smaller quantity than the solvent originally used.

It has been found that the formation of the polystyrene gel always requires the presence of water in an organic solvent. With a relatively small amount of water, it is possible to form a gel which has a low viscosity but which can be seen as a separate phase in the solvent sys- tern. By increasing the amount of water in the solvent it is possible to produce a thicker gel which is further separated into a phase of its own in the solvent system and which can be easily isolated from the rest of the solvent for further processing.

The gel obtained from the polystyrene is very viscous, which can be utilized in the regeneration step, in which the shape of the obtained polystyrene can be influenced by the way of performing this step. When the regeneration is performed with water, the gel can be subjected to shearing forces e.g. by agitating the water-gel mixture, or in another way. If the gel is isolated as a phase of its own from the solvent system, such a separated plastic "lump" can be worked mechanically, wherein the solvent residue of the polystyrene gel is simultaneously evaporated. By all these methods it is possible to produce solid polystyrene with a desired shape or size distribution or density.

The polystyrene gel in a suitable solvent provides a significant reduction in volume (compacting). The polystyrene can be dissociated in the solvent to form a saturated gel, preferably with the help of agitation. If the water content of the organic solvent is not sufficient, water is added into it, if necessary, to control the gelling speed and to obtain a gel with a suitable viscosity. In the form of a compact gel, the polystyrene is easy to handle, for example to transport within a plant or from collecting points to the treatment plant.

In regeneration, the polystyrene gel and water are contacted with each other, wherein the solvent is transferred from the gel into the aqueous

phase and the polystyrene is separated as a solid phase. The regeneration is preferably carried out by adding the gel into water and by agitation so that the gel is dissociated in the water to form parts which will later form into polystyrene particles. By the intensity or the duration of the agitation, it is thus possible to affect the size of the forming polystyrene particles. The more effectively or the longer time the mixture is agitated, the smaller the gel forms when dispersing into water, and the smaller the particle size of the solid polystyrene is generated. The gel can be added into water, for example, by pouring it in one go to a pre-measured quantity of water and stirring. Another alternative is to extrude the gel through a special nozzle or nozzles into water under simultaneous agitation. The water used in the regeneration has preferably a temperature from 15 to 35 degrees (Celsius). The quantity of water used for the regeneration in relation to the quantity of the gel is greater, preferably more than double, in weight parts.

The regenerated solid polystyrene, which is chemically the same polymer than the polystyrene of the FPS waste but which no longer has a cellular structure, can be isolated from the remaining aqueous solution of the solvent (liquid phase) by a suitable method of separating solids from a liquid, for example by filtering. The solvent used for the gel formation can be recovered from the water by distillation and, if necessary, it can be recirculated in the process. In this case, the heat treatment is directed to the aqueous solution of the solvent and not to the polystyrene, whose regeneration temperature is the temperature of the precipitation water, which is relatively low.

Alternatively, the polystyrene gel can be prepared by increasing the water content of the solvent used so that it is separated from the sol- vent system as a separate phase of its own with a high viscosity. This phase can be easily separated from the solvent system without particular separation methods, and after that, it can be processed by the above-mentioned water regeneration or it can be worked mechanically. For example, it can be fed as such into an extruder or a calender, at which stage the solvent residue is also evaporated. In this process, it is not necessary to evaporate large solvent quantities from

polystyrene, but only the solvent residue remaining in the gel phase after the removal from the solvent system.

In the following, the invention will be described with reference to the appended drawings and the examples below. In the drawings,

Fig. 1 shows the method according to a first embodiment in a schematic view, and

Fig. 2 shows the method according to a second embodiment in a schematic view.

The schematic view of Fig. 1 shows the method according to the invention in principle. Polystyrene waste PS, which consists of foamed polystyrene (FPS), i.e. cellular polystyrene, and which may originate from packages, construction waste or other sources, is mixed in step 1 with a suitable aqueous organic solvent S in a vessel or a container, whereafter a viscous gel PSG is formed. The solvent may be ready in the vessel or container into which the pieces of polystyrene are fed. In the case of cellular polystyrene, air is removed and the polystyrene collapses, that is, is compacted. The solvent also refers to a mixture of two or more organic solvents that also contains dissolved water. Next, the gel PSG and water are brought into contact with each other (step 2), resulting in an aqueous solvent solution S + H ₂ O, in which the polystyrene particles are separated as solid matter. In other words, a heterogeneous mixture S + H ₂ O + PS is formed, containing a solid phase and a liquid phase. After this, the polystyrene PS is isolated from the liquid phase (aqueous solvent solution S + H ₂ O), which may be performed by filtering off the polystyrene (step 3). The polystyrene can then be dried and packed.

The solvent S is recovered from the aqueous solution (step 4) for example by distillation and is recirculated to the mixing step 1 to dissociate new PS waste.

In the method of Fig. 2, the water content dissolved in the organic solvent is greater than in the case of Fig. 1. The compacting of cellular

polystyrene is now obtained in the same way as above, but now a polystyrene gel phase and a solvent phase (PSG + S) are obtained, from which the polystyrene gel can be separated e.g. as a "lump" without laborious separation methods. The gel can be processed further in the same way as the polystyrene gel in Fig. 1 , or the gel phase is led as such to mechanical working, for example into an extruder or a calender, after which the polystyrene can still be comminuted to a desired particle size, if desired. In this context, the solvent residue in the polystyrene is also evaporated and the polystyrene is converted from the gel into a solid. The solvent S remaining after the separation of the polystyrene can be re-used for the formation of a polystyrene gel. Similarly, the solvent S removed from the separated gel by evaporation can be recovered.

In practical recycling methods, the small volume of the polystyrene gel PSG can be utilized for example so that the polystyrene is transported from the collection points to the treatment plant in a state where it is mixed into the solvent and, in the treatment plant, the polystyrene is separated according to either alternative and the solvent is recovered.

Consequently, in step 1 of the method according to the invention, FPS waste is compacted by introducing the material into such an organic solvent (a substance consisting of one organic solvent or a mixture of two or more organic solvents), in which the polystyrene forms a gel without any polystyrene being present in solution form. A solvent that is particularly suitable for the purpose is acetone which, in commercial forms, always contains a small amount of water (normally 0.5 wt-%). By one liter of acetone, it is easy to compact, for example, about 515 g of foamed polystyrene. The subsaturated or nonsaturated gel thus formed has a very high surface tension, and it does not adhere to metal or glass surfaces at all.

Acetone is a common, easily available solvent which also has few safety risks, because acetone is known to have a low toxicity compared with many other organic solvents. Acetone may also be the main substance in a solvent mixture containing an auxiliary solvent.

In step 2, polystyrene is isolated by introducing a suitably selected quantity of PS gel into a vessel equipped with an integrated shearing propeller agitator and containing water. After the agitator has been started, the gel phase is dissociated and separated in the form of grains whose particle size can be adjusted simply by the operating time of the agitator. The agitation power can also be used as a variable for controlling the particle size. The capability of the agitator to operate at high rotation speeds is advantageous in the process. Thus, for example two liters of water can be used to process a gel containing about 85 g polystyrene. The PS grains are recovered, for example, by pouring the grain mixture through a screen filter (step 3), after which the grains are dried and used, for example, for thermal insulation. If desired, acetone can be separated from the water by distillation.

Alternatively, the step 2 can be implemented so that the saturated PS gel is introduced into an extruder equipped with a granulation nozzle, preferably a double-screw extruder, and is extruded at room temperature directly into water. The formed granulate is washed with water to remove the solvent, removed from the solution, dried, and utilized e.g. for melt processing.

When operating according to the chart of Fig. 2, water can be added into acetone before it is mixed into the polystyrene so that the water content is increased to a level desired in view of the gelling speed and the properties of the obtained gel. The water content may be, for example, in the range from 0.5 to 10 wt-%, preferably from 0.5 to 5 wt-%. When the water content increases from the value 0.5 wt-% up to the value of 10 wt-%, the gel becomes thicker; that is, its viscosity is increased, and simultaneously it is separated as a phase of its own in the solvent system. Such a gel can be isolated as a separate "lump" of its own from the acetone-water solution and be processed further by any of the above-mentioned ways.

Other possible organic solvents that contain a suitable quantity of water dissolved in it include glycol ethers or propylene glycol ethers, preferably methyl ethers. Examples to be mentioned are diethylene glycol dimethyl ether (diglyme) and dimethyl glycol. Diglyme forms a perfect

solution with cellular polystyrene even in the water content of about 5 wt-%. Gel formation takes place in the water content of 7.5 to 15 wt-%. Diglyme gel is generally "thinner" (having a lower viscosity) than acetone gel, and it is adhesive, a property that a gel formed with acetone does not have.

The recycled polystyrene obtained by the method can be used as a raw material for the manufacture of polystyrene products by the same methods that are used for manufacturing products by using, as the starting material, fresh polystyrene made by polymer synthesis. If necessary, it can even be used for manufacturing cellular polystyrene again, or polystyrene pieces without a cellular structure.

The invention will be described in more detail in the following embodi- ment examples which do not limit the invention.

Example 1. Preparation of a gel into acetone, and regeneration with water

85 g of FPS waste are added into 165 ml of acetone containing 0.5 wt-% of water. A nonsaturated gel is obtained, having a PS content of about 39.4 wt-% (calculated according to the acetone density of 0.79). A calculated saturated acetone gel can be obtained with a ratio of about 1600 g of polystyrene per 1 I of acetone (containing 0.5 wt-% of water). Such a gel is in a compact form for transportation but it is not easily pumpable.

The obtained nonsaturated gel is transferred to a blender containing 2.0 I of water. The blender is started, and the gel phase is dissociated into grains as the acetone is transferred to the aqueous phase. The particle size and bulk density of the forming grains depend on the operating time of the blender. With operating times of 5 s and 10 s, the bulk densities for this blender type are 80 g/dm ³ and 215 g/dm ³ , respectively. The grain/water mixture is poured through a screen filter and dried.

According to another alternative, the gel prepared with said mixing ratio is transferred into an extruder equipped with a granulating nozzle and is extruded at room temperature directly into water. The granulate/ water mixture is agitated for Vz hour for transferring the acetone into water, the granulate is isolated and dried in a tower drier.

Example 2. Preparation of a gel into acetone, and regeneration by evaporation

The solvent system is 165 ml of acetone according to example 1 , into which still 8.25 ml of water are added. 64 g of FPS waste are mixed into the solvent. A relatively thick gel lump is obtained that weighs 104 g and contains a solvent residue of about 4O g. After the separation of the gel, the remaining solvent quantity is about 140 ml. Thus, the quantity of the solvent to be evaporated remains smaller.

Example 3. Preparation of a gel into diglyme, and regeneration by evaporation

The solvent system is 165 ml of diglyme, into which still 16.5 ml of water are added. 64 g of FPS waste are mixed into the solvent. A relatively thin gel lump is obtained, weighing 138 g and containing a solvent residue of 74 g. After the separation of the gel, the remaining solvent quantity is about 107 ml.

Even though batch processes have been described in the examples above, the method can also be applied in continuous regeneration of polystyrene, in which polystyrene gel and regeneration water are con- tinuously fed into a reaction tank and polystyrene is continuously separated from the outflowing heterogenic mixture of acetone solution and polystyrene, or polystyrene gel is continuously fed into processing for the evaporation of the solvent residue from the gel.