Solvent for reducing volume and for recycling of polystyrene foam

Abstract: A solvent for polystyrene, having an excellent dissolving ability and chemical stability with no influence on the operating environment, is disclosed. A relatively simple recycling or reutilizing process of both the polystyrene foam and the solvent is also disclosed. The solvent comprises at least one member selected from the group consisting of such a glycol ether compound as diethylene glycol dimethyl ether, diethylene glycol diethyl ether or dipropylene glycol dimethyl ether, and such a fatty acid dialkyl ester compound as dimethyl succinate, dimethyl glutarate or dimethyl adipate. These compounds have a flash point of 100°C or higher, and can be made nonflammable by addition of water.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a solvent for dissolving polystyrene and more particularly a solvent enabling reduction in the volume and recycling of polystyrene foam or expanded polystyrene, as well as a method for recycling polystyrene foam materials. 2. Description of the Prior Art In recent years, it has become desirable from the viewpoint of waste-disposal, to establish a recycling technology for polystyrene foam used as a buffer or shock-absorber material. Disposal of polystyrene foam has hitherto generally been performed by heat-treatment. The reduction of the volume of the polystyrene foam by such heat-treatment however suffers from the disadvantage that the physical properties of the polystyrene material deteriorate due to the applied heat and the resultant contraction of the material. Research on new methods for recycling the polystyrene foam by means of dissolution is being undertaken to replace the heat-treatment, because the new methods are considered to have a relatively small influence on the deterioration of the physical properties of the material. At present, it is proposed to use d-limonene, which is one of citrus plant essential oils, as a solvent for the dissolution method. The solvent, d-limonene, used in the above-mentioned dissolution method has excellent ability to dissolve the polystyrene foam. The d-limonene however has the disadvantage that it has a strong citrus odor and a low flash point of 47°C, and thus influences the operating environment. It is also liable to decompose and lacks stability, because it is a natural product. US 5,278,282 (Rensselaer Polytechnic Institute) relates to a method of separating polymers from a physically commingled solid mixture containing a plurality of polymers. The method centres on polymer recycling by selective dissolution. By way of example, polystyrene can be recycled using 2-(2-butoxyethoxy)ethanol and the solvent can be eliminated by flash volatization. JP 3076706A relates to the production of a hydrogenated styrene resin in mild conditions by hydrogenating the aromatic ring of styrene resin using a palladium catalyst carried on silica. Diethylene glycol dimethyl ether is cited as a suitable solvent for the resin. SUMMARY OF THE INVENTION The present invention is proposed in order to overcome the above-mentioned disadvantages of the prior art. In particular there was a need to provide a solvent for polystyrene having excellent dissolving ability and excellent chemical stability, with no influence on the operating environment. There was also a need to provide a method for reducing the volume of the polystyrene foam with the use of the above-mentioned solvent, and a method for recycling polystyrene foam with the use of the above-mentioned solvent. The present invention relates to a solvent for polystyrene e.g. polystyrene foam. In particular, the invention relates to the use of at least one compound selected from a glycol ether compound represented by the general formula (1) where m represents an integer of 1 ― 4, n represents 0 or an integer of 1 ― 4, and R represents a methyl group, for dissolving polystyrene. As the above-mentioned glycol ether compound, it is preferable to use dipropylene glycol dimethyl ether. In a preferred embodiment of the present invention, the above-mentioned solvent is selected from glycol ether compounds having a flash point of 100°C or higher, and may have water added to it. A further aspect of the invention provides a method for recycling the polystyrene foam which comprises the steps of dissolving the polystyrene foam in the above-mentioned solvent, and distilling the obtained solution at a temperature at which polystyrene does not decompose, thereby to separate the above-mentioned solvent from the polystyrene. Another aspect of the invention provides a method for recycling the polystyrene foam which comprises the steps of dissolving the polystyrene foam in the above-mentioned solvent, precipitating polystyrene from the obtained solution by adding water to the solution, and separating the polystyrene from the solution. Preferably, the polystyrene is dissolved in dipropylene glycol dimethyl ether. Yet another aspect of the invention provides a method for recycling the polystyrene foam which comprises the steps of dissolving the polystyrene foam in the above-mentioned solvent, precipitating polystyrene from the obtained solution by cooling the solution to a temperature lower than the temperature at which polystyrene dissolves, and separating the polystyrene from the solution. In the above-mentioned methods for recycling the polystyrene foam, it is preferable to perform a filtrating process for separating the precipitated polystyrene from the solution. In a further preferred aspect, the present invention provides a method for recycling the polystyrene foam, wherein the above-mentioned solvent is regenerated by distillation and used again as the solvent for the polystyrene foam. More generally the recycling method comprises dissolving polystyrene in the solvent and thereafter separating the polystyrene and the solvent. Yet another aspect of the invention provides a method for reducing the volume of polystyrene foam comprising the step of dissolving at least a major portion of the polystyrene foam of which it is intended to reduce the volume, in a solvent which comprises as its sole or main ingredient at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1-4, n represents 0 or an integer of 1 - 4 and R represents a methyl group. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The glycol ether compounds represented by the above-mentioned formula (1), wherein the alkyl groups represented by C m H 2m+1 and C n H 2n+1 are any of methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group and tert-butyl group, can be exemplified as dipropylene glycol monoalkyl ether and dipropylene glycol dialkyl ether. In the above-mentioned dialkyl ether compounds, the two alkyl groups may be the same or may be different from each other. Since the ability of the solvent to dissolve polystyrene increases with increase in the concentration of these glycol ether compounds in the solvent, it is most desirable to configure the solvent only with the glycol ether compounds represented by the above-mentioned formula (1). Although depending on the compound constituting the solvent in accordance with the present invention, the ability of the solvent to dissolve polystyrene increases in general if the solvent is heated and the dissolving process proceeds more smoothly, accordingly. The temperature required for dissolving polystyrene in the solvent in accordance with the present invention is usually in a range from room temperature to 70°C. In order to promote the dissolving process, any physical means such as ultrasonic radiation or stirring may be added to the process. It is desirable that the solvent contains dipropylene glycol dimethyl ether among the glycol ether compounds represented by the formula (1), because the ability of the solvent to dissolve polystyrene further increases if this compound is incorporated. In the glycol ether compounds represented by the above-mentioned formula (1), if m and n in the alkyl groups represented by C m H 2m+1 and C n H 2n+1 exceeds 4, the ability to dissolve polystyrene becomes poor. If the additional mole number of the oxypropylene represented by the formula (2) where R represents a methyl group, is 1, the glycol ether compound has a toxicity and a low flash point, and thus is not suitable from the viewpoint of preserving the operating environment. On the other hand, if the additional mole number is 3 or more, the compound has a poor ability to dissolve polystyrene and is not preferable as the solvent. If the polystyrene foam is dissolved in the solvent containing the above-defined glycol ether compounds and then the obtained solution is distilled, it is possible to separate polystyrene from the solvent. In this case, it is more desirable to perform the distillation under reduced pressure so that the calorimetric value required for the separation becomes low. If the polystyrene foam is dissolved in the solvent containing the above-defined glycol ether compounds and then water is added to the obtained solution, it is possible to easily precipitate polystyrene again from the solvent. Although the amount of water required for the precipitation varies with the composition, it is usually in a range of 10 ― 50 parts by weight for 100 parts by weight of the solvent with dissolved polystyrene. In addition, polystyrene may alternatively be precipitated by lowering the temperature of solvent which has dissolved a saturated amount of polystyrene. According to these methods, since the precipitated polystyrene tends to float on the solvent, it is possible to separate polystyrene from the solvent by simply scooping it from the liquid phase but it is also possible to perform a filtration process if necessary. After the precipitated polystyrene is removed by filtering off from the liquid phase, it is possible to recover the solvent to use it again by removing water from the solvent by distillation. This manner of separation requires a lower calorimetric input than the method of separation by distilling the solvent which liquid phase, it is possible to recover the solvent to use it again by removing water from the solvent by distillation. This manner of separation requires a lower calorimetric input than the method of separation by distilling the solvent which dissolves polystyrene and thus is considered to be more preferable as a recycling method. Among the glycol ether compounds represented by the above-mentioned formula (1), monoalkyl ethers wherein the alkyl groups represented by C m H 2m+1 are any of butyl group, isobutyl group and tert-butyl group have a flash point of 100°C or higher. By adding water to these glycol ether compounds, they can be made nonflammable. The amount of water required for making the solvent nonflammable is at least 3% by weight and, more preferably, 5% by weight or more for securing operational safety. By making the solvent, nonflammable, it is possible to reduce the volume of the polystyrene foam at recycling factories where the nonflammable solvent is more preferable. In addition, the glycol ether compounds in accordance with the present invention are thermally stable and thus it is possible to use the solvent repeatedly. According to the present invention, it is possible to provide a solvent for polystyrene having an excellent dissolving ability and stability with no influence on the operating environment. It is also possible to realize a relatively simple recycling process of both the polystyrene foam and the solvent for polystyrene by precipitation of polystyrene by distillation or addition of water. In the following Examples 1 and 2 various additives may be added to the solvent unless the recycling method in accordance with the present invention is harmed by their presence. As an additive for the glycol ether compounds, there may be exemplified any glycol ethers other than the glycol ether compounds represented by the formula (1), water, aliphatic alcohols, aliphatic ketones, glycols, hydrocarbons and various surfactants. In the following paragraphs, the present invention will be described more specifically with reference to specific Examples. EXAMPLE 1, COMPARATIVE EXAMPLES 1 ― 2 Table 1 and Table 2 below summarize compositions of the solvents to be used in Example 1 and Comparative Examples 1 ― 2, respectively. Numerals in these tables represent ingredients in the compositions in percent by weight. Classification 1 2 Example 1 70 30 Classification 3 4 Comparative Example 1 100 Comparative Example 2 100 Each of the glycol ether compounds indicated by numerals 1 ― 4 in Table 1 and Table 2 represents the following compounds: 1 : dipropylene glycol monomethyl ether 2 : dipropylene glycol dimethyl ether 3 : diethylene glycol monopentyl ether 4 : triethylene glycol monoethyl ether A cube of polystyrene foam (length: 5 cm, width: 5 cm, height: 5 cm) was immersed in 1,000 ml of each of the solvents listed in Table 1 and Table 2 above, and the temperature and the time period until the cube was dissolved in the solvent was measured. The results of the measurements are summarized in Table 3 below. Example 1 Comparative Examples 1 - 2 Result ○ x Symbols for evaluation criteria ○ : Dissolved in 1 hour at room temperature. x : Not dissolved in 1 hour at 70°C. As clearly shown by the above-mentioned Example 1, the solvents in accordance with the present invention have an excellent ability to dissolve the polystyrene foam. In contrast to this, the solvents in accordance with Comparative examples are not suitable for the following reasons. Comparative Example 1: The solvent, diethylene glycol monopentyl ether, has an alkyl group C m H 2m+1 in the formula (1) having 5 or more carbon atoms, and has a remarkably poor ability to dissolve the polystyrene foam. Comparative Example 2: The solvent, triethylene glycol monoethyl ether, has a unit number of 3 or more for oxypropylene as shown in the general formula (2), and has a remarkably poor ability to dissolve the polystyrene foam. EXAMPLE 2 In this example, an evaluation was made on the amount of the polystyrene foam which can be dissolved in the solvents in accordance with the present invention. A hundred (100) g of the polystyrene foam was able to be dissolved in 100 g of dipropylene glycol dimethyl ether. In this manner, the solvent in accordance with the present invention can dissolve a large quantity of the polystyrene foam.

Claims: Use of at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1 - 4, n represents 0 or an integer of 1 - 4, and R represents a methyl group, for dissolving polystyrene. Use in accordance with claim 1, wherein said at least one glycol ether compound is dipropylene glycol dimethyl ether. Use in accordance with claim 1 or 2, wherein said glycol ether compound is a compound having a flash point of 100°C or higher and further contains water. A method for recycling polystyrene foam comprising the steps of: dissolving the polystyrene foam in a solvent which comprises at its sole or main ingredient at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1 - 4, n represents 0 or an integer of 1 - 4, and R represents a methyl group, and distilling the obtained solution at a temperature at which polystyrene does not decompose, thereby to separate said solvent from polystyrene. A method for recycling polystyrene foam comprising the steps of: dissolving the polystyrene foam in a solvent which comprises as its sole or main ingredient at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1 - 4, n represents 0 or an integer of 1 - 4, and R represents a methyl group, precipitating polystyrene from the obtained solution by adding water to the solution, and separating the precipitated polystyrene from said solution. A method for recycling polystyrene foam comprising the steps of: dissolving the polystyrene foam in a solvent which comprises as its sole or main ingredient at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1 - 4, n represents 0 or an integer of 1 - 4, and R represents a methyl group, precipitating polystyrene from the obtained solution by cooling the solution to a temperature lower than the temperature at which polystyrene dissolves, and separating the precipitated polystyrene from said solution. The method for recycling polystyrene foam in accordance with claim 4, 5 or 6, wherein said solvent is regenerated by distillation and used again as the solvent for the polystyrene foam. A method for recycling polystyrene foam comprising the steps of: dissolving the polystyrene foam in at least dipropylene glycol dimethyl ether, precipitating polystyrene from the obtained solution by adding water to the solution, and separating the precipitated polystyrene from said solution. A method for reducing the volume of polystyrene foam comprising the step of: dissolving at least a major portion of the polystyrene foam of which it is intended to reduce the volume, in a solvent which comprises as its sole or main ingredient at least one compound selected from glycol ether compounds represented by the general formula (1) wherein m represents an integer of 1 - 4, n represents 0 or an integer of 1 - 4 and R represents a methyl group.

Inventors: NOBUTA KENICHI, ONISHI HIROSHI, SHIINO TORU, NOBUTA, KENICHI, ONISHI, HIROSHI, SHIINO, TORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: C08J3
IPC codes: C08J3, C08J11