WO2002038660A1

WO2002038660A1 - Method for collapsing and recycling foamed polystyrene

Info

Publication number: WO2002038660A1
Application number: PCT/US2001/032367
Authority: WO
Inventors: Nicholas Edward Kob, Iii
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2000-11-07
Filing date: 2001-10-18
Publication date: 2002-05-16
Anticipated expiration: 2003-05-07
Also published as: TW581781B; AU2002213307A1

Abstract

An improved method of recycling polystyrene foam involving collapsing the polystyrene foam in a solvent/cosolvent system without dissolving the polystyrene and then separating and recovering the softened polystyrene typically floating of the liquid solvent system phase. The solvent system useful for collapsing foamed polystyrene comprises a dibasic organic acid ester solvent selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof and a cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, isopropyl alcohol, and mixtures thereof wherein the cosolvent is present in said solvent/cosolvent system from at least 10 weight percent up to 40 weight percent. Such a process is particularly useful for isolating and recovering both polystyrene and PET in that PET is denser than the liquid solvent system phase and is soluble in the liquid solvent phase at elevated temperatures.

Description

TITLE OF THE INVENTION

METHOD FOR COLLAPSING AND RECYCLING FOAMED

POLYSTYRENE

BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates to an improved method of recycling polystyrene foam involving collapsing the polystyrene foam in a solvent/cosolvent system without dissolving the polystyrene. More specifically but not by way of limitation, the present invention relates to the use of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof in combination with at least one cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, and isopropyl alcohol to collapse polystyrene foam and thus physically separate the collapsed softened polystyrene and the liquid solvent system into two different immiscible phases. Such a method is particularly useful in separating and recovering polystyrene from a mixture containing polystyrene and polyethylene terephthalate (PET). 2. Description of the Related Art:

Due to increasing environmental concern/awareness, establishing a safe cost-effective technology to recycle polystyrene and PET is of interest. Disposal of polystyrene foam has traditionally been performed by incineration or heat treatment. Incineration, although it is a disposal method, does not recycle the polystyrene material. Reducing the volume of polystyrene by heat treatment causes deterioration of the polystyrene, resulting in inferior recycled material.

Another method of recycling polystyrene described in the known art (see for example U.S. Patent Nos. 5,629,352; 4,031,039; 5,223,543; 5,891,403; and 4,517,312) is to dissolve polystyrene into an organic solvent. In particular, dimethyl adipate, dimethyl glutarate, dimethyl succinate and mixtures of these diesters have been used commercially m Japan to dissolve foamed polystyrene. Such technology requires that the dissolved polystyrene be recovered from the organic solvent using one of several techniques. Such techniques include but are not limited to precipitation, extrusion, and flash evaporation. Because dissolving the polystyrene into an organic solvent at low temperatures does not degrade the polystyrene it is a preferred method. Having to recover the dissolved polystyrene from the solvent results in an extra step being needed, requiring more energy input into the process.

BRIEF SUMMARY OF THE INVENTION The present invention is an improvement over the current methods in the prior art since the solvent/cosolvent system does not dissolve the polystyrene. Rather the solvent system in the present invention causes the polystyrene lattice to collapse. The lattice collapse results in the desired reduction in polystyrene volume and simple filtration can separate the collapsed polystyrene from the solvent without precipitation, or flash evaporation. In the presence of polyethylene terephthalate, the PET sinks and is subsequently separated and recovered by simple decantation. . In the presence of polyethylene terephthalate and another insoluble polymer such as polyolefins, the PET sinks with the polyolefϊns and is subsequently re- dissolved by heating and then separated and recovered by precipitation from solution. The solvent system comprising mainly dimethyl esters of succinate, glutarate, and adipate with a cosolvent that is preferably propylene glycol has the right solvency properties to collapse the polystyrene structure without dissolving it. The present invention provides a safe more cost-effective recycling method for polystyrene using the above-mentioned solvent system.

Thus the present invention provides a method for recovering polystyrene and polyethylene terephthalate from a mixture of plastics wherein said mixture of plastics include foamed polystyrene and polyethylene terephthalate comprising the steps of: (a) collapsing foamed polystyrene in a mixture of foamed polystyrene and polyethylene terephthalate without dissolving polystyrene or polyethylene terephthalate thus producing three separate immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising for every 100 parts by weight of a dibasic organic acid ester selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms, at least one cosolvent selected from the group consisting of;

(i) from 10 to 66.7 parts by weight ethylene glycol, (ii) from 10 to 66.7 parts by weight propylene glycol,

(in) from 17.6 to 42.8 parts by weight propylene carbonate, (iv) from 53.8 to 66J parts by weight dimethyl sulfoxide, (v) from 53.8 to 66.7 parts by weight diacetone alcohol, and (vi) from 42.8 to 66J parts by weight isopropyl alcohol; and (b) separating and recovering a softened collapsed polystyrene phase floating on the top of a liquid solvent system phase.

The present invention also provides a method for recycling foamed polystyrene comprising the steps of:

(a) collapsing foamed polystyrene without dissolving polystyrene thus producing two immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising a dibasic organic acid ester solvent selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms and a cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, isopropyl alcohol, and mixtures thereof wherein the cosolvent is present in said solvent/cosolvent system from at least 10 weight percent up to 40 weight percent of said solvent/cosolvent system; (b) separating a softened collapsed polystyrene phase from a liquid solvent system phase; and

(c) recovering polystyrene.

In one specific embodiment wherein a mixture of foamed polystyrene and polyethylene terephthalate are contacted with a solvent system according to the instant invention such as to collapse the foamed polystyrene, the step of separating a softened collapsed polystyrene phase from a liquid solvent system phase is followed by the further steps of:

(c) heating the liquid solvent system phase with remaining plastics produced in step (b) to a temperature of 160 to 225°C such as to dissolve said polyethylene terephthalate phase;

(d) separating said liquid solvent system phase with dissolved polyethylene terephthalate from undissolved plastic; and

(e) recovering polyethylene terephthalate phase from said liquid solvent system phase.

In another specific embodiment wherein a mixture of plastics including foamed polystyrene, polyethylene terephthalate and at least one other insoluble polymer are contacted with a solvent system according to the instant invention such as to collapse the foamed polystyrene, the step of separating a softened collapsed polystyrene phase from a liquid solvent system phase is followed by the further steps of:

(c) heating the liquid solvent system phase with remaining plastics produced in step (b) to a temperature of 160 to 225°C such as to dissolve said polyethylene terephthalate phase; (d) separating said liquid solvent system phase with dissolved polyethylene terephthalate from undissolved plastic; and (e) recovering polyethylene terephthalate phase from said liquid solvent system phase.

Preferably the solvent/cosolvent polymer mixture useful in the above processes comprises from 1 to 40 weight percent polystyrene being collapsed in from 99 to 60 weight percent of a solvent/cosolvent system. Based on the relative densities of the respect phases involved, the softened collapsed polystyrene phase will float on the liquid solvent system phase while the polyethylene terephthalate phase does not float.

It is an object of the present invention to provide an improved process for recovering polystyrene using a novel solvent system for collapsing polystyrene foam without dissolving the polystyrene. It is an associated object of the present invention to provide such a solvent system based on a mixture of at least one polystyrene solvent categorically having Hansen solubility parameters within the corresponding polystyrene Hansen solubility envelope and at least one companion cosolvent categorically having Hansen solubility parameters outside the corresponding polystyrene Hansen solubility envelope. Fulfillment of these objects and the presence and fulfillment of other objects will be apparent upon complete reading the attached specification and claims taken in view of the drawing. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING FIGURE 1 is a two dimensional representation of a Hansen solvent map illustrating selected solvents relative to a polystyrene polymer solubility envelope.

FIGURE 2 is a flow diagram illustrating a reclaiming process for polystyrene and PET from a mixed plastic waste stream.

DETAILED DESCRIPTION OF THE INVENTION The liquid phase solvent system useful in the present invention involves a blend or combination of two different miscible components. The first component consists of the dialkyl esters of dibasic organic acids (DBE's), either used individually or as a mixture.

Categorically these so-called DBE's are (by themselves) solvents for polystyrene, in that DBE's dissolve polystyrene over a broad range of conditions and concentrations. Preferably the organic esters useful in the present invention are formed or produced by reacting dibasic organic acids such as adipic acid, glutaric acid, succinic acid, Ci to C alkyl substituted derivatives of these dibasic acids, mixtures thereof and the like with alcohols or mixtures of alcohols having from 1 to 12 carbons. Of particular interest are the dimethyl esters of adipic, glutaric, and succinic acid, mixtures of these dimetyl esters and esters of 2-ethyl succinic acid and 2- methyl glutaric acid and mixtures of diisobutyl esters of adipic, glutaric, and succinic acids. Such solvents are commercially available under the product name DBE from E. I. Du Pont de Nemours and Company, Inc. located in Wilmington, Delaware.

The second component of the liquid phase solvent system useful in the present invention consists of an organic liquid cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, and isopropyl alcohol. For purposes of this invention these second components are referred to as cosolvents in that they are miscible with the DBE first solvent component over a broad range of conditions and concentrations. However, in contrast to the DBE's, these second components (by themselves) are categorically non-solvents for polystyrene. As such and for purposes of this invention the liquid phase solvent system is also referred to herein as a "solvent/cosolvent" with the implicit understanding that by definition what is being referred to is a liquid phase that is capable of collapsing foamed polystyrene into a softened polymeric phase (perhaps better viewed as a plastisized polystyrene) but does not dissolve the polystyrene. In other words and as an alternate perspective, the liquid phase solvent system comprises a solvent component capable of dissolving polystyrene and a cosolvent component which does not dissolve polystyrene wherein the amount of cosolvent present in the multi component solvent/cosolvent combination is sufficient to prevent polystyrene from dissolving but insufficient to prevent collapse of the foam polystyrene lattice or cellular foam structure.

The novel liquid phase solvent system of the present invention, how it functions, and how it differs unexpectedly from prior art predicted compositional behavior can perhaps be better explained and understood by reference to Figure 1 and to the Hansen Solubility Parameter System. In principle, each solvent has a unique set of solvency characteristics described by their Hansen parameters: _D = dispersive or "non-polar" parameter; p = polar parameter; and _H ⁼ hydrogen bonding parameter. Each of these parameters describes the bonding characteristic of a solvent in terms of polar, non-polar, and hydrogen bonding tendencies. It is customary to plot solvents on a Hansen map where the x-axis is polar bonding and the y-axis is hydrogen bonding. Such a plot reveals that each solvent has a unique place on the map, which results in different solvency towards polymers. Lists of polymer solubilities in solvents have been developed with emphasis on room temperature and pressure conditions. It is also known that increasing the temperature will increase solvency. What is a non-solvent at a low temperature may become a solvent with increased temperature. Component parameters for polymers may also be assigned and plotted on a Hansen map thus creating a coherent region (or envelope) of solubility on the Hansen map such that any solvent whose parameters reside within this space should dissolve the polymer in question. Also, according to the Hansen Solubility Parameter System a simple mathematical mixing rule can be applied in order to derive or calculate the respective Hansen parameters for a blend of solvents from knowledge of the respective parameters of each component of the blend and the volume fraction of the component in the blend. Thus according to this mixing rule _Vbten ⁼ i _ft- and _H««rf ⁼ ;^• _H where , is the volume fraction of the z* component in the blend and where summation is over all i components. For further details and explanation of the Hansen Solubility Parameter System see for example Hansen, C. M. and Beerbower, Kirk-Othmer, Encyclopedia of Chemical Technology, (Suppl. Vol. 2^nd Ed), 1971, pp 889-910 and "Hansen Solubility Parameters A User's Handbook" by Charles Hansen, CRC Press, 1999. Figure 1 is a two dimensional Hansen map as described above with the x-axis representing the polar bonding, _P, and the y-axis representing the hydrogen bonding, _H- It should be appreciated that this Figure 1 is an approximation in that the third parameter is not being illustrated. Typically this common practice is rationalized on the basis that the non-polar parameter, _D, for most commonly employed organic solvents

5 is invariant and very similar to each other. As illustrated in Figure 1, the _P verses _H position for many well-known organic solvents is plotted using various indicia to designate the unique position of the respective solvent along with a common name or acronym for the compound. Also plotted on the lower left of Figure 1 is a curved line labeled "PS solubility envelope".

[0 This line represents the demarcation between solvency and insolvency for polystyrene. On the inside of the closed area made by the intersection of the x-axis and y-axis and the PS solubility envelope are found the solvents that dissolve polystyrene. On the outside of this area are found the non- solvents that categorically do not dissolve polystyrene. Thus and as

.5 illustrated, the cosolvents ethylene glycol, propylene carbonate (PC), and dimethyl sulfoxide (DMSO) are outside the polystyrene solubility envelope while DBE is within the polystyrene solubility envelope.

Conceptually and in principle (but not in fact for the present invention) a tie line drawn between the position of DBE and the position of

10 a cosolvent (not shown in the drawing) would intersect the polystyrene solubility envelope such as to quantitatively divide this tie line into two line segments the lengths of which would be proportional to the volume fractions of solvent and cosolvent necessary to make a blend that would represent the transition from solvent to non-solvent (as predicted by the

15 above mixing rule equations). In contrast, it has now been discovered that there is an alternate behavior manifested relative to foamed polystyrene for combinations of DBE solvent and selected cosolvents that does not conform to the previously known Hansen parameter mixing rule. Literally the novel compositions of the instant invention exhibit a lack of solubility for i0 polystyrene yet the mixing rule may or may not predict (and frequently does predict) that the blend should dissolve polystyrene. Further, the solvent/cosolvent systems of the present invention exhibit the ability to collapse the foamed polystyrene lattice and/or cellular structure. Thus inherently releasing entrained blowing agent or gaseous phase and resulting in a softened or plastisized polystyrene that tends to float on the liquid phase solvent/cosolvent without dissolving therein.

The relative amount of cosolvent and DBE solvent employed in the liquid phase solvent system will vary predominantly according to the selection of cosolvent and to a lesser degree according to the selection of DBE or DBE mixture. Typically the lower limit of the amount of cosolvent employed relative to DBE will correspond to the onset of insolubility of polystyrene in the blend. In other words, there needs to be sufficient cosolvent present in the blend to achieve the desired lack of dissolving of polystyrene. Preferably, there must be at least 10% by weight cosolvent or mixture of cosolvents to observe the desired lack of solublizing the polystyrene. The upper limit of amount of cosolvent employed relative to the amount of solvent/cosolvent blend will correspond to the onset of the lack of the ability of the blend to collapse the foamed polystyrene. In other words, there must be sufficient solvent present to collapse the foam structure but not enough cosolvent to inhibit this process. Typically weight percentages of cosolvent numerically in excess 40 % of the blend will lead to rates of collapse of foamed polystyrene that are impractical. As shown in the examples below, the weight percent of cosolvent necessary to achieve the desired results varies from cosolvent to cosolvent with ethylene glycol requiring from 10 to 40 weight % of the liquid phase system; propylene glycol requiring from 10 to 40 weight %, propylene carbonate requiring from 15 to 30 weight %, dimethyl sulfoxide requiring from 35 to 40 weight %, diacetone alcohol requiring from 35 to 40 weight %, and isopropyl alcohol requiring from 30 to 40 weight %. On a 100 parts by weight of a dibasic organic acid ester basis these weight percent ranges correspond to from 10 to 66.7 parts by weight ethylene glycol; from 10 to 66.7 parts by weight propylene glycol; from 17.6 to 42.8 parts by weight propylene carbonate; from 53.8 to 66J parts by weight dimethyl sulfoxide; from 53.8 to 66J parts by weight diacetone alcohol; and from 42.8 to 66.7 parts by weight isopropyl alcohol.

The actual forming or making of the liquid solvent phase can be by any of the well-known methods generally practiced in the art for adding one miscible liquid to another. It should be appreciated that the choice of concentration of each cosolvent will be influenced by the above details associated with individual cosolvents. Preferably the amount of the cosolvent employed is in the mid portion of the range unless the ultimate use inherently leads to selective consumption of the solvent or cosolvent thus influencing the preference towards one or other limit of operability. It should also be appreciated that the liquid phase does not have to be free of water and that some water will be present in commercial grades of solvents. For purposes of this invention the presence of some water can in fact be considered an equivalent to a cosolvent and utilized as a supplemental cosolvent. The use of the solvent systems in the process of the present invention is generally viewed as being particularly applicable and advantageously employed to collapse polystyrene foam and subsequently to separate, recover and recycle the collapsed polystyrene. The improved separation and recycle process according to the invention is viewed as being effective in collapsing generally any low density polystyrene foam including in particular both open and closed cellular foams as well the so- called beaded polystyrene foams. Because of the fact that the softened or plastisized polystyrene produced from the collapsed foam floats on the liquid phase, the resulting collapsed polystyrene and solvent mixture is amenable to simple phase separation, isolation and recovery. Such a process is effective at compositional ranges as low as from 1 weight percent foamed polystyrene and 99 weight percent of a solvent/cosolvent system up to 40 weight percent foamed polystyrene and 60 weight percent of a solvent/cosolvent system. The recovered softened polystyrene is also amenable to reprocessing and removal and recovery of residual solvent by such subsequent processes as injection and/ or extrusion molding and the like. The volatilized solvents withdrawn from such subsequent processing can be advantageously recycled to the foamed polystyrene contact step.

Figure 2 of the drawing schematically illustrates the basic concept of collapsing foamed polystyrene as well as the subsequent or simultaneous step of separating and recovering the resulting softened polystyrene in the presence of PET using the liquid phase solvent system of the present invention. As illustrated, the foamed polystyrene when in contact with the liquid solvent/cosolvent phase collapses into a buoyant softened state and thus can be readily separated from the top of the liquid solvent system. It should be appreciated that the collapsing and floating of polystyrene from the mixed plastic waste stream can be further facilitated by an optional grind or chopping step prior to liquid phase contact. Also, any non-foamed polystyrene will tend to be removed along with the collapsed polystyrene. This softened or plastisized polystyrene can then be conveniently directed to an extruder for recovery and recycle of entrained solvent/cosolvent (not shown in Figure 2) simultaneously with fabrication as a manufactured polystyrene product or the like.

It should be further appreciated that in the specific embodiment of the invention wherein foamed polystyrene is recovered in the absence of other plastics the schematic illustration of Figure 2 still applies except the recovery of PET from the bottom of the solvent/cosolvent solution is omitted. Also in a mixed plastic waste stream consisting for example of foamed polystyrene, PET, low density polyethylene (LDPE), high density polyethylene (HDPE), and polypropylene (PP), the polystyrene can be separated from the other plastics at room temperature and pressure by immersing or contacting the mixed plastics waste stream with the liquid solvent/cosolvent system. In such an embodiment, the liquid solvent system along with the PET, LDPE, HDPE, and PP, after the separation and removal of polystyrene, is then heated to a temperature in the range of 160 to 225°C. The PET remaining in the mixed plastics dissolves at these temperatures into the liquid solvent/cosolvent phase at atmospheric pressure. An advantage of the present invention is that the dimethyl ester solvent system typically has a boiling point above 200°C therefore no pressure (extra energy input) is required on the system to keep the solvent in its liquid state. This also makes the separation scheme less complicated since the separations always involve a liquid stream. Since degradation for some polymers starts to occur at about 250°C the present solvent system is ideal. The actual separation of the PET can be accomplished by hot filtration (or other similar methods of separation well known in the art) to separate the dissolved PET from the undissolved other plastics. This step (not illustrated in Figure 2) is followed by cooling of the dissolved PET solution to 160°C to precipitate out the PET. Preferably, from 1% to 40% by weight polystyrene and/or PET can be separated from a mixed plastic stream.

The following examples are presented to more fully demonstrate and further illustrate various individual aspects and features of the present invention and the showings are intended to further illustrate the differences and advantages of the present invention. As such the examples are felt to be non-limiting and are meant to illustrate the invention but are not meant to be unduly limiting. Example 1

A series of individual mixtures involving 20 wt. % foamed polystyrene in contact with 80 wt % liquid phase solvent/cosolvent were prepared and observed at room temperature and pressure by adding the foamed polystyrene to the solvent/cosolvent. The DBE solvent employed was a blend of dimethyl esters of succinate, glutarate, and adipate (i.e., 20 wt. % succinate, 60 wt. % glutarate, and 20 wt. % adipate). The relative amount of the cosolvent to DBE (i.e., relative weight percentages in the liquid phase) was varied such as to establish the concentration range for operability. The following results were obtained:

Example 2: Comparative example In a manner analogous to Example 1, the behavior of a series of individual mixtures involving 20 wt. % foamed polystyrene in contact with 80 wt % liquid phase single solvents at room temperature and pressure was observed with the following results:

Example 3 Foamed polystyrene was added to a mixture of 80 wt% dimetyl esters (which was 20% succinate, 60% glutarate, 20% adipate) and 20 wt% propylene glycol solvent to determine the capacity of the solvent for collapsing foamed polystyrene. It was found that from 1 wt% to 40 wt.% foamed polystyrene could be added to the solvent/cosolvent. Example 4 Foamed polystyrene was added to 3 separate solvent systems: 80 wt% dimethyl succinate/20 wt% propylene glycol; 80 wt% dimethyl glutarate/20 wt% propylene glycol; and

80 wt% dimethyl adipate/20 wt% propylene glycol. The foamed polystyrene was collapsed when placed in all three of the solvent systems. The recovery of the polystyrene was 100% in all three solvent systems.

Example 5 20 wt.% PET was added to a solution containing dimethyl esters of succinate, glutarate, and adipate and the resulting mixture was heated. At 180°C the PET begins to "soften" and at 200°C the PET dissolved. Upon cooling of the solution to a range of 160 to 170°C the PET re-precipitated out of the DBE solution.

Thermal analysis of the precipitated PET by differential scanning calorimetry shows that the thermal properties of the reclaimed PET are equal to that of the virgin. Both PET samples show a characteristic endotherm at 245°C and an exotherm at 453°C.

Comparative Example 6 PET was not soluble in the following other solvents tested up to 225°C (solvents systems were under pressure where necessary in order to keep them in the liquid state): Heptane, diethylene glycol butyl ether, ethylene glycol, diisobutyl ketone, toluene, propylene glycol, propylene glycol methyl ether, and diacetone alcohol.

Example 7: separation of polystyrene from PET A mixed plastics stream of 4 grams of foamed polystyrene, 4 grams of PET, was added to 100 grams of a solvent consisting of dimethyl esters of succinate, glutarate, and adipate with 20% of cosolvent propylene glycol. The polystyrene collapsed at room temperature and the collapsed polystyrene was separated and recovered by decantation (i. e., in this case floating off of the collapsed polystyrene with some of the solvent). The PET was then dissolved in the remainder of the solvent by heating to 205°C and recovered as a recycle solution. The resulting solution was cooled to 160°C and PET was filtered from recycle solvent. Recovery of polystyrene from mixed plastics was 100% and recovery of PET was 98%. Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following claims are not to be so limited but are to be afforded a scope commensurate with the wording of each element of the claim and equivalents thereof.

Claims

CLAIMS We claim:

1. A method for recycling foamed polystyrene comprising the steps of: (a) collapsing foamed polystyrene without dissolving polystyrene thus producing two immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising for every 100 parts by weight of a dibasic organic acid ester selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms, at least one cosolvent selected from the group consisting of;

(vii) from 10 to 66.7 parts by weight ethylene glycol, (viii) from 10 to 66.7 parts by weight propylene glycol, (ix) from 17.6 to 42.8 parts by weight propylene carbonate, (x) from 53.8 to 66.7 parts by weight dimethyl sulfoxide, (xi) from 53.8 to 66.7 parts by weight diacetone alcohol, and

(xii) from 42.8 to 66J parts by weight isopropyl alcohol;

(b) separating a softened collapsed polystyrene phase from a liquid solvent system phase; and

(c) recovering polystyrene.

2. A method of Claim 1 wherein solvent/cosolvent polymer mixture is from 1 to 40 weight percent polystyrene and from 99 to 60 weight percent solvent/cosolvent system.

3. A method for recycling foamed polystyrene comprising the steps of: (a) collapsing foamed polystyrene without dissolving polystyrene thus producing two immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising a dibasic organic acid ester solvent selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms and a cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, isopropyl alcohol, and mixtures thereof wherein the cosolvent is present in said solvent/cosolvent system from at least 10 weight percent up to 40 weight percent of said solvent/cosolvent system;

(c) recovering polystyrene.

4. A method of Claim 3 wherein solvent/cosolvent polymer mixture is from 1 to 40 weight percent polystyrene and from 99 to 60 weight percent solvent/cosolvent system.

5. A method for recovering polystyrene and polyethylene terephthalate from a mixture of foamed polystyrene and polyethylene terephthalate comprising the steps of:

(a) collapsing foamed polystyrene in a mixture of foamed polystyrene and polyethylene terephthalate without dissolving polystyrene or polyethylene terephthalate thus producing three separate immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising for every 100 parts by weight of a dibasic organic acid ester selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms, at least one cosolvent selected from the group consisting of; (xiii) from 10 to 66J parts by weight ethylene glycol,

(xiv) from 10 to 66.7 parts by weight propylene glycol, (xv) from 17.6 to 42.8 parts by weight propylene carbonate, (xvi) from 53.8 to 66.7 parts by weight dimethyl sulfoxide, (xvii) from 53.8 to 66.7 parts by weight diacetone alcohol, and (xviii) from 42.8 to 66.7 parts by weight isopropyl alcohol;

(b) separating and recovering a softened collapsed polystyrene phase floating on the top of a liquid solvent system phase; and

(c) separating and recovering polyethylene terephthalate phase from a liquid solvent system phase.

6. A method for recovering polystyrene and polyethylene terephthalate from a mixture of foamed polystyrene and polyethylene terephthalate comprising the steps of: (a) collapsing foamed polystyrene in a mixture of foamed polystyrene and polyethylene terephthalate without dissolving polystyrene or polyethylene terephthalate thus producing three separate immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising a dibasic organic acid ester solvent selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms and a cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, isopropyl alcohol, and mixtures thereof wherein the cosolvent is present in said solvent/cosolvent system from at least 10 weight percent up to 40 weight percent of said solvent/cosolvent system;

7. A method for recovering polystyrene and polyethylene terephthalate from a mixture of plastics wherein said mixture of plastics include foamed polystyrene and polyethylene terephthalate and at least one other insoluble polymer comprising the steps of:

(a) collapsing foamed polystyrene in a mixture of foamed polystyrene and polyethylene terephthalate without dissolving polystyrene or polyethylene terephthalate thus producing three separate immiscible phases by contacting said polystyrene foam at a temperature from

0 to 50°C with a solvent system comprising for every 100 parts by weight of a dibasic organic acid ester selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms, at least one cosolvent selected from the group consisting of; (xix) from 10 to 66J parts by weight ethylene glycol, (xx) from 10 to 66.7 parts by weight propylene glycol, (xxi) from 17.6 to 42.8 parts by weight propylene carbonate,

(xxii) from 53.8 to 66.7 parts by weight dimethyl sulfoxide, (xxiii) from 53.8 to 66.7 parts by weight diacetone alcohol, and (xxiv) from 42.8 to 66.7 parts by weight isopropyl alcohol;

(b) separating and recovering a softened collapsed polystyrene phase floating on the top of a liquid solvent system phase;

8. A method for recovering polystyrene and polyethylene terephthalate from a mixture of plastics wherein said mixture of plastics include foamed polystyrene and polyethylene terephthalate and at least one other insoluble polymer comprising the steps of:

(a) collapsing foamed polystyrene in a mixture of foamed polystyrene and polyethylene terephthalate without dissolving polystyrene or polyethylene terephthalate thus producing three separate immiscible phases by contacting said polystyrene foam at a temperature from 0 to 50°C with a solvent system comprising a dibasic organic acid ester solvent selected from the group consisting of dialkyl adipate, dialkyl glutarate, dialkyl succinate and mixtures thereof, wherein said alkyl groups are either identical or different and have from 1 to 12 carbon atoms and a cosolvent selected from the group consisting of ethylene glycol, propylene glycol, propylene carbonate, dimethyl sulfoxide, diacetone alcohol, isopropyl alcohol, and mixtures thereof wherein the cosolvent is present in said solvent/cosolvent system from at least 10 weight percent up to 40 weight percent of said solvent/cosolvent system; (b) separating and recovering a softened collapsed polystyrene phase floating on the top of a liquid solvent system phase; (c) heating the liquid solvent system phase with remaining plastics produced in step (b) to a temperature of L60 to 225°C such as to dissolve said polyethylene terephthalate phase;