WO2002059189A2

WO2002059189A2 - Method for separating polystyrene and polyethylene terephthalate

Info

Publication number: WO2002059189A2
Application number: PCT/US2001/050739
Authority: WO
Inventors: Nicholas Edward Kob, Iii
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2000-11-08
Filing date: 2001-10-22
Publication date: 2002-08-01
Anticipated expiration: 2003-05-08
Also published as: TW558562B; WO2002059189A3; AU2002246886A1

Abstract

An improved method of recovering and recycling polystyrene and polyethylene terephthalate from a mixed plastic waste stream involving dissolving the polystyrene in a dibasic organic acid ester solvent (e.g., a mixture of dimethyl adipate, dimethyl glutarate, and dimethyl succinate) at a temperature range of 0 °C to less than 160 °C without dissolving the polyethylene terephthalate and then after separating and recovering the polystyrene solution, dissolving the polyethylene terephthalate in the same solvent at a temperature range of from 160 °C to 225 °C. The polystyrene can be conveniently isolated from solution by evaporation of solvent while the polyethylene terephthalate can be isolated from solution by cooling and precipitation.

Description

TITLE OF THE INVENTION

METHOD FOR SEPARATING POLYSTYRENE AND

POLYETHYLENE TEREPHTHALATE

BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates to the use of dialkyl esters of dibasic organic acids (DBE's), either individually or as a mixture, as solvents to selectively separate and recover both polystyrene and the polyethylene terephthalate (PET) from a mixed plastic stream. More specifically but not by way of limitation, the present invention relates to the use of dimethyl adipate, dimethyl glutarate, dimethyl succinate and mixtures thereof to selectively dissolve polystyrene and PET sequentially at two different temperature ranges.

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 in 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.

BRIEF SUMMARY OF THE INVENTION The present invention provides a method for recovering polystyrene and polyethylene terephthalate from a mixture of plastics wherein said mixture of plastics include polystyrene and polyethylene terephthalate in combination with at least one other plastic comprising the steps of:

(a) dissolving polystyrene from a mixture of plastics containing polystyrene and polyethylene terephthalate without dissolving polyethylene terephthalate and other plastics thus producing two separate immiscible phases by contacting said mixture of plastics at a temperature from 0°C to less than 160°C with a solvent comprising 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;

(b) separating the solvent with dissolved polystyrene phase produced in step (a) from the undissolved polyethylene terephthalate and others plastics thus recovering polystyrene; (c) dissolving polyethylene terephthalate from said undissolved polyethylene terephthalate and others plastics produced in step (b) without dissolving said other plastics thus producing two separate immiscible phases by contacting said undissolved polyethylene terephthalate and others plastics at a temperature from 160°C to 225°C with a solvent comprising 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; and

(d) separating the solvent with dissolved polyethylene terephthalate phase produced in step (c) from the undissolved other plastics thus recovering polyethylene terephthalate. Preferably the polyethylene terephthalate isolated and recycled by cooling the solvent containing the dissolved polyethylene terephthalate to a temperature below 160°C thus precipitating polyethylene terephthalate from solution. Similarly, the polystyrene is isolated and recycled by evaporating the solvent from the solution containing the dissolved polystyrene.

Conveniently final traces of DBE solvent can be removed from the recycled plastics during subsequent melt extrusion associated with product fabrication or the like and the DBE can be reused as solvent. Preferably the solvent is selected is dimethyl adipate, dimethyl glutarate, dimethyl succinate and mixtures of these diesters.

It is an object of the present invention to provide an improved process for recovering both polystyrene and polyethylene terephthalate from a mixed plastic waste stream using a single DBE solvent system at two different temperature ranges. It is a further object of the present invention to provide economic process for recycling polystyrene and PET from a mixed plastic stream also containing polyolefins such as LDPE, HDPE, and PP. 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 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 useful in the present invention involves the dialkyl esters of dibasic organic acids (DBE's), either individually or as a mixture. Categorically these so-called DBE's are 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 C₄ 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 solvent systems according to the present invention are viewed as being particularly useful to separate, recover and recycle polystyrene from PET in the presence of polyolefins. Typically, this dissolving of polystyrene is accomplished at a temperature range from 0°C to less than 160°C at essentially atmospheric pressure (although higher pressures are contemplated as being effective but not necessary). The solvent systems are also viewed as being effective in collapsing and subsequently dissolving generally any low density polystyrene foam including in particular both open and closed cellular foams as well the so-called beaded polystyrene foams. The process of selectively dissolving polystyrene is generally effective at compositional ranges as low as from 1 weight percent polystyrene and 99 weight percent of the solvent system up to 40 weight percent polystyrene and 60 weight percent of solvent. The solution phase containing the dissolved polystyrene can be conveniently separated from the undissolved plastic by methods generally known in the art, including by way of example but not by way of limitation; filtration, decantation. Evaporation and the like. The recovered softened polystyrene is also amenable to reprocessing and removal and recovery of residual solvent by such subsequent processes such as during injection and/or extrusion molding. The volatilized solvents withdrawn from such subsequent processing can be advantageously recycled to the solvent/polystyrene contact step (i.e., the dissolving of polystyrene) or the like. The solvent systems according to the present invention are also viewed as being particularly useful to separate, recover and recycle PET from the polyolefins. However, this dissolving of PET is typically to be accomplished at an elevated temperature range from 160°C to 225°C again at essentially atmospheric pressure (although higher pressures are again contemplated as being effective but not necessary). The process of selectively dissolving PET at elevated temperature is generally effective at compositional ranges as low as from 1 weight percent PET and 99 weight percent of the solvent system up to 40 weight percent PET and 60 weight percent of solvent. The solution phase containing the dissolved PET can be conveniently separated from the undissolved plastic by merely cooling the solution and allowing the PET to precipitate. The precipitated PET can then be recycled by methods generally known in the art, including by way of example but not by way of limitation; filtration, decantation, evaporation and the like. Similarly to the polystyrene, the recovered softened PET is also amenable to reprocessing and removal and recovery of residual solvent by such subsequent processes as injection, extrusion, and/or blow molding and the like. The volatilized solvents withdrawn from such subsequent processing can be advantageously recycled to the solvent/polystyrene or PET contact step (i.e., the dissolving of polymer).

Figure 1 of the drawing illustrate the basic concepts involving the more economical process for separating and recovering polystyrene and PET from a mixed plastics stream using the liquid phase solvent system of the present invention. In a mixed plastic waste stream, consisting for example of foamed polystyrene and 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.

As illustrated the foamed polystyrene when in contact with the liquid solvent phase collapses and dissolves in the liquid solvent system. It should be appreciated that the collapsing and dissolving of foamed 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 dissolved and removed along with the collapsed polystyrene. This polystyrene solution can then be optionally subjected to an evaporation/distillation step and/or conveniently directed to an extruder for recovery and recycle of entrained solvent simultaneously with fabrication as a manufactured polystyrene product or the like.

As further illustrated in Figure 1, the undissolved solids remaining after the removal of the polystyrene/DBE solution, i.e., the PET, LDPE, HDPE, and PP, is then heated in the presence of additional DBE solvent 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 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 isolation and recovery of the PET/DBE solution can be accomplished by hot filtration (or other similar methods of separation well known in the art) thus separating the dissolved PET from the undissolved other plastics. This step 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 organic solvents were prepared and observed at room temperature and pressure by adding the foamed polystyrene to the organic solvent. 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 following results were obtained:

Example 2

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 3 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 4: mixed plastics A mixed plastics stream of 4 grams of foamed polystyrene, 4 grams of PET, 4 grams of high density polyethylene (HDPE), 4 grams of low density polyethylene (LDPE), and 4 grams of polypropylene was added to 100 grams of a solvent consisting of dimethyl esters of succinate, glutarate, and adipate. The polystyrene dissolved at room temperature and the polystyrene was separated by filtration of the dissolved polystyrene from other plastics. The polystyrene was recovered from solution by evaporation of the solvent and precipitation of the polystyrene. The undissolved other plastics were placed into 100 grams of DBE solvent. Upon heating to 205° the PET dissolved. The mixture was then hot filtered to separate the hot solution containing the dissolved PET from the other undissolved plastics. Cooling the PET solution to 160°C precipitated the PET. The PET was recovered by filtration. Recovery of polystyrene from mixed plastics was 100% and recovery of PET was 98%.

Example 5: mixed plastics A mixed plastics stream of 0.5 grams of polystyrene, 0.5 grams of

PET, 6.3 grams of high density polyethylene (HDPE), 6.3 grams of low density polyethylene (LDPE), and 6.3 grams of polypropylene was added to 100 grams of a solvent consisting of dimethyl esters of succinate, glutarate, and adipate. The polystyrene dissolved at room temperature and the polystyrene was separated by filtration of the dissolved polystyrene from other plastics. The polystyrene was recovered from solution by evaporation of the solvent and precipitation of the polystyrene. The undissolved other plastics were placed into 100 grams of DBE solvent. Upon heating to 205° the PET dissolved. The mixture was then hot filtered to separate the hot solution containing the dissolved PET from the other undissolved plastics. Cooling the PET solution to 160°C precipitated the PET. The PET was recovered by filtration. Recovery of polystyrene from mixed plastics was 60% and recovery of PET was 100%.

Example 6: mixed plastics A mixed plastics stream of 9.25 grams of polystyrene, 9.25 grams of PET, 0.5 grams of high density polyethylene (HDPE), 0.5 grams of low density polyethylene (LDPE), and 0.5 grams of polypropylene was added to 100 grams of a solvent consisting of dimethyl esters of succinate, glutarate, and adipate. The polystyrene dissolved at room temperature and the polystyrene was separated by filtration of the dissolved polystyrene from other plastics. The polystyrene was recovered from solution by evaporation of the solvent and precipitation of the polystyrene. The undissolved other plastics were placed into 100 grams of DBE solvent. Upon heating to 205° the PET dissolved. The mixture was then hot filtered to separate the hot solution containing the dissolved PET from the other undissolved plastics. Cooling the PET solution to 160°C precipitated the PET. The PET was recovered by filtration. Recovery of polystyrene from mixed plastics was 99% and recovery of PET was 82%.

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

CLAIMSWe claim:

1. A method for recovering polystyrene and polyethylene terephthalate from a mixture of plastics wherein said mixture of plastics include polystyrene and polyethylene terephthalate in combination with at least one other plastic comprising the steps of:

(b) separating the solvent with dissolved polystyrene phase produced in step (a) from the undissolved polyethylene terephthalate and others plastics thus recovering polystyrene;

(c) dissolving polyethylene terephthalate from said undissolved polyethylene terephthalate and others plastics produced in step (b) without dissolving said other plastics thus producing two separate immiscible phases by contacting said undissolved polyethylene terephthalate and others plastics at a temperature from 160°C to 225°C with a solvent comprising 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; and (d) separating the solvent with dissolved polyethylene terephthalate phase produced in step (c) from the undissolved other plastics thus recovering polyethylene terephthalate.

2. A process of Claim 1 further comprising the step of isolating polyethylene terephthalate by cooling the solvent with dissolved polyethylene terephthalate phase of step (d) to a temperature below 160°C thus precipitating polyethylene terephthalate from solution.

3. A process of Claim 1 further comprising the step of isolating polystyrene by evaporating the solvent from the solvent with dissolved polystyrene phase of step (b).

4. A process of Claim 1 wherein said solvent is selected from the group consisting of dimethyl adipate, dimethyl glutarate, dimethyl succinate and mixtures of these diesters.