EP1791899A1

EP1791899A1 - Process for the treatment of a mixture of abs and of ps

Info

Publication number: EP1791899A1
Application number: EP20050784664
Authority: EP
Inventors: Jean-Raphaël CAILLE; Rachida Ramdane
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2004-09-17
Filing date: 2005-09-15
Publication date: 2007-06-06
Also published as: JP2008513559A; FR2875504B1; WO2006030020A1; FR2875504A1

Abstract

Process for the treatment of a mixture of ABS (acrylonitrile-butadiene- styrene terpolymer) and of PS (polystyrene), according to which the said mixture is brought into contact with a liquid medium comprising water and a water-miscible common solvent in proportions and under pressure and temperature conditions such that the ABS is soluble therein and the PS insoluble.

Description

Process for the treatment of a mixture of ABS and of PS

The present patent application relates to a process for the treatment of a mixture of ABS and of PS and more particularly to a process for the separation of ABS and of PS by selective dissolution/precipitation.

Plastics are widely used in the manufacture of varied flexible or rigid articles, such as, for example, tarpaulins, coated fabrics and other components for the interior trim of vehicles, pipes, window frames, electrical cables possessing polymeric insulation, housings and other electrical and electronic equipment parts.

Due to the low biodegradability of these materials, their recycling has been studied for a long time. This is in particular the case with styrene polymers (PS, HIPS, SAN, ABS, and the like) present in some plastic waste streams, such as Waste Electrical and Electronic Equipment (WEEE). Conventionally, these polymers can be concentrated by fractions by density (flotation) starting from the complete waste stream. Nevertheless, by this means, some fractions are obtained which are difficult to make use of as they are composed of incompatible polymers (for example, ABS and (HI)PS). This is because the various grades of these resins have density values which are too close to one another to be suitably separated by simple flotation.

A few processes are found in the literature which make it possible, despite everything, to carry out this separation, and patents US 5 653 867 and WO 99/61158 constitute a reference in the field. The processes disclosed are based on flotation techniques and separation is obtained by virtue of the modification of the bulk density of one of the two polymers, either by modification of the pH and of the surface tension of the flotation medium (US 5 653 867) or by heat treatment of the mixture of polymers in the presence of the flotation medium (WO 99/61158). The 1st process exhibits the disadvantage of operating in an acidic medium and thus associated problems of corrosion and of safety. The 2^nd exhibits the disadvantage of a poor yield (poor selectivity) in the case of the pair of polymers with which we are concerned.

Furthermore, it is known to be able to separate a mixture of polymers by selective dissolution/precipitation of one of the polymers using an appropriate solvent. Thus, for example, the Applicant Company has developed a process for the recovery of vinyl polymers, known as the Vinyloop® process, which consists in treating articles comprising PVC with a solvent which is miscible and miscible which forms an azeotrope with water, so as to dissolve the PVC, in removing possible insoluble materials from the solution and in precipitating the PVC in solution by injection of water, at least partly in the form of steam, to carry out an azeotropic distillation of the solvent. This process is disclosed in particular in patents EP 945 481 and 1 232 204 and in application EP 1 268 628. It can be carried out batchwise (that is to say, noncontinuously, by a sequence of solubilizations/precipitations) or continuously but, whatever the version chosen, one advantage of this process is that it can operate in a loop, with complete reuse of the solvent and without discharges to the atmosphere.

The Applicant Company had the idea of applying the principle of this process to styrene polymers and has thus discovered, unexpectedly, a simple separation process which makes it possible to obtain polymers of high purity with a high yield. This discovery is based on the surprising observation that these, ABS and (HI)PS, have common « solvents » which are miscible with water but that their tolerance with respect to water is different. In reality, although PS is indeed soluble, ABS and HIPS are not strictly speaking soluble as they exhibit an insoluble crosslinked elastomeric phase. It is therefore more a matter of stable colloidal suspensions of ABS and of HIPS than of true solutions. With regard to the water, it is indeed a common nonsolvent of the targeted polymers but its « nonsolvating » power (or its precipitating effect, so to speak) appears more pronounced for (HI)PS than for ABS.

Consequently, the present invention relates to a process for the treatment of a mixture of ABS (acrylonitrile-butadiene- styrene terpolymer) and of PS (polystyrene), according to which the said mixture is brought into contact with a liquid medium comprising water and a water-miscible common solvent in proportions and under pressure and temperature conditions such that the ABS is soluble therein and the PS insoluble.

The process according to the invention is applicable to all commercial grades of ABS and of PS. In particular, the latter can be HIPS (High Impact PS), that is to say a grafted styrene-butadiene copolymer obtained by grafting polystyrene to nodules of partially crosslinked polybutadiene rubber in order to obtain impact strengthening with respect to the PS homopolymer. The mixture in which they are present can be composed solely of these polymers. Alternatively, they can comprise other polymers or substances which are preferably insoluble in the solvent chosen.

According to the invention, the common solvent has to be miscible with water (that is to say, must form a homogeneous phase with water over at least one range of concentration), an essential condition in order for water to be able to interact with the molecules of polymer(s) in solution. In order for the liquid medium to indeed have a « solvating » power with respect to the ABS, water is generally present therein as a minor component. The liquid medium preferably consists of a single-phase medium given that, with two-phase media (having respectively a water-rich phase and a solvent-rich phase), only the latter will be effective in the process (since the polymers will be dissolved in this phase alone).

The common solvent (capable of dissolving both the (HI)PS and the ABS) used in the process according to the invention can consist of a pure substance or a mixture of substances. Solvents which are highly suitable are polar organic solvents. Examples of such solvents are THF (tetrahydrofuran), MIBK (methyl isobutyl ketone), cyclohexanone and MEK (methyl ethyl ketone). The latter gives good results. At ambient temperature and pressure, an MEK/water mixture remains single-phase up to a content of water in the MEK of approximately 12 % by weight.

In the process according to the invention, the proportions of solvent and of water can be variable. However, in view of the above, it is advantageous to determine, according to the solvent, the maximum possible amount in order to retain a single-phase medium capable of dissolving the ABS. In the case where the solvent chosen is MEK, the water content of the mixture is advantageously in the region of 12 % by weight.

Several methods are possible for bringing the mixture of polymers into contact with the liquid medium.

A 1st possibility consists in bringing the mixture directly into contact with the liquid medium under conditions (in particular temperature, pressure and concentrations) such that the ABS is soluble therein and the PS insoluble, after which the undissolved PS is separated from the ABS solution by any appropriate means. Thus, according to a 1st preferred alternative form of the present invention, an ABS and (HI)PS mixture in the form of granules is introduced into a solvent composed of MEK comprising 12 % of water. The combined mixture is stirred and heated for a few minutes. Stirring is subsequently halted in order to - A -

promote separation by settling and recovery of the undissolved (HI)PS (centrifuging is possible in order to accelerate the separation by settling). Finally, the supernatant is withdrawn, the solvent is evaporated under reduced pressure and the ABS is collected. The extraction can be repeated with a fresh MEK/water mixture in order to extract the maximum amount of ABS.

A 2^nd way of proceeding consists in dissolving the two polymers of the mixture in solvent comprising little or no water and under conditions such that the two polymers dissolve and in subsequently selectively precipitating the PS by addition of water. Thus, according to a 2^nd preferred alternative form of the present invention, an ABS and (HI)PS mixture in the form of granules is « dissolved » with stirring in MEK. After complete « dissolution », the water is added to saturation and the (HI)PS selectively precipitates. Stirring is then halted to promote separation by settling and the supernatant is withdrawn. After the evaporation of the solvent under reduced pressure, the ABS is collected. The polymer separated by settling can be rinsed with a fresh MEK/water mixture in order to extract the maximum amount of ABS.

A 3rd way of proceeding consists in dissolving the ABS and the PS in a solvent/phase-separation agent mixture optionally comprising water and in subsequently removing the phase-separation agent by distillation and addition of water, if necessary (that is to say, if the liquid medium does not comprise a sufficient amount thereof). This in fact amounts, in other words, to dissolving the two polymers in a solvent/water/phase-separation agent ternary mixture (a phase-separation agent is a substance compatible with the solvent and not compatible with the water which renders the solvent more hydrophobic) or in a solvent/phase separation agent binary mixture and in subsequently removing the phase-separation agent by distillation, water being added to the medium if necessary (optional in the case of a ternary mixture but obligatory in the case of a binary mixture).

This is because the removal of the phase-separation agent renders the solvent-rich phase of the mixture richer in water and, by doing this, brings about the precipitation of the PS. In this case, it is advisable to choose a starting ternary mixture which is sufficiently rich in water and/or to gradually enrich it in water in order to be able to indeed bring about the precipitation of the PS. The latter way of proceeding advantageously involves recourse to the injection of steam or of water/solvent azeotropic vapour. A starting solvent having approximately the following composition by weight gives good results : MEK 80 %, isohexane 15 %, water, 5 %.

Thus, according to a 3rd preferred alternative form of the present invention, an ABS and (HI)PS mixture in the form of granules is dissolved with stirring in an MEK/water/hexane mixture. After dissolution, the hexane is distilled off by injection of water/solvent azeotropic vapour. The MEK is then found to be saturated with water and the (HI)PS selectively precipitates. Stirring is then halted in order to promote separation by settling and the supernatant is withdrawn. After evaporation of the solvent under reduced pressure, the ABS is collected. The polymer separated by settling can be rinsed with a fresh MEK/water mixture in order to extract the maximum amount of ABS. The phase-separation agent (hexane) can also be removed by injection of steam but, in this case, it is possible for the medium at the end of precipitation to be three- phase (presence of a third denser aqueous phase). The presence of this aqueous phase generally does not make possible effective centrifuging of the solid phase, which has a tendency to become located at the interface of the two liquids, which is unfavourable.

The last two methods are preferred in the case of mixtures comprising other polymers or substances than ABS and PS which are insoluble in the chosen solvent as they make it possible to more easily purify the solution of the polymers from the said substances before the precipitation of the PS.

Once the PS has precipitated from the ABS solution, any conventional method can be used to separate it therefrom : separation by settling, centrifuging, and the like. The ABS solution is then treated by any appropriate way (separation by settling at a lower temperature and/or pressure; devolatilization under vacuum; azeotropic distillation of the solvent/water mixture (when a solvent is chosen which forms an azeotrope with water, which is the case with the abovementioned solvents, namely : THF, MIBK, cyclohexanone and MEK); atomization; and the like).

Advantageously, in order to increase the yield of the process, the dissolution/precipitation operations can be repeated and the various solutions collected can be combined for the final treatment for recovery of the ABS.

During the dissolution(s), appropriate heating and/or stirring means are advantageously resorted to.

The separated polymers collected can be subjected to appropriate treatments (rinsing, drying, and the like) for the purpose of purifying them. The process according to the invention exhibits the advantage of being able to be easily made use of on existing plants designed for the abovementioned Vinyloop^® process. This is because the solvents which are used in this process (and in particular MEK) are also common solvents of ABS and of (HI)PS. Consequently, the present invention also relates to a process according to which :

(a) an article based on at least one vinyl chloride or vinylidene chloride polymer is shredded into fragments with a mean size of 1 cm to 50 cm, in the event of these sizes being exceeded;

(b) the fragments of the article are brought into contact with an azeotropic or quasiazeotropic mixture of water and of a solvent capable of dissolving the polymer ;

(c) the precipitation of the polymer dissolved in the solvent is brought about by reduction in pressure and by injection of steam into the solution thus obtained, which additionally brings about the entrainment of a solvent/water azeotrope and thus leaves remaining a mixture essentially composed of water and of solid polymer particles ;

(d) the polymer particles, on the one hand, and the vapours resulting from the azeotropic distillation, on the other hand, which are condensed, are collected ;

(e) at least a fraction of the vapours condensed in the preceding stage is used as all or part of the liquid medium in a process for the separation of ABS and of (HI)PS as described above.

Stages (a) to (d) of this process are described in more detail in the abovementioned patents EP 945 481 and 1 232 204, the contents of which to this effect are introduced by reference into the present patent application.

As mentioned above, a phase-separation agent can be present in the liquid medium and thus in the condensed vapours used. This is the case when dissolution of the polymer takes place in the presence of this agent as disclosed in the abovementioned Application EP 1 268 628 (and the content of which to this effect is introduced by reference into the present patent application).

The present invention is illustrated without implied limitation by the following examples. Example 1 (illustrative of the 1st method according to the invention) :

400 g of PS and 400 g of ABS were introduced into a reactor containing 3520 g of MEK and 480 g of water. The reactor was closed and the stirred medium was brought to 90°C for 20 minutes and then cooled down to 53 °C. Stirring was then halted and the combined mixture was separated by settling for 5 minutes. As much supernatant as possible (which supernatant was present in the form of a milky liquid) was withdrawn, i.e. 3690.30 g.

1760 g of MEK and 240 g of water were added to the polymer, separated by settling, in the reactor. The stirred medium was again brought to 90°C for 10 minutes and was then cooled to 53°C. Stirring was halted and the combined mixture was separated by settling for 15 minutes. As much supernatant as possible was withdrawn, i.e. 1951.75 g.

1760 g of MEK and 240 g of water were added to the polymer, separated by settling, in the reactor. The stirred medium was again brought to 90°C for 10 minutes and was then cooled to 53°C. Stirring was halted and the combined mixture was separated by settling for 15 minutes. As much supernatant as possible was withdrawn, i.e. 1651.06 g.

The isolated supernatants were vigorously stirred in order to obtain a homogeneous mixture. A fraction of the supernatants was withdrawn and, for each, the solvent was removed under reduced pressure. The resulting polymer was dried under vacuum at 110°C to constant weight. The results relating to the amount of polymer extracted in the supernatant are collated in the following table :

The residue present in the reactor was present in the form of a mass of swollen polymer and of a small amount of supernatant. The combined product weighed 1491.9 g. The material balance is given in the following table :

The combined product of the balance was stirred and a homogeneous sample was withdrawn. The solvent was evaporated under reduced pressure and the resulting polymer was dried under vacuum at 110°C to constant weight. The dried polymer was subsequently dissolved in THF and it was determined, by ultracentrifuging the solution, that it comprised 0.12 % of insoluble entities. It was thus composed of approximately 99.88 % by weight of PS. Furthermore, the quantitative determination of the nitrogen present in this fraction by the Kjeldahl method (standard method well known to a person skilled in the art, described in particular in the following reference : Kjeldahl, J., 1883 ; « A new method for the determination of nitrogen in organic matter" ; Z. Anal. Chem., 22, 366) revealed that it comprised only 0.4 g of acrylonitrile per kilogram. In point of fact, commercial ABSs comprise from 20 to 35 % by weight of acrylonitrile and thus the PS collected comprised only from 0.1 to 0.2 % by weight of ABS. The order of magnitude of 99.8 % for the purity of the PS recovered is thus confirmed.

The balance of this separation method is presented in the following table :

The method thus exhibits the advantage of high selectivity for separation of the two products (pure to more than 98.5 %), combined with a high yield (greater than 99.8 %). Example 2 (illustrative of the 2^nd method according to the invention) :

100 g of ABS and 100 g of PS were dissolved in 880 g of MEK. Subsequently, 120 ml of water were added to the solution at ambient temperature and with vigorous stirring. The combined mixture was allowed to separate by settling for 5 minutes and the supernatant (906.4 g) was withdrawn. 880 g of MEK and 120 g of water were then added and the medium was stirred. 1067.0 g of supernatant were withdrawn after separation by settling. The fraction separated by settling in the form of a solid swollen with solvent was dried under vacuum at 110°C to constant weight. The results relating to the supernatants are collated in the following table :

The material balance is given in the following table :

101.43 g of a fraction rich in ABS and 98.3 g of a fraction rich in PS were thus isolated by this method, i.e. 99.87 % of polymer recovered.

Example 3 (illustrative of the 3rd method according to the invention) :

400 g of HIPS and 400 g of ABS were introduced into a reactor comprising 3200 g of MEK, 600 g of isohexane and 200 g of water. The reactor was closed and the stirred medium was brought to 90°C for 45 minutes. The pressure was approximately 2.5 bar. A homogeneous milky liquid was obtained.

4000 g of MEK and 800 g of water were introduced into another reactor and the combined mixture was heated under pressure up to 90°C. This reactor acted as generator of MEK/water azeotrope vapour.

The milky mixture was cooled down to 40°C, then introduced into a reactor equipped with a condenser and vigorously stirred. The pressure was set at 600 mbar and the azeotrope vapours were injected into the reactor. The isohexane was then distilled off, thus making it possible to increase the water content of the MEK, which consequently became a poor solvent of the HIPS.

When the temperature reached 59.9°C, i.e. the boiling point of the MEK/water azeotrope for this pressure, the injection of azeotrope vapour was halted. The contents of the reactor were collected and centrifuged to give a milky supernatant and a viscous solid. The solid was dried under vacuum at 110°C to constant weight to give 375 g of a fraction rich in HIPS.

The solvent present in the supernatant was evaporated under reduced pressure and the resulting polymer was dried under vacuum at 110°C to constant weight. 399 g of a fraction rich in ABS were thus isolated. The latter comprises 91 % by weight of ABS (versus 3.6 % of ABS for the phase rich in HIPS).

Claims

C L A I M S

1 - Process for the treatment of a mixture of ABS (acrylonitrile-butadiene- styrene terpolymer) and of PS (polystyrene), according to which the said mixture is brought into contact with a liquid medium comprising water and a water- miscible common solvent in proportions and under pressure and temperature conditions such that the ABS is soluble therein and the PS insoluble.

2 - Process according to the preceding claim, characterized in that the PS is HIPS (High Impact PS).

3 - Process according to either one of the preceding claims, characterized in that the solvent is MEK (methyl ethyl ketone).

4 - Process according to any one of the preceding claims, characterized in that the liquid medium is single-phase.

5 - Process according to the preceding claim, characterized in that the water content of the liquid medium is the maximum amount tolerated for the ABS to remain soluble therein.

6 - Process according to any one of the preceding claims, characterized in that the mixture of ABS and of PS is brought directly into contact with the liquid medium under conditions such that the ABS is soluble therein and the PS insoluble and that the undissolved PS is separated from the ABS solution.

7 - Process according to any one of Claims 1 to 5, characterized in that the ABS and the PS are dissolved in solvent comprising little or no water and under conditions such that the two polymers dissolve and in that the PS is subsequently selectively precipitated by addition of water.

8 - Process according to any one of Claims 1 to 5, characterized in that the ABS and the PS are dissolved in a solvent/phase-separation agent mixture optionally comprising water and in that the phase-separation agent is subsequently removed by distillation and addition of water, if necessary (that is to say, if the liquid medium does not comprise a sufficient amount thereof). 9 - Process according to the preceding claim, characterized in that the distillation of the phase-separation agent takes place by injection of water/solvent azeotropic vapour.

10 - Process according to which :

(e) at least a fraction of the vapours condensed in the preceding stage is used as all or part of the liquid medium in a process according to any one of the preceding claims.