How to recover SAN from ABS

Abstract

A method is provided for recovering styreneacrylonitrile from acrylonitrile butadiene styrene plastic, comprising: a dissolution step of heating the plastic in acetonitrile to obtain a suspension containing polybutadiene particles dispersed in acetonitrile and dissolved styreneacrylonitrile; and separating the polybutadiene particles and the dissolved styreneacrylonitrile by either a density difference-based separation technique or microfiltration.

Description

Technical Field

[0001] The present invention relates to a method for recovering styrene acrylonitrile (SAN) from acrylonitrile butadiene styrene (ABS) plastic, and to the use of this method in the recycling of ABS plastic.

Background Art

[0002] Introduction Acrylonitrile butadiene styrene (ABS) plastic is a hard, tough, heat-resistant plastic. ABS plastic is widely used as a plastic material for manufacturing toys, automotive interiors, personal care products, and electronic devices.

[0003] ABS is a polymer of acrylonitrile, styrene, and butadiene. ABS consists of styrene acrylonitrile (SAN) and polybutadiene. ABS contains a continuous phase of a styrene-acrylonitrile copolymer and polybutadiene particles dispersed (and preferably uniformly distributed throughout the continuous phase) in the continuous phase. The continuous phase of SAN is sometimes also referred to as the SAN matrix. ABS has three monomer units, namely, a butadiene unit, an acrylonitrile unit, and a styrene unit. ABS typically consists of 15% to 35% monomer units of acrylonitrile, 5% to 30% monomer units of butadiene, and 40% to 6% monomer units of styrene.

[0004] ABS is produced by polymerizing styrene and acrylonitrile in the presence of polybutadiene. ABS can be produced by bulk polymerization (also known as mass polymerization) or emulsion polymerization. ABS obtained by the mass polymerization process is sometimes called mass polymerized ABS or m-ABS, while ABS obtained by the emulsion polymerization process is sometimes called emulsion polymerized ABS or e-ABS. More specifically, m-ABS is produced by polymerizing styrene and acrylonitrile in the presence of dissolved polybutadiene to obtain SAN-grafted polybutadiene particles dispersed in a continuous SAN phase. e-ABS is typically produced by compounding SAN-grafted polybutadiene particles produced by the emulsion process with bulk polymerized SAN.

[0005] ABS plastics are typically mixtures of ABS polymer and one or more additives. When preparing ABS plastics, plastic formulations are typically used. In plastic formulations, SAN is mixed in molten state with polybutadiene and optionally one or more additives. In the case of m-ABS, additives may be added by formulation after ABS polymerization. In the case of e-ABS, additives are typically incorporated during ABS polymerization. The properties of ABS plastics can be adjusted by temperature and the additives used.

[0006] A disadvantage of using ABS plastic to manufacture plastic products is that its material properties, particularly in terms of shock absorption, can degrade over time. The reason for this degradation is the aging decomposition and / or crosslinking of polybutadiene particles. In particular, polybutadiene can crosslink, creating a polymer network within the polybutadiene rubber, which can make ABS plastic more brittle. Therefore, any properties of ABS that depend on the presence of polybutadiene particles are degraded by the decomposition or crosslinking of polybutadiene. For example, one function of polybutadiene in ABS is to provide good shock absorption properties. ABS plastic's shock absorption properties deteriorate after several years.

[0007] Therefore, a method for recycling ABS is needed. In particular, it is necessary to separate polybutadiene and SAN so that the polybutadiene particles can be broken down and / or replaced with new particles.

[0008] For example, it is known from European Patent Application Publication No. 4019576(A1) and International Publication No. WO2015076868(A1) that ABS can be recycled using dissolution techniques to separate different components.

[0009] European Patent Application Publication No. 4019576(A1) describes a method for recycling a reinforced thermoplastic polymer reinforced with elastomer particles. This method comprises (a) dissolving the thermoplastic polymer in a solvent to form a solution in which the elastomer is dispersed in the solution as fine particles; (b) separating the elastomer particles from the solution by applying centrifugal force to form a separation system; and (c) recovering the thermoplastic polymer and the elastomer polymer. Embodiments are described in which the recovered thermoplastic polymer may be a SAN containing a small amount of elastomer polymer (e.g., polybutadiene or polyisoprene). The solvent used is acetone, or chloroform, or a mixture thereof. A second solvent (chloroform) may be used to swell the elastomer particles.

[0010] A drawback of European Patent Application Publication No. 4019576(A1) is that, when used to separate SAN from polybutadiene particles, a co-solvent (chloroform) is required to enable the removal of the polybutadiene particles. The inventors were aware that in the absence of chloroform, the polybutadiene particles could not be effectively removed by sedimentation or filtration.

[0011] International Publication WO2020082184(A1) describes a process for recycling thermoplastic (co)polymer waste, comprising: dissolving the polymer waste in a suitable solvent to obtain a mixture of liquid and solid; heating the mixture under acidic conditions, then cooling the mixture to obtain a supernatant containing the thermoplastic (co)polymer in solution and a solid waste residue; and separating the supernatant from the solid waste residue. ABS is mentioned as an example of a thermoplastic resin. When the thermoplastic resin is ABS or SAN, chlorinated solvents are mentioned as suitable solvents, particularly dichloroethane and optionally alcohol non-solvents. International Publication WO2020082184(A1) teaches that particles having a size or diameter of less than 1 micrometer are removed by flocculation.

[0012] A drawback of International Publication No. WO2020082184(A1) is that aggregation requires the use of additional chemicals.

[0013] International Publication WO2015076868(A1) describes a method for recovering a purified polymer composition. This method uses a first solvent to achieve liquid-solid phase separation of the first component from the target (polycarbonate) polymer and the second component. Subsequently, this method uses a second solvent to achieve liquid-solid phase separation of the target polycarbonate polymer from the second component. Acrylonitrile butadiene styrene polymer is mentioned as an example of the second component. Acetonitrile is mentioned as an example of a solvent or poor solvent (first or second). International Publication WO2015076868(A1) does not mention how to separate ABS and polybutadiene. [Overview of the Initiative]

[0014] The object of the present invention is to provide a method for improving the material properties of old ABS.

[0015] A further object of the present invention is to provide a method for recycling ABS, which involves replacing decomposed or crosslinked polybutadiene particles with new particles.

[0016] A further object of the present invention is to recover SAN from ABS without the presence of decomposed or crosslinked polybutadiene.

[0017] One or more of these objectives are achieved by providing a method for recovering styreneacrylonitrile from acrylonitrile butadiene styrene plastic, the method comprising: a dissolution step of heating the plastic in acetonitrile to obtain a suspension containing polybutadiene particles dispersed in acetonitrile and dissolved styreneacrylonitrile; and separating the polybutadiene particles and styreneacrylonitrile by either a density difference-based separation technique or microfiltration.

[0018] The inventors found that while SAN can be dissolved by exposing ABS to a specific solvent, polybutadiene particles do not dissolve and remain dispersed in the SAN solution. However, the inventors also found that removing dispersed polybutadiene particles from the SAN solution is extremely difficult to achieve. These particles typically do not settle in the solvent in which the SAN is dissolved, but rather remain dispersed throughout the solvent. Furthermore, the application of microfiltration to SAN solutions containing dispersed polybutadiene particles was unsuccessful because the dispersed polybutadiene particles passed through the filter bed along with the SAN solution.

[0019] The above problems apply to organic solvents typically used in dissolution techniques (e.g., acetone and ethyl acetate), but not to acetonitrile. When ABS was dissolved in acetonitrile at high temperatures, the SAN polymer dissolved, but the polybutadiene particles did not, and were initially observed to be dispersed in the SAN solution. However, after being left to stand for a while, the polybutadiene particles settled to the bottom of the container. [Modes for carrying out the invention]

[0020] In a first aspect, the present invention relates to a method for recovering SAN from ABS plastic, comprising a dissolution step and a separation step. More specifically, the present invention may relate to a single-solvent process for recovering styrene acrylonitrile, the method comprising a dissolution step, a separation step and a precipitation step.

[0021] As used herein, the term "ABS" refers to the ABS polymer. Therefore, ABS consists of styrene acrylonitrile (SAN) and polybutadiene. The term "ABS" does not include any additives that may be present in ABS plastics.

[0022] As used herein, the term "ABS plastic" refers to a plastic made primarily of ABS. In addition to SAN and polybutadiene, ABS plastic may contain one or more additives. ABS plastic contains more than 50% by weight, preferably at least 80% by weight of ABS, more preferably at least 90% by weight, and even more preferably at least 95% by weight, for example, at least 98% by weight or at least 99% by weight of ABS.

[0023] ABS plastic may be a mixture of ABS polymer and one or more additives. For example, ABS plastic may be a compounded plastic. One or more additives may be selected from the group consisting of pigments, flame retardants, minerals, lubricants, stabilizers, fibers, antioxidants, and fillers. ABS plastic may contain one or more stabilizers, such as heat stabilizers, hydrolysis stabilizers, and UV stabilizers. Such additives may be incorporated into ABS plastic by plastic compounding. In plastic compounding, the plastic is formed by mixing a molten polymer with additives. Upon cooling, a plastic material, which is a mixture of polymer and additives, is obtained. The plastic obtained by plastic compounding is referred to herein as compounded plastic.

[0024] The amount of additives in the ABS plastic may be in the range of 0.1 to 40% by weight, preferably 0.5 to 25% by weight, more preferably 1.0 to 15% by weight, for example, 3 to 10% by weight. When no flame retardant is present, the amount of additives in the ABS plastic may be in the range of 0.1 to 10% by weight, preferably 0.5 to 5% by weight, for example, 1 to 3% by weight.

[0025] The ABS plastic may contain a flame retardant as an additive. For example, the ABS plastic may contain a brominated flame retardant (BFR) and antimony trioxide. The amount of flame retardant present in the ABS plastic may be 1 to 30% by weight, for example, 5 to 20% by weight.

[0026] The additives present in the ABS plastic are typically, preferably non-polymer additives. The polymer material may not dissolve easily in acetonitrile, which may prevent the release of polybutadiene particles from the plastic. In one embodiment, at least 95% by weight, preferably at least 98% by weight, more preferably at least 99% by weight, still more preferably at least 99.9% by weight of the polymer material present in the ABS plastic is ABS. In a preferred embodiment, no polymers other than ABS are present in the ABS plastic.

[0027] As described in the introduction, ABS is a polymer that includes a continuous phase of SAN and in which polybutadiene particles are dispersed in that continuous phase.

[0028] The term SAN refers to styrene acrylonitrile. SAN is a copolymer of styrene and acrylonitrile. SAN may contain 70 to 80% by weight of styrene monomer and 20 to 30% of acrylonitrile monomer. As described in the introduction, SAN forms a continuous phase (also called the SAN matrix) in the ABS, and polybutadiene is dispersed in that continuous phase.

[0029] Polybutadiene exists in the form of rubber particles in ABS. The size of the polybutadiene particles may be in the range of 0.05 to 3.0 μm, preferably in the range of 0.1 to 2.0 μm. Preferably, at least 95% by weight, more preferably at least 99% by weight of all the polybutadiene particles present in the ABS plastic are within the above size range. The ABS plastic may be m-ABS or e-ABS. The average particle size of the polybutadiene particles in e-ABS is smaller than the average particle size of the polybutadiene particles in m-ABS. Furthermore, the distribution of different particle sizes in m-ABS is wider compared to e-ABS. Most of the polybutadiene particles in e-ABS (for example, at least 90% by weight, preferably at least 95% by weight) have a size in the range of 0.1 to 0.3 μm. Most of the polybutadiene particles in m-ABS (for example, at least 90% by weight, preferably at least 95% by weight) have a size in the range of 0.1 to 2.0 μm.

[0030] The polybutadiene present in ABS may be grafted with SAN. This means that the polybutadiene chains can contain side chains of SAN. The polybutadiene particles are formed from a plurality of grafted polybutadiene chains. The use of SAN grafted onto polybutadiene can improve the dispersion of polybutadiene in the SAN bulk matrix during its preparation. Good results were obtained when the method of the present invention was carried out using grafted polybutadiene. The SAN grafted onto polybutadiene may remain at least partially present during the method of the present invention. After separation, some or all of the polybutadiene particles may still contain an outer layer of SAN. Therefore, the term "polybutadiene particles" as used herein can refer to both particles formed from ungrafted polybutadiene and particles containing polybutadiene grafted with SAN.

[0031] The ABS in ABS plastic is preferably not crosslinked. It is assumed that the lower the degree of crosslinking, the better the separation. Dissolution of SAN can be adversely affected in crosslinked ABS, especially if the degree of crosslinking is high. Preferably, the degree of crosslinking of SAN chains (i.e., the percentage of SAN chains that are crosslinked with other SAN chains) is less than 5%, preferably less than 2%, and more preferably less than 1%.

[0032] ABS plastic may be in the form of powder, pellets, particles, or chips. Powder is preferred because this form provides a large surface area for efficient dissolution. ABS plastic may also be obtained from a plastic object, for example, by crushing or cutting the object. Therefore, the method may further include the step of providing ABS plastic by, for example, crushing or cutting an object made of ABS plastic. This object may contain or consist of ABS plastic.

[0033] This method may include a step of mixing a solvent with ABS plastic to obtain a mixture of ABS plastic and solvent. The solvent is acetonitrile. In this step, no solvent other than acetonitrile should be mixed with the ABS plastic. The entire method of the present invention can be successfully carried out using a single solvent. The mixture of ABS plastic and solvent may be called a dissolution mixture. The dissolution mixture can then be supplied to the dissolution step.

[0034] In the dissolution process, a dissolution mixture containing ABS plastic and a solvent is heated. The ABS plastic is heated in acetonitrile, thereby dissolving the SAN present in the ABS. SAN is soluble in acetonitrile, but polybutadiene is not. As a result, the dissolution process yields a suspension containing polybutadiene particles dispersed in acetonitrile and dissolved styrene acrylonitrile. In particular, the resulting suspension is a suspension of polybutadiene particles dispersed in a SAN solution.

[0035] The dissolution process may be carried out at a temperature of at least 30°C, preferably in the range of 30 to 150°C. The dissolution process may be carried out under high pressure or atmospheric pressure.

[0036] If rapid dissolution is desired, the dissolution step is preferably performed at a temperature of 80-150°C, preferably 100-120°C. Since the boiling point of acetonitrile is 82°C, such a dissolution step is preferably performed under high pressure. For example, the pressure may be in the range of 1.5-10 bar, preferably 3-5 bar. The dissolution step may be performed until all SAN is dissolved. The dissolution step has a duration of 1-60 minutes, preferably 2-30 minutes, more preferably 5-20 minutes. Subsequent process steps of the present invention (e.g., filtration and separation) are preferably performed at a temperature lower than the boiling point of acetonitrile, preferably under atmospheric pressure. Therefore, the suspension obtained after the dissolution step may then be cooled to a temperature below 100°C, preferably below 80°C.

[0037] Alternatively, the dissolution process may be carried out at a temperature slightly higher or lower than the boiling point of acetonitrile. The dissolution process may be carried out at a temperature of 40–80°C, preferably 40–60°C. In this case, the dissolution process is preferably carried out at atmospheric pressure. By heating the ABS plastic under these conditions, the SAN dissolves in acetonitrile.

[0038] The amount of acetonitrile used in the dissolution process is 10 to 100 grams, preferably 30 to 50 grams, of acetonitrile per gram of ABS plastic.

[0039] The dissolution process may be carried out in a container in which the dissolved mixture can be stirred. Stirring keeps the particles suspended and / or makes them more easily soluble. The container can be stirred, for example, by stirring, pulsating, or oscillating. For example, the container may be equipped with a stirrer. Examples of containers suitable for oscillating or pulsating stirring are disc and donut-shaped columns or containers.

[0040] The acetonitrile used in the method of the present invention preferably has a purity of at least 99.0% by weight, more preferably at least 99.4% by weight. For example, technical grade acetonitrile can be suitably used, which typically has a purity of 99.5% by weight. Furthermore, it has been found that the presence of other solvents adversely affects the separation of SAN and polybutadiene. Therefore, no solvents other than acetonitrile are essentially present in the solvent used in the method of the present invention. Accordingly, the solvent used in the method of the present invention consists of at least 98% by weight of acetonitrile, preferably at least 99.0% by volume of acetonitrile, more preferably at least 99.9% by volume of acetonitrile.

[0041] Preferably, during dissolution, no solvents other than acetonitrile are added to the dissolution mixture. Acetonitrile may be added during dissolution, for example, if the dissolution step is carried out as a continuous process. In particular, no solvents other than acetonitrile are to be added to the ABS plastic during the dissolution step, or preferably in any step of the method of the present invention prior to the dissolution step. Therefore, acetonitrile is the only organic solvent present in the dissolution mixture during dissolution. While multiple solvent embodiments may be theoretically possible, it has been found that for the most successful separation of polybutadiene and SAN, no solvents other than acetonitrile should be present in the dissolution mixture.

[0042] The dissolution step dissolves SAN in acetonitrile, yielding a suspension in which polybutadiene is dispersed in the SAN matrix. The suspension may have a SAN concentration of at least 0.01 g / mL, preferably at least 0.05 g / mL, more preferably 0.1 to 0.5 g / mL, and more preferably 0.1 to 0.3 g / mL.

[0043] The suspension obtained in the dissolution process may contain substances other than SAN and polybutadiene. For example, the suspension may contain solid substances, particularly additives insoluble in acetonitrile. Such solid substances are dispersed in acetonitrile. An example of an additive insoluble in acetonitrile is a pigment.

[0044] Optionally, the method further includes a step of removing solid substances (e.g., additives or contaminants), which is performed after the dissolution step and before the separation step. Such a step may be used to remove solid substances other than polybutadiene from the suspension. Preferably, this step is performed by filtration (in which case this step may be called a pre-filtration step). The pore size of the filter should be such that polybutadiene particles pass through the filter. In this way, the solid substances remain in the filter, while polybutadiene and SAN are contained in the filtrate. Such a filtration step can remove, for example, certain additives. Decantation and centrifugation may be used as alternatives to the use of filtration for removing solid substances before the separation step.

[0045] Optionally, the method further includes a step of removing dissolved substances (e.g., additives or contaminants), which is performed after the dissolution step and before the separation step. The method may also include, for example, an sorption step (e.g., adsorption). An example of an sorption step is to bring the suspension obtained in the dissolution step into contact with activated carbon by passing the suspension through an activated carbon sorbent bed.

[0046] This method further includes a separation step. In the separation step, polybutadiene particles are separated from dissolved styrene acrylonitrile. The separation step can be carried out by a density difference-based separation technique. The separation step can also be carried out by microfiltration. The separation step yields a solid fraction containing polybutadiene and a liquid fraction containing dissolved SAN. In addition to polybutadiene, the solid fraction may further contain other insoluble substances, such as insoluble additives, such as pigments. The amount of additives is typically relatively small. Polybutadiene particles may account for more than 95% by weight of the solid material present in the suspension processed in the separation step. The liquid fraction is a solution of SAN dissolved in acetonitrile, and is also referred to herein as the SAN solution. Thus, the separation step yields a solution of SAN in acetonitrile. Further, the SAN can be recovered as a slurry or solid by removing the solvent (e.g., by precipitation, filtration, and / or evaporation).

[0047] When the separation process is carried out by a separation technique based on density difference, styrene acrylonitrile and polybutadiene particles may be separated using one or more separation techniques selected from sedimentation (e.g., gravity sedimentation), cyclone separation, centrifugation, and lamella separation. The separation process may be carried out, for example, in a sedimentation machine, cyclone separator, decanter, or lamella separator. In some cases, for example, in the case of sedimentation or gravity sedimentation, the separation technique may be further combined with a decantation step. The decantation step may be effective in removing the SAN solution from the settled solid.

[0048] The separation step may include allowing the suspension to settle for a duration of at least 5 minutes, preferably at least 30 minutes. Such a settling step yields a bottom fraction containing polybutadiene particles and an upper fraction (which is a SAN solution) containing dissolved styrene acrylonitrile. The bottom fraction is a slurry and contains a solid fraction. Since the polybutadiene settles at the bottom, the upper fraction may be a clear solution. The settling step is preferably carried out at a temperature lower than the boiling point of acetonitrile, for example, below 70°C or below 50°C. It is not necessary to add a coagulant or flocculant to successfully settle the polybutadiene particles.

[0049] The separation step may further include a decantation step, for example, if sedimentation is used. In the case of sedimentation, the separation step can be carried out by first allowing the suspension to settle (as described above), and then separating the upper fraction from the slurry by decantation. This yields a solution in which styrene acrylonitrile is dissolved in acetonitrile.

[0050] In another embodiment of the present invention, separation may be carried out by cyclone separation, centrifugation, and lamellar separation, with or without a sedimentation step.

[0051] Furthermore, separation can be carried out in multiple stages. For example, the upper fraction obtained after sedimentation and decantation may be subjected to a second separation step. This second separation step may be one of the separation techniques described herein, for example, microfiltration.

[0052] It has been found that separation is successful even when the separation process is carried out by microfiltration. The pore size of the microfilter should be less than 5 μm, preferably less than 3 μm, and more preferably less than 2 μm. In the case of e-ABS, even smaller pore sizes, for example, less than 1.0 μm or less than 0.5 μm, can be used. The sedimentation process described above may be performed before microfiltration.

[0053] Microfiltration can be performed using bed filtration or membrane filtration. In bed filtration, filtration is carried out using a filter bed. The filter bed may contain filter aids. Examples of filter aids that can be used to make a filter bed suitable for microfiltration are Celite or diatomaceous earth. Such filter aids may have particle sizes in the range of 0.02 to 0.1 mm. Methods for preparing a suitable filter bed using filter aids will be known to those skilled in the art. Celite is commercially available, for example, under the trademark Celite-545. In membrane filtration, filtration is carried out using a membrane. Microfiltration can be performed using cross-flow filtration or dead-end filtration.

[0054] Optionally, the method further includes a step of removing dissolved substances (e.g., additives or contaminants), which is performed after the dissolution step and before the separation step. The method may also include, for example, an sorption step (e.g., adsorption). An example of an sorption step is to bring the suspension obtained in the dissolution step into contact with activated carbon by passing the suspension through an activated carbon sorbent bed.

[0055] The method of the present invention can be carried out continuously. It may also be carried out in a batch or semi-continuous manner. The SAN solution obtained in the separation step may still contain dissolved additives and / or dissolved contaminants. Therefore, the method may include a purification step to remove dissolved substances from the SAN solution. The method may include, for example, an sorption step (e.g., adsorption) or nanofiltration. In a preferred embodiment, the SAN solution is subjected to a step in which the SAN solution is padded over an activated carbon bed. The sorption step may be carried out in a column packed with an adsorbent or a packed-bed column.

[0056] This method may further include a concentration step to increase the concentration of SAN. Since acetonitrile has a relatively low boiling point, this can be easily achieved by evaporation.

[0057] This method may further include a precipitation step in which dissolved styrene acrylonitrile is precipitated from acetonitrile. Precipitation is preferably carried out at a temperature lower than the boiling point of acetonitrile, for example, below 70°C or below 50°C. Precipitation may be carried out under atmospheric pressure or high pressure.

[0058] SAN may be precipitated, for example, by removing the solvent by evaporation or flash evaporation.

[0059] In principle, it is also possible to recover SAN from a SAN solution by poor solvent precipitation. However, poor solvent precipitation is undesirable because it unnecessarily adds extra solvent to the SAN solution, making the entire process more complex and costly. In such embodiments, a mixture of solvents is formed, and the poor solvent must be separated into a pure solvent fraction and a poor solvent fraction, for example, by distillation.

[0060] The method of the present invention is preferably a single-solvent process. As used herein, the term “single-solvent process” refers to a process in which the same solvent is used in all process steps and does not involve the addition of a second solvent (i.e., a solvent other than the same solvent). In the single-solvent process according to the present invention, SAN is precipitated from a first solvent without the addition of a second solvent (i.e., a solvent different from the first solvent), and this first solvent is the same solvent in which SAN was dissolved in the dissolution step. According to a first aspect of the present invention, the first solvent is acetonitrile.

[0061] More preferably, the method of the present invention is a single-solvent dissolution and precipitation process. Such a process refers to a process of recovering the compound by dissolving the compound (i.e., SAN) in a solvent to form a solution, and then precipitating the compound from the solution. Optionally, the solution may be subjected to one or more purification steps (e.g., steps to remove insoluble substances, such as filtration) between dissolution and precipitation.

[0062] The precipitation step is preferably a single-solvent precipitation step. In single-solvent precipitation, the compound is recovered by precipitation from the solution by cooling. In single-solvent precipitation, a poor solvent is not required or used to precipitate the compound. The solvent in which the compound precipitates is typically the same solvent used for dissolution.

[0063] The SAN recovered by the method of the first embodiment of the present invention may be reused in plastic manufacturing, for example, to prepare ABS and / or ABS plastics. Recycled acrylonitrile butadiene styrene can be prepared by mixing the styrene acrylonitrile recovered by the method of the first embodiment with new polybutadiene particles.

[0064] In a second embodiment, the present invention relates to a method for recycling ABS plastic. In this embodiment, SAN is recovered from ABS plastic in the same manner as described above in the first embodiment of the present invention. The method may further include a step of preparing a plastic by combining the recovered SAN with one or more polymers and optionally one or more additives. For example, the method of the second embodiment may include a plastic compounding step, in which the recovered SAN is mixed with one or more polymers and optionally one or more additives. The SAN recovered in the precipitation step of the recovery method of the first embodiment has good purity and quality and is suitable for direct or indirect use in plastic manufacturing. Preferably, ABS is prepared in a plastic compounding step. Thus, SAN is first recovered from ABS as described in the method of the first embodiment and then used to prepare new ABS. When preparing ABS, the method includes a step of mixing the recovered SAN with new polybutadiene particles. In the case of plastic compounding, such a step may be performed before or during the plastic compounding step.

[0065] In a further embodiment, the present invention relates to a method of the first embodiment, differing only in that a different solvent is used instead of acetonitrile. While most solvents are unlikely to be useful, the inventors assume that the objective can be achieved to some extent by using 2-ethoxyethanol, dimethylacetamide, dimethylformamide, and 1-butanol as solvents.

[0066] In a further embodiment, the present invention relates to a method for recovering styreneacrylonitrile from acrylonitrile butadiene styrene plastic, comprising a dissolution step of heating the plastic in a bio-derived solvent to obtain a suspension containing polybutadiene particles dispersed in the bio-derived solvent and dissolved styreneacrylonitrile. The bio-derived solvent is selected from methyl 2-furate and methyl levulinate. The inventors have found that these two solvents can successfully dissolve SAN. Since these solvents are bio-derived, they are preferred from the viewpoint of sustainability and recyclability. A further advantage is that dissolution in these two bio-derived solvents is an exothermic process, which makes the process more energy-efficient. To dissolve SAN in these solvents, they may be heated to a temperature above 60°C, preferably above 90°C. The method in this embodiment may further include a step of recovering SAN. This step may be carried out, for example, by separation or precipitation techniques known in the art. [Examples]

[0067] Herein, the present invention is further illustrated by the following non-limiting embodiments.

[0068] Example 1: Dissolution and Separation 5 g of ABS was dissolved in 100 mL of organic solvent. The mixture was stirred and heated at 50°C for 2 hours. The resulting suspension was then cooled and allowed to settle overnight. Acetonitrile, acetone, ethyl acetate, chlorobenzene, and benzonitrile were used as organic solvents. The results are shown in the table below.

[0069] [Table 1]

[0070] After a night of sedimentation and visual observation, the suspension was centrifuged for 1 hour. In experiments 2, 3, 6, and 7, a densified suspension layer was observed at the bottom, with a turbid liquid layer above it in all cases.

[0071] Samples were taken from the upper liquid layer and the lower suspension layer to determine the composition of the different layers after overnight sedimentation, obtained by dissolving ABS in acetonitrile (Experiment 1) and acetone (Experiment 2). The solvent was evaporated from both the upper and lower samples, and the resulting dried solid samples were subjected to Fourier-transform infrared spectroscopy (FTIR) analysis.

[0072] FTIR results from samples obtained from Experiment 1 showed that the precipitated solid obtained in Experiment 1 (acetonitrile) consisted almost entirely of PBD, while the solid obtained from the transparent upper layer consisted almost entirely of SAN.

[0073] The solid obtained from Experiment 1 was further analyzed by differential scanning calorimetry (DSC) combined with thermogravimetric analysis (TGA). DSC / TGA analysis of the sample from Experiment 1 (acetonitrile) showed that the precipitated solid had a high PBD content, while the solid obtained by evaporating the solvent from the liquid layer above it had a low PBD content. The latter solid sample consisted mostly of SAN.

[0074] Example 2: Comparison of m-ABS and e-ABS: Experiment 1 of Example 1 was repeated for different types of ABS. - ABS (e-ABS) obtained by emulsion polymerization, - ABS (m-ABS) obtained by bulk polymerization.

[0075] Separation was found to be less pronounced for m-ABS than for e-ABS. However, SAN and polybutadiene were successfully separated for both types of ABS. Similar FTIR results were observed for both e-ABS and m-ABS samples. Furthermore, similar DSC / TGA results were observed for both e-ABS and m-ABS samples.

[0076] Example 3: Microfiltration 25 g of clear (natural color) e-ABS (virgin grade) samples and 25 g of recycled black ABS (a mixture of m-ABS and e-ABS) samples were separately dissolved in 500 mL of acetonitrile according to Experiment 1 of Example 1. In both cases, the SAN polymer dissolved, but the PBD particles settled to the bottom of the container. The solution obtained from the clear ABS was only slightly turbid, while the solution obtained from the black ABS was colored yellowish / orange and turbid with suspended black particles. In general, it was observed that bot PBD particles and pigment particles tended to settle in the ABS solution in acetonitrile.

[0077] Liquid samples were taken from these solutions, and the solvent was evaporated to obtain dissolved polymers as dry samples for analysis by DSC / TGA.

[0078] Next, a microfilter with a pore size smaller than 5 μm was provided by preparing a filter bed of Celite filtration aid. Then, the suspension was filtered at a constant rate through the filter bed by applying a slight overpressure.

[0079] Next, a portion of the liquid layer above the settled solid was filtered through a filter bed, and the filtrate was collected. The filtrate obtained from the clear ABS solution was slightly less turbid compared to the supplied liquid. On the other hand, the filtrate obtained from the black ABS solution was clearly less turbid, but still had a yellowish / orange tint. Samples were taken from the filtrate for analysis by DSC / TGA.

[0080] [Table 2]

[0081] A portion of the solution was resuspended, then filtered through a filter bed, and the filtrate was collected again. In both cases, the filtrate was very similar to the filtrate obtained in the previous step from the clear liquid on top of the settled solid. It was concluded that both PBD particles and pigment particles could be removed by microfiltration, and that PBD particles and pigment could be removed together by microfiltration. Samples were taken from the filtrate for analysis by DSC / TGA.

[0082] The DSC / TGA analysis results of the obtained samples were compared with those of the raw material e-ABS and recycled ABS. These results showed that the samples obtained from the liquid layers above the settled solid (liquids B and C) had a PBD content 10 to 20 times lower than that of the raw material. The samples obtained from the filtrates (filtrates 1B and 1C) showed a further reduction in PBD, corresponding to a PBD content 20 to 40 times lower than that of the raw material. It was concluded that a significant amount of PBD could be removed from the acetonitrile suspension obtained by the method of the present invention by microfiltration.

[0083] Example 4: Mixed solvent This example demonstrates the effect of the presence of a second solvent in acetonitrile.

[0084] The ABS solution was prepared in a solvent mixture. 0.5 grams of ABS was dissolved in 10 mL of the solvent mixture as shown in the table below. The combinations of the two solvents used in the solvent mixture were:

[0085] [Table 3]

[0086] After stirring at 50°C for 3 hours, complete dissolution of all solvent components was not achieved. The solution was heated to 65°C for 1 hour. Then, the solution was stirred at 40°C overnight. Finally, the solution was cooled to room temperature and allowed to settle.

[0087] After 60 minutes of sedimentation, only mixture #1 (i.e., acetonitrile) achieved separation into a two-phase system with a clear liquid at the top and a slurry at the bottom. The other mixtures did not sediment and remained in suspension. In mixtures 2, 3, 6, and 7, some rubber particles appeared to be floating at the top. After a further 15 hours of sedimentation, all mixtures except #1 remained in stable suspension.

Claims

1. A method for recovering styreneacrylonitrile from acrylonitrile butadiene styrene plastic, The plastic is heated in acetonitrile to obtain a suspension containing polybutadiene particles dispersed in acetonitrile and dissolved styrene acrylonitrile; this is a dissolution step. A method comprising separating the polybutadiene particles from the dissolved styrene acrylonitrile by either a density difference-based separation technique or microfiltration.

2. The method according to claim 1, wherein the styreneacrylonitrile and the polybutadiene particles are separated using one or more separation techniques selected from sedimentation, gravity sedimentation, cyclone separation, centrifugation, and lamellar separation.

3. The method according to claim 2, wherein the separation step comprises allowing the suspension to settle for at least 5 minutes, preferably at least 30 minutes, in the absence of a flocculant and a coagulant, to obtain a bottom fraction containing polybutadiene particles and a clear upper fraction containing dissolved styrene acrylonitrile.

4. The method according to claim 3, wherein, after sedimentation, the upper fraction is decanted to obtain a solution in which styrene acrylonitrile is dissolved in acetonitrile.

5. The method according to claim 1, wherein the styrene acrylonitrile and the polybutadiene particles are separated by microfiltration.

6. The method according to any one of claims 1 to 5, wherein the dissolution step is carried out at a temperature of at least 30°C, preferably 30 to 90°C, preferably 40 to 70°C, and more preferably 40 to 60°C.

7. The method according to any one of claims 1 to 6, wherein the dissolution step is carried out under high pressure at a temperature of 80 to 150°C, and the suspension is subsequently cooled to a temperature of less than 80°C before separating the polybutadiene particles from the dissolved styrene acrylonitrile.

8. The method according to claim 1, wherein the method for recovering styreneacrylonitrile is a single-solvent dissolution and precipitation process.

9. The method according to any one of claims 1 to 8, further comprising a precipitation step of precipitating the dissolved styreneacrylonitrile from acetonitrile.

10. The method according to claim 9, wherein the dissolved styrene acrylonitrile is precipitated by cooling precipitation, evaporation of acetonitrile, or a combination of the two.

11. The method according to any one of claims 1 to 10, further comprising a filtration step performed prior to the separation step, wherein the filter used has a pore size large enough for the polybutadiene particles to pass through.

12. The method according to any one of claims 1 to 11, further comprising an sorption step for removing dissolved additives or contaminants from the suspension or from the solution obtained in the separation step, in which styrene acrylonitrile is dissolved in acetonitrile.

13. The method according to any one of claims 1 to 12, wherein the amount of acetonitrile used in the dissolution step is 10 to 100 grams, preferably 30 to 50 grams, of acetonitrile per gram of ABS plastic.

14. A method for recycling acrylonitrile butadiene styrene plastic, The plastic is heated in acetonitrile to obtain a suspension containing polybutadiene particles dispersed in acetonitrile and dissolved styrene acrylonitrile; this is a dissolution step. The polybutadiene particles and the dissolved styrene acrylonitrile are separated by either a density difference-based separation technique or microfiltration. A method for preparing recycled plastic, comprising combining the separated styreneacrylonitrile with one or more polymers and optionally one or more additives.

15. The method according to claim 14, wherein the recycled plastic is prepared by a plastic compound, and the separated styrene acrylonitrile is mixed with new polybutadiene particles.

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