METHOD FOR RECYCLING PET PLASTIC WASTE

Description

[0001] The present invention relates to the field of recycling PET plastic waste.

[0002] Polyethylene terephthalate (PET) is a semi-crystalline polymer known for its use in the manufacture of consumer bottles. Its advantages are that it has good mechanical and chemical properties, as well as good thermal stability, excellent transparency combined with good barrier properties (permeability to gases and contaminants).

[0003] Polyethylene terephthalate (PET) is a linear polyester whose repeating unit is shown below

[0004] The presence of a rigid group in the chain skeleton ensures the cohesion of the polymer. Its chemical structure thus gives it great rigidity.

[0005] PET can be produced by different synthetic routes: the esterification of terephthalic acid and ethylene glycol; the Trans esterification of dimethyl terephthalate and ethylene glycol more commonly used because the purification of the polymer is easier.

[0006] These two reactions take place in two stages: pre-polycondensation and polycondensation.

[0007] The recycling of PET plastic waste from both industrial offcuts and post-consumer waste is a major ecological issue.

[0008] Today, recycling techniques are of four types: primary, secondary, tertiary and quaternary (incineration) which we will not detail here. The first two relate to mechanical recycling, currently the most widespread. Their disadvantages lie in the fact of the limit of operability since the recycled material loses its mechanical properties after a few recycling operations due to thermal and mechanical degradation.

[0009] In particular, opaque PETs pose a serious problem for recyclers since they cannot be assimilated into the same recycling chain as other PE or PP plastics. Tertiary or chemical recycling has the advantage of being able to recover, through depolymerization, the virgin starting monomers, which are terephthalic acid (PTA) and ethylene glycol (EG). This recycling route, through partial or complete depolymerization of PET into monomers or oligomers, allows recycling in the form of the production of new PET with a large number of cycles.

[0010] The depolymerization agents known in the prior art are multiple: water (hydrolysis), methanol (methanolysis), ethylene glycol (glycolysis) and butanediol (diolysis). The most common depolymerization processes today are glycolysis and methanolysis.

[0011] Hydrolysis and methanolysis regenerate monomers (terephthalic acid, dimethyl terephthalate, and ethylene glycol), while glycolysis and other solvolysis simply produce oligomers. It is also possible to chemically convert waste by modifying its structure for specific applications, such as the production of polyurethanes (aminolysis) or unsaturated polyesters. Aminolysis consists of depolymerizing PET in the solid state in an aqueous solution containing primary amines, and allows the formation of diamides, terephthalic acid, and ethylene glycol. The diamides formed are then used as monomers for the production of polyester amides or polyurethanes via reactions with isocyanates. The use of PET as a source of monomers (by depolymerization and chemical modification) for the production of chemical intermediates is limited in volume.However, the marketing of these intermediaries seems to be experiencing a relatively significant increase according to recent economic studies.

[0012] After depolymerization, the monomers and / or oligomers are purified by vacuum distillation and repolymerized with ethylene glycol to produce PET. The resulting polymer can then be used to manufacture food packaging. The advantage is that it is not necessary to sort the PET before processing, and it is possible to use different grades of PET without this having any influence on the resulting products. State of the art

[0013] Known in the state of the art are solutions described in US patent US9550713 concerning the depolymerization of polymers and the recovery of the starting substances used for the production of the polymer. This prior art document relates more specifically to the depolymerization of polyethylene terephthalate (PET) and the recovery of terephthalic acid and ethylene glycol, using an apolar and halogenated solvent, preferably dichloromethane.

[0014] The disadvantage of this known solution is that dichloromethane is a solvent classified in category 2 for carcinogenic and mutagenic effects and is subject to restrictions by the REACH directive, which makes industrial application very problematic. In addition, from an operability point of view, manufacturers do not favor the use of chlorinated solvents in their equipment due to their reactive nature and harmfulness to the environment when released into stacks. Finally, this application mainly describes the production of terephthalic acid and not its ester.

[0015] Also known is Chinese patent CN108359126 describing another process for recycling PET (polyethylene glycol terephthalate) protective film waste comprising the following steps: fragmenting the film waste, adding the pieces into two prepared degumming reagents and gradually degumming three types of adhesives, adding and washing the degummed pieces into a high-temperature washing reagent, and finally blowing for drying, thus obtaining clean film pieces.

[0016] The recycling process described in this prior art document is intended to efficiently remove and separate all kinds of adhesives in PET protective film waste without using water for washing. This patent does not describe the use of this pretreatment for the subsequent chemical depolymerization of these films. Solution provided by the invention

[0017] In order to overcome these drawbacks, the present invention relates to a method for recycling PET plastic waste comprising a step of crushing the waste to produce fragmentary fragments and a step of depolymerization treatment of said fragmentary fragments for the recovery of the virgin starting monomers which are terephthalic acid (PTA) or its ester (DMT) and ethylene glycol (EG) characterized in that it comprises: a pretreatment step by soaking without dissolution in a container ensuring continuous stirring, of said fragmentary fragments in a solution comprising at least one mixture of solvents comprising at least one reagent chosen from the components comprising DMAc (dimethyl acetamide), DMF (dimethyl formamide), MEK (Methyl Ethyl Ketone), phenolic esters or biphenyl ethers at a temperature below 50° and for a duration of less than 18 hours, followed by a decantation and spinning step followed by a step of treatment of the pretreated PET fragments in a container containing a monoalcohol or a diol and a base at a temperature below 50° followed by an operation of recovery of the suspended solid and filtering.

[0018] According to a variant, said reagent of the pretreatment step consists of DMAc or DMF, the decantation step being followed by a step of washing the pretreated fragments with an alcohol before the addition of the base.

[0019] According to a variant, said decantation step comprises a treatment for recovering the reagents for reuse for subsequent pretreatment of new PET plastic waste.

[0020] Preferably, the bases are potassium hydroxide KOH or sodium hydroxide NaOH.

[0021] The bases are introduced either in pellets or in aqueous solution.

[0022] For alkoxides, sodium or potassium methoxide, the medium must be anhydrous. In this case it is possible to advantageously obtain the corresponding terephthalic acid ester (DMT).

[0023] Advantageously, the said recovered solid is a monoester or a methyl diester of terephthalic acid.

[0024] According to a variant, said recovered solid is subjected to filtration and distillation to recover said dimethanolic terephthalate (DMT) ester.

[0025] According to another variant, said recovered solid is dissolved in an aqueous solution to give rise to the basic form of terephthalic acid soluble in water.

[0026] Preferably, the solution obtained is subjected to the addition of a mineral or organic acid to obtain terephthalic acid by precipitation. Detailed description of a non-limiting example of embodiment

[0027] The present invention will be better understood upon reading the following description concerning a non-limiting example of embodiment of the invention.

[0028] The recycling process according to the example of the invention comprises a succession of steps presented below.

[0029] The first step is a mechanical preparation of the PET materials to be recycled by crushing them, to reduce them to thin fragments of a few square centimeters in size. These fragmented wastes can be supplied directly since they exist on the market, which facilitates the process.

[0030] The second step consists of pretreating the PET fragments without proceeding with immediate dissolution. This pretreatment step aims to promote the subsequent step of depolymerization of the polymer, without total solubilization of the polymer to facilitate the subsequent penetration of the molecules within the matrix notwithstanding the hydrophobic character due to the nature of the PET monomers. It should be noted that without this pretreatment step, depolymerization under the mild conditions mentioned above is impossible for reasonable contact times.

[0031] In the case of very thin PETs such as trays, it is possible to have the presence of another polymer that goes into solution. However, this step does not cause the dissolution of the PET but only, optionally, that of the polymer forming the coating.

[0032] This pretreatment step is carried out at room temperature, and preferably below 50°C.

[0033] This pretreatment step is carried out by soaking the crushed or cut pieces of PET to be recycled in a tank containing a reagent or a mixture of reagents chosen from: solvents such as DMAc (dimethyl acetamide), DMF (dimethyl formamide), Methyl Ethyl Ketone or (MEK) commercial compositions comprising phenolic esters, biphenyl ethers mixed with solvents.

[0034] This mixture of solvents modifies the ordering of the polymer chains within the material and makes the ester functions more accessible in order to break them during the subsequent depolymerization step.

[0035] The soaking time in this reagent mixture is between 1 minute and 18 hours, preferably 30 minutes (for example for trays) to 6 hours, more precisely between 30 minutes and 3 hours, at a temperature ranging from 20°C to 100°C, preferably from 23 to 50°C, more precisely at 25°C. The mixture of pieces in the solvent is subjected to continuous stirring to substantially improve the efficiency of this step.

[0036] The next step is to decant the pre-treated pieces. The advantage of this approach is that it eliminates the subsequent step of recycling the high-boiling solvent, which requires distillation operations.

[0037] The solvent can be reused repeatedly in separate tanks, reactors, or other devices. The treated PET pieces are then drained, optionally dried, and transferred to a glass reactor.

[0038] The next step is to treat the decanted fragments in a bath containing an alcohol. This alcohol is a monoalcohol or a diol. For the monoalcohol, it can be methanol, preferably ethanol, or any other linear or branched alcohol with a hydrocarbon chain. For the diol, it can be ethylene glycol, tetraethylene glycol, etc.

[0039] A base is then added to this bath and the whole thing is stirred.

[0040] Advantageously, the base used can be KOH or NaOH. This base can be added in the form of granules or in the form of an aqueous solution (50% commercially available). NaOH, commercially sold in the form of drain unblocker granules, also gives excellent results.

[0041] After a few minutes, the pieces of PET begin to cloud and twist.

[0042] When the reaction is complete, a suspended solid is obtained.

[0043] The latter is filtered. The filtrate contains the alcohol used and the ethylene glycol EG obtained.

[0044] The filter cake contains terephthalic acid in its esterified form of DMT alcoholate or its ionized form of sodium or potassium terephthalate as well as the unreacted material. The excess alcohol and the ethylene glycol obtained can be recovered by means known to those skilled in the art, namely distillation, centrifugation, etc.

[0045] In the case where terephthalic acid is desired, the addition of water to the cake causes the dissolution and hydrolysis of the terephthalate ester.

[0046] A clear solution is obtained after dissolving in water. In the particular case of opaque PET, an additional centrifugation / filtration step is required since the pigment used is insoluble in water. The treatment will vary depending on whether the pigment is titanium oxide or calcium carbonate.

[0047] The unreacted pieces are retained in the filter. Terephthalic acid PTA is obtained in solution by precipitation with concentrated acid (sulfuric, hydrochloric, acetic) and then by filtration of the precipitated PTA.

[0048] Depending on the industrial need, some PET resin manufacturing technologies resort to the use of dimethanolic ester of terephthalic acid (DMT). Advantageously, the use of a [wu1]alkoxide base such as sodium or potassium methoxide makes it possible to obtain DMT. The latter can be recovered by washing with methanol and has the advantage of avoiding the use of water. Using techniques known to those skilled in the art, it is possible to purify DMT by recrystallization and distillation from the cake obtained.

[0049] Unexpectedly, other types of solvents usually used in applications aimed at separating different types of thermoplastic materials also gave good results.

[0050] More specifically, the use of Methyl Ethyl Ketone MEK, which is known to dissolve PVC (polyvinyl chloride) without dissolving PET, makes it possible to separate the latter two in composite materials.

[0051] The treatment of PET with MEK following the process cited above provided similar results. Indeed, longer contact times depending on the nature of the PET used (between 3h and 10h versus 3h for DMAc) are necessary to give equivalent yields and depolymerization times. Examples of implementation of the invention

[0052] Example 1: A quantity (2 g) of polyethylene terephthalate PET pieces from plastic bottles, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) so that all the plastic pieces are immersed. The latter are stirred for 4 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 20 mL of methanol is added to the pre-treated pieces followed by 1.5 g of KOH pellets. As the base dissolves, the PET pieces undergo a physical transformation and become opaque. The medium begins to heat slightly under the combined effect of the dissolution of the base in the medium and the start of the reaction. After 2 hours, all the PET pieces have disappeared, leaving a white solid in solution.The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and ethylene glycol EG product of the depolymerization reaction. The white solid is recovered and is dissolved by adding a volume of water (30 mL). A clear solution is obtained. The unreacted material remains retained in the filter. To the solution obtained is added concentrated sulfuric acid (97%) until a pH of approximately 1.5. The terephthalic acid precipitates in the form of a white solid and is filtered and dried to give 1.45 g of product (99% yield).

[0053] Example 2 A quantity (2 g) of polyethylene terephthalate PET pieces from plastic bottles, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) in such a way that all the plastic pieces are immersed. The latter are stirred for 18 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 20 mL of methanol is added to the pretreated pieces followed by 1.5 g of KOH pellets. As the base dissolves, the PET pieces undergo a physical transformation and become opaque. The medium begins to heat slightly under the combined effect of the dissolution of the base in the medium and the start of the reaction. After 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution.The reaction crude is filtered through filter paper or Buchner funnel; the recovered liquid contains residual methanol and ethylene glycol produced by the depolymerization reaction. The white solid is recovered and dissolved by adding a volume of water (30 mL). A clear solution is obtained. The unreacted material remains retained in the filter. Concentrated sulfuric acid (97%) is added to the resulting solution until a pH of approximately 1.5 is reached. The terephthalic acid precipitates as a white solid, which is filtered and then dried to give 1.45 g of product (99% yield).

[0054] Example 3: A quantity (1 g) of polyethylene terephthalate PET pieces from plastic bottles, after being washed with water, is placed in a container containing Methyl Ethyl Ketone MEK (10 mL) in such a way that all the plastic pieces are immersed. The latter are stirred for 18 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 20 mL of methanol is added to the pre-treated pieces followed by 1.5 g of KOH pellets. As the base dissolves, the PET pieces undergo a physical transformation and become opaque. The medium begins to heat up slightly under the combined effect of the dissolution of the base in the medium and the start of the reaction. After 5 hours, all the PET pieces have disappeared, leaving a white solid in solution.The reaction crude is filtered through filter paper or Buchner funnel; the recovered liquid contains residual methanol and ethylene glycol produced by the depolymerization reaction. The white solid is recovered and dissolved by adding a volume of water (30 mL). A clear solution is obtained. The unreacted material remains retained in the filter. Concentrated sulfuric acid (97%) is added to the resulting solution until a pH of approximately 1.5 is reached. The terephthalic acid precipitates as a white solid, which is filtered and then dried to give 1.44 g of product (99% yield).

[0055] Example 4: A quantity (2 g) of polyethylene terephthalate PET pieces from food trays, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) so that all the plastic pieces are immersed. The latter are stirred for 1 hour 30 minutes. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 15 mL of methanol is added to the pretreated pieces followed by 2.05 g of KOH pellets. After 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction. The white solid is collected and dissolved by adding a volume of water (30 mL).A clear solution is obtained. The unreacted material remains retained in the filter. To the resulting solution is added concentrated sulfuric acid (97%) until a pH of approximately 1.5. The terephthalic acid precipitates as a white solid, which is filtered and then dried to give 1.40 g of solid (97% yield).

[0056] Example 5: A quantity (1 g) of opaque polyethylene terephthalate PET pieces from detergent bottles, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) so that all the plastic pieces are immersed. The latter are stirred for 18 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 12 mL of methanol is added to the pretreated pieces followed by 1.5 g of NaOH pellets previously dissolved in 2 mL of distilled water. After 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction.The white solid is added to a water solution (30 mL). A cloudy solution is obtained. The white pigment is separated by centrifugation and the supernatant is recovered. To the supernatant solution obtained is added concentrated sulfuric acid (97%) until a pH of approximately 1.5. The terephthalic acid precipitates in the form of a white solid which is filtered and then dried to give 0.62 g.

[0057] Example 6: A quantity (2.65 g) of transparent polyethylene terephthalate PET in the form of uncrushed peels from water bottles, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (15 mL) in such a way that all the plastic pieces are immersed. The latter are stirred for 18 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 50 mL glass reactor. 15 mL of methanol and 5 mL of distilled water are added to the pretreated plastics followed by 2.65 g of KOH pellets dissolved in 5 mL of water. After 1 hour, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction.The white solid is added to a water solution (30 mL). A cloudy solution is obtained. The white pigment is separated by centrifugation and the supernatant is recovered. To the supernatant solution obtained is added concentrated sulfuric acid (97%) until a pH of approximately 1.5. The terephthalic acid precipitates in the form of a white solid which is filtered and then dried to give 2 g (99% yield).

[0058] Example 7: A quantity (2 g) of polyethylene terephthalate PET pieces from food trays, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) so that all the plastic pieces are immersed. The latter are stirred for 1 hour 30 minutes. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 15 mL of dry methanol is added to the pretreated pieces followed by 500 mg of KOH pellets. After 1 hour 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction.The white solid that is collected and washed with methanol is dimethanolic terephthalate (DMT) ester.

[0059] Example 8: A quantity (2 g) of polyethylene terephthalate PET pieces from food trays, after being washed with water, is placed in a container containing Dimethylacetamide DMAc (10 mL) so that all the plastic pieces are immersed. The latter are stirred for 1 hour 30 minutes. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 15 mL of dry methanol is added to the pretreated pieces followed by 560 mg of sodium methoxide powder. After 1 hour 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction.The white solid that is collected and washed with methanol is dimethanolic terephthalate (DMT) ester.

[0060] Example 9: A quantity (5 g) of polyethylene terephthalate PET pieces from food trays, after being washed with water, is placed in a container containing MEK (15 mL) so that all the plastic pieces are immersed. The latter are stirred for 2.5 hours. Then, the treated PET pieces are drained, optionally washed with alcohol and dried before being transferred to a 25 mL glass reactor. 30 mL of dry methanol is added to the pretreated pieces followed by 1.5 g of sodium methoxide powder. After 30 minutes, all the PET pieces have disappeared, leaving a white solid in solution. The reaction crude is filtered through filter paper or Buchner, the recovered liquid contains the residual methanol and the ethylene glycol produced by the depolymerization reaction. The white solid that is collected and washed with methanol is dimethanolic terephthalate (DMT) ester.

Claims

1. Method for recycling PET plastic waste comprising a step of grinding the waste to produce fragmentary pieces and a step of depolymerization treatment of said fragmentary pieces to recover virgin starting monomers which are terephthalic acid (TPA) or its dimethanol ester (DMT) and ethylene glycol (EG), characterized in that it comprises: - a step of pre-treating by soaking, without dissolving, in a container ensuring continuous stirring, said fragmentary pieces in a solution comprising at least one mixture of solvents comprising at least one reagent selected from components comprising DMAc (dimethylacetamide), DMF (dimethylformamide), MEK (Methyl Ethyl Ketone), phenolic esters or biphenyl ethers at a temperature less than 50° and for a period of less than 18 hours, - followed by a step of decanting - followed by a step of treating the pretreated PET fragments in a container containing a monoalcohol or a diol and a base at a temperature less than 50° - followed by an operation for recovering the solid in suspension and filtering.

2. Method for recycling PET plastic waste according to the preceding claim, characterized in that the reagent of the pretreatment step consists of DMAc or DMF, the decanting step being followed by a step of washing the pretreated fragments with an alcohol before adding the base.

3. Method for recycling PET plastic waste according to the preceding claim, characterized in that said decanting step comprises a treatment for recovering reagents for reuse in a subsequent pretreatment of new PET plastic waste.

4. Method for recycling PET plastic waste according to the preceding claim, characterized in that said recovered solid is a monoester or a methyl diester of terephthalic acid.

5. Method for recycling PET plastic waste according to claim 3, characterized in that said recovered solid is washed or subjected to distillation to purify said dimethanol terephthalate ester (DMT).

6. Method for recycling PET plastic waste according to claim 3, characterized in that said recovered solid is dissolved in an aqueous solution to produce the basic form of water-soluble terephthalic acid.

7. Method for recycling PET plastic waste according to claim 3, characterized in that the solution obtained is subjected to the addition of a mineral or organic acid to obtain terephthalic acid by precipitation.