TW202424104A - Recycling methods for mixed polyester materials in plastic products - Google Patents

Abstract

The present invention provides a method for recycling mixed polyester materials in plastic products, comprising: a crushing step, crushing the recycled plastic products to obtain recycled plastic fragments; a sorting step, separating mixed polyester material fragments from the recycled plastic fragments; and a chemical depolymerization step, subjecting the mixed polyester material fragments to a chemical depolymerization reaction with a multifunctional alcohol containing at least two hydroxyl groups in the presence of an esterification reaction catalyst or an ester exchange reaction catalyst to obtain polyester polyols; wherein the recycling method further comprises a cleaning step, which is performed between the sorting step and the chemical depolymerization step, to remove surface dirt and printing on the mixed polyester material fragments.

Description

Recycling methods for mixed polyester materials in plastic products

The present invention provides a method for recycling mixed polyester materials in plastic products, which can use chemical treatment methods to recycle mixed polyester materials such as PET, PETG, PLA, PBS and PBSA in recycled plastic products.

The materials used in plastic packaging products generally include PET, PETG, PP, PE, PVC, PS, PLA, PBS, PBSA, etc. Recently, due to the improvement of environmental awareness, in order to reduce the burden on the environment, how to recycle and reuse plastic products that have been used and discarded has become an important issue.

In the production process of flat plastic products, the raw plastic pellets are generally first molten through a screw heating device, then extruded from the opening of the mold and cooled to form plastic sheets of various widths and thicknesses. Finally, the plastic sheets are hot-pressed to form plastic products of various shapes. This method can be used for rapid mass production with low manufacturing costs, so the plastic products produced are widely used in the disposable packaging demand market, including various beverage cups (lids), plastic lunch boxes (lids), plastic cookie boxes, plastic product blister shells, and packaging of small factory parts products.

In the production process of plastic bottles and cans, molten plastic is directly processed by extrusion blow molding or injection blow molding to obtain plastic bottles and cans of various shapes for use as food or non-food containers.

Different plastic products need to have different characteristics according to the needs, such as transparency, printability, heat resistance, environmental protection, drop resistance, load deformation ability, etc. For example, if plastic products such as lunch boxes or drink cups need to be heat-resistant, PP materials will be used; if transparency is required, PET or PS materials will be used; if environmental protection is required, biodegradable materials such as PLA or PBS will be used; if soft and bendable properties are required, PE materials will be used; and if cost and processability are considered, PVC materials will be used.

Due to the above factors, there are many types of materials used in flat plastic products and bottle and can plastic products on the market. The resulting problem is that when these disposable plastic products are discarded after use, all plastic materials cannot be efficiently separated by type during recycling, which will cause the following problems during subsequent processing.

In traditional physical recycling methods, recycled plastic products are generally crushed and cleaned to obtain clean plastic fragments, which are then heated and melted to make plastic pellets, which are then provided to plastic product manufacturers to use injection, extrusion and other processing equipment for recycling to make various plastic products. However, different plastic materials have different melting temperatures, cracking temperatures and polarities. Therefore, if all materials from recycled plastic products cannot be completely separated after recycling, the various mechanical strengths of the plastic pellets made from melt granulation, including impact strength, tensile strength, elongation, etc., may be significantly reduced, or foreign matter may be produced on the surface of plastic products made from such plastic pellets. The above problems will affect the final recyclable market of plastic pellets.

At present, due to cost considerations, the amount of PET and PETG materials used in polyester plastic products on the market is far greater than that of PLA, PBS and PBSA. Among them, since bottle and can plastic products are usually made of only a single material, different materials can be separated only through manual sorting with electrostatic or optical sorting machines. However, flat plastic products contain many materials. In particular, in recent years, due to environmental protection demands, PLA, PBS, and PBSA, as biodegradable materials, have begun to be used in the production and application of flat plastics and plastic bottles and cans.

In the traditional physical recycling method, the temperature in the screw heating equipment must be heated to above 250°C to melt PET and PETG for processing. However, the aforementioned biodegradable materials are not resistant to high temperatures. Therefore, if the PET or PETG materials in the recycled plastic products are mixed with PLA, PBS or PBSA materials, the PLA, PBS or PBSA materials will be thermally cracked at the high temperature of the melting process, resulting in a decrease in the molecular weight of the recycled plastic and cannot be reused.

In view of the problems encountered in the prior art, the present invention aims to provide a method for recycling mixed polyester materials in plastic products, wherein a technology for using chemical treatment to mix any polyester materials, such as PET, PETG, PLA, PBS and/or PBSA, etc., when two or more polyester materials are mixed, can be directly chemically recycled and reused without completely sorting out a single material before reuse, so as to get rid of the disadvantage of the traditional physical recycling method that mixed multiple plastic materials cannot be reused.

The present invention provides a method for recycling mixed polyester materials in plastic products, comprising: a crushing step of crushing the recycled plastic products to obtain recycled plastic fragments; a sorting step of separating mixed polyester material fragments from the recycled plastic fragments; and a chemical depolymerization step of subjecting the mixed polyester material fragments to a chemical depolymerization reaction with a multifunctional alcohol containing at least two hydroxyl groups in the presence of an esterification reaction catalyst or an ester exchange reaction catalyst to obtain polyester polyols.

The recycling method of the present invention further comprises a cleaning step, wherein the cleaning step is performed before the crushing step to remove surface dirt and printing of the recycled plastic product; or, the cleaning step is performed between the crushing step and the sorting step to remove surface dirt and printing of the recycled plastic fragments; or, the cleaning step is performed between the sorting step and the chemical depolymerization step to remove surface dirt and printing of the mixed polyester material fragments.

In one embodiment of the present invention, the chemical depolymerization reaction is carried out at a temperature higher than 190°C and lower than 250°C.

In one embodiment of the present invention, the mixed polyester material chips include at least one of PET, PETG, PLA, PBS and PBSA.

In one embodiment of the present invention, the weight ratio of the mixed polyester material fragments to the multifunctional alcohol is in the range of 0.8 to 2.5.

In one embodiment of the present invention, the multifunctional alcohol comprises at least one of 2-methyl-1,3-propanediol, diethylene glycol, 2,2-dimethyl-1,3-propanediol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, pentanediol, hexanediol, glycerol and trihydroxymethylpropane.

In one embodiment of the present invention, the weight ratio of the mixed polyester material fragments/the multifunctional alcohol is in the range of 0.8 to 2.5, and wherein, when the multifunctional alcohol comprises at least ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof, the total addition amount of the ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof does not exceed 20 phr.

In one embodiment of the present invention, the esterification reaction catalyst or the transesterification reaction catalyst comprises at least one of acetic acid metal salts, metal oxides, and titanium esters.

In one embodiment of the present invention, the sorting step further includes: a density flotation step, placing the recycled plastic fragments into a liquid with a specific gravity greater than 1.05 and less than 1.15 to separate the bottom-sinking plastic fragments containing PVC and polyester materials from the recycled plastic fragments; and an electrostatic or optical sorting step, separating the PVC in the bottom-sinking plastic fragments by an electrostatic sorting device or an optical sorting device to obtain the mixed polyester material fragments.

In one embodiment of the present invention, in the cleaning step, at least one of a strong alkaline compound and an organic solvent is used at a temperature of 25° C. to 100° C. to remove surface dirt and printing from the recycled plastic product, the recycled plastic fragments or the mixed polyester material fragments.

In one embodiment of the present invention, the recycling method further comprises a polyurethane synthesis step, using the polyester polyol obtained in the chemical depolymerization step to synthesize polyurethane resins (PU) or polyurethane foam materials.

In summary, the present invention first obtains a mixed polyester plastic material, including PET, PETG, PLA, PBS and/or PBSA, through the above-mentioned plastic material sorting step. This mixed material does not need to undergo any material sorting step. After surface cleaning and decontamination treatment, it can be directly subjected to a chemical depolymerization step. Under high temperature, a catalyst and the presence of a multifunctional alcohol, the recycled mixed polyester material fragments are converted into a polyester polyol with a medium molecular weight. The polyester polyol obtained by the depolymerization reaction can be reused in various applications, for example, as a raw material for synthesizing PU resin (polyurethane resin) or PU foam material (polyurethane foam).

The effects of the present invention are not limited to the above-mentioned effects, and a person having ordinary knowledge in the relevant technical field can clearly understand other effects not mentioned from the description of the patent application scope.

According to the described embodiment with reference to the accompanying drawings below, the advantages and features of the present invention and its implementation method will be more clearly understood. However, the present invention is not limited to the following embodiments, but can be implemented in various different forms.

In order to solve the problems encountered in the prior art, the present invention provides a method for recycling mixed polyester materials in plastic products, wherein first, PET (polyethylene terephthalate), PETG (Poly (ethylene terephthalateco-1,4-cylclohexylenedimethylene terephthalate), PLA (polylactic acid), PBS (polybutylene succinate-co-butylene ester) and PBSA (Poly (butylene succinate-co-butylene ester)) are recycled from the plastic products. The polyester materials such as poly(butylene succinate-co-butylene adipate) are separated, and then the separated mixed polyester materials are chemically depolymerized to obtain recycled polyester polyols, so that the polyester polyols can be reused, for example, as raw materials for synthesizing PU resins or PU foam materials.

Table 1. Structure and melting point of various polyester materials Name Structure Melting point PET ＞250℃ PETG ＞250℃ PLA 180℃ PBS 115℃

As shown in the structure in Table 1 above, polyester materials can generally be divided into: polymers obtained by polycondensation of polyacids (such as dibasic acids) and polyols (such as diols), such as PET, PETG, PBS, PBSA; and polymers obtained by polycondensation of carboxylic acids containing hydroxyl groups, such as PLA. For example, PET is a polymer obtained by polycondensation of terephthalic acid (TPA) and ethylene glycol (EG); and generally speaking, PETG is a copolyester composed of TPA, EG and 1,4-cyclohexanedimethanol (CHDM).

As shown in Table 1 above, the melting points of PET and PETG are both higher than 250°C, while the melting points of PLA and PBS are much lower than 250°C, and the melting point of PBSA is even lower than PBS. Therefore, the traditional physical recycling method cannot meet the melting conditions of these materials at the same time. To solve this problem, the present invention develops a recycling method that uses chemical depolymerization technology instead of the traditional physical melting method, thereby being able to simultaneously process mixed polyester recycled materials.

As shown in the following reaction formula 1, the principle of the depolymerization reaction is that the ester bond of the polyester material undergoes a depolymerization reaction (i.e., alcoholysis reaction) with the OH bond of a multifunctional alcohol (e.g., a difunctional alcohol) to obtain a polyester polyol, which is a type of oligomer. The depolymerization reaction can be accelerated in the presence of a catalyst and/or at a high temperature. [Formula 1]

Therefore, referring to Figures 1 to 3, the present invention provides a method for recycling mixed polyester materials in plastic products, comprising a crushing step S10, a sorting step S20 and a chemical depolymerization step S30; wherein the recycling method further comprises a cleaning step C before the crushing step S10, between the crushing step S10 and the sorting step S20, or between the sorting step S20 and the chemical depolymerization step S30.

First, in the crushing step S10, the recycled plastic products, such as plastic cups (lids), plastic boxes (lids), plastic bottles and cans, etc., can be crushed by crushing equipment known in the technical field of the present invention, such as a crusher, a shredder or a crusher, to produce recycled plastic fragments.

Specifically, the materials of recycled plastic products may include, for example, PET, PETG, PP, PE, PVC, PS, PLA, PBS, PBSA and the like, but are not limited thereto.

Next, in the sorting step S20, mixed polyester material fragments including at least one of PET, PETG, PLA, PBS and PBSA are separated from the recycled plastic fragments.

Specifically, the sorting step S20 may further include: a density flotation step S21; and an electrostatic or optical sorting step S22.

In the density flotation step S21, the recycled plastic fragments produced by the crushing step S10 are placed in a liquid with a specific gravity greater than 1.05 and less than 1.15. At this time, materials with a specific gravity less than 1.05-1.15 in the recycled plastic fragments, such as PP, PE and PS, will float on the liquid, while materials with a specific gravity greater than 1.15, such as PVC materials and/or polyester materials such as PET, PETG, PLA, PBS, PBSA, etc., will sink into the liquid. Therefore, the plastic fragments containing PVC and polyester materials that sink to the bottom of the liquid can be separated from the recycled plastic fragments.

Next, in the electrostatic or optical sorting step S22, the PVC material is separated from the bottom plastic fragments by an electrostatic sorting device or an optical sorting device to obtain the mixed polyester material fragments including at least one of PET, PETG, PLA, PBS and PBSA.

Specifically, the electrostatic sorting device may be, for example, a plastic electrostatic sorting machine, which utilizes the different electrostatic attraction between various plastics to sort materials; and the optical sorting device may be, for example, a near infrared (NIR) optical sorting machine, which utilizes the significantly different absorption and vibration effects of near infrared light on chemical bonds in plastics to sort materials. In addition, the above-mentioned electrostatic or optical sorting device may also be an electrostatic or optical sorting device known in the art to which the present invention belongs.

In addition, the mixed polyester material recycling method of the present invention may further include a cleaning step C to remove surface dirt and printing from the recycled plastic products, recycled plastic fragments or mixed polyester material fragments. The cleaning step C may be performed at any time before the chemical depolymerization step S30, as long as clean mixed polyester material fragments free of impurities can be obtained before the chemical depolymerization step S30.

For example, referring to FIG1 , according to one aspect of the present invention, the cleaning step C may be performed before the crushing step S10. Alternatively, referring to FIG2 , according to another aspect of the present invention, the cleaning step C may be performed between the crushing step S10 and the sorting step S20. Alternatively, referring to FIG3 , according to yet another aspect of the present invention, the cleaning step C may be performed between the sorting step S20 and the chemical depolymerization step S30.

Specifically, in the cleaning step C, the recycled plastic product, recycled plastic fragments or mixed polyester material fragments are placed in a liquid containing at least one of a strong alkaline compound and an organic solvent at a temperature of 25°C to 100°C for stirring and cleaning to remove surface dirt and printing to obtain clean recycled plastic products, recycled plastic fragments or mixed polyester material fragments.

The strong alkaline compound may include at least one of sodium hydroxide and potassium hydroxide, and the organic solvent may include small molecule organic solvents such as ketones, alcohols, esters, etc. with a lower carbon number, or organic solvents such as benzene, for example: acetone, butanone, ethanol, isopropanol, ethyl acetate or toluene, but not limited thereto.

In addition, in the cleaning step C, a surfactant can be further used as an auxiliary cleaning liquid. Due to the hydrophilic and hydrophobic properties of the surfactant, surface dirt and printing can be effectively dispersed to promote the cleaning effect.

However, the cleaning technology used in the cleaning step C is not limited to the above-mentioned ones, and any cleaning technology known in the technical field to which the present invention belongs may also be used.

Next, in the chemical depolymerization step S30, the clean mixed polyester material fragments and a multifunctional alcohol containing at least two hydroxyl groups are subjected to a chemical depolymerization reaction in the presence of an esterification reaction catalyst or an ester exchange reaction catalyst to obtain a polyester polyol.

Specifically, the chemical depolymerization reaction is carried out at a temperature higher than 190°C, such as 195°C or higher, preferably higher than 210°C, and lower than 250°C, preferably lower than 230°C. When the depolymerization temperature is lower than 190°C, the depolymerization effect is poor. When the depolymerization temperature is higher, the color of the product becomes yellower, and the acid value becomes higher, which will affect the appearance and properties of the product. Therefore, the depolymerization temperature should be lower than 250°C.

In addition, since the present invention adopts a chemical depolymerization method to recycle the polyester material, it can allow the polyester material to undergo partial thermal cracking reaction compared to the traditional physical melting method, as long as the depolymerization temperature is lower than 250°C, preferably lower than 230°C.

Specifically, the weight ratio of the mixed polyester material fragments to the multifunctional alcohol is in the range of 0.8 to 2.5, and the preferred range is 1 to 2. When the amount of the multifunctional alcohol added is too little, the viscosity of the product obtained by the depolymerization reaction is too high, and it is difficult to flow and the foaming reaction is not easy; when the amount of the multifunctional alcohol added is too much, the molecular weight of the product obtained by the depolymerization reaction is too low, and although it is easy to flow, the strength of the foamed product is insufficient and it is easy to crack.

Specifically, the multifunctional alcohol may be a difunctional alcohol, such as at least one of 2-methyl-1,3-propanediol, diethylene glycol, 2,2-dimethyl-1,3-propanediol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, pentanediol and hexanediol; or a trifunctional alcohol, such as at least one of glycerol and trihydroxymethylpropane. Among them, butanediol, pentanediol and hexanediol generally refer to 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol, respectively.

Preferably, in the depolymerization reaction, by mainly using branched alcohols (such as 2-methyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, glycerol and trihydroxymethylpropane) as the polyfunctional alcohol, and optionally using straight-chain alcohols (such as ethylene glycol, 1,3-propanediol, butanediol, pentanediol and hexanediol) in combination, a polyester polyol that is a free-flowing liquid at room temperature can be obtained, which is beneficial for subsequent recycling.

Preferably, the concentration of 2-methyl-1,3-propanediol or 2,2-dimethyl-1,3-propanediol added may be 50-70 phr. Preferably, the concentration of diethylene glycol added may be 20-70 phr.

Preferably, when the multifunctional alcohol comprises at least ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof, the total amount of ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof added is not more than 20 phr.

Among them, phr is defined as parts per hundred parts of resin, that is, the weight (g) added to every 100 g of (mixed) polyester material chips.

Specifically, the esterification reaction catalyst or the transesterification reaction catalyst may include at least one of metal acetate salts, metal oxides, and titanium esters. The metal acetate salts may be, for example, zinc acetate, potassium acetate, and lithium acetate, the metal oxides may be, for example, dibutyltin oxide and antimony trioxide, and the titanium ester may be, for example, tetrabutyl titanium.

The polyester polyol obtained by the chemical depolymerization step S30 of the present invention can be reused in various applications, for example, as a raw material for synthesizing PU resin or PU foaming material.

Therefore, the mixed polyester material recycling method of the present invention may further include a polyurethane synthesis step S40, wherein the polyester polyol obtained in the chemical depolymerization step S30 is reacted with isocyanate to synthesize a polyurethane resin. The reaction formula is shown in Formula 2 below: [Formula 2]

The polyurethane (3) is a polymer containing a urethane structure (4) in the main chain. The polyester polyol obtained in the chemical depolymerization step S30 of the present invention can be used as the polyol (1), thereby reacting with the isocyanate (2) to obtain the polyurethane (3) resin.

In addition, in the polyurethane synthesis step S40, a proper amount of foaming agent may be further added to the above-mentioned raw materials such as polyol and isocyanate and mixed evenly, and then the mixture is injected into a mold to perform polymerization reaction and foaming reaction to synthesize a polyurethane foam material.

There is no limitation on the synthesis method of the polyurethane resin and the polyurethane foam material, and any synthesis method known in the art to which the present invention belongs can be used.

In order to verify the efficacy of the mixed polyester material recycling method of the present invention, chemical depolymerization experiments were conducted on polyester material fragments of various materials under different conditions, and the appearance and/or fluidity of the obtained polyester polyols were observed. The experimental conditions and results of various examples of the present invention are shown in Tables 2 to 6.

Table 2 shows the depolymerization experimental conditions (including the amount of each component added) and results using PET as the polyester material, wherein Examples 1 to 5 are cases where a larger amount of polyfunctional alcohol is added (the weight ratio of polyester material fragments/polyfunctional alcohol is 1.43); Examples 6 to 7 are cases where a smaller amount of polyfunctional alcohol is added (the weight ratio of polyester material fragments/polyfunctional alcohol is 2.0).

Table 2. Depolymerization experiment of PET polyester material Example PET Ethylene glycol 2-Methyl-1,3-propanediol Diethylene glycol Catalyst Depolymerization conditions Appearance at high temperature Appearance at room temperature 1 100g 70g A 200℃*4hr Transparent liquid White solid 2 100g 70g A 220℃*2hr Transparent liquid Free flowing liquid 3 100g 70g A 220℃*2hr Transparent liquid Difficult-flowing liquids 4 100g 70g B 220℃*2hr Transparent liquid Free flowing liquid 5 100g 70g C 200℃*4hr Transparent liquid Free flowing liquid 6 100g 50g A 200℃*4hr Transparent liquid White solid 7 100g 50g A 220℃*3hr Transparent liquid Difficult-flowing liquids

Among them, catalyst A is zinc acetate, catalyst B is tetrabutyl titanium (TBT), and catalyst C is dibutyltinoxide.

Among them, referring to the experimental results of Examples 1 to 5, it can be seen that the polyester polyol obtained by using 2-methyl-1,3-propanediol as a multifunctional alcohol to depolymerize PET is an easy-flowing liquid at room temperature, which is conducive to the subsequent recycling of the polyester polyol, such as the synthesis of PU resin or PU foam material.

In addition, in Example 3, the polyester polyol obtained by depolymerization reaction using diethylene glycol as the multifunctional alcohol is a difficult-to-flow liquid at room temperature and is not easy to undergo a foaming reaction, which is not conducive to the subsequent recycling of the polyester polyol.

Furthermore, the experimental results of Examples 1 and 6 show that when ethylene glycol is used as a multifunctional alcohol to depolymerize PET, the resulting polyester polyol may be solid at room temperature, which is not conducive to the subsequent recycling of the polyester polyol.

In addition, referring to the experimental results of Examples 6 and 7, it can be seen that when the amount of polyfunctional alcohol added is small, the polyester polyol obtained by the depolymerization reaction is a solid and a difficult-to-flow liquid at room temperature, which is not conducive to the subsequent recycling of the polyester polyol. In addition, when the amount of polyfunctional alcohol added is small, the depolymerization reaction rate will also be slow.

As shown below, Table 3 shows the depolymerization experimental conditions (including the amount of each component added) and results using PLA and PBS as polyester materials, among which Examples 8~9 are the cases where PLA is used as the polyester material and different multifunctional alcohols are added; Examples 10~11 are the cases where PBS is used as the polyester material and different multifunctional alcohols are added.

Table 3. Depolymerization experiments of PLA and PBS polyester materials Example PLA PBS Ethylene glycol 2-Methyl-1,3-propanediol Catalyst Depolymerization conditions Appearance at high temperature Appearance at room temperature 8 100g 70g A 200℃*4hr Transparent liquid Free flowing liquid 9 100g 70g A 210℃*2hr Transparent liquid Free flowing liquid 10 100g 50g A 200℃*4hr Transparent liquid Free flowing liquid 11 100g 50g A 210℃*2hr Transparent liquid Free flowing liquid

Among them, referring to the experimental results of Examples 8 to 11, it can be seen that when PLA and PBS are used as polyester materials for depolymerization reaction, the obtained polyester polyols are all easy-to-flow liquids at room temperature, which is conducive to the subsequent recycling of polyester polyols.

As shown below, Table 4 shows the depolymerization experimental conditions (including the amount of each component added) and results using glycerol and trihydroxymethylpropane as part of the multifunctional alcohol, among which Example 12 is the case of using glycerol as part of the multifunctional alcohol; Example 13 is the case of using trihydroxymethylpropane as part of the multifunctional alcohol.

Table 4. Depolymerization experiments of PET and PLA polyester materials Example PET PLA 2-Methyl-1,3-propanediol Glycerol Trihydroxymethylpropane Catalyst Depolymerization conditions Appearance at high temperature Appearance at room temperature 12 90g 10g 50g 20 A 220℃*2hr Transparent liquid Free flowing liquid 13 90g 10g 50g 20 A 220℃*2hr Transparent liquid Free flowing liquid

Among them, referring to the experimental results of Examples 12 and 13, it can be seen that when propylene glycol and trihydroxymethylpropane are used as multifunctional alcohols for depolymerization reaction, the obtained polyester polyols are all easy-flowing liquids at room temperature, which is conducive to the subsequent recycling of polyester polyols.

From the experimental results in Tables 2 to 4 above, it can be seen that in the presence of an esterification reaction catalyst or an ester exchange reaction catalyst, the polyester material PET, PLA and/or PBS and a multifunctional alcohol having at least two hydroxyl groups are heated to a temperature higher than 190° C. (e.g., higher than 200° C.) to carry out a depolymerization reaction, and after a period of time (e.g., 2 to 4 hours), a liquid and transparent polyester polyol at a high temperature can be obtained.

However, the polyester polyol obtained by depolymerization of PET and ethylene glycol is solid after cooling to room temperature, which is not conducive to the subsequent reuse of polyester polyol. This is because BHET (bis(hydroxyethyl) terephthalate) and BHET oligomers are obtained after depolymerization of PET materials and ethylene glycol, which are linear unbranched structures that are easily stacked and arranged, so they are highly crystalline and solid at room temperature.

In comparison, 2-methyl-1,3-propanediol or 2,2-dimethyl-1,3-propanediol with a branched chain structure, because of the presence of methyl groups on the side chains, will hinder the molecular arrangement of the depolymerization product during the cooling process after the depolymerization reaction with PET. Therefore, the product is not easy to crystallize and has fluidity. Therefore, it can be used in the production process of PU foaming, for example.

Among the polyfunctional alcohols mentioned above, 2-methyl-1,3-propanediol has a similar structure to the above-mentioned 2,2-dimethyl-1,3-propanediol, glycerol, and trihydroxymethylpropane. Therefore, using these alcohols as the polyfunctional alcohol for the reaction will obtain a depolymerization effect similar to that of 2-methyl-1,3-propanediol.

In addition, ethylene glycol has a similar structure to the above-mentioned 1,3-propylene glycol and butanediol, so using these alcohols as the polyfunctional alcohol for the reaction will obtain a depolymerization effect similar to that of ethylene glycol.

In addition, although the above-mentioned 1,2-propylene glycol has a branched structure, it will vaporize in the chemical depolymerization reaction due to its too low boiling point (~188°C), and is therefore not suitable for use alone in the depolymerization reaction.

In particular, the use of pentanediol or hexanediol as the polyfunctional alcohol for the reaction can also produce a depolymerization effect similar to that of 2-methyl-1,3-propanediol.

Therefore, in order to speed up the experiment, 2-methyl-1,3-propanediol will be used as the main material to carry out the depolymerization experiment of mixed polyester plastic materials.

Table 5 shows the depolymerization experimental conditions (including the amount of each component added) and results of a mixed polyester material using PET and PLA and/or PBS, wherein 2-methyl-1,3-propanediol was mainly used and diethylene glycol and/or ethylene glycol was selectively added as a polyfunctional alcohol.

Table 5. Depolymerization experiment of mixed polyester materials Example PET PLA PBS 2-Methyl-1,3-propanediol Diethylene glycol Ethylene glycol Depolymerization conditions Appearance at room temperature 14 50g 50g 70g 220℃*2hr Free flowing liquid 15 90g 10g 70g 220℃*2hr Free flowing liquid 16 50g 50g 70g 220℃*2hr Free flowing liquid 17 90g 10g 70g 220℃*2hr Free flowing liquid 18 50g 50g 50g 220℃*3hr Free flowing liquid 19 90g 10g 50g 220℃*3hr Free flowing liquid 20 90g 10g 50g 220℃*3hr Free flowing liquid twenty one 90g 10g 50g 20g 220℃*3hr Free flowing liquid twenty two 90g 10g 50g 20g 220℃*3hr Free flowing liquid twenty three 80g 10g 10g 50g 220℃*3hr Free flowing liquid twenty four 50g 25g 25g 50g 220℃*3hr Free flowing liquid Zinc acetate was used as the catalyst in all the above experiments.

Among them, referring to the experimental results of Examples 14 to 24, it can be seen that when the PET content in the mixed polyester material fragments is 50 to 100 wt%, the PLA content is 0 to 50 wt%, and the PBS content is 0 to 50 wt%, 2-methyl-1,3-propanediol is used as the main multifunctional alcohol for depolymerization reaction, a polyester polyol that is easy to flow at room temperature can be obtained, which is beneficial to the subsequent recycling of the polyester polyol.

Table 6 shows the experimental conditions (including the amount of each component added) and results of the depolymerization experiment for separating polyester materials from mixed recycled plastic products, wherein 1 kg of mixed recycled plastic products were crushed, and the plastic products included bottles, cans and flat plastic products; then the crushed recycled plastic fragments were poured into a solution with a density of 1.1, slowly stirred and allowed to stand, the plastic fragments sunk below the liquid were taken out and drained, and PVC was removed by electrostatic sorting or optical sorting equipment to obtain plastic fragments containing only mixed polyester materials, and the plastic fragments were mainly PET materials, with a small amount of PLA and PBS materials. ; then the fragments are placed in a high temperature washing device (e.g., a metal tank containing a stirring device and a heating device), and the dirt and printing ink on the surface of the fragments are washed away using the aforementioned washing step (washing step C) to obtain clean mixed polyester material fragments; finally, the clean mixed polyester material fragments are subjected to a chemical decomposition polymerization reaction under the experimental conditions listed in Table 5 to obtain polyester polyols, and the fluidity of the obtained polyester polyols is observed.

Table 6. Depolymerization experiment of mixed recycled plastic products Example Mixed polyester material scraps 2-Methyl-1,3-propanediol Diethylene glycol Depolymerization conditions Appearance at room temperature 25 1kg 700g 220℃*2hr Free flowing liquid 26 1kg 500g 230℃*3hr Free flowing liquid 27 1kg 500g 200g 220℃*2hr Free flowing liquid 28 1kg 300g 200g 220℃*2hr Free flowing liquid 29 1kg 700g 195℃*5hr Free flowing liquid All of the above catalysts use zinc acetate.

Among them, referring to the experimental results of Examples 25 to 29, it can be seen that for actual mixed recycled plastic products, by adding 2-methyl-1,3-propanediol as the main multifunctional alcohol for depolymerization reaction, the obtained polyester polyols are all easy-to-flow liquids at room temperature, which is conducive to the subsequent recycling of polyester polyols.

Table 7 shows the addition amount of each component, experimental conditions and results of depolymerization experiments under different conditions using PET and PETG as polyester materials, among which Example 30 uses PET as the polyester material, and Examples 31 and 32 use PETG as the polyester material.

Table 7. Depolymerization experiments of PET and PETG polyester materials Example PET PETG 2,2-Dimethyl-1,3-propanediol 2-Methyl-1,3-propanediol Depolymerization conditions Appearance at room temperature 30 100g 50g 220℃*2hr Free flowing liquid 31 100g 50g 220℃*2hr Free flowing liquid 32 100g 50g 220℃*2hr Free flowing liquid All of the above catalysts use zinc acetate.

Referring to the experimental results of Example 30, it can be seen that when 2,2-dimethyl-1,3-propanediol, which also has a branched structure, is used instead of 2-methyl-1,3-propanediol to carry out a depolymerization reaction with PET, the obtained polyester polyol is also an easy-flowing liquid at room temperature, which is also conducive to the subsequent recycling of the polyester polyol.

In addition, referring to the experimental results of Examples 31 and 32, it can be seen that when PETG is used instead of PET as the polyester material for depolymerization reaction, the obtained polyester polyol is also a free-flowing liquid at room temperature. This means that if PETG is included in the polyester material, its depolymerization result should be the same as the depolymerization result of the polyester material containing PET. This is because, compared with the polymer structure of PET, the structure of PETG only replaces part of the ethylene glycol unit with 1,4-cyclohexanedimethanol unit, and the structures of the two are similar, so PET and PETG should have similar depolymerization results.

Similarly, it can be inferred from the above results that if PBSA is included in a polyester material, its depolymerization result should be similar to the depolymerization result of a polyester material containing PBS. This is because, compared with the polymer structure of PBS, the structure of PBSA only replaces part of the succinic acid units with adipic acid units. The structures of the two are similar, so PBS and PBSA should have similar depolymerization results.

In summary, through the mixed polyester material recycling method of the present invention, polyester materials including PET, PETG, PLA, PBS and/or PBSA can be separated from non-polyester materials in plastic products, and then the mixed polyester materials are chemically degraded to obtain recycled polyester polyols, so that the obtained polyester polyols can be reused, especially as raw materials for synthesizing PU resins or PU foam materials.

Those having ordinary knowledge in the technical field to which the contents of the present invention relate will understand that various modifications and changes in form may be made without departing from the basic characteristics of the contents of the present invention, such as combination, separation, replacement and change of configuration.

Therefore, the embodiments of the present invention are intended to illustrate the scope of the technical concept of the present invention, and the scope of the present invention is not limited by the embodiments. Therefore, any modifications or changes made to the present invention under the same spirit of the invention should still be included in the scope of protection intended by the present invention.

S10: Crushing step S20: Sorting step S21: Density flotation step S22: Electrostatic or optical sorting step S30: Chemical depolymerization step S40: Polyurethane synthesis step C: Cleaning step

Figure 1 is a flow chart illustrating the steps of a mixed polyester material recycling method according to one embodiment of the present invention; Figure 2 is a flow chart illustrating the steps of a mixed polyester material recycling method according to another embodiment of the present invention; Figure 3 is a flow chart illustrating the steps of a mixed polyester material recycling method according to yet another embodiment of the present invention.

S10: Crushing step

S20: Sorting step

S21: Density flotation step

S22: Electrostatic or optical sorting step

S30: Chemical depolymerization step

S40: Polyurethane synthesis step

C: Cleaning steps

Claims (10)

A method for recycling mixed polyester materials in plastic products, comprising: a crushing step, crushing the recycled plastic products to obtain recycled plastic fragments; a sorting step, separating mixed polyester material fragments from the recycled plastic fragments; and a chemical depolymerization step, subjecting the mixed polyester material fragments to a chemical depolymerization reaction with a multifunctional alcohol containing at least two hydroxyl groups in the presence of an esterification catalyst or an ester exchange catalyst to obtain polyester polyols; wherein the recycling method further comprises a cleaning step, and wherein, the cleaning step is performed before the crushing step to remove surface dirt and printing on the recycled plastic product; or The cleaning step is performed between the crushing step and the sorting step to remove surface dirt and printing from the recycled plastic fragments; or The cleaning step is performed between the sorting step and the chemical depolymerization step to remove surface dirt and printing from the mixed polyester material fragments. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the chemical depolymerization reaction is carried out at a temperature higher than 190°C and lower than 250°C. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the mixed polyester material fragments include at least one of PET, PETG, PLA, PBS and PBSA. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the weight ratio of the mixed polyester material fragments to the multifunctional alcohol ranges from 0.8 to 2.5. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the multifunctional alcohol comprises at least one of 2-methyl-1,3-propanediol, diethylene glycol, 2,2-dimethyl-1,3-propanediol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, pentanediol, hexanediol, glycerol and trihydroxymethylpropane. A method for recycling mixed polyester materials in plastic products as claimed in claim 5, wherein the weight ratio of the mixed polyester material fragments to the multifunctional alcohol is in the range of 0.8 to 2.5, and wherein, when the multifunctional alcohol comprises at least ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof, the total amount of ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, butanediol or a combination thereof added does not exceed 20 phr. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the esterification reaction catalyst or the ester exchange reaction catalyst comprises at least one of metal acetate salts, metal oxides, and titanium esters. The method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein the sorting step comprises: A density flotation step, placing the recycled plastic fragments in a liquid with a specific gravity greater than 1.05 and less than 1.15 to separate the bottom-sinking plastic fragments containing PVC and polyester materials from the recycled plastic fragments; and An electrostatic or optical sorting step, separating PVC from the bottom-sinking plastic fragments by an electrostatic sorting device or an optical sorting device to obtain the mixed polyester material fragments. A method for recycling mixed polyester materials in plastic products as claimed in claim 1, wherein, in the cleaning step, at least one of a strong alkaline compound and an organic solvent is used at a temperature of 25°C to 100°C to remove surface dirt and printing on the recycled plastic product, the recycled plastic fragments or the mixed polyester material fragments. The method for recycling mixed polyester materials in plastic products as in claim 1 further comprises: A polyurethane synthesis step, using the polyester polyol obtained in the chemical depolymerization step to synthesize polyurethane resin or polyurethane foam material.