Regeneration preparation method of PETG or PCTG copolyester

Abstract

The invention relates to the technical field of PETG or PCTG copolyesters, and discloses a regeneration preparation method of a PETG or PCTG copolyester, which comprises the following steps: (1) in the presence of an antioxidant, carrying out a regeneration reaction on PETG or PCTG copolyesters, the preparation method comprises the following steps: carrying out esterification reaction on a mixture containing bis (2-hydroxyethyl) terephthalate, a bis (2-hydroxyethyl) terephthalate dimer and cyclohexanedimethanol terephthalate, terephthalic acid and cyclohexanedimethanol; and (2) sequentially carrying out first polycondensation and second polycondensation on the esterification reaction product in the presence of a stabilizer. According to the method, the regenerated PETG (or PCTG) copolyester product can be directly polymerized and prepared by utilizing the residual catalyst in the high-purity monomer obtained by chemical recovery under the condition of not adding the catalyst, so that the method is green and environment-friendly, the production cost is saved, and good economic benefits are achieved.

Description

Technical Field

[0001] This invention relates to the field of PETG or PCTG copolyester technology, and specifically to a method for preparing recycled PETG or PCTG copolyester. Background Technology

[0002] PETG or PCTG copolyester is a copolyester copolymerized from terephthalic acid (PTA), ethylene glycol (EG), and 1,4-cyclohexanediethanol (CHDM) monomers. Its full name is ethylene terephthalate-1,4-cyclohexanediethanol ester. Compared to ethylene terephthalate (PET), the random insertion of 1,4-cyclohexanediethanol (CHDM) into the polymer chains hinders the regular arrangement of the molecular chains, significantly reducing crystallinity and ultimately resulting in an amorphous form. Copolyesters with a CHDM content of less than 50% are called PETG copolyesters, while those with a CHDM content greater than 50% are called PCTG copolyesters. By adjusting the ratio of CHDM to EG, PETG or PCTG copolyesters can have different properties. PETG copolyester has high transparency, excellent impact resistance and toughness, excellent processing and molding properties, does not whiten when cold-bent, does not crack, and is easy to print and finish. It is widely used in sheets, films, shrink films, bottles, and profiles where high transparency is required.

[0003] PETG has excellent secondary processing performance and can be processed by various molding methods such as injection molding, extrusion, compression molding, blow molding, and vacuum forming, making it widely used.

[0004] Since PETG copolyester itself is not biodegradable, physical recycling methods can only downgrade the product. While these methods are simple to operate, they cannot produce stable recycled polyester products, and after multiple cycles, they are even less able to meet performance requirements. Summary of the Invention

[0005] The purpose of this invention is to overcome the limitations of existing technologies where physical recycling methods cannot yield stable recycled PETG or PCTG copolyester products, and where the recycled products have poor performance. This invention provides a method for preparing recycled PETG or PCTG copolyesters.

[0006] To achieve the above objectives, the present invention provides a method for preparing recycled PETG or PCTG copolyesters, the method comprising the following steps:

[0007] (1) In the presence of an antioxidant, a mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediol terephthalate is subjected to an esterification reaction with terephthalic acid and cyclohexanediol.

[0008] (2) In the presence of a stabilizer, the esterification reaction product undergoes a first polycondensation and a second polycondensation in sequence.

[0009] Through the above technical solution, the present invention can achieve at least the following beneficial effects:

[0010] This invention utilizes the residual catalyst in high-purity monomers obtained through chemical recycling to directly polymerize and prepare recycled PETG (or PCTG) copolyester products without adding external catalysts. This method is green, environmentally friendly, and saves production costs, resulting in good economic benefits. Detailed Implementation

[0011] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0012] This invention provides a method for preparing recycled PETG or PCTG copolyesters, the method comprising the following steps:

[0013] (1) In the presence of an antioxidant, a mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediol terephthalate is subjected to an esterification reaction with terephthalic acid and cyclohexanediol.

[0014] (2) In the presence of a stabilizer, the esterification reaction product undergoes a first polycondensation and a second polycondensation in sequence.

[0015] According to the present invention, preferably, the amount of terephthalic acid is 0.03-9 mol and the amount of cyclohexanediol is 0.04-9 mol, based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediol terephthalate in the mixture.

[0016] According to the present invention, preferably, the conditions for the esterification reaction include: a temperature of 200-260°C and a pressure of atmospheric pressure.

[0017] According to the present invention, preferably, the esterification reaction is carried out for such a time that the actual amount of the small molecule product generated by the reaction reaches more than 90 wt% of the theoretical amount (e.g., 90 wt%, 98 wt%). Herein, the small molecule product refers to the byproduct generated by the esterification reaction (e.g., methanol).

[0018] According to the present invention, preferably, the esterification reaction is carried out under an inert atmosphere, preferably, the inert atmosphere comprising nitrogen and / or an inert gas. In the present invention, the inert atmosphere may be provided by at least one of argon, helium, and neon.

[0019] According to the present invention, preferably, the antioxidant comprises at least one selected from 4,4'-butylenebis(6-tert-butyl-3-methylphenol), pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,6-hexanediol 3,5-bis(1,1-dimethylethyl)-4-hydroxyphenylpropionate, and triethylene glycol ether-bis(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate.

[0020] According to the present invention, preferably, the amount of antioxidant added is 10-200 ppm of the total weight of the esterification reaction monomers. In the present invention, the esterification reaction monomers refer to: bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, cyclohexanediethanol terephthalate, terephthalic acid, and cyclohexanediethanol in the mixture.

[0021] According to the present invention, preferably, the stabilizer is a phosphorus-containing compound; more preferably, the stabilizer includes at least one selected from triphenyl phosphate, triphenyl phosphite, trimethyl phosphate, trimethyl phosphite, triethyl phosphate, triethyl phosphite, and triethyl phosphate.

[0022] According to the present invention, preferably, the amount of stabilizer added is 20-200 ppm of the total weight of the monomers fed in the esterification reaction.

[0023] According to the present invention, preferably, the conditions for the first polycondensation reaction include: a temperature of 235-280°C, a time of 1-2 hours, and a system pressure of 1000-20000 Pa.

[0024] According to the present invention, preferably, the conditions for the second polycondensation reaction include: a temperature of 255-285°C, a time of 0.4-3 h, and a system pressure of ≤300 Pa, preferably, the system pressure is ≤200 Pa, and more preferably, the system pressure is ≤60 Pa.

[0025] According to the present invention, preferably, the mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate is obtained by:

[0026] (I) In the presence of a catalyst and a depolymerizing agent, a copolyester with an average particle size of 0.1-5 cm is subjected to a depolymerization reaction to obtain a depolymerization solution containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate; wherein the copolyester is polyethylene terephthalate-1,4-cyclohexanediethanol terephthalate, the water content in the copolyester is ≤2% by weight, and the catalyst is a titanium-containing catalyst;

[0027] (II) Purify the product obtained from the depolymerization reaction.

[0028] According to the present invention, preferably, the catalyst is a titanium-containing compound and / or a titanium-containing composition, more preferably at least one of an organotitanium catalyst, a titanium-silicon composite catalyst, and a titanium-rare earth composite catalyst. The organotitanium catalyst may have 1-5 carbon atoms and may include at least one of titanium glycolate, tetrabutyl titanate, and tetraisopropyl titanate. The titanium-rare earth composite catalyst includes a titanium component and a rare earth component. The titanium component may be at least one of titanium glycolate, tetrabutyl titanate, and tetraisopropyl titanate. The rare earth component may contain at least one of lanthanum, yttrium, neodymium, dysprosium, and terbium; the rare earth component may also contain at least one of lanthanum acetylacetonate and yttrium acetylacetonate. The titanium component content in the titanium-rare earth composite catalyst may be 50-90% by weight, and the rare earth component content may be 10-50% by weight. Using a titanium-rare earth composite catalyst can further improve the product depolymerization rate and yield.

[0029] According to the present invention, preferably, the amount of catalyst used is 0.1-0.5g, more preferably 0.1-0.3g, relative to 100g of copolyester. When the amount of catalyst is limited to the above range, the depolymerization rate of the copolyester can be further improved.

[0030] According to the present invention, preferably, the depolymerizing agent is ethylene glycol. More preferably, the weight ratio of the depolymerizing agent to the copolyester is 2-6:1, and even more preferably 2.5-5:1.

[0031] According to the present invention, in order to further improve the depolymerization rate and reduce the energy consumption of depolymerization, preferably, the temperature of the depolymerization reaction is 190-240°C, more preferably 200-230°C. Preferably, the pressure of the depolymerization reaction is 0.2-0.6 MPa, more preferably 0.35-0.5 MPa. Preferably, the time of the depolymerization reaction is 1-10 h, more preferably 2.5-5 h.

[0032] According to the present invention, preferably, the oxygen content in the reaction system during the depolymerization reaction is less than 3.5% by weight; more preferably, the depolymerization reaction is carried out under an inert atmosphere; even more preferably, the inert atmosphere includes nitrogen and / or an inert gas. In the present invention, the inert atmosphere may be provided by at least one of argon, helium, and neon.

[0033] According to the present invention, preferably, the average particle size of the copolyester is 0.1-3 cm, more preferably 0.1-2 cm. When the average particle size of the copolyester is within the above-mentioned preferred range, it can not only further improve the depolymerization rate of the copolyester, increase the content of the target product, and reduce the content of the polymer, but also further improve the depolymerization rate.

[0034] According to the present invention, preferably, the water content in the copolyester is ≤1% by weight. When the water content in the copolyester is within the above-mentioned preferred range, the depolymerization rate of the copolyester can be further improved, thereby increasing the content of the target product.

[0035] According to the present invention, the purification process includes the following steps:

[0036] (a) The depolymerization solution is subjected to a first solid-liquid separation to obtain a first filtrate;

[0037] (b) The first filtrate is mixed with a diluent, and then a second solid-liquid separation is performed to obtain the second filtrate;

[0038] (c) The second filtrate is mixed with activated carbon, and then a third solid-liquid separation is performed to obtain the third filtrate;

[0039] (d) The third filtrate is crystallized and dried to obtain a mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate.

[0040] In this invention, the first solid-liquid separation, the second solid-liquid separation, and the third solid-liquid separation can be separation methods commonly used in the art, such as filtration, centrifugal separation, etc.

[0041] According to the present invention, preferably, the first solid-liquid separation is carried out at 80-105°C. The first solid-liquid separation mainly removes non-depolymerizable components and impurities from PETG (or PCTG), such as blends, fillers, or dust introduced during the PETG (or PCTG) processing.

[0042] According to the present invention, washing the first filtrate with a diluent can increase the content of BHET, BHET dimer, and BHCT in the product obtained from the depolymerization reaction, while reducing the content of other impurities (e.g., BHET oligomers with a degree of polymerization greater than 3, polyurethane, titanium dioxide, etc.). Preferably, the diluent is water, more preferably deionized water. Preferably, the mass ratio of the depolymerization solution to the diluent is 1:1-10, more preferably 1:2-5. When the mass ratio of the depolymerization solution to the diluent is limited to the above range, the content of BHET, BHET dimer, and BHCT in the product obtained from the depolymerization reaction can be further increased, and the content of impurities can be reduced, thereby obtaining a depolymerized product with high purity and low degree of polymerization (BHET degree of polymerization is only 1 and 2). Preferably, the mixing conditions of the first filtrate and the diluent include: a temperature of 70-90°C and a time of 1-3 hours. The second method of solid-liquid separation is filtration, in which the filter paper used has a pore size of 0.5-20μm, preferably 2-15μm.

[0043] According to the present invention, when activated carbon is used to adsorb the second filtrate, not only can the purity of the depolymerization product be further improved, but also the L value and b value of the depolymerization product can be further increased, while the residual amount of catalyst in the depolymerization product can be effectively controlled. Preferably, the amount of activated carbon used is 1-10g relative to 100g of copolyester. Preferably, the mixing conditions of the second filtrate and activated carbon include: a temperature of 70-90℃ and a time of 0.1-2h.

[0044] According to the present invention, preferably, the crystallization conditions include a temperature of 2-10°C and a time of 4-12 hours. Preferably, the drying conditions include a temperature of 50-90°C and a time of 1-3 hours.

[0045] This invention does not impose particular limitations on the content of ethylene glycol terephthalate structural units and 1,4-cyclohexane terephthalic acid structural units in PETG or PCTG polyester raw materials. The method of this invention can be applied to the depolymerization of PETG or PCTG polyesters with different structural contents. For example, the content of 1,4-cyclohexane terephthalic acid structural units in PETG polyester is 0.01-50 mol%; the content of 1,4-cyclohexane terephthalic acid structural units in PCTG polyester is 50-100 mol%. The embodiments of this invention use waste PETG copolyester raw materials as examples, but the invention is not limited thereto.

[0046] In preferred embodiments, the depolymerization products of the present invention have high purity, good color, L value ≥ 90, and b value ≤ 2. Moreover, the esterification time for directly using them as esterification raw materials to prepare recycled PETG (or PCTG) is short, and the b value is low.

[0047] In this invention, L value represents whiteness; the larger the L value, the higher the brightness; the smaller the L value, the lower the brightness. b value represents yellow-blue tint; a positive b value indicates a higher yellow tint, a negative b value indicates a lower bluish tint, and b = 0 indicates neither yellow nor blue tint. The larger the L value, the closer the b value is to 0, indicating a good hue for the object.

[0048] According to a particularly preferred embodiment of the present invention, the method for depolymerizing the copolyester includes the following steps:

[0049] (1) The depolymerization process of waste PETG copolyester is as follows:

[0050] Waste PETG copolyester is crushed into particles with an average particle size of 0.5-0.8 cm and dried until the water content in the copolyester is 0.4-0.5% by weight. The crushed PETG copolyester and depolymerizing agent (ethylene glycol) are added to the depolymerization reactor at a weight ratio of 1:4-5. A catalyst is added, and the air in the reactor is replaced with nitrogen to ensure that the oxygen weight fraction in the reactor is less than 3.5%. After checking the airtightness, a certain amount of nitrogen is introduced to make the pressure in the reactor reach 0.4-0.45 MPa. Stirring is started, and the temperature in the reactor is set to 205-210℃. Once the reactor temperature reaches the set temperature, the reaction time is started and the reaction time is 3-3.5 hours to obtain the depolymerization liquid, which contains bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate. The catalyst is titanium glycolate and / or tetraisopropyl titanate, wherein the amount of the catalyst is 0.1-0.35g (e.g., 0.1-0.15g or 0.3-0.35g) relative to 100g of copolyester.

[0051] (2) The purification process of the depolymerization solution is as follows:

[0052] (a) The depolymerization solution is filtered while hot at 100-105°C to obtain the first filtrate.

[0053] (b) Then add the diluent at a weight ratio of 1:2-2.2 for the first filtrate: diluent (deionized water), stir in a water bath at 70-75°C for 1.5-1.8 hours, and filter with filter paper with a pore size of 10-15 μm to obtain a clear second filtrate.

[0054] (c) The activated carbon added to the second filtrate is stirred at 70-80℃ for 20-60 min, filtered, and the third filtrate is obtained; wherein, the amount of activated carbon used is 8-10g relative to 100g of copolyester.

[0055] (d) Cool the third filtrate at 4-5℃ for 10-11 h to crystallize, filter to obtain a solid, and dry in an oven at 60-65℃ for 2-3 h to obtain a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT).

[0056] The present invention will now be described in detail through preparation examples and embodiments. In the following preparation examples and embodiments,

[0057] The waste PETG copolyester raw material contains 70 mol% ethylene glycol terephthalate structural units and 30 mol% 1,4-cyclohexane terephthalic acid structural units.

[0058] The formula for calculating the depolymerization rate of PETG copolyester is:

[0059]

[0060] The purity and composition of the depolymerized and purified products were determined by high-purity liquid chromatography (HPLC). The elution times of BHET, BHET dimer, and BHCT were determined by HPLC, and the content of each component was calculated by peak area.

[0061] The L and b values ​​of the purified product and the L and b values ​​of the copolyester were determined by spectrophotometer.

[0062] The residual amount of titanium (or Zn) in the mixture was determined using an Agilent inductively coupled plasma optical emission spectrometer (ICP-OES).

[0063] The method for testing the CHDM:EG ratio in copolyesters is: proton nuclear magnetic resonance spectroscopy.

[0064] The intrinsic viscosity of copolyesters is tested using the capillary viscometer method (phenol: tetrachloroethane).

[0065] =1:1 solvent).

[0066] The test method for the content of terminal carboxyl groups in copolyesters is volumetric titration.

[0067] Preparation Example 1

[0068] (I) The depolymerization process of waste PETG copolyester is as follows:

[0069] Waste PETG copolyester was crushed into particles with an average particle size of 0.5 cm and dried until the water content in the copolyester was 0.5% by weight. The crushed PETG copolyester and depolymerizing agent (ethylene glycol) were added to a depolymerization reactor at a weight ratio of 1:5, along with a catalyst (ethylene glycol titanium, wherein the amount of catalyst was 0.1 g relative to 100 g of copolyester). The air in the reactor was replaced with nitrogen to ensure the oxygen weight fraction was less than 3.5%. After checking the airtightness, a certain amount of nitrogen was introduced to bring the pressure inside the reactor to 0.4 MPa. Stirring was started, and the reactor temperature was set to 210℃. The reaction time was 3 hours after the reactor temperature reached the set temperature, yielding a depolymerization solution containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate. The depolymerization rate of the waste PETG copolyester is shown in Table 1.

[0070] (II) The purification process of the depolymerization solution is as follows:

[0071] (a) The depolymerization solution is filtered while hot at 100°C to obtain the first filtrate.

[0072] (b) Then add the diluent at a weight ratio of 1:2 for the first filtrate: diluent (deionized water), stir in a 70°C water bath for 1.5 h, and filter with filter paper with a pore size of 10-15 μm to obtain a clear second filtrate.

[0073] (c) The activated carbon added to the second filtrate is stirred at 70°C for 20 min and filtered to obtain the third filtrate; wherein, the amount of activated carbon used is 8 g relative to 100 g of copolyester.

[0074] (d) The third filtrate was cooled and crystallized at 4°C for 10 h, filtered to obtain a solid, and dried in an oven at 60°C for 2 h to obtain a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediol terephthalate (BHCT). The total weight percentage of BHET, BHET dimer, and BHCT in the mixture (i.e., the purity of the mixture), the weight percentage of BHCT in the mixture, the L value and b value of the mixture, and the Ti content of the mixture are shown in Table 1.

[0075] Preparation Example 2

[0076] (I) The depolymerization process of waste PETG copolyester is as follows:

[0077] Waste PETG copolyester was crushed into particles with an average particle size of 1 cm and dried until the water content in the copolyester was 0.8% by weight. The crushed PETG copolyester and depolymerizing agent (ethylene glycol) were added to a depolymerization reactor at a weight ratio of 1:5, along with a catalyst (tetrabutyl titanate, wherein the amount of catalyst was 0.3 g relative to 100 g of copolyester). The air in the reactor was replaced with nitrogen to ensure the oxygen weight fraction was less than 3.5%. After checking the airtightness, a certain amount of nitrogen was introduced to bring the pressure inside the reactor to 0.4 MPa. Stirring was started, and the reactor temperature was set to 210℃. The reaction time was 3 hours after the reactor temperature reached the set temperature, yielding a depolymerization solution containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate. The depolymerization rate of the waste PETG copolyester is shown in Table 1.

[0078] (II) The purification process of the depolymerization solution is as follows:

[0079] (a) The depolymerization solution is filtered while hot at 100°C to obtain the first filtrate.

[0080] (b) Then add the diluent at a weight ratio of 1:3 for the first filtrate: diluent (deionized water), stir in an 80°C water bath for 1.5 h, and filter with filter paper with a pore size of 10-15 μm to obtain a clear second filtrate.

[0081] (c) The activated carbon added to the second filtrate is stirred at 75°C for 40 min and filtered to obtain the third filtrate; wherein, the amount of activated carbon used is 2 g relative to 100 g of copolyester.

[0082] (d) The third filtrate was cooled and crystallized at 10°C for 12 h, filtered to obtain a solid, and dried in an oven at 70°C for 2 h to obtain a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT). The total weight percentage of BHET, BHET dimer, and BHCT in the mixture, the weight percentage of BHCT in the mixture, the L value and b value of the mixture, and the Ti content of the mixture are shown in Table 1.

[0083] Preparation Example 3

[0084] (I) The depolymerization process of waste PETG copolyester is as follows:

[0085] Waste PETG copolyester was crushed into particles with an average particle size of 3 cm and dried until the water content in the copolyester was 0.8% by weight. The pulverized PETG copolyester and depolymerizing agent (ethylene glycol) were added to the depolymerization reactor at a weight ratio of 1:5. A titanium-rare earth composite catalyst (ethylene glycol titanium-lanthanum acetylacetone composite catalyst, wherein the titanium-rare earth composite catalyst contains 50% by weight of titanium glycol and 50% by weight of lanthanum acetylacetone; the amount of catalyst used is 0.2g per 100g of copolyester) was added. The air inside the reactor was replaced with nitrogen to ensure the oxygen weight fraction was less than 3.5%. After checking the airtightness, a certain amount of nitrogen was introduced to bring the pressure inside the reactor to 0.4MPa. Stirring was started, and the reactor temperature was set to 210℃. The reaction time was 3 hours after the reactor temperature reached the set temperature, yielding a depolymerization solution containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate. The depolymerization rate of the waste PETG copolyester is shown in Table 1.

[0086] (II) The purification process of the depolymerization solution is as follows:

[0087] (a) The depolymerization solution is filtered while hot at 90°C to obtain the first filtrate.

[0088] (b) Then add the diluent at a weight ratio of 1:4 for the first filtrate: diluent (deionized water), stir in an 85°C water bath for 3 hours, and filter with filter paper with a pore size of 10-15 μm to obtain a clear second filtrate.

[0089] (c) The activated carbon added to the second filtrate is stirred at 80°C for 60 min and filtered to obtain the third filtrate; wherein, the amount of activated carbon used is 5 g relative to 100 g of copolyester.

[0090] (d) The third filtrate was cooled and crystallized at 2°C for 24 h, filtered to obtain a solid, and dried in an oven at 70°C for 2 h to obtain a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediol terephthalate (BHCT). The total weight percentage of BHET, BHET dimer, and BHCT in the mixture, the weight percentage of BHCT in the mixture, the L value and b value of the mixture, and the Ti content of the mixture are shown in Table 1.

[0091] Preparation Example 4

[0092] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that...

[0093] In step (I), the catalyst added is tetraisopropyl titanate, wherein the amount of the catalyst is 0.3g relative to 100g of copolyester;

[0094] In step (II), (c) the activated carbon added to the second filtrate is stirred at 80°C for 60 min and filtered to obtain the third filtrate; wherein, the amount of activated carbon used is 10 g relative to 100 g of copolyester.

[0095] Preparation Example 5

[0096] (I) The depolymerization process of waste PETG copolyester is as follows:

[0097] Waste PETG copolyester was crushed into particles with an average particle size of 1.5 cm and dried until the water content in the copolyester was 1% by weight. The pulverized PETG copolyester and depolymerizing agent (ethylene glycol) were added to the depolymerization reactor at a weight ratio of 1:5. A titanium-rare earth composite catalyst (ethylene glycol titanium-yttrium acetylacetone composite catalyst, wherein the titanium-rare earth composite catalyst contains 80% titanium glycol by weight and 20% yttrium acetylacetone by weight; the amount of the catalyst is 0.2g per 100g of copolyester) was added. The air in the reactor was replaced with nitrogen to ensure that the oxygen weight fraction in the reactor was less than 3.5%. After checking the airtightness, a certain amount of nitrogen was introduced to make the pressure in the reactor reach 0.4MPa. Stirring was started, and the temperature in the reactor was set to 210℃. The reaction time was 3h after the reactor temperature reached the set temperature to obtain a depolymerization liquid containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediol terephthalate. The depolymerization rate of waste PETG copolyester is shown in Table 1.

[0098] (II) The purification process of the depolymerization solution is as follows:

[0099] (a) The depolymerization solution was filtered while hot at 80°C to obtain the first filtrate.

[0100] (b) Then add the diluent at a weight ratio of 1:2.5 for the first filtrate: diluent (deionized water), stir in an 80°C water bath for 2 hours, and filter with filter paper with a pore size of 10-15 μm to obtain a clear second filtrate.

[0101] (c) The activated carbon added to the second filtrate is stirred at 80°C for 80 min and filtered to obtain the third filtrate; wherein, the amount of activated carbon used is 7 g relative to 100 g of the second filtrate.

[0102] (d) The third filtrate was cooled and crystallized at 4°C for 16 h, filtered to obtain a solid, and dried in an oven at 70°C for 2 h to obtain a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT). The total weight percentage of BHET, BHET dimer, and BHCT in the mixture, the weight percentage of BHCT in the mixture, the L value and b value of the mixture, and the Ti content of the mixture are shown in Table 1.

[0103] Preparation Example 6

[0104] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that step (II) did not include step (c), that is, the second filtrate was directly subjected to the operation of step (d).

[0105] Preparation Example 7

[0106] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that in step (I), the waste PETG copolyester was crushed into particles with an average particle size of 4 cm.

[0107] Preparation Example 8

[0108] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that in step (I), it was dried until the water content in the copolyester was 2% by weight.

[0109] Preparation Example 9

[0110] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that a diluent was added at a weight ratio of 1:10 for the first filtrate to diluent (deionized water).

[0111] Preparation Example 10

[0112] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that the catalyst was replaced with an equal weight amount of zinc acetate catalyst.

[0113] Preparation Example 11

[0114] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that in step (I), the waste PETG copolyester was crushed into particles with an average particle size of 10 cm.

[0115] Preparation Example 12

[0116] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 1, except that in step (I), it was dried until the water content in the copolyester was 5% by weight.

[0117] Comparative Preparation Example 1

[0118] The waste PETG copolyester was depolymerized and purified according to the method of Preparation Example 3, except that the catalyst was replaced with an equal weight amount of lanthanum acetylacetone. The copolyester did not depolymerize.

[0119] Table 1

[0120]

[0121] Example 1

[0122] (1) An antioxidant was added to the esterification reactor, along with a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 1, and terephthalic acid (PTA) and cyclohexanediethanol (CHDM) were added for esterification. The amount of terephthalic acid and cyclohexanediethanol was 0.35 mol, based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediethanol terephthalate in the mixture. The amount of antioxidant (4,4'-butylenebis(6-tert-butyl-3-methylphenol)) added was 100 ppm of the total weight of the monomers used in the esterification reaction. The esterification reaction conditions included: a nitrogen atmosphere, a temperature of 220°C, and a pressure of atmospheric pressure. When the amount of small molecule product generated by the esterification reaction reaches more than 90 wt% of the theoretical output, the esterification reaction is stopped. The time t taken for the esterification reaction is shown in Table 2.

[0123] (2) The esterification reaction product obtained in step (1) is introduced into a polycondensation reactor, and a stabilizer is added to the reactor to perform a first polycondensation and a second polycondensation sequentially to obtain a copolyester. The conditions for the first polycondensation are: a temperature of 255°C, a vacuum degree maintained at 10000 Pa (absolute pressure), and a time of 2 hours. The conditions for the second polycondensation are: a temperature of 270°C, a vacuum degree controlled below 100 Pa (absolute pressure), and a time of 1 hour. The amount of stabilizer (trimethyl phosphate) added is 100 ppm of the total weight of the monomers fed into the esterification reaction.

[0124] The proportion of CHDM:EG, intrinsic viscosity, end carboxyl group content, L value, and b value of the recycled copolyester are shown in Table 2.

[0125] Example 2

[0126] (1) An antioxidant was added to an esterification reactor, along with a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 2, and terephthalic acid (PTA) and cyclohexanediethanol (CHDM) were added for esterification. The amount of terephthalic acid and cyclohexanediethanol was 0.18 mol, based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediethanol terephthalate in the mixture. The amount of antioxidant (pentaerythritol tetratetra(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)) added was 100 ppm of the total weight of the monomers used in the esterification reaction. The esterification reaction conditions included: a nitrogen atmosphere, a temperature of 240°C, and a pressure of atmospheric pressure. When the amount of small molecule product generated by the esterification reaction reaches more than 90 wt% of the theoretical output, the esterification reaction is stopped. The time t taken for the esterification reaction is shown in Table 2.

[0127] (2) The esterification reaction product obtained in step (1) is introduced into a polycondensation reactor, and a stabilizer is added to the reactor to perform a first polycondensation and a second polycondensation sequentially to obtain a copolyester. The conditions for the first polycondensation are: temperature of 250°C, vacuum degree maintained at 6000 Pa (absolute pressure), and time of 1.5 h; the conditions for the second polycondensation are: temperature of 275°C, vacuum degree controlled below 80 Pa (absolute pressure), and time of 0.45 h. The amount of stabilizer (triphenyl phosphate) added is 100 ppm of the total weight of the monomers fed into the esterification reaction.

[0128] The proportion of CHDM:EG, intrinsic viscosity, end carboxyl group content, L value, and b value of the recycled copolyester are shown in Table 2.

[0129] Example 3

[0130] (1) An antioxidant was added to an esterification reactor, along with a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 3, which was then added to terephthalic acid (PTA) and cyclohexanediethanol (CHDM) for esterification. The amount of terephthalic acid and cyclohexanediethanol was 0.04 mol, based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediethanol terephthalate in the mixture. The amount of antioxidant (3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate n-octadecyl alcohol ester) added was 150 ppm of the total weight of the monomers used in the esterification reaction. The esterification reaction conditions included: a nitrogen atmosphere, a temperature of 230°C, and a pressure of atmospheric pressure. When the amount of small molecule product generated by the esterification reaction reaches more than 90 wt% of the theoretical output, the esterification reaction is stopped. The time t taken for the esterification reaction is shown in Table 2.

[0131] (2) The esterification reaction product obtained in step (1) is introduced into a polycondensation reactor, and a stabilizer is added to the reactor to perform a first polycondensation and a second polycondensation sequentially to obtain a copolyester. The conditions for the first polycondensation are: temperature of 260°C, vacuum degree maintained at 6000 Pa (absolute pressure), and time of 1.5 h; the conditions for the second polycondensation are: temperature of 280°C, vacuum degree controlled below 60 Pa (absolute pressure), and time of 0.4 h. The amount of stabilizer (triphenyl phosphate) added is 120 ppm of the total weight of the monomers fed into the esterification reaction.

[0132] The proportion of CHDM:EG, intrinsic viscosity, end carboxyl group content, L value, and b value of the recycled copolyester are shown in Table 2.

[0133] Example 4

[0134] (1) An antioxidant was added to the esterification reactor, and a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 4 was added to carry out an esterification reaction with terephthalic acid (PTA) and cyclohexanediethanol (CHDM). The amount of terephthalic acid and cyclohexanediethanol was 0.58 mol based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate in the mixture. The amount of antioxidant (3,5-di(1,1-dimethylethyl)-4-hydroxyphenylpropionic acid-1,6-hexanediol ester) added was 200 ppm of the total weight of the monomers fed into the esterification reaction. The conditions for the esterification reaction included: a nitrogen atmosphere, a temperature of 230℃, and a pressure of atmospheric pressure. The esterification reaction was stopped when the amount of small molecule product produced reached more than 90 wt% of the theoretical output. The time t taken for the esterification reaction is shown in Table 2.

[0135] (2) The esterification reaction product obtained in step (1) is introduced into a polycondensation reactor, and a stabilizer is added to the reactor to perform a first polycondensation and a second polycondensation sequentially to obtain a copolyester. The conditions for the first polycondensation are: temperature of 235°C, vacuum degree maintained at 5000 Pa (absolute pressure), and time of 1 h; the conditions for the second polycondensation are: temperature of 280°C, vacuum degree controlled below 50 Pa (absolute pressure), and time of 1.2 h. The amount of stabilizer (triethyl phosphite) added is 140 ppm of the total weight of the monomers fed into the esterification reaction.

[0136] The proportion of CHDM:EG, intrinsic viscosity, end carboxyl group content, L value, and b value of the recycled copolyester are shown in Table 2.

[0137] Example 5

[0138] (1) An antioxidant was added to an esterification reactor, along with a mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer, and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 5, and terephthalic acid (PTA) and cyclohexanediethanol (CHDM) were added for esterification. The amount of terephthalic acid and cyclohexanediethanol was 0.90 mol, based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediethanol terephthalate in the mixture. The amount of antioxidant (triethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate) added was 180 ppm of the total weight of the monomers used in the esterification reaction. The esterification reaction conditions included: a nitrogen atmosphere, a temperature of 230°C, and a pressure of atmospheric pressure. When the amount of small molecule product generated by the esterification reaction reaches more than 90 wt% of the theoretical output, the esterification reaction is stopped. The time t taken for the esterification reaction is shown in Table 2.

[0139] (2) The esterification reaction product obtained in step (1) is introduced into a polycondensation reactor, and a stabilizer is added to the reactor to perform a first polycondensation and a second polycondensation sequentially to obtain a copolyester. The conditions for the first polycondensation are: a temperature of 250°C, a vacuum degree maintained at 4000 Pa (absolute pressure), and a time of 1 h. The conditions for the second polycondensation are: a temperature of 270°C, a vacuum degree controlled below 50 Pa (absolute pressure), and a time of 1.2 h. The amount of stabilizer (triethyl phosphate acetate) added is 140 ppm of the total weight of the monomers fed into the esterification reaction.

[0140] The proportion of CHDM:EG, intrinsic viscosity, end carboxyl group content, L value, and b value of the recycled copolyester are shown in Table 2.

[0141] Examples 6-9

[0142] The copolyester was prepared according to the method of Example 1, except that "the mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 1" was replaced with "the mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Examples 6-9".

[0143] Example 10

[0144] The copolyester was prepared according to the method in Example 3, except that...

[0145] In step (1), based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate in the mixture, the amount of terephthalic acid used is 1.33 mol and the amount of cyclohexanediethanol used is 1.33 mol.

[0146] In step (2), the stabilizer is trimethyl phosphite.

[0147] Example 11

[0148] The copolyester was prepared according to the method in Example 3, except that...

[0149] In step (1), based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate in the mixture, the amount of terephthalic acid used is 2.23 mol and the amount of cyclohexanediethanol used is 2.23 mol.

[0150] In step (2), the stabilizer is triethyl phosphite.

[0151] Example 12

[0152] The copolyester was prepared according to the method in Example 3, except that...

[0153] In step (1), based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate in the mixture, the amount of terephthalic acid used is 3.9 mol and the amount of cyclohexanediethanol used is 3.9 mol.

[0154] Example 13

[0155] The copolyester was prepared according to the method in Example 3, except that...

[0156] In step (1), based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate in the mixture, the amount of terephthalic acid used is 8.7 mol and the amount of cyclohexanediethanol used is 8.7 mol.

[0157] Examples 14-16

[0158] The copolyester was prepared according to the method of Example 1, except that "the mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Example 1" was replaced with "the mixture of bis(2-hydroxyethyl) terephthalate (BHET), BHET dimer and cyclohexanediethanol terephthalate (BHCT) obtained in Preparation Examples 10-12".

[0159] Comparative Example 1

[0160] This comparative example is used to illustrate the method of preparing PETG using native monomers.

[0161] Terephthalic acid (PTA), cyclohexanediol (CHDM), and ethylene glycol (EG) were added in a molar ratio of 1.14:0.43:1. 50 ppm (by weight of total monomers) of tetrabutyl titanate and 150 ppm (by weight of total monomers) of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate were added to the esterification reactor. The reaction was carried out at 230°C under atmospheric pressure and an inert gas atmosphere until the amount of the small molecule product discharged reached a certain level. The esterification reaction time t is shown in Table 2 until the theoretical output liquid volume reaches more than 90 wt%. The product is then introduced into a polycondensation reactor, and 120 ppm (of the total weight of monomers) of triphenyl phosphite is added. The temperature is raised to 260°C, and vacuum reaction conditions are gradually established so that the absolute pressure of the vacuum degree is maintained at 6000 Pa for pre-polymerization reaction. After 1 hour of reaction, the product is introduced into a final polycondensation reactor, and the absolute pressure of the vacuum degree is controlled below 60 Pa. The product is then reacted at 280°C for 0.4 hours to obtain PETG copolyester.

[0162] Comparative Example 2

[0163] This comparative example illustrates the method of regenerating PETG using physical recycling.

[0164] After the pulverized waste PETG polyester is dried to a moisture content of 0.5% by weight, it is added to the feeder of a twin-screw extruder. The material enters the twin screw through the feeder. The processing temperature is maintained at 215-260℃. After extrusion and granulation by the twin screw, PETG chips are obtained. The intrinsic viscosity of the chips is 0.42dL / g, the L value is 51, and the b value is 8.4.

[0165] Table 2

[0166]

[0167] As can be seen from the results in Table 2, compared with PETG prepared using the virgin monomers of Comparative Example 1, the PETG copolyester obtained by the physical recycling method of Comparative Example 2 has lower intrinsic viscosity and lower hydroxyl end content.

[0168] It has a high base content and poor color.

[0169] Compared to PETG prepared using virgin monomers in Comparative Example 1, the PETG copolyester prepared using the recycling method of this invention exhibits smaller differences in intrinsic viscosity, terminal hydroxyl content, L value, and b value. Preferably, the recycling method of this invention can prepare recycled PETG copolyesters with higher intrinsic viscosity, lower terminal hydroxyl content, and better hue. Furthermore, the recycling method of this invention not only achieves the recycling of waste PETG copolyesters.

[0170] Furthermore, the regeneration method of the present invention can flexibly adjust the molar ratio of CHDM:EG to meet the needs of different products.

[0171] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing recycled PETG or PCTG copolyester, characterized in that, The method includes the following steps: (1) In the presence of an antioxidant, a mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediol terephthalate is subjected to an esterification reaction with terephthalic acid and cyclohexanediol. (2) In the presence of a stabilizer, the esterification reaction product undergoes a first polycondensation and a second polycondensation in sequence.

2. The method according to claim 1, wherein, Based on the total amount of bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediethanol terephthalate in the mixture, the amount of terephthalic acid used is 0.03-9 mol, and the amount of cyclohexanediethanol used is 0.04-9 mol.

3. The method according to claim 1, wherein, The conditions for the esterification reaction include: a temperature of 200-260℃ and a pressure of atmospheric pressure; And / or, the esterification reaction is carried out for such that the actual amount of the small molecule product produced by the reaction reaches more than 90 wt% of the theoretical amount of product. And / or, the esterification reaction is carried out in an inert atmosphere, preferably, the inert atmosphere comprising nitrogen and / or an inert gas.

4. The method according to claim 1, wherein, The antioxidants include at least one of 4,4'-butylenebis(6-tert-butyl-3-methylphenol), pentaerythritol tetra(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,6-hexanediol 3,5-bis(1,1-dimethylethyl)-4-hydroxyphenylpropionate, and triethylene glycol ether-bis(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate; And / or, the amount of antioxidant added is 10-200 ppm of the total weight of the monomers fed in the esterification reaction.

5. The method according to claim 1, wherein, The stabilizer is a phosphorus-containing compound; preferably, the stabilizer includes at least one selected from triphenyl phosphate, triphenyl phosphite, trimethyl phosphate, trimethyl phosphite, triethyl phosphate, triethyl phosphite, and triethyl phosphate. And / or, the amount of stabilizer added is 20-200 ppm of the total weight of the monomers fed in the esterification reaction.

6. The method according to claim 1, wherein, The conditions for the first polycondensation reaction include: a temperature of 235-280℃, a time of 1-2 hours, and a system pressure of 1000-20000 Pa; And / or, the conditions for the second polycondensation reaction include: a temperature of 255-285°C, a time of 0.4-3 h, and a system pressure of ≤300 Pa, preferably ≤200 Pa, and more preferably ≤60 Pa.

7. The method according to claim 1, wherein, The mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer, and cyclohexanediol terephthalate is obtained by: (I) In the presence of a catalyst and a depolymerizing agent, a copolyester with an average particle size of 0.1-5 cm is subjected to a depolymerization reaction to obtain a depolymerization solution containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate; wherein the copolyester is polyethylene terephthalate-1,4-cyclohexanediethanol terephthalate, the water content in the copolyester is ≤2% by weight, and the catalyst is a titanium-containing catalyst; (II) Purify the product obtained from the depolymerization reaction.

8. The method according to claim 7, wherein, The average particle size of the copolyester is 0.1-3 cm; And / or, the water content in the copolyester is ≤1% by weight.

9. The method according to claim 7, wherein, The catalyst is at least one of organotitanium catalyst, titanium-silicon composite catalyst, and titanium-rare earth composite catalyst. And / or, relative to 100g of copolyester, the amount of catalyst used is 0.1-0.5g, preferably 0.1-0.3g; And / or, the depolymerizing agent is ethylene glycol; And / or, the weight ratio of the depolymerizing agent to the copolyester is 2-6:1, preferably 2.5-5:1; And / or, the conditions for the depolymerization reaction include: a temperature of 190-240°C, preferably 200-230°C; The pressure is 0.2-0.6 MPa, preferably 0.35-0.5 MPa; the time is 1-10 h, preferably 2.5-5 h.

10. The method according to claim 7, wherein, The purification process includes the following steps: (a) The depolymerization solution is subjected to a first solid-liquid separation to obtain a first filtrate; (b) The first filtrate is mixed with a diluent, and then a second solid-liquid separation is performed to obtain the second filtrate; (c) The second filtrate is mixed with activated carbon, and then a third solid-liquid separation is performed to obtain the third filtrate; (d) The third filtrate is crystallized and dried to obtain a mixture containing bis(2-hydroxyethyl) terephthalate, bis(2-hydroxyethyl) terephthalate dimer and cyclohexanediethanol terephthalate.

11. The method according to claim 10, wherein, The first solid-liquid separation was carried out at 80-105℃; And / or, the diluent is water, preferably deionized water; the weight ratio of the depolymerization solution to the diluent is 1:1-10, more preferably 1:2-5; the conditions for mixing the first filtrate with the diluent include: a temperature of 70-90°C and a time of 1-3 hours.

12. The method according to claim 10 or 11, wherein, The amount of activated carbon used is 1-10g relative to 100g of copolyester; The conditions for mixing the second filtrate with activated carbon include: a temperature of 70-90℃ and a time of 0.1-2h; And / or, the crystallization conditions include: a temperature of 2-10°C and a time of 4-12 hours; And / or, the drying conditions include: a temperature of 50-90°C and a time of 1-3 hours.

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