Preparation method of low-color-value recycled polyester

Abstract

The invention relates to a preparation method of low-color-value recycled polyester, and belongs to the technical field of new materials. The invention aims to solve the problems of high color value and poor quality of products caused by thermal degradation and oxidative degradation in the processing process of regenerated polyester in the prior art. The method comprises the following steps: melting the polyester waste in a single-screw extruder, and sequentially carrying out two-stage vacuum devolatilization on the material so as to respectively remove moisture, low-molecular organic matters and subsequently generated oligomers; then, the polyester melt and a compound additive are subjected to melt blending in a double-screw extruder, and the compound additive comprises a phosphite antioxidant, a hindered phenol heat stabilizer and a bluish violet toner; and finally, carrying out rapid cooling and pelletizing by adopting an underwater pelletizing system. Through the synergistic effect of a unique process flow and a specific auxiliary agent system, the degradation reaction and the formation of chromophore are effectively inhibited, and the finally obtained regenerated polyester product has the advantages of low color value, good quality and stable and controllable intrinsic viscosity.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of new materials, and specifically relates to the recycling and high-quality preparation of regenerated polyester, aiming to solve the problems of high color value and unstable quality of regenerated polyester in the process of heat processing, and to realize the efficient recycling and high-value utilization of polyester waste such as polyester waste silk and polyester tape. BACKGROUND

[0002] With the promotion of global plastic circular economy, the demand for recycling and utilization of regenerated polyester is increasing due to its ability to effectively reduce resource consumption and environmental pollution. The preparation of regenerated polyester usually requires heat processing of polyester waste (such as polyester waste silk, polyester tape, polyester melt block, etc.) such as melt granulation and extrusion. However, in this process, the waste inevitably faces high temperature and strong shear action, and it is difficult to completely isolate oxygen during the melting stage, which leads to the easy breaking of polyester macromolecular chains in various forms: on the one hand, hydrolysis, anaerobic thermal degradation, thermal oxidative degradation and mechanical degradation coexist, damaging the structure of polyester molecular chains; on the other hand, if there are trace amounts of non-polyester impurities in the waste, it will further induce the generation of macromolecular radicals, hydrogen peroxide groups and carbonyl compounds, changing the types and concentrations of polyester macromolecular chain end groups, and even causing carbonization and coking in severe cases.

[0003] The above problems directly lead to a significant increase in the color value index of regenerated polyester (especially the b value representing yellowness), and the product often presents a yellowish, grayish or grayish appearance, which is quite different from the color of virgin polyester, making it difficult to meet the strict requirements of high-end fields (such as food packaging and high-quality textile fibers) on color value.

[0004] To improve this situation, existing technologies use two methods: one is to downgrade regenerated polyester and only apply it to fields with very low color value requirements (such as low-end packing tape and filler materials), which greatly limits its application range and economic value; the other is to improve the color value by adding a whitening agent and mixing it with a small amount of virgin polyester melt, but this method not only has limited whitening effect and poor weather resistance, but also is prone to color value rebound over time, and the "small amount of doping" mode cannot realize large-scale high-value recycling of regenerated polyester, failing to fundamentally solve the core problems of degradation and color generation in the process of regenerated polyester processing. SUMMARY

[0005] The application aims to solve the problems of low devolatilization efficiency, insufficient antioxidant, poor heat resistance of toning agent, and continuous oxidation in the cutting stage of traditional regenerated polyester process, which leads to high color value, low whiteness, and many impurities, and cannot meet the requirements of high-end applications. The application provides a preparation method of low-color-value regenerated polyester, which realizes the high-value recycling of waste polyester to high-end textile-grade chips through the system-level synergy of "head and tail double vacuum devolatilization-phenol / phosphorus synergistic antioxidant-in-situ pigment purple 23 toning-oxygen isolation cutting by underwater grinding head", and one-step preparation of low-color-value regenerated polyester with b value ≤5.2, L value ≥70, and acetaldehyde ≤100 ppm.

[0006] To achieve the above application purposes, the application adopts the following technical solutions: a preparation method of low-color-value regenerated polyester comprises the following steps: (a) melting the polyester waste in a single screw extruder, and sequentially performing first stage vacuum devolatilization and second stage vacuum devolatilization on the material in the single screw extruder; wherein the first stage vacuum devolatilization is performed in the plastic melting section of the material to remove water and low molecular organic matter; the second stage vacuum devolatilization is performed in the mixing and homogenizing section of the material to remove oligomers; (b) melt blending the polyester melt treated in step (a) with a composite additive comprising the following components in a twin screw extruder: at least one phosphite antioxidant; at least one hindered phenolic heat stabilizer; a blue-purple toning agent; (c) cutting the polyester melt blended in step (b) using an underwater cutting system to obtain low-color-value regenerated polyester.

[0007] Further, before step (a), a step of pretreating the polyester waste is further included, and the pretreatment includes shredding, hot nitrogen drying, and compaction.

[0008] Further, the first stage vacuum devolatilization is performed within the first 1 / 3 of the length of the single screw extruder screw, and the vacuum pressure is not less than 1000 Pa; the second stage vacuum devolatilization is performed within the last 1 / 3 of the length of the single screw extruder screw, and the vacuum pressure is not higher than 1000 Pa.

[0009] Further, the length-diameter ratio of the single screw extruder screw is not less than 30, and the material temperature is controlled between 250-290℃; the length-diameter ratio of the twin screw extruder screw is not less than 40, and the material temperature is controlled between 270-290℃.

[0010] Further, in the composite additive, the total addition amount of the phosphite antioxidant and the hindered phenolic heat stabilizer is not more than 2 wt% based on the weight of the regenerated polyester, and the addition amount of the blue-purple toning agent is not more than 0.5 wt%.

[0011] Further, the phosphite antioxidant is tris (decylphenyl) phosphite.

[0012] Further, the hindered phenolic heat stabilizer is tetraethylene glycol ether-di (3-tert-butyl-4-hydroxy-5-methylphenyl) propionate.

[0013] Further, the blue-purple toner is pigment violet 23.

[0014] Further, the color value b of the obtained regenerated polyester is not higher than 5.2, and the L value is not lower than 70.

[0015] Further, the end carboxyl content of the obtained regenerated polyester is not higher than 50 mmol / kg, the diethylene glycol content is not higher than 3.2 wt%, the acetaldehyde content is not higher than 100 ppm, and the iron content is not higher than 10 ppm.

[0016] Compared with the prior art, the present application has the following remarkable beneficial effects through the unique process design and chemical formula synergy: 1. The product quality is significantly improved, and the color value is low: The problem of high color value, yellowing or ashing of traditional regenerated polyester due to degradation is solved. The color value b of the final product is not higher than 5.2, the L value is not lower than 70, the color quality is good, the acetaldehyde content is not higher than 100 ppm, the end carboxyl content is not higher than 50 mmol / kg, and the impurity content is low. The characteristic viscosity is effectively controlled, and the mechanical properties of the product are stable.

[0017] 2. Effectively inhibit thermal degradation and oxidative degradation: Two sets of vacuum devolatilization systems are adopted, which can effectively remove water, organic impurities and oligomers before and after melting, respectively, reducing the occurrence of hydrolysis, thermal degradation and oxidative degradation. The underwater grinding head cutting system makes the polyester melt rapidly cool while being isolated from oxygen, further inhibiting the degradation reaction at high temperature.

[0018] 3. Synergistic effect of chemical additives: The addition of various antioxidants and heat stabilizers can effectively capture free radicals and inhibit the formation of color bases, reducing the color value from the chemical level. The blue-purple toning master batch used in the present application directly improves the visual whiteness of the product by using the principle of color complementation to neutralize yellow.

[0019] 4. Wide applicability of raw materials: The preparation method of the present application has wide applicability and can be used to process polyester waste in various forms such as polyester melt block, waste silk, white cloth and polyester tape, realizing efficient recycling. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is the structural formula of tris (decylphenyl) phosphite of the present application; Figure 2 is the structural formula of the tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate of the present application. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art belong to the scope of protection of the present application.

[0022] The present application provides a preparation method of low-color-value regenerated polyester, which significantly improves the color value and comprehensive quality of the regenerated polyester by combining a unique physical processing flow with a chemical additive system (antioxidant, heat stabilizer and blue-purple toning agent). The antioxidant is at least one of phosphite, diphosphite, benzoic anhydride or sulfite, such as tris(decylphenyl) phosphite, tris(2,4-di-tert-butylphenyl) phosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, preferably tris(decylphenyl) phosphite; the heat stabilizer is at least one of hindered phenol compounds, such as tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate, 2,6-di-tert-butyl-p-cresol, hexanediol bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], preferably tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl) propionate; and the blue-purple toning agent is at least one of dioxazine compounds, such as pigment violet 23, pigment violet 37, preferably pigment violet 23.

[0023] The core technical solutions of the present application will be described in detail below in conjunction with specific embodiments. It is worth noting that all the devices used in the following embodiments (such as a shredder, a hot air drying and water removing device, a compression device, a single-screw extruder with vacuum suction, a filter, a double-screw extruder for particles or powder, a mixer, and a pelletizer with an underwater grinding head pelletizing system, including a connecting device, a die head, a pelletizer, a bypass pipeline (pneumatic valve), a centrifugal dehydration system, a circulating water tank and a heating system. In the underwater grinding head pelletizing system, the high-temperature and high-viscosity polyester melt strand extruded from the die hole is immediately surrounded and rapidly cooled by process cooling water. The temperature drop and the isolation of oxygen can inhibit the occurrence of thermal degradation and thermal oxidative degradation of the polyester, and at the same time, through water bath extraction, other water-soluble small molecules such as acetaldehyde will be washed out) are all prior art (the device models are all conventional commercially available models, which can be adjusted according to the production capacity), and the present application only improves the preparation method.

[0024] Example 1 The present embodiment provides a method for preparing low-color-value regenerated polyester by using undyed polyester tape waste.

[0025] 1. Pretreatment: The recycled undyed polyester tape waste (intrinsic viscosity 0.75 dL / g, belonging to the range of 0.65-0.80 dL / g) was sent into a shredder for shredding treatment to obtain shredded materials with a length of 50 mm, a width of 20 mm, and a thickness of 5 mm. Subsequently, the polyester shredded materials were sent into a hot air drying and water removing device, advanced in vibration, and dried by passing hot nitrogen gas with a temperature of 95 ℃ and a flow rate of 110 L / min, while removing dust. The dried shredded materials were sent into a compression device and compacted into blocks with a density of 0.35 g / cm 3 for the subsequent stable feeding of the extruder.

[0026] 2. Double vacuum devolatilization melting: The compacted polyester blocks were sent into a single screw extruder with a screw length-diameter ratio of 32. The extruder was provided with 6 heating and cooling sections, and the material temperature was set between 250-280 ℃. A first vacuum devolatilization port was arranged at about 1 / 4 of the screw length (i.e., the plasticizing and melting section), connected with a vacuum device, and the vacuum pressure was controlled at 1200 Pa, mainly for removing the moisture and volatile small organic molecules remaining in the raw materials. A second vacuum devolatilization port was arranged at about 3 / 4 of the screw length (i.e., the mixing and homogenizing section), and the vacuum pressure was controlled at 800 Pa, for removing the volatile components such as oligomers generated by degradation or reaction in the melt.

[0027] 3. Melting and blending: After the treatment of the single screw extruder, the regenerated polyester melt was filtered by a filter to remove possible carbides or inorganic impurities, and then sent into a coaxial counter-rotating fully meshing double screw extruder with a screw length-diameter ratio of 45. The material temperature was set between 270-290 ℃. The composite additives were added to the polyester melt through the hopper arranged on the double screw extruder, and fully melted and blended.

[0028] The components and contents of the composite additives (based on the total weight of the polyester) are as follows: 1.0 wt% of phosphite antioxidant: tris (decylphenyl) phosphite, the structural formula of which is shown in Figure 1 .

[0029] 0.8 wt% of hindered phenolic thermal stabilizer: tetraethylene glycol ether-di (3-tert-butyl-4-hydroxy-5-methylphenyl) propionate, the structural formula of which is shown in Figure 2 .

[0030] 0.3 wt% of blue-violet toner: pigment violet 23.

[0031] 4. Mixing and pelletizing: The polyester melt, after being blended in a twin-screw extruder, is further homogenized in a dynamic mixer to ensure a more uniform distribution of additives. The homogenized melt is then fed into an underwater pelletizing system. The high-temperature melt bundles extruded from the die head are immediately surrounded by circulating process water and cut into pellets by a rotating cutter. This process achieves rapid cooling of the material, isolation from oxygen, and extraction and elution of water-soluble small molecules such as acetaldehyde.

[0032] Product performance: The final recycled polyester granules were tested (using existing technology, which will not be described in detail here), and their performance indicators are as follows: color value b is 4.8, L is 72; terminal carboxyl group content is 45 mmol / kg; diethylene glycol content is 2.8 wt%; acetaldehyde content is 85 ppm; iron content is 8 ppm; intrinsic viscosity (0.56 dL / g) decreased by 0.19 dL / g compared with its raw material (0.75 dL / g).

[0033] Example 2 This embodiment provides a method for preparing low-color-value recycled polyester using waste polyester filaments, wherein the intrinsic viscosity of the raw material is 0.90 dL / g.

[0034] The main difference from Example 1 lies in the adjustment of process parameters and additive ratios.

[0035] Pretreatment: During the drying process, the hot nitrogen temperature is 100℃ and the flow rate is 120L / min. The density after compaction is 0.4g / cm³. 3 .

[0036] Dual vacuum devolatilization melting: The single-screw extruder has a screw length-to-diameter ratio of 35, and the material temperature is set at 260~290℃. The pressure at the first vacuum port is 1500Pa, and the pressure at the second vacuum port is 700Pa.

[0037] Melt blending: The twin-screw extruder has a screw length-to-diameter ratio of 50. The components and contents of the composite additives are: 1.2 wt% of tri(decylphenyl) phosphite, 0.8 wt% of tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate, and 0.4 wt% of pigment violet 23.

[0038] Product performance: The obtained recycled polyester granules have a color value b of 5.0 and an L value of 71; the end carboxyl group content is 48 mmol / kg; the diethylene glycol content is 3.0 wt%; the acetaldehyde content is 90 ppm; the iron content is 9 ppm; and the intrinsic viscosity (0.66 dL / g) is 0.24 dL / g lower than that of its raw material (0.90 dL / g).

[0039] Example 3 This embodiment provides a method for preparing low-color-value recycled polyester using polyester melt bulk material, with an intrinsic viscosity of 1.08 dL / g.

[0040] The main difference from Example 1 lies in the adjustment of process parameters and additive ratios.

[0041] Pretreatment: After the material is shredded, the hot nitrogen gas in the drying process is at a temperature of 90℃ and a flow rate of 100L / min. The density after compaction is 0.3g / cm³. 3 .

[0042] Dual vacuum devolatilization melting: The single-screw extruder has a screw length-to-diameter ratio of 30, and the material temperature is set at 250~290℃. The pressure at the first vacuum port is 1000Pa, and the pressure at the second vacuum port is 1000Pa.

[0043] Melt blending: The twin-screw extruder has a screw length-to-diameter ratio of 40. The components and contents of the composite additives are: 0.5 wt% of tri(decylphenyl) phosphite, 0.5 wt% of tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate, and 0.1 wt% of pigment violet 23.

[0044] Product performance: The obtained recycled polyester granules have a color value b of 5.2 and an L value of 70; the end carboxyl group content is 50 mmol / kg; the diethylene glycol content is 3.2 wt%; the acetaldehyde content is 100 ppm; the iron content is 10 ppm; the intrinsic viscosity (0.73 dL / g) is 0.35 dL / g lower than that of its raw material (1.08 dL / g), which shows that the decrease varies with the viscosity of the raw material, and all meet the preset requirements.

[0045] In summary, the color value (b) of low-color-value recycled polyester should not exceed 5.2, and the L value should not be lower than 70; in addition, the end carboxyl group content of low-color-value recycled polyester should not exceed 50 mmol / kg, the diethylene glycol content should not exceed 3.2 wt%, the acetaldehyde content should not exceed 100 ppm, and the iron content should not exceed 10 ppm. When the intrinsic viscosity of polyester waste is not less than 1.05 dL / g, the intrinsic viscosity reduction of low-color-value recycled polyester compared to it shall not exceed 0.35 dL / g; when the intrinsic viscosity of polyester waste is not less than 0.95 dL / g and less than 1.05 dL / g, the intrinsic viscosity reduction of low-color-value recycled polyester compared to it shall not exceed 0.3 dL / g; when the intrinsic viscosity of polyester waste is 0.80~0.95 dL / g, the intrinsic viscosity reduction of low-color-value recycled polyester compared to it shall not exceed 0.25 dL / g; when the intrinsic viscosity of polyester waste is 0.65~0.80 dL / g, the intrinsic viscosity reduction of low-color-value recycled polyester compared to it shall not exceed 0.20 dL / g.

[0046] In summary, this invention, through systematic optimization of the recycled polyester preparation process, particularly the combination of innovative dual-vacuum devolatilization technology, synergistic composite additive system, and advanced underwater pelletizing process, successfully solves the technical problems of high color value and unstable quality of recycled polyester in the prior art, providing a practical and effective technical approach for the high-value recycling of waste polyester resources.

[0047] The parts of this invention not described in detail are prior art, therefore they are not described in detail here.

[0048] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0049] Although this document uses a considerable amount of technical terminology, the possibility of using other terms is not excluded. These terms are used merely for the convenience of describing and explaining the essence of the invention; interpreting them as any additional limitation would contradict the spirit of the invention.

[0050] This invention is not limited to the preferred embodiments described above. Anyone can derive other products in various forms under the guidance of this invention. However, regardless of any changes made to their shape or structure, any technical solution that is the same as or similar to this invention falls within the protection scope of this invention.

Claims

1. A method for preparing low-color-value recycled polyester, characterized in that, Includes the following steps: (a) Polyester waste is melted in a single-screw extruder, and a first-stage vacuum devouring and a second-stage vacuum devouring are performed sequentially on the material in the single-screw extruder; wherein the first-stage vacuum devouring is carried out in the plasticizing and melting zone of the material to remove moisture and low-molecular-weight organic matter; and the second-stage vacuum devouring is carried out in the mixing and homogenizing zone of the material to remove oligomers. (b) The polyester melt treated in step (a) is melt-blended in a twin-screw extruder with a composite additive containing the following components: At least one phosphite antioxidant; At least one hindered phenolic heat stabilizer; A blue-purple tint; (c) The polyester melt after blending in step (b) is pelletized using an underwater pelletizing system to obtain the low color value recycled polyester.

2. The preparation method according to claim 1, characterized in that, Prior to step (a), the process includes a pretreatment step of the polyester waste, which includes shredding, hot nitrogen drying, and compaction.

3. The preparation method according to claim 1, characterized in that, The first stage of vacuum devolatilization is carried out within the first 1 / 3 of the screw length of the single-screw extruder, with a vacuum pressure of not less than 1000 Pa; the second stage of vacuum devolatilization is carried out within the last 1 / 3 of the screw length of the single-screw extruder, with a vacuum pressure of not more than 1000 Pa.

4. The preparation method according to claim 1, characterized in that, The single-screw extruder has a screw length-to-diameter ratio of not less than 30 and a material temperature controlled between 250 and 290°C; the twin-screw extruder has a screw length-to-diameter ratio of not less than 40 and a material temperature controlled between 270 and 290°C.

5. The preparation method according to claim 1, characterized in that, In the composite additives, the total amount of the phosphite antioxidant and the hindered phenolic heat stabilizer added, based on the weight of the recycled polyester, does not exceed 2 wt%, and the amount of the blue-purple colorant added does not exceed 0.5 wt%.

6. The preparation method according to claim 1, characterized in that, The phosphite antioxidant is tri(decylphenyl) phosphite.

7. The preparation method according to claim 1, characterized in that, The hindered phenolic heat stabilizer is tetraethylene glycol ether-di(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate.

8. The preparation method according to claim 1, characterized in that, The blue-purple tinting agent is Pigment Violet 23.

9. The preparation method according to any one of claims 1-8, characterized in that, The resulting recycled polyester has a color value (b) not higher than 5.2 and an L value not lower than 70.

10. The preparation method according to claim 9, characterized in that, The resulting recycled polyester has a terminal carboxyl group content of no more than 50 mmol / kg, a diethylene glycol content of no more than 3.2 wt%, an acetaldehyde content of no more than 100 ppm, and an iron content of no more than 10 ppm.

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