Method for recycling waste plastic

Abstract

To provide a method for simply and efficiently thermally decomposing waste plastic and recycling it as oxidized polyethylene wax, which is useful as a molding aid, lubricant, release agent for plastics and rubber, an ink and coating additive, a pigment dispersant, a hot-melt adhesive, a compatibilizer between polyolefins and polar polymers, and the like.SOLUTION: The invention provides a method for recycling waste plastic as an oxidized wax by feeding waste plastic satisfying the following conditions (1)-(3) into an extruder and thermally decomposing it within the extruder:(1) the number average molecular weight (Mn) measured by the GPC method is 5,000 or more and 100,000 or less; (2) the number of double bonds per molecule is 2.0 or less; and (3) the acid value is 0.1 mgKOH / g or more and 4.0 mgKOH / g or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for recycling waste plastics. [Background technology]

[0002] Due to its usefulness, plastics are used in a wide range of products and containers / packaging. Meanwhile, with calls for achieving carbon neutrality and strengthening efforts to address the problems of marine plastic waste and climate change, promoting the recycling of plastics is becoming increasingly important. Chemical recycling, which uses naphtha synthesized by thermal decomposition of waste plastic (hereinafter sometimes abbreviated as "waste plastic") as a raw material to produce polyethylene (PE) and polypropylene (PP) using naphtha crackers and polymerization plants, can even recycle degraded waste plastic. Furthermore, because plastics are synthesized from monomer and oligomer raw materials, recycled products have the advantage of being of the same quality as virgin products. However, the lengthy and complex recycling process has raised concerns about its lack of economic rationality.

[0003] Low-molecular-weight polymers (molecular weights below 10,000) exhibit different physical and chemical properties from typical polymers with molecular weights in the tens of thousands to hundreds of thousands. Polyolefin waxes, such as low-molecular-weight polyethylene and low-molecular-weight polypropylene, are widely used not only in the production of polyolefins, which account for the majority of plastics, but also in a wide range of applications, including pigment dispersants, molding processing aids, ink and paint additives, and hot-melt adhesive additives. Furthermore, oxidized polyolefin waxes, obtained by oxidizing polyolefin waxes, are highly compatible with polar group-containing components such as hydroxyl groups. Therefore, they are useful not only as compatibilizers in polymer alloys composed of polyolefins and synthetic resins such as polyvinyl alcohol, polyester, polyamide, and polycarbonate, but also as dispersants for compounding cellulose-based powders such as wood flour and paper powder, and glass fiber with polyolefin resins.

[0004] Known methods for synthesizing oxidized polyolefin wax include a method of oxidizing a polymerized polyolefin wax in a molten state (see, for example, Patent Documents 1 to 3), a method of oxidizing a thermally decomposed polyolefin wax (see, for example, Patent Documents 4 and 5), and a method of producing an oxidized polyolefin wax by oxidizing a polyolefin in a solid state and then decomposing it (see, for example, Patent Document 6). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2004-501246 [Patent Document 2] Patent No. 3813554 [Patent Document 3] Patent No. 4799716 [Patent Document 4] Special Publication No. 43-9367 [Patent Document 5] Special Publication No. 47-49313 [Patent Document 6] Patent No. 5410991 Summary of the Invention [Problem to be solved by the invention]

[0006] In the methods proposed in Patent Documents 1 to 5, oxidized wax is synthesized by oxidizing wax, and the oxidation requires pressurized conditions, long reaction times, or the use of harmful oxidizing agents such as ozone. In the method proposed in Patent Document 6, it is necessary to control the particle size of the polyethylene resin in order to uniformly oxidize the polyethylene, which increases the production cost. Furthermore, these methods do not mention environmental considerations such as the use of plant-derived ethylene or waste plastics.

[0007] Therefore, the present invention aims to provide a method for simply and efficiently recycling waste plastics, particularly discarded polyethylene resins, into oxidized wax, and more specifically, to provide a method for recycling into oxidized wax that is useful as a wax for applications such as molding aids for plastics and rubber, lubricants, mold release agents, ink and paint additives, pigment dispersants, and hot melt adhesives, as well as a compatibilizer for polymer alloys and a dispersant for fillers, etc. Increasing the efficiency of reuse, regeneration, conversion into raw materials, conversion into oil, etc. of molded and used resins and molded bodies contributes to the promotion of inclusive and sustainable industrialization and is one of the technologies necessary for a sustainable society, such as the SDGs that have been attracting attention in recent years. [Means for solving the problem]

[0008] As a result of extensive research to solve the above problems, the inventors discovered that waste plastics can be recycled simply and efficiently as high-quality oxidized wax by feeding them into an extruder and thermally decomposing them within the extruder, which led to the completion of the present invention.

[0009] That is, the embodiments of the present invention are [1] to [5] shown below. [1] A method for recycling waste plastics that satisfy the following (1) to (3) by feeding them to an extruder and thermally decomposing them in the extruder to produce oxidized wax. (1) The number average molecular weight (Mn) measured by the GPC method is 5,000 or more and 100,000 or less. (2) The number of double bonds per molecule is 2.0 or less. (3) Acid value is 0.1 mg KOH / g or more and 4.0 mg KOH / g or less. [2] The recycling method according to [1], wherein the waste plastic is waste polyethylene. [3] The recycling method according to [2], wherein the waste polyethylene is used polyethylene generated in extrusion lamination molding. [4] The recycling method according to any one of [1] to [3], wherein the extruder is a twin-screw extruder. [5] A recycling method according to any one of [1] to [4], which includes the steps of crushing the waste plastics using a crusher to an average particle size of 1 mm or more and 30 mm or less, and feeding the plastics into an extruder with an inert gas flowing through the cylinder, and pyrolyzing the plastics at a temperature of 350°C or more and 480°C or less. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a method for simply and efficiently pyrolyzing and recycling waste plastics into oxidized wax that is useful as a molding aid for plastics and rubber, a lubricant, a mold release agent, an ink and paint additive, a pigment dispersant, a hot melt adhesive, a compatibilizer for polymer alloys, a dispersant for fillers, etc., and the industrial value of this method is extremely high. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below.

[0012] In the present invention, waste plastics that have been oxidized during molding processing or due to the environment in which they are used are thermally decomposed, making it possible to recycle them as oxidized wax without the need for oxidation steps during or after pyrolysis.

[0013] The waste plastics include those referred to as used products, non-standard products, waste products, etc., and have a number average molecular weight (Mn) of 5,000 to 100,000 as measured by gel permeation chromatography (GPC). If the number average molecular weight (Mn) is less than 5,000, a large amount of smoke is generated during thermal decomposition. On the other hand, if the number average molecular weight (Mn) is more than 100,000, the fluidity during melting is poor and the stirring efficiency is reduced.

[0014] The number of double bonds per molecule of the waste plastic is preferably 2.0 or less. If it exceeds 2.0, the color number of the oxidized wax deteriorates. Here, the molecular weight per molecule is the number average molecular weight measured by GPC, and the number of double bonds is the number average molecular weight measured by nuclear magnetic resonance spectroscopy ( 1The number of trans vinylenes, trisubstituted olefins, and terminal double bonds determined by H-NMR is the sum of the number of terminal vinyls and vinylidenes.

[0015] Gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy ( 1 More detailed examples of the measurement method using H-NMR will be given in the Examples section below.

[0016] The acid value of the waste polyethylene is 0.1 mgKOH / g or more and 4.0 mgKOH / g or less. If it is less than 0.1 mgKOH / g, the reactivity will be poor when recycled into oxidized wax and mixed with polar group-containing components as a compatibilizer for polymer alloys and a filler dispersant. It also functions poorly as an internal lubricant during molding of engineering plastics. Conversely, if it exceeds 4.0 mgKOH / g, thermal decomposition will proceed rapidly, resulting in problems such as the generation of coke, odor, and coloration.

[0017] Examples of waste plastics include polyolefin resins such as polyethylene (e.g., high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, ethylene-vinyl acetate copolymer (EVA), saponified EVA, and polypropylene; polyester resins such as polyethylene terephthalate and butylene terephthalate; chlorinated resins such as polyvinyl chloride; polyamide resins such as nylon 6 and nylon 66; styrene resins such as polystyrene; and polycarbonate resins, as well as mixtures of these waste plastics. Among these waste plastics, polyethylene resins primarily composed of polyethylene resin are preferred because they can be used in a wide range of applications, including as pigment dispersants, molding aids, ink or paint additives, and hot-melt adhesive additives, and because their molecular weights are easily controlled. The polyethylene resin content is preferably 70% by weight or more, and more preferably 80% by weight or more, because it produces a stable, high-quality pyrolysis wax.

[0018] Furthermore, the waste plastics are not limited to pellets or powder, but may also be in the form of films, sheets, bottles, fibers, pipes, injection-molded products, and other crushed products. However, since air oxidation during molding is significant, used polyethylene generated during the extrusion lamination molding process is particularly preferred. These waste plastics are preferably crushed to an average particle size of 1 mm to 30 mm using a crusher. A particle size of 1 mm or more results in minimal dust scattering and is easy to handle. A particle size of 30 mm or less eliminates the risk of bridging when the plastics are fed into the extruder.

[0019] The extruder used in the present invention is not particularly limited, but is preferably a twin-screw extruder because of its high self-cleaning properties.

[0020] In addition, the ratio (L / D) of the screw length (L) to the screw diameter (D) of the extruder is preferably 30 or more, and particularly 40 or more, because this allows the waste plastic to be fully pyrolyzed and then oxidation controlled by cooling to obtain oxidized wax of stable quality.

[0021] In the recycling method of the present invention, the conditions for feeding the waste plastics into the extruder and pyrolyzing them therein should be appropriately adjusted depending on the type of raw waste plastics and extruder, the molecular weight of the desired pyrolysis wax, and the like. However, to facilitate rapid pyrolysis and reduced odor of the resulting pyrolysis wax, the cylinder temperature in the pyrolysis zone can be, for example, in the range of 350°C to 480°C, more preferably 365°C to 470°C, and particularly preferably 380°C to 460°C. Furthermore, the pyrolysis time in the pyrolysis zone, i.e., the residence time of the polyethylene resin in the extruder, can be in the range of 0.5 to 30 minutes, more preferably 1 to 20 minutes, and particularly preferably 1.5 to 15 minutes. Furthermore, the tip temperature of the extruder when extruding the pyrolyzed wax can be in the range of 120°C to 300°C, more preferably 130°C to 280°C, and particularly preferably 140°C to 250°C, to facilitate more precise control of the molecular weight. In addition, in order to suppress the odor of the resulting oxidized wax and to facilitate control of the molecular weight, it is preferable that the atmosphere inside the extruder during pyrolysis be purged with an inert gas such as hydrogen, helium, argon, nitrogen, or carbon dioxide, and nitrogen gas is particularly preferable. The oxidized wax extruded from the extruder after pyrolysis can be molded into a desired shape by a method such as hot cutting, mist cutting, or underwater cutting. [Example]

[0022] EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these. (1) GPC measurement Apparatus: HLC (registered trademark)-8321GPC / HT (detector: RI method) (manufactured by Tosoh Corporation) Columns: One column of (i) and three columns of (ii) below are used in series. (i) TSKgel® guardColumuH(HR)(30)HT (7.5mm I.D.) x 7.5cm) (Tosoh Corporation) x 1 (ii) TSKgel® GMH(HR)-H(20)HT (7.5mm I.D.) x 30cm) (Tosoh Corporation) x 3 Eluent: 1,2,4-trichlorobenzene (containing 0.05 wt% BHT) (Purchased from Fujifilm Wako Pure Chemical Industries, Ltd.) Flow rate: 1.0mL / min Injection volume: 0.3mL Column temperature: 140℃ System temperature: 40°C Sample concentration: 1 mg / mL Calibration curve: quintic approximation curve using standard polystyrene manufactured by Tosoh Corporation. However, the molecular weight was converted to PE equivalent molecular weight using the Q factor. (2) Acid value measurement Measurement was carried out in accordance with JIS K2501 under the following conditions.

[0023] Equipment: 50 mL burette (minimum graduation 0.1 mL), Erlenmeyer flask Reagents: toluene, methanol, dimethylformamide, pure water, phenolphthalein indicator, N / 20KOH-n-propanol / toluene solution (F=0.9758) Melting conditions: 165℃×30min Reaction conditions: room temperature x 30 min Sample amount: approx. 1g (3) 1 H-NMR measurement Equipment: ECZ 400 (manufactured by JEOL Ltd.) Solvent: orthodichlorobenzene-d 4 Temperature: 120℃ Number of times accumulated: 128 ~Calculating the number of double bonds~ The number of double bonds per molecule is calculated by the GPC number average molecular weight (Mn) and 1 The signal intensity of the H-NMR spectrum was used to calculate the value from the following equation.

[0024] Number of double bonds (pieces / Mn)=(Iva+Ivb+Ivc+Ivd)×(Mn / 14000) Here, Iva, Ivb, Ivc, and Ivd are the integrated intensities of transvinylene (intramolecular double bond), trisubstituted olefin (intramolecular double bond), terminal vinyl (terminal double bond), and vinylidene (terminal double bond), respectively, and are quantities expressed by the following formula.

[0025] Iva=(I5.5~5.3) / 2 Ivb=(I5.3~5.1) Ivc=(I5.1~4.85) / 2 Ivd=(I4.85~4.6) / 2 I denotes the integrated intensity, and the subscripts of I indicate the range of chemical shifts.

[0026] For example, I5.5-5.3 indicates the integrated intensity of the proton signal detected between 5.5 ppm and 5.3 ppm.

[0027] The integrated intensity was determined by setting the integrated intensity of the signal derived from the main chain methylene protons detected between 1.8 ppm and 1.0 ppm to 2000. The chemical shift was set to the proton signal of 1,2-dichlorobenzene at 6.95 ppm, and the chemical shifts of the signals derived from other protons were based on this.

[0028] [Example 1] The extruder used was a co-rotating intermeshing twin-screw extruder with a screw diameter of 11 mm and a screw length L to screw diameter D ratio (L / D) of 40. The extruder had a total of 8 temperature zones, with each zone corresponding to L / D = 5, and zones 1 to 8 were defined from the root of the screw to the die exit, with zones 1 to 3 being the plasticization zone, zones 4 to 7 being the thermal decomposition zone, and zone 8 being the cooling zone. The waste plastic was used polyethylene (number average molecular weight 15,000, 0.8 double bonds per molecule, acid value 0.35 mgKOH / g) generated during resin switching in extrusion lamination molding of low-density polyethylene (Petrothene® 205, manufactured by Tosoh Corporation). The waste was crushed to an average particle size of 10 mm or less and reduced in volume using a crusher. The resulting mixture was then fed into the main feeder of the extruder at a feed rate of 200 g / hr together with a constant flow of nitrogen gas. The melt-kneaded mixture was then pyrolyzed by heating the cylinder temperature in the pyrolysis zone of the extruder to 450°C and the cooling zone to 200°C. The melt-kneaded mixture was then extruded onto a steel plate placed under a nitrogen atmosphere, cooled, and crushed to obtain a powdered oxidized polyethylene wax. GPC analysis of the resulting oxidized polyethylene wax revealed a number average molecular weight of 2,900 and an acid value of 0.63 mgKOH / g.

[0029] [Comparative Example 1] A polyethylene wax was obtained in the same manner as in Example 1, except that a low-density polyethylene (Petrothene (registered trademark) 203 manufactured by Tosoh Corporation (number average molecular weight 16,000, number of double bonds per molecule 0.4, acid value 0 mgKOH / g) was used as the polyethylene. GPC measurement was then carried out using the obtained polyethylene wax, and the number average molecular weight was found to be 2,800 and the acid value was found to be 0.00 mgKOH / g. [Industrial Applicability]

[0030] The present invention provides a method for simply and efficiently pyrolyzing waste plastics and recycling them into oxidized polyethylene wax, which is useful as a molding aid for plastics and rubber, a lubricant, a mold release agent, an ink and paint additive, a pigment dispersant, a hot melt adhesive, a compatibilizer for polyolefins and polar polymers, etc.

Claims

1. A method for recycling waste plastics that satisfy the following (1) to (3) by feeding them to an extruder and thermally decomposing them in the extruder to produce oxidized wax. (1) The number average molecular weight (Mn) measured by GPC is 5,000 or more and 100,000 or less. (2) The number of double bonds per molecule is 2.0 or less. (3) The acid value is 0.1 mg KOH / g or more and 4.0 mg KOH / g or less.

2. 2. The recycling method according to claim 1, wherein the waste plastic is waste polyethylene.

3. 3. The recycling method according to claim 2, wherein the waste polyethylene is post-consumer polyethylene generated in extrusion lamination molding.

4. 2. The recycling method according to claim 1, wherein the extruder is a twin-screw extruder.

5. 2. The recycling method according to claim 1, comprising the steps of: pulverizing the waste plastics using a pulverizer to an average particle size of 1 mm or more and 30 mm or less; and feeding the waste plastics into an extruder having an inert gas flowing through the cylinder thereof, and pyrolyzing the waste plastics at a temperature of 350°C or more and 480°C or less.