Process for preparing recycled polyolefin resins

Abstract

The present invention provides a method for preparing a recycled polyolefin resin, the method comprising: adding a base resin containing a polyolefin and wash water to a scrubber equipped with a rotating member, and washing the base resin in a wet friction manner by high-speed rotation of the rotating member; and supplying the washed base resin to an extruder and extruding it, wherein the washing water includes water, a nonionic surfactant, and chitosan. According to the present invention, in the washing step of the base resin, separation of foreign matter can be promoted by using a nonionic surfactant, and reattachment of the separated foreign matter to the base resin can be prevented by using chitosan in combination, thereby improving the effect of removing foreign matter. Thus, it is possible to prepare a recycled polyolefin resin in which the occurrence of odor and the deterioration of physical properties are suppressed.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Korean Patent Application No. 10-2023-0051538 filed on April 19, 2023 and Korean Patent Application No. 10-2024-0047361 filed on April 8, 2024, the entire contents of which are incorporated herein as part of the specification.

[0004] The present invention relates to a method for preparing a recycled polyolefin (PO) resin, and more particularly, to a method for preparing a recycled PO resin having an improved effect of removing foreign matter and odor. Background Art

[0005] Recently, the use of plastics is increasing as plastics with physical properties required for various uses and purposes have been developed. Generally speaking, plastics use a lot of energy from crude oil collection to manufacturing, and a large amount of carbon is discharged in the process. In addition, when the final product is discarded and the plastic contained in the product is also discarded, the social cost of plastic pollution to the environment and its treatment is consumed. Therefore, it is necessary to recycle discarded plastics to reduce energy consumption, reduce carbon emissions and prevent environmental pollution. Therefore, plastics manufactured by recycling plastics discarded by final consumers after use are called post-consumer recycled (PCR) resins, and attempts and efforts to obtain PCR resins with desired purity and physical properties are continuously being carried out.

[0006] Meanwhile, polyolefin resin is a thermoplastic polymer prepared by polymerization of olefins such as ethylene and propylene (hydrocarbons containing a double bond per molecule), which shows different properties according to density, and is used as a raw material for various products based on its density. For example, when density is high, plasticity, firmness, uniformity, etc. are shown, when density is low, elasticity, transparency, flexibility, etc. like liquid or rubber are shown, therefore, high-density polyethylene (HDPE) is mainly used for products requiring high strength, low-density polyethylene (LDPE) is mainly used for products requiring high transparency, and linear low-density polyethylene (LLDPE) has excellent moldability and is mainly used for film production. Since polyolefin resin is widely used commercially because of its low cost, it accounts for most of the waste plastics currently occurring.

[0007] Generally, the methods for recycling waste plastics (resins) include mechanical recycling, chemical recycling and thermal recycling. Mechanical recycling is a method of preparing resin products by grinding and sorting collected waste plastics and separating them by type, and mixing with new materials at a certain ratio or strengthening with functional additives when melting and granulating using an extruder. Chemical recycling is a method of extracting only specific polymers from waste plastics using various chemical means or recycling them as pure monomers and repolymerizing them. Thermal recycling is a method of burning waste plastics and recovering them as heat energy.

[0008] When foreign matter is included in the process of treating waste polyolefin resin by mechanical recycling, the foreign matter is oxidized during the melting process using an extruder to produce an odor, and the residual foreign matter may deteriorate the physical properties. For example, since agricultural mulch films have a high content of various foreign matter, such as dirt, straw, and paper, and involve a fermentation process through the natural growth of microorganisms, an unknown odor may be generated during the granulation process. In addition, in the case of a stretch film made of LLDPE material commonly used for packaging, paper foreign matter such as express labels and other types of film foreign matter (e.g., PVC, PET, etc.) may generally flow in. In particular, it is difficult to remove the cellulose component that forms the paper foreign matter, and when the residual cellulose flows into the extruder, it is oxidized due to the high temperature, thereby causing an odor, and also discolored due to carbonization.

[0009] Generally, when post-consumer recycled (PCR) resin is produced by recycling waste plastics, washing is performed in a pretreatment process for preparing raw material resin (i.e., base resin) before being supplied to an extruder, but the effect is not significant and foreign matter and odor may become worse in the final product.

[0010] Therefore, in order to produce high value-added recycled resin, additional wet washing of the raw resin is required, and at this time, additives that cause chemical reactions to remove various contaminants can be used to improve the washing effect. The additives can be selected in consideration of washing performance, activity against impurities, rinsability, price, etc., and as a representative washing additive, a surfactant is used.

[0011] Surfactants relax the boundaries of the interface by having a chemical structure with a hydrophobic group and a hydrophilic group, and effectively separate pollutants on the surface by allowing the hydrophobic group to penetrate into the pollutants. According to the charge of the hydrophilic part when dissociated in water, surfactants can be divided into anionic, cationic, amphoteric and nonionic surfactants. Among them, nonionic surfactants do not produce ionic repulsion, which allows easy oil / water separation, and are therefore suitable for removing organic pollutants remaining in resins used as agricultural mulch films.

[0012] In addition, surfactants can have properties that change with deformation of the chemical structure, and among nonionic surfactants, the EO / PO type formed by a block copolymer of ethylene oxide (EO) and propylene oxide (PO) can adjust the washing effect, rinsability and foaming rate according to the temperature according to the ratio of EO and PO as repeating units.

[0013] When the base resin is added to water to which the above-mentioned nonionic surfactant is added and washed by high-speed rotation of the rotating member, foreign matter separated from the resin floats in the water. Most of the separated foreign matter is a cellulose component, and relatively large floating foreign matter may settle naturally, but small foreign matter may remain suspended in the water and then attach to the resin. Therefore, bad odor and deterioration of physical properties may be caused in the extrusion step.

[0014] Therefore, there is a need for a technology for improving the washing effect by causing small floating foreign matter separated in the wet washing step of a base resin to settle to prevent it from being reattached to the resin. Summary of the invention

[0015] [Technical issues]

[0016] The present invention is to solve the problems mentioned in the background art, and an object of the present invention is to provide a method for preparing a recycled polyolefin resin, which can prevent the re-adhesion of foreign matter separated in the washing step of a raw material resin prepared by pre-treating waste polyolefin, thereby improving the effect of removing foreign matter and odor and suppressing the deterioration of physical properties.

[0017] Another object of the present invention is to provide a recycled polyolefin resin comprising one or more selected from the group consisting of a nonionic surfactant, chitosan, and decomposition products thereof.

[0018] [Technical solution]

[0019] In a general aspect, a method for preparing a recycled polyolefin resin comprises: adding a base resin comprising polyolefin and washing water into a washer equipped with a rotating member, and washing the base resin in a wet friction manner by high-speed rotation of the rotating member; and supplying the washed base resin to an extruder for extrusion, wherein the washing water comprises water, a nonionic surfactant, and chitosan.

[0020] [Beneficial Effects]

[0021] According to the present invention, when washing a base resin obtained by pre-treating a waste polyolefin resin, separation of foreign matters is promoted by using a nonionic surfactant, and re-adhesion of the separated foreign matters to the base resin is prevented by using chitosan in combination, thereby improving the effect of removing foreign matters, and it is also possible to prepare a recycled polyolefin resin in which odor is removed by the unique antimicrobial effect of chitosan such as microbial growth inhibition, etc.

[0022] In particular, the effect of removing foreign matter and odor can be maximized by adjusting the weight ratio of the nonionic surfactant to chitosan to a range of 0.2:1 to 2:1 while using 100 ppm to 150 ppm of the nonionic surfactant based on the total weight of the wash water. DETAILED DESCRIPTION

[0023] The terms and words used in the specification and claims of the present invention should not be interpreted as having general or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical ideas of the present invention based on the principle that inventors can appropriately define the concepts of the terms in order to describe their own inventions in the best manner.

[0024] In the present invention, the polyolefin resin is a concept that collectively refers to homopolymers, copolymers and / or block (co)polymers containing ethylene and / or propylene monomers, and compositions containing these polymers.

[0025] Meanwhile, the base resin in the present disclosure is a polyolefin resin that has been separated and recovered from waste products and then pre-processed, and may contain impurities or foreign matter, and the resin prepared from the base resin by the method for preparing a recycled resin of the present invention is defined as a recycled polyolefin resin. In addition, a resin in a state of a mixture of a recycled resin and a new material for applying the recycled polyolefin resin to a product or in a state of being added with an additive for imparting desired physical properties is referred to as a post-consumer recycled (PCR) polyolefin resin.

[0026] Hereinafter, for better understanding of the present invention, the present invention will be described in more detail.

[0027] The method of preparing a recycled polyolefin resin according to an exemplary embodiment may include washing a base resin and supplying the washed base resin to an extruder for extrusion.

[0028] The base resin may include a polyolefin resin that has been separated and recovered from waste products and then pre-treated.

[0029] Polyolefin resins include homopolymers, copolymers and / or block (co)polymers containing ethylene and / or propylene monomers, and representative examples thereof include polyethylene. Polyethylene is classified into high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ultra-low-density polyethylene (VLDPE), etc. according to density, and is used after being appropriately selected according to the properties of the product. In particular, low-density polyethylene (LDPE) resin, linear low-density polyethylene (LLDPE) resin or a mixture thereof has excellent transparency and plasticity, and can effectively control weeds, moisture and temperature in the soil, so it has been used as an agricultural mulch.

[0030] Polyolefin resins used in various products including agricultural mulch films can be recovered from products discarded after use and recycled as raw materials for post-consumer recycled (PCR) resins. In order to use the recycled waste polyolefin resin as a base resin for the raw material of PCR resin, the waste polyolefin resin should undergo pretreatment processes such as sorting, crushing, washing and drying.

[0031] Sorting is a process of separating waste polyolefin resin from waste products and then classifying by material, size, etc., and methods such as classification using wet specific gravity or dry specific gravity, classification using near-infrared spectroscope, and electrostatic separation using electrostatic properties can be used. In such a sorting process, impurities other than resin such as metal components or fiber components can be separated first, and as large foreign matter in the waste resin is removed, the total foreign matter content is reduced.

[0032] The sorted waste polyolefin resin can be crushed into small sizes for smooth operation of subsequent processes. For example, the waste polyolefin resin can be added to a large-capacity crusher equipped with a knife and cut into uniform certain sizes, such as a size of less than 60 mm. In addition, washing water can be supplied during the crushing of the waste polyolefin resin for wet crushing.

[0033] Since the pulverized waste polyolefin resin is generally contaminated or mixed with foreign matter such as chemicals, organic matter, sand and stone, it is necessary to remove the foreign matter for subsequent processes. For example, the pulverized waste polyolefin resin is added to a washing device and pre-washed with water.

[0034] Pre-washing can be performed in a centrifugal manner, wherein materials having a heavy specific gravity are removed by settling them, and materials floating on the water are recovered. Thus, large foreign matter in the waste resin can be removed to reduce the total foreign matter content.

[0035] Thereafter, the waste polyolefin resin from which impurities are removed by pre-washing as described above can be ground into particles suitable for supplying to an extruder. For example, the waste polyolefin resin can be ground into flakes or particles (pellets or fine powder) of suitable size using a device such as a jaw crusher, an impact crusher, a cross-jet mill, a roll mill or a rod mill.

[0036] The ground waste polyolefin resin may be flakes or particles having an average particle size of 4 mm to 10 mm, specifically 5 mm to 8 mm, and when satisfying this average particle size range, supply to an extruder in a subsequent process may be well performed.

[0037] The waste polyolefin resin after grinding may still contain some foreign matter. Therefore, in order to remove some fine foreign matter contained in the waste resin in the ground state, a device using, for example, magnetic force, wind force, centrifugal force, gravity, etc. may be used to separate and remove the fine foreign matter. At this time, a wet method of supplying washing water may be applied.

[0038] The pretreatment method may be appropriately selected and modified as needed, and is not particularly limited as long as it can provide the base resin in an appropriate state to be supplied to the extruder.

[0039] The pretreated waste polyolefin resin itself can be used as the base resin for producing PCR resin.

[0040] Meanwhile, even after pretreatment, the base resin may still contain trace amounts of foreign matter, and in particular, the resin recovered from agricultural mulch films involves a fermentation process due to its nature and may cause odor.

[0041] Therefore, after the pretreatment, further washing in a wet friction manner is performed on the base resin. Specifically, washing can be performed by adding the base resin and washing water to a washer equipped with a rotating member and rotating the rotating member at a high speed.

[0042] The wash water may include nonionic surfactants and chitosan, which together with the water act as detergency enhancers.

[0043] Generally, surfactants relax the boundary of the interface by having a chemical structure of a hydrophobic group and a hydrophilic group, and effectively separate pollutants on the surface by penetrating the hydrophobic group into the pollutants. In particular, nonionic surfactants do not generate ionic repulsion, which allows easy oil / water separation, and can therefore be advantageously used to remove organic pollutants remaining in resins used as agricultural mulch films.

[0044] In an exemplary embodiment of the present invention, the nonionic surfactant may include a propylene oxide-ethylene oxide block copolymer.

[0045] More specifically, the propylene oxide (PO)-ethylene oxide (EO) block copolymer can be represented by the chemical formula (EO): x (PO) y (EO) x It means that the polyoxyethylene at both ends forms a hydrophilic chain and the polyoxyethylene at the center can be a nonionic triblock copolymer forming a hydrophobic chain. The molecular weight and PO ratio of the copolymer can be determined according to the chemical formula (EO) x (PO) y (EO) x The x and y values ​​vary, and in particular, the PO ratio is a factor that determines the toxicity and solubility of the copolymer in applications.

[0046] For example, the nonionic PO / EP block copolymer used in the present invention has a weight average molecular weight (hereinafter referred to as "weight average molecular weight (Mw)") in terms of polystyrene measured by gel permeation chromatography (GPC; using tetrahydrofuran as an elution solvent) of 9000 to 15000, specifically 11000 to 13000, and the ratio of propylene oxide (PO) of the copolymer is 20% to 40% of the total molecular weight, specifically 25% to 35%, and when the above range is met, low toxicity and high solubility are shown, and thus the washing effect, rinsability and foaming rate of the polyolefin-based film can be effectively adjusted. When the PO ratio of the PO / EO block copolymer is too high, hydrophobicity is greatly shown, and the solubility in water becomes poor, making it difficult to obtain a sufficient washing effect.

[0047] The nonionic surfactant with the properties can effectively cause foreign matter to be separated from the resin in the wet friction washing process of the polyolefin base resin, even if a very small amount is used. For example, based on the gross weight of the wash water, the nonionic surfactant can be used with 50ppm to 200ppm, preferably 100ppm to 150ppm, and can show excellent washing effect. When the content of the nonionic surfactant is less than 50ppm, the washing enhancement effect may not be significant. When the content is greater than 200ppm, excessive bubbles will appear, resulting in process difficulties, the possibility of oxidation itself producing odors may be increased, and the effect of other additives used together may be hindered.

[0048] When the base resin is added to water to which a nonionic surfactant is added and washed by high-speed rotation of a rotating member, foreign matter separated from the resin floats in the water. Most of the separated foreign matter is a cellulose component, and relatively large floating foreign matter may settle naturally, but small foreign matter may remain suspended in the water and then adhere to the resin. Therefore, bad odor and deterioration of physical properties may be caused during extrusion.

[0049] Therefore, in the present invention, chitosan, which is a natural polymer, is used in combination with a nonionic surfactant in order to prevent impurities separated during washing from floating in water and reattaching to the polyolefin base resin and to improve the effect of removing odor.

[0050] Generally, chitosan is a natural biopolymer having a linear polysaccharide structure formed of D-glucosamine and N-acetylglucosamine, and since it has functional groups such as hydroxyl and amino groups, it is known to have antibacterial effects, sterilization effects, etc. to bind to the cell membrane of microorganisms to inhibit the growth of microorganisms. When chitosan is used to wash waste polyolefin resin recovered from agricultural mulch, it can effectively remove odor caused by the fermentation process of microorganisms.

[0051] In addition, since chitosan is similar in structure to cellulose, it may aggregate with foreign matter separated from the resin in washing. That is, since most of the foreign matter separated in the washing water is a cellulose component, which has many hydroxyl groups on the surface, and various functional groups of chitosan form bonds with the hydroxyl groups and aggregate, thereby settling down without floating in the water. When washing waste polyolefin resins recovered from stretch films for packaging, the foreign matter settling effect of chitosan is also beneficial in removing paper foreign matter.

[0052] Meanwhile, although chitosan can show hydrophilicity through the presence of many hydroxyl groups on the surface, it forms strong bonds with adjacent molecules through hydrogen bonds, cannot dissolve well in water, and shows the characteristics of dissolving in dilute organic and inorganic acids. The solubility of chitosan is inversely proportional to the molecular weight. Therefore, when chitosan is added to washing water, it is advantageous to select a molecular weight range to ensure water solubility.

[0053] In this regard, the weight average molecular weight (Mw) of chitosan suitable for the present invention may be 4000 to 700000, preferably 5000 to 500000, more preferably 10000 to 200000. When the molecular weight of chitosan is greater than 700000, water solubility is very low, so it is difficult to use as an additive for removing odors and foreign matter, and when it is less than 4000, the aggregation and sedimentation degree of foreign matter may be reduced.

[0054] As described above, since the chitosan used in the present invention is similar in structure to cellulose, it can aggregate with foreign matters separated from the resin in washing. That is, since most of the foreign matters separated in the washing water are cellulose components, which have many hydroxyl groups on the surface, and various functional groups of chitosan form bonds with the hydroxyl groups and aggregate, thereby settling down without floating in the water. Therefore, foreign matters accumulated in the washer equipped with a rotating member can be settled down and easily removed, and clean washing water in the upper layer can be recovered and reused.

[0055] In an exemplary embodiment of the present invention, chitosan can be used with 50ppm to 1000ppm or 100ppm to 1000ppm based on the gross weight of washing water. In addition, chitosan can be directly added to washing water in a powder state, or added to washing water in a state dissolved in an acid solution such as acetic acid to increase water solubility.

[0056] At the same time, when used with a nonionic surfactant in a suitable combination ratio, chitosan can induce effective aggregation and sedimentation of foreign matter. When chitosan is used alone as an additive in the washing of a base resin, even when chitosan combines with the cellulose component of foreign matter and forms aggregates, chitosan has a reduced tendency to settle and is very likely to exist in a suspended state throughout the washing water, so it is difficult to separate the aggregates of chitosan and foreign matter from the base resin, and the foreign matter may flow into the extrusion process together.

[0057] For example, the weight ratio of the nonionic surfactant to chitosan may be 0.05: 1 to 4: 1, preferably 0.2: 1 to 2: 1, and more preferably 1: 1 to 2: 1. In particular, when the content of the nonionic surfactant based on the total weight of the wash water is 100 ppm to 150 ppm and the weight ratio of the nonionic surfactant to chitosan is 0.2: 1 to 2: 1, the effect of removing foreign matter and odor can be maximized.

[0058] When the nonionic surfactant is used at more than 4 times the amount of chitosan, in severe cases, difficulties in the process are caused by foaming, odor may be generated due to oxidation of the remaining nonionic surfactant in the extrusion process, and the foreign matter settling effect of chitosan may be hindered. In particular, when the wash water is used consistently due to reduced foreign matter settling, the possibility of re-contamination by unstable foreign matter may increase. However, when the content of chitosan is excessively increased compared to the nonionic surfactant, chitosan itself may act as a foreign matter and impair the physical properties of the recycled resin finally prepared, and cause odor.

[0059] Even in the case of adding water as well as washing during the pretreatment of the waste polyolefin resin, the combination of the nonionic surfactant and chitosan can be used.

[0060] In an exemplary embodiment of the present invention, washing can be performed by adjusting the temperature of the washing water to 30° C. to 50° C., such as 40° C. to 45° C., and rotating a rotating member provided in the washer at 1000 rpm to 2000 rpm, such as 1200 rpm to 1500 rpm for 1 to 5 minutes, such as 2 to 3 minutes. When the temperature and rotation speed ranges are met, the effect of removing foreign matter / odor can be achieved while preventing damage to the polyolefin base resin.

[0061] If necessary, the washing water may further include bleach, bactericide, antibacterial agent, disinfectant, dispersant, oxidant, pH regulator, defoamer, etc. to further enhance the washing effect of the base resin. In particular, the defoamer is used to remove bubbles that may be generated by the nonionic surfactant, and defoamers such as silicone, mineral oil and polymer can be used.

[0062] Thereafter, the polyolefin base resin washed by the above method is supplied to an extruder and extruded, thereby preparing a recycled polyolefin resin.

[0063] The extruder has the functions of melting, kneading and extruding the base resin, and may be, for example, a twin-screw extruder. In addition, the extruder may include: a supply port to which the base resin is supplied; an outlet located at the other end of the supply port through which the extrudate is discharged; a barrel located within the housing of the extruder, in which the extrusion feed is extruded; a screw located inside the barrel and pushing the extrusion feed or extrudate in the outlet direction; and a heating device disposed outside the barrel and heating the barrel.

[0064] The temperature inside the extruder can be 150°C to 260°C, specifically 190°C to 230°C. In the case where the extruder has a barrel inside the extruder housing, the temperature inside the extruder can refer to the temperature inside the barrel. The temperature range in such an extruder is the melting point or higher and the degradation temperature or lower of the polyolefin base resin. However, this temperature range is higher than the extrusion temperature of the conventional preparation of recycled polyolefin resins. When the polyolefin base resin stays for a long time in an extruder with this temperature range, the resin is prone to deterioration and discoloration due to heat, which is directly related to the deterioration of the physical properties of the recycled resin. Therefore, when the temperature in the extruder is maintained in the range of 150°C to 260°C, it is important to shorten the residence time of the extrusion feed in the extruder to minimize the side effects from heat. For example, the residence time of the polyolefin base resin in the extruder can be 0.5 to 10 minutes, specifically 1 to 5 minutes.

[0065] Meanwhile, the pressure in the extruder according to the exemplary embodiment of the present invention may be normal pressure to 100 bar or less. Within this pressure range, the temperature in the extruder is easily controlled, and the melting and kneading of the polyolefin base resin may be preferably performed.

[0066] In an exemplary embodiment of the present invention, the extruded composition discharged from the extruder is supplied to a pelletizer for pelletization. For example, the extruded composition discharged through the outlet of the extruder is formed into a desired size by an underwater pelletizer, and can finally be produced into a recycled polyolefin resin.

[0067] According to the present invention as described above, when washing the base resin obtained by pre-treating the discarded polyolefin resin, the separation of foreign matter is promoted by using a nonionic surfactant, and the aggregation and sedimentation of the separated foreign matter are induced by using chitosan in combination to prevent the separated foreign matter from re-attaching to the base resin, thereby improving the effect of removing foreign matter, and it is also possible to prepare a recycled polyolefin resin in which odor is removed by the antibacterial effect such as the unique microbial growth inhibition of chitosan.

[0068] In particular, by adjusting the weight ratio of the nonionic surfactant to chitosan to a range of 0.2:1 to 2:1 while using 100 ppm to 150 ppm of the nonionic surfactant based on the total weight of the wash water, the effect of removing foreign matter and odor can be maximized.

[0069] The recycled polyolefin resin prepared by the method may include one or more residues selected from the group consisting of a nonionic surfactant used in washing and chitosan and a decomposition product thereof.

[0070] For example, in the present invention, when 50 ppm to 200 ppm of a nonionic surfactant and 50 ppm to 1000 ppm of chitosan are used in the washing of the base resin, the finally prepared recycled resin may contain less than 25 ppm, for example, 4 ppm to 21 ppm of the nonionic surfactant or its decomposition product, or less than 100 ppm, for example, 4 ppm to 99 ppm of chitosan or its decomposition product as residues.

[0071] Hereinafter, the present invention will be described in more detail by examples. However, the following examples are provided to illustrate the present invention, and it will be apparent to those skilled in the art that various modifications and changes may be made without departing from the scope and spirit of the present invention, and the scope of the present invention is not limited thereto.

[0072] Example 1

[0073] Waste linear low-density polyethylene (LLDPE) resin recovered from agricultural mulch films is pretreated through a process including sorting, pulverizing, pre-washing, granulation and classification, and large impurities are separated and removed during the process, and then ground to prepare an LLDPE base resin in the form of pellets having an average particle size (D50) of 8 mm or less.

[0074] 400 g of the base resin and 23 L of washing water (45° C.) were added to a washer equipped with a rotating member, and the rotating member was rotated at 1200 rpm to wash the base resin for 2 minutes, wherein the washing water was prepared by adding 100 ppm of Pluronic having a PO-EO block copolymer structure as a nonionic surfactant. TMF127 (PF-127, Mw: about 12500) and 100 ppm chitosan (Mw: 120000) were added to water. After washing, the base resin and washing water were recovered.

[0075] The washed base resin was supplied to a twin-screw extruder including a barrel having an inner diameter of 25 mm and a screw length of 800 mm, and melt-kneading was performed. At this time, the internal temperature of the extruder was sequentially increased from 150° C. in the hopper to 190° C. in the die head, and the rotation speed of the screw was set to 300 rpm and the residence time was controlled to 120 seconds. Thereafter, the composition discharged from the extruder was supplied to a pelletizer to prepare an LLDPE recycled resin having an average diameter of 2 mm.

[0076] Comparative Example 1

[0077] A LLDPE recycled resin was prepared in the same manner as in Example 1, except that the base resin was not washed.

[0078] Examples 2 to 23 and Comparative Examples 2 to 10

[0079] A LLDPE recycled resin was prepared in the same manner as in Example 1, except that the type and content of the additive used in the washing water of the base resin were changed to those in Table 1.

[0080] [Table 1a]

[0081]

[0082] [Table 1b]

[0083]

[0084] Experimental Example 1: Foreign matter and odor evaluation

[0085] Evaluation of foreign matter and odor of the recycled resins prepared in Examples and Comparative Examples was performed by the following method, and the results are shown in Table 2 below.

[0086] <Foreign matter evaluation-Measurement of the amount of thin film gel>

[0087] The prepared LLDPE recycled resin was extruded using a separate extruder to prepare a film sample having a thickness of 0.04 mm, and the number of gels in the film was counted using an optical device (Q-film software, COLLIN). At this time, gels and fibers having a diameter of 100 μm or more in the prepared film sample were regarded as gels and counted, and the foreign matter score of the film was calculated by scoring a gel having a diameter of 100 μm to 300 μm as 0.3 points, a gel having a diameter of 300 μm to 500 μm as 0.5 points, and a gel having a diameter of 500 μm or more as 1 point.

[0088] <Odor Evaluation>

[0089] The recycled resin was provided to 50 panelists and given a score on a scale of 0-5, with the standard being "3"; the less odor the resin had, the closer the score was to "0." The average of the scores given by the 50 panelists was calculated.

[0090] [Table 2a]

[0091]

[0092] [Table 2b]

[0093]

[0094] From Table 2, it is confirmed that Examples 1 to 23 using washing water containing 50 ppm to 200 ppm of PF-127 as a nonionic surfactant and 50 ppm to 1000 ppm of chitosan having a molecular weight of 4000 to 700000 in washing the base resin obtained by pretreatment of the waste polyolefin resin have a better effect of removing foreign matter than Comparative Example 1 in which washing is not performed or Comparative Examples 2 to 10 in which no additive is used or washing water containing other types of additives is used.

[0095] More specifically, compared with Comparative Example 8 using PF-127 alone and Comparative Example 10 using chitosan (Mw 120000) alone, in Examples 1 to 5, due to the use of 100 ppm to 150 ppm of PF-127, while adjusting the combination ratio thereof with chitosan (Mw 10000 to 200000) to 0.2: 1 to 2: 1, a significantly improved effect of removing foreign matter was shown, and the evaluation of odor was comparable or better. In particular, by adjusting the combination ratio of PF-127 and chitosan (Mw 120000) to 1: 1 or 2: 1, Examples 1 to 5 had the lowest foreign matter / odor evaluation score.

[0096] In Examples 11 and 13, since 100 ppm to 150 ppm of PF-127 was used while increasing the content of chitosan (Mw 120000) to adjust the combination to 0.1:1 to 0.15:1, the foreign matter score was similar to that of Examples 1 to 4, but the odor evaluation score was slightly increased. Since chitosan was used in an excess amount of 1000 ppm, this is considered to be due to the oxidation of chitosan itself to generate odor. Meanwhile, Example 12 using a reduced content of chitosan (Mw 120000) had a slightly increased odor evaluation score.

[0097] In Example 6 and Examples 8 to 10, since PF-127 was reduced to 50 ppm while the combination ratio with chitosan (Mw 120000) was changed to 0.01:1 to 1:1, the foreign matter score and the evaluation score of odor were slightly increased compared with Examples 1 to 4. Since less PF-127 was added, the effect of separating foreign matter was reduced, wherein foreign matter aggregated by chitosan was not induced to settle due to insufficient use of PF-127 and remained suspended in the washing water, resulting in poor separation of the base resin and foreign matter.

[0098] In Examples 7 and 14 to 16, PF-127 was increased to 200 ppm, while the combination ratio with chitosan (Mw 120000) was also changed to 0.2:1 to 1:4, and the foreign matter score and the evaluation score of odor increased compared with Examples 1 to 4. This is considered to be because too much PF-127 acted as a foreign matter and self-oxidation occurred to cause odor.

[0099] Meanwhile, in Examples 17 to 23 in which the molecular weight of chitosan was changed under the condition that the combination ratio of PF-127 and chitosan was 1:1, it was confirmed that the effect of removing foreign matter and odor was excellent when the molecular weight of chitosan was within the range of 10,000 to 200,000.

[0100] As described above, the combined use of PF-127 and chitosan in a polyolefin base resin improves the effect of removing foreign matter compared to the case where PF-127 is used alone, and when used in combination in this way, when the concentration of PF-127 in wash water is 100 ppm to 150 ppm and the combined ratio between PF-127 and chitosan is 0.2:1 to 2:1, there is even an excellent additional effect of removing odors.

[0101] Experimental Example 2: Determination of additive residues in recycled resin

[0102] The residual content of the components used as detergent additives in the recycled resins prepared in the Examples was analyzed by liquid chromatography-mass spectrometry (LC-MS).

[0103] <Preparation of sample solution>

[0104] 1 g of a recycled resin solution dissolved in xylene was added to ethanol and soaked, and the supernatant obtained by centrifugation was filtered to prepare a sample solution.

[0105] <Preparation of standard solution>

[0106] Standard solutions were prepared by completely dissolving PF-64 standard and PF-127 standard, which are nonionic surfactants, in acetonitrile or water, respectively.

[0107] The chitosan standard was dissolved in 0.5% acetic acid aqueous solution to prepare a standard solution.

[0108] <LC analysis conditions>

[0109] System: Waters HPLC

[0110] Column: C18 column (inner diameter 4.6 mm × length 50 mm, average particle size 3.0 μm)

[0111] Mobile phase: Acetonitrile / H2O (containing 0.1% TFA), concentration gradient from 10 / 90 (v / v) to 100 / 0 (v / v)

[0112] Flow rate: 1.0 ml / min

[0113] Column temperature: 40 °C

[0114] Injection volume: 10 μl

[0115]

[0116] SIM (Selective Ion Monitoring mode)

[0117] PF-64: m / z = 694, PF-127: m / z = 801, Chitosan: m / z = 892

[0118] Calculate the areas of the peaks corresponding to PF-127 and chitosan used as additive components in the HPLC / MS chromatogram obtained by the calculation analysis when washing the recycled resin or its decomposition products, and then substitute them into the calibration curve to analyze the residual content of the additive components contained in 1 g of the recycled resin sample solution. The results are shown in Table 3 below.

[0119] [Table 3]

[0120]

[0121] It is confirmed from Table 3 that in the examples of the present invention, due to the use of 50 ppm to 200 ppm of PF-127 (non-ionic surfactant) and 50 ppm to 1000 ppm of chitosan in the washing of the base resin, 4 ppm to 21 ppm of PF-127 components (including decomposition products) and 4 ppm to 99 ppm of chitosan components (including decomposition products) remained in the finally prepared recycled resin. That is, in the final recycled resin, PF-127 and chitosan respectively retained about 5% to 15% of the content used in the washing.

Claims

1. A method for preparing a recycled polyolefin resin, the method comprising: adding a base resin comprising polyolefin and washing water into a washer equipped with a rotating member, and washing the base resin in a wet friction manner by high-speed rotation of the rotating member; and supplying the washed base resin to an extruder for extrusion, The washing water comprises water, a nonionic surfactant and chitosan.

2. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The nonionic surfactant includes a propylene oxide-ethylene oxide block copolymer.

3. The method for preparing a recycled polyolefin resin according to claim 2, wherein: The weight average molecular weight (Mw) of the propylene oxide (PO)-ethylene oxide (EO) block copolymer is 9,000 to 15,000, and the proportion of propylene oxide (PO) in the propylene oxide (PO)-ethylene oxide (EO) block copolymer accounts for 20% to 40% of the total molecular weight.

4. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The weight average molecular weight (Mw) of the chitosan is 4,000 to 700,000.

5. The method for preparing recycled polyolefin resin according to claim 4, wherein: The weight average molecular weight (Mw) of the chitosan is 5,000 to 500,000.

6. The method for preparing a recycled polyolefin resin according to claim 5, wherein: The weight average molecular weight (Mw) of the chitosan is 10,000 to 200,000.

7. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The nonionic surfactant is used at 50 ppm to 200 ppm based on the total weight of the wash water.

8. The method for preparing recycled polyolefin resin according to claim 1, wherein: The chitosan is used at 50 ppm to 1000 ppm based on the total weight of the wash water.

9. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The nonionic surfactant and the chitosan are used in a weight ratio of 0.05:1 to 4:

1.

10. The method for preparing a recycled polyolefin resin according to claim 8 or 9, wherein: The nonionic surfactant is contained in an amount of 100 ppm to 150 ppm based on the total weight of the wash water, and The nonionic surfactant and the chitosan are used in a weight ratio of 0.2:1 to 2:

1.

11. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The base resin is obtained through a pretreatment process including sorting, pulverizing and pre-washing.

12. The method for preparing a recycled polyolefin resin according to claim 1, wherein: The washing is performed at a temperature of 30° C. to 50° C. and a speed of 1000 rpm to 2000 rpm for 1 minute to 5 minutes.

13. The method for preparing recycled polyolefin resin according to claim 1, wherein: The base resin is selected from low density polyethylene (LDPE) resin, linear low density polyethylene (LLDPE) resin or a mixture thereof.

14. A recycled polyolefin resin comprising one or more selected from the group consisting of a nonionic surfactant, chitosan, and decomposition products thereof.

15. The recycled polyolefin resin according to claim 14, wherein The nonionic surfactant or its decomposition product is contained in an amount of 4 ppm to 21 ppm based on the weight of the recycled polyolefin resin; and the chitosan or its decomposition product is contained in an amount of 4 ppm to 99 ppm based on the weight of the recycled polyolefin resin.

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