A weather-resistant masterbatch UV-3864MB and its application in PE wood-plastic board surface material

Abstract

The present invention belongs to the field of polymer material processing aids, and discloses a weather-resistant masterbatch UV-3864MB and its application in PE wood-plastic board surface materials. The weather-resistant masterbatch UV-3864MB includes the following components in percentage by mass: triazine ultraviolet absorber: 1% to 5%; benzotriazole ultraviolet absorber: 1% to 5%; light stabilizer UV-3853: 5% to 15%; light stabilizer UV-2908: 5% to 15%; hindered phenol main antioxidant: 1% to 3%; phosphite auxiliary antioxidant: 1% to 3%; HDPE resin: 70% to 85%. The formula of the present invention focuses on the adverse effects of the UVB band in sunlight on the weather resistance of PE wood-plastic board surface materials, and the formula is close to neutral, which is completely different from the conventional alkaline formula in the wood-plastic board industry, and improves the degree of freedom in the design of the formula of PE wood-plastic board surface materials, and has broad application prospects in related industries such as wood-plastic boards.

Description

Technical Field

[0001] The invention belongs to the field of polymer material processing aids, and specifically relates to a weather-resistant masterbatch UV-3864MB and application thereof in PE wood-plastic board surface layer materials. Background Art

[0002] Wood plastic board is a high-tech green and environmentally friendly new decorative material mainly made of wood (wood cellulose, plant cellulose) as the basic material and thermoplastic polymer materials (plastics) and processing aids, which are mixed evenly and then heated and extruded by mold equipment. It has the performance and characteristics of both wood and plastic and is a new composite material that can replace wood and plastic. Wood plastic composite materials originated in North America. With the great progress and rapid development of new material development technology and processing technology, in recent years, wood plastic composite materials have been increasingly used in automotive interior materials, real estate industry, public facilities and other aspects. It is a new type of composite material that has been very popular in the domestic and foreign markets in recent years. Because wood plastic composite materials contain plastics and plant fibers, they have wood-like processing properties, and are significantly superior to wood and other synthetic materials in terms of mechanical properties, nail holding force, durability, etc. The material has many advantages such as strong plasticity and can be recycled and reproduced, which gives wood plastic composite materials a broad market application space. The use of wood plastic composite materials has also alleviated the current shortage of natural resources such as wood to a certain extent, and is also of great significance to the protection of forest resources.

[0003] Wood plastic panels are usually produced in the form of multi-layer co-extrusion, with the core layer in the middle and the surface layers on both sides. Generally speaking, the core layer does not need to be anti-aging. As for the surface layer materials, they are prone to light aging due to exposure to sunlight and ultraviolet rays. It is inevitable that there will be fading, cracking, powdering and other problems caused by light, high temperature or atmospheric pollutants. Therefore, adding antioxidants and anti-light aging additives to plastic wood materials has become a must to solve the above problems.

[0004] The first generation of surface plastic wood products in the wood-plastic industry have low technical requirements and low service life requirements for wood-plastic boards. In order to reduce costs, many practitioners in the plastic wood board industry often do not add anti-aging additives or only add a very small amount of light stabilizers. However, as the requirements of the wood-plastic board industry become higher and higher, industry leaders such as Mexin, Greenco, Guofeng and other well-known companies will put forward higher weather resistance requirements for their plastic wood products, and often require the service life of wood-plastic boards to be more than 10 years or even decades. Therefore, it is very meaningful to develop a new generation of more applicable weather-resistant masterbatch.

[0005] Regarding the anti-aging problem of the wood-plastic board industry, from the analysis of publicly published papers, public patents, industry conferences and other related materials, the main technical idea is to add certain UV absorbers and light stabilizers to the cortical materials to solve the aging problem of wood-plastic boards in outdoor use environments. However, there is currently no in-depth research on the action mechanism of UV absorbers and light stabilizers in the anti-aging process of wood-plastic boards in the industry, resulting in blind anti-aging formula design in the existing wood-plastic board industry, low anti-aging performance, and high production costs, which is not conducive to the market application of anti-aging masterbatch in the wood-plastic board industry.

[0006] Patent CN111231469A discloses a method for preparing PE wood-plastic board, which gives the wood-plastic board certain anti-aging properties by adding a certain amount of ultraviolet absorber and light stabilizer. However, the patent embodiment mentions that the ultraviolet absorber is UV-531 and the light stabilizer is UV-944. The selection of these two weathering additives actually has some defects. First of all, UV-944 has a strong alkalinity and will react with certain weakly acidic components in the PE wood-plastic board formula, causing the light stabilizer to gradually fail. Secondly, the ultraviolet absorber UV-531 may be more suitable for use in PVC wood-plastic board. For PE wood-plastic board, the standard for finding the most suitable ultraviolet absorber is to see which ultraviolet absorber has an absorption band that is closer to HDPE. We can know from the literature that the band that PE material is most sensitive to ultraviolet rays in sunlight is 300-340nm, which includes the two bands of UVA and UVB. Patent CN107933028A discloses a PE double-layer co-extruded surface material formula, which also solves the aging problem of the wood-plastic board by adding a UV absorber. This formula has greater defects than patent CN111231469A. The reason is that the patent CN107933028A formula only contains UV absorbers and lacks light stabilizers. As we know, UV absorbers are small molecular weight additives. After a few years, they may precipitate and cause the weather resistance of the PE wood-plastic board surface layer to be greatly reduced. Then, due to the breakage of the PE molecular weight caused by sunlight exposure, the aging of the PE wood-plastic board surface layer cannot be prevented because there is no light stabilizer to capture free radicals.

[0007] In our previous patent CN114479247A, the anti-aging performance of PE wood-plastic board is improved by the synergistic effect of the reinforcing agent TAIC and specific ultraviolet absorbers and light stabilizers, but its formula combination still has room for further optimization and improvement. In particular, how to achieve better anti-aging performance and be applicable to a wider range of application scenarios through the synergistic effect of specific ultraviolet absorbers, light stabilizers and antioxidants without adding specific reinforcing agents is the goal pursued by those skilled in the art. Summary of the invention

[0008] In view of the shortcomings and deficiencies of the above prior art, the primary purpose of the present invention is to provide a weather-resistant masterbatch UV-3864MB.

[0009] Another object of the present invention is to provide a method for preparing the weather-resistant masterbatch UV-3864MB.

[0010] Another object of the present invention is to provide application of the weather-resistant masterbatch UV-3864MB in PE wood-plastic board surface material.

[0011] The present invention provides a basically neutral anti-aging technical solution, improves the anti-aging performance of PE wood-plastic panels in outdoor application scenarios, and expands the application range of PE wood-plastic panels.

[0012] The purpose of the present invention is achieved through the following technical solutions:

[0013] A weather-resistant masterbatch UV-3864MB, comprising the following components in percentage by weight:

[0014]

[0015] Furthermore, the triazine ultraviolet absorber is one or more of the ultraviolet absorber UV-479, ultraviolet absorber UV-1577, and ultraviolet absorber UV-1164. This type of triazine ultraviolet absorber has good absorption of the UVA and UVB bands (280-320nm) of ultraviolet rays in sunlight, and can effectively give the polyolefin material excellent outdoor weather resistance.

[0016] Furthermore, the benzotriazole ultraviolet absorber is one or more of the ultraviolet absorber UV-326, ultraviolet absorber UV-328, and ultraviolet absorber UV-5411. This type of benzotriazole ultraviolet absorber is a common high-performance ultraviolet absorber on the market, has good absorption performance for the UVA band in the ultraviolet rays in sunlight, and can give the polyolefin material excellent outdoor weather resistance.

[0017] The light stabilizer UV-3853 is a new type of highly efficient hindered amine light stabilizer, and its chemical name is 2,2,6,6-tetramethyl-4-piperidinyl stearate. It is suitable for most polymer materials, especially polyolefin polymer materials and products, and can also be used as a light stabilizer in polymer materials such as polyacetal, polyamide, styrene, TPO, etc.

[0018] The light stabilizer UV-2908 is a hindered benzoate light stabilizer and a highly efficient UV-resistant free radical scavenger. Its chemical name is 3,5-di-tert-butyl-4-hydroxybenzoic acid hexadecyl, which has the advantages of anti-gas fading, low alkalinity, low volatility, low coloring, good chemical stability, and acid and alkali resistance. It has broad application prospects in many fields such as polyolefin products, flat wires, round wires, plates, and pipes.

[0019] Furthermore, the hindered phenolic main antioxidant is one or more of antioxidant 1010, antioxidant 1076, and antioxidant 245, and its main function is to prevent the masterbatch from thermal oxidative aging due to the high temperature processing environment during processing, and the problem of yellowing due to gas fumigation during storage and use of the masterbatch.

[0020] Furthermore, the phosphite auxiliary antioxidant is one or more of antioxidant 168 and antioxidant 626, and its main function is to enhance the processing stability of the polymer material during the masterbatch processing and provide color retention.

[0021] Furthermore, the HDPE resin is 5502 from Maoming Petrochemical, which serves as a carrier of the UV-3864MB masterbatch.

[0022] The preparation method of the weather-resistant masterbatch UV-3864MB comprises the following preparation steps:

[0023] The raw material components except the light stabilizer UV-3853 are stirred and mixed, and then added to the hopper of the twin-screw extruder. The light stabilizer UV-3853 is added through a liquid refueling pump, and the twin-screw extruder is used for extrusion granulation, screening, and magnetic separation to obtain the weather-resistant masterbatch UV-3864MB.

[0024] Furthermore, the stirring and mixing refers to stirring at a stirring speed of 600 to 800 rpm for 2 to 10 minutes in a high-speed mixer under normal temperature conditions.

[0025] Furthermore, the conditions for extrusion granulation by the twin-screw extruder are as follows: the temperature of the rear section of the barrel of the extruder is set to 200-240°C, the temperature of the middle section of the barrel of the extruder is set to 190-230°C, and the temperature of the front section of the barrel of the extruder is set to 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0026] Furthermore, the screening refers to screening the obtained particles through a vibrating screen to select particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm as finished products, and particles of other sizes are recycled; the magnetic separation refers to passing the obtained particles of qualified size through a magnetic separator to remove unqualified particles containing trace metals.

[0027] The application of the weather resistant masterbatch UV-3864MB in the PE wood plastic board surface layer material is as follows: adding the weather resistant masterbatch UV-3864MB in an amount of 3% to 5% by mass into the production process of the PE wood plastic board surface layer material.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The raw materials of the weather-resistant masterbatch formula of the present invention are basically neutral, which will allow the formula of the present invention to have a higher degree of freedom in formula design during the subsequent improvement process.

[0030] (2) The present invention proposes for the first time the adverse effect of the UVB band in ultraviolet rays on the weather resistance of the PE wood-plastic board surface material. By combining specific ultraviolet absorbers in the formula, the weather resistance effect of the entire formula can be improved.

[0031] (3) For the first time in the wood-plastic industry, light stabilizers UV-3853 and UV-2908 were introduced into the wood-plastic board industry as weather-resistant technical solutions, providing a new direction for subsequent weather-resistant technical solutions in the wood-plastic industry.

[0032] (4) The cost of the present invention is relatively low, and the weather resistance effect is excellent. It is a cost-effective weather resistance product applied to PE wood-plastic board surface material, and has broad market prospects. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0034] Example 1

[0035] A weather-resistant masterbatch UV-3864MB used for PE wood-plastic board surface material in this embodiment includes the following components in percentage by weight:

[0036]

[0037] The weather-resistant masterbatch UV-3864MB used for the surface layer material of PE wood-plastic board is prepared by the following method:

[0038] (1) The components of the light-removing stabilizer UV-3853 are mixed and stirred in a high-speed mixer at a stirring speed of 600 to 800 rpm for 2 to 10 minutes at a temperature of 25°C.

[0039] (2) After the light stabilizer UV-3853 is heated to become liquid, it is added through a liquid refueling pump and enters a twin-screw extruder for granulation. The temperature of the rear section of the barrel of the extruder is set to 200-240°C, the temperature of the middle section of the barrel of the extruder is set to 190-230°C, and the temperature of the front section of the barrel of the extruder is set to 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0040] (3) The obtained particles are passed through a vibrating screen, and particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm are finished products, and particles of other sizes are recycled.

[0041] (4) The particles of qualified size obtained in step (3) are passed through a magnetic separator to remove unqualified particles containing trace metals, thereby obtaining a weather-resistant masterbatch UV-3864MB for use in PE wood-plastic board surface materials.

[0042] Example 2

[0043] A weather-resistant masterbatch UV-3864MB used for PE wood-plastic board surface material in this embodiment includes the following components in percentage by weight:

[0044]

[0045] The weather-resistant masterbatch UV-3864MB used for the surface layer material of PE wood-plastic board is prepared by the following method:

[0046] (1) The components of the light-removing stabilizer UV-3853 are mixed and stirred in a high-speed mixer at a stirring speed of 600 to 800 rpm for 2 to 10 minutes at a temperature of 25°C.

[0047] (2) After the light stabilizer UV-3853 is heated to become liquid, it is added through a liquid refueling pump and enters a twin-screw extruder for granulation. The temperature of the rear section of the barrel of the extruder is set to 200-240°C, the temperature of the middle section of the barrel of the extruder is set to 190-230°C, and the temperature of the front section of the barrel of the extruder is set to 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0048] (3) The obtained particles are passed through a vibrating screen, and particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm are finished products, and particles of other sizes are recycled;

[0049] (4) The particles of qualified size obtained in step (3) are passed through a magnetic separator to remove unqualified particles containing trace metals, thereby obtaining a weather-resistant masterbatch UV-3864MB for use in PE wood-plastic board surface materials.

[0050] Example 3

[0051] A weather-resistant masterbatch UV-3864MB used for PE wood-plastic board surface material in this embodiment includes the following components in percentage by weight:

[0052]

[0053] The weather-resistant masterbatch UV-3864MB used for the surface layer material of PE wood-plastic board is prepared by the following method:

[0054] (1) The components of the light-removing stabilizer UV-3853 are mixed and stirred in a high-speed mixer at a stirring speed of 600 to 800 rpm for 2 to 10 minutes at a temperature of 25°C.

[0055] (2) After the light stabilizer UV-3853 is heated to become liquid, it is added through a liquid refueling pump and enters a twin-screw extruder for granulation. The temperature of the rear section of the barrel of the extruder is set to 200-240°C, the temperature of the middle section of the barrel of the extruder is set to 190-230°C, and the temperature of the front section of the barrel of the extruder is set to 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0056] (3) The obtained particles are passed through a vibrating screen, and particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm are finished products, and particles of other sizes are recycled.

[0057] (4) The particles of qualified size obtained in step (3) are passed through a magnetic separator to remove unqualified particles containing trace metals, thereby obtaining a weather-resistant masterbatch UV-3864MB for use in PE wood-plastic board surface materials.

[0058] Comparative Example 1

[0059] This comparative example simulates the weathering-resistant formula of a well-known functional masterbatch company in East China, including the following components in percentage by weight:

[0060]

[0061] The weather-resistant masterbatch of this comparative example is prepared by the following method:

[0062] (1) All components are fully mixed and stirred in a high-speed mixer at a stirring speed of 600-800 rpm for 2-10 minutes at a temperature of 25°C. Then the mixture is fed into a twin-screw extruder for granulation. The temperature of the rear section of the barrel of the extruder is set at 200-240°C, the temperature of the middle section of the barrel of the extruder is set at 190-230°C, and the temperature of the front section of the barrel of the extruder is set at 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0063] (2) The obtained particles are passed through a vibrating screen, and particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm are finished products, and particles of other sizes are recycled.

[0064] (3) The particles of qualified size obtained in step (2) are passed through a magnetic separator to remove unqualified particles containing trace metals, thereby obtaining a weather-resistant masterbatch used in the surface layer material of the PE wood-plastic board in this comparative example.

[0065] Comparative Example 2

[0066] This comparative example simulates the weathering formula of a well-known functional masterbatch company in South China, including the following components in percentage by weight:

[0067]

[0068] The weather-resistant masterbatch of this comparative example is prepared by the following method:

[0069] (1) All components are fully mixed and stirred in a high-speed mixer at a stirring speed of 600-800 rpm for 2-10 minutes at a temperature of 25°C. Then the mixture is fed into a twin-screw extruder for granulation. The temperature of the rear section of the barrel of the extruder is set at 200-240°C, the temperature of the middle section of the barrel of the extruder is set at 190-230°C, and the temperature of the front section of the barrel of the extruder is set at 180-210°C. The strips extruded by the twin-screw extruder are cooled by tap water at room temperature, and pelletized after hot air drying.

[0070] (2) The obtained particles are passed through a vibrating screen, and particles with a diameter of 0.1 to 0.3 mm and a length of 0.5 to 0.7 mm are finished products, and particles of other sizes are recycled;

[0071] (3) The particles of qualified size obtained in step (2) are passed through a magnetic separator to remove unqualified particles containing trace metals, thereby obtaining a weather-resistant masterbatch used in the surface layer material of the PE wood-plastic board in this comparative example.

[0072] The weather-resistant masterbatch prepared in the above embodiments and comparative examples for PE wood-plastic board surface materials was added to the wood-plastic board surface material at an addition amount of 4% to extrude the finished product, and the color was standard iron red. The blank sample is a finished product directly extruded from the wood-plastic board surface material without adding any weather-resistant masterbatch, and the color is also standard iron red. Then, for all wood-plastic board samples, according to the xenon lamp aging conditions in the artificial accelerated aging experiment recommended by the national standard GB / T24508-2020 for the wood-plastic board industry, after 2000 hours, the color difference value test was carried out, and the results are shown in Table 1 below:

[0073] Table 1

[0074]

[0075]

[0076] It can be seen from the above results that the weather-resistant masterbatch UV-3864MB applied to the surface layer of PE wood-plastic board of the present invention has better anti-aging performance in the surface layer of PE wood-plastic board than similar products on the market. The main reason is that the selection of ultraviolet absorber (it needs to have good absorption of both UVA and UVB bands of ultraviolet rays in sunlight) and the use with specific light stabilizers UV-3853 and UV-2908 are very critical. Comparative Example 1 uses different light stabilizers UV-944 in combination and Comparative Example 2 uses different ultraviolet absorbers UV-531 and light stabilizers UV-944 in combination, but the weather-resistant effect of the present invention cannot be achieved. The preparation and addition operation of the weather-resistant masterbatch involved in the present invention are simple and low in cost, and can be widely used in outdoor application scenarios of PE wood-plastic boards.

[0077] In order to further demonstrate the synergistic effect of the ultraviolet absorber and the light stabilizer of the present invention, the following comparative example is set:

[0078] Comparative Example 3

[0079] Compared with Example 1, in this comparative example, light stabilizers UV-3853 and UV-2908 are not added, and the amounts of ultraviolet absorbers UV-1164 and UV-326 are adjusted to 3.5% and 10.5%, respectively.

[0080] Comparative Example 4

[0081] Compared with Example 1, in this comparative example, the ultraviolet absorbers UV-1164 and UV-326 are not added, and the amounts of the light stabilizers UV-3853 and UV-2908 are adjusted to 7% and 7%, respectively.

[0082] The xenon lamp aging test results of the weather-resistant masterbatch obtained in the above comparative examples 3 and 4 applied to the surface layer of PE wood-plastic board are shown in Table 2 below:

[0083] Table 2

[0084]

[0085] From the results in Table 2, it can be seen that under the same addition amount conditions, only adding the ultraviolet absorber of the present invention and only adding the light stabilizer of the present invention in combination with the antioxidant cannot achieve the weathering effect of the present invention, which indicates that the specific ultraviolet absorber of the present invention and the light stabilizer have a good synergistic effect.

[0086] In order to further demonstrate the effect of the combination of the ultraviolet absorbers of the present invention on the weathering effect of the entire formulation, the following comparative examples are set: Comparative Example 5

[0087] Compared with Example 1, this comparative example does not add the ultraviolet absorber UV-1164, and adjusts the amount of the ultraviolet absorber UV-326 to 4%.

[0088] Comparative Example 6

[0089] Compared with Example 1, this comparative example does not add the ultraviolet absorber UV-326, and adjusts the amount of the ultraviolet absorber UV-1164 to 4%.

[0090] The xenon lamp aging test results of the weather-resistant masterbatch obtained in the above comparative examples 5 and 6 applied to the surface layer material of PE wood-plastic board are shown in Table 3 below:

[0091] Table 3

[0092]

[0093] It can be seen from the comparison of the results in Table 3 with the results in Example 1 that the present invention can improve the weather resistance of the entire formula by adopting a combination of specific ultraviolet absorbers that have good absorption of both the UVA and UVB bands of ultraviolet rays in sunlight.

[0094] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. Application of weather-resistant masterbatch UV-3864MB in PE wood-plastic board surface material, characterized in that: The weather-resistant masterbatch UV-3864MB is composed of the following components in percentage by mass: Triazine UV absorber UV-1164: 1%~5%; Benzotriazole UV absorber UV-326: 1%~5%; Light stabilizer UV-3853: 5%~15%; Light stabilizer UV-2908: 5%~15%; Hindered phenolic primary antioxidant: 1%~3%; Phosphite auxiliary antioxidant: 1%~3%; HDPE resin: 70%~85%; The application method is: adding the weather-resistant masterbatch UV-3864MB in an amount of 3% to 5% by weight into the production process of the PE wood-plastic board surface material.

2. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to claim 1, characterized in that: The hindered phenol main antioxidant is one or more of antioxidant 1010, antioxidant 1076, and antioxidant 245.

3. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to claim 1, characterized in that: The phosphite auxiliary antioxidant is one or more of antioxidant 168 and antioxidant 626.

4. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to claim 1, characterized in that: The HDPE resin is 5502 from Maoming Petrochemical.

5. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to any one of claims 1 to 4, characterized in that: The weather-resistant masterbatch UV-3864MB is prepared by the following method: The raw material components except the light stabilizer UV-3853 are stirred and mixed, and then added to the hopper of the twin-screw extruder. The light stabilizer UV-3853 is added through a liquid refueling pump, and the twin-screw extruder is used for extrusion granulation, screening, and magnetic separation to obtain the weather-resistant masterbatch UV-3864MB.

6. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to claim 5, characterized in that: The stirring and mixing refers to stirring at a stirring speed of 600-800 rpm for 2-10 minutes in a high-speed mixer under normal temperature conditions; the conditions for extrusion granulation by the twin-screw extruder are: the temperature of the rear section of the barrel of the extruder is set to 200-240°C, the temperature of the middle section of the barrel of the extruder is set to 190-230°C, and the temperature of the front section of the barrel of the extruder is set to 180-210°C, and the strip material extruded by the twin-screw extruder is cooled by tap water at normal temperature, and pelletized after hot air drying.

7. The use of a weather-resistant masterbatch UV-3864MB in a PE wood-plastic board surface material according to claim 5, characterized in that: The screening refers to screening the obtained particles through a vibrating screen to select particles with a diameter of 0.1-0.3 mm and a length of 0.5-0.7 mm as finished products, and particles of other sizes are recycled; the magnetic separation refers to passing the obtained particles of qualified size through a magnetic separator to remove unqualified particles containing trace metals.

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