A high barrier modified polylactic acid material and its preparation method and application

Abstract

The present invention provides a high-barrier modified polylactic acid material and a preparation method and application thereof. The high-barrier modified polylactic acid material, the raw materials include polylactic acid, a grafting monomer, a chain extension monomer, and a filler. First, the present invention utilizes the grafting monomer to react with the terminal carboxyl group and the hydroxyl group of polylactic acid to improve the barrier property to small molecule gases. Secondly, the present invention further introduces the grafting monomer to form a dendritic structure around multiple benzene rings to improve the melt strength and gas barrier property. In addition, the introduction of multiple benzene rings can also improve the overall stiffness of the material, which is beneficial to the compression rebound of the material. The high-barrier modified polylactic acid material of the present invention can be foamed using gases such as carbon dioxide, and after foaming, it can be widely used in the preparation of packaging cushioning materials, disposable lunch boxes, etc.

Description

Technical Field

[0001] The invention relates to a modified polylactic acid material, in particular to a high-barrier modified polylactic acid material and a preparation method thereof, and application of the modified polylactic acid material in carbon dioxide extrusion foaming. Background Art

[0002] Polylactic acid (PLA) is a polyester polymer obtained by polymerization of lactic acid as raw material. It is a completely biodegradable material. Polylactic acid has good thermal stability, a wide processing window of 170-230°C, good solvent resistance, and can be processed in a variety of ways, such as extrusion, spinning, biaxial stretching, injection blow molding, etc. It has broad application prospects in many fields such as daily necessities, packaging, medical treatment, textiles, and electronic appliances. However, due to the presence of side methyl groups in the PLA molecular structure, the polylactic acid molecular chains are not easy to entangle with each other, the melt strength is low, and it is difficult to foam, especially the barrier property to small molecules such as carbon dioxide is poor, which limits the selection of its foaming agent and its application in packaging materials, lunch boxes and other fields.

[0003] In order to solve the problem of low melt strength of polylactic acid, commonly used methods include introducing side groups of long molecular chains, crosslinking polylactic acid molecular chains, and blending with polymers with high melt strength. However, the introduced long molecular chains are often difficult to degrade or the degradation rate is affected due to the increase in molecular weight. The crosslinking of polylactic acid molecular chains will slow down the degradation rate. Currently known high melt strength polymers are mainly non-degradable materials. The introduction of non-degradable materials will cause the degradation problem to still exist. In addition, although the above methods can solve the problem of low melt strength of polylactic acid, they do not contribute much to improving the barrier properties of polylactic acid using small molecule foaming gases such as carbon dioxide. Therefore, there is currently no good solution in the industry for how to solve the problem of using carbon dioxide foaming agents in modified PLA.

[0004] Patent CN 111286070 A improves the crystallization and melt properties of polylactic acid materials by using cellulose nanofibers as nucleating agents, but the preparation method is complicated and the foaming ratio is only 2 to 20 times.

[0005] Patent CN 109762313 B uses polylactic acid, nucleating agent, foaming agent and auxiliary agent through the steps of primary extrusion, secondary extrusion and extruded foam sheet, effectively solving the difficulties of low foaming ratio and low strength of carbon dioxide in industrial production. However, its foaming ratio is only a dozen times, which is difficult to meet the requirements of high foaming ratio.

[0006] Patent CN 114957947 A provides the polymer with higher bonding properties by utilizing phenolic resin g-PDLA rich in ether bonds, which is beneficial to heterogeneous nucleation and bubble generation during the foaming process. The cross-linked structure is beneficial to locking the gas while providing rigidity. However, its preparation process is cumbersome and difficult to industrialize. In addition, the introduction of a large amount of phenolic resin makes it difficult to completely degrade the prepared modified PLA.

[0007] In summary, the modified polylactic acid material prepared by the above patented technology fails to solve the problem of poor barrier properties to small molecule foaming gases such as carbon dioxide, and it is necessary to find a new solution to the above problem so that gases such as carbon dioxide can be used for foaming. Summary of the invention

[0008] In view of the above problems existing in the prior art, the present invention provides a high-barrier modified polylactic acid material and a preparation method thereof. The modified polylactic acid material can be used for carbon dioxide extrusion foaming.

[0009] First, the present invention utilizes grafted monomers to react with the terminal carboxyl and hydroxyl groups of polylactic acid, so that the prepared modified polylactic acid has an amide group, and utilizes the strong intermolecular forces between the amide groups to form a structure similar to nylon, thereby improving the barrier properties to small molecular gases such as oxygen and carbon dioxide. Secondly, as a preferred embodiment, the present invention further introduces grafted monomers to form a dendritic structure around multiple benzene rings, which on the one hand increases the molecular weight of PLA, improves the melt strength, and is conducive to foaming. On the other hand, as the length of the molecular chain increases, it is conducive to the entanglement of the polylactic acid molecular chain, which can further improve the melt strength and reduce the gap between the polylactic acid molecular chains, thereby improving the gas barrier properties. In addition, the introduction of multiple benzene rings, on the one hand, improves the overall stiffness of the material, which is conducive to the compression and rebound of the material. On the other hand, multiple benzene rings are connected together to further improve the barrier properties of the material to gases, so that carbon dioxide and other gases can be used for foaming.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] The present invention provides a high barrier modified polylactic acid material, which is prepared from raw material components including the following weight percentages:

[0012] Polylactic acid, 70-94%, preferably 75-88%;

[0013] Grafting monomer, 0.5-3%, preferably 1-2%;

[0014] Chain extension monomer, 0.1-0.5%, preferably 0.2-0.4%;

[0015] Filler, 5-30%, preferably 10-25%;

[0016] Among them, the grafting monomer is a compound containing an epoxy group and an amide group, preferably one or more of N-(2-(oxiran-2-ylmethoxy)phenyl)acetamide, 4-[(2S)-2-oxiranylmethoxy]phenylacetamide, 3-(benzoyl)oxirane-2,2-dicarboxamide, and N-(3-acetyl-4-(2,3-epoxypropoxy)phenyl)butanamide.

[0017] In the present invention, the chain extension monomer is a polyphenyl ring compound containing epoxy functional groups or isocyanate functional groups, wherein the epoxy functional groups or isocyanate functional groups are 2 to 4, preferably 3, and the benzene rings are 2 to 5, preferably 3;

[0018] Preferably, the chain extension monomer is one or more of tris(4-hydroxyphenyl)methane triglycidyl ether and tris(p-isocyanatephenyl)amine.

[0019] In the present invention, the melt index (190° C., 2.16 kg) of the polylactic acid (PLA) is 3 to 30 g / 10 min, preferably 5 to 25 g / 10 min.

[0020] In the present invention, the filler is selected from one or more of talc, calcium carbonate, kaolin and mica.

[0021] The present invention also proposes a method for preparing a high barrier modified polylactic acid material as described above, comprising the following steps:

[0022] 1) mixing polylactic acid, grafting monomer, chain extension monomer and filler in a high-speed mixer to obtain a premix;

[0023] 2) The premix of step 1) is extruded and granulated by a twin-screw extruder to obtain high-barrier modified polylactic acid.

[0024] In the present invention, the mixing conditions of the high speed mixer in step 1) are: temperature of 10-40° C., rotation speed of 50-400 rpm, preferably 100-300 rpm, and mixing time of 3-8 min, preferably 4-7 min.

[0025] In the present invention, the operating conditions of the twin-screw extruder in step 2) are: screw temperature 170-200° C., preferably 175-190° C., and rotation speed 150-500 rpm, preferably 200-400 rpm.

[0026] The high barrier modified polylactic acid material of the present invention can be used in the fields of extrusion foaming, autoclave foaming, etc., and is particularly suitable for carbon dioxide extrusion foaming. The foamed product can be widely used in the preparation of packaging cushioning materials, disposable lunch boxes, etc.

[0027] In some specific examples of the present invention, the high barrier modified polylactic acid material is used in the application of carbon dioxide extrusion foaming to prepare a foamed product, and the specific preparation method comprises the following steps:

[0028] S1: adding high barrier modified polylactic acid raw material into a twin-screw extruder, heating and melting to form a melt;

[0029] S2: Supercritical carbon dioxide and freon are injected into the compression section of the twin-screw extruder, mixed evenly with the melt, and then transferred to the single-screw extruder and extruded through the die to obtain a foamed product.

[0030] Further, in S1, the temperature of the twin-screw extruder is 170-220°C, preferably 175-210°C, and the screw speed is 100-500rpm, preferably 200-400rpm;

[0031] Furthermore, in S2, the sum of the injection amounts of supercritical carbon dioxide and freon is 2-7% of the mass of the melt, preferably 3-6%;

[0032] The supercritical carbon dioxide injection amount is 10-40% of the mass of the freon, preferably 15-30%;

[0033] The temperature of the single screw extruder is 170-200° C., preferably 175-195° C., and the screw speed is 100-500 rpm, preferably 200-400 rpm.

[0034] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0035] The modified polylactic acid material prepared by the present invention utilizes the strong intermolecular force between amide groups to form a nylon-like structure, which effectively solves the problem of poor barrier properties when using carbon dioxide as a foaming agent. The present invention also utilizes grafted monomers to form a dendritic structure around multiple benzene rings, increases the molecular weight of PLA, improves the melt strength, is conducive to the entanglement of polylactic acid molecular chains, can further improve the melt strength and reduce the gap between polylactic acid molecular chains, and improves gas barrier properties. In addition, the introduction of multiple benzene rings, on the one hand, improves the overall rigidity of the material, is conducive to the compression and rebound of the material, and on the other hand, multiple benzene rings connected together can further improve the material's barrier properties to gases, and improve the barrier properties to small molecular gases such as oxygen and carbon dioxide, so that carbon dioxide and other gases can be used for foaming. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a comparison of the appearance of the foamed products prepared from the polylactic acid foam material of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0037] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present invention.

[0038] The sources of the main raw materials used in the examples and comparative examples are shown in Table 1. Other materials were obtained from common commercial channels unless otherwise specified:

[0039] Table 1 Information on the sources of main raw materials

[0040]

[0041]

[0042] The main performance testing methods used in the present invention are as follows:

[0043] (1) Expansion ratio: The expansion ratio is calculated by density, which is tested according to standard ISO 1183.

[0044] (2) Compression deformation rate and compression rebound rate: Place a 5 kg weight on a 1 cm thick foam board (20 cm*20 cm) and measure the deformation rate after 24 hours. Remove the weight and measure the recovery of the compressed part of the board 10 minutes later.

[0045] (3) Oxygen permeability: Oxtran 2 / 21ML oxygen permeability tester, Mocon, USA. The test uses a disc-shaped sample with an area of ​​50 cm 2 , thickness is 80μm.

[0046] The main equipment information used in this invention is as follows:

[0047] High-speed mixer: 120L pot-type high-speed mixer, Suzhou Songyuan Environmental Protection Technology Co., Ltd.;

[0048] Twin-screw extruder: The twin-screw extruder used is a product of Coperion Machinery Co., Ltd., model ZSK32Mc;

[0049] Extrusion foaming equipment: twin-screw + single-screw extruder, a product of Shandong Tongjia Machinery Co., Ltd., model TJ-PLA35 / 65, the foaming agent is supercritical carbon dioxide and freon.

[0050] Example 1

[0051] To prepare a high barrier modified polylactic acid material, prepare 100 kg of raw materials according to the following weight percentages:

[0052] Polylactic acid PLA L130, 79.7%;

[0053] Grafting monomer N-(2-(oxiran-2-ylmethoxy)phenyl)acetamide, 1.5%;

[0054] Chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether, 0.3%;

[0055] Filler talc, 18.5%.

[0056] The weighed PLA, grafting monomer, chain extension monomer and filler were mixed uniformly in a high-speed mixer at an operating temperature of 25° C., a rotation speed of 200 rpm and a mixing time of 6 min to obtain a premix;

[0057] The premix is ​​extruded through a twin-screw extruder and granulated to obtain a high-barrier modified polylactic acid material for carbon dioxide extrusion foaming, wherein the screw temperature is set to 170°C, 170°C, 200°C, 200°C, 180°C, 180°C, 175°C, 175°C, 175°C, 170°C, 190°C from the feed port to the die, and the speed is 300rpm.

[0058] Example 2

[0059] To prepare a high barrier modified polylactic acid material, prepare 100 kg of raw materials according to the following weight percentages:

[0060] Polylactic acid PLA FY401, 74.6%;

[0061] Grafting monomer 4-[(2S)-2-oxiranylmethoxy]phenylacetamide, 1%;

[0062] Chain extension monomer tri(p-isocyanatophenyl)amine, 0.4%;

[0063] Filler calcium carbonate, 24%.

[0064] The weighed PLA, grafting monomer, chain extension monomer and filler were mixed uniformly in a high-speed mixer at an operating temperature of 30° C., a rotation speed of 300 rpm and a mixing time of 7 min to obtain a premix;

[0065] The premix is ​​extruded through a twin-screw extruder and granulated to obtain a high barrier modified polylactic acid material for carbon dioxide extrusion foaming. The screw temperature is set to 170°C, 170°C, 200°C, 200°C, 185°C, 180°C, 180°C, 175°C, 175°C, 170°C, 190°C from the feed port to the die head, and the speed is 400rpm.

[0066] Example 3

[0067] To prepare a high barrier modified polylactic acid material, prepare 100 kg of raw materials according to the following weight percentages:

[0068] Polylactic acid PLA REVODE110, 87.8%;

[0069] Grafting monomer 3-(benzoyl)oxirane-2,2-dicarboxamide, 2%;

[0070] Chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether, 0.2%;

[0071] Filler kaolin, 10%.

[0072] The weighed PLA, grafting monomer, chain extension monomer and filler were mixed uniformly in a high-speed mixer at an operating temperature of 20° C., a rotation speed of 100 rpm and a mixing time of 4 min to obtain a premix;

[0073] The premix is ​​extruded through a twin-screw extruder and granulated to obtain a high barrier modified polylactic acid material for carbon dioxide extrusion foaming, wherein the screw temperature is set to 170°C, 170°C, 190°C, 190°C, 190°C, 190°C, 175°C, 175°C, 175°C, 170°C, 180°C from the feed port to the die, and the speed is 200rpm.

[0074] Example 4

[0075] To prepare a high barrier modified polylactic acid material, prepare 100 kg of raw materials according to the following weight percentages:

[0076] Polylactic acid PLA LX575, 91.9%;

[0077] Grafting monomer N-(3-acetyl-4-(2,3-epoxypropoxy)phenyl)butanamide, 3%;

[0078] Chain extension monomer tri(p-isocyanatophenyl)amine, 0.1%;

[0079] Filler mica, 5%.

[0080] The weighed PLA, grafting monomer, chain extension monomer and filler were mixed evenly in a high-speed mixer at an operating temperature of 10°C, a rotation speed of 50 rpm and a mixing time of 3 min to obtain a premix;

[0081] The premix is ​​extruded through a twin-screw extruder and granulated to obtain a high barrier modified polylactic acid material for carbon dioxide extrusion foaming, wherein the screw temperature is set to 170°C, 170°C, 190°C, 190°C, 190°C, 180°C, 175°C, 175°C, 175°C, 170°C, 175°C from the feed port to the die, and the speed is 150rpm.

[0082] Example 5

[0083] To prepare a high barrier modified polylactic acid material, prepare 100 kg of raw materials according to the following weight percentages:

[0084] Polylactic acid PLA L105, 70%;

[0085] Grafting monomer N-(2-(oxiran-2-ylmethoxy)phenyl)acetamide, 0.5%;

[0086] Chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether, 0.5%;

[0087] Filler kaolin, 29%.

[0088] The weighed PLA, grafting monomer, chain extension monomer and filler were mixed uniformly in a high-speed mixer at an operating temperature of 40° C., a rotation speed of 400 rpm and a mixing time of 8 min to obtain a premix;

[0089] The premix is ​​extruded through a twin-screw extruder and granulated to obtain a high-barrier modified polylactic acid material for carbon dioxide extrusion foaming, wherein the screw temperature is set to 170°C, 170°C, 200°C, 200°C, 200°C, 180°C, 175°C, 175°C, 175°C, 170°C, 200°C from the feed port to the die, and the speed is 500rpm.

[0090] Comparative Example 1

[0091] The modified polylactic acid material was prepared by referring to the method of Example 1, except that the grafting monomer N-(2-(oxirane-2-ylmethoxy)phenyl)acetamide was not added during the preparation process, that is, the raw materials only included PLA, talc, and tris(4-hydroxyphenyl)methane triglycidyl ether of the same mass as in Example 1.

[0092] Comparative Example 2

[0093] A modified polylactic acid foam material was prepared according to a method substantially the same as in Example 1, except that during the preparation process, the grafting monomer N-(2-(ethylene oxide-2-ylmethoxy)phenyl)acetamide and the chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether were simultaneously replaced with 0.5% of the chain extender ADR4468 (conventional method).

[0094] Comparative Example 3

[0095] The modified polylactic acid foam material was prepared by referring to the method of Example 1, except that the grafted monomer N-(2-(oxirane-2-ylmethoxy)phenyl)acetamide was replaced by an equal mass of chain extender ADR4468 during the preparation process.

[0096] Comparative Example 4

[0097] The modified polylactic acid foam material was prepared by referring to the method of Example 1, except that the grafted monomer N-(2-(oxirane-2-ylmethoxy)phenyl)acetamide was replaced with an equal mass of 4,4'-diacetamidodiphenylmethane during the preparation process.

[0098] Comparative Example 5

[0099] The modified polylactic acid material was prepared by referring to the method of Example 1, except that the chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether was not added during the preparation process, that is, the raw materials only included PLA, N-(2-(2-(oxirane-2-ylmethoxy)phenyl)acetamide and talc of the same mass as in Example 1.

[0100] Comparative Example 6

[0101] The modified polylactic acid foam material was prepared by referring to the method of Example 1, except that the chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether was replaced by single benzene ring styrene oxide during the preparation process.

[0102] Comparative Example 7

[0103] The modified polylactic acid foam material was prepared by referring to the method of Example 1, except that the chain extension monomer tris(4-hydroxyphenyl)methane triglycidyl ether was replaced by triglycidyl p-aminophenol, an epoxy compound not containing a benzene ring, during the preparation process.

[0104] Application Examples

[0105] The modified polylactic acid foam materials prepared in Examples 1-5 and Comparative Examples 1-7 were subjected to carbon dioxide and freon extrusion foaming experiments respectively, and the steps were as follows:

[0106] S1: Add the polylactic acid raw materials prepared in the embodiment and the comparative example into a twin-screw extruder respectively, heat and melt to form a melt, and set the screw temperature from the feed inlet to the die to 170°C, 200°C, 200°C, 220°C, 220°C, 210°C, 210°C, 200°C, 200°C, 180°C, and 175°C in sections, and the rotation speed is 300rpm;

[0107] S2: In the compression section of the twin-screw extruder, supercritical carbon dioxide and freon with a melt mass of 5% are injected, wherein the supercritical carbon dioxide injection amount is 30% of the mass of freon, and the melt is evenly mixed, and then enters the single-screw extruder, and the screw temperature is set to 200°C, 190°C, 180°C, 180°C, 170°C from the feed port to the die, and the speed is 300rpm to obtain a foamed product, wherein the modified polylactic acid material prepared in Comparative Example 1 cannot be foamed and the foam is broken. The performance test results of other foamed products are shown in Table 2:

[0108] Table 2 Product performance test results

[0109]

[0110] It can be seen from the test results of various properties in Table 2 that the modified polylactic acid foam material prepared by the present invention has the characteristics of easy foaming, high foaming ratio, and can use carbon dioxide as a foaming agent.

[0111] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A high barrier modified polylactic acid material, characterized in that: It is prepared from the following raw material components in weight percentage: Polylactic acid, 70-94%; Grafting monomer, 0.5-3%; Chain extension monomer, 0.1-0.5%; Filler, 5-30%; Wherein, the grafting monomer is a compound containing epoxy group and amide group; The chain extension monomer is a polyphenyl ring compound containing an epoxy functional group or an isocyanate functional group.

2. The high barrier modified polylactic acid material according to claim 1, characterized in that: It is prepared from the following raw material components in weight percentage: Polylactic acid, 75-88%; Grafting monomer, 1-2%; Chain extension monomer, 0.2-0.4%; Filler, 10-25%.

3. The high barrier modified polylactic acid material according to claim 1, characterized in that: The grafting monomer is one or more of N-(2-(oxiran-2-ylmethoxy)phenyl)acetamide, 4-[(2S)-2-oxiranylmethoxy]phenylacetamide, 3-(benzoyl)oxirane-2,2-dicarboxamide, and N-(3-acetyl-4-(2,3-epoxypropoxy)phenyl)butyramide.

4. The high barrier modified polylactic acid material according to claim 1, characterized in that: The number of the epoxy functional groups or isocyanate functional groups is 2 to 4, and the number of the benzene rings is 2 to 5.

5. The high barrier modified polylactic acid material according to claim 4, characterized in that: The number of the epoxy functional groups or isocyanate functional groups is 3 and the number of the benzene rings is 3.

6. The high barrier modified polylactic acid material according to claim 1, characterized in that: The chain extension monomer is one or more of tris(4-hydroxyphenyl)methane triglycidyl ether and tris(p-isocyanatephenyl)amine.

7. The high barrier modified polylactic acid material according to claim 1, characterized in that: The polylactic acid has a melt index of 3 to 30 g / 10 min.

8. The high barrier modified polylactic acid material according to claim 7, characterized in that: The polylactic acid has a melt index of 5 to 25 g / 10 min.

9. The high barrier modified polylactic acid material according to claim 1, characterized in that: The filler is selected from one or more of talc, calcium carbonate, kaolin and mica.

10. A method for preparing the high barrier modified polylactic acid material according to any one of claims 1 to 9, characterized in that: The following steps are involved: 1) mixing polylactic acid, grafting monomer, chain extension monomer and filler in a high-speed mixer to obtain a premix; 2) The premix of step 1) is extruded and granulated by a twin-screw extruder to obtain high-barrier modified polylactic acid.

11. The preparation method according to claim 10, characterized in that: Step 1) The mixing conditions of the high-speed mixer are: temperature of 10-40° C., rotation speed of 50-400 rpm, and mixing time of 3-8 min.

12. The preparation method according to claim 11, characterized in that: The rotating speed is 100-300 rpm, and the mixing time is 4-7 min.

13. The preparation method according to claim 10, characterized in that: Step 2) The operating conditions of the twin-screw extruder are: screw temperature 170-200° C., and rotation speed 150-500 rpm.

14. The preparation method according to claim 13, characterized in that: The temperature of the screw is 175-190° C., and the rotation speed is 200-400 rpm.

15. Use of the high barrier modified polylactic acid material according to any one of claims 1 to 9 or the high barrier modified polylactic acid material prepared by the method according to any one of claims 10 to 13 in the fields of extrusion foaming and autoclave foaming.

16. The use according to claim 15, characterized in that The method for preparing a foamed product by extruding and foaming the high barrier modified polylactic acid material using carbon dioxide comprises the following steps: S1: adding high barrier modified polylactic acid raw material into a twin-screw extruder, heating and melting to form a melt; S2: Supercritical carbon dioxide and freon are injected into the compression section of the twin-screw extruder, mixed evenly with the melt, and then transferred to the single-screw extruder and extruded through the die to obtain a foamed product.

17. The use according to claim 16, characterized in that In S1, the temperature of the twin-screw extruder is 170-220°C, and the screw speed is 100-500rpm; In S2, the sum of the supercritical carbon dioxide and freon injected is 2-7% of the melt mass; The supercritical carbon dioxide injection amount is 10-40% of the mass of freon; The temperature of the single-screw extruder is 170-200° C., and the screw speed is 100-500 rpm.

18. The use according to claim 17, characterized in that The temperature of the twin-screw extruder is 175-210° C., and the screw speed is 200-400 rpm.

19. The use according to claim 17, characterized in that: The sum of the injected amounts of supercritical carbon dioxide and freon is 3-6% of the mass of the melt.

20. The use according to claim 17, characterized in that The injection amount of the supercritical carbon dioxide is 15-30% of the mass of the freon.

21. The use according to claim 17, characterized in that The temperature of the single-screw extruder is 175-195° C., and the screw speed is 200-400 rpm.

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