Hollow product with microporous wall and preparation method thereof

Abstract

The invention discloses a hollow product with a microporous wall, a large number of micropores are formed in the wall part and the bottom of the hollow product, the micropores are uniform in size, and the diameter of the micropores is within 150 microns; and the inner and outer wall surfaces of the wall part and the bottom part are of solid structures. The invention also discloses a preparation method of the hollow product with the microporous wall, which comprises the following steps: (1) preparing a foaming master batch: uniformly mixing the pretreated foaming agent, nano nucleating agent, dispersing agent, activity regulator and matrix resin, and preparing the foaming master batch through a plastic extruder; (2) melting, extruding and pelletizing the dried plastic raw material through a plastic extruder to form a main material of a to-be-molded hollow product; (3) adding the foaming master batch in the step (1) into the main material in the step (2), and uniformly mixing to form a mixed material; (4) the mixed material is subjected to a heating gas release experiment, a gas release time curve of the mixed material at the corresponding temperature T is determined, and characteristic values of the curve comprise peak time tP, starting time tS and ending time tE; (5) based on the gas release time curve obtained in the step (4), the mixed material is added into a three-in-one injection-blow molding machine provided with a bottle blank mold and a bottle blowing mold, in the injection-blow process, the highest temperature of a charging barrel of the injection-blow molding machine is set to be T, and the time interval between the time when the mixed material enters the charging barrel of the machine from a hopper and the time when the melt starts to be blown is between tS and tP; a hollow product with a microporous wall is obtained.

Description

Technical Field

[0001] The invention relates to a hollow product with microporous walls and also to a method for preparing the hollow product with microporous walls. Background Art

[0002] Hollow blow molding is a typical molding method for hollow polymer products. It uses gas pressure to inflate the hot melt parison closed in the mold and finally form a hollow product. According to the production characteristics of the parison, it can be divided into extrusion blow molding and injection blow molding. Compared with traditional extrusion blow molding, secondary injection blow molding or injection stretch blow molding, the parison production, blow molding and demolding of the one-step injection blow molding are completed in one step without secondary heating. Its products have perfect mouth, bottom, seam line, transparency and higher rigidity, no flash, no sprue, and fewer defects. Therefore, hollow products of various shapes obtained by one-step injection blow molding are now widely used in chemical industry, transportation, agriculture, food, beverage, cosmetics, medicine, washing products, children's toys and other fields.

[0003] Introducing micropores into polymer products can reduce weight, insulate, soundproof, reduce shock, eliminate shrinkage marks, and stabilize size. Introducing micrometer-sized bubbles into the wall of one-step injection-blowing products is expected to give such hollow products some excellent properties. Compared with the ordinary injection process, the one-step injection-blowing process does not cool and shape the preform after injection, and then enters the blow molding stage. This process has its own particularity. At present, it is still difficult to form hollow products with microporous structures. This is mainly manifested in that it is difficult to quickly introduce gas after the preform injection is completed and then enters the blow molding process. If the gas is introduced into the preform very early, the gas-containing preform will often have foamed during blow molding or there will be no signs of bubbles forming, which makes it difficult to synchronize blow molding and foaming, and the product wall itself and the micropores in the wall cannot be shaped at the same time. Therefore, the existing one-step injection-blowing process cannot produce hollow products with microporous walls. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a hollow product with microporous walls, the wall thickness of the hollow product is higher than that of conventional hollow products (hollow products without microporous walls-solid wall hollow products) by more than 10%, and the longitudinal and transverse compressive strength of the hollow product is significantly improved compared with conventional hollow products; another purpose of the present invention is to provide a method for preparing the above-mentioned hollow product with microporous walls.

[0005] Technical solution: The hollow product with microporous walls described in the present invention has a large number of micropores in the wall and bottom of the hollow product, the micropores are uniform in size, the micropore diameter is within 150 microns, and the micropores are slightly oriented along the upper and lower directions of the container; the inner and outer wall surfaces of the wall and bottom are solid structures.

[0006] The wall density of the hollow product is 50-90% of the raw material density, that is, the foaming ratio is 10% to 100%.

[0007] The method for preparing the hollow product with microporous walls comprises the following steps:

[0008] (1) Preparing a foaming masterbatch: uniformly mixing a pretreated (dried) foaming agent, an active regulator and a base resin, and obtaining a foaming masterbatch by a plastic extruder;

[0009] (2) melting, extruding and pelletizing the dried plastic raw material through a plastic extruder to form the main material of the hollow product to be formed;

[0010] (3) adding the foaming masterbatch of step (1) to the main material of step (2), and mixing them uniformly to form a mixed material;

[0011] (4) Perform a heating gas release experiment on the mixed material to determine the gas release time curve of the mixed material at the corresponding temperature T. The characteristic values ​​of the curve include the peak time t P , start time t S and end time t E ;

[0012] (5) Based on the time curve obtained in step (4), the mixed material is added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold. During the injection blow process, the maximum temperature of the injection blow machine barrel is set to T, and the time interval from the time when the mixed material enters the machine barrel from the hopper (which means when the mixed material starts to be injected into the barrel and starts to be heated) to the time when the melt starts to be blown is between t S With t P Between, that is, less than t P , greater than t S ; A hollow product with microporous walls is obtained.

[0013] Wherein, in step (1), the foaming masterbatch is mixed by the following components in parts by weight: 10 to 40 parts of foaming agent, 1.5 to 5 parts of active regulator and 50 to 100 parts of base resin.

[0014] The foaming agent is azodicarbonamide or bicarbonate, and its decomposition temperature is 130-220°C; the base resin is LDPE-g-MA with moderate fluidity, its maleic anhydride grafting rate is about 1%, its melt flow rate is 0.3-10g / 10min, and its melting point is below 120 degrees.

[0015] The active regulator is composed of an activator, a passivator and a cross-linking agent; the activator can catalyze the gas decomposition of the foaming agent, which is zinc oxide (ZnO); the passivator can delay the penetration of the foaming agent in the melt, which is dimethyl silicone oil; the cross-linking agent makes the polymer matrix properly cross-linked to improve its gas carrying capacity, which is diisopropylbenzene peroxide. By adjusting the mixing mass ratio of the three components of the active regulator in the foaming masterbatch, it is possible to avoid excessive concentration of gas release or too long a delay in time.

[0016] Wherein, in step (1), during extrusion, the extrusion temperature is 120-125° C. If the temperature is too high, the foaming agent may decompose prematurely, and if the temperature is too low, it is not conducive to plasticization.

[0017] Wherein, in step (2), the plastic raw material includes one or more of PE and PP, and the melt flow volume rate of the obtained main material is 0.3 to 5 cm 3 / 10min.

[0018] Wherein, in step (3), the mass percentage of the foaming masterbatch in the mixed material is 2% to 10%.

[0019] Among them, in step (4), the heating and gas release experiment uses a torque rheometer with a sealed cavity, a vent is opened on one side of the sealed cavity, a sensor capable of testing the gas flow is provided at the vent, and the sensor is connected to an external monitoring terminal.

[0020] The heating and gas release experiment is specifically as follows:

[0021] (4.1) Preheating the torque rheometer to a temperature T, adding 85-95% of the mixed material of the fixed capacity of the torque rheometer through the feed port of the torque rheometer, sealing the feed port, and starting the rotor of the torque rheometer to rotate at a determined speed;

[0022] (4.2) The gas release time curve is recorded by the gas sensor at the vent, including the gas release peak time t P , Start time (the time interval from sealing the feed port to the start of gas generation) t S and end time t E；

[0023] (4.3) Repeat the above experiment (steps (4.1) to (4.2)) three times and take the average of the three times as the result.

[0024] Wherein, in step (4.1), the temperature T is set to be 10 to 30 degrees higher than the peak temperature of gas released by the foaming masterbatch (this temperature is the peak temperature of gas released by the foaming agent; the foaming masterbatch is taken and scanned at 5 to 10 degrees / minute using a differential scanning calorimeter (DSC) to obtain the peak temperature of gas released by the foaming masterbatch) and exceeds the melting point of the mixed material; the rotation speed of the torque rheometer is 40 to 80 rpm.

[0025] The method of the present invention can prepare a microporous wall container with a hollow volume of 20ML to 2L. The hollow product wall and bottom have a large number of micropores, the micropore size is uniform, the diameter is within 150 microns, the micropores are slightly oriented along the upper and lower directions of the container, and the inner and outer wall surfaces of the product wall and bottom are solid structures. The surface quality is similar to that of conventional one-step injection-blown hollow products, and the product wall density is between 50 and 90% of the raw material density. The present invention can make the product pore formation process consistent with the blow molding process, avoid the problem of premature or late pore formation, and effectively solve the problem that the molding and foaming shaping of one-step injection-blown products are difficult to synchronize.

[0026] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) Under the condition of the same amount of materials, the wall thickness of the hollow product prepared by the method of the present invention is more than 10% higher than that of the conventional product, and the longitudinal and transverse compressive strength are significantly improved; (2) Under the condition of the same volume, there are a large number of micropores inside the hollow product prepared by the method of the present invention, resulting in low density and light weight of the product; (3) The method of the present invention can obtain thick-walled hollow products with lower pressure and shorter injection time, effectively reducing equipment energy consumption and improving production efficiency; at the same time, the presence of micropores makes the product easier to shape, and ultimately improves the dimensional accuracy of the hollow product; (4) The uniform micropores existing in the wall of the hollow product can improve the performance of the product in terms of sound insulation, heat insulation, shock resistance, etc.; and under the condition of the same amount of materials, the microporous wall hollow product prepared by the method of the present invention has better barrier properties and is suitable for the packaging field. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a structural schematic diagram of the device used in the gas release experiment of the present invention;

[0028] Figure 2 This is the appearance diagram of the injection blow molded product in Example 1;

[0029] Figure 3 The photo of the hollow product obtained in Example 1 (a) and the scanning electron microscope image of the micropore morphology of the wall (b);

[0030] Figure 4 This is a diagram showing the appearance change of the hollow product obtained in Example 1 after being compressed;

[0031] Figure 5The typical results of the compression force of the hollow product obtained in Example 1 changing with displacement;

[0032] Figure 6 This is a diagram showing the appearance change of the hollow product obtained in Comparative Example 1 after being compressed;

[0033] Figure 7 The following are typical results of the change in compression force of the hollow product obtained in Comparative Example 1 with displacement;

[0034] Figure 8 This is the appearance diagram of the injection blow molded bottle in Comparative Example 2;

[0035] Fig. 9 This is the appearance diagram of the injection blow molded bottle in Comparative Example 3;

[0036] Fig.10 This is a scanning electron microscope image of the micropore morphology of the wall of the hollow product obtained in Comparative Example 4. DETAILED DESCRIPTION

[0037] In terms of equipment, a three-in-one one-step injection blow molding machine, extruder and torque rheometer are required. The injection blow molding machine can be a one-step production equipment for conventional 20ML to 2L hollow containers without any modification. The molds used include one or more hot runner injection preform molds and blowing molds. The extruder used for raw material mixing can be single-screw or twin-screw.

[0038] In terms of raw materials, the PP grade used is domestic T30S, with a melt flow index of 3.2g / 10min (230℃, 21.6N), the LDPE is domestic 951-000, with a melt flow index of 2.18g / 10min (190℃, 21.6N), and the HDPE is domestic 5502, with a melt flow index of 0.35g / 10min (190℃, 21.6N). All other additives used are commercially available.

[0039] Example 1

[0040] This embodiment uses LDPE as the main raw material for producing microporous wall hollow products, including the following steps:

[0041] (1) After drying the required raw materials, weigh the following parts by mass: 40 parts of azodicarbonamide, 2 parts of zinc oxide, 2 parts of dimethyl silicone oil, 1 part of dicumyl peroxide and 55 parts of LDPE-g-MA; after fully mixing these raw materials, prepare a foaming masterbatch through a plastic extruder; during extrusion, the temperature is set to 120-125° C.;

[0042] (2) melting, extruding and pelletizing the dried LDPE through a plastic extruder to form the main material of the hollow product to be formed;

[0043] (3) adding 2% by weight of the foaming masterbatch obtained in step (1) to the main material obtained in step (2), and mixing them evenly to form a mixed material;

[0044] (4) The mixed material is heated to release gas. The specific process is as follows:

[0045] (4.1) First, for the existing fixed capacity of 60cm 3 The torque rheometer is modified, and the structure after modification is as follows Figure 1 As shown, on the one hand, the torque rheometer can be sealed so that the gas cannot escape from between the feeding port 1 and the cavity 2 (specifically: a sealing ring 5 is arranged outside the feeding port 1, so that the cover body 4 at the feeding port 1 can achieve a sealing effect when the device is covered), and on the other hand, a vent hole 3 is punched to connect the cavity 2 with the outside, and a sensor capable of testing the gas flow is placed at the outlet of the vent hole 3, and the sensor is connected to an external monitoring device;

[0046] (4.2) Preheat the modified torque rheometer to a certain temperature of 220 degrees, and add 55 cm 3 After the feed port 1 is sealed, the rotor of the torque rheometer starts to rotate at 60 rpm;

[0047] (4.3) The gas release time curve is obtained through the gas sensor at the outlet of the vent 3, including the gas release peak time t P , start time t S and end time t E ;

[0048] (4.4) Repeat the above experiment three times and take the average value to obtain the gas release peak time t P (19.6s), start time t S (15.8s) and end time t E (27.1s);

[0049] (5) Based on the time curve obtained in step (4), the mixed material is added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold, and a microporous wall hollow product can be produced by a one-step injection blow molding method similar to conventional hollow products. During the injection blow molding process, the maximum temperature of the injection blow molding machine barrel is set to 220 degrees, and the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt starts to be blown is 16 seconds, at t S With t P At this time, the shape of the preform after injection is as follows Figure 2 As shown, the bottle blank at this time has obvious signs of foaming.

[0050] The products obtained through the above process are as follows Figure 3The wall thickness is shown in a. The scanning electron microscope characterization results are as follows Figure 3 The quality, wall thickness and performance of the hollow product of Example 1 are tested and listed in Table 1. The results of the longitudinal compression process of the product are shown in Table 1. Figure 4 and Figure 5 middle.

[0051] Comparative Example 1

[0052] Comparative Example 1 is a conventional LDPE injection blow molding hollow product. The same LDPE as in Example 1 is used. The dried LDPE is directly added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold, and an unfoamed hollow product is processed at a time and temperature similar to that in Example 1 (the maximum temperature of the injection blow molding machine barrel is set to 220 degrees, and the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt begins to be blown is 16s).

[0053] The quality, wall thickness and performance of the hollow product of Comparative Example 1 are tested and listed in Table 1. The results of the longitudinal compression process of the product are shown in Table 1. Figure 6 and Figure 7 middle.

[0054] Comparing the results of comparative example 1 with those of example 1, it can be seen that when using the same raw materials, the average wall thickness of the product obtained in example 1 increases by 20%, the compressive strength increases by more than 20%, the ring compression stiffness increases by more than 15%, and the gas barrier performance improves (the carbon dioxide gas permeation decreases within 48 hours). Figure 7 It can be seen that the unfoamed product is unstable when under pressure and it is difficult to Figure 5 The performance enhancement is shown after compression.

[0055] Comparative Example 2

[0056] Comparative Example 2 The method for preparing hollow products is basically the same as that of Example 1, except that in step (5), the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt begins to be blown is 20 seconds, and the appearance of the obtained bottle blank when opened is as follows: Figure 8 As shown, the preform has a large degree of foaming, and the preform is blown apart during the subsequent blow molding process, making it impossible to carry out subsequent testing.

[0057] Comparative Example 3

[0058] Comparative Example 3 The method for preparing hollow products is basically the same as that of Example 1, except that in step (5), the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt begins to be blown is 14 seconds, and the appearance of the obtained bottle blank when opened is as follows: Fig. 9 As shown, there is no sign of foaming at this time, and the product obtained from the bottle blank after blow molding is basically similar to that of Comparative Example 1.

[0059] Comparative Example 4

[0060] The method for preparing the hollow product in Comparative Example 4 is basically the same as that in Example 1, with the only difference being that in step (1), no active regulator is added during the preparation of the foaming masterbatch.

[0061] The specific steps of Comparative Example 4 are:

[0062] (1) After drying the required raw materials, weigh the following parts by mass: 40 parts of azodicarbonamide and 60 parts of LDPE-g-MA; after fully mixing these raw materials, prepare a foaming masterbatch through a plastic extruder; during extrusion, the temperature is set to 120-125° C.;

[0063] (2) melting, extruding and pelletizing the dried LDPE through a plastic extruder to form the main material of the hollow product to be formed;

[0064] (3) adding 2% by weight of the foaming masterbatch obtained in step (1) to the main material obtained in step (2), and mixing them evenly to form a mixed material;

[0065] (4) The mixed material is heated to release gas. The specific experimental process is the same as in Example 1, and the gas release peak time t is obtained. P (11.1s), start time t S (9.8s) and end time t E (15.1s);

[0066] (5) Based on the time curve obtained in step (4), the mixed material is added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold, and a microporous wall hollow product can be produced by a one-step injection blow molding method similar to conventional hollow products. During the injection blow molding process, the maximum temperature of the injection blow molding machine barrel is set to 220 degrees, and the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt starts to be blown is 10 seconds, at t S With t P At this time, the shape of the preform after injection is also Figure 2 similar.

[0067] The scanning electron microscope image of the wall thickness of the product obtained in Comparative Example 4 is as follows Fig.10 As shown, through Figure 1 , Fig.10 As can be seen from Table 1, the foaming effect of Comparative Example 4 is significantly different from that of Example 1, and the improvement of the performance of the hollow product is not obvious.

[0068] Example 2 - HDPE + LDPE

[0069] The method for preparing the hollow product in Example 2 is basically the same as that in Example 1, except that a mixed plastic of LDPE and HDPE in a mass ratio of 1:1 is used, and specifically comprises the following steps:

[0070] (1) After the required raw materials are dried, the following weight parts are weighed: 10 parts of azodicarbonamide, 0.5 parts of zinc oxide, 0.7 parts of dimethyl silicone oil, 0.3 parts of dicumyl peroxide and 82 parts of LDPE-g-MA; these raw materials are fully mixed and then used to prepare a foaming masterbatch through a plastic extruder; during extrusion, the temperature is set to 120-125° C.;

[0071] (2) Mixing the dried HDPE and LDPE in a mass ratio of 1:1, melting, extruding, and pelletizing through a plastic extruder to form the main material for the hollow product to be molded;

[0072] (3) adding 8% by weight of the foaming masterbatch obtained in step (1) to the main material obtained in step (2), and mixing them evenly to form a mixed material;

[0073] (4) The mixed material is heated to release gas. The specific process is as follows:

[0074] (4.1) First, for the existing fixed capacity of 60cm 3 The torque rheometer is modified, and the structure after modification is as follows Figure 1 As shown, on the one hand, the torque rheometer can be sealed so that the gas cannot escape from between the feeding port 1 and the cavity 2, and on the other hand, a vent hole 3 connected to the cavity 2 is punched, and a sensor capable of testing the gas flow is placed at the outlet of the vent hole 3, and the sensor is connected to an external monitoring device;

[0075] (4.2) Preheat the modified torque rheometer to a certain temperature of 220 degrees, and add 55 cm 3 After the feed port 1 is sealed, the rotor of the torque rheometer starts to rotate at 60 rpm;

[0076] (4.3) The gas release time curve is obtained through the gas sensor at the outlet of the vent 3, including the gas release peak time t P , start time t S and end time t E ;

[0077] (4.4) Repeat the above experiment three times and take the average value to obtain the gas release peak time t P (23.2s), start time t S (18.7s) and end time t E (38.6s);

[0078] (5) Based on the time curve obtained in step (4), the mixed material is added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold, and a microporous wall hollow product can be produced by a one-step injection blow molding method similar to conventional hollow products. During the injection blow molding process, the maximum temperature of the injection blow molding machine barrel is set to 220 degrees, and the time interval from the time when the material enters the machine barrel from the hopper to the time when the melt starts to be blown is 20 seconds, at t S With t P At this time, the preform after injection has obvious signs of foaming.

[0079] The quality, wall thickness and performance of the hollow products obtained through the above process were tested and are also listed in Table 1.

[0080] Comparative Example 5 - HDPE + LDPE

[0081] Comparative Example 5 is a conventional LDPE+HDPE injection blown hollow product. The same LDPE and HDPE as in Example 2 are used. The dried HDPE and LDPE are mixed in a mass ratio of 1:1, and then directly added to a three-in-one injection blow molding machine equipped with a preform mold and a bottle blowing mold. The unfoamed hollow product is processed at a time and temperature similar to that in Example 2 (the maximum temperature of the injection blow machine barrel is set to 220 degrees, and the time interval from the time when the material enters the machine barrel from the hopper to the time when the molten material begins to be blown is 20s).

[0082] The quality, wall thickness and performance of the hollow product obtained in Comparative Example 5 are tested and are also listed in Table 1.

[0083] By comparing the results of Comparative Example 5 with those of Example 2, it can be seen that when the same raw materials are used, the average wall thickness of the product obtained in Example 2 increases by 20%, the compressive strength increases by more than 20%, the ring compression stiffness increases by more than 15%, and the gas barrier performance improves (the carbon dioxide gas permeability decreases within 48 hours).

[0084] Example 3 - PP + LDPE

[0085] This embodiment is for foaming injection blowing, and the plastic raw materials used are PP and LDPE, and the mass ratio thereof is 1:1. The other implementation steps are consistent with the preparation concept of Example 2.

[0086] Comparative Example 6-PP+LDPE

[0087] Comparative Example 6 is used for comparison with Example 3. Conventional LDPE+PP injection blown hollow products are used. The plastic raw materials used are PP and LDPE in a mass ratio of 1:1. The injection blown hollow products are produced at a time and temperature similar to that of Example 3.

[0088] Example 4 - PP + HDPE

[0089] This embodiment is for foaming injection blowing, and the plastic raw materials used are PP and HDPE, and the mass ratio thereof is 1:1. The other implementation steps are consistent with the preparation concept of Example 2.

[0090] Comparative Example 7-PP+HDPE

[0091] Comparative Example 7 is used for comparison with Example 4. Conventional HDPE+PP injection blown hollow products are used. The plastic raw materials used are PP and HDPE in a mass ratio of 1:1. The injection blown hollow products are produced at a time and temperature similar to that of Example 4.

[0092] Table 1 shows the properties of hollow products

[0093]

Claims

1. A hollow product with microporous walls, characterized in that: The wall and bottom of the hollow product are provided with a large number of micropores, the micropores are uniform in size, and the diameter of the micropores is within 150 microns; the inner and outer wall surfaces of the wall and bottom are solid structures.

2. The method for preparing a hollow product with microporous walls according to claim 1, characterized in that: The steps include: (1) Preparing a foaming masterbatch: uniformly mixing the pretreated foaming agent, the active regulator and the base resin, and obtaining the foaming masterbatch by a plastic extruder; (2) melting, extruding and pelletizing the dried plastic raw material through a plastic extruder to form the main material of the hollow product to be formed; (3) adding the foaming masterbatch of step (1) to the main material of step (2), and mixing them uniformly to form a mixed material; (4) Perform a heating gas release experiment on the mixed material to determine the gas release time curve of the mixed material at the corresponding temperature T. The characteristic values ​​of the curve include the peak time t P , start time t S and end time t E ; (5) Based on the gas release time curve obtained in step (4), the mixed material is added to a three-in-one injection blow molding machine equipped with a bottle preform mold and a bottle blowing mold. During the injection blow process, the maximum temperature of the injection blow machine barrel is set to T, and the time interval from the time when the mixed material enters the machine barrel from the hopper to the time when the melt starts to be blown is between t S With t P Between; a hollow product with microporous walls is obtained.

3. The preparation method according to claim 2, characterized in that: In step (1), the foaming masterbatch is mixed by the following components in parts by weight: 10 to 40 parts of a foaming agent, 1.5 to 5 parts of an active regulator, and 50 to 100 parts of a base resin.

4. The preparation method according to claim 3, characterized in that: The foaming agent is azodicarbonamide or bicarbonate, and its decomposition temperature is 130-220°C; the base resin is LDPE-g-MA, and its melt flow rate is 0.3-10g / 10min, and its melting point is below 120 degrees; the activity regulator is composed of an activator, a passivator and a cross-linking agent; wherein the activator is zinc oxide; the passivator is dimethyl silicone oil; and the cross-linking agent is diisopropylbenzene peroxide.

5. The preparation method according to claim 2, characterized in that: In step (1), during extrusion, the extrusion temperature is 120-125°C.

6. The preparation method according to claim 2, characterized in that: In step (2), the plastic raw material is one or more of PE and PP.

7. The preparation method according to claim 2, characterized in that: In step (3), the mass percentage of the foaming masterbatch in the mixed material is 2% to 10%.

8. The preparation method according to claim 2, characterized in that: In step (4), the heating and gas release experiment uses a torque rheometer with a sealed cavity, a vent is opened on one side of the sealed cavity, a sensor capable of testing the gas flow is provided at the vent, and the sensor is connected to an external monitoring terminal.

9. The preparation method according to claim 8, characterized in that: The heating and gas release experiment is specifically as follows: (4.1) Preheating the torque rheometer to a temperature T, adding 85-95% of the mixed material of the fixed capacity of the torque rheometer through the feed port of the torque rheometer, sealing the feed port, and starting the rotor of the torque rheometer to rotate at a determined speed; (4.2) The gas release time curve is recorded by the gas sensor at the vent, including the gas release peak time t P , start time t S and end time t E； (4.3) Repeat steps (4.1) to (4.2) three times and take the average of the three results as the result.

10. The preparation method according to claim 9, characterized in that: In step (4.1), the temperature T is 10 to 30 degrees higher than the peak temperature of the released gas of the foaming masterbatch and exceeds the melting point of the mixed material; the rotation speed of the torque rheometer is 40 to 80 rpm.

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