Polypropylene microporous foam material and its preparation method and application

Abstract

The present invention relates to the field of plastic processing technology, and discloses a polypropylene microporous foam material, a preparation method thereof, and an application thereof. The preparation method comprises: (S1) injecting high-pressure carbon dioxide into the polypropylene melting process, and obtaining a polypropylene gas-containing material through extrusion molding and cooling treatment; (S2) drying the polypropylene gas-containing material and transferring it to a mold cavity, heating it to a foaming temperature, and injecting supercritical carbon dioxide into the mold cavity, maintaining the supercritical fluid state for at least 30 minutes, and then quickly releasing the pressure to obtain a polypropylene microporous foam material. This method shortens the saturation time, widens the subsequent solid-state foaming window, and improves operational flexibility and production efficiency; at the same time, the prepared polypropylene microporous foam material has a pore diameter of less than 150 μm and a pore density of more than 10 7 pieces / cm 3 , has good mechanical properties and can be used in automotive interior and exterior decoration, product packaging, building materials and other fields.

Description

Technical Field

[0001] The present invention relates to the technical field of plastic processing, and in particular to a polypropylene microporous foam material and a preparation method and application thereof. Background Art

[0002] The microporous structure of polypropylene foam material gives it the characteristics of low density, high specific strength, good thermal insulation and sound insulation, and low dielectric constant. It is widely used in emerging fields such as lightweight automobiles, 5G communications, and new energy vehicles.

[0003] Carbon dioxide has the advantages of wide sources, environmental friendliness, and safe use. Microporous foaming polymer technology using supercritical carbon dioxide as a foaming agent meets the requirements of improving material performance and green manufacturing. It is a key common technology in the industry that my country prioritizes development and has received increasing attention.

[0004] At present, the physical foaming of polypropylene mostly adopts the pressure reduction foaming method. According to the foaming temperature, the process can be divided into two categories: melt foaming and solid foaming.

[0005] Typical melt foaming processes include extrusion foaming and injection foaming. Polypropylene and the foaming agent are fully mixed at the melt temperature, eliminating the influence of the crystalline region and allowing more foaming agent to dissolve. However, due to the low melt strength of polypropylene, the melt cannot completely encapsulate the gas, resulting in large cell size and a low expansion ratio in the foamed product, leading to poor mechanical properties.

[0006] Solid-state foaming production processes, represented by autoclave foaming and mold compression foaming, involve dissolving and diffusing the foaming agent below the melting temperature, resulting in high foaming ratios, small and uniform pores, and improved mechanical properties. However, both are intermittent foaming processes, requiring long saturation times and resulting in low production efficiency. Furthermore, solid-state foaming has a narrow foaming window. At too low a temperature, the foaming agent can only enter the amorphous region. At too high a temperature, the foaming gas easily breaks through the pore walls, creating a certain degree of open-cell structure.

[0007] Therefore, a method for foaming polypropylene materials is needed to strengthen the dissolution and diffusion process of the foaming agent to widen the foaming range and improve the production efficiency of solid-state foaming. Summary of the Invention

[0008] The present invention aims to overcome the defects of low production efficiency and narrow foaming window in the intermittent foaming process of polypropylene in the prior art, and provides a polypropylene microporous foam material and a preparation method and application thereof. The method shortens the saturation time, broadens the subsequent solid-state foaming window, and improves operational flexibility and production efficiency. At the same time, the prepared polypropylene microporous foam material has a smaller pore diameter and a larger pore density, and the product has better mechanical properties.

[0009] In order to achieve the above object, the first aspect of the present invention provides a method for preparing a polypropylene microporous foam material, wherein the preparation method comprises:

[0010] (S1) injecting high-pressure carbon dioxide into the polypropylene melting process, and performing extrusion molding and cooling treatment to obtain a polypropylene gas-containing material;

[0011] (S2) drying the polypropylene gas-containing material and transferring it to a mold cavity, heating it to a foaming temperature, and injecting supercritical carbon dioxide into the mold cavity. After maintaining the supercritical fluid state for at least 30 minutes, the pressure is quickly released to obtain a polypropylene microporous foam material.

[0012] The second aspect of the present invention provides a polypropylene microporous foam material prepared by the above-mentioned preparation method.

[0013] The third aspect of the present invention provides an application of the aforementioned polypropylene microporous foam material in one or more of automobile interior and exterior decoration, product packaging and building materials.

[0014] Through the above technical solution, the preparation method of the polypropylene microporous foam material of the present invention has the following advantages:

[0015] (1) The present invention adopts the operation mode of foaming gas-containing materials, combining the advantages of melt foaming and solid-state foaming, and strengthens the dissolution and diffusion process of carbon dioxide in polypropylene materials without lengthening the process, shortens the solid-state foaming saturation time by half, widens the foaming window, and improves operational flexibility and production efficiency;

[0016] (2) The polypropylene foam material prepared by the present invention has a better pore structure than traditional solid foam products, with a pore diameter of less than 150 μm and a pore density of more than 10 7 pieces / cm 3 Under the optimal conditions, the pore diameter of the polypropylene microporous foam material is 5-50 μm, and the pore density is 3×10 10 pieces / cm 3 to 9×10 10 pieces / cm 3 , which improves the tensile strength and compressive strength of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a structural schematic diagram of the equipment for the method for preparing the polypropylene microporous foam material of the present invention.

[0018] Description of Reference Numerals

[0019] 1-First carbon dioxide tank; 2-Regulating valve; 3-Extruder; 4-Water tank; 5-Constant temperature drying oven; 6-Second carbon dioxide tank; 7-Carbon dioxide injection pump; 8-Ball valve; 9-Mold cavity; 10-Ball valve. DETAILED DESCRIPTION

[0020] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0021] As mentioned above, the first aspect of the present invention provides a method for preparing a polypropylene microporous foam material, wherein the preparation method comprises:

[0022] (S1) injecting high-pressure carbon dioxide into the polypropylene melting process, and performing extrusion molding and cooling treatment to obtain a polypropylene gas-containing material;

[0023] (S2) drying the polypropylene gas-containing material and transferring it to a mold cavity, heating it to a foaming temperature, and injecting supercritical carbon dioxide into the mold cavity. After maintaining the supercritical fluid state for at least 30 minutes, the pressure is quickly released to obtain a polypropylene microporous foam material.

[0024] The inventors of the present invention have discovered that, on the one hand, the premixing of molten polypropylene and high-pressure carbon dioxide enhances the dissolution and diffusion of carbon dioxide in the polypropylene material without lengthening the process. Furthermore, after cooling, the resulting polypropylene gas-containing material, which is injected into the material, exists as tiny pores in both the crystalline and amorphous regions. This not only broadens the subsequent solid-state foaming window and improves yield, but also shortens the carbon dioxide diffusion path during solid-state foaming, saving saturation time and increasing production efficiency. Therefore, the present invention can be used to produce polypropylene microporous materials with higher cell density and smaller cell diameters, improving the mechanical properties of the product and finding wider applications in automotive interior and exterior trim, product packaging, and building materials.

[0025] According to the present invention, in step (S1), the pressure of the high-pressure carbon dioxide is 2-10 MPa, preferably 4-6 MPa. In the present invention, the high-pressure carbon dioxide can be injected into the gas injection port of a single-screw extruder. Under the mixing action of the single-screw extruder, the high-pressure carbon dioxide dissolves into the polypropylene melt to form a carbon dioxide / polypropylene homogeneous material.

[0026] According to the present invention, in step (S1), the amount of carbon dioxide used is 1-6 g, preferably 2-3 g, relative to 1000 g of the polypropylene.

[0027] According to the present invention, in step (S1), the melting conditions include: a temperature of T m ~(T m +35℃), where T m is the melting temperature of polypropylene raw material, T m The temperature is 151-155°C. If the temperature is too low, the melt fluidity in the extruder is poor. If the temperature is too high, the solubility of carbon dioxide and the melt strength of the material will be affected, and the heat transfer load during the cooling process will also be increased. In the present invention, the polypropylene melt and high-pressure carbon dioxide are mixed in the extruder in advance. The purpose is to shorten the diffusion path, destroy the crystalline area of ​​polypropylene, and improve the solubility and diffusion efficiency of carbon dioxide.

[0028] According to the present invention, the melt treatment is carried out in a single-screw extruder; wherein the melting section of the single-screw extruder includes five zones, wherein the temperature of zone 1 is 140-150°C, the temperature of zone 2 is 160-175°C, the temperature of zone 3 is 180-190°C, the temperature of zone 4 is 170-180°C, the temperature of zone 5 is 160-170°C, and the head temperature is 155-165°C. In the present invention, zone 1 is a process of heating from room temperature, and polypropylene does not melt in zone 1, but begins to melt after entering zone 2.

[0029] According to the present invention, the extruder temperature is set at 140-190°C.

[0030] According to the present invention, the step (S2) is performed in a mold cavity.

[0031] According to the present invention, in step (S2), the temperature increasing conditions include: a temperature increasing rate of 10-20°C / min, preferably 10-11°C / min.

[0032] According to the present invention, the solid state foaming temperature is (T m -18℃) to (T m -7°C), preferably the foaming temperature is 137-148°C, more preferably 138-143°C.

[0033] According to the present invention, the time for maintaining the supercritical fluid state is 30-60 minutes, preferably 40-45 minutes; in the present invention, the inventors of the present invention found that the traditional method of the prior art takes more than 90 minutes, while in the present invention, the time of the supercritical fluid state is controlled within the aforementioned range, which can shorten the preparation time by half, complete the carbon dioxide dissolution and diffusion process in a shorter time, and greatly improve production efficiency.

[0034] According to the present invention, in step (S1), high-pressure carbon dioxide is injected into the polypropylene melting process, and the carbon dioxide / polypropylene homogeneous material obtained by extrusion molding is rapidly cooled at the die head outlet. Under the action of thermodynamic instability and polypropylene melt solidification, small pores are generated, thereby preparing a polypropylene gas-containing material, which is a gas-containing sheet or plate. When pressurized and saturated in the mold cavity in step (S2), the small pores shorten the diffusion path, allowing supercritical carbon dioxide to diffuse into the polypropylene material faster, significantly shortening the saturation time. At the same time, the small pores also destroy the original crystalline region of polypropylene, widening the foaming window.

[0035] According to the present invention, the cooling can be performed in an ice-water bath; the drying can be performed in a drying oven, and the drying conditions include: a drying temperature of 25-50° C. and a drying time of 20-80 min.

[0036] According to the present invention, in step (S2), the pressure of described supercritical carbon dioxide is 5-20MPa, is preferably 12-15MPa.In the present invention, polypropylene is placed in the atmosphere that has been filled with supercritical carbon dioxide, the consumption of described supercritical carbon dioxide is relevant with container condition and pressure, and main process condition is the pressure of supercritical carbon dioxide, and supercritical carbon dioxide is environment, and the carbon dioxide in the last product micropore will be replaced with air.Therefore in the application, the consumption of supercritical carbon dioxide is not limited, only mention the pressure of the supercritical carbon dioxide that environment gives.According to the present invention, in step (S2), the speed of described quick pressure relief is at least 100MPa / s, is preferably 400-600MPa / s.

[0037] According to the present invention, the polypropylene is selected from homopolypropylene and / or polyolefin copolymers; wherein the polyolefin copolymer is ethylene propylene copolymer and / or ethylene propylene butene copolymer; preferably, the polypropylene is ethylene propylene copolymer and / or ethylene propylene butene copolymer.

[0038] According to the present invention, at a temperature of 230° C. and a pressure of 2.16 kg, the polypropylene has a melt index MFR of 1-8 g / 10 min.

[0039] In the present invention, the polypropylene raw material is purchased from one or more commercially available products, such as E02ES and E07ES, from the Sinopec Zhenhai Refining and Chemical Company. At 230°C and 2.16 kg, the melt index (MFR) of E02ES is 1.8 g / 10 min and the melting temperature is 155°C; the melt index (MFR) of E07ES is 7.1 g / 10 min and the melting temperature is 152°C. Carbon dioxide is a product of Shanghai Chenggong Gas Co., Ltd. and has a purity of 99.9%.

[0040] The second aspect of the present invention provides a polypropylene microporous foam material prepared by the above-mentioned preparation method.

[0041] According to the present invention, the pore diameter of the polypropylene microporous foam material is less than 150 μm, and the pore density is greater than 10 7 pieces / cm 3 Preferably, the pore diameter of the polypropylene microporous foam material is 5-50 μm, and the pore density is 3×10 10 pieces / cm 3 to 9×10 10 pieces / cm 3 .

[0042] The third aspect of the present invention provides an application of the aforementioned polypropylene microporous foam material in one or more of automobile interior and exterior decoration, product packaging, and building materials.

[0043] According to a particularly preferred embodiment of the present invention, Figure 1 As shown, Figure 1 1 is a schematic structural diagram of an apparatus for preparing a polypropylene microporous foam material according to the present invention, wherein the method for preparing a polypropylene microporous foam material comprises:

[0044] (S-1) heating the polypropylene raw material to a molten state in an extruder (3);

[0045] (S-2) injecting high-pressure carbon dioxide (1) into the gas injection port of the extruder (3) to achieve dissolution and mixing with the polypropylene melt under high pressure;

[0046] (S-3) extruding the carbon dioxide / polypropylene homogeneous material obtained in step (S-2) at a die head and rapidly cooling it in an ice water bath (4) to prepare a polypropylene gas-containing material in the shape of a sheet or plate;

[0047] (S-4) drying the polypropylene gas-containing material in a drying oven (5);

[0048] (S-5) heating the polypropylene gas-containing material in the mold cavity (9) to a foaming temperature;

[0049] (S-6) injecting supercritical carbon dioxide (6) into the mold cavity (9) and maintaining the supercritical state for at least 30 minutes;

[0050] (S-7) Rapidly decompressing the mold cavity (9) at a rate of at least 100 MPa / s to obtain a polypropylene foam material.

[0051] In addition, in the present invention, it should be noted that Figure 1 middle:

[0052] The first carbon dioxide tank 1 and the second carbon dioxide tank 6 are commercially available carbon dioxide gas cylinders. The regulating valve 2 is connected to the outlet of the first carbon dioxide tank 1, and its function is to control the amount of carbon dioxide injected. The extruder 3 completes the mixing of carbon dioxide and polypropylene melt, and after being fixed and formed in the cooling water tank 4, the segments are cut into gas-containing materials. The drying oven 5 is used to dry the moisture in the gas-containing material, and the mold cavity 9 is used to accommodate the polypropylene gas-containing material required for foaming. The gas in the second carbon dioxide tank 6 is pressurized by the carbon dioxide injection pump 7 to form supercritical carbon dioxide, and is injected into the mold cavity 9. The function of the ball valve 8 is to control the injection of carbon dioxide and open or close it as needed. The ball valve 10 is connected to the outlet of the mold cavity 9, and its function is to quickly relieve the pressure of the mold cavity 9.

[0053] The present invention will be described in detail below through examples.

[0054] In the following examples and comparative examples:

[0055] The expansion ratio of polypropylene foam material is calculated according to formula (1):

[0056]

[0057] In formula (1), ρ is the density of polypropylene before foaming (g / cm 3 ), ρ f is the density of the foamed sample (g / cm 3 ), measured by ASTM D792 test standard.

[0058] The cell structure was obtained by analyzing the scanning electron microscope photos using Image-Pro software. The cell diameter is the average diameter of the cells in the foamed sample, which can be directly calculated by the software. The cell density was calculated according to formula (2):

[0059]

[0060] In formula (2), n is the number of cells counted in the electron microscope photo, and A is the statistical area selected in the electron microscope photo (cm 2 ).

[0061] Mechanical properties test of polypropylene foam material:

[0062] Tensile testing was conducted according to ASTM D3575. Samples were first cut into standard bar sizes, with no fewer than five specimens per set. Tensile testing was performed on the standard bars using an MTS universal testing machine at a rate of 20 mm / min. The stress at the sample break was defined as the breaking strength, and the corresponding strain as the breaking elongation.

[0063] Compression testing is conducted according to ASTM D3575. Samples are cut into standard sizes using a foam cutter, with at least five specimens per set. Compression testing is performed on standard specimens using an MTS universal testing machine at a compression rate of 12.5 mm / min. The stress at 25% strain is the material's compressive strength.

[0064] Example 1

[0065] This embodiment is to illustrate the use of the method of the present invention in Figure 1 The polypropylene foam material is prepared in the equipment structure process shown.

[0066] (S1) E02ES raw material (MFR = 1.8 g / min, T m =155°C) was melt-extruded through a single-screw extruder 3, with the heating temperatures of each section being 150, 170, 190, 180, and 170°C, and the die head temperature being 160°C. After the extrusion stabilized, the regulating valve 2 was opened, and high-pressure carbon dioxide was injected into the melting section of the extruder at a mass ratio of polypropylene to carbon dioxide of 1000:5 at an injection pressure of 3 MPa. After extrusion through the die, the sheet was rapidly extruded into a water tank 4 for an ice-water bath, thereby obtaining a polypropylene gas-containing sheet.

[0067] (S2) The polypropylene air-containing sheet is dried in a drying oven 5 at 40°C for 40 minutes and then placed in a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 148°C, and then kept warm; the ball valve 8 is opened, the supercritical carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 20 MPa, the ball valve 8 is closed, and the pressure is maintained (maintaining a supercritical state) for 60 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 500 MPa / s to obtain a polypropylene foam sheet.

[0068] Example 2

[0069] This embodiment is to illustrate the use of the method of the present invention in Figure 1 The polypropylene foam material is prepared in the equipment structure process shown.

[0070] (S1) E02ES and E07ES blended raw materials (MFR = 5.5 g / min, T m =155°C) was melt-extruded through a single-screw extruder 3, with the heating temperatures of each section being 140, 165, 180, 170, and 165°C, and the die head temperature being 160°C. After the extrusion stabilized, the regulating valve 2 was opened, and high-pressure carbon dioxide was injected into the melting section of the extruder at a mass ratio of polypropylene to carbon dioxide of 1000:1 at an injection pressure of 5 MPa. After extrusion through the die, the sheet was rapidly extruded into a water tank 4 for an ice water bath, thereby obtaining a polypropylene gas-containing sheet.

[0071] (S2) The polypropylene air-containing sheet is dried in a drying oven 5 at 40°C for 40 minutes and then placed in a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 148°C, and then kept warm; the ball valve 8 is opened, the supercritical carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 12 MPa, the ball valve 8 is closed, and the pressure is maintained for 35 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400 MPa / s to obtain a polypropylene foam sheet.

[0072] Example 3

[0073] This embodiment is to illustrate the use of the method of the present invention in Figure 1 The polypropylene foam material is prepared in the equipment structure process shown.

[0074] (S1) E02ES raw material (MFR = 1.8g / min, T m =155°C) was melt-extruded through a single-screw extruder 3, with the heating temperatures of each section being 150, 170, 190, 180, and 170°C, and the die head temperature being 160°C. After the extrusion stabilized, the regulating valve 2 was opened, and high-pressure carbon dioxide was injected into the melting section of the extruder at a mass ratio of polypropylene to carbon dioxide of 1000:2 at an injection pressure of 5 MPa. After extrusion through the die, the sheet was rapidly extruded into a water tank 4 for an ice water bath, thereby obtaining a polypropylene gas-containing sheet.

[0075] (S2) The polypropylene air-containing sheet is dried in a drying oven 5 at 40°C for 40 minutes and then placed in a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 138°C, and then kept warm; the ball valve 8 is opened, the supercritical carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 15MPa, the ball valve 8 is closed, and the pressure is maintained for 45 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400MPa / s to obtain a polypropylene foam sheet.

[0076] Example 4

[0077] This embodiment is to illustrate the use of the method of the present invention in Figure 1 The polypropylene foam material is prepared in the equipment structure process shown.

[0078] (S1) E02ES and E07ES blended raw materials (MFR = 5.5 g / min, T m=155°C) was melt-extruded through a single-screw extruder 3, with heating temperatures of 140, 165, 180, 170, and 165°C in each section, and a die head temperature of 160°C. After stable extrusion, a regulating valve 2 was opened, and high-pressure carbon dioxide was injected into the melting section of the extruder at a mass ratio of 1000:3 of polypropylene to carbon dioxide at an injection pressure of 6 MPa. After extrusion through a sheet die, the sheet was rapidly extruded into a water tank 4 for an ice water bath, thereby obtaining a polypropylene gas-containing sheet.

[0079] (S2) The polypropylene air-containing sheet is dried in a drying oven 5 at 40°C for 40 minutes and then placed in a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 138°C, and then kept warm; the ball valve 8 is opened, the supercritical carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 15MPa, the ball valve 8 is closed, and the pressure is maintained for 45 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 600MPa / s to obtain a polypropylene foam sheet.

[0080] Comparative Example 1

[0081] (S1) E02ES raw material (MFR = 1.8g / min, T m =155°C) was melt-extruded through a single-screw extruder 3, with the heating temperatures of each section being 150, 170, 190, 180, and 170°C, and the die temperature being 160°C. After extrusion through the die, it quickly entered a water tank 4 for an ice water bath to obtain a polypropylene raw material;

[0082] (S2) The raw material obtained in step (S1) is dried in a drying oven 5 at 40°C for 40 minutes and then placed into a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 148°C, and then kept warm; the ball valve 8 is opened, the carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 20 MPa, the ball valve 8 is closed, and the pressure is maintained for 90 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 500 MPa / s to obtain a polypropylene foam sheet.

[0083] Comparative Example 2

[0084] (S1) E02ES and E07ES blended raw materials (MFR = 5.5 g / min, T m =155°C) is melt-extruded through a single-screw extruder 3, with heating temperatures of 140, 165, 180, 170, and 165°C in each section and a die head temperature of 160°C. After extrusion through a die, it quickly enters a water tank 4 for an ice water bath to obtain a polypropylene raw material;

[0085] (S2) The raw material obtained in step (S1) is dried in a drying oven 5 at 40°C for 60 minutes and then placed into a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 148°C, and then kept warm; the ball valve 8 is opened, the carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 12 MPa, the ball valve 8 is closed, and the pressure is maintained for 45 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400 MPa / s to obtain a polypropylene foam sheet.

[0086] Comparative Example 3

[0087] (S1) E02ES raw material (MFR = 1.8g / min, T m =155°C) was melt-extruded through a single-screw extruder 3, with the heating temperatures of each section being 150, 170, 190, 180, and 170°C, and the die temperature being 160°C. After extrusion through the die, it quickly entered a water tank 4 for an ice water bath to obtain a polypropylene raw material;

[0088] (S2) The raw material obtained in step (S1) is dried in a drying oven 5 at 40°C for 40 minutes and then placed into a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 140°C, and then kept warm; the ball valve 8 is opened, the carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 15 MPa, the ball valve 8 is closed, and the pressure is maintained for 45 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400 MPa / s to obtain a polypropylene foam sheet.

[0089] Comparative Example 4

[0090] (S1) E02ES, E07ES blended raw materials (MFR = 5.5g / min, T m =155°C) is melt-extruded through a single-screw extruder 3, with heating temperatures of 140, 165, 180, 170, and 165°C in each section and a die head temperature of 160°C. After extrusion through a die, it quickly enters a water tank 4 for an ice water bath to obtain a polypropylene raw material;

[0091] (S2) The raw material obtained in step (S1) is dried in a drying oven 5 at 40°C for 60 minutes and then placed into a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 140°C, and then kept warm; the ball valve 8 is opened, the carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 15 MPa, the ball valve 8 is closed, and the pressure is maintained for 90 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400 MPa / s to obtain a polypropylene foam sheet.

[0092] Comparative Example 5

[0093] (S1) E02ES, E07ES blended raw materials (MFR = 5.5g / min, T m =155°C) is melt-extruded through a single-screw extruder 3, with heating temperatures of 140, 165, 180, 170, and 165°C in each section and a die head temperature of 160°C. After extrusion through a die, it quickly enters a water tank 4 for an ice water bath to obtain a polypropylene raw material;

[0094] (S2) The raw material obtained in step (S1) is dried in a drying oven 5 at 40°C for 40 minutes and then placed into a mold cavity 9; the mold cavity 9 is temperature-controlled and heated at a heating rate of 10°C / min to a foaming temperature of 140°C, and then kept warm; the ball valve 8 is opened, the carbon dioxide injection pump 7 is started, the supercritical carbon dioxide injection pressure is controlled to 15 MPa, the ball valve 8 is closed, and the pressure is maintained for 120 minutes; the ball valve 10 is opened, and the mold cavity 9 is quickly depressurized at a depressurization rate of 400 MPa / s to obtain a polypropylene foam sheet.

[0095] Comparative Example 6

[0096] The foaming material was prepared in the same manner as in Example 3, except that:

[0097] In step (S2), “heating to a foaming temperature of 138°C and keeping warm” is modified to “heating to a foaming temperature of 150°C and keeping warm”.

[0098] A polypropylene foam sheet is obtained.

[0099] Comparative Example 7

[0100] A polypropylene foam material was prepared in the same manner as in Example 4, except that:

[0101] In step (S2), “heating to a foaming temperature of 138°C and keeping warm” is modified to “heating to a foaming temperature of 135°C and keeping warm”.

[0102] A polypropylene foam sheet is obtained.

[0103] Before the foaming process begins, commercially available polypropylene raw materials must be re-melted and extruded before forming. This invention does not add any additional steps; it only adds a gas injection process during the melt extrusion process, which has a negligible impact on the extrusion process time. Therefore, the dwell time within the mold cavity is used as the saturation time.

[0104] Table 1

[0105]

[0106] Note: Saturation time refers to the time to maintain the supercritical fluid state;

[0107] “-” means that no microporous structure was obtained, so there is no test data.

[0108] From the results in Table 1 we can see that:

[0109] (1) Examples 1 and 2 use a foaming temperature of 148°C, which can achieve a larger foaming ratio.

[0110] (2) Examples 3 and 4 can still foam normally at 138° C. to obtain a fine cell structure, and the cell density is much higher than that of Examples 1 and 2 and Comparative Example 1 using a direct foaming method.

[0111] (3) Comparative Examples 1 and 2 did not inject carbon dioxide during the melting process and used a direct solid-state foaming method, requiring a saturation time of 90 minutes. A saturation time of 45 minutes was insufficient to obtain a microporous structure. The gas-containing material foaming method employed in the present invention, however, achieved a saturation time of approximately 35-60 minutes, significantly shortening the polypropylene foaming time and improving production efficiency. Furthermore, the cell density of Comparative Example 1 was similar to that of Examples 1 and 2, and significantly lower than that of Examples 3 and 4.

[0112] (4) Comparative Examples 3, 4, and 5 employ a direct solid-state foaming method. Even with extended saturation time, the polypropylene material cannot foam because 140°C is below the foaming window. However, Examples 3 and 4 can still foam normally at 138°C, demonstrating that the foaming method of the present invention can broaden the foaming window and improve operational flexibility.

[0113] (5) Comparative Example 6: Solid-state foaming was performed at 150°C using the method provided by the present invention. However, due to the low melt strength of the polypropylene matrix, a microporous structure could not be obtained.

[0114] (6) Comparative Example 7: Solid-state foaming was performed at 135°C using the method provided by the present invention. However, due to the low temperature, a microporous structure could not be obtained.

[0115] (7) Comparative Example 2-7 did not obtain a microporous structure and was a non-foamed material, which had no comparative significance, so no mechanical property test was performed on it.

[0116] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for preparing a polypropylene microporous foam material, characterized in that: The preparation method comprises: (S1) Injecting high-pressure carbon dioxide into the polypropylene melting process, and then extruding and cooling to obtain a polypropylene gas-containing material; wherein the melting conditions include: a temperature of T m to (T m +35℃), where T m is the melting temperature of polypropylene; wherein the pressure of the high-pressure carbon dioxide is 2-10 MPa; and the amount of the high-pressure carbon dioxide is 1-6 g relative to 1000 g of the polypropylene; (S2) drying the polypropylene gas-containing material and transferring it to a mold cavity, heating it to a foaming temperature, and injecting supercritical carbon dioxide into the mold cavity, maintaining the supercritical fluid state for at least 30 minutes and then quickly releasing the pressure; wherein the foaming temperature conditions include: the temperature is (T m -18℃) to (T m -7℃); obtain polypropylene microporous foam material.

2. The preparation method according to claim 1, wherein In step (S1), the polypropylene gas-containing material is a gas-containing sheet or plate.

3. The preparation method according to claim 2, wherein In step (S1), the pressure of the high-pressure carbon dioxide is 4-6 MPa; And / or, in step (S1), the amount of the high-pressure carbon dioxide used is 2-3 g relative to 1000 g of the polypropylene.

4. The preparation method according to claim 1, wherein In step (S1), T m It is 151-155℃.

5. The preparation method according to claim 1, wherein The foaming temperature is 137-148℃.

6. The preparation method according to claim 5, wherein The foaming temperature is 138-143℃.

7. The preparation method according to claim 1, wherein In step (S2), the pressure of the supercritical carbon dioxide is 12-15 MPa; And / or, the time for maintaining the supercritical fluid state is 30-60 minutes.

8. The preparation method according to claim 7, wherein The supercritical fluid state is maintained for 35-60 minutes.

9. The preparation method according to claim 1, wherein In step (S2), the rapid pressure relief rate is at least 100 MPa / s.

10. The preparation method according to claim 9, wherein In step (S2), the rapid pressure relief rate is 400-600 MPa / s.

11. The preparation method according to claim 1, wherein The polypropylene is selected from homopolypropylene and / or copolymers of polyolefins; And / or, at a temperature of 230° C. and a pressure of 2.16 kg, the polypropylene has a melt index (MFR) of 1-8 g / 10 min.

12. The preparation method according to claim 11, wherein The polyolefin copolymer is an ethylene propylene copolymer and / or an ethylene propylene butene copolymer.

13. A polypropylene microporous foam material prepared by the preparation method according to any one of claims 1 to 12.

14. The polypropylene microporous foam material according to claim 13, wherein: The pore diameter of the polypropylene microporous foam material is less than 150 μm, and the pore density is greater than 10 7 pieces / cm 3 .

15. The polypropylene microporous foam material according to claim 14, wherein: The pore diameter of the polypropylene microporous foam material is 5-50 μm, and the pore density is 3×10 10 pieces / cm 3 to 9×10 10 pieces / cm 3 .

16. Use of the polypropylene microporous foam material according to any one of claims 13 to 15 in one or more of automobile interior and exterior decoration, product packaging and building materials.

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