Preparation method of fluororesin foam with high foaming ratio

Abstract

The invention provides a preparation method of fluororesin foam with high foaming ratio, and relates to the technical field of foaming materials, and the preparation method comprises the following steps: S1, carrying out melt blending on raw materials containing 90-100 parts of polyvinylidene fluoride (PVDF) and 0-5 parts of an inorganic nucleating agent to prepare a precursor; s2, performing supercritical fluid foaming treatment on the precursor to obtain a micro-foaming nucleation sample; s3, carrying out irradiation crosslinking treatment on the micro-foaming nucleation sample; s4, the irradiated sample is subjected to secondary supercritical fluid foaming treatment, and fluororesin foam is prepared; the nucleating agent is added to cooperate with a low-temperature and high-pressure infiltration process, so that the nucleating efficiency of a PVDF matrix is improved, the nucleation density is improved, PVDF molecular chains can be subjected to a cross-linking reaction through irradiation cross-linking to form a net structure, meanwhile, the melt strength of a micro-foaming precursor is improved, cell collapse and combination are inhibited in the foaming process in the second step, and the foaming performance of the PVDF matrix is improved. And a uniform and compact foam structure is prepared.

Description

Technical Field

[0001] This invention relates to the field of foaming materials technology, and in particular to a method for preparing high-expansion-ratio fluoropolymer foam. Background Technology

[0002] Polyvinylidene fluoride (PVDF) foam materials have excellent temperature resistance, resistance to photo-oxidative aging, heat insulation, and slow-delay properties, and are widely used in aerospace, semiconductor, medical and other fields. Supercritical fluid foaming is an effective foaming method for preparing PVDF-based foamed materials. During the foaming process, the matrix first nucleates the cells, and then the cells merge and grow to form a foam. However, the preparation of high-expansion-ratio PVDF materials requires a high-temperature and high-pressure foaming process. Excessive temperature will cause the cells to collapse and merge, resulting in poor properties such as rough cells and dimensional shrinkage in the prepared foamed material. Therefore, this invention proposes a method for preparing high-expansion-ratio fluoropolymer foam to solve the problems existing in the prior art. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes a method for preparing high-expansion-ratio fluoropolymer foam. This method combines the addition of a nucleating agent with a low-temperature, high-pressure impregnation process to improve the nucleation efficiency of the PVDF matrix and increase the nucleus density.

[0004] To achieve the objectives of this invention, the following technical solution is provided: a method for preparing high-expansion-ratio fluoropolymer foam, comprising the following steps: S1: A precursor is prepared by melt blending raw materials containing 90-100 parts of polyvinylidene fluoride (PVDF) and 0-5 parts of inorganic nucleating agent; S2: Supercritical fluid foaming treatment is performed on the precursor to obtain micro-foamed nucleated samples; S3: Irradiation crosslinking treatment of micro-foamed nucleated samples; S4: The irradiated sample is subjected to a secondary supercritical fluid foaming treatment to obtain fluoropolymer foam.

[0005] A further improvement is that: in S1, PVDF is homopolymer PVDF, copolymer PVDF, or a blend of both; the inorganic nucleating agent is selected from at least one of phosphate nucleating agents, aromatic amide nucleating agents, quaternary phosphonium salt nucleating agents, silicon dioxide, titanium dioxide, and calcium carbonate.

[0006] A further improvement is that the particle size of the inorganic nucleating agent is 50nm-10μm, and when a blended nucleating agent is used, the mass ratio of the phosphate nucleating agent to silicon dioxide is 1:(0.5-2).

[0007] Further improvements include: the melt index of the PVDF is 1-30 g / 10 min, and the content of vinylidene fluoride monomer in the copolymerized PVDF is ≥85 mol.

[0008] A further improvement is made in S1, where the melt blending process is either injection molding or extrusion, and the processing temperature is 140-220℃. When using injection molding, the temperatures of the three zones of the injection molding machine are 190℃-210℃, and the mold temperature is 70-90℃. When using extrusion, the extruder screw temperature is 140-200℃, and the die temperature is 190-210℃.

[0009] A further improvement is that, in step S2, the supercritical fluid foaming process includes the following steps: The precursor is placed in the foaming chamber, and the temperature is controlled at 130-160℃. Control the immersion pressure to 10-30MPa, treat under constant temperature and pressure for 2-16 hours, and then release the pressure to normal pressure. Micro-foamed nucleation samples with a foaming ratio of 1.5-3 times were obtained.

[0010] A further improvement is that, in S3, the irradiation crosslinking treatment is performed using electron beam irradiation, with an irradiation dose of 10-80 kGy and a crosslinking degree controlled at 10-70%.

[0011] A further improvement is made in that the equipment parameters for electron beam irradiation are: equipment energy of 1.5-2.5MeV, titanium pump current of 0.1-1mA, and electron beam power of <100kW.

[0012] A further improvement is made in S4, where the secondary supercritical fluid foaming includes the following steps: The irradiated sample was placed in the foaming chamber, and the temperature was controlled at 140-180℃. Control the immersion pressure to 0.5-10 MPa and treat at a constant temperature and pressure for 0.5-2 hours; Depressurize to atmospheric pressure.

[0013] A further improvement is made in S4, where the average pore diameter of the fluororesin foam is 10-100 μm and the pore density is ≥10. 6 The foaming ratio is 5-50 times, with a volume of 1 piece per cm3.

[0014] The beneficial effects of this invention are as follows: 1. This invention improves the nucleation efficiency and increases the nucleus density of PVDF matrix by adding nucleating agents in combination with a low-temperature and high-pressure impregnation process.

[0015] 2. This invention enables cross-linking reactions between PVDF molecular chains through irradiation, forming a network structure. It also improves the melt strength of the microfoaming precursor, inhibits cell collapse and merging during the second foaming process, and prepares a uniform and dense cell structure. Furthermore, it can effectively suppress the shrinkage of PVDF-based foamed materials after foaming.

[0016] 3. The second step of the foaming process of this invention has relatively simple conditions, low foaming pressure, and low requirements for foaming equipment, which effectively reduces equipment investment costs and is easy to scale up production. Attached Figure Description

[0017] Figure 1 This is a flowchart of the present invention; Figure 2 This is a schematic diagram of the bubble structure in Comparative Example 1 of the present invention; Figure 3 This is a schematic diagram of the bubble structure in Embodiment Six of the present invention. Detailed Implementation

[0018] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0019] Example 1 according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: The material comprises the following components: 90-100 parts PVDF and 0-5 parts nucleating agent. The PVDF is either homopolymer PVDF or copolymer PVDF, or a blend of both; the nucleating agent is an inorganic nucleating agent, including phosphate nucleating agents, aromatic amide nucleating agents, quaternary phosphonium salt nucleating agents, silica, titanium dioxide, and calcium carbonate, etc.

[0020] PVDF is melt-blended with a nucleating agent to prepare a precursor. The melt-blending process includes injection molding or extrusion, with a processing temperature of 140-220℃. Preferably, the injection molding machine has three temperature zones of 190℃-210℃ and a die temperature of 70-90℃; the extruder screw temperature is 140-200℃ and the die head temperature is 190-210℃.

[0021] The precursor is placed in the foaming cavity, the temperature is controlled below the melting point (130℃-160℃), the wetting pressure is 10-30MPa, and the temperature and pressure are kept constant for 2-12 hours. Then the pressure is released to normal pressure, and the mold is opened to obtain the micro-foamed nucleation sample with a foaming ratio of 1.5-3 times.

[0022] The precursors for microfoaming nucleation are irradiated and crosslinked to induce crosslinking between PVDF molecules (irradiation dose 10-80 KGy), further improving their melt strength. The degree of crosslinking is controlled between 10-70%, preferably 30-60%. The irradiation crosslinking type is electron beam, the equipment energy is 2.0±0.5M, the titanium pump current is 0.1-1mA, and the electron beam power is <100KWT.

[0023] The irradiated precursor is placed in the foaming chamber, the temperature is controlled near the melting point (140℃-180℃), the wetting pressure is 0.5-10MPa, and the temperature and pressure are kept constant for 0.5-2h. Then the pressure is released to normal pressure, and the mold is opened to obtain the foamed sample.

[0024] Example 2 according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: Three parts of nucleating agent were melt-blended with PVDF using an extruder to prepare a 10 mm precursor product. The screw zone temperatures of the extruder were 140℃, 180℃, 185℃, 200℃, 200℃, 195℃, 195℃, 195℃, and 195℃, respectively, while the die temperature was 200℃. The product was then placed in a sealed cavity and impregnated at 140℃ and 15 MPa for 10 hours until saturated. After depressurization and mold opening, a micro-foamed nucleation precursor was obtained. This precursor was then crosslinked by irradiation at a dose of 10 kGy. The irradiated precursor was placed in a sealed cavity and held at 163℃ and 5 MPa for 1 hour. After depressurization and mold opening, the PVDF foamed product was obtained.

[0025] Example 3 according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: Three parts of nucleating agent and 90 parts of PVDF were melt-blended using an extruder to prepare a 10 mm precursor product. The screw zone temperatures of the extruder were 140℃, 180℃, 185℃, 200℃, 200℃, 195℃, 195℃, 195℃, and 195℃, while the die temperature was 200℃. The product was then placed in a sealed cavity and impregnated at 140℃ and 15 MPa for 10 hours until saturated. After depressurization and mold opening, a micro-foamed nucleation precursor was obtained. This precursor was then crosslinked by irradiation at a dose of 30 kGy. The irradiated precursor was placed in a sealed cavity and held at 165℃ and 5 MPa for 1 hour. After depressurization and mold opening, the PVDF foamed product was obtained.

[0026] Example 4 according to Figure 1As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: Three parts of nucleating agent and 90 parts of PVDF were melt-blended using an extruder to prepare a 10 mm precursor product. The screw zone temperatures of the extruder were 140℃, 180℃, 185℃, 200℃, 200℃, 195℃, 195℃, 195℃, and 195℃, respectively, while the die temperature was 200℃. The product was then placed in a sealed cavity and impregnated at 140℃ and 15 MPa for 10 hours until saturated. After depressurization and mold opening, a micro-foamed nucleation precursor was obtained. This precursor was then crosslinked by irradiation at a dose of 50 KGy. The irradiated precursor was placed in a sealed cavity and held at 166℃ and 5 MPa for 1 hour. After depressurization and mold opening, the PVDF foamed product was obtained.

[0027] Example 5 according to Figure 1 As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: Three parts of nucleating agent and 90 parts of PVDF were melt-blended using an extruder to prepare a 10 mm precursor product. The screw zone temperatures of the extruder were 140℃, 180℃, 185℃, 200℃, 200℃, 195℃, 195℃, 195℃, and 195℃, while the die temperature was 200℃. The product was then placed in a sealed cavity and impregnated at 140℃ and 15 MPa for 10 hours until saturated. After depressurization and mold opening, a micro-foamed nucleation precursor was obtained. This precursor was then crosslinked by irradiation at a dose of 80 kGy. The irradiated precursor was placed in a sealed cavity and held at 168℃ and 5 MPa for 1 hour. After depressurization and mold opening, the PVDF foamed product was obtained.

[0028] Example 6 according to Figure 1 , 3 As shown in the figure, this embodiment proposes a method for preparing high-expansion-ratio fluoropolymer foam, including the following steps: Five parts of nucleating agent and 90 parts of PVDF were melt-blended using an extruder to prepare a 10 mm precursor product. The screw zone temperatures of the extruder were 140℃, 180℃, 185℃, 200℃, 200℃, 195℃, 195℃, 195℃, and 195℃, while the die temperature was 200℃. The product was then placed in a sealed cavity and impregnated at 142℃ and 15 MPa for 10 hours until saturated. After depressurization and mold opening, a micro-foamed nucleation precursor was obtained. This precursor was then crosslinked by irradiation at a dose of 50 KGy. The irradiated precursor was placed in a sealed cavity and held at 168℃ and 5 MPa for 1 hour. After depressurization and mold opening, the PVDF foamed product was obtained.

[0029] Comparative example:

[0030] The test results of various properties of the fluororesin foam materials prepared in Examples 2-6 and Comparative Examples 1-4 are shown in the table below:

[0031] The difference between Example 4 and Comparative Example 3 is whether or not there is irradiation. The melt strength of the cross-linked substrate without irradiation is poor, and the cell collapse and merging are prone to occur during the secondary foaming process, resulting in larger cell size and higher shrinkage rate.

[0032] The difference between Example 4 and Comparative Example 4 lies in the presence or absence of a nucleating agent. The substrate without a nucleating agent has a low cell nucleation density during the first foaming, and the final foaming result in a larger cell size and a higher shrinkage rate.

[0033] As shown in Examples 4, 6, and Comparative Example 4, the nucleating agent increases the nucleation density, further affecting the cell size and dimensional shrinkage rate. The higher the content of the nucleating agent, the smaller the cell diameter and dimensional shrinkage rate. This is because the nucleating agent increases the cell nucleation density, and the dense nucleation density reduces the cell size and dimensional shrinkage rate during the subsequent foaming process.

[0034] As can be seen from Examples 2-5, the cell diameter and dimensional shrinkage rate decrease with increasing irradiation dose. This is because irradiation crosslinking improves melt strength and reduces the risk of cell merging and collapse during secondary foaming, resulting in smaller cell size and dimensional shrinkage rate.

[0035] The difference between Comparative Example 1 and Comparative Example 2 is that Comparative Example 1 is completed in one foaming step, while Comparative Example 2 is completed in two foaming steps. By controlling nucleation and cell growth step by step, the cell size and shrinkage rate are reduced.

[0036] Under the same conditions of "nucleating agent + secondary foaming", simply adding "irradiation crosslinking in the order of this application" reduced the pore diameter from 131.2 μm to 64.3 μm (a decrease of 50.9%) and the dimensional shrinkage rate from 6.5% to 2.4% (a decrease of 63.1%), proving that this irradiation sequence can effectively inhibit pore coalescence and reduce shrinkage, solving the core pain points of existing technologies. With the same "irradiation sequence", the absence of a nucleating agent leads to an increase in pore diameter (119.5 μm vs 64.3 μm) and an increase in shrinkage rate (5.1% vs 2.4%), proving that the "nucleating agent + specific irradiation sequence" in this application is a synergistic effect, rather than a simple superposition of existing technologies. This synergistic effect is not disclosed in existing technologies (which only disclose nucleating agents). The irradiation sequence is not "accidentally effective," but rather its effect can be optimized through dosage control, further confirming the scientific validity of the "nucleation before crosslinking" mechanism and ruling out the possibility of "random effects".

[0037] In this invention, the first step is micro-foaming. At this stage, the PVDF contains many dense bubble nuclei, achieving a certain foaming ratio and completing the initial foaming. Irradiation is not performed in this step because irradiation would result in excessively high melt strength, hindering bubble nucleus formation. The second step, irradiation crosslinking, increases melt strength, providing high melt strength for the third foaming step and preventing bubble collapse and merging during cell growth. The third foaming step, based on the micro-foaming and dense bubble nuclei formed in the first step, allows for rapid wetting with supercritical gas. Pressure relief and mold opening enable rapid bubble nucleus growth, completing the foaming process. This effectively addresses the core pain points of existing technologies—cell collapse and dimensional shrinkage—and demonstrates repeatability and significant results. This invention pioneers a sequence of "microfoaming nucleation (S2) → irradiation crosslinking (S3) → secondary foaming (S4)". The core logic is as follows: First, microfoaming under medium temperature and high pressure in S2 forms a large number of uniform microfoam nuclei in the PVDF matrix (foaming ratio of 1.5-3 times, high nucleus density but no growth); then, through irradiation crosslinking in S3, a network structure is constructed in the matrix containing the formed nuclei. At this time, the crosslinking reaction does not destroy the existing microfoam nuclei, but can instead "encapsulate" the nuclei, preventing the nuclei from collapsing during the subsequent secondary foaming; finally, through low-pressure foaming in S4, the nuclei protected by the crosslinked structure grow steadily (rather than merged randomly), ultimately achieving a balance between high foaming ratio (5-50 times) and uniform pore size (10-100μm in diameter). The advantages of the "nucleation followed by crosslinking" mechanism of this invention are as follows: Stage 1 (S2): In the uncrosslinked PVDF matrix, the nucleating agent and low-temperature high-pressure wetting work synergistically to efficiently form a large number of micro-bubble nuclei (without crosslinking obstacles, maximizing nucleation efficiency); Stage 2 (S3): Irradiation crosslinking occurs only in the molecular chains between the bubble nuclei, and the formed network structure acts like a "scaffold" to fix the position of the bubble nuclei, while retaining a certain degree of elasticity in the matrix (crosslinking degree controlled at 10-70%, rather than complete crosslinking); Stage 3 (S4): During secondary foaming, the bubble nuclei expand smoothly under the "scaffold protection," avoiding both the collapse of the bubble pores (insufficient melt strength) when there is no crosslinking and the inhibition of the bubble nuclei (overly rigid matrix) when there is crosslinking first, achieving a chain of advantages of "high bubble nuclei density → uniform bubble pores → small size shrinkage." This invention can prepare bubble pores with a diameter of 10-100 μm and a bubble pore density ≥10 6 pcs / cm 3 The fluoropolymer foam, with a foaming ratio of 5-50 times, solves the pain point of existing technologies such as "high temperature and high pressure causing cell collapse and size shrinkage". Moreover, the second foaming pressure is only 0.5-10MPa (far lower than the high temperature and high pressure of existing technologies), which reduces equipment costs and has industrial application value.

[0038] This method for preparing high-expansion-ratio fluoropolymer foam improves the nucleation efficiency and density of the PVDF matrix by combining the addition of a nucleating agent with a low-temperature, high-pressure impregnation process. Furthermore, the invention utilizes irradiation crosslinking to induce crosslinking reactions between PVDF molecular chains, forming a network structure. This simultaneously enhances the melt strength of the microfoaming precursor, inhibits cell collapse and merging during the second foaming step, and produces a uniform and dense cell structure. It also effectively suppresses shrinkage of the PVDF-based foam material after foaming. Moreover, the second-step foaming process of this invention is relatively simple, with lower foaming pressure and less demanding requirements for foaming equipment, effectively reducing equipment investment costs and facilitating large-scale production.

[0039] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-expansion-ratio fluoropolymer foam, characterized in that, Includes the following steps: S1: A precursor is prepared by melt blending raw materials containing 90-100 parts of polyvinylidene fluoride (PVDF) and 0-5 parts of inorganic nucleating agent; S2: Supercritical fluid foaming treatment is performed on the precursor to obtain micro-foamed nucleated samples; S3: Irradiation crosslinking treatment of micro-foamed nucleated samples; S4: The irradiated sample is subjected to a secondary supercritical fluid foaming treatment to obtain fluoropolymer foam.

2. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In S1, the PVDF is homopolymer PVDF, copolymer PVDF, or a blend of both; the inorganic nucleating agent is selected from at least one of phosphate nucleating agents, aromatic amide nucleating agents, quaternary phosphonium salt nucleating agents, silicon dioxide, titanium dioxide, and calcium carbonate.

3. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 2, characterized in that: The inorganic nucleating agent has a particle size of 50 nm to 10 μm, and when a blended nucleating agent is used, the mass ratio of the phosphate nucleating agent to silica is 1:(0.5-2).

4. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 2, characterized in that: The PVDF has a melt index of 1-30 g / 10 min and the copolymer PVDF contains ≥85 mol of vinylidene fluoride monomer.

5. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In S1, the melt blending process is either injection molding or extrusion, and the processing temperature is 140-220℃. When using injection molding, the temperatures of the three zones of the injection molding machine are 190℃-210℃, and the mold temperature is 70-90℃. When using extrusion, the extruder screw temperature is 140-200℃, and the die head temperature is 190-210℃.

6. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In step S2, the supercritical fluid foaming process includes the following steps: The precursor is placed in the foaming chamber, and the temperature is controlled at 130-160℃. Control the immersion pressure to 10-30MPa, treat under constant temperature and pressure for 2-14 hours, and then release the pressure to normal pressure. Micro-foamed nucleation samples with a foaming ratio of 1.5-3 times were obtained.

7. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In S3, the irradiation crosslinking treatment is performed by electron beam irradiation with an irradiation dose of 10-80 kGy and a crosslinking degree controlled at 10-70%.

8. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 7, characterized in that: The equipment parameters for electron beam irradiation are as follows: equipment energy is 1.5-2.5 MeV, titanium pump current is 0.1-1 mA, and electron beam power is <100 kW.

9. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In step S4, the secondary supercritical fluid foaming includes the following steps: The irradiated sample was placed in the foaming chamber, and the temperature was controlled at 140-180℃. Control the immersion pressure to 0.5-10 MPa and treat at a constant temperature and pressure for 0.5-2 hours; Depressurize to atmospheric pressure.

10. The method for preparing a high-expansion-ratio fluoropolymer foam according to claim 1, characterized in that: In step S4, the average pore diameter of the fluororesin foam is 10-100 μm, and the pore density is ≥10. 6 pcs / cm 3 The foaming ratio is 5-50 times.

Images