A high beta-crystal content polypropylene microcellular foam material and a method for preparing the same

Abstract

The application discloses a kind of polypropylene microcellular foaming material with high beta crystal content and a preparation method thereof.The nano calcium carbonate or partially organicized nano calcium-based montmorillonite is mixed with pimelic acid to obtain nano-particle loaded calcium pimelate, then polypropylene resin and nano-particle loaded calcium pimelate are mixed, extruded, granulated, and molded to obtain a sample to be foamed, the sample to be foamed is sealed in an autoclave, and a two-step temperature control foaming is used to obtain a polypropylene microcellular foaming material with high beta crystal content.The polypropylene microcellular foaming material prepared by the method has stable cell quality and a beta crystal content of more than 50%.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, specifically relating to a polypropylene microporous foam material with high β crystal content and its preparation method. Background Technology

[0002] Polypropylene (PP) foam materials have been widely used in construction, automotive, and packaging industries due to their advantages such as lightweight, excellent heat resistance, good processability, and economical cost. However, there is still considerable room for improvement in the cell uniformity, mechanical strength, and dimensional stability of existing PP foam materials. β-crystalline polypropylene (β-PP), due to its unique crystal morphology, typically exhibits superior toughness compared to the traditional α-crystalline form. Therefore, inducing the formation of the β-crystalline phase to prepare high-performance PP foam materials has become an important modification direction.

[0003] However, in existing technologies, conventional β-nucleating agents (such as inorganic compounds, dicarboxylate salts, and amide compounds) often exhibit poor dispersibility in polypropylene matrices, leading to insufficient efficiency and stability in induced crystallization. In contrast, supported β-nucleating agents (such as montmorillonite-supported calcium pimecronate and wollastonite-supported calcium pimecronate) demonstrate more efficient crystallization nucleation, inducing higher β-crystal content at lower addition levels. However, in processes such as extrusion foaming, injection foaming, and autoclaving, thermodynamically metastable β-crystals readily transform into α-crystals, resulting in a significant reduction in the β-crystal content of the final foamed product, making it difficult to fully leverage the toughening advantages of β-crystals. Furthermore, the function of supported β-nucleating agents in existing technologies is mainly focused on inducing crystallization, with limited promotion of bubble nucleation during foaming. This makes it difficult to achieve precise control over cell size, density, and distribution uniformity, posing a challenge in preparing high-performance PP foams with both high β-crystal content and a uniform, dense microporous structure.

[0004] Therefore, developing a new method to synergistically regulate the crystallization and cell nucleation processes and effectively stabilize the β-crystal phase during foaming, thereby preparing polypropylene foam materials with high β-crystal content, uniform microporous structure and excellent comprehensive properties, has significant technical value and application prospects. Summary of the Invention

[0005] The purpose of this invention is to provide a polypropylene microporous foam material with high β crystal content and its preparation method. The resulting foam material has high crystallization and nucleation efficiency of the nucleating agent and significant cell nucleation effect, resulting in an ideal cell structure and excellent mechanical properties.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A polypropylene microporous foam material with high β-crystal content has the following characteristics: β-crystal content greater than 50%, average cell size of 2~50μm, and cell density of 1×10⁻⁶.8 ~1×10 10 pcs / cm 3 .

[0007] The preparation method of the polypropylene microporous foam material with high β crystal content includes the following steps: (1) Mix and grind nano-calcium carbonate or partially organically modified nano-calcium-based montmorillonite with pimelic acid, and then heat and react to obtain nanoparticle-loaded calcium pimelic acid. (2) After mixing polypropylene resin with calcium pimecronate loaded with nanoparticles, the mixture is extruded and granulated to obtain a blend. The blend is then molded to obtain a sample to be foamed. (3) The obtained sample to be foamed is sealed in an autoclave, high-pressure carbon dioxide is injected at room temperature and the temperature is raised, and the temperature and pressure are maintained for a period of time to allow the CO2 to dissolve fully. Then the temperature is lowered and the pressure is released quickly to foam the polypropylene microporous foam material with high β crystal content.

[0008] Further, the partially organicated nano-calcium-based montmorillonite in step (1) is obtained by dispersing inorganic nano-calcium-based montmorillonite in deionized water, adding octadecyltrimethylammonium bromide, stirring and reacting for 3 hours to partially organicate it, and then filtering, drying and grinding it.

[0009] Furthermore, the amount of octadecyltrimethylammonium bromide added is calculated to be 0.7 to 1.4 times the cation exchange capacity (CEC) of the inorganic nano-calcium-based montmorillonite.

[0010] Furthermore, the mass ratio of nano-calcium carbonate or partially organicated nano-calcium-based montmorillonite to pimelic acid used in step (1) is 5:1 to 10:1.

[0011] Furthermore, the heating reaction in step (1) is carried out at a temperature of 110~120℃ for 1~2h.

[0012] Further, the polypropylene resin in step (2) is one or more of homopolymer polypropylene, block copolymer polypropylene or random copolymer polypropylene, and its melt index is 2~10g / 10min (230℃, 2.16kg).

[0013] Furthermore, based on the mass of polypropylene resin, the amount of calcium pimecronate loaded with nanoparticles added in step (2) is 0.2-2%. Nano-montmorillonite and nano-calcium carbonate have large specific surface areas and high surface activity, which can effectively load and disperse calcium pimecronate, and they can also serve as bubble nucleation sites.

[0014] Furthermore, a compatibilizer was added during mixing in step (2), which can improve the compatibility between the nanoparticles and the polypropylene matrix.

[0015] Furthermore, the compatibilizer is preferably maleic anhydride-grafted polypropylene (PP-g-MAH). Based on the mass of the polypropylene resin, the amount of compatibilizer added is 0.4-4%.

[0016] Furthermore, an antioxidant was added during the mixing process in step (2) to prevent thermal oxidative degradation during processing.

[0017] Furthermore, based on the quality of the polypropylene resin, the amount of antioxidant added is 0.1~1%.

[0018] Furthermore, the operating temperature of the compression molding in step (2) is 185~195℃, the operating pressure is 5~10 MPa, and the operating time is 5~10 min.

[0019] Furthermore, the temperature for heat preservation and pressure maintenance in step (3) is 175~185℃, the pressure is 5~15MPa, and the maintenance time is 20~120 minutes, so as to ensure that carbon dioxide is fully dissolved and diffused.

[0020] Furthermore, in step (3), the temperature is lowered to 115~120℃.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses nanoparticles loaded with calcium pimecronate to enable them to simultaneously function as both β-crystal nucleating agents and pore nucleating agents in a polypropylene matrix, thereby significantly improving nucleation efficiency through interfacial synergy and solving the problem of low crystallization induction efficiency without nucleating agents.

[0022] 2. This invention uses a two-step temperature-controlled foaming process with supercritical carbon dioxide. First, carbon dioxide is fully dissolved under high temperature and high pressure, and then the temperature is lowered to a specific temperature range for depressurization foaming. The synergistic effect of this process and the nanoparticle-loaded calcium pimedate can effectively inhibit the transformation of β crystals to α crystals during the foaming process, so that the β crystals in the final product are >50%, and the high β crystals are effectively retained in the foamed product.

[0023] 3. This invention achieves a microporous structure with small, uniformly distributed pores and high pore density through the synergistic effect of β-crystal stabilization and pore control. The average pore size is 2~50μm, and the pore density is 1×10⁻⁶. 8 ~1×10 10 pcs / cm 3 The uniformity and density of the foam cells are significantly better than those of conventional foaming systems, which is the physical basis for materials to obtain excellent mechanical properties (such as toughness and energy absorption) and functional properties (such as heat insulation and sound insulation). Attached Figure Description

[0024] Figure 1The image shows a scanning electron microscope (SEM) image of the polypropylene microporous foam material prepared in Example 1, which demonstrates that it has a uniform and fine pore structure.

[0025] Figure 2 The image shows a SEM image of the polypropylene foam material prepared in Comparative Example 1, which demonstrates that the foam cells are large and uneven.

[0026] Figure 3 The figure shows a comparison of the differential scanning calorimetry (DSC) heating curves of the polypropylene foam materials obtained in the examples and comparative examples. The DSC curve of the material prepared in the examples shows a significant β-crystal melting peak at about 150~160℃, while the β-crystal melting peak of the material prepared in the comparative examples is significantly weakened or even disappears. This proves that the present invention can significantly increase the relative content of β-crystals in polypropylene foam materials through the synergistic effect of nanoparticle-loaded calcium pimecrolate and precise foaming process. Detailed Implementation

[0027] A polypropylene microporous foam material with high β-crystal content is prepared by the following steps: (1) Mix and grind nano-calcium carbonate or partially organicated nano-calcium-based montmorillonite with pimelic acid at a mass ratio of 5:1 to 10:1, and then heat and react at 110 to 120°C for 1 to 2 hours to obtain nanoparticle-loaded calcium pimelic acid. (2) After mixing polypropylene resin with nanoparticle-loaded calcium pimecronate, compatibilizer and antioxidant, the mixture is extruded and granulated to obtain a blend. The blend is then molded at 185~195℃ and 5~10 MPa for 5~10 min to obtain a foaming sample. The amount of nanoparticle-loaded calcium pimecronate added is 0.2~2%, the amount of compatibilizer added is 0.4~4%, and the amount of antioxidant added is 0.1~1%, based on the mass of polypropylene resin.

[0028] (3) The obtained sample to be foamed is sealed in an autoclave, and high-pressure carbon dioxide is injected at room temperature and then heated. When the temperature is 175~185℃ and the pressure is 5~15MPa, the temperature and pressure are maintained for 20~120 minutes. Then the temperature is slowly reduced to 115~120℃, and then the pressure is quickly released to foam, so as to obtain the polypropylene microporous foam material with high β crystal content.

[0029] The polypropylene resin is one or more of homopolymer polypropylene, block copolymer polypropylene, or random copolymer polypropylene, and its melt index is 2~10g / 10min (230℃, 2.16kg).

[0030] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0031] The organic calcium-based montmorillonite used in the examples was prepared by ultrasonically dispersing 10 parts by weight of calcium-based montmorillonite in 350 parts by weight of deionized water, and then adding 1.4 CEC of octadecyltrimethylammonium bromide based on the cation exchange capacity (CEC) of calcium-based montmorillonite. After ultrasonic dispersion and mechanical stirring for 3 hours, the mixture was filtered, dried and ground to obtain the organic calcium-based montmorillonite. Example 1

[0032] (1) Mix 10 parts by weight of organic calcium-based montmorillonite with 1 part by weight of pimelic acid and grind thoroughly. Then heat and react in an oven at 120°C for 1 hour to obtain calcium pimelic acid supported on montmorillonite nanoparticles (CaHA-MMT).

[0033] (2) 100 parts by weight of polypropylene (PP, block copolymer, k8003, MI=2.5g / 10min), 2 parts by weight of CaHA-MMT, 2 parts by weight of maleic anhydride grafted polypropylene (PP-g-MA, compatibilizer) and 0.2 parts by weight of antioxidant 1010 are premixed, melt-blended in a twin-screw extruder (temperature 170-205℃) and then granulated to obtain modified PP granules.

[0034] (3) The modified PP granules were molded on a hot press at 190°C and 10 MPa for 5 minutes. After cold pressing and shaping, a plate with a thickness of about 2 mm was obtained. Then it was cut into sample pieces with a size of 10 mm × 10 mm as samples to be foamed.

[0035] (4) Place the sample to be foamed into the autoclave, fill it with CO2 at room temperature to 4 MPa, and then heat it to 180°C at a rate of 5 °C / min. At this time, the pressure is about 5 MPa. Keep it at this temperature and pressure for 20 min. Then, cool the system to 115°C at a rate of 5 °C / min. After the temperature stabilizes, quickly open the pressure relief valve to allow the sample to complete the foaming in the autoclave and obtain polypropylene foam material.

[0036] Performance testing: Differential scanning calorimetry (DSC) analysis showed that the β-crystal content of the obtained foamed material was 81%. Scanning electron microscopy (SEM) analysis showed that the average cell diameter was 8 μm, the standard deviation of the cell size was 4 μm, and the cell density was approximately 3.3 × 10⁻⁶. 9 pcs / cm 3 . Example 2

[0037] In step (2), the amount of CaHA-MMT is 0.6 parts by weight and the amount of PP-g-MAH is 1.2 parts by weight. Other operations are the same as in Example 1 to obtain polypropylene foam material.

[0038] Performance testing: The β-crystal content of the foamed material is 73%, the average cell diameter is 18 μm, the standard deviation of cell size is 8 μm, and the cell density is approximately 3.6 × 10⁻⁶. 9 pcs / cm 3 . Example 3

[0039] After CO2 is introduced in step (3), the temperature is raised to 120°C and the pressure is 5MPa for heat preservation and pressure maintenance. Other operations are the same as in Example 2 to obtain polypropylene foam material.

[0040] Performance testing: The β-crystal content of the foamed material is 68%; the average cell diameter is 5 μm, the standard deviation of cell size is 2 μm, and the cell density is approximately 5.5 × 10⁻⁶. 9 pcs / cm 3 .

[0041] Comparative Example 1 In step (2), the 2 parts by weight of CaHA-MMT used were replaced by a mixture of 1.8 parts by weight of organic calcium-based montmorillonite and 0.2 parts by weight of pimelic acid. Other operations were the same as in Example 1, and polypropylene foam material was obtained.

[0042] Performance testing: The β-crystal content of the foamed material is 21%; the average cell diameter is 72 μm, the standard deviation of cell size is 79 μm, and the cell density is approximately 3.5 × 10⁻⁶. 7 pcs / cm 3 It is evident that when a mixture of organic calcium-based montmorillonite and calcium pimecronate is used directly, the β-crystal content and cell quality of the foaming material are significantly lower than those achieved using CaHA-MMT.

[0043] Comparative Example 2 After CO2 is introduced in step (3), the temperature is raised to 130°C. Other operations are the same as in Example 2 to obtain polypropylene foam material.

[0044] Performance testing: The β-crystal content of the foamed material is 13%, the average cell diameter is 173 μm, the standard deviation of cell size is 76 μm, and the cell density is approximately 1.9 × 10⁻⁶. 6 pcs / cm 3 The β-crystal content and cell quality of the foamed material prepared at this foaming temperature were significantly lower than those in Example 2.

[0045] Comparative Example 3 In step (2), 0.6 parts by weight of calcium pimecronate were used instead of CaHA-MMT, and other operations were the same as in Example 3 to obtain polypropylene foam material.

[0046] Performance testing: The β-crystal content of the foamed material is only 33%, the average cell diameter is 16μm, the standard deviation of cell size is 7μm, and the cell density is approximately 9.0×10⁻⁶. 8pcs / cm 3 Therefore, it can be seen that the β-crystal nucleation efficiency and bubble nucleation efficiency of directly using calcium pimecrolate are significantly lower than those of Example 3.

[0047] The first heating curve of the foamed sample was tested using DSC, and the melting peak areas of β-crystals and α-crystals were obtained by peak fitting. and The β crystal content (β) was calculated using formula (1). c ): (1), in , The enthalpy values ​​of melting for 100% crystalline α-crystals and β-crystals are reported in the literature as 177 J / g and 168.5 J / g, respectively.

[0048] After immersing the foamed sheet in liquid nitrogen and cooling for 2 hours, the cell structure was observed using a scanning electron microscope, and the cell density (N) was calculated using formula (2). c ): (2), Where n is the number of bubbles in area A measured in the scanning electron microscope image, A is the area measured in the scanning electron microscope image, and M is the magnification of the scanning electron microscope image.

[0049] The performance test results of the polypropylene foam materials in the above embodiments and comparative examples are shown in Table 1.

[0050] Table 1

[0051] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing a polypropylene microporous foam material with high β-crystal content, characterized in that, Includes the following steps: (1) Mix and grind nano-calcium carbonate or partially organicated nano-calcium-based montmorillonite with pimelic acid, and then heat and react to obtain nanoparticle-loaded calcium pimelic acid. (2) After mixing polypropylene resin with calcium pimecronate loaded with nanoparticles, the mixture is extruded, granulated, and molded to obtain the sample to be foamed. (3) The obtained sample to be foamed is sealed in an autoclave, high-pressure carbon dioxide is injected at room temperature and the temperature is raised, and the temperature and pressure are maintained for a period of time. Then the temperature is lowered and the pressure is released quickly to foam, so as to obtain the polypropylene microporous foam material with high β crystal content.

2. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, The mass ratio of nano-calcium carbonate or partially organicated nano-calcium-based montmorillonite to pimelic acid used in step (1) is 5:1 to 10:

1.

3. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1 or 2, characterized in that, The partially organicated nano-calcium montmorillonite was prepared by treating calcium montmorillonite with octadecyltrimethylammonium bromide.

4. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, The heating reaction in step (1) is carried out at a temperature of 110~120℃ for 1~2h.

5. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, Based on the mass of polypropylene resin, the amount of calcium pimecronate loaded with nanoparticles added in step (2) is 0.2-2%; the operating temperature of the compression molding is 185-195℃, the operating pressure is 5-10 MPa, and the operating time is 5-10 min.

6. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1 or 5, characterized in that, The polypropylene resin is one or more of homopolymer polypropylene, block copolymer polypropylene, or random copolymer polypropylene, and its melt index is 2~10g / 10min.

7. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, In step (2), compatibilizers and antioxidants were also added during mixing.

8. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, The temperature for heat preservation and pressure maintenance in step (3) is 175~185℃, the pressure is 5~15MPa, and the holding time is 20~120 minutes.

9. The method for preparing polypropylene microporous foam material with high β-crystal content according to claim 1, characterized in that, In step (3), the temperature is lowered to 115~120℃.

10. A polypropylene microporous foam material with high β-crystal content prepared by the method described in claim 1, characterized in that, The polypropylene microporous foam material has a β-crystal content greater than 50%, an average pore size of 2~50μm, and a pore density of 1×10⁻⁶. 8 ~1×10 10 pcs / cm 3 .

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