High-melt-strength polypropylene blend and preparation method thereof

Abstract

The invention relates to the technical field of high polymer material modification, discloses a high-melt-strength polypropylene blend and a preparation method thereof, and aims to solve the problem that common polypropylene is low in melt strength and difficult to form a uniform and stable foaming structure. According to the invention, long-chain branch polypropylene is used as a main body and is blended with graft modified polypropylene prepared by grafting linear polypropylene with a polyfunctional group monomer, the long-chain branch polypropylene provides physical entanglement and strain hardening effects, and the graft modified polypropylene constructs intramolecular and intermolecular branched connection to form a quasi-three-dimensional network structure with synergistic physical entanglement and chemical crosslinking; the method is simple in process and low in cost, the prepared blend can effectively resist cell wall breakage in the foaming process, and the high-quality polypropylene foaming material with uniform and fine cells and low density is obtained.

Description

Technical Field

[0001] This invention relates to the field of polymer material modification technology, and in particular to a high melt strength polypropylene blend and its preparation method. Background Technology

[0002] Polypropylene (PP) foam materials are widely used in packaging, automotive, and building insulation due to their advantages such as being lightweight, heat-resistant, impact-resistant, and recyclable. However, ordinary linear polypropylene has low melt strength and poor melt elasticity, making it easy for the cell walls to break during the foaming process, which makes it difficult to form a uniform and fine pore structure, severely limiting the application of polypropylene materials in high-end foaming fields.

[0003] Existing technologies for improving the melt strength of polypropylene mainly employ synthesis, irradiation, or blending methods. For example, CN118184846A discloses a high melt strength propylene polymer and its preparation method. In the polymerization process, propylene is first polymerized with hydrogen using a polymerization catalyst to obtain polypropylene-I component. Then, a hydrogenation catalyst is added to generate ultra-high molecular weight polypropylene component. Although this method is effective, it has high technical barriers, complex processes, and high costs. CN110483829A discloses a method for preparing high-strength polypropylene foam beads using low irradiation intensity. Copolymerized polypropylene, glycidyl methacrylate, and a stabilizer are blended and granulated. After high-energy irradiation, chain extenders containing carboxyl / hydroxyl / amino groups and functional additives are added for secondary extrusion granulation. Finally, foam beads are prepared using a physical foaming agent in an intermittent batch process. This method can effectively improve the melt strength of polypropylene, but it may produce odors and degradation, thus affecting the stability of the product. Summary of the Invention

[0004] The technical problem to be solved by this invention is: how to develop a polypropylene blend with simple process, low cost and effective melt strength improvement and its matching modification method. To this end, we propose a high melt strength polypropylene blend and its preparation method.

[0005] To achieve the above objectives, this application adopts the following technical solution: a high melt strength polypropylene blend, composed of long-branched polypropylene, graft-modified polypropylene, free radical initiator and other additives, wherein the graft-modified polypropylene is prepared by grafting multifunctional monomers onto a linear polypropylene matrix, and the long-branched polypropylene and the graft-modified polypropylene interact during melt blending to form a quasi-three-dimensional network structure containing physical entanglement and chemical crosslinking, wherein the quasi-three-dimensional network structure is used to improve the melt strength and strain hardening effect of the blend.

[0006] Preferably, by weight percentage, it comprises 20%-50% long-branched polypropylene, 30%-60% graft-modified polypropylene, 0.01%-0.5% free radical initiator, and 0%-5% other additives.

[0007] Preferably, the melt index of the long-branched polypropylene is 0.5-5 g / 10min, the melt index of the linear polypropylene matrix is ​​1-50 g / 10min, and the grafting rate of the grafted modified polypropylene is 0.5%-2.5%.

[0008] Preferably, the long-branched polypropylene is a homopolymer long-branched polypropylene or a random copolymer long-branched polypropylene, wherein the isotacticity of the homopolymer long-branched polypropylene is ≥96%, and the ethylene content in the random copolymer long-branched polypropylene is 3-5 wt%.

[0009] Preferably, the multifunctional monomer is a monomer containing at least two unsaturated bonds that can participate in free radical reactions.

[0010] Preferably, the multifunctional monomer is selected from at least one of trimethylolpropane triacrylate, pentaerythritol triacrylate, and divinylbenzene.

[0011] Preferably, the multifunctional monomer is an unsaturated monomer containing hydroxyl, amide, carboxyl, sulfonic acid, or dialdehyde groups.

[0012] Preferably, the multifunctional monomer is selected from at least one of acrylamide, hydroxyethyl acrylate, acrylic acid, methacrylic acid, sodium styrene sulfonate, and monomers containing furanyl or maleimide groups.

[0013] Preferably, the free radical initiator is an organic peroxide, selected from dicumyl peroxide or 1,3-bis(tert-butylperoxyisopropyl)benzene.

[0014] A method for preparing a high melt strength polypropylene blend includes the following steps: S1: premixing a linear polypropylene matrix, a multifunctional monomer, a free radical initiator, and an antioxidant to obtain a premix; S2: subjecting the premix to melt reaction extrusion, granulation, and drying to obtain a graft-modified polypropylene masterbatch; S3: mixing the graft-modified polypropylene masterbatch, long-chain branched polypropylene, and other additives to obtain a blend; S4: subjecting the blend to melt blending, extrusion granulation, and obtaining a high melt strength polypropylene blend.

[0015] The technical effects and advantages of this invention are as follows: In this invention, long-branched polypropylene is used as the melt skeleton to provide physical entanglement and strain hardening effects. At the same time, multifunctional monomers are grafted onto linear polypropylene through reactive extrusion to form grafted modified polypropylene with chemical crosslinking points. After blending the two, the long branches and grafted branches entangle with each other and undergo chemical bridging, constructing a quasi-three-dimensional network structure that combines physical entanglement and chemical crosslinking. In the molten state, this structure can effectively restrict molecular chain slippage, improve melt strength and melt elasticity. The blend exhibits excellent tensile fracture resistance during foaming, achieving uniformity and stability of the cell structure. Meanwhile, the preparation process is simple and cost-controllable, making it suitable for the production of polypropylene foamed products with high melt strength requirements. Attached Figure Description

[0016] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts: Figure 1 This is a graph showing the change in melt strength of the polypropylene blend of the present invention as a function of stretching rate. Figure 2 This is a scanning electron microscope image of the cell structure of the polypropylene foam material in Example 1 of the present invention; Figure 3 This is a scanning electron microscope image of the cell structure of the polypropylene foam material of Comparative Example 1 of the present invention. Detailed Implementation

[0017] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention.

[0018] This invention provides a high melt strength polypropylene blend, the raw material components of which, by weight percentage, include 20%-50% long-chain branched polypropylene (LCB-PP), 30%-60% graft-modified polypropylene, 0.01%-0.5% free radical initiator, and 0%-5% other additives.

[0019] Specifically, the long-branched polypropylene can be divided into homopolymer long-branched polypropylene and random copolymer long-branched polypropylene according to the polymerization type. Both are prepared by metallocene catalytic polymerization process or post-modified free radical long-chain branching process, and the melt flow index (MFR) is uniformly controlled at 0.5-5 g / 10 min to adapt to the processing fluidity and melt strength requirements of the blend system.

[0020] The homopolymer long-branched polypropylene has an isotacticity of ≥96%, and its high stereoregularity makes its molecular chains more compact and the physical entanglement density between long branches is higher, which can provide a more stable melt skeleton support for the blend. The random copolymer long-branched polypropylene uses propylene as the main monomer and ethylene as the comonomer. Its ethylene content is controlled at 3-5wt%. The introduction of appropriate ethylene units can improve the flexibility of long-branched polypropylene, avoid the increase in brittleness caused by excessive rigidity of homopolymer long-branched polypropylene, and at the same time, do not damage the entanglement effect brought by long branches.

[0021] The long-branched polypropylene serves as the melt skeleton of the blend, and its core function is to provide basic melt strength and a significant strain hardening effect, providing key support for the stability of the cell structure during foaming. The long branches on the molecular chain can form dense physical entanglements with its own molecular chain and the grafted modified polypropylene molecular chain. In the molten state, these entanglements are like network nodes, which can restrict the free slip of the molecular chain, thereby improving the melt's ability to resist flow deformation. When the melt is subjected to stretching, the long branches will align in a direction of stretching, further increasing the entanglement density. The melt viscosity increases with the increase of tensile strain, i.e., a strain hardening effect is generated, thereby resisting the tensile rupture of the cell wall.

[0022] The grafted modified polypropylene uses linear polypropylene as the matrix, which has a melt index of 1-50 g / 10 min. It is obtained by grafting multifunctional monomers onto its molecular chain through a reactive extrusion process. The grafting rate is controlled between 0.5% and 2.5%. If the grafting rate is too low, the branched network will be incomplete and unable to form an effective synergistic effect. If the grafting rate is too high, gelation will occur, which will hinder processing. Neither of these conditions can achieve the expected technical effect.

[0023] The grafting monomer is a polyfunctional monomer containing at least two unsaturated bonds that can participate in free radical reactions, preferably one of trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), and divinylbenzene (DVB), used to construct intramolecular and intermolecular branched connections. To further enhance the interaction between molecular chains, the grafting monomer may also be an unsaturated monomer containing hydroxyl, amide, carboxyl, sulfonic acid, or dialdehyde groups, specifically one of acrylamide (AM), hydroxyethyl acrylate (HEA), acrylic acid (AA), methacrylic acid (MAA), sodium styrene sulfonate (SSNa), furanyl, or maleimide monomers.

[0024] When the grafted monomer is a polyfunctional monomer containing at least two unsaturated bonds that can participate in free radical reactions, it couples with the linear polypropylene molecular chain through unsaturated bonds to form a branched structure. It can also undergo directional addition reactions with active sites on the long branched polypropylene molecular chain, such as terminal double bonds and chain structure defects, to form intermolecular chemical bridges, connecting the dispersed long branched polypropylene backbone segments and constructing a quasi-three-dimensional branched network that combines physical entanglement and chemical cross-linking, thereby enhancing the melt elasticity and resistance to flow deformation. When the grafted monomer is an unsaturated monomer containing hydroxyl, amide, carboxyl, sulfonic acid, or dialdehyde groups, in addition to forming a basic branched structure through unsaturated bonds, its characteristic functional groups can also form through hydrogen bonds, ionic bonds with metal salts, or reversible covalent bonds, such as Zn. 2+ Salt, Ca 2+ Salt, together with polypropylene segments or additive molecules, forms a dynamic reversible network. During processing, this dynamic network dissociates and recombines with changes in shear force or temperature, reducing melt viscosity to ensure smooth processing and maintaining high melt strength after molding. During foaming, the high melt strength and strain hardening effect of the synergistic network can resist the tensile stress of cell expansion, preventing cell wall rupture and merging, while providing stable sites for foam nucleation and forming a uniform and dense cell structure.

[0025] The free radical initiator is used to trigger the grafting reaction of linear polypropylene with multifunctional monomers. It is preferably an organic peroxide whose decomposition temperature matches the processing temperature. Specifically, it can be selected from dicumyl peroxide (DCP) or 1,3-bis(tert-butylperoxyisopropyl)benzene (BIBP). Among them, DCP has a decomposition temperature of 175℃ and BIBP has a decomposition temperature of 180℃. The residual amount of both is extremely low and has no adverse effect on the material properties. At the extrusion processing temperature of 170-220℃, organic peroxide molecules break to generate free radicals. The free radicals attack the tertiary carbon atoms on the linear polypropylene molecular chain, making the polypropylene chain form active free radical sites. The unsaturated bonds of the multifunctional monomers undergo addition reactions with the active free radical sites of polypropylene, grafting the monomers onto the polypropylene molecular chain. At the same time, the remaining unsaturated bonds of some grafted monomers can crosslink with other active polypropylene chains to form a branched structure.

[0026] Other additives are added according to actual application requirements, including antioxidants, nucleating agents, lubricants and metal salts. It should be noted that metal salts are only added when the grafted monomer contains carboxyl or sulfonic acid groups.

[0027] The antioxidant is preferably a compound system of hindered phenolic antioxidant 1010 and phosphite antioxidant 168, with a mass ratio of 1:1 and a purity of ≥99%. Its function is to inhibit the thermal oxidative degradation of polypropylene during melt processing. The hindered phenolic antioxidant 1010 captures degradation free radicals, and the phosphite antioxidant 168 decomposes hydrogen peroxide. The two work together to terminate the free radical chain degradation reaction. The nucleating agent can be selected from one or more of talc, organophosphate and montmorillonite, wherein the talc particle size is ≤2μm and the organophosphate purity is ≥98%. As heterogeneous nucleation sites, it reduces the crystallization nucleation energy barrier, so that polypropylene forms more and finer grains during the cooling process, avoids stress concentration inside the material caused by large grains, and makes the distribution of foaming nucleation sites more uniform. The preferred lubricant is ethylene bis-stearamide (EBS), which reduces the frictional resistance during the melt processing of the blend, improves the processing fluidity, prevents the material from adhering to the equipment, and migrates to the material surface and the contact surface between the material and the equipment in the molten state to form a lubricating film, thereby reducing the internal friction between molecular chains and the external friction between the material and the screw and barrel. The metal salt can be selected from zinc oxide (ZnO), calcium stearate, or a combination thereof, with a particle size ≤5μm. It is added only when the grafted monomer contains carboxyl or sulfonic acid groups to promote the formation of ionic crosslinking sites. 2+ Ca 2+ It forms carboxylate / sulfonate ion pairs with carboxyl / sulfonic acid groups, enhancing interchain interconnection and optimizing the dynamic network structure.

[0028] This invention also provides a method for preparing a high melt strength polypropylene blend, specifically including the following steps: S1: Weigh the linear polypropylene matrix, multifunctional monomer, free radical initiator and antioxidant, put them into a high-speed mixer, and premix them at a speed of 1200-1500 rpm for 4-5 minutes to keep the materials uniformly dispersed and obtain the premix. S2: The premix is ​​added to a twin-screw extruder for melt reaction extrusion. The extrusion temperature range is controlled between 170-220℃, with zone 1 temperature set at 170-180℃, zone 2 at 180-195℃, zone 3 at 195-210℃, and zone 4 at 210-220℃. The screw speed is 200-250 r / min, and the material residence time is 1.8-2.5 min. After being pelletized by an underwater pelletizer, the pellets are sent to a vacuum drying oven and dried at 80℃ for 2 hours to remove moisture, thus obtaining grafted modified polypropylene masterbatch. S3: Weigh the grafted modified polypropylene masterbatch, long-chain branched polypropylene and other additives, put them into a high-speed mixer, mix at 1200 r / min for 3-5 min, and 5 min for metal salts to ensure that the metal salts and grafted groups are fully neutralized to obtain a mixture; S4: Add the mixture to a twin-screw extruder for melt blending and granulation. The extrusion temperature range is controlled between 170-220℃, with zone 1 temperature set at 175-185℃, zone 2 at 185-195℃, zone 3 at 195-205℃, and zone 4 at 205-220℃. The screw speed is 150-180 rpm, and the material residence time is 1.5-2 minutes. After cooling and pelletizing, the finished product is obtained.

[0029] It is important to note that in S2, if the temperature is too low, the initiator will not decompose sufficiently and the grafting reaction will be incomplete; if the temperature is too high, it will easily lead to polypropylene degradation and monomer volatilization.

[0030] To further verify the technical effect of the high melt strength polypropylene blend of the present invention, the technical solution and the achieved technical effect of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0031] Example 1 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 35% homopolymer long-branched polypropylene, 58% linear polypropylene, 1.5% trimethylolpropane triacrylate, 0.2% dicumyl peroxide, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.2% talc. This embodiment also provides a method for preparing a high melt strength polypropylene blend, which specifically includes the following steps: S1: Weigh linear polypropylene, grafted monomer, initiator and antioxidant, put them into a high-speed mixer, and premix at 1300 r / min for 4.5 min to obtain a premix; S2: The premix is ​​added to a twin-screw extruder for melt reaction extrusion. The extrusion temperature is set to 175℃ in zone 1, 188℃ in zone 2, 200℃ in zone 3, and 215℃ in zone 4. The screw speed is 220r / min and the material residence time is 2min. After underwater pelleting, it is dried in a vacuum drying oven at 80℃ for 2h to obtain grafted modified polypropylene masterbatch. S3: Weigh the grafted modified polypropylene masterbatch, homopolymer long-chain branched polypropylene, ethylene bis-stearamide and talc, put them into a high-speed mixer and mix at 1200 r / min for 4 min to obtain a mixture. S4: Add the mixture to a twin-screw extruder for melt blending and granulation. Set the extrusion temperature to 180℃ in zone 1, 190℃ in zone 2, 200℃ in zone 3, and 210℃ in zone 4. Set the screw speed to 160 r / min and the material residence time to 1.8 min. After cooling and pelletizing, the finished product is obtained.

[0032] Example 2 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 40% random copolymer long-branched polypropylene, 52% linear polypropylene, 2.0% acrylic acid, 0.3% 1,3-bis(tert-butylperoxyisopropyl)benzene, 0.4% antioxidant, 0.6% ethylene bis-stearamide, 2.7% zinc oxide, and 1.3% organophosphate. This embodiment also provides a method for preparing a high melt strength polypropylene blend, which differs from Example 1 in that, in S2, the temperature in zone four is increased to 220°C, and in S3, the mixing time is extended to 5 min.

[0033] Example 3 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 25% homopolymer long-chain branched polypropylene, 58% linear polypropylene, 1.8% methacrylic acid, 0.15% dicumyl peroxide, 0.3% antioxidant, 0.4% stearate, 2.35% calcium stearate, and 1.0% montmorillonite. This embodiment also provides a method for preparing a high melt strength polypropylene blend, the difference from Example 1 being that in S3, the mixing time is extended to 5 min.

[0034] Example 4 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 50% random copolymer long-branched polypropylene, 42% linear polypropylene, 1.2% divinylbenzene, 0.4% 1,3-bis(tert-butylperoxyisopropyl)benzene, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.1% organophosphate. This embodiment also provides a method for preparing a high melt strength polypropylene blend. The difference from Example 1 is that in S2, the screw speed is 230 r / min, the material residence time is 2.2 min, and the temperature in the fourth zone is adjusted to 198℃.

[0035] Example 5 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 20% homopolymer long-chain branched polypropylene, 58% linear polypropylene, 1.0% pentaerythritol triacrylate, 0.1% dicumyl peroxide, 0.2% antioxidant, 0.3% ethylene bis-stearamide, and 4.4% talc, with the remainder consistent with that of Example 1.

[0036] Example 6 This embodiment provides a high melt strength polypropylene blend, which, by weight percentage, comprises 38% random copolymer long-branched polypropylene, 50% linear polypropylene, 1.6% acrylamide, 0.25% 1,3-bis(tert-butylperoxyisopropyl)benzene, 0.3% antioxidant, 0.45% stearate, and 2.4% montmorillonite, with the remainder consistent with that of Example 1.

[0037] Comparative Example 1 This comparative example provides a polypropylene blend, which, by weight percentage, comprises 95% linear polypropylene, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.2% talc. This comparative example also provides a method for preparing a polypropylene blend, which differs from Example 1 in that the grafting modification steps S1 and S2 are omitted, and the raw materials are directly melt-blended and granulated according to steps S3 and S4 of Example 1.

[0038] Comparative Example 2 This comparative example provides a polypropylene blend, which, by weight percentage, comprises 93% linear polypropylene, 1.5% trimethylolpropane triacrylate, 0.2% dicumyl peroxide, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.5% talc, with the remainder being consistent with Example 1.

[0039] Comparative Example 3 This comparative example provides a polypropylene blend, which, by weight percentage, comprises 35% homopolymer long-branched polypropylene, 59.8% linear polypropylene, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.4% talc. This comparative example also provides a method for preparing a polypropylene blend, which differs from Example 1 in that the grafting modification steps S1 and S2 are omitted, and the raw materials are directly melt-blended and granulated according to steps S3 and S4 of Example 1.

[0040] Comparative Example 4 This comparative example provides a polypropylene blend, which, by weight percentage, comprises 35% homopolymer long-chain branched polypropylene, 58% linear polypropylene, 1.5% methyl methacrylate, 0.2% dicumyl peroxide, 0.3% antioxidant, 0.5% ethylene bis-stearamide, and 4.5% talc, with the remainder consistent with Example 1.

[0041] Test Example 1 Referring to ISO 16790:2005 "Plastics – Determination of tensile properties of melts of thermoplastics", the finished products of Examples 1 and 2 and Comparative Example 1 were selected. A capillary rheometer with a melt tensile testing device was used. The test temperature was set to 180℃, the capillary diameter to be 2mm, and the length-to-diameter ratio to be 20:1. After the melt was held for 5 minutes, it was stretched at a rate gradient of 0–20 m / min. The melt tensile force at different stretching rates was recorded, and a curve of melt strength versus stretching rate was plotted. The results are shown in [Figure 1]. Figure 1 As shown; The finished products of Examples 1-6 and Comparative Examples 1-4 were selected and tested under the same equipment and conditions. The peak melt strength and strain hardening index during the tensile process were tested. The results are shown in Table 1.

[0042]

[0043] Table 1 according to Figure 1 Data shows that the polypropylene molecular chains in Comparative Example 1 have a linear structure, lacking branching and entanglement. When stretched, the molecular chains are prone to slippage and cannot produce strain hardening effect. Therefore, the melt tensile force is low and does not increase. In Examples 1 and 2, the long branches of the long-branched polypropylene and the grafted modified polypropylene form a quasi-three-dimensional network of physical entanglement and chemical bridging. When the stretching rate increases, the entanglement density increases synchronously, showing obvious strain hardening. The melt tensile force increases with the increase of the rate, which can effectively support the cell expansion during the foaming process. According to the data in Table 1, the melt strength and strain hardening index of the examples are generally higher than those of the comparative examples. Comparative example 2 relies only on grafting and lacks the support of long-chain skeletons, resulting in a melt strength of less than 2N. Comparative example 3 relies only on the physical entanglement of long-chain polypropylene without chemical cross-linking reinforcement, resulting in a low strain hardening index. Comparative example 4 cannot form an effective branched network, has poor melt elasticity, and its performance is far inferior to that of the examples.

[0044] Test Example 2 The finished granules from Examples 1-6 and Comparative Examples 1-4 were molded using an injection molding foaming machine. The foaming agent was supercritical CO2, the injection pressure was 15 MPa, the barrel temperature was set to 185°C, the mold temperature was 40°C, and the holding time was 10 s. Foamed samples of 50 mm × 50 mm × 20 mm were prepared. Cross-sections of the foamed samples from Example 1 and Comparative Example 1 were taken, treated with gold sputtering, and the cell structure was observed using a scanning electron microscope. Figure 2 This is a SEM image of the bubble structure in Example 1. Figure 3 Here is a SEM image of the bubble structure in Comparative Example 1; Five SEM fields were randomly selected from the foamed samples of Examples 1-6 and Comparative Examples 1-4. The cell density and average cell diameter were statistically analyzed, and the apparent density was determined by the water displacement method. The results are shown in Table 2.

[0045]

[0046] Table 2 according to Figure 2 and Figure 3 The results showed that the cell structure of Example 1 was dense and uniform, which was due to the high melt strength network formed by long-branched polypropylene and grafted modified polypropylene. During foaming, this network could resist the tensile stress of cell expansion and maintain the integrity of the cell wall. In contrast, Comparative Example 1 had no long-branched polypropylene and graft modification, and the melt strength was insufficient. During the cell expansion process, the cell wall could not support the deformation, resulting in cell merging and rupture, and a loose structure. The data in Table 2 show that the strain hardening effect of high melt strength provides stable support for the foam cells. At the same time, the dynamic network constructed by graft modification, combined with the nucleating agent, increases the foam nucleation sites and refines the foam cells. Comparative Example 2 lacks a melt skeleton, Comparative Example 3 has no chemical cross-linking enhancement, and Comparative Example 4 has an ineffective branched network. None of them can form a dense and uniform foam structure, and their foaming performance is significantly worse than that of the examples.

[0047] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A high melt strength polypropylene blend, characterized in that, Composed of long-chain branched polypropylene, graft-modified polypropylene, free radical initiator and other additives, the graft-modified polypropylene is prepared by grafting multifunctional monomers onto a linear polypropylene matrix. The long-chain branched polypropylene and the graft-modified polypropylene interact during melt blending to form a quasi-three-dimensional network structure containing physical entanglement and chemical crosslinking. The quasi-three-dimensional network structure is used to improve the melt strength and strain hardening effect of the blend.

2. The high melt strength polypropylene blend according to claim 1, characterized in that: By weight percentage, it includes 20%-50% long-chain branched polypropylene, 30%-60% graft-modified polypropylene, 0.01%-0.5% free radical initiator, and 0%-5% other additives.

3. A high melt strength polypropylene blend according to claim 1 or 2, characterized in that: The melt index of the long-branched polypropylene is 0.5-5 g / 10min, the melt index of the linear polypropylene matrix is ​​1-50 g / 10min, and the grafting rate of the grafted modified polypropylene is 0.5%-2.5%.

4. A high melt strength polypropylene blend according to any one of claims 1-3, characterized in that: The long-branched polypropylene is either homopolymer long-branched polypropylene or random copolymer long-branched polypropylene. The isotacticity of the homopolymer long-branched polypropylene is ≥96%, and the ethylene content in the random copolymer long-branched polypropylene is 3-5 wt%.

5. A high melt strength polypropylene blend according to claim 1, characterized in that: The multifunctional monomer is a monomer containing at least two unsaturated bonds that can participate in free radical reactions.

6. A high melt strength polypropylene blend according to claim 6, characterized in that: The multifunctional monomer is selected from at least one of trimethylolpropane triacrylate, pentaerythritol triacrylate, and divinylbenzene.

7. The high melt strength polypropylene blend according to claim 1, characterized in that: The multifunctional monomer is an unsaturated monomer containing hydroxyl, amide, carboxyl, sulfonic acid, or dialdehyde groups.

8. A high melt strength polypropylene blend according to claim 1, characterized in that: The multifunctional monomer is selected from at least one of acrylamide, hydroxyethyl acrylate, acrylic acid, methacrylic acid, sodium styrene sulfonate, and monomers containing furanyl or maleimide groups.

9. A high melt strength polypropylene blend according to claim 1, characterized in that: The free radical initiator is an organic peroxide, selected from dicumyl peroxide or 1,3-bis(tert-butylperoxyisopropyl)benzene.

10. A method for preparing a high melt strength polypropylene blend, used to prepare the polypropylene blend as described in any one of claims 1-9, characterized in that, Includes the following steps: S1: Linear polypropylene matrix, multifunctional monomer, free radical initiator and antioxidant are premixed to obtain a premix; S2: The premix is ​​subjected to melt reaction extrusion, granulation and drying to obtain grafted modified polypropylene masterbatch; S3: The grafted modified polypropylene masterbatch, long-chain branched polypropylene and other additives are mixed to obtain a blend material; S4: The blended materials are melt-blended and extruded into granules to obtain a high melt strength polypropylene blend.

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