A polyolefin foamed bead and a method for producing the same, a foamed article

Abstract

The application provides a polyolefin foamed bead and a preparation method and a foamed product thereof, and belongs to the technical field of foamed materials. The preparation method of the polyolefin foamed bead comprises the following steps: mixing polyethylene, an alpha-olefin elastomer and a crosslinking agent, and then adding the mixture into an open mill to perform melt extrusion and granulation, so as to obtain a polyolefin composite; mixing the polyolefin composite with polypropylene, adding a supercritical fluid in a plasticizing stage of a first-order extruder, and adopting a cooling process in a second-order extruder, so as to obtain the polyolefin foamed bead. The polyolefin foamed bead has low forming pressure, high heat resistance and high foaming ratio.

Description

Technical Field

[0001] This application relates to the field of foamed materials technology, specifically to a polyolefin foamed bead and its preparation method, and foamed products. Background Technology

[0002] Expanded polypropylene (EPP) molded beads, with their ultra-light density and excellent impact resistance, are widely used in automotive cushioning components, cushioning packaging, cold chain transportation, furniture, toys, and many other fields. With the expansion of applications, EPP molded products face direct market competition from EPS molded products, necessitating further reductions in processing costs. The processing costs of EPP molded products include the manufacturing cost of EPP beads, the transportation costs of EPP beads and molded products, and the molding cost of EPP beads. With the localization of PP resin production and the expansion of the application market, the selling price of PP resin for autoclaving foaming has gradually decreased and reached a low level, with limited room for further price reductions. Currently, the main factor affecting the cost of EPP molded products is the steam molding cost of polypropylene foam beads.

[0003] The PP resin used to manufacture EPP is generally a binary or terpolymer of propylene and α-olefins other than propylene. The presence of α-olefins can reduce the crystallinity and melting point of homopolymer PP, such as from 165°C for homopolymer PP to 140-145°C for copolymer PP. However, if the α-olefin content is further increased, the melting point of the copolymer will drop significantly to 120°C, or even below 100°C. Although this can reduce the steam molding cost of polypropylene foam beads, the foamed products are difficult to meet the requirements for heat resistance.

[0004] Blending copolymerized PP resin with polyethylene and polyolefin elastomers can reduce the crystallinity of the blend and provide low-melting-point components, potentially lowering the molding temperature of EPP beads. However, PP, polyethylene, and polyolefin elastomers are all non-polar polymers, and there are no suitable compatibilizers to significantly improve the compatibility between PP and α-olefin elastomers. Therefore, it is difficult to achieve blending of copolymerized PP with high-content polyethylene and polyolefin elastomers, making it challenging to reduce the steam molding temperature of EPP beads through blending with polyethylene and polyolefin elastomers. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a polyolefin foamed bead, a method for preparing the same, and foamed products.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows: Firstly, a method for preparing polyolefin foamed beads is provided, comprising the following steps: S1: After mixing polyethylene, α-olefin elastomer and crosslinking agent, the mixture is added to a two-roll mill and melt-extruded to obtain a polyolefin composite. S2: After mixing the polyolefin composite, polypropylene and additives obtained in step S1, a premix is ​​obtained; S3: The premix obtained in step S2 is added to a first-stage extruder for mixing, melting, and plasticizing to obtain a homogeneous mixture of the premix and supercritical fluid; the homogeneous mixture is added to a second-stage extruder for mixing and extrusion to obtain polyolefin composite microparticles; wherein, the supercritical fluid is injected during the plasticizing stage of the first-stage extruder, the mass content of the supercritical fluid in the melt is 0.5~3.0%, and the temperature of the second-stage extruder from the feed inlet to the die head shows a decreasing trend; S4: The polyolefin composite microparticles obtained in step S3 are impregnated in a pressure vessel with supercritical fluid at a pressure of 10-50 MPa, and then the pressure is released to atmospheric pressure to obtain polyolefin foamed beads.

[0007] In some embodiments, the temperature of the second-stage extruder from the feed inlet to the die is set to decrease from 200°C to 170°C.

[0008] In some embodiments, the temperature of the two-stage extruder from the feed inlet to the die is set as follows: Zone 1 temperature: 185~195℃, Zone 2 temperature: 175~185℃, Zone 3 temperature: 172~175℃, and Die temperature: 170~172℃.

[0009] In some embodiments, the ratio of the melt index of the polypropylene at 230°C and 2.16 kg to the melt index of the α-olefin elastomer at 190°C and 2.16 kg is (1:0.15) to (1:4.5).

[0010] In some embodiments, the polyolefin composite comprises, by weight, 40-55 parts of polyethylene, 20-40 parts of α-olefin elastomer, and 0.005-0.05 parts of crosslinking agent.

[0011] In some embodiments, the polyolefin composite further includes 0.5 to 5 parts by weight of nanoparticles.

[0012] In some embodiments, the premix comprises, by weight, 40-60 parts of polypropylene, 25-40 parts of the polyolefin composite, and 0.5-6.5 parts of additives.

[0013] In some implementations, at least one of the following (A) to (D) is satisfied: (A) The nanoparticles include at least one of layered silicates, fumed silica, and calcium carbonate; (B) At least one of the following: ethylene-octene copolymer, ethylene-butene copolymer, ethylene propylene rubber, and ethylene propylene diene monomer (EPDM) rubber; (C) The crosslinking agent is a peroxide crosslinking agent; (D) The supercritical fluid is at least one of supercritical carbon dioxide, supercritical nitrogen, and supercritical propane.

[0014] Secondly, a polyolefin foamed bead is provided, which is prepared by the method for preparing polyolefin foamed beads.

[0015] Thirdly, a foamed article is provided, the material of which includes the aforementioned polyolefin foamed beads.

[0016] Fourthly, a method for preparing the foamed product is provided, comprising the following steps: placing polyolefin foamed beads in a finished product mold and steam molding to obtain the foamed product; wherein the steam molding pressure is 1.0~1.6 bar.

[0017] Compared with the prior art, the beneficial effects of this application are as follows: 1. This application successfully solves the core contradiction in the field of polyolefin foamed beads by adding a polyolefin complex to polypropylene, adding supercritical fluid in the plasticizing stage of the first-stage extruder, and adopting a cooling process in the second-stage extruder. It also solves the long-standing problem of the difficulty in achieving both "low water vapor molding pressure (corresponding to low molding temperature)" and "high heat resistance and high foaming ratio". 2. When the polyolefin foam beads of this application are steam foamed, the steam molding pressure is significantly reduced from the conventional 1.8-2.6 bar to 1.0-1.6 bar, a reduction of 30%-50%; this significantly reduces the processing energy consumption and cost of polyolefin foam beads. 3. The polyolefin foam beads of this application achieve low-pressure molding while maintaining the heat resistance and dimensional stability required for foamed products. The heat shrinkage rate of the foamed products is less than 2% after 1 hour at 70°C, which fully meets the requirements of mainstream applications. 4. The polyolefin foam beads of this application have good industrial compatibility. This technical approach can be smoothly connected with existing mainstream autoclaving and steam molding equipment and processes, making it easy to achieve large-scale production and promotion. Attached Figure Description

[0018] Figure 1 This is a photograph of the polyolefin foam beads used in this application. Detailed Implementation

[0019] To facilitate understanding of this application, a more complete description will be provided below. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0020] As used in this article: "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0021] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0022] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1-5” is disclosed, the described range should be interpreted as including ranges “1-4”, “1-3”, “1-2”, “1-2 and 4-5”, “1-3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0023] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0024] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has 'a' parts by mass and component B has 'b' parts by mass, it means that the mass ratio of component A to component B is a:b. It is important to understand that, unlike mass percentage content, the sum of the mass parts of all components is not limited to 100 parts.

[0025] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0026] A first aspect of this application provides a method for preparing polyolefin foamed beads, comprising the following steps: S1: After mixing polyethylene, α-olefin elastomer and crosslinking agent, the mixture is added to a two-roll mill and melt-extruded to obtain a polyolefin composite. S2: After mixing the polyolefin composite, polypropylene and additives obtained in step S1, a premix is ​​obtained; S3: The premix obtained in step S2 is added to a first-stage extruder for mixing, melting, and plasticizing to obtain a homogeneous mixture of the premix and supercritical fluid; the homogeneous mixture is added to a second-stage extruder for mixing and extrusion to obtain polyolefin composite microparticles; wherein, the supercritical fluid is injected during the plasticizing stage of the first-stage extruder, the mass content of the supercritical fluid in the melt is 0.5~3.0%, and the temperature of the second-stage extruder from the feed inlet to the die head shows a decreasing trend; S4: The polyolefin composite microparticles obtained in step S3 are impregnated in a pressure vessel with supercritical fluid at a pressure of 10-50 MPa, and then the pressure is released to atmospheric pressure to obtain polyolefin foamed beads.

[0027] Although the mechanism is not yet clear, the applicant unexpectedly discovered that mixing polyethylene (PE), α-olefin elastomer (POE), and a crosslinking agent, followed by melt blending and extrusion granulation using an open mill, yields a polyolefin composite with a micro-crosslinked structure. When the polyolefin composite is blended with polypropylene, the polyolefin composite provides a uniform and stable low-melting-point "island phase." The micro-crosslinked structure of the polyolefin composite achieves morphological stability of the "island phase" itself and interfacial compatibilization with the polypropylene (PP) "sea phase." Injecting supercritical fluid (SCF) during the plasticizing stage of a first-stage extruder reduces melt viscosity, increases free volume, and enhances plasticization, achieving initial homogenization of the melt under low shear stress. The homogeneous mixture is stably pumped to the second-stage extruder. SCF generates a strong "cavitation" effect and turbulence in the mixing section of the second-stage extruder, providing an extremely high local shear rate, thereby breaking the dispersed phase of the polyolefin composite to the micron or even submicron scale. Subsequently, through the cooling process of the screw, SCF is gradually and controllably released, while the already formed ultrafine phase morphology is "frozen". Uniform polyolefin composite microparticles are obtained by underwater pelletizing.

[0028] Polyolefin composite microparticles are impregnated in a pressure vessel with supercritical fluid at a pressure of 10-50 MPa. Then, by rapidly depressurizing, the thermodynamic instability of the polymer / fluid homogeneous system is utilized to induce cell nucleation and growth, resulting in polyolefin foamed beads. Step S3 forms polyolefin composite microparticles with an ultrafine and stable two-phase structure, providing a large number of uniform heterogeneous nucleation sites for foaming. This gives the polyolefin foamed beads the characteristics of fine pores, high foaming ratio (usually up to 20-50 times), and high closed-cell ratio.

[0029] Specifically, α-olefin elastomer (POE), as a low-melting-point flexible component, can reduce the molding temperature of polyolefin foam beads. During the preparation of polyolefin foam beads, the crosslinking agent induces slight crosslinking of the PE / POE matrix to form a micro-crosslinking network, thereby significantly enhancing the morphological stability of the dispersed phase of the polyolefin composite during subsequent high-temperature blending and foaming processes, preventing it from being excessively sheared and broken or undergoing phase merging.

[0030] Although the mechanism is not yet clear, the applicant unexpectedly discovered that when the mass content of supercritical fluid in the melt is within the above-mentioned range, the dispersed phase size of the polyolefin composite can be stabilized within the range of 1-5 μm, which is beneficial to improving the overall performance of polyolefin foam beads. If the mass content of supercritical fluid in the melt is less than 0.5%, the effect of supercritical fluid in reducing melt viscosity and increasing melt pressure is insufficient, and the dispersed phase size of the polyolefin composite is too large, resulting in a decrease in the overall performance of polyolefin foam beads. If the mass content of supercritical fluid in the melt is greater than 3%, it will cause premature foaming of the melt in the second-stage extruder, resulting in extrusion fluctuations and residual bubbles in the polyolefin composite microparticles, affecting the uniformity of the polyolefin composite microparticles in autoclave foaming.

[0031] Specifically, in step S3, the mass content of supercritical fluid in the melt can be a range of one or any two of the following: 0.5%, 0.7%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, and 3.0%; preferably 0.8 to 2.5%, more preferably 1.2 to 2.0%.

[0032] In some embodiments, the temperature of the second-stage extruder from the feed inlet to the die is set to decrease from 200°C to 170°C.

[0033] In some embodiments, the temperature of the two-stage extruder from the feed inlet to the die is set as follows: Zone 1 temperature: 185~195℃, Zone 2 temperature: 175~185℃, Zone 3 temperature: 172~175℃, and Die temperature: 170~172℃.

[0034] Specifically, Zone 1 is the pre-mixing section, Zone 2 is the mid-conveying section, and Zone 3 is the final metering section. Although the mechanism is not yet clear, the applicant unexpectedly discovered that: when the temperature from the feed inlet to the die in the second-stage extruder is within the above-mentioned range, it can further ensure the stable removal of supercritical fluid and the solidification of the phase size and phase distribution state of polyolefin in polypropylene; if the die temperature is higher than 180°C, the supercritical fluid escapes too quickly, which may form bubbles and the phase structure is prone to rebound and coarsening; if the die temperature is lower than 160°C, the melt viscosity increases sharply, which may lead to excessive screw torque, extrusion difficulties, and incomplete escape of SCF.

[0035] In some embodiments, in step S1, the temperature of melt extrusion is 130~160°C, for example, it can be a range of one or any two of 130°C, 133°C, 135°C, 138°C, 140°C, 143°C, 145°C, 148°C, 150°C, 152°C, 155°C, 157°C, and 160°C.

[0036] In step S1, the temperature of melt extrusion is within the above range, which can prevent the crosslinking agent from decomposing prematurely and in large quantities, thereby obtaining a polyolefin composite with uniform composition and micro-crosslinked structure.

[0037] In some embodiments, the polyolefin composite comprises, by weight, 40-55 parts of polyethylene, 20-40 parts of α-olefin elastomer, and 0.005-0.05 parts of crosslinking agent.

[0038] In this application, the crosslinking agent, within the above-mentioned weight range, can cause micro-crosslinking of PE / POE, thus avoiding excessive crosslinking of PE / POE and resulting in processing difficulties.

[0039] Specifically, the weight parts of polyethylene can be a range of 40 parts, 43 parts, 45 parts, 47 parts, 49 parts, 51 parts, 53 parts, 55 parts or any combination of both.

[0040] Specifically, the weight parts of the α-olefin elastomer can be within the range of one or any combination of 20 parts, 22 parts, 24 parts, 26 parts, 28 parts, 30 parts, 32 parts, 34 parts, 36 parts, 38 parts, and 40 parts.

[0041] Specifically, the weight parts of the crosslinking agent can be one or any combination of 0.005 parts, 0.007 parts, 0.01 parts, 0.015 parts, 0.02 parts, 0.025 parts, 0.03 parts, 0.035 parts, 0.04 parts, 0.045 parts, and 0.05 parts.

[0042] In some embodiments, the polyolefin composite further includes 0.5 to 5 parts by weight of nanoparticles; for example, it can be a range of one or any combination of 0.5 parts, 0.7 parts, 1 part, 1.3 parts, 1.5 parts, 1.8 parts, 2 parts, 2.2 parts, 2.5 parts, 2.7 parts, 3 parts, 3.3 parts, 3.5 parts, 3.8 parts, 4 parts, 4.2 parts, 4.5 parts, 4.7 parts, and 5 parts.

[0043] In this application, nanoparticles can improve the melt strength of polyolefin composites, restrict the chain segment movement of α-olefin elastomers, and act as physical crosslinking points at the phase interface, further improving the morphological stability of the "island phase" and the interfacial compatibility with the PP "marine phase". The weight of nanoparticles within the above range can effectively exert their performance and is not prone to agglomeration.

[0044] In some embodiments, the particle size D50 of the nanoparticles is 50~500 nm, for example, it can be a range of one or any two of 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, and 500 nm; preferably 80~200 nm.

[0045] In some embodiments, the polyolefin composite further includes 0.5 to 1 part by weight of a lubricant. Adding a lubricant to the polyolefin composite can improve the dispersibility of the nanoparticles.

[0046] Specifically, the lubricant includes stearic acid.

[0047] In some embodiments, the ratio of the melt index of the polypropylene at 230°C and 2.16 kg to the melt index of the α-olefin elastomer at 190°C and 2.16 kg is (1:0.15) to (1:4.5); for example, it can be a range of values ​​consisting of one or any two of 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.2, 1:2.4, 1:2.6, 1:2.8, and 1:3.

[0048] In this application, the ratio of the melt index of polypropylene at 230°C and 2.16 kg to that of the α-olefin elastomer at 190°C and 2.16 kg is within the aforementioned range. This is beneficial for matching the melt viscosity of the continuous polypropylene phase and the low-melting-point dispersed phase containing the α-olefin elastomer, allowing the polyolefin composite to form a small-sized and uniformly distributed dispersed phase in the polypropylene. If the melt index of the α-olefin elastomer is too low, its viscosity will be too high, making it difficult to fully break down and disperse during blending. This can easily lead to an increase in the size of the dispersed phase and a reduction in cell nucleation points, resulting in a decrease in the foaming ratio and an increase in molding pressure. If the melt index of the α-olefin elastomer is too high, the viscosity of the dispersed phase will be too low, making it prone to flow, bubble formation, or phase merging during subsequent foaming and steam molding, leading to a decrease in cell stability and the heat resistance and dimensional stability of the product. Therefore, the above-mentioned melt index ratio can balance low molding pressure, high foaming ratio, and good heat resistance and dimensional stability.

[0049] In some embodiments, the melt index of the polypropylene at 230°C and 2.16 kg is 1 to 10 g / 10 min, for example, it can be a range of one or any two of 1 g / 10 min, 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, 9 g / 10 min, and 10 g / 10 min.

[0050] In some embodiments, the melt index of the α-olefin elastomer at 190°C and 2.16 kg is 0.5 to 15 g / 10 min, for example, it can be a range of one or any two of 1 g / 10 min, 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, 9 g / 10 min, and 10 g / 10 min.

[0051] In some embodiments, the melt index of polyethylene at 190°C and 2.16 kg is 1 to 10 g / 10 min, for example, it can be a range of one or any two of 1 g / 10 min, 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, 9 g / 10 min, and 10 g / 10 min.

[0052] In some embodiments, the premix comprises, by weight, 40-60 parts of polypropylene, 25-40 parts of the polyolefin composite, and 0.5-6.5 parts of additives.

[0053] In this application, polypropylene is used as the continuous phase of polyolefin foam beads, and the polyolefin composite is used as the dispersed phase. The weight percentage of the polyolefin composite affects the molding temperature of the polyolefin foam beads.

[0054] Specifically, the weight parts of polypropylene can be a range of 40 parts, 42 parts, 44 parts, 46 parts, 48 ​​parts, 50 parts, 52 parts, 54 parts, 56 parts, 58 parts, and 60 parts, or any combination of both.

[0055] Specifically, the weight parts of the polyolefin composite can be a range of 25 parts, 27 parts, 30 parts, 32 parts, 34 parts, 36 parts, 38 parts, 40 parts, or any combination thereof.

[0056] Specifically, the weight parts of the adjuvant can be a range of 0.5 parts, 0.7 parts, 1 part, 1.2 parts, 1.5 parts, 1.7 parts, 2 parts, 2.3 parts, 2.5 parts, 2.8 parts, 3 parts, 3.2 parts, 3.5 parts, 3.7 parts, 4 parts, 4.3 parts, 4.5 parts, 4.8 parts, 5 parts, 5.3 parts, 5.5 parts, 5.8 parts, 6 parts, 6.2 parts, and 6.5 parts, or any combination of both.

[0057] In some embodiments, the polypropylene is at least one of homopolymer polypropylene and copolymer polypropylene.

[0058] In some embodiments, the additives include at least one of color masterbatch and antioxidant.

[0059] In some embodiments, the nanoparticles include at least one of layered silicates, fumed silica, and calcium carbonate; the nanoparticles have better dispersibility in polyolefin foam beads, which is more conducive to the melt strength of the premix, restricts the movement of α-olefin elastomer chain segments, and acts as a physical crosslinking point at the phase interface, thereby improving the compatibility between polypropylene and other components.

[0060] In some embodiments, the polyethylene is at least one of low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE); LDPE and LLDPE have lower melting points and greater flexibility, which is more conducive to reducing the melting temperature of polyolefin foam beads.

[0061] In some embodiments, the α-olefin elastomer comprises at least one of ethylene-octene copolymer (POE), ethylene-butene copolymer (POP), ethylene propylene rubber (EPR), and ethylene propylene diene monomer (EPDM).

[0062] In some embodiments, the crosslinking agent is a peroxide crosslinking agent.

[0063] In some embodiments, the supercritical fluid is at least one of supercritical carbon dioxide, supercritical nitrogen, and supercritical propane.

[0064] A second aspect of this application provides a polyolefin foamed bead, which is prepared by the method for preparing polyolefin foamed beads.

[0065] A third aspect of this application provides a foamed article, the material of which includes the aforementioned polyolefin foamed beads.

[0066] A fourth aspect of this application provides a method for preparing the foamed product, comprising the following steps: placing polyolefin foamed beads in a finished product mold and steam molding them to obtain the foamed product; wherein the steam molding pressure is 1.0~1.6 bar.

[0067] The aforementioned polyolefin foam beads are placed in a mold cavity, and the beads are initially compressed using air pressure technology to make them tightly packed together. Then, low-temperature saturated steam is introduced for heating, softening, secondary expansion, and mutual fusion. After cooling and shaping, a three-dimensional foamed product is obtained. Because a large number of low-melting-point polyolefin composite dispersed phases are uniformly distributed in the polyolefin foam beads, these "island phases" soften and melt first when heated by steam, acting as "bonding points" and "energy concentration points," thereby significantly reducing the steam temperature and pressure required for good fusion between the polyolefin foam beads.

[0068] The molding pressure of traditional EPP beads is typically between 1.8 and 2.6 bar; the steam molding pressure of polyolefin foam beads prepared using the method described in this application can be significantly reduced to 1.0 to 1.6 bar. Preferably, the molding pressure is 1.2 to 1.5 bar. This reduction in molding pressure directly decreases steam energy consumption, improves production safety, and allows for the use of simpler molds and steam systems, resulting in a significant decrease in overall cost. Furthermore, although the steam molding temperature is lowered, the heat distortion temperature of the foamed product does not decrease proportionally because the melting point of the PP continuous phase itself is unaffected, and the nanoparticles and / or micro-crosslinked structures provide support and constraint for the dispersed phase. A heat shrinkage test conducted at 70°C for 1 hour showed that the heat shrinkage rate of the foamed product of this application is less than 2%, fully meeting the heat resistance requirements of most applications such as automotive interior parts and packaging materials. Preferably, the heat shrinkage rate is less than 1.5%.

[0069] This application, through the synergistic effect of a unique "nanocomposite compatibilizing masterbatch" design and a "supercritical fluid-assisted ultrafine dispersion" process, successfully solves the long-standing core contradiction in the field of polypropylene foam materials: the difficulty in simultaneously achieving "low molding temperature" and "high heat resistance and high foaming ratio." This results in significant comprehensive benefits: First, it fundamentally and substantially reduces molding energy consumption and costs, significantly lowering the steam molding pressure from the conventional 1.8-2.6 bar to 1.0-1.6 bar. First, the pressure reduction can reach 30%-50%. Second, while achieving low-pressure molding, it excellently maintains the necessary heat resistance and dimensional stability of the product. The heat shrinkage rate of its molded products is less than 2% after 1 hour at 70℃, fully meeting the requirements of mainstream applications. Third, it ensures excellent foaming processability. The high-shear, low-viscosity environment created by supercritical fluid allows for the uniform dispersion of high-content elastomers at the micro-nano scale. This not only provides abundant heterogeneous nucleation sites for foaming, thus producing high-ratio, fine-pore high-quality foamed beads, but also ensures the stability of the cell structure during the foaming process through the "morphology locking" effect of nanoparticles and trace cross-linking. Fourth, it has good industrial compatibility. This technology can be smoothly integrated with existing mainstream autoclaving and steam molding equipment and processes, making it easy to achieve large-scale production and promotion. Therefore, this application provides a practical and feasible innovative solution for preparing high-performance, low-cost polypropylene foamed products.

[0070] Specifically, the steam forming pressure can be a range of one or any combination of 1.0 bar, 1.1 bar, 1.2 bar, 1.3 bar, 1.4 bar, 1.5 bar, and 1.6 bar.

[0071] The raw materials used in the embodiments and comparative examples are described below, but are not limited to these materials: Polypropylene (PP): a random copolymer of propylene and ethylene, with a melt index (230℃, 2.16kg) of 2.4 g / 10min and a melting point of 163℃; the grade is Daploy WB135HMS, and the manufacturer is Borealis; Polyethylene (PE): Linear low-density polyethylene (LLDPE), melt index (190℃, 2.16kg) is 2 g / 10min; grade is TotalEnergies Q2018 C, manufacturer is TOTAL Petrochemicals, France; α-Olefin elastomer (POE-1): Ethylene-octene copolymer, melt index (190℃, 2.16kg) is 0.5 g / 10min; grade is Engage 8150, manufacturer is Dow Chemical Company, USA; α-Olefin elastomer (POE-2): Ethylene-octene copolymer, melt index (190℃, 2.16kg) is 5 g / 10min; grade is Engage 8200, manufacturer is Dow Chemical Company, USA; α-Olefin elastomer (POE-3): ethylene-octene copolymer, melt index (190℃, 2.16kg) is 13 g / 10min; grade is Engage 8137, manufacturer is Dow Chemical Company, USA; Nanoparticles: Organically modified montmorillonite (OMMT), with a particle size D50 of 100 nm; grade: Nanomer® I.44P, manufacturer: Sigma-Aldrich; Crosslinking agent: dicumyl peroxide (DCP), commercially available; Antioxidant: Antioxidant 1010, BASF Irganox® 1010.

[0072] Example 1 <Preparation of Polyolefin Foamed Beads> S1: Add 50 parts by weight of polyethylene, 30 parts by weight of POE-1, 2 parts by weight of nanoparticles and 0.02 parts by weight of crosslinking agent to a high-speed mixer and mix at 800 r / min for 5 min to obtain a first mixture; add the first mixture to a two-roll mill and melt-mix at 150°C for 8 min, then sheet, cool and granulate to obtain a polyolefin composite; S2: Add 30 parts by weight of the polyolefin composite obtained in step S1, 55 parts by weight of polypropylene, 4 parts by weight of color masterbatch and 0.1 parts by weight of antioxidant 1010 into a high-speed mixer and mix at 800 r / min for 5 min to obtain a premix. S3: The premix obtained in step S2 is added to a first-stage extruder for mixing, melting, and plasticizing to obtain a homogeneous mixture of the premix and supercritical fluid; the homogeneous mixture is added to a second-stage extruder for mixing, extrusion granulation, and polyolefin composite microparticles are obtained; wherein, the temperature of the first-stage extruder is: zone 1 160℃, zone 2 170℃, zone 3 180℃, zone 4 190℃, zone 5 195℃, and die head 195℃; the screw speed is 120 r / min; the supercritical fluid is injected into zone 3 during the plasticizing stage of the first-stage extruder, and the mass content of the supercritical fluid in the melt is 1.5%; the temperature of the second-stage extruder from the feed inlet to the die head is: zone 1 195℃, zone 2 185℃, zone 3 175℃, and die head 170℃; the screw speed is 80 r / min; S4: Place the polyolefin composite microparticles obtained in step S3 into a high-pressure foaming kettle, introduce CO2 to a pressure of 15 MPa and a temperature of 155℃, and impregnate for 4 hours under constant temperature and pressure; then depressurize to atmospheric pressure at a depressurization rate of 3 MPa / s, discharge, crush, and dry to obtain polyolefin foamed beads.

[0073] <Preparation of Foamed Products> Polyolefin foam beads are filled into a 200mm×200mm×20mm mold. Using a steam molding machine, the mold is first pressed at 0.5 bar air for 10 seconds, then saturated steam is introduced and the pressure is maintained at 1.3 bar gauge pressure for 15 seconds. After cooling, the foamed product is demolded.

[0074] Example 2 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 2%; the rest are the same as in Embodiment 1.

[0075] Example 3 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 1.2%; the rest are the same as in Embodiment 1.

[0076] Example 4 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 0.9%; the rest are the same as in Embodiment 1.

[0077] Example 5 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 2.5%; the rest are the same as in Embodiment 1.

[0078] Example 6 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 3%; the rest are the same as in Embodiment 1.

[0079] Example 7 The difference between this embodiment and Embodiment 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 0.5%; the rest are the same as in Embodiment 1.

[0080] Example 8 The difference between this embodiment and Example 1 is that the composition of the polyolefin composite in step S1 of the <Preparation of Polyolefin Foamed Beads> is different; the polyolefin composite in this embodiment includes the following components by weight: 45 parts of polyethylene, 35 parts of POE-1, 3 parts of nanoparticles and 0.03 parts of crosslinking agent; the rest are the same as in Example 1.

[0081] Example 9 The difference between this embodiment and Example 1 is that the composition of the polyolefin composite in step S1 of the <Preparation of Polyolefin Foamed Beads> is different; in this embodiment, the polyolefin composite includes the following components by weight: 40 parts of polyethylene, 40 parts of POE-1, 3 parts of nanoparticles and 0.02 parts of crosslinking agent; the rest are the same as in Example 1.

[0082] Example 10 The difference between this embodiment and Embodiment 1 is that the temperature from the feed inlet to the die head in step S3 of the <Preparation of Polyolefin Foamed Beads> is different; all other aspects are the same as in Embodiment 1. In this embodiment, the temperatures of the two-stage extruder from the feed inlet to the die are: Zone 1 185℃, Zone 2 180℃, Zone 3 175℃, and Die 170℃.

[0083] Example 11 The difference between this embodiment and Embodiment 1 is that the temperature from the feed inlet to the die head in step S3 of the <Preparation of Polyolefin Foamed Beads> is different; all other aspects are the same as in Embodiment 1. In this embodiment, the temperatures of the two-stage extruder from the feed inlet to the die are: Zone 1 200℃, Zone 2 180℃, Zone 3 170℃, and Die 165℃.

[0084] Example 12 The difference between this embodiment and Embodiment 1 is that the temperature from the feed inlet to the die head in step S3 of the <Preparation of Polyolefin Foamed Beads> is different; all other aspects are the same as in Embodiment 1. In this embodiment, the temperatures of the two-stage extruder from the feed inlet to the die are: Zone 1 205℃, Zone 2 185℃, Zone 3 175℃, and Die 170℃.

[0085] Example 13 The difference between this embodiment and Example 1 is that the weight proportions of nanoparticles and crosslinking agent in step S1 of the <Preparation of Polyolefin Foamed Beads> are different; all other aspects are the same as in Example 1. In this embodiment, the nanoparticles are 0 parts by weight and the crosslinking agent is 0.04 parts by weight.

[0086] Example 14 The difference between this embodiment and Example 1 is that the weight proportions of nanoparticles and crosslinking agent in step S1 of the <Preparation of Polyolefin Foamed Beads> are different; all other aspects are the same as in Example 1. In this embodiment, the nanoparticles are 0.05 parts by weight and the crosslinking agent is 0.04 parts by weight.

[0087] Example 15 The difference between this embodiment and Example 1 is that the weight proportions of nanoparticles and crosslinking agent in step S1 of the <Preparation of Polyolefin Foamed Beads> are different; all other aspects are the same as in Example 1. In this embodiment, the nanoparticles are 4 parts by weight and the crosslinking agent is 0.02 parts by weight.

[0088] Example 16 The difference between this embodiment and Example 1 is that the weight proportions of nanoparticles and crosslinking agent in step S1 of the <Preparation of Polyolefin Foamed Beads> are different; all other aspects are the same as in Example 1. In this embodiment, the nanoparticles are 5 parts by weight and the crosslinking agent is 0.01 parts by weight.

[0089] Example 17 The difference between this embodiment and Example 1 is that POE-2 is used instead of POE-1 in step S1 of the <Preparation of Polyolefin Foamed Beads>; the rest are the same as in Example 1.

[0090] Example 18 The difference between this embodiment and Example 1 is that POE-3 is used instead of POE-1 in step S1 of the <Preparation of Polyolefin Foamed Beads>; the rest are the same as in Example 1.

[0091] Comparative Example 1 The difference between this comparative example and Example 1 is that step S1 is not included in the <Preparation of Polyolefin Foamed Beads>, and step S2 is different; the rest are the same as in Example 1. Step S2 of this comparative example is as follows: 50 parts by weight of polyethylene, 30 parts by weight of POE-1, 2 parts by weight of nanoparticles and 0.02 parts by weight of crosslinking agent were added to a high-speed mixer and mixed at 800 r / min for 5 min to obtain the first mixture. 30 parts by weight of the first mixture, 55 parts by weight of polypropylene, 4 parts by weight of color masterbatch and 0.1 parts by weight of antioxidant 1010 were added to a high-speed mixer and mixed at 800 r / min for 5 min to obtain a premix.

[0092] Comparative Example 2 The difference between this comparative example and Example 1 is that the weight of the crosslinking agent in step S1 of the <Preparation of Polyolefin Foamed Beads> is different; all other aspects are the same as in Example 1. In this embodiment, the crosslinking agent has a weight of 0 parts.

[0093] Comparative Example 3 The difference between this comparative example and Example 1 is that the mass content of supercritical fluid in the melt in step S3 of <Preparation of polyolefin foamed beads> is 0.4%; the rest are the same as in Example 1.

[0094] Comparative Example 4 The difference between this comparative example and Example 1 is that the mass content of supercritical fluid in the melt in step S3 of the <Preparation of Polyolefin Foamed Beads> is 3.5%; the rest are the same as in Example 1.

[0095] Comparative Example 5 The difference between this comparative example and Example 1 is that the temperature of the second-stage extruder from the feed inlet to the die head is different in step S3 of the <Preparation of Polyolefin Foamed Beads>; all other aspects are the same as in Example 1. In this embodiment, the temperatures of the two-stage extruder from the feed inlet to the die are: Zone 1 185°C, Zone 2 185°C, Zone 3 185°C, and Die 185°C.

[0096] Comparative Example 6 The difference between this comparative example and Example 1 is that in step S3 of the <Preparation of Polyolefin Foamed Beads>, the first-stage / second-stage tandem extrusion and supercritical fluid-assisted dispersion process is not used, but a conventional twin-screw extruder is used for direct extrusion granulation; the rest are the same as in Example 1. Step S3 of this comparative example is as follows: The premix obtained in step S2 is added to a twin-screw extruder, mixed, melted, extruded and granulated to obtain polyolefin composite microparticles; wherein, the temperature of the twin-screw extruder from the feed inlet to the die is: 195℃ in zone 1, 185℃ in zone 2, 175℃ in zone 3, and 170℃ at the die; the screw speed is 80 r / min.

[0097] Comparative Example 7 The difference between this comparative example and Example 1 is that commercially available EPP beads were used instead of polyolefin foam beads in the <Preparation of Foamed Products>. The manufacturer of the commercially available EPP beads is Wuxi Huitong Lightweight Materials Co., Ltd., the grade is EPP-30, and the apparent density is 30 g / L.

[0098] Performance testing The polyolefin foamed beads and foamed products obtained in the examples and comparative examples were subjected to performance tests.

[0099] (1) Dispersed phase size: The polyolefin composite particles were brittlely fractured with liquid nitrogen, and the fracture surface was etched (removing the POE / PE phase) and sputtered with gold. The average diameter (Dn) of at least 100 dispersed phase particles was observed using a scanning electron microscope (SEM). (2) Apparent density: The apparent density of polyolefin foam beads was tested according to GB / T 6343-2009 standard; (3) Minimum molding pressure: During the process of <preparation of foamed products>, record the lowest gauge pressure (bar) when the mold cavity reaches a stable saturated steam pressure and the product is successfully molded. (4) Heat shrinkage rate: Cut the foamed product into 100mm×100mm×10mm samples, place them in a 70℃ forced-air drying oven for 1 hour, take them out and cool them to room temperature, measure the dimensional changes, and calculate the area shrinkage rate; (5) Expansion ratio: The expansion ratio is calculated according to formula (1): Expansion ratio = ρ0 / ρ f Where ρ0 is the density of the unfoamed polyolefin composite microparticles, measured using a densitometer or the water displacement method; ρ f The apparent density of the polyolefin foamed beads was tested according to GB / T 6343-2009. In the examples and comparative examples, the density of the polyolefin composite microparticles was 0.90 g / cm³. 3 That is, 900 g / L.

[0100] The test results are shown in Table 1.

[0101] Table 1 As can be seen from the experimental data in Table 1, the minimum molding pressure of the polyolefin foam beads of this application is ≤1.6 bar, the heat shrinkage rate at 70℃ is ≤2.2%, and the foaming ratio is ≥26%. This shows that this application has successfully solved the core contradiction in the field of polyolefin foam beads that has long existed: it is difficult to balance "low water vapor molding pressure (corresponding to low molding temperature)" and "high heat resistance and high foaming ratio".

[0102] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A method for preparing polyolefin foamed beads, characterized in that, Includes the following steps: S1: After mixing polyethylene, α-olefin elastomer and crosslinking agent, the mixture is added to a two-roll mill for melt extrusion granulation to obtain a polyolefin composite. S2: After mixing the polyolefin composite, polypropylene and additives obtained in step S1, a premix is ​​obtained; S3: The premix obtained in step S2 is added to a first-stage extruder for mixing, melting, and plasticizing to obtain a homogeneous mixture of the premix and supercritical fluid; the homogeneous mixture is added to a second-stage extruder for mixing and extrusion to obtain polyolefin composite microparticles; wherein, the supercritical fluid is injected during the plasticizing stage of the first-stage extruder, the mass content of the supercritical fluid in the melt is 0.5~3.0%, and the temperature of the second-stage extruder from the feed inlet to the die head shows a decreasing trend; S4: The polyolefin composite microparticles obtained in step S3 are impregnated in a pressure vessel with supercritical fluid at a pressure of 10-50 MPa, and then the pressure is released to atmospheric pressure to obtain polyolefin foamed beads.

2. The method for preparing polyolefin foamed beads as described in claim 1, characterized in that, The temperature of the two-stage extruder from the feed inlet to the die head is set to decrease from 200°C to 170°C.

3. The method for preparing polyolefin foamed beads as described in claim 2, characterized in that, The temperature settings for the two-stage extruder from the feed inlet to the die are as follows: Zone 1 temperature: 185~195℃, Zone 2 temperature: 175~185℃, Zone 3 temperature: 172~175℃, and Die temperature: 170~172℃.

4. The method for preparing polyolefin foamed beads as described in claim 1, characterized in that, The polyolefin composite comprises, by weight, 40-55 parts polyethylene, 20-40 parts α-olefin elastomer, and 0.005-0.05 parts crosslinking agent.

5. The method for preparing polyolefin foamed beads as described in claim 4, characterized in that, The polyolefin composite also includes 0.5 to 5 parts by weight of nanoparticles.

6. The method for preparing polyolefin foamed beads as described in claim 1, characterized in that, The ratio of the melt index of the polypropylene at 230°C and 2.16 kg to the melt index of the α-olefin elastomer at 190°C and 2.16 kg is (1:0.15) to (1:4.5). And / or, by weight, the premix comprises the following components: 40-60 parts of polypropylene, 25-40 parts of the polyolefin composite, and 0.5-6.5 parts of additives.

7. The method for preparing polyolefin foamed beads as described in claim 1, characterized in that, Satisfy at least one of the following (A) to (D): (A) The nanoparticles include at least one of layered silicates, fumed silica, and calcium carbonate; (B) At least one of the following: ethylene-octene copolymer, ethylene-butene copolymer, ethylene propylene rubber, and ethylene propylene diene monomer (EPDM) rubber; (C) The crosslinking agent is a peroxide crosslinking agent; (D) The supercritical fluid is at least one of supercritical carbon dioxide, supercritical nitrogen, and supercritical propane.

8. A type of polyolefin foamed beads, characterized in that, The polyolefin foamed beads are prepared by the method described in any one of claims 1 to 7.

9. A foamed product, characterized in that, The material of the foamed product includes the polyolefin foamed beads as described in claim 8.

10. A method for preparing a foamed product as described in claim 9, characterized in that, The process includes the following steps: placing polyolefin foam beads in a finished product mold and steam molding them to obtain foamed products; wherein the steam molding pressure is 1.0~1.6 bar.

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