A method for preparing one-time foamed polypropylene beads

Abstract

This invention relates to a method for preparing one-time foamed polypropylene beads. Polypropylene microparticles, water, dispersant, and co-dispersant are added to a high-pressure reactor in a weight ratio of 1:1–2.5:0.0015–0.01:0.0002–0.002 and sealed. Under continuous stirring, the material inside the high-pressure reactor is heated to the foaming temperature. Simultaneously, carbon dioxide is injected into the reactor until the pressure inside reaches the foaming pressure. The material is processed under foaming temperature and pressure conditions. The high-pressure reactor is then opened, and the material is discharged into a foamer with a steam atmosphere for foaming and expansion, yielding foamed EPP beads. The one-time foamed polypropylene beads provided by this invention have higher efficiency in the production process of high-ratio EPP beads, and the molded parts have low energy consumption, excellent appearance, and excellent mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of foamed materials technology, and in particular to a method for preparing and applying a type of one-time foamed polypropylene beads. Background Technology

[0002] Expanded polypropylene (EPP) beads and their molded parts are lightweight and have diverse structural shapes, making them suitable for irregularly shaped components such as weight reduction, energy absorption and cushioning, sound insulation, and heat insulation. They have been widely used in automotive parts, packaging materials, building insulation materials, sound insulation and noise reduction materials, and other fields, making them a very promising type of foamed material.

[0003] Currently, the industrial production of EPP beads is mainly achieved through high-temperature and high-pressure autoclave foaming. This involves heating the particle material at high temperatures to soften it, then impregnating it with foaming gas under high pressure. During depressurization, the foaming gas explodes and separates within the material phase, forming a foam structure – this is the primary foaming process. To ensure the closed-cell characteristics of the EPP beads, the minimum density of primary foamed EPP beads is generally higher than 35 g / L. To obtain lower-density EPP beads, a secondary foaming process is required, where the primary foamed beads are subjected to air pressure and heated to expand, resulting in lighter-density secondary foamed beads. This process, producing high-ratio EPP beads with a density of 20-34 g / L, requires a long cycle and has low production efficiency. Traditional autoclave foaming processes involve a time interval between the primary and secondary foaming stages. The secondary foaming stage requires further high-pressure air pre-compression to ensure the product is foamable, resulting in a long time interval, wasted manpower and resources, and overall low efficiency. Furthermore, after the first foaming stage, the cell structure is already formed. Under high-pressure air pressure and in a heated environment, the high-pressure air within the cell structure expands, causing the foamed beads to expand overall, thus achieving secondary foaming. This secondary foaming process only expands the cell size of the primary foamed beads to a certain extent, having little impact on the number of cells or the uniformity of cell distribution inside and outside the beads. Therefore, the primary foaming process essentially determines the distribution of cells inside and outside the foamed beads; the more uniform the cell distribution in the inner and outer layers of the beads, the better their molding performance.

[0004] Furthermore, PP resin is a semi-crystalline material with a relatively high melting point. Compared to polystyrene (EPS) foam, EPP beads require higher steam energy consumption for shaping and sintering during molding. High-temperature molding also leads to longer water-cooling and setting times for the molded parts, and the parts are more prone to shrinkage after demolding. This results in a longer molding cycle for single-molded parts, and a longer drying and curing process to eliminate shrinkage, further increasing the overall production cycle of the EPP molding process. Therefore, reducing molding energy consumption and shortening the molding cycle have become development demands for the EPP bead industry.

[0005] Chinese patent CN106674583B discloses a foamed polypropylene bead and its preparation method. The preparation method of the foamed polypropylene bead includes: preparing polypropylene microparticles by mixing a first polypropylene resin with a crystallinity of 25%-40% and a second polypropylene resin with a melting point of 126℃-140℃ under the action of a nucleating agent; subjecting the polypropylene microparticles to a first foaming treatment by heating and pressurizing to obtain first foamed beads; subjecting the first foamed beads to a pre-compression treatment to obtain pre-compressed beads; and subjecting the pre-compressed beads to a second foaming treatment to obtain foamed polypropylene beads.

[0006] Chinese patent CN117209900A discloses a novel polypropylene foam material, its preparation method, and its application. By weight, the material comprises: 60-90 parts of random copolymer polypropylene, 10-35 parts of polybutene, 1-5 parts of abrasion resistant agent, 0.5-3 parts of compatibilizer, and 0.1-0.5 parts of cell regulator. This novel polypropylene foam material exhibits excellent UV resistance, a yellowing resistance rating of 4 or higher, a bond strength ≥1.3 MPa, and an abrasion resistance ΔL ≤1.1. It can be applied to the field of foam materials for outdoor structures.

[0007] However, the above preparation method requires two foaming processes, which lengthens the production cycle of high-ratio EPP beads and reduces production efficiency.

[0008] Chinese patent CN113831647A discloses a method for preparing foamed polypropylene beads, comprising the following steps: (1) Polypropylene microparticles, dispersion medium, dispersant, and dispersion aid are added to a sealed high-pressure reactor in a certain weight ratio, and a dispersion system is formed under continuous stirring; (2) The dispersion system in the reactor is heated to a certain temperature, and a certain amount of CO2 is introduced and maintained for a certain time; (3) The dispersion is released into atmospheric pressure to obtain expanded foamed polypropylene beads, and then the foamed polypropylene beads are rinsed with washing water. The dispersant adhering to the surface of the polypropylene beads is then dehydrated and dried to obtain the finished foamed polypropylene beads. The polypropylene microparticles comprise the following components by mass percentage: 94-99.94% polypropylene, 0.01-2% crystallizing nucleating agent, 0.02-4% organophosphate compound, and 0.03-0.3% cell nucleating agent. The organophosphate compound is at least one of tris(2,3-dibromopropyl) phosphate, tricresyl phosphate, lauryl ether phosphate, and tritert-butylphenyl phosphate. In the above method, the organophosphate compound mainly inhibits the agglomeration of microparticles inside the reactor or the agglomeration of beads outside the reactor, reducing the amount of dispersant required. The final foamed polypropylene beads have a bulk density higher than 50 g / L and do not have a thickening effect. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing one-time foamed polypropylene beads and its application, which has higher efficiency in the production process of high-ratio EPP beads, and produces molded parts with low energy consumption, excellent appearance, and excellent mechanical properties.

[0010] To achieve the above objectives, the present invention provides foamed polypropylene beads, wherein the weight parts of each component are: polypropylene microparticles, water, dispersant, and co-dispersant in a weight ratio of 1:1 to 2.5:0.0015 to 0.01:0.0002 to 0.002.

[0011] Preferably, the weight parts of each component of the polypropylene microparticles are: polypropylene 87.5-99.77%, cell nucleating agent 0.03-0.5%, regulator 0.2-2%, and other plastic additives 0-10%.

[0012] Preferably, the modifier includes sodium polyacrylate with a relative molecular mass of 200,000 to 600,000.

[0013] Preferably, other plastic additives include at least one of antioxidants, lubricants, antistatic agents, and colorants.

[0014] The antioxidant is selected from at least one of hindered phenolic compounds, thiopropionate compounds, and phosphite compounds.

[0015] The lubricant is selected from at least one of erucamide, oleamide, glyceryl monostearate, polyethylene wax, and polypropylene wax.

[0016] When selecting one of the other plastic additives, its weight percentage in the microparticles is 0-5%.

[0017] Preferably, the dispersant is kaolin with an average particle size of 0.1-1 micrometer.

[0018] The dispersant is sodium dodecylbenzenesulfonate.

[0019] The electrical conductivity of water is less than 10 mS / m.

[0020] Polypropylene is a random copolymer polypropylene with a melt index of 6-9 g / 10 min (230℃, 2.16 kg) and a melting point of 138-144℃.

[0021] The cell nucleating agents include one or more of talc, zinc borate, zinc oxide, and calcium carbonate, with an average particle size of 1-15 micrometers.

[0022] This invention provides a method for preparing foamed polypropylene beads, comprising the following steps:

[0023] S1, polypropylene, cell nucleating agent, regulator, and other plastic additives are uniformly mixed and then melt-mixed in an extruder, drawn into fibers and granulated to obtain polypropylene microparticles with a length of 0.8-2.5 mm and a single particle weight of 0.3-2 mg.

[0024] S2. Add polypropylene microparticles, water, dispersant, and co-dispersant to a high-pressure reactor in the specified proportions and seal the reactor. Under continuous stirring, heat the material in the high-pressure reactor to the foaming temperature, while simultaneously injecting carbon dioxide into the reactor until the pressure inside reaches the foaming pressure. Maintain the material in the high-pressure reactor under the conditions of foaming temperature and foaming pressure. Open the high-pressure reactor and discharge the material into a foamer with a water vapor atmosphere for foaming and expansion to obtain foamed EPP beads. The bulk density of the EPP beads is 20-34 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads ranges from 13-21 J / g.

[0025] Preferably, in step S1, the temperature of the single-screw extrusion section is 200-240℃; the length-to-diameter ratio of the extruder screw is (20-40):1; and the screw speed is 40-80 rpm.

[0026] Preferably, in step S2, the time for which the material in the high-pressure reactor is treated under the conditions of foaming temperature and foaming pressure is 10-60 min, the foaming temperature of the material is 137-152℃, the foaming pressure of the material is 2-4 MPa, and the pressure range of water vapor in the foamer is 0.01-0.1 MPa; the treatment time for the material to be discharged into the foamer in the water vapor atmosphere for foaming and expansion is 8-30 s.

[0027] This invention provides a method for preparing molded parts. Polypropylene beads are pressurized with air in a sealed container and then vacuum-filled into a mold. After molding by heating with steam, the molded parts are cooled by water to obtain ejected parts. The ejected parts are then treated in a hot air atmosphere to obtain finished molded parts.

[0028] Preferably, polypropylene beads are pressurized in a sealed container under 0.5 MPa air pressure for 8 hours, and the molded parts are treated in a hot air atmosphere at 70-90℃ for 5-10 hours to obtain finished molded parts. EPP molded parts have excellent appearance, i.e., the surface has few or no gaps and pits; the compressive stress at 10% compressive strain of EPP molded parts is not less than 80 kPa, and the tensile strength is not less than 600 kPa.

[0029] Preferably, the minimum molding pressure for EPP bead molding is less than 2.2 bar, and the water cooling time for single-mold EPP heated molding is less than 80 seconds when the cooling water temperature is 35°C.

[0030] The present invention has the following beneficial effects:

[0031] Unlike the traditional two-stage foaming process of EPP, the particles of this invention are directly discharged from the autoclave into the foamer in a steam atmosphere, and ultra-light EPP beads are obtained by undergoing only one foaming process. This avoids the time interval between the first and second foaming in the traditional two-stage foaming process, thus improving the production efficiency of ultra-light EPP beads. Moreover, there is no need to pre-press the particles again, which can easily achieve the preparation of ultra-low density EPP beads.

[0032] During the foaming process, the foam particles are gradually forming and not yet solidified. The steam in the foamer assists in the foaming process, which helps to make the size of the surface and internal pores of the foam beads more uniform. This has a significant effect on improving the molding expansion of EPP, reducing the steam pressure of EPP bead molding, shortening the water cooling time of EPP molding, and reducing the gaps and pits on the surface of EPP parts.

[0033] In addition, the regulator has a strong water absorption and thickening effect; during the autoclave foaming process, the thickening effect increases the viscosity of the material melt, which is conducive to the formation of closed-cell structure of foam beads; the absorbed water exists in the material in the form of water vapor, which, as a foaming agent, is conducive to the volume expansion and foaming uniformity of the material, and it is easy to obtain foam beads with high foaming ratio and excellent molding performance. Attached Figure Description

[0034] Figure 1 This is a SEM image of the cross-section of the foamed beads obtained in Example 3.

[0035] Figure 2 This is a SEM image of the cross-section of the foamed beads obtained in Comparative Example 9. Detailed Implementation

[0036] The technical solution of the present invention will be further described below through embodiments.

[0037] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0038] Table 1. Main material information in the embodiments.

[0039]

[0040] Example 1

[0041] The materials, with a weight percentage of 98.45% polypropylene 1, 0.15% talc, 0.2% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0042] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.5 J / g.

[0043] Example 2

[0044] The materials, with a weight percentage of 96.65% polypropylene, 0.15% talc, 2% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0045] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151.3℃, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.4MPa. After maintaining this temperature and pressure inside the reactor for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04MPa for 14 seconds. The bulk density of the foamed EPP beads was 21g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.2J / g.

[0046] Example 3

[0047] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0048] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 16.4 J / g.

[0049] Example 4

[0050] The materials, with a weight percentage of 93.15% polypropylene, 0.15% talc, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, 0.5% polypropylene wax, and 5% carbon black, were uniformly mixed and then processed through a single-screw extruder with a length-to-diameter ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0051] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145.5℃, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.2MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04MPa for 12 seconds. The bulk density of the foamed EPP beads was 25g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 15.2J / g.

[0052] Comparative Example 1

[0053] The materials, with a weight percentage of 98.65% polypropylene, 0.15% talc, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a weight of 1.2 mg per particle.

[0054] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 30 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.7 J / g.

[0055] Comparative Example 2

[0056] The materials, with a weight percentage of 98.5% polypropylene 1, 0.15% talc, 0.15% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a weight of 1.2 mg per particle.

[0057] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 27 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.5 J / g.

[0058] Comparative Example 3

[0059] The materials, with a weight percentage of 96.65% polypropylene, 0.15% talc, 2% tricresyl phosphate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a weight of 1.2 mg per particle.

[0060] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151.3℃, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.4MPa. After maintaining this temperature and pressure inside the reactor for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04MPa for 14 seconds. The bulk density of the foamed EPP beads was 30g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.4J / g.

[0061] Comparative Example 4

[0062] The materials, with a weight percentage of 96.15% polypropylene, 0.15% talc, 2.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a weight of 1.2 mg per particle.

[0063] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151.3℃, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.4MPa. After maintaining this temperature and pressure inside the reactor for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04MPa for 14 seconds. The bulk density of the foamed EPP beads was 21g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.1J / g.

[0064] Comparative Example 5

[0065] The materials, with a weight percentage of 98.65% polypropylene, 0.15% talc, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a weight of 1.2 mg per particle.

[0066] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 151°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. This temperature and pressure were maintained for 20 minutes. Then, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.1 MPa for 30 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 18.3 J / g.

[0067] Comparative Example 6

[0068] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.2% antioxidant, 0.5% polyethylene glycol, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0069] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 16.2 J / g.

[0070] Comparative Example 7

[0071] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.2% antioxidant, 0.5% glycerol, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0072] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a steam pressure of 0.04 MPa for 14 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point in the first DSC melting curve of the foamed beads was 16.2 J / g.

[0073] Comparative Example 8

[0074] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.2% antioxidant, 0.5% sodium polyacrylate, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0075] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 3.6 MPa. After maintaining this temperature and pressure inside the reactor for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a foamer with a hot air atmosphere and an internal temperature of 95°C for 14 seconds. The bulk density of the foamed EPP beads was 21 g / L, and the endothermic enthalpy value above the intrinsic melting point melting peak in the first DSC melting curve of the foamed beads was 16.5 J / g.

[0076] Comparative Example 9

[0077] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0078] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145.6°C, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 2.7 MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a breathable collection bag at room temperature and pressure. The bulk density of the EPP beads obtained after one-stage foaming was 47 g / L. After being placed in a breathable collection bag at room temperature and pressure for 24 hours, the primary foamed beads were drawn into a sealed pre-pressurization tank. After being pressurized by air in the pre-pressurization tank, they were placed in a foaming chamber with a water vapor atmosphere of 0.085 MPa for secondary foaming for 14 seconds, resulting in a final foamed bead bulk density of 21 g / L. The endothermic enthalpy value above the intrinsic melting point peak in the first DSC melting curve of the foamed beads was 16.4 J / g. The pressurization process in the pre-pressurization tank involved uniformly increasing the air pressure from atmospheric pressure to 0.7 MPa over 24 hours, then maintaining it at 0.7 MPa, which could then be used for secondary foaming.

[0079] Comparative Example 10

[0080] The materials, with a weight percentage of 98.15% polypropylene 2, 0.15% talc, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax, were uniformly mixed and then processed through a single-screw extruder with an aspect ratio of 30:1 at a processing temperature of 220°C and a screw speed of 60 rpm. The mixture was then drawn into fibers and granulated to obtain polypropylene microparticles with a length of 2.0 mm and a single particle weight of 1.2 mg.

[0081] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added to a high-pressure reactor in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the material in the high-pressure reactor was heated to 145.6℃, and carbon dioxide was injected into the reactor until the pressure inside the reactor reached 2.7MPa. After maintaining this temperature and pressure for 20 minutes, the high-pressure reactor was opened, and the material was discharged into a breathable collection bag at room temperature and pressure. The bulk density of the EPP beads obtained after one-stage foaming was 47g / L. The collected primary foamed beads were immediately placed in a foaming barrel with a water vapor atmosphere of 0.1MPa for secondary foaming for 30 seconds, resulting in a final foamed bead bulk density of 45g / L. The endothermic enthalpy value above the intrinsic melting point melting peak in the first DSC melting curve of the primary foamed beads was 16.2J / g.

[0082] Because the bead material and cell structure have solidified after the first foaming, and there is insufficient internal pressure within the cell structure, further foaming and expansion are impossible even in a steam atmosphere. Furthermore, the steam atmosphere has minimal impact on the number of cells in the foamed beads and the uniformity of cell distribution between the inner and outer layers of the beads. However, when the collected first-foamed beads are immediately placed in a foaming chamber with a steam atmosphere exceeding 0.1 MPa for secondary foaming, the excessively high temperature causes significant agglomeration between the beads, making it impossible to obtain foamed beads with a bulk density below 34 g / L.

[0083] Example of molded part preparation:

[0084] EPP beads are pressurized in a sealed container at 0.5MPa air pressure for 8 hours, and then vacuum-filled into a rectangular mold (450mm long, 350mm wide, and 60mm thick). The molded parts are then steam-molded. The resulting molded parts are baked in a hot air atmosphere at 80℃ for 5 hours, and then slowly cooled to room temperature within 2 hours to obtain the final molded parts used for performance testing.

[0085] Among them, the minimum molding pressure is the minimum steam sintering pressure required for the proportion of particles with destroyed pores on the fracture surface of the foamed polypropylene granule molded part to be above 95%.

[0086] Water cooling time: The optimal time for cooling the mold with water after heating the mold with steam during the molding process under the lowest molding pressure (this water cooling process ensures that the molded part can be easily removed from the mold and that the appearance does not shrink at the moment of demolding).

[0087] Determination of performance parameters of molded parts:

[0088] Surface quality of the parts: "□" indicates a large number of pits or gaps on the surface of the part; "□" indicates a small number of pits or gaps on the surface of the part. This indicates that the surface of the part has no or very few or very small pits or gaps.

[0089] The compressive stress of the molded part at 10% strain is tested according to GB / T 8813-2020.

[0090] The tensile strength of molded parts was tested according to GB / T 6344-2008.

[0091] Table 2 Performance of EPP molded parts in the examples and comparative examples.

[0092]

[0093] Examples 1-4 are all within the technical requirements. The resulting foamed bead molding parts with a bulk density range of 21-25 g / L have low molding energy consumption, excellent appearance, and excellent mechanical properties.

[0094] Compared to Example 1, Comparative Example 1 does not contain sodium polyacrylate, thus lacking the expansion-enhancing effect. In the same foaming process, it is impossible to obtain lighter-density EPP beads, and the bead foaming uniformity is worse, resulting in poorer molding expansion, increased molding pressure, increased water cooling time, and more gaps and pits in the molded parts. Compared to Example 1, Comparative Example 2 has a lower sodium polyacrylate content, resulting in a weaker expansion-enhancing effect. In the same foaming process, it is impossible to obtain lighter-density EPP beads, and the bead foaming uniformity is relatively poor, resulting in poor molding expansion, increased molding pressure, increased water cooling time, and a small number of gaps in the molded parts. Compared to Example 2, Comparative Example 3 uses tricresyl phosphate instead of sodium polyacrylate, lacking the expansion-enhancing effect. In the same foaming process, it is impossible to obtain lighter-density EPP beads, and the bead foaming uniformity is worse, resulting in poorer molding expansion, increased molding pressure, increased water cooling time, and more gaps and pits in the molded parts. Compared to Example 2, Comparative Example 4, with the addition of excessive sodium polyacrylate, showed an increased tendency for aggregation in polypropylene. In the same foaming process, this was detrimental to the uniformity of bead foaming, resulting in relatively poor molding expansion, increased molding pressure, increased water cooling time, and the appearance of a small number of gaps and pits in the molded parts. Compared to Example 1, Comparative Example 5, lacking sodium polyacrylate, lacked the expansion-promoting effect during the foaming process. By increasing the steam pressure in the foamer and extending the foaming time of the beads, forced foaming was achieved to obtain beads with a bulk density of 21 g / L. This increased the degree of damage to the closed-cell structure of the foamed particles, resulting in poor molding properties and a decrease in the compressive stress and tensile strength of the parts at 10% strain. Compared to Example 3, in Comparative Examples 6 and 7, polyethylene glycol and glycerin were used instead of sodium polyacrylate, respectively. Although foaming was achieved, the thickening effect was lacking. During foaming, the beads were more prone to rupture, the uniformity of the foamed beads' pores deteriorated, the molding properties worsened, the molding pressure increased, the water cooling time increased, and the compressive stress and tensile strength of the molded part at 10% strain decreased. Compared to Example 3, in Comparative Example 8, continuous foaming was performed in a 95°C hot air atmosphere in the foamer. Hot air has weak thermal penetration into the material. Although foaming was achieved, the pore structure of the surface and inner layers of the foamed beads showed significant differences, resulting in poor molding properties and reduced compressive stress and tensile strength of the molded part at 10% strain.Compared with Example 3, Comparative Example 9 did not use the single-stage foaming process of this technology, but instead adopted the traditional process of two foaming steps, namely, the first stage of autoclaving and the second stage of high-pressure air foaming. It also obtained ultralight beads with a density of 21 g / L, but the preparation process took at least 48 hours longer and the production efficiency was low. In addition, the cell structure distribution trend was already formed in the first stage of foaming in Comparative Example 9. Although the second stage of foaming was also heated with a steam atmosphere, it only expanded the cell size of the first-stage foamed beads to a certain extent. It had little effect on the number of cells and the uniformity of the cell distribution in the inner and outer layers of the beads. Therefore, it only achieved the effect of second-stage foaming. The cell structure of the surface and inner layers of the foamed beads was significantly different, resulting in poor molding properties and reduced compressive stress and tensile strength of the molded parts at 10% strain.

[0095] Figure 1 The image shown is a cross-sectional SEM image of the foamed beads obtained in Example 3. It can be seen that the size of the pores inside the beads is generally close to the size of the pores on the surface of the beads, indicating that the pore size of the inner and outer layers of the beads is highly uniform, which is beneficial for molding. Figure 2 The image shows a cross-sectional SEM image of the foamed beads obtained in Comparative Example 9. It can be seen that the internal pore size of the beads is relatively large, while the pore size on the surface of the beads is relatively small. In other words, there is a large difference in pore size between the inner and outer layers of the beads, which is not conducive to molding.

[0096] Therefore, the present invention employs the above-mentioned one-time foamed polypropylene beads and preparation method and application, which has higher efficiency in the production process of high-ratio EPP beads, and the molded parts have low energy consumption, excellent appearance and excellent mechanical properties.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A process for the preparation of one-shot foamed polypropylene beads, characterized in that, The method comprises the following steps: S1, uniformly mixing polypropylene, cell nucleating agent, adjusting agent, other plastic additives, melting and mixing through an extruder, drawing, and granulating to obtain polypropylene particles; In step S1, the temperature of the extrusion section is 200-240℃; the length-diameter ratio of the screw of the extruder is (20-40):1; the rotation speed of the screw is 40-80rpm; wherein the length of the polypropylene particles is 0.8-2.5mm, and the weight of a single polypropylene particle is 0.3-2mg; in step S2, the time for maintaining the material in the autoclave to be treated under the foaming temperature and the foaming pressure is 10-60min, the foaming temperature of the material is 137-152℃, the foaming pressure of the material is 2-4MPa, and the pressure range of the water vapor in the foaming device is 0.01-0.04MPa; the time for discharging the material into the foaming device in the water vapor atmosphere for foaming and expansion is 8-30s; S2, adding polypropylene particles, water, dispersing agent, and auxiliary dispersing agent into an autoclave in proportion and sealing; under the action of continuous stirring, heating the material in the autoclave to the foaming temperature, while injecting carbon dioxide into the autoclave until the pressure in the autoclave reaches the foaming pressure, and maintaining the material in the autoclave to be treated under the foaming temperature and the foaming pressure; opening the autoclave body, discharging the material into the foaming device in the water vapor atmosphere for foaming and expansion to obtain foamed EPP beads; The EPP beads are prepared from polypropylene particles, water, dispersing agent, and auxiliary dispersing agent in a weight ratio of 1:1~2.5:0.0015~0.01:0.0002~0.002; The polypropylene particles at least comprise the following components in the following weight percentages: polypropylene 87.5-99.77%, cell nucleating agent 0.03-0.5%, adjusting agent 0.2-2%, and other plastic additives 0-10%. The adjusting agent is sodium polyacrylate with a relative molecular mass of 200-600 thousand.

2. The process for the preparation of one-shot foamed polypropylene beads according to claim 1, characterized in that, The polypropylene is random copolymer polypropylene with a melt index of 6-9g / 10min and a melting point of 138-144℃.

3. The process for the preparation of one-shot foamed polypropylene beads according to claim 1, characterized in that, The cell nucleating agent is one or more of talc, zinc borate, zinc oxide, and calcium carbonate with an average particle size of 1-15 microns.

4. The process for the preparation of one-shot foamed polypropylene beads according to claim 1, characterized in that, The other plastic additives are at least one of antioxidants, lubricants, antistatic agents, and coloring agents; the antioxidants are at least one of hindered phenolic compounds, thio propionic acid ester compounds, and phosphite ester compounds; the lubricants are at least one of erucic acid amide, oleic acid amide, glycerol monostearate, polyethylene wax, and polypropylene wax; when one kind of other plastic additive is selected, the weight percentage in the particles is 0-5%.

5. A molded article characterized by, The product contains the foamed polypropylene beads prepared by the method of any one of claims 1-4.

6. A method of producing a molded article according to claim 5, wherein The polypropylene beads are loaded into a mold under vacuum after being loaded into a closed container under air pressure, and the mold is heated by water vapor to obtain a molded part, which is treated in a hot air atmosphere to obtain a finished molded part.

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