Preparation method of once-foamed polypropylene beads

Abstract

The invention relates to a preparation method of once-foamed polypropylene beads, which comprises the following steps: adding polypropylene particles, water, a dispersing agent and an auxiliary dispersing agent into a high-pressure-resistant kettle according to a weight ratio of 1: 1-2.5: 0.0015-0.01: 0.0002-0.002, sealing, heating the materials in the high-pressure-resistant kettle to a foaming temperature under the action of continuous stirring, injecting carbon dioxide into the high-pressure-resistant kettle until the pressure in the kettle reaches a foaming pressure at the same time, and continuously stirring to obtain the once-foamed polypropylene beads. The materials in the high-pressure-resistant kettle are treated under the conditions that the foaming temperature and the foaming pressure are maintained; and opening the high-pressure-resistant kettle body, discharging the material into a foam maker in a water vapor atmosphere, and foaming and expanding to obtain the foamed EPP beads. The one-time foaming polypropylene bead provided by the invention has higher efficiency in the production process of the high-magnification EPP bead, and a molded part is low in molding energy consumption, excellent in appearance and excellent in mechanical property.

Description

Technical Field

[0001] The present invention relates to the technical field of foamed materials, and particularly to a preparation method and application of a one-step foamed polypropylene bead. Background Art

[0002] Foamed polypropylene (EPP) beads and their molded parts are light in weight and have diverse structural shapes, and are suitable for special-shaped structural parts such as weight reduction, energy absorption and buffering, sound insulation, and heat insulation. They have been widely used in fields such as automotive parts, packaging materials, building thermal insulation materials, and sound insulation and noise reduction materials. They are a very promising type of foamed material.

[0003] At present, the industrial production of EPP beads is mainly achieved by the high-temperature and high-pressure autoclave foaming method. By heating the particle material at high temperature to soften it and impregnating it with a foaming gas under high pressure, and then during the pressure relief process, the foaming gas bursts and separates in the material phase to form a foaming structure to achieve one-step foaming. In order to ensure the closed-cell characteristics of EPP beads, the lowest density of one-step foamed EPP beads is generally higher than 35 g / L; if lower-density EPP beads are required, on the basis of one-step foamed beads, they need to be heated and expanded under air pressure to achieve secondary foaming to obtain lighter-density secondary foamed beads; this process for producing high-ratio EPP beads with a density of 20 - 34 g / L requires a long cycle and low production efficiency. In the traditional autoclave pressure foaming process, there is a certain time interval between one-step and secondary foaming, and secondary foaming requires re-high-pressure air pre-pressurization to make the product have the foamability for secondary foaming. The time interval is long and it wastes manpower and material resources, and the overall efficiency is low. Moreover, after one-step foaming, the cell structure has been formed. After being pressurized by high-pressure air and in a heated environment, the high-pressure air in the cell structure expands due to heat, resulting in the overall expansion of the foamed beads, that is, secondary foaming is achieved. This secondary foaming process only expands the cell size of one-step foamed beads to a certain extent, and has little effect on the number of cells and the uniform distribution of cells inside and outside the beads. Therefore, the one-step foaming process basically determines the distribution of cells inside and outside the foamed beads. The more uniform the cell distribution in the inner layer and the surface layer of the beads, the better its molding performance.

[0004] In addition, PP resin is a semi-crystalline material and has a relatively high melting point. Compared with polystyrene (EPS) foam, EPP beads require higher steam energy consumption to shape and sinter the molded parts during the molding process. At the same time, high-temperature molding will result in a longer time for the molded parts to be shaped by water cooling and the molded parts are prone to shrinkage after demolding. The molding cycle of a single molded part is longer, and it takes longer to eliminate the shrinkage of the molded parts during the subsequent drying and curing process, increasing the production cycle of the entire EPP molding process. Therefore, reducing molding energy consumption and shortening the molding cycle have become the development demands of 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 fine particles from a first polypropylene resin with a crystallinity of 25%-40% and a second polypropylene resin with a melting point of 126°C-140°C under the action of a nucleating agent; performing a first foaming treatment on the polypropylene fine particles by heating and pressurizing to obtain first foamed beads; performing a pre-pressing treatment on the first foamed beads to obtain pre-pressed beads; and performing a second foaming treatment on the pre-pressed beads to obtain the foamed polypropylene beads.

[0006] Chinese Patent CN117209900A discloses a novel polypropylene foaming material, its preparation method and application. By weight, the raw materials of this material include: 60-90 parts of random copolymer polypropylene, 10-35 parts of polybutene, 1-5 parts of anti-friction additive, 0.5-3 parts of compatibilizer, 0.1-0.5 parts of cell regulator. This novel polypropylene foaming material has excellent anti-ultraviolet performance, a yellowing resistance grade of 4 or above, a bonding strength ≥ 1.3 MPa, and an anti-friction performance △L ≤ 1.1, and can be applied to the field of foaming materials for outdoor structures.

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

[0008] Chinese Patent CN113831647A discloses a preparation method of foamed polypropylene beads, which includes the following steps: (1) putting polypropylene fine particles, a dispersion medium, a dispersant, and a dispersion aid into a closed high-pressure resistant kettle in a certain weight ratio, and forming a dispersion system under continuous stirring; (2) heating the dispersion system in the kettle to a certain temperature, and filling a certain amount of CO2 and maintaining for a certain time; (3) releasing the dispersion to atmospheric pressure to obtain expanded foamed polypropylene beads, then rinsing the surface of the foamed polypropylene beads attached with the dispersant with washing water, and then performing dehydration and drying to obtain the finished product of foamed polypropylene beads; the polypropylene fine particles include the following components in mass percentage: 94-99.94% of polypropylene, 0.01-2% of a crystalline nucleating agent, 0.02-4% of an organophosphate compound, and 0.03-0.3% of a cell nucleating agent; the organophosphate compound is at least one of tris(2,3-dibromopropyl) phosphate, tricresyl phosphate, lauryl alcohol ether phosphate, and tri-tert-butylphenyl phosphate. In the above method, the organophosphate compound is mainly used to inhibit the caking of fine particles in the kettle or the caking of beads outside the kettle, which can reduce the dosage of the dispersant. Finally, the obtained foamed polypropylene beads have a bulk density higher than 50 g / L and have no thickening effect. Summary of the Invention

[0009] The purpose of the present invention is to provide a preparation method and application of foamed polypropylene beads, which have higher efficiency in the production process of high magnification EPP beads, and the molded parts have low molding energy consumption, excellent appearance and excellent mechanical properties.

[0010] To achieve the above object, the present invention provides a kind of foamed polypropylene beads. The weight parts of each component are as follows: the feeding weight ratio of polypropylene microparticles, water, dispersant and co-dispersant is 1: 1-2.5: 0.0015-0.01: 0.0002-0.002.

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

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

[0013] Preferably, the other plastic auxiliaries include at least one of antioxidant, lubricant, antistatic agent and colorant.

[0014] The antioxidant is selected from at least one of hindered phenol compounds, thio propionate compounds and phosphite compounds.

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

[0016] When only one kind of other plastic auxiliaries is selected, its weight proportion in the microparticles is 0-5%.

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

[0018] The co-dispersant is sodium dodecyl benzene sulfonate.

[0019] The conductivity of water is lower than 10 ms / m.

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

[0021] The cell nucleating agent includes one or several of talcum powder, zinc borate, zinc oxide and calcium carbonate, and the average particle size is 1-15 microns.

[0022] The present invention provides a preparation method of foamed polypropylene beads, which includes the following steps:

[0023] S1. After uniformly mixing polypropylene, a cell nucleating agent, a regulator, and other plastic auxiliaries, they are melt-mixed by an extruder, drawn into filaments and cut into pellets 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, a dispersant, and a co-dispersant into a high-pressure resistant autoclave in proportion and seal it; under continuous stirring, heat the materials in the high-pressure resistant autoclave to the foaming temperature, and at the same time inject carbon dioxide into the high-pressure resistant autoclave until the pressure in the autoclave reaches the foaming pressure, and maintain the treatment of the materials in the high-pressure resistant autoclave under the conditions of the foaming temperature and the foaming pressure; open the autoclave body, and discharge the materials into a foaming device with a water vapor atmosphere for foaming and expansion to obtain foamed EPP beads with a bulk density of 20 - 34 g / L, and the endothermic enthalpy value range of the melting peak higher than the inherent melting point in the first DSC melting curve of the foamed beads is 13 - 21 J / g.

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

[0026] Preferably, in step S2, the time for maintaining the treatment of the materials in the high-pressure resistant autoclave under the conditions of the foaming temperature and the foaming pressure is 10 - 60 min, the foaming temperature of the materials is 137 - 152 °C, the foaming pressure of the materials is 2 - 4 MPa, the pressure range of the water vapor in the foaming device is 0.01 - 0.1 MPa; the treatment time for discharging the materials into the foaming device with a water vapor atmosphere for foaming and expansion is 8 - 30 s.

[0027] The present invention provides a preparation method of a molded part. After the polypropylene beads are pressurized by air in a closed container, they are vacuum-filled into a mold, heated and molded by water vapor, cooled by water, and an out-of-mold part is obtained. The out-of-mold part is treated in a hot air atmosphere to obtain a finished molded part.

[0028] Preferably, the polypropylene beads are pressurized at 0.5 MPa air pressure in a closed container for 8 h, and the out-of-mold part is treated in a hot air atmosphere at 70 - 90 °C for 5 - 10 h to obtain a finished molded part. The EPP molded part has excellent appearance, that is, there are no or few gaps and pits on the surface; the compression stress of the EPP molded part at 10% compression strain is not less than 80 kPa, and the tensile strength is not less than 600 kPa.

[0029] Preferably, the minimum molding pressure of EPP bead molding is lower than 2.2 bar, and when the cooling water temperature is 35 °C, the water cooling time for single-mode EPP hot molding is less than 80 s.

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

[0031] Different from the traditional EPP secondary foaming process, the particles of the present invention are directly discharged from the autoclave into the foamer in a water vapor atmosphere for foaming, and only go through one foaming process to obtain ultra-light EPP beads, avoiding the certain time interval between the first and second foaming processes in the traditional secondary foaming process, improving the production efficiency of ultra-light EPP beads; and there is no need to pre-press the particles again, and it is easy to prepare ultra-low density EPP beads.

[0032] During the process of discharging the material under the autoclave pressure of the first foaming into the foamer, the pores inside the particles are gradually forming and not solidified. The water vapor in the foamer assists in foaming, which is conducive to making the sizes of the pores on the surface layer and inside of the foamed beads more uniform. It has a significant effect on improving the molding expansibility of EPP, reducing the molding steam pressure of EPP beads, 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 strong water absorption and thickening effects; during the autoclave foaming process, the thickening effect increases the melt viscosity of the material, which is conducive to the formation of the closed-cell structure of the foamed beads; the absorbed water exists in the material in the form of water vapor, which serves as a co-foaming agent to facilitate the volume expansion and foaming uniformity of the material, and it is easy to obtain foamed beads with a high foaming ratio and excellent molding properties. Brief Description of the Drawings

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

[0035] Figure 2 It is a SEM picture of the cross-section of the foamed beads obtained in Comparative Example 9. Detailed Embodiments

[0036] The technical solutions of the present invention are further described below through examples.

[0037] Unless otherwise defined, the technical terms or scientific terms used in the present invention should have the ordinary meanings understood by those with ordinary skills in the field to which the present invention belongs.

[0038] Table 1 Main material information in the examples

[0039]

[0040] Example 1

[0041] After uniformly mixing the materials of 98.45% polypropylene 1, 0.15% talcum powder, 0.2% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax by weight, passing through a single-screw extruder with a length-diameter ratio of 30:1, at a processing temperature of 220°C, a screw speed of 60 rpm, melting and mixing, and drawing and pelletizing, polypropylene microparticles are obtained. The length of the microparticles is 2.0 mm, and the single-particle weight of the microparticles is 1.2 mg.

[0042] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151°C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body and discharge the materials into a foamer with a steam pressure of 0.04 MPa for treatment for 14 s. The bulk density of the foamed EPP beads obtained is 21 g / L, and the endothermic enthalpy value range of the melting peak higher than the inherent melting point in the first DSC melting curve of the foamed beads is 18.5 J / g.

[0043] Example 2

[0044] Mix the materials of 96.65% polypropylene 1, 0.15% talcum powder, 2% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly. Then, through a single-screw extruder with a length-diameter ratio of 30:1, at a processing temperature of 220°C and a screw speed of 60 rpm, melt and mix, draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0045] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151.3°C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.4 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body and discharge the materials into a foamer with a steam pressure of 0.04 MPa for treatment for 14 s. The bulk density of the foamed EPP beads obtained is 21 g / L, and the endothermic enthalpy value range of the melting peak higher than the inherent melting point in the first DSC melting curve of the foamed beads is 18.2 J / g.

[0046] Example 3

[0047] Mix the materials of 98.15% polypropylene 2, 0.15% talcum powder, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly. Then, through a single-screw extruder with a length-diameter ratio of 30:1, at a processing temperature of 220°C and a screw speed of 60 rpm, melt and mix, draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0048] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into a high-pressure resistant autoclave in a weight ratio of 1:1.6:0.007:0.001 and seal it; under continuous stirring, heat the materials in the high-pressure resistant autoclave to 145°C, and at the same time inject carbon dioxide into the high-pressure resistant autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, and discharge the materials into a foaming device with a steam pressure of 0.04 MPa for treatment for 14 s. The bulk density of the foamed EPP beads obtained is 21 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 16.4 J / g.

[0049] Example 4

[0050] Mix the materials with a weight percentage of 93.15% polypropylene 2, 0.15% talcum powder, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, 0.5% polypropylene wax, and 5% carbon black evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220°C and a screw speed of 60 rpm, melt and mix them, and then draw and pelletize them to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0051] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into a high-pressure resistant autoclave in a weight ratio of 1:1.6:0.007:0.001 and seal it; under continuous stirring, heat the materials in the high-pressure resistant autoclave to 145.5°C, and at the same time inject carbon dioxide into the high-pressure resistant autoclave until the pressure in the autoclave reaches 3.2 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, and discharge the materials into a foaming device with a steam pressure of 0.04 MPa for treatment for 12 s. The bulk density of the foamed EPP beads obtained is 25 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 15.2 J / g.

[0052] Comparative Example 1

[0053] Mix the materials with a weight percentage of 98.65% polypropylene 1, 0.15% talcum powder, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220°C and a screw speed of 60 rpm, melt and mix them, and then draw and pelletize them to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0054] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, discharge the materials into a foamer with a steam pressure of 0.04 MPa for 14 s of treatment, and obtain foamed EPP beads with a bulk density of 30 g / L. The endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 18.7 J / g.

[0055] Comparative Example 2

[0056] Mix the materials with a weight percentage of 98.5% polypropylene 1, 0.15% talcum powder, 0.15% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix them, draw and pelletize them to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0057] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, discharge the materials into a foamer with a steam pressure of 0.04 MPa for 14 s of treatment, and obtain foamed EPP beads with a bulk density of 27 g / L. The endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 18.5 J / g.

[0058] Comparative Example 3

[0059] Mix the materials with a weight percentage of 96.65% polypropylene 1, 0.15% talcum powder, 2% tricresyl phosphate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix them, draw and pelletize them to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0060] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave in a weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151.3 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.4 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body and discharge the materials into a foamer with a steam pressure of 0.04 MPa for treatment for 14 s, obtaining foamed EPP beads with a bulk density of 30 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 18.4 J / g.

[0061] Comparative Example 4

[0062] Mix the materials with a weight percentage of 96.15% polypropylene 1, 0.15% talcum powder, 2.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix, draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0063] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave in a weight ratio of 1:1.6:0.007:0.001 and seal it. Under continuous stirring, heat the materials in the autoclave to 151.3 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.4 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body and discharge the materials into a foamer with a steam pressure of 0.04 MPa for treatment for 14 s, obtaining foamed EPP beads with a bulk density of 21 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 18.1 J / g.

[0064] Comparative Example 5

[0065] Mix the materials with a weight percentage of 98.65% polypropylene 1, 0.15% talcum powder, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix, draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0066] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added together to a high-pressure resistant autoclave in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the materials in the high-pressure resistant autoclave were heated to 151 °C, and at the same time, carbon dioxide was injected into the high-pressure resistant autoclave until the pressure in the autoclave reached 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, the autoclave body was opened, and the materials were discharged into a foamer with a steam pressure of 0.1 MPa for treatment for 30 s. The bulk density of the foamed EPP beads obtained was 21 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads was 18.3 J / g.

[0067] Comparative Example 6

[0068] Materials with a weight percentage of 98.15% polypropylene 2, 0.15% talcum powder, 0.2% antioxidant, 0.5% polyethylene glycol, 0.5% light stabilizer, and 0.5% polypropylene wax were uniformly mixed, and then passed through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, they were melt-mixed, drawn into strands, and pelletized to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0069] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added together to a high-pressure resistant autoclave in a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the materials in the high-pressure resistant autoclave were heated to 145 °C, and at the same time, carbon dioxide was injected into the high-pressure resistant autoclave until the pressure in the autoclave reached 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, the autoclave body was opened, and the materials were discharged into a foamer with a steam pressure of 0.04 MPa for treatment for 14 s. The bulk density of the foamed EPP beads obtained was 21 g / L, and the endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads was 16.2 J / g.

[0070] Comparative Example 7

[0071] Materials with a weight percentage of 98.15% polypropylene 2, 0.15% talcum powder, 0.2% antioxidant, 0.5% glycerol, 0.5% light stabilizer, and 0.5% polypropylene wax were uniformly mixed, and then passed through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, they were melt-mixed, drawn into strands, and pelletized to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0072] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it; under continuous stirring, heat the materials in the autoclave to 145 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, and discharge the materials into a foaming device with a steam pressure of 0.04 MPa for treatment for 14 s to obtain foamed EPP beads with a bulk density of 21 g / L. The endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 16.2 J / g.

[0073] Comparative Example 8

[0074] Mix the materials with a weight percentage of 98.15% polypropylene 2, 0.15% talcum powder, 0.2% antioxidant, 0.5% sodium polyacrylate, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix them, and then draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0075] Add polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate into an autoclave according to the weight ratio of 1:1.6:0.007:0.001 and seal it; under continuous stirring, heat the materials in the autoclave to 145 °C, and at the same time inject carbon dioxide into the autoclave until the pressure in the autoclave reaches 3.6 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, open the autoclave body, and discharge the materials into a foaming device with a hot air atmosphere and an internal temperature of 95 °C for treatment for 14 s to obtain foamed EPP beads with a bulk density of 21 g / L. The endothermic enthalpy value range of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 16.5 J / g.

[0076] Comparative Example 9

[0077] Mix the materials with a weight percentage of 98.15% polypropylene 2, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax evenly, and then pass them through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, melt and mix them, and then draw and pelletize to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0078] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added into an autoclave at a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the materials in the autoclave were heated to 145.6 °C, and at the same time, carbon dioxide was injected into the autoclave until the pressure in the autoclave reached 2.7 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, the autoclave body was opened, and the materials were discharged into a breathable collection bag at normal temperature and pressure, obtaining EPP beads with a bulk density of 47 g / L after the first foaming. After the first-foamed beads were left standing at normal temperature and pressure in the breathable collection bag for 24 h, they were suctioned into a closed pre-pressurization tank. After being pressurized by air in the pre-pressurization tank, they were put into a foaming barrel with a steam atmosphere of 0.085 MPa for the second foaming for 14 s, obtaining final foamed beads with a bulk density of 21 g / L, and the endothermic enthalpy value range of the melting peak higher than the inherent melting point in the first DSC melting curve of the foamed beads was 16.4 J / g. Among them, the pressure-bearing process of the first-foamed beads in the pre-pressurization tank was that the air pressure in the tank uniformly increased from normal pressure to 0.7 MPa in 24 h, and then remained at 0.7 MPa and could be used for the second foaming.

[0079] Comparative Example 10

[0080] Materials with a weight percentage of 98.15% polypropylene 2, 0.15% talcum powder, 0.5% sodium polyacrylate, 0.2% antioxidant, 0.5% light stabilizer, and 0.5% polypropylene wax were uniformly mixed, and then passed through a single-screw extruder with a length-diameter ratio of 30:1. At a processing temperature of 220 °C and a screw speed of 60 rpm, they were melt-mixed, drawn into strands, and cut into pellets to obtain polypropylene microparticles with a particle length of 2.0 mm and a single-particle weight of 1.2 mg.

[0081] Polypropylene microparticles, water, kaolin, and sodium dodecylbenzenesulfonate were added into an autoclave at a weight ratio of 1:1.6:0.007:0.001 and sealed. Under continuous stirring, the materials in the autoclave were heated to 145.6 °C, and at the same time, carbon dioxide was injected into the autoclave until the pressure in the autoclave reached 2.7 MPa. After maintaining this temperature and pressure in the autoclave for 20 min, the autoclave body was opened, and the materials were discharged into a breathable collection bag at normal temperature and pressure, obtaining EPP beads with a bulk density of 47 g / L after the first foaming. The collected first-foamed beads were immediately put into a foaming barrel with a steam atmosphere of 0.1 MPa for the second foaming for 30 s, obtaining final foamed beads with a bulk density of 45 g / L, and the endothermic enthalpy value range of the melting peak higher than the inherent melting point in the first DSC melting curve of the first-foamed beads was 16.2 J / g.

[0082] Because the bead material and pore structure have solidified after the first foaming and there is not enough internal pressure in the internal pore structure, it cannot further foam and expand even in the water vapor atmosphere, and the water vapor atmosphere has little effect on the number of pores in the foamed beads and the uniform distribution of pores in the inner and outer layers of the beads during this process. When the above collected primary foamed beads are immediately put into a foaming barrel with a water vapor atmosphere higher than 0.1MPa for secondary foaming, a large number of lumps appear between the beads due to the high atmosphere temperature, and it is impossible to obtain foamed beads with a bulk density lower than 34g / L.

[0083] Example of molding preparation:

[0084] The EPP beads were pressurized in a closed container at 0.5 MPa air pressure for 8 h, vacuum filled into a rectangular mold (450 mm long, 350 mm wide, 60 mm thick) and steam molded; the resulting module was baked in a hot air atmosphere of 80°C for 5 h, and then slowly cooled to room temperature within 2 h to obtain the final molded part for performance testing.

[0085] Among them, the minimum molding pressure is the minimum steam sintering pressure required for the proportion of particles with destroyed cells on the broken surface of the foamed polypropylene particle molding to be more than 95%.

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

[0087] Determination of performance parameters of molded parts:

[0088] Apparent quality of parts: Indicates that there are many pits or gaps on the surface of the workpiece; "□" indicates that there are few pits or gaps on the surface of the workpiece; It means that there are no or very few or very small pits or gaps on the surface of the workpiece.

[0089] The compressive stress of molded parts 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 Properties of EPP molded parts of Examples and Comparative Examples

[0092]

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

[0094] Compared with Example 1, in Comparative Example 1, sodium polyacrylate is not contained, lacking the swelling assisting effect. In the same foaming process, EPP beads with a lighter density cannot be obtained, and the foaming uniformity of the beads is worse, resulting in worse molding expansibility, increased molding pressure, increased water cooling time, and more gaps and pits appearing on the molded parts. Compared with Example 1, in Comparative Example 2, the content of sodium polyacrylate is low, and the swelling assisting effect is weak. In the same foaming process, EPP beads with a lighter density cannot be obtained, and the foaming uniformity of the beads is relatively poor, resulting in poor molding expansibility, increased molding pressure, increased water cooling time, and a small number of gaps appearing on the molded parts. Compared with Example 2, in Comparative Example 3, tricresyl phosphate is used to replace sodium polyacrylate, lacking the swelling assisting effect. In the same foaming process, EPP beads with a lighter density cannot be obtained, and the foaming uniformity of the beads is worse, resulting in worse molding expansibility, increased molding pressure, increased water cooling time, and more gaps and pits appearing on the molded parts. Compared with Example 2, in Comparative Example 4, an excessive amount of sodium polyacrylate is added, and its agglomeration tendency in polypropylene increases. In the same foaming process, it is instead not conducive to the foaming uniformity of the beads, resulting in relatively poor molding expansibility, increased molding pressure, increased water cooling time, and a small number of gaps and pits appearing on the molded parts. Compared with Example 1, in Comparative Example 5, sodium polyacrylate is not contained, and there is a lack of swelling assisting effect during the foaming process. By increasing the steam pressure in the foamer and prolonging the foaming time of the beads in the foamer, beads with a bulk density of 21 g / L are forcibly foamed, increasing the degree of damage to the closed-cell structure of the foamed particles. Not only is the molding property poor, but also the compressive stress and tensile strength at 10% strain of the molded parts are reduced. Compared with Example 3, in Comparative Examples 6 and 7, polyethylene glycol and glycerol are used to replace sodium polyacrylate respectively. Although the foaming effect is also achieved, the thickening effect is lacking, and the beads are more likely to have pores during the foaming process. The cell uniformity of the foamed beads becomes worse, the molding property becomes worse, the molding pressure increases, the water cooling time increases, and the compressive stress and tensile strength at 10% strain of the molded parts are reduced. Compared with Example 3, in Comparative Example 8, continuous foaming is carried out in a hot air atmosphere of 95 °C in the foamer. The heat penetration of the hot air to the material is weak. Although the foaming effect is also achieved, the cell structure difference between the surface layer and the inner layer of the foamed beads is relatively large, the molding property is poor, and the compressive stress and tensile strength at 10% strain of the molded parts are reduced.Compared with Example 3, Comparative Example 9 did not adopt the primary foaming process of the present technology, but adopted the traditional process that undergoes two foaming procedures, namely the traditional process of primary autoclave pressure foaming and secondary high-pressure air carrier pressure post-foaming. Finally, ultra-light beads with a density of 21 g / L were also obtained, but the preparation process took at least 48 hours more time-consuming, and the production efficiency was low. In addition, during the primary foaming process of Comparative Example 9, the distribution trend of the cell structure had been formed. Although the secondary foaming was also heated in a water vapor atmosphere, the secondary foaming process only enlarged the cell size of the primary foamed beads to a certain extent, and had little effect on the number of cells and the uniform distribution of the cells inside and outside the beads. Therefore, only the secondary foaming effect was achieved, and the cell structure difference between the surface layer and the inner layer of the foamed beads was relatively large, the molding characteristics were poor, and the compressive stress and tensile strength at 10% strain of the molded parts decreased.

[0095] Figure 1 Figure Figure 2 is a SEM picture of the cross-section of the foamed beads obtained in Example 3. It can be seen that the cell size inside the beads is generally close to the cell size on the surface layer of the beads, that is, the uniformity of the cell size inside and outside the beads is excellent, which is beneficial for molding. Figure 2 Figure is a SEM picture of the cross-section of the foamed beads obtained in Comparative Example 9. It can be seen that the cell size inside the beads is larger and the cell size on the surface layer of the beads is smaller, that is, the difference in the cell size inside and outside the beads is large, which is not beneficial for molding.

[0096] Therefore, by adopting the above-mentioned primary foamed polypropylene beads, preparation method and application, the present invention has higher efficiency in the production process of high magnification EPP beads, and the molded parts have low molding 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 are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing foamed polypropylene beads at one time, characterized in that, It includes the following steps: S1. After uniformly mixing polypropylene, a cell nucleating agent, a regulator, and other plastic auxiliaries, melt-mix them through an extruder, draw and pelletize to obtain polypropylene microparticles. S2. Add the polypropylene microparticles, water, a dispersant, and a co-dispersant into a high-pressure resistant autoclave in proportion and seal it. Under continuous stirring, heat the materials in the high-pressure resistant autoclave to the foaming temperature, and at the same time inject carbon dioxide into the high-pressure resistant autoclave until the pressure in the autoclave reaches the foaming pressure. Maintain the materials in the high-pressure resistant autoclave under the conditions of foaming temperature and foaming pressure for treatment. Open the high-pressure resistant autoclave body, and discharge the materials into a foamer with a steam atmosphere for foaming and expansion to obtain foamed EPP beads.

2. The preparation method of the primary foamed polypropylene beads according to claim 1, wherein, In step S1, the temperature of the single-screw extrusion section is 200 - 240 °C; the length-diameter ratio of the extruder screw is (20 - 40):1; the screw speed is 40 - 80 rpm; among them, the length of the microparticles is 0.8 - 2.5 mm, and the weight of a single microparticle is 0.3 - 2 mg; in step S2, the time for maintaining the materials in the high-pressure resistant autoclave under the conditions of foaming temperature and foaming pressure for treatment is 10 - 60 min, the foaming temperature of the materials is 137 - 152 °C, the foaming pressure of the materials is 2 - 4 MPa, the pressure range of the steam in the foamer is 0.01 - 0.1 MPa; the treatment time for discharging the materials into a foamer with a steam atmosphere for foaming and expansion is 8 - 30 s.

3. The preparation method of the primary foamed polypropylene beads according to claim 1, characterized in that The beads are prepared from polypropylene microparticles, water, a dispersant, and a co-dispersant according to a weight ratio of 1:1 - 2.5:0.0015 - 0.01:0.0002 - 0.

002.

4. The preparation method of the once-foamed polypropylene beads according to claim 1, characterized in that, The polypropylene microparticles at least include the following components in weight percentages: 87.5 - 99.77% of polypropylene, 0.03 - 0.5% of a cell nucleating agent, 0.2 - 2% of a regulator, and 0 - 10% of other plastic auxiliaries.

5. The preparation method of the primary foamed polypropylene beads according to claim 1, characterized in that, The polypropylene is random copolymer polypropylene, with a melt index of 6 - 9 g / 10 min (230 °C, 2.16 kg) and a melting point of 138 - 144 °C.

6. The preparation method of the once-foamed polypropylene beads according to claim 1, characterized in that, The regulator is sodium polyacrylate, selected from one or more with a relative molecular weight of 200,000 - 600,000.

7. The preparation method of the once-foamed polypropylene beads according to claim 1, characterized in that The cell nucleating agent is one or several of talcum powder, zinc borate, zinc oxide, calcium carbonate, with an average particle size of 1 - 15 microns.

8. The preparation method of the primary foamed polypropylene beads according to claim 1, characterized in that The other plastic auxiliaries are selected from at least one of an antioxidant, a lubricant, an antistatic agent, and a colorant; the antioxidant is selected from at least one of hindered phenol compounds, thioester compounds, and phosphite compounds; the lubricant is selected from at least one of erucamide, oleamide, glycerol monostearate, polyethylene wax, and polypropylene wax; when one kind of other plastic auxiliaries is selected, its weight proportion in the microparticles is 0 - 5%.

9. A molded part, characterized in that, It contains foamed polypropylene beads prepared by using the method according to any one of claims 1 - 8.

10. A method for preparing a molded part according to claim 9, characterized in that, After the polypropylene beads are pressurized by air in a closed container, they are vacuum-filled into a mold, heated and molded by steam, then cooled by passing water to obtain a demolded part, and the demolded part is treated in a hot air atmosphere to obtain a finished molded part.

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