Gray foamed polypropylene beads, preparation method thereof, and molded parts

Abstract

The present invention provides gray foamed polypropylene beads, a preparation method thereof, and molded parts, relating to the technical field of polymer materials. The gray foamed polypropylene beads are prepared by kettle foaming modified polypropylene particles. The modified polypropylene particles comprise the following components, by weight: 72-95 parts of polypropylene A; 1-10 parts of a color masterbatch; 1-3 parts of a cell nucleating agent masterbatch; 1-5 parts of a foaming agent masterbatch; 1-5 parts of a lubricant masterbatch; and 1-5 parts of an antioxidant masterbatch. By utilizing the synergistic effect of the various masterbatches, the present invention utilizes only a single carbon black color matching system with a relatively low amount of carbon black added to obtain gray foamed polypropylene beads with uniform color. Furthermore, the surface of the molded parts produced therefrom has no or only minimal or very small pits or cracks, thereby significantly improving the surface quality of the molded parts.

Description

Technical Field

[0001] The invention relates to the technical field of polymer materials, in particular to gray foamed polypropylene beads and a preparation method and molded parts thereof. Background Art

[0002] Expanded polypropylene (EPP) beads and their molded parts are widely used in automotive parts, turnover packaging, cushioning packaging, lightweight home furnishings, children's entertainment and other fields due to their light weight, excellent mechanical properties and excellent energy absorption.

[0003] In recent years, gray, as a light color between white and black, is more likely to make packaging products reflect a sense of luxury. Therefore, packaging users are increasingly favoring gray EPP packaging products. In particular, more and more EPP packaging products are no longer just used as lining cushioning materials, but more and more use integrated packaging solutions, so that the appearance of EPP products can be directly presented in various application scenarios. Based on this, packaging users have increasingly higher requirements for the appearance quality of gray EPP products.

[0004] Currently, the gray EPP beads and molded parts thereof available in the domestic market have always had problems such as uneven color to varying degrees. In view of this, providing a gray foamed polypropylene bead with uniform color and its molded parts is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: in order to solve the problem of poor surface color uniformity of gray foamed polypropylene beads and molded parts prepared therefrom in the prior art, the present invention provides gray foamed polypropylene beads. Through formula optimization, gray foamed polypropylene beads with uniform color can be obtained by using only a single carbon black coloring material, and the molded parts prepared therefrom have no or only very few or very small pits or cracks on the surface, thereby solving the problem of poor surface color uniformity of molded parts prepared therefrom in the prior art.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] Gray foamed polypropylene beads are prepared by kettle foaming of modified polypropylene particles;

[0008] The modified polypropylene particles include the following components in parts by weight:

[0009]

[0010] The masterbatch comprises the following components in parts by weight:

[0011] 15 parts of carbon black;

[0012] 5 parts of maleic anhydride modified polypropylene wax;

[0013] Polypropylene A 80 parts.

[0014] Optionally, the carbon black has an average particle size range of 30-80 nm and a DBP oil absorption value of (60-120) ml / 100 g.

[0015] Optionally, the cell nucleating agent masterbatch comprises the following components in parts by weight:

[0016] 10 parts of cell nucleating agent powder;

[0017] 3 parts of maleic anhydride modified polypropylene wax;

[0018] Polypropylene A 87 parts;

[0019] The cell nucleating agent powder is selected from at least one of polytetrafluoroethylene powder, zinc borate, talc, barium sulfate, calcium carbonate, calcium stearate, and silicon dioxide; and the average particle size of the cell nucleating agent powder is 3-10 microns.

[0020] Optionally, the co-foaming agent masterbatch includes the following components in parts by weight:

[0021] 10 parts of auxiliary foaming agent;

[0022] 3 parts of maleic anhydride modified polypropylene wax;

[0023] Polypropylene A 87 parts;

[0024] The auxiliary foaming agent is selected from at least one of sodium chloride, potassium chloride and alum.

[0025] Optionally, the lubricant masterbatch comprises the following components in parts by weight:

[0026] 10 parts of lubricant;

[0027] Polypropylene A 90 parts;

[0028] The lubricant is selected from at least one of erucamide and glyceryl monostearate.

[0029] Optionally, the antioxidant masterbatch comprises the following components in parts by weight:

[0030]

[0031] Optionally, the number average molecular weight of the maleic anhydride modified polypropylene wax is 5000-10000.

[0032] Optionally, the polypropylene A is a random copolymer polypropylene with a melt index of 6-9 g / 10 min and a melting point of 130-150° C., preferably 138-148° C.

[0033] Another object of the present invention is to provide a method for preparing the gray foamed polypropylene beads as described above, comprising the following preparation steps: adding modified polypropylene microparticles together with a dispersant and water into an autoclave, sealing the autoclave, adding CO2, and stirring; raising the temperature of the autoclave to a foaming temperature, wherein the temperature in the autoclave is uniformly raised from 90°C to the foaming temperature over a period of 60-90 minutes; adjusting the pressure in the autoclave to a foaming pressure; maintaining the pressure at the foaming temperature and pressure for 5-30 minutes; and releasing the pressure to discharge the material into a foaming pipe at normal pressure for foaming, wherein the atmosphere in the foaming pipe is at a temperature of 85-110°C and the material remains in the foaming pipe for 4-15 seconds, thereby obtaining gray foamed polypropylene beads.

[0034] Another object of the present invention is to provide a molded article obtained by steam molding the gray foamed polypropylene beads as described above.

[0035] The beneficial effects of the present invention are:

[0036] The gray expanded polypropylene beads provided by the present invention are characterized by the fact that the color uniformity of the gray expanded polypropylene beads and molded parts is generally affected by multiple factors such as the uniformity of the dispersion of the coloring material, the uniformity of the density or expansion ratio between the beads, and the uniformity of the bead pores. The carbon black colorant is introduced in the form of a masterbatch, which helps to achieve uniform dispersion of the carbon black colorant. Furthermore, the combination of a pore nucleating agent and a foaming aid is beneficial to achieving uniform expansion ratio and pores between the expanded beads. Furthermore, through the synergistic effect of the masterbatches, gray expanded polypropylene beads with uniform color can be obtained by using only a single carbon black color matching system and a relatively low amount of carbon black added. While improving the surface color uniformity of the molded parts prepared from the gray expanded polypropylene beads, the surfaces of the molded parts prepared have no or only very few or very small pits or cracks, thereby greatly improving the surface quality of the molded parts. DETAILED DESCRIPTION

[0037] The present invention will now be described in further detail. The embodiments described below are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0038] To address the problem of poor surface color uniformity of gray foamed polypropylene beads and molded parts prepared therefrom in the prior art, the present invention provides gray foamed polypropylene beads, which are prepared by autoclave foaming of modified polypropylene particles. Specifically, the modified polypropylene particles comprise the following components, calculated in parts by weight:

[0039]

[0040] Wherein, according to parts by weight, the masterbatch includes the following components:

[0041] 15 parts of carbon black;

[0042] 5 parts of maleic anhydride modified polypropylene wax;

[0043] Polypropylene A 80 parts.

[0044] The weight of a single modified polypropylene microparticle of the present invention is preferably in the range of 0.5-1.8 mg, and the preparation process of the modified polypropylene microparticles is further preferably as follows: after the above materials are homogenized, they are melt-mixed in a twin-screw extruder at a processing temperature of 160-200°C, passed through a cylindrical die with a pore size of 1-3 mm, drawn, water-cooled and pelletized to obtain modified polypropylene microparticles with uniform single weight.

[0045] The gray expanded polypropylene beads provided by the present invention are characterized by the fact that the color uniformity of the gray expanded polypropylene beads and molded parts is generally affected by multiple factors such as the uniformity of the dispersion of the coloring material, the uniformity of the density or expansion ratio between the beads, and the uniformity of the bead pores. The carbon black colorant is introduced in the form of a masterbatch, which helps to achieve uniform dispersion of the carbon black colorant. Furthermore, the combination of a pore nucleating agent and a foaming aid is beneficial to achieving uniform expansion ratio and pores between the expanded beads. Furthermore, through the synergistic effect of the masterbatches, gray expanded polypropylene beads with uniform color can be obtained by using only a single carbon black color matching system and a relatively low amount of carbon black added. While improving the surface color uniformity of the molded parts prepared from the gray expanded polypropylene beads, the surfaces of the molded parts prepared have no or only very few or very small pits or cracks, thereby greatly improving the surface quality of the molded parts.

[0046] In order to further improve the uniformity of the color distribution on the surface of the molded part, the present invention preferably has an average particle size of carbon black in the range of 30-80 nm and a DBP oil absorption value of (60-120) ml / 100 g.

[0047] The preferred preparation process of the masterbatch of the present invention is as follows: according to the formula of the masterbatch, the masterbatch is melted and granulated by a twin-screw extruder to obtain a masterbatch with uniform single particle weight, preferably a single particle weight range of 20-30 mg.

[0048] The present invention further improves the uniformity of carbon black dispersion by using specially selected carbon black colorants in combination with various masterbatch formulation processes, thereby helping to further improve the uniformity of color distribution on the surface of molded parts.

[0049] In order to improve the uniformity of the expansion ratio of the expanded polypropylene beads and the uniformity of the bead cells, the present invention preferably comprises the following components in parts by weight:

[0050] 10 parts of cell nucleating agent powder;

[0051] 3 parts of maleic anhydride modified polypropylene wax;

[0052] Polypropylene A 87 parts;

[0053] The cell nucleating agent powder is selected from at least one of polytetrafluoroethylene powder, zinc borate, talc, barium sulfate, calcium carbonate, calcium stearate, and silicon dioxide; and the average particle size of the cell nucleating agent powder is preferably 3-10 microns.

[0054] The preferred preparation process of the cell nucleating agent masterbatch of the present invention is as follows: according to the formula of the cell nucleating agent masterbatch, the cell nucleating agent masterbatch is melt-granulated by a twin-screw extruder to obtain cell nucleating agent masterbatch with uniform single particle weight, preferably with a single particle weight range of 20-30 mg.

[0055] In the present invention, the foaming agent masterbatch preferably comprises the following components in parts by weight:

[0056] 10 parts of auxiliary foaming agent;

[0057] 3 parts of maleic anhydride modified polypropylene wax;

[0058] Polypropylene A 87 parts;

[0059] Preferably, the auxiliary foaming agent is selected from at least one of sodium chloride, potassium chloride and alum.

[0060] The preparation process of the foaming agent masterbatch is as follows: according to the formula of the foaming agent masterbatch, the foaming agent masterbatch is melted and granulated by a twin-screw extruder to obtain the foaming agent masterbatch with uniform single particle weight, and the single particle weight range is 20-30 mg.

[0061] The present invention can greatly improve the uniformity of the expansion ratio and the uniformity of the pores of the gray foamed polypropylene beads by combining the pore nucleating agent masterbatch with the auxiliary foaming agent masterbatch, thereby helping to improve the uniformity of the color distribution on the surface of the molded parts. Furthermore, gray foamed polypropylene beads and molded parts with uniform color distribution can be prepared by adding a small amount of a single carbon black coloring material.

[0062] The present invention preferably comprises the following components in parts by weight:

[0063] 10 parts of lubricant;

[0064] 90 parts of polypropylene A;

[0065] The lubricant is selected from at least one of erucamide and glyceryl monostearate.

[0066] The lubricant masterbatch preparation process is as follows: according to the formula of the lubricant masterbatch, the lubricant masterbatch is melted and granulated by a twin-screw extruder to obtain lubricant masterbatch with uniform single particle weight, and the single particle weight range is 20-30 mg.

[0067] The present invention preferably comprises the following components in parts by weight:

[0068]

[0069] The preparation process of the antioxidant masterbatch is as follows: according to the formula of the antioxidant masterbatch, the antioxidant masterbatch is melted and granulated by a twin-screw extruder to obtain the antioxidant masterbatch with uniform single particle weight, and the single particle weight range is 20-30 mg.

[0070] The preferred number average molecular weight of the maleic anhydride modified polypropylene wax of the present invention is 5000-10000; the preferred polypropylene A is a granular raw material from a petrochemical plant; specifically, the preferred polypropylene A is a random copolymer polypropylene with a melt index of 6-9 g / 10 min and a melting point of 130-150° C., more preferably 138-148° C.

[0071] Another object of the present invention is to provide a method for preparing the gray expanded polypropylene beads described above, comprising the following steps: adding modified polypropylene microparticles, a dispersant, and water to an autoclave, sealing the autoclave, adding CO2, and stirring; raising the temperature of the autoclave to a foaming temperature, wherein the temperature in the autoclave is uniformly raised from 90°C to the foaming temperature over a period of 60-90 minutes; adjusting the pressure in the autoclave to a foaming pressure; maintaining the pressure at the foaming temperature and pressure for 5-30 minutes; and releasing the pressure to discharge the material into a foaming pipe at atmospheric pressure for foaming, wherein the atmosphere in the foaming pipe is at a temperature of 85-110°C and the material remains in the foaming pipe for 4-15 seconds, thereby obtaining gray expanded polypropylene beads. The gray expanded polypropylene beads have a bulk density of 15-120 g / L, and an endothermic enthalpy of 12-22 J / g for the melting peak above the intrinsic melting point in the first DSC melting curve.

[0072] Preferably, the dispersant is kaolin, and the mass ratio of the modified polypropylene particles, the dispersant and water is 100:(0.03-1):(100-200).

[0073] The method for preparing gray foamed polypropylene beads provided by the present invention, through a special temperature control step during the foaming process, combined with the formula of each masterbatch, is conducive to the uniformity of the foaming ratio and the foam cells among the foamed beads, thereby helping to improve the uniformity of the color distribution on the surface of the molded parts. Furthermore, gray foamed polypropylene beads and molded parts with uniform color distribution can be prepared by adding a small amount of a single carbon black coloring material.

[0074] Another object of the present invention is to provide a molded article obtained by steam molding the gray foamed polypropylene beads as described above.

[0075] Specifically, the molding process is preferably as follows: the gray foamed polypropylene beads described above are pressurized with air in a pre-pressurized tank, filled into a metal mold, and molded and sintered with high-temperature steam to obtain a gray molded part with uniform appearance.

[0076] In summary, the present invention improves the uniformity of material dispersion through the selection of special carbon black and the formulation of each masterbatch, which is beneficial to improving the color uniformity of the foamed beads; the average particle size range of carbon black is selected to be 30-80nm, which is beneficial to coloring and dispersion; if the carbon black particle size is less than 30nm, it is difficult to evenly disperse the carbon black, which is not conducive to improving the uniformity of color distribution; on the contrary, if the carbon black particle size is greater than 80nm, the coloring effect becomes worse; at the same time, the addition of maleic anhydride modified polypropylene wax with a specific molecular weight to the masterbatch helps to further improve the uniformity of carbon black dispersion, thereby helping to improve the uniformity of color distribution; in addition, the unit weight of each masterbatch in the present invention is within the range of 20-30mg, which is beneficial to the uniform distribution of the formula material during mechanical stirring, reducing the influence of the specific gravity difference of each masterbatch on mechanical stirring and dispersion, thereby helping to improve the uniformity of color distribution.

[0077] The combination of the cell nucleating agent masterbatch and the auxiliary foaming agent masterbatch in the present invention, along with the specialized temperature control step of uniformly increasing the temperature from 90°C to the foaming temperature during the foaming process, imparts a uniform density and cell structure to the foamed beads, thereby improving color uniformity between different foamed beads and across the entire surface of a single foamed bead, thereby also improving the appearance quality of molded parts. The specifically selected auxiliary foaming agent masterbatch and the cell nucleating agent masterbatch in the present invention allow for more uniform water vapor to act as the foaming gas during the autoclave foaming process, facilitating a uniform cell structure for the foamed beads. This uniform cell structure improves the uniformity of the color distribution of the foamed beads and the excellent appearance quality of the molded parts. During the autoclave foaming process, the time for the autoclave temperature to be uniformly increased from 90°C to the foaming temperature is preferably 60-90 minutes, so that the carbon dioxide can be more evenly impregnated into the polypropylene during the heating process, which is conducive to obtaining foamed beads with more uniform density distribution and more uniform pores, and is conducive to improving the uniformity of the color distribution of the foamed beads and the excellent appearance quality of the molded parts; if the time for the autoclave temperature to be uniformly increased from 90°C to the foaming temperature is less than 60 minutes during the foaming process, the density and pore structure of the obtained foamed beads tend to be more uneven; if the time for the autoclave temperature to be uniformly increased from 90°C to the foaming temperature is greater than 90 minutes during the foaming process, the foaming production efficiency is affected.

[0078] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below.

[0079] Example 1

[0080] The performance parameters of each raw material and the preparation methods of each material in this embodiment are as follows:

[0081] Polypropylene A1, melt index 7g / 10min, melting point 138℃.

[0082] Carbon black, average particle size 50nm, DBP oil absorption value 80ml / 100g.

[0083] Maleic anhydride modified polypropylene wax, number average molecular weight 8000.

[0084] The cell nucleating agent powder is silicon dioxide, and its average particle size is 5 microns.

[0085] The auxiliary foaming agent is sodium chloride, and its average particle size is 10 microns.

[0086] The lubricant is glyceryl monostearate.

[0087] Preparation of masterbatch: According to the weight parts, 15 parts by weight of carbon black, 5 parts by weight of maleic anhydride modified polypropylene wax, and 80 parts by weight of polypropylene A1 were stirred uniformly in a high-speed mixer and granulated by twin-screw mixing at 180°C to obtain a masterbatch with an average weight of 25 mg per particle.

[0088] Preparation of cell nucleating agent masterbatch: According to the weight parts, 10 parts by weight of silicon dioxide powder, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A1 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain cell nucleating agent masterbatch with an average weight of 25 mg per particle.

[0089] Preparation of foaming agent masterbatch: According to the weight parts, 10 parts by weight of sodium chloride particles, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A1 were stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a foaming agent masterbatch with an average weight of 25 mg per particle.

[0090] Preparation of lubricant masterbatch: According to parts by weight, 10 parts by weight of glyceryl monostearate and 90 parts by weight of polypropylene A1 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain lubricant masterbatch with an average single particle weight of 25 mg.

[0091] Preparation of antioxidant masterbatch: According to parts by weight, 10 parts by weight of antioxidant 1010, 5 parts by weight of antioxidant 1076, 5 parts by weight of antioxidant DLTP, and 80 parts by weight of polypropylene A1 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain antioxidant masterbatch with an average single particle weight of 25 mg.

[0092] Preparation of modified polypropylene microparticles: According to parts by weight, 91.5 parts by weight of polypropylene A1, 1 part by weight of color masterbatch, 1.5 parts by weight of cell nucleating agent masterbatch, 2 parts by weight of foaming agent masterbatch, 2 parts by weight of lubricant masterbatch, and 2 parts by weight of antioxidant masterbatch were stirred uniformly in a high-speed mixer, melt-mixed in a twin-screw extruder at a processing temperature of 180°C, passed through a cylindrical die with a pore size of 1.5 mm, drawn, water-cooled, and pelletized to obtain modified polypropylene microparticles with an average single particle weight of 1.1 mg and an average length of 2.0 mm.

[0093] Preparation of gray foamed polypropylene beads: modified polypropylene particles are added to an autoclave together with a dispersant and water; the dispersant is kaolin, and the mass ratio of modified polypropylene particles, dispersant and water is 100:0.6:160; CO2 is added after sealing and stirring continuously; the temperature of the reactor is gradually increased, and the time for the temperature in the reactor to uniformly increase from 90°C to the foaming temperature of 144°C is 65 minutes; the pressure in the reactor is adjusted to a foaming pressure of 2.4 MPa; the material in the reactor is maintained at 144°C and 2.4 MPa for 20 minutes; the pressure is released to discharge the material into a foaming pipe at normal pressure for foaming, the atmosphere temperature in the foaming pipe is 98°C, and the time the material spends in the foaming pipe is 10 seconds, and finally gray foamed polypropylene beads with a bulk density of 39 g / L are obtained. According to tests, the cell size of the gray foamed polypropylene beads ranges from 150 to 200 microns, and the endothermic enthalpy value of the melting peak above the intrinsic melting point in the first DSC melting curve of the gray foamed polypropylene beads is 14.5 J / g.

[0094] The cell size test method is as follows: 50 gray expanded polypropylene beads are randomly selected and cut open with a thin, sharp blade. The flat, uncut side is then gold-sprayed onto the surface. The cross-section cell structure is then observed and analyzed using a scanning electron microscope. Cell size is calculated using Image Pro Plus software. The cell size range is defined as the range between the minimum and maximum cell sizes measured for the 50 gray expanded polypropylene beads. A narrower cell size range indicates a more uniform cell profile.

[0095] Molding: Gray foamed polypropylene beads are subjected to an air pressure of 0.25 MPa for 10 hours in a pre-pressurized tank, then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a molded part with a uniform gray and glossy appearance is obtained.

[0096] Visual inspection revealed no, minimal, or very small pits or cracks on the surface of the molded part. Testing revealed that 95% of the particles on the cross-section of the molded part in this example had destroyed cells, demonstrating a good sintering state. Color analysis of the molded part revealed surface L values ​​of 55.11 to 59.15, a values ​​of 0.93 to 2.42, and b values ​​of 1.14 to 2.76.

[0097] Method for judging the sintering state of parts: during the breaking process of parts, if only the skin of the foamed beads is separated from each other and no destruction of the pores is observed, it indicates that the sintering between the beads is poor; during the breaking process of parts, if the skin between the foamed beads is torn or the internal pores are torn, and then the destroyed pores of the beads can be observed with the naked eye, it indicates that the sintering between the beads is strong; break the molded parts, observe and count the morphology of no less than 200 foamed beads on the cross section, and the value of (number of beads with destroyed pores / number of beads counted) × 100% is called the proportion of particles with destroyed pores on the broken surface of the molded parts. The higher the proportion, the more firmly the molded parts are sintered.

[0098] The color analysis method is as follows: Using a DS-700D spectrophotometer from Hangzhou Caipu Technology Co., Ltd., 100 test points were randomly selected on the surface of the molded part in the same environment to read the part's "L," "a," and "b" chromaticity values. "L" represents lightness, with a range of 0 to 100, indicating increasing brightness; "a" represents red and green, with a range of 127 to -128, indicating a gradual transition from magenta to green; and "b" represents yellow and blue, with a range of 127 to -128, indicating a gradual transition from yellow to blue. Within these 100 data sets, the ranges between the minimum and maximum L values, the minimum and maximum a values, and the minimum and maximum b values ​​are defined as the L, a, and b value ranges, respectively. Generally, the narrower the L, a, and b value ranges in the test cases, the more uniform the part's color.

[0099] Example 2

[0100] The difference between this embodiment and embodiment 1 is that the preparation process of the modified polypropylene particles is as follows:

[0101] According to parts by weight, 84.5 parts by weight of polypropylene A1, 8 parts by weight of color masterbatch, 1.5 parts by weight of cell nucleating agent masterbatch, 2 parts by weight of auxiliary foaming agent masterbatch, 2 parts by weight of lubricant masterbatch, and 2 parts by weight of antioxidant masterbatch are stirred uniformly in a high-speed mixer, melt-mixed at a processing temperature of 180°C in a twin-screw extruder granulator, passed through a cylindrical die with a pore size of 1.5 mm, drawn, water-cooled, and pelletized to obtain modified polypropylene particles with an average single particle weight of 1.1 mg and an average length of 2.0 mm.

[0102] Testing according to the method described in Example 1 revealed that the expanded beads produced in this example had a cell size range of 150-200 microns. The molding process produced a uniform gray molded part with minimal or no pits or cracks on the surface. The percentage of particles with destroyed cells on the cross-section of the molded part in this example was 95%, demonstrating a well-sintered state. The surface L values ​​of the molded part ranged from 23.02 to 26.33, a values ​​from -0.93 to 0.48, and b values ​​from -0.97 to 0.63.

[0103] Example 3

[0104] The performance parameters of each raw material and the preparation methods of each material in this embodiment are as follows:

[0105] Polypropylene A2, melt index 7g / 10min, melting point 142℃.

[0106] Carbon black, average particle size 50nm, DBP oil absorption value 80ml / 100g.

[0107] Maleic anhydride modified polypropylene wax, average molecular weight 8000.

[0108] The cell nucleating agent powder is silicon dioxide, and its average particle size is 5 microns.

[0109] The auxiliary foaming agent is sodium chloride, and its average particle size is 12 microns.

[0110] The lubricant is glyceryl monostearate.

[0111] Preparation of masterbatch: According to the parts by weight, 15 parts by weight of carbon black, 5 parts by weight of maleic anhydride modified polypropylene wax, and 80 parts by weight of polypropylene A2 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a masterbatch with an average single particle weight of 25 mg.

[0112] Preparation of cell nucleating agent masterbatch: According to the weight parts, 10 parts by weight of silicon dioxide powder, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A2 are stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a cell nucleating agent masterbatch with an average weight of 25 mg per particle.

[0113] Preparation of foaming agent masterbatch: According to the weight parts, 10 parts by weight of potassium chloride particles, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A2 are stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a foaming agent masterbatch with an average weight of 25 mg per particle.

[0114] Preparation of lubricant masterbatch: According to parts by weight, 10 parts by weight of glyceryl monostearate and 90 parts by weight of polypropylene A2 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain lubricant masterbatch with an average single particle weight of 25 mg.

[0115] Preparation of antioxidant masterbatch: According to the weight parts, 10 parts by weight of antioxidant 1010, 5 parts by weight of antioxidant 1076, 5 parts by weight of antioxidant DLTP, and 80 parts by weight of polypropylene A2 were stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain antioxidant masterbatch with an average weight of 25 mg per particle.

[0116] Preparation of modified polypropylene microparticles: According to parts by weight, 89.5 parts by weight of polypropylene A2, 3 parts by weight of color masterbatch, 1.5 parts by weight of cell nucleating agent masterbatch, 2 parts by weight of auxiliary foaming agent masterbatch, 2 parts by weight of lubricant masterbatch, and 2 parts by weight of antioxidant masterbatch are stirred uniformly in a high-speed mixer, melt-mixed at a processing temperature of 180°C in a twin-screw extruder granulator, passed through a cylindrical die with a pore size of 1.5 mm, drawn, water-cooled, and pelletized to obtain modified polypropylene microparticles with an average single particle weight of 1.1 mg and an average length of 2.0 mm.

[0117] Gray expanded polypropylene beads were prepared by adding modified polypropylene microparticles, a dispersant, and water to an autoclave. The dispersant was kaolin clay, and the mass ratio of modified polypropylene microparticles, dispersant, and water was 100:0.6:160. After sealing, CO2 was added with continuous stirring. The autoclave temperature was gradually increased from 90°C to a foaming temperature of 147°C at a constant rate of 65 minutes. The pressure in the autoclave was adjusted to a foaming pressure of 2.6 MPa. The contents were maintained at 147°C and 2.6 MPa for 20 minutes. The pressure was then released and the contents were discharged into a foaming pipe at atmospheric pressure for foaming at 102°C for 10 seconds. The resulting gray expanded polypropylene beads had a bulk density of 32 g / L. Testing revealed that the pore size of the gray expanded polypropylene beads ranged from 170 to 225 microns, and the endothermic enthalpy of the melting peak above the intrinsic melting point in the first DSC melting curve of the expanded beads was 16.2 J / g.

[0118] Molding: Gray foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurized tank, then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a molded part with a uniform gray and glossy appearance is obtained.

[0119] Visual inspection revealed that the molded part had no, or very few, pits or cracks on its surface. Testing showed that 90% of the particles on the cross-section of the molded part in this embodiment had destroyed cells, demonstrating a well-sintered state. The L value of the molded part's surface was 34.05 to 37.65, the a value was -1.39 to 0, and the b value was -1.14 to 0.46.

[0120] The test method in this embodiment refers to the relevant records in Example 1.

[0121] Example 4

[0122] The performance parameters of each raw material and the preparation methods of each material in this embodiment are as follows:

[0123] Polypropylene A2, melt index 7g / 10min, melting point 142℃.

[0124] Carbon black, average particle size 70nm, DBP oil absorption value 65ml / 100g.

[0125] Maleic anhydride modified polypropylene wax, average molecular weight 8000.

[0126] The cell nucleating agent powder is silicon dioxide, and its average particle size is 5 microns.

[0127] The auxiliary foaming agent is alum, and its average particle size is 15 microns.

[0128] The lubricant is glyceryl monostearate.

[0129] Preparation of masterbatch: According to the parts by weight, 15 parts by weight of carbon black, 5 parts by weight of maleic anhydride modified polypropylene wax, and 80 parts by weight of polypropylene A2 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a masterbatch with an average single particle weight of 25 mg.

[0130] Preparation of cell nucleating agent masterbatch: According to the weight parts, 10 parts by weight of silicon dioxide powder, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A2 are stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a cell nucleating agent masterbatch with an average weight of 25 mg per particle.

[0131] Preparation of foaming agent masterbatch: According to the weight parts, 10 parts by weight of alum particles, 3 parts by weight of maleic anhydride modified polypropylene wax, and 87 parts by weight of polypropylene A2 are stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain a foaming agent masterbatch with an average weight of 25 mg per particle.

[0132] Preparation of lubricant masterbatch: According to parts by weight, 10 parts by weight of glyceryl monostearate and 90 parts by weight of polypropylene A2 were stirred uniformly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain lubricant masterbatch with an average single particle weight of 25 mg.

[0133] Preparation of antioxidant masterbatch: According to the weight parts, 10 parts by weight of antioxidant 1010, 5 parts by weight of antioxidant 1076, 5 parts by weight of antioxidant DLTP, and 80 parts by weight of polypropylene A2 were stirred evenly in a high-speed mixer, and granulated by twin-screw mixing at 180°C to obtain antioxidant masterbatch with an average weight of 25 mg per particle.

[0134] Preparation of modified polypropylene microparticles: According to the weight parts, 89.5 parts by weight of polypropylene A2, 3 parts by weight of color masterbatch, 1.5 parts by weight of cell nucleating agent masterbatch, 2 parts by weight of auxiliary foaming agent masterbatch, 2 parts by weight of lubricant masterbatch, and 2 parts by weight of antioxidant masterbatch are stirred evenly in a high-speed mixer, melt-mixed at a processing temperature of 180°C in a twin-screw extruder granulator, passed through a cylindrical die with a pore size of 1.5 mm, drawn, water-cooled, and pelletized to obtain modified polypropylene microparticles with an average single particle weight of 1.1 mg and an average length of 2.0 mm.

[0135] Gray expanded polypropylene beads were prepared by adding modified polypropylene microparticles, a dispersant, and water to an autoclave. The dispersant was kaolin clay, and the mass ratio of modified polypropylene microparticles, dispersant, and water was 100:0.6:160. After sealing, CO2 was added with continuous stirring. The autoclave temperature was gradually increased from 90°C to a foaming temperature of 147°C over 65 minutes. The pressure in the autoclave was adjusted to a foaming pressure of 2.6 MPa. The contents were maintained at 147°C and 2.6 MPa for 20 minutes. The pressure was then released and the contents were discharged into a foaming pipe at atmospheric pressure for foaming at 102°C for 10 seconds. The resulting gray expanded polypropylene beads had a bulk density of 32 g / L. Testing revealed that the pore size of the gray expanded polypropylene beads ranged from 177 to 229 microns, and the endothermic enthalpy of the melting peak above the intrinsic melting point in the first DSC melting curve of the expanded beads was 16.4 J / g.

[0136] Molding: Gray foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurized tank, then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a molded part with a uniform gray and glossy appearance is obtained.

[0137] Visual inspection revealed no, minimal, or extremely small pits or cracks on the surface of the molded part. Testing revealed that 90% of the particles on the cross-section of the molded part in this example had destroyed cells, demonstrating a well-sintered state. The L value of the molded part's surface ranged from 39.21 to 42.45, a value from -1.52 to -0.13, and a value from -0.97 to 0.53.

[0138] The test method in this embodiment refers to the relevant records in Example 1.

[0139] Comparative Example 1

[0140] The difference between this comparative example and Example 3 is that the preparation process of the foamed polypropylene beads is as follows:

[0141] Modified polypropylene microparticles, a dispersant, and water were added to an autoclave. The dispersant was kaolin clay, and the mass ratio of modified polypropylene microparticles, dispersant, and water was 100:0.6:160. After sealing, CO2 was added with continuous stirring. The autoclave temperature was gradually increased from 90°C to a foaming temperature of 147°C over 50 minutes. The pressure in the autoclave was adjusted to a foaming pressure of 2.6 MPa. The contents were maintained at 147°C and 2.6 MPa for 20 minutes. The pressure was then released and the contents were discharged into a foaming pipe at atmospheric pressure for foaming at an atmosphere temperature of 102°C. The contents remained in the pipe for 10 seconds, resulting in foamed beads with a bulk density of 33 g / L. Testing revealed that the pore size of the foamed beads ranged from 120 to 287 microns, and the endothermic enthalpy of the melting peak above the intrinsic melting point in the first DSC melting curve of the foamed beads was 15.8 J / g.

[0142] Molding: After the foamed beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurized tank, they are filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0143] Visual inspection revealed a few darker "black spots" and lighter "white spots" on the surface of the molded part, as well as a small number of pits and cracks. In this comparative example, the proportion of particles with damaged cells on the cross-section of the molded part was 70%, indicating a low sintering degree. The L value of the molded part's surface ranged from 27.76 to 48.02, the a value ranged from -1.64 to 0.22, and the b value ranged from -1.41 to 0.69.

[0144] The test method in this comparative example refers to the relevant description in Example 1.

[0145] The solution provided in this comparative example has a tendency to be uneven in the impregnation effect of carbon dioxide on the particles because the uniform heating time from 90°C to the foaming temperature during the autoclave pressure foaming heating process is less than 60 minutes, that is, the heating speed is relatively fast. The expansion ratio of a few foamed beads is significantly lower than the average foaming ratio of most beads, the color is darker, and they become "black spots" on the molded parts; the expansion ratio of a few foamed beads is significantly higher than the average foaming ratio of most beads, the color is lighter, and they become "white spots" on the molded parts; in addition, a few foamed beads also have uneven pores, the parts with large pores are dark in color and become "black spots" on the parts, and the parts with small pores are light in color and become "white spots" on the parts; the uneven pores and expansion ratio of a few foamed beads lead to a decrease in the sintering degree of the molded parts, and a small number of pits or gaps appear on the appearance of the parts.

[0146] Comparative Example 2

[0147] The difference between this comparative example and Example 3 is that the average particle size of the carbon black is 20 nm and the DBP oil absorption value is 160 ml / 100 g.

[0148] The molding process is as follows: gray foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurization tank, and then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0149] Visual inspection revealed a few darker "black spots" and lighter "white spots" on the surface of the molded part, as well as a small number of pits and cracks. Testing revealed that 80% of the cells on the cross-section of the molded part in this comparative example were damaged, resulting in a reduced degree of sintering. The cell size of the expanded beads ranged from 164 to 221 microns. The L value of the molded part surface was 28.12 to 39.13, the a value was -1.54 to 0.11, and the b value was -1.28 to 0.57.

[0150] The test method in this comparative example refers to the relevant description in Example 1.

[0151] In the solution provided in this comparative example, the carbon black particles are easy to agglomerate as the size becomes smaller, and are difficult to disperse, resulting in uneven color of the foamed beads. The parts with more carbon black are darker in color and become "black spots" in the molded parts; the parts with less carbon black are lighter in color and become "white spots" in the molded parts. The uneven dispersion of carbon black also causes uneven pores in some foamed beads, resulting in a decrease in the sintering degree of the molded parts and a small number of pits or gaps on the appearance of the parts.

[0152] Comparative Example 3

[0153] The difference between this comparative example and Example 3 is that the average particle size of the carbon black is 90 nm and the DBP oil absorption value is 42 ml / 100 g.

[0154] The molding process is as follows: gray foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurization tank, and then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0155] Visual inspection of the molded part reveals a very light surface color with a few relatively lighter "white spots." The surface exhibits few, if any, or very small pits or cracks. Testing indicates that 90% of the particles in the cross-section of the molded part in this comparative example have destroyed cells, indicating a good sintering state. The cell size of the expanded beads ranges from 167 to 225 microns. The L value of the molded part surface is 66.91 to 73.56, the a value is 1.56 to 3.01, and the b value is 2.51 to 4.14.

[0156] The test method in this comparative example refers to the relevant description in Example 1.

[0157] In the solution provided in this comparative example, the carbon black particle size becomes larger and the coloring effect becomes worse, resulting in the overall color of the foamed beads and their molded parts being lighter, and the parts with poor local coloring effect are relatively lighter in color, appearing as "white spots", which is not conducive to the overall color uniformity of the molded parts.

[0158] Comparative Example 4

[0159] The difference between this comparative example and Example 3 is that the modified polypropylene microparticles are prepared as follows: 91.5 parts by weight of polypropylene A2, 3 parts by weight of color masterbatch, 1.5 parts by weight of cell nucleating agent masterbatch, 2 parts by weight of lubricant masterbatch, and 2 parts by weight of antioxidant masterbatch are mixed uniformly in a high-speed mixer, melt-mixed in a twin-screw extruder at a processing temperature of 180°C, passed through a cylindrical die with a pore size of 1.5 mm, drawn, water-cooled, and pelletized to obtain modified polypropylene microparticles with an average single particle weight of 1.1 mg and an average length of 2.0 mm.

[0160] The molding process is as follows: gray foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurization tank, and then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0161] Visual inspection revealed a few darker "black spots" and lighter "white spots" on the surface of the molded part, as well as a small number of pits and cracks. Testing revealed that 80% of the cells on the cross-section of the molded part in this comparative example were damaged, resulting in a reduced degree of sintering. The cell size of the expanded beads ranged from 141 to 259 microns. The L value of the molded part surface ranged from 29.02 to 43.41, the a value ranged from -1.59 to 0.14, and the b value ranged from -1.32 to 0.61.

[0162] The test method in this comparative example refers to the relevant description in Example 1.

[0163] The solution provided in this comparative example lacks the auxiliary foaming agent masterbatch, and the unevenness of the pores and the unevenness of the foamed polypropylene beads are increased. The parts with large pores are dark in color, and the beads with lower foaming ratio are dark in color, becoming "black spots" on the parts; the parts with small pores are light in color, and the beads with high foaming ratio are light in color, becoming "white spots" on the parts; the unevenness of the pores and foaming ratio of the foamed beads leads to a decrease in the sintering degree of the molded parts, and a small number of pits or gaps appear on the appearance of the parts.

[0164] Comparative Example 5

[0165] The difference between this comparative example and Example 3 is that the auxiliary foaming agent is polyethylene glycol with an average molecular weight of 2000.

[0166] The molding process is as follows: the foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurized tank, and then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0167] Visual inspection revealed a few darker "black spots" and lighter "white spots" on the surface of the molded part, with minimal pits or cracks. Testing revealed that 85% of the cells on the cross-section of the molded part in this comparative example were damaged, resulting in a slightly reduced sintering degree. The cell size of the expanded beads ranged from 155 to 243 microns. The L value of the molded part surface was 31.65 to 39.87, the a value was -1.49 to 0.05, and the b value was -1.21 to 0.54.

[0168] The test method in this comparative example refers to the relevant description in Example 1.

[0169] In the solution provided in this comparative example, the effect of polyethylene glycol on promoting the uniformity of foam cells and foaming ratio is not as good as that of the auxiliary foaming agent specified in this technology, which results in that the auxiliary foaming agent masterbatch prepared with polyethylene glycol as the auxiliary foaming agent is difficult to be combined with the foam nucleating agent masterbatch to improve the uniformity of the foaming ratio and the uniformity of the foam cells of the gray foamed polypropylene beads, so that the foam cells and the foaming ratio of the foamed polypropylene beads are still in an uneven state. The parts with large foam cells are dark in color, and the beads with lower foaming ratio are dark in color, and become "black spots" on the parts; the parts with small foam cells are light in color, and the beads with higher foaming ratio are light in color, and become "white spots" on the parts; the unevenness of the foam cells and foaming ratio of the foamed beads leads to a slight decrease in the sintering degree of the molded parts.

[0170] Comparative Example 6

[0171] The difference between this comparative example and Example 3 is that the auxiliary foaming agent is glycerol.

[0172] The molding process is as follows: the foamed polypropylene beads are subjected to an air pressure of 0.3 MPa for 10 hours in a pre-pressurized tank, and then filled into a metal mold (the internal cavity is a rectangular parallelepiped, 460 mm long, 350 mm wide, and 50 mm thick). After molding and sintering with high-temperature water vapor at 0.22 MPa, a gray molded part is obtained.

[0173] Visual inspection revealed a few darker "black spots" and lighter "white spots" on the surface of the molded part, with minimal pits or cracks. Testing revealed that 85% of the cells on the cross-section of the molded part in this comparative example were damaged, resulting in a slightly reduced sintering degree. The cell size of the expanded beads ranged from 157 to 239 microns. The L value of the molded part surface was 32.23 to 39.52, the a value was -1.46 to 0.02, and the b value was -1.16 to 0.52.

[0174] The test method in this comparative example refers to the relevant description in Example 1.

[0175] In the solution provided in this comparative example, the effect of glycerol in promoting the uniformity of foam cells and foaming ratio is not as good as the auxiliary foaming agent specified in this technology, which results in that the auxiliary foaming agent masterbatch prepared with glycerol as the auxiliary foaming agent is difficult to be combined with the foam nucleating agent masterbatch to improve the uniformity of the foaming ratio and the uniformity of the foam cells of the gray foamed polypropylene beads, so that the foam cells and the foaming ratio of the foamed beads are still in an uneven state. The parts with large foam cells are dark in color, and the beads with lower foaming ratio are dark in color, and become "black spots" on the parts; the parts with small foam cells are light in color, and the beads with high foaming ratio are light in color, and become "white spots" on the parts; the unevenness of the foam cells and foaming ratio of the foamed beads leads to a slight decrease in the sintering degree of the molded parts.

[0176] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A gray foamed polypropylene bead, characterized in that: Prepared by autoclave foaming of modified polypropylene particles; The modified polypropylene particles include the following components in parts by weight: Polypropylene A 72-95 parts; Masterbatch 1-10 parts; 1-3 parts of cell nucleating agent masterbatch; 1-5 parts of foaming agent masterbatch; 1-5 parts of lubricant masterbatch; 1-5 parts of antioxidant masterbatch; The masterbatch comprises the following components in parts by weight: 15 parts of carbon black; 5 parts of maleic anhydride modified polypropylene wax; 80 parts of polypropylene A; The carbon black has an average particle size range of 30-80 nm and a DBP oil absorption value of (60-120) ml / 100 g; The cell nucleating agent masterbatch comprises the following components in parts by weight: 10 parts of cell nucleating agent powder; 3 parts of maleic anhydride modified polypropylene wax; Polypropylene A 87 parts; The cell nucleating agent powder is silicon dioxide; the average particle size of the cell nucleating agent powder is 3-10 microns; The co-foaming agent masterbatch comprises the following components in parts by weight: 10 parts of auxiliary foaming agent; 3 parts of maleic anhydride modified polypropylene wax; Polypropylene A 87 parts; The auxiliary foaming agent is selected from at least one of sodium chloride and alum; The preparation method of the gray foamed polypropylene beads comprises the following preparation process: adding modified polypropylene microparticles together with a dispersant and water into an autoclave, sealing the autoclave, adding CO2, and stirring; raising the temperature of the autoclave to a foaming temperature, and uniformly raising the temperature in the autoclave from 90°C to the foaming temperature over 60-90 minutes; adjusting the pressure in the autoclave to a foaming pressure; maintaining the pressure under the foaming temperature and pressure conditions for 5-30 minutes; and releasing the pressure to discharge the material into a foaming pipe at normal pressure for foaming, wherein the atmosphere temperature in the foaming pipe is 85-110°C and the material stays in the foaming pipe for 4-15 seconds, thereby obtaining gray foamed polypropylene beads.

2. The gray expanded polypropylene beads according to claim 1, characterized in that: The lubricant masterbatch comprises the following components in parts by weight: 10 parts of lubricant; 90 parts of polypropylene A; The lubricant is selected from at least one of erucamide and glyceryl monostearate.

3. The gray expanded polypropylene beads according to claim 1, characterized in that: The antioxidant masterbatch comprises the following components in parts by weight: 10 parts of antioxidant 1010; Antioxidant 1076 5 parts; 5 parts of antioxidant DLTP; Polypropylene A 80 parts.

4. The gray expanded polypropylene beads according to any one of claims 1 to 3, characterized in that: The number average molecular weight of the maleic anhydride modified polypropylene wax is 5000-10000.

5. The gray expanded polypropylene beads according to any one of claims 1 to 3, characterized in that: The polypropylene A is a random copolymer polypropylene with a melt index of 6-9 g / 10 min and a melting point of 130-150° C.

6. The gray expanded polypropylene beads according to claim 5, characterized in that: The melting point of the polypropylene A is 138-148°C.

7. A molded part, characterized in that: The gray foamed polypropylene beads are obtained by steam molding according to any one of claims 1 to 6.

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