An Intrinsic Flame-Retardant Polypropylene Foaming Material and Its Preparation Method

Abstract

The present invention discloses an intrinsic flame-retardant polypropylene foaming material and a preparation method thereof, comprising the following steps: uniformly mixing polypropylene powder, a peroxide initiator, and polymerization monomer A, and obtaining grafted polymer 1 by a solid-phase grafting method; mixing grafted polymer 1 and ungrafted polypropylene powder and extruding them through a twin-screw extruder, adding an amino phosphate flame retardant through a side feeding system at the middle position of the extruder for grafting to obtain grafted polymer 2; subjecting grafted polymer 2 to a rigidifying reaction with excessive isocyanate groups through high-temperature steam; and subjecting the obtained material to supercritical foaming to obtain the foaming material. The present invention improves the melt strength of the material through two grafting steps plus a rigidifying reaction, and through supercritical fluid foaming, the overall preparation process has the advantages of being clean and environmentally friendly. The prepared foaming material contains both nitrogen and phosphorus elements, and has a high halogen-free flame retardancy efficiency after testing, solving the problems of low melt strength and low flame retardancy efficiency of polypropylene.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and specifically, to an intrinsic flame-retardant polypropylene foaming material and a preparation method thereof. Background Art

[0002] Polypropylene, abbreviated as PP, as one of the four general-purpose plastics, has the advantages of low VOC, light weight, excellent mechanical properties, and stable chemical properties. After foaming, on the basis of light weight, the material still has good thermal stability and high mechanical properties. The common foaming method of PP is supercritical foaming, which requires PP to have a high melt strength. As a linear polymer, the melt strength of PP decreases linearly with the increase of temperature. Therefore, in order to foam the PP-based material, it is necessary to first modify its high melt strength.

[0003] Foamed PP is currently widely used in fields such as new energy batteries, household appliances, and cushioning materials. In these application fields, it is required that PP has good flame-retardant properties. Restricted by the long carbon chain structure of PP itself, its body is not flame-retardant and needs to be flame-retardant modified.

[0004] Currently, the commonly used high melt strength modification methods include broadening the molecular weight distribution and carbon chain branching technology. Broadening the molecular weight distribution requires the introduction of copolymer monomers such as ethylene, which will reduce the physical properties of the material; when the material modified by the carbon chain branching technology is post-processed, it is easy to degrade after being sheared by a screw.

[0005] Currently, the flame-retardant modification technology mainly adopts the method of adding external flame retardants. Flame retardants can be divided into halogen-containing flame retardants and halogen-free flame retardants. Common halogen-containing flame retardants include decabromodiphenylethane, octabromoether, etc. Due to their high molecular polarity and incompatibility with the PP matrix, there is a risk of precipitation after long-term use. And with the enhancement of people's environmental awareness, halogen-containing flame retardants are gradually replaced. Halogen-free flame retardants include intumescent flame retardants, nitrogen-phosphorus powders, etc. To achieve the flame-retardant effect, a large dose of addition is required, and their particle size is generally large and not easy to disperse in the PP matrix, resulting in large pores after foaming. Summary of the Invention

[0006] The purpose of the present invention is to provide an intrinsic flame-retardant polypropylene foaming material and a preparation method thereof in view of the deficiencies in the prior art.

[0007] The preparation method of the intrinsic flame-retardant polypropylene foaming material of the present invention includes the following steps:

[0008] (1) Mix polypropylene powder, peroxide initiator, and polymerization monomer A containing both double bonds and isocyanate groups in a ball mill, and obtain grafted polymer 1 by the method of solid-phase grafting. The temperature of the ball mill is controlled at 80-140 °C, and the ball milling time is 0.5-2 h.

[0009] The polypropylene powder described above is preferably homopolymer polypropylene, and the particle size of the powder is controlled within 1 - 100 microns. The mass ratio of the polypropylene powder to the peroxide initiator is 100∶(0.2 - 3), and the molar fraction of double bonds in the polymerization monomer A used per 100 g of the polypropylene powder is 0.1% - 5%.

[0010] The peroxide initiator is selected from dodecanoyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4 - tert - butylcyclohexyl) peroxydicarbonate, tert - butyl peroxybenzoate, tert - butyl peroxy pivalate, di - tert - butyl peroxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, p - menthane hydroperoxide, bis(2 - phenoxyethyl) peroxydicarbonate.

[0011] The polymerization monomer A is selected from vinyl isocyanate, 3 - isocyanatopropene, isocyanatomethyl methacrylate, 2 - butene isocyanate, styryl isocyanate, isoprene isocyanate, cyclopentene isocyanate, norbornene isocyanate, methylstyrene isocyanate.

[0012] The principle of obtaining the grafted polymer 1 by the reaction of polypropylene, peroxide, and polymerization monomer A (vinyl isocyanate) is as follows:

[0013] ;

[0014] The monomer of the polypropylene molecule is grafted with an isocyanate group.

[0015] (2) After mixing the grafted polymer 1 and the ungrafted polypropylene powder in a certain proportion, it is extruded through a twin - screw extruder. An amino - phosphate - ester - type flame retardant is added through a side - feeding system at the middle position of the extruder, and a secondary grafting reaction is carried out by heating and shearing with the screw, that is, the amino group of the amino - phosphate - ester - type flame retardant reacts with the isocyanate group in the grafted polymer 1 for grafting to obtain the grafted polymer 2. The reaction principle is as follows:

[0016] ;

[0017] The mass ratio of the polypropylene powder used in step (1) to the ungrafted polypropylene powder used in step (2) is 100∶(0 - 50).

[0018] The amino - phosphate - ester - type flame retardant is selected from diphenyl amino phosphate, tris(2 - aminoethyl) phosphate, tris(2 - hydroxyethyl) aminomethyl phosphate, diethylamine phosphate. The dosage of the amino - phosphate - ester - type flame retardant is such that the molar ratio of the amino group in the flame retardant to the isocyanate group in the grafted polymer 1 is (55 - 90)∶100, ensuring that the isocyanate group is in excess by 10% - 45% to provide isocyanate groups for the subsequent rigidification reaction.

[0019] (3) The grafted 2 polymer is subjected to a rigidifying reaction with an excessive amount of isocyanate groups through high-temperature steam to obtain a polyurea group, thereby enhancing the melt strength of the material. This rigidifying reaction generates CO2 gas, and the reaction principle is as follows:

[0020] ;

[0021] (4) The material obtained in step (3) is subjected to supercritical foaming to obtain an intrinsic flame-retardant polypropylene foamed material. The preferred supercritical foaming conditions are as follows: the temperature of the molding and foaming cavity is controlled at 140 - 160 °C, the pressure for injecting supercritical fluid CO2 or / and nitrogen is 8 - 20 MPa, the temperature and pressure are kept constant for 1 - 4 h, and then the pressure is rapidly released at a rate of 1 - 10 MPa / s. After the pressure is released to atmospheric pressure, the mold is opened to obtain the foamed material.

[0022] The intrinsic flame-retardant polypropylene foamed material of the present invention is obtained by the above preparation method.

[0023] The advantages of the present invention are as follows:

[0024] 1. Through solid-phase grafting by ball milling, the grafting temperature is controlled below the Tm of polypropylene, avoiding the degradation of polypropylene during high-temperature grafting.

[0025] 2. The reaction rate between the amino group of the amino phosphate ester flame retardant and the isocyanate group in the grafted 1 polymer is fast, the grafting efficiency in the screw is high, and the side reactions are few.

[0026] 3. The high melt grafting of polypropylene is carried out by reacting isocyanate and water to generate polyurea for rigidification on the extrusion production line after passing through the extrusion screw. The rigidifying efficiency is high, and the formed material will not be sheared by the screw, avoiding the degradation problem of branched materials. Moreover, CO2 is generated during the rigidifying process, providing a gas passage for subsequent supercritical foaming and improving the saturation efficiency during supercritical foaming.

[0027] 4. The foaming precursor is foamed by supercritical fluid CO2 or / and nitrogen, and the overall preparation process has the advantages of environmental protection and cleanliness.

[0028] 5. The present invention provides a new flame-retardant polypropylene foamed material, which contains both nitrogen and phosphorus elements. After testing, it has high halogen-free flame-retardant efficiency, and the physical and mechanical properties of the foamed material are excellent. The preparation method of the present invention is novel, solving the problems of low melt strength and low flame-retardant efficiency of polypropylene. Specific Embodiments

[0029] This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. Unless otherwise specified, the materials used are all commercially available conventional products.

[0030] In the examples, the polypropylene grades used can be T30S, C1608, CP80M, CP80S, EP1X37F, F401, S1004, CF-501, EPS30R, F3002, F400-H, F800E, F800E(DF), FC801, FC801M, FC801MX, JF300, T36F, X37F, X47F, YPF-3003 or YPF-3008. They are all homopolymer polypropylenes, and the powder particle size is 1 - 100 microns.

[0031] The peroxide initiators are selected from dilauroyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate, di-tert-butyl peroxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, p-menthane hydroperoxide, bis(2-phenoxyethyl) peroxydicarbonate. Their usage effects are the same, and only some peroxides are specifically listed in the examples.

[0032] The polymerization monomer A is selected from vinyl isocyanate, 3-isocyanatopropene, isocyanatomethyl methacrylate, 2-butene isocyanate, styryl isocyanate, isoprene isocyanate, cyclopentene isocyanate, norbornene isocyanate, methylstyrene isocyanate. They contain both double bonds and isocyanate groups, and their usage effects are the same. Only some polymerization monomers A are listed in the examples.

[0033] The amino phosphate flame retardants are selected from diphenyl amino phosphate, tris(2-aminoethyl) phosphate, tris(2-hydroxyethyl)aminomethyl phosphate, diethylamine phosphate.

[0034] Example 1

[0035] 100 g of polypropylene powder, 0.5 g of benzoyl peroxide, and 20.7 g of vinyl isocyanate are placed in a ball mill and mixed evenly. The temperature of the ball mill is controlled at 100 °C, and the ball milling time is 0.5 h. The grafted 1 polymer is obtained by the solid-phase grafting method.

[0036] After mixing the obtained grafted 1 polymer with 20 g of ungrafted polypropylene powder, it is extruded through a twin-screw extruder. 54.82 g of diphenyl amino phosphate is added through the side feeding system at the middle position of the extruder, and a secondary grafting reaction is carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0037] The grafted 2 polymer is subjected to a rigidification reaction with excessive isocyanate groups through high-temperature steam and a CO2 gas path is established. The vehicle speed is 3 m per minute, and the oven length is 12 m to obtain a polyurea group, which improves the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material is respectively injection molded into solid sheets with a thickness of 4 mm.

[0038] The solid sheet is subjected to supercritical foaming. The temperature of the die-pressing foaming cavity is controlled at 145 °C, the pressure of the injected supercritical fluid CO2 is 10 MPa, and it is kept at a constant temperature and pressure for 2 h. Then, the pressure is rapidly released at a rate of 1 - 10 MPa / s. After the pressure is released to atmospheric pressure, the mold is opened to obtain the intrinsic flame-retardant polypropylene foaming material.

[0039] Example 2

[0040] 100 g of polypropylene powder, 1.5 g of dodecanoyl peroxide, and 166.2 g of 3-isocyanatopropene are placed in a ball mill and mixed evenly. The temperature of the ball mill is controlled at 120 °C, and the ball milling time is 1 h. The grafted 1 polymer is obtained by the method of solid-phase grafting.

[0041] After mixing the obtained grafted 1 polymer and 30 g of ungrafted polypropylene powder, it is extruded through a twin-screw extruder. 91.84 g of tris(2-aminoethyl) phosphate is added through a side feeding system at the middle position of the extruder, and a secondary grafting reaction is carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0042] The grafted 2 polymer is subjected to a stiffening reaction with excessive isocyanate groups through high-temperature steam and the establishment of a CO2 gas path. The vehicle speed is 3 m per minute, and the oven length is 12 m to obtain the polyurea group, which improves the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material is respectively injection-molded into solid sheets with a thickness of 4 mm.

[0043] The solid sheet is subjected to supercritical foaming. The temperature of the die-pressing foaming cavity is controlled at 150 °C, the pressure of the injected supercritical fluid nitrogen is 15 MPa, and it is kept at a constant temperature and pressure for 3 h. Then, the pressure is rapidly released at a rate of 1 - 10 MPa / s. After the pressure is released to atmospheric pressure, the mold is opened to obtain the intrinsic flame-retardant polypropylene foaming material.

[0044] Example 3

[0045] 100 g of polypropylene powder, 3 g of diisopropyl peroxydicarbonate, and 635 g of isocyanatomethyl methacrylate are placed in a ball mill and mixed evenly. The temperature of the ball mill is controlled at 140 °C, and the ball milling time is 0.8 h. The grafted 1 polymer is obtained by the method of solid-phase grafting.

[0046] After mixing the obtained grafted 1 polymer and 50 g of ungrafted polypropylene powder, it is extruded through a twin-screw extruder. 894 g of tris(2-hydroxyethyl)aminomethyl phosphate is added through a side feeding system at the middle position of the extruder, and a secondary grafting reaction is carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0047] The grafted 2 polymer was subjected to a stiffening reaction with excess isocyanate groups through high-temperature steam and a CO2 gas path was established. The vehicle speed was 3 m per minute and the oven length was 12 m to obtain a polyurea group, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was injection-molded into solid sheets with a thickness of 4 mm.

[0048] The solid sheets were supercritically foamed. The temperature of the mold pressing and foaming cavity was controlled at 160 °C, the pressure of the injected supercritical fluid nitrogen was 8 MPa, and it was kept at a constant temperature and pressure for 4 h. Then, the pressure was rapidly released at a rate of 1 - 10 MPa / s. After the pressure was released to atmospheric pressure, the mold was opened to obtain the intrinsic flame-retardant polypropylene foamed material.

[0049] Example 4

[0050] 100 g of polypropylene powder, 0.2 g of dicyclohexyl peroxydicarbonate, and 13.3 g of styryl isocyanate were placed in a ball mill and mixed evenly. The temperature of the ball mill was controlled at 80 °C and the ball milling time was 2 h. The grafted 1 polymer was obtained by the method of solid-phase grafting.

[0051] After mixing the obtained grafted 1 polymer and 40 g of ungrafted polypropylene powder, it was extruded through a twin-screw extruder. 10.2 g of diethyl phosphate amine was added through a side feeding system at the middle position of the extruder, and a secondary grafting reaction was carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0052] The grafted 2 polymer was subjected to a stiffening reaction with excess isocyanate groups through high-temperature steam and a CO2 gas path was established. The vehicle speed was 3 m per minute and the oven length was 12 m to obtain a polyurea group, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was injection-molded into solid sheets with a thickness of 4 mm.

[0053] The solid sheets were supercritically foamed. The temperature of the mold pressing and foaming cavity was controlled at 140 °C, the pressure of the injected supercritical fluid CO2 and supercritical fluid nitrogen mixture was 20 MPa, and it was kept at a constant temperature and pressure for 1 h. Then, the pressure was rapidly released at a rate of 1 - 10 MPa / s. After the pressure was released to atmospheric pressure, the mold was opened to obtain the intrinsic flame-retardant polypropylene foamed material.

[0054] Example 5

[0055] 100 g of polypropylene powder, 2 g of tert-butyl peroxybenzoate, and 146 g of methylstyrene isocyanate were placed in a ball mill and mixed evenly. The temperature of the ball mill was controlled at 110 °C and the ball milling time was 1.5 h. The grafted 1 polymer was obtained by the method of solid-phase grafting.

[0056] The obtained grafted 1 polymer was extruded through a twin-screw extruder, and 198 g of diphenyl aminophosphate was added through a side feeding system at the middle section of the extruder. The second grafting reaction was carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0057] The grafted 2 polymer was subjected to a rigidifying reaction with high-temperature steam and excessive isocyanate groups and a CO2 gas path was established. The vehicle speed was 3 m per minute and the oven length was 12 m to obtain a polyurea group, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was respectively injection-molded into solid sheets with a thickness of 4 mm.

[0058] The solid sheets were supercritically foamed. The temperature of the mold pressing and foaming cavity was controlled at 155 °C, the pressure of the supercritical fluid CO2 injected was 13 MPa, and it was kept at a constant temperature and pressure for 2.5 h, and then quickly depressurized. The depressurization speed was 1 - 10 MPa / s. After depressurizing to atmospheric pressure, the mold was opened to obtain the intrinsic flame-retardant polypropylene foamed material.

[0059] Example 6

[0060] 100 g of polypropylene powder, 1 g of tert-butyl peroxy pivalate, and 440 g of cyclopentene isocyanate were placed in a ball mill and mixed evenly. The temperature of the ball mill was controlled at 90 °C and the ball milling time was 1.8 h. The grafted 1 polymer was obtained by the method of solid-phase grafting.

[0061] After mixing the obtained grafted 1 polymer and 10 g of ungrafted polypropylene powder, it was extruded through a twin-screw extruder. 186 g of tris(2-aminoethyl) phosphate was added through a side feeding system at the middle section of the extruder. The second grafting reaction was carried out by heating and shearing with the screw to obtain the grafted 2 polymer.

[0062] The grafted 2 polymer was subjected to a rigidifying reaction with high-temperature steam and excessive isocyanate groups and a CO2 gas path was established. The vehicle speed was 3 m per minute and the oven length was 12 m to obtain a polyurea group, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was respectively injection-molded into solid sheets with a thickness of 4 mm.

[0063] The solid sheets were supercritically foamed. The temperature of the mold pressing and foaming cavity was controlled at 145 °C, the pressure of the supercritical fluid CO2 injected was 17 MPa, and it was kept at a constant temperature and pressure for 1.5 h, and then quickly depressurized. The depressurization speed was 1 - 10 MPa / s. After depressurizing to atmospheric pressure, the mold was opened to obtain the intrinsic flame-retardant polypropylene foamed material.

[0064] Example 7

[0065] 100 g of polypropylene powder, 2.5 g of cumene hydroperoxide, and 242.5 g of 2-butene isocyanate were placed in a ball mill and mixed evenly. The temperature of the ball mill was controlled at 130 °C, and the ball milling time was 1.2 h. Grafted 1 polymer was obtained by the method of solid-phase grafting.

[0066] After mixing the obtained grafted 1 polymer and 25 g of ungrafted polypropylene powder, it was extruded through a twin-screw extruder. 342.7 g of diphenyl aminophosphate was added through a side feeding system at the middle position of the extruder, and a secondary grafting reaction was carried out by screw heating and shearing to obtain grafted 2 polymer.

[0067] The grafted 2 polymer was subjected to a rigidifying reaction with excess isocyanate groups through high-temperature steam and the establishment of a CO2 gas path. The vehicle speed was 3 m per minute, and the oven length was 12 m to obtain polyurea groups, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was respectively injection-molded into solid sheets with a thickness of 4 mm.

[0068] The solid sheets were subjected to supercritical foaming. The temperature of the die-pressing foaming cavity was controlled at 150 °C, the pressure of injecting supercritical fluid nitrogen was 11 MPa, and it was kept at a constant temperature and pressure for 3.5 h. Then, the pressure was quickly released, and the pressure release rate was 1 - 10 MPa / s. After releasing the pressure to atmospheric pressure, the mold was opened to obtain the intrinsic flame-retardant polypropylene foaming material.

[0069] Example 8

[0070] 100 g of polypropylene powder, 1.2 g of p-menthane hydroperoxide, and 249 g of 3-isocyanatopropene were placed in a ball mill and mixed evenly. The temperature of the ball mill was controlled at 120 °C, and the ball milling time was 1 h. Grafted 1 polymer was obtained by the method of solid-phase grafting.

[0071] After mixing the obtained grafted 1 polymer and 15 g of ungrafted polypropylene powder, it was extruded through a twin-screw extruder. 672.8 g of diphenyl aminophosphate was added through a side feeding system at the middle position of the extruder, and a secondary grafting reaction was carried out by screw heating and shearing to obtain grafted 2 polymer.

[0072] The grafted 2 polymer was subjected to a rigidifying reaction with excess isocyanate groups through high-temperature steam and the establishment of a CO2 gas path. The vehicle speed was 3 m per minute, and the oven length was 12 m to obtain polyurea groups, which improved the melt strength of the material. The well-grafted high-melt-intrinsic polypropylene material was respectively injection-molded into solid sheets with a thickness of 4 mm.

[0073] The solid sheet is subjected to supercritical foaming. The temperature of the mold pressing foaming cavity is controlled at 150 °C, the pressure of the injected supercritical fluid CO2 is 14 MPa, and it is kept at a constant temperature and pressure for 3 h. Then, the pressure is rapidly released at a rate of 1 - 10 MPa / s. After releasing the pressure to atmospheric pressure, the mold is opened to obtain the intrinsic flame-retardant polypropylene foaming material.

[0074] Comparative Example 1

[0075] 100 g of polypropylene powder, 0.5 g of benzoyl peroxide, and 20.7 g of vinyl isocyanate are placed in a ball mill and mixed evenly. The temperature of the ball mill is controlled at 100 °C, and the ball milling time is 0.5 h. The grafted 1 polymer is obtained by the method of solid-phase grafting.

[0076] After mixing the obtained grafted 1 polymer and 20 g of ungrafted polypropylene powder, it is extruded through a twin-screw extruder, and then subjected to a rigidification reaction with high-temperature steam and a CO2 gas path is established. The vehicle speed is 3 m per minute, and the oven length is 12 m. The well-grafted high-melt polypropylene material is respectively injection-molded into solid sheets with a thickness of 4 mm.

[0077] The solid sheet is subjected to supercritical foaming. The temperature of the mold pressing foaming cavity is controlled at 145 °C, the pressure of the injected supercritical fluid CO2 is 10 MPa, and it is kept at a constant temperature and pressure for 2 h. Then, the pressure is rapidly released at a rate of 1 - 10 MPa / s. After releasing the pressure to atmospheric pressure, the mold is opened to obtain the polypropylene foaming material.

[0078] The difference from Example 1 is that no flame retardant grafting is carried out.

[0079] Comparative Example 2

[0080] 100 g of polypropylene powder is extruded through a twin-screw extruder, passed through high-temperature steam, with a vehicle speed of 3 m per minute and an oven length of 12 m. Then, the polypropylene material is respectively injection-molded into solid sheets with a thickness of 4 mm. Finally, the solid sheet is subjected to supercritical foaming. The temperature of the mold pressing foaming cavity is controlled at 145 °C, the pressure of the injected supercritical fluid CO2 is 10 MPa, and it is kept at a constant temperature and pressure for 2 h. Then, the pressure is rapidly released at a rate of 1 - 10 MPa / s. After releasing the pressure to atmospheric pressure, the mold is opened to obtain the polypropylene foaming material.

[0081] The differences from Example 1 are that no isocyanate group grafting, flame retardant grafting, and rigidification reaction are carried out.

[0082] Performance Test

[0083] The foamed materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 were tested. The horizontal and vertical flame retardancy test method was UL94, the tensile strength was ASTM-D3574-08, the bending measurement method was GB-T 8812.2-2007, and the surface adhesive strength test was GB / T 33332. The results are shown in Table 1.

[0084] Table 1 Test Results of the Foamed Materials in Example 1, Comparative Example 1, and Comparative Example 2

[0085] From the test results, it can be seen that in Example 1, through two complete grafting reactions and one rigidifying reaction, a halogen-free flame retardant containing nitrogen and phosphorus elements was grafted onto the molecular chain. The solid sheet could pass the V-0 vertical burning test, and after foaming, the flame retardancy grade of the material could reach V-1. Compared with Comparative Example 2, the tensile properties, melt strength, and flame retardancy performance increased. In Comparative Example 1, since the raw material still contained nitrogen elements during the first grafting, the solid sheet still had a certain flame retardancy grade. After the rigidifying reaction, due to the high content of isocyanate groups in the molecular chain, the rigidifying effect was good, and the tensile strength and melt strength were higher than those in Example 1. Under the same foaming conditions, the density of the obtained material was higher. Both Example 1 and Comparative Example 1 underwent foaming reactions, and there were more bubbles on the surface. Therefore, the surface adhesive peel force showed an upward trend after foaming. Comparative Example 2 did not undergo grafting and rigidifying reactions, its physical and mechanical properties were poor, and it could not meet the melt strength required for supercritical foaming. The pure material did not have any flame retardant effect.

[0086] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the method of the present invention, several improvements and supplements can be made, and these improvements and supplements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of an intrinsic flame-retardant polypropylene foaming material, characterized in that, It includes the following steps: (1) Mix polypropylene powder, peroxide initiator, and polymerization monomer A containing both double bonds and isocyanate groups in a ball mill, and obtain grafted polymer 1 by solid-phase grafting method; (2) Mix the grafted polymer 1 and ungrafted polypropylene powder in a certain proportion, extrude through a twin-screw extruder, add an amino phosphate flame retardant through a side feeding system at the middle position of the extruder, and conduct a secondary grafting reaction through screw heating and shearing, that is, the amino group of the amino phosphate flame retardant reacts with the isocyanate group in the grafted polymer 1 for grafting to obtain grafted polymer 2; (3) React the grafted polymer 2 with high-temperature steam and excessive isocyanate groups to conduct a stiffening reaction to obtain a polyurea group and improve the melt strength of the material; (4) Supercritically foam the material obtained in step (3) to obtain an intrinsic flame-retardant polypropylene foam material; In step (1), the polymerization monomer A is selected from vinyl isocyanate, 3-isocyanatopropene, isocyanatomethyl methacrylate, 2-butene isocyanate, styryl isocyanate, isoprene isocyanate, cyclopentene isocyanate, norbornene isocyanate, methylstyrene isocyanate; in step (2), the amino phosphate flame retardant is selected from diphenyl amino phosphate, tris(2-aminoethyl) phosphate, tris(2-hydroxyethyl)aminomethyl phosphate, diethylamine phosphate; the molar ratio of the amino group to the isocyanate group is (55-90):

100.

2. The preparation method according to claim 1, characterized in that, The polypropylene powder is homopolymer polypropylene, and the powder particle size is controlled at 1-100 microns.

3. The preparation method according to claim 1, wherein, In step (1), the mass ratio of the polypropylene powder to the peroxide initiator is 100:(0.2-3), and the molar fraction of double bonds in the polymerization monomer A used per 100 g of polypropylene powder is 0.1%-5%.

4. The preparation method according to claim 1, characterized in that, In step (1), the peroxide initiator is selected from dilauroyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate, di-tert-butyl peroxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, p-menthane hydroperoxide, bis(2-phenoxyethyl) peroxydicarbonate.

5. The preparation method according to claim 1, wherein In step (1), the ball mill controls the temperature at 80-140°C and the ball milling time at 0.5-2 h.

6. The preparation method according to claim 1, wherein In step (1), the mass ratio of the polypropylene powder to the ungrafted polypropylene powder in step (2) is 100:(0-50).

7. The preparation method according to claim 1, characterized in that, The supercritical foaming conditions in step (4) are: the temperature of the compression molding foaming cavity is controlled at 140-160°C, the pressure of injecting supercritical fluid CO2 or / and nitrogen is 8-20 MPa, keep the temperature and pressure constant for 1-4 h, then quickly release the pressure, the pressure release rate is 1-10 MPa / s, after releasing the pressure to atmospheric pressure, open the mold to obtain the foamed material.

8. An intrinsic flame-retardant polypropylene foam material, characterized in that, It is obtained by using the preparation method described in any one of claims 1-7.

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