Method for preparing thermoplastic polyimide material by interfacial polymerization-reactive extrusion process

Abstract

The invention belongs to the technical field of high polymer materials, and relates to a method for preparing thermoplastic polyimide by an interfacial polymerization-reactive extrusion process, and the preparation method comprises the following steps: firstly, respectively preparing a diamine monomer and an aromatic tetracarboxylic dianhydride monomer into a water phase solution and a hydrophobic organic phase solution, then mixing the two solutions, and heating to carry out an interfacial reaction; drying to obtain a prepolymer; and carrying out reactive extrusion on the prepolymer to obtain the thermoplastic polyimide. Compared with the prior art, the method has the advantages that an initial product is prepared by utilizing interfacial polymerization, and monomers are partially polymerized in the interfacial polymerization stage, so that the monomers are fully mixed; and further reacting the interfacial polymerization product by using a reactive extrusion process to improve the molecular weight and imidization degree of the product and finally prepare the target thermoplastic polyimide, and no high-boiling-point organic solvent exists in the reactive extrusion process.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and relates to a method for preparing thermoplastic polyimide materials by an interfacial polymerization-reactive extrusion process. Background Art

[0002] Thermoplastic polyimide is a polymer material that combines the excellent mechanical properties and high temperature resistance of polyimide materials, as well as the excellent thermoplastic processability of thermoplastic polymer materials. It is widely used in the fields of electronic communication, aerospace, and new energy.

[0003] As a polymer material with excellent performance and wide applications, the production and manufacturing of such materials have always been a research hotspot. Patent CN102492141A reports a method for manufacturing thermoplastic polyimide materials. 2,3,3,4-Diphenyl ether tetracarboxylic dianhydride (a-ODPA) is used as the main dianhydride monomer to react with diamine monomers such as 4,4'-diaminodiphenyl ether to obtain thermoplastic polyimide molding powder through a two-step method. However, this method is only applicable to the preparation of insoluble thermoplastic polyimide materials and is not suitable for the preparation of soluble thermoplastic polyimide materials that have a certain solubility in high-boiling solvents such as dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF). The development of an efficient and concise synthesis process for such soluble polyimides has always been the focus of research by various institutions.

[0004] Interfacial polymerization is a special polymerization system. In this method, the polymerization monomers undergo polymerization reactions at the interface between two phases, and the resulting products usually have low solubility in both phases, that is, they precipitate in the form of solid powders. For example, as reported in patent CN100432120C, an aromatic amide polymer material is prepared by interfacial polymerization.

[0005] Reactive extrusion is an efficient polymer material production process. This method can produce thermoplastic polymer material particles in one step through in-situ polymerization, simplifying the production process and improving production efficiency. Patents CN 1001235198A and CN 1006700040A report the application of this method in the production of polyamide and polylactic acid. Scholars have also developed a technology for manufacturing thermoplastic polyimide using a reactive extrusion process: Prior art CN 1252137C introduces that bisphenol A diether dianhydride (BPADA), 4,4'-diaminodiphenyl ether (ODA) and other dianhydride diamine monomers are blended with phthalic anhydride in a solid state and then added to a reactive extruder for reaction. In this method, the corresponding monomers are difficult to fully mix in a solid state, which can easily cause molecular weight fluctuations in the resulting product and affect product performance; Prior art CN 1890293A discloses another reactive extrusion production technology, in which dianhydride diamine monomers are first polymerized in an organic solvent to form a polyamic acid copolymer, and then the reaction solution is added to a reactive extruder for reaction and the reaction solvent is removed by the extruder. In this method, a large amount of organic solvent is added to the extruder, which has a new impact on equipment requirements and safe production. Summary of the invention

[0006] Based on the problems existing in the existing process for preparing thermoplastic polyimide materials as described in the background art, the present invention provides a method for preparing thermoplastic polyimide materials by interfacial polymerization-reactive extrusion process.

[0007] The interfacial polymerization-reaction extrusion process of the invention can be used to prepare thermoplastic polyimide and blended modified thermoplastic polyimide.

[0008] The purpose of the present invention can be achieved by the following technical solutions: A method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process comprises the following steps: S1: preparing a diamine monomer and an aromatic tetraacid dianhydride monomer into an aqueous phase solution and a hydrophobic organic phase solution respectively, then mixing and heating the two solutions to perform an interfacial reaction, and drying to obtain a prepolymer; S2: The prepolymer is subjected to reaction extrusion to obtain a thermoplastic polyimide material.

[0009] In one embodiment of the present invention, in step S1, the diamine monomer is selected from at least one of m-phenylenediamine (mPDA), p-phenylenediamine, and 4,4-diaminodiphenyl ether.

[0010] In one embodiment of the present invention, in step S1, the aromatic tetracarboxylic dianhydride monomer is selected from at least one of 4,4'-diphenyl ether dianhydride (s-ODPA), bisphenol A diether dianhydride or pyromellitic acid dianhydride (PMDA).

[0011] In a preferred embodiment of the present invention, the aromatic tetracarboxylic dianhydride monomer is selected from one or a combination of two of bisphenol A type diether dianhydride, 4,4'-biphenylene ether dianhydride, and pyromellitic dianhydride.

[0012] In a further preferred embodiment of the present invention, the aromatic tetracarboxylic dianhydride monomer is bisphenol A type diether dianhydride.

[0013] In an embodiment of the present invention, the mass ratio of the diamine monomer to the aromatic tetracarboxylic dianhydride monomer is (210 - 220):(950 - 1050).

[0014] In an embodiment of the present invention, the mass ratio of the aqueous solution to the hydrophobic organic phase solution is 2160:(6800 - 6950).

[0015] In an embodiment of the present invention, the solvent of the aqueous solution is water or a mixture of water and a hydrophilic organic solvent.

[0016] In a preferred embodiment of the present invention, the hydrophilic organic solvent is selected from at least one of tetrahydrofuran, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, or N-methylpyrrolidone; the mass ratio of water to the hydrophilic organic solvent is (3.5 - 4.5):1.

[0017] In an embodiment of the present invention, the solvent of the hydrophobic organic phase solution is selected from at least one of dichloromethane, chloroform, ethyl acetate, ether, toluene, or xylene.

[0018] In an embodiment of the present invention, in the interfacial reaction, the reaction temperature is 40 - 110°C, and the reaction time is 4 - 6 h.

[0019] In an embodiment of the present invention, in the interfacial reaction, the obtained reaction product mixture is successively subjected to heating to remove the organic solvent, filtration, and drying to obtain a prepolymer.

[0020] In an embodiment of the present invention, the drying is carried out by vacuum drying, and the drying temperature gradient is successively 55 - 65°C, 85 - 95°C, 110 - 140°C. The drying time for each gradient is 2 - 4 h, and the vacuum degree is -0.08 MPa to -0.1 MPa.

[0021] In some specific embodiments, step S1 includes the following steps: S1-1: Weigh the diamine monomer and add it to the aqueous phase, stir and dissolve to obtain an aqueous solution; S1-2: Weigh the aromatic tetracarboxylic dianhydride monomer and add it to the organic phase, stir to obtain an organic phase solution; S1-3: While rapidly stirring the aqueous solution obtained in step S1-1, slowly dropwise add the organic phase solution or suspension obtained in step S1-2 into the system; S1-4: Maintain rapid stirring, and the system presents a milky white suspension; S1-5: Heat the reaction system to remove the organic phase; S1-6: Filter the aqueous phase, and the initial product of interfacial polymerization is a white powdery solid; S1-7: Dry the initial product in a vacuum oven to remove low-boiling volatile components; S1-8: Obtain a prepolymer after drying, which is used for the next reaction extrusion process.

[0022] In some more specific embodiments, step S1 includes: Under vigorous stirring conditions, slowly dropwise add the hydrophobic organic phase solution or suspension formed by the aromatic tetracarboxylic dianhydride monomer and the organic solvent into the aqueous solution. A large amount of white solids slowly appear in the aqueous phase. After the organic phase is completely dropwise added, heat the system. During heating, the temperature in the system is 70 - 110 °C, and slowly distill out the added organic solvent to obtain a white suspension. Filter the suspension to obtain a white solid, which is washed three times with water and then dried in a vacuum oven. The treatment temperature is 60 °C, 90 °C, and 120 °C for 3 hours each, and during this period, maintain the pressure in the vacuum oven at -0.08 MPa to -0.1 MPa. The dried white solid is the prepolymer.

[0023] In an embodiment of the present invention, in the reaction extrusion, the extruder used includes a feeding section, a mixing section, a reaction section, and a discharging section. The temperature of the feeding section is 25 - 200 °C, the temperature of the mixing section is 200 - 320 °C, the temperature of the reaction section is 320 - 350 °C, and the temperature of the discharging section is 320 - 350 °C.

[0024] In a preferred embodiment of the present invention, the temperature of the feeding section is 100 - 180 °C; more preferably, the temperature of the feeding section is 150 - 180 °C.

[0025] In a preferred embodiment of the present invention, the temperature of the mixing section is 250 - 320 °C; more preferably, the temperature of the mixing section is 280 - 320 °C.

[0026] In a preferred embodiment of the present invention, the temperature of the reaction section is 320 - 350 °C; more preferably, the temperature of the mixing section is 330 - 350 °C.

[0027] In some specific embodiments, step S2 includes the following steps: S2-1: Add the prepolymer to the reactive twin-screw extruder through a loss-in-weight feeder at a feeding rate of 0.2 - 3 kg / h; S2-2: The reactive twin-screw extruder is a co-rotating intermeshing twin-screw extruder with a length-diameter ratio of 28. It is divided into a feeding section, a mixing section, a reaction section, and a discharging section according to its functions. The temperature of the feeding section is set at 25 - 200 °C, the temperature of the mixing section is set at 200 - 320 °C, the temperature of the reaction section is set at 320 - 350 °C, and the temperature of the discharging section is set at 320 - 350 °C; S2-3: The rotational speed of the reactive twin-screw extruder is set in the range of 50 - 300 rpm; S2-4: The reactive twin-screw extruder has a set of exhaust holes, which are respectively located in the reaction section; S2-5: The thermoplastic polyimide and its blend-modified composition are in the form of granules after extrusion and pelletization.

[0028] Furthermore, the method further includes: mixing the prepolymer with the modified filler and then carrying out reactive extrusion together to prepare a thermoplastic polyimide blend-modified product, so as to achieve in-situ blend modification with the modified filler; Among them, the mass content of the modified filler is 1 - 99%.

[0029] In an embodiment of the present invention, the die form of the reactive twin-screw extruder is a single hole, and the die aperture is 1 mm - 3 mm.

[0030] In an embodiment of the present invention, the mass content of the modified filler is 5 - 80%.

[0031] In a preferred embodiment of the present invention, the mass content of the modified filler is 10 - 50%.

[0032] In an embodiment of the present invention, the modified filler is one or a combination of two of inorganic fillers or polymers.

[0033] In an embodiment of the present invention, the inorganic filler is selected from at least one of glass fiber, carbon fiber, graphite, and carbon nanotube.

[0034] In an embodiment of the present invention, the polymer is selected from one or a combination of two of polytetrafluoroethylene and polyimide molding powder.

[0035] In an embodiment of the present invention, the degree of polymerization of the thermoplastic polyimide is 3 - 1000.

[0036] The present invention provides a method for preparing thermoplastic polyimide material by interfacial polymerization-reactive extrusion process, which utilizes the advantages of interfacial polymerization, simplifies the production process, reduces the amount of highly polluting organic solvents while improving the mixing efficiency of raw materials; and has the characteristics of convenient and efficient reactive extrusion process. The obtained thermoplastic polyimide and the blended modified thermoplastic polyimide have excellent thermoplastic processability and mechanical properties, and the thermoplastic polyimide has a melt flow rate of 0.1~20g / 10min, a tensile strength of 95~150MPa, a flexural strength of 100~200MPa, a glass transition temperature of 200~300℃, and a heat deformation temperature of 185~240℃.

[0037] Compared with the prior art, the present invention has the following characteristics: 1) The present invention uses interfacial polymerization to first prepare the initial product, and the monomers are partially polymerized in the interfacial polymerization stage, ensuring that the monomers are fully mixed; 2) The product obtained by interfacial polymerization in the present invention is in a solid suspension state and can be directly filtered and separated in the system, and the production process is simple; 3) The present invention utilizes a reactive extrusion process to further react the interfacial polymerization product, thereby increasing the product molecular weight and imidization degree, and finally obtaining the target thermoplastic polyimide, and no high-boiling point organic solvent is present during the reactive extrusion process. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A thermoplastic polyimide prepared in Example 1 1 H NMR spectrum.

[0039] Figure 2 for Figure 1 A partial enlarged view of . DETAILED DESCRIPTION

[0040] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0041] The following are more detailed implementation cases, which further illustrate the technical solutions of the present invention and the technical effects that can be obtained.

[0042] In the following examples, unless otherwise specified, raw materials, reagents or processing techniques are all conventional commercial products or conventional processing techniques in the art.

[0043] Example 1

[0044] A method for preparing thermoplastic polyimide by interfacial polymerization-reactive extrusion process comprises the following steps: S1: Weigh 216 g of m-phenylenediamine and add it to 1944 g of water, stir to dissolve to prepare an aqueous solution. Then add 1040 g of bisphenol A diether dianhydride to 5890 g of dichloromethane, stir to dissolve, and slowly add the resulting solution dropwise into the aqueous solution over 75 min, while maintaining vigorous stirring and nitrogen purge protection. During the dropping process, the reaction system gradually becomes a white suspension. After the dropping is completed, heat the reaction system. Start timing from a system temperature of 40 °C and continue heating for 5 h. At this time, the system temperature gradually rises to 60 °C, and the dichloromethane used is collected through a condensation recovery device. The distilled reaction system is filtered using a Buchner funnel to obtain a white cake crude product. This product is washed 4 times with water and then placed in a vacuum oven for drying. The drying temperatures are 60 °C, 90 °C, and 120 °C for 3 h each, while maintaining the pressure in the vacuum oven at -0.08 MPa to -0.1 MPa. The dried white solid is defined as the prepolymer, and a total of 1218.3 g is obtained.

[0045] S2: Add the obtained prepolymer to a reactive twin-screw extruder through a loss-in-weight feeder at a feeding speed of 0.4 kg / h; set the rotational speed of the reactive twin-screw extruder at 75 rpm; set the feeding section temperature of the reactive twin-screw extruder at 100 °C, 170 °C, the mixing section temperature at 270 °C, 290 °C, the reaction section at 320 °C, 330 °C, and the discharging section temperature at 340 °C; after the material is extruded from the extruder, it is cooled by air and pelletized to obtain thermoplastic polyimide pellets. The obtained pellets are light yellow transparent granular.

[0046] The nuclear magnetic characterization spectrum of the obtained pellets is shown in Figure 1 and Figure 2 . The glass transition temperature is 214 °C (DSC, N2, 20 °C / min), and the thermal decomposition temperatures in both nitrogen and air atmospheres exceed 500 °C (TGA, 10 °C / min). The mechanical properties of the molded plastic samples are: flexural strength 157 MPa (GB / T 9341 - 2008), flexural modulus 3.1 GPa (GB / T 9341 - 2008), tensile strength 107 MPa (GB / T 1040.1 - 2006), elongation at break 21% (GB / T 1040.1 - 2006), and notched impact strength 4.3 kJ / m 2 (GB / T 1043 - 1993).

[0047] Comparative Example 1: A method for preparing thermoplastic polyimide by a reactive extrusion process, which is only different from Example 1 in that: Weigh 2000 g of bisphenol A diether dianhydride (BPADA), 432 g of m-phenylenediamine, and 35 g of phthalic anhydride (PA), and add them to a high-speed mixer for mixing. The mixed material is added to a reactive twin-screw extruder through a loss-in-weight feeder at a feeding speed of 12 kg / h. The rotational speed of the reactive twin-screw extruder is set at 140 rpm. The temperature of the feeding section of the reactive twin-screw extruder is set at 60 °C to 340 °C, and the temperature of the discharging section is set at 330 °C to 320 °C. After the material is extruded from the extruder, it is cooled by air, water, and pelletized to obtain thermoplastic polyimide particles.

[0048] In this comparative example, since the monomers are physically mixed, the monomers in the obtained mixture are partially oxidized during the reactive extrusion stage, and the obtained pellets are black-red to black granular. The glass transition temperature of the obtained pellets is 216 °C, and the mechanical properties of the plastic sample after molding are: flexural strength 154 MPa (GB / T 9341-2008), tensile strength 96 MPa (GB / T 1040.1-2006), and elongation at break 6.3% (GB / T 1040.1-2006).

[0049] Example 2

[0050] A method for preparing thermoplastic polyimide by an interfacial polymerization-reactive extrusion process, which is only different from Example 1 in that: In step S1, the dichloromethane organic solvent is replaced with chloroform, and the drying temperatures of the obtained crude product are 60 °C, 90 °C, and 120 °C for 4 hours each. In step S2, the material obtained after pelletizing is dark yellow and transparent.

[0051] The mechanical properties of the plastic sample after molding of the obtained pellets are: flexural strength 153 MPa (GB / T 9341-2008), flexural modulus 3.1 GPa (GB / T 9341-2008), tensile strength 106 MPa (GB / T 1040.1-2006), and elongation at break 16% (GB / T 1040.1-2006).

[0052] Example 3

[0053] A method for preparing thermoplastic polyimide by an interfacial polymerization-reactive extrusion process, comprising the following steps: S1: Weigh 216 g of m-phenylenediamine and add it to a mixed solvent composed of 1556 g of water and 388 g of N,N-dimethylformamide. Stir to dissolve to prepare an aqueous solution. Then, add 1037 g of bisphenol A diether dianhydride to 5890 g of dichloromethane and stir to dissolve. The resulting solution is slowly added dropwise to the aqueous solution over 39 minutes while maintaining vigorous stirring and nitrogen purging for protection. During the dropwise addition, white primary products gradually precipitate from the system. After the dropwise addition is completed, heat the reaction system. Start timing from a system temperature of 40 °C and continue heating for 5 hours. At this time, the system temperature gradually rises to 110 °C. The dichloromethane and DMF used are collected through a condensation recovery device. The distilled reaction system is filtered using a Buchner funnel to obtain the white filter cake primary product. This product is washed with water 5 times and then placed in a vacuum oven for drying. The drying temperatures are 60 °C, 90 °C, and 130 °C for 3 hours each, while maintaining the pressure in the vacuum oven at -0.08 MPa to -0.1 MPa. The dried white solid is defined as the prepolymer, and a total of 997.1 g is obtained.

[0054] S2: Add the obtained prepolymer to a reactive twin-screw extruder through a loss-in-weight feeder at a feeding speed of 0.4 kg / h. Set the rotation speed of the reactive twin-screw extruder to 65 rpm. Set the feeding section temperature of the reactive twin-screw extruder to 80 °C, 150 °C, the mixing section temperature to 270 °C, 290 °C, the reaction section to 320 °C, 330 °C, and the discharging section temperature to 330 °C. After the material is extruded from the extruder, it is cooled by air and pelletized to obtain thermoplastic polyimide particles. The obtained pellets are dark yellow transparent granules.

[0055] The glass transition temperature of the obtained pellets is 209 °C (DSC, N2, 20 °C / min). The mechanical properties of the plastic sample after molding are: flexural strength 157 MPa (GB / T 9341-2008), tensile strength 97 MPa (GB / T 1040.1-2006), elongation at break 8% (GB / T 1040.1-2006), notched impact strength 2.7 kJ / m 2 (GB / T 1043-1993).

[0056] Example 4

[0057] A method for preparing thermoplastic polyimide by an interfacial polymerization-reactive extrusion process, which is only different from Example 3 in that: In step S1, replace 1040 g of BPADA with a mixture composed of 936 g of BPADA and 62 g of 4,4'-biphenylene ether dianhydride (s-ODPA). At this time, the organic phase is partially dissolved. The drying temperatures of the obtained primary product are 60 °C, 90 °C, and 130 °C for 4 hours each.

[0058] In step S2, the obtained material after granulation presents a dark yellow transparent state.

[0059] The glass transition temperature of the obtained pellets is 220 °C (DSC, N2, 20 °C / min). The mechanical properties of the plastic sample after molding are as follows: flexural strength 159 MPa (GB / T 9341-2008), flexural modulus 3.1 GPa (GB / T 9341-2008), tensile strength 108 MPa (GB / T 1040.1-2006), and elongation at break 9% (GB / T 1040.1-2006).

[0060] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process, characterized in that, It includes the following steps: S1: Prepare aqueous solutions and hydrophobic organic phase solutions of diamine monomers and aromatic tetracarboxylic dianhydride monomers respectively, then mix the two solutions and heat them for interfacial reaction, and dry to obtain a prepolymer; S2: Subject the prepolymer to reactive extrusion to obtain a thermoplastic polyimide material.

2. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, In step S1, the diamine monomer is selected from at least one of m-phenylenediamine, p-phenylenediamine, and 4,4-diaminodiphenyl ether; The aromatic tetracarboxylic dianhydride monomer is selected from at least one of 4,4'-biphenyl ether dianhydride, bisphenol A type diether dianhydride, or pyromellitic dianhydride; The mass ratio of the diamine monomer to the aromatic tetracarboxylic dianhydride monomer is (210~220):(950~1050).

3. The method for preparing a thermoplastic polyimide material by the interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, The mass ratio of the aqueous solution to the hydrophobic organic phase solution is 2160:(6800~6950).

4. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, The solvent of the aqueous solution is water, or a mixture of water and a hydrophilic organic solvent.

5. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 4, wherein, The hydrophilic organic solvent is selected from at least one of tetrahydrofuran, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, or N-methylpyrrolidone; the mass ratio of water to the hydrophilic organic solvent is (3.5~4.5):

1.

6. The method for preparing a thermoplastic polyimide material by the interfacial polymerization-reactive extrusion process according to claim 1, wherein The solvent of the hydrophobic organic phase solution is selected from at least one of dichloromethane, chloroform, ethyl acetate, ether, toluene, or xylene.

7. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, In the interfacial reaction, the reaction temperature is 40~110°C, and the reaction time is 4~6 h.

8. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, The drying is carried out by vacuum drying, and the drying temperature gradient is successively 55~65°C, 85~95°C, 110~140°C, the drying time for each gradient is 2~4 h, and the vacuum degree is -0.08 MPa~-0.1 MPa.

9. The method for preparing a thermoplastic polyimide material by an interfacial polymerization-reactive extrusion process according to claim 1, characterized in that, In the reactive extrusion, the extruder used includes a feeding section, a mixing section, a reaction section, and a discharging section. The temperature of the feeding section is 80~150°C, the temperature of the mixing section is 270~290°C, the temperature of the reaction section is 320~330°C, and the temperature of the discharging section is 320~340°C.

10. The method for preparing a thermoplastic polyimide material by the interfacial polymerization-reactive extrusion process according to claim 1, the method further comprising: Mix the prepolymer with a modified filler and then carry out reactive extrusion together; Among them, the mass content of the modified filler is 1~99%; the modified filler is selected from one or a combination of two of glass fiber, carbon fiber, graphite, carbon nanotube, polytetrafluoroethylene, or polyimide molding powder.

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