Flame-retardant, fire-resistant and high-temperature resistant nylon and preparation method thereof

Abstract

The present invention relates to the technical field of nylon, and discloses a kind of flame retardant, fireproof and high temperature resistant nylon and preparation method thereof; The present invention extrude granulation by nylon 6, montmorillonite-copper complex flame retardant etc. in a twin-screw extruder, obtain flame retardant, fireproof and high temperature resistant nylon.Montmorillonite, after surface organic modification, becomes better in compatibility with nylon 6, the tensile strength of material becomes larger, and the flame retardant contains phosphate group and more nitrogenous urea group, phenanthroline structure, forms nitrogen-phosphorus flame retardant, contains salicylic acid-phenanthroline-copper complex structure simultaneously, can catalyze nylon matrix into carbon at high temperature, thus forming a stable carbon barrier layer, significantly improves the flame retardancy of nylon material, UL94 grade reaches V0 level, limiting oxygen index becomes larger, and residual carbon amount increases at high temperature, is conducive to improving the high temperature resistance of material.

Description

Technical Field

[0001] The present invention relates to the technical field of nylon, in particular to flame-retardant, fire-proof and high-temperature resistant nylon and a preparation method thereof. Background Art

[0002] Flame retardants are common functional additives. Traditional flame retardants include inorganic flame retardants such as magnesium hydroxide and metal complexes, and organic flame retardants such as nitrogen-phosphorus and bromine-based flame retardants. Nylon 6, a high-performance polyamide resin material, can be made into plastics and fibers, and is widely used in automotive manufacturing, instrumentation, electronics, luggage, and fabrics. However, nylon 6 is flammable, and its application in fire protection and flame retardancy generally requires the addition of flame retardants.

[0003] Montmorillonite is a cheap and readily available natural silicate mineral with advantages such as a large specific surface area and excellent mechanical properties. It can be used as a reinforcing agent and flame retardant carrier, and has important applications in materials such as nylon, epoxy resin, polystyrene, and polypropylene. Chinese invention patent publication number CN116004095B discloses a fire-retardant coating, its preparation method, and its application. Using silane-modified copper- and nickel-loaded montmorillonite, hexaphenoxycyclotriphosphazene, and samarium oxide as fillers and flame retardants, the coating improves the adhesion and flame retardancy of epoxy resin materials. Compared to that invention, the present invention introduces a phosphate, phenanthroline, and copper complex structure onto the montmorillonite surface to form a composite flame retardant, improving the mechanical strength and flame retardancy of nylon materials. This expands the practical application of nylon materials in flame-retardant plastics, fiber fabrics, and luggage. Summary of the Invention

[0004] The invention solves the problem that the nylon material has poor flame retardant and fireproof performance and high temperature resistance.

[0005] The technical solution of the present invention is a method for preparing flame-retardant, fire-resistant and high-temperature resistant nylon, comprising the following steps:

[0006] Step S1: Add ethanol and 100 parts by weight of phenanthroline-modified montmorillonite to a reactor and disperse them by ultrasonication. Then, add water, 25-45 parts by weight of copper salt, and 40-70 parts by weight of phenyl salicylate phosphate flame retardant, carry out a hydrothermal reaction, filter, wash with water and ethanol in sequence, and dry to obtain a montmorillonite-copper complex flame retardant. The reaction formula is:

[0007]

[0008] Step S2: 100 parts by weight of nylon 6, 1-8 parts by weight of montmorillonite-copper complex flame retardant, and 0.3-0.4 parts by weight of antioxidant 1010 are added to a mixer and mixed for 10-20 minutes. The mixture is then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder are 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed is 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0009] Furthermore, the copper salt is copper sulfate or copper nitrate.

[0010] Furthermore, the preparation method of phenyl salicylate phosphate flame retardant is as follows: add ethanol, 100 parts by weight of 5-aminosalicylic acid, and 26-30 parts by weight of sodium hydroxide to a reaction container, stir, add 230-240 parts by weight of 4-formylphenyl diphenyl phosphate, heat to 70-80°C, react for 4-6 hours, cool, add hydrochloric acid to adjust the pH to 5-6, filter, and recrystallize the filter cake with ethanol to obtain phenyl salicylate phosphate flame retardant.

[0011] The reaction formula is as follows:

[0012]

[0013] Furthermore, the preparation method of phenanthroline-modified montmorillonite includes the following steps: adding toluene and 100 parts by weight of montmorillonite to a reaction vessel, stirring and dispersing, adding 30-100 parts by weight of toluene-2,4-diisocyanate in a nitrogen atmosphere, heating to 80-90°C, reacting for 1-3 hours, cooling, filtering, washing the filter cake with dichloromethane, drying, adding the product to a solvent, wherein the solvent includes toluene, 1,4-dioxane, and tetrahydrofuran, adding 50-160 parts by weight of 1,10-phenanthroline-5-amino, heating to 60-90°C, reacting for 6-8 hours, cooling, filtering, washing the filter cake with ethanol, and drying to obtain phenanthroline-modified montmorillonite. The reaction formula is:

[0014]

[0015] Furthermore, the temperature of the hydrothermal reaction in step S1 is 100-120° C., and the reaction time is 6-10 h.

[0016] Furthermore, in step S1, the volume ratio of ethanol to water is 1:(0.6-1).

[0017] Technical effect: The present invention sequentially utilizes toluene-2,4-diisocyanate and 1,10-phenanthroline-5-amino to modify the surface of montmorillonite, introduces a phenanthroline structure, and then carries out a high-temperature hydrothermal reaction with a phenyl salicylate phosphate flame retardant and a copper salt to obtain a montmorillonite-copper complex flame retardant, which is then melt-blended with nylon 6 to obtain a flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0018] After the surface of the montmorillonite of the present invention is organically modified, its compatibility with nylon 6 is improved, the tensile strength of the material is increased, and the flame retardant contains phosphate groups and a large number of nitrogen-containing urea groups and phenanthroline structures, forming a nitrogen-phosphorus flame retardant with excellent condensed phase flame retardancy, which has a synergistic flame retardant effect with the montmorillonite. At the same time, the flame retardant contains a salicylic acid-phenanthroline-copper complex structure, which can catalyze the nylon matrix into carbon at high temperatures, thereby forming a stable carbon barrier layer with flame retardant effects such as isolating oxygen, hindering heat transfer, and reducing heat release. The flame retardancy of the nylon material is significantly improved, the UL94 rating reaches V0, the limiting oxygen index becomes larger, and the residual carbon content at 800°C increases, which is conducive to improving the high temperature resistance of the material. The practical application of nylon materials in flame-retardant plastics, fiber fabrics, luggage, etc. is expanded. DETAILED DESCRIPTION

[0019] The above-mentioned invention of the present invention is further described in detail below in conjunction with specific embodiments. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following embodiments. Various substitutions and modifications may be made based on common technical knowledge and customary means in the art without departing from the above-mentioned technical concept of the present invention, and all such substitutions and modifications are intended to be within the scope of the present invention.

[0020] The nylon 6 of the present invention was purchased from Dongguan Gaochuang Plastic Raw Materials Co., Ltd. The montmorillonite was purchased from Xinyang Licheng New Materials Technology Co., Ltd.

[0021] Example 1:

[0022] (1): Add 700 mL of toluene and 20 g of montmorillonite to a reaction vessel, stir and disperse, add 6 g of toluene-2,4-diisocyanate in a nitrogen atmosphere, heat to 90 ° C, react for 1 hour, cool, filter, wash the filter cake with dichloromethane, and dry, add the product to 800 mL of 1,4-dioxane solvent, add 10 g of 1,10-phenanthroline-5-amino, heat to 90 ° C, react for 6 hours, cool, filter, wash the filter cake with ethanol, and dry to obtain phenanthroline-modified montmorillonite.

[0023] (2) 150 mL of ethanol, 0.5 g of 5-aminosalicylic acid, and 0.15 g of sodium hydroxide were added to a reaction vessel, and after stirring, 1.2 g of 4-formylphenyl diphenyl phosphate was added. The mixture was heated to 70° C. and reacted for 6 h. After cooling, hydrochloric acid was added to adjust the pH to 5, and the mixture was filtered. The filter cake was recrystallized with ethanol to obtain salicylic acid phenyl phosphate flame retardant.

[0024] (3): Add 150 mL of ethanol and 10 g of phenanthroline-modified montmorillonite into the reactor and disperse it by ultrasound. Then add 90 mL of water, 2.5 g of copper nitrate, and 4 g of phenyl salicylate phosphate flame retardant. Place it in a heating box and carry out hydrothermal reaction at 100 ° C for 10 hours. Filter, wash with water and ethanol in turn, and dry to obtain a montmorillonite-copper complex flame retardant.

[0025] (4): 5 kg of nylon 6, 50 g of montmorillonite-copper complex flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0026] Example 2:

[0027] (1): Add 800 mL of toluene and 20 g of montmorillonite to a reaction vessel, stir and disperse, add 12 g of toluene-2,4-diisocyanate in a nitrogen atmosphere, heat to 80 ° C, react for 3 h, cool, filter, wash the filter cake with dichloromethane, and dry, add the product to 900 mL of toluene solvent, add 22 g of 1,10-phenanthroline-5-amino, heat to 60 ° C, react for 8 h, cool, filter, wash the filter cake with ethanol, and dry to obtain phenanthroline-modified montmorillonite.

[0028] (2) 120 mL of ethanol, 0.5 g of 5-aminosalicylic acid, and 0.13 g of sodium hydroxide were added to a reaction vessel, and after stirring, 1.15 g of 4-formylphenyl diphenyl phosphate was added. The mixture was heated to 80° C. and reacted for 4 h. After cooling, hydrochloric acid was added to adjust the pH to 6. The mixture was filtered and the filter cake was recrystallized with ethanol to obtain salicylic acid phenyl phosphate flame retardant.

[0029] (3): Add 150 mL of ethanol and 10 g of phenanthroline-modified montmorillonite into the reactor and disperse it by ultrasound. Then add 150 mL of water, 4.5 g of copper sulfate, and 7 g of phenyl salicylate phosphate flame retardant. Place it in a heating box and carry out hydrothermal reaction at 120°C for 6 hours. Filter, wash with water and ethanol in turn, and dry to obtain a montmorillonite-copper complex flame retardant.

[0030] (4): 5 kg of nylon 6, 200 g of montmorillonite-copper complex flame retardant, and 15 g of antioxidant 1010 were added to a mixer and mixed for 20 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0031] Example 3:

[0032] (1): Add 800 mL of toluene and 20 g of montmorillonite to a reaction vessel, stir and disperse, add 20 g of toluene-2,4-diisocyanate in a nitrogen atmosphere, heat to 80 ° C, react for 3 h, cool, filter, wash the filter cake with dichloromethane, and dry, add the product to 900 mL of tetrahydrofuran solvent, add 32 g of 1,10-phenanthroline-5-amino, heat to 60 ° C, react for 6 h, cool, filter, wash the filter cake with ethanol, and dry to obtain phenanthroline-modified montmorillonite.

[0033] (2): Add 150 mL of ethanol and 10 g of phenanthroline-modified montmorillonite into a reactor and disperse it by ultrasound. Then add 120 mL of water, 3.8 g of copper sulfate, and 5.6 g of phenyl salicylate phosphate flame retardant (prepared by Example 1). Place the mixture in a heating box and perform a hydrothermal reaction at 110° C. for 6 h. Filter the mixture, wash it with water and ethanol in turn, and dry it to obtain a montmorillonite-copper complex flame retardant.

[0034] (3): 5 kg of nylon 6, 400 g of montmorillonite-copper complex flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 20 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0035] Comparative Example 1:

[0036] (1): 5 kg of nylon 6 and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 min. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain a nylon material.

[0037] Comparative Example 2:

[0038] (1): 5 kg of nylon 6, 50 g of montmorillonite, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 min. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain a nylon material.

[0039] Comparative Example 3:

[0040] (1) 5 kg of nylon 6, 50 g of phenanthroline-modified montmorillonite (prepared in Example 1), and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 min. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220° C., 235° C., 250° C., 260° C., and 260° C., and the screw speed was 100 r / min to obtain a nylon material.

[0041] Comparative Example 4:

[0042] (1): Add 150 mL of ethanol and 10 g of montmorillonite into a reactor and disperse it by ultrasonication. Then add 90 mL of water, 2.5 g of copper nitrate, and 4 g of phenyl salicylate phosphate flame retardant. Place it in a heating box and carry out hydrothermal reaction at 100 ° C for 10 hours. Filter, wash with water and ethanol in turn, and dry to obtain a montmorillonite composite flame retardant.

[0043] (2): 5 kg of nylon 6, 50 g of montmorillonite composite flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain a nylon material.

[0044] Comparative Example 5:

[0045] (1): Add 150 mL of ethanol and 10 g of phenanthroline-modified montmorillonite into a reactor and disperse it by ultrasonic. Then add 90 mL of water and 2.5 g of copper nitrate. Place it in a heating box and carry out hydrothermal reaction at 100 ° C for 10 hours. Filter, wash with water and ethanol in turn, and dry to obtain a montmorillonite composite flame retardant.

[0046] (2): 5 kg of nylon 6, 50 g of montmorillonite composite flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0047] Comparative Example 6:

[0048] (1): Add 150 mL of ethanol and 10 g of phenanthroline-modified montmorillonite into a reactor and disperse it by ultrasonication. Then add 90 mL of water and 4 g of phenyl salicylate phosphate flame retardant. Place it in a heating box and carry out hydrothermal reaction at 100 ° C for 10 hours. Filter, wash with water and ethanol in turn, and dry to obtain a montmorillonite composite flame retardant.

[0049] (2): 5 kg of nylon 6, 50 g of montmorillonite composite flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0050] Comparative Example 7:

[0051] (1): Add 150 mL of ethanol and 2.06 g of 1,10-phenanthroline to a reactor and disperse it by ultrasonication. Then add 90 mL of water, 2.5 g of copper nitrate, and 4 g of phenyl salicylate phosphate flame retardant. Place it in a heating box and carry out hydrothermal reaction at 100 ° C for 10 hours. Filter, wash with water and ethanol in turn, and dry to obtain a copper complex flame retardant.

[0052] (2): 5 kg of nylon 6, 50 g of copper complex flame retardant, and 20 g of antioxidant 1010 were added to a mixer and mixed for 10 minutes. The mixture was then extruded and granulated in a twin-screw extruder. The temperatures of sections 1-5 of the extruder were 220°C, 235°C, 250°C, 260°C, and 260°C, and the screw speed was 100 r / min to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon.

[0053] The flame-retardant, fire-proof and high-temperature resistant nylon is injected into a standard specimen through an injection molding machine. The temperatures of sections 1-4 of the injection molding machine are 230°C, 240°C, 245°C and 250°C, and the pressure is 80 MPa.

[0054] Tensile properties are tested according to the national standard GB / T 1040.1-2018. Flame retardancy is tested according to the UL94 method and the national standard GB / T2406.1-2008.

[0055] The nylon material was placed in a thermogravimetric analyzer and heated to 800°C at a heating rate of 10°C / min in a nitrogen atmosphere to test the residual carbon content.

[0056] Table 1 Nylon performance test

[0057]

[0058]

[0059] As shown in Table 1, the nylon material of Comparative Example 1 has good tensile strength, but has no UL94 grade, has very low limiting oxygen index and residual carbon content at 800°C, and has very poor flame retardancy.

[0060] Comparative Example 2 added montmorillonite, which has poor compatibility with nylon 6 and poor dispersion in the nylon matrix, and cannot effectively improve the tensile strength of the nylon material. It also does not improve the flame retardant properties of the nylon material.

[0061] Comparative Example 3 added phenanthroline-modified montmorillonite. After surface organic modification, the compatibility of montmorillonite with nylon 6 improved, and its dispersion in the nylon matrix was excellent, which was beneficial to improving the tensile strength of the nylon material. However, it did not significantly improve the flame retardancy of the nylon material.

[0062] In Example 1-3, a montmorillonite-copper complex flame retardant is added. After the surface of the montmorillonite is organically modified, its compatibility with nylon 6 is improved, and the tensile strength of the material is increased. In addition, the flame retardant contains phosphate groups and a large number of nitrogen-containing urea groups and phenanthroline structures to form a nitrogen-phosphorus flame retardant, which has a good condensed phase flame retardant effect and a synergistic flame retardant effect with the montmorillonite. At the same time, the flame retardant contains a salicylic acid-phenanthroline-copper complex structure, which can catalyze the nylon matrix into carbon at high temperature, thereby forming a stable carbon barrier layer, which has flame retardant effects such as isolating oxygen, hindering heat transfer, and reducing heat release, significantly improving the flame retardancy of the nylon material, and the UL94 grade reaches V0. The limiting oxygen index becomes larger, and the residual carbon content at 800°C increases, which is beneficial to improving the high temperature resistance of the material.

[0063] The montmorillonite in Comparative Example 4, which was not organically modified, resulted in a low tensile strength for the nylon material. The absence of a phenanthroline structure prevented the nylon material from forming a salicylic acid-phenanthroline-copper complex with copper ions and the phenyl salicylate phosphate flame retardant. This resulted in a low limiting oxygen index and residual carbon content for the nylon material, resulting in a UL94 rating of only V-1. Although flame retardancy deteriorated, it was still better than that of Comparative Example 1, primarily because the salicylic acid structure contained in the phenyl salicylate phosphate flame retardant could form a complex structure and complex precipitate with copper ions, catalyzing carbon formation.

[0064] Comparative Example 5 does not add phenyl salicylate phosphate flame retardant, the composite flame retardant does not contain phenyl phosphate flame retardant structure, and the flame retardancy of the nylon material is poor.

[0065] In Comparative Example 6, copper ions were not added, and the phenanthroline-modified montmorillonite and the phenyl salicylate phosphate flame retardant could not react with each other. After washing, the phenyl salicylate phosphate flame retardant was washed away, leaving only the phenanthroline-modified montmorillonite, and the flame retardancy of the nylon material was poor.

[0066] Comparative Example 7 uses 1,10-phenanthroline instead of phenanthroline to modify montmorillonite, and forms a copper complex flame retardant with copper ions and phenyl salicylate phosphate flame retardant. The flame retardancy of the nylon material is improved, but without the addition of montmorillonite, the tensile strength and flame retardancy of the material are lower than those of the embodiments.

Claims

1. A method for preparing flame retardant, fireproof and high temperature resistant nylon, characterized in that: The preparation method comprises the following steps: Step S1: Add ethanol and phenanthroline modified montmorillonite to the reactor, disperse them by ultrasonic, and then add water, copper salt, and the structural formula salicylic acid phenyl phosphate flame retardant, performing hydrothermal reaction, filtering, washing, and drying to obtain a montmorillonite-copper complex flame retardant; Step S2: adding 100 parts by weight of nylon 6, 1-8 parts by weight of a montmorillonite-copper complex flame retardant, and 0.3-0.4 parts by weight of an antioxidant into a mixer and mixing for 10-20 minutes, and then extruding and granulating the mixture in a twin-screw extruder to obtain flame-retardant, fire-resistant, and high-temperature resistant nylon; The preparation method of phenanthroline-modified montmorillonite comprises the following steps: adding toluene and 100 parts by weight of montmorillonite into a reaction container, stirring and dispersing, adding 30-100 parts by weight of toluene-2,4-diisocyanate in a nitrogen atmosphere, heating to 80-90° C., reacting for 1-3 hours, cooling, filtering, washing the filter cake, and drying, adding the product into a solvent, adding 50-160 parts by weight of 1,10-phenanthroline-5-amino, heating to 60-90° C., reacting for 6-8 hours, cooling, filtering, washing the filter cake, and drying to obtain the phenanthroline-modified montmorillonite.

2. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 1, characterized in that: The dosage of the phenanthroline modified montmorillonite is 100 parts by weight, the copper salt is 25-45 parts by weight, and the phenyl salicylate phosphate flame retardant is 40-70 parts by weight.

3. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 2, characterized in that: The copper salt is copper sulfate or copper nitrate.

4. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 2, characterized in that: The preparation method of the salicylic acid phenyl phosphate flame retardant comprises: adding ethanol, 5-aminosalicylic acid, and sodium hydroxide into a reaction container, adding 4-formylphenyl diphenyl phosphate after stirring, heating to 70-80° C., reacting for 4-6 hours, cooling, adding hydrochloric acid to adjust the pH to 5-6, filtering, and recrystallizing the filter cake with ethanol to obtain the salicylic acid phenyl phosphate flame retardant.

5. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 4, characterized in that: The dosage of the 5-aminosalicylic acid is 100 parts by weight, the sodium hydroxide is 26-30 parts by weight, and the 4-formylphenyl diphenyl phosphate is 230-240 parts by weight.

6. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 1, characterized in that: In the preparation method of phenanthroline-modified montmorillonite, the solvent is selected from toluene, 1,4-dioxane, and tetrahydrofuran.

7. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step S1 is 100-120° C., and the reaction time is 6-10 hours.

8. The method for preparing flame retardant, fireproof and high temperature resistant nylon according to claim 1, characterized in that: The volume ratio of ethanol to water in step S1 is 1:(0.6-1).

9. A flame-retardant, fire-resistant and high-temperature resistant nylon obtained by the preparation method according to any one of claims 1 to 8.

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