PA6-based carbon dot flame-retardant modified polyamide compound and preparation method thereof

Abstract

The invention provides a PA6-based carbon dot flame-retardant modified polyamide compound and a preparation method thereof, and relates to the technical field of flame-retardant materials. The preparation method of the PA6-based carbon dot flame-retardant modified polyamide compound comprises the following steps: mixing PA6-based carbon dots, polyamide and formic acid, and freeze-drying to obtain polyamide master batches containing the PA6-based carbon dots; and mixing polyamide with the polyamide master batch containing the PA6-based carbon dots, carrying out melt blending, and curing to obtain the PA6-based carbon dot flame-retardant modified polyamide compound. Due to the addition of the PA6-based carbon dots, polyamide molecules are easier to crystallize, and the formed crystal structure is more complete. By adding the PA6-based carbon dots, the composite material can generate a carbon layer to cover the surface of a matrix in the middle and later stage combustion process, the heating speed of the polyamide matrix is delayed, the thermal degradation process of the polyamide matrix is slowed down, and a good flame retardant effect is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of flame retardant materials, and particularly relates to a PA6-based carbon dot flame retardant modified polyamide composite and a preparation method thereof. Background Art

[0002] Synthetic polymer materials are gradually replacing traditional metals and woods and are widely used in industry, commerce, agriculture and households in modern society. Since these synthetic polymers are highly flammable, especially those with foam and fiber structures having a large surface area, improvements are needed to enhance their flame retardant properties. Flame retardants have become one of the largest chemical additives in the plastics industry today. In this regard, flame retardant additives containing halogens (brominated and chlorinated compounds), phosphorus and nitrogen have been developed successively. Unfortunately, current commercial halogen, phosphorus or composite additives pose a great hazard to the environment and human health, and emit toxic fumes during combustion, threatening human lives. Therefore, those skilled in the art are committed to developing an environmentally friendly and non-toxic flame retardant for polymer materials.

[0003] In addition, due to the large-scale use of PA66, a large amount of plastic waste is generated every year. Although plastic chemical recycling technology provides a new way to prepare new plastics from waste plastics, the recycling conditions are harsh and often need to be carried out under high temperature or supercritical conditions. How to recycle and reuse it is also a challenge faced by those skilled in the art. Summary of the Invention

[0004] The purpose of the present invention is to provide a PA6-based carbon dot flame retardant modified polyamide composite and a preparation method thereof, so as to solve the problems that existing flame retardants will produce toxic fumes during combustion and the difficulty in recycling waste PA6 materials.

[0005] In order to achieve the above invention purpose, the present invention provides the following technical solutions:

[0006] The present invention provides a preparation method of a PA6-based carbon dot flame retardant modified polyamide composite, comprising the following steps:

[0007] 1) Mix PA6-based carbon dots, polyamide and formic acid, and then freeze-dry to obtain a polyamide masterbatch containing PA6-based carbon dots;

[0008] 2) Mix polyamide with the polyamide masterbatch containing PA6-based carbon dots, and then carry out melt blending and curing to obtain a PA6-based carbon dot flame retardant modified polyamide composite.

[0009] Preferably, in step 1), the mass ratio of the PA6-based carbon dots, polyamide and formic acid is 1:4:15-25.

[0010] Preferably, in step 2), the temperature of the melt blending is 260-300 °C.

[0011] Preferably, the mass fraction of the PA6-based carbon dots in the PA6-based carbon dot flame-retardant modified polyamide composite is 1-5%.

[0012] Preferably, the preparation method of the PA6-based carbon dots in step 1) is as follows:

[0013] Mix pyromellitic acid, polyamide 6 and water and react to obtain PA6-based carbon dots.

[0014] Preferably, the mixing ratio of the pyromellitic acid, polyamide 6 and water is 0.3 g: 1.0-3.5 g: 20 mL;

[0015] The temperature of the reaction is 180-260 °C, and the reaction time is 12-30 h.

[0016] The present invention also provides a PA6-based carbon dot flame-retardant modified polyamide composite prepared by the above preparation method.

[0017] The present invention has at least the following beneficial effects:

[0018] The present invention uses PA6-based carbon dots as a flame retardant and adds them to the polyamide material. The heating and cooling curves of the prepared composite material are similar to those of the pure polyamide material, indicating that the PA6-based carbon dots do not affect the crystal morphology of the polyamide. The thermal crystallization temperature of the PA6-based carbon dot flame-retardant modified polyamide composite is higher than that of the pure polyamide. This is because during the heat treatment process, the breaking of the amide bond leads to the breaking of the polymer molecular chain and the decrease in molecular weight, and the promoting effect on the crystallization ability is greater than the inhibitory effect of the cross-linking reaction on the crystallization ability caused by the destruction of the hydrogen bonds inside the polymer. The addition of the PA6-based carbon dots makes the crystallization of the polyamide molecules easier, and the crystal structure of the final product is more complete, and the mechanical properties and high-temperature resistance are improved.

[0019] The addition of the PA6-based carbon dots can cause a carbon layer to form on the surface of the matrix during the combustion process of the composite material in the middle and late stages, delaying the heating rate of the polyamide matrix and slowing down the thermal degradation process of the polyamide matrix, achieving a good flame retardant effect. Description of the Drawings

[0020] Figure 1 It is a differential scanning calorimetry test result diagram of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5. Figure 1 In (a) is the heating curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5. Figure 1 In (b) is the cooling curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5.

[0021] Figure 2Thermogravimetric curves of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5 Figure 2 In (a) is the thermogravimetric TG curve of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5 Figure 2 In (b) is the derivative thermogravimetric DTG curve of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1-5

[0022] Figure 3 Is the heat release rate curve of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Example 5

[0023] Figure 4 Is the total heat release curve of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Example 5

[0024] Figure 5 Is the total smoke production curve of PA6 and PA6-based carbon dot flame-retardant modified PA6 composites prepared in Example 5

[0025] Figure 6 Is the differential scanning calorimetry test result diagram of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10 Figure 6 In (a) is the heating curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10 Figure 6 In (b) is the cooling curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10

[0026] Figure 7 Is the thermogravimetric curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10 Figure 7 In (a) is the thermogravimetric TG curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10 Figure 7 In (b) is the derivative thermogravimetric DTG curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6-10

[0027] Figure 8 Is the heat release rate curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Example 10

[0028] Figure 9 Is the total heat release curve of PA66 and PA6-based carbon dot flame-retardant modified PA66 composites prepared in Example 10

[0029] Figure 10Total smoke production curve of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10;

[0030] Figure 11 SEM images of the PA6-based carbon dot flame-retardant modified PA6 composite prepared from PA6 and Example 5 after cone calorimeter test, Figure 11 In (a) is the SEM image of the PA6 surface magnified 200 times, Figure 11 In (b) is the SEM image of the PA6 surface magnified 1000 times, Figure 11 In (c) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 magnified 200 times, Figure 11 In (d) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 magnified 1000 times;

[0031] Figure 12 SEM images of the PA6-based carbon dot flame-retardant modified PA66 composite prepared from PA6 and Example 10 after cone calorimeter test, Figure 12 In (a) is the SEM image of the PA6 surface magnified 200 times, Figure 12 In (b) is the SEM image of the PA6 surface magnified 1000 times, Figure 12 In (c) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 magnified 200 times, Figure 12 In (d) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 magnified 1000 times;

[0032] Figure 13 Raman spectra of the residual carbon on the surface of the PA6-based carbon dot flame-retardant modified PA6 composite prepared from PA6 and Example 5 after cone calorimeter test, Figure 13 In (a) is the Raman spectrum of the residual carbon on the PA6 surface, Figure 13 In (b) is the Raman spectrum of the residual carbon on the surface of the PA6-based carbon dot flame-retardant modified PA6 composite;

[0033] Figure 14 Raman spectra of the residual carbon on the surface of the PA6-based carbon dot flame-retardant modified PA66 composite prepared from PA66 and Example 10 after cone calorimeter test, Figure 14 In (a) is the Raman spectrum of the residual carbon on the PA66 surface, Figure 14 In (b) is the Raman spectrum of the residual carbon on the surface of the PA6-based carbon dot flame-retardant modified PA66 composite;

[0034] Figure 15The infrared spectrum of the surface residual carbon of the PA6-based carbon dot flame-retardant modified PA6 composite prepared from PA6 and Example 5 after performing a cone calorimeter test;

[0035] Figure 16 The infrared spectrum of the surface residual carbon of the PA66-based carbon dot flame-retardant modified PA66 composite prepared from PA66 and Example 10 after performing a cone calorimeter test;

[0036] Figure 17 The XPS full spectrum of the surface residual carbon of the PA6-based carbon dot flame-retardant modified PA6 composite prepared from PA6 and Example 5 after performing a cone calorimeter test;

[0037] Figure 18 The XPS fine spectrum of the surface residual carbon of the PA6-based carbon dot flame-retardant modified PA6 composite prepared from PA6 and Example 5 after performing a cone calorimeter test, where Figure 18 in (a), Figure 18 in (c), Figure 18 in (e) is the XPS fine spectrum of the residual carbon of the PA6 material, Figure 18 in (b), Figure 18 in (d), Figure 18 in (f) is the XPS fine spectrum of the residual carbon of the 5% 6CDs-PA6 material.

[0038] Figure 19 The XPS full spectrum of the surface residual carbon of the PA66-based carbon dot flame-retardant modified PA66 composite prepared from PA66 and Example 10 after performing a cone calorimeter test;

[0039] Figure 20 The XPS fine spectrum of the surface residual carbon of the PA66-based carbon dot flame-retardant modified PA66 composite prepared from PA66 and Example 10 after performing a cone calorimeter test, where Figure 20 in (a), Figure 20 in (c), Figure 20 in (e) is the XPS fine spectrum of the residual carbon of the PA66 material, Figure 20 in (b), Figure 20 in (d), Figure 20 in (f) is the XPS fine spectrum of the residual carbon of the 5% 6CDs-PA66 material. Detailed implementation mode

[0040] The present invention provides a method for preparing a PA6-based carbon dot flame-retardant modified polyamide composite, comprising the following steps:

[0041] 1) Mix PA6-based carbon dots, polyamide, and formic acid, and then freeze-dry to obtain a polyamide masterbatch containing PA6-based carbon dots;

[0042] 2) Mix the polyamide with a polyamide masterbatch containing PA6-based carbon dots, and then perform melt blending and curing to obtain a PA6-based carbon dot flame-retardant modified polyamide composite.

[0043] In the present invention, the polyamide includes one or more of PA6, PA66, PA610, PA1010, and PA12.

[0044] In the present invention, the mass ratio of the PA6-based carbon dots, polyamide, and formic acid in step 1) is 1:4:15 - 25, preferably 1:4:17 - 23, further preferably 1:4:19 - 21, and more preferably 1:4:20.

[0045] In the present invention, the temperature of the melt blending in step 2) is 260 - 300 °C, preferably 265 - 295 °C, further preferably 270 - 290 °C, and more preferably 275 - 285 °C.

[0046] In the present invention, the melt blending is preferably carried out using a screw extruder, and the temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively.

[0047] In the present invention, the mass fraction of the PA6-based carbon dots in the PA6-based carbon dot flame-retardant modified polyamide composite is 1 - 5%, and can be 1%, 2%, 3%, 4%, or 5%.

[0048] In the present invention, the preparation method of the PA6-based carbon dots in step 1) is as follows:

[0049] Mix pyromellitic acid, polyamide 6, and water and react to obtain PA6-based carbon dots.

[0050] In the present invention, the mixing ratio of pyromellitic acid, polyamide 6, and water is 0.3 g:1.0 - 3.5 g:20 mL, preferably 0.3 g:1.5 - 3.0 g:20 mL, and further preferably 0.3 g:2.0 - 2.5 g:20 mL.

[0051] In the present invention, the temperature of the reaction is 180 - 260 °C, preferably 190 - 250 °C, further preferably 200 - 240 °C, and more preferably 210 - 230 °C; the reaction time is 12 - 30 h, preferably 15 - 28 h, further preferably 18 - 25 h, and more preferably 20 - 22 h.

[0052] The present invention also provides a PA6-based carbon dot flame-retardant modified polyamide composite prepared by the above preparation method.

[0053] The technical solution provided by the present invention will be described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0054] Example 1

[0055] (1) Preparation of PA6-based carbon dots (6CDs):

[0056] 3 g of PA6, 0.3 g of pyromellitic acid, and 20 mL of deionized water were mixed and reacted at 260 °C for 18 h to obtain the 6CDs stock solution. The 6CDs stock solution was filtered through a 0.2 μm filter membrane, and the filtrate was freeze-dried by a vacuum freeze dryer to obtain solid 6CDs.

[0057] (2) 6CDs solid, polyamide (PA6, waste generated during the production process in a spinning factory), and formic acid were mixed in a mass ratio of 1:4:21, and the formic acid in the system was removed by freeze-drying to obtain a polyamide masterbatch with a 6CDs mass percentage of 20%.

[0058] (3) After mixing PA6 with the polyamide masterbatch containing 6CDs, it was dried in a vacuum oven at 120 °C for 12 h, and then extruded and molded by a screw extruder. The temperatures of zones I-IV of the screw extruder were set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure was set at 2 MPa to obtain a PA6-based carbon dot flame-retardant modified polyamide composite with a 6CDs mass fraction of 1%, denoted as 1% 6CDs-PA6.

[0059] Example 2

[0060] Steps (1) and (2) are the same as those in Example 1

[0061] (3) After mixing PA6 with the polyamide masterbatch containing 6CDs, it was dried in a vacuum oven at 120 °C for 12 h, and then extruded and molded by a screw extruder. The temperatures of zones I-IV of the screw extruder were set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure was set at 2 MPa to obtain a PA6-based carbon dot flame-retardant modified polyamide composite with a 6CDs mass fraction of 2%, denoted as 2% 6CDs-PA6.

[0062] Example 3

[0063] Steps (1) and (2) are the same as those in Example 1

[0064] (3) Mix PA6 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then extrude and mold it using a screw extruder. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA6 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 3%, denoted as 3% 6CDs - PA6.

[0065] Example 4

[0066] Steps (1) and (2) are the same as in Example 1.

[0067] (3) Mix PA6 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then extrude and mold it using a screw extruder. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA6 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 4%, denoted as 4% 6CDs - PA6.

[0068] Example 5

[0069] Steps (1) and (2) are the same as in Example 1.

[0070] (3) Mix PA6 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then extrude and mold it using a screw extruder. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA6 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 5%, denoted as 5% 6CDs - PA6.

[0071] Example 6

[0072] (1) Preparation of PA6 - based carbon dots (6CDs):

[0073] Mix 3 g of PA6, 0.3 g of pyromellitic acid, and 20 mL of deionized water, react at 260 °C for 18 h to obtain a 6CDs stock solution. Filter the 6CDs stock solution through a 0.2 - μm filter membrane, and freeze - dry the filtrate using a vacuum freeze - dryer to obtain solid 6CDs.

[0074] (2) Mix the 6CDs solid, polyamide (PA66, waste generated during the production process in a spinning factory), and formic acid with a mass ratio of 1:4:21, and remove the formic acid in the system by freeze - drying to obtain a polyamide masterbatch with a 6CDs mass percentage of 20%.

[0075] (3) Mix PA66 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then use a screw extruder for extrusion molding. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA66 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 1%, denoted as 1% 6CDs - PA66.

[0076] Example 7

[0077] Steps (1) and (2) are the same as those in Example 6.

[0078] (3) Mix PA66 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then use a screw extruder for extrusion molding. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA66 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 2%, denoted as 2% 6CDs - PA66.

[0079] Example 8

[0080] Steps (1) and (2) are the same as those in Example 6.

[0081] (3) Mix PA66 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then use a screw extruder for extrusion molding. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA66 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 3%, denoted as 3% 6CDs - PA66.

[0082] Example 9

[0083] Steps (1) and (2) are the same as those in Example 6.

[0084] (3) Mix PA66 with the polyamide masterbatch containing 6CDs, dry it in a vacuum oven at 120 °C for 12 h, and then use a screw extruder for extrusion molding. The temperatures of zones I - IV of the screw extruder are set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure is set at 2 MPa to obtain a PA66 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 4%, denoted as 4% 6CDs - PA66.

[0085] Example 10

[0086] Steps (1) and (2) are the same as those in Example 6.

[0087] (3) After mixing PA66 with the polyamide masterbatch containing 6CDs, it was dried in a vacuum oven at 120 °C for 12 h, and then extruded and molded using a screw extruder. The temperatures of zones I - IV of the screw extruder were set at 280 °C, 275 °C, 275 °C, and 270 °C respectively, and the injection pressure was set at 2 MPa to obtain a PA66 - based carbon dot flame - retardant modified polyamide composite with a 6CDs mass fraction of 5%, denoted as 5% 6CDs - PA66.

[0088] Differential scanning calorimetry (DSC) tests were carried out on PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5. The atmosphere was N2, the heating rate and the cooling rate were both 10 °C / min. It was heated from room temperature to 250 °C and then cooled back to room temperature. The test results are as Figure 1 shown, Figure 1 in which (a) is the heating curve of PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5, Figure 1 and (b) is the cooling curve of PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5. The heating and cooling curves of 6CDs - PA6 and pure PA6 are similar, indicating that the addition of 6CDs has little effect on the crystal morphology of PA6. The melting point (T m ) of 6CDs - PA6 changes little compared with that of pure PA6. The cold crystallization temperature (T cc ) of 6CDs - PA6 increases compared with that of pure PA6. This is because the addition of 6CDs promotes the heterogeneous nucleation of PA6, so crystallization becomes easier. ΔT mc is the degree of supercooling, which is the difference between T m and the thermal crystallization temperature (T mc ). The smaller the value of ΔT mc , the easier it is to crystallize and the faster the crystallization rate. Table 1 shows the DSC parameters of PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5. It can be seen from the table that as the mass fraction of 6CDs increases, the value of ΔT mc shows a downward trend, that is, 6CDs - PA6 is easier to crystallize than pure PA6.

[0089] Table 1 DSC parameters of PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5

[0090]

[0091] Thermogravimetric tests (TG) were carried out on PA6 and the PA6 - based carbon dot flame - retardant modified PA6 composites prepared in Examples 1 - 5. The atmosphere was N2, the heating rate was 10 °C / min, and it was heated from room temperature to 700 °C. The test results are as Figure 2 shown, Figure 2(a) in it is the thermogravimetric (TG) curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1 to 5. Figure 2 (b) in it is the derivative thermogravimetric (DTG) curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1 to 5. The relevant parameters are shown in Table 2. Compared with pure PA6, the T 5wt％ decreases, while the T max changes little. This is because 6CDs preferentially absorb heat and degrade rapidly. Therefore, the initial decomposition temperature of 6CDs-PA6 is lower than that of pure PA6. In terms of the char residue amount, compared with pure PA6, when the mass fraction of 6CDs is 5%, the char residue amount of 6CDs-PA6 increases from 0.4% to 0.8%. This is because the addition of 6CDs causes 6CDs-PA6 to form a char layer covering the surface of the matrix during the combustion process in the middle and late stages, delaying the heating rate of the PA6 matrix and slowing down the thermal degradation process of the PA6 matrix.

[0092] Table 2 TG-DTG data of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composites prepared in Examples 1 to 5

[0093]

[0094] Figure 3 is the heat release rate (HRR) curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5. It can be seen from the figure that the addition of 6CDs reduces the PHRR (peak heat release rate) of PA6. When the mass fraction of 6CDs is 5%, the PHRR of 5% 6CDs-PA6 decreases by 2.0% compared with pure PA6.

[0095] Figure 4 is the total heat release (THR) curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5. It can be seen from the figure that the addition of 6CDs reduces the total heat release (THR) of PA6. When the mass fraction of 6CDs is 5%, the THR of 5% 6CDs-PA6 decreases by 3.1% compared with pure PA6.

[0096] Figure 5 is the total smoke production curve of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5. It can be seen from the figure that the addition of 6CDs reduces the total smoke production (TSP) of PA6. When the mass fraction of 6CDs is 5%, the TSP of 5% 6CDs-PA6 is 5.7 m2, which is 6.6% lower than that of pure PA6.

[0097] Differential scanning calorimetry (DSC) was performed on the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10. The atmosphere was N2, the heating rate and the cooling rate were both 10 °C / min. It was heated from room temperature to 300 °C and then cooled to room temperature. The test results are as Figure 6 shown, Figure 6 in (a) is the heating curve of the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10, Figure 6 in (b) is the cooling curve of the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10. The corresponding thermodynamic data are shown in Table 3. The results show that the heating and cooling curves of 6CDs-PA66 and pure PA66 are similar, indicating that the addition of 6CDs has little effect on the crystal morphology of PA66. The T cc and T m of 6CDs-PA66 change little compared with pure PA66. However, the thermal crystallization temperature of 6CDs-PA66 increases compared with pure PA66. This is because the promotion of the crystallization ability caused by the breakage of amide bonds and the breakage of polymer molecular chains and the decrease of molecular weight during the heat treatment process is greater than the inhibitory effect of the destruction of hydrogen bonds inside the polymer caused by the cross-linking reaction on the crystallization ability. The addition of 6CDs makes the molecular structure of PA66 more regular, the crystal structure of 6CDs-PA66 more complete, and the crystallization becomes easier. ΔT mc is the degree of supercooling, which is the difference between T m and T mc . The smaller the value of ΔT mc , the easier the crystallization and the faster the crystallization rate. As can be seen from Table 3, with the increase of the mass fraction of 6CDs, the value of ΔT mc shows a downward trend, that is, 6CDs-PA66 is easier to crystallize than pure PA66.

[0098] Table 3 DSC parameters of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 5 to 10

[0099]

[0100] Thermogravimetric test (TG) was performed on the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10. The atmosphere was N2, the heating rate was 10 °C / min, and it was heated from room temperature to 700 °C. The test results are as Figure 7 shown, Figure 7 in (a) is the thermogravimetric TG curve of the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10, Figure 7In (b), the thermogravimetric DTG curves of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 6 to 10 are shown. The specific data are shown in Table 4. Compared with pure PA66, the T 5wt％ decreases, while the T max changes little. This is because 6CDs preferentially absorb heat and degrade rapidly. Therefore, the initial decomposition temperature of 6CDs-PA66 is lower than that of pure PA66. In terms of the char residue amount, the addition of 6CDs has little effect on the char residue amount at 700 °C. The R max of 6CDs-PA66 decreases. When the mass fraction of 6CDs is 5%, the R max of 6CDs-PA66 decreases from 21.3% to 18.3%. This is because the addition of 6CDs inhibits the combustion process in the middle and late stages, delays the heating rate of the PA66 matrix, and slows down the thermal degradation process of the PA66 matrix.

[0101] Table 4 TG-DTG data of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 5 to 10

[0102]

[0103]

[0104] The flame retardant properties of PA6, PA66, and the composite materials prepared in Examples 1 to 10 were tested. The limiting oxygen index (LOI) was tested according to GB / T 2406.2-2009, the vertical burning rating test (UL-94) was carried out according to GB / T 2408-2008, and the cone calorimeter test (CONE) was carried out according to the ISO 5660-1 standard. The external heat flux was 50 kW / m 2 , the temperature was 768 °C, and each sample was tested 5 times. The test results are shown in Tables 5 and 6.

[0105] Table 5 Limiting oxygen index values (LOI) and UL-94 ratings of PA6 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 1 to 5

[0106]

[0107] Table 6 Limiting oxygen index values (LOI) and UL-94 ratings of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 5 to 10

[0108]

[0109]

[0110] a Average combustion duration after the first and second flame ignitions of the sample;

[0111] b Total combustion time of the sample for 5 times;

[0112] As can be seen from Table 5, pure PA6 is easy to burn, with its LOI value only being 23%. As the mass fraction of 6CDs increases, it is found that the LOI value of 6CDs-PA6 gradually increases; when the mass fraction of 6CDs is 5%, the LOI value of 6CDs-PA6 reaches 30%. The UL-94 test of pure PA6 has no rating, and the total combustion time is 237.7 s; when the mass fraction of 6CDs added is 1%, UL-94 reaches V-2 level, and the total combustion time is 144.4 s; as the mass fraction of 6CDs continues to increase, the UL-94 rating of 6CDs-PA6 is still V-2 level, but the total combustion time gradually decreases. When the mass fraction of 6CDs added is 5%, the total combustion time is the lowest at 24.4 s.

[0113] As can be seen from Table 6, pure PA66 is easy to burn, with its LOI value only being 26%. As the mass fraction of 6CDs increases, it is found that the LOI value of 6CDs-PA66 gradually increases; when the mass fraction of 6CDs is 5%, the LOI value of 6CDs-PA66 reaches 35%. The UL-94 test of pure PA66 has no rating, and the total combustion time is 263.6 s; when the mass fraction of 6CDs added is 1%, UL-94 reaches V-2 level, and the total combustion time is 30.1 s; as the mass fraction of 6CDs continues to increase, the UL-94 rating of 6CDs-PA66 is still V-2 level, but the total combustion duration gradually decreases. When the mass fraction of 6CDs added reaches 5%, the total combustion duration decreases to 12.9 s.

[0114] Figure 8 Figure [ID number not provided] is the heat release rate curve of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10. As can be seen from the figure, the addition of 6CDs reduces the PHRR of PA66. When the mass fraction of 6CDs is 5%, the PHRR of 5% 6CDs-PA66 is reduced by 7.3% compared to pure PA66.

[0115] Figure 9This is the total heat release curve of the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10. It can be seen from the figure that the addition of 6CDs reduces the THR of PA66. When the mass fraction of 6CDs is 5%, the THR of 5% 6CDs-PA66 decreases by 9.7% compared with that of pure PA66. This is because 6CDs promote the formation of a shielding carbon layer, reducing the heat release and the release amount of combustible volatiles, thus reducing the PHRR and THR of 6CDs-PA66. The main influencing factors of the ignition time (TTI) are the thermal conductivity and heat absorption rate of the matrix. Good thermal conductivity and better heat absorption are beneficial to the increase of TTI. The addition of 6CDs increases the TTI. When the mass fraction of 6CDs is 5%, the TTI of 5% 6CDs-PA66 is 77 s, which is increased compared with that of pure PA66. This is because 6CDs promote the formation of a thermal conduction network inside the matrix, prolonging the TTI of 6CDs-PA66.

[0116] Figure 10 This is the total smoke production curve of the PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10. It can be seen from the figure that the addition of 6CDs reduces the TSP of PA66. When the mass fraction of 6CDs is 5%, the TSP of 6CDs-PA66 is 9.2 m 2 , which is reduced by 17.9% compared with that of pure PA66.

[0117] Refer to GB / T 1040-92 to test the mechanical properties of PA6, PA66 and the composite materials prepared in Examples 1-10. The test results are shown in Tables 7 and 8.

[0118] Table 7 Mechanical data of PA6 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 1-5

[0119]

[0120] Table 8 Mechanical data of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composites prepared in Examples 5-10

[0121]

[0122]

[0123] Take pictures of the surface morphologies of the char residues after the cone calorimetry of PA6, 5% 6CDs-PA6, PA66, and 5% 6CDs-PA66, as shown in Figure 11 , Figure 12 respectively. Figure 11 This is the SEM image of the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 of PA6 after the cone calorimetry test. Figure 11In (a) is the SEM image of the PA6 surface magnified 200 times, Figure 11 In (b) is the SEM image of the PA6 surface magnified 1000 times, Figure 11 In (c) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 magnified 200 times, Figure 11 In (d) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 magnified 1000 times.

[0124] The char layer morphology is an important factor for evaluating the quality of the protective barrier, and an effective protective barrier can prevent mass and heat transfer. By Figure 11 It can be seen that there are porous char layers in the char of pure PA6, while the char layers in the 5% 6CDs-PA6 char are continuous and dense. These continuous and dense char layers act as a barrier during the combustion process of the PA6 matrix, preventing the volatilization of combustible gases and heat transfer. The PHRR, THR, and TSP of 5% 6CDs-PA6 during the combustion process are all lower than those of pure PA6, which is consistent with the cone calorimeter test results. From Figure 11 it can be seen that under the same area, the number of holes on the surface of the 5% 6CDs-PA6 char layer is less than that of pure PA6, and the hole area is also smaller than that of pure PA6.

[0125] Figure 12 are the SEM images of PA6 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 after cone calorimeter test, Figure 12 In (a) is the SEM image of the PA6 surface magnified 200 times, Figure 12 In (b) is the SEM image of the PA6 surface magnified 1000 times, Figure 12 In (c) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 magnified 200 times, Figure 12 In (d) is the SEM image of the surface of the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 magnified 1000 times.

[0126] By Figure 12 it can be seen that there are porous char layers in the char of pure PA66, while the char layers in the 5% 6CDs-PA66 char are more dense. These dense char layers act as a barrier during the combustion process of the PA66 matrix, preventing the volatilization of combustible gases and heat transfer. When the mass fraction of 6CDs reaches 5%, the PHRR, THR, and TSP of 6CDs-PA66 during the combustion process are all lower than those of pure PA66, which is consistent with the cone calorimeter test results. From Figure 12 it can be seen that under the same area, the number of holes on the surface of the 5% 6CDs-PA66 char layer is less than that of pure PA66, and the hole area is also smaller than that of pure PA66.

[0127] The graphitization degree of the char residue after the cone calorimeter test of 5% 6CDs-PA6 and 5% 6CDs-PA66 was measured using a Raman spectrometer (Renishaw) with an excitation wavelength of 532 nm. The results are as Figure 13 , Figure 14 shown. Figure 13 In (a) of Figure 13 is the Raman spectrum of the char residue on the surface of PA6, Figure 13 and in (b) of -1 is the Raman spectrum of the char residue on the surface of the PA6-based carbon dot flame-retarded modified PA6 composite. -1 The D peak at 1360 cm -1 belongs to amorphous carbon, while the G peak at 1595 cm -1 represents the C═C stretching vibration peak of ordered carbon. By calculating the ratio of the integrated areas of the D peak and the G peak (I D / I G ), the graphitization degree of the char residue can be evaluated. The smaller the value of I D / I G , the higher the graphitization degree of the char residue. I G / I All represents the proportion of graphitized carbon in the total carbon. Compared with pure PA6, the value of I D / I G of 5% 6CDs-PA6 did not change, and the value of I G / I All decreased from 28.6% to 28.4%. Generally speaking, the changes in I D / I G and I G / I All were not significant, indicating that 6CDs had a poor effect on promoting matrix carbonization in PA6 and there was no obvious change in the graphitization degree, which was also consistent with the cone calorimeter test results of 6CDs-PA6.

[0128] Figure 14 In (a) of Figure 14 is the Raman spectrum of the char residue on the surface of PA66, Figure 14 and in (b) of D is the Raman spectrum of the char residue on the surface of the PA6-based carbon dot flame-retarded modified PA66 composite. It can be seen that compared with pure PA66, the value of I G / I D G / I All of 5% 6CDs-PA66 decreased from 2.6 to 2.4, and the value of I

[0129] increased from 28.0% to 29.4%. The results show that 6CDs can promote matrix carbonization in PA66 and increase the graphitization degree, which is consistent with the cone calorimeter test results of 6CDs-PA66.The structure of the char residue was analyzed by Fourier transform infrared spectroscopy (FT-IR) in the scanning range of 4000 - 400 cm -1 , and the number of scans was 64 times to obtain the functional group information. The results are as Figure 15 , Figure 16 shown. Figure 15 Figure Figure 16 is the infrared spectrum of the surface char residue of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 after the cone calorimeter test. Figure 15 Figure Figure 16 is the infrared spectrum of the surface char residue of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 after the cone calorimeter test. It can be seen from -1 and -1 that compared with pure PA6, the absorption band intensity of C=O at 1710 cm -1 of 5% 6CDs-PA6 becomes less obvious, indicating that 6CDs promotes the detachment of C=O and forms a carbon layer. Compared with pure PA66, the absorption band intensity of C=O at 1710 cm -1 of 5% 6CDs-PA66 becomes less obvious, indicating that 6CDs promotes the detachment of C=O and forms a stable and dense carbon layer.

[0130] XPS was used to analyze the structure of the char residue to obtain the elemental composition, content and chemical bond information. Among them, Figure 17 is the full XPS spectrum of the surface char residue of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5 after the cone calorimeter test. Figure 18 is the XPS fine spectrum. It can be seen from Figure 17 and Figure 18 that compared with pure PA6, the contents of carbon and oxygen elements in the char residue of 6CDs-PA6 decrease. Combining the cone calorimeter data of PA6 and 6CDs-PA6 in Table 9, the yield of CO does not change, while the yield of CO2 increases. This is because 6CDs promotes the thermal cracking of PA6 to produce more CO2, diluting the combustible gas and oxygen and playing a flame-retardant role in the gas phase.

[0131] Table 9 Cone calorimeter test data of PA6 and the PA6-based carbon dot flame-retardant modified PA6 composite prepared in Example 5

[0132]

[0133] Figure 19 is the full XPS spectrum of the surface char residue of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10 after the cone calorimeter test. Figure 20 is the XPS fine spectrum. It can be seen from Figure 19 and Figure 20It can be seen that compared with pure PA66, the carbon element content in the char residue of 6CDs-PA66 has increased. This is because 6CDs promote the carbonization of PA66 to form a dense carbon layer. The peak at 288.1 eV in the C1s spectrum belongs to C=O in the char residue of 6CDs-PA66. The increase in C element content and the decrease in O element content indicate that during the combustion process, the original structure of PA66 is damaged, forming a benzene-containing carbon layer structure.

[0134] Table 10 Cone calorimetry test data of PA66 and the PA6-based carbon dot flame-retardant modified PA66 composite prepared in Example 10

[0135]

[0136] 6CDs were physically blended with PA6 and PA66 respectively to prepare 6CDs-PA6 and 6CDs-PA66 composites, and their flame retardancy and mechanical properties were investigated. The results show that when the mass fraction of 6CDs reaches 5%, the LOI value of 6CDs-PA6 reaches 30%, UL-94 is V-2 grade, the peak value of the heat release rate decreases by 2.0% compared with pure PA6, the total heat release decreases by 3.1%, the total smoke release decreases by 6.6%, and the addition of 6CDs has little effect on the tensile strength and elastic modulus of PA6. When the mass fraction of 6CDs reaches 5%, the LOI value of 6CDs-PA66 reaches 35%, UL-94 is V-2 grade, the peak value of the heat release rate decreases by 7.3% compared with pure PA66, the total heat release decreases by 9.7%, the total smoke release decreases by 17.9%, and the addition of 6CDs has little effect on the tensile strength and elastic modulus of PA66. In addition, the flame retardancy mechanism of 6CDs was investigated. The results show that 6CDs promote the thermal cracking of the PA6 matrix to generate more CO2, and the char residue mass does not change significantly, achieving the flame retardant effect; while 6CDs promote the formation of a continuous, dense and highly graphitized shielding carbon layer in the PA66 matrix, achieving the effects of reducing heat, suppressing smoke and flame retardancy.

[0137] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite, characterized in that, It includes the following steps: 1) Mix PA6-based carbon dots, polyamide, and formic acid, and then freeze-dry to obtain a polyamide masterbatch containing PA6-based carbon dots; 2) Mix polyamide with the polyamide masterbatch containing PA6-based carbon dots, and then carry out melt blending and curing to obtain a PA6-based carbon dot flame-retardant modified polyamide composite.

2. The preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite according to claim 1, characterized in that In step 1), the mass ratio of the PA6-based carbon dots, polyamide, and formic acid is 1:4:15 - 25.

3. The preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite according to claim 1 or 2, characterized in that, In step 2), the temperature of the melt blending is 260 - 300 °C.

4. The preparation method of a PA6-based carbon dot flame retardant modified polyamide composite according to claim 3, characterized in that, In the PA6-based carbon dot flame-retardant modified polyamide composite, the mass fraction of the PA6-based carbon dots is 1 - 5%.

5. The preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite according to claim 4, characterized in that, The preparation method of the PA6-based carbon dots in step 1) is as follows: Mix pyromellitic acid, polyamide 6, and water and react to obtain PA6-based carbon dots.

6. The preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite according to claim 5, characterized in that The mixing ratio of the pyromellitic acid, polyamide 6, and water is 0.3 g:1.0 - 3.5 g:20 mL; The temperature of the reaction is 180 - 260 °C, and the reaction time is 12 - 30 h.

7. A PA6-based carbon dot flame-retardant modified polyamide composite prepared by the preparation method of a PA6-based carbon dot flame-retardant modified polyamide composite according to any one of claims 1 - 6.

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