Patent Publication Number: US-2022227935-A1

Title: Flame-retardant semi-aromatic polyamide and preparation method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to the technical field of novel polymer materials, and particularly relates to a flame-retardant semi-aromatic polyamide and a preparation method thereof. 
     DESCRIPTION OF RELATED ART 
     Polyamide is the most widely used engineering plastic, which has important applications in fields of electronic appliances, home appliances, etc. However, flammability and water absorption of polyamide itself fluctuate greatly, which greatly limits the application and promotion of the polyamide. Therefore, for the polyamide a flame-retardant modification and at the same time reduction of the water absorption has a very important significance. 
     Flame-retardant modification methods of the polyamide mainly include copolymerization flame-retardant modification, blending flame-retardant modification and post-treatment flame-retardant modification. At present, the method of blending flame-retardant modification is widely used. However, because a flame retardant is usually a small molecule, it is prone to have a micro phase separation in a blending system with the polyamide, which damages a mechanical property and flame-retardant property of the material, and limits the application of the flame-retardant polyamide especially in the fields of precision components, such as electronic appliances. The method of post-treatment flame-retardant modification is only limited to polyamide fiber, and the application field is very limited. The method of in situ copolymerization flame-retardant modification means that a reactive flame retardant is connected to a polyamide chain through a chemical bond, achieving a molecular-level modification, where there is no migration, agglomeration and precipitation of the flame retardant, and it has an intrinsic flame-retardant characteristic. 
     The following four Chinese patents disclose a method of in situ copolymerization flame-retardant modification of the polyamide by a reactive flame retardant. 
     Chinese patent 2013102079850 discloses a halogen-free flame-retardant nylon 66 polymer, wherein in this method, a reactive flame retardant DOPO derivative is copolymerized with a diacid or a diamine to form a salt, and then the resulting salt is copolymerized with a nylon 66 salt, to obtain the halogen-free flame-retardant nylon 66 polymer. Chinese patent 2015106245312 discloses a bi-phosphorous flame-retardant copolymerized nylon, wherein an intermediate, a bi-phosphorous organophosphoryl ammonium salt, which is synthesized by a diamine of phosphine oxides and a dihydric alcohol of phosphine oxides, is adopted to copolymerize with a nylon 66 salt to prepare the bi-phosphorous flame-retardant copolymerized nylon. However, the technology of the above-mentioned patents is limited to the synthesis of in situ copolymerized flame-retardant material of nylon 66, where there is no development of any in situ copolymerized flame-retardant material suitably used in a semi-aromatic polyamide system and the corresponding high-temperature resistant polymerization method. Moreover, the above-mentioned in situ copolymerized flame-retardant nylon 66 has much lower mechanical properties than ordinary nylon 66, which cannot meet the demand for a high-strength flame-retardant product. 
     Chinese patent 2015106604531 discloses a flame-retardant high-temperature resistant nylon copolymer, wherein a nylon salt obtained by a reaction with a reactive flame retardant bis(p-carboxyphenyl)phenyl phosphine oxide is used to copolymerize with an ordinary nylon salt, to obtain a flame-retardant polyamide product. Chinese patent 2017112299959 discloses a permanent halogen-free flame-retardant PA10T polyamide resin, wherein by salt formation, pre-polymerization, and solid phase polymerization, 1,1,3,5-tetraphenylamine cyclotriphosphazene is copolymerized to polyamide. Although in these two patents, a flame-retardant semi-aromatic polyamide material has been developed, a mechanical property and a water absorption resistance of the developed flame-retardant material are significantly reduced compared with those before copolymerization, which limits the application of this type of material in the industries of electronic appliances. 
     From the perspective of the preparation method, the above process requires that a reactive flame-retardant monomer is first synthesized into a flame-retardant precursor to improve stability of the reactive flame-retardant monomer during the polymerization process. However, an additional pretreatment step will lead to an increase in process cost and reduce market competitiveness of the product. 
     SUMMARY 
     An objective of the present invention is to provide a flame-retardant semi-aromatic polyamide, which has advantages of an excellent flame-retardant property, a good mechanical property and low water absorption. 
     Another objective of the present invention is to provide a preparation method of the above flame-retardant semi-aromatic polyamide, which has simple process steps and low production cost. 
     The present invention is realized through the following technical solutions. 
     A flame-retardant semi-aromatic polyamide, derived from the following monomers: 
     a diacid monomer A: where A1 is terephthalic acid or terephthalic acid and other diacid, terephthalic acid accounts for 50 to 100 mol % of A1, and A2 is [(6-oxido-6H-dibenzo-(c,e)(1,2)-oxaphosphorin-6-ketone)-methyl]-butanedioic acid, A1+A2=100 mol %, A1=90 to 99 mol %, A2=1 to 10 mol %; and 
     a diamine monomer B: one or more of diamine monomers containing 4 to 36 carbon atoms. 
     A structural formula of [(6-oxido-6H-dibenzo-(c,e)(1,2)-oxaphosphorin-6-ketone)-methyl]-butanedioic acid (DDP) is as follows: 
     
       
         
         
             
             
         
       
     
     DDP is a kind of a flame-retardant monomer, which dicarboxylic acid with a phosphate ester structure can be polymerized with a diamine monomer. The in situ copolymerized flame-retardant semi-aromatic polyamide has a good flame-retardant effect and a good mechanical property. Moreover, the present invention has found that, contrary to an expected effect, compared to other flame-retardant monomers (even for a flame-retardant monomer with a phosphate ester structure similar to DDP, a water absorption property after addition is not very satisfactory), the in situ copolymerized flame-retardant semi-aromatic polyamide containing a phosphate ester structure DDP has a reduced water absorption. 
     Preferably, in the diacid monomer A, a content of A2 is 2.5 to 7.7 mol % of all the diacid monomers. 
     At a preferred A2 content, mechanical properties can be further improved and water absorption can be reduced. 
     The other diacid monomer is selected from at least one of an aliphatic diacid and an aromatic diacid. 
     The aliphatic diacid is selected from one or more of oxalic acid, malonic acid, 1,4-succinic acid, 1,5-glutaric acid, 1,6-adipic acid, 1,7-pimelic acid, 1,8-suberic acid, 2-methyl suberic acid, 1,9-azelaic acid, 1,10-sebacic acid, 1,11-undecanedioic acid, 1,12-dodecanedioic acid, 1,13-tridecanedioic acid, 1,14-tetradecanedioic acid, and cyclohexanedicarboxylic acid; the aromatic diacid is selected from at least one of isophthalic acid and naphthalenedicarboxylic acid. 
     The diamine monomer B is selected from one or more of 1,4-butanediamine, 1,5-pentanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, 1,14-tetradecanediamine, 1,16-hexadecandiamine, 1,18-octadecanediamine, 1-butyl-1,2-ethylenediamine, 1,1-dimethyl-1,4-butanediamine, 1-ethyl-1,4-butanediamine, 1,2-dimethyl-1,4-butanediamine, 1,3-dimethyl-1,4-butanediamine, 1,4-dimethyl-1,4-butanediamine, 2,3-dimethyl-1,4-butanediamine, 2-methyl-1,5-pentanediamine, 3-methyl-1,5-pentanediamine, 2,5-dimethyl-1,6-hexanediamine, 2,4-dimethyl-1,6-hexanediamine, 3,3-dimethyl-1,6-hexanediamine, 2,2-dimethyl-1,6-hexanediamine, 2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine, 2,4-diethyl-1,6-hexanediamine, 2,2-dimethyl-1,7-heptanediamine, 2,3-dimethyl-1,7-heptanediamine, 2,4-dimethyl-1,7-heptanediamine, 2,5-dimethyl-1,7-heptanediamine, 2-methyl-1,8-octanediamine, 3-methyl-1,8-octanediamine, 4-methyl-1,8-octanediamine, 1,3-dimethyl-1,8-octanediamine, 1,4-dimethyl-1,8-octanediamine, 2,4-dimethyl-1,8-octanediamine, 3,4-dimethyl-1,8-octanediamine, 4,5-dimethyl-1,8-octanediamine, 2,2-dimethyl-1,8-octanediamine, 3,3-dimethyl-1,8-octanediamine, 4,4-dimethyl-1,8-octanediamine, and 5-methyl-1,9-nonanediamine; and preferably selected from at least one of 1,10-decanediamine and 1,6-hexanediamine. 
     Preferably, the other diacid is selected from at least one of isophthalic acid and 1,6-adipic acid. 
     A preparation method of the above flame-retardant semi-aromatic polyamide includes the following steps: weighing the diacid monomer A, the diamine monomer B, a catalyst (which may be sodium hypophosphite), and deionized water into a high-temperature high-pressure reactor, after gas aerating and exchange to make atmosphere in the reactor as nitrogen, heating up to 160 to 180° C., reacting at a constant temperature for 0.5 hours, continuing to heat up to 200 to 210° C., and reacting at a constant temperature for 0.5 hours, continuing to heat up to 240 to 250° C., reacting at a constant temperature for 1 hour, draining for about 0.5 hours, discharging, and granulating; putting the granulated material into a rotating drum, evacuating to make a pressure lower than 3000 Pa, heating up to 230 to 260° C., reacting for 3 hours, stopping heating, and after the temperature returning to room temperature, discharging the material to obtain the flame-retardant semi-aromatic polyamide. 
     Compared with the prior art, the present invention has the following beneficial effects. 
     For the flame-retardant semi-aromatic polyamide of the present invention, by an in situ copolymerization with a specific flame-retardant monomer DDP in a chain segment, the semi-aromatic polyamide has a good flame-retardant property and at the same time has a good mechanical property and low water absorption. By adjusting a molar ratio of DDP in the diacid monomer A, a proportion of an aromatic ring in a main chain structure of the semi-aromatic polyamide is optimized, and the water absorption is further reduced. 
     Different from the prior art, the preparation method of the present invention can ensure a smooth progress of the polymerization reaction, without pretreatment of the reactive flame-retardant monomer DDP, where a gradient heating process can improve stability of the reactive flame-retardant monomer during the polymerization process, and an in situ copolymerized flame-retardant semi-aromatic polyamide having an intrinsic flame-retardant property, an excellent mechanical property and low water absorption is obtained. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention is further illustrated by the following embodiments, but which shall not be understood as limitation of the protection scope of the present invention. 
     Raw materials used in the present invention are derived from commercially available products.
     BCPPO: bis(4-carboxyphenyl)phenyl phosphine oxide;   CEPPA: 3-hydroxyphenylphosphinyl propanoic acid   CEMPO: bis(2-carboxyethyl)methyl phosphine oxide; and   MCA: cyanuric acid.   

     A preparation method of a polyamide in Embodiments and Comparative Examples: according to a ratio of each component in Tables 1 to 4, weighing a diacid monomer A, a diamine monomer B, sodium hypophosphite, and deionized water into a high-temperature high-pressure reactor, after gas aerating and exchange to make atmosphere in the reactor as nitrogen, heating up to 160 to 180° C., reacting at a constant temperature for 0.5 hours, continuing to heat up to 200 to 210° C., and reacting at a constant temperature for 0.5 hours, continuing to heat up to 240 to 250° C., reacting at a constant temperature for 1 hour, draining for about 0.5 hours, discharging, and granulating, putting the granulated material into a rotating drum, evacuating to make a pressure lower than 3000 Pa, heating up to 230 to 260° C., reacting for 3 hours, stopping heating, and after the temperature returning to room temperature, discharging the material to obtain the semi-aromatic polyamide. 
     Test Methods of Various Properties 
     (1) End groups: a potentiometric titrator was used to determine an amino end group content and a carboxyl end group content of the polymer. A sample of 0.45 g polyamide was weighed, 50 mL of preheated and dissolved o-cresol was added and heated to reflux until the sample was dissolved. After cooling to 50° C. in a water tank at 50° C., 0.5 mL of formaldehyde solution was added, a magnetic stirrer was put in to stir the solution, an electrode test section of the automatic potentiometric titrator was immersed into the solution, and carboxyl end group data were measured by titration with a calibrated KOH-ethanol solution. 0.45 g of the sample was weighed, 45 mL of phenol and 3 mL of anhydrous methanol were added, heated to reflux until the sample was dissolved. After cooling to 50° C. in the water tank at 50° C., the magnetic stirrer was put in to stir the solution, the electrode test section of the automatic potentiometric titrator was immersed into the solution, and amino end group data were measured by titration with a calibrated hydrochloric acid solution. 
     (2) Relative viscosity: with reference to a standard GB/T 12006.1-1989, a Ubbelohde viscometer was used to measure a relative viscosity of a product with a concentration of 0.25 g/dL in 98% concentrated sulfuric acid at (25±0.01°) C. 
     In the present invention, a molecular weight of the polyamide is determined according to an end group content and a relative viscosity of the polyamide. Under the premise of the same monomer, if the relative viscosity and the end group content are similar, the molecular weight of the polyimide is similar. 
     (3) Tensile strength: with reference to ISO 527, tensile strength of a resin material was tested. 
     (4) Bending strength: with reference to a standard ISO 178, bending strength of a resin material was tested. 
     (5) Notched impact strength/unnotched impact strength: with reference to a standard ISO 180, impact strength of a resin material was tested. 
     (6) UL94 flame-retardant rating: measured with reference to GB/T2408-1996, with a test sample size of 13 cm×1.3 cm×0.3 cm. 
     (7) Limiting oxygen index (LOI): measured with reference to a standard GB/T5454-1997, with a test sample size of 12 cmxl cm×0.4 cm. 
     (8) Water absorption: with reference to a standard ISO 62:2008, water absorption of a resin material was tested. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Ratio of each component of the polyamide and test results in Embodiments 1 to 4 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Embodiment 1 
                 Embodiment 2 
                 Embodiment 3 
                 Embodiment 4 
               
               
                   
                 10T10DDP 
                 10T10DDP 
                 10T10DDP 
                 10T10DDP 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 mol % of DDP accounting 
                 1 
                 2.5 
                 5 
                 7.7 
               
               
                 for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 99 
                 97.5 
                 95 
                 92.3 
               
               
                 and/or other diacid monomer 
               
               
                 Amino end group mol/t 
                 72 
                 74 
                 77 
                 75 
               
               
                 Carboxyl end group mol/t 
                 108 
                 138 
                 133 
                 136 
               
               
                 Relative viscosity 
                 1.88 
                 1.87 
                 1.87 
                 1.87 
               
               
                 Tensile strength MPa 
                 73.2 
                 76.0 
                 75.2 
                 76.8 
               
               
                 Bending strength MPa 
                 92.5 
                 95.3 
                 94.2 
                 95.7 
               
               
                 Notched impact strength kJ/m 2   
                 5 
                 6 
                 6 
                 6 
               
               
                 Unnotched impact strength kJ/m 2   
                 35 
                 39 
                 39 
                 38 
               
               
                 UL flame-retardant rating 
                 V0 
                 V0 
                 V0 
                 V0 
               
               
                 LOI 
                 32 
                 35 
                 35 
                 35 
               
               
                 Water absorption % 
                 0.6 
                 0.3 
                 0.3 
                 0.3 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Ratio of each component of the polyamide and test results in Embodiments 5 to 8 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Embodiment 5 
                 Embodiment 6 
                 Embodiment 7 
                 Embodiment 8 
               
               
                   
                 10T10DDP 
                 6T/66/6DDP 
                 6T/66/6DDP 
                 6T/6I/6DDP 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 mol % of DDP accounting 
                 10 
                 1 
                 10 
                 1 
               
               
                 for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 90 
                 99 (T:6 = 
                 90 (T:6 = 
                 99 (T:I = 
               
               
                 and/or other diacid monomer 
                   
                 50:50) 
                 50:50) 
                 80:20) 
               
               
                 Amino end group mol/t 
                 71 
                 76 
                 89 
                 70 
               
               
                 Carboxyl end group mol/t 
                 130 
                 132 
                 140 
                 110 
               
               
                 Relative viscosity 
                 1.86 
                 1.87 
                 1.85 
                 1.88 
               
               
                 Tensile strength MPa 
                 74.3 
                 73.1 
                 72.2 
                 76.3 
               
               
                 Bending strength MPa 
                 92.7 
                 95.9 
                 93.8 
                 96.8 
               
               
                 Notched impact strength kJ/m 2   
                 5 
                 5 
                 6 
                 5 
               
               
                 Unnotched impact strength kJ/m 2   
                 35 
                 38 
                 40 
                 36 
               
               
                 UL flame-retardant rating 
                 V0 
                 V0 
                 V0 
                 V0 
               
               
                 LOI 
                 34 
                 31 
                 34 
                 32 
               
               
                 Water absorption % 
                 0.6 
                 0.7 
                 0.7 
                 0.6 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Ratio of each component of the polyamide and test results in Embodiments 9 to 11 
               
            
           
           
               
               
               
               
            
               
                   
                 Embodiment 9 
                 Embodiment 10 
                 Embodiment 11 
               
               
                   
                 6T/6I/6DDP 
                 10T/66/10DDP 
                 10T10I10DDP 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 mol % of DDP accounting 
                 10 
                 5 
                 5 
               
               
                 for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 90 (T:I = 
                 95 (T:6 = 
                 95 (T:I = 
               
               
                 and/or other diacid monomer 
                 80:20) 
                 80:20) 
                 80:20) 
               
               
                 Amino end group mol/t 
                 85 
                 88 
                 81 
               
               
                 Carboxyl end group mol/t 
                 135 
                 137 
                 136 
               
               
                 Relative viscosity 
                 1.86 
                 1.85 
                 1.86 
               
               
                 Tensile strength MPa 
                 74.6 
                 75.0 
                 75.0 
               
               
                 Bending strength MPa 
                 94.5 
                 94.8 
                 105 
               
               
                 Notched impact strength kJ/m 2   
                 6 
                 6 
                 8 
               
               
                 Unnotched impact strength kJ/m 2   
                 39 
                 38 
                 42 
               
               
                 UL flame-retardant rating 
                 V0 
                 V0 
                 V0 
               
               
                 LOI 
                 35 
                 35 
                 35 
               
               
                 Water absorption % 
                 0.7 
                 0.6 
                 0.5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Ratio of each component of the polyamide and test results in Comparative examples 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 example 1 
                 example 2 
                 example 3 
                 example 4 
               
               
                   
                 10T 
                 10T10I 
                 PA66 
                 PA66/6DDP 
               
               
                   
               
               
                 mol % of a flame-retardant monomer 
                 0 
                 0 
                 0 
                 5 
               
               
                 accounting for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 100 
                 100 (T:I = 
                 100 
                 95 
               
               
                 and/or other diacid monomer 
                   
                 80:20) 
               
               
                 Amino end group mol/t 
                 68 
                 71 
                 69 
                 72 
               
               
                 Carboxyl end group mol/t 
                 104 
                 112 
                 100 
                 110 
               
               
                 Relative viscosity 
                 1.89 
                 1.88 
                 1.88 
                 1.86 
               
               
                 Tensile strength MPa 
                 78.1 
                 78.0 
                 63.7 
                 41 
               
               
                 Bending strength MPa 
                 97.9 
                 96.1 
                 80.5 
                 58.5 
               
               
                 Notched impact strength kJ/m 2   
                 5 
                 5 
                 5 
                 4 
               
               
                 Unnotched impact strength kJ/m 2   
                 37 
                 35 
                 35 
                 30 
               
               
                 UL flame-retardant rating 
                 — 
                 — 
                 — 
                 V0 
               
               
                 LOI 
                 26 
                 26 
                 21 
                 30 
               
               
                 Water absorption % 
                 1.2 
                 1.2 
                 3.0 
                 3.5 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 example 5 
                 example 6 
                 example 7 
               
               
                   
                 10T10BCPPO 
                 10T10CEPPA 
                 10T10I10DDP 
               
               
                   
               
               
                 mol % of a flame-retardant monomer 
                 5 
                 5 
                 5 
               
               
                 accounting for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 95 
                 95 
                 95 (T:I = 
               
               
                 and/or other diacid monomer 
                   
                   
                 20:80) 
               
               
                 Amino end group mol/t 
                 73 
                 75 
                 81 
               
               
                 Carboxyl end group mol/t 
                 121 
                 119 
                 136 
               
               
                 Relative viscosity 
                 1.89 
                 1.89 
                 1.86 
               
               
                 Tensile strength MPa 
                 74.6 
                 72.9 
                 63.0 
               
               
                 Bending strength MPa 
                 93.7 
                 92.8 
                 47.0 
               
               
                 Notched impact strength kJ/m 2   
                 5 
                 6 
                 9 
               
               
                 Unnotched impact strength kJ/m 2   
                 33 
                 33 
                 40 
               
               
                 UL flame-retardant rating 
                 V0 
                 V0 
                 V0 
               
               
                 LOI 
                 32 
                 32 
                 30 
               
               
                 Water absorption % 
                 1.5 
                 1.6 
                 1.1 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 example 8 
                 example 9 
                 example 10 
               
               
                   
                 10T10DDP 
                 10T10MCA 
                 10T10CEMPO 
               
               
                   
               
               
                 mol % of a flame-retardant monomer 
                 30 
                 5 
                 5 
               
               
                 accounting for a diacid monomer 
               
               
                 mol % of terephthalic acid 
                 70 
                 95 
                 95 
               
               
                 and/or other diacid monomer 
               
               
                 Amino end group mol/t 
                 90 
                 78 
                 75 
               
               
                 Carboxyl end group mol/t 
                 143 
                 115 
                 117 
               
               
                 Relative viscosity 
                 1.84 
                 1.89 
                 1.89 
               
               
                 Tensile strength MPa 
                 62.5 
                 60.7 
                 55.6 
               
               
                 Bending strength MPa 
                 83.3 
                 78.2 
                 65.4 
               
               
                 Notched impact strength kJ/m 2   
                 6 
                 4 
                 4 
               
               
                 Unnotched impact strength kJ/m 2   
                 45 
                 29 
                 27 
               
               
                 UL flame-retardant rating 
                 V0 
                 V2 
                 V2 
               
               
                 LOI 
                 36 
                 25 
                 24 
               
               
                 Water absorption % 
                 1.3 
                 1.5 
                 1.9 
               
               
                   
               
            
           
         
       
     
     It can be seen from Embodiments 1 to 5 that at a preferred DDP content range, a flame retardant effect and mechanical properties are good, and water absorption is low. 
     It can be seen from Comparative Example 1 and Table 1 that for an in situ polymerization of DDP into 10T, not only an excellent flame-retardant property is obtained, but also mechanical properties are maintained and even improved and water absorption is reduced. 
     It can be seen from Comparative Example 1 or 6 that when the in situ copolymerized flame-retardant monomer is CEPPA, although there are good mechanical properties and flame-retardant rating, water absorption is relatively high. 
     It can be seen from Comparative Example 1 or 5 that when the in situ copolymerized flame-retardant monomer is BCPPO, mechanical properties are decreased and water absorption is high. 
     It can be seen from Comparative Example 3 or 4 and Table 1 that after an in situ copolymerization of DDP in an aliphatic polyamide PA66 system, mechanical properties are significantly reduced; while after an in situ copolymerization of DDP in a semi-aromatic polyamide, relatively good mechanical properties are maintained, a flame-retardant property and a water absorption property are significantly improved. 
     It can be seen from Embodiment 11 and Comparative Example 7 that when a content of terephthalic acid accounting for the diacid monomer A is too low, it not only reduces a flame-retardant property to a certain extent and causes water absorption to rise, and leads to very poor mechanical properties, making it difficult to achieve application values. 
     It can be seen from Comparative Example 8 that when a content of DDP accounting for the diacid monomer A is too high, it also causes a decrease in mechanical properties and an increase in water absorption. 
     It can be seen from Comparative example 9 or 10 that when the in situ copolymerized flame-retardant monomer is MCA or CEMPO, a flame-retardant effect and mechanical properties are both relatively poor, and water absorption is relatively high.