Patent Publication Number: US-2023145884-A1

Title: Halogen free flame-retardant materials and method for manufacturing the same

Description:
TECHNICAL FIELD 
     The disclosure relates to a halogen-free flame-retardant material formed by using a twin-screw extruder with different zones to mix a silane-modified nano-silica aqueous suspension and a molten mixture and a method of forming the same. 
     DESCRIPTION OF RELATED ART 
     Electrical wires and cables can be seen anywhere in life. Cable sheathing materials thereof mostly include PVC, while the rest are mainly cross-linked plastic rubber. However, the above-mentioned materials are often regarded as not environmentally friendly due to toxic substances that may be produced during processing or difficulty in recycling and reuse. Accordingly, there is a tendency toward using thermoplastic elastomers, which are easier to be recycled, together with the application of halogen-free flame retardants. Nonetheless, when the thermoplastic elastomer is applied to the electrical wires and cables that emphasize flexibility, insufficiency in wear resistance remains a problem. Therefore, the integration of nano powder with the thermoplastic elastomer is considered as an important means to improve the wear resistance and mechanical strength of the cable sheathing materials, but nanopowder is inherently unlikely to disperse in the thermoplastic elastomer. 
     In summary of the foregoing, there is currently an urgent need for a new material composition and manner of processing to overcome the above-mentioned disadvantage of poor dispersion so as to meet the requirements for environmentally friendly materials with highly wear resistant, halogen-free, and highly flame-retardant properties. 
     SUMMARY 
     An embodiment of the disclosure discloses a forming method of a halogen-free flame-retardant material, and the method includes the following. A twin-screw extruder including a first zone and a second zone is used. A mixture in the first zone is mixed, melted and heated to form a molten mixture. The molten mixture includes a halogen-free flame retardant, a wear-resistant modifier, a thermoplastic elastomer, and an antioxidant. In addition, a silane-modified nano-silica aqueous suspension is introduced into the second zone to mix the silane-modified nano-silica aqueous suspension with the molten mixture from the first zone. The first zone and the second zone are continuously connected regions. 
     Another embodiment of the disclosure discloses a halogen-free flame-retardant material prepared by using the above-mentioned forming method. The halogen-free flame-retardant material has an wear resistance of less than 40 mg, a hardness (shore A) of less than 85, a tensile strength of greater than 120 kgf/cm 2 , an elongation of greater than 300%, and a flame retardancy achieving a UL94 V-0 rating at a specimen thickness of 0.8 mm. 
     Still another embodiment of the disclosure discloses a wear-resistant halogen-free flame-retardant cable sheathing material, including the above-mentioned halogen-free flame-retardant material. 
     To make the aforementioned features of the invention more obvious and comprehensible, embodiments accompanied with drawings are particularly described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       None. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the disclosure provides a forming method of a halogen-free flame-retardant material, and the method includes the following. A twin-screw extruder including a first zone and a second zone is used. The first zone and the second zone are continuously connected regions. In addition, a mixture in the first zone is mixed, melted and heated to form a molten mixture. The mixture includes a halogen-free flame retardant, a wear-resistant modifier, a thermoplastic elastomer, and an antioxidant. Then, a silane-modified nano-silica aqueous suspension is introduced into the second zone to mix the silane-modified nano-silica aqueous suspension with the molten mixture from the first zone. 
     In some embodiments, the silane-modified nano-silica aqueous suspension includes silane, nano-silica powder, and water. 
     In some embodiments, the halogen-free flame retardant includes a phosphorus nitrogen flame retardant, aluminum phenylphosphinate, aluminum hypophosphite, melamine cyanurate, a phosphate amine salt, or phosphate esters. For the wear-resistant modifier, a siloxane polymer, vinyl polydimethylsiloxane, or a methicone silsesquioxane crosspolymer may be adopted. The thermoplastic elastomer includes polyether type thermoplastic polyurethane, polyester type thermoplastic polyurethane, a polyether type thermoplastic polyester elastomer, or a polyester type thermoplastic polyester elastomer. The antioxidant includes a hindered phenolic antioxidant, bis[3-(1,1-dimethylethyl)-4-hydroxy-5-methylbenzenepropionic acid] tripolyethylene glycol, or a hindered phenolic composite type antioxidant. 
     In some embodiments, the mixture may also include a volume resistivity enhancer modified elastomer, such as a styrene-ethylene/butylene-styrene copolymer thermoplastic elastomer or a polyolefin elastomer. 
     In some embodiments, based on 100 parts by weight of the total weight of the mixture, a content of the halogen-free flame retardant is 15 to 45 parts by weight, for example, about 15 to 40 parts by weight, about 15 to 30 parts by weight, and about 15 to 20 parts by weight, but not limited thereto. 
     In some embodiments, based on 100 parts by weight of the weight of the thermoplastic elastomer, a content of the wear-resistant modifier is 1 to 10 parts by weight, for example, about 1 to 7 parts by weight and about 1 to 5 parts by weight, but not limited thereto. 
     In some embodiments, based on 100 parts by weight of the weight of the thermoplastic elastomer, a content of the antioxidant is 0.2 to 1.5 parts by weight, for example, about 0.2 to 0.7 part by weight and about 0.2 to 0.5 part by weight, but not limited thereto. 
     In some embodiments, based on 100 parts by weight of the total weight of the mixture, a content of the nano-silica powder is 0.5 to 12 parts by weight, for example, about 0.5 to 10 parts by weight and about 0.5 to 5 parts by weight, but not limited thereto. 
     In some embodiments, based on 100 parts by weight of the total weight of the mixture, a content of the volume resistivity enhancer modified elastomer is 5 to 30 parts by weight, for example, about 5 to 20 parts by weight and about 5 to 10 parts by weight, but not limited thereto. 
     In some embodiments, a forming method of the silane-modified nano-silica aqueous suspension may include the following. Under a room temperature condition, nano-silica powder is added into deionized water and stirred to form a nano-silica mixed aqueous solution. Then, an alkaline solution of 20 ml of 1M KOH and an acid solution of 20 ml of 1M HCl are sequentially added to adjust the pH value of the nano-silica mixed aqueous solution to neutral. After filtering and drying, the treated nano-silica are mixed and stirred with 7.5 g of silane, 2.5 of suspending agent, and 500 g of water to form a silane-modified nano-silica aqueous suspension. For example, the silane may be (3-glycidyloxypropyl)trimethoxysilane (GPTMS), hexamethyldisilane (HMDS), vinyltriethoxysilane (VTES), and polydimethylsiloxane (PDMS). 
     In some embodiments, a temperature of the first zone of the twin-screw extruder is set to 120° C. to 200° C., and a temperature of the second zone of the twin-screw extruder is set to 120° C. to 200° C. 
     In some embodiments, a pump pressure for introducing the silane-modified nano-silica aqueous suspension into the second zone may be 20 to 50 bars. An excessively high pump pressure causes the twin-screw extruder to have a comparatively high torque value and be relatively energy-consuming. An extremely low pump pressure causes failure in maintaining a stable flow of the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica aqueous suspension into the twin-screw extruder. 
     In some embodiments, the screws have a rotation speed of 60 rpm to 300 rpm. In some embodiments, a halogen-free flame-retardant material manufactured using the above-mentioned forming method has a wear resistance of less than 40 mg, a hardness (shore A) of less than 85, a tensile strength of greater than 120 kgf/cm 2 , an elongation of greater than 300%, and a flame retardancy achieving a UL94 V-0 rating at a specimen thickness of 0.8 mm. Therefore, a wear-resistant halogen-free flame-retardant cable sheathing material including the halogen-free flame-retardant material also has the technical efficacy of low hardness, high wear resistance, and high flame retardancy. 
     Experiments are listed below to verify the efficacy of the invention, but the invention is not limited to the following content. 
     Preparation Example 
     Preparation Example 1 
     100 grams of nano-silica powder (purchased from Trump Chemical, Aerosil 200, 100 to 500 nm) was obtained, and after the nano-silica powder was stirred and mixed with 100 ml of 1M KOH for 2 hours, 10 to 50 ml of 1M HCl was added to adjust the pH value of the nano-silica mixed aqueous solution to neutral. After filtering and drying, the treated nano-silica was stirred with 1.5 grams of (3-glycidyloxypropyl)trimethoxysilane (GPTMS) (purchased from Echo Chemical), 1.25 grams of a suspending agent, and 250 grams of deionized water to form a (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica aqueous suspension with a concentration of about 40 wt %. It was expected that, after the modification, grafted-silane will be formed on the surface of the nano-silica. 
     Preparation Example 2 
     A modified nano-silica aqueous suspension was prepared using the method of Preparation Example 1, but GPTMS therein was changed into hexamethyldisilane (HMDS) (purchased from Chongxin Trading), to form a hexamethyldisilane-modified nano-silica aqueous suspension with a concentration of about 40 wt %. It was expected that, after the modification, grafted-silane will be formed on the surface of the nano-silica. 
     Comparative Preparation Example 1 
     A nano-silica aqueous suspension was prepared using the method of Preparation Example 1, but silane was not added for modification. 
     Experimental Example 
     Experimental Examples 1 to 6 
     Raw materials: thermoplastic polyurethane (purchased from Coating Chemical, ER-85A), halogen-free flame retardant (purchased from Anchem Technology, SONGFLAM-203), siloxane polymer (purchased from DuPont, model MB50-017), antioxidant (purchased from BASF, model Irganox-245), and the suspension of Preparation Example 1. 
     According to the proportions in Table 1, a mixture containing thermoplastic polyurethane (TPU), halogen-free flame retardant, siloxane polymer, and antioxidant was directly introduced from the main inlet of the twin-screw extruder into the first zone of the twin-screw extruder, and was mixed, melted and heated at a temperature of 120 to 180° C. and a screw rotation speed of 250 rpm to form a molten mixture. 
     With a high-pressure pump under a pressure of 40 bars, according to the proportions in Table 1, the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica (nano-SiO 2 ) suspension obtained in Preparation Example 1 was introduced from the side inlet of the twin-screw extruder into the second zone of the twin-screw extruder, and was mixed with the molten mixture from the connected first zone, where the side inlet had a temperature of 180° C., and the second zone had a temperature of 200° C. and a screw rotation speed of 250 rpm. During the process, a flow of the suspension of Preparation Example 1 was controlled by a flow controller, so that the proportions of raw materials in the mixture retained after passing through the second zone were in accordance with Table 1. After extruding the mixture as a strand through water, and pelletizing it with the twin-screw extruder, the mixture was then placed in an oven at 105° C. for 4 hours to obtain a halogen-free flame-retardant material. A mechanical test, a flame retardancy test, and a wear resistance test were performed on the manufactured halogen-free flame-retardant material, and the results are also shown in Table 1. 
     Comparative Example 1 
     A halogen-free flame-retardant material was manufactured using the method of Experimental Example 1, but the suspension of Preparation Example 1 was not added, and the rest of the raw materials were in the proportions as shown in Table 1. 
     Then, a mechanical test, a flame retardancy test, and a wear resistance test were performed on the halogen-free flame-retardant material of Comparative Example 1, and the results are also shown in Table 1. 
     Comparative Example 2 
     A halogen-free flame-retardant material was manufactured using the method of Experimental Example 1, but a content of t(3-glycidyloxypropyl)trimethoxysilane-modified nano-silica of Preparation Example 1 was above 5 parts by weight. 
     Then, a mechanical test, a flame retardancy test, and a wear resistance test were performed on the halogen-free flame-retardant material of Comparative Example 2, and the results are also shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Experimental 
                 Experimental 
                 Experimental 
                 Experimental 
                 Experimental 
                 Experimental 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 TPU (parts by weight) 
                 82 
                 81.5 
                 81 
                 80 
                 79 
                 78 
                 77 
                 72 
               
               
                 Halogen-free 
                 18 
                 18 
                 18 
                 18 
                 18 
                 18 
                 18 
                 18 
               
               
                 flame retardant 
               
               
                 (parts by weight) 
               
               
                 Nano-SiO 2   
                 0 
                 0.5 
                 1 
                 2 
                 3 
                 4 
                 5 
                 10 
               
               
                 (parts by weight) 
               
               
                 Antioxidant (phr) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 Siloxane polymer (phr) 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Hardness (shore A) 
                 82 
                 82 
                 82 
                 84 
                 84 
                 84 
                 85 
                 87 
               
               
                 Tensile strength 
                 218 
                 230 
                 235 
                 212 
                 200 
                 189 
                 165 
                 97 
               
               
                 (kgf/cm 2 ) 
               
               
                 Elongation (%) 
                 576 
                 556 
                 596 
                 569 
                 589 
                 543 
                 307 
                 310 
               
               
                 Flame retardancy 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
               
               
                 (UL94 0.8 mm) 
               
               
                 Wear resistance (mg) 
                 45 
                 27 
                 26 
                 22 
                 23 
                 22 
                 21 
                 120 
               
               
                   
               
               
                 phr: parts by weight added per hundred parts by weight of thermoplastic resin. 
               
            
           
         
       
     
     As can be seen in Table 1, the halogen-free flame-retardant material of the disclosure has excellent wear resistance and also superior flame retardancy. 
     Experimental Examples 7 to 10 
     A halogen-free flame-retardant material was manufactured using the method of Experimental Example 1. According to the proportions in Table 2, the contents of thermoplastic polyurethane, halogen-free flame retardant, suspension of Preparation Example 1, and antioxidant were fixed, and the content of the siloxane polymer of Preparation Example 1 was changed. 
     Then, the halogen-free flame-retardant materials manufactured in Experimental Examples 7 to 10 were subjected to a mechanical test, a flame retardancy test, and a wear resistance test, and the results together with the values of Experimental Example 3 are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Experimental 
                 Experimental 
                 Experimental 
                 Experimental 
                 Experimental 
               
               
                   
                 Example 3 
                 Example 7 
                 Example 8 
                 Example 9 
                 Example 10 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TPU (parts by weight) 
                 80 
                 80 
                 80 
                 80 
                 80 
               
               
                 Halogen-free 
                 18 
                 18 
                 18 
                 18 
                 18 
               
               
                 flame retardant 
               
               
                 (parts by weight) 
               
               
                 Nano-SiO 2   
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 (parts by weight) 
               
               
                 Antioxidant (phr) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 Siloxane polymer (phr) 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                 Hardness (shore A) 
                 84 
                 82 
                 82 
                 81 
                 81 
               
               
                 Tensile strength 
                 212 
                 190 
                 189 
                 162 
                 147 
               
               
                 (kgf/cm 2 ) 
               
               
                 Elongation (%) 
                 569 
                 596 
                 622 
                 589 
                 543 
               
               
                 Flame retardancy 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
                 V-0 
               
               
                 (UL94 0.8 mm) 
               
               
                 Wear resistance (mg) 
                 22 
                 28 
                 27 
                 23 
                 22 
               
               
                   
               
            
           
         
       
     
     As can be obtained from Table 2, the disclosure having the halogen-free flame-retardant material with 1 to 5 phr siloxane can achieve excellent wear resistance, and also superior in flame-retardancy and other mechanical properties. 
     Experimental Examples 11 and 12 
     A halogen-free flame-retardant material was manufactured using the method of Experiment 1, and according to the proportions in Table 3, a styrene-ethylene/butylene-styrene copolymer (SEBS) thermoplastic elastomer (purchased from TSRC, taipol 6150) was also added into the first zone of the twin-screw extruder. 
     Then, the halogen-free flame-retardant materials manufactured in Experimental Examples 11 and 12 were subjected to a mechanical test, a flame retardancy test, and a wear resistance test, and a volume resistance was detected. The results are also shown in Table 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Experimental 
                 Experimental 
               
               
                   
                 Example 11 
                 Example 12 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 TPU (parts by weight) 
                 75 
                 70 
               
               
                 Halogen-free flame retardant 
                 18 
                 18 
               
               
                 (parts by weight) 
               
               
                 SEBS (parts by weight) 
                 5 
                 10 
               
               
                 Nano-SiO 2  (parts by weight) 
                 2 
                 2 
               
               
                 Antioxidant (phr) 
                 0.3 
                 0.3 
               
               
                 Siloxane polymer (phr) 
                 2 
                 2 
               
               
                 Hardness (shore A) 
                 83 
                 83 
               
               
                 Tensile strength (kgf/cm 2 ) 
                 210 
                 183 
               
               
                 Elongation (%) 
                 556 
                 569 
               
               
                 Flame retardancy (UL94 0.8 mm) 
                 V-0 
                 V-0 
               
               
                 Wear resistance (mg) 
                 38 
                 35 
               
               
                 Volume resistance (Ω · cm) 
                 1.2E+13 
                 1.3E+13 
               
               
                   
               
            
           
         
       
     
     As can be seen in Table 3, by adding SEBS into the halogen-free flame-retardant material of the disclosure, the volume resistance can be further improved, and the wear resistance is still better than Comparative Examples 1 and 2. 
     Comparative Examples 3 to 5 below are for investigating the effects of nano-silica on mechanical strength and wear resistance, and thus do not contain halogen-free flame retardant and siloxane polymer. 
     Comparative Example 3 
     Raw materials: thermoplastic polyurethane (purchased from Coating Chemical, ER-85A), nano-silica (purchased from Trump Chemical, Aerosil 200, 100 to 500 nm), and antioxidant (purchased from BASF, Irganox-245). 
     According to the proportions in Table 4, the thermoplastic polyurethane (TPU), nano-silica, and antioxidant were directly introduced from the main inlet of the twin-screw extruder into the first zone and the connected second zone of the twin-screw extruder, and were mixed, melted and heated at a temperature of 120 to 200° C. and a screw rotation speed of 250 rpm to form a molten mixture. 
     After extruding the mixture as a strand through water, and pelletizing it with the twin-screw extruder, the mixture was then placed in an oven at 105° C. for 4 hours to obtain a composition without halogen-free flame retardant and siloxane polymer. A mechanical test and a wear resistance test were performed on the manufactured halogen-free composition, and the results are also shown in Table 4. 
     Comparative Example 4 
     A halogen-free composition was manufactured using the method of Comparative Example 3, but nano-clay (purchased from nanocor, PGN, 5 to 20 nm) was used to replace the nano-silica. 
     Then, a mechanical test and a wear resistance test were performed on the manufactured composition without halogen-free flame retardant and siloxane polymer, and the results are also shown in Table 4. 
     Comparative Example 5 
     Raw materials: thermoplastic polyurethane (purchased from Coating Chemical, ER-85A), the nano-silica aqueous suspension without silane modification of Comparative Preparation Example 1, and antioxidant (purchased from BASF, Irganox-245). 
     According to the proportions in Table 4, the thermoplastic polyurethane (TPU) and antioxidant were directly introduced from the main inlet of the twin-screw extruder into the first zone of the twin-screw extruder, and were mixed, melted and heated at a temperature of 120 to 200° C. and a screw rotation speed of 250 rpm to form a molten mixture. 
     With a high-pressure pump under a pressure of 40 bars, according to the proportions in Table 4, the nano-silica aqueous suspension obtained in Comparative Preparation Example 1 was introduced from the side inlet of the twin-screw extruder into the second zone of the twin-screw extruder, and was mixed with the molten mixture from the connected first zone, where the side inlet had a temperature of 180° C., and the second zone had a temperature of 200° C. and a screw rotation speed of 250 rpm. During the process, a flow of the suspension of Comparative Preparation Example 1 was controlled by a flow controller, so that the proportions of raw materials in the mixture retained after passing through the second zone were in accordance with Table 4. After extruding the mixture as a strand through water, and pelletizing it with the twin-screw extruder, the mixture was then placed in an oven at 105° C. for 4 hours to obtain a composition without halogen-free flame retardant and siloxane polymer. 
     Then, a mechanical test and a wear resistance test were performed on the manufactured halogen-free composition, and the results are also shown in Table 4. 
     Comparative Example 6 
     A composition without halogen-free flame retardant and siloxane polymer was manufactured using the method of Comparative Example 5, but the HMDS-modified nano-silica aqueous suspension of Preparation Example 2 was used to replace the nano-silica aqueous suspension of Comparative Example 5. 
     Then, a mechanical test and a wear resistance test were performed on the manufactured composition without halogen-free flame retardant and siloxane polymer, and the results are also shown in Table 4. 
     Comparative Example 7 
     A composition without halogen-free flame retardant and siloxane polymer was manufactured using the method of Comparative Example 5, but the GPTMS-modified nano-silica aqueous suspension of Preparation Example 1 was used to replace the nano-silica aqueous suspension of Comparative Example 5. 
     Then, a mechanical test and a wear resistance test were performed on the manufactured composition without halogen-free flame retardant and siloxane polymer, and the results are also shown in Table 4. 
     Comparative Example 8 
     A composition without halogen-free flame retardant and siloxane polymer was manufactured using the method of Comparative Example 7, but the content of the suspension of Preparation Example 1 was increased. 
     Then, a mechanical test and a wear resistance test were performed on the manufactured composition without halogen-free flame retardant and siloxane polymer, and the results are also shown in Table 4. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 7 
                 Example 8 
               
               
                   
                 Molten 
                 Molten 
                 Aqueous 
                 Aqueous 
                 Aqueous 
                 Aqueous 
               
               
                   
                 dispersion 
                 dispersion 
                 dispersion 
                 dispersion 
                 dispersion 
                 dispersion 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 TPU (parts by weight) 
                 95 
                 95 
                 95 
                 95 
                 95 
                 90 
               
               
                 Nano-SiO 2   
                 5 (commercial) 
                 5 (clay) 
                 5 (untreated) 
                 5 (HMDS- 
                 5 (GPTMS- 
                 10 (untreated) 
               
               
                 (parts by weight) 
                   
                   
                   
                 treated) 
                 treated) 
               
               
                 Antioxidant (phr) 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 Hardness (shore A) 
                 82 
                 82 
                 80 
                 82 
                 82 
                 87 
               
               
                 Tensile strength 
                 384 
                 212 
                 342 
                 327 
                 413 
                 127 
               
               
                 (kgf/cm 2 ) 
               
               
                 Elongation (%) 
                 569 
                 530 
                 565 
                 575 
                 560 
                 569 
               
               
                 Wear resistance (mg) 
                 38 
                 57 
                 45 
                 42 
                 8 
                 95 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 4, Comparative Examples 5 to 7 with aqueous dispersion have better mechanical strength, while Comparative Examples 6 to 7 added with silane modification have better wear resistance. Therefore, it can be inferred that the halogen-free flame-retardant material of the disclosure can achieve better wear resistance if silane-modified nano-silica aqueous suspension is used. 
     Comparative Example 9 
     95 parts by weight of thermoplastic polyurethane (purchased from Coating Chemical, ER-85A) was obtained, and was introduced from the main inlet of the twin-screw extruder into the first zone of the twin-screw extruder, and was melted and heated at a temperature of 120 to 200° C. and a screw rotation speed of 250 rpm. Next, with a high-pressure pump under a pressure of 20 to 50 bars, the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica aqueous suspension obtained in Preparation Example 1 was introduced from the side inlet of the twin-screw extruder into the second zone of the twin-screw extruder, and mixed with the molten thermoplastic polyurethane from the connected first zone, where a temperature of the side inlet was set to 180° C., and the second zone had a temperature of 200° C. and a screw rotation speed of 250 rpm. During the process, a flow of the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica aqueous suspension of Preparation Example 1 was controlled by a flow controller, so that after the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica retained after passing through the second zone was mixed with the molten thermoplastic polyurethane from the connected first zone, 100 parts by weight in total of a material may be obtained, where the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica was 5 parts by weight. 
     It was observed from the experiment that the (3-glycidyloxypropyl)trimethoxysilane-modified nano-silica suspension of Preparation Example 1, introduced from the side inlet of the twin-screw extruder into the second zone of the twin-screw extruder exhibited a stable flow. Moreover, a torque value obtained from converting the current value measured from the twin-screw extruder revealed a stable value of 55%. 
     Comparative Examples 10 and 11 
     The method of Comparative Example 9 was used, but the setting pressures of the high-pressure pump were respectively changed to 18 bars and 52 bars. 
     It was observed in the experiment that Comparative Example 10 exhibited a large and unstable flow, and a torque value of about 48%. Therefore, when the pressure of the high-pressure pump is less than 20 bars, a stable flow of nano-silica aqueous suspension into the twin-screw extruder can not be maintained. 
     As for Comparative Example 11, a stable flow was exhibited, but with a relatively high torque value (of about 60%). It means that when the pressure of the high-pressure pump is greater than 50 bars, the current value of the twin-screw extruder increases, showing greater energy consumption. 
     In summary of the foregoing, the silane-modified nano-silica aqueous suspension of the disclosure has good dispersion. By integrating the process technology of introducing aqueous dispersible nano-silica with the technical knowledge of twin-screw design, the surface-modified nano-silica in a form of pressurized aqueous solution is introduced into the twin-screw extruder, and then is blended with a finely dispersed molten mixture (including a halogen-free flame retardant, a wear-resistant modifier, a thermoplastic elastomer, and an antioxidant). Accordingly, a halogen-free flame-retardant material with comprehensively excellent performance, such as low hardness, high wear resistance, and high flame retardancy is obtained, and it is applicable for a wear-resistant halogen-free flame-retardant cable sheathing material. 
     The invention is disclosed in, but not limited to, the above embodiments. It is possible for anyone with ordinary skills in the related art to make some alterations and modifications without departing from the spirit and scope of the invention. Therefore, the protection scope of the invention shall depend on the appended claims.