Anti-yellowing mobile phone rear cover material and preparation method thereof

Abstract

The invention relates to the technical field of anti-yellowing plastic materials, in particular to an anti-yellowing mobile phone rear cover material and a preparation method thereof. The anti-yellowing mobile phone rear cover material is prepared by the following steps: preparing modified bio-based polyamide, synthesizing a benzotriazole anhydride grafting agent, preparing functionalized mesoporous silica nanoparticles, and finally carrying out extrusion and injection molding by virtue of a specific step-by-step charging process. The problems of yellowing, surface stickiness and gloss reduction caused by easy migration and precipitation of a traditional physical additive in a thin-wall injection molded part are thoroughly solved; the prepared material has excellent thin-wall mold filling flowability, high impact toughness, lasting scratch resistance and extremely low yellowness index increment, and is particularly suitable for manufacturing high-end smart phone rear covers which have the thickness of 0.4-0.8 mm and still keep bright appearance and dry touch feeling under the working conditions of outdoor and human body sweat contact.

Description

Technical Field

[0001] This invention relates to the field of anti-yellowing plastic materials technology, specifically to an anti-yellowing mobile phone back cover material and its preparation method. Background Technology

[0002] With the rapid development of 5G communication technology and smart mobile terminals, smartphone back covers, as key components protecting the precision internal components and directly determining the grip and appearance, face extremely high demands on their material properties. To meet the demands for thinness, wireless charging penetration, and aesthetic appeal, polycarbonate (PC), polyamide (PA), and their blended modified materials have become the mainstream choice for non-metallic back covers due to their excellent toughness, colorability, and processing performance. In particular, with increasing environmental awareness, the application of bio-based engineering plastics has become an industry trend.

[0003] In practical applications, mobile phone back covers are constantly exposed to outdoor sunlight, the high temperatures of in-car navigation systems, and frequent contact with human hand sweat and sebum. This complex coupling of light, heat, and chemical media (sweat / oil) easily induces photo-oxidative degradation of polymer materials. Polycarbonate readily undergoes Fries rearrangement under ultraviolet light to generate colored groups, while the amide bonds in polyamide molecular chains are susceptible to chain breakage and cross-linking due to moisture, heat, and oxygen attack. These changes in microstructure macroscopically manifest as severe yellowing and discoloration of the material surface (increased ΔE), not only damaging the product's aesthetic appearance but also causing the material to become brittle, crack, and shorten its lifespan.

[0004] To suppress photoaging, conventional techniques typically involve physically blending small-molecule UV absorbers (such as benzotriazoles and benzophenones) and hindered amine light stabilizers (HALS) into the matrix resin. While this physical addition method is simple, it suffers from thermodynamic incompatibility because the small-molecule additives and the polymer matrix are only bound by weak van der Waals forces, and their molecular weights and polarities differ significantly. Furthermore, in thin-walled (0.4-0.8 mm) injection-molded parts, the large surface area, coupled with the extraction effect of high temperature and humidity or human sweat, allows these small-molecule additives to easily migrate from the material's interior to the surface, forming blooms or oil films. This not only causes the matrix to yellow and age rapidly due to the lack of protective agents, but the migrated additives also mix with sebum, resulting in a sticky surface and a significant decrease in gloss on the phone back cover, which seriously affects the user's tactile experience and the product's hygiene and safety. At the same time, in order to pursue an ultra-thin design, the material must have extremely high fluidity, which often requires reducing the molecular weight of the matrix or adding lubricants, further exacerbating the tendency of additives to migrate and the loss of mechanical properties.

[0005] Therefore, how to provide a back cover material that still meets the requirements of low yellowing, high surface gloss, high scratch resistance, and beneficial mechanical strength under high-light conditions in outdoor / in-vehicle navigation is a technical bottleneck that urgently needs to be overcome in the field of mobile phone exterior component materials. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose an anti-yellowing mobile phone back cover material and its preparation method, so as to solve the problems of yellowing, surface stickiness and loss of gloss, and difficulty in maintaining mechanical properties in the long term caused by the migration and failure of small molecule additives in existing thin-walled mobile phone back covers under photothermal and sweat coupling conditions.

[0007] To achieve the above objectives, the present invention provides a method for preparing an anti-yellowing mobile phone back cover material, comprising the following steps:

[0008] S1: Deionized water is added to the reactor, heated and stirred, and then 1,10-decanediamine, sebacic acid and citric acid are added in sequence and stirred to form a homogeneous nylon salt slurry. The reaction is carried out in a stepwise heating polymerization reaction under a nitrogen atmosphere: first, prepolymerization is carried out at the first temperature section, then the temperature is raised to the second temperature section and the pressure inside the reactor is maintained at the temperature, then the temperature is raised to the third temperature section and maintained at the temperature, and finally the pressure is slowly released to the atmospheric pressure and maintained at the temperature under vacuum. After discharge, water cooling and pelletizing, modified bio-based polyamide particles are obtained.

[0009] S2: Under nitrogen protection, UV absorber UV-328 was dissolved in anhydrous dichloromethane, triethylamine and 4-dimethylaminopyridine were added, and the system was cooled to 0°C before adding trimesoyl chloride dropwise. After the addition was complete, the reaction was stirred and then heated to room temperature to continue the reaction. After the reaction was completed, the crude product was obtained by filtration and solvent removal, followed by precipitation, washing, and drying to obtain benzotriazole anhydride grafting agent; S3: An alcohol-water solution containing a cationic surfactant was prepared and the pH was adjusted to 10-11 to form a mixture; Toluene, UV absorber UV-328, 3-(triethoxysilyl)propyl isocyanate, and 4-dimethylaminopyridine were mixed and reacted under heating conditions to obtain an intermediate solution. Subsequently, the intermediate solution, tetraethyl orthosilicate, and 3-glycidoxypropyltrimethoxysilane were mixed and added dropwise to the mixture to carry out a hydrolysis-condensation reaction. After the reaction was completed, the resulting reaction product was centrifuged, washed, and dried. The template agent was removed by reflux with an acidic alcohol solution, and then washed and dried to obtain functionalized mesoporous silica nanoparticles.

[0010] S4: Modified bio-based polyamide particles, polycarbonate, benzotriazole anhydride grafting agent, and functionalized mesoporous silica nanoparticles are dried separately; melt blending extrusion is performed using a co-rotating twin-screw extruder. Modified bio-based polyamide particles and polycarbonate are added through the main feed port, benzotriazole anhydride grafting agent is added through side feeding in the third temperature zone of melt conveying, and functionalized mesoporous silica nanoparticles are added in the fifth temperature zone of melt conveying; poly(dimethylsiloxane) is added at the end of the extruder barrel, and the resulting extrudate is water-cooled, stretched, and pelletized to obtain composite particles;

[0011] S5: After drying the composite particles, they are injection molded using an injection molding machine to obtain an anti-yellowing mobile phone back cover material.

[0012] Preferably, the ratio of 1,10-decanediamine, sebacic acid and citric acid used in step S1 is 500-550g:580-620g:1-3g.

[0013] Preferably, the specific process of the stepped heating polymerization in step S1 is as follows: first, heat to 200-220℃ and hold for 1-2 hours; then heat to 230-250℃ and maintain the pressure inside the reactor at 1.4-1.6MPa for 1-2 hours; finally, heat to 255-265℃ and depressurize within 30 minutes.

[0014] Preferably, the vacuum degree of the vacuum insulation is 5 kPa, and the insulation time is 1-2 hours.

[0015] Preferably, the ratio of ultraviolet absorber UV-328, triethylamine, 4-dimethylaminopyridine to trimesoyl chloride in step S2 is 170-190g:60-80g:4-6g:100-120g.

[0016] Preferably, the dripping time in step S2 is within 1 hour.

[0017] Preferably, the stirring reaction time after the dropwise addition in step S2 is 2 hours.

[0018] Preferably, the room temperature reaction conditions in step S2 are a stirring reaction at 25°C for 4-8 hours.

[0019] Preferably, the precipitation washing in step S2 is performed using anhydrous ethanol.

[0020] Preferably, the drying conditions in step S2 are vacuum drying at 60°C for 12 hours.

[0021] Preferably, the cationic surfactant in step S3 is hexadecyltrimethylammonium bromide.

[0022] Preferably, the pH adjustment in step S3 is performed using ammonia water (25% by mass).

[0023] Preferably, in step S3, the ratio of the cationic surfactant, ultraviolet absorber UV-328, 3-(triethoxysilyl)propyl isocyanate, 4-dimethylaminopyridine, tetraethyl orthosilicate, and 3-glycidoxypropyltrimethoxysilane is 8-12g:8-12g:6-8g:0.8-1.2g:90-110g:8-12g.

[0024] Preferably, the reaction conditions for obtaining the intermediate solution under heating conditions in step S3 are 60°C and the reaction time is 3-5 hours.

[0025] Preferably, the hydrolysis-condensation reaction temperature in step S3 is 40°C and the reaction time is 4-8 hours.

[0026] Preferably, the template removal agent in step S3 is carried out in a mixture of anhydrous ethanol and concentrated hydrochloric acid (mass ratio 10:1) under the condition of reflux at 75-85°C for 5-7 hours.

[0027] Preferably, the ratio of modified bio-based polyamide particles, polycarbonate, benzotriazole anhydride grafting agent, functionalized mesoporous silica nanoparticles to poly(dimethylsiloxane) in step S4 is 600-680g:300-360g:8-12g:12-18g:4-6g.

[0028] Preferably, the process parameters of the co-rotating twin-screw extruder in step S4 are set as follows: the barrel temperature zones are sequentially 235-245℃ for zone 1, 245-255℃ for zone 2, 250-260℃ for zone 3, 255-265℃ for zone 4, and 255-265℃ for zone 5; the die head temperature is 260-270℃; and the screw speed is 180-220 r / min.

[0029] Preferably, the drying conditions in step S5 are hot air drying at 85-95°C.

[0030] Preferably, the injection molding process parameters in step S5 are as follows: the barrel temperature is set sequentially from the feeding section to the nozzle to 255-265℃, 265-275℃, and 270-280℃; the mold temperature is 85-95℃; the holding pressure is 75-85MPa; the holding time is 5-7s; and the cooling time is 15-25s.

[0031] Furthermore, the present invention also provides a material for a mobile phone back cover that resists yellowing.

[0032] The beneficial effects of this invention are:

[0033] This invention achieves permanent fixation of UV-stabilizing groups, fundamentally solving the problems of yellowing and additive precipitation. Abandoning traditional physical blending methods, it uses molecular design to graft benzotriazole groups with strong UV absorption capabilities onto the polymer backbone in a stable covalent bond form. Even under harsh conditions such as high-temperature baking, strong UV radiation, or immersion in sweat, these groups overcome thermodynamic migration tendencies and remain permanently within the material matrix, providing light shielding. This not only significantly reduces the increase in yellowness index after long-term use, maintaining the purity of the base color, but also completely eliminates surface stickiness, blooming, and decreased gloss caused by additive migration.

[0034] This invention constructs a microstructure that combines rigidity and flexibility, achieving both thin-wall molding fluidity and high strength and toughness. By introducing a multifunctional monomer (citric acid) during the polymerization stage of bio-based polyamide, the melt is endowed with excellent shear-thinning rheological properties, enabling it to smoothly fill ultra-thin mold cavities of 0.4mm-0.8mm. At the same time, the terminal enriched active groups enhance the interfacial compatibility between polyamide, polycarbonate, and inorganic particles, forming a dense stress transmission network. Thus, while ensuring high fluidity, it also endows the material with excellent impact resistance and tensile strength.

[0035] This invention establishes a multi-dimensional surface protection system, achieving durable high gloss and scratch-resistant feel. By innovatively introducing surface-functionalized mesoporous silica nanoparticles and reactive polydimethylsiloxane, the functionalized silica forms a strong chemical anchor with the matrix through surface epoxy groups, and constructs a UV-blocking layer at the interface. This utilizes the hardness of inorganic particles to enhance the surface's scratch resistance and prevents interfacial photoaging. Combined with epoxy-terminated polydimethylsiloxane, a lubricating chain is chemically locked to the material surface, forming a molecular-level lubricating film that cannot be wiped or washed away by sweat.

[0036] In summary, this multi-layered protection mechanism significantly reduces the surface friction coefficient, giving the phone back cover a long-lasting dry and delicate feel, as well as an extremely high gloss retention rate in artificial sweat resistance tests, meeting the stringent requirements of high-end electronic products for appearance quality. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0038] The sources or properties of the raw materials used in the embodiments and comparative examples of this invention are as follows:

[0039] Polycarbonate: Makrolon 2407; Poly(dimethylsiloxane): diglycidyl ether capped, Merck 480282, average molecular weight 1000.

[0040] Example 1: A method for preparing an anti-yellowing mobile phone back cover material, the specific steps of which are as follows:

[0041] (1) Take 700g of deionized water and add it to the reactor. Heat it to 75℃ and keep the stirring speed at 200r / min. Then add 500g of 1,10-decanediamine, 580g of sebacic acid and 1g of citric acid in sequence. Continue stirring for 40min to form a uniform nylon salt slurry. Heat it to 200℃ in a nitrogen atmosphere and keep it at that temperature for 1h. Then heat it to 230℃ and keep the pressure inside the reactor stable at 1.4MPa for another 1h. Then heat it to 255℃ and slowly depressurize it to atmospheric pressure within 30min. Then turn on the vacuum to 5kPa and keep it at that temperature for 1h. After discharge, water cooling and pelletizing, modified bio-based polyamide particles are obtained.

[0042] (2) Take a flask, purge with nitrogen for protection, add 1500g of anhydrous dichloromethane and 170g of UV absorber UV-328 to dissolve, then add 60g of triethylamine and 4g of 4-dimethylaminopyridine, control the temperature of the system to 0℃, add 100g of pyromellitic acid chloride dropwise over 1h and continue stirring for 2h, then raise the temperature to 25℃ and continue stirring for 4h. After the reaction is complete, filter to remove triethylamine salt, remove dichloromethane from the filtrate under reduced pressure to obtain crude product, then precipitate and wash with anhydrous ethanol, and finally dry under vacuum at 60℃ for 12h to obtain benzotriazole anhydride grafting agent;

[0043] (3) Add 450g deionized water, 180g anhydrous ethanol, and 8g hexadecyltrimethylammonium bromide to a reaction flask and stir at 40°C until completely dissolved. Then add ammonia to adjust the pH to 10 to form a mixture. Separately, take 280g toluene, 8g ultraviolet absorber UV-328, 6g 3-(triethoxysilyl)propyl isocyanate, and 0.8g 4-dimethylaminopyridine and stir at 60°C for 3 hours to obtain an intermediate solution. Then, add the intermediate solution, 90g tetraethyl orthosilicate, and 8g... After mixing 3-glycidyl etheroxypropyltrimethoxysilane, it was added dropwise to the mixture within 30 min. The mixture was stirred at 40 °C for 4 h. After the reaction was completed, the product was centrifuged and filtered, washed three times with anhydrous ethanol, dried at 80 °C for 8 h, and then refluxed at 75 °C for 5 h in a mixture of 400 g anhydrous ethanol and 40 g concentrated hydrochloric acid to remove the template agent. After washing until neutral, the product was vacuum dried at 60 °C for 12 h to obtain functionalized mesoporous silica nanoparticles.

[0044] (4) 600g of modified bio-based polyamide particles were vacuum dried at 80℃ for 8h, 300g of polycarbonate was vacuum dried at 110℃ for 6h, 8g of benzotriazole anhydride grafting agent was vacuum dried at 60℃ for 4h, and 12g of functionalized mesoporous silica nanoparticles were vacuum dried at 120℃ for 4h. The materials were fed into a co-rotating twin-screw extruder, and the barrel temperature zones of the co-rotating twin-screw extruder were set as follows: Zone 1: 235℃, Zone 2: 245℃, Zone 3: 250℃, Zone 4: 255℃, Zone 5: 255℃, and the die head temperature was 260℃. The screw speed is 180 r / min. First, 600 g of modified bio-based polyamide particles and 300 g of polycarbonate are added through the main feed port to form a homogeneous melt. When the melt enters the third temperature zone, 8 g of benzotriazole anhydride grafting agent is added through the side feed and the total residence time is maintained for 2 min in the third and fourth temperature zones. Then, 12 g of functionalized mesoporous silica nanoparticles are added in the fifth temperature zone. Finally, 4 g of poly(dimethylsiloxane) is added at the end of the extruder barrel (near the die head) and the vacuum is turned on to exhaust to -0.09 MPa. The resulting extrudate is water-cooled, stretched, and pelletized to obtain composite particles.

[0045] (5) After drying the composite particles with hot air at 85°C, they are injection molded. The barrel temperature is set to 255°C, 265°C, and 270°C (increasing from the feeding section to the nozzle), the mold temperature is 85°C, the injection speed is medium to high speed to ensure thin-walled mold filling, the holding pressure is 75MPa, the holding time is 5s, and the cooling time is 15s to obtain the anti-yellowing mobile phone back cover material.

[0046] Example 2: A method for preparing an anti-yellowing mobile phone back cover material, the specific steps of which are as follows:

[0047] (1) Take 800g of deionized water and add it to the reactor. Heat it to 80℃ and keep the stirring speed at 300r / min. Then add 530g of 1,10-decanediamine, 600g of sebacic acid and 2g of citric acid in sequence. Continue stirring for 40min to form a uniform nylon salt slurry. Heat it to 220℃ in a nitrogen atmosphere and keep it at that temperature for 2h. Then heat it to 240℃ and keep the pressure inside the reactor stable at 1.5MPa for 2h. Then heat it to 260℃ and slowly depressurize it to atmospheric pressure within 30min. Then open the vacuum to 5kPa and keep it at that temperature for 1h. After discharge, water cooling and pelletizing, modified bio-based polyamide particles are obtained.

[0048] (2) Take a flask, purge with nitrogen for protection, add 1600g of anhydrous dichloromethane and 180g of UV absorber UV-328 to dissolve, then add 70g of triethylamine and 5g of 4-dimethylaminopyridine, control the temperature of the system to 0℃, add 110g of pyromellitic acid chloride dropwise within 1h and continue stirring for 2h, then raise to 25℃ and continue stirring for 6h. After the reaction is completed, filter to remove triethylamine salt, remove dichloromethane from the filtrate under reduced pressure to obtain crude product, then precipitate and wash with anhydrous ethanol, and finally dry under vacuum at 60℃ for 12h to obtain benzotriazole anhydride grafting agent;

[0049] (3) Add 500g deionized water, 200g anhydrous ethanol, and 10g hexadecyltrimethylammonium bromide to a reaction flask and stir at 40°C until completely dissolved. Then add ammonia to adjust the pH to 11 to form a mixture. Separately, take 300g toluene, 10g UV absorber UV-328, 7g 3-(triethoxysilyl)propyl isocyanate, and 1g 4-dimethylaminopyridine, and stir at 60°C for 4 hours to obtain an intermediate solution. Then, add the intermediate solution, 100g tetraethyl orthosilicate, and 10g... After mixing 3-glycidyl etheroxypropyltrimethoxysilane, it was added dropwise to the mixture within 30 min. The mixture was stirred at 40 °C for 6 h. After the reaction was completed, the product was centrifuged and filtered, washed three times with anhydrous ethanol, dried at 80 °C for 8 h, and then refluxed at 80 °C for 6 h in a mixture of 400 g anhydrous ethanol and 40 g concentrated hydrochloric acid to remove the template agent. After washing until neutral, the product was vacuum dried at 60 °C for 12 h to obtain functionalized mesoporous silica nanoparticles.

[0050] (4) 640g of modified bio-based polyamide particles were vacuum dried at 80℃ for 8h, 330g of polycarbonate was vacuum dried at 110℃ for 6h, 10g of benzotriazole anhydride grafting agent was vacuum dried at 60℃ for 4h, and 15g of functionalized mesoporous silica nanoparticles were vacuum dried at 120℃ for 4h. The materials were fed into a co-rotating twin-screw extruder, and the barrel temperature zones of the co-rotating twin-screw extruder were set as follows: Zone 1: 240℃, Zone 2: 250℃, Zone 3: 255℃, Zone 4: 260℃, Zone 5: 260℃, and the die head temperature was 265℃. The screw speed is 200 r / min. First, 640 g of modified bio-based polyamide particles and 330 g of polycarbonate are added through the main feed port to form a homogeneous melt. When the melt enters the third temperature zone, 10 g of benzotriazole anhydride grafting agent is added through the side feed and the total residence time is maintained for 2 min in the third and fourth temperature zones. Then, 15 g of functionalized mesoporous silica nanoparticles are added in the fifth temperature zone. Finally, 5 g of poly(dimethylsiloxane) is added at the end of the extruder barrel (near the die head) and the vacuum is turned on to exhaust to -0.09 MPa. The resulting extrudate is water-cooled, stretched, and pelletized to obtain composite particles.

[0051] (5) After drying the composite particles with hot air at 90°C, they are injection molded. The barrel temperature is set to 260°C, 270°C, and 275°C (increasing from the feeding section to the nozzle), the mold temperature is 90°C, the injection speed is medium to high speed to ensure thin-walled mold filling, the holding pressure is 80MPa, the holding time is 6s, and the cooling time is 20s to obtain the anti-yellowing mobile phone back cover material.

[0052] Example 3: A method for preparing an anti-yellowing mobile phone back cover material, the specific steps of which are as follows:

[0053] (1) Take 900g of deionized water and add it to the reactor. Heat it to 85℃ and keep the stirring speed at 400r / min. Then add 550g of 1,10-decanediamine, 620g of sebacic acid and 3g of citric acid in sequence. Continue stirring for 40min to form a uniform nylon salt slurry. Heat it to 220℃ in a nitrogen atmosphere and keep it at that temperature for 2h. Then heat it to 250℃ and keep the pressure inside the reactor stable at 1.6MPa for 2h. Then heat it to 265℃ and slowly depressurize it to atmospheric pressure within 30min. Then open the vacuum to 5kPa and keep it at that temperature for 2h. After discharge, water cooling and pelletizing, modified bio-based polyamide particles are obtained.

[0054] (2) Take a flask, purge with nitrogen for protection, add 1700g of anhydrous dichloromethane and 190g of UV absorber UV-328 to dissolve, then add 80g of triethylamine and 6g of 4-dimethylaminopyridine, control the temperature of the system to 0℃, add 120g of pyromellitic acid chloride dropwise within 1h and continue stirring for 2h, then raise to 25℃ and continue stirring for 8h. After the reaction is completed, filter to remove triethylamine salt, remove dichloromethane from the filtrate under reduced pressure to obtain crude product, then precipitate and wash with anhydrous ethanol, and finally dry under vacuum at 60℃ for 12h to obtain benzotriazole anhydride grafting agent;

[0055] (3) Add 550g of deionized water, 220g of anhydrous ethanol, and 12g of hexadecyltrimethylammonium bromide to a reaction flask and stir at 40°C until completely dissolved. Then add ammonia water to adjust the pH to 11 to form a mixture. Separately, take 320g of toluene, 12g of ultraviolet absorber UV-328, 8g of 3-(triethoxysilyl)propyl isocyanate, and 1.2g of 4-dimethylaminopyridine, and stir at 60°C for 5 hours to obtain an intermediate solution. Then, add the intermediate solution, 110g of tetraethyl orthosilicate, and 12g of... After mixing 3-glycidyl etheroxypropyltrimethoxysilane, it was added dropwise to the mixture within 30 min. The mixture was stirred at 40 °C for 8 h. After the reaction was completed, the product was centrifuged and filtered, washed three times with anhydrous ethanol, dried at 80 °C for 8 h, and then refluxed at 85 °C for 7 h in a mixture of 400 g anhydrous ethanol and 40 g concentrated hydrochloric acid to remove the template agent. After washing until neutral, the product was vacuum dried at 60 °C for 12 h to obtain functionalized mesoporous silica nanoparticles.

[0056] (4) 680g of modified bio-based polyamide particles were vacuum dried at 80℃ for 8h, 360g of polycarbonate was vacuum dried at 110℃ for 6h, 12g of benzotriazole anhydride grafting agent was vacuum dried at 60℃ for 4h, and 18g of functionalized mesoporous silica nanoparticles were vacuum dried at 120℃ for 4h. The materials were fed into a co-rotating twin-screw extruder, and the barrel temperature zones of the co-rotating twin-screw extruder were set as follows: Zone 1: 245℃, Zone 2: 255℃, Zone 3: 260℃, Zone 4: 265℃, Zone 5: 265℃, and the die head temperature was 270℃. The screw speed is 220 r / min. First, 680 g of modified bio-based polyamide particles and 360 g of polycarbonate are added through the main feed port to form a homogeneous melt. When the melt enters the third temperature zone, 12 g of benzotriazole anhydride grafting agent is added through the side feed and the total residence time is maintained for 2 min in the third and fourth temperature zones. Then, 18 g of functionalized mesoporous silica nanoparticles are added in the fifth temperature zone. Finally, 6 g of poly(dimethylsiloxane) is added at the end of the extruder barrel (near the die head) and the vacuum is turned on to exhaust to -0.09 MPa. The resulting extrudate is water-cooled, stretched, and pelletized to obtain composite particles.

[0057] (5) After drying the composite particles with hot air at 95°C, they are injection molded. The barrel temperature is set to 265°C, 275°C, and 280°C (increasing from the feeding section to the nozzle), the mold temperature is 95°C, the injection speed is medium to high speed to ensure thin-walled mold filling, the holding pressure is 85MPa, the holding time is 7s, and the cooling time is 25s to obtain the anti-yellowing mobile phone back cover material.

[0058] Comparative Example 1: The difference from Example 2 is that citric acid is not added in step (1), and the amount of 1,10-decanediamine is kept at a molar ratio of 1,10-decanediamine to sebacate of 1:1. The other conditions are the same as in Example 2.

[0059] Comparative Example 2: The difference from Example 2 is that benzotriazole anhydride grafting is not prepared in step (2) and the benzotriazole anhydride grafting agent is replaced with an equal amount of ultraviolet absorber UV-328 in step (4). The other conditions are the same as in Example 2.

[0060] Comparative Example 3: The difference from Example 2 is that UV absorber UV-328 and 3-(triethoxysilyl)propyl isocyanate are not added in step (3). The specific steps are as follows: 500g of deionized water, 200g of anhydrous ethanol and 10g of hexadecyltrimethylammonium bromide are added to the reaction flask and stirred at 40°C until completely dissolved. Then, ammonia water is added to adjust the pH to 11 to form a mixture. 100g of tetraethyl orthosilicate and 10g of 3-glycidyl etheroxypropyltrimethoxysilane are mixed and added dropwise to the mixture within 30 minutes. The subsequent reaction, washing, template removal and drying steps are the same as in Example 2.

[0061] Comparative Example 4: The difference from Example 2 is that in step (4), the modified bio-based polyamide particles, polycarbonate, benzotriazole anhydride grafting agent, functionalized mesoporous silica nanoparticles and poly(dimethylsiloxane) are directly mixed and fed, and the other conditions are the same as in Example 2.

[0062] Comparative Example 5: The difference from Example 2 is that in step (4), poly(dimethylsiloxane) is replaced with an equal amount of polydimethylsiloxane (Merck 469319), and the other conditions are the same as in Example 2.

[0063] Performance testing

[0064] All samples were taken from the anti-yellowing mobile phone back cover materials obtained by injection molding in Examples 1-3 and Comparative Examples 1-5 of this invention, or standard test strips corresponding to national standards. Before testing, all samples were placed in a standard environment (temperature 23℃, relative humidity 50%) for 48 hours to adjust their condition. To ensure the accuracy and repeatability of the test data and eliminate random errors, a strict number of parallel tests was set for each performance test, with 5 parallel tests per test. The recorded results were all average values.

[0065] Melt flow rate: According to GB / T 3682.1-2018 "Determination of melt mass flow rate (MFR) and melt volumetric flow rate (MVR) of thermoplastic plastics - Part 1: Standard method", the dried composite particles of each example and comparative example were used. A melt flow rate meter was set with a test temperature of 260℃, a load of 2.16kg, and a cutting time interval of 10s. The melt mass flow rate (g / 10min) was calculated.

[0066] Mechanical properties: Tests were conducted according to GB / T 1040.2-2022 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics" and GB / T 1043.1-2008 "Determination of impact properties of simply supported beam plastics - Part 1: Non-instrumental impact testing". The composite particles obtained in the examples and comparative examples were injection molded into standard dumbbell-shaped specimens (Type 1A) and notched impact specimens. In the tensile test, the tensile speed was set to 50 mm / min, and the yield tensile strength (MPa) was recorded. In the impact test, a Type A notch (0.25 mm radius) was prepared in the middle of the specimen using a notching machine, and the notched impact strength (kJ / m²) was recorded using a simply supported beam impact testing machine. 2 );

[0067] Scratch resistance: The anti-yellowing mobile phone back cover materials obtained in each embodiment and comparative example were tested using a scratch tester (equipped with a spherical corundum probe with a diameter of 1 mm). A constant load of 10 N was applied, the scratch speed was 5 mm / s, the scratch length was 20 mm, and the average depth of the scratch groove (μm) was measured using a laser confocal microscope.

[0068] Resistance to sweat / sebum exudation: Tested according to GB / T 8807-1988 "Test Method for Mirror Gloss of Plastics" and the industry-standard artificial sweat resistance. First, acidic artificial sweat was prepared (according to ISO 105-E04 standard formula: components are L-histidine hydrochloride, sodium chloride, sodium dihydrogen phosphate, and pH adjusted to 5.5). Then, the anti-yellowing mobile phone back cover materials obtained in each example and comparative example were immersed in artificial sweat at (55)℃ for 168h. After being taken out, they were washed with deionized water and dried. The gloss of the sample surface before and after immersion was measured using a 60° angle gloss meter, and the gloss retention rate (%) was calculated.

[0069] Artificial accelerated aging test: The test was conducted according to GB / T 16422.2-2022 "Laboratory Light Source Exposure Test Methods for Plastics - Part 2: Xenon Arc Lamp" and GB / T 2409-1980 "Test Method for Yellow Index of Plastics". Specifically, the anti-yellowing mobile phone back cover material (0.8mm thick) obtained from each example and comparative example was selected. The surface needed to be smooth and free of defects. A xenon lamp aging test chamber equipped with a Hitachi filter was used. The blackboard temperature was set to 65℃, the relative humidity of the chamber was 50%, and the irradiance was set to 0.51W / (m²). 2 The test periods were set to 1000h and 2000h, respectively. The tristimulus values ​​X, Y, and Z of the sample were measured using an integrating sphere spectrophotometer under a D65 light source and a 10° field of view. The yellowness index YI was calculated according to the formula.

[0070] The test results are shown in Table 1.

[0071] Table 1 Performance Test Results

[0072]

[0073] Data Analysis:

[0074] As can be seen from the data in Examples 1-3 of Table 1, the anti-yellowing mobile phone back cover material prepared by the present invention exhibits excellent comprehensive performance in terms of melt flow rate, tensile strength, notched impact strength, scratch resistance, gloss retention rate, and anti-yellowing properties. This indicates that by covalently fixing ultraviolet absorbing groups, functionalizing mesoporous silica, and using reactive extrusion processes, a multi-level light-stabilized system is effectively constructed, significantly improving the material's weather resistance and appearance stability during long-term use. This further demonstrates that by chemically bonding ultraviolet absorbing groups and lubricating segments to the resin matrix and inorganic particle surface, not only is the photo-oxidative degradation pathway completely blocked at the molecular level, but the migration and precipitation of functional additives during long-term use are also effectively prevented. This endows the mobile phone back cover with durable mechanical reliability and appearance integrity under complex working conditions, solving the industry pain points of traditional materials being prone to yellowing and stickiness.

[0075] As can be seen from the data in Example 2 and Comparative Example 1 in Table 1, the introduction of microbranched structures is the decisive factor in balancing the processing flowability and impact toughness of the modified bio-based polyamide / polycarbonate alloy system. It is speculated that this is because the microbranched structure is introduced during the polymerization stage using citric acid as a multifunctional monomer. This special topology endows the polymer melt with superior rheological response characteristics during processing, allowing the melt to smoothly fill the mold cavity and reducing orientation stress and micro-defects. Simultaneously, the terminal amino groups enriched at the ends provide sufficient sites for subsequent compatibilization with polycarbonate and functional particles, enhancing the interfacial bonding force, thereby achieving a synergistic improvement in both high flowability and high toughness.

[0076] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, the strategy of covalently immobilizing ultraviolet absorbing groups using benzotriazole anhydride grafting agents is crucial for improving the color stability and surface quality of the material under long-term photothermal environments. It is speculated that this is because the benzotriazole groups are firmly grafted onto the polyamide molecular chain in the form of amide bonds through a chemical reaction, transforming them into part of the polymer structure. Even under harsh conditions such as high temperature, high humidity, or contact with sweat, they can remain permanently within the material and continuously absorb harmful ultraviolet rays, thus fundamentally solving the problem of appearance deterioration and yellowing caused by additive precipitation.

[0077] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, the surface functionalization design of mesoporous silica particles provides a dual guarantee for achieving high scratch resistance and deep UV resistance of the material surface. This is presumably because by simultaneously modifying the surface of the mesoporous silica with UV-absorbing groups and epoxy active groups, on the one hand, the epoxy groups react with the matrix resin to form a strong chemical anchor, significantly improving surface hardness and shear resistance; on the other hand, the UV absorber on the particle surface and the physical scattering effect of the particles themselves form a synergistic shielding effect, effectively protecting the deep matrix from UV radiation damage, thus achieving excellent scratch resistance and aging resistance.

[0078] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, the timing control of the stepwise reactive extrusion process plays an irreplaceable role in constructing the ideal microstructure and optimizing macroscopic mechanical properties. This is presumably because precise control of the feeding position and residence time ensures that the grafting modification and compatibilization of the matrix are completed first, followed by the introduction of inorganic particles for interfacial anchoring, and finally surface lubrication modification. This orderly chemical reaction process guarantees that each functional component plays its role at its optimal spatial location, thereby constructing an alloy morphology with tight interfacial bonding and few defects, maximizing overall performance.

[0079] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, the reactivity of the poly(dimethylsiloxane) end groups is key to ensuring a long-lasting dry feel and high gloss retention. It is speculated that the diglycidyl ether-terminated poly(dimethylsiloxane) covalently bonds the long-chain siloxane segments to the material surface. This bound lubricating layer provides durable low surface energy to resist external scratches and also resists the erosion and extraction by sweat and sebum, ensuring that the phone back cover maintains a high-gloss, non-sticky texture even after long-term use.

[0080] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing an anti-yellowing mobile phone back cover material, characterized in that, Includes the following steps: S1: Using 1,10-decanediamine and sebacic acid as polymerization monomers, citric acid is added to form nylon salt slurry, which is then polymerized to form modified bio-based polyamide particles. S2: Using UV absorber UV-328 and pyromellitic tricarboxylic acid chloride as raw materials, a reaction is carried out under the action of triethylamine and 4-dimethylaminopyridine to form benzotriazole anhydride grafting agent; S3: UV absorber UV-328 and 3-(triethoxysilyl)propyl isocyanate were reacted with 4-dimethylaminopyridine to form an intermediate, which was then hydrolyzed and polycondensed with tetraethyl orthosilicate and 3-glycidoxypropyltrimethoxysilane in an alkaline system containing a cationic surfactant. The resulting product was then detemplated to obtain functionalized mesoporous silica nanoparticles after the template agent was removed. S4: In a co-rotating twin-screw extruder, modified bio-based polyamide particles are melt-blended with polycarbonate. During the melt conveying process, benzotriazole anhydride grafting agent and functionalized mesoporous silica nanoparticles are added sequentially, and poly(dimethylsiloxane) is added at the end of the extrusion. After vacuum degassing, the mixture is extruded and granulated to form composite particles. S5: The composite particles are dried and then injection molded to obtain an anti-yellowing mobile phone back cover material; The benzotriazole anhydride grafting agent described in step S4 is added in the third temperature zone during the melt conveying process; The functionalized mesoporous silica nanoparticles described in step S4 are added in the fifth temperature zone during the melt transport process.

2. The preparation method according to claim 1, characterized in that, The ratio of 1,10-decanediamine, sebacic acid and citric acid used in step S1 is 500-550g:580-620g:1-3g.

3. The preparation method according to claim 1, characterized in that, The specific polymerization process described in step S1 is as follows: first, heat to 200-220℃ and hold for 1-2 hours; then heat to 230-250℃ and maintain the pressure inside the reactor at 1.4-1.6MPa for 1-2 hours; finally, heat to 255-265℃ and depressurize within 30 minutes.

4. The preparation method according to claim 1, characterized in that, In step S2, the ratio of ultraviolet absorber UV-328, triethylamine, 4-dimethylaminopyridine to pyromellitic trimethylol chloride is 170-190g:60-80g:4-6g:100-120g.

5. The preparation method according to claim 1, characterized in that, In step S3, the ratio of the cationic surfactant, ultraviolet absorber UV-328, 3-(triethoxysilyl)propyl isocyanate, 4-dimethylaminopyridine, tetraethyl orthosilicate, and 3-glycidoxypropyltrimethoxysilane is 8-12g:8-12g:6-8g:0.8-1.2g:90-110g:8-12g.

6. The preparation method according to claim 1, characterized in that, The hydrolysis-condensation reaction temperature in step S3 is 40°C, and the reaction time is 4-8 hours.

7. The preparation method according to claim 1, characterized in that, In step S4, the ratio of modified bio-based polyamide particles, polycarbonate, benzotriazole anhydride grafting agent, functionalized mesoporous silica nanoparticles to poly(dimethylsiloxane) is 600-680g:300-360g:8-12g:12-18g:4-6g.

8. The preparation method according to claim 1, characterized in that, The process parameters of the co-rotating twin-screw extruder in step S4 are set as follows: the barrel temperature zones are 235-245℃ in zone 1, 245-255℃ in zone 2, 250-260℃ in zone 3, 255-265℃ in zone 4, and 255-265℃ in zone 5; the die head temperature is 260-270℃; and the screw speed is 180-220 r / min.

9. The preparation method according to claim 1, characterized in that, The injection molding process parameters in step S5 are as follows: the barrel temperature is set sequentially from the feeding section to the nozzle to 255-265℃, 265-275℃, and 270-280℃; the mold temperature is 85-95℃; the holding pressure is 75-85MPa; the holding time is 5-7s; and the cooling time is 15-25s.

10. A material for a mobile phone back cover that resists yellowing, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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