A method for preparing anti-mildew polypropylene composite material based on gradient temperature-controlled extrusion

Abstract

A method for preparing anti-mildew polypropylene composite material based on gradient temperature-controlled extrusion is disclosed. This invention relates to the field of polymer composite material preparation technology. The method involves mixing pretreated polypropylene substrate, a synergistic anti-mildew system, a special compatibilizer, an anti-aging compound system, and a lubricant in a high-speed mixer according to a preset mass ratio. The mixture is then fed into a twin-screw extruder and extruded and granulated using a segmented gradient temperature-controlled mode. After extrusion, the material is water-cooled, pelletized, and air-dried to obtain anti-mildew polypropylene composite material particles. The extruded and granulated composite material particles are then sieved in a vibrating screen and dried a second time to obtain the finished anti-mildew polypropylene composite material. This method achieves a synergistic improvement in the long-term anti-mildew effect, mechanical stability, processing adaptability, and safety and environmental friendliness of the composite material.

Description

Technical Field

[0001] This invention relates to the field of polymer composite material preparation technology, specifically to a method for preparing anti-mildew polypropylene composite material based on gradient temperature-controlled extrusion. Background Technology

[0002] Polypropylene (PP) is widely used in small kitchen appliances due to its advantages such as low density, excellent mechanical properties, easy processing, and low cost, including rice cooker inner pots, soy milk maker shells, blender bases, and dish storage boxes. However, the kitchen environment is characterized by high temperature, high humidity, and easy contamination with oil and food residue, making it highly susceptible to the growth of molds such as Aspergillus niger, Aspergillus flavus, and Penicillium. Molds not only cause mold spots, discoloration, and odors on the surface of polypropylene composite materials, affecting the product's appearance and user experience, but they also damage the material's molecular structure, leading to decreased mechanical properties, accelerated aging, and shortened lifespan of small kitchen appliances. Furthermore, they may produce mycotoxins, which can harm human health.

[0003] Currently, the anti-mold treatment of polypropylene composite materials used in small kitchen appliances suffers from the following technical defects: First, most methods use a single anti-mold agent, which has poor dispersibility and is prone to agglomeration, resulting in uneven anti-mold effects and insufficient long-term effectiveness. Furthermore, excessive dosage of a single anti-mold agent can severely affect the mechanical properties and processing flowability of the polypropylene substrate. Second, extrusion molding processes often employ constant-temperature extrusion, but there is a mismatch between the melting temperature of polypropylene and the thermal stability of the anti-mold agent—excessive temperature can lead to thermal decomposition and failure of the anti-mold agent, while insufficient temperature results in incomplete melting of the polypropylene and uneven dispersion of the anti-mold agent. Additionally, existing constant-temperature extrusion methods struggle to address insufficient heat in the feeding section and excessive shear heat in the metering section, further exacerbating material performance fluctuations. Third, existing composite materials are not optimized for the high-temperature, high-humidity, and oily environment of the kitchen, making it difficult to simultaneously achieve anti-mold performance with aging resistance, oil resistance, and mechanical properties. This can lead to problems such as anti-mold agent precipitation and material cracking during use. Fourth, some anti-mold systems use toxic and harmful components, failing to meet the food contact safety requirements for small kitchen appliances. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings and deficiencies of existing technologies by providing a method for preparing anti-mildew polypropylene composite materials based on gradient temperature-controlled extrusion. This method solves the technical problems of uneven anti-mildew effect, poor long-term effectiveness, unstable mechanical properties, unreasonable extrusion process, high similarity to existing technologies, and difficulty in adapting to the usage environment of small kitchen appliances in existing polypropylene composite materials. It achieves a synergistic improvement in the anti-mildew long-term effectiveness, mechanical stability, processing adaptability, and safety and environmental protection of composite materials.

[0005] To achieve the above objectives, the present invention adopts the following technical solution, which includes the following operational steps: Step 1, Raw material pretreatment: Polypropylene substrate, synergistic composite anti-mildew system, special compatibilizer, anti-aging compound system, and lubricant were placed in a vacuum drying oven and dried at 80-100℃ for 2-4 hours until the moisture content of each raw material was ≤0.1wt%. The synergistic composite anti-mildew system was composed of organic anti-mildew agent, surface-modified inorganic anti-mildew agent, and anti-mildew synergist in a mass ratio of 1:2-4:0.3-0.8. Step 2, Mixing raw materials: The pretreated polypropylene substrate, synergistic composite anti-mildew system, special compatibilizer, anti-aging compound system, and lubricant are mixed according to the preset mass ratio and placed into a high-speed mixer. The mixing speed is adjusted to 800-1200 r / min, the mixing temperature is 60-80℃, and the mixing time is 5-10 min. After uniform mixing, the mixture is obtained. Step 3: Gradient temperature controlled extrusion granulation: The mixture is fed into a twin-screw extruder and extruded and granulated using a segmented gradient temperature control mode. After extrusion, it is water-cooled, pelletized, and air-dried to obtain mildew-resistant polypropylene composite material granules. The temperature settings for each segment of the gradient temperature control mode are as follows: feeding section 140-160℃, melting section 170-190℃, metering section 180-200℃, and die head section 175-195℃. The screw speed of the twin-screw extruder is 100-200 r / min, the length-to-diameter ratio is 30-40:1, and the extrusion pressure is 10-20 MPa. Step 4, Post-processing: The extruded and granulated composite material particles are placed in a vibrating screen for sieving to remove impurities and unqualified particles. After sieving, they are placed in a vacuum drying oven for secondary drying at a temperature of 70-80℃ for 1-2 hours. After drying, they are sealed and packaged to obtain the finished anti-mildew polypropylene composite material.

[0006] Further, in step 1, the polypropylene substrate is homopolymer polypropylene or copolymer polypropylene with a melt index of 2-10 g / 10 min (test conditions: 230℃, 2.16 kg) and a dosage of 80-90 wt%. Selecting a polypropylene substrate with this melt index can take into account both the processing fluidity and mechanical properties of the material, and is suitable for the injection molding and extrusion molding processes of small kitchen appliances.

[0007] Furthermore, in step 1, the organic antifungal agent is a compound of isothiazolinone antifungal agent and benzimidazole antifungal agent in a mass ratio of 1:1-2, and the dosage is 0.5-2wt%. Isothiazolinone antifungal agents have a fast onset of action and can quickly kill surface molds, while benzimidazole antifungal agents have good long-lasting effect and can continuously exert antibacterial effects. The compound of the two achieves a synergistic effect of rapid and long-lasting action, and has low toxicity and meets food contact safety requirements.

[0008] Further, in step 1, the surface-modified inorganic antifungal agent is a mixture of nano-zinc oxide and nano-titanium dioxide modified with a silane coupling agent at a mass ratio of 2-3:1, with a particle size of 50-200 nm and a dosage of 1-4 wt%. The silane coupling agent is selected from γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane, and the modification dosage is 1-3 wt% of the inorganic antifungal agent. The modification process is as follows: the inorganic antifungal agent is dispersed in anhydrous ethanol, the silane coupling agent is added, and the mixture is stirred and reacted at 60-70℃ for 2-3 hours. After the reaction, the mixture is centrifuged, dried, and pulverized to obtain the surface-modified inorganic antifungal agent. Surface modification can significantly improve the dispersibility of the inorganic antifungal agent, avoid agglomeration, and enhance its compatibility with the polypropylene substrate.

[0009] Further, in step 1, the antifungal synergist is tributyl citrate or epoxidized soybean oil, with an amount of 0.1-0.5 wt%; its function is to reduce the surface tension of the composite antifungal system, promote the dispersion of the antifungal agent in the polypropylene substrate, enhance the antibacterial activity of the antifungal agent, improve the synergistic antifungal effect, and improve the processing fluidity of the material.

[0010] Furthermore, in step 1, the special compatibilizer is maleic anhydride-grafted polypropylene (MAH-g-PP), with a grafting rate of 0.5-2.0 wt% and a dosage of 1-5 wt%. The polar groups of maleic anhydride-grafted polypropylene can form chemical bonds with the hydroxyl groups of the surface-modified inorganic antifungal agent, while the non-polar groups can be compatible with the polypropylene substrate, significantly improving the interfacial compatibility between the composite antifungal system and the substrate, and solving the problems of easy precipitation of antifungal agent and decline in material mechanical properties.

[0011] Furthermore, in step 1, the anti-aging compound system is composed of hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:1-3, with a dosage of 0.5-2wt%. The hindered phenolic antioxidants can inhibit the thermal oxidative aging of the material, while the phosphite antioxidants can assist in anti-oxidation and improve the processing stability of the material. The combination of the two can specifically improve the aging resistance of the material in the high temperature and high humidity environment of the kitchen, and extend the service life of the material.

[0012] Furthermore, in step 1, the lubricant is a mixture of stearic acid and butyl stearate in a mass ratio of 1:1, with a dosage of 0.3-1.5 wt%. This can improve the processing fluidity of the mixture, reduce friction between the material and the extruder screw and barrel, prevent the material from sticking to the wall, and at the same time improve the surface smoothness of the composite material particles, facilitating subsequent molding and processing.

[0013] Furthermore, the specific steps of step 3 are as follows: Step 3.1, Equipment Debugging: Select a twin-screw extruder (screw diameter 30-65mm). Beforehand, check the cleanliness of the screw, barrel, die head, and pelletizer, removing any residual materials and impurities. Install a spinneret with a suitable orifice diameter (2-3mm). Start the equipment and set the temperature for each section according to the gradient temperature control mode. Preheat the feeding section, melting section, metering section, and die head section sequentially for 30-40 minutes. During this period, check the actual temperature of each section every 10 minutes to ensure that the actual temperature deviates from the set temperature by ≤±2℃. Simultaneously, adjust the twin-screw extruder's screw speed to 100-200 r / min, length-to-diameter ratio to 30-40:1, extrusion pressure to 10-20 MPa, and pelletizer speed to 300-500 r / min to ensure stable equipment operation. Step 3.2, Feeding and Extrusion: The mixture is fed into the feed section of a twin-screw extruder at a uniform speed through a metering feeder (feeding speed 5-15 kg / h, matched with screw speed). During the feeding process, the feeding is kept uniform to avoid material interruption or excessive feeding, which would cause fluctuations in extrusion pressure. After the material is preheated and softened in the feed section (140-160℃), completely melted in the melting section (170-190℃), and uniformly plasticized and stably metered in the metering section (180-200℃), it is extruded from the die head section (175-195℃) to form a continuous melt filament. Step 3.3, Cooling and pelletizing: After the melt filaments are extruded, they are immediately fed into a water cooling tank (water temperature 20-30℃, water depth 15-20cm) for 10-15 seconds to ensure that the filaments are completely cooled to room temperature (after cooling, the filaments feel hard and are not sticky). The cooled filaments are then drawn to a pelletizer by a traction machine (traction speed 1-2m / min, matched with the extrusion speed) for pelletizing. The pellet length is controlled at 2-3mm, and the pellets are uniform in size, without burrs or clumping. Step 3.4, Air-drying and screening: The pelletized composite material particles are fed into a dryer (wind speed 5-8m / s, temperature 40-50℃) and dried for 5-8 minutes to remove surface moisture. After drying, the particles are passed through a 10-mesh standard sieve to remove unqualified clumps, large particles and impurities, thus obtaining preliminary anti-mildew polypropylene composite material particles.

[0014] Compared with the prior art, the beneficial effects of the present invention are: the present invention provides a method for preparing anti-mildew polypropylene composite material based on gradient temperature control extrusion, which solves the technical problems of uneven anti-mildew effect, poor long-term effect, unstable mechanical properties, unreasonable extrusion process, high similarity with the prior art, and difficulty in adapting to the use environment of small kitchen appliances in the prior art. It achieves a synergistic improvement in the anti-mildew long-term effect, mechanical stability, processing adaptability and safety and environmental protection of composite material. Attached Figure Description

[0015] Figure 1 This is a data table of test results for the embodiments and control group of the present invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. The preferred embodiments described are only examples. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example 1: This example includes the following steps: Step 1, Raw material pretreatment: Homopolymer polypropylene with a melt index of 5 g / 10 min (230℃, 2.16 kg) was selected as the base material (85 wt%). The synergistic composite antifungal system consists of an organic antifungal agent (isothiazolinone and benzimidazole compounded in a 1:1 mass ratio, 1 wt%), a surface-modified inorganic antifungal agent (nano-zinc oxide modified with γ-aminopropyltriethoxysilane compounded with nano-titanium dioxide in a 2:1 mass ratio, particle size 100 nm, 2 wt%, silane coupling agent modified by 2 wt% of the inorganic antifungal agent mass), and an antifungal synergist (tributyl citrate, 1 wt%). The following components were used: 0.3wt% of maleic anhydride-grafted polypropylene (grafting rate 1.0wt%, dosage 3wt%); the anti-aging compound system consisted of 2,6-di-tert-butyl-p-cresol (hindered phenol) and tris(2,4-di-tert-butylphenyl) phosphite (phosphite) in a mass ratio of 1:2, dosage 1wt%; the lubricant was stearic acid and butyl stearate in a mass ratio of 1:1, dosage 0.7wt%; all the above raw materials were placed in a vacuum drying oven and dried at 85℃ for 3 hours until the moisture content was ≤0.1wt%, and then set aside for later use. Step 2, Mixing raw materials: The pretreated raw materials were placed into a high-speed mixer according to the above proportions. The mixing speed was adjusted to 1000 r / min, the mixing temperature to 70℃, and the mixing time to 8 min. After the mixture was uniformly mixed, the mixture was obtained. Step 3: Gradient temperature controlled extrusion granulation: A twin-screw extruder (model TE-35) with a screw diameter of 35mm was selected. The screw, barrel, and die head were cleaned, and a 2.5mm orifice spinneret was installed. The equipment was started, and the gradient temperatures were set as follows: feed section 150℃, melt section 180℃, metering section 190℃, and die head section 185℃. Preheating was performed for 35 minutes, and the temperature deviation of each section was checked to be ≤±2℃. The screw speed was adjusted to 150 r / min, the length-to-diameter ratio to 35:1, the extrusion pressure to 15 MPa, the pelletizer speed to 400 r / min, and the traction machine speed to 1.5 m / min. A trial run of 5 minutes was conducted, and feeding began after the equipment stabilized. A quantitative feeder is used to uniformly feed the mixture into the feeding section at a speed of 10 kg / h to avoid material interruption. After the material is preheated and softened in the feeding section, completely melted in the melting section, and uniformly plasticized in the metering section, it is extruded into melt filaments by the die head. The melt filaments are immediately sent to a water cooling tank (water temperature 25℃, water depth 18cm) and cooled for 12s. After cooling, they are pulled to a pelletizer by a traction machine and cut into pellets with a length of 2.5mm. The pellets are then sent to a dryer with a wind speed of 6m / s and a temperature of 45℃ for 6 minutes to remove surface moisture. The pellets are then passed through a 10-mesh sieve to remove unqualified pellets, resulting in preliminary composite material pellets. Step 4, Post-processing: The composite material particles are placed in a vibrating screen for sieving to remove impurities and unqualified particles. After sieving, they are placed in a vacuum drying oven and dried at 75°C for 1.5 hours. After drying, they are sealed and packaged to obtain the finished product.

[0018] Example 2: This example includes the following steps: Step 1, Raw material pretreatment: Copolymer polypropylene with a melt index of 2 g / 10 min (230℃, 2.16 kg) was selected as the base material (80 wt%). The synergistic composite antifungal system consisted of an organic antifungal agent (isothiazolinone and benzimidazole compounded at a mass ratio of 1:2, 0.5 wt%), a surface-modified inorganic antifungal agent (nano-zinc oxide modified with γ-glycidoxypropyltrimethoxysilane compounded with nano-titanium dioxide at a mass ratio of 3:1, particle size 50 nm, 1 wt%, silane coupling agent modified at 1 wt% of the inorganic antifungal agent mass), and an antifungal synergist (epoxidized soybean oil, 0.1 wt%). The mixture is composed of a mass ratio of 1:2:0.2; the special compatibilizer is maleic anhydride-grafted polypropylene (grafting rate 0.5wt%, dosage 1wt%); the anti-aging compound system is composed of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (hindered phenol) and tris(2,4-di-tert-butylphenyl) phosphite (phosphite) in a mass ratio of 1:1, dosage 0.5wt%; the lubricant is composed of stearic acid and butyl stearate in a mass ratio of 1:1, dosage 0.3wt%; all the above raw materials are placed in a vacuum drying oven and dried at 80℃ for 4 hours until the moisture content is ≤0.1wt%, for later use; Step 2, Mixing raw materials: The pretreated raw materials were placed into a high-speed mixer according to the above proportions. The mixing speed was adjusted to 800 r / min, the mixing temperature to 60℃, and the mixing time to 10 min. After mixing evenly, the mixture was obtained. Step 3: Gradient temperature controlled extrusion granulation: A twin-screw extruder (model TE-30) with a screw diameter of 30mm was selected and a 2mm orifice spinneret was installed after cleaning. The gradient temperatures were set as follows: feed section 140℃, melting section 170℃, metering section 180℃, and die head section 175℃. Preheating was performed for 30 minutes, with a temperature deviation ≤ ±2℃. The screw speed was adjusted to 100 r / min, the length-to-diameter ratio to 30:1, the extrusion pressure to 10 MPa, the pelletizer speed to 300 r / min, and the traction machine speed to 1 m / min. Stable trial operation was achieved. A quantitative feeder was used to feed material at a rate of 5 kg / h to avoid fluctuations. After processing in each section, the material was extruded as melt filaments at the die head. The melt filaments were immediately fed into a water cooling tank (water temperature 20℃, water depth 15cm) for 15 seconds, then traction to the pelletizer, with a pellet length of 2mm. The air dryer was used at a wind speed of 5 m / s and a temperature of 40℃ for 8 minutes, followed by screening through a 10-mesh sieve to obtain preliminary pellets. Step 4, Post-processing: The composite material particles are placed in a vibrating screen for sieving to remove impurities and unqualified particles. After sieving, they are placed in a vacuum drying oven and dried at 70°C for 2 hours. After drying, they are sealed and packaged to obtain the finished product.

[0019] Example 3: This example includes the following steps: Step 1, Raw material pretreatment: Homopolymer polypropylene with a melt index of 10 g / 10 min (230℃, 2.16 kg) was selected as the base material (90 wt%). The synergistic composite antifungal system consisted of an organic antifungal agent (isothiazolinone and benzimidazole compounded at a mass ratio of 1:1.5, 2 wt%), a surface-modified inorganic antifungal agent (nano-zinc oxide modified with γ-aminopropyltriethoxysilane compounded with nano-titanium dioxide at a mass ratio of 2.5:1, particle size 200 nm, 4 wt%, silane coupling agent modified at 3 wt% of the inorganic antifungal agent mass), and an antifungal synergist (tributyl citrate, ...). The mixture consists of 0.5 wt% of maleic anhydride-grafted polypropylene (grafting rate 2.0 wt%, dosage 5 wt%), and 2 wt% of tris(2,4-di-tert-butyl-p-cresol, a hindered phenol) and tris(2,4-di-tert-butylphenyl) phosphite (a phosphite) in a mass ratio of 1:3. The lubricant is 1.5 wt% of stearic acid and butyl stearate in a mass ratio of 1:1. All the above raw materials are placed in a vacuum drying oven and dried at 100°C for 2 hours until the moisture content is ≤0.1 wt%. Step 2, Mixing raw materials: The pretreated raw materials were placed into a high-speed mixer according to the above proportions. The mixing speed was adjusted to 1200 r / min, the mixing temperature to 80℃, and the mixing time to 5 min. After the mixture was uniformly mixed, the mixture was obtained. Step 3: Gradient temperature controlled extrusion granulation: A twin-screw extruder (model TE-40) with a screw diameter of 40mm was selected and a 3mm orifice spinneret was installed after cleaning. The gradient temperatures were set as follows: feed section 160℃, melting section 190℃, metering section 200℃, and die head section 195℃. Preheating was performed for 40 minutes, with a temperature deviation ≤ ±2℃. The screw speed was adjusted to 200 r / min, the length-to-diameter ratio to 40:1, the extrusion pressure to 20 MPa, the pelletizer speed to 500 r / min, and the traction machine speed to 2 m / min. The test run was stable. The metering feeder fed material at a rate of 15 kg / h. After processing in each section, the material was extruded as melt filaments at the die head. The melt filaments were immediately fed into a water cooling tank (water temperature 30℃, water depth 20 cm) for 10 seconds, then traction to the pelletizer, with a pellet length of 3mm. The air dryer was set at 8 m / s and 50℃ for 5 minutes, and the pellets were screened through a 10-mesh sieve to obtain preliminary particles. Step 4, Post-processing: The composite material particles are placed in a vibrating screen for sieving to remove impurities and unqualified particles. After sieving, they are placed in a vacuum drying oven and dried at 80°C for 1 hour. After drying, they are sealed and packaged to obtain the finished product.

[0020] Compared with the prior art, the beneficial effects of the present invention are: 1. The synergistic composite anti-mold system of this invention adopts a ternary compound structure of "organic-modified inorganic-synergist", which solves the problems of poor anti-mold effect, insufficient long-term effect and easy aggregation of existing single anti-mold agents. It achieves synergistic effect of fast antibacterial and long-term antibacterial. The composite material has been tested and found to have an antibacterial rate of ≥99% against Aspergillus niger, Aspergillus flavus and Penicillium, and the anti-mold effect can last for more than 12 months, which is significantly better than the existing technology. At the same time, the total amount of anti-mold agent is low, which will not affect the mechanical properties and processing flow of polypropylene substrate, and all components meet the food contact safety requirements, making it suitable for the use of small kitchen appliances. 2. The segmented gradient temperature-controlled extrusion process in this invention optimizes the temperature gradient of each segment based on the melting characteristics of polypropylene and the thermal stability of the antifungal agent. This avoids the thermal decomposition and failure of the antifungal agent caused by excessively high constant temperature, and solves the problems of insufficient polypropylene melting and uneven dispersion of the antifungal agent caused by excessively low constant temperature. At the same time, it alleviates the industry pain points of insufficient heat in the feeding section and excessive shear heat in the metering section, improves the dispersion uniformity of the antifungal agent in the substrate and the molding quality of the material, reduces internal defects in the material, and makes the mechanical properties of the composite material more stable. 3. The optimized raw material ratio system in this invention improves the interfacial compatibility between the composite anti-mildew system and the polypropylene substrate through a special compatibilizer, solving the problems of poor compatibility and easy precipitation of the anti-mildew agent and the substrate. The anti-aging compound system improves the material's high temperature and humidity resistance, making it suitable for the high temperature and humidity environment of the kitchen. It achieves a synergistic improvement in "anti-mildew performance - mechanical properties - aging resistance - processing fluidity". The tensile strength of the composite material is ≥30MPa, the impact strength of a simply supported beam is ≥11kJ / m², and the mechanical property retention rate after the aging test is ≥95%. 4. The preparation method of the present invention is simple, highly controllable, and has low production cost, enabling large-scale production. The prepared anti-mildew polypropylene composite material can be widely used in the outer shell, inner liner, and accessories of small kitchen appliances, with a long service life and good user experience, and has significant practical value and market competitiveness.

[0021] To verify the superiority of the present invention, three control groups were set up. Control groups 1 to 3 were prepared using existing methods and their performance was compared with that of Examples 1 to 3 of the present invention.

[0022] Control group setup: Control Group 1: No antifungal agent was added. The existing conventional constant temperature extrusion process was used (extrusion temperature was fixed at 180℃). The remaining preparation steps (raw material pretreatment, mixing, cooling and pelletizing, post-treatment) were the same as in Example 1 (only all antifungal-related components were removed). That is, the same homopolymer polypropylene (melt index 5g / 10min, 230℃, 2.16kg), maleic anhydride grafted polypropylene, anti-aging compound system, and lubricant were selected as in Example 1. The components were weighed, mixed, extruded at constant temperature, and post-treated according to the non-antifungal component ratio of Example 1 to simulate the existing preparation process of polypropylene composite materials without antifungal function.

[0023] Control Group 2: A single organic antifungal agent was added (only the isothiazolinone antifungal agent from Example 1 was used, and the dosage was the same as the total amount of organic antifungal agent in Example 1, which was 1 wt%). The "organic-modified inorganic-synergist" ternary compound system of the present invention was not used. The existing conventional constant temperature extrusion process was used (the extrusion temperature was fixed at 180℃). The remaining preparation steps (raw material pretreatment, mixing, cooling and pelletizing, post-treatment) were the same as those in Example 1, simulating the conventional preparation process of existing single antifungal agent modified polypropylene composite materials. However, the defects of poor long-term effect and uneven dispersion of single antifungal agents were not solved.

[0024] Control Group 3: A composite anti-mold system was added (using the same type of isothiazolinone anti-mold agent, benzimidazole anti-mold agent, nano zinc oxide, and nano titanium dioxide as in Example 1, with the same ratio of organic anti-mold agent and surface-modified inorganic anti-mold agent as in Example 1, i.e., 1 wt% organic anti-mold agent and 2 wt% inorganic anti-mold agent), but the anti-mold synergist of the present invention was not used (no tributyl citrate was added), and the inorganic anti-mold agent was not subjected to silane coupling agent surface modification treatment (only simple physical mixing was performed, without ultrasonic dispersion, coupling agent reaction, centrifugal drying and other modification steps); the extrusion process adopted the existing conventional constant temperature extrusion process (extrusion temperature fixed at 180℃), and the segmented gradient temperature control mode of the present invention was not adopted; the remaining preparation steps (raw material drying, mixing, cooling and pelletizing, post-treatment) were consistent with Example 1, simulating the improved preparation process of simple composite anti-mold agent + constant temperature extrusion in the prior art, but the technical defects of anti-mold agent agglomeration, thermal decomposition, and poor synergistic effect were not solved.

[0025] The test items included: anti-mildew performance (tested according to GB / T 24128-2009 "Test Method for Anti-mildew Performance of Plastics", with Aspergillus niger, Aspergillus flavus, and Penicillium as the test strains, a test time of 28 days, and calculation of the antibacterial rate), mechanical properties (tested according to GB / T 1040.1-2006 for tensile strength, and according to GB / T 1043.1-2008 for impact strength of simply supported beams), aging resistance (tested according to GB / T 16422.2-2014, an aging time of 1000 hours, and calculation of the retention rate of mechanical properties), and long-term anti-mildew effect (tested after 12 months of natural placement, with an antibacterial rate). The test results are as follows: Figure 1 As shown.

[0026] Depend on Figure 1 The comparative experimental data show that: 1. The composite materials prepared in Examples 1-3 of this invention have an antibacterial rate of ≥99% against Aspergillus niger, Aspergillus flavus, and Penicillium. After 12 months, the antibacterial rate is still ≥98.2%. The anti-mold effect is significantly better than that of control group 2 (single organic anti-mold agent, constant temperature extrusion) and control group 3 (composite anti-mold agent without modification or synergist, constant temperature extrusion). This shows that the synergistic composite anti-mold system of this invention (organic-modified inorganic-synergist ternary compound) combined with gradient temperature control extrusion process can significantly improve the anti-mold effect and long-term effectiveness of composite materials, and effectively solve the defects of anti-mold agent agglomeration, thermal decomposition, and poor synergistic effect in the prior art. 2. The tensile strength of Examples 1-3 of the present invention is ≥30.5MPa, the impact strength of simply supported beam is ≥11.2MPa, and the retention rate of mechanical properties after aging is ≥95.1%. The mechanical properties and aging resistance are better than those of control groups 2 and 3, and slightly lower than those of control group 1 (without antifungal agent). This shows that the formulation design of the present invention (special compatibilizer + anti-aging compound system) can maximize the retention of the mechanical properties and aging resistance of polypropylene substrate while ensuring antifungal performance, thus solving the problem of decreased mechanical properties caused by the addition of antifungal agents in the prior art. Among them, the mechanical properties of control group 3 are slightly better than those of control group 2 because the inorganic antifungal agent is not modified and is prone to agglomeration, but are still far lower than those of the embodiments of the present invention, further demonstrating the necessity of the surface modification process of the inorganic antifungal agent of the present invention. 3. Control groups 2 and 3, due to the use of constant temperature extrusion process, resulted in uneven dispersion or partial thermal decomposition of the antifungal agent. Furthermore, control group 2 lacked inorganic antifungal agents and synergists, while control group 3 lacked antifungal synergists and the inorganic antifungal agent was not modified. Consequently, their antifungal effect and long-term effectiveness were poor, and their mechanical properties and aging resistance were also inferior to those of the embodiments of the present invention. This further demonstrates the innovation and superiority of the gradient temperature control extrusion process, the synergistic composite antifungal system, and the inorganic antifungal agent surface modification process of the present invention. Control group 1 had no antifungal effect and could not meet the usage requirements of small kitchen appliances.

[0027] For those skilled in the art, modifications can be made to the technical solutions described in the foregoing embodiments, and equivalent substitutions can be made to some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A method for preparing a mildew-resistant polypropylene composite material based on gradient temperature-controlled extrusion, characterized in that it includes the following steps: Step (1), Raw material pretreatment: The polypropylene substrate, synergistic composite anti-mildew system, special compatibilizer, anti-aging compound system, and lubricant were separately placed in a vacuum drying oven and dried at 80-100℃ for 2-4 hours until the moisture content of each raw material was ≤0.1wt%. These were then set aside for later use. The synergistic composite anti-mildew system consists of organic anti-mildew agent, surface-modified inorganic anti-mildew agent, and anti-mildew synergist in a mass ratio of 1:2-4:0.3-0.8; Step (2), mixing raw materials: The pretreated polypropylene substrate, synergistic composite anti-mildew system, special compatibilizer, anti-aging compound system, and lubricant are mixed according to the preset mass ratio and placed into a high-speed mixer. The mixing speed is adjusted to 800-1200 r / min, the mixing temperature is 60-80℃, and the mixing time is 5-10 min. After uniform mixing, the mixture is obtained. Step (3), gradient temperature controlled extrusion granulation: The mixture is fed into a twin-screw extruder and extruded and granulated using a segmented gradient temperature control mode. After extrusion, it is water-cooled, pelletized, and air-dried to obtain mildew-resistant polypropylene composite material granules. The temperature settings for each segment of the gradient temperature control mode are as follows: feeding section 140-160℃, melting section 170-190℃, metering section 180-200℃, and die head section 175-195℃. The screw speed of the twin-screw extruder is 100-200 r / min, the length-to-diameter ratio is 30-40:1, and the extrusion pressure is 10-20 MPa. Step (4), Post-processing: The extruded and granulated composite material particles are placed in a vibrating screen for sieving to remove impurities and unqualified particles. After sieving, they are placed in a vacuum drying oven for secondary drying at a temperature of 70-80℃ for 1-2 hours. After drying, they are sealed and packaged to obtain the finished anti-mildew polypropylene composite material.

2. The method for preparing a mildew-resistant polypropylene composite material based on gradient temperature-controlled extrusion according to claim 1, characterized in that: In step (1), the polypropylene substrate is homopolymer polypropylene or copolymer polypropylene with a melt index of 2-10 g / 10 min and a dosage of 80-90 wt%.

3. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the organic antifungal agent is a compound of isothiazolinone antifungal agent and benzimidazole antifungal agent in a mass ratio of 1:1-2, and the dosage is 0.5-2wt%.

4. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the surface-modified inorganic antifungal agent is a mixture of nano zinc oxide modified with silane coupling agent and nano titanium dioxide at a mass ratio of 2-3:1, with a particle size of 50-200 nm and a dosage of 1-4 wt%. The silane coupling agent is selected from γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane, and the modification dosage is 1-3 wt% of the inorganic antifungal agent. The modification process is as follows: the inorganic antifungal agent is dispersed in anhydrous ethanol, the silane coupling agent is added, and the mixture is stirred and reacted at 60-70℃ for 2-3 h. After the reaction, the mixture is centrifuged, dried, and pulverized to obtain the surface-modified inorganic antifungal agent.

5. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the antifungal synergist is tributyl citrate or epoxidized soybean oil, and the dosage is 0.1-0.5wt%.

6. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the special compatibilizer is maleic anhydride-grafted polypropylene with a grafting rate of 0.5-2.0 wt% and a dosage of 1-5 wt%.

7. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the anti-aging compound system is composed of hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:1-3, and the amount used is 0.5-2wt%.

8. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: In step (1), the lubricant is a mixture of stearic acid and butyl stearate in a mass ratio of 1:1, and the amount used is 0.3-1.5 wt%.

9. The method for preparing a gradient temperature-controlled extrusion anti-mildew polypropylene composite material according to claim 1, characterized in that: The specific operation steps of step (3) are as follows: Step (3.1), Equipment Debugging: Select a twin-screw extruder. Beforehand, check the cleanliness of the screw, barrel, die head, and pelletizer, removing any residual materials and impurities. Install a spinneret with the appropriate orifice diameter. Start the equipment and set the temperature for each section according to the gradient temperature control mode. Preheat the feeding section, melting section, metering section, and die head section sequentially for 30-40 minutes. During this period, check the actual temperature of each section every 10 minutes to ensure that the deviation between the actual temperature and the set temperature is ≤±2℃. Simultaneously, adjust the twin-screw extruder's screw speed to 100-200 r / min, length-to-diameter ratio to 30-40:1, extrusion pressure to 10-20 MPa, and pelletizer speed to 300-500 r / min to ensure stable equipment operation. Step (3.2), Feeding and Extrusion: The mixture is fed into the feeding section of the twin-screw extruder at a uniform speed through a metering feeder. During the feeding process, the feeding is kept uniform to avoid material interruption or excessive feeding, which would cause fluctuations in extrusion pressure. After the material is preheated and softened in the feeding section, completely melted in the melting section, and uniformly plasticized and stably metered in the metering section, it is extruded from the die head section to form a continuous melt filament. Step (3.3), cooling and pelletizing: After the melt filaments are extruded, they are immediately fed into a water cooling tank for 10-15 seconds to ensure that the filaments are completely cooled to room temperature. After cooling, the filaments are pulled to a pelletizer by a traction machine for pelletizing. The pellet length is controlled at 2-3 mm, and the pellets are uniform in size, without burrs or clumping. Step (3.4), air drying and screening: The pelletized composite material particles are fed into an air dryer and air-dried for 5-8 minutes to remove surface moisture. After air drying, they are passed through a 10-mesh standard sieve to remove unqualified pellets, large particles, and impurities, thus obtaining preliminary anti-mildew polypropylene composite material particles.

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