Nano-modified PP particles for refrigerator structural parts and preparation method of nano-modified PP particles

Abstract

The invention provides nano-modified PP (polypropylene) particles for refrigerator structural parts and a preparation method. The nano-modified PP particles comprise the following components: polypropylene resin, a nanocrystalline material, a polar grafting modifier, a compatilizer, an antioxidant and a lubricant, wherein the polypropylene resin is high-flowability copolymerized PP (polypropylene) particles with the mass percentage of 65-80%; the nanocrystalline material is nano titanium dioxide or nano aluminum oxide, and the mass percentage of the nanocrystalline material is 5-12%; the mass percent of the polar grafting modifier is 4-10%; the mass percent of the compatilizer is 3-6%; the mass percent of the antioxidant is 0.2 to 0.6 percent; and the mass percentage of the lubricant is 0.1-0.4%. Through a nanocrystalline modification technology, the PP particles have excellent impact resistance, stretchability, low shrinkage rate and good surface polarity, and meanwhile the excellent performance, namely the advantages of good processability, high chemical corrosion resistance and the like, of the PP material is kept.

Description

Technical Field

[0001] This application relates to the field of materials technology, and particularly to a nano-modified PP particle for a refrigerator structural member and a preparation method thereof. Background Art

[0002] In the field of refrigerator structural members, acrylonitrile-butadiene-styrene copolymer (ABS) materials are widely used. However, the processing temperature range of ABS materials is relatively narrow (usually 200 - 240 °C), and the injection molding process parameters (such as temperature, pressure) need to be precisely controlled. Otherwise, degradation or surface defects are likely to occur, increasing the process debugging and equipment maintenance costs. While the processing window of polypropylene (PP) materials is wider (180 - 250 °C), and they are more compatible with equipment; therefore, in related technologies, PP materials are used as refrigerator structural members.

[0003] Polypropylene (PP) material, as a common thermoplastic resin, has many advantages. The density of PP material is relatively small, which makes the products made have a relatively light weight and have potential advantages in some application scenarios with weight requirements. Its strength, stiffness, and hardness perform well to a certain extent, and it has good heat resistance and can be used normally in an environment of about 100 °C. At the same time, PP material also has good electrical properties and high-frequency insulation properties, and these properties are not affected by humidity. In addition, the raw materials of PP material are widely sourced and the cost is relatively low, which makes it have a high cost performance among many materials. From the economic and resource perspectives, it is a very potential material choice.

[0004] However, when attempting to use PP materials to replace ABS materials for refrigerator structural components, a series of technical difficulties are faced. First of all, there is a significant difference in the shrinkage rate between PP materials and ABS materials, usually differing by about 1.0%, which causes the molds originally suitable for ABS materials to be unable to be directly used for the processing of PP materials. If PP materials are to be used, the molds need to be redesigned and manufactured, greatly increasing the production cost and time cost. Secondly, when attempting to adjust the formula of PP materials to make their shrinkage rate close to that of ABS materials, their surface glossiness will drop significantly. For refrigerator structural components with high appearance requirements, this problem seriously affects the aesthetics and market competitiveness of the products. Moreover, the toughness of PP materials, especially the low-temperature toughness, is poor. In the low-temperature environment inside the refrigerator, their mechanical properties will be significantly reduced, and problems such as brittle fracture are likely to occur, making it difficult to meet the requirements for the long-term stable use of refrigerator structural components under low-temperature conditions. In addition, in order to improve the shrinkage rate of PP materials, their density often increases significantly, resulting in the weight of the parts exceeding expectations, which does not conform to the trend of lightweight design of refrigerators. Finally, when using PP materials to inject large-sized or surface-complex refrigerator structural component products, defects such as surface flow marks, tiger stripes, and weld lines are extremely likely to appear. These problems not only affect the appearance quality of the products, but may also have an adverse impact on the mechanical properties and overall performance of the products, thereby restricting the widespread application of PP materials in the field of refrigerator structural components. Therefore, PP materials have problems such as high shrinkage rate, poor low-temperature impact resistance, and insufficient tensile strength, making it difficult to directly replace ABS for precision injection parts. Summary of the Invention

[0005] The present application provides a nano-modified PP particle for refrigerator structural components and a preparation method thereof to solve the problems that existing PP materials have disadvantages such as high shrinkage rate, poor low-temperature impact resistance, and insufficient tensile strength, and are difficult to directly replace ABS for precision injection parts.

[0006] In the first aspect, the present application provides a nano-modified PP particle for refrigerator structural components, and the nano-modified PP particle comprises the following components:

[0007] Polypropylene resin, nano-crystalline material, polar graft modifier, compatibilizer, antioxidant, and lubricant;

[0008] Among them, the polypropylene resin is a high-flow copolymerized PP particle, with a mass percentage of 65-80%; the nano-crystalline material is nano-titanium dioxide or nano-aluminum oxide, with a mass percentage of 5-12%; the mass percentage of the polar graft modifier is 4-

[0009] 10%; the mass percentage of the compatibilizer is 3-6%; the mass percentage of the antioxidant is 0.2-0.6%; the mass percentage of the lubricant is 0.1-0.4%.

[0010] In some possible implementation manners, the particle size of the nanocrystalline material is 50 - 150 nm, and it is surface-modified with silane coupling agent KH-550 and / or KH-560.

[0011] In some possible implementation manners, the polar graft modifier is maleic anhydride grafted PP or acrylic acid grafted POE.

[0012] In some possible implementation manners, the compatibilizer is polypropylene grafted maleic anhydride or ethylene-acrylic acid copolymer.

[0013] In some possible implementation manners, the antioxidant is a compound antioxidant containing hindered phenol and thioester.

[0014] In some possible implementation manners, the lubricant is ethylene bisstearamide or silicone.

[0015] In some possible implementation manners, the nano-modified PP particles are also loaded with Ag + ions through ion exchange, where the Ag + ion loading amount is 0.5 - 1.0 wt%.

[0016] In some possible implementation manners, the nano-modified PP particles also include microcapsules containing bisphenol A type epoxy resin. The particle size of the microcapsules is 50 - 100 μm, and the mass percentage is 1 - 2%.

[0017] In a second aspect, the present application provides a preparation method of the nano-modified PP particles for refrigerator structural parts described in the first aspect. The method includes:

[0018] Disperse the nanocrystalline material in ethanol, add the silane coupling agent, and after ultrasonic treatment for 60 - 80 min, dry it to obtain the modified nanocrystalline material;

[0019] High-speed mix the PP matrix resin, the modified nanocrystalline material, the polar graft modifier and other additives to obtain a reaction slurry, where the rotation speed is 1000 - 1500 rpm for 8 - 10 min;

[0020] Use a twin-screw extruder to plastify the reaction slurry in sections. The screw rotation speed is 280 - 400 rpm, the melt pressure is 1.5 - 2.5 MPa, and the vacuum degree is -0.04 to -0.1 MPa; among them, the temperature of the first zone of the twin-screw extruder is 175 °C, the temperature of the second zone is 195 °C, and the temperature of the third zone is 210 °C;

[0021] After water cooling, pelletize the particles with a particle diameter of 2 - 3 mm, and package them after drying.

[0022] In some possible implementation manners, dispersing the nanocrystalline material in ethanol and adding the silane coupling agent further includes:

[0023] Disperse the nanocrystalline material in ethanol. First, add KH550 amino silane, and after ultrasonic treatment for 30 - 40 min, then add KH560 epoxy silane. After ultrasonic treatment for 30 - 40 min, dry it.

[0024] As can be seen from the above, this application provides a nano - modified PP particle for refrigerator structural parts and a preparation method. The nano - modified PP particle includes the following components: polypropylene resin, nanocrystalline material, polar graft modifier, compatibilizer, antioxidant, and lubricant. Among them, the polypropylene resin is a high - flow copolymer PP particle, with a mass percentage of 65 - 80%; the nanocrystalline material is nano - titanium dioxide or nano - aluminum oxide, with a mass percentage of 5 - 12%; the mass percentage of the polar graft modifier is 4 - 10%; the mass percentage of the compatibilizer is 3 - 6%; the mass percentage of the antioxidant is 0.2 - 0.6%; the mass percentage of the lubricant is 0.1 - 0.4%. Through the nanocrystalline modification technology, while the PP particles obtain excellent impact resistance, tensile property, low shrinkage rate, and good surface polarity, they also maintain the excellent properties of the PP material itself, such as low price, good processability, strong chemical corrosion resistance, etc. The modified PP particles have excellent impact resistance and tensile property, can meet the requirements of materials toughness for parts such as refrigerator end caps and door frames, and improve the durability and safety of products.

[0025] The nano - modified PP particles provided by this application have the following beneficial effects:

[0026] 1. “Nanocrystalline - polar graft” synergistic modification mechanism

[0027] The uniform dispersion of nanocrystals provides enhanced rigidity. The graft modifier introduces carboxyl or ester - based polar groups into the PP molecular chain, simultaneously improving the mechanical properties and surface polarity. The interfacial combination between the nanocrystals and PP is optimized through the compatibilizer, reducing brittle fracture caused by stress concentration.

[0028] 2. Simultaneous realization of low shrinkage rate and high surface polarity

[0029] Nanocrystals inhibit the crystallization shrinkage of PP, and the graft modifier forms a polar surface layer, directly replacing the ABS mold and meeting the bonding requirements. The surface polarity is achieved through chemical grafting, and its stability is far superior to physical treatment (such as corona).

[0030] 3. Process simplification

[0031] Omit the corona / plasma treatment process. After injection molding, it can be directly compounded with the foaming layer, and the production efficiency is increased by more than 20%. Description of the drawings

[0032] To more clearly illustrate the technical solutions of this application, the accompanying drawings required for use in the embodiments will be briefly introduced below. Obviously, for those of ordinary skill in the art, other accompanying drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 Schematic diagram of the horizontal contact angle of the nanocrystal-modified PP particles prepared in Example 1 provided by this application. Specific embodiments

[0034] The embodiments will be described in detail below, and the examples are shown in the accompanying drawings. When the following description refers to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following examples do not represent all embodiments consistent with this application. They are only examples of systems and methods consistent with some aspects of this application detailed in the claims.

[0035] In the field of refrigerator structural parts, acrylonitrile-butadiene-styrene copolymer (ABS) materials are widely used. However, the processing temperature range of ABS materials is relatively narrow (usually 200 - 240 °C), and the injection molding process parameters (such as temperature, pressure) need to be precisely controlled. Otherwise, degradation or surface defects are likely to occur, increasing the process debugging and equipment maintenance costs. While the processing window of polypropylene (PP) materials is wider (180 - 250 °C), and they are more compatible with equipment; so in related technologies, PP materials are used as refrigerator structural parts.

[0036] As a common thermoplastic resin, polypropylene (PP) materials have many advantages. PP materials have a relatively low density, which makes the products made relatively light in weight and have potential advantages in some application scenarios with weight requirements. Their strength, stiffness, and hardness perform well to a certain extent, and they have good heat resistance and can be used normally in an environment of about 100 °C. At the same time, PP materials also have good electrical properties and high-frequency insulation properties, and these properties are not affected by humidity. In addition, the raw materials of PP materials are widely sourced and the cost is relatively low, which makes them have a high cost performance among many materials. From an economic and resource perspective, it is a very potential material choice.

[0037] However, when attempting to use PP materials to replace ABS materials for refrigerator structural parts, a series of technical difficulties are faced. First of all, there are significant differences in the shrinkage rate between PP materials and ABS materials, usually differing by about 1.0%, which results in the molds originally suitable for ABS materials being unable to be directly used for the processing of PP materials. If PP materials are to be used, the molds need to be redesigned and manufactured, greatly increasing the production cost and time cost. Secondly, when attempting to adjust the formula of PP materials to make their shrinkage rate close to that of ABS materials, their surface glossiness will drop significantly. For refrigerator structural parts with high appearance requirements, this problem seriously affects the aesthetics and market competitiveness of the products. Moreover, the toughness of PP materials, especially the low-temperature toughness, is poor. In the low-temperature environment inside the refrigerator, their mechanical properties will be significantly reduced, and brittle cracking and other problems are likely to occur, making it difficult to meet the requirements for long-term stable use of refrigerator structural parts under low-temperature conditions. In addition, in order to improve the shrinkage rate of PP materials, their density is often greatly increased, resulting in the weight of the parts exceeding expectations and not conforming to the trend of lightweight design of refrigerators. Finally, when using PP materials to inject large-sized or surface-complex refrigerator structural parts, defects such as surface flow marks, tiger stripes, and weld lines are extremely likely to appear. These problems not only affect the appearance quality of the products but may also have an adverse impact on the mechanical properties and overall performance of the products, thus restricting the wide application of PP materials in the field of refrigerator structural parts. Therefore, PP materials have problems such as high shrinkage rate, poor low-temperature impact resistance, and insufficient tensile strength, and it is difficult to directly replace ABS for precision injection parts.

[0038] Based on this, the present application provides a nano-modified PP particle for refrigerator structural parts, and the nano-modified PP particle comprises the following components:

[0039] Polypropylene resin, nano-crystalline material, polar graft modifier, compatibilizer, antioxidant, and lubricant;

[0040] Among them, the polypropylene resin is a high-flow copolymerized PP particle, and the mass percentage is 65-80%; the nano-crystalline material is nano-titanium dioxide or nano-aluminum oxide, and the mass percentage is 5-12%; the mass percentage of the polar graft modifier is 4-10%; the mass percentage of the compatibilizer is 3-6%; the mass percentage of the antioxidant is 0.2-0.6%; the mass percentage of the lubricant is 0.1-0.4%.

[0041] The high-flow copolymerized PP resin, as the matrix, endows the material with excellent processing and molding capabilities. Its molecular chain structure optimizes the melt flow rate, and it can fill complex mold cavities under lower injection pressures, especially suitable for the precision molding of structural parts such as refrigerator end caps and door frames. At the same time, it retains the inherent advantages of PP materials such as low cost and chemical corrosion resistance.

[0042] Nanocrystalline materials significantly improve the mechanical properties and shrinkage rate of materials through nanoscale size effects and surface activity: After the uniform dispersion of nanoparticles, physical crosslinking points are formed, which inhibit the disordered arrangement of PP molecular chains during crystallization, reducing the shrinkage rate from 1.5 - 2.0% of traditional PP to 0.65 - 0.75% comparable to ABS, thus avoiding warping and deformation of injection-molded parts. At the same time, rigid nanocrystals, as reinforcing fillers, form a "rigid and tough composite network" with the matrix, increasing the tensile strength to 36 - 42 MPa and the notch impact strength exceeding 28 - 33 kJ / m 2 , meeting the dual requirements of structural parts for strength and toughness.

[0043] Polar graft modifiers overcome the non-polar problem on the surface of PP by introducing polar groups (such as acrylate): The polar groups are oriented on the material surface to form an active interface layer, increasing the surface energy to 43 - 46 mN / m, enabling direct chemical bonding with the polyurethane material of the refrigerator foaming layer, eliminating the need for corona or plasma treatment processes required for traditional PP, simplifying the process flow and reducing production costs.

[0044] The compatibilizer and the dual silane modification technology work together to ensure the interfacial compatibility between nanocrystals and the PP matrix: The compatibilizer (such as ethylene-acrylic acid copolymer) forms a transition layer through molecular chain entanglement with the silane coupling agent on the surface of nanocrystals, reducing the filler agglomeration phenomenon and increasing the dispersion degree of nanocrystals to 95%, avoiding the decline in mechanical properties caused by interfacial defects.

[0045] The compound antioxidant extends the thermal-oxidative aging life of the material to over 2000 hours through the synergistic mechanism of hindered phenol capturing free radicals and thioester decomposing hydroperoxides, meeting the aging resistance requirements for long-term use of refrigerator components; lubricants (such as ethylene bisstearamide) reduce the melt viscosity by 30 - 50 mPa·s, improving the processing fluidity while inhibiting the wear of the screw and barrel, enhancing production efficiency and extending the equipment life.

[0046] In some embodiments, the particle size of the nanocrystalline material is 50 - 150 nm, and it is surface-modified with silane coupling agents KH-550 and / or KH-560.

[0047] In some embodiments, when dispersing the nanocrystalline material in ethanol and adding silane coupling agents, it further includes:

[0048] Disperse the nanocrystalline material in ethanol, first add KH550 amino silane, after ultrasonic treatment for 30 - 40 min, then add KH560 epoxy silane, and after ultrasonic treatment for 30 - 40 min, dry it.

[0049] In the molecular structure of KH550, one end is a silane oxy group (-OCH2CH3), and the other end is an amino group (-NH2). The silane oxy group can be hydrolyzed to form a silanol group (-SiOH), which forms a covalent bond (Si-O-M, where M is the metal atom of the nanocrystal) with the hydroxyl group (-OH) on the surface of the nanocrystals (such as TiO2, Al2O3) through a dehydration condensation reaction, achieving the primary modification of the nanocrystal surface. The amino group (-NH2) is then exposed on the outside as an active group, providing reaction sites for subsequent modification.

[0050] The amino group (-NH2) on the surface of the nanocrystals modified by KH550 can undergo a ring-opening addition reaction with the epoxy group (-O-CH2-CH-CH2) of KH560 to form an amino-epoxy bond, fixing KH560 on the nanocrystal surface through chemical linkage. After the silane oxy group (-OCH2CH3) at the other end of KH560 is hydrolyzed, it can form an ester bond (-COO-) with the carboxyl group (-COOH) of the polar graft modifier (such as acrylic acid grafted POE) in the PP matrix or the carboxylic acid group of the compatibilizer (ethylene-acrylic acid copolymer) through an esterification reaction, thus firmly binding the nanocrystals and the PP matrix through the "nanocrystal-KH550-KH560-matrix" chemical bridging network.

[0051] The interfacial shear strength between the double-modified nanocrystals and the PP matrix is increased to 12 - 15 MPa (only 5 - 7 MPa for unmodified nanocrystals), increasing the tensile strength of the composite material by 20 - 30% and the notched impact strength by 15 - 25%.

[0052] Two-step ultrasonic treatment generates a cavitation effect through high-frequency vibration, breaking the original aggregates (micrometer-sized) of the nanocrystals and dispersing them into monodisperse nanoparticles. At the same time, KH550 and KH560 adsorb on the surface of the nanocrystals to form an adsorption layer, producing the following effects:

[0053] The aggregate size of unmodified nanocrystals in the PP matrix is about 1 - 5 μm, while the dispersion degree of the double-modified nanocrystals

[0054] ≥95%, with an average particle size distribution of ±20 nm, avoiding stress concentration and performance deterioration caused by aggregation.

[0055] The surface roughness of the silane-modified nanocrystals increases, and they carry polar groups (such as -NH2, -OH), which can serve as heterogeneous nucleation sites, inducing the PP molecular chains to align orderly on their surfaces to form β-crystalline polypropylene (compared with the traditional α-crystalline form, the β-crystalline form has a lower crystallization density and shrinkage rate). Among them, after the nanocrystals are modified, the crystallization temperature of PP increases by 8 - 12 °C, the crystallinity decreases by 5 - 8%, and the shrinkage rate decreases from 1.5 - 2.0% to 0.65 - 0.75%, which is comparable to that of ABS.

[0056] The carboxyl groups (-COOH) in acrylic acid grafted POE can form hydrogen bonds (bond energy is about 20 - 40 kJ / mol) with the amino groups (-NH2) on the surface of nanocrystals, further enhancing the stability of the polar interface layer and avoiding the timeliness defect (the treatment effect decays with time) of traditional physical treatments (such as corona).

[0057] The organic long chains of KH550 and KH560 act as molecular-level lubricants, reducing the frictional resistance between nanocrystals and the PP matrix. After testing, the melt viscosity of PP filled with modified nanocrystals decreases by 25 - 35% (at 190 °C, shear rate 100 s -1 -1), from 800 mPa·s to 500 - 600 mPa·s, reducing the torque of the twin-screw extruder by 15 - 20% and the energy consumption by about 10%, while avoiding screw wear caused by filler agglomeration. The amino groups (-NH2) of KH550 have a certain free radical capture ability and can form a synergistic antioxidant network with the hindered phenolic antioxidants in the system, keeping the nanocrystals stable during high-temperature extrusion (210 °C) and avoiding the thermal degradation of the PP matrix caused by the hydroxyl groups on the surface of nanocrystals (the thermal weight loss rate decreases by 0.05% / min).

[0058] Surface modification of nanocrystalline materials with a particle size of 50 - 150 nm by KH550 and KH560 can significantly improve the compatibility and interaction between the nanocrystals and the PP matrix: The dual silane coupling agent constructs an "amino - epoxy" gradient interface layer on the surface of the nanocrystals through step - by - step hydrolysis, condensation, and ring - opening reactions. On the one hand, it forms a covalent bond (Si - O - M) with the hydroxyl groups on the surface of the nanocrystals through silane oxy groups to achieve firm anchoring. On the other hand, using the active groups of amino and epoxy groups, they respectively form a chemical bridging network with the polar graft modifiers and compatibilizers in the PP matrix through esterification reactions and hydrogen - bond interactions, increasing the interfacial shear strength between the nanocrystals and the matrix to 12 - 15 MPa, and increasing the tensile strength and notched impact strength by 20 - 30% and 15 - 25% respectively. The ultrasonic treatment, combined with the steric hindrance and electrostatic repulsion effects of the silane coupling agent, can increase the dispersion degree of the nanocrystals to more than 95%, and control the average particle size distribution within ±20 nm, avoiding stress concentration caused by agglomeration. The introduction of polar groups (-OH, -NH2) on the surface of the nanocrystals increases the surface energy of the material to 43 - 46 mN / m. When bonding with the foaming layer, urethane bonds can be formed, and the peel strength ≥4.2 N / cm. At the same time, as a heterogeneous nucleation site, it induces the formation of low - shrinkage β - crystalline form in PP. Combining with the physical constraint effect of the interface network, the shrinkage rate is reduced from 1.5 - 2.0% to 0.65 - 0.75%. In addition, the organic long chains of the silane coupling agent reduce the melt viscosity by 25 - 35%, improving the processing fluidity. The amino group and the antioxidant synergistically enhance the thermal stability, reducing the extrusion energy consumption by about 10%. This modification process systematically solves the problems of traditional PP materials such as insufficient mechanical properties, high shrinkage rate, and poor surface polarity through the multi - mechanism synergy of chemical bridging, physical dispersion, and functional regulation, providing key technical support for its replacement of ABS.

[0059] In some embodiments, the polar graft modifier is maleic anhydride - grafted PP or acrylic acid - grafted POE.

[0060] Maleic anhydride - grafted PP and acrylic acid - grafted POE systematically solve the three major problems of traditional PP materials: "difficult adhesion of non - polar surface, difficult balance of mechanical properties, and narrow processing window" through chemical bridging of polar groups, rigid - flexible regulation of the interface layer, and permanent optimization of surface energy. Through chemical bond and hydrogen - bond interactions, the dispersion degree of the nanocrystals ≥90% and the interfacial bonding strength is increased.

[0061] In some embodiments, the compatibilizer is polypropylene - grafted maleic anhydride or ethylene - acrylic acid copolymer.

[0062] Through multi-dimensional effects such as chemical bridging, physical entanglement, polarity regulation, and processing optimization between polypropylene grafted maleic anhydride and ethylene-acrylic acid copolymer, the key problems of "weak filler-matrix interface, uneven distribution of polar groups, and poor processing stability" in nanocomposites are solved. The dispersion degree of nanocrystals is ≥95%, and the interfacial bonding strength is improved; a single addition achieves multiple effects of reinforcement, toughening, and polarization, replacing the traditional multi-component compounding process.

[0063] In some embodiments, the antioxidant is a compound antioxidant containing hindered phenol and thioester.

[0064] The compound antioxidant (hindered phenol + thioester) comprehensively improves the thermal-oxidative stability of the material during processing and long-term use through a synergistic antioxidant mechanism in the nanocrystal-modified PP particles. The specific effects are as follows:

[0065] Dual mechanism for synergistic antioxidant: The hindered phenol captures alkyl radicals (R) and alkoxy radicals (ROO) by transferring active hydrogen through phenolic hydroxyl groups, inhibiting the growth of the oxidation chain; the thioester decomposes hydroperoxides (ROOH) into alcohols / ketones through thioether bonds, blocking the initiation of the oxidation chain from the source. The combination of the two forms a "radical capture-hydroperoxide decomposition" synergistic network, and the antioxidant efficiency is increased to 2-3 times that of a single antioxidant, extending the half-life of the tensile strength of the material in the 120°C oven aging test from 500 hours for pure PP to more than 2000 hours.

[0066] Thermal stability protection during the processing stage: In the high-temperature (175-210°C) environment of a twin-screw extruder, the hindered phenol quickly captures the radicals generated by high-temperature shearing, maintaining the retention rate of the weight-average molecular weight of PP ≥95%; the thioester decomposes ROOH, controls the melt flow rate (MFR) fluctuation within ±5%, and at the same time adsorbs on the surface of the nanocrystals to block their catalytic oxidation of PP, reducing the thermal weight loss rate from 0.3% / min to 0.15%, ensuring the uniformity of particle quality.

[0067] In some embodiments, the lubricant is ethylene bisstearamide or silicone.

[0068] Ethylene bisstearamide (EBS) forms a "molecular-level lubricating layer" by entangling with the PP molecular chains through non-polar long carbon chains, reducing the friction between chain segments, decreasing the melt viscosity by 20-30%, and at the same time, the polar amide groups adsorb on the surface of the equipment to form a physical adsorption film, reducing the screw torque by 15-20%, decreasing the injection pressure, and reducing the energy consumption by about 10%; silicone lubricants form a low-friction slip layer at the melt-equipment interface with their low surface energy and flexible chain segments, significantly reducing the filling resistance, especially suitable for complex structural parts, and at the same time assisting in the dispersion of nanocrystals, reducing the aggregate size to ≤150 nm and increasing the dispersion degree to 97%.

[0069] The amide groups of EBS are adsorbed on the surface of the nanocrystals through hydrogen bonds to form a "lubricant - nanocrystal" protective layer, reducing the collision wear during high - speed mixing and extrusion, with the tensile strength loss ≤ 5%; the flexible silicone segments entangle the nanocrystals, inhibiting agglomeration through steric hindrance, and the elongation at break increases to 65% at high filling levels. Both can form a low - stress transfer layer between the nanocrystals and the matrix, alleviating the interfacial stress concentration, making the failure mode turn to ductile yield, and the elongation at break increases from 30% to 80%.

[0070] In some embodiments, the nano - modified PP particles are also loaded with Ag + ions through ion exchange, where the Ag + ion loading is 0.5 - 1.0 wt%.

[0071] Loading Ag + ions (loading 0.5 - 1.0 wt%) in the nano - modified PP particles mainly endows the material with multiple functions such as high - efficiency antibacterial, long - term mildew prevention, and antioxidant synergistic enhancement. Its action mechanism and technical effects are as follows:

[0072] Ag + destroys the cell membrane permeability through charge adsorption when contacting the surface of bacteria, causing the cytoplasm to flow out and leading to the death of bacteria; at the same time, Ag + slowly releases in a humid environment (release rate about 0.01 - 0.05 ppm / day) and kills bacteria through the following ways:

[0073] Ag + combines with groups such as sulfhydryl (-SH) and amino (-NH2) in bacteria to inhibit enzyme activity (such as pyruvate dehydrogenase) and block energy metabolism; Ag + embeds into the double - helix structure of bacterial DNA, inhibits the replication of genetic material, and the antibacterial rate against common pathogenic bacteria such as Escherichia coli and Staphylococcus aureus can reach more than 99.9%.

[0074] Nano - titanium dioxide or alumina has a high specific surface area (50 - 100 m 2 / g). After being loaded with Ag + through ion exchange, a "nanocrystal - Ag + " composite antibacterial unit is formed, increasing the effective antibacterial sites per unit mass of the material to 3 - 5 times that of traditional silver - based antibacterial agents. For example, the antibacterial active surface area of nano - modified PP particles loaded with 1.0 wt% Ag + can reach 5000 m 2 / g, significantly improving the antibacterial efficiency.

[0075] In some embodiments, the nano-modified PP particles further include microcapsules containing bisphenol A epoxy resin. The particle size of the microcapsules is 50 - 100 μm, and the mass percentage is 1 - 2%.

[0076] The microcapsules are uniformly dispersed in the PP matrix. When the material generates microcracks under impact, friction, or long-term stress, the crack propagation will pierce the capsule shell (usually made of urea-formaldehyde resin or gelatin), releasing the internal bisphenol A epoxy resin to the damaged area. For example, a 0.1 - 0.5 mm crack generated in the collision of the refrigerator door panel can instantly destroy the microcapsules on the path, and the repair agent can be diffused within 5 minutes.

[0077] The epoxy resin can undergo an esterification reaction with the carboxyl group (-COOH) of the polar graft modifier (such as acrylic acid grafted POE) in the PP matrix, or form hydrogen bonds or covalent bonds with the silane coupling agent (KH550 / KH560) on the surface of the nanocrystals, ensuring the long-term stability of the repair interface.

[0078] After the repair agent is cured (usually completed within 24 hours), the tensile strength at the crack can be restored to 85 - 90% of the original material, and the notched impact strength can be restored to 90 - 95%. For example, the refrigerator drawer guide rail made of PP particles containing 1.5% microcapsules, after 100,000 times of sliding wear, the deviation of the friction coefficient after repair from the initial value is ≤5%, and the wear amount is reduced by 60%.

[0079] In some embodiments, for the preparation method of the nano-modified PP particles for refrigerator structural parts in the above embodiments of the present application, the method includes:

[0080] Disperse the nanocrystal material in ethanol, add the silane coupling agent, and after ultrasonic treatment for 60 - 80 min, dry it to obtain the modified nanocrystal material;

[0081] High-speed mix the PP matrix resin, the modified nanocrystal material, the polar graft modifier, and other additives to obtain a reaction slurry, with a rotation speed of 1000 - 1500 rpm for 8 - 10 min;

[0082] Use a twin-screw extruder to carry out segmented plasticization on the reaction slurry, with a screw rotation speed of 280 - 400 rpm, a melt pressure of 1.5 - 2.5 MPa, and a vacuum degree of -0.04~-0.1 MPa; among them, the temperature of the first zone of the twin-screw extruder is 175 °C, the temperature of the second zone is 195 °C, and the temperature of the third zone is 210 °C;

[0083] After water cooling, pelletize, with a particle diameter of 2 - 3 mm, and package after drying.

[0084] Extend the ultrasonic treatment to 60 - 80 min, completely break the original agglomerates (micrometer - scale) of nanocrystals through continuous cavitation effect, and combine with the step - by - step adsorption of silane coupling agents (KH550 / KH560) to increase the dispersion degree of nanocrystals from 80% in the traditional process to over 98%, control the average particle size distribution within ±15 nm, and avoid the uneven mechanical properties caused by agglomeration.

[0085] Adopt a two - step modification method of "amino - silane first and epoxy - silane later" to form a gradient structure of "strong - bonding layer - active cross - linking layer" on the surface of nanocrystals: the amino group of KH550 firmly anchors the nanocrystals through covalent bonds (Si - O - M), and the epoxy group of KH560 is exposed on the outside to form chemical bonds with the polar groups of the PP matrix (such as the carboxyl group of acrylic - grafted POE), increasing the interfacial shear strength from 8 MPa in the traditional single - silane modification to 15 MPa and raising the tensile strength of the composite material by 25%.

[0086] Example 1:

[0087] A preparation method of nanocrystal - modified PP particles, comprising the following steps:

[0088] 1. Double - silane surface modification of nanocrystals: Disperse 60 - nm nano - titanium dioxide nanocrystal materials in ethanol, first add KH550 amino - silane, after ultrasonic treatment for 30 min, then add KH560 epoxy - silane, and dry after ultrasonic treatment for 30 min.

[0089] 2. Preparation of premix: High - speed mix PP matrix resin (40%), modified nanocrystals (40%), acrylic - grafted POE (7%), ethylene - acrylic copolymer (4%), antioxidant (0.4%), and lubricant (0.2%) (1000 rpm, 8 min).

[0090] 3. Use a twin - screw extruder for sectional plasticization (zone 1: 175 °C, zone 2: 195 °C, zone 3: 210 °C), screw speed 320 rpm, melt pressure 2.0 MPa, and vacuum degree - 0.1 MPa.

[0091] 4. Pelletizing and post - treatment: Water - cooled pelletizing, particle diameter 2 - 3 mm, and package after drying.

[0092] Example 2:

[0093] A preparation method of nanocrystal - modified PP particles, comprising the following steps:

[0094] 1. Double - silane surface modification of nanocrystals: Disperse 100 - nm nano - titanium dioxide nanocrystal materials in ethanol, first add KH550 amino - silane, after ultrasonic treatment for 30 min, then add KH560 epoxy - silane, and dry after ultrasonic treatment for 30 min.

[0095] 2. Preparation of premix: Mix PP matrix resin (50%), modified nanocrystals (30%), acrylic-grafted POE (8%), ethylene-acrylic copolymer (5%), antioxidant (0.5%), and lubricant (0.3%) at high speed (1000 rpm, 8 min).

[0096] 3. Use a twin-screw extruder for staged plasticization (zone 1 at 175°C, zone 2 at 195°C, zone 3 at 210°C), screw speed 320 rpm, melt pressure 2.0 MPa, and vacuum degree -0.1 MPa.

[0097] 4. Pelletizing and post-treatment: Water-cooled pelletizing, particle diameter 2 - 3 mm, and packaging after drying.

[0098] Example 3:

[0099] A method for preparing nanocrystal-modified PP particles, comprising the following steps:

[0100] 1. Dual-silane surface modification of nanocrystals: Disperse 150 nm nano-titanium dioxide nanocrystal materials in ethanol, first add KH550 amino-silane, after ultrasonic treatment for 30 min, then add KH560 epoxy-silane, and dry after ultrasonic treatment for 30 min.

[0101] 2. Preparation of premix: Mix PP matrix resin (60%), modified nanocrystals (25%), acrylic-grafted POE (9%), ethylene-acrylic copolymer (6%), antioxidant (0.6%), and lubricant (0.4%) at high speed (1000 rpm, 10 min).

[0102] 3. Use a twin-screw extruder for staged plasticization (zone 1 at 175°C, zone 2 at 195°C, zone 3 at 210°C), screw speed 320 rpm, melt pressure 2.0 MPa, and vacuum degree -0.1 MPa.

[0103] 4. Pelletizing and post-treatment: Water-cooled pelletizing, particle diameter 2 - 3 mm, and packaging after drying.

[0104] Figure 1 Schematic diagram of the horizontal contact angle of the nanocrystal-modified PP particles prepared for Example 1.

[0105] Perform performance analysis on Examples 1 to 3, and the specific test results are shown in Table 1.

[0106] Table 1

[0107]

[0108] As can be seen from Table 1, the nanocrystal-modified PP particles prepared in the above embodiments have the following properties: tensile strength of 36 - 42 MPa, notched impact strength greater than 28 - 33 kJ / m 2 , shrinkage rate of 0.65 - 0.75%, density less than 0.97 g / cm 3 , good surface polarity, and can be directly bonded to the foaming layer. This material not only has excellent impact resistance, tensile property and low shrinkage rate, but also has good surface polarity, does not require corona or plasma treatment, simplifies the process flow, reduces the production cost, and is suitable for manufacturing components such as refrigerator end covers and door frames.

[0109] As can be seen from the above embodiments, the present application provides a nanomodified PP particle for refrigerator structural parts and a preparation method thereof. The nanomodified PP particles include the following components: polypropylene resin, nanocrystal material, polar graft modifier, compatibilizer, antioxidant and lubricant; wherein, the polypropylene resin is a high-flowability copolymer PP particle, with a mass percentage of 65 - 80%; the nanocrystal material is nanometer titanium dioxide or nanometer aluminum oxide, with a mass percentage of 5 - 12%; the mass percentage of the polar graft modifier is 4 - 10%; the mass percentage of the compatibilizer is 3 - 6%; the mass percentage of the antioxidant is 0.2 - 0.6%; the mass percentage of the lubricant is 0.1 - 0.4%. Through the nanocrystal modification technology, while the PP particles obtain excellent impact resistance, tensile property, low shrinkage rate and good surface polarity, they also maintain the excellent properties of the PP material itself, such as low price, good processability, strong chemical corrosion resistance, etc. The modified PP particles have excellent impact resistance and tensile property, can meet the requirements of components such as refrigerator end covers and door frames for material toughness, and improve the durability and safety of the products.

[0110] For the similar parts between the embodiments provided in the present application, reference can be made to each other. The specific embodiments provided above are only several examples under the general concept of the present application, and do not constitute a limitation to the protection scope of the present application. For those skilled in the art, any other embodiments expanded based on the solution of the present application without creative efforts fall within the protection scope of the present application.

Claims

1. A nano-modified PP particle for a refrigerator structural member, characterized in that, The nano-modified PP particles include the following components: Polypropylene resin, nano-crystalline material, polar graft modifier, compatibilizer, antioxidant and lubricant; Among them, the polypropylene resin is a high-flow copolymer PP particle with a mass percentage of 65-80%; the nano-crystalline material is nano-titanium dioxide or nano-aluminum oxide with a mass percentage of 5-12%; the polar graft modifier has a mass percentage of 4-10%; the compatibilizer has a mass percentage of 3-6%; the antioxidant has a mass percentage of 0.2-0.6%; the lubricant has a mass percentage of 0.1-0.4%.

2. The nano-modified PP particles for the refrigerator structural parts according to claim 1, characterized in that, The particle size of the nano-crystalline material is 50-150 nm and is surface-modified with silane coupling agent KH-550 and / or KH-560.

3. The nano-modified PP particles for the refrigerator structural parts according to claim 1, wherein, The polar graft modifier is maleic anhydride grafted PP or acrylic acid grafted POE.

4. The nano-modified PP particles for refrigerator structural parts according to claim 1, characterized in that The compatibilizer is polypropylene grafted maleic anhydride or ethylene-acrylic acid copolymer.

5. The nano-modified PP particles for the refrigerator structural parts according to claim 1, characterized in that, The antioxidant is a compound antioxidant containing hindered phenol and thioester.

6. The nano-modified PP particles for the refrigerator structural parts according to claim 1, characterized in that, The lubricant is ethylene bis-stearamide or silicone.

7. The nano-modified PP particles for the refrigerator structural parts according to claim 2, wherein, The nano-modified PP particles are also loaded with Ag ions through ion exchange, where the Ag ion loading is 0.5 - 1.0 wt%. + ions, where Ag + ion loading is 0.5 - 1.0 wt%.

8. The nano-modified PP particles for the refrigerator structural member according to claim 2, wherein, The nano-modified PP particles also include microcapsules containing bisphenol A epoxy resin, the particle size of the microcapsules is 50-100 μm, and the mass percentage is 1-2%.

9. A preparation method of the nano-modified PP particles for the refrigerator structural parts according to any one of claims 1-8, characterized in that, The method includes: Disperse the nano-crystalline material in ethanol, add a silane coupling agent, and after ultrasonic treatment for 60-80 min, dry to obtain the modified nano-crystalline material; High-speed mix the PP matrix resin, modified nano-crystalline material, polar graft modifier, compatibilizer, antioxidant and lubricant to obtain a reaction slurry, where the rotation speed is 1000-1500 rpm for 8-10 min; Use a twin-screw extruder to carry out staged plasticization of the reaction slurry, with a screw rotation speed of 280-400 rpm, a melt pressure of 1.5-2.5 MPa, and a vacuum degree of -0.04 to -0.1 MPa; among them, the temperature of the first zone of the twin-screw extruder is 175 °C, the temperature of the second zone is 195 °C, and the temperature of the third zone is 210 °C; Cool with water and then pelletize, with a particle diameter of 2-3 mm, and package after drying.

10. The method according to claim 9, characterized in that, Dispersing the nano-crystalline material in ethanol and adding a silane coupling agent also includes: Disperse the nano-crystalline material in ethanol, first add KH550 amino silane, after ultrasonic treatment for 30-40 min, then add KH560 epoxy silane, and after ultrasonic treatment for 30-40 min, dry.

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