High-weather-resistance UV (ultraviolet) stable PP (polypropylene) cardboard box material and preparation method thereof

Abstract

The invention relates to a high-weather-resistance UV stable PP cardboard box material and a preparation method thereof, and relates to the technical field of high polymer material modification, and the material is prepared from block copolymerized COPP, homopolymerized PP, a hindered phenol antioxidant, a phosphite ester auxiliary antioxidant, a thioester antioxidant, a hindered amine light stabilizer, a high-molecular-weight hindered amine light stabilizer and a UV-resistant toner. The preparation method comprises the steps of raw material drying, high-speed mixing, twin-screw extrusion granulation and injection molding. According to the invention, the dosage of the thioester antioxidant is accurately controlled, and the double light stabilizers are compounded, so that the problem of resistance to inactivation of the acidic thioester auxiliary and the alkaline hindered amine light stabilizer is effectively solved, and meanwhile, a rigid-tough balanced matrix is constructed by using COPP and homopolymerized PP. The obtained material has excellent ultraviolet aging resistance, long-term thermal stability and good impact strength, and is suitable for outdoor logistics turnover scenes.

Description

Technical Field

[0001] This application relates to the field of polymer material modification technology, and in particular to a high weather-resistant UV-stable PP cardboard box material and its preparation method. Background Technology

[0002] Polypropylene (PP) is widely used in the logistics packaging industry, especially as a primary raw material for pallet boxes, due to its advantages such as low density, good chemical resistance, ease of processing and molding, and recyclability. However, pallet boxes often face complex outdoor environments during logistics handling, being exposed to sunlight, high temperatures, and humidity for extended periods. The polypropylene molecular chain contains a large number of tertiary carbon atoms with low bond energy, making it extremely sensitive to light and heat. Under ultraviolet radiation and thermo-oxidative conditions, it readily undergoes auto-oxidation, leading to molecular chain breakage and degradation. Macroscopically, this manifests as powdering, cracking, severe discoloration on the product surface, and a sharp decline in mechanical properties such as impact strength, thus shortening the lifespan of the pallet box and increasing logistics costs.

[0003] To improve the weather resistance and thermal stability of polypropylene materials, existing technologies typically add antioxidants and light stabilizers to the resin matrix. For resistance to thermo-oxidative aging, thioester antioxidants, as highly efficient auxiliary antioxidants, are often used in combination with hindered phenolic primary antioxidants to significantly improve the long-term thermal stability of the material. For resistance to UV aging, hindered amine light stabilizers are widely recognized as the most effective anti-aging additives due to their highly efficient free radical scavenging ability. However, in practical applications, there is a significant chemical antagonism between thioester antioxidants and hindered amine light stabilizers. Thioester antioxidants produce acidic byproducts during the decomposition of peroxides, while hindered amine light stabilizers are usually alkaline. When the two coexist, an acid-base neutralization reaction easily occurs, generating inactive ammonium salts. This reaction not only consumes the effective components of the additives but also directly leads to the deactivation of the light stabilizers, making it impossible for the material to simultaneously achieve excellent thermal and light stability.

[0004] Furthermore, as large load-bearing containers, pallet boxes have special requirements for the mechanical properties of materials. The materials need to possess sufficient rigidity to support the weight of stacked goods and prevent deformation, while also exhibiting excellent toughness to withstand drops, collisions, and forklift operations during transportation. While homopolymer polypropylene has good rigidity, it suffers from poor low-temperature toughness and is prone to brittleness; whereas block copolymer polypropylene, while possessing good toughness, lacks sufficient rigidity and is susceptible to creep. Therefore, how to construct a matrix resin system with a balance of rigidity and toughness while addressing the antagonistic effects of additives is a crucial technical challenge that urgently needs to be solved in the development of high-performance pallet box materials.

[0005] To address the aforementioned related technologies, a high weather-resistant UV-stable PP cardboard box material and its preparation method are provided. Summary of the Invention

[0006] The purpose of this application is to provide a high weather-resistant UV-stable PP pallet box material and its preparation method, aiming to improve the existing polypropylene pallet box material's photothermal degradation problem during outdoor use, as well as the technical defects of sulfur ester antioxidants and hindered amine light stabilizers failing due to acid-base antagonism, and the difficulty of achieving a balance between rigidity and toughness with a single matrix.

[0007] By adopting the above technical solution, a high weather-resistant UV-stable PP pallet box material is obtained, which is made of the following components in parts by weight:

[0008] Block copolymer COPP: 70.0–80.0 parts;

[0009] Homopolymer PP: 20.0–28.0 parts;

[0010] Hindered phenolic antioxidants: 0.15–0.35 parts;

[0011] Phosphite-based auxiliary antioxidants: 0.15–0.35 parts;

[0012] Thioester antioxidants: 0.20–0.40 parts;

[0013] Hindered amine light stabilizer: 0.15–0.35 parts;

[0014] High molecular weight hindered amine light stabilizer: 0.15–0.35 parts;

[0015] UV-resistant pigment: 0.30-0.60 parts.

[0016] Preferably, the hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]; and the phosphite auxiliary antioxidant is tris(2,4-di-tert-butylphenyl) phosphite.

[0017] Preferably, the thioester antioxidant is octadecyl 3,3'-thiodipropionate, CAS number 693-36-7.

[0018] Preferably, the hindered amine light stabilizer is bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, CAS number 52829-07-9.

[0019] Preferably, the UV-resistant pigment includes titanium dioxide, black pigment, yellow pigment, and green pigment.

[0020] Preferably, the material is made of the following components in parts by weight: 75 parts block copolymer COPP, 23 parts homopolymer PP, 0.25 parts hindered phenolic antioxidant, 0.25 parts phosphite auxiliary antioxidant, 0.3 parts thioester antioxidant, 0.25 parts hindered amine light stabilizer, 0.25 parts high molecular weight hindered amine light stabilizer, and 0.45 parts UV resistant pigment.

[0021] A method for preparing a high weather-resistant UV-stable PP pallet box material includes the following steps:

[0022] S1. Raw material pretreatment: The block copolymer COPP and homopolymer PP are dried.

[0023] S2. Mixing: The dried block copolymer COPP, homopolymer PP, hindered phenolic antioxidants, phosphite auxiliary antioxidants, thioester antioxidants, hindered amine light stabilizers, high molecular weight hindered amine light stabilizers and UV-resistant pigments are added to a high-speed mixer and mixed evenly to obtain a premix.

[0024] S3. Extrusion granulation: The premixed material is added to a twin-screw extruder for melt extrusion, and then cooled and granulated to obtain modified PP granules;

[0025] S4. Injection Molding: After drying, the modified PP granules are added to the injection molding machine, and the finished product is obtained after injection molding, pressure holding and cooling.

[0026] Preferably, the drying temperature in step S1 is 80℃~90℃ and the time is 2.0h~3.0h; the speed of the mixer in step S2 is 1000rpm~1400rpm and the mixing time is 5min~8min.

[0027] Preferably, the temperature setting range of the twin-screw extruder in step S3 is: 160℃~170℃ for the feeding section, 170℃~180℃ for the compression section, 180℃~190℃ for the homogenization section, and 175℃~185℃ for the die head; the screw speed is 200rpm~300rpm.

[0028] Preferably, in step S4, the injection temperature is 175℃~195℃, the mold temperature is 45℃~55℃, the injection pressure is 85MPa~95MPa, and the holding time is 10s~20s.

[0029] In summary, this application includes at least one of the following beneficial technical effects:

[0030] 1. This application effectively solves the antagonistic problem between acidic thioester antioxidants and basic hindered amine light stabilizers in traditional technologies by precisely controlling the amount of thioester antioxidants and introducing high molecular weight hindered amine light stabilizers. This design retains the auxiliary strengthening effect of thioester additives on thermal stability while avoiding the failure of light stabilizers, thus achieving a balance between thermal stability and light stability.

[0031] 2. This application constructs a matrix resin system with a balance of rigidity and toughness by using a high proportion of block copolymer COPP as a tough matrix and an appropriate amount of homopolymer PP as a rigid support; COPP gives the pallet box excellent impact resistance and prevents drop damage; while homopolymer PP ensures the load-bearing capacity and stacking strength of the box, meeting the stringent mechanical requirements of logistics turnover.

[0032] 3. This application constructs a ternary synergistic antioxidant system composed of hindered phenols, phosphites, and thioesters; this system can comprehensively inhibit the thermal oxidative degradation of polypropylene during high-temperature processing and outdoor use, effectively prevent yellowing and embrittlement of the material due to thermal aging, and ensure the processing stability and long-term heat resistance of the material. Detailed Implementation

[0033] Example 1:

[0034] A high weather-resistant UV-stable PP pallet box material includes:

[0035] Block copolymer polypropylene: 75.0 parts

[0036] Homopolymer polypropylene: 23.0 parts

[0037] Hindered phenolic antioxidant: 0.25 parts

[0038] Phosphite-based auxiliary antioxidant: 0.25 parts

[0039] Thioester antioxidant: 0.30 parts

[0040] Hindered amine light stabilizer: 0.25 parts

[0041] High molecular weight hindered amine light stabilizer (model 944): 0.25 parts

[0042] UV-resistant pigment: 0.45 parts

[0043] Preparation steps:

[0044] Raw material pretreatment: Block copolymer COPP granules and homopolymer PP granules are placed in a forced-air drying oven, and the drying temperature is set to 85℃ for 2.5 hours to remove moisture from the raw materials.

[0045] Mixing: Weigh each component according to the above formula. Add the dried resin, antioxidant, light stabilizer, and colorant to a high-speed mixer. Set the mixer speed to 1200 rpm and the mixing time to 6 minutes to ensure the additives are evenly adhered to the resin surface, thus obtaining the premix.

[0046] Extrusion granulation: The premixed material is added to the main feed hopper of a twin-screw extruder. The temperature settings for each zone of the extruder are as follows: feed section 165℃, compression section 175℃, homogenization section 185℃, and die head 180℃. The screw speed is set to 250 rpm. After melting and mixing, the material is extruded into strips, cooled by circulating water, air-dried, and then granulated to obtain modified PP granules.

[0047] Injection molding: Modified PP granules were dried again at 85℃ for 2 hours and then added to the injection molding machine. The injection molding process parameters were set as follows: melt temperature 185℃, mold temperature 50℃, injection pressure 90MPa, and holding time 15 seconds. After cooling and solidification, the mold was opened to obtain test samples of the pallet box material.

[0048] Example 2:

[0049] A high weather-resistant UV-stable PP pallet box material includes:

[0050] Block copolymer polypropylene: 70.0 parts

[0051] Homopolymer polypropylene: 20.0 parts

[0052] Hindered phenolic antioxidant: 0.15 parts

[0053] Phosphite-based auxiliary antioxidant: 0.15 parts

[0054] Thioester antioxidant: 0.20 parts

[0055] Hindered amine light stabilizer: 0.15 parts

[0056] High molecular weight hindered amine light stabilizer (model 944): 0.15 parts

[0057] UV-resistant pigment: 0.30 parts

[0058] Preparation steps:

[0059] Raw material pretreatment: Place block copolymer COPP granules and homopolymer PP granules in a forced-air drying oven, set the drying temperature to 80℃, and the drying time to 2.0 hours.

[0060] Mixing: Weigh each component according to the above formula. Add all raw materials to a high-speed mixer. Set the mixer speed to 1000 rpm and the mixing time to 5 minutes to obtain the premix.

[0061] Extrusion granulation: The premixed material is added to a twin-screw extruder. The temperature settings for each zone of the extruder are as follows: feed section 160℃, compression section 170℃, homogenization section 180℃, and die head 175℃. The screw speed is set to 200 rpm. The material is extruded, water-cooled, and pelletized to obtain modified PP granules.

[0062] Injection molding: The modified PP granules were dried and then added to the injection molding machine. The injection molding process parameters were set as follows: melt temperature 175℃, mold temperature 45℃, injection pressure 85MPa, and holding time 10 seconds. After cooling and solidification, the mold was opened to obtain test samples of the pallet box material.

[0063] Example 3:

[0064] A high weather-resistant UV-stable PP pallet box material includes:

[0065] Block copolymer polypropylene: 80.0 parts

[0066] Homopolymer polypropylene: 28.0 parts

[0067] Hindered phenolic antioxidant: 0.35 parts

[0068] Phosphite-based auxiliary antioxidant: 0.35 parts

[0069] Thioester antioxidants: 0.40 parts

[0070] Hindered amine light stabilizer: 0.35 parts

[0071] High molecular weight hindered amine light stabilizer (model 944): 0.35 parts

[0072] UV-resistant pigment: 0.60 parts

[0073] Preparation steps:

[0074] Raw material pretreatment: Place block copolymer COPP granules and homopolymer PP granules in a forced-air drying oven, set the drying temperature to 90℃, and the drying time to 3.0 hours.

[0075] Mixing: Weigh each component according to the above formula. Add all raw materials to a high-speed mixer. Set the mixer speed to 1400 rpm and the mixing time to 8 minutes to obtain the premix.

[0076] Extrusion granulation: The premixed material is added to a twin-screw extruder. The temperature settings for each zone of the extruder are as follows: feed section 170℃, compression section 180℃, homogenization section 190℃, and die head 185℃. The screw speed is set to 300 rpm. The material is extruded, water-cooled, and pelletized to obtain modified PP granules.

[0077] Injection molding: The modified PP granules were dried and then added to the injection molding machine. The injection molding process parameters were set as follows: melt temperature 195℃, mold temperature 55℃, injection pressure 95MPa, and holding time 20 seconds. After cooling and solidification, the mold was opened to obtain test samples of the pallet box material.

[0078] Comparative Example 1:

[0079] Compared with Example 1, the difference is that the amount of thioester antioxidant is increased from 0.30 parts to 1.0 parts, while the other components and preparation process parameters are the same.

[0080] Comparative Example 2:

[0081] Compared with Example 1, the difference is that no sulfur ester antioxidant was added, while the other components and preparation process parameters are the same.

[0082] Comparative Example 3:

[0083] Compared with Example 1, the difference is that the high molecular weight hindered amine light stabilizer is removed and the amount of hindered amine light stabilizer is increased to 0.50 parts, while the other components and preparation process parameters are the same.

[0084] Comparative Example 4:

[0085] Compared with Example 1, the difference is that 75.0 parts of block copolymer polypropylene were completely replaced with an equal amount of homopolymer polypropylene, while the remaining components and preparation process parameters were the same.

[0086] Comparative Example 5:

[0087] Compared with Example 1, the difference is that the raw material pretreatment in the preparation step is omitted, and the undried raw material is directly put into the mixer for subsequent operations. The remaining components and preparation process parameters are the same.

[0088] Test Example 1: Basic Mechanical Property Test

[0089] This test case aims to determine the tensile strength, flexural modulus, and notched impact strength of the materials prepared in each embodiment and comparative example, in order to characterize their basic mechanical properties.

[0090] Experimental steps:

[0091] Sample preparation: The injection-molded standard samples prepared in Examples 1-3 and Comparative Examples 1-5 were placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for conditioning for 48 hours.

[0092] Tensile strength test: A universal testing machine was used to perform tensile tests on dumbbell-shaped specimens. The tensile speed was set to 50 mm / min, and the maximum stress value at which the specimen broke was recorded. Five parallel specimens were tested in each group, and the arithmetic mean was taken as the final result.

[0093] Bending modulus test: A universal testing machine was used to perform a three-point bending test on the long strip specimen. The span was set to 50 mm, the loading speed was 2 mm / min, the bending modulus was calculated, and 5 parallel specimens were tested in each group, and the arithmetic mean was taken.

[0094] Impact strength test of cantilever beam with notch: An impact testing machine was used to conduct impact tests on pre-notched specimens. The pendulum energy was set to 2.75 J, the test temperature was 23℃, and the energy absorbed when the specimen fractured was recorded. Ten parallel samples were tested in each group, and the arithmetic mean was taken.

[0095] Test results:

[0096] The basic mechanical property test results of each embodiment and comparative example are shown in Table 1.

[0097] Table 1: Basic Mechanical Property Test Data of Materials in Each Group

[0098] Group Tensile strength Flexural modulus Notched impact strength of cantilever beam (kJ / m²) Example 1 27.4 1285 42.6 Example 2 26.8 1210 46.2 Example 3 28.1 1340 38.4 Comparative Example 1 27.2 1290 42.1 Comparative Example 2 27.5 1280 41.9 Comparative Example 3 27.3 1288 42.3 Comparative Example 4 34.6 1580 4.2 Comparative Example 5 24.1 1150 28.5

[0099] Results analysis:

[0100] Table 1 shows that Examples 1-3 maintained tensile strength, flexural modulus, and notched impact strength within specific ranges, without any obvious performance shortcomings. Example 1 exhibited a tensile strength of 27.4 MPa, a flexural modulus of 1285 MPa, and a cantilever beam notched impact strength of 42.6 kJ / m², indicating that the formulation system achieved a balance between rigidity and toughness. In Example 2, the impact strength increased to 46.2 kJ / m² with increasing block copolymer polypropylene content, while in Example 3, the flexural modulus increased to 1340 MPa with increasing homopolymer polypropylene content, consistent with the component variation pattern of polypropylene matrix materials.

[0101] Comparative Example 4 used homopolymer polypropylene as the matrix resin. Although its tensile strength and flexural modulus were significantly higher than those of the Example, its cantilever beam notched impact strength was only 4.2 kJ / m². The toughness of polypropylene materials mainly comes from the energy absorption effect of the ethylene-propylene rubber phase in the block copolymer. Due to the lack of a rubber phase structure, homopolymer polypropylene cannot effectively dissipate energy under high-speed impact, exhibiting brittle fracture characteristics. This invention introduces a specific proportion of block copolymer polypropylene, utilizing the rubber microregions to induce crazes and shear bands, significantly improving the material's impact resistance to meet the needs of pallet boxes in outdoor drop scenarios.

[0102] Comparative Example 5 omitted the raw material drying step, resulting in significantly lower mechanical properties compared to Example 1, with the impact strength decreasing to 28.5 MPa. This is because residual moisture in the raw material vaporizes during high-temperature melt extrusion, forming micropores or bubbles within the polymer matrix, disrupting the material's continuity and density, and becoming stress concentration points. Furthermore, moisture may induce hydrolysis and other degradation reactions in polypropylene or additives, leading to a decrease in molecular weight and consequently weakening the material's macroscopic mechanical strength. Therefore, rigorous raw material pretreatment is essential to ensuring stable material performance.

[0103] Test Example 2: Artificial Accelerated UV Aging Performance Test

[0104] This test case aims to determine the aging resistance of the materials prepared in each example and comparative example under ultraviolet irradiation and condensation environment, and to evaluate the long-term weather resistance stability of the materials by color difference change and mechanical property retention rate.

[0105] Experimental steps:

[0106] Sample preparation: Injection-molded color plates prepared for each embodiment and comparative example were selected for color difference testing, and cantilever beam notched impact specimens were selected for mechanical retention testing. Five parallel specimens were prepared for each group.

[0107] Initial value determination: The initial values ​​of each group of color plates before aging were determined using a spectrophotometer. Value. Perform notched impact strength tests on the impact spline and record the initial impact strength value.

[0108] Aging Test: The samples are fixed on the sample rack of the UV aging test chamber. The test program is set, using a UV lamp as the light source. Each aging cycle includes: 8 hours of UV irradiation at 60°C, followed by 4 hours of condensation at 50°C. The total test run time is 500 hours.

[0109] Post-aging test: After the test, the samples were removed and conditioned for 24 hours under standard conditions. The color of the aged sample was measured using a spectrophotometer. Values, and calculate color differences according to the formula. The impact strength of the aged impact specimens was tested with notched impact strength, and the impact strength retention rate was calculated as (aged value / initial value) × 100%.

[0110] Test results:

[0111] The results of the artificial accelerated UV aging performance tests for each embodiment and comparative example are shown in Table 2.

[0112] Table 2: Test data of accelerated UV aging performance of materials in each group

[0113] Group Initial color difference Color difference after 500 hours of aging Initial impact strength (kJ / m²) Impact strength (kJ / m²) after 500 hours of aging Impact strength retention rate Example 1 0 1.12 42.6 40.5 95.1 Example 2 0 1.25 46.2 43.1 93.3 Example 3 0 1.08 38.4 36.8 95.8 Comparative Example 1 0 3.84 42.1 24.6 58.4 Comparative Example 2 0 1.15 41.9 39.8 95 Comparative Example 3 0 2.65 42.3 31.4 74.2 Comparative Example 4 0 1.05 4.2 3.9 92.9 Comparative Example 5 0 1.88 28.5 16.2 56.8

[0114] Results analysis:

[0115] Table 2 shows that after 500 hours of UV aging, the color difference in Examples 1-3 was... All values ​​were controlled below 1.3, and the impact strength retention rate was above 93%, demonstrating excellent weather resistance. In contrast, in Comparative Example 1, the amount of sulfur ester antioxidant added was increased to 1.0 part, resulting in color difference after aging. The impact strength retention rate dropped sharply to 58.4%, surging to 3.84. Thioester antioxidants are peroxide decomposers, producing acidic byproducts during the decomposition of hydrogen peroxide. Hindered amine light stabilizers are basic compounds with a secondary or tertiary amine group on the piperidine ring as their active center. When the concentration of thioester antioxidants in the system is too high, the acidic byproducts react with the basic hindered amines in an acid-base neutralization reaction, forming ammonium salt precipitates with no photostable activity. This inhibits the hindered amines' ability to capture free radicals, leading to rapid photo-oxidative degradation of the polypropylene matrix under ultraviolet light, macroscopically manifested as discoloration and embrittlement. This invention effectively avoids the acid-base antagonistic effect by strictly controlling the amount of thioester antioxidants within a low range of 0.20–0.40 parts, thus assisting in antioxidant function without sacrificing photostable stability.

[0116] Comparative Example 3 removed the high molecular weight hindered amine light stabilizer and used only a low molecular weight hindered amine light stabilizer. Its color difference after aging was 2.65, and its impact strength retention rate was 74.2%, significantly lower than Example 1. Although low molecular weight hindered amines migrate quickly in the polymer matrix and can rapidly replenish the surface to protect against initial aging, their small molecular weight, volatility, and poor extraction resistance mean they are easily lost from the matrix under prolonged light exposure and condensation. High molecular weight hindered amines, due to their long molecular chains, have a strong anchoring effect in the matrix, exhibiting excellent anti-migration and extraction resistance, and providing long-lasting protection. This invention employs a technical solution of combining low molecular weight and high molecular weight hindered amines. Utilizing their synergistic effect, the low molecular weight component provides rapid surface protection, while the high molecular weight component ensures durable bulk protection, thereby achieving ultra-stability of the pallet box material throughout its entire life cycle.

[0117] Comparative Example 2, without the addition of thioester antioxidants, exhibited photoaging performance comparable to Example 1, indicating that the light stabilizer was not affected in the absence of thioester antioxidants. However, data from Test Example 3 shows that the lack of thioester antioxidants leads to insufficient thermal stability of the material. The formulation design of this invention aims to find a balance point: introducing an appropriate amount of thioester antioxidants to construct a complete "phenol-phosphorus-sulfur" ternary antioxidant system, while limiting the amount of thioesters below the threshold that does not induce hindered amine deactivation, thereby balancing the material's heat resistance and weather resistance.

[0118] Test Example 3: Thermal Stability Test

[0119] This test case aims to determine the thermal decomposition behavior of the materials prepared in each embodiment and comparative example under high temperature conditions. The 5% thermogravimetric temperature index obtained by thermogravimetric analysis is used to quantitatively evaluate the thermal oxidation stability of the materials.

[0120] Experimental steps:

[0121] Sample preparation: Small pieces weighing approximately 5 mg to 10 mg were cut from the middle portion of the injection-molded specimens prepared in each example and comparative example as test samples. The samples must be clean and free of oil or impurities.

[0122] Equipment calibration: Use standard materials to calibrate the temperature and weight axes of the thermogravimetric analyzer to ensure that baseline drift is within the instrument's allowable error range.

[0123] Test procedure: Place the prepared sample in an alumina crucible and then place it inside the furnace of the thermogravimetric analyzer. High-purity nitrogen is introduced as a protective atmosphere, with a gas flow rate set to 50 mL / min. After the system stabilizes, start the heating program, raising the sample temperature from room temperature to 600℃ at a heating rate of 10℃ / min.

[0124] Data recording: The instrument automatically records the curve of sample weight change with temperature. The software analyzes the data to determine the temperature point at which the sample weight loss reaches 5% of the initial weight, denoted as T-5%.

[0125] Test results:

[0126] The thermal stability test results of each embodiment and comparative example are shown in Table 3.

[0127] Table 3: Thermal stability test data of each group of materials

[0128] Group T-5%(℃) Example 1 378.5 Example 2 376.2 Example 3 380.1 Comparative Example 1 379.4 Comparative Example 2 358.7 Comparative Example 3 377.9 Comparative Example 4 382.4 Comparative Example 5 345.6

[0129] Results analysis:

[0130] Table 3 shows that the 5% thermogravimetric temperature (T-5%) of Examples 1-3 remained stable above 376℃, with Example 1 reaching 378.5℃, indicating that the formulation system has good thermal stability. In contrast, Comparative Example 2, without the addition of thioester antioxidants, showed a significant reduction in T-5% to 358.7℃, nearly 20℃ lower than Example 1. During thermal processing and high-temperature use, the carbon chain of polypropylene materials is prone to breakage, generating active free radicals and hydroperoxides. Hydroperoxides are unstable intermediates; their thermal decomposition triggers new free radical chain reactions, accelerating the auto-oxidative degradation of the material. Thioester antioxidants, as auxiliary antioxidants, function primarily as peroxide decomposers, reducing hydroperoxides to inactive alcohols, thereby interrupting the branching reactions that promote free radical proliferation. Example 1 constructed a ternary synergistic antioxidant system containing hindered phenols, phosphites, and thioester antioxidants. Compared to the binary system of Comparative Example 2, this system more comprehensively inhibits the thermal oxidative degradation process and significantly improves the material's thermal decomposition temperature.

[0131] Although Comparative Example 1 added an excessive amount of thioester antioxidants, its T-5% was 379.4℃, a slight improvement compared to Example 1. This indicates that, within a certain range, the improvement in thermal stability by thioester antioxidants exhibits diminishing marginal returns. Combined with the results of Test Example 2, it can be seen that while excessive thioester antioxidants maintained high thermal stability, it came at the significant cost of sacrificing weather resistance. This invention, through experimental screening, determined a specific dosage range of 0.20–0.40 parts, aiming to achieve the optimal balance between thermal stability and weather resistance. This ensures that the material can withstand injection molding and high-temperature outdoor environments while avoiding performance degradation caused by additive antagonism.

[0132] Comparative Example 5, having omitted the drying step, exhibited the lowest T-5%, at only 345.6℃. This further confirms the destructive effect of residual moisture on the polymer matrix. At high temperatures, moisture not only triggers hydrolysis leading to a decrease in molecular weight, but also causes antioxidants and other additives to partially fail during the melting process due to hydrolysis, weakening the protective efficiency of the antioxidant system. Therefore, a rigorous dehumidification and drying process is the physical basis for ensuring the thermal stability of materials and is of equal importance to chemical formulation design.

[0133] Test Example 4: Evaluation of Appearance and Processing Quality

[0134] This test case aims to conduct a macroscopic evaluation of the surface condition of the injection-molded specimens and finished cardboard boxes prepared in each embodiment and comparative example through visual observation, so as to qualitatively analyze the processing adaptability and appearance defects of the materials.

[0135] Experimental steps:

[0136] Sample selection: Ten tensile specimens after injection molding and three actual injection-molded pallet box products were randomly selected from each group's preparation process as test samples.

[0137] Observation environment: Place the sample in a standard light source box, set the illuminance to 1000 Lux, and keep the observation distance between 30 cm and 50 cm.

[0138] Evaluation indicators and methods: Three trained inspection personnel independently conducted a visual inspection of the sample surface, focusing on observing for the following defects:

[0139] Silver streaks: Silvery-white stripes that appear on the surface following the direction of flow.

[0140] Bubbles: Tiny voids that exist inside or on the surface of a part.

[0141] Flow marks: wavy or fingerprint-like marks that appear on the surface.

[0142] Uneven color: The surface color varies in shade or there are clusters of colored spots.

[0143] Comprehensive judgment criteria:

[0144] Excellent (A): The surface is smooth and flat, the color is uniform, and no of the above defects were found.

[0145] Good (B): The surface is free of obvious silver streaks and bubbles. Slight flow marks or very minor differences in gloss are allowed in the microscopic area, but they do not affect the overall appearance.

[0146] Poor (C): The surface has visible silver streaks, bubbles, obvious flow marks or discoloration, affecting the appearance quality.

[0147] Test results:

[0148] The evaluation results of the appearance and processing quality of each embodiment and comparative example are shown in Table 4.

[0149] Table 4: Evaluation data of material appearance and processing quality for each group

[0150] Group Silver thread phenomenon Bubble phenomenon Flow marks / uneven color Comprehensive assessment level Example 1 none none none A (Excellent) Example 2 none none none A (Excellent) Example 3 none none none A (Excellent) Comparative Example 1 none none none A (Excellent) Comparative Example 2 none none none A (Excellent) Comparative Example 3 none none none A (Excellent) Comparative Example 4 none none slight flow marks at the gate B (good) Comparative Example 5 obvious small amount Local color difference C (Poor)

[0151] Results analysis:

[0152] Table 4 shows that Examples 1-3 and most comparative examples achieved excellent quality in terms of appearance, indicating that the formulation components of this invention have good compatibility with the polypropylene matrix, and the extrusion and injection molding process parameters are reasonably matched, enabling uniform plasticization and mold filling of the material. Comparative Example 4 was rated as good, mainly because its matrix was entirely homopolymer polypropylene, and its melt viscosity characteristics differed from those of copolymer polypropylene. Under the same injection molding process, the crystallization rate of homopolymer polypropylene was faster, which led to slight condensation marks at the mold gate, but the overall appearance was still acceptable.

[0153] Comparative Example 5 was rated as having poor appearance quality, exhibiting obvious silver streaks and a small number of bubbles. The mechanism of silver streaks is mainly related to the volatile components in the raw materials. When the polypropylene raw material is not sufficiently dried, the adsorbed moisture instantly vaporizes in the high-temperature barrel of the injection molding machine, forming high-pressure water vapor. These gases mix into the melt, and when injected into the mold cavity and contacting the cooled mold wall, the bubbles are stretched and burst, forming silvery-white thread-like streaks on the surface of the part. At the same time, the presence of gases also interferes with the normal flow of the melt and the dispersion of pigments, leading to localized color differences.

[0154] This result verifies the indispensability of the drying step in the preparation process from a macroscopic physical perspective. While optimizing the chemical formulation is key to achieving high material performance, proper physical processing is a prerequisite for ensuring the functionality of the chemical formulation. Moisture not only causes appearance defects but, as shown in Test Examples 1 and 3, weakens the mechanical strength and thermal stability of the material through hydrolytic degradation mechanisms. Therefore, the drying process parameters defined in this invention are one of the key technical features for ensuring the overall quality stability of pallet box materials.

Claims

1. A high weather-resistant UV-stable PP pallet box material, characterized in that, The material is made from the following components in parts by weight: Block copolymer COPP: 70.0–80.0 parts; Homopolymer PP: 20.0–28.0 parts; Hindered phenolic antioxidants: 0.15–0.35 parts; Phosphite-based auxiliary antioxidants: 0.15–0.35 parts; Thioester antioxidants: 0.20–0.40 parts; Hindered amine light stabilizer: 0.15–0.35 parts; High molecular weight hindered amine light stabilizer: 0.15–0.35 parts; UV-resistant pigment: 0.30-0.60 parts.

2. The high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, The hindered phenolic antioxidant is pentaerythritol tetrakis; the phosphite auxiliary antioxidant is tris(2,4-di-tert-butylphenyl) phosphite.

3. The high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, The thioester antioxidant is octadecyl 3,3'-thiodipropionate.

4. The high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, The hindered amine light stabilizer is sebacic acid diester.

5. The high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, The UV-resistant pigments include titanium dioxide, black pigments, yellow pigments, and green pigments.

6. The high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, This material is made from the following components in parts by weight: The composition includes 75 parts block copolymer COPP, 23 parts homopolymer PP, 0.25 parts hindered phenolic antioxidant, 0.25 parts phosphite auxiliary antioxidant, 0.3 parts thioester antioxidant, 0.25 parts hindered amine light stabilizer, 0.25 parts high molecular weight hindered amine light stabilizer, and 0.45 parts UV resistant pigment.

7. A method for preparing a high weather-resistant UV-stabilized PP pallet box material, as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Raw material pretreatment: The block copolymer COPP and homopolymer PP are dried. S2. Mixing: The dried block copolymer COPP, homopolymer PP, hindered phenolic antioxidants, phosphite auxiliary antioxidants, thioester antioxidants, hindered amine light stabilizers, high molecular weight hindered amine light stabilizers and UV-resistant pigments are added to a high-speed mixer and mixed evenly to obtain a premix. S3. Extrusion granulation: The premixed material is added to a twin-screw extruder for melt extrusion, and then cooled and granulated to obtain modified PP granules; S4. Injection Molding: After drying, the modified PP granules are added to the injection molding machine, and the finished product is obtained after injection molding, pressure holding and cooling.

8. The method for preparing a high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, In step S1, the drying temperature is 80℃~90℃, and the time is 2.0h~3.0h; In step S2, the mixer speed is 1000 rpm to 1400 rpm, and the mixing time is 5 min to 8 min.

9. The method for preparing a high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, The temperature setting range for the twin-screw extruder in step S3 is as follows: Feeding section 160℃~170℃, compression section 170℃~180℃, homogenization section 180℃~190℃, die head 175℃~185℃; The screw speed is 200 rpm to 300 rpm.

10. The method for preparing a high weather-resistant UV-stable PP pallet box material according to claim 1, characterized in that, In step S4, the injection temperature is 175℃~195℃, the mold temperature is 45℃~55℃, the injection pressure is 85MPa~95MPa, and the holding time is 10s~20s.