Testing method for evaluating whiteness of expandable microspheres for polymer and application of testing method

Abstract

The invention relates to a test method for evaluating whiteness of expandable microspheres for polymers and application of the test method, and relates to the technical field of high polymer material processing and performance testing. The method aims at solving the problems of distortion and deviation caused by dependence on expandable microsphere powder test or subjective visual evaluation in the prior art. The testing method comprises the following steps: blending the expandable microspheres to be tested with a polymer base material, and carrying out mold pressing at a specific temperature and pressure to prepare a standard sample so as to simulate an actual processing process. And then measuring the L value, the a value and the b value of the sample by using a colorimeter, and carrying out quantitative evaluation based on a Hunter whiteness calculation formula WH = 100-[(100-L) + a + b]. According to the method, a standardized evaluation system for simulating real processing and objectively quantifying results is established, the functions of whiteness performance transverse screening and yellowing trend longitudinal evaluation are achieved, and reliable data support is provided for accurate model selection of the expandable microspheres, polymer formula optimization and processing technology determination.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing and performance testing technology, specifically to a standard test method for simulating real processing conditions and quantitatively evaluating the whiteness performance of expandable microspheres in polymers. Background Technology

[0002] Expandable microspheres, as important polymer additives, are widely used in foamed products made of PVC, EVA, and PP to reduce product density, improve touch and appearance, and enhance thermal and sound insulation performance. After foaming and expanding at high temperatures, microspheres form a uniform microporous structure within the matrix. These micron-sized bubbles are ideal light scattering centers, theoretically capable of significantly improving the whiteness and hiding power of the product. However, in practical applications, it has been found that different brands and formulations of expandable microspheres, under the same polymer matrix and processing conditions, often exhibit significant differences in the whiteness of the final product. Some products even show obvious yellowing, severely affecting the product's appearance quality.

[0003] Currently, industry evaluations of expandable microspheres primarily focus on physical parameters such as foaming ratio and initial foaming temperature. However, systematic research and standardized evaluation methods are lacking regarding their color and whiteness performance in specific polymer matrices (such as PVC). This is because the color of expandable microspheres in the final product is a result of their own thermal stability and the complex interactions with the polymer matrix and formulation additives under specific processing temperatures, pressures, and times. Existing technologies often employ a high-temperature oil bath foaming method to test the microsphere powder itself. This method exposes the microspheres directly to a high-temperature fluid medium, resulting in a fundamental difference in their thermal history and chemical environment compared to their actual state encapsulated in the polymer melt. Consequently, such test results cannot accurately reflect the actual color changes of the microspheres in the final product, leading to significant discrepancies between the evaluation results and practical application effects. Furthermore, the whiteness evaluation of microsphere-containing products currently relies heavily on manual visual comparison. This method is highly subjective, lacks repeatability, and lacks objective quantitative comparison standards, thus failing to achieve accurate quality judgment and effectively distinguishing performance differences between different grades of microspheres, posing numerous difficulties for raw material selection and quality control. Although existing technologies have addressed the yellowing problem by improving material formulations (such as developing anti-yellowing microspheres or optimizing PVC stabilization systems), there is still a lack of a standardized method for scientifically evaluating and comparing the actual whiteness performance of different microspheres under specific PVC formulations and processing conditions. This results in a lack of efficient and reliable data support and screening criteria for both material research and process optimization.

[0004] Therefore, developing a standard evaluation method that can simulate the real processing environment, achieve objective quantitative measurement, and directly and accurately reflect the final whiteness performance of expandable microspheres in a polymer matrix is ​​of great industrial value and urgent practical significance for accurately screening high-performance microsphere materials, scientifically guiding the optimization of polymer processing technology, and improving the appearance quality of end products. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing technologies and provide an evaluation method that can accurately and quantitatively assess the whiteness performance of expandable microspheres in polymer (especially PVC) matrices after simulated high-temperature processing. This method can also indirectly reflect the foaming effect by testing the density of sample pieces, achieving comprehensive performance evaluation. The method of this invention aims to solve problems in existing technologies such as misaligned evaluation objects, reliance on subjective visual inspection, and inability to quantitatively compare the performance differences of different microspheres, providing objective and reliable data support for microsphere screening, polymer formulation optimization, and processing technology determination.

[0006] It should be noted that the core of the testing method described in this invention lies in simulating the processing and performing quantitative evaluation, and its principle is universally applicable. The "polymer base material" is not limited to PVC, but can be various thermoplastics commonly found in the field, as shown in Example 3 below. This method can also effectively distinguish the whiteness differences of different microspheres in polypropylene (PP) systems. When applied to different polymers, only the molding temperature, pressure, and time parameters in step S2 need to be adaptively adjusted according to the specific processing characteristics of the polymer (such as melting temperature and thermal stability), and its evaluation logic remains consistent with the standard.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a test method for evaluating the whiteness of expandable microspheres for polymers, characterized by comprising the following steps: S1. Sample premixing: The polymer base material and the expandable microspheres to be tested are uniformly mixed at a mass ratio of 0.1-5 wt% to obtain a mixture. S2. Sample preparation: The mixture obtained in step S1 is placed in a molding device and hot-pressed at a temperature of 160℃~240℃ and a pressure of 0.5 MPa~6.0 MPa for 15 seconds to 5 minutes. Then it is transferred to a cold press for cold pressing and shaping. After demolding, it is made into a standard test sample. S3. Data Measurement: Use a colorimeter to measure the color parameters of the standard test sample described in step S2, and obtain its lightness value L, red-green value a, and yellow-blue value b; S4. Result Evaluation: Based on the yellow-blue hue value b mentioned in step S3 and / or the Hunter whiteness WH calculated according to the formula WH = 100 - √[(100 - L)² + a² + b²], evaluate the whiteness performance of the expandable microspheres under the corresponding processing conditions. To more comprehensively evaluate the overall performance of expandable microspheres in foamed products, step S4 may also include measuring the density of the standard test sample. By comprehensively comparing the density (reflecting foaming and weight reduction effects) and Hunter whiteness WH value (reflecting color and thermal stability) of samples made from different microspheres, a more comprehensive evaluation of their overall application performance under specific processing conditions can be achieved. Generally speaking, microspheres with lower density and higher whiteness index have better overall performance.

[0008] Preferably, the polymer base is polyvinyl chloride (PVC) base. In the testing of the PVC base, the mass ratio of the expandable microspheres is more preferably 1–3 wt%, and the hot-pressing temperature is more preferably 180℃–220℃. The thickness of the standard test sample is preferably controlled within 1.0 mm ± 0.1 mm. This range of microsphere mass ratios ensures that the whiteness and yellowing trend of the expandable microspheres are significantly observed in the test, while avoiding excessive alteration of the polymer matrix color due to excessive addition. Furthermore, excessively high hot-pressing temperatures can easily cause significant yellowing of the PVC base, affecting the whiteness test of the microspheres; excessively low temperatures may result in incomplete melting of the PVC base.

[0009] In step S3, the data measurement further includes: preparing at least three parallel samples under the same processing conditions, measuring at least five different locations randomly selected for each sample, and using the average value as the final color data of the sample.

[0010] In step S4, one or a combination of the following two modes can be used for evaluation based on different evaluation purposes. (1) Horizontal screening mode: Multiple expandable microspheres to be tested are prepared into samples under the same matrix formulation and process parameters. By directly comparing their Hunter whiteness (WH value) or b value, the microsphere grade with the best whiteness performance is screened out. (2) Yellowing trend assessment mode: For the same expandable microsphere, samples are prepared by changing the processing temperature or time. By analyzing the change range of its Hunter whiteness (WH value) and / or b value (ΔWH and / or Δb), its yellowing trend is quantitatively assessed. Beneficial effects

[0011] This invention, by standardizing the molding of expandable microspheres within a real polymer matrix, simulates the actual high-temperature and high-pressure processing, fundamentally overcoming the problem of distorted results in traditional expandable microsphere powder testing methods. Objective measurements are performed using a colorimeter, replacing subjective visual judgment with precise quantitative indicators such as b-value and Hunter whiteness, establishing a repeatable and comparable unified evaluation standard. Compared to the traditional high-temperature oil bath foaming method described in Comparative Example 1, oil bath foaming exposes the microspheres directly to a high-temperature fluid medium, leading to severe and homogeneous thermal oxidation on their surface, resulting in distorted and indistinguishable whiteness. This invention, by encapsulating and isolating the microspheres within the polymer melt during molding, simulates their encapsulated and heated state during actual processing, thus avoiding distortion caused by self-thermal oxidation due to direct exposure to high temperatures. It accurately reflects the color performance of the microspheres within the polymer melt environment. This method not only allows for horizontal screening of the instantaneous whiteness of different microsphere grades but also provides sensitive evaluation of their thermal stability through multi-temperature point testing, offering efficient and reliable data support for precise material selection and optimized processing techniques. Furthermore, this invention has strong versatility. As shown in the following embodiments, this method has been successfully applied to PVC, PP and other systems. By adjusting the process window, it can be widely applied to a variety of thermoplastic plastics. Moreover, it is easy to operate, low in cost, and easy to promote and implement. Attached Figure Description

[0012] Figure 1 This is a flowchart illustrating the testing method described in this invention.

[0013] Figure 2 These are comparative photos of the whiteness of the four expandable microspheres (A, B, C, and D) in PVC in Example 1.

[0014] Figure 3 The images show the yellowing trend of microsphere B in Example 2 at different processing temperatures (180°C to 220°C).

[0015] Figure 4 These are photographs of the four expandable microspheres (A, B, C, and D) in Comparative Example 1 after being foamed in an oil bath. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, a test method for evaluating the whiteness of expandable microspheres for polymers, as described in this invention, will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0017] Example 1: Comparative Screening of Whiteness Performance of Different Grades of Expandable Microspheres in PVC Matrix

[0018] Step 1: Sample Premixing: Add 100 parts PVC (Junzheng SG5), 50 parts plasticizer DBP, and 20 parts epoxidized soybean oil to a kneader set at 80℃ and stir for 1 hour, then allow to cool naturally to room temperature. Next, add the cooled PVC premix to the kneader, along with 5 parts calcium carbonate, 1.5 parts calcium-zinc stabilizer, 0.5 parts stearic acid, and 3 parts expandable microspheres. Mix at room temperature for 1 hour. The expandable microspheres to be tested were selected from four different commercially available brands, labeled as microsphere A, microsphere B, microsphere C, and microsphere D.

[0019] Step 2 Sample preparation: The temperature of the flat vulcanizing machine is set to 220℃. The PVC premix from Step 1 is loaded into the mold and then hot-pressed under a pressure of 6MPa for 30 seconds. After that, it is transferred to a cold press for cold pressing and shaping. After demolding, a PVC test sample with a thickness of 1mm is obtained.

[0020] Step 3 Data Measurement: Before measurement, calibrate the colorimeter using a white plate according to the manufacturer's guidelines. Using the calibrated colorimeter (Sanenh, model 3nh NR110), measure the color space parameters (L, a, b values) at five different locations on the surface of each sample, and take the average value. Calculate the Hunter whiteness (WH) using the formula WH = 100 - √[(100 - L)² + a² + b²]. Density testing is performed according to GB / T 1033.1-2008.

[0021] Table 1 Comparison of results for different expandable microspheres Expandable microspheres Average L value Average a value Average b value Hunter Whiteness WH Density (g / cm3) Appearance Microsphere A 95.0 0.1 2.0 94.6 0.55 Whiter Microsphere B 90.0 0.1 4.0 89.2 0.45 Whiter Microsphere C 85.0 0.5 7.0 83.4 0.65 Slightly yellow Microsphere D 83.0 0.5 11.0 79.7 0.70 Yellower

[0022] The results of Example 1 are shown in Table 1. Under the same processing conditions of 220℃, the Hunter whiteness (WH=89.2) of microsphere B is slightly lower than that of microsphere A (WH=94.6), but its density (0.45 g·cm⁻³) is significantly lower than that of microsphere A (0.55 g·cm⁻³), indicating that it has higher foaming efficiency and better weight reduction effect. In actual selection, if the product has extremely high requirements for lightweighting, microsphere B may be a better choice; if there are extreme requirements for whiteness, microsphere A should be selected. The results of this example show that the method of the present invention can effectively and quantitatively distinguish the differences in whiteness performance of different grades of microspheres under the same processing conditions, providing direct data basis for selecting raw materials with better whiteness and better weight reduction. The above test results are consistent with... Figure 2 The visual comparison of the sample images shown is highly consistent, intuitively demonstrating the effective distinguishing ability of the method of the present invention.

[0023] Example 2: Evaluation of the yellowing trend of the same microspheres at different processing temperatures

[0024] Step 1: Sample Premixing: Add 100 parts PVC (Junzheng SG5), 50 parts plasticizer DBP, and 20 parts epoxidized soybean oil to a kneader set at 80℃, stir for 1 hour, and allow to cool naturally to room temperature. Then, add the cooled PVC premix to the kneader, along with 5 parts calcium carbonate, 1.5 parts calcium-zinc stabilizer, 0.5 parts stearic acid, and 3 parts expandable microspheres (brand B), and mix at room temperature for 1 hour.

[0025] Step 2 Sample preparation: The temperature of the flat vulcanizing machine was set to 180℃, 190℃, 200℃, 210℃ and 220℃ respectively. The PVC premix from Step 1 was loaded into the mold and then hot-pressed at 6 MPa for 1 min. After that, it was transferred to the cold press for cold pressing and shaping. After demolding, a PVC test sample with a thickness of 1 mm was obtained.

[0026] Step 3 Data Measurement: Following the method described in Example 1, use a colorimeter to measure the color parameters (L, a, b values) of the sample and calculate the Hunter whiteness (WH), while simultaneously measuring the sample density.

[0027] Table 2 Comparison of results at different processing temperatures Processing temperature Average L value Average a value Average b value Hunter Whiteness WH Density (g / cm3) Appearance 180℃ 95.0 0.1 3.0 94.2 0.50 Whiter 190℃ 94.0 0.1 4.0 92.8 0.48 Whiter 200℃ 93.0 0.1 5.0 91.4 0.45 Whiter 210℃ 89.0 0.5 7.0 87.0 0.48 Slightly yellow 220℃ 85.0 0.5 9.0 82.5 0.50 Yellower

[0028] The results of Example 2 are shown in Table 2. As the processing temperature increased, the b-value of the sample increased significantly, while the WH-value decreased, indicating that the thermal yellowing of microspheres B and the PVC matrix intensified. From 180℃ to 200℃, the decrease in whiteness was relatively gradual (ΔWH = -1.4); however, from 200℃ to 220℃, the whiteness decreased sharply (ΔWH ≈ -8.9). This indicates that microspheres B maintain good whiteness when processed at temperatures at or below 200℃; when the temperature exceeds 200℃, the yellowing trend is obvious, and their use is not recommended. The results of this example demonstrate that the method of the present invention can accurately quantify the yellowing trend of microspheres, providing crucial guidance for determining the safe processing temperature window. Figure 3 As shown, the color of the sample gradually deepens and turns yellow as the temperature rises, intuitively demonstrating the above-mentioned quantitative law.

[0029] Example 3: Testing of different microspheres in polypropylene (PP) systems

[0030] Step 1 Sample Premixing: Mix 100 parts PP powder (Maoming Petrochemical PP Powder 040), 0.1 parts antioxidant 168, and 1.5 parts expandable microspheres at room temperature for 1 hour. The expandable microspheres to be tested were selected from three different brands of commercially available expandable microspheres, labeled as microsphere E, microsphere F, and microsphere G, respectively.

[0031] Step 2 Sample preparation: The temperature of the flat vulcanizing machine is set to 190℃. The PP premix from Step 1 is loaded into the mold and then hot-pressed at 6 MPa for 2 minutes. After that, it is transferred to a cold press for cold pressing and shaping. After demolding, a PP test sample with a thickness of 1 mm is obtained.

[0032] Step 3 Data Measurement: Following the method described in Example 1, use a colorimeter to measure the color parameters (L, a, b values) of the sample and calculate the Hunter whiteness (WH), while simultaneously measuring the sample density.

[0033] Table 3 Comparison of results for different expandable microspheres Expandable microspheres Average L value Average a value Average b value Hunter Whiteness WH Density (g / cm3) Appearance Microsphere E 91 0.1 5.0 89.7 0.78 Whiter Microsphere F 89 0.1 5.6 87.7 0.83 Slightly yellow Microsphere G 87 0.5 8.0 84.7 0.98 Yellower

[0034] The results of Example 3 are shown in Table 3. Under the same processing conditions of 190°C, the PP sample using microsphere E exhibited the highest L and WH values, and the lowest b value, indicating optimal whiteness and minimal yellowing. Microsphere F was the second best, while microsphere G showed the worst whiteness and the highest density. This example clearly demonstrates that the method of the present invention can effectively and quantitatively distinguish the whiteness differences of different grades of microspheres in a PP matrix. These results confirm that the method of the present invention, by adjusting the process window, can be effectively applied to PP and other thermoplastic plastic systems.

[0035] Comparative Example 1: Whiteness comparison of different brands of expandable microspheres foamed in high-temperature oil bath test tubes.

[0036] Step 1 Sample preparation: The expandable microspheres and commercially available conventional silicone oil are loaded into a glass test tube at a mass ratio of 2:1. The expandable microspheres to be tested are the same brand as those described in Example 1 and are labeled as microsphere A, microsphere B, microsphere C and microsphere D respectively.

[0037] Step 2: Oil bath foaming: Place the glass test tube containing expandable microspheres from Step 1 into a 190°C constant temperature oil bath containing silicone oil. The oil bath is equipped with a stirring paddle and a thermometer to ensure that the microspheres are heated evenly and to maintain a stable oil bath temperature.

[0038] Step 3: Data Measurement: Record the foaming height of the expandable microspheres according to the graduation lines in the glass test tube, and visually record the appearance and color of the foamed microspheres.

[0039] Table 4 Comparison of results for different expandable microspheres Expandable microspheres Maximum foaming height Final height of foam Appearance Microsphere A 56 43 Yellower Microsphere B 55 55 Yellower Microsphere C 53 50 Slightly yellow Microsphere D 69 68 Slightly yellow

[0040] The results of Comparative Example 1 are shown in Table 4. When tested in silicone oil at 190°C, the four types of microspheres all appeared slightly yellow or yellowish, and it was difficult to effectively distinguish the differences in whiteness between them by visual inspection. Figure 4 After foaming in a medium-oil bath, the appearance of the microspheres becomes more similar to that of the others. Figure 2The samples prepared by the method of this invention, with distinct whiteness levels, present a stark contrast. This directly demonstrates the distortion and limitations of the traditional oil bath method in evaluating whiteness. Furthermore, the operating temperature of silicone oil should generally not exceed 200℃. Excessive temperature can easily lead to oxidation of the silicone oil, resulting in increased viscosity and unstable temperature of the silicone oil in the oil bath.

[0041] Comparative Example 2: Comparison and verification of the results of the compression molding method and the traditional injection molding method

[0042] Step 1: Sample Premixing: Add 100 parts PVC (Junzheng SG5), 50 parts plasticizer DBP, and 20 parts epoxidized soybean oil to a kneader set at 80℃, stir for 1 hour, and allow to cool naturally to room temperature. Then, add the cooled PVC premix to the kneader, along with 5 parts calcium carbonate, 1.5 parts calcium-zinc stabilizer, 0.5 parts stearic acid, and 3 parts expandable microspheres (brand B), and mix at room temperature for 1 hour.

[0043] Step 2 Sample Preparation: The PVC premix from Step 1 is foamed in an injection molding machine at an injection temperature of 200℃ and a holding time of 20 s. Other injection parameters are performed according to conventional injection molding processes for footwear materials. After cooling, PVC sole material is obtained. Simultaneously, the PVC premix from Step 1 is loaded into a mold, and then the mold is placed in a flat vulcanizing machine at a temperature of 200℃ and a pressure of 5MPa for 1 min. After hot pressing, the mold is transferred to a cold press for cold pressing and shaping. After demolding, PVC test samples are obtained.

[0044] Step 3 Data Measurement: Before measurement, calibrate the colorimeter using a white plate according to the manufacturer's guidelines. Using the calibrated colorimeter (Sanenh, model 3nh NR110), measure the color space parameters (L, a, b values) at five different locations on the surface of each sample, and take the average value. Calculate the Hunter whiteness (WH) using the formula WH = 100 - √[(100 - L)² + a² + b²]. Density testing is performed according to GB / T 1033.1-2008.

[0045] Table 5 Comparison of results between compression molding and injection molding Sample preparation method Average L value Average a value Average b value Hunter Whiteness WH Density (g / cm3) Appearance Molding method 93.0 0.1 5.0 91.4 0.45 Whiter Injection molding 92.0 0.1 5.2 90.5 0.48 Whiter

[0046] The results of Comparative Example 2 are shown in Table 5. Under the same processing temperature, the L value, b value, WH value, and density of the samples obtained by compression molding and injection molding are very close, and the color trends are consistent, indicating a high degree of agreement in whiteness performance. Because the compression molding method yields thinner samples and has a more controllable thermal history, it can more quickly and sensitively reflect the color change trend of microspheres under laboratory conditions. Furthermore, the results show good consistency with the actual injection molding process, verifying the effectiveness and practicality of the method of this invention.

Claims

1. A test method for evaluating the whiteness of expandable microspheres for polymer applications, characterized in that, Includes the following steps: S1. Sample premixing: The polymer base material and the expandable microspheres to be tested are uniformly mixed at a mass ratio of 0.1~5 wt% to obtain a mixture. S2. Sample preparation: The mixture obtained in step S1 is placed in a molding device and hot-pressed at a temperature of 160℃~240℃ and a pressure of 0.5~6.0 MPa for 15 seconds to 5 minutes. Then it is transferred to a cold press for cold pressing and shaping. After demolding, it is made into a standard test sample. S3. Data Measurement: Use a colorimeter to measure the color parameters of the standard test sample described in step S2, and obtain its lightness value L, red-green value a, and yellow-blue value b; S4. Result Evaluation: The whiteness performance of the expandable microspheres under the corresponding processing conditions is evaluated based on the yellow-blue hue value b obtained in step S3, or the Hunter whiteness WH calculated from the L, a, and b values ​​according to the formula WH = 100 - √[(100 - L)² + a² + b²].

2. The test method according to claim 1, characterized in that, The polymer base material is polyvinyl chloride (PVC) base material.

3. The test method according to claim 1, characterized in that, The polymer base material is polypropylene (PP) base material.

4. The test method according to claim 2, characterized in that, In step S1, the mass ratio of the expandable microspheres is 1 to 3 wt%.

5. The test method according to claim 2, characterized in that, In step S2, the temperature of the hot pressing is 180℃~220℃.

6. The test method according to claim 1, characterized in that, In step S2, the thickness of the standard test sample is 1.0 mm ± 0.1 mm.

7. The test method according to claim 1, characterized in that, In step S3, the data measurement includes: preparing at least three parallel samples under the same processing conditions, measuring at least five different locations on each sample and taking the average value as the final color data of the sample.

8. The test method according to claim 1, 2 or 3, characterized in that, In step S4, the evaluation method includes a horizontal screening mode, specifically: multiple expandable microspheres to be tested are made into samples under the same polymer matrix formulation and molding process parameters, and the microsphere grade with the best whiteness performance is screened by directly comparing their Hunter whiteness WH value and / or b value.

9. The test method according to claim 1, 2 or 3, characterized in that, In step S4, the evaluation method includes a yellowing trend assessment mode, which specifically involves preparing samples of the same expandable microspheres at at least two different processing temperatures or times, and quantitatively assessing their yellowing trend by analyzing the variation range (ΔWH and / or Δb) of their Hunter whiteness WH value and / or b value.

10. The test method according to claim 1, characterized in that, Step S4 also includes measuring the density of the standard test sample, and evaluating its overall performance by comprehensively comparing the density and WH value of samples made from different microspheres.

11. The application of the test method as described in any one of claims 1-10 in screening high-whiteness expandable microspheres for polymer articles.

12. The application of the test method as described in any one of claims 1-10 in optimizing the processing parameters of polymer materials containing expandable microspheres.

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