Discussion on different test methods for carbon black content in polyethylene
The carbon black content in polyethylene was determined by GB13021 "Determination of Polyethylene Pipe and Tube Carbon Black Content (Thermal Weight Loss Method)" and Thermogravimetric Analysis. The results of the two methods were compared. The results show that both methods have good repeatability and accuracy. The results are basically the same. The results obtained by different methods can be used for mutual reference.
Key words GB13021, thermogravimetric analysis, carbon black content
Carbon black content in polyethylene was determined by two methods of GB13021, polyethylene pipe and tube carbon black content determination (thermal gravimetric method) and thermo gravimetric analyzer. Compared with the measurement results of the two methods of the surface, the two methods have good repeatability And accuracy. The measurement results are basically the same, the determination results obtained by different methods can reference each other
Key words GB13021, thermal gravimetric analysis, carbon black content
In recent years, polyethylene pipe has become the second largest plastic pipe product in the world after PVC, widely used in water supply, agricultural irrigation, gas transportation, sewage, oil fields, chemicals, communications and other fields. Polyethylene without additives is poor in weathering resistance and solar exposure, so carbon black is added in practical use [1]. Carbon black can make the material have sufficient UV aging resistance, and it can effectively prevent the influence of ultraviolet rays when the carbon black content is 2.0% to 3.0% [2]. Since the carbon black content has an important influence on the polyethylene pipe, many standards stipulate the carbon black content in the polyethylene. For the purpose of research and development, production and sales, the carbon black content is an indicator that the polyethylene pipe must be tested. At present, the test method for carbon black content in plastics for pipelines mainly implements GB13021–1991 [3]. The use of thermogravimetric analyzers is a commonly used thermal analysis method. It is very convenient to measure the composition of high polymers. The commonly used standard is ASTM E1131–2008 [4]. Thermogravimetric analyzers can also be used to determine the carbon black content in polyethylene. . At present, these two methods coexist, and different methods are often used in different laboratories. There is a problem that the results of carbon black content analysis cannot be directly compared. The author used the above two methods to determine the carbon black content in the same batch of polyethylene pellets, and discussed the advantages and disadvantages of different test methods, the measurement repeatability and the consistency of the test results of the two methods. The selection provides a reference.
Experimental part
1.1 Main instruments and materials
Carbon black content analyzer: HS-TH-3500, Shanghai Heyi Instrument Technology Co., Ltd.; mechanical analysis balance: accuracy 0.0001g, Shanghai Tianping Instrument Factory; thermogravimetric analyzer: STA449C type; Germany NETZSCH; electronic balance: M2P type, Sartorius, Germany; Polyethylene: commercially available.
1.2 Experimental methods
1.2.1 GB13021 Act
Weigh the sample mass m1 (1±0.05)g into the sample boat, put the sample boat into the carbon black content analyzer, adjust the nitrogen flow rate to 130mL/min, raise the temperature to 600 °C under nitrogen protection, and sterilize for 30 minutes at constant temperature. After taking out, it was placed in a desiccator and cooled to room temperature. The mass m2 was weighed, placed in a muffle furnace at 950 ° C for 10 min, taken out in a desiccator and cooled to room temperature, and the mass m3 was weighed. Carbon black content c (%)
Calculated according to formula (1).
1.2.2 Thermogravimetric Analyzer Method
Weigh the sample quality (10 ± 0.05) mg into the sample holder, close the heating furnace, set the heating program, raise the temperature to 550 ° C at room temperature under nitrogen atmosphere, convert to oxygen, and raise the temperature to 750 ° C in an oxygen atmosphere. Automatically collect sample quality changes during the heating process.
2. Results and discussion
2.1 Comparison of measurement results
The content of carbon black in the polyethylene was measured in accordance with 1.2.1, and the measurement results are shown in Table 1.
The thermogravimetric curve of the polyethylene sample was determined according to 1.2.2 (see Figure 1). According to the percentage of weight loss in each step on the curve, the decomposition mechanism of the sample and the content of each component can be judged. As the temperature increases, the polyethylene cracks and continues to a constant mass of 550 ° C. Because the carbon black does not react in high purity nitrogen, the gas is switched, oxygen is introduced, the carbon black is reacted to completion, and the sample quality is again Constant. The mass reduction that occurs when the oxygen is switched from 550 ° C to 650 ° C is the carbon black content in the polyethylene. The remaining material after stabilization at 650 ° C is the ash in the polyethylene. The results of the measurement of the carbon black content in the polyethylene sample are shown in Table 1.
From the test results, the relative standard deviations of the two test methods are less than 3%, indicating that both methods have good repeatability, and the relative standard deviation of the thermogravimetric analyzer method is slightly larger than the relative standard deviation of GB13021. It is related to the sample volume of the thermogravimetric analyzer and the unevenness of the sample. The consistency of the test results of the two methods can be performed by the following method [5]: Assume that the test results of the two test methods are x11, x12...x1n, the average value is x1, the standard deviation is S1; x21, x22...x2n, average The value is x2 and the standard deviation is S2. If xx12- is regarded as a random variable, the basic rules according to the variance are:
Therefore, if xx2S12(x1x2)-G-, the two sets of data are considered to be consistent. Substituting the data in Table 1 into the formula can be calculated as: xx0.8212-=, 2S(x1-x2)=0.83, and the calculation results show that the two sets of data are consistent. The results of the two methods are consistent and can be used to compare each other.
2.2 Thermogravimetric Analyzer Accuracy
The thermogravimetric analyzer automatically records the real-time quality of the sample during the analysis. The human factor is small, and the accuracy of the thermal weight loss can be verified by the standard CaC2O4. CaC2O4·H2O will undergo the following three-step chemical reaction with increasing temperature: CaC2O4·H2O (solid) = CaC2O4 (solid) + H2O (gas) (3) CaC2O4 (solid) = CaCO3 (solid) + CO (gas) ( 4) CaCO3 (solid) = CaO (solid) + CO2 (gas) (5) Gas is released in each reaction, so that the weight loss occurs in the solid. According to the chemical reaction equation and molecular weight, the chemical reaction of each step can be calculated. Theoretical weight loss. The chemical reaction of each step of CaC2O4·H2O can be reflected on the thermogravimetric curve. The weight loss and theoretical value of CaC2O4·H2O obtained by thermogravimetric analyzer are listed in Table 2.
It can be seen from Table 2 that the relative deviation of the measurement of the thermogravimetric analyzer at 550 to 750 ° C is 1.3%, and the measurement accuracy is high. The results of the thermogravimetric analyzer method and the GB13021 method for measuring carbon black content are reliable. The thermogravimetric analyzer method is quick and convenient, but the relative standard deviation of the measurement is larger than that of the GB13021 test method. The reason is that the sample amount used in the thermogravimetric analysis is only 10 mg. If the carbon black in the sample is unevenly distributed, it is measured by a thermogravimetric analyzer. The carbon black content of the polyethylene increases the test standard deviation. It is recommended to use the thermogravimetric analysis to sample carbon black as much as possible and to increase the sample size as appropriate.
3. Conclusion
From the experimental process and analysis results, it can be seen that the two different methods of carbon black content analysis have the following characteristics: (1) Both test methods can be used to determine the carbon black content in polyethylene, and the results are basically consistent and comparable. (2) The GB13021 method for measuring carbon black content has good repeatability, but the carbon black analyzer and the muffle furnace are used. The cooling and three weighings are required during the experiment, and the operation is more complicated than the thermogravimetric analyzer. (3) The thermogravimetric analysis method is convenient and fast, and the result is intuitive. However, due to the small amount of sample used, the standard deviation of the test results is large, and abnormal values are likely to occur during the test. Samples should be sampled from multiple particles to increase the sample amount as much as possible. The number of tests is at least 2 times. When there are two deviations, the number of tests is increased.
