Melt flow index method is the most popular method

Failure analysis of thermoplastic elastomers by melt flow index method

for example, the location and direction of sampling, the shape and size of the sample, the force application characteristics during the experiment, including the loading speed, the composition and temperature of the environmental medium, etc., will affect the results of the experiment. For rubber and plastic component manufacturers, the problem of the production process is a serious factor that affects the size of the benefit, which can cover the lower jaw and increase waste. A rapid evaluation method to check production process problems is particularly important for maintaining production efficiency. In addition, third party material assessments are often used to quickly identify raw material compliance issues. On the contrary, based on scientific data, commercial problems originating from raw materials can be properly solved and the production process can be put back on track

as a typical case, polymer solutions received a pair of samples marked "good" and "bad". We should use objective and scientific measurement to find out whether the problem is caused by raw materials for our customers as soon as possible, so that they can resume the normal production and processing process. The control test of two samples has been completed. From the perspective of root cause analysis, we have adopted three analysis methods for comparative test. This is considered to be a cost-effective first step in line with technical specifications. These three analysis methods include:

● determination of inorganic filler content by thermogravimetric analysis (TGA)

● determination of thermal phase transition by differential scanning calorimetry (DSC)

● melt flow index (MFI) comparative test of melt flow performance

melt flow index

cut a small number of "good" and "bad" samples with scissors and blades, and then dry them in a convection constant temperature oven at 120 ℃ for melt flow index test, And evaluate the test results

first, after the initial drying for 3 hours, the sample showed a noticeable bubbling phenomenon when tested by the melt flow index. Therefore, the sample was dried at 120 ℃ for 24 hours. Secondly, it is noted that under the standard conditions of 224 ℃ and 1.2kg melt flow index, the resin flow is too fast to accurately test the melt flow. Therefore, the test temperature was reduced to 199 ℃, and the test protocol was modified with the consent of the customer, including part of the formal test report

Table 1. Melt flow index (MFI)

according to the measured value of melt flow index, it is found that there is a significant difference between the two samples. The difference between the measured values of samples is obvious. See Table 1 for details. Compared with the "good" sample, the "bad" sample has a higher melt flow index, which indicates that the molecular weight of the "bad" sample is lower than that of the "good" sample. The actual molecular weight distribution can be detected by gel permeation chromatography (GPC) or volume exclusion chromatography (SEC), which can evaluate molecular weight parameters (such as weighted average molecular weight and degree of polymerization dispersion)

thermogravimetric analysis (TGA)

in order to compare the degradation of the two samples, Ta instruments Q500 instrument was used for thermogravimetric analysis. Place about 15mg of the sample in a platinum pan and heat it from 30 ℃ to 900 ℃ at a heating rate of 10 ℃ per minute at 1 atmospheric pressure. Figure 1 depicts the thermogravimetric analysis curve of two samples

compare the two data backups: the test data can be saved in the degradation phenomenon and inorganic residue content of samples in any hard disk partition, and it is found that there is no obvious difference

different thermal degradation properties can indicate different polymer or additive formulations. If different thermal degradation is observed, the chemical structure identification of the basic polymer will be carried out. The analytical methods include Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR)

Figure 1. The difference in the residual amount of inorganic matter in thermogravimetric analysis (TGA)

will indicate that the amount or type of filler filled in thermoplastic elastomer is different. If the test results of inorganic residues show significant differences, further chemical composition, physical properties and concentration results of fillers are required. Scanning electron microscopy (SEM) combined with additional data provided by energy dispersive X-ray spectroscopy (EDS) and digital optical microscopy (OM) are required

differential scanning calorimetry (DSC)

TA instruments Q200 instrument was used for differential scanning calorimetry analysis. Pack about 7Mg of the sample into an aluminum pan and heat it from -80 ℃ to 250 ℃ at a heating rate of 10 ℃ per minute. Then cool the sample to -80 ℃, and then heat it again to 250 ℃ at a heating rate of 10 ℃ per minute. The differential scanning calorimetry curve of the two samples is shown in Figure 2

Figure 2. Differential scanning calorimetry (DSC)

test shows that the glass transition temperature of the two samples is very small in the first heating scanning curve. The melting point of both thermoplastic polyurethane is 155 ℃. In addition, the two samples have the same melting peak characteristics (peak start temperature, minimum peak, final peak), and the crystallinity level depending on the melting peak area (melting heat △ HF) is also very similar

summary

melt flow index, thermogravimetric analysis and differential scanning calorimetry were applied to examine the causes of thermoplastic polyurethane extrusion process variation. The test results of thermal degradation performance and inorganic residue of thermogravimetric analysis showed that there was no significant difference between "good" and "bad" samples; There is no significant difference between the glass transition temperature and isothermal melting characteristics of the two samples; The most significant difference between the two samples is the melt flow index value. Compared with the "good" sample, the melt flow rate of the "bad" sample is about twice higher. This significant difference in melt flow rate indicates that the molecular weight distribution of the two thermoplastic polyurethane samples is significantly different