When a medical pvc granules manufacturer used S-1000 PVC resin to produce medical pvc granules, it found the following problems: The color of the 815th batch of PVC resin powder was poor, and the color of the medical PVC pellets produced with this batch was relatively poor. Yellow; while the color is normal when the 812th batch and the 816th batch of PVC resin powder are used for production; the 814th batch can also be used for normal production, but the color is slightly worse than the 812th and 816th batches, which is better than Batch 815. Observing the four batches of powder with naked eyes, it can be clearly seen that the color of the 815th batch of materials is poor, while the color of the 812th batch, the 814th batch, and the 815th batch is not much different. The author has analyzed these 4 batches of materials, and summarized them as follows for the reference of colleagues.

Cause Analysis:

1. Analysis of whiteness, aging whiteness and thermal stability time
The test results of the whiteness, aging whiteness and thermal stability time of the 4 batches are shown in Table 1.

It can be seen from Table 1: that the 815th batch of material is significantly lower than the other three batches in terms of self-degree and aging whiteness, and the aging whiteness is slightly lower than the index of SG5 resin superior grade in GB/T 5761-2006 (≥ 78%), the whiteness and aging whiteness of the remaining three batches are not much different; in addition, the thermal stability time of the 815th batch is the shortest. It can be seen that the poorer color of batch 815 has lower aging whiteness, shorter thermal stability time, and quality problems.

2. Optical performance analysis
Optical performance is one of the important indicators of transparent products. The light transmittance, haze and yellow index of PVC resin are greatly affected by the product formula and processing technology, so there is a certain gap between the results of laboratory tests and actual production. The author uses the same process conditions as the manufacturer to prepare transparent sheets through mixing, double-roll milling, and compression molding. The measured optical properties are shown in Table 2.

It can be seen from Table 2: The optical performance of batch 815 is significantly worse than that of batch 812 and batch 816, especially the yellow index and haze are much higher than the latter; while the color of batch 814 is slightly worse. The haze is higher than batches 812 and 816, but lower than batches 815.

The yellow index has a corresponding relationship with the thermal stability of PVC resin. The poor thermal stability and low aging whiteness of PVC resin will cause high yellow index. The test results in Table 2 are consistent with Table 1. Through the analysis of Table 1 and Table 2, it can be seen that the yellow color of the medical PVC pellets produced from the 815th batch is caused by the quality problems of the 815th batch. The author made further analysis on this.

3. Molecular mass and its distribution
The molecular mass and distribution of PVC resin is one of the basic factors that affect the processing performance and use characteristics, and it is also the main basis for dividing PVC resin types and uses. Gel Permeation Chromatography (GPC) is a commonly used method to determine the molecular mass and distribution of polymers. It has the advantages of high accuracy, high speed, and small sample volume. This method was used to test the molecular mass and distribution of these 4 batches of PVC resin, and the results are shown in Table 3.

It can be seen from Table 3 that the molecular masses of the four batches of samples are similar, and the molecular mass distribution is basically the same, with no significant difference. It can be seen that the poor color of batch 815 has nothing to do with molecular mass and its distribution.

4. Analysis of metal ion content
During the polymerization process, the impurities contained in VCM (especially iron and hydrogen ions) enter the polymerization kettle, and they will catalyze the degradation of PVC macromolecules at higher temperatures and increase the rate of dehydrochlorination of PVC molecules; at the same time, hydrogen chloride After the content is increased, the pH value of the reaction system is further reduced, which greatly increases the rate of dehydrochlorination, thereby increasing the probability of the formation of double bonds by PVC macromolecules, and worsening the thermal stability and appearance of PVC. The results of testing the iron ion content in the 4 batches were: batch 812 2.9 μg/g, batch 814 3.6 μg/g, batch 815 13.0 μg/g, and batch 816 1.2 μg/g. It can be seen that the iron ion content in batch 815 with the worst aging whiteness is significantly higher than the other three batches, while the iron ion content in batch 814 is slightly higher than that in batches 812 and 816 . It can be seen that the poor color of the 815th batch has a certain relationship with its higher iron ion content.

According to reports, impurities in monomers (such as aldehydes, acetylene and iron ions, etc.) are the main factors causing the decrease in PVC thermal stability. Heavy metal ions can catalyze the degradation of PVC molecular chains. Therefore, the author believes that the high iron ion content is the main reason for the poor thermal stability and poor color of batch 815.

5. Saturated double bond analysis
PVC resin is very unstable, and dehydrochlorination and other reactions occur at about 150 oC. The study found that PVC analog compounds-2,4-dichloropentane and 2,4,6-trichlorohexane are still very stable at about 200 degrees Celsius. Therefore, it is believed that the normal chain structure of PVC resin is very stable, while the molecular chain The abnormal structure is the root cause of PVC instability. Studies have found that there are more unstable abnormal structures in PVC than PS and PMMA. This is due to the low reactivity of non-conjugated VCM, and the high reactivity of free radicals during chain extension, so the reaction selectivity is poor; a variety of basic reactions compete, and abnormal structures are easily formed.
Conjugated double bond is one of the abnormal structures in PVC molecules, and this structure is rare in PVC molecules in the early processing stage. The data shows that the initial colorability of PVC resin is not directly related to long-term thermal stability. The initial coloring behavior of PVC resin during processing is related to the content of conjugated double bonds in its molecular chain, and the content of isolated double bonds has an important influence on the long-term thermal stability of the resin[]. When the length of the conjugated double bond segment exceeds 7, its absorption electron energy level is close to the visible light region, resulting in color, which will color the PVC resin in the early stage of processing. The isolated double bond is only an active center, and its electron absorption energy level is in the far ultraviolet region. As the PVC resin ages, the isolated double bond gradually becomes a conjugated double bond, which affects the long-term thermal stability of the PVC resin. Therefore, it is necessary to detect the content of unsaturated double bonds in PVC resin. The test sample is cold-pressed with powder at room temperature to form a sheet. After the pressure is relieved, with the extension of the standing time, the stress in it is gradually released, and the sample gradually becomes thicker and tends to be constant. In order to eliminate the influence of the small difference in thickness on the integrated area of ​​the unsaturated double bond absorption peak and improve the repeatability of the test, the author adopts the internal standard peak method, which is represented by the ratio of the integrated area of ​​the unsaturated double bond absorption peak to the internal standard peak The content of unsaturated double bonds is shown in Table 4.

It can be seen from Table 4 that the content of conjugated double bonds, carbonyl groups, and isolated double bonds in batch 815 PVC resin with poor color is higher than batches 812 and 816, which is consistent with the aging whiteness and thermal stability in Table 1. The results of time have very good consistency. The higher the double bond content of PVC resin, the lower the aging whiteness and the worse the thermal stability. Although the aging whiteness of the 814th batch is higher, the content of unsaturated double bonds in the molecule is higher than that of the 812th and 816th batches, and lower than the 815th batch. Reasons for medium quality. It can be seen that the high content of unsaturated double bonds in the PVC resin molecule is the inherent reason for the yellowing of medical PVC pellets. In the PVC production process, in addition to fundamentally changing the polymerization aid system, it is also necessary to improve the drying process of the resin and reduce the residual amount of polymerization aids to reduce the content of unsaturated double bonds in the PVC resin molecule and increase the PVC The aging whiteness and thermal stability of the resin.

six. in conclusion
(1) Batch 815 with poor color has low aging whiteness, short thermal stability time, poor light transmittance, haze, and high yellow index.
(2) The poor quality of batch 815 has nothing to do with the molecular mass and its distribution.
(3) The high iron ion content of the 815th batch is the main reason for the yellow color of medical PVC pellets, because the high iron ion content will lead to the high content of unsaturated double bonds in the molecule, which makes the quality of PVC resin poor .

Seven. Suggest
Unstable quality of PVC resin will affect the quality of medical PVC pellets and rigid transparent PVC products. Therefore, companies that produce high-end products such as rigid transparent PVC pellets should adopt relatively stable PVC resins and formulate standards for companies to purchase raw materials. , To test the different batches of resin purchased to ensure quality.

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