When a medical rigid clear PVC granules manufacturer used S-1000 PVC resin to produce medical PVC pellets, it found the following problems: The color of the 815th batch of PVC resin powder was poor, and the medical PVC pellets produced with this batch of materials The color of the material is 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 produced normally, but the color is slightly worse than the 812th and 816th batches, Better than the 815th batch. Observing the four batches of powder with naked eyes, it can be clearly seen that the color of batch 815 is poor, while the colors of batch 812, batch 814, and batch 815 are not much different. The author has analyzed these 4 batches of materials and summarized them as follows for the reference of the peers.

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 laboratory test results and the 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 batch 814 with a slightly worse color The haze is higher than the 812th batch and the 816th batch, but lower than the 815th batch.

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 a 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 known that the yellow color of the medical rigid clear PVC granules produced with the 815th batch is caused by the quality problems of the 815th batch. The author made a 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 the type and use of PVC resin. 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 ions and hydrogen ions) enter the polymerization vessel, which can 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 rigid clear PVC granules. The results of testing the iron ion content of 4 batches are: batch 812 is 2.9μg/g, batch 814 is 3.6μg/g, batch 815 is 13.0 μg/g, and batch 816 is 1.2μg/g. It can be seen that the iron ion content of batch 815 with the worst aged whiteness is significantly higher than that of the other three batches, while the iron ion content of batch 814 is slightly higher than that of batches 812 and 816. . It can be seen that the poor color of the 815th batch of material 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, and 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 the 815th batch.

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 result of time has a good consistency. The higher the double bond content of the 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 batch and the 816th batch, and lower than the 815th batch. This is also the 814th batch of materials reported by users. Reasons for medium quality. It can be seen that the high content of unsaturated double bonds in PVC resin molecules is the inherent reason for the yellowing of medical PVC pellets. In the rigid clear PVC granules production process, in addition to fundamentally changing the polymerization aid system, methods such as improving the drying process of the resin and reducing the residual amount of polymerization aids should also be adopted 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 molecular mass and its distribution.

(3) The high iron ion content of the 815th batch is the main reason for the yellowing of medical rigid clear PVC granules 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

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