Discussion on quality control method of PVC special light calcium carbonate

Discussion on quality control method of PVC special light calcium carbonate

Hard PVC products in the processing process of light calcium carbonate demand is very large, whether PVC ~ U profile or PVC - U pipe, pipe fittings, light calcium carbonate is an essential filler. Adding light calcium carbonate can increase the volume, reduce the cost, but also improve the dimensional stability, hardness and rigidity of hard PVC products, improve the processing performance of plastics. But, the light calcium carbonate with uneven good and bad on the market has adverse effect to product performance, if product rigidity is poor, white product appearance color slants grey slants the problem such as green. Therefore, it is very important to choose high-quality light calcium carbonate for PVC ~ U products manufacturers.

For light calcium carbonate in the standard of technical indicators, due to the detection project, detection method is more complex, more trouble, even the light calcium carbonate production enterprises, some can only be partial index detection, at the same time, most of the light calcium carbonate using the enterprise is not equipped with enough test conditions or to carry out all testing program to monitor the quality of light calcium carbonate. Therefore, from the actual situation of hard PVC products production enterprises, it is very important to find a more convenient and practical method to evaluate the quality of light calcium carbonate for the quality control of hard PVC products. This study proposed to evaluate the quality of light calcium carbonate by measuring the sedimentation volume, hue and apparent density, providing reliable reference for the selection and detection of light calcium carbonate.

1.Test method for light calcium carbonate

1.1 Settlement volume detection

Sedimentation volume testing is based on the principle of the different particle size and density of the settling velocity of particles in the liquid, the basic process is to put the sample into a liquid made from a certain concentration of suspension, suspension of particles sedimentation in under the action of gravity and centrifugal force, big, big density of particles sedimentation rate faster, settlement is small; The sedimentation rate is slow and the sedimentation volume is large for those with small particle size and density. Hard PV C products are commonly used in the processing of light calcium carbonate and heavy calcium carbonate, light calcium carbonate is produced by chemical methods, the density of 2.4 -- 2.7g/cm3; Heavy calcium carbonate is produced by physical-mechanical methods such as crushing and grinding of calcium carbonate ore, with a density of 2.7 -- 2.95 g/cm3. In the process of plastic forming, the wear of heavy calcium carbonate is more serious than that of light calcium carbonate, especially the wear of extruder barrel and screw. By measuring the sedimentation volume of calcium carbonate, it can be simply determined as light or heavy calcium carbonate, and also indirectly characterize the particle shape and size distribution of calcium carbonate. Calcium carbonate produced by 6 manufacturers was selected for the settlement volume test, and the results are shown in Table 1.

Seen from Table 1, the lowest settling volume of calcium carbonate in manufacturer 4 and manufacturer 5 is 1.9m g/L and 2.1m g/L respectively, while the requirements for qualified products in H g/T 2226-2000 << Industrial precipitated calcium carbonate >> are as follows: The sedimentation volume is ≥ 2.4m g/L, which indicates that the calcium carbonate of manufacturers 4 and 5 May be heavy calcium or partially mixed with heavy calcium. The settling volume of calcium carbonate in factory 1, 2 and 3 all meet the requirements of qualified products in HG/T 2226-2000. The maximum settling volume of calcium carbonate produced by Manufacturer 6 is 3.9m g/L, far exceeding the requirement of H G/T 2226-2000 superior product > 2.8m g/L. The sedimentation volume of light calcium carbonate is high, which indicates that the particles are small, the particles are loose and the gap is large. This kind of calcium carbonate is dispersed quickly when mixed with PVC.

1.2 Detection of calcium carbonate color

In the processing process of hard PVC products, in addition to the whiteness of calcium carbonate powder, the hue of calcium carbonate is also a key problem, especially for some white product formula system with more calcium carbonate, the hue of calcium carbonate directly affects the color and color difference of the product, resulting in the color of plastic products is not bright, poor gloss, For example, in the production of white PV C-U pipe and profile, the production factory should ensure that the color of the product is uniform in the process control system. White products should be controlled and the appearance of the product is not allowed to have obvious green, gray, red and other phenomena. Some PV C - U white products manufacturers in order to improve the whiteness of products, often add fluorescent whitening agent, in order to achieve the effect of increasing the white degree. However, because the fluorescent whitener often contains double bonds, it is easy to cause the formation of unstable molecular chain structure in the resin, and it is easy to decompose under high temperature and high shear. If the fluorescent whitener is improperly selected or the number of copies added is excessive, it will not only accelerate the aging of PVC products, accelerate the decomposition and yellowing, and reduce the aging performance of the products. There will also be PVC processing process is not easy to disperse or agglomerate agglomeration phenomenon. Light calcium carbonate has red phase, yellow phase, green phase, gray phase, red and yellow equal, light calcium carbonate. Both the hue and the whiteness of the powder are related to the production process and control process of light calcium carbonate, and generally have nothing to do with the particle size and particle size distribution of light calcium carbonate powder.

Calcium carbonate hue detection is to PVC, calcium carbonate, composite stabilizer according to 100:25:4 ratio thermal mixing to make the dry mixture, and then the dry mixture was minced by the torque rheometer. After the plasticization peak appeared, the test was stopped to observe the appearance color of PVC mixture plasticizer, whether there were obvious green, gray or red phenomena. Table 2 shows the whiteness of light calcium carbonate powder from 5 manufacturers and the hue when 25 parts are added. The whiteness of calcium carbonate powder from manufacturers 2 and 4 is the highest, but its hue is obviously gray. The whiteness of powder from manufacturers 1, 3 and 5 is lower than that of manufacturers 2 and 4, but its hue is better. It shows that there is no direct relationship between the whiteness of calcium carbonate powder and the background color of calcium carbonate.

In HG/T 2226 -- 2000<< Industrial precipitated calcium carbonate >>, the whiteness requirement for superior products is ≥90.0 degrees, which cannot really meet the requirements of white PV C -- U products manufacturers, as shown in Table 2: The whiteness of light calcium carbonate powder of 5 manufacturers all met the requirements of H G/T 2226-2000, but the calcium carbonate hue of manufacturers 2, 4 and 5 were all unqualified. Calcium carbonate phase color green, gray or red will directly affect the appearance of white PVC hard products correspondingly green, gray or red. Therefore, manufacturers should monitor the hue of light calcium carbonate.

1.3 Detection of apparent density

 Powder apparent density refers to the mass of uncompressed unit powder volume. Light calcium carbonate powder belongs to polycrystalline granular aggregate, and its apparent density reflects the state of particle aggregation and pore state. Calcium carbonate processing performance often has relationship with calcium carbonate particle properties, and calcium carbonate particle properties directly related to the particle size distribution, porosity, pore diameter and pore size distribution, specific surface area, density distribution, particle morphology, such as data, the use of light calcium carbonate enterprise test has certain difficulty, but related to calcium carbonate particles indirect features of apparent density, Its detection is based on the apparent density detection method of polyvinyl chloride resin, its detection equipment is simple, easy to operate, PVC hard products can be relatively easy to detect enterprises.

Table 3 shows the apparent density of inactive calcium carbonate from 6 manufacturers, among which calcium carbonate from manufacturers 5 and 6 are heavy calcium. It can be seen from Table 3 that the apparent density of heavy calcium produced by manufacturers 5 and 6 is the highest, which is much higher than other manufacturers. The apparent density of light calcium carbonate from manufacturers 1, 2 and 3 is relatively close. The apparent density of manufacturer 1 is the smallest, which is close to the apparent density of nano calcium carbonate of 0.28g/cm3, indicating that the particle size is small and relatively uniform. However, the apparent density of calcium carbonate from manufacturer 4 is significantly higher than that of light calcium carbonate from manufacturer 1, 2 and 3, indicating that the volume of calcium carbonate is relatively small and there may be a certain amount of heavy calcium carbonate particles mixed with it.

For PVC hard products production and processing, calcium carbonate added to PVC capacity, price reduction, should ensure that the apparent density of calcium carbonate in a certain range, can reduce particle aggregation porosity, absorption of additives, good processing performance, is good for improving the quality of PVC hard products. Therefore, PVC hard products production enterprises should consider the processing performance of materials, production technology and equipment, determine the suitable range of light calcium carbonate apparent density index control.

2. Conclusion

It can be seen that the sedimentation volume, hue and apparent density of calcium carbonate can simply distinguish light calcium carbonate from heavy calcium carbonate, affect the degree and effect of various additives and their dispersion, and affect the processing performance and appearance color of PVC hard products.

Light calcium carbonate chemical industry standard H G/T 2226-2000 in calcium carbonate content, hydrochloric acid insoluble matter, sieve residue, whiteness, water soluble matter, iron content and other testing items are relatively more, for PVC hard products manufacturers, not easy to directly convenient application. And light calcium carbonate, the settlement of the test items, such as volume, hue, apparent density of easy operation, convenient and practical, rigid PVC products production enterprises should be monitored for production and processing, at the same time should be different manufacturer production of light calcium carbonate, sedimentation volume, apparent density, color index, respectively the archive, as reference in monitoring the quality of light calcium carbonate.