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Research and Analysis of Factors Affecting Melt Index of Polypropylene

Jan 08, 2019

1 Basic summary of melt index


The melt index, MFI, also known as the melt flow rate, refers to the weight of the polymer melt passing through a standard die within ten minutes at a certain temperature and load. The temperature is generally 230 degrees Celsius, the load is 2160 grams, and the standard die is 2.095 mm. The greater the melt index, the better the fluidity of the polymer melt and the lower the average molecular weight.


The main operation process of the test is as follows: First, the polymer raw material to be tested, that is, plastic, is placed in a small tank, and a thin tube having a diameter of 2.095 mm and a length of 8 mm is connected at the end of the tank. Then, after heating to 230 degrees, squeeze down and calculate the weight of the material extruded in ten minutes, which is the flow index of the plastic.

2 polypropylene basic overview

Polypropylene, or PP, is a typical thermoplastic polymer. It can be divided into three types based on the position of the methyl group: isotactic polypropylene, random polypropylene, and syndiotactic polypropylene. Its properties mainly include physical properties, mechanical properties, thermal properties, chemical stability, electrical properties and weatherability. The main features of polypropylene include the following:


Advantages: (1) The relative density is small, only between 0.89 and 0.91, which is one of the lightest plastic varieties.

(2) Good mechanical properties, strong impact resistance, and good molding processing properties.

(3) The use temperature can be as high as 110 degrees Celsius to 120 degrees Celsius, and has good heat resistance.

(4) It has good electrical insulation and is not easy to react with chemicals and does not absorb water.

(5) Better transparency, pure texture, non-toxic and harmless.


Disadvantages: (1) poor cold resistance, susceptible to the effects of light, heat and oxygen.

(2) It is not easy to color, and the ignition point is low.

(3) Poor toughness.

3Influencing factors of polypropylene melt index


3.1 Factors affecting the melt index of polypropylene


3.1.1 Exploring the effect of hydrogen on the melt index of polypropylene


Under the action of Ziegler-Natta, propylene produces a polymerization phenomenon, resulting in chain termination and chain transfer in the active center of polypropylene. On the basis of the ideal chain termination to achieve chain transfer, the catalyst activity was not destroyed, and the polymerization characteristics of the original catalytic system did not change. There are two cases of common chain termination: one is that chain termination occurs under the action of a chain terminator. Among them, water, sulfur, arsenic and other related substances that can cause catalyst deactivation will lead to chain termination. The second is the transfer of β-H. In the process of chain transfer, the active center undergoes monomer transfer in the direction of alkyl aluminum and olefin, and in this process, it is necessary to add appropriate hydrogen as a chain transfer agent to achieve molecular weight control. the goal of.


3.1.2 The melt index of polypropylene is affected by the hydrogenation method


The hydrogenation mode mainly includes two types of parallel hydrogenation and distributed hydrogenation.


Parallel hydrogenation: Hydrogen can be uniformly dispersed in the polymerization vessel, and the diffusion effect is better, so that the molecular weight in the reactor is very close and the distribution rate is narrow. At the same time, parallel hydrogenation is difficult to accurately grasp the amount of hydrogenation.


Distribution hydrogenation: easy to operate, the process is simple, only need to add an appropriate amount of hydrogen to the reactor. However, hydrogen addition in the latter two reactors by means of slurry entrainment easily affects the amount of hydrogenation and the diffusion effect of hydrogen.


According to the analysis of the practical results, there is no difference in the products of the melt index between the parallel hydrogenation and the distributed hydrogenation. The main difference lies in the problem of the narrow distribution of the molecular weight.


3.1.3 The melt index of polypropylene is affected by the degree of hydrogen diffusion


In this process, hydrogen diffusion and hydrogenation reactions are achieved by agitation and gas circulation. The stirring speed is faster, and the diffusion effect of hydrogen is better. However, in actual cases, the degree of hydrogen dispersion is generally improved by the circulation of the gas within the allowable range of the process. When entering the kettle, the circulating gas continuously moves upward from the bottom of the kettle through the form of bubbling, thereby increasing the contact surface of hydrogen and liquid phase propylene, increasing the uniformity of diffusion, promoting the chain transfer reaction, increasing the effect of heat removal, and benefiting high. The production of melt index polypropylene products reduces the fluctuating frequency of the melt index and achieves the purpose of increasing the melt index.


3.2 Exploring the effect of raw materials on the melt index of polypropylene


In this process, the device uses propylene as a polymerization monomer, hydrogen as a chain transfer agent, and an appropriate amount of Ziegler-Natta as a catalyst to help the polymerization reaction. The basic components of the raw material propylene include: propylene purity, oxygen, carbon monoxide, arsenic, total sulfur, alkanes, water, carbon dioxide, etc., wherein carbon monoxide, sulfur, arsenic, oxygen, water, unsaturated olefins, and water and oxygen in hydrogen, It is possible to cause the active center of the catalyst to be inactivated and inactivated. In particular, the high-efficiency catalyst contains TiCl4, although the occupancy rate is low, but it has a serious influence on trace impurities in the reaction medium, which is easy to cause poisoning. If the catalyst is deactivated due to severe poisoning, it will make it difficult for the polymerization product to reach the specified melt index. In addition, a certain inert gas exists in propylene, although it does not affect the activity of the catalyst, but if the content exceeds a certain range, it will occupy a large amount of reaction space, and reduce the partial pressure of hydrogen in the kettle, resulting in difficulty in controlling the melt index.

It can be seen that the purification of hydrogen and the purification of propylene contribute to the stability of the melt index.


3.3 Exploring the effect of catalyst on the melt index of polypropylene


Table 1 Relationship between different catalyst types and melt index (same amount of hydrogenation)


From the analysis of Table 1, in the case of the same amount of hydrogenation, different catalysts result in different melt indices of the products. Strictly speaking, there are differences in hydrogen sensitivity due to the manner in which the catalyst is formulated and the composition of the catalyst. Therefore, if it is necessary to replace the catalyst during the production process, the amount of hydrogenation must be adjusted so that the melt index is kept within a stable range.


When the melt index of the produced product is low, the difference between the melt index and the hydrogenation amount of the first reaction product is not large. However, when the melt index of the produced product is high, the melt index and hydrogenation amount of the first reaction product are high. There are big differences. Therefore, when producing products, different hydrogenation amounts should be selected according to the specific conditions of the products, and the catalyst should be used reasonably.


4 Conclusion


Based on a brief introduction of the melt index and polypropylene, the article explores the factors affecting the melt index of polypropylene, and uses relevant experiments to analyze the data and draw relevant conclusions. According to the previous experimental research, the amount of hydrogen and the type of catalyst have the greatest influence on the melt index of polypropylene during the polymerization. At the same time, the increase of hydrogenation amount will increase the melt index of polypropylene. The effective mixing of antioxidants with antioxidants and powders will also affect the change of polypropylene melt index. The raw materials and catalysts will cause changes in melt index. Related conclusions


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