Based on the actual production and part of the tests, the quality problems of the small-bulk polypropylene powder products were analyzed and discussed. The phenomenon of product quality was analyzed and discussed, and methods to solve these problems were put forward.
The development of China's polypropylene process technology began in the 1960s and has achieved certain results for more than 30 years. In particular, the batch bulk polymerization process developed and researched by us has become a unique and mature polypropylene production process. However, with the rapid development of bulk polypropylene, the scope of application of its products has not been greatly developed. This is due to the characteristics of its production, ie raw materials, equipment, processes, operating conditions, storage and transportation environment and many other factors. Fluctuations lead to insufficient product quality and many quality problems. On the one hand, they limit the scope of product use. Most of the products are only used for the production of low-grade products such as woven bags, binding ropes, and daily necessities; on the other hand, they cause certain processing for users. Difficulties, users put forward many deficiencies.
Factors Affecting Melt Index MFR: The control index for controlling product quality is the melt index MFR. Other indicators cannot be analyzed at present. In the future, with the development of the company and the transition to formality, indicators such as ash, isotacticity, volatiles, apparent density, and tensile yield strength must be improved. MFR is regulated by hydrogenation. Only by studying the variation of MFR can we effectively control the MFR distribution of products. With the same isotactic index, the MFR increases, the average molecular weight of polypropylene decreases, the yield tensile strength increases, and the impact strength decreases. When the MFR decreases, the elongation at break of polypropylene increases. Therefore, the distribution of MFR should be controlled in production so as to improve the quality and processability of polypropylene. Factors affecting MFR, catalyst performance, process parameters, ratio of raw materials, operation of hydrogen adjustment, etc., are the main factors affecting MFR. The adoption of high-efficiency catalysts has achieved a high level of polymerization stability, catalyst yield, and anti-impurity interference, providing convenient conditions for further understanding the main factors affecting MFR changes. The actual production proves that the key to affecting the MFR control depends on the hydrogen adjustment means of operation. The hydrogen adjustment is achieved through the dissolution, diffusion and adsorption of hydrogen in the gas phase and then participating in the polymerization. As the mass ratio of hydrogen to propylene (ie, the ratio of hydrogen to alkylene) increases, the relative molecular weight of polypropylene decreases accordingly, and the MFR increases accordingly.
Main factors affecting ash: harmful impurities in raw material propylene, trace amounts of water, oxygen, sulfur, arsenic, carbon monoxide, acetylenes, diolefins, nitrogen, and the use of recycled propylene in constant oxygen, etc. are all harmful polymeric impurities that severely damage the catalyst. The activity and orientation capability requires an additional activator to eliminate impurities to protect the activity of the catalyst, which leads to increased ash content. The amount of activator added is small, the activity of the catalyst is difficult to play, the reaction is weak, the yield per single pot is low, and the product has a high ash content. With the increase of the amount of catalyst added, the single-pot production increased and the product ash content decreased. When the catalyst is increased to a certain extent, the output of single-pot reactors decreases and the ash content increases. This is because when the catalyst addition amount is too low, the polymerization is not normal due to the limitations of the catalyst itself and the destruction of impurities. There was even no reaction in the later period, resulting in a decline in the output of the single-pot reactor. The excessive addition of catalyst will cause violent reaction. At this time, it can only be forced to adopt emergency recovery methods to reduce the reaction temperature and pressure, which will ultimately lead to a decrease in the output of the single-pot reactor and increase the ash content of the product. Therefore, the proper amount of catalyst must be strictly controlled.
The product's stability is poor and its product's isotacticity is poor. The influencing factors are:
(1) The influence of propylene feedstock, such as other unsaturated hydrocarbons.
(2) The effect of hydrogen regulation, high melt index isotacticity decreased.
(3) The influence of the aluminum alkyl component, such as the impure aluminum component, may affect the isotacticity.
The high and low isotacticity mainly affects the strength of processed products. Polypropylene with low isotacticity has low strength, but isotacticity is not as high as possible. Above 96% is acceptable. In addition, isotacticity also influences the flowability of the processing. The irregularities in the products with low isotacticity are more likely to clog the filter screen, the feeding of the extruder is easily interrupted, and the feed is uneven, which affects the quality of the processed products. Therefore, in the production process, it is more critical to strictly control the various indicators of the product and to ensure the quality of the product.
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