Why is the melt index of polypropylene different? Read on to know how to find the PP you want in a polymer plant.

The Melt Index is a value that represents the flow of aplastic material during processingand is based on the DuPont method of identifying the properties of plastics, otherwise known as the melt flow rate.

Polypropylene is a thermoplastic resin obtained through the polymerization of propylene and is divided into three main categories: isotactic polypropylene, random polypropylene and inter-routine polypropylene. Strict control of the melt index of polypropylene, so that the melt index is within the corresponding allowable values, is conducive to good processing performance and quality assurance of polypropylene products.

Basic overview of the Melt Index

The Melt Index, or MI, also known as the melt flow rate, refers to the weight of a polymer melt passing through a standard mouth mould in ten minutes at a given temperature and load. It is typically 230 degrees Celsius with a load of 2160 grams and a standard orifice mould of 2.095 mm. The higher the melt index, the better the flow of the polymer melt will be and the lower the average molecular weight will be.

The main operation of the test is as follows: first the polymeric material to be tested, i.e. the plastic, is placed in a small trough with a thin tube of 2.095 mm diameter and 8 mm length attached to the end of the trough. Then, after heating to 230°C, the material is squeezed downwards and the weight of the material squeezed out in ten minutes is calculated, which is the flow index of the plastic.

Study of factors influencing the melt index of polypropylene

I. Factors influencing the size of the melt index of polypropylene

1. Investigating the effect of hydrogen on the melt index of polypropylene

The polymerization of propylene in the presence of Ziegler-Natta leads to chain termination and chain transfer in the active centre of polypropylene. Ideal chain termination achieves chain transfer on the basis that the catalyst activity is not destroyed and the polymerization characteristics of the original catalytic system remain unchanged. Two common scenarios for chain termination exist: firstly, chain termination occurs in the presence of chain terminating agents. Where water, sulphur, arsenic and other relevant substances that can cause catalyst deactivation, all cause chain termination to occur. The second is the transfer of β-H. In the process of chain transfer occurring, the active centre undergoes monomer transfer in the direction of alkyl aluminium and olefins, and care needs to be taken in this process to add appropriate hydrogen as a chain transfer agent for the purpose of molecular weight control.

2. Polypropylene melt index as influenced by hydrogenation method

The two main types of hydrogenation are parallel hydrogenation and distributed hydrogenation.

Parallel hydrogenation: The hydrogen can be dispersed evenly in the polymerisation kettle and diffuses well, resulting in very close molecular weights and narrow distribution rates in the reactor. At the same time parallel hydrogenation makes it difficult to accurately determine the amount of hydrogen to be added.

Distributed hydrogenation: easy to operate, simple process, only requires the addition of hydrogen via the appropriate amount to the reactor. However, hydrogen addition to the latter two reactors by means of slurry entrainment tends to affect the amount of hydrogen added as well as the hydrogen diffusion effect.

The analysis of the practical results shows that there is no difference between parallel and distributed hydrogenation in terms of melt index products, the main difference being the width of the molecular weight distribution.

3. The melt index of polypropylene as influenced by the degree of hydrogen diffusion

In this process, the diffusion of hydrogen and the hydrogenation reaction are achieved using stirring and the circulation of the gas. The faster the stirring speed, the better the hydrogen dispersion. However, in practice, the higher the hydrogen dispersion is achieved by the circulation of the gas, generally to the extent that the process allows. During the feeding of the kettle, the circulating gas constantly moves upwards from the bottom of the kettle in the form of bulging bubbles, thus increasing the contact surface between the hydrogen and the liquid phase propylene, increasing the homogeneity of the diffusion, promoting the chain transfer reaction, increasing the heat withdrawal effect, benefiting the production of polypropylene products with a high melt index, reducing the frequency of fluctuations in the melt index and achieving a higher melt index.

II. Exploring the effect of raw materials on the melt index of polypropylene

In this process, the plant uses propylene as the polymerisation monomer, treats hydrogen as the chain transfer agent and adds an appropriate amount of Ziegler-Natta as a catalyst to aid the polymerisation reaction. The basic components of the feedstock propylene include: propylene purity, oxygen, carbon monoxide, arsenic, total sulphur, alkanes, water and carbon dioxide, among which carbon monoxide, sulphur, arsenic, oxygen, water, unsaturated olefins as well as water and oxygen in hydrogen may cause the catalyst active centre to be attacked and inactivated.

In particular, the high efficiency catalyst contains TiCl4 which, although less occupied, has a serious effect on trace impurities in the reaction medium and can easily lead to its poisoning. If the catalyst is deactivated due to severe poisoning, it will make it difficult to achieve the required melt index for the polymerisation product. In addition, there is a certain amount of inert gas in propylene, which does not affect the catalyst activity, 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, making it difficult to control the melt index. It follows that purifying the hydrogen with refined propylene helps the melt index to remain in a stable state.

III. Investigating the effect of catalysts on the melt index of polypropylene
The analysis in Table 1 shows that different catalysts, for the same amount of hydrogen addition, result in different melt indices for the product, strictly due to the way the catalyst is formulated and the different components within the catalyst that make the hydrogen adjustment sensitivity different. Therefore, if the catalyst needs to be changed during production, the amount of hydrogen added must be adjusted to keep the melt index within a stable value.

When the melt index of the product is low, the difference between the melt index of the first reaction kettle product and the amount of hydrogenation is not large, however, when the melt index of the product is high, the difference between the melt index of the first reaction kettle product and the amount of hydrogenation is large. Therefore, when producing products, different hydrogenation amounts should be chosen according to the specific situation of the product and the catalyst should be used reasonably.