(315b) Diffusion Measurements of Isopentane, Hexene, Cyclohexane In Polyethylene Particles

After polymerization, the un-reacted hydrocarbons must be purged to comply with environmental regulations and ensure the safety of our products. The diffusion behaviors of hydrocarbons in PE have been studied extensively. However, it’s still difficult to find appropriate values needed. There are several reasons; first, the values measured by different methods may differ largely. Second, in some literatures, D is the self-diffusion coefficients, not the mutual ones. Diffusion in PE films has been studied a lot, in the background of membrane separation, while diffusion in PE particles is seldom studied.

In the article, D of i-C₅, C₆H₁₂, C₆H₁₄ in PE particles were measured with an advanced microbalance—intelligent gravimetric analyzer, N₂ as a bypassing gas. The first step was choosing the appropriate rate of N₂, making sure the external diffusion was eliminated. The correlation between D and temperature was studied according to the Arrhenius equation, showing that diffusion was faster under higher temperature. It was also found that the smaller molecular, such as i-C₅ didn’t always diffuse faster than the larger ones. The relationships of D and the crystallinity of PE were agreed with the Flory free volume theory. Considering that the crystallinity got by different methods may differ, so D was further connected with the density and melt index values. With the increasing of density, D decreased. D seemed have no relationship with melt index. As we known, the real PE particles’ sizes are not uniform. So we also measured D of PE particles with different sizes. It was showed that D was linear with the square of particle size, nothing to do with the size distribution. The pressure effect could be neglected under the purging conditions.