(651e) Degassing of Porous and Compact Polypropylene Particles

The main objective of this work is an experimental study of mass transport processes involved in the process of polyolefin production. Our investigation is focused on the mass transport in fully-grown particles with already developed porous structure. Moreover, we report on interesting observations of propylene transport in compact polypropylene.

We have utilized gravimetric measurements to obtain the dynamics of degassing and to determine basic morphology characteristics of porous polypropylene (PP) particles. We have found that a simple Fick's diffusion model is not generally capable to fit the shape of experimentally measured degassing curves of porous particles. Therefore we proposed the particle model consisting from two sizes of compact polymer granules and demonstrated that the dynamics of degassing can be described by this simplified model and that the model is capable to estimate fractions of large and small compact polymer zones in particle and the size of large compact zones (Bobak et al., 2008).

We have found the limiting step of transport through the fully-grown polyolefin particle to be the slow diffusion through polymer. Therefore we have focused our experimental investigations also on diffusion in compact polymer films and particles. Different types of experiments were performed, e.g., degassing caused by either large- or low-step pressure change. Measured degassing curves exhibit behavior which cannot be explained by simple Fick's diffusion even with concentration dependent diffusivity. We have tested number of models describing diffusion in compact semi-crystalline polymers, but none of these models was able to completely explain the observed transport behavior, although the models included swelling, swelling-enhanced flow, thermodynamic driving forces, temperature effects, etc.

Bobak M., Gregor T., Bachman B., Kosek J.: Marcomol. React. Eng. 2, 176-189 (2008).

Novak A., Bobak M., Kosek J., Banaszak B.J., Lo D., Widya T., Ray W.H., de Pablo J.J.: J. App. Pol. Sci. 100, 1124-1136 (2006).