Gas diffusion in and through polypropylene

Polypropylene (PP) is the most used commercially tactic polymer. The packaging industry uses about 25% of the PP output, mainly as films. This is due to its remarkable barrier characteristics. Its notable resistance against heat and solvents is mainly due to the fact that it is a partially crystalline polymer, which is dense and rigid, fuses near 165°C and has a low glass transition temperahne (Tg = -18°C). 

The main qualitative features concerning the transport of gases through polymer films can be described as the condensation and solution of gas at the one surface followed by diffusion through and evaporation to the gaseous state at the other surface. 

The nature of the penetrant and the nature of the polymer are the main factors affecting the diffusivity or permeability of gases. Thus if the operating temperature is lower than or equal to the critical temperature of the penetrant vapour, there appears a strong concentration and 

If the polymer is above its glass transition tempetature, Tg, it responds rapidly to changes in its physical condition and we have Fickian or Case I diffusion. This is the simplest case, and for T T, Henry s law is valid for sorption and the diff ion coefficient is a constant (ideal Fickian diffusion). Its temperature dependence is well approximated by a simple Arrhenius expression with a constant activation energy. 

In contrast, at temperatures below Tg, we have the so-called Case II and Super Case II transport, the other extreme, in which diffusion is very rapid compared with simultaneous relaxation processes. Sorption processes may be complicated by a strong dependence on swelling kinetics. Finally we have anomalous diffusion, which occurs when the diffusion and relaxation rates are comparable. 

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