Water Vapour Permeability of PLA

Siparsky et al. [121] showed that the increase in the L-lactide content in PLA induced no significant modification of the water vapour permeability whatever the temperature or relative humidity, except a decrease in the permeability coefficient at 90% RH between 50 and 90% of L-lactide content. This was not confirmed by Tsuji et al. and Auras et al. who showed a constant permeability, around 1.8-1.9xl0 kg.m.m ls .Pa at 20°C and 40-90% RH, despite an increase in the L-lactide content [106,136]. 

The crystallinity degree does not reveal any change in the solubility coefficient whatever the temperature and the relative humidity are [120]. Consequently the permeability coefficient seems to be controlled by the diffusion coefficients which increase with the crystallinity degree at certain temperature and relative humidity [121]. This is in contradiction with the decrease in permeability coefficients, reported by Tsuji et al. and Shogren [120, 136]. Indeed, in their study the diffusion and solubility coefficient decrease slightly with the crystallinity degree of PLLA [138]. 

Furthermore, the effect of the temperature on measured water vapour permeability changes in function of the study. Auras et al. showed a decrease of the permeability with the temperature, whereas Shogren highlighted an increase [5,120]. Siparsky et al. [121] showed that the diffusion coefficients increase and the solubility coefficients decrease with the temperature at 90% RH, which is contradicted by Holm et al. who showed higher values of moisture sorption at a higher temperature [139]. The various and contradictory results show that the mechanism of water transport in PLA is not completely understood yet [5]. In particular the presence of water clusters in the PLA matrix and their potential to diffuse in cluster form are still controversial. 

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