And within the soft segment phase, permeability decreases with increased crystallinity. As an example, Schultze notes, if you want higher barrier properties, use ester materials, but if you want a breathable material, such as for a mattress cover, use ether-based urethane. Elevated temperatures also increased permeability, showing that TPU materials, as with most plastics, are less effective barrier materials at higher temperatures. Schultze said this could be especially important in gasket applications. His paper concluded additional study is needed to better understand complex transport behavior related to activation energy but, in general, confirmed the applicability of standard equations.
Moisture also affects permeability.
Types of TPU that tend to absorb more moisture have decreased barrier properties compared to TPU materials that tend not to absorb moisture, although the variations are small, Schultze said. He noted that chemistries that produce increased soft segment in the molecule have a larger impact on the effect of moisture. But in general, TPU films have stable barrier properties over a wide range of humidity. Schultze noted that his research confirmed existing information about the difference in gas transport rates for various gases. This is dependent on the diffusion and sorption characteristics of each gas.