Multilayer thin films to enhance stratospheric balloons

A multilayer film composed of outside layers of polyethylene (PE) and an inside layer of ethylene vinyl alcohol (EVOH) is being investigated for improved helium permeability in stratospheric balloons. 

These are high-altitude, uncrewed balloons for collecting scientific and technical data.

A paper in the Journal of Polymer Science explains how thin films of linear low-density polyethylene (LLDPE) 'have been used for many decades as the envelope of zero-pressure balloons'. 

Now, 'LLDPE is being considered as the envelope structure for other types of stratospheric balloons. These balloons would have a pressurised structure with helium inside and must have a low helium permeability (i.e. high gas barrier) to retain the supporting gas longer and hence increase the flight time. [But] LLDPE is not particularly efficient in retaining helium'.

The paper on Dielectric and mechanical relaxations contribution to the microstructure understanding of PE/EVOH/PE multilayer thin films for stratospheric balloons applications explores the use of EVOH to improve helium permeability. 

The scientists from France have mimicked the multilayers composed of PE-EVOH-PE used in food packaging. Polyethylene protects EVOH from moisture, while EVOH brings the gas-barrier property PE lacks. 

The contribution of each component to the molecular mobility of PE/EVOH/PE has been observed and analysed by dynamic mechanical analysis and dielectric dynamic spectroscopy. 

The aim was to measure the improvement in helium permeability of two multilayer PE/EVOH/PE compositions (A and B) in comparison with pure PE. Multilayer B reportedly had a higher EVOH weight percentage, with the helium permeance of PE/EVOH/PE B divided by four by EVOH compared with pure PE.
 
The paper reads, 'Its crystalline phase as well as melting enthalpy has not changed with the addition of EVOH, apart from the presence of a small melting peak associated with EVOH melting. 

'The tensile mechanical properties of PE/EVOH/PE have been improved compared to pure PE due to the highly cohesive structure of the EVOH layer.'

These findings aim to enhance the development of stratospheric balloons.