Improved Barrier Properties in Flexible Plastic Substrates

The influence of a mica thin film barrier on gas permeation properties was investigated. A mica barrier decreases the permeation of helium through polymer substrates by factor of up to 45. Bending these samples causes an increase in the permeation through the sample. Earlier attempts were made using small size mica flakes to reduce the permeability of flexible polymer films. Aligning these flakes by casting a suspension of flakes in a polymer solution resulted in permeability drop by a factor of 10. Experiments done in this work showed a decrease in the permeability by up to a factor of 45 using a single layer of a thin film of mica thickness up to 30 microns. A bent PEN-mica sample shows comparatively higher permeation, even when it is shown that mica can be easily bent without damaging the material. A possible answer is that on making a stack, the mica layer is sandwiched between PEN samples using adhesive. The stacking arrangement is not perfect and there are a significant number of air bubbles between the layers, causing uneven stress points on the mica. The misfit strain and the shear stress induced by the forced bending may cause a break in the thin mica film. More studies on the effects of mechanical stress on making PEN-mica stacks and thus their barrier performance are required. Also, a better way to stack the different layers is required to minimize the number of air bubbles present between the layers, thus increasing the transmittance. It is evident from the results that the permeation through mica films is defect driven. Other barrier layers can be used in conjunction with mica to improve the barrier properties. Mica is a promising candidate as a permeation barrier.