X-Ray analysis of structural evolutions in MDO-PE films

PE (Polyethylene) based mono-material flexible packaging design presents a new alternative to conventional design as it has the potential to be mechanically recycled efficiently. Conventionally outer layer of multilayer flexible packaging is usually made from non-PE materials which can provide adequate stiffness and barrier properties to the overall package. Stiffness of PE films can be enhanced
significantly by MDO (Machine Direction Orientation) process. During MDO process the polymer film is stretched in machine direction at a certain orientation temperature which depends on polymer being stretched.

X-Ray diffraction is a powerful non-destructive characterization tool which requires only small amount of sample and can reveal critical microstructural information. Our work presented X-Ray analysis of structural evolutions in MDO (Machine Direction Oriented)-PE films which results in enhancement of stiffness.
5 different PE resins and blends chosen for study were processed in a lab scale setup to produce MDO-PE films. One selected resin was further processed at different orientation temperatures. X-Ray analysis was performed for the analysis of shape, size, and orientation of the crystals.

WAXS and SAXS profile revealed average crystallite dimension and long period of orthorhombic crystal structure after stretching and it increased with increase in orientation temperature indicating higher partial melting and strain induced re-crystallization. Apparent crystallinity increased after stretching and increase was higher for comparatively higher density resin films. Crystallite dimension decreased more after stretching for relatively higher density resin films indicating relatively higher fibrillation of the lamella. Orientations quantified by Herman orientation factors presented highly oriented structure towards stretch direction.

Our study concluded that enhancement in crystallinity due to fibrillation and re-crystallization of initial crystalline structure combined with highly oriented structure in stretch direction leads to enhancement of stiffness of MDO-PE films which can be further used as outer layer in PE based design for flexible packaging applications.