Machine direction orientation of high density polyethylene (HDPE): Barrier and optical properties

- HDPE films were uniaxially stretched in MD direction using commercial technologies.
- Barrier and optical properties were studied as a function of draw ratio (DR).
- As a function of DR, water vapor transport rate goes through a maximum.
- Properties are a strong function of the inherent resin physical properties.

In this study, blown films were produced from three different high density polyethylene (HDPE) resins. Further, these blown films were oriented along the machine direction using either traditional machine direction orientation (MDO) or compression roll drawing (CRD MDO) technology. The stretched/oriented films were compared for their water vapor transport and optical properties as a function of the extent of orientation (i.e., draw ratio, DR). In addition to that, these properties were directly compared between the oriented and unoriented (blown) films of the same thickness. The present study revealed that the water vapor transmission rate (WVTR) as a function of draw ratio (DR) went through a maxima. Detailed morphological characterization showed that initially, at low DR, stretching resulted in lamellar organization (along the MD). Consequently, the tortuosity in the diffusion path decreased and the overall WVTR increased. However, beyond a critical DR, lamellar crystals transformed to microfibrillar crystals. During this process the amorphous content of the HDPE chains decreased and geometric redistribution of the amorphous phase occurred along with some orientation. As a combination of all these effects, the WVTR was found to decrease beyond the critical DR. Unlike transport properties, optical properties monotonically improved as a function of DR. Further, for both, the barrier and optical properties, the absolute values were found to be a strong function of the resin physical properties. Finally, compression roll drawing technology (CRD MDO) utilizes a patented process that simultaneously applies a stretching and compressive force to orient polymer chains homogeneously without any tiger striping (uneven deformation). Consequently, CRD MDO stretched samples showed a lower surface roughness compared to their traditional MDO stretched counterpart. Therefore, for the same resin and DR, CRD MDO stretched samples displayed markedly better barrier properties and optics, especially lower total haze and higher gloss.

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