TABLE OF CONTENTS The optical properties of interest are: refractive index and molecular refraction, polarisation, double refraction (birefringence) of isotropic and oriented PE, clarity, transparency, haze, and gloss (see Tables 3.1 and 3.9). PE granules are white and translucent, but the final appearance varies from hard, rigid, brightly coloured, glossy, flexible or transparent film to high-tenacity fibre. In the melt state the refractive index is 1.4297 and 1.4432 and the molar refractivity is 0.3297 and 0.32986 for HDPE and LDPE, respectively.
Transparency may be defined as the state permitting perception of objects through or beyond the specimen. It is often assessed as that fraction of the normally incident light transmitted with a deviation of less than 0.1 from the primary beam direction. A material with good transparency will have high transmittance and low haze. Thin films can be transparent or opaque depending on the grade and the processing conditions. The transparency can be improved by control of crystallinity and bi-orientation. The single-phase random copolymers that suppress the formation of crystal structure due to their irregular structure usually have better clarity than homopolymers. It is also important that the refractive index is constant throughout the sample in the line of direction between the object in view and the eye. The presence of interfaces with different refractive indices will cause scattering of light.
Using moulds or dies that provide a very good surface finish improves transparency. Further improvements can be made by choosing processing conditions that restrict the formation of...
