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News

DARTS cracks the crystal haze

CCLRC Daresbury Laboratory : 01 November, 2004  (New Product)
The cause of the haziness affecting a transparent polymer food packaging material was discovered by DARTS by combining two complementary techniques available on the synchrotron.
Transparent packaging suits both consumers, who can see what they are buying, and manufacturers who can use the visual appeal of the product to entice the customer. However, if the package looks imperfect or deteriorates in storage then this advantage is negated.

The polymer film in question consisted of a core of polypropylene with a 0.6 micron surface layer of random copolymer of propylene and ethylene. The haze effect was known to be caused by the scattering of light from crystallites, but more information about the exact nature and size of these crystallites was needed in order to eradicate the problem.

The use of Grazing Incidence X-ray Diffraction (GIXD) allowed the molecular arrangement of the coating layer alone to be examined. The X-rays are directed at a very shallow angle through the surface and are diffracted without penetrating the core material. The results revealed that despite the presence of ethylene in the coating film there was still a significant degree of crystallisation occurring with crystal sizes large enough to scatter visible light.

Small Angle X-ray Scattering (SAXS) used on the bulk material showed that hazy samples contained crystallites of around 200 Angstrom in size.
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