DARTS CRACKS THE CRYSTAL HAZE

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DARTS CRACKS THE CRYSTAL HAZE
01 November 2004 - CCLRC Daresbury Laboratory

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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About: CCLRC Daresbury Laboratory
DARTS is a unique service offering solutions to materials characterisation problems that are unattainable in the conventional laboratory. This is possible because it makes use of the Synchrotron Radiation Source (SRS). Advantages of using the SRS include:

• The ability to study small or weakly diffracting samples. Improved resolution, enabling structures intractable in the home laboratory to be solved.

• The ability to select the optimum wavelength for a particular application.

• Additional specialised techniques only possible using synchrotron radiation can be exploited.

The DARTS team's ability to access and combine a range of complementary techniques provides information to help manufacturers control processing conditions and modify manufacturing techniques to improve a wide range of products. Even the everyday crisp packet has benefited from DARTS leading edge technology.

Applications can cover materials from agrochemicals, pigments, polypeptides, microporous materials, organometallics, catalyst materials, minerals and samples from processing plants.

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