Improved Quality Control for Plastics Industry

(NIST: Gaithersburg, Maryland) -- In a tour de force of measurement science, researchers at the National Institute of Standards and Technology (NIST) have developed and issued for sale a new test material for calibrating quality control equipment used extensively by the polymer (plastics) industry. The new material, NIST Standard Reference Material (SRM) 2881, “Polystyrene Absolute Molecular Mass Distribution Standard,” provides the absolute molecular masses of 43 different lengths of polystyrene molecules, and their cumulative contribution to the total mass of the sample from 1 to 99 percent.

Developing the new SRM involved a long process of analyzing and optimizing all aspects of the sample preparation, an international comparison of results from different laboratories, and developing improved tools for measuring individual molecular masses using a technique called "matrix-assisted laser desorption/ionization time of flight" (MALDI TOF) mass spectrometry.

Almost all industrial plastics are manufactured in continuous processes. Raw ingredients are fed into chemical reactors that cause the individual base units, called monomers, to link together into the long repeating chains that make them polymers. By its nature, the process produces plastic molecules that have a variety of lengths (and therefore different molecular weights), varying from a few tens of monomers long to hundreds or thousands, depending on a variety of process variables. This molecular mass distribution is tricky to control, but the product’s physical properties heavily depend on getting the correct distribution, so manufacturers are intensely interested in ways to accurately monitor it.

Typically, this is done by periodically sampling the product using chromatography, a measurement technique that separates molecules by length. For years, NIST has provided polymer mass reference materials for use in calibrating these instruments, but the earlier reference materials were certified by the laboratory only for an average distribution. Essentially, they would pin down only a couple of values across the total mass distribution. The SRM is the product of a five-year effort, offering manufacturers a test sample with a known and verified distribution of molecular masses.

For further information, visit www.nist.gov/public_affairs/techbeat/tb2009_0310.htm#polymers.

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Founded in 1901, the National Institute of Standards and Technology (NIST) is a nonregulatory federal agency within the U.S. Department of Commerce. Headquartered in Gaithersburg, Maryland, NIST’s mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.