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CARBOXYLATED PHOSPHONATES Carboxylated phosphonates represent a newer class of compounds which combine the effective carboxylic acid functionality of polymers such as PMA and polyacrylate with the proven capabilities of other organophosphorus compounds such as AMP and HEDP. One material that is frequently used is phosphono butane tricarboxylic acid (PBTC). As the name suggests, the structure of this material, which is shown in the below diagram, is a 3:1 ratio of carboxylic acid to phosphorus.
There are similar materials to PBTC that have carboxylic acid to phosphorus ratios as high as 16:1. One of the first uses of PBTC was as a zinc stabilizer in zinc based corrosion inhibitor programs. Because it is able to sequester the zinc ion, PBTC is used to prevent zinc precipitation in waters where the pH is controlled above 8.0. In this manner, PBTC acts to transport zinc to the metal surface where the localized high pH will cause zinc to precipitate and form its protective film of zinc hydroxide. PBTC has also proven to be an effective iron sequesterant agent and is more resistant to strong oxidizing agents such as chlorine. As a calcium carbonate antiscalant, PBTC functions well via the mechanisms of dispersion and scale crystal distortion. |
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