DETERMINING PERMEABILITY OF COATINGS OF POLYMER-COATED UREA

Permeability of polymer coatings on urea varies greatly with the type of polymer. A conventional test of measuring the effectiveness of coating involves a 7d static dissolution rate of coated urea into concentrated urea solution, but the results are only qualitative. Our approach was, instead, to make quantitative measurements of permeability, and so make more accurate predictions of release rate of urea across a membrane. A simple device, consisting of a container attached to vertical pipes at the bottom, was constructed to determine permeability of coats on urea granules. A turbulent flow of water ran over a 2 cm pack of coated-urea granules so urea did not accumulate at the outer surface of the coated granules. Separate determinations with two thicknesses of coats (8.8 and 14.7 mum) were conducted with water at 12 or 31-degrees-C. Permeability and activation energy of permeability were calculated. A comparison was also made between release rate of urea calculated from permeability and that determined by 7d dissolution rate method at 23-degrees-C. Nearly 100 h were required for 100% release with the thick coating, but only 20% urea was released after 168 h with the 7d dissolution rate method.