Slow and controlled-release property of C18 polyol coated urea modified by C36 polyol

Abstract: In order to study the slow and controlled-release properties of dimeric fatty acid polyol (namely C36 polyol) on C18 polyol (considering as conventional coating material consisting of castor oil and soybean oil) and its potential application value as coating material, C18 polyol was used as co-coating material mixed and polymerized with C36 polyol by 1:2, 2:3, 1:1 proportion to produce three kinds of new C36 coating materials (NC36). Three kinds of NC36 and the conventional coating materials were used to manufacture four types coated urea as slow and controlled-release urea, named PCU1, PCU2, PCU3 and PCU4. The PCU1 was the control. The average coating thickness is 2.5%, calculated by mass percentage of coating material out of substrate. The pure water incubation and refractive index method was used to test the release rate and longevity of the slow and controlled-release urea before and after impacting test to evaluate the controlled release property of the C36 polyol. The results showed that the appearances of the coating for the three NC36 were homogeneous and smooth, and the coating exhibited more flexible compared to the coating of the conventional coating material (PCU1). Furthermore, the surface area of coated fertilizer particle of PCU3 after incubation was increased by 23% in average compared to PCU1. The longevity retention rate (tR) of coated fertilizer, expressed as longevity of the coated fertilizer after impacting 20 times divided by that without impacting, was applied to evaluate the controlled release property of the new coating material. The longevity retention rate(tR) of coated urea by use of the new material could be as high as 88%, which was 2.5 times higher than that of the control. When the ratio of C36 to C18 was 2 to 3 (PCU3), no significant differences in preliminary solubility rate and differential solubility of coated urea were observed compared to control, and coated urea possessed the best ability to resist the impacting to reduce the longevity under this condition. However, when the ratio of C36 to C18 beyond 2 to 3, the initial and differential release rate of coated urea were lower than that of control. The nutrient release period or the longevity of the coated urea was significantly affected by both of the proportion of C36 to C18 and the amount of the new coating material applied to produce coated urea. Although the initial and differential release rate of the controlled release urea was not influenced by adding C36 to C18 with a ratio of 2:3. The coating layer of the mixture polymerid coating material was homogeneous and more flexible shown by significant increase of diameter of the coated urea exposed to water. However, the longevity was not lasted when the amount of coating material applied was less than conventional 3% to 4%. The research results suggested that rational application of C36 polyol to C18 with a ratio of 2:3 could significantly improved the property of flexibility of the coating and maintain the longevity of coated fertilizer under condition of strong crashing. It could be concluded that combination of C36 with C18 was an effective measure to improve the controlled release ability of the C18 and maintain longevity of coated urea when the fertilizer suffering strong impacting.