Revestimento de ureia com materiais poliméricos para produção de fertilizantes de liberação controlada

Abstract

The present study reports an economically attractive improvement in the production of controlled release nitrogen fertilizer using Kraft lignin, acetylated Kraft lignin, polyvinyl alcohol (PVA) and polystyrene (PS), as well as mixtures between Kraft-PVA lignin and Kraft-PS as coating materials. Controlled release fertilizer was produced by depositing a thin layer of polymer material using a rotary drum, with urea granules coated with 10%, 15% and 25% polymeric coating percentages being produced. The materials were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), grain size, contact angle, traction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and analysis thermal differential (DTA). The fertilizers produced were placed in aqueous medium, the amount of nitrogen released in water being quantified by the spectrophotometric method. In addition, they were submitted to leaching tests using lysimeters for this purpose. The insertion of Kraft lignin in the PVA films increased the contact angle for the same, suggesting an improvement in the hydrophobicity of the outer surface of the films formed in the urea granules. In addition, an improvement in the mechanical properties of the Kraft-PVA lignin containing 25% Kraft lignin and 75% PVA lignin can be observed in relation to pure polymers with Kraft lignin contents higher than 25% the film has become more elastic and tensile strength. MEV images showed a dense cover for all the fertilizers produced, and the coatings containing Kraft lignin and blends with mass contents of 50% and 75% of lignin presented light fissures on their surfaces. All the polymers used reduced the time of release of nitrogen in aqueous medium from the urea granules compared to the uncoated urea. Considering the release of 90% of the nitrogen present in the urea, lignin acetylation increased the release time from 65 min to 44 h compared to the lignin coating in natura. This increase was due to the increased hydrophobicity of the lignin after the acetylation reaction, which was evidenced by the reduction of the contact angle measured with water drop on the surface of the polymer films. Due to its relative hydrophilicity, PVA showed a release time of 11 h. However, mixing of PVA with 25% lignin in natura increased the release time to 12 h. Already the coating with pure PS presented a release time superior to all the other polymers (more than 6 days). The mixture of PS with 25% lignin did not increase the time of nitrogen release. The fertilizers coated with acetylated lignin and blends with a 25% Kraft lignin mass content and 75% PS reduced soil leaching losses in relation to uncoated urea, since in the period of 51 days the amount of nitrogen leached in the lysimeters containing uncoated urea granules was 34.09%, whereas for fertilizers coated with acetylated lignin and 25L75PS blends the values were lower than 14%. In this study, it was possible to conclude that the use of acetylated lignin, PS and PS-lignin blends are suggested for the urea coating with times of total nitrogen release in water over 40 h. In addition, the use of these materials reduced the nitrogen leaching rate in soil. These results are significant for the process of production of controlled release fertilizers, having as justification the ease of conduction and compliance with the strategies of sustainable development, minimization and treatment of residues, in an evident approximation with the green technologies.