Acrylic-kaolinite composite hydrogels for controlled-release nitrogen fertilizers application
Supervisors: PERRIN François-Xavier
This thesis is part of the development of new granular fertilizers that can be used in agriculture and that combine a slow release of organic nitrogen fertilizer and a high water absorption and retention capacity. Urea was first incorporated into acrylic superabsorbent hydrogels by two methods, either by incorporation during polymerization or by a post-loading step with the preformed polymer. The first method leads to a more homogeneous polymer network with reduced pore sizes and leading at the same time to a slow release of the fertilizer and to excellent water retention capacity under pressure and swelling/reswelling capacity. Kaolinite-urea intercalates were prepared in a second part of this work by dry grinding kaolin KGa-1b with urea using a laboratory-scale planetary ball mill. A degree of intercalation close to 100% with very little amorphization of the kaolinite was obtained by grinding KGa-1b with a small excess of urea for only 15 minutes. The slow release of urea from composites consisting of a poly (acrylic acid-co-acrylamide) matrix and kaolinite-urea intercalates is linked to the coating of the kaolinite-urea complex in the hydrogel matrix as well as at the location of the urea in the interlayer space of the kaolinite. In a final part, polyacrylamide-kaolinite hydrogels were prepared in the absence of organic crosslinking agent and at room temperature. The polymerization of acrylamide is initiated at the surface of the kaolinite previously functionalized with 2-(2-dimethylaminoethoxy) ethanol. The grafting reaction was studied by X-ray diffraction, thermogravimetric analysis, IRTF spectrometry, 13C NMR and elemental analysis. The XRD results revealed an expansion of the interlayer during polymerization with a low stacking order and partial exfoliation of kaolinite. Kaolin acts as a multifunctional crosslinking agent in this composite which has good elastic properties and a high swelling rate. The polyacrylamide-kaolinite composites showed good elastic recovery properties and a high swelling capacity.
Hydrogels, kaolinite, urea, composite, slow release of fertilizers.