Natural rubber-lignin composites modified with natural rubber-graft-polyacrylamide as an effective coating for slow-release fertilizers

Biomaterials-based coating for slow-release fertilizer (SRFs) was prepared from a composite of lignin and natural rubber (Li/NR) modified with natural rubber-graft-polyacrylamide (NR-g-PAM). A series of NR-based coating films, i.e., NR-g-PAM, Li/NR and Li/NR modified with NR-g-PAM (Li/NR-g) were prepared by direct casting from their dispersions. The casted films were tested for the water absorbency which revealed an increase in water absorption with increasing of NR-g-PAM content in Li/NR. SEM results showed a dense composite film with less defects of Li/NR-g coating films which is attributed to an enhanced dispersibility and compatibility of lignin and NR. Urea SRFs were prepared by coating urea pellets with Li/NR-g dispersions. The slow-release efficiency was determined by monitoring cumulative N amount (%) with time both in water as well as in soil. Compared with a complete N release in water within 1 h of urea precoated with ethyl cellulose, the N release from Li/NR-g coated SRFs substantially prolonged to more than 700 h. The N release kinetics in water were satisfactorily fitted with Korsmeyer-Peppas (KP) model. Accordingly, n values of KP model were found to be 0.60-0.70 suggesting the non-Fickian transport of N through Li/NR-g coatings in which the transport is affected by not only the diffusion but also the relaxation. Barens and Hopfenberg (BH) model, therefore, was employed to explain the transport of N in Li/NR-g coated urea. The fitting of experimental N releases to BH model indicated that NR-g-PAM content in Li/NR-g coatings influenced the extent of diffusion and relaxation. For NR-g-PAM content less than 25 phr, N transport through Li/NR-g coatings was dominated by the relaxation leading to relatively slow N releases. Once NR-g-PAM in Li/NR-g coatings higher than 25 phr, the diffusion became more significant than the relaxation resulting in N release to increase. For N release in soil, Li/NR-g10 coated urea revealed substantially slow releasing of N reaching only 60% of total N release after 112 days.