Table 3.
Natural polymer based CRFs.
| Natural Polymer-Based | ||||
|---|---|---|---|---|
| Material | Modifier | Research Findings | Release Duration a | References |
| Biobased Polyurethane (PU) | Isocyanate, acrylonitrile modification, superabsorbent from chicken feather meal; nano fumed silica | Double layer polymer coating significantly retards the release rate. Castor oil-based PU has better adherence as the coating material. Nano fumed silica reduces porosity and pore size. Isocyanate affects the structure of PU which affects the release rate. | 14–77 days | [53,54,56,57,58,59] |
| Bio-based modified alkyd resin | Cassava Starch | Using castor oil reduces coating requirement compared to rubber oil. | NA | [55] |
| Polysulfone (SO2 and eugenol based) | - | Increasing Mw of polymer reduces the rate of degradation, slowing down the release. | 3–30 days | [60] |
| Latex | - | Urea content affects swelling degree which greatly affects the release rate. | 30 days | [61] |
| Natural rubber | Cassava starch; attapulgite/NR and NR-g-Polyacrylic acid | Hydrophobic NR can retard release rate with enhanced hydrophilicity through grafting. Multicoated CRF with NR and hydrogel shows great controlled release. | >24 h b | [62,63] |
| Starch | bentonite; cellulose nanofibril from bagasse; natural char NP; bagasse, melamine, polyvinylacetate; EC | Urea can act as a plasticizer. Modification of starch to increase hydrophobicity and the use of reinforcing agent can improve controlled release. Starch-based hydrogel shows excellent water holding capacity and retention in soil. Using an appropriate filler creates interactions which slow down the release. | 6–30 days | [4,64,65,66,67,68,69,70,71,72] |
| Cellulose | Silica NP, bentonite, montmorillonite (MMT) | Incorporation of filler into cellulose-based coating material promotes tortuous path and compactness which slows down diffusion. | 6 days–30 days; >30 days (w/MMT)c | [73,74,75,76,77] |
| Lignin | Alkenyl succinic anhydride | Water-repelling properties shows great potential to retard nutrient release | 10–30 min | [75] |
| Alginate | Κ-Carrageenan/celite superabsorbent; MMT; biogenic silica | Incorporation of filler increases porosity which improves water absorption and slows down the release. | 18–50 days; >60 days (w/MMT) | [15,80,81] |
| Chitosan | Humic substances; starch+allicin; salicylaldehyde; magnesium+natural rubber | Smaller urea crystals can be better encapsulated in the matrix for slow release. Chitosan does not provide strong effects but incorporation with other materials may promote interactions that retard release. | 7–13 days | [20,82,83,84] |
a Time required to reach 75% release; b Release experiment only conducted until 30% release; c Release experiment only conducted until 60.8% release.