Reactive extrusion of ammonium polyphosphate in a twin-screw extruder: polydispersity improvement

Reactive extrusion has been widely applied to polymerization processes with high-viscosity materials. In this work, the co-extrusion of monoammonium phosphate (MAP) and urea for the synthesis of ammonium polyphosphate (APP) was performed in a co-rotation twin-screw extruder (TSE). APP with a maximal polymerization degree less than 10, was obtained for the potential application as fertilizers where the polydispersity index (PDI, defined as the ratio of weight-average molecular weight to number-average molecular weight M-w/M-n) is a key parameter for controlled release of the phosphorus in soil. The properties of samples along the screws, such as molecular weight distributions (MWD), were characterized via ion chromatography (IC), Fourier-transformed infrared spectrum (FT-IR), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A pyrophosphate-controlled growth mechanism of PDI is revealed, and viscous dissipation in TSE could extend the urea-assisted polycondensation stage. The resulting PDI value is increased to 1.272 at a screw rotation speed of 150 rpm. This reactive extrusion for inorganic polymers is highly efficient and is flexible in controlling the polymer PDI.