Synergistic effects of microencapsulated ammonium polyphosphate with melamine-formaldehyde resin/ethyl cellulose on flame retardancy and hydrophobicity of cellulosic paper.

In this study, ammonium polyphosphate (APP) was sequentially microencapsulated with melamine-formaldehyde resin (MFR) and ethyl cellulose (EC). The results indicated that the pristine APP exhibited a smooth surface, and its water solubility and water contact angle (WCA) were 0.23 g/100 mL and 29.28°, respectively. After microencapsulation with MFR, the surface roughness of MFAPP slightly increased, while the solubility declined to 0.16 g/100 mL, and WCA rose to 83.46°. Upon further microencapsulation with EC, the EC@MFAPP surface became significantly rougher, with solubility dropping to 0.04 g/100 mL and WCA increasing to 135.89°. The char residue at 800 °C was improved from 25.27% for APP to 50.49% for EC@MFAPP. In addition, when the three APPs were incorporated separately into the pulp, the retention rate of MFAPP increased from 26.65% (APP) to 31.14%, while that of EC@MFAPP reached 74.97%. For paper containing MFAPP, the limiting oxygen index (LOI) rose from 28.70% to 29.60%, and the WCA increased from 98.08° to 103.14°. After double encapsulation, the WCA of EC@MFAPP/Pulp showed a further enhancement of 119.98° and an LOI of 34.70%, and a slight decrease in tensile index. Notably, the EC@MFAPP/Pulp exhibited self-extinguishment upon flame removal and formed a denser char layer. These char residues exhibited higher nitrogen and phosphorus content, along with a greater graphitization degree, which effectively hindered heat and gas transfer. Therefore, the EC@MFAPP significantly enhanced both the hydrophobicity and flame retardancy of cellulose paper.