Preparation of melamine–formaldehyde resin-microencapsulated ammonium polyphosphate and its application in flame retardant rigid polyurethane foam composites

Abstract

Melamine–formaldehyde resin-microencapsulated ammonium polyphosphate (MFAPP) was prepared by in situ polymerization using melamine–formaldehyde (MF) resin as the shell material. MFAPP was characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), which confirmed its successful fabrication. MFAPP was further introduced to prepare rigid polyurethane foam/microencapsulated ammonium polyphosphate composites (RPUF/MFAPP). The flame retardancy, water resistance, physical properties, and thermal stability of RPUF/MFAPP were compared with virgin RPUF and RPUF/APP composite. RPUF/MFAPP30 possessed excellent flame retardancy. Even after immersion in water for 15 days, RPUF/MFAPP30 achieved V-1 rating in UL-94 test with limiting oxygen index (LOI) of 21.3 vol%, which was better than that of RPUF/APP30 with equivalent APP loading. The compressive strength of RPUF/MFAPP30 was 0.295 MPa, which was 13.5% higher than that of RPUF/APP30. Thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR) was applied to investigate gaseous products of the decomposition process for RPUF/APP and RPUF/MFAPP. It was found that the intensities of CO2, isocyanate compounds, and CO for RPUF/MFAPP were lower than the values for RPUF/APP, indicating superior fire safety of RPUF/MFAPP. SEM and Raman spectra confirmed that RPUF/MFAPP30 possessed more compact char residue with higher thermal resistance, which was thus better able to inhibit mass and heat transmission in combustion. Consequently, a possible gas–solid flame-retardant mechanism of the RPUF/MFAPP composite was proposed.