Flame retardancy of poly(styrene-co-acrylonitrile) by the synergistic interaction between clay and phosphomolybdate hydrates

A combination of montmorillonite (MT) clay and catalysts was used to explore possible synergistic effects in reducing the flammability of poly(styrene-co-acrylonitrile) (SAN). Several catalysts, including ammonium phosphomolybdate hydrate (NHPMo), melamine phosphomolybdate hydrate (MEPMo), zinc phosphomolybdate hydrate (ZnPMo) and sodium phosphomolybdate hydrate (NaPMo), were used. The combination of MT (Cloisite 20A) with NHPMo showed synergistic enhancement in improving the char formation and reducing the peak mass loss rate of SAN40 as compared with SAN40 composites containing MT or NHPMo alone, while similar synergistic performance was not obtained between MT and NaPMo or ZnPMo or MEPMo. The better flame retardancy with this combination is probably due to the two primary aspects. One is the increased catalytic activity of the combination of Cloisite 20A and NHPMo probably due to the overlapping degradation temperature range of the combination of 20A/NHPMo and SAN40, which probably results in more char formation; the other is more NHPMo being around or on the clay stacks while less is in the galleries of the clay; this probably bridges the clay stacks and strengthens the clay network with char formed during the degradation process. High temperature rheological data confirmed the formation of a stronger network structure in SAN40/20/NHPMo; this benefits flame retardancy by allowing fewer cracks to form in the outer char layer on the polymer mass. The more continuous char layer suppresses bubbles transport of fuel vapors and heat transfer through the char layer, thereby reducing the mass loss rate.