Thermal melt processing to prepare halogen-free flame retardant poly(vinyl alcohol)

A thermal melt processing method to prepare halogen-free flame retardant poly(vinyl alcohol) (PVA) was successfully established by using water and triphenyl phosphate (TPP) as the internal plasticizer and external lubricant respectively, both of which also played the roles of flame retardants. Series of processing parameters including the melt point, crystallinity, torque and die pressure of the composites showed that water greatly decreased the melting point and crystallinity of PVA by internal plasticization. Meanwhile, during extrusion, hydrophobic TPP could migrate to the surface of the extruded strips, forming a lubricating film that can effectively retard the quick volatilization of water and also prevent serious adherence between the polymer melt and the screw. Consequently, the incorporation of both the modifiers guaranteed a favorable thermal melt processing of PVA. Vertical combustion (UL94), limiting oxygen index (LOI), thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), micro-scale combustion calorimetric (MCC) and cone calorimetric analysis were employed to evaluate the flame retardance as well as to reveal the corresponding mechanisms. It indicated that the plasticizer water and lubricant TPP remarkably increased the LOI and decreased the heat release rate (HRR). However, both of them mainly functioned in the gaseous phase due to low evaporation temperatures. When combined with another nitrogen-phosphorus flame retardant with higher thermal stability, melamine polyphosphate (MPP), the charring behavior in the condensed phase was greatly reinforced, thus endowing PVA with satisfactory flame retardance.