A Study On Ammonium Polyphosphate/Red Phosphorus Compound As Flame Retardant In Polypropylene

Polypropylene(PP) as one of the largest output general resin has been widely used in various fields with its excellent electrical insulation, corrosion resistance, lightness and other characteristics. But due to the low oxygen index of polypropylene, it is combustible with releasing poisonous gas, so it is of great significance to give polypropylene a certain flame retardant property without changing the mechanical properties. With the development of the flame retardant technology, halogen-free flame retardant has become a new trend. Among those inorganic flame retardants, the ammonium polyphosphate and phosphorus are two kinds of environmentally friendly flame retardant with significant effect. But they are easy to absorb moisture thus resulting deterioration when exposed to the air for a long time, which would reduce the flame retardant efficiency, therefore, they can be modified by surface coating method.In this paper, the ammonium polyphosphate coated by melamine formaldehyde resin(MFAPP) was prepared by means of in-situ polymerization, the best coating conditions were obtained by measuring the water solubility of MFAPP as follows:in the performed process,the ratio of melamine and formaldehyde was 1:3,the time was 30 min, the pH was 9, and the temperature was 80℃;in the process of coating, the pH was 5, the temperature was 80℃, the ratio of prepolymer and ammonium polyphosphate was 1:3, the reaction time was 2 h. The effect of coating was characterizedby infrared spectroscopy(FTIR) and scanning electron microscopy(SEM), and the flame retardant mechanism of the coated ammonium polyphosphate flame retardant was analyzed by thermogravimetry(TG). Through PP compounded with the coated ammonium polyphosphate, the influence of the coated ammonium polyphosphate dosage on the properties and mechanical properties of PP was studied. The experimental results showed that the surface of the coated ammonium polyphosphate was obviously different from that of the uncoated ammonium polyphosphate, and melamine formaldehyde resin coating on the surface of the ammonium polyphosphate would decompose first thus increasing the charring amount in the process of combustion, which slowed down the decomposition rate of ammonium polyphosphate.In addition, with increasing the content of the coated ammonium polyphosphate, the flame retardant ability of PP composite was enhanced gradually, and when the concentration of the coated ammonium phosphate was 25%, the PP composite could pass through the horizontal burning test; but the vertical burning test level was UL 94 V-2 grade; the mechanical properties was similar with pure PP, and the tensile strength of it decreased more.In order to increase the charring amount of flame retardant, improve the flame retardant effect of flame retardant, red phosphoruscoated by magnesium hydroxide was prepared.Through the determination of moisture absorption rate, the optimal coating conditionswere obtained as follows: the temperature was 80℃, the reaction time was 2h, the pH was 12, the stirring speed was 600r/min. The coating effect was characterized by means of SEM; the decomposition processand ultimate charring amount of flame retardant were characterized by TG; the flame retardant properties and mechanical properties of the composite compounded by Mg(OH)2-en-P, MFAPP and PP were investigated.The results indicated that the surface of Mg(OH)2-en-P had a lot of small Mg(OH)2 particles; after the thermal decomposition of the flame retardant, the charring amount of Mg(OH)2-en-P was very high playing the role of synergistic flame retardant with MFAPP; when the dosage of MFAPP and Mg(OH)2-en-P were 20% and 5% respectively, the vertical burning test level of the composite increased to UL 94 V–1 grade, the flexural strength and notched impact strength were respectively 42 MPa and 6.18 kJ·m-2 changing little compared with the pure PP; the tensile strength decreased to 24.15 MPa.