Preparation And Properties Of Flame Retardant Polyoxymethylene Composites

Polyoxymethylene(POM)is one of the five major engineering plastics.Because of its high strength,high modulus,the performance characteristics of POM is often used instead of copper,iron,aluminum as well as many other metals.POM also has very good fatigue resistance,self-lubrication and dimensional stability,which makes POM widely applied in the industrial machinery,automobile,electrical and electronic industry,sports equipment,etc.Because of high degree of crystallinity of POM,POM is very sensitive to low notched impact strength,and thermal stability of POM.Its limit oxygen index is only 15%and easy burning,and its burning releases large amounts of toxic formaldehyde gas.The above two characteristics(big brittleness and easy-burning)greatly limit POM’s wide applications.In recent years,the requirement of flame retardant performance becomes more and more serious.So far the domestic and foreign researchers never stop exploring of POM flame retardant,but POM flame retardant and toughening problems still have not been able to get a good solution.So this kind of topic has great significance.This thesis mainly studies the ammonium polyphosphate flame retardant modification and their modified flame retardant flame retardant POM,including the APP particles coating modification and the particles of the flame retardant effect.Firstly,the synthesis process of flame retardant ammonium polyphosphate(APP)was studied,and PUAPP particles with thermoplastic polyurethane and thermoset polyurethane were prepared,respectively.The absorption characteristics of PUAPP particles were characterized by infrared absorption spectroscopy,transmission electron microscopy(TEM)and so on.The infrared absorption peaks of PUAPP showed obvious carbonyl absorption peaks,indicating that PUAPP had polyurethane composition.In addition,the morphology of PUAPP particles was observed by transmission electron microscopy(TEM).It was observed that PUAPP particles had perfect core-shell structure.The water solubility and hydrophobicity of PUAPP particles were also studied,which indicated that PUAPP particles had lower water solubility and strong hydrophobicity than APP particles.Strong hydrophobicity indicated that PUAPP flame retardant particles have better compatibility with the polyoxymethylene matrix.In addition,the thermal stability of PUAPP particles was compared.The thermal stability of PUAPP was better.The two different polyurethane coating ammonium polyphosphate particles have different effects,in which the thermosetting polyurethane resin modified ammonium polyphosphate particles have better hydrophobicity,thermal stability.The surface modification of APP particles led to improved compatibility between APP particles and polymers,which has certain practical significance in further expanding the application of APP flame retardants.Secondly,the flame-retardant properties of modified poly-ammonium phosphate particles are better than those of unmodified ammonium polyphosphate particles.Our results showed that UL-94 test can achieve the V-O level(APP flame retardant composite material is V-1),limiting oxygen index(LOI)can reach 66.5%.The APP flame retardancy of the same proportion of early PUAPP particles is obviously better than that of flame retardant effect.PUAPP-PU layer on the surface of the particles can act as a carbon agent,as the behavior of the composites formed by the carbon layer can also be effective separation of acidic substances and contact of POM matrix,and slow down the acid catalysis of POM matrix decomposition.Coated modified ammonium polyphosphate particles in the combustion process of flame retardant POM composites have a low heat release rate(HRR),total heat release(THR)and higher carbon rate.Two kinds of modified PUAPP in flame retardant were both superior to APP.Polyoxymethylene(POM),because of its structural characteristics,makes it difficult for the flame retardant composite materials.During the processing operation,a small amount of acid and alkali will lead to a large number of POM’s decomposition.In addition,the collective group is also very obvious temperature,processing temperature range of about 20 degree Celsius.Although polyphosphoric acid has good flame-retardant effect,but the decomposition of polyphosphoric acid in the combustion process produces acidic substances will catalyze the decomposition of the POM matrix,accelerating the combustion of POM.