synopsisThe ability of selected phosphorus-containing compounds and nitrogen-containing compounds to act as flame retardants for cotton cellulose was investigated using flame tests, thermal gravimetric analysis (TGA), and differential thermal analysis (DTA). The effectiveness of the organophosphorus compounds was found to correlate with the changes which they induced in the pyrolytic degradation of the cellulose as indicated by DTA. Many of the samples containing nitrogen were found to exhibit DTA curves having pronounced exothenns. The presence of nitrogen-containing compounds in conjunction with the organophosphorus systems enhanced their ability to retard flame propagation but did not produce significant changes in t.he pyrolytic endotherm as observed by DTA. More conclusive evidence for a chemically based phosphorus-nitrogen synergism was obtained by the use of TGA. These data can be interpreted in terns of the phosphorusnitrogen synergistic effects and flame retardance mechanisms which have been proposed in the literature.
SynopsisA series of poly( 1,4-cyclohexylene dimethylene terephthalate) (PCHDT) fabrics treated with selected phosphorus-and halogen-containing flame retardants has been studied by static oxygen bomb calorimetry. The amount of heat evolved when these fabrics are burned in the open atmosphere has been determined indirectly using calculations based on Hess' law of summation. This heat evolution when corrected for contributions due to burning of the flame retardant appeared to correlate with the efficiency of the flame retardant treatment and was interpretable in terms of mechanisms of flame retardant action. Also, this technique has been applied to blends containing polyester. INTRODUCTIONCalorimetry has been a useful tool in the study of the combustion of polymeric materials. Oxygen bomb techniques have been used successfully to study the thermochemistry of treated cotton cellulose.' In this study, the amount of heat evolved by fa.brics burning in the open atmosphere was determined indirectly using calculations based on Hess' law of heat summation and assuming complete combustion of all volatilized materials. This heat evolution was found to correlate inversely with the efficiency of the flame retardant treatment and was interpretable in terms of mechanisms of flame retardant action. method is given in Figure 1, where (AH,"') polymer = total standard specific heat of combustion of the polymer; (AHpyr"') polymer = standard specific heat of pyrolysis of the polymer; (AH,"') gases = standard specific heat of combustion of the flammable gases; (AH,"') char = st>andard specific heat of combustion of the polymer char; A = heat liberated per gram in normal atmospheric burning of the polymer; and R = weight fraction of the treated polymer converted into char. In the initial stages, heat is supplied to the nonvolatile polymer substrate initiating an endothermic degradation reaction which is generally conceded to be predominantly pyrolytic in nature. There seems to be little evidence of significant oxidative processes occurring in the condensed phase.2 The products of this polymer pyrolysis diffuse to the surface of the fabric and are released into the atmosphere immediately above the fabric. It is at this point that these flammable gaseous products begin to mix with the oxygen of the air so that combustion can take place. This is an exothermic process, of course, and the heat thus liberated can be returned in part to the fabric surface to continue the polymer pyrolysis, assuring a continuous supply of fuel for further flame propagation. This leads to thc conclusion that the heat flux a t the surface of the condensed phase is one of the most critical factors in determining whether a material will exhibit self-extinguishing characteristics. Unfortunately, this is not an easy quantity to measure unambiguously. However, the heat flux should be related to the total heat liberated during the burning of a fabric, and it would be logical to expect that good flamc retardants should lowcr the amount of heat liberated during the combu...
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