Investigation on the thermal degradation of flexible poly(vinyl chloride) filled with ferrites as flame retardant and smoke suppressant using TGA-FTIR and TGA-MS

A series of ferrites (ZnFe2O4, MgFe2O4, NiFe2O4, and CuFe2O4) was prepared by wet air oxidation and their structure and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The prepared ferrites were added to flexible poly(vinyl chloride)(PVC) as flame retardants and smoke suppressants. Subsequently, thermal degradation properties of flexible PVC with ferrite additives were investigated by using TGA-DTG, TGA-FTIR, and TGA-MS. The main mechanism underlying the flame-retarding activity was shown to be caused by the catalyzed the dehydrochlorination of PVC at the first stage and breaking of the char residue at higher temperature. The results showed that addition of ferrites, especially MgFe2O4, at the first stage, not only decreased T5% (defined as the temperature at which 5% of the initial weight has been lost) and increased the weight loss rate, but also promoted cross-linking and stability of char residue. The main gas pyrolysis products were HCl and benzene. When compared with neat PVC, PVC/MgFe2O4 and PVC/CuFe2O4 generated more HCl and less benzene, and consequently retained more carbon in the matrix with respect to the increased flame-retarding and smoke-suppressing. At the second stage, the weight loss of flame-retarding PVC increased obviously, with simultaneous increase in the yield of main gaseous pyrolysis products, CO2 and aliphatic fragments. PVC/CuFe2O4 generated a high yield of CO2, because of the cationic cracking reactions in the presence of Lewis acid, fragmenting of the char residue at higher temperature. Additionally, the inner surface morphology of the char residue derived from PVC/MgFe2O4 was investigated by SEM and EDS, which showed the existence of alveolate and breakable structures, as well as needle-like structures, composed of a mixture of iron chloride and magnesium chloride.