Submitted to: Polymer Degradation and Stability
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/11/2012
Publication Date: 3/1/2012
Citation: Nam, S., Condon, B.D., White, R.H., Zhao, Q., Fei, Y., Santiago Cintron, M. 2012. Effect of urea additive on the thermal decomposition kinetics of flame retardant greige cotton nonwoven fabric. Polymer Degradation and Stability. 97(5):738-746. Interpretive Summary: Urea has widely been used as an additive to impart synergistic flame retardancy with phosphorus-based flame retardants to cotton, but the effect of urea on the thermal decomposition kinetics of cotton has rarely been studied. The objective was to elucidate how or why urea can enhance the flame-retarding efficacy of diammonium phosphate (DAP) on greige cotton nonwoven fabrics from the determination of activation energy of decomposition using three methods: Kissinger, Friedman, and Flynn-Wall-Ozawa methods. The results produced by these methods were consistently in agreement, revealing that urea has dual functions. Up to a certain concentration, which depends on the amount of phosphorus of DAP, urea improves the thermal stability of cotton cellulose by assisting the DAP reactions. The other function, which was predominant at high concentrations of urea, is to facilitate the decomposition of cellulose at temperatures lowered by the dehydration of DAP. These functions were further supported by the analyses on the charred fabrics using 31P MAS NMR, ATR-FTIR, and EDAX. The information derived from this study is expected to help develop new nitrogen-containing flame retardants, which makes cotton flame retardant in an efficient manner.
Technical Abstract: Urea is well known to have a synergistic action with phosphorus-based flame retardants such as diammonium phosphate (DAP) in enhancing the flame retardant performance of cellulosic materials, but its effect on their thermal decomposition kinetics has not been thoroughly studied. In this study, the activation energy (Ea) of the thermal decomposition of greige cotton nonwoven fabrics, which were treated with different concentrations of urea at fixed contents of DAP, was measured using Kissinger, Friedman, and Flynn-Wall-Ozawa methods. With a good agreement between these methods, the addition of urea at low concentrations increased the Ea above the Ea obtained from DAP treatment alone, but the excess of urea significantly lowered the Ea. These effects of urea were associated with a continuous increase of its synergistic function observed as increasing the addition of urea. Up to a certain concentration, which depends on the amount of phosphorus, urea improves the thermal stability of cellulose by assisting the DAP reactions involving pyrophosphate formation and phosphorylation. The other effect, which was predominant at high concentrations of urea, is to facilitate the decomposition of cellulose at temperatures lowered by the dehydration of DAP. These explanations were supported by the analyses on the thermal decomposition process of greige cotton fabrics and charred fabrics using thermogravimetry, 31P MAS NMR, ATR-FTIR, and EDAX.