Effect of nitrogen additives on flame retardant action of tributyl phosphate: Phosphorusenitrogen synergism Sabyasachi Gaan a , Gang Sun a, * , Katherine Hutches a , Mark H Engelhard b a Division of Textiles and Clothing, University of California, One Shields Avenue, Davis, CA 95616, USA b Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA Received 20 May 2007; received in revised form 6 October 2007; accepted 18 October 2007 Available online 25 October 2007 Abstract The effects of three nitrogen additives (urea, guanidine carbonate, and melamine formaldehyde) on the flame retardant action of cotton cel- lulose treated with tributyl phosphate (TBP) were investigated in this research. The limiting oxygen index (LOI) of treated cotton cellulose clearly revealed the synergistic interactions of TBP and nitrogen compounds. The Kissinger method was used to evaluate the kinetics of thermal decomposition on treated cellulose. The results show that adding nitrogen additives increases the activation energy at a higher degree of deg- radation, thus indicating better thermal stability at higher temperatures. Scanning electron microscope pictures of chars formed after a LOI test show the formation of protective polymeric coatings on char surfaces. Evaluating char surfaces using attenuated total reflectance-Fourier trans- form infrared spectroscopy and X-ray photoelectron spectroscopy suggests that these coatings are composed of species containing phosphoruse nitrogeneoxygen. Possible chemical interactions of phosphorus and nitrogen compounds during the burning process and the formation of a protective coating could be the reason for the observed synergism. Potential reaction pathways contributing to the formation of this protective polymeric coating have also been proposed. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Phosphorusenitrogen synergistic action; XPS; Activation energy of decomposition; Char; SEM; ATR-FTIR 1. Introduction The means of combustion and the flame retardancy of poly- meric materials are very complex processes. Despite tremen- dous progress in studying the flame retardancy of polymeric materials, ideal flame retardants (FRs) for cellulosic materials are still under development, and new technologies are urgently needed. Increased environmental concerns regarding the use of current FRs have raised standards for developing future FRs. In addition, the existing theories of combustion processes and the mode of action of FRs are continuously being revised. Recent findings by Taatjes et al. regarding the pyrolysis behav- ior of hydrocarbons have changed the theory of the pyrolysis mechanism of hydrocarbons that was accepted for a long time [1]. New reaction mechanisms of phosphorus FRs on cellulosic fibers have been proposed as well [2,3]. It has been well established that there is a phosphoruse nitrogen (PeN) synergistic action in the flame retardancy of cellulosic fibers. Many N-containing compounds, such as urea (UR), cyanamides, dicyandiamide, guanidine salts, and melamine and its derivatives, can dramatically improve the FR efficacy of P-FRs, even though they have limited flame re- tardancy by themselves [4e6]. In general, N compounds can interact with P-FRs either positively or negatively, i.e., syner- gistic and antagonistic FR effects, respectively. When the combined action of the FR and the N compound exceed the sum of their individual actions, the phenomenon is known as ‘‘synergism’’. ‘‘Antagonism’’ refers to the case when the combined actions of the two compounds are less than the sum of their individual actions. N compounds like nitriles have been reported to have an antagonistic effect on P-FRs on cellulose [7]. * Corresponding author. Tel.: þ1 530 752 0840; fax: þ1 530 752 7584. E-mail address: gysun@ucdavis.edu (G. Sun). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.10.013 Available online at www.sciencedirect.com Polymer Degradation and Stability 93 (2008) 99e108 www.elsevier.com/locate/polydegstab