Using TGA/FTIR TGA/MS and cone calorimetry to understand thermal degradation and flame retardancy mechanism of polycarbonate filled with solid bisphenol A bis(diphenyl phosphate) and montmorillonite
Introduction
With its high strength, thermal stability and outstanding optical transparency, polycarbonate (PC) is the second large consumed engineering plastic widely used in many areas. Although PC can form char during combustion and is less flammable than many polyolefins, more strict flame retardancy is required under certain circumstances.
Conventional flame retardants used in PC include halogenated, phosphorus-containing, sulfur-containing, siloxane compounds etc. [1], [2], [3]. Among these flame retardants, aryl phosphates such as triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP) and bisphenol A bis(diphenyl phosphate) (BDP) are commonly used in PC because of their excellent compatibility with the matrix. During combustion such aromatic phosphates can generate PO radicals to cause flame inhibition in the gaseous phase, as well as promoting char formation in the condensed phase. BDP is an aromatic phosphate with high hydrolytic resistance and it is considered to have slight gaseous effect but mainly to act in the condensed phase [4], [5], [6]. Besides conventional flame retardants, nanocomposites technology has been intensively studied recently.
Polymer nanocomposites, as one field of the cutting edge science of nanotechnology, have drawn huge interest from both academia and industry around the world due to their dramatically improved properties [7], [8], [9], [10], [11], [12]. Carbon nanotubes [13], [14], [15], [16], [17], graphene [18], clays [19], [20], [21], [22], [23], polyhedral oligomeric silsesquioxane (POSS) [24], [25], [26] and other nano-scaled materials [27], [28], [29] have been introduced into PC with the purpose of enhancing various properties. Although polymer nanocomposites exhibit much lower heat release rate than the matrices, usually they fail some protocols such as UL-94 burning tests. Thus there were some attempts to combine polymer nanocomposites technology and conventional organophosphates to improve the flame retardancy of polymers. Chigwada et al. developed flame retarded polystyrene nanocomposites filled with organo-montmorillonite (OMMT) and several organophosphates [30]. Synergy of flame retardancy between organophosphates and OMMT did occur, and the authors suggested that this methodology could be promisingly extended to other polymeric systems. Pack et al. prepared RDP-coated sodium montmorillonite (RDP MMT-Na+), and incorporated it into several polymer blends [31]. They found that RDP MMT-Na+ could not only compatibilize polycarbonate/poly(styrene-co-acrylonitrile) (PC/SAN) blend but also enhance the flame retardancy of the blend. Moreover, the flame retardant effect of RDP MMT-Na+ was more effective than ammonium-modified clays in this kind of blend.
In our previous work, we employed the melt extrusion method, a method which is favored in industrial manufacturing, to prepare flame retarded polycarbonate nanocomposite filled with solid BDP (S-BDP) and OMMT [32]. S-BDP was more convenient during transportation and processing than the conventional liquid BDP. The PC nanocomposite, with a mainly intercalated and otherwise exfoliated morphology, showed improved flame retardancy highlighted by the synergistic effect of S-BDP and OMMT. Flame retardancy mechanism of this system was primarily explained by the enhanced thermal-oxidative stability of the char residue with the assistance of OMMT. However, whether the thermal degradation of PC would be altered by S-BDP and OMMT is yet unknown.
It is believed that detailed investigation of thermal degradation of a polymer is a rather important issue while dealing with flame retardancy. It could assist us to design or optimize the formulations by understanding the thermal decomposition pathways of the flame retardants and the matrices. Thermal degradation of PC is proposed to have complex reactions including chain scission of isopropylidene linkages, hydrolysis/alcoholysis and Fries rearrangement of carbonate linkages, and cross-linking reaction to form char residue [33], [34], [35], [36], [37].
In this work, we first conducted quantitative investigation of combustion behavior of the PC nanocomposite by cone calorimetry. Then, thermogravimetric analysis coupled with Fourier transform infrared spectrometry (TGA/FTIR) and thermogravimetric analysis coupled with mass spectrometry (TGA/MS) were employed due to their powerful ability for in situ analyzing the pyrolysis products from degradation. These investigations would surely help us to further understand the flame retardancy mechanism of the PC nanocomposite.
Section snippets
Materials
Bisphenol A polycarbonate (Makrolon 2805) was purchased from Bayer (Shanghai), and solid bisphenol A bis(diphenyl phosphate) (S-BDP) was provided by Yoke Chemical Co., Ltd. The organo-montmorillonite (OMMT) was Nanomer I.44P from Nanocor Inc., which was cation-exchanged modified by dimethyl dihydrogenated tallow ammonium. PC and OMMT were dried for 12 h at 100 °C prior to process, whereas S-BDP was used as received.
Sample preparation
Dehumidified PC was melt blended with addictives by a co-rotating twin-screw
Dispersion of OMMT
According to our previous study [32], we chose three samples for investigations in this work (listed in Table 1). Polytetrafluoroethylene (PTFE) was added as an anti-dripping agent to help the samples reach V-0 rating in the UL-94 burning tests. Since the TGA/FTIR and TGA/MS tests were scheduled herein, the composition of the samples in this research should preferably be as simple as possible. Thus, PTFE was excluded as can be seen from Table 1.
With absence of PTFE, dispersion of OMMT in the
Conclusions
Thermal degradation and flame retardancy mechanisms of polycarbonate filled with solid bisphenol A bis(diphenyl phosphate) and montmorillonite were comprehensively cross-examined by cone calorimetry, TGA/FTIR and TGA/MS.
Firstly the polycarbonate nanocomposite morphology was confirmed with TEM to be mainly intercalated and partially exfoliated. Under an inert atmosphere, the main pyrolysis products evolved from thermal degradation of polycarbonate are carbon dioxide, methane, carbonates, benzene
Acknowledgment
This work was sponsored by the National High Technology Research and Development Program of China (“863” Program) (No. 2007AA03Z538). Also the authors would like to express great appreciations to Mr. Hongqiang Qu from Heibei University, for the testing and fruitful discussion of TGA/MS.
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