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

Abstract

Investigation of thermal degradation is essential for understanding flame retardancy mechanism and further tailoring of materials. In this work, polycarbonate was compounded with solid bisphenol A bis(diphenyl phosphate) (S-BDP) and organo-montmorillonite (OMMT) to form a nanocomposite with mainly intercalated and partially exfoliated morphology, and the main flame retardancy activity of the nanocomposite was shown to be in the condensed phase as revealed by cone calorimetry, thermogravimetric analysis coupled with Fourier transform infrared spectrometry (TGA/FTIR) and thermogravimetric analysis coupled with mass spectrometry (TGA/MS). Although the main gaseous pyrolysis products of polycarbonate can't be greatly altered by S-BDP and OMMT, carbonate linkage would be stabilized and vigorous decomposition at higher temperature would be delayed, thereby char residue formation could be promoted. S-BDP also shows slight gaseous phase effect as proved by the detection of phosphorus–oxygen species in TGA/MS. Moreover, the relatively enhanced evolution of PO radicals in the sample filled with only S-BDP suggests that S-BDP alone exhibits a slightly stronger gaseous phase effect than the combination of S-BDP and OMMT. This enhanced condensed phase effect of S-BDP in the presence of OMMT could be associated with the delayed vigorous decomposition at higher temperature due to the barrier effect of OMMT. The peak heat release rate of polycarbonate could not be significantly reduced by substituting S-BDP with OMMT, yet it would prolong the time to peak heat release rate and reduce the smoke toxicity with a smaller release of carbon monoxide. The reduced carbon monoxide release was probably caused by further oxidation of carbon monoxide in the hotter char surface due to the barrier effect of OMMT.

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.