###### Abstract  
Recently, nanocomposites of polymers have attracted attention due to their advanced features compared to their complement polymer microcomposites. In this study, thermoplastic polyurethane (TPU) was used as a matrix; antimony trioxide (primary flame retardant) and montmorillonite organo-clay (secondary flame retardant), along with benzoflex (plasticizer), were used as fillers to examine their synergistic effect. Nanocomposites of various compositions (TPU-1 to TPU-6) were prepared via the melt-mixing method and compressed to form sheets of the desired dimensions with a compression molding hydraulic press machine. Characterization of the samples was conducted with Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). A tensile test was performed through a universal testing machine (UTM) which showed that the Young's Modulus improved from 147.348 MPa for the pure sample (TPU-1) to 244.568 MPa for TPU-6. A UL-94 test was executed to observe flame retardance. The sample of interest (TPU-6) achieved V-0 classification in UL-94. All these results confirmed the synergistic effect of primary and secondary flame retardants. An optimum increase in fire resistance and mechanical strength was observed for TPU-6. thermoplastic polyurethanes; nanocomposites; flame retardants; synergic effect; organo-clay +
Footnote †: journal: Applied Sciences  
Furthermore, the degraded products and toxic smoke may give rise to environmental pollution and health risks. Although several efforts have been made to address this issue, the flame-retardant efficiency of TPU still needs to be improved [3]. TPUs are linear block copolymers with alternating soft and hard segments. The soft segments are comprised of long-chain diols (commonly polyesters, polyether, silicon, polycaprolactone, or hydrocarbon) with 1000-4000 gmol\({}^{-1}\) molecular mass, while the hard segments typically consist of di-isocyanates, and the sequences are short-chain extenders. The hydrogen bonds break, and the linear chains are disintegrated at melting temperature. In the meantime, the urethane linkage (-NHCOO-) of the hard segment becomes unstable and is decomposed reversibly to alcohol and free isocyanates (Figure 1). The TPU properties can easily be adjusted by fluctuating the components and ratios of the hard and soft fragments to achieve an extensive range of PUs (polyurethanes) varying from thermosetting foams to thermoplastic elastomers [5, 6, 7]. Different processing techniques have been used to make polymer nanocomposites from polymer beads and nano additives. These include melt blending, the solution method, and in situ polymerization. From an industrial standpoint, melt blending is the most economical mass-production method as it is fast, and a solvent is not involved [8]. In recent years, montmorillonite (MMT) has attracted the most interest because of its catalytic and barrier effects [9]. It is reported that in the case of well-dispersed nanocomposites, the burning process of the polymer is slowed down, while charring is promoted due to the physical barrier effect enhanced by the ablative reassembling of the silicate [10]. The research results, however, pointed out that clays are not sufficient for commercial applications, since they fail to act as stand-alone flame retardants in important regulatory fire tests, such as UL [11]. Antimony trioxide Sb\({}_{2}\)O\({}_{3}\) (an inorganic compound) has heat stability at higher temperatures up to 570 \({}^{\circ}\)C and has application in polymeric matrices as a flame retardant [12]. Almost 20,000 metric tons of Sb\({}_{2}\)O\({}_{3}\) were consumed in the USA in 1990 for the flame retardancy of polymers. Although it is found in nature, it is difficult to use due to its impurities. To manufacture flame retardant grade Sb\({}_{2}\)O\({}_{3}\), other oxides of antimony, antimony metal or sulfide, and ore (b oxidation in air at 600-800 \({}^{\circ}\)C) are needed [13]. Many researchers have reported the flame retardant and synergistic properties of antimony trioxide and montmorillonite.