Preparation of thermally reduced graphene oxide and the influence of its reduction temperature on the thermal, mechanical, flame retardant performances of PS nanocomposites

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Abstract

In this article, we investigated the influence of thermally reduced graphene oxides (TGOs) at different reduction temperatures on the thermal, mechanical and flame retardant performances of polystyrene (PS). The results indicated that disordered expanded layer structure can be obtained as the reduction temperature increases from 200 to 500 and 800 °C (the resulted composites are named as PS/TGO2, PS/TGO5 and PS/GTO8, respectively), which could lead to better dispersion of TGO sheets in PS matrix. Dynamic mechanical thermal analysis showed that both the storage modulus and Tg of PS/TGO5 and PS/TGO8 nanocomposites are significantly improved compared with that of neat PS. Noticeable improvement in flame retardant performance were achieved with the addition of TGO5 and TGO8, particularly TGO8, due to the removal of the functional oxygen groups from GO and the barrier effect of intumescent and loosely structure of char layers.

Introduction

Polystyrene (PS) is a widely used general plastics for numerous applications in our daily life. For many applications, particularly transportation and electrical appliances, it is very necessary to use flame retardants to reduce the flammability of PS. Among various flame retardants, halogenated compounds are good flame retardant additives for PS. However, they suffer some serious disadvantages, such as evolution of toxic gases and corrosive smoke. For environmental concerns, the introduction of halogen-free flame retardants, such as phosphorus, nitrogen, silicon, intumescent flame retardant as well as inorganic fillers, has been an effective way of improving their flame retardancy [1], [2], [3], [4], [5].

Recently, various kinds of nanoparticles have been investigated as flame retardants differing in both their chemical compositions and morphologies. Compared with micron-sized additives, nanomaterials offer several advantages, especially with respect to significantly reducing heat release rate at quite low loadings, usually less than 5 wt%. In sharp contrast, the contents of inorganic fillers such as Mg(OH)2 or Al(OH)3 are much higher and vary between 40 wt% and 65 wt% [1]. Currently, carbon nanofillers, including carbon nanotube, graphene or graphene oxide, have begun to be explored for enhancement of the flame-retardant properties of various polymer systems.

Graphene, a single-atom-thick two-dimensional carbon layer, comprised of sp2 hybridized carbon, has aroused considerable interest in developing a variety of novel composites due to its high surface area, electrical conductivity, high flexibility, and mechanical strength and exhibits great promise for potential applications in the fields of nanoelectronics, sensors, batteries, super-capacitors, hydrogen storage, and nanocomposites. The addition of graphene, even at a very low concentration level (usually less than 5%), into a polymeric matrix can significantly improve polymer properties such as mechanical, thermal, and flame retardancy. For example, incorporating graphene alone into polypropylene [6], epoxy [7], [8],waterborne polyurethane [9] matrix exhibited improved flame retardant and mechanical properties. Noticeable reduction in flammability and improvement in thermal stability of the polymer nanocomposites are achieved with the addition of graphene and other flame retardants, such as intumescent flame retardants [10], [11], metal hydroxides [12], layered double hydroxide [13], [14], montmollronite [15], and melamine polyphosphate [16], [17]. It is a novel strategy to prepare functionalized graphene oxide (FGO) grafted with other flame retardants. The prepared polymer/grafted FGO nanocomposites showed excellent flame retardance even at low additive concentrations [18], [19], [20], [21], [22], [23], [24], [25]. In our previous work, the comparison of different oxidation degrees of GOs with graphene on both flame retardancy and dynamic viscoelastic properties of the PS composites have been investigated. We found that the decreasing content of oxygen groups of GOs or graphene is benefit for the improvement of the flame retardancy and thermal stability [26]. Considered that different reduction temperature might cause the change of both chemical compositions and morphologies of GO, it is of significance to investigate the reduction degree of GO on the flame retardancy of polymer composites.

In this work, TGOs with different reduction temperatures have been prepared. PS/TGO nanocomposites have also been prepared by melt blending. The structure, morphology, dynamic mechanical properties, thermal stability, and flame retardancy of the nanocomposites were investigated.

Section snippets

Materials

GO was synthesized from natural graphite (1000 mesh) that was kindly supplied Shandong Pingdu Graphite Company (Qingdao, China). Concentrated sulfuric acid (H2SO4) (A.R.), concentrated hydrochloride acid (HCl) (A.R.) and potassium permanganate (KMnO4) (A.R.) were purchased from Nanjing Chemical Reagent Company (Nanjing, China). Sodium nitrate (A.R., NaNO3) and H2O2 (A.R., 30%) was purchased from Shanghai Renyu Chemicals Company. All these commercial chemicals were used as received without

Structure and morphology

FTIR was used to confirm the structure of GO, TGO and PS/TGO nanocomposites. Fig.1 shows the FTIR spectra of GO, TGO2, TGO5 and TGO8, respectively. In the FTIR spectrum of GO, peak at about 3410 cm−1 is assigned to the stretching vibration of COsingle bondH. Peak at 1731 cm−1 is assigned to the Cdouble bondO stretching vibration; the absorption bands at 1624, 1052 and 1224 cm−1 are assigned to the stretching vibration of double bondC, Csingle bondO and Csingle bondOH, respectively [28]. After thermal reduction, the intensity of these peaks decreased

Conclusions

Different reduction degrees of TGO were prepared at different thermally reduced structure. PS/TGO nanocomposites were prepared by melt blending method. The results from FTIR, Raman, XRD, XPS and SEM showed that TGO2, TGO5 and TGO8 provided different structure and morphology. The increase of the thermally reduction temperature is benefit for the removal of the oxygen groups form GO layers, and thus lead to an expanded disordered structure. DMA showed that both the storage modulus and Tg of

Acknowledgement

This work is supported by Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2015RCJJ002) and Jiangsu Province Science Foundation for Youths (BK20140841).

References (39)

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