Facile preparation of phosphorus containing hyperbranched polysiloxane grafted graphene oxide hybrid toward simultaneously enhanced flame retardancy and smoke suppression of thermoplastic polyurethane nanocomposites
Introduction
TPU has been widely utilized in the areas of automotive, aerospace, coatings, and electronics industries due to its excellent soft elasticity, abrasive resistance, and desirable break strength [1], [2], [3]. However, the high flammability, heavy smoke, and accompanied release of toxic gases during combustion such as CO, HCN, and NOx restrict the further applications of TPU [4], [5]. Therefore, it is important to search for highly efficient and halogen-free flame retardants to inhibit the fire hazards of TPU [6], [7]. It has been demonstrated that the incorporation of a small amount of two-dimensional (2D) nanofillers can improve the fire safety of polymers and restrain the release of toxic gases during combustion due to the layered structure and high aspect ratio [8], [9], [10]. However, the problem with the heterogeneous distribution of 2D nanofillers in polymer matrices needs to be resolved urgently because of the serious aggregation and incompatibility between 2D nanofillers and the polymer matrices, which will deteriorate the mechanical properties of polymers [11]. The surface functionalization of 2D materials as multifunctional filllers for TPU can improve the flame retardant efficiency as well as enhance the compatibility between nanofillers and polymers. The peak heat release rate (PHRR) of TPU nanocomposite was dramatically reduced by 40% with the addition of 3.0 wt% melamine cyanurate (MCA) functionalized MXene (Ti3C2TX) due to the superior barrier effect of MXene nanosheets as well as the catalytic effect of TiO2 nanoparticles [12]. The TPU composites containing 2.0 wt% tannin modified black phosphorous (TA@BP) showed a significant suppression in the release of toxic CO gas from TPU, which was attributed to free radical scavengers generated by TA@BP nanosheets that could react with flammable fragments to cut off the combustion reaction [13]. It was proved that the PHRR and the smoke product rate (SPR) of TPU nanocomposite with h-BN@SiO2@PA were reduced dramatically [14]. During the combustion, the surface of h-BN produced an electron hole to oxidize and reduce toxic gases, and the Si element-enhanced char layer served as thermal insulation to slow down the penetration of combustion products.
Recently, graphene oxide (GO) has been employed as a flame retardant nanofiller for polymers due to its superior barrier effect and large specific surface adsorption capability [15]. The heat conduction and char residue barrier produce a labyrinth effect in which the heat and combustible gas must follow a flexural path to the fuel, and this effectively prevents the flame from spreading [16], [17], [18]. In addition, the continuous and compact carbon layer can restrain heat and oxygen from entering into the underlying polymer [19]. Because the surface of GO is rich in oxygen-containing functional groups, it is more active than graphene and can react with them to improve its properties [20]. The PHRR and total heat release (THR) of carbon fiber/epoxy resin (CF/EP) with 3 wt% GO-DOPO were reduced by 38.9% and 24.7%, respectively. The 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) modified GO nanosheets dispersed uniformly in the CF/EP composites to hinder the transformation of heat flow and promote the formation of char residues [21]. The GO nanosheets preprocessed by phosphorus-nitrogen-containing dendrimers (PND-GO) reduced PHRR and total smoke production (TSP) of PU resin. Under the action of heat flow, the phosphoric acids produced by PND-GO promote the process of cross-linking carbonization, increase the char yields, and improve the fire safety of PU composites [22]. The PU foam containing polydopamine modified GO (PDA/GO) exhibited a significantly reduced PHRR and an average heat release rate (AHRR), which is attributed to the multiple effects of the anti-oxidation function of PDA during the coating process and the formation of a porous intumescent structure of PDA/GO composites [23]. However, the agglomeration tendency of GO in the polymer matrix and the easy decomposition of GO during the combustion process result in low flame retardant efficiency.
Hyperbranched polysiloxane (HBPSi) is a special type of three-dimensional random halogen-free silane with superior flame retardancy, good thermal stability, excellent toughness, low viscosity, surface energy and interface tension [24]. Because HBPSi has a high concentration of terminal multi-reactive functional groups (OH, NH2 CC), it is well suited for surface modification of inorganic fillers to improve interface compatibility. A fully capped amino HBPSi (Am-HPSi) flame retardant was synthesized, which effectively reduced the flammability of bismaleimide/diallyl-bisphenol A resin (BDM/DBA). This improvement in fire safety can be ascribed to the synergistic effect that the N element generates multiple non-combustible gases to dilute the surrounding oxygen concentration and the Si element aggregates in the gap to generate a char layer with a continuous and compact structure [25]. Functionalized graphene oxide nanosheets (FGO) with phosphorus-nitrogen-containing hyperbranched flame retardants as nano-additives effectively reduce the PHRR and THR values of polystyrene (PS) materials and inhibit the release of flammable volatiles [26]. The uniform dispersion of FGO enhanced the barrier effect and char formation during the pyrolysis process, resulting in the enhancement of flame retardancy of PS composites. It has also been reported that the unique cross-linked network of HBPSi combined with a low phosphorus content load helps to improve the flame retardancy, thermal stability, toughness and stiffness of the cyanate ester/GO hybrid resin [27]. Up to now, the combination of HBPSi and GO as flame retardant additives has not been utilized in the TPU matrix yet.
In this work, a novel organic-inorganic flame retardant P-HBPSi was synthesized via a covalent chemical reaction between DOPO and vinyltrimethoxysilane (VTMS). The P-HBPSi@GO was then prepared and mixed with TPU using the melt compounding method. The presence of HBPSi contributed to achieving the uniform dispersion of GO in the TPU matrix with highly efficient flame retardancy. TPU nanocomposites simultaneously combine flame retardant elements phosphorus, nitrogen and silicon. It was expected that P-HBPSi forms an inorganic ceramic carbon layer through a strong Si-O-Si structure and catalyzes polymer carbonization in the condensed phase, and generates ·PO2, ·PO free radical scavengers in the gas phase [28]. Furthermore, a new chemical covalent bond is formed between the three-dimensional Si-O-Si network structure in P-HBPSi and the polyarylene carbon structure in GO, improving the carbonthe char's thermal stability [29], [30]. The effects of P-HBPSi@GO on the thermal stability, flame retardancy, and volatile gaseous products of TPU nanocomposites were systematically analyzed and the synergistic flame-retardant mechanism was explained based on comprehensive evaluation.
Section snippets
Materials
DOPO and 2, 2-azobi-sisobutyronitrile (AIBN) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). VTMS was bought from Hubei Jianghan New Materials Co., Ltd. (Jingzhou, China). GO was supplied by Soochow Tanfeng Graphene Co. Ltd. (Soochow, China). Thermoplastics polyurethane (TPU, Elastollan 1195A) was purchased from BASF Co. Ltd. (Shanghai, China). Methanol, ethanol, and methyl ethyl ketone were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ion exchange resin
Structural characterizations of P-HBPSi
The FT-IR spectra of DOPO, VTMS, P-VTMS and P-HBPSi are shown in Fig. 1. It is observed that there are three peaks at 921, 1118 and 1210 cm−1, which correspond to the stretching vibration of the POPh, PPh and PO groups, respectively. The results confirm the presence of the phospaphenanthrene group in P-VTMS. The stretching vibration peak of PH for DOPO at 2437 cm−1 disappears in the spectra of P-VTMS and P-HBPSi. The peak at 1103 cm−1 is assigned to the stretching of SiOC bonds in the spectrum
Conclusions
In this work, a novel organic–inorganic P-HBPSi flame retardant was synthesized via a covalent chemical reaction, and subsequently grafted onto GO (P-HBPSi@GO) as an effective flame retardant for TPU. SEM observation showed that P-HBPSi@GO had good compatibility with the TPU matrix. Moreover, the addition of [email protected] increased the stiffness without deteriorating of the toughness of TPU nanocomposites. The cone calorimeter results indicated that the PHRR and THR of TPU/[email protected] were
CRediT authorship contribution statement
Wenjie Huang: Writing – original draft, Investigation. Jingshu Huang: Investigation. Bin Yu: Resources, Investigation. Yuan Meng: Data curation. Xianwu Cao: Methodology, Supervision, Funding acquisition. Qunchao Zhang: Methodology, Writing – review & editing, Supervision. Wei Wu: Conceptualization, Writing – original draft, Methodology, Project administration, Funding acquisition. Dean Shi: Supervision, Writing – review & editing. Tao Jiang: Supervision, Writing – review & editing. Robert K.Y.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2016YFB0302300), the Natural Science Foundation of Guangdong Province (2021A1515012425), International collaboration research fund of Guangdong Province (2020A0505100010), the Overseas Famous Scholar Funds of Guangdong Province (2020A1414010372), and the Opening Project of Key Laboratory of Polymer Processing Engineering (South China University of Technology), Ministry of Education of China (KFKT1904).
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