Comparative Studies on Thermal , Mechanical , and Flame Retardant Properties of PBT Nanocomposites with Functionalized Amino-Carbon Nanotubes Modified by Different Oxidation State Phosphorus-containing Agents

High-performance poly(1,4-butylene terephthalate (PBT) nanocomposites have been developed via the consideration of phosphorus-containing agents and amino-carbon nanotube (A-CNT). One-pot functionalization method has been adopted to Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 17 January 2018 doi:10.20944/preprints201801.0161.v1 © 2018 by the author(s). Distributed under a Creative Commons CC BY license. Peer-reviewed version available at Nanomaterials 2018, 8, 70; doi:10.3390/nano8020070


Introduction
Owing to the superior dimensional stability and heat resistance, poly(1,4-butylene terephthalate) (PBT) has been extensively applied in the electrical and electronics industries [1,2].Nevertheless, PBT resins without flame retarded modification are easily ignitable either by an electric spark or short circuitry within the electrical network which could lead to serious melt-dripping, rapid flame propagation and large production of smoke in an untenable fire condition.With attempts being made to fire-proofing PBT, only few nanoadditives, e.g.nano-clay [3][4][5], and graphene [6] have been explored to improve the flame retardancy of PBT, as well as other properties.A decrease (66%) of the pHRR values for PBT with the presence of 5% organoclay was reported by Camino et al. [3] which was prepared by the ion exchange of original alkaline cations with dimethyl hydrogenated tallow benzyl quaternary ammonium chloride.Wang and co-workers [6] prepared MnCo2O4-graphene (GNS) hybrids, which were then added into the PBT matrix via a masterbatch-melt blending method.The peak heat release rate and smoke production rate values of MnCo2O4-GNS/PBT composites were decreased by 39.4 and 35.7%, respectively.Apart from the reported flame retardant additives, aluminium phosphinate and aluminium hypophosphite with similar molecular structure and relatively high oxidation state of phosphorus atom have proven to be very effective flame retardants for PBT, due to a combination of gas-phase flame inhibition effect and barrier effect of char layers in the condensed phase [4,[7][8][9].
Carbon nanotubes (CNTs) have attracted enormous attention due to their outstanding mechanical, electrical, and thermal properties [10][11][12].Various applications from nanodevices to nanocomposites have been considered.One promising application of CNTs lies in the development of polymer nanocomposites since the incorporation of CNTs into polymers at a very low loading can lead to substantial enhancement in the thermal, electrical, mechanical, and flame retardant properties [13][14][15].However, CNTs have poor dispersion characteristics in common solvents and polymeric materials [11,16].Covalent functionalization of CNTs has been considered as an effective method to overcome the shortcoming of CNTs [11,[16][17][18].For example, CNTs covalently functionalized with pyrrolidine exhibited a solubility of 50 mg/mL in chloroform [17].CNTs grafted with intumescent flame retardant (PDSPB) promoted the distribution in acrylonitrile-butadiene-styrene copolymer, leading to improved flame retardancy [11].
Functionalization of CNTs with tri(1-hydroxyethyl-3-methylimidazolium chloride) phosphate (IP) improved the dispersion of CNTs in polylactide (PLA), and thus the thermal stability and flame retardancy of CNTs/PLA nanocomposites were significantly enhanced [18].Consequently, CNTs covalently functionalized with suitable organic modifiers contribute to the improvement of the overall properties of the corresponding polymer nanocomposites.
Although lots of efforts have been made to achieve the functionalization of CNTs, most of the methods generally involve complex reactions during the treatment process.To the best of our knowledge, a systematic investigation of CNTs functionalization by phosphorus-based flame retardant units with different oxidation state has not been reported.
Also, there is still a great need to purposefully develop a simpler approach to achieve a multi-functional CNTs-based polymer.In reinforcing the flame retardant effect via the use of phosphorus-containing compounds, a simple one-pot functionalization method has been utilized to prepare the covalently functionalized CNT via the reaction between amino-carbon nanotube (A-CNT) and different oxidation state phosphorus-containing agents.In this study, A-CNT reacts with chlorodiphenylphosphine (DPP-Cl,), diphenylphosphinic chloride (DPP(O)-Cl), and diphenyl phosphoryl chloride (DPP(O3)-Cl) to prepare the different functionalized CNTs.More importantly, the current study is aimed

Preparation of functionalized CNTs
Functionalized CNTs was prepared based on the method reported in our study [19].

Preparation of PBT/functionalized CNTs nanocomposites
Before melt processing, PBT, A-CNT, DPP-A-CNT, DPP(O)-A-CNT, and DPP(O3)-A-CNT were continuously dried at 80 o C for 12 hours.In a typical experiment, 0.5 g A-CNT was blended with 49.5 g PBT to prepare the nanocomposites using a XK-160 twin-roll mill (Jiangsu, China) at 235 o C at a constant rotation speed of 100 rpm for 10 mins.Tensile properties were evaluated by a WD-20D universal testing machine according to the standard ASTM D-638.The width and thickness of specimens were 4.0 ± 0.1 mm and 2.0 ± 0.1 mm, respectively.The crosshead speed was set as 20 mm/min.Five runs for each sample were measured, and the average value was recorded.
Flame retardant properties were investigated on a FTT cone calorimeter (UK) based on the ISO 5660-1 standard.The sample size was 100 × 100 × 3.0 mm 3 .All samples were wrapped by a layer of aluminum foil, and they were then irradiated under a heat flux of 35 kW/m 2 .Residues were analyzed by SEM coupled with energy dispersive X-ray (EDX).
The surface elements were attained from EDX on an EMAX energy spectroscopy (HORIBA, Ltd., Japan).grafted with these organic species clearly demonstrated better dispersion characteristic in DMF, which is consistent with the reported results [11,19].

Thermal properties
The melt and non-isothermal crystallization behaviors of neat PBT and PBT/DPP(Ox)-A-CNT nanocomposites were characterized by DSC. Figure 5 depicts the thermal behavior curves recorded for all samples at the heating and cooling scan of 10 o C/min.The thermal parameters obtained from the thermograms are summarized in Table S1.In the heating scan, the influence of DPP(Ox)-A-CNT on melting temperature (Tm) of PBT can be seen to be negligible.As can be seen from Figure 5 (a), the multiple melt behavior is observed for neat PBT.The multiple melt behavior is caused by the fusion of a certain amount of original crystals, followed by the recrystallization and final melting of more perfect crystals, partly formed during primary crystallization and through the recrystallization process occurring during the heating scan [30][31][32].However, the overlapping of two peaks of PBT mix forms a new peak with the introduction of DPP(Ox)-A-CNT nanoparticles.This indicates that the presence of DPP(Ox)-A-CNT nanoparticles improve the crystallization process, leading to the formation of more perfect and stable crystals, which becomes melt at higher temperature [32][33][34].Therefore, the first melting peak shifts to high temperature region, resulting in the reduction of multiple melt behavior.reported in our previous work [18,35].Functionalized CNTs show more significant heterogeneous nucleation effect on the crystallization process of PBT.On the other hand, the Tc value of the three nanocomposites increases with the increasing quantity of oxygen atom, due to the different hydrogen-bond interaction: (PBT/DPP(O3)-A-CNT > PBT/DPP(O)-A-CNT > PBT/DPP-A-CNT).The strong hydrogen-bond interaction promotes the PBT macromolecular chains to envelope the CNT nanoparticles threreby resulting in the formation of non-covalent crosslinking points, which can strongly restrict the segmental motion of PBT chains [27][28][29].Consequently, the crystallization process of PBT is accelerated.When the crystallization begins at higher temperature, more perfect and stable crystals will be formed, which is beneficial to the improvement of mechanical strength as well as the reduction of multiple melt behavior.

Thermal decomposition behaviors
The thermal decomposition behaviors of neat PBT and its nanocomposites under nitrogen condition are shown in Figure 6, and the corresponding data are summarized in Table 2. From Figure 6 (a), it is seen that the thermal decomposition behavior of each sample exhibits a one-stage degradation process.Neat PBT leaves only 2.7 wt% char residue at 700 o C. In the thermal decomposition process, the main volatiles, composed of butadiene, carbon dioxide, tetrahydrofuran, benzoic acid and ester derivatives, are released, leaving small solid residues with acidic and anhydride structures [17].PBT/DPP(O3)-A-CNT shows however inferior thermal stability due to the unstable organic phosphate grafted on the surface of DPP(O3)-A-CNT, while the thermal stability is still higher than that of neat PBT.The results show that the presence of DPP(Ox)-A-CNT nanoparticles can significantly enhance the thermal stability of the PBT nanocomposites, which aligns with findings in our previous studies [18,35].The improved thermal stability is attributed to the excellent thermal conductivity and homogeneous dispersion of functionalized CNTs [36,37].there is no substantial influence on the maximum mass loss rates (MMLR).These results demonstrate that the introduction of DPP(Ox)-A-CNT can improve the thermal stability and char yields, but does not alter the decomposition pathway of PBT.

Tensile properties
The tensile properties of neat PBT and its nanocomposites are illustrated in Figure 7.
The corresponding data are summarized in Table 3. From the stress-strain curves, it is seen that the introduction of the three DPP(Ox)-A-CNT nanoparticles into PBT improves the tensile strength due to nano-reinforcing effect of CNTs with ultra-high aspect surface area [27,38].PBT/DPP(O3)-A-CNT shows the highest tensile strength of 62.1 MPa As shown in Figure 7 and Table 3, the elongation at break for the PBT nanocomposites decreases with the introduction of DPP(Ox)-A-CNT nanoparticles (PBT/DPP-A-CNT > PBT/DPP(O)-A-CNT > PBT/DPP(O3)-A-CNT), while the tensile strength values show the opposite trend.PBT nanocomposites become brittle in comparison with neat PBT, because of the increased stiffness of the PBT nanocomposites and the micro-voids formed around the nanotubes during the tensile measurement [27,39].The elongation at break for PBT nanocomposites is in relation with the interfacial interaction between CNTs and PBT matrix.The stronger interfacial adhesion, the more difficult segmental stretching and motion of PBT chains, the lower elongation at break [39].PBT is a kind of aromatic polymers which releases lots of smoke and toxic gases during burning process [42][43][44].The reduction of smoke and toxic products during the combustion process is a very important consideration in view of the tenability condition for

a
constant weight.These functionalized CNTs are denoted as DPP-A-CNT, DPP(O)-A-CNT, and DPP(O3)-A-CNT.The preparation route of the functionalized carbon nanotubes is depicted in Figure 1.

Figure 1 .
Figure 1.Schematic representation of the preparation route for covalently functionalized

Figure 4
Figure 4 shows the schematic pattern of the hydrogen-bond interaction between

Figure 4 .
Figure 4. Schematic representation of the hydrogen-bond interaction between

Figure 5 .
Figure 5. DSC curves of heating scan (a) and nonisothermal crystallization (b) at a rate of The addition of DPP(Ox)-A-CNT nanoparticles results in the improvement of thermal stability and char yields of PBT, and PBT/DPP(O)-A-CNT is seen to be more efficient than the other two functionalized CNTs.For example, the T-5% value is increased from 367 o C for neat PBT to 388 o C for PBT/DPP(O)-A-CNT, and the Tmax value is improved from 408 o C to 419 o C.

Figure 7 .
Figure 7. Tensile stress-strain curves of neat PBT, PBT/DPP-A-CNT, The HRR and THR curves under a heat flux of 35 kW/m 2 are shown in Figure 8 (a) and (b), and the related data are listed in Table 4 respectively.Neat PBT burns at 120 s (TTI value) with a high PHRR value (944 kW/m 2 ).The incorporation of DPP(Ox)-A-CNT nanoparticles lead to the slightly reduced TTI values and increased full width at half maximum for the HRR curves.The PHRR value decreases from 944 kW/m 2 for neat PBT to 759 kW/m 2 for PBT/DPP-A-CNT, 668 kW/m 2 for PBT/DPP(O)-A-CNT, and 710 kW/m 2 for PBT/DPP(O3)-A-CNT, with reductions of 20%, 29%, and 25%, respectively.From the THR curves shown in Figure 8b, it can be observed that PBT/DPP(O)-A-CNT demonstrates the lowest THR value, which is reduced by 5% compared to neat PBT.The results show that the introduction of DPP(Ox)-A-CNT nanoparticles inhibits the heat release through the barrier effect of the char residues and nanoparticle networks, of which DPP(O)-A-CNT exhibits the highest reduction on HRR and THR.

Preprints
(www.preprints.org)| NOT PEER-REVIEWED | Posted: 17 January 2018 doi:10.20944/preprints201801.0161.v1Peer-reviewed version available at Nanomaterials 2018, 8, 70; doi:10.3390/nano8020070occupants in enclosed environments.Figure 8 (c) and (d) exhibits the SPR and total smoke production (TSP) curves of neat PBT and its nanocomposites.The corresponding data are summarized in Table 4. Neat PBT shows high peak SPR (PSPR) and TSP values.The presence of DPP-A-CNT results in the occurrence of wide and flat SPR curve.However, the PSPR and TSP values of PBT/DPP-A-CNT are slightly increased.The incorporation of DPP(O)-A-CNT and DPP(O3)-A-CNT reduces the PSPR value.It decreases from 0.207 m 2 /s for neat PBT to 0.189 m 2 /s for PBT/DPP(O)-A-CNT and 0.183 m 2 /s for PBT/DPP(O3)-A-CNT with reductions of 9% and 12%, respectively.The TSP values of PBT are also reduced by the addition of DPP(O)-A-CNT and DPP(O3)-A-CNT.The results indicate that the introduction of DPP(O)-A-CNT and DPP(O3)-A-CNT effectively inhibit the smoke production during the combustion process of PBT nanocomposites.

From Figure 9 ,
it is clearly seen that the highest char yields for PBT/DPP(O)-A-CNT are left after cone calorimeter tests.Results of TGA indicate that DPP(O)-A-CNT shows more outstanding carbonization effect on PBT, which is consistent with the higher residual yield for PBT/ DPP(O)-A-CNT (7.4 wt%) in cone calorimeter tests.Hence, the incorporation of DPP(O)-A-CNT particles can preferably promote the carbonization of PBT matrix during the combustion process.

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 17 January 2018 doi:10.20944/preprints201801.0161.v1
Peer-reviewed version available at Nanomaterials 2018, 8, 70; doi:10.3390/nano8020070at fabricating high-performance PBT nanocomposites filled with functionalized CNTs.The dispersion of different functionalized CNTs in PBT matrix is evaluated, and the influence of these functionalized CNTs on the mechanical, thermal and flame retardant properties of the resultant PBT nanocomposites is subsequently assessed.

Table 1 .
Calorimetric data of the melting and nonisothermal crystallization processes for

Table 3 .
Tensile properties for each sample.

Table 4 .
Cone calorimeter data for each sample at 35 kW/m 2. (TTI: time to ignition; PHRR: peak heat release rate; THR: total heat release; PSPR: peak smoke production rate; TSP: total smoke production; PCO2P: peak CO2 production; PCOP: peak CO production.)In a fire scenario, CO and CO2 are the main toxic gases generated from the burning of Preprints (www.

Table 5 .
EDX data of the residues for PBT nanocomposites after cone calorimeter tests.