Titanate Nanotubes Decorated Graphene Oxide Nanocomposites: Preparation, Flame Retardancy, and Photodegradation
© The Author(s). 2017
Received: 21 April 2017
Accepted: 22 June 2017
Published: 5 July 2017
Most polymers exhibit high flammability and poor degradability, which restrict their applications and causes serious environmental problem like “white pollution.” Thus, titanate nanotubes (TNTs) were adopted to decorate graphene oxide (GO) by a facile solution method to afford TNTs/GO nanocomposites with potential in improving the flame retardancy and photodegradability of flexible polyvinyl chloride (PVC). Results show that the as-prepared TNTs/GO can effectively improve the thermal stability and flame retardancy than TNTs and GO, especially, the peak heat release rate and total heat release were reduced by 20 and 29% with only 2.5 wt.% loading. And more, the TNTs/GO also improve the photodegradability of PVC compared with the neat PVC. The reasons can be attributed to synergistic flame-retardant and photocatalytic effects between TNTs and GO. The present research could contribute to paving a feasible pathway to constructing polymer-matrix composites with desired flame retardancy and photodegradability, thereby adding to the elimination of white pollution caused by polymers.
Polymer-based materials are widely used in our daily lives and many industrial fields, due to their good properties such as low weight to strength ratio, relatively low cost, and good physical and chemical stability. However, most polymers are flammable and could cause potential hazard to human’s life and property, owing to their organic nature [1–4]. In the meantime, they usually exhibit chemical inertness and non-biodegradability, thereby producing severe white pollution to contaminate soil and water [5–8]. To deal with these issues, many researchers have made efforts to construct novel flame retardants in order to improve the flame retardancy and reduce the waste pollution of polymers.
For overcoming the flammability of polymer, researchers have explored a variety of strategies in the past decades . It has been found that the introduction of nano-fillers is effective in improving the flame retardancy of polymer matrix, and non-toxic and environmentally friendly flame-retardant additives are of special significance in responding to people’s environmental concern. Among a variety of non-toxic and environmentally friendly additives, graphene-based materials are potentially attractive, because graphene and graphene oxide (GO) with layered structure and high specific surface area can act as barriers to inhibit heat release and prevent combustion gases from contact with flame [10–12]. Particularly, graphene or GO as a significant adjuvant can be combined with inorganic nanomaterials to afford promising candidates of flame retardants [13–15]. This is ascribed to the fact that the combination of two or more components can often present a synergism or integrate different flame retarding models, thereby offering an unexpected enhancement in the properties of composites. For example, inorganic nano-fillers, metals or metal derivative-based nanomaterials, such as Ce-MnO2 , TiO2 [17, 18], MoS2 , layered double hydroxide , and ZnSn(OH)6  can be readily combined with graphene to provide graphene-based flame retardants.
The abovementioned synergistic strategy makes sense in improving the flame retardancy of polymer. However, it would still be infeasible in engineering unless the white pollution of polymer is simultaneously reduced or even eliminated. Currently available routes to dealing with the white pollution of polymer cover landfill and incineration. Landfill and incineration, nevertheless, can often cause a serious secondary pollution, such as contamination of soil and water by landfill as well as the release of toxic gas during incineration. This bottleneck, fortunately, could be overcome by applying sunlight to photodegrade waste polymer in an efficient and environmentally acceptable mode . For example, TiO2, an important solid-phase photocatalyst, can be incorporated in polystyrene to afford polystyrene-TiO2 nanocomposite film that can be efficiently photocatalytically degraded under ultraviolet (UV) illumination in air [22, 23]. Vitamin C (VC)-modified TiO2 can endow photodegradable polystyrene-TiO2 nanocomposite films with a high photodegradation efficiency, which is attributed to the formation of a TiIV–VC charge-transfer complex with five-member chelate ring structure that can prolong the separation of rapidly photogenerated charge .
In the present research, therefore, we try to combine proper flame-retardant additive with phtodegradation additive in order to simultaneously improve the flame retardancy and photodegradability of flexible polyvinyl chloride (PVC), a thermoplastic widely used in the fields of electronic industry, household electrical appliances, and building materials. We pay special attention to one dimensional titanate nanotubes (TNTs) rather than titania nanoparticles with a relatively small specific surface area, because TNTs combined with GO could have desired flame-retardant properties and photocatalytic activity towards polymer [17, 24]. Such a combination strategy might be feasible, because TNTs could catalyze charring and form a net-work structure which acts as an effective barrier to resist the release of flammable gases and change degradation pathway [25, 26]. In the meantime, TNTs with radical adsorption effect exhibit excellent smoke suppression ability as well as excellent photocatalytic activity towards Rhodamine B or waste water treatment. This article reports the preparation of TNTs decorated graphene oxide nanocomposites (TNTs/GO) by a facile solution reaction route. It also deals with the flame retardancy and photodegradation of TNTs/GO-PVC composites, with the emphasis being placed on the strategy to simultaneously improve the flame retardancy and reduce the white pollution of polymer.
PVC (for injection molding) was purchased from Tianjin Botian Chemical Company Limited (Tianjin, China). Commercial sodium titanate nanotubes (NaTA) were supplied by Engineering Technology Research Center for Nanomaterials (Jiyuan, China). Graphite powder (spectrally pure) was purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China). Ethanol (C2H5OH) was purchased from Anhui Ante Food Company Limited (Suzhou, China). Reagent grade concentrated sulfuric acid (98%), 30% H2O2 solution, hydrochloric acid, and 1, 2-ethanediamine (C2H4(NH2)2) were provided by Tianjin Kermel Chemical Reagent Company (Tianjin, China). Deionized water was prepared at our laboratory. All reagents were used as received without further purification.
Preparation of GO Nanosheets and TNTs/GO Nano-filler
GO nanosheets were prepared from purified natural graphite through the method reported by Hummers and Offeman [27, 28]. TNTs/GO nano-fillers were prepared by a simple and practical solution method. In a typical procedure, 1.5 g of NaTA was added to 150 mL of H2O under mild stirring with the assistance of sonication, and the pH of the solution was adjusted to 1.6 with hydrochloric acid. After 30 min of stirring, 0.1 g of the as-prepared GO was added to the solution and sonicated for 1 h to afford a uniform suspension. The suspension was transferred into a 250-mL flask and maintained at 70 °C for 5 h. Upon completion of reaction, the precipitate was collected by filtration and washed several times with distilled water and ethyl alcohol to remove remnant impurities. The as-obtained precipitate was dried at 60 °C for 18 h to provide the TNTs/GO nano-filler.
Preparation of TNTs/GO-PVC Composites
TNTs/GO-PVC composites filled with different contents of TNTs/GO nano-fillers were prepared with the method reported in our previous research . A series of PVC composites denoted as PVC 0.5, PVC 1.5, PVC 2.5, and PVC 3.5 (mass fraction; the same hereafter except for explanation) were prepared in the same manners except that different dosages of TNTs/GO were incorporated. In addition, PVC composites with 2.5% of TNTs and GO (TNTs-PVC and GO-PVC) were also prepared under the same condition for comparative studies.
Preparation of TNTs/GO-PVC Film
PVC powder (39 g) was suspended in 30 mL of tetrahydrofuran under 2 h of ultrasonic vibration; then, TNTs/GO (1 g) was dissolved in the suspension under 24 h of continuous vigorous stirring. Upon completion of stirring, the mixture was spread on a glass plate and dried for 72 h in an airtight vacuum vessel to afford the TNTs/GO-PVC film.
X-ray powder diffraction (XRD) patterns were collected with an X′ Pert Pro diffractometer (Cu Kα radiation; λ = 0.15418 nm, operation voltage 40 kV, current 40 mA). A JEM-2010 transmission electron microscope (TEM) was performed to observe the morphology and microstructure of various products. X-ray photoelectron spectroscope (XPS) analysis was performed on an Axis Ultra multifunctional X-ray photoelectron spectrometer, using Al Kα excitation radiation (hv = 1486.6 eV). Raman spectra were recorder on a Renishaw inVia spectrometer, laser excitation light at 532 nm. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were conducted with a DSC6200 thermal analyzer at the scanning rate of 10 °C/min. A JF-3 oxygen index meter was employed to measure the LOI values of the specimens with dimensions of 100 × 6.5 × 3 mm3. A WDW-10D microcomputer control electronic universal testing machine (Jinan Test Machine Company Limited; Jinan, China) was performed to determine the tensile strength of PVC-matrix composites. Cone calorimeter (Fire Testing Technology, UK) tests were conducted following the procedures described in ISO5660. Each specimen with the dimensions of 100 × 100 × 3 mm3 was exposed to 35 kW/m2 heat flux. The dispersion state of the additives in PVC matrix and the topography of residue chars were observed with a Nova Nano SEM 450 scanning electron microscope (SEM). An UV accelerated weathering tester (UV-II, Shanghai Pushen Chemical Machinery Co. Ltd.; Shanghai, China) was run to evaluate the photocatalytic degradation behavior of the rectangular PVC-matrix nanocomposite film (size 7.5 cm × 15 cm) under 45 °C and 40% humidity.
Results and Discussion
Microstructure of TNTs/GO Nano-filler
Raman scattering spectra were recorded to investigate the changes in the structure of the as-prepared carbonaceous materials. Figure 1b shows the Raman spectra of GO, NaTA, and TNTs/GO nanocomposite. GO and TNTs/GO nanocomposite exhibit two typical peaks of GO at about 1588 cm−1 (G band; derived from the in-plane vibration of sp2-bonded carbon atoms) and a peak at 1338 cm−1 (D band; associated with the vibrations of carbon atoms with sp3 electronic configuration of disordered graphene.) . Besides, the peak intensity ratio of the D band to G band (I D/I G) is 1.2 for GO but 1.6 for TNTs/GO nanocomposite, which also proves that TNTs are successfully incorporated into GO nanosheets to provide TNTs/GO nanocomposite through the formation of Ti–C and Ti–O–C bonds and the reduction of GO, and they are will observed in FTIR data (see Additional file 1: Figure S1) [35, 36]. Moreover, aside from the predominant Raman peaks of GO, the TNTs/GO nanocomposite shows the characteristic peaks of NaTA, and these further indicates that TNTs have been successfully incorporated into GO nanosheets, which well conforms to relevant XRD.
Peak area (A) ratios of the oxygen-containing bonds to the total carbon bonds (obtained by XPS)
Peak area ratio
A CC /A
A CO /A
A C(O) /A
A TiC /A
The possible formation process of Ti–C bonds can be described as following. TNTs have a scroll-type nanotube structure, and their (100) facets are of a stepped surface structure consisting of Ti and exposed O atoms . In an acidic solution of pH = 1.6, the walls of TNTs will undergo dehydration and structure transformation to afford defects [32, 41]. As a result, Ti–C bonds are formed between GO and TNTs to provide TNTs/GO nanocomposite through a stable chemical attachment rather than a physical absorption. Since Ti–C bonds can facilitate the interfacial charge transfer between TiO2 and graphene , the high proportion of Ti–C bonds could be of special significance for the application of TNTs/GO nanocomposite in the photodegradation catalysis.
Thermal Stability and Mechanical Properties of Flexible PVC Composites
Thermogravimetric data of flexible PVC and PVC-matrix composites in air atmosphere
Flame Retardancy of Flexible PVC Composites
Cone calorimetric data of pure PVC and its composites
pHRR (kW m−2)
THR (MJ m−2)
TSR (m2 m−2)
AMLR (g s−1)
Photodegradation of TNTs/GO-filled PVC Film
In summary, TNTs/GO nanocomposites were prepared through a facile solution method. The as-prepared TNTs/GO nano-filler can simultaneously improve the flame retardancy and photodegradability of PVC, which could be attributed to the synergistic effects between TNTs and GO. On the one hand, TNTs can suppress the re-stack of GO and promote the uniform dispersion of TNTs/GO in the PVC matrix; GO nanosheets can act as electron acceptors to reduce the charge resistance and charge recombination rate, and the GO skeleton can also act as a template for the carbonaceous char and promote the formation of multiple char under the help of TNTs. On the other hand, titanium dioxide transformed from TNTs during combustion can be anchored in the char layers to enhance the thermal stability of the char layers and accelerate the photodegradation of PVC matrix under UV irradiation. As a result, TNTs/GO-PVC composites exhibit enhanced flame retardancy and photodegradability than TNTs-PVC and GO-PVC counterparts. The present research, hopefully, would help to provide a promising strategy for constructing polymer-matrix composites with simultaneously improved flame retardancy and photodegradability, thereby shedding light on dealing with the white pollution of commonly used polymers. Further researches are to be conducted concerning the enhancement in the flame-retardant and photodegradation efficiencies of the TNTs/GO nano-filler.
This work was supported by the Ministry of Science and Technology of China (project of “973” Plan; grant no. 2015CB654703), the Scientific Innovation Talent of Henan Province (grant no. 164200510005), the Science and Technology Research Program of Henan Educational Committee (grant no. 16A430001), and the Program for Innovative Research Team from the University of Henan Province (grant no. 17IRTSTHN004).
ZL, XL, and ZZ conceived and designed the experiments. BS performed the experiments and analyzed the data. LY contributed the analysis tools. BS and ZL wrote the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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