Impact of Spherical Anatase TiO2 Nanoparticles on Thermal Properties of Polypropylene Resin

Ceramics nanometric reinforced polymer composite is a signicant material for catalysis, solar cells, production of hydrogen and energy applications, etc. In order to take benet from the interesting mechanical properties and thermal stability of TiO 2 , these ceramic nanomaterials was synthesized by the Sol-Gel process in attempt to study the thermal stability, structure, and morphology of the resulting nanoparticles powders. The obtained results revealed that, the sphere is composed of 20-30 nm nanoparticles with excellent thermal stability of nano-TiO 2 . This work focused on the thermal characterization and the study of nanocomposite xWt. %TiO 2 /PP (x=0, 2.5, 5, 7.5 mol%). In this study, the obtained results revealed that the molar ratio of TiO 2 inuences the nal thermal stability and degree of crystallinity of the composite. It was found that the use of TiO 2 seems to be an effective and very promising way to increase the thermal properties of the resulting composite. The greatest degree of crystallinity (54.80%) and thermal degradation stability are obtained for composite reinforced by 7.5Wt. and XRD analysis determined the thermal, structural and morphology behavior of the TiO 2 nanoparticles. In this research study, details of nanocomposites synthesis were presented. MEB analysis revealed the nanoparticle dispersion in the nanocomposites. Thermal degradation stability and degree of crystallinity were observed using DSC and TG/DTG.


Introduction
Polymer composite materials are attractive for several technological applications because of their enhanced speci c properties (Harizi et al., 2015). Thermal degradation, in combinations with their degree of crystallinity in uence the physical and thermal behavior of polymer materials (Zhang and Kraus, 2017). The correlation between structural and mechanical properties is one of the approaches to develop new nano-materials for various applications. Recent overwhelming use of composites has motivated the reinforcement by nanoparticles for improving the interfacial bonding strength and thermal performances.
According to several researchers, polypropylene has been used in several elds of application because of its excellent chemical stability (Hansen et al., 2019;Khaskhoussi et al., 2017). It is widely used in advanced technologies due to its low density, low processing temperature and high stiffness (Kumar et al., 2017).
In the present work, synthesis revealed the impact spherical anatase TiO 2 nanoparticles on the thermal performances of polypropylene resin. DTA/TG, SEM, TEM and XRD analysis determined the thermal, structural and morphology behavior of the TiO 2 nanoparticles. In this research study, details of nanocomposites synthesis were presented. MEB analysis revealed the nanoparticle dispersion in the nanocomposites. Thermal degradation stability and degree of crystallinity were observed using DSC and TG/DTG.

Elaboration of TiO 2 nanopowder
In order to synthesize TiO 2 nanopowder, methanol and acetic acid (CH 3 COOH) used as catalysts and titanium tetra isopropoxide (TTIP) precursor were mixed in the order shown. The mixture was stirred for 30 minutes at a constant speed to obtain the TiO 2 sol.
Then, the resulting sols were introduced into autoclave heated up to 243°C and pressurized to overcome the critical point of ethanol (T c =243°C, P c = 63bar). After maintaining temperature at 243°C for 1h, the sol gelation occurred. To evacuate the interstitial solvent, depressurization for 1 h down to room temperature was conducted with nitrogen gas. To avoid cracking of the sample due to thermal strain, the autoclave was opened after 24 h to slowly achieve thermal equilibrium. Finally, TiO 2 , Al 2 O 3 and SiO 2 aerogels was obtained and then annealed in air at 500°C for 1 h

Nanocomposites preparation
For TiO 2 /PP nanocomposite preparation, commercially available isotactic PP (density = 0.9 g.cm − 3 ) was purchased from National Industrialization Company JSC (TASNEE). Then, nanocomposites were prepared with different nano-TiO 2 contents (0wt.%, 2.5wt.%, 5wt.% and 7.5% wt.% TiO 2 ). To do so, TiO 2 nanopowder was added to PP resin and dispersed by using a mixer (30 min at room temperature). Then, the different nanocomposites were prepared using a twin screws extruder MEG DS32-1A (screw diameter of 35/80 mm and T°=210°C). Then, the solidi ed nanocomposites were pelletized with a crusher, and traction specimen were obtained by injection molding (SM-120TSCE, T°=30°C°). In order to study the structural properties and phase identi cation,TiO 2 nanopowder was analyzed using PANalyticalX'Pert Pro MPD diffractometer with Cu Kα radiation (λ = 1.5418Å) for X-Ray Diffraction (XRD) measurements recorded in the 20-85° 2θ range at room temperature with an incidence angle of 0.05.
From XRD patterns, average crystal sizes were calculated using Scherrer's formula.
The thermal stability of the nano-TiO 2 was studied by DTA and TG analysis. The DTA and TG analyzes were performed in a atmosphere from tempeture to 800°C with a heating rate of 20°C/min. For the analysis, it was used approximately 10mg of sample.

Nanocomposites characterization
Differential scanning calorimetry (DSC, TA Instruments model Q200) was used to measure the effect of nano ller on thermal properties of PP such as melting temperature and melt crystallization. The DSC analysis were performed in a temperature range from − 60°C to 180°C in two cycles (heating rate of 10°C/min and a cooling rate of 20°C/min). The thermal analyses of the different nanocomposites are submitted to thermogravimetric analysis (TGA, TA Instruments Model TGA 500). The TGA analyses were performed from ambient temperature to 650°C with a heating rate of 10°C/min.
The morphological observation of the samples (surface and cross section) were conducted according to scanning electron microscope (SEM) with accelerating voltage of 15KV.

TiO 2 nanopowder study
The thermal analyses (DTA/TG) of TiO 2 nanoparticle are given in gure.1 (a). The endothermic peaks observed near 224°C are related of the evaporation and the release of free water is the phenomenon of dehydration of the sample so it also re ects to the removal of hydroxyl groups it is the phenomenon of deshydroxylation. The second exothermic peak, located at 288°C, corresponds to the partial decomposition and departure of organic matter (carbon dioxide...), and it can be attributed to the oxidation elimination of the Ti precursor (alcoxide). The corresponding mass loss are localized at 138°C of the order of 3.84%. This loss is characterized by the endothermic peak, which re ects the phenomenon of dehydration and the phenomenon of dehydroxylation. The second mass loss characterizes an exothermic accident. It is fast and important located between 138°C and 450°C is of the order of 5.7%. It corresponds to the partial decomposition of titanium dioxide. The recorded mass loss could be related by the evaporation of absorbed water which corresponds to a signi cant endothermic effect (Salhi and Deschanvres, 2016). It is worth ion to mention that the excellent stability of TiO 2 anatase phase from 288°C to 800°C.
The morphology study of nano-TiO 2 with the SEM shows that the prepared nanoparticle composed of ne particles with heterogeneous size and shape before the treatment ( Fig.1-b). In addition to the presence of the agglomerates (is formed by primary crystallites connected together) that they are consolidated by the formation of a crystalline neck.
To determine the size and morphology of the TiO 2 nanopowder TEM analyses have been performed ( Fig.1-c). Furthermore, the TEM images of the nanopowders TiO 2 with diameters of about 10 nm and 20 nm. As a conclusion, the TiO 2 nanoparticles were prepared by sol gel method (10-20 nm) with spherical morphology through hydrothermal assisted sol-gel method. The crystallite average size was calculated from the full width at half maximum of the diffraction peaks using the Scherrer formula: Where D is the crystallite size in nm, λ is the X-ray wavelength of Cu-Kα radiation in nm, K is the shape constant (0.9), θ is the Bragg's angle in degrees and β is the line broadening at half the maximum intensity (FWHM) in radians. It can be estimated the crystallite size of TiO 2 is 12 nm.

Polypropylene nanocomposites study
3.2.1. Morphology analysis of surface and fracture for nanocomposites The micrograph of polypropylene surfaces (Fig.2-a)  The degree of crystallinity rst increased with the addition of 2.5 Wt%TiO 2 , and then reached a maxima of 53.23 %. At these conditions, the nanoparticles are acting as nucleating agents for PP and consequently increased the crystallization rate. Thus, the polymer crystals switched from homogeneous nucleation to heterogeneous nucleation, which facilitated crystallization. The addition of 5 Wt% nano-TiO 2 was found to increase the endothermic and exothermic peaks and delayed the end of the melting point during the phase change. Table-1  The crystallization temperature (T p ), the melting peak temperature (T mp ) and the degree of crystallinity (X c ) of PP and derived nanocomposites are indicated in Table 1. The results suggest that the incorporation of nano llers TiO 2 accelerates the crystallinity of polypropylene. The TiO 2 was considered to be important in increasing the thermal properties because it's a heterogeneous nucleation agent. In order to calculate the degree of crystallinity (X c ) of the samples the enthalpy area of the melting peak (∆H m ) was divided by the literature value (∆H L ) (equation 2). These results are consistent with previous ndings (Sun et al., 2019). The thermal degradation stability and mechanisms of the nanocomposites are an important property for various application areas due to the changes in the viscoelastic behaviour and viscosity (Aydemir et al., 2016). In this study, the thermal properties of the nanocomposites have been investigated using TGA. Fig.4 shows the TGA curves of TiO 2 /PP nanocomposites. According to the obtained results, the addition of nano-TiO 2 slightly improved thermal stability of the neat PP, when TiO 2 nanocharges were added up to 5 Wt%. The DTG curve of nanocomposite suggested the presence of peaks located between 325 and 475°C . This peak is attributed to the rupture of the C-C chain's bonds along with H-abstraction at the site of rupture (Aydemir et al., 2016).
Incorporation of 2.5Wt% nano-TiO 2 into the polypropylene matrix increases the thermal stability with the decomposition temperature was found to be 433.42°C. The decomposition temperature was decreasing with nano-TiO 2 content increasing. At 7.5Wt% TiO 2 , the result indicates that the decomposition temperature for the nanocomposites is around 429°C (Table-2). It was reported that the incorporation of nano-TiO 2 with molar ratio in the range of 1%-3%, the degradation temperature for nanocomposites shifts to high temperature (above 54°C) (Maharramov et al., n.d.; Ramazanov et al., 2018). It should be noted that interaction degree between polymer matrix and nano llers is connected to the concentration of base phase, crystallite sizes of nano llers and acquisition condition of nanocomposites. This con rms the presence of an interphase or border layer between nano llers and matrix with properties that differ from matrix properties in the bulk (Maharramov et al., n.d.). The crystalline structure in the transition regions of the polymer boundary layer are form in the nanocomposites, at this time is formed the structural activity of nanoparticles and whereby the thermodynamic conditions of the crystallization of molecular chains in the boundary layer are likely to improve (Ramazanov et al., 2018).

Conclusion
In this study, the nano-TiO 2 with spherical morphology and nanometric crystallite size has been synthesized using hydrothermal-assisted sol-gel technique. Diffraction peaks reveal the anatase TiO 2 phase. From the TEM images, the sphere is composed of 20-30 nm nanoparticles. This article aimed at presenting the impact of the nano-TiO 2 powders in the morphology and thermal performances of the polypropylene resin. The thermal degradation stability of the polypropylene increased with the addition of nano-TiO 2 . The results obtained show that the incorporation of nano-TiO 2 into the polypropylene matrix, the degree of crystallinity were relatively increased. Therefore, the interfacial interactions increased with the presence of anatase nano llers in polyester matrix.  Thermogravimetric (TG) and rst-order derivative (DTG) curves of/ (a) Polypropylene; (b) 2.5Wt%TiO2; (c) 5Wt%TiO2; (d) 7.5Wt% TiO2