Antioxidant functionalized silica-coated TiO2 nanorods to enhance the thermal and photo stability of polypropylene

Accepted Manuscript Full Length Article Antioxidant functionalized silica-coated TiO2 nanorods to enhance the thermal and photo stability of Polypropylene Gen Li, Feng Wang, Peng Liu, Chong Gao, Yanfen Ding, Shimin Zhang, Mingshu Yang PII: DOI: Reference:

S0169-4332(19)30120-5 https://doi.org/10.1016/j.apsusc.2019.01.116 APSUSC 41515

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

22 October 2018 2 January 2019 12 January 2019

Please cite this article as: G. Li, F. Wang, P. Liu, C. Gao, Y. Ding, S. Zhang, M. Yang, Antioxidant functionalized silica-coated TiO2 nanorods to enhance the thermal and photo stability of Polypropylene, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.01.116

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Antioxidant functionalized silica-coated TiO2 nanorods to enhance the thermal and photo stability of Polypropylene Gen Li a, b, Feng Wang

a,

*, Peng Liu a, Chong Gao a, Yanfen Ding a, Shimin Zhang a,

Mingshu Yang a, b, * a Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding Authors * Beijing National Laboratory for Molecular Science, CAS Key Laboratory of

Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, Zhongguancun North First Street 2, 100190, China. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Yang).

1

Abstract Rutile TiO2 is commonly used as UV shielding agent because of the absorption of UV light. In this research, we integrated the function of rutile TiO 2 nanoparticles and antioxidants (AO), and prepared a kind of multifunctional TiO2 nanoparticles with the capacity of both UV absorption and antioxidation. Firstly, rutile TiO2 nanorods were prepared and encapsulated by SiO2 to decrease its photocatalytic activity. Then, antioxidant functionalized silica-coated TiO2 nanorods (AO-KH550-SiO2-TiO2) were successfully synthesized by using aminosilane coupling agent (KH550) as a bridge. By melt blending method, AO-KH550-SiO2-TiO2 were incorporated into PP matrix. The results indicated that AO-KH550-SiO2-TiO2 could improve both photo stability and thermal stability of PP. PP/AO-KH550-SiO2-TiO2 film broke after 220 h UV irradiation, and the broken time of PP/TiO2 film was only 80 h. And the oxidation induction time of PP/AO-KH550-SiO2-TiO2 sample was 4.7 min, while PP/TiO2 sample showed no antioxidation

effect.

In

addition,

AO-KH550-SiO2-TiO2

exhibited

excellent

anti-extraction property. This novel design of AO-KH550-SiO2-TiO2 may open up a new avenue for fabricating multifunctional nanoparticles and facilitating their practical application. Keywords: TiO2; SiO2; antioxidant; PP; antiaging

2

1. Introduction Polymeric materials are commonly exposed to heat, oxygen, radiation and mechanical stress during processing and application. These external effects cause degradation of the polymers which results in deterioration of their chemical, physical and mechanical properties [1]. Additives, such as antioxidants (AO) and light stabilizers, are usually used to reduce the degradation of polymers. The commonly used antioxidants include hindered phenols (e.g. Irganox®1010) and organophosphorus compounds (e.g. Irgafos®168) [2-4]. And the commonly used light stabilizers include ultraviolet (UV) shielding agents (e.g. rutile TiO2) and hindered amines radical scavenger (e.g. Tinuvin770®). Rutile TiO2 is commonly used as UV shielding agent because of the absorption of UV light [5, 6]. When exposed to sunlight, the electrons on the valence band of TiO 2 can be excited and transfer to the conduction band of TiO2, so the electron-hole pairs with strong redox ability are generated. The organics adsorbed on the surface of TiO2 will be decomposed into carbon dioxide and water through oxidation-reduction reaction[7, 8]. Therefore, the photocatalytic activity of TiO2 limits its application in some polymers and cosmetics because TiO2 can accelerate the aging of polymers and skin [9, 10]. Over the past decades, considerable efforts have been made to decrease the photocatalytic activity of TiO2 under UV light irradiation, such as grafting with coupling agents, coating with inorganic minerals. Zhao et al. [11] applied silane coupling agents to graft commercial TiO2 nanoparticles, which suppressed the photocatalytic activity of the TiO2 nanoparticles without damaging the UV shielding property. Li et al. [6] coated TiO2 with SiO2 successfully and found that SiO2 layer could obviously suppress the photocatalytic

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activity of TiO2. Regarding to antioxidants，most of the antioxidants are small molecules, and the adding amount is often less than 0.5 wt% [12]. Hindered phenols are the most commonly used class of antioxidants. These stabilizers act by efficient trapping of peroxyl radicals, and thus preventing the hydrogen abstraction from polymer chains by the peroxyl radicals [13]. However, antioxidants with low molecular weight are sensitive to physical loss through volatilization, migration and extraction during polymer processing and long-term service [14], and these events also occur when the products contact with skin, food, water and so on [15, 16]. Physical loss of antioxidants therefore constitutes a major concern in environmental issues and safety regulation, as well as in long-term use of polymers [17]. Over the past decades, considerable efforts have been made to reduce the physical loss of antioxidants, such as increasing the molecular weight [2, 18], grafting onto the polymer backbone [19, 20], and immobilizing onto the surface of nanoparticles [21, 22]. Kasza et al. [1] synthesized a kind of hyperbranched poly(ethyleneimine) based macromolecular antioxidant and found that this macromolecular antioxidant exhibited excellent anti-extraction property. Zeng et al. [23] synthesized hindered phenol functionalized graphene oxide which significantly improved the thermo-oxidative aging resistance of natural rubber. In the last decade, there is a growing interest in preparation of multifunctional nanoparticles. In this research, we integrated the function of rutile TiO2 nanoparticles and antioxidants, and prepared a kind of multifunctional TiO2 nanoparticles with the capacity of both UV absorption and antioxidation. Firstly, Rutile TiO2 nanorods were prepared and encapsulated by SiO2 to decrease the photocatalytic activity. Then, antioxidant

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functionalized silica-coated TiO2 nanorods (AO-KH550-SiO2-TiO2) were successfully synthesized by using aminosilane coupling agent (KH550) as a bridge. By melt blending method, AO-KH550-SiO2-TiO2 were incorporated into PP matrix. We investigated the thermal and photo stability of PP, and anti-extraction property of AO-KH550-SiO2-TiO2. To the best of our knowledge, there are no reports concerning antioxidant functionalized silica-coated TiO2 nanorods. In addition, a possible mechanism of the improved antiaging properties of PP/AO-KH550-SiO2-TiO2 nanocomposite was proposed. 2. Experimental 2.1. Materials Titanium trichloride solution (TiCl3) and Iron trichloride (FeCl3) were purchased from Sinopharm Chemical Reagent Co., Ltd. China. Tetraethoxysilane (TEOS) was purchased from Xilong Chemical Co., Ltd. China. Absolute ethanol, toluene, thionyl chloride, dichloromethane were obtained from Beijing Chemical Reagent Co., Ltd. China. Aminosilane coupling agent 3-aminopropyltriethoxysilane (KH550) and antioxidant 3,5-bis(1,1-dimethylethyl)-4-hydroxy benzoic acid (AO, a typical small molecule antioxidant) were bought from Alfa Aesar. PP powder without stabilizer was supplied by Harbin Petrochemical Co., Ltd. China. All chemicals were used without further purification. 2.2. Synthesis of functionalized silica-coated TiO2 nanorods 2.2.1. Synthesis of TiO2 nanorods TiO2 nanorods were synthesized by hydrothermal method of TiCl3 solution [24]. Firstly, 3.5 L deionized water was added to a 5 L round-bottom flask, when the

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temperature reached 80 oC, and then 500 mL FeCl3 aqueous solution (2 wt%) was poured into the deionized water under continuously stirring (50 r/min). Finally, 700 mL TiCl3 solution was poured into the system when the temperature reached 95 oC. After reaction at 100 ºC for 24 h, the product was filtered and washed 4 times with deionized water. Subsequently, the filter cakes were dried at 80 oC in a vacuum oven for 12 h. 2.2.2. Preparation of KH550 modified silica-coated TiO2 nanorods (KH550-SiO2-TiO2) In order to inhibit the catalytic activity of TiO2, silica-coated TiO2 nanorods (SiO2-TiO2) were prepared by using TEOS as silica source [25]. 7.5 g TiO2 nanorods, 150 mL deionized water, 300 ml ethanol and 15 g TEOS were added to a 1 L round-bottom flask, then the pH was adjusted to 10 by ammonia under continuously stirring (300 r/min). After reaction at 30 oC for 24 h, the product was filtered and washed 4 times with ethanol. Subsequently, the filter cakes were dried at 80 oC in a vacuum oven for 12 h. The product was further calcined at 800 oC for 1 h to perfect the silica shell on the TiO2 core and is termed as silica-coated TiO2 nanorods. Aminosilane coupling agent can be used as a bridge for grafting antioxidant on the surface of nanoparticles. 4 g silica-coated TiO2 nanorods, 300 mL toluene were added to a 500 mL round-bottom flask, then 4 g aminosilane coupling agent KH550 was slowly added into the flask under continuously stirring (300 r/min). After reaction at 110 oC for 24 h, the product was filtered and washed 4 times with ethanol. Subsequently, the filter cakes were dried at 80 oC in a vacuum oven for 12 h. 2.2.3.

Preparation

of

antioxidant

functionalized

silica-coated

TiO2

nanorods

(AO-KH550-SiO2-TiO2) AO-KH550-SiO2-TiO2 were prepared by the reaction between KH550-SiO2-TiO2

6

and acylating chlorination of AO (AO-Cl), the synthesis route was illustrated in Scheme 1. AO-Cl was synthesized by the following method. 5 g AO was dissolved in 20 ml thionyl chloride under nitrogen atmosphere. After reaction at 30 oC for 24 h, the color of the mixture changed from light yellow to red. The unreacted thionyl chloride was removed through vacuum distillation, then 50 ml toluene was added to the system and AO-Cl solution was obtained. 25 ml AO-Cl solution was added to the suspension of KH550-SiO2-TiO2 (4 g) in toluene (200 mL) under continuously stirring (300 r/min). After reaction at 30 oC for 24 h, the solution was filtered and washed with ethanol for two times to remove the toluene, then the pH of the solution was adjusted to 9 by 1 mol/L NaOH solution, and the product was filtered and washed with ethanol until no Cl- was detected by 0.1 mol/L AgNO3 solution. After dried at 60 oC in a vacuum oven for 12 h, the AO-KH550-SiO2-TiO2 were obtained.

Scheme 1. Synthesis route of antioxidant functionalized silica-coated TiO2 nanorods (AO-KH550-SiO2-TiO2). 2.3. Preparation of PP composites The PP composites were prepared by melt blending using an internal mixer

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(HAAKE Polylab OS) at 190 oC with a screw speed of 50 rpm for 6 min. The content of TiO2, KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 was 2 wt% in the corresponding PP composites. In PP/AO-KH550-SiO2-TiO2 composite, the content of AO is 0.06 wt%, equivalent to the molar fraction of the antioxidant in PP/AO. The samples were pressed into films and sheets using a plate vulcanizer (Labtech LP-20B) at 190 oC. The preparation conditions were as follows. After preheating for 4 min, the samples were kept at the pressure of 10 MPa for 2 min, and then the samples were cooled with water at the pressure of 10 MPa for 2 min. Films with thickness of about 40 μm and sheets with thickness of about 500 μm were used for accelerated aging and extraction test. 2.4. Characterization The powder X-ray diffraction (XRD) was carried out on a Rigaku D/max-2500 diffractometer equipped with Cu Ka radiation at a scan rate of 8 o/min, about 0.3 g sample was evenly laid on a grooved glass sheet and compacted with another glass slide. Morphology of nanoparticles and PP composites was characterized by a transmission electron microscopy (TEM, HT7700, Hitachi) and a field emission scanning electron microscope (SEM, SU8020, Hitachi). About 5 mL ethanol and 5 mg nanoparticles were mixed together after ultrasonic, then copper-mesh with support carbon film was immersed into the solution for about 1 min, after drying, TEM was used to observe the sample on copper-mesh. Before SEM observation, PP and the composites were broke in liquid nitrogen and fracture sections were coated with gold-palladium. The X-ray photoelectron spectroscopy (XPS) spectra were collected on ESCA-Lab220i-XL electron spectrometer with 300 W Mg Ka radiation. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. A piece of double-sided tape (0.5 cm 0.5

8

cm) was attached to a piece of aluminium foil (3.0 cm 3.0 cm), about 0.1 g powder was evenly spread on the tape, then another piece of aluminium foil was covered on the sample, the sample was pressed by a pressure machine (5 MPa) for 1 min. Uncover the above aluminum foil and XPS was used to test the sample. Thermal gravimetric analysis (TGA) was carried out using Perkin-Emler Pyris 1 under air atmosphere with air flow of 40 mL/min. About 3 mg sample was heated up to 700 oC at a heating rate of 20 oC/min. Isothermal TGA was also carried out under air atmosphere, about 3 mg sample was heated up to 200 oC at a heating rate of 50 oC/min, then the sample was hold at 200 oC for 45 min. The UV-Vis spectra of the PP films were recorded with a UV-Vis spectrophotometer (Lambda 35, Perkin Elmer) by using air as background. The test range of UV-Vis spectra was 200-800 nm, the resolution was 1 nm, and scans were repeated 3 times. Water contact angle was measured by using a DSA-100 contact angle meter (Germany). A piece of double-sided tape (0.5 cm 5.0 cm) was attached to a piece of glass slide (2.6 cm 7.6 cm), about 0.2 g powder was evenly spread on the tape and compacted with another glass slide, then this specimen was used to test water contact angle for 5 times and average the results. The oxidation induction time (OIT) measurement was performed on a differential scanning calorimetry (DSC, Perkin Elmer) according to the standard method (ISO 11357-6-2008), which specified the gas flow and temperature ramping. Firstly, the sample was held at 60 oC for 5 min with a nitrogen flow of 50 mL/min. Subsequently, the sample was heated to 170 oC at a rate of 20 oC/min. After held for 5 min under nitrogen flow, the nitrogen was switched to oxygen at a flow rate of 50 mL/min. The oxidation of the samples was observed as a sharp increase in heat flow due to the exothermic nature of

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the oxidation reaction. Tangent method was employed to calculate the OIT value. PP and the composites were extracted with dichloromethane at 30°C, and the samples were dried in an air-circulating oven at 60 °C for 2 hours before OIT test. The photodegradation tests of PP and the composites were carried out under UV-B irradiation (280-320 nm) at 40 °C in a UV Crosslinker (CL-1000, UVP LLC). The composites were placed 12 cm away from the light source (8 W, 302 nm). Surface cracks of samples were observed using an optical microscope equipped with a reflectance device (BX53, Olympus). Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet 6700 spectrophotometer equipped with an attenuated total reflectance device (Smart Orbit), the resolution of the spectra was 4 cm−1, and scans were repeated 32 times. The FTIR spectra of nanoparticles were recorded by reflectance device, and the FTIR spectra of PP films were recorded by transmission device. Carbonyl index (CI), calculated from FTIR spectrum, was used to characterize the extent of degradation of PP. CI= AC/AR, where AC is the area of the carbonyl absorption band (1670-1800 cm-1) and AR is the area of the reference band (2700-2750 cm-1). 3. Results and discussion 3.1. Characterization of nanoparticles TEM image of TiO2 is shown in Fig. 1(A), demonstrating that the prepared TiO2 nanoparticles were all nanorods with a length of about 70 nm and a width of about 9 nm. The prepared TiO2 nanoparticles were slightly agglomerated because of physical adsorption during evaporation of water. Fig. 1(B) shows the TEM images of silica-coated TiO2 nanorods. Obviously, TiO2 nanorods were coated by SiO2 layer which was uniform and dense. The width of silica-coated TiO2 nanorods was about 15 nm, indicating that the

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thickness of the SiO2 coating layer was about 3 nm.

Fig. 1. TEM images of TiO2 nanorods (A) and silica-coated TiO2 nanorods (B) (inset: HRTEM image of silica-coated TiO2 nanorods).

Fig. 2. XRD patterns of TiO2 nanorods, KH550-SiO2-TiO2, AO-KH550-SiO2-TiO2 and the standard rutile TiO2. Fig. 2 shows XRD patterns of TiO2, KH550-SiO2 -TiO2 and AO-KH550-SiO2-TiO2. Compared with the standard card of rutile TiO2 (JCPDS No.86-0148), all the diffraction peaks of TiO2, KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 can be indexed to the standard rutile TiO2, indicating that the prepared TiO2 nanoparticles were complete rutile form and the surface modification did not alter the crystalline form. The weak diffraction

11

peaks of KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 at 22.3o are attributed to the diffraction of amorphous SiO2 [26], suggesting that TiO2 nanoparticles were coated by SiO2 successfully.

Fig. 3. FTIR spectra of silica-coated TiO2 nanorods, KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2. Fig. 3 shows FTIR spectra of silica-coated TiO2 nanorods, KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2. For silica-coated TiO2 nanorods, the peaks at 3365 cm−1 and 1621 cm−1 corresponding to the stretching and bending vibration of OH bands were observed, which resulted from the physically adsorbed water and surface hydroxyl groups [5]. Compared with silica-coated TiO2 nanorods, several new peaks appearing at 2933 cm−1, 1565 cm−1 and 1482 cm-1 in KH550-SiO2-TiO2 are attributed to CH2 stretching, NH stretching and CH2 bending vibration absorption respectively, suggesting that the aminosilane coupling agent KH550 was successfully grafted on the surface of silica-coated TiO2 nanorods [27]. Besides, the absorption peaks of AO-KH550-SiO2-TiO2 at 2964 cm−1 and 1439 cm−1 are corresponding to CH3 vibration absorption which can be assigned to the tert-butyl groups of the hindered phenol antioxidant [28]. In particular, the

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absorption of amide II at 1541 cm−1 was observed, indicating that the chemical reactions took place between the acyl chloride groups of AO and the -NH2 groups on the surface of KH550-SiO2-TiO2 [29].

Fig. 4. XPS spectra (A) and N1s XPS high resolution spectra (B, C) of KH550-SiO2-TiO2 (B) and AO-KH550-SiO2-TiO2 (C). XPS technique is a useful tool for surface analysis and can provide elemental information on the surface of samples. As shown in Fig. 4(A), Ti, Si, C, O and N elements were present in KH550-SiO2-TiO2 sample, indicating that aminosilane coupling agent was successfully grafted on the surface of silica-coated TiO2 nanorods. The XPS spectra comparison shows that the intensity of C element in AO-KH550-SiO2-TiO2 is significantly higher than that in KH550-SiO2-TiO2, implying that antioxidant was grafted on the surface of KH550-SiO2-TiO2. The N1s XPS spectra of KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2

are shown in

Fig.

4(B,

C). The N1s spectrum of

KH550-SiO2-TiO2 can be divided into two peaks, a major peak at 399.5 eV and a lower peak at 401.3 eV. The low binding energy and the high binding energy are attributed to free amine groups (-NH2) and protonated amines (-NH3+) respectively [14]. For

13

AO-KH550-SiO2-TiO2, the N1s spectrum can also be divided into two peaks. The low binding energy at 399.5 eV is attributed to residual -NH2 groups, and the high binding energy at 401.4 eV is attributed to amide groups (-NH-(C=O)-) [28]. Therefore, XPS results further confirmed that AO was chemically grafted on the surface of KH550-SiO2-TiO2.

Fig. 5. TGA curves of silica-coated TiO2 nanorods, KH550-SiO2-TiO2, AO-KH550-SiO2-TiO2 and AO (the inset curve) (Air, 20 oC/min). TGA was used to evaluate the content of AO in AO-KH550-SiO2-TiO2. As shown in Fig. 5, silica-coated TiO2 nanorods were very stable, revealing almost unchanged weight under 700 oC. The initial degradation temperature of AO was about 200 oC and it was totally

decomposed

around

250

o

C,

while

for

KH550-SiO2-TiO2

and

AO-KH550-SiO2-TiO2, the thermal decomposition process could be separated into two stages. The stage below 300 oC was attributed to the dehydration condensation of silanol groups and the slow decomposition of organic groups, and the stage above 300 oC could be related the fast decomposition of organic groups. This result indicates that the AO which is grafted on KH550-SiO2-TiO2 has better thermal stability than pure AO. In

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addition, the residue weight of KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 were 96.4 wt% and 93.4 wt% under 700 oC respectively. Therefore, about 3.6 wt% coupling agent and 3 wt% antioxidant was grafted on the surface of silica-coated TiO2 nanorods. 3.2. Dispersion of nanoparticles in PP Uniform dispersion is an important aspect for sufficiently exerting the role of nanoparticles in polymer. SEM and TEM were used to characterize the dispersion of nanoparticles in PP. Fig. 6(A) shows the SEM image of PP/TiO2 composite, and TiO2 nanoparticles tended to form large agglomerates with the size of 2-30 μm in PP. In Fig. 6(B), most of the agglomerates of KH550-SiO2-TiO2 had a size below 500 nm, but there were also a few agglomerates with a size of about 1 μm. Therefore, surface modification significantly improved the dispersion of TiO2 in PP. Fig. 6(C, D) show the SEM and TEM images of PP/AO-KH550-SiO2-TiO2 nanocomposite, AO-KH550-SiO2-TiO2 dispersed uniformly in PP with the size around 200 nm, which indicates that AO further increased the dispersion of KH550-SiO2-TiO2 in PP. In addition, the water contact angle of KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 were 54.56.1° and 138.47.6 ° respectively. Such a significant change suggests clearly that the surface of KH550-SiO2-TiO2 transformed from hydrophilic to hydrophobic after AO modification. Therefore, AO-KH550-SiO2-TiO2 were uniformly dispersed in PP.

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Fig. 6. SEM imagines of PP/TiO2 (A), PP/KH550-SiO2-TiO2 (B), PP/AO-KH550-SiO2-TiO2 (C) and TEM imagine of PP/AO-KH550-SiO2-TiO2 (D). 3.3. Thermal stability of PP and the composites TGA and DSC were used to study the thermal stability of PP and the composites. Isothermal TGA of PP and the composites in air atmosphere are shown in Fig 7, and the weight of PP and the composites increased slightly in the first 10 min due to oxidation [13]. About 10 min later, the weight of the samples began to decrease, indicating that the samples were degraded. Compared with PP, the weight loss rate of PP/TiO2 composite was obviously accelerated, and this is probably due to catalytic effect of TiO2 [30]. Due to the inhibited catalytic effect of TiO2 by SiO2 coating layer, the weight loss rate of PP/KH550-SiO2-TiO2 composite was slower than that of PP [31]. After degradation at 200 oC for 45 min, the weight loss of PP, PP/AO, PP/AO-KH550-SiO2-TiO2 was 16.7,

16

13.5, 11.7 wt% respectively, indicating that hindered phenol structure of AO could scavenge free radicals in PP, and the AO grafted on nanoparticles still have the effect of scavenging free radicals [29].

Fig. 7. Isothermal TGA of PP and the composites under air atmosphere at 200 oC.

Fig. 8. Oxidation induction time curves of PP and the composites (DSC measurement, 170 oC, oxygen). The oxidation induction time (OIT) determined by DSC has been considered as an effective measurement to evaluate the thermal stability of polymer. Fig. 8 shows the OIT

17

curves of PP and the composites, and the oxidation of PP and PP/TiO2 composite were detected almost immediately after the gas switched from nitrogen to oxygen. The OIT curve of PP/KH550-SiO2-TiO2 composite was a little longer than that of PP, indicating that KH550-SiO2-TiO2 had weak free radicals scavenging ability [27]. The OIT value of PP/AO and PP/AO-KH550-SiO2-TiO2 nanocomposite is 3.8 and 4.7 min respectively, much higher than that of PP, suggesting that AO can scavenge free radicals in PP [29]. Due to the free radicals scavenging effect of aminosilane coupling agent and AO, the thermal stability of PP/AO-KH550-SiO2-TiO2 nanocomposite was better than that of PP/AO.

Fig. 9. Oxidation induction time curves of PP/AO and PP/AO-KH550-SiO2-TiO2 after different extraction time in dichloromethane. The resistance to extraction is an important indicator for evaluating antioxidant [32]. So PP/AO and PP/AO-KH550-SiO2-TiO2 nanocomposite were extracted with dichloromethane, and the OIT values versus extraction time are shown in Fig. 9. After extraction for 140 h, the OIT of PP/AO reduced sharply from 3.8 min to about 1.7 min, while the OIT of PP/AO-KH550-SiO2-TiO2 nanocomposite reduced from 4.7 min to

18

about 3.7 min. It is obvious that the AO which was grafted on the surface of nanoparticles had better anti-extraction property than pure AO [14, 21], and this is favorable for resolving environmental and health problems. 3.4. Photo stability of PP and the composites

Fig. 10. UV-Vis spectra of PP and the composites films (air as background). Polymers are inevitably exposed to light during long-term service. Therefore, the anti photoaging property of polymers is of critical importance. Fig. 10 shows UV-Vis spectra of PP and the composites, the transmittance of PP/AO film in visible light region (400-800 nm) was nearly the same as that of pure PP, and the absorption peak of the PP/AO film at 260 nm corresponds to the absorption of AO. Due to the agglomeration of pure TiO2, the visible light transmission and UV shielding performance of PP/TiO 2 film was poor. After surface modification, the dispersion of KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 had been significantly improved in PP. So PP/KH550-SiO2-TiO2 and PP/AO-KH550-SiO2-TiO2 films not only had good visible light transmission, but also had excellent UV shielding property. It is noteworthy that the transmission of PP, PP/TiO2,

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PP/KH550-SiO2-TiO2, PP/AO-KH550-SiO2-TiO2 and PP/AO films at 300 nm were 67.2%, 25.6%, 1.3%, 1.3% and 67.2%, respectively. During practical application, the photo degradation of polymer products is mainly caused by UV-B (280-320 nm) irradiation in sunlight. Therefore, UV-B was used to study the photoaging behavior of PP and the composites.

Fig. 11. Optical micrographs showing evolution of the surface cracks of PP and the composites sheets under UV-B irradiation (280-320 nm).

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Fig. 12. SEM imagines of surface cracks in PP and the composites sheets after 300 h UV-B irradiation (A: PP, B: PP/TiO2, C: PP/KH550-SiO2-TiO2, D: PP/AO-KH550-SiO2-TiO2, E: PP/AO). Surface cracks can reflect the aging degree of polymers [33]. The number and depth of cracks could be observed by optical microscope and SEM. As shown in Fig. 11, the surface of the samples was smooth and no cracks before UV-B irradiation. After irradiation for 60 h, several big and random cracks appeared on the surface of PP sheet, while some small cracks were also observed on the surface of PP/TiO2 sheet. Cracks began to appear on the surface of the PP/KH550-SiO2-TiO2 and PP/AO sheets at about 80 h, and cracks appearing time of PP/AO-KH550-SiO2-TiO2 sheet was about 100h. Obviously, with increasing irradiation time, the size of the cracks increased and lots of small new cracks formed. For all samples, coarse mesh-like surface formed after irradiation for 200 h, but the cracks on the surface of the PP/AO-KH550-SiO2-TiO2 were smallest and uniform. Fig. 12 shows more details of cracks, especially their depth. These cracks formed and propagated toward the interior of the samples during UV-B irradiation.

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The observed

maximum crack depth in PP, PP/TiO2, PP/KH550-SiO2-TiO2,

PP/AO-KH550-SiO2-TiO2 and PP/AO was 164, 121, 31, 25 and 114 μm, respectively, suggesting that KH550-SiO2-TiO2 and AO-KH550-SiO2-TiO2 possess excellent UV shielding effect and can prevent UV light from entering and damaging PP matrix. This result is consistent with that of UV-Vis. In addition, the crack depth of PP/AO-KH550-SiO2-TiO2 was shallower than that of PP/KH550-SiO2-TiO2, indicating that AO-KH550-SiO2-TiO2 have better anti photoaging effect than KH550-SiO2-TiO2.

Fig. 13. Carbonyl index (obtained from FTIR spectra) of PP and the composites films versus aging time under UV-B irradiation. Carbonyl index (CI) obtained from FTIR spectrum was used to further explore the photoaging behavior of PP composites. As shown in Fig.13, the CI of PP film increased linearly with increasing irradiation time. Due to the catalytic effect of TiO2, the CI of PP/TiO2 film increased faster than that of PP film, and PP/TiO2 film was completely broken after 80 h irradiation. The degradation rate of PP/AO film was slow in the beginning of irradiation, and the rate was basically parallel to that of PP after 30 h，which indicates that AO can capture radicals during photo aging process. Due to the inhibited

22

catalytic effect of TiO2 by SiO2 coating layer and the excellent UV shielding effect of KH550-SiO2-TiO2, PP/KH550-SiO2-TiO2 exhibited better stability than PP. However, the degradation rate of PP/KH550-SiO2-TiO2 accelerated constantly after 100 h, this is probably due to the active sites of TiO2 were exposed in the process of irradiation. After 140 h irradiation, PP/KH550-SiO2-TiO2 film was totally broken. In contrast, the degradation rate of PP/AO-KH550-SiO2-TiO2 film was slowest in all samples, and this film was

broken after

220 h irradiation.

The marked difference between

PP/AO-KH550-SiO2-TiO2 and PP/KH550-SiO2-TiO2 indicates that AO can greatly improve the anti photoaging property of KH550-SiO2-TiO2 in PP. Scheme 2 shows the antiaging mechanism of AO-KH550-SiO2-TiO2 in PP. UV light, like scissors, can enter and damage pure PP, but AO-KH550-SiO2-TiO2 in PP can prevent UV light from entering and damaging the PP matrix because of uniform dispersion and excellent UV shielding effect. SiO2 coating layer, aminosilane coupling agent and AO on the surface of TiO2 can significantly inhibit the catalytic activity of TiO2. In addition, the hindered phenolic antioxidant on the surface of silica-coated TiO2 nanorods can capture radicals which produced by PP matrix during thermal and photo aging process.

Scheme 2. Antiaging mechanism of AO-KH550-SiO2-TiO2 in PP.

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4. Conclusions In summary, we successfully prepared hindered phenolic antioxidant functionalized silica-coated TiO2 nanorods for the

first time. By melt

blending method,

AO-KH550-SiO2-TiO2 were uniformly dispersed in PP matrix. Isothermal TGA and oxidation induction time were used to evaluate the thermal stability of PP composites. Surface cracks and carbonyl index were used to evaluate the photo stability of PP composites. The results showed that AO-KH550-SiO2-TiO2 could not only significantly improve the thermal stability and photo stability of PP, but also exhibit excellent anti-extraction property. The antiaging mechanism of AO-KH550-SiO2-TiO2 in PP is as follows. SiO2 coating layer, aminosilane coupling agent and hindered phenolic antioxidant on the surface of TiO2 can significantly inhibit the catalytic activity of TiO2. AO-KH550-SiO2-TiO2 can prevent UV light from entering and damaging the PP matrix because of uniform dispersion and excellent UV shielding effect. In addition, the hindered phenolic antioxidant on the surface of silica-coated TiO2 nanorods can capture radicals which produced by PP matrix during thermal and photo aging process. This novel design of AO-KH550-SiO2-TiO2 may open up a new avenue for fabricating multifunctional nanoparticles and facilitating their practical application. Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant No. 51650110503, 51534007 and 51403217], the Yunnan Provincial Science and Technology Department [Project No. 2018IB025], the Youth Innovation Promotion Association of CAS [Grant No.2017041] and the "Strategic Priority Research Program" of the Chinese Academy of Sciences [Grant No. XDA09030200].

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Highlights 

Antioxidant functionalized silica-coated TiO2 nanorods were prepared for the first time.



AO-KH550-SiO2-TiO2 enhanced the thermal and photo stability of PP.



AO-KH550-SiO2-TiO2 exhibited excellent anti- extraction property.



AO-KH550-SiO2-TiO2 could block UV light and capture radicals.

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Graphical Abstract (for review)

Graphical Abstract