Effect of Surface-Modified TiO2 Nanoparticles on the Anti-Ultraviolet Aging Performance of Foamed Wheat Straw Fiber/Polypropylene Composites

Surface modification and characterization of titanium dioxide (TiO2) nanoparticles and their roles in thermal, mechanical, and accelerated aging behavior of foamed wheat straw fiber/polypropylene (PP) composites are investigated. To improve the dispersion of nanoparticles and increase the possible interactions between wheat straw fiber and the PP matrix, the surface of the TiO2 nanoparticles was modified with ethenyltrimethoxy silane (A171), a silane coupling agent. The grafting of A171 on the TiO2 nanoparticles’ surface was characterized by Fourier transform infrared spectroscopy (FTIR). The wheat straw fibers treated with A171 and modified TiO2 nanoparticles were characterized by FTIR and thermogravimetric analysis (TGA). FTIR spectra confirmed that the organic functional groups of A171 were successfully grafted onto the TiO2 nanoparticles and wheat straw fibers, and the modified TiO2 nanoparticles were adsorbed onto the wheat straw fibers. Thermogravimetric analysis showed that a higher thermal stability of the wheat straw fiber was obtained with the modified TiO2 nanoparticles. The flexural, tensile, and impact properties were improved. A higher ultraviolet (UV) stability of the samples treated with modified TiO2 nanoparticles was exhibited by the study of the color change and loss in mechanical properties.


Introduction
Most agricultural residues are currently burned off in the field. The burning off of wheat straw and other agricultural residues is responsible not only for a series of environmental problems, but also for the sustainable development of precious resources. Therefore, using a waste biomass resource has been the main objective, scientifically and sufficiently, of many studies. Recently, many studies have been carried out on composite panels produced using various agricultural residues and plastic [1][2][3]. The most commonly investigated mechanical properties of agricultural fiber plastic composites (AFPC), such as tensile, flexural, and impact properties, are comparable to wood-plastic composites (WPC). Similar to WPC, AFPC can be extensively used for furniture, packaging, transportation, decorative materials, and so forth. However, several deficiencies, such as low strength-to-weight ratio, poor toughness, weak brittleness, and inconvenience of installation, have limited the application of AFPC [4].
Incorporating a blowing or foaming agent in the production of the AFPC could effectively avoid some of these disadvantages [5,6]. ZnO-modified azodicarbonamide (AC) has recently been widely used in research as a blowing agent owing to its compatible decomposition temperature range with the processing temperature of the plastic matrix in the AFPC [7]. The application of the AFPC as a building material necessitates that they are used outdoors and are often exposed to sunlight, which has resulted in concerns about their short service life. Of particular concern is the durability of AFPC after UV exposure. To overcome the inclination of AFPC to aging caused by UV, various approaches have been used to impart an anti-aging agent to the composite materials [8,9]. TiO 2 is of interest for various applications owing to its numerous merits, such as low cost, nontoxicity, high photocatalytic activity, optical and electronic properties, antibacterial activity, UV protection, and environmental friendliness. When the TiO 2 particle size is reduced to the nanoscale, a significant effect on the anti-ultraviolet aging performance, thermal stability, and mechanical properties in the polymer materials has been demonstrated [10]. Novel properties of nanocomposites can be obtained by successfully incorporating TiO 2 nanoparticles into matrix materials. However, owing to their nanoscale size, extremely high surface area, and the ability to form highly-reactive hydroxyl and superoxide radicals, TiO 2 nanoparticles agglomerate very easily in various media and show a poor dispersion capacity in materials, which may reduce the resultant correlation properties of the nanocomposite materials. Therefore, it is necessary to control the nanoparticle morphology by dispersion modification to make them uniformly combine with other components. Many efforts have been made to overcome this problem and to enhance the filler-matrix interaction [11]. Surface modification of TiO 2 nanoparticles with silane coupling is an effective way to reduce its surface energy and to improve its dispersion properties. Moreover, grafting active functional groups onto the nanoparticle surface can increase the possibility of forming chemical bonds between modified nanoparticles and matrix materials. Surface-modified TiO 2 nanoparticles have been widely applied to polymer nanocomposites. However, research to study the effects of TiO 2 nanoparticles on foamed fiber/plastic composites is relatively rare, even though TiO 2 nanoparticles exhibit obvious UV absorption that may cause an increase in the UV resistance of composites.
Thus, the aim of this study was to investigate the practical feasibility of using TiO 2 nanoparticles as an additive in the manufacturing of foamed wheat straw fiber/PP composites. To achieve an appropriate dispersion of TiO 2 nanoparticles and to yield a better compatibility between the wheat straw fiber and PP, a silane coupling agent, A171, was used for surface modification of the TiO 2 nanoparticles. The effects of the modification with TiO 2 nanoparticles on the mechanical properties and resistance to UV aging of the composites were evaluated.

Modification of TiO 2 Nanoparticles with the A171 Coupling Agent
Although the commercial and functionalized TiO 2 nanoparticles can be obtained from many suppliers nowadays, there are some problems affecting the control quality and the stability of the coupling agent. Thus, the TiO 2 nanoparticles used in this study were functionalized in the laboratory. The silane coupling agent, A171, was used for the modification of nano-sized TiO 2 particles. The TiO 2 nanoparticles were dispersed in a certain amount of ethanol, and acetic acid was added into the mixture to achieve a pH of 4. The mixture was mechanically stirred at 500 rpm at room temperature for 30 min and was then ultrasonicated for 20 min. Then, the dispersion was transferred into a 250 mL three-necked round-bottom flask fitted with a condenser and a thermometer and A171 (15% of the mass of the TiO 2 nanoparticles) was added. The reaction for the functional groups of A171 to the TiO 2 nanoparticle surfaces was performed at 60 • C and allowed to proceed for 1 h. The dispersion was purified of the free A171 by five cycles of centrifugation for 20 min (10,000 rpm), redispersion of the sediment in absolute ethanol, and ultrasonication for 10 min. A part of the dispersion was oven-dried at 100 • C for 24 h and was cooled in a vacuum for 1 h to obtain a modified TiO 2 nanoparticle powder for back-up detection.

Modification of Wheat Straw Fiber with the A171 Coupling Agent and Modified TiO 2 Nanoparticle Powder
Ninety-five percent ethanol was prepared by mixing pure ethanol and distilled water. A171 was added into the prepared 95% ethanol with stirring to reach a 4% concentration. Acetic acid was added into the mixture to reach pH 4 and mixed with mechanical stirring at room temperature for some hours for hydrolysis. The wheat straw fiber was placed into a high-speed mixer. Under the condition of 60 • C, 1% prepared silane solution and 0%-3% modified TiO 2 nanoparticles were sprayed on the surface of the fiber and then the fibers were stirred for 30 min. The fiber samples were taken out and then dried at room temperature to fully evaporate the ethanol (Figure 1). nanoparticles were dispersed in a certain amount of ethanol, and acetic acid was added into the mixture to achieve a pH of 4. The mixture was mechanically stirred at 500 rpm at room temperature for 30 min and was then ultrasonicated for 20 min. Then, the dispersion was transferred into a 250 mL three-necked round-bottom flask fitted with a condenser and a thermometer and A171 (15% of the mass of the TiO2 nanoparticles) was added. The reaction for the functional groups of A171 to the TiO2 nanoparticle surfaces was performed at 60 °C and allowed to proceed for 1 h. The dispersion was purified of the free A171 by five cycles of centrifugation for 20 min (10,000 rpm), redispersion of the sediment in absolute ethanol, and ultrasonication for 10 min. A part of the dispersion was oven-dried at 100 °C for 24 h and was cooled in a vacuum for 1 h to obtain a modified TiO2 nanoparticle powder for back-up detection.

Modification of Wheat Straw Fiber with the A171 Coupling Agent and Modified TiO2 Nanoparticle Powder
Ninety-five percent ethanol was prepared by mixing pure ethanol and distilled water. A171 was added into the prepared 95% ethanol with stirring to reach a 4% concentration. Acetic acid was added into the mixture to reach pH 4 and mixed with mechanical stirring at room temperature for some hours for hydrolysis. The wheat straw fiber was placed into a high-speed mixer. Under the condition of 60 °C, 1% prepared silane solution and 0%-3% modified TiO2 nanoparticles were sprayed on the surface of the fiber and then the fibers were stirred for 30 min. The fiber samples were taken out and then dried at room temperature to fully evaporate the ethanol (Figure 1).

Blend Design and Sample Fabrication
Through a comprehensive analysis of the relevant formulations of foamed composites and nanocomposites, the design for various formulations is shown in Table 1. According to Table 1, modified wheat straw fiber, PP, AC, ZnO, CaCO3, and industrial paraffin were uniformly mixed in a high-speed mixer. The extrudates were prepared in the Engineering Composite Laboratory of Northeast Forestry University using a SJSH30/SJ45 twin-screw extruder (Nanjing Rubber Machinery Plant, Nanjing, China) and then granulated using a granulator. The blending temperature profile was controlled at 100, 130, 160, 165, 165, 180, and 175 °C from the feeding zone to the die. After the granulation was masticated by a two-roll mill, the composites were fabricated by prepressing for 15 min, hot-pressing under 5 MPa for 10 min, and cold-pressing for 5 min, in that order.

Blend Design and Sample Fabrication
Through a comprehensive analysis of the relevant formulations of foamed composites and nanocomposites, the design for various formulations is shown in Table 1. According to Table 1, modified wheat straw fiber, PP, AC, ZnO, CaCO 3 , and industrial paraffin were uniformly mixed in a high-speed mixer. The extrudates were prepared in the Engineering Composite Laboratory of Northeast Forestry University using a SJSH30/SJ45 twin-screw extruder (Nanjing Rubber Machinery Plant, Nanjing, China) and then granulated using a granulator. The blending temperature profile was controlled at 100, 130, 160, 165, 165, 180, and 175 • C from the feeding zone to the die. After the granulation was masticated by a two-roll mill, the composites were fabricated by prepressing for 15 min, hot-pressing under 5 MPa for 10 min, and cold-pressing for 5 min, in that order. The treated and untreated wheat straw fiber and TiO 2 nanoparticles were ground and FTIR absorption data were obtained using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Science, Agawam, MA, USA) with an ATR unit (Universal ATR Diamond Zn/Se) at a resolution of 4 cm −1 for 32 scans in the spectral range of 600-4000 cm −1 .

Thermogravimetric Analysis
Thermal properties of the untreated and treated wheat straw fiber samples were measured by using a TGA 309F3 thermal analyzer (TA Instruments, New Castle, DE, USA) at a heating rate of 10 • C/min from room temperature up to 600 • C under a nitrogen atmosphere.

Accelerated Aging Test
Fifteen replicates of each of the six formulations were placed in a xenon arc-type light exposure apparatus operated according to the American Society for Testing Material (ASTM) G 154-04 standard practice, with treatment for up to 600 h ( Figure 2). To understand the effect of UV exposure, the samples were removed for colorimetric analysis at an interval of 100 h. A NF333 photometer (Nippon Denshoku Co., Tokyo, Japan) was used to measure the color according to the International Commission on Illumination (CIE). The standard colorimetric parameters (brightness L*, red-green chromaticity index a*, and yellow-blue chromaticity index b*) were measured at five locations on each sample, and the color change (∆E) was determined with the procedure outlined in ASTM D 2244: where ∆L*, ∆a*, and ∆b* represent the differences between the initial and final values of L*, a*, and b*, respectively.

FTIR Study
The treated and untreated wheat straw fiber and TiO2 nanoparticles were ground and FTIR absorption data were obtained using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Science, Agawam, MA, USA) with an ATR unit (Universal ATR Diamond Zn/Se) at a resolution of 4 cm −1 for 32 scans in the spectral range of 600-4000 cm −1 .

Thermogravimetric Analysis
Thermal properties of the untreated and treated wheat straw fiber samples were measured by using a TGA 309F3 thermal analyzer (TA Instruments, New Castle, DE, USA) at a heating rate of 10 °C/min from room temperature up to 600 °C under a nitrogen atmosphere.

Accelerated Aging Test
Fifteen replicates of each of the six formulations were placed in a xenon arc-type light exposure apparatus operated according to the American Society for Testing Material (ASTM) G 154-04 standard practice, with treatment for up to 600 h ( Figure 2). To understand the effect of UV exposure, the samples were removed for colorimetric analysis at an interval of 100 h. A NF333 photometer (Nippon Denshoku Co., Tokyo, Japan) was used to measure the color according to the International Commission on Illumination (CIE). The standard colorimetric parameters (brightness L*, red-green chromaticity index a*, and yellow-blue chromaticity index b*) were measured at five locations on each sample, and the color change (ΔE) was determined with the procedure outlined in ASTM D 2244: where ΔL*, Δa*, and Δb* represent the differences between the initial and final values of L*, a*, and b*, respectively.

Mechanical Properties
Samples were oven-dried at 103 °C for 24 h before testing to ensure the same conditioning for samples before and after UV exposure. Tensile tests were conducted in accordance with ASTM D 638.

Mechanical Properties
Samples were oven-dried at 103 • C for 24 h before testing to ensure the same conditioning for samples before and after UV exposure. Tensile tests were conducted in accordance with ASTM D 638.
The size of each sample was 165 mm length and 19 mm width. Flexural (three-point bending) strength was measured according to ASTM D 790 using a support span of 56 mm and a cross head speed of 2 mm/min. Five replicates were run for each formulation. All tests were performed on a universal testing machine (Shenzhen REGER Instrument Co., Ltd., Shenzhen, China). The impacting strengths were measured according to ASTM D 6110 on a XJ-T0G composited impacting tester (Hebei Chengde Mechanical Instrument Co., Ltd., Chengde, China). Ten replicates for each composition were tested for impact strength. The mean value of parallel specimens was used as the test result. Specimens of the mechanical properties test are shown in Figure 3. The size of each sample was 165 mm length and 19 mm width. Flexural (three-point bending) strength was measured according to ASTM D 790 using a support span of 56 mm and a cross head speed of 2 mm/min. Five replicates were run for each formulation. All tests were performed on a universal testing machine (Shenzhen REGER Instrument Co., Ltd., Shenzhen, China). The impacting strengths were measured according to ASTM D 6110 on a XJ-T0G composited impacting tester (Hebei Chengde Mechanical Instrument Co., Ltd., Chengde, China). Ten replicates for each composition were tested for impact strength. The mean value of parallel specimens was used as the test result. Specimens of the mechanical properties test are shown in Figure 3.

FTIR Analysis
FTIR spectra of unmodified TiO2 nanoparticles and A171-modified TiO2 nanoparticles are presented in Figure 4. In the spectrum for the unmodified TiO2 (curve-4a), the stretching vibration of the absorbed water, as well as surface -OH groups, which are present in the TiO2 nanoparticles, were confirmed by the broad absorption band between 3400 cm −1 and 3200 cm −1 and the peak at around 1630 cm −1 [12]. This peaks were mainly observed because the TiO2 nanoparticles were easily polarized in the presence of water and produced a large quantity of hydroxyl groups on the surface. Furthermore, the electrons in the valence band of nano TiO2 were excited to generate electron-hole pairs, and part of the electron-hole pairs migrated to the surface, forming the hydroxyl groups. For the A171-treated TiO2 (curve-4b), the broad absorption peak between 3400 cm −1 and 3200 cm −1 was weakened. This indicated that a condensation reaction between the silanol groups of hydrolyzed A171 and the surface hydroxyl group of TiO2 nanoparticles occurred [13]. In addition, a broad band at around 1276.16 cm −1 corresponding to a Si-O-Si bond was observed, which confirmed a condensation reaction between silanol groups hydrolyzed by A171 [11]. Compared with pure TiO2 nanoparticles, the characteristic bands of 2958.43 cm −1 and 1620 cm −1 were attributed to the -CH3 stretching and C=C stretching, respectively, demonstrating that the functional groups in A171 were grafted onto the TiO2 nanoparticle surface. Zhao et al. [11] showed that according to the IR spectra of modified nano TiO2, the peak at 940 cm −1 -910 cm −1 was assigned to the stretching vibration band of Ti-O-Si. Chen and Yakovlev [14] also reported that the Si-O-Ti bonds confirmed the bonding of organosilane functional groups on the particle surface. Since the residual (non-reacted) and physically-adsorbed A171 molecules were removed by extraction in ethanol and centrifugal separation, the peaks mentioned showed that the chemical bonding of A171 onto the nanoparticle surfaces was realized through Ti-O-Si bonds, which could improve the dispersion of TiO2

FTIR Analysis
FTIR spectra of unmodified TiO 2 nanoparticles and A171-modified TiO 2 nanoparticles are presented in Figure 4. In the spectrum for the unmodified TiO 2 (curve-4a), the stretching vibration of the absorbed water, as well as surface -OH groups, which are present in the TiO 2 nanoparticles, were confirmed by the broad absorption band between 3400 cm −1 and 3200 cm −1 and the peak at around 1630 cm −1 [12]. This peaks were mainly observed because the TiO 2 nanoparticles were easily polarized in the presence of water and produced a large quantity of hydroxyl groups on the surface. Furthermore, the electrons in the valence band of nano TiO 2 were excited to generate electron-hole pairs, and part of the electron-hole pairs migrated to the surface, forming the hydroxyl groups. For the A171-treated TiO 2 (curve-4b), the broad absorption peak between 3400 cm −1 and 3200 cm −1 was weakened. This indicated that a condensation reaction between the silanol groups of hydrolyzed A171 and the surface hydroxyl group of TiO 2 nanoparticles occurred [13]. In addition, a broad band at around 1276.16 cm −1 corresponding to a Si-O-Si bond was observed, which confirmed a condensation reaction between silanol groups hydrolyzed by A171 [11]. Compared with pure TiO 2 nanoparticles, the characteristic bands of 2958.43 cm −1 and 1620 cm −1 were attributed to the -CH 3 stretching and C=C stretching, respectively, demonstrating that the functional groups in A171 were grafted onto the TiO 2 nanoparticle surface. Zhao et al. [11] showed that according to the IR spectra of modified nano TiO 2 , the peak at 940 cm −1 -910 cm −1 was assigned to the stretching vibration band of Ti-O-Si. Chen and Yakovlev [14] also reported that the Si-O-Ti bonds confirmed the bonding of organosilane functional groups on the particle surface. Since the residual (non-reacted) and physically-adsorbed A171 molecules were removed by extraction in ethanol and centrifugal separation, the peaks mentioned showed that the chemical bonding of A171 onto the nanoparticle surfaces was realized through Ti-O-Si bonds, which could improve the dispersion of TiO 2 nanoparticles, mechanical properties, and UV aging resistance of composites containing TiO 2 nanoparticles [15]. nanoparticles, mechanical properties, and UV aging resistance of composites containing TiO2 nanoparticles [15].  Figure 5 shows the FTIR spectra of pure wheat straw fiber and modified wheat straw fiber. The pure wheat straw fiber was characterized by the absorption bands (shown in curve-5a) appearing at between 3598.7 cm −1 and 3257.18 cm −1 , which were associated with the stretching vibration band of intramolecular -OH of cellulose, hemicellulose, polysaccharide, and monosaccharide in the wheat straw fiber. The C-H stretching vibrations of -CH3, as well as -CH2, which were present in the cellulose, were confirmed by the peaks at 2853.17 cm −1 and 2918.25 cm −1 , respectively. The cellulose was characterized by a high-intensity peak at 1034.14 cm −1 for -OH bending and shoulder peaks appeared at around 1158.53 cm −1 and 897.70 cm −1 for C-O-C stretching. The absorption peak at 896.25 cm −1 was ascribed to the intramolecular hydrogen bonding [16]. After surface modification by organosilane, as presented in spectra curve-5b, the characteristic peaks for wheat straw fiber and organosilane were observed. The broad absorption peaks at 2853.17 cm −1 and 2918.25 cm −1 were significantly weakened, indicating the quantity of C-H presented in cellulose was changed by the silane coupling agent. The peak that appeared at 1134.42 cm −1 for the Si-O-Si bond was due to the condensation reaction of silanol groups hydrolyzed by A171. The absorption peak at 1200.47 cm −1 was ascribed to the Si-O-cellulose bond, indicating that a strong polycondensation grafting reaction between silanol groups and wheat straw fiber occurred [12]. No characteristic peaks for Ti-O-cellulose or hydrogen bonds formed between the TiO2 surface -OH and the wheat straw fiber surface -OH were observed, showing that the chemical reaction between the wheat straw fiber and the modified TiO2 nanoparticles did not occur. This proved that the main bonding mechanism of the modified TiO2 nanoparticles and wheat straw fiber was physical sorption. However, Sun et al. showed that TiO2 was chemically bonded to the wood surface through the hydrogen groups during the hydrothermal process [17]. This may arise from the different reaction conditions.  Figure 5 shows the FTIR spectra of pure wheat straw fiber and modified wheat straw fiber. The pure wheat straw fiber was characterized by the absorption bands (shown in curve-5a) appearing at between 3598.7 cm −1 and 3257.18 cm −1 , which were associated with the stretching vibration band of intramolecular -OH of cellulose, hemicellulose, polysaccharide, and monosaccharide in the wheat straw fiber. The C-H stretching vibrations of -CH 3 , as well as -CH 2 , which were present in the cellulose, were confirmed by the peaks at 2853.17 cm −1 and 2918.25 cm −1 , respectively. The cellulose was characterized by a high-intensity peak at 1034.14 cm −1 for -OH bending and shoulder peaks appeared at around 1158.53 cm −1 and 897.70 cm −1 for C-O-C stretching. The absorption peak at 896.25 cm −1 was ascribed to the intramolecular hydrogen bonding [16]. After surface modification by organosilane, as presented in spectra curve-5b, the characteristic peaks for wheat straw fiber and organosilane were observed. The broad absorption peaks at 2853.17 cm −1 and 2918.25 cm −1 were significantly weakened, indicating the quantity of C-H presented in cellulose was changed by the silane coupling agent. The peak that appeared at 1134.42 cm −1 for the Si-O-Si bond was due to the condensation reaction of silanol groups hydrolyzed by A171. The absorption peak at 1200.47 cm −1 was ascribed to the Si-O-cellulose bond, indicating that a strong polycondensation grafting reaction between silanol groups and wheat straw fiber occurred [12]. No characteristic peaks for Ti-O-cellulose or hydrogen bonds formed between the TiO 2 surface -OH and the wheat straw fiber surface -OH were observed, showing that the chemical reaction between the wheat straw fiber and the modified TiO 2 nanoparticles did not occur. This proved that the main bonding mechanism of the modified TiO 2 nanoparticles and wheat straw fiber was physical sorption. However, Sun et al. showed that TiO 2 was chemically bonded to the wood surface through the hydrogen groups during the hydrothermal process [17]. This may arise from the different reaction conditions.

Morphology and Dispersibility of Surface-Modified TiO2 Nanoparticles
SEM analysis was conducted to examine the morphology and dispersibility of surface-modified TiO2 nanoparticles. Typical surface SEM images of pure TiO2 nanoparticles and surface-modified TiO2 nanoparticles are illustrated in Figure 6. As can be seen from Figure 6a, the spherical shape, as well as agglomerates, comprised the unmodified TiO2 nanoparticles, indicating a poor dispersion in the control group. From Figure 6b, an improved dispersion was observed in the surface-modified TiO2 nanoparticles, showing that the surface modification prevented the aggregation of TiO2 nanoparticles to some extent. Certainly, there were still a small amount of agglomerates in the surface-modified TiO2 nanoparticles, which might have an impact on some properties.

Thermogravimetric Analysis
To estimate the optimum amount of modified TiO2 nanoparticles required to enhance the properties of the wheat straw fiber, the wheat straw fiber treated by various percentages of modified TiO2 nanoparticles were analyzed by the TGA technique. The TGA curves of the wheat straw fiber with different percentages of modified TiO2 nanoparticles is shown in Figure 7. Three main mass loss regions were observed. In the thermograms obtained from the TiO2 nanoparticle-treated wheat straw

Morphology and Dispersibility of Surface-Modified TiO 2 Nanoparticles
SEM analysis was conducted to examine the morphology and dispersibility of surface-modified TiO 2 nanoparticles. Typical surface SEM images of pure TiO 2 nanoparticles and surface-modified TiO 2 nanoparticles are illustrated in Figure 6. As can be seen from Figure 6a, the spherical shape, as well as agglomerates, comprised the unmodified TiO 2 nanoparticles, indicating a poor dispersion in the control group. From Figure 6b, an improved dispersion was observed in the surface-modified TiO 2 nanoparticles, showing that the surface modification prevented the aggregation of TiO 2 nanoparticles to some extent. Certainly, there were still a small amount of agglomerates in the surface-modified TiO 2 nanoparticles, which might have an impact on some properties.

Morphology and Dispersibility of Surface-Modified TiO2 Nanoparticles
SEM analysis was conducted to examine the morphology and dispersibility of surface-modified TiO2 nanoparticles. Typical surface SEM images of pure TiO2 nanoparticles and surface-modified TiO2 nanoparticles are illustrated in Figure 6. As can be seen from Figure 6a, the spherical shape, as well as agglomerates, comprised the unmodified TiO2 nanoparticles, indicating a poor dispersion in the control group. From Figure 6b, an improved dispersion was observed in the surface-modified TiO2 nanoparticles, showing that the surface modification prevented the aggregation of TiO2 nanoparticles to some extent. Certainly, there were still a small amount of agglomerates in the surface-modified TiO2 nanoparticles, which might have an impact on some properties.

Thermogravimetric Analysis
To estimate the optimum amount of modified TiO2 nanoparticles required to enhance the properties of the wheat straw fiber, the wheat straw fiber treated by various percentages of modified TiO2 nanoparticles were analyzed by the TGA technique. The TGA curves of the wheat straw fiber with different percentages of modified TiO2 nanoparticles is shown in Figure 7. Three main mass loss regions were observed. In the thermograms obtained from the TiO2 nanoparticle-treated wheat straw

Thermogravimetric Analysis
To estimate the optimum amount of modified TiO 2 nanoparticles required to enhance the properties of the wheat straw fiber, the wheat straw fiber treated by various percentages of modified TiO 2 nanoparticles were analyzed by the TGA technique. The TGA curves of the wheat straw fiber with different percentages of modified TiO 2 nanoparticles is shown in Figure 7. Three main mass loss regions were observed. In the thermograms obtained from the TiO 2 nanoparticle-treated wheat straw fibers, all of the samples showed an initial mass loss in the region of 50-100 • C, which was attributed to the evaporation of water and residual solvents. It was evident that the mass loss was greater in the untreated wheat straw fibers compared with the treated fibers in the first stage, which arose from the untreated wheat straw fibers not being completely dry. The weight loss at this stage depends mainly on the initial moisture content of the fiber, therefore, the thermal properties should be analyzed from the second stage of thermal degradation. The second main degradation region was located between 200 • C and 350 • C, and the third stage mass loss occurred above 350 • C. The second degradation region was attributed to the cellulose and also to the degradation of lignin and other low molecular weight substances, which agreed with the results from the study of Huang et al. [18]. As can be seen, the treated wheat straw fiber had a higher decomposition temperature than the untreated wheat straw fiber, indicating the thermal stability of the fiber was enhanced by adding the modified TiO 2 nanoparticles. The degradation temperature of the treated wheat straw fiber was slightly increased with the TiO 2 nanoparticle content. Compared with the untreated wheat straw fiber, a reduced rate of the degradation of the wheat straw fiber became evident with the gradual increase in the additional amount of modified TiO 2 nanoparticles. This further demonstrated that the modified TiO 2 nanoparticles played a role on the thermal stability of the wheat straw fiber. Another slight weight loss after 350 • C may be attributed to the water formed from the condensation of the hydroxyl group on the particle surface [15]. In the thermal degradation process, 61% and 77% weight loss were observed for the untreated and treated wheat straw fiber, respectively. The percentages of modified TiO 2 particles had no obvious effect on the thermal properties of wheat straw fiber within the range selected for this experiment. For higher 1 wt.% content, the effect of TiO 2 nanoparticles on the thermal properties of wheat straw fiber was small. Hence, it was concluded that the thermal stability of the obtained wheat straw fiber was slightly improved with the dispersion of modified TiO 2 nanoparticles. fibers, all of the samples showed an initial mass loss in the region of 50-100 °C, which was attributed to the evaporation of water and residual solvents. It was evident that the mass loss was greater in the untreated wheat straw fibers compared with the treated fibers in the first stage, which arose from the untreated wheat straw fibers not being completely dry. The weight loss at this stage depends mainly on the initial moisture content of the fiber, therefore, the thermal properties should be analyzed from the second stage of thermal degradation. The second main degradation region was located between 200 °C and 350 °C, and the third stage mass loss occurred above 350 °C. The second degradation region was attributed to the cellulose and also to the degradation of lignin and other low molecular weight substances, which agreed with the results from the study of Huang et al. [18]. As can be seen, the treated wheat straw fiber had a higher decomposition temperature than the untreated wheat straw fiber, indicating the thermal stability of the fiber was enhanced by adding the modified TiO2 nanoparticles. The degradation temperature of the treated wheat straw fiber was slightly increased with the TiO2 nanoparticle content. Compared with the untreated wheat straw fiber, a reduced rate of the degradation of the wheat straw fiber became evident with the gradual increase in the additional amount of modified TiO2 nanoparticles. This further demonstrated that the modified TiO2 nanoparticles played a role on the thermal stability of the wheat straw fiber. Another slight weight loss after 350 °C may be attributed to the water formed from the condensation of the hydroxyl group on the particle surface [15]. In the thermal degradation process, 61% and 77% weight loss were observed for the untreated and treated wheat straw fiber, respectively. The percentages of modified TiO2 particles had no obvious effect on the thermal properties of wheat straw fiber within the range selected for this experiment. For higher 1 wt.% content, the effect of TiO2 nanoparticles on the thermal properties of wheat straw fiber was small. Hence, it was concluded that the thermal stability of the obtained wheat straw fiber was slightly improved with the dispersion of modified TiO2 nanoparticles.

Color Change during UV Aging
The values of ΔL*, Δa*, Δb*, and ΔE at different exposure times for UV-aged composites made from six different formulations are shown in Figure 8. From the figure, it was obvious that the brightness factor ΔL* and the calculated total ΔE increased for all the samples with an increasing exposure, which was indicative of color fading of the UV-aged composite samples. Although all of the samples experienced an increase in the value of ΔL* and ΔE, the increment of the value was not proportional to the increase of exposure. Within the 300-400 h block, there was an apparent growth rate, and then the growth rate gradually decreased. These findings were consistent with the study of

Color Change during UV Aging
The values of ∆L*, ∆a*, ∆b*, and ∆E at different exposure times for UV-aged composites made from six different formulations are shown in Figure 8. From the figure, it was obvious that the brightness factor ∆L* and the calculated total ∆E increased for all the samples with an increasing exposure, which was indicative of color fading of the UV-aged composite samples. Although all of the samples experienced an increase in the value of ∆L* and ∆E, the increment of the value was not proportional to the increase of exposure. Within the 300-400 h block, there was an apparent growth rate, and then the growth rate gradually decreased. These findings were consistent with the study of Fabiyi [19]. The samples without TiO 2 nanoparticles exhibited larger ∆L* and ∆E values than samples with TiO 2 nanoparticles, indicating the enhanced UV aging-resistant performances for TiO 2 nanoparticle-added materials, which arose from the UV shielding ability of TiO 2 nanoparticles. Du et al. [9] and Hou et al. [20] reported that the color fading of wood-plastic composites could be attributed to the photodegradation nature of the fiber. Therefore, nano TiO 2 could inhibit color fading by absorbing some of the UV radiation, which resulted in less UV radiation being available to degrade the color of the samples. Furthermore, surface modification of TiO 2 nanoparticles by silane has been found to be effective in improving its dispersion. The TiO 2 particles with better dispersion enhanced the UV protection and improved the weathering performance [21]. As for the effect of the TiO 2 nanoparticle content on the color fading of composites, no significant differences were recorded among the TiO 2 nanoparticle-incorporated formulations, showing that the TiO 2 nanoparticle content level had little effect on the anti-ultraviolet aging performance within the selected range of this experiment. This may be related to the presence of a small amount of agglomerates after the surface modification of the TiO 2 nanoparticles. When comparing the effect of TiO 2 nanoparticles on the values of ∆a* and ∆b*, it can be seen that these values decreased for all samples. Compared with ∆L*, the absolute values of ∆a* and ∆b* were relatively small, showing the ∆a* and ∆b* were not the main influencing factors of ∆E. Since the trend of ∆E followed that of ∆L*, ∆L* was the most important parameter influencing the total ∆E. A similar observation was reported by Hou [20]. This color fading pattern, therefore, revealed that composite formulations with moderated weathering performance could be achieved by incorporating TiO 2 nanoparticles. Du et al. [9] also reported that the UV stability of wood-polymer composites was increased by incorporating ultraviolet absorbers. Fabiyi [19]. The samples without TiO2 nanoparticles exhibited larger ΔL* and ΔE values than samples with TiO2 nanoparticles, indicating the enhanced UV aging-resistant performances for TiO2 nanoparticle-added materials, which arose from the UV shielding ability of TiO2 nanoparticles. Du et al. [9] and Hou et al. [20] reported that the color fading of wood-plastic composites could be attributed to the photodegradation nature of the fiber. Therefore, nano TiO2 could inhibit color fading by absorbing some of the UV radiation, which resulted in less UV radiation being available to degrade the color of the samples. Furthermore, surface modification of TiO2 nanoparticles by silane has been found to be effective in improving its dispersion. The TiO2 particles with better dispersion enhanced the UV protection and improved the weathering performance [21].  [20]. This color fading pattern, therefore, revealed that composite formulations with moderated weathering performance could be achieved by incorporating TiO2 nanoparticles. Du et al. [9] also reported that the UV stability of wood-polymer composites was increased by incorporating ultraviolet absorbers.

Mechanical Properties Study
The results of flexural, tensile, and impact properties of foamed wheat straw fiber/PP composites with different formulations before and after UV weathering are summarized in Figure 9. It was observed that the composites containing modified TiO 2 nanoparticles exhibited higher mechanical properties than those of composites without fillers. This might be a result of the nanoscale particles having a high aspect ratio, which offers an active surface area, and could play a role on the adsorption of hydroxyl groups on the surface of the fiber and chemical bonds in PP; hence, the mechanical properties were reinforced. An improvement in the mechanical properties arising from the incorporation of nanoparticles into the composites has already been reported [12,22]. Xiao [23] showed that the mechanical properties for wood fiber/PP composite improved owing to the incorporation of Al 2 O 3 nanoparticles. The trends of the mechanical properties were similar to each other, which showed a rising tendency along with the increasing concentration of modified TiO 2 nanoparticles, except for a small increase at a low level of modified TiO 2 nanoparticle loading (1%-2.5%). As clearly seen from Figure 9, the control samples showed lower mean mechanical values of 11.02 MPa, 5.02 MPa, and 0.53 KJ/m 2 for flexural, tensile, and impact properties, respectively. When the modified TiO 2 nanoparticle content was increased to 3%, the mechanical values correspondingly increased to 18.13 MPa, 10.29 MPa, and 3.28 KJ/m 2 . In general, the addition of modified nano TiO 2 affected the mechanical properties. However, in the range of the modified TiO 2 nanoparticle additions selected in this experiment, no optimal values were obtained. The effect of higher modified TiO 2 nanoparticle contents (above 3 wt.%) on the mechanical properties of epoxy nanocomposites were examined in previous studies [24]. The influence of a high modified TiO 2 nanoparticle content on the mechanical properties of the foamed wheat straw fiber/PP composites remains to be studied.

Mechanical Properties Study
The results of flexural, tensile, and impact properties of foamed wheat straw fiber/PP composites with different formulations before and after UV weathering are summarized in Figure 9. It was observed that the composites containing modified TiO2 nanoparticles exhibited higher mechanical properties than those of composites without fillers. This might be a result of the nanoscale particles having a high aspect ratio, which offers an active surface area, and could play a role on the adsorption of hydroxyl groups on the surface of the fiber and chemical bonds in PP; hence, the mechanical properties were reinforced. An improvement in the mechanical properties arising from the incorporation of nanoparticles into the composites has already been reported [12,22]. Xiao [23] showed that the mechanical properties for wood fiber/PP composite improved owing to the incorporation of Al2O3 nanoparticles. The trends of the mechanical properties were similar to each other, which showed a rising tendency along with the increasing concentration of modified TiO2 nanoparticles, except for a small increase at a low level of modified TiO2 nanoparticle loading (1%-2.5%). As clearly seen from Figure 9, the control samples showed lower mean mechanical values of 11.02 MPa, 5.02 MPa, and 0.53 KJ/m 2 for flexural, tensile, and impact properties, respectively. When the modified TiO2 nanoparticle content was increased to 3%, the mechanical values correspondingly increased to 18.13 MPa, 10.29 MPa, and 3.28 KJ/m 2 . In general, the addition of modified nano TiO2 affected the mechanical properties. However, in the range of the modified TiO2 nanoparticle additions selected in this experiment, no optimal values were obtained. The effect of higher modified TiO2 nanoparticle contents (above 3 wt.%) on the mechanical properties of epoxy nanocomposites were examined in previous studies [24]. The influence of a high modified TiO2 nanoparticle content on the mechanical properties of the foamed wheat straw fiber/PP composites remains to be studied.  From Figure 9 and Table 2, the flexural, tensile, and impact properties were reduced after UV treatment. The reduction in the mechanical properties was greater in the control samples. The modified TiO 2 nanoparticle-added formulations exhibited less of a reduction in all flexural, tensile, and impact properties. This might be a result of the modified TiO 2 nanoparticles inhibiting the rate of photodegradation. Related research has shown that ultraviolet light absorbed by TiO 2 nanoparticles is converted into heat, and the remaining UV energy is not sufficient to destroy the composites. Therefore, the resistance of foamed wheat straw fiber/PP composites to UV instability could be significantly improved with the addition of modified TiO 2 nanoparticles. However, the loss of mechanical properties for the composites incorporating any level of modified TiO 2 nanoparticles was unremarkable for the studied period of UV irradiation. These findings were consistent with the above analysis, where the range of modified TiO 2 nanoparticle additions selected in this experiment was limited. Moreover, the effect of a small amount of agglomerates formed in the surface modified TiO 2 nanoparticles could not be ignored. A study for a lower reduction in the mechanical properties and better UV resistance of these composites by adding more TiO 2 nanoparticles is still needed. As with the composites without UV aging, the mechanical properties were consistently high for all composites containing modified TiO 2 nanoparticles compared with the control specimens. This may arise from the interfacial interaction between the PP matrix, modified TiO 2 nanoparticles, and fibers, which is powerful enough to delay aging resulting from exposure to UV irradiation. Furthermore, the UV aging resulted in a relatively higher reduction of flexural properties in all composites with different formulations when compared with the tensile and impact properties, indicating the ultraviolet radiation had a greater influence on the flexural properties. This was mainly because the PP was oxidized under ultraviolet light and the aerobic environment and, as a result, the PP chains were fractured. The chain scission of the PP increased the microcracks on the composite surfaces, which led to a decrease in the flexural properties and hindered the stress transfer from the filler to the matrix, causing a decrease in the composite strength. In addition, since flexural strength depends on the tensile strength of the composite, the decreased flexural strength can be attributed to the decreased tensile modulus.

Conclusions
TiO 2 nanoparticles modified by A171 were used as the active fillers for foamed wheat straw fiber/PP composites imparting an anti-UV performance to the materials. Surface modification of the TiO 2 nanoparticles with a silane coupling agent and an interaction between the wheat straw fiber and the silane coupling agent were successfully achieved. Thermogravimetric analysis showed that a higher thermal stability was achieved for the wheat straw fiber treated with modified TiO 2 nanoparticles. Improvements in the flexural, tensile, and impact properties were obtained, which could be attributed to the active surface area of the modified TiO 2 nanoparticles. The samples treated with modified TiO 2 nanoparticles were shown to exhibit a higher UV stability than that of untreated nanoparticles by studying the color change and loss in mechanical properties. Basic research on the application of TiO 2 nanoparticles incorporated into foamed wheat straw fiber/PP composites was conducted and will aid in future research in this field.