Effect of titanium dioxide on the thermal ageing of

Polymer Degradation and Stability 97 (2012) 615e620

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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Effect of titanium dioxide on the thermal ageing of polypropylene Saisy Kudilil Esthappan a, Suma Kumbamala Kuttappan b, Rani Joseph a, * a b

Department of Polymer Science and Rubber Technology, Cochin University of Science & Technology, Kochi 682 022, India Department of Chemistry, Maharajas College Ernakulam, Cochin 682 011, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 November 2011 Received in revised form 29 December 2011 Accepted 9 January 2012 Available online 21 January 2012

Titanium dioxide nanoparticles were prepared by wet synthesis method and the particle size was calculated by transmission electron microscopic technique. Polypropylene/titanium dioxide nanocomposites were prepared using 0e3 wt% of titanium dioxide by melt mixing. It was then compression moulded in to films at 180 C. Thermal ageing was carried out in an air oven at 100 C for 24 h. Mechanical properties of the samples were determined before and after thermal ageing. Infrared spectroscopy was used to study the degradation products. Morphology of the tensile fractured surfaces was analysed using scanning electron microscope showed stability of polypropylene/titanium dioxide nanocomposites after thermal ageing. Good dispersion of titanium dioxide in the PP matrix was evident from SEM images. Change in torque with mixing time was noted out using Thermo Haake Rheocord 600 mixing chamber. Thermogravimetric analysis of the samples indicated that thermal stability of PP was increased by the addition of titanium dioxide. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Polypropylene Titanium dioxide Nanomaterials Thermal stability

1. Introduction Polymers have become incredibly important in modern industrial society. Introduction of inorganic nanoparticles in to organic polymers has attracted much attention because organiceinorganic nanocomposites offer an effective way to improve the physical properties of conventional polymers such as mechanical properties, thermal stability, flame retardancy, decreased vapour permeability, and chemical reagent resistance [1e4]. Furthermore, improvements in physical properties are achieved at very low concentrations, compared to micron-sized fillers [5]. This enhancement was attributed to the unique characteristics of the nanofillers [6]. There is a growing trend in using nanostructured versions of conventional inorganic fillers in plastic composites. The major findings in polymer nanocomposites are reported by Toyoto researchers on nylon nanocomposites, for which moderate inorganic loadings resulted in concurrent and remarkable enhancements of the thermal and mechanical properties and Grannelis demonstrated the possibility of melt mixing polymers and clays without organic solvents. Since then, the high promise of industrial applications have motivated vigorous research, which has revealed dramatic enhancements of many material properties of the nanocomposites. Much effort has been devoted to improve the properties of polymers by the

* Corresponding author. Tel.: þ91 484 2575723; fax: þ91 484 2577747. E-mail address: [email protected] (R. Joseph). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.01.006

addition of inorganic nanofillers such as SiO2 [7], ZnO [8,9] and CaCO3 [10]. Nowadays, nanocomposites based on polypropylene constitute a major challenge for industry since they represent the route to substantially increase the mechanical and physical properties of polypropylene (PP) [11,12]. PP is one of the most widely used and fastest growing thermoplastics. PP continues to be an important engineering polymer because of its several advantageous in cost and performance. PP film is used for packaging of a variety of products. In addition to the film, this plastic is moulded in to domestic hollow ware, toys, bottles, automotive components such as distributor caps, disposable syringes for medical and veterinary uses, battery cases, rope and carpeting [13]. PP is a crystalline polymer which is very versatile in nature. Its high crystallinity imparts high tensile strength and stiffness. However PP has some limitations when used as engineering material. It has low service temperature and is sensitive to heat, light and oxidation. PP is easily degraded by a stimulus such as elevated temperature or sunlight [14], which causes it to become brittle. PP has tertiary carbon atoms and is known to be very vulnerable to oxidative degradation under influence of elevated temperature and sunlight. PP degradation chemistry has been recognized as a free radicalechain reaction, which leads to polymer chain scission [15]. It is generally accepted that this chain scission is responsible for the brittleness. The change of morphology by degradation surely has a great influence on the mechanical properties and also changes its appearance. There are a number of recent studies on the thermal stability and degradation behaviour of polymeric nanocomposites. A good

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knowledge on these issues will be critical for assessing the lifetime for components fabricated using nanocomposites. The ageing process induced by photooxidation in polymer matrix nanocomposites has been studied by several researchers. These studies reveal that the incorporation of nanoclays and nano zinc oxide does not change the ageing mechanism of the composite [16e22]. There are a number of studies on the thermal stability, thermal degradation kinetics and mechanisms of nanocomposites [17,18,20,21,23e26]. Various factors affecting the degradation of PP and its composites with various fillers are found in the literature [27e34]. In this work we report the synthesize titanium dioxide nanoparticles and the preparation of its nanocomposites with PP. In particular, the effectiveness of titanium dioxide nanoparticles on the thermal ageing of PP. 2. Experimental Isotactic PP homopolymer (REPOL H200MA) with melt flow index of 25 g/min was supplied by M/s.Reliance Industries limited. Aqueous solution of TiCl4, (purity > 99.9%), HNO3 and aqueous ammonia used were analar grade. 2.1. Preparation of nano TiO2 Titanium dioxide nanoparticles were prepared using wet synthesis method. Both TiCl4 solution (200 g/l), and NaOH solution (64.5 g/l) were added drop wise to water with stirring. After the resulting solution reaches the pH to 7, the slurry was filtered, and the filter cake of TiO2 was washed and redispersed in water to prepare 1 M of TiO2 slurry. Resulting TiO2 slurry and an aqueous solution of HNO3 were refluxed at 95 C for 2 h, cooled to room temperature and neutralized with 28% of aqueous ammonia. Then, it was filtered, washed and dried at 400 C [35].

Fig. 2. Effect of thermal ageing on tensile strength of PP with the addition of TiO2 nanoparticles.

were prepared. The samples were compression moulded at 180 C for 6 min in an electrically heated hydraulic press. 2.3. Mechanical properties using universal testing machine The mechanical properties of the composites were studied using a Shimadzu Universal Testing Machine (Model-AG1) with a load cell of 10 kN capacities. The gauge length between the jaws at the start of each test was adjusted to 40 mm and the measurements were carried out at a crosshead speed of 50 mm/min. Mechanical properties were studied before and after 24 h of thermal ageing.

2.2. Polypropylene/TiO2 nanocomposites by melt mixing

2.4. Thermogravimetric analysis

The melt mixing was done using a Thermo Haake Rheocord 600 mixing chamber with a volume capacity of 69 cm3 fitted with a roller type rotors operating at 40 rpm for 8 min at 170 C. Nanocomposites with varying concentration (0e3wt %) of TiO2

Thermogravimetric analyzer (TGA Q-50, TA instruments) was used to study the effect of TiO2 on the thermal stability of PP. Approximately 10 mg of the samples were heated at a rate of 20 C/ min from ambient to 800 C in nitrogen atmosphere. The

Fig. 1. Transmission electron micrograph of titanium dioxide.

Fig. 3. Effect of thermal ageing on modulus of PP with the addition of TiO2 nanoparticles.


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3.2. Mechanical properties of PP/TiO2 nanocomposites

The morphology of the tensile fractured surfaces of the samples before and after thermal ageing was studied using scanning electron microscope (JOEL model JSM 6390LV).

The mechanical properties of the nanocomposites, such as tensile strength, modulus and elongation at break, before and after 24 h of thermal ageing have been evaluated and the results are shown in Figs. 2e4. The incorporation of titanium dioxide in the PP resulted in an increase in the tensile strength and modulus. Incorporation of 1 wt % of nano TiO2 results in 23.41% increase in tensile strength and 26.14% increase in modulus. It is known that the interface between nanoparticles and a polymer matrix can transfer stress, which is beneficial for the improvement of the tensile strength of composite films. However, with increasing content of nanoparticles, aggregation occurs, which leads to a decrease in the contact area between the nanoparticles and polymer matrix causing defects in the composites. This results in reduced interfacial interaction as a result of which the tensile strength of the films drops [9]. The mechanical properties also depend on the dispersion of nanoparticles in the matrix. Analyzing the trend of mechanical properties reveals information about the effect of different concentration of TiO2 in the composites. The improvement of tensile modulus and strength of PP/TiO2 nanocomposites in the present study is related to the inherent stiffness and quality of the dispersion of TiO2 and also due to adhesion between the matrix and nanoparticles in each loading [36,37]. From the SEM photograph (Fig. 5b) it is clear that the nanoparticles are well dispersed in the PP matrix, thus improving mechanical properties. Evaluation of mechanical properties after thermal ageing shows that elongation at break increases significantly. This indicates an increase in toughness of the composite after ageing. Increase in elongation at break is observed in HDPE by the addition of ZnO [9]. Similar observation is found by Hongxia Zhao in the photodegradation of PP/ZnO nanocomposites. They reported improvement in the resistance of PP to photodegradation embrittlement by the addition of ZnO [19]. Thermal ageing does not improve the tensile strength and modulus of the composites. The variation of these properties with loading is same as that observed before ageing.

3. Results and discussion

3.3. Morphology of the fractured surface

3.1. Transmission electron microscopic analysis of titanium dioxide

SEM images of the fractured surfaces of the neat PP and its composites with 1.5 wt% of TiO2 before thermal ageing are shown in Fig. 5a and b respectively. As can be seen from Fig. 5b, the nanoparticles are well dispersed in the polymer matrix resulting in improved mechanical properties discussed earlier. Mechanical properties of a composite strongly depend on the compatibility between filler and matrix. It is a key for practical application that polymer composite is of good mechanical properties [38]. F.G.

Fig. 4. Effect of thermal ageing on elongation at break of PP with the addition of TiO2 nanoparticles.

corresponding weight changes were noted with the help of an ultra sensitive microbalance. 2.5. IR spectroscopy IR spectroscopy (Thermo Nicolet, iS10 Fourier Transform Infrared, KBr Pellets transform mode) was used to analyse degradation products after 24 h of thermal treatment at 100 C. 2.6. Scanning electron microscopy

Transmission electron microscopy (TEM) gives an idea about the size and shape of the particles on the scale of atomic diameters. Fig. 1 shows the TEM of titanium dioxide. It can be observed that most of the particles are in the range 0e100 nm and are mostly of elongated shape. Particle size of the smallest particle is found to be as 20 nm.

Fig. 5. Scanning electron micrographs of fractured surface of (a) Neat PP and (b) 1.5 wt% TiO2 filled PP before thermal ageing.

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Fig. 6. Scanning electron micrographs of fractured surface of (a) Neat PP and (b) 1.5 wt% TiO2 filled PP after thermal ageing.

Ramos Filho et al. reported the similar fractured surface of PP/ modified bentonite nanocomposites. They also observed good dispersion of modified bentonite in PP [39]. The SEM images of the fractured surface of the neat PP and 1.5 wt% of nano TiO2 filled PP after thermal ageing is shown in the Fig. 6a and b. A well dispersed fillerematrix system is observed after ageing also. 3.4. IR spectroscopy Degradation products of neat PP and its composites with TiO2 after thermal ageing is studied by IR spectroscopy. IR spectrum of neat PP, PP with 0.5 wt% TiO2, 1.5 wt% TiO2 and 3 wt% TiO2 are shown in Fig. 7. The consequence of degradation of PP is the formation of hydroperoxides and carbonyl species such as ketones, esters and acids. These degradation products give absorption peaks in the wave number range 3200e3600 and 1600e1800 cm1 in the FTIR spectrum [19]. IR spectrum of neat PP, PP with 0.5 wt% TiO2, 1.5 wt% TiO2 shows a peak in the range of 3200e3600 cm1 indicating the

Fig. 7. IR spectrum of neat PP and composites after thermal ageing.

formation of hydroperoxides. The intensity this peak is less in case of PP composites with 3wt% TiO2. The IR peak in the range 1600e1800 cm1 corresponds to carbonyl group, the intensity of which decreases with increasing TiO2 concentration, which supports the fact that incorporation of nano TiO2 reduces the thermal degradation of PP. The characteristic peaks for PP in the wave number range of 2800e3000 cm1, are related to the asymmetric and symmetric C-eH stretching vibration. The intensity of this peak increases with increasing the TiO2 concentration. This indicates an increase in thermal stability of PP in presence of TiO2. 3.5. Thermogravimetric analysis (TGA) Most of the polymers are generally subjected to degradation of the mechanical and physical properties with an increase in temperature. Fundamental information regarding the thermal stability of the composites to be processed is obtained from TGA. In this technique the mass of the sample is monitored as a function of temperature or time, while the substance is subjected to a controlled temperature programme. Degradation behaviour of the composites and neat PP was studied using TGA is shown graphically in the Fig. 8 and the data tabulated in Table 1.

Fig. 8. Thermogram of PP/TiO2 nanocomposites.


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Table 1 Thermogravimetric analysis of PP/TiO2 nanocomposites. Concentration of TiO2

Temperature at which maximum degradation take place ( C)

Residue (%)

Degradation rate (%)

Onset of degradation ( C)

Endset of degradation ( C)

0 0.5 1.5 3

472 474 477 477

1.4 1.5 2.1 3.4

56.3 52.0 49.3 50.2

391 419 421 416

497 499 504 500

As shown in Fig. 8 and Table 1, TiO2 filled composites show enhanced thermal stability. The onset of degradation is increased by 29.88 C at 1.5 wt% of TiO2 loading in the nanocomposites. The increased endset of degradation also points to the improved thermal stability of the composites. The rate of degradation decreased by 13.42% upon 1.5 wt% of TiO2. Residue at 800 C increases substantially with loading of nano TiO2. The temperature at which maximum degradation take place is increased by 5.36 C at 1.5 wt% of TiO2 nanoparticles. This shows enhanced thermal stability of the composites compared to neat PP. Improvement in thermal stability of polymers in presence of fillers is due to the hindered thermal motion of polymer molecular chains [40]. An increase in onset of degradation and maximum decomposition temperature is reported for PP by the addition of intumescent flame retardant [41]. The literature reported that the addition of organoclay to a polymer matrix is expected to slow down the release rate of decomposed products within the nanocomposites, thus increasing the thermal stability [42]. Shahryar Jafari Nejad et al. reported an increase in thermal stability of PP with modified bentonite clay. They observed an increase in onset temperature by 22 C at 3 wt% clay loading [43]. An increase in thermal stability of PP/TiO2 nanocomposite fibres is observed by Hassan M. et al. [44]. 3.6. Torque studies Fig. 9 shows the variation of torque with mixing time for the neat PP and PP/nano-TiO2. Solid lines (Neat PP) shows the curve for mixing of PP, dotted line (1.5NTO) represents mixing of PP/1.5 wt% of TiO2 and dashed line (3NTO) indicate mixing for PP/3 wt% of TiO2. Torque is increased rapidly during initial mixing and then dropped to stabilize a line with increasing mixing time. This indicates good level of mixing at the specified conditions. Also the

Fig. 9. Variation of torque with time during mixing.

torque value of the PP/nano-TiO2 composites is higher than that of neat PP. This is mainly caused by the interfacial interaction between the nanoparticles and polymer [45]. In case of 3NTO, the torque value is decreased when compared to 1.5NTO, indicating a decrease in interfacial interaction due to the agglomeration of nanoparticles. 4. Conclusions TiO2 nanoparticles could be prepared by wet synthesis method and most of the particles are in 0e100 nm range and are mostly of elongated shape. Tensile test measurements before and after thermal ageing indicated that tensile strength and modulus significantly increased by the addition of TiO2. Elongation at break decreased for PP/TiO2 nanocomposites before thermal ageing while it was increased after thermal ageing. Uniform distribution of TiO2 nanoparticles in PP was indicated in the morphology studies of the tensile fractured surface of the nanocomposites. Thermogravimetric analysis showed an increase in thermal properties by the addition of TiO2. Maximum degradation temperature, onset of degradation and residue were increased while degradation rate decreased by the addition of TiO2 in PP. Torque studies carried out during mixing indicated the good mixing of TiO2 in the PP. Torque increased rapidly during initial mixing and then dropped to stabilize a line with increasing mixing time. IR spectroscopic studies indicated an increase in thermal stability of PP matrix with increasing the concentration of TiO2. References [1] Saujanya C, Radhakrishnan S. Structure development and crystallization behaviour of PP/nanoparticulate composite. Polymer 2001;42(16):6723e31. [2] Mishra S, Sonawane SH, Singh RP, Bendale A, Patil K. Effect of nano-Mg(OH)2 on the mechanical and flame-Retarding properties of polypropylene composites. J Appl Polym Sci 2004;94:116e22. [3] Zhang Ming Qiu, Rong Min Zhi, Zhang Hai Bo, Fried Rich Klaus. Mechanical properties of low nano-silica filled high density polyethylene composites. Polym Eng Sci 2003;43(2):490e500. [4] He Jiang-Ping, Li Hua-Ming, Wang Xia-Yu, Gao Yong. In situ preparation of poly(ethylene terephthalate)eSiO2 nanocomposites. Eur Polym J 2006;42: 1128e34. [5] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng Rep 2000; 28(1e2):1e63. [6] Yang J, Lin Y, Wang J, Mingfang Lai, Jing Li, Liu J, et al. Morphology, thermal stability, and dynamic mechanical properties of atactic polypropylene/carbon nanotube composites. J.Appl Polym Sci 2005;98:1087e91. [7] Garcia M, Van Vilet G, Jain S, Schrauwen BA, Sarkissov A, Van Zyl WE, et al. Polypropylene/SiO2 nanocomposites with improved mechanical properties. Rev Adv Mater Sci 2004;6:169e75. [8] Chae Dong Wook, Kim Byoung Chul. Characterization on polystyrene/zinc oxide nanocomposites prepared from solution mixing. Polym Adv Technol 2005;16:846e50. [9] Li Shu-Cai, Li Ya-Na. Mechanical and antibacterial properties of modified nano-ZnO/high-density polyethylene composite films with a low doped content of nano-ZnO. J Appl Polym Sci 2010;116(5):2965e9. [10] Avella Maurizio, Errico Maria Emanuela, Gentile Gennaro. PMMA based nanocomposites filled with modified CaCO3 nanoparticles. Macromol Symp 2007;247:140e6. [11] Garcıa-López D, Merino JC, Pastor JM. Influence of the CaCO3 nanoparticles on the molecular orientation of the polypropylene matrix. J Appl Polym Sci 2003; 88:947e52. [12] Ellis TS, D’Angelo JS. Thermal and mechanical properties of a polypropylene nanocomposite. J Appl Polym Sci 2003;90:1639e47.

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