Nano-Sized Titanium Dioxides as Photo-Catalysts in Degradation of Polyethylene and Polypropylene Packagings

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1 SCIENCE JOURNAL Ubon Ratchathani University Sci. J. UBU, Vol. 1, No. 2 (July-December, 2010) Research Article Nano-Sized Titanium Dioxides as Photo-Catalysts in Degradation of Polyethylene and Polypropylene Packagings T. Manangan 1,2 *, S. Shawaphun 1,2, D. Sangsansiri 1, J. Changcharoen 1, S. Wacharawichanant 3 1 Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut s University of Technology North Bangkok, Bangkok 10800, Thailand. 2 Research Center of Nano Industries and Bio-plastics, King Mongkut s University of Technology North Bangkok, Bangkok 10800, Thailand. 3 Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand. Received 10/03/10; Accepted 22/12/10 Abstract In present, packaging plastics such as PE and PP have become a major environmental problem. While production of biodegradable plastics is still expensive, various photo-catalytic additives especially titanium dioxide have been used as pro-oxidants in order to make conventional packaging plastics degradable after working period. Various particle-sized titanium dioxides were thermally blended with PE and PP and casted into 80-micron thick films. During the processing period, nano-sized TiO 2 significantly induced auto-oxidation of the PE films only. The carbonyl formation in PP films showed the opposite trend possibly due to other mechanical pathways. The films then were exposed under 254nm and 366nm UV light mimicking solar light profile. In most cases, TiO 2 catalyzed photodegradation occurred under the shortwave UV-254nm irradiation several folds higher than under long wave UV irradiation. The carbonyl index of the nano-sized TiO 2 (1%wt) blended PE films increased continuously over irradiation period and their tensile strength reduced to 35-38% after 28 days up to 42 days before total ruptured. The films have also lost weight about % in 14 days. The nano-sized TiO 2 blended PP films showed a dramatic increasing of carbonyl index in the first few days and then continuously dropped as they become fragmented with in 2 weeks. This also caused PP films to lose weight by 22% in 42 days. This study also suggested that titanium dioxide nano-sized particle showed favorable activity and results over the commercial micron-sized. Keywords: Photo-catalysts, Degradation, Plastic packaging, Titanium dioxide. *Corresponding author. address: jxudel@yahoo.com

2 T. Manangan et al., Sci. J. UBU, Vol. 1, No. 2 (July-December, 2010) Introduction Polyethylene and polypropylene are the most popular plastics in packaging industries due to their useful mechanical properties and physical properties for foods and household products packaging. However, these plastics take several years to decompose. Therefore, gigantic amount of the petroleum based plastic packaging has been disposed into the environment and caused serious natural resource contamination every year [1]. Many countries have urged to apply various regulations, policies and managements to overcome these problems e.g. recycle reuse and reduce protocol, yet it is far from success. Moreover, bio-plastics and biodegradable plastics have also become worldwide main research area in order to solve such waste problems [2]. However, large scale production and marketing of such plastics are still very expensive. So many plastic producers have developed oxodegradable plastics from the conventional plastics and made them degradable after used. In this communication, the oxodegradability has been introduced in situ using photo-catalytic additives that can also act as the thermal oxidative catalyst during the thermal plastic film processing. Typically, metal oxides or metal salts such as Mn 2+ /Mn 3+, Fe 3+, Zn 2+, Zr 2+, Ce 2+ and Ti 4+ have been used in such purpose [3-6]. In our previous work, thermal oxidative activity of TiO 2 compared with other metal oxides was preliminarily studied by monitoring carbonyl index of the blended plastic films and hydrocarbon solvents. Only ZnO 2 and TiO 2 were found to give good activity. In addition, TiO 2 showed approximately 2 folds higher activity than ZnO 2 [7]. Due to its superb characteristics such as inexpensiveness, nontoxicity, stability and highly photo-activeness, TiO 2 has become the excellent choice for photo-catalyst in order to make the conventional packaging plastics such as PE, PP and PS become degradable after working period, especially for food contacting plastics [8-9]. Since, TiO 2 can also be thermal catalyst during the processing period and induce the auto-oxidation of plastics. It is often referred as a pro-oxidant or sometimes called pro-degradants. Plus, several groups of the nano-sized TiO 2 have been synthesized and proven to give superior catalytic behavior over the micro-sized TiO 2 in many reactions. Hence, it is possible to be used effectively in the plastic degradation. 2. Theory Typically, a natural pathway of autooxidative degradation of polyethylene and polypropylene is normally initiated by light (photo-oxidative degradation) or heat (thermo -oxidative degradation). This step usually is the slowest process and becomes the rate determining step, because plastics have to combine with oxygen in the air and react to give the unstable peroxides which can then decompose to carbonyl and free radical intermediates e.g. alkyl radicals or hydroxyl radicals [10]. Moreover, the carbonyl intermediates can be photolyzed to more free radicals [11]. The radicals then undergo propagation step in the chain reaction of PE and PP degradation and give more of oxygenated carbon skeletons in the plastics in form of hydroxyls, aldehydes, ketones and carboxylic acids which then can be further biodegraded at low molecular weight ultimately resulting carbon dioxide and water [12]. The objective of this research is to accelerate the rate determining step by using TiO 2 prooxidants which can generate carbonyl intermediates during the thermoplastic processing step and catalyze degradation of plastics under solar light exposure after used and casted away. Moreover, the thermal oxidative and photolytic reactivity of nanosized TiO 2 particle was also investigated as well as crystalline types e.g. anatase and rutile. The TiO 2 with various particle sizes, crystalline types, grades, and additive concentrations were blended into LDPE and PP, casted into a thin film, irradiated under ultraviolet light at 254nm and 366 nm. Tensile strength, modulus and elongation at break of the films and carbonyl index were measured thoroughly to understand their catalytic degradation pathways.

3 16 Nano-Sized Titanium Dioxides as Photo-Catalysts in Degradation 3. Materials and Methods Virgin grade LDPE (IRPC) and PP (IRPC) were used for the preparation of films. Commercial analytical grade TiO 2 (predominantly rutile, referred as TiO 2 -com), nanosized rutile TiO 2 (rutile, particle size < 100 nm, SSA > 14 m 2 /g), nano-sized anatase TiO 2 (anatase, particle size < 25 nm, SSA m 2 /g), benzene (HPLC grade) and hexanes (HPLC grade) were purchased from Sigma Aldrich. The micron-sized TiO 2 (rutile, particle size < 63 µm) was prepared from TiO 2 -com by particle size sieving at 230 Mesh and referred as TiO 2-63µm. Most catalysts were oven dried for 24 hours and kept in desiccators before use. Catalytic Thermal Oxidation of PE and PP Films during Processing. The powders of TiO 2 -com, TiO 2-63µm, nano-sized rutile TiO 2 and nano-sized anatase TiO 2 were blended into both LDPE and PP at 0%, 1%, 2%, 3%, and 5% w/w concentrations using a Twin-Screw Extruder (TSE 16 TC; Intro enterprise Co., Ltd.). The heating profile was set as followed: feed zone 120 o C, compression zone 140 o C and metering zone 160 o C. Screw speed was set at 40 rpm. Then the obtained plastics were casted to 80- micron thick films using a Chill Roll Cast Film Machine (LE 25-30/C; Labtec Engineering Co., Ltd.) with screw speed at 50 rpm and six heating zones set at 170 o C, 190 o C, 200 o C, 210 o C, 210 o C and 210 o C. Tensile strength of the films was measured using a Universal testing machine (Tensile H5K-T; Calserve Thailand Co., Ltd) to compare with the virgin film. The film carbonyl index (CI) was measured via ATR-IR spectroscopy using a Perkin Elmer spectrum 2000 FTIR spectrometer and calculated by the ratio of the peak area between cm -1 (C=O stretching band) and the peak area between cm -1 (C-H bending band). were cut to 1 x 5 cm 2 size and exposed under a 20-watt shortwave UV lamp (average wavelength at 254 nm) and a 20-watt long wave UV lamp (average wavelength at 366 nm) at 30 cm in distance. The plastic films then were taken out daily or weekly to measure tensile strength, elongation at break, modulus, carbonyl index and % weight loss to determine their degree of degradation. 4. Results and Discussion Thermal Oxidation of PE Films. The effect of particle size in thermal oxidative degradation of LDPE films, only TiO 2 with rutile crystalline were used as catalysts in this investigation. After the powders of TiO 2 - com, TiO 2-63µm and the nano-sized rutile TiO 2 were blended into LDPE at various concentrations and then casted to 80-micron thick films, it was found that the film without TiO 2 showed very low carbonyl index at This indicated that the heat as high as 210 o C during plastic film processing did not induce auto-oxidation. However, the LDPE film blended with TiO 2 -com 1%w/w via the same process showed carbonyl index at similarly to the film blended with the nanosized rutile TiO 2 1%w/w. It is important to note that the size-sieved TiO 2-63µm gave slightly higher catalytic oxidative activity than TiO 2 -com at higher concentrations as shown in Figure 1. The particle size effect became so clear when the nano-sized rutile TiO 2 was blended with LDPE under the same Catalytic Photo-Oxidative Degradation of PE and PP Films under UV Lights. The obtained plastic films containing various types of TiO 2 which possess about the same carbonyl index Figure 1. Effect of TiO 2 particle size, structure and concentration on catalytic thermal oxidation of LDPE films during processing.

4 T. Manangan et al., Sci. J. UBU, Vol. 1, No. 2 (July-December, 2010) processing condition at high concentration 3% and 5% w/w. The nano-sized TiO 2 provided thermal oxidative activity by folds over TiO 2 -com. This is probably due to dispersion effect only. Because the LDPE films blended with 1% and 2% w/w of most rutile TiO 2 catalysts have nearly the same carbonyl index. Though, the nano-sized rutile catalyst has particle size less than 100 nm and a specific surface area more than 14 m 2 /g. In addition, the nano-sized anatase TiO 2 blended LDPE film under the same processing condition showed significantly higher carbonyl index than the film blended with rutile crystalline in all catalyst concentrations. The nano-sized anatase TiO 2 not only processes particle size less than 25 nm, but also process specific surface area of m 2 /g or about 15 times greater than the nano-sized rutile specific surface area. Hence, its catalytic oxidative activity has been superb. concentration increased. This is possibly due to the degradation via scission mechanism already occurred in this high temperature film processing period. Again, nano-sized anatase catalyst showed better reactivity than rutile catalysts in all concentrations. Thermal Oxidation of PP Films. The effect of particle size in thermal oxidative degradation of PP films was carried out exactly the same as in LDPE film. After the powders of the TiO 2 -com, TiO 2-63µm and the nano-sized TiO 2 with rutile crystalline were blended into PP at various concentrations and film casted, it was found that the pure PP film without TiO 2 also showed very low carbonyl index at indicating that the heat as high as 210 o C during plastic film processing did not induce auto-oxidation. However, the film blended with TiO 2-63µm showed carbonyl index about similar to the film blended with TiO 2 -com as shown in Figure 3. Figure 2. Tensile strength of LDPE films containing rutile TiO 2 at various particle sizes after thermal oxidative processing. Similar to carbonyl index, TiO 2 -com and TiO 2-63µm gave nearly identical results on the LDPE film tensile strength during the film processing period in all additive concentrations as shown in Figure 2. However, most of the blended films have slightly lower tensile strength than pure LDPE. This suggested that these LDPE films still possess normal mechanical properties for packaging uses. The LDPE film blended with both nano-sized TiO 2 also showed lowering of the film tensile strength as the additive Figure 3. Effect of TiO 2 particle size on thermal oxidation of PP films during processing. Figure 4. Tensile strength of PP films containing various sized rutile TiO 2 and nano-sized anatase TiO 2 after processing.

5 18 Nano-Sized Titanium Dioxides as Photo-Catalysts in Degradation Surprisingly, the nano-sized TiO 2 both anatase and rutile showed lower oxidative activity in PP film than the commercial micron-sized rutile especially at low catalyst content. Furthermore, the tensile strength also has become even greater at high catalyst content as shown in Figure 4. This is possibly due to some recombination of the generated radicals instead of scission or oxidation [9]. Catalytic Photo-Oxidative Degradation of PE and PP Films under UV Lights. In order to minimize the difference of the carbonyl content, the blended LDPE and PP films containing 1% w/w of various TiO 2 which process about the same initial carbonyl index were cut to 1 x 5 cm size and exposed under a 20-watt shortwave UV lamp (254 nm) and a 20-watt long wave UV lamp (366 nm) at 30 cm in distance. Moreover, the films were also exposed under both lamps to mimic the solar light profile. The plastic films then were taken out daily to measure tensile strength, carbonyl index and % weight loss to determine their degree of degradation. It was found that most of these catalysts were highly active only under 254-nm ultraviolet irradiation. The results under dual lamp condition gave almost identical to the 254-nm ultraviolet irradiation alone (results not shown here). Hence, most of the photo-oxidative degradation was done under a 20-watt shortwave UV lamp (254 nm) only. Most LDPE films blended with TiO 2 slowly increased their carbonyl index in the first 24 hour and then dramatically increased later after. The carbonyl index kept on rising until the film surface became brittle and ruptured. The cloudy films were usually observed after 72 hours and the carbonyl index tended to drop due to ATR-IR scattering interference as shown in Figure 5. These results fitted perfectly with the tensile strength profile as shown in Figure 6. Most of the film tensile strength appeared to remain constant up to 48 hour of irradiation and then drop drastically right after as the film fragmentation occurred and the carbonyl index started to drop. Figure 5. Initial effect of TiO 2 (1%w/w) on carbonyl index of LDPE films under 254-nm UV irradiation. Among these TiO 2 catalysts, the nano-sized showed the highest photo-oxidative degradeation of LDPE films especially the nanosized anatase form which can lower the film tensile strength by 38% in 96 hours under UV irradiation compared to that of pure LDPE under the same condition as depicted in Figure 6. Figure 6. Initial effect of TiO 2 (1%w/w) on tensile strength of LDPE films under 254-nm UV irradiation. In the other hand, PP films blended with the nano-sized TiO 2 did not show much increase of their carbonyl index, while the commercial TiO 2 -com and the micron-sized TiO 2-63µm blended films showed similar trend as in LDPE films as shown in Figure 7. Furthermore, the film tensile strength appeared to increase under UV irradiation as shown in Figure 8. The catalytic photo-degradation of these PP films seemingly underwent via a different mechanism.

6 T. Manangan et al., Sci. J. UBU, Vol. 1, No. 2 (July-December, 2010) with in 2 weeks. This also caused PP films to lose weight by 22% in 42 days. 5. Conclusions Figure 7. Initial effect of TiO 2 (1%w/w) on carbonyl index of PP films under 254-nm UV irradiation. Figure 8. Initial effect of TiO 2 (1%w/w) on tensile strength of PP films under 254-nm UV irradiation. To further understand and predict total rupture of these LDPE and PP films under solar light irradiation, the films then were exposed under 254-nm ultraviolet for longer period. The films then were taken out weekly to determine their degree of degradation by measuring carbonyl index, tensile strength, modulus, % elongation at break and % weight loss. It was found that the carbonyl index (using FTIR, KBr) of both nano-sized TiO 2 (1%wt) blended LDPE films increased continuously over irradiation period and their tensile strength reduced to 35-38% after 28 days up to 42 days before total ruptured. The films have also lost weight about % in 14 days. Both nano-sized TiO 2 blended PP films showed a dramatic increasing of carbonyl index in the first few days and then continuously dropped as they became fragmented Titanium dioxide catalysts can be used as packaging plastic pro-oxdiants or prodegradants which can actively catalyze thermal oxidation generating carbonyl intermediates during the film processing and also act as the photo-oxidative degradation catalysts along with the carbonyl intermediates under the UV and solar lights irradiation. During the processing period, both nanosized TiO 2 significantly induced autooxidation of the PE films. However, carbonyl formation in PP films suggested other mechanical pathways. After an exposure under 254nm and 366nm UV light mimicking solar light profile, in most cases, TiO 2 catalyzed photo-degradation occurred under the shortwave UV-254nm irradiation several folds higher than under long wave UV irradiation. The carbonyl index of the nanosized TiO 2 (1%w/w) blended PE films increased continuously over the irradiation period and their tensile strength reduced to 35-38% after 28 days up to 42 days before total ruptured. The films have also lost weight about % in 14 days. The nanosized TiO 2 blended PP films showed a dramatic increasing of carbonyl index in the first few days and then continuously dropped as they become fragmented with in 2 weeks. This also caused PP films to lose weight by 22% in 42 days. This study also suggested that titanium dioxide nano-sized particle showed significantly more favorable in both thermo- and photo-oxidative degradation catalytic activity and results than the commercial and micron-sized TiO 2. Acknowledgements The authors would like to acknowledge the financial support from the Research, Development and Engineering (RD&E) Fund through National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Thai-

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