Oxidative Degradation of Polymers (Part 4)

石油学会誌 J. Japan Petrol. Inst., 21, (1), 63-67 (1978) 63 Oxidative Degradation of Polymers (Part 4) Photooxidation of Isotactic Polypropylene Powder Takeo SHIONO*, Etsuo NIKI*, and Yoshio KAMIYA* Isotactic polypropylene powder was found to be quite rapidly oxidized at low temperatures when it was irradiated with a low pressure mercury lamp. About a half of the absorbed oxygen appeared as carbon dioxide independent of the extent of oxidation. The formation of other volatile products and incorporation of oxygen into the polymer chain were small as compared to the absorbed oxygen. At low oxygen pressures, the formation of hydrogen and methane was also observed, but it was not at atmospheric pressure of oxygen. The rate of oxidation increased with increasing surface area of polypropylene powder. Possible mechanism of this photooxidation is discussed. 0.607, and 0.91, respectively. Polypropylene pow- 1. Introduction ders of various particle size were prepared by sieving. Photochemical oxidations of polymers have been Water was purified by passing through an ion- studied by numerous investigators mostly on films exchange resin. from both fundamental and practical viewpoints1)-7). A 30W low pressure mercury lamp was used as Although pure polypropylene does not show any an ultraviolet light source, the main emission being absorption beyond 300nm light, it deteriorates the resonance lines of 184.9 and 253.7nm. during weathering by terrestrial sunlight. It is now 2.2 Procedures generally accepted that ultraviolet light contributes Two procedures were employed. In one pro- to the generation of free radicals by homolytic cedure, 100mg of isotactic polypropylene powder cleavage of the carbonyl groups and hydroperoxides was weighed into an about 50ml quartz vessel and and that subsequent chain oxidation is similar to then 10ml of water was introduced. The vessel thermal autoxidation. However, only scattered was connected to a vacuum line and degassed by data seem available on oxygen balance, i. e., the alternately freezing and thawing after which oxygen selectivity of various products based on the oxygen was introduced at approximately 1 atmosphere unless absorbed. Above all, quantitative product analyses otherwise noted. The composition of the initial are lacking in the photooxidation of polymers ir- and that of the final gases were determined by radiated with 253.7nm light. In the course of Toepler pump and by subsequent gas analysis with gas our study on the oxidation of polymers by molecular chromatography8). Molecular sieve 13X and active oxygen, we have undertaken photooxidation of Iso- charcoal were used as a column for the analyses of tactic polypropylene powder floated on the water oxygen, carbon monoxide, hydrogen, methane and surface using a low pressure mercury lamp as a carbon dioxide. A Teflon valve equipped with light source, and we found it was oxidized remarka- Swagelok fittings, type 6VD of Nupro Company, bly fast to produce carbon dioxide exclusively. was used for connecting and disconnecting the vessel We wish to report here the results of this interesting to the Toepler pump. In another procedure, pho- and characteristic photooxidation reaction. tooxidation was carried out under atmospheric pressure of pure oxygen, and the rate of gas absorption 2. Experimental was followed using a buret9). In both procedures, 2.1 Materials oxidation was started by irradiation with a low pres- Isotactic polypropylene powder free of any addi- sure mercury lamp after the reaction mixture had tives was kindly supplied by Mr. Fujita of Mitsui attained the reaction temperature. The polypro- Petrochemical Industries. The molecluar weight, pylene powder was floating on the water at first, fraction of crystallinity, and tacticity were 3.5×105, but part of it became suspended as oxidation pro- ceeded. Received July 2, 1977. * Department of Reaction Chemistry, Faculty of Engi- After oxidation and analysis of the final gases neering, University of Tokyo (7-3-1, Bunkyo-ku, Hongo, noncondensables at -80℃, the vessel was opened Tokyo 113) and the reaction mixture filtered with a glass filter 石油学会誌 J. Japan Petrol. Inst., Vol. 21, No. 1, 1978 64 Table 1 Photooxidatioii of Isotactic Polypropylene Powder under Atmospheric Oxygen a) By monomer unit. Powder smaller than 48 mesh. b) (O2 absorbed/propylene monomer unit)×100. c) See text for calculation. 1G-4. The isotactic polypropylene powder recovered was washed several times with water, dried under vacuum at 70℃, weighed, and finally analyzed by infrared spectroscopy and elementary analysis. The filtrate was analyzed as described elsewhere9),10). Organic peroxides and hydrogen peroxide were measured separately by iodometric titration in the absence and presence of catalasell). It was confirmed that hydrogen peroxide was de- composed quantitatively by catalase under the analytical conditions employed. 3. Results Pertinent results of photooxidation of the isotactic polypropylene powder at different reaction times and temperatures are summarized in Table -▲-: Run 8, on neutral water, 51hr 1. Polypropylene powder used in Table 1 was -●-: Run 9, on alkaline water, 16hr (--▲--: Corrected rate of Run 8 (see text) smaller than 48 mesh in size. Although pure poly- Fig. 1 Photooxidation of Polypropylene Powder at 70℃ propylene should be transparent to 253.7nm light, Table 1 shows that the rate of oxidation is re- propylene powder is oxidized quite readily without markably fast and that approximately a half of the any noticeable induction period. The higher rate absorbed oxygen is found as carbon dioxide even of oxidation in the alkaline solvent must have at low levels of conversion. No other gaseous pro- stemmed from the evolution of carbon dioxide in ducts such as carbon monoxide, methane, and hy- neutral water. The dotted line in Fig. 1 is the drogen were observed at atmospheric pressure of corrected rate of oxidation assuming that a half of oxygen. The presence or absence of water did not the absorbed oxygen evolved as carbon dioxide in give any appreciable difference in the rate and pro- a neutral solvent. This assumption may be justi- ducts of oxidation (Runs 3 and 5). Table 1 also fied from the results in Table 1 and from the shows that the effect of temperature is small. After coincidence shown between the corrected rate and oxidation, the product solution was filtered and the the rate in the alkaline solvent. Conversion based recovered polypropylene was dried and weighed. on monomer unit was as high as 90% in 16 hours Although the amount of oxygen absorbed per mono- and 240% in 51 hours, and the recovery of polymer unit (conversion) is large, much of the polypropylene powder was 64% and 10%, respectively. propylene powder could be recovered. The signi- The observed rate of oxidation is measured from ficance of this observation will be discussed later. Fig. 1 as 4.0×10-4mol O2/kg PP・sec, which is Figure 1 shows the apparent rate of oxygen ab- faster by a factor of about 102 than the thermal sorption for longer reaction times in neutral water and γ-irradiated oxidation of bulk polypropy- (Run 8, 51 hours) and in an alkaline aqueous sol- lene12)-14). vent (Run 9, 16 hours). It can be seen that poly- The infrared absorption spectrum of the recovered 石油学会誌 J. Japan Petrol. Inst., Vol. 21, No. 1, 1978 65 Table 2 Effect of Particle Size in the Photooxidation of Polypropylene Powder in 10ml Water at 70℃ for 3 hours -: Initial polypropylene -●-: Run 4, neutral water, 4.5hr ----: Run 9, alkaline water, 16hr -● ●-: Run 10, neutral water, 16hr a) By monomer unit, b) See text for calculation. Fig. 2 Infrared Spectra of Photooxidized Polypropylene Powder, 2.5wt% in KBr effect of particle size was examined. Powders of polypropylene is shown in Fig. 2, which indicates different sizes were prepared by sieving. The re- the formation of carbonyl (1,710cm-1) and hy- sults in Table 2 show that the rate of oxidation is droxy(3,400cm-1) groups. The gas liquid chro- higher for smaller powders but the formation of matographic analysis of the filtrate showed the carbon dioxide is similar. As mentioned previously, presence of acetone, methanol and acetic acid, but polypropylene powder is floating on the surface of their formation was quite small. Hydroperoxide water and its surface is covered more with decreas- and hydrogen peroxide were also found among the ing particle size; thus, the higher rate of oxidation products. If it is assumed that one molecule of for smaller powders must be ascribed to their large water is formed per carbon dioxide formed, then surface areas. When the powder is smaller than 80-90% of oxygen is accounted for by these pro- 48 mesh, the water surface is covered completely ducts. with polypropylene powder and this must be the reason as to why the rates are similar for Runs 13 and 14. Table 3 summarizes the results of elementary analysis of the initial and photooxidized polypropylene powders. It shows that, although the a- mount of oxygen absorbed per monomer unit is quite high, the oxygen incorporated into the polymer chain is surprisingly small and that the ratio of carbon to hydrogen remains constant. Further- more, no discoloration of polypropylene powder was observed after oxidation, suggesting that no con- jugated double bonds were formed. ●: Hydrogen, ▲: Methane, ■: Carbon monoxide Fig. 3 Effects of Oxygen Pressure on Rate and Products Table 3 Elementary Analysis of Initial and of the Photooxidation of Isotactic Polypropylene Photooxidized Polypropylene Powder at 70℃ for 1 hour Figure 3 shows the effect of oxygen pressure on the rate and gaseous products of the photooxidation of isotactic polypropylene powder. It shows that the rate of oxidation increases with increasing oxygen pressure and that hydrogen and methane are formed at low oxygen pressure but that their formation a) Oxygen content by difference, b) Oxygen introduced is suppressed by sufficiently high oxygen pressure.

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