THE EVALUATION OF DEGRADATION MECHANISM OF POLYOLEFINS, AND DEVELOPMENT OF NON-DESTRUCTIVE EVALUATING METHOD

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1 THE EVALUATION OF DEGRADATION MECHANISM OF POLYOLEFINS, AND DEVELOPMENT OF NON-DESTRUCTIVE EVALUATING METHOD Toshio Igarashi, Yusuke Hiejima, Takumitsu Kida, Koh-hei Nitta, Division of Material Sciences, Kanazawa University, Japan Yutaka Yonezawa, Adeka Corporation, Japan Abstract A comprehensive investigation of the degradation mechanism of polyolefin products and evaluation of their deterioration state using non-destructive methods are essential. The photodegradation of low-density polyethylene was investigated with various methods in a wide range of scales from macroscopic to microscopic. The ability to characterize the initial stage of degradation of these materials using non-destructive Raman spectroscopic measurements is also demonstrated. Introduction With the expected end of the era of petroleum production in the 2020s, the prominence of reducing, reusing, and recycling countermeasures in the plastics industry is expected to increase [1]-[3]. In rotational molding, degradation research is important because many products are used for a long time in the field such as water and chemical tanks [4]. Polymer product degradation in the field is caused by various factors such as UV light, elevated temperature, fatigue, shock, biological deterioration, and water; however, the mechanisms for this degradation have not been sufficiently elucidated. A non-destructive method for correctly evaluating the degree of degradation of plastic products is thus required, and investigation of the mechanisms leading to the initiation and progression of material degradation from both chemical and physical viewpoints is critical [5]. To comprehensively study these deterioration issues, we have created a research and development consortium in 2016 [6]. Four universities in Japan, three public research institutes, and four private companies are scheduled to participate. The project "Comprehensive Science for Degradation of Polymer Materials" is focused on the following three targets: Target I: Elucidation of the degradation mechanism of polymeric materials Target II: Development of nondestructive degradation evaluation methods Target III: Development of a prototype for a nondestructive deterioration evaluation device and application for detection in the field. The main motivations for this project are the following: 1) Polymers are widely adopted as one of three major materials (metals, inorganics, resins); however, their properties undergo various changes with time depending on the use environment. 2) Durability evaluation and life expectancy are important factors for reliable product design and the design of highperformance materials; however, data accumulation is insufficient. Consequently, polymers are scarcely used in products for long-term use. 3) Only a few detailed and quantitative studies of degradation processes based on changes in the internal structure of resinous polymers have been conducted. The development of a polyolefin with a structure that is resistant to deterioration and the development of a molding method, new additive systems, and secondary processing methods are required. In this paper, we describe the goals and background of the degradation study and present research results on the degradation of rotational molded products. Raman spectroscopy measurements are performed in addition to conventional degradation evaluation techniques such as the determination of the yellowness index (YI), gloss values and microscopic evaluation. Finally, we investigate the initial stage of ultraviolet (UV) degradation of lowdensity polyethylene (LDPE) based on the relative intensity changes and peak shifts during UV exposure. Material Experiments and Procedures LDPE (Prime Polymer Co. Ltd.) with a density of 0.92 g/cm 3 and melt index (MI) of 4 g/10 min was used in this study. Based on the rotational molding process described in Ref. [7], samples with a thickness of 0.5 mm were prepared. Using a test press molding machine, the raw material powder was first preheated at 150 C for 3 min. After degassing at 150 C for 2 min, the sample was pressurized at 10 MPa for 1 min. The sample was then heated up to 200 C in 8 min, and then gradually cooled to 180 C in 8 min, followed by quenching to room temperature. SPE ANTEC Anaheim 2017 / 1207

2 Preparation of deteriorated samples The photodegraded LDPE samples were prepared with a Xenon Weather Meter (Suga Test Instruments, SX2-75). A Xenon fade lamp was used as a light source, and the samples were irradiated at an irradiance of 60 W/m 2 in a wavelength range of 300 to 400 nm under a black panel temperature of 89 C and no rain condition. The irradiation time was set from 120 to 1200 h. Evaluation of deteriorated samples The YI value was measured by using a spectrocolorimeter (Suga Test Instruments, SC-T (P)) under a reflection condition with a diffuse lighting for 8 times, where positive reflection component was excluded. The wavelength range was nm with a resolution of 5 nm. The gloss value was measured with a gloss meter (Nippon Denshoku Industries, VG-2000) at an angle of 60 with a hole size of mm 2, where a halogen lamp (9 W) was used as a light source. The surface of the sample was observed with an optical microscope (Keyence, VHX-1000) at a magnification of 100x. Raman spectroscopy The apparatus for Raman spectroscopy developed in our laboratory was used [8], [9]. A DPSS laser (LASOS) was used as the excitation light at a wavelength of 638 nm. SpectraPro2300i and PIXIS100 (Princeton Instruments) were used as the spectrometer and the detector, respectively. Each Raman spectrum was accumulated 20 times with an exposure time of 2 s. Results and Discussion YI, gloss values measurements and microscopic observation The irradiation-time dependence of the YI values is presented in Fig. 1. The YI value decreased gradually from 7.7 to 6.8 until ~500 h of irradiation and then began to increase. The point at which the YI value began to increase was considered the starting point of degradation. The gloss values are plotted against the irradiation time in Fig. 2. The gloss value decreased with increasing the irradiation time, and the slope becomes appreciably steeper after 500 h. Because a decrease of the gloss value represents an increase of the surface roughness, the formation of microcracks was thought to occur after approximately 500 h. The light microscopy images after 0 to 1080 h of irradiation are presented in Fig. 3. Partial cracking was observed in the sample after 840 h of irradiation (as indicated by the circle on the right side of the image). After 960 h of irradiation, the cracks appeared to grow, and after 1080 h. the cracks spanned the entire surface. Based on the YI and gross value measurements, it is suggested that deterioration starts approximately 500 h after irradiation. The microscopic examination of the surface also suggests that cracks formed after 840 h of irradiation and deterioration progressed, and after 1080 h, the surface cracks spanned the entire surface, leading to material destruction. Raman spectroscopy analysis The Raman spectra of LDPE after various exposure times are presented in Fig. 4. The spectral assignments are listed in Table 1 [10]. The intensities of the 1063 and 1130 cm 1 bands assigned to the C C stretching modes of the crystalline chains increased with exposure time. The spectral shape in the 1400 cm 1 region assigned to the CH 2 bending modes of the crystalline and amorphous chains also changed with exposure time. The relative intensities normalized by the sum of the intensities of the 1298 and 1313 cm 1 bands [9],[10] are plotted as a function of exposure time in Fig. 5. The relative intensities of the 1063 and 1130 cm 1 bands increased linearly with increasing exposure time, suggesting an increase in the number of trans chains. As observed in Fig. 6, the 1063 and 1130 cm 1 bands shifted toward smaller wave number, and the red shifts gradually increased with increasing exposure time, suggesting that a stretching stress is imposed on the polymer chain [8],[9], in accordance with the formation of trans chains. The relative intensity of the 1080 cm 1 band assigned to the C C stretching mode of the amorphous chains decreased after 500 h, suggesting a decrease in the number of amorphous chains because of chemicrystallization. As observed in Fig. 7, the relative intensity of the 1440 cm 1 band assigned to the CH 2 bending mode of the amorphous trans chains increased with increasing exposure time, whereas that of the 1460 cm 1 band assigned to the amorphous chains decreased. These symmetrical changes of the 1440 and 1460 cm 1 bands are considered to represent the formation of trans chains in the amorphous phase. These two bands shifted toward higher wave number and the blue shift gradually increased with increasing exposure time. In addition, it is suggested that the CH 2 bending motions were further hindered with increasing exposure time, which is explained by the reduced flexibility of trans chains. As shown in Fig. 8, the red shift of the 1418 cm 1 band increased with increasing exposure time, suggesting shrinkage of the crystalline lattice by ~1% [9],[10]. These microscopic changes indicate that the conversion of the amorphous to trans chains occurs during the early stage, followed by an increase of the crystallinity at ~500 h. It is plausible that the increase of crystallinity triggers macroscopic changes of the specimen such as microcrack formation. SPE ANTEC Anaheim 2017 / 1208

3 Conclusions It is concluded that degradation of semicrystalline polymers such as LDPE progresses from microscopic to macroscopic scales, as illustrated in Fig. 9. The conventional methods for evaluation of the degradation of polymeric materials, such as microscopic observation of microcracks and characterization of the YI and gloss values, detect the deterioration after the degradation process reaches larger scales than the size of spherulites. We have found that Raman spectroscopy is suitable to probe molecular level degradation of LDPE subjected to UV exposure. Changes of the relative intensities and peak shifts enabled detection of microstructural changes in the amorphous phase before chemicrystallization. In addition, the initial stage of UV degradation of LDPE was detected at shorter times using this approach compared with other techniques. Figures and Tables Fig. 1 Temporal change of YI values of LDPE as a function of UV irradiation time. References 1. The-twilight-of-petroleum.html T. Igarashi, Next Generation Polyolefins, 9, 115 (2015) 4. T. Igarashi, Structure and Properties of Polyolefin Materials, Ch.7, Transworld Research Networks, J.-L.Gardette et al., Polym. Degrad. Stab., 128, 200 (2016). 6. T.Igarashi, Next Generation Polyolefins, 10, 95 (2016) 7. A. Spencer et al., SPE-ANTEC Tech.Papers, 49, 1241 (2003). 8. T. Kida et al., Polymer, 58, 88 (2015). 9. T. Kida et al., Int J. Exp. Spectrosc. Tech., 1, 1 (2016). 10. M. Gall et al., Spectrochim. Acta A Mol. Spectrosc. 28A, 1485 (1972). 60 Fig. 2 Gloss values of LDPE as a function of UV irradiation time. SPE ANTEC Anaheim 2017 / 1209

4 0 h 120 h 240 h 360 h 480 h 600 h Fig. 4 Raman spectra of LDPE after various UV irradiation times. Table 1 Raman spectra peak assignments for polyethylene [10] 720 h 840 h Wave Mode Phase number (cm -1 ) 1063 C-C Anti-symmetric stretching Trans chain 1080 C-C Stretching Amorphous 1130 C-C Symmetric stretching Trans chain 1298 C-C Twisting Crystalline 1313 C-C Twisting Amorphous 1418 CH 2 Bending,Wagging Crystalline 1440 CH 2 Bending Amorphous trans 1460 CH 2 Bending Amorphous 960 h 1080 h Fig. 3 Microscopic examination of LDPE after various UV irradiation times. SPE ANTEC Anaheim 2017 / 1210

5 Fig. 5 Exposure time dependence of the relative intensities of Raman bands at 1063, 1081, and 1130 cm 1 (C C stretching mode). Fig. 8 Time evolution of the peak shifts for 1418, 1440, and 1460 cm 1 bands (CH 2 bending mode). Fig. 9 Degradation mechanism of semicrystalline polymers. Fig. 6 Exposure time dependence of the peak shifts for the nonirradiated samples for the 1063, 1080, and 1130 cm 1 bands (C C stretching mode). Fig. 7 Time evolution of the relative intensities of the 1440 and 1460 cm 1 bands (CH 2 bending mode). SPE ANTEC Anaheim 2017 / 1211