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Conversion among photo-oxidative products of polypropylene in solid, liquid and gaseous states

Conversion among photo-oxidative products of polypropylene in solid, liquid and gaseous states During aging of polymers, oxidized species on macromolecular chains in solid state, volatile degradation products in liquid state and gaseous degradation products in gaseous state are often investigated separately. The conversion among these products is not especially concerned and biased results may be obtained based on the products in a single state. In this paper, photo-oxidative products of commercial polypropylene (PP) and unstabilized PP in solid, liquid and gaseous states were investigated by using Fourier transform infrared spectroscopy (FTIR), pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) and gas chromatography (GC). By comparing the formation profiles, conversion among the photo-oxidative products in three states was traced. During photo-oxidative aging, the main chains of PP were first oxidized to form carbonyl species in solid state, or fractured to form volatile alkenes as liquid. With the proceeding of aging, the oxidized main chains fractured to form small molecules, resulting in the conver- sion of oxidized species from solid state to liquid and gaseous states. When the aging degree was extremely high, the accumulation of liquid oxidized products was limited due to migration and condensation. Therefore, both the car- bonyl index (CI) and the concentrations of volatile oxidized products were increased first and then decreased, while the concentrations of gaseous products kept increasing all along. Keywords: Polypropylene, Photo-oxidative aging, Conversion, Aging degree Introduction molecules. The dynamic process results in continuous During the aging process of polymers, oxidation and deg- change of products in three states. Therefore, studying radation of macromolecular chains happened, leading the products in a single state is not enough to reflect the to the deterioration of mechanical properties [1–3]. The whole aging process. aging products of polymers exist in solid, liquid and gase- Most researches focused on the changes of solid poly- ous states. Solid products are resulted from the oxidation mers before and after aging. For instance, changes of of main chains [4, 5]. Liquid products are small molecular functional groups, chromophores, molecular weight, degradation products, which can be absorbed in the aged crystallinity and morphology were often used to evaluate polymers or volatilize [6]. Gaseous products are smaller the aging degree of polymers [8–12]. Some researchers molecules, which are in gaseous state at room tempera- turned their eyes to small molecular degradation prod- ture and easily migrate out [7]. Oxidized main chains ucts [13]. Carlsson, Wiles and Philippart contributed continuously fractured to generate liquid and gaseous to the identification of volatile and gaseous products of polypropylene (PP) in various aging conditions [14–16]. Combination of selective isotopic labeling with solid- *Correspondence: yangr@mail.tsinghua.edu.cn phase microextraction or cryotrapping gas chromatog- Department of Chemical Engineering, Tsinghua University, raphy/mass spectroscopy made it possible to confirm the Beijing 100084, People’s Republic of China © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Liu and Yang BMC Chemistry (2020) 14:44 Page 2 of 8 original position of carbons on the main chains entering the volatile products [17]. Egerton et al. [18–21] analyzed the generation of C O in real time during photo-oxida- tion of a series of polymers, such as acrylic paint, low density polyethylene (LDPE), poly(vinyl chloride) and polyethylene terephthalate. When the stabilities of polymers were compared based on macromolecular products or small-molecular deg- radation products separately, there might be conflicting results. For example, rare volatile oxidized products were Fig. 1 Schematic diagram of reaction device generated during photo-oxidation of high density poly- ethylene composites, but the carbonyl index (CI) of the solid sample was rather high [22]. Another example was was extracted for the gas chromatography (GC) meas- found in LDPE nanocomposites. LDPE/ZnO nanocom- urement. Then the film was taken out for the Fourier posites had lower CI but much higher concentration of transform infrared spectroscopy (FTIR) and pyrolysis- CO than LDPE/TiO nanocomposites after photo-oxida- 2 2 gas chromatography/mass spectrometry (Py-GC/MS) tive aging [23]. These facts remind us that the products measurements. in a single state can only provide information related to aging properties from a single aspect. However, there FTIR measurement are few works on the whole aging procedure, including FTIR measurement was conducted to detect the oxidized changes of solid, liquid and gaseous states together. In macromolecules in solid state. Transmission spectra were order to understand aging mechanism comprehensively, obtained by using a Thermo-Nicolet iS10 FTIR spec - studying the conversion among aging products in three trometer. For attenuated total reflection (ATR) mode, an states is of great importance. accessory with a diamond crystal was used. CI was calcu- In this paper, photo-oxidative products of two PPs with −1 lated by dividing the peak area at 1714 cm to the peak different stabilities in solid, liquid and gaseous states −1 area at 2722  cm [24]. Three measurements were car - were studied. Different induction periods indicated dif - ried out for each film and the results were averaged. ferent generation times of these products. Conversion among the products in three states was traced. Overview of the aging products in three states was an effective way Py‑GC/MS measurement to obtain comprehensive information of the whole aging Py-GC/MS measurement was conducted in a gas chro- process and avoid possible biased evaluation of the aging matography/mass spectrometer (GC/MS-QP2010 SE, degree and relative stability of polymers. Shimadzu) installed with an EGA/PY-3030D multi-shot pyrolyzer (Frontier Laboratories) to detect the vola- tile degradation products absorbed by the film. 10.0  mg Experimental film or ethanol solution of the condensed liquid droplets Materials on the tube wall was heated at 300  °C for 30  s and the Two kinds of PPs were used: commercial PP pellets evaporated components went through an Ultra Alloy-5 (F401, purchased from Sinopec, with stabilizers’ package column. Limited by the stationary phase of the column, containing UV stabilizers and/or antioxidants, marked as components with carbon number less than six might not CPP) and unstabilized PP powders (supplied by Petro- be separated completely. The peak area of a component China Panjin Petrochemical Company, without any addi- in the flash evaporation-gas chromatogram was calcu - tives, marked as UPP). Both pellets and powders were lated to represent the relative concentration of the com- hot-pressed at 190 °C into films (thickness of 0.3 mm). ponent. Two measurements were carried out for each film and the results were averaged. Photo‑oxidative aging Photo-oxidative aging was conducted in a Q-Sun Xe-3 chamber (Q-Lab Corporation). A film (40 mm × 15  mm) GC measurement was put into a sealable quartz tube before aging GC measurement was conducted in a 7890B GC system (Fig.  1). The intensity of UV irradiation was 0.35  W/ (Agilent Technologies) to detect the gaseous degradation m /nm@340  nm. The temperature was 60  °C and the products around the film in the tube. 200 μL gas were time intervals were 48, 96, 144, 288, 384 and 480  h. extracted from a tube by an injector and injected into Before aging, the valve was closed to seal the tube. After the GC system, through a G3591-81023 column. Accord- aging, the valve was opened and the gas in the tube ing to the separation capacity of the column, only small Liu and Y ang BMC Chemistry (2020) 14:44 Page 3 of 8 −1 gaseous molecules like H, N, O , CO, CO , and alkanes that of UPP before 288 h (0.04 h ). After 288 h, the CI of 2 2 2 2 and alkenes with carbon number less than three could UPP decreased. be detected. A flame ionization detector and a thermal The decrease of the CI in Fig.  3b seemed unreasonable, conductivity detector were used. The peak area of a com - since the oxidation of PP was an accumulation process ponent in the gas chromatogram was calculated to repre- of the oxidized species. Therefore, some highly oxidized sent the relative concentration of the component. species might be lost. Considering that UPP was easier to be oxidized than CPP, it reached a high oxidation degree first. At the late stage of aging (after 288 h), severely oxi - Results and discussion dized fragments might drop from the surface and cause Macromolecular products the decrease of CI [25–27]. The change of CI of UPP in CI mainly reflected the accumulation of carbonyl prod - Fig.  3a supported the speculation. The surface oxidation ucts on the macromolecular chains in solid state. CI cal- degree of UPP after aging for 288 h also showed a lower culated from ATR spectra showed the local oxidation increasing rate than that of CPP. In this case, although degree on the surface of the film, while CI calculated the aging degree of UPP was evidently higher than that from transmission spectra showed the average oxidation of CPP, the CI of UPP was close to or even lower than degree in the bulk polymer. that of CPP after 288 h. Thus, the biased evaluation of the ATR spectra, transmission spectra and corresponding aging degree was obtained. CI of two PPs are shown in Figs. 2 and 3. On the surface In addition, different sensitivities of ATR spectra and (Figs.  2a, b and 3a), CPP had an induction period of at transmission spectra were observed. Despite the same least 96 h, while UPP was oxidized from the very begin- difference between the oxidation degrees of two PPs after ning of aging. After the induction period, CPP was oxi- the same aging time, the differences between CI of two −1 −1 dized at a rate (0.10 h ) similar to UPP (0.11 h ) before PPs were not the same from ATR spectra and transmis- 288 h. After that, the increase of CI in UPP slowed down. sion spectra. For instance, after aging for 288  h, the CI Two PPs seemed to reach the similar oxidation degree from ATR spectra of UPP was about 1.4 times of that of after 480 h. In the bulk (Figs. 2c, d and 3b), UPP was also CPP, while the CI from transmission spectra of UPP was oxidized from the very beginning of aging, while CPP about 4.2 times of that of CPP. Obviously, transmission exhibited an induction period of at least 96 h. The oxida - spectra were more sensitive to the different oxidation −1 tion rate of CPP after 288  h (0.04  h ) was the same to degrees. Fig. 2 Infrared spectra of CPP and UPP during aging. a ATR spectra of CPP; b ATR spectra of UPP; c transmission spectra of CPP; d transmission spectra of UPP) Liu and Yang BMC Chemistry (2020) 14:44 Page 4 of 8 The peak areas of volatile oxidized products were expected to keep increasing with aging time due to the continuous oxidation of polymer chains. In CPP, they kept increasing indeed. In UPP, however, the peak areas stopped increasing and even began to decrease from 288 h. This was caused by the evaporation of these prod - ucts and then condensation on the tube wall. There were visible droplets on the tube wall in which UPP was aged for 384 and 480 h. The droplets were washed by ethanol and the obtained solution was analyzed by Py-GC/MS. The flash evaporation-gas chromatogram of the drop - lets was compared with the result of the corresponding film (Fig.  5). Acetic acid and the lactone were identified in both the droplets and the corresponding film. Once the volatile oxidized products were evaporated and con- densed on the tube wall, they could not come back to the film, so the peak areas in UPP were decreased obviously (Fig. 4c, d). For CPP, no visible droplets were observed even after aging for 480  h, due to the relative low concentration of volatile oxidized products. There was a maximal con - centration of volatile oxidized products that could be retained in the film. If the concentration did not exceed the maximum, the volatile oxidized products would accu- mulate in the solid polymer. When the concentration exceeded the maximum, the excessive parts began to be desorbed and some condensed on the tube wall. In this case, the biased evaluation of the aging degree of two PPs according to volatile degradation products would also be obtained as according to macromolecular products. Similar phenomenon can be expected if the polymer is Fig. 3 Carbonyl index of CPP and UPP with aging time. a calculated photo-oxidized in unsealed atmosphere like in practical from ATR spectra; b calculated from transmission spectra use. Gaseous degradation products Gaseous degradation products included H , CO, CO , Volatile degradation products 2 2 alkanes and alkenes. They were in gaseous state and Volatile degradation products were absorbed in PP film could be identified directly by using GC measurement. as liquid. They could be desorbed through flash evapo - The peak areas of four typical gaseous products with ration and detected by Py-GC/MS. Four typical volatile aging time are shown in Fig. 6. These products were gen - products, i.e. two alkenes, acetic acid and a lactone, were erated from 48 h in UPP and from 144 h in CPP respec- identified in the flash evaporation-gas chromatograms tively. After that, the peak areas kept increasing in both of two PPs. The peak areas of these products with aging PPs and the peak areas in UPP were higher than in CPP time are shown in Fig.  4. In Fig.  4a, b, the alkenes were all along, showing the higher aging degree of UPP with- generated from the very beginning of aging in UPP, while out the protection of stabilizers. later than 96  h in CPP. After a period of time, i.e. 96  h in UPP and 288  h in CPP, the peak areas of two alkenes turned to decreasing, corresponding to the increasing of Overview of photo‑oxidative products in three states two oxidized products in Fig. 4c, d at the same time. This Induction periods of the generation of photo-oxidative indicated that: (1) The formation of alkenes was prior products in three states were extracted from Figs.  3, 4 to the formation of oxidized products, in other words, and 6, listed in Table  1. In CPP, the induction periods of the chain scission of PP happened before the formation carbonyl products and volatile alkenes were the same, of volatile oxidized products; (2) The oxidized products shorter than acetic acid, the lactone and gaseous deg- might be from the further oxidation of alkenes. radation products, indicating the earlier generation of Liu and Y ang BMC Chemistry (2020) 14:44 Page 5 of 8 Fig. 4 Peak area of four typical volatile degradation products in CPP and UPP with aging time. a and b two alkenes; c acetic acid; d lactone oxidation species in macromolecular chains and volatile were decreased at the late stage of aging, due to the loss alkenes. Thereafter volatile alkenes might be further oxi - of severely oxidized species in solid state and the migra- dized and the oxidized main chains fractured to gener- tion and condensation of the liquid products. The peak ate volatile oxidized products and gaseous degradation areas of gaseous degradation products kept increasing for products. As shown in Figs.  4c, d and 6, the peak areas as long as 480 h. of acetic acid, the lactone and gaseous degradation prod- Conversion among photo-oxidative products in ucts were increased rapidly from 288  h. At the same three states is illustrated in Fig.  7. During the photo- time, the peak areas of volatile alkenes were decreased oxidation of PP, the main chains were first oxidized rapidly (Fig.  4a, b). This phenomenon suggested the fur - and the oxidized species remained in the film as solid ther oxidation of volatile alkenes and the corresponding products. In the meanwhile, the main chains fractured accumulation of volatile oxidized products and gaseous to generate volatile alkenes, absorbed by the film as degradation products. liquid. Then the oxidized main chains fractured along Compared with CPP, UPP exhibited much shorter with the further oxidation of volatile alkenes, to gen- induction periods. Without the protection of stabilizers, erate volatile oxidized products absorbed by the film oxidation species in macromolecular chains and volatile as liquid and gaseous degradation products in the alkenes were generated from the very beginning of aging, atmosphere around the film. The conversion took followed by volatile oxidized products and gaseous deg- place throughout the aging process. When the conver- radation products within 48 h. As shown in Figs.  3b, 4c, sion was severe as in UPP, the CI and the concentra- d, CI and the peak areas of volatile oxidized products tions of volatile products were increased first and then Liu and Yang BMC Chemistry (2020) 14:44 Page 6 of 8 Fig. 5 Flash evaporation-gas chromatograms of UPP and corresponding droplets in the tube after aging for 384 h Fig. 6 Peak area of four typical gaseous degradation products in CPP and UPP with aging time. a H ; b CO; c CH ; d C H 2 4 2 4 decreased, due to the loss of severely oxidized species aging of the polymer. In CPP, the similar conversion in solid state and the migration and condensation of was observed, although the aging process was retarded liquid droplets in the late stage, despite the continuous owing to the presence of stabilizers. Therefore, the Liu and Y ang BMC Chemistry (2020) 14:44 Page 7 of 8 Table 1 Induction period of generation of photo-oxidative Conclusions products in CPP and UPP The photo-oxidative products of CPP and UPP in solid, liquid and gaseous states were detected by FTIR, Py-GC/ Products Induction period Induction in CPP/h period MS and GC respectively, and their formation profiles in UPP/h were compared. During photo-oxidative aging, the car- bonyl products in macromolecular chains and volatile CI on the surface (solid) 96 0 alkenes were generated first. Then the volatile oxidized CI in the bulk (solid) 96 0 products and gaseous degradation products were gener- Alkenes (liquid) 96 0 ated from the further oxidation of volatile alkenes and Acetic acid (liquid) 144 48 the fracture of oxidized main chains. At the late stage Lactone (liquid) 144 48 of aging, CI and the concentrations of volatile oxidized H , CO, CH , C H (gas) 144 48 2 4 2 4 products were decreased, due to the loss of severely oxi- dized species in solid state and the migration and con- densation of liquid droplets, while the concentrations of products in a single state, especially only in solid state, gaseous degradation products maintained increasing. could not reflect the comprehensive aging process and Overview of the aging products in three states provides might lead to biased results. It was more reliable to comprehensive information and overall understanding of consider the photo-oxidative products in three states the aging mechanism. It offers an effective way to evalu - as a whole when evaluating the aging degree and rela- ate the aging degree and relative stability of polymers tive stability of polymers. accurately. Fig. 7 Schematic diagram of conversion among photo-oxidative products of PP in three states Liu and Yang BMC Chemistry (2020) 14:44 Page 8 of 8 Abbreviations 9. Milichovsky M, Milichovska S (2008) Characterization of oxidized cellulose PP: Polypropylene; CPP: Commercial PP; UPP: Unstabilized PP; FTIR: Fourier with ultraviolet-visible spectroscopy. J Appl Polym Sci 107:2045–2052 transform infrared spectroscopy; ATR : Attenuated total reflection; Py-GC/MS: 10. Ahn K, Rosenau T, Potthast A (2013) The influence of alkaline reserve on Pyrolysis–gas chromatography/mass spectrometry; GC: Gas chromatography; the aging behavior of book papers. Cellulose 20:1989–2001 CI: Carbonyl index. 11. Komatsu LGH, Oliani WL, Lugao AB et al (2014) Environmental ageing of irradiated polypropylene/montmorillonite nanocomposites obtained in Acknowledgements molten state. Radiat Phys Chem 97:233–238 The authors gratefully acknowledge Jing Li, Chunsong Li and Prof. Qi Lu for 12. Elanmugilan M, Sreekumar PA, Singha N et al (2014) Natural weather their kind help in GC measurements. The authors also thank Guangyan Chen aging of low density polyethylene: effect of prodegradant additive. Plast for his kind support in experimental materials. Rubber Compos 43(10):347–353 13. Schwarzinger C, Hintersteiner I, Schwarzinger B et al (2015) Analytical Authors’ contributions pyrolysis in the determination of the aging of polyethylene. J Anal Appl XL carried out the experiments and drafted the manuscript. RY proposed the Pyrol 113:315–322 research idea, improved the data interpretation and revised the manuscript. 14. Carlsson DJ, Wiles DM (1969) The photodegradation of polypropyl- Both authors read and approved the final manuscript. ene films II Photolysis of ketonic oxidation products. Macromolecules 2(6):587–597 Funding 15. Philippart JL, Posada F, Gardette JL (1995) Mass spectroscopy analysis This study was financially supported by the National Natural Science Founda- of volatile photoproducts in photooxidation of polypropylene. Polym tion of China (NSFC, Project No. 51673111). NSFC was not involved in design Degrad Stab 49:285–290 of the study and collection, analysis, and interpretation of data and in writing 16. Commereuc S, Vaillant D, Philippart JL et al (1997) Photo and thermal the manuscript. decomposition of iPP hydroperoxides. Polym Degrad Stab 57:175–182 17. Bernstein R, Bernstein SM, Assink RA et al (2007) The origins of volatile oxi- Availability of data and materials dation products in the thermal degradation of polypropylene, identified The datasets used and/or analyzed during the current study are available from by selective isotopic labeling. Polym Degrad Stab 92:2076–2094 the corresponding author on reasonable request. 18. Christensen PA, Dilks A, Egerton TA (1999) Infrared spectroscopic evalua- tion of the photodegradation of paint. J Mater Sci 34:5689–5700 Competing interests 19. Jin CQ, Christensen PA, Egerton TA et al (2006) Rapid measurement of The authors declare that they have no competing interests. Rui Yang is cur- polymer photo-degradation by FTIR spectrometry of evolved carbon rently acting as an Editorial Board Member for BMC Chemistry. dioxide. Polym Degrad Stab 91:1086–1096 20. Jin CQ, Christensen PA, Egerton TA et al (2006) Rapid measurement of Received: 22 April 2020 Accepted: 15 July 2020 photocatalytic oxidation of poly(vinyl chloride) by in situ FTIR spectrom- etry of evolved CO . Mater Sci Tech 22(8):908–914 21. Fechine GJM, Christensen PA, Egerton TA et al (2009) Evaluation of poly(ethylene terephthalate) photostabilisation using FTIR spectrometry of evolved carbon dioxide. Polym Degrad Stab 94:234–239 22. Yang R, Yu J, Liu Y et al (2005) Eec ff ts of inorganic fillers on the natural References photo-oxidation of high-density polyethylene. Polym Degrad Stab 1. Hamid SH (2000) Handbook of polymer degradation. Marcel Dekker, New 88:333–340 York, 2nd ed 23. Yang R, Christensen PA, Egerton TA et al (2010) Degradation products 2. Allen NS, Edge M (1992) Fundamentals of polymer degradation and formed during UV exposure of polyethylene–ZnO nano-composites. stabilisation. Elsevier Applied Science, New York Polym Degrad Stab 95:1533–1541 3. Celina MC, Wiggins JS, Billingham NC (eds) (2009) Polymer degradation 24. Rabello MS, White JR (1997) The role of physical structure and morphol- and performance. American Chemical Society, Washington DC ogy in the photodegradation behaviour of polypropylene. Polym Degrad 4. Jelle BP, Nilsen TN (2011) Comparison of accelerated climate ageing Stab 56:55–73 methods of polymer building materials by attenuated total reflectance 25. Yang R, Wang KH, Yu J et al (2006) Depth profiles of high-density polyeth- Fourier transform infrared radiation spectroscopy. Constr Build Mater ylenes filled with different inorganic fillers during natural photo-oxidation 128:2122–2132 by infrared microscopy. Spectrosc Spect Anal 26(5):821–824 5. Mowery DM, Assink RA, Derzon DK et al (2005) Solid-state C NMR inves- 26. Li JF, Yang R, Yu J (2008) Study on natural photo-oxidation degradation of tigation of the oxidative degradation of selectively labeled polypropylene polypropylene nanocomposites by infrared microscopy. Spectrosc Spect by thermal aging and γ-Irradiation. Macromolecules 38:5035–5046 Anal 28(8):1758–1762 6. Hakkarainen M, Albertsson A (2005) Indicator products: A new tool for 27. Yakimets I, Lai DW, Guigon M (2004) Eec ff t of photo-oxidation cracks on lifetime prediction of polymeric materials. Biomacromol 6:775–779 behaviour of thick polypropylene samples. Polym Degrad Stab 86:59–67 7. James SL, Robinson AJ, Arnold JC et al (2013) The effects of humidity on photodegradation of poly(vinyl chloride) and polyethylene as measured by the CO evolution rate. Polym Degrad Stab 98:508–513 Publisher’s Note 8. Rouillon C, Bussiere PO, Desnoux E et al (2016) Is carbonyl index a quanti- Springer Nature remains neutral with regard to jurisdictional claims in pub- tative probe to monitor polypropylene photodegradation? Polym Degrad lished maps and institutional affiliations. Stab 128:200–208 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Chemistry Central Journal Springer Journals

Conversion among photo-oxidative products of polypropylene in solid, liquid and gaseous states

Chemistry Central Journal , Volume 14 (1) – Jul 18, 2020

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Abstract

During aging of polymers, oxidized species on macromolecular chains in solid state, volatile degradation products in liquid state and gaseous degradation products in gaseous state are often investigated separately. The conversion among these products is not especially concerned and biased results may be obtained based on the products in a single state. In this paper, photo-oxidative products of commercial polypropylene (PP) and unstabilized PP in solid, liquid and gaseous states were investigated by using Fourier transform infrared spectroscopy (FTIR), pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) and gas chromatography (GC). By comparing the formation profiles, conversion among the photo-oxidative products in three states was traced. During photo-oxidative aging, the main chains of PP were first oxidized to form carbonyl species in solid state, or fractured to form volatile alkenes as liquid. With the proceeding of aging, the oxidized main chains fractured to form small molecules, resulting in the conver- sion of oxidized species from solid state to liquid and gaseous states. When the aging degree was extremely high, the accumulation of liquid oxidized products was limited due to migration and condensation. Therefore, both the car- bonyl index (CI) and the concentrations of volatile oxidized products were increased first and then decreased, while the concentrations of gaseous products kept increasing all along. Keywords: Polypropylene, Photo-oxidative aging, Conversion, Aging degree Introduction molecules. The dynamic process results in continuous During the aging process of polymers, oxidation and deg- change of products in three states. Therefore, studying radation of macromolecular chains happened, leading the products in a single state is not enough to reflect the to the deterioration of mechanical properties [1–3]. The whole aging process. aging products of polymers exist in solid, liquid and gase- Most researches focused on the changes of solid poly- ous states. Solid products are resulted from the oxidation mers before and after aging. For instance, changes of of main chains [4, 5]. Liquid products are small molecular functional groups, chromophores, molecular weight, degradation products, which can be absorbed in the aged crystallinity and morphology were often used to evaluate polymers or volatilize [6]. Gaseous products are smaller the aging degree of polymers [8–12]. Some researchers molecules, which are in gaseous state at room tempera- turned their eyes to small molecular degradation prod- ture and easily migrate out [7]. Oxidized main chains ucts [13]. Carlsson, Wiles and Philippart contributed continuously fractured to generate liquid and gaseous to the identification of volatile and gaseous products of polypropylene (PP) in various aging conditions [14–16]. Combination of selective isotopic labeling with solid- *Correspondence: yangr@mail.tsinghua.edu.cn phase microextraction or cryotrapping gas chromatog- Department of Chemical Engineering, Tsinghua University, raphy/mass spectroscopy made it possible to confirm the Beijing 100084, People’s Republic of China © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Liu and Yang BMC Chemistry (2020) 14:44 Page 2 of 8 original position of carbons on the main chains entering the volatile products [17]. Egerton et al. [18–21] analyzed the generation of C O in real time during photo-oxida- tion of a series of polymers, such as acrylic paint, low density polyethylene (LDPE), poly(vinyl chloride) and polyethylene terephthalate. When the stabilities of polymers were compared based on macromolecular products or small-molecular deg- radation products separately, there might be conflicting results. For example, rare volatile oxidized products were Fig. 1 Schematic diagram of reaction device generated during photo-oxidation of high density poly- ethylene composites, but the carbonyl index (CI) of the solid sample was rather high [22]. Another example was was extracted for the gas chromatography (GC) meas- found in LDPE nanocomposites. LDPE/ZnO nanocom- urement. Then the film was taken out for the Fourier posites had lower CI but much higher concentration of transform infrared spectroscopy (FTIR) and pyrolysis- CO than LDPE/TiO nanocomposites after photo-oxida- 2 2 gas chromatography/mass spectrometry (Py-GC/MS) tive aging [23]. These facts remind us that the products measurements. in a single state can only provide information related to aging properties from a single aspect. However, there FTIR measurement are few works on the whole aging procedure, including FTIR measurement was conducted to detect the oxidized changes of solid, liquid and gaseous states together. In macromolecules in solid state. Transmission spectra were order to understand aging mechanism comprehensively, obtained by using a Thermo-Nicolet iS10 FTIR spec - studying the conversion among aging products in three trometer. For attenuated total reflection (ATR) mode, an states is of great importance. accessory with a diamond crystal was used. CI was calcu- In this paper, photo-oxidative products of two PPs with −1 lated by dividing the peak area at 1714 cm to the peak different stabilities in solid, liquid and gaseous states −1 area at 2722  cm [24]. Three measurements were car - were studied. Different induction periods indicated dif - ried out for each film and the results were averaged. ferent generation times of these products. Conversion among the products in three states was traced. Overview of the aging products in three states was an effective way Py‑GC/MS measurement to obtain comprehensive information of the whole aging Py-GC/MS measurement was conducted in a gas chro- process and avoid possible biased evaluation of the aging matography/mass spectrometer (GC/MS-QP2010 SE, degree and relative stability of polymers. Shimadzu) installed with an EGA/PY-3030D multi-shot pyrolyzer (Frontier Laboratories) to detect the vola- tile degradation products absorbed by the film. 10.0  mg Experimental film or ethanol solution of the condensed liquid droplets Materials on the tube wall was heated at 300  °C for 30  s and the Two kinds of PPs were used: commercial PP pellets evaporated components went through an Ultra Alloy-5 (F401, purchased from Sinopec, with stabilizers’ package column. Limited by the stationary phase of the column, containing UV stabilizers and/or antioxidants, marked as components with carbon number less than six might not CPP) and unstabilized PP powders (supplied by Petro- be separated completely. The peak area of a component China Panjin Petrochemical Company, without any addi- in the flash evaporation-gas chromatogram was calcu - tives, marked as UPP). Both pellets and powders were lated to represent the relative concentration of the com- hot-pressed at 190 °C into films (thickness of 0.3 mm). ponent. Two measurements were carried out for each film and the results were averaged. Photo‑oxidative aging Photo-oxidative aging was conducted in a Q-Sun Xe-3 chamber (Q-Lab Corporation). A film (40 mm × 15  mm) GC measurement was put into a sealable quartz tube before aging GC measurement was conducted in a 7890B GC system (Fig.  1). The intensity of UV irradiation was 0.35  W/ (Agilent Technologies) to detect the gaseous degradation m /nm@340  nm. The temperature was 60  °C and the products around the film in the tube. 200 μL gas were time intervals were 48, 96, 144, 288, 384 and 480  h. extracted from a tube by an injector and injected into Before aging, the valve was closed to seal the tube. After the GC system, through a G3591-81023 column. Accord- aging, the valve was opened and the gas in the tube ing to the separation capacity of the column, only small Liu and Y ang BMC Chemistry (2020) 14:44 Page 3 of 8 −1 gaseous molecules like H, N, O , CO, CO , and alkanes that of UPP before 288 h (0.04 h ). After 288 h, the CI of 2 2 2 2 and alkenes with carbon number less than three could UPP decreased. be detected. A flame ionization detector and a thermal The decrease of the CI in Fig.  3b seemed unreasonable, conductivity detector were used. The peak area of a com - since the oxidation of PP was an accumulation process ponent in the gas chromatogram was calculated to repre- of the oxidized species. Therefore, some highly oxidized sent the relative concentration of the component. species might be lost. Considering that UPP was easier to be oxidized than CPP, it reached a high oxidation degree first. At the late stage of aging (after 288 h), severely oxi - Results and discussion dized fragments might drop from the surface and cause Macromolecular products the decrease of CI [25–27]. The change of CI of UPP in CI mainly reflected the accumulation of carbonyl prod - Fig.  3a supported the speculation. The surface oxidation ucts on the macromolecular chains in solid state. CI cal- degree of UPP after aging for 288 h also showed a lower culated from ATR spectra showed the local oxidation increasing rate than that of CPP. In this case, although degree on the surface of the film, while CI calculated the aging degree of UPP was evidently higher than that from transmission spectra showed the average oxidation of CPP, the CI of UPP was close to or even lower than degree in the bulk polymer. that of CPP after 288 h. Thus, the biased evaluation of the ATR spectra, transmission spectra and corresponding aging degree was obtained. CI of two PPs are shown in Figs. 2 and 3. On the surface In addition, different sensitivities of ATR spectra and (Figs.  2a, b and 3a), CPP had an induction period of at transmission spectra were observed. Despite the same least 96 h, while UPP was oxidized from the very begin- difference between the oxidation degrees of two PPs after ning of aging. After the induction period, CPP was oxi- the same aging time, the differences between CI of two −1 −1 dized at a rate (0.10 h ) similar to UPP (0.11 h ) before PPs were not the same from ATR spectra and transmis- 288 h. After that, the increase of CI in UPP slowed down. sion spectra. For instance, after aging for 288  h, the CI Two PPs seemed to reach the similar oxidation degree from ATR spectra of UPP was about 1.4 times of that of after 480 h. In the bulk (Figs. 2c, d and 3b), UPP was also CPP, while the CI from transmission spectra of UPP was oxidized from the very beginning of aging, while CPP about 4.2 times of that of CPP. Obviously, transmission exhibited an induction period of at least 96 h. The oxida - spectra were more sensitive to the different oxidation −1 tion rate of CPP after 288  h (0.04  h ) was the same to degrees. Fig. 2 Infrared spectra of CPP and UPP during aging. a ATR spectra of CPP; b ATR spectra of UPP; c transmission spectra of CPP; d transmission spectra of UPP) Liu and Yang BMC Chemistry (2020) 14:44 Page 4 of 8 The peak areas of volatile oxidized products were expected to keep increasing with aging time due to the continuous oxidation of polymer chains. In CPP, they kept increasing indeed. In UPP, however, the peak areas stopped increasing and even began to decrease from 288 h. This was caused by the evaporation of these prod - ucts and then condensation on the tube wall. There were visible droplets on the tube wall in which UPP was aged for 384 and 480 h. The droplets were washed by ethanol and the obtained solution was analyzed by Py-GC/MS. The flash evaporation-gas chromatogram of the drop - lets was compared with the result of the corresponding film (Fig.  5). Acetic acid and the lactone were identified in both the droplets and the corresponding film. Once the volatile oxidized products were evaporated and con- densed on the tube wall, they could not come back to the film, so the peak areas in UPP were decreased obviously (Fig. 4c, d). For CPP, no visible droplets were observed even after aging for 480  h, due to the relative low concentration of volatile oxidized products. There was a maximal con - centration of volatile oxidized products that could be retained in the film. If the concentration did not exceed the maximum, the volatile oxidized products would accu- mulate in the solid polymer. When the concentration exceeded the maximum, the excessive parts began to be desorbed and some condensed on the tube wall. In this case, the biased evaluation of the aging degree of two PPs according to volatile degradation products would also be obtained as according to macromolecular products. Similar phenomenon can be expected if the polymer is Fig. 3 Carbonyl index of CPP and UPP with aging time. a calculated photo-oxidized in unsealed atmosphere like in practical from ATR spectra; b calculated from transmission spectra use. Gaseous degradation products Gaseous degradation products included H , CO, CO , Volatile degradation products 2 2 alkanes and alkenes. They were in gaseous state and Volatile degradation products were absorbed in PP film could be identified directly by using GC measurement. as liquid. They could be desorbed through flash evapo - The peak areas of four typical gaseous products with ration and detected by Py-GC/MS. Four typical volatile aging time are shown in Fig. 6. These products were gen - products, i.e. two alkenes, acetic acid and a lactone, were erated from 48 h in UPP and from 144 h in CPP respec- identified in the flash evaporation-gas chromatograms tively. After that, the peak areas kept increasing in both of two PPs. The peak areas of these products with aging PPs and the peak areas in UPP were higher than in CPP time are shown in Fig.  4. In Fig.  4a, b, the alkenes were all along, showing the higher aging degree of UPP with- generated from the very beginning of aging in UPP, while out the protection of stabilizers. later than 96  h in CPP. After a period of time, i.e. 96  h in UPP and 288  h in CPP, the peak areas of two alkenes turned to decreasing, corresponding to the increasing of Overview of photo‑oxidative products in three states two oxidized products in Fig. 4c, d at the same time. This Induction periods of the generation of photo-oxidative indicated that: (1) The formation of alkenes was prior products in three states were extracted from Figs.  3, 4 to the formation of oxidized products, in other words, and 6, listed in Table  1. In CPP, the induction periods of the chain scission of PP happened before the formation carbonyl products and volatile alkenes were the same, of volatile oxidized products; (2) The oxidized products shorter than acetic acid, the lactone and gaseous deg- might be from the further oxidation of alkenes. radation products, indicating the earlier generation of Liu and Y ang BMC Chemistry (2020) 14:44 Page 5 of 8 Fig. 4 Peak area of four typical volatile degradation products in CPP and UPP with aging time. a and b two alkenes; c acetic acid; d lactone oxidation species in macromolecular chains and volatile were decreased at the late stage of aging, due to the loss alkenes. Thereafter volatile alkenes might be further oxi - of severely oxidized species in solid state and the migra- dized and the oxidized main chains fractured to gener- tion and condensation of the liquid products. The peak ate volatile oxidized products and gaseous degradation areas of gaseous degradation products kept increasing for products. As shown in Figs.  4c, d and 6, the peak areas as long as 480 h. of acetic acid, the lactone and gaseous degradation prod- Conversion among photo-oxidative products in ucts were increased rapidly from 288  h. At the same three states is illustrated in Fig.  7. During the photo- time, the peak areas of volatile alkenes were decreased oxidation of PP, the main chains were first oxidized rapidly (Fig.  4a, b). This phenomenon suggested the fur - and the oxidized species remained in the film as solid ther oxidation of volatile alkenes and the corresponding products. In the meanwhile, the main chains fractured accumulation of volatile oxidized products and gaseous to generate volatile alkenes, absorbed by the film as degradation products. liquid. Then the oxidized main chains fractured along Compared with CPP, UPP exhibited much shorter with the further oxidation of volatile alkenes, to gen- induction periods. Without the protection of stabilizers, erate volatile oxidized products absorbed by the film oxidation species in macromolecular chains and volatile as liquid and gaseous degradation products in the alkenes were generated from the very beginning of aging, atmosphere around the film. The conversion took followed by volatile oxidized products and gaseous deg- place throughout the aging process. When the conver- radation products within 48 h. As shown in Figs.  3b, 4c, sion was severe as in UPP, the CI and the concentra- d, CI and the peak areas of volatile oxidized products tions of volatile products were increased first and then Liu and Yang BMC Chemistry (2020) 14:44 Page 6 of 8 Fig. 5 Flash evaporation-gas chromatograms of UPP and corresponding droplets in the tube after aging for 384 h Fig. 6 Peak area of four typical gaseous degradation products in CPP and UPP with aging time. a H ; b CO; c CH ; d C H 2 4 2 4 decreased, due to the loss of severely oxidized species aging of the polymer. In CPP, the similar conversion in solid state and the migration and condensation of was observed, although the aging process was retarded liquid droplets in the late stage, despite the continuous owing to the presence of stabilizers. Therefore, the Liu and Y ang BMC Chemistry (2020) 14:44 Page 7 of 8 Table 1 Induction period of generation of photo-oxidative Conclusions products in CPP and UPP The photo-oxidative products of CPP and UPP in solid, liquid and gaseous states were detected by FTIR, Py-GC/ Products Induction period Induction in CPP/h period MS and GC respectively, and their formation profiles in UPP/h were compared. During photo-oxidative aging, the car- bonyl products in macromolecular chains and volatile CI on the surface (solid) 96 0 alkenes were generated first. Then the volatile oxidized CI in the bulk (solid) 96 0 products and gaseous degradation products were gener- Alkenes (liquid) 96 0 ated from the further oxidation of volatile alkenes and Acetic acid (liquid) 144 48 the fracture of oxidized main chains. At the late stage Lactone (liquid) 144 48 of aging, CI and the concentrations of volatile oxidized H , CO, CH , C H (gas) 144 48 2 4 2 4 products were decreased, due to the loss of severely oxi- dized species in solid state and the migration and con- densation of liquid droplets, while the concentrations of products in a single state, especially only in solid state, gaseous degradation products maintained increasing. could not reflect the comprehensive aging process and Overview of the aging products in three states provides might lead to biased results. It was more reliable to comprehensive information and overall understanding of consider the photo-oxidative products in three states the aging mechanism. It offers an effective way to evalu - as a whole when evaluating the aging degree and rela- ate the aging degree and relative stability of polymers tive stability of polymers. accurately. 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Published: Jul 18, 2020

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