Pyrolysis behaviorFigure 1 shows the weight loss behavior of each plastic. The start and end temperatures were defined by the DTG value of 5 wt.%/min. The temperature ranges varied among the plastic samples, with biodegradable plastics exhibiting lower ranges than those of common plastics. Among the common plastics, HDPE and PS exhibited the highest (458–499 °C) and lowest (386–444 °C) temperature ranges, respectively, with the range of PP being 422–478 °C at a heating rate of 10 °C/min. In comparison, PLA and PHBH exhibited lower temperature ranges of 325–378 and 273–298 °C, respectively. Table S1 outlines the start, end, and peak-top temperatures of all samples. All plastics exhibited one-step weight loss without any residue production. Notably, Das and Tiwari37 reported the degradation temperatures of HDPE (433–493 °C), PP (409–469 °C), and PLA (323–374 °C) at a heating rate of 10 °C/min. Saad et al.38 indicated that the pyrolysis temperature ranges of HDPE, PP, and PS are 464–497, 441–483, and 420–452 °C, respectively, at a heating rate of 10 °C/min. Kopinke et al.24 reported that PLA degradation initiates at 225 °C and ends at 370 °C, while PHB degradation initiates at 255 °C and ends at 305 °C under a heating rate of 5 °C/min. Thus, the pyrolysis temperatures observed in this work were consistent with these reports, validating the TG measurements.Figure 1(a) TG and (b) DTG curves of each plastic.As shown in Fig. 2a, the main pyrolyzates of PLA were acetaldehyde; D-, L-, and meso-lactides; and their oligomers. Meso-lactide is typically produced through a free radical reaction, and D- and L-lactides are produced by transesterification reactions24,39. The produced oligomers were considered cyclic lactide, with reference to the works of Shao et al. 26 and Arrieta et al.40. The main pyrolyzates of PHBH were propylene, isocrotonic acid, crotonic acid, 2-hexenoic acid, and their dimers (Fig. 2b). HDPE yielded aliphatic hydrocarbons with a wide range of carbon numbers through random radical scission. Under the current analytical conditions, C1–C35 hydrocarbons were observed (Fig. 2c). PP pyrolyzates were more complex than those of HDPE because of the presence of a methyl unit. Through a comparison with the NIST17 library, propylene, pentane, 2-methyl-1-pentene, and 2,4-dimethyl 1-heptene were identified, with a matching score exceeding 90% (Fig. 2d). Moreover, with reference to the Py-GC/MS Data Book of Synthetic Polymers41, propylene, n-pentane, 2-,ethyl-1-pentene, 2,4-dimethyl-1-pentene, and propylene oligomers with various lengths were identified. The main PS pyrolyzates were styrene, styrene dimers, and styrene trimers (Fig. 2e). The pyrogram pattern was consistent with that reported in the Data Book41. The volatile emission behavior of each sample was investigated through EGA-MS analysis, and the peak-top temperatures of total ion chromatograms (TICs) obtained from each sample are summarized in Table S2. These temperatures were slightly lower than the DTG peak-top temperatures (Table S1) owing to the superior heat conductivity in the Py-GC/MS method attributable to the smaller amount of sample loading. Details of the evolution behavior of selected compounds are presented in the following sections.Figure 2Pyrograms of (a) PLA, (b) PHBH, (c) HDPE, (d) PP, and (e) PS.Co-pyrolysis of PLA and common plasticsThe co-pyrolysis behavior of PLA and common plastics with a mixing ratio of 50:50 was analyzed by TGA (Fig. 3). The solid and dotted lines show the experimental curves and calculated curves (using Eq. (1)), respectively. PLA50HDPE50 exhibited a two-step weight loss with temperature ranges of 315–389 and 434–509 °C, consistent with those of PLA and HDPE (Fig. 1). The experimental TG/DTG curves exhibited a good fit with their calculated curves. Similar trends were observed for other mixing ratios (PLA:HDPE = 20:80 and 10:90), as shown in Figures S1 and S2. Thus, no pyrolytic synergistic interactions were identified through the weight loss behavior. Py-GC/MS revealed the type of pyrolyzates obtained by co-pyrolysis of PLA50HDPE50 (Fig. 4a). Meso-lactide; D-, L-lactides; and their oligomers were obtained from PLA, while HDPE produced hydrocarbons with different carbon numbers. No new compounds were observed that were not obtained from PLA and HDPE. Figure 5a shows the TIC and extracted ion chromatograms (EICs) obtained by the EGA-MS analysis of PLA50HDPE50. Results for other mixing ratios can be found in Figure S5. The peak-top temperatures for the first and second gas evolution were 357–363 and 476–479 °C, respectively, consistent with those of PLA and HDPE (Table S2).Figure 3(a) TG and (b) DTG curves of PLA50HDPE50, PLA50PP50, and PLA50PS50.Figure 4Pyrograms of (a) PLA50HDPE50 (PLA:HDPE = 50:50), (b) PLA50PP50 (PLA:PP = 50:50), and (c) PLA50PS50 (PLA:PS = 50:50).Figure 5TICs and EICs of (a) PLA50HDPE50, (b) PLA50PP50, and (c) PLA50PS50, obtained by EGA-MS analysis.To examine the evolution behavior of the main pyrolyzates from PLA, specific ions for selected for monitoring: m/z 28 (molecular ion of carbon monoxide), m/z 29 (main fragment ion of acetaldehyde), m/z 44 (molecular ion of carbon dioxide), and m/z 56 (main fragment ion of lactides). For HDPE, m/z 57, 67, and 83 (fragment ions of hydrocarbons, i.e., C4H9+, C5H7+, and C6H11+, respectively) were monitored. Notably, the ions (m/z 29 and 56) selected for PLA pyrolyzates were also produced from HDPE pyrolyzates. Moreover, the hydrocarbons selected from HDPE (m/z 57) were also produced by PLA. The peak-top temperatures of the selected ions were consistent with those observed in the TIC. Thus, no pyrolytic synergistic interactions occurred during the EGA-MS analysis. In conclusion, PLA and HDPE independently decomposed under this slow heating condition and did not influence the pyrolysis behavior.During the co-pyrolysis of PLA/PP, a two-step weight loss similar to that for PLA/HDPE co-pyrolysis was observed. However, PP degradation occurred at a lower temperature range (410–489 °C) compared with that of HDPE (Fig. 1). Nevertheless, the PLA pyrolysis temperature during the co-pyrolysis of PLA50PP50 was similar to that for PLA50HDPE50. The calculated TG/DTG curves were consistent with the experimental curves. The same trends were observed for PLA20PP80 and PLA10PP90. According to Fig. 4b, i.e., the pyrogram of PLA50PP50, both PLA and PP pyrolyzates were observed, with no new compounds observed during the co-pyrolysis of PLA and PP. Figure 5b shows the evolved gas profiles during co-pyrolysis of PLA50PP50. The peak-top temperatures of the first and second peaks were the same as those obtained from the pyrolysis of PLA and PP. The same ions (m/z 28, 29, 44, and 56) from PLA were monitored, while ions m/z 29 and 56 were also produced from PP pyrolyzates including 2,4 dimethyl-1-heptene and 2-methyl-1-petene. The selected ion m/z 69 was a fragment ion of hydrocarbons derived from PP, and m/z 126 corresponded to the molecular ion of 2,4-dimethyl-1-heptene. The peak-top temperatures from the TIC and selected EICs were the same as those observed in the pyrolysis of PLA and PP. Thus, no pyrolytic synergistic interaction was identified during EGA-MS analysis. In conclusion, PLA and PP independently decomposed under this slow heating condition and did not influence the pyrolysis behavior.Because the PS degradation temperature was lower than those of HDPE and PP, the temperature range of PS and PLA is the closest combination in this study. Although PLA50HDPE50 and PLA50PP50 exhibited clear two-step weight loss, PLA50PS50 displayed a slightly overlapped weight loss region (Fig. 1), with the first-step weight loss initiating from 311 °C and the second-step weight loss terminating at 451 °C. Regardless of this overlapped weight loss region, the experimental TG/DTG curves fit the calculated curves. Thus, no pyrolytic interaction between PLA and PS was observed through TGA. The pyrogram of PLA50PS50 is shown in Fig. 4c. Both PLA and PS pyrolyzates were identified, and no compound was newly produced during the co-pyrolysis of PLA50PS50. Figure 5c shows the EGA-MS profiles of PLA50PS50. Consistent with TGA, a two-step gas evolution was observed (with the first and second steps corresponding to PLA and PS, respectively). The temperature ranges of the two peaks, 363–370 and 413–414 °C, were similar to those obtained from PLA and PS pyrolysis. The extracted ions of m/z 104, 208, and 312 corresponded to molecular ions of styrene, styrene dimer, and styrene trimer, respectively. As in the case of the co-pyrolysis of PLA/HDPE and PLA/PP, the peak-top temperatures of the selected ions from PLA (m/z 28, 29, 44, and 56) and PS (m/z 104, 208, and 312) were identical to those obtained from PLA and PS pyrolysis. Thus, no pyrolytic interaction occurred between PLA and PS during co-pyrolysis under the current heating method.Co-pyrolysis with PHBHDuring the co-pyrolysis of PHBH/HDPE, a two-step weight loss behavior was observed by TGA (Fig. 6). The first weight loss (267–319 °C) corresponded to PHBH degradation, while the second (428–507 °C) pertained to HDPE degradation. The experimental and calculated curves exhibited a good fit, indicating no pyrolytic synergistic interaction under this condition. The pyrogram of PHBH50HDPE50 is shown in Fig. 7a. Both PHBH and HDPE pyrolyzates were observed, while no new compound was produced during the co-pyrolysis of PHBH/HDPE. The EGA-MS results for PHBH50HDPE50 are shown in Fig. 8a. In addition to PHBH50HDPE50, PHBH20HDPE80 and PHBH10HDPE90 showed two-step gas evolution (Figure S8). The peak-top temperatures of the first and second gas evolution were 289–290 and 477–479 °C. The extracted ions of m/z 86 and 114 from PHBH corresponded to the molecular ions of (iso)crotonic acid and 2-hexenoic acid, respectively. Notably, m/z 114 was a fragment ion of hydrocarbons (C8H18+) produced from HDPE. The extracted ions, m/z 57 (C4H9+), 67 (C5H7+), and 83 (C6H11+), were representatives fragment ions of hydrocarbons derived from HDPE, whereas m/z 57 was a fragment ion of crotonic acid and 2-hexenoic acid derived from PHBH. The fragment ion behavior demonstrated that no pyrolytic interactions occurred during this co-pyrolysis under the selected conditions.Figure 6(a) TG and (b) DTG curves of PHBH50HDPE50, PHBH50PP50, and PHBH50PS50.Figure 7Pyrograms of (a) PHBH50HDPE50 (PHBH:HDPE = 50:50), (b) PHBH50PP50 (PHBH:PP = 50:50), and (c) PHBH50PS50 (PHBH:PS = 50:50).Figure 8Evolved gas profiles of (a) PHBH50HDPE50, (b) PHBH50PP50, and (c) PHBH50PS50.The co-pyrolysis of PHBH/PP and PHBH/PS showed the same trends as those for PHBH/HDPE. The two-step weight loss corresponded to PHBH degradation and PP or PS degradation, and the experimental and calculated curves fit well. No new compound was produced during the co-pyrolysis of PHBH50PP50 and PHBH50PS50, as indicated by the pyrograms of PHBH50PP50 (Fig. 7b) and PHBH50PS50 (Fig. 7c). Additionally, the EGA-MS profiles of the TICs for PHBH50PP50 (Fig. 8b) and PHBH50PS50 (Fig. 8c) showed two-step gas evolution corresponding to PHBH degradation and PP or PS degradation, with temperatures consistent with those observed in the TGA. Moreover, the temperatures were identical, as confirmed by neat pyrolysis of PHBH, PP, and PS. The emission temperatures of the selected ions, m/z 86 and 114 for PHBH; m/z 69 and 126 for PP; and m/z 104, 208, and 312 for PS, were the same as those observed in the neat pyrolysis of PHBH, PP, and PS. Thus, no pyrolytic interactions occurred during the co-pyrolysis of PHBH/PP and PHBH/PS under the current conditions.
