DOI: 10.3303/CET25115008 Paper Received: 18 October 2024; Revised: 20 December 2024; Accepted: 21 January 2025 Please cite this article as: Goto S., Watanabe Y., 2025, A Study on Evaluation Methods for Preventing Recurrence of Accidents Involving Mixtures of Monomers and Initiators, Chemical Engineering Transactions, 115, 43-48 DOI:10.3303/CET25115008 CHEMICAL ENGINEERING TRANSACTIONS VOL. 115, 2025 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Carlo Pirola, Antonio Espuña, Sabrina Copelli Copyright © 2024, AIDIC Servizi S.r.l. ISBN 979-12-81206-16-8; ISSN 2283-9216 A Study on Evaluation Methods for Preventing Recurrence of Accidents Involving Mixtures of Monomers and Initiators Shinichiro Goto*, Yusuke Watanabe Mitsui Chemicals, Inc., Japan shinichirou.gotou@mitsuichemicals.com, yusuke.watanabe@mitsuichemicals.com It is well known that radical polymerization reactions can release a large amount of energy. In recent years, unintended polymerization had progressed during storage and transportation, causing accidents. Therefore, chemical companies are examining evaluation methods. Since conventional thermal analysis equipment with temperature ramping measurements cannot adequately assess these monomers, a method using Polymerization Induction Time (PIT) obtained from isothermal measurements with high-sensitivity calorimeters is being established in recent years. However, these proposed methods target the evaluation of monomers alone. This study compared the results of isothermal measurements using a conventional high-sensitivity thermal analyzer and AKTS simulation analysis using DSC data for mixtures consisting of monomers, inhibitors, and initiators. The PIT estimation method by AKTS is a simpler method than the conventional isothermal method, but some samples were applicable while others were not. The difference is also discussed in terms of radical stability by computational chemistry. 1. Introduction Thermally polymerizable substances are one of the polymer materials used to manufacture various everyday products and are highly useful substances, but they also have high potential energy. Therefore, it should be handled with care. In fact, an explosion of an acrylic acid storage tank (NIPPON SHOKUBAI, 2013) and an accident involving a container ship transporting divinylbenzene have been reported (Federal Bureau of Maritime Casualty Investigation, 2014). In 2017, regulations on thermally polymerizable substances were established under the UN transport recommendations, and the measurement of the Self-Accelerating Polymerization Temperature (SAPT) was made mandatory for transportation. Since SAPT testing requires large sample volumes and lengthy periods of time, previous studies have introduced a simpler evaluation method that shows a good correlation with SAPT. This method estimates the Polymerization Induction Time (PIT) using isothermal measurements with a high-sensitivity calorimeter (Min Sheng et al., 2019). Additionally, it has been reported that the estimated PIT is influenced not only by temperature but also by factors such as dissolved oxygen, the concentration of polymerization inhibitors (Min Sheng et al., 2019). On the other hand, in the polymer manufacturing process, to produce products with the desired functions, there are cases where various additives, including polymerization initiators, are pre-mixed with thermally polymerizable monomers in a mixing tank before being sent to the polymerization section. Attention must be paid to the fact that such mixtures exhibit different heat generation behaviour compared to pure thermally polymerizable monomers. In 2022, at one of our overseas affiliates, an incident occurred where a drum containing a mixed waste liquid of styrene and other substances ruptured. This mixed waste liquid contained not only monomers and polymerization inhibitors but also polymerization initiators. As a result of being stored outdoors in temperatures reaching around 40 °C, the waste liquid polymerized and underwent thermal runaway after about nine days. As part of the investigation into the cause of the accident, a simulation using the kinetic analysis software AKTS, based on DSC heating measurement data, was conducted. This successfully reproduced the conditions of the accident in the previous report. As a recurrence prevention measure for similar accidents, although it is suggested that the drums be managed at low temperatures, it’s also necessary to evaluate a wide variety of combinations of different monomers, polymerization inhibitors, and initiators. 43 However, it takes a lot of time to conduct isothermal measurements using a high-sensitivity calorimeter for all brands. This study conducted a comparative analysis of isothermal measurements using a previous high- sensitivity calorimeter, along with the simpler evaluation methods of DSC measurements and AKTS simulation analysis, on mixtures of monomers, polymerization inhibitors and initiators. 2. Materials & experiment 2.1 Samples The monomers used in the experiment were styrene(St), methyl methacrylate(MMA), glycidyl methacrylate(GMA) and methyl acrylate(MA). All of which were purchased from reagent manufacturers and used without further purification. The type of polymerization inhibitors used were 4-Methoxyphenol(MEHQ), 6- tert-Butyl-2,4-xylenol(6TBX), and 4-tert-Butylcatechol(TBC), with some added in a concentration range of 30 to 300 ppm. The initiators were t-Butyl peroxy-2-ethylhexanoate(PB-O) and di-t-butyl peroxide(PB-D), and the addition amount was fixed at 1 wt% for all samples. The atmosphere when placing the samples into the measurement cell was either air or nitrogen. Seven prepared samples are summarized in Table 1. Table 1: List of prepared samples No. Monomer Inhibitor Initiator Condition type ppm Type % 1 St TBC 30 PB-O 1 Air 2 PB-D 3 MMA 6TBX 4 MEHQ 5 GMA 100 6 MA 300 7 N2 2.2 Measurement conditions Table 2: Measurement conditions DSC C80 Sample mass [mg] 2 600 Crucible(cell) material Stainless steel Glass Heat rate [°C/min] 0.5 to 8 - (isothermal) Temp. range [°C] 30 to 300 70 to 100 Determination method of PIT Isothermal prediction of AKTS (0.5% conversion point) See below The point at which the cell is inserted into C80 and measurements are started is defined as the PIT starting point, while the 0.5 % reaction point calculated from the average heat of reaction in the DSC measurement results is defined as the PIT endpoint. Examples of C80 measurement results and determination of the 0.5% reaction point are shown in Figure 1. Figure 1(a): Examples of C80 measurement results (b): The area enclosed by the dashed line in (a) is enlarged. 44 3. Result and discussion 3.1 DSC measurement and AKTS results DSC measurements were performed under the conditions described above, and the average heat (QAVE), standard deviation (SD), and SD/QAVE values were calculated by AKTS. They were summarized in Table 3. For No.1 (St, TBC, PB-O), the SD/QAVE is greater than 0.1, while for No.2 (St, TBC, PB-D), the SD/QAVE was 0.03. To confirm this difference, the details of the AKTS results were summarized in Table 4. For No.1 and No.2, their respective DSC charts were shown in Figures 2 (a) and (b). The half-life temperature of PB-O(10 hours half- life:72.1 °C) is lower than that of PB-D(10 hours half-life:123.7 °C), which manifest as a difference in the onset of the mixture (approximately 20 to 40 °C). Additionally, in No.1, the shape of the heat flow chart changed with the heating rate, showing that as the heating rate decreased, the heat generation tended to increase. Table 3: Summarize of DSC measurement and AKTS results No. Monomer Inhibitor Initiator R QAVE. J/g SD SD/ QAVE. type ppm Type 1 St TBC 30 PB-O 0.960 503.8 107.2 0.213 2 PB-D 0.999 703.7 13.8 0.020 3 MMA 6TBX 0.927 588.1 41.0 0.070 4 MEHQ 0.924 577.3 41.1 0.071 5 GMA 100 0.993 425.1 13.6 0.032 6 MA 300 0.981 963.0 32.2 0.033 7 0.991 892.0 39.6 0.044 Figure 2: DSC chart of No.1 (a), and No.2 (b) respectively. Table 4: AKTS results in detail No. 8 °C /min 4 °C /min 2 °C /min 1 °C /min 0.5 °C /min Onset °C Q J/g Onset °C Q J/g Onset °C Q J/g Onset °C Q J/g Onset °C Q J/g 1 98.5 440.1 103.2 440.5 98.3 512.0 80.2 606.1 73.7 634.3 2 130.0 701.8 124.0 700.4 118.8 688.9 115.2 731.5 113.5 723.8 When polymerization is initiated using PB-O, the polymerization progresses from a lower temperature. Additionally, when the temperature is increased at a relatively fast rate, polymerization and thickening occur rapidly, hindering the diffusion of the monomer and polymerization inhibitors. Furthermore, the termination of polymer growth radicals by two molecules is inhibited, as is the termination of polymer growth radicals by polymerization inhibitors. These phenomena were referred to as the gel effect (Yu.L.Kuznetsova et al 2020), and it is presumed that the increased amount of unreacted monomer contributes to the variability in heat generation. According to the manufacturer's technical information, if the SD/QAVE is 0.1 or higher, the data is deemed unsuitable for use in AKTS. Therefore, No.1 was excluded from further evaluations, and PB-O was not used in combination with other monomers. 45 3.2 Comparison of AKTS Estimation Results and C80 Isothermal measurement results From the literature by Min Sheng et al. assuming that the rate of consumption of the inhibitor at isothermal temperature is constant, the PIT can be shown in Equation (1) (1) (2) PIT: min., preexponential factor A: 1/min., inhibitor concentration C: mol/L, activation energy Ea: J/mol The PIT for each sample obtained from the C80 measurement was substituted into Equation (1) and plotted in Figure 3a with the inverse of temperature as the x-axis. The approximate equation for each plot, the activation energy(Ea) calculated from the slope of the equation, and the activation energy obtained by AKTS estimation were summarized in Table 5. The activation energy of AKTS was set in the range of 0 to 0.5% reaction rate, which was the initial stage of the reaction. The activation energy of PB-D is 155 KJ/mol, which is relatively close for all samples in the C80 measurement. The activation energy calculated from AKTS for MMA as monomer, especially for No. 3, tended to be lower than that of PB-D. This may have affected the differences in each PIT, which will be discussed later. Figure 3: (a) plotted the PIT obtained from the C80 measurement substituted into Equation (1). (b) No.2’s conversion and activation energy calculated from AKTS. Table 5: Summary of calculated activation energies No. Monomer C80 AKTS Approximate equation Ea kJ/mol Range* of Ea kJ/mol 2 St -15.979x + 39.092 132.8 146.6 to 155.0 3 MMA -19.661x + 49.374 163.4 87.3 to 94.6 4 -19.142x + 48.056 159.1 112.6 to 115.2 5 GMA -22.076x + 55.256 183.5 94.8 to 158.9 6 MA -19.389x + 46.521 161.2 112.1 to 134.0 7 ** 136.2 to 159.9 *Conversion between 0 and 0.5% **Not calculated due to 2-point data A plot of No.2’s conversion and activation energy calculated from AKTS is also shown in Figure 3b. The initial stage of the reaction was dominated by the influence of the initiator, with high activation energies, which gradually became lower as the reaction progressed. The activation energy of the styrene chain growth reaction is estimated to be about 33 kJ/mol (Masami Kamigaito, 2009), and it is assumed that the energy was close to that value. This trend was observed in all samples. The PIT obtained from each method is then compared. 46 Figure 4(a) shows comparison of the PIT from C80 and AKTS. Although some variability is observed, PIT from AKTS and C80 are approximately the same except No.3. Figure 4(b) is Table 6 summarizing the PIT results for No. 3 and No. 4 in detail. In PIT calculated from C80, No.3 tended to be longer than No.4. Generally, the lower the electron-donating property of phenolic inhibitors, the more effective they are as inhibitors, hence 6TBX > MEHQ is assumed (Ali Darvishi et al, 2019). On the other hand, the AKTS results were completely opposite. In addition, No.3's AKTS estimated PIT showed a linear correlation with the C80 isothermal measured PIT, it exhibited a shorter trend. This may be due to the smaller activation energy mentioned above. Figure 4(c) is a Table 7 summarizing the PIT results for No. 6 and No. 7. These results indicate that oxygen is necessary for the inhibitor to take effect even if an initiator is present (Sergio S. Cutie et al, 1996). (b) Table 6: Summary of PIT for No.3,4 C80 AKTS No.3 No.4 No.3 No.4 75 °C 20.2 17.1 6.3 14.7 80 °C 9.5 7.7 3.6 8.3 85 °C 4.1 3.7 2.1 2.8 (c) Table 7: Summary of PIT for No.6,7 at 85 °C C80 AKTS No.6 No.7 No.6 No.7 85 °C 40.9 1.8 45.6 4.9 Figure 4: (a) Comparison of the PIT from the C80 and AKTS. (b) Table 6 summarizing the PIT results for No. 3 and No. 4 (c) Table 7 summarizing the PIT results for No. 6 and No. 7 at 85 °C 3.3 Approach to Further Analysis Using Computational Chemistry As showed in Figure 4(b), the strengths of inhibitors 6TBX and MEHQ in the C80 and AKTS results were exactly opposite. To consider these in more detail, an approach to analysis was implemented from the estimation of the stability of each radical by computational chemistry. Calculated reactions of No.3 or No.4 are shown in Figure 5(a). Rea1: Cleavage of initiator, Rea2: Addition of Rea1’s product to monomer, Rea3: polymerization by Rea2’s product, Rea4: Peroxidation of the Rea2’s product, Rea5: polymerization of Rea4’s product. Rea6: inhibition of Rec4’s product, Rea7: inhibition of Rea2’s product, Rea8: inhibition of Rea1’s product. The stabilization energies for inhibition reaction for Product6,7 and 8 in Figure 5(a) were summarized in Table 8. (b) Table 8 Summary of stabilization energies for inhibition reaction. value kJ/mol Product6 6TBXr. MEHQr. -37.1 -33.9 Product7 6TBXr. MEHQr. -52.3 -49.0 Product8 6TBXr. MEHQr. -100.3 -97.0 Figure 5: (a) Calculated reaction of the No.3 or No.4. (b) Table 8 summarizing the stabilization energies for inhibition reaction. 47 The calculation program used was Gaussian 16 B.01 (M.J. Frisch et al, 2016), and the calculation level was set to D3-B3LYP/6-31+G(d). The larger the negative value of the calculation result, the more stable the product is. The rate constants for inhibition reaction (Rea6,7,8) were summarized in Table 9. Table 9: Summary of rate constants for inhibition reaction (/s) inhibitor 25 °C 100 °C 200 °C Forward Reverse Forward Reverse Forward Reverse Rea6 6TBX MEHQ 2.0×107 1.1×108 2.8×103 5.8×104 3.8×106 1.5×107 4.7×103 4.8×104 9.8×105 2.9×106 7.7×103 4.3×104 Rea7 6TBX MEHQ 3.4×101 2.3×102 1.9×10-8 4.8×10-7 3.9×101 5.5×102 2.5×10-6 9.1×10-5 4.5×101 1.2×103 1.6×10-4 7.7×10-3 Rea8 6TBX MEHQ 1.9×1011 6.2×1012 5.1×10-7 2.2×10-4 7.6×109 9.0×1011 8.3×10-5 2.5×10-2 5.3×108 6.2×1010 6.3×10-3 1.4×100 As shown in the Table 9, the rate constants of 6TBX were smaller than those of MEHQ in all three reactions at all temperature range. A similar trend was observed in the reverse reaction. And as summarized in Table 8, the stability of the reaction products was higher for 6TBX. Therefore, 6TBX were less likely to be inactivated. AKTS results of No.3 was not explained by any of the calculations in this study. At high temperatures, product 6 is known to decompose to per oxyradical X. The difference in reactivity of the inhibitor to per oxyradical X may have an effect. Comparing the results of Rea6 and 7, Rea6 is by far the faster. It indicated that even in 6TBX that didn't need oxygen, the inhibitor was more effective in the presence of oxygen. 4. Conclusions As a simpler method for determining the PIT of mixtures of monomers, initiators, and inhibitors, the AKTS method was attempted. When PB-D was used as the initiator, it was found that the PIT estimation method using AKTS could be applied. Considering the electron donation and C80 measurements confirm that 6TBX was more effective than MEHQ as an inhibitor, but AKTS results were the opposite totally. It should be noted that the case of 6TBX can be overestimated in AKTS estimation. Alternatively, it is possible that per oxyradical X, which is not calculated in this study, is involved in the reaction during the temperature increase process. As well as the gel effect, this effect would be one of the factors affecting the applicability of the PIT estimation method by AKTS. More details will be discussed in the future. Acknowledgements This research was accomplished with significant support from colleagues and other departments. We thank all of them. References Ali Darvishi et al, 2019, A theoretical and experimental study for screening inhibitors for styrene polymerization, Processes 2019, 7, 677 Federal Bureau of Maritime Casualty Investigation, 20144, Fire and explosion on board the MSC FLAMINIA on 14 July 2012 in the Atlantic and the ensuing events Investigation report 255/12 Masami Kamigaito, Kotaro Sato, 2009, Network polymer, Vol.30, No.5, radical polymerization Min Sheng et al., 2019, Calorimetric Method to Determine Self-Accelerating Polymerization Temperature (SAPT) for Monomer Transportation Regulation: A Heat Balance Approach, Organic process research & Development 2019, 23, 750-761 Min Sheng et al., 2019, Calorimetric method to determine self-accelerating polymerization temperature (SAPT) for monomer transportation regulation: kinetics and screening criteria, Organic process research & Development 2019, 23, 737-749 M.J. Frisch et al, Gaussian 16, Revision B.01, Gaussian, Inc., Wallingford, CT, 2016. NIPPON SHOKUBAI Co., Ltd, Himeji Plant 2013, Explosion and Fire at Acrylic Acid Production Facility Investigation Report Sergio S. Cutie et al, 1996, The Effects of MEHQ on the Polymerization of Acrylic Acid in the Preparation of Superabsorbent Gels, Journal of Applied Polymer Science, vol 64, Issue 3 Takayuki Otsu, 1975, on the function of polymerization inhibitors, organic synthetic chemistry, vol 33, No.8 Yu. L. Kuznetsova et al, 2020, Polymerization of methyl methacrylate presence of 2,5-di-tert-p-benzoquinone, Russian chemical bulletin, international edition, vol 69, No.4, 763-767 48 418Goto-PAGATO.pdf A Study on Evaluation Methods for Preventing Recurrence of Accidents Involving Mixtures of Monomers and Initiators