HAZARDOUS EVALUATION OF ORGANIC BLOWING AGENTS FOR TIRES

Transcription

1 # 27 IChemE HAZARDOUS EVALUATION OF ORGANIC BLOWING AGENTS FOR TIRES Yusaku Iwata, Momota Michihiko and Hiroshi Koseki National Research Institute of Fire and Disaster, , Jindaijihigashi-machi, Chofu, Tokyo , Japan; Two kinds of organic blowing agents for tires were involved in the fire in a tire production plant in Japan in September, 23. One organic blowing agent had N,N -dinitrosopentamethylenetetramine () and the other one had azodicarbonamide (ADCA). and ADCA play an important role in the recent automobile industry. Though they are known as typical self-reactive substances, the overall hazardous evaluation has not been discussed in the addition of the view of the thermal stability, the combustion and the pressure rise characteristics. Their thermal behaviors and the combustion characteristics were studied using some thermal analyses and the hazardous evaluation methods in this paper. The data in the adiabatic condition is important to predict and evaluate the intensity of decomposition in a runaway reaction. The decomposition in the adiabatic condition was investigated on the basis of the heat rate and the pressure rise measured by the adiabatic calorimetric equipment. The thermal stability of organic blowing agents containing and ADCA in addition to and ADCA was examined using a differential scanning calorimeter (DSC). A small flame test and a flash point test regarding to solid substances were conducted to examine the combustion characteristics of organic blowing agents. The reaction hazards of and ADCA were investigated using an auto pressure tracking adiabatic calorimeter () in the adiabatic condition. The adiabatic control system of the is the same as the accelerating rate calorimeter (ARC). However, the pressure rise data by the is not so familiar in the chemical hazards evaluation. In this paper, the pressure data obtained by the were discussed using di-tert-butylperoxide (DTBP) before the hazardous evaluation method of the was applied to and ADCA. DTBP was used as the standard substance because there were several references regarding to DTBP decomposition. The main conclusions were made as follows: 1. The heat onset temperatures of organic blowing agents containing and ADCA were158c and 248C. The organic blowing agent containing was easy to ignite in the small flame test. The flash points of organic blowing agents containing and ADCA were 1278C and 1758C by the Seta closed-cup flash point test. 2. The thermal stability and the intensity of the decomposition were higher than those of ADCA on the basis of data of the. When the pressure rise maximum per unit mass of and unit volume was compared, that of was approximately eight times than that of DTBP2 wt.%/toluene solution of 5 g. The activation energy of was 215 kj/mol. The heat onset temperature of was 1458C and lower than that of ADCA. KEYWORDS: hazardous evaluation, pressure rise,, ADCA, INTRODUCTION The tire production plant fire occurred in Japan on September, 23. Because the fire continued for about forty-seven hours it became a big social problem. Two kinds of organic blowing agents were involved in its fire in a tire production plant. One organic blowing agent contained N,N -dinitrosopentamethylenetetramine () and the other one contained azodicarbonamide (ADCA). The organic blowing agent is used to make the bubble structure in natural rubber and synthetic rubber by using the generated gas in thermal decomposition at a certain temperature. Though their substances are known as typical self-reactive substances, the overall hazardous evaluation has not been discussed in the addition of the view of the thermal stability, the combustion and the pressure rise characteristics. The thermal analysis and the ignition evaluation test were conducted to examine the hazardous properties of organic blowing agents, and ADCA. An adiabatic pressure tracking calorimeter () was used to evaluate the reactivity of and ADCA in thermal decomposition. EXPERIMENT SAMPLES Two kinds of blowing agents were supplied by the tire production company. The blowing agent 1 (BW 1) containing of 4 5 wt.% and the blowing agent 2 (BW 2) containing ADCA of wt.%. and ADCA were provided by Wako Pure Chemical Industries, Ltd. is white powder. ADCA is pale yellow powder. Chemical formula of DTP is C 5 H 1 N 6 O 2 and molecular weight is Chemical formula of ADCA is C 2 H 4 N 4 O 2 and molecular weight is

2 # 27 IChemE MEASUREMENT METHOD DSC and TG-DTA Samples for DSC measurement were BW 1, BW 2, and ADCA. The closed stainless cell was used. Sample mass was from.9 to 1.1 mg. Heating rate was 1 K/min. Each measurement run for BW 1 and BW 2 were more than five times. Samples for TG-DTA measurement were BW 1 and BW 2. The open aluminum cell was used. Sample masses were 6.5 mg of BW 1 and 4.7 mg of BW 2. Heating rate was 2 K/min. Each measurement run for BW1 and BW 2 was one time. Small flame test The small flame test was conducted according to the Japanese Fire Service Law. Samples for the small flame test were BW 1 and BW 2. The purpose of this test was to classify the flammable solid. Sample volume was 3cm 3. The sample was put on an adiabatic board, molded like the hemisphere. Fuel of igniter was propane/butane mixture gas. Flame height was 7 cm approximately. The flame contacted within ten seconds. Measurement run for BW 1 were three times and measurement run for BW 2 were five times. Flash point test The Seta flash point test was conducted according to the Japanese Fire Service Law. Samples for the Seta flash point test were BW 1 and BW 2. The purpose of this test was to evaluate the flammability by measuring the flash point of a flammable solid. Sample mass was 2 g. The sample was put inside a sample cup of the tester for the five minute at a set temperature. The test flame was applied inside the sample cup. A schematic of the calorimeter is shown in Fig. 1. Three type N thermocouples are used to measure the temperature inside the sample, the surface temperature of the sample cell wall and the ambient temperature. The maintains a sample in an adiabatic condition once an exothermic reaction is detected. The top, side, bottom and tube heaters are used to control the temperature inside the sample adiabatically. The adiabatic control system is the same as the accelerating rate calorimeter (ARC) system. Reactions can be followed up to about 4K/min [1]. The pressure outside the sample cell is controlled to equal to the pressure inside the sample cell. Normal volume of the sample cell of was 13 cm 3. The sample cell of ARC was used to measure DTP and ADCA instead of the vessel. The volume of sample cell of ARC cell was 9 cm 3. Samples of the test were mixtures of or ADCA and a-alumina (Al 2 O 3 ) powder. Because the heat rate of pure was beyond 4K/min in the, it was diluted by a-alumina powder. The heat rate was so large in the decomposition of that the shut down for safety. ADCA was also diluted by a-alumina because the heat rate of pure ADCA may be beyond 4 K/min. The sample of generated much soot and many combustion products even if it was diluted. As a result, the piping system of the measurement equipment was stopped by the soot and the combustion products. The concentration range of DTP and ADCA was from 1 wt.% to 15 wt.%. Masses of pure DTP and ADCA were from.1 g to.15 g. Sample mass of mixture of or ADCA and a-alumina powder put in the sample cell was 1. g. The ARC sample cell was used because the heat capacity of the ARC cell was small compared with the normal cell. The experiment condition of the small thermal inertia was good for N2 cylinder thermocouple pressure transducer and tube heater sample drop-outpot single shot injection vaccum pressure valve control relief valve exhaust Figure 1. Outline of apparatus 2

3 # 27 IChemE the measurement for the hazardous evaluation. Measurement run for were three times and measurement run for ADCA was one time. Heat of reaction of was calculated the following equations (1) and (2). Heat of reaction of ADCA was obtained by the same calculation method. The w-factor is the dimensionless thermal inertia factor for the sample cell and Al 2 O 3. The w-factor is defined as the following equation. H R, ¼ w C (T max T o ) (1) w ¼ (C v W v þ C Al2O3 W Al2O3 þ C W )=(C W ) (2) H R, : Heat of reaction of (J/g), C : Specific heat of (J/g/K), W : weight (g), C Al2O3 : Specific heat of Al 2 O 3 (J/g/K), W Al2O3 : Al 2 O 3 weight (g), C v : Specific heat of vessel (J/g/K), W v : Vessel weight (g), T o : Heat onset temperature (8C), T max : Maximum temperature (8C). The specific heats of and ADCA were measured by DSC. They were 1.43J/g/K and 1.65J/g/K, respectively. The sample cell was made of titanium. The averaged specific heats of titanium and Al 2 O 3 between heat onset temperature and the maximum temperature were used. All experiments of the were performed in a closed cell environment with ambient air above the sample. The ARC sample cells made of titanium were used in the measurement. The threshold to detect an exothermic reaction was.5k/min of the heat rate. The sample temperature was automatically incremented by 1K. The pressure rise inside the sample cells during the reactions was followed up to about 7,5 kpa of the pressure and 75, kpa/min of the pressure rate. ACTIVATION ENERGY Activation energy was calculated by the following equations (3) and (4), assumed that reaction number was first. The reaction was regarded as the first order, because [2log(k)] was on the straight line against (1/T) in the plot of (1/T) versus [ log(k)]. The activation energy was obtained from the slope of the straight line [2]. k ¼ (dt=dt)=(tmax T) (3) log (k) ¼ log (A) (E=2:33=R) (1=T) (4) T: sample temperature (8C), k: kinetic rate (1/s), E: activation energy (J/mol), A: frequency factor (1/s), R: gas constant (8.314 J/mol/K). RESULTS AND DISCUSSION DSC AND TG-DTA Heat of reaction and heat onset temperature are summarized in Table 1. Number is the averaged values. The heat onset temperature of BW 1 and 2 were 158C and 248C. The heat onset temperature is the cross point between the Table 1. Results of DSC T o1 (8C) T o2 (8C) H R (J/g) H R (kj/mol) BW BW ADCA BW 1: blowing agent 1, BW 2: blowing agent 2, T o1,t o2 : heat onset temperature of peak, H R : heat of reaction. maximum slope of peak and baseline. The peak value of BW 2 was about three times than that of BW 1. In contrast, heat of reaction of BW 1 was slightly larger than that of BW 2. The DSC curves of and ADCA are shown in Figs. 2 and 3. There were two exothermic peaks in the DSC curve of. The lower and the higher heat onset temperatures of were 168C and 1948C, respectively. There is one exothermic peak in the DSC curve of ADCA. The heat onset temperature of ADCA was 1948C. Because heat of reaction per mole of was 4.8 times larger than that of ADCA, there was the possibility that the intensity of the decomposition of was higher than that of ADCA. Because heat onset temperature of was lower than that of ADCA, the thermal stability of was lower than that of ADCA. The experimental results of DSC show that has more thermal hazards than ADCA. The weight reduction onset temperatures in TG- DTA curve of BW 1 and BW 2 were 128C and 178C. The weight reduction rates were.4 wt.%/s and.3 wt.%/s at the heat onset temperatures. SMALL FLAME TEST BW 1 was ignited and continued to burn after the pilot flame contacted with the piled test sample of BW 1 within three seconds in all run. BW 1 was judged as the first flammable solid according to the Japanese Fire Service Law. BW 2 was not ignited after the pilot flame contacted with the HEAT FLOW (mw) C 23.6 C Figure 2. Relationship between temperature of and heat flow measured by DSC 3

4 # 27 IChemE 1 25 HEAT FLOW (mw) ADCA C ADCA ADCA C Figure 3. Relationship between temperature of ADCA and heat flow measured by DSC piled test sample of BW 2 within ten seconds. BW 2 was not judged as the flammable solid according to the Japanese Fire Service Law. FLASH POINT TEST Flash points of BW 1 and 2 were 1278C and 1758C. The generated gas inside the sample cup was ignited at 1278C in the test of BW 1. There was the case that BW 1 was not ignited at 138C because BW 1 generated the large amount of gas and blew the flame. BW 2 did not generate the large amount of gas beyond the flash point. The weight reduction onset temperatures in TG-DTA curve of BW 1 and BW 2 were 128C and 178C. The weight reduction rates were.4 wt.%/s and.3 wt.%/s at each heat onset temperature. These temperatures almost corresponded to the flash points in the flash point test of the BW 1 and BW 2. This experiment results explained the generated decomposition gas was immediately ignited after the beginning of decomposition. TEST The temperature history of and ADCA are shown Figs. 4 and 5. The heat onset temperature and heat of reaction of and ADCA obtained by are shown in Table 2. Number for is the averaged values TIME(min) Figure 4. Time history of temperature of measured by TIME(min) Figure 5. Time history of temperature of ADCA measured by The both of heat onset temperatures measured by the were lower than the values measured by the DSC because the thermal stability of the was higher than that of the DSC. Both of heats of reaction measured by the were close to the values measured by the DSC. The time history of the wall temperatures of the vessel were the same as that of the sample temperature in the measurements. These results showed the measurement method with the dilution by a- alumina was appropriate. Fig. 6 shows the relationship between [1/T] and [2 log(k)]. Because the straight lines were obtained in 1/ T versus [2 log(k)] diagrams of and ADCA, the decomposition reactions of and ADCA could be regarded as the first order reaction. The activation energy does not depend on the concentration. The activation energy can be applied to the hazardous evaluation considering the kinetic rate. It is the index of easiness of the reaction. The smaller the activation energy is, the easier the decomposition reaction progresses. Di-tert-butyl peroxide (DTBP)/toluene solution is used as the standard sample of the ARC [3]. The results of and ADCA measured by the were compared with those of DTBP measured by the. The glass vessel of 13 cm 3 was used in the measurement of DTBP by the. The activation energy of DTBP2 wt.%/ toluene was 157 kj/mol. With regarding to DTBP2 wt.%/ toluene solution of 5 g (w ¼ 7.6), the maximum of the heat rate and pressure rate.23k/min and 3.1 kpa/min, respectively. T o (8C) Table 2. Results of H R (kj/mol) E (kj/mol) HRM (K/min) P max (kpa) PRM (kpa/min) ADCA T o : heat onset temperature, H R : heat of reaction, E: activation energy, HRM: maximum heat rate, P max : maximum pressure, PRM: maximum pressure rate. 4

5 # 27 IChemE -logk ADCA /T (1/K) Figure 6. Relationship between [1/T] and [ 2 log(k)] The maximum heat rate (HRM) of 12 wt.% in a- alumina of 1. g (w ¼ 6.4) was 6 times larger than that of DTBP2 wt.%/ toluene of 5 g approximately when HRM of the unit weight was compared. The maximum pressure rate (PRM) of 12 wt.% in a-alumina was 8 times larger than that of DTBP2 wt.%/ toluene of 5 g approximately when PRM of the unit weight was compared correcting the difference of vessel volume. The amount of the decomposition gas was investigated on the basis of the maximum pressure. The of 1 mole generated the decomposition gas of 1.8 moles at the maximum temperature of 218C. The ADCA of 1 mole generated the decomposition gas of.64 moles at the maximum temperature of 1878C. These experimental results are important because the amount of the decomposition gas generated can be estimated from this from an arbitrary sample mass. The heat of reaction, HRM and PRM of were larger than those of ADCA. The decomposition of was more intense that of ADCA. The heat onset temperature of was lower than that of ADCA. The thermal stability of was lower than that of ADCA. The activation energy of and ADCA was could be regarded as almost the same values because the measurement error was considered. The decompositions of and ADCA began in almost the same kinetic rate at each heat onset temperature. CONCLUSIONS Two kinds of organic blowing agents were involved in a fire in a tire production plant in Japan in September, 23. One organic blowing agent had N,N -dinitrosopentamethylenetetramine () and the other one had azodicarbonamide (ADCA). The thermal stability of blowing agents containing and ADCA in addition to and ADCA was investigated by a differential scanning calorimeter (DSC). A small flame test and a flash point test were conducted to examine combustion characteristics of organic blowing agents. The reaction hazards of and ADCA were investigated using an auto pressure tracking adiabatic calorimeter (). The thermal properties of DTP and ADCA were studied on the basis of the thermodynamic parameters and the pressure data. The conclusions were made as follows: 1. The heat onset temperatures of organic blowing agents containing and ADCA were158c and 248C. The organic blowing agent containing was easy to ignite in the small flame test. The flash points of organic blowing agents containing and ADCA were 1278C and 1758C by the Seta closed-cup flash point test. 2. The thermal hazards of DTP with regard to the thermal stability and the intensity of decomposition were higher than those of ADCA on the basis of the heat rate and the pressure rate data obtained by the. The activation energy of and ADCA were 215 kj/mol and 164 kj/mol. The heat onset temperature of was 1458C and lower than that of ADCA. REFERENCES 1. Iwata, Y., Koseki, H., 25, Risk evaluation by pressure rise measured using adiabatic calorimeter, Science and Technology of Energetic Materials 66 No.5: Kikuchi, T., Thermal hazard evaluation of reactive chemicals by the ARC, 1989, Technical Report of Sumitomo Chemical, 1989-_: Iizuka, Y., Fujita, A., Akiba, K., Tomita, Y., 2, A study on reliability of adiabatic kinetic equation from ARC data, Journal of Japan Society for Safety Engineering, 39. No.2: 91 97(in Japanese). 5