Research on the dynamics and thermodynamics of the explosive process of powder coatings.

Authors Wen/Qin Wenjing 1, Fan Bing 1, 2, Liu Jiawei 1, 2, Wang Yao 1, 2, Xiao Qiuping 1,2,3
1. Shanghai Chemical Research Institute Co., Ltd.; 2. Shanghai Chemical Safety Engineering Technology Research Center; 3. Nanjing University of Technology
Summary:
for understanding The explosive hazard of powder coating in the process of spraying process, the explosion dynamics and thermodynamic parameters of epoxy, polyester and acrylic were studied by Siwek20L spherical explosive device, and the risk of explosion process of different kinds of powder coating was compared with thermal weight and DSC2 analysis methods. The results showed that of the three powder coatings, the maximum explosion pressure and pressure increase rate of acrylic powder coating was 0.730MPa and 76.0MPa/s, and the explosion hazard level was St2, while the maximum explosion pressure and pressure increase rate of polyester powder coating was the smallest, 0.617MPa and 37.9MPa/s, and the explosion hazard level was St1. The glass transition temperature of epoxy, polyester and acrylic powder coatings was 62 degrees C, 57 degrees C and 58 degrees C, respectively, and the mass loss rate in inert atmosphere was 52.9%, 98.0% and 97.3%, respectively, and the loss rate in air atmosphere increased by .
0 Foreword
In recent years, electrostectric spraying process due to low energy consumption, high recovery rate characteristics are widely used in industrial production, epoxy, polyester and acrylic powder coatings with good leveling, film-forming fast, good coloring force and other advantages to become the largest use of powder coatings. Spraying dust in the spray workshop is combustible dust, the air in the workshop as an oxidant, static sparks formed during the spraying operation can be used as a source of ignition, in the spraying operation area to detonate dust clouds, dust explosion occurred. According to 10 "major production safety accident hazards", the invention summarizes the dust explosion hazards existing in powder electrostectrectrelectrelectration spraying enterprises and puts forward corresponding corrective measures. Le Zhonglin based on a series of standards developed by the American Fire Association, put forward that in powder production, storage and transportation operations, there must be dust hazard analysis (DHA) process; It is suggested to use explosion-proofing method for prevention and control, Yan Xinxin and so on analyze the effect of mass concentration on the lower explosion limit, maximum explosion pressure and maximum pressure rise rate of phenolic resin dust, the results show that there is the most dangerous explosion concentration of dust explosion, Li and so on studied the powder coating dust cloud minimum fire temperature, dust layer minimum fire temperature and minimum ignition energy, and suggested the use of explosion insulation and explosion technology in the spraying process through practical cases. Powder paint-related powder explosion accidents occur frequently, but most of the research on dust explosion in spraying focuses on process research, and very little research on material hazards. In this paper, combined with thermal analysis methods, the explosive dynamics and thermodynamic parameters of different types of powder coatings are studied in order to provide a basis for safety protection in the granulation production, storage, transportation and application of powder coatings.
1 Experiment
. 1. 1 The experimental
select three powder coatings of epoxy, polyester and acrylic from a spraying plant as the experimental raw material. In order to avoid the effect of particle size on the experimental results, 3 powder coatings were screened with 250 mesh and 500 purpose screens before the experiment, and 3 powder coatings with particle size range of 32 to 63 m were obtained. The powder coating after screening was heat-dryed to ensure that its moisture quality score was less than 5% during the experiment.
1. 2 Experimental equipment and methods
particle size analyzer: Mastersizer 3000, Malvern, UK; differential scanning thermal instrument: DSC 204 F1, German Niech; thermal weight analyzer: Pyris 1 TGA, Perkin Elmer, UNITED States; explosion test system: Swiss Siwek 20L spherical explosion test system ("20L").
in the course of the experiment, the ball description device vacuum to -0. 06MPa, the tank began to enter the air, when the pressure value of the tank reached 0.06MPa, began to spray powder, the experimental sample was blown away by compressed air in the container to form a dust cloud, with a certain amount of chemical ignition head to detonate the dust cloud, while the pressure sensor records the pressure change curve of the process, ignition delay selection ISO 80079-20-2:2016 recommended 60 ms.
2 Results and discussions
. 2. 1 Volatility and particle size analysis
Particle size analysis of samples treated after dry screening, D90 of all three powder coatings is less than 63 m, the specific particle size results are shown in Table 1.
the particle size distribution range is indicated by discreteness
, the smaller the discrete degree indicates that the narrower the particle size distribution range, the smaller the number of particles that are too large or too small, and the more concentrated the particle size. Table 1 shows that the dispersion of the three powder coatings is small and the particle size distribution is narrow. All three powder coatings are micron powders, and the ultrafine powder content in polyester and acrylic powders is greater than that of epoxy powder coatings.
according to the relevant experimental standards, the same quality epoxy powder coating, polyester powder coating and acrylic powder coating, tested the volatile score of the test sample after screening at 105 degrees C, the results were 1.26%, 1.58% and 0.81%, respectively.
2. 2 Explosion parameter
2.2.1 Maximum explosion pressure
Maximum explosion pressure (Pmax) is a typical thermodynamic characteristic parameter that characterizes the total energy released by the explosion, and the explosion pressure curve of the three powder coatings is shown in Figure 1.
as can be seen from Figure 1, Pmax is in the order of acrylic powder coating, epoxy powder coating, polyester powder coating. In the mass concentration of 125 to 2250g/m3, the explosive pressure showed a tendency to increase and decrease. At a mass concentration of 125g/m3, the explosive pressure of epoxy, polyester and acrylic powder coatings was 0.32MPa, 0.513MPa and 0.585MPa, respectively. At 750g/m3, the explosive pressure of acrylic powder coatings first peaks at 0.730MPa, and at 1000g/m3, the explosive pressure of epoxy and polyester powder coatings also peaks at 0.704 MPa and 0.617MPa, respectively. Then, with the increase of mass concentration, the explosion pressure of different kinds of powder coatings decreased slowly and leveled off, and at 2250g/m3, the explosion pressure of epoxy, polyester and acrylic powder coatings was 0.656MPa, 0.469MPa and 0.530 MPa, respectively, which decreased by 6.8%, 24.0% and 27.4% compared to the peak pressure.
Under certain spatial conditions, when the powder concentration is low, the oxygen quantity is sufficient, heat transfer and reaction is sufficient, the main constraint of the explosive pressure is the concentration of the powder coating, the explosion pressure is low;
2.2.2 Maximum pressure rise rate
Maximum pressure rise rate (dP/dt)max is a dynamic characteristic parameter that characterges the rate of energy release during an explosion, i.e. combustion rate, and the pressure rise rate curve of three powder coatings is shown in Figure 2.
as can be seen from Figure 2, the order of (dP/dt) max is: acrylic powder coating, epoxy powder coating, polyester powder coating, epoxy and polyester powder coating experimental values are not much different. Within the mass concentration of 125 to 2250 g/m3, the pressure increase rate showed a trend of increasing and then decreasing. At a mass concentration of 125g/m3, the pressure increase rate of epoxy, polyester and acrylic powder coatings was 10. 0 MPa/s, 20.0MPa/s, and 32.8MPa/s, followed by an increase in the rate of pressure increase as the dust mass concentration increases. At 500g/m3, the pressure rise rate of acrylic powder coatings was the first to peak at 76. 0 MPa/s; at 1250g/m3, the pressure increase rate of polyester powder coating reaches a peak of 37. 9 MPa/s; at 1500g/m3, the pressure increase rate of epoxy powder coating reaches a peak of 40. 0 MPa/s. Then, with the increase of mass concentration, the pressure increase rate of different kinds of powder coatings decreased slowly and stabilized, and at 2250g/m3, the pressure increase rate of epoxy, polyester and acrylic powder coatings was 38.0MPa/s, 28.2MPa/s and 52.2MPa/s, respectively, which decreased by 5.0%, 25.6% and 30.9% compared to the peak.
Under certain spatial conditions, the low dust concentration is in an oxygen-rich state, the reaction is sufficient, more volatile points are released, the pressure rise rate increases with the concentration, when the optimal concentration range is reached, the pressure rise rate reaches a peak, and when the powder concentration is high, due to the limitation of oxygen concentration, the explosive reaction is incomplete, the heat transfer slows down, and the pressure rise rate is getting smaller and smaller.
2.2.3 Explosion Rating
The German Explosion Index grading method is related to the maximum pressure rise rate and the volume of the explosive container, Kst (dP/dt) maxV1/3 (MPa.m)/s, the evaluation criteria are shown in Table 2, 3, the explosion index of powder coatings is shown in Figure 3.
according to the explosion index, the Kst values of epoxy and polyester powder coatings are 10.8 (MPa.m)/s, 10.3 (MPa.m)/s, respectively, for St1, the explosion hazard strength is weak;
2.3 DSC analysis
3 powder coatings in the nitrogen inert atmosphere and air oxidation atmosphere respectively in the differential scanning volume thermal analysis, temperature range of 25 to 600 degrees C, heating rate of 10 degrees C / min, the results are shown in Figures 4 and 5.
as can be seen from Figure 4, in the nitrogen atmosphere, epoxy, polyester and acrylic powder coatings appear between 50 and 100 degrees C heat absorption peak, may have reached Tg. Tg is directly related to the storage stability of powder coating, powder coating Tg is generally 50 to 80 degrees C. According to DSC results, the Tg of epoxy powder coating is 62 degrees C, the Tg of polyester powder coating is 57 degrees C, and the Tg of acrylic powder coating is 58 degrees C. After continuous heating, heating in the range of 500 degrees C is not complete.
figure 5 shows that in the air atmosphere, 3 powder coatings continue to oxidize and heat due to oxygen participation in the reaction. From the thermal peak area, acrylic peak area is the largest, epoxy powder area is larger than polyester powder coating, the corresponding thermal value can represent the total energy released by the powder. Combined with the experimental values of the maximum explosion pressure of the thermodynamic parameters, the conclusions are consistent. And the heat peak with the participation of oxygen, moving to the low temperature zone, indicating that oxygen can accelerate the occurrence of the reaction.
2.4 Thermal Weight Analysis
Thermal Reanalyzes 3 powder coatings in the nitrogen inert atmosphere and air oxidation atmosphere respectively, with a temperature range of 25 to 600 degrees C and a heating rate of 10 degrees C/min, as shown in Figures 6 and 7.
As can be known from Figure 6, in the nitrogen atmosphere, epoxy powder has significantly lost mass from 360 degrees C, with a loss of mass of 52.9%, polyester powders have significantly lost mass from 230 degrees C, a loss of mass of 98.0%, and acrylic powders have significantly lost quality from 250 degrees C, with a loss of mass of 97.3%.
As can be known from Figure 7, in the air atmosphere, epoxy powder has significantly lost mass from 350 degrees C, with a loss rate of 77.8%, polyester powders have significantly lost mass from 230 degrees C, with a loss of mass of 98.8%, and acrylic powders have significantly lost mass from 250 degrees C, with a loss of mass of 98.7%. Because oxygen reacts with polymers in the air atmosphere, accelerating the oxidation and decomposition of polymers, the loss rate of the three powder coatings in the air atmosphere is faster and the loss rate is higher than in the nitrogen atmosphere.
generally the higher the organic volatile score, the more likely it is to explode. Polyester powder coatings have the highest volatile content under the same heating conditions, followed by acrylic powders and epoxy powder coatings. Figure 8 shows the explosion process of three powder coatings in an airtight container.
can be seen from Figure 8, polyester powder coating explosion pressure is the largest, epoxy powder coating explosion pressure is the smallest, consistent with the above conclusions.
Because the main components of epoxy, polyester and acrylic powder coatings are various types of resins, resins are large molecules formed by different alcohols or acids through different bi-bonds, which are cracked into small molecule volatiles at high temperatures. During the heating process, the polymer large molecules analyze the gas and mix it with the air to form an explosive gas environment. It can be guessed that the powder coating in the process of combustion and explosion of gas combustion behavior.
3 Conclusion
(1) According to DSC results, the glass transition temperature of epoxy powder coating is 62 degrees C, polyester and acrylic powder coatings are 57 degrees C and 58 degrees C, according to TG results, in the inert atmosphere and oxidation atmosphere, polyester powder coating loss rate is the highest, and air can accelerate the loss of mass rate. Therefore, powder coating in storage and transportation, the use of the process to prevent excessive temperature leading to changes in coating performance.
(2) mass concentration range from 125 to 2250g/m3, the maximum explosive pressure (Pmax) value is acrylic powder coating, 0.730MPa, epoxy and polyester powder coating maximum explosion pressure is 0.704MPa and 0.617MPa, respectively. Therefore, in the actual industrial production process, the concentration of dust should be controlled in a limited space to prevent the occurrence of explosion accidents.
(3) mass concentration range from 125 to 2250g/m3, the maximum pressure rise rate value is acrylic powder coating, 76.0MPa/s, epoxy and polyester powder coating maximum pressure rise rate is 40.0MPa/s, 37.9MPa/s. Acrylic powder coatings have the highest explosive risk, at St2, and epoxy and polyester powder coatings at St1, according to the Kst Explosion Index rating. In the actual process, the corresponding explosion control measures can be set according to the explosion level of different powder coatings.
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