Effect of the Plasticizer Dibutyl Phthalate on the Thermal Behaviours of Nitrocellulose

增塑劑臨苯二甲酸二丁酯對硝化棉熱行為的影響研究

Student thesis: Doctoral Thesis

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Award date1 Aug 2019

Abstract

Nitrocellulose (NC), which tends to be spontaneously ignited and deflagrable, has been extensively employed in military and civilian industry to produce explosives, lacquers, films and celluloid products. This material easily self-ignites when directly exposed to a hot and/or humid environment. A typical disastrous accident that involved NC occurred in Tianjin Port in China on August 12, 2015. Two severe explosions were triggered and caused 165 deaths, 798 injuries, and 8 missing persons. The direct economic loss was CNY 6.866 billion. Thus, the study of thermal behaviours of NC is necessary, and the thermal hazards of these materials must be evaluated. To ensure the safety performance, stabilizers or humectants are often added to NC. The plasticizer dibutyl phthalate (DBP, C16H22O4) is also commonly employed as an additive to make the NC mixture high gloss, less dusty and flexible.

In this thesis, a combination of experimental and theoretical methods was employed to study the critical safe temperatures, ageing characteristics, decomposition characteristics and combustion characteristics of pure NC and NC with the plasticizer DBP. The principle focused areas of this study are listed follows:

(1) Evaluating the critical safety temperature of NC is necessary to reduce the possibility of NC accidents (thermal explosion and fire) and implement effective loss prevention. In this study, isothermal and non-isothermal experiments were performed on different forms of NC using a differential scanning calorimeter. A scanning electron microscope was utilized to detect the ageing process of NC for different constant temperatures. Thermodynamic parameters were simulated by thermal safety software; the results indicated that autocatalytic simulation rather than nth order simulation was applicable to assess the apparent activation energy of NC samples. Iso-conversional methods, including Kissiger–Akahira–Sunose, Ozawa–Flynn–Wall, Friedman, Tang et al., and distributed activation energy model methods, were employed to validate the simulated activation energy. The average activation energy calculated by the five iso-conversional methods was lower than that simulated by autocatalytic simulation. Critical storage temperatures, including the time to the maximum rate and the time to the conversion limit, were evaluated. The critical storage temperatures of NC with plasticizer DBP were lower than that of pure NC. The critical thermal explosion temperatures of two samples were calculated; the results concluded that the temperature of pure NC was approximately 5.0 ℃ higher than that of NC with plasticizer DBP.

(2) To further understand the ageing process of energetic materials such as NC, ageing treatment of NC samples was performed by a constant temperature chamber. Non-isothermal experiments were performed with a TG-DSC analyser. A scanning electron microscope was utilized to detect the morphological changes between two NC samples. With an increase in the ageing time, the decomposition properties, including mass loss, decomposition temperature, heat production rate and heat of decomposition, exhibited significantly different trends for the two NC samples. The thermal kinetic parameters of pure NC were calculated based on the Kissinger method. Pure NC has the highest thermal instability at the ageing time of 24 days at the ageing temperature of 90 ℃, and the stability of NC will increase when the ageing time exceeds 32 days.

(3) The effect of the plasticizer DBP on the thermal decomposition of NC was investigated using a series of analytical apparatuses. In this study, the detailed structures of pure NC and NC with DBP were revealed by SEM. The fibres in NC with DBP are more closely aligned than those in pure NC, which causes the thermal behaviours of NC with DBP to differ from pure NC. The thermal stability of two samples was examined by a simultaneous TG- DSC apparatus (STA). Three different thermal kinetic methods (Kissiger-Akahira-Sunose method, Ozawa-Flynn-Wall method, and Friedman method) were applied to determine the activation energy E of these two NC samples. The experimental data were compared with sigmoidal models, and a pre-exponential factor was calculated by a compensation effect. In situ Fourier transform infrared (FTIR) and a TGA instrument coupled with a FTIR spectrometer were employed to investigate the characteristic functional groups of decomposition residues and gaseous products, respectively, at different temperatures. The results indicate that the two samples have similar decomposition products and decomposition mechanisms.

(4) To ensure the safety of inflammable and explosive chemical substances, such as NC mixtures, in the process of handing, storage, and usage, the fire properties of NC with different exterior structures need to be determined. In this study, the fire properties of the two commonly employed NCs with a soft fibre structure and a white chip structure were investigated and analysed with an ISO 5660 cone calorimeter. The experimental findings revealed that the most important fire properties such as ignition time, mass loss rate and ash content exhibited significant differences between the two structures of NC. Compared with soft fibre NC, chip NC possesses a lower fire hazard, and its heat release rate intensity (HRRI) is primarily affected by the sample mass. In addition, the oxygen consumption (OC) calorimetry method was compared with the thermal chemistry (TC) method, which is based on the stoichiometry for the HRRI calculation. The HRRI results of NC with two structures obtained by these two methods showed consistency.

    Research areas

  • nitrocellulose, dibutyl phthalate, thermal behaviours, critical safe temperatures, ageing characteristics, decomposition characteristics, combustion characteristics, thermal kinetic methods