Ling-ke Zhang,Xiang-yang Zheng

School of Energy and Power Engineering,Nanjing University of Science&Technology,Nanjing 210094,China

Keywords:Base bleed technology AP/HTPB composite propellant Natural ageing Decomposition kinetics DSC experiment

ABSTRACT The kinetics of the thermal decomposition for a naturally ageing ammonium perchlorate(AP)and hydroxyl-terminated-polybutadiene(HTPB)base bleed composite propellant were investigated using a differential scanning calorimetry(DSC).The naturally ageing AP/HTPB base bleed propellant samples have been stored in a sealed plastic bag at room temperature(5-25oC)for more than 20 years.The experimental DSC results were obtained by placing samples(each about 1.5 mg)in a sealed pan under non-isothermal condition under different heating rates,5.0,10.0,15.0,20.0 and 30.0oC?min-1.The activation energy and pre-exponential factor were estimated based on the relationship between the exothermic peak temperature and the heating rate by Ozawa and Kissinger methods,respectively.The decomposition kinetic parameters is lower the values under laboratorial aging condition.

1.Introduction

Base bleed extending range projectiles(BBERP)are used widely on large caliber artillery system.Compared with traditional projectile without base bleed unit,the base bleed projectile could increase firing range by high increasing ratio(10-30%)due to the gas generatorhoused in the tail of projectiles[1].The major principle of base bleed extending range is to reduce the base drag by injecting low speed and high temperature combustible gases into the base region.AP/HTPB composite propellant is an important energetic material burning slowly and producing subsonic flow[2].In the last five decades,many researchers devoted them selves to investigating the decomposition and combustion mechanism of AP/HTPB composite propellant for rocket motor to improve the performance[3,4].Usually,AP is the main components as fuel whose thermal decomposition of AP is rather complex and exhibits several unusual characteristics[5].In general,there are three stages for the decomposition of AP.Firstly,the AP crystal experiences a transition from orthorhombic structure to a cubic structure at 513 K.Secondly,the low-temperature decomposition of AP occurs below approximately587 K and results only in 30%decomposition.Thirdly,it then undergoes equilibrium dissociative sublimation and degradation when the temperature increases to around 830 K[6].Vyazovkin and Wight summarized the relatively results of the AP thermal decomposition and reported the activation energies vary from69 to 141.2 kJ?mol-1(in the one case even of 37.6 kJ?mol-1)which are determined under the different experimental conditions[7].As a long-chain,cross-linked and high-molecular-weight polymer,HTPB is chemically inactive and using as a kind of binder.At low heating rate,the decomposition of HTPB undergoes a 2-stage kinetic mechanism.The first is endothermic and 10-15%mass weight loss occurs during this stage.In the second stage,it is an exothermic process and the remaining residue cyclyzes and crosslinks undergoes further degradation[7].

Aging is an important issue in base bleed unit application during storage and handing as well as in rocket motor.In general,the aging process of AP/HTPB composite solid propellant is very complex and causes a combination of the physical and chemical damage in the initial properties of the propellant[8].The purposes of studying aging behavior of AP/HTPB composite propellant mainly including developing failure criteria,estimating service life,researching thermal decomposition characteristics, finding the changes in mechanical properties,observing the burning characteristics and so on[9].The activation energy of composite propellant,as a key parameter of properties,reduces during the aging process via oxidative crosslinking of the polymeric binder,migration of additives and recrystallization of the oxidizer[10].

Thermal analysis techniques,such as DSC and thermosgravimetry(TG)have been widely used today to describe the thermal decomposition kinetics of energetic materials[11].Analyzing DSC curves and obtaining the linear relationship between peak temperatures and heating rates could estimate activation energy and pre-exponential factor by using integral isoconversional method of Ozawa[12].

The primary objective of this work is to study experimentally AP/HTPB composite propellant thermal decomposition process by DSC.The second objective is to determine how the kinetic parameters,such as activation energy and pre-exponential factor changes due to the naturally aging.The third objective is to compare the processed kinetic parameters to the laboratory aging results and analyze the changing trend.

2.Experimental

Samples were derived from a real AP/HTPB base bleed composition propellant grain,which has been sealed in a plastic bag and kept in storage for more than 20 years at room temperature(5-25oC).We assume this history process as a naturally aging process,which is different from the accelerating aging method.The composite propellant grain consists of 76%AP oxidizer with bimodal particle sizes and 24%HTPB binder.We placed the samples(1.35±0.02mg)in a sealed aluminum pan and conduct the DSC measurements.The heating rates were controlled at 5.0,10.0,15.0,20.0 and 30.0oC?min-1respectively under nitrogen atmosphere(purity above 99.99%)with flow rate of 50 ml?min-1.The DSC curves were obtained by sampling rate of 12-20 points per centigrade in nonisothermal mode,which is enough sufficient to eliminate errors the evaluation of both enthalpy and kinetic parameters.The DSC curves at different heating rates for natural ageing AP/HTPB based bleed composite propellant is shown in Fig.1.

3.Kinetic analysis

The kinetics of thermal decomposition of solid composite propellant are usually described by a single-step kinetic equation[7].

Where t is the time,T is the temperature,αis the extent conversion,and f（α）is the reaction model associated with a certain reaction mechanism.The explicit temperature dependence of the rate constant is introduced by replacing k（T）with the Arrhenius equation,which is

where A is the pre-exponential factor,E is the activation energy,R is the gas constant,8.314J/mol·K.

For the nonisothermal condition,the sample is heated at a constant rate.Hence the Eq.(1),has an expression as

whereβ=dT/dt is the heating rate.The integral of Eq.(3)can be written as

where x=E/RT and p（x）is an approximation of the temperature integral.Eq.(5)expressed in logarithmic form is

The approximation of log p（x）was given when x ≥ 20 as

Submitting Eq.(7)to Eq.(6)and differentiating at constant degree of conversion leads to linear equations of Ozawa[12].A linear relationship between the heating rate(logβi)and the reciprocal of the absolute temperature Tpi(the temperature of exothermic peak)based on Ozawa models was given by Rocco[13]

where a is the slope and a=-0.457E/R,b is the intercept.So,the activation energy E can be determined according to the slope a which could be obtained by a plot of logβvs 1/Tpbased on the DSC curves.Eq.(8)was derived by assuming the constant activation energy throughout a wide range of the temperatures and the extents of conversion.The assumption obviously introduces some systematic error in estimating E because it is dependent on the extent conversionαin multi-step reaction process.

For the first-order reaction model,

where n is the reaction constant.Submitting Eq.(9)into Eq.(2)and differentiating on both sides,one can obtain d2α/dt2=0 when T is equal to Tp.Hence

It has been proved that the term n exp（1- α）n-1is independent on heating rateβand the value is equal to 1 approximately.Eq.(10)expressed in logarithmic form is

Activation energyEcan be estimated from the slope of the linear relationship ln（β/）vs.1/Tpas

So,pre-exponential factor A can be calculated in terms of the equation

4.Results and discussion

The DSC curves of natural ageing AP/HTPB base bleed composite propellant were obtained by placing samples in a closed sample pan under nonisothermal conditions with different heating rates:5.0,10.0,15.0,20.0oC?min-1.The DSC curve shows an obvious endothermic peak with the maximums at about 245oC and a following exothermic peak at about 326.2oC for the lowest heating rate 5.0oC?min-1.This endothermic event corresponds to the transition from the orthorhombic to the cubic crystalline phase for AP.For other three higher heating rates,the endothermic events do not occur,or at least,they are imperceptible from the DSC curves.The exothermic stage(the second peaks)occurs at 353.7,376.8 and 393.2oC for different heating rates of 10.0,15.0,20.0oC?min-1.Such experimental results may be changed because the shape of DSC curves of AP depending strongly on the experimental conditions whether samples are in a closed pan or an openpan.In an openpan,the exothermic decomposition is always following by an endothermic process.

In order to use Eq.(8)and Eq.(12),and calculate the activation energy of experimental samples,the plots of logβivs.1/Tpiand log[βi/T2pi]vs.1/Tpiwere shown in Fig.2 and Fig.3 respectively.The slopes of linear fitting function for Ozawa and Kissinger methods were known as-3565.8 and6948.9 from both figures.The values of the activation energy were estimated to be 64.9 kJ?mol-1and 57.8 kJ?mol-1,respectively.The difference of the both values is 7.1 kJ?mol-1,which is acceptable in the error region.

The decomposition kinetics of HTPB binders were studied by many researchers.Cohen[17]studied the kinetics of the surface pyrolysis of HTPB based on the zero-order model and determined the activation energy of 71 kJ?mol-1,and the other activation energy values of 78.69 kJ?mol-1,115.5 kJ?mol-1,192.5kJ?mol-1,100.5kJ?mol-1were reported based on the TGA experiment under the wide temperature range of 320-470oC[7].Numerous studied results show the weight loss of HTPB decomposition at lower heating rates are mainly found above 400oC(weight loss of 80%-90%),even though the DSC exothermic peak temperature in the range of 360-390oC,which is close to the exothermic peak temperature of cubic AP.Comparing the activation energy for AP/HTPB composite propellant decomposition with individual AP and HTPB,it is suggested to synthesize the overall kinetics of the mass loss for the reaction between AP and HTPB[13].In practice,the weight fraction of AP oxidizer is always over 75%,even more than 80%,and the thermal decomposition contribution of AP is a dominant factor below 400oC at lower heating rates.So it is reasonable to estimate the pre-exponential factor A based on the TGA data for the extent conversion of AP.The thermogravimetric analysis experiments for cubic AP under nonisothermal conditions at different heating rates and the TGA data was shown in Fig.4.

It can be seen the extent conversion of AP changes from about 0.4 to 0.7 when the temperature increases from 325 to 400oC for a constant heating hate.The factor Aiin logarithmic form log Aiwere calculated by linear fitting for the peak temperature Tpiand the heating rate βibased on Ozawa method Eq.(6)to be 4.8113,4.9557,5.0507 and 5.0507 min-1,and the average value is 4.9942 min-1(9.87?104min-1).Using Kissinger method Eq.(6)the values of log Aiwerecalculatedtobe 4.0248,4.0657,4.0391 and 4.0280 s-1,and the average value is 4.0394 s-1.

Rocco and Goncalves[13]reported the energy of aging AP/HTPB composite propellant under the synthetic aging process by exposing the propellant formulation at 338K(65oC)for 300 days is 79.0kJ?mol-1,and for the for non-aged propellant,the values are 134.5kJ?mol-1(Ozawa)and 126.2kJ?mol-1(Kissinger),which is very close also.Meanwhile,the factor Aiin logarithmic form log A was found to be 10.3096 s-1for non-aged samples and 6.1106 s-1for aged samples.Comparing the above studied results,it is easily to find the activation energy and pre-exponential changes small with aging degree.

5.Conclusions

In this work,the thermal decomposition experiment for natural aged AP/HTPB base bleed composite propellant with bimodal AP particles was carried out by DSC.The DSC curves were obtained under non-isothermal conditions at different heating rates.The decomposition kinetic was analyzed by using Ozawa and Kissinger methods based on the assumption of constant kinetic parameters in a wide range of temperature.The decomposition kinetic parameters of the aged AP/HTPB composite propellant in this paper are smaller than the values reported by Rocco's obtained by DSC curves under laboratorial aging condition and obtained by the same numerical methods for similar ingredients of AP/HTPB propellant but with different AP particles.