Effect of TPB on curing reaction of HTPB-TDI

OU Ya-peng， CHANG Shuang-jun， ZHANG Bai-lei

(School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, China)

EffectofTPBoncuringreactionofHTPB-TDI

OU Ya-peng， CHANG Shuang-jun， ZHANG Bai-lei

(SchoolofChemicalandEnvironmentalEngineering,NorthUniversityofChina,Taiyuan030051,China)

Abstract：Catalysis effect of triphenyl bismuth (TPB) on kinetics of hydroxyl terminated polybutadiene-toluene diisocyanate (HTPB-TDI) curing reaction was studied by non-isothermal differential scanning calorimetry (DSC).The characteristic temperature of curing system was measured for calculating kinetic parameters and establishing curing reaction kinetic equations.The results show that activation energy (Ea) of uncatalyzed HTPB-TDI curing system is 51.29 kJ·mol-1, and TPB decreasesEato 46.43 kJ·mol-1.Catalyst lowers reaction temperature and shortens curing time through decreasing activation energy of curing reaction and accelerating reaction rate.TPB can increase the reaction rate at 27 ℃ to the value of uncatalyzed system at 80 ℃.The catalytic activity reaches the maximum when concentration is 0.5%.

Key words：hydroxyl-terminated polybutadiene (HTPB)； catalyst； thermal analysis； curing reaction kinetics

CLDnumber： V512Documentcode： A

Hydroxyl terminated polybutadiene (HTPB) is extensively used as binder in composite solid propellants and cast polymer bonded explosive (PBX) due to its convenience for casting and manufacturing[1-3].The binder imparts dimensional stability and structural integrity to the grain for appropriate mechanical properties.The low reaction rate of the urethane forming cure reaction of HTPB binder with isocyante curative causes high curing temperature and long manufacturing period.The curing temperature of composite solid propellants and cast PBX using HTPB as binder is about 60 ℃ in actual production[4].High manufacturing temperature reduces the safety of propellants and cast PBX productive process and increases the production cost.The grain gets residual stress which reduces the mechanical properties of PBX and propellants after cooled to ambient temperature.Considering the above-mentioned reasons, catalysts are widely used for reducing the curing temperature and curing time[5].Dibutyltin dilaurate (DBTDL), ferric tris (acetyl acetonate)(Fe(AA)3) and triphenyl bismuth (TPB) are commonly used as catalysts.The catalytic activity and mechanism have been investigated using various techniques[6-9].But the reason for high catalytic activity, especially from the perspective of reaction kinetic, is still obscure.

This paper focuses on the reaction kinetics of TPB catalyzed hydroxyl terminated polybutadiene-toluene diisocyanate (HTPB-TDI) curing reaction, calculation of kinetic parameters and establishment of curing reaction kinetic equations.The catalysis effect of catalyst concentration on curing reaction is also presented.

1 Experiment

1.1 Materials

HTPB resin was manufactured at Luoyang Liming Institute of Chemical Technology of China.The important properties include hydroxyl value of 44.13 mg KOH/g and average molecular mass of 2 800 g/mol.TDI and TPB were obtained from commercial source and used as received.

1.2 Method

HTPB and TDI were mixed in equivalent ratio 1∶1.The curing reaction was studied at varied heating rates of 5,10,15 and 20 ℃/min for determining the kinetics by Kissinger and Crane methods.Catalyst concentration of 0.3% was reached by mass of the resin.The mixture of about 20 mg was taken in alumina pans and subjected to differential scanning calorimeter (DSC) analysis in a Setaram DSC131 in nitrogen atmosphere at a flow rate of 50 mL/min with heating temperature range of 20-300 ℃.Influence of catalyst concentration was investigated at four different catalyst concentrations of 0.1%， 0.3%， 0.5% and 0.7% (wt%).

2 Results and discussion

2.1 DSC test results

The DSC curves of uncatalyzed and TPB catalyzed HTPB-TDI curing systems at 15 ℃/min are depicted in Fig.1.It can be seen that the HTPB curing reaction is a single exothermic reaction, and catalyst decreases the reaction temperature and makes exothermic peak of DSC curve antedisplaced.

Fig.1 DSC curves of different curing systems at 15 ℃/min

The TPB catalyzed HTPB-TDI curing system has two apparent exothermic peaks.According to the research on catalytic mechanism，TPB was shown to be able to associate with hydroxyl.The most operative mechanism is through activating the isocyanate-hydroxyl complex, which increases as reactions proceed.The first exothermic peak, which the association of TPB with hydroxyl hydrogen increases the nucleophilicity of the adjacent oxygen in the hydroxyl.Therefore, the catalyzed reactions of hydroxyl with isocyanate may be suggested to involve the initial formation of a TPB-hydroxyl association and a subsequent reaction of the formed associate with isocyanate, in which the urethane is formed.After that, the catalyst is regenerated, which results in the second exothermic peak.

Tables 1 and 2 show the initial curing temperature(Ti), peak curing temperature(Tp) and final curing temperature(Tf) of DSC curves of uncatalyzed and TPB catalyzed HTPB-TDI curing systems.There are early and later stages in the DSC curve of TPB catalyzed HTPB-TDI curing system.But the later stage is not apparent at other heating rates, which indicates that TPB-isocyanate-hydroxyl association or complex is not quite stable and the TPB regenerating reaction nearly happens after the hydroxyl-isocynate reaction.Thus, only the early stage is discussed in this paper.The peak temperature of the early stage was reduced by 80 ℃ compared to uncatalyzed curing system.And the curing reaction peak temperature gradient(Tf-Ti) was decreased from 50-20 ℃.The above data show that TPB can decrease the curing reaction temperature and shorten reaction time.The high catalytic activity of TPB is cincident with the reaction mechanism.

Table 1 DSC data of HTPB-TDI curing reaction

Table 2 DSC data of TPB catalysed HTPB-TDI reaction

2.2 Kinetics of TPB catalyzed HTPB-TDI curing system

Kinetic parameters of curing reaction are calculated by Kissinger and Crane methods.

Kissinger formula:

(1)

Crane formula:

BecauseEa/nR?2TP,

(2)

whereβis the heating rate,Eais the activation energy of curing reaction,Ris the universal gas constant,Tis the absolute temperature andnis the order of reaction.

The HTPB-TDI curing system abidesnorder reaction model according to the research on the curing reaction mechanism of HTPB[10-11]，and reaction kinetic model equation is

(3)

whereαis the degree of reaction,tis the reaction time, dα/dtis the reaction rate andkis the reaction rate constant which follows the Arrhenius formulak=Aexp(-Ea/RT).Eq.(3) is for all values ofnexceptn=1.

Fig.2 Linear fitting of -lnβ and vs.1 000/TP for uncatalyzed curing system

Fig.3 Linear fitting of -lnβ and vs.1 000/TP for TPB catalyzed system

The kinetic parameters of uncatalyzed HTPB-TDI curing system areEa=51.29 kJ/mol，A=7.7×104s-1， andn=0.86.The values of TPB catalyzed HTPB-TDI curing system areEa=46.43 kJ/mol，A=2.4×105s-1，andn=0.87.The above parameters were substituted into Eq.(3).

The kinetic equation of HTPB-TDI curing system is calculated as

The kinetic equation of TPB catalyzed HTPB-TDI-TPB curing system is calculated by

TPB decreases the activation energy of the curing reaction.This is the reason why TPB lowers the reaction temperature.The reaction orders of two curing systems are similar and approximate 1, which shows that catalyst does not change the elementary reaction of HTPB-TDI and curing reaction abidesn-order reaction model correspondingly.

The kinetic equations show that it is linear relationship between reaction rate constants and reaction temperatures.Reaction rate constants determine the reaction rate at the specified temperatures.Some reaction rate constants of uncatalyzed and TPB catalyzed curing reactions at significant temperatures are shown in Table 3.The curing temperature of HTPB is about 60-80 ℃ in actual production, and the reaction rate constant at 60 ℃ is barely 6.93×10-4s-1without catalyst.It causes the long curing period and low production efficiency of HTPB based on grain.The reaction rate constant of the TPB catalyzed curing system at 27 ℃ is 1.98×10-3s-1， which reacheskvalue of the uncatalyzed system at 80 ℃.High temperature promotes isocyanate groups reacting with carbamic acid ester or urea linkages and generating cross bond.It decreases the properties of HTPB.High manufacture temperature reduces the safety of propellant and cast PBX productive process.It is the reason why catalysts are added to decrease tie manufacture temperature.TPB can make the curing reaction rate reach the value at high temperature without catalyst at low temperature.The above data prove that TPB is available for efficient catalyst based on reaction kinetics.

Table 3 Raue constants at different temperatures

2.3 Effect of TPB concentrations on catalytic activity

Fig.4 is the DSC curves of different TPB concentration catalyst systems at heating rate of 15 ℃/min, and concentrations of TPB are 0.1%，0.3%，0.5% and0.7%.The curing temperatures decrease with the increasing of concentration.Trend of temperature decreasing is slowing down after the concentration reaching a rather rich value.

Fig.4 DSC curves of different TPB concentrations catalyst system

Table 4 shows the peak temperatures of the DSC curves.It can be seen that it is linear relationship between temperature decreasing and the concentration increasing range from 0.1% to 0.5%.The curng temperature peak decreases by 15 ℃ with the concentration increasing by 0.1%.After the concentration reaching 0.5%, the peak value just decreases by 8 ℃ with the concentration increasing by 0.2%.It indicates that the catalytic activity reaches the maximum when concentration is 0.5%.

Table4DSCdataofdifferentTPBconcentrations'catalystsystem

TPBconcentration 0.1%0.3%0.5%0.7%TP/℃193149120112

3 Conclusion

BecauseEaof uncatalyzed HTPB-TDI curing system is 51.29 kJ/mol andEaof the TPB catalyzed system is 46.43 kJ/mol， catalyst decreases the activation energy of curing reaction and reaction temperature.HTPB-TDI-TPB system curing reaction kinetic equation is dα/dt=240 000(1-α)0.87×exp (-5 584/T), and the reaction rate constant at 27 ℃ reaches the value of the uncatalyzed system at 80 ℃.TPB is able to catalyze HTPB-TDI curing reaction at low temperature.The catalytic activity of TPB increases with the concentration and reaches the maximum when concentration is 0.5%.The appropriate concentration of TPB is 0.5%.

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TPB對HTPB-TDI固化反應(yīng)的影響

歐亞鵬， 常雙君， 張百磊

(中北大學(xué) 化工與環(huán)境學(xué)院， 山西 太原 030051)

摘 要:采用非等溫示差掃描量熱法(DSC)研究了三苯基鉍(TPB)對HTPB-TDI體系固化反應(yīng)動力學(xué)的影響。 測定了其固化峰溫， 以Kissinger法和Crane法計算其活化能等動力學(xué)參數(shù)， 列出了固化反應(yīng)動力學(xué)方程。 結(jié)果表明， 未加催化劑時活化能為51.29 kJ/mol， 加入TPB后活化能降至46.43 kJ/mol。 因此， TPB能有效地降低固化反應(yīng)的活化能， 加快反應(yīng)速率， 降低反應(yīng)溫度， 縮短固化時間， 并使固化體系在27 ℃時的反應(yīng)速率達到未加催化劑時80 ℃的值， 當(dāng)催化劑濃度為0.5%時， 催化活性達到最大。

關(guān)鍵詞:端羥基聚丁二烯(HTPB)； 催化； 熱分析； 固化反應(yīng)動力學(xué)

引用格式：OU Ya-peng， CHANG Shuang-jun， ZHANG Bai-lei.Effect of TPB on curing reaction of HTPB-TDI.Journal of Measurement Science and Instrumentation, 2014, 5(4): 89-92.[doi: 10.3969/j.issn.1674-8042.2014.04.017]

Article ID：1674-8042(2014)04-0089-04

10.3969/j.issn.1674-8042.2014.04.017

Receiveddate： 2014-09-15

Foundation item：Natural Science Foundation of Shanxi Province

Corresponding author：OU Ya-peng (ouyapeng@126.com)