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Fuel 87 (2008) 120–124

Flammability and rheological behavior of mixed flameretardant modified asphalt binders

Shaopeng Wu a, Liantong Mo a,b,*, Peiliang Cong a, Jianying Yu a, Xiaofeng Luo a

a Key Laboratory of Silicate Materials Science, Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, Chinab Road and Railway Engineering, Civil Engineering and Goesciences, Delft University of Technology, Delft 2600GA, The Netherlands

Received 8 May 2006; received in revised form 15 October 2006; accepted 22 March 2007Available online 4 May 2007

Abstract

The flammability and rheological behavior of asphalt modified by mixed flame retardants was investigated using differential scanningcalorimetry and dynamic shear rheometer, respectively. The mixed flame retardants used consisted of decabromodiphenyl ether, anti-mony trioxide and aluminium trihydroxide with the ratio of 3:1:1 by mass. Experimental results indicated that the inclusion of 6 wt%mixed flame retardants improves the flame retardancy significantly by consuming the released heat of asphalt in a temperature rangeof 300–500 �C. The close values of the activation energy obtained by fitting Arrhenius model demonstrate flame retardant modifiedasphalt exhibits similar thermal susceptibility to base asphalt in the temperature range of 30–100 �C. It can foresee that the contentof flame retardant is expected to increase with respect to better flame resistance if required based upon this research.� 2007 Published by Elsevier Ltd.

Keywords: Asphalt; Flame retardant; Rheological property; Dynamic mechanical analysis

1. Introduction

Asphalt binders with excellent flame retardancy havebeen prepared successfully by adding various flame retar-dants such as antimony trioxide, decabromodiphenyl ether,aluminium trihydroxide and zinc borate [1,2]. Generally,different flame retardants exhibit different mechanisms offlame resistance. Thus, the synergistic effect of mixed flameretardants has been an interesting approach to improve theflame retardancy [3,4]. Currently, these flame retardantmodified asphalt (FRMA) binders above mainly aim tomeet the increasing concerns on the safety of asphalt appli-cation, particularly the flammability in tunnels [5–7].Researchers previously have demonstrated that the addi-tion of fillers, such as limestone powers, rubber crumb,silica and plastic have a significant influence on the rheo-logical properties [8–10]. Therefore, it is foreseen that the

0016-2361/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.fuel.2007.03.039

* Corresponding author. Address: Key Laboratory of Silicate MaterialsScience, Engineering of Ministry of Education, Wuhan University ofTechnology, Wuhan 430070, China. Tel./fax: +86 27 87162595.

E-mail address: molt@whut.mail.edu.cn (L. Mo).

addition of flame retardants would greatly affect the rheo-logical properties of asphalt binders. It is also believed thata fundamental understanding on the rheological propertieswill be vital of importance in the practical application offlame retardant modified asphalt binders. Thus, besidesthe improved flammability, this paper will focus on investi-gation of the rheological properties of unaging and agingFRMA binders using dynamic shear rheometer, which isconsidered to an effective and common approach to obtainthe desired purpose mentioned above. For the purpose ofcomparison, the changes of rheological properties ofasphalt binder added flame retardants will be evaluatedin term of temperature shift factor using the activationenergy obtained by fitting Arrhenius model.

2. Experimental

2.1. Materials

The used bitumen was based Bitumen 60/80 modified bySBS and the modified binder is in the range of Superpave

Fig. 1. Effect of flame-retardant content on LOI value.

S. Wu et al. / Fuel 87 (2008) 120–124 121

performance grade PG76-22T with penetration (ASTM D5-61) 69 dmm and Ring & ball softening point (ASTM O36-26) 81.8 �C, obtained from Koch Asphalt Co. Ltd. inHubei province, China.

All of flame retardants were made in Taixing Industry ofFine Chemicals Co. Ltd. in Jinan, China. The related prop-erties of these flame retardants are as follows:

• Aluminium trihydroxide (ATH): density 2.42 g/cm3,maximum particle size 10 lm, pH < 8.5.

• Zinc borate (ZB): density 2.67 g/cm3, maximum particlesize 5 lm, maximum ignition loss 15.5%; Antimony tri-oxide: melting point 656 �C, maximum particle size1.6 lm.

• Decabromodiphenyl ether (EBPED): density 3.25 g/cm3,maximum particle size 5 lm, minimum bromine con-centration of 82% and maximum free bromine concen-tration of 10%. The melting temperature rangesfrom 300 to 310 �C and the pyrolysis temperature is320 �C.

2.2. Methods

FRMA was prepared with 0–8 wt% mixed flame retar-dants with a weight ratio of EBPED/ATH/ZB (3:1:1).The blends were prepared by using Silverson L4R mixerat a high frequency of 30 Hz. The mentioned mixed flameretardants were added into bitumen at 170 �C during halfan hour.

Limiting oxygen index methods are widely used to mea-sure the flammability of polymers and to investigate theeffectiveness of flame retardants. Flame retardancy ofasphalt binders was assessed by the limiting oxygen indexaccording to ASTM D-2863. Test procedures were as fol-lows: the top of the sample was ignited by a gas flamewhich was stopped once ignition had occurred, and thenthe lowest oxygen concentration in a flowing mixture ofnitrogen and oxygen which just supports sustained burningcan be determined. The effectiveness of flame retardantswas measured by the changes in the critical oxygenconcentration.

DSC analysis in this study was performed using a PL-DSC made in USA with a power compensation head.Approximately 10 mg of sample was weighted using niceelectric scales in close condition and then placed in theDSC cell. The sample was heated from room temperatureto 600 �C at a heating rate of 10 �C/min under atmosphericcondition supporting with flowing air of 10 ml/min.

Both base asphalt and FRMA were aged using the roll-ing thin film oven test (RTFOT) according to ASTM D1754. Dynamic shear properties were measured withAR2000 dynamic shear rheometer (TA Inc., USA) in a par-allel plate configuration with a gap width of 1 mm. Mea-surements were conducted at fixed frequencies 10 rad/sand temperature from 30 to 100 �C with rate of heating1 �C/min.

3. Results and discussion

3.1. Flame retardancy of FRMA

The LOI measurements as a qualitative method fordetermining the flammability of blends have been shownin Fig. 1 as the function of increasing concentrations ofmixed flame retardants. It is clear that the base asphaltused exhibits a high tendency to propagate flame with aLOI value of 19.8 since oxygen concentration in air is justabout 20.95% by volume. However, the oxygen index ofasphalt binder increases obviously with the addition ofthe mixture of flame retardants increases. LOI value largerthan 26, which is expected to be self-extinguished in air fora material [11,12], is obtained when the amount of flameretardants is beyond 6 wt%. Hereafter, all FRMA bindersare referred to 6 wt% flame retardant modified asphalt inthis study because of a satisfied flame resistance.

Fig. 2 presents DSC test results of base asphalt, FRMAand pure mixture of flame retardants at a wide temperaturerange from room temperature to 600 �C. The peak of theheat release rate and the total heat release were utilizedto characterize the flame retardant properties. At the begin-ning, a small exothermic effect was observed around 40 �C,which is probably linked with the crystallization on warm-ing, of species staying amorphous on cooling [13]. In gen-eral, FRMA exhibits a similar trend compared to baseasphalt when test temperature is below 250 �C. That meansflame retardants shows excellent thermal stability in such atemperature range. Therefore, no significant decomposi-tion of flame retardants will occurs during mixing in plantand paving and compacting in field because the relatedworking temperatures, for instant, mixing temperature of160–180 �C, are much less than 250 �C. The base asphaltappears exothermic peaks at temperature range of 300–415 �C, but there are no remarkable exothermic peaks forFRMA until temperature beyond 500 �C; The mechanismof the flame retardation can be explained by comparison

Fig. 2. Results of DSC test of base asphalt and FRMA.

Fig. 3. Log–log plot of temperature vs G* curves.

122 S. Wu et al. / Fuel 87 (2008) 120–124

of exothermic effect of basic bitumen with endothermiceffect of the retardant mixture due to the decompositionwithin the temperature range of 300–550 �C. In combina-tion with the two effects, DSC values of FRMA are belowor around zero line (see in Fig. 2) when temperature isbelow 550 �C. From comparison analysis, it can be seenthat the temperature range of 300–415 �C is vital of impor-tance for controlling the flame resistance of asphalt becauseinitial exothermic peaks. Fortunately, the added flameretardants also start to work at such a temperature rangeso that the released heat of asphalt can be consumed bythe decomposition of flame retardants. Based upon analy-sis above, it can be concluded that 6 wt% FRMA hasenough flame retardancy and this selected concentrationis considered to be reasonable and economic because thereleased and consumed heat reaches a well balance in widetemperature range. It is also foreseen that greater concen-tration will lead to minus DSC values that related to betterflame retardancy. As a result of this, DSC test could beused as an effective tool for evaluation the flammabilityof FRMA binders.

3.2. Dynamic shear properties

Fig. 3 shows the temperature dependency of complexmodulus G* for each binder in a range of temperature30–100 �C. The obtained results indicate that flame retar-dant modified asphalt binders show an increase in the com-

Table 1Effects of ageing and flame retardant on complex modulus

Temperature(�C)

G* (Pa)

BA (1) BA + age (2) FRMA (3) F

35 162,300 198,800 644,400 945 38,570 53,130 174,500 355 11,640 16,750 51,330 965 3804 5903 17,440 375 1576 2520 5758 185 695 1017 1710 495 332 471 542 1

Average – – – –

plex modulus compared to the origin asphalt binder,especially at relatively low temperatures. At higher temper-atures, the increase in G* is relatively small. As can beobserved in Fig. 1, the slope of G* curve of flame retardantmodified binder is slightly larger than the one of the neatasphalt. The effects of short-term aging of RTFOT indicatethat the simulating aging in the laboratory increases G*.Generally, there is not a significant difference betweenunaged and aged binders in their G* values for both baseasphalt and FRMA. A more illustrative way of comparisonis to represent by the relevant factors in changes of G* forboth base asphalt (BA) and FRMA after/before ageing, aslisted in Table 1. Several important observations can bederived from these values listed. The influences of ageingon increase of G* follows the average factors of 1.44 and2.01 for base asphalt and FRMA, respectively; moreover,the effect of the addition of flame retardants results inincrease of G* by an average factor of 3.61 and 4.76 forunage and age.

Fig. 4 illustrates the changes of phase angle d for theabove-mentioned asphalt binder as the function of temper-ature, which also indicates that the addition of flame retar-dants to asphalt binder decreases the phase anglesignificantly at relatively low temperature region. However,when the temperature is beyond a certain value, forinstance 75 �C, the unaging FRMA exhibits a higher d

Factors

RMA + age (4) (2)/(1) (3)/(1) (4)/(3) (4)/(2)

47,500 1.22 3.97 1.47 4.7707,200 1.38 4.52 1.76 5.7858,000 1.44 4.41 1.87 5.722,190 1.55 4.58 1.85 5.450,550 1.60 3.65 1.83 4.19366 1.46 2.46 2.55 4.29484 1.42 1.63 2.74 3.15

1.44 3.61 2.01 4.76

Fig. 4. Phase angle vs temperature curves. Fig. 5. Log–log plot of temperature vs G*/sind curves.

S. Wu et al. / Fuel 87 (2008) 120–124 123

value compared with the base one. Furthermore, theasphalt aging using RTFOT will decrease d and this trendseems to be similar for base and modified asphalts. It isfound that the original asphalt shows a plateau region tophase angle over the intermediate temperature rangeapproximately from 35 to 70 �C, but such a well-definedtrend is not obvious for flame retardant modified binder,even subjected to short-term aging simulation.

It is known that the flame retardant modified binder canbe treated as a type of two-phase composite material withflame retardant particles dispersed in as asphalt matrix,with is similar to particulate-filled composite materials.Consequently, the increasing complex modulus of modifiedasphalt binder can be expected based on the rule-of-mix-ture’s methods. Because no chemical interaction betweenbitumen and flame retardants occurs, the G* values of mod-ified binder should be in the same order of magnitude asthose of the base asphalt when the addition of flame retar-dant is relatively small, which has be demonstrated by theexperimental data obtained. In combination with reducingd and increasing G*, it is concluded that the inclusion offlame retardant into base asphalt evokes a remarkableamount of increase in elastic component, which impliesthat a significant shift of rheological behavior from visco-like to a more elastic-like body would occurs.

Fig. 5 shows the effect of temperature on rutting param-eter G*/sind, which indicates to be similar to complex mod-ulus. It is known that the temperature of the asphaltbinders when G*/sind is equal to 1 kPa is defined as a cri-terion for the high temperature performance of asphalt[14,15]. The resulting temperature of base asphalt andmodified asphalt are determined as 78 �C and 87 �C fromthe testing data obtained. This indicates that the FRMAhas a higher performance grade than base asphalt. It is alsoknown that higher G*/sind values were found to correlatewith higher rutting resistance. As has been indicated in thisstudy before, flame retardant increases the complex modu-lus and reduces phase angle, thus changes the elasticityof its modified binder. It is observed that the increase inG*/sind by flame retardant was mainly caused by G*

increase between the two affecting factors above. Probably,the use of G*/sind in practice would make the importanceof d much less compared to that of G*. The reason for thisis that, at such a test temperature region, sind was observedto be in a relatively narrow range between 0.5 and 1 with aphase angle of 30–90, while G* values vary over a muchwider range.

3.3. Analytical model for temperature shift factor

It is known the temperature dependence of the viscoelas-tic behavior of asphalt, as indicated by the shift factors canbe represented well by an Arrhenius equation

aT ðT Þ ¼ expEa

R1

T� 1

T 0

� �� �ð1Þ

where aT(T) is the shift factor relative to the reference tem-perature; Ea is the activation energy; R is the universal gasconstant, 8.314 J/(mol K); T and T0 are test temperatureand reference temperature, K.

To test the experimental data for temperature shift fac-tors with Arrhenius models, the equations for complexmodulus and rutting parameters is given by

G�ðT Þ ¼ aT ðT ÞG�0 ð2ÞG�ðT Þ= sin dðT Þ ¼ aT ðT ÞG�0= sin d0 ð3Þ

where G�0 and d0 are complex modulus and phase angle atthe reference temperature.

The model parameters in the proposed model will beobtained by least error fitting so that the model can be usedto explain the changes of complex modulus and ruttingparameters. The selected fitting range provides useful infor-mation about in-service asphalt performance at hightemperature.

To do this, the prediction error is defined as

Percent error PEi ¼MVi � EVi

MVi� 100 ð4Þ

Table 2Arrhenius model parameters

Sample G* G*/sind

Ea

(kJ/mol)%MAE R2 Ea

(kJ/mol)%MAE R2

Base asphalt 102.43 18.38 0.994 104.15 19.35 0.991Base asphalt after

RTFOT96.64 7.11 0.998 93.64 6.75 0.999

FRMA 105.41 14.58 0.998 98.80 18.06 0.996FRMA after

RTFOT97.41 9.80 0.997 89.04 16.01 0.991

124 S. Wu et al. / Fuel 87 (2008) 120–124

The percent of mean absolute error %MAE is calculatedfor each loading type using

%MAE ¼Pn

i¼1jPEijn

ð5Þ

where MVi and EVi is the model value and experimentalvalue. i is the ith test in the total number n of tests.

The error %MAE is important and do not allow tounderline an eventual dependence of shift factor upon thetemperature. In this study, the lower mean absolute percenterror corresponds to higher accuracy of proposed modelfor predicting the obtained data, that is, the model valuesare closer to the true behavior of asphalt binder.

The model statistics are shown in Table 2 for Arrheniusmodels, respectively, in which all of the reference tempera-ture for modeling analysis are 30 �C. It is found that theexperimental data of complex modulus and G*/sind followthe analytical model above quite well.

As can be observed in Table 2, the activation energy forall binders ranges from 89.04 to 105.41 kJ/mol. In previousresearches, the activation energy was found approximately70–200 kJ/mol for various asphalt binders [8,16]. FRMAhas higher activation energy than origin asphalt binderwith respect to complex modulus, but not rutting parame-ter G*/sind. Aged asphalts have lower activation energythan the relevant unaged one, and the difference is about5–10 kJ/mol. This trend is in agreement with Ruan’s report[16]. Table 1 shows the fitting results with similar activationenergy for both base asphalt and FRMA, even subjected toage. Therefore, because the activation energy is linked withthe thermal susceptibility, it can be concluded that flameretardant has a slight influence on the thermal susceptibil-ity, that is, the thermal susceptibility is not improved by theinclusion of flame retardants, but mainly depends upon onbase asphalt.

4. Conclusions

The flammability and rheological behavior of bitumenmodified with mixed flame retardants was investigated.The inclusion of 6 wt% mixed flame retardants improvesthe flame retardancy significantly by consuming the

released heat of asphalt in a temperature range of 300–500 �C. Besides the increased flame resistance, flame retar-dants also quadruples the complex modulus, however affectphase angle significantly. The close values of the activationenergy obtained by fitting Arrhenius model demonstrateflame retardant modified asphalt exhibits similar thermalsusceptibility to asphalt binder in the temperature rangeof 30–100 �C. It can foresee that the content of flame retar-dant is expected to increase with respect to better flameresistance if required based upon this research.

Acknowledgements

Support for this work by the Department of Transpor-tation in Hubei Province, China and Headquarters ofHurong-Xi Expressway in Hubei Province is greatlyappreciated.

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