88 Vol.28 No.1 GAO Ying et al: Thermal Oxidative Aging Characterization of SBS Modif...

Thermal Oxidative Aging Characterization of SBS Modifi ed Asphalt

GAO Ying, GU Fan, ZHAO YongLi ( Transportation School, Southeast University, Nanjing 210096, China)

Abstract: Both macro and micro-methods were introduced to study the physical and chemical properties of thermal oxidative aging of SBS (styrene-butadiene-styrene) modifi ed asphalt. The physical properties of SBS modifi ed asphalt before and after aging were analyzed by normal tests. The structure and quality variation of SBS modifi ed asphalt during the aging process was analyzed by FTIR (Fourier transform infrared spectrum). FTIR result shows that the degeneration of SBS modifi ed asphalt is mainly caused by oxidative reaction and rupture of C=C double bond. The molecular weight variations of asphalt function groups and SBS polymer were studied by GPC (Gel Permeation Chromatography). GPC result shows that small molecules transform into larger one in asphalt and SBS polymer molecule degrade during the aging process. SBS polymer may lose its modifying function after long time aging.

Key words: thermal oxidative aging; SBS modifi ed asphalt; FTIR; GPC

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013(Received: June 19, 2011; Accepted: Sept. 23, 2012)

GAOYing(高英): Assoc. Prof.; Ph D; E-mail: gy@seu.edu.cnFunded by the National Natural Science Foundation of China (Nos.

50878054, 51108081)

DOI 10.1007/s11595-013-0646-0

1 Introduction

Asphalt pavement become hardening and lose its durability with time goes by. Many works were done on the aging of asphalt and its affection on pavement performance. SBS modified asphalt is widely used in surface and intermediate layers of asphalt pavement. It is quite necessary to study the characterization of aged SBS modifi ed asphalt and the cause of aging so that to understand the pavement performance.

Many researches were done on the performance change of aged asphalt. Traxler thought that the hardening of the asphalt is generally caused with time and increased by volatilization and oxidation[1]. Herrington P R aged the fi lm of road asphalt at different temperatures and found that the increase in viscosity fi t a hyperbolic function of time[2]. The polymer modifi ed asphalt has improved quality because of the existence of polymers. The aging process of SBS asphalt includes both the aging of asphalt binder and the aging of SBS polymer[3, 4]. But few studies were done on the aging of SBS polymer and its influence on the asphalt’s aging

process. It is wondering whether SBS polymer keep working after aging. This paper focus on the aging of SBS polymer and SBS modified asphalt at the same time.

FTIR is commonly used to study the molecular structure of asphalt binder and polymers[5]. It is used here to evaluate the characterization of aged and unaged SBS modifi ed asphalt.

The GPC method is used to separate components of compounds based on molecular size or volume. The molecular weight of SBS polymer is about 400 000 Daltons, while the molecular weight of large molecule in asphalt is just 10 000[6]. The GPC method is used to distinguish the variation of molecular weight of both SBS polymer and asphalt binder before and after aging to evaluate the characteristic of aged SBS modified asphalt.

2 Experimental

2.1 MaterialsTable 1 and 2 show the chemical composition

and physical properties of base asphalt binder. Table 3 shows the physical properties of SBS polymer. SBS modifi ed asphalt was made by adding SBS into asphalt binder through shearing machine. The mass of SBS is 4.5% of that of asphalt binder.

Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2013 89

SBS modifi ed asphalt was aged under short-term and long-term aging condition respectively.2.2 Measurements

FTIR test of SBS modified asphalt was done by Nicolet740, whose resolution is 4 cm1, scanning frequency is 32 times and test range is 400-5000 cm1. The samples were prepared by casting a film onto a sodium chloride (NaCl) window from a 5% w/v solution in chloroform.

GPC test of SBS modifi ed asphalt was performed at room temperature on a Waters 515 equipped with a Waters 2487 Ultraviolet absorbance detector and Wyatt Technology Optilab REX refractive index detector.

Test samples include original, short term aged and long term aged SBS modifi ed asphalt. Short term aged SBS modifi ed asphalt was gotten by 5 h ,163 ℃ TFOT(Thin Film Oven Test). Long term aged SBS modifi ed asphalt was gotten by 5 h ,163 ℃TFOT and 20 h PVA(Pressure Aging Vessel).

3 Results and discussion

3.1 Normal test of aged SBS modifi ed asphaltSBS modifi ed asphalt and base binder were aged

under short-term aging condition and long-term aging condition respectively. Table 4 shows the normal test results of all unaged and aged samples.

Similar with former studies, Table 4 shows that both softening point and viscosity of base asphalt and SBS modified asphalt increase during the aging process. But the ductility of asphalt declines sharply at

the same time. SBS modifi ed asphalt and base binder display similar aging characteristic in normal test. More work should be done to study the aging of SBS modifi ed asphalt and SBS polymer itself.3.2 FTIR analysis of aged SBS modified

asphaltThe FTIR analysis of aged and unaged SBS

modified asphalt was given in Fig.1. In the figure, horizontal axis is the wavenumber and vertical axis is the transmission rate.

The major band at the frequency of 2 920 cm1 is identified as typical hydrocarbon stretching vibration. C-H bond’s deformation vibration occurs at the frequency of 1 460 cm1. The peak at 1 600 cm1 frequency corresponds to C=C bond in benzene ring and C-H bond’s stretching vibration, and C=C bond in non-benzene ring appears at 966 cm1 peak[7,8]. Peak at 966 cm1 also corresponds to C=C bond in butadiene and peak at 723 cm1 shows the existence of styrene[9-11].

90 Vol.28 No.1 GAO Ying et al: Thermal Oxidative Aging Characterization of SBS Modif...

Fig.1 shows that a new characteristic peak at 1 030 cm1 appears in both short term and long term aging SBS modified asphalt. This identifies the S=O bond in sulfoxide. Characteristic peak at 1 650 cm1 in short term aging SBS modifi ed asphalt and peak at 1 700 cm1 in long term aging sample identifi es C=O bond in carboxyl. This means that oxidative reaction occurred in the aging process of SBS modifi ed asphalt. These newly produced C=O bond in carboxyl is due to the absorption of oxygen in the unsaturated carbon chain and S=O bond is generated by the absorption of oxygen in sulfur element.

The C=O bond enhances obviously with the increase of aging time. Carbonyl is generated on the whole aging process of asphalt. Oxidation occur in the unsaturated carbon chain due to the absorption of oxygen with the generation of C=O bond.

The S=O bond in short term aged asphalt is much more than that in original SBS asphalt. While the bond keeps little difference between short and long term aging asphalt. This means that the generation of sulfoxide mainly occurs in short-term aging stage.

The C=C bond declines with the increase of aging time. This shows that C=C bond has been fractured in the aging process and C=O bond was generated on the thermal oxidation aging condition. Meanwhile, the decrease of C=C content means the declines of butadiene content in SBS which causes the aging of SBS polymer.

Fig.1 shows that the modifying effect of SBS polymer declines severely after long term aging process. 3.3 GPC analysis of aged SBS modified

asphaltThe molecular weight of SBS polymer can

reach 400 000 Daltons, while the molecular weight of macromolecule in asphalt is just 10 000. Thus, SBS polymer and asphalt function groups can be distinguished obviously from the GPC (Gel Permeation Chromatography) chromatogram.

The GPC chromatogram for SBS modifi ed asphalt is shown in Fig.2. In the fi gure, horizontal axis is the elution volume and vertical axis is the refractive index, which can show the distribution of molecular weight.

There are two peaks in GPC chromatogram for SBS modifi ed asphalt. In terms of the principle of GPC, the large molecules are eluted fi rstly. The left peak in the figure indicates the distribution of SBS polymer, and the right one shows that of asphalt functional groups.

The GPC analysis of aged and unaged SBS modified asphalt was done to study the variation of SBS polymer and asphalt phase during aging process. The chromatogram of these samples is shown in Fig.3.

The results of number average molecular weight Mn, weight average molecular weight Mw and dispersion degree d (Mn /Mw) were given in Table 5.

It is generally known that number average molecular weight Mn tends to refl ect the trend of small molecule and weight average molecular weight Mw shows the variation of SBS polymer molecule[12]. Table 5 indicates that number average molecular weight increases gradually during the aging process of SBS modifi ed asphalt. This means that the small molecules of asphalt, such as saturates and aromatics, transform into large molecules, such as resin and asphaltenes. It is shown in Table 5 that weight average molecular weight reduces during aging. This means that large molecules of SBS polymer trend to degrade to smaller one. The shift from smaller molecule to larger one among asphalt compositions occurs and SBS polymer

Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2013 91

is degraded during the aging process.In order to study the variation of SBS polymer in

asphalt during the aging process, the distribution of the molecular weight of SBS polymer was separated from Fig.3. The results of SBS polymer’s number average molecular weight Mn, weight average molecular weight Mw and dispersion degree d were given in Table 6.

Table 6 shows that the molecular weight of SBS polymer in asphalt declines sharply during the aging process. The weight average molecular weight of SBS polymer decreased about 18% under short-term aging condition while it declined by 41% under long-term aging condition. This indicates that SBS polymer in asphalt degrades obviously during the aging process. The molecular weight of SBS polymer after long-term aging was just half of that of the original one. This shows that SBS polymer degrade severely and may lose its modifying function after long time aging.

The dispersion degree of SBS polymer increased during the aging process which means that both degradation and crosslink reaction occurred in SBS polymer.

In o rder to know more about the ag ing characteristics of SBS polymer itself, GPC tests were done on original and long term aging SBS polymer. The results of SBS polymer’s number average molecular weight Mn, weight average molecular weight Mw and dispersion degree d were given in Table 7.

Similarly, the molecular weight of SBS polymer decreased sharply after long term aging. It is only 21% of that of original SBS polymer. The results indicate that SBS polymer degrade as it does in asphalt during aging. But separate SBS polymer shows faster aging than SBS polymer in asphalt.

4 Conclusions

a) The aged SBS modified asphalt shows the increase of viscosity and the decline of deformability.

b) In the thermal oxidative aging process of SBS modified asphalt, C=C bond in SBS polymer is fractured and C=O bond in asphalt is generated due to large absorption of oxygen.

c) The shift from smaller molecule to larger one among asphalt compositions occurs and SBS polymer is degraded during the aging process.

d) SBS polymer degrades severely and may lose its modifying function after long time aging.

References[1] Traxler R N. Relation Between Asphalt Composition and Hardening by

Volatilization and Oxidation[C]. Proceedings, Association of Asphalt

Paving Technologists, 1961, 30:359-377

[2] Herrington P R, Atrick J E, George F A B. Oxidation of Roading

Asphalts[J]. Ind. Eng. Chem. Res., 1994, 33:2 801-2 809

[3] Qian Chun-xiang, Xie Jie-guang, Wang Hong-bo. Anti-aging Capabi-

lity of SBS and SEBS Modified Asphalt and Mixture[J]. Journal of

Southeast University (Natural Science Edition), 2005, 35: 945-949

[4] Lu X, Isacsson U. Modifi cation of Road Bitumens with Thermoplastic

Polymers[J]. Polymer Testing, 2001, 20: 77-86

[5] Ali M F, Siddiqui M N. Changes in Asphalt Chemistry and Durability

during Oxidation and Polymer Modification[J]. Petroleum Sci.

Technol., 2001, 19:1 229-1 249

[6] Martin McCann, Joseph F Rovani, Kenneth P Thomas. Instrumental

Method Suitable for the Detection of Polymers in Asphalt Binders [CD].

Transportation Research Board, No.0907, Washington, D.C., 2008

[7] Maria da Conceicao Cavalcante Lucena, Sandra de Aguiar Soares,

Jorge Barbosa Soares. Characterization and Thermal Behavior of

Polymer Modifi ed Asphalt[J]. Materials Research, 2004, 4:529-534

[8] Codrin Daranga. Characterization of Aged Polymer Modifi ed Cements

for Recycling Purposes[D]. Louisiana: Louisiana State University,

2005

[9] Diego O Larsen, Jose Luis Alessandrini, Alejandra Bosch. Micro-

structural and Rheological Characteristics of SBS-asphalt Blends

During Their Manufacturing[J]. Construction and Building Materials,

2009, 23: 2 769-2 774

[10] Sengoz B, Isikyakar G. Evaluation of Properties and Microstructure

of SBS and EVA Polymer Modified Bitumen[J]. Construction and

Building Materials, 2007, 22:897-905

[11] Wang S, Chang J, Tsiang R. Infrared Studies of Thermal Oxidative

Degradation of Polystyrene-Block-Polybutadiene-block-Polystyrene

Thermoplastic Elastomers[J]. Polymer Degradation and Stability,

1996, 52:51-57

[12] Brule B. Characterization of Bituminous Compounds by Gel

Permeation Chromatography (GPC) [J] . Journal of Liquid

Chromatography, 1979, 2:165-192