V. O. Bulatović* 1, V. Rek 1 and K. J. Marković 2

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1 REVIEW V. O. Bulatović* 1, V. Rek 1 and K. J. Marković 2 Standard methods of characterisation of bitumen (BIT) and polymer modified bitumen (PMB) do not provide sufficient information. We propose to rely on determination of rheological properties which are much more informative. Along theses lines, we have determined rheology before and after the rolling thin film oven test. The materials studied were BIT, BITzlinear (L), styrene butadiene styrene (SBS), block copolymer, BITzradial (R) and SBS copolymer. The rheological properties of the modified binders are characterised using a dynamic shear rheometer, over wide ranges of temperatures at a fixed traffic frequency of 10 rad s 21. The SBS block copolymers increase the elasticity of the BIT at high temperatures and contribute to a better flexibility at low temperatures. s have higher resistance to permanent deformation. After aging, the hardening of BIT and PMBs occurred and the elastic response decreases as a consequence of degradation. The determined rheological properties are in agreement with the results of conventional tests. Keywords:, Rheological properties, DSR, SBS-L, SBS-R, Thermooxidative degradation Introduction Bitumen (BIT) is used in road industry as a binder for asphalt. Roads must withstand changes in temperature in the range of.80uc and handle heavy loads without cracking at low temperatures, and also must provide resistance to the occurrence of permanent deformation of asphalt mixtures at high temperatures. 1 Today s climate in addition to the increases demands that are placed on the BIT as a binder affect the configuration and geometry of the vehicle tire. As BIT has not the temperature resistance for high and low temperatures, it softens in summer and cracks in winter. 2 In order to produce BIT with improved properties, i.e. wider temperature range in use as well as better rheological properties, polymers are used. Polymers with high viscoelasticity, higher creep and recovery under the traffic frequency, enhance BIT resistance to permanent deformation. s (PMBs) are generally considered to provide a prolonged life or enhanced pavement performance of BITs. 2 6 In Europe BIT and PMBs are still defined on conventional tests such as penetration, ring and ball temperature. It is not enough to carry out conventional BIT tests to give an insight into rheological properties. 7 The USA adopted rheological specifications at the end of the Strategic Highway Research Program (SHRP) in the early 1990s, well known as the performance grade. 7 9 The performance grade classification includes two methods: one characterising the binder in the high temperature range, 1 Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, Zagreb, Croatia 2 Institute of Civil Engineering Croatia d.d, J. Rakuše 1, Zagreb, Croatia *Corresponding author, vocelic@fkit.hr the other is for low temperature range. The idea behind this criterion is that too fluid a binder leads a risk of permanent deformation, and that too rigid a binder leads a risk of cracking for the pavement. Within the SHRP, the permanent deformation of BIT and PMB is correlated to viscoelastic functions, shear complex modulus, G* and phase angle d with the equation G*/ sin d. 10 Therefore, BITs with a high complex modulus and a high degree of elasticity produced pavements with a low tendency for permanent deformation. The most commonly used method of rheological testing of BIT and PMB, included in SHRP is the dynamical mechanical method using the oscillatory type testing with dynamic shear rheometers (DSRs). 4,11,12 Polymers with the higher glass transition compared with BIT, thermoplastic, enhance BIT resistance to high temperatures, while polymers with the lower glass transition, elastomers, enhance BIT resistance to low temperatures. Bitumen modified with thermoplastic elastomers and styrenic block copolymers due to their ability to combine both elastic and thermoplastic properties has better temperature resistance to high and low temperatures than BIT. One of the most important modifiers in the styrenic block copolymers is styrene-butadiene-styrene (SBS) block copolymer. 2 6,11,12 The structure of SBS copolymers consists of SBS three block chains, having a twophase morphology of spherical glassy polystyrene block domains within a matrix of rubber polybutadiene. 12,13 The formation of a PMB is based on the dissolution and/ or fine dispersion of polymer in BIT and on the compatibility of the polymer BIT blend. The elastomeric phase of the SBS copolymer swells in the maltene phase of BIT and a continuous polymer network is formed through the PMB, which influences the viscoelastic properties of PMB ß W. S. Maney & Son Ltd Received 30 May 2011; accepted 17 September 2011 DOI / X11Y Materials Research Innovations 2012 VOL 16 NO 1 1

2 Bitumen aging is one of the principal factors causing the deterioration of asphalt pavements. BIT is aging during production, application and their service life. 3 Aging is a very complex process in BIT and the complexity increases when polymer bitumens are involved. In BIT aging oxidation and physical hardening are present. 13 The factors affecting BIT and PMB aging include characteristics of BIT and composition of BIT and PMB, polymer content, structure and phase interactions. 4 The same factors influence the rheological properties and temperature range in use. In this paper, the influence of the content and structures of SBS copolymers, as a BIT polymer modifier on the rheological properties of PMBs in a set range of temperatures under defined traffic frequencies with a DSR, was researched. The rheological properties of unaged PMBs and PMBs after artificial termooxidative aging in the rolling thin film oven test (RTFOT) were determined. Two SBSs were used: one with a linear structure (SBS-L), and the other with a radial structure (SBS-R). Experimental Materials The investigations are carried out with the 70/100 penetration grade base BIT. The source of BIT is INA Refinery Croatia, Rijeka, Croatia. Two SBS block copolymers are used in this study: one is a linear SBS (SBS-L), commercial grade Kraton D 1101, with a content of 31 wt-% polystyrene and the other is a branched block copolymer (SBS-R), commercial grade Kraton D 1184, with a content of 30 wt-% polystyrene, fabricated by the Shell Chemicals Company, Wesseling, Germany. Table 1 Sample SBS-L mark Composition of samples SBS-R mark BIT 70/ 100 wt-% 2L 2R L 3R L 4R L 5R L 7R 93 7 SBS content/ wt-% Five SBS-L PMBs and five SBS-R PMBs are produced with various contents of polymer (Table 1). Sample preparation All the PMBs were prepared using a Silverson L4R mixer of 1500 min 21. First, the base BIT is thoroughly heated (160uC) and stirred for y2 h to obtain homogeneity and is then poured into 1 L aluminium cans. The cans of BIT were then heated to uC and stirred for 10 min before adding the polymer. A given part of SBS was then added slowly into the BIT under high speed stirring for 4 h until the blend became essentially homogenous. A constant temperature was kept while the mixing process continued. After completion, the blends were removed from the aluminium cans and divided into small containers, covered with aluminium foil and stored for testing at ambient temperature. Measurements Conventional tests The base BIT and PMBs were subjected to the following conventional BIT tests according to standards: the penetration test (HRN EN1426), the ring and ball technique to determine the softening point temperature (HRN EN1427), the elastic recovery test (HRN EN13398) and the Fraass breaking point test (HRN EN12593/01). The results of these are listed in Table 2. Furthermore, the penetration index (PI) was calculated from the penetration test and the softening point was determined from the ring and ball test to predict temperature susceptibility. A classical approach to the PI calculation is given in Shell bitumen handbook, which is shown by the following equation PI~1952{500 log (pen 25 ){ 20SP=50 log (pen 25 ){SP{120 (1) where pen 25 is the penetration at 25uC and SP is the softening point temperature of PMB. 16 Rheological measurements Rheological properties, i.e. viscoelastic parameters such as the complex modulus G*, the complex viscosity g*, and the phase angle d, were determined with a DSR, MCR 301, Anton Paar, using the Peltier temperature Table 2 Conventional properties of modified BIT Property %SBS content 2 4 Before RTFOT 0 SBS-L 3 SBS-R 5 7 Penetration 25uC/dmm Ring and ball temperature/uc PI , Fraass breaking point/uc Elastic recovery 25uC/% After RTFOT Penetration 25uC/dmm Ring and ball temperature/uc Change of mass/% Retained penetration/% Fraass breaking point/uc Elastic recovery 25uC/% Materials Research Innovations 2012 VOL 16 NO 1 2

3 control system. A dynamic rheometer is a type of testing equipment which applies sinusoidal, oscillatory loading on a material sample. The tests are conducted over a range of temperatures and loading frequencies in order to provide a complete characterisation of the viscoelastic properties of the binder. The sinusoidally varying shear stress can be expressed as 17,18 t(t)~t 0 sin ð$tzdþ (2) and the resulting strain as c(t)~c 0 sin $t (3) where t 0 is the stress amplitude (Pa), c 0 is the strain amplitude, v is the angular frequency (rad s 21 ), t is the time (s), and d is the phase angle of the measured material response (u). The angular frequency v, also known as the rotational frequency, is defined as $~2pf (4) where f is the frequency (Hz). The sinusoidally varying stress and strain can also be represented by the following complex notation as c ~c 0 e i$t (5) t ~t 0 e i($tzd) (6) The complex shear modulus G* is then defined as G ~ t c ~ t 0 e id (7) c 0 With equation (7) also being written as G ~ t 0 t 0 cos dzi sin d~g zig (8) c 0 c 0 where G* is the complex shear modulus (Pa), G9 is the storage modulus (Pa) related to the energy stored and G0 is the loss modulus (Pa) related to the energy lost at deformation. The in-phase component of G*, or the real part of the complex shear modulus, is defined as G ~G cos d (9) and the out-of-phase component, or the imaginary part of the complex shear modulus as G ~G sin d (10) It can be shown that G9 is the ratio of the stress, in phase with the strain to the strain, whereas G0 is the ratio of the stress, 90u out of phase with the strain to the strain. These two components (G9 and G0) are related to the complex shear modulus and to each other through the phase angle d, which indicates the viscoelastic character of the materials and defined as tan d~ G (11) G If d50u, the material is entirely elastic, and if d590u, completely viscous. Between these two extremes the material behaviour can be considered to be viscoelastic in nature with a combination of viscous and elastic responses. 15,17 The complex shear viscosity g* (Pa s) is defined as 18 g ~ t : c (12) where c : is the shear rate function, the time derivative of the sinusoidal strain (or deformation) function c(t). The DSR tests were performed under controlled strain loading conditions using temperature sweep tests. The temperature sweep was applied in the range 25 to80uc at a fixed traffic frequency of 10 rad s 21 (y85 km h 21 ) and variable strain. Preliminary tests were carried out at different temperatures in order to determine the strain range in which the BIT remains in the linear viscoelastic range (LVN). 19 In this range, the stress response is directly proportional to the strain value and the complex modulus is independent of the strain level. 19 In the terms for defining the LVN range, the chosen strain value is where the complex modulus does not differ by.5% from its initial value. 19,20 The temperature sweep tests for low temperatures between 25 and 30uC were carried out with a parallel plate testing geometry of 8 mm diameter and a 2 mm gap, and for the medium and higher temperatures, the tests were carried out with a parallel plate testing geometry of 25 mm diameter and a 1 mm gap. The sample was put onto the lower plate of the DSR and was heated to flow. Then, the top plate was lowered to come into contact with the sample, and the sample was trimmed. Before the temperature sweep test, all the samples were tempered at 80uC for 10 min and then the measurements started. 21 To provide a more profound insight into the rheological properties, the critical temperature without permanent deformation (rutting) was determined according to the SHRP. 9 The critical temperature is both the temperature at which G*/sin d is equal to or less than 1 kpa at a frequency of 10 rad s 21 (the strain is the constant 10%) before aging and that at which G*/sin d is equal to or less than 2?2 kpa (the strain value is 12%) after aging. 9 The critical temperature was determined automatically by the DSR software. Aging procedure Accelerated thermooxidative aging of base BIT and PMBs was performed using the RTFOT, according to ASTM D2872. The RTFOT procedure simulated the short term aging of BIT and PMBs. The BIT and PMBs were exposed to elevated temperatures to simulate the conditions during the production, mixing and lying of asphalt mixes. Samples of a specific weight were placed into glass containers heated to 163uC for y15 min and then were placed into a rotating oven at 163uC for 85 min with the air on, and with the flowrate of 4 L min 21. All measurements were performed before and after the simulated aging in the laboratory, i.e. before and after the RTFOT. Results and discussion The results of rheological behaviour of the base BIT and the PMBs modified with SBS-L and SBS-R in dependence on temperature are presented in Figs Changes in G*, g* and d in the temperature range from 25 to 80uC under the traffic frequency of 10 rad s 21 are noted. As the temperature increases, the G* of the base BIT and PMBs decreases (Figs. 1 and 3). The decrease is Materials Research Innovations 2012 VOL 16 NO 1 3

4 1 Complex modulus as function of temperature for BIT 70/100 and SBS-L PMB 4 Complex modulus as function of temperature for BIT 70/100 and SBS-R PMB 2 Complex viscosity as function of temperature for BIT 70/100 and SBS-L PMB 5 Complex viscosity as function of temperature for BIT 70/100 and SBS-R PMB 3 Phase angle as function of temperature for BIT 70/100 and SBS-L PMB smaller for the PMBs modified with SBS-L (Fig. 1). The G* and g* values of PMBs are higher than those of the base BIT and increase with the polymer content (Figs. 2 and 4). It means that the polymer gives stiffness to the BIT and act as the filler. 4 However, sometime filler can decrease g* which is connected with the influence of molecular structure on rheology. 22 The formation of elastic plateau is noted on the G*/T and g*/t curves of PMBs with higher polymer contents. 15 It indicates the development of dominant polymer network. 11 The plateaus are more expressed on the curves of PMBs modified with SBS-R (Figs. 4 and 5). It is due to various interactions between SBS-R and BIT. At lower temperatures in the range of 25 to10uc, the PMB values of G* and g* are the same as the pure BIT values. As the 6 Phase angle as function of temperature for BIT 70/100 and SBS-R PMB temperature increases, the increase in G* and g* values of PMBs is more expressed in comparison with the same BIT values. This increase is more expressed in the PMB with the higher polymer content. The changes in the phase angle d related with changes in temperature (Figs. 3 and 6) are more expressed than those in G* in the same temperature range. The base BIT shows predominantly viscous behaviour with increasing temperature and the phase angle approaches 90u. The d/ T curves of PMBs (Figs. 3 and 6) are shifted to lower d values. The PMBs with higher SBS contents have lower d values and the formation of plateau on the d/t curve is noted. This is in agreement with the plateau formation on the G*/T curves of PMBs and indicates the formation Materials Research Innovations 2012 VOL 16 NO 1 4

5 7 Complex modulus as function of temperature for BIT 70/100 and SBS-L PMB after RTFOT 10 Complex modulus as function of temperature for BIT 70/100 and SBS-R PMB after RTFOT 8 Complex viscosity as function of temperature for BIT 70/100 and SBS-L PMB after RTFOT 11 Complex viscosity as function of temperature for BIT 70/100 and SBS-R PMB after RTFOT 9 Phase angle as function of temperature for BIT 70/100 and SBS-L PMB after RTFOT 12 Phase angle as function of temperature for BIT 70/100 and SBS-R PMB after RTFOT Table 3 Critical temperature according to SHRP Temperature where G*/sin d51 kpa before RTFOT and G*/sin d52. 2 kpa after RTFOT/uC %SBS content SBS-L SBS-R Materials Research Innovations 2012 VOL 16 NO 1 5

6 of polymer network as a consequence of the swelling of SBS in the maltene phase in BIT. 14 Lower d and plateau formation mean that PMBs have better elastic behaviour. At the low and higher temperatures the BIT with radial SBSs (Fig. 6) has a reduced phase angle and a more pronounced plateau region than the BIT modified with the same content of SBS-L (Fig. 3). This increase in elastic response of the PMBs at higher temperatures can be attributed to the viscosity of the base BIT being low enough to allow the elastic network of the polymer to influence the mechanical properties of the modified BIT. 4 This is evident for the PMBs with higher polymer contents and is more expressed in the PMB modified with SBS-R. This may be assigned to different compatibilities of SBS-L and SBS-R with BIT. The changes in rheological properties, i.e. in G* and g*, and in d are noted after aging under RTFOT conditions (Figs. 7 12). After aging, the d values of BIT on the d/t curve (Figs. 9 and 12) are evidently lower as well as the d values of PMBs with lower contents of SBS- L and SBS-R. In PMBs with higher SBS contents (5 and 7%), the decrease in d is smaller after aging for both polymers and the elastic plateaus are destroyed to a lesser degree. s aged with SBS-R as a modifier exhibit fewer changes in d after aging. At higher temperatures d values of PMBs increase after aging; this results in less elastic behaviour. Bitumen and PMBs show the increase in G* after aging (Figs. 7 and 10). It means that chemical processes which include degradation reactions as well as secondary processes of cross-linking take place during the aging of BIT and PMBs. As a consequences of aging, the viscosity g* of BIT and PMBs increases (Figs. 8 and 11) and the elastic response decreases (Figs. 9 and 12). In the degradation processes, the polymer network is affected The critical SHRP temperature values for permanent deformation, when the value of G*/sin d>1 kpa before aging, and the value of G*/sin d>2?2 kpa after aging, are presented in Table 3. The critical temperatures of PMBs are higher than that of the base BIT. Polymer modified bitumens with higher polymer contents have higher critical temperatures, which means better temperature resistance to permanent deformation. Better rheological properties and resistance to temperature can be also noted in conventional tests, such as the penetration test, the ring and ball temperature test, and the elastic recovery test (Table 2). The addition of polymers SBS-L and SBS-R decreases the BIT penetration and increases its ring and ball temperature and elastic recovery. The ring and ball temperature increases with the polymer content and this is more pronounced in PMB modified with SBS-R. Aged BIT and PMBs have lower values of penetration and elastic recovery (Table 2) and higher ring and ball temperature. It indicates that hardening occurs during aging, as a consequence of secondary reaction of degradation. 13,26 Elastic recovery decrease is smaller in the SBS-R modified BIT. These changes are in agreement with the changes in rheological properties of BIT and PMBs after aging. Conclusions The rheological properties of road BIT are improved by means of SBS-L and SBS-R polymer modification as proven by both conventional and rheological parameters G*, g* and d obtained by DSR measurements. With the addition of SBS, G* and g* are increased as well as the elastic response. These increases are more pronounced with changes in the temperature and the higher polymer content. Styrene butadiene styrene with a radial structure shows a great degree of polymer modification. After aging, the hardening of BIT and PMBs occurred and the elastic response decreases as a consequence of degradation. These changes are less pronounced in PMBs with a higher polymer content and PMBs modified with SBS-R. Acknowledgement The financial support of the Ministry of Science, Education and Sports of the Republic of Croatia (grant no ) is acknowledged. References 1. U. Isacsson and H. Y. Zeng: Constr. Build. Mater., 1997, 2, X. Lu and U. Isacsson: Constr. Build. Mater., 1997, 11, V. Rek and Z. M. Barjaktarović: Mater. Res. Innov., 2002, 6, X. Lu and U. Isacsson: Polym. Test., 2001, 20, U. Isacsson and X. Lu: J. Mater. Sci., 1999, 34, G. D. Airey: Constr. Build. Mater., 2002, 16, D. Lesueur: Adv. Colloid Interface Sci., 2009, 145, Y. Yildirim: Constr. Build. Mater., 2007, 21, T. W. Kennedy, G. A. Hubert, E. T. Harrigan, R. J. Cominsky, C. S. Hughes, H. L. von Quintus and J. S. 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