Rheological properties of styrene butadiene styrene polymer modified road bitumens☆☆
Introduction
The use of synthetic polymers to modify the performance of conventional bituminous binders dates back to the early 1970s [1], with these binders subsequently having decreased temperature susceptibility, increased cohesion and modified rheological characteristics [2], [3], [4], [5], [6], [7]. Globally, approximately 75% of modified binders can be classified as elastomeric, 15% as plastomeric with the remaining 10% being either rubber or miscellaneously modified [8], [9]. Within the elastomeric group, styrenic block copolymers have shown the greatest potential when blended with bitumen [10]. Other examples of elastomers used in bitumen modification include natural rubber, polybutadiene, polyisoprene, isobutene isoprene copolymer, polychloropren and styrene butadiene rubber.
Styrenic block copolymers, commonly termed thermoplastic rubbers due to their ability to combine both elastic and thermoplastic properties, can be produced by a sequential operation of successive polymerisation of styrene butadiene styrene (SBS) [9]. Alternatively, a di-block precursor can be produced by successive polymerisation of styrene and the mid-block monomer butadiene, followed by a reaction with a coupling agent [11]. Therefore, not only linear copolymers but also multi-armed copolymers (known as star-shaped, radial or branched copolymers) can be produced. The structure of a SBS copolymer therefore consists of SBS tri-block chains, having a two-phase morphology of spherical polystyrene block domains within a matrix of polybutadiene [4], [11].
SBS copolymers derive their strength and elasticity from physical cross-linking of the molecules into a three-dimensional network. The polystyrene end-blocks impart the strength to the polymer while the polybutadiene, rubbery matrix mid-blocks give the material its exceptional elasticity. The effectiveness of these cross-links diminishes rapidly above the glass transition temperature of polystyrene (approximately 100 °C), although the polystyrene domains will reform on cooling restoring the strength and elasticity of the copolymer [4], [11].
When SBS is blended with bitumen, the elastomeric phase of the SBS copolymer absorbs the maltenes (oil fractions) from the bitumen and swells up to nine times its initial volume [4], [12]. At suitable SBS concentrations (commonly 5–7% by mass), a continuous polymer network (phase) is formed throughout the PMB, significantly modifying the bitumen properties. As thermoplastic rubbers have molecular weights similar to or higher than that of the asphaltenes, they compete for the solvency power of the maltene phase and phase separation can occur if insufficient maltenes are available. This phase separation is an indication of the incompatibility of the base bitumen and polymer and care should be taken when blending thermoplastic rubber PMBs. The compatibility of the SBS-bitumen blend can be improved through the addition of aromatic oils. However, too high an aromatic content in the blend will dissolve the polystyrene blocks and destroy the benefits of the SBS copolymer.
Although considerable research has been undertaken in this area, SBS PMBs have still to be comprehensively characterised, due to the complex nature and interaction of the bitumen and polymer system [2], [13]. This paper presents a laboratory evaluation of the rheological characteristics of a number of unaged and laboratory aged base bitumens and SBS PMBs with the primary aim of quantifying the differences in rheological behaviour of the PMBs produced using two compositionally different base bitumens. The SBS PMBs have been produced by laboratory mixing the two base bitumens with a linear SBS copolymer at three polymer contents. The effects of bitumen source, polymer content, bitumen–polymer compatibility and ageing on the rheological (viscoelastic) properties of the PMBs have been determined using conventional rheological binder tests as well as dynamic (oscillatory) mechanical analysis with the data being presented in the form of temperature and frequency dependent rheological parameters.
Section snippets
Materials
Two base bitumens (A and B) from paraffinic (Russian) and naphthenic (non-paraffinic) (Venezuelan) crude sources were used to produce a number of laboratory blended, block copolymer SBS PMBs. Although the two base bitumens have similar consistencies (penetrations of 73 and 81 dmm and softening points of 47.0 and 46.8 °C), they differ in terms of their chemical, fractional composition (saturates, aromatics, resins and asphaltenes (SARA) fractions), as shown in Table 1, and high and low
Conventional binder properties
The effect of SBS polymer modification on the conventional binder properties of the two PMB groups can be seen in Table 3 as a decrease in penetration and an increase in softening point with increasing polymer content. Although the decrease in penetration is relatively uniform with increasing polymer content, there is a significantly larger increase in softening point temperature at the high polymer contents of 5 and 7%. In addition to the increase in hardness (stiffness), the increased
Conclusions
The rheological properties of road bitumens are improved by means of SBS polymer modification as identified by both conventional and more fundamental rheological parameters. The mechanism associated with SBS polymer modification consists of a swelling of the polymer through the absorption of the light fractions of the base bitumen and the establishing of a rubber–elastic network within the modified binder. The nature of the network and its influence on polymer modification is a function of the
Acknowledgements
Some of the research reported in this paper formed part of a Brite Euram research project entitled: ‘Quality Analysis of Polymer Modified Bitumen Products by Microscopic Image Analysis with Fluorescent Light’. The Author acknowledges the work of the Danish Road Institute, Dansk Vejteknologi Ramboll, Jean Lefebvre and Ooms Avenhorn Holdings, partners in this project, in producing the chemical and conventional binder test data presented in this paper.
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