EFFECT OF MOLECULAR STRUCTURE AND CONCENTRATION OF STYRENE BUTADIENE STYRENE POLYMER ON THE PERFORMANCE PROPERTIES OF MODIFIED ASPHALT BINDER

Abstract

In the past few decades, there has been a significant increase in traffic volume and load, leading to premature failures in asphalt pavements such as rutting, fatigue cracking, raveling, pothole creation, etc. Additionally, harsher climatic conditions are responsible for the reduced service life of asphalt pavements. The majority of the pavement failures in the asphalt pavements can broadly be classified according to temperature conditions, e.g., upper service temperature failure (40-60 °C) includes rutting, intermediate temperature failures (0-40 °C) includes fatigue cracking and low-temperature failures (-20-0 °C) includes low temperature cracking and thermal cracking. Furthermore, asphalt binders are subjected to elevated temperatures (120-170 °C) during their storage in large heating tanks, transportation, pumping, and pavement construction process such as mixing with aggregates, rolling, lying and compaction, etc. Finding and designing a suitable asphalt binder that can sustain elevated temperature processes and provide a pavement that shows resistance against all types of permanent failures is practically not possible. Therefore, asphalt binders are modified using suitable modifiers to improve the performance properties of flexible pavements. The utilization of polymers for binder modification is among the most widely accepted practice. Among the different polymers available commercially, styrene-butadiene-styrene (SBS) polymer, a thermoplastic elastomer, is considered highly efficient for binder modification due to its ability to impart improved viscoelastic properties to the asphalt pavements with excellent resistance against permanent deformation, fatigue cracking, low-temperature thermal cracking, and moisture-induced damage. SBS is a versatile polymer commercially available in different molecular structures, molecular weights, variable styrene-butadiene (SB) ratios, etc. The Polystyrene (PS) segment of SBS polymer is a hard domain that imparts rigidity to asphalt binder, beneficial at upper service temperature conditions. Polybutadiene (PB) is the rubbery part of the SBS polymer that provides flexibility at low and intermediate service temperatures desirable to prevent low temperature cracking in asphalt pavements. Properties of the SBS modified binders (SBS-MBs) strongly depend on the concentration, molecular structure, and chemical composition of the SBS polymer. A comprehensive comparison of the properties of SBS-MBs as a function of molecular structure (linear, branched, high vinyl, and diblock) and concentrations (1-8 wt.%) over a wide temperature range (0-170 °C) is necessary. For this purpose, four commercial-grade of SBS polymers (linear triblock: L-SBS, branched triblock: B-SBS, linear high vinyl triblock: HV-SBS, and linear diblock: DB-SB) with a similar composition (styrene/butadiene ratio: 30/70) were utilized in this study to evaluate performance properties of SBS-MBs. To cover a wider range of SBS polymer concentrations, binders were modified by the addition of 1-8 wt.% of the polymer at an increment of 1 wt.%. The analysis of the SBS-MBs was divided into three temperature ranges; intermediate and low service temperature (0-40 °C), upper service temperature (40-80 °C), and elevated temperature (120-170 °C), according to temperature experience by the asphalt binder. The characterization of the four pristine SBS polymers, asphalt binders, and SBS-MBs was carried out using respective techniques such as FTIR, NMR, XRD, GPC, DSC, melt rheology, softening point, penetration, Brookfield and capillary, ductility, and rheology. The upper service temperature analysis was carried out between 40-80 °C. SBS-MBs were examined by softening point, viscosity, morphology, and rheological study through temperature sweep from 40-120 °C and frequency sweep (0.01-100 rad/s) at 60 °C. The results demonstrate that the properties of the SBS-MBs are strongly influenced by SBS content > 2.5-3 wt.%, owing to the establishment of a network structure in the modified binder. For the three triblock SBS polymers, merely increasing the SBS content from 3 to 4 wt.%, the softening point (SP) and elastic modulus (at 0.1 rad/s and 60 °C) of the SBS-MBs desirably increases by 15-20 °C and 100-200 Pa, while phase angle (δ) decreases by 20-25°. Compared to linear SBS-MBs, the SP and elastic recovery (ER) of the branched SBS-MBs were higher by 5-10 °C and 5-10 %, while δ was lower by 15-25°. The highest viscosities, softening point, and % elastic recovery of branch SBS-MBs might result in the highest rutting resistance of asphalt mixes and enhance the service life of flexible pavements. On the other hand, linear and high vinyl grades of SBS polymer show quite similar rheological and conventional properties. Since the high vinyl grade of SBS polymer has a double bond in the side chain, it offers resistance against oxidation and provides a thermal stable modified binder. Among the four SBS polymers investigated, the properties of the diblock SBS-MBs were the lowest. Our study suggests that the branched SBS polymer will produce PMBs that perform better at upper pavement design temperature due to higher network density. FTIR spectra of SBS-MBs indicate no shift/change in the peak position of asphalt binder and SBS polymer, suggesting physical interaction instead of chemical interaction. Further, the performance properties of SBS-MBs at elevated pavement construction temperatures (120-170 °C) were evaluated using rotational and oscillatory deformation modes, and results show that combination of radial branches and higher molecular weight of branched SBS polymer results in SBS-MBs with a stronger interconnected network. Compared to the other three SBS polymers, the viscosity of the branched SBS-MBs binders was 2-5 times higher at elevated temperatures, while the phase angle values were lower. Above 3 wt.% of SBS content, the influence of molecular structure becomes more prominent. Hence, at a fixed SBS content, the modified binder with branched SBS polymer needs to be maintained at higher temperatures during pavement construction related to the additional three SBS polymers. Results show that two distinct rheological behavior was observed from the temperature sweep measurements, one between 120 to 140 °C and another above 140 °C based on the response of modified binders. Finally, an intermediate temperature (0-40 °C) analysis of SBS-MBs was carried out of four SBS polymers using 15 mm diameter parallel-plate (PP15) geometry in the DSR instrument to predict its response. The results indicate that asphalt binders play a crucial role at intermediate temperature conditions, and performance properties improve with the increase in the SB polymer concentration. On the other hand, the contribution of four SBS molecular structures becomes less effective at lower temperatures (< 15 °C). In this temperature range, the role of asphalt binder becomes prominent with increased hardening nature. Overall this thesis work comprehensively evaluates the efficacy of SBS polymers with four different molecule structures with the modification of asphalt binder. A wide range of conventional and rheological characterization was carried out. Results indicate that SBS molecular structure significantly affects the upper service (40-80 °C) and elevated temperature (120-170 °C) properties of SBS-MBs. In the intermediate temperature conditions (0-40 °C), the role of the molecular structure of SBS polymer becomes insignificant. This study provides significant findings for the pavement engineers to use appropriate grades and concentrations according to different field conditions.SBS polymer results in SBS-MBs with a stronger interconnected network. Compared to the other three SBS polymers, the viscosity of the branched SBS-MBs binders was 2-5 times higher at elevated temperatures, while the phase angle values were lower. Above 3 wt.% of SBS content, the influence of molecular structure becomes more prominent. Hence, at a fixed SBS content, the modified binder with branched SBS polymer needs to be maintained at higher temperatures during pavement construction related to the additional three SBS polymers. Results show that two distinct rheological behavior was observed from the temperature sweep measurements, one between 120 to 140 °C and another above 140 °C based on the response of modified binders. Finally, an intermediate temperature (0-40 °C) analysis of SBS-MBs was carried out of four SBS polymers using 15 mm diameter parallel-plate (PP15) geometry in the DSR instrument to predict its response. The results indicate that asphalt binders play a crucial role at intermediate temperature conditions, and performance properties improve with the increase in the SB polymer concentration. On the other hand, the contribution of four SBS molecular structures becomes less effective at lower temperatures (< 15 °C). In this temperature range, the role of asphalt binder becomes prominent with increased hardening nature. Overall this thesis work comprehensively evaluates the efficacy of SBS polymers with four different molecule structures with the modification of asphalt binder. A wide range of conventional and rheological characterization was carried out. Results indicate that SBS molecular structure significantly affects the upper service (40-80 °C) and elevated temperature (120-170 °C) properties of SBS-MBs. In the intermediate temperature conditions (0-40 °C), the role of the molecular structure of SBS polymer becomes insignificant. This study provides significant findings for the pavement engineers to use appropriate grades and concentrations according to different field conditions.