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Characterization of Crumb Rubber Modifiers (CRM) after Dispersion in Asphalt Binders William H. Daly, Sreelatha Balamurugan, Ioan I. Negulescu, Moses Akentuna, Louay Mohammad, Samuel B Cooper, Samuel B Cooper, and Gaylon L. Baumgardner Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03559 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 15, 2019
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Characterization of Crumb Rubber Modifiers (CRM)
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after Dispersion in Asphalt Binders
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William H. Daly*, Sreelatha S Balamurugana, Ioan Negulescub,, Moses Akentunac, Louay
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Mohammadd , Samuel B. Cooper, IIIe, Samuel B. Cooper, Jr.f, and Gaylon L. Baumgardnerg
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* Corresponding
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70803, Email:
[email protected] 9
aResearch
Author, Dept. of Chemistry, Louisiana State University, Baton Rouge, LA
Associate, Dept. of Chemistry, Louisiana State University, Baton Rouge, LA 70803,
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Email:
[email protected] 11
bGrace
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Merchandising, Louisiana State University and LSU AgCenter, Baton Rouge, LA 70803, Email:
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[email protected] 14
cResearch
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Email:
[email protected] 16
dIrma
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Engineering and Louisiana Transportation Research Center, Louisiana State University, Baton
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Rouge, LA 70803, Email:
[email protected], Tel: 225-767-9126.
Drews
Lehmann Distinguished Professor, Department of Textiles, Apparel and
Associate, Louisiana Transportation Research Center, Baton Rouge, LA 70808,
Louise Rush Stewart Distinguished Professor, Dept. of Civil and Environmental
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eMaterials
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Email:
[email protected] 3
fDirector,
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[email protected] 5
gExecutive
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[email protected], Tel: 601-933-3217.
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Research Administrator, Louisiana Transportation Research, Baton Rouge, LA 70808,
Louisiana Transportation Research Center, Baton Rouge, LA 70808, Email:
Vice President, Paragon Technical Services, Inc., Jackson MS 39218, Email:
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Keywords Crumb rubber modified binder, ambient ground crumb rubber, cryogenic ground
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crumb rubber, E-rubber, rubber morphology, thermogravimetric analysis, gel permeation
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chromatography, scanning electron microscopy, Fourier transform infrared, SARA analysis
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Abstract: Blending ground crumb rubber (CR) with asphalt binder is an economical and
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sustainable method of binder modification. The objective of this paper was to evaluate the
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interaction between asphalt binder and the three crumb rubber types at 170 and 190°C. The three
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crumb rubber types are ambiently ground, cryogenically ground, and Ecorphalt (E-rubber), were
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blended with a Louisiana conventional PG 67-22 asphalt at two temperatures, 170°C and 190°C.
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The composition of the crumb rubber before and after treatment with asphalt was studied using
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thermogravimetric analysis (TGA). Gel permeation chromatography (GPC) was used to study
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the molecular weight changes to the asphalt before and after rubber treatment. Fourier Transform
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Infrared (FTIR) Spectroscopy was used to study the aging characteristics of the CR modified
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asphalt binder prepared at the two temperatures. Scanning Electron microscopy (SEM) shows the
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morphology of the rubbers before and after dispersion in asphalt binder. Performance grade tests
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of the CR/asphalt binder blends was used to characterize the rheological properties. E-rubber 2 ACS Paragon Plus Environment
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additive did allow better dispersion of particles in asphalt binder at lower temperature than the
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other additives evaluated. Ground crumb rubber (CR) particles were comprised of favorable
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polyisoprene contents (minimum natural rubber content was 50%) for blending with asphalt
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binder. An increase in blending temperature from 170°C to 190°C resulted in a minimal increase
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in the favorable polyisoprene contents of CR-asphalt binder blends containing 5 and 10 % E-
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rubber which is likely to influence performance. Favorable polyisoprene contents of the CR–
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asphalt blends containing 10% ambiently ground CR did not change with blending temperature
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whereas that of the CR-asphalt blend containing 10% cryogenically ground increased with
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blending temperature. The CR particles isolated from asphalt blends prepared at 190 oC were
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more swollen than those separated from asphalt blends prepared at 170 oC as measured by SEM.
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Approximetely 73-87% of the E-rubber particles dissolved in the asphalt binder during blending
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confirming that E-rubber had great compatibility with the binder chemistry and hence can
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improve performance.
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INTRODUCTION
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Background. Polymer modification of asphalt binder is a common practice to improve physical
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properties and enhance performance. Incorporating polymers in asphalt decreases temperature
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susceptibility enabling asphalt to withstand more load and more severe environments.1 Styrene-
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butadiene polymers, styrene-butadiene rubber (SBR), styrene-butadiene block copolymers (SB)
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and styrene-butadiene-styrene block copolymers (SBS), have been used to modify asphalt to
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improve pavements in the United States for more than four decades. It is estimated that styrene-
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butadiene polymers account for over 90% of the polymers used in asphalt in the United States.2
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Use of synthetic polymers is limited by cost and commercial availability, with recent shortages in supply and increased costs prompting use of alternative modifiers. Blending ground 3 ACS Paragon Plus Environment
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tire rubber (crumb rubber, CR) with the binder is an economical and environmental friendly
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method of recycling waste tires while improving the asphalt’s physical and mechanical
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properties.3 According to the Rubber Manufacturers Association (RMA), there were
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approximately a billion scrap tires in stockpiles throughout the United States. Extensive efforts to
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reduce this waste have born fruit, by 2015, over 93% of those tires have been cleaned up. Only
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67 million more stock- piled tires remain. However, hundreds of millions of waste tires are
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generated annually (RMA). The RMA estimates that approximately 248.8 million light duty tires
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and 30.9 commercial truck tires were generated in 2015. In 2015, markets for scrap tires were
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consuming 3551 thousand tons, 87.9%, of the estimated 4038.8 thousand tons of scrap tires
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generated annually. Only 12.1 % of the tires are sent to landfill. (“U.S. Scrap Tire Management
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Summary” August 2016).
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In the early 1990’s the Federal Highway Administration (FHWA) identified crumb rubber,
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obtained from waste tires, as a priority additive for use in highway pavements.4 Incorporation of
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recycled tire rubber in asphalt pavement eliminates landfilling of solid waste, and the resultant
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asphalt concrete exhibits increased skid resistance under icy conditions, improved flexibility and
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crack resistance, and reduced traffic noise.5 Blending natural rubber with asphalt has been
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practiced as early as 1843. During the 1960’s, researchers and road engineers started blending
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crumb rubber with asphalt (for pavement applications) to reduce the environmental burden of
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their disposal. McQuillen et. al. reported a life-cycle economic analysis that showed a the rubber
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modified asphalt mix is more cost effective than a conventional mix.6
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A typical tire composition by weight is shown in Figure 1. The steel wire and the fabric
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components are removed before the ground tire rubber is dispersed into the asphalt binder. Thus
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the GTR contributes both natural rubber and synthetic rubber, carbon black filler, oil, and other 4 ACS Paragon Plus Environment
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additives including antioxidants antiozonants, and residual accelerators to the binder/ GTR
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blend. These additional ingredients should enhance the properties and the stability of the
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GTR/asphalt blends.
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Figure 1. Typical tire composition7.
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Incorporation of scrap rubber into asphalt requires processing waste tires to produce crumb
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rubber particles (CR). Crumb rubber is produced by either ambient or cryogenic grinding.8
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Ambient grinding typically produces irregularly shaped torn rubber particles with relatively large
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surface areas which promote interaction with the liquid binder. In contrast, cryogenic grinding
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uses liquid nitrogen to freeze the scrap tire before it is shattered with a hammer mill. The
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resultant smooth rubber particles have a lower surface area than ambient ground crumb rubber.
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After removal of synthetic fibers and steel wire in the ground tires, the remaining CR is generally
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comprised of natural rubber (NR) and synthetic rubber (SR); cross-linked with sulfur and 5 ACS Paragon Plus Environment
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reinforced with carbon black. Other additives like processing oils (aromatic hydrocarbons)
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curing catalysts, antioxidants and fatty acids added to improve rubber workability and prevent
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rubber aging, respectively, are also present. NR and SR polymers are like virgin styrene-
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butadiene copolymers (SB) and styrene-butadiene-styrene (SBS) block copolymers currently
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used to modify asphalt binders. A detailed analysis of the CR composition which includes both
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the actual rubber content, as well as the ratio of natural (NR) vs. synthetic (SR) components may
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provide insight on the interaction of the CR with the binders. Therefore, one of the objectives of
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this study was chemical characterization of ambient ground (AMB) and cryogenic ground (CRY)
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CR particles and the changes that occur when these particles are dispersed in asphalt.
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Techniques used to mix CR with asphalt binder include wet, dry, and terminal blending.9 In the
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wet process, an asphalt rubber or CRM asphalt binder is produced.10 The CRM is pre-blended
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with the asphalt binder at high temperature for a finite time and is then mixed with the aggregate.
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In the dry process, CRM is blended with the aggregate before the asphalt binder is added into the
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mix.11, 12 The CRM particles in this process are generally coarser than those in the wet process
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and are considered as part of the aggregate gradation.13 Particle size impacts the efficacy of
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blending and the binder properties.14 In Louisiana the maximum size of rubber particles is 30
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mesh crumb (90-100 percent passing the No. 30 sieve) and a maximum replacement of 10
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percent by weight of asphalt material is specified.15
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Currently there are two primary methods used to analyze tire rubber composition, ASTM D297-
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13, “Standard Methods for Rubber Products Chemical Analysis”16 and ASTM E1131-08,
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“Standard Test Method for Compositional Analysis by Thermogravimetry.”17 D297 is a two part
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collection of general wet chemistry test methods for quantitative and qualitative analysis of the
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composition of rubber products; part A, methods used in determination of some or all of the 6 ACS Paragon Plus Environment
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major constituents of a rubber product while part B covers indirect determination of specific
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polymers present in the rubber product. D297 is an extensive, and relatively involved, process.
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E1131is a simpler empirical technique using TGA where mass loss over specific temperature
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ranges in specific atmospheres provide compositional analysis of that substance.
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Thermogravimetric analysis (TGA). TGA is a simple method used to study the thermal
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degradation of single or multicomponent materials. In this work TGA was used to understand
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CR composition before and after dispersion in asphalt at different temperatures. TGA has been
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used for precise and accurate composition analysis and identification of polymers from their
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decomposition pattern. Lee et al 18 have studied the model ternary blend system of natural rubber
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(NR), styrene-butadiene rubber (SBR) and butadiene rubber(BR) using TGA, along with
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differential scanning calorimetry (DSC), FTIR and pyrolysis GC/MS. Ghavibazoo et al19 used
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TGA to study the changes in CR composition after partial dissolution in asphalt.
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In TGA, the change in mass of a material is measured as a function of temperature or time. TGA
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facilitates acquisition of information on properties of a material and its composition. When a
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sample is heated it often loses mass. Loss of mass may be caused by vaporization or chemical
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reactions which evolve gaseous products from the sample. In material decomposition because of
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chemical reactions, the mass of the sample often changes in a stepwise manner. The temperature
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at which steps occur provides information on the stability of the material in the atmosphere used.
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Typically, this mass change is exhibited in the form of mass loss, however, in cases such as
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oxidation there may be a gain in mass. Composition of a material can be determined by
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analyzing the temperatures and the heights of the individual weight loss steps. By programming
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the temperature heating sequence, the differences between decomposition temperatures
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corresponding to specific polymer decompositions can be elucidated. The potential of TGA for 7 ACS Paragon Plus Environment
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quantitative analysis of rubber compositions based on binary elastomer blends of natural rubber
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(NR) and styrene-butadiene rubber (SBR) has been previously reported.20
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In a multicomponent system, each component will have a specific decomposition temperature
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and a specific amount of decomposing mass, at that temperature.21 In conventional thermal
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analysis, the sample is heated at a pre-determined heating rate to obtain a decomposition pattern.
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Sometimes the thermal decomposition pattern of the different components will overlap, and it
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will be difficult to quantitatively analyze the components. Rubber is an example of a
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multicomponent material, which contains natural rubber, synthetic rubber, oil and fillers.
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Ghavibazoo et al 19, 22-24 have investigated the dissolution pattern of CR in the asphalt as a
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function of mixing temperature, time and mixing speed. Their investigation using TGA has
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shown that the dissolution of CR happens at all interaction conditions and that TGA can
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quantitatively measure the components of the crumb rubber remaining after each treatment. They
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used a method known as Stepwise Isothermal Thermogravimetric (SITG) analysis which
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prevented the overlapping between decomposition temperatures of different components of
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crumb rubber by using a programmed heating rate.
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Most of the reports on the impact of CRM on asphalt binders have focused on the concentration,
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particle size and blending temperature used to prepare asphalt binder/CRM blends5. Limited
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reports examining the relationship of CRM composition to binder properties have been
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published. Using CRM compositional data determined by ASTM D297, Geiger et al. 25
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studied the relationship of composition of different GTR sources, to physical properties of
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GTR modified asphalt binders including: softening point, dynamic viscosity and storage
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stability.25 They reported that a poly(isoprene) (NR) content of least 25 weight percent was
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required to produce suitable quality CRM. Willis et. al examined the effects of particle size , 8 ACS Paragon Plus Environment
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chemical composition, particle surface area, tire type and grinding temperature on the properties
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of the resultant CRM/binder blends.26 Based upon performance grading(PG), multiple stress
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creep recovery(MSCR), cigar tube separation test, and softening point methodologies, these
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workers concluded that CRM particle size was the most influential parameter of the four tests.
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The smaller particle sizes improved the high- and low-temperature PG, particle separation, and
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the softening point. However, among the 12 CRM sources the composition was very similar, i.e.
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extractables, 9 ± 1.4%; rubber, 54.8. ± 3.5%; carbon black, 30.14±1.4%; ash, 5.96±0.3%. No
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effort was made to determine the ratio of NR to SBR in the samples.
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Impact of CRM on binder properties. Blending of CRM using the wet method produces
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asphalt rubber (AR). During the blending process, an interaction occurs as the oils in the CRM
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equilibrate with the maltenes in the asphalt binder and the particles swell.10, 27 The outer sphere
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of the rubber particle is more swollen than the inner sphere because the initial swelling
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accelerates the degradation of the rubber matrix and decreases the crosslink density.28 The
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increased viscosity induced by swelling results in a thicker film coating on the aggregate
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particles in asphalt mixtures. The increase in film thickness provides for a more durable hot mix
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asphalt (HMA) mixture showing increased resistance to oxidative ageing.29 The interaction
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between the CRM and the base binders through exchange of components between asphalt and
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CRM is influenced by the magnitude of light fractions in the binder, the temperature and time
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duration of the blending process, and the CRM production method as well as the chemical
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characteristics of the rubber as described above.27, 30-33 The interaction temperature is the main
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factor affecting the mechanism of dissolution of the CRM particles and consequently defines the
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role of the released components of CRM in asphalt matrix. At 160ºC CRM absorbs aromatics of
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asphalt and swells; however, at 220°C, CRM dissolves into the asphalt; this process leads to the 9 ACS Paragon Plus Environment
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release of different components of CRM including carbon black, fillers, and polymeric and oily
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components. Interactions at 190°C (intermediate interaction temperature), released components
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that are more effective on physical properties of asphalt than those released during interactions at
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220°C (high interaction temperature).34 A schematic illustration of crumb rubber swelling and
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degradation in asphalt is shown in Scheme 1.
At high temp
At low temp
(a) Crumb Rubber (CR)
Natural rubber
(b) Swollen CR, discharging oil and fillers to asphalt Synthetic rubber
(c) Breaking of CR
Oil
Fillers and carbon black
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Scheme 1. Schematic Illustration of Crumb Rubber Swelling and Degradation in Asphalt.
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If components of the CRM dissolved in the binder, evidence for their presence should be readily
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detected using GPC. Gel permeation chromatography (GPC) identified the changes that occur in
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virgin asphalt blends upon the addition of crumb rubber and other additives. The use of GPC is a
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well-established procedure for following these modifications.35-37 GPC provides a quantitative
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distribution of all species present in a binder, such as maltenes, asphaltenes, and polymers.
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Efforts to predict the properties of asphalts using GPC have been reported. Rather than estimate
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the actual molecular weight of the eluting fractions, the GPC chromatograms have been
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arbitrarily divided into three regions: large molecular size (LMS), medium molecular size
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(MMS), and small molecular size (SMS). The LMS and SMS regions are significant with respect 10 ACS Paragon Plus Environment
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to predicting pavement performance.38-42 The consensus of these reports is that the LMS fraction
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of asphalt exhibits the best correlation with the final properties of the blended asphalt binders. In
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lieu of the arbitrary division of the chromatograms into arbitrary regions, we prefer to calibrate
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the GPC chromatograms and identify the maltenes, asphaltenes and polymer components on the
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basis of their apparent molecular weight ranges. The three fractions are: apparent molecular
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weights greater than 19,000 (representing polymers and associated asphaltenes)), apparent
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molecular weights from 19,000 to 3,000 (representing asphaltenes)), and apparent molecular
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weights less than 3000 (representing maltenes). Using apparent molecular weight regions, it is
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possible to divide the LMS fraction into a distribution of molecular species which changes when
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the asphalt ages or is modified. Figure 2 shows the GPC chromatogram of a conventional PG76-
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22M. Quantitative data can be obtained (using Origin software) by determining the area under
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the curve.43 Deconvolution of the GPC chromatogram more precisely determines the
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contributions of the asphalt components embedded in the curve as shown in Figure 2. Both types
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of quantification were used to understand various parameters.
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Energy & Fuels
1000 100 50 20 10
5
2
1 0.5
0.2
73.6% MW 1,000 RI (Relative Units)
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17.8% MW 7,5K 3.4% 0.3% MW 125K MW 185K
4.9% MW 3,150
0
1000 100 50 20 10
5
2
1 0.5
0.2
-3
MW (Daltons x 10 )
1 2
Figure 2. GPC elution curve of a PG76-22M binder containing SBS polymer
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Fourier Transform Infrared Spectroscopy (FTIR). Characterization of oxidative asphalt
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aging with FTIR has been studied extensively in the past few years. The formation of carbonyl
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(C=O) containing molecules¸ which can be identified in the FTIR spectrum, has been correlated
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with standard asphalt binder ageing techniques, RTFO and PAV.44 Carbonyl (C=O) groups
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(esters, ketones, aldehyde, acids etc.) are the major functional groups formed during oxidative
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aging and appear in the region of 1650 cm-1 -1800cm-1. It is well established that the main
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process occurring during this period is the oxidation of asphalt molecules which then leads
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aggregation due to the strongly interacting oxygen containing molecule.45, 46 Since the primary
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aging process is oxidation, quantification of the oxidized species can be related to the extent of
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aging. FTIR spectra of the aged samples exhibit a peak around 1700 cm-1 which is a
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characteristic of a suite of C=O containing species.
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In our previous investigations related to aging of styrene-butadiene-styrene (SBS) copolymer
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modified asphalt cements, we have used FTIR to estimate the degree of oxidation, which is
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directly related to asphalt binder aging.47 The area of the carbonyl absorbance occurring at 1695
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cm-1 increased as compared to that of the C-C absorbance occurring at 1455 cm-1. The ratio of
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the C=O and C-C vibrations (carbonyl index) gave a relative comparison of extent of oxidation.
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As the C=O index increased, there was a higher level of oxidation in the asphalt binder, and a
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stiffening of the binder was observed.47
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Scanning electronic microscopy (SEM). Scanning electron microscopy (SEM) reveals details
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of the rubber–asphalt binder interaction.48 In this study, SEM was used to investigate the
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physical nature of the mixture and whether these crumb rubbers effectively mix with the asphalt
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binder. Scanning electronic and fluorescence microscopy were used to characterize the
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microstructures of crumb rubber particles before and after dispersion of the CRM in asphalt
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binders respectively.49-53 The physical nature of the blends and the extent that CRM with and
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without modifiers effectively mix with the asphalt has been reported.54
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Most of the CR studies reported in the literature use either cryogenic rubber or ambient rubber
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with a few studies comparing the effect of both.54 There are reports on the use of engineered
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rubber products including chemically modified rubber and devulcanized rubber from waste tires
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on the modification of asphalt.55-57. “Devulcanized tire rubber” refers to ground tire rubber from
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used passenger and truck tires, free of fiber and metal, devulcanized with peptizers and binder,
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and made into free flowing pellets comprising 50 to 85% by weight of Rubber Hydrocarbon, 15
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to 30% by weight of Carbon Black, 10 to 15% by weight of softeners, plasticizers and aromatic
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Oil, and 5 to 20% by weight of virgin polymer binder.56 In this work we study the interaction of
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cryogenic, ambient and a processed rubber called Ecorphalt, which is a devulcanised waste tire
23
rubber pressed into a soft pellet. 13 ACS Paragon Plus Environment
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Objective and Scope
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The objective of this paper was to evaluate the interaction between asphalt binder and the three
3
crumb rubber types at 170 and 190°C. Three types of crumb rubbers: ambiently ground,
4
cryogenically ground, and Ecorphalt, were blended with a Louisiana conventional PG 67-22
5
asphalt at two temperatures, 170 and 190°C. The composition of the crumb rubber before and
6
after treatment with asphalt was studied using thermogravimetric analysis (TGA). Gel
7
permeation chromatography (GPC) was used to study the molecular weight changes to the
8
asphalt before and after rubber treatment. Fourier Transform Infrared (FTIR) Spectroscopy was
9
used to study the aging characteristics of the CR modified asphalt binder prepared at the two
10
temperatures. Scanning Electron microscopy (SEM) shows the morphology of the rubbers before
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and after dispersion in asphalt binder. Performance grade tests of the CR/asphalt binder blends
12
was used to characterize the rheological properties.
13
Experimental Section
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Materials. A conventional asphalt binder PG 67-22 (unmodified) meeting Louisiana’s
15
Specification was selected58 (LADOTD, 2016). Three different crumb rubber types, namely,
16
ambient ground crumb rubber (AMB), cryogenic ground crumb rubber (CRYO), and Ecorphalt
17
rubber (E-rubber) were considered in this study. E-rubber is a round free flowing rubber pellet
18
from Full Circle Technologies, LLC. (Pepper Pike, OH) produced by chemical-mechanical
19
processing of waste tire rubber. Also, a styrene-butadiene-styrene (SBS) polymer modified asphalt
20
binder meeting Louisiana’s specification for (LADOTD, 2016) PG 76-22M was evaluated as a
21
control binder.
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Table 1 presents a summary of the asphalt binder and the CR-asphalt binder blend samples used 14 ACS Paragon Plus Environment
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1
in this study. Each crumb rubber type was blended with PG 67-22 at two temperatures, 170°C and
2
190°C. A high shear mixer was used to prepare the blend per manufacturers’ recommendation at
3
a blending period of 45 minutes. Each crumb rubber type was blended with the asphalt binder at
4
10% of the weight of the binder. Further, additional blend was prepared with E-rubber at 5% level.
5
The performance grade of all the asphalt binders was determined according to AASHTO M320,
6
“Standard Specification for Performance-Graded Asphalt Binder.” The results are presented in
7
Table 2.
8
Table 1. Summary of asphalt binder samples Sample ID
9
CRM, %
CRM Type
Blending Blend PG Temp., °C Grade AMB-R, 170 Ambient 170 76-22 ground 10 AMB-R, 190 190 76-22 CRYO-R, 170 Cryogenic 170 76-22 ground 10 CRYO-R, 190 190 82-22 E5-170 170 70-22 E-Rubber 5 E5-190 190 70-22 E10-170 170 76-22 10 E10-190 190 76-22 AMB-R, 170, AMB-R, 190: Ambiently ground crumb rubber modified binder blend prepared at
10
170°C and 190°C, respectively; CRYO-R, 170, CRYO-R, 190: Cryogenically ground crumb
11
rubber modified binder blend prepared at 170°C and 190°C, respectively; E5-170, E5-190: 5%
12
E-Rubber modified binder blend prepared at 170°C and 190°C, respectively; E10-170, E10-190:
13
10% E-Rubber modified binder blend prepared at 170°C and 190°C, respectively.
14
15
16
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Table 2. Performance grading of binder blends Aging Level Unaged Binder
Unaged Binder
RTFO Aged Binder
PAV Aged Binder
Parameter Test Temp. (°C) Viscosity (Pa.S) Test Temp. (°C) |G*| (kPa) δ (°) |G*|/sinδ (kPa) Test Temp. (°C) |G*| (kPa) δ (°) |G*|/sinδ (kPa) Test Temp. (°C) m-value S(MPa) PG Grade
Specification
AMB-R, 170
AMB-R, 190
135
CRYO-R, 170
CRYO-R, 190
135
3.0 Pa.S max.
2.0
2.0
1.9
2.2
NA NA NA
1.54 80.0
76 1.59 78.3
1.69 81.2
82 1.03 81.1
1.00 kPa min.
1.56
1.62
1.71
1.04
NA NA NA
3.23 72.1
76 3.08 71.3
3.68 69.9
82 2.10 71.0
2.20 kPa min.
3.39
3.25
3.91
2.22
NA 0.300 min. 300 MPa max. NA
0.324 122 PG 76-22
0.334 127 PG 76-22
0.314 131 PG 76-22
0.312 123 PG 82-22
-12
2
AMB-R, 170, AMB-R, 190: Ambiently ground crumb rubber modified binder blend prepared at
3
170°C and 190°C, respectively; CRYO-R, 170, CRYO-R, 190: Cryogenically ground crumb
4
rubber modified binder blend prepared at 170°C and 190°C, respectively; Temp.:Temperature;
5
G*: Dynamic shear modulus; δ: Phase angle; S: Creep stiffness; NA: Not applicable; max.:
6
Maximum; min: Minimum; RTFO: Rolling thin-film oven; PAV: Pressure aging vessel
7
8
9
10
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1
Energy & Fuels
Table 2. Performance grading of binder blends (continued) Aging Level
Parameter Specification E5-170 E5-190 E10-170 E10-190 Test Temp. (°C) 135 135 Unaged Binder RV (Pa.S) 3.0 Pa.S max. 0.9 0.9 1.6 1.6 Test Temp. (°C) NA 70 76 |G*| (kPa) NA 1.60 1.87 1.30 1.26 Unaged Binder δ (°) NA 81.2 81.0 79.1 79.4 |G*|/sinδ (kPa) 1.00 kPa min. 1.61 1.89 1.32 1.28 Test Temp. (°C) NA 70 76 |G*| (kPa) NA 3.22 3.50 2.43 2.39 RTFO Aged Binder δ (°) NA 77.9 77.6 76.35 76.4 |G*|/sinδ (kPa) 2.20 kPa min. 3.29 3.58 2.50 2.45 Test Temp. (°C) NA -12 m-value 0.300 min. 0.313 0.313 0.318 0.319 PAV Aged Binder S(MPa) 300 MPa max. 184 195 163 162 PG Grade NA PG 70-22 PG 70-22 PG 76-22 PG 76-22 2
E5-170, E5-190: 5% E-Rubber modified binder blend prepared at 170°C and 190°C,
3
respectively; E10-170, E10-190: 10% E-Rubber modified binder blend prepared at 170°C and
4
190°C, respectively; Temp.:Temperature; G*: Dynamic shear modulus; δ: Phase angle; S: Creep
5
stiffness; NA: Not applicable; max.: Maximum; min: Minimum; RTFO: Rolling thin-film oven;
6
PAV: Pressure aging vessel
7
Determination of soluble components of crumb rubber (CR). One gram of the rubber
8
particles was slurried in 100 mL of THF and stored at room temperature overnight to maximize
9
dissolution of the soluble components. The resultant slurry was filtered using a pre-weighed
10
0.45µ PTFE filter. The insoluble components that were collected in the filter were washed with
11
THF until the filtrate was colorless. The filter was dried in a vacuum oven at 60 ºC and weighed
12
to determine the mass of the insoluble component. The soluble components were estimated by
13
the difference in weight. The filtered solution was used for GPC analysis.
14
Separation of dispersed crumb rubber from the CR-asphalt blend: A CR-asphalt blend
15
(~5g) was slurried in 250 mL tetrahydrofuran (THF) solvent and stirred for 2 hours. The slurry 17 ACS Paragon Plus Environment
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Page 18 of 54
1
was filtered using a number 200 (75 µ) sieve size. The retained crumb rubber particles were
2
thoroughly washed with an excess amount of THF until the filtrate becomes colorless. The
3
extracted rubber particles were dried in a vacuum oven at 60°C for 4 hours to ensure the removal
4
all residual solvent. The weight of the dried product corresponds to the amount of undissolved
5
rubber in the asphalt. The soluble components were estimated by difference. This process yields
6
the THF soluble parts in the blends which include asphalt binder, soluble polymers, and additives
7
present in the crumb rubber, and soluble polymers like SBS added to the blend.
8
Gel Permeation Chromatography (GPC). GPC was performed using an EcoSEC high-
9
performance GPC system (HLC-8320GPC) of Tosoh Corporation, equipped with a differential
10
refractive index detector (RI) and UV detector. A set of four microstyragel columns of pore sizes
11
200 Å, 75 Å (2 columns) and 30 Å from Tosoh Bioscience was used for the analysis.
12
Tetrahydrofuran (THF) at a flow rate of 0.35 mL/ min. was used as the solvent. Columns were
13
calibrated using polystyrene standard mixtures PStQuick B (MW= 5480000, 706000, 96400,
14
10200, and 1000 daltons), PStQuick E (MW= 355000, 37900, 5970, and 1000 daltons), and
15
PStQuick F (MW= 190000, 18100, 2500, and 500 daltons) from Tosoh Bioscience. The asphalt
16
binder in the blend samples were obtained by preparing a 0.5% slurry of asphalt-crumb rubber
17
blend in THF solvent. This slurry was stored at room temperature overnight to maximize
18
dissolution of the soluble components. The resultant slurry was filtered using a 0.45µ PTFE
19
filter, and the filtered solution was directly used for the GPC analysis.
20
Thermogravimetric analysis (TGA). Thermal degradation studies of CR materials were
21
performed on a TA Instruments 2950 thermogravimetric analyzer using 30-50 mg samples
22
(enough sample to eliminate any sample size effect, considering either the fine powder nature of
23
ground rubbers, 30 mesh, or the uniformity in composition. Since the E rubber additive is a 18 ACS Paragon Plus Environment
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rubber-type material compounded with carbon black, the TGA analysis was performed according
2
to the same experimental protocol followed for AMB and CRYO rubbers. The method protocol,
3
adapted from earlier publications59, 60 for determination of natural rubber (NR) and synthetic
4
rubber (SBR) content of ground tire rubbers (CR), was as follows. 1: Select Gas (Nitrogen). 2:
5
Equilibrate at 40°C. 3: Data storage: Off. 4: Isothermal for 3.00 min. 5: Data storage: On. 6:
6
Ramp 20°C/min to 300°C. 7: Isothermal for 20.00 min. 8: Ramp 20°C/min to 550°C. 9:
7
Isothermal for 5.00 min. 10: Ramp 20°C/min to 750°C. The output from the thermal degradation
8
analysis method was presented graphically by plotting the change in mass versus change in time
9
to produce the stepwise TGA thermal degradation curve, TG. A curve representing the
10
derivative (DTG) of the stepwise TG was also plotted. The weight change in steps 6 and 7 (to
11
and at 300°C), comprising the volatiles and acetone extracts, was labeled as volatiles and oils,
12
respectively. In steps 8 and 9, the region of 550°C, thermal degradation of Hydrocarbon Rubber
13
Content (HRC), viz., NR and SBR, occurred and was labeled as such. The weight change to
14
750°C (step 10) was labeled as fillers. Since the analysis was carried out under inert atmosphere
15
(nitrogen), carbon black was a major part of the ash composition. Deconvolution of DTG curves
16
recorded in steps 8 and 9 allowed the determination of NR and SBR content of GTR.
17
Alternatively, the calibration curves developed by59 allowed the application of eq. 1 to estimate
18
the natural rubber content directly from the maximum % weight loss/min DTG point:
19
% NR = [(max DTG)+0.2602)/0.15836]
[1]
20
Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra for the samples was
21
obtained using a Bruker Alpha FT–IR spectrometer (Alpha), which uses a diamond single
22
reflection attenuated total reflectance (ATR). An OPUS 7.2 data collection program was used for
23
the data analysis. The following settings was used for data collection: 32 scans per sample, 19 ACS Paragon Plus Environment
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Page 20 of 54
1
spectral resolution 4 cm−1, and wave number range 4000 – 500 cm−1. Approximately 1%
2
solutions of mix samples were prepared in carbon disulfide (CS2) and filtered using a 0.45 μ
3
filter. A few drops of the solution were coated on the diamond crystal and the solvent was
4
allowed to evaporate. Spectrum was collected after complete evaporation of the solvent.
5
Scanning Electron microscopy (SEM). Crumb rubber particles were examined using a focused
6
beam ionized scanning electron microscopy (FIB/SEM) model FET Quanta 3D FEG.). A small
7
amount of crumb rubber particles was placed on an adhesive tape attached to the sample holder.
8
It was then sputter-coated with gold and was used for the analysis. Magnification ranging from
9
20× to approximately 30,000×, spatial resolution of 50 to 100 nm was obtained.
10
Iatroscan Analysis. Each binder was deasphaltened according to ASTM Method D-3279
11
“Standard Test Method for n-Heptane Insolubles”61 to yield asphaltenes (As) and maltenes which
12
are dissolved in the n-heptane soluble portion. The maltenes were further fractionated on an
13
Iatroscan TH-10 Hydrocarbon Analyzer to yield the composition in saturates (S), aromatics (Ar),
14
and resins (R). n-Pentane was used to elute the saturates, and a 90/10 toluene/chloroform mixture
15
was used to elute the aromatics. The resins were not eluted and remained at the origin.
20 ACS Paragon Plus Environment
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Energy & Fuels
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Asphalt Binder Extraction and Recovery from Mixtures. An aged asphalt mixture was obtained; the
2
details of the mix design are discussed in a related paper.62 Asphalt binders from the long-term aged CR-
3
modified mixtures were extracted with trichloroethylene(TCE) according to AASHTO T164.63 The
4
extracted binders from the mixtures containing trichloroethylene were distilled to a point where most of it
5
was removed and then carbon dioxide gas introduced to remove all traces of trichloroethylene following
6
the procedure described in AASHTO R59.64 Comparable rheology was observed on binders recovered
7
using either TCE or THF to extract the binder.65
8
RESULTS AND DISCUSSION
9
Characterization of crumb rubbers using SEM. Size and morphology of cryogenically and
10
ambiently ground rubber were studied using SEM is shown as Figure 3 (a) and 3(b),
11
respectively. The results show that AMB-R is larger in size (µm to 2 mm) compared to CRYO-R
12
(µm). The main difference between ambiently and cryogenically ground rubber is their shape
13
and morphology. Cryogenically ground rubber particles have a smooth surface because the
14
rubber was cooled below the glass transition to make it brittle, so it shattered like a glass. For a
15
given particle size the resultant particles have less surface area compared to the those produced
16
by the ambient process. The ambiently ground particles are formed by grinding waste tire at
17
room temperature which creates a particle with rough surfaces The SEM of ambient rubber in
18
figure 3(b), clearly shows a very rough surface. Figure 3(c) shows a photograph of E-rubber
19
particles which are soft rubber pellets of ~5mm size.
21 ACS Paragon Plus Environment
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(a)
(b)
Page 22 of 54
(c)
Figure 3. SEM images of (a) CRYO-R (b) AMB-R (c) photo of E-rubber (the coin is placed to show the size of the rubber pellets) 1
2
3
Rubber Particle Composition. The composition of the crumb rubber particles was determined
4
using TGA. The thermogram observed for the cryogenically ground rubber particles (CRYO-R)
5
is shown in Figure 4a. The time/temperature program used for analysis of all rubber samples of
6
this investigation allows an accurate determination of the CR components. The x-axis of the
7
plots is the time of analysis; the corresponding temperature is plotted on the figure. The left axis
8
is the sample weight loss, and the right axis is the derivative (DTG) of the weight loss curve.
9
Comparing Figures 4a and 4b clearly shows that the shape of the DTG curve changes
10
correspondingly with respect to the ratio of NR to SBR. The maxima in the DTG appearing in
11
rubber decomposition region can be correlated to the rubber composition using a calibration
12
curve generated with known binary mixtures of natural rubber and SBR. 59
13
The thermogram can be divided into four sections as labeled: volatiles, oils, rubber, carbon
14
black, and ash. When a sample is heated under nitrogen the volatiles, including water and other
15
organics, evolving at temperatures below 300ºC. The oils trapped in the rubber evolve or
16
decompose around 300ºC. The combination of rubber components decomposes between 32522 ACS Paragon Plus Environment
Page 23 of 54
1
500ºC with the natural rubber decomposition peaking around 420 ºC followed by the SBR
2
peaking around 490 ºC. If the atmosphere is switched to air above 600 ºC, the carbon black will
3
oxidize and evolve as carbon dioxide and the residue is identified as ash. In this work the
4
nitrogen atmosphere was retained through the entire temperature protocol, so the residue is
5
combination of carbon black and ash. Resolution of the rubber components is achieved using the
6
programmed temperature profile shown in the figures. Sample: SB_CA_18_AMB_45 100
WEIGHT (%) TG
5.516% VOLATILES
File: C:...\SB_CA_18_AMB_45.txt Operator: Ju Run Date: 14-Aug-2018 08:22 Instrument: TGA Q50 V6.7 Build 203
MAX. DTG SBR RUBBER
7.726% OILS
80
600
Temperature (°C)
5
MAX. DTG NATURAL RUBBER
48.72% RUBBERS
60
TEMPERATURE 300C
400 3
200
DTG
CARBON BLACK AND ASH
TIME (min) 0
10
20
30
1
1.475% FILLERS
40C 20
7
550C
40
7 8
750C
Deriv. Weight (%/min)
TGA
Size: 12.7830 mg
Weight (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
40
50
(min) Rubber (CRYO-R) Figure 4a. Thermogram of Cryogenically Time Ground
60
70
-1
Universal V4.3A TA Instruments
9
23 ACS Paragon Plus Environment
Energy & Fuels Sample: SB_CA_18_CRYO_45 Size: 10.8150 mg
100
File: C:...\SB_CA_18_CRYO_45.txt Operator: Ju Run Date: 13-Aug-2018 15:46 Instrument: TGA Q50 V6.7 Build 203
TGA
TG (WEIGHT %)
12
750C
2.537% VOLATILES 6.894%OILS
10
NR 80
600
550C
8
RUBBERS
60
55.86% TEMPERATURE 300C
40
6
4 300
FILLERS 1.252%
DTG
20
Deriv. Weight (%/min)
Temperature (°C)
SBR Weight (%)
2
0
40C
1 2
0
0
ASH 33.457%
TIME (min) 10
20
30
40
50
60
Time (min) Figure 4b. Thermogram of Ambiently Ground Rubber (AMB-R)
70
-2
Universal V4.3A TA Instruments
12
NR ORIGINAL
NR THF EXTRACTED 10
8
6
4
Deriv. Weight (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 54
2
DTG 0
TIME (min)
3
25
35
45
55
Time (min)
-2 Universal V4.3A TA Instruments
4
Figure 4c. Derivatives of thermograms showing the ground rubber decomposition domain of
5
ambient ground rubber before after THF extraction
6
An expanded plot of the rubber decomposition range showing the derivatives (DTG) of the
7
thermograms of the original ambient ground CR and the recovered CR of the sample after 24 ACS Paragon Plus Environment
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Energy & Fuels
1
extracting it with THF is shown in Figure 4c. The maxima of the peak in the DTG curves
2
associated with the NR fraction is used to calculate the percentage of NR in the rubber mixture
3
using the calibration eq. 1. In Table 3, the total % rubber is shown on the top line of each entry,
4
and the relative % of natural rubber and % of styrene-butadiene rubber is shown on the line
5
directly below the total rubber concentration. The original ambient ground rubber contained
6
48.73% rubber with a composition of 50% NR and 50% SBR. The soluble components content
7
was estimated gravimetrically by extracting the particles with THF at room temperature
8
overnight. If the volatiles and oils determined by TGA are directly related to the THF solubles
9
fraction, only a fraction of the solubles and oils observed by TGA are actually removed by the
10
THF extraction. The residual oily components remained trapped in the crosslinked rubber matrix
11
because the THF did not penetrate the internal regions of the rubber particles.
12
Subjecting the particles to THF extraction removes all the accessible extractables and changes
13
the NR/SBR ratio in the residual particles because SBR is more soluble in THF and is selectively
14
extracted. Thus, for the AMB-THF entry, the total rubber content increases to 53.9% and the
15
SBR contribution to the insoluble CR particles decreased from 50% to 37%.
16
Similar thermograms were observed with all ground particles and E-rubber samples separated
17
from mixtures. The CYRO samples contained 55.96% rubber with a NR content of 65%. The
18
higher NR content should make the CYRO samples more compatible with asphalt binders.25 This
19
sample shows clearly the selective extraction of SBR in THF; the composition of particles
20
recovered from the THF slurry was 84% NR and only 16% SBR.
21
TGA analysis confirms that Ecorphalt (E-rubber) has a higher percentage of rubber components
22
than either cryogenically or ambiently ground CR. E-rubber is soft pellet, produced by the 25 ACS Paragon Plus Environment
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Page 26 of 54
1
devulcanization followed by processing, which compresses the powdered rubber into pellets.
2
Cryogenically ground CR contains a higher a rubber component than ambiently ground CR.
3
Cryogenic processing does not expose the rubber to any heat leading to less degradation of
4
rubber. During the cryogenic grinding, almost all fiber and steel detach from the crumb rubber
5
resulting in a cleaner product with little loss of rubber.66 In ambient grinding, the process
6
generates a significant amount of heat, which can degrade the rubber. The fiber and steel are
7
removed by air separation and magnetic separation, which give a fairly clean material but still
8
may contain 0.5% fiber and 0.15 steel.66
9
The initial composition of E-polymer as shown in Table 3 indicated that 7.36% volatiles and oils
10
were present along with 64.75% rubber. The THF extraction of E-Rubber was unique in that
11
more than the TGA estimated solubles (18.3%) were dissolved. The initial particles contained
12
64.6% rubber with a composition ratio of NR/SBR equal to 51/49. In this case, the modification
13
of natural rubber substantially increased its solubility in THF; the recovered particles contained
14
59.7% rubber with a NR/SBR equal to 31/69. The modification was successful in making the
15
isoprene (NR) content more accessible to solvent and presumably also to an asphalt binder.
16
Table 3. Composition of crumb rubber particles before and after extraction with THF Sample ID
% Dissolved In THF
% Volatiles and Oils
% Rubber Composition % NAT
AMB-R
13.27
48.73 50
AMB-R, THF CRYO-R
5.40
CRYO-R
6.69
6.36
50 53.93
63 9.43
39.71 37
55.96 65
5.06
%SBR
% Carbon Black, Ash and Fillers 38.01
34.61 35
59.03
35.91 26
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Energy & Fuels
THF E-Rubber
84 7.20
16 64.61
51
34.87 49
1
E-Rubber, 18.32 5.37 59.71 34.92 THF 31 69 AMB-R: Ambient ground CRM; AMB-R, THF: Ambiently ground CRM extracted from THF;
2
CRYO-R: Cryogenically ground CRM; CRYO-R, THF: Cryogenically ground CRM extracted
3
from THF; E-Rubber: Ecorphalt CRM; E-Rubber, THF: Ecorphalt CRM extracted from THF;
4
THF: Tetrahydrofuran
5
Characterization of crumb rubber particles after interaction with base asphalt binder. The
6
aim is to characterize the changes in the different crumb rubbers particles when they are blended
7
with asphalt at a high shear and at two different temperatures. The dissolution pattern of crumb
8
rubbers at 170°C and 190°C may impact the performance characteristic of the CR modified
9
binder. A compilation of asphalt binder samples and crumb rubbers is shown in Figure 5.
10
Asphalt was blended with each crumb rubber (10%) at two different temperatures at shear rate of
11
3600 rpm for 45 minutes.
12
Figure 5 shows the percentage of different components present in each rubber particle before and
13
after dispersion in asphalt binder at both 170°C and 190°C. At both temperatures, dissolution of
14
some rubber occurred; the amount of dissolution increased as the temperature was increased
15
from 170°C to 190°C. A significant amount of rubber was dissolved in the case of E-rubber at
16
170°C. Since E-rubber is a processed product, the pellets disintegrate easily under the high shear
17
rate stirring (3600 rpm) at either 170°C or 190°C. The resultant fine particles are well dispersed
18
and impossible to recover with a 200 mesh screen.
27 ACS Paragon Plus Environment
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Page 28 of 54
1
2
A
A1
A2
C
C1
C2
E
E1 E2
3
Figure 5. Composition of crumb rubbers A-ambient, C-cryogenic and E- rubbers before and
4
after interaction with asphalt binder. A, initial AMB-R; A1,and A2 AMB-R dispersed in asphalt
5
at170°C and 190°C respectively; C, initial CRYO-R, C1and C2 CRYO-R dispersed in asphalt
6
at170°C and 190°C respectively; E, initial E-rubber; E1andE2, E-rubber dispersed in asphalt
7
at170°C and 190°C respectively.
8
Characterization of CRM after dispersion in asphalt using SEM. After interaction with asphalt at two
9
different temperatures, the undissolved crumb particles were separated by filtration, washing and drying.
10
More evidence about the structure and morphology of the crumb rubbers after treatments with asphalt was
11
obtained using a scanning electron microscope (SEM). Earlier studies28, 67 on the interaction of crumb
12
rubbers with asphalt indicated that two of the important factors influencing final properties of modified
13
asphalt are the extent of CR crumb rubber swelling and the presence of swollen rubber particles. Crumb
28 ACS Paragon Plus Environment
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1
particles will lose some components to the asphalt depending on the different mixing parameters, but the
2
presence swollen particles is an important factor because they can serve as elastic fillers in asphalt, which
3
play a dominant role in defining the final properties.31, 32 Abdelrahman et.al. noted that the stiffness of the
4
asphalt increased through the swelling of crumb particles.68 The stiffness decreased when crumb rubber
5
start dissolving in to the asphalt. Higher temperatures, mixing speeds and longer interaction times can
6
devulcanize and depolymerize the rubber to produce low molecular weight molecules which cannot
7
participate in a reinforcing network structure.
8
The morphology of asphalt extracted crumb rubbers is shown in Figure 6. SEM images of the recovered
9
cryogenic and ambient rubbers shows that the main structure of the ambient and cryogenic crumb has not
10
changed much even if some part of it is extracted by the asphalt. However, the particles recovered from
11
190ºC blends are clearly more swollen than those from 170ºC blends. Since the particles retained their
12
structure they can serve as elastic fillers.
13
During interaction with asphalt, E-rubber completely lost its pellet nature and became very small particles
14
and only ~25 % of these particles were retained on the 200 mesh sieve. Whereas ambiently and
15
cryogenically ground rubbers retained their shape at 170°C, and become larger at 190°C, the residual
16
associated rubber particles in E-rubber disintegrated further at the higher temperature. Most of the E-
17
rubber dissolved in the asphalt during its interaction at 170°C. The disintegration of pellets of E-rubber is
18
promoted by the devulcanization of rubber during the E-rubber production process. The corresponding
19
depolymerization of the rubber matrix allows more complete dissolution of polyisoprene fragments in the
20
binder. The properties of E-rubber blends are controlled by the presence of the soluble components in the
21
asphalt. The TGA and SEM analysis clearly shows that the mode of interaction of E-rubber with asphalt
22
binder is completely different from that of ambiently and cryogenically ground CR
29 ACS Paragon Plus Environment
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b
a
c
b1
a1
Page 30 of 54
c1
1 2
Figure 6 SEM images of crumb rubbers obtained after interaction with asphalt.
3
Images. a and a1, CRYO-R dispersed at 170°C and 190°C respectively; b andb1, AMB-R
4
dispersed at 170°C and 190°C respectively; c and c1, the E- rubber dispersed at 170°C and
5
190°C respectively
6
Characterization of crumb rubber extracts using GPC. A GPC chromatogram of the soluble
7
fractions of cryogenic, ambient and E-rubber in tetrahydrofuran (THF) solvent is shown in figure
8
7. These soluble components of the rubber particles consist of the low molecular weight(LMW)
9
additives such as curing catalysts, processing oils, antioxidants and fatty acids27. These additives
10
appear in the apparent molecular weight species below 1000 daltons. Quantification of the
11
extracted solubles in the GPC chromatograms in figure 7 appear in Table 4. The extracts from
12
ambiently or cryogenically ground CR are composed predominantly of LMW species. In
13
contrast, the E-rubber extract is dominated by moderate molecular weight(MMW) to high
14
molecular weight(HMW) fractions. 30 ACS Paragon Plus Environment
Page 31 of 54
E-rubber Cryogenic Ambient
RI response
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Energy & Fuels
1000000
100000
10000
1000
100
Molecular Weight 1 2
Figure 7. GPC chromatograms of THF extracts of CR materials.
3
The natural and synthetic rubber molecules present in the crumb rubber, which are either not
4
cross-linked to the network matrix or are partially degraded by the grinding process, can be
5
extracted. Factions appear in the MMW-HMW ranges amount to 5.4% ambiently ground rubber,
6
6.7% of cryogenically ground rubber and 18.3 % E-rubber. This MMW-HMW fraction had a
7
broad molecular weight distribution, with a mean MW of 20,000 (AMB-R and E-Rubber) to
8
30,000 (CRYO-R) daltons (Figure 7). These soluble molecules should be dispersed in the binder
9
phase of the CRM binder blends and be detected in the binder extracted from these mixtures.
10
Molecules in this average apparent molecular weight range would contribute to the asphaltene
11
fraction. These molecules are not large enough to enhance the elasticity of the blend unless they
12
can be re-cross-linked during the blending process with sulfur or reactive polymer additives.
13
14
31 ACS Paragon Plus Environment
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1
Page 32 of 54
Table 4. Composition of THF soluble extracts from CR particles GPC analysis data (%) Rubber
THF Soluble Fraction (%)
High MW 100K-45K
45K-3K
>500K daltons. Crosslinking to insoluble polymer
9
matrices, which cannot be extracted from the mixtures may explain the failure to detect HMW
10
components in the extracts from the other mixtures.
50
500
20 10
5
2
1
100
1000
20
10
0.5 0.2 0.1
MW 950 63.19%
30
RI (Relative Units)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
MW 9,240 MW 3,690 16.15% 11.19% MW 22,950 8.98% MW >>500K 0.49%
0 1000
100
500
50
20 10
5
2
1 0.5 0.2 0.1
-3
10 MW (Dalton) 11 12
Figure 9. Molecular weight distribution for binder extracted from long-term aged CRYO-190
13
CRM-binder blend mixture.
39 ACS Paragon Plus Environment
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1
Page 40 of 54
Table 7. Distribution of asphalt species in binders extracted from long-term aged mixtures Sample ID AMB-R 170 AMB-R 190 CRYO-R 170 CRYO-R 190 E5-170
GPC Analysis Asphaltenes Maltenes MW % MW (%) 7,650 19.31 955 66.68 3,115 6.31 17,650 10.05 970 65.30 7,050 17.47 3,155 7.18 22,820 10.60 955 61.12 9,190 12.32 2,725 15.16 22,950 8.98 950 63.19 9,240 11.19 3,690 16.15 13,000 10.38 840 59.26 3,500 30.36
E5-190
13,000 3,800
15.52 20.66
910
63.82
E10-170
33,500 13,200 4,050 15,000 3,700
2.50 12.24 21.90 16.26 23.92
950
63.36
900
59.82
E10-190
2
Sample designations defined in Table 4
3
FTIR analysis of the samples. The blends were further analyzed by FTIR to determine if aging
4
occurred during the blending process at different temperatures. FTIR is a spectroscopic method
5
used to identify the chemical functional groups present in the sample. Carbonyl (C=O) groups
6
(esters, ketones, aldehyde, acids etc) are the major functional groups formed during oxidative
7
aging and appear in the region of 1650 cm-1 -1800cm-1. Negulescu et al72 studied the aging of
8
polymer modified asphalt binder using FTIR. They observed that the area of the carbonyl
9
absorbance peak, occurring at 1700 cm-1, increased compared to that of the C-C absorbance
40 ACS Paragon Plus Environment
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Energy & Fuels
1
occurring at 1455 cm-1. The carbonyl index is a ratio of the C=O and C-C absorbance provides
2
an indication of the extent oxidation and indirectly reflects the extent of sample aging. The
3
carbonyl index (CI) was calculated from the band areas measured from valley to valley 73, by Eq.
4
3.
𝐶𝑎𝑟𝑏𝑜𝑛𝑦𝑙 𝐼𝑛𝑑𝑒𝑥 =
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑎𝑟𝑏𝑜𝑛𝑦𝑙 𝑐𝑒𝑛𝑡𝑒𝑟𝑒𝑑 𝑎𝑟𝑜𝑢𝑛𝑑 1700 𝑐𝑚 ―1 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑡𝑟𝑎𝑙 𝑏𝑎𝑛𝑑𝑠 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 1490 𝑎𝑛𝑑 1320 𝑐𝑚 ―1
(3)
5
FTIR analysis of the asphalt before and after interaction with crumb rubbers showed no signs of
6
aging as there was no increase in the carbonyl peak of theses samples. No peak at 1700 cm-1
7
appears regardless of the blending temperature of the ambiently ground CR blends. Similar
8
results were observed with samples treated with cryogenically ground CR and E-rubber. FTIR of
9
extracts from long term aged mixtures contained a weak carbonyl peak and a carbonyl index was
10
calculated and reported in Table 8. Generally, the extent of oxidation of the CR blends was
11
comparable to that observed with the extract from a mixture prepared with a conventional PG67-
12
22. Mixtures containing CRYO-R did exhibit a slightly greater extent of oxidation. These
13
results suggest that there is minimal impact of crumb rubber on the aging characteristics of the
14
mixtures.
15
16
17
18
41 ACS Paragon Plus Environment
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Page 42 of 54
1
Table 8. Carbonyl index of binders and mixtures
2
Sample name Binder CI Mixture Extract CI PG67-22 0 0.0727 AMB-R, 170 0 0.0712 AMB-R, 190 0 0.0950 CRYO-R, 170 0 0.1131 CRYO-R, 190 0 0.1116 E5-170 0 0.0756 E5-190 0 0.0695 E10-170 0 0.0983 E10-190 0 0.0913 CI: Carbonyl Index; PG-7-22, Asphalt binder extracted from aged mixture; AMB-R,170, AMB-
3
R,190: Asphalt binder extracted from aged AMB-R mixtures; CRYO-R, 170, CRYO-R, 190:
4
Asphalt binder extracted from aged cryogenically ground CRM mixtures respectively; E5-170,
5
E5-190: 5% E-Rubber modified binder extracted from aged mixtures; E10-170, E10-190: 10%
6
E-Rubber modified binder extracted from aged mixtures;
7
Conclusion
8
This study evaluated the impact of crumb rubber modifier type and blending temperature on
9
asphalt binders. Three types of crumb rubber: ambiently ground, cryogenically ground, and
10
Ecorphalt rubber (free flowing rubber pellets) were blended with a base asphalt binder meeting
11
Louisiana specification for PG 67-22 (LADOTD, 2016) at 170º and 190ºC. Each asphalt binder
12
blend was used into a 12.5mm Level 2 asphalt mixture design. Asphalt binder rheology was
13
conducted to determine the performance grade for each asphalt binder/CR blend. Five chemical
14
characterization procedures: gel permeation chromatography, thermogravimetric analysis,
15
Iatroscan analysis, Fourier transform infrared, and scanning electron microscopy were used to
16
characterize the binder and the CR particles respectively.
17
Specific observation included the following: 42 ACS Paragon Plus Environment
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1
Energy & Fuels
temperature than the other additives evaluated.
2 3
Ground crumb rubber (CR) particles were comprised of favorable polyisoprene contents (minimum natural rubber content was 50%) for blending with asphalt binder.
4 5
E-rubber additive did allow better dispersion of particles in asphalt binder at lower
An increase in blending temperature from 170°C to 190°C resulted in a minimal increase
6
in the favorable polyisoprene contents of CR-asphalt binder blends containing 5 and 10
7
% E-rubber (E5-R E10-R) which is likely to influence the intermediate temperature
8
performance.
9
Favorable polyisoprene contents of the CR–asphalt blends containing 10% ambiently
10
ground CR (AMB-R) did not change with blending temperature whereas that of the CR-
11
asphalt blend containing 10% cryogenically ground CR (CRYO-R) increased with
12
blending temperature.
13
The selective dissolution of natural rubber component in asphalt binder was estimated by
14
the change in the natural rubber/styrene butadiene ratio using thermogravimetric analysis
15
of particles recovered from dispersions of CR prepared at either 170ºor 190ºC. In the case
16
of the cryogenically ground CR, the NR/SBR ratio changed from 65/35 to 35/65.
17
The changes in the morphology of the CR particles was examined using scanning
18
electron microscopy. Particles isolated from asphalt blends prepared at 190 oC were more
19
swollen than those isolated from asphalt blends prepared at 170 oC
20
Tetrahydrofuran(THF) extracts from either ambient or cryogenic ground crumb rubber
21
contained less than 7 % of soluble species. Only 70 wt.% of CR particles were recovered
22
after dispersion of 10 wt.% of either ambient ground or cryogenic ground crumb rubber
23
in asphalt binder at 190oC. The particle weight loss was assumed to have dissolved in the 43 ACS Paragon Plus Environment
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asphalt binder.
1 2
Tetrahydrofuran extracts from E-rubber contained 18% soluble species, but dispersion of
3
either 5% or 10 wt.% E-rubber in asphalt binder at 190oC reduced the amount of
4
recovered CR particles to 13-27 wt. %
5
Page 44 of 54
Approximetely 73-87% of the E-rubber particles dissolved in the asphalt binder during
6
blending confirming that E-rubber had great compatibility with the binder chemistry and
7
hence can improve performance.
44 ACS Paragon Plus Environment
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Energy & Fuels
1
AUTHOR INFORMATION
2
Corresponding Author
3
*Telephone: 225-802-5790, E-mail:
[email protected] 4
Acknowledgments
5
The research work reported in this paper was supported by LADOTD through the Louisiana
6
Transportation Research Center under contract Project Number 15-1B. The authors would like to
7
express their appreciation to all those who provided valuable help in the conduct of this project.
8
45 ACS Paragon Plus Environment
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Page 46 of 54
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