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Ind. Eng. Chem. Res. 2006, 45, 3044-3049
Molecular Interactions between Rubber and Asphalt Irena Gawel,*,† Robert Stepkowski,‡ and Franciszek Czechowski§ Faculty of Chemistry, Department of Fuels Chemistry and Technology, Wroclaw UniVersity of Technology, 7/9 Gdanska Street, Wroclaw 50-344, Poland, Rubber Research Institute “Stomil”, 30 Harcerska Street, Piastow 05-820, Poland, and Institute of EnVironment Protection Engineering, Wroclaw UniVersity of Technology, 9 Grunwaldzki Pl., 50-377 Wroclaw, Poland
The analysis of the interactions between rubber and asphalt was carried out. The swelling rate and equilibrium of rubber penetration with the selected asphalt components were found to be dependent on these components and the rubber content in asphalt. At fixed conditions of stirring asphalt with rubber, the equilibrium swell value decreases with the increase in rubber content. From the gel permeation chromatography (GPC) analysis, it follows that the lighter asphalt components penetrate more readily into the internal matrix of the polymer. The gas chromatography-mass spectrometry (GC-MS) study allowed us to assess which components penetrated from asphalt into the rubber and which moved from rubber to asphalt. It has been found that of the nonpolar components, the n-alkanes and n-alkylbenzenes, possess the highest propensity to penetrate into rubber particles. Preferential absorption of the compounds with linear aliphatic chains into the rubber suggests that these components have a good compatibility with the linear polymeric skeleton of the rubber. During immersion of the rubber in hot asphalt, the fatty acids, which are the components of the curing system of the polymer, move from rubber to asphalt, and they are most probably concentrated in the naphthene-aromatic fraction of the asphalt. The electron paramagnetic resonance (EPR) study confirmed the effect of the immersion time of rubber in asphalt and of the shear on the swelling rate. Introduction The amount of waste tires generated in the world is 5 × tons/year which is 2% of the total waste production.1 The disposal of such a large amount of waste tires raises environmental problems since the waste tire stockpiles are burned out of control. Of the several methods for rubber scrap utilization, the modification of asphalt with ground rubber seems to be a very promising application for these wastes. The addition of ground rubber to asphalt brings about an appreciable improvement in the performance-related properties of the binder. Stiffness can be enhanced greatly, leading to improvement in the rutting resistance of mixtures containing asphalt-rubber binders.2,3 Mixtures obtained with asphalt-rubber binder exhibit a higher stability than do similar mixtures containing asphalt cement.4 The benefits of using asphalt-rubber binder also include improved temperature susceptibility,3,5 reduction in the reflective cracking of the pavement, noise reduction,6 and reduced propensity for failure at low pavement temperatures.7,8 Two methods have been used to add crumb rubber into the asphalt-aggregate mix: the dry method and the wet method. In the dry method, rubber is used as an aggregate replacement. The more common method, referred to as the wet process, consists of the addition of the rubber to the asphalt binder. Crumb rubber is mixed with asphalt at elevated temperature (170-220 °C) for 45 to 120 min.9,10 The addition of rubber ranges from several to 20 percent. The optimum addition falls between 3 and 7% of rubber, which is sufficient to produce markedly enhanced properties without raising the high temperature viscosity excessively.2,3 The rheological properties of the 106
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax.: (+48-71) 3221580. † Faculty of Chemistry, Department of Fuels Chemistry and Technology, Wroclaw University of Technology. ‡ Rubber Research Institute “Stomil”. § Institute of Environment Protection Engineering, Wroclaw University of Technology.
asphalt-rubber binder depend on the rubber content, rubber particle size, and chemical composition of asphalt, as well as the temperature and length of mixing time. It is generally believed that the asphalt-rubber interaction is of a physical type. When immersed in hot asphalt, rubber particles absorb the components of similar solubility parameters and swell quickly. Strong cross-links between the elastomer chains prevent the rubber particles from being completely dissolved in liquid asphalt. With the extension of the immersion time, the liquid penetrates into the internal matrix of the polymer and swelling increases. It has been found, however, that swelling follows a linear rate for the first 90 s and then increases at a decreasing rate.11 Rubber particles might swell to 3-5 times their original volume.12 The extent of rubber swelling in asphalt varies according to the temperature and time of rubber-asphalt contact, chemical composition of asphalt, rubber type, and particle size. The chemical nature of asphalt determines the equilibrium swell value whereas the asphalt viscosity determines the rate of swell.13 The rate of swell increases as the viscosity of the liquid decreases. The rate of asphalt penetration throughout the rubber particles increases with the decrease in particle size.13,14 Compatibility of liquid and rubber may be assessed by comparing the solubility parameters of the components. Asphalt components differ in solubility parameters.15 As for the asphalt used in this study, the solubility parameters of the components are the following: saturates δ ) 17.5, aromatics δ ) 18.2, resins δ ) 19.5, asphaltenes δ ) 22.3 MPa1/2. However, for asphalts of a different chemical nature, the solubility parameters of these fractions may take values deviating from those given above.16 The solubility parameter for rubber ranges from 17.8 to 20.8 MPa1/2.17 Rubber from car tires is primarily a combination of styrene-butadiene rubber (δ ) 17.5) and natural rubber (δ ) 16.8).18 Comparing the solubility parameters of asphalt components and rubber, it may be expected that, of the asphalt fractions, saturates are the most compatible with rubber. Stroup-
10.1021/ie050905r CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006
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Gardiner et al. have concluded that asphalt-rubber interactions are a function of the molecular weight of asphalt.13 They have found that lower molecular weight asphalts are more interactive with crumb rubber. The penetration of lighter asphalt components into the rubber may, however, result in the deterioration of the colloidal stability of asphalt binder. If the asphalt is poor in light components and exhibits a high content of asphaltenes, the addition of ground rubber may bring about the coagulation of asphaltenes. To prevent this, use is made of asphalts of a higher penetration grade. Also, the pretreatment of rubber with various organic compounds and their mixes inhibits the penetration of asphalt components into the rubber.13,19,20 The scrap tire rubber used as an asphalt modifier is in a vulcanized state. Swelling accounts for a partial degeneration of the polymer properties.21,22 By using high temperature and high shear, the rubber particles can be devulcanized and depolymerized in the asphalt. It has been found that the devulcanization and depolymerization of rubber is higher when asphalts are rich in aromatics.23 If the rubber is not partially devulcanized, it produces a heterogeneous binder with the rubber acting mainly as a flexible filler.24 However, excessive devulcanization brings about the deterioration of binder properties. An improvement in the elasticity of the binder may be achieved by additional cross-linking of the polymer in the asphalt-rubber binder.25 Little effort has been made to identify the asphalt-rubber interactions, and different data on rubber swelling in asphalt have been reported.11,12 This should be attributed to the change in the solubility parameters of the components with the rise in temperature.25 The study presented in this paper was undertaken to clarify the type of asphalt-rubber interaction. It is expected that the better understanding of the asphalt-rubber interaction will be essential to optimizing the composition of, and the production technology for, asphalt-rubber binders and, thus, to accelerating the acceptance of these materials in road construction. Experimental Section Materials. The asphalt used in this study was 70/100 penetration grade and was produced from Russian crude oil by air-blowing of the vacuum residue. The vacuum residue (penetration 165 × 10-1 mm) from the same crude oil was applied as a softening agent. The ground rubber samples were obtained from scrap car tiretreads. Two fractions of particle size distribution, 0.3-0.7 and 0.7-1.0 mm, were isolated by sieving. The proportion of styrene-butadiene to natural rubber in the crumb rubber was 80:20 wt %. Swelling experiments were carried out with rubber blocks obtained from vulcanized rubber of the same composition as that of the ground rubber. The thickness of the blocks was 0.5 and 0.85 mm, respectively, which is equivalent to the average particle size of the rubber used for asphalt modification. The diameter of the blocks was 25 mm, and the mass averaged to 0.3 and 0.5 g, respectively. Procedures. (A) Samples Preparation. The asphalt-rubber binder was prepared by mixing asphalt, vacuum residue, and rubber at 200 °C. The mixture was processed for 2 h under high shear. The blend comprised 72 wt % of asphalt, 23 wt % of vacuum residue, and 5 wt % of rubber (particle size 0.71.0 mm). The blend was composed so as to obtain a binder of a penetration grade corresponding with that of the base asphalt (70 × 10-1 mm). The proportion of particular components in the binder was determined from the binder penetration-to-
composition plot. To obtain samples for generic composition determination and spectroscopic examinations, the binder obtained was centrifuged at elevated temperature to remove rubber. To evaluate the changes in chemical composition of the asphalt-vacuum residue blend caused by rubber addition, the blend was prepared, where the proportion of the asphalt and vacuum residue was identical to that in the asphalt-vacuum residue-rubber blend. The preparation of the asphalt-vacuum residue blend consisted of stirring the components at 200 °C for 2 h under high shear. (B) Swelling. Tests were carried out at 180 and 200 °C, with rubber contents of 5 and 10% by the mass of asphalt or vacuum residue. The rubber block was immersed in hot asphalt or in the residue. After a given time, the block was taken off the liquid, wiped, and then immersed in toluene for a few seconds in order to remove the persisting asphalt from the block surface. Following evaporation of toluene and weight stabilization, the rubber block was weighed. The mass of the block was measured for 480 min, each time with a fresh rubber block and a fresh asphalt sample. The extent of swelling was calculated as the percent of rubber mass increment after a set time of immersion in the asphalt. Methods of Analysis. (A) Generic Composition. The generic composition of the asphalt-rubber binder (rubber removed) and the asphalt-vacuum residue blend was determined according to the ASTM D4124-91 standard. The procedure involved the precipitation of asphaltenes with a 100fold volume of n-heptane followed by separation of maltenes by liquid chromatography into saturated components, naphthenearomatics and polar-aromatics (resins). (B) Gel Permeation Chromatography (GPC). Binder samples (100 mg) were dissolved in 10 mL of tetrahydrofurane. For separation of the components, use was made of two Plgel 3 µm MiniMix 4.6 mm × 250 mm columns. The chromatographic column was calibrated with a polystyrene standard of a molecular weight ranging from 208-165 000 u. A total of 20 µL of the solution was dosed into the column at a flow rate of 0.3 mL min-1. UV detection (at 350 nm) and also refractometric detection were carried out. (C) Gas Chromatography-Mass Spectrometry (GC-MS). GC-MS was used to analyze the following samples: asphaltrubber binder (rubber removed), toluene extract from rubber, and toluene extract from the rubber separated by centrifugation from the asphalt-rubber binder. This made it possible to determine which of the components present in the toluene extract from the rubber separated from the asphalt-rubber binder penetrated from asphalt-vacuum residue blend into the rubber. To ensure comparable thermal conditions for rubber treatment, the raw crumb rubber was heated at 200 °C for 2 h. GC-MS was used to analyze nonpolar fractions (eluted from the silicagel column with n-hexane) from the above-mentioned samples. Analysis was carried out with an HP5890 II gas chromatograph equipped with a fused silica capillary column (30 m × 0.25 mm i.d.) coated with diphenylpolysiloxane phase (HP-5, 0.25 µm film thickness). Helium was used as the carrier gas. The GC oven was programmed from 35 to 300 °C at a rate of 3 °C min-1. The gas chromatograph was connected to the HP 5971A mass spectrometer detector. The MS was operated with an ion source temperature of 200 °C and an ionization energy of 70 eV. Samples were analyzed using full scan data acquisition (mass range m/z 40-600 with cycle time of 1 s) and selected ion monitoring (SIM) modes. Respective groups of compounds were monitored using selective ions: m/z 71 for n-alkanes; m/z
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Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006 Table 1. Percent of Rubber Swelling in Asphalt and Vacuum Residue (Rubber Size 0.85 mm) temperature (°C) percent of swelling
180
180
200
200
rubber content (wt %) rubber swelling in asphalta (%) rubber swelling in vacuum residuea (%)
5 55 70
10 50 65
5 70 80
10 65 75
a
Swelling is given as the percent of rubber mass increase.
Table 2. Generic Composition of the Asphalt-Vacuum Residue Blend before and after Rubber Immersion fraction content (wt %)
Figure 1. Swelling rate of rubber in asphalt at 200 °C: (4 on solid line) 5% of 0.50 mm thickness rubber in asphalt; (O on broken line) 10% of 0.50 mm thickness rubber in asphalt; (2 on solid line) 5% of 0.85 mm thickness rubber in asphalt; (b on broken line) 10% of 0.85 mm thickness rubber in asphalt.
92 for n-alkylbenzenes; m/z 60 for free fatty acids; m/z 191 for hopanes; m/z 217, 253, and 231 for steranes and their monoaromatic and triaromatic derivatives, respectively. (D) Electron Paramagnetic Resonance (EPR). EPR spectra were obtained with Radiopan SE and Bruker ESP300E spectrometers operating at X-band frequencies at room temperature. The measurements were performed on solid samples placed in 5 mm diameter glass tubes sealed under vacuum. The Bruker ER 035M spectrometer with a 100 kHz magnetic field modulation was equipped with a Bruker NMR gaussmeter and a Hewlett-Packard HP 5350B microwave frequency counter. The Li/LiF standard was used for g-value calibration; 4-hydroxyTEMPO and Reckitt’s ultramarine were applied as spin concentration standards. Use was made of the quantitative EPR technique (microwave power 1 mW, modulation amplitude 1 G, 20.0 mg sample, etc.). EPR analysis was carried out with the asphalt-vacuum residue blend, asphalt-rubber binders, the raw crumb rubber, and the rubber heated at 200 °C for 2 h. To determine the effect of heating time and shear on the asphalt-rubber interactions, the asphalt-vacuum residue and asphalt-vacuum residuerubber blends were prepared by mixing the components at 200 °C for 1 min only. The free radicals that formed in the obtained samples were analyzed by EPR. To evaluate the stability of the free radicals, the samples were analyzed immediately after preparation and after two months of storage at ambient temperature. Results and Discussion Swelling. It can be concluded from the experimental results that the rate and extent of rubber swelling in asphalt and in the vacuum residue depended principally on the rubber particle size. The extent of swelling was approximately twice as high with 0.5 mm thick rubber blocks than with 0.85 mm thick rubber blocks (Figure 1). After 180 min of immersion in hot asphalt or vacuum residue, the cohesion of the thinner rubber samples dropped dramatically so that the procedure of swelling determination had to be discontinued. The rate of swelling of the thicker rubber samples was notably slower; equilibrium was established after approximately 300 min of immersion in hot asphalt (vacuum residue) (Figure 1). It is likely that the longer swelling time of the thicker samples has promoted additional cross-linking of the rubber, thus hindering further swelling.26
fraction
before rubber immers
after rubber immers
saturates naphthene-aromatics polar-aromatics (resins) asphaltenes
12.75 33.88 41.42 11.41
10.52 37.46 39.53 11.89
Table 1 includes the equilibrium swelling values for the 0.85 mm sized rubber block immersed in hot asphalt or vacuum residue. As shown by these data, the extent of swelling is greater when the rubber content in asphalt is lower. This finding suggests that it is not the whole asphalt material but only some of the components, those occurring in comparatively small amounts, that contribute to the swelling of the rubber. The noticeably greater extent of rubber swelling in the vacuum residue than in asphalt (Table 1) under the same conditions demonstrates that primarily lighter fractions are involved. Generic Composition. The generic composition of asphaltvacuum residue blend, determined before and after rubber immersion, is given in Table 2. From the data in Table 2, it follows that the immersion of the rubber in the hot asphalt-vacuum residue blend brought about a decrease in the saturates and the polar-aromatics content. The reduction in the saturates content can be explained in terms of penetration of the components into the internal matrix of the polymer due to the similarity between the solubility parameters of the saturates and the rubber. However, the observed decrease in the polar-aromatic components of the asphalt-vacuum residue blend after its modification with rubber is difficult to rationalize. Darmstadt et al. have reported that polar components from pyrolytic oil, which have fewer alkyl groups than do respective components from petroleum residue, are stronger adsorbed on the carbon black present in the rubber.27 It is not very probable that the polar-aromatic fraction of asphalt underwent adsorption on carbon black since asphaltenes, which generally have shorter alkyl substituents than do polar-aromatics from the same asphalt,28 would have been adsorbed first. We assumed that resins (polar-aromatics) undergo partial polycondensation with the styrene, which forms as a result of degradation of the polymer in hot asphalt, and that the zinc oxide present in the rubber acts as a polycondensation catalyst. The catalytic activity of zinc oxide in the polycondensation of aromatics has been reported by other investigators.29 Further research is needed, however, to support the hypothesis. GPC. The GPC chromatograms of asphalt-vacuum residue blend and asphalt-rubber binder (rubber removed) are shown in Figure 2. The molecular weight distributions have two maximums which provide strong evidence for the existence of two distinct types of asphalt particle aggregates in the solution.30 From the molecular weight distribution of the samples analyzed, it follows that the asphalt-rubber binder (rubber removed) is richer in higher molecular weight components (solid line in Figure 2). The average molecular weights, calculated from the GPC, are 5440 and 6680 for the asphalt-vacuum residue blend
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Figure 2. GPC chromatogram of asphalt-rubber binder (rubber removed) and asphalt-vacuum residue blend (UV detection).
and asphalt-rubber binder (rubber removed), respectively. Comparing the data obtained, it can be concluded that the lighter components of the asphalt-vacuum residue blend penetrated into the internal matrix of the rubber polymer. GC-MS. The molecular composition of the selected types of nonpolar components extracted from the scrap tire rubber heated at 200 °C for 2 h, asphalt-vacuum residue blend, and rubber removed from the hot asphalt-rubber binder containing 10% rubber was analyzed. The comparison of composition of the components extracted made it possible to assess which components penetrated from the asphalt-vacuum residue blend into the rubber polymeric matrix and which moved from the rubber into the asphalt-residue blend. Homological distributions of n-alkanes extracted from rubber, asphalt-rubber blend (rubber removed), and rubber separated from asphalt-rubber blend are shown in Figure 3 (traces A, B, and C, respectively). In the rubber, homologues are present in the range from n-C13 to n-C32; a narrow Gaussian-type elevation in relative abundance of homologues around the most dominant n-C26 can be observed (Figure 3, trace A). As for the n-alkanes from asphalt-rubber binder (rubber removed), their homological distribution is wider, from n-C12 to n-C40 (Figure 3, trace B; homologues of higher molecular weight were also present but could not be analyzed under conditions used by GC-MS). Differences in the relative abundance of n-alkane homologues are not so sharply pronounced as in the rubber. The n-alkanes contained in the rubber separated from the hot asphalt-rubber binder show an intermediate homological composition (Figure 3, trace C). Stirring of the rubber in asphalt-vacuum residue blend at 200 °C for 2 h resulted in penetration of the whole spectrum of n-alkanes from asphalt into the rubber matrix, which is demonstrated by the widening of their homological range and diminishing of the pronounced dominance of homologues around n-C26 (compare traces A and C in Figure 3). It is worth noticing that n-alkylbenzenes present in the asphalt-vacuum residue blend analyzed by GC-MS possess a similar ability to penetrate rubber. The same change in the homological composition as for n-alkanes (Figure 3) is observed for n-alkylbenzenes (not illustrated). The effective penetration of the compounds with linear aliphatic chains into the rubber suggests that these
Figure 3. Mass chromatograms of n-alkanes from (A) rubber, (B) asphaltvacuum residue blend, and (C) rubber separated from asphalt-rubber binder.
components have better skeletal compatibility with the linear polymeric skeleton of the rubber. Changes in the composition of pentacyclic triterpanes (hopanes) in the rubber separated from asphalt-rubber binder (Figure 4, traces A, B, and C) allow us to conclude that also the hopanes contained in the asphaltvacuum residue blend penetrate into the rubber matrix. Namely, extended (long chained C31-C35) hopanes are present in the asphalt-vacuum residue blend (Figure 4, trace B), while they are present in the rubber at relatively small concentration (Figure 4, trace A). Rubber separated from asphalt-rubber binder exhibits the presence of extended hopanes (Figure 4, trace C), which must derive from the asphalt-vacuum residue blend and hence indicate penetration of these components into rubber. The molecular constitution of steranes and their mono- and triaromatic derivatives in the samples analyzed was very similar, and it was difficult to conclude whether they move between rubber and asphalt-vacuum residue blend. In the case of fatty acids, we observed the opposite effect. The myristic (C14), dominant palmitic (C16), and stearic (C18) acids are abundant in the rubber as they are components of the curing system of the polymer, while they are lacking in the asphalt-vacuum residue blend. Processed rubber contains only traces of these compounds, and we suggest that during immersion of the rubber in the hot asphalt-vacuum residue blend these acids penetrate from the rubber matrix into asphalt. During separation of asphalt-rubber binder (rubber removed) into generic components, these acids probably concentrate in the naphthene-aromatic fraction, as evidenced by the increase in the fraction content after modification with rubber. EPR. The crumb rubber has a single line EPR spectrum, which indicates the presence of one type of paramagnetic center. The determined g-parameter value is 2.0027, and it is associated with the existence of an unpaired electron in the condensed polyaromatic system.31 As for the rubber, such structural systems are present in the carbon black used as one of the rubber
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Figure 4. Mass chromatograms of hopanes from (A) rubber, (B) asphaltvacuum residue blend, and (C) rubber separated from asphalt-rubber binder: (Ts) 18R(H)-22,29,30-trisnorhopane; (Tm) 17R(H)-22,29,30-trisnorhopane; (C29NH) norhopane; (C30H) hopane; ((C31-C35)H(S/R)) extended hopanes
and, consequently, reducing the observed number of free radicals. The addition of rubber to the asphalt-vacuum residue blend brought about an approximately 2-fold rise in the amount of free radicals (Table 3, samples 3 and 5 or samples 4 and 6). An increase in free radicals centered on the carbon atom after mixing of asphalt with rubber at an elevated temperature has been reported by Ibrahim and Seehra.32 The penetration of asphalt-vacuum residue blend components into the rubber (also between the sheets of the aromatic crystallites of the carbon black) accounted for the separation of the interacting unpaired electrons, thus increased number of the isolated unpaired electrons. The rise in the free radical concentration also may be due to the catalytic interaction of oxide present in the rubber with the asphalt-vacuum residue blend components. The prolongation of the immersion time of rubber in hot asphalt and shearing forces acting on the asphalt-Vacuum residue blend and asphalt-rubber binder accounted for a 4-fold rise in the concentration of free radicals (Table 3, compare data for samples 3 and 4 and samples 5 and 6). The concentration of free radicals in the samples analyzed, determined after 2 months of storage at room temperature, has risen (doubled) only in the asphalt-rubber binder heated at 200 °C for a short time, i.e., 1 min (sample 5 in Table 3). This may be due to the further slow swelling of the rubber in the asphaltvacuum residue blend during the 2 months of aging, which resulted in a further separation of primarily recombined unpaired electrons in the aromatic structures of the carbon black present in the rubber. The presented aging data on the free radical concentration (Table 3) allowed us to conclude that, in the asphalt-rubber binder heated at 200 °C for 2 h, the swelling of rubber is close to equilibrium.
Table 3. Concentrations of Free Radicals in the Samples Analyzed
no.
sample
1 2
crumb rubber crumb rubber (heated at 200 °C for 2 h) asphalt-vacuum residue blend (heated at 200 °C for 1 min) asphalt-vacuum residue blend (stirred at 200 °C for 2 h) asphalt-rubber binder (heated at 200 °C for 1 min) asphalt-rubber binder (stirred at 200 °C for 2 h)
3 4 5 6
concentration of concentration of free radicals free radicals after two months (spins/g) of storage (spins/g) 0.55 × 1017 0.41 × 1017
0.55 × 1017 0.42 × 1017
0.61 × 1017
0.60 × 1017
2.30 × 1017
2.30 × 1017
1.10 × 1017
2.20 × 1017
4.64 × 1017
4.30 × 1017
components. In the EPR spectrum of asphalt-rubber binder, two signals are present: one associated with the unpaired electron of the vanadyl ion, VO2+, and the other one with the paramagnetic centers in the polyaromatic systems. Vanadyl complexes are common constituents of asphalt (mainly as vanadyl porphyrins), while polyaromatic systems are present in asphalt, as well as in carbon black. Table 3 shows the values of free radical concentrations in the crumb rubber, asphaltvacuum residue blend, and asphalt-rubber binder. From the data presented in Table 3, it can be concluded that heating the rubber crumb brings about a slight decrease in the free radical density. This can be interpreted as the result of rubber heating at 200 °C where some amount of liquid substances was released as volatile compounds (for instance elasticises). This reduced the interspaces between the crystallites that contained free radicals in the aromatic sheets of the carbon black included in the rubber, thus leading to a recombination of the spins of unpaired electrons at the opposite crystal lattices
Conclusions The study confirmed the effect of temperature, shear rate, rubber particle size, and rubber content on the equilibrium swell value of rubber in asphalt. At fixed conditions of stirring asphalt-vacuum residue blend with rubber, the equilibrium swell value decreases with the increase in the rubber content. It is suggested that only the selected asphalt-vacuum residue blend components penetrate throughout the rubber particles. The swelling rate and equilibrium of rubber penetration with the selected asphalt-vacuum residue blend components is a summary effect of these components and the rubber content in asphalt. Based on the results of GPC analysis, it may be concluded that the lighter asphalt-vacuum residue blend components penetrate more readily into the internal matrix of the polymer. It has been found that, of the nonpolar components, the n-alkanes and n-alkylbenzenes possess the highest propensity to penetrate into rubber particles. Preferential absorption of the compounds with linear aliphatic chains into the rubber suggests that these components have good compatibility with the linear polymeric skeleton of the rubber. The substantial decrease in the fatty acid content of the rubber (the component of the curing system of the polymer) after immersion in hot asphalt-vacuum residue blend gives evidence supporting the movement of these components from the rubber into the asphalt. They are most probably concentrated in the naphthene-aromatic fraction, as it may by inferred from the increased content of this fraction in asphalt-rubber binder (rubber removed). The increase in the free radical concentration with prolongation of the rubber immersion time in hot asphalt shows that the
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penetration of asphalt into the rubber particles leads to the separation of interacting unpaired electrons. The rise in the free radical concentration in the asphalt-rubber binder which was not stirred at 200 °C for 2 h, after two months of storage, substantiates further swelling of the rubber which brought about the isolation of the previously recombined unpaired electrons in the aromatic sheets of the carbon black. Literature Cited (1) Galvano, S.; Casu, S.; Casablanca, T.; Calabrese, A.; Cornacchia, G. Pyrolysis Process for the Treatment of Scrap Tyres: Preliminary Experimental Result. Waste Manage. 2002, 22, 917. (2) Daly, W. H.; Mohammad, L. N.; Negulescu, I.; Paul, H. R.; Youngblood, J. Fundamental Properties of Asphalt Rubber Blends. Proceedings of the Third Materials Engineering Conference. New Materials and Methods of Repair, San Diego, 1994; p 425. (3) Beaty, A. N. Latex-Modified Bitumen for Improved Resistance to Brittle Fracture. Highways Transportation 1992, 9, 32. (4) Khedaywi, T. S.; Tamimi, A. R.; Al-Masaeid, H. R.; Khamaiseh, K. Laboratory Investigation of Properties of Asphalt-Rubber Concrete Mixtures. In Transportation Research Record; National Academy Press: Washington, D.C., 1993; Vol. 1417, p 93. (5) Billiter, T. C.; Davison, R. R.; Glover, C. J.; Bullin, J. A. Physical Properties of Asphalt-Rubber Binder. Pet. Sci. Technol. 1997, 15, 205. (6) Way, G. B. Asphalt Rubber-Research and Development 35 Years of Progress and Controversy. Presented at the European Tire and Recycling Association Meeting, Brussels, 2000. (7) Bahia, H. B. Effects of Crumb Rubber Type and Content on Performance Related Properties of Asphalt Binders. Proceedings of the Third Materials Engineering Conference. New Materials and Methods of Repair, San Diego, 1994; p 449. (8) Hui, J. C.; Morrison, G. R.; Hesp, S. A. Improved Low-Temperature Fracture Performance for Rubber-Modified Asphalt Binders. In Transportation Research Record; National Academy Press: Washington, D.C., 1994; Vol. 1436, p 83. (9) Morrison, G. R.; Hesp, S. A. A New Look at Rubber-Modified Asphalt Binders. J. Mater. Sci. 1995, 30, 2584. (10) Coomarasamy, A.; Hesp, S. A. Performance of Scrap Rubber Modified Asphalt Paving Mixes. Rubber World 1998, 218, 26. (11) Joseph, T. M.; Li, Z.; Beatty, C. L. Kinetic of the Sweeling of Ground Rubber Tyre Particles. Proceedings of the 52nd Annual Technical Conference ANTEC 94; San Francisco, 1994; p 1865. (12) Massucco, J. Asphalt Rubber a Federal Perspective. Proceedings of the Third Materials Engineering Conference. New Materials and Methods of Repair, San Diego, 1994; p 467. (13) Stroup-Gardiner, M.; Newcomb, D. E.; Tanquist, B. Asphalt-Rubber Interactions. Transportation Research Record National Academy Press: Washington, D.C., 1993; Vol. 1417, p 99. (14) Gawel, I.; Stepkowski, R. Dobo´r optymalnego sposobu modyfikowania asfaltu gum. Drogownictwo 2003, 6, 181.
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ReceiVed for reView August 3, 2005 ReVised manuscript receiVed January 20, 2006 Accepted March 7, 2006 IE050905R
