11. Descripción del Manual
11.3. Oficina de Gestión de Proyectos
11.3.1. Tipos de Oficina de Proyectos
fractions of the binder. The authors concluded that the four rejuvenators that were tested only really worked on the outermost layers of the RAP. Those layers were tested to be closest to the AC-20 penetration grade binder in terms of the penetration test. The authors also note that the innermost layers were not significantly affected by the aging process.
1.4.3 Sequential Dissolution
Asphalt binder is commonly divided into different fractions. Petersen (2009) provided an insightful literature review on the oxidation characteristics of asphalt binders. In doing so he highlighted the division of asphalt into four fractions known as the Corbett fractions (Corbett 1969). These four fractions are saturates, naphthene aromatics, polar aromatics, and asphaltenes, each respectively increasing in molecular polarity. Each of these fractions reacts differently when introduced to an environment where oxidation can take place.
A more basic fractionation of the asphalt binder is its separation into two fractions, asphaltenes and maltenes. In this case, the maltene component is all non-asphaltene related fractions, which essentially consolidates three of the four Corbett fractions (Petersen 2009). Asphaltenes and maltenes are easily separated into fractions based on solubility in heptane. The n-heptane-soluble (C7H16) fraction is the maltene and n-heptane-insoluble fraction is the asphaltene. Liu et al. (1998) points out that an increase in asphaltene occurs due to the oxidation of some portion of the maltene phase. Many of
15 the studies with GPC have found similar results, with a decrease in the maltene fraction of the molecular weight distribution and an increase in the asphaltene fraction (Lu and Isacsson 2002, Siddiqui and Ali 1999, Kim and Burati 1993, Churchill et al. 1995, Lee et al. 2009a).
Work by Wang and Zhang (2010) voiced concern over the impact of solubility parameters, an indicator of how miscible materials are with each other, on the introduction of polymers into polymer modified bitumen. One effort made within their research is to evaluate the solubility of asphalt binder in 27 different solvents and evaluate the solubility parameter. Among these solvents were toluene and tetrahydrofuran. They tested the solvents on six different binders with penetration values ranging from 67 to 233 mm/10 at 25°C. Tests were performed by placing 0.5mg of binder in 5mL of solvent in a sealed test tube for 48 hours and then evaluating the solubility by way of microscopy. The researchers found that tetrahydrofuran was a good solvent for all binders and that toluene was a good solvent for all but one binder.
Concern has been raised over whether these solvents would preferentially dissolve certain binder constituents in a staged extraction scenario. This concern will henceforth be referred to as sequential dissolution. For example, when the initial wash is performed on the mixture, the lighter maltene fraction may wash out first, leaving behind asphaltenes which eventually break down from the outermost layer after successive washes. If this is the case, then the staged extraction method, while still useful, may not be truly removing
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“layers” of binder, but rather fractions at different rates. This could influence data interpretation when comparing each layer. An example of the two cases is shown in Figure 1.4. Case 1 is that the binder is dissolved in layers, not sequentially. Case 2 is that the binder is dissolved sequentially by dissolving the maltene fraction prior to the asphaltene fraction.
Figure 1.4 Removal of asphalt binder by layers versus sequential dissolution in solvent
One approach for evaluating the possibility of sequential dissolution is to consider the solubility parameters of the solvents and the bitumen along with its fractions. Hansen solubility parameters are used in chemistry as a 3 dimensional method of determining whether a material is soluble in a solvent. The three dimensions considered are the dispersion forces between molecules (δd), energy from dipolar intermolecular force between molecules (δp), and energy from hydrogen bonds between molecules (δh). This method is based largely on the common “like-dissolves-like” premise from basic chemistry. In 2004, Redelius set out to determine the solubility parameters of a
17 Venezuelan bitumen and its subsequent asphaltene and maltene fractions. Redelius calculated these parameters by using a large number of solvents with known solubility parameters. Redelius’ work can be used to calculate an (RA) 2 value, which is defined as the distance between parameters in Hansen space. This equation (Equation 1.1), as defined by Hansen (2000) is:
(RA)2 = 4(δd1 − δd2)2+ (δp1− δp2)2+ (δh1− δh2)2 (1.1)
The parameter denoted by “1” is the parameter for the solute, while the parameter denoted by “2” is the parameter of the solvent. Contextually, the (RA)2 value can be further used to calculate the relative distance between parameters, or RED. However, this requires a maximum solubility parameter difference which was not predefined. The concept of (RA)2 is still relevant in the context that the lower the (RA)2 value, the more soluble a material is in a given solvent. These values were calculated in the following Table 1.1 on the basis of Redelius’ solubility parameters for bitumen, asphaltenes, and maltenes and Hansen’s solubility parameters for the solvents of interest.
Table 1.1 (RA)2 values calculated from Hansen solubility parameters provided in Redelius (2004) and Hansen (2000)
Solvent Bitumen Asphaltenes Maltenes
Trichloroethylene (TCE) 4.2 11.1 15.5
Tetrahydrofuran (THF) 32.8 49.6 33.5
Toluene 9.5 20.0 20.0
Decahydronaphthalene (Decalin) 28.3 34.9 43.1
n-heptane 66.6 104.9 62.9
18 On the basis of Hansen solubility parameters, it appears that trichloroethylene is the best solvent for the bitumen with a (RA)2 value of 4.2. Without considering n-heptane, the solvents rank as toluene, decahydronaphthalene, and tetrahydrofuran from best to worst respectively. A fractionation of the asphalt binder into asphaltenes and maltenes is typically conducted using n-heptane, which is why it is provided in this table. Examining the (RA)2 values for n-heptane gives a relative difference between the asphaltene and maltene fraction, with a difference greater than 40 with the insoluble asphaltene fraction being the higher of the two values. With respect to the other common asphalt solvents, it appears that TCE may have a slightly higher dissolution rate for the asphaltenes than the maltene fraction and toluene has no difference between fractions. However, decahydronaphthalene appears to yield higher solubility for asphaltenes than maltenes, and THF has a large difference between the maltene and asphaltene fraction. Based solely on the solubility parameters for the given asphalt, it appears that THF would be a candidate for sequential dissolution of the asphalt binder, dissolving the maltene fraction prior to dissolving the asphaltene fraction.