1. Introduction

Bitumen is a material manufactured or refined from crude oil. These materials have been derived from organic materials produced in processes that have spanned several million years in Earth’s history.

Organic material and mud accumulated in layers hundreds of meters thick. The weight of the layers compressed the mud into sedimentary rock. The application of heat from the earth, biochemical and bacterial processes as well as pressure converted the organic materials into oil.

The composition of this crude oil is thus very dependent on the source. There are approximately 1500 sources of crude in use in the world today from places as far apart as the North Sea. Middle east, South America. Asia, Micronesia and even Australia. Some have significant bitumen fractions, some have very little bitumen fractions figure 1 shows the extremes (1).

Although there is still some debate as how to characterize bitumen adequately in chemical terms it can be stated categorically that different crude sources produce bitumens of different properties. (see figure 2). Control of the bitumen chemistry is the key to creation of consistent bitumens of consistent properties.

SHRP work has underlined what has been known for many years, bitumens ain’t bitumens as oils ain’t oils, what works in one locale may well not work in another. Different climatic conditions require bitumens of different properties.

Care must be exercised therefore in dealing with the concept of chemistry as a determining factor, bitumen is chemically so complex and has so many different chemical combinations possible that the same physical result may be possible with quite different chemical compositions. Chemical composition as a tool for control of properties rests with the refiner.

It is useful to review some basic aspects of bitumen chemistry.

2. Basic Bitumen Chemistry

Bitumen is made up of polar and non-polar compounds in complex association. The interaction of polar compounds determines bitumen structure and mechanical properties (2,3).

The chemistry of the bitumen produced depends on two main parameters, the crude source and the manufacturing process. The crude source is most influential for processes that involve no chemical modification of the final bitumen blend. In such systems the bitumen may be optimized for properties but not improved on. For systems where a chemical modification of the bitumen can be carried out or components from other feed sources can be introduced an unsatisfactory bitumen may be made satisfactory.

Bitumen can be conveniently viewed on two levels, the molecular and the inter- molecular. The molecular is important to determine the potential for structure formation and the inter-molecular for the types of structuring that exist.

2.1 Elemental and Molecular Composition

Elemental analysis is an averaging, it gives little information on molecular arrangements and so sheds little light on physical properties.

Bitumens do contain trace amounts of metals, mainly vanadium and nickel. Figure 3 shows several asphalts that give an indication of the range observed in elemental analysis.

Although all asphalts are predominantly carbon and hydrogen most of the molecules contain at least one hetero (S,N,0) atom.

The hetero atoms are often present in sufficient amounts such that on average every molecule has one. These may be in the rings, in non ring components or as functional groups attached to compounds. These together with polarisable aromatic rings contribute polarity.

It is the arrangement of different elements / atoms , into molecules that determines the interactions of these molecules and hence the physical properties. Because the hetero atoms often impart functionality and polarity they have a disproportionate effect on the properties. This includes effects such as aging, the more reactive the hetero atoms age faster ( e.g. sulfur).

The organic origin ensures a wide range of different molecules. These may be Classified into the following three categories: (figure 4).

1. aliphatics (linear or chain like molecules)
2. cyclics (napthenic ring type materials)
3. aromatics ( ring materials that are unsaturated, ie different atoms share electrons).

Aliphatic and cyclics are three-dimensional and form shapes that keep them apart. Aromatics are flat and can stack.

These types of molecules, combined with the presence of hetero atoms determine the molecular interactions and hence the bitumen’s properties. Carbon in aromatic ring systems is about 25- 35 of the total carbon. The aromatic carbons are incorporated in condensed ring systems containing 1-10 rings . These ring systems may be associated with napthenic (saturated) ring systems and they both may have attachments of various branched and linear hydrocarbons. Carbon associated with napthenic ring systems are of the order of 15-30 of total carbon.

Non aromatic or napthenic hydrocarbons are present alone or as side branches ( as above) and account for 35-60 of the carbon content. Other structures are present in different bitumens. The types of interactions available are: (see figure 5).pi- pi bonding ( stacks of aromatic rings) polar or hydrogen bonding ( polar interactions of hetero atoms)Van Der Waals forces (interactions due to aliphatic chains intertwining).

Bitumen thus consists of only two families polar and non-polar. Polar molecules differ in:

1. Strength and number of polar groups
2. Molecular weight
3. Degree of aromaticity The non-polar molecules differ in:
1. Molecular weight
2. Degree of aromaticity

The compatibility of these two families i.e. the degree to which they can disperse in each other, is controlled by the relative aromaticity.

Correlations with discrete chemical groups are close to impossible due to the huge diversity, this has lead to a treatment based on fractionation of groups of materials to characterize these types of interactions.

2.2 Bitumen Characterization by Fractionation

It is simpler and more useful to determine interactions in the bitumen. Fractionating important components may do this.

There have been many systems proposed for the fractionation of bitumen.

2.2.1 Simple Fractionation

Bitumens can be classified most simply as two major fractions, Maltenes and Asphaltenes. (5)

1. Asphaltenes
   
   The Asphaltenes consists of highly condensed planar and heteroatom polar groups, Polarisable aromatic ring systems and large amounts of hetero atom polar functional groups.(4.6)
   
   The fraction is defined as the proportion of material precipitated when a straight chain alkane is added to the bitumen. It has been shown that the portion of the bitumen that is precipitated varies with the alkane used.(7)
   
   Asphaltenes are a product of resin development by geological or processing effect. They need not be high molecular weight but are the most polar of the fractions.
   
   It has been found that the asphaltenes are agglomerations of the most polar molecules in the bitumen and as such can only be dissociated from one another by dilution or some energy source such as heat or ion emission. When molecular weight is determined by field ionization mass spectroscopy of this fraction the levels found are much lower than those traditionally recorded.( 1000 c/f 20,000). This indicates that the asphaltenes are not large molecules but highly interactive polar molecules .(5)
   
   The polarity of the asphaltene component is derived by the presence of hetero-atoms (S,0,N ). These are functionalized and the functional groups are the polar groups.(5,8)
   
   In recent times it has been established that the alkane extraction process also may precipitate higher molecular weight straight chain hydrocarbons that have a wax like behavior that may cause low temperature cracking due to a free volume collapse in cooling (of the hole in the fat that appears on cooling). (9). These are not waxes per se as they have much greater molecular weights.
   
2. Maltenes (Petrolenes)
   
   This is the remained portion of the bitumen. It consists of two fractions, OILS and RESINS. Separation to these fractions can be easily carried out by Clay/gel separation.
   
   1. Oils
      These oils are the liquid part of the bitumen and consist of n, iso and cyclo paraffins and condensed napthenes with some alkyl aromatics. The aromatic portion is mostly naptho-aromatic hydrocarbons with three or four napthenic rings per molecule. The fraction is non-polar.(10)
   
   
   
   2. Resins
      The resins are chemically very similar to the asphaltenes. That is they are a transition from oils to asphaltenes. The resins consist of mainly polycylic molecules containing saturated aromatic and hetero-aromatic rings and hetero atoms in various functional groups, the resins are not as polar as the asphaltenes and hence are not as interactive (11).
      
      The asphaltenes are the thickening agent or structuring agent, the polar aromatics also contribute structure and ductility. The fluidity is imparted by the saturate and napthene aromatic fractions, which , in combination with the asphaltenes produce complex flow behavior.(1,2).

2.2.2 Complex Fractionations

Many attempts have been made to fractionate asphalt to its active components, as they exist in nature. The techniques illustrate the main methods used in the last 50 years.(12) These are:

1. Corbett Selective Adsorption/ Desorption
2. Roestier/ Sternberg Chemical Precipitation
3. Traxler Partitioning with Partial Solvents
4. Boduszynski Chemical Reactivity
5. Jennings HPLC
6. Brule GPC
7. Clay/ Gel

All of the fractionation techniques essentially show the same thing. in that they separate out the influential fractions in asphalt structuring and the compatibility of solvent type effects of the oil or neutral phases. It is beyond the scope of this presentation to consider these methods in detail and the reader is referred to the literature.

The SHRP program on fractionation of bitumens has refined the above concepts to separate out two important fractions, a solvent (oils) fraction and an associated fraction (asphaltenes/ resins - i.e. polar materials). Size Exclusion Chromatography is the first step in fractionation. (13) This is carried out in simple glassware columns packed with swollen beads. The pore structure of the gel beads determines the rate of flow through the column. The solvent phase flows straight through and the associated phase is slowed. The two fractions derived from this method are termed SEC I and SEC II. The former is the associated phase and the latter the dispersing or solvent phase.

The SEC I phase give bitumen its viscoelastic properties. This can be observed by measurement. That is the tan delta (ratio of viscous to elastic modulus) is the same for the SEC I fraction as for the whole bitumen. Gel type bitumens thus have low values of tan delta and high SEC I fractions and sol type bitumens have high values of tan delta and low SEC I fractions. In field terms a high SEC I fraction means greater structure, less thermal susceptibility and better performance at high temperatures. Associations are confirmed by fluorescence experiments. Associations quench fluorescence.

The SEC II or solvent phase is the oil fraction that the associated phases are dispersed in. The better this fraction is as a solvent the less brittle the bitumen will be and the viscoelasticity is suppressed. The SEC II fraction is a viscous oil and the SEC I fraction is a black solid.

In practical terms the fractions here could be considered as SEC I fraction equals asphaltenes and the resins (polar). The SEC II fraction consists of the oils. The SEC I fraction may be further fractionated by ion exchange Chromatography (14) to an add fraction, basic fraction, and amphoterics. The amphoteric fractions are black solids, the basic fractions are intractable tacky semi solids and the add fractions are viscous liquids.

The SEC II fractions are liquids and may be fractionated by supercritical fluid Chromatography (15) further to the base saturates and aromatic oils.

3. Application Of Bitumen Chemistry

3.1 Models

The above fractions and their nature may be used to infer a bitumen structure or model. this model can then be used to discuss the mechanical properties and physical behavior of bitumen in relation to different fractions. The major models are as follows:

1. Colloidal Model
   
   This is the traditional model where solid asphaltene particles are dispersed in the maltenes fraction. The asphaltene is the center of a micelle and these are peptized by the polar aromatic fractions absorbed from the Maltenes. (See figure 6).
   
   
   
   A sol type bitumen has the asphaltenes fully dispersed .In a gel type the micelles are not fully dispersed. Sol bitumens are Newtonian, gel bitumens are non Newtonian. Real bitumens exhibit some character of both sol and gel. Generally most bitumens are Newtonian at temperatures higher than 60°C, at lower temperatures they exhibit non Newtonian behaviour. This fact makes the bitumen set as it cools. This setting appears to be related to structuring of the bitumen, this structure is due to molecular orientation at aggregate surfaces and ordering in the bulk bitumen due to interactions of polar species.
   
   The colloidal model is inadequate to explain the flow and elastic properties of bitumen under these conditions of setting.


2. Microstructural
   
   A variant on the colloidal model that extends the understanding is the microstructural. (see figure 7).
   
   
   
   Bitumen is a continuum of polar and non- polar material i.e. a homogeneous, self compatible mixture consisting of a variety of molecular species that are mutually dissolved or dispersed. This creates areas of order or structure depending on the concentrations of polar material. The only differential between asphaltenes and resins is in polarity and thus the degree of potential associations. Thus structuring depends as much on asphaltene chemistry as concentration. The resinous proportion is less interactive and the oils are not interactive at all. It has been found . for example that add and base fractions and especially the amphoterics are viscosifiers, this is due to the interactive capability of such species.
   
   This has been correlated this with the rheology of the solid bitumen. The quality of the asphaltenes could be described as the ability to interact with each other and the resinous components to produce order and hence complex flow properties. The SHRP fractionation technique, as described above, splits the bitumen into two fractions with SEC (GPC) and then the SEC II by ion exchange Chromatography into strong adds, strong bases and amphoterics. The strong bases and amphoterics have been found to associate most strongly and hence control the mechanical properties to the strongest extent. The measurement of asphaltenes could be considered a rough approximation of this material. It can be asserted therefore that bitumen is a complex mixture of chemical compounds in a complex equilibria, hence the proportions of each chemical species is critical to performance. This is summarized in figure 8.

3.2 The Triangular Composition Diagram

What the fractionation and the model theories give us is the capability to conceptualize the way bitumen flows and reacts to stress. It can thus allow a rational approach to crude selection and manufacturing methods.

The composition of bitumen as shown by the different fractionation techniques and conceptualized in the microstructural model is good evidence that the basic approach of measurement of polarity differences by asphaltene/ resin and oil measurement is sound. These components may be easily and directly measured using either clay- gel analysis or using the latroscan instrumented Chromatography method (in its latest MARK V version ) can be used directly to plot a triangular diagram of composition .Then, by either empirical or theoretical means, a compositional region may be established for performance optimization. This performance/compositional area will be crude dependent to the extent that the exact chemistry of the fractions is not considered. However, for chemically manipulated systems where the bitumen chemistry is, to some extent, controlled it will be practically independent. Thus the system would be expected to operate best on low asphaltene crudes that have been modified to higher asphaltenes bitumens. Put plainly, once a suitable correlation has been found for a crude source then the composition can be used to check performance.

This is not as exact as the SHRP approach and has some inherent difficulties associated with some high molecular weight saturates and the crossover in chemistry between the asphaltenes and resins, however correlations for low asphaltene crudes both in PPA bitumen blends and air rectified systems have been shown to be satisfactory in a Mobil Oil study in 1990. The diagram is also useful in defining the end uses of products.(see figure 9).

3.2.1 Quality Of Asphaltenes

This is an important concept in reconciling the compositional analysis with the observed properties. A bitumen may have the same percentage of asphaltenes and have quite different physical properties, this is because the properties depend on the whole chemistry, not just one component.

It is also because the chemistry of asphaltenes will vary from crude to crude. Thus the manner in which they are able to form associations will vary( higher associations- higher pen index, higher viscosity, at high temperatures, less shear susceptibility).

This variation will be in the form of different levels and ratios of amphoterics, strong bases and strong acids . Also the presence of high molecular weight non polar compounds that may be precipitated in the asphaltene separation (that contribute nothing to associations and hence structure ) will cause variation in asphaltene quality.

For these reasons composition using the asphaltenes/ resins/ oils approach must be related to a field correlation. However for manipulation by chemical reaction such as air rectification asphaltenes of high associative potential and specific chemistry are formed and composition will be a good indicator of improved properties.

3.3 VSS Technology

Over the last 20 years VSS internationally have been developing the application of the above principles into a system for assessment of crude sources for bitumen manufacture and the improvement of so called poor crudes into premium bitumens. These techniques have been applied in many countries to allow production of quality bitumen from a range of light and extra light crudes. It has also been used to develop compatible bitumen base stocks for polymer modification.

The principles and techniques are also applicable to engineering bitumens with specific properties for particular applications. This induces industrial and roadpave bitumens.

The specifics of the techniques used are proprietary and will not be discussed in detail here. However they include the use of controlled chemical reactor systems to create tailored bitumen products.

4. Performance Requirements For Bitumen

Passing a specification is rarely a guarantee of field performance. This is because most specifications are grading specifications that have evolved over time often having tests introduced to exclude poor performing materials rather than true performance tests that predict success.

True performance based testing does not currently exist. The correlation with field performance is still the final arbitrator of what is acceptable. It is possible to frame, in general terms the requirements for performance of a given binder in a given application. The bitumen must be pure and safe, it must have a suitable consistency at the temperatures of service to prevent plastic flow or cracking failure and it must adhere to the substrates. The properties should not change detrimentally with aging or exposure to other environmental factors such as moisture.

All of these factors may be controlled by control of the chemistry of the system.

Field performance of bitumen largely depends on its rheology, more specifically the fact that bitumens are viscoelastic. That is the response to stress or strain will be time and temperature dependent (figure 10 shows a typical temperature dependency) Control of these properties will lead to control over performance.

The shear and temperature dependency of the binder may be assessed by use of penetration index and shear index.

5. Multi – Quality-Bitumens

Bitumens made using the VSS process can be termed “Multi” or” Multigrade” bitumens The multigrade bitumen concept is not new , there are a number of patents and proprietary methods for achieving the result of a bitumen that has a rheology that is less temperature sensitive.

This means that such controlled composition bitumens will be less stiff at low temperatures displaying a lower Fraas point and more stiff at high temperatures displaying a better resistance to deformation. The differences are shown in figures 11,12 and 13.

Multi bitumens also have better fatigue resistance in hotmix, better aging resistance and are more useful in polymer bitumens and emulsions.

6. Conclusions

To produce bitumens that is suitable for environmental conditions in any given area requires a control of the rheology of the bitumen. This may be achieved by control of the chemical composition of the bitumen. Chemical composition may be controlled by crude source , by manufacturing processes or by additive systems.

The VSS process takes the most economic light or heavy crudes and adjusts the chemical composition of the bitumen produced to create the desired properties. As the rheology of the bitumen is a key determinant of performance the premium bitumen produced could be classed as a multigrade type. This can be achieved for any given grade.

This has the effect of improving performance to a desired optimum based on field or performance design.

Multi bitumens are not a new class of quality controlled bitumens, they take the excellent performance of bitumen as a road surfacing medium and tailor its performance to the customers and climates needs.

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