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Handbook of Size Exclusion Chromatography and Related ...

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esulting very hard asphalt may be cut back with lighter fractions. Regardless <strong>of</strong><br />

howitisproduced,theresultisasticky,nearsolidcontainingavastarray<strong>of</strong>highmolecular-weight<br />

compounds varying from paraffins to highly condensed<br />

aromatics. Included within these compounds, especially in the more condensed<br />

material, are the so-called heteroatoms, O, N, S, <strong>and</strong> metals, especially Ni <strong>and</strong> V.<br />

Tosimplifyasphaltanalysis,acommonpracticeist<strong>of</strong>ractionatethematerial<br />

todivideitintogroupings<strong>of</strong>simplerconstitution.Alargenumber<strong>of</strong>methodshave<br />

been proposed, but most are based on either selective solvent extraction or<br />

chromatographic separation or, frequently,acombination <strong>of</strong> solvent precipitation<br />

<strong>and</strong> chromatographic separation.<br />

One <strong>of</strong> the most used procedures, an ASTM st<strong>and</strong>ard, D4124, was<br />

developed by Corbett (97) <strong>and</strong> separates asphalt into four fractions. Asphaltenes<br />

are precipitated by heptane, <strong>and</strong> the remaining solution is divided into saturates,<br />

naphthene aromatics, <strong>and</strong> polar aromatics by aseries <strong>of</strong> successively more polar<br />

solvents on an alumina column. Similar procedures produce fractions variously<br />

known as asphaltenes, resins, <strong>and</strong> oils or saturates, aromatics, resins, <strong>and</strong><br />

asphaltenes, for example. Although similar, the methods are not identical <strong>and</strong><br />

produce fractions that overlap those <strong>of</strong> other methods.<br />

Corbett (97,98) used a densometric procedure coupled with molecular<br />

weight determination by VPO at 378C to determine the structure <strong>of</strong> his fractions,<br />

asshowninTable1.Asphaltenescouldnotbecharacterizedcompletelybecause<strong>of</strong><br />

the difficulties in molecular weight determination as a result <strong>of</strong> asphaltene<br />

molecular association.<br />

Table 2(99) shows additional structural data estimated for the fractions.<br />

These results are all dependent on the composition <strong>of</strong> the source crude oil,<br />

particularly heteroatom content <strong>and</strong> metals. Both Ni <strong>and</strong> Vare found primarily in<br />

the heptane-precipitated asphaltenes <strong>and</strong> are evenly distributed without regard to<br />

molecular size.Theyseemtobeinterchangeableinstructureinthatinfractions<strong>of</strong><br />

agiven asphalt the ratio <strong>of</strong> Vto Ni is constant over wide ranges <strong>of</strong> composition.<br />

These metals <strong>of</strong>ten exist in porphyrin structures <strong>and</strong> have been implicated in<br />

higher rates <strong>of</strong> asphalt oxidation.<br />

Heteroatoms are important because <strong>of</strong> an inordinate contribution to<br />

properties. Large increases in asphalt hardening occur with the uptake <strong>of</strong> only<br />

1wt% oxygen. Petersen (100,101) has carried out extensive work on heteroatom<br />

analysis. Atypical analysis is shown in Table 3(101). When asphalt oxidizes, the<br />

principle increase is in ketones <strong>and</strong> sulfoxides. Carboxylic acids <strong>and</strong> anhydrides<br />

tend to concentrate at the aggregate surface in asphalt concrete <strong>and</strong> may produce<br />

sensitivity to water damage.<br />

Studies have shown that increases in asphalt viscosity with oxidation can be<br />

correlated with increases in carbonyl formation (28) which has been shown to be<br />

proportional to oxygen uptake (102). Almost certainly this hardening results from<br />

hydrogen bonding between heteroatom groups in asphaltene molecules <strong>and</strong> also<br />

© 2004 by Marcel Dekker, Inc.

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