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Handbook of Size Exclusion Chromatography and Related ...

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Apparently more polar compounds are shielded by less polar <strong>and</strong> less reactive<br />

material(127).Itisclearthatthetendency<strong>of</strong>bothasphaltenes<strong>and</strong>polararomatics<br />

to associate, which is affected by other asphalt constituents <strong>and</strong> the polarity <strong>of</strong><br />

carrier solvents, has anumber <strong>of</strong> implications for SEC analysis.<br />

3 APPLICATIONS OF SEC TO ASPHALTS<br />

3.1 Asphalt Fingerprinting, Compositional Analysis, <strong>and</strong><br />

Aging<br />

Asphalt from each source crude oil has its own characteristic chromatogram that<br />

usually changes only slightly with grade. For this reason SEC is avery effective<br />

tool for detecting changes in asphalt as aresult <strong>of</strong> processing changes, crude<br />

source, or contamination. Glover et al. (9) ran monthly SEC chromatograms on<br />

11 asphalts for aperiod <strong>of</strong> ayear. Each asphalt exhibited its characteristic shape,<br />

but some <strong>of</strong> these showed considerable seasonal change, probably reflecting<br />

processing changes. It must be emphasized that characterizations <strong>of</strong> this kind<br />

require that all SEC parameters be held constant. This is amajor disadvantage,<br />

makingcomparisonsdifficultbetweenlaboratories<strong>and</strong>evenovertime.Anasphalt<br />

st<strong>and</strong>ard should be run periodically to confirm constant operating parameters.<br />

Garrick (61,63) divided asphalts into groups depending on the shape <strong>of</strong> the SEC<br />

pr<strong>of</strong>ile. This was based on width, location, <strong>and</strong> height <strong>of</strong> the peak maximum, <strong>and</strong><br />

so on, <strong>and</strong> showed that properties such as temperature susceptibility <strong>and</strong> viscosity<br />

ratiobefore<strong>and</strong>after thinfilmoventest(TFOT)oxidationtendedt<strong>of</strong>allintothese<br />

groups.<br />

Low-molecular-weight contaminants, or any material having a narrow<br />

molecular weight range, produce apeak on the chromatogram <strong>and</strong> are easily<br />

detected, <strong>of</strong>ten at very low concentrations. Before asphalts from roads or hot-mix<br />

plantscanbestudiedchemically,theymustbeseparatedfromtheaggregate.There<br />

are st<strong>and</strong>ard ASTM procedures for extracting the asphalt <strong>and</strong> then removing the<br />

extracting solvent. Burr et al. (25) showed that the st<strong>and</strong>ard procedures <strong>of</strong>ten left<br />

sufficient solvent in the asphalts to affect properties significantly.The literature is<br />

repletewith work that has been marred in this manner. By using SEC, the solvent<br />

canbedetectedatlowconcentrations,<strong>and</strong>Burretal.developedmethodstoassure<br />

complete solvent removal. It is prudent to use SEC routinely to assure complete<br />

solvent removal from recovered asphalt.<br />

SEC analysis can be used very effectively in combination with Corbett<br />

separation, solvent or supercritical solvent fractionation, <strong>and</strong> other fractionation<br />

procedures for the purpose <strong>of</strong> underst<strong>and</strong>ing asphalt composition <strong>and</strong><br />

aging. Figure 2shows chromatograms for an asphalt cut into a60% top fraction<br />

<strong>and</strong> a40% bottom fraction by supercritical pentane (15). The top 60% was<br />

fractionated into four fractions by supercritical pentane (Fig. 3), <strong>and</strong> the bottom<br />

© 2004 by Marcel Dekker, Inc.

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