asphalt chemistry - good

7/30/2019 Asphalt Chemistry - Good

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ASPHALT CHEMISTRY

MOLECULAR STRUCTURE

- Diverse molecular structures

- Predominantly composed of carbon and

hydrogen.

- Most molecules contain one or more

heteroatoms, such as sulfur, nitrogen and oxygen.

- The type of molecular structure is more

important than the total amount of each element

present.

TYPICAL ELEMENTAL ANALYSIS(Petroleum Asphalts)

ELEMENT PERCENT

Carbon 83 - 87

Hydrogen 10 -11

Sulfur 1 - 5

Nitrogen 0.3 - 1,1

Oxygen 0.2 - 0.8

Vanadium, ppm 4 - 1400

Nickel, ppm 0.4 - 110

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FUNCTIONAL OR POLAR GROUPS

- Heteroatoms (e.g. sulfur, nitrogen, oxygen) are attached

to carbon atoms in asphalt molecules in different

configurations and form different polar molecules or

functional groups.

- These molecules are polar because of imbalance of

electrochemical forces which produces a dipole.

- Molecular interactions among these polar molecules and

other asphalt molecules strongly influence the physical

properties and performance of the asphalt.

- A small amount of polar molecules can have a great

effect.

- Resistance to stripping is thought to be affected by the

adsorption of polar molecules by aggregate surface.

CONCEPTUAL COMPOSITION OF ASPHALT

ASPHALT

N-Heptane

Precipitate

ASPHALTENE

Soluble

MALTENES

OIL RESIN

Liquid Propane

Soluble Precipitate


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ASPHALTENES

- Dark brown, friable solids- Highest polarity

- Insoluble in nonpolar solvents (e.g. heptane,

pentane)

- Strong tendency to associate in conglomerates

- Highest molecular weights

- Carbon to hydrogen ratio greater than 0.8

- Higher asphaltene content is related to higher

viscosity of asphalt.- Low asphaltene content has been linked to the

problem of tender asphalt mixtures.

RESINS

- Dark semi-solid

- Fluid when heated & brittle when cold

- Soluble in n-heptane or n-pentane

- Have a lower molecular weight than that of

asphaltene.

- Carbon to hydrogen ratio of greater than 0.6

- Act as dispersing agents to the asphaltenes(disperse the asphaltenes throughout the oils to

provide a homogeneous liquid).


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OILS

- Colorless or white liquid

- Soluble in most solvents

- Lowest molecular weight among the three

components

- Carbon to hydrogen ratio of less than 0.6

- Oxygen and nitrogen usually not present

- Can change into resins or asphaltenes upon

oxidation.

AsphaltenesResins Oils


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ASPHALT AS A COLLOIDAL SYSTEM

(1) Asphaltenes are peptized (dispersed) by the resins

throughout the oils.

(2) Relative amounts and characteristics of asphaltenes,

resins and oils influence the physical properties and

performance of an asphalt.

(3) When the asphaltenes are highly peptized, the asphalt

acts as a SOL, which has a highly Newtonian behavior.

(4) When the resins are not effective in peptizing the

asphaltenes, the asphalt acts as a GEL, which has a non-

Newtonian behavior.

(5) SOL-GEL is intermediate between a SOL and a

GEL.

Maltenes Asphaltenes (A)

Bitumen

Nitrogen

Bases(N)

First

Acidaffins(A1)

Second

Acidaffins(A2)

Paraffins

(P)

Insoluble in n-pentaneSoluble in n-pentane

Precipitates

with 85%

sulfuric acid

Precipitates

with 98%

sulfuric acid

Precipitates

with fuming

sulfuric acid

(with 30% SO3)

CHEMICAL PRECIPITATION METHOD

(developed by Rostler and Sternberg)


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USES OF THE RESULTS OF CHEMICAL

PRECIPITATION METHOD

Compatibility Ratio = N / P

( >0.5 considered to be good)

Durability Parameter = (N+A1) / (P+A2)

= (most reactive) / (least reactive)

Durability rating of asphalt binders from 60 to 100

penetration gradeAverage Abrasion Loss2

Group

N + A11

P + A2 Percent milligram/rev Durability

03 1.7 >50 >2.00 Inferior

1Based on original composition.

2Average of abrasion at 25C (pellet method) before and after aging.

3Cheesy and putty-like products of consistency not associated with highway asphaltbinders.

(After Rostler)


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SELECTIVE ADSORPTION-DESORPTION

METHOD (Corbett Method)

CORBETT METHODDISSOLVING ASPHALT IN N-HEPTANE


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CORBETT METHOD

FILTERING OUT ASPHALTENE

CORBETT METHOD

Chromatographic Column


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SATURATES (from Corbett Method)

- The first fraction to emerge from the

chromatographic column when eluted with n-

heptane .

- Non polar

- Liquid at ambient temperatures

- Hardly change with time

- Increase temperature susceptibility of asphalt

NAPHTHENE AROMATICS(from Corbett Method)

- Emerge as the second fraction when eluted with

a more polar aromatic solvent (benzene or

toluene).

- Molecules may contain the heteroatoms oxygen,

nitrogen and sulfur.

- Liquid at ambient temperatures.- Softening component in asphalt.

- Aging fraction in asphalt.


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POLAR AROMATICS (from Corbett Method)

- The final fraction to emerge when eluted with a

mixture of methanol and benzene.

- Molecules contain more functional groups.

- Solid or semi-solid at ambient temperatures.

- Aging fraction in asphalt.

CHARACTERIZATION BY MOLECULAR SIZE

DISTRIBUTION

- HP-GPC (High Pressure Gel Permeation

Chromatography)

- Analogous to sieving of aggregate.

- Asphalt is dissolved in THF (tetrahydrofuran) and the

solution is passed through gel permeation columns.

- Large molecules are eluted first and smallest molecules

are eluted last.- Plots of relative molecular size distributions are obtained

as a result of GPC tests.


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RESULTS OF

GPC TEST ONAN ASPHALT

CHARACTERIZATION BY INFRAREDABSORPTION SPECTRUM

- FTIR (Fourier Transform Infrared) Spectroscopy.

- The infrared absorption spectrum (absorption intensity

versus wavelength plot) is related to the type and amount

of functional groups present..

- Can be used to measure changes in functional groups due

to oxidation of asphalt.

- The functional groups, Sulfoxide, Anhydride, Carboxylic

Acid and Ketone, are formed on oxidative aging and canbe characterized by FTIR spectroscopy.


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INFRARED ABSORPTION SPECTRUM

of an asphalt

Ketones &

Anhydrides

Carbon-Carbon

Double bonds

Peterson

identifiedPolar

functional

groups

Natural

By Aging

asphalt chemistry - good

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