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Influence of Composition of Paving Asphalt on Viscosity,

Viscosity-Temperature Susceptibility, and Durability

R.

L. GRIFFIN, W. C. SIMPSON and

T. K.

MILES

Shell Development Co., Emeryville Calif.

T h e epara t ion of pe t ro leum residues on the basis of molecu-

lar size and chemical type is a complex study in which many

problems remain unsolved. Th e residues may be s ubdivided

into bro ad groups of compoun ds at various molecular weight

levels. Th e composition of paving asphalts is described in terms

of molecular weight an d mo lecular type distrib ution; composi-

tion

is

related to viscosity, viscosity-temperature susceptibility,

and durabi l i ty or resistance to ha rdening .

SEPARATION

OF

ASPHALTS

Th e four asphalts studied were separated in terms of m olecu-

lar size and definite chemical classes of compounds. Most

methods of separation into chemical types lose sharpness when

applied to mixtures with a bro ad distribution of moleculai

weights . Th e aspha l ts s tudied conta in m ater ia l ranging from

350 to 5000 molecular weight. To enhance the sharpness of

separation into chemical types, a separation o n the basis of mo-

lecular size

is

first made by molecular disti l lation. The individ-

ual disti l lation fractions having fairly narrow molecular weight

spreads are separated by chrom atograph y over sil ica gel into

three type fractions: saturated hydrocarbons, arom atic hydro -

carbons, and he te rocycl ic and polar compounds conta in ing

most of the sulfur, nitrogen, a nd oxygen which has been desig-

nated as resins for convenience. T h e residue from the m o-

lecular disti l lation

is

generally a very ha rd m aterial of high

molecular weight; it is separated into an oil fraction and a solid

pow der (designated as asph altenes ) by solvent precip itation

with

40

volumes of 2-methylbutane (isopentane). The oil frac-

tion may be furth er m olecularly disti lled or separated directly

by silica gel chromatography into saturates, aromatics, and

resins as were the disti l lation fractions. Th e asphaltenes consist

in a large part of high molecular weight aromatic and hetero-

cyclic compoun ds whose furth er se paration into distinct classes

of compounds is beyond the scope of a routine separation.

A

schematic diagr am of the separation is shown in Figure

1.

Th e experimental details of these sep aration p rocedures hav e

been published

(7) ,

and only a brief description is given here.

Th e molecular disti l lations are carrie d out in 14-

or

5-inch

cyclic sti lls manufactu red by Distil lation Products, Inc ., which

permit disti l lation of high m olecular weight material w ith mini-

mu m the rma l hazard . I n the chromatographic separa t ion , s il ica

A S P H A L T

I

MOLECULAR

DISTILLATIO N

I

I

DISTILLATION

RESIDUE

I

DISTILLATE F RACTIO NS

ISO P ENTANE

P RECIP ITATIO N

EXTRACT P RECIP ITATE

I

( A S P H A L T E N E S )

SILICA G EL CH RO MATOCRAP H Y O F EACH F RACTIO N

I

ELUTION WITH ELUTION WITH ELUTION WITH

I S O PE N T A N E B E N Z EN E E T H A N O L A N D B E N Z E N E

S A T U R A T E S

I

RESINS

AROMATICS

I

Figure

1

Schematic diag ram of separat ion scheme

gel to oil ratios u p to 50 to 1 are needed to obtain sharp separa-

tion, the highest ratios being required for the high molecular

weight materials. Th e split between saturate and arom atic frac-

tions is made by observing the ultraviolet absorption spectrum

of the eluate a t a wave len gth of 2650

A .

When aromat ics begin

to be eluted, a ra pid rise in absorption at this wave length oc-

curs. By making the split when the extinction coefficient of the

effluent is 0.1 l i ter per gr am cm ., an aro matic content of less

tha n 0.2Yc

is

assured in the saturates.

Th e arom atics are removed w ith benzene, eluting until the

benzene comes through colorless. Then a mixture of equal vol-

umes of ethanol a nd benzene is used to remove the resins. Total

recoveries generally amoun t to 98 to 10070by weight of charge.

During the chromatographic separation of fractions above

about 500 molecular weight, the eluate is protected from ex-

cessive exposure to oxygen and light to prevent oxidation of the

aroma tic and resin fractions.

In m any cases when th e separated fractions have been recom-

bined; examination of the reconstituted product confirms that

these separation procedures do not lead t o changes in the pro p-

erties of the asphalt.

MOLECULAR WEIGHT DISTRIBUTION

Ebullioscopic determ ination of the a verage m olecular weights

of molecular disti l lation fractions in ben zene reveals interesting

molecular weight distribution differences between asphalts of

the same viscosity but from different crude sources. Figure 2

shows the molecular weight distribution in four asp halts having

a nominal penetration of 200 mm ./ lO a t 77 F . The se a spha lt s

a re from a Cal ifornia coastal c rude f rom the S anta M aria a rea ,

a California valley crud e from the Bakersfield region of th e San

Joaqu in Valley, a Venezuelan crude from the east coast of L ake

Maraca ibo, and a mid-cont inent lube c rube f rom West Texas.

All were manufactur ed by disti l lation of the cr ude without fur-

ther processing

or

blending an d a re commerc ia l products wi th

the exception of the mid-continent a sphalt . T he averag e mo-

lecular weight of the first distillatio n fraction from these asphalts

ranges from about 350 for the C alifornia coastal cru de to about

1150 for the mid-continent crude, while the average m olecular

weight of the asphaltenes ranges from 2500 for the California

valley asphalt to 4800 for the Venezuelan asphalt . This large

difference in the molecular weight to which various crudes must

be distilled to yield a n as phaltic residue of a given viscosity is a

result of differences in concentrations of the components which

ar e reflected in the viscosity of th e mate rials.

2 1600

5 1400

4791

1 = 4 0 1 9

I

3 5 0 0

,

e 2 5 3 0

V A L L E Y

2 0 0

0 10 20 30 40 50 60 70 80

90

100

w

O F A S P H A L T

Figure 2. Molecular weight distribution

VOL.

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Elem ental Composi t ion.

Th e sulfur content rises rapidly with

increasing molecu lar weight in th e case of the C aliforn ia coastal

asphalt , less rapidly in the V enezuelan asphalt a nd hardly at

all in the other two asphalts, as shown in Figure 3. Xitrogen

conten t rises with increasing de pth of distillatio n except in th e

case of the mid-continent asphalt as shown in Figure 4. The

atomic ratio of hydrogen to carbon decreases from a level of

1.5 to 1 .6 rather slowly with increasing de pth of disti l lation,

but drops to a level of 1.1 o 1.2 n the highest molecular weight

fraction (disti l lation residue).

Viscosi ty

of

Asphalts.

Th e flow properties of asphalts at vari-

ous tempe ratures are among the m ost impor tant characteristics

of these m aterials with respect to con ditions required for ap -

plication as well as per form anc e in use. Specification tests which

are common ly used in asphalt testing describe the dependence

of viscosity on temperature. Table

I

shows the results on the

four aspha lts discussed.

All of these specification tests give results in different units at

different temperatures; hence,

it

becomes difficult to compare

the flow properties of asphalt in this ma nne r. Therefore, the a u-

thors measured viscosity in fundamenta l units over the tempe ra-

ture range from

32

to 285 F. At temperatures up to 140 F.,

all measurements were made with the sliding plate microvis-

cometer

5)

manufactured by Hallikainen Instruments, Berke-

ley, Calif. At high temperatures, \ziscosities were determined in

a Lantz -Zeitf uchs reverse Row capilla ry viscometer (

7 .

Ta b le

I1

shows the viscosities of the four asphalts between

32

and 285

F.

T he variation of asphalt viscosity over this w ide

tempera ture range

is

plotted using the Walth er

9 )

quat ion:

Log- log

(1;

+ 0.7)

=

If 105 +

where li

is

viscosity in centistokes, 1

s

temperature in degrees

absolu te , and

it1

a n d

b

are constants of the liquid. T his equ a-

tion is the basis for the A ST M viscosity-temperature charts for

petroleum p roducts which extend to 2 x 10' stokes.

By

extend-

ing the charts to 10 '' stokes they are suitable for asphalts. The

absolute viscosity in poises is obtained by introducing the

density.

T he viscosities of the four asph alts shown in T ab le I1 were

plotted on the viscosity-temperature charts a nd the slope of the

straight l ine obtained for each asphalt is shown in Table 111.

Th e California coastal asph alt , with a slope of -3 .52 is the least

tempe rature susceptible, and the C alifornia valley, with a slope

of -4.20, is the m ost susceptible.

Viscosity

of

Dist i l lat ion Fract ions. Th e extremely wide range

of viscosities covered by the distillation fractions of asphalts is

demonstrated by the data for the fractions

of

the California

coastal asphalt shown in Figure

5

which range from 0 .3 poise to

130 billion poises at 140 F . The linear relation between log vis-

cosity and molec ular weight for the fractions of a given asph alt

has been found to hold rea sonab ly well for all asphalts ex-

amined , the slope and intercept of the line depending on the

composition of the asphalt . Th is type of relation also holds for

mem bers of a hom ologous series of organ ic compo unds 3) .

On ly large differences in chem ical constitution of the fractions

with the same molecular weight but from different crudes can

account

for

the variation in viscosity (Figure 5 ) .

2

9

8

ir

f

2 -

Table

I.

Asphal t Speci f icat ion Tests Deal in g wi th Flow

Properties

at

Various Temperatures

Venc- Caliiornra

Caliiornia

Mid-

zuelan

Coastal

\.a l lev

C on t i nm i

Pene t r at ion a t 77 k

, rnm 10

100 g for 5

xc

183 194 224 200

Softening point, ' r ing and ball 103 105 97

5

102

Penetration index - 0 6 0

1

-1 2 - 0 5

Penetration at 39 2 F mm IO

200e for60 sec 67 71 63 60

6 -

5

4 -

VENEZUELAN

3 -

e

CALIF O RNIA VALLEY

0-c I

\

I ' , I I ,

P e n . a t 3 9 . 2 F. x 100

Pen at 7 7 F .

Pen. ratio

= -

36.6 36 .6 28 .0 30 .0

r - - j

1 .8 -

1 . 6

-

I

CALIF O RNIA CO ASTAL

I

Savbolt-Furol

\iscosity

at 275'

F.,

seconds 125 129 58 175

8

7

I

I

g 1.4

1 . 2

1 . 0

8 0 . 8

0 . 6

0 . 4

0 .2

t

V E N E Z U E L A N

MID-CO NTINENT

0

2 0 40

60 80 100

%w

O F A S P H A L T

Figure

4.

Ni t rogen d is t r ibu t ion

Table

It.

Measu red Viscosi t ies of Asphal ts Over

a Wi de Temperature Range

(Viscosities in p oises)

O F .

Venrzuelan Coasta l Valley Continent

Tem p. , California California Mid -

32 4.17 x 10'

1.09

x 10' 1.95

x 10' 4.16

x

l o 8

77

3.65

x 10

1.95

x

10' 9.4

x

10' 1.70

x

10'

140 1.60 x

10'

1.21

x

10'

3.09

x

10' 1.00 x 10'

225 11.9

12.0 5 .09 15.9

285 2.04

2.11 0.93

2.80

Viscosities at 140

F.

and

lower

by sliding plate microviscometer at a

Viscosities at higher te mp era tur e by reverse flow Lantz-Zeitfuchs

shear rate of

5 x

10.'

set.-'.

capillary viscometer.

Table

Ill.

Slopes

of

the Viscosity-Temperature

Plots Between 32 a n d 285

F.

Venezuelan Coastal Valley Con tinent

California California Mid -

.L/ from

Walther

equatio n -3.59 -3.52 -4.20 -3.64

1 0 4 P

I

CALIF O RNIA

f 1 0 1

.

MID-CO NTINENT

/

n

I t I I I

1 1

200

600

1,000 1,400 1,800 2.200

MOLECULAR WEIGHT

Figure

5 .

Viscosity of dist i l lat ion f ract ions

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Th e s lopes of the viscosity-temperature plots calculated by

the Wal the r equatio n for the molecular distillation fractions

from these four asphal ts are shown in F igure 6. I n general , the

susceptibility of viscosity to ch ange with tem per atu re decreases

with increasing molecular weight, the most striking variance

with molecular weight being shown by the Venezuelan asphalt

fractions. The distillation fractions from the mid-continent

asphalt show a low viscosity temperatu re susceptibility with

practically no chan ge with increasing m olecular weight.

Micro f i lm Du rab i l i t y Test .

O n e of the main factors determin-

ing the life of a paving asphalt

is

i t s ability to resist hardening

as a result of oxidation an d loss

of

volatile components. The de-

gree to which an asphalt resists such changes is an indicat ion

of its durability relative to other asphalts when used under the

same conditions. As a result of theoretical and experim ental

studies of the phen omen a which lead to hard ening of asph alts,

V a n O o r t (8)devised an accelerated du rability test designed to

allow predictio n of the harden ing w hich a n asphalt m ay be ex-

pected to unde rgo in use. Th e ratio of the final viscosity to the

origi nal viscosity is used as an aging index to express the dura-

bility of th e mater ial. A mo difica tion of this durab ility test

(4,

correlates reasonably well with field performance 6 )of asphalts

unde r identical conditions, and h as been used to study the con-

tribution of molecular size and ty pe distrib ution to th e du rabil-

ity of asp halts. T he test consists of aging films of asphal t 5 mi-

crons thick o n glass plates in an oven at 225 F. for 2 hours ; the

hardening which occurs is determined by measuring the vis-

cosity of the material before and after with the sliding plate

microviscometer. If the aging is carried ou t in a nitrogen atmos-

phere, the hardening observed is a t t r ibu tab le to loss of vola-

tiles only, while aging in air gives a measure of harden ing due

to oxidation as well as to loss of volatiles. The aging is con-

duc ted in the absence of light, because m ost of the aspha lt films

between mineral part ic les in road construct ion are in a dark

environment .

Durab i l i t y o f Asphal t s an d D is t i l l a t ion Fract ions.Application

of the microfilm dur abil ity test to four aspha lts yields th e results

shown in Ta ble IV . Aging of the asphalts in nitrogen caused the

viscosity to increase by only a very small factor in all but the

California coastal asphalt in which it increased by a factor of 20,

indicat ing that hardening by

loss

of volatiles is a serious prob-

lem in this asphalt. T he actual viscosity of this a sphalt increased

durin g the test from 1.95

x

l o 5 t o 4.03

x

l o 6

poises at

77F.

T h e relative vulnerability of this asphalt to hard ening by

loss

of volatiles is indicated by the molecular weight distribution

da ta .

Results of the microfilm durability test in air show that the

California coastal asphalt hardens by a factor of 84 because of

the combined effects of

loss

of volatiles and oxidation, but the

others har den by a factor of 2.5 to 3.

Th e ra tio of the a ir aging index to the ni t rogen aging index

is a measure of the harden ing du e to oxid ation. This r atio for

the California coastal asphalt

is

4 .1 while for the other three

asphalts i t varies from

1 .9

to

2 . 5

showing that the California

coas tal asphal t a lso hardens more from oxidat ion than the

others do.

Th e tendency of the individual mo lecular distillation frac-

tions to volatilize un der th e conditions of the microfilm du rab il-

ity test was measured by determ ining th e weight loss of the frac-

tions of the C alifornia Coastal asph alt duri ng the test in n itro-

gen as shown in Figure

7 .

The weight loss due to volatility

amounts to

2 5

to

30%

in the lowest mole cular weight fractions.

T o demo nstrate the influence of the initial molecular weight

on the durab ility of asphalt the molecular distillation residue of

the California coastal asphalt w as blended b ack w ith the dis-

tillation f raction s to yield a series of pro duc ts all w ith viscosities

of about 8 x l o 5 poises but with successively higher initial

molecular weights as a result of leaving out low molecular

weight fractions. Th e aging indexes of these asphalts in ai r as a

function of initial molecular weight a re shown in F igure 8.

Fro m da ta of this type, it is concluded that the hardening of

asphalts by the loss of volatile components may be effectively

-2.

C A L I F O R N I A

Table IV . Harden in g o f Asphalts in the

Micro f i lm Durab i l i t y Test

California Cal i fornia Mid-

Venezuelan Coastal Val ley Continent

Aging index, ni t rogen 1 . 1 7 20.7

1 .35

1.40

Aging index in air 2.91

8 4 . 5

2.50

2 no

Air aging index

nitrogen aging index

'Film

5

microns thick aged

2

hr. at 225' F.; viscosi tydetermined at

O

F. in s l id-

ing plate microviscometer. Rat io of viscosity after aging to initial viscosity desig-

nated as aging index.

2 5 4.1

1 9

2 0

MOLECULAR W E I G H T OF FRACTION

Figure 7. Weigh t loss o f d is t i l l a t ion f rac t ions o f

Cal i fo rn ia coastaI asp ha1

3 5 0

4 0 0 4 5 0 5 0 0

LOWEST MOLECULAR WEIGHT FRACTION

IN

THE ASPHALT

Figure 8. In f luence o f in i t i a l molecu lar we ight on d urab i l i t y

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z AROMATICS

8 6 0 /

3

IC

5 0

d l r I

I

I < I

L- --- -

ALIF O RNIA VALLEY

4 LATURATES

0

20

40 6 0 8 0 100

%w O ASP H ALT

Figure

9

H ydroca rbon t y pe com pos i ti on

of

Cal i fo rn ia

va l ley aspha l t

nitrogen, and oxygen which make u p the resins. Althou gh these

trends a re generally found, there are large differences between

asphalts as demonstrated by the distribution curves for four

asphalts shown in Figures 10 to

12.

Note especially the low

saturate content and high concentration of aromatics in th e

initial fractions of the mid-continent asphalt relative to the

others.

From the data shown in Figures 10 to 12, a summary of the

composition

of

each asphalt may be derived as shown in Fig-

ure

13.

Th e outstandin g differences between these four asph alts

are the wide spread in asphaltene content ranging from

29.9y0

in the California coastal asphalt to 7.3% in the mid-continent

asphalt, the predominan tly arom atic character of the mid-

continent asph alt, and the high saturate an d resin content of th e

California valley asph alt.

Th e hydrogen-carb on ratio of the sa turate fractions proved

to be uniform over the wide molecular w eight range represented

in each asphalt. Th e averag e values of hydrogen-carbon ra t io of

saturate fractions

f rom

the individual asphalts are also very

similar as shown in Tab le V. The only source of d eviation from

0 1

I

0

20

40 6 0 8 0 100

w O F A S P H A LT

Figure 10. D i s t r i bu t i onof saturates

1

MID-CONTINENT

~ ~ ~~

Table V. Average H yd rogen -C arbon R a t i o (A t omi c )

of

Chemical Type Fract ions

Chemical

Type California California Mid-

Fraction Venezuelan coastal valley continent

Saturates

1 8 8

1

92 1 8 9 1 8 8

Aromatics

1 4 5 1 4 9 1 4 3

1

52

Resins

1 4 6 1 4 5 1 44 1 4 6

Hydrogen-Carbon R atio

the hydrogen -carbon ratio indicated by C,H( , , would be

expected to arise from cyclic alkanes an d their alkyl derivatives

(CnH2,J. The data indicate that the California coastal asphalt

would be expected to have a somewhat lower content of cyclic

alkanes (naphthenes) than the other asphalts.

Th e average values of the hydrog en-carbon atom ic ratio for

the aromatic fractions are also shown in Table V. The mid-

continent aromatics must contain more carbon atoms in satu-

rated structures than the aromatics from the other asphalts

studied.

O n e of the interesting findings from the ultraviolet absorp-

tion spectra of the arom atic fractions is th at the presence of

cataco nden sed tricyclic an d tetracyclic arom atic molecules is

limited to a few

per

cent of the arom atic fractions.

The average hydrogen-carbon ratio of the resin fractions

shown in Table V is about the same as the corresponding

arom atic fractions in th e case of the California valley and Vene-

zuelan asphalts, thus ind icating similar structures. Howe ver,

the resin fractions of the California coastal and mid-continent

asphalts have fewer saturated substituents th an th e arom atics

from corresponding fractions. Th e average numbers of various

heteroatoms per molecule in the resins shown in T abl e V I indi-

cate the high sulfur conte nt in the California coastal asphal t,

high nitrogen content in the California valley asphalt, a nd high

oxygen content in both the Venezuelan and mid-continent

asphalts.

Th e elemental composition of the asphaltenes

is

shown in

:

I

CALIFORNIA

CO ASTAL '

MID-CONTINENT

CALIFORNIA

0

2 0 40 60

8 0 100

% w O F ASP H ALT

Figure

12.

Dist r ibu t ion

of

resins

CALIFORNIA

CO ASTAL

YENEZUE

15.8%

5 3 . 1 %

IO.

0

0 . 5 %

CALIFORNIA

411

VALLEY

1 6 .9 %

54 .0

1 7 .8 %

d1 .3 %

M I D-

CO NTINENT

7 3 .3 %

1 3 . 7 %

352

Figure

13.

Chemica l t ype d is t r ibu t ion

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10

rA

w

5 -

4 -

3 -

d 2 -

I

k :

4 -

c 0 5 :

I

::;

A

5

0 . 2

Table VI. Heteroatom Content

of

Resin Fractions

Average

Hetero atoms California California M id-

per Molecule Venezuelan Coastal Val ley Cont inent

Sulfur

0 .54 1.11

0.21

0.44

Nitrogen

0.55 0.78 0.98

0.95

Oxygen

1.50 0.77 0.89 2.03

Total

2.59 2.66 2.08 3.42

c

1 -

-

-

able VII. Composition of Asphaltenes

T o t a l

M o l e c u l ar H / C H e t e r o at o m s

Weigh t Empi r i cal Fo rmu las (Atomic) per Mo lecu le

Venezue lan 4719

C 3 , , H 3 , , N 4 3 8 S s72 3

1 .13 13 .90

California

coastal 3500 CL~4H295N4 e 0 3

3 3

1.26 16.54

California

Mid-

2530

C , 8 4 H 2 0 8 N 3

6 f i s 0

7 f i 0 1

9 3

1.13 6.25

con t inen t

4019 C, , ,H, , , ,N, 9 5 S ,

ofi02

6 1.12 5.77

T a b l e V I I . Very l it t le is kno hn abo ut the degree of complexity

of the aro mat ic r ing systems involved, but it is clear from the

relat ively high hydrog en-carbon rat ios that a co nsiderable frac-

t ion of the carbon atom s present must be nona roma tic.

Viscosity of Chemical Type Fractions.

The viscosities at 77 F.

of

the various type fractions obtaine d from the sep aration of the

California valley asphalt shown in Fig ure 14 are typical of data

generally obtaine d. At a given molecular weight level, the

saturates ar e lowest in viscosity, the aromatics are interme diate,

and the resins the highest . With such large differences in vis-

cosity of the three types

of

components ,

it

is easy to understand

how varying concentrations could lead to different viscosities at

a given molecular weight level (Figure

5)

for the molecular dis-

t i l lation cu ts from various asphalts.

Th e viscosity of the saturates from the fo ur asphalts r ises with

increasing molecular w eight; the data for al l the saturate frac-

t ions

fit

within a narrow band as shown in Figure 1 5 . Th e vis-

cosity-temperature slopes in Figure 16 over the range

77

to

210 F. ar e also similar. The susceptibil ity of viscosity to change

with tem per atu re decreases with increasing molecular weight of

the saturate fraction.

In contrast to the similari ty of the satu rate fractions, the

arom atic fractions from the four a sphalts have widely different

viscosit ies at the same molecular weight as shown in Figure 17.

The general level

of

viscosity of aromatics from the various

sources appea rs to be associated with the am ounts of saturated

substi tuents, the California valley aro matics with the least alkyl

substi tut ion havin g the highest viscosity an d the mid-continent

arom atics with the greatest amo unt of saturated grou ps having

the lowest viscosity. T he viscosity

of

the aromatic fractions be-

comes less susceptible to change with temp erature at higher mo-

lecula r weights, as shown by th e plots of slope of the viscosity-

300 400 500 600 700

800

9 0 0 1 000

MOLECULAR WEIGHT

Figure

14.

Viscosity of chemical type fractions

of

a

California valley asphalt

CALIFORNIA VALLEY

ID-CONTINENT

0. 1 I

I

I I

I

I

I I

200

600

1,000 1,400 1,800

MOLECULAR WEIGHT

Figure

15.

Viscosity of saturate fractions

CALIFORNIA

't /

C O A S T A L

d U

w w

-4.0 1

I I I

I

I I I

g zoo 600 1,000 1,400 1.800

s

M O L E C U L A R W E I G H T

Figure 16. Viscosity-temperature slopes of saturate fractions

temp erature l ines (Walther equation) vs. molecu lar weight in

Figure

18.

The decrease of slope with increasing molecular

weight is only sl ight in the mid-continent aromatics in which

the atomic hydrogen-carbon rat io decreases sl ightly with in-

creasing molecular weight. I n the oth er asphalts there is a gen-

eral upw ard trend of the hydrogen-carbon rat io w ith increasing

molecular weight which may account in part for the gene ral de -

crease in viscosity-temperature slopes.

IO

10'

z

E1ozl

0

/

1

I I

I I I I 1 1

200 6

1,000

1.400 1,800

MOLECULAR WEIGHT

Figure 17. Viscosity of aromatic fractions

- 2 . 5

MID-CONTINENT

CALIFORNIA COASTAL

V A L L E Y

/VENEZUELAN

-5.51 I / I I I I I I I I

200

600 1,000 1,400 1,800

M O L E C U L A R W E I G H T

Figure

18.

Viscosity-temperature slopes of aromatic fractions

VOL.

4, No.

4

OCTOBER

1959

353

8/12/2019 1959 Influence of Composition of Paving Asphalt on Viscosity, Viscosity-Temperature Susceptibility, And Durability

6/6

Differences in viscosity of the resin fractions of the same mo-

lecular weight from vario us sources shown in Figure 19 are even

greater than in the case of the aromatic fractions. The depend-

ence of viscosity level on source

is

essentially the same as in the

case of the aromatics , the resins from the C alifornia valley

asphalt be ing the most viscous and those from the m id-contin ent

asphalt t he least viscous. Th e decrease

of

the viscosity-tempera-

tur e slope with increasing mo lecular weight is greatest in the

case

of

the resins from the Ca lifornia valley asp halt an d least

for

the mid -continen t resins as shown in Figure 20.

The viscosity of a mixture

of

ideal l iquids can be calculated

from the viscosities of the components and their mole fractions

( 2 ) .

Using the measured molecular weights and viscosities of

satur ate, aromati c, and resin fractions, calcu lations of th e vis-

cosity of the mixture (the molecular disti l lation fraction) have

been mad e. T he calculated viscosities are in reasonable agree-

ment with observed values at low molecular weight levels, but

at higher molecular weights, the calculated viscosities are al-

ways lower than the observed values. This discrepancy between

calculated and observed values is generally attributed to the oc-

currence of association between some of the various components .

/ / MID-CO NTINENT

1

10

>

L l

I

_ -

200 600 1,000

1 400

1 800

MOLECUL AR WEIGHT

Figure 19. Viscosity of resin fract ion s

In spite of this nonideal behavior of the components in mix-

tures,

it is

possible to contro l the flow properties of asphalts by

blending of selected fractions chosen on the basis of hy droc ar-

bon-type composition.

A t

the same time, the durabili ty or re-

sistance of the asphalt to change can be controlled to a very

large degree by pro per attention to initial molecular weight.

Durab i l i t y of Chemical Type Fractions.

The hiqh aging index

of all fractions of less than 400 molecular weight

is

due to vola-

t i l i t y , and this overshadows any effect of chemical type. Above

400 molecular weight, volatility

is

insignificant, and oxidation

is

the important factor effecting the aging index. Accordingly,

the aging index in ai r of the tvp e fractio ns above

400

molecular

weight was determ ined in the microfilm d urab ility test and

is

summarized in Table VII I .

Saturates from the Venezuelan a nd California coastal asphalt

showed no hardening (aging index

l . O ,

while saturates from

- 2 . 0

I

CALIF O RNIA VALLEY

I

I I I I I

I

I I

1 1 1

200 600 1 ,000 1,400

1,800

MOLECULAR WEIGHT

Figure 20. Viscos i ty - temperatureslopes o f res in fract ions

Table VIII. Harden ing

of

Hydrocarb on Types above 400

Molecu lar Weigh t in the Mic rof i lm Durabi l i ty Tes t

A c h e Inde x in A i r

California California Mi d-

Ty pe Fraction Venezuelan coastal valley continent

Satura tes 1 o 1 o 1.4-2.8 . . .

Aromatics

1.3-1.8 1.2-1.9 1.8-4.0 1 o-2.0

Resins

1.3-4.1 1.6-2.6 1.6-4.2 1.0-2.2

To o fluid for test

the California valley asphalt had aging indexes which varied

from 1 .4 o 2 .8 .

Aromatics from the California valley asphalt had air aging

indexes which varied from 1 to 4 while those from all the other

asphalts had air aging indexes below 2.0 .

In the type fractions from these asphalts, th e resins had the

highest aging indexes. Th e values for the Venezuelan and Cal-

ifornia valley resins varied from 1.3 to 4.2, while th e mid-

continen t and Califo rnia coastal had a maxim um of 2.6.

Th e dura bility of the various asphalts as shown by th e m icro-

film du rabilit y test

is

reasonably well accounted for by the data

from the type fractions from the distillation cuts with the excep-

tion

of

the California coastal asphalt . Th e ratio of the air aging

index to the nitrogen aging index for the California coastal

asphalt was 4.1 (Ta ble V), while the comparable ratio for the

other three asphalts varied from

1 . 9

to

2.5.

Because the air

aging index for the type fractions from the C alifornia coastal

asphalt is lower tha n that for some others, t he question arises as

to what

is

responsible for the high air aging index of the whole

asphalt . The answer

is

found in the effect of the asphaltenes.

Th e California coastal asphaltenes proved to be more sus-

ceptible to hardening by oxidation than any of the other as-

phaltenes. Synthetic asphalts prepared by blending the asphalt-

enes from one crude with the fractions from another crude

showed high aging indexes whenever the asphaltenes from the

Californ ia coastal asphalt were used. Th e flow properties of

such asphalts prepared by blending of selected fractions are o ut-

side the scope of this articl e.

C O N C L U S I O N

Composition data of the type shown here can be a valuable

aid to the refiner in choice of crudes and blending stocks for the

manufacture of asphalt and can provide flexibility in the con-

trol of flow properties and durability of the products. The

chemical constitution of asphalts is far too complex to be used

as a basis for simple specification tests,

so

the user would specify

the desired flow properties an d dur ability in the m ost precise

and meaningful terms possible and rely on the refiner to apply

his knowledge of composition of crudes and blending stocks as

well as processing methods to produce materials meeting these

well-defined p roperties.

ACKNO WLEDG M ENT

Fredenburg for their assistance with the experimental work.

LITERATURE CITED

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

R E C E I V E Dor review Septemb er 2, 1958. Accepted F ebruary 18, 1959.

The authors express appreciation to C.

E.

Creely and A. D.

Am. SOC. es t ing Mater ia ls , AS TM Sta ndard s

D

4 4 5 53T.

Bingham, E .

C.,

Fluidity and Plasticity, p . 81, McGraw-Hil l ,

New York, 1922.

Gilma n, H . , Orga nic Che mist ry , 2nd e d . ,

11,

1747, Wiley, New

York, 1943.

Griffin, R . L . , Miles,

T.

K . , Penther, C. J.,

Proc

4ssoc. Asphalt

Paving Technologists

24, 31 (1955).

Griffin, R. L . , Miles,

T.

K. Penther, C. J . , Simpson, W. C . , Am.

SOC. es ting M ater ia ls Specia l Tech. P ubl . No. 212,36 (1957).

Heithaus,

J .

J . , ohnson, R. W.,

Proc

Assoc. Asphalt

Pauzng

Tech-

nohgists 27, 17 (1958).

ODonnell , Gordon, Anal. Chem. 23,894 (1951).

Va n O or t , W. P . , I d .

Eng

Chem. 48, 1196 (1956).

Wa l the r , C . , Oel u . Kohl 11, 684 (1953).

354

JOURNAL

OF

CHEMICAL AND ENGINEERING DATA