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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
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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
VOL.
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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