VINYL POLYMERIZATION IN A S P H A L T I C MEDIA ALLEN E. LEYBOURNE I l l 1AND HERBERT E. SCHWEYER Uniuersitj of Florida, Gainesville, Fla.
The presence of free radicals in asphalts has been established b y electron spin resonance measurements. This evidence has then been used in proposing a mechanism for vinyl polymerization in an asphalt medium. The proposed mechanism indicates a second order rate at constant concentration of asphalt present. It i s proposed further that the polymerization rate constant passes through a maximum at low concentrations of asphalt. It i s implied that the study of the presence of free radicals may explain the reactions that asphalts undergo in processing and in service performance.
HE PRESESCE OF STABILIZED FREE RADICALS (hereafter Tdesignated SFR) in asphalts has been suggested in the literature ( 4 ) . I t does not appear to be generally recognized that these free radicals may be capable of reaction, especially with vinyl monomers. I t is proposed herein that the kinetics of homogeneous vinyl polymerization as influenced by asphaltic material may be interpreted if certain of the reactive properties of asphalts are attributed to the SFR contained therein. Once the reactive properties of asphaltic free radicals are thoroughly understood, it may be possible to explain partially such phenomena as the behavior of asphalts in air-blowing processes and hardening under service conditions. I t has been the purpose of this investigation to contribute to a better understanding of the influence of these SFR.
Theoretical
Evidence of the Existence of SFR. Many naturally occurring materials exhibit electron spin resonance, indicating the presence of free electrons ( 9 ) . Because of the observed longevity of the species with free electrons, they have been designated as stabilized free radicals. There are also free radicals which may be present as a result of what may be visualized as mechanical trapping in the lattice. This type of free radical is not considered to be important here. Electron spin resonance has been observed for the high molecular weight species present in crude oils precipitated by ultracentrifugation and for the high molecular weight fraction of asphalts (4. 7). The asphalts used in this study were subjected to analysis by electrcn spin resonance. I t was determined that the concentration of unpaired electrons varied from 10'6 to 1017 SFR per gram. By far, the greatest concentration of free radicals was present in the asphaltene fraction of these asphalts. Because of the complexity of the molecular species present in asphalts, little detailed knowledge is available to elucidate Present address, American Oil Co., Texas City, Tex.
their chemical compositions. However, there is sufficient evidence to point up the similarity of the structures of known free radicals. which have a high degree of resonance capability, to the structures of the high molecular weight species present in asphalts (3. 5, 9, 73, 74. 16). I t has been proposed that highly condensed aromatic nonhydrocarbon systems comprise the central portion of the asphaltene micelle occurring in asphalt systems. There are materials present in petroleum products, such as vanadium, which give rise to electron spin resonance absorption other than free radicals. Several investigators (6. 7, 72) have obtained data which indicate that electron spin resonance arising from this source is less than 10% of the observed absorption. Proposed Polymerization Mechanism. From a consideration of the principles of thermal polymerization of vinyl monomers, such as discussed by Bamford and others (7), and several assumptions as to the characteristics of the properties of stabilized free radicals in interactions with these monomers, a mechanism has been proposed to represent the intermediate steps of the polymerization process. Chain transfer with the solvent, which is often considered in many vinyl polymerizations, has been neglected in this development. It is reasonable to assume, as concluded from the work of others (2, 77), that this does not substantially contribute to the over-all rate of the polymerization process. The mechanism for vinyl polymerization in the presence of SFR is as follows. If M represents monomer and R* is a free radical of species defined by a subscript, then a proposed mechanism might be as in Table I. If the polymer chain length is relatively great, the over-all polymerization rate will be governed by the rate of monomer reacting in the propagation steps. From the preceding statement of the assumed mechanism it can be shown analytically that the over-all polymerization rate may be expressed as:
+
~
=
-d(M)/dT = kpc(R*c)M
n
k,,(R*,)M
(1)
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127
r = l
VOL. 1
NO. 2
Table I.
If the term preceding MZin Equation 6 is set equal to K , the over-all polymerization rate constant, the result is :
Proposed Polymerization Mechanism
(7)
Equations Governing Rate Initiation Phase
+M R*, + M + M R*, + M M
By separation of the variables and integrating between the limits, T = 0, T, and M = Mo, M, the following equation may be obtained:
+ 2R*, +M +
+ R*,
-1= K T + x 1 M
R*l
Propagation Phase
+M R*z + M + AI R*1
R*3
+
R*2
+
R*3
+
R*4
A plot of 1/M us. Twill be a straight line for a process having this type of rate dependence. The slope of this line will be identically equal to K. From the substitution to obtain Equation 7 , K is:
K = A[(R*,'
+M
R*,A
(7)
+
+ C)'.'
- R**]
(9)
The constants A , B, C, and D have been evaluated by systematically varying them and comparing the calculated values of K with experimental ones. I t has been found expedient to use the IBM 650 digital computer for this evaluation.
R*,
Termination Phase
-
+
With active free radicals:
R*,
(8)
+ R*,
Experimental
Dead polymer
With stabilized free radicals:
+
+
R*, R*, M + Dead polymer (9) a Concentration dependence of rate in this step assumed nonlinear in R*,. Therefore, forward rate equation for this step is arbitrarily written as: d(R*,)/dTrornsrd = kdR*,)D(M)2 Exponent D has been introduced to represent a simple nonlinear function.
Utilizing the assumption that thc k,'s are identical and noting that:
1 = I
by definition: k,(R*,).M
-d(zM)/dT
(3)
The rate of change of concentration of any free radical species may be stated in general as: Rate of change of species R*i = formation rate of species R*, - propagation rate to species - termination rate of species R*, Utilizing the principle of the steady state-the concentration of any given free radical species remains constant with time-it is possible to determine R*, as a function of the stabilized free radical concentration, R*,. The following expression results: R*,
=
(
f
M(kta/2kto)
fl-( 2 k c + k, R*,D)10.5- R*,) (4)
4k
Combining Equations 3 and 4 yields: -d(M)/dT
=
kpkt,/2kia x
( [R*82+
4k
(2ko
Results
+ ksR*BD)]OJ- R*8).li'
(5)
By a suitable grouping of the individual reaction step rate ocnstants and rearranging, a n equation of the following form results:
128
Several asphaltic rrsidua, representing asphalts of high and low asphaltene content, werc used in this study. The principal asphalts investigated were a Gulf Coast naphthenic residuum (S119) of l o ~ vsulftir and low asphaltene content and a n East Central Texas residuum (S120) of high sulfur and high asphaltene content. Asphaltene fractions were prepared from these asphalts by precipitation with n-pentane. Two other residua were used in these studies: S117 was an East Texas asphalt base material and S118 was a South Texas heavy residuum. Several monomers were investigated, but this report is confined to results using styrene. The polymerization experiments were conducted in a n American Instrument Co. high pressure rocking autoclave, equipped with automatic temperature and pressure control. The charge concentration of styrene was maintained a t a constant value of 10 weight % in all experiments. The remaining 90 weight % of charge material comprised the asphaltene or asphalt used and xylene, which was used as a diluting solvent. The asphalt or asphalt-xylene mixture was charged to the reactor, the reactor sealed, and then brought to the desired temperature of the polymerization experiment prior to introducing the monomer. Analysis of the monomer a t any given time was achieved with a selective microhydrogenation technique. This hydrogenation was conducted in a quantitative manner by hydrogenating the olefin over a palladium on charcoal catalyst after dissolving the sample in a suitable solvent (reagent grade xylene). Measurements of electron spin resonance were made by Schlumberger Corp. using a Strand Labs Model No. 600 spectrometer a t 9.47 kmc. and 3380 gauss. Samples were prepared by means of scooping the sample into 1.5-mm. diameter borosilicate glass capillary tubes.
l&EC PROCESS DESIGN A N D DEVELOPMENT
It was found that the polymerization rate is second order with respect to monomer concentration at a constant concentration of asphalt or asphaltene present. Typical polymerization data are plotted in Figure 1, with reciprocal monomer concentration us. time. These data, determined a t different temperatures, are adequately represented by a straight line. In these polymerizations and others the values
0.9
*
0.8
3 z- 0.7 0
k f
0.6
z u W 0.5 0
Figure 1 .
W
E a
Typical polymerization data
0.4
> + 07 -1
0.3
9
8P
0.2
0 W
a
0.I
0
200
400
600
800
1000
1200
1400
1600
1600
POLYMERIZATION RUN TIME, MINUTES
of the slope of the data plotted in this manner have been determined. These values may be interpreted as the over-all polymerization rate constant. Figures 2 and 3 show the Arrhenius-type temperature dependence. An over-all activation energy of 20,700 cal. per gram mole has been calculated from the data for the styreneasphalt systems shown in Figure 2. This is about the same as determined for styrene by previous investigators in other solvent systems ( I ) . The air blowing of the naphthenic residuum has caused an increase in the polymerization rate, as shown in Figure 2. The data plotted in Figure 3 show the effect of the different asphaltene contents of several asphalts and summarize the data reported here. Components present in the asphaltene fraction have been found to be of primary importance. A maximum accelera-
- 7,0001 5,000. 3 x
3,000
tion of the polymerization process occurred at low concentration of the asphalts studied. These data are presented in Figure 4. The asphaltene contents are given in Table 11. This same effect of maximum acceleration was observed at much lower concentrations of the asphaltenes, thus indicating the diluting effects of the other asphalt fractions in the whole asphalt which presumably contain a smaller amount of the reactive species per unit mass. Plots of these asphaltene data, shown in Figure 5 , have the shape characteristic of the data of Figure 4 u p to 40% asphalt. In Table 11, data are given for values of the free radical concentrations of the asphalts and asphaltenes investigated in this study. As indicated, these determinations have been made by electron spin resonance techniques. Calibration has
I
'0 \
I I
I I
I
1
I
1 I
W-
0
1,000 700 500
X Y
sF z
u)
8
I
300
IO0 70
50
3
I
I
2
Figure 3.
Summary chart of rate constants
A - HIGH ASPHALTENE CONTENT CONTENT
- 8 - LOW ASPHALTENE
b VOL.
1
NO. 2 A P R I L 1 9 6 2 129
been by comparison with a 0.002M aqueous solution of MnC12. I t is apparent that the majority of the SFR are concentrated in the asphaltene fraction of these asphalts.
Table II. Free Radical Concentration of Asphalts and Asphaltenes Calculated Concentra. Relative tion,e Signal,b Free Instrument Radicals1 Asphaltenes,a Value/ Gram Sample % Gram 70-10 Asphalts S 117 S 118 S 119 21.4 510 4.4 s 120
x
wt.
Asphaltenes S 61-6 (from S 119) S 61-9 (from S 120)
Discussion
-100 -100
5.7 11
660 1281
Method of Hubbard and Stanfield ( 8 ) . Vaiues estimated accurate to within about 70% of value determined; average of duplicate determinations. Determined by comparison with a 0.002M aqueous solution of MnCiz.
Constants Determined for Theoretical Polymerization Rate Constant Equation
Table 111.
Systema
A
Styrene-xylene-S 119 asphalt or asphaltene Styrene-xylene4 120 asphalt or asphaltene
Values of the Constantsb C X 7015
B X 709
D
290
9.5
1.90
0.7
547
8.1
5.3
0.7
a Values determined by visually comparing calculated curves with experiValues for constants cited to be used when R * , zr mental data for K. expressed as gram moles S F R per kg. of system and K as ( % monomer-’) (min.-l).
The maximum acceleration of polymerization rate, as indicated by the data of Figures 4 and 5, is believed to be the result of a competition between the accelerating influence of SFR on chain initiation and termination. This conclusion follows from an analysis of Equation 5, by alternately setting k , or k , equal to 0. A zero value of k,, the monomer selfinitiation rate constant, allows the influence of SFR initiation of polymer chains to be evaluated. The zero value of k,, the monomer SFR-initiation rate constant, allows for consideration of the separate effect of SFR-termination. Termination of peroxide-induced polymerization by SFR and Gilsonite asphaltenes has been observed by Wright (75). No acceleration was observed in this study. This failure to observe acceleration is attributable to the overwhelming chain initiation by active peroxide catalysts. In the present study, however, it is obvious that polymerization acceleration has occurred. Figure 6 compares the polymerization rate constants calculated from Equation 9 a t a given R*, with those determined from the experimental correlation implied by Equation 8. The free radical concentration in Figure 6 was determined from the asphalt concentration in the solution and the SFR concentration in the asphalts, as shown in Table 11. Values of the constants of Equation 9 used to calculate the curves of Figure 6 are given in Table 111. It has been possible to correlate with one set of values for these constants the effect of SFR from the asphalt S 119 and asphaltene S 61-6 (asphaltene from S 119) and with another set of values for these constants
20 D
P
SYSTEMS (300OF
X
x
5
16
>
bg 0
ASPHALT- XYLENE-IO% STYRENE
12
w x
Figure 4. Influence of asphalt concentration on rate constants
t?
=z I z
e t 3 8
10
4
5
g 0
0
IO
20
30
40
50
60
70
80
90
WEIGHT PER CENT ASPHALT
20 n
P X X
16
12
Figure 5. Influence of asphaltene concentration on rate constants
8
4
0
0
2
4
6
8
IO
12
WEIGHT PER CENT ASPHALTENES
130
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
14
16
I8
20 n
2
SYSTEMS (?OO°F)
X
Y
16
10% STYRENE A S 120 - XYLENE STYRENE A S 61-9-XYLENE-STYRENE 0 S I I 9 XYLENE -STYRENE 0 S 61-6-XYLENE-STYRENE CALCULATE0 CURVES
-
-
-
Figure 6. Comparison of theory with experimental results
,
I
I
A
I
” 0
I .o
0.5
I .5
2.0
FREE RADICAL CONCEKTRATION, Rs* X IO’,
the effect of SFR from the asphalt S 120 and asphaltene S 61-3 (asphaltene from S 120). The ability of Equation 9 to correlate the influence of the SFR variable is believed to constitute a strong argument for the theory presented. In addition, the observed Arrheniustype temperature dependence of the polymerization rate constant in these systems is indicative of reaction rate control via the chemical reaction mechanism rather than one where mass transfer is important. At high asphalt concentration, the data shown in Figure 4 indicate that the trend in polymerization rate constant is to increase. This is probably caused by aggregation and selective solvation of the active asphaltene SFR in this concentration range. The effective free radical concentration of the SFR in the mobile portion of the solution will then become limited. This effect is described in greater detail by Leybourne (70). It is ascribed to a n increased initiation rate with respect to termination rate, although both rates decrease. Conclusions
A reaction mechanism for polymerization of styrene has been presented considering the influence of SFR derived from asphalts. An equation has been derived which correlates the effects of monomer and SFR concentrations. This is offered as ajustification of the probable validity of the theory presented. Principal aspects of this theory are that free radicals present in asphalts initiate polymerization via a termolecular reaction involving the SFR and two monomer molecules and that they terminate chains via a termolecular reaction involving a n SFR, active free radical, and monomer molecule. This investigaticn has shown that asphalts containing SFR may have catalytic and inhibiting properties. At 300’ F., competition between these two effects results in maximum acceleration of the polymerization rate a t low concentration of the asphalts containing the free radicals. T h e heretofore unnoticed reactivity of SFR present in asphalts may offer a means of obtaining greater insight into the problems of asphalt processing. Nomenclature
A , B, C? D = empirical constants in kinetic equation for polymerization rate constant, Equation 9 k = reaction mechanism rate constant K = polymerization rate constant
2.5
3.0
3.5
GM. MOLES F. R . PER KG.
M R*
= monomer molecule or monomer concentration = free radical or free radical concentration identified by
T
= time
subscript
Sub scripts a c
i 0
p
s
t
active free radicals copolymer ith item initial conditions polymer species = stabilized free radical = termination = = = = =
Acknowledgment
T h e courtesy of Texaco Corp. in supplying asphalt samples is gratefully acknowledged. literature Cited
(1) Bamford, C. H., Barb, W. G., Jenkins, A. D., Onyon, P. F., “The Kinetics of Vinyl Polymerization by Radical Mechanisms,” Academic Press, New York, 1958. (2) Breitenbach, J. W., Maschin, A. Z., Z.physik. Chem. A187, 175 (1940). (3) Clerc, R. J., O’Neal, M. J . , Anal. Chem. 33, 380 (1961). (4) Corbett, L. W., Swarbrick, R. E., Proc. Assoc. Asphalt Paving Technologists 27, 107 (1958). (5) Corradini, G., Giona, A. R., Mariani, E., Riv. combustibili 13, hTo. 3, 187 (1959). (6) Eldib, I. A., Dunning, H. N., Bolen, R. J., J. Chem. Eng. Data 5 , 550 (1960). (7) Gutowsky, H. S., Ray, B. R., Rutledge, R. L.. Unterberger, R. R., J . Chem. Phys. 28, 744 (1958); Brown, T. H., Gutowsky, H. S.,Van Holde, K. E., J . Chem. Eng. Data 5 , 181, 1960. (8) Hubbard, R. L., Stanfield, K. E., Analyst 74, 470 (1949). (9) Ingram, D. J. E., “Free Radicals as Studied by Electron Spin Resonance,” Butterworths, London, 1958. (10) Leybourne, A. E., Ph.D. thesis, University of Florida, 1961. (11) M,ayo, F. R., J . A m . Chem. SOC.65, 2324 (1943). (12) 0 Reilly, D. E., J . Chem. Phys. 29, 1188 (1958). (13) Schweyer, H. E., Univ. Florida Eng. Industrial Expt. Sta. Bull. Ser., No. 89, June 1957. (14) Traxler, R. N., Romberg, J. W., Petrol. Engr. 30, No. 10, C-37 (1958). (15) Wright, R. D., Ph. D. thesis, University of Utah, 1960. (16) Yen, Teh Fu, Erdman, J. G., Pollack, S.S., ACS Division of Petroleum Chemistry Preprints, General Papers, 6, No. 1, 22 (March 1961). RECEIVED for review September 5, 1961 ACCEPTEDJanuary 22, 1962 Research supported by Ethyl Corp. fellowship and assistance under National Science Foundation grant NSF-GI 4249. VOL. 1
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