December 1948
1NDU.STRIAL AND E N G I N E E R I N G CHEMISTRY ACKNOWLEDGMENTS
The writer wishes t o thank L. A. Mikeska, S. B. Lippincott, and L. T. Eby for the synthesis of the many compounds required during the course of this study. H e is also grateful t o W. C. Smith and R. L. Zapp for carrying on much of the evaluation work involved, and t o others in the organization with whom it was his pleasure to discuss many of the points involved. LITERATURE CITED (1) Am. SOC. Testing Materials, Standards on Rubber Products, D 824-411'. (2) Farmer, H. H . , Trans. Faraday SOC.,3 8 , 3 4 0 (1942). (3) Fisher, H. L., IND.ENG.CHEM.,31, 1381 (1939): U. S. Patents 1,918,328 (1933); 2,170,191 (1939).
2319
(4) Flory, P. J., and Rehner, J., Jr., IND. ENG. CHEM.,38, 500 (1946). (6) Haworth, J. P., U. S. Patent 2,393,321 (1946). (6) Lessig, E. T., IND. ENQ.CHEM.,ANAL.ED., 9, 582 (1937) ; Rubber Chem. and Technol., 11, 249 (1938). (7) Mooney, M., IND.ENQ.CHEM.,ANAL.ED.,6, 147 (1934). (8) Nietaki, R., and Kehrmann, F., Ber., 20, 613 (1887). (9) Ostromislensky, I. I., J.Russ. Phys. Chem. SOC.,44, 204 (1912). (10) Rehner, J., Jr., and Gray, P., IND. ENQ.CHEM.,ANAL.ED.,17, 367 (1945). (11) Thomas, R. M., and Sparks, W. J., Australian Patent 112,876 (1941).
RECEIVED April 3, 1947. The work reported here was carried out during the past five years. Earlier publication was withheld in accordance with secrecy orders of the United States Patent Office.
RESIN-PLASTICIZER SYSTEMS Analysis of Log Stiffness-Temperature and Viscosity Characteristics G. J. DIENES AND F. D. DEXTER Bakelite Corporation, Bound Brook, N . J .
A
q u a n t i t a t i v e method of c o m p a r i n g resin-plasticizer
systems in t w o important temperature regions is de-
scribed. The characteristics in the temperature region of application are evaluated f r o m log stiffness-temperature curves, and those in the processing temperature region by parallel-plate plastometer measurements. A simple and practical empirical equation is developed w h i c h describes m a t h e m a t i c a l l y the inverted S-shape log stiffness-temperature curves. The equation is characterized by four physical constants: the limiting stiffness a t low and high temperatures, the temperature, T,, at w h i c h the slope of the log stiffness-temperature c u r v e has its m a x i m u m value, and the m a g n i t u d e , S,, of this m a x i m u m slope. This equation is s h o w n to be applicable to a l a r g e variety of materials. The variation of T, w i t h pbr cent plasticizer, w h i c h is approximately linear i n the 15-50qo concentration
I
N ORDER t o describe theessentialeffects of the addition of a plasticizer to a resin, it is necessary to consider the mechanical
and flow properties in two general temperature regions. The first, which covers a fairly extended range, is t h a t region in which the compounds may find ultimate use and is described as the application temperature region. The second is the region of processing temperatures. T o be useful such evaluation must be expressible in terms of some simple, physically meaningful constants which are indices of such important properties as plasticizer efficiency and temperature sensitivity. One of the most useful methods for evaluating the mechanical property of elastomers in the application temperature region is the determination of stiffness as a function of temperature. I n a stiffness measurement, usually carried out in flexure or torsion, a constant load is applied to the specimen for a given time interval ( 5 to 10 seconds), the resulting deformation is measured, and the stiffness calculated from well-known formulas of the theory of elasticity. Since for plastic materials the deformation, in general, is complex and contains elastic, delayed elastic, and viscous components, the resulting stiffness is but one point on the deformation-time or creep curve. The stiffness-temperature curve is, therefore, arbitrary but nevertheless useful. Log stiffness-temperature curves show a characteristic inverted S-shape. It is possible, in principle, to derive an equation
range, is s h o w n to be a good measure of plasticizing efficiency. The m a g n i t u d e of the slope, S,, w h i c h is approximately independent of the plasticizer concentration in the 15-50% region, measures the temperature sensitivity of the resin-plasticizer system. The most useful q u a n t i t y for c o m p a r i n g resin-plasticizer s y s t e m in the vicinity of processing temperature is the absolute viscosity obtainable b y parallel-plate plastometer techniques. It is s h o w n that the log viscosity-per cent plasticizer c u r v e is linear in the 15-50% concentration range. The slope of this c u r v e is a measure of the h i g h temperature plasticizer efficiency. Thus, t w o s u p p l e m e n t a r y analyses permit q u a n t i t a t i v e comparison of various resin-plasticizer systems in the t w o impofiant temperature regions-the general temperature region of application and the region of processing t e m p e r a t u r e . .
for these curves from a mathematical expression for creep deformation as a function of time and temperature. However, the theory of creep phenomena is not far enough advanced t o carry out such a treatment. Tuckett (6) obtained a stiffnesstemperature relation based on a simple mechanical model which predicts a curve of the correct general shape but does not describe the experimental data. Thus a need exists for describing the stiffness-temperature curves in terms of some simple function containing constants which can be easily interpreted and have physical meaning. The development of such a n empirical equation makes i t possible to describe quantitatively such propertim as temperature sensitivity of the resin-plasticizer system and concentration efficiency of the plasticizer. The effect of plasticizers at elevated temperatures is most conveniently evaluated by means of a previously developed parallelplate plastometer technique (4, 6). The principal feature of these tests is that elastic, delayed elastic, and viscous components may be separated and evaluated. Of these, viscosity is the most convenient constant t o employ in determining compound characteristics in the processing region of temperatures. The activation energy for viscous flow measures the high temperature sensitivity. T h e slope of the log viscosity-per cent plasticizer curve determines the high temperature plasticizer efficiency.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2320
Yol. 40, No. 12
N
'j m
wLo-
B
l2z 1w,ooo
\ E X ~ R I M E N T A LSTIFFNESS' ' -CALCUUTEO CURVE FROM
FITTED EQUATION
I
:
-Eo
-50
j
DEG.CENT
;EMPERATURE,
-32
-40
-
2 .e2
237
c.31
I
a
I I
0 1
10
1o.ax
10 2.47
Figure 2.
1
1
il l l
-80
-60
*\
VALUES
40
-
TEMPERATURE, E O . CENT.
Stiffness-Temperature Curve for Polyethylene
TIIEORETICAL COIVSIDERATIQNS
Log stiffness-temperature curvcs of clastomers exhibit the well known inverted S-shape which suggest a n integral distribution function, as pointed out by Clash and Berg (1). The obvious distribution functions, such as t'he normal curve and the Gompert,z gromh curve, cannot, be fitted t o the experimental data. It was found, horn-ever, that a generalized Cauchy type distribution function is applicable t o a wide variety of stiffncsstemperature curves. The proposed function is:
stiffness (or apparent modulus of elasticity) at an!' temperature E L = limiting value of stiffness at low temperature EH = limiting value of stiffness at hizh temperature il' = temperature. degrees absolute A,n = constants
nhere E
=
Any stiffness-temperature curve is characterized theiefore, on the basis of Equation 1, by four experimentally deteiminable const an ts. For fitting experimental curvcs, Equation 1 may be put in the mole convenient form,
Taking logarithms of both sides gives: log Z
=
log A
+ n log T
-?a
(3)
d
I
2p
40
TEMPERATURE. DEG. CENT.
Thus, plotting log Z against log T should result in a straight line, from which A and 1~ are easily calculated, provided the correct
Figure 3.
Stiffness-Temperature Curves for Vinyli te T'YNW f Dioctyl Phthalate
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2321
Equations 6 and 7 permit the calculaticn of T , and the maximum slope from A and n. The stiffness-temperature curve is completely determined now, in addition to EL and E H , by T , and S, instead of A and n. The reverse transformation-the calculation of A and n from given valucs of TfSand S,-can be carried out uniquely by solving Equations 6 and 7 simultaneously, provided n is positive. This is always the case since stiffness decreases with increasing temperature. These constants, therefore, determine completely the stiffness-temperature curve. The determination of the characteristic constants EL,E x , A , and n is based on Equation 3. The correct value of EL and EH are determined by trial and error. The log 2 vs. log T plots are very sensitive to changes in EL and E H . EL changes the low temperature end of the curve radically but leaves the high temperature end almost unchanged; EH does just the opposite. These two factors are balanced against each other, until a good straight line of log Z us. log 2' is obtained. This procedure is illustrated in Figure 1 for the stiffness-temperature curve of a vinyl chloride-acetate resin (Vinylite VYNW)-45% dioctyl phthalate compound. Once a good straight line is obtained, constant 72 is given by the slope of this line. T , and S, are easily obtained, particularly if Equations 6 and 7 are transformed as follows: Since A T ; = 2, = value of Z at, T,, Equation 6 can be written:
n - 1 z, = n+l Since a 2 us. T plot is already available, temperature Tm can be read off the graph directly as the value corresponding to By the same substitution the expression for S, becomes: Z,.
s,
TEMPERATURE, DEQ. CENT.
Figure 4.
Stiffness-Temperature Curves for ButadieneAcrylate Rubber Compounds
valucs of EL and EH have been chosen. This is readily done by trial and error, as will be shown later. From the form of the equation, constants EL and EN 'are immediately interpreted as the limiting values of stiffness a t low and high temperature, respectively. A physical interpretation of A and n is not evident; hence, it is preferable to transform them into two other constants. A striking feature of a log stiffness-temperature curve is its steep rise over a rather small temperature interval. Two useful constants are the temperature at which the log stiffness-temperature curve has its maximum slope and the magnitude of the slope a t this point. The transformation from A and n to the two new constants is easily carried out. Differentiating Equation 1 with respect to the temperature yields
@(log E ) dT2
-
(log E ~ H (1 A$a
d2(log E ) Setting ___ dT2
[(i
+
=
0 gives the relation, T" =
where T ,
=
n - 1
A(%
+ 1)
value of T a t
Tm from Equation maximum slope, Sm
+
=
(log EH
- log EL)nZ,
T , (1
+
z 7 4 2
which is more convenient to use in practice than the corresponding relation, Equation 7. APPLICABILITY O F STIFFNESS-TEMPERATURE EQUATION
The stiffness-temperature relation, Equation 1, has been applied successfully to a variety of resin-plasticizer systems. These compounds exhibited log stiffness-temperature characteristics ranging from very flat and extended curves (polyethylene) to very steep ones (butadiene-acrylate rubber compounds). Figures 2, 3, and 4 give typical examples where experimental stiffness values are shown in comparison with the curve obtained from the fitted equation. These figures indicate that the proposed equation describes the stiffness-temperature curves within the accuracy of experimental error. The esamples cover the extremes in shape encountered in practice. No difficulty has been experienced in fitting any stiffness-temperature curves falling in the intermediate region. The data of Figures 2 and 3 were taken from the work of Clash and Berg ( I ) , who employed Bakelite Corporation's torsion tester, and those of Figure 4 from 7 AT") n ~ ( n 1) T"-2 - 2 n 2 ~ 2 ~ 2 n - 2 (5) the data of Conant and Liska ( 5 ) , who used a flexural method of stiffness measurement. I n connection with this work, some new stiffness-temperature data have been obtained on two sets of plasticized cellulose acetate compounds using the torsion tester. The plasticizers (6) were dimethyl phthalate and Santicizer M-171. Figures 5 and 6 give these results as well as those for two Vinylite resin VYNS compounds plasticized with dioctyl phthalate for comparison.
into Equation
gives for the
1 Obtained from Celanese Corporation of America and Monsanto Chemical Company, respectively. These were samples prepared for our experimental
work and do not necessarily represent production formulations.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2322
Vol. 40, No. 12
\
\
\
100,060
100,00( N
'.' m'
N
fg
W
m'
Lo v)
(VYNS
ui
+
2 v
Lo-
In
u1
W
lL
k
i
o)
10,000
l0,OOo
. I
1000 0
20
1 0
I
too0 40
-20
60
Figure 5. Acetate
Curves for + Stiffness-Temperature Dimethyl Phthalate and Vinylite Dioctyl Phthalate
40
LU
TEMPERATURE,DEG,
T E M P E R A T U R E , DEG. C.
Cellulose VYNS
+
The stiffness-temperature equation is evidently applicable to these compounds which are not usually considered to be elastomers, and its usefulness is therefore extended. Table I gives detailed values of the constants of the stiffness equation for a few representative compounds. PLASTICIZER CONCENTRATION EFFICIENCY
The characteristic constants of the stiffness-temperature equation, EL. Ex, T,, and S,, are functions of the resin-plasticizer system. Examination of the numerical data shows t h a t in any given resin-plasticizer system the limiting stiffnesses, EL and E=, are approximately constant-that is, independent of plasticizer concentration. The results of Vinylite resin VYNW-plasticizer systems shov that the value of maximum slope S , decreases markedly in the 0 to 25y0 plasticizer region but remains approximately constant for any given plasticizer in the 25-4570 range. Varying the plasticizer concentration in the 25-4570 region affects, therefore, primarily the characteristic temperature, T,. The principal effect of the plasticizer is t o shift the log stiffness-temperature curve along the temperature scale. T mwas found to vary linearly with the concentration in this region. Typical examples are shown in Figure 7 . Plasticizer dioctyl phthalate extrapolates as a straight line t o 0% concentration (i.e., T, of unplasticized VYNW resin). This behavior is the exception rather than the rule. All T , us. concentration curves must converge to a single point-namely, t o T , of the unplasticized resin; therefore, the lines of Figure 7 cannot remain linear below about, 15-20yc plasticizer content.
Figure 6.
0
60 C.
Stiffness-Temperature Curves for CelluIose Acetate Santicizer M-17
+
The slope of the T m us. concentration lines may be used as an index of plasticizing efficiency (with respect to plasticizer concentration). Since the lines are not linear at low concentrations, this slope will not determine the amount of plasticizer necessary t o give a certain value of Tm. This number will determine, however, how much the stiffness curve is shifted along the
TABLE 1. TYPICAL STIFFNESS-TEMPERATURE CONSTANTS T m , C.
Plasticizer, Sone
Vinylite VYNW 74 -0.1381 +14.2 -0,0466 +1 7 -0,0478 -11 3 -0.0520 -21 8 -0.0560 -33.4 -0.0632
+
None Dioctyl phthalate 25 Dioctyl phthalate: 30 Dioctyl phthalate, 35 Dioctyl phthalate, 40 Dioctyl phthalate, 45 None
iisrdol, 20
Dibutyl phthalate, 20 Dibeneyl sebacate, 20
Sm
Polyethylene -59.7 -0.0154
Butadiene-Acrylate Rubber -13 3 -0 179 - 17 -0.i83 -0,130 -26.9 -31.2 -0,112 Cellulose Acetate
Dimethyl phthalate, 21 Dimethyl phthalate, 2 6 . 5 Dimethyl phthalate, 31 Santicizor NI-17, 20 Santicizer M-17, 30 Santicizer M-17, 40 Banticiser M-17, 50
+ +++ +
I-81 69 60 69 63 60 +54.7
L o ~ E L LogEn 6.60
3.80
5.40
2 .oo 2.40 2.40
5.71
5.68 5.68 5.71 5.70
on 6.00 6
6.20 5.98
2.50 2.60
2.43 3 . 70. 3.39 3.27 3.18
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948
1 1 q5,
CELLULOSE ACETATE + DMP, ~ F € ~ 2 . l 0
80
1
2323
1
v) Y
E 0 I
F
c
H
E
-60
I I
I
0
10
'
1
I
1
20
I
I
30
40
6
VYNW +TOP, SLOPE.; 3.76
I
4
i
I
50 IO4
1
I
140
150
-
PERCENT PLASTICIZER
160 I70 180 TEMPERATURE, DEG. C. , 2.3 2.2
2 ,e
I 190
l/T IIO'
Figure 7 .
Variation of T , with Plasticizer Concentration
temperature axis by varying the plasticizer content in the 25This is the region of major practical interest. Similar conclusions apply to the plasticized cellulose acetate compounds. These Tm vs. concentration curves, as well as the results for Vinylite resin VYNS plasticized with dioctyl phthalate, are also shown in Figure 7. The plasticizer efficiency of Santicizer M-17 is much lower than that for dimethyl phthalate in the 20-50y0 region. Dioctyl phthalate in Vinylite resin VYNS is somewhat more efficient than either of the plasticizers in cellulose acetate. However, the difference between the absolute values of T, is more significant. I t is evident that, in this respect, the effect of plasticizer in cellulose acetate differs greatly from t h a t of dioctyl phthalate in resin VYNS. The range of T mvalues for the latter include room temperature while those for the former do not approach 25" C., the lowest T, found for any of these compounds being about 55" C. This is particularly significant since many of these
45oj, range.
Figure 8.
Viscosity-Temperature Curves for Cellulose Dimethyl Phthalate Acetate
+
materials find use a t or about room temperature. This difference is emphasiaed if the various stiffnesses at 25" C. are considered (Figures 5 and 6). This comparison shows that the stiffness of the plasticized acetates varies only by a maximum factor of about 2.5, while the stiffness of Vinylite resin VYNS plasticized with 10 and 30% dioctyl phthalate differs by a factor of approximately 1000. It is therefore apparent that the stiffness of cellulose acetate at room temperature is largely independent of plasticizer concentration. TEMPERATURE SENSITIVITY
Temperature sensitivity, as expressed by maximum slope
S, of the log stiffness-temperature curve, is approximately con-
stant for any of the resin-plasticizer systems studied in the 2045% concentration range. It has already been pointed out that in Vinylite VNYW-plasticizer s y s t e m S, decreases greatly in the low plasticizer region but changes very little above 25%. The magnitude of S,, (averaged at various plasticizer conTABLE11. MAXIMUMLOG STIFFNESS-TEMPBRATURE SLOFES centrations) is characteristic of a given resin-plasticizer system, FOR VARIOUS SYSTEMS and changes markedly for different plasticizers and resins. Max. Slope of Table I1 shows typical results which bring out elearly the large Log E us. T Curve Resin Plasticizer, % ' (Av.1 increase in stiffness-temperature sensitivity in going from poly-0,015 Polyethylene DE3401 (natural) ethylene through the vinyl chloride-acetate resin and plasticized -0.138 Vinylite VYNW -0,032 Vinylite VYNW cellulose acetate compounds t o the butadiene-acrylate rubber -0,051 Vinylite VYNW compounds. -0.081 Vinylite VYNW Vinylite VYNS Vinylite VYNS Cellulose acetate Cellulose acetate Butadiene-acrvlate Butadiene-ncrylatc Butarlicane-arrylutr Butadi~?nr-arry!atc
None Dioetyl phthalate Santioiaer M-17 Dimethyl phthalate None Bardol 20 Dibuty'l phthalate 20 Dibenzyl sebacate: 20
-0,080 -0,081 -0,043 -0.063 -0,179 -0.183 -0.130 -0.112
PLASTICIZER EFFICIENCY A T HIGH TEMPERATURES
The analysis contained in previous sections evaluates one of the principal functions of a plasticizer. Another important function is that of changing the high temperature characteristics
INDUSTRIAL A N D ENGINEERING CHEMTSTRY
2324
TEYPFRATURE. DEG. C.
2.4
'IT
Figure 9.
, IO
2.2
2.3
+
-Le., in the region of processing tempmatures. Both of these functions must be considered in studying the action of any plasticizer. The high temperature properties were determined by using the parallel-plate plastometer. The mechanics of running these tests and the complete analysis of the resulting deformation time curves have been covered in detail in previous papers (4, 5). Of the several visco-elastic constants calculable by these methods, absolute viscosity has proved most suitable for the evaluation of high temperature plasticizing efficiency. I n the case of parallel-plate plastometer measurements, the plate separation, h , is measured as a function of time. It has been ahown (5) that absolute viscosity is inversely proportional to the slope of the linear portion of the l / h * us. time (or deformation-time) curve is calculable from relation:
where q
28
30
PLASTICIZER
10'
Viscosity-~emperatiire Curves for Cellulose Santicizer RI-17 Acetate
4 =
Vol. 40, No. 12
8.21 X lo6 X __ W V 2X m
( 10)
= viscosity, poises
W = applied load, kg. V = volume of sample at temperature of test, cc. m = slope of straight line portion of l l h e t i m e curve, em.-* see.-'
The sample volume, V , which is determined from the weight and room temperature density, must be corrected for thermal volumetric expansion. These correction factors were obtained froni published data (2). Figures 8 and 9 show the viscosity temperature characteristics of tlvo plasticized cellulose acetates and Vlnylite 1-YA-S 30% dioctyl phthalate. From these curves the activation energies for vlscoug flow, which are proportional to the slopes, can be calculated. Typical values are:
+
40
50
06)
Figure 10. Viscosity-Per Cent Plasticizer Curves for Dioctyl Plasticized Cellulose -4cetate (Vinylite VYNS Phthalate Shown for Comparison)
+
Activation Energy for Viscous Flow, Cal./Molo
Material
Polyethylene ( 4 )
+
The activation energy for viscous floir- of Vinylite resin V Y S S dioctyl phthalate is considerably higher than those for t8he plasticized cellulose acetates. This is true of most Vinylite compounds, while polyethylene represents a n extreme on the loir- end. The variation of log viscosity with per cent plasticizer was previously shoKn to be approximately linear above 10% plasticizer concentration ( 5 ) for plasticized Vinylite VYKS compounds. Figure 10 shows that linearity apparently holds for the plasticized cellulose acetates. Mathematically, this is equivalent to : 7 = Be-b P
where q
(11)
= viscosit'p, poises = constant
B 6 = slope, log, viscosity-per cent plasticizer line P = plasticizer concentration, %
Accoidingly, slope b is a measure of the high temperature plasticizer efficiency. Table 111 lists the various high tempcraturc plasticizer efficiencies and the viscosities at 30y0 plasticizer concentration. This shows that there is little difference in the relative cffect bet- een Santicizer hI-17in cellulose acetate and dioctyl phthalate in S'inylite YYSS, ~xhiledimethyl phthalate is a more efficient
December 1948
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
TABLE111. HIGH TEMPERATURE PLASTICIZER EFFECT Plasticizer (30%) Temp., ' C. b Viscosity, Poises Dimethyl phthalate Dimethyl phthalate Santicizer M-17 Santicizer M-17 Dioctyl phthalate Dioctyl phthalate
Cellulose Acetate 170 190 170 190
0.35 0.30 0.22 0.23
6 2 X 105 9 . 6 x 104 5 . 6 x 10s
Vinylite VYNS 160 170
0.23 0.20
3 . 7 x 104 7 . 1 X 108
6 . 7 X 10'
2325
many other effects (toughness, brittleness, etc.) on the mechanical and flow properties which are not taken into account in this relatively simple analysis. ACKNOWLEDGMENT
The writers wish to express their appreciation for the cooperation of J. H. Teeple, of Celanese Corporation of America, and H. W. Mohrman, of Monsanto Chemical Company, in supplying the samples of plasticized cellulose acetate. LITERATURE CITED
high temperature plasticizer. The absolute values of the viscosities are widely different. The viscosity values for the acetates are lo2to lo5 poises higher, depending on the plasticizer. The comparative curve for Vinylite VYNS in Figure 10 shows this difference graphically. Thus, a t processing temperatures the more significant difference lies in the relative values of the viscosities. The essential feature of the evaluation presented here is t h a t quantitative comparison of various plasticizer-resin systems can be made in the two important temperature regions-the general temperature range of application and the region of processing temperatures. It is recognized that a plasticizer may have
Clash, R. F., Jr., and Berg, R. M., M o d e r n Plastics, 21, 119 (1944). ( 2 ) Clash, k. IT., Jr., and Rynkiewicz, L. M., IND.ENG.CHEM., 36,279 (1944). (3) Conant, F. S., and Liska, J. W., J . A p p l i e d Phys., 15, 767 (1944). (4) Dienes, G. J., J. CoZZoid Sci., 2 , 131 (1947). (5) Dienes, G. J . , and Klemm, H. F., J. Applied Phw., 17, 458 (1946). ( 6 ) Tuckett, R. F., Tians. Faraday Soc., 39, 158 (1943); 40, 448 (1944). (1)
RECEIVED October 24, 1947. Presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 112th Meeting of the AMERICAN CHCIIICAL SOCIETY,New York, N. Y .
Carbon Black Dispersion in Rubber Effect of Fatty Acids ROSS E. MORRIS AND JOSEPH W. HOLLISTER Rubber Laboratory, Mare Island Naval Shipyard, Vallejo, Calif. Adsorption isotherms for stearic acid and other longchain aliphatic acids on various carbon blacks were determined. The solvent employed was heptane in most cases, although some work was done with benzene and cyclohexane. Considerably more acid is adsorbed by channel black from cyclohexane and heptane solutions than from benbene solutions. Adsorption isotherms on a molal basis for the acids on channel black coincide fairly closely. Stearic acid adsorption is generally higher for the finer particle size blacks, but the degree of adsorption is influenced also by the surface condition of the blacks. Cal-
culations indicate t h a t adsorbed stearic acid covers less than 20Yo of the surface of the hlaclc particles a t saturation. The effect of stearic acid on the dispersion of conducting furnace black and channel black in Butyl rubber was determined by a series of objective experiments. These experiments involved measuring the rate of incorporatioll of the blacks on a mill and determining the viscosity, plasticity, electrical resistivity, bound rubber, and tensile strength of Banbury-mixed batches. Paraffin was used as a control softener in all cases. Stearic acid apparently has little effect on dispersion of these blacks in Butyl rubber.
H E T H E R or not stearic acid assists in dispersing carbon black in rubber has not been settled. Blake (3)was the first to claim that fatty acids are dispersing agents for carbon black. He postulated a mechanism based on Langmuir's work with monomolecular films of fat acids on water (15) and calculated that the fatty acids naturally present in Hevea rubber, which he stated to be present to the extent of 2%, are just sufficient to form a monomolecular film on each particle of 30 volumes of channel black in 100 volumes of rubber. H e assumed that this monomolecular film serves to facilitate wetting the individual particles with rubber and prevents agglomeration or flocculation of the particles. Later developments, however, have invalidated Blake's calculations. Blake assumed that the diameter of the average channel-black particle is 2 0 0 , ~whereas ~ electron photomicrographs ( 7 ) have shown t h a t the actual average particle diameter is 2 8 , ~ . Therefore, 30 volumes of channel black have about seven times the
surface area calculated by Blake. The fatty acids in Hevea rubber, even if present to the extent of 27, and entirely concentrated on the surface of the particles, are insufficient to cover completely the surface of this quantity of channel black. Blake neglected to take into account the solubility of fatty acids in rubber. If the acids are soluble in rubber, not all of the acid molecules will concentrate at the rubber-black interface; some will remain in solution in the rubber. Morris (18) showed t h a t the solubility of stearic acid in Hevea rubber is aboui 1.5 parts in 100 parts of rubber at room temperature and increases rapidly as the temperature rises. The fatty acids naturally present i n Hevea rubber are known to consist of linoleic, oleic, and stearic acids (26), and the total amount is about 1 part per 100 parts rubber hydrocarbon (5'2). Linoleic and oleic acids probably are more soluble in rubber than stearic acid, so that these acids are present in quantities well below their respective solubility limits. It does not appear, therefore, that the fatty acids will
