Cure of Carbon Black-Unsaturated Polyester Mixtures J
C. W. SWEITZER, F. LYON, AND T. S. GRABOWSKI Columbian Carbon Co., Brooklyn 32, N. Y .
R
ECENT investigations (3, 6) have indicated that unsaturation in elastomers is required for carbon black to be fully reinforcing. The unsaturated nature of the polyester in commercial polyester-vinyl monomer mixtures has suggested, therefore, the use of carbon black as a reinforcing filler, Industry reports have indicated, however, that carbon black could not be evaluated for this purpose because of its severe cure retardation effect. Cure studies on polyester-carbon black mixtures in this laboratory confirmed the cure inhibiting characteristics of carbon black. This paper discusses the role of carbon black in peroxide initiated cures and presents practical curing systems which eliminate inhibition, and provide greater cure control by using carbon black as the filler. Qualitative mechanisms of carbon black-peroxide interactions are presented with particular reference to the role of carbon black in the inhibition and acceleration of polymerization. Cure charmteristics of each mixture were followed by measuring the temperature with a pencil thermocouple during the course of reaction. Since these polymerization reactions are exothermic, the cure time and extent of cure can be estimated from temperature versus time plot. The measurements of cure time and the exotherms have been described previously in detail (4). CARBON BLACK AS AN INHIBITOR
In a preliminary experiment, to determine the effect of carbon on the cure of polyester resins-i.e., general purpose resins dissolved in approximately 20y0 styrene-10 parts of a channel carbon( Micronex W-6) were mixed by hand into 100parts of Bakelite polyester resin, BRSQ-147. To this mix 1 part of Luperco ATC (50% benzoyl peroxide in tricresylphosphate) was added and the sample cured in a constant temperature bath at 80" C. (176' F.). Although the control with no carbon cured in 15 minutes, the mix with carbon black showed no cure after 4 hours. Similar results were obtained with two furnace blacks, Statex B and Statex 93, which were dispersed in uncatalyzed Cyanamid Laminac 4128 resin. The Luperco ATC catalyst was added to the mix immediately before the start of the test. Although carbon black inhibits cure when normal catalyst dosages are used, a cure can be obtained if the catalyst concentration is increased above normal as shown in Figure 1. Similarly, total carbon surface is an important factor in cure rate as revealed in Figure 2. CARBON BLACK AS AN ACCELERATOR
A large number of compounds were tried in carbon-resin mixtures to overcome the strong cure inhibition effect of carbon black. Since diethylaniline and dimethylaniline are known t o accelerate polymerization when benzoyl peroxide is used as initiator they were tried first. The addition of these two compounds in low concentrations (0.2% on resin-all concentrations are expressed as yoon resin) t o resin-carbon-peroxide mixtures yielded acceleration effects greater than those produced in the mixtures without carbon, thus changing the role played by carbon black from that of an inhibitor to that of an accelerator (Figure 3). Numerous aromatic amines and resorcinol were effective in accelerating cure in the presence of carbon black. Primary aromatic amines were relatively ineffective a t high temperature,
which may be associated with the inability of aniline and methylaniline to accelerate polymerization of styrene (8),while tertiary aromatic amines were effective over a wide temperature range. Aliphatic amines did not promote cure. Although lauryl mercaptan is a promoter of unfilled resin, it was ineffective in carbon black loaded polyester resins. Differences in the ability of the various additives to promote cure with carbon black is evident from the exotherms in Figure 4. The samples with dimethylaniline and diethylaniline reach peak exotherm in 2 to 4 minutes at room temperature while those with resorcinol and p-dimethylaminobenzaldehyde remain unchanged for 40 minutes. However, these latter two compounds cure in 5 to 7 minutes at 80' C., indicating the importance of cure temperature in these carbon-resin-catalystpromoter systems. By employing appropriate combinations, cure time can be varied within wide limits a t room temperature (Figure 5). EFFECT O F PROdMOTER AND CATALYST CONCENTRATION
The effect of varying the concentration of promoter in a carbon black loaded resin was studied in two series. In the first series the concentration of diethylaniline was varied from 0.025 to 0.2'% in a recipe consisting of Laminac No. 4123, 10% furnace carbon (Furnex), and 201, Luperco. In this series the carbon black was mixed by hand. I n the second series the concentration of p-dimethylaminobenzaldehyde was varied from 0.05 to 0.2% in a compound containing Resin BRSQ 193, 15% furnace carbon (Statex 93), and 4y0 Luperco. The Statex 93 wam milled in the resin. From the results of these tests (Figure 6), i t is concluded that cure time decreases with increase in concentration of promoter. Furthermore, the peak exotherm is lowered as promoter concentration is decreased. In a similar study with diethylaniline the same effect of concentration was observed. When the promoter concentration is held constant and the catalyst concentration is varied between 2 and 5%, the cure time is affected to a minor degree while the peak exotherm is increased 60 ' F. (Figure 7). EFFECT OF CARBON LOADING
A 70-part carbon resin mix was prepared by milling Furnex into Laminac 4128. Portions of this stock were diluted with additional resin to obtain the desired carbon loadings. Before each test was started 2% Luperco ATC, 0.370 dimethylaminobenzaldehyde, and 0.03% diethylaniline were thoroughly mixed in. The results (Figure 8) show that as carbon loading is increased the cure time as well as the peak exotherm decrease. Without carbon black no cure was obtained within the time limit of the test (80 min. a t 80" F.). The loading of carbon black needed to obtain a particular cure time can be determined readily from data in Figure 8. EVALUATION OF VARIOUS FILLERS
The cure characteristics of six fillers were investigated and tested in the same recipe and the exotherms compared with that for the nonpigmented resin. The results (Figure 3) show that
2380
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1955
~~
fine powdered silica cures more slowly than the pure resin while zinc oxide and carbon black are faster curing. As a group the carbon blacks are faster curing than the inorganic pigments and, in addition, exhibit lower exotherms. Within the carbon family Furnex has the highest exotherm and Thermal 140 the lowest. Micronex W-6, a channel carbon black which has approximately 5% volatile matter largely in the form of chemisorbed oxygen on the surface, had a cure rate equal to Furnex carbon but a lower exotherm. If Micronex W-6 is heat treated at elevated temperatures to remove the volatile matter, the treated carbon has a lower exotherm but essentially the same cure rate as the untreated control. In addition to pigment loading, previously shown to be a factor in cure rate, the nature of the pigment surface is equally important. Factors other than surface area often exert an overwhelming influence in cure as noted by the position of the various exotherms in Figure 3. The surface area values of the various pigments used in these studies are set forth in Table I.
Table I.
Pigment Surface Areas by Nitrogen Adsorption
Pigment Carbon black Carbon black Carbon black Carbon black Zinc oxide Silica
Trade Name Furnex Statex 93 Ststex B Micronex W-6
Surface Area, Nl.z/Grrtm 23 38 50
Class SRF HMF FF EPC
103 6 111
... ...
...
...
EFFECT OF PEROXIDE STRUCTURE
In general carbon black inhibits cure of polyester resin when organic peroxides are used as initiators. The degree to which a particular carbon inhibits cure depends largely on the type
2381
~
~
Carbon black can now be evaluated as a filler in unsaturated polyester resins Fabricators concerned with weathering, high speed drying, and low specific gravity of polyester castings will be interested in the findings reported here
peroxide used. The effect of carbon black on the cure of polyester resin with various peroxides in the absence of a promoter is presented in Table 11. Chlorine substitution in the phenyl ring increases enormously the inhibiting characteristic of carbon black, as evidenced by the greater inhibition effect with p-chlorobenzoyl peroxide and 2,4-dichlorobenzoyl peroxide. A comparison of the cure behavior of tert-butylperbenzoate with benzoyl peroxide shows that substitution of the alkyl group for one benzoyl group greatly diminishes the inhibiting effect of carbon black. If aromatic amines are employed to accelerate cure of polyestercarbon black-peroxide mixes, the effect of the carbon is reversed as shown by data in Table 111. In aromatic amine promoted systems carbon black accelerates cure of polyester resin catalyzed with 2,4-dichlorobenzoyl peroxide, p-dichlorobenzoyl peroxide and benzoyl peroxide, while slightly inhibiting cure with tertbutyl perbenzoate and strongly inhibiting di-tert-butyl peroxide
-
ERSQ
193 100 iuperco A T C I C a r b o n - as ~ n d ~ c o t e d
-
Lominoc R e s i n 4128 I00 Stater E FF I O Luperco A T C - a s indicated
Cure
30 Cure
Temp.
&NO
1 7 6 ' F.
Temp
176'
F.
Ccrbon
27
L*
24
-
o
a
a
i\
0 b
a E
I
0
L
al
I
L
0,
21
0
0 I
I-
I
E W
F-
\
1
220
\-.?
phr
Stater
93
3 o/oLuperco ATC I90
\r" I60-
15 IO
20
Time
Figure 1.
30 in
0
20
40
Minutes
Effect of catalyst concentration on cure without promoters
40
Time
Figure 2.
in
60
80
Minutes
Effect of carbon black loading on cure without promoters
INDUSTRIAL AND ENGINEERING CHEMISTRY
2382
Vol. 47, No. 11
Brookfield Synchro-Lectric viscometer a t 80" F. for zero time and after 7 days aging. The results (Table IV), show no measurable change in the viscosity of the pure Fil101 resin after the 7-day period while the catalyzed resin is fully cured. The addition of i 201 carbon black to the uncatalyzed resin in0 creases the viscosity but does not significantly change aging. When catalyst is L 3 introduced to the carbon-resin mix, the 0 viscosity drops sharply at first and con0 151 tinues to decrease slowly over the 7-day a aging period. Interaction between carbon E black and benzoyl peroxide over this 7-day 42 t period is indicated. IO It was thought that loss in polymerizability should result from such interaction and the following experiment was devised in an effort to detect such a loss. Resin and carbon-resin samples were prepared with 5 10 IS 20 25 4y0,2%, and 1% Luperco ATC concentrations and placed in an air oven a t 85" C. T i m e in M i n u t e s At 20-minute intervals a, portion from each Figure 3. Effect of various fillers on cure with promoter sample was removed from the oven, cooled, and the time to cure a t room temuerature was determined after the addition of promoter. With the 4% and 2% catalyst concentrations the cure time, Table 11. Effect of Carbon Black on Cure with Various Peroxides-No Promoter determined a t room temperature with promoter present, deRecipe: Laminac 4123 Resin 100 Catalyst (varied) 1 creased with heating time at 85" C. for both the resin and the Carbon black (as shown) carbon black samples. With the 1 yo catalyst concentration the Cure temperature: 100" C. cure time for the resin decreased with heating time whereas the Cure Time, Min. cure time for the carbon loaded resin drastically increased. This Catalyst No carbon 10 parts Furnex Lominac 4 1 2 3 Luperco ATC Dirthylanlline
-
100 2 0.2 IO
Cure T e m p
80°F.
P
0)
L
L
0)
2,4-Dichlorobenzoyl peroxide Benzoyl peroxide p-Chlorobenzoyl peroxide tert-Butyl perbenzoate
>60 33 >60
16
22
25 42
44
cures. This effect of carbon black on the cure of polyester resin with benzoyl peroxide] tert-butyl perbenzoate and di-tert-butyl peroxide] in promoted systems, is shown in Figure 9.
L
Resin Laminoc 4 1 2 3 Carbon SRf
P'
Promo t e r s
Cure T e m p .
- IO0
-
10
0.2
25'C. 77'F.
Table 111. Effect of Carbon Black on Cure with Various Peroxides-with Promoter Recipe:
Laminac
4123
Resin
100
Catalyst (varied) Cure temperature: 100' C. 2,4-Dichlorobenzoyl peroxide p-Chlorobenzoyl peroxide Benzoyl peroxide tert-Butyl perbenzoate Di-tert-butyl peroxide ~~~
~
1
Promoter N,N-dimethyl p-t oluidine Carbon black (as shown)
Nocarbon 0.5 1.5
2.5
12
20 ~
-diet
0.1
Cure Time, Min. 10 parts S R F 20 parts S R F 0.1 .. 1 0.5 .. 17 18 27 >40 ~~
h ylanillne
Z - d i m e thylanlline
~~
DECOMPOSITION OF PEROXIDE B Y CARBON BLACK
Braden, Fletcher, and McSweeney ( 1 ) have shown that carbon black promotes the decomposition of benzoyl peroxide. Some type of interaction between carbon black and benzoyl peroxide in the polyester systems is indicated by the cure inhibition effects observed in the present study. The following experiments were designed to obtain additional evidence on the interaction of carbon black and peroxide in polyester resins. Statex 93 (15%) and Statex B (10%) were milled into Laminac 4128 resin, and each mix was divided into two portions. T o one portion 1% Luperco ATC was added while the other portion was kept as a control. Besides these carbon samples, unfilled resin was treated in an identical manner. The viscosities of the various mixtures were determined with a
I
e
p - d i methylami nobenraldehyde
c 5
Figure 4.
IO
15
T i m e in M i n u t e s Effect of promoters on cure of carbon loaded resin
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1955
2383
34
r L o m i n a c Resin Luperco Stotex
ATC
B
FF
Promoter
i/
0.2 p - D M A B
+
C u r e Temp.
-
4128
100
-
2
-
15
1.2 p
3c
a s indicated
methylominobenza ldehyde
-
BRSQ 193
015p-dm0b
- os
Luperco
0 03 OEA
+
0 . 2 ~D M A B I
Prornoler
0.02DEA
1 7 6 O F.
2E
100 15 4 indicate
OO5p-dmab
LL.
21 LA' 0 0
L
¶
ie
0
b
k
0)
0
E 0
+
I
C u r e Temp. 8 0 ° F . IO
20 Time
Figure 5.
30
40
Effect of promoter mixtures on cure of carbon loaded resjn
Sample Pure resin Resin 1% Luperco Resin 15% Statex93 Resjn 15% Statex 93 4- 1%Luperco Resin 10 Statex B Resin 10% Statex B 1%Luperco
++ ++ +
+
I C
i
/
in M i n u t e s
Table IV. Effect of Storage Time on Viscosity of Laminae 4128 with and without Carbon Black and Luperco -4TC
Table V.
14
Viscosity after Standing a t 80" F., Poises 0 time 7 days 10 10 10 Cured 330 370 160 95 320 305 170 75
Effect of Carbon Loading on Hardness of Polyester Resine
Figure 6.
5 Time
in
10 Minutes
3
15
Effect of promoter concentration on cure of carbon loaded resin
mined (Table V). Hardness increases with carbon loading; the decrease a t higher loadings in this series is ascribed to insufficient catalyst for the increased carbon surface developed with the higher carbon loadings. In a second series the catalyst concentration was varied in resin and resin-carbon mixtures; the hardness results are shown in Table VI. In the straight resin compound, catalyst concentration has no observed effect on hardness. In the carbonresin compounds, on the other hand, hardness increases with increasing catalyst concentration, the level being lower than the control a t low catalyst concentrations but higher than the control at high catalyst concentrations. DISCUSSION
Recipe:
100 Laminac 4128 2 Luperco ATC 0.3 p-Dimethylaminobenzaldel~yde 0.033 DiethylanPine Vaned Carbon SRF Cure temperature: SOo C. Post cure temperature: 120' C. Hardness, Barco Units Carbon Loading, %
n 6 10 15 20 25 30 50
en __ 63
64 65 66 68 60 55
The literature discloses several instances in which carbon black is considered to inactivate free radicals. Rhodes and Goldsmith (6) in 1926 attributed the retarding effect of carbon black on the drying of oil paints to adsorption of intermediate oxidation products. Sweitzer and Lyon (6) suggested the inhibiting effect of carbon black on rubber oxidation results from free radical interaction with chemical groups on the carbon surface. Watson
Table VI. Recipe:
Effect of Catalyst Concentration on Hardness Laminac 4128 Luperco ATC
result with the 1% Luperco carbon-resin mixture is strong evidence of the interaction between carbon black and peroxide or its decomposition products.
Diet hslaniline Carbon Cure temperature: SOo C.
HARDNESS PROPERTIES
Catalyst Concentration, % 2 3
Hardness properties were measured with a Barco Impressor on carbon loaded polyester resins. In one series using Laminac 4128 resin the effect of carbon loading on hardness was deter-
100 Varied 0.3 0,033 As shown Post cure temperature: 120' C. Hardness, Barco Units No carbon 20% FF 50% SRF 60 15 55
p-Dimethylaminobenzaldehyde
4 6 10
60
66
54
66
62 62
72
62
72
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Vol. 47, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY 3251
-
Br s q 193 Luperco ATC
,elo/o
p
-
I00 voried dimelhylominobenzoldehyde - 0 2
Carbon Black
HMF
-
-
Lominoc 4128 Luperco ATC p -dimethylominobenrolde hyde
-
I5
-
-
d l e t h yloniline C a r b o n Black
260-
Cure
-
SRF
-
Temp
I00 2
0.3 0.033 voried
B O o F.
220-
I3
%
l i
0
70
OO /
5 0 Yo
al
a E 140
e -e
e-0
2
4
Time
Figure 7.
6
a
20
10
Time
in M i n u t e s
Effect of catalyst concentration on cure
(9) recently considered carbon black as a radical acceptor of a special polyfunctional type, while Garten and Sutherland ( 8 ) presented evidence for the interaction of carbon black with several types of free radicals. Braden, Fletcher, and McSweeney (1) showed that carbon black inhibits the vulcanizing action of peroxides in natural rubber. Inhibition. A probable mechanism to explain the cure inhibiting action of carbon black in benzoyl peroxide catalyzed polyester resins is that of adsorption and inactivation on the carbon surface of the free radicals produced by thermal decomposition of the benzoyl peroxide, This inactivation could occur through transfer of radical activity to the carbon black particle, which would presumably be stabilized by resonance or by combination of the radical fragment with a carbon black particle. Besides acting as an inhibitor, carbon black has a high absorptive activity for benzoyl peroxide which is accompanied by accelerated decomposition of the peroxide. Assuming that this decomposition is homolytic, the free radicals formed would in turn be inactivated on the carbon surface. Carbon blacks which Sweitzer and Lyon (7) found to be strong antioxidants for unvulcanized rubber are also the most powerful inhibitors for benzoyl peroxide polyester cures. In both cases carbons with a highly oxygenated surface were most effective in free radical inactivation although all carbon blacks regardless of differences in surface. chemistry inhibited cure. The exceptional inhibiting properties of carbon black in benzoyl and substituted benzoyl peroxide cures are related to the diaroyl structure of the peroxide. The inhibiting characteristic of carbon black is accentuated also by the substitution of chlorine, an electron withdrawing group, in the phenyl ring of the peroxide. The decreased inhibiting effect of carbon black on the cure of polyester resin catalyzed with alkyl peroxides, due to a decreased rate of termination on the carbon surface, is aecribed to a reduced adsorption activity by the carbon black for the alkyl peroxides. Evidence for this is provided with tert-butyl-perbenzoate cures
30
Figure 8.
50
40
Minutes
in
Effect of carbon loading on cure with promoter
Laminoc Carbon Catolyst
-
4123
SRF
N,N-dimethyl-p- toluidine T e m p of C u r e
-
I00
-uoried 2
-
0 I
2 1 2 " F.
0
I-
o
t-butyl
.-E t-
perbenzoote
to-
0
I
1 0 Carbon
10
Looding
20
phr
Figure 9. Effect of carbon black on cure of polyester with various catalysts
November 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
which are less inhibited by carbon black than are diaroyl peroxide cures. Acceleration. Polyester cures catalyzed with benzoyl and chlorine-substituted benzoyl peroxides (diaroyl peroxides), in combination with aromatic amines or resorcinol, are accelerated by carbon black. Increasing the carbon black concentration increases rate of cure in these systems. tert-Butyl perbenzoate (an aroyl-alkyl peroxide) cures, which are slightly inhibited by carbon black in systems without promoters, are also inhibited by carbon black in mixtures promoted by aromatic amines. Increasing the carbon loading in these cures, with or without aromatic amines as promoters, decreases cure rate. Di-tert-butyl peroxide (a dialkyl peroxide) is very strongly inhibited by carbon black in promoted systems, with the inhibition increasing with higher carbon loading. These accelerating and inhibiting effects may be due to the peroxides being adsorbed in varying degrees on the carbon surface. Aromatic peroxides would be expected to be adsorbed to a greater extent than alkyl peroxides. In a diaroyl peroxidearomatic amine-carbon black system both the peroxide and amine, therefore, are strongly adsorbed on the carbon surface. The high adsorption of peroxide and amine on the carbon surface produces decomposition a t an extremely rapid rate which is far beyond the rate a t which the carbon surface can inactivate the radicals formed. In addition, the adsorbed amine may decrease the termination reaction by blocking reactive sites on the carbon surface-the net effect is an accelerated cure. If lauryl mercaptan, an aliphatic promoter, is added to resin-carbon-aroyl peroxide mixtures, inhibition is severe indicating that adsorption of promoter as well as peroxide is essential for acceleration with carbon black. With tert-butyl perbenzoate and particularly di-tert-butyl peroxide the specific adsorption of the peroxide is much lower. The aromatic amine is, nevertheless, adsorbed on the carbon surface which effectively reduces the concentration of amine in the system. Increasing the carbon black loading a t constant
2385
amine concentration decreases the effective concentration of amine. The rate of free radical formation is, therefore, much lower than in the benzoyl peroxide systems. The rate of termination on the carbon surface of the free radicals formed by thermal decomposition and the amine reaction is still sufficient to inhibit cure. In such cases cure rate decreases as carbon black loading increases. CONCLUSIONS
1. Practical curing systems are described for carbon blackpolyester resin mixtures. 2. Carbon black in these systems prolongs the shelf life of catalyzed resins but accelerates the polymerization when promoter is added. This combination of properties, should interest the fabricators of these resins. 3. Qualitative mechanisms of these carbon black-peroxide interactions are presented. LITERATURE CITED
Braden, M., Fletcher, W. P., and McSweeney, G. P., TTan6. I n s t . Rubber Ind., 30, 44 (1954). Garten, V. A., and Sutherland, G. K., "Xature of Chemisorption hlechanisms in Rubber Reinforcement," presented a t 3rd Rubber Technology Conference, London, June 22-25, 1954. Kolthoff, I. M., Guimacher, R. G., and Kahn, A . , J . Phys. & Colloid Chem., 55, 1240 (1951). Nichols, F. S.,and Bliss, C. H., Modern Plastics, 29, 124 ( M a y 1952). Rhodes, F. H., and Goldsmith, H. E., IND.ENG.CHEx., 18, 566 (1926). Sweitzer, C. W.. Rubber Age (N. Y.), 72, 55 (1952). Sweitzer, C. W., and Lyon, F., IND.ENG.CHEM.,44, 125 (1952). Tobolsky, A. V., and Rlesrobian, R. B., "Organic Peroxides," Interscience, New York, 1954. Watson, W. F., "Interaction of Rubber and Fillers during Cold RIixing," presented a t 3rd Rubber Technology Conference, London, June 22-25,1954. RECEIVED for review January 18, 1955. ACCEPTED June 15, 1955. Division of Paint, Plastics, and Printing Ink Chemistry, 126th Meeting, ACS, New York, September 14, 1954.
Isobaric Heat Capacities at Bubble Point Two Trimethylbenzenes and n-Heptane P. F. HELFREY, D. A. HEISER, AND B. H. SAGE California Institute of Technology, Pasadena, Calif.
L
IMITED experimental data are available for the heat c(tr pacity of 1,3,5-trimethylbenzene (10,19,$I), but there do not appear to be any recent heat capacity measurements relating to 1,2,4-trimethylbenzene a t temperatures above 70" F. (19). However, as part of an early investigation, Schiff (19)reported heat capacities for this compound at temperatures below 70" F. The critical constants of both the trimethylbenzenes were measured by Altschul ( 1 ) . In addition, the critical temperature of 1,3,5-trimethylbenzene was determined by Prud'homme (16). Mair (11)reported the index of refraction and density of these trimethylbenzenes, and Smith and coworkers (2'4,86)determined the vapor pressure and the index of refraction of 1.2,4-trimethylbenzene a t relatively lower temperatures. Because of the absence or limited availability of heat capacity data above room temperature for these trimethylbenzenes, the isobaric heat capacities of these compounds were measured at temperatures from 80" t o 220" F. I n addition, measurements were made at temperatures from 70" to 220" F. of the heat capacity of n-heptane, which has been selected by the Fourth Conference on Calorimetry ( 4 ) as one of the reference substance6
for calorimetric work. The heat capacity of n-heptane was studied by Parks (IS),and critically chosen values were reported by Ginning8 ('7). Beattie and coworkers (2'3)determined the critical constants and Smith ( 2 2 ) the vapor pressure of this compound. Rossini ( 1 7 ) summarized the properties of these three hydrocarbons and his values were used in establishing a measure of the purity of these compounds. MATERIALS
The 1,2 4-trimethylbenzene was obtained from Project 44 of the American Petroleum Institute a t the Ohio State University and was reported to contain less than 0.006 mole yoof impuritiee. The specific weight was 54.4263 pounds per cubic foot a t 77" F., and the refractive index as measured with the D-lines of sodium a t 77" F. was 1.5024. These values may be compared with a specific weight of 54.4250 pounds per cubic foot and an index of refraction of 1.50237 under the same conditions reported by Rossini (17). The 1,3,5-trimethylbenzene was reported by Project 44 to contain 0.0022 mole fraction of impurities. The
