Epoxy Casting Resins Modified with Polysulfide Liquid Polymer KEITH
R.
CRANKER AND ALAN J. BRESLAU
Jhiokol Chemical Corp., Trenton 7, N. 1.
Epoxy resins are inherently brittle in the cured state. Coreaction at room temperature with polysulfide liquid polymer results in flexible compounds which have excellent properties for casting and embedment applications. These include high impact strength, good stressstrain properties, good electrical properties, dimensional stability, low moisture vapor transmission, and resistance to most chemicals and solvents. The effects of the rate and state of cure of these compounds on the mechanical, chemical, and electrical properties are examined. The advantage o f a liquid formulation which cures to a tough solid with negligible shrinkage a t room temperature suggests the use of these materials in plastic tooling, electrical casting and embedment, sealing applications, and the like.
ERCAF'TAK-terminated polysulfide liquid polymers are M known to react with epoxy resins in the presence of many catalysts. The state of cure is extremely critical in the development of optimum properties, particularly where room temperature cures are used. Although the liquid polymer/epoxy resin combinations have become increasingly important in recent years, little has been published covering this subject. The purpose of this article is to shed further light on the effects of rate and extent of cure on the chemical, physical, mechanical, and electrical properties of liquid polymer/epoxy resin systems.
Chemistry
primary, secondary, and tertiary amines of all types, and various resins such as urea- , phenol- , and melamine-formaldehyde resins. The amine cures appear to be the most important; much of the patent literature is concerned with methods of curing these resins. Narracott ( d ) , Shechter, and others (4,6) go into considerable detail as to the curing mechanisms involved. The epoxy resins cure to a rigid state having a strained network and, hence, the impact resistance is inadequate for many applications particularly in the casting field. On the other hand, the polysulfide liquid polymers can be cured a t room or elevated temperatures t o a flexible, rubbery state; they have the ability to flexibilize epoxy resins. The polysulfide liquid polymers are mercaptan-terminated saturated elastomeric chains prepared from bis(2-chloroethy1)formal and cross linked with trichloropropane (S), represented by
According to the bulk of the patents covering epoxy resins, their DreDaration involves the condensat ion of epichlorohydrin - with a polyhydroxy compound such as 2,2-bis(4-hyHS(CHZCHZ-O-CHZ-O-CH~CH~SS ) nCHzCH2-O-CHz-O-CH&HzSH droxy phenyl)-propane (Bis-Phenol A). This condensation may be represented as follows: where n may vary from 2 to 26, ~~
H O - C > - b - DI - O H
/ \ + CH2-CH-CH2-Cl
Aq. NaOH 75-80" C.
I
CHI
r
where m usually varies from 0 to about 12. The product may be either a liquid or a solid depending on the value of m. Since excess epichlorohydrin is generally employed, the chains are shown terminated by epoxide groups. The final resin possesses two types of reactive functional groups-hydroxyls and epoxides-which can be utilized in cross linking the resin and which may do so competitively. Cross linking or curing of epoxy resins may be carried out by mineral and organic acids and anhydrides, bases, particularly
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Side mercaptan groups occur occasionally along the chain of repeating formal units and segments are crosa linked at various points. These polymers, when cured, are outstanding for their excellent oil, water, and solvent resistance and excellent aging properties. The polysulfide polymer used throughout this study was LP-3 (Thiokol Chemical Corp.). It appears that the polysulfide liquid polymers, particularly the lower molecular weight ones, coreact with the epoxy resins to form block polymers of the form
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
THERMOSETTING RESINS
Table 1.
Mechanical Properties of LP-3/Epoxy
Resin Mixtures All Samples Aged 48 Hours at 80' F. Plus 60 15 30 3 0 days days days days days at at at at at 212O F. 212O F. 212' F. 212' F. 212' F.
All Samples Aged 48 Hours a t 80" F. Plus 0 3 15 30 60 days days days days days at at at at at 212O F. 212O F. 212' F. 212O F. 212' F.
Cured with DMP-30
Cured _ _ - with T E T With 33% LP-3 BR-18794 Tensile, lb./s inch Elongation, Hardness, Shore D
2325 20 58
2790 0 63
5240 0 67
4810 0 73
6000" 0
80
With 33% LP-3 BR-18794 Tensile, lb./s .inch Elongation, Hardness, Shore D
Epon 828 Tensile, lb./s .inch Elongation, Hardness, Shore D
2875 20 59
4580 0 61
5940 0 71
5450
0 70
6000 0 77
With 50% LP-3 BR-18794 Tensile, lb./s inch Elongation, Hardness, Shore D
670 50 31
1600 20 43
1880 20 45
1600 40 41
Epon 828 Tensile, lb./s .inch Elongation, Hardness, Shore D
925 40 30
1785 20 45
1975 20 47
1750 40 53
4. 8
.
8 4
" Tensile strength
6000" 0 80
6000
Epon 828 Tensile, lb./s .inch Elongation, Hardness, Shore D
6000 0 84
2170 30 61
With 50% LP-3 BR-18794 Tensile, lb./s inch Elongation, Hardness, Shore D
1860 40 54
Epon 828 Tensile, lb./s inch Elongation, Hardnesa, Shore D
4
8.
8.
6000 0 80
6000 0 80
6000
6000 0 80
6000 80
0
6000 0 80
6000 0 81
3370 20 70
630 50 29
580 60 30
575 60 35
660 60 40
3090 20 70
570 110 30
490 130 30
425 120 41
675 75 41
0
80
0
80
of 6000 lb./sq. inch is the limit of the test apparatus.
-A-BS-A-Bz~-A-Bz~~where A represents the polysulfide polymer; B, the epoxy resin; and subscripts 2, z', d',etc., may be 1, 2, 3, etc. This is borne out bv the toluene extraction data. Toluene is an excellent solvent for polysulfide liquid polymers. Thus, with this coreaction, the whole gamut of properties from brittle epoxy resin to flexible polymer may be obtained. The particular curing agent used is critical and to a large extent will control the final properties.
Mechanical Properties Polysulfide/epoxy compounds offer their most important advantages in the casting field when the compounds have reached their maximum cure. The final properties of such compounds may be controlled by proper selection of polysulfide polymer/epoxy resin ratio, the type of amine used for curing, and/or the temperature and time for curing. For example, a compound designed to give high tensile does not necessarily give the best impact and thermal shock properties. There are several ways of determining the final state of cure for optimum performance properties. Data are presented here on some of these tests as a function of the state of cure. Stress-Strain Properties. Two typical fastcuring amines were selected for checking the stress-strain properties of the epoxy resins when the compounds contained 33 and 50% LP-3. The resin combinations were cured from 2 to 5 days a t room temperature (75' to 80" F.) a t which time the hardness values began to level off and then were aged a t 212' F. over a period of days. The data indicate that vastly different properties may be obtained from compounds cured with different amines-vis., primary and tertiary. January 1956
8
Tridimethylaminomethyl phenol (DMP-30, Rohm & Haas Co.) is one of the most active tertiary amines for curing these compounds. However, this amine is not recommended for applications in which the cured compounds are exposed t o elevated temperatures above 150' F. because the physical properties are
Table II. Cure Data on Blends LP-3 with Araldite CN-503
LP-3 CN-503 TET DMP-30
100 100 10
..
100 100
Working properties Brookfield viscosity at SOo F., poises 34.5 34 5 Pot life at 80" F., min., 50-gram mass 27 16 Max.temp.,°F.,50-grammassa 226 202 Stress-strain properties b After 6 days' cure a t SOo F. Tensile, lb./s inch Elongation, Hardness Shore D e After aging ?O hr. a t 212O F. Tensile, lb./s .inch Elongation Hardness, dhore D
8. 2
950 60 42
100
.10 200 ..20
LP-3 with Epiphen 823 100 200
.
20
52
51
51
14 300
14 250
14 246
8 277
2175 3425 4680 5 5 30 75 60 70
Stress-strain properties b After 5 days' cure at SOo F. Tensile, lb./s inoh Elon ation, Hardgness, Shore D After 70 hr. a t 212O F. Tensile, lb./s , inch Elongation Hardness, dhore D
8. 4
b C
100 200 20
..
13 275
1625 1425 3700 2400 10 20 0 10 51 71 76 54 1900 0 83
LP-3 with BR-18794
100 100
100 100 10
100 100
200
100 200
..
10
..
20
20 40 251
20 26.5 26 5 16 5 30 35 20 20 263 310 304 269
16 5 20 290
27 15 318
27 15 325
1785 2800 4580 4800 1600 2200 2800 30 5 10 30 30 20 30 41 63 60 78 36 56 65
5300 5 64
1800 30 43
..
100 200
20
100 100 10
.. 10
100 200 20
100 200
105 105 16 288
1550 1700 5200 6000 2225 2425 5500 10 0 0 10 30 40 0 76 56 62 53 56 80 80
..
Working roperties Brookfieli viscosity a t SOe F., poises Potlifeat80' F., min., 50-gram mass Max. temp., F., 50-gram mass'
..
100 100 .. 10
52
LP-3 with Epon 828 LP-3 Epon 828 TET DMP-30
100 100 10
.. 20
..
100 20
..
900 5000 4200 1650 1450 4400 3400 80 0 5 20 40 5 20 40 76 40 43 63 59 65
Starting a t 80" F. and 35% relative humidity. ASTM D-412. ASTM-49T-Method B.
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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Co.) is a very fast-curing aliphatic epoxy resin but because of its high viscosity is not entirely satisfactory for applications where high fluidity of the compound is required. Tables I1 through V give the working properties and stress-strain data on cures of Epon 828, BR-18794, CN-503, and Epiphen 823 epoxy resins containing 33 and 50% LP-3. These tables again point up the fact that DMP-30 cures the blends extremely fast a t room temperature, but these cures are sensitive to physical degradation a t high temperature aging. In addition the data on maximum temperpture due to exotherm indicate how extremely reactive these systems are and that the temperature due t o exotherm serves to boost the cure. I n the 1:l blends of CX503 with LP-3, the maximum temRESULTS WER€ ABOVE perature due t o exotherm is appreciably lower and as a result, the tensile after 5 days' curing a t 80" F. is low. The stress-strain properties we1 e run on a Scott tensile tester with a jaw separation speed of 20 inches Figure 1. Effect of aging time on impact resistance per minute. The samples were tested at room temperature. Since the tensile values of straight lowered. This is clearly shown in Table I for 1 : 1 blends of epoxy epoxy compounds were above the limit of our Scott tester, resins, Epon 828 (Shell Chemical Co.) and BR-18794 (Bakelite samples of cast Epon 828 cured 5 days a t 80" F. with DMP-30 Corp.), with LP-3. (Although the tensile strength values for were run on an Instron tester a t a jaw separation speed of 0.2 inch per minute. Ten parts of DhlP-30 were used to cure the 1:2 blends of LP-3 to epoxy resin with DMP-30 were above the maximum range of our test equipment, the elongation and 100 parts of Epon 828. The final tensile, elongation, and Shore D Shore hardness were not, and these remained constant over the hardness values were 8600 pounds per square inch, 5%, and 87, entire aging period.) respectively. The low elongation is indicative of the rigidity of Triethyleiietetramine (TCT, Carbide and Carbon Chemical Co.) the compound. ia a primary amine and is slower acting than the DMP-30 but Impact Resistance. Impact resistance tests point up the conmaintains a more uniform tensile strength and hardness over a siderable improvement that is imparted to epoxy resins by the wider temperature range (Table I). The final cured state of addition of LP-3. This improvement in impact resistance is resin combinations with this curing agent does not reach the exhibited at low temperatures as well a8 a t room temperature values attained vith DMP-30 initially, but the tensile values and is independent of curing agent, cure temperature, or rate of increase with aging to exceed those for DhIP-30 and which are cure, although these factors also exert an effect. The present satisfactory for a wide range of casting and potting applications. commercial epoxy resins cure to glasslike materials developing Compounds cured with TET resist thermal shock over a temperature range of -60" to + B O 0 F. T E T allows a slightly longer pot life than DMP-30 when mixed with blends of LP-3 and epoxy 0 %LP-3 33XLP-3 resins. Hovever, DbIP-30 can be added t o the LP-3 to make a stable mixture while TET cannot since it will separate and cause gelation a t the interface. Two other amines that impart good physical properties with polysulfide-epoxy combinations at room and elevated temperature cures are dimethylaminomethpl phenol (DhIP-IO, Rohm IO & Haas Co.) and diethylenetriamine. Approximately 10 to 12 parts of these amines per 100 parts of epoxy resin give cures with high thermal shock resistance. The DMP-10 is also compatible with the LP-3. Diethylenetriamine cures the blends t o a reasonably high tensile strength, (4000 t o 5000 pounds per square 1.0 inch for 1 : 2 blends) and maintains 10 t o 20% elongation. Epon 828 and BR-18794 are two fast-curing epoxy resins which can be combined with LP-3 for casting applications. I These resins cure to a good state within 1 to 2 days a t 80" F. I I I 0.11 I I I Araldite CN-503 (Ciba Corp., now designated Araldite 6020) 0.1 1 30 60 90 0 30 60 90 0 is another fluid epoxy resin which will cure well with polysulfide M/NUT€S AGING AT212FAFTER 2 DAYS ATROOM TEMR polymers but needs an elevated temperature cure t o develop Figure 2. Effect of heat aging on impact resistance high mechanical properties. Epiphen 823 (Borden Chemical
t
100
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 48, No. 1
THERMOSETTING RESINS
-
KU I33 '
O
io a a
m
€ . M E N 61e3 I
I
I
I
TIME, DAYS
I
1
I
I
TiME, DAYS
BR- I8794
EPON-828
Figure 3. Water absorption of LP-3/ epoxy resins cured with DMP-30 E
high internal strains. The elastomeric a polysulfide polymer, when interspersed along the epoxy chains relieves these 2 stresses, thus imparting high impact resistance. I n addition to improvement of low temperature and room temperature impact properties, there is marked improvement in low temp4 erature flexibility and thermal shock TIME. DAYS TIME, DAYS resistance, These properties are generally critical in casting and potting applications. Figures 1 and 2 show DMP-30 is used. The stress-strain properties obtained over this the large improvement in impact resistance obtained by the short heat-aging period are superior for the DMP-30 cure. addition of increasing amounts of LP-3. The test method However, these facts show that a short heat treatment will used was the falling ball test described in U. S. Navy Specscagive a product of high physical strength and shock resistance. tion MIL-(2-16923 (Ships), Par. 4.6.4, for plastics. (The m x i Extraction Test. Data for Soxhlet extraction in boiling toluene mum force obtainable with our apparatus was 78.2 foot pounds.) clearly indicate the difference in the state of cure of LP-3/epoxy Average values only are shown in these plots. An interesting resin combinations with DMP-30 and TET. phenomenon exhibits itself in the cure rate study-in every case The 24hour-extraction values indicate that both amines cure a maximum was observed a t approximately the seventh day of blends of the polymers to a satisfactory state, but the lower room temperature aging. values for DMP-30 suggest a tighter cure. These data indicate that higher impact resistance is obtained A typical set of data on blends of Epon 828 and LP-3 cured on LP-3/epoxy resins blends with TET as the curing agent. with the two amine catalysts is shown in Table 111. It also appears that at least 30 days' aging a t room temperature Water Absorption and Moisture Vapor Transmission. The is necessary before the optimum cure is reached. This may not water absorption and the moisture vapor transmission (MVT) be the maximum value obtainable, since heat-aged samples tests generally indicate an increase in water absorption with a (Figure 2) reached higher values in a matter of hours. corresponding decrease in moisture vapor transmission as the polysulfide liquid polymer concentration in the product is increased. This is shown in Figures 3 and 4 for the water absorption data and in Table I V for the moisture vapor transmission Table 111. Soxhlet Extraction-24 Hours in Boiling Toluene data. Several theories have been propounded ( 1 ) for this phe-
8
E s
RatioLP-3/Epon828 Extracted, %
(ASTM D297-43T) TET Cures 2:l 1:2 1:l 4.3 8 7 4.5
DMP-30 Cures 1:Z 1:1 2:l 0.4 1.0 0.9
Table IV.
Both the heat-aged and room temperature-aged data show how one resin may appear to have higher impact strength a t one time and a lower value a t another, depending on how and when the sample was aged. It should also be kept in mind that when the LP-3 content of the test samples was increased to 50%, the impact resistance after 2 days of room temperature aging was above the maximum test value of 78.2 foot pounds of the apparatus used. Figure 2 indicates that the impact resistance may be improved severalfold by short periods of heat aging. The final value obtained when TET is the curing agent is again greater than when January 1956
Moisture Vapor Transmission Data
(ABTM D 814-46T) MVT, Fluid Oz./Sq. Ft./Day LP-3 Concentration, % 0 33 50 67 DMP-30 Curing Agent With AralditeCN-503
With BR-18794 With Epon 828 With Epiphen 823
0.136 0.182 0.057 0.000
0.045 0.037 0.057 0.000
0.009 0.004
0.008
0.001
0 005
0.008
0.003 0.003
TET Curing Agent WithAraldite CN-503 With BR-18794 With Epon 828 With Epiphen 823
0 014 0.007 0.151 0.004
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
0 009 0.059 0 014 0.039
0.000
0.005 0.057 0.067
Toosoft 0.038 0.042 0.031
101
CN-L3.3
rc 0.
I
I
4
O
I
U
I
N
I
t
O
W
TIME. DAYS
TIME. DA7S
EM-828
Figure 4. Water absorption of LP-3/ epoxy resins cured with TET
&-I8794
E nomenon, but the experimental evidence is insufficient to substantiate them a t this time. In several cases there appear to be deviations from the general trend, such as with Araldite CN-503 where both water absorption and moisture vapor transmission both decrease with increased amounts of 4 a I I I . e a U polysulfide polymer when DMP-30 is TIME, DAYS TIME, DAYS the catalyzing reagent. The MVT data for Epiphen 823 also seems to behave contrary to the trend. quencies such as 1 kc. the measurements are quite accurate since However, there is a certain amount of incompatibility a t polythey are not greatly influenced by the test lead length nor by sulfide polymer concentrations of 50% and higher. This inthe effect of extraneous fields. compatibility also has an effect on the electrical properties. The data presented in the stress-strain section lead the authors Here, too, the differencebetween aliphatic and aromatic structure to believe that at optimum cure the best electrical and stressmay be showing up. strain properties coincide. Usually it takes more than 8 days for the water absorption to approach a constant rate. This time lag is probably more than that req&ed -for the attainment of apEPON 828 BR 18794 ARALDITE CN 503 EPlf"EN 823 parent equilibrium and reflects the 1 6.5 rate of cure of the samples. I 1 " 6.0 Electrical Properties. Many of the 5.5 polysulfide/epoxy compounds are used for electrical potting applica4.5 tions. Therefore, it is important that 4.0 the optimum cure be obtained for 3.5 the best electrical properties. ComIO" s plete polymerization must be achieved. This can be accomplished by a proper stoichiometric proportion of polysulfide/epoxy resin and by extended cure IO" of these combinations at room temperature or postcures a t elevated temperatures. Theoretically, a comg IO" 3 plete cure may be described as the P state when all the cross linking of the molecules is complete, and there is no unreacted material trapped within the network. Until maximum cure is reached there is a limited amount of shrinkage which subjects the physical and electrical property measurements to error. Electrical measurements, especially 0 E 4 6 8 lOI214 of dielectric loss, are reasonable indiTIME, DBYS cations of the molecular reactivity of Figure 5. Electrical properties of LP-/3epoxy resins cured with DMP-30 these systems during cure. At low fre-
4 Hs
8
9
k-
Q
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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THERMOSETTING RESINS Measurements of the electrical properties were made on cured compounds according to ASTM tests D 150-471' and D 25734T. The rate of cure of epoxy/amine systems is accelerated when these systems are modified with polysulfide polymer. This increase is not a linear function of polysulfide polymer concentration; a n optimum concentration exists. This effect is shown in Table V. Once the reactions reach equilibrium, there is no substantial degradation in electrical properties for the 1:2 and 1 :1 ratios of polysulfide polymer to epoxy resin.
A typical plot of volume resistivity was made when LP-3/Epon 828 blends were cured with T E T (Figure 6). These graphs indicate that a fair state of cure was obtained after a 4-day cure a t 80" F.; an extended cure up to 14 days shows a gradual improvement of these properties to a point where the values are approaching their optimum. With T E T the optimum cure at 80" F. is approached rather rapidly, but the values are lower than with the DMP-30 cure. The data on mechanical properties support the fact that TET-cured epoxy resin blends do not cure as tightly as those with DMP-30. Epon 828 and BR-18794 epoxy resins with LP-3 are fast curing as indicated by the rapid leveling off of the curves. The incorporation of 33 and 50% of LP-3 in all the resins does not appreciably increase the electrical loss as shown by the dissipation factor values. The electrical properties of LP-31 Epiphen 823 blends are poorer than the other resins because LP-3 is slightly incompatible with this particular resin. Blends of LP-3 with CN-503 are slower curing, and as a result the optimum electrical properties are not reached until 2 to 3 weeks at an 80" F. cure cycle. Normally, for the best electrical properties, a heat cure is recommended for blends of LP-3 with Araldite CN-503.
Table VI.
IOloj
0 2 4 6 8 101214 TIME, DAYS Figure 6. Effect of time on volume resistivity of LP-3/Epon 828 cured with TET
Electrical Properties of LP-3/Epoxy Blends
Resin
(33% LP-3 in blends cured with DMP-30 for 4 hr. at 212O F.) Epoxy resin Epon 828 CN-503 BR-18794 Epiphen 823 Vol. resistivity, ohm-cm. 1 . 2 X 1018 8 X 101: 9 X 1018 5 X 101% Dielectric constant, 1 kc. 4.0 4.2 4 2 4.4 Dissipationfactor, 1 kc. 0.011 0.013 0.008 0.017
Summary Table V.
Effect of Polysulfide Liquid Polymer on Rate of Cure of Epoxy Resin Compounds
(Samples cured with 10 parts of DMP-30/100 parts Epon 828) 0 33 50 67 LP-3 oonon., % Gel time, m h o 41 15 21 72 Cure time, min.0, b 225 27 38 1000 Max. temp. due to reaction exotherm, F. 114 266 218 152 a Room temperature was 80 I 3O F,;relative humidity, 55%; samples had a 50-gram mass in paper cups. b,Hard, nontaoky.
Since it is impractical to give the resins a heat cure in many applications, electrical properties were measured after various periods of room temperature aging. Four commercial epoxy resins were blended with LP-3 a t ratios of 1:l and 1:2 of LP3/epoxy resin. These blends were cured using DMP-30 and TET, and test sheets were cast a t 80" F. The sheets were cured over a period of days at 80" F., and the electrical measurements were taken at various stages of the curing period. Plots are shown for dielectric constant ( K ) , dissipation factor (D), and volume resistivity ( p ) for each resin with these different blends when cured with DMP-30 (Figure 5). Here it is illustrated that the electrical properties of these compounds approach optimum values after 3 to 4 days' curing at 80" F. if the polysulfide polymer is added to a fast-curing epoxy resin and a fastacting curing agent is used. However, for most of these systems ti short heat cure at elevated temperature may be substituted for a lengthy room temperature cure. An example of the effect of an elevated temperature cure is shown in Table VI where the electrical properties presented are a t the highest values for these copolymer systems. January 1956
Incorporation of polysulfide liquid polymer into the epoxy resin chain offers a liquid diluent for the epoxies which chemically combines to give permanent flexibility. The resultant products have lower shrinkage, fewer internal strains, lower moisture vapor transmission, and increased impact resistance. Through proper selection of LP3/epoxy resin ratio and catalyst a wide range of properties may be obtained The compounds will give ultimate stress-strain and electrical properties when fully cured. However, best impact and thermal shock properties are obtained from compounds which do not necessarily exhibit the highest tensile strength. The electrical properties of these combinations are satisfactory for a great number of applications which require intermediate insulation resistance, low dielectric loss, and a moisture seal, These combinations are relatively new and it is hoped that data submitted here will help make these resins useful for new applications in the casting field.
Acknowledgment The authors wish to express their appreciation to the staff of the Technical Service and Research Departments of the Thiokol Chemical Corp. for their assistance in the preparation of this paper.
literature Cited (1) Jorcsak, J. S., and Belisle, J., SPE Journal 1 0 , 2 , 23 (1954). (2) Narracott, E. S., Brit. Plastics 24, October 1951. (3) Patrick, J. C., and Ferguson, H. R. (to Thiokol Chemical Corp.) U. S. Patent 2,466,963 (April 12, 1949). CHEM.48, 86 (1956). (4) Shechter, L., and Wynstra, J., IND.ENQ. (5) Shechter, L., Wynstra, J., and Kurkjy, R., Ibid., p. 94. RECEIVED for review April 21, 1955.
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ACCEPTED October 27, 1955.
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