Cracking of Stress’edPolyethylene EFFECT OF CHEMICAL ENVIRONMENT J. B. DECOSTE, F. S. MALR.1,
AND V.
T. WALLDER
Bell Telephone Laboratories, Inc., Murray Hill,N . J . I n a number of applications for polyethylene, particularly cable sheaths and cosmetic containers, it has been found t h a t under certain conditions failure of the polyethylene results in a cracking of the plastic. Considerable information is available to show t h a t in a n unstressed condition polyethylene is highly resistant to a wide variety of chemical environments such as alcohols, soaps, and fatty oils. However, when polyethylene is exposed to these environments under polyaxial stress i t fails by cracking. The work described in this paper was undertaken to determine the factors involved in polyethylene cracking. A qualitative laboratory test was developed to evaluate this property and the effect of a variety of organic and nonorganic materials was studied. It was found t h a t the higher the molecular weight of a polyethylene the more resistant i t becomes to cracking, t h a t the degree of orystalIinity affects its readiness to crack, and t h a t the addition of polyisobutylene or Butyl rubber improves crack resistance. This paper shows t h a t useful end products, which are resistant to cracking, can be made from polyethylene.
Figure 1. Environmental Stress Crack in Polyethylene Cable Sheath
cracking may be almost instantaneous for the low molecular weight resins, while for regular extrusion grades it may take a matter of hours or days depending upon the activity of the environment. T h e use of the term “embrittlement” is not considered appropriate, €or it implies t h a t a general stiffening has taken place which would make the plastic more susceptible to cracking by impact. No over-all stiffening has been observed and the properties of the resin adjacent t o a crack are usually identical t o those possessed by the resin before exposure t o a n environment. Since embrittlement appears t o be something of a misnomer, the term “environmental cracking” is advanced as being preferable for describing this phenomenon. The environmental cracking of polyethylene is analogous in several respects t o the familiar solvent crazing ( 2 , I f ) observed in polymethyl methacrylate and polystyrene. Crazing in these polymers is favored by the same factors that produce cracking in polyethylene. Crazing can be produced in these polymers b y a polyaxial stress in the presence of a solvent. Also, the cracks seen in crazed resins and those produced in polyethylene occur most frequently in a direction that is perpendicular to the major stress. An example of environmental cracking in polyethylene is shown in Figure 1 . This is a photograph of a polyethylene sheathed cable t h a t has developed a crack as a result of the combined action of polyaxial stress and a surface active agent. The polyaxial
T
HE natural flexibility possessed by polyethylene is one of the properties of the resin t h a t has recommended i t for many practical applications. When a regular extrusion grade of polyethylene is stressed uniaxially in tension, i t elongates to about 600%, including cold drawing, and ultimately fails forming a fibrous fracture. Carey, Schulz, and Dienes ( 4 )have reported the results of a recent study on the s!ress-strain properties of polyethylene with particular emphasis on its behavior below the yield point. Their study confirms and extends the previous information available on the performance of polyethylene under essentially uniaxial stress. Under biaxial stress, however, i t has recently been shown b y Hopkins, Baker, and Howard (6) that in selected samples of the resin only a small fraction of the elongation in tension is obtainable, and the resin can fail forming a sharp conchoidal fracture. T h e conditions necessary to produce a sharp fracture in this manner are rather critical and are not expected to be encountered frequently. If,however, polyethylene is stressed biaxially or polyaxially in the presence of a surface active environment, the conditions for producing conchoidal fractures or cracks become much less critical. I n the presence of an active environment, cracking occurs under stresses t h a t the resin might ordinarily resist indefinitely. Carey (3) has recently investigated this phenomenon which he calls “stress cracking” by analogy with the behavior of certain metals and shows t h a t in contact with certain environments polyethylene fails a t stresses and strains less than those normally reported. This cracking is believed t o be similar to the embrittlement, reported by Richards (IO), for low molecular weight resin in the presence of mobile polar liquids such as alcohol or acetone. H e also pointed out that higher molecular weight grades were immune t o this embrittlement. Richards is believed to be referring t o materials having Williams plasticities ( I ) of 20 or 30 for the low molecular weight polymers and about 50 or above for the higher molecular weight products. T h e present authors have found that for polyethylene currently produced in this country,
Figure 2.
117
Magnified View of Face of a Sheath Crack
INDUSTRIAL AND ENGINEERING CHEMISTRY
118
stress was introduced when the cable was bent during installation and the surface active agent was known to have been soap which was used as a lubricating agent when the cable was drawn into a duct. Nougey ( 7 ) first established that failure of polyethylene telephone cable sheaths was due to environmental cracking. When closely examined, the crack in the sheath was found to be sharp and conchoidal and to have superficial features analogous FOIL COVER W I T H R E A G E N T 0.50'' A B O V E TOP OF
SAMPLE\
.
I5OMM.lISMM PYREX
Lo.,-.I
llml wl
Vol. 43, No. 1
The appearance of a bent specimen before and after cracking is shown in Figure 4. Tj-hen the specimen is bent the razor slit opens up and resin a t the base of the slit is strained polyaxiallv. It is in this part of the specimen a t the base of the slit that cracking is initiated. After initiation, cracks generally propagate a t right angles to the slit. This may occur directly across the specimen or, as shown in the example, cracking may take place a t either end of the slit. An examination of the surfaces of cracks produced in this manner shows that they have the characteristic ribs and hackles found in the magnified cable crack of Figure 2. Since the nature of the resin has a considerable effect on its tendency to crack, the resins selected for study were chosen on the basis of their Williams ( 1 ) melt plasticity values a t 130' C. These values are approximately proportional to the niolecular weight of the resins as determined by the Staudinger solution viscosity method ( 5 ) . Unless otherwise specified polyethylene having a \Tilliams plasticity of 55 was used. Resins were compounded as for cable sheath with 2% of a fine particle size carbon black and 0.07y0 of a n antioxidant. This compounding has not been observed to have any significant effect on the environmental crack resistance of regular grades of polyethylene. CRACKING EKVIRONMEKTS
TEST SAMPLE TEST ASSEMBLY
Figure 3. Test Specimen and Apparatus for Determining Crack Resistance
to those of a glass fracture. These can be seen in the magnified view in Figure 2 of a small section of the face of a sheath crack, B y analyzing the surface characteristics of the crack according to the method outlined by Oughton (9) for glass it is possible t o determine the point of origin, the direction in which the crack propagated, and areas of high shearing stress. The fine almost indistinguishable marks that appear to radiate from a point in the dark area are known as "hackle marks." The point from which these marks radiate is the origin of the crack and the general area in which the marks appear has been one of high shearing stress. The clearly distinguishable curved marks on either side of the hackle area are known as "rib marks" and indicate the direction of propagation of a crack. The propagation of a crack takes place in a direction which approaches the rib marks on the concave side and leaves on the convex side. Since the possibility of cracking must be taken into account in the engineering applications of polyethylene where polyaxial stresses may be developed, as in cable sheathed with the material, a n investigation was undertaken to determine: the types of environments that can cause cracking; how polyethylene might be modified to improve its crack resistance; and the effect oi thermal history on crack resistance.
I n order to determine the types of environments that produce cracking, compounds representative of different classes of materials were tested for their effect on regular grades of polyethylene using the test method previously described. The majority of the environments tested were liquids which could be added directly to tubes containing the stressed specimens. Where a compound was a water-soluble solid it was tested as a 10% aqueous solution. Solid or semisolid environments were applied as coatings directly to the specimens before bending. The results of these tests are summarized in Table I. The materials listed as active environments produced cracking, while those listed as inactive had n o effect on the stressed resin. There is no doubt about an active classification but it is difficult t o decide when a material is inert. Before classifying a material as inert, tests were run from 100 to 200 days before being discontinued.
TABLE I. EFFECTO F VARIOUS MATERIALS ON 55 GRADE POLYETHYLENE (Crack resistance test) Active Environments -4liphatic and aromatic liquid Metallic soaps hydrocarbons Sulfated and sulfonated alcohols Alcohols Alkanolamines Organic acids Polyglycol ethers Sodium and potassium hydroxide Ester-type plasticizers Vegetable oils Depolymerized rubbers Animal oils Polybntenes Mineral oils Silicone fluids m7ater Polyhydric alcohols Sugars Selected saponins Hydrolyzed protein
Inactive Environments Rosin Selected asphalts Paraffin wax Bentonite Acid and neutral inorganic salts
TEST METHOD
I n order to study the phenomenon a qualitative laboratory test has been developed that effectively reproduces the type of cracking observed in cable sheath. A specimen having a thickness equivalent to t h a t of the sheath is used and a controlled im erfection in the form of a razor slit is introduced along wgich localized polyaxial stresses may be concentrated. A drawing of the test specimen and the method of holding it in a stressed position are given in Figure 3. The specimen measures 0.5 X 1.5 inches with a thickness of 0.125 * 0.005 inch and is punched from carefully molded sheets low in residual stress. The test is performed by bending the specimens with the slit on the outside and then inserting them in the 150 18-mm. test tube as shown. The cracking agent is added to the tube until all specimens are completely covered. Multiple specimens, usually five or more, are tested and crack resistance is reported as the percentage of the specimens tested resisting cracking after an arbitrary test period of sufficient duration to represent an approximate equilibrium condition. Tests are made a t 50" C., except in the case of volatile solvents and gases where a temperature of 25" C. is used.
+
Specific examples of materials that produced cracking included hexane and toluene among the hydrocarbon liquids, methyl and ethyl alcohols, and such plasticizers as dioctyl phthalate and tricresyl phosphate. Linseed, soya, and castor oils as well as light and medium viscosity mineral oils also produced cracking. Among the mineral oils examined were samples from Pennsylvania, mid-continent, and coastal crudes; no essential difference was observed in their cracking tendency. The metallic soaps, sulfated and sulfonated alcohols, alkanolamines, and polyglycol ethers were among the most active cracking agents found. Materials of this nature find application in industry as emulsifiers, detergents, and wetting agents, and may be classified together as surface active agents. Examples of surface active agents that produced cracking included ordinary soap, the sodium salts of lauryl and myristyl alcohols, polyoxyethylene sorbitan fatty acid esters, and Igepal (General Dyestuff Corp.,
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1951
119
have the characteristic ribs and hackles of a crack produced by a polar surface active agent. From this evidence i t is believed that polar and nonpolar materials are capable of producing the same type of cracking in polyethylene. I n the application of polyethylene as a sheath on cables to be installed in underground ducts, the gaseous environments that may be encountered must be considered for their tendency to cause cracking. High concentrations of carbon dioxide and hydrogen sulfide are found in some locations, while in others the ground may be saturated with natural gas. Cracking tests were made in the presence of methane, propane, hydrogen sulfide, carbon monoxide, carbon dioxide, and air under a pressure of 3 pounds per square inch a t 25" C. All tests were negative and i t was concluded that these gaseous environments d o not represent a major cracking hazard. The only gases among those tested that might have been expected t o cause cracking were methane and propane. These gases are still viewed with suspicion, as it is felt t h a t under somewhat different conditions, such as higher temperatures, cracking might occur. Figure 4. Appearance of Stressed Specimen
HIGH MOLECULAR WEIGHT RESINS
A . Before cracking
The first means considered for obtaining improved crack resistance was through the use of resins of higher molecular weight than the regular grade. These resins were expected t o be more resista n t t o cracking because of increased toughness imparted by raising the molecular weight. Special samples of high molecular weight resins, ranging up t o a Williams plasticity value of 90, were examined for their crack resistance to Igepal at 50" C.
B.
~
-
After cracking
which haa been described as an alkyl aryl polyethylene glycol ether. This latter material was standardized as a reagent for comparing the crack resistance of resins and compounds, T h e selection of Igepal for this purpose was made on the basis t h a t i t waa a n easily handled stable liquid producing rapid cracking. About 2 hours at 25" C. are required t o crack commercial polyethylene having a Williams plasticity of approximately 55 and about one quarter that time at 50" C. Experience with Igepal has shown t h a t results obtained with it are a good index of the resistance of a polyethylene to other cracking agents. Igepal has ordinarily been used in full concentration, but it is still active when diluted t o 1part in 1000 with water. The destructive action of the surface active agents is unique in t h a t they are absorbed in amounts of only a few tenths of a per cent at temperatures where they are very active. For example, from the data given in Table 11, a 0.5-inch cube of polyethylene increases only 0.08% in weight after 14 days' immersion in Igepal at 25 O C. For triethanolamine oleate, another extremely active cracking agent, the increase is only 0.16%. As such amounts of these materials uniformly distributed in polyethylene would not be expected to have any noticeable effect on the general physical properties of the resin i t may be assumed that these environments function by attacking the surface of the resin (6). Tests on natural resin with a cracking agent containing a red dye tended to confirm this conclusion, for cracking took place in this test without a n y detectable penetration of the dyed cracking agent. Although Richards (IO)has mentioned only mobile polar liquids as embrittling polyethylene, certain nonpolar materials have also been observed to crack the resin. Examples of such materials are rt-dodecane, hexadecane, and octadecane. Examination of the crack patterns made with these nonpolar hydrocarbon compounds has shown that they are conchoidal in nature and
TABLE 111. NUMBEROF COMPOUNDS AND SPECIMENS TEBTED AT EACH PLASTICITY RANGE No. of Compounds 13 28 8 3
Plasticity, Mils 65 65 75 85
AVG. MOL.WT.
Figure 5.
22.000
24,000
No. of Specimens 152
27,000
250 112 60
29,000
Effect of Molecular Weight on Crack Resistance
Cracking test; compounds contain 2% carbon black and 0.07 Yo antioxidant
TABLE 11. ABBORPTION OF SURFACE ACTIVEAQENTSBY POLYETHYLENE
Water (control) Green soap 10 soln. Aquarex D ' l O z soln. Igepal CA 'extra high conc. Ethyl aloo'hol Triethanolamine oleate Mineral oil, medium vis. n-Dodecane
% Weight Change of 55 Grade Polyethylene after 14 Days' Immersion 25' C. 60' C. Nil Nil Nil 0.02 0.21 0.04 0.30 0.08 Nil 0.10 0.73 0.16 0.16 3.20 30.7 9.29
Data are furnished in Table I11 covering the number of compounds and the total number of specimens tested in each plasticity range. As small differences in plasticity do not have a significant effect the results have been averaged for each increase of 10 mils. T h e height of each column in Figure 5 shows the percentage of the total number of specimens tested in each plasticity range t h a t resisted cracking. T h e commercial grade of resin, which is represented b y the 55 plasticity column, shows a crack resistance
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
120
Vol. 43, No. L
in all proportions tested. The nonuniform results obtained with the 120,000 and 180,000 grades probably resulted from the incomplete dispersion of these higher moIecular weight polymers. The improved crack resistance is undoubtedly related t o the lower moduli of the blends and their greater tendency to relax under stress than the st,raight resin. While blends such as these reduce environmental cracking, on the basis of Newberg's (8) data, increasing concentrations of polyisobutylene lower tensile strength, modulus, stiffness, tear resistance, Shore hardness, heat softening, and heat resistance, and increase elongation. EFFECT OF THERMAL HlSTORY
Figure 6.
Examples of Twisted Cable Treated with Igepal Top. Sheath of 17 grade resin B u t t o / n . Sherth of 5.5 grade rosin
of 0.7% after 2 days. Thus only 0,7y0of all the specimens tested in this plasticity range resisted cracking, or 99.3% failed. I n the 65 range over 40% resistance was obtained while essentially 100% resistance was obtained with 75 and 85 grade material. The result,s obtained after the 2-week period are believed to be equilibrium values and no additional cracking is expected beyond this period of exposure. I n view of the fact t h a t broad ranges in plasticity must. be esamined to demonstrate the effect of increasing molecular weight, it is evident that other compositional variables such as the molecular weight distribution or chain branching may also play an important part in crack resistance. Despite the effect that these uncontrolled variables may have, however, it appears that if the average chain length of the polyethylene molecule is made sufficiently long a high level of crack resistance will be obtained. I n Figure 6 are shown two lengths of experimental polyethylene sheathed cable that demonstrate the good correlation that has been obtained between the laboratory cracking test and r e s u h obtained on cable. The upper cable was sheathed with a 77 grade resin, while the l o r ~ e rone WRS sheathed with a 55 grade resin. Both cables were t,wisted 540" around their longitudinal axes and Igepal applied at room temperature. T h e cable made with the 55 grade resin cracked within a few days while no cracks have developed in the cable sheathed with the 77 grade resin after 6 months. POLYISOBUTYLENE-POLYETHYLENE BLENDS
T h e next method considered for improving the crack resistance of polyethylene was by blending i t with polyisobutylene. Blends of this type are well known, and Sewberg, Young, and Evans (8) have described their general properties. Polgisobutylene has a plasticizing effect on polyethylene and when used in large enough percentages produces highly flexible, rubberlike materials. Compositions of this type have had a wide use in England, one unique example of which is a submarine cable described by Wilson (16) that is insulated with a compound consisting of 12.5% of a polyisobutylene and 87.5y0 of polyethylene. The effect of the addition of a number of grades of polyisobutylene in varying amounts on the crack resistance of polyethylene was investigated. Grades of polyisobutylene, having Staudinger molecular weights of' 40,000, 80,000, 120,000, and 180,000, and Butyl rubber were added in amounts of 5, 10, 25, and 50% t o a 54 grade resin on a hot two-roll mill. T h e results of these tests are summarized graphically in Figure 7 . From these results it appears that the addition of polyisobutylene is particularly effective for imparting crack resistance t o polyethylene. Butyl rubber and the 40,000 and 80,000 molecular weight grades of polyisobutylene were essentially 100yo effective
In processing a thermoplastic resin such as polyethylerie thta thermal history includes a heating a.nd cooling cycle. The heating cycle involves bringing the polyethylene to some temperature: above its 110' C. melting point. Observations on sheets molded between 135 O and 175 C. have shown that varying the temperature between these limits makes no appreciable difference in. crack resistance.
WSTANEX
Figure 7.
RUBBER
VIST4NEX
VISTANEX
VISTA'VEX
Effect of Addition of Polyisobutylene on Crack Resistance of Polyethylene
Contains 2 % carbon black and 0.07 o+'
antioxidant
The effect of the cooling cycle wa9 next considered by taking epecimens t h a t represented extremes in their cooling rates. Rapidly cooled samples were prepared by quenching sheet8 of regular grade resin a t 150" C. in liquid nitrogen (-196" C.), A4nnenled specimens were produced by allowing sheets to cool sloa~lyto room temperature from 150" C. a t a n average rate of 0.5' C. per minute. The crack resistance of the quenched samples was found in general to be somewhat, better initially than those that had been annealed. The improvement obtained was temporary and disappeared after 35 days of shelf aging. Data illustrating the in.stability of the quenched resin are shown in Figure 8. Similar experiments performed with high molecular weight resins aliw
U 4 J
200
4 a w
u 5
150
x
Y
"
100
I-
3 3
J
50
L
'0
I
5
I
10
I
15
1
20
I
25
I
30
I
35
J
40
D A Y S SHELF A C I N G A T ROOM T E M P E R A T U R E ( 2 8 O C )
Figure 8.
Effect of Shelf -4ging on Crack Resistance. of Quenched and Annealed Resin
January 1951
showed the quenched samples to have somewhat better crack resistance. The differences in this case were more permanent than those found for the regular grade of resin, and the quenched samples remained unchanged after prolonged shelf aging. The difference observed between quenching and annealing probably is associated with the degree of crystallinity obtained. When the resin is cooled slowly from the melt a greater degree of crystallinity results and the product is noticeably stiffer (6). This would introduce higher stresses which explains a t least partially the greater tendency for the annealed specimens to crack.
many helpful discussions, and to H. G. Johnstone of the Western Electric Co. for his valuable observations. The authors also extend their thanks to G. F. Brown and F. R. Criger for performing much of the experimental work and to R. D. Mindlin and W. T. Read for their analysis of the fracture patterns, and to the Bakelite Development Laboratories for features incorporated in the test method. Both the Bakelite Corporation and the Du Pont Co. were of great assistance in supplying the special grades of polyethylThe Dolvisobutvlene resins were obtained through the courof the-Standard Oil Company of New Jersey.
-
LITERATURE CITED
SUMMARY
*
121
INDUSTRIAL AND ENGINEERING CHEMISTRY
A qualitative laboratory test suitable for studying environmental cracking has been described which gives results that correlate reasonably well with those obtained on cable sheath. Polyaxially stressed polyethylene has been shown to crack conchoidally in the presence of a variety of organic substances. Surface active agents are particularly conducive toward producing such cracks. Crack resistance can be obtained in polyethylene by the selection of suitable high molecular weight resins. Blends of polyethylene with polyisobutylene have also been shown to have good resistance to environmental cracking.
Am. SOC.Testing Materials, A.S.T.M. Designation D 926-47T. Baker, W.O.,U. S. Patent 2,373,093(1945). Carey, R.H., A.S.T.M. Bull., 167,56 (1950). Carey, R.H., Schulz, E. F., and Dienes, G. J., IND.ENG.CHEM., 42,842(1950).
Clarke. W. J.. Trans. Am. Inst. Elec. Ennrs.. 64. 919 (1945). Hopkina, I. L., Baker, W. O., and Howard, J.'B., i.Applied Phya., 21,206(1950).
Mougey, W. E., Bell Telephone Laboratories, Inc., private communication. Newberg, R. G., Young, D. W., and Evans, H. C., M o d e r n Plastics, 26, 119 (1948). Oughton, C. D.,Glass Ind., 26, 72 (1945). Richards, R.B.,Trans. Faradag SOC.,42, 10 (1946). Russell, E. W.,Nature, 165, 91 (1950). Wilson, H.F.,Brit. Plastics, 20,20 (1948).
ACKNOWLEDGMENT
The authors wish to extend their appreciation t o the staff of the Bell Telephone Laboratories, Inc., for their suggestions and
RECEIVED July 12, 1960. Presented before the Division of Paint, Varnish. CHEMICAL and Plastics Chemistry a t the 117th Meeting of the AMERICAN SOCIETY, Detroit, Mich.
Nitriles as Selective Solvents KENNETH W. SAUNDERS American Cyanamid Co., S t a m f o r d , Conn.
-
T h e object of this work was to find solvents that are highly selective and have a reasonable capacity for the extraction of aromatics from petroleum hydrocarbons. All highly selective solvents for the extraction of aromatics from petroleum hydrocarbons were found to be nitriles having the general formula, CN--(CH&P-X, where n = 1 to 5 , X = CN, NR1R2, OR, SR, and the nature of the R groups is limited. The results indicate that solvents exist which are much more selective in their ability to extract aromatics from petroleum hydrocarbons than solvents now used for such purposes. Their proved selective nature in this field suggests they may be equally useful in the extraction of unsaturated or aromatic compounds from vegetable oils, tall oils, and similar materials. The results also indicate'the critical effect of chemical structure on selectivity.
T
HE importance of solvent extraction asananalytical tool and in commercial applications has served to stimulate the search for better solvents of high selectivity and capacity. The use of solvents in the petroleum field to obtain raffinates with a low
aromatic content for lubricating oils and Diesel fuels is now well established. Friedman (6) has observed that ethers and amines containing nitrile groups were superior selective solvents for aromatics from hydrocarbon mixtures. It was confirmed that in this general classification @,@'-oxydipropionitrile(dicyanoethyl ether) and @,p'-iminodipropionitrile (dicyanoethylamine) did possess an unusual selectivity and that these solvents may have broad applications (1, IO). These preliminary indications have
been extended on a quantitative basis, with ternary diagrams, for the two solvents mentioned as well as for other nitriles which were discovered to have equivalent selective capacity. They denionstrate that in selectivity for extracting aromatics certain nitriles far surpass presently used solvents such as furfural, sulfur dioxide, nitrobennene, phenol, Chlorex [bis(%chloroeth;yl)ether1, and DuoSol (propane-phenol mixture) (8). Some observations in the literature (4, 9, 12) are consistent. SCREENING TESTS FOR SELECTIVE SOLVENTS
In the search for selective solvents which might be comparable to @$'-oxydipropionitrile and p, p'-iminodipropionitrile a qualitative screening program was set up. Early work with these two compounds as well as with succinonitrile and adiponitrile indicated that the key for high selectivity appeared to be associated with dinitrile compounds or the CN-CH2-CH2-N-, or CN-CH%-CHS-Ostructures. Hence most of the compounds tested were nitriles. For a compound to have selective solvent powers the compound should have the following properties: 1. Liquid at room temperature, or at least have a meltingpoint below 50" C. 2. Miscible with an aromatic compound (benzene used). 3. Incompletely miscible with paraffins (heptane used). 4. Two-phase system formed when equal volumes of paraffin, aromatic, and solvent were mixed.
The purpose of test 4, Table I, was to eliminate all but the most highly selective solvents, and again, high selectivity implies that the major area of the ternary diagram is a two-phase area. Actually, of the 31 compounds passing the first three tests, twelve were eliminated by test 4, which means that the two-phase area in
