OXIDATIVE DEGRADATION OF T H E
POLYPHENYL ETHERS WESLEY
L. A R C H E R A N D K E I T H 6 . B O Z E R
Chemicals Department Research, T h e D a w Chemical Co., M i d l a n d , M i c h .
Polyphenyl ethers which are thermally stable at temperatures in excess of 800" F. undergo significant oxidative degradation a t 600" F. in the presence of air. This paper discusses the isolation and identification of these oxidation products and presents a systematic series of equations proposing a mode of oxidative degradation consistent with the experimental results. A significant difference in the oxidative stability of the meta and para isomers has been noted and i s also accounted for in the proposed mechanism.
use of the polyphenyl ethers as experimental high temperature lubricants is a fairly recent development. Bis(phenoxyphenoxy)benzene (5P4E) has been used as a high temperature (500' to 600' F.) lubricant in experimental jet engines. Such polyphenyl ether lubricants are normally prepared and used as isomeric mixtures in order to extend their liquid range a t low temperatures. T h e presence of a high (80 to 85y0)proportion of meta-ether linkages gives the optimum low pour point-Le., some 40' F. for bis(phenoxyphenoxy)benzene. Structures and properties of the typical polyphenyl ethers are shown in Tables I and 11. T h e polyphenyl ethers are thermally stable to 800' F. (2) but prone to oxidative degradation above 550' to 600' F. The oxidative breakdown of the polyphenyl ethers was studied in this laboratory in order to determine the mode of breakdown and to find possible means of combating this degradation. HE
Experimental
T h e experimental procedures employed were very similar for the various polyphenyl ether homologs and their isomers. I n a typical experiment 250 grams (0.7054 mole) of mixed isomer 4P3E was placed in a 1-liter, 3-necked, round-bottomed flask equipped with a gas inlet tube, stirrer, and reflux condenser. Dry carbon dioxide-free air was bubbled through the fluid a t a flow rate of 5 liters per hour a t 600' F. The degradation products swept through the reflux condenser were trapped in two dry ice-acetone-cooled traps and the carbon di-
Table 1. Polyphenyl Ether Structures M e t a Isomer Para Isomer
Bis(m-phenoxyphenyl) ether 4P3E. n = 2
Discussion of Results
p-Bis(p-phenoxyphenoxy)-
benzene 5P4E. n = 3
Isomer Distribution in D o w Polyphenyl Ethers
5P4E
oxide was adsorbed in an Ascarite trap. The flask residue and both phases of the cold trap condensate were analyzed by mass spectrometry and vapor-phase chromatography (VPC). A sample of meta-rich 4P3E treated under these conditions, in which the air was replaced by nitrogen, showed very little viscosity or color change, in contrast to a large viscosity increase and darkening with air. The VPC analyses were performed on a Model 500 F and M Scientific Corp. thermal conductivity unit using a '/4 x 60 inch stainless steel column containing 2OY0 silicone grease on a 30- to 60-mesh, white, acid-washed Chromosorb support. Typical analysis conditions used a 5-pl. sample injected onto a 100' C. column, temperature-programmed a t a rate of 1 1' per minute to a final temperature of 30C' C. Helium flow was controlled a t 47 ml. per minute and the injection port and detector block temperature were 350' and 325' C., respectively. T h e quantity of each degradation product was determined by calculating the peak area using maximum peak height and width a t half height and subsequently dividing by the sum of the peak areas in the chromatograph. Any of the original PPE fluid in the cold traps was subtracted from the total before calculating the percentage of the degradation products. The rate of oxygen absorption by the PPE fluids was determined by using a modified Dornte oxidation apparatus. In a typical case 50 grams of the test fluid were placed in the test cell and heated to 600' F. while oxygen was recirculated through the fluid with a Fisher Dynapump. The decrease in oxygen volume was measured by means of a gas buret. A mercury manometer was used to maintain a pressure of 1 atm. in the system. Any carbon dioxide and water formed during the oxidation were removed from the gaseous phase by the appropriate traps, while an acetone-dry ice trap was used to remove the organic degradation products.
8 63 27 2 2 56
36 5 1
Analysis of the pot residue and overhead fractions with a mass spectrometer gave the following degradation products for the meta-rich 4P3E fluid : bis(phen0xy)benzene (3P2E), hydroxydiphenyl ether, diphenyl ether, phenol, and water. Substantial amounts of carbon dioxide were also formed during the degradation. T h e average amounts of the degradation products for four 4P3E runs are given in Table 111. The products were also examined with the aid of vapor-phase
Table II. Properties of Typical Isomeric Mixtures Meta-Rich Polyphenyl Ether 4P3E 5P4E
Boiling range, ' C./l.O mm. Pour point, O F. Kinematic viscosity, cs. At loODF. At 400' F. Initial thermal decomp., ' F.
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2181286 40 382 2.2 >900
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chromatography (VPC), and several new products identified. Trace amounts of dibenzofuran and p-dihydroxybenzene were found. Examination of a basic extract of the reaction products revealed that nearly half of the diphenyl ether product reported by mass spectrometry was o- and p-phenylphenol. Small amounts of three unidentified products were also indicated by the VPC analysis. Oxidative degradation of meta-rich bis(phenoxy)benzene (3P2E) and bis(phenoxyphenoxy)benzene (5P4E) proceeds in a manner similar to the 4P3E reaction. The products from the 3P2E and 5P4E degradations are given in Table IV. The isolated products indicate that the degradation of the polyphenyl ethers must involve free radical species. The initial formaLion of a free radical site involves oxygen, since the use of a nitrogen atmosphere prevents excessive degradation of the fluid a t 600' F. The first product of oxidation would be a hydroperoxide, Ivhich could then decompose to give the phenoxy and hydroxy free radicals. The next major change in configuration would involve a cleavage of the ether linkage. T h e amount of water found in the degradation studies would indicate that some of the water is derived from reaction(s) other than the normal oxidation of a -CHstructure-eg., termination of a hydroxyl radical.
,OH
+ RH
-+
breakdown, since it is not found in the degradation productse.g., Equations 2 and 3. All of the suggested compounds
-
A
0
Cleavage atA
-
Cleavage
PHENOL
+
HO
- DPO (3)
p-BENZOQUINONE 8 D P O
at B
resulting from the degradation of HO-3P2E have been found. Termination of the phenoxy or aryl free radicals would involve abstraction of a hydrogen from another molecule of 4P3E. This termination step in turn furnishes a new radical site for further degradation of the fluid. Many of the resultant products of lower molecular weight are distilled from the hot fluid before further degradation can occu-. An attack of oxygen on an inner ring and cleavage of the inner ether linkage would give the structures shown in Equations 4 and 5.
HzO f .R
If oxygen attacks an inner ring of the bis(phenoxypheny1) ether (4P3E), the structures shown in Equation 1 are possible:
(4)
-
A B
Cleavage
A
at A
B
Cleovage at B * D P O Oxygen attack on the ring should be favored in the positions ortho and para to the ether linkages. Cleavage may not occur a t position A, since neither benzene nor biphenyl has been isolated from the degradation products. Immediate oxidation of a phenyl radical to the phenolic structure could also account for the absence of benzene. The cleavage products a t B in Equation 1 would be phenol and hydroxybis(phen0xy)benzene, HO-3P2E. The diradical must undergo further
Table 111. Degradation Products from Meta-Rich 4P3E as Determined by Mass Spectrographic Analysis (Average of 4 experiments) Weight 70 of Total Equivalent Degradation M m o l e s of Organic Mmoles of Product" Product Product 4P3E ... 685.0 4P3E 685.0 30.5 9.8 3P2E 9.8 14.4 4.2 HO-DPO 4.2 7.1 2.3 DPO 2.3 47.9 1 .7b Phenol 13.3 ... 4.4 COa 104.0 HnO] 186.0 ... ... -
Total 707.4 3P2E is bis(phenoxy)benrene, H O - D P O is hydroxydiphenyl ether, and T w o mmoles of phenol formed per mmole of D P O is d2phenyl ether. hydroxydiphenyl ether. Therefore, mmoles of 4P3E equivalent to phenol product is calculated f r o m expression: a
M m o l e s phenol - 2 m m d e s H O - D P O 4
Direct weight measurements. ~
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+
2MOLES of H O - D P O
aoaie* (5)
T h e phenoxy-0-benzoquinone probably remains in the pot residue as a part of the polymeric component. Equation 5 is the only simple means of explaining the formation of the diphenyl ether. T h e large amount of bis(phen0xy)benzene suggests that oxygen also attacks the terminal phenyl ringe.g., Equation 6. Benzoquinone has been found in trace amounts.
3P2E Table IV.
+ p-BENZOQUINONE
Degradation Products from Meta-Rich 3P2E and 5P4E as Determined by VPC Analysis Weight yo of Total Organic Products Products Found . in Ouerhead 3P2E 5P4E 62 Phenol 34 19 10 DPO 2 Dibenzofuran 5 2 1 Unknown 1 4 HO-DPO ? 14 3P2E 12 26 2-Phenoxvdibenzofuran 9 4P3E 81 , 4 mmoles 8 3 . 6 mmoles COS 1 7 5 . 0 mmoles 1 8 8 . 0 mmoles Hz0
T h e occurrence of small amounts of dibenzofuran can be explained by the intramolecular condensation of a diphenyl ether free radical :
T h e corresponding six-membered ring closure does not occur, since no dibenzo-p-dioxane was found in the reaction products. T h e superior oxidative stability of the para isomer over the meta isomer was first suggested by the oxygen consumption experiments shown in Figure 1. Experiments with the metarich 5P4E and pIp',p"-5P4E confirmed the outstanding stability of the para configuration as compared to the meta linkage. Heating 100-gram samples of the two isomers a t 600' F. in the presence of approximately 1.0 ml. of air per minute per gram of fluid gave a vivid demonstration of these stability diflerences. T h e oxidation of the meta-rich isomeric mixture was terminated a t the end of 150 hours because of excess tar formation, while the para isomer showed little evidence of degradation after 336 hours of oxidation. T h e meta isomer exhibited a 730% viscosity increase (at 340' F.) after 150 hours of oxidation as compared with a 577, viscosity increase (340' F.) for the pure para isomer after 336 hours. Carbon dioxide evolution for the para isomer was 1.11 grams over 336 hours, while the metarich isomer gave 2.25 grams in 150 hours. T h e para isomer had changed from a white solid to a light tan solid during this oxidation period. T h e oxidation stability of the para isomer can be explained by a n intramolecular termination reaction. This termination would depend on the formation of the diene-one free radical structure shown in Equation 7 ; while attack of oxygen on the meta isomer would give no ring closure, as shown in Equation 8. The para structure would give the substituted dibenzo-
or
Ring Closure
No Ring Closure furan structure and thus hinder further degradation of this portion of the 4P3E molecule. The meta isomer, hoivever, would give a structure with the free radical site on the carbon adjacent to the ether linkage. This structure could then undergo cleavage a t the ether linkage and degradation into products of lower molecular weight. The ability of the free radical site to terminate itself immediately into the adjacent phenyl ring illustrates one means by which the degradation process can be suppressed. Chain transfer may also occur between a terminal phenyl free radical site in the para isomer and a n internal para-substituted ring, followed by chain termination of the latter via dibenzofuran formation. A
0
I
2
4
3
5
6
T I M E IN HOURS
Figure 1 .
Tests run at
600" F.
phenoxydibenzofuran structure has been found in the 3P2E and 5P4E degradations. As little as 100 p.p.m. of cobalt as the benzoate salt effectively suppresses the viscosity increases of the meta-rich PPE fluids under typical test conditions. The effectiveness of the cobalt benzoate inhibitor is shown by the greatly reduced yields of the degradation products from a n inhibited meta-rich 3P2E fluid (see Table V). Two 300-gram samples of metarich 3P2E, one containing no additives and the second containing 1200 p.p.m. of cobalt as the benzoate salt, were tested a t 600' F. for 168 hours. The pot residue from the uninhibited 3P2E oxidation contained a fair amount of black insoluble material. Extraction of pot residue with several portions of hot toluene (total of 1400 ml.) gave a n extract suitable for VPC analysis. T h e toluene-insoluble material represented 26% of the original 3P2E. No insoluble material was found in either the cobalt-containing fluid or another uninhibited run which was heated for only 96 hours. T h e cobalt-containing fluid shows an increase in oxygen uptake during the first 2 hours of oxidation. However, the over-all oxygen consumption is much lolver (7.6 grams during 168 hours), compared to 28.0 grams for the uninhibited fluid. These facts rule out the possibility of cobalt's acting as a n oxidation catalyst for some intermediate structure which is responsible for the viscosity increase of the fluid. T h e initially fast oxygen uptake probably oxidizes the cobalt to the higher valence state of 3. Reduction of C O + ~ to a lower valence state-e.g., CoL2 or metallic cobalt-by transfer of a n electron(s) from a free radical structure could explain the inhibi~ R+. tion-e.g., C O + ~ R .+ C O +f Any method for reducing the degradation and resultant viscosity increases of the polyphenyl ethers must be concerned with reducing the total number of free radical species present in the polyphenyl ethers. The improved oxidative stability of the p,p'-4P3E isomer illustrates this point nicely, where intramolecular attack of a free radical site on the adjacent phenyl ring gives a stable dibenzofuran structure. T h e
+
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antioxidant activity of certain aryl amides, imides, aryl esters, and anthraquinone must also be related to their ability to react or coordinate with the free radical sites ( 7 ) . Figure 2 illustrates the yields of the various degradation products as a function of oxidation time a t 600" F. Each point on the plot represents the total yield of a product in grams obtained from 300 grams of meta-rich 3P2E. Diphenyl ether is the only product which increases continuously with time. The initial yield of phenol (1.4 grams a t 55 hours) decreases to 0.37 gram a t 96 hours with a final yield of 1.0 gram a t 168 hours. The yields of dibenzofuran and 2-phenoxydibenzofuran reach a maximum value a t 96 hours and then begin to decline. The higher yield of the 2-phenoxydibenzofuran structure is not surprising, since the appearance of the first free radical site on the 3P2E molecule could be quickly followed by an intramolecular condensation to give the phenoxy-substituted dibenzofuran structure. T h e decreased yields of phenol, dibenzofuran, and 2-phenoxydibenzofuran after 96 hours may be related to the appearance of the insoluble material in the pot residue. Further degradation of the first generation of breakdown products is also possible and reflected as a constant increase in carbon dioxide yield. The structures responsible for the large viscosity increases are probably due to structures of higher molecular weight derived directly from the polyphenyl ether or their degradation products. Simple dimerization of free radical species of the polyphenyl ethers \vould be one possibility. Some of the postulated quinone structures from the degradation could also be arylated by a polyphenyl ether radical-e.g.,
GMS. OF P R O D U C T S
2.50
[,{
I
2.00+
\n
I I
.'V\
0 0
Table V.
Degradation Products from Meta-Rich 3P2E and 3P2E Containing Cobalt Benzoate Weight 70 of Total M m o l e s of Organic Degradation Products Conditions of Run Product Products
Uninhibited 3P2E at 600" F. for 96 hours
For 168 hours
Phenol Diphenyl ether Dibenzofuran 2-Phenoxydibenzofuran COa Hz0 Phenol Dipheny' ether Dibenzofuran 2-Phenoxydibenzofuran
coz
€320
3P2E with 1200 p.p.m, Phenol Diphenyl ether cobalt at 600' F. Dibenzofuran for 168 hours 2-Phenoxydibenzofuran
cos
Hz0
Neut. Cpd. 1" Neut. Cpd. 2
3.9 5.9 7.4 9.3 150.0
380.0 10.6 8.8 4.8 6.9 410.0 850.0 1.2 1.5
0.2 1.4 150.0 40.0
... ...
7.4 19.9 24.7 48.0
...
...
19.5 29.4 16.0 35.2
... ...
12.9 30.6 3.5 43.5
... ...
7.1 2.4
4 h'eutral compounds not found i n V P C analysis of basic extract of cold tra# organics.
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PRODUCT RESEARCH A N D DEVELOPMENT
J-DIBENZOFURAN I
100 150 T I M E IN H O U R S
50
200
Figure 2. Degradation of 300 grams 3P2E a t 600" F. with 5 liters of air per hour
250
of
suggests higher molecular weight structures similar to the following formula, where R equals phenyl.
I
Both the C and 0 alkylation type reactions have been reported ( 3 ) . Infrared analysis of a typical insoluble fraction
\
I
OR
R
PHENOXY D i BENZOF uR A N
OR
Wilson and coworkers' report (4) on the oxidative degradation of the polyphenyl ethers suggests the addition of molecular oxygen to the pi electron system of a phenyl ring and a disproportionation to give phenoxy and hydroxyl free radicals. T h e electron spin resonance spectra of a higher molecular weight oxidation fraction has revealed a stable free radical species. The proposed phenoxy free radical could then add to another polyphenyl ether molecule and with subsequent ring closures and dehydrogenations give the high molecular weight oxidation products responsible for viscosity increases. The volatile oxidation products of phenol, diphenyl ether, hydroxydiphenyl ether, m- and p-3P2E, carbon dioxide, and water have been reported from the oxidation of meta-rich 4P3E. LVilson and coworkers do not, however, suggest any possible difference in the oxidative stability of the m- and p polyphenyl ether isomers. Conclusions
The oxidative degradation of the polyphenyl ethers is a free radical reaction. The isolation and identification of the principal degradation products have suggested a series of equations to explain this breakdokvn of the polyphenyl ethers. The degradation involves the attack of oxygen on a terminal or inner phenyl ring, followed by cleavage of one or more ether linkages. Aryl ethers, aryl phenols, benzofuran structures, and quinones, along with carbon dioxide and water, are the typical oxidation products. The large viscosity increases shown by the oxidized fluid are undoubtedly due to higher molecular weight structures derived from dimerizations of free radical species or perhaps from arylation of quinone intermediates. A comparison of the para and meta isomers of the polyphenyl ethers has shown the para linkage to have superior oxidative stability. The stability of the para isomer may be due to a n
intramolecular attack of the free radical site on the adjacent phenyl ring and formation of a dibenzofuran structure. T h e experimental evidence suggests that the antioxidant activity of cobalt benzoate in the polyphenyl ethers is related to its ability to terminate the free radical species found in the fluid. T h e amounts of volatile organic products from the cobalt-containing fluid are much less than those from the uninhibited fluid as well as the yields of carbon dioxide and water. Further evidence of inhibition is seen in the fourfold decrease in over-all oxygen consumption of the cobalt benzoatecontaining fluid. Initially the cobalt-containing fluid experiences a brief and rapid uptake of oxygen, probably for oxidizing the cobalt ion to a higher valence state.
Acknowledgment
T h e authors thank Earl A. Ebach for samples of the polyphenyl ethers. Literature Cited (1) Archer, W. L., U. S. Patents 3,151,079,3,151,080,3,151,081,
3,151,082(Sept. 29, 1964). (2) Blake, E. S., Hammann, W. C., Edwards, J . IV., Beichard, T . E., Ort, M. R., J . Chem. E n g . Datu 6, 87-98 (1961). (3) Walling, C., “Free Radicals in Solution,” 1st ed., pp. 166-7, I$’iley, Yew York, 1957. (4) LVilson, G. R., Stemniski, J. R., Smith, J. O., Proceedings of USAF Aerospace Fluids and Lubricants Conference (April 16-19, 1963), P. M. Ku, ed., pp. 274-81, prepared under Contract AF 33(657)-11088 by Southwest Research Institute, San Antonio, Tex. RECEIVED for review July 22, 1965 ACCEPTED December 30, 1965
COMPRESSIVE STRENGTH OF POLYMERMODIFIED HYCRAULIC CEMENTS H E R M A N
B. W A G N E R
Drexel Institute of Technology, Philadelphia, Pa.
The principal variables determining compressive strength of polymer-modified hydraulic cements are the “gel-space” ratio and the degree of air entrainment, Secondary variables, of significance under particular conditions, are rate of evaporative water loss during the hardening period and degree of wetness. Polymer type can influence rate of evaporative water loss and also the magnitude of the compressive strength decrease due to wetting. A quantitative correlation of compressive strength i s made with the two principal variables.
THE general characteristics of polymer-modified
hydraulic cements have been discussed (7). T h e present paper treats specifically of one important physical property of such compositions, compressive strength. When a polymer latex is incorporated in the cement composition, a large number of potentially significant new variables are introduced. Among these are effects on hydration rate, adhesion effects, effects upon elastic properties, workability a t a given cement-water ratio, air entrainment, and chemical reactions involving the polymers and cement constituents. The objective here has been to identify the most significant variables and to ascertain the extent to which “conventional” theory must be modified to accommodate the polymer. Compressive Strength of Conventional Cement Compositions
T h e ultimate particles in conventional, hardened portland cement paste have a high specific surface area and a crystalline structure. There is some disagreement as to the mechanism of bonding. Bernal (7) and Jeffery (4) picture a fine meshwork of calcium silicate hydrate crystals, growing outward from the cement grains as hydration proceeds. T h e crystalline products of this growth interlock and fill the voids among the original cement grains and between cement grains and aggregate particles, binding these together. Electron diffraction diagrams of transitional states of hydration indicate the presence of structures related to tobermorite and hillebrandite. Complete hydration, however, shows some
coagulated mass of very small particles having a crystal structure very similar to afwillite ( 6 ) . Brunauer (2) regards the adhesion of tobermorite particles to each other as the most important factor in the strengths of hardened portland cement pastes and concrete ; to produce failure in compression one must work against valence forces within the tobermorite crystallites, perhaps aided by imperfections within the crystallites. Relation of Compressive Strength to “Gel-Space’’ Ratio
Powers and Brownyard (5): from extensive measurements, find that the compressive strength of hardening portland cement pastes is a linear function of the ratio of the volume of the cement gel existent a t any time to the original space available. They term this quantity the “(cement) gel-space ratio,” and find the relationship to hold regardless of age, original cementwater ratio, or identity of the cement. The quantity of cement gel formed a t any time during hydration is considered to be measured by the surface area of the gel; this, in turn, is proportional to the quantity V , in the BET equation, and is evaluated from the experimentally determined water vapor pressure isotherm. Thus,
f c = M s + B
(1)
w,
where fc is the compressive strength, w, is the weight of water VOL. 5
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