I
GORDON G. KNAPP and HAROLD D. ORLOFF Ethyl Corporation, Ferndale 20, Mich.
Improved Lube Oil Antioxidants Mixtures of hindered phenols and alkyl phosphonate provide superior antioxidant effectiveness as well as good antiwear properties to lubricating oils
T H E EFFECTIVEKESS of phenolic antioxidants, which normally act by a free radical inhibition mechanism, can be greatly improved when used simultaneously with a peroxide decomposer (3.5). T h e phenols thus tested were 2.6-di-ter-tbutyl-p-cresol and 2.2 '-methylenebk(6tert-butyl-@-cresol), With the discovery of the "ortho alkylation reaction'' a new series of phenols can be readily prepared ( 7 , 2, 7) with a wide variety of ortho and para substituents. I n this report, some of these new phenols are evaluated in combination with various dialkyl phosphonates (dialkyl phosphites) which act primarily as peroxide decomposers. Dialkyl phosphonates are isomeric with dialkyl phosphites, (RO),P(O)HS (R0)zPOH. However. most chemical and physical evidence supports the phosphonate structure; therefore: "dialkyl phosphonate" is used throughout this report. DialkyI phosphonates and sterically hindered alkylphenols have a marked synergistic effect when tested as antioxidant mixtures in lubricating oils. T h e effectiveness of phenols in such mixtures is directly related to the steric hindrance of groups ortho ta the hydroxyl and not to any electronic effect from strongly electropositive or electronegative substituents in the para position. T h e effectiveness of the phosphonate is also related to the nature of the alkyl groups. 6-tert-Butyl-n-cresol derivatives possess a n optimum structure in syner. gistic mixtures. Phosphonates appear to exert their effect on phenols through a "regeneration" mechanism whereby a hydrogen transfers from the phosphonate to oxidized forms of the phenol.
the oil sample was measured continuously and plotted on the chart of a Leeds and Korthrup Speedomax. For each run, the glass bulb was cleaned by rinsing with chloroform, acetone and water, and then heated with concentrated nitric acid, followed by rinsing in the reverse order, and finally dried in vacuum. '4 saturated aliphatic white mineral oil. PenolaBayol 85C (viscosity at 100' F.! 87.1 S.U.S.; viscosity index, 106.5). was used without further purification. Ferric 2ethylhexoate (0.05 weight % as Fe20s) was used as the catalyst. I n general, 10 inl. of oil containing both catalyst and antioxidant was made u p and 1.O ml. \vas introduced into the cell which previously had been flushed with oxygen. T h e runs were carried out a t 150' C. T h e concentration of antioxidants used gram-moles per liter each was 1 .O X of the phenolic inhibitor and dialkyl phosphonate except as otherivise indicated. These molar concentrations correspond to about 0.5 weight %, of bisphenolic types and about 0.1 weight % of dimethyl phosphonate. As a run proceeds, little or no oxygen absorption by the oil sample takes place initially. At some point, however, oxidation proceeds at a n accelerated rate and this results in a break in the oxygen absorption curve. T h e time (minutes) from this point to the start of the run was taken as the induction time I,.
These Data Confirm the Synergy between Hindered Phenols and Dialkyl Phosphonates .Idditive Concn.
x
10-2,
Compound
hfole/L.
Experimental
Base oil Dimethyl phosphonate, A
1.00
T h e general design of the reaction cell was similar to that of Krager ( 8 ) . It consisted of a glass bulb connected to a mercury-filled manometer. T h e change in oxygen volume (or pressure) above
4,4'-Methylenebis (2,6di-tert-butylphenol), B A S B a Of each.
... 2.00 1 .OO 2 .OO 1.00'
I& JIiri. 3 3 3 85 121 350
Results and Discussion
Induction Tests. T h e effectiveness of phenol-phosphonate mixtures can be changed markedly by altering the structure of both the phenol and phosphonate. PHENOLSTRUCTURE.I n the first series of runs? dimethyl phosphonate was used throughout while the phenol was changed. O n the basis of structure: the phenols were divided into three groups: monocyclic phenols, methylene-bridged bisphenols, and sulfur-bridged bisphenols. I n each series, the steric hindrance of the substituents ortho to the hy-droxyl group was varied from hydrogen to trrt-but) 1. Figure 1 shows that the effectiveness of the phenol-phosphonate mixture is directly related to the degree of steric hindrance existing in such a phenol. A meta methyl group imparts little effectiveness to phenol itself, whereas an ortho methyl results in a tenfold increase in induction time. Ortho-tert-butylphenol was the most cfl'ective monccyclic phenol with a single alkyl substituent. I n Figure 2: the highly hindered 2.6-ditert-butylphenol was the least effective. T h e 2,6-dimethyi compound, ivhich has maximum ortho substitution but not much ortho bulk? was somewhat more effective. ,4 medium hindered phenol, the 2,6-diisopropyl derivative, u as much more effective than either of the phenok with small or large ortho hindrance. And the most effective in this series \vas the 2-methyl-6-tert-butyl Fhenol u hich might also be considered as a phenol \vith medium hindrance. Condensation of these ortho-su bstituted monocyclic phenols with formaldehyde leads to the corresponding 4:4'methylenebisphenols which were also run with dimethyl phosphonate (Fig-ure 3). T h e methylenebisphenol with no ortho alkyl substituents, that is, where R = hydrogen, was the least effective. This confirms that ortho hindrance is important. VOL. 53, NO. 1
JANUARY 1961
63
I 2 6-Dl-!-0UTIL
1-1
I
1-1
;(p-METrY.
2 6-01 SOPROPYL
2-METH”.-6-i-
1 - 1 0
2 6-0 METHYL
I
1
500
1000
BUTYL
I
e-I-BJTIL
1
1
1
I503
C
1000
500
INGUCTIO\I TINE, M’h
lhDJCliCN
5
Mlh
Figure 1 . Dimethyl phosphonate with various phenols. Induction time depends on ortho bulk in the phenol
Figure 2. Dimethyl phosphonate with 2,6-dialkyl phenols. Maximum effectiveness i s obtained when the phenol has medium ortho hindrance
The medium hindered diisopropyl and methyl-tert- butylmethylene-bridged phenols sholzed maximum effectiveness. Of these t\\o compounds, the unsymmetrically substituted methyl-tert-butylbridged phenol \vas the more effective by a b m t a thousand minutes. These facts substantiate those found for the monocyclic phenols-namely, that antioxidant protection from phenol-phosphonate mixtures is a t a maximum when the phenol posscsses medium hindrance adjacent to the hydroxyl group. I n addition, the methylenebisphenols are in the same relative order \vith respect to ortho substituent effects as they were for monocyclic phenols. If the 2.6-dialkyl phenols mentioned earlier were allo\ved to react with sulfur chloride. structures of the type shown in Figure 4 resulted. These compounds are the 4,4 ’-thiobisdialkylphenols. The unhindered thiobisphenol. \ diisopropyl > di-tert-butyl ; tut-butyl > methyl > hydrogen; and in para positions, -S> -CX- > H. ELECTROSICEFFECTSO N PHEIOLS. The phenols used were derivatives of 2,6di-tert-butylphenol Icith various substituents in the para position varying
from nitro to mcthos!.. A11 phenols tested failed to show a correlation be. nveen the electronic effect (acidity of the phenolic hydrogen) and the antioxidant action of such mixtures. EFFECTO F PHOSPHOSATE STRUCTURE. I n a number of runs. the phenol was held constant and the phosphonate !vas varied (Table I ) , In this study. the nature of the alkyl groups of the phosphonate molecule appears to be involvcd. Among the lo\ier alk)-l phosphonates. effectiveness as synergists decreased in the ordrr td-butyl > isopropyl > methyl, although the values for the isopropyl and methyl compounds were very close. A possible explanation for the high effectiveness of di-tert-butyl phosphonate may be attributed to the bulk of the alkyl groups. This bulk may cause ireakening of the P-H bond with concomitant ease of expulsion of this hydrogen. There appears to be a change in mechanism for phosphonates containing four or more carbons in the alcohol. For these compounds, effectiveness decreased as the alkyl chain length increased. If
64
Table I. Induction Tests Were Made Using Different Dialkyl Phosphonates with the Same Amount of the Same Phenol‘‘
Dialkyl I’hosphotiate fed-Butyl Isopropyl Methyl sec-Butyl Isobutyl 2-Methyl-4-pentyl 2-Ethylhexyl Z,6-Dimethylheptyl
I’henolI’hoarhosphonate phoConcn. n a t e , x JIolar 11, MolelL. Ratio Min. 0.526 0.572 0.573 0.582 0.532 0.528 0.545 0.578
0.97 0.90 0.90 0.88 0.96 0.97 0.94 0.89
208 170 160 143 142 134 120 100
* 0.512 X 1 0 P inole/liter of 1’,4-thiobis(G-fe,.~-hut~-l-o-crejoi).
INDUSTRIAL AND ENGINEERINGCHEMISTRY
availability of the P-H hydrogen tu the phenol is involved in the synergy mechanism, larger alkyl groups I\ hich merely shield would be expected to result in a lo\ver synergy. Changes in the alkyl substituents appear to be more critical with phenols than lvith phosphonates-induction time could not be increased b>- more than a factor of two by altering the alkyl in the phosphonate. For phenols. a factor of 10 was easily realized. h f E C H A s I s b f . The function of the phosphonate in phenol-phosphonate synergy is to react lrith an oxidized (quinoid) form of the phenol molecule to regenerate the original phenol. *4ccording to current theory. a phenolic antioxidant arrests oxidation of hydrocarbons in 1\70 steps: RH
+
0
+ .O?H
R.
2 -+
OH
K’ I
R”
‘(?”
\\A,/
- V U
l i
(1 I
0. R’ K” I
I/
1
In Step 2, phenol I gives u p a proton to the hydrocarbyl free radical R . and forms an intermediate free radical 11. The latter may be consumed directly by reaction with another radical. I n the presence of a phosphonate molecule, (Step 3) intermediate I1 probably forms a complex 111, tvhich facilitates transfer of a proton from
R”’
OH
R’
R”
‘/\/’
,i
phosphoius
+ compound
(3’)
I
A t , ,
the phosphonate to I I ? thereby regenerating phenol I . A phosphorus-containing moiety i s left and can undergo further reaction with various components in the system. This mechanism is consistent with the \ride range of synergistic effects as a function of the amount of steric hindrance surrounding the phenolic hydroxyl group. The more hindered the phenol, the less easily phosphonate
LUBE O I L A N T I O X I D A N T S could approach and the more difficult it \vould be for hydrogen transfer to take place. T h e stability of the complex [vould also be controlled by the phosphonate structure, although presumably to a smaller eytent than for the phenolic molecule.
3-VETHYL-6-1-
0:)
~ U T I L
on
I
1
2 -METHYL-B-I-BUTYL
7 1
l
IO00
0
2000
3030
3
o:#-+ods
2000
IND.ICTION TIME MIN
INDUCTION TIME, MIN.
+ RH
(4)
Step 4 shoivs yer another path whereby the intermediate free radical I1 can reform the hydrocarbon R H from R . . I n this case, the phenol ends u p as the quinoid oxidation product IV. For this reaction to take place. R " ' in I1 must be capable of losing a hydrogen atom or accommodating a double bond. Depending upon the nature and steric requirements of the alkyl groups in the phenol molecule. the intermediate free radical I1 may interact with a phos0
Figure 3. Dimethyl phosphonate with ortho-substituted monocyclic phenols. Induction time i s longest when the phenol has medium hydroxyl hindrance
phonate (Step 5). Formation of alkyl aryl phosphates has been demonstrated ( 9 ) where chloranil has been converted to the corresponding phosphate ester by heating with phosphonate. Here it is shown that hindrance plays a role-treatment of 3,3 ',5,5 '-tetra-tert-butyldiphenoquinone in a similar manner gave the bisphenol as the major product.
phonare moiety (RO)ZP. to form a dialkyl aryl phosphate. This has been demonstrated b>-passing a stream of oxy%en gas for 6 hours at 80' C. into a mixture of 1 mole each of dimethyl phosphonate and m-cresol, in the presence of 1 gram of benzoyl peroxide. Dimethyl m-tolyl phosphate \\-asobtained in a 307, yield. The quinoid IV can also react with phosphonate (Step 5). The nature of the resulting products \vi11 depend on the amount of steric hindrance provided by the alkyl groups in IV or in the phosplionatt
I'
R"' 0
OH
R'"
+ phosphorus compound
P(OR)!
0
RI ' 4
(5) For highly hindered phenols, the free radical may sholr. considerable stability. IVith such compounds, regeneration takes place to give predominantly the original phenol (,Step 3 ) . With phenols of low hindrance, the regeneration product to a large extent is the corresponding phosphate ester \- of rhe phenol. T h e latter reaction can also take place bet\veen the quinone I\. and the phos-
X
X
I
11
Figure 4. With dimethyl phosphonate, thiobisphenols, particularly those with medium ortho hindrance, were most effective
. = ( l __ -,=a=O 0
I
I
X
+H
I
+
X
X
HC)
1
I
20,00c,
05 I3 15 P H G S F H O h 4 T E l FHEPiOL MOLAR RATIO
X
O - b - 0 ' X some
X 0
20
Figure 5. Dimethyl phosphonate with 4,4'-thiobis(6-terf-butyl-o-cresol). Induction time increases sharply as the molar ratio of phosphonate to phenol i s increased Phenol concentration, 0.5
X 10 moles p e r liter
OP(OK)!
I n i.his case the phosphate ester was unexpected. Since the latter compound has antioxidant properties, its formation may be considered a form of regeneration. If the proportion of phosphonate to phenol is increased above equimolar amounts, a tremendous additional synergism results. With 0.5 X I 0-2 moles per liter of phenol (Figure 5) and phosphonate alone, the induction times were only 6 3 and 3 minutes, respectively. With a phosphonate-phenol ratio of 2.0, however, the induction time increased to about 20,000 minutes. Another example of the extreme effectiveness obtained by increasing phosphonare concentration is shown in Figure 6 where the induction period was increased from about 99 minutes for the phenol itself to over 1000 minutes at a phosphonate-phenol ratio of 1.2. The oxidation product represented by curve 13 in Figure 6 is about as effective as the parent phenol without phosphonate. However, this difference may be within experimental error and therefore not significant. The low induction time (Curve C) for the free-radical
77/'"""'
I
0
___
10 15 05 mUbvmUNPTE/PHENOL M O L 4 R RATIO
-
-
2ri
Figure 6. Dimethyl phosphonate with a phenol and two phenolic oxidation products. Phosphonates not only improve antioxidant activity of phenols, but they can also convert phenolic oxidation products info compounds which protect the medium A.
E. C.
4,4'-methylenebir(2,6-di-ferf-butyl phenol) (parent compound) 2,6,3',5/-tetra-ferf-butyl-4/-phenoxy-4methylene-2,5-cyclohexadiene-l -one Same compound as 5, except with free radical
VOL. 53, NO. 1
JANUARY 1961
65
stability of the phosphonates \vas investigated because excessive insrabiliiy could preclude the use of some of the esters for commercial application (Table Viscosity 111). In one test 0.01 mole of phosIncrease, Visual Acid % at Sludge phonate \vas refluxed for 0.5 hour in Compound Concn. Number l o O D F. Ratinga 100 ml. of water and the amount of acid None 5.7 107 C formed was titrated. I n the other, a Dimethyl Phosphonate 0.05% P 4.4 60 C solution of phosphonate in 50 grams of oil 4,4’-Thiobis(6-tert-butyl-o-cresol) 0 . 5 wt. % 4.6 112 E (approximately 0.04% phosphorus) was Mixture of phenol and phosphonate As above 1.2 17 A mixed with 12 ml. ofwater. Forty-eight a Range of visual sludge ratings: A = clean; E = worst condition. liters of air per hour was introduced for 4 hours a t 65’ F. and the phosphorus remaining in the oil layer was determined Although the phosphonates from 2These results show that phosphocompound (15 minutes) results from the methyl-4-pentanol and 2,6-dimethylnates a r e not only effective in promotabsence of a n OH group as such. T h a t heptanol were outstanding from the ing the antoxidant action of phenols, curves B and C break a t about the same standpoint of hydrolytic stability, they b u t that they can also convert oxidaphosphonate-phenol ratio may be bewere somewhat less synergistic than the tion products of the phenols into comcause both compounds go through the lower alkyl analogs--e.g.. di-sec-butyl pounds which can protect the medium. same intermediate. phosphonate. The final choice was Polyveriform Apparatus. T h e mixFor synergism to appear, the parent based on antioxidant rating and cost as ture of 4,4’-thiobis(6-tert-butyl-o-cresol) well as resistance to hydrolysis. O n this phenol requires only half as much phosand dimethyl phosphonate, one of the phonate as its two oxidation products. basis, the sec-butyl ester \vas selected. most effective systems evaluated in the T h e reason for the drop in curve B has I n Polyveriform tests i t \vas extremely induction test, was formulated in a solnot been elucidated. At phenol-phoseffective with 4,4’-thiobisI6-tert-butyl-ovent-refined midcontinental neutral oil phonate ratios above 1.0, the oxidation cresol) as well as Jvith other hindered (201.9 S.U.S. at 100’ F.; viscosity index, products presumably give identical phenols. 95) and evaluated in the Polyveriform species, because their curves approach Bench Tests with Copper-Lead Bearapparatus ( 4 ) . This test was conducted each other closely. Dimethyl phosi n g Shells, To ascertain whether the for 20 hours a t 300’ F. by passing 48 phonate with the free radical compound phosphonate component might be detriliters of air per hour through 100 grams is less effective, roughly by a factor of 10. mental to engine bearings. a series of of oil containing 0.1 gram of lead bromide Polyveriform tests \vas carried out in the and ferric 2-ethylhexoate (0.05 weight % presence of copper-lead bearing shells, as FesOs) as the catalyst. T h e results in addition to rhe standard soluble lead given in Table I1 show again the striking and iron salts (Table I\.). synergism between phenols and phosExcellent results were obtained using Table 111. sec-Butyl Esters Have the phonates. O,2jyGphenol in combination with the Best Hydrolytic Stability A number of other hindered phenols phosphonate at either 0.05 or 0.075Yc Mole NaOH/l\lole with dimethyl phosphate were tested, phosphorus. At a lower concentration of Ester on Hydrolysis and each mixture was much more effecphosphonate, the used oil properties and ReAfter tive than the phenol alone. I n these bearing weight losses were unfavorable. Compound tainedY Initialb Reflux tests, the results of changing the phosI n identical tests. other phenols showed 1.8 0 0.97 Methyl phonate structure were similar to those excellent antioxidant action with loxv 0.5-0.6 14 0.36 Isopropyl for the induction test, except for the bearing weight loss. 0.8 64 0.03 n-Butyl dimethyl analog. T h a t this compound 0.24 45 0.03 sec-Butyl was less effective may be because the 0.64 55 0.02 iso-Butyl 2.0
