Ind. Eng. Chem. Prod.
357
Res. Dev. 1983,22, 357-302
Llpinsky, E. S. "Hydrolysis of Cellulose: Mechanisms of Enzymatic and AcM Catalysis"; Advances in Chemlstry Series No. 181; American Chemlcal Soclety: Washington. DC, 1979. Lora. J. H.; Wayman, M. TAPPI 1978, 61, 08. Lowendahl, L.; Samwlson, 0. Svensk PapperstMn. 1974, 16, 593. Mlllet, M. A.; Baker, A. J.: Felst, W. C.; Mellenberger, R. W.; Sattler, L. D. J . Anim. Sci. 1970. 31, 701. Prltchard, 0. I.; PMgen, W. J.; Minson, D. J. Can. J . Anim. Sci. 1982, 42, 215.
Schaieger, L. L.; Brink, D. L. OxMative Hydrolysis of Lignocellulose; Tappi Conference, Forest BioiogyIWood Chemlstry, Madison, WI, 1977. Schaleger, L. L.; Brlnk, D. L. TAPPI 1978, 61, 85. Tarkow, H.; Felst, W. C. "Mechanism for Improvingthe Dlgestibility of Lignocellulosic Materlals with Dilute Alkali and LlquM Ammonia"; Advances in Chemistry Serles No. 95; American Chemical Society: Washlngton, DC, 1969. Worthy, W. Chem. Eng. News 1981, 59(49), 35. Zimmermann, F. J.; DMdams, D. 0.TAPPI 1980, 43, 710.
Salvesen, J. R.; Brink, D. L.; Diddams, D. G.; Owzarskl, P. Vanlllln, U S . Patent 2434626, 1952. Sarkanen, K. V. "Progress in Biomass Conversion"; Academic Press: New York, 1980.
Received for review July 26, 1982 Accepted December 14, 1982
Esters from Branched-Chain Acids and Neopentylpolyols and Phenols as Base Fluids for Synthetic Lubricants Tal S. Chao,' Manley Kjonaas, and James DeJovlne ARC0 Petroleum products Company, Division of Atlantic Richfieid Company, Harvey, Illinois 60426
Esters providing greater oxidation resistance than neopentylpolyol esters of straight-chain acids were prepared and evaluated as base fluids for synthetic lubricants. Acids employed included 2,2-, 3,3-, and 4,4dimethylpentanoic acids, 4,4dimethylhexanoic acid, and acids having single branchings. Polyols and phenols employed included trimethylolpropane, pentaerythrltol, resorcinol, and dihydroxybenzophenones. As shown by oxygen absorption test, oxidation resistance of these esters was improved when both hydrogen atoms in a methylene group were replaced by methyl groups. This improvement is substantlated by Erdco bearing head tests showing lower viscosity increase, lower acid number, and reduced deposits. Further improvement in oxidation resistance was seen when the neopentylpolyols were replaced by polyhydric phenols. Esters of 2,2- and 3,3dimethylpentanoic acids with dihydric phenols have induction periods ten times those of Type 2 base fluids. However, most of them are deficient in low-temperature fluidii. Data showing variations of crltical physical properties and oxidation resistance with structure are presented.
Introduction Esters of straight-chain carboxylic acids with neopentylpolyols such as pentaerythritol (PE), dipentaerythritol (diPE), and trimethylolpropane (TMP) are widely used as base fluids for Type 11/2and Type 2 lubricants for aircraft gas turbine engines (Yaffee, 1965; Dukek, 1964; Robson, 1971). Lubricants formulated from these base fluids are more thermally and oxidatively stable than Type 1lubricants based on dibasic acid esters (Dukek, 1964; Barnes and Fainman, 1957). Lubricants of even greater stability than Type 2 are required for advanced aircraft engines such as those equipped for supersonic transports. Aerodynamic heating raises the skin temperature of the aircraft and denies air cooling to the lubricant. The possible extent of cooling with fuel is limited (Dukek, 1964). Considerable research and development efforts have been carried out, both in the United States (Yaffee, 1965) and abroad (Dukek, 1964; Byford and Edginton, 1971; Robson, 1971; Byford, 1971) to search for lubricants which have better high-temperature characteristics than Type 2 lubricants but are less expensive than Type 3 lubricants such as polyphenyl ethers, fluorine compounds, pyrazines, etc. This paper describes some of the efforts made in our laboratory toward the development of what can be called Type 21/2 lubricants. Such lubricants are described by one jet engine manufacturer as having a temperature capability 50 O F higher than that of a Type 2 lubricant. Our efforts in developing these Type 21/2 lubricants involved both base oil and additive studies. For the base oil our effort was concentrated on the development of branched-chain acid esters of neopentylpolyols and phe0196-432118311222-0357$01.50/0
nolic compounds. The branched-chain acids studied included neo-acids, e.g., 2,2-dimethylpentanoic acid, gemdimethyl acids, e.g., 3,3-dimethylpentanoic acid, and acids containing single branching such as 2-ethylhexanoic acid. The phenols included resorcinol, dihydroxybiphenyls, dihydroxybenzophenones,etc. The esters and many of the branched-chain acids were prepared in our laboratories. The esters were evaluated as base fluids based on their physical characteristics and oxidative resistance as determined by oxygen absorption testing. The choice of esters of branched-chain acids is based upon their potential for improved oxidative stability. Earlier work by Crouse and Reynolds (1963) and by Metro (1964) indicated that oxidative and thermal stability can be improved by replacing straight-chain acids with pivalic, 2,2-dimethylpentanoic, and 2,2-dimethylhexanoic acids. These esters, however, are often solids and therefore unsuitable as lubricant base fluids. Our work was devoted to the optimization of the physical properties of these esters through selection of structural forms, while at the same time maintaining a suitable high level of oxidation stability. This was accomplished through the use of acids containing double branching remote from the 2- or CYposition and through the incorporation of n-valeric acid. Of the C, through C9 acids previously investigated in our laboratories, n-valeric was found to provide PE esters with the highest oxidation resistance.
Experimental Section Preparation of Branched Chain Acids. Many of the acids used in this work were prepared in our laboratories by established methods. 2,2-Dimethylbutyric acid was 0 1983 American Chemical Society
358
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
prepared by carboxylation of a Grignard reagent of tertamyl chloride, followed by hydrolysis. 3,3-Dimethylpentanoic acid was prepared by the procedure of Bott (1965),which involves the alkylation of tert-amyl alcohol with vinylidene dichloride in concentrated sulfuric acid, followed by hydrolysis. 4,4-Dimethylpentanoic acid was prepared by treating neohexyl chloride with NaCN, followed by hydrolysis. This method was also used in the preparation of 4,4-dimethylhexanoic acid. 5,5-Dimethylhexanoic acid was prepared by the addition of acetic acid to neohexene, with benzoyl peroxide as catalyst. The acids thus prepared in our laboratories were purified by distillation and their purity was confirmed by GC analysis. Sources of Other Materials. 2,2-Dimethylpentanoic and 2,2-dimethylhexanoic acids, commonly called neoheptanoic and neooctanoic acids, were purchased from Exxon. Phenols and single branched-chain acids such as 2- and 3-methylbutyric and 2- and 4-methylpentanoic, 2-ethylhexanoic acid as well as a number of straight chain fatty acids were obtained from laboratory chemical suppliers such as Aldrich, Eastman, Kodak, etc. n-Valeric acid was supplied by Union Carbide. The acids were purified by distillation prior to use. Pentaerythritol and trimethylolpropane were supplied by Hercules and Celanese, respectively. N-Phenyl-anaphthylamine was purchased from Du Pont. Preparation of Esters. The esters (except those designated as otherwise) employed in this study were prepared in our laboratories. Esters of straight-chain acids were prepared by heating a slight excess of the acid with the respective neopentylpoly01 with stirring under an N2 atmosphere. A solvent such as xylene was used to continuously remove by azeotropic distillation the water formed. Catalysts such as sulfuric and p-toluenesulfonic acid form byproducts which can act as antioxidant and deposit precursors and were, therefore, avoided. Typical reaction time was 24 h with completion being indicated by the cessation of water formation. Esters of branched-chain acids were prepared in a similar manner. However, longer reaction times were required. Hindered acids such as 2,2-dimethylpentanoic acid required a reaction time of 150-300 h for completion. Other acids such as 3,3-dimethylpentanoic and 2-ethylhexanoic were much easier to esterify than 2,2-dimethylpentanoic and only required 20-48 h reaction times for completion. Esters of branched-chain acids with dihydric phenols were prepared by first converting the branched-chain acid to its acid chloride, using PC1, or S0Cl2. The acid chloride was then purified by distillation and then treated with the respective phenol in the presence of a small amount of dimethylformamide, according to the procedure of Taylor et al. (1961), who first prepared esters such as resorcinol dineoheptanoate. Dihydric phenols which have been successfully esterified include resorcinol, p,p'-biphenol, o,o'-biphenol, Bisphenol A, 4,4'-dihydroxydiphenyl ether, 2,4-dihydroxybenzophenone,4,4'-dihydroxybenzophenone, etc. The branched-chain acids used include 2,2-dimethyland 3,3-dimethylpentanoic acids. Esters of mixtures of straight- and branched-chain acids were prepared by a two-step process. A partial ester of the branched-chain acid was prepared first, the extent of the reaction being monitored by the yield of water collected in a water separation trap. The reaction mixture was then treated with an appropriate quantity of the straight-chain acid and the reaction was carried to completion. The synthesized esters were generally purified by washing with 5% aqueous Na2C03after removing the
Table I. Temperature Dependence of Bearing and Lubricant classification Type 1 Type 11/* Type 2 Type 21/2 lubricant inlet 300 350 400 450 temp, "F lubricantsump 340 390 440 490 temp, "F bearing temp, 500 500 500 550
"F
excess acid by distillation. The ester was then washed with water, dried under vacuum, and filtered through Hyflo Super Cel. A post-treatment with aqueous NaBH, and/or clay was also applied in some cases. Solid esters were subjected to recrystallization, melting point determination, and O2 absorption as a check of their purity. For esters of mixtures of straight- and branchedchain acids the proportion of e.ach acid in the final ester was verified by NMR. Oxygen Absorption Test. This test is a modified Domte (1936) O2 absorption test and involves the bubbling of oxygen at a rate of 1ft3/h through a 75-g sample of test oil maintained at a specified temperature. The time and rate of absorption of an established quantity of O2 are monitored. Details of this apparatus were described in a previous publicatih by Chao et al. (1970). The parameters of this test, such as oil temperature, duration, catalyst, etc., can be adjusted to suit the nature of the lubricants to be tested. For Type 2 and Type 2lI2 synthetic lubricants, the test temperature is maintained at 450 O F and the test is run until 2500 mL of O2(measured at normal temperature and pressure) is consumed. For fully compounded lubricants, the test was run in the presence of 12 ppm of soluble iron catalyst. For evaluating base fluids, the test was run in the presence of 1% by weight of N-phenyl-a-naphthylamine(PANA), an antioxidant used in many Type 2 synthetic lubricants. The results of the test are reported as Ti, the induction period, which is the time elapsed prior to rapid change of rate of O2 absorption, T,,the time required to absorb 2500 mL of 02,and Vi, the volume of O2 absorbed during the induction period. Properties of the fluid following the O2 absorption test are compared to those of the original fluid to determine the level of viscosity increase and change in acid number (TAN by ASTM D-664). Bearing Head Test. The bearing head test was conducted according to the procedure established by Wright Air Development Division (WADD) and Pratt and Whitney (P&W). The test employs an Erdco Universal Tester equipped with a 100-mm roller bearing which rotates at a speed of 10000 rpm under a radial load of 500 lb. Two gallons of the test lubricant are circulated through the bearing head at a rate of 600 mL/min while moist air is supplied at a rate of 9900 mL/min. The temperature of the bearing and lubricant is varied dependent on the classification of the test lubricant. (See Table I.) The test is run with 15-h intervals, each followed by a 9-h cooling off time until a total runping time of 100 h is accumulated. Properties of the oil including kinematic viscosity at 210 and 100 OF,TAN, pentane and benzene insolubles, and ppm of Fe are monitored during the test. A t the end of the test the bearing head is examined and rated by both the WADD and P&W systems. The filter deposit is weighed. Results and Discussion Physical Properties of Branched-Chain Acid Esters. Physical properties, including viscosity, pour point, melting point, and flash point, of the branched-chain acid esters prepared in this study are shown in Table 11.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 359 Table 11, Physical Properties of Esters of Branched-Chain Acids polY01 PE PE PE PE PE PE
PE PE PE PE TMP TMP TMP TMP
acid
no. of C atoms
2-methylbutyric acid 3-methylbutyric acid 2-methylpentanoic acid 4-methylpentanoic acid 2,2dimethylbutyric acid 2,2-dimethylpentanoic acid 3,3dimethylpentanoic acid 2.2 dimethylhexanoic acid 4;4dimethylhexanoic acid 5,5dimethylhexanoic acid 2,2dimethylpentanoic acid 2ethylhexanoic acid 4,4dimethylhexanoic acid 5,5dimethylhexanoic acid
5 5 6 6 6 7 7 8 8 8 7 8 8 8
Liquid at room temperature.
melting point, "F
pour flash point, "F point, "F -45 -45 -7 5
a
a a a
300-310 189-1 92 80-82 149-153 a a a a
a a
Solid at room temperature.
It can readily be seen that most of the branched-chain acid esters have rather poor low-temperature fluidity. For example, a typical Type 2 synthetic lubricant should have a pour point below -75 O F and a maximum kinematic viscosity of 13000 cSt at -40 O F . Only a few of the esters in Table I1 have pour points close to the pour target, and none can meet the -40 O F viscosity target. These results were not unexpected. Crouse and Reynolds (1963) gave a melting point of 258-262 OF for the PE esters of 2,2dimethylpropionic (or pivalic) acid. Bohner (1962) reported a pour point of 178 O F for this ester, but indicated it to be crystalline at this temperature. He also concluded that branching results in increased viscosity and viscosity-temperature slope. It was our belief that improvements in low-temperature properties could be achieved through judicious selection of ester structure based on increased knowledge of the relationship between chemical structure and low-temperature properties, such as melting point, pour point, low-temperature viscosity, etc. Structural features which may affect these properties include molecular size, symmetry, extent and position of branching, interaction of functional groups, etc. Some of these relationships have been reported by previous workers, while others remain to be developed or ascertained. One relationship which underlines this investigation is the one between the low-temperature properties of these esters with the position of branching of the gem-dialkyl acids. The question was raised as to whether and how the low-temperature properties of these esters can be improved by removing the two alkyl groups in the acid position of the molecule away from the carboxyl group. This question was answered by preparing PE esters of 2,2-dimethylbutyric, 2,2- and 3,3-dimethylpentanoic,and 2,2-, 44-, and 5,5-dimethylhexanoic acids and comparing their physical properties. Data in Table I1 indicate that for the 2,2-dimethylsubstituted acids, the melting point of the PE ester decreases as the chain length increases. Further reductions in melting point are seen when the methyl groups are moved away from the ester group. Some of the esters have a tendency to exist in a supercooled state. This tendency is especially strong for the P E ester of 3,3-dimethylpentanoic acid. The ester as prepared remained as a liquid at ambient temperature for more than 2 years. However, when it is was seeded with the PE ester of 2,2-dimethylpentanoic acid and placed in a refrigerator, crystallization took place in a few hours. The ester remained as a solid upon subsequent storage at room temperature. This suggests the potential of using an additive to inhibit crystallization of such esters and extend their usefulness.
b b -2oc b -1 0 +5 -55 -7 0 -35 -2 5
450 465 440 49 0 47 0 530 480 42 5 430 465 47 5
kin. vis., cSt 210 "F
100 O F
4.687 5.431 4.269 5.689 b b 15 -09 13.19 17.01 11.16 4.333 4.460 9.042 6.828
30.40 43.00 23.98 35.56 b b 273.2 88.4OC 340.3 169.3 28.31 33.97 103.7 65.64
Supercooled.
Another relationship which we intended to clarify is the effect of symmetry and molecular size of esters on their low-temperature properties. This was done by comparing properties of TMP esters with PE esters of the same acids and by comparing esters having single branching with those having gem-type double-branching. Data in Table I1 show that esters of TMP have better low-temperature fluidity than the corresponding esters of PE. The TMP ester of 2,2-dimethylpentanoic acid has a pour point of -55 O F vs. a melting point of 189-192 O F for the PE ester. TMP esters also have lower viscosity than the corresponding PE esters. Fluidity and viscosity differences are believed to be due to the lower degree of symmetry as well as the lower molecular weight of the TMP esters. This behavior in pour point and viscosity is also observed in the comparison of the esters of single and double-branched acids. For example, the PE ester of 2methylpentanoic acid has a pour point of -75 OF, while that of the 2,2-dimethylpentanoic acid is a solid at room temperature. Here the role of molecular symmetry appears to be more important than that of molecular weight. Duling (1965), stated that crystallization is least likely to occur with structures which have a low degree of symmetry. Salomon (1973), comparing the melting points and liquid range of saturated hydrocarbons, concluded that a high degree of symmetry will result in a higher melting point and a shorter liquid range. It has also been mentioned in literature that low-temperature properties of synthetic ester lubricants can be improved through the use of multicomponent mixtures of straight- and branched-chain acids and mixtures of polyols. This was demonstrated by work by Warman (1967) and Niedzielski (1976). Our work on esters of mixtures of straight- and branched-chain acids confirms this relationship. Table I11 shows properties of a number of these esters. It can be seen that pour point, low temperature viscosity, and fluidity improved as the concentration of the straight-chain acid increased. Data in Table I1 also show that the volatility of esters, as measured by their flash point, generally decreases as the site of branching moves away from the ester group. This comparison is true for the PE esters of 2,2- and 3,3dimethylpentanoic acid, the P E esters of 2,2- and 4,4-dimethylhexanoic acid, and between TMP esters of 4,4- and 5,5-dimethylhexanoic acids. Oxidation Resistance of Straight- and BranchedChain Acid Esters. Figures 1, 2, and 3 illustrate the oxidation resistance of P E and TMP esters of selected straight- and branched-chain acids as determined by the O2 absorption test.
360
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
Table 111. Properties of PE Esters of Mixtures of Straight- and Branched-Chain Acids mol % of acid f melting flash fire POW point, "C point, O F point, O F point, "F 210 "F 0 26.1 39.2 56.8 72.9 100 0 22.1 28.1 54.8
100 73.9 60.8 43.2 27.1
0 0 0 0 0
0 0
0
189-192 135 -144 86-96
a
-45 -70 secondary > tertiary > allylic or benzylic and that the relative reactivity of secondary hydrogen atoms toward oxidation is 12-27 times that of primary hydrogen atoms (Sniegoski, 1976). The relationship between rate of oxidation and the number of carbon atoms shown in Figure 1 can readily be -+
a'2"
/I0
0
-C H 2 [ C H 2 ) n
C H3 I4
a2" 2"
1"
where lo,2 O , a2O, and a'2O denote primary, secondary, a-acyl secondary, and a-alcohol secondary H atoms. In going from n-valeric to n-caprylic, the only changes are the value of n and the number of 2O C-H bonds. Sniegoski recently showed that the ordinary 2O C-H bonds in esters are the most reactive toward oxidation, while the a2' and a ' 2 O C-H bonds are much less reactive. Therefore, it is not surprising that by reducing the length of the carbon chain and hence the number of more reactive C-H bonds, the oxidation resistance of these esters can be substantially improved. The improvement in oxidation resistance through the reduction of secondary C-H bonds has been used by Snead and Gisser (1963) in the development of fluoroalcohol esters. The fact that oxidation resistance can be improved by reducing the number of 2O C-H bonds prompted the investigation of esters of branched-chain acids. Here, when a CH2 group is replaced by a C(CHJ2 group, the number of 2O C-H bonds is reduced by 2. The number of loC-H bonds is simultaneously increased by 6. Since the 2" C-H bond is 12-27 times as reactive as a lo C-H bonds, a net gain in oxidation stability can be expected. Results of the O2absorption tests confirmed this expectation. Figure 3 shows that induction period of PE and TMP esters of both straight- and branched-chain acids. The scattering of data toward the top is believed to result from a combination of factors such as test repeatability and variations in purity of esters. The existence of the obvious
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 381 Table IV. Bearing Head Test Data Showing Effect of Branched-Chain Acids oil D oil C
PE ester of PE ester of mixt. mixt. of Oil PE ester of of valeric and valeric and 2,2-dimethyl2,2-dimethyl- 3,3dimethylpentanoic acid pentanoic acids pentanoic acids plus same plus oil A plus oil A additives as oil A additives additives
oil A approved Type 2 synthetic oil using commercial PE and DiPE esters of straight-chain fatty acids test conditions hours completed mode oil consumption, lb/h final acid no. (D664) filter deposit, g WADD demerit rating P&W rating viscosity, increase, % KV at 210 "F KV at 100 "F
Type 2 100 normal 0.07 0.87 1.22 37 1-1-1
Type 2112 75 continuous 0.1 4.39 10.85 190.5 2-2-2
5 P e 2112 100 continuous 0.23 1.70 31.66 194 4-4-3
25 37
419 829
63.94
Type 2112 100 normal 0.17 3.06 29.89 149 5-4-3
5 P e 2112 100 normal 0.13 1.88 3.78 104 1-2-1
165 250
98 177
Table V. Properties of Phenolic Esters of Branched-Chain Acids ester E
H
F G 2,2-dimethylpentanoic acid
J
I
3,3-dimethylpentanoic acid phenol OH
OH
kinematic viscosity, cSt 210 OF 100 O F 0 "F -40 "F pour point, "F flash point, "F fire point, O F 0, absorption test (450 "F, 1% PAN, 2500cm3 0,) Ti, min Tt,min Vi, mL A(KV at 100 O F ) ,
6.164 82 .OO
16.07 522.8
C
C
-40 39 5 420
- 20
+10 49 5
>2095 2095
> 1928 1928
>2006 2006
> 2882
42.2
91 .o
59.5
96.0
21.5
4.57
6.00
20.4
3.079 16.87 1231.
405 47 0
4.556 29.14 1814. 39 584 -70 435 48 0
2882
OH
OH
8.778 174.1
21.16 733.8
C
C
+1 0
KU
L*
4.291 20.68
8.752 56.83
4 544 1493 1493
>1598 1598
223 269 448 48.3
161 222 39 7 41.8
35 .O
10.64
11.92
%
A(TAN)
Commercial PU ester of C,-C, acids.
11.4
Commercial diPE ester of C,-C, acids.
correlation does point out the importance of reducing the total number of secondary C-H bonds. The shape of the curve also points out that the relationship is not linear. As the number of secondary C-H bonds decreases from about 32 to 20, much greater increases in induction period were observed. For example, the induction period of P E esters increased from 260 min to 358 min when n-caproic acid was replaced by n-valeric acid, decreasing the 2 O C-H bonds from 40 to 32. But when 4,4-dimethylhexanoic acid was replaced by 2,2- or 3,3-dimethylpentanoic acids, decreasing 2 O C-H bonds from 32 to 24, the induction period of PE esters increased from 271 to 723-795 min. Our data also indicate that the position of the gem-dimethyl groups is relatively unimportant to the induction period of the O2absorption test. This was shown by the closeness (723-795 min) of the induction periods of the PE esters of 2,2-, 3,3-, and 4,4-dimethylpentanoic acids. This is contrary to what was found by Sniegoski (1976), who
Too viscous to measure.
showed that secondary C-H bond at the a-acyl position is about eight times less reactive than those at other positions. It is possible that other effects, such as steric hinderance exerted by the methyl groups, may have reduced the difference between these two types of secondary C-H bonds. Esters prepared from mixtures of straight- and branched-chain acids have oxidation resistance in between these two types. As the percent of straight-chain acid decreases, the induction period increases and the viscosity rise decreases. However, this is accompanied by a corresponding loss of low-temperature fluidity and a proper balance is therefore required. It should be pointed out that all the esters discussed above contain no single branchings which form tertiary C-H bonds. The latter type of esters generally have shorter induction periods in the O2absorption test, especially if the 3O H atom is at the a-position; e.g., the induction period of the P E ester of 2-methylpentanoic acid
382 Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
was 175 min vs. 260 min for that produced from n-caproic acid. Erdco High-Temperature Bearing Head Test. Table IV shows results of bearing head test data obtained under Type 2 and Type 2 l i 2 conditions. Oil A is an approved Type 2 synthetic oil based on PE and diPE esters of straight-chain acids. Under standard Type 2 conditions, this oil gave a viscosity increase of 37%, a final TAN of 0.87, a WADD demerit rating of 37, and a P&W rating of 1-1-1.When this oil was tested under Type 2l/, conditions (temperature of oil and bearing 50 O F higher than that specified for Type 2), the oil gave a viscosity increase of 829% at 75 h and the test was aborted. The WADD demerit rating was 190.5, and the final TAN was 4.39. Furthermore, the test was run continuously, which as experience has demonstrated, is less severe than the normal sequence of 15 h running and 9 h of cooling off for a total of 100 running hours. When the same additives for Oil A were added to a PE ester of 2,2-dimethylpentanoic acid, Oil B was obtained. This oil was run continuously (due to its relatively high melting point) for 100 h under Type 2l/, conditions. When the test was completed, the used oil had a KV at 210 O F which was only 64% higher than that of the new oil. The final TAN was 1.70. These results reflect the improved oxidation resistance of the base fluid. Oils C and D were based on PE esters of mixtures of n-valeric with 2,2- and 3,3-dimethylpentanoic acids. Further improvement in oxidation resistance under Type 2 l i 2 conditions was reflected in lower viscosity increases, lower final TANS, improved WADD demerit ratings, and for Oil D, improved P&W ratings and lower filter deposits than obtained with Oil A. Phenolic Esters of Branched-ChainAcids. Table V shows some of the critical physical properties for esters prepared from three dihydric phenols and two branchedchain acids as well as their O2 absorption test data. These esters are typified by the fact that their induction periods were longer than the time required to absorb 2500 mL of 02. To compare their oxidation resistance with other base fluids, it is necessary ta use T,, the time required to absorb 2500 mL of 02. As shown in Table V, the six esters gave Tt values of 1493-2822 min. In comparison, the two commercial P E and diPE esters of C5-C9 acids gave Ti of 223 and 161 min and Tt of 269 and 222 min. Thus the phenolic esters are almost ten times as resistant to oxidation in the O2absorption test as the commercial PE and diPE esters used for Type 2 lubricants. However, this drastic improvement in oxidation resistance was not achieved without penalty of other properties. Data in Table V indicate that these esters are often deficient in low-temperature fluidity. Some of these esters do have pour points of -40 to -70 O F , considerably better than the 40 OF pour point seen with mixed bis (mixed phenoxyphenoxy) benzenes (Gunderson and Hart, 1962) which are considered as base fluids for Type 3 or Type 4 lubricants. Considerable improvement in low-temperature properties can be achieved through the use of multicomponent mixtures of phenols and branched-chain acids while at the same time maintaining a high level of oxidative resistance.
Acknowledgment The authors wish to acknowledge their appreciation to the management of Atlantic Richfield Company for permission to publish this paper, to Dr. B. W. Turnquest, Dr. F. J. Chloupek, and Dr. J. P. Kuebrich for their encouragement and guidance, to Dr. W. D. Hoffman (deceased) for his initiation of the synthesis of the branched-chain acids, and to Dr. J. L. Bach and Dr. A. L. Mueller for the synthesis and preparation of these acids. Registry No. Pentaerythritol tetrakis(2-methylbutyrate), 25811-385;pentaerythritol tetrakis(%methylbutyrate),26086-25-9; pentaerythritol tetrakis(2-methylpentanoate,25811-39-6;pentaerythritol tetrakk(4methylpentanoate, 26086-27-1;pentaerythritol tetrakis(2,2-dimethylbutyrate),24449-50-1;pentaerythritol tetrakis(2,2-dimethylpentanoate),24449-51-2;pentaerythritol tetrakis(3,3-dimethylpentanoate),24499-81-8;pentaerythritol tetrakis(2,2-dimethylhexanoate),24499-83-0;pentaerythritol tetrakis(4,4-dimethylhexanoate),24449-52-3;pentaerythritol tetrakis(5,5-dimethylhexanoate,24449-48-7;trimethylolpropane tris(2,2-dimethylpentanoate), 14217-47-1;trimethylolpropanetris(2-ethylhexanoate),26086-33-9;trimethylolpropanetris(4,4-dimethylhexanoate),24449-46-5; trimethylolpropane tris(5,5-dimethylhexanoate), 24449-45-4; o-phenylene bis(2,2-dimethylpentanoate), 85553-39-5; 2,2'-biphenylene bis(2,2-dimethylpentanoate), 29262-66-6; 4-benzoyl-1,3-phenylene bis(2,2-dimethylpentanoate),35713-52-1;o-phenylene bis(3,3-dimethylpentanoate), 85553-40-8; 2,2'-biphenylene bis(3,3-dimethylpentanoate), 29111-41-9; 4-benzoyl-1,3-phenylenebis(3,3-dimethylpentanoate),35713-55-4. Literature Cited Barnes, R. S.; Fainman, M. 2 . Lubr. Eng. 1957, 73,454-8. Bohner, G. E.; Krimmel, J. A,; Schmidt-Colierus, J. J.; Stacy, R. D. J . Chem. Eng. Data 1062, 7(4), 547-553. Eon, K. Angew. Chem. 1965, 77(21), 967. Byford, Q. C. Chem. Ind. (London) 1071, 1082-4. Byford, D. C.; Edginton, P. 0. Roc. of 8th World Pet. Congr. Moscow 1971, 5, 101-110. Chao, T. S.; Kjonaas, M.; Vitchus, B. C. SAE Paper 700890, SAE National Combined Fuels and Lubricants and Transportation Meeting, Philadelphia, PA, NOV 4-6, 1970. Crouse, E. F.; Reynolds, W. W. US. Patent 3 115 519, 1963. Dewar, M. J. S.; Dougherty, R. C. "The PMO Theory of Organic Chemistry"; Plenum Publishing Co., New York, 1975; p 277. Dornte, R. W. Ind. Eng. Chem. 1936, 28, 26-30. Dukek, W. G. J. Inst. Pet. 1964, 50(491) 273-296. Duiing, I. N.; Grlffith, J. Q.; Stearns, R. S. ASLE Paper 65LC-1, ASLEIASME Lubrication Conference, San Francisco, Oct 18-20, 1965. Emanuel, N. M.; Denlson, E. T.; Meizus. 2 . K. "Liquid Phase Oxidation of Hydrocarbons"; Plenum Press, New York, 1967; p 134. Gunderson, R. C.; Hart, A. W. "Synthetic Lubricants"; Reinhold Publishing Corp.: New York, 1962; p 411. Metro, S. J. French Patent 1349 100, 1964. Niedzielski, E. L. Ind. Eng. Chem. Prod. Res. Dev. 1976, 75, 54-8. Robson, R. Ind. Lubr. Trlbol. 1971, 2 3 , 1082-4. Salomon, T. Discussion of paper by R. E. Hutton on "Synthetic Oils," NASA Special Pub. 318, National Aeronautic and Space Administration, Washington, DC, 1973. Snead, J. J.; Gisser, H. ASLE Trans. 1063, 6(4) 316-323. Sniegoski, P. J. ASLE Trans. 1976, 20(4), 282-286. Taylor, W. E.; Witt, E. R.; Osborn, C. L.; Huguet, J. L.; Thigpen, H. H. "The Synthesis and Evaluation of Aromatic Esters as Potential Base Stock Fluids for Gas Turbine Engine Lubricants"; WADD Tech. Report 60-913, Wright Air Development Divlsion, March 1961. Warman, M. U.S.Patent 3360465. 1967. Yaffee, M. L. Avht. Week Space Techno/. 1965, 82(1)55-62
Received f o r review September 2, 1982 Accepted February 4, 1983
Presented at the 178th National Meeting of the American Chemical Society, Washington, DC, Sept 9-14, 1979, in the Synthetic Lubricants and Additives Symposium sponsored by Division of Petroleum Chemistry.
