I
R. 0.BOLT and J. G. CARROLL California Research Corp., Richmond, Calif.
Radiolysis and Radiolytic Oxidation of Lubricants L U B R I C A N T S for use in and around atomic power plants must be resistant to deterioration by nuclear radiation. The objective of this work, begun in 1948, was to develop radiation-resistant oils. Studies in about 1930 of effects of radiation on hydrocarbons indicated that compounds containing benzene rings are more stable than aliphatic compounds (70, 73). This and other work on chemical effects of radiation on organic materials (9) led to further investigation of influences of molecular structure on radiolysis-for example, the radiolysis of carboxylic acids (76), alcohols ( 7 7 ) , and benzene (7) was studied. The role of benzene in protecting less stable materials from radiation damage in mixtures was also investigated (72). The present work involved an initial survey of various organic classes and species which might be suitable for lubricant use. Included was a study of the relative stabilities of organic fluids, effects of additives on these, and effects of irradiation variables. Work with nuclear radiation was supplemented with tests using gamma radiation alone. Resistance to oxidation is also an important requirement for modern lubricants, as exclusion of air is not generally practical. Chemical reactions, such as oxidation, are accelerated by radiation; so resistance to oxidation is even more critical in a radiation environment. For these reasons, oxidation tests, common in work on conventional lubricants, were selected as a second step in the development of radiation-resistant oils. Experiments were conducted, without radiation, on selected base oils and base oil-additive combinations. In-pile oxidations were then performed under comparable conditions on the most promising formulations. I n another phase of the work, oxidation tests on previously irradiated fluids were conducted without radiation. T h e base oils and base oiladditive combinations used in all this work were those found promising in the initial survey. Lubrication qualities of the oils developed were also studied (6).
Static Irradiations
Test Procedures and Equipment, Viscosity change was taken as the principal criterion of damage, because viscosity is the principal property which governs the choice of a lubricant for a given application. Viscosity values were obtained by the ASTM method ( 2 ) . IN-PILETESTS.All exposures in the nuclear reactor were made in quartz
ampoyles of about 10-ml. volume (74, in the shape of a test tube topped with a short section of 7-mm. tubing. This open ampoule, containing 7 ml. of fluid, was placed in an aluminum can (isotope can), which had a rounded bottom and a flat cap with a crimped closure. A vent was made in the edge of the cap after closure. The samples were exposed in graphite blocks (stringers) in the Oak Ridge graphite pile. The duration of the irradiations was set by the weekly pile shutdown schedule. Thus, exposures were for one week or multiples of one week. No attempt was made to control the temperature of irradiation except in exposures of about 285" F. and above, where electrically heated ovens were used without graphite blocks. After the samples were taken from the reactor, they were allowed to stand for about a week to permit induced radioactivity to decay. The outside aluminum containers were then re-
In designing a nuclear power plant, the effects of radiation must be known for every component-down to the last drop of lubricating oil. This survey shows how radiation affects lubricants and opens the door on a field of expanding interest moved, and the quartz vessels were capped and shipped from the pile site for evaluation. TESTS WITH GAMMA RADIATION.The gamma exposures were made in borosilicate glass tubes which were sealed in air. About 7 ml. of fluid was used, and an air space of about 1.25 ml, remained. Gamma radiation from discharged pile fuel elements at Hanford, Wash., was used. The elements were changed periodically to maintain a high radiation flux. These radiation sources were placed symmetrically under water around a canister containing the glass ampoules. In the experiments at elevated temperatures the canister was heated electrically. After removal from the radiation field, the glass containers were opened and the fluids removed for evaluation. Materials Used. Some experimental materials were synthesized especially for this development program, but the majority were purchased products used
generally without further purification. Thus, materials ranged from commercial mixtures to highly purified organics, depending upon the source. In-Pile Test Results a n d Discussion. Pile radiation dosage is expressed here as slow neutrons per square centimeter, a single component of the incident combined radiation. Values were obtained from pile operating data with which correlations had been made with previous measurements. The fast neutron and gamma ray components, which are the more damaging to organics, occur in direct proportion to the slow neutrons ( 5 ) . Techniques for determining the fast neutron and the gamma components of combined reactor flux are still experimental. The temperature values below about 285' F. reported were also obtained from correlations of pile operating data with previous measurements. These ambient temperatures in the reactor were subject to pile operational changes. INDEXOF DAMAGE, Ala. Irradiations for various periods were conducted in the pile over a span of several years, during which the pile operating cycle varied. Exposures had to be conducted in several positions in the pile with slightly different radiation fluxes (intensities). Thus, samples received many different radiation dosages. To intercompare the viscosity changes which occurred, a relationship between this criterion of damage (for exposures in air) and radiation dosage was needed. It was found that within certain limits of dosage, the log of a function of viscosity increase (am) varied linearly with the pile dosage raised to the power. The empirical equation for the straight line and its slope, Ala, are expressed as follows: log 710 = A10 (pile dosage)s/2 log 10 (1)
+
'lope
=
-
=
log 710 log 10 (pile dosage)W2 (2 )
The 710 value of a fluid is the viscosity of the irradiated material a t the temperature at which the viscosity of the original material was 10 centistokes. Through 710 values, all viscosity changes resulting from irradiation are referred to a given base line, 10 cs. Figure 1 shows how the 710values were determined from measured viscosities a t 100" and 210' F. for both the original and irradiated fluids. The coordinates employed are those used in ASTM charts D 341 for petroleum products (2). Ala, the slope of the line, is characteristic of the material irradiated. This VOL. 50, NO. 2
FEBRUARY 1958
221
Table I.
Radiation Resistance of Types of Organic Fluids Exposed in a Nuclear Reactor" Fluid
Initial Viscosity a t loo0 F.,
Source and Structure
Mineral oil Solvent refined western paraffinic lubricating oil n- Alkane Hexadecane
CS.
Comm. 150 neutral; arom. C = 2 % ; aliph. C = 77%; naph. C = 21%
31.9
Phillips cetane
Alkyl aromatics Alkylbenzene 350
9
84.0
0.30
5
2.0
0.14
3
1.4
Solid
13
57.4 12.4 38.9 13.1
0.55 0.65 1.4 Solid
7 12 13
Synthesized; ~ - C * H ~ - ~ - C ~ H I Z ~ ( - C ~ R ~ O84.0 )~H Synthesized ; ~ - C ~ H ~ O ( - C ~ H O O - ) (mol. ~-CHS 12.9 wt. -450) Synthesized (mol. wt. -540) 27.6
0.09 0.12
2
Solid
13
Hooker Fluorolube Monsanto Arochlor 1232
0.26 0.45
4
Solid Solid
13 13
0.90
11
Humphrey Wilkinson
Esters Didecyl terephthalate Mixed tetraaryl silicates Tricresyl phosphate* Di(2-ethylhexyl) sebacate
Synthesized; branched alkyl group Synthesized ; 60% phenyl, 40% cresyl Eastman T 4420 Rohm & H a a s Plexol 201
Poly(but0xyethene) Halohydrocarbon and halocarbons Poly(ch1orotrifluoroethene) Dichlorobiphenyl Silicones Dimethyl siliconeb Phenyl methyl silicone
Dow Corning 200 Dow Corning 710
81.4 47.3 282 238
Ketone Phenyl n-propyl ketoneC
*
Eastman 1908 1.8 Exposure temperature = 142-72" F.; dosage = 1.41-1.78 X 101s slow neutrons/sq. cm.; 4-week exposure. Dosage = 0.85 t o 0.94 X 10'8 slow neutrons/sq. cm. Exposure temperature = 248-73" F.
holds true for pile dosages up to about 2.5 x 10l8slow neutrons per. sq. cm. and temperatures below the order of 285' F. The influence of temperature and dosage is discussed further in later sections. AI0 values were calculated for all samples subjected to pile radiation and are used in the figures and tables. As this index was developed from results of experiments in the presence of air, it is a measure of total deterioration-i.e., that caused by radiolysis, oxidation, and cracking-in the radiation atmosphere. Errors in determining 7710 and differences in pile exposure conditions make Ala values reproducible only to 5 1 5 7 6 . RADIATION RESISTANCE OF ORGAXIC CLASSES. ~ N D SPECIES. Table 1 lists data for compounds of several different classes. The beneficial influence of aromatic rings is apparent except with the silicones, where all species solidified under the test conditions. The low viscosity increase of ethers of the poly(propene oxide) type also stands out, even though this may have been caused by low molecular weight radiolysis products as diluents. Although halogenated materials ranked well in these tests, they corroded iron and copper in similar tests in the presence of these metals. Thus, they were not considered practical lur further development as lubricant bases. However, certain halo-organics were found satisfactory as additives.
222
10
0.66
Alkene l-DodecenebnC
Ethers Poly(propene oxide) Poly(propene oxide) -B
0.73
3.1
Oronite Aromatics ABH; RR'CeH4 (mol. wt. -350) Eastman P 2415
1-Methylnaphthalene
Index of Damage A10 Rank
Table II.
8
1
6
Variation of Radiation Resistance with Chemical Structure" Original Carbon Atoms __ \ ~ i % at1OO"F.. No. aromatic Cs.
~
~
~
I "
Fluid 1-Methylnaphthalene (Eastman P 2415) Dodecylnaphthalene" Octadecylnaphthalened Octadecyl-l,2,3,4-tetrahydr~naphthalene~ 9-n-Dodecylanthracene (PSU 124') 9-(2-Phenylethyl)heptadecane (PSU 871) 9-(2-Cyclohexylethyl)heptadecane(PSU 88f) 9-(3-Cyclopentylpropyl)heptadecane (PSU 110') n-Heptadecane (PSU 5351)
11
91
2.0
22 28 28
45
19.6 30.3 29.1
36
,410 0.14 0.25< 0.43
0.59 26 54 77.5 0.27 0.56 25 24 9.7 0.92 25 0 14.8 1.3 25 0 11.5 0.84 17 0 3.6 Pile dosage = 1.41-1.153 X lO18slow neutronsisq. c m . ; 4-week exposure at, 149-65' 1;. From HF alkylation of naphthalene with 1-dodecene, B.P.1o.e = 434--i0 F.;irradiation a t 248' F. Exposure temperabure = 248' F. From 41C13 alkylation of naphthalene with 1-chloro-octadecane, B.P.3 = 410-40' I,', From AlC18 alkylation of Tetralin with 1-chloro-ortadecane. f Pure compound from American Petroleum Institute Project 42, Pennsylvania State Uniyersity.
Typical data on variations of radiation resistance with chemical structure are given in Table 11. The detrimental effect of decreasing the aromatic ring content is illustrated by the results from the first four compounds listed. This is emphasized by comparison of the remaining five compounds, three of which each contain the same number of carbon atoms. The fluids described in Tables I and I1 \$'ere also heated in ovens. The time and temperature of the pile exposures were duplicated, except that radiation
INDUSTRIAL AND ENGINEERING CHEMISTRY
21
was not present. IYithout exception, the viscosity changes were minor for the oven tests as compared with those for the pile exposures. This conclusion was confirmed by reference tests throughout this work. EFFECTS OF ADDITIVES. As lubricants are normally used in the presence of air, an investigation was made to discover effective oxidation inhibitors. Three base oils of interest in the formulation of lubricants were chosen for the test work (Table 111). Didodecyl selenide was consistently beneficial. Aromatic
~
i
~
O X I D A T I O N OF LUBRICANTS B Y R A D I A T I O N 1.0 -
0
a,0.8 W
c3 4
B LL
2
0.6
-
0.4 -
W
Q
z
- 0.2. 3.01
'
100
'
120
'
I
1
I50
*
180
'
'
'
'
PI0
TEMPERATURE,%
I
01 0
4
2
Figure 1. Concept of qlc for irradiated fluids of different initial viscosity
Table 111. Effects of Oxidation Inhibitor Types on Radiation Resistance of Fluids" Additive Aio Di(2-ethylhexyl) Sebacate Base None Solid 2% Didodecyl selenide (Oronite OLOA 250) 0.90 2% 2,6-Di(te~t-butyl)-4-methylphenol (Enjay Paranox 441) Solid 6Yo Didodecyl selenide 0.57 10% Didodecyl selenide 0.44 10% Pinene-P& product (Monsanto Santolube 394C) 0.45 2% Pinene-P& product Solid 2% Sulfurized wax thiomer (Oronite OLOA 231) Solid 1% Zinc dibutyl dithiocarbamate (synthesized, m.p. 2216' F.) Solid Alkylbenzene 350 Base None 0.34 2% 2,6-Di(tert-butyl)-4-methylphenol 0.34 2% Pinene-PzSs product 0.29 1% Zinc dibutyl dithiocarbamate 0.28 2% Dihexacosyl polysulfide 0.26 2% Didodecyl selenide 0.15 Poly(propene Oxide) None 0.16 2% Pinene-P& product 0.11 2y0 Didodecyl selenide 0.08 1% Zinc dibutyl dithiocarbamate 0.11 2% n',N'-diphenyl-p-phenylenediamineb (Goodrich) 0.36c a Pile dosage = 1.44-1.80 X 10l8 slow neutrons/sq. cm.; 4-week exposure a t 68176' F. All conventional amine-type inhibitors tried had adverse effects. Dosage = 1.14 X 101* slow neutrons/ sq. om.
IO
12
14
16
18
Figure 2. Effect of iodobenzene concentration on radiation damage to di(2-ethylhexyl) sebacate exposed at 145" to 150" F. Pile dosage, 1.2 to 1.63
materials containing sulfur also gave favorable results. Amine-type antioxidants, which are frequently used in conventional lubricants, generally catalyzed the radiolysis of base oils. Certain compounds termed free radical scavengers, which are not conventional oxidation inhibitors but are known to react with free radicals, were investigated in several base oils. The only materials
8
6
P E R CENT IODOBENZENE BY WEIGHT
found effective in retarding viscosity increase were iodine and compounds containing iodine. Work was principally with the latter in view of their more favorkble solubility and volatility properties. Table IV summarizes pertinent data on the effects of halogenated materials. All were beneficial, but none was as beneficial in reducing viscosity increase as iodobenzene. Figure 2 shows that an effective concentration of iodobenzene in di(2-ethylhexyl) sebacate is about 5% by weight. Infrared spectrometry on samples before and after irradiation showed that iodobenzene was consumed in performing its function. Thus, with alkyl aromatic, ester, and ether base oils, iodobenzene practically disappeared after a 1-week exposure at 150" F. to a pile
Table IV. Effect of Halogen Compounds on Radiation Resistance of Oils"
Additive Di(2-ethylhexyl) sebacate base
Index of Damage, Aio
X 1 O I 8 slow
dosage of about 3 X 1017slow neutrons per sq. cm. The reaction products are not known, although iodine remained in the samples as shown by chemical analysis. I t is often desirable in lubricant development to increase the viscosity and viscosity index (V.I.) of base oils by the use of polymeric viscosity index improvers ( 8 ) . Effects of radiation on oils containing such additives were investigated in the present work. Large viscosity decreases generally occurred at a low radiation dosage and were followed by viscosity increases, the magnitude of which depended on the particular base oil used. This suggests an early depolymerization of the additive which initially overshadows the continuous radiolytic thickening of the base oil. Figure 3 summarizes data on alkylbenzene 250 thickened with poly(alky1 methacrylate) (see Table VI1 for source and structure). The normal behavior of Table V. Effects of 1-Methylnaphthalene in a Less Radiation-Resistant Base Fluid"
-11
U11
Solid iodobenzene (Eastman 152) 0.29 5% bromobenzene (Eastman P 43) ' 0.62 5% bromoform (Eastman P 45) 0.64 5% 4-fiuoranisole (Eastman 3141) 0.80 5% dichlorobiphenyl 1.1 Octadecylbenzeneb base oil None 0.59c 2% iodobenzene 0.27 Poly(propene oxide) base oil None 0.16 2% iodobenzene 0.10 a Pile dosage = 1.12-1.60 X slow neutrons/sq. om.; 4-week exposure a t 14969" F. Largely 2-phenyloctadecane from H F alkylation of benzene with 1-octadecene, b.p.15 392-94' F. Exposure temperature = 265' F.
neutrons per rq. cm.
Aromatic Carbon Index of Atoms, Damage,
None 5%
Fluid % Aiq Octadecylbenzene 25 0.5Qb Solvent refined western paraffinic lubricating oil 20% I-methylnaphthalene 20 0.56 I-Methylnaphthalene 91 0.14 Solvent refined western paraffinic lubricating oil 2 0.73 Solvent refined western paraffinic lubricating oil 20% I-methylnaph20 0.61 thalene, calcd.c Pile dosage = 1.46-1.70 X 1OI8 slow neutrons/sq. cm.; 4-week exposure a t 1405 3 O F. Exposure temperature = 265' F. From weighted arithmetic averages of Am's of components.
+
+
VOL. 50, NO. 2
FEBRUARY 1958
223
a n alkyl aromatic is illustrated by the curve for alkylbenzene 350 (Table I). The negative Ala values for the oil containing the polymer illustrate the initial radiolytic depolymerization of the additive. This compound alkylbenzene 250polymer blend behaved similarly in oxidation tests in the pile described later. Octadecylbenzene and a mixture of paraffinic mineral oil with l-methylnaphthalene were each irradiated to determine if physical mixtures of aromatic and aliphatic compounds behave in the same way as "chemical" mixtures. The aromatic carbon atom content was the same order for each fluid (20 to 25%). Table V gives the results of these exposures. The Ala values show the physical mixture to be the equivalent of the chemical mixture, octadecylbenzene. The data for 1-methylnaphthalene and for the mineral oil are also given, as is a calculated value for the 2OY0 mixture derived from the Ala values of the components. This calculated value also agrees with the experimental values for octadecylbenzene and for the mixture of 1-methylnaphthalene and mineral oil. Although the equivalence of the physical and chemical mixtures is important, the use of this principle in lubricant development is limited by such considerations as required volatility and viscosity index. The low molecular weight aromatic compounds which are soluble in nonaromatic oils have detrimental effects on both volatility and viscosity index. EFFECTSOF IRRADIATION VARIABLES. Viscosity change was practically independent of exposure temperature below about 285' F. This was shown in irradiation work in the pile a t temperatures from 68' to 428" F. and dosages u p to about 2.5 X IO1* slow neutrons per sq. cm. Comparable conditions ofradiation dosage and exposure period were used. Typical results obtained for various oils are summarized in Table V I . Negligible differences in Ala values exist below about 285' F. Increases
Table VI.
Effect of Temperature on Oils Irradiated in the Pile for Four Weeks"
Alkylbenzene 350 selenide
+ 2%
+
Didecyl terephthalate f 2% decyl selenide
0.16 0.16
0.26
Coked
0.18
0.29
0.43
...
0.90
0.79
0.90
Coked
0.38
...
...
0.39
0.44
Solid
...
0.17
...
0.12
0.52
Coked
... ...
ZY0 dido-
+
Poly(propene oxide)-B 2% iodobenzene 5 % didodecyl selenide 0.01% quinizarid I,
... ...
0.15
Di(2-ethylhexyl) sebacate didodecyl selenide
+
428
68
didodecyl
Amylbiphenyl
+
Index of Damage, A N , after Exposure at a F. 148 218 285 356
...
Dosages = 1.5-1.9 X 1018 slow neutrons/sq. em. Eastman T-3054.
generally were observed above this temperature, and a t 428' F. all of the compounds except amylbiphenyl were solids. The linear relationship between radiation dosage and radiolysis, as expressed by Equation 1, was found valid for various radiation dosages u p to about 2.5 x 1018 slow neutrons per sq. cm. Data for typical oils are given in Figure 4. For pile dosages beyond about 2.5 X 1O18, the slopes of the curves increased with increasing dosages. In a study of the effects on organics of much larger pile dosages, both compounded oils and oils without additives were exposed in the reactor for six months a t a nominal temperature of 150' F. Radiation dosages of 9 to 10 X 1018 slow neutrons per sq. cm. were received by these oils, which included amylbiphenyl, poly(propene oxide), octadecylbenzene, and di(2-ethylhexyl) sebacate base materials, together with additives. Amylbiphenyl containing 2% didodecyl selenide was a tarlike semisolid after this exposure. All others were hard, brittle, transluscent solids which could not be dissolved in any common solvents. This work established that a pile dosage of this order is too severe for such oils in the presence of air. Equation 1 predicts
the viscosity of the compounded amylbiphenyl sample to be about 6.5 X 108 centistokes after this long-term exposure. This is about 30 times the viscosity of a fluid a t its pour point. Gamma Irradiation Test Results and Discussion. The principal fluids used in the pile exposure work were irradiated with gamma rays alone. Table VI1 summarizes the data from these exposures a t two dosages and two temperatures. The data show that alkyl aromatics are generally the least affected by radiation. The ester containing an aromatic ring was also more resistant than the aliphatic ester. An alkyl aromatic containing a viscosity index improver exhibited a large viscosity decrease, again indicating depolymerization of the polymeric additive. These effects were the same as those which occurred in the exposures to pile radiation. The difference in irradiation conditions between the pile and the gamma exposures probably accounts for the relatively large viscosity increases observed with the poly(propene oxide) materials in Table VII. Such ethers decomposed in the pile exposures to give low molecular weight products and viscosity decreases. This phenomenon can be attributed to
__.
400
100
P
2.0
-
4
1.0
w
1
2 00
-
r
r
I
sLL
ALKYLBENZENE 350
-- -*------*,. --
r--
0
a
DI(2-ETHYLHEXYL) SEBAC t2% DIDODECYL SELENI
0 -
l
n
OLVENT REFINED ESTERN PARAFFINIC LUBRICATING O I L
100 80 60 50
40
1.0 -
0
4% POLY(ALKYL METHACRYLATE)
/
t
2% DIDODECYL SELENIDE
/
30
YLBENZENE 350
20
z IO
0
-4 0
0.5
I .o
1.5
2.5
2.0
PILE DOSAGE,SLOW NEUTRONS/ C M . x~
I O Is
Figure 3. Polymerization of viscosity index-improved oil in pile exposure at 68" to 285" F.
224
lNDUSTRlA1 AND ENGlNEERlNG CHEMISTRY
0.5
I O
15
2 .o
(PILE DOSAGE, SLOW NEUTRONS /cM.'
2 5
x 10'~)yz
Figure 4. Effect of pile dosage on oils exposed at 68" to 266" F. Pile nux, 7 X 10" slow neutrons per sq. cm. per second
O X I D A T I O N OF LUBRICANTS B Y R A D I A T I O N oxidation which took place in the pile irradiations, In the gamma exposures only a very small amount of air was present, thus limiting the possible oxidation and, thereby, the formation of low viscosity diluents.
Oxidation Tests Test Procedures a n d Equipment.
TESTS WITHOUT RADIATION.Two types of oxidation tests were used. The first employed the cell described in ASTM Method D 943-53 (3). Oxygen, a t 3 liters per hour, was bubbled through the oil, and samples were taken periodically for viscosity and acidity determinations. .4water-cooled condenser returned volatile acids to the reaction mixture. The copper and iron wire catalysts of the ASTM method were omitted in all but one experiment. No attempt was made in the work to relate oxygen flow rate to oxygen saturation of the oils under test. Experiments were conducted a t 285' F., a temperature in the range expected in many applications. An oil bath was used to maintain this temperature. In the second type of test, oils which had been irradiated in an atomic reactor were oxidized in a Zeitfuchs (77) viscometer tube. This was placed in an oil bath maintained a t 210' F. Oxygen was bubbled through the material in the viscometer tube, and it was necessary only to stop the flow of oxygen to permit a viscosity determination. I n this manner the viscosity change criterion of oxidation could be followed closely. The lack of large quantities of irradiated oils made desirable the use of this small-scale test, which required only 3 to 5 ml. of fluid. IN-PILE TESTS.T h e single procedure used was similar to the modified ASTM method, except that three temperatures, 175', 195', and 285" F., were employed. Some equipment differences also existed because of the peculiar requirements of the reactor facilities. The oxidation cell consisted of an aluminum cylinder of 275-ml. volume. The container was heated, where necessary, by the use of resistance wire wound on the outside of the cell. Aluminum inlet and outlet lines, thermocouples, and heater leads extended from the cell in the pile to the control apparatus outside. After placing the cell in the pile, about 150 ml. of the test fluid was introduced through the inlet tube. Oxygen a t the rate of about 3 liters per hour was then bubbled through the oil by use of the same tube. Exhaust gases passed through a cold trap and an alkaline scrubber and were then vented. Periodic samples of the test oil were taken by reversing the oxygen flow so that the oil was forced out the normal inlet tube under slight pressure. Piping arrange-
ments were such that oxidized samples could be removed, the cell flushed with a suitable solvent, the excess solvent removed by flushing with nitrogen, and a new sample introd,uced for subsequent oxidation, all while the test cell remained in the pile. The effect of insoluble lacquers possibly deposited on the walls of the cell was neglected. Copper and iron wire catalysts (3)were placed in the cell in one in-pile experiment but were omitted in other experiments to facilitate the reuse of the apparatus without removal from the reactor after each oil test. Materials Used. Table VI11 lists the fluids employed. T h e base oils and additives were all commercial-type products generally used without further purification. Further identity information is given in tables. Octadecylbenzene, di(2-ethylhexyl) sebacate, poly(propene oxide)-B, and solvent refined paraffinic lubricating oil
Table VII.
were used in comparative test work as representing the alkyl aromatic, ester, ether, and mineral oil classes of lubricants. Each of these species possessed a good viscosity index (8) relative to other members of its respective class. Alkylbenzene 250 was also used as an alkyl aromatic in certain experiments. This material had a relatively low viscosity index. Various additives were used for the reasons discussed under Static Irradiations. Quinizarin was added in selected cases because in certain lubricant applications this material had been found to protect metals, especially copper, from attack (75). In-Pile Oxidation Test Results a n d Discussion. All irradiations were conducted in the Oak Ridge graphite pile. Radiation dosages are expressed in slow neutrons per square centimeter as described earlier. TESTS WITH AND WITHOUT NUCLEAR
Effect of Gamma Radiation Alone on Oils" Original Viscosity at 100° F.,
cs.
Fluid Solvent refined western paraffinic lubricating oil (SAE 30 base) Octadecylbenzene Di(2-ethylhexyl) sebacate Di(2-ethylhexyl) sebacate didodecyl selenide Poly(pr0pen.e oxide) Poly(propene oxide)-B Didecyl terephthalate decyl selenide
+
109 11.0 12.8
2% 12.8 57.. 4 12.8
+ 2% dido-
52.1
+
Alkylbenzene 250b 4 40 Poly(alkyl methacrylate)c -I-2% didodecyl selenide Alkylbenzene 350 selenide
14.0
+ 2% didodecyl
77.1
Irradiated Viscosity, 710 after Exposure to 100° F. 285' F. 4.1 X 108 r, 3 4 X 10%r. 0 6 X 108 r. 12.8
...
39.0
... 19.0
... ... ... ...
... ... ... 15
... 22 12.9
10.3
10.2
...
11
..' ...
10.5
...
5.3d
10.5
9.gd
Flux about 5 X lo4 roentgens per hour. Oronite Alkane-monoalkylbenzene of average mew. 246 from HF alkylation of benzene with polypropene. C Rohm & Haas Acryloid H F 855 minus carrier. d Indicates that viscosity is lower than original.
Table Oil Designat,ion
A B C
D
VIII. Identities of Test Oils
Base Oil Octadecylbenzene Di(2-ethylhexyl) sebacate Octadecylbenzene
Identity ' Reference AdditiveQ,Wt. % Table S-1 0-1 R-2 D-2 S-14 5 0.01 IV 5 2 0.01 I 20 IV 2 (4% more added during test) 2 4 VI1 5 2 0.01 I 6 0.01 5 IV
Alkylbenzene 250 Poly(propene oxide)-B Octadecylbenzene Solvent refined western parafanic G 5 6 0.01 lubricating oil I 20 5 0.01 Di(2-ethylhexyl) sebacate I 20 H I I Di(2-ethylhexyl) sebacate L VI1 Alkylbenzene 250 J a Additive code. S-1 = 1-methvlnaDhthalene (Table I) : 0-1 = didodecvl selenide (Table 111) ; R-2 = iodobenzene (Table I?) : D-2 = quinisarin (Table VI) ; 5-14 =- poly(alky1 methacrylate) (Table VII).
E F
,.
VOL. 50, NO. 2
FEBRUARY 1958
225
Table IX.
Oil Designation (Table I)
Relative Performance of Oils on Oxidation in Pile Original Propertiesb viscosity at 1000 F . , cs.
Oil Identitya
Properties after Pile Dosage of 1.4 iTeutrons/Sq. Cm. Viscosity Acidiiy, at 1000 F . , mg. KOH/ cs. gram fluid
X 1017Slom
-
~~~~~
Viscosity increase i t 100
O
F.,%
Tests at 285' F.
+ 5% 0 - 1 + 0.01% D-2 + 4+ + Octadecylbenzene + 5% 0-1 + 6% R-2 +
A
Octadecylbenzene
R
Di(2-ethylhexyl) sebacate 20% S-1 57, 0-1 2% R-2 0.01% D-2
F
0 . 0 1 % D-2
G
Solvent refined western paraffinic lubricat20% S-1 5% 0 - 1 6% R-2 ing oil 0.01% D-2
H
Di(2-ethylhexyl) sebacate 5% 0 - 1 0.01% D-2
+
+
+
+ 20% S-1 +
+
11.1
13.5
3.2
22
8.9
12.6
3.4
42
9.8
12.8
3.4
31
10.4
26.3
3.2
153
8.7
12.2
6.3
40
8.9
10.3
2.0
16
13.9
18.5
3.3
33
Tests at 195' F.
+
B
Di(2-ethylhexyl) sebacate 20% S-1 $. 5% 0 - 1 2% R-2 0.01% D-2
E
Poly(propene oxide)-B R-2 O.Olyo D-2
+
+
+
+ 5%
0-1
+ 2%
Additive code. S-1 = 1-methylnaphthalene; 0-1 = didodecyl selenide; R-2 = iodobenzene; D-2 = quiniearin. Acidity of original oils was minor.
I n Figure 5 curves are given for acidity developed in the oxidation of a compounded octadecylbenzene (oil A, Table VIII). The data are typical of those obtained with various oils tested. The effects due to oxidation in the absence of radiation were minor when compared to those due to oxidation in its presence. This is also shown by Figure 6, which presents data on the viscosity changes observed in these typical experiments. Viscosity changes with irradiated materials can occur both from oxidation in the presence of radiation and from irradiation alone without oxidation (4). The latter effect is believed small with the oil of Figure 6, particularly in the low maximum pile dosage range (about 1.4 x 1017slow neutrons per sq. cm.) involved. The major accelerating effect of radiRADIATION.
ation on the oxidation of oils was also demonstrated when pile shutdown periods occurred during the tests. Here the radiation intensity was drastically reduced while the oxidation proceeded. Viscosity and acidity changes continued to occur but at a reduced rate as a result of the curtailed radiation flux. This effect is shown by the change in slope in the curve of Figure 7 where acidity is plotted against oxidation time. Such a change was always observed as a result of pile shutdown irrespective of the oil used. However, the magnitude of the effect varied with the susceptibility of the oil to oxidation in the presence of radiation. This susceptibility, with viscosity change as the criterion, is discussed later. Similar discontinuities resulting from pile shutdown are evident in the curves of Figures 8 and 9.
EFFECT O F z4DDITIVES. Because O f thr great acceleration of oxidation by radiation, as illustrated by the data of Figures 5, 6, and 7, there was some question as to the magnitude of the beneficial effect of oxidation inhibitors in this accelerated test. In one experiment, results of which are given in Figures 8 and 9, an additional 4% of didodecyl selenide was introduced about midway in the test (2YGwas present at the start). The discontinuity in the curves with the resulting lower slopes illustrates the benefit derived from the additive. In is common practice in lubricant development to use certain polymeric additives to increase the viscosity of base oils and improve their viscosity indexes (8). I n the present work, alkylbenzene 250 containing 2% didodecyl selenide was improved in this manner with 4%
=1
0 3.0
I a a
Y1 2 . 0
2 cj
2
i
k
k
e0
