ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT
esistance HIGH TEMPERATURE APPLICATIONS W. J. MUELLER
AND
R. A. CLARK
Battelle Memorial Institute, Columbus I , Ohio
M
ILITARY aircraft flying a t very high speeds are known to develop operating temperatures of 350' F. and higher. The usual petroleum-base lubricants and hydraulic fluids are not satisfactory a t temperatures in this range. As a result of considerable work by the Armed Services and others, a number of more heat-resistant fluids have been developed for this purpose (1-6, 9, 16-15). Of the various synthetic lubricants that have been developed, the first one that was approved for purchase by the Armed Services under Specification MIL-L-7808B was based primarily on diiso-octyl sebacate ( 5 ) and sold as Turbo oil-15. This same fluid also has some potential as a hydraulic fluid, but its current chief interest to the military is as a lubricant. Man)- diesters make excellent plasticizers for rubber. As such, they are necessarily compatible with rubber and tend to swell it, At higher temperatures, this is a serious problem. Coupled with this are the severe oxidation conditions that are associated with aircraft lubrication systems that pump air with the lubricant. Thus, the object of the present study was to develop a rubber that nould endure long exposures to the diester fluid Turbo oil-15 a t 350" F. in containers open to the air. The specific target properties sought in the rubber are: Property Unrtged IOOOb Tensile strength, Ib./sq. in. 200b Elongation, 70 50-80 Hardness. Shore A2 Swelling, 95 Cracking For 500 hours in T u r b o oil-I5 a t 350' F b Minimum. c Maximum.
.. .. ..
Aged" 500b 1006 50-90
30c (maximum) Pass 180' flat bend
These are the minimum properties considered essential for rubber components in an aircraft lubrication system, such as seals, gaskets, and hose. The rubber compounding studies presented in this paper are limited to an investigation of acrylate-type rubbers for this application. This class of rubbers is known to be outstanding for heat and oxidation resistance, but deficient in its resistance to diester fluids. Thus, one of the principal objectives of this investigation was to reduce the swell of this rubber. Other rubbers were investigated for use in diester fluids, such as the nitrile type and poly-FBA (poly-1, 1-di-hydroperfluorobutyl acrylate), but research on these is not included in this article. Those interested in the complete investigation may refer to the institute's reports submitted to the Wright Air Development Center (16). Hycar 4021 was milled, cured, and dumbbell specimens were prepared, following ASTM Methods D 15-52T and D 412-51T as closely as possible. During milling, cooling water was circulated through one roll only t o minimize the tendency for the stock t o split and run on both rolls. After the polymer banded, the compounding ingredients were added in the following order: fillers, stearic acid, plasticizer, and vulcanizing agents. Amine-type vulcanizing agents Rere always added last because they often caused the stock to stick to the rolls. 982
All stocks r e r e cured in standard 6 X 6 X 0.075 inch molds at 310" F. Dumbbell test specimens were cut out with ASTM die
Rubber specimens aged in hot oil
The hot-oil aging of rubber specimens was carried out in an aluminum-block type heater, drilled to contain 60 test tubes of 38 X 200 mm. The block was maintained a t a temperature of 350" 2" F. The individual specimens were prepared for aging by suspending them in the test tubes on a stainless steel wire (0.0475 inch in diameter) which hung from the lip of the test tube. The test tube was covered with the top of a Petri dish (size 30 X 50 mm.). The wire caused a gap between the test tube and the Petri dish, which allowed free access of air into the tube during aging. Two test tubes were used for each sample to be tested. One contained three dumbbell specimens for the stress-strain and hardness tests, while the second contained two specimens approximately 1 X 1 inch for determination of swell and crack resistance. During aging, the specimens were suspended in test tubes containing 140 ml. of Turbo oil-15. When the various aging periods were completed, the tubes nere removed from the heater and allowed to cool for 1 hour. The rubber specimens were then removed from the oil, dipped quickly in acetone to remove oil from the surface, and blotted dry before being tested. At the beginning of the program, the plan was to run aging tests of 72 and 168 hours on all samples. Those with promising properties after 168 hours' aging were aged for 500 hours. As compositions were gradually improved during the program, many of the 72-hour tests were omitted. Stress-strain properties TI ere determined with a Scott tensile tester, Model L6, run a t 20 inches per minute. Hardness was determined by a Shore A2 Durometer, according to ASTN Method D314-52T. &ell measurements were made with a Kraus-Jolly balance. The degree of cracking was rated visually by examining the 1 X 1 inch specimens after they were bent 180' and then rolled. The arbitrary scale employed was (1) no cracking, (2) cracking, and (3) edge cracking. A sample was rated as cracked when the crack extended completely across the sample. When the sample cracked only a t the edges and the crack did not grow as the sample was rolled, the condition was described as edge cracking. Vulcanizing systems studied with Hycar 4021
Several authors have reported briefly on the best vulcanizing system for an ethyl acrylate-chloroethyl vinyl ether copolymer, such as Hycar 4021. This copolymer vulcanizes through the chlorine group, and six types of curing systems have been recog-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 6
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ing agent, several of the batches had lesser amounts. This resulted in no ap--- Optimum Optimum tensile strength parent discrepancies in the data and inelonqation _ hordneM dicates, at least qualitatively, that the - - - -- - Optimum Optimum swelling ratio of vulcanizing agents is more important than the total amounts. Comparison of Other Vulcanizing Systems. After investigating the TETA-sulfur-TMTD system, four additional vulcanizing systems were briefly evaluated (Table 11). Recipe PA-82 was the best TETA-sulfurT M T D system, while recipe PA-88 was a TETA-TMTD system-i.e., no sulfur-used in early work, but which had good aged properties. Recipe PA115was a direct substitution of Trimene base for TETA I n recipe PA-116, the Trimene base was increased so that the amount of amine present was the same as in recipe PA-82, as Trimene base contains a lower percentage of amine than TETA. ReciTMTD TriethYiow pes PA-78 and PA-86 xere a direct substitution of tetramethylthiuram after aging in Fiqure 1. Effect of TETA-sulfur-TMTD ratio on properties . . monosulfide ( T M T M ) and Polvac Turbo oil- 15 f o r t e t r a m e t h y l t h i u r a m disulfide (TMTD). 168 hours at 350' F. Data from Table I1 show that the TETA-sulfur-TMTD vulcanizing system was the most outstanding of those evaluated. This system nized which can perform in this manner (11). These are quinone produced stocks that swelled the least and were most effecdioxime and red lead; sulfur, alone or in combination with sulfurtive in maintaining hardness and tensile strength during aging, liberating compounds; peroxides; dinitrobenzene and lead The composition cured with Trimene base-sulfur-TMTD system oxides; polymerized dinitrosobenzene (Polyac); and amines. is considered the poorest, as it became very soft and swelled Systems most extensively studied were sulfur, alone and in comexcessively during aging. The remaining systems are interbination with sulfur-liberating compounds, and amines. Amines mediate between these two. give rapid cures which revert on heat aging, while sulfur comEffect of Curing Time. The effects of curing time are well pounds give slower cures which are good a t maintaining properties known for unsaturated polymers, such as natural rubber or after heat aging. The common practice has been to use a balGR-S. For each stock there is an optimum curing time, and anced system of an amine and a sulfur compound to obtain good overcuring generally degrades properties. Saturated polymers heat aging and still have a reasonable curing time (8). such as the acrylates, which are inherently more oxidation Effect of TETA-Sulfur-TMTD Ratio. After reviewing the resistant, might be expected t o behave differently. Since data literature, the most promising vulcanizing system consisted of a in the literature on this subject were very limited, the effect of mixture of triethylenetetramine (TETA), sulfur, and tetracuring time was determined, The results given in Table I1 are methylthiuram disulfide (TMTD). The optimum ratios of these typical of those obtained throughout the entire work. The best three components were investigated in a Hycar 4021 composition properties were obtained with the longest cures, ranging up to containing Philblack A as a reinforcing filler material. 120 minutes a t 310° F. Increasing the curing time increased both The results in Table I show that the original properties, parunaged and aged tensile strength, decreased unaged elongation ticularly elongation, varied over a considerable range, depending (but had little effect on aged elongation), increased hardness upon the ratio of the vulcanizing agents. Vulcanizates conslightly, and decreased swelling. Thus, longer cures led to a net taining less than 1.5 parts per hundred rubber (p.h.r.) TETA gain in desirable properties. No evidence has been found that either did not cure or were undercured, as indicated by a very high extending the cure time is harmful, a t least for cures as long as elongation. 240 minutes at 310' F. The cure conditions are relatively mild While the unaged properties varied with the ratio of the vulcompared to the aging conditions that the samples later endure, canizing agents, the aged properties varied even more. The Tempering of acrylate-type polymers involves heating them areas marked off in the triangular graph in Figure 1 show the in air after they have been vulcanized in a compression mold. compositions, selected from Table I, which produced optimum This postcure treatment is recommended by the B. F. Goodrich values for tensile strength, elongation, hardness, and swell resistChemical Co., t o reduce compression set of Hycar 4021 stocks ance. The superimposed area represents a general range of compositions which showed the best balance of these physical (8). Presumably, during tempering the vulcanization continues and produces a tighter cure. Other properties are also affected properties. Further examination of the data indicated that the to various degrees. best aged properties were obtained with the following ratio of I n an investigation of tempering, two Hycar 4021 stocks convuhanizing agents: taining different fillers were evaluated. One of these stocks was Tricthglenetetramine 2.1 filled with Philblack A, while the other one contained Silene EF. Tetramethylthiuram disulfide 1.0 The tempering was conducted in a circulating-air oven for 7 hours Sulfur 0.9 at 350' F., following one of the recommended procedures (8). The results in Table I V indicated t h a t this procedure improved the While much of the data plotted in Figure 1 resulted from vuloriginal tensile strength of these stock4 but a t considerable sacricanizates having a total of 4.0 partsjlO0 parts rubber vulcanizSulfur
__
June 1956
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
983
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
:
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Vol. 48, No. 6
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT fice to their elongation. While tempering did not alter the hotoil aging properties of the stocks loaded with Philblack A, it did change the aging characteristics of those filled with Silene EF. The extent of these changes is best shown in Figures 2 and 3. The tempered Silene E F stock had a net gain in properties with a higher tensile strength and a lower swell. Therefore, in subsequent studies, tempering was adopted as a standard practice for stocks of this type.
Fillers play importanf role on physical properties The selection of fillers has a n important bearing on the properties obtained with the acrylate polymers. This is because amine-type vulcanizing systems depend upon the maintenance of a basic pH in the uncured stock and makes it desirable to avoid the use of acidic fillers. IS00
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To compare fillers fairly, it was considered desirable to examine them in stocks having the same initial hardness: Estimated Filler Volume Loading t o Produce 80 Durometer Stock, ' P.H.R. by Vol. 25 40 35 70 63 23
Estimated Swellinga of 80 Durometer Vulcaniaates, 7% 50 44 32 30 29 25 25
Filler Hi-Si1 Philblack A Silene E F P-33 oarbon black Calcene NC Valron Estersil Thermax 76 Swelling after aging i n Turbo Oil-15, 168 hours a t 350" F
The extrapolated data indicate that stocks containing Philblack A and Hi-Si1 would swell excessively; those containing Silene EF, Calcene NC, and P-33 carbon black would swell approximately 30%; and those containing Valron Estersil and Thermax would swell approximately 25%. Unfortunately, these latter two filler materials were then found to be unsuitable a t this filler loading because stocks containing them cracked during extended hot-oil aging (500 hours at 350' F.), The filler volume necessary to produce an 80 Durometer stock varied over a range of 23 to 75 parts/100 parts rubber by volume. Cracking after aging, during the 180" bend test, has been a problem with some fillers. As shown in Tables I11 and IV, Valron Estersil stocks cracked after aging only 168 hours. Those made with Philblack A, thermal black, and high loadings of Calcene NC were entirely free from this difficulty for aging times up to 168 hours, but cracked when examined a t the end of 500 hours. Silene EF, Hi-Sil, and lower loadings of Calcene NC produced cracking only in occasional samples.
000
600
400
Silenc EF, phr by volume
Figure 2. Effect of tempering on tensile strength of Silene EF-filled Hycar 402 1 vulcanizates (1)
(2)
Tempered 7 hours in air at 350' F. prior to aging Aged 168 hours in Turbo oil-1 5 a t 350' F.
Comparison of Fillers at Equal Volume. Several fillers were evaluated a t loading levels corresponding to 40 and 60 partsjlO0 parts rubber of carbon black (Tables I11 and IV). The original tensile strengths of about 1600 pounds/square inch were obtained for stocks containing Philblack A, Hi-Sil, and Valron Estersil. However, after hoboil aging, only the stocks filled with Valron Estersil and Hi-Si1 retained a reasonably good tensile strength. The original elongation varied considerably with the type and amount of filler. With the exception of the stock containing Hi-Sil, increasing the filler loading reduced the original elongation. The elongation of all stocks dropped severely after aging, with the greatest drop being obtained for the stock containing Valron Estersil. Considering the high aged tensile strength and hardness of this latter stock, there appears to have been a considerable increase in modulus during aging. The hardness of most stocks declined during aging. However, the data show that those reinforced with Valron Estersil or Cab0-Si1 tended to harden during hot-oil aging. The trend for the Valron Estersil stock to harden and retain good tensile strength probably accounts for its lower swell during aging. The excessive hardening of the Valron Estersil stock may be offset by combining this filler with others that show the opposite tendency,
June 1956
30
40
Silene EF, phr by volume
Figure 3. (1) (2)
Effect of tempering on volume change Silene EF-Hycar 402 1 vulcanizates
of
Tempered 7 hours in air at 350' F. prior to aging Aged 168 hours in Turbo oil-1 5 at 350' F.
Effect of Filler Volume. Filler volume loadings have an important effect on physical properties of synthetic polymers. This is especially true with GR-S, which has very poor gum strength. Studies were made with Hycar 4021 to determine the importance of filler loading for Silene EF, Calcene NC, and Philblack A, with results shown in Tables I11 and IV and in Figures 4 and 5. The effect of filler volume on tensile strength is shown in Figure 4. Philblack A produced the highest unaged tensile strength,
INDUSTRIAL AND ENGINEERING CHEMISTRY
985
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT while Silene EF was intermediate and Calcene NC the poorest in this regard. For the range studied, the lowest loadings did not give the lowest temile strengths. ?To data were obtained for extremely low loadings of these fillers, as this would have been out of the range of practical compounds for this application. A comparison of the data for unaged and aged tensile strength indicates that each of the three fillers acted differently. Philblack A, while it produced the highest unaged tensile strength, suffered the greatest loss in tensile strength during aging. At low loadings, Silene EF vulcanizates lost considerable tensile strength during aging, but this loss hecaiiie negligible at high loadings Most of the stocks filled with Calcene NC did very well in retaining their tensile strength during aging. These data demonstrate conclusively that a stock n-ith high original tensile strength is not necessarily the best able to endure severe aging conditions. Tables I11 and IV indicate the effect of filler content on the elongation of the rubber. Calcene NC stocks were least affected by change in loading, while Silene EF stocks were most affected. After aging, the elongation differences between stocks containing these various filler8 became negligible. Hardness was affected by the filler volume. At a given filler loading, Calcene NC stocks had unaged hardnesses which nere 25 to 30 Durometer units below the hardnesses for stocks of Philblack A and Silene E F During aging, stocks of Philblack A lost about 20 Durometer units compared t o only about 5 Durometer units loss for stocks of Silene E F and Calcene NC. Sn-elling during aging was found t o be influenced to a large extent by the amount of filler in the composition. The data in Figure 5 show that increasing the filler loading decreased the swell of stocks containing Silene EF more rapidly than those containing Calcene X C or Philblack A However, the best balanceof other pioperties is not realized a t the lowest amount of swell. Properties after Aging for 500 Hours. The goal of this research was to produce a stock that wiII meet certain minimum properties after aging of 500 hours in Turbo oil-15. Because of the time involved, this lengthy test was conducted only on those stocks showing promise after aging of 168 hours. The data in Tables I11 and IT' reveal that certain characteristic changes take place during the period between 168 and 500 hours. During this aging interval, the tensile strength of nonblack stocks either was unaffected or increased slightly, 'i?hile carbon black-filled vulcanizates decreased approximately 60% in this propcrty. A t the same time, both black- and nonblack-filled stocks increased slightly in elongation, lost from 5 to 10 Durometer hardness units, and sKelled 5 t o 1554 iiiore than before. The results for swell indicate that a stock should preferably have a swell below 209?0 after aging of 168 hours in order to not exceed 30y0 swell after aging of 500 hours. I n comparing the results for Hycar 4021 stocks containing various fillers, those reinforced with Silene E F came the closest to meeting the desired target properties, while those containing Calcene NC were a close second. If a higher svell (50%) would be permissible, Hi-Sil-filled stocks might be the best. Philblack A, Valron Estersil, and theimal black did not perform too well as fillers for this application, because stocks containing them either had too high a swell or
986
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 48, No. 6
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ticizers may have been the result of partial or even complete extraction, though the experience with Flexol R2H indicates that extraction is not necessarily beneficial in reducing swell. Among the three plasticizers compared, Hycar 4001Xll appeared to produce the best balance of rubber physical properties after aging. However, the results for the compositions containing this plasticizer were no better than those for the control containing no plasticizer. Thus, except for the limited reduction in swell, these brief results reveal no clear-cut method for enhancing the hot-oil aging characteristics of Hycar 4021. The true advantages, if any, for employing a plasticizer in compositions of this type would be more evident by comparing stocks having the same initial hardness. This implies that adjustments would have t o be made in the amounts of plasticizer and/or filler material. Because of the apparent limited gains contributed by these few plasticizers, a detailed study along this line was not undertaken.
5 5 300L
20
,
I
I
1
30
40
50
60
Filler Loading, phr by volume
Figure 4. Effect of filler volume loading on tensile strength o f Hycar 402 1 vulcanizates
were prone to cracking during hot-oil aging. Further improvements in the performance of all these fillers, of course, might be realized by more exhaustive compounding studies. Plasticizers offen employed to reduce rubber swelling
In the compounding of oil-resistant rubbers, extractable plasticizers can often be employed to reduce rubber swell. Sometimes this is done by jointly increasing the filler and plasticizer content to maintain a desired initial hardness. Since swell varies to some extent with both the type and amount of filler, an increase in the filler loading is sometimes desirable by this method. A brief study was made to determine whether improvements could be made in the performance of Hycar 4021 by employing an extractable plasticizer, such as Flexol R2H, or a polymeric type that is more likely to remain in the rubber, as is represented by DPR N-27 (a depolymerized butadiene-acrylonitrile copolymer) or Hycar 4001Xll (a liquid acrylate polymer). The results of these studies are given in Table V. The three plasticizers mentioned above decreased the tensile strength and hardness, and increased the elongation of the unaged rubber specimens. Most plasticizers perform in this general manner to various extents. More significant, however, are the results obtained on the oil-aged samples. These show that the extractable plasticizer (Flexol R2H), which was expected to reduce swell, actually did the opposite. On the other hand, modest amounts of the polymeric plasticizers reduced swell several per cent. Thus these particular polymeric plasticizers were found to increase the oil resistance of acrylate rubber. I n the case of DPR N-27, this resistance presumably is derived from the acrylonitrile portion of the polymer. Since the chemical identity of Hycar 4001Xll is not known, the reason for its action is obscure. However, it may have covulcanized with the base polymer to produce a more swell-resistant network. The extent to which the polymeric plasticizers may have been extracted was not determined. Consequently, some of the reduction in swell afforded by these plasJune 1956
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280 280
30 30
40 40
”
IP 50
60
i 70
Filler Loading, phr by volume
Figure
5. Effect of filler volume on volume change after aging in Turbo oil-15 168 hours at 350’
F.
Without a lubricant, Hycar 4021 stocks are difficult to mill because of sticking or splitting. To overcome this, a small amount of stearic acid (1.0 parts/100 parts rubber) is usually added as a lubricant. Therefore, a short study was made t o determine how processing aids of this type influence the hot-oil aging characteristics of the resulting composition. The data in Table VI indicate that increasing the amount of stearic acid impaired the hot-oil aging characteristics of Hycar 4021. Neither Acrowax CT nor lanolin appeared to be adequate substitutes for stearic acid. As a result of this effort, one part/ 100 parts rubber of stearic acid was adopted as the standard minimum amount of lubricant for the satisfactory processing of Hycar 4021. Since Hycar 4021 is a saturated copolymer, antioxidants are usually considered t o be of little or no value. However, because of the very severe aging conditions, a protective agent of this type might be of value in retarding degradation of this polymer. AgeRite resin D was selected from among the better commercial antioxidants for evaluation in this application. I n both Philblack A and Silene EF stocks, a t levels of 1 and 5 parts/100 parts
INDUSTRIAL AND ENGINEERING CHEMISTRY
987
ENGINEERING, DESIGN, AND PROCESS DEVELQPMENT
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INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
Vol. 48, No. 6
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
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June 1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
989
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Table VI.
Effect of Lubricants on Aging Properties of Hycar 4021 Base recipe:
Ingredients
P a r t s b y Weight
All samples untempered.
Recipe NO. PA43 PA-2 PA-23 PA-26 PA-27 PA-28
Physical Properties after Aging i n Turbo Physical .Properties after Aging in Original Physical Propertiee Oil-15, 72 Hours a t 350° F. Turbo 011-15, 168 Hours a t 350' F. TenElonTenElonTen- ElonCure, sile, gaHardsile, GaHardsile gaHardMilling 310' F.. Ib./sq. tion, ness. lb:/sq. tion, ness, Swell. Crack- lb,/sh. tion ness Swell. CrackLubricants, P.H.R. % Shore A in. % Shore A % ing in. % ' Shore'A % ing Behavior Minutes in. 0.5 stearic acid Bfldsticking 60 1580 490 61 710 130 44 4 7 . 4 None 700 150 41 51 8 Xone Sticking 30 1540 350 63 640 70 57 3 9 . 1 None 640 80 51 4 1 . 9 Kone 1.0 KOsticking 30 1620 520 58 700 110 45 5 2 . 4 None 310 80 39 5 8 . 5 None 2.0 3.0 K Osticking 30 1550 510 56 500 100 44 44 5 None 400 100 44 57.4 None 30 1490 700 55 2.0 acrawax C T Slightsti,cking 630 140 38 5 3 . 5 None 490 130 35 76 1 None Bad splitting 2.0 lanolin andsticking 30 1570 620 52 660 140 39 52.7 None 290 120 36 61.3 None
rubber, this antioxidant was found to contribute nothing to the retention of rubber physical properties during hot-oil aging. It was, therefore, considered rather remote that other antioxidants would provide the considerably greater protective action that was needed to be effective. Other studies were made with Acrylon EA-5 Compounding studies also were made with Acrylon EA-5, a copolymer of ethyl acrylate and acrylonitrile. While little is known about the mechanism by which copolymers of this type vulcanize, it has been estabhhed that essentially the same vulcanizing agents that are effective with Hycar 4021 are also effective n i t h this t l p e ( 7 , l O ) As in the case of Hycar 4021, the effects of various vulcanizing systems and filler weie investigated. The results of the work indicated that Hycar 4021 and Acrylon EA-5 both had advantages and limitations The most serious disadvantage of the Acrylon EA-5 was that it was consistently prone to cracking after aging for 500 hours. Though many attempts were made t o reduce or eliminate this deficiency. cracking still persisted. For this reason, the stocks made Fith Hycar 4021 came closer t o meeting the target requirements than those made uith Aciylon EA-5. Aside from its cracking, hon ever, Acrylon Eli-6 displayed
Table VIII.
several outstanding advantages. The most important of these was its greater resistance to swelling. After aging for 500 hours in Turbo oil-15 a t 350" F., stocks made with this polymer swelled only about 15% compared t o about 4OY0 for similar stocks made with Hycar 4021. Compositions containing Acrylon EA-5 attained original tensile strengths of 2500 lb. per square inch, while those containing Hycar 4021 reached only 1700 lb. per square inch. For aging periods up to 168 hours, vulcanizates of Acrylon EA-5 were free from cracking and retained an elongation of a t least 20070. Under these same conditions, vulcanizates of Hycar 4021 had only about lo070 elongation and likewise did not crack. An attempt was made to overcome the tendency for Acrylon EA-5 to crack by blending it with Hycar 4021. It was hoped that the product would be both more swell resistant and free from cracking. While blends containing 20y0 or less of Acrylon EA-5 did not crack, they were no more swell resistant than Hycar 4021 alone. When blends were made with 40% or more of Acrylon EA-5, they did develop more sn-ell resistance than Hycar 4021 alone, but then they became prone to cracking during hotoil aging. Therefore, there appeared to be no particular advantage to employing blends of this type. Acrylon EA-5 might very well be a superior polymer to employ for resistance t o these particularly severe aging conditions, provided a means could be found for overcoming its tendency to
Summary of Compounds Having Best Aged Properties
P a r t s by Weight Ingredients PA-184 PA-256 PA-225 PA-95 Hycar 4021 100 100 100 100 Silene E F io 70 70 ... Calcene N C ... .. 120 D P R li-27 ... i6' ... Hycar 400 1X 11 ... ... 26' ... Stearic acid 1 1 1 1 Tetramethylthiuram disulfide 2 2 2 2 Triethylenetetramine 1.5 1.5 1.5 1 5 Tempering, 7 hours a t 350" F. Physical Properties after .4ging in Turbo Physical Properties after Aging in Turbo Oil-15, 168 Hours a t 350' F. Original Physical Properties Oil-15, 500 Hours a t 350° F. TenElonTenElonTenElonsile, gaHardsile, saHardsile gaHardlb:/sq. tion, ness lb,/sq. tion. ness, Swell, Cfacklb../sq. tion, nem Swell, in. % Shore A2 in. % Shore A2 % ing in. % S h o r e h 2 rr/o Cracking 85 64 4 2 . 7 h-one 1080 430 800 90 71 3 7 . 6 I\'one 1000 100 72 3 5 . 7 None 93 940 80 79 30 8 None 1240 90 1260 100 800 50 72 2 9 . 4 None 77 75 800 330 1100 io 3 7 . 0 None 76 28 4 None 80 1060 78 3 4 . 1 h-one 890 210 40 1150 60 1030 1210 100 86 50 78 2 8 . 5 None 1130 60 80 3 4 . 2 None 1010 83 3 3 . 3 Cracked 1320 90 88 40 79 28 8 None 1240 60 60 3 7 . 8 None 890 110 90 900 540 io 800 64 31 0 None 80 71 2 7 . 9 None 750 400 74 880 960 90 69 34 4 None 63 55 4 2 . 6 None 60 3 6 . 3 None 810 530 1020 120 710 70 69 880 60 40.6 Kone 770 390 io 61 34.2 Sone 980 100 io 59 4 2 . 8 None 860 65 3 4 . 5 None 860 100 730 310 71 62 65 32 4 Xone 38.8 None 1020 120 980 90 84 880 io 85 67 3 2 . 5 None 1080 110 880 io 860 80 65 3 7 . 9 E d g e o m:king 1010 100 85 800 60 68 31 6 Kone 980 90 65 3 7 . 0 None Recipes used:
I
Cure, Treatment 3?0° F. after Minutes Cure 30 None Tempered 30 Kone PA-184 60 Kone 120 Tempered 60 Tempered 120 None PA-256 60 120 Xone None 30 PA-225 None 60 I20 None Tempered 30 Tempered 60 120 Tempered
Recipe No. PA-95
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 6
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT crack. Since it is a saturated polymer, as also is Hycar 4021, the cracking tendency appears to be the result of the acrylonitrile portion of t h e polymer. It is possible that cracking may be tied up in some manner with the tendency for this polymer to decline in swell during extended aging periods, while Hycar 4021 continues to swell slowly during its entire aging period. This may be evidence that Acrylon EA-5 continues to cross link during aging. The loss in tensile strength and elongation during aging indicates that chain scission occurs a t the same time. Thus, cracking might be reduced by either minimizing the apparent tendency for excessive cross linking or by retarding the chainscission reactions (Table VII). Summary
r
Studies were made to develop an acrylate rubber composition which would resist degradation and swelling when aged 500 hours at 350' F. in a synthetic diester lubricant, Turbo oil-15. The acrylate rubbers investigated included Hycar 4021 and Acrylon EA-5. Although none of the compositions developed completely met the minimum target requirements of the Air Force, four made with Hycar 4021 were found to be most promising (Table VIII). These met the target requirements, except for having a swell that was 5 to 10% higher than the 30% maximum desired. The high points of the research can be summarized as follows. Among several vulcanizing systems investigated, the best rubber aging properties were obtained with a combination of triethylenetetramine, sulfur, and tetramethylthiuram disulfide. Long cures, and in some instances tempering, were beneficial to the aging characteristics of the rubber. Acknowledgment
The data reported in this paper were obtained in connection with a research project sponsored by the Materials Laboratory, Wright Air Development Center, Wright-Patterson Air Force Base. The authors are grateful t o the Wright Air Development Center for permission to publish this work and to E. R. Bartholomew of the Materials Laboratory for monitoring this research. Any opinions expressed here are those of the authors and do not necessarily represent those of the sponsor. Literature cited
(1) Atkins, D. C., Jr., Baker, H. It., Murphy, C. M., Zisman, W. A., IND.ENG.CHEM.39,491 (1947). (2) Bartholomew, E. R., Rubber Age (N. Y.) 72,64 (1952). (3) Bried, E. M., Kidder, H. F., Murphy, C. M., Zisman, W. A , IND. ENG.CHEM.39, 484 (1947). (4) Carlotta, E. L., Hobein, E. M., Rubber Age (N. Y.)74, 8590, 134 (1953). (5) Chem. Week, p. 74 (Nov. 27, 1954). (6) Christian, G. L., Avzation Week, p. 72 (April 27, 1953). (7) Filachione, E. M., Fitspatrick, T. J., Rehberg, C. E., Woodward, C. F., Palm, W. E., Hansen, J. E., Rubber Age (Ai. y.) 72 631 (1953). (8) Goodrich Chemical Co., B. F., Akron, Ohio, "Polyacryhc Rubber," Service Bull. H-11, March 1953. (9) Keller, G. R., S.A.E. Journal 61, 71 (August 1953). (10) Mast, W. C., Rehberg, C. E., Diets, T. J., Fisher, C. H., IND. ENG.CHEM.36, 1022 (1944). (11) Mast, W. C., Rehberg, C. E., Fisher, C. H. (to U. S. of America as represented by the Sec. of Agr.), U. S. Patent 2,492,170, Dec. 27, 1949. (12) Mitten, F. C., S.A.E. Journal 61, 71 (August 1953). (13) Mosteller, J. C., King, J. A., Ibid., 61, 71 (August 1953). (14) Postelnek, W., Chem. Eng. News 31, 1958 (1953). (15) Rubin, B., Glass, E. M., S.A.E. Quart. Trans. 4, 287 (April 1950). (16) Wright Air Development Center, Dayton, Ohio, Rept. TR54-190, Parts 1, April 1954; 2, December 1954; and 3, August 1955. RECEIVED for review May 16, 1955. ACCEPTEDMarch 6, 1956. Rubber Division, AC8 Meeting, Detroit, Mich., May 1955. June 1956
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