R. 8. FEllD and C. F. ROBINSON Eastern Laboratory, Explosives Department,
E. 1.
du Pont de Nemoursffi Co., Inc., Gibbstown, N. J.
Pyromellitic Dianhydride in Curing of Epoxy Resins Significant retention of physical strength may make it possible to utilize epoxy resins cured by pyromellitic dianhydride mixtures at or above heat distortion temperature
IN
A STUDY of the resins obtained by curing polyglycidyl ethers of Bis-PhenolA, the so-called epoxy resins, by carboxylic anhydrides of various structures, Dearborn and others ( 3 ) reported that the resistance of the cured resin to stress at elevated temperatures increases as the functionality or the compactness of the anhydride curing agent increases. Although the effect of the compactness of the anhydride molecule on the resistance to stress at elevated temperatures was confirmed by Robitschek and Nelson (4), only monoanhydrides have been used ordinarily as cross-linking agents for epoxy resins. Pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic dianhydride, PMDA)
has the increased functionality as well as the compactness of structure that would be expected to produce cured epoxy resins having exceptionally high resistance to stress a t elevated temperatures. The results obtained in the present study of the use of mixtures of pyrometallic dianhydride and other carboxylic anhydrides, particularly phthalic anhydride (PA) and maleic anhydride (MA), as cross-linking agents for epoxy resins confirm the theoretical observations of the effects of increased functionality and compactness on resistance of cured resins to stress a t elevated temperatures. The data show the effects of changing the dianhydride-monoan-
hydride ratio, the curing time, the curing temperature, and the anhydride-epoxide ratio on the resistance of the cured resin to stress at elevated temperatures. In addition, information on the physical, electrical, and chemical properties of epoxy resins cured by mixtures of pyromellitic dianhydride and phthalic anhydride or maleic anhydride are reported. Experimental Procedure
Initial attempts to cure epoxy resins by pyromellitic dianhydride alone were unsuccessful because of the limited solubility of the dianhydride in the resin system. However, when phthalic anhydride or maleic anhydride was used as a flux for pyromellitic dianhydride, as much as 657, of the anhydride groups used could be present, as pyromellitic dianhydride and clear castings still were obtained. Samples were prepared for casting by heating the epoxy resin to the desired temperature (usually 120' C.), and then adding the cross-linking agent at a rate such that the temperature remained constant during addition. The stirred reaction mixture was held a t the desired temperature until the cross-linking agent was completely dissolved. The mixture was then poured into preheated molds, which consisted of two stainless steel plates of the desired size separated by a Teflon spacer of appropriate thickness. The molds were coated on the inside with a mold-release agent and held together by C clamps. The filled molds were then placed in conventional laboratory ovens set a t the desired temperature and heated. Heat distortion temperature (7) is accepted generally as a measure of the resistance of a plastic material to stress at elevated temperatures and is reported as the temperature a t which a plastic material under a fiber stress of 264 pounds per square inch was deflected I
10 mils. In this study, the heat distortion temperature was used to evaluate the degree of cure and the resistance of the cured resin to stress, and also to compare the effectiveness of mixtures of pyromellitic dianhvdride and phthalic anhydride or maleic anh)dride with that of other known cross-linking agents for epoxy resins. Various physical and electrical properties of epoxy resin castings cross-linked with mixtures of pyromellitic dianhydride and phthalic anhydride or maleic anhydride also were determined. The test pieces used in the heat aging inch. The studies were 5 X '/2 X test pieces were placed in 4-ounce bottles in a conventional convection oven. The temperature of the oven was controlled within 1.2' C. and measured by means of an iron-constantan thermocouple. Test pieces for use in the chemX 11'2 X ical resistance tests were g1', 1/2 inch. The test pieces were immersed in the test solutions in 2-ounce capped bottles at room temperature. Discussion
When it was found that phthalic anhydride or maleic anhydride could be used as a flux to incorporate pyromellitic dianhydride in a liquid epoxy resin, the effect of increasing the amount of pyromellitic dianhydride in the anhydride mixture on the heat distortion temperature was investigated. Typical results are shown in Figure 1, which shows the effect of increasing the amount of pyromellitic dianhydride used in combination with phthalic anhydride, expressed as per cent of the total anhydride groups, on the heat distortion temperature of Araldite GO20 (Ciba Chemical Co.). The data used were obtained from cured resins in which 0.55 anhydride group per epoxide group of the resin was used, and cure was effected by heating the composition in stages-Le., 4 hours at 120' C. followed by 20 hours at 160' C. VOL. 49, NO. 3
MARCH 1957
369
3
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0 % PYROMELLITIC DlANHYDRiDE IN PMDAIPA
MIXTURE
Figure 1 . Effect of pyromellitic dianhydride on heat distortion temperature of cured Araldite 6020
The heat distortion temperature increased from 8.5' to 210" C. \\.hen the total anhydride units supplied by pyre~nellitic dianhydride increased from 0 to 65c;. Larger proportions of pyromellitic dianhydride ctcrc not used because t h e resulting mixtures gelled before they could be cast. Similarly shaped curves lvere obtained from cured resins in which other ratios of anhylride to the epoxide groups \ \ e r e used: and from resins cured by mixtures of pyromellitic dianhydride and maleic anh!.dride in various anhydride epoxide ratios. Since Dearborn and others (,3) had sholvn that the rrsistarice co stress at elevated temperatures of cured rpox!resins \vas affected by the racio of anhydride grou1:r of the curing agent to
epoxide groups of the resin. the rffixt of this ratio on rhe heat clistortiun temperature of epoxy rcsins cured by mixtures of p)-roniellitic dianh>dride and phthalic atih!.dride or maleic anh!.dride \\-as determined. 'I'lie mixtures [vert' made on the basis of the per cent of rota1 anhydride proups supplied by each anhydride. Figure 2 sho\vs the results obtained \\.lien .\raldite 6020 \vas cured aL variour anhydridcepoxide ratios b!. mixtures of (1) 50 pyrornellitic dianhydride \vith 50 phthalic anhydride and ( 2 ) 2.jCi pyrumelliric dianhydride \\.it11 'i"; phthalic anhydride by liratirig for 4 hours a t 120" C.. follo\ved by 20 hours a1 160' C:. \\-hen the anhydride-cpoxide ratio of the .iOC;l pyroniellitic dianli\.di.idc-.jij!';
-
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40 12 16 20 24 28 CURING TIME (HR) Figure 4. Effect of stage curing on heat distortion temperature of Araldite 6020 0
CURING TIME (HRS.) Figure 3. Effect of curing time on heat distortion temperature Anhydride-epoxide ratio 0.85 to 1
370
INDUSTRIAL AND ENGINEERING CHEMISTRY
4
8
Anhydride-epoxide ratio 0.55 to 1 50% PMDA-50% PA (anhydride basis)
Table 1.
Effect of Heat Aging on Cured Araldite 6020 % Weight Loss after Heat Aging at 210' C. Curing Agenta 340 hr. 500 hr. 750 hr.
507, MA-507, PMDA 1.95 2.26 3.62 50% PA-50% PMDA 14.86 23.65 49.60 100% chlorendic 17.86 34.67 anhydride
...
Anhydride-epoxide ratio for maximum heat distortion temperature. a
Table II. Effect of Temperature of Heat Aging on Cured Araldite 6020 % ' Weight Loss after 500 Hours 220' 230" Curing 200' 210' C. C. C. Agent C. 50% MA50% PMDA 1.76 2.28 3.63 10.00 Chlorendic anhydride 3.97 14.51 39.52 42.70
The effects of curing time and curingoven temperature on the heat distortion temperature of epoxy resins cured by mixtures of pyromellitic dianhydride and phthalic anhydride are shown in Figure 3. The data were obtained from Araldite 6020 cured by a 507? pyromellitic dianhydride-50% phthalic anhydride mixture a t an 0.85 to 1 anhydrideepoxide ratio. Figure 4 shows that increasing the oven temperature in the range covered, 120' to 200' C., produced higher heat distortion temperatures for curing times up to about 8 hours. The highest heat distortion temperatures, however, were obtained when the curing time exceeded 8 hours and the oven temperature was 160' C. Stage curing has been reported ( 2 ) to increase the heat distortion temperature and to improve other physical properties of cured epoxy resins considerably. For the 0.85 to 1 anhydride-epoxide ratio, the formulation which gave maximum heat distortion temperatures, stage curing was not beneficial. At a lower anhydride-epoxide ratio (0.55 to l ) , stage curing was found to be beneficial for mixtures of pyromellitic dianhydride and phthalic anhydride, as is shown in Figure 4. When single-stage curing was used, the maximum heat distortion temperature that could be obtained for a resin cured by a 50% pyromellitic dianhydride-50a/, phthalic anhydride mixture was 120' C. Heat distortion temperatures of about 150' C. were obtained by stage curing when a proper selection of times and curing temperatures was made. No data on the effect of stage curing mixtures of pyromellitic dianhydride and maleic anhydride a t the lower anhydride-epoxide ratio were collected. While qualitatively similar results
were obtained when either maleic anhydride or phthalic anhydride was used as a flux, resins cured by the maleic anhydride mixtures appeared to have much higher heat distortion temperature than those cured by the phthalic anhydride mixtures. A possible explanation for the superiority of the pyromellitic .dianhydride-maleic anhydride mixture may be the compactness of the maleic anhydride molecule compared with that of the phthalic anhydride molecule. Figure 5 shows the higher heat distortion temperature of pyromellitic dianhydride-maleic anhydridecured resin compared with pyromellitic dianhydride-phthalic anhydride-cured resin a t various anhydride-epoxide ratios. A second important difference between maleic anhydride and phthalic anhydride as a flux is apparent when the results of heat aging tests are considered. Table I shows the results of heat-aging experiments on Araldite 6020 cured by (1) a mixture of 50% pyromellitic dianhydride and 50% maleic anhydride, (2) a mixture of 50% pyromellitic dianhydride and 50% phthalic anhydride, and (3) commercially available chlorendic anhydride (1,4,5,6,7,7 hexachlorobicyclo - [2.2.1]5 heptene 2,3 - dicarboxylic anhydride). A 0.85 to 1 anhydride-epoxide ratio was used for both the mixtures containing pyromellitic dianhydride. The samples containing pyromellitic dianhydride were cured for 24 hours a t 160' C.
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The chlorendic anhydride was used in a manner which should give a high heat distortion temperature ( 3 ) . A 0.55 to 1 anhydride-epoxide ratio was used, and the samples were cured for 24 hours a t 180' C. The marked superiority of the resin cured by the mixture of pyromellitic dianhydride and maleic anhydride is evident. Further evidence of the superiority of the pyromellitic dianhydride and maleic anhydride mixture is shown in Table 11. Here the effect on the weight loss of increasing the temperature a t which the specimens were aged is shown. Comparisons were made between the mixture of pyromellitic dianhydride and maleic anhydride and chlorendic anhydride, because chlorendic anhydride appeared to be superior to the pyromellitic dianhydride and phthalic anhydride mixtures. Curing conditions and anhydride-epoxide ratios were the same as in the previous comparison, but different test specimens were used. Most of the data were obtained with Araldite 6020 resin, because of the favorable solubility ofpyromellitic dianhydride in the resin and the high heat distortion temperatures obtained. However, with certain lots of Araldite 6020, heat distortion temperatures 20' to 30' c. lower than those reported were obtained. Table I11 shows that the heat distortion temperatures obtained from other liquid epoxy resins cross-linked by a 50%
Anhydride-epoxide ratio0.85 to 1 VOL. 49,
NO. 3
MARCH 1957
371
Table 111. Heat Distortion Temperature of Epoxy Resins Cured by PAPMDA Mixtures Resinajb
Heat Distortion Tenip., C.
Epon 828 E R L 3794 ERL 2774
196 218 201
m -PHENYLENE-
)IO0
DEFLECTION A 7 WHICH HEAT DISTORTION TEMPER -ATURE I S -
1Ie:it
0.00 0.35 0.47 0.54 0.58 0.78 0.95 1.07 1.52 1.82 1.91 2.65 2.82 3.13 3.59 5.78
100.0 99.53 98.43 98.80 98.80 98.43 98.23 98.14 98.00 97.68 97.69 97.33 97.62 96.72 95.66 93.31
D1.to1 Tenip
REPORTED^,
tlOll
,
'C
195 200 200 197 196 197 200 193 196 198 193 205 197 203 191 193
80
I20
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PMDA 2 0 PHR
t-
160
200
240
280
320
TEMPERATURE ("C.1 Figure 6.
Resistance of cured Araldite 6020 to stress at elevated tempercfure 2 6 4 Ib./sq. inch fiber stress
Table V.
pyrornellitic dianhydride-jOr phthalic anhydride mixture at an anli!drideepoxide ratio of 0.85 to 1 ivcre also around 200' C. The heat distortion temperature. as measured b!. the XSTLI procedure. is a n arbitrary standard ivhicli is assumcd to be the critical trrnperature at ivliich the physical properties of the curcd rcsin start to fall off rapidl!.. Thennal !-ield point is generall!. considered r o be coniparable to the hear distortion ternperature and is defined as the trniperature a t d i i c h the rate of' the deflection of a plasric material under load incrrasrs sharply. The thermal !.ield p o i r i r and the heat distortion temperature of eimxy resin cured by various kno\vn cross-linking agents were comparable. as is shoivn in Figure 6. Surprisin castings cured by mixtu pyromellitic dianhydride and phthalic or maleic anhydride apparently did not exhibit a thermal yield point at temperatures near the heat distortion temperature shown by this curve. Therefore, the behavior of castings in ivhich epoxy resins \Yere cross-linked rr.ith various amounts of pyromellitic dianhydride were studied a t temperatures above the heat distortion temperature. Tests a t a fiber stress of 264 pounds per square inch and a temperature rise of 2' C. per minute were run on -4raldite 6020 cross-linked with various mixtures of pyromellitic dianhydride and phthalic
372
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Table IV. Effect of PMA on Heat Distortion Temperature of Cured Araldite 6020
70 PMD.1
ANHYDRIDE PHR
Ln
' Anhydride-epoxide ratio 0.86,'l. Epon 828 from Shell C;lieniiral C o . ERL 3i94 and 27i4 from Bakelite Co.
% PM.IA
__ C H LOR END IC
DIAMINE 14.2 P H R
Physical Properties of Cured Araldite 6020
Proycrties of C'ured Itesitl 5,000 7,000 Tensile strength, Ib. 'sq. inch Modulus of elasticity, lb. sq. inch 0.48 X 10' 21,000 22,000 Compressive strength, Ib. 'sq. inch Flexural strength, lb. sq. inch 15,000 16,000 At 20' C. At 100' C. ... At 150' C. ... Flexural modulus, Ib.,'sq. inch a t 20" C. ... Impact strength (Izod), (it. Ib. inch o f notch) 0.46 Rockwell hardness M 100 H e a t distortion temp., ' C. 86 Coefficient of linear thermal expansion, in. in. ' C . ...
Table VI.
Dielectric strength, v. 'mil Surface resistivity, ohms Volume resistivity, ohmscm.
4,230 0.54 x 10' 43,000
4,240 0.47 Y 10' 40,100
13,000 8,500 4,800 0.53 X 10'
12,400 9,100
5,200 0.49 X IOfi
0.20
0.13 M 115 250
E 84.2 200
5.2 Y 10
4.56 X 10
Electrical Properties of Cured Araldite 6020
415
>5.7 X 10': >8.0
x
10'3
398 2.51 X 1016 1.27 X l o 6
324 >5.66 X
loL6
1.59 X 10l6
Dielectric constant 60 cycles lo3 cycles 106 cycles
3.64 3.65
3.75 3.70 3.39
4.06 3.98 3.56
Power factor 60 cycles I O 3 cycles 106 cycles
0.007
0.002
0.0076 0.0077 0.0325
0.0129 0.0144 0.0295
INDUSTRIAL AND ENGINEERING CHEMISTRY
...
anhydride a t several pyromellitic dianhydride-phthalic anhydride ratios. The results obtained, plotted as shown in Figure 7, indicate that resins cured by mixtures of pyromellitic dianhydride and phthalic or maleic anhydride do not suffer complete loss of resistance to stress a t temperatures a t or above the heat distortion temperature and that resistance to stress increases as the amount of pyromellitic dianhydride increases. The practical significance of this surprising retention of physical strength above the heat distortion temperature is being investigated. Pyromellitic dianhydride, like most acid anhydrides, reacts with water of the atmosphere to form the corresponding acid. The presence of pyromellitic acid (PMA) in pyromellitic dianhydride does not appear to affect the heat distortion temperature to any noticeable degree until the pyromellitic acid concentration exceeds 6% (Table IV). Data a t higher acid concentrations were not obtained because of the greatly reduced pot life a t higher acid concentrations. All the data reported elsewhere in this paper were obtained from cured resins in which the pyromellitic dianhydride used contained more than 987, pyromellitic dianhydride. While the present study was concentrated on determining the conditions required to obtain maximum heat distortion temperatures for resins cured by mixtures of pyromellitic dianhydride and phthalic or maleic anhydride, some data on other physical and electrical properties generally considered important for epoxy resins were determined for Araldite 6020 cured by mixtures of pyromellitic dianhydride and phthalic or maleic anhydride. Table V shows that physical properties of Araldite 6020 cross-linked with phthalic anhydride, pyromellitic dianhydride-maleic anhydride, and pyromellitic dianhydridephthalic anhydride are essentially comparable. Table VI shows that the electrical properties of Araldite 6020 cross-linked with pyromellitic dianhydride-phthalic anhydride and pyromellitic dianhydride-maleic anhydride are superior to those of Araldite 6020 crosslinked with phthalic anhydride alone. The curing conditions and the formulations used were designed to give maximum heat distortion temperatures. Coupons of Araldite 6020 cured by a mixture of 50y0 phthalic anhydride and 50% pyromellitic dianhydride at a 0.55 to 1 anhydride-epoxide ratio were subjected to the action of solvents and solutions for 90 days a t room temperature. The coupon weights were checked after 90 days. The solvents used and the changes in weight are reported in Table V I I . Even though the testing conditions were not extremely severe, the results indicated that the mixed anhydride-cured resin had good resist-
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Table VII.
120
140
160 180 TEMPERATURE (“C.)
200
220
Effect of PMDA on resistance to stress at elevated temperature
Chemical Resistance a t Riom Temperature of Araldite 6020 Cured by PMDAIPA Mixture
(90 days’ exposure) 30% HzSOi 70% %SO4
35% HCl 10% €IC1 30% HNOs
HzO (distilled) 1% NaOH 50% NaOH
Butyl Cellosolve
Less Than 1% Weight Change +0.53 ’ 30% Na2SOn -0.10 20% NaC1 f0.37 88% Hap04 10% HaPo4
+0.47 +0.40 f0.40 +0.56 -0.04 +0.20
Acetic acid (glacial) 10% acetic acid Ethylene glycol Oxalic acid (satd.) 30% Ha02
+0.47 $0.21 -0.09 +0.54 f0.21 +0.54 -0.04 f0.56 +0.61
Less Than 2% Weight Change 50% HNOs
Ethylene dichloride
+1.23 1.73
+
Ethyl acetate
Less Than 5% Weight Change Acetone
ance to the common solvents and very good resistance to acetone and 70% sulfuric acid.
Literafure Cited (1) Am. SOC.Testing Materials, Method D 648-45T. ( 2 ) Bakelite Co., “Epoxy Resins,” Epoxy Tech. Release 3 (February 1955). ( 3 ) Dearborn, E. C., Fuoss, R. M., MacKenzie, A. K., Shepherd, R. G.,
f1.54
f4.04
Jr., IND. END. CHEM.45, 2715 (1953). (4) Robitschek, P., Nelson, S. J., Abstracts of Papers, 128th Meeting, ACS,
Minneapolis, Minn., Sept. 11-16, 1955, p. 17P.
RECEIVED for review May 8, 1956 ACCEPTEDOctober 11, 1956 Division of Paint, Plastics, and Printing Ink Chemistry, 129th Meeting, .4CS, Dallas, Tex., April 1956. VOL. 49, NO. 3
MARCH 1957
373
