HIGH-TEMPERATURE EPOXY RESINS BASED ON 3,3’,4,4’-BENZOPHENONETETRACARBOXYLlC WALTER
P .
B A R I E ,
JR.,
DIANHYDRIDE (BTDA) A N D
N O R M A N
W .
F R A N K E
Gulf Research & Development Co., Pittsburgh, Pa. 15230 The increasing demand of the aerospace and other industrial applications for materials having high-temperature stability has led to increased research for n e w resin formulations. BTDA (3,3’,4,4’-benzophenonetetracarboxylic dianhydride)-epoxy resin mixtures have been studied with emphasis on formulation, curing conditions, and testing of the cured products. BTDA can be used alone and in admixture with maleic anhydride ( M A ) to give homogeneous castings and an empirical equation has been developed to aid in formulating BTDA/ M A mixtures. BTDA imparts high temperature properties to the cured resin, as shown by heat distortion temperatures in the 280’ to 300’ C. range. Excellent chemical and solvent resistance and stability to heat aging a t high temperatures show that optimum curing and crosslinking have taken place.
THEaerospace industry and other industrial applications have been demanding materials having good hightemperature stability. The amine- or monoanhydridehardened epoxy resins have been used in many applications which utilize their superior strength, chemical and solvent resistance, electrical properties, and temperature stability. However, in some applications the high-temperature stability is not good enough and new hardening agents are being sought to improve this property. Organic acid dianhydrides, which contain either cyclic or aromatic structures and have high functionality, can impart these desirable properties to the cured epoxy resin. Reportedly, pyromellitic dianhydride (PMDA) (Field and Robinson, 1957) and cyclopentane dianhydride (CPDA) (Van Volkenburg and Johnson, 1966) give excellent hightemperature stability. The success of both as epoxy hardening agents is attributed t o their tetrafunctionality, which leads to higher density of crosslinks, thus higher heat distortion temperature ( H D T ) and increased chemical and solvent resistance (Skeist, 1958). The H D T may be improved by increasing the functionality of either the epoxy resin or the anhydride hardening agent (Dearborn, etal., 1963). A new anhydride, 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA), has become available and has one
anhydride group on each benzene ring o f t h e benzophenone molecule, whereas PMDA and CPDA have both anhydride groups on the same ring. Considering these structural differences, a program was undertaken to study the formulation, curing, and testing of BTDA-epoxy systems. 72
I & E C PRODUCT RESEARCH AND DEVELOPMENT
The main objective was to study the effect of many variables on the high-temperature resistance, as determined by H D T , and also the mechanical and chemical properties of the cured resins. Discussion and Results
The organic dianhydrides, generally high-melting solids which are insoluble in epoxy resins a t room temperature, require special handling procedures to incorporate them into the epoxy resins so that clear homogeneous castings can be prepared. Therefore. the evaluation of BTDA as an epoxy hardener included studying solubility, use of BTDA alone and BTDA mixtures with maleic anhydride (MA) and other anhydrides, and the effect of changing the anhydride-epoxide (A: E ) and BTDA! MA ratios, as well as the curing times and temperatures. I n the study of these variables, emphasis was on determining their effect on the heat distortion temperature ( H D T ) of the cured epoxy resins. The H D T is generally used to measure the efficiency of epoxy resin hardening agents. This test (ASTM D 648-56) measures the resistance of a plastic material to deformation under stress a t elevated temperatures, and is reported as the temperature a t which the material, under a fiber stress of 264 p.s.i., is deflected 10 mils. The physical and mechanical properties and the resistance to chemicals and solvents were also determined on the castings. First, a method was needed for mixing BTDA with liquid epoxy resins to obtain homogeneous castings. BTDA, a white solid, melts a t about 226’ C. and is insoluble in liquid epoxy resins a t room temperature. I t s solubility can be increased by raising the temperature, but when BTDA was used alone a t A i E ratios above 0.65, premature gelation occurred without complete solubility. The A:E ratio is defined as the ratio of anhydride equivalents to epoxy equivalent and the BTDA/ MA ratio is defined as the ratio of BTDA anhydride equivalents to MA anhydride equivalents. BTDA can be used as the only anhydride hardening agent a t A / E ratios between
Table I. Formulation of BTDA and BTDA/MA Epoxy Mixtures
Formulation I Anhydride equivalents 'epoxide equivalents BTDA equivalents' MA equivalents '-c of total anhydride as BTDA Weight of anhydride based on 100 grams of Elpon 828 BTDA MA (maleic anhydride)
300
g
-
2
40
-13
27.4 23.0
30.2 18.4
39.3 8.0
2:3
I
i
c
I
280
I W K
2
260
K
n
I
?
240
z
2 I-
Ei z
G
220
200
180 200 220 CURE T E M P E R A T U R E , ' C .
160
240
Figure 1. Effect of cure t e m p e r a t u r e on HDT
0.4 and 0.65 a t mixing temperatures of 170" to 175°C. These formulations have pot lives of 5 minutes or less and must be used very quickly. BTDA could be handled more easily by using a monoanhydride as a flux. Maleic anhydride was more efficient than either phthalic or succinic anhydride. Formulations containing up to 90Lc of the anhydride equivalents furnished by BTDA were prepared. The following mixing procedure was used to prepare homogeneous castings. The epoxy resin was heated to approximately 150°C.; 170°C. when I3TDA was used alone. The BTDA or B T D A / M A mixtures were added with stirring to the hot epoxy resin. The time for complete solution, depending on the particle size, is from 5 to 10 minutes. The pot life for formulations with low B T D A / M A ratios is from Table II. Effect of Accelerator on Heat Distortion Temperature (HDT) BTDA MA = 2 , .
Anhydride epoxide = 0 85 1
Accelerator PHHO
Cure Temp , C
Cure Time, H r
HDT, C IASTM U 648)
0.1
150
16 20
Kone
~
LO0
24 16
138 148 153 147
160 180 200
24 24 24
220 234 266
B D M A = benzyldimet hylamine.
4
9
0.73, 1 1;l 50
0.85.1
0.651 1 3/1
0.60. I
...
100 48.3
...
5 to 10 minutes; for the high BTDA/MA ratios and BTDA alone it is usually less than 5 minutes a t the mixing temperatures. The heated mixture was poured into heated molds and cured for the specified time a t elevated temperature. Since the BTDAi MA mixtures gave H D T values from 240" to 290"C., this system was studied in greater detail. The formulations in Table I were selected as representative of many others in which the A;E ratios can be varied using either BTDA alone or BTDA/MA mixtures. Particle size of BTDA and acid content were important variables-for example, samples of BTDA put through a 325-mesh screen reacted more quickly than those put through a 30-mesh screen. Usually both samples mix easily with the epoxy resin if the acid content is below 5'-~. In samples containing larger amounts of acid, the solution time increased and premature gelation occurred. Thus, for the greatest ease of formulation. BTDA of small particle size and low acid content should be used when possible. The formulations in Table I and others were used t o evaluate the BTDA and BTDA:MA epoxy systems a t various cure temperatures and times. Figure 1 shows that as the BTDA content and cure temperature increase, the H D T increases, passing through a maximum a t 220°C. The best cure time was 24 hours. Two series are shown in Figure 1, but other mixtures including those using BTDA alone behaved similarly. Most of the formulations were cured a t 200" C., a few a t 220" C. Significantly, HDT's are extremely high, the highest in the 290" C. range. Table I1 shows the effect of an accelerator a t different curing temperatures and curing times on the H D T of BTDAi MA-hardened epoxy resins. Tertiary amine accelerators gave very dark cured resins low in H D T . The accelerator used was benzyldimethylamine (BDMA). Similar results-Le., low H D T and premature gelationwere obtained when tertiary amine accelerators were added to epoxy mixtures cured with BTDA alone. Curing BTDA and BTDA.; MA-epoxy resin mixtures without accelerators gave the best results. The complete range of A I E ratios using various accelerators was not studied in detail and perhaps the use of higher ratios would have produced castings having optimum properties. I t was important to avoid fast gelation, so that homogeneous castings could be prepared. Because two systems-one involving BTDA alone and the other BTDAi MA mixtures with epoxy resins-were available for preparing homogeneous castings of high H D T , a more comprehensive study of A / E ratio, BTDA/ MA ratios, and BTDA when used alone was undertaken. Four significant trends are shown in Figure 2. BTDA can be used alone only when the A / E ratio is less than 0.65. There is a maximum H D T a t some A / E ratio for VOL. 8 NO. 1 M A R C H 1 9 6 9
73
each BTDAiMA mixture. The B T D A / M A ratio can be increased, but the AIE ratio must be decreased a t the same time. Homogeneous, hardened epoxy resins having HDT's from 238' to 293°C. can be prepared with from 33.3 to 1 0 0 5 BTDA present, These data show a few of the possible combinations of BTDA/MA or BTDA alone which can be used in various formulations. An empirical formula developed from these data for BTDA/ MA mixtures is:
230
1
050
I
I
1
I
055 0 6 0 0 6 5 0.70 075 ANHYDRIDE EOUIVALENTS/EPOXIDE
1
I
I
1
080 0 8 5 0 9 0 095 EOUIVALENTS ( A / E l
Figure 2. Effect of anhydride-epoxide rotio on HDT
~~
Table Ill. Correlation of HDT, A/E, and BTDA Content Formula. A E
['. BTDA anhydride equivalents] = K Bl'DA Anhldride Eq
l C
Maximum HUT, = C 278 280 288 286
270 252 238
A E 0.50 0.55 0.85 0.65 0.75 0.85 0.95 0.95
K
90.0 90.0 90.0
45.0 49.5 76.5 49.0 45.0 42.5 47.5 31.3
-is.O
60.0 50.0 50.0 33.3
' Mixture gels before anhydrides dissolve.
BTDA
-1
I
MA 2084~
070 075 0.65 ANHYDRIDE / EPOXIDE RATIO
4
080
085
Figure 3. Weight of anhydrides necessary for maximum HDT 74
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
A / E ['; BTDA anhydride equivalents] = K where A ' E = anhydride-epoxide ratio i C c BTDA anhydride equivalents] = '> of total anhydride equivalents furnished by BTDA K = constant between 40 and 30 Table I11 shows a compilation using the data shown in Figure 2. The 'i BTDA anhydride equivalents a t the maximum HDT's for the various A / E ratios show good correlation when substituted in the above equation. With this equation BTDA: MA mixtures can be formulated using A ' E ratios from 0.5 t o 0.95 and BTDA,/MA ratios from 1 / 2 to 911-for example, a mixture of BTDA,'MA containing 90"c BTDA anhydride equivalents at an Ai E ratio of 0.55 and one containing 505 BTDA anhydride equivalents a t an A!E ratio of 0.95 would have K values of 49.5 and 47.5 and HDT's of 280" and 252"C., respectively. An anhydride mixture containing 90' BTDA anhydride equivalents a t an A / E ratio of 0.85 would have a K value of 76.5. This value is out of the 40 to 50 range needed for good formulation and maximum HDT. From experience, this formulation gels and hardens before the anhydrides become completely soluble. Likewise, an anhydride mixture containing 33.3'; BTDA anhydride equivalents a t an A / E ratio of 0.95 would have a K value of 31.3. This would be on the low side of the 40 to 50 range needed for maximum H D T . This formulation has been tried and an H D T of 238" C. found. Although rather high, the better formulation for maximum H D T a t this A;E ratio would contain 50'; BTDA anhydride equivalents. Figure 3 shows a comparison, on a weight basis, of the requirements necessary to give hardened resins of very high HDT. Five formulations a t various weights of BTDA and MA are shown-for example, to prepare a hardened epoxy resin having an H D T of 280°C. we can use either 39.9 grams of BTDA + 2.79 grams of MA with 100 grams of Epon 828 (BTDAIMA = 911, A i E = 0.55) or 30.2 grams of BTDA + 18.4 grams of MA with 100 grams of Epon 828 (BTDA/MA = 111, A / E = 0.75) mixtures as the hardening agents. On a cost basis the latter formulation would be preferred because of the lower cost of maleic anhydride. The physical properties of BTDA- and BTDAIMAhardened epoxy resins in Table I V are representative of the many formulations prepared. The HDT's range from 240" to 290" C. Flexural strengths are between 8 and 10,000 p.s.i. Izod impact strengths are from 0.2 to 0.5 footpound per inch notch. Rockwell hardness on the M scale is from 100 to 117; Barcol hardness, from 45 t o 49. The cured epoxy resins obtained using BTDA have excellent high temperature properties because this tetrafunctional hardener gives products having a high density of crosslinks. Crosslinking contributes most significantly to the retention of chain rigidity and integrity, leading to useful mechanical
Table IV. Physical Properties of Hardened Epoxy Resins Epoxy resin. Epon. 828 Cure conditions. 24 hours. 200' C
BTUA
Irod Impact, Ft. Lb. in. h'otch ( A S T M U 256)
A E
Heat Distortion Temp., C . (ASTM D 6 4 8 )
Flexural Strength, P . S . I . IASTM U 790)
0.50 0.60
263 283
8500 8800
0.338 0.520
0.95 0.85 0.75 0.75 0.65 0.55
238 258 280 286 288 280
8700 9500 9800 9900 7900 8200
0.330 0.389 0.413 0.391 0.508 0.214
BTDA MA
12 2 3 1 1 .3 2 3 1 91
108 102 113 108 106 117
48 46 46 45 48 48
Table V. Effect of Heat Aging at 200OC. on Thermal Stability and Flexural Strength Resin cure conditions. 24 hours a t 200"C. ( (
Flexural Strength, AST.M D 790, P . S . I . , after L)a)'s
Weight Loss after Da.~s
BTUA ,ZIA
A E
7
14
43
0
-
1 2 23 1 1 32 3.1 91
0.95 0.85 0.75 0.75 0.65 0.55
1.06 1.88 0.83 0.69 0.58 0.43
1.63 1.62 1.30 1.17 0.99 0.76
2.78 2.78 2.26 2.02 2.80 1.60
12,500 9,500 9,800 9,900 7,900 8,200
10,100 9.500 8,400 4,000 8,800 8,000
8,650 13,200 9,600 8,800 11,700 10,800
10.900 11.300 9,800 7,200 5,200 8,900
BTDA
0.50 0.60 0.65
0.53 0.57 0.58
1.10 0.97 1.60
2.20 1.10 1.60
8,500 8,800 7,000
15,700 9,100 15,000
9.400 10,000 11,100
10,000 7,000 8,700
42
II
i
Table VI. Chemical and Solvent Resistance of Hardened Epoxy Resins
ASTM Method D 543 Cure. 24 hours at 200"C. i c
P Y
1 %
BT'U.4 A21.A
I 1
Weitht ChaMe
3 2
.'I I
9 1
0.75
0.6.5
O.,?.?
.A E
B Tl1'4
0. if5
0.0'5
0.95 --
L)a>s
10'r h a c 1 3'c H SO1 10'r HaOH Acetone H.0 (dist.) 5 ( sodium ~ hypochlorite Merusol oil Jet fuel A Hexane ( i
7
28
7
0.63 1.3 0.48 0.47 0.76
1.3 1.7 1.4 1.5 1.5
0.59 0.67 0.56 0.09 0.76
0.69 0.08 0.16 0.12
1.3 0.40' 0.39' 0.42?
28
i
0.54 0.58 1.0 0.33 0.46 -1.2 1.3 0.61 1.1 1.2
0.69 0.04 0.02 0.11
28
7
30'
-I
1.0 0.85 1.5 0.6 0.64 1.2 0.90 1.9 0.47 0.83 0.13 1.2 0.20 -0.39 0.004 -0.09 0.68 1.2 0.80 1.9 1.3" 0.33 0.55 0.42
-0.02
0.01 0.13 0.04
38
7
28
1.2
0.48 0.58 0.35 -0.01 0.61
1.0 1.7 0.8 0.2 1.2
1.3
1.0 0.2
1.4
i3T~L~'A
0..50
i 0.47 0.57 0.39 0.03 0.61
28
1.0 1.1 0.9 0.4 1.2
0.60
7
28
0.59 0.67 0.56 0.09 0.76
1.3 1.4 1.1 0.35 1.5
-0.21'' 0.14' 0.31'' 0.06"
ueight change after 43 d a j s
properties a t high temperatures. This crosslinking generally produces a harder, more brittle product with lower flexural strengths than those obtained using amines or monoanhydride hardeners. However, the combination of physical properties, especially the high HDT's. should make the BTDA- and BTDA/MA-hardened epoxy resins of commercial value in applications requiring unusually high temperature performance. How thermal degradation affects BTDA- and BTDA / MA-hardened epoxy resins was determined by measuring both weight loss and flexural strength after various periods
of heat aging. Table V shows the results obtained with nine different formulations after heat aging a t 200°C. The per cent weight loss increases with time, attaining 1.1 t o 2 . 8 ' ~in 42 days (1008 hours). As the BTDA content increases, the thermal stability appears to improve. However, for both BTDA- and BTDA / MA-hardened resins the thermal stability was good. Per cent weight loss and flexural strength are not directly correlated. I n some cases the flexural strength increases; in others it decreases. Most significantly, there was no severe breakdown or loss of strength even after 1000 hours a t 200°C. VOL. 8 NO. 1 M A R C H 1969
75
Table VII. BTDA-EPON 828 High Temperature Adhesive Anhydride1 epoxide = 0.6 100 P H R atomized aluminum 3 P H R Cab-0-Si1 Alclad aluminum substrate Cure. 2 hours a t 200' C.
"F
Tensile Lap Shear Strength, P.S.I. iASTM D 1002)
73 300 500
2480 1600 1220
Table VIII. Effect of Heat Aging at 482' F. on Tensile lap Shear Strength of BTDA-EPON 828 High Temperature Adhesive
Tensile Lap Shear Strength,
P.S.I. ut 500" F. ( A S T M D 1002) Initial After 500 hours' aging (904; retention) After 1000 hours' ~ aging ( 8 5 cretention)
1220 1090
One of the principal uses of epoxy resins is as adhesives. Homogeneous castings of excellent high-temperature stability could be prepared using BTDA or BTDAIMA mixtures as the hardening agents. Therefore, a hightemperature epoxy adhesive was formulated using BTDA as the hardening agent. Many different fillers were examined, but the best system was one involving atomized aluminum and Cab-0-Sil. BTDA and liquid epoxy resin (Epon 828) were mixed on a three-roll mill, and the aluminum and Cab-0-Si1 were added. At room temperature, the mixed formulation was applied as a paste to the Alclad aluminum substrate. The adhesive mixture was cured a t 200°C. for 2 hours, and then samples were tested a t 73", 300",and 500°F. (Table VII). Significantly, the tensile lap shear (TLS) strength was over 1200 p.s.i. when tested a t 500°F. To determine the effect of heat aging, adhesive samples were exposed to 250" C. (482" F . ) for various periods of time up to 1000 hours and then tested a t 500" F. (Table VIII). The excellent retention of TLS strength after both 500 and 1000 hours' heat aging a t 250°C. (482°F.) is attributed to the increased crosslinking density imparted by the tetrafunctional BTDA molecule.
1040 Acknowledgment
Generally, hardened epoxy resins of BTDA and BTDA/ MA maintain good flexural properties with small weight losses a t high temperatures. T o determine the chemical and solvent resistance of BTDA- and BTDA/ MA-hardened epoxy resins, the procedure described in ASTM Method D-543 was used (Table VI). Castings prepared from eight formulations were examined in acid, alkali, salt, and bleach solutions, and in five solvents. The weight change after 7 days was less than 1% and all samples had less than 2% change in weight, with most less than 1 . 5 5 , after 28 and 42 days. All formulations showed excellent chemical and solvent resistance with the wide variety of reagents described. Both the BTDA and BTDA/ MA castings showed excellent resistance to acetone, one of the more severe organic solvents for epoxy resins. This chemical resistance-as is its thermal stability-is due to the extensive crosslinking by the tetrafunctional BTDA with the epoxy resin during cure.
The authors gratefully acknowledge the assistance of James R . Feudale, who performed much of the experimental work. Literature Cited
Dearborn, D. C., Fuoss, R . M., MacKenzie, A. K., Shepherd, R . C., Jr., Ind. Eng. Chem. 45, 2715 (1953). Field, R . B., Robinson, C.F., Ind. Eng. Chem. 49, 309 (1957). Skeist, I., "Epoxy Resins," p. 51, Reinhold, New York, 1958. Van Volkenburg, R., Johnson, W. C., 21st Annual Meeting of SPI Reinforced Plastics Division, Section 1-A, Feb. 8-10. 1966.
RECEIVED for review May 9, 1968 ACCEPTED October 14, 1968 Division of Organic Coatings and Plastics Chemistry, l53rd Meeting, ACS, Miami Beach, Fla., April 1967.
Correction PREPARATION OF SYNTHETIC MALACHITE
In this article by H. S. Parekh and A. C. T. Hsu [IND. ENG.CHEM.PROD.RES. DEVELOP.7 , 2 2 2 (196811, the caption for Figure 1 should be: Figure 1. X-ray powder diffraction picture of samples Upper. Na2CO4uSOd reactant molecular ratio = 1.10 Lower. N ~ L O Y C U Sreactant O~ molecular ratio = 0.75
76
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
