P.
ROBITSCHEK and S. J. NELSON
Hooker Electrochemical Co., Niagara Falls, N.
Y.
Flame- and Heat-Resistant Epoxy Resins
M O S T epoxy resins now commercially available are condensation polymers of 4,4'-isopropylidenediphenol and epichlorhydrin. The linear chain compounds contain terminal epoxy groups as well as a number of secondary hydroxyl groups. According to well established commercial practice, the linear polymers are generally cross-linked by reaction with amines (7) or carboxylic acid anhydrides (2). The nature of the cross-linking agent influences to a considerable extent the properties of the cured polymers. There are substantial differences in the properties of epoxy resins cross-linked by various amines or by various carboxylic acid anhydrides. A study on the effect of carboxylic acid anhydrides of different structures indicated that thermal yield point is increased by increasing the compactness of the molecules of the anhydrides and by increasing dipole-dipole forces. Taking phthalic anhydride as a reference point, the thermal yield point was increased by about 10" to 20' C. by employing 1,5-dimethyl-2,3,4,6,7,8-hexahydronaphthalene - 3,4,7,8 tetracarboxylic acid dianhydride, or dichlorophthalic anhydride (4). Data are presented herein which show that dibasic acid anhydrides obtained by means of the diene reaction of hexachlorocyclopentadiene and some of its derivatives with organic unsaturated dicarboxylic acids harden epoxy resins, and that the polymers so produced have thermal yield points considerably higher than have been heretofore obtained. Also, the resinous compositions show a high degree of flame resistance. The effect of changing the ratio of epoxy resin to the dicarboxylic anhydride, or the effect of curing temperature and curing time, have been examined in detail on epoxy resins hardened by the reaction product of hexachlorocyclopentadiene with maleic anhydride (1,4,5,6,7,7-hexachlorobicyclo- [2.2.1]-5- heptene-2,3 dicarboxylic anhydride), hereafter referred to as HET
CUQE
4 H R 5 . AT t8O'C. I
,-
CURE
20 H R S . AT 160-C.
U
&
180
I
z
0
m E -W N828. M-PHENYLENEDIAMINE (14.5 PARTS PER 100 E P O X Y RESIN) =I-EPON 8 2 8 - HET ANHYDRIDE (120 PARTS PER too EPOXY RESIN)
Figure 1. Heat distortion temperatures of Epon 828 cured with m-phenylenediamine and HET anhydride
roo
-
-
-
2GC
I90
v I80
n
3 170
w 1 160
Y
6
150
t;
et?
140
k
130
w
I I20
I10 100
45
50 55 % HET ANHYDRIDE BY WEIGUT
65
Figure 2. Heat distortion temperature vs. composition for Epon 828-HET anhydride castings VOL. 48, NO. 10
OCTOBER 1956
1951
anhydride (Hooker Electrical Co.).
Hexachlorocyclopentadiene
Maleic Anhydride
c1
1,4,5,6,7,7-Hexach1orobicyclo[2.2.1] 5-Heptene-2,3-DicarboxylicAnhydridr %
HET ANHYDRIDE BY WEIGHT
Figure 3. Heat distortion temperature vs. composition for Araldite 6020-HET anhydride castings Table I.
Thermal Data for Araldite 6020 Cured with Anhydrides Derived from Halocyclopentadienes ( C u r e t i m e , 17 h o u r s at 160' C.)
Dienophilic Diene Hexachlorocyclopentadiene Hexachlorocyclopentadiene Hexachlorocyclopentadiene Difluorotetrachlorocyclopentadienea Dimethoxytetrachlorocyclopentadiene" Dihydrotetrachlorocyclopentadienea a b
Anhydride
Maleic Citraconic Itaconic
Maleic Maleic
Maleic
Substituted in t h e 5 5- position. Only melting points'of corresponding acids reported.
Heat MeZtinr/ Point Distortion ' C. Lit. Czted Temp., C. (7) 178 238 (11) 158 243.5 131.0-4.6 (11)* 149 (9) 178 165-7 184-5.5 (10); 149 (8) 167 172.2-3 Those of anhydrides a r e new data.
Since the high temperature properties and flame resistance of epoxy resins are becoming increasingly important in many uses such as electrical components, tooling, and glass reinforced structural materials, some of thc compounds described here are of practical interest. Carboxylic dibasic acid anhydrides readily derived from halocyclopentadienes by the diene reaction, which have been used to cross-link epoxy resins, are listed in Table I.
Preparation of Casting Compositions Casting compositions were prepared by stirring the appropriate finely divided anhydride into epoxy resins a t temperatures \vhere rapid solution occurred I n most instances, the epoxy resin was heated to about 120O C , the anhydride added, and the mixture stirred a t 100' to 110' C. until a solution was formed The liquid compositions were poured into a glass container, sized with DriFilm (General Electric Co.) as a release agent, and then heated in a thermostatically controlled oil bath or a mechanical air convection oven, both controlled to within 11' C., in order to carry out curing or post curing. The epoxy resins used in this investigation are :
200 190
I80
I70
160
I50 140
I30
Same
120
Epon 828 ( 1 2 )
Araldite 6020 (3) E p o n 834 (12)
I10
228 258
a By analysis using the pyridine hydrochloride method ( 4 ) .
I00
45
Figure 4. castings
1952
Epoxy Equivalentn 200
50
55 60 % UET A N H Y D R I D E BY W E I G H T
65
Heat distortion temperature vs. composition for Epon 834-HET anhydride
INDUSTRIAL AND ENGINEERINGCHEMISTRY
Preparation of laminates Laminates were prepared as follows :
120 parts of HET anhydride and 100
parts of Araldite 6020 were dissolved in 132 parts of methyl ethyl ketone and 15 parts of toluene, all parts by weight. Individual plies of commercial 181 glass cloth containing Volan finish (E. I. du Pont de Nemours 8L Co.) were dipped in the solution and dried in a forced draft oven until tack free. A satisfactory drying time was generally, 7 to 12 minutes a t 140' C. The laminates were molded from 12 plies of the impregnated fabric at 100 lb./sq. inch and 180' C. Post cure was carried out in a forced draft oven without pressure.
2ooE 190
!
!
CURED 24 HPS. AT 180.C. CURED Z 4 U R S . AT 120.C.
Testing Procedures The relative rates of reaction on curing were followed by recording the temperature rise in the castings as follows: 100 grams of epoxy resin containing the appropriate hardener was introduced into a n unwaxed three-ounce paper container with an average diameter of l 5 / ~ inches. The container was placed in a circulating air oven and a thermocouple junction positioned at the center of the casting. Temperatures were recorded a t 48-second intervals on a recording potentiometer. ASTM methods were used. For heat distortion, D 648-45T at a fiber stress of 264 lb./sq. inch; for tensile strength and tensile modulus, D 63852T; for flexural strength and flexural modulus D 790-49T (for measurements at high temperatures the procedure was the same, except that the specimen was held a t the test temperature for one half hour before testing) ; for compression strength, D 695-49T; and for flame resistance, ASTM D 75749.
Comparison of Heat Distortion Temperatures Obtained with Various Hardeners The heat distortion temperatures of cast epoxy resins hardened by the various derivatives of halocyclopentadienes were determined after a curing cycle of 17 hours a t 160' C. The comparative distortion temperatures using Araldite 6020 in a weight ratio of 1 to 1 is shown in the last column of Table I Also, a comparison of such temperatures for Epon 828, hardened by various curing agents (the proportion of phthalic anhydride is equivalent on a molar basis to the lower proportion of HET anhydride) is shown as follows : EPOXY,
Curing A g e n t
Pt./100
HET anhydride 120 HET anhydride 100 Phthalic anhydride 40 m-Phenylene diamine 14.5 1 hr. at 80' C . ; Postcured 24 hr. at 180° C.
-.+!+-;.
J
hl
Q !
210 220 230 240 GRAMS R E S I N PER MOLE O F E P O X Y GROUPS
250
c
J
Figure 5. Effect of epoxide equivalent on heat distortion temperature (HET anhydride, 54y0)
The effect of post cure on epoxy resins hardened by H E T anhydride and m-phenylene diamine, is shown in Figure
1. Epoxy Resins Hardened with HET Anhydride
Castings. The properties of cast epoxy resins hardened with H E T anhydride have been examined in detail. Figures 2, 3, and 4 describe the variations in heat distortion temperatures produced in castings cured at 120" C. and 180' C. by varying the proportion of anhydride. Figure 5 illustrates the effect of epoxy equivalent of cast resins on heat distortion temperatures at a 54% concentration of anhydride. Figure 6 illustrates the effect of temperature and post cure; and Figure 7 Cure Cycle, HT. at 180° C. 24 24 24 (i
Heat Distortion Temp., O C. 202
197 81 151
shows the effect of time and temperature of cure on heat distortion of Araldite 6020 castings. Figure 8 shows that the presence of H E T acid, produced by hydrolysis of the anhydride, accelerates the rate of cure. However, a n acid content up to about 570 in the H E T anhydride has no significant effect on heat distortion of the cured cast epoxy resins (Figure 9). H E T anhydride cured epoxy resins are unusually flame resistant. They also exhibit low shrinkage, high mechanical strength, and other properties commonly associated with epoxy resins. The properties of Araldite 6020 castings hardened with this anhydride combined in 1 : l weight proportions and cured for 24 hours at 140' C. are: Heat distortion temperature, O C. Tensile strength, lb./sq. inch Tensile modulus, lb./sq. inch Flexural strength, lb./sq. inch Flexural modulus, lb./sq. inch Compression strength, lb./sq. inch Shrinkage, volume % Flame resistance Inch burned/minute Time t o ignite, sec. Time flame out, sec. VOL. 48, NO. 10
168 12,000 4.6 x 105 17,600 5.2 x 105 20,500 1.3 0.15 65 115
OCTOBER 1956
1953
200 I90 IO0 U IJ
170
3
5
v 5
I60 150
F 2
Q
I40
L 30
6
5
I20
I I O
35
40
45 M ANHYDRIDE
55
% HET
Figure 6. Effect of post cure (24 hours at 180" C.) on samples cured for 24 hours at 120' C. compared to those cured at 180' C. for 24 hours
Glass Fabric Laminates. Table I1 describes flexural strength and flexural modulus of glass cloth laminates bonded by H E T anhydride-cured liquid epoxy resins.
Discussion H E T anhydride cured epoxy resins have, compared to epoxy resins cured with phthalic anhydride and m-phenylenediamine, unusually high heat distortion temperatures. The range of their highest heat distortion temperatures when cured at 180" C. lies in the region of about 50% to 5770 anhydride in the composition, by weight, with the optimum around 54%. Except as dis-
cussed subsequently, their heat distortion temperature increases with increasing temperatures and extent of cure (Figure 7), but decreases slowly as the epoxy equivalent of the epoxy resin increases, when the cure is carried out at 180°C. (Figure 5). Above about 509;b H E T anhydride content. curing a t 120' C. gives substantially lower heat distortion temperatures than curing a t 180" C.; at the lower temperature the proportion of anhydride is much less critical. Post curing at 180' C. of a casting cured at 120" C. raises the heat distortion temperature; however. below about 46% of anhydride, the 120" C. cure gives the higher heat distortion temperatures. This discrepancy becomes even more
v
Figure 7. Cure rate of Araldite 6020 hardened with HET anhydride (HET resin weight ratio, 1 :1)
1 954
INDUSTRIAL AND ENGINEERING CHEMISTRY
pronounced if castings cured at 120' C. are post cured at 180" C. (Figure 6). It can be assumed that the thermal yield points expressed as yield temperatures in the previous study (4) are qualitatively comparable to the heat distortion temperatures shown here; both methods depend on measurement of flexural deflection under load while gradually raising the temperature of the sample. A comparison of thermal yield points obtained when hardening epoxy resins with phthalic anhydride (4) and the data presented here on epoxy resins hardened with H E T anhydride indicate that, within the optimum range of the anhydride content, phthalic anhydride gives highest yield points after prolonged heating a t the lower hardening temperatures, whereas, H E T anhydride produces optimum yield points after prolonged heating a t higher hardening temperatures. However, the latter anhydride seems to behave similarly to phthalic anhydride if its content is reduced to below about 46%. The dissimilarity in behavior between phthalic and H E T anhydride may indicate that the mechanisms of the reaction, or That the relative rates of polyesterification and polyetherification, believed to be the dominant reactions in anhydride hardening of epoxy resins ( 6 ) , are different. One of the possible causes of different behavior may reside in the different acid strength of H E T acid and phthalic acid. The accelerating action of H E T acid present in the anhydride is in accord with the recorded accelerating effect conferred by benzoic or succinic acids (5). U p to about 5%, it does not significantly lower heat distortion temperatures of castings cured at 180' C., and at a 120" C . curing temperature even a larger percentage has no substantial lowering effect. High percentages, however, substantially impair the pot life of the compositions. As can be noted from Figures 2, 3, and 4, considerably less H E T anhydride is needed for optimum heat distortion temperatures than corresponds to an equivalent amount of anhydride groups to epoxy groups. Flexural strength and modulus data on glass cloth laminates (Table 11) show that these properties are retained to the extent of about 80% at 177" C. At 204" C., flexural strength retention was of the order, 40%, and modulus retention of the order 6070-some differences were caused by differences in cure time. A casting composition comprising the laminating resin was found to have a heat distortion point of 186 " C. after two hours and 196" C. after 24 hours cure. I n this instance, the heat distortion point appears a reasonably accurate indication of the temperature at which the corresponding laminate begins to lose strength rapidly.
Conclusion
T I M E I N MINUTES
Figure 8. Effect of acid content on exotherms of 1 00-gram castings of Epon 828 and HET anhydride (weight ratio, 1 .O: 1.2), oven temperature, 120' C.
Dicarboxylic acid anhydrides derived from halocyclopentadienes are effective hardeners for epoxy resins. The rate of hardening is rapid and the cured products are characterized by unusually high thermal yield points as well as fire resistance. The product obtained by the reaction of hexachlorocyclopentadiene and maleic anhydride (HET +anhydride),used as a hardener, gives optimum thermal yield points when present in amounts of approximately 54y0 by weight. The thermal yield points were found to increase, within the range of 50 to 5770 of HET anhydride, with increasing cure temperature. Curing a t 180' C. produced castings which gave a n ASTM D 648-45T heat distortion temperature in the vicinity of 200" C. and glass cloth laminates which retained about 80% of their room temperature flexural strength and flexural stiffness a t 177' C. The very high thermal yield points and excellent flame resistance of these compositions open new fields of application for epoxy resins.
Acknowledgment The authors are indebted to Doctors
C. T. Bean, S. M. Creighton, S. Gelfand, and B. 0. Schoepfle for the preparation of some of the adducts used in this work.
Literature Cited
5
IO
15
20
25
% H E T ACID Figure 9. Effect of HET acid on heat distortion temperature of HET epoxies (epoxy resin, Epon 828. Resin-anhydride ratio, 1 :0: 1.2)
Table II. High Temperature Flexural Strength and Modulus of HET AnhydrideCured Epoxy Resin Glass Cloth Laminates Resin: Araldite 6020 100 pts. HET anhydride 120 pts., cured 2 hr. at 180° C. Glass Fabric: Type 181, Volan finish Resin in Laminate: 29% by weight Post-Cure, 88 hr. at 180' C. Flexural Modulus of Flexural Modulus of Strength, Elasticitg, strength, elasticity, Temperature, O C. Ab./&. In. Lb./Sq. In. X 10-6 lb./sq. in. lb./sq. in. X 10-6 23 87,900 4.17 86,700 4.01 138 75,500 3.28 76,600 3.36 177 73,500 3.33 74,000 3.35 204 29,800 2.22 39,300 2.71 260 9,700 1.20 9,800 1.20
(1) Castan, P. (to de Trey Freres S.A.), U. S. Patent 2,444,333 (June 29, 1948). (2) Castan, P., Zbid., 2,324,483 (July 20, 1943). (3) Ciba Co., Inc., plastics division, New York, Provisional Tech. Data Bull. 6, 1955. (4) Dearborn, E. C., Fuoss, R. M., MacKenzie, A. K., Shepherd, R. G., Jr., IND.ENG. CHEM.45, 2714 (1953). ( 5 ) Dearborn, E. C., Fuoss, R. M., White, A. F., J . PolymerSci. 16,201 (1955). (6) Fisch, W., Hofmann, W. J., Zbid., 12, 497 (1954). (7) Herzfeld, S. H., Lidov, R. E., Bluestone, H. (to Velsicol Corp.), U. S. Patent 2,606,910 (Aug. 12, 1952). (8) McBee, E. T., Meyers, R. K., Baranauckas, C. F., J. Am. Chem. Soc. 7 7 , 86 (1955). (9) McBee, E. T., Smith, D. K., Ungnade, H. E., J . Am. Chem. Soc. 77. 387 (1955). . . Newcomer; J. 'S.,McBee, E. T., J . Am. Chem. SOG.71, 948 (1949). Robitschek, P., Bean, C. T., IND. ENC. CHEM.46, 1629 (1954). Shell Chemical Co., New York, Bull. SC-52-31, 1952. ~
RECEIVED for review December 15, 1955 ACCEPTEDMay 28, 1956 Presented at Division of Paint, Plastics, and Printing Ink Chemistry, 128th Meeting of the American Chemical Society, Minneapolis, Minn., September 1955. VOL. 48, NO. 10
OCTOBER 1956
1955
