I
HANS
K. WEISS'
National Aniline Division, Allied Chemical & Dye Corp., Buffalo, N. Y.
Anhydride Curing Agents for Epoxy Resins Low-melting anhydrides have clarity, light color, long pot life, ease of handling, and impart heat distortion temperatures up to 143O C.
Mom
commercially available epoxy resins are condensation products of bisphenol A (4,4'-isopropylidenediphenol) and epichlorohydrin, usually cured or cross-linked with amines ( 2 ) or anhydrides (7). This discussion is limited to anhydride curing agents. The basic structure of a bisphenol Aepichlorohydrin resin ( 9 ) is:
(4). An exception is the case of highly chlorinated anhydrides, such as chlorendic anhydride, where the optimum compositions correspond to 0.60 to 0.65 mole of anhydride per epoxy equivalent (8). T h e reaction of epoxy resins with anhydrides is relatively slow, even a t elevated temperatures. Addition of tertiary amines increases the rare of cure ( 7 ) and
CHI
CH 2-CH-CH '0'
O-C>-+-C)-G-CN~--CH-~HCH J
AH
crp,
ing the following structures, show escep-
In--
0 --c)-+-(z)-O-CH-CH-CH~ CH3 Anhydrides react with this epoxy resin in three steps (6, 7) : monoesterification, diesterification, and etherification.
Initially, the predominating reaction is the formation of a monoester (Reaction I). The carboxy group thus formed reacts with the epoxy group to form diesters (Reaction 11). Simultaneously, Reaction 111, catalyzed by unreacted anhydride and carboxyl groups, takes place, resulting in a cured resin consisting principally of diester and ether linkages but also of anhydride, monoester, and hydroxy groups. Generally, when an anhydride reacts with an epoxy resin, 0.85 mole of anhydride per epoxy equivalent is necessary to obtain maximum thermal yield points Present address, National Aniline Division, Allied Chemical & Dye Corp., 40 Rector St., New York 6 , N. Y .
'0'
does not change the thermal yield points appreciably. When a n amine is used as a promoter, the optimum composition is
changed, so that 1.0 mole of anhydride per epoxy equivalent is required. During the past few years the anhydrides have found increasing use as hardening agents. The more common anhydrides are phthalic anhydride (7, 4, 6, 7), pyromellitic dianhydride ( 5 ) , and chlorendic anhydride ( 8 ) . They melt a t 131°, 286', and 238' C., respectively, to give heat distortion temperatures ranging from 145' to 260' C. However, because of their high melting points they have poor compatibility with epoxy resins a t room temperature, and the epoxyanhydride blends have a relatively short working or pot life. Considerable color is developed when castings are cured. Recently developed anhydrides, hav-
All three are monoanhydrides-dodecenylsuccinic anhydride (DDSAA) is aliphatic, and methylbicyclo[2.2.1 lhept-5ene-2,3-dicarboxylic anhydride and cis1,2-cyclohexanedicarboxylic anhydride (methyl Nadic anhydride and hexahydrophthalic anhydride or HHPAA, respectively) are alicyclic. T h e first two are liquid well below room temperature ; the third melts slightly above room temperature. Because they are liquid or melt slighthy above room temperature, these anhydrides can be mixed with liquid resins by simply stirring them together. If the resin is a solid, it need be heated only above its melting point and stirred into the anhydride. T h e anhydrides are compatible with liquid resins at room temperature. Preparation of Casting Compositions
Casting compositions were prepared by stirring the liquid anhydrides into the epoxy resin a t 25' C. When hrxahydrophthalic anhydride was used. the liquid resin was stirred into the molten hexahydrophthalic anhydride (at 40' to 50' C.) and then cooled to 25' C. BenVOL. 49, NO. 7
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JULY. 1957
1089
zyldimethylamine (BDMA) was then added to the resin-anhydride si-stem. Mixtures containing phthalic anhydride were prepared by adding the anhydride to the liquid resin, raising the temperature to 120" to 130' C., and stirring until a clear solution was obtained. Epcxy resins using rn-phenylenediamine as curing agent were prepared by stirring together equal quantities of resin and m-phenvlenediamine, previously heated to 80' C., until homogeneous and then blending with the required amount of resin. The epoxy resins used were Epon 828 ( 9 ) and Araldite 6020 ( 3 ) .
Table 1.
Thermal Properties of Epon 828 Used with Anhydrides
Curing Agent M.P., C. % BDMA < 12 1.0 Dodecenylsuccinic anhydride < 12 0.5 Methyl Nadic anhydride Hexahydrophthalic anhydride 35-36 0.5 Phthalic anhydride 130.8 0.2 Chlorendic anhydride (8) 238 None m-Phenylenediamine 62.8 Parts of curing agent per 100 parts of resin. 2 hours at 85' C . ; postcured 24 hours at 150° C. 2 hours at 115' C.; postcured 8 hours at l5O0 C. 24 hours at 180° C. e 2 hours at 86' C . ; postcured 4 hours at 150' C.
...
PHR"
Heat Distortion Temp., C .
130 85
78* 143b 133b 14SC 202d 153e
80 75 120 16.7
Thermal Properties of Araldite 6020 Used with Anhydrides Heat ~.
Test Methods
Curing Agent
Heat distortion test bars were cast in 0.5 x 0.5 X 7 inch seamless, square brass tubing. sized with Dow-Corning 20 (diluted with toluene), one end stoppered with a silicone rubber stopper. Sheets inch thick) were cast by pouring the mixtures between two 10-inch square glass plates sized with Dow-Corning 20 and separated by a ',-inch Teflon rod spacer. The specimens were cured in a forced-circulating, air-tvpe oven. ASTM test methods were used : tensile strength and tensile modulus (D 63852T) ; impact resistance (D 256-54T) ; heat distortion (D 648-45T) ; flexural strength and flexural modulus (D 79049T) ; compressive strength and compressive modulus (D 695-54). Results and Discwssion
Table I shows the thermal properties of Epon 828 reacted with various anhydrides, arranged in order of increasing melting points. For comparison with a typical amine curing agent, rn-phenylenediamine is included. With the exception of dodecenylsuccinic anhydride, these monoanhydrides, which have a functionality of 2, give heat distortion temperatures comparable to rn-phenylenediamine, which has a functionality of 4. Although phthalic anhydride is aromatic and melts 96" C. higher than hexahydrophthalic anhydride, an alicyclic anhydride, a heat distortion temperature of only 12" C. higher is obtained with phthalic anhydride. The heat distortion temperatures obtained with Araldite 6020 are, in general, lower than those obtained with Epon 828. Replacement of 25 mole yo of hexahydrophthalic anhydride with chlorendic anhydride gives an anhydride mixture w-hich is liquid at room temperature. Using this mixture, the heat distortion is raised 18" c. over that of hexahydrophthalic anhydride alone. Preliminary work indicates that hexahydrophthalic anhydride can be used to make sheet laminates having flexural strengths of 45,000 pounds per square
1090
INDUSTRIAL
M.P.,
C.
% BDMA
PHR
Distortion Temp., C.
0.5 0.5
71 82
116.7a 122b
0.5
53 42 49 100
134. 5a
Hexahydrophthalic anhydride 35-36 Methyl Nadic anhydride < 12 Hexahydrophthalic anhydride plus Liquid at 30 Chlorendic anhydride Phthalic anhydride 130.8 238 Chlorendic anhydride (8) 16 hours at 130' C.; postcured 1 hour at 180" C. 16 hours at 120' C.; postcured 1 hour a t 180' C. 7 hours at 160" C. 24 hours at 140' C. ~
None None
81' I 6Sd
Physical Properties of Araldife 6020 Cured with Anhydrides "Pail + Methyl Chlorendic Chlorendic HHPAA Nadic A4nhgdride Anhydride 116.7a Heat distortion temp., O C. Izod impact, ft. Ib./inch notch 0.25 Flexural strength, Ib./sq. inch 15,000 3.16 Flexural modulus ( X l o 6 ) , lb./sq. inch 16,500 Compressive strength, lb./sq. inch Compressive modulus ( X l o 6 ) , lb./sq. inch 4.2 11,900 Tensile strength, lb./sq. inch 3.6 Tensile modulus ( X lo6), lb./sq. inch Cured 16 hours a t 130' C.: postcured 1 hour a t 180' Cured 16 hours a t 120' C.; postcured 1 hour a t 180' Cured 24 houis at 140' C.
122b 0.295 11,200 3.60 18,800 4.8 12,500 3.9
134.P 0.31 17,200 3.52 16,600 2.8 9,100 3.4
16gC
...
17,600 5.2 20,500
...
12,000 4.6
C. C.
inch and flexural moduli of 19.9 X l o 5 pounds per square inch. Sixty-five per cent glass laminates of chrome-finished. cut strand mat give flexural properties equal to or better than Epon 828 hardened with rn-phenylenediamine. Improved clarity and color are also evident in formulations containing hexahydrophthalic anhydride.
distortion temperatures up to 143" C. are required.
Conclusions
(4) Dearborn, E. C., Fuoss, R. M., MacKenzie, A. K., Shepherd, R. G., Jr., IND. EXG. CHEM. 45, 2714 (1953). (5) du Pont de Nemours & Co., Inc., E. I., Wilmington, Del., Tech. Bull. PMDA (1955); Supplement 2 (1956). ( 6 ) Fisch, W., Hofmann, W.,J . Polymer Sci. 12, 497 (1954). (7) Fisch, W., Hofmann, W., Koskikallio, J., J . A p j l . Chem. 6, 429 (1956). (8) Robitschek, P., Nelson, S . J., IND. ENC.CHEM.48, 1951 (1956). ( 9 ) Shell Chemical Corp., "Epon" Tech. Bull. SC:50-40,2 (1950).
Literature Cited
(1) Castan, P., U. S. Patent 2,324,483 (July 20, 1943). (2) Castan, P. (to de Trey Freres S. A ) , Ibid., 2,444,333 (June 29, 1948). ( 3 ) Ciba Go., Plastics Division, New York, Provisional Tech. Data Bull. 6, (1955).
Methyl Nadic, dodecenylsuccinic, and hexahydrophthalic anhydrides are effective curing agents for epoxy resins. Depending on the resin, cure schedules of 2 hours a t 85" C. plus 24 hours at 150' C. or 16 hours at 120' to 130" C. followed by an additional hour a t 180" C. produce resins with heat distortion temperatures of 78' to 143" C. when 1.0 mole of anhydride per epoxy equivalent and a tertiary amine, such as benzyldimethylamine, is used as a promoter. These low-melting anhydrides should find use where clarity, light color, long pot life, ease of handling, and/or heat
A N D ENGINEERING CHEMISTRY
RECEIVED for review January 8, 1957 ACCEPTEDApril 28, 1957 Ninth National Chemical Exposition, Cleveland, Ohio, November 27-30, 1956.
