T a b l e XXXII. Water Vapor Transmission Hydrolyzed O.!LMil Mylar Film Treatment
Grams/Square Meter 24 Hours
Film a s received
15.6 16.2 18.5
Film exposed t o 95% RH s t 9OoC. 12 d a y s 18 d a y s 25 days
16.4 17.4 16.3 17.9 15.4 16.3
alcohols than with water. When the polymer i s cold-drawn and exposed to hydrolysis, i t retains t e n s i l e properties longer then undrawn polymer under t h e same conditions. I n t h e form of 10-mil thick s h e e t s , Mylar hydrolyzes less rapidly t h a n 0.5-mil thick f i l m s . However, Dacron filaments hydrolyze much more slowly than t h e s h e e t material, even though their diameter is only 0.5 mil. It is evident that t h e outward form of the polymer s a m p l e s influences i t s resistance to hydrolysis, and also that t h e structure of t h e polymer on a molecular scale is equally important in t h i s respect.
Flgure 22. to
A log --= A-X
Degradation of 0.5-,mil Mylar film 0.003 (confidence limit 0.95)
ACKNOWLEDGMENT
The authors w i s h t o thanK N. P. Dery, who made many of the physical measurements, John Demchik, who made many of the viscosity determinations, J. H. H e i s s , who gave helpful a d v i c e on viscosity measurements, and Dorothy Angel1 and John Moshman, who made s t a t i s t i c a l a n a l y s e s of t h e data. They would a l s o l i k e t o thank Irene Dion, who provided t h e photomicrographs i n Figure 17, and Norma Nelson, who made t h e measurements of insulation r e s i s t a n c e given in T a b l e XXVI. LITERATURE CITED (1) Flory, P. J., "Principles of Polymer Chemistry," Cornell University P r e s s , Ithsca, N. Y., 1953. (2) Hudson, R. A., Brit. P l a s t i c s 26, 6 (1953). (3) Keller. A., Lester, G. R., Morgan, L. E?., Phil. Trans. Roy. SOC.(London) A 247. 6 (1954). (4) Kitson, R. E., E. I. du Pont d e Nemours 8 Co., Inc., private communication. (5) Kolb, H. J., Izard, E. F., J. Appl. Phys. M , 564, 571 (1949). (6) Marshall, I., Todd, A, Trans. Faraday SOC.49, 67 (1953). (7) Pohl, H. A., J . Am. Chem. SOC. 73, 5660 (1951). (8) Reddlsh, W., Trans. Faraday SOC.46, 459 (1950). (9) Schildknecht, C. E., 'aPolymer P r o c e s s e s ; Chemical Technology of P l a s t i c s , Resins, Rubbers, Adhesives, and Fibers," Interscience. New York. 1956.
Figure 23. to
A log --= A-X
Degradation of 10-mil Mylar sheet 0.0022 (confldence limlt 0.95)
(10) Taylor, R. L., €lerrmann, D. n., Kernp. A. R., l n d . E n g . Chpm. 28. 1255 (1936). (11) Thompson, A. B., Woods, D. W., Nature 176, 78 (1955). (12) Whinfield, J. R.. Endeavour 11. 29 (1952). (13) Wood, D. L., Dodd, D. M., Bell Telephone Laboratories, Murray Hill, N. J., unpublished data. (14) Wooley, M. C., Kohman, G. T.. McMahon, W., Elec. Eng. 71, 715 (1952). Received for review May 5, 1958. Accepted September 17, 1958.
Some Anhydride Curing Agents for Epoxy Resins F. W. MICHELOTTI, C. J. KNUTH, and A. BAVLEY Chas. Pfizer and Co., Inc., Brooklyn, N. Y. I n h i s original patent, C a s t a n (2) described t h e curing of polyolepichlorohydrin condensation products with dicarboxylic acid anhydrides, and i n recent y e a r s t h e use of anhydrides in t h i s application h a s continued t o grow. Although they lack t h e high reactivity of polyamine curing a g e n t s and generally cannot b e employed i n room-temperature curing, anhydrides find u s e with t h e epoxy r e s i n s of low molecular weight b e c a u s e of their low exotherms, freedom from skin VOL. 4.
NO. 1, JANUARY 19S9
sensitization, and t h e good chemical and electrical propert i e s which they u s u a l l y impart. T h e availability of itaconic and citric a c i d s from fermentation s o u r c e s affords an interesting opportunity t o compare t h e effect of various modifications of a b a s i c succinic anhydride StNCtUre on t h e properties of epoxy-resin curing agents. Itaconic (I), citraconic (II), and methylsuccinic (111) anhydrides may b e derived from itaconic acid, while 79
citric acid affords acetylcitric (IV), tricarballylic (V, R = H), and the aconitic (VI a and b) anhydrides.
C-
I
COOH
CH,COOH Itaconic Acid
an unequivocal synthesis, tricarballylic anhydride w a s treated with anhydrous diazomethane i n ether t o give t h e same product. P,yanhydroaconitic acid (VIa) and a,yanhydroaconitic acid (VIb) were prepared by treatment of trans-aconitic acid with a c e t i c anhydride (6). Upon filtration of t h e reaction slurry, VIa is recovered from the filtrate and VIb from t h e cake. T h e structures shown a r e t h o s e assigned i n t h e reference. Chlorocitraconic anhydride w a s prepared by chlorination and dehydrochlorination of citraconic anhydride according t o the procedure of Michael and T i s s o t (7).
7” ,/ C-
II
\o
I VI1 H,C -COOH
I I H,C -COOH
HOC -COOH
Citric Acid
H,C
/ I\
H,C -COOR
-COOH
I
I /p CH,COOC -C
HC-C
H,C-C
H,C-C NO
IV
& VI
/o
%
Dodecenylsuccinic anhydride w a s used a s received from National Aniline. Epon r e s i n s 562 and 828, containing 0.65 and 0.54 epoxy equivalent and 0.25 and 0.08 hydroxyl equivalent per 100 grams, respectively, were obtained from the Shell Chemical cop. Benzyldimethylamine w a s obtained from t h e F i s h e r Scientific Co. For physical testing %-inch s h e e t s were cast i n a chromeplated mold coated with Dow Coming 20 emulsion a s release agent. After curing in a laboratory oven t h e s h e e t s were removed from t h e mold and postcured for 1 hour a t 100°C. t o r e l e a s e s t r e s s e s . Optimum curing schedu l e s were not determined.
V RESULTS AND DISCUSSION
Molecular weights and melting points of t h e anhydrides used in t h i s study a r e given i n T a b l e 1. T h e low melting points of methylsuccinic, dodecenylsuoa n i c , and citraconic anhydrides permit incorporation i n liquid epoxy resins at room temperature or slightly above with the result that low viscosity casting liquids of long pot life are obtained. S o m e pot l i v e s are given i n T a b l e 11. a
b Toble I. Dicorboxylic Anhydrides
T h e present article describes t h e effect of s o m e of t h e s e anhydrides i n c a s t epoxy resins. Dodecenylsuccinic anhydride w a s included in the study b e c a u s e of i t s structural relationships t o the other anhydrides. EXPERIMENTAL
Citraconic and methylsuccinic anhydrides a r e Pfizer development chemicals. Itaconic anhydride w a s prepared by treating t h e acid with acetyl chloride, concentrating t h e resulting solution in vacuum, and recrystallizing from ether. Acetylcitric anhydride w a s prepared from citric acid by treatment with a c e t i c anhydride using sulfuric acid a s catalyst. Tricarballylic anhydride w a s prepared from the acid in a manner similar t o t h e preparation of acetylcitric anhydride. T h e product w a s recrystallized from chloroform-acetic acid. An attempt t o duplicate Emery’s preparation (4). treatment with acetyl chloride followed by evaporation in vacuum, w a s unsuccessful, and t h e melting point reported by t h i s author could not b e duplicated. Tricarballylic anhydride methyl e s t e r (V, R CH,) w a s prepared by treating tricarballylic anhydride with anhydrous methanol t o obtain t h e a-monomethyl ester, which on distillation s p l i t s out water t o give the desired product. A s
-
80
Melting Point, Anhydride
Molecular Weight
Methylauccinic Dodecenyl succinic Itaconic Citraconic Tricarball ylic Tricarballylic, methyl ester Acetylcitric A yanhydrokonitic acid U, yanhydroaconitic acid
114.1 266.4 112.1 112.1 158.1 172.1 216.1 156.1 156.1
O C
36-37
< 12
68 ~. 6-7 143-144 69-69.5 123-125 76 135
Tobla II. Pot L i f e of Epoxy Resins ot 2 5 O C . O Anhydride
Epon Resin
Methylsuccinicb 562 Methylauccinicc 562 Methylsuccinicb 828 Methyleuccinie 828 Citraconicb 562 CitraconicC 562 Citraconicb 828 Citraconicc 828 .One mole of anhydride per epoxy equivalent. b 1% benzyldimethylamine. CNo added catalyst.
Days to Gelation
8 >40 7
>40 7 22 5
40
JOURNAL OF CHEMICAL AND ENGINEERIWQ DATA
Side Reactions. Some of the anhydrides may possibly have application i n t h e preparation of foamed r e s i n s bec a u s e of a tendency t o form g a s e o u s by-products under certain curing conditions. While i t h a s been reported (3 that citraconic anhydride decarboxylates readily at temperatures above 16OoC., t h i s h a s not been found t o b e correct. When pure citraconic anhydride w a s heated for an hour a t 190°C. no carbon dioxide evolution w a s detected. However, i n t h e presence of 0.4% of benzyldimethylamine approximately half a mole of carbon dioxide w a s evolved per mole of anhydride at 150°C. Attempts to i s o l a t e other decomposition products yielded only a black base-soluble resin. A s a consequence of t h i s behavior, dark foamed p l a s t i c s result under a l l but t h e mildest of curing conditions when citraconic anhydride i s used together with a tertiary amine c a t a l y s t in curing epoxy resins. Preparation of t h e citraconic anhydride-Epon 562 casting described in T a b l e VI required curing with 0.3% of benzyldimethylamine a t 55OC. for 48 hours and at 100°C. for 24 hours t o avoid bubbling. With Epon 828 it was not possible t o avoid bubbles, even a t 46OC., with 0.08% of amine. Dark, foamed r e s i n s a l s o resulted where itaconic anhydride w a s substituted for citraconic anhydride. T h e itaconic anhydride probably first rearranges t o t h e thermodynamically more s t a b l e citraconic anhydride, which then partially decarboxylates. Chlorocitraconic anhydride (VII) decarboxylates even more readily, while maleic dnhydride, dichloromaleic anhydride, and methylsuccinic anhydride a r e a l l s t a b l e i n t h e presence of t h e amine. T h e evidence indicates that in the presence of b a s e , citraconic anhydride l o s e s an allylic hydrogen and undergoes a self-condensation of unknown nature, eliminating carbon dioxide i n the process. T h e tricarboxylic acid anhydrides [IV, V(R = H) and VIa and b] do not require t h e tertiary amine catalyst t o cure epoxy resins. In s p i t e of this, acetylcitric anhydride (IV) and aconitic anhydride (VIa and b) a l s o resulted in foamed castings. B e c a u s e tricarballylic anhydride (V) g i v e s bubble-free, light-colored products with Epon 828, i t is believed that t h e foaming with IV r e s u l t s from t h e elimination of a c e t i c acid, and with VI, from decarboxylation (I). P h y s i c a l Properties of Cured Resins. T h e effect of methylsuccinic anhydride on heat distortion temperature w a s determined with Epon 828, and results a r e . given in T a b l e 111. A s expected, best results a r e obtained a t , or c l o s e to, a value of 1 mole of anhydride per epoxy equivalent. T h i s i s in agreement with previous reports of work with other anhydrides (8). Benzyldimethylamine c a t a l y s t w a s employed. Tricarballylic anhydride i s unusual, in that it contains a free carboxyl group. N o tertiary amine c a t a l y s t i s required, and optimum molar concentration i s remarkably low (Table IV). While t h e s e experiments were conducted a t 150°C., homogeneous c a s t i n g s have a l s o been prepared at ternperatures a s low a s 115" by careful stirring at the s t a r t t o ens u r e complete sojution of t h e anhydride in the resin. T h e participation of t h e carboxyl group in t h e curing i s evidenced by t h e fact that when tricarballylic anhydride methyl e s t e r (V, R = CH,) was employed at a level at 0.5 Table
111.
Effect of Anhydride Concentrotion on Epon 828 Cured with Methylruccinic Anhydride0
Moles Anhydride per Epoxy Equivalent
Heat Distortion Temp.. OC.6
0.5 0.75 1.0 1.25
65 80 90 75
'Curing conditions. 1% of benzyldimethylamine; 2 hours at 8S0C. and 24 hours at 150°C. bASTM D648-45-T, 264 p. s i .
VOL. 4, NO. 1, JANUARY 1959
Table
IV.
Effect of Anhydride Concentration on Epon Cured with Tricarballylic Anhydride0
828
Mole Anhydride per Epoxy Equivalent
Gel Time, Mia
Heat Distortion Temp., O C . b
0.2 0.3 0.4 0.5 0.75 1.0
75 70 55 45 25 5
85 139 147 108 99 65
eCuring conditions: 5 hours at 150°C. b ASTM D648-45T, 264 p.s.i. Table
V. Physical Properties of Epon 828 Cured with Various Anhydrides
Anhydride PHRc Cure shrinkage, linear, %, i n . / i n Polymer density, grams/cc. Flexural strength, p. s,Ld Flexural modulus, p.s.i. X Rockwell hardness Heat distortion temp., OC.' Dielectric constant1 60 c p s . 10' Power facto# 60 cps. 10'
Methyl Dodecenyl TricarSuccinicd Succinic' ballylicb
90
144 0.6 1.08 12,000 3.3 L89.0 69
6.12 5.57
5.08 4.76
3.95 3.45
0.00321 0.02040
0.00368 0.00935
0.0 144 0.0257
62 0.2 1.22 15,500 4.1 L98.5
34
...
1.25 16,400 4.3
M105.5 147
'Anhydride per epoxy equivalent 1.0 mole. Curing conditions: 1% BDMA; 2 hours at 85' C. and 24 hours at 150' C. bAnhydride p e r epoxy equivalent, 0.4 mole. Curing conditions: 150° for 5 hours without added catalyst. CParts by weight of anhydride per 100 parts of epcxy resin. dASTM D790-49T, flatwise. eASTM D785-51. ASTM D648-45T, 264 p. s. i. e ASTM ~ 1 5 0 - 4 7 ~ .
mole per epoxy equivalent with Epon 828, t h e resulting casting had a heat distortion temperature of only 48°C. In addition (Table V), considerably harder resins with higher heat distortion temperatures a r e obtained with tricarballylic anhydride than with methylsuccinic anhydride. Yet i t may b e maintained that the "abnormality" of tricarballylic anhydride i s more probably associated with t h e fact that no amine catalyst w a s used rather than with the presence of a free carboxyl group. T h e authors a r e not in agreement with this point of view, b e c a u s e it h a s been shown (3) that t h e addition of free carboxylic a c i d s to their resin system (Epon 834-phthalic anhydride) accelerated t h e rates of cure. In addition, a c i d s react directly with epoxide groups to produce secondary alcohols, which in turn r e a d with t h e anhydride, regenerating carboxylic acid groups. It appears, therefore, that t h e free carboxyl group in tricarballylic anhydride i s acting both a s a catalyst and a s a center for further c r o s s linking. When temperatures c l o s e t o the anhydride melting point a r e employed to homogenize tricarballylic anhydride with Epon 828, curing i s rather rapid, particularly at the higher anhydride concentrations (see T a b l e IV). T h e c a s t i n g s obtained, a s with methylsuccinic anhydride, a r e very light in color. Subble-free c a s t i n g s could not b e prepared with Epon 562 and tricarballylic anhydride, perhaps because t h e higher hydroxyl content of t h i s resin r e a c t s with the free carboxyl groups t o produce appreciable quantities of w a t e r . T h i s explanation i s being investigated further. P h y s i c a l t e s t results obtained with a number of the anhydride-epoxy resin systems are given in T a b l e s V and VI. Comparison of t h e data for dodecenylsuccinic anhydride and methylsuccinic anhydride demonstrates the plasticizing e f f e c t of t h e large hydrocarhon substituent: Flexural 81
Table VI. Physical Properties of Epon 562 Cured with Various Anhydridesa Methyl Dodecenyl Succinic b Succinic b C
Anhydride
c
l21 I II
I-
EPON 828
METHYLSUCCINIC ANHYDRlDE
w 2
80
PHR' Cure shrinkage. linear, 70,,in./& Polymer density. grams/cc. Flexural strength, p.s.i. F l e x u r d modulus, p. s. i. X 10" Rockwell hardness Heat distortion temp., 'C. Dielectric constant 60 cps. 10" Power factor 60 cps. 106
74
Citraconic d
186 1.6 1.11 7800 2.6
1.0 1.30
9100 3.1 L79.0 41
73 0.2 1.33 18, 200
5.1 L105.5
... 38
7.29 6.51 0.00592 0.02950
55.5
... ... ... ...
4.58 4.12
0.00762 0.03200
per epoxy equivalent, 1.0 mole. 1% R D W 4 hours a t lW0C C Shell 9ulletin SC. 56-7. dCuring conditions. 0.3% BDMA, 48 hours. a t 55OC. and 24 hours a t lOOoC e P a r t s by weight of anhydride per 100 parts of epoxy resin. a Anhydride
bCming conditions.
I Toble VII. Solvent Resistonce of Anhydride-Cured Epoxy Resins a
70 1
.
.
.
.
.
.
.
.
,
.
.
.I .2 .3 4 5 .6 .7 .8 .9 1.0 1.1 12 1.3 MOLES ANHYDRIDE PER EPOXY EQUIVALENT Figure 1. Plot of heat distoflion temperature o s a function of anhydride concentration for Epon 828 cured with tricarballylic ond methylsuccinic anhydrides
strength, modulus, hardness, and heat distortion temperature a r e a l l reduced. T h e rigidity of t h e citraconic anhydride molecule is c l e a r from a comparison of t h e values obtained with citraconic and methylsuccinic anhydrides (Table VI). Solvent r e s i s t a n c e properties a r e reported i n Table VII. It is not surprising that increasing hydrocarbon substitution i n t h e anhydride reduces r e s i s t a n c e to organic solvents and improves resistance t o aqueous media, although t h e difference of behavior between methylsuccinic- and dodecenylsucciniccured r e s i n s in benzene i s indeed rem arkab 1e. ACKNOWLEDGMENT
T h e authors w i s h to acknowledge t h e contributions of A. E. Timreck t o certain a s p e c t s of t h i s work. They a r e a l s o indebted t o Nancy DeFilippi for her very capable a s s i s t a n c e during t h i s investigation.
82
% Weight Increase in
.
Epon Anhydride
Resin
Water
Citraconic b Methylsuccinic b Dodecenylsilccinicb Dodecenylsuccinic Tricarballylic d
562 828 562 828 828
1.3 0.68 1.3 0.28 1.5
50% H,SO,
-0.24 0.12
1.8 0.07 0.23
10% NaOH
-11 0.50
0.65
0.22 3.2
Benzene
0.04 0.34 c
c
0.22
eSamples, 1%x 1 x % inch, immersed i n 60 ml. for 30 days. Values calculated by measuring weight increases both immediately after removing specimens from t h e solvent and after reaching equilibrium upon drying. bAnhydride per epoxy equivalent, 1.0 mole. CDisintegrated. dAnhydride per epoxy equivalent, 0.4 mole.
LITERATURE CITED (1) Bentley, R , Proc. Chem. SOC. (London) 1957, 21. (2) Castan, P.. U. S P a t e n t 2,324.483 (July 20, 1943). (3) Dearborn, E. C ,Fuoss, R. M., White. A. F., J. Polymer Scl. XVI, 201 (1955). (4) Emery, W. O.,Ber. 24, 569 (1891). (5) Fittig, R., Ann. 188, 67 (1877). (6) Malachowski, R., others Ber. 61 (n) 2525 (1928). (7) Michael, A., Tissot. G, 1. prakt. Chem. (2) 46, 385 (1892). (8) Weiss, H. K., Ind. Eng. Chem. 49, 1089 (1957). Feceived for review February 17, 1958. Accepted July 30, 1958 New York Meeting-&Miniature, ACS. March 1958.
JOURNAL O F CHEMICAL AND ENCilNEERlHG DATA
