EPOXY R E S I N S band at about 1300 cm.-’ which appears in all simple quinones is present in the resin, but not in Phenolphthalein.
Curing The epoxy resin of phenolphthalein can be cured by conventional hardeners such as the anhydrides or the amines. The values of heat distortion temperature, measured according to ASTM-D648, and solvent resistance are described in Table 11.
Co-epoxy Resin One method of modifying the properties of phenolphthalein resin is preparation of its co-epoxy resins. T h a t is, instead of phenolphthalein, a mixture of phenolphthalein with another phenol is used. Bisphenol A, phenol, resorcinol, diphenol sulfone, chlorinated phenols, or the like, may be used in combination with phenolphthalein. By varying the amount of these phenols, resins from
viscous liquids to high melting solids were obtained. These can also be cured by amines or anhydrides.
Coating
A high molecular weight phenolphthalein epoxy resin was reacted with soybean oil acid at 235” to 240” C. for 3’/z hours. The ester formed (acid value about 10) was mixed with a small amount of cobalt naphthenate in xylene, coated on a glass plate, and baked at 150” C. for ‘/z hour. A hard, tough and continuous film was obtained. Experimental Phenolphthalein (318 grams), epichlorohydrin (925 grams) and isopropanol (100 grams) were placed in a flask and brought to reflux with stirring. Sodium hydroxide pellets (81 g r a m ) were added slowly to the solution and followed by 12 grams of water. Reflux
Polyesters from Epoxides and Anhydrides
*
E
I N THE PRESENCE of a tertiary amine, a terminal epoxide such as allyl glycidyl ether (AGE), reacts cleanly at 70’ to 100’ C . with an acid anhydride (such as phthalic) giving a linear polyester. If equimolar quantities are used, residual activity at the end of the reaction is generally less than 1% of the starting value, proving that the reaction is at least 99% specific-Le., that at least 99% of the epoxide molecules react with anhydride molecules rather than with each other.’ The anhydride requirement with terminal epoxides is 99%; with vinylcyclohexene and cyclohexene oxides the requirement is 75 to SO%, and with such internal epoxides as the alkyl epoxy stearates it is only 60 to 65%. This means that 20 to 40% of these epoxides are unavailable for anhydride reaction, due to homopolymerization, isomerization, or other side reactions. This decreasing order of anhydride demand somewhat parallels the decreasing quality of cure obtained with polyepoxides containing these types of epoxides. Even with terminal epoxides not all catalysts are specific. Tetrabutyl titanate for example, gave 40% homopolymerization of AGE in a phthalate esterification. The reaction has several features which result in important advantages over conventional polyester formation :
1. Volatile products are not formed, eliminating the need for distillation during the reaction. 2. Molecular weights, at least to 12,000, are controlled by the presence of
active hydrogen impurities (alcohol, carboxyl, or water). Agreement between molecular weights by end-group and ebullioscopic analyses is very good to about 12,000. Above this value the discrepancy ranges to 30 to 40%. However, in this region both ebullioscopy and conventional end-group analysis are approaching their detection limits. I t is possible, therefore, that chain termination is largely or even completely caused by stray active hydrogen impurities in the reaction mixture. 3. Within certain limits the reaction is zero order-Le., is independent of monomer concentration. The zero order portion of the curves at 1 to 1 mole ratios persists to about 60% conversion. At this level, the excess of monomers is no longer large, and the over-all rate drops as the growing chain must alternately seek anhydride and epoxide molecules. If an excess of one reagent is present, the rate remains zero order with respect to the other monomer to 8O+Y0 reaction. 4. The zero order rate persists over a wide range of catalyst concentrations, and this rate is proportional to the catalyst concentration. Mathematically the equation takes the form:
R,
= u
+ k 5 (R, -
u)
CY
where R, and R, represent the rates a t catalyst concentrations x and y , respectively; a is the intercept a t 0 catalyst concentration; k is a constant Practically, a may be taken as the rate of the uncatalyzed reaction and k an efficiency factor representing the portion of the amine which is catalytically effective. For the system studied in
was continued for an additional hour. The solution turned from deep purple to pale yellow, indicating the end of the reaction. The resin solution was filtered to separate sodium chloride. Isopropanol and excess epichlorohydrin were removed by vacuum distillation. A pale yellow resin (about 400 grams) left in the flask had an epoxy equivalent of about 0.42 per 100 grams of resin (4).
literature Cited (1) Fieser, L. F., Fieser, M., “Organic Chemistry,” 3rd ed., p. 896, Reinhold,
New Ynrk. (2) Green, A. G., King, P. E., Ber. 40, 3724-35 (1907). (3) Meyer, R., Marx, K., Ibid., ai, 2446-53 (1908). (4) Mitchell, J., Jr., “Organic Analysis,” vol. I, p. 136, Interscience, New York.
ELIZABETH S. LO Exploratory Research Department, Permacel, New Brunswick, N.J.
detail (AGE-phthalic anhydride a t 100’ C . ) , a = 0.0770 reaction per minute and k = 96y0. T h a t is, doubling the catalyst concentration increases the rate by a factor of 1.92, taking into account a small constant uncatalyzed rate. 5. The reaction is so specific that an excess of epoxide may be used. I t serves merely as a solvent, and when the anhydride has been consumed, the excess epoxide may be distilled quantitatively. Within the limits of the analyses, the products are indistinguishable from those in which the reagents are carefully calculated at l to lM, both in structure and molecular weight. The use of excess epoxide has the advantage that the rate remains high to the last removal of anhydride. With 1 to 1 M mixtures the rate becomes very low as both anhydride and epoxide concentrations become low. 6. Another difference from conventional esterification is that end-groups tend to be the same-Le., with excess epoxide, the polymers can be better than 90% hydroxyl terminated, and with a slight excess of anhydride, carboxyl termination predominates.
Mec ha niIm The following mechanism is consistent with the facts previously described. 1. Activation of anhydride by the amine : 0
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There is present in the system a large excess of anhydride over amine, which can shift the equilibrium strongly to the right. The k factor of 0.96 might result from 4% of the reverse reaction. I t is important to note that the activation of the anhydride causes deactivation of the amine. This helps explain the actual suppression of epoxide homopolymerization by the anhydride-amine combination. 2. Reaction of the carboxyl anion with the epoxide: 0
n
0
0
This is a widely accepted reaction (2) in normal acid or anhydride reactions with epoxides. 3. Reaction of alkoxide anion with the anhydride: 0
0
I1
0
x
I n a typical case, a product having a molecular weight of 11,500 (degree of polymerization, 48 of each of epichlorohydrin and phthalic anhydride) was obtained at a catalyst concentration of 0.2 mole This represents about 110 chains per molecule of amine, indicating that the amine is quite labile. This mechanism and the kinetic data associated with it bear a formal resemblance to the findings of Fontana and Kidder (7), who studied quite a different system. The amine-anhydride equilibrium corresponds to their “associative complex equilibrium,” and the carboxy anion-epoxide reaction corresponds to their “associated monomer rearrangement.” Other workers, notably Schechter and Wynstra (2) have proposed a stepwise mechanism involving initiation by small quantities of alcohols, which react with anhydrides to form carboxy esters, which in turn react with the epoxides giving hydroxy esters. In this case the hydroxyl impurities control the molecular weight of the product by chain initiation as well as termination. The rate moreover should be dependent on hydroxyl concentration. In an experiment designed to test this condition, the hydroxyl content of a typical epoxide anhydride system was varied systematically. The rate was found to be constant, and, therefore, independent of hydroxyl concentration. This implies that hydroxyl is not strictly necessary for polyester formation. Any hydroxyl present. however, could start chains in the manner proposed by Shechter and Wynstra.
it is necessary to precipitate the product at least once (in ether). This seems to remove a trace of low molecular weight impurity. The polyesters are colorless to tan, depending on the starting materials, and from most epoxides have been organic glasses. This is expected, since most epoxides are unsymmetrical, and undirected opening of the ring will give a random distribution of configurations along the chain. The product will be a mixture of closely related stereoisomers. Softening points have ranged from 35’ to 40’ C. for poly(AGE-phthalates) through 80’ to 90’ C. for poly(epichlorohydrin phthalates) to 115’ to 125’ C. for poly(cyc1ohexene oxide phthalates). Ethylene oxide is symmetrical and has therefore given higher-melting, truly crystalline products. From ethylene oxide and phthalic anhydride has been obtained, apparently for the first time, a crystalline poly(ethy1ene phthalate) having a melting point of 108’ to l l O o C. Cyclohexene oxide is also symmetrical, and although its phthalate polyesters have not crystallized, they soften well above 100’ C. The cyclohexene polyesters are also brilliantly clear and very light colored. They are surprisingly brittle. however. Among the anhydrides. phthalic anhydride has been most used, because of its cheapness, high commercial purity, satisfactory rate, and light color of products. Hexaand tetrahydrophthalic anhydrides have also been very satisfactory. Aliphatic anhydrides, especially succinic, have been slower reacting and have given dark products. A number of polymeric noncyclic anhydrides, notably adipic, isophthalic, and terephthalic have been used successfully. Maleic anhydride turns very dark with tertiary amines. In the epoxide system it also gels when very little anhydride has reacted. Gelling probably occurs when an anion adds to one of the resonance forms of maleic anhydride :
Products
Continuation of these alternating steps would give rise to the polyester. A displacement of the amine by alkoxide ion would also continue polyester formation: 4.
0
-OCH?CH--0!4---
x
322
___f
As indicated earlier the products are linear polyesters. When prepared from ordinary reactants and analyzed without purification, molecular weights of the total product have ranged from 2500 to 4000. Simple distillation of the starting epoxide has usually removed enough impurity to give products ranging from 6000 to 8000 in molecular weight. More careful fractionation of epoxide and precautions to exclude water ordinarily raised the molecular weights to the level of 10,000 to 12,000. In one case where extraordinary precautions were taken, however, and a molecular weight of 27,000 was expected, t h e molecular weight of the product was only 18,400. In most cases above 6000,
INDUSTRIAL AND ENGINEERING CHEMISTRY
CO-
/I
+
fCH-
0
CH=C ‘0-
,
//O
C-0-CHC 0
c=c / \o-
H
,
COCH-C //O CH-C
-
\o
EPOXY RESINS This product can then further react bifunctionally, giving cross-linked materials. U p to 30 mole % of maleic anhydride has been incorporated by keeping the reaction temperature below
90' C. and adding maleic anhydride in the later reaction stages. literature Cited (1)Fontana, c, M., Kidder, G. A.,
J.
Am.
Chem. Sod. 70, 3748 (1948).
These Are Typical Properties for Solutions of Bisphenol A in Epoxidized Soybean Oil Cured with HETAnhydride"
Specific gravity Water absorption (24 hr.), % Fire resistance, in./ min. Heat distortion temp., O C., 264 p.8.i. Flexural strength, p.s.i. Flexural modulus, p.s.i. Tensile strength, psi. Tensile modulus, p.s.i. Compressive strength, p.s.i. Hardness
Plasticizers
?.
I n a typical epoxidized vinyl plasticizer, based on soybean oil, the unsaturated components are epoxidized to an average of about 3.6 equivalents of epoxy oxygen out of a theoretical of about 4.6 equivalents. Materials of this type have an average molecular weight of about 1000, an epoxide number of about 260, and an epoxide content of about 6.0%. These epoxidized soybean oils can be considered as potential epoxy resins which are somewhat similar in basic structure to those derived from glycerol and epichlorohydrin ; however, these
Table 1. Y
Hardening Agent Diethylenetriamine m-Phenylenediamine HET anhydride
Dielectric
D-570
1.340
D-635
0.09 0.26 Self-ext.
D-648
85
0-790
11,320
D-790
3 . 0 X 106
D-638
6,390
D-638
4 . 3 X IO6
D-695 9,630 Rockwell M-66 Barcol 10 D-149
(WT) (WS)
410 3 94
Dielectric constant (IMC) (Cond A)
R. F. FISCHER: Shell Development Co., Emeryville, Calif.
Table I shows that typical amine hardeners, when compounded with epoxidized soybean oil, do not yield materials of sufficient rigidity for most epoxy applications. O n the other hand, H E T anhydride provides compositions having a linear increase in heat distortion from 45' to 63' C. as its content is reduced from 200 to 100 parts per hundred of resin (p.h.r.). As the H E T content is reduced further to 80 and to 6 0 p.h.r., brittleness and excessive shrinkage become apparent.
Modifler Additives
strength
Volts/mil
D-149 2.87
2.96 " HET anhydride (100 p.h.r.) 1:2 molar ratio of bisphenol A in epoxidized soybean oil, cured for 24 hours at 140° C. (D48-50)
materials have less epoxy oxygen for a given molecular weight and more of a long chained plasticizing-type structure.
I n an attempt to reduce the brittleness and shrinkage of the H E T anhydrideepoxidized soybean oil systems, the difunctional coreactants listed in Table I1 were evaluated for their effect on the heat distortion characteristics of the system. These studies suggested that bisphenol A is one of the better modifiers, in that it afforded an optimum heat distortion of 91' C. us. 63' C. for the nonmodified system. This optimum heat distortion was obtained at a 1 to 2 molar ratio of bisphenol A to epoxidized soybean oil and 100 p.h.r. H E T anhydride at a cure temperature of 140" C. (Table 111). The viscosity stability characteristics of various solutions of bisphenol A in an epoxidized soybean oil indicate that the systems are quite stable at moderate temperatures-Le., 24' to 60' C. However, their general stability decreases as the temperature of the system is increased and the molar ratio of bisphenol A is increased.
Epoxidized Soybean Oil with Epoxy Resin Hardeners
Quantity, P.H.R. 7.8" 10.2" 60 80 100
120 140 160 180 200
ASTM Method 0-792
I N D . END.
Epoxy Resin Hardeners
Epoxy Resin Systems Based on Epoxidized Soybean Oil and HET Anhydride T H E RELATIVELY HIGH cost of conventional epoxy resins has frequently stimulated searches for lower cost systems containing inexpensive epoxidized derivatives. The current use of epoxidized vinyl plasticizers, compounded with poly(viny1 chloride) resins and selected epoxy hardeners, represents one procedure by which the low cost epoxycontaining plasticizers can perform the dual function of a vinyl plasticizer and an epoxy resin. The work presented herein provides a system by which homogeneous epoxy resin formulations can be derived from epoxidized plasticizers and the epoxy resin hardener, chlorendic anhydride (HET; Hooker Chemical Corp.)
(2) Shechter, L., W~nstra,J., CHEM.48, 86 ( 1 9 5 6 ) .
Gel Time, Hr.
... ...
3 . 5 -4.0 2 . 0 -2.5 2 . 0 -2.5 1.0 0.75-1.0 0.75 0.75 0.5
Control (Araldite 6020)f HET anhydride 100 0.5 e Ratio of amine hydrogen equivalents to epoxy equivalents, 1:l. Rubbery. 180' C. Hard, cracked. e Cracked. I Ciba Co., Inc.
Temp.,
... ... 120
120 120 140 140 140 140 140
C.
Cure Temp, a C. 180 1so 150 18OC 150 18OC 150 180c 150 180c 150 18OC 150 180c 150 18OC 150 f 180°
+ + +
+ +
+ +
Heat Distortion, 'c . h b
b d
63 58 55 a
48 45
120 180 196 Stage cured for 24 hours at 150' C. followed by 24 hours a t
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