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DWIGHT E. PEERMAN, WESLEY TOLBERG, and DON E. FLOYD General Mills Research Laboratories, Minneapolis, Minn.
Reaction of Polyamide Resins and Epoxy Resins Infrared analysis gives new information on curing conditions in thermoset compositions used industrially
A m v o - c o n t a i n i n g polyamides, which are essentially thermoplastic resins, may be converted to thermosetting substances by reaction with a number of materials, including the epoxy resins. T h e amino polyamide resins have interspersed along the molecule, amino groups which may be primary, secondary, or tertiary and which are free to react with, or catalyze the curing of, epoxy resins (73). The thermoset compositions now find industrial usage as adhesives, coatings, and potting resins, and other applications,
such as plastics tooling (5, 6, 8, 9, 77). The reaction between amines and epoxy resins is rather complex (2, 3, 70). The complexity increases when polymeric coreactants such as the amino polyamides are utilized in place of simple amines to obtain cross-linked structure. I n addition to the primary, secondary, and tertiary amine group in the polyamide, there may be heterocyclic nitrogen-containing terminal groups such as the imidazolines or tetrahydropyrimidines (7).
This article concerns a study of the rates and degree of reaction of amino polyamide resins and epoxy resins. For comparison, data on commonly used short-chain amine curing agents are included. Thereactionofepoxy resins with amino polyamides is only mildly exothermic in nature, compared with that of epoxy resins with aliphatic short-chain amines. This suggests a slower rate of reaction. T h e short-chain amines generally harden epoxy resins rapidly, while polyamides
. ,
TIME
TIME IN MINUTES
Figure 1 . Relation of time to disappearance of epoxy at various temperatures
Figure 2.
- HOURS
Relation of time to disappearance of epoxy a t
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figure 3. Disappearance of epoxy at 200" F. Vertical scale displaced
require longer curing times a t room temperature. The amino polyamides probably react with epoxy resins more slowly because the high initial viscosity slows down the reaction. Spectroscopic studies of epoxy resins in Great Britain ( 4 ) have shown that ethylenediamine reacts more fully in a given time a t room temperature than the amino polyamide. Therefore, the relation of residual epoxy content to cure conditions as well as to the physical properties of the thermoset reaction product was studied. Obtaining accurate analytical data is complicated by the fact that a t a certain stage in the reaction of amine curing agent with epoxy resin, the viscous resins gel to a solid matrix. This solid is no longer soluble in organic solvents and cannot be melted. T h e apparent solution to the problem was the use of infrared spectroscopic analysis. T h e infrared spectroscopic techniques employed were very satisfactory. despite considerable mechanical difficulties, Prior infrared work on products of this nature \vas helpful in developing the techniques used ( 7 , 72). A new concept of optimum curing conditions and of the interrelationship of viscosity, temperature, and reaction rate was developed from data provided by the infrared analysis presented here. T h e concept of molecular orientation is postulated to play a part in the degree of reaction between amino polyamides and epoxy resins which takes place under various curing conditions. T h a t
temperature and viscosity play a n important part in the rearrangement of the molecules is also obvious. Viscosity reduction a t elevated temperature promotes mobility and thus increases the rate of cross linking. Thus, although increase of curing temperature tends to reduce the viscosity of the system and allow more complete orientation of the resin matrix, it also accelerates gelation and promotes immobility. The latter eventually is predominant and a t this stage the polymer has little further chance for reaction. The thermoset reaction product can thus become immobilized before all of the epoxy groups are used up. When a certain percentage of the amine groups available for reaction have reacted with epoxy groups, the mixture becomes a solid and is extensively cross-linked. This inhibits the reaction of the remaining amino groups, either in the polymeric curing agent or in the short-chain ethylenediamine. As the polymeric curing agent is more viscous than ethylenediamine, for instance, and reacts more sloiz-ly, it follows that the optimum tempcrature for reaction must be higher. If the temperature is too high, however, the matrix becomes solid or nonmobile before all of the epoxy groups present are able to react fully. This finding is shown graphically in Figures 1 to 3 and in Table I. I n the work reported here, unless otherwise indicated, blends of 35 parts of Versamid 125 (trade name of thermoplastic resin sold by General
100
80 I-
z w !-
z 0 0
>. X
60
g w -I
4
f 40 0 LL 0
fl
20
Figure 4. Infrared spectra of Versamid 125(35)/AraIdite 601 O(65) before and after 10 minutes at 300" F. Vertical scale displaced
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2
3
4
TIME
Figure 5.
IN HOURS
Rate of cure of Araldite 6010 at
300' F.
EPOXY R E S I N S
lo0l 80
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Figure 6. cure time
*(
1
1
40 TIME
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80
Table 1. Disappearance of Epoxy Groups in Blends on Curing at 75' F.
Time Interval, Hours 0
2 26.5 31 95 124 165
100
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120
% of
Origmal Epoxy Content 100 ...
92.5 45.0 43.9 41.7 41.2 40.8
5
10
RESIDUAL
Heat distortion temperature as a function of
Mills, Inc.) and 65 parts of Araldite 6010 (Ciba Co.) were used. I t is concluded that if the optimum temperature of reaction is selected, the competing factors of decrease in viscosity due to heat and increase in immobility due to reaction can be balanced successfully, so that nearly all of the epoxy groups in the epoxy resins react with amine groups in the amino polyamide. Some of the more commonly used amine-type curing agents are not cured in an optimum manner a t 300' F., a frequently suggested curing temperature. Figures 4 and 5 show that at 300' F. the blend of amino polyamide and epoxy resin contains essentially no residual epoxy groups after 10-minute cure, but appreciably epoxy content is left in epoxy and amine hardener systems under the same conditions. Although the shortchain amine-cured epoxies have good physical properties when cured a t 300°F., this is obviously not the temperature at which all epoxy content disappears. Perhaps a more suitable curing temperature would give improved properties in these systems. I t was not within the
'O0I 80
1
30
I5 20 25 EPOXY CONTENT %
-
35
Figure 7. Effect of residual epoxy content on heat distortion temperature
scope of the present work to determine these optimum temperatures. The disappearance of epoxy with other typical amine curing agents has been discussed (7) ; estimation of residual epoxy is in essential agreement with the findings in the present study. Disappearance of oxirane oxygen continued even when insufficient amounts of coreactant were present. I n demonstrating this phenomenon, three ratios of amino polyamide to epoxy resin were chosen. When excess polyamide was used in the blend, the rate of disappearance of epoxy was approximately the same as when the optimum ratio of amino polyamide to epoxy was used. (The optimum ratio of amino polyamide to epoxy resins is defined arbitrarily as the blend having the highest heat distdrtion temperature.) Thus, a t 300' F., both of these ratios left essentially no oxirane oxygen content at the end of a 10-minute heat-up period, during which time the temperature attained 300" F. When a deficiency of polyamide in relation to epoxy was used, the disappearance of epoxy assumed an entirely different 'course. Epoxy initially disappeared rapidly, but when the available amine had been exhausted, there was a continuing dissapparance of epoxy, which was attributed to either degradation or catalyzed eelf-polymerization. T h e disappearance of epoxy with the various ratios at constant temperature is shown in Figure 6. Epoxy continues to disappear up to and through 12 hours, even though the amino groups of the polyamide have been used u p quickly in the initial portion of the reaction.
The optimum ratio of amino polyamide to epoxy resin was determined by extensive physical testing, including such tests as heat distortion temperature, tensile, flexural, and compressive strength, hardness, and impact resistance. Heat distortion temperature has been shown to be a sensitive indication of degree of cure. T h e present work shows clearly that heat distortion temperature is related very closely to the disappearance of epoxy groups in the polymer (Figures 7 and 8). T h e heat distortion is a measure of the resistance to flexural 100
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2 B
D
x 2.
2 50 -1
r" %
40
b r(
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6
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0
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Figure 8. Effect of polyamide-epoxy ratio on disappearance of epoxy VOL. 49, NO. 7 '
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1 1 7 0 HOURS
23 HOURS
2 HOURS
0 TIME
Figure 9. Disappearance of epoxy a t 75”F. Vertical scale displaced
bending under the influence of elevated temperature. Other physical measurements, such as hardness, ultimate flexural strength, and compressive strength, are less closely related to the degree of reaction. Many partly cured specimens show considerable physical strength. The relationships of heat distortion temperature, hardness, and flexural strength in cured blends of amino polyamides and epoxy resins are shown in Table 11. Heat distortion values are critical to the elements of the study and determinations are repeated no less than four times. Barcol hardness is routinely measured ten times. Flexural strength data are obtained in duplicate as they are not critical. The infrared curves of blends cured a t room temperature (75’ F.) (Figure 9) show the epoxy to react only partially
It.
Table
Temp., O
F.
400 400 300 300 200
200 200 200 200
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a t 75’ F. Full cure does not take place even after a postcure a t 300’ F. This finding substantiates the concepr of formation of a solid matrix a t other than optimum temperatures, which greatly hinders complete disappearance of epoxy in the system. The oxirane oxygen content of the samples studied was determined by observing an absorption peak in their infrared spectra a t about 10.9 microns. The samples were prepared as thin films (0.002 inch) between potassium chloride plates. The spectra were recorded a t essentially zero time and after approximate intervals, during which the samples were cured a t various temperatures. Initially, the most obvious approach in preparing resin samples for determination of their infrared spectra was to form a thin film of known thickness between rock salt plates separated by a standard spacer. O n curing, the resin blend creates a strong bond between the plates and prevents recovery of the rock salt plates for re-use. Difficulty was encountered in preparing suitable spacers. An alternative to this method was developed. I t was relatively simple to prepare clear plates, 1 inch in diameter and about ”18 inch thick, from powdered potassium chloride. These were made in a pellet die under 50-ton pressure. O n e of each pair of plates was made with a ram which had a 0.001 inch rise across the center ”4 inch of its face. This formed a 0.002-inch depression between the plates, which allowed resin thickness to be maintained even under pressure. Plates made with this ram had what amounted to a built-in spacer. T h e resin blend was placed between potassium chloride plates and installed in a special holder which firmly compressed the sandwich so that sample thickness would not change on heating. As the samples were of constant thickness, the consecutive readings of absorbance were compared directly with the reading a t zero time to obtain per cent oxirane oxygen. The sandwich methods thus permitted determination of epoxy content on samples too soft for conventional potassium chloride plate solid state dispersions and too insoluble for solution spectra. No data other
Heat Distortion Values for Blends under Various Curing Conditions Heat Distortion, Flexural Time, Barcol ’F. Strength Hardness Min. 174 13,600 5 79 60 10 90 10 30 60 120 480
80 77 86
55 69 68 72 77
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171 187 131 156
154 162 171
14,900 14,600 14,800 13,400 11,500 12,000 9,100
11,000
PLUNGER
R
DIE
Die for potassium chloride plates
than absorbance of the epoxy band were required. As the same sample was observed throughout cure, errors in manipulation, sample thickness, and time of cure were avoided. At least two samples were studied under each set of conditions and agreement was obtained in each case. Literature Cited (1) Dannenberg, H., Harp, W. K., Anal. Chem. 28,86 (1956). (2) Greenlee, S. 0. (to Devoe-Raynolds), U. S. Patent 2,585,115 (1952). (3) Zbid., 2,589,245. (4) O’Neil, L.A., Cole, C. P., “Chemical and Spectroscopic Studies of Epoxy Resin Reactions in the Surface Coating Field,” Birmingham Paint Club, Birmingham, England, 1956. (5) Paint, Oil C3 Chem. Rev. 113 ( 2 3 ) , 15 (1950). (6) Peerman, D. E., Floyd, D. E., Mitchell, W. S.. Plastics Technol. 2, 25 (January 1956). (7) Peerman, D. E., Tolberg, W, E., Wittcoff, H., J. Am. Chem. Soc. 76, 6085 (1954). (8) Renfrew, M. M., Wittcoff, H., FIoyd, D. E., Glaser, D. W., IND.ENC. CHEW46,2226 (1954). (9) Riley, M. W., “Plastics Tooling,” Reinhold, New York, 1955. (10) Shechter, Leon, Wynstra, John, Kurkjy, R., IND. END. CHEM. 48,74 (1956). (11) Silver, I., Atkinson, H. B., Jr., Modern Plastics 28 ( 3 ) , 113 (1950). (12) U. S. Dept. Commerce, Office of Technical Services, Bull. C. B. 1111438 (1954). (13) Wittcoff, H., Renfew, M. M. (to General Mills, Inc.), U. S. Patent 2,705,223 (1955).
RECEIVED for review October 19, 1956 ACCEPTED February 13, 1957 Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Epoxy Resins, 130th Meeting, ACS, AtIantic City, N. J., September 1956. Journal Series 203, General Milk, Inc.
