319
Ind, Eng. Chem. Prod. Res. Dev. 1980, 79,319-326
the incorporation of high molecular weight homopolymeric silicone fluid. This is not the case in this spreading application. PDMS homopolymer is detrimental, probably because of adsorption on the substrate producing a low surface energy material over which the resin cannot spread. Literature Cited Fort, T., Jr.; Patterson, H. T. J . Colloid Sci. 1963, 78, 217. Hill, M. P. L.; Millard, P. L.; Owen, M. J. "Polymer Science and Technology", Lee, L-H, Ed.; Plenum Prees: New York, 1974, Vol. 5B, p 469. Kaneiiopoulos, A. G.; Owen, W.J. J. Colloid Interface Sci. 1971, 35, 120.
Kendrick, T. C.; Kingston. 6.M.; Lloyd, N. C.; Owen, M. J. J . ColbH Interface Sci. 1967, 24, 135.
Received for review April 10, 1980 Accepted May 22, 1980 This paper was originally presented at a symposium on Mechanisms of Film Formation from Powders, Melts, and Solutions a t the 179th National Meeting of the American Chemical Society, Houston, Texas, March 23-28,1980, Division of Organic Coatings and Plastics Chemistry.
IV. Symposium on New Concepts in Coatings and Plastics Chemistry R. H. Lalk, Chairman 178th and 179th National Meetings of the American Chemical Society, Washington, D.C., September 1979, and Houston, Texas, March 1980
Diethanolamine as a Hardener for Epoxy Resins C. V. Lundberg Bell Laboratories, Murray Hi//, New Jersey 07974
The chemical changes occurring during the reaction between diethanolamine and epoxy resin are followed by infrared analysis; and physical, mechanical, and electrical properties are related to these chemical changes. Properties critically examined are glass transition temperature ( Tg),hardness, linear shrinkage, dissipation factor, and insulation resistance. Complete cure does not occur at 40 O C within 100 days, and yet, after 3 days at 40 OC, the system is sufficiently cured so that on postcuring at 100 OC no additional shrinkage occurs. A T of 60 OC is developed in 3 days at 40 O C and slowly rises to 75 "C in 100 days. Curing at 100 "C results in a h l cure and a T, in the vicinity 'of 100 "C. The dissipation factor at lo3 Hz increases as time of cure increases and doubles in value at full cure. Hardness and insulation resistance increase in value as the epoxy converts from liquid to solid, but they quickly reach constant values and become insensitive to additional cure.
Introduction Diethanolamine has been used as a hardener for epoxy resins for 15 or more years. Within the past 2 years we have had occasion to examine some of the physical, mechanical, and electrical properties of diethanolamine-cured epoxy compounds and have uncovered some interesting information. In addition we have attempted to follow the chemical changes occurring during cure at several temperatures by means of infrared analysis and have attempted to correlate these changes with the mechanical and electrical property changes. Epoxy resin cured with diethanolamine, whose structural formula and properties are shown in Figure 1,converts to a solid a t room temperature in approximately 1 day. It was observed that tranaformers impregnated and encapsulated with this compound and cured at room temperature developed corona extinction voltages an order of magnitude higher than when cured at 100 "C. It was quite natural to attribute the improved corona extinction voltage to the low shrinkage associated with the low-temperature cure. Low shrinkage decreases the likelihood of the formation of internal voids and cracks--the most likely source of low corona extinction voltages aside from trapped air.
The state of cure of the epoxy was determined by measuring its electrical resistance. When this measurement reached a plateau value, it was thought that a full epoxy cure had been obtained. By this method a full cure is obtained in 8 days at 24 "C, 3 days a t 40 O C and 18 h at 75 " C . In the present study we have used the glass transition temperature ( T ) as an indicator of chemical change or degree of cure. tures similar to the 40 "C cure above yield T , values in the 55-60 "C range. On continued exposure at 100 " C for 3 days, the Tgincreases to 97 " C , and, on further heating at 125 "C, it advances to 103 "C. If the transformers referred to above had been tested after temperature exposures sufficiently long to increase the T,of the epoxy to the 97-103 " C range, other property changes would most likely have taken place along with the chemical change indicated by the increase in the Tg, and the corona extinction voltage may have been decreased. The word "may" is used because changes associated with the increase in T, could result in stress buildup in the epoxy without formation of internal cracks. This transformer was small in size-ca. 2 in. in diameter by ll/*-in. high-and small castings resist cracking more effectively than large castings.
0196-432 1180112 19-0319$01.OO/O 0 1980 American Chemical Society
320
Ind. Eng. Chem. Prod. Res. Dev., Vol, 19, No. 3, 1980 DIETHANOLAMINE
PH2CH20H
H-N
'CH2-
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1.09
VISCOSITY
AT
BOILING
20°C
POINT
CH20H
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DAYS
Figure 2. Tgvs. time of cure at 40 and 100 O C . VAPOR PRESSURE AT 20°C < 0.01nWI HG Figure 1. Structure and properties of diethanolamine.
It is always best to expose epoxy compounds to temperatures somewhat above their final use temperatures before measuring their mechanical, physical, and electrical properties as these properties can change on exposure to temperatures higher than their processing temperatures. Provided the products made from diethanolamine-epoxy cured at or below 40 "C operate at normal room temperature, electrical and mechanical measurements will probably remain fairly stable. But if the products operate at elevated temperatures, as do many transformers, additional cure will take place with a possible concurrent shift in properties. In the early part of this study, aside from the glass transition temperature measurements, dissipation factors as a function of cure were measured at room temperature (23 "C), and it was found that the lowest values were obtained as soon as the specimens became hard and essentially nonindentable. As further cure was induced by exposure to elevated temperatures, the dissipation factors more than doubled. This was surprising as we had believed that the lowest dissipation values were associated with the most thorough cure. Del Monte (1959) studied dielectric properties as a function of epoxy cure and noted that the dissipation factor of diethylenetriamine-cured epoxy showed a "slight rise after prolonged aging". This might be expected as entities are likely to be produced during aging which could have an adverse effect on electrical properties. Thus, it appears that electrical properties (at least minimum dissipation factor and maximum electrical resistance) are not necessarily indicative of thorough cure and that thermal measurements, namely, glass transition temperatures, are more sensitive and truly indicative of internal chemical changes. We decided to prepare new sets of specimens and systematically follow insulation resistance, dissipation factor, and glass transition temperature a t several cure temperatures. We also decided to follow the cures by means of infrared analysis to see whether indicated chemical changes could be correlated with changes in the properties mentioned. Specimen Preparation In our use of diethanolamine as an epoxy curing agent, it has been customary to use an adduct consisting of 80% diethanolamine and 20% of a relatively pure diglycidyl ether of bisphenol A epoxy resin, such as Dow Epoxy Resin 332. This epoxy resin is "pure" in the sense that its listed epoxy equivalent weight is 172-176 compared to 170 for
a resin which is exclusively diepoxide. One hundred grams of this mixture in an 8-02 paper cup at a room temperature of 24 "C generates about 16 O C exotherm in about 31 min. This mixture is cloudy and overnight separates into two layers, the lower layer being about lj4-in. thick and very viscous. When this mixture is stirred and heated at 75 "C, it clears and no exotherm develops. This adduct is now used to make the final compound by adding 15 parts of it to 100 parts of Dow Epoxy Resin 332, yielding a compound containing 11.65 parts of diethanolamine on 100 parts of epoxy resin. The two ingredients are preheated at 75 "C. On mixing, cloudiness develops which clears on further heating at 75 "C for 30 min. Specimens were cast in flat-bottom, 21/4-in. diameter, aluminum dishes for electrical testing. These 0.080-in. thick specimens were cured at various temperatures and periodically tested so that the same specimens were continually being retested. Specimens 0.100-in. thick were cast in 11/4-in.diameter aluminum dishes for Tgmeasurements; one specimen was prepared for each planned cure period. Drops of the liquid mixture were placed between sodium chloride plates, and the plates were squeezed together to reduce the epoxy compound thickness so that suitable infrared spectra could be obtained. The same salt plate assembly was periodically retested after exposure to a particular cure schedule. Specimens for physical, mechanical, electrical, and infrared tests were cured concurrently at 40 and 100 "C, and one set of specimens was cured for 3 days at 40 "C and postcured at 100 "C, and one set of specimens was cured for 3 days at 40 "C and postcured for 1day at 60 " C plus 1 day at 80 "C and further postcured at 100 "C. Some of the cures a t 100 "C were carried out for as long as 1year and might better be described as aging a t 100 "C rather than curing at 100 "C. The purpose of the 40 "C cures was to follow the properties developed at the temperature which has become the standard cure temperature used in our transformer manufacture. The purpose of the 100 "C cures was to find out how much the properties change after exposure to this higher temperature-a temperature which is at or near the operating temperature of some of the larger transformers. Results Glass Transition Temperature. The T, values are shown in Figures 2, 3, and 4 and essentially duplicate similar data obtained earlier. Curing for 1 week at 40 "C develops Tgvalues in the vicinity of 60 "C, and further cure at 40 "C (i.e., 3-5 weeks) increases the Tgvalues to the vicinity of 70 "C. Curing at 100 "C for 1 week or more
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
O
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,b
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30
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Figure 6. Dissipation factor at 23 "C vs. time of cure at 40 "C plus a postcure at 100 "C.
Figure 3. T, vs. time of cure at 40 "C plus a postcure at 100 "C.
012
p 0
"O
2
t
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io
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@,roo O C Figure 4. Tgvs. time of cure at 40, 60, 80, and 100 "C. DAYS
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g
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8
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Figure 8. Insulation resistance at 23 "C vs. time of cure at 40 and 100 "C.
"C.
develops Tgvalues hetween 93 and 103 "C. Step curing a t 40,60,80, and 100 "C increases the Tgin steps to values approximating 60, 80, 90, and 100-110 "C. Electrical Properties. Dissipation factors measured a t room temperature are shown in Figures 5, 6, and I . Again, as in the preliminary study, the dissipation factors at 23 "C increase with additional cure. In cures at 40 "C the dissipation factor increases 23% as cure time increases from 3 to 28 days. When the 40 "C cure is supplemented by additional curing at 100 "C, the dissipation factor increases by as much as 130%. When the 40 "C cure is supplemented by a 1-day cure a t 60 "C, the dissipation
factor increases by 39%. One day at 80 "C increases it by 90% over the 40 "C value, and one day at 100 "C increases it by 114% over the 40 "C value. For specimens cured solely at 100 "C the dissipation factor increases by more than 70% between 1 and 7 days. Thus, for all four cure schedules, one cannot use minimum dissipation factor as an indicator of optimum cure. The electrical resistance at 100-V dc, measured at 23 "C through the 0.080-in. thick specimens, increases during the first and second days and becomes greater than l O I 5 Q after 3 or 4 days for all four cure schedules. Figure 8 presents the data for the 40 and 100 "C cures. Thus, electrical resistance at 100 V as measured on equipment capable of
322 Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 WVELENGTH (pmj
WAVELcYGTU ( p m 25
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measuring up to Q is not discriminatory between different degrees of cure, except at a low degree of cure. Infrared. From Figure 9, the infrared transmittance spectrum of D.E.R. 332, we see that it is essentially monomeric epoxy; i.e., few, if any, higher molecular weight fractions are present, as hydroxyl groups are not evident in the vicinity of 2.9 pm. The epoxy band followed during cure is at 10.95 pm. Figure 10 shows the infrared transmittance spectrum of diethanolamine. Infrared scans were obtained on the diethanolamine-epoxy adduct, 80:20 ratio by weight, 35 min after mixing, 3 h after mixing, and subsequent to heating 2.5 h at 75 "C. As previously mentioned, 100 g of this in an 8-oz paper cup at a room temperature of 24 "C generates about 16 "C exotherm in about 31 min. Most likely the amino hydrogen of the diethanolamine reacts at the epoxy ring in the epoxy resin, opens it, and forms a hydroxy group on one carbon atom of the epoxy group. The increase in hydroxyls and the decrease in amino hydrogens is not evident in this series of infrared spectra because they appear in the same region of the spectrum. A high concentration of hydroxyls is present in the hardener (32%) and in the adduct (25%). Figure 11 shows the infrared transmittance spectrum of the uncured epoxy-diethanolamine compound at the top and of the 1-day 100 "C cure specimen at the bottom. Figure 12 shows the 9.05- and 10.95-pm regions of the two scans in detail, and we can see that the transmittance at 10.95 pm has changed considerably after the 1-day cure at 100 "C. We have converted the transmittance values into absorbance by using the following relationships T = 1/10 A = - log T = log 1 / T = log 1 0 / 1 A = log Io - log I where T = transmittance, I = intensity of radiation transmitted, Io = intensity of radiation at the base line (determined here by the tangent line method), and A = absorbance. The ratios of the absorbance values at 10.95 pm to the absorbance values at 6.32 pm are plotted in Figure 13 for the uncured material and for the I-, 2-, and 3-day 100 "C cures. Dividing by the phenyl absorbance at 6.32 pm corrects for thickness variations as proposed by Dannenberg and Harp (1956). Figure 14 shows the 9.05- and 10.95-pm regions of the transmittance spectrum for the uncured and for the 3-day
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at 40 "C cured specimen. It is readily apparent that the intensity of the 10.95 pm band for the cured epoxy is less than that for the uncured epoxy. The absorbance values were calculated from the transmittance values, and Figure 15 is a plot of the absorbance ratios at 10.95 pm to those at 6.32 pm vs. cure time for the 40 "C cures. We can see that in 3 days at 40 "C the absorbance ratio levels out and remains constant for at least 6 weeks. Figure 16 shows the absorbance ratios at 10.95 pm to those a t 6.32 pm for the 3-day 40 "C cures postcured at 100 "C. The absorbance ratio decreased uniformly the first two days and then leveled out through the third day, at which time the specimen was placed in a 100 " C oven and
Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No. 3, 1980 323
0
1 DAYS
@
" 2 3 iOOaC
Figure 15. Infrared absorbance ratio at 10.95 pm/6.32 pm vs. time of cure at 40 "C.
Figure 13. Infrared absorbance ratio at 10.95 pm/6.32 um vs. time of cure at 100 "C. E
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the absorbance ratio decreased to a negligible value within 1 day. A significant infrared change occurred at 9.05 pm, a band associated with allphatic ether according to Lee and Neville (1967, Chapter 6, pp 9 and 10) and Bellamy (1975), and resulted from an increase in aliphatic ether. Figures 17 and 18 show the absorbance ratios a t 9.05 pm to those a t 6.32 pm vs. cure time for the 100 "C cures, the 40 "C cures, and the 40 "C cures postcured at 100 "C. The two cures a t 40 "C check each other very closely, and when the 40 "C cure is postcured a t 100 "C, it very closely checks the speciment cured solely at 100 "C. The aliphatic ether content levels out in 2-3 days at 40 "C and remains constant for a t least 6 weeks. The aliphatic ether content
0 1 2 3 4
PAYS Figure 17. Infrared absorbance ratio at 9.05 pm/6.32 pm vs. time of cure at 40 and 100 "C.
levels out in 1-2 days a t 100 "C, and within 1 day when the 40 "C cure is postcured a t 100 "C. Aliphatic ether increases slightly on prolonged heating a t 100 "C. The absorbance ratios of the hydroxyl band at approximately 2.95 pm to the phenyl band at 6.32 pm for the 100 "C cures, the 40 "C cures, and the 40 "C cures postcured at 100 "C are shown in Figures 19 and 20. As with the
324
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
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Figure 22. Linear shrinkage vs. time of cure at 40 "C plus a postcure at 100 "C, and time of cure solely at 100 "C.
CAYS @4CPC\D
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Figure 20. Infrared absorbance ratio at 2.95 pm/6.32 pm vs. time of cure at 40 "C plus a postcure at 100 "C.
9.05-pm ether band, the two cures at 40 "C check each other very closely, and when the 40 "C cure is postcured at 100 "C, it very closely checks the specimen cured solely a t 100 "C. The 2.95 pm/6.32 pm absorbance ratios drop about 20% for the two 40 "C cures, and both level out at a ratio value of ca. 0.9. The 40 "C specimen during postcure at 100 "C increases from the 40 "C cure value and levels out at about the uncured value, indicating a restoration of the hydroxyl content. Likewise, the hydroxyl content of the specimen cured solely at 40 "C increases between 42 and 105 days to about its original value. The hydroxyl content, like the aliphatic ether content, increases modestly on prolonged heating a t 100 "C.
Hardness. Durometer D hardness was measured to see whether it is sensitive to changes in degree of cure, but, as can be seen in Figure 21, the hardness measured after a 3-day cure at 40 "C remained constant through 24 days of curing a t 40 "C and even during a postcure of 7 days at 100 O C . Thus, hardness a t room temperature is not sensitive to degree of cure once the epoxy has gelled and developed its initial cure. It is possible that hardness measurements at elevated temperature might be more sensitive to degree of cure. Linear Shrinkage. The linear shrinkage per ASTM D 2566 was measured on semicircular cross-section bar specimens cast in a mold 13/4in. in diameter and 10 in. long. Figure 22 shows the linear shrinkage of a bar cured for 3 days at 40 "C and postcured at 100 "C vs. the linear shrinkage of a bar cured at 100 "C. The bar cured 3 days at 40 "C shrinks less than 1 mil/in. and is sufficiently cross-linked so that on postcuring at 100 "C it does not show additional shrinkage. The bar cured at 100 "C shrinks ca. 8 mils/in. when cured 1 day and shows no additional shrinkage when cured 2,3,4, and 7 days at 100 "C. The low degree of shrinkage obtained for the 40 "C cures may explain the 10-fold advantage in corona extinction voltage obtained when transformers are cured at or near
Ind. Eng. Cham. Prod. Res. Dev., Vol. 19, No. 3, 1980 325 Table I. Epoxy Reactivity during Various Stages of Cure A cure % epoxy groups reacted during
43 mixing and prior t o heat curing % epoxy groups reacted during 40" heat curing 3 days at 40 C % epoxy groups reacted during heat curing a t 100 C Above values based on following data:
B cure
C cure
49
43 33b
51
24
absorbance ratio a t 10.95 pmc
D.E.R.332 epoxy resin A uncured B uncured C uncured A cured 3 days a t 40 C B cured 3 days at 100 C C cured 3 days a t 40 C C cured 3 days a t 40 'C, postcured 3 days a t 100 " C a
1.76 1.01 0.89 1.00 0.31 0 0.42 0
17% of original epoxy groups remain unreacted.
b 24% of original epoxy groups remain unreacted.
Absorbance a t 10.95 pm divided by absorbance a t 6.32 pm.
room temperature compared to those cured at 100 "C. Discussion T h e infrared study indicates that in 3 days at 40 "C about two-thirds of the epoxy groups present at the beginning of the heat cure react, and for the next 6 weeks a t 40 "C none of the remaining epoxy groups react. Essentially all of the epoxy groups present at the beginning of the heat cure react within 1 day at 100 "C. When the 3-day 40 "C cure is postcured 1 day at 100 "C, the free epoxy groups essentially all react. As the epoxy groups are consumed, aliphatic ether groups are produced. At 40 "C, the hydroxyl content decreases about 20%; postcuring at 100 "C restores it to its original value. The epoxy-diethanolamine compounds were labeled "uncured" at the time the first infrared spectra were obtained, which was within several hours of mixing the adduct and the epoxy resin together. But we know that reaction takes place when the adduct is prepared as exotherm develops and cloudiness clears during exposure at 75 "C. Also, when the adduct is mixed with additional epoxy resin to form the final compound, further reaction occurs between diethanolamine and epoxy resin and again cloudiness clears during heating at 75 "C. Epoxy groups decrease during the preparation of the adduct and the final compound as evidenced by the absorbance ratio at 10.95 pm to that a t 6.32 p m dropping from 1.76 for the epoxy resin itself to 1.01 for the mixed "uncured" infrared specimen prepared for subsequent cure at 40 "C, to 0.89 for the mixed "uncured" infrared specimen prepared for subsequent cure at 100 "C, and to 1.00 for the mixed "uncured" infrared specimen prepared for subsequent cure at 40 "C followed by a postcure at 100 "C. Thus, 40-50% of the epoxy groups present in the epoxy resin react during the preparation of the adduct and the final compound and before the beginning of the heat cures. Table I shows epoxy reactivity during various stages of cure. If we were to provide one amino hydrogen for each epoxy group, we would need 60 phr (parts per hundred of resin) of diethanolamine based on equivalent weights of 175 for the epoxy and 105 for the amine. We have used only 11.65 phr, as the mechanical properties decrease when more than 14 phr are used, according to Quant (1977) and Lee and Neville (1967, Chapter 9, p 13). The 11.65phr of hardener
contain 11.65160 or 19.490 of the amino hydrogens required for reaction with all of the epoxy groups. If the hydrogens of the hydroxyls present in the hardener react with epoxy groups on a one-to-one basis, 2 X 19.4 or 38.8% of the remaining epoxy groups will react. Thus, the amine and two hydroxyl groups, if fully reactive, will react with 3 X 19.4 or 5890 of the epoxy groups. All of the epoxy groups present at the beginning of the heat cures react in the cures at 100 "C, in the 40 "C cures during the postcure at 100 "C, and presumably in the 40 "C cures during postcure a t 60, 80, and 100 "C. Since insufficient amino and hydroxyl hydrogens are present to react with all of the epoxy groups, homopolymerization most likely occurs under the catalytic influence of the tertiary amines formed when diethanolamine, a secondary amine, reacts with epoxy groups, as reported by Lee and Neville (1967, Chapter 5 , pp 4 and 9; Chapter 9, pp 2,9, and 14). Also, at 40 "C the hydroxyl content decreases about 20% followed by a return to its original value during additional curing at 40 " C as well as during postcuring at 100 "C. A decrease in hydroxyl groups followed by an increase has been reported by Lee and Neville (1967, Chapter 6, p 10) for tertiary amine cures. Aliphatic ether groups form at 40 and 100 "C and should form when hydroxyl hydrogens react with epoxy groups and during homopolymerization. At 40 "C additional groups react between 6 and 15 weeks, and the Tgand dissipation values increase. Also, within 14 or 15 weeks the aliphatic ether absorbance ratios of all cures increase about 10% and the hydroxyl absorbance ratios increase by 10% or more. The ether and hydroxyl changes a t 15 weeks for the 40 "C cure are probably related to the decrease in the epoxy group at 15 weeks. Conclusions It is apparent from measurements of glass transition temperature, dissipation factor, and infrared absorbances at 10.95, 9.05, and 2.95 pm that epoxy resin containing 11.65 phr of diethanolamine does not fully cure at 40 "C within 100 days. Also apparent is that some of these measurements are changing; i.e., cure is still progressing after this long exposure a t 40 "C. Postcuring the 40 "C cure at 60,80, and/or 100 "C brings about additional cure, and all of the properties reach relatively level values at 100 "C, indicating that a full cure is obtained at this temperature. The glass transition temperature of 3-day cures a t 40 "C is about 60 "C, and, when fully cured at 100 "C, it increases to about 100 "C. The dissipation factor at lo3Hz is at a minimum as soon as the epoxy is gelled sufficiently so that it is not readily indentable, i.e., in ca. 1-2 days at 40 "C, and increases in value with increased cure so that it doubles on postcuring at 100 "C. Insulation resistance increases during the first 3 days of cure and then exceeds the measuring capability of the equipment so that this test is not satisfactory for determining the completeness of cure. Likewise, hardness at room temperature is unsatisfactory for determining degree of cure as it increases during the first 3 days a t 40 "C and then remains constant even during postcuring a t 100 "C. Infrared absorbance at the 10.95-pm epoxy band indicates that about 17% of the original epoxy groups are unreacted during exposure at 40 "C for as long as 6 weeks and that cure is not complete in 15 weeks. Most of the epoxy groups are reacted within 1 day of curing at 100 "C. Aliphatic ether groups are produced as epoxy groups react. Hydroxyl groups are reduced about 20% during cure at
326
Ind. Eng. Chern. Prod. Res. Dev. 1980, 79, 326-329
40 "C and increase to their original content during postcure at 100 "C. Curing for 3 days at 40 "C results in a linear shrinkage of less than 1mil/in., and this shrinkage does not increase during postcuring at 100 "C even though 17-24% of the original epoxy groups react during the 100 "C postcure. When the liquid epoxy compound is placed in a 100 "C oven and cured for 1 day a t this temperature, it shrinks 8 mil/in. and no additional shrinkage occurs during 7 days at 100 "C. Acknowledgment The author is grateful to E. W. Anderson and G. E. Johnson for assistance in performing the electrical tests, to L. T. Pappalardo for advice and help in preparing the infrared specimens and in operating the spectrometer, and
to J. P. Luongo and Mrs. G. A. Pasteur for valuable advice concerning interpretation and presentation of the infrared data.
Literature C i t e d Bellamy, L. J., "The Infrared Spectra of Complex Molecules", 3rd. ed.,Halsted Press, New York, 1975, p 131. Dannenberg, H., Harp, W. R., Jr., Anal. Chem., 28, 86-90 (1956). Del Monte, J., J. App!. Polm. Sd., 2 , 106-13 (1959). Lee, H., Neville, K., Handbook of Epoxy Resins", McG-aw-Hill, New York, 1967. Quant, A., Sandia Corp., Alberquerque, NM, private communication, 1977.
Received for reuiew March 10, 1980 Accepted April 9, 1980 Presented at the 178th National Meeting of the American Chemical Society, Washington, D.C.,Sept 1979.
Polymeric Coatings Effect on Surface Activity and Mechanical Behavior of High Explosives John K. Bower, John R. Kolb, and Cesar 0. Pruneda' Organic Materials Division, Lawrence Livermore National Laboratory, Livermore, California 94550
There is a continuing effort at Lawrence Livermore National Laboratory to formulate new plastic-bonded explosives (PBX) with enhanced energy and improved safety characteristics. We have found that these parameters depend mainly upon the polymer or plastic chosen as binder. Typically, these materials have 5-10 vol % polymer and the explosive may be treated as a solid coated with a polymeric film. In an effort to determine which properties of a polymer are advantageous here, we have examined in depth three polymers (Kel-F 800, Phenoxy PKHJ, and Kraton G 1650)and an insensitive explosive (1,3,5-triamino-2,4,6-trinitrobenzene (TATB)). We determined mechanical and rheological properties, surface characteristics, and acid-base behavior of the pure materials and composites (PBX). It was found that extensibility, glass transition temperature, and Lewis acidity surface wettability of the components of a PBX are governing factors in safety characteristics.
Introduction Chemical high explosives (HE) have a continuing and multitudinous role in industrial applications and in the Department of Energy (DOE) complex. Historically, the most common base HEs for DOE applications have been cyclic nitramines denoted as RDX and HMX. As one
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might imagine, these explosives are subject to a number of problems related to their sensitivity and stored energy, Le., shock sensitivity and relative chemical instability. These materials, replete with both physical and chemical problems, render it incumbent upon the user to adopt means of stabilizing the materials in order to obtain the maximum safe, useful work from them. For roughly 35 years, the most common behavior modifier has been a polymer (with or without a concomitant plasticizer) which acts as a binder for the individual HE particles. The main advantages of the binders are: (1)desensitization of the HE to unwanted external stimuli, (2) enhancement of the 0196-4321/80/1219-0326$01 .OO/O
maximum possible energy which may be extracted from the explosive, and (3) the imparting of structural integrity to the PBX composite. Some of the polymers which are utilized as H E binders are fluorocarbon, urethane, and silicone polymers. It was recognized, previously, that for the sensitive RDX and HMX compositions, it was necessary to use relatively "soft" (low modulus), rubbery polymers to minimize shock sensitivity because high modulus binders yield stiff, crack-susceptible, highly sensitive composites. To incorporate these binders it was necessary to exclude enhancement of energy and the ability to design structural parts from these Plastic Bonded explosives (PBX) due to their compliant makeup. Recently, however, intense interest has arisen in two chemicals that were initially synthesized long ago, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and nitroguanidine (NQ). These materials are
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possessed of extreme insensitivity and have spawned a new generation of PBXs of greater relative safety char0 1980 American Chemical Society
