Ind. Eng. Chem. Prod. Res. Dev. 1982, 27, 290-296
290
National Fertilizer Solutions Association. "Liquid Fertilizer Manual"; NFSA: Peoria, IL, 1980 Chapter 27. Newsom, W. S. U.S.Patent 3000 170, 1963. Sawyer, E. W. Solutbns 1878, No. 3 . Sawyer, E. W. "Tank Mix Carrbr Systems". presented at ASTM Symposium on Tank Mix Applications, Philadelphia, PA, Nov 1980. Silverberg, J.; Dixon, A. J. "Lime Suspensions"; TVA, Muscle Shoals, AL, 1969. Sparks, R. W.; Sawyer, E. W. "Utilization of Attapuigite as a Stabilizing, Thickening, and Suspending Agent in Liquid Feeds"; presented at American Feed Manufacturers Association, 5th Annual Symposium, Omaha, NE, 1975. Sparks, R. W.; Sawyer, E. W. "Attapulgite-Stabilized Liquid Animal Feed Supplements Containing Fat Emulsions"; presented at American Feed Manu-
facturers Association 7th Annual Symposium, St. Louis, MO, 1977. Trask, 0. J. Solutions 1976, No. 3 . Wasp, E. J. Trans. Tech. Pub/. 1977, l(4). Wolford, J. R.;Sawyer, E. W. Solutions 1977, No. 6 . Wolford, J. R.; Sawyer, E. W. "The Production and Use of Liquid Clay"; Fioridin Company, Norcross, GA, 1981.
Received for reuiew September 30, 1981 Accepted January 4, 1982
Presented at the 182nd National Meeting of the American Chemical Society, Fertilizer and Soil Chemistry Division, New York, NY, Aug 1981.
Structure-Property Relationships in Neat and Reinforced Epoxy Resins Exposed to Aggressive Environment Jovan MlJovl6 Polytechnic Institute of New York, Department of Chemical Engineering, Brooklyn, New York 1 120 1
Several neat and reinforced epoxy resin formulations were prepared and investigated. Solid glass microspheres, with and without coupling agent, were used as reinforcement. All samples were exposed to an aggressive environment by immersion in acetone for various lengths of time. Dynamic mechanical and fracture measurements were used to evaluate the effect of acetone on mechanical properties of different formulations. Electron microscopic evidence was obtained for the existence of inhomogeneous morphology in all cured systems. Acetone-induced changes in dynamic mechanical parameters have been described in terms of the model of inhomogeneous thermoset morphology.
Introduction At the present time there exists a steadily increasing interest in epoxy resins due to their extensive use as structural adhesives and matrix material in high performance composites. In actual service, such materials are always used in an aggressive environment, and it is therefore of primary concern to evaluate their environmental resistance. In the case of epoxy resins, one is primarily concerned with the effect of moisture on their mechanical properties. Particularly within the past five years, a number of reports appeared in the literature describing the effect of various hygrothermal treatments on neat and reinforced epoxies. In all reinforced systems the effect of water on the properties of matrix-reinforcement interphase is of utmost importance. Consequently, many high resolution (mostly spectroscopic) techniques have been used to reveal the nature of interactions within the interphase, and an excellent review of the state of the art of this subject has been written by Ishida and Koenig (1978). Changes in mechanical properties of various epoxy formulations upon exposure to moisture have been studied t y p i d y by comparing dry and wet flexural, tensile, and/or fracture characteristics (Gledhill and Kinloch, 1974; Mostovoy and Ripling, 1976; DeIasi and Whiteside, 1978; Moy and Karasz, 1980; Peyser and Bascom, 1981). A number of analytical and experimental investigations of sorption and desorption of moisture in composite materials have been compiled in a recently published monograph (Springer, 1981). However, in spite of the existing publications, there is no report in the literature in which a correlation has been established between the mechanism of action of an ag0196-4321/82/1221-0290$01.25/0
gressive environment and the epoxy resin morphology. The latter has only recently become a subject of many investigations which have shown that the morphological model of cured epoxies is best described by the regions of higher cross-link density (nodules), immersed in a lower cross-link density matrix. Therefore, our primary objective was to correlate aggressive environment induced changes in mechanical properties of neat and reinforced epoxies to their morphology. Acetone was chosen as the aggressive environment in this study. Although polar, like water, acetone is known to display more rapid attack on cured epoxies. Thus, in essence, the use of acetone is equivalent to an accelerated test, which is the basic tool of analyses for the prediction of long-term performance of all materials. The effect of immersion in acetone on mechanical properties was evaluated by nondestructive (dynamic mechanical) and destructive (fracture) measurements. Dynamic mechanical analysis offers a distinct advantage over other mechanical property tests, for it provides the most sensitive response to morphological inhomogeneities and various physical and chemical transitions in polymers over a wide temperature range. Ultimate mechanical properties have been determined from linear elastic fracture mechanics (LEFM) analysis (Irwin, 1958). A critical value of the strain energy release rate (GIJ, at which a preexisting crack extends in the cleavage mode (mode I), was calculated as described elsewhere (Ripling et al., 1970; MijoviE, 1980). Experimental Section Chemical Systems. Epon 825, Shell's liquid diglycidyl ether of bisphenol A (DGEBA) resin was cured with di0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table
291
I. Chemical Structure of Epoxy Resin and Curing Agent
a. typical diglycidyl ether o f bisphenol A (DGEBA) resina H,N-CH, -CH ,-NH-CH, -CH, -NH, b. diethylene traimineb (DETA) curing agent Epon 825, Shell's liquid DGEBA resin used in this study, is a purified form of commercially available Epon 826. DETA was supplied by Aldrich Chemical Co. Table 11. Various Formulations Studied formula tion or system no.
composition
type of surface treatment on reinforcement
cure schedule 24 h a t R T t 4 h a t 1 2 8 ° C
1
Epon 825 t 11 phr DETA
2
Epon 825 t 11 phr DETA t 2 5 phr (by wt) glass spheres no. 3000. average diameter 25 pm
none
same as 1
3
Epon 825 + 1 1 phr DETA + 25 phr (by wt) glass spheres no. 3000 CP-0 2
monomolecular layer of a silane type coupling agent recommended for use with epoxies
same as 1
Table 111. Procedure for Cleaning of Aluminum B e a m s Prior to the Application of Adhesive 1.
2.
3.
4.
5.
6.
solvent treatment a. ethyl alcohol-removes blueing b. perchloroethylene-removes grease, fingerprints, oil alkaline treatment a. "Oakite" no. 1 6 4 , 4 4 9 g in 6 0 0 0 mL of deionized a t 180 f 5 "F ( 8 2 i 3 "C) for 15 min followed by immediate water rinse ( 5 min) FPL etch a. 165 g NalCr20,, 895 mL H,SO,, 4940 mL distilled water a t 150 * 5 "F ( 6 2 i 3 "C) for 15 min followed by immediate water rinse ( 5 min) phosphoric acid anodize bath a. 6 0 0 mL H,PO, (85% ortho, s.g. = 1.436 g/cm3) in 5400 mL water a t 65-85 "F for 20-25 min a t an applied potential of 10 t 1 V, followed by a cold water rinse (within 2 min) for 10-15 min drying a. blow off the water (with air), dry the beams by hanging them up with stainless steel wire; prime within 3 h priming a. BR-127 corrosion inhibiting adhesive primer (a modified epoxy phenolic primer manufactured by American Cyanamid Co.); warm primer t o room temperature, mix thoroughly, continuously agitate during application; spray t o a primer thickness of 0.0001 to 0.0004 in.; good results have been obtained using a Devilbiss Spray Gun with Fluid Needle MBC-44F, Fluid Tip AV-15F and Air Cap no. 3 6 ; airline pressure of 4 0 psi is satisfactory; air dry for 30 min and oven cure for 6 0 min at 250 "F; clean the primed surfaces with acetone prior t o the application of the adhesive
ethylene triamine (DETA). The chemical structure of resin and curing agent is shown in Table I. Potters Ind. Inc. glass microspheres, used as reinforcement, are also described in Table 11. Composition and cure schedule of systems are described in Table 11. Cured samples were immersed in acetone, a t room temperature, for 0, 1, 2 , 3 , 7, 12, 16, 20, 30, and 60 days and were then tested. Techniques. Specimens for dynamic mechanical measurements were cast in silicone rubber molds. A Silastic E RTV rubber (Dow Corning) cured with 10 phr (parts per hundred parts of resin, by weight) of Silastic E curing agent was used for the preparation of molds. Weight changes as a function of immersion time were determined with an analytical balance (Mettler Instrument Corp.). Dynamic mechanical measurements were performed in DuPont 981 DMA connected to a 1090 thermal
analyzer. All tests were run a t the oscillation amplitude of 0.2 mm peak-to-peak and heating rate of 5 "C min-'. Tapered double cantilever beam (TDCB) specimens were used for fracture energy measurements. The exact methods of manufacturing TDC aluminum beams and beam dimensions are given in ASTM D 3433-75. Preparation of surfaces of TDCB speciments is described in Table 111. The subsequent application of the resin on the beams has been detailed elsewhere (Ripling et al. 1971). An Instron Tensile Tester was used for fracture measurements, at room temperature and a crosshead speed of 0.127 cm/min (0.05 in./min). One-stage carbon-platinum (C-Pt) replicas of various fracture surfaces were made and studied by transmission electron microscopy (TEM). A detailed description of the sample preparation method is given elesewhere (Mijovii:
292
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
2
1
I
"e" 1 -
-40
-20
0
20 W
40
60
80
Kx)
120
140 M
Tamprratura
5
ib
PC)
Figure 2. Loss modulus (E") as a function of temperature for untreated formulations 1, 2, and 3.
0 d 0
B
'b
000 O -SO -60
A 0
I5
2G
25
30
'(days1
Figure 1. Weight gain, based on resin weight, as a function of immersion time for all three formulations (A = 1; 0 =2; 0 = 3).
and Koutsky, 1979). A Philip EM 200 model transmission electron microscope was used to investigate the fracture surfaces.
Results and Discussion Optimum mechanical properties of a cured thermoset are achieved when its glass transition (T,) reaches the value of T , the highest obtainable value for a given formulation. Suca a thermoset network is then said to be fully cured. There are two molecular phenomena encountered during thermoset cure, gelation and vitrification, whose interrelation has been discussed by Gillham (1979). In the case of cross-linked thermosets, one must realize that as long as the temperature of cure (and/or post-cure) remains below T,,,vitrification will prevent completion of chemical reactions and such networks will not be fully cured. The T., of an incompletely cured network will depend on the highest temperature achieved during cure (and/or postcure), and hence, optimum mechanical properties will not be developed. Many a study of thermosets reported in literature has overlooked or simply left out this important fact. Consequently, in the fmt series of experiments, designed to establish a set of reference processing conditions, post-curing was done at various elevated temperatures, and the T ,of the system (as defined by the location of loss modufus peak in dynamic mechanical spectra) was found to be 5120 "C. In order to develop optimum mechanical properties in fully cured systems, in addition to post-curing a t T 2 Tg,(128 "C)one must determine the minimum post-cure time needed to reach T,,. The value of T, increased during the first 4 h of post-cure at 128 "C and then began to drop. Hence, post-cure time of 4 h and post-cure temperature of 128 "Cwere chosen as optimal processing Conditions and have been used throughout this study. The glass transition of each specimen will be hereafter defined by the location of loss modulus peak (E'? in the dynamic mechanical spectrum. The percentage weight gain, based on resin weight, as a function of length of exposure to acetone is plotted in Figure 1,for all three formulations. In each case, an abrupt initial weight gain is followed by a slow increase a t longer exposure times. At this juncture, it is instructive to offer a brief qualitative description of the molecular mechanism of penetration of small molecules into cross-linked thermosets and their composites. To elucidate the phenomena of diffusion and absorption of small molecules in ther-
moset-matrix composites, the inhomogeneous morphology of the thermosetting resin and the presence of an interphase must be considered. As a consequence of an inhomogeneous morphology, the rate of acetone penetration would be different in the regions of different cross-link density. On the other hand, albeit the thickness of an interphase is on the order of several thousand angstroms, its importance is paramount, particularly with respect to its response to an aggressive environment. Hence every study of diffusion in thermoset-matrix composites should address the questions of penetration and absorption of small molecules in (1) the inhomogeneous resin matrix and (2) the interphase. At this point, we revert to the discussion of our results. As seen in Figure 1,the most rapid weight gain is observed in formulation 2 (reinforced system without the coupling agent). Since the thermal history (curing schedule) of the epoxy resin is identical in all formulations and since the glass spheres are impermeable to acetone, it follows that the excess acetone must be accomodated within the glass-matrix interphase. Interestingly, the smallest amount of acetone is absorbed by formulation 3, indicating that, a t room temperature, the coupling agent decreases the affinity of interphase toward acetone, thereby reducing the overall weight gain. We next consider dynamic mechanical properties of all systems. Figure 2 shows loss modulus as a function of temperature for all three formulations prior to exposure to acetone. Whereas the neat resin has a single T peak (curve l),reinforced formulations are characterized by the appearance of two overlapping peaks in the Tg region (curves 2 and 3). Such behavior is believed to be a direct consequence of the existence of inhomogeneous morphology in cross-linked epoxy resins. Interestingly, though, dynamic mechanical measurements did not distinguish nodular regions from internodular matrix in the neat resin, but did so in reinforced formulations. Recently, it has been suggested that, within the matrix-reinforcementinterphase there exists a morphological gradient, i.e., variation in morphology as a function of distance from reinforcement extending over a length of several hundred angstroms. The idea of morphological gradients has been advanced by several authors (Cuthrell, 1968; Kardos, 1973; Racich, 1977), and evidence has been shown for the existence of smaller nodules within the interphase. Simultaneously, the size of nodules was shown to be inversely proportional to their cross-link density (MijoviE and Koutsky, 1979; MijoviE and Tsay 1981). Thus the presence of reinforcement may lead to the formation of smaller nodules, characterized by higher cross-link density, which contribute to an enhanced average cross-link density difference between nodular and internodular regions. Hence, it is
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 293
Table IV. Changes in T, and T,' of Various Formulations as a Function of Length of Exposure to Acetonea days in acetone formulation 0 1 2 3 4 7 12 16 20 30 1
Tg, 113
116
116
T, 104, 1 2 0
2
111,125
102,119
3
125
112
116
110 very weak
120
116 30
weak
119 114
115 30
112 32
116 34
111 34
111
122 38
122
127 49
118 48
129
127
128
shoulder
shoulder
125 28
121
very weak 121
128 32
127 32
126 50
120
very weak
60
30
41
All temperatures are in "C.
1
% 024
0.0ol -80
"
-60
-40
'
-20
'
0
'
'
20 40 Temperature
I
60
rC I
'
80
'
100
'
120
I
I40
0.251
0.001
J
Figure 3. Loes modulus (E'? as a function of temperature for formulation 1 upon immersion in acetone for 1, 7, 16, and 30 days.
possible that this difference is large enough to be detected by dynamic mechanical measurements, as shown in Figure 2. More specifically, each reinforced formulation is characterized by two peaks, which appear at 104 and 120 "C and 102 and 119 "C in formulations 2 and 3, respectively. The first peak a t (lower temperature) corresponds to the onset of molecular motion in the internodular matrix, and the higher peak to the onset of molecular motion in more highly cross-linked nodules. Several interesting observations have been made of variations in dynamic mechanical properties upon immersion in acetone. Changes in Tgfor all formulations as a function of length of exposure to acetone, are summarized in Table IV. In each case, a slight initial increase in Tg is followed by an almost insignificant decrease at longer immersion time. After a 4-day exposure to acetone, an indication of a very weak shoulder has been detected in the neat resin. The onset of this shoulder appeared at approximately 20 "C. As the exposure time was increased, a distinct peak appeared, whose location shifted to higher temperature, as shown in Figure 3 and Table IV. In reinforced formulations, the two original peaks (correspondingto the Tis of matrix and nodules) observed in untreated specimens, merge after a 2-day exposure to acetone. Analogously to changes observed with the neat resin, after 4 days in acetone another peak appears: first as a weak shoulder, which with further exposure progressively transforms into a more prominent peak. The development of this peak as a function of length of immersion in acetone is shown in Figures 4 and 5 for formulations 2 and 3, respectively. If acetone were to plasticize the resin immediately, one would intuitively expect its preferential absorption within the matrix and consequently widening of the gap between the glass transition of plasticized matrix (hereafter referred to as 5";) and the glass transition of, as yet unaffected, more highly cross-linked nodules. Such behavior, however, has not been observed. Instead, in reinforced systems
-80
'
-60
I
'
-40 -20
'
I
0
20
1
40
1
60
'
80
1
100
1
120
'
140
Temperature ('CI
Figure 4. Loss modulus (E'? as a function of temperature for formulation 2, upon immersion in acetone for 3, 7, 12, and 30 days.
-g
0.24
0
-
0.20
n
z
0.16
3 0.12
A e
0.08
0.04
t
0.ool
-BO
'\ '
1
'
-60 -40 -20
1
0
'
20
I
40
'
60
I
I
80
100
'
120
I
140
TemperOtUN CCI
Figure 5. Loss modulus (E'? as a function of temperature for formulation 3, upon immersion in acetone for 2, 7, 16, and 20 days.
(formulations 2 and 3), the cross-link density difference between the two morphological regions becomes smaller upon short immersion times. It is possible that the initially absorbed acetone may relieve some residual stresses and increase the local mobility of few unreacted groups (even though T,, 2 T,,), thus actually increasing the Tgof the matrix. Furthermore, acetone may fill voids introduced during sample preparation, in the bulk or along the interface, or may diffuse to locally large free volume spaces within the resin network. Hence, during short immersion times, the primary role of acetone is not that of a true plasticizer which occupies space between polymeric chains and facilitates their movement against one another. This initial sorption of acetone is relatively fast and is described by the rapid weight gain in all formulations as shown in Figure 1. The true plasticizing action of acetone occurs next. Molecules of acetone diffuse into the three-dimensional resin network and penetrate preferentially the less highly cross-linked internodular matrix. It is possible that simultaneous limited degradation of the network also takes
294
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
4 t
n A
IO
I
4t
60
3
n
A
20
30 t
40
50
(days)
Figure 6. Room temperature Young's modulus (EkT)as a function of immersion time for formulation 1.
place. The onset of the process of matrix plasticization is detected by weight gain measurements after approximately 4 days, and it occurs more slowly than the initial sorption, as is clearly seen in Figure 1. After a 4-day exposure to acetone, its plasticizing effect is also detected by dynamic mechanical measurements, as judged by the appearance of a T i (the glass transition of plasticized matrix) peak which gains in intensity at longer immersion times (Figures 3-5). This is due to the fact that the penetration of acetone continues slowly into nodular regions which, even when partly plasticized, are of higher cross-link density, thus contributing to an increase in the average T i of the plasticized regions. The glass transition of the parts of the nodules still unaffected by acetone penetration remains almost unchanged, as shown in Figures 3-5; however, its intensity (the intensity of E" peak) decreases significantly as the exposure time is increased. This decrease, also seen in Figures 3-5, is caused by the slow penetration of acetone into nodules thereby leaving smaller and smaller amounts of unaffeded highly cross-linked sites in the specimen. By comparing the results shown in Figure 1and Table IV, one clearly sees that the formulation with the largest acetone pickup does not have the lowest TB' An explanation of this result is again offered in terms of the composite morphology. In all formulations, the changes in Tgand T; are caused by the penetration and absorption of acetone within the resin matrix. The total weight gain, however, also takes into account the additional (excess) acetone absorbed within the interphase. The Tgand Tgl of each formulation depend only on the amount of acetone absorbed within the resin. The asymptotic value of Tgfor each formulation is determined by the equilibrium acetone level absorbed within the bulk resin. In addition to Tgand T i , a low temperature relaxation, commonly referred to as @ transition, has been observed in all dynamic mechanical spectra a t around -40 "C. This @ relaxation is believed to represent the onset of the crankshaft motion of the glyceryl group. Its location did not vary as a function of exposure time but its magnitude did. Broadening and decrease in intensity of /3 relaxation at longer immersion times probably indicate a decrease in the total number of glyceryl groups due to some chain degradation. Simultaneously with shifts in Tg,we have observed the changes in storage modulus as a function of length of exposure to acetone. In Figure 6, the room temperature values of storage modulus (ELT)of formulation 1 are plotted as a function of immersion time. Only slight fluctuations in E $T were observed for the neat resin, as well as for the reinforced formulations. EkT is not the most useful indicator of changes in the system on the molecular level due to the fact that at room temperature, for all immersion times, all systems are below the lowest
1
0
2
0
3
0
4
0
5
0
t (days1
Figure 7. Young's modulus at 50 "C (E'%)as a function of immersion time for formulation 1.
.pi
IO
3
20 ?
40
50
1
(days1
Figure 8. Young's modulus at 50 "C (E'%)as a function of immersion time for formulation 2.
IO
20
30
40
50
t (days1
Figure 9. Young's modulus at 50 "C (E'm) as a function of immersion time for formulation 3.
observed T i (the glass transition of the plasticized matrix) and therefore in the glassy state. A more prominent change in stiffness occurs above the Ti,and hence it is crucial to evaluate mechanical properties at a temperature at least equal to Tl. Consequently, in Figures 7-9, we have shown plots of storage modulus at 50 "C ( E b )as a function of immersion time. For each formulation, the change in modulus (E'M)follows two different patterns; an initially abrupt drop is followed by a rather smooth decrease. Experimentally obtained points could be approximated by either an exponential function or two straigh lines of different slopes. In either case, however, there is a distinct change in pattern after approximately 5 days of exposure to aggressive environment. This coincides with the time a t which the rate of weight gain changes (Figure l),indicating how the initial absorption of acetone has the most drastic effect on elastic modulus. Almost all fracture specimens exhibited an apparent cohesive [center of bond (COB)]failure. Also, an unstable
ind. Eng. Cham. prod. Res. Dev., VoI. 21. No. 2. 1982 295
0
IW
t 20
10
40
33
60
Y)
ilmrr,
Figure 10. GI, a8 a function of immersion time for formulation 1.
I O
i
0 ~
O
x
I
y
i
4
0
Y
I
Ib
o
Figure 11. G I , an a function of immersion time for formulation 2.
Figure 13. Transmission electron micrograph of B one atage C-Pt replica of fracture surface of nest epoxy (formulation 1) prior to immersion in acetone. Note characteristic nodular morphology.
crack propagation, characterized by the "saw-toothed" appearance in load-displacement diagrams, was observed in all cases. Occasionally,a crack was found to propagate (over a short distance) along the resin-metal interface but such points were not considered for the calculations of strain energy release rate. Figures l(t12 show the critical strain energy release rate for crack initiation G (), as a function of immersion time, for formulations 1, 2 and 3, respectively. In spite of the certain amount of data scatter, all three curves follow a similar pattern. In each case, GI, reaches a maximum value after approximately 2 days, followed by a sudden drop and finally leveling off. Although fracture testa at room temperature (RT < Ti for all formulations) are not the most sensitive indicator of acetone induced changes on the molecular level, the initial trend displayed by GI,, at least qualitatively, corresponds to that observed in dynamic mechanical and absorption measurements. An initial diffusion of acetone into voids introduced during processing or into locally large free volume spaces within the network would lead to reduction of potentially high stress concentration sites, thereby increasing the GI, volume. The crack propagation path was observed to migrate between the aluminum adherends, although the fracture remained almost invariably within the adhesive. Each initiation occurred in a plane different from that in which the crack had arrested. Similar experimental results have been also reported by other researchers (Wang et al., 1976; Mijovii., 1980). A recent analytical study of the effect of crack elevation in TDCB adhesive test configuration
showed that the crack growth angle increases with respect to ita original plane, as the crack approaches the adhesive-adherend interface (Wang et al., 1976). Finally, a transmission electron microscope (TEM) was used to obtain TEM photomicrographs of fracture surfaces of untreated samples. Each crack jump was characterized by the presence of three distinct zones on the fracture surface. The initiation and arrest zones were typified by the presence of ridges which are the consequence of local plastic flow. Nevertheless, those zones were confined to a small length in comparison to the propagation zone, which appeared smooth to the naked eye but showed a characteristic nodular morphology under the microscope. One such TEM photomicrograph is shown in Figure 13. Fracture proceeds around nodules, indicating that the nodules are indeed the sites of higher cross-link density in the system. Hence, the morphological model (composed of higher cross-link density nodules immersed in a lower cross-link density matrix) which was used to explain acetone-induced changes in dynamic mechanical properties of cross-linked epoxies, is corroborated by the TEM study. A chronological development of the entire concept of inhomogeneous thermoset morphology has been considered elsewhere (MijoviE and Koutsky, 1970). TEM Investigation of fracture surfaces of samples exposed to acetone is currently continuing in our laboratories. Conclusions The effect of immersion in acetone on dynamic mechanical and fracture properties of various epoxy resin formulations was evaluated. After 4 days of exposure to
,11",,
206
Ind. Eng. Chem. Prod. Res. Dev. 1082, 21, 296-299
acetone, the glass transition of the plasticized resin ( T i ) , became detectable in dynamic mechanical spectra. Upon further exposure, the intensity of T i peak increased, while the intensity of the glass transition of fully cured resin (TgJdecreased simultaneously. The Tgand T i were found to be determined by the amount of acetone absorbed in the resin, while the total weight gain also includes the excess acetone absorbed within the interphase. An explanation for acetone-induced changes in dynamic mechanical parameters was offered in terms of an inhomogeneous thermoset morphology. Electron microscopic study corroborated the morphological model of higher cross-link density nodules immersed in a lower cross-link density matrix. Changes in storage modulus were found to follow two distinctly different patterns as a function of length of exposure to acetone. Such measurements are particularly important a t temperatures above T ’. In general, environmentally induced changes in meckanical properties of composites, as determined a t room temperature (RT), do not provide a complete picture of the effect of aggressive environment, if RT < T i . Above Ti,the difference in the response of nodules and internodular matrix to various external forces becomes more prominent, resulting in a large variation of properties within the same specimen. Hence, in order to establish processing-structure-property-durability relationships in reinforced thermosets, one must elucidate the resin morphology.
Acknowledgment The author is grateful to Mr. L.-Y. C. Lin, who performed most of the experimental work, and Mr. R. Tong (American Cyanamid), who supplied the primer. Partial support of this research, provided by the Engineering Foundation, is gratefully acknowledged. Literature Cited Cuthrell, R. E. J. Appl. Polym. Sci. 1988, 12, 955. DeIasl, R.; Whlteside, J. B. I n “Advanced Composite MaterialsEnvironmental Effects”, ASTM STP 658; Vlnson, J. R., Ed., ASTM: 1978; pp 2-20. (3lliham, J. K. Polym. f n g . Scl. 1979, 19, 676. Gledhill, R. A.; Kinloch, A. J. J. Adhesbn 1974, 6 . 315. Irwln, G. R. “Handbuch der Physk“; Springer: Berlin, 1958; Vol. 6. Ishlda, H.; Koenlg, J. L. Polym. Eng. Scl. 1978, 18, 128. Kardos, J. L. Trans. N . Y . Aced. Sci. I I 1073, 35, 136. MIJoviE,J.; Koutsky, J. A. J. Appl. folym. Scl. 1979, 2 3 , 1037. MljovlE. J.; Koutsky, J. A. Polymer 1979, 20, 1095. MIJovIE, J. J. Appl. Pdym. Sei. 1980, 2 5 , 1179. MIJovlE. J.; Tsay. L. Polymer 1981, 22, 902. Mostovoy, S.;Borsch. C. R.; Rlpling, E. J. J. Adhesion 1971, 3 , 125. Moatovoy, S.; Ripllng, E. J. In “Adheslon Science and Technology”; Lee, L. H., Ed.; Plenum: New Ywk, 1976 Voi. 98. Moy, P.; Karasz, F. E. Pdym. Eng. Sci. 1980, 2 0 , 315. Peyser, P.; Besmm, W. D. J. Mater. Sci. 1981, 16, 75. Raclch, J. L. Ph.D. Thesis, Unhrerstty of Wlsconsln: Madlson. WI, 1977. Rlpling, E. J.; Mostovoy, S.; Corten, H. T. J. Adhesion 1971, 3 . 107. Sprlnger, 0. S., Ed. “Environmental Effects on Composfie Materiels”; Techmmlc Publ. Comp. Inc.: Westport, 1981. Wang, S. S.; Mandell, J. F.; McGarry, F. J. Research Report R 76-3, MIT: Boston. 1976.
Received for review July 10, 1981 Accepted December 23, 1981
Role of Stabftlzer-Aluminum Reactions in Methylchloroform Stabilization Barry Van Gemert PPG
Industrks, Chemical Divlsion Technical Center, Barberton, Ohio 44203
The mode of action of 1,l, 1-trichioroethane stabilizers has been elucidated. Most additives appear to have a dual function. They can react or complex with aluminum chloride, thereby removing this destructive species. They also appear to have the ability to prevent the formation of aluminum chlorkle by reacting more rapidly with aluminum than 1, 1,l-trkhloroethane does. Epoxides and alcohols are most important in inactivating aluminum chloride while ethers and nitroparaffins aid in retarding its formation.
Introduction l,l,l-Trichloroethane (methylchloroform) is a largevolume industrial chemical used primarily in vapor degreasing, cold cleaning, and aerosol applications. Because this chlorinated solvent is subject to catastrophic decomposition reactions in the presence of aluminum, the common. material of trade contains on the order of 5% stabilizers. This composition allows for its safe use with all metals, including aluminum. The mechanism of the decomposition reaction has been studied in some detail, but little is known about the function of the stabilizer system. This paper presents our findings in this area. It now appears that chemical additives previously thought to act only by forming stable aluminum chloride complexes have a more primary function of competing with the solvent for active aluminum sites, thereby reducing the formation of destructive amounts of the metal halide. A new technique 0 196-4321/82/ 1221-0296$01.25/0
involving the use of amalgamated aluminum has been used to study this alternative mechanism of methylchloroform stabilization. Experimental Section A. Identification of the Products of AluminumMethylchloroform Reactions. The reactions were carried out in a 200-mL, round-bottom flask, equipped with a Soxhlet extractor and topped with a reflux condenser. The outlet arm of the Soxhlet extractor was modified so siphoning could not occur. Instead, a steady drainingequal to the volumn condensed-resulted once the capacity (about 50 mL) of the chamber was reached. A pure aluminum coupon (1100 series) weighing 1.5 g was placed in a thimble in the extractor. Methylchloroform (150 mL) and methanol (7 mL) were added to the round-bottom flask and brought to reflux. After three days, all but 0.1 g of the aluminum had been consumed. A white powdery
0 1982 American
Chemical Society
