11 An Alternative Liquid Rubber for Epoxy Resin Toughening Improving Poly(n-butyl acrylate) RubberEpoxy Compatibility by Use of Acrylonitrile and Acrylic Acid Copolymers and Terpolymers S. L.
KIRSHENBAUM , 1
S. GAZIT , and J. P. B E L L 2
Institute of Materials Science, University of Connecticut, Storrs, CT 06268 The solubility (compatibility) of poly(n-butyl acrylate) (PnBA) rubber in a bisphenol A-methylene dianiline epoxy resin has been increased by copolymerization and terpolymerization of acrylonitrile (AN) and acrylic acid (AA) with n-butyl acrylate (nBA) monomer. The general effect of rubber-epoxy compatibility on the precipitated rubber particle size and number has been investigated. Rubbers containing substantial amounts of AN or AA precipitate later from solution (during curing), and the particles are smaller in size. Some rubber remains dissolved in the matrix, and the total volume fraction of precipitated rubber decreases. The most compatible rubber (PnBA-19% AA), which did not precipitate from solution before gelation, resulted in a single-phasesystem. Tensile impact measurements (test speed ~ 8000 in./min) indicate that impact strengths for the rubber-modified epoxies are affected by rubber-epoxy compatibility, but impact strength improvements are limited by the brittleness of the high glass transition temperature matrix.
I N A N Y R U B B E R - M O D I F I E D S Y S T E M , the compatibility between the rubber and the epoxy before and during cure is an important chem ical criterion for toughening of the matrix (I). Initially, the rubber should be completely soluble i n the epoxy resin. This solubility de pends upon the initial molecular weight of the rubber, the chemical composition and amount of the functional groups, and the solubility Current address: A V C O Specialty Materials Division, 2 Industrial Avenue, Lowell, MA 01851. Current address: Rogers Corporation, Lurie Research and Development Center, Rogers, C T 06263. 1
2
0065-2393/84/0208-0163/$06.00/0 © 1984 American Chemical Society
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parameters of the r u b b e r and epoxy (2, 3). W i t h o u t good initial rubber dissolution, the rubber does not contribute to the improve ment of impact strength; i n the case of poly(n-butyl acrylate) (PnBA) modified, methylene dianiline ( M D A ) cured epoxy systems (4), the rubber has a detrimental effect. D u r i n g epoxy cure, the compatibility of the rubber and epoxy should be such that the initially dissolved rubber precipitates as a discrete, randomly dispersed rubbery phase with a definite size and shape (3, 5). The degree of phase separation (i.e., volume fraction, domain size, and number of particles) is determined by the com peting effects of the incompatibility between the rubber and the curing epoxy, the rate of particle nucleation and domain growth, and the quenching of morphological development by gelation (6). These effects in turn are controlled interdependently by the conditions of cure, the type and concentration of curing agent and resin, the type and c o n c e n t r a t i o n of r u b b e r , a n d the r u b b e r m o l e c u l a r w e i g h t (3, 6 - 9 ) . The toughening of epoxy resins has been attempted with l i q u i d rubbers. The major area of interest is low molecular weight carboxylor amine-terminated butadiene-acrylonitrile ( C T B N and A T B N , re spectively) rubber modifiers. For the most part, use of these rubber modifiers i n low glass transition temperature (T ) epoxy systems has been successful in improving impact strength. Recently, a low mo lecular weight, carboxyl-terminated P n B A rubber has been synthe sized and successfully incorporated as a toughening agent in a 2,5dimethyl-2,5-hexanediamine-cured diglycidyl ether of bisphenol A ( D G E B A ) type system (10,11). Unfortunately, i n other e p o x y - c u r i n g agent systems, the P n B A rubber exhibited poor compatibility, which limits its role as a toughness modifier. P n B A rubber, w h i c h consists of a saturated backbone structure, is more stable chemically than the butadiene-based C T B N modifier. If the P n B A - e p o x y compatibility could be improved, applications could become possible that could not be obtained with C T B N . F o r this reason, we wanted to develop a P n B A rubber type whose solu bility parameter could be easily altered to fit different epoxy—curing agent systems. O u r goal was to obtain a rubber that at 120 °C was completely soluble in the epoxy resin. This rubber would stay i n solution with the addition of the curing agent and would not precip itate until the epoxy began to cure. F o r this study we used acrylonitrile (AN) and acrylic acid (AA) separately, as copolymers, and in combination as terpolymers with the P n B A rubber. A N was considered as possible copolymer material because of its high intermoleeular bonding forces and high solubility parameter. W h e n copolymerized with butyl acrylate in small quan tities ( 0 - 2 5 % by weight), A N considerably alters the solubility pa rameter of the rubber. Addition of A A to the P n B A rubber not only g
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changes the r u b b e r s o l u b i l i t y p a r a m e t e r , but also increases the number of reactive groups along the rubber chains. This effect allows greater rubber—epoxy interfacial bonding. C o m b i n i n g A N and A A as a terpolymer with the P n B A rubber allows us even greater control of rubber solubility and interfacial bonding. Experimental Copolymerization Method and Characterization of Rubber. The proce dure for the copolymerization and terpolymerization of the P n B A - A N , P n B A AA, and P n B A - A N - A A rubbers is similar to the procedure for the PnBA rubber (4). A N (0-42% by weight) and AA (0-20% by weight) were polymerized to gether and separately with butyl acrylate by using 4,4'-azobis(4-cyanovaleric acid) as the initiator and dithiodiglycolic acid as the chain transfer agent. The reaction proceeded by bulk polymerization under continuous mixing in a reac tion vessel purged with nitrogen gas. The polymerization temperature was main tained at 72 °C because A N has a relatively low boiling point (77 °C). A reflux column above the reactor served as a condenser for possible evaporation. After polymerization, the rubbers were washed with an acetone-water separation technique (12) to remove all unreacted materials. Low polydispersity molecular weight fractions were obtained by slowly varying the acetone-to-water ratio during the wasjb^cycle. Number (M ) and weight (MJ average molecular weights of the rubbers were measured with gel permeation chromatography (GPC). The GPC was cal ibrated with low polydispersity P n B A - A N - A A rubber fractions. Actual M values for PnBA fractions were measured by using a Knauer vapor pressure osmometer. Rubber functionality was measured by K O H titration (4). Preparation of Scanning Electron Microscope and High-Speed Tensile Test Samples. Scanning electron microscope (SEM) and high-speed tensile test samples were made by using a two-step cure cycle. Ten weight parts Epon 828 with a reaction catalyst, 1% tetra(n-butyl)ammonium iodide, was mixed with one weight part rubber and placed in an oven for 2 h at 120 °C. This cure step ensures the formation of the epoxy-rubber intermediate (4, 12) and pro motes blending of epoxy and rubber prior to the final cure. In the second step of the curing cycle, a stoichiometric amount of melted (approximately 100 °C) M D A was added to the epoxy-rubber mixture. After hand stirring, the solution was poured into small (Vs x /s x V2 in.) silicone rubber molds or cast between two glass plates (4) sprayed with Miller-Ste phenson MS-136 Hot Mold Release agent and placed in the oven at 120 °C for 1 h. The temperature was raised to 150 °C, and the samples were postcured for 2 h before slowly cooling to room temperature. The cured S E M samples were fractured at room temperature in a vise and coated with a layer of gold, ap proximately 200 A thick. All samples were observed under the S E M , and the number of rubber particles, their size and distribution, and rubber area were determined from the photographs. High-Speed Tensile Impact Tester. The tensile impact tester used in this study is a modified version of the Plastechon Universal Tester (Plas-Tech Equip ment Company) (see Reference 4 for a detailed description of instrumentation). Ram speeds were from 8000 to 10,000 in./min. Load and displacement were measured by a 1000-lb capacity load cell (Tyco) and a noncontacting displace ment measuring system (Kaman Multi-Vit model DK-2300-10cu), respectively. Signals from the load and displacement transducers were stored on magnetic discs via an Explorer III digital oscilloscope (Nicolet Instrument Corporation). With the aid of a table-top computer interface (Hewlett Packard HP 9826), n
n
l
166
RUBBER-MODIFIED THERMOSET RESINS
ultimate stress, ultimate strain, modulus, energy to break, and strain rate were calculated for all samples. To ensure statistical integrity, 6-12 samples from each group were broken. Average values, along with standard deviations, are re ported. Results
and
Discussion
C o m p a t i b i l i t y S t u d y — P n B A - A N R u b b e r S e r i e s . Table I shows characterization data for the P n B A - A N series of rubber sam ples used i n this solubility study. T h e solubility parameters, calcu lated b y using a group contribution method (13, 14), for P n B A , P A N , E p o n 828, and M D A are 1.23, 12.78, 9.78, and 10.05 (cal/cm )i, respectively. W i t h the addition of 20% A N by weight to the P n B A , the calculated rubber solubility parameter is raised from 9.23 to 9.94 (cal/cm )i This value theoretically means the rubber should be more soluble i n the epoxy. A d d i t i o n of the c u r i n g agent ( M D A ) raises the calculated solubility parameter of the epoxy mixture, but not suffi ciently to precipitate the rubber from solution before cure. U n d e r experimental conditions, the calculated predictions h o l d true for the P n B A - 2 0 % A N - E p o n 828 mixture. A t 120 °C, the rubber readily dissolves and, w i t h the addition of curing agent, initially remains i n solution. D u r i n g the first 5 m i n of cure the rubber begins to precip itate, and the system becomes cloudy. I n the P n B A - A N ( 0 - 4 2 % A N by weight) copolymer series, the rubber precipitated before gelation. The average precipitated particle size and size distribution as a function of percent A N i n the rubber are shown i n F i g u r e 1. W i t h increasing A N composition, the average particle size decreases. T h e P n B A rubber alone (molecular weight 6500) is not completely com patible w i t h the epoxy before or after the addition of the curing agent. The resultant modified epoxy, as shown i n the S E M micrograph i n F i g u r e 2, exhibits a broad distribution of particle sizes. T h e larger particles are the result of rubber that d i d not dissolve, and the n u merous small particles are rubber that partially dissolved and precip itated d u r i n g cure. The P n B A - 5 % A N and P n B A - 1 0 % A N rubber-modified sam3
3
Table I. Characterization Data for PnBA-- A N Rubber Series
AN in Rubber (%)
M (GPC)
M /M
0 5 10 20 26 42
6,500 6,690 7,672 5,614 5,715 14,412
7.00 6.03 7.12 6.54 8.54 4.56
n
w
n
Functionality (acid groups per chain, or eq/mol) 1.60 1.12 1.66 1.23 1.14 1.76
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pies also exhibit a wide particle-size distribution. T h e 5 and 10% A N rubber samples initially dissolved i n the epoxy but precipitated out of solution shortly after the addition of M D A . T h e 2 0 - 4 2 % A N rub bers, all soluble i n the E p o n 8 2 8 - M D A mixture, exhibit small par ticle size and narrow size distribution. A n S E M micrograph of the 20% A N rubber-modified sample (Figure 3) is a good example of an initially compatible system.
Figure 2. SEM micrograph of PnBA rubber-modified Epon 828 at stoichiometric equivalence of amine (MDA) to epoxy.
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R U B B E R - M O D I F I E D T H E R M O S E T RESINS
Figure 3. SEM micrograph of PnBA-20% AN rubber-modified Epon 828 at stoichiometric equivalence of amine (MDA) to epoxy. In F i g u r e 4, area fractions of the rubber phase are plotted on the basis of the total area of the fractured surface, related to i n creasing A N concentration i n rubber. F o r all rubber-modified sam ples, the area fraction of the rubber exceeds the 10 weight parts p e r
% AN IN RUBBER
Figure 4. Area fraction of precipitated rubbery phase as a function of percent AN in the rubber (determined from SEM micrographs). Note: dotted line portion of curve is due to uncertainty because of the higher molecular weight for the PnBA-42% AN rubber.
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Epoxy Resin Toughening
Table II. S E M Data for P n B A - A N Rubber Series AN in Rubber (%)
Particles per cm ( x 10 )
0 10 20 26 42
3.0 3.1 14.8 15.1 15.1
2
6
h u n d r e d (pph) of rubber initially added to the epoxy. In agreement w i t h previous reports (3, 6, 15-18), this observation may indicate the presence of epoxy inclusions that were trapped w i t h i n the rubber particles. W e must be careful, however, w h e n making assumptions comparing the measured area fraction to the added weight or volume fraction. W i t h the S E M , we cannot determine where i n the rubber sphere the particle fractured, and thus cannot directly compare our measured rubber area to the actual volume fraction of precipitated rubber. T h e measured area fractions can only be compared relative to each other. T h e n u m b e r of particles per unit area of fracture surface is pre sented i n Table II. Initially, as compatibility increases, the rubber particles become smaller i n size and increase i n number (0—20% A N in rubber). C o p o l y m e r rubbers, w i t h A N concentrations above 20%, level off i n the n u m b e r of precipitated particles. Compatibility S t u d y — P n B A - A A Rubber Series. Char acterization data for the P n B A - A A rubber series are shown i n Table III. W i t h the A A copolymer rubbers, particle size and distribution again decrease as the A A concentration increases (Figure 5). A t a concentration of 19% A A , the rubber remains i n solution throughout the cure and produces a single-phase system. T h e total number of precipitated particles (Table IV) related to A A concentration goes through a m a x i m u m at a concentration of 12% A A . T h e total area of r u b b e r o n t h e f r a c t u r e surface, w h i c h is r e l a t e d to the size a n d Table HI. Characterization Data for PnBA--AA Rubber Series.
AA in Rubber (%)
M (GPC)
M /M
0 5 9 12 16 19
6,500 10,000 10,300 9,000 12,400 12,300
7.00 6.20 4.90 4.60 5.00 5.40
n
w
n
Functionality (acid groups per chain, or eq/mol) 1.60 8.16 11.60 12.60 22.40 26.20
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R U B B E R - M O D I F I E D T H E R M O S E T RESINS
6c o o 5s :
§' * • • • • * • * § * • • * • » • • • i
0
5
* ' *
1
• * • • • J • * * * * * * '
10
15
*§
20
% A A in R u b b e r
Figure 5. Rubber particle size vs. percent AA in the rubber (determined from SEM micrographs). number of particles, decreases sharply with an increase i n A A con centration (Figure 6). In the P n B A - A A rubber composition studies, the amount of p r e c i p i t a t e d r u b b e r is more sensitive to small A A concentration changes. A n S E M m i c r o g r a p h of a f a i r l y c o m p a t i b l e P n B A - A A rubber-modified system (Figure 7) shows small and uniform size par ticles, w h i c h appear to be better bonded to the matrix. C o m p a r e d to the P n B A - 2 0 % A N micrograph (Figure 3), the interface between rubber and epoxy is more difficult to distinguish. W h e n compatibility is c o m p l e t e , t h e r u b b e r r e m a i n s i n s o l u t i o n , a n d a single-phase system after cure is obtained (Figure 8). Effect of R u b b e r M o l e c u l a r W e i g h t on Compatibility. F i g u r e 9 shows a plot of particle size for four molecular weight fractions of P n B A - 1 5 % A N - 2 % A A terpolymers. A s expected, the average parTable IV. S E M Data for P n B A - A A Rubber Series AA in Rubber (%) 0 5 9 12 16 19
Particles per cm
2
3.00 5.98 17.49 42.80 8.83 0
(xlO ) 6
ticle size and total area of rubber (Figure 10) decrease w i t h a decrease i n molecular weight. T h e n u m b e r of particles exhibits an inverse relationship w i t h molecular weight; the number increases with a de crease i n molecular weight (Table V). Interestingly, the effect of mo lecular weight on the particle size and area is greater at the lower molecular weights. Below average molecular weights of 4000, small decreases drastically affect the rubber precipitation characteristics.
Figure 7. SEM micrograph of PnBA-12% AA rubber-modified Epon 828 at stoichiometric equivalence of amine (MDA) to epoxy.
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R U B B E R - M O D I F I E D T H E R M O S E T RESINS
Figure 8. SEM micrograph of one-phase PnBA-19%AA rubber-modified Epon 828 system at stoichiometric equivalence of amine (MDA) to epoxy. Effect of R u b b e r Compatibility on Tensile Impact Proper ties. B y varying the composition of the rubber, we have shown that size and distribution, area, and number of the precipitated rubber particles are all related to the r u b b e r - e p o x y compatibility. Next, we examined the effect of compatibility on the impact strength of the rubber-modified epoxies. F o r this study we chose to break the sam ples by using a high-speed tensile impact tester. B y breaking u n -
i.ooh
0.75-
0.50-
*
i
0.25-
% 0.00-1
1
1
1
i
1
4000
1 1
1
1
I
1
1
1
MOLECULAR
1
1
I
1
1
1
12000
8000 WEIGHT
Figure 9. Rubber particle size as a function of rubber molecular weight for PnBA-15% AN-2% AA weight fractions (from SEM micrographs).
173
Epoxy Resin Toughening
notched A S T M dog-bone samples i n tension at high speeds, we ob tain a measure of the materials' combined strength to resist crack initiation and crack propagation. This test method gives a better i n dication of the total energy absorbed during impact failure than many other common measures. The impact test results for the P n B A - A N series of rubber-mod ified epoxies are shown i n F i g u r e 11. T h e pure P n B A rubber-modi fied sample with an M of 6500 is not a very compatible system. It has a l o w impact strength, as illustrated by Figure 11, at 0% A N . A d d i t i o n of A N to the rubber improves rubber—epoxy compatibility and impact strength. Although compatibility may still be good, at high A N concentrations ( P n B A - 4 2 % A N ) the rubber begins to lose its rubbery nature (the T of polyacrylonitrile is approximately 100 n
Table V. Characterization and SEM Data for PnBA-15% AN-2% AA Weight Fractions AN-AA
in Rubber (%) 15-2 15-2 15-2 15-2
M (GPC)
M /M
15,416 7,041 3,298 1,140
3.28 2.10 2.02 1.53
n
w
n
Particles per cm
2
22.6 34.1 38.5 50.1
(xlO ) 6
174
RUBBER-MODIFIED THERMOSET RESINS
Control
9
-
•
0
MM.)
MM
5
.MM,
10
MM.
MM,...
15 %
....|
20 AN IN
25
|
30
»
35
.
40
RUBBER
Figure 11. High-speed tensile impact test, energy to break vs. percent AN in the rubber; ram speed, 8000 in./min.
°C), and impact strength decreases. Interestingly, impact strengths of the rubber-modified samples can only approach or slightly exceed the impact strength of their equivalent control sample. This result indicates that poor rubber—epoxy compatibility w i l l be detrimental to impact strength. Nevertheless, a compatible system does not en sure i m p r o v e d toughening. Possibly, the matrix viscoelasticity and ability to f l o w are the determining factors when trying to improve the impact strengths of brittle, high T epoxies. Results by O c h i and B e l l (12) seem to further support this matrix viscoelasticity theory. The high-speed tensile m o d u l i for the P n B A - A N formulations (Figure 12) show no change i n modulus with an increase in A N con centration i n the rubber. M o d u l i for all rubber-modified compositions were slightly reduced compared to that of the control. The elonga tion-to-break curve (Figure 13) appears almost identical i n shape to the impact-strength curve. Elongation of the rubber-modified sam ples is not too d i f f e r e n t f r o m that of the c o n t r o l s , b u t u l t i m a t e stress values (Figure 14) for the rubber-modified samples are 10% less than the controls. Impact strength, ultimate stress, elongation to break, and m o d ulus results from our P n B A - A A series of rubber-modified epoxies (not shown) exhibit the same trend as that for the P n B A - A N - e p o x y compositions. B y varying the A A content of the rubber, we were able to vary the impact strength. However, the strength of the rubbermodified samples again only approached those of the controls at best. g
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Epoxy Resin Toughening
8H
< Q. LU
°
6-
Control
CO -I Q
i
4-
LU -J CO
z UJ
2T
10
T
T
T
T
1
rrrrpi
rrrrpi
20
25
15
m r r p i
rrrrpi
30
35
40
% AN IN RUBBER
Figure 12. High-speed tensile impact test, modulus vs. percent AN in the rubber; ram speed, 8000 in. Imin. Interestingly, the impact strength of samples with high-functionality, chemically well-bonded rubbers and single-phase systems (no pre cipitate) were virtually identical to the controls. T values for the E p o n 828-stoichiometric equivalent controls and a few of the rubber-modified systems are shown i n Table V I . W i t h increased r u b b e r - e p o x y compatibility, the T of the matrix de creases. This decrease is a result of an increased amount of rubber remaining i n solution. g
g
6S
:
X
AN
IN
RUBBER
Figure 13. High-speed tensile impact test, elongation to break vs. percent AN in the rubber; ram speed, 8000 in. Imin.
176
R U B B E R - M O D I F I E D T H E R M O S E T RESINS
1 .25H Control 0. y*% m
g
1 .00-^
in LU QC
0.751
0.501 1
0
1 1
1 1
1
1 1
1
1
1
1 1
1
1 1
1
1 1 1
1 1
1
i *' i * i ' i ' * i * i ' 1 * < ' r 5 10 15 2 0 25 30 35 40 45
7. AN IN RUBBER Figure 14. High-speed tensile impact test, ultimate stress vs. percent AN in the rubber; ram speed, 8000 in. Imin. Conclusions P n B A rubber w i t h an M of 6500 is not completely soluble in E p o n 828. Rubber—epoxy compositions containing P n B A have a wide par ticle-size distribution and exhibit poor impact strength. F o r tough ening the rubber must dissolve completely i n the resin before curing. B y copolymerizing A A or A N with n-butyl acrylate, the rubber solubility parameters can be altered. E p o x y - r u b b e r compatibility, the precipitated rubber particle size, number of particles, and total precipitated rubber area can be controlled. U s i n g small concentrations of A N and A A with n-butyl acrylate as a terpolymer allows further control over the wide range of the rubber solubility parameters and functionality. Varying r u b b e r - e p o x y compatibility w i l l affect the ultimate i m pact strength of the compositions. However, with our high T matrix system, impact strength improvement was insignificant at best. n
g
Table VI. D M A Glass Transition Temperatures for 10 pph Rubber-Modified Epon 828 PnBA-AN-AA in Rubber (%)
CO
0-0-0 74-26-0 58-42-0 91-0-9 88-0-12
160 158 155 155 152
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Literature Cited 1. Drake, R.; Siebert, A. SAMPE Q. July 1975. 2. Sultan, T. N.; McGarry, F. J. Polym. Eng. Sci. 1973, 13, 29. 3. Bucknall, C. B.; Yoshii, T. Br. Polym. J. 1978, 10, 53. 4. Gazit, S. "Toughening of Epoxy Resin by Acrylic Elastomer", Ph.D. Thesis, Univ. of Connecticut, Sept. 1980. 5. Rowe, E. H.; Riew, C. K. Plast. Eng. March 1975. 6. Manzione, L. T.; Gillham, J. K.; McPherson, C. A. J. Appl. Polym. Sci. 1981, 26, 889. 7. Siebert, A. R.; Riew, C. K. "The Chemistry of Rubber Toughened Epoxy Resin I"; presented at the 161st Natl. Meet. American Chemical Society, Washington, D . C . , March 1971. 8. McGarry, F. J.; Willner, A. M . "Toughening of An Epoxy Resin by an Elas tomeric Second Phase"; R68-8, MIT, March 1968. 9. Meeks, A. C. Polymer 1974, 15, 675. 10. Gazit, S.; Bell, J. P. In "Epoxy Resin Chemistry II"; Bauer, R. S., E d . ; ACS SYMPOSIUM SERIES No. 221; American Chemical Society: Wash ington, D . C . , 1983; p. 69. 11. Ibid, p. 55. 12. Bell, J. P.; Ochi, M . Polym. Mat. Sci. Eng. Prepr. 1983, 49, 393. 13. Small, P. A. J. Appl. Chem. 1953, 3, 71. 14. Hoy, K. L. J. Paint Technol. 1970, 42, 76. 15. Manzione, L . T.; Gillham, J. K.; McPherson, C. A. Org. Coat. Plast. Chem. Prepr. 1979, 41, 371. 16. Rowe, E. H . Annu. Tech. Conf., Reinf. Plast./Compos. Div. SPI, Sec. 12E, 26th 1971, 1. 17. Kunz-Douglass, S.; Beaumont, P. W. R.; Ashby, M . F.J. Mater. Sci. 1980, 15, 1109. 18. Manzione, L. T.; Gillham, J. K.; McPherson, C. A. J. Appl. Poly. Sci. 1981, 26, 907. RECEIVED
for review November 18, 1983.
ACCEPTED
March 14, 1984.
