allow the use of MCS in expandable castings, expandable sandwich structures, and foam core moldings. In addition, the significant improvements in physical and /or chemical characteristics inherent in MCS and PMCS lend themselves to several other unique applications for which styrene is not completely satisfactory. Acknowledgment
The authors gratefully acknowledge the technical assistance of D. H. Clarke, W .K. Glesner, W. N . DeLano, and T. 0. Ginter in obtaining the information presented. literature Cited
Boundv, R. H., Boyer, R . F., “Styrene.” Reinhold, Yew York, 1952.
Glesner, W. K., Nowak, R. M., Rexer, J. K., Mod. Plast. 43, 37 (1966). Rubens, L. C., Gordon Conference on Chemistry and Physics of Cellular Materials. New Hampton. N. H., 1969; J . Cell. Plast 6, No. 1, 1 (1970). Rubens, L. C., J . A p p l . Polym. Sci 9, 1473 (1965). Rubens, L. C., Thompson, C. F., Kowak, R. M., SPI 20th Reinforced Plastics Technical and Management Conference, Chicago, Ill., February 1965. RECEIVED for review December 16, 1969 ACCEPTED April 20, 1970 Symposium on Eew Halogenated Monomers, Division of Organic Coatings and Plastics Chemistry, 158th Meeting, ACS, Xew York, N.Y., September 1969.
Cycloaliphatic Epoxy Resins with Improved Strength and Impact Coupled with High Heat Distortion Temperature Anthony C. Soldatos a n d Allison S. Burhans Chemicals and Plastics Division, Research and Development Department, Union Carbide Corp., Bound Brook, N . J . 08805 The toughness of cycloaliphatic epoxy resins, measured by the impact strength and area under the stress-strain curve, can be significantly improved through modification with several elastomeric materials containing functional groups. The toughness of these systems is proportional to the concentration of the elastomer. Twofold to greater than tenfold improvements in impact have been obtained by the addition of 5 t o 35% carboxyl-terminated butadiene-acrylonitrile copolymer, without significantly degrading the heat-distortion temperature.
Simultaneous improvements were obtained in tensile
strength and elongation. These systems have produced glass cloth reinforced composites with high tensile strength, under both static and dynamic conditions.
THERMOSETTIN(: POLYMERS in general are brittle and susceptible to crack initiation and propagation. Because of this behavior, the usefulness of crosslinked epoxy resins in some of the more critical applications is sometimes limited. This deficiency is often corrected by improving the toughness of the resin through modification with plasticizers or flexible hardeners. As a rule, however, this improvement is accompanied by a severe degradation of some other mechanical properties-modulus, strength, and heat-distortion temperature. In the case of glassy thermoplastic polymers, such as polystyrene. the problem of brittleness has been effectively corrected by the inclusion of elastomeric particles suspended throughout the polymer matrix. More recently, McGarry et ai. (1967) have shown that the toughness of aromatic epoxy resins can be improved under certain limited conditions by incorporating a specific carboxyl-
296
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
terminated elastomer in concentrations up to 10 pph resin. Our investigation extended this concept to the area of cycloaliphatic epoxies, which have a wide spectrum of outstanding properties. Our goals were to define the parameters affecting the toughness of these resins and develop resin compositions of improved toughness without degrading strength and heat-distortion temperature. This investigation has shown that the toughness of cycloaliphatic epoxy resins can be significantly improved through modification with several elastomeric materials containing carboxyl and mercaptan groups and varying in molecular configuration. The resin selected for this study was the 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate (ERL-4221).
Elastomers
The following materials were tested and found effective in toughening ERL-4221. Carboxyl-terminated butadiene-acrylonitrile random copolymers varying in molecular weight and acrylonitrile content (Drake and McCarthy, 1968). The 80120 butadiene-acrylonitrile copolymer, having a molecular weight of 3300, designated CTBN. HOOC
t
(CH~-CH=CH-CH~),-(CHE-CH
x.5
Y= I
I CN
yf,.OOH
2.10
A 1700 molecular weight mercaptan-terminated 70130 butadiene-acrylonitrile random copolymer, designated MTBN (Drake and McCarthy, 1968). HS
f
(CHe-CH=CHCH,
x.3
)x-(CHz-CH)y I CN
2.7
Y=l
A 2800 molecular weight mercaptan-terminated-linear copolymer of 70% n-butyl acrylate and 30% ethyl acrylate. A carboxyl-terminated, low molecular weight = 3000) polybutadiene designated C-3000. This polymer is mainly of the 1,2 configuration (Masatoshi, 1967).
(mn
a-o POLYBUTADIENE DICARBOXYLIC ACID HOOC- CH-CH,-(CH2-CH)n-CH,-CH-COOH I I I CH CH /I I1 CH2 CH2 CH2
FH
Mechanical Properties
The toughness of the cured polymers was measured by the impact strength and the area under the stressstrain curve. The impact energy, expressed in inch pounds, was measured by a simple but reliable test, the Gardner impact, which consists of striking cured resin specimens with a 4-pound round-nosed rod, 0.5 inch in diameter, from various distances; all test specimens were disks 0.1 inch thick by 2.0 inches in diameter. The room temperature impact of ERL-4221 cured with hexahydrophthalic anhydride ("PA) was doubled when modified with 10 pph resin of each of the above elastomers
(Table I ) , which is actually only 4.7% by weight based on the total formulation. A cure cycle of 2 hours a t 120°C plus 4 hours at 160°C was used in all cases. At the 10-pph level of modification, in general, the cast tensile strength and elongation of the above elastomermodified systems were increased, with the exception of carboxyl-terminated polyhutadiene, without severely degrading the modulus or heat-distortion values (Table I ) . The tensile strength and elongation were increased in the case of CTBN from 7500 psi and 1.9% to 12,000 psi and 5%, which represents improvements of 60 and 250%, respectively. The maximum drop in modulus was only 7(A (from 435,000 to 401,000 psi) and the heatdistortion temperature was decreased by a few degrees (from 191" to 187"C),with the exception of MTBN, which degraded this property more excessively, by 22" C. Comparison of CTBN with Other Elastomers
Of all the elastomers tested in this study, CTBN was found the most effective modifier for ERL-4221 (Table I). I t is evident that the improvements in impact, and particularly in strength, are influenced by the chemical structure and composition of the elastomer. The nature of the functional groups and the composition of the polymeric backbone affect its rate of reaction, as well as its compatibility with the epoxy resin, which in turn affects the molecular structure of the matrix resin. MTBN, the mercaptan-terminated polybutadiene-acrylonitrile copolymer, is much less effective as far as tensile strength, modulus, and heat-distortion temperature are concerned than its carboxyl-terminated counterpart (CTRN). Mercaptan groups are considerablv less reactive with cycloaliphatic epoxides than carboxyls, even in the presence of catalysts. This difference in the rate of reaction between the epoxy and the functional groups of the modifier may influence the extent of the elastomer precipitation as a distinct second phase. Similar conclusions can be drawn with MTA, the mercaptan-terminated n-butyl acrylate-ethyl acrylate copolymer. This material is also significantly inferior to CTBN in all properties. The carboxyl-terminated butadiene homopolymer (C-3000) is about equally effective to CTRN in impact and heat-distortion temperature, but considerably less effective in other properties. The tensile strength and per cent elongation are very significantly lower. Along the same lines, a butadiene-acrylonitrile elastomer with only 10' acrylonitrile did improve the Gardner impact, but failed to increase the tensile strength and elongation of ERL-4221 (Table I ) . The strength properties
Table I. Cast Resin Properties of ERL-4221 /"PA
Modified with Various Elastomers 10 Parts Elastomer/100 Parts Resin
Gardner impact, inch Ib. Heat distortion temperature, C Tensile strength, psi Tensile modulus, psi 5 elongation Formulation
EKL-4221 Hexahydrophthalic anhydride ("PA) Ethylene glycol Benzyldimethylamine (BDMA) Modifier (as indicated)
None
CTBN
MTBN
MTA
C-3000
CTBN 10% V C N
40 191 7,500 436.000 1.9
80 187 12,000 401,000 5.0
70 169 9,600 4 12.000 3.0
70 177 8,070 437,000 2.6
75 188 4,300 401,000 1.5
80 185 7,300 387,000 2.3
Parts
100 100 1.5 1.0 10
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970 297
Table II. Cast Resin Properties of ERL-422 1 /"PA
Modified with Carboxyl-Terminated Elastomer (CTBN) Parts CTBN/100 Parts Resin
0
Gardner impact a t RT, inch lh Gardner impact a t -70' C, inch lb Heat distortion temp., ' C Tensile strength, psi Tensile modulur, psi ' C elongation 24-hr water boil, wt. ' c increase Formation
ERL-4221
HHPA Ethylene glycol
BDMA CTBS
10
40
20
80
110
...
... 191 7,500 435.000 1.9 0.94
187 12,000 401,000 5 0.94
Parts
Cure Cycle, Hr.
100 100 1.5 1.0 As indicated
2 , 120'C 4, 160'C
30
... 188 10,600 3R4,000 5.3 0.96
40
90
... 168 9,390 308.000 5.5 0.98
100
> 320n 159 8,500 276,000 8
...
80
50
100
120 120O 158 8.050 270,000 8.5
168
>320
127 5,850 189,000 20
...
...
92 4,980 142,000 30
...
'Specimen 0.5 inch thick. "Specimen 0.1 inch thick
of this compound are intermediate between those of 20% acrylonitrile copolymer (CTBK) and 0% acrylonitrile (polybutadiene C-3000). Properties of ERL-422 1 -CTBN Systems
CTBK (the carboxyl-terminated 80120 butadieneacrylonitrile copolymer) is the most effective modifier and was therefore selected for further study. The concentration of CTBN was increased to levels up to 1 to 1 by weight ratio with ERL-4221. Hexahydrophthalic anhydride ("PA) was chosen as the hardener and benzyldimethylamine (BDMA) as the catalyst. The ethylene glycol functions as an initiator by reacting with the anhydride t o yield acid groups, which in turn react with the epoxide. As shown in Table 11, the room
% STRAIN
Figure 1. Cast resin tensile stress-strain curves of ERL-4221/ CTBN systems Inch Pounds,
Area under Curve 1. 2. 3. 4. 5.
298
+ +
ERL-4221 ERL-4221 ERL-4221 + ERL-4221 ERL-4221 +
+
0 CTBN 10 20 30 40
CTBN CTBN CTBN CTBN
per Cu. Inch 86 382 370 342 458
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 3, 1970
temperature impact of ERL-4221 was increased from 40 to 80 inch pounds by the addition of 10 pph or 4.7% by weight of CTBN, and to greater than 320 inch pounds by the addition of 100 pph or 33cr by weight of CTBN. At -70°C, the impact of the system modified with 50 pph of CTBN was 120 inch pounds. Thicker cast resin disks, 0.5 inch thick, passed 320 inch pounds at -70" C even a t the 40-pph, or 16.5'5, modification level. The degree of impact strength appears to be directly proportional to the concentration of the elastomer. As the concentration of CTBN increases above the 60-pph level, impact increases more rapidly. Reinforcing Effect of CTBN
Additional cast resin properties of the various ERL42211CTBN blends has further shown that CTBN, in addition to improving the toughness of ERL-4221, increases the strength of the resin a t the lower, 10- to 50-pph, levels of modification. As shown in Table 11, the most significant improvement was obtained a t the 10-pph level. The tensile strength and per cent elongation were increased from 7500 psi and 1.9% to 12,000 psi and 5';, respectively, a t the expense of somewhat reduced modulus. The heat-distortion temperature of the system was decreased by only 4" C (from 191" to 187" C). Higher concentrations of elastomer failed to increase the tensile strength any further, and a t the 50-pph level the strength was only slightly higher than the unmodified system. At 80 pph CTBN, the tensile strength was already lower than the control. As the elastomer was increased, the heat-distortion temperatures and moduli were decreased, but the elongation continued to increase, as might have been expected. The boiling water resistance of the modified systems was essentially unaffected, at least up to the 30-pph level of modification. This substantial toughening effect of CTBN on the cycloaliphatic epoxide (ERL-4221), coupled with significant increase of the strength of the resin without seriously lowering the heatdistortion temperature, is rather unique for thermosetting systems. In the case of aromatic epoxides, cured with tertiary amine catalysts. other investigators have shown (McGarry et a l , 1967) that the addition of 10 pph of CTBK decreased the tensile strength, which may suggest a different mechanism of reaction.
Figure 2. Electron micrograph of ERL-4221/HHPA system Area
Figure 3. Electron micrograph with 10 pph of CTBN
of
ERb422l/HHPA modified
under Stress-Strain Curve
A resin system with high elongation and high tensile strength should, in general, have greater toughness as measured by the energy to break (area under the stressstrain curve) (Soldatos et al., 1969). The tensile stressstrain curves for the cast resin system ERL-4221iHHPA modified with 0, 10, 20, 30, and 40 pph of CTRN are shown in Figure 1. The areas under the curves of the modified systems are considerably larger (370 to 458 inch pounds per cu. inch) than that of the unmodified system, which is only 86 inch pounds per cu. inch. These improvements are the result of the increased elongation, from 1.9 to 870, and the increased tensile strength, from 7500 to 12,000 psi. Microstructural Features of CTBN Modified Systems The carboxyl-terminated elastomer, which is soluble initially in the liquid phase, partially precipitates as distinct particles during crosslinking. The cured specimens are opaque, in contrast to the uncured liquid solutions, which _. are clear. hlectron microscoDv was used to demonstrate
system is clearly a two-phase structure. The continuous phase (white) consists of epoxy resin, and the disperse phase (dark) consists of rubber, with an average particle size of approximately 7000 A. Some rubher is finely dispersed in the resin matrix. The electron micrographs were prepared from castings stained by treatment with osmium tetroxide, which reacts preferentially with the rubber. Effect of Hardeners and Catalysts on ERC4221/CTBN Systems
The extent of the previously mentioned improvements in cycloaliphatic epoxies obtained with CTBN depends greatly upon the nature of the crosslinking agent. Our work has shown that only certain acid-type hardeners, such as hexahydrophthalic anhydride, are effective. More strongly acidic anhydrides are much less effective. As shown in Table 111, the tensile strength, and particularly the heat-distortion temperature of maleic anhydridecured, CTBN-modified ERL-4221, are substantially lower than those of the unmodified systems as well as those of the HHPA cured system. No improvements in tough. . ness were obtained. Furthermore. Iaole 111 shows tnat acidic h;ardeners are more desirahle than catalysts. The t e nsile niodulus, tensile strength, and heat-distortion tempe rature of ERL-4221 modified with CTBN (30 pph) DO ?.“VA ... W I Y ~WIL: considerably lower than anId cure” .W..I:LLl.U YI
- ..
__1
.
II_
^_^
Pe,rties of ERL-4221 /CTBN Cured
?ic Anhydrides a n d BFaMEA ERL-422l/Moleic Anhydride
ERL-4221/8F3MEA
Parts CTBN/lOO Ports Resin
Paris CTBNilOO Ports Resin
0
10
Formulation
Ports
ERL-4221 BFAMEA CTBN
100 6 As Indicated
Ind. Eng. Chem. Prod.
0
Cure Cycle,
30
HI
1.5, EO*C +1, 90°C +2, 120’ C +4, 160’ C
Res. Develop., Vol. 9, No. 3, 1970 299
the unmodified system and the corresponding anhydride (”PA) cured systems. The only property improved was elongation (from 3% to 6.3%). Effect of Molecular Weight of CTBN
20,000
17,000
Using the same curing hardener, hexahydrophthalic anhydride, three low molecular weight CTBK elastomers = 3300, 5800, and 10,000) were all found effective toughening and reinforcing agents for ERL-4221. The data in Table IV show improvements in impact and tensile strength in all three systems. The lowest molecular weight elastomeric copolymer ( M , = 3300) is somewhat more effective than the higher molecular weight tk’, = 5800, 10,000) homologs in all properties, which suggests that the effectiveness of the butadieneacrylonitrile copolymer is a function of its molecular weight. I t appears that copolymers with molecular weight much greater than about 10,000 would not be effective.
(mn
laminates
An important prerequisite to the design of composite structures is retention of strength under dynamic fatigue stressing. Since previous work has shown that matrix resins with improved toughness provide composites with improved fatigue endurance (Soldatos et al., 1968), glasscloth laminates based on ERL-4221iHHPA and modified with CTBN were fabricated and tested. Wet layup technique was used in fabrication, with 181-Volan-treated glass cloth as the reinforcement. The properties of two laminates modified with zero (control) and 10 pph of CTBN tested under static loading are shown in Table V. The flexural strength of the laminates was decreased from 77,600 to 57,800 psi as the elastomer concentration was increased from 0 to 10 pph. The tensile strength, however, which is a more important property, remained essentially unchanged.
15,000
12,000 11,000 2
4
6
2x10‘
8 IO’
IO6
2x10’
CYCLES TO FAILURE
Figure 4. Fatigue results on ERL-4221 laminates modified with CTBN
0
0
ERL-4221 ERL-4221
+
10 pph CTBN
T o establish the relationship between the toughness of the matrix resin and the retention of mechanical properties under dynamic loading, the above two laminates were further tested under tensile stressing using the Sonntag 1800 cpm fatigue testing machine, Model SF-1-U. Tensile fatigue was chosen instead of flexural fatigue because the tensile strength values of the laminates were essentially equivalent. The data shown in Figure 4 clearly demonstrate that the composite based on the elastomer modified epoxy system is superior to the unmodified system. The number of cycles to fail for all specimens tested under various stress levels show that the fatigue endurance of the tougher system is approximately 100% greater than the unmodified resin. For example, a t the 15,000-psi stress level the laminate based on ERL-4221 failed a t 178,000 cycles, while the laminate based on ERL-4221110 pph CTBX failed a t 490,000 cycles. Acknowledgment
Table IV. Effect of CTBN Molecular Weight on Cured Resin Properties CTBN
Heat distortion temp., ’ C Tensile strength, psi Tensile modulus, psi % elongation Gardner impact, inch Ib Formulation
Parts
ERL-4221 HHPA Ethylene glycol BDMA
100 100 1.5 1.0 10
CTBN
3300
5800
mol. wt.
mol. wt.
10,000 mol. wt
187 12,000 401,000 5.0 80
182 11,000 369,000 4.3 50
18Q 10,800 379,000 4.2 50-60
Table V. Properties of Glass Cloth Laminates of ERL-4221/“PA Modified with CTBN Parts CTBN/100 Parts Resin
Tensile strength, psi Tensile modulus, psi Flexural strength, psi, R T Flexural modulus, psi, RT
300 Ind. Eng. Chem. Prod.
0
10
54,100 2,920,000 77,600 3,190,000
52,800 2,860,000 57,800 3,080,000
Res. Develop., Vol.
The authors express appreciation for the contributions of C. M. Eichert in conducting the experimental portion of this work, and to W. D. Niegisch for the electron microscopy studies. literature Cited
Drake, R. S., McCarthy, W. J., Rubber World 159, 51-6 (1968). McGarry, F. J., Willner, A. M., Sultan, J. N., “Relationships between Resin Fracture and Composite Properties,” Technical Report AFML-TR-67-381 (1967). Masatoshi, F., J a p . Chern. Quart. 3 , 2 (1967). Soldatos, A. C., Burhans, A. S., Cole, L. F., “Correlation between High Performance Epoxy Cast Resin Properties and Composite Performance,” SPI, 24th Annual Technical and Management Conference, Washington, D. c., February 1969. Soldatos, A. C., Burhans, A. S., Cole, L. F., Mulvaney, W. P., “High Performance Cycloaliphatic Epoxy Resins for Reinforced Structures with Improved Dynamic Flexural Properties,” 155th Meeting, ACS, San Francisco, Calif., April 1968. RECEIVED for review March 6, 1970 ACCEPTED May 7 , 1970 SPI Meeting, Washington, D. C., February 1970.
9, No. 3, 1970
