5 Rubber Toughening of Oxazolidinone-Modified Epoxy Novolacs Downloaded by PURDUE UNIVERSITY on March 19, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch005
J. A. C L A R K E The Dow Chemical Company, Freeport, TX 77541 Multifunctional resins do not respond as dramatically to elastomer toughening as do their difunctional counterparts. One route to improvement is by partial "advancement" of the resin itself using reactants that provide oxazolidinone linkages. This approach minimizes the downgrading of the thermal properties in cured systems that would occur with bisphenol A advancement. These advanced resins will provide a measurable but not strongly significant improvement in fracture toughness when cured with methylene dianiline. Their main advantage lies in the response to incorporation of carboxyl-terminated nitrile rubber into multifunctional resins. This incorporation brings the measured toughness (via the fracture energy) into the same performance range as typical commercial difunctional epoxy resins.
Ill
P O X Y R E S I N S F O R M A N I M P O R T A N T P O L Y M E R C L A S S within the family of thermoset materials. Most types of epoxies exhibit an inherent brittleness that is due to the typical glassy cross-linked nature of ther mosets. In many applications, this brittleness is of no importance; i n another major application, that of coatings for metals, brittleness is overcome by an advancement reaction of the liquid epoxy with bis phenol A . This advancement produces a solid resin that, upon curing, is substantially tougher than its liquid counterpart. H e r e , toughness is gained at the expense of high-temperature performance. In other areas, such as structural adhesives, neither brittleness nor loss in glass transition temperature (T ) can be tolerated. A means of mod ifying the resin to alleviate brittleness must be found without de grading the important thermal and environmental qualities of the cured epoxy, reasons for selection of this particular polymer type i n the first place. Rubber toughening techniques have reached this goal with commercially proven success (i). Increases i n fracture toughness g
0065-2393/84/0208-0051/$06.00/0 © 1984 American Chemical Society
In Rubber-Modified Thermoset Resins; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
52
R U B B E R - M O D I F I E D T H E R M O S E T RESINS
on an order of magnitude have become commonplace for the typical, difunctional, bisphenol A based epoxies such as D . E . R . 331. The solution to the brittleness problem becomes more difficult for multifunctional resins such as D . E . N . 438 epoxy novolac. These high-temperature performance resins (T > 200 °C) usually combine multifunctionality with partial elimination of flexible linkages to at tain their desired properties. The hardener must also be selected to follow these guides in order to develop a highly cross-linked rigid cured network. As a result, the viscoelastic response of the network that is required for energy dissipation (2), whatever specific mecha nism is applied, is greatly restricted. Although toughening of this class of epoxies is an important need, the ability to respond to the presence of rubber inclusions is much more limited (3). In this case, rubber alloying in itself is becoming generally recognized as an i n adequate toughening technique. A practical answer might be to com bine this alloying with certain principles used in high-temperature thermoplastics. A way to loosen the tight cross-linking to simulate thermoplastic behavior without a simultaneous reduction i n the T should improve the ability of the network to react in a viscoelastic rather than brittle fashion i n the presence of the rubber inclusions. A n application of such a principle to a well-established high-temper ature epoxy, D . E . N . 438, is the subject of this chapter. 1
1
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g
g
Description
of Resin
Two features distinguish D . E . N . 438 epoxy novolac from the bis phenol A based epoxies. First, D . E . N . 438's broad molecular weight d i s t r i b u t i o n is a c c o m p a n i e d b y an e q u a l l y b r o a d d i s t r i b u t i o n i n e p o x i d e f u n c t i o n a l i t y of the i n d i v i d u a l m o l e c u l e s . F u n c t i o n a l i t y varies from 2 to over 20, as indicated i n Figure 1. This variation is expected from the condensation reaction of phenol with formalde hyde to produce an average 3.5-functional novolac after removal of unreacted phenol. C o u p l i n g of the phenolic hydroxyls with epichlorohydrin produces the epoxy novolac. The resin makeup includes about 25% diepoxide (diglycidyl ether of bisphenol F), which does not contribute to high-temperature performance, and another 25 w t % with functionality greater than 6. This observation suggests that ge lation w i l l occur early in the curing reaction or in any other reaction involving epoxide that we may use to modify the resin. Second, the solubility parameter (8) of D . E . N . 438 is higher than that for l i q u i d diglycidyl ether of bisphenol A ( D G E B A ) epoxies. Small's method of calculation (4) estimates 8 to be about 1.0 unit higher (10.2 vs. 9.15 if the calculation is based on the pure structural formula). This result suggests a different interaction with solvents and with the nitrile rubber modifier, an important consideration. 1
Trademark of The Dow Chemical Company.
In Rubber-Modified Thermoset Resins; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
5.
Oxazolidinone-Modified Epoxy Novolacs
CLARKE
53
Downloaded by PURDUE UNIVERSITY on March 19, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch005
Molecular Weight Distribution
N-MER
WEIGHT FRACTION
1
(stripped out)
2
.25
3
.175
4
.137
5
.105
6
.080
7+
.25
wt. avg. = 3.5f
THEORETICAL STRUCTURE
A
A
O - C H 2 - C H - C H 2
O - C H 2 - C H - C H 2
6 — — 6 —
c
h
A
0 - C H 2 - C H - C H 2
— 6
Average value for n: D.E.N. 431 = 0 . 2 D.E.N. 438 = 1.6 D.E.N. 439 = 1.8
Figure 1. D.E.N. 438 epoxy novolac molecular weight distribution and theoretical structure.
Experimental
Approach
The objective of this study is to compare the performance of cured D . E . N . 438 epoxy novolac to a chemically modified D . E . N . 438 epoxy novolac of lower cured cross-link density. This comparison is to be made both w i t h and without rubber toughening to determine the effectiveness of the rubber i n each case. T h e principal criterion is fracture energy, ^ , w h i c h indicates relative resistance to crack propagation. The obvious way to reduce cross-link density i n an epoxy is through "advancement"—reaction of a portion of the epoxide content with a diphenolic such as bisphenol A . This advancement converts a l i q u i d resin into an i n h e r e n t l y tougher solid product of r e d u c e d epoxide content. D . E . R . 661 solid epoxy resin, epoxide equivalent weight ( E . E . W . ) = 4 7 5 - 5 7 5 , is an example. W h e n cured w i t h 4,4'methylenedianiline ( M D A ) , it has over twice the fracture toughness of its l i q u i d counterpart, D . E . R . 331 epoxy resin, (
1760 cm-i Band Configurations
Figure 3. IR scans of modified epoxy novolac. 399 for metals was used (9). This test is one of a variety of crack-propagation tests that are becoming popular for evaluating variations in toughness in polymer systems (10). Test pieces, 1 x 1.04 x 1/8 in., were cut from the castings, notched (9), and then precracked by tapping with a heavy-duty razor blade. The crack length (a) varied from piece to piece over a range of 0.06 to 0.25 in. The preliminary test result, K , was calculated by combining the force required to start propa gation of the precrack with specimen dimensions in the equation given in ASTM q
In Rubber-Modified Thermoset Resins; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
5.
57
Oxazolidinone-Modified Epoxy Novolacs
CLARKE
Table I. D . E . N . 438: Comparisons Before and After Cure Parameter Epoxide, % Softening point, °C (Durran) DMS Analysis (MDA cure) K
Modified with ICA
24.0
17.2
18.4
40
85
80
243 80 +
222 170
231 145
M is average molecular weight between cross-links
NOTE:
Downloaded by PURDUE UNIVERSITY on March 19, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch005
Modified with TDI
Unmodified
c
E-399-82. This result was converted to &} by using the measured tensile mod ulus (E) and an assumed Poisson's ratio (|x) of 0.34 lc
_ (K f (1 - p.*) ^ q
9
lc
where
