Rubber-Modified Thermoset Resin - American Chemical Society

12 Morphology and Dynamic Mechanical Behavior of Rubber-Toughened Epoxy Resins Downloaded by COLUMBIA UNIV on June 20, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch012

A L A N R. SIEBERT Research and Development Center, The BFGoodrich Company, Brecksville, O H 44141

The use of either a carboxyl- or amine-terminated butadiene-acrylonitrile copolymer to modify an epoxy resin leads to a more crack-resistant (tougher) epoxy resin. This "toughness" is brought about by the formation of a second rubbery phase [with 5-15 phr (parts per hundred parts of resin) of the reactive liquid polymer] during cure and occurs with a variety of cure agents and epoxy resins. Generally, the carboxyl-terminated polymer is prereacted with the epoxy resin prior to cure. The phase separation is determined by a number of variables including initial compatibility, catalyst, and cure temperature. Both volume fraction and particle-size distribution of the dispersed phase are important in determining the optimum degree of toughness in a given system. Above 15 phr of liquid polymer a phase inversion occurs. A variety of techniques have been used to measure the presence of the second phase, including transmission and scanning electron microscopy, light microscopy, light scattering, cloud point measurement, X-ray scattering, and dynamic mechanical behavior.

I N C O R P O R A T I O N O F L O W L E V E L S of a l i q u i d carboxyl-terminated buta d i e n e - a c r y l o n i t r i l e copolymer ( C T B N ) to a normally brittle epoxy resin significantly improves the crack resistance and impact strength without a reduction i n other thermal and mechanical properties (1). This enhancement i n crack resistance and impact strength (increased toughness) is brought about b y the separation d u r i n g cure of a pre dominately r u b b e r y second phase. T h e size of this second phase is usually between 0.1 and 5 |xm.

0065-2393/84/0208-0179/$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.

Downloaded by COLUMBIA UNIV on June 20, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch012

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These modified thermoset resins have found wide application i n structural film adhesives for m e t a l - m e t a l bonding i n aircraft, i n paste adhesives for automotive and industrial application, i n electronic en capsulation, i n epoxy solvent and powder coatings, and i n advanced aircraft and aerospace composites. In most epoxy applications, the final properties are strongly de pendent on the morphology generated during cure of these systems. The morphology is determined by a large number of variables, such as the compatibility of the rubber—resin system prior to cure, the cure agent, the time and temperature of cure, and prereaction of C T B N w i t h the epoxy prior to cure. In some cases, o p t i m u m tough ness is provided w i t h a bimodal distribution of particle sizes. In the m i d 1970s, l i q u i d amine-terminated butadiene-acrylonitrile copoly mers ( A T B N ) were produced. These copolymers provided another way to introduce rubber modification into a cured epoxy network (2). This chapter presents a systematic review of the work to date on these two-phase systems i n neat resin systems, adhesives, and com posites. This review is somewhat complicated by several factors. The amount of literature on these two-phase systems has increased i n number and scope. F o r example, the work includes different types of cure agents (catalytic, primary amine, latent systems, and anhy drides) as w e l l as a variety within each type. In admixed systems, the type of cure agent is important because of the reactivity and selectivity of the carboxyl—epoxy reaction. M a n y authors have re solved this p r o b l e m by prereacting the C T B N and the epoxy resin in an alkylhydroxy esterification reaction. These acid adducts can now be c u r e d w i t h any c u r e agent because t h e y o n l y c o n t a i n epoxy groups. A discussion of these prereactions and the catalysts used is given by D r a k e and Siebert (3). Different techniques are used to e x a m i n e the m o r p h o l o g y of these t w o - p h a s e systems. S c a n n i n g ( S E M ) and transmission electron microscopy ( T E M ) have been used. In addition, some authors have measured morphology i n the areas of fast-crack growth, and others have concentrated primarily i n the area of crack initiation where considerable stress whitening occurs. A n attempt w i l l be made to highlight these differences.

Early

Work

The pivotal work of M c G a r r y and coworkers (4-6) demonstrated the necessity of developing a discrete, well-dispersed, rubbery second phase to provide enhanced crack resistance as measured by fracturesurface energy ( F S E ) measurements. They used a l i q u i d diglycidyl ether of bisphenol A ( D G E B A ) epoxy resin, C T B N , and an amine catalyst. The authors then determined compositional and morpho logical effects related to F S E improvement of glassy, cross-linked



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resins. They also investigated the effect of C T B N molecular weight, relative solubility of rubber and resin, and rubber-phase particle size. Figure 1 is a T E M micrograph showing the presence of the second phase. Rowe et al. (7) demonstrated a maximum i n fracture energy i n a l i q u i d D G E B A epoxy resin—piperidine system w h e n the bound acrylonitrile (AN) content of the C T B N was between 12 and 18%. They also showed a general decrease i n average particle size from 3 (Jim at 12% b o u n d A N to Q.2 \xm at 25% b o u n d A N . Siebert and R i e w (8) first described the chemistry of rubberparticle formation i n an admixed model involving C T B N , a D G E B A l i q u i d epoxy resin, and a piperidine catalyst. They proposed that the composition of the rubber particles i n the dispersed phase critically depended upon the i n situ formation of the e p o x y - C T B N - e p o x y adduct, w h i c h is then further chain-extended and cross-linked with additional epoxy resin. This progression provides a chemical bond between the dispersed rubber phase and the matrix resin and occurs with piperidine, a selective catalyst. Most other cure agents, how ever, favor either the epoxy—epoxy reaction or an e p o x y - a m i n e reac tion, and the carboxyl-epoxy reaction is suppressed. Siebert and R i e w s h o w e d that a n o n r e a c t i v e b u t a d i e n e - a c r y l o n i t r i l e l i q u i d rubber does show a second phase on cure even with a selective cat alyst. However, the fracture energy d i d not improve for this system. This result demonstrated the need for chemical bonding between the dispersed phase and the matrix. R i e w and Smith (9) developed a new O s 0 staining technique for optical or electron microscopy that aided i n demonstrating the rubbery nature of the dispersed second phase. 4

Figure 1. Electron micrograph showing liquid HYCAR rubber precipitated in the epoxy resin. (Magnification X4800.)


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Bimodal

Particle

Systems

Sultan and M c G a r r y (10) demonstrated that different particle sizes of the dispersed phase i n the epoxy matrix promote different defor mation mechanisms. E l e c t r o n micrographs of the fracture surfaces of systems with only large particles ( 0 . 5 - 5 |mm) show microcavitation around the particle. A c c o r d i n g to the authors, this result is similar to the crazing observed i n thermoplastic systems. However, no re search has p r o v i d e d definitive evidence to date of pure crazing i n a cross-linked epoxy resin. These authors also show that systems with only smaller particles (0.01-0.03 |xm) exhibit shear banding. R i e w et al. (11) described an admixed model system based on C T B N - b i s p h e n o l A - D G E B A l i q u i d epoxy r e s i n - p i p e r i d i n e i n which toughness synergism appears through the inclusion of a d i phenol. This inclusion resulted in a bimodal distribution of rubber particles, and microscopy showed multiple failure sites (a term used by the authors to show that m u c h more surface is created during failure of these systems). Figures 2 and 3 are micrographs showing the bimodal distribution (Os0 -stained T E M ) and multiple failure sites ( T E M taken from the stress-whitened area), respectively. The physical properties of this system are given i n Table I. They also define a set of chemical, morphological, and thermal-mechanical cri teria for the toughened n i t r i l e - e p o x y systems (see Box). C r i t e r i o n C - 3 was not directly demonstrated by the authors with F S E measurements. Subsequent work by Bascom and Cottington (12) and H u n s t o n (13) shows the temperature and rate of test depen dence of F S E for rubber-modified adhesives. W o r k by Bascom et al. 4

Figure 2. Negative of an electron micrograph of an Os0 -stained microsection of a bisphenol A modified, CTBN-toughened epoxy resin. (Magnification x 29,700.) (Reproduced from Ref. 11. Copyright 1976, American Chemical Society.) 4



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Figure 3. Electron micrograph of a bisphenol A modified, CTBN-toughened epoxy resin. (Magnification x7000.) (Reproduced from Ref. 11. Copyright 1976, American Chemical Society.) (14) shows the dependence of F S E on rate of test for rubber-modified bulk systems (see Figure 4). Rowe and R i e w (15) examined the fracture surface of a tensile specimen after stressing. T h e area examined showed considerable stress whitening and necking. T h e i r micrograph (Figure 5) shows microvoid development around the rubbery particles. This microvoid area is primarily responsible for the stress whitening, and the microvoid regions disappear on heating above the heat distortion temper ature ( H D T ) of the sample. Table I. Thermal-Mechanical Properties of Bisphenol A Modified C T B N Epoxy System

Property Tensile strength , MPa Elongation at break , % Modulus, GPa Tensile strength^, MPa Elongation at break , % Heat distortion temp, °C Fracture energy, kj/m Gardner impact , J Izod impact, J/m of notch 0

0

fo

2

2

Unmodified Epoxy Resin System (Control)

Bisphenol A Modified CTBNEpoxy System

65.5 4.8 2.8 73.1 7.3 83 0.18 6 0.68

64.1 9.0 2.7 95.8 11.3 83 5.3-8.8 23-34 3.5

0.12 mm/s 6.35 m/s Nominal 0.635 cm thick samples S O U R C E : Reproduced from Re£ 11.

a

b

c


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Toughening Criteria for Rubber-Toughened Thermosets


A. Chemical Criteria 1. Two reactive end groups 2. Chemical bonds between rubbery particles and the epoxy matrix 3. Compatibility B. Morphological Criteria 1. Second particulate rubbery phase 2. Definite size and shape 3. Rubbery particles must be dispersed C. Thermal-Mechanical Criteria 1. Improved crack and impact resistance 2. Retention of thermal-mechanical properties 3. Insensitive to the rate of loading (or temperature) SOURCE:

Reproduced with permission from Ref. 3.

Yee and Pearson (16) made volumetric measurements on rubbermodified specimens similar i n composition to those used by Rowe and R i e w (IS). These samples were subjected to constant crosshead rate u n i a x i a l t e n s i l e tests. O p t i c a l m i c r o s c o p y r e v e a l e d that the rubber particles had cavitated but not fractured and that shear bands connected the cavitated particles. They propose "that the toughening effect is c a u s e d b y a s e q u e n c e of m e c h a n i s m s : sharp cracks are blunted by voiding; then the voids enhance shear localization be tween themselves. Thus, the rubber particles serve as void nucleants, but the toughness is d e r i v e d largely from the shear plastic z o n e . " Bascom et al. (14, 17) demonstrated that the addition of a high molecular weight rubber ( H Y C A R 1472) along with C T B N does en-

(Sec ) Figure 4. Fracture energy vs. strain rate for epoxy polymers. Key: O , Sample no. 205; O, 206; x , 207; A, 210; and •, 185. (Reproduced with permission from Ref. 14. Copyright 1981, Journal of Material Science.) H



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Figure 5. Micrograph of fracture surface. Microvoids are seen in association with rubber particles. (Magnification x2580.) (Reproduced with permission from Ref. 12. Copyright 1976, Journal of Adhesion.). hance the base fracture energy over that attained with the C T B N alone. Table II shows the compositions and fracture energies for these systems. These samples were prepared by the H e x c e l Corporation; Hexcel F-185 is a commercial epoxy resin formulation. A latent cat alyst (modified urea-accelerated dicyanodiamide) was used. In these systems both the C T B N and 1472 are prereacted with the epoxy resin to produce an acid-free adduct. The enhanced toughness was observed over a wide range of strain rates (including impact testing), and the effect of adding the solid rubber was greatest at the lowest strain rate. This effect is demonstrated i n F i g u r e 4. Phase

Inversion

The work of Burhans and Soldatos (IS, 19) showed enhanced tough ness with C T B N i n a cycloaliphatic epoxy resin cure w i t h hexahydrophthalic anhydride. T h e y also reported that, as the amount of C T B N was increased i n the C T B N - E R L - 4 2 2 1 - H H P A system, a phase inversion occurred at about 50 phr of C T B N . Their ( T E M ) micrographs showed that the C T B N - e p o x y adduct became the con tinuous phase w i t h domains of epoxy resin. Table II. Fracture Energy (

Rubber-Modified Thermoset Resin - American Chemical Society

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