Toughening of Epoxy Resin Networks with ... - ACS Publications

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 20, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0233.ch011

11 Toughening of Epoxy Resin Networks with Functionalized Engineering Thermoplastics Jeffrey C. Hedrick , Niranjan M. Patel , and James E. McGrath* 1

1

Department of Chemistry and National Science Foundation and Technology Center: High Performance Polymeric Adhesives and Composites, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Epoxy resin thermosets are traditionally toughened by the incorporation of an elastomeric component that phase-separates during cure to form a multiphase network. The toughness enhancement in these systems is considerable. However, in most instances, toughness enhancement is achieved at the expense of stiffness and high-temperature performance. An alternative approach to increasing the toughness and impact strength involves the use of functionalized engineering thermoplastics as toughening agents. The improvement in toughness is accomplished in this case without significant sacrifice of properties

at

elevated temperatures. In this chapter, a brief review of the state of the art of thermoplastic-modified epoxy resin networks is presented. Emphasis is placed on the types of modifiers, morphological character, and the various mechanisms proposed to be responsible for the toughening behavior.

EPOXY RESIN NETWORKS ARE CURRENTLY USED as coatings, structural adhesives, and advanced composite matrices in many applications involving both the aerospace and electronics industries. In addition to their outstanding adhesive properties, these highly cross-hnked networks possess excellent Current address: IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY 10598. * Corresponding author.

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0065-2393/93/0233-0293$06.00/0 © 1993 American Chemical Society

In Toughened Plastics I; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


294

RUBBER-TOUGHENED PLASTICS

thermal and dimensional stability as well as high modulus and strength ( I , 2). The widespread use of epoxies, however, is limited in many high-perfor mance applications because of their inherent brittleness. Several methods have been proposed to improve on this attribute of epoxy networks. The most common method involves the addition to the system of a second polymeric component that phase-separates upon curing (3, 4). Traditional modifiers include functionalized rubbers such as the widely known carboxyl-terminated butadiene-acrylonitrile copolymers (CTBNs). A two-phase morphology con sisting of relatively small (O.l-ΐμιη) rubbery particles dispersed in and bonded to an epoxy resin network is produced by incorporating C T B N s into epoxy resins. The toughness of the modified networks is dependent on the properties of the original epoxy, the particle size, particle volume fraction, interfacial bonding, and the properties of the elastomeric component (5). A major limitation to toughening epoxy resins with elastomers such as C T B N s is that increased toughness can be achieved only at the expense of high-temperature performance. Because of the low glass-transition tempera ture (T ) of the rubbery phase, rubber modification often lowers both the use temperature and the thermoset modulus. With the growing demands of the aerospace industry for materials that display high thermal stability as well as toughness, alternative methods of toughening epoxy resins based on the incorporation of functionalized thermoplastics have emerged. Thermoplastic modifiers are tough, ductile engineering polymers possessing high T s. Net work systems based on this technology are toughened without negatively affecting their high-temperature performance. In this chapter, the aim is to provide a general overview of this relatively new class of material systems with a focus on the variations in the modifier chemistry, aspects of phase separation and the resulting morphology, and some of the mechanisms that have been proposed to explain the toughening behavior. g

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Thermoplastic

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Initial reports in the literature concerning the use of thermoplastics to toughen epoxy resins appeared in the early 1980s. Raghava (6) and Bucknall and Partridge (7) were the first to publish the results of their investigations in which a commercial poly(ether sulfone) (Vitrex 100P, Chart 1), was employed as a toughening agent in epoxy resins. Although the existence of a two-phase structure was evident from both dynamic mechanical analysis ( D M A ) and scanning electron microscopy (SEM), Bucknall and Partridge (7) observed no toughness improvement in systems in which both chemistry and stoichiometry of the resin and curing agent were varied. The lack of toughening agreed with the findings of Raghava (6), who, in a later publication (8), attributed it to poor adhesion between the phase-separated components. The importance of chemically prereacting a thermoplastic polymer into the epoxy resin to control the compatibility and interfacial adhesion of the



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phase-separated network was realized by McGrath and co-workers (9-11). In their approach, a phenolic hydroxyl-terminated bisphenol A polysulfone (PSF; Chart 1) was incorporated into an epoxy resin system based on diglycidyl ether of bisphenol A and diaminodiphenyl sulfone (DDS) as depicted in Scheme I. The PSF was prereacted with a large excess of epoxy resin using a quaternary ammonium catalyst to functionalize the PSF oligomers with epoxide groups prior to the addition of the D D S curing agent. The concentration and molecular weight of the PSF modifier were varied to investigate their effect on the properties of the resulting networks. Mechanical property results demonstrated that the fracture toughness (K ) of the cured networks increased substantially (from 0.6 to 1.7 N / m ) when both the concentration and the molecular weight of the PSF were increased. Moreover, the increase in fracture toughness was accomplished without significant reduction in the modulus, thermal stability, or solvent resistance (9-11). Chu et al. (12) reported in patent literature similar results utilizing amino-functionalized thermoplastics. Specifically, they demonstrated that amine-terminated aromatic polyethers, polysulfones, and poly(ether sulfone) of molecular weights in the range of 2000-10,000 g/mol and T s between 125 and 250 °C produced multiphase morphologies and significant enhancements in the fracture toughness over the neat resin. In addition, Chu et al. (12) reported that the modified epoxy compositions resulted in stiff, tough thermoset composites with increased compression-after-impact (CAI) strengths. Other thermoplastics such as amino-functional poly(arylene ether ketone)s (PEKs) have been tested as toughening agents for epoxy resins. The P E K oligomers, however, appear to be less compatible with epoxy resins; inclusions of greater than 10-15 wt % (depending on the molecular weight) resulted in macrophase separation (13, 14). Recently, Bucknall and Gilbert (15) demonstrated that simple physical blends of poly(ether imides) (PEI; Chart 1) also can be used for effective toughening of epoxy resin networks. They showed that fracture toughness increased linearly with P E I content up to 25 wt % with only modest reductions in Young's modulus. Whether the linear correlation between fracture toughness and modifier concentration is valid in the entire composition range has been of some interest. For instance, Recker et al. (16, 17) proposed that an optimum morphology exists at intermediate compositions at which a maximum in fracture toughness is achieved. The work of Recker et al. involved correlating the fracture toughness of modified epoxy networks to damage tolerance in composite laminates. Fracture toughness as measured by fracture energy ( G ) and the strain energy release rate under shear ( G ) was studied as a function of modifier molecular weight and concentration. The G was directly proportional to the number-average molecular weight up to about 10,000 g/mol, beyond which there was little or no change. Additionally, the toughness appeared to have a sharp maximum at an interlc

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Toughening of Epoxy Resin Networks with ... - ACS Publications

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