Rajesh Turakhia The Dow Chemical Company, Epoxy R & D

Transcription

1 . Novel Epoxy Toughening for Coatings and Composites Applications Rajesh Turakhia The Dow Chemical Company, Epoxy R & D Presented at a meeting of the Thermoset Resin Formulators Association at the Hilton Suites Chicago/Magnificent Mile in Chicago, Illinois, September 15 through 16, 2008 This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA.

2 Novel epoxy toughening for coatings and composites applications Rajesh Turakhia, George Jacob, Marv Dettloff, and Ha Pham The Dow Chemical Company, Epoxy R&D, Freeport, Texas, 77541, USA Epoxy resins were first commercialized in 1946 and are now used in a wide variety of structural and protective coatings applications. 1 Epoxy resins are typically used as two-part systems that combine ease of application (e.g., low viscosity and wetting characteristics) with a variety of desirable product properties such as high modulus, excellent adhesion and corrosion resistance and elevated glass transition temperatures (T g ). However, the above benefits come at the cost of brittleness, which often limits applications, rendering them unsuitable in many cases without some modification to improve toughness (Figure 1). Figure 1: Epoxy resins in coatings and composites Positive Attributes: excellent adhesion thermal resistance chemical resistance corrosion resistance Can & Coil UV Cure Marine & Protective Powder Limitation: ductility/fracture toughness or flexibility Automotive Composites A couple of different approaches have been adopted (Figure 2) to provide ductility to these materials. The first is the use of plasticizers, reactive diluents, chain extenders and flexibilizers and flexible curing agents. These fall into the general category of flexiblization which often results in compromises in key properties such as T g, viscosity and modulus while achieving higher elongations at break. The second approach is the addition of toughening agents to the epoxy thermoset systems. These toughening agents form a separate phase 2 from the epoxy matrix enabling ductile deformation to occur via deformation and cavitation in an otherwise linear elastic, brittle polymer. Most often this improvement in ductility comes by preserving T g, viscosity and

3 modulus while achieving higher elongations at break. Some of these toughening technologies depend on mixing and cure kinetics and comes at the expense of some loss in T g and increase in viscosity. Figure 2: Epoxy flexiblization/toughening technologies Plastisizer Modulus Aliphatic backbone CTBN Chem Resist Tg Corrosion Core Shell Rubbers Viscosity Dow s novel toughening technology (FORTEGRA Epoxy Tougheners) attempts to address that issue and is based on the addition of amphiphilic block copolymers to the uncured epoxy thermoset formulation. 3, 4 By adding a low concentration (typically <5 weight%) of an amphiphilic block copolymer to an epoxy resin, novel nano-scale morphologies can be formed in the liquid state and are for the most part preserved through the curing process (Figure 3). These morphologies give rise to significant improvements in toughness (resistance to crack propagation) without increasing the formulation viscosity and sacrificing modulus and T g of these crosslinked epoxy thermoset systems. It is established that using block copolymers as the toughener can alleviate these processing concerns because the morphology develops spontaneously upon blending with the uncured resin. Trademark of the Dow Chemical Company ( Dow ) or an affiliated company of Dow

4 Figure 3: Formation of nano-meter length scale morphology with amphiphilic block copolymer toughening technology. Creation of rough spherical micelles before cure and refinement of spherical micelles during cure Copolymer Cured epoxy curing ~ nm Bifunctional epoxy Curing agent This new toughened epoxy technology is targeted and used in high performance applications such as marine coatings, protective coatings, fusion coatings for solid bonded epoxy, rebar, and oil and natural gas pipes, powder coatings, composites, and in adhesive applications. Coatings The presentation will discusses the use of the toughening approach to enhance the performance of epoxies used in coatings applications. The improvements seen (Figure 4) include: Improved impact resistance Significant adhesion improvement No impact on corrosion resistance Improved flexibility at sub-ambient temperatures

5 Figure 4: Improved Coating Properties Composites The presentation will also discuss the toughening of epoxies used as matrices in fiber reinforced composite materials. Traditional toughening agents in the micron size range tend to get filtered by the fiber reinforcements during the composite fabrication technique (for example during the infusion of the resin through the fiber reinforcements). This kind of filtration of the toughening agent can lead to one part of the composite to be devoid of any toughening at all. Dow s novel toughening agent because of its ability to form morphologies in the nano-meter length scale does not get filtered by the fibers that are usually separated from each other by distances in the micrometer length scale. The Scanning Electron Microscopy (SEM) picture below illustrates the presence of the novel toughening agent in between the fibers (Figure 5). Figure 5: SEM of a toughened glass fiber reinforced epoxy composite Fiber Epoxy Epoxy Fiber FORTEGRA in Epoxy

6 Composite materials are increasingly finding use in structural applications that could be subjected to impact at high rates (for examples as crashworthy structures in automobiles). This would require them to be tough not only at quasi-static rates of loading but also at higher impact rates. Hence it is important that the technologies employed to toughen the epoxy matrix in these composites be able to perform well even at higher loading rates. The Table 1 shows the toughened anhydride cured epoxy system having a glass transition temperature of about 145 o C and maintaining a superior fracture toughness over the control (about twice as much as the control) over 4 orders of loading rates. Table 1: Variation of fracture toughness with loading rate Conclusions The new novel toughening technology based on amphiphilic block copolymer enhances the mechanical performance of the epoxy resin without compromising other key properties such as T g and process ability (viscosity). This novel toughening technology has broken the conventional Viscosity-Tg-Toughness paradigm in conventional toughening technologies by improving toughening and lowering the viscosity and without affecting the final T g of the system. Acknowledgements The authors would like to acknowledge the contributions of Bill Dellinger, Carol O Connell, David Reuschle, Nikhil Verghese, Steve Hoyles, and Fabio Aguirre.

7 REFERENCES 1 Ashcroft, W. R. In Chemistry and Technology of Epoxy Resins, Ellis, B., Ed.; Blackie Academic & Professional: London, 1993, Yee, A. F.; Pearson, R. A. In Fractography and Failure Mechanisms of Polymers and Composites, Roulin-Moloney, A. C., Ed.; Elsevier Science Publishers: London, 1989, WO A1; Amphiphilic block copolymer-toughened thermoset resins; University of Minnesota, Bates Frank s; Dow Global Technologies inc; Pham Ha Q, Verghese Nikhil Eapen; White Jerry E 4 Verghese, N. E., Pham, H. Q. and Bates, F., Macromolecules, 2003, 36, 9267