Failure Analysis of Rubber Toughened Epoxies Utilizing Light and Transmission Electron Microscopies E. I. Garcia-Meitin,* Hua Liu,** Nikhil Verghese*** *Analytical Sciences, Building B-1470, The Dow Chemical Company, Freeport, TX 77541 ** Materials Eng. Center, Building 1616, The Dow Chemical Company, Midland, MI 48667 *** Thermosets R&D, Building B-1608, The Dow Chemical Company, Freeport, TX 77541 Dow Chemical is a leading supplier of epoxy based composites and adhesives having a broad range of applications from automotive to wind energy. In order to develop and improve our product portfolio, it is essential to implement a systematic approach in understanding failure mechanisms in toughened epoxy systems to support development of next generation products. A key development variable for these materials is having the ability to study and understand the role of toughening agents and the sequence of events during failure. By utilizing a double notch-four point bend (DN-4PB) method [1], a sub-critically propagated crack can be isolated for microscopic investigation to examine the role of tougheners and the sequence of events during failure. This work will focus on how light and transmission electron microscopies can be utilized to systematically examine a sub-critically propagated crack. Having the ability to correlate fracture toughness with toughening mechanisms is critical in material design and development [1,2,3]. In order to examine a sub-critically propagated damage zone produced by DN-4PB, the arrested crack tip is cut along the mid-plane sub-fracture surface zone (SFSZ) for microscopic analysis. Detailed sample preparation procedures are reported elsewhere [1]. One half of the mid-plane is carefully sanded and placed at the tip of an embedding mold with the mid-plane core region facing outward. The sample is embedded in epoxy and further trimmed for microtomy with the crack tip roughly in the center of the block. A dual staining approach requiring block pre-staining with osmium tetraoxide followed by a ruthenium tetraoxide post staining is used to elucidate the phases in the formulation. Ultrathin sections of approximately 100nm in thickness are collected at ambient temperature using a Leica UC6 microtome. TEM analyses are carried out on a JEOL 1230 operated at an accelerating potential of 100 kilovolts and images are captured using Gatan MultiScan digital cameras. The remaining half of the mid-plane is then placed into a ring-form with the mid-plane core region facing downward on tape. The ring-form is filled with epoxy, cured, and polished to a 0.3μm finish. The polished sample is bonded to a petrographic glass slide and back cut to approximately 1mm in thickness. Further polishing of the sample is performed until a 0.3μm finish is achieved, resulting in an optically transparent thick section of approximately 60μm in thickness. An Olympus Vanox research microscope is used to examine the SFSZ under cross polarized light and images are captured using a Nikon DXM-1200 digital camera. Upon cutting through the sample mid-plane and exposing the SFSZ, the damage zone and crack tip can be investigated (Figure 1). TEM analyses of the arrested crack (Figures 2-3) revealed that the toughening process began with cavitation of sub-micron rubber particles followed by matrix shear yielding. The process also facilitated interfacial debonding at boundaries between larger rubber domains and the epoxy matrix. The combined mechanisms led to crack blunting and ultimately arrested the advancing crack.

[1] H.-J. Sue, et al, Elastomer Technology Handbook 661-700 (1993) CRC Press. [2] E.I. Garcia-Meitin., H-J. Sue, Polymer Composite 165-173 (1994) 15. [3] E.I. Garcia-Meitin, et al, Microscopy and Microanalysis 14 (Suppl. 2) (2008) 630-631 [4] W.D. Bascom, et al, Journal of Applied Polymer Science 1975, 19, 2545.

Fig.1. LM images of A). Mid-plane damage zone below notch. B). Polished thick section of stress whitened damage zone C). Below crack arrest showing bright rubber particle boundaries (arrowed).

Fig. 2. TEM images of A). Grid with crack tip region. B).Crack arrest zone at large rubber domain. C). Crack bridging in large rubber domain (blue), and sub-micron rubber particle cavitations (red).

Fig. 3. TEM images of A). Shear yielding, debonding of epoxy from rubber (blue) and sub-micron rubber cavitation (red) within epoxy domains. B). Interfacial cavitation (red) along larger rubber and matrix and C). Crack above arrest (dotted red) showing matrix shear-yielding and rubber cavitation.

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