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TRODUCTION

EPOXY is a naturally brittle material, which makes it vulnerable to flaws and microcracks that limit its application. Epoxy resins may be reacted as cross linked curing either with themselves through catalytic homopolymerization, or with a wide range of co-reactants. After curing epoxy forms a thermosetting polymer with strong mechanical properties as well as high temperature and chemical resistance. Epoxy has a wide range of imperative applications, including metal coatings, electronics and electrical components, high tension electrical insulators, fiber-reinforced plastic materials, and structural adhesives.[1–4] In order to extend the epoxy applications, it is essential to be toughened and enforced with other materials. One of the early attempts to toughen epoxy by the incorporation of rubber spheres with ~1 lm diameter was carried out by Laible and McGarry.[5] The rubbery epoxy particles were applied as a toughening agent for glassy epoxy matrices. However, the resultant composites were more complex, as toughening routes, liquid rubbers limit the usage of these composites.[6] Since then extensive investigations have been conducted for toughening epoxy using a number of tougheners as liquid rubbers,[7,8] thermoplastics,[9] copolymers,[10,11] core shell particles,[12] silica hollow nanoparticles,[13] and combinations of these. Lately, clay particles were used to toughen epoxy,[14] fracture toughness increased to 50 to 100 pct, but tensile strength decreased as well as the failure strain. Oztuirk et al.[15] reported the effects of adding small quantities of liquid rubber on the tension and the impact behavior of epoxy resin. They found that the small addition of liquid rubber increases the plastic strain failure approximately to the double, as well as relative MEDHAT AWAD EL-HADEK, Associate Professor, is with the Department of Production & Mechanical Design, Faculty of Engineering, Port-Said University, Port-Fouad, Port-Said 42523, Egypt. Contact e-mails: [email protected]; [email protected] Manuscript submitted January 6, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A

low improvements to the impact strength. Later Chikhi et al.[16] reported that the addition of 16 wt pct of liquid rubber decreases the glass transition temperature, failure stress, as well as the tensile modulus that decreased from 1.85 to about 1.3 GPa. Interestingly, they also reported the drastic increase of the Izod impact strength from 0.85 to 2.8 J/m2 and from 4.19 to 14.26 kJ/m2 for notched and unnotched specimens, respectively, while the critical stress intensity factor (SIF; KIC) varies from 0.91 to 1.49 MPa m1/2 with the addition of 16 wt pct of liquid rubber. The same observation was earlier identified by Kim et al.[17] that confirmed that the fracture toughness determined by the potential energy release rates and the stress intensity factors were significantly higher by up to 80 pct for the limited modified rubber epoxy matrix system. Furthermore, the impact energy threshold was evaluated based on a simple residual strengt