Organic toughening agents for epoxy resins

Thermosetting polymers based on epoxy resins often display superior tensile strength and chemical resistance when compared with their thermoplastic counterparts. Such attributes make epoxy polymers ideal matrices for adhesives and composites. However the applicability of epoxy polymers as matrices for adhesives and composites are often limited by an inherent weakness - low flaw tolerance. Ironically, the same crosslinked chemical structure that imparts high strength and superior chemical resistance also promotes brittle behavior. 

Although it is not the intention to provide a comprehensive thesis on the origins of LEFM, it is useful to review the test method that is so often used to characterize the fracture toughness of epoxy polymers. The most common geometry to characterize the fracture toughness of epoxy polymers is the single edge notched, three-point bend test 

LEFM uses the load at fracture and the starting crack length to define a critical stress intensity factor, Xjc. The critical stress intensity factor is an inherent material property and does not suffer from the geometry dependence of other mechanical characterizations, provided that plane strain conditions are met. It is common practice to use the term fracture toughness in place of the critical stress intensity factor, which is rather cumbersome. The fracture toughness of a material can be calculated from the load displacement diagram of a 3PB-SEN specimen as follows  

It is common to use a 65 mm x 12.7 mm x 6.4 mm specimen supported by a 50.8 mm span to perform these tests. The fracture toughness can also be converted to the critical strain energy release rate Gic using equation 3  

As mentioned previously, epoxy polymers possess low flaw tolerance. Typically the fracture toughness values are below 1 MPa m or, in terms of fracture energy, below 200 J/m. These values are extremely low when one compares them with a ductile thermoplastic polymer such as polycarbonate, which has a Kic = 2.4 MPa m and a Gjc = 2400J/m. However, there are organic based materials that can be added to epoxy polymers that increase their fracture toughness such that it even exceeds the fracture toughness of common thermoplastic resins  

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Impact modifiers (4) Organic toughening agents for epoxy resins... 

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