Internal stress of modified epoxy resins with polyester

The internal stresses of modified epoxy resins with 4-24 phr of polyester were measured by the steel ring method. The shear moduli and dynamic viscositiy of the cured epoxy resins were tested by dynamic mechanical analysis. A scanning electric microscope was used to observe the morphologies of the fracture surface of the cured epoxy resins. The results showed that the internal stress of cured epoxy resins greatly depended on the addition of polyester and the minimum value of internal stress emerged with the addition of 16 phr polyester with no sacrifice of mechanical properties and heat-resistance. The internal stress of cured resins was almost absent during the curing period and was less from the curing temperature to the glass transition temperature and drastically increased from near the glass transition temperature to room temperature.