Gel Time vs. Cure Speed Gel Time is defined as the required time for a system to make an exothermic state change from liquid to solid. Cure speed is the time it takes for a system to actually cross link with itself in order to form a lattice structure.
Low Temperature Curing Phenalkamines - excellent for low temperature curing POPDA – gives excellent properties Accelerators such as benzyl alcohol, salicylic acid, and dimethylaminopropyl- amine PropertyAmine A Phenalk- amine Gel time, min., 25°C 6650 Pencil Hardness 3H Cure through time (5°C) >24 hours16 hours Direct Impact Test (in/lb) 1412
Cracking of Epoxies in Structural Applications Epoxies crack in many electrical apparatus due to sudden changes in temperature. Cracks usually start in areas of high stress High stress areas include places where a metal or ceramic insert is placed.
Fracture Toughness This is measured by calculating K Ic and G Ic of a material. The above figure illustrates different modes of fracture testing The below figure illustrates a double torsion method used on filled materials [ K1c] 2 = E* G 1c * (1-ν)
Core-shell Toughening Incorporates a fine dispersion of soft particles as a second phase within the epoxy matrix Such particles, with sizes less than 1 micron have a core structure that absorbs energy and a shell that provides for good adhesion to the epoxy matrix.
Conclusions Epoxies noted for: Excellent mechanical strength Outstanding dielectric properties Excellent chemical resistance Increased usage in medium and high voltage applications where subject to hostile environments
Conclusions Different approaches are available to formulators to improve toughness critical in low temperature applications Matrix flexibilization Multiphase toughening