Thermal and mechanical characterization of epoxy resins toughened using preformed particles

Preformed, multilayer particles have been used to toughen an epoxy resin. T he particles were formed by emulsion polymerization and consist of alternat e glassy and rubbery layers, the outer layer having glycidyl groups to give the possibility of chemical bonding of the particles in the cured resin. T wo variants of this type of particle were used, termed GM(47/15) and GM(47/ 37); both types have an overall diameter of 0.5 mum, but the former have a thicker rubbery layer. For comparison, acrylic toughening particles (ATP) w ith no surface functionality and a liquid carboxyl-terminated butadiene-acr ylonitrile (CTBN) rubber were used as toughening agents. The epoxy resin sy stem consisted of a commercial diglycidyl ether of bisphenol A (Shell Epon 828) with diamino-3,5-diethyl toluene as hardener, two commercial sources o f which were used, namely Ethacure-100 (Albemarle SA) and DX6509 (Shell Che micals). These hardeners contain a mixture of two isomers, namely 2,6-diami no-3,5-diethyltoluene and 2,4-diamino-3,5-diethyltoluene Thermogravimetry in nitrogen shows that the preformed toughening particles begin to degrade at 230 degreesC, whereas the cured resin begins to degrade rapidly at 350 degreesC. Thus, even though the particles are less thermall y stable than the cured resin, their degradation temperature is well above the glass transition temperature of the resin, and their use does not affec t the thermal stability of the toughened materials at normal use temperatur es. The performance of the toughening agents was compared using Ethacure-100 as the hardener. The GM(47/15) and GM(47/37) toughening particles gave rise t o a greater toughening effect than the ATP and the CTBN. For example, the f racture energies were: 0.26 kJ m(-2) for the unmodified resin; 0.60 kJ m(-2 ) for the resin toughened with CTBN; and 0.69 kJ m(-2) for the resin toughe ned with the GM(47/15) particles. The ultimate tensile stress of the unmodi fied epoxy resin was 43 MPa, which increased to 55 MPa when 20wt% of GM(47/ 15) toughening particles were added. The toughness of resins cured with the DX6509 hardener were superior to tho se obtained with the Ethacure-100 hardener, most probably due to DX6509 pro ducing a less-highly-crosslinked network. This highlights the sensitivity o f the toughening process to the hardener used, even for hardeners of a simi lar nature. (C) 2001 Society of Chemical Industry.