Amino-cured epoxy resins arc, prepared containing an aliphatic phosphonate additive which aids in the processing of the epoxy by reducing the Viscosity or the resin mixture. The additive also acts as an antiplasticizer in the cured epoxy by increasing the modulus and yield strength under uniaxial tension and compression. Additionally, phosphonates are known to behave as flame retardants and dimethyl methyl phosphonate (DMMP) demonstrates a reduction in the rate of thermal degradation and the heat-release rate upon pyrolysis. Addition of the antiplasticizer reduces the T-g and suppressed beta-relaxations, while effectively increasing the density of the material.