The effects of molecular fortification on the compression and shear properties of a commercial epoxy resin system ​

This study seeks to determine the viability of molecular fortification of a commercially available epoxy system based on the RS-M135 epoxy with the RS-MH137 hardener. Four possible fortifiers, dimethyl carbonate (DMC), dimethyl sulfoxide (DMSO), bis(4-hydroxyphenyl)phenylphosphine oxide (BHPPO), and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were studied. DMC and DMSO have previously been used as fortifiers while BHPPO and DOPO were selected due to their reduced toxicity compared with dimethyl methyl phosphonate (DMMP). Mechanical properties of the fortified resins are investigated in compression and shear. Significant increases in the compression properties (11% increase in compression modulus and 24% in peak yield stress with 2 mol% DMSO) and the shear properties (16% increase in shear modulus and 15% in peak yield stress with 4 mol% DMSO) respectively are observed following fortification with DMSO. The improvement in the compressive properties (13% increase in compression modulus and 13% in peak yield stress with 2 mol% DMC) is also observed in the blends containing DMC, although to a lesser degree. Differential scanning calorimetry analysis reveals that there is evidence of modest levels of co-reaction and a very slight drop in the glass transition temperature on addition of the fortifiers (i.e. a reduction of up to 3 °C in the case of DMSO and 8 °C in the case of DMC, when 16 mol% of either fortifier was added), while thermogravimetric analysis confirms that the onset of thermal degradation is practically unchanged. Molecular dynamics modelling confirms the compatibility of DMSO and DMC with the epoxy blend but indicates that the BHPPO and DOPO undergo strong self-association, reducing their ability to be dispersed effectively within the epoxy blend.