Effect of Conversion on the Structure−Property Relationships of Amine-Cured Epoxy Thermosets

Abstract

The effects of conversion beyond the gel point on the structure-property relationships of epoxy thermosets using formulations representative of the most commonly used epoxy resin and amine curing agents at balanced stoichiometry with an emphasis on the thermal, tensile, and fracture properties were studied. The range of T(g) from just beyond the gel point to full conversion typically is >100 degrees C. Fracture toughness (as K(1c)) of the epoxy thermosets cured with relatively flexible amines such as ethylenediamine (EDA), diethylenetriamine (DETA), and m-xylylenediamine (MXDA) reaches near its full cure value at only approximately 65-70% conversion. The maximum in K(1c) for these types of epoxy thermosets is at approximately 90% conversion for EDA and DETA but just below its full cure for MXDA. Isophoronediamine represents a special case for fracture behavior because of its apparent substantial cyclization during cure. In the 4,4'-diaminodiphenylsulfone series, K(1c) generally increases with conversion as the concentrations of their strongly antiplasticizing soluble and pendant fractions decrease. A uniform trend of decreasing tensile modulus with increasing conversion was observed in each formulation and is consistent with the expected decrease in the cohesive energy density as monomer glass is transformed into polymer glass.

Simultaneous reinforcement and toughness improvement in an aromatic epoxy network with an aliphatic hyperbranched epoxy modifier

“…Materials that contain opposing glassy-state E' and T g values are said to be antiplasticized, where at temperatures below the glass transition additives can restrict short-range molecular motions and thus stiffen the material. Such hindrances may arise from the filling of free volume and/or an increase in secondary molecular interactions and have been extensively reported for epoxy networks [44][45][46]. The E' trend observed herein is attributed to the network-level incorporation of HBE which provides additional functionality to enhance the network's cohesive forces.…”

“…Hydrogen-bonding networks within polyester hyperbranched polymers have been previously found to impact material properties, and it has been shown that the concentration of these secondary interactions is the key to the observed increases [27]. An increase in non-bond interactions is understood to enhance the network's cohesive energy density, which has been previously shown to improve room temperature modulus [46]. FTIR spectra were taken to confirm that the additional hydroxyl-functionality remains after the HBE-modifier is cured into the network ( Figure 6).…”

Chemorheology investigation of a glassy epoxy thermoset on tensile plastic flow and fracture morphology

“…These parameters drive the final bulk properties of the material through a variety of interactions. Marks and Snelgrove measured decreasing mechanical properties of epoxy‐amine thermosets with increasing conversion . Literature reported results have been consistent in that properties, such as modulus and fracture toughness, diminish as conversion increases from 0.8 to 1.0.…”

Control of reactions and network structures of epoxy thermosets

“…Thus diaminodiphenylsulfone [126,127] (DDS, Fig. 23) can present pot-life of several months, while for diethylenetriamine [127] (DETA, Fig. 23), it could hardly be longer than twenty minutes.…”