The influence of the amount of hardener on the tensile mechanical behavior of an epoxy system

Abstract

The tensile mechanical behavior of an epoxy/amine system, with varying amounts of hardener, was evaluated. The results showed that a brittle to ductile transition occurs as the amount of hardener is increased. This behavior was associated with the change in the number of molecular segments that present mobility in the macromolecular network. The epoxy‐rich formulations are brittle due to the total consumption of the functional groups on the hardener molecule. On the other hand, the amine‐rich formulations showed a fracture behavior characteristic of materials with large deformation capacity. This behavior was associated with the presence of unreacted points on the hardener molecule which introduce a new mobile molecular segment in the macromolecular network formed. © 1998 John Wiley & Sons, Ltd.

Ageing of the DGEBA/TETA epoxy system with off-stoichiometric compositions

“…First, there remains a great number of unreacted epoxy rings, as shown by infrared analysis 18 . Second, a rigid and tight macromolecular structure is developed, were the only expected mobile group is the dimethylene ether linkage of bisphenol-A 20,27,30 . These characteristics are a direct consequence of the complete exhaustion of all the reactive sites on the hardener molecule, giving way to a rigid and brittle structure 30 .…”

“…For the particular system made of the triethylene tetramine, TETA, hardener and the DGEBA monomer the variation of the hardener to monomer ratio promotes strong changes on the mechanical behavior [18][19][20][21] . Of particular interest with respect to these changes is the very sharp increase in the impact strength, when amine rich mixtures were used 21 .…”

“…The epoxy system based on the reaction of the difunctional epoxy monomer diglycidyl ether of bisphenol-A, DGEBA, with aliphatic amines is such an example. The properties of this and other epoxy systems can be varied as a function of the molecular weight of the hardener molecule [7][8][9][10] , by variations in processing conditions [11][12][13] or by the use of different hardener to monomer ratios 8,11,[13][14][15][16][17][18][19][20][21] . This last variable introduces boundary condition (i.e., when a variable like temperature or the amount of monomers is changed).…”

Effect of the hardener to epoxy monomer ratio on the water absorption behavior of the DGEBA/TETA epoxy system

“…Besides the chemical nature of the hardener, other variables like the time and temperature of cure [3,4] and the hardener to epoxy monomer ratio [5] could produce very different macromolecular structures. For the epoxy system formed by the difunctional epoxy monomer, diglycidyl ether of bisphenol-A (DGEBA), and the hexafunctional aliphatic amine, triethylenetetramine (TETA), the hardener to epoxy ratio was shown to strongly affect mechanical [6,7] and thermal properties [8] .…”

“…Earlier studies on the DGEBA/TETA system showed that the degree of cure, and therefore the presence of latent unreacted sites, is greatly affected by the hardener to epoxy ratio and that, for room temperature cured resins without post-curing, the consume of epoxy rings go to completion only for off-stoichiometric hardener rich mixtures [7] . Also, thermo-gravimetric analysis showed that some off-stoichiometric mixtures could have a two phase like microstructure, where very crosslinked domains could be embedded on a less crosslinked matrix [8] .…”