posted on 2012-11-05, 13:39authored bySomkiat Thitipoomdeja
The aim of the work reported here was to determine the influence of an amine
curing agent, and postcure cycle on the mechanical and thermal properties of
diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The results of this initial study
were then used as the basis for selecting material to obtain optimum toughness in
epoxy/glass fibre systems. These basic materials were further used to make
comparisons with the properties of modified resin systems which contained commercial
elastomers. Differential Scanning Calorimetry (DSC), Dynamic Mechanical Thermal
Analysis (DMTA), Fourier Transform Infrared Spectroscopy (FTIR), flexural and
interlaminar shear tests, Instrumented Falling Weight Impact (IFWI), visual
observation, Scanning Electron Microscopy (SEM), and Transmission Electron
Microscopy (TEM) were all used to investigate various properties and the structures
which gave rise to them. The properties of cured products were found to be affected
by the amounts of curing agent, curing times and temperatures, and the structure of the
elastomers. Not surprisingly the maximum thermal and mechanical properties tended to
be found in the stoichiometric (standard) mix systems. However, postcuring at higher
than room temperature, which was used as the basic curing temperature, led to more
conversion. This effect improved the thermal and mechanical properties of both the
unmodified and modified resin systems. The maximum flexural strength of 104 MPa of
the unreinforced resins was found in the stoichiometric mix ratio after postcure at
150°C for 4 hr. However, the maximum flexural modulus and glass transition
temperature (Tg) were found after postcuring at the same temperature for 48 hr. This
was believed to be due to increased crosslinking, but unfortunately the longer curing
time led to degradation of the resins. In the systems modified with -20 phr of
polyetheramine elastomers, the one modified with the lowest molecular weight (2000)
was found to have the highest flexural strength (85.8 MPa) and modulus (2.5 GPa).
The impact properties of all the composites with modified resin matrices were found to
be higher than the unmodified resin matrix composites. The best impact properties
were, however, obtained with the elastomer modifier with a molecular weight of 4000.
The impact energy at maximum force increased from 11.9 to 16.4 J, and energy at
failure increased from 18.7 to 21.6 J. This increase in impact properties was due to the
increase in areas of phase separated elastomer particles over similar systems with lower
molecular weight modifier.
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Aeronautical, Automotive, Chemical and Materials Engineering