Use of Rosin Acids as Rigid Building Blocks in the Synthesis of Curing Agents for Epoxies - Rosin-based Chemicals and Polymers - page 18

Varying the stoichiometric ratio of epoxy to curing agent also provides a means of
regulating the properties. Figure 1.12 shows the representative tensile stress-strain
curves of the epoxy resins cured with MPA-PCL530 at different epoxide/anhydride
equivalent ratios. At the epoxide:anhydride ratios of 3:2 and 4:2, the cured epoxy
resins exhibited clear stress yield, followed by shear yielding of the samples. On
further increasing the epoxide:anhydride ratio to 5:2, the cured resin still displayed
a stress yield but was not able to undergo a stable shear yielding. On the other
hand, the tensile strength and modulus of the cured resins increased continuously
with an increase in the epoxide:anhydride equivalent ratio ranging from 3:2 to 5:2.
The changes of the tensile properties with increasing epoxy:curing agent ratio were
probably due to the increasing content of rigid polyether segments formed from the
ring-opening polymerisation of the epoxide.
DER332/MPA-PCL530 (3:2)
DER332/MPA-PCL530 (5:2)
DER332/MPA-PCL530 (2:1)
Strain (%)
Figure 1.12 Effect of epoxide:anhydride equivalent ratio on the tensile properties
of the cured resins. Reproduced with permission from H. Wang, X. Liu, J. Zhang
and M. Xian, Polymer International, 2009, 58 , 1435.2009, John Wiley and Sons
[11].
Figure 1.13 shows the nonisothermal DSC thermograms of DER332 curing with MPA-
PCL530 at different epoxide:anhydride equivalent ratios. At the epoxide:anhydride
equivalent ratio of 3:2, there was only one broad exothermic peak noted. However,
two exothermic peaks were noted when the equivalent ratio was increased to 4:2