Use of Rosin Acids as Rigid Building Blocks in the Synthesis of Curing Agents for Epoxies - Rosin-based Chemicals and Polymers - page 19

or 5:2. Theoretically, one anhydride group can react with two oxirane groups to
form a hydroxyl diester. However, if the epoxide:anhydride equivalent ratio is
large than 1:1 in the curing system (i.e., excessive epoxy is available) the hydroxyls
formed can initiate the ring-opening of oxirane in the epoxy molecules, resulting in
homopolymerisation of the epoxy and the formation of crosslinked polyether segments
in the network. The exothermic peak at the higher temperature was attributed to the
homopolymerisation of epoxy. The occurrence of homopolymerisation of the epoxy
was detected in the cured samples using Fourier transform infrared (FTIR) analysis.
Figure 1.14 shows that the intensity of the characteristic absorption peak (1040
cm -1 ) attributed to the -C-O-C- stretch increased with increasing epoxide:anhydride
equivalent ratio, suggesting that more polyether segments were formed. Because the
accelerator, 2-ethyl-4-methylimidazole, also reacted with epoxide, there was only
a slightly excess of epoxy in the system when the epoxide:anhydride equivalent
ratio was 3:2. Therefore, in that case, the peak in the DSC thermogram which was
attributed to the homopolymerisation of epoxy could not be distinguished from that
attributed to the accelerated curing between the epoxy and the anhydride. Similar
multiple exothermic peaks were also observed in other epoxy systems which contained
a large excess of epoxy.
DER332/MPA-PCL530
5:2
4:2
3:2
Temperature (ºC)
Figure 1.13 DSC thermograms of cured DER332/MPA-PCL530 systems with
different epoxide:anhydride equivalent ratios. Reproduced with permission from
H. Wang, X. Liu, J. Zhang and M. Xian, Polymer International, 2009, 58 , 1435.
2009, John Wiley and Sons [11].