# Synthesis and Characterization of Unsaturated Succin Acid Biobased Polyester Resins
## 3 Results and Discussion
### Synthesis and Characterization of Cross-Linked Resins  
#### 3.3.1 DSC Results  
From the above studies it was proven that UPR resins were successfully prepared and should be further cross-linked using AA in the presence of different initiators and accelerators. In order to better understand the curing procedure, DSC investigation was implemented. Typically, the two most frequently chosen initiators for thermal-induced cross-linking are BPO and MEKP, with their main difference being the temperature of homolytic dissociation [38; 39]. For the measurements, UPRs were mixed with AA at a 2:1 ratio, as well as the respective initiator, along with 1% Co(II) octoate, which served as the accelerator [40]. In Figure 5, the influence of the accelerator and both the two initiators is presented, regarding the thermal curing evaluation of the UPRs by means of DSC measurements, using the PESM 50-50 resin as representative. The strong influence of the initiator type was noticed in the curing process, as is shown by the two thermographs of Figure 5 with the indication \"Initiator only.\" The cross-linking of the resins presented two exothermic peaks with maxima at 110.9 and 126.5 \({}^{\circ}\)C when using BPO, and at 98.8 and 157.3 \({}^{\circ}\)C when using MEKP as the initiator. These two exothermic peaks show that the cross-linking is a two-stage reaction, as also reported for the cross-linking of UPRs with the use of accelerator and initiator in previous studies [41; 42; 43]. These two stages are attributed either to redox decomposition of the initiator initially, followed by its thermal decomposition [41; 44], or to the radical copolymerization of AA with the resin followed by the radical homopolymerization of AA [45; 46]. Nevertheless, the attribution of these two peaks is a matter still under consideration [40]. After the addition of the accelerator, the two stages of cross-linking using BPO (Figure 5a) were shifted to 103.7 and 121.4 \({}^{\circ}\)C, whereas in the case of MEKP (Figure 5b) the two stages were difficult to separate, as the first stage at 90.5 \({}^{\circ}\)C was the dominant one. Figure 4: Viscosity of the synthesized resins (**a**) containing EG, (**b**) containing PEG200 as a function of temperature. MEKP and BPO are among the most commonly used peroxides for the cross-linking of monomers in the wood industry [47], whereas it has been reported that polyester resins present better thermal stability when using MEKP as the initiator, instead of BPO [48, 49]. On the other hand, although BPO has been reported as more efficient than MEKP for the polymerization of the UPR of PEG-MA with styrene [50], the UPRs of the present study presented better cross-linking behavior with MEKP. Regarding the use of accelerator, the addition of cobalt(II) octaate enhances the curing, adding better control of the reaction as it provides a higher cross-linking rate at lower temperatures and also shortens the duration of the reaction. On the other hand, the use of MEKP as the initiator provides the cross-linking of the resin at more than 10 \({}^{\circ}\)C lower than BPO, and the whole phenomenon is also completed faster. For the abovementioned reasons, MEKP was exclusively used as the initiator for the cross-linking of the experimental resins, with the additional use of cobalt(II) octaate as the accelerator. The influence of the amounts of both the accelerator and the initiator were examined. In Figure 6, the curing of PPEGSM 25-75 is presented, with the use of 3 wt% Co(II) octaate. Once again, the cross-linking was described as a two-stage reaction. The radical copolymerization of AA with the UPR was initiated at 61, 55, and 48 \({}^{\circ}\)C for the three different amounts of MEKP, with exothermic peak maxima at 80, 68, and 66 \({}^{\circ}\)C, respectively, showing that the temperature dependence of the reaction with the initiator stabilized after the use of 3 wt% of MEKP.