*3.5. Mechanisms of the Effects of the Diamine*

Figure 15 illustrates the aminolysis of PET using a diamine [34], where the reaction led to a decrease in the PET molar mass. However, the amine terminated PET chain could react further with a second component (WG or another PET chain). Based on the IR data, it was observed that the ester groups of the PET reacted with the amine (decrease in the ester peak in the PET component of the Brushed sample). The good bond between WG and PET indicated that the un-reacted amine on the "aminated" PET reacted with WG at the interface. The presence of the diamine is a prerequisite for any bonding to occur at all between PET and WG. However, at high temperature, depolymerization involving amide containing polymers occurs in the presence of diamine [27]. Hence, the diamine reacts with the main chain protein amide causing a cleavage of the protein chain. Cross-linking reactions with the diamine as a coupling agent may also occur between the protein chains, however, the cleavage mechanism seems to dominate.

**Figure 15.** Aminolysis of PET.

### **4. Conclusions**

Through the current study, an innovative step has been taken to apply a continuous, strong, and transparent layer aided by a coupling agent on WG plastics to impart water resistance. The diamine cross-linker (i.e., Jeffamine) that was used created a strong bond between the WG polymer and the

PET layer. The two methods used to laminate the protein plastic with PET had different advantages and drawbacks. In the Ground method, the strongest WG/PET interface was obtained but the protein was to some extent negatively affected (lower strength). The lower strength was due to the bulk WG material and not related to the adhesion with the PET layer. No delamination occurred between the WG and the PET layer in the Ground or the Brushed samples. In the Brushed method, the protein was less affected, but the interfacial strength and the hydrophobicity of the PET layer, containing migrated diamine, was lower, also leading to a somewhat poorer water vapor transmission rate. Nevertheless, both methods of lamination with PET led to significantly higher water vapor barrier properties than that of neat WG. The PET layered samples also remained undistorted during the water exposure. One of the biggest issues with WG is its dimensional instability under high humidity conditions. The PET layers, having a strong adherence to the WG, eliminated this issue. Still, there is room for improvement regarding the negative side-effects of the diamine and future work should include the optimization of the content of the added diamine as well as the processing conditions.

**Author Contributions:** Conceptualization, O.D. and M.S.H.; Methodology, O.D. and M.S.H.; Formal Analysis, O.D., T.A.L., and A.J.C.; Investigation, O.D. and I.L.; Data Curation, O.D., T.A.L., A.J.C., and M.S.H.; Writing–Original Draft Preparation, O.D.; Writing–Review & Editing, O.D., T.A.L., A.J.C., and M.S.H.; Supervision, O.D. and M.S.H.; Project Administration, M.S.H.; Funding Acquisition, M.S.H.

**Funding:** This research was funded by the Lantmännen Research Foundation (No. 2016H010).

**Acknowledgments:** The authors are grateful to the Lantmännen Research Foundation for providing the funding for this project. We also thank the University of Auckland, New Zealand for allowing us to use their nanoindentation instrument for this research.

**Conflicts of Interest:** The authors declare no conflict of interest.
