Preparation And Properties Of Dendritic Poly(Propylene Imine) Curing Agents For Epoxy Resins

In this dissertation, on the basis of the extensive review of the research progress of epoxy curing agents, we intent to develop new curing agents based on poly(propylene imine)s (PPIs) to overcome the high volatile of conventional low-molecular-weight aliphatic amines and the inferior thermal stability of poly(amido amine) dendrimers, simultaneously. Five PPIs of the different molecular structure and functionality are prepared in high purity, with optimized the synthesis and separation methods obtained, and their molecular structures are characterized with FTIR, NMR and MS.By using differential scanning calorimetry (DSC), the nonisothermal reactions of the different epoxy resin (DGEBA) cured with different PPIs are systematically investigated. The results indicate that PPIs are able to well cure the epoxy resin with the high reactivity and the reaction exotherm in accord with the typical values (98-122 kJ·mol-1) for a majority of epoxy-amine polymerizations. Meanwhile, the molecular structure, functionality, and molecular weight of PPIs have moderate influences on their reactivity. Furthermore, the global activation energies of the different epoxy systems are determined to be 55-60 kJ-mol"1 with the Kissinger method, and the reaction kinetic analysis with the Malek method confirms the SB(m, n) model and its parameters to establish the corresponding rate equations with which to model the reaction rate, with a good match achieved.The DGEBA/PPI reactions are inspected in detail with an isothermal DSC technique. The results show that reaction enthalpies of the isothermal and nonisothermal reactions differ slightly (<10%), and for all cases the reactions exhibit the apparent characteristic of autocatalysis with the maximum curing rate observed at conversion of 0.2-0.3. Moreover, the Kamal model is justified to be able to well model the reaction rates in the reaction-controlled stage, whereas the reaction rates over the entire conversion range can be satisfactorily simulated with the extended Kamal containing a diffusion term. Furthermore, an analysis of the reaction mechanisms with the advanced isoconversional method (AICM) reveals that the effective activation energy of the reaction closely relates to the fractional conversion. This finding mirrors the change of the microscopic reaction mechanisms during the cure. Additionally, the tertiary amine of PPIs is likely to be able catalyze the epoxy-amine reactions, as indicated by the decrease in the reaction activation energy.The viscoelasticity, thermal stability and other properties of the epoxy resins cured with the two representative PPIs:N,N,N',N'-tetra(3-aminopropyl)-1,2-ethanediamine and N,N,N',N'-tetra(3-aminopropyl)-1,6-diaminohexane are examined using DMA, TGA, static mechanical analysis, etc., respectively. The results demonstrate that the cured epoxy networks exhibit the glass and secondary relaxations, the former having much higher activation energy, enthalpy, and entropy than the latter. Furthermore, compared to the controlled linear aliphatic amines, PPIs can impart the cured epoxy resins with the sufficiently good thermal stability, improved thermal resistance, excellent shear strength, and reduced gel time.The preliminary attempt is made of using glycidylether (BGE) and acrylonitrile (AN) to chemically modify one representative PPI (N,N,N',N'-tetra(3-aminopropyl)-1,2-ethanediamine) to yield the novel modified-aliphatic-amine curing agents for epoxy reins. Subsequently, nonisothermal reactions, thermal stability, dynamic mechanical properties, and other performances of the corresponding epoxy systems are examined in depth with DSC, TGA, DMA, and other means, respectively. The results show that these modified curing agents can considerably decrease the reactivity, moderately suppress the reaction exotherms, appreciably lower the glass and secondary relaxation temperatures, and substantially prolong the gel time, in particular, AN being much more effective in lowering the reactivity of the modified curing agent. Therefore, they show a great promise as the new hardeners for epoxy resins, particularly, for high-performance epoxy adhesives and related technical fields.