Modificació química de reïnes epoxi amb lactones

Abstract

The curing of thermosetting materials is generally accompanied by shrinkage because covalent bonds form between chains and increase the density of the materials. This shrinkage leads to internal stress in the material, reduces adhesion to the substrate, and produces microvoids and microcracks, which reduce the durability of the material to worse the properties. Shrinkage during curing could be reduced or eliminated using monomers that polymerize without contraction or even with expansion. Ring-opening polymerization leads to less shrinkage than that produced by polycondensation or polyaddition, because not only small molecules are not eliminated in the polymerization, but for every bond that changes from a van der Waals distance to a covalent distance there is another bond that changes front a covalent to a van der Waals distance. Thus, the ring-opening polymerization of bicyclic monomers [(spiroorthoesters (SOE)] is a good strategy for obtaining non-shrinkable resins that can be applied in adhesives, coatings or composites. The classical synthetic procedure for obtaining SOEs is to react lactones with epoxides in the presence of a Lewis acid as a catalyst. In this way, cationic crosslinking of mixtures of epoxy resins with lactones could take place with little shrinkage because SOE groups are formed during this process. The cationic catalysts were lanthanide triflates because in previous studies, we have demonstrated that lanthanide triflates are Lewis acid that can completely cure diglycidylic and cycloaliphatic epoxy resins. The results of this study have been: - Using FTIR/ATR, we have shown that there were four elemental reactive processes during the copolimerization of epoxy groups and lactones of five members: homopolymerization of epoxy groups, formation of SOE by reaction of epoxy groups and lactones, copolimerization of SOE and epoxy groups and homopolimerization of SOE. The initiation process takes place through two active species. - The copolimerization of epoxy groups and lactones led to increase ester groups in the polymeric networks and this led to increase its degradability. - The addition of lactones to the curing of epoxy groups reduced the contraction after gelation, and this led to lower internal stress. - When Lewis acidity of lanthanide cation is increased, the catalyst becomes more active and the curing process accelerates.