Research And Application Of Metal-Organic Framework In Latent Curing Agent Of Epoxy Resin

Epoxy resin has a variety of excellent properties,so it is used in various fields.Typically,epoxy resin is liquid at room temperature and has limited its utilities.Only after adding the curing agent,the epoxy resin can solidify and exhibits excellent performance to apply in diversified fields.The explicit curing agent has some shortcomings,such as tedious preparation,transportation trouble and short storage life,which has been unable to meet current demands,so the latent curing agent is favored and arouses much attention of researchers.Due to the strong chemical activity of2-methylimidazole,choosing 2-methylimidazole（2MI）as a latent curing agent for solidifying epoxy resin epoxy resin is easily cured and the storage life of the mixed system is short and the use efficiency is low.Therefore,a better approach is to temporarily block the 2MI activity with the aiding of external carriers,where the shelf life of curing system can be extended,and then release activity under the stimulus of external conditions and trigger the curing reaction.Metal-Organic Frameworks（MOFs）have the characteristics of unsaturated metal active sites,large specific surface area,adjustable pore sizes,and designable structures.These properties are the favorable advantages of MOFs as carrier-loaded curing agents.There are few research reports on MOFs as carrier-loaded curing agents,and it is a great challenge to develop new latent curing agents with strong stability.First of all,MOFs need to use its three-dimension interpenetration structure to encapsulate the curing agent in the pore channel and simultaneously inhibit the activity of the curing agent.Then,stimulated by external conditions,MOFs can open the pore channels,and the curing agent activity can be released rapidly,thus stimulating the curing reaction of epoxy resin.In this paper,the new latent curing agent for epoxy resin was prepared by using Zn-based MOF（ZIF-8）and Zr-based MOFs（Ui O-66,Ui O-66-（COOH）₂）as carriers and encapsulating curing agent 2MI.The purpose is to prolong the storage and shelf life of the curing system,explore the reaction kinetics of the curing process,improve the properties of the cured products,and expand the practical application range of epoxy resin.The research process and conclusions are as follows:（1）Preparation of a novel latent curing agent 2MI@ZIF-8 by one-step synthesis method.By loading 2MI into the channel of ZIF-8,the curing temperature of epoxy resin was increased while the reaction activity was inhibited.The unsaturated metal sites of ZIF-8 enhance the cross-linked interaction with epoxy resin and improve the stability of the cured product.The results show that the curing reaction of E-51/2MI@ZIF-8 system requires higher energy,that is,the curing reaction needs to start at a higher temperature.Compared with the E-51/2MI system,the storage life of the curing system with 2MI@ZIF-8 at 40℃is increased by 2-6 times,and the storage stability at different temperatures is improved as well.In addition,the preparation method of 2MI@ZIF-8 is effortless to operate,which provides the possibility for volume-produce.（2）The effect of different MOFs as curing agent carrier on the curing system was studied,and Zr-based MOF was used as external carrier.Synthesis of Ui O-66 by condensation reflux,2MI was loaded into the pores of MOF by solution impregnation method.According to the different loading of 2MI,the curing agents with different curing effects were obtained.Compared with 2MI@ZIF-8 curing agent,the curing system using 2MI@Ui O-66 has a higher curing temperature,which is approximately15℃higher.At the same time,the energy needed for curing reaction increased correspondingly,and the storage life of E-51/2MI@Ui O-66 system was prolonged again.（3）In order to further broaden the scope of application of Zr-based MOF as the curing agent carrier,adopting pyromellitic acid hydrate to functionalize Ui O-66 and further synthesize Ui O-66-（COOH）₂.After two carboxyl groups modification,the latent curing agent 2MI@Ui O-66-（COOH）₂has small aperture and high loading density of2MI in the pore channels.In comparison with 2MI@Ui O-66 curing agent,the increase of the number of-COOH reduces the activation energy needed for curing reaction,reduces the curing temperature and expands the scope of application.Correspondingly,the storage time of E-51/2MI@Ui O-66-（COOH）₂system was shortened.In addition,compared with the E-51/2MI system,the thermal stability of the solidified products of the E-51/2MI@Ui O-66-（COOH）₂system was improved.