Study On Sequential Thermal Dual-Curing And Properties Of Thiol-Acrylate-Epoxy Resin 3D Network

Curing of thermosetting resins is usually a continuous,temperature-sensitive and irreversible process,which limits their processing and shape design.Sequential dual-curing can be used to control the structure and properties of intermediate and final materials during machining.However,the existing sequential-dual curing systems have some problems,such as high curing temperature,overlapping curing reaction of two curing stages and difficult to control the properties of intermediate materials.In addition,the preparation of thermosetting resin foams by two-step chemical foaming has been widely studied,but it is difficult to match the increase in viscosity caused by resin crosslinking with the growth of bubbles.Therefore,in this study,the thiol-acrylate-epoxy sequential dual-curing system was constructed by taking advantage of the great difference in the reaction activity of thiol-acrylate and thiol-epoxy catalyzed by tertiary amine at room temperature.In this systems,acrylate and thiol are cured at room temperature to form intermediate materials,providing adjustable entropy elasticity,mechanical properties and process operation.Epoxy and thiol are cured at high temperature to form final materials,providing a complete cross-linking network and final properties.In addition,the application of shape memory actuator,high performance pressure sensitive adhesive film and UV foam adhesive film by regulating the network composition of intermediate materials respectively.And the influence of hydrogen bonding on the curing process and properties of the system was also investigated.The main research results of this paper are as follows:First and foremost,1,4-bis[（2-oxiranylmethoxy）methyl]-benzene（BOB）and 2,5-bis[（2-oxiranylmethoxy）methyl]-furan（BOF）were used as two epoxy resin building blocks of the network respectively,1,4-cyclohexane dimethanol diacrylate（CHDMDA）was used as the acrylate building blocks of the network,tetra（3-mercaptopropionate）pentaerythritol ester（SH4）was used as crosslinking agent and 4-dimethylaminopyridine（DMAP）was used as nucleophilic catalyst to construct two thiol-epoxy pure formulation system,one thiol-acrylate pure formulation system and one thiol-acrylate-epoxy double formulation system.The results of isothermal DSC and FTIR monitoring of the curing of the three pure formulations showed that the thiol-acrylate reaction ended in 60 min at 30℃,while the thiol-epoxy system did not change significantly for at least 600 min.The initial reaction temperature of thiol-epoxy was about 100℃,which was 70℃different from the curing temperature of thiol-acrylate.At the same time,it was found that the hydrogen bond in thiol-epoxy system would lead to the increase of activation energy of curing reaction,which was beneficial to the regulation of sequential curing.What’more,DMA,DSC and mechanical test results show that hydrogen bonding can also improve mechanical properties.Through 30℃isothermal FTIR tracing and temperature rheological tracing of the dual formulation system,it was also found that the thiol-acrylate reaction and the thiol-epoxy reaction did not overlap at 30℃,which was consistent with the results of the pure formulation system.Therefore,it proved the successful construction of the sequential thermal dual-curing system of thiol-acrylate-epoxy,and there is a wide operating window between the two stages of curing reaction.Secondly,in the dual formulation system,the rheological,thermal,thermomechanical and mechanical properties of intermediate and final materials were precisely regulated by controlling the ratio of acrylate to thiol group r_acrylate under the premise of keeping the total reaction group and thiol group equal mole of epoxy and acrylate.When r_acrylate>0.2,the intermediate material changes from liquid to gel,and with the increase of r_acrylate,the thermal,thermomechanical and mechanical properties of the intermediate and final materials gradually improve.When r_acrylate was between 0.4 and 0.6,the system had good operability at room temperature.The applications of shape memory actuator and pressure sensitive adhesive film were investigated by using r_acrylate0.6 and 0.4 systems respectively.By monitoring the changes of the residual reaction heat and hardness of the intermediate materials with time at room temperature,the application period of the intermediate materials at room temperature was evaluated as 24 h.Finally,a two-step UV foaming system was constructed in the thiol-acrylate-epoxy sequential thermal dual-curing system by introducing the diacridine compound which can produce nitrogen under 365 nm UV light as the foaming agent（BA）.By increasing the content of BA,the cell size of UV foaming materials can be further controlled from 700μm to 100μm;relative density from 1.8667 to 0.3910;thermal conductivity from 0.2181 to 0.0915W/（m·K）;the degradation time at 90℃was reduced from 60 min to 40 min.Moreover,because the diacridine structure in BA has no selective insertion of molecular chain after UV irradiation,it can increase steric resistance and make the T_g of foaming material rise instead of decrease.In addition,the application of this system as flexible UV foam adhesive film was explored by using the intermediate material with self-adhesive property and machinability.