| The syntactic foam has excellent properties such as high strength,low water absorption,and low dielectric constant,et al.It has a very wide application in the fields of aerospace and marine.Due to its good curing activity,mechanical properties and dielectric properties,the epoxy resin-based syntactic foams have become a hotspot in this field,but the viscosity of syntactic foam before curing is relatively high,which limits its forming performance.The imidazole curing accelerator has the characteristics of high activity,low dosage,low volatility and high heat distortion temperature of the cured product.It is an important curing agent for epoxy resin.However,imidazole curing accelerator also has a high melting point,short shelf life and some other issues.Therefore,this paper mainly studies from the following aspects:Firstly,the naphthalene diisocyanate reacts with imidazole to obtain the passivating NI which is a kind of imidazole curing agent,and its structure and curing mechanism is studied.The results showed that the allophanyl group had formed and the reaction is complete.The deblocking temperature of NI is 190? which is determined by DSC(Differential Scanning Calorimetry)and TG(Thermogravimetry).The factors which have effects on the curing activity,curing kinetics and conversion rate of epoxy resin/dicyandiamide/NI system(EDN)are also investigated.The results show that the this system exhibits a good curing activity,and its characteristic temperatures are 50~60 ? lower than the system without accelerator.The relationship between accelerator content,curing time and conversion rate is not obvious.However the curing temperature has a great effect on the conversion rate.The apparent activation energy Ea is 89.33kJ/mol,and the curing reaction order is 1.The shelf life of EDN system is about 12 days at 40?and above 170 days at room temperature respectively.Secondly,the heat resistance,mechanical properties,hygrothermal aging performance and water absorption performance of the cured product of EDN system are investigated.The results show that the Tg increases with the rising content of NI,and when the content of NI is 2phr,the Tg of the cured product of the system is up to 149?.The NI content has little effect on the thermal decomposition temperature and bending property of the cured material.The retention rate of the bending strength after hygrothermal aging is between 80%~95%.The water absorption of the system is increasing with the rising amount of accelerator.When the equilibrium is reached,the water absorption is between 9% and 12%.Thirdly,the effects of microspheres type and volume fraction on the density,mechanical properties and water absorption of syntactic foam are studied.The results show that the increase of microsphere content lead to a decrease in density of syntactic foam,and the actual density is lower than the theoretical density.For the tensile and bending properties,they gradually decrease with the increase of microsphere content,but the modulus shows an increasing trend.The compressive strength of the syntactic foam with A type microspheres gradually increases with the increase of microsphere content;and for the one with B type,it remains almost the same;and for the one with C type,it decreases with the increase of the microspheres content.The water absorption rate of the syntactic foams increases firstly and then decreases with the increasing content of microspheres.The microstructures of syntactic foam with different contents of A type microspheres are analyzed with SEM.The results show that when the content of microspheres is low,the main failure mode is the shear failure of matrix resin;when the content of microspheres is relatively large,the main failure mode is the interface destruction and damage of microspheres.The effects of the volume fraction of B type microspheres on the mechanical properties of the syntactic foams with two kinds of microspheres are also studied.The results show that the tensile strength will be the maximum when the volume fraction of B type microspheres is 16.1 vol%;and bending and compressive strength will reach maximum when the volume fraction of B type microspheres is 23.7 vol%.In general,the mechanical properties of syntactic foam with the mixed microspheres are not as good as the one which uses B type microspheres singly.Finally,an epoxy resin-based syntactic foam film(SF)is prepared and its curing performance,rheological property,forming method and the heat resistance are studied.The results show that the SF film has a good medium temperature curing property.And its low viscosity temperature range is 50? before curing,which means it has a long processing time.The mechanical properties of sample prepared by press curing are better than those of the one prepared by vacuum bags curing.The results of heat loss show that the 5%,10% and 20% weight loss of the SF film corresponds to the temperature of 351?,373? and 395? respectively.When the temperature rises to 800?,it still has a weight retention rate of 35%. |
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