Study On The Properties Of Recyclable Cycloaliphatic Epoxy Resin Containing Acetal Dynamic Covalent Bond And Its Carbon Fiber Reinforced Composites

As a class of general epoxy resins,cycloaliphatic epoxy resins have excellent processability,heat resistance,weather resistance and mechanical properties.Cycloaliphatic epoxy resins have been widely used in electronics packaging,adhesives,mold making and advanced composite materials.Cycloaliphatic epoxy resins would form three-dimensional cross-linked structures constructed by irreversible covalent bonds after curing.Therefore,the cured cycloaliphatic epoxy resins are insoluble and infusible,making them difficult to recycle and reuse.As the applications of epoxy resin-based carbon fiber reinforced composites(CFRC)keep increasing in recent years,the recycling problems of CFRC have attracted more and more attention.It is an effective way to develop epoxy resins with recyclable functions to realize recycling CFRC as useful resources.A novel cycloaliphatic epoxy resin containing acetal dynamic covalent bond was designed and synthesized in this thesis,and its rheological,thermal,mechanical,and recyclable properties were studied through a variety of test methods.Finally,the prepared epoxy resin is used as the matrix for CFRC,and non-destructive recycling of carbon fibers is realized by degrading the resin matrix.The main research contents of this thesis are as follows:(1)The liquid cycloaliphatic epoxy monomer containing acetal dynamic covalent bonds(CVP)was synthesized using 3-cyclohexene-1-methanol and 1,4-cyclohexanedimethanol divinyl ether as raw materials.The chemical structure of the synthesized CVP was characterized by infrared spectroscopy(FT-IR),~1H-NMR and ¹³C-NMR.The curing kinetics of the methylhexahydrophthalic anhydride/CVP system was studied by Kissinger and Isoconversional methods,and the apparent activation energy was calculated.The curing conditions of CVP determined by the epitaxy method are as follows:120?-2 h,150?-1 h,180?-1 h.(2)The thermal,mechanical,structural dynamics,and reprocessability of the CVP resin cured by methylhexahydrophthalic anhydride were studied through dynamic mechanical analyzer(DMA),thermogravimetric analyzer(TGA)and electronic universal testing machine.The results show that the glass transition temperature(T_g)of CVP epoxy resin is 119?,the initial thermal decomposition temperature(T_d5%)is 250? and the flexural strength,flexural modulus and fracture toughness parameter critical stress intensity factor(K_IC)of epoxy resin are 53.8 MPa,2639 MPa and 0.68 MPa·m^1/2,respectively.CVP epoxy resin exhibits stress relaxation behavior due to the dynamic reversible exchange reaction of the acetal bond when heated,and the network topology freezing transition temperature(T_v)is calculated to be 88?.The introduction of the acetal dynamic covalent bond allows the epoxy resin to be repeatedly processed and molded under hot-pressing conditions.After repeated molding,the retention rate of the flexural strength and flexural modulus of the epoxy resin can reach 81.3%and88.8%,respectively.In addition,CVP epoxy resin also has repairable,weldable and shape memory functions.(3)The degradation processes of CVP epoxy resin under different types of acid solutions(p-toluenesulfonic acid,hydrochloric acid,phosphoric acid,acetic acid),different acid concentrations(0.5 M,1 M,1.5 M),and different temperatures(25??60?)were studied.The experimental results show that the CVP epoxy resin can degrade fastest in a 1.5 M hydrochloric acid(water/acetone=1/9,v/v)solution at 60?(time used 0.23 h).Finally,using methylhexahydrophthalic anhydride/CVP as the matrix,CFRC was fabricated by hand lay-up and hot pressing.The prepared CFRC has excellent mechanical properties,and its tensile strength and tensile modulus are 678.1 MPa and 17.4 GPa,respectively.Under the optimized acid solution,the carbon fiber in CFRC can be recovered by degrading the epoxy resin matrix.The results of Raman spectroscopy and scanning electron microscopy showed that there was no residual resin matrix on the surface of the recycled carbon fiber and the structure was not damaged.