Research On The Aging Properties Of Epoxy Resin In Hygrothermal Environment

With excellent electrical properties,epoxy resin is widely used in electronic packaging and pouring power equipment.The existence of moisture and heat factors in the environment would affect the insulation resistance of epoxy resin seriously,accelerate its aging rate,and endanger the safe operation of equipment.Therefore,it is important to study the aging characteristics of epoxy resin in hygrothermal environment.After hygrothermal aging experiment,the mass change rates,infrared spectrum,tensile cross-sectional images,dielectric properties and space charge characteristics of the samples were tested.The changing laws of hygroscopicity and aging characteristics were analyzed.And molecular simulation software was used to research the influence of hygrothermal environment on the epoxy resin system from the micro level to supplement the experimental conclusions.The main contents are shown below:(1)The variations of different moisture absorption contents with time are gained in consideration of thermal loss,and the hygroscopic process is divided into three stages.At the first stage,hygroscopicity is mainly achieved by the physical diffusion caused by the different concentration of water molecules.At the second stage,cracks and other absorbent channels begin to appear and hygroscopicity is mainly achieved through physical diffusion and absorption channels.Chemical moisture absorption occurs at the third stage.Chemical moisture absorption is converted by physical moisture absorption.The appearance of chemical moisture absorption leads to moisture absorption increasing rapidly.After hygrothermal aging,the mass change rate of samples shows“increase-decrease-increase”changing trend,which does not follow the Fick's second law.(2)The internal cracks of aged samples are processed and the processing result indicates the perimeter and area of cracks increase exponentially with aging time increasing.The non-uniformity of cracks increases first and then decreases slowly,which indicates that cracks and water molecules are distributed more and more evenly inside material during the late moisture absorption period.The cracks undergo the change process from little to many and shallow to deep.With the increase of moisture absorption content,the number of cracks increases,and they are constantly deepened and broadened on the basis of the original cracks.The appearance of lots of cracks causes that water molecules are fully in contact with the epoxy hydrophilic groups and chemical moisture absorption appears under high temperatures.(3)It is found that the carbonyl content of the samples increases after hygrothermal aging,which indicates oxidation reaction is caused inside the material during hygrothermal aging process.The contents of free small molecules produced by the materials cracking and water molecules are constantly changing,which causes that the dielectric constant real and imaginary parts of aged samples are not monotonously increasing.Hygrothermal aging introduces more traps and the dissipation rate of residual charges inside the epoxy resin is suppressed.The maximum value of trap depth increases exponentially with time,and has a similar functional relationship with crack perimeter.More cracks mean more deep traps.(4)The movement ability of water molecules increases with water molecule content or ambient temperature increasing.When the water molecule content is higher than 1.32%,water molecules preferentially bind hydrogen bonds with polar groups inside epoxy resin and chemical absorption appears,which is close to 1.36%of the experimental results.At the initial aging stage,the binding force between water molecules and epoxy hydrophilic groups increases,which causes that the structure of material becomes tight,thus glass transition temperature increases.When water molecule content is greater than 1.32%,system swells significantly and glass transition temperature decreases.At the beginning of aging,water molecules mainly occupy free volume,and the swelling effect in the later stage is greater than the occupation effect,which causes the free volume fraction to decrease first and then increase.(5)The chain mobility of the epoxy resin increases after hygrothermal aging,which results in system stability decreasing.The elastic modulus of the epoxy crosslinking system gradually decreases,the ability of the system to resist deformation decreases,and the growth of cracks inside material is accelerated.The cross-linked epoxy resin chains undergo decrosslinking reaction in hygrothermal environment,and the cleavage product contains intermediate reaction product of oxidation reaction,C3H6,CO2,H~+radicals and so on.According to the calculation results of the density of states,the band gap of the system becomes narrow after hygrothermal aging,and new traps are generated.The traps are mainly provided by the 2p orbits of C atoms.