Research On Modification And Adhesion Properties Of Epoxy Resin Adhesive

As a desirable solution for joining the heterogeneous materials,adhesive-bonding technology has been widely applied to the automobile manufacturing.However,high-strength structural adhesives can usually be cured only at high temperature,which results in both the increase of costs and a great thermal stress at the adhesive joint.The great thermal stress may lead to a warpage and joint cracking.To alleviate this problem,a study on “Adhesive bonding of Homogeneous and Heterogeneous Materials” was conducted by Zhengzhou University and General Motors Corporation.In this study,the effects of environmental exposure on the joint strength of AA6061-T4 aluminum alloy bonded with two modified adhesives(4.0%C1+1.0%CA1+5089 and 2.5%C2+4.0%CA2+ 5089)were studied.Besides,the differential scanning calorimetry(DSC)and dynamic thermomechanical analysis(DMA)were used to analyze the curing behavior and heat resistance of the modified adhesives.The results of the environmental exposure tests showed that the air exposure of modified adhesives,hot and humid environmental exposure(40? & R.H 96%)of the AA6061-T4 aluminum alloy,and the water soak(54?+7 days)of the bonded joints had little effects on the shear strength.However,the working life of the modified adhesives 4.0%C1+1.0%CA1+5089 and 2.5%C2+4.0%CA2+5089 was shorten to 3h and 5h,respectively.If the exposure time was longer than the working life,it was difficulty to apply uniformly the modified adhesive to the aluminum alloy sheets.This results indicated that a partial curing reaction of the modified adhesives should be occurred with the prolonging of exposure time,which resulted in a significant increase of viscosity.The joint strength of the modified adhesive 4.0%C1+1.0%CA1+5089 increased with the increase of curing temperature(100-120?)for curing time of 20 min while the shear strength of 2.5%C2+4.0%CA2+5089 adhesive-bonded joints remained almost the same(~21MPa),which was only 5MPa lower than that of Henkel 5089 adhesive(~26MPa)cured at 177? for 20 min and significantly higher than that of 4.0%C1+1.0%CA1 +5089 adhesive(15-19MPa).It suggested that the combined addition of curing agent and curing accelerator can markedly lower the curing temperature of Henkel 5089 adhesive with a little sacrifice in strength.If the curing time was reduced to 10 min,the joint strength of two modified adhesives decreased with decreasing curing temperature.Nevertheless,the decrease of joint strength was more apparent for 2.5%C2+4.0%CA2+5089 adhesive.The high temperature(82?)tensile tests showed that the shear strengths of adhesive-bonded joints made with Henkel 5089,4.0%C1+ 1.0%CA1+5089 and 2.5%C2+4.0%CA2+5089 adhesives were significantly reduced by 49%,69% and 8.7% respectively compared to that obtained at room temperature,which indicated that a much higher joint strength at 82 oC can be obtained with 2.5%C2+4.0%CA2+5089 adhesive.In addition,the room temperature shear strength of the bonded joints increased slightly by 1.7-7.1% after a retention at 82? for 30 min.It may be related to the decrease of the water content in the bonding joints.During heating,an apparent exothermic peak associated with curing reaction can be observed for the Differential Scanning Calorimetry(DSC)curves of the modified adhesives 2.5%C2+4.0%CA2+5089,1.0%C1+4.0%CA2+5089(formulated with the optimized composition of the modified adhesive 4.0%C1+1.0%CA1+5089 and 2.5%C2+4.0% CA2+5089)and Henkel 5089 adhesive Comparing to the exothermic peak temperature(162oC)of Henkel 5089 adhesive,the peak temperature(~125oC)was evidently lower.It indicated that the combined addition of curing agent and curing accelerator can apparently decrease the curing temperature of Henkel 5089 adhesive.However,two exothermic peaks were observed for the DSC curve of the modified adhesive 4.0%C1+ 1.0%CA1+5089.One exothermic peak appeared at a much lower temperature(87?)while the other exothermic peak was in well agreement with that of Henkel 5089 adhesive but with an apparent lower peak height.It indicated that the addition of 4.0%C1+1.0%CA1 cannot change the curing reaction process of Henkel 5089 adhesive,but simply increased an extra curing process caused by the curing additives.The relaxation behavior of the modified adhesives cured at different curing temperature for 20 min was investigated by dynamic mechanical analysis(DMA).It showed that the glass transition temperature Tg of the modified adhesive 4.0%C1+1.0%CA1+5089 increased with the increase of curing temperature,while the Tg of the other two modified adhesives was independent of the curing temperature.Among them,the modified adhesive 2.5%C2+4.0%CA2+5089 had the best heat resistance with a higher Tg.In addition,the activation energy of glass transition of all modified adhesives was significantly lower than that of Henkel 5089.It indicated that the combined addition of curing agent and curing accelerator weakened the heat resistance of the modified adhesives.The comprehensive results showed that the combination addition of curing agent C1/C2 and curing accelerator CA1/CA2 to Henkel 5089 significantly lowered the curing temperature with only a little sacrifice in strength.Besides,the modified adhesives had good water resistance and thermal cycling performance.As a whole,the modified adhesive 2.5%C2+4.0%CA2+5089 had the best modified effect on Henkel 5089 adhesive.