| UV cationic curing of cycloaliphatic epoxy resin system combines the advantages of epoxy resins and cationic photopolymerization features,such as rapidly-curing,much more deeper curing,no oxygen inhibition,good adhesion,excellent dimensional stability as well as heat resistance.It is these advantages that greatly satisfy the industrial needs of high-performance electronic encapsulation materials.However,it still has the defects of brittleness and poor impact resistance of the epoxy resin.Introduction of flexible component into epoxy resin could solve above problems in a great certain degree to provide good modified epoxy composites for electronic encapsulation.Usually,hydroxyl-terminated hyperbranched polyether was used to modify the UV cationic curing epoxy resins,but glass transition temperature of cured material will decrease,and impact strength still needs to be improved.Linear hydroxyl-terminated polyethers have a toughening effect in thermosetting epoxy resins,but it has not been reported in UV cationic curing systems.In this paper,a series of linear polyether(PTMG2000,PTMG1000,PPG2000,PPG4000)were chosen as flexible component in order to improve the impact strength of cationic UV-curable epoxy resin(ERL4221)composites while triarylhexafluorophosphonium thiophonium salt(PAG202)as cationic photoinitiator.Comparing effects of molecular weight and molecular structure of the toughening component on the performance of cationic UV-curable epoxy resin,the following conclusion can be drawn:1.Curing behavior and properties of UV cured ERL4221/PTMG1000 were studied.Real time-FTIR showed that PTMG1000 has a catalytic effect on the rate of ring-opening polymerization and reaction degree of epoxy resin.Thermal properties of the cured material show that PTMG 1000 could improve the heat resistance and increase Tg of the cured material.When the dosage of PTMG 1000 was 20wt%,the Tg and heat resistance increased by 17%and 1%,reaching 129.2℃ and 291.3℃ respectively.Impact strength increased significantly with increasing the amount of PTMG1000.SEM images also show that the toughness of cured material gradually increased with the continuous improvement of epoxy cross-linking network.2.Polymerization behavior of ERL4221/PTMG2000 system consisting of a higher molecular weight of PTMG shows that PTMG2000 could increase the ring-opening polymerization rate of epoxy groups.Tg of the solidified material increased from 110.4 ℃ up to 136.1℃with increasing PTMG2000 dosage.Howener,addition of PTMG2000 to ERL4221 will negatively affect heat resistance.Impact strength of cured material will increase significantly with increasing PTMG2000 dosage,and reached a maximum of 10.44 kJ/m2,which is 9 times that of cured ERL4221.Scanning electron microscope image showed that the fracture behavior changed from plastic fracture to ductile fracture,which provided a good evidence for the toughness of the cured material.3.PPG2000 was used as toughening component of UV curable epoxy resin.The results of photopolymerization kinetics show that PPG2000 has a limited effect on the rapid polymerization of epoxy resin.Tg and heat resistance of solidified materials showed a trend of decreasing first and then gradually increasing with increasing the amount of PPG2000,and eventually exceeding ERL4221.Tg and T5 could eventually reach 114.9℃ and 290.2℃,respectively.Impact strength of cured material increased with increasing the amount of PPG2000,and reached a maximum value of 10.01 kJ/m2.4.UV cationic photopolymerization of ERL4221/PPG4000 system with a larger molecular weight of PPG was studied.Results of polymerization kinetics show that PPG4000 also has a limited effect on polymerization.After testing thermal properties of cured material,it is known that the introduction of PPG4000 will reduce Tg and heat resistance of cured material.As the amount of PPG4000 increases,Tg and heat resistance of cured material willrecover and eventually approach the pure EL4221.Water absorption increased after addition of PPG4000,reaching 2.4%,but moisture resistance didn’t change significantly with increasing of PPG4000 dosage.Impact strength of ERL4221/PPG4000 could reach 12.13 kJ/m2,more than 10 times of pure ERL4221,which revealed that PPG4000 could effectively toughen the epoxy resin.5.By comparing four groups of linear polyether modified cycloaliphatic epoxy resin,it can be found that polymerization rate of epoxy group could be accelerated by reducing molecular weight of polyether.When the molecular weight of polyether components reaches a certain degree,the rate of ring-opening polymerization of epoxy groups in autoaccelerating stage will decrease.Tg and heat resistance of cured material are greatly affected by toughening component,and decrease in molecular weight of polyether is beneficial to increase Tg and heat resistance of cured material.The relatively low water absorption of toughening component itself helps to increase moisture resistance of cured material.As the molecular weight of toughening component increases,the moisture resistance and impact strength will gradually increases. |
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