| Cationic UV curing cycloaliphatic epoxy resin that possesses advantages of rapid-curing, absence of oxygen inhibition, dark-curing, low shrinkage, good adhesion and excellent thermal stability is considered to have a bright application future in electronic encapsulation, while the inherent brittleness and poor moisture resistance of epoxy resin caused by high crosslinking density have always played as negative roles in developing application. So it is necessary to enhance toughness of epoxy resin by introducing flexible chain and improve thermal stability and moisture resistance via adding inorganic fillers, which would be of great significance to broaden application of epoxy resin for electronic encapsulation and prolong life of encapsulating device. With mixed triarylsulfonium hexafluorophosphate salt (PAG202) as cationic photoinitiator, four different toughening strategies for UV-cured cycloaliphatic epoxy resin (ERL4221) were carried out to prepare electronic encapsulation materials with excellent comprehensive properties. Following systems to improve thermal and mechanical properties of UV-cured cycloaliphatic epoxy resin were studied:(1) organic-organic composites prepared by ERL4221 and hyperbranched polyester (H2004); (2) organic-inorganic nanocomposite prepared by ERL4221 and nano-silica particles (nm-SiO2); (3) organic-inorganic composite prepared by ERL4221 and micro-silica particles (μm-SiO2); (4) organic-inorganic multi-composites prepared by ERL4221, H2004, nm-SiO2 and μm-SiO2. The main work and results of this research are as follows:1. Based on the unique dendritic structure of hyperbranched polyemrs (HBPs) and plentiful terminal functional groups, effects of different H2004 contents (5-20 wt%) on UV curing kinetics, moisture resistance, thermal and mechanical properties of UV cured ERL4221 were investigated in detail. Results revealed that epoxy group conversion, coefficient of thermal expansion (CTE) and impact strength were improved by adding H2004, accompanying with thermostability decreased to a certain degree. Glass transition temperature (Tg) decreased with increasing H2004 content, while impact strength showed an opposite tendency. The composites contained 5 wt% H2004 had an relative low CTE of 139 ppm/℃ and water absorption of 2.1%. Scanning electron microscope (SEM) images of fracture surfaces were turned out to be evidence of toughness enhancement. So it is possible to suggest that epoxy system contained 5 wt% H2004 should be a good choice for preparation of electronic encapsulation materials.2. Nm-SiO2 prepared by sol-gel technique was blended with ERL4221 to prepare UV curable organic-inorganic nanocomposites. Effects of different nm-SiO2 contents (5-20 wt%) on UV curing kinetics, moisture resistance, thermal and mechanical properties of UV cured nanocomposites were studied. Results indicated that epoxy group conversion increased with nm-SiO2 content increased to 10 wt%, at which maximum epoxy group conversion of 75% was gotten, water absorption was 2.6% and thermostability showed a small decrease of 6% and was still above 300℃(T5=305℃). Subsequently, epoxy group conversion decreased with increasing nm-SiO2 content. CTE decreased with increasing nm-SiO2 content, where an opposite tendency was found in impact strength. It is worth noting that epoxy system contained 10wt% nm-SiO2 showed balance between thermal and mechanical properties.3. Aiming at the unwanted increase in viscosity and difficulty in processing, a new strategy was put forward to prepare ERL4221/μm-SiO2 composites, where μm-SiO2 was firstly modified with silane coupling agents. Effects of different modified μm-SiO2 contents (10-40 wt%) on properties of composites were discussed. Epoxy group conversion was found to increase with μm-SiO2 content increased to 20 wt%, at which maximum epoxy group conversion and minimum CTE of 62 ppm/℃ were obtained, water absorption decreased to 1.8% and impact strength reached to 3.12 kJ/m (increased by 174%, compared with pristine ERL4221). Subsequently, epoxy group conversion decreased with μm-SiO2 increasing from 20wt% to 40wt%. All these information demonstrated that 20wt% μm-SiO2 was proper to prepare epoxy composites as electronic encapsulation materials.4. Based on toughness behavior of H2004, nano-size effect of nm-SiO2, high loading ability of μm-SiO2 and those conclusions above, epoxy-based multi-composites for electronic encapsulation was prepared by mixing epoxy resin,5wt% H2004,10wt% nm-SiO2 and 20wt% μm-SiO2, properties of which were fully characterized. The results indicated that the modifiers had a little effect on thermal properties, where Tg and T5 decreased to 99℃ and 309℃, respectively. Compared with pristine ERL4221 system, epoxy-based multi-composites possessed a low CTE of 88 ppm/℃, a high impact strength of 3.47 kJ/m2 and water absorption of 1.9%, which meet the requirement of electronic encapsulation. |
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