Curing Behavior and Characterization of Dual Curable Adhesives Based on Azo-initiator with High Reactivity for Touch Screen Panel in Display

Abstract

Dual curing was introduced to improve the conversion of optically clear resin (OCR) on shadowed area of display bonding process. The application of UV curing is increased due to the distinct advantages such as fast curing rate, low ambient temperature and solvent-free process. However, UV curing can cause curing problem in shadowed area or curved area, which inhibits UV light penetration. Up till now, only UV curing process with side UV irradiation was applied in touch screen panel bonding process. Curing problem was an issue especially in display bonding process because UV light could not penetrate black matrix (BM) area of the window. Therefore, dual curing system was researched to improve the conversion at shadowed area. In this study, 2,2'-azobis(4-methoxy-2,4-dimethyl valeronitrile), the thermal radical initiator with 30°C of 10 hour half-life decomposition temperature, was used for granting ability of thermal curing in the low curing temperature. Compared to other dual curing methods, the radical dual curing system did not remain the unreacted functional group after curing. Also, changing thermal initiator easily controlled the reaction temperature. Acrylic prepolymer was synthesized by UV bulk polymerization using 2-ethylhexyl acrylate (2-EHA), isobornyl acrylate (IBA) and N-vinyl caprolactam (VC) with 1phr of hydroxydimethyl acetophenone as photo radical initiator. Dual curable adhesives were prepared by blending prepolymer with poly(ethyleneglycol (200) dimethacrylate) and thermal radical initiator. The thermal radical initiator, 2,2'-azobis(4-methoxy-2,4-dimethyl valeronitrile), was added under 0.1phr of the resin without problems such as stability, yellowing and bubbling issue. Also, dual curable adhesives was cured with various UV doses (0, 400, 800 and 1600 mJ/cm2) because progress of UV light was hindered by FPCB or BM area of the display in the bonding process. The curing behavior of dual curable adhesives was investigated by using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), photo-differential scanning calorimetry (photo-DSC) and UV-advanced rheometric expansion system (UV-ARES). Thermal curing efficiency was increased as the concentration of thermal initiator increased. Also, 0.1phr of thermal initiator contents provided enough conversion of the adhesives. Thermal curing efficiency was decreased as the UV dose increases due to the decreasing mobility of the polymer. However, thermal initiation was accelerated in low UV dose condition. Gel fraction results showed that network structure was formed additionally by thermal curing, but none of the network structure of the dual cured adhesive was formed over 800 mJ/cm2 UV dose. Conversion and degree of crosslinking could be obtained by accelerating thermal curing efficiency. However, adhesive with 800 mJ/cm2 UV dose which the thermal curing was suppressed had lower crosslinking density and conversion than adhesive with 400 mJ/cm2 UV dose. In addition, thermal resistance and glass transition temperature was increased by thermal initiator reaction. It is assumed that the formation of a temporary crosslinking structure was produced through the reaction of residual monomer and thermal initiator after the complete UV curing through the thermal curing. UV curing behavior, thermal curing behavior and viscoelastic property were investigated sequentially by UV-ARES. Addition of thermal initiator did not affect to storage modulus change in UV curing process. Thermal curing efficiency was decreased as UV dose increased. Especially, the addition of thermal initiator could not influence the storage modulus after 800 mJ/cm2 of UV dose. The results indicated that thermal curing was suppressed by UV curing because the mobility of the polymer was decreased. The thermal curing efficiency was affected to the storage modulus at plateau area. Viscoelastic property results indicated that dual curable adhesives with low UV dose had higher crosslinking density. Adhesion performance was increased due to the formation of network structure with high crosslinking density in low UV dose condition at the shadow area. Based on the result, formation of molecular structure in accordance with dual curing condition was suggested.