Characterization and aging behavior of optically clear acrylic adhesives for touch screen panel

Abstract

Optically clear adhesive (OCA) for touch screen panel (TSP) means highly transparent pressure sensitive adhesive (PSA) which has more than 90% of visible light transmittance. This name indicates especially PSA applied for TSP assembly, for example, bonding between transparent electrode and cover window or display module. OCA needs high transparency because contents from display module reach to users eye through it. Along with this transparency, required properties for OCA are suitable adhesion strength, anti-corrosive property, cloud point-resistance, and durability. Acrylic polymer is mainly used as OCA because of its excellent transparency. UV-curing method has employed for OCA preparation due to its high efficiency and productivity. In capacitive TSP structure, which is used mostly in these days, OCA is contacted to transparent electrode. Therefore, OCA should not damage transparent electrode since it is closely related to TSP driving. General acrylic adhesives include acrylic acid for improving cohesion but this component can corrode transparent electrode due to carboxyl group in it. Accordingly, acid-free PSA should be prepared with excluding acidic component. In this study, N-vinyl caprolactam was chosen instead of acrylic acid. Cloud point-resistance of PSA mean no haze appearance on cured PSA film regardless of environmental change. This property can be given to PSA as incorporating hydrophilic moiety into PSA matrix. Hydrophilic ingredient helps equal distribution of absorbed moisture in PSA matrix under humid condition thus cloudy state of PSA do not occur. Oligomeric (Methoxylated Polyethyleneglycol Acrylate, PEGA) and monomeric (2-hydroxyethyl acrylate, HEA) hydrophilic component were incorporated to the PSA in this experiment. Their effects on adhesion property and aging behavior were compared. As a result, monomeric ingredient, HEA showed better cloud point-resistance assignment to the PSA and more stable adhesion property although it passed hygrothermal aging than those of oligomeric material incorporated one. PEGA contains ether groups in its structure and this functional group is weak to oxidation hence adhesion property of PEGA blended PSA declined when it was aged. On the other hand, HEA was well distributed owing to its low molecular weight. This phenomenon and hydroxyl group in HEA structure had relevance to better cloud point-resistance, adhesion property, and hygrothermal aging stability of prepared PSA by HEA incorporation. Both hydrophilic component was also effective to prevent corrosion of conductive substrate.