New Light-Curing Instant Adhesives (ThreeBond 1771E, 1773E, and ...

2. Characteristics of light-curing instant adhesives2-1. Curing mechanismAs shown in Figure 1, a light-curing instantadhesive cures on irradiation by ultraviolet orvisible light, thanks to a photo polymerizationinitiator contained in the adhesive that decomposesand generates anion species (Nu - (nucleophile)),which then advances anion polymerization.Conventional light-curing resins include radicalpolymerization and cation polymerization typeresins. Much technological expertise hasaccumulated for radical polymerization type resinsin particular, resulting in a wide range of adhesivesbased on various resin combinations.Among these, anion types (including instantadhesives) that use anion species as polymerizationinitiators remain at the stage of basic research dueto their instability and handling difficulty. Onlyseveral studies have been reported to date. Kutaland Yamaguchi et al. (University of Georgia, 2))reported that 2-cyanoacrylic acid esters undergophoto-induced polymerization with the addition ofmetallocene derivatives such as benzoylferrocene.However, the report addresses only aspects such as:"increased viscosity" and "gelation," with nodiscussion of preservation stability. In short, thereport is not useful in advancing practical adhesiveapplications.Our research has shown that radical generatorssignificantly activate the reactivity of the aboveferrocene derivative. We have succeeded indeveloping light-curing instant adhesives withexcellent light-curing properties and goodpreservation stability through concurrent use ofadhesives and radical generators.As shown in Figure 3, light-curing instantadhesives are activated by light irradiation, whichcures excess adhesive that laps from the adherendsof bonded materials. Based on instant adhesives, theportion between the adherends cures instantly bymoisture curing, as do conventional instantadhesives.AdherendsUltraviolet or visible lightCuring as a conventional instant adhesiveFigure 3: Curing mechanismCuring by opticalanion polymerization2-2. Characteristics of light-curing instantadhesivesLight-curing instant adhesives combine both lightcuring and moisture curing. This combinationreduces bleaching effect and improves surfacecuring and filling bonding properties, and vastlyexpands applications.Table 3 compares the characteristics oflight-curing instant adhesives, conventional instantadhesives, and typical light-curing resin.Table 3:Bonding of light-blocking materialsCuring of excess portionsSurface curing propertyBleachingTemporary retaining jigsCharacteristics of light-curing instantadhesivesInstantadhesivesNot requiredLight-curing instantadhesivesNot requiredLight-curingresinsRequired(1) Good bonding properties for light-blockingmaterialsAlthough transparent materials pose no problems,ordinary light-curing resins fail to cure when usedto bond light-blocking materials, as shown in Figure3, since the materials block light. In contrast,light-curing instant adhesives cure instantly viamoisture curing in the gaps between adherends.Light-curing instant adhesives are thus associatedwith lower incidence of curing failures thanlight-curing resins due to insufficient lightirradiation, even for transparent adherends.(2) Good surface curing of excess adhesivesprevents bleaching effectWhile conventional instant adhesives cureinstantly in the gaps between adherends, excessadhesive remaining as a liquid cures slowly,reacting with airborne moisture. This behavior isdue to an imbalance between the acid stabilizer andthe alkaline initiator, with too much stabilizer forthe amount of moisture. Under these conditions, themonomer gas evaporates readily and promotesbleaching effect. In contrast, light-curing instantadhesives and light-curing resins require only lightto cure.It should be noted that oxygen inhibits the curingof light-curing resins that cure by radicalpolymerization, resulting in poor surface curingcharacteristics.(3) No fixing jig required at bonding4