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

While light-curing resins require fixing jigs untilexposed to light, most light-curing instant adhesiveshave the advantage of adhering when applied to theadherends, eliminating the need for fixing jigs. Thisextremely useful characteristic reduces labor costsand increases speed of production lines.2-3. Properties of light-curing instantadhesivesTable 4 shows the properties for ThreeBond1771E, 1773E, and 1776E.Table 4: Properties of light-curing instant adhesivesItems Unit TB1771E TB1773ETB1776EMonomerColor and appearanceMain componentViscosity--mPa sTransparent yellow liquid2-cyanoacrylic acid ester2 150 1000Linear expansion coefficient (0 -100 )10 6 /81-103 75-99 70-99Glass transition temperature Tg124 123 117Standard curingconditionsUltraviolet curingVisible light curingkJ/m 2kJ/m 210 10 100.4 0.2 0.2PolymerDielectric breakdown voltageVolume resistivitykV/mmm27.0 26.0 25.85.4 10 13 5.9 10 13 5.4 10 13Surface resistivity1.2 10 13 5.9 10 12 1.2 10 13Dielectricconstant1MHz1MHz--3.27 2.69 2.904.05 3.34 3.61Dielectrictangent1MHz1MHz--0.0511 0.0529 0.05550.0522 0.0534 0.05482-4. Setting time and bonding strength oflight-curing instant adhesivesLight-curing instant adhesives offer good bondingproperties for various substrates, similar toconventional instant adhesives. Table 5 shows anexample of the setting time and bonding strength oflight-curing instant adhesives. The tension shearingadhesive strength in the table is assessed in atension shear test after applying adhesives underconditions of 25°C and 50% humidity, followed by24 hours of curing. Except for materials (such asmetals) that are stronger than the adhesivesthemselves, most adherends exhibit material break(i.e., the material breaks before the adherends due tothe strength of the bond).Not shown in Table 5 is another advantage oflight-curing instant adhesives, this being thatnonpolar or highly crystalline resins, generallyregarded as difficult to bond, can bond strongly. Forexample, in combination with a primer (ThreeBond1797), light-curing instant adhesives create strongbonds when used with polyethylene, polypropylene,polyacetal, soft polyvynyl-chlorides including largeamounts of plasticizer, silicone rubber, and EPDMrubber.Figure 4 shows the curing speed for moisturecuring and light curing. As previously discussed,conventional instant adhesives are moisture-curingtype. However, moisture-curing adhesives do notcure fully when initially applied to adherends.Although bonding strength increases rapidly at first,to complete curing generally it requires 12 to 24hours at room temperature.Light irradiation allows a bond to attain finalstrength in approximately 10 seconds. Thus, theoptimal curing method is to temporarily fix the5

adherends by moisture curing, then to curecompletely by light irradiation.Instant adhesives generally function also asreactive "solvents" capable of dissolving thesurfaces of certain plastic materials, includingacrylic, polycarbonate, and ABS. Table 5 shows thatthe setting times for acrylic and polycarbonate arerelatively slow. This is due the mutual solution ofinstant adhesive and the plastic in the bonding area.Since they cure instantly, light-curing instantadhesives can prevent the dissolution of plasticsurfaces in such cases.Table 5: Setting time and bonding strength of light-curing instant adhesivesAdherendsPolycarbonateABS6-nylonPETAcrylicNBRPaper (woody paper)IronStainless steelAluminumCopperBrassSetting time (seconds)Tension shearing adhesive strength (MPa)TB1771E TB1773E TB1776E TB1771E TB1773E TB1776E12 15 20 11.0 (*) 8.9 (*) 9.1 (*)2 3 7 7.4 (*) 6.7 (*) 7.0 (*)5 7 15 7.1 (*) 7.1 (*) 7.2 (*)2 7 10 12.2 (*) 12.9 (*) 12.8 (*)15 25 45 5.8 (*) 8.4 (*) 8.5 (*)2 2 2 0.5 (*) 0.5 (*) 0.5 (*)5 15 30 - (*) - (*) - (*)3 5 10 15.1 15.9 16.83 7 15 11.9 11.3 12.43 10 20 10.6 11.2 11.43 7 20 11.5 13.5 11.53 5 15 9.2 10.3 10.9Test method: 3TS-220-04 (Setting time)(*) indicates material breaking.3TS-301-11 (Tension shearing adhesive strength) [Tension speed 10 mm/min]TB1771E TB1773E TB1776ETension shearing adhesive strength (MPa)1210864200 6 12 18 24Time (h)Tension shearing adhesive strength (MPa)1210864200 6 12 18 24Time (h)(Note) The light irradiation condition is 3.5 kJ/m 2 with a 4-kW high-pressure mercury-vapor lamp.Moisture curing is performed in shade at 25°C and 50% humidity.Figure 4: Comparison of curing speed for moisture curing and light curing (Adherends: polycarbonate)6

2-5. Applications of light-curing instantadhesivesLight-curing instant adhesives offer a vast rangeof applications, including electric and electroniccomponents, fields in which conventional instantadhesives are not used, and precision components.They are used not just to create surface bondingwith various adherends, but for applicationsinvolving sealing, potting, and fixing. Additionally,since light-curing instant adhesives have theadvantage of curing at lower illumination thantypical light-curing resins, that is, since they curefaster, they should improve productivity in fastproduction lines. Major applications are listedbelow.(1) Electric, electronic, optical, and generalcomponents requiring non-bleaching properties(2) Fixing and bonding of supports around opticallenses(3) Fixing and bonding of various light-blockingadherends(4) Reinforcement of IC attachment(5) Bonding of fitting sections such as printer headsand bearings(6) Bonding of the fitting section of a drum motorand main shaft(7) Fixing and bonding of ornamental materials(8) Assembly of hypodermic needles and syringes(9) Creation of Braille characters and graphicsAdvancing miniaturization and driving to reduceweight has resulted in single components havingextremely fine structures. Even the portions thatcould conventionally be irradiated with sufficientlight, now the irradiation often causes problems. Insuch cases, light-curing instant adhesives that useboth light and moisture curing are effective inproviding reliable curing and bonding.With increasing miniaturization, printed circuitboards use less solder. But since bonding strengthmust be maintained, light-curing instant adhesivesare used to reinforce solder.For metal plated plastic components,conventional light-curing or anaerobic resinssometimes fail to bond the adherends. In some cases,light-curing instant adhesives are used, due to theireven broader bonding capabilities for a wide rangeof materials.Components such as hypodermic needles andsyringes have direct effects upon the human body.Reliable complete bonding and curing is critical intheir assembly. Light-curing instant adhesives arealso used for Braille characters and graphics, sincethe surface curing of light-curing resins is relativelypoor due to oxygen-induced curing inhibition, asalready discussed, and touching incompletely curedportions with hand may result in skin irritations.Light-curing instant adhesives are not affected bycuring inhibition with oxygen. It can be said a betteradhesive.7

3. SummaryIn view of recent trends toward international green technologies, the effectiveness and benefits of light-curingtechnology (including reduced use of organic solvents) in the areas of environment and energy are beingrecognized once again. Adding light curing properties overcomes the disadvantages of instant adhesives. Therange of potential applications for light-curing adhesives continues to expand rapidly.We plan to pursue the commercialization of shock-resistant and highly peel-resistant products, with the goal ofovercoming the remaining disadvantages of the materials discussed, thereby exploiting new applications and newmarkets.Masakazu MotokiIndustrial Materials Development DivisionDevelopment DepartmentResearch LaboratoryThree Bond Co., Ltd.1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, JapanTel: 81-426-61-13338