Light Cure CyanoacrylatesAdvantages• One-part• Solvent-free• Rapid fixture and cure with low intensitylight exposure• Rapid room temperature fixture• Cure in shadowed areas• Not inhibited by oxygen• Excellent adhesion to many substrates• Superior bond strength in shearand tensile modes• Primers available <strong>for</strong> polyolefins anddifficult-to-bond plastics• Wicking grade availableConsiderations• Poor peel strength• Limited gap cure in areas shielded from light(
Select AdhesiveCandidatesAppendix 1 presents the key per<strong>for</strong>manceattributes <strong>for</strong> 19 Loctite ® needle bonding adhesives.This in<strong>for</strong>mation should be used to select adhesivecandidates <strong>for</strong> further consideration.Several important selection variables are:ViscosityViscosity is the resistance of a fluid to flow. Fluidsthat flow easily, like water, have a low viscosity.Fluids that do not flow easily, such as windowcaulk, have high viscosities. Viscosity is typicallymeasured in centipoise (cP).The adhesive’s viscosity must be selected based onthe part design and manufacturing process used. Ifthe cannula will be assembled with the hub be<strong>for</strong>ethe adhesive is dispensed, a low viscosity (< 3,000cP) adhesive will ensure fast, consistent fill of thewell and core. If the adhesive will be dispensed onthe cannula be<strong>for</strong>e it is assembled with the hub orthere are large gaps between the cannula and thehub, a higher viscosity adhesive (3,000 to 30,000cP) will ensure that the product does not migrateprior to assembly and/or ensure optimum gap fill.Table 2 presents the time required to fill the hubwell and core as a function of adhesive viscosity.These flow times were determined using a highspeedcamera when dispensing Loctite ® light cureacrylic adhesives on standard specimens that had a0.002" diametrical gap.TABLE 2: Viscosity-Flow Time Relationshipwith 0.002" Diametrical Gap (minute:second)VISCOSITY(cP)MAX. TIMETO FILL WELLMAX. TIMETO FILL COREOther factors such as the thixotropic ratio andsurface tension of a particular adhesive candramatically effect flow time. There<strong>for</strong>e, the data inTable 2 should only be used as a general guideline.On-part testing should determine actual flow timesrequired <strong>for</strong> a specific manufacturing process.Depending on the needle joint design,manufacturers may want to quickly fill the annularspace in the core. If the cannula protrudes from thecore as shown in Figure 1A, the surface tension ofthe adhesive will typically prevent the adhesive fromflowing out of the core. Most needle assemblies aredesigned with a 0.002" diametrical gap. With thisgap, even very low viscosity adhesives (20 cP) willnot migrate after prolonged delays be<strong>for</strong>e cure. Byfilling the annular space in the core quickly, needlepull strength will be maximized.If the cannula ends in the core as shown in Figures1B and 1C, it is more likely that the adhesive willflow beyond the end of the cannula. Capillary actioncan then pull the adhesive into the inner diameter ofthe cannula, obstructing the needle and limiting fluidflow. As the flow time increases, adhesive is morelikely to migrate. Table 3 illustrates the potential <strong>for</strong>cannula clogs as a function of adhesive viscosityand the flow time between dispense and cure.This testing was conducted using a seatedcannula with a 0.002" diametrical gap anda light cure acrylic adhesive.TABLE 3: Clog-Flow Time RelationshipFLOWAdhesive Viscosity (cP)TIME 100 500 1,0007 sec – – –14 sec Clogs – –5 min Clogs Clogs –As an adhesive is heated, its viscosity drops rapidly.For example, an epoxy that has a viscosity of20,000 cP at room temperature may have aviscosity of 3,000 cP at 65 o C (150 o F). Conversely, ifan adhesive is cooled, its viscosity will rise.This viscosity-temperature relationship createsan interesting advantage <strong>for</strong> one-part heat cureepoxies. Their viscosity will initially drop as theadhesive is heated and will then begin to risesharply as the adhesive cures, allowing heat cureepoxies to achieve full coverage that would not bepossible with light cure acrylic adhesives of thesame viscosity dispensed at room temperature.FluorescenceFluorescent dyes absorb UV light.The energy they absorb excites thedye molecule, emitting a fluorescentlight that is typically blue in color. Thislight can be visually observed with ablack light if held over the adhesive ina darkened environment. Detectors,such as the Loctite ® FluorescenceDetector, part number 98083, canbe used alone or in conjunctionwith vision systems to per<strong>for</strong>min-line confirmation that theadhesive is present in the bondline.When fluorescent dye is added tolight curing adhesives, the dyedecreases the light cure responseand can also decrease needle pullstrengths. By quantitativelydetermining the minimumlevel of fluorescencenecessary <strong>for</strong> robustin-line detection, Loctite ®needle bondingadhesives are <strong>for</strong>mulatedwith the optimumbalance of fluorescence,light cure response, andpull strength.Cure SpeedCure speed defines howlong the adhesive mustbe exposed to a givencondition be<strong>for</strong>e it is fullyconverted from a liquid to a solid. Cure speed willdictate work-in-process levels. Typically, the rapidcure of light cure acrylics and light curecyanoacrylates results in minimal work-in-process,whereas the 15 to 45 minute cure times of onepartheat-cure epoxies necessitates more inprocesstime.100 0:05 0:20500 0:07 0:30121,000 0:15 1:206,000 0:30 3:109,000 0:40 4:00A. Protruding CannulaFigure 1: Common <strong>Needle</strong> <strong>Design</strong>sB. Recessed Cannula C. Seated Cannula13
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