TROUBLE WITH CYANOACRYLATE
The problems most people have with cyanoacrylates are tame by comparison: merely poor or failed bonds. So why does this happen?
There can be many reasons. Sometimes the trouble starts in the plant. Inferior drugstore cyanoacrylates often are of poor quality to begin with. Most are imported from Japan or Taiwan, where producers don’t spend enough time in refining.
If cyanoacrylate isn’t properly prepared, it will have a short shelf life. So the makers of the low quality stuff add excessive stabilisers to keep it from curing in the tube. The result is poor performance. The stabilisers are acidic materials. If the concentration is too high, it will overcome the catalytic effect of the minute amount of moisture on the substrate and nothing will happen.
Good CAs - used properly...are used every day by hundreds of different industries. Museums soak brittle bones and fossils with the glue; it helps bind them together and give them strength. Burt Rutan used CA extensively in the construction of the Voyager, the lightweight airplane that flew around the world.
CA can have a tensile strength of 4,000 to 5,000 psi, or roughly four times that of white oak. For all practical purposes that’s overkill.
Getting good results is more than a matter of buying a quality glue, however. You also need the right formula for the job you’re doing, and you have to use it correctly. Rule one is this: Don’t expect the water-thin CAs to do every job. The gap filling is directly related to the viscosity. You use the water-thin products on parts that are smooth, tight fitting, and relatively non-porous. These glues set fast; so when gluing a joint you must assemble the parts dry, and then apply the adhesive around the edges of the seam. It wicks deep onto the joint by capillary action and cures in seconds.
Because these CAs are so thin, they will not wick into loose joints, and they won’t bridge gaps. They’re not much good on porous materials either. They get soaked up before they can wick throughout the bond area.
That’s where the higher-viscosity formulas come in. Makers offer medium viscosities (like syrup) and thick glues (like a mixed epoxy). These are thick enough to bridge small gaps and to resist being sucked into the joint.
You apply them to the surfaces first, and then assemble the joint. Consequently they have slower cure rates. The thickest usually take a minute to cure.
But if you are using them on sloppy joints with wide gaps to fill, cure time may extend to minutes or even hours. This is one case where the spray-on accelerators are indispensable. Just mist a light coat on one of the mating surfaces. It will dry almost instantly, but remain active for several minutes. Apply your glue to the other part then assemble the joint. The accelerator will kick the glue over in seconds.

STICKY BUT TRICKY
Cyanoacrylates are odd beasts. So sometimes, despite your efforts, you may still have problems with them. Common causes are:
1. Poor fit. Even though thick formulas can fill small gaps, the better the fit the better the bond. Always check mating surfaces before bonding. Smooth them up and remove any burrs or rough spots. Kickers help, but it’s best to aim for a good fit in the first place.
2. Too much glue. Never use more than necessary. Optimum results are obtained with the minimum quantity of adhesive required to fill the joint. In general one free-falling- drop spreads over one square inch. It takes some experience to know how much glue is enough so it’s a good idea to experiment on scraps of your material.
3. Premature curing. Do not spread your glue before you assemble the parts. This encourages it to start curing. Instead, lay down a serpentine bead, then assemble the parts, letting pressure spread the bead out into a thin film.
4. Premature stressing. Although CAs cure in a matter of seconds, this initial cure is only about 20 percent of full strength, which is reached in 8 to 24 hours. Give the bond ample time to cure before subjecting it to much stress.
5. Surface contamination. CAs are more tolerant of this than most glues, but they still work best on clean surfaces. Waxes, oils, and excess moisture can act as barriers between glue and substrate, and this can lower bond strength.
6. Acidic surfaces. Since alkalinity triggers the cure, it’s not surprising that acidity inhibits it. To solve this problem, you can use a kicker on one of the mating surfaces. Kickers are organic amines that supply a heavy dose of alkalinity.
7. Low shelf life. Store unopened bottles in the freezer. Frozen, the adhesive will last at least two years. Once the bottle is opened, however, shelf life drops to about six months. Moisture in the air gets in, starts the curing process and the glue gets progressively thicker until it’s too gummy to use. Experts do not recommend refrigerating or freezing bottles that have been opened.
8. Cold. Users who store it in the fridge or freezer may take it out and use it cold. They apply it as usual, and the joint simply falls apart. What happened? The polymerization is not terribly sensitive to cold but when the glue is colder it is thicker, and it may not get squeezed into a thin enough film to expose it adequately to the surface moisture it needs to catalyse.
PS: Bomond Trading in Brookvale keep the range of Loctite CAs in handy sized bottles, refrigerated too.