More than you ever wanted to know about CA's......
The basic CA molecules are all very similar. Most of what we use in finishing are ethyl cyanoacrylates. The low odor CA's are things like ethoxyethyl cyanoacylate. They are low odor because the basic molecule is a higher molecular weight, and so it is less volatile.
Manufacturers of CA adjust the viscosity of the CA glues by adding in polymethylmethacrylate (Plexiglas) in small amounts. The CA materials themselves have a very low viscosity, so the PMMA is used to tailor the glues for a variety of applications. Sometimes they also add in microscopically small silica particles. The effect of agingon the shelf is just like the effect of adding PMMA -- instead of adding PMMA intentionally, polycyanoacrylate forms in the CA liquid due tothe residual moisture.
CA cures via a process known as anionic polymerization. The reaction is triggered by the presense of a weak base. Water is a weak base, and every surface has a small amount of water present. Low humidity conditions (like winter up north) will slow down the cure of a CA. Very small amounts of moisture are needed to initiate the cure -- the water is not "used up" in each step of the reaction. This also means that exposure to very low leves of water during storage will reduce the shelf life of the CA. On the shelf, the CA cures due to the presence of water. every time you squeeze a drop of CA out of the bottle, the bottle breaths in a small amount of air (containg moisture), and the shelf life diminishes. When the tip clogs, using a pin to open up the clog can introduce more watter into the bottle, diminishing shelf life further. Always immediately replace the cap on your CA -- there is some diffusion of air/moisture in and out of the bottle even just sitting there. The reduced temperature of a refrigerator slows the aging, but if you take a bottle of CA out of the refrigerator and use it while the CA is cold and leave the opened bottle sitting near the lathe, yoou run the risk of condensation occuring within the bottle just due to the diffusion described above. With cold CA, it is very important to replace the cap right away. The thickened, old CA may still work for bonding, or even as a finish. If thin has aged to medium or thick, I wouldn't use it. A slight increase in viscosity is no big deal -- it is just like adding more Plexiglas to the original CA material just like the manufacturers do.
I store my CA bottles inside of a glass jar, and I keep a dessicant (like those little bags of silica gel you find in newly purchased electronics) inside the jar as well to scavenge any residual moisture.
Because CA cure is base-catalyzed, sometimes we have problems with the CA curing on wood. Many wood surfaces are slightly acidic, so the CA may not cure well right away, and sometimes not at all. The acid on the surface of the wood neutralizes the base required to catalyze the CA polymerization. It is a good practice to wipe your blanks with acetone to remove any surface oils, and then wipe with an accelerator for the CA. The accelerators are tyipcally amines, and amines are somewhat basic. It is the presence of the additional base that catalyzes the faster CA cure.
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