CA (Cyanoacrylate) is a family of substances, the simplest molecule is methyl 2-cyanoacrylate and the most complex is 2-octyl cyanoacrylate. The simpler molecules are the superglues and the more complex are used in medical applications. The primary curing of CA is not by evaporation, evaporation of a solvent, or heat but is triggered from the presence of moisture in the air assuming there is even just a small amount of relative humidity present.
Unopened Bottles: The general guidelines for CA is that unopened bottles are best kept refrigerated (between 35F and 45F) and will last about 1 year. They should be brought to room temperature before opening to prevent condensation from forming inside the bottle.
Opened Bottles: After opening however, it should no longer be refrigerated as the condensation will prematurely cure the adhesive. Every time a drop of glue exits the package the volume of a drop of air enters. That air contains moisture. If the air is refrigerated, the moisture will condense and accelerate the curing process. According to general guidelines, open CA has a shelf life of 1 to 2 months. To prolong shelf life of opened bottles the guidelines indicate that the nozzle should be kept clean and securely sealed to insure the glue is air-tight. Storing open bottles in an air-tight bag or container with a desiccant like silica gel to remove any internal moisture will help prolong the workable life.
This information came from Gluegun, a 25 year old company specializing in the application of adhesives and Permabond a well known adhesive manufacturer.
Even though storing opened bottles without caps is outside the guidelines, depending on the environment (average relative humidity), with a small opening very little ambient air should enter the bottle other than when glue is being dispensed. And since evaporation of a volatile (solvent) is not a curing factor there should be little evaporation out of the bottle which would again be replaced by air entering the bottle. So, although it seems to be counter intuitive I can understand why the cap might not be necessary. The primary function of the cap may be more to prevent spillage if the bottle gets dropped or tipped over.
Regards,
Dave
