The cyanoacrylate glue, popularly referred to as superglue is one of those additions to the workbench which brought a small technological revolution in modelling. Not only did it allow to bond different materials such as metal to plastic, but also opened up the way for entirely new modelling materials such as resin. Used on plastic, it provides non-shrinking, non-destructive seams that are just great for sanding. Applied as filler, it takes scribing unlike any other.
Since there seems to be a degree of confusion about how cayanoacrylate glue works, and what it is best used for, some research was conducted and the results are presented here.
What is Superglue?
The correct chemical designation for the CA glue is ethyl 2-cyanoacrylate. An acronym ECA is also used in chemistry. There are also numerous trade names, including superglue (of course), permabond, pro grip, black max, krazy glue, cyanolite, superbonder and so on.
The actual composition of most commercial glues is typically ca. 91% ethyl 2-cyanoacrylate, 9% polymethylmethacrylate, <0.5% hydroquinone and a trace of organic sulfonic acid. Mercury Adhesives is 99.9% pure CA. Cheap Chinese CA’s can be as much as 95% CA, with 5% toxic fillers, such as Benzene and Chloroform (yeah, like the stuff you see in movies that knock people out). Some Chinese CA’s have been seen with as much as 50% filler. It is believed that these CA’s were old, and chemicals were added to thin, or “refresh” the glues to be marketable.
A common source of confusion is mixing up CA glue with thread locking glue. The main reason for this seems to be historical. In the thread locking glue, polymerisation starts in the absence of oxygene (air), while CA bonds with the aid of humidity.
How it Works
The cyanoacrylate glue hardens very quickly when trapped between two surfaces. The reaction is caused by the condensed water vapour on the surfaces (namely the hydroxyl ions in water). The water comes from the surrounding air, so obviously the air humidity is a factor that may affect bonding capabilities, or cause them to differ from application to application.
The curing reaction starts at the surface of the bonded material and develops towards the centre of the bond. Because of this, thick seams or large blobs of glue may harden less satisfactorily than surface-to-surface bonds with good fit. In a thick blob of glue, a polymerisation reaction may stop before it reaches the centre of the blob. A rule of thumb is that seams thicker than 0,25 mm should be avoided. Thick seams will also take longer time to cure.
The described relation between seam width and curing time can be used to advantage: a thick superglue-filled seam will allow adjustment of the parts, but will bond instantly and definitely when they are pressed together, so that the gap decreases below 0,25 mm. Pressing the parts harder against each other will make the glue cure instantly.
The best use for CA glues is undoubtedly attaching small details, where small amount of glue would cater for thorough polymerisation and advantage can be taken of the extremely fast bonding time.
CA glue will provide strong bonds on a wide variety of materials. The shearing and pulling resistance are very good. However, it should not be used on glass or on parts that are exposed to water.
Curing time and slow-setting inhibitors
The hardening reaction can be described like this. The cyanoacrylate is a polymer which contains its own hardener compound. However, a weak acid is added acting as an inhibitor, preventing the reaction and “holding apart” the molecules which accounts for the liquid consistency of the compound. When exposed to water, the acid is dissolved. It triggers a chain reaction and the compound cures to the solid state.
Manufacturers use the inhibitor to control the curing time of the glue. Slow-setting superglues have a larger proportion of inhibiting acid in the basic mixture.
Besides water, cyanoacrylate polymerises also in presence of alcohol and basic compounds (including weak amines). The latter can be used to produce a superglue “kicker” – a compound which triggers quick polymerisation of the glue.
Baking soda is one well-known substance with this effect. If you apply a layer of superglue to a seam and gently pour baking soda over it, the glue will cure very quickly. It makes for most effective filler for smaller jobs, and the baking soda results in a slightly rough surface which is good for sanding.
The great advantage of using superglue as filler is the total absence of shrinking which plagues most solvent-based fillers on the market.
Mercury Adhesives provides an Accelerator for our CA’s, called MH16. It is Heptane based, to produce a less violent reaction. This slower reaction is by design to reduce bubbles in the bond to ensure maximum strength. If there are bubbled in the bond, then it is weakened and may fail when under stress. Also, the slower reaction time reduces heat that can melt or damage foam.
Like water, the accelerator also affects the reaction through surface contact, so it will be much less effective on thick layers of glue. When filling larger recesses with superglue, it is therefore advisable to build up the volume in several thin layers rather than applying a large volume of glue at once.
With these precautions (i.e. working with small amounts at a time), CA can also be used for moulding smaller detail parts.
A word of health warning
Be mindful that superglue comes with its own set of health hazards.
The glue has a distinctive, strong, acid odour. Breathing cyanoacrylate fumes is irritating for your breathing organs. For some individuals, repeated or extended exposure to fumes may prompt chronic allergic reaction. In dry air (less than 50% humidity), fumes may be also be irritating to eyes, stimulating tears. With the cheaper generic/Chinese brands, the fumes can also contain toxins that can be outright harmful to your health. Mercury Adhesives are 99.9% pure CA. As such, there are fewer fumes and absolutely no toxins.
In contact with the skin, the primary risk with the CA is bonding fingers or other body parts together. This goes also for eyelids, so remember to never ever poke your hands into the eyes while working with superglue!
In skin or eye contact, CA is deemed to be non-toxic, so don’t panic, assess the situation, and seek medical help if necessary. Never try to tear the apart the bonded body parts!
It is also ascertained that CA cannot trigger allergic reactions through skin contact.