Too Much Accelerator?

December 9th, 2020

I see this over and over again on social media and YouTube; woodworkers applying cyanoacrylate (CA) glue followed by dousing the adjoining pieces of wood with ‘accelerator’. For most applications CA glue isn’t really necessary, and I’d wager that most use accelerator out of habit.

But, what’s the big deal? Can the use of an accelerator cause any harm to your project (or you)? Actually, yes! Let me explain. Cyanoacrylate (CA glue) exists as a monomer in the bottle. When cyanoacrylate is applied, the individual monomers of CA glue react with each other to form chains (polymers). The longer the chain, the better! The longer the chains are that form, the stronger the glue joint will be. The monomers soak into the wood surface (and adhere to surface nooks and crannies) of both pieces being glued together and when the monomers form a chain, both sides become glued together.

When an accelerator is used (usually a solution in acetone), it helps to speed up the process (see top part of figure below) of forming the chains. In the absence of accelerator, water will start the chain reaction and because water is ubiquitous it’s usually nearby. Accelerators are usually an organic molecule such as an aniline; which is a molecule with a nitrogen atom. Nitrogen atoms are basic (nucleophilic) and will react with the double bond of a CA glue monomer (the negative charge that forms is stabilized by resonance, but don’t get bogged down by this) which starts a chain reaction. The ‘accelerator-CA glue monomer‘ will then react with another CA glue monomer and so on (see top section of figure below). The more times the process happens, the longer the chain will be.

Now, here’s where we can get ourselves into trouble with accelerator. If too much accelerator is used, it can limit the number of available unreacted CA glue monomers that are available to react next in the chain. In the bottom section of the figure above, the accelerator has already reacted with a CA glue monomer and one other monomer (bottom section of the figure in the middle). If another unreacted monomer were around, it could react with it, extending the polymer chain by one more. But, in the example above there aren’t any unreacted monomers nearby because they have all reacted with accelerator; this causes the chain to terminate (stop).

In theory, too much accelerator will prevent the CA glue polymers from growing very long. And as I mentioned above, the longer the CA glue polymer chain, the stronger the glue joint will be.

Personally, I don’t like to use accelerator. CA glue cures pretty fast already and I don’t find that it saves a meaningful amount of time. Maybe it’s not worth the few seconds it saves in exchange for a weaker joint? I don’t think the tradeoff is worth it.

Here’s another thought to consider: CA glue, after it cures, is relatively non-toxic (oral). But, how toxic is the accelerator you’re using? I’ve watched videos of woodworkers dousing cutting boards with the stuff. Is your accelerator food-safe? You likely don’t even know what the accelerator is AND if you ask the manufacturer, they will likely tell you that it’s proprietary and won’t reveal it to you. The accelerator I use in the example above is a popular accelerator and it’s called N,N-dimethyl-p-toluidine. And guess what? It’s known to be carcinogenic and toxic (oral). Whether or not it makes you sick is going to depend on how much you use and how much you ingest. Another factor to consider when you want to save a few seconds of drying time for CA glue.

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