Archive for the ‘Science’ Category

Too Much Accelerator?

Wednesday, 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.

How Two Part Epoxy Works

Saturday, November 18th, 2017

Every attempt was made to write the article below so that anyone with a basic general knowledge of chemistry could follow along.  If anything is not clear please ask below.

Two part epoxies work by combining an epoxy resin and a hardener in equal amounts.  The “equal amounts” is important because the reaction between the two is one to one.  The hardener does NOT work as a catalyst, but as a component of the reaction (it gets consumed in the reaction).  The epoxy resin is composed of an epoxide functional group separated by a spacer (a bunch of atoms) and another epoxide.  Interestingly, the word “epoxide” is where epoxy resin gets its name.

An epoxide is a very reactive species that is composed of a three-membered ring (two CH2 and one oxygen).  Epoxides are very reactive to nucleophiles which causes the ring to open.  A nucleophile is an atom in a chemical that is rich in electron density and is attracted to positive charge.  Nucleophiles (Nu) differ in their nucleophilic strength and examples include the oxygen atom of water, and nitrogen atoms of amines.  And to take the strength point one step further, the nitrogens of different chemicals can also differ in nucleophilic strength.  That is, not all chemicals that contain an amine have the same nucleophilic strength.  The two dots (..) above the -Nu denote the unshared pair of electrons of the nucleophile and it is the unshared pair of electrons that “seek out” positive charge (there really isn’t any “seeking” per se – positive and negative charges attract each other and obey Coulomb’s law).  In the figure below, the carbon adjacent to the oxygen has a partial positive charge because of the neighboring oxygen atom (inductive effect).

Below is an example of an epoxide resin (A) with an epoxide functional group on either end (three membered ring).  Epoxide resins can vary in structure and the differences will impact the epoxy’s properties such as strength, cure time and appearance.  The structure of epoxy resins and their hardeners are often proprietary and covered by patents.

The amine chemical above that is labeled “B” is the hardener in a two part epoxy.  It usually contains a primary amine (-NH2) as the nucleophile (Nu).  Primary amines will have a nitrogen atom and two hydrogens connected to them followed by a carbon chain (or singe carbon in the case of the primary amine methylamine – Breaking Bad anyone?).  The hardener will often contain two nucleophiles per molecule (B has two primary amines).  A hardener with two nucleophiles can cross link two molecules of resin (one nucleophile reacts with one epoxide per molecule).  The product of the cross linking can continue with another molecule of B because it has available epoxides remaining.  This is what leads to the polymerization or chain forming process.

In the schematic below, the hardener contains a primary amine, but it only has one amine per molecule (the -R simply denotes any organic fragment).  The primary amine B can react with one molecule of A to give C which contains a secondary amine (a secondary amine will only have one hydrogen atom connected to it).  Secondary amines are also capable of reacting with epoxides and will give D in the example below.  The problem with secondary amines is that the nitrogen will be more crowded (steric hindrance) and this can slow down its reacting with another molecule of A and slow down the curing process.

An interesting point to make with epoxides is that woodworkers often like to darken their epoxy using a dye.  And if your dye contains a primary or secondary amine (like an aniline dye) it could actually react with your epoxy resin.  This shouldn’t interfere with the properties of your epoxy because only a very small amount of dye is usually used.

substituted aniline

Music, it Calms the Savage Woodworker

Tuesday, March 1st, 2016

Do you listen to music while you work in the shop?  Maybe you should if you’re not already.  Studies have demonstrated that music can have the effect of reducing stress and improving focus.  If you are ever sedated and put under the knife of a surgeon, chances are he is doing the cutting to the sounds of music.  Aside from its calming attributes, music has also been shown to enhance learning and memory when played during an activity; I find this fascinating.  As a woodworker these benefits have the potential to be profound.  When learning to cut dovetails for example, playing music while cutting them has the potential to relax you and help to ingrain the required procedural memory.  Any time a new task is learned or practiced, any woodworker could potentially gain more from the experience by playing music.  Googling the association between music and learning/memory will reveal dozens of articles, studies and anecdotes.

Unfortunately for me, as a video content creator, most of the time I can’t listen to music as I work in the shop; it would muck up the video production pretty good.  But, I do like to listen to music when the camera isn’t rolling.

I’d LOVE to get a little feedback from you guys.  Do you listen to music in the woodshop?  Do you find it helpful?  What kind of music do you play and does it change with the type of task you are doing?