Archive for the ‘Wood Chem’ Category

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

How does “Polyurethane” work?

Tuesday, September 8th, 2015

(**Every attempt was made to make this easy to read by the non-chemist woodworker.  My fellow chemists please keep this in mind.)

*Images enlarge when clicked*

 

Polyurethane is a polymer (a long chain molecule with repeating units) made by combining a dual isocyanate (this is just the group R-C=N=O, a very reactive species) with a poly (more than one – this is where the POLY in polyurethane comes from) alcohol.

urethane

The straight arrows on the left and right (above) indicate where the polymer could continue on, resulting in a larger polymer.  So, a polyurethane is a long chain polymer of an isocyanate and a polyol (alcohol with more than one -OH).  As soon as the two are mixed the reaction will start.  So, as soon as they are mixed the curing process begins.  This isn’t very useful for a woodworker (some professions might use a two part polyurethane).  If the manufacturer mixed them in a can and sold them to you, by the time you opened the can it would be a solid.

Another way to make a film finish is to have everything in one part with the curing process a little slower and controllable.  This can be achieved with a uralkyd finish.  A uralkyd finish is really what you’re most likely buying at the home center when you buy “polyurethane”.  A uralkyd finish combines features of a urethane (above) and an alkyd.  What the heck is an alkyd?

An alkyd is a polyester with fatty acids sticking off of it.  The fatty acids are what give an alkyd finish the ability to polymerize (with oxygen).  The process is the same one that gives drying oils, like tung oil, their ability to cure in the presence of oxygen.

Below is a uralkyd resin that I drew.  The resin below is only an example and the version of uralkyd that you are using will be different (the differences are going to be proprietary by the company that sells it) but the principle is the same.  A uralkyd can be viewed as a fatty acid modified polyurethane.

uralkyd3

After a uralkyd finish is applied to your project and the solvent evaporates, it will begin to cure.  This happens at the double bonds  (shown with arrows above) which react with a molecule of oxygen (below).  After this, it will react with another uralkyd molecule (the R-OOH will first break down into R-O radical and OH radical, but don’t worry about this – see fat arrow below).  The process of the double bonds reacting with oxygen, and cross linking, can be accelerated with metal driers which is usually added by the manufacturer.  I might dive into the effect of metal driers in more detail in another posting (depends on how this one goes down).  The section of the uralkyd shown in red below is linoleic acid.  Linoleic acid is a component in a lot of drying oils, like tung oil.

polymerization2

Neat huh?   Please let me know if you have questions.

 

** EDIT (9/9/2015) a uralkyd is still considered a polyurethane because it has poly (many) urethane (also called carbamate)  linkages.

 

Super Glue is ‘Super’

Saturday, May 21st, 2011

Cyanoacrylate or ‘super glue’ is a very useful glue to have around the shop. It works by forming a polymer or ‘chain of molecules’ with another molecule of cyanoacrylate.

The Nu- in the drawing above stands for ‘Nucleophile’ and is what starts the chain reaction. It is normally water. In super glue accelerators the Nu is the nitrogen atom of an aniline which is a much stronger nucleophile than water (that’s what makes it an accelerator). Using too much accelerator will make shorter polymers and potentially a weaker bond. Use sparingly!

Cyanoacrylate comes in handy for me most often when fixing small imperfections in wood (cracks, knots, etc.) by applying a dilute (thin) solution of cyanoacrylate in the crack and applying a small amount of sawdust. After it’s sanded you can barely see the crack when done carefully. For this I usually buy a thin solution already pre-made from ‘INSERT FAVORITE SUPPLY WOODWORKING STORE’ and use as is.

Cheers,
Brian