Lab benches - treatment for wood
(repost with better image quality)
Introduction and Background:
This post is on a suitable coating for wood surfaces for use in the wet chemistry lab, but is suitable for use in all kinds of things such as engineering benches or electronics benches too.
The most common material used to construct benches- especially in the amateur setting where cost is a restriction- is wood. Wood is cheap but presents the issue that it is porous in nature and absorbs spills readily. This is especially problematic when the spill is acidic, as the wood will continue to be attacked even after wiping the surface down with a wet rag. Protection against aqueous spills can be furnished by the use of paint, but it raises the problem that paint is not solvent-proof, and usually not acid-proof either.
In the past we used ceramic tile and float glass to cover our wooden benches, but they are difficult to drill for the installation of stand rods and very heavy, plus would require replacement when broken.
We were tipped off by a friend to a neat recipe (DOI 10.1021/ed002p353), which we relay here plus our own experience with it.
Warnings:
Potassium chlorate is an oxidizer, and may not be legal to own in certain jurisdictions. Copper sulfate is toxic if ingested. Aniline is possibly a carcinogen. Hydrochloric acid is corrosive. This reaction may produce a trace of chlorine. Boiled linseed oil presents a pyrophoric hazard if the rags used with it are throw in a trash receptacle.
Theory:
The treatment consists of two solutions:
\- Potassium chlorate and cupric sulfate
\- Aniline hydrochloride and hydrochloric acid
The first solution is applied to the clean wood surface while boiling hot in order to allow for expansion of the pores to allow for as much solution to absorb as possible. Two coats are used in total. When the second solution of aniline hydrochloric and hydrochloric acid is applied, the acid allows for the oxidation of aniline by chlorate (as chloric acid) to proceed, many intermediates are formed such as quinones, but the end result is the formation of polyaniline. This is an insoluble navy-blue to black compound that is inert. The only purpose of this is to stain the wood a black color, matching proper phenolic and epoxy benches. The reason for this complex chemical treatment to stain the wood is because in this way, the wood pores are unoccupied and allow for the final treatment with linseed oil to work. If black paint was used instead, the oil could not penetrate the wood properly.
Boiled linseed oil is oil from the flax plant (linseed) that has transition metal catalysts added to accelerate "drying". The name is derived from the very old process where the raw oil was heated with lead oxide to form salts with the fatty acids, resulting in catalysts. In modern times, the catalysts are usually cobalt, nickel, or iron instead of lead. The oil has a high content of linolenic acid esters, approximately 40-50%. Linolenic acid is one more unsaturated bond than linoleic acid, and is liable to air oxidation. On oxidation it polymerizes to form what amounts to linoleum. Due to this polymerization, the oil sets into a solid that is highly resistant to chemical attack and solvents. This is the same principle behind seasoning cast-iron cookware. With ordinary linseed oil, it would take several months to fully "dry", with boiled linseed, several days to weeks, and the process can be accelerated with a "Japan dryer", which is essentially a higher concentration solution of the metal catalysts in a solvent. I suspect the leftover copper sulfate in the wood also acts as a catalyst to some extent.
This oxidation reaction is also why rags soaked with the oil may spontaneously combust if throw into a trash can. The rag acts like a wick, with a large amount of surface area to air, and in a trash can the heat from the oxidation can result in a runaway and eventually a fire. Rags used with linseed oil must be laid flat on a non-flammable surface and let "dry" before disposal, or alternatively soaked in a bucket of water.
Notes from experience:
The wood must be entirely free of paint and oil. Even if you use a solvent to strip the paint off, the pores of the wood are still occupied by polymer, and thus the board must be planed or sanded down.
The solutions need-not be at a rolling boil, but near boiling is sufficient. For the coats of the chlorate-copper solution, you do not need the first coat to completely be dry before the second is applied. It only has to be dry to the touch. The application of the aniline solution however requires the wood to be fully dry, otherwise it will not penetrate as thoroughly. At this stage, the wood should turn slightly blacker, but we found that the complete darkening only happens on heating or drying. Be patient.
After the aniline solution is applied and developed properly, the wood board is washed with a wet soapy rag a few times, then a rag with fresh water a few times. This only serves to remove the remaining copper sulfate and potassium chloride from the surface of the wood. There is no aniline present anymore as it is consumed in the polymerization, nor is there chlorate for it has been reduced to chloride, thus there is no severe hazard at this stage. The rag will likely get colored blue or black as some surface polyaniline rubs off, but it is easily washed free from the rag as the polyaniline exists as particles just like dirt does.
The board must be let FULLY dry before boiled linseed oil is applied. We simply poured a good quantity of the oil onto the board, spread it around with a paper towel, and let it sit for an hour or two. If all of the oil gets absorbed, apply more oil. Eventually we gave up trying to saturate the board after using about half a liter. Vigorously rubbing the oil into the board can speed up the process. The wood is entirely usable as a bench surface once the oil is taken up, but keep in mind it has not fully dried yet at this stage so try to not spill anything on it.
Conclusion:
The result is a very nice looking surface that is entirely waterproof and solventproof (after drying for a week or longer), it may be polished to a shine but we did not choose to do so. Compared to our phenolic lab bench in our fumehood, it looks very similar other than the wood texture, but we quite like the appearance. We would describe the appearance as satin. The obvious advantage over a phenolic or epoxy bench is that wood is cheap, and also more heat resistant than the phenolic bench. Our phenolic bench spalled when we put hot glassware on it whereas a wood bench would singe, but otherwise remain unaffected. Wood is also easier to work with than phenolic resin.
Overall this treatment for wood is quite ingenious in its simplicity yet effectiveness, and while one could simply treat the wood with only linseed oil if they do not desire a nice appearance, we think it is worth going the extra effort to stain the wood with polyaniline as the chemicals used are quite cheap, the process is simple, and the end result is gorgeous.
If you are going to spend much of your free time at the lab bench, why not make it something you are proud of?