Urethane and Polyurethanes

Re: Urethane and Polyurethanes

Bill Tindall, E.Tn.

somebody put this in the Articles so I don't have to keep repeating it.......

"Urethane" is the name for the chemical linkage used to hook together small molecules to make bigger ones. "Poly(urethane)" is a polymer(resin) made by hooking together small molecules to make a polymer with this linkage. As it pertains to varnish this name reveals that some or all of the polymer in the formulation is a polyurethane, and/or the finish is crosslinked with urethane linkages.

Confused? The name urethane on the can tells us nothing about what got linked together or how much of the finish components are polyurethane resin. However, in general polyurethane varnishes are tougher and harder than previous varnish formulations. All the other varnish properties as well as these two vary depending on what got linked, how much , etc. Bottom line, the name offers no reliable predictive properties.

"Years ago I saw examples of polyurethane finishes and they were a rather thick film that felt like plastic" Such a result is a result of varnish, any varnish, application and has nothing to do with the chemicals in the can. Apply it thick and it will look thick. When urethane resins began showing up in consumer varnish "experts" stuck in their ways disparaged these new products. Followers repeated the claims and these varnish products acquired a reputation that only people that didn't know better would use them.

Some example:

One of the top Period furniture makers displayed a pie crust table at a SAPFM meeting. The audience asked what it was finished with, anticipating the answer to be French Polish, to confirm their prejudices. Nope. He replied that this was a table that got used regularly and it was finished with durable poly(urethane). Nobody could tell the difference.

A friend sells jewelry boxes that you may have seen in Southern Highland craft shops. They go for $500. You have never seen a better finish on wood. It's poly(urethane).



And finally, the picture shows our used daily soft maple dining table, stained cherry, and finished with poly(urethane) about 40 years ago.

Re: Urethane and Polyurethanes

Clint Searl

Well said, Bill; plus, poly can be manipulated to produce any effect from a dull oil to an almost high gloss. With the introduction of waterbornes, it becomes a universal finish, whether brushed or sprayed.

Waterborne Urethane

Bill Tindall, E.Tn.

I have been told by someone that could know, that chemically the waterborne urethanes are chemically not at all the same as the solvent formulation. They are related only in each has some component hooked up with urethane linkages but what is hooked up not the same. Makes sense because I see no way of turning the resins in solvent formulations into the latex essential for a waterborne.

So if one hates solvent based urethane varnish because ........, or visa versa, this experience may not translate to the other formulation. Bottom line- experiment and once a finish schedule is optimized hope the manufacture doesn't change it. They never give notice with these changes. It's always SURPRISE!

As usual . . .

Yonak

Bill, a well reasoned and informative answer. Thank you.

semi-tangent

David Weaver

This is of some interest to me because I buy and make some guitars, and if anyone thinks furniture is bad with misinformation, go to guitars.

The difference with guitars is that the highly opinionated people are buying, and they just want to be sold something that lines up with what they think (which manufacturers cater to).

Supposedly, "modern" finishes (which polyurethane is included in) can't make for a big open sounding guitar. Same folks tend to love how a 60s stratocaster would sound, or some of the vintage american guitar copies made in japan in the 70s and early 80s. A large number of each of those is polyurethane finish. On guitars, polyurethane will sometimes blush over time (at least on some of the desirable japanese guitars) - not sure why. Nitro is more repairable, but it's a no-no to repair a valuable older guitar, anyway, and the techs repairing guitars are moving toward using tinted CA products (which is a farce to me, but nobody will stop it - you can sell the gadget CA repair kits for huge money if you're an instrument specialty retailer).

Buyers for guitars have no idea what french polish is - they generally believe nitro finishes are the original guitar finishes (shellac and violin type varnishes probably were), and that nothing is harder (nitro for stringed instruments is softened quite a bit so that it doesn't craze and break). They're also sure that water based finishes are soft and look blue (I quite like target's "WB lacquer", which is no such thing, but it works well as a substitute, especially if used with crosslinker and shellac under - I can't see the difference with it).

Is it apparent what finish this mahogany is? (This is the butt end of an electric guitar blank that I've finished recently).

https://imgur.com/gallery/xsCNjbz

Re: semi-tangent - french polish (acrylic?)

David Weaver

This post will probably be lost to time (or the prior post).

Before it is, the finish in the picture is buttonlac, french polished. I can't really tell the difference between it and lacquer at a glance except that it's a little bit more wavy on the polished surface than lacquer or acrylic sprayed...

Here is a picture of the neck that will go with the guitar after a first light go-around with french polish. It's indian rosewood. guitars wouldn't be made commercially this way, but the french polish method allows a hand builder a lot of luxuries that spraying doesn't, and takes the same time or less than spraying, leveling and buffing (probably about four hours of total finish time for a guitar and neck). The cost is excellent, too - about $6-8 in finish materials, and what it may lack in durability, it makes up for easily in extreme ease of repair (this last part is what's missing for current customers - difficult but less frequent repair is probably preferable to ease).

Speaking of, a question for bill - I was led to believe at some point (without reading further, I'll admit) that all of the water based finishes are very closely related, much more so than solvents, and that they're some type of acrylic enamel.

Being less attached to chemistry or tradition, I don't much care what they are as long as they work in a way that I can manage and look like I want them to look. Waterbase target with crosslinker (which they tell you not to use for instruments, but I do- I like the hardness) looks and works excellent for me, just as french polish does, and I've had fairly good luck with my own made true varnish, as long as I can find the sun somewhere so that it can dry.

Not Universal...

David Weaver

i don't have a dog in this fight and have no problem with polyurethane, even if I don't use it that much myself.

But looking around for beaumontage (sticks?), I came across a post from someone who linked their site at the bottom.

http://www.jpkfinefurniture.com/BombeFoot.html

The finish is a chemical treatment of potassium dichromate and seed-lac shellac.

I have to admit that if I ever was able to make a piece of furniture as fine as that one, I wouldn't be able to bring myself to use aniline dyes and polyurethane.

(this maker does use aniline dye on other pieces that are finished with shellac - maybe I'm out of line with the dye comment!)

Potassium dichromate?!

Henry (Raleigh NC)

David wrote: "... I came across a post from someone who linked their site at the bottom.

http://www.jpkfinefurniture.com/BombeFoot.html

The finish is a chemical treatment of potassium dichromate and seed-lac shellac. ... I wouldn't be able to bring myself to use aniline dyes and polyurethane."

And I as someone who has used potassium dichromate at work, under safe conditions and with safe disposal, would NEVER consider using it at home - for anything.

Henry

that is a wise plan.....

Bill Tindall, E.Tn.

For if you used it at home and your neighbor was pissed off at you they would call the various state and federal agencies that deal with the workplace and the environment and you would be fined, jailed, both because if anyone is ignorant enough to use this stuff they are probably too ignorant to use it according to the multitude of state and federal requirements imposed on it.

It is one of the few proven carcinogens by respiration. I have shown that it persists in the sanding dust after staining with it. (It raises grain so sanding is required after application)

For what it is worth, I tested its light fastness on mahogany vs premetalized dye and the dye won. There is no valid reason to use this stuff.

Less about the..

David Weaver

..Less about the potassium dichromate and more to the point that some of the period makers do actually use linseed oil and shellac.

potassium dichromate isn't something I'd use (or even try to acquire), but I'm not a period maker and perhaps some period makers will use it because it's correct.

I figured that might cause a sidetrack, but it's not what I was making a point of - the point is that the piece isn't polyurethane or general finishes rub on water based stain and "varnish".

Deliberate sidetrack

Henry (Raleigh NC)

David:

I did deliberately sidetrack this thread (apologies) because of your mention of potassium dichromate.

I want to highlight that this is NOT a home brew material to be used. Period. Thanks to Bill for underlining my point.

I am not on a crusade, but I do want to put warnings on the record so that if anyone reads this thread they become aware that this material (potassium dochromate - K2Cr2O7) is NOT just another way to color wood - it is a HAZARDOUS way to do so and there is no 'safe way' to handle and use it in a home or wood shop environment.

Henry

And...it is not period correct

Bill Tindall, E.Tn.

Given the difficulty making it and the fact there is no natural source for it, I doubt it was used before 1850 and likely not before 1900. There is no mention of it in a 1925 book I have on wood finishing.

Speculating....I think it simulates the look of old mahogany(fact) and therefore may have been used to make "modern" Period pieces look like authentic ones.

Side track justified by prevailing ignorance

Bill Tindall, E.Tn.

I was at a SAPFM meeting some years ago where a high school teacher shared work of his talented students. Then he mentioned potassium dichromate was used for staining, oblivious to the chemical and legal danger. In the break I warned him of the peril, then asked the meeting organizers if I could have a few minutes to explain that using this chemical on wood is a very bad idea, legally and toxicology wise. Incredibly my concern was dismissed and I was unable to warn others.

About 2 years ago Fine Woodowrking published an article using potassium chromate. The editor got an ear full from me and a bunch of other chemists. Wisely, the next issue provided appropriate warning.

Mostly I'm railing against the fear of "Chemicals", driers in linseed oil or finishing solvents for example, but not in this case.

A more general rule...

David Weaver

...aside from reading MSDS or SDS for various products. For any woodworker or toolmaker (me not being a chemist, so this is all trivial knowledge to me - there's no understanding of the machine, so to speak), any time chromium or chromate ever comes up in anything, some research is in order.

I learned that after being told errantly years ago that the chrome ox III powder that i was using on razors was hexavalent chromium.

The good news with shellac is that it will stick to about anything, so you can be far more reasonable about coloring. I've polished into wet stain using the stain as an oil for french polish with great results. Same with waterborne and alcohol-based dyes. As soon as the color is tolerable, I've gone straight to shellac - no wait.

Bill's interest is different than mine. I prefer to work by hand if possible, and I like to fill pores. I never make something identical, so there's never going to be batching, and something like a french polish is about as quick for a small maker on a guitar or small piece of furniture as surface prep, spray/bulld/leveling and then polishing.

In line with your suggestions, it's usually fairly easy (health-wise) on the user and other occupants in the house, too.

I tend to just do things, experiment and see how they work. I read when something doesn't work rather than too much before. Bill's understanding of chemistry leads him on to something like polyurethane to probably say (about the things that I do in some cases) - "why do you do that - it's outdated".

a search shows it's easily found..

David Weaver

....and the combination of it being easily found plus a youtube video or 12 showing its ability to age wood quickly is probably why people use it.

The ebay listings don't say much except they do mention that it's toxic, carcinogenic and a potential fire hazard.

More mundane - but illustration..

David Weaver

http://www.jpkfinefurniture.com/QAChairDetail.html

The point here is still (ignoring the potassium dichromate) that every other piece listed on that maker's site is something plus some type of shellac.

The chair is linseed oil and seed lac.

I would suspect if I looked over various makers' pages, I'd find a lot more than one person still using shellac instead of polyurethane, but I'm not a buyer of these types of things, so I have no idea who is dictating use of one or the other. Is it the maker, or is it the buyer?

submitting an article

John K Jordan

>>>somebody put this in the Articles so I don't have to keep repeating it.......

According to http://www.woodcentral.com/cgi-bin/articles.pl

"To submit an article for possible inclusion here, please e-mail it to us. Please include any photos, drawings or other information that helps illustrate your article."

The email address is:

webmaster@woodcentral.com

It's the makers who obsess

Shellac is a common furniture finish

Bill Tindall, E.Tn.

Our club speaker last night makes and sells Shaker inspired cherry furniture, $4 to $8K pieces. It is all French Polished. Another professional in the club French Polishes some of his furniture, and lacquers the rest. Stuff from the Krenov school is typically shallaced. In the Headley shop some things are shellaced, others lacquered. As I think about it most of the Period furniture makers I have met shellac their product.

Re: Shellac is a common furniture finish

Dick Coers

I suppose that if someone purchases an $8K piece of furniture, they will listen to the instructions of care and use of a piece finished with shellac. But that was not my experience near the end of my custom woodworking business. The general public did not differentiate conversion varnish from shellac. In appearance nor care.

Warned

Bill Howatt

When I started work years ago in a research lab this stuff was needed for a step in the preparation of a lab item. I was given a very serious lecture on its dangers before we started the process.

Bill

cleaning glass?

David Weaver

it looked when I was reading about it yesterday like it was quite popular when cleaning lab glassware.

Wiki says it's still present in cement and is one of the reasons for cement rash (or whatever that would be called). Apparently, it slows the set of cement.

My dad ended up with a big ring of cement rash decades ago, and I could never understand why cement would give someone a rash.

It's still common for...

David Weaver

...gloss finish on some very high end instruments.

I asked George about it after seeing several classical guitar makers using french polish, and he said that it's appropriate for nylon string guitars because it actually makes a difference in sound vs. a heavier lacquer finish (less string energy in shellac, I'm sure).

That whole tone effect thing is a can of worms - you can hear it on steel string guitars, too, but the range of irate opinions about it goes from "it makes no difference" to "I can hear differences in finish type even when they're the same thickness and hardness".

But I can easily believe it on a nylon string guitar. One of the few types of guitars left that's still bought from master makers instead of factories and production custom shops.

Re: cleaning glass?

Bill Howatt

From what I recall, my case was for cleaning very fine glass tubes before parts of them underwent vacuum deposition of silver.

Bill