Difference between Polyurethane and Urethane varnish.

[Tricia]

I was given two tins of varnish one marked Polyurethane and the other Urethane. The varnish was decanted into the tins so there are no labels or instructions. What is the difference? Are they suppose to be two pack? And can I use them over "normal" varnish?

Thanks

Tricia

 

[MM5AHO]

Urethane is a family of chemicals. Polyurethane is when lots of these molecule combine together to form a longer chain molecule.
So there's a technical difference, but that probably isn't the reason you came across both names.
Polyurethane coating is sometimes just called Urethane.

There's no such thing as "normal" varnish. There are a whole variety of chemistries involved, and many different coating preparations could be called "varnish".
Originally the stuff was made of tree sap resin, thinned with a solvent which evaporated leaving a hard dry coating. That's where the term "drying" came from - in reference to coatings drying.

Today most coating systems don't "dry" they "cure". A chemical reaction starts which causes the hardening. In many cause this is activated by absorption of moisture from the atmosphere. That's the case with most paints. It's why they will set in the tin left open, and are slow to cure in really low humidity places.
Other curing mechanisms are to add another chemical just before using. These are so called "two pack" coatings. This is similar to say Araldite glue. Mix the hardener then use before it hardens. So with 2 pack paints/coatings - mix and use before it cures.

There's not necessarily better performance by using a 2 pack system, it's just a different curing mechanism. Some people use the term "two pack" as if it were a quality label. ("it must be good its a 2 pack system"). That's misleading, and probably wrong.

Different coatings can sometimes be used together, sometimes not. It depends on the chemistry, and the average user has no idea, and the only way to tell is to read the label or ask the manufacturer / seller. The end result of mixing unidentified coatings could range from at best they seem as if they were a single pack, to at worst, a reaction that causes bubbles and foaming, or no adhesion, or in other words a total mess.

In the case of 2 unlabelled packs, don't take the risk of mixing them. Unless compatibility is stated, don't mix any paint systems like this.

 

[Robin B]

Top answer.

 

[Robert Wilson]

Top answer.

+1

And I add "Well whadaya know - I didn't!"
Thank you.

 

[25931]

One of the most useful replies that I have seen for a long time.

 

[alant]

Urethane is a family of chemicals. Polyurethane is when lots of these molecule combine together to form a longer chain molecule.
So there's a technical difference, but that probably isn't the reason you came across both names.
Polyurethane coating is sometimes just called Urethane.

There's no such thing as "normal" varnish. There are a whole variety of chemistries involved, and many different coating preparations could be called "varnish".
Originally the stuff was made of tree sap resin, thinned with a solvent which evaporated leaving a hard dry coating. That's where the term "drying" came from - in reference to coatings drying.

Today most coating systems don't "dry" they "cure". A chemical reaction starts which causes the hardening. In many cause this is activated by absorption of moisture from the atmosphere. That's the case with most paints. It's why they will set in the tin left open, and are slow to cure in really low humidity places.
Other curing mechanisms are to add another chemical just before using. These are so called "two pack" coatings. This is similar to say Araldite glue. Mix the hardener then use before it hardens. So with 2 pack paints/coatings - mix and use before it cures.

There's not necessarily better performance by using a 2 pack system, it's just a different curing mechanism. Some people use the term "two pack" as if it were a quality label. ("it must be good its a 2 pack system"). That's misleading, and probably wrong.

Different coatings can sometimes be used together, sometimes not. It depends on the chemistry, and the average user has no idea, and the only way to tell is to read the label or ask the manufacturer / seller. The end result of mixing unidentified coatings could range from at best they seem as if they were a single pack, to at worst, a reaction that causes bubbles and foaming, or no adhesion, or in other words a total mess.

In the case of 2 unlabelled packs, don't take the risk of mixing them. Unless compatibility is stated, don't mix any paint systems like this.

Some polyurathanes are waterbased & still as tough.

 

[MM5AHO]

Most polyurethanes are tough. That they're "water based", (in itself a bit misleading, but very common term, though we all know what's meant), doesn't mean not tough, and nowadays doesn't mean poor performance, or any accusation of low quality.
There are now "water based" automotive paints. Most trains are now painted with these types of paint.

There's legislation that restricts the use of "volatile organic compounds" (VOCs for short), so when government penalises the use of something, some bright spark invents something else. And good on them. It would be good if all government regulations worked like that - moving us form polluting, dangerous substances to less hazardous ones. But without a push industry would keep on doing the same as before.

So we'll see an increasing number of these new chemistries including "water based". Don't use your experience of 25 years ago when these were akin to putting household plasterboard emulsion type paint on things. But equally there's still a lot of trash paints out there making outrageous claims, just as there's low profile ordinary boring sounding formulations that really do perform. So: find out, get references from others, don't assume anything.