2K polyurethane could catch on soon in the U.S.

Polyurethane might be best known as a varnish-type finish available at paint stores and home centers. But polyurethane is a versatile resin with other common uses as well. For example, moisture-cured polyurethane is used in polyurethane adhesives and heat-cured polyurethane forms the basis for many plastics and foams.

There’s also two-part polyurethane, which is used as a finish on wood and metal and consists of two components that are mixed just before application. For whatever reason, two-part or “2K” polyurethane has never really caught on in the United States for finishing wood, but it is available.

In Europe, 2K polyurethane is very popular. In fact, catalyzed finishes such as conversion varnish have been almost totally replaced by two-part polyurethane.

Polyurethane finishes

Polyurethane resin has been around since the mid-20th century.

The single-pack polyurethane finish sold in home centers is not a true polyurethane. It’s an alkyd varnish or water-based dispersion in which some isocyanate (a primary chemical in polyurethane) has been woven into the molecular structure. This finish applies and performs more like common varnishes and water-based finishes than it does like the purer types of polyurethane.

Two-part polyurethanes are pure polyurethanes. Unlike catalyzed finishes, which consist of the finish and an acid catalyst that doesn’t add solids to the film, both parts in 2K polyurethanes add solids. The part that is added is the isocyanate, which usually makes up a quarter to a third of the mix. Therefore, 2K polyurethanes, which can be sprayed with all common spray systems, resemble epoxy finishes in their mixing characteristics more than catalyzed finishes.

There are two large categories of these polyurethane finishes: aromatic and aliphatic. Aromatics cost less and dry faster, but they yellow more and have a shorter pot life (meaning that you have to be more careful to clean out your spray equipment often or the finish will cure and ruin it). By contrast, aliphatics cost more, but they have a longer pot life and they are fairly non-yellowing and resist destruction by UV light better than aromatics, so they are used in exterior situations.

Most polyurethanes made for interior use are a combination of the two types, which combines their characteristics and leaves you to choose products based on tradeoffs of cost, pot life and yellowing. It’s important to know these tradeoffs when dealing with your supplier.

Two-part polyurethanes are usually formulated for thinning before spraying and manufacturers sell reducers for this purpose. These reducers resemble lacquer thinners in some of the solvents that are included, but you should be sure to use the solvents specified by the manufacturer of the finish because common lacquer thinners won’t work properly.

Beyond these considerations, 2K polyurethanes offer the same decorative application possibilities as other finishes. They can be applied with stains and glazes, can be made into toners, can be rubbed out and are available in all sheens from gloss to flat. Pore fillers, however, aren’t often used with 2K polyurethanes, because the finishes build so well that it’s usually less time-consuming just to sand the finish back if you want a mirror-flat surface.

Adding to the versatility, 2K polyurethanes are also available as water-based finishes.

Advantages

Compared to conversion varnish — the most durable of the catalyzed finishes — 2K polyurethane has a number of advantages, including the following:

• The solids content (that which remains after the thinner has evaporated) of 2K polyurethane is about 50 percent, while that of conversion varnish is about 35 percent after the catalyst is added. Therefore, you might be able to get away with just two coats when you would apply three with conversion varnish.

• Besides saving a step, you will reduce the amount of VOCs you are exhausting, which could be an important factor depending on where you live.

• Two-part polyurethane is even more durable than conversion varnish. According to the Architectural Woodwork Institute, it is more wear-, heat-, solvent-, moisture- and stain-resistant.

In addition, 2K polyurethane is easier to apply than conversion varnish because it is more forgiving in three areas.

The two parts of 2K polyurethane are usually mixed at a 2:1 or 3:1 ratio compared to conversion varnish where you add just 3 to 10 percent catalyst. So if you are off 1 or 2 percent, the effect on the curing isn’t nearly as great with 2K polyurethane.

You can apply the next coat of 2K polyurethane at any time without fear of wrinkling the previous coat, which contrasts sharply with the tight conversion-varnish recoat window. You just might need to sand the surface to create a mechanical bond if you wait too long.

There’s almost no practical limit to the amount of build you can get with 2K polyurethane and still achieve a stable film. With conversion varnish you are limited to 5 mils (about three coats) to avoid the risk of the film cracking.

Disadvantages

Of course, there are also downsides.

The pot life of 2K polyurethane can be as short as 3 or 4 hours. This is considerably shorter than with most conversion varnishes, which means that you have to be more careful to mix only as much finish as you can use in that shorter time and you might have to clean your equipment twice a day instead of once.

In addition, the better durability translates to 2K polyurethane being more difficult to repair invisibly and more difficult to strip if that becomes necessary at some point.

Two-part polyurethane also costs more than conversion varnish. Even though the higher solids and better durability largely make up for the added expense, the perception of higher cost is still there.

Health issues

Both formaldehyde, contained in most catalyzed finishes, and isocyanates, contained in 2K polyurethanes, are toxic substances. But formaldehyde is emitted from the finish for a considerable time after application, while isocyanates become totally reacted with the rest of the finish very quickly.

Though manufacturers have been working hard to reduce and even eliminate (in some cases) the formaldehyde in their catalyzed finishes, most of these finishes still contain some formaldehyde.

This is a big reason for 2K polyurethanes being more popular in Europe, where catalyzed finishes aren’t even allowed in some areas because of the health risks to consumers.

Of course, both the isocyanates and the formaldehyde pose health risks to spray operators, but in neither case is this true if he or she is working in a proper spray booth and wearing an appropriate respirator.

As with any case when trying a new and unfamiliar finish, be sure to read all data sheets and maybe even talk to a manufacturer’s representative if you do decide to try two-part polyurethane.

Bob Flexner is author of “Understanding Wood Finishing” and “Flexner on Finishing.”

This article originally appeared in the July 2015 issue.

Polyurethane might be best known as a varnish-type finish available at paint stores and home centers. But polyurethane is a versatile resin with other common uses as well. For example, moisture-cured polyurethane is used in polyurethane adhesives and heat-cured polyurethane forms the basis for many plastics and foams.
There’s also two-part polyurethane, which is used as a finish on wood and metal and consists of two components that are mixed just before application. For whatever reason, two-part or “2K” polyurethane has never really caught on in the United States for finishing wood, but it is available.
In Europe, 2K polyurethane is very popular. In fact, catalyzed finishes such as conversion varnish have been almost totally replaced by two-part polyurethane.

Polyurethane finishes
Polyurethane resin has been around since the mid-20th century.
The single-pack polyurethane finish sold in home centers is not a true polyurethane. It’s an alkyd varnish or water-based dispersion in which some isocyanate (a primary chemical in polyurethane) has been woven into the molecular structure. This finish applies and performs more like common varnishes and water-based finishes than it does like the purer types of polyurethane.
Two-part polyurethanes are pure polyurethanes. Unlike catalyzed finishes, which consist of the finish and an acid catalyst that doesn’t add solids to the film, both parts in 2K polyurethanes add solids. The part that is added is the isocyanate, which usually makes up a quarter to a third of the mix. Therefore, 2K polyurethanes, which can be sprayed with all common spray systems, resemble epoxy finishes in their mixing characteristics more than catalyzed finishes.
There are two large categories of these polyurethane finishes: aromatic and aliphatic. Aromatics cost less and dry faster, but they yellow more and have a shorter pot life (meaning that you have to be more careful to clean out your spray equipment often or the finish will cure and ruin it). By contrast, aliphatics cost more, but they have a longer pot life and they are fairly non-yellowing and resist destruction by UV light better than aromatics, so they are used in exterior situations.
Most polyurethanes made for interior use are a combination of the two types, which combines their characteristics and leaves you to choose products based on tradeoffs of cost, pot life and yellowing. It’s important to know these tradeoffs when dealing with
your supplier.
Two-part polyurethanes are usually formulated for thinning before spraying and manufacturers sell reducers for this purpose. These reducers resemble lacquer thinners in some of the solvents that are included, but you should be sure to use the solvents specified by the manufacturer of the finish because common lacquer thinners won’t work properly.
Beyond these considerations, 2K polyurethanes offer the same decorative application possibilities as other finishes. They can be applied with stains and glazes, can be made into toners, can be rubbed out and are available in all sheens from gloss to flat. Pore fillers, however, aren’t often used with 2K polyurethanes, because the finishes build so well that it’s usually less time-consuming just to sand the finish back if you want a mirror-flat surface.
Adding to the versatility, 2K polyurethanes are also available as water-based finishes.

Advantages
Compared to conversion varnish — the most durable of the catalyzed finishes — 2K polyurethane has a number of advantages, including the following:

? The solids content (that which remains after the thinner has evaporated) of 2K polyurethane is about 50 percent, while that of conversion varnish is about 35 percent after the catalyst is added. Therefore, you might be able to get away with just two coats when you would apply three with conversion varnish.
? Besides saving a step, you will reduce the amount of VOCs you are exhausting, which could be an important factor depending on where you live.
? Two-part polyurethane is even more durable than conversion varnish. According to the Architectural Woodwork Institute, it is more wear-, heat-, solvent-, moisture- and stain-resistant.

In addition, 2K polyurethane is easier to apply than conversion varnish because it is more forgiving in three areas.
The two parts of 2K polyurethane are usually mixed at a 2:1 or 3:1 ratio compared to conversion varnish where you add just 3 to 10 percent catalyst. So if you are off 1 or 2 percent, the effect on the curing isn’t nearly as great with
2K polyurethane.
You can apply the next coat of 2K polyurethane at any time without fear of wrinkling the previous coat, which contrasts sharply with the tight conversion-varnish recoat window. You just might need to sand the surface to create a mechanical bond if you wait too long.
There’s almost no practical limit to the amount of build you can get with 2K polyurethane and still achieve a stable film. With conversion varnish you are limited to 5 mils (about three coats) to avoid the risk of the
film cracking.

Disadvantages
Of course, there are also downsides.
The pot life of 2K polyurethane can be as short as 3 or 4 hours. This is considerably shorter than with most conversion varnishes, which means that you have to be more careful to mix only as much finish as you can use in that shorter time and you might have to clean your equipment twice a day instead of once.
In addition, the better durability translates to 2K polyurethane being more difficult to repair invisibly and more difficult to strip if that becomes necessary at some point.
Two-part polyurethane also costs more than conversion varnish. Even though the higher solids and better durability largely make up for the added expense, the perception of higher cost is still there.

Health issues
Both formaldehyde, contained in most catalyzed finishes, and isocyanates, contained in 2K polyurethanes, are toxic substances. But formaldehyde is emitted from the finish for a considerable time after application, while isocyanates become totally reacted with the rest of the finish very quickly.
Though manufacturers have been working hard to reduce and even eliminate (in some cases) the formaldehyde in their catalyzed finishes, most of these finishes still contain
some formaldehyde.
This is a big reason for 2K polyurethanes being more popular in Europe, where catalyzed finishes aren’t even allowed in some areas because of the health risks to consumers.
Of course, both the isocyanates and the formaldehyde pose health risks to spray operators, but in neither case is this true if he or she is working in a proper spray booth and wearing an appropriate respirator.
As with any case when trying a new and unfamiliar finish, be sure to read all data sheets and maybe even talk to a manufacturer’s representative if you do decide to try two-part polyurethane.

Bob Flexner is author of “Understanding Wood Finishing” and “Flexner on
Finishing.”