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2K Polyurethanes: What Wood Finishers Should Know – Part 1

Have you noticed “2K polyurethanes” or simply “2K urethanes” are “having a moment” on social media lately?

I certainly have, and I must admit, I have some concerns about what is being said (or not said) about these products, particularly the ones being marketed as “safer” because they are water based.

If you haven’t heard of them, 2K or “two-part” polyurethanes (also called “2-pack,” or “2K-PU”) consist of two components that are mixed together just prior to being used.

The benefits, in general, are superior durability and higher solids, so you can obtain high levels of physical performance and sometimes get away with using fewer coats.

2K waterborne polyurethanes and their cousins, 2K waterborne conversion varnishes, certainly have their place in high-performance environments.

However, it is the use of a specific type of catalyst – a highly toxic and reactive polyisocyanate based on hexamethylene diisocyante (HDI) – that is being used by untrained finishers, in poorly ventilated environments, that is raising my concerns.

It’s these downsides (the health and safety profile) that I’m concerned is not being talked about enough – at least not on the blogs and social media posts I’ve been seeing…and the ones you might be seeing, too.

That’s why I am in the middle of a bit of a research project.

Unlike some of the people showing up in our inboxes and social media feeds, I am trying to get the FACTS (not just clicks and attention) before I give you my better-informed opinion about these products in an upcoming post.

I’m waiting on a few expert sources to answer some specific questions I have, and I’d like to incorporate yours, too, if you have them.

I’d love to know what YOU think about 2K polyurethanes. Do you have any experience with or questions about them?

Please share your thoughts or read what others are saying below in the comments section.

14 thoughts on “2K Polyurethanes: What Wood Finishers Should Know – Part 1”

  1. I would love to see a water bourne 2k clear, but if it is using iso’s I don’t care about it. I use 2k clears and it is not the solvents that concern me, it’s the iso’s.

  2. After they started getting popular I wondered by the American companies didn’t make them; why only other countries. Our paint manufacturers certainly have the ability to make any product the foreign companies make. I contacted a person in Benjamin Moore that I met at a trade show who knows pretty much everything about the BM products, and he said Benjamin Moore made a decision as a company not to make products with isocyanates, to not expose their employees or customers to those chemicals, so they were not making or planned to make 2K polys. I don’t think Sherwin Williams carries them, but I could be wrong on that. So the 2Ks have gotten extremely popular on social media, and no one is talking about the isocyanates. I did see where one person said they started developing a rash on their arms from handling the dry cabinet doors that were sprayed with 2K, and they never had this before when using modified urethane products. At one point I did specifically look at the 2K TDS and SDS, and was surprised to see that they show no or very low hazards, especially after what the Benjamin Moore person said. I am mystified at this point because the BM person said the isocyanates are dangerous, but the SDS says they’re ok. And yes, I did look at the catalyst, not just the base. I would really like to know what the true picture is.

  3. I’m glad you are looking into this Jeff. I cannot get a definitive answer. Professionals using 2k water base do not agree. Some say full suit with outside air required. One popular manufacturer I’m told says once the hardener is mixed, isocyanates are bonded with the product and only a regular chemical filter respirator is required. Most professionals who have posted videos only wear a respirator. I’ve never seen anyone in a full suit with outside air.

  4. Scott Schaeffer

    Used 2K Poly (not waterbased) for many years as a luthier. Very forgiving, lays down like glass; also one of the most toxic substances known to man. And not just through inhalation; it is also absorbed through the skin and eyes. So, a proper spraying environment and PPE is a must!

  5. I really appreciate your covering this. I admit I use Bona floor finishes for many things in both cabinets and furniture, both for its very effective matte sheen (which is popular right now with architects and designers) and for its palette of sealers (6 tomes ranging from mild pickled effect to simulated oil-based effect). I never spray it, and it claims to do very little out-gassing, which is great for clients. But I think it uses isocyanate catalyst, and is some kind of urethane emulsion. If I am not spraying, am I subjecting myself to harmful chemicals simply from exposure during drying? I am trying to make the full switch to EmTech, which offers other significant advantages. I just need that matte sheen, and that range of tones in the sealers.

    1. Erling – Thank you for adding your comments to this thread, they are greatly appreciated. I am surprised that Bona is offering an HDI catalyst for one of their floor coatings. While it is listed under their professional line and not for consumer retail sale, it offers lower toxic channels of entry when the catalyzed finish is applied by pad or hand-applicator, but it still presents a hazard to the handler while mixing. One thing that I did notice is that Bona eliminated Aziridine as the crosslinking option for their finishes. With more self-crosslinking polymers coming into use by coatings formulators the need for Aziridine has dropped way off the usage charts. Target Coating is engineering a new line of finishes that are self-crosslinking and do not require an external additive to meet KCMA chemical test standards. More to come on this development in the Emtech Lab in near-future posts.


  6. Robert McGovern

    Both professionals and amateurs have been using solvent-based 2-pack linear urethane paints (and now clear finishes) on boats for years; those made by Awlgrip and Interlux are most commonly used. HOWEVER. They are extraordinarily difficult to work with, with steep learning curves and requiring perfect thinning and surface decontamination. Furthermore, they are very sensitive to conditions, like rising vs falling temperatures and ambient humidity. But their biggest danger is finely-atomized crosslinker, typically methyl isocyanate. It pretty much dissolves lungs, eyes, and mucus membranes and was the chemical that killed ~20,000 people in Bhopal, India. The MSDS flat-out requires use of a positive pressure hood and remote air supply. Do not count on a cartridge respirator to keep you safe, even if rated for isocyanates.

    It is considered safe to apply these post-catalyzed urethanes outdoors by brush and roller. Many amateurs paint their boat hulls in this fashion, safely and with acceptable results. Fair risk of failed application tho (fish eyes, orange peel, runs) and the products are obscenely expensive, with proprietary everything (thinners, retarders, wiping solvents, primers….)

    For once-every-ten-years boat painting, maybe it makes sense. My instinct is that pre-cat products are much easier to use for the average person and small-shop professional. Most of the crosslinking has been done for you under controlled conditions. Pot life is not an issue, so you are not constantly batch-mixing and either running short or throwing away hot product. The best pre-cats are within maybe 10-15% of the performance of the best 2-packs. There are applications where you really want that last 10-15%, but I’m happy with Target’s line of 1-packs. IIRC, you can buy thru Target additional crosslinker (amine silane?) to boost the water, chemical, and physical toughness of Emtech topcoats, if you really want to go that way. ;)

    1. Robert — Thank you for sharing your notes about our isocyanate blog. Yes, I certainly recall 2K’s being used in public and private boatyards by weekend painters. I heard more than one story of individuals becoming ill while painting their boats with 2K urethanes. Many yards banned the use of these paints by unsupervised boat owners. Likewise, yards pressed their painters into training seminars (at great expense) to certify that they were compliant in the use, safety and storage of these types of coatings. While the newer waterborne 2K’s eliminate the flammable issue of the older 2K solvent paints, the use of HDI (isocyanate) in these formulas takes the safety factor of WB’s a full step backwards when being spray or brush applied. Yes, the fumes are lower with the WB/2K’s, but the compounding toxicity of repeat exposure to MDI leaves the shop or the user wide open to future health and liability issues.


  7. I don’t see what the big deal is with isocyanates. After all, some of the biggest chemical companies in the world manufacturer them, and they certainly wouldn’t make something that could harm your health,. Just ask Monsanto about DDT or Roundup, or Dupont about Teflon, or Dow Chemical about agent orange.
    As for isocyanates, what could be bad about a chemical that killed 15000 people in India in 1984?
    What really bothers me is that companies like Milesi and Renner are pushing 2k poly without any regard for their customers health. Most people have no idea what precautions are necessary to prevent skin problems, asthma and even death. I can see massive lawsuits in the future for these companies.
    I’ve been using 1k products for kitchen and bathroom cabinets for about 8 years now, and have never gotten one call back. I would never risk my health just to make paint a little bit harder.

    1. Jay – Yes, the continued push of various types of isocyanates into small shops is a head-scratcher. I would like to point out that the term “2K” can be applied to the use of other, less toxic crosslinkers such as carbodiimides, siloxanes and certain diols – and can and will be found here at Target Coatings with the use of our CL100 Crosslinker. However, offering isocyanates and aziridines for use in waterborne 2K’s is foolish and irresponsible.


  8. I appreciate that you didn’t come out of the gate swinging and totally discount isocyanates as viable coating. You are 100% correct in saying they are quite hazardous, especially in the hands of anyone untrained or under equipped. Isocyanates are a type of chemical sensitizer which means they act like an allergen upon subsequent exposures with further exposures leading to more adverse effects. Safety data sheets (formerly material data safety sheets) are usually a great source for hazard information and protective controls, but I find manufacturers error on the side of extreme caution so you will frequently see air supplied respirators or SCBAs recommended. This is partially due to the fact the manufacturer can’t possibly foresee every scenario of use and partially CYA on the manufacturer’s part. The National Institute for Occupational Safety and Health (NIOSH) pocket guide (https://www.cdc.gov/niosh/npg/default.html) is also a great source of information on chemical hazards. You may have to search by the chemical abstract (CAS) number found in the SDS due to the fact chemicals can go by many different names or fall under a class of chemicals.

    When it comes to PPE for isocyanates, you should wear a Tyvek or similar full body suit, gloves, and respirator. Since the eyes, skin and respiratory system are all target organs for isocyanates you don’t want to have any part of your body exposed when using the chemical. As far as respirator cartridges go, ensure the cartridges are effective for isocyanates (or any chemical you are using for that matter), this information can usually be found in the literature provided with the cartridges, on the manufacturer’s website or by simply giving the manufacturer a call. With that in mind not all respirator configurations are equal and different configurations have different protection factors. For example, a full-face respirator with cartridges does not provide the same protection as that same full-face respirator with air supply. The level of protection you need is based on the level of exposure you encounter, measured in mg/m3 or ppm. To know your exposure level, you would need to perform time weighted average air sampling in the breathing zone, which for most of us is cost prohibitive.

    With all that being said elimination or substitution are the most effective controls, if you don’t have to use isocyanates use another chemical that will provide adequate results. I wouldn’t recommend using isocyanates unless you can accurately determine your exposure level and provide appropriate controls to the process. Your best resource is an industrial hygienist, this topic is the bread and butter of that profession.

    Disclaimer: I am an industrial hygienist; this posting is strictly informational and in no way my professional opinion and should not be used as such.

    1. Matt – Thank you for your insightful reply. I have more information on this topic ready to share with my subscribers. Being as the world of 2K is a large one, I want to ensure that the history of water-based two component systems is touched upon. Stay tuned for a Part 2 and Part 3 of this post.


  9. Christina Broja

    I there concern about polyurethane being harmful for the end user? For example if it’s used as a finish coating on kitchen cabinets? Or is the main concern about the health of the person who would apply the coat?

    1. Christina – Thank you for your inquiry. Once a urethane coating is fully cured the film is inert and is not dangerous to the environment or the occupant of the space.
      Contact the coating mfg to learn more about the full cure time for the finish in question.


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