What are the two most common types of polyurethane and which is best for me?
Polyurethane is a versatile and durable finish, but PU is not one material. It is the result of a chemical reaction, and the resin that reaction starts from can be one of several. The two you are most likely to meet in wood finishing are alkyd and acrylic.
They behave differently on the bench and they are labelled differently. Here is what separates them, and where the difference is smaller than it is often made out to be.
What is the difference between alkyd and acrylic PU?
Alkyd PUs are built on an alkyd resin base and acrylic PUs on an acrylic resin base. Both are two-pack systems, and in both cases an isocyanate hardener drives the reaction that forms the polyurethane.
Worth being precise about one thing, because it is often blurred: the resin and the hardener are separate choices. An acrylic lacquer is not automatically supplied with an aliphatic hardener, and an alkyd is not automatically supplied with an aromatic one. In practice the pairings tend to run that way, but what determines the isocyanate in your system is section 3 of the hardener sheet, not the resin type on the front of the lacquer tin.
Health and safety
This is where the two families most often diverge, and it is worth stating what the difference actually is rather than reaching for the word safer.
Alkyd PUs: commonly supplied with aromatic isocyanate hardeners. Aromatic isocyanates such as TDI and MDI carry a carcinogenicity classification. They are also typically mixed at 2:1 or 1:1, so more hardener goes into the cup for a given volume of lacquer.
Acrylic PUs: commonly supplied with aliphatic hardeners based on HDI, which carries no carcinogenicity classification. Mix ratios are usually much lower — 10:1 is typical — so less hardener is used.
That is a real and checkable difference, and it shows up on the label. What it does not mean is that either type is safe to breathe. All isocyanates are respiratory sensitisers, and the controls your COSHH assessment sets out apply to both. The diisocyanate training requirement makes no distinction between them either.
So: aliphatic chemistry is a carcinogenicity argument and, as below, a yellowing argument. It is not a reason to relax anything about how the product is sprayed.
Yellowing
Alkyd PUs: more prone to yellowing over time, particularly under UV light. The benzene ring in an aromatic system breaks down under ultraviolet and the breakdown products are coloured. That is inherent to the chemistry rather than a fault in a particular product.
Acrylic PUs: do not yellow in the same way, which makes them the better choice for pale colours and pale substrates.
Worth noting that no coating keeps a pale timber looking exactly as it did on day one. Timber extractives bleeding through and the ageing of the lacquer film itself both play a part. Choosing an acrylic removes one cause of discolouration, not all of them.
Durability
Both can be equally durable. This comes down to what the manufacturer formulated the product to do rather than to the resin family, so it is not a useful basis for choosing between them.
Drying time
Alkyd PUs: tend to dry faster for a given solids level, though this is a tendency rather than a rule.
Acrylic PUs: tend to dry more slowly for a given solids level.
In practice you may see the opposite, because solids levels are not equal across the two. A general clear multicoat acrylic will often dry faster than the average alkyd equivalent because its solids level is lower. A high solids acrylic base coat will be considerably slower than its alkyd counterpart. Compare like for like, or the comparison tells you nothing.
Odour
Alkyd PUs: a sweeter solvent odour, often quite noticeable during application.
Acrylic PUs: a more fruity character, which comes largely from butyl acetate.
Neither tells you anything about how hazardous the product is. Smell tracks how quickly a solvent evaporates and how low its odour threshold is, and some of the more hazardous solvents are the subtle ones. If you want to know what you are handling, the classification in section 2 of the safety data sheet will tell you and your nose will not.
The pros and cons of alkyd
Alkyd PUs remain popular, particularly in the Italian market where much of the production sits.
Pros
- Very high build.
- Clarity marginally better than acrylic.
Cons
- Commonly supplied with aromatic isocyanate hardeners, which carry a carcinogenicity classification.
- Higher mix ratios, typically 2:1 or 1:1, so more hardener per litre of mixed product.
- Yellowing over time, and discolouration of pale wood.
Although most of the PU coatings we stock are acrylic, there are jobs where an alkyd is the right answer — particularly where build and clarity matter more than colour stability. For those we would point you at the Hesse Optibase.
The pros and cons of acrylic
Everything in our Hesse PU range is acrylic with the exception of DG 4750 and DG 4720.
Pros
- Supplied with aliphatic hardeners, which carry no carcinogenicity classification.
- Lower mix ratios, so less hardener is used for a given volume of mixed product.
- No yellowing of the film, so pale colours and pale substrates hold their appearance better.
- Longer pot life once mixed — up to three days on some products, against a working day or less on many alkyd systems.
Cons
- Lower solids content than alkyd, so more coats may be needed to achieve the same build.
One thing that is often claimed and does not hold up: a lower mix ratio does not make a product lower in emissions. Solvent-based PU sits at around 700 grams of VOC per litre whichever family it comes from, and the ratio changes how much hardener you use rather than how much solvent goes into the air. If VOC is what you are trying to reduce, the answer is a water-based system rather than a different solvent-based one.
Which should you choose?
For pale finishes, anything that will see daylight, and any job where colour stability matters, acrylic with an aliphatic hardener is the straightforward answer. For maximum build and clarity where yellowing is not a concern, an alkyd still earns its place.
Whichever you use, the control measures are the same. Read the hardener sheet, know which isocyanate you have, and make sure your COSHH assessment covers spraying rather than just storage.
If you would like help choosing, our team is always happy to talk it through. Call 023 9223 3310 or email sales@redwood-uk.com.