what's the difference (other than price)? we have been spraying 2k urethane for about a year and i just don't know anything about acrylic urethanes. i think i understand that acrylics have better clarity, but that's about it.
Acrylic PU is also more flexible than the aromatic PU. It normally has lower solids which gives better pore definition on open pore finishes. As Rick said, yes it has better clarity, dries slower and is also non yellowing. Of course it is more expensive.
Acrylics are used in a wide variety of coatings.... either as the primary resin or co-polymer with other resins. You’ll find them in waterborne finishes (e.g., Minwax polycrylic), aerosol spray can clear finishes, latex paints, non-yellowing lacquers and conversion varnishes as well as 2K-PU.
In addition to their clarity, acrylics are known for their color stability, gloss retention, adhesion and ability to perform exceptionally well in exterior use without damage from UV radiation or water. Acrylics are the binder of choice for exterior latex paints, sign paints and automotive finishes (2K urethane).
There are 2 main types of 2K urethanes, determined by the polyisocyanate used - aromatic or aliphatic.
Polyurethane based on aromatic polyisocyanates cost less and turn yellow with exposure to sunlight. They are usually formulated with polyester resins which allows for a higher solids finish with excellent solvent resistance and improved adhesion to metals.
Aliphatic isocyanates are more expensive and are usually teamed with acrylic polyols to produce a polyurethane with the characterics listed above.
Aliphatic PUs do cure slowly compared to aromatic isocyanates which are much more reactive. It's not unusual for them to be subject to printing within the first 24 hours and even then you have to be careful. The only way to improve production rates has been to put the finished items through an oven cycle.
A while back I found another solution though. I switched to an acrylic polyurethane that cuts dry to handle time down to 4 hours by adding an accelerator. Of course it also shortens the pot life from 8 hours to 2 hours so you have to take that into account if you're not using plural spray equipment.
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