[mrcleanr6]

now i remeber going over this with barry k a couple years ago and i remember him saying there were distinct differences. dont remember exactly what they were except that polyurethanes are more chemical resistant and more flexible. hopefully he will pop in here and remind me of our conversation.

kenseth17: thats funny about the buffing. i usually see just the opposite. most urethanes i see say to buff within a few days and poly's within 30 or so.

[baddbob]

I agree, the polyurethanes seem to stay green longer but when they do reach full cure they are harder to buff IMO. Like Barry has said a few times here, mix ratios can somewhat be an indicator of quality, 2-1 and 3-1 clears are usually higher quality than the 4-1 mixes. It's all rocket science IMO , but if anyone wants to shed some more light on the subject I'm all ears.

[powderbill]

Well, first off, it's much harder to make predictions about the quality of one formulation versus another if you don't have access to the formulas. Typically, one can get a few tidbits of information about the composition of the products from either labels or MSDS, but the information is typically very limited. For example, one who is familiar with the various commercially available isocyanates can typically pretty well guess what isocyanate curing agent is being used by looking at the solvent composition, VOC, and wt./gal. on an MSDS or even some labels. Information on the non-hazardous components such as acrylic resin solids is typically much more limited.

For a few broad generalizations, with all other factors being equal, mix ratios that use more isocyanate are typically higher in chemical resistance, and may often have an edge in gloss retention. However, comparison of higher solids (lower VOC) products to a lower solids product may make such predictions less reliable. The main factors that ultimately determine mixing ratios are the hydroxyl equivalent weights of the resins used and the quantity of resin solids present.

With respect to buffing behavior, I would expect that rate of hardness development would be affected by the molecular weight and monomer composition of the acrylic resin used, the solvent blend, and most especially by the accelerator package used in the formulation (eg concentration of certain metal salts). A tradeoff in this case is potlife versus cure rate.

Kenseth's experience using the isocyanate for a primer with a topcoat may work fine in some cases, and not in others depending on the individual formulations. There are some good reasons for a manufacturer to utilize a relatively small number of different isocyantes in a product line, and sometimes two entirely different products may use exactly the same isocyanate curative. Other times, they may be very different. Again, without access to the specific formulations or doing some analytical work, it would boil down to a calculated risk. Availability of good labeling information or MSDS info could definitely tilt the odds in your favor of guessing correctly, however.

With respect to toxicity of isocyanates, the ones used in 2K paints are typically selected to have much, much less toxicity than the monomeric isocyanates they were derived from due to their vastly reduced vapor pressure. Still, they should be handled with respect, particularly owing to the possibility of sensitization if an aerosol containing them is inhaled into the lungs.

I hope these comments are interesting to readers. Let me know if comments on a particular aspect are wanted.

[grouch]

You can't always go by the names things are sold under. The foam boards I bought for some fiberglass work were called "urethane foam" in the advertisement. They were actually polyisocyanurate foam, a.k.a. rigid insulation without any facing material. Another example: ever bought vinyl siding or tore a vinyl top off a car? Those are not "vinyl" but rather a polymer, most likely polyvinylchloride or plain old PVC.

As a follow-up to powderbill's organic chemistry lesson (thanks for that, powderbill!), see http://scifun.chem.wisc.edu/chemweek.../Polymers.html

[Cruzin90]

I'm a LINE-X spray-on truck bedliner dealer. Our product is a blend of polyurethane and polyurea. A few points to make:

1. Polyester urethanes are more durable than acrylic urethanes.
2. There are two-part urethanes (polyol resin mixed with a isocyanate (a hardener) and there are one-part urethanes (also called solvent based urethanes) which is urethane suspended in a solvent. Long story short, two-part urethanes are typically harder and more durable than one-part urethanes.
3. There are aromatic urethanes and aliphatic urethanes (depends on the position of the hydrocarbon chains). Aliphatic urethanes are more UV resistant but usually are more expensive.

[powderbill]

[QUOTE=Cruzin9
1. Polyester urethanes are more durable than acrylic urethanes.


It may be helpful to define our terms here. When Cruzin9 refers to durability in this context, I'm sure he means impact resistance, flexibility, and toughness. I would agree that polyester urethanes generally have better impact resistance, flexibility, and toughness than acrylic urethanes.

The reason for my post is, when reading coatings literature, you'll often come across references to exterior durability referring to gloss and color retention on exterior exposure. So as not to cause any confusion, acrylic urethanes as a class will frequently exhibit better gloss and color retention than polyester urethanes. So, don't be surprised if you read statements to the effect that acrylic urethanes typically have better gloss and color retention, which may in some contexts also be referred to as exterior durability.

It's not that either statement is wrong, they are both right if the term durability is understood to be used in the correct context. I hope this might help clarify any possible confusion for interested readers.

[baddbob]

I've seen some durability ratings in pencil hardness, what's this referring to? Impact resistance?

[BarryK]

Deleted.

Changed my mind.

[Cruzin90]

powderbill: Great explanation! I'm a LINE-X spray-on truck bedliner dealer and I find few people who know this info.

A couple of side notes just to confuse everyone: Polyester resins are better (more durable) than acrylic resins. Also, isocyanate linked with amine terminated resins (as opposed to hydroxl terminated resins) gives you polyurea.

[mluckett4410]

In this forum you are talking about paint. I am a paint chemist and I do dabble a little with auto primers and clear coats. Polyurethanes can be rock hard or a elastic soft touch. Polyurethane is what you get when you mix a 1. Isocyanate with a 2. Polyol. Urethane or really polyurethanes can be acrylic, polyester,alkyd, vinyl, Nitrocellulose, cellulose acetate butyrate and on and on. All of thiese items in the second group are the polyol. Water is a polyol H-OH, this will cause CO2 bubbles in your paint. This is why there are urethane grade solvents which are very low in moisture. We also use moisture scavengers to consume water. A polyurethane can be lacquer solids of 15-20% to 100% solids. There are so many products out there and so many claims it is hard to keep it straight. Japaneses, German, and American automakers all have different approaches to clear coat hardness and they all have advantages and disadvantages. Polyurethane is the predominate clear coat chemistry hands down. Where I work we still make Nitrocellulose Lacquers just like the old days.