Digital BW, The Print

Paul,

One thing to remember here...is that the technology that you are being
handed from these companies is for coating wood...a rigid, hard surface.
The demands on a coating for a flexible, porous substrate like paper are
very different.  That's one reason why no artist's materials companies use
polyurethanes.  

Regardless of what you are being told about the aliphatic water-based
polyurethanes...no significant crosslinking occurs after the original
emulsion polymerization reaction...that's marketing hype.  The film cure is
very similar between the polyurethanes and the acrylics.  The difference is
that the polyurethane polymer is harder and more light sensitive than some
acrylics.  Because of the huge range of acrylics polymers, products can be
formulated with a considerable variance in hardness.

The key in this application is to get something that is just hard enough
that there isn't a tack problem, but not as hard as polyurethane so that
there is still flexibility.  Of course this isn't an issue with wood.

Robert

On 12/10/02 9:59 AM, "Paul Roark" <paul.roark@...> wrote:

> Robert wrote:
> 
>> Hydrocote is a polyurethane that is already polymerized!...
>> that's why it is called "poly".  There are coatings which you
>> add a cross-linker to but they are not available in
>> water-based formula.
> 
> My understanding is that the water-based, single-component polyurethanes do
> cross-link after application, whereas the acrylics do not.  Both are (I've
> been told) soft "golf balls" in suspension.  While the best cross-linking is
> with the 2-component, solvent-based polyurethanes, the single-component
> water-borne version is said to be about half way between the acrylics and
> the industrial-strength PURs.
> 
> As I understand it, the importance of the cross-linking is that without it
> the coating is porous and cannot achieve as high hardness relative to
> brittleness.  Many seem to recommend that a water-borne acrylic be coated
> with a solvent-based polymer to seal it and reduce the "tackiness" of the
> surface.  My hope with the PUR is to get a single-step coating procedure
> that is tough enough to withstand some abuse.
> 
> For a tough, durable coating, the aliphatic PURs are the standard, as far as
> I can tell.  The 2-component, solvent-borne versions just seem like they'd
> be too much of a hassle to use.
> 
> One company has said it will send a formula to me/us that will make a
> 2-component water-based PUR.  This is the next step even for industrial
> PURs, in part due to the pressure on VOCs.  This would probably be the
> strongest water-based coating possible today, but I have yet to receive the
> materials.
> 
> Paul
> http://www.PaulRoark.com
> 
> 
> 
> 
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