Digital BW, The Print

I have found a U.S. Army Corps of Engineers Engineering and Design manual on
line that is an excellent source of information on coatings.  Chapter 4 is
devoted to "Coating Types and Characteristics."  See
www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-3400/c-4.pdf

The two types of coatings that are of most interest to me now are the
water-borne acrylics and aliphatic polyurethanes.  With respect to the
water-borne acrylics the manual notes that they "provide coatings with
unexcelled color retention and have excellent exterior weathering and
durability properties." (p. 4)

On p. 9 the polyurethanes are discussed.  These cross-linking co-polymers
work due to the affinity of their isocyanate groups for active hydrogens.
The isocyanates can be either aromatic (containing the benzene ring) or
aliphatic (straight chain or cyclical) hydrocarbons.  "Aromatic
polyurethanes are prone to darkening and yellowing on exposure to sunlight
because of the chromophoric nature of the benzene ring.  Because aliphatic
polyurethanes, by definition, do not contain the benzene right, they do not
yellow or darken and are preferred for exterior use."

The aliphatic isocyanates are more expensive, but they "allow the
formulation of non-yellowing, light stable, high gloss finish coats. The
appearance of polyurethane coatings formulated with aliphatic isocyanates
are unsurpassed in this regard by any of the epoxies, acrylics, or other
coating materials."

I currently favor the polyurethanes because they are tougher and non-porous.
The non-cross-linked acrylics are porous and relatively soft/tacky.  The
Golden technical representative led me to Hydrocote Polyshield, which is
currently what I am using. It is a water-borne, single-solution, aliphatic
polyurethane that, from what I have read and what the Golden technical
person reports from their tests, does not yellow.  Long term experience will
probably be needed to be sure of this.

While Hydrocote appears to be excellent product (one coat on Eclipse kicks
the dMax to over 2, doesn't yellow the whites, and provides a satin/pearl
finish that is tough enough to clean with a damp paper towel), I am
searching for an aliphatic, water-borne polyurethane that has a higher
solids content.  So far, I have not had any luck.  All the higher-solids
aliphatic polyurethanes that I've found so far are solvent-based.  (If a
company doesn't or won't say whether it's polyurethane is aliphatic, assume
that it uses the cheaper aromatic [yellowing] isocyanates [read Varathane].)

If anyone knows of a high-solids water-borne aliphatic poly, please let me
know.

Paul
http://www.PaulRoark.com