Two Part Linear Aliphatic Poly Urethane Coatings
Available from:
Progressive Epoxy Polymers, Inc.
Your Host and Tour Guide:
Paul Oman, MS, MBA - Progressive Epoxy Polymers,
Inc.
Member: NACE (National Assoc. of
Corrosion Engineers), SSPC (Soc. of Protective Coatings), Dept of Defence Corrosion Exchange (www.dodcorrosionexchange.org)
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Two-Part Urethanes & Clear-Coats
The following is an introduction into 2-part urethanes and 2 part urethane clear-coats. These 2-part paints are
perhaps the best performing coatings (low yellowing, high gloss, durable and tough physically and chemically) available
for brush, roller or conventional spray application.
Most, of not all, 2-part urethanes are either acrylic polyurethanes or polyester polyurethanes. Sometimes the prefix
“poly” is left off. These are also called linear (or aliphatic) urethanes, or LPUs (linear poly-urethanes). In
any case, lots of ‘keywords’ for generally two kinds of 2-part
polyurethanes.
Polyester (poly) urethanes are considered the ‘best’. Compared to acrylic (poly) urethanes the polyesters are more
abrasion resistant, stain resistant, and more chemical resistant. You’ll find polyester urethanes on jet airplanes,
and on the floors of the hangers these airplanes live in. Boat owners should
note that operating a boat in water, especially seawater, is very much a chemical environment. Two very well known
2-part marine paints, (we will not say their names) are polyester urethanes.or acrylic/polyester urethane blends.
Acrylic urethanes are a bit cheaper and generally one notch down from the polyester urethanes in terms of toughness
and chemical resistance, but still above ‘regular’ paints. Acrylic urethanes are found in clear-coats used in the
automobile industry and ‘city water towers’ that grace many
small towns in America. Boat owners should note that Awlcraft 2000 (tm) is an acrylic urethane, as probably are
most of the 2 part urethanes sold in marine catalogs that don’t specify if they are polyester urethanes or acrylic
urethanes.
APPLICATION PROPERTIES: Both kinds of urethanes contain large amounts of solvents and thus have a strong solvent
smell. That said, additional solvents are often added during application. In the urethane world solvents are called
reducers. There are ‘fast reducers’ for spray application. These speed up the time it takes for the urethane to
‘gel’ on the surface. “Slow reducers” slow down the gel time for more working time when applying by brush.
Brushing on a two-part urethane is not like brushing on a varnish or oil based enamel. The thin, almost watery
urethane (you’ll probably need two coats or more to cover) starts to ‘gel’ on the surface quickly. Unlike varnish,
you’ll get 2 or 3 brush strokes and then, like it or not, it’s time to move on. An application method called ‘tip
and roll’ gets almost sprayer like results by applying the urethane with a roller and then gently removing the
roller marks with the tip of a brush. If is likely you will not be happy with the results of a brushed on application
on such things as boat hulls, epoxy table tops,etc...
In my experience, both the acrylic and polyester urethanes go on about the same, but the general view is that the
acrylics are slightly easier and friendlier to apply and, perhaps, repair.
Urethane’s weakest link is their adhesion. Because of that they are often applied over an epoxy primer. Besides
priming the surface the epoxies tend to ‘level the surface’ too, important because the high gloss urethanes will
show every flaw in the subsurface.
Recoat window for two-part urethanes is about 6 to 16 hours. Beyond that, sand lightly. Apply only in good, dry
weather, as urethanes are moisture sensitive during application and curing. The coating will become dry overnight,
hard in about 3 days, with maximum hardness in 7-10 days.
EVALUATING YOUR TWO-PART URETHANE: Obviously, the first question is, is it an unmodified polyester polyurethane
or an acrylic urethane? Next, what is the price (two part urethanes sold in marine catalogs are priced sinfully
high)? Finally, look at percent solids.
Percent Solids: these coats, like most other coatings, consist of some amount of solvents which evaporate away
(called VOCs - volatile organic compounds) and what is left behind is the ‘solids’ of the coating. A coating with
40% VOC has 60% solids. Apply a 10 mil (1/1000 times 10 in inch units) coating of this product and when dry you
will have 6 mils on the surface. Some manufacturers describe it in terms of coverage for 1 dry mil of their product.
A coating with 0% VOC (most epoxies) will have a dry 1 mil coverage amount of 1604 square feet. A coating with
50% VOC will have a 1 mil dry thickness coverage rate of 802 square feet ( you applied 2 mils over the 800 square
feet and 50% of it evaporated
away).
The higher the solids (the lower the VOC) the more paint you are actually getting on your surface. One of the leading
vendors of ‘boat hull’ 2-part urethanes does a wonderful job of providing their technical product information on
their web site. Their web site reports 1 mil dry film thickness of 570 square feet for their original polyester
urethane and 846 square feet for their version 2 polyester polyurethane. Their acrylic urethane product (introduced
around 2000) reports 512 square feet at 1 mil dry coverage.
NOTE OUR PIGMENTED 2 PART POLYS CANNOT BE SOLD IN CALIFORNIA
AND ARE ONLY APPROVED FOR APPLICATION ON MOBILE EQUIPMENT SUCH AS CARS, BOATS AND PLANES. SEE OUR VOC PAGE FOR DETAILS.
URETHANE CLEAR-COATS:
Urethane Clear-coats are almost a different topic from pigmented urethanes and much of that is due to UV blocking.
UV rays damage and fade coatings. It yellows and damages epoxies. One of the best blockers of UV rays is pigmented
paint. The pigments block the UV, limiting their affect to the very surface only. Clear coatings, of course, have
no pigments to block UV, hence, while they add additional gloss and ‘depth’ to a fine paint job, they are generally
considered to be performance inferior to the pigment coatings they commonly go over.
There are UV blockers that can be added to clear 2-part urethanes, but surprisingly a very minimal ‘UV Package”
is generally added to these clear urethanes (I’ve heard that automobile Clear-coats are an exception). The additives
are expensive (about $8 per gallon) and since most clear urethanes are applied over pigmented urethanes the manufacturers
tend to let the pigments in the bottom coating perform the UV blocking.
The Acrylic Poly UV Plus™ offered by Progressive Epoxy Polymers (mentioned above) is also an exception. This clear-coat
acrylic polyurethane has the maximum amount of UV blockers that can be added ($78 gallon), while their regular
Acrylic Poly - available in white only - ($70 per gallon) has the tradition minimal amount of UV blockers found
in most other pigmented acrylic urethanes. Progressive Epoxy also sells both ‘fast’ and ‘slow’ urethane reducers,
as well as moisture cured urethanes and, of course, all kinds of epoxies! SEE LINKS BELOW.
This product compares directly to Awlcraft 2000™ clear coat but is sell
for 30% of the Awlcraft price (70% discount!). We are fairly certain our product has much more UV protection (UV
blockers and UV absorbers) than Awlcraft 2000 clear. AWLGRIP WEB SITE.
THIS PRODUCT IS CONSIDERED A WATERPROOFING
SEALER. IT CANNOT BE SOLD IN SOUTHERN CALIFORNIA.
URETHANE COLORED COATS:
We offer polyester polyurethane (50% VOC) available in white only (LPU
Marine) - for use on boat hulls, cars, planes. Not for sale in Southern California.
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NOTES: 2 part poly paints are really sensitive to surface contamination
problems etc. Guys that paint cars use 2 part poly paints. They use a product made by Du Pont and sold at auto
part stores called 259S Paint Additive (quote: an additive for topcoats and clears to eliminate fisheyes, craters
and other surface defects). I paid $22 for a pint of it in 2003.
LPU coatings like to go on thin (multi thin coats!) - many folks sand between all or some of the coats. The coats
are very thin so here is a link to a site that sells 'sandpaper' in the 400 - 12,000 super/super fine range (thanks
for this info from one of our customers!) https://www.micro-surface.com/
Find the Polyester or Acrylic Polyurethane in our Marine Catalog - click here
Find the Polyester or Acrylic Polyurethane in our Industrial Catalog
- click here
Solvents:
Products have low viscosity, but if thinning (reducing) is still desired use our:
SU 93 REDUCER (Thinner) - slow - used mostly for brush/roll application
SU 94 REDUCER (Thinner) - fast - for spray applications
Knowledge is Power - We like informed consumers!
Learn the basics of epoxy at our educational EPOXY 101 page -
Click Here.
Finally, email us back with your questions or comments before
you buy - EMAIL HERE
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