Acrylic Latex Primer vs Latex Primer
In North America, latex house paints and latex primers are made of two different kinds of plastic resins:
1. Polyvinyl acetate
Polyvinyl acetate is also what white wood glue is made from. The polyvinyl acetate resins used to make paints and primers are harder and much less sticky than the ones used to make white wood glue.
2. Polymethyl methacrylate
Polymethyl methacrylate is what Plexiglas, Lucite and Perspex are made of (depending on whether it was made by Rohm & Haas, DuPont or ICI of Britain).
Basically, polymethyl methacrylate makes for a better binder for paint in every respect than polyvinyl acetate, except for price. PMMA is more expensive than PVA.
Where PMMA stands out, however, is in the following three categories:
A) Adhesion to moist or damp substrates. PMMA will stick much better to damp wood or concrete much better than PVA.
B) Moisture resistance. PVA will become soft and lose it's adhesion if it gets wet. This often causes PVA paints to peel on bathroom ceilings, with the result that the problem is often incorrectly attributed to poor prep work prior to painting the bathroom ceiling.
C) Blocking resistance. Blocking is the tendancy of PVA paints to remain slightly sticky even when they're completely dry. PMMA paints don't remain sticky at all once they're fully dried.
PMMA resins are also much more resistant to strong alkali substrates, which is why you can buy primers made of PMMA resins meant for use directly over new concrete. PVA and alkyd primers wouldn't stand up on new concrete because of it's high alkalinity.
PMMA resins are also more UV, mildew, and acid resistant than PVA resins, and they're harder than PVA resins so paints and primers made from PMMA will dry to a harder film than PVA paints and primers.
NOW, paints and primers that use PVA resins will call themselves "Vinyl Acrylic" paints and primers. Or, in the case of primers, just "PVA primer". PMMA paints and primers will call themselves "100% Acrylic" paints and primers.
So, look on the can or it's MSDS sheet and look for the words "Vinyl Acrylic" and "100% Acrylic". If the Acrylic latex primer says it uses a 100% Acrylic binder, then the resins in it are made of PMMA. If they both say that they contain a Vinyl Acrylic binder, then there is little difference between them.
Quite frankly, the reason why they recommend using a PVA primer over bare drywall is NOT because PVA is superior to PMMA in any way. It's because all of the things that make PMMA resins superior to PVA resins in making an interior or exterior paint simply aren't of any advantage in making a general purpose primer. For example, there's little disadvantage in having the primer remain slightly sticky after it dries if you're going to be top coating over it with paint anyway. Similarily, the ability of PMMA to stick to a damp surface better is of little benefit when priming over drywall. Also, the fact that PMMA doesn't soften up and lose it's adhesion if it gets wet is of little benefit in a primer if the paint over top of it prevents it from ever getting wet in the first place.
Where you need the special benefits of PMMA, you can get special primers for this purpose, like the Acrylic primers meant for use over new concrete mentioned previously.
So, there's a good chance the one that calls itself an "Acrylic" Latex is a PMMA primer, but you won't know that for sure unless you see the wording "100% Acrylic" on the can or on the MSDS sheet or Product Data Sheet.
Last edited by Nestor_Kelebay; 08-02-2008 at 10:03 PM.