Leland,
I'm not sure to which point you are addressing the "why", but in general modern waterborne paints are a delicate balance of tiny binder emulsion particles, pigment particles, cosolvents, fillers and complicated surfactant packages. It's kind of amazing that the stuff works at all, and it is a really under-appreciated technical triumph, if you think about how far all of this has come in just our lifetimes.
Pigment and filler particles can carry little electrical charges that tend to make them stick to each other. The binder particles are little globs of polymer surrounded by a surfactant film that allows them to form stable emulsions in water.
Dispersants/surfactants are there keep them apart. Freezing can easily upset the surfactant chemistry.
Binder particles are supposed to coalesce and form a film after the paint is applied and starts to dry out. The paint doesn't know the difference between the air surrounding the walls of your house and the air in the head space of your half-consumed can of paint. It coalesces and doesn't easily go back to being dispersed little particles. (Note that I did not say half-EMPTY can of paint - as paint containers, like glasses of water, are never empty, a portion of their liquid contents simply being replaced by an odorless, colorless gas).
Most paints include ingredients that can act as food microbes of one sort or another. There are anti-microbial additives that keep them in check for a while, but once the can is opened and nature gains entree, the anti-microbial package can eventually lose the battle.
Edison Coatings, Inc.
Michael P. Edison
President
3 Northwest Drive
Plainville, CT 06062
Phone: (860) 747-2220 or (800)341-6621
Fax: (860)747-2280
Internet: www.edisoncoatings.com
www.rosendalecement.net
E-Mail: [log in to unmask]
---------- Original Message -----------
From: Leland Torrence <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, 9 Sep 2010 06:26:23 -0400
Subject: Re: [BP] anything wrong with using 5 year old paint...
> Michael,
> Why? I remember something about freezing in Hell, but thechemistry eludes me.
> Best,
> Leland
>
>
> From: The listserv wherethe buildings do the talking [mailto:[log in to unmask]] OnBehalf Of Edison Coatings
> Sent: Wednesday, September 08, 2010 12:20 PM
> To: [log in to unmask]
> Subject: Re: [BP] anything wrong with using 5 year old paint...
>
> Maybe. Degradation can definitely occur in the can, even if properly stored.Acrylic latex polymers may tend to coagulate after some period of time. Openedand reclosed containers are subject to skinning over due to air in the headspace, or biological degradation. Pigments may hard-pack at the bottom of thecan and not fully redisperse. Some of the se issues are obvious when you seethem.
> On the other hand some problems are not readily evident. We had a shipmentof waterborne polyurethane coating frozen in transit on its way to Minneapolisin February in a supposedly heated trailer. (The trucking company claimed thiswas an "act of God" when denying our damage claim and we let themknow it would take an act of God to get their freight bills paid - but that'sanother story). In any case, the coating was quite rightly rejected by thecustomer when it arrived as a solid frozen block. By the time it was returnedto us it has fully thawed, and just stirring it up again it all looked OK. Butwhen we did bond strength testing on the thawed paint it had lost half itsoriginal performance.
>
> Edison Coatings, Inc.
> Michael P. Edison
> President
> 3 Northwest Drive
> Plainville, CT 06062
> Phone: (860) 747-2220 or (800)341-6621
> Fax: (860)747-2280
> Internet: www.edisoncoatings.com
> www.rosendalecement.net
> E-Mail: [log in to unmask]
>
> ---------- Original Message -----------
> From: [log in to unmask]
> To: [log in to unmask]
> Sent: Wed, 8 Sep 2010 12:10:41 EDT
> Subject: [BP] anything wrong with using 5 year old paint...
>
> >
> > ...as long as it is adequately mixed? 10 year old paint?
> >
> > c
> > --
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