For Chemist: Solvents

[Scott Rosen]

When using an oil-based paint or varnish, what are some solvents or additives that will slow down the cure and help the brush marks level out on a hot and windy day? I need something slower than turps or so-called mineral spirits.

I know about Penatrol. It's okay for paint, but kills the gloss and UV protection of varnish.

[Peter Duck]

You could try laying the paint on a little thicker, so that there is enough thickness in the paint film for the molecules to move around more easily, and thus find a common level [i.e. smoothing the brush marks out.] A little extra paw linseed oil may also help, but I'd be pretty careful not to make it too much. How much is enough is pretty much a trial and error thing, using a scrap panel to practice on.
Peter.

[Wayne Jeffers]

Scott,

A couple of thoughts:

1) avoid painting when it is too hot and windy (okay, not always easy)

2) for paint, a little raw linseed oil will retard drying and provide some of the benefits of Penetrol

3 for varnish, don't be afraid of thinning it too much, if the need arises. Non-photochemically reactive mineral spirits will evaporate and will not noticeably change the characteristics of the varnish. (Turpentine will increase yellowing from light.) You'll just have to put on more coats, because each will be thinner. Penetrol or linseed oil, on the other hand, will dilute the UV absorbers in the varnish so that it degrades faster in sunlight. I've occasionally thinned varnish maybe as much as 50/50 with mineral spirits, with no apparent harm, other than the extra coats that were required.

Wayne

[thechemist]

You won't slow down the cure without screwing up the catalysts ["dryers"] that make the stuff cure. Using different amounts of different dryers, complexed with different chelating agents, gives different cure profiles. That's something the manufacturer does in the formulating, and it's out of your hands.

What I think you actually care about is how it brushes and levels, and there one needs solvents with different evaporating rates, as well as an additive set that does various sorts of magic.....leveling, crater elimination, and so forth. Those are developed specifically for the particular resin formulation and I can't help you there, either.

What I can help you with is the solvents , and that may be enough. The usual distribution channels for varnish solvents are not likely to be too helpful....unless you get lucky and find some place that has Aromatic 150. Some polyurethane brushing reducers, while more expensive, may have an evaporation rate that suits your conditions. Evaporation rate is usually specified as relative to n-Butyl Acetate, arbitrarily given as one [or 100...ya gotta watch for that ]. Odorless mineral spirits is about 0.2....... You probably want an evaporation rate of something in the 0.1 to 0.02 range. Ask around.

[Wild Wassa]

The only way to slow the cure is to add more binder. Linseed. You can also use higher quality (seed) oils, such as poppy seed oil. This has one advantage, non yellowing.

When I have really wanted to retard the curing, I have used 'Stand' oil, which increases the gloss markedly. Not an easy one to mix.

Warren.

[ 06-19-2002, 04:16 AM: Message edited by: Wild Wassa ]

[capt jake]

Glad to see you back, Mr. Chemist!! [img]smile.gif[/img] Long time no hear from.
I have no say in this thread, only want to say welcome back!!!!! [img]smile.gif[/img] [img]smile.gif[/img] [img]smile.gif[/img]

[htom]

Welcome back, Chemist; I've missed your posts, too. Hope you enjoyed your downtime.

I'll contribute a question: I've got some of those cheap green poly tarps. Careful examination reveals that the "rope" in the hem is held there by what appears to be glueing the tarp to itself, first by folding under the cut edge, and then gluing that tab to the rest of the tarp.

Is this a "real glue", some substance applied to the tarp, like the sticky layer on tape, or is it (as it appears to be) a solvent that dissolves the surface of the tarp, allowing the two parts to "melt" into each other, and then evaporates?

If the latter, what would that solvent be?

[thechemist]

Not likely glue, unless some hot-melt thing...if they are really cheap polyethylene my first guess is heat-welded.

[htom]

Thank you, sir. Now where's that old iron? I want to try an experiment ... maybe I should start wtih the soldering iron.

[thechemist]

If you're gonna heat-weld polyethylene, you will likely make a mess, as it melts through easily. Lay a sheet of teflon film above and below the area to be heated, and connect the soldering iron [with a wide chisel tip] to a variable transformer, to reduce the heat from its typical 500-800 degrees to soemthing close to the softening temperature of polyethylene. Even teflon thread tape might work, although I would use 5-mil teflon or FEP. Move the chisel tip over the release film [not too slow, not too fast] and ,with the temperature set [not too high, not too low] you might get lucky and weld the stuff.

[Wayne Jeffers]

Originally posted by thechemist:
You won't slow down the cure without screwing up the catalysts ["dryers"] that make the stuff cure. . . .

In oil-based paint, I believe the catalysts are not required for the paint to cure, but rather accelerate the cure (especially in cool, damp weeather.)

Raw linseed oil will dry (slowly) even in the absence of additives. My layman's experience has been that a bit of linseed oil will retard the drying of oil paint with no apparent ill effects to the dried paint (except that "flat" paint acquires some gloss.)

Wayne

[ 06-20-2002, 06:34 PM: Message edited by: Wayne Jeffers ]

[thechemist]

Ummmmm...not exactly.

It is true that there are some oils, notably Linseed and Tung, that seem to "dry" without dryers.

These oils have a high percentage of linolenic acid, an 18-3 unsaturated fatty acid. 18-3 means it is 18 carbons long and has three double bonds. These are conjugated, meaning alternate with single bonds. Such triple-bond arrangements are highly reactive [due to resonances in electron distributions...it's technical], and will oxidize without catalysts/dryers.

Tung oil has about thirteen percent 18-3 and about sixty percent 18-2. That's enough for it to self-dry. Linseed has about 53% 18-3, an extraordinarily high amount. It of course does self-dry. Sunlight weathering characteristics are of course lousy, and small life can easily biodegrade it.

Unfortunately, such arrangements are so sterically hindered that all three double bonds do NOT react, leaving at least one unreacted double-bond at each reaction site.

Unreacted double-bonds in a cured varnish ARE the source of yellowing and premature embrittlement.

That's why varnish formulators use oils that have inherently little or no triple bonds, and why dryers are necessary for them to cure.

When you see a varnish advertising that it contains tung oil, you know it was formulated by the marketing department, not the chemistry department, and you should expect it will darken prematurely. That's fine, as long as that is what you want.

It also follows that adding linseed oil to varnish degrades the quality of the cured polymer.

[Wayne Jeffers]

Ahhh, thanks, Chemist, but a point of order!

I suggested linseed oil to retard drying of paint, wherein UV degradation is a minimal issue.

I've never used it in varnish, nor would I recommend it for varnish. I don't recall ever using anything in varnish except non-photochemically reactive mineral spirits. These seem to evaporate without ill effect, except that building up a proper film thickness requires more coats.

What are the issues with adding linseed oil to oil-based paint?

Wayne

[Wild Wassa]

The (My) issue is 'fat over lean' (adding a drop more oil per coat) vrs reducing your solvent amount (per coat) as you paint your way to the top, often balancing both. In an attempt to find a usable blend that suits the surface you are after. As a painter this is part of the skill. I've killed surfaces with a dash too much solvent. The amount of oil added is a small amount. Even drops. Do not splash or pour it into the paint. Measure your changes well.

Warren.

ps, I was taught to paint by a Dutchman, Mr Weston. He was taught by a Dutchman as well. Who was taught by a Dutchman as well.

[thechemist]

Point taken.

Added to oil-base paints, it will reduce flexibility of the cured system.