Digital BW, The Print

From: "Robert Morrison" <rmorrison@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Tuesday, December 10, 2002 6:16 PM
Subject: Re: [Digital BW] Coating (was Faux Meyer Rod - first dMax
testresults)


> On 12/10/02 7:59 AM, "Ernst Dinkla" <E.Dinkla@...> wrote:
>
> > Paul,
> >
> >>>> I am down to near the bottom of the quart can of Hydrocote I first
> >>>> purchased, and I think I've noticed a  viscosity increase.  ...
> >>
> >>> Is there any chance the viscosity increase is actually 'Hydrocote
> >>> polymerised' or just solids sinking / solvents evaporating ? One would
> >>> expect polymerisation to happen on top of the liquid of course.
> >>
> >> I think some polymerization is a distinct possibility.  I'd be most
> >> concerned if it occurred in clumps -- making a smooth application
tough.
> >>
> >> I'm wondering if we ought to stir the mix very well when first
purchased
> > and
> >> pour it into several smaller bottles.  The 4 oz./ 110 ml. ink bottles
come
> >> to mind as possible candidates.
> >
> > I have not the faintest idea how fast it polymerises. Whether they have
done
> > any stuff in it to prevent polymerisation in the can itself. Mixing the
> > varnish first with the Fish Eye/Leveling additive and then pouring it in
> > smaller polyethylene bottles where you can squeeze out the extra air may
be
> > a good idea. Put a marble in too. Then you can shake the bottle without
> > mixing air in the liquid. But I have not the fainte ........
>
> Hydrocote is a polyurethane that is already polymerized!...that's why it
is
> called "poly".  There are coatings which you add a cross-linker to but
they
> are not available in water-based formula.  The problem with viscosity is a
> universal problem with water-based polyurethanes.  They are not really
> soluble in water so people add cosolvents to them.  If the cosolvent
(which
> is more volatile than water) begins to evaporate the polymer starts to
drop
> out of solution.  The polymer itself is much more viscous than the
solution.
> Personally, I don't fine polyurethanes to be a good solution for print
> coating...there are just too many variables that can go wrong.  Acrylics
are
> a much better solution and they are commonly used for artists
> materials...polyurethanes...are not.

There are polymers, prepolymers, monomers, dispersions, copolymers etc etc.
What is written on the can isn't exactly what is in the can but could as
well indicate what it will be when it is cured into a coating. And you could
be right that it is just a dispersion that will level and link after the
solvents disappear. I do not even know what is written on the can but got my
information from the thread.

In dispersions the trick is to get the water out before the cosolvent goes.
The gloss and strength are improved that way. So it is better to use a
cosolvent which is less volatile than water (if possible) or make the bond
to the resin better than water. The two component (poly) urethane itself
isn't known for its easy solvability, that's why they use it for silkscreen
squeegee blades and to seal concrete floors where solvents are spilled. In
the health safety data sheet for Polyshield is written: "solubility in
water: complete". "Waterborne" is written in relation to aliphatic
urethanes. That means it should be resolvable with water after the coating
has "dried", that isn't  so. Then two alternatives are there: it is a
dispersion and resolving the "dried" coating can be done by the cosolvent or
the coating is cured one way or another and even the cosolvent can't resolve
it afterwards. The Google search on aliphatic urethanes says "dispersions"
most of the time.

Acrylics have a history in art for fifty years max. Any new material will
have to be judged on its merits. Oil paints in art were not new when the
Flemish introduced them but they made them usable.

Ernst