Digital BW, The Print

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.

Robert