Some clarifications. (I hope these clarify ...) What John says is
basically true; here are a few more details. (Anyone with better
knowledge, please correct me anywhere. I don't mind learning more or
unlearning any erroneous "knowledge.")
Lacquer actually dries -- i.e. the vehicle (lacquer thinner)
evaporates, leaving the pigments behind. When painting lacquer, thin
layers is the rule, since each new layer actually melts a little of
the previous layers and bonds with them to form a full-thickness
coating eventually. Cellulose is the actual plastic part which is
left behind [along with the colourants] when the thinner evaporates.
Lacquers are environmentally bad because all of the liquid eventually
must evaporate and fills our atmosphere with unburnt hydrocarbons,
leading to ground-level ozone and smog.
An enamel actually cures (mostly); some of the vehicle evaporates upon
application, but much of the liquid in the material remains behind and
polymerises [rather slowly] over the course of time, forming the final
coat. This coat is resistant to many solvents because of the cure
process. Needs to be applied in as thick a layer as possible (without
runs) because subsequent layers don't adhere like the lacquer layers
do. Or at least there is no good reason to apply thin layers. But
you do have to let each coat dry sufficiently (allow the volatiles to
evaporate) before recoating, otherwise the volatiles in the lower coat
have no way to evaporate except by lifting the upper coat. Enamels
have less volatiles than lacquers, generally, so are environmentally
more friendly.
The acrylic enamels (and latexes) are specific types of enamels in
which the liquid which will eventually polymerise into the body of the
paint is an acrylic (or latex) monomer (or shorter-chain polymer)
material which polymerises further on exposure to oxygen. (I believe
most of these polymerised materials form their chains when the free
ends of the shorter molecules oxidise. That is the way that linseed
oil, for instance, gets all gummy, and why oil paint "dries" even in
the can -- the oxygen that gets in while the can is open cures the oil
in the paint.)
The two-part paints are a kind of enamel which use isocyanates as a
catalyst to speed the polymerisation of the vehicle (which otherwise
would take some years to proceed to completion,) so very little extra
liquid (and not a lot of time) is required for the process. The
vehicle is a urethane monomer which is converted to polyuerthane by
the polymerisation reaction. This kind of paint is very resistant to
solvents and environmental attack because of the chemical stability of
the polyurethane. This stuff is very often applied as a base (colour)
coat with an overcoat of clear to give "depth" to the finish.
Epoxy enamels are another form of two-part paint. This stuff's
monomer is an epoxide resin, just like the adhesive. I think the
expense of epoxies precludes their use except where their cured
characteristics (adhesion, toughness) are really necessary.
HTH, and I'm sorry I got so long-winded.
Donald.
> Date: Thu, 13 Sep 2001 18:43:57 -0400
> From: "John T. Blair" <jblair@exis.net>
>
> At 10:58 AM 9/13/01 +0100, Dean Rayner wrote:
> >Hi,
> >
> >Can some one clarify what Enamel is?
> >and
> >What Laquer is?
> >
> >As a UK resident I would call the origional paint technology on our cars
> 'Celulose' and moden paint technology 'Two Pack'. There is also the touch
> up sprays which I would call 'Acrylic' (Very soft).
> >
> >"Celulose" Mix 50/50 with thiners and shoot with a mask.
> >"Two Pack" Contains IsoCyanates (sp) and kills you!!
>
>
> Dean,
>
> I haven't seen anyone reply to your question. I'm not a chemist so
> I don't really know if I can answer your question with specifics, but
> I'll try.
>
> Lacquer - an older type paint. It was thinned with lacquer thinner, and
> was very fast drying. It was usually applied in several coats,
> then "wet sanded". Several more coats were applied and the
> "wet sanding" repeated. This would go on for many many coats.
>
> It is said to be very brittle, but I've never really had a problem
> with it.
>
> It did have a tendency to blush - or go whitish if there was too
> much moisture in the air as it dried from the outside in, which
> allowed it to trap the moisture.
>
> Enamel - A newer paint. Doesn't go on as wet - has a greater tendency to
> run. So places actually use heat lamps to help speed its curing.
> It has a tendency to fish eye - form small circular areas where
> the paint is rejected due to oils being on the material being
> painted. It also has a tendency to "orange peel". Get dimples
> due to the paint curing too fast.
>
> This paint usually only has a couple of coats applied. Then if
> it is dull from overspray, or orange peeled, it can be wet sanded
> with 2,000 grit paper and then buffed back out to a luster.
>
> There is a "hardener" additive that contains IsoCyanates that
> can be used with this paint.
>
> The "Two Pack" as you call them are called "Base coat Clear coat" here.
> The base coat is the color coat and is very fast drying - like lacquer.
> The clear coat contains the IsoCyanates and gives the paints their depth.
>
> This is a real laypersons explination. But I hope it helps.
>
> John
/// spitfires@autox.team.net mailing list
/// or try http://www.team.net/cgi-bin/majorcool
|