> Didn't think of that one... Now I am correct in saying that refractive
> indices don't apply for solid objects right? Where the light hits the
> object and heads directly back the way it came?

Well, say you pot things made of various resins in polyester. A piece
of cured, clear polyester, made from the same polyester as that you're
potting with, will largely disappear. It won't completely because the
bonding on the surfaces will be imperfect. For example, if you want
to "float" an object in an encapsulation, you'd put down a layer of
clear, then place the object on that layer when it's gelled. But gelled
only enough to hold the object up. Then fill the remaining space with
clear. Pouring one layer onto a softly gelled layer will make the boundary
of the pours virtually invisible

LEDs are made from epoxy, AIUI. The epoxy they are made from
is water-clear. Polyester potting resin is not water-clear, so the body
of the LED will be observed as clearer. Coloured LEDs of course you
wouldn't worry about. Other plastics, like acrylic, are water-clear. The
problem with polyester is that it has coloured additives. Generally this
would be the promoter, which an be anything from straw to purple.
Uncured clear encapsulating polyester is often pale blue

Mentioning epoxy - one thing I found with coaster experimenting is
that polyester doesn't like improperly cured epoxy or anything that
releases something. For example, objects glued together with the 5
minute or 24 hour epoxy you'd use on the bench stops polyester curing.
There's probably an excess of one of reagents, amine or acid, that
interferes with the catalyst or cross-linking. The one plastic polyester
doesn't mind is laminating sheet. Just won't stick to it. Or glass/metal
if you use release wax (sparingly). All are good for making a hard
smooth surface. Air inhibits polyester curing and any exposed surface
will be the last bit to cure and will not be smooth, and may be sticky
for quite some time. Polyester also doesn't like going on things like
paint (styrene is quite a strong solvent), polystyrene, the glue on
adhesive tape etc

> > The same also happens if the object has any liquid (eg moisture) in
> > it, as the exothermy of the curing causes a vapour layer if you don't
> > pre-coat
>
> Ahh thanks, I'll keep that in mind then. I recently embedded a watch
> along with 6 batteries in parallel in polyester resin, I didn't have any
> problems with silvering, but the plastics studio at school tends to be
> really hot and dry.

The main factors are temperature, catalyst and bulk. The higher the
resin temperature or higher the catalyst, the faster the resin will
gel/cure. And the faster it gels/cures, the higher the temperature of
the resin will get through exothermy. This is also influenced by the bulk
of the resin. For example, you can have a fairly high catalyst level
when fibreglassing because there's a large surface area and heat can
escape. The same catalyst level in a 1kg block might probably cause
it to split. It'll easily boil water. In the lab we even had the rare case
of polyester igniting

Our cavalier sales rep learned a valuable lesson. We warned him
about the dangers of naphthenates and peroxides. But would he listen ?
Would he heck. Then one day he threw a couple of litres each of MEKP
and cobalt naphthenate promoter into a box in the boot of his car. Maybe
the bottles had reagent on the outside or one or the other leaked. By the
time he'd popped in the office and come back out, the wall had well and
truly been written on. The fire brigade were hopelessly late. I'll say this
for a Toyota Corolla - it don't half burn well, even with a sales rep
shouting at it. And us lads thinking "told you so" but deciding this was
probably an inappropriate time to utter that out loud

During winter I'd have to warm all the ingredients up in a drier cabinet or
I'd be waiting all day for it to gel. In the summer you'd have to pop them
in the fridge for a while. Summer heat is a real pain for storage too. The
styrene in polyester self-polymerises very easily in summer and a can on
the shelf might last a couple of months

One thing to watch for in relation to curing heat is post-cure shrinkage.
In extreme cases this will cause cracking, but managing the temperature
and gel time properly (as above) should result in the resin just shrinking
away from the mould. You might expect 0.5 - 1%. Odd-shaped
encapsulations might go a bit funny, because of the heat distribution. In
that case you'd use a lower level of catalyst so the gel time is longer and
heat has time to escape and distribute more evenly. The other advantage
with a longer gel time is that air bubbles have longer to rise. You can go
too low, resulting in a soft cure because of incomplete immediate
polymerisation (would still hurt if someone threw it at you !!)

> A minute or two with a heat gun set on low should be enough to
> prevent that right? At least for the surface moisture?

You can also clean it in meths or acetone, as both are miscible with
water and will dry it. If you wanted to pot something like a leaf, you'd
pre-coat it with resin, using a fairly high (5% ?) peroxide level because
of the air exposure and rapid heat loss. That will trap the moisture, then
you can pot this when gelled

> > OTOH, using heat, including a sensor could make the coaster
> > light up and even change colour (RGB LEDs) as the temperature
> > changes. Might be too rough on small cells

> Do you mean that the cells, as in batteries, would be permenently
> installed?

I'm sure you could mould a coin-cell (eg 2032) holder into a coaster,
or even use adhesive aluminium foil as the contacts, with wires going
to the device from a battery compartment. You can use self-adhesive
felt for the bottom

> I've got a project on the backburner that would involve a flat disk
> with embedded murcury tilt-sensors and solar-cells to run everything.
> I'd throw in a bunch of super-capacitors for night operation. Just
> gotta find a way to get sufficient power, it'd need a radio link to
> communicate with a similar module inside, and super-capacitors
> just don't store much energy.

Watch the styrene-as-sovent issue, especially with thermo-setting
plastics. The el cheapo solar cells I've seen look to have what might
be polystyrene covers/lenses, which free styrene will attack before
polymerising

> > Good hobby, bit smelly and messy though
>
> Not to mention the wonders of what methel-ethel ketone does in
> your body...

Yeah, you have to watch most organic chemicals as they're fat-soluble
and can be absorbed through the skin. I well remember making my first
batch of polyurethane and getting a minute splash of toluene di-isocyanate
on my hand. Even before that, just looking at that big brown bottle gave
my sphincter plenty of exercise. So actually getting a drop on me was....

It was hysterical and wasted agonising is what it was. Just 2 seconds to
get to the tap and rinse it off. The time I got a drop of chromic acid on
my finger was something real though. Such a powerful oxidiser (it's
potassium dichromate dissolved in hot conc sulphuric acid - nasty) that
in just 2 seconds tapward bound I got burned and still have a 3mm
round scar


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