Homebrew PCBs

--- Steve <alienrelics@...> wrote:
> --- In Homebrew_PCBs@yahoogroups.com, mpdickens
> <md30022@y...> wrote:
> > 
> > --- Steve <alienrelics@y...> wrote:
> >  
> > > I  may just pot the high impedance part of the
> > circuit > if I can find an epoxy encapsulant that
> has
> > very little 
> > > conductance.
> > 
> > Polyester resin (Epoxy is the trade name for
> polyester
> > resin) has an extremely low conductance. In fact,
> the
> > conductance of expoxy is close to that of glass.
> I think you've said this before, but I've never
> heard that. And epoxy
> resin is always listed separately from polyester
> resin.
> 
> Like this site:
> http://www.shopmaninc.com/resins.html

Your right. You nailed me ;) Most of the stuff they
sell in stores that is labeled "Epoxy" is really
polyester resin. In fact, true epoxies have to be
heated in order to cure (In an autoclave). For our
purposes (At least mine...), polyester resin is just
fine. I happen to know a little chemistry. Here is the
difference between the two:

Polyester resin is a simplier molecule than an epoxy
molecule. Polyester is made by reacting dibasic acids
such as maleic anhydride or phthalic anhydride with
dihydric alcohols such as ethylene glycol (antifreeze)
in equal amounts.  These resins are short chain
polymers  and are set into amorphous solids by cross
linking the polyester chains to each other (This
results in an exothermic reaction which is why it get
hot when it cures).

OTOH, Epoxy polymers are made by reacting
epichlorohydrin with bisphenol-A in an alkaline
solution. Each polymer chain has one epoxide grouping
at each end of the chain (but none within the polymer
chain). The hardners that are used are short chain
diamines such as ethylene diamine. Once they are
mixed, the reaction to produce an epoxy molecule has a
high heat of reaction and requires the introduction of
energy (Heat) into the system. Also, hcl is produced
as  a by product (along with water) during this
synthesis. Both the hcl and the water must be burned
off (By heat) as it is produced in order to completely
cure.

Epoxy is the strongest and most flexible of the two
(And also real expensive because you need an
autoclave).

Best regards

Marvin Dickens
Alpharetta, Georgia 

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