Acrylic or Polycarbonate substitutes for Glass Tops

["S. Hieber" <shieber at yahoo_com>]

Some plastics have over 95% light transmissivity.  But they
all absorb water.  Depending on the material as high a
0.15% in 24 hours for optical grade, which seems like a
miniscule number, but the saturation is usually about
double that figure.  Okay, that's still a miniscule number
but the water absorption tends to cause the sheet to swell
on the wet side.  Some acrylics are as high as 0.4%
absorbtion over 24 hours.  Lucite L (brand name product) is
only 0.03% over 24 hours and 92% light tramssion -- it's a
very costly form of the stuff.  You can prevent warpage of
even the cheapest sheet plastic with adequate thickness or
structural bracing (for example, box sides).

For a sump lid about 10" x 13", a 1/8 inch thick piece or
ordianry plexiglass can be expected to deflect about a
quarter inch from center to corner.  A 1/2" piece will not
deflect much at all and a 5/8' thickness won't noticably
deflect.  For those thicknesses, you usually need to cement
two pieces together.  [Cement this stuff requires, gloves
(preferably nitrile), very good ventilation or a carbon
filter mask -- the cement gives of some goshawful dangerous
fumes.  Maybe working with acrylic is why I made those
mistakes in the Horsepower=Btu discussion :-0  ]

Acryclic can be worked much like glass.  You can score it
with a sharp edge (say a sharp utility knife run along a
straight edge) and crack the sheet right down the score by
applying pressure on each side of the score.

You can cut acrylic and polycarbonate with saws.  But if
the saw blade gets too hot, the sheet will melt around it. 
In fact, the cut can close up right behind the blade.  And
if you stop mid cut, you might find it hard to retrieve
your blade from the material.  A circular saw, therefore,
works better than a "sabre" saw because the large blade,
and the air stream it produces over itself reduces heat
build up.  Tuffak brand polycarbonate, for example, melts
at 270F. 

Polycarbonate doesn't work with scoring as well ;-)  --
once you get thicker than about 1/200ths of an inch,
terrific force is needed to get the crack started.  On the
plus side you can put polycarbonate in a vice, grab an edge
with a vice-grips and bend the material over on itself and
it will hold together.  It's great for durable workshop
jigs and templates.  Polycaronbates are susceptible to
yellowing with exposure to ultraviolet -- but many types
are coated one side to prevent or reduce this.

All the versatility comes at a premium; generally, the
plastic sheets are much more expensive than plain glass. 
And polycarbonate is even more costly than acrylic.  If you
can find a dealer that sells scraps, you can work
inexpensively.  I think Erik Olson built an entire sump out
of such scraps -- and very nice job he did:

http://www.thekrib.com/Plants/People/Darn/darn4.html

I wish I could have found affordable material to build my
sump :-\

Scott H.

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