In view of the overwhelming concern here with materials, I wonder if
anyone has tried accelerated testing of rubberized fabrics, leathers,
and other sheet materials.

It shouldn't be all that difficult to set up a test cell.  You'd need
one of those 'air ionizer' devices to produce some ozone, a strong lamp
to provide light and heat, an ultra-violet (UV) light source, and some
sort of an oscillating pump to inflate and deflate small bags of the
competing materials under test several times per second.

At 86,400 seconds per day and three inflation cycles per second, you'd
put 7,776,000 cycles on the samples in thirty days under conditions of
high heat and ozone.  And of course you could glue the test bags to
a suitable sheet of wood to test various sorts of glue as well.  And
while you're at it, you might add different sorts of air tubing in the
cell as well.  If you used a brush-type motor for motive power, your
ozone source would be the motor itself if you placed it inside your
airtight test cell.  You'd want a window in the thing so you could
watch the progress of your tests.

While there is no such thing as artificial time, accelerated testing
techniques are often very helpful in eliminating poor materials.
Materials that pass accelerated aging may still fail with time, but
your odds are better.

Since pianos are closed up, it's possible that the UV light source
might seem unnecessary, but I would include it because UV light can
break down weak long-chain molecules that are stressed from heat and
constant flexing.

And that is, in general, the objective of this test.  Rubber and
plastics consist of very large molecules or "chains" of carbon atoms.
These molecules are long and stringy, and they tend to tangle up.  But
they can also slide along each other.  When they're in this condition,
they form a gum or a very thick liquid.

In order to turn these compounds of long-chain molecules into a useful
plastic or rubber, it is necessary to turn them from gums into stable
solids.  We do this by adding stabilizing materials that form strong
bonds between the various stringy molecules wherever they cross each
other.  In rubber, we make these cross-links by adding a bit of sulfur
and heat.  (The sulfur is what makes burned rubber smell so bad.)
Other plastics use different chemicals and techniques to provide
cross-links.

We've all seen rubber products and fabrics turn back into gummy
liquids.  This happens when the cross-links rot out, leaving the
original carbon chains without anything to hold them together.  They're
then able to slide over each other once again and the whole thing turns
into a gummy mess.

Someone who knows what he's doing will have to tell us why, in this
context, hide glue works as well as it does.  My guess is that it
doesn't have cross-links at all and is thus a super-cooled liquid like
glass, so there aren't any cross-links to rot out.  I believe that
polyethylene is much the same, but of course glue won't stick to it.

Now, we also have to consider the bonds that hold the various carbon
atoms together in each long chain.  If these bonds fail, then the
rubber or plastic will dry out and crack.  Mechanical stress is
important here, as are heat, ozone, and UV light.

(If anyone has access to XLPE, or cross-linked polyethylene, I'll bet
that it would be a great sheet material for player piano work.  We use
it to insulate large electric power cables, but it's extruded and I've
never seen it in sheet form.)

Corrections invited, please.

Mark Kinsler
Lancaster, Ohio
http://www.mkinsler.com/