Fellow MMDers,  I don't know if the discussion on synthetic materials
is still open, but here is my two cents worth.  I didn't join in on
the discussion earlier since it's difficult to defend most synthetic
materials.  I have redone enough player actions with gooey plastic
tubing and Perflex pouches to cure me of using "un-natural" materials.

However, there is one exception to the rule, as far as I am concerned.
It is a nylon pneumatic material that American Piano Supply Company
used to market as Bilon or Bylon (I believe) and is now marketed by
Schaff Piano Supply Company.  I am continually servicing actions that
were rebuilt 30-plus years ago with this material and there is nary
a pinhole that has developed, whereas I continually service players
that were rebuilt as few as 12 years ago where the rubber cloth is
dried out and cracked.

Yes, you have to use PVC-E glue; however, 40 years from now when the
player requires rebuilding, the price of a sanding belt or a bottle of
hydrogen peroxide(*) will be a small price to pay for 20 extra years
of longevity.

Don Barton
Barton Player Piano Co.

* Footnote: hydrogen peroxide breaks down PVC-E glue.