The Piano is full of Polymers
Most of the insides of this piano are made of

Outside In

The outside casing of a piano and the frame that the keyboard, action and harp sit in is made of strong dense wood like oak. Wood is a form of cellulose, a natural polymer. The cabinet of the piano is what makes it look pretty, especially if the finish is kept shiney and scratch free. Most pianos have a coat of clear laquor made from a synthetic polymer. Some of the more popular choices for making a pinao shiny and protecting the wood are nitrous cellulose, polyurethane, or (more rarely) polyester, which is very durable and smooth, and also expensive due to the time it takes to polish it to a glassy shine.

A typical upright soundboard
The spruce soundboard in the back
of an upright is what generates
the sound you hear.
Though the strings do all the vibrating that initiates the sound, the sound that you really hear from a piano comes from the soundboard - in the back of an upright or the underside of a grand. It is made of spruce wood - more
cellulose - and composed of a set of boards about 3/8" thick. They are held together by a set of "ribs" on the back. The strings are connected to the spruce via a set of hardwood bridges that conduct the vibrations of the strings into the wood and cause the entire soundboard to vibrate. Without it, a piano would sound like... well not much at all. Pure tone from a vibrating string isn't really very loud