They are Teflon sheets with poly tetra flour ethylene anti-stick properties or a protective sheet that can be used at high temperature and pressure.

These types of sheets are highly resistant to corrosion and hydrolysis, have a wide operating temperature range, and are chemically inert (except for some alkali metals). PTFE sheet also has approved materials and properties and is resistant to ultraviolet rays, PTFE is composed of carbon and fluorine atoms that form very strong chemical bonds with each other, that's why Teflon is very non-reactive and this It makes the coefficient of friction very low. This issue makes everything slide easily on its surface and for this reason it provides the possibility of proper sealing to the consumer.
The specifications of Teflon are:

Superior corrosion resistance
 
Non-flammable (limited oxygen index below 90)
 
Low friction coefficient
 
Non-sticky and self-lubricating
 
Superior high and low temperature endurance (usable from -200~260℃)
 
High electrical insulation
 
Resistance to the atmosphere
 
Low permeability, property of preventing gas leakage
 
Appropriate physiological adaptation and anti-thrombosis