Lowest friction of
any solid material.
Chemically inert.

PTFE stock costs more than most engineering plastics (raw virgin PTFE is ~USD 25–40/kg vs $5/kg for nylon), and machinability is only fair — the material is soft and prone to deformation under tool pressure, requiring sharp tools and careful fixturing. Waste is also higher as chips cannot be remelted into new stock (unlike thermoplastics).

Nearly. Exceptions: molten alkali metals (sodium, potassium) at high temperature, elemental fluorine gas above 150 °C, and some fluorinated solvents at elevated temperature. For all practical chemistry including concentrated acids (HF, HCl, H2SO4, HNO3), bases, and most organic solvents, PTFE is effectively inert.

Fusion welding: no. PTFE does not melt in the conventional sense — it sinters. Adhesive bonding: very difficult. The non-stick surface that makes PTFE useful resists all adhesives. For bonding, surface must first be chemically etched (sodium naphthalene) or plasma-treated, then epoxy or cyanoacrylate can achieve modest bond strength. For assemblies, mechanical fasteners and press-fits are preferred.

PTFE wins on pure chemical resistance (inert to more chemicals than PEEK) and low friction. PEEK wins on mechanical strength (5× PTFE tensile), dimensional stability, and wear resistance under load. For low-load gaskets and seals, PTFE. For structural chemical parts under load, PEEK.

Virgin PTFE is FDA 21 CFR 177.1550 compliant for food contact. USP Class VI grades exist for medical fluid handling. However, PTFE decomposes to toxic fluorinated compounds above 260 °C, so do not expose to open flame or cooking surfaces above that temperature. For repeated high-temperature medical sterilization, PEEK is preferred over PTFE.