This is a EPDM 5/8 Washer which is suitable for sealing hose tails on Keg Couplers and Tap Shanks.

Specifications:
15mm ID x 21.5mm OD x 3mm

What is EPDM?
EPDM = Ethylene Propylene Rubber which has excellent chemical resistance including being compatible with ozone, oils, wide variety of alkaline and acid compounds, water, alcohol, ketone and beverages.

EPDM is also resistant to aging, weather and will not perish easily like other rubber compounds.   When compared with silicon and vinyl EPDM has better gas transmission rates, better wear resistance, is BPA free and is the number one choice for many long lasting food grade applications.