Thermo Scientific. ™. National PTFE/Silicone Septa for Storage Vial Caps. • Thick PTFE, high density silicone septum is ideal for storage applications. • Thinner PTFE layer is suitable for fragile needles†. • Thick Silicone layer provides extra resealing ability. • Natural color PTFE laminated to clear silicone. • Extremely clean.