Q: Need to replace a standard steel washer with a non-conductive material.
A: After speaking with this customer, I learned that he needed to replace a standard steel washer with a non-conductive material for a critical bolted joint. These washers needed to withstand 70°C temperatures while maintaining preload. He had been experiencing creep with non-metal washers at elevated temperatures and needed to minimize this issue.
While Vespel (polyimide) fasteners may have superb creep properties, it may be overkill for your application. Vespel is also very expensive. I think you may be better off considering PEEK fasteners. It is much more affordable that Vespel fasteners and could hold up to the 70C temperatures easily. It is also very well known for excellent creep resistance (as well as chemical resistance and temperature resistance). To further improve the creep resistance, glass and carbon fibers can be added. Glass fiber and carbon fiber will improve stiffness and creep resistance.