Why Do Stainless Steel Thermos Flasks Last Longer Than Plastic Ones?

The metal retains heat better. If the container is at the same level of heat the contents are it takes that much longer for the cold air outside to defuse into the cup. It also slows the diffusion rate inside of the cup due to metal being more dense.

Because of expansion and contraction, plastic material get weird out sooner than metals and this is why steel thermos flasks last longer than plastic ones.

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