We use a tested #5 food-grade polypropylene (PP) because of its durability and non-leaching characteristics.
Some bottles are made of Polycarbonate which is one of the highest quality polymer which is also food-safe.
Our food containers are made of either PP or Styrene Acrylonitrile (SaN) as they are BPA free and food-safe.
Some flask models also use caps made of Eastman Tritan whose safety reports can be freely read online.