These "leak resistant" type bottles are very nice because the design of the closure on the top of the bottle as well as the design of the lid itself creates an excellent seal without the need for overtightening them or the need for a separate seal piece in the lid (which might fall out and get lost). Effectively, a conical seal integral to the lid (like part of a stopper) is driven downward into a mating tapered area in the top of the bottle as you screw the cap on. As long as the two mating surfaces are free of grit, they form a perfect seal as they mate.If you keep the mating areas free of grit, they can be used over and over with confidence, and without the need for cranking the lids down hard.These are HDPE. Check chemical compatibility charts if you have any questions about its suitability for your intended use.