Silicones exhibit many useful characteristics, including:[1]
>Low thermal conductivity
>Low chemical reactivity
>Low toxicity
>Thermal stability (constancy of properties over a wide temperature range of −100 to 250 °C).
>The ability to repel water and form watertight seals.
Does not stick to many substrates, but adheres very well to others, e.g. glass.
>Does not support microbiological growth.
>Resistance to oxygen, ozone, and ultraviolet (UV) light. This property has led to widespread use of silicones in the construction industry (e.g. coatings, fire protection, glazing seals) and the automotive industry (external gaskets, external trim).
Is also be concerned about residual chemicals from the manufacturing process, and this goes for metal too. For example I'm sure the molds have release spray that can get passed into the bottle, same with stainless steel machine spray, oils, and greases. And the thing with metal containers is that it's slightly porous and the chemicals can permeate a bit which would then leach back into the water. Another concern is low grade recycled metals with previous chemical contact used to manufacture the bottles of produced in places like China.
silicone doesn't require mold release on a nicely machined mold. If it IS required, such as with complex shapes, food-grade oils are used.
I suppose you could get a newly machined mold that may have cutting oil or something impregnated in the metal, but i'm not sure how you'd avoid that anywhere ,really.
(Just wash everything thoroughly when you receive it and buy from reputable dealers, that seems about as good as you can get.)
And yes, I agree it's something that's a concern with everything and I hate that I can't ignore it and it's always at the back of my mind. I used to work in FDA related manufacturing and I know how things operate.
https://www.amazon.ca/Collapsible-Silicone-Water-Bottles-Cam...