Innertube Porosity?

Steve Sr. wrote:
I was wondering if anyone has any information on the porosity of
various kinds / brands of innertubes. By porosity I mean the very slow
leakdown rate that occurs with sound tubes over days and weeks. You
know... The reason you are always topping up your tires to prevent
pinch flats.

My current interest is any differences between the regular and
"lightweight" road tubes of the same size. I am also wondering if
"brand" makes any difference as well.

Thanks,

Steve

there's two factors: one is actual porosity, the other is diffusion.
rubbers are subject to both, with the former being a factor of quality,
the latter is natural and can't be stopped. [condom production is a
fine example of porosity control.] for quality, stick to name brands
and look for country of manufacture.

for air, butyl rubber offers the best air diffusion/mechanical
properties ratio, hence it's common use.