Lots of hardware stores sell PTFE tubing, and essentially it
is Teflon, not just coated on the inside, but
all the way through. There's a few different sub-types but they are overall very similar. The type that I mostly see in hardware stores is milky-white, where bike shops seem to use the black colored stuff. The two kinds feel identical, held in your hands. And the hardware store white stuff is even often in the same diameter/thickness that is used for chain tubes.
Below from
http://www.zeusinc.com/teflon_tubing.asp
....PTFE tubing is commonly referred to as "TeflonŽ tubing". TeflonŽ is a registered trademark of DuPont and refers to three fluoropolymer plastic resins: PTFE, FEP, and PFA. These three plastics share common properties such as lubricity, chemical resistance, high temperature resistance and more. There are slight mechanical differences that make each tube uniquely suited for different applications. ...
...the main problems I have seen with chain tubes is where the chain enters and exits the tubing ends. Often the chainline does not line up with the tube perfectly and so the chain is rubbing on the end-edge of the tube. This causes a significant amount of drag especially where the chain is entering the tube.
I can see where one might want a bit of tubing on a commuter/general short-run utility bicycle, but for a recreational machine I prefer to do without if at all possible. They don't keep the chain from getting dirty, and don't make the whole thing any easier to clean either.
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I'd guess the tubing is pretty close to frictionless when it's brand-new and clean, but the problem is that it doesn't stay clean for long. And it's probably acceptable on the lower-run of chain that is not under much tension and the tube ends are lined up well with the chain. It's when it's used on the upper-run that it eats into the efficiency.
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