What are LEGO rubber bands made of?

Question

I have noticed that LEGO rubber bands (along with just about everything else LEGO) are fantastically durable. They last ages without getting crusty and dying like most commonly available rubber bands. They can even be held in stretched positions for extended lengths of time without showing any signs of wear, or even any noticeable hysteresis.

So anyhow, what are they made of to make them so superior to common rubber bands?

Clarifications:

• By "standard"/"common" rubber bands I mean those which are commonly available to purchase at the store, like these.
• I am talking about the newer "round cross section" type, not the old "square cross section" type which are more similar to standard rubber bands. Part #'s x1106, x90, x89, x37, and x71.

Answer 1

They are made of SBS rubber.

According to user "B. Lister" on eurobricks.com, these bands are silicon rubber, used in medical and dental work for long lasting connections and tension.

Source

However, a LEGO representative at a toy safety site indicated otherwise:

The colors and materials in ABS plastic do not rub off or leach and, in fact, meet the same material standards for safety that are required of plastic eating utensils. Tires and elastic materials found in some LEGO sets are manufactured from Styrene-Butadiene Styrene (SBS), a material commonly found in vehicle mud guards and shoe soles.

Source

SBS has some very similar traits to silicon rubber, including sterilization and long lasting properties. I noticed in the other answer that bike tire cuttings worked almost identically. They should! Those inner tubes also consist of a Butyl base.

Answer 2

I cannot be certain about this, but I think it could be one of these two material options, (1) Urethane: i.e. it's not rubber at all, and it is moulded, and not extruded. That's probably also why they have colour options. (2) Butyl rubber: this is the rubber used in tyre tubes. I've used cut sections from inner bicycle tubes, and they stretch and seem to last forever. I'd be mildly surprised if I saw them in round section, or in colour, though.