In reply to Jon:
> Unfortunately I am looking for slightly more technical information than "under-cooked"! Hopefully there is someone out there who works in the rubber industry!?
My guess - although it's been ages since I did any polymer chemistry - is that by "under-cooked" they mean "not highly cross-linked".
(Apologies if you already know all of this).
A rubber gets its properties from is structure - a 3D tangle of long-chain polymers. In synthetic rubbers, these chains are joined together at random points along their length by "cross-linking" - molecular bonds formed between chains. The commonest process for doing this is called Vulcanization, and uses sulphur to form these links. Natural rubbers are generally not crosslinked, but the molecules are so tangled that the effect is the same.
The elastic properties arise from the ability of the tangled molecules to straighten and slide past each other under stress, and re-crinkle (to a higher-entropy form) on relaxation. Obviously this is limited by the amount of cross-linking, and if a rubber is very highly cross-linked it is hard and will crack easily (tyre rubber, for example, will increasingly (although slowly) cross-link on exposure to UV, which is why tyres go crumbly eventually). It is also affected by the amount of "free space" inside the rubber - how tightly packed the molecules are together. If they're very tightly packed, they cannot move easily and untwist. To mould rubber, then, it is heated (which gives the molecules more energy to move) and often a plasticiser is added. This is a chemical which sits around the molecules and effectively provides lubrication. The plasticiser should evaporate out at some point in the process.
So my guess is that these rubbers have been "under-cooked" during the moulding process, resulting in incomplete cross-linking and residual plasticiser. This would leave the rubber a little "tarry" to the touch.
Hopefully that helps (and is vaguely right).
