oxygen transmission rate (OTR) is a key property of wine bottle closures because the interaction between wine and oxygen is critical to the way a wine changes post bottling (see bottle ageing). It used to be widely thought that corks and alternatives such as screwcaps and synthetic closures provided an airtight seal that allowed no oxygen into the bottle, except when the closure had failed. But with the advent of the first generation of synthetic corks, it quickly became clear that even though these were sealing bottles tightly so that they did not leak, they were allowing oxygen into the bottle, resulting in wines that showed signs of oxidation after a fairly short period of time. This is because the structure of plastic allows oxygen to diffuse through it. It also became clear that the screwcaps with a tin layer in the liner (almost all those used in Australia and New Zealand), were allowing virtually no oxygen transmission at all, and that natural corks were variable in their OTR but were mostly somewhere in between the OTR of synthetic closures and that of tin-lined screwcaps.
