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DETAILED DESCRIPTION OF THE INVENTION Before we proceed to describe the present invention in more detail (including by means of Examples and an Experiment) it is appropriate to deal with the question of how one may determine permeance or permeability that a wall or composition would have in the absence of scavenging (this permeance or permeability being referred to several times above).
The ratio of permeances or permeabilities in the presence and absence of scavenging are one (reciprocal) measure of the size of the scavenging effect, and it is for this reason that various upper limits on this ratio are suggested above.
(Another measure might be the ratio of the quantities of oxygen emerging and entering the wall under test, but this is less practically convenient.
) Four methods of determining the permeances or permeabilities in question will now be described with particular reference to determining whether a particular preferred ratio (3/4, 1/2, 1/10 etc.
as described above) is exceeded: (1) The wall under test is exposed to oxygen for a time sufficiently long that the oxygen permeance or permeability begins to rise as the oxidisable organic component is consumed.
It is of course not necessary to continue the exposure until no further rise occurs (i.
e.
until the scavenging is totally absent).
Whenever the exposure is terminated for a particular sample one can confidently set a lower limit on permeance or permeability in the absence of scavenging, and therefore an upper limit on the ratio in question.
(2) A wall is prepared for comparison free of catalyst, and the effect of the catalyst on pure permeation is estimated or (more likely) reasonably ignored.
Some scavenging activity in the absence of catalyst will not preclude the establishment of the lower and upper limits referred to in (1).
(3) In some cases, as will be discussed in more detail later, the oxygen-scavenging property is still undeveloped until some time after the forming of a wall, in which case one may take the largest P