Pagina:PPKS 0006 Mededeelingen van het Algemeen Proefstation der AVROS - Rubber Serie No. 31.pdf/12

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 It is apparent that, while the outer layer showed a constant moisture content in one day, the inner rubber continued for several days longer to absorb moisture from the atmosphere. This is due to the presence of hygroscopic serumsubstances. On the sixth day both moisture contents are slightly decreased. Since the weather had been very dry on that and the preceding day, this may show how close a relationship exists between the moisture content of rubber and the state of weather. Though in neither of the samples the original moisture content was reached again, that of the inner rubber remained markedly higher than that of the outer rubber.
 It is evident from these tests that there is a recovery of the moisture content in rubber after drying and that is especially true, and in a higher degree, for the rubber from the inner parts of a sheet. Hence in the inner layers the moisture content can rise so high, that the rubber becomes opaque while the outer layers remain transparent, and thus we have the origin of the stripe.
 That the disappearance of the stripe has not been caused by the heat in the stove, but by the drying process can be proved by drying the rubber over lime in a vacuum desiccator. The stripe disappears in the same way, as can be seen from the behavior of four examples tested. The results of these tests are to be found in Table II.

Table II

Disappearance of the phenomenon of the white stripe in sheet

rubber, when dried under different circumstances.

Sample I. Sample II. Sample III. Sample IV.
Dried in heated stove: Stripe disappears. Dried in heated stove: Stripe disappears Dried in desiccator at a low temperature: Stripe disappears Dried in desiccator at a low temperature: Stripe disappears
Cooled and left in desiccator for some days: No stripe Cooled and left in desiccator for several weeks: No stripe Kept in the open air: The stripe comes back after several days Kept in desiccator for several weeks: No stripe
Kept in the open air: The stripe comes back in a few days Kept in the open air: The stripe comes back in a few days Kept in the open air: The stripe comes back in a few days