Water Transport Properties of Radiation-cured Coatings and their Effect on Adhesion

Organic polymers are, unlike ceramics or metals, permeable to a greater or
lesser extent to water vapour. Thus, it is important to bear in mind how the water
transport properties will affect their performance in the environment in which
they will operate.

Use of organic polymers to protect products from degradation is widespread in
industry. In the food packaging industry, control of moisture, oxygen, light and
odours are important in order to prolong the shelf-life of the . To prevent
or inhibit mould growth, products can be subjected to a variety of preservation
processes such as storage at below ambient temperatures, addition of suitable
chemicals, heat treatments such as pasteurisation or sterilisation and moisture
removal. The latter technique is aimed at reducing the moisture content of the
product to about 30-40% relative humidity, and thus well below the lower limit for
mould growth of 60% relative humidity; Bacterias and yeasts generally require
relative humidities in excess of 80% in order for significant growth to occur.
Packaging for dry foods must therefore meet the need, among many others, of
reducing the ingress of moisture and maintaining the relative humidity above the
product at below the minimum levels required for sustaining growth of moulds,
bacterias or yeasts. Storage of fruits also requires packaging with low
permeabilities to water vapour in order to maintain the high moisture levels in the
products to prevent wilting.

In the electrical and electronic industries, organic polymers provide electrical
insulation and dielectric isolation as well as protection against moisture, heat,
salt spray, oxygen and . However as opposed to the hermetically
sealed ceramic packages previously used, moisture will eventually permeate the
polymer coating. In combination with the salt impurities remaining in the polymer
from the manufacturing process and the high potential gradients typically found
in modern electrical devices, corrosion of the aluminium tracks will take place
eventually leading to device failure. The polycrystalline tellurium films that have
been investigated as a record and playback optical recording medium, though
exhibiting many excellent properties, have the unfortunate drawback of
degrading rapidly under conditions of high humidity in the presence of oxygen.
On exposure to air, the surface of the film is oxidised forming a passive layer
impervious to any further degradation. However in the presence of moisture, this
passivating layer is destabilised allowing the remaining material to be oxidised. It
has been proposed that the films could be protected from oxygen and moisture
by either direct overcoating with a polymer or by using the polymer as a sort of
window separated from the optically sensitive films by annular spacers.