CONTAINER BREATHING

No container is airtight. If the seals are good and the vents are taped shut, air will move in and out more slowly, but any pressure differential between inside and outside will certainly be equalized in a matter of hours.

The air pressure outside a container will vary for metrological reasons over the course of a voyage. When the barometer falls, air and moisture will move out from the container, and when it rises the reverse will happen.

This effect becomes much more significant if the container is subject to repeated cycles of large temperature variations. When the container cools, the pressure inside it goes down. Air and moisture from the outside will move in until the pressure is equalized. When the container heats up, the reverse happens.

While moisture can move both in and out of the container, it is not a balanced process. Under very common circumstances, cycles of temperature variations will lead to a buildup of moisture within the container.

If the container contains absorbent packaging material, that build-up can be very
significant indeed.

Moisture Exchange of Packages within the Container

A package is like a container in miniature. Even where it completely sealed, there could still be moisture damage inside as a result of temperature changes alone. In fact, most packages exchange a lot of moisture with the air inside the container. Almost all common plastics, except alu-foil, let moisture diffuse through to a significant degree as will coated or uncoated cardboard. The least mistake in sealing a plastic package will anyway leave it subject to “breathing” processes.

For a plastic wrapped package, including a pallet liberally shrink wrapped, the most
important process of exchange is diffusion through the plastic. The diffusion rate is
proportional to the surface area of a package. Thus it is important to note that a bigger package has a smaller surface area in relation to its volume, than does a small package. When you put many boxes into a pallet, or stuff many pallets closely together, you lessen the significance of moisture diffusion.

For a wooden crate, diffusion as such may be of less importance in the timeframe of a typical voyage, but the natural breathing of the wood may be a dominant mechanism. If not, the “breathing” will be the most important aspect. The breathing is proportional to the amount of free air inside the crate and it is exponentially dependent on the temperature outside at constant relative humidity.

It is worth noting that moisture will not only move into the packages, but also out of them if the container environment is sufficiently dry. In practice it is often found that it makes a more sense to install moisture protection in the container and leave the pallets open at top and bottom to breathe, than to attempt to seal out the moisture.