A container is a closed system with it’s own ”weather” inside. It differs from the warehouse in that the variation in temperature is much greater. It is not unusual to see containers wherein temperatures range from freezing to 60-70C during the course of a single voyage.
The central fact is that warm air can hold more moisture than cold air. That means that if warm air is cooled, it becomes more humid. And if it is cooled enough, some of the moisture must rain out – condense. That is exactly the same phenomenon that causes dew in the grass or fog on a cool autumn.
In a container a fast temperature change of 5-10C is often enough to cause problems. Water will condense on the coolest available surface, which is often the container ceiling or walls. From there it may drip down onto the cargo and cause damage – “container rain”. At other times it condenses on the cargo, – “cargo sweat”-, which is usually even more damaging
Even without any condensation, elevated humidity over a period of time is sufficient to cause damage. Many metals will corrode or discolor at a rather modest level of humidity , 60-70%. At higher levels of humidity, 80%-90%, moulds will grow, labels will peel and corrugated boxes will start to soften.
The Relative Humidity (RH) is a percentage measure of how much moisture the air holds as compared to the maximum mount of moisture air at that temperature can hold. That means that completely dry air has a RH of 0%. The RH can never be more than 100%, or any excess moisture will rain out. There is little danger of damage to anything if the RH is below 50% or so.
The Humidity Changes when the Temperature does
The important thing to realize is that the humidity of the air changes only as a result of the change in temperature. When air cools it becomes more humid, – even though the moisture content in the air remains the same.
The Humidity in a container will go up and down throughout the voyage, as a result of changing temperature only. If the temperature changes rapidly enough there is sure to be moisture trouble, even if the container may be fairly dry by reasonable standards.
In a container, moisture evaporates into the air during periods when the container is warm. The warm dry air can accept a lot of moisture. Or warm moisture containing air enters from the outside through “Container Breathing”. When the container cools down, that air becomes very humid. And it is then the troubles start.
But the temperature doesn’t have to vary in time to create a difference. It is equally bad when different parts of a container are at different temperatures. When warm air moves into a colder part it becomes humid and perhaps even condenses moisture. Tons of moisture can be redistributed within a container during a voyage through such processes. Mysterious patterns of damage may arise, such as mold only in certain parts of the cargo.
Temperature changes in a container may arise because one side of the container is exposed to the elements and another is not. Or it may arise simply as a result of a great thermal inertia in the cargo as outside temperatures change. It is common that it takes weeks for the temperature to equalize through a densely stuffed cargo.
It should be noted that all the basic processes outlined above are strongly nonlinear. A small difference in conditions may cause a grate difference in outcome. That is why the pattern of damage may seem unpredictable.
Where Does the Moisture in the Container Come From?
The moisture in the container:
* Is in the air when the container doors are closed
* Is contained in the cargo and packaging and is evaporated throughout the voyage
* Enters from the outside through so called container breathing.
The amount of air contained in the air at loading depends at the temperature and the humidity at loading. If loading at cool temperatures the amount is seldom significant, at most a few hundred grams. At loading in the tropics, however, the total amount of moisture could be a Kg or more.
Most cargo and packaging materials can both absorb and evaporate moisture. What happens depends on the temperature and how humid the surrounding air is. It is common that the cargo will evaporate during one part of the voyage and absorb during a different part.
No container is airtight. Moisture can move both into and out of the container as a result of temperature variations. Unfortunately, common circumstances will lead to a gradual build up of moisture within the container.
It could very well happen that you start with a very dry cargo, but at some later time the cargo has absorbed a lot of moisture which may be released in a very destructive way. If there is a temperature difference within the cargo, very substantial amounts of moisture may be re-distributed within the cargo. The moisture will always move from the warmer to the colder part.
Any absorbers put in the container are of course expected to be part of the solution and not the problem. Alas, that is not so. Unfortunately almost all kinds of absorbers, other than Absorpole and Absorbag, will re-evaporate moisture under some circumstances, usually in connection with a period of elevated temperature some time into the voyage.
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 build-up 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.