The root cause of moisture damage in container transport is the simple fact that warm air can hold more moisture than cold air. Take the dewy grass in the morning after a cool summer night as an example. Moisture gets into the air in the container from the outside or by evaporation from the cargo. When the temperature in the container changes or there is a difference in temperature between different parts of the cargo, damaging moisture conditions arise
Moisture is great for chocolate cake, but not for Sea Containers!
Moisture damage happens even where there is no condensation. Many grades of steel will start to corrode at a relative humidity of about 70%. Mould growth could begin after even a short period over 80%.
The only remedy is to keep the air inside the container dry. The first thing to do is to ensure that the cargo and all the packaging are as dry as possible. A wet container floor or some pallets stored in the rain may be enough to ruin a cargo.
No container is airtight whatever you do; – it will “breathe” as a result of temperature cycles. When the air inside the container cools, the pressure drops. Air – and moisture – moves in from the outside to equalize the pressure. The opposite happen when the air inside the container heats up, but it is easy to show how a repeating cycle of breathing can cause a buildup of moisture inside he container, especially if there is absorbing packing materials. Using a container with good seals and vents taped shut will slow down, but not stop- the “container breathing”.
Substances that remove moisture from the air are called “Desiccants”. The most widely used desiccants are probably “Silica Gel”, a kind of porous glassy substance that adsorbs moisture well under the right conditions. When used in containers they are fatally flawed in that they work best at room temperature, and not at all at the much higher temperatures often found in containers. Other widely used desiccants based on Clay work to a little higher temperature, but then similarly fail in an even more dramatic way.
The worst case is when the desiccant is already fully charged and then meet high temperature, followed by low temperature, e.g. as a result of a day and night cycle when the container is on the quayside. Much of the moisture absorbed is then first re-evaporated and then rained out. Sometimes the container will look as someone threw a bucket of water inside the container, and wet moldy desiccant bags are a common sight.
Desiccants based on Calcium Chloride, have a vigorous absorption over a large temperature range. Desiccants based on a mixture of clay and calcium chloride or Tyvec pouches with calcium chloride are very good absorbers, but easily “over saturate”. If an “over-saturated” absorbent meet dry conditions, e.g. as a result of a sudden increase in temperature, it will re-evaporate the moisture already absorbed in a very destructive way. Only calcium chloride absorbers that sequester the absorbed moisture to keep it from contact with the air are free of this problem.
The important thing to remember is that there is always a risk of moisture damage in the next shipment, and one needs to implement a moisture protection program that will prevent the build up of moisture in the air to levels where it may cause damage.To design an efficient moisture protection requires finding the most economic balance between packaging, container desiccants and in-packaging desiccants, taking into account not only the individual package, but how it is stuffed and combined throughout the logistic chain.