(Editor’s Note: This is part one of a two-part column. Part one discusses the problem of moisture contamination in shipping containers. Part two, scheduled to be published in the July issue, will focus on how to prevent moisture contamination.)

            Water is abundantly present in all parts of a living tree. The weight of water in green wood is commonly close to or greater than the weight of dry wood material (per unit of volume). In other words, the moisture content of living ‘green’ wood often approaches or exceeds 100%. A typical 48x40-inch newly manufactured hardwood pallet can contain as much as three to four gallons of water.

            When a tree dies or a freshly cut log is processed into lumber or pallet parts, the wood immediately begins to dry and lose some of its moisture to the surrounding environment. Assuming liquid water is not re-introduced to the wood, by being left in the rain, for example, the wood will continue to dry until it approaches the equilibrium moisture content of the surrounding environment — typically around 10-12% in the temperate regions of the U.S.

            The rate of drying in wood is relatively slow under natural ambient conditions and depends on factors such as wood species (density), wood volume and dimensions, and environmental conditions (temperature, humidity, wind, etc). It can take as long as six months or more for the thicker components in a green wood pallet to approach the equilibrium moisture content of the natural environment.

            To accelerate the drying rate in wood, an external energy source is required that can apply heat and-or air flow, as with a dry kiln or fan shed. Furthermore, wood pallets should be maintained under cover in dry, well ventilated conditions. Accelerated drying methods can reduce drying times from six months or more down to one month or less depending on energy input.

            Water is considered a major contaminant for hardwood pallets because it adversely affects the material properties and performance of the wood. Furthermore, when moisture from wood pallets is introduced into the distribution environment, it can also contaminate the products (and packaging materials) that are carried on the pallet. This is particularly the case when green wood pallets are loaded into sealed trailers and cargo containers. Inside these containers, a microclimate exists that allows water vapor, released from the pallets, to condense on the walls and ceilings of the container. This condensation then drips (rains) down onto the cargo and puddles on the floor.

            Water originating from the wood pallets can significantly damage or destroy the contents of a shipping container and impose huge costs in lost property, liability and insurance claims. Yet, in the U.S. hardwood pallet industry, it is estimated that 90% of the roughly 400 million new wood pallets manufactured per year are manufactured and delivered in ‘green’ condition. The primary reason hardwood pallets are not introduced to some form of drying during manufacturing is because of the additional energy and inventory costs that are required. Furthermore, with the recent implementation of heat-treatment requirements for hardwood pallets used in international trade (ISPM-15), there is a misconception that heat-treating dries the pallet. That fact is that heat-treating alone (56 degrees-C temperature for 30 minutes) does little to dry hardwood pallets.

            While the relationships and problems caused by water in wood are well understood in the academic community, they are often underappreciated or altogether ignored in pallet manufacturing and product distribution environments. Removing water from hardwood pallets greatly improves the material properties and performance of the pallet. The following relationships illustrate this point:

            1. Removing water from pallets reduces pallet weight.

            2. Water in wood pallets reduces pallet strength and stiffness.

            3. Water in pallets can corrode or degrade products in direct or indirect contact with the pallets.

            4. Wood shrinks as it dries and swells when it gets wet.

            5. Water in wood pallets supports the growth of mold and certain insects.

            6. Wet wood pallets may stain products being shipped on them.

            7. Wet wood in pallets may smell and contaminate odor-sensitive products.

            8. The surfaces of green wood pallets are slippery.

            9. Pallet parts in pallets that are not properly dried will split and warp excessively.

            Moisture in hardwood pallets hinders the competitiveness of the U.S. hardwood pallet industry, and manufacturers struggle when dealing with this issue. Moisture in hardwood pallets can contaminate consumer goods and can force customers to switch to pallets made from alternative materials. There is direct evidence that the reluctance of hardwood pallet manufacturers to dry pallets is affecting hardwood markets due to kiln dried pine, wood composite (plywood) pallets, and plastic pallets being used as substitutes.

Shipping Containers

            Sea containers provide a practical means for shipping almost any kind of cargo. But putting cargo into a closed strong box also entails a constant risk of moisture damage for every kind of cargo on every voyage. The following are examples of problems that can arise as a result of moisture contamination inside a shipping container:

            • Metals corrode, discolor and lose their shine.

            • Cargo and packaging get moldy, soft, crumbled and discolored.

            • Bad smell.

            • Physical damage from water, ice, things gluing together, caking etc.

            Losses and costs that occur due to moisture damage inside shipping containers can be significant. Of course, not every shipment suffers moisture damage, and most of those that do suffer only limited damage. In fact, a lot of moisture damage remains unrecognized because it is considered ‘normal.’ Very few shippers have a good system of feedback from the receivers of their goods. It is important for both shippers and receivers to become educated on moisture related issues that affect the quality of the cargo during shipment.

            The pattern of moisture damage may seem random because small differences in the cargo and voyage conditions can have a huge effect on moisture conditions inside a container. A shipper who typically doesn’t seem to have any problems may suddenly experience a disaster. In other words, there is always the risk of moisture damage in the next shipment even if no problems occurred previously.

            When deciding whether a particular container and its contents are suitable for shipping —sometimes great distances around the world — those involved should consider the anticipated climatic conditions that will prevail inside the container during transit. Because the contents of a shipping container can be sealed inside for long durations — from several days to several months — the conditions inside the container must be controlled to prevent surfaces from wetting.

            The three major factors that impact the microclimate environment inside a closed shipping container are: 1) external climatic conditions, 2) the cargo, and 3) the type and condition of the container. The following discussion applies most specifically to closed (sealed) containers.

External Conditions

            The external climatic conditions outside and around a shipping container will have a significant impact on the microclimatic conditions inside the container. Factors such as the transportation route, season of year, time of day, and weather (rain, sun, etc.) all influence conditions inside the container. The microclimate inside a shipping container is characterized by temperature, humidity, the dew point, and the temperature-dew point difference.

The Cargo

            The microclimatic conditions inside a container can be strongly influenced by the cargo in terms of both the mass of the cargo and the surface area of the cargo that is readily accessible to the air inside the container. Hygroscopic cargos, such as those containing wood pallets and corrugated packaging materials, largely determine the water vapor balance (relative humidity) inside a container due to their sorption behavior (hygroscopicity). Hygroscopic materials inside the container will adjust the relative humidity of the residual air inside the container in an effort to achieve equilibrium. In other words, a decrease in relative humidity inside the container — due to high daytime temperatures — will cause moisture contained in wood pallets to be released into the air.

            Any additional liquid water that has entered a container through leaks or because the container was packed during wet weather will exacerbate moisture and potential condensation conditions.

            If external water or water vapor is not present in a container, the largest sources of moisture and resulting condensation almost always come from the cargo. In particular, green hardwood pallets are often the largest source of this moisture, but wood fiber based packaging, wood blocking and dunnage, wooden flooring, and any other hygroscopic materials can contain various levels of moisture as well.

            Depending on the temperature and humidity conditions inside a container, hygroscopic materials (pallets) will either release or absorb moisture. Assuming no external moisture has entered or leaked into a container, rising air temperatures in the container will evaporate water from the hygroscopic cargo. At constant absolute humidity, a rise in air temperature in the container expands the capacity of the air to hold moisture and results in a drop in relative humidity. At lower humidity, moisture contained in hygroscopic materials gets released into the air (evaporated) as the microclimate inside the container strives for equilibrium. The relative humidity inside the container increases such that the dew point is easily achieved with cooling temperatures (that typically occur at night). At dew point, the water vapor inside the container will condense on the cold container walls and ceilings or any other materials in the cargo that have cooled.

            Day and night temperature changes inside a shipping container can be significant, but even small changes can be enough to cause condensation. Condensation is most severe on the container ceilings and causes what is known as ‘container rain.’ Cargo often is loaded and sealed into what initially appeared to be a dry container and is opened weeks later at its destination and found to be drenched with water, covered with mold, and having damaged products and packaging.

            (For more information, please contact Peter Hamner at the Virginia Tech – Center for Unit Load Design at phamner@vt.edu.)