(Editor’s Note: This is the second column in a two-part series dealing with preventing moisture contamination in sea containers.)
As previously discussed, the water that is naturally contained in wood — from when it was part of a living tree — is considered a major moisture contaminant for wood pallets and the products and packaging materials that are stored on them.
The weight (volume) of water in a freshly cut piece of wood can approach or even exceed the weight of the wood material itself. In fact, a single 48×40 ‘green’ hardwood pallet can contain as much as three to four gallons of water.
When moisture from wood pallets is introduced into the distribution environment, it has the potential to significantly contaminate the products and packaging materials that are carried on the pallets.
The cavity inside a sealed shipping container or trailer provides perhaps the most moisture sensitive distribution environment where the use of ‘green’ wooden pallets is concerned. Inside these containers, a microclimate exists that allows water vapor, released from the pallets, to condense on the walls and ceilings of the container and drip (rain) down onto the cargo. 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.
Unfortunately, many pallet manufacturers and their customers either do not appreciate the relationships and problems caused by water in wood pallets or they ignore them altogether. Moisture contamination caused by the use of ‘green’ wooden pallets in shipping containers is preventable.
Wood Pallets Must Be Dry
When utilizing wood pallets to load cargo inside shipping containers, the key to preventing potential moisture contamination is to ensure that all the contents of the container are adequately dry. This includes the wood pallets! Wood pallets need to be, at a minimum, less than 20% moisture content (MC). Even at 20% MC there is still enough moisture in the wood to potentially contaminate the contents of the container — depending on the duration of travel, the number of pallets used, and the temperature and humidity conditions experienced inside the container. Using wood pallets with moisture contents closer to 15% should eliminate most moisture problems that could be attributed to the pallets.
Because the cargo inside a container and, more specifically, any newly manufactured ‘green’ hardwood pallets that are included are so frequently the cause of moisture contamination, it is imperative that the pallet supplier, the shipper, and all parties involved understand and appreciate the problems that can be caused by putting green pallets into a sealed container.
To simply specify that wood pallets must be ‘dry’ is too vague and not adequate. A wood pallet can look and feel dry and still contain copious amounts (gallons) of moisture. Therefore, when procuring wood pallets for use inside shipping containers, the specification should include explicitly that wood must be dried to a maximum of 15% MC.
Without some form of accelerated kiln or fan shed drying process, hardwood pallets take anywhere from one to three months when properly air dried to achieve a moisture content less than 20%.
It is important to note that repaired and recycled pallets can be good substitutes for newly manufactured pallets since they usually have had time to dry naturally and adjust closer to the equilibrium moisture content in the environment (around 12% MC) — provided they have been stored and maintained in dry conditions.
Desiccants
Container desiccants have long been instrumental to the global transport industry and have prevented countless insurance claims by protecting shipments from condensation damage. The moisture that desiccants can capture can amount to many gallons when relative humidity is high. It is by preventing the release of this moisture as liquid water condensation — or in the worse instance as ‘container rain’— that desiccants have proven most effective and gained increased acceptance with the majority of the world’s container traders. The argument favoring the use of container desiccants has become even more compelling because of their affordability and the development of improved formulations.
Anhydrous calcium chloride is perhaps the most popular desiccant available today since it is efficient in mopping up free moisture released within a container load as it experiences changes in temperature, and it can remain active for many weeks at a time. This particular desiccant is also effective at preventing leakage and re-releasing moisture back into the environment.
By contrast, silica gel is generally less effective in treating a whole container load and remains far more suited for smaller scale applications, such as providing more localized protection inside cartons or other packaging material.
Vapor Barriers
Since their introduction some years ago, various forms of vapor barriers have revolutionized the packaging of materials as diverse as moisture sensitive military equipment and perishable foodstuffs. Vapor barriers are typically available in some form of film, foil, paper, or spray-on material that are applied directly to a product or used within the packaging materials to prevent or retard moisture from contaminating a product directly. They are useful when transporting or storing moisture sensitive products when shipping over great distances and for long durations of time — sometimes as long as two years. While vapor barriers are effective at preventing damage due to moisture occurring on the product directly, they do nothing to limit the volume of moisture present inside a shipping container.
Alternative Materials for Shipping Pallets
Wood pallets dominate the U.S. pallet market simply because they offer the best bang for the buck. Wood pallets are relatively inexpensive, allow for versatility in design, and have good strength, stiffness, and durability. However, they can also contain significant amounts of water.
When potential moisture contamination is a concern in a particular distribution environment, such as inside a shipping container, the use of wood pallets in that environment poses a realistic threat — unless they are sufficiently dried to less than 20% MC.
If obtaining dry solid wood pallets is not an option, pallets made from other ‘moisture free’ materials should be used. Materials used to make pallets that will not contribute to moisture related problems include wood composites (plywood, OSB, etc.), plastic and metal.
Floor Loaded Cargo Containers
At present, researchers at Virginia Tech’s Center for Unit Load Design are attempting to compare the costs and benefits associated with the two primary methods of loading shipping containers:
• Unitized loading – products loaded into containers on pallets
• Floor Loading – products loaded into containers without using pallets
A recent examination of U.S. ports and distribution centers along the Eastern Seaboard revealed that most cargo arriving in shipping containers from Europe is not palletized but floor loaded instead. Floor loading a shipping container provides more room inside the container for product storage due to the absence of pallets. Also, by not using pallets, the potential for moisture contamination that is often caused by wood pallets is eliminated. It is important to note, however, that floor loading a container requires more time and labor.
(For more information, please contact Peter Hamner at the Virginia Tech – Center for Unit Load Design at phamner@vt.edu).
