Best Practices for Kiln Drying Wooden Pallets
01 Aug 2014
Any business that deals with moisture-sensitive goods, and food products for export, is obliged to follow specialised packaging standards. The most economic method of complying with this standard is to use kiln-dried wooden pallets to ship the goods.
All wooden packaging materials used to export goods must comply with International Standards of Phytosanitary Measure 15 (ISPM 15); a standard developed under the aegis of the International Plant Protection Convention (IPPC). This standard stipulates the need to treat all wooden materials of thicknesses greater than 6 mm in order to prevent the spread of diseases and pests carried in the wood.
This means that all wooden pallets must be either heat-treated or fumigated to kill or halt the development of mould and fungi. Heat treatment is a complete sterilisation of the wood. The material is heated to a temperature of 56 degrees C for 30 minutes. The timber emerges from the treatment lighter, stronger and stained due to the heating of the wood’s sap.
Kiln-drying reduces the moisture content of the wood. The pallets are stacked inside large kilns and heated until the excess moisture evaporates. It is more analogous to a curing process than the complete sterilisation. The timber does not emerge from the process fully sterilised. In addition, the moisture content in separate pieces of wood of the same species emerging from one drying session can vary significantly
Concern about the varying moisture content in wooden pallets, as well as about the fact that mould will still survive in kiln-dried wood packaging materials, led the UK’s pallet association, the Timber Packaging and Pallet Confederation (TPPC), to commission an in-depth kiln-drying project to determine the best practice for this procedure.
The study produced recommendations for best practices in kiln-drying procedures that will enable pallet manufacturers to reduce the variability of moisture content in the dried wood.
The first step is to allow the raw timber to dry for 14 days in a well-ventilated covered storage area before being placed in the kiln. Pre-drying cuts down on the time needed in the kiln. Once inside the kiln, the pallets can be dried for between 19 and 25 hours to achieve a moisture content of between 15 and 22 per cent. The standard practice by most manufacturers has been to dry untreated and not pre-dried pallets in the kiln for between 12 and 36 hours at a temperature of 94 degrees Celsius, depending on the moisture content required. The aim is to get the moisture content low enough to prevent mould growth — ideally about 15 per cent.
Once removed from the kiln, the wood must be allowed to air-dry. It is not enough to store it in a warehouse away from contaminants such as mud and bird droppings. The wood is best kept indoors away from any weather conditions. It has to be appropriately spaced and ventilated with large fans to maintain an air flow as it dries out for about two weeks.
The wood can later be transported to its final users on specially fitted covered trailers that allow air to flow through the load and keep it dry.