The issue of mold on pallets continues to be a major concern for both pallet manufactures and users. Much information has been presented and published about mold prevention strategies for domestic pallets, the most common being the use of chemical preventatives, drying the pallets to a moisture content below 25% or a combination of both. As part of a larger project, researchers at Virginia Tech completed a pilot study to determine if a heat treatment/drying schedule could be developed that would dry pallets to 25% rapidly, using standard kiln temperatures, yet not lead to significant pallet degrade.
The study was conducted on oak and yellow poplar, to represent common materials used in pallet manufacture and to represent both an easy and difficult to dry species. The pilot study consisted of two heat treating/drying tests per species. A small test kiln that held 20 pallets was used for the study. Thermocouples were inserted into the center of several stringers to accurately measure the core temperature. While the results will be discussed for a pallet all moisture content and drying times are based on the pallet stringer, since being larger in size, it would be the limiting material for reaching a 25% moisture content.
The pallets were first heated until the core temperature of the pallet stringer reached 133F for a minimum of 30 minutes. After the heat treating time period had been accomplished, the kiln set points were adjusted to dry the pallets. Pallets were measured for drying degrade (checks and splits) prior to and after testing. Pallet degrade was determined using the guidelines set forth by the National Wood and Pallet Container Association (NWPCA) component grades lumber characteristic restrictions.
All the pallets were manufactured from pallet cants and exhibited a lower moisture content than would be expected from freshly harvested logs, average moisture contents were 42 to 49% for yellow poplar and 49 – 52% for oak. The schedules were controlled based on the time required to heat treat the pallets (measured with a thermocouple) and moisture content of the pallet stringer. Future tests will be conducted to develop a time based schedule for the entire process.
Poplar Pallets
The first test was developed by modifying a standard kiln schedule for lumber. The conditions are reported in Table 1. The pallets in this test had an average moisture content of 43.8%. After testing, the pallets showed no signs of splits; therefore no drying or pallet degrade occurred. The pallets were heat treated and dried from 43.8% moisture content to 23.5% moisture content in 17 hours.
Since the first test resulted in no drying or pallet degrade, a second test was conducted at a higher temperature to accelerate the process. The pallets for this test had an average moisture content of 42.1%. Four minor splits occurred on the 20 pallets tested. None of these splits were severe enough to degrade the pallet. The pallets were heat treated and dried from 42.1% moisture content to 23.5% moisture content in 16 hours.
Oak Pallets
Again test schedules were developed by modifying a standard kiln schedule for lumber. The conditions are reported in Table 3. The pallets had an average moisture content of 49.2%. The pallets showed no signs of splits caused by the heat treatment and following drying. This test resulted in heat treating and drying oak pallets from 49.2% moisture content to 23.4% moisture content in 60 hours.
A second test was conducted at a slightly higher temperature to accelerate the process. The average moisture content of these pallets was slightly higher, 55.6%. The pallet stringers had an average initial moisture content of 55.6% that was measured using the oven dry method. This test did result in some minor splitting of the boards, which are presented in Table 5. However, none of the splits were severe enough to degrade the pallet. The pallets were heat treated and dried from 55.6% moisture content to 24.9% moisture content in 41 hours.
In both of the oak tests, there was some slight warp that occurred on the top two pallets in the kiln, but the NWPCA pallet component grades do not limit warp. Once this pallet was inspected, and other pallets were placed on top of the warped pallet, the warp eliminated.
The results of this small scale pilot test indicate that a heat treatment and drying schedule to reduce the moisture in pallets to 25% moisture content to assist in the prevention of mold is possible with standard kiln temperatures in a reasonable amount of time. Yellow poplar can be heat treated and dried from 42.1% moisture content to 23.5% moisture content in 16 hours and oak pallets from 55.6% moisture content to 24.9% moisture content in 41 hours with minor warp and splits, none significant enough to lead to a pallet degrade.
When interpreting these results it must be considered that the pilot study used a limited number of pallets, the initial pallet moisture contents ranged between 40 and 50% and that the moisture contents reported are for the stringer of the pallet. We hope to conduct work in the future to demonstrate the effectiveness of these schedules on a larger sample size and to develop time based drying schedules for each species. Funds from the Wood Education and Resource Center were used to support this project.
Dr. Brian Bond is an associate professor, Department of Wood Science and Forest Products at Virginia Tech and Thomas Blount is a graduate research assistant in the Department of Wood Science and Forest Products at Virginia Tech.
