Water In Wood Pallets: The Facts Behind the Opportunity Available In Dry Pallets

by Marshall S. White, Director, Sardo Pallet & Container Research Laboratory, Virginia Polytechnic Institute and State University

Publisher's Introduction:

More details on analyzing moisture levels are available from the Pallet Lab at 540/231-7134.
There is very little that is good about water in wood pallets. Water in wood pallets presents the following list of undesirable characteristics:

1) reduces pallet strength and stiffness;

2) adds useless weight;

3) corrodes metal products;

4) causes odor, which contaminates products;

5) causes staining of products;

6) leads to wood shrinkage;

7) causes splitting and warping of pallet parts;

8) loosens joints; and

9) causes mold and mildew growth

In a competitive pallet market, manufacturing new or used dry pallets can be an excellent way to differentiate your product. A dry wood pallet will compete better against dry plastic or corrugated paperboard pallets which advertise this advantage.

Important Moisture Levels of Wood Pallet Parts
1) Fiber saturation point (FSP). Water in wood is contained within the cell wall and within the cell opening called a (Figure 1) lumen. When wood dries, most of the water in the opening is removed before the water trapped in the cell wall. This is because it takes less energy to remove the loosely held water in the openings of the cells. The level of moisture in wood at which all lumens are void of water, and the cell wall remains saturated, is called the "fiber saturation point" (FSP). This is an important moisture level that varies between 25 to 30 percent moisture depending on the wood species.
Properties of wood containing levels of moisture greater than FSP are unchanged by changes in MC. The physical and mechanical properties of wood change only when wood dries below FSP. Shrinkage only occurs below FSP. Pallets made of wood with maximum MC less than FSP will not contaminate most moisture sensitive products. When corrosion sensitive metal is in contact with wood, the maximum safe moisture content is 20 percent.
2) Green. Wood is typically called green when the MC level is equal to or greater than the FSP. The MC of wood in a tree varies somewhat throughout the stem, but typically averages 50 to 100 percent depending on the wood species and season of the year.
3) Equilibrium Moisture Content (EMC). This is the final moisture level that wood will reach in any environment. This can vary from eight to 10 percent in very dry, arid environments or in heated homes in winter. It can be 16 to 18 percent in humid, tropical conditions. However, the EMC of most wood in temperate climates is 10 to 16 percent and fluctuates seasonally, with highest levels being in the spring and summer. In a warehouse, multiple-use wood pallets will eventually stabilize at moisture levels between 10 to 12 percent. Wood at EMC below 20 percent will not support mold, mildew and decay fungal attack. EMC of wood can be predicted from the dry bulb temperature and relative humidity, using Table 1.
4) Kiln Dried. For grade-marked softwood structural lumber, this refers to lumber having a maximum MC of 15 percent or 19 percent, and 23 percent for cants. For hardwood lumber, this is typically a maximum MC of seven to nine percent, but not necessarily at EMC.
5) Air Dried. For grade-marked softwood structural lumber, this refers to lumber having a maximum MC of 19 percent. For hardwood lumber, usually this is approximately 18 percent.

How to Measure the Moisture Content of Wood Pallet Parts
Dry wood is an electrical insulator. Water is a conductor. Therefore, electric resistance moisture meters can be used to estimate the moisture content of wood in pallets between four and 30 percent. Moisture meters which can be used for measuring the moisture content of pallet parts are available from: Delmhorst Instrument Company, 51 Indian Lane E, P.O. Box 68, Towaco, NJ 07082, Ph: (201) 334-2557, FAX (201) 334-2657; Lignomat, 14345 NE Morris Court, Portland, OR. 97230, Ph: (503) 257-8957, FAX (503) 255-1430; and Wagner Electronic Products, 326 Pine Grove Rd, Rogue River, OR, 97537, Ph: (800) 585-7614.
Wood pallet parts not at equilibrium will be drier on the ends and on the surface than at the center of the pieces. Sensor type meters electromagnetically measure the moisture level in wood up to a one-inch depth. To determine the average moisture content of the piece, place the sensors at 1/3 points along the length parallel to the grain. With a pin type meter, use the probes to penetrate to about 1/3 the thickness of the pieces, again at 1/3 points along the length parallel to the grain.

Estimating the Weight of a Wood Pallet
The weight of the wood portion of a pallet will depend on the volume of lumber used in the pallet, the specific gravity of the wood, and the MC of the wood. Once the MC is known, two simple (although sometimes intimidating) formulas can be used to estimate the weight of the pallet at any moisture level. This is often of significant importance when unit-load shipping is weight limited and the weight of the pallet becomes an issue. Also, human ergonomics limit the weight that humans can lift (often pallets are lifted by hand).

where:

Sg (MC%) in the density of the wood at a certain moisture content

Sg (green) in the density of wood at or above fiber saturation point (varies with species)

MC% is the moisture content of the wood pallet parts

Vol. shr% is the wood volumetric shrinkage found for different wood species in Table 3. (Only a few common pallet lumber species are listed.)
All of these values are available in the literature. The best source is the ASTM D-2555 Standard methods of Establishing Clear Wood Strength Values. American Society of Testing and Materials (ASTM). Bank of Standard Volume 04.09. ASTM, West Conshokochen, PA 19428. Ph. (610) 832-9500. To help with rapid estimates of pallet weight, Table 2 contains some weights as a function of common moisture levels and common wood species used in pallet construction.

How Much will Wood Pallet Parts Shrink as They Dry?
Wood shrinks more tangentially than radially (Figure 2). The percent shrinkage also depends on the species of wood. Representative shrinkage values are found in Table 3 for commercially important woods of the US and Canada. The differential shrinkage between tangential and radial within species causes boards to warp and flat sawn boards to cup. Wood shrinks very little in the longitudinal direction, which is end-to-end in a board.
One can estimate the percent shrinkage in pallet parts using Table 3. This issue is important in pallets assembled with green parts because the customer may inspect dry pallets and compare these measurements to pallet specifications based on green dimensions. Since most lumber is bastard sawn, such that the shrinkage will not be truly tangential and radial, it is best to use an average of both tangential and radial shrinkage for the given species and then take the average shrinkage and substitute it into the equation below.

Sm = So (30-MC) / 30
where Sm is the percent shrinkage from green to the designated MC and So is the average of the tangential and radial shrinkage in Table 3. (The 30 is FSP above which no shrinkage occurs). Let us estimate the percent shrinkage to 12% EMC for yellow-poplar.

Sm = 6.4 (30-12) / 30 = 3.8% shrinkage
A 3/4 x 3-3/4-inch wide green yellow-poplar board will shrink 0.75 x 0.038 = 0.029-inches in thickness, or approximately 1/32 inch. It will shrink 3.75 x 20.038 = 0.143-inches in width, or between 1/8 and 5/32 inch.
Boards dry more rapidly from the ends than from the sides or edges. Therefore, in a pallet deckboard or stringer, the ends will tend to shrink more rapidly than the interior of the boards. The resistance of the interior of the board to shrinkage will result in high stresses in the ends and eventually cause the ends to split. Splits reduce the integrity of nailed joints. Most splits will occur near nails since these are pre-stressed regions caused by assembly of the pallet.
To roughly estimate how much wood pallet parts will shrink when air-dried to (18 percent), multiply the green part size by 0.02 or two percent, and at long-term equilibrium in warehouses by 0.04 or four percent.

Air Drying Wood Pallets or Pallet Parts
Whether dried in the assembled pallet or as a part prior to assembly, green lumber can be air dried if warranted by the customer's needs. As indicated, dry wood will not decay or mold. Moisture-sensitive products shipped on dry pallets will have a greater success. And dry pallets may weigh 25 to 30 percent less than green. If drying is handled properly, and assembly is of a quality nature, pallets assembled with dry lumber are more durable than pallets assembled with green lumber because of the improved joint integrity. Further, less degradation will occur during the drying process if some pre-drying is performed on the parts.
Generally, the more dense the wood, the more slowly the lumber will dry. The rate of drying will depend on atmospheric conditions as well as the MC of the lumber. Air drying times for stickered stacks of one-inch thick lumber in temperate climates for selected different species are found in Table 4. Note that the final moisture content is 20 percent and also that one can estimate the drying time for different thicknesses of lumber by using the ratio of thicknesses. For example, 3/4 inch thick material would drying 25 percent less time than specified. Also note the variation. The longer periods of time in this table refer to fall and winter. When drying starts in the fall or winter, the material will not be dry until the next spring.
When air drying pallet parts or pallets, the top of the piles should be covered to protect pallet components from direct exposure from the sun. Direct exposure to sun will result in excessively rapid drying and the exposed lumber will warp, split, and check to the point the pallets may not be useful. Orient the stickers, or in the case of stacked stringer pallets, stringers parallel to the predominant wind direction.
Figure 3 is a plot of the change in average pallet moisture content of 15-high stacks of hardwood GPC specification pallets as a function of air drying time. These pallets contain 13/16-inch thick dense hardwood deckboards and 1-3/4 x 3-3/4 inch dense hardwood stringers. In one stack, the pallets were separated by 3/4-inch thick stickers. The other stack was dead-piled.
This study was conducted between June and August in western Virginia. The initial drying rate of the stickered pallets was twice that of the dead piled pallets. Between green and fiber saturation point, the average drying rate of the stickered pallets was 40 percent greater than dead piled. To air dry the stickered pallets took about 50 days. The dead piled pallets will take 80 to 90 days during the summer and twice as much in the winter in temperate climates.
Pallets with thinner deckboards will dry more rapidly. Figure 4 represents the average summertime air drying of 5/8-inch hardwood deckboards in stacked GMA pallets. These deck-boards reached equilibrium in about one month from the very green condition. It appears such pallet deckboards, fabricated with parts that have already dried to 30 to 40 percent moisture, would air dry in about 20 days. Tests have shown that pallets properly air dried do not mechanically degrade.
From experience in drying hardwood lumber, if a fan shed was used, this drying time could be cut by half, and if the pallets were dried in a kiln operating at 100-110 degrees F, the deck-board moisture content change of 85 percent to 20 percent could be achieved in about five days.
What is the best method for drying wood pallets? Should one dry parts before assembly or dry assembled pallets? The answer to these questions are not known, and are the subjects of proposed research before the NWPCA Research Steering Committee.