Dimensional stability in particleboard & MDF.

Balance construction to reduce moisture-related problems with engineered wood products.

Water is always present in wood or wood products. Driven by physical forces, water enters and leaves wood - changing the wood's volume and most of its properties.

Since there is no guaranteed method for keeping water out of wood, appropriate design measures must be taken when building with wood products.

Balance of moisture in wood and air

When wood is green, it is saturated with water in both the cell cavities and the cell walls. The water in the cell cavities is called "free water" and the water in the cell walls is called "bound water" (Figure 1).

Normally, free water is removed completely during the drying process. Bound water remains and is in equilibrium with the relative humidity of the air. This equilibrium is defined by the "sorption isotherm" (Figure 2).

All wood species more or less follow this curve. Particleboard and MDF follow a somewhat modified curve as indicated by the dashed line.

As the relative humidity of the air changes, all wood products gain or lose water, including wood-based laminating materials like resin impregnated papers or high pressure laminates (HPL).

Water's effect on dimensions

Residing in the cell wall, bound water affects wood's bulk. This in turn affects the gross dimensions of the wood. As a general rule, wood swells and shrinks in proportion to the volume of water gained or lost.

For solid wood, swelling and shrinking is quite different in longitudinal versus tangential or radial directions (Figures 3 and 4).

These differences are serious and could result in real practical problems.

Control dimensional changes with

cross lamination 

One important method for reducing dimensional changes is cross lamination, a key characteristic of plywood, particleboard and MDF products.

Cross lamination is accomplished by alternating the grain direction in the panel or in the case of particleboard and MDF, using randomly placed particles and fibers.

Restraining expansion

To see how cross lamination controls dimensional changes, let's use the example of a veneered lumber panel (Figure 5):

* When there's an increase in moisture, an edge-glued lumber panel freely expands.

* When restraining members, such as cross veneers, are applied to both sides of the panel - prior to moisture gain - they act like steel straps nailed to the panel (Figure 6). They are strong enough to greatly reduce, if not totally eliminate, the expansion of the panel.

* As the moisture of the lumber panel increases, the restraining members will be stressed in tension, the lumber panel in compression. Although significant stress occurs, the panel will stay flat as long as the forces in the restraining members are exactly equal or balanced.

* An extreme imbalance occurs when the top strap is cut, resulting in a warp that is easy to see.

The problem with cross lamination is that sometimes it can be difficult to maintain balance. For exact balance, the two restraining members must be identical in:

* thickness

* Resistance to deformation, such as stretching

* MOE (modulus of elasticity)

* Expansion characteristics.

Even relatively minor imbalances in the characteristics of the restraining members can cause significant warping. The greater the potential expansion of the lumber panel, the greater the warp when the restraint is removed on one side.

Alternating grain direction

Consider a furniture panel where the veneer layers serve the same function as steel straps, except that they absorb moisture and expand.

By design, the veneer grain is arranged at 90 degrees relative to the grain of the core lumber - pairing the minimum expansion of the veneer along its grain with maximum expansion of the core across its grain.

This way, the veneer layers effectively restrain the lumber core because of:

* Very high resistance to stretching (MOE) of the veneer along the end grain

* Relatively low resistance to compression (also MOE) of the lumber core across the grain.

As before, balance is vital. if the bottom veneer was only half as thick as the top veneer, it would not restrain the core as effectively and the panel would warp concavely upwards.

Relative expansion and direction of stresses are reversed when considering the other principal direction of the panel, but the mechanism is the same.

Using randomly placed particles

or fibers

Particleboard and MDF also benefit from cross lamination because of random orientation of the elements.

Expansion of the particles or fibers in the plane of the board is greatly reduced and is substantially the same in both directions. This property makes particleboard and MDF a superior core material (Figure 7).

Providing additional restrains

Laminating particleboard provides additional restraint, but even here allowances must be made for dimensional change as the potential for warping still exists.

Consider particleboard or MDF overlaid with high pressure laminate, with and without backing (Figure 8):

* Equal thickness of HPL on both sides provides maximum stability.

* Thinner backing cannot restrain the panel with the same force as the HPL, making it vulnerable.

* The restraint is completely one-sided, equivalent to the snapped steel strap in our prior example.

Both of the unbalanced constructions would warp concavely upwards upon moisture gain.

Need for a backing sheet

The lack of a backing sheet can also cause an imbalance.

The top overlay could be a complete moisture barrier, like a vinyl overlay, or it could be a material which absorbs water at a much slower rate than the substrate.

As relative humidity increases, moisture is absorbed by the particleboard or MDF from the back of the panel only. A moisture content gradient develops and the back of the board swells more than the top, causing the panel to warp concavely upwards.

Eventually the moisture content gradient disappears. The panel may straighten out if the overlay applies no mechanical restraint, such as vinyl. Other more rigid one-sided overlays with expansion characteristics different from that of the substrate would still result in permanent warp.

These examples deal with expansion due to moisture adsorption. On the other hand, if moisture is lost through desorption, panel warp may still result, but in the opposite direction.

Balanced construction the key

The most certain way to minimize the degree of warp is to use balanced construction practices.

Consider the moisture-related expansion and shrinkage characteristics of wood products during design and construction to help eliminate these and other potential moisture-related problems.