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Unique Features

Effect of Temperature

The strength of wood structural panels is less at elevated temperatures than at normal temperatures. In the range of 0° F to 200° F, the strength of the panel at 12 percent moisture content or more will increase or decrease approximately 1/2 percent for each one degree increase or decrease in temperature from 70° F. However, such panels exposed to temperatures up to 200° F for a year or more may not experience any significant or permanent loss in strength. If drying occurs, the increase in strength due to drying may offset the loss in strength due to elevated temperature.

The thermal expansion of wood is much smaller than expansion due to absorption of water. Because of this, thermal expansion can be neglected in cases where wood is subject to considerable swelling and shrinking.

Keep reading an advanced discussion of the thermal properties of wood structural panels, including information on conductivity, resistance, exposure to extreme heat, thermal degradation and ignition point and cryogenic temperatures.

Fire-Resistant Properties

Wood structural panels are real wood, a natural insulator, and therefore provide good protection against heat loss and condensation. Large panel size also minimizes the number of joints that can "leak'' heat, airborne noise or flames. Tests conducted in 1974 by Underwriters Laboratories showed flame-spread ratings from 75 to 200 for common plywood grades. Variables in the U.L. tests included species group, glue type, thickness and surface texture. All plywood tested exhibited a flame spread of 200 or less. A decrease in flame spread with increasing panel thickness was noted for thicknesses from 1/4 to 5/8 inch.

Plywood pressure-impregnated with fire-retardant salts is able to inhibit combustion. Precisely defined, FRT plywood has been impregnated with fire-retardant chemicals in accordance with American Wood Preservers Association Standard UCFA to have a flame spread of not over 25 when subjected to a 30-minute test. Since fire-retardant treatment involves proprietary chemical formulations that generally reduce the structural properties of plywood, technical recommendations should be sought from the company that provides treating and redrying services.

Keep reading an advanced discussion of the fire-resistant properties of wood structural panels.

Permeability

The permeability of plywood is different from solid wood in several ways. The veneers from which plywood is made generally contain lathe checks from the manufacturing process. These small cracks provide pathways for materials to pass into when entering through the panel edge. When permeability is measured through the panel thickness, a number of variables will affect the actual flow rate. The anatomy of the species, consistency of the glue line, number of void spaces, and growth characteristics all affect permeability. Exterior-type plywood, and especially High Density Overlaid plywood, is a relatively efficient barrier.

Keep reading an advanced discussion of the permeability of wood structural panels.

Electrical Properties

The single most useful electrical property of plywood is its resistivity, or inability to conduct electrical current. Plywood is a highly effective insulator as long as it is dry. The resistivity is very sensitive to water absorption and drops drastically as moisture content increases. The glueline in plywood is not as effective an insulator as the wood itself. For applications in electric fields in the range of household voltages it may be neglected, but it could be important on certain test benches of sensitive electrical instruments.

The mechanism of electrical conduction depends upon the presence of ions in the wood. In the moisture content range from 0 to more than 20 percent, the number of charge carriers becomes a major factor. The DC resistance of wood and plywood is approximately 10¹⁰ ohm per cm at a moisture content of 12 percent, and drops to about 10⁷ at 20 percent.

The alternating-current (AC) resistivity is considerably lower at high frequencies than the DC resistivity. For oven-dry wood in the transverse direction a value of 18 x 10⁶ ohm per cm at 2 MHz has been reported. Moisture content affects the AC resistivity, but not as dramatically as for DC current.

Resistance to Chemical Agents

Plywood is highly resistant to many chemicals. It is effectively used in many industrial applications involving contact with chemicals, including dilute acids, alkalies, organic chemicals, and many neutral and acid salts, both hot and cold. Exterior glue lines are more chemically resistant to most acids and bases, solvents and petroleum than the wood. Plywood does, however, accept pressure treatments such as preservatives and fire-retardant solutions. OSB can be treated to resist biological attack with proprietary processes.

Keep reading about wood structural panels' resistance to chemical agents.

Acoustical Properties

Wood has some unique properties that allow it to contribute to noise reduction in buildings and in industrial applications. Industrial noise control has gained considerable importance due to the requirements established by OSHA.

Audible sound is a propagation of energy which is commonly measured in terms of decibels (dB). The lower threshold of human hearing is 1 dB, and 130 dB is considered the threshold of pain. Examples of typical sound levels are: Rustle of leaves, 10 dB; ordinary conversation, 40 dB; and heavy traffic, 90–95 dB.

Since sound in air is energy in motion, sound waves may be absorbed by a surface or reflected. Plywood, like other materials, will absorb some of the sound energy and reflect the remainder. Relative coefficients of sound absorption are given in the table below. A material that exhibits perfect absorptivity is rated as 1.0; a perfect reflector of sound would have a coefficient of sound absorption of 0.0.

Sound Absorption Coefficients of Various Building Materials
Material	Coefficient
Open window	1.0
Special acoustic materials	0.2 - 0.8
Brick	0.03
Window glass	0.03
Plywood	0.04
Varnished wood	0.03