Thermal Degradation and Ignition Point
When the temperature of dry wood is raised above 212° F (100° C) a slow exothermic decomposition takes place. This degradation involves the loss of carbon dioxide and volatile materials such as extractives, in the form of gases or vapors. The rate depends upon temperature and air circulation.
The thermal degradation and ignition point of wood and plywood may be generalized by the following:
230° to 302° F (110° C to 150° C): The wood will char over time with the formation of charcoal. If the heat is not dissipated there is some possibility of spontaneous combustion. Examples of the thermal degradation of maple blocks are: 1050 days at 225° F (107° C): 10 percent loss in weight and slight discoloration. 1235 days at 248° F (120° C): 30 percent weight loss and a chocolate color. 320 days at 284° F (140° C): 60 percent weight loss and charcoal appearance. 302° to 392° F (150° to 200° C): Charring takes place at a somewhat greater rate. If the heat source is close to the wood, the surface temperature may be higher than the temperature of the surrounding air due to radiant heating. Gases released at these temperatures are not readily ignited by an outside flame source. A greater chance for spontaneous combustion is present if the heat is not dissipated. In tests, after 165 days at 302° F (150° C) maple blocks showed a 60 percent weight loss, and the samples had the appearance of charcoal. 392° to 536° F (200° to 280° C): The formation of charcoal takes place at a rapid rate. Spontaneous combustion is probable. 536° F (280° C) and greater: Spontaneous combustion will occur in a short period of time. A number of attempts have been made to measure a definite ignition temperature of wood, with little success. A specific temperature is hard to define because there are so many contributing factors, such as size and shape of the material, air circulation, rate of heating, moisture content of the wood and so on. Estimates range from 510° to 932° F (270° to 500° C), but no value should be accepted as an absolute.
Investigations of wood in low temperatures, down to -300° F (-184° C), have shown mechanical strength increases. The increase is up to three times the property measured at room temperature, depending on the strength property and moisture content. This increase is consistent with other materials that exhibit increased resistance to changes in form as the temperature drops. Cycling of freezing and thawing do not seem to affect the properties of the wood itself, but may reduce the strength of some fastenings by as much as 10 percent.
In practical applications of wood products, the increase in strength due to exposure to subnormal temperature will tend to offset strength losses caused by other factors. With regard to glue performance, studies have shown that the joint strength of plywood made with phenolic, urea and casein glues is not affected by a temperature of -68° F (-56° C).
On the basis of available test information, published stresses for plywood are considered applicable at temperatures down to -300° F (-184° C).
Plywood has been successfully used as part of an insulation jacket for ship hulls transporting liquid natural gas (LNG). This gas is maintained in the liquid state at approximately -250° F (-157° C). The plywood is used in conjunction with insulating foam, and it reaches a service temperature of approximately -150° F (-101° C). Design engineers are well satisfied with the plywood performance for this purpose.
For more information on wood structural panels' thermal properties, consult ICC Evaluation Service ICC-ES Evaluation Report ESR-2586, and APA Performance-Rated Wood Structural Panels as Thermal Barriers, Form TT-060.
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