Madera

General

Heat treatment and tempering are optional processing
steps that follow the hotpress. They are used only in
the manufacture of thin medium- and high-density fiberboards.
Insulation board, thick MDF, and particle board
are not heat treated or tempered.

Heat treatment is the exposure of pressed fiberboard
to dry heat. Tempering is the heat treatment of pressed
board, preceded byaddition of drying oils. The major
function of the heat treatments is to improve dimensional
stability and to enhance some important mechanical
board properties.

Humidification of fiberboard is the addition of
water to bring the board moisture content into
equilibrium with air conditions expected in service.
Where heat treatment or tempering is used, humidification
immediately followsthose treatments.

In general, heat treating and tempering are more effective
on wet-formed than on dry-formed board. Heattreated
boards and particularly tempered boards are
substantially more expensive than untreated boards.

Improvement of Board Properties

Dimensional stabilization

Dimensional change in wood products from adsorption
and desorption of water in the cell wall is animportant
hazard, particularly in high-density products. This
follows because the volumetric expansion of wood approximately
equals the volume of water adsorbed by the
cell walls. As wood is densified by reducing the pore
volume, the same amount of water will still be adsorbed
under a given exposure condition (unless, of course, the
hygroscopicity of the cell wall has been modified in theprocess), resulting in a greater relative volumetric expansion.

This is illustrated in figure 240, where 1 cm^of cell
wall substance (specific gravity = 1.46) at 100 percent
relative humidity adsorbs 28 percent of its weight in water
(1.46 X 0.28 = 0.40 g). This is the maximum amount of
water the cell wall can adsorb. Additional water uptake
would fill the pore volume without furtherswelling. The
expansion of the cell wall by 0.40 cm^ results in different
relative volumetric expansion values, depending upon the
total volume (cell wall plus pore volume) of the product.
In the case of solid wood with a specific gravity of 0.49
(1 part cell wall, 2 parts pore volume) the volumetric expansion
is 13 percent; in the case of hardboard with a
specific gravity of 0.97 (1 part cellwall, Vi part pore
volume), it would be 27 percent; and in the case of a
paper with no pore volume at all, the volumetric expansion
would be 40 percent.

Densified products, generally, swell in the direction
of densification, which in fiberboard is in the direction of
board thickness. Swelling in the plane of the board is very
small, due to a mutual restraint of the *'cross-laminated"
fibers.In addition to the increased relative expansion
resulting from the reduction of pore volume, densification
causes another swelling component called springback.
It is due to swelling forces causing partial failures
of bonds between fibers, which in turn creates additional
void space. Part or all of this additional void space
created during the swelling process is permanent and will
notdisappear upon redrying of the board (fig. 241). This
adds substantially to the swelling of densified board such
as particle board and hardboard and is often accompanied
by a permanent strength reduction (fig. 242).
Heat treatment improves this behavior in two ways;
it reduces the water adsorption by the cell wall, and it improves
the bond between fibers, which in turn helps resist
thecreation of voids during swelling. Figure 243 shows
the reduction of the permanent thickness swelling
(springback) after heat treatment at various temperatures
and for various times. See also figure 244.

The linear expansion—that is, changes in dimension
in the plane of the board that occur because of change in
moisture content—and its modification by heat treatment
are more complicated....