Lumber, Plywood, and Other Wood Products - Carpenter's and Builder's Math, Plans, and Specifications - page 81

It has become a successful and economical panel product because
of the availability and economy of thermosetting synthetic resins,
which permit blends of wood particles and the synthetic resin to be
consolidated and the resin set (cured) in a press that is heated.
Thermosetting resins used are primarily urea-formaldehyde and
phenol-formaldehyde. Urea-formaldehyde is lowest in cost and is the
binder used in greatest quantity for particleboard intended for inte-
rior or other nonsevere exposures. Where moderate water or heat
resistance is required, melamine-urea-formaldehyde resin blends are
being used. For severe exposures, such as exteriors or where some
heat resistance is required, phenolics are generally used.
The kinds of wood particles used in the manufacture of parti-
cleboard range from specially cut flakes an inch or more in length
(parallel to the grain of the wood) and only a few hundredths of
an inch thick to fine particles approaching fibers or flour in size.
The synthetic resin solids are usually between 5 and 10 percent by
weight of the dry wood furnish. These resins are set by heat as the
wood particle-resin blend is compressed in flat-platen presses.
As floor underlayment, particleboard provides (1) the leveling, (2)
the thickness of construction required to bring the final floor to ele-
vation, and (3) the indentation-resistant smooth surface necessary as
the base for resilient finish floors of linoleum, rubber, vinyl tile, and
sheet material. Particleboard for this use is produced in 4- by 8-foot
panels commonly

1 3

/ 8 ,or 5

/ 8 inch thick. Separate use specifica-
tions cover particleboard floor underlayment. In addition, all man-
ufacturers of particleboard floor underlayment provide individual
application instructions and guarantees because of the importance
of proper application and the interaction effects of joists, subfloor,
underlayment, adhesives, and finish flooring. Particleboard under-
layment is sold under a certified quality program where established
grade marks clearly identify the use, quality, grade, and originating
mill.
Other uses for particleboard have special requirements, as for
phenol-formaldehyde, a more durable adhesive, in the board. Par-
ticleboard for siding, combined siding sheathing, and use as sof-
fit linings and ceilings for carports, porches, and the like requires
this more durable adhesive. For these uses, type 2 medium-density
board is required. In addition, such agencies as the Federal Housing
Administration have established requirements for particleboard for
such use. The satisfactory performance of particleboard in exterior
exposure depends not only on the manufacture and kind of adhesive
used but also on the protection afforded by the finish. Manufacturers

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