ORIENTED STRAND BOARDS (OSB): WHAT IS IT AND HOW IS IT
MANUFACTURED?
This article is reduced version of Fact Sheet entitled “Oriented
Strand Board as a Building Material. Salim Hiziroglu. FAPC -145.
Oklahoma State University. www.fapc.biz.
Printed in The Cedar Leader Oct-Dec 2007and Jan-Mar 2008
Oriented Strand Board (OSB) is one of the most commonly used
engineered wood-based panels for structural purposes in construction
especially in residential sector in North America since 1980’s. OSB
was first produced in Canada in 1964 but it did not find its solid
status in the market until mid 1980s. The number of OSB mills
increased by more than 50% between 1990-1997. Today total OSB
production in North America is approximately is 706 million ft3 (20
million m3) and there are around 20 companies which manufacture OSB
in the United States, Canada, and Europe. It is fact that forest
products companies are aware of utilization of raw material and more
efficiently with better technologies and environmentally friendly
way. With this approach on management of forests, engineered wood
composite products such as OSB gained a significant role in the
world market. A decline in plywood manufacturing in many countries
due to limited large log supply and environmental concerns will
increase production of OSB in future.
OSB is manufactured from fast growing small trees. West Coast inland
mills producing OSB primarily use lodgepole pine logs averaging
10-12 in diameter at the breast height (DBH). Mills in the south use
pine plantation thinning and Southern Pine and soft hardwoods
species such as aspen with an average DBH of 8 in. Eastern redcedar
can also be used to manufacture OSB. However currently there is no
commercial manufacture of such panels from eastern red cedar. Short
segments of the logs are washed and debarked prior strand production
by using disk or ring type flakers. Generally strand size is
approximately 0.5 in by 3 in by 0.02 in depending on type of process
and raw material. Size of the strands can be modified based on
target specifications of the panel. Since moisture content of the
strands ranges from 40% to 75% they are dried to 5% to 10% moisture
in a large rotating single-pass or triple-pass dryers with an inlet
temperature of as high as 1,250oF. Dried strands are classified into
two sections using various sizes of screen. Larger strands are used
for the face layers while the smaller strands are laid out in the
core layer. Face and core layer strands are mixed separately with an
exterior binder in a drum type rotating blender. Depending on
process certain percentage of wax and other chemicals can also be
applied during the blending process. OSB is manufactured using two
types of water resistant binders, phenol formaldehyde (PF) and
diphenylmethane diisocyanate (MDI). Simple design of spinning disks
is used in forming of the strands. Face and core layers of the mat
are oriented in opposite to each other and sequentially dropped on
the conveyor belt. The mat thickness depends on the final thickness
of pressed panel and its density. In a general OSB plant forming
lines are around 6-8 ft wide. A combination of pressure and
temperature is applied to cure resin and giving stiffness to the
panel. A typical press parameters include a temperature of 400-425
oF and a pressure of 650-800 psi for approximately 4-6 minutes.
OSB is extensively used for wall sheating, floor underlayment, roof
cover, and I joist in both commercial and residential building. It
is also used in furniture, reels, trailer liners, and recreational
vehicle floors. OSB is prone to decay due to biological
microorganisms such as fungus when conditions are optimum. Zinc
borate and oxine copper are sprayed as fungicides to enhance
resistance of OSB against biological deterioration. In general OSB
is not used for applications where there is direct contact with a
moisture source such as soil. However brief periods of exposure to
rain during the installation of OSB in residential construction
should not cause some problems in terms of its dimensional
stability.
Some of the advantages of OSB over other wood based products are as
follows:
OSB can be used for both structural and non-structural applications
such as furniture frames, decorative wall paneling, shelving,
packaging, and crating, pallet manufacture, dry storage pallets, and
industrial tabletops. Opposite direction of the face and core layers
strands give a very high bending properties to OSB which is almost
comparable to plywood.
Table 1. Some of physical and mechanical properties of a typical OSB.
| PROPERTY |
PLYWOOD |
OSB |
| Bending strength |
|
|
| Modulus of elasticity |
1,0-1,9×106 psi |
0.7-1,2×106 psi |
| Modulus of rupture |
3,000-7,000 psi |
3,000-4,000 psi |
| Tensile strength |
1,500-4,000 psi |
1,000-1,500 psi |
| Compression strength |
3,000-5,000 psi |
1,500-2,500 psi |
| Shear strength |
600-1,000 psi |
1,000-1,500 psi |
Linear expansion
(From 30% to 90% Relative humidity) |
0.15 % |
0.15 % |
This article is reduced version of Fact Sheet entitled “Oriented
Strand Board as a Building Material. Salim Hiziroglu. FAPC -145.
Oklahoma State University. www.fapc.biz.
|
