Theatre Design & Technology - Fall 2009 - 27

lumber (PSL), glued-laminated timber (Glulam), as well as pre-manufactured wooden I-beam assemblies. The expense of SCL products typically
excludes it from the shopping list of average theatres, however the mere
fact that SCL products are superior to traditional lumber in dimensional
stability (resistance to warping, cupping, checking, bowing) suggests
that these products will continue to gain the attention of technical directors and scenery designers. However, "unlike lumber [and plywood], no
standard grades or design stresses have been established for SCL. Each
manufacturer may have unique design properties and procedures. Thus,
the designer should consult information provided by the manufacturer."
[Wood Handbook: wood and an Engineering Material, 11-3, 1999, FPLGTR-113]. The fact that no unified design stresses exists can increase the
effort necessary to make safe and efficient use of SCLs in stage scenery
applications. Equations necessary to safely engineer and construct shopbuilt I-Beams and Box Beams can be found in chapters 8 and 11 of the
Wood Handbook, as well as Structural Design for the Stage, by Alys
Holden and Ben Sammler, Consult the specific manufacture of any SCL
product before incorporating it into a structural stage application.
All wood composite panels have five manufacturing steps in common:
1) raw material is milled into the desired sizes of uniform particles and
screened to exclude non uniform product; 2) the sized wood particles are
dried to carefully control moisture content and prepare the material to better accept the next step; 3) the dried particles are blended with synthetic
resins and other additives; 4) the blended particles and resin is formed into
a mat; and finally 5) high heat and pressure is applied to press the mat to
slightly thicker than the desired final thickness. The cured panel is then
sanded to the desired thickness and cut to size for packing and shipment.
Since composite wood panels are largely industrial panels, not only are they
available in four by eight standard size, but also industrial sizes including
49-inches by 97-inches, five by nine feet, and larger.
The adhesive used to hold wood fibers together in most wood composite panels is urea formaldehyde. Under certain service conditions, formaldehyde can be released from wood composite panels. This toxic gas can "react
with proteins of the body to cause irritation and, in some cases, inflammation of membranes of eyes, nose, and throat, and it is a suspected carcinogen." [Wood Handbook: Wood as an Engineering Material]. A widely used
alternative adhesive is PMDI (Polymeric methylene diphenyl diisocyanate).
Similar to polyurethane adhesives including Gorrilla Glue, and Elmer's Ultimate Glue, PMDI bonds well to wood and is water resistant. Panel products
that state "no added formaldehyde" will most likely incorporate no formaldehyde adhesive and instead use PMDI. Specifically, wood naturally contains
low amounts of formaldehyde, making it almost impossible to produce a
"formaldehyde free" product. Other panels may state they possess either
"low" or "ultra low emission formaldehyde," in which case the panel has
some formaldehyde adhesives and possibly additives to reduce emissions.
As emission standards become stricter over the next three years, the cost
of wood composite panels will likely rise. Urea formaldehyde adhesive cost
$0.24 per pound verses PMDI at $1.40 per pound. The increased cost to
produce a no-added formaldehyde panel while maintaining a panels physical properties is unavoidable. [CARB: A Challenge and an Opportunity for
Composite Panels, by Bradfield, John, presented at the International Wood
Composite Symposium, April, 2008].
Wood composite panels are classified by the size of wood particles
used in the final product. Hardboard (Masonite) is comprised of very small

wood fibers, while the manufacturing of structural panels such as oriented
strand board (OSb) requires larger wood flake particles. Wood particles
vary from large veneers, chips and flakes, strands, particles, fibers, down to
wood flour and cellulose. Manufacturers carefully control particle size, adhesive and resin proportions, and heat and pressure to mass produce panel
products in enormous quantities with very predictable properties.
Conditioning is important to all wood composite panels. Wherever
possible, panels should be stood up on their long side for 48 hours to allow the panel to "breath" and acclimate to the temperature and humidity
conditions of the room. Conditioning will help thinner panels warp less and
improve overall reliability of the installed panel. Much more information is
available through two sources, the Wood Handbook: Wood and an Engineering Material, a publication of the Forest Products laboratory, and The
Composite Panel Association (CPA), founded in 1960, that represents the
North American composite panel industry on technical, regulatory, quality
assurance and product acceptance issues [www.pbmdf.com].

Physical Properties Relevant to Wood Composites
IB - Internal Bond

Internal bond (Ib) is the force that it takes to pull a material apart, perpendicular to its face. Ib relates directly to the adhesive bond between
wood fibers and the adhesives in the panel. Ib is the measure of force
necessary to delaminate the test sample. Panels are engineered to break
apart at or near the panel's middle plane where both material density and
adhesive content are less than the face layers due to the pressing process.
The pressing process incorporates high heat, from to 284°F to 325°F,
and pressure, from 199 to 498 lbs/in2, to activate and set the adhesives
and compress the wood fibers together. As a panel is pressed it becomes
denser at the face layers as the adhesives migrate to the face surfaces. This
is best illustrated by placing a piece of foam rubber padding between your
hands and pushing your hands together to squeeze the foam. The outer
cells of foam are compressed first while the cells at the center of the foam
are compress less, if at all. This same reaction is found in the pressing
process of wood composites and results in smooth face layers.
Materials with lower Ib values will be less strong due to a weaker
bond between wood fibers and the adhesive. Higher values mean stronger
materials. Note the range of Ib values on the mechanical properties chart.
For example, the Ib for PbU (particle board underlayment) grade commonly found at local lumber yards is 58 pounds-per-square inch, while
grade d-2, particleboard that is engineered specifically for manufactured
home decking, is 80 pounds-per-square inch [Wood Handbook, 10-18]
and MdF is 109 psi. This illustrates the relative strength of the bond between wood particles and adhesive in these panels
MOE - Modulus of Elasticity

Modulus of Elasticity (MOE) is the ratio of stress to corresponding strain
on a material, below its elastic point. This important but intricate concept is the measurement of the force placed on a material which, when
the force is removed, the material will return to its original shape. This
means that the force was within the materials elastic range. If the force
increases above the elastic range of a material and passes the material's
elastic point, then the material will deform permanently either bending
or breaking. The Modulus of Elasticity (MOE) represents how a material

2009

theatre

design

technology

http://www.pbmdf.com

Table of Contents for the Digital Edition of Theatre Design & Technology - Fall 2009