By Gregory Havel
The adhesives used in manufactured lumber have evolved and become more sophisticated since plywood was first introduced more than 100 years ago. At that time, there were no consensus standards on the raw materials used, the performance of the adhesives used in assembly of the plywood, or the performance of the plywood when exposed to moisture or fire.
Some of the adhesives were not water-resistant, so plywood was rated for exterior use (water-resistant) or interior use (less water-resistant). Some adhesives were thermoplastic resins and could soften when exposed to fire.
In 1932, a group of manufacturers of Douglas fir plywood met and developed a standard for the grading and performance of plywood. This standard included sections on the wood, adhesives, curing, and moisture resistance. In 1933, this standard was adopted by the U.S. Department of Commerce as Commercial Standard CS 45-33. In the following years, this standard was revised several times to reflect the knowledge obtained by manufacturers and users during the World War II and postwar building boom. These revisions included the development of moisture-resistant and exterior-grade adhesives, test methods for adhesive bond performance, use of wood species other than Douglas fir, and increased strength of adhesives.
The original plywood used animal glues and screw presses and required several hours for the glue to cure. In the 1920s, casein and soybean glues were used. These solved the odor problem from the animal glues, but were no more moisture-resistant and delaminated when wet.
In 1933, a process was developed using heated presses and phenolic resins. This speeded up the manufacturing process and resulted in moisture-resistant plywood that could be sold as “exterior grade.”
In 1966, the CS 45 standard was replaced with the PS-1 standard for plywood manufacturing and performance. This standard was revised in 1974, 1983, 1995, and 2009 reflecting new best practices and additional species of wood that can be used. Photo 1 shows a plywood subfloor in a multistory building of wood-frame (Type V) construction. Standard PS-2, with the 2010 revision being current, is a parallel standard for other types of structural wood panels.
(1) Photos by author.
Phenol-formaldehyde liquid resins have been used almost exclusively as adhesives in exterior-grade plywood since the 1960s. The less-moisture-resistant adhesives that were used for interior-grade plywood have been removed from the standards because they were no longer available and no longer in use.
In 1947, a heat performance test was added to the CS45 standard to evaluate adhesive bond resistance to fire; this has been part of all standards revisions since. This requirement applies to all adhesive types used in manufactured lumber products and ensures that the adhesives do not lose their bond strength during fires. The interior-grade adhesives did not reliably meet the performance test and were phased out of production.
The adhesives used in exterior and structural plywood manufactured according to the standard since 1947 are thermosets. This means that once they are cured with heat, they have a permanent shape. They will not melt when exposed to heat or flame, although they can char and burn. The adhesive manufacturers state that when cured, the adhesives have ignition temperatures and burning characteristics similar to the wood that they bond. However, the products of combustion and the heat content will be different since the adhesives are synthetic resins and not cellulose.
Other types of manufactured lumber, their adhesives, and the standards under which they are manufactured include the following:
- Plywood, which uses phenol-formaldehyde adhesive resins, and is made to PS 1.
- Oriented strand board (OSB), which uses phenol-formaldehyde or polymeric Methylene Diphenyl Diisocyanate (pMDI) (isocyanate) adhesives and is made to PS 2.
- Laminated veneer lumber (LVL), which uses phenol formaldehyde adhesives and is made to ASTM D5456 (Photo 2).
- Glulam, which uses melamine or phenol resorcinol formaldehyde (PRF) adhesives and is made to ANSI A190.
- I-joists, which uses PRF, melamine, or isocyanate adhesives as well as PS 2 webs (OSB) and are made to ASTM D5055.
All of these products and adhesives have to pass the heat durability requirements in ASTM D7247, Standard Test Method for Evaluating the Shear Strength of Adhesive Bonds in Laminated Wood Products at Elevated Temperatures.
The PS 1 and PS 2 standards are free downloads from APA: The Engineered Wood Association’s Web site www.apawood.org. You may be required to register as a user of the Web site before accessing the documents library.
American Society for Testing and Materials (ASTM) standards and American National Standards Institute (ANSI) standards can be purchased from http://webstore.ansi.org.
Photo 3 shows a panel of plywood roof sheathing from a 1960s building that has delaminated. This is a failure of adhesive bonds between the layers of wood. This could be the result of a defective sheet of plywood (wrong amount of adhesive, wrong amount of moisture in the wood, or wrong curing temperature) installed as part of the roof. This could also be the result of a sheet of interior-grade plywood having been used on the roof, which requires exterior-grade plywood and has been affected by moisture. This could also be the result of condensation or another high-moisture condition because of inadequate ventilation of the attic space.
The kind of delamination shown in Photo 3 is unlikely to occur during a fire since the adhesives are thermoset resins that burn but do not soften and melt. However, it is possible for delamination to occur before or during a fire but only if the plywood is original construction material from before the CS 45 standard was established and in common use, if the plywood was not manufactured according to the APA standards, or if the plywood had been so damaged by long-term exposure to moisture that the bonds between the wood fibers have broken.
As a result of the concern for the health effects of the formaldehyde and other toxic or irritating vapors that can be released from the adhesives used in manufactured wood products, chemists are revisiting the soy bean and have developed an interior grade of soy-based adhesive that uses no formaldehyde and whose performance surpasses that of the soy-based adhesives from the past. Chemists hope that future soy-based adhesives will replace the formaldehyde-based adhesives presently in use for interior and exterior grades of manufactured lumber.
As firefighters, we must be aware that formaldehyde and other toxic and irritating vapors are present in the products of combustion, that they condense onto our personal protective equipment (PPE), and that this will happen even when all new manufactured lumber uses adhesives of a nonpetroleum base. The older types of manufactured lumber will still be present in older buildings. This is an argument in favor of gross decontamination of our PPE at our exit from fire buildings, with a low-pressure water hose, before we remove any parts of our PPE.
As firefighters, we will also be unaware if the spongy feel or sound of a floor or roof is because of its burning through from below or from delaminating. In either case, this finding must be immediately communicated to the incident commander, as we are removing ourselves from that area so that the floor or roof does not collapse under us.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.
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