Edge sealant formulation for wood-based panels

Details Edge sealant formulation for wood-based panels Edge sealant formulation for wood-based panels

2000-07-19

2003-08-19

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S073000, C524S074000, C524S488000, C428S194000

Reexamination Certificate

active

06608131

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a formulation for sealing oriented strandboard edges to prevent edge swelling.
BACKGROUND OF THE INVENTION
Oriented strandboard (OSB) panels are commonly used as subfloor sheathing in residential homes. These panels are installed directly on top of floor joists prior to installation of the walls and roof of the structure. Thus, the subfloor is exposed to external environmental conditions for a period of time during the general process of building a house. It is common for the subfloor panels to be subjected to rain during this process. Sill plates, which vertically protrude from the perimeter of the floor, can literally convert the floor into a basin. An uncovered subfloor can accumulate as much as two inches of water during a rainstorm. In some cases the accumulated water will be left to absorb into the subfloor panels for several days during the home-building process.
Unfortunately, exposure to water causes most OSB panels to undergo severe, irreversible thickness swell. Panels, which are manufactured at a thickness of 720 mils (0.720 inch), can actually swell to edge thickness values in excess of 1000 mils. Upon drying, these same panels will typically have an edge thickness of approximately 900 mils. The worst aspect of the swelling behavior is that the OSB swells to a greater extent on the edge of the panel than it does in regions towards the center of the panel. Panels subjected to a wet and redry cycle can be 20 to 150 mils thicker at the panel edges than they are 4 inches proximal to the edges. This phenomenon is typically referred to as differential edge swell. For the purpose of this application differential edge swell is defined as the edge thickness of a water-swollen OSB panel minus the caliper at a location that is 4 inches proximal to the edge point:
DIFFERENTIAL EDGE SWELL=(THICKNESS AT PANEL EDGE)−(THICKNESS 4 INCHES PROXIMAL TO THE EDGE)
There are several factors that effect OSB differential edge swell. It is helpful to review some of the factors that are believed to effect differential edge swell.
Consider a subfloor comprised of OSB panels at a home construction site. Builders are instructed to leave small gaps between the panels in the floor system in order to accommodate linear expansion. During a rainstorm there is a natural tendency for the accumulated rainwater to flow into these gaps or seams in the floor. Floor joists or protruding tongues reside directly beneath the seams, thus the water that flows into the seams can not readily drain. In this manner the edges of the OSB panels in a wet floor system are exposed to just as much water as the major, top-side surfaces of the panels.
The orientation of the strands in OSB is almost exclusively parallel to the plane of the panel. This orientation results in relatively nonporous major faces and highly porous edges. Thus, the porous edges of OSB panels absorb water faster than do the relatively nonporous major surfaces. An interesting consequence of the anisotropic pore structure of OSB is that brief exposure to water actually produces maximum differential edge swell. When OSB is subjected to water for a relatively long period of time, the interior regions of the panel have time to fully hydrate, and swell to become nearly as thick as the perimeter of the panel.
Most strands in OSB have been compressed to density values that are significantly greater than that of the virgin wood. Generally, when compressed wood is exposed to water it springs back to its original dimensions. Thus, compressed strands will tend to increase in thickness to at least their original dimensions as they absorb water. Upon drying, the dimensions of these strands do not return to the compressed state.
Another significant factor, which effects thickness swell in an OSB panel, relates to the wet strength of the strand-to-strand bonds. Strands in an OSB panel are held together with adhesives, such as phenol/formaldehyde (PF) resins or methylene-diphenyldiisocyanate (MDI). As adjacent strands in an OSB panel undergo dramatic dimensional change, there are considerable stresses placed on the strand-to-strand bonds. Some of the water that penetrates an OSB panel can absorb into the adhesive glue-lines and weaken them. Phenolic glue-lines can be especially susceptible to water absorption. The combination of physical stresses and low wet strength causes a number of these strand-to-strand bonds to rupture. In many cases, strands in the panel are bent over each other like a loaded catapult. As bonds rupture, strands are able to relax into a more linear shape, which increases the thickness of the panel. This part of the thickness swelling process is also not reversible with drying. It should be noted that strand-to-strand bonds near the edges of the panel will have fewer neighboring strands for load sharing as compared to strand-to-strand bonds in the interior region of the panel. Thus, more strand-to-strand bonds would be expected to rupture at the edge of a panel than in the interior regions of the panel.
In summary, excessive thickness swell, and especially, excessive differential edge swell in OSB panels are facilitated by (1) the seams in a floor system that trap rainwater against the edges of OSB panels; (2) the relatively porous nature of the OSB edges; (3) the compressed state of strands in OSB; and (4) the residual stresses in flexed strands and the rupturing of wet strand-to-strand bonds.
The consequences of differential edge swell can be significant. When differential edge swell occurs during residential home construction it manifests itself as ridges along the seams in the subfloor. Builders are often required to sand the seams in the subfloor in order to remove these ridges and create a flat, smooth subfloor. Obviously, the practice of sanding the subfloor is costly, time-consuming, and frustrating to the builder.
There are available solutions to the problem of differential edge swell. In wet environments the builder can avoid the differential edge swell problem by using plywood as the subfloor panel. The thickness swell associated with plywood when it is subjected to water is usually so subtle that sanding is not required. Unfortunately, plywood is more expensive than OSB. A desirable panel for the builder to use would be one that is as inexpensive as OSB, but has the thickness swelling properties of plywood.
OSB manufacturers have recognized this opportunity for years. Essentially all North American manufacturers of OSB subfloor panels attempt to improve the dimensional stability of the panel by applying a paint-like formulation to all four edges of the OSB subfloor panel. Subsequent to application this type of formulation dries into a hydrophobic film, which binds strongly to the OSB substrate and inhibits the absorption of water into the edge of the panel. Thus, the edge sealant helps to reduce the degree of differential edge swell experienced by the panel when it is exposed to water during the construction process.
The edge sealant technology is not the only method that can be used by OSB manufacturers to make the panel more resistant to differential edge swell. Addition of wax to the individual strands makes them more hydrophobic and significantly decreases the rate at which an OSB panel absorbs water. Apparently, all OSB manufacturers apply wax to the strands in order to make them more hydrophobic. Unfortunately, the addition of wax beyond a level of about 1% by weight significantly interferes with the strand-to-strand adhesive bonds. Thus, OSB manufacturers are limited in the amount of wax that can be added to OSB to improve thickness swell.
It is also known that increasing the amount of bonding resin in the board can significantly improve the dimensional stability of OSB. Unfortunately, the cost of using higher levels of adhesive is significantly greater than the cost of applying an edge sealant. Thus, application of an edge sealant is a low-cost method for improving the dimensional stability properties of the OSB.
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