Plywood weathering

Plywood, The weathering of plywood is related directly to the quality of the veneer exposed and to the adhesives used. Because of its tendency to check, most exterior plywood is protected with a finish or with overlay material. Such plywood weathers and performs similarly to solid wood (J, 2, 71). 

Adhesives (qv) used to make plywood are classified as either the exterior adhesive or the lesser quaHty interior adhesive. The terms relate to the abUity of the adhesive to survive exposure to moisture and weather. PhenoHc resins (qv) are commonly used as adhesives to make plywood. The difference between interior and exterior phenoHc resin adhesive is the filler level. Exterior rated plywood uses higher resin content adhesives. Interior rated plywood uses either highly extended (below 24% resin soHds) or protein-based adhesives. The adhesive is appHed to the veneer by roU coating, spraying, curtain coating, or foam extmsion. 

Below-grade walls may be constructed of poured concrete, masonry blocks, or other materials such as all-weather wood or stone. This chapter discusses details for use of poured concrete and masonry foundation because these are the materials most commonly used for new construction. Recently, trade associations such as American Plywood Association (APA) and the National Forest Products Association (NFoPA) have issued publications on designing radon resistance permanent wood foundations. Information on these types of foundations can be found by contacting the appropriate trade association.21... 

Articles made from amino resins are water clear, hard, and strong, but they can crack. They have good electrical properties, and they have better colorability than phenolic resins. Amino resins are used as adhesives for plywood and particleboard but only in interior grades. They have low weather resistance and deteriorate when exposed to sun, heat, cold, and moisture. 

The most prominent wood adhesives used over the last quarter of a century have been aminoplast and polyphenolic types (2). In the United States, polyphenolic adhesives continue to be predominantly used for production of weather-resistant wood products, such as structural plywoods and flake boards (3). Phenolic resin prices have increased over the past decade, generally paralleling phenol prices. This increase has occurred in part due to a continuing erosion of United States phenol manufacturing capacity and the corresponding increase in availability of phenol from other countries. Any significant increase in the price of oil (the source of phenol) itself or interruption in supply will only compound the problem and raise phenol prices even higher. 

Effective fire-retardant treatments for wood for exterior uses under conditions of leaching and weathering have been needed for many years. For wood shingle or shake roofing, a commercial treatment system has been developed (61) in the United States that meets acceptance requirements of Underwriters Laboratories, Inc. Lumber and plywood are also available with this exterior-type treatment. 

Face checking of plywood and parallel laminates, with phenolic resin treated faces, is practically eliminated on indoor exposure. Under out-of-doors weathering conditions face checking and erosion are materially reduced (9). 

Douglas fir plywood with acetylated faces, when exposed to the weather on a test fence for two years without a surface finish developed only a slight roughening and checking whereas the controls weathered and checked badly (76). The weathering of exterior paints on panels with acetylatecT faces were considerably better than on the controls. Presurface acetylation also seemed to improve the weathering properties of painted wood (36, 37). 

Perhaps the most extensive area of research in simple pretreatment systems to enhance the weathering properties of wood has involved application of inorganic salt solutions. Much of the early work in this field was undertaken at the U.S. Forest Products Laboratory in Madison, Wisconsin and was aimed at improving the performance of transparent finishes. Black [25] described an experimental chromate-based wood finish and Black and Mraz [26] discovered that both acid and ammoniacal-copper chromate treatments significantly improved the performance of clear finishes on western red cedar, redwood (Sequoia sempervirens), and Douglas fir plywood. Later research [27] demon-... 

The weathering process described thus far has been for solid wood. The introduction of another variable, the adhesive, in the weathering of wood-based materials such as plywood and particle board creates additional complications. Wood substance is still ex-... 

Plywood undergoes many visible changes in appearance during the weathering process. These changes can be described according to the followdng sequence (72) ... 

Figure 18. Cross-sectional views of plywood illustrating the effects of weathering on face veneers with selected grain patterns.

Latewood also erodes away through weathering. Its erosion rate for most softwood species is slow (Table IV). Eventually, however, the face veneer of unprotected plywood will erode away, regardless of the grain pattern or wood species. 

The earliest wood adhesive for mass produetion of panels was phenol formaldehyde (PF). This is widely used for products designed to perform in severe weathering eonditions and dominates the exterior plywood market. Resorcinol formaldehyde is a somewhat similar resin system that cures at ambient temperatures, but the cost is much higher, limiting its use in high-volume applications. These systems, although formaldehyde-based, do not have the continuing formaldehyde emission problem that is associated with other formaldehyde-based systems. 

While the perforator method also can be used for production control of MDF-boards it is questionable weather the method is feasable for plywood and other laminated wood panels. The two tested UF-bonded plywood boards e.g. although equal perforator values shows large difference in emission. 

Kauramin . [BA AG] Melamine-formaldehyde based glues fcv prod, of weather-resistant chipboard and plywood impregnating resins for decorative and overiay papers. 

Kauresin . [BASF AG] Phenol-formaldehyde based glues for prod, of weather-resistant plywood and chipboard. 

Penetrating stains are suitable for both smooth and rough surfaces. However, their performance is markedly improved if applied to roughsawn, weathered, or rough-textured wood 4, 96, 99, 112, 136, 137, 141-144) because more material can be applied to such surfaces. They are especially effective on lumber and plywood that does not hold paint well, such as flatgrained and weathered surfaces, or dense species. Penetrating stains can be used effectively to finish such exterior surfaces as siding, trim, exposed... 

Poly(vinyl fluoride) is a highly crystalline polymer available commercially as a tough, flexible film sold under the trade name Tedlar by DuPont. It has excellent chemical resistance like other fluoropoly-mers, excellent outdoor weatherability, and good thermal stability, abrasion, and stain resistance. It maintains useful properties between -180°C to 150°C. It is used as protective coatings for materials like plywood, vinyl, hardboard, metals, and reinforced polyesters. These laminated materials find applications in aircraft interior panels, in wall covering, and in the building industry. 

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