Plywood grades

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in stmctural appHcations. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

Classification—a review of exposure durabiHty in terms of interior and exterior grades of plywood... 

Plywood requirements—includes wood species used, synthetic repair requirements, veneer grades, veneer layers and thicknesses, panel grades with respect to end-use, adhesive bond requirements, panel constmetion and workmanship, scarf and finger-jointed panels, dimensional tolerances, moisture content, and packaging and loading... 

Common grades of laminates tend to be thin materials ranging from 0.5—1.5 mm in thickness, therefore for most appHcations they must be supported. In the manufacture of furniture, cabinetry, and countertops the laminates are bonded to particle board or plywood. Since the laminates consist largely of ceUulosic paper, their dimensional stabiHty is similar to wood, particularly to particle board. 

Plywood is also divided into softwood and hardwood plywood products. The classification depends on the type of wood the face pHes are made from. The principal appHcation for 1 cm and thicker hardwood plywoods are cabinets and furniture. The thinner grades are used to make paneling and doorskins, which represent approximately 56% of the total hardwood plywood products. The total market in 1989 was estimated to be 2.1 biUion. 

Phenol, in its various purity grades, is used for phenol—formaldehyde resins to bond constmction materials like plywood and composition board (40% of the phenol produced), for the bisphenol A employed in making epoxy resins (qv) and polycarbonate (qv) (30%), and for caprolactam (qv), the starting material for nylon-6 (20%). Minor amounts ate used for alkylphenols (qv) and pharmaceuticals (10). 

In other parts of the world, plywood adhesive fillers are obtained from local sources and may be quite different than those used in North America. In Southeast Asia, banana flour is quite important. In Europe, calcium carbonate (chalk) is often used. Nearly any fibrous material or fine particulate material capable of forming a functionally stable suspension can be made to work if the formulator is sufficiently skillful. However, the mix formulator will be very specific about the type and grade of filler to be used in a particular mix. Substitutions may lead to serious gluing problems. 

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... 

Other applications for phenolics are switchgears, handles, and appliance parts, such as washing machine agitators (that s why they re usually black). Phenolics are widely used to bond plywood, particularly exterior and marine grades. Although urea-formaldehyde resins are cheaper for this purpose, they were not nearly as water-resistant and have been limited to interior grades. Abrasive wheels and brake linings also are bonded with phenolic adhesives. 

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. 

Lower formaldehyde emissions were found for the plywoods (all exterior grades) than for particleboard and MDF, but organic acid emissions still occurred, especially for the hardwood-based plywood (black butt) since such woods are... 

In the hot pressing of exterior grade plywood, phenolic glues behave quite differently. Veneers must be dried to less than 5% moisture content and glueline temperatures must be 100 to 150°C. 

Attempts to make adhesive formulations by direct reaction of formaldehyde or its equivalent resulted in products that were excessively viscous, and the working time was too short for commercial application (57). It was concluded that formaldehyde, although readily reactive with the tannin molecule, provided much too short linkages to connect the bulky tannin molecules. This problem was circumvented by the preparation of a polymethylolphenol reagent that, when put in solution with the bark extract, formed a combination that was stable for several weeks at room temperature. When heated, the polymethylolphenol and bark extract reacted rapidly to form an infusible resin. Commercial trials were made to produce exterior-grade Douglas-fir plywood. Widespread use of the extracts for this purpose, however, was inhibited by a drop in the price of phenol below what the bark extracts could be manufactured for. (The best extract for adhesive purposes was an ammonia extract of hemlock bark converted to a sodium derivative prior to spray drying, a more costly extraction procedure than simple sodium hydroxide extraction of bark.)... 

Wattle tannin resins are also used to manufacture other resins, such as foams comparable to phenolics, as waterproofing additives, and binders for corrugated cardboard or charcoal briquettes. This discussion, however, deals only with particleboard, plywood, glulam, and finger-jointing exterior-grade wood adhesives. Formulations of the adhesives will be mentioned ad hoc, if at all necessary, as they have already been extensively discussed in articles and reviews in the relevant literature.(7)... 

This type of system may be used as a bonding agent for marine-grade plywood. Other applications are for laminated wood beams and wood doors. 

Techniques to deal with the cracks, instability and other problems caused by this dry condition have been developed in recent years by wood tank manufacturers. One procedure is the application of marine grade plywood sheets, with adequate nailing, to the entire inside surface of the wood tank. This may not be required when properly designed prefabricated PVC bag liners are to be installed, and such design may be used for other types of membranes. 

When selecting the wood, there are choices to be made to achieve a suitable and economical tank for the specified service. Some species are better than others for specific conditions. The grade, or quality, of the lumber chosen will affect the cost a lower grade can be considered if the tank is to be lined with plywood and a membrane. The thickness of the lumber varies with the tank size, but the judgement of the manufacturer, based on previous experience, may be influenced by such operating conditions as excessive agitation, super-imposed loads from machinery, etc. 

Melurac [Cytec]. TM for urea-melamine-formaldehyde condensation products used mainly as adhesives for bonding of veneers for the production of exterior-grade plywood. 

Hardwoods are used for plywood, for structural applications including pallets, in furniture and in the remanufaeturing sector. It is important not to discount the lower grade pallet and eontainer market as this sector absorbs about half of all US... 

Traditionally plywood has required a much higher grade of log than was necessary for the manufacture of other wood panels, so those nations with an unsuitable wood supply have had to import plywood although now there are manufacturing construction grades that use a poorer and smaller log type. 

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