Plywood

Plywood is a panel made from wood veneers (thin shces or sheets) bonded to one another. Generahy each ply is oriented at right angles to the adjacent ply, and the two face pHes should have the grain direction parahel to each other. Thus most plywood wih have an uneven number of pHes, such as 3, 5, 7, or more. An exception to this is a four-ply constmction in which the two core pHes are oriented parahel to one another and perpendicular to the two face phes. 

Plywood has essentiahy equal stability in both panel directions and is almost as stable as the parent wood in the direction of the wood grain. Strength properties in bending are roughly proportional in each panel direction to the amount of wood in those layers closest to the surface which are parahel to the wood grain direction. Thus as the number of phes increases, these bending properties become more equalized in both panel directions. 

Fig. 1. The hardwood plywood process using sHced decorative veneers.

The primary adhesive used ia hardwood plywood is urea—formaldehyde (UF) mixed with wheat flour as an extender to improve spreadabiUty, reduce penetration, and provide dry-out resistance. A catalyst may also be added to UF resias to speed the cure or to cause the UF to cure. Scavengers also may be added to reduce formaldehyde emissions from finished panels. If more water-resistance is requited using a UF bond, small amounts of melamine maybe added, producing a melamine—urea—formaldehyde (MUF) adhesive. 

For exterior appHcations, where water exposure is expected, phenol—formaldehyde (PF) or phenol—resorcinol—formaldehyde (PRF) adhesives are used. Only small quantities of this type of hardwood plywood are made, primarily for marine use. 

Lay-up proceeds by laying down the veneer which is to be the back surface of the panel. Then a sufficient number of pieces of core veneer are passed through the glue spreader to form the next layer of cross-oriented veneer. The glue spreader commonly used in hardwood plywood manufacture is a roU coater in which a pair of opposing mbber roUs are coated with a thin layer of adhesive. As the veneer is passed between the roUs, the adhesive is transferred to the surfaces of the veneer. Adhesive is appHed only to the cross-pfles and in sufficient quantity to provide a continuous layer on both opposing faces of veneer. Thus, in the case of a three-ply panel, only the core layer is spread with adhesive and in that of a five-ply panel, the second and fourth layers both of which are cross-pfles, are spread with adhesive. Then the top surface veneer, which is normally the decorative surface, is placed on the assembly. 

Uses and Treatments of Hardwood Plywood. Most early appHcations of hardwood plywood were those where the hardwood plywood was better adapted to the use than soHd wood. One of the most important early uses was in curved or formed parts, an appHcation particularly suited to the use of veneers which could be molded into intricate shapes during the pressing and bonding process. Then, as furniture manufacturers realized the inherently superior stabiHty of plywood compared to soHd wood, lumber-core or plywood panels began to be used for most flat-panel constmctions in furniture. 

Plywood furniture core panels, also about 19 mm (3/4 in.) thick, were normally made of a number of layers of relatively thick, 1.5—3.0 mm (1 /16—1 /8 in.) lower value wood veneers combined with thin surface pHes of the decorative veneer. These assembhes were laid-up from glued veneers and then pressed while the bonding occurred. Both lumber core and plywood core have been almost totally displaced in recent years by particleboard or medium-density fiberboard, both discussed herein. This change resulted from the increasing availabiHty and improved finishing characteristics of composites and from decreasing suppHes of core lumber or veneer of suitable quaHty. 

One type of thick hardwood plywood stiU available is imported from the northern Scandinavian countries and is generally known as Finnish birch. Characteristically, these plywoods are manufactured using multiple layers of veneer of the same thickness, about 1.5 mm (1 /16 in.), and bonded with a urea—formaldehyde or melamine—urea—formaldehyde adhesive. 

Thin hardwood plywood in the range of 4.5—6.0 mm (3/16—1 /4 in.) was normally a three-ply constmction with a thin, medium-quaHty back ply, a thicker lower value core, and another thin, high quaHty decorative face veneer. These panels were used as wall paneling, door facings, or for fumiture/cabinet appHcations requiring thin panels. Currently, only relatively small quantities of these types of panels are produced in the United States. 

Economic Aspects. The hardwood and decorative plywood industry has decreased in size and production significantly in the past few years. In 1994, there were an estimated 100 mills operating in the United States having a production volume of 1.135 x 10 m (2). The doUar value of this production is extremely difficult to estimate because of the very wide range of prices for the products. 

Softwood is generally considered to be that coming from a coniferous tree, ie, an evergreen tree having needle-like or scale-like leaves. There are exceptions to the evergreen rule, however. In addition, many hardwoods also may now be used in softwood plywood as core veneers. 

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. 

There is a hybrid product available which has a veneer back, a layer of PF-coated wood particles, core veneer cross-ply, another layer of wood particles, and a top veneer. This assembly is pressed into a panel, trimmed to size, and sold into the stmctural-use panel market where it competes with plywood and oriented strand board. 

Economic Aspects. The stmctural plywood industry now has (ca 1997) about 105 operating mills, representing a significant decrease over the past several years. Production in 1994 was about 17.4 x 10 m (2), also representing a marked decrease over previous years. This decrease is a result of several factors, two of the most important being a decrease in availabiHty of suitable veneer logs, especially in the western states, and competition from the newer oriented strand board stmctural panel industries. 

Specifications, Standards, Quality Control, and Health and Safety Factors. APA-The Engineered Wood Association represents the softwood plywood and oriented strandboard industries in the areas of specification, standards, and quaHty control (QC). An APA product standard, PSl-95 (6), discusses the above areas in detail. The following listing summarizes plywood characteristics covered in PSl-95. 

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... 

Testing—includes test specimen preparation, bond durabiHty tests, and stmctural performance tests. It should be noted that formaldehyde emission tests of phenoHc bonded products such as stmctural plywood are not required because emissions are normally about 0.02—.03 pl/L (ppm), weU below the previously noted safe level of 0.10 p.L/L (ppm). 

Then, in the early 1980s the concept of OSB was realized in the constmction and operation of large-size mills. OSB is a panel product made from wood strands and somewhat like plywood in that the strands on the two faces are oriented in the long direction of the panel and the core strands are oriented in the cross-panel direction. The use of orientation yields panels having exceUent directional properties, much like plywood, and thus an exceUent and economical stmctural sheathing material is created. 

Secondary Treatment and Uses. The vast majority of OSB panels are used "as is," without further processing or treatment. Primary uses are as wall and roof sheathing, floor decking, and other constmction panel uses in home and commercial constmction. OSB products are effectively filling in for the decline in plywood production. Small amounts of OSB are used in furniture, primarily as frame stock, and in other uses in which plywood might be used. 

Economic Aspects. There were 31 OSB plants in the United States in 1994 and these produced 6.625 x 10 m of OSB products (2). This industry is growing rapidly in both Canada and the United States. In fact, many of the composite mills currently under constmction are designed to produce OSB or similar products based on strands. Outside of North America, where building practices are not yet extensively utilizing the distinct advantages of the stud wall and plywood/OSB sheathing, there are only a few operating OSB plants. There are also small export markets for OSB products in Europe and the Far East. 

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