Veneer panels, bonding

In an earlier paper (2), we determined that carbohydrates could replace a significant portion of the phenol-formaldehyde resin used for bonding plywood veneer. Carbohydrates from renewable resources such as wood can replace up to 50% of the phenol and formaldehyde in resins formulated under basic conditions without significant loss of bond quality. Two-ply, Douglas-fir-veneer panels bonded with these carbohydrate-modified resins have shear strengths approximately equivalent to those for panels bonded with unmodified phenol-formaldehyde resin. 

Figure 1 compares the dry- and wet-shear strengths of two-ply, Douglas-fir veneer panels bonded with a commercial phenol-formaldehyde resin (basic), a phenol-formaldehyde resin prepared in the laboratory under basic conditions, and an unmodified neutral resin prepared in the laboratory. The shear strengths obtained with these three resins served as control data for further experiments. The dry-shear strengths of panels bonded with the unmodified neutral resin are lower than those for panels bonded with the resins cured under basic conditions however, the wet-shear strengths of panels bonded with the three resins are all... 

In this study, up to about 50% of the phenol-formaldehyde was replaced with carbohydrates and the modified resins used to bond wood veneer panels. The carbohydrate modified resins were formulated and cured under neutral conditions. The resins bond wood with acceptable dry- and wet-shear strengths, and wood failures. Reducing as well as nonreducing carbohydrates can be used as modifiers. The carbohydrate modifiers are being incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and methylol groups in the phenol-formaldehyde resin. The resins formulated under neutral conditions are very light in color. 

Phenol-formaldehyde resins modified directly with reducing sugars successfully bond wood veneers at neutral conditions. The dry- and wet-shear strengths of two-ply panels bonded with xylose-modified resins are not adversely affected until the amount of xylose is increased to between 0.6 and 1.0 moles xylose per mole of phenol (Figure 2). However, even resin with 2 moles xylose per mole of... 

Veneer and solid wood composite products are manufactured for building and construction commodity markets and for specialty markets such as marine applications and aesthetic decor. Bonding systems vary based upon the service demands placed upon the final product, interior versus exterior. Construction and decorative plywood are veneered panel products. Laminated veneer lumber (LVL) is a veneer product which competes with lumber. Lumber laminates form majestic beams often for dual structural and aesthetic applications. 

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. 

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. 

Lumber core is a five-ply panel, usually about 19 mm (3/4 in.) thick, in which the bulk of the thickness, about 16 mm (5/8 in.) is edge-glued lumber. Yellow poplar and red gum are desired species for lumber core. Cross-pHes of lower value wood veneers are laid at right angles to the core grain direction, followed by two thin surface pHes of the decorative face veneer in a parallel direction to the core. This assembly is pressed and bonded to form a panel of exceptional quaHty, provided all steps are accompHshed in a desirable manner. 

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. 

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... 

Lumber banding consists of gluing lumber strips, 1/2 to 2 inches in width, on the particleboard edges. These strips are normally used in applications where the particleboard is to be covered with wood veneers. The solid wood strip can be machined to decorative edges and, with the veneer surfaces, the panel is fully as functional and attractive as a solid wood panel, but at a lower cost. The lumber bands are normally bonded to the particleboard with polyvinyl acetate or urea-formaldehyde adhesives, cured rapidly by either contact or high frequency heating. 

Wood is an important structural material consisting of a cellulosic composition with a highly porous nature. Adhesives are commonly employed to bond wood in the furniture industry. They are increasingly being used in laminating and veneering of wood-based products onto composite panels. Sealants are commonly applied to wood framing members in the construction industry. Several properties are unique to these materials that will affect their ability to be joined. 

Veneer. Rotary cut Douglas-fir veneer (3 mm thick) was conditioned to equilibrium moisture content at 27 °C and 30% relative humidity. Pieces 150 x 150 mm2 were cut and bonded into panels. 

Bonding. About 2 to 3 grams of the phenol-formaldehyde adhesive was spread as evenly as possible with a spatula on one piece of veneer, which was then dried at room temperature for 10 minutes. The coated piece of veneer was assembled into a two-ply panel with an uncoated piece of veneer with the grain in both plies parallel and held at room temperature for 15 minutes. The panel was placed into a heated press and pressed at 1 MPa (145 psi) for five minutes at 170 °C. 

Neutral resins formulated with various xylose contents were used to bond Douglas-fir veneers into two-ply panels at 170 °C as opposed to 140 °C used for the basic resins. This temperature was chosen for bonding since differential scanning calorimetry (DSC) showed that the unmodified and modified neutral resins produce a major exotherm at about this temperature, whereas, resol resins cured under basic conditions produce an exotherm at about 140 °C. 

Use Hot-press bonding of furniture veneers, premium wall paneling. 

Traditional composite panels are made from veneers and from mat-formed eomposites bonded by adhesive. More recently wood has also been combined (eompression moulded or extruded) with synthetic polymers, e.g. thermoplastic polymers, to make wood-polymer composites (WPC). WPC products have been growing very rapidly in the recent years, especially in the deeking market, where Woleott (2004) observed that their market share has grown from 2% in 1997 to 14% in 2003. Further, much research work has explored the use of fibre-reinforced polymers (FRP) to enhance the structural performance of engineered wood eomposites, ealled FRP-wood hybrid composites (Dagher et ai, 1998 Shi, 2002). 

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