Laminated timber

Resorcinol is to phenol as melamine is to urea. Resorcinol—formaldehyde (RF) is very expensive, produces dark and waterproof gluelines, but will cure at room temperature. As with melamine and urea, resorcinol is often combined with phenol to produce phenol—resorcinol—formaldehyde (PRF) adhesives, thus producing an exceUent adhesive with some of the economy of phenol. These adhesives are the mainstay of the laminated timber industry which generally requites a room-temperature cure with durable, waterproof gluelines. 

In order to establish control values for the adhesives formulated using tannins, the initial work was done with phenol-resorcinol-formaldehyde (PRF) or resorcinol-formaldehyde (RF) resins on both surfaces, but modified for the honeymoon principle. The PRF resin chosen for this work was Borden s resin LT-75 with Borden s hardener FM-260. The RF resin used for a comparison was Chembond s RF-900. These resins have been used for wood gluing in the United States for more than two decades, especially for the manufacture of structural laminated timbers. 

The reactivity of resorcinol with formaldehyde is essential for developing the cohesive strength of the interlayer and its bonding characteristics. Condensed tannins are known to be very reactive with formaldehyde (7-0), so these renewable phenolic polymers are good candidates as resorcinol replacements. Indeed, condensed tannins from wattle and pine bark extracts have been successfully used in cold-setting, wood-laminating adhesives, and the former are used extensively in the commercial production of laminated timbers in South Africa (Pizzi, A., National Timber Research Institute, Pretoria, South Africa, personal communication, 1982) (10-13). 

The variety of wood products has increased enormously over the past decades as more adhesives have been found for bonding. The most important products in terms of volume are plywood, particle board, and fiber board. But adhesively bonded products range from tiny articles of jewelry to giant laminated timbers spanning hundreds of feet (5). The modification of adhesive properties to suit the different application requirements requires a sound understanding of the basic chemistry of adhesives. 

Glued-laminated timber (glulam) is an engineered stress-rated produet that eonsists of two or more layers of timber in whieh the grain of all layers is oriented parallel to the length of the timber. 

Various countries have standard practices established to determine allowable properties for glued-laminated timber. In the United States, ASTM D3737 (ASTM, 2005g) guides the allowable stress development. 

AITC (1990) Standard for preservative treatment of structural glued-laminated timber. AITC 109. American Institute of Timber Construction, Englewood, Colorado AITC (2001) Standard appearance grades for structural glued-laminated timber. AITC 110. 

Anonymous (1996) Best Management Practices for the Use of Treated Wood in Aquatic Environments. USA version. Western Wood Preservers Institute, Vancouver, BC ANSI/AITC (2002) Structural glued laminated timber. ANSEAITC A190-1. American National Standards Institute and American Institute of Timber Construction, Englewood, Colorado... 

In Canada, PCP is used primarily to treat wood poles, piles, bridge timbers, exterior laminated timbers, bridge decking, and fence posts. 

Crystic Fireguard. [Scott Bader] Thermoset polyester resin surface coating resin giving fire protection to GRP laminates, timber, porous substrates. 

CEN/TC 124 Timber Structures , SN EN 408 2003 Timber structures — construction timber and glued laminated timber — Determination of some physical and mechanical properties, SIA Schweizerischer Ingenieur- und Architektenverein Zurich, Switzerland (2003). 

EPI, Emulsion Polymer Isocyanate, MDI, pMDI, wood adhesives, solid wood panel, parquet, window frame, furniture, plywood, finger joint, glulam beam, I-betim, cross-laminated timber... 

In Europe the term multilayer board includes cross-laminated timber elements used for construction parts in buildings and shuttering boards. Both these types of boards are mainly produced with MUF adhesives in gluing processes with relatively long pressing times or in processes with high temperatures. Polyurethane (PUR) adhesives are already in use as an alternative to MUF adhesives for production of building elements whereas EPI adhesives are in the introduction phase. 

Building elements (cross-laminated timber) require moisture resistant gluelines. The elements are large and consist of many layers of wood (Fig. 9). Thus, it is very energy consuming to produce these constructions by hot pressing. The elements also contain a large amount of adhesive and the formaldehyde emission has to be taken into account when MUF adhesives are in use. This is no concern when EPI and PUR adhesives are used. 

Since an EPI adhesive was first developed in Japan, the Japanese were the first to include EPI adhesives in their standards. The Japanese Industrial Standard (JIS) K 6806-1985 [7] deals with Water Based Polymer — Isocyanate Adhesives for Wood . The Standard describes the quality of the adhesive, the performance properties and the requirements which should be fulfilled to be classified as Class 1 or 2 or used for laminate timber, furniture, decorative veneer, or plywood. 

On their own, adhesives have been used as an attempt to repair minor delamination of glued laminated timber or drying fissures (Fig. 2). Besides safety considerations, fissures and delamination may contribute to increased risk of biological attack, therefore, it may be advisable to have them sealed for the sake of durability. 

One particular structural application of adhesives is in the manufacture of glued laminated timber or glulam members. This is normally manufactured by glueing together at least four planks of timber with their grain essentially parallel. The laminations are usually machined from 38 or 50 mm thick timber although thinner sections may be necessary in the production of curved members. End... 

Useful mono-material structural elements may be formed by bonding together a number of individual pieces of the same material. Glued laminated timber (glulam) members as described in Chapter 7 represent a development in which structural elements of large cross-section may be created. Adhesive bonding, as opposed to mechanical fastening, leads to reduced stress concentrations as well as the elimination of joint slip. The same principle may be applied to other materials where appropriate, such as aluminium extrusions and... 

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