Timber adhesives

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. 

Standard specification for phenolic and aminoplastic adhesives for laminating and fingerjointing of timber, and for furniture and joinery. SABS 1349-1981, South African Bureau of Standards, 1981. 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

Reference was made earlier to difficulties of wetting plastics surfaces, and to problems in this regard compared with hydrophilic materials like glass, metals, and timber. With some plastics very effective bonds are possible, but as a group, mainly for this reason, they are amongst the most difficult to join by adhesives. 

Subsequent work was, therefore, directed to substitution of resorcinol, a much more expensive phenol, in cold-setting, waterproof adhesives (38). Formulations based on 30 to 60% of extract mixed with a resorcinol-formaldehyde condensate and additional formaldehyde met pot-life and assembly time requirements for timber lamination. Test bonds passed requirements of the major performance standards in the United States, but competition from lower cost, phenol-modified resorcinol resins and the lack of longterm commercial perfor-... 

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

Although preliminary studies suggest that face-laminations can be obtained using adhesives made from condensed tannins that pass the standards of the American Institute of Timber Construction (< ), less is known about how to formulate adhesives using these materials for end-jointing of wood. This study, funded by the USDA Small Business Innovation Research Program, was undertaken to develop adhesive formulations containing 50% of sulfite extracts obtained from the bark of southern pine trees and demonstrate their use in... 

If this method proves to be commercially feasible, a renewable and abundant waste product of forest products manufacture could replace expensive petrochemicals. Additionally, use of an inexpensive waste product from processing of forest products could materially reduce adhesive costs and expand opportunities for manufacture of structural materials from low-quality wood. This benefit is particularly important since the difficulty in producing large, strong, structural members from timber resources of declining quality is growing exponentially with time. The cost-benefit ratios of replacing PRF resins with extracts from conifer barks are, therefore, quite favorable in a honeymoon system. 

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). 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

In conclusion, it is worthwhile to note that in 1970-71, when I started to be involved with tannin adhesives, there were no more than three laboratories, academic or industrial, conducting research on tannin adhesives. By 1986, I have had direct or indirect contact with 42 different laboratories in many countries that have carried out at some time, or are now carrying out, research on these bonding materials. The increase in industrial usage has also been fast but for various reasons has lagged behind the technical and scientific interest. However, after practically zero consumption in 1970, indications are that about 12,000 tons of exterior-grade tannin resin solids per year are now produced and consumed in the timber industry based on reliable information from several... 

Resin synthesis, adhesive formulation, and evaluation techniques are described by Clark et al. (3,4) Plywood shear specimens were prepared according to U.S. Product Standard PS 1-83 for exterior plywood (5). Glulam shear specimens were tested according to the American Institute of Timber Construction (AITC) standards AITC-T107 and AITC-T110 for dry shear and vacuum-pressure soak (3). Glulam test specimens were also subjected to a 2-hour boil treatment (not an AITC test) prior to shear (7,8). 13C-NMR spectra were recorded at 100.6 MHz on a Bruker AM-400 NMR spectrometer. Gel permeation chromatography... 

Hot asphalt applications had been used for many years in concrete tanks, inner lined with brick for similar service, and also, of course, unlined wood tanks made from timber, and small pickling tanks made by hollowing out cavities in granite blocks. But the use of hot asphalt as a liner for steel tanks had been unsatisfactory due to the erratic cold flow of the asphalt which demonstrated selective adhesion to steel and so would tear and open cracks in the membrane system in the areas of cold flow below the points where it adhered. To overcome this difficulty, a number of inventive persons experimented with the manufacture of asphalt sheet lining materials, similar in form to the sheets of natural rubber, in which the asphalt was compounded with various admixtures, including rubber. These asphaltic compound sheets were then warmed sufficiently to make... 

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. 

The raw material for the particle board is scrap timber that has been cut into chips, and its final properties are to a large extent due to the nature of the adhesive used to bond the chips together, it is believed that for this reason the amount of dispersion of values is relatively small. In the case of lauan plywood on the other hand, the raw material is natural wood, so the dispersion in strength is relatively large but the wood fibers give only little dispersion in elongation results. 

Improved adhesives and manufacturing techniques have allowed prefabricated I-joists to replace larger timber sizes in floor and roof applications for both residential and commercial buildings (Figure 10.19). 

Lehman WF (1986) Outline of a fast durability test for UF and PF adhesives in composite materials. In Maloney T (ed) Proceedings of the 20th Washington State University Particleboard Symposium, Pullman, Washington, 105-22 Leicester RH (1988) Timber engineering standards for tropical countries. Proceedings of 1988 International Conference of Timber Engineers, Seattle. Forest Products Society, Madison, Wisconsin, Vol. 1 177-85... 

The input of pesticides in an indoor environment may result from either a direct appUcation (fighting, e.g., insects hke flies, mosquitoes, and fleas) or pesticides used in the preservation of wood and timber, pesticides of textile finishing, and finishing of leather, carpets, fabric, etc., pesticides (mainly fimgicides) in varnishes, colors, adhesives, or pesticides brought in by foot traffic, through pets, etc. from outdoors. Pesticides then spread in indoor air and in house dust... 

The emission of formaldehyde fumes from particleboard manufactured using urea-formaldehyde resins, and its decrease, have now been topics of interest in the timber and wood adhesives industry for a long time. Many solutions, some very effective, to this problem have already been advanced by many authors. In this brief article we do not pretend to present yet another successful or less successful method to control HCHO emission but to show the decrease in the amount of formaldehyde emitted by UF-bonded particleboard, over a period of time, to which tannin extract has been added in small amounts. Tannin extract is an inexpensive commodity in Southern Africa as well as in many other countries in the southern emisphere such as Brazil, Argentina and New Zealand. The method presented, if not completely effective may be an inexpensive system of control of HCHO emission over a limited period of time. 

Timber Wood dust, adhesives, solvents, resins, asbestos, arsenic and compounds, chrome compounds, formaldehyde, isocyanates, phenol, teipenes, aldehydes, esters, ketones... 

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