Wood adhesives -phenolics

In 1932, the first plywood hot press was installed in the United States. This marked the advent of the large market for phenolic wood adhesives [51]. By 1962, the volume of phenolic wood adhesives had reached about 33 kt (solids) in the U.S. Growth was accelerated in 1962 with the development of Southern pine plywood. By 1979, the consumption of phenolic plywood adhesives exceeded 220 kt or about 25% of phenolic resin production [51]. Phenolic adhesive demand for wood products took another jump in 1964 with the commencement of waferboard production. The first oriented strandboard (OSB) plants were built in 1981 [52]. OSB soon replaced most of the waferboard production and began a period of... 

Before lignin sulfonates can be usefully incorporated into phenolic wood adhesives, these shortcomings must be remedied in a cost-effective way relative to the price of phenol. The remedial approach selected in this work has been to investigate the phenolysis of commercial lignin sulfonates with commercial grade phenol. 

The use of phenolic resins for wood bonding has been reviewed by Pizzi, Dinwoodie, Kollmann and coworkers, " the U.S. Department of Agriculture, and Barth. Tapered double cantilever beam specimens have been recently used to test the effect of bondline thickness and cure time on the fracture energy of phenolic-wood adhesive joints. ... 

These adhesives are suitable for exterior use and are unaffected by water (even boiling water), molds, grease, oil, and most solvents. Their applications primarily include wood, pljrwood, plastics, paper, and fiberboard. Resorcinol-formaldehydes are excellent marine-plywood adhesives. Curing at room temperature normally takes 8—12 h, whereas phenolic wood adhesives require a high-temperature cure. The adhesives are also used for indoor appli-... 

In 1993, worldwide consumption of phenoHc resins exceeded 3 x 10 t slightly less than half of the total volume was produced in the United States (73). The largest-volume appHcation is in plywood adhesives, an area that accounts for ca 49% of U.S. consumption (Table 11). During the early 1980s, the volume of this apphcation more than doubled as mills converted from urea—formaldehyde (UF) to phenol—formaldehyde adhesives because of the release of formaldehyde from UF products. Other wood bonding applications account for another 15% of the volume. The next largest-volume application is insulation material at 12%. 

Wood Bonding. This appHcation requires large volumes of phenoHc resins (5—25% by weight) for plywood, particle board, waferboard, and fiberboard. Initially, phenoHc resins were used mainly for exterior appHcations, whereas urea—formaldehyde (UF) was used for interiors. However, the concern over formaldehyde emission has caused the replacement of UF by phenol-formaldehyde adhesives. 

Clarke, M., Steiner, P.R. and Anderson, A.W., United States patent USP 4,824,896. Phenol formaldehyde adhesives for bonding wood pieces of high moisture content and composite board and veneers bonded with such adhesive. Assigned to the inventors, 1989. 

The major disadvantage associated with urea-formaldehyde adhesives as compared with the other thermosetting wood adhesives, such as phenol-formaldehyde and polymeric diisocyanates, is their lack of resistance to moist conditions, especially in combination with heat. These conditions lead to a reversal of the bond-forming reactions and the release of formaldehyde, so these resins are usually used for the manufacture of products intended for interior use only. However, even when used for interior purposes, the slow release of formaldehyde (a suspected carcinogen) from products bonded with urea-formaldehyde adhesives is observed. 

Recent work has concentrated on the use of ALCELL lignin as a substitute for phenol-formaldehyde resins in wood adhesives, particularly wafer-board. Some of the results obtained when a PF resin (Bakelite 9111) was replaced with different levels of hardwood ALCELL lignin in waferboard manufacture will be briefly discussed below. Table III shows the conditions used for waferboard manufacture. 

The most prominent wood adhesives used over the last quarter of a century have been aminoplast and polyphenolic types (2). In the United States, polyphenolic adhesives continue to be predominantly used for production of weather-resistant wood products, such as structural plywoods and flake boards (3). Phenolic resin prices have increased over the past decade, generally paralleling phenol prices. This increase has occurred in part due to a continuing erosion of United States phenol manufacturing capacity and the corresponding increase in availability of phenol from other countries. Any significant increase in the price of oil (the source of phenol) itself or interruption in supply will only compound the problem and raise phenol prices even higher. 

It is becoming apparent that wood components, especially lignin, are chemically modified by solvents during wood dissolution, and that the resulting wood tars or pastes become highly reactive. Attempts have therefore been made to prepare effective adhesives, moldable resins and other products from wood after dissolution in phenols or polyhydric alcohols. This review presents recent progress on wood dissolution, and on the preparation of epoxy and phenol resin adhesives from kraft lignin. 

Resorcinol is used primarily in the rubber industry for tyres and reinforced rubber products (conveyer belts, driving belts) and in high-quality wood adhesives, which are made from resorcinol, phenol and fonnaldehyde. It is also used in the preparation of dyes and pharmaceuticals, as a cross-linking agent for neoprene and a rubber tackifier, and in cosmetics (Lewis, 1993 Schmiedel Decker, 1993 Krumenacker et al., 1995). 

The first demonstration of the industrial importance of heme peroxidases in grafting applications has been the development of hybrid resins from renewable sources to replace phenol-formaldehyde based resins. Phenolic resins are widely used in surface coatings, adhesives, laminates, molding, friction materials, abrasives, flame retardants, carbon membranes, glass fiber laminates, fiberboards, and protein-based wood adhesives, [5]. Table 7.1 and Fig. 7.2 summarize some of the... 

It is worthwhile to review the U.S. market size for the four principal resins currently used in wood-panel products today (4 )- These are phenol-formaldehyde (PF), urea-formaldehyde (UF), melamine-formaldehyde (MF), and resorcinol-formaldehyde (RF) (Table III). When these production figures are compared to the quantities of lignin potentially available (Table II), it is immediately obvious that all wood adhesives could be replaced by only a very small fraction of the lignin produced annually during chemical woodpulping processes. 

Large volumes of wood composites are bonded with phenol-formaldehyde adhesives. The U.S. output of phenol in 1987 will likely set a record of more than 3 billion pounds, and approximately 40% of this will be used as a comonomer with formaldehyde in adhesive applications (1). 

The partial replacement of some of this phenol with lignins has long represented an apparently attractive market opportunity because the phenol used in wood adhesives is worth about 528 MM per year when phenol is 44 cents a pound, which was the August 1987 bulk price of phenol with all allowances taken into account (2). 

These calculations show that if renewable resources such as lignins are to find use as substitutes for phenol in wood adhesives, the cost differential between these materials must be maximized to attract investment capital. Clearly, the pulp lignins already being produced are likely to be the lowest cost renewable resource available for this purpose. Those generated in the kraft process are already recycled for their fuel value in the recovery furnace and are not easily accessible. There is only one U.S. company selling kraft lignin, and its lowest bulk price is 40 cents a pound (3). 

Why then are these lignin sulfonates not used as a partial replacement for phenol in phenol-formaldehyde-based wood adhesives The first reason is that the presence of the sulfonate groups confers a water sensitivity to the adhesive. This sensitivity is exacerbated by the presence of water-soluble carbohydrates. A second reason is the low reactivity of the lignin sulfonates with formaldehyde and the consequent low level of crosslink density achieved in the final adhesive. A third reason is the molecular size of some of the lignin sulfonates. Large molecular weight material cannot penetrate the cell walls of the wood to form an adhesive continuum between contiguous wood particles. 

The rapid growth of the pulp and paper industry following World War II coupled with a renewed scientific interest in utilization of bark and wood residues led to investigative programs on bark and wood tannins. The leather industry was continuing to decline in importance, so other alternatives were needed. One of these was replacement of phenol in whole or in part in phenol-formaldehyde adhesive formulations. This work progressed to the point where commercial quantities of polyphenolic extractives were made and sold for adhesive application. Excessive capacity and low petrochemically derived phenol prices in the 1960 s led to the demise of this effort in the United States (2,5). 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

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)... 

Modified Synthetic Adhesives. Phenol-formaldehyde (68) and urea-formaldehyde (69) are important synthetic adhesives. Phenol-formaldehyde adhesives (PF) find a variety of applications including bonded abrasives, foundry applications, fiber bonding, and wood bonding. Urea-formaldehyde adhesive resins (UF) are used generally to bond wood products. I will illustrate the modification of synthetic adhesives with carbohydrates using both these general types of adhesives. 

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