Phenol formaldehyde adhesives

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. 

Using this bending-creep test on aspen flakeboards, control boards made with phenol-formaldehyde adhesive failed in an average of 71 days with T. palustris and 212 days with T. versicolor (4l). 

Flakeboards bonded with 5 percent phenol-formaldehyde adhesive. 

The disadvantages of the urea-formaldehyde adhesives lie in their lack of durability and in their characteristic pungent formaldehyde odor. For particleboard applications subject to high temperature and moisture exposure, phenol-formaldehyde adhesives are required, since the urea-formaldehyde polymer is hydrolyzable and hydrolysis is enhanced with moisture and heat. 

Phenol-formaldehyde adhesives are the only other adhesive system used in significant quantity in particleboard production. The increased durability of this class over that of the ureas results in phenolics as the adhesive of choice for exterior particleboard. However, phenolic adhesives are only used where the additional durability is required since they are more expensive and require longer curing times. 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

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 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). 

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. 

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. 

Several chapters also demonstrate the use of smaller molecular-weight carbohydrates (i.e., monomers) in adhesives. Tony Conner and his colleagues (Chapter 25) explore the partial replacement of phenol-formaldehyde adhesives used to bond wood with various wood-derived carbohydrates. A1 Christiansen (Chapter 26) and Joe Karchesy and his coworkers (Chapter 27) investigate the very complicated chemistry and the practical application of adhesives based on the reaction of a carbohydrate with urea and phenol. Tito Viswanathan (Chapter 28) describes his attempts to utilize a very large carbohydrate waste stream, whey permeates from the processing of cheese, for the production of wood adhesives. 

An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formalde-hyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resorcinol-formal-... 

In early operations LVL was made in multidaylight presses using 3.2 to 2.5 mm (1/8 to 1/10 in.) veneer, which were hot pressed with phenol-formaldehyde adhesive into lengths from 2.4 to 18.3 m (8 to 60 ft) or more. 

Lines of demarcation between hardwood plywood, softwood plywood and certain other wood based panel products have become less distinct in recent years. One of the most important distinctions in respect to formaldehyde emission potential is that softwood plywood is typically bonded with phenol-formaldehyde while hardwood plywood is typically bonded with urea-formaldehyde. Phenol-formaldehyde adhesives are more stable and have less tendency to emit formaldehyde than do urea-formaldehyde adhesives. Some important features of hardwood plywood ... 

Formaldehyde Release from Wood Panel Products Bonded with Phenol Formaldehyde Adhesives... 

Both the published literature and previously unpublished information obtained by the structural panel industry indicate that formaldehyde levels associated with panel products glued with phenol formaldehyde adhesives are extremely low. Large dynamic chamber tests which simulate conditions that might be found in tightly sealed residences indicate consistently that formaldehyde levels associated with freshly manufactured phenolic panel products are less than 0.1 parts per million. The data, as well as theoretical considerations, also indicate that the amount of formaldehyde contributed to the environment by phenolic panel products should rapidly approach zero as the small quantity of formaldehyde initially present in the products is released. 

Virtually all wood panel products such as plywood and particleboard are manufactured using either urea formaldehyde or phenol formaldehyde adhesives. Urea formaldehyde adhesives are used in hardwood plywood and in certain types of particleboards. These adhesives are not waterproof, and products made with them are normally used indoors for paneling, furniture, shelving and floor underlayment. 

EMERY Wood Panel Products Bonded w ith Phenol Formaldehyde Adhesives 31... 

Phenolic and Other Tar Acid Resins Adhesives. Annual figures for phenolic adhesives are disclosed in 1947 3) and in 1949 and 1950 (11), Figures for phenol-formaldehyde adhesives are disclosed ifrom 1943 through 1946 and annual resorcinol-formaldehyde adhesives are disclosed in 1945 and 1946 11). Monthly figures on phenolic adhesives are available from June 1945 to the present (4, 9). 

A major use of phenol is in phenol-formaldehyde adhesives for wood. Tannins are polyphenols from plants, as in bark,46 that may be able to replace phenol in such adhesives. A typical repeating unit is shown in (12.18). 

An industrialist wants to set up a phenol-formaldehyde adhesive plant. He has approached you with the following phenolic compounds. 

---