Phenol-resorcinol-formaldehyde adhesives

PRF adhesives in which a liquid phenol-resorcinol-formaldehyde adhesive and a powder or liquid hardener are used are currently the most commonly used industrially. Pure resorcinol-formaldehyde (RF) adhesives were used extensively... 

Selection of the End-Joint Profile. Because the purpose of this work was to determine whether a portion of the phenol-resorcinol-formaldehyde adhesive used in structural end-joints could be replaced by tannin extracts from southern pine bark, it was necessary to use a structural end-joint profile design. The profile chosen was the design used by many U.S. plants for the manufacture of structural end-joints. A reproduction in natural size with a drawing showing the exact dimensions is shown in Figure 1. 

A particularly interesting system now used extensively in several southern hemisphere countries is the so-called honeymoon fast-setting, separate-application system [66,67]. In this system one of the surfaces to be mated in the joint is spread with a standard synthetic phenol-resorcinol-formaldehyde adhesive plus paraformaldehyde hardener. The second surface is spread with a 50% tannin solution at pH 12. When the two surfaces are... 

B. Adhesive 2 Phenol-Resorcinol-Formaldehyde Adhesive and Powder,... 

Phenol-formaldehyde resin is the most common adhesive for exterior applications due to its water resistance, low initial viscosity and its ability to bond various types of wood substrates [1], Because of its resemblance to phenolic moieties, studies on tannin have been oriented towards an alternative formulation to replace the current synthetic phenol-formaldehyde or phenol-resorcinol-formaldehyde adhesives [2-4], A few suitable alternative natural resources such as oil palm shell, pecan shell nut, lignin, starch, rice bran and tannin are also available for this purpose. Among these materials, tannins represent the best immediate substitute for phenol in wood adhesive production [1],... 

The deformation of the joint will normally be that of the adherends, but torsional shear of the gusset plate joints in light wooden trusses may lead to an appreciable increment in the total deflection. A phenol-resorcinol-formaldehyde adhesive joining Douglas fir adherends showed a modulus of 0-93 GN.m (Krueger, 1962). 

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. 

For exterior appHcations, where water exposure is expected, phenol—formaldehyde (PF) or phenol—resorcinol—formaldehyde (PRF) adhesives are used. Only small quantities of this type of hardwood plywood are made, primarily for marine use. 

Resorcinol-formaldehyde (RF) and phenol-resorcinol-formaldehyde (PRF) coldsetting adhesives are used primarily in the manufacture of structural, exterior-... 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

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. 

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. 

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-... 

Strands are coated with a waterproof structural adhesive, e.g. phenol-resorcinol formaldehyde, and laid-up using special equipment to ensure proper orientation and distribution. The microwave continuous press both densifies the material and cures the adhesive. LVL is commonly produced in 0.28 by 0.48 m (11 by 19 in.) section -much thicker than that of LVL. Final product can be sawn into smaller dimension, if desired, while the length is limited only by freight and handling restrictions. 

Furfural has replaced formaldehyde in phenol-resorcinol-formaldehyde resins.These formaldehyde-containing resins have been used as cold-set, exterior grade structural wood adhesives for almost half a century. The use of furfural to prepare these resins has several advantages. In addition to longer resin shelf life, when furfural is used, the emission of formaldehyde is lowered. To enhance the reaction rate when furfural is used, a small amount of formaldehyde is added to the furfural system. Hence, the furfural system is not formaldehyde-free, but nearly so. Finally, furfural has a higher molecular weight than formaldehyde. This results in a smaller amount of the expensive resorcinol being required in the system. 

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. Formulation of Phenol-Resorcinol-Formaldehyde Cold-Set Adhesives... 

Resorcinol-based adhesives are used extensively to bond structural grade, exterior laminated beams for building construction (see Chap. 29 on Resorcinol Adhesives). The coldsetting adhesives which dominate this field are based on phenol-resorcinol-formaldehyde (PRF) resins. The adhesive itself is composed of the PRF resin and a hardener that includes formaldehyde, often in the form of paraformaldehyde mixed with inert fillers. The performance of the resin is resorcinol dependent. The cost of the resin is also resorcinol dependent as this is a very expensive chemical produced industrially in only three locations in the world. The research work on these resins from their inception has then been based on the optimization of their bonding performance coupled with the decrease in the relative percentages of resorcinol used. It has the been a long and successful work of empirical research and development which still continues. 

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