U. S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE • FOREST PRODUCTS LABORATORY • MADlSON, WIS. 

U.S. FOREST SERVICE 

RESEARCH NOTE 

FPL-082 

December 1964 

ADHESIVES FOR BONDING WOOD TO METAL1 

Forest Products Laboratory, Forest Service 

U.S. Department of Agriculture 

During recent years considerable research and development work have been 

done on high-strength, durable adhesives for use in bonding metals. Much of 

this is also applicable for gluing wood to metal. The woodworking adhesives, 

such as animal, vegetable, casein, urea resin, phenol resin, and resorcinol 

resin, have not been found entirely suitable for this purpose. They usually are 

not elastic enough to distribute the stresses encountered in service when two 

dissimilar materials, such as wood and metal, are bonded together. Also, they 

generally do not adhere well to the surfaces of metals. Therefore, more elastic 

materials, such as vinyl resins and natural and synthetic rubbers, have been 

used in combination with such thermosetting resins as phenol-formaldehyde to 

produce adhesive formulations that have the proper elasticity and adhere well 

to both wood and metals. 

A number of these adhesives are on the market. Many fabricators of wood 

parts have used them for combining wood and metal to utilize the best properties 

of both materials. In general, the results obtained have been good. Some 

fabricators, however, have been cautious about designing parts where wood-tometal bonds are required, believing that the use of wood-metal adhesives is 

1

This Note is a revision of Forest Products Laboratory Report TP-120, of the same title, revised 

in 1962. 
difficult, or that the results obtained with these adhesives are often erratic. 

This short review describes briefly some of the types of wood-metal adhesive 

processes that are available. 

Types of Wood-Metal Adhesives 

A number of adhesives have been developed that are reported to be suitable 

for bonding wood to metal. They vary from types formulated for ease of use at 

room temperature, for quick setting under field conditions, and for low cost, to 

those formulated for use where highest strengths are required in the joint and 

where joints must be resistant to elevated temperatures, high humidities, salt 

sprays, and general outdoor service conditions. Generally, it has been impossible 

to incorporate all desirable properties in a single adhesive formulation, and 

thus each adhesive has its own distinct advantages and limitations. 

Work on the bonding of metal to wood at the Forest Products Laboratory has 

been directed primarily to aircraft applications and has been conducted largely 

in cooperation with the armed services. For this reason, information available 

at this Laboratory involves mainly those adhesives that have been developed to 

produce high-strength durable joints. Consequently, this report is largely 

limited to this type of adhesive. 

It is difficult to classify adhesives used for bonding wood to metal into the 

various chemical types, as has been done for the adhesives used in gluing wood to 

wood. The chemical composition of these wood-metal adhesives is more complex. 

Therefore, these high-strength metal-bonding adhesives have been broadly 

classified according to the curing temperatures and techniques by which they are 

used rather than by chemical types. At this time, the three main groups of woodmetal adhesives to be discussed are: (1) the direct-bonding, high-temperaturesetting adhesives, '(2) the two step-bonding systems with both an adhesive 

primer and a secondary adhesive, and (3) the direct-bonding, room-temperaturesetting adhesives. 

Direct-Bonding, High-TemperatureSetting Processes (One-StageBonding) 

A large group of the metal-bonding adhesives is included in the directbonding type that requires curing the bond under pressure at temperatures 

ranging from 250° to 350° F. Adhesives of this group are usually composed of 

at least two resins--one a thermoplastic resin or elastomer (such as polyvinyl 

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formal, ployvinyl butyral, polyvinyl acetate, a polyamide, or a synthetic rubber) 

and the other a thermosetting resin (such as phenol formaldehyde). These resins 

are combined in solvents in such proportions as will provide the necessary 

characteristics of adhesion to metal, elasticity, flow during cure, heat resistance, 

and durability of the resultant bonds. In some instances, the two resin ingredients 

may be applied separately. For example, a liquid resin may first be applied to 

the surfaces and then a powdered resin is sprinkled into the wet adhesive spread. 

In addition to the liquid adhesives for direct bonding to metals, a number of 

film or tape adhesives are now available. 

In bonding wood to metal with the direct-bonding, high-temperature-setting 

adhesives, high stresses are developed as the bonds are cooled because of the 

different coefficients of thermal expansion of the two materials. Stresses are 

also developed when the wood expands as it regains moisture. These high 

stresses tend to distort the metal-wood construction and may even rupture the 

bond if the adhesive is not sufficiently elastic and strong. In hot pressing, 

blistering of the bonds can also occur. This blistering is caused by the tendency 

of the entrapped adhesive solvents to escape. This process does not produce 

stable, flat panels if metal is bonded to only one surface of the wood. 

Two-Stage Bonding 

To avoid the limitations of the direct-bonding, high-temperature-setting 

adhesive, as well as to avoid the need for hot-press equipment, a two-step 

adhesive process was developed. In this process a priming adhesive is applied 

to the metal in much the same way as a paint finish might be applied. Many of 

the high-temperature-setting, direct-bonding adhesives may be used as primers. 

The primer is cured on the metal without pressure at elevated temperatures in 

an oven, on the platens of a hot press, or under infrared heating. The cured 

primer is then bonded to the wood under pressure with room- or intermediatetemperature-setting urea-, resorcinol-, or phenol-resin adhesives of the 

woodworking type. 

Room-Temperature Bonding Systems 

lone-Stage Bonding) 

The third kind of metal-bonding adhesives includes those adhesives capable 

of forming direct bonds to metals at room temperatures. At the present stage 

of development, joints of metal to wood made with some of the available roomtemperature-setting adhesives do not have the overall strength at room and 

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elevated temperatures, and after soaking in water and aircraft fluids, of those 

made with the hot-setting and two-step adhesives. The casein-rubber-latex 

adhesives have been used successfully at room temperatures for bonding wood 

to metal for those applications that are not so critically stressed as aircraft 

construction. However, these adhesives usually have the disadvantage of losing 

a large part of their strength when the bonds become wet or are exposed to high 

humidity. 

There are a number of mastic and solvent-type adhesives for bonding wood to 

metal at room temperature. These are mainly rubber-base adhesives of which 

the neoprene-base “contact” cements are most prominent. These liquid adhesives 

are commonly applied to both the wood and metal surfaces to be joined and are 

then air dried or force dried under infrared or oven heating conditions to drive 

off most of the solvent. The two surfaces are then carefully assembled and 

pressed momentarily either by tapping or rolling. The nip-roll bonding presses 

are particularly useful here. These roll presses provide a rather high instantaneous pressure as the assembled panel passes through. The bond continues to 

develop further strength for several weeks at room temperature as residual 

solvent diffuses from the glueline. These contact cements develop a generally 

lower level of strength than do the one- and two-stage processes mentioned 

previously. This level may be adequate for many nonstructural joints, as �or 

metal faces on plywood. Their durability characteristics have not yet been 

adequately established. 

The epoxy-resin adhesives, originally developed for metal-to-metal bonding, 

can be used for bonding wood to metal at room temperature. These are liquid 

resins, to which curing agents are added just before use. These adhesives 

typically are exothermic and have a short pot life but under the proper conditions 

can be used for bonding panels of reasonable size. These adhesives appear to 

produce joints, under the proper bonding conditions, that are as strong as the 

wood initially, Their water resistance and durability have not yet been adequately 

investigated in metal-to-wood joints. 

General Comments 

As in gluing wood to wood, the choice of the proper adhesive and the adequate 

control of gluing conditions are equally important to assure uniformly satisfactory joint quality. Since different adhesives for bonding wood to metal vary widely 

in their use conditions, it is difficult to generalize. The manufacturer’s instructions should be followed closely for each adhesive. Of special importance is the 

preparation of the metal surfaces for bonding. This may vary from a simple 

degreasing and cleaning with organic solvents to rather elaborate chemical treatments, depending on the type of metal, type of adhesive, and the level of strength 

and durability required of the joint. More specific recommendations on metal 

preparation for bonding should be obtained from the adhesive manufacturer. 

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