U.S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE FOREST PRODUCTS LABORATORY • MADISON, WIS. In Cooperation with the University of Wiscsonsin U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0138 (Revised) OCTOBER 1968

1 SELECTION AND PROPERTIES OF WOODWORKING GLUES

By

Forest Products Laboratory, Forest Service U.S. Department of Agriculture

The great variety of types and trade-name brands of glues available to the woodworker often makes selection of the proper glue appear difficult. This brief discussion should be helpful in making the proper selection. It is intended for the small shopworker and home craftsman as well as others who may not have the benefit of technical training in the use of adhesives.

In general, woodworking glues can be divided into two main groups: (a) Glues formulated from materials of natural origin, and (b) synthetic-resin glues, developed over the past four decades by the chemical industry. Within each group, different types of glues have varying properties which determine their suitability for different products and use conditions. These properties are described briefly later in this report.

Selecting a Woodworking Glue

The choice of glue depends first of all on the service conditions intended for the glued product. These can generally be classified as (a) normally dry interior use, or (b) service under damp or wet conditions, as is more or less common under direct exposure to the weather. 1 This note summarizes and updates information originally published by the Laboratory in 1953 in three Technical Notes: No. 256, "How to Select a Woodworking Glue"; No. 257, "Woodworking Glues of Natural Origin"; and No. 258, "Synthetic Resin Glues for Wood." This information was previously summarized in a Note of the same title in June 1966. If the product will be exposed to the weather, a glue not affected by moisture (waterproof) or other severe climatic influences is required. For normal interior use (temperature and humidity not exceeding the human comfort range for appreciable or repeated periods), a waterproof bond is not required and glues with various degrees of moisture resistance are usually satisfactory, depending on such factors as product construction, wood species, and type of finish. Available plant facilities and use characteristics of the glue must, of course, also be considered.

Since these factors play very important roles in the performance of glue joints, detailed discussions of the effect of each follow:

1. Product Construction

Wood shrinks and swells with changes in moisture content--an appreciable amount across the grain but generally only an extremely small amount along the grain. These dimensional changes across the grain are almost twice as large for flat-grain stock as for vertical-grain stock. Consequently, the stresses on the glue joint between two flat-grain pieces of lumber glued together with the grain at right angles (cross-laminated) will be very large with appreciable changes in moisture content. The stresses are cut almost in half if the pieces are vertical grain; and if the grain in the two pieces is parallel (laminated construction), the stresses are generally insignificant in comparison to cross- laminated construction.

If the layers of wood are thin (veneers), such as in plywood, the stresses exerted are greatly reduced; and glue bonds can be made that will perform satisfactorily even under severe moisture changes.

As an example of the effect of veneer thickness, a flush door having 1/8-inch­ oak face veneers is more apt to develop glue failure between faces and cross- bands than one having 1/24-inch faces of oak, because the magnitude of the stresses on the joints will be greatly reduced when the thinner veneer is used.

It is very important, therefore, to design the joint so that the stresses will be minimum under changing environmental conditions. If this is not possible, a glue that suffers the least damage under continued or repeated stress must be selected.

FPL-0138 -2- 2. Wood Species

It is a well-known fact that there is a wide variation both in density and shrinkage characteristics between the different species of wood. A combination of high density and high shrinkage results in the greatest stresses on the glue joints. Conversely, a low-density species having small shrinkage develops low stresses on the joints.

A criterion for a good glue joint in which side grain is bonded to side grain is that it should be fully as strong as the wood (fail in the wood when the joint is tested). It is readily understandable that, based on this criterion, a weaker glue bond would suffice for a weak- or low-density species than for a higher density species. For example, a glue that performs well on a medium-density softwood might not be adequate for a high-density hardwood.

3. Surface Protection or Finish

Deterioration or weakening of glue bonds may be caused by a variety of factors. Exposure to moisture, heat, and shrinking and swelling stresses are all known to cause degradation of certain types of glues. A good finish (having high moisture-excluding effectness) can significantly retard moisture changes in the wood, and thus, reduce shrinking and swelling stresses. At the same time, it provides appreciable protection for the glue bonds. The benefit from good finishes or treatments in reducing the rate of moisture changes in glued wood products is often overlooked as a means of significantly prolonging the useful service life of glue joints. In selecting the proper glue, the type and durability of the finish is an important consideration.

4. Plant Facilities Available

Nearly all glues are suitable for industrial use if plants are provided with the necessary equipment. The major difference in equipment requirements for different glues involves the curing or setting of the glue joints. Some glues will set satisfactorily at ordinary room or even lower temperatures, while others require from moderate to elevated temperatures. High-temperature setting glues are common in plywood production (employing multiple-opening hot presses), particularly of the exterior softwood type. A large percentage of hardwood plywood also is hot pressed. High-frequency dielectric heating is also widely used in industry to obtain rapid cure of glue joints in wood.

FPL-0138 -3- The choice of glues for small shops or hobby work is usually more restricted than for large-scale operations because of the limited equipment available, particularly such items as hot presses and high-frequency heating equipment. But glues from the most durable to the lower ranges in resistance, that set at room temperatures, are available.

5. Use Characteristics of Glues

The temperature at which a glue sets (room, lower than room, or elevated) is an important factor in proper selection of glues. Pot life (usable life after mixing), storage life, ease of spreading, and ease of equipment cleanup are other factors that must be considered, Permissible assembly period (time between spreading and pressing) and length of time the joint must be under pressure are other factors that generally play an important role in the selection of a glue.

6. How Important is Cost?

Since prices of glues are continually changing and often depend largely on the quantities ordered, it is impractical to list exact figures. Costs increase, however, from the relatively low-cost soybean and vegetable glues to the animal, casein, urea-resin, and phenol-resin glues, to the melamine-resin glues, and finally to the straight resorcinol-resin glues, which are the most expensive on the pound basis. Cost must be considered, however, in terms of the actual cost per pound of mixed glue, and of the amount of glue spread required; that is, the cost to spread a certain joint area. Waste due to short pot life, costs of rejected material due to lack of control of the gluing process, and costs of the equipment and manpower needed must also be weighed in comparing glue costs. Production rates feasible with a particular glue must also be considered.

But the most important consideration in the selection of a glue is this: Will it give satisfactory performance for the intended use?

Glues of Natural Origin

The glues formulated from materials of natural origin include the animal, vegetable, casein, soybean, and blood glues. Properties of these glues are summarized in table 1.

FPL-0138 -4- 1. Animal glues, also called hide glues or hot glues, are probably the oldest known type of wood glue. They are made from the hides, bones, sinews, and hide fleshings of cattle. Most animal glues come in a dry form and are prepared for use by soaking in water, melted, and applied while hot. Liquid animal glues, ready to use at room temperature, are also available. Animal glues are avail­ able in different grades. The higher grades are preferred for joint work, and the lower grades are suitable for veneering. Hot animal glues develop strength first by cooling and gelling and later by drying, and are often preferred for hand spreading on irregularly shaped joints and for assembly work, as for furniture. The chief disadvantages of these glues are their low moisture resistance, the importance of temperature control in their use, and their relatively high cost.

2. Vegetable (starch) glues are usually made from cassava starch. They are sold in powder form and may be mixed cold with water and alkali, but heat is commonly applied in their preparation for use. These glues are relatively cheap, can be used cold, and remain in good working condition (long pot life), free from decomposition, for many days. The normal vegetable glues prepared from powder are extremely viscous and it is not practical to spread them by hand. Special liquid starch glues, although easier to spread, are more expensive, and their other characteristics are not well established.

Vegetable glues set relatively slowly, mainly by loss of water to the wood. In the past vegetable glues were widely used, particularly for veneering, because the time between spreading and pressing (assembly time) could be varied without affecting the quality of the joint. The use of vegetable glues is limited today because they lack moisture resistance, like the animal glues, and because they cause staining in thin veneer. These glues have not been used extensively for joint work.

3. Casein glues are made from casein curd (protein) precipitated from milk either by natural souring or by the addition of acids or enzymes. Lime and other chemical ingredients are added to the casein to prepare the glue for use. Formulas are available for the user to compound his own glue from raw casein, but it is generally more convenient to use prepared glues supplied in powder form, which require only the addition of water before use. The prepared glues are available in small retail packages. The many casein glues available include glues with long pot life but relatively low moisture resistance, and glues with good moisture resistance but a definitely limited pot life.

Casein glues have sufficient strength for either veneer or joint work, although with high density wood species the glue bonds are not as strong as the wood; that is, when joints are tested in shear, the failure is largely in the glue. They are used cold (although they may be hot pressed), and when properly mixed can

FPL-0138 -5- be spread with a brush. The moisture-resistant casein glues are intermediate in moisture resistance; they are superior to vegetable and animal glues but poorer than most synthetic-resin glues. Disadvantages of casein glues are their tendency to stain veneers, the relatively short working life of some types, and the dulling effect of the gluelines on tools. Each of these limitations may be minimized separately by special formulations, but generally at the sacrifice of some other property.

4. Soybean (vegetable protein) glues, which are similar in general composition, properties, and use characteristics to casein glues, are formulated from dried protein of soybeans from which the oil has been extracted. Soybean glues are cheaper and generally produce lower-strength joints than casein glues and are therefore mainly used for veneer gluing, primarily of the softwood species. Soybean glues are not normally suitable for handwork or small shop operations. Their moisture resistance is generally somewhat lower than that of casein glues.

5. Liquid dues include those glues of natural origin offered in ready-to-use form. Originally they were made from heads, skins, bones, and swimming bladders of fish, but more recently they have also been prepared from animal- glue bases by special treatments. These liquid glues tend to vary considerably in quality from sample to sample, but the better types produce joints comparable to those of hot animal glues. Liquid glues are more expensive than other nonresin glues and are used primarily in small-scale operations, such as assem­ bly or hobby work. They are also used in place of hot animal glues if convenience of use is important.

6. Blood glues are generally made from powdered, dried blood from meat- packing operations, usually by addition of caustic soda and lime. The process is similar to that used to make casein and soybean glues. Powdered blood is also used with soybean and other proteins in so-called "protein blend" glues to improve moisture resistance. The various blood glues are commonly used as hot-press plywood glues, primarily for interior-type softwood plywood. Blood glues are often modified by incorporating small amounts of phenol resin, or of organic preservatives to improve resistance to micro-organisms. Sometimes addition of these materials significantly reduces the pot life and makes the glue more difficult to use.

Production of an older type of blood glue involved dispersing the dry blood in ammonium hydroxide, with the addition of some lime. Small amounts of para- formaldehyde were sometimes added to improve moisture resistance. These earlier glues were sometimes referred to as "blood albumin" glues. This type is seldom used at present.

FPL-0138 -6- Synthetic-Resin Glues

Synthetic-resin glues, or simply resin glues of various types have been introduced as woodworking glues since the early 1930's, but the greatest progress in their development and acceptance on a large scale began during World War II and is still increasing. These resin glues are products of the chemical industry and originate from such raw materials as are derived from coal, air, petroleum or natural gas, and water. Although the intermediate raw materials are available, the complex production methods required for the resins and the fact that some are covered by current patents makes the small-scale manufacture of resin glues by the user ordinarily impractical.

All resin glues described here, except the conventional polyvinyl-resin emulsion glues, are thermosetting types. They undergo irreversible chemical curing reactions to produce insoluble, infusible glue films in the joint. The polyvinyl-resin emulsion glues are thermoplastic; that is, they do not undergo any chemical curing during the gluing process but remain in a reversible state and soften on subsequent heating.

Most of the thermosetting resins (such as urea-formaldehyde and phenol- formaldehyde) include formaldehyde as a major ingredient or reactant, but this term is often omitted from names of the resin glues for simplicity. The chemical curing or hardening reactions of the thermosetting resin glues, by which their joint strength is developed, are accelerated by adding catalysts to certain glues, additional formaldehyde in some form, or by increasing the glueline temperature by various methods. Any resin glue may also be modified with fillers, such as walnut-shell flour, to improve working properties (increase viscosity), and control penetration, or with extenders, such as wheat or other cereal flours, used largely to reduce glueline costs.

Some resin glues are sold in a single package ready to use or as a powder to be mixed only with water. Many others, however, must be prepared for use by mixing resin, catalyst, fillers, extenders, and water or other solvent at the time of use. In any case, the manufacturer's instructions should be followed closely.

Common types of resin glues are described here, and properties of the principal types are summarized in table 1.

1. Urea-resin dues are available in powder and liquid forms to be used with or without added catalysts, fillers, and extenders. These glues are formulated for curing at room temperature (usually considered as about 70° F.) or at

FPL-0138 -7- hot-press temperatures of 240° to 260° F. Special formulations are available for tapeless splicers, high-frequency curing, and other specific applications. The gluelines are colorless to light tan and have only a moderate dulling effect on tools.

Unextended urea-resin glues have high water and moisture resistance at normal room temperature, but are sensitive to elevated temperatures and degrade more rapidly as the temperature increases to such levels as 150° F. or higher, particularly at high-humidity conditions. These glues are generally not recommended for exterior service. Extension with cereal flours reduces their resistance to moisture and water. Resistance to elevated temperatures and exterior conditions can be somewhat improved, however, by modification with special fortifiers. When properly formulated, the unmodified urea-resin glues give high initial strength and are suitable for both veneering and joint work. They are generally not recommended where poorly fitted joints and thick gluelines are involved.

Several urea-resin glues which set at room temperature are available in small retail packages for small-scale shop work, and require only the addition of water to prepare them for use. Special liquid resins have been developed for economical tank-car or truck shipment for large-scale users, but these resins require addition of catalysts and usually also extenders before use.

2. Phenol-resin dues are normally dark-reddish liquids and require hot pressing at 260° F. or higher temperatures. Some, however, are supplied in powder form to be mixed with water or other solvent. At least one phenol- resin glue is available in film form, which eliminates mixing and spreading operations. Because they add no moisture to the glueline, film glues are particularly suitable for gluing thin, figured, fragile veneers.

Special formulations are available for curing at intermediate temperatures, and certain acid-catalyzed resins are capable of curing at temperatures as low as 75° F. for some applications. The nonacid phenol-resin glues are one of the most durable types of wood glues and, when properly used, give joints that are as durable as the wood under severe exposures. There are indications that the acid-catalyzed phenol-resin glues give joints that are not so durable at elevated temperatures as joints made with conventional phenol-resin glues, but are more durable than joints made with ordinary urea-resin glues.

Because they require high curing temperatures, phenol-resin glues are mainly suitable for plywood production. The phenol-resin glues are not ordinarily suitable for small-scale shop or hobby work.

FPL-0138 -8- 3. Resorcinol, and phenol-resorcinol resin glues are dark-reddish in color and are generally supplied as liquids to which a liquid or powdered curing agent is added before use. These glues have much the same performance character­ istics, including high durability, as the phenol-resin glues. They have the added advantage of curing sufficiently for many applications at temperatures as low as about 70° F. They are, however, the more expensive of the current Wood­ working glues.

Recent formulations of phenol-resorcinol resin glues are much lower in price than straight resorcinol-resin glues and appear to retain most of the desirable characteristics of the resorcinol resins. They are formulated for different rates of setting; the faster ones for use in cool weather and the slower setting for use in hot weather. Intermediate setting types are also available. They are all used for laminating or assembly of articles where a high degree of durability to exterior or other severe service is required. Their relatively high cost prevents their use as a veneering glue except for special applications. Several brands of resorcinol-resin glue are available in small retail packages for small-scale shop work and are of particular interest to the amateur boat builder.

4. Melamine-resin glues are available as colorless hot-press glues at a price between that of urea-resin and phenol-resin glues and that of straight resorcinol-resin glues. Although the high-temperature-setting, uncatalyzed melamine-resin glues are about as durable as the phenol- and resorcinol-resin glues on certain wood species and under certain conditions of use, their high cost and high curing temperature requirement limit use of the straight melamine resins to a few special applications. On dense woods such as oak, melamine- resin glue bonds have been found to deteriorate under long-term salt water soaking. These resins, however, are often used to fortify and thus improve the durability of urea-resin glues. Melamine-urea resin combinations lend themselves well to high-frequency curing.

5. Polyvinyl-resin emulsion glues are available in a ready-to-use liquid form that sets at room temperature to a colorless glueline. Unlike the other resin glues described, these glues do not cure by a chemical reaction, but set by losing water to the wood. They remain somewhat elastic and thermoplastic after setting, which makes their use in highly stressed joints inadvisable. The polyvinyl-resin emulsion glues do appear promising, however, for certain types of assembly joints where their greater elasticity is an advantage over the con­ ventional rigid woodworking glues, Within recent years special vinyl copolymers have been introduced. These have some thermosetting properties and greater durability than conventional polyvinyl resin emulsions.

FPL-0138 -9- 6. Other adhesives. A number of new types of adhesives, developed originally for bonding other materials than wood, are finding some application in wood gluing. Among these types are the epoxy resins, which have been found useful in certain repair applications because they do not shrink appreciably on curing (can be used in thick gluelines). A wide variety of epoxy adhesives is available. Information on their long-term durability is generally lacking.

FPL-0138 -10- 2.-10 1 Table I.--Summary of some general properties of woodworking glues

1These properties are based on average glues of each type. There are numerous exceptions for glues of each type. Two or more values for the same property indicate several types of glues are available. 2Except as noted, all types of these glues can be spread by hand as well as by machine and are applied cold to the wood at normal room temperature. 3Certain types of glues can be cured at 75° F. but will cure faster at higher temperatures. 4Will cure at lower temperatures if given sufficient time. 5Applied hot at 140° to 150° F. 6None of the synthetic-resin glues stain wood directly, although discoloration of wood surfaces from penetration of dark-colored glues may be objectionable.