Selection and Properties of Woodworking Glues
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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.