.. DRYING AND CONDITIONING GLUED JOINTS Revised August 1948 LIBRAF ATLANTA •KANCH • TLANTA, GEORGIA No. D475 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST S ERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin Digitized by the Internet Archive in 2013 http://archive.org/details/condiiOOunit DRYING AND CONDITIONING GLUED JOINTS In order to obtain strong joints and to prevent warping, checking, sunken joints, and other defects in the finished article, it is essential that wood after gluing be brought to the moisture content most suitable for the subsequent use of the article and that the moisture be evenly distributed throughout. The moisture content of the wood may be either increased or decreased during gluiiig, by an amount depending on (l) the process used, (2) the form and composition of the glue, (3) the amount of glue spread, and (h) the dimensions of the wood parts glued. In general, hot-press methods reduce the moisture content and room -temperature gluing increases it. Glues of high water content add more moisture to the wood than glues of low water content, and heavy spreads add more than light spreads . More water is added by the glue to a construction made of thin plies than to one of equal thickness made of thick plies. Furthermore, the percentage Increase in moisture content from a given amount of glue spread will be greater in woods of low specific gravity than in woods of high specific gravity. The older types of woodworking adhesives such as animal, vegetable, and casein glues add considerably to the moisture content of the wood, and frequently effect an uneven distribution of moisture within the construction,, The moisture added with the more recently developed synthetic -res in glues is much less and sometimes practically negligible. Table 1 illustrates the approximate effect on moisture content of gluing with several room- temperature-setting glues. The table is based on the assumption that the wood absorbs all of the water added by the glue. This is not strictly correct, since some of the glue squeezes out of the joints and some water evaporates during the pressure period. The calculated percentages, however, are reasonably close to the results obtained in actual gluing. It is evident from these values that regardless of the type of room- temperature -setting glue used, a panel consisting of plies of thin veneer throughout will require a drying period following the gluing operation, ev though the veneers were previously dried to 3 or k percent moisture content. Far less moisture will need to be removed, however, when one of the resin glues is used rather than casein, animal, or vegetable glue. Conditioning Glued Thick Stock When the moisture increase in the wood is small, as in thick lamina- tions dried to a suitable moisture content before gluing, only conditioning to a uniform moisture content is necessary. Sunken joints are common defects in the manufacture of thick edge- glued lumber glued at room temperature. They are caused by surfacing the stock too soon after gluing. The wood at the joint absorbs more water from Rept. No. Dl+75 -1- the glue than the remainder of the piece and therefore swells more. If the piece is surfaced before this excess moisture is distributed, more wood is removed along the joints than at intermediate points. Then, during subse- quent drying and conditioning, greater shrinkage occurs at the joints than elsewhere, and permanent depressions are formed. Such depressions along the glue line may show very conspicuously in the finished panel when viewed under a side light. To avoid sunken joints in edge-glued lumber 1 inch thick glued with casein, animal, or vegetable glue, it should be piled on stickers and dried for a period of 2 days in a kiln at 100° F., or for 5 to 7 days at 70° F. If the gluing is done with room- temperature- setting urea or resorcinol resins, the added moisture will be much less and a conditioning period of 1 to 2 days at 70° F. may be sufficient. Drying Plywood It is necessary to dry a part of the glue moisture from plywood and veneer panels pressed at room temperature. For example, assuming a moisture content of 3 percent in the veneer and an increase of Ik percent from the glue, the panels when removed from the clamps or press would contain about 17 percent of moisture. Such percentages are common in many types of plywood immediately after gluing, especially if made with casein, animal, or vegetable glueo For use in cabinets, in furniture, or in the interior of buildings, more than one-half of this moisture should be removed before the panels are ready to be put into the finished article. For use outdoors or in unheated buildings, plywood containing about 12 percent moisture will generally prove satisfactory. Where veneer is glued over a lumber core, the increase in moisture content of the whole panel at the time of removing the pansls from the press is not so large as with thin plywood. In thick core panels, however, the moisture from the glue is largely confined to the out- aide of the core and to the veneer. Therefore, the excess moisture of these parts is as great as in thin plywood and must be dried out or allowed to equalize through the core. The moisture content of panels made with lumber core is also affected by the type of glue used, but to a less extent than those made of thin veneers throughout. Table 2 shows the amount of moisture added by the glue to two particular lumber core panel constructions . This table also shows that the moisture added by the resin glues is considerably less than the amount added with casein, animal, or vegetable glue, and this fact must be taken into consideration both in drying the core lumber and in conditioning the glued panels. If, for instance, the core in the first panel listed in table 2 is dried to 5 to 6 percent moisture content and a urea or resorcinol glue is used, final drying will not be required if an opportunity is afforded for the moisture to become equalized among the plies. If thick cores ( 1-1/2 inches) are dried to a low moisture content before gluing, the water added in gluing the veneer onto the core may not bring the whole panel above 7 or 8 percent moisture content, even when casein, animal, or vegetable glues are used. Under such a circumstance, the panels are sometimes stacked solid in piles and allowed to condition. This Kept. No. D475 -2- practice requires a long conditioning period, and the absorption of moisture by the core after the cross bands have been glued to it subjects the whole panel to severe stresses. A better and rapidly increasing practice for conditioning thick-core panels that contain excessive moisture, is to place the panels on stickers and allow them to dry in panel kilns or in factory workrooms., This practice allows the excess moisture to be dried from the panel faces, where it is largely concentrated, and does not necessitate drying the thick-core stock to an extremely low mo: sture content before gluing. Panel kilns permit more rapid drying than factory workrooms, give a better means for controlling the conditions during drying, and save factory space. In panel kilns it is very easy to dry most three-ply and five- ply panels satisfactorily in 2k hours. Thick stock and low drying temperatures increase the required drying time. Results of tests at "Che Forest Products Laboratory in panel kilns show that under normal conditions the moisture \dded in gLuing three-ply panels, 3/l6 of an inch thick, can be dried out satisfactorily in from 8 to 16 hours. These tests also indicate that the desired essentials in drying can be met by maintaining a constant temperature and relative humidity throughout the drying. To save time in such kiln operations, it is advantageous to maintain conditions that correspond to a moisture content slightly below that to which the panels are to be dried. Table 3 shows several combinations of temperatures and relative humidities that will bring the stock to approximately the desired moisture content, but that will not allow an appreciable amount of drying beyond this point. Panels are usually open-piled on strips called stickers. The stickers should be made from dry, straight-grained wood, entirely free fro ; .ain or decay. Moreover, the stickers should be dressed to a uniform thickners. Seven-eighths by l-l/4-inch stickers should be used in drying the usual run of panels . In loading a kiln truck, stickers should be placed at the extreme ends of ^he panel and the intervening space so divided that the distance between stickers will not exceed 18 inches. Where there is danger of warping, the stickers may be spaced a foot apart. It is important that the stickers in each tier be placed in vertical alignment on solid foundations to prevent panels from sagging. The possibility of warping in the upper panels may be further reduced by placing a cover board on stickers on top of the pile. Sometimes the piles are weighted, but experiments indicate the application of pressure to panels during drying does not reduce warp as much as commonly thought. Whenever practicable, plywood should be so piled as to provide flues from the top to the bottom, of the load in order that air may readi mov<^ in a vertical plane through it. Drying panels to an excessively low moisture content materially in- creases warping, checking, opening of joints, and other defects. Tests show that the amount of warping on three-ply veneer panels is approximately pro- portional to the percentage of moisture removed from the panel in drying, Rept. No. DV75 -3- In a few instances, plywood has been dried on mechanical veneer driers and on hot-plate presses. These methods, however, have been confined to ply- wood of a high moisture content that was glued with water-resistant glue. Plywood dried in this way is usually comparatively thin and not of the highest quality. The use of mechanical driers and hot-plate presses results in quick drying, but involves more expensive equipment than the other methods „ Plywood and other members glued on hot presses commonly contain some ■: or 3 percent moisture when removed from the press. Such material should be conditioned to about 8 percent if intended for interior use and to about 12 percent if made for exterior service. This may be done in conditioning rooms in which a relative humidity is maintained that is approximately equal to or slightly in excess of that corresponding to the desired moisture content. Another method is to apply sufficient water to the hot-pressed panels to bring them to the required moisture content and then to stack them solidly, allowing the moisture to equalize throughout. Care should be used to apply only sufficient water to bring the panels to the desired moisture content. The correct amount of water can be readily calculated after determining the moisture content and weight of the dry panels. The moisture is conveniently applied by passing the panels between water-covered rolls, such as in a glue spreader, or by spraying. By weighing a number of panels before and after the application of the water, the amount and uniformity of the application can be checked. The time required for equalization in the solid piles again varies with the thickness of the individual panels. While the panels are usually warm when the water is applied, a circumstance that aids equalization, the glue lines, especially of synthetic -res in glues, retard diffusion. Con- ditioning periods for plywood of different thickness and number of plies should be based on actual moisture- content determinations of both the interior and exterior plies . Kept, No. DV75 -k- Table 1.- - Calculated percentages— of moisture added to vood in gluing five-ply constructions with room- temperature-setting glues Thickness . o : Moisture added in gluing.r with Species of : each ply Casein, : Room- temperature- Room-temperature- : or animal, or setting urea- setting resorcinol- lamination vegetable: glues resin glues resin glues : Inch Percent : Percent : Percent Yellow-poplar 1/32 62.0 : 23.6 : 17.4 Yellow- poplar : 1/16 : 31.0 11.8 ; 8.7 Yellow birch 1/32 : . 1+0.0 : 15.1 ii„i Yellow birch ! 1/16 : 20.0 I 7.6 i 5*6 Sitka spruce ; i/8 ; 15.5 : 5.9 ; 4.3 Sitka spruce : 3/8 : 5.2 : 2.0 : 1.5 Sitka spruce ! 3A 2.6 : 1.0 0.7 Yellow birch : 1/8 : 10.0 : 3.8 2.8 Yellow birch I 3/8 3-3 : 1.3 0.9 Yellow birch 3A 1.7 ! O06 : 0.5 -Calculated percentages are based on oven-dry weight of wood and volume •it 12 percent. In the calculations it is assumed that all the surplus solvent added by the glue is absorbed by the wood. This assumption is known to be somewhat in error, but it nevertheless affords a satis- factory basis for comparison. 2 —Spreads of 75 pounds of wet casein, animal, or vegetable glue and k"J pounds of wet area- and resorcinol-resin glues per 1,000 square feet of single glue line are assumed in these calculations . It is assumed that the casein, animal, or vegetable glue is mixed one part dry glue to two parts of water (solids content 33 percent), the cold-setting urea resin one part dry glue to O.65 parts of water (solids content 60 percent), and that the mixed resorcinol-resin glue has a solids content of 70 percent. Rept. No. DJ+75 Table 2 . - -Calculated percentages— of moisture added to wood in gluing five- ply lumber core panels with room- temperature-setting glues Face : Cross : and : bands back : : Core : Moisture added in gluingf: with Casein, : animal, . : or : vegetable. : glues : Room-temperature - : setting urea- : resin glues : Room- temp erature- setting re sore in ol- resin glues : Percent . Percent : Percent l/28- inch: 1/20- inch black : sweetgum walnut ; 15/16 -inch : sweetgum 7.0 : 2.6 2.0 1/20- inch :l/20- inch mahogany: yellow- or : poplar sweetgum: :5/8-inch : ches tnut 10.7 : 4.0 1 5-0 —Calculated percentages are based on oven-dry weight of wood and volume at 12 percent. In the calculations it is assumed that all the surplus solvent added by the glue is absorbed by the wood. This assumption is known to be somewhat in error, but it nevertheless affords a satisfactory basis for comparison, o —Spreads of 75 pounds of wet casein, animal, or vegetable glue and ^7 pounds of wet urea and resorcinol -resin glues per 1,000 square feet of single glue line are assumed in these calculations. It is assumed that the casein- animal, or vegetable glue is mixed one part dry glue to two parts of water (solids content 35 percent), the cold setting urea resin one part dry glue to O.65 parts of water (solids content 60 percent), and that the mixed resorcinol -res in glue has a solids content of 70 percent. Sept. No. Dl+75 Table 3 • - - Combinations of temperatures and relative humidities suitable for "" "drying plywood panels to moisture -content values of 6 to 12 percent, inclusive Moisture Percentage of relative humidity for use with stated temperature— content : desired (percent) 70° F. ' 80° F. I 90° f. ; 100° F. : not f. ; 120° F. : 140° F. 6 ' 19 19 20 : 2i : 22 : 2k | 26 7 2k ! 26 i 27 ' 28 '. 29 : 31 34 8 30 31 32 1 33 : 35 : 37 : 1*1 10 hi kk : *5 k6 \ 1+8 : 50 53 12 55 56 57 ! 58 i 59 61 65 —The relative humidities shown for the lower temperatures and moisture- content values are obtainable ordinarily only during the winter season, Where a low moisture content is necessary during warm, humid weather, it can be obtained by raising the temperature. Kept. No. DV75 UNIVERSITY OF FLORIDA 3 1262 08927 9201