• 
 
 
 
 
 FOREST PRODUCTS LABORATORY 
 
 LREA-RLASTICIZED WOOD 
 
 (LRAEECy) 
 
 Revised August 1943 
 
 LIE 
 
 •• • 
 
 i, cii 
 
 
 U.S. DtrOSi lOr'K 
 
 No. R1277 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 FOREST SERVICE 
 FOREST PRODUCTS LABORATORY 
 
 Madison, Wisconsin 
 
 In Cooperation with the University of Wi«con*in 
 
FORE ST PRODUCTS LABORATORY UREA- PL AS TIC I Z ED WOOD 
 
 (UR ALLOY) 
 
 Discovery of the wood plasticizing properties of urea, a low-cost chemical 
 now produced synthetically, has opened a new field of research at the 
 Forest Products Laboratory. Two of the earliest lines of experimentation 
 followed in developing the possibilities of the new treatment are described 
 in this mimeograph. Simple impregnation of wood with urea, followed by 
 drying and bending with the aid of heat, comprises one branch of investi- 
 gation. For the sake of convenience, the Laboratory designated the 
 product made by this process as Uralloy A. The second line of research 
 involves a thermosetting process and, by overcoming certain limitations 
 of the simple urea treatment, has resulted in a markedly different 
 product, designated Uralloy C. 
 
 It should be emphasized that in neither phase has development proceeded 
 beyond the laboratory stage of experimentation. No extensive commercial 
 applications have been made, and it is recommended that such applications 
 be undertaken only after careful study of the properties of Uralloy and 
 instructions for processing the material as set forth on the following 
 pages. However, patents have been granted for both processes. Licenses 
 for their free use in the United States may be granted by the Secretary 
 of Agriculture to persons or companies who are prepared to make use of 
 the process commercially. Patent No. 2,2925,017 relates to Uralloy A and 
 Patent No. 2,313,953 relates to Uralloy C. 
 
 UREA- IMPREGNATED THERMOFLAST IC WOOD 
 (Uralloy A) 
 
 Solid Wood 
 
 The original disclosure that wood becomes thermoplastic upon soaking with 
 a solution of urea came as a result of experiments in chemical seasoning 
 of wood at the Forest Products Laboratory. It was found that green 1-inch 
 oak squares so soaked, and subsequently dried, became plastic and easily 
 bendable with the hands upon application of heat. With cooling, the bent 
 wood became rigid again, retaining its new shape. 
 
 Essentially, the simple urea treatment involves nothing more than this 
 procedure. A technique worked out for its application on a laboratory 
 scale can be adapted easily to other conditions, such as the home workshop 
 or small woodworking plant. 
 
 Greenwood is required for treatment. An aqueous solution of urea is 
 formed, using equal amounts by weight of water and the chemical. The 
 wood is placed in this solution for a length of time that varies with the 
 species being plasticized. For oak, the soaking period may be generally 
 
 Mimeo. No. R1277 (revised) -1- 
 
stated as 10 days for each inch of thickness — the time needed for 
 absorption of an amount of urea equal to 25 percent of the oven-dry weight 
 of the wood. 
 
 After this treatment, the wood should be dried to a moisture content of S 
 to 12 percent. The drying time needed likewise varies with the drying 
 temperature used, wood species, and similar considerations. In no case, 
 however, should the temperature be higher than l^+G F. , and for oak it 
 should not exceed the optimum kiln temperature of 115'' F * during the 
 initial stages of drying. Prolonged drying at temperatures higher than 
 1^0° F. will cause changes of the urea in the wood and thus destroy the 
 plasticizing effect. The drying time for urea-soaked wood can be shorter 
 than that of untreated wood going through the same moisture content changes; 
 for example, if the kiln-drying period of 3-inch Sitka spruce is 30 days, 
 this time can be reduced to 15 days with urea- impregnated wood. This 
 accelerated drying rate in the kiln is brought about by more severe drying 
 conditions permitted by the treatment and should largely offset the time 
 consumed by the soaking process. 
 
 ■n properly dried, the wood is ready for heating and bending. A temper- 
 ature range of from 212° to 220° F. is the optimum for bending operations. 
 Thin wood can be twisted to the desired shape easily by hand. For larger 
 pieces, mechanical aids to bending are necessary. Properly designed 
 equipment will give much sharper bends than can be achieved with ordinary 
 bending methods and with far less breakage to stock.— Best bending re- 
 sults will be obtained if, after drying, temperature of the wood is 
 raised to the optimum temperature for bending by boiling in the regular 
 urea solution for a period of 15 or 20 minutes per inch of thickness. 
 Good bending can, however, be accomplished by heating the dry treated 
 wood to the desired temperature in dry air. Upon cooling, the wood, as 
 stated earlier, will retain its new shape, provided it is dried to a 
 sufficiently low moisture content before bending* In some cases, as in 
 thick stock, it is desirable to predry to a relatively high moisture con- 
 tent of 25 to 30 percent. In this event the bent products will have to 
 be dried on the form somewhat after bending in order to preserve the 
 desired shape. 
 
 Sawdus t 
 
 Urea plasticization can also be applied to sawdust for the formation of 
 solid sheets or panels. It is necessary, however, to use considerable 
 pressure in the molding operation. For this, machinery is needed. 
 
 The compressing technique employed by the Laboratory for molding urea- 
 treated sawdust consisted of the following steps: 
 
 ""See "Wood Bending (Oct. I929) with Appendix (l^l) on Apparatus for Bend- 
 ing Boat Ribs," by T. R. C. Wilson. FPL Mimeo. R966. 
 
 Mimeo. No. R1277 (revised) 
 
lc Determine oven- dry weight of sawdust. 
 
 2. Add dry urea -- 25 percent of the oven-dry weight of the sawdust. 
 
 3c Add enough water to make the mass a paste. 
 
 k. Dry the sawdust mix to a moisture content of 2 to 3 percent, 
 
 5« Add a suitable lubricant, such as 1 to 5 percent zinc stearate, to the 
 mix. 
 
 6. Place mix in mold. 
 
 7» Exert pressure of 1,000 to 1,500 lbs. per sq. in. 
 
 S. Raise press temperature to 35^~3c , 5° F» 
 
 9- Subject to temperature-pressure 3 to 5 minutes. 
 
 10. Cool under pressure by running water through press platens. 
 
 Hi When cooled to 1^-0° P., remove product from press. 
 
 The technique used here is not necessarily the optimum for all conditions 
 of pressing, nor the only way pressing can be done. Products manufactured 
 at the Laboratory by following these steps were, however, of good strength 
 properties. 
 
 Pr ope rties of th e Pr od uct s 
 
 Solid Wood 
 
 From limited tests it seems that lumber or dimension stock impregnated by 
 the simple urea treatment has strength properties at room temperature 
 about equal to those of the normal untreated wood. In color it is 
 characteristically gray or black. It can be handled and used about as 
 ordinary wood is used. 
 
 Sawdust 
 
 Molding of the urea-impregnated sawdust in a hot press results in consider- 
 ably changed properties. The color is black or gray. The material becomes 
 so hard that it is often difficult to nail. Sawing and machining proper- 
 ties are good. Veneer facings can be applied, but some work will be re- 
 quired to perfect suitable glues. The material is noncorrosive, so far as 
 metal fastenings are concerned. Tests thus far made indicate that 
 strength qualities of the treated, compressed sawdust are comparable to 
 other common plastics. 
 
 The molded material is not as resistant to water as is ordinary untreated 
 wood. It will withstand soaking for a comparatively few hours before 
 losing strength through disintegration. Its water resistant properties 
 can be greatly improved by the addition of '-such chemicals as furfural, 
 carbon tetrachloride, or thiourea. 
 
 Limitations 
 
 Two important deficiencies affect the usefulness of Uralloy A, however, 
 whether it be of solid wood or sawdust. One of them has already been 
 
 meo. l T o. R1277 (revised) -3- 
 
mentioned with relation to compressed sawdust -- its poor water resistance. 
 Impregnation with urea, in fact, actually increases the hygroscopicity of 
 wood unless the soaking solution be modified with chemicals which increase 
 water resistance. The other deficiency involves the return -of the material 
 to a plastic condition when reheated to 212° F. Softened thus, it tends 
 to lose shape in response to any pressures or stresses it may be under. 
 
 These two deficiencies naturally place limitations upon the practical 
 usefulness to which the simple urea treatment can be turned. Those who 
 developed it are therefore inclined to regard it as more important from 
 a scientific than from a commercial standpoint. The discovery that urea 
 plasticizes wood is an interesting development in the study of basic wood 
 properties. Wide commercial applications are not expected because of the 
 thermoplastic nature of the complex and because urea tends to darken wood 
 and increases its hygroscopicity. Conceivably, however, the process may 
 have some important uses where these properties are not objectionable. 
 
 UREA- FORMALDEHYDE THERMOSETTING 7-fOOD 
 (Uralloy C) 
 
 The twin problems of preventing resoftening of bent urea-treated wood and 
 improving its water resistance were in great degree solved by addition of 
 other chemicals, notably formaldehyde, to the soaking solution. It was 
 found that this substance, uniting chemically with urea within the wood 
 oell walls, formed a synthetic resin which gave the wood considerable re- 
 sistance to water. Swelling and shrinking were thereby materially reduced. 
 As soon as the resin was set by heat (polymerized), the wood assumed a 
 stiffness which could not be noticeably affected by subsequent heating, at 
 least at 300° F. It thus became permanently thermoset. To obtain good 
 diffusion into the wood before the resin-forming action of the formalde- 
 hyde and urea takes place, this chemical reaction must be retarded by 
 buffers which maintain the solution in the slightly alkaline range. 
 
 A buffered urea-aldehyde solution developed at the Laboratory accomplished 
 these results. Either of the following formulas can be used to make this 
 solution,, The ingredients should be added in the order shown with the mix- 
 ture heated to not higher than l40° F. until all the urea is dissolved. 
 The mixture should be cooled to room temperature before adding formaldehyde 
 
 Formula A 
 
 Water 231 parts 
 
 Sodium hydroxide, . . . 6 " 
 
 Borax 39 " 
 
 Urea 3S0 " 
 
 Formaldehyde (37 
 
 percent solution). . Jkk " 
 
 1,000 " 
 
 Formula B 
 
 Water ............... hl6 parts 
 
 Sodium hydroxide ... . 6 
 
 Borax. 39 
 
 Dimethylol^urea. . . . . 236 
 Urea, 253 
 
 1,000 
 
 All proportions in these formulas are by weight. 
 
 Mimeo. No. R1277 (revised) 
 
 -k- 
 
To secure best results, the solution formula must be modified and adjusted 
 to suit various species. Thus the above formulas, which have a pH of 
 about 10, are admirably adapted to plasticizing oak, a highly acid wood. 
 For other woods it may be desirable to adjust the pH nearer to 3. This 
 may be done by omitting the sodium hydroxide in the formulas, or by adding 
 10 parts of glacial acetic acid, or by other suitable means. When the 
 solution is used to resinify wood in applications which do not involve 
 bending, the proportion of formaldehyde can be increased so as to obtain 
 greater water resistance and increased strength properties. 
 
 The soaking period necessary to get complete diffusion of the chemicals 
 within the wood is about the same as that for the simple urea solution. 
 From this point on the steps necessary in the entire bending and molding 
 process are different from those suitable for Uralloy A. After the 
 treated wood is removed from the urea-formaldehyde solution, it is not 
 dried, but is promptly heated and bent in the wet condition. If the 
 heating and bending are not done promptly, resin will begin to form, and 
 the plasticity of the wood will decrease. When this happens, reheating 
 will be of no avail in resoftening the wood. Heating is done by boiling 
 the treated wood in a 50 percent solution of urea in water at a tempera- 
 ture of 212 to 220° F. for a period of 15 to 20 minutes per inch of 
 thickness. The wood is then bent to the desired shape. Under such con- 
 ditions, the bending properties appear to be as good as those of Uralloy 
 A. The r/ood is then dried while held to the bent form in an oven, kiln, 
 or by simple exposure to air, after which it is heated to about 300° F. 
 in order to fully polymerize the resin. Prolonged exposure to temperatures 
 of I5O F. or above also will set the resin sufficiently for most purposes. 
 
 When the thermosetting process is used to resinify wood for applications 
 which do not involve bending, such as for modification of hardness or mois- 
 ture resistance, the heating in urea and the bending are omitted. 
 
 Properties of the Product 
 
 As experimentation with the materials produced by the process has been 
 limited, all the properties of the new substances have been by no means 
 established. In general, however, the tests made thus far indicate that 
 the strength qualities are not materially affected by temperatures up to 
 the boiling point of water. Uralloy is stiffer than normal wood, that 
 is, it does not bend as much under the same load. It is considerably 
 harder. However, it can be readily worked with ordinary woodworking tools 
 It takes a high polish when buffed. Unlike urea treatment, the urea- 
 aldehyde treatment tends to bleach rather than to darken the wood. 
 
 The formation of a water-resistant urea-formaldehyde resin in the fine 
 swelling structure of the wood cell walls destroys some of the natural 
 hygroscopic properties of the wood. The treatment thus imparts true water 
 resistance. The rate of moisture movement through the wood and the ulti- 
 mate amount of shrinking and swelling in response to moisture changes are 
 considerably decreased compared with those of normal wood. 
 
 Mimeo. No. R1277 (revised) -5- 
 
GENERAL 
 
 The mechanics of successfully bending wood treated by either method depends 
 largely upon good technique in handling properly designed apparatus, as 
 mentioned earlier. As yet, research has not been carried far enough to 
 establish the shortest radius upon which wood of a given thickness may be 
 bent. Stock l-l/2 inches thick has been bent through ISO degrees on an 
 inside radius of 5 inches, and l/k inch stock was similarly bent on a 
 radius of j/k inch. While this degree (3A inch) of upset is not neces- 
 sarily the maximum obtainable, it will be noted that the ratio of the 
 length of the inside radius to the thickness of the stock is about J:l 
 in both instances. It can be assumed, however, that thicker wood requires 
 a proportionately longer radius. 
 
 It has not yet been possible to establish whether the treatment adds to 
 the resistance of wood to destructive insect and decay attack. 
 
 Research has been carried out on only a limited number of wood species. 
 It appears, however, that the hardwoods respond better than do softwoods to 
 the plasticizlng action of both processes. Of the hardwoods, oak and 
 sweet gum give better results than the other species tried. Resinif ication 
 of the wood by the urea-aldehyde solution probably can be accomplished 
 with all hardwoods and softwoods, although this has not been verified by 
 research. 
 
 Urea, the synthetic chemical which is the basis for this plasticizing 
 treatment, is manufactured commercially by combining liquid carbon 
 dioxide and liquid ammonia under pressure. It is commonly sold in 
 crystalline form at £80 to ^90 a ton. It is not sold by the manufacturers 
 in quantities less than 100 pounds, but smaller quantities can be obtained 
 from chemical and drug supply houses at a considerably higher price. 
 
 It should be pointed out to those interested in undertaking commercial 
 applications of either process that existing limitations upon domestic 
 sales of critical chemicals and pressure machinery, molds, dies, etc, 
 caused by the war must be taken into consideration. 
 
 Mimeo. Ho. R1277 (revised) -6- 
 
UNIVERSITY OF FLORIDA 
 
 llllllllllllllllll 
 
 3 1262 08866 6390