I I GLUES FOR USE IN AIM FAN October 1941 PCJCu'^,-/.; U.S. DEPOSITORY THIS II I < I I IS ONE CF A SERIES ISSUED TO AID THE NATION'S WAR PROGRAM No. 1337 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin SCHOOL OF FORCSThY 1 IRRARY UNIVERSITY 01 GLUES FOH USE IK AIHCKAEtI The glues that are adapted for gluing wood in aircraft may conveniently be divided into five classes, as follows: (1) synthetic- resin glues, (2) casein glues, (3) blood albumin glues, (U) marine glues, and (5) miscellaneous adhesives. Synthetic-resin and casein glues are most important for air- craft. Blood-albumin glues were formerly of importance in producing water-resistant plywood but they have been largely displaced by syn- thetic-resin glues. Blood albumin may be encountered, however, a? a filler or extender of synthetic-resin glues-. Synthetic-resin >Tlues are used chiefly in the production of plywood. In the fabrication of other parts and in the assembly of the finished craft, casein and cold- press resin glues will find use. Marine glues are used in wooden float and hull construction but not for making joints where high stren^t'-; is required. Synthetic-resin glues are based on resins synthesized from various chemical compounds. Of the many types of synthetic resins, only two have found wide use as woodworking adhesives. Of the two, the most highly resistant to moisture is the condensation product formed by the reaction of an aldehyde (usually formaldehyde) and a phenol (usually phenol or cresol). The product of the reaction between urea and formaldehyde is not so highly resistant to all types of exposure as the phenol-aldehyde product but, in a glue line, its resistance is superior to that of casein glue and it is widely used in woodworking. In preparing these products for use as adhesives, the reaction between the chemicals is stopped at an. intermediate stage in which the product may be applied on the veneer, the plies assembled, and the reaction completed under simultaneous application of heat and pressure (hot pressing). Some are applied in the form of a film, some as aqueous suspensions or solutions, and others in alcohol solution. A few are available that can be applied and pressed at room temperatures (cold pressing). These depend on the addition of 3 "catalyst" or "hardener" to cause the condensation reaction to proceed at room temperatures. Casein glues are a mixture of casein and other materials (usually lime and one or more sodium salts) combined in such propor- tions as to dissolve the casein and produce a mixture of satisfactory properties. The casein itself is a variable product, and the use of different ingredients in varying proportions resiilts in glues of widely -This mimeograph is one of a series of progress reports issued by the Eorest Products Laboratory to aid the Nation's defense effort. iAnimal, vegetable, liquid, soybean, and flour-extended resin glues, which 8.re extensively used in other wood-working industries, are not suited for aircraft work. Kimeo. No. 1337 differing properties* A marked difference exists in rater resistance -- a property of very erea.t importance ir. aircraft, since only the more water-rer.istant glues should be used for this uuroose. ^l ood ->lbumin flues ?re madn with ar albuminous base obt from the blool of animals, combined with chemicals, stich as lime, caus- tic soda, sodium silicate, paraformaldehyde, etc. The albumin is usually bought in dry form. Marine <-.-luc used in aircraft construction is solely for t purpose of making hulls and floats, constructed of w ood planking, ater- proof. The glue, therefore, must have sood adhesive qualities, remain tacky ir all climates, and be of such consistency as to penetrate fabric of high thread count. In general, marine glue should contain the follow- ing ingredients: Rosin, oine tar, denatured alcohol, and a drying oil (Tun 3; oil, rosin oil, or linseed oil), the proportions of which are left entirely to the manufacturer. L'iscellaneous aibesives . Vegetable proteins, such as soy- bean and peanut meal, serve as bases for adhesives, which in gener- L properties resemble casein glues. Some of them have a good degree of water resistance, moderate dry strength, and are relatively cheap. In their present stage of development they do not quite equal the strength of the better-quality casein glues* Other materials, including cellu- lose cements, asphalts, gums, and rubber have been tried as adhesives for vood. In their present forms these materials are not suitable as glues Tor wood joints, although some of them may have possibilities of development , and an improvement in one or more respects may make their use in aircraft desirable. rroporti.es of glues . A comparison of synthetic resin, casein, and blood albumin -lues is made in the accompanying t Le. 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