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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y GLUE FOR USE ON AEROPLANES By PERCYA. HOUSE MAN^ Received December 18, 1916
There are many parts of a n aeroplane t h a t require t h e use of a n adhesive material. Of these may be mentioned “airscrews” (the collective name for propellers a n d tractors), hollow spars, booms, ribs, plywood, etc. For some of t h e small parts, such as t h e ribs, which are subsequently covered b y “doped” fabric, i t is not necessary t o use t h e best quality of glue. For other parts, however, i t is of vital importance t h a t only t h e strongest glue be used. This applies particularly t o airscrews, which rotate a t a very high speed. Airscrews are built u p in laminae of walnut or mahogany, usually 7 in number, a n d each I in. thick. It is usual t o glue two laminae together, a n d allow t h e m t o remain in clamps for a period of from 6 t o 24 hrs. before gluing on t h e next lamina. The finished airscrew is shaped out after t h e total number of layers have been glued together a n d have been allowed t o set. T h e glue is applied hot, a n d usually by at least two workmen, in order t o cover t h e surface as quickly as possible. T h e temperature of t h e gluing room is kept above 70’ F. It would, of course, be a n economy t o glue at one operation all of t h e laminae of a n airscrew, b u t this is not possible when a cake glue is used, on account of t h e rapidity with which t h e hot solution sets. Attempts have been made t o overcome this difficulty b y t h e use of “liquid” glues, b u t these have usually proved unsatisfactory. One French liquid glue contained some hygroscopic material, a n d failed t o set in 1 4 days. “Liquid” glues should be used only on work in which great strength under widely varying conditions of temperature a n d humidity is not of prime importance. Bone glue should never be used on aircraft work. T h e best results are obtained from skin or hide glues. I n working out a method for t h e examination of glue some attention was given t o such chemical a n d physical tests as moisture, ash, nitrogen, strength of jelly, viscosity, percentage precipitable b y alcohol (Stelling’s test),2 etc., b u t while t h e information furnished b y these tests was of interest, i t was not found possible t o correlate definitely t h e results obtained, with t h e value of t h e glue for aeroplane work. A method was therefore developed t o test t h e strength of a glued joint under various conditions. Experiments were made with blocks of biscuit-ware, as recommended by Rideal in his book on “Glue a n d Glue Testing.” Messrs. Doulton, of London, kindly made t h e necessary blocks, which h a d a base ( a b ) I in. square, similar in design t o t h a t shown in Fig. I. Two such blocks were glued together under various conditions of strength of glue solution, temperature, method of application, pressure of clamping, etc. T h e blocks were pulled a p a r t in a cement testing machine with automatic application of t h e load, b u t as uniform results could not be obtained, t h e stone blocks were abandoned. More uniform results were obtained 1 Formerly Chief Examiner, Aeronautical Inspection Laboratories, London, England. 9 Chem.-Zlg, 20, 461.
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with wood, which also has t h e advantage of giving results more nearly comparable with those obtained under actual service conditions. Pieces of carefully selected straight-grained walnut were used, and from t h e m a number of different types of joints were prepared, and used t o illustrate different kinds of stresses. T h e t y p e of joint finally adopted is t h a t shown in Fig. 11. This joint gives b y no means a t r u e shear, as was easily seen by examining under polarized light a celluloid model made t o scale. A stress more nearly approaching pure shear would probably be obtained with a joint like t h a t shown in Fig. 111, made u p from pieces 9 in. X 2 in. X 1/4 in. b u t for t h e purpose of obtaining comparative results, t h a t shown in Fig. I1 answers well a n d has t h e advantage of simplicity. BLOCKS AND JOINTS FOR
GLUE T E S T / NG
L7 F / GI
i f / G . z
fi0.n
The wood is exposed for 2 4 hrs. t o a temperature of 3 5 ’ C. before making t h e joint. The test pieces ( A ) are 9 in. X 2 in. X 3 / g in. These were roughened by a toothed plane, as it was found t h a t a roughened surface gives a stronger joint t h a n a smooth one. The distance pieces ( B ) are 3 in. X 2 in. X 3 / 8 in. The area of t h e glued surface of t h e joint t o be pulled a p a r t is 4 sq. in. T o prepare t h e glue, i t is broken into small pieces a n d is soaked over night in t h e requisite amount of water. For t h e gelatine types of glue, I p a r t b y weight of glue t o 2 parts b y weight of water gives a desirable concentration, while for “Scotch” glues I part of glue t o 1 . 2 5 parts of water gives t h e best results. For other types of glue t h e water required usually falls within these limits. T h e soaked glue is dissolved b y warming t o 60’ C., a n d t h e solution is applied a t t h a t temperature. T h e joints are clamped under moderate pressure for 48 hrs., a n d are tested after a further 24 hrs. For each sample of glue, t h e following three tests, each in duplicate, are carried out on t h e joints prepared as described above. ( I ) R E G U L A R TEST-The joints are pulled apart in a Buckton a n d Wicksteed or in a Riehl& (Philadelphia) Testing Machine a n d t h e breaking strain per sq. in. of glued surface is recorded. T h e experimental variation is less t h a n * I O per cent, most of which may be ascribed t o unavoidable variations in t h e wood. ( 1 ) HEAT TEST-The joints are subjected t o dry heat in a n electrically heated oven for 2 days a t 4 5 ’ C., a n d t h e n pulled apart, and t h e breaking strain recorded.
