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Aviation History
1942
1942 - 2271.PDF
OCTOBER 2QTH, 1942 FLIGHT 475 PLASTIC PROGRESS t Some Recent Developments : Bonding Metal to Plastics By W. NICHOLS, A.R.Ae.S. IT is pleasing to be able to record, when the authorities permit, items of progress made with plastics in aircraft production. Actually the plastics sections of the British aircraft industry has made great strides, and it is now probably the equal of that of any other country in the world. One of the latest developments, of which particulars may now be divulged, is a method of bonding metal to plastics, carried out by Aero-Research, Ltd., directed by Dr. N. A. de Bruyne, M.A., Ph.D. The process, known as " Plastel," ingeniously makes use of metal stabbed all over with small buried holes. The re-entrant edges of these burred holes engage with the plastic so that the metal and plastic are firmly locked together. An illustration shows the nature of the material in the form of a hinge between two pieces of cellulose acetate made with "Plastel." Although not apparent from the photograph, the surface of the "Plastel" is actually quite smooth. It should be pointed out that '' Plastel'' may be used in conjunction with any other plastic (thermosetting or thermoplastic). The objects of bonding metals to plastics fall under two main headings: (a) improvement in flexural stiffness; (b) improvement in dimensional stability. The improve ments obtained are such as to offer a wide range of applica tions of the composite material. Improvement in Flexural Stiffness Under certain conditions the flexural stiffness of any plastic material can be considerably increased, without a proportionate rise in weight, by bonding a thin metal coating to the two sides of the plastic sheet. The fiexural stiffness of a "sandwich " panel of cellulose acetate between thin steel covers will, if correctly proportioned, be greater than that of a sheet of duralumin of the same surface area and same weight, and is far greater than the flexural stiff ness of either steel alone, or of cellulose acetate alone, of the same weight and surface area as the "sandwich." All plastics have about the same specific gravity, and the stiffness is primarily determined by the separation between the metal covers ; roughly speaking it is, therefore, only the specific gravity of the filling that is the decisive factor. If plywood is used, a material less dense than plastics, the stiffness is much increased. Outer Layers ^^ Steel Steel Steel Steel Duralumin Duralumin Inner Layers Formvar Cellulose acetate Laminated fabric plastic Plywood Laminated fabric plastic Plywood Maximum stiffness of Strip lin. Wide 2,505 xio8x W3 1,086 Xio'xW3 3.035 Xlo*xW 5,064 X io6 X W3 2,220 x ro6 X Ws 6,700 X TO* x W The above table gives values for the maximum stiffness of strips iin. wide. They have been confirmed experiment ally and should be compared with the values for the materials by themselves, given below. Material Maximum Stiffness of Strip lin. Wide 111.5X lo8x\\'3 8ooxio*x\Y; astic . . .. .. 731 X 10s xW3 .. 5,125 xio'xW " 38IXIO«X"\\ ' 56 X ioc x W* W -weight of 1 sq. inch of " sandwich." A steel panel has a very poor flexural stiffness compared Steel I >uralumin . . Laminated fabric Hvwood Formvar Cellulose acetate A simple demonstration of flexural stiffness. The steel, duralumin and "Plastel " strips are all of the same weight and area. with a duralumin panel of same weight and area. Accord ing to Dr. de Bruyne, the duralumin panel is seven times as stiff as the steel, but a sandwich of steel with a plastic filling will be two and a half times as stiff as a duralumin panel. Plywood, a very stiff material by itself, gives 10 per cent, increase in stiffness when used as the inner-layer in a correctly proportioned sandwich with duralumin outer layers. Dr. de Bruyne points out, however, that the above analysis assumes that the bonding between the layers of the "sandwich" are complete or continuous. With bis '' Plastel'' process the bonding is not continuous, and experiment shows that the values for maximum stiffness are only half the values given above. Thus a "Plastel" sandwich of steel on plastic has a stiffness of about 1,000 x io6 x W3. This stiffness is, however, greater than that of steel alone or of any plastic (except Gordon Aerolite) alone, and "Plastel" has a stiffness somewhat greater than duralumin alone. The accompanying photograph, supplied by Dr. de Bruyne, clearly demonstrates these facts. All the strips shown in the photograph are of the same length, same width, and all equally loaded. The steel strip is deflected most. Because of its high density it has to be very thin in comparison with the other materials to be of the same weight. Since flexural stiffness is proportional to the cube of the thickness, the steel strip deflects the most because its high Young's Modulus is nullified by the small depth of the beam. The "Plastel" strip is of stainless steel with a plastic middle layer made of paper—reinforced phenol-formaldehyde. It can be regarded as a steel strip expanded to a reasonable thickness by the use of a lightweight filling. This gain in stiffness without increase in weight is of importance in the use of plastics for instrument panels and where large fiat surfaces occur. The gain in resistance to abrasion and heat is also an important factor. Dimensional Stability Another purpose in bonding metal to plastics is to obtain dimensional stability despite changes of humidity and tem perature. Practically all plastics (the chief exception being polystryene) change their dimensions considerably with changes in moisture content. These changes are extremely marked in cellulose-acetate, and they are quite significant in Bakelite materials. The graph given herewith shows swelling and shrinkage corves in fabric-reinforced laminated Bakelite sheet in comparison with the behaviour of the same sheet covered with stainless steel by the "Plastel"
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