RESEARCH
Monolayer Bonds Metals to Thermoplastics Stearic acid joins aluminum to polyethylene; resulting bond is more resistant to tearing than is polyethylene itself
BOND. Stearic acid molecules adhere to a metal plate as it's pulled up and through the monolayer on the water's surface (top diagram). The acid portion, represented by the circles, dissolves in the water just below the surface. The hydrocarbon portion (perpendicular lines) tends to stand straight up when the molecules are pushed together. The lower diagram shows the final chemical bond (metallic stéarate) between the metal plate and acid portion of the molecule, and the physical bond between the hydrocarbon part of the molecule and the thermoplastic 28
C&EN
DEC.
2 4,
1962
A method of bonding metals to thermoplastics with a single layer of molecules as an adhesive has been developed at Bell Telephone Laboratories, Murray Hill X.J. (C&EX, Dec. 17, page 39). The method, worked out by Bell's Dr. Harold Schonhorn, produces a bond between aluminum and polyethylene that is more permanent than any previously achieved. The bond is more resistant to tearing and pulling than is the plastic itself. Dr. Schonhorn bonds the materials together with a monomolecular layer of stearic acid. Details of his method will appear soon in the Journal of Polymer Science. Up to now, it hasn't been possible to form a direct metal-to-polyethylene bond that will withstand mechanical stress at high humidities and temperatures. An aluminum-polyethylene-aluminum bond, prepared by Dr. Schonhorn's method, has held up for months under 600 p.s.i. tensile-shear stress at U)(Y/t relative humidity (at 80° to 120° F . ) . In binding metals to plastics, the Bell scientist uses a technique developed in 1935 by Dr. Irving Langmuir and Dr. Katherine Blodgett at General Electric Research Laboratory, Schenectady, X.Y. The technique enables scientists to deposit a monomolecular layer of a substance on a water surface. The technique has been used by chemists to study the structure of single molecules. But it has never been applied to adhesion, Dr. Schonhorn points out. Long-chain hydrocarbon acids such as stearic acid can serve as the monomolecular layer in the metal-plastic bond. The acid end of the molecule forms a chemical bond with the metal, and the hydrocarbon chain forms a physical bond with the thermoplastic. One end of the stearic acid molecule forms aluminum stéarate with the metal plate; the other end becomes immersed in the polyethylene. These reactions explain the bond's permanence.
Analytical Scheme Separates Hafnium and Zirconium National Bureau of Standards method is quantitative and accurate to within 2 to 5 parts per thousand
BOND PREPARATION. Dr. Harold Schonhorn prepares an aluminum-to-polyethylene bond. The bond is more permanent than any such bond made up to now
Monomolecular. To prepare the monomolecular layer, Dr. Schonhorn dissolves the stearic acid in benzene and spreads the solution on water contained in a long trough. The volatile benzene evaporates and leaves a monomolecular stearic acid layer on the surface. He then pushes these molecules together, using a float. The hydrocarbon portion of the stearic acid is insoluble in water and tends to stand up straight when the film is compressed. The acid portion dissolves and lies just below the water's surface. Next, Dr. Schonhorn lowers an aluminum plate through the monolayer into a rectangular well at one end of the trough. During its descent, the plate contacts the hydrocarbon chain portion of the monolayer. Since this part of the stearic acid molecule has no affinity for aluminum, it doesn't adhere to the plate. But when Dr. Schonhorn raises the aluminum plate, it contacts the acid part of the molecule to form a chemical bond. The molecules adhere to the plate's sides so that the hydrocarbon chain faces out. Once coated with such a monomolecular layer, metals can't adsorb atmospheric water or gases in appreciable amounts. And they can be
stored for months before they are bonded to thermoplastics. The atmosphere contaminates unprotected metals in a short time. Thus the metals must be specially treated before they can be bonded. In the final experimental step, Dr. Schonhorn melts polyethylene onto the monomolecular layer. The stearic acid molecule's hydrocarbon chain becomes immersed in the polyethylene. This completes the bond. Stearic acid and other long-chain hydrocarbon acids usually are used in thicker films to release parts from molds. However, as long as these compounds are used in a monomolecular layer, they may act as adhesives. Dr. Schonhorn has also bonded aluminum, stainless steel, and copper to polypropylene and polystyrene. He has used octadecylamine and octadecylphosphonate as adhesives for the bonds. By varying the adhesive, he can bond other thermoplastics and metals together, he says. Dr. Schonhorn's method could find a number of applications in the electronics industry. For example, a permanent bond between polyethylene insulators and copper conductors could improve mechanical properties of telephone cables. It also could increase the reliability of printed circuits.
A method for quantitatively analyzing hafnium and zirconium has been devised at the National Bureau of Standards, Washington, D.C. The analytical method was worked out by L. A. Machlan and J. L. Hague of NBS. The two elements are similar enough in many respects to allow using one in place of the other except in nuclear reactor applications. Their nuclear properties are different; so for reactor uses, the relative amounts of each have to be known. Hafnium is found only in the presence of zirconium (usually in zircon ore). The two react with the same materials, thus are difficult to separate by usual chemical methods. Earlier, NBS workers separated hafnium from zirconium by dissolving the mixed oxides or the mixed metals in hydrofluoric and sulfuric acids. Hydrofluoric acid was then fumed off, the solution diluted with water, and put through an anion exchange resin column. Separation with this technique is successful. The new work refines the separation of the two elements when they occur in various ratios. The hafnium and zirconium solutions collected separately from the columns are treated with hydrochloric acid and cooled. Cupferron reagent is added to the samples, and the resulting precipitates filtered, washed, dried, and ignited to constant weight (in platinum crucibles) to form the dioxides. To learn how accurate the method is, the NBS chemists first checked hafnium and zirconium dioxide samples for purity with a spectrograph. They then mixed known ratios of the dioxides and analyzed the mixture with their technique. The analysis is accurate to within 2 to 5 parts per thousand. With the method, separation and quantitative analysis of 0.5-gram samples containing up to 20% zirconium can be made in about three days. More time is needed for samples containing 20 to 80% zirconium. In concentrations higher than 80% zirconium, the metals form complexes that prevent quantitative analysis. Work is now under way at NBS to solve the problem. DEC.
2 4, 1962 C & E N
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