Aluminum alkyl deposits film of metal from liquid phase A process for liquid-phase deposition of aluminum films by thermal decomposition of an aluminum alkyl has been developed at Continental Oil Co. The process is competitive with tinplating for sheet steel and may be competitive on a performance basis with zinc galvanizing and dipped aluminum coatings, Dr. George J. Kostas of Continental's organometallics division, Houston, Tex., told the International Symposium on Decomposition of Organometallic Compounds to Refractory Ceramics, Metals, and Metal Alloys, in Dayton, Ohio. (See page 50 for more on the symposium. ) Dr. Kostas points out that some applications of the process are close to commercialization. In fact, Continental, which now produces pilot quantities of aluminum diethyl hydride, the alkyl used in the liquid-phase process, is planning to scale up to commercial production next year. To date, most work on deposition of metallic coatings by decomposition of aluminum alkyls has been with vaporphase processes. Many patents have issued, but extensive market research studies by manufacturers of aluminum alkyls have turned up few applications. For example, at the symposium, Dr. J. J. Crosby of the Air Force Materials Laboratory disclosed test results showing that aluminum-coated aircraft fasteners outperform conventional cadmium-plated fasteners in corrosive environments. These fasteners can be economically coated by decomposing triisobutyl aluminum in the vapor phase. Other potential applications cited by Dr. Crosby for the vaporphase process are production of ceramic-metal structures with controlled dielectric properties, electronic components, and in composite structures with boron filaments. But such specialized uses are being considered on a performance basis. In large-volume markets, vapor-phase decomposition of aluminum alkyls has not been economically competitive with hot-dip coating or direct vaporization of aluminum or other corrosionproof films, such as tinplate or galvanizing. Vapor-state handling leads to high process costs. Furthermore, the use of relatively expensive alkyls results in a high cost per pound of aluminum coated. Dr. Kostas points out that liquid phase decomposition of metal alkyls offers several advantages. Aluminum diethyl hydride can be decomposed from a 25% solution in a suitable highmolecular-weight solvent. Such a solution overcomes the pyrophoric tendency of the alkyl. 24 C&EN DEC. 11, 1967
Because the alkyl is decomposed in the liquid phase, materials handling is simplified, the process can be better controlled, and is more amenable to continuous operation. Deposition rates are faster, 4 to 6 mils per minute, and the desired thickness can be deposited in single or multiple passes. In the simplest operation, the coating of strip steel, existing tinplate lines can be converted to the process. Other techniques, such as spraying the alkyl on the heated substrate, may be possible, depending on substrate geometry and thickness desired. According to Dr. Kostas, the liquidphase process is economically competitive with tinplating of strip steel, on a basis of $2.00 per pound of aluminum coated. This figure reflects a raw material cost of about 60 cents per pound of alkyl used. Although this raw material cost is higher than for zinc galvanizing and dipped aluminum, such factors as performance and potentially higher coating rates may make it competitive with these also. Other possible applications for the process are wire coating, printed circuits, coatings for glass fibers to improve adhesion to polyester resins, and heat exchanger tubes.
Tributyl tin salts basis for rodent-repellent cable coating Scientists at M&T Chemicals have developed a rodent-repellent cable coating. Almost a year of field testing by the Department of Interior's Wildlife Research Center has proved that the coating works, M&T's Charles Anthony said at the International Wire and Cable Symposium, in Atlantic City,
N.J. The plastic-based coating, called bioMeT 12, has proved about 95% effective in repelling rodent attack. Containing a mixture of tributyl tin salts as the active ingredient, the coating can be machine applied and it has no adverse effect on other cable properties, the M&T chemist points out.
Rat chewing cable Preliminary nibble
Impetus behind development of the coating, Mr. Anthony explains, is increasing rodent damage to helicopter cabling, field telephone lines, and other military equipment in Vietnam. However, potential uses by civilian electrical and telephone companies—which are now suffering many millions of dollars of rodent damage per year— also offer considerable markets. In addition, waste containers, shipping bags, and electrical tape are other possible outlets for bioMeT systems. M&T, a subsidiary of American Can Co., hopes to see its new coating in production use for army field telephone cable by early next year. It will be applied at about 15 pounds per mile of cable. With the Army using several hundred thousands of miles of this cable per year, and with the coating price between $1.00 and $1.50 per pound, this potential market alone would be a multimillion-dollar one. Mr. Anthony notes that it isn't known how the coating works. It does not kill the rodents. But apparently they can't stand the taste or smell of tributyl tin—a widely used antifoulant, fungicide, and bactericide— and strictly avoid anything coated with it after a preliminary nibble. Rodents often bite right through uncoated cable, but they don't eat it. They have to constandy gnaw on something to sharpen and wear down their incisors that never stop growing. Quite often a cable is the most convenient thing to gnaw on. The new coating will be the first effective and practical chemical rodent repellent coating, Mr. Anthony says. Earlier systems have been too difficult to apply or too evil smelling to be of much use. Another answer to rodent damage is metal cladding. But this is expensive and it is limited to heavier cables. In developing the cable coating, Mr. Anthony points out, it was necessary to produce a formulation that gave the necessary physical and mechanical properties without reducing the repellency of the organotin ingredients. The coating had to adhere well to the polyethylene surface of the cable. It also had to be flexible, transparent, stable to temperature extremes, and effective against rodents in the field for six months or more. The company will not reveal the full formulation as it hopes to obtain a patent. In all tests to date the uncoated cables have been bitten by rodents a total of nearly 1200 times. Forty-two of these bites have been severe enough to require cable repairs. The coated cables have suffered only 79 bites and none of these need repair. The sealed tests are being carried out in Arizona, New Mexico, Massachusetts, and the Panama Canal Zone.
