country now are offering n-radiography services. Beam. For an η-radiograph image to be sharp and the processing time reasonably short, the neutron beam that strikes the object must have a high flux, preferably on the order of 10 6 to 10 7 neutrons per sq. cm. per second. Nuclear reactors are the most intense source of thermal neutrons, and rapid acceptance of n-radiography has been hindered by the inconveni ence of sending objects to a reactor facility for testing. The thermal neutron beam emerges from a moderator, is collimated, and then impinges on the object being imaged. On the far side of the ob ject, the attenuated neutron beam strikes a plate or sheet of foil coated with neutron-sensitive elements—such as gadoHnium, lithium, or boron. When neutrons hit the nuclei of these elements, secondary radiation is gen erated. It is this radiation from the converter plate that forms an image on a piece of adjacent x-ray film. Some elements—such as rt odium, silver, gold, indium, and dysprosiumcontinue to emit beta and gamma ra diation after removal from the neutron beam. Such delayed emitters permit exposure of the x-ray film at a loca tion other than the site of neu tron bombardment, an advantage when radioactive materials—them selves gamma emitters—are studied. Alternate. Many groups are inves tigating alternate neutron sources. One approach involves use of charged particle accelerators. Another uses combinations of isotopes that generate neutrons. However, in the present state of development, the collimated neutron beams from these alternate neutron sources are weak. Californium-252 as a neutron source promises to be an important factor in the future development of n-radiogra phy. This synthetic element, with a half-life of 2.65 years, produces neu trons by spontaneous nuclear fission. While working at Battelle Northwest in Richland, Wash., John Cason de signed a unit that incorporates 268 micrograms of 252Cf, and has a thermal neutron flux of about 10 4 neutrons per sq. cm. per second. Being portable, it opens the way to expanding n-ra diography into new fields. The big drawback in use of califor nium-252 right now is its short sup ply and high price, points out Dr. Gary Dau, manager of Battelle's applied physics and instrumentation depart ment. But the price is coming down. On Feb. 1, the U.S. Atomic Energy Commission is making the element available for bulk shipment at $10 per microgram, a sharp change from the current price of $100 for 0.1 micro gram (C&EN, Jan. 11, page 31).
New thermoset puts acrylics into powder-coating uses Acrylic resins are moving into a rela tively new but fast growing corner of the paint market—powder coating of metal products. Japan's Mitsui Toatsu Chemicals, Inc., has developed a thermosetting acrylic resin suited to processing by paint producers into coating powder, and is supplying de velopment quantities from a 400 metric-ton-a-year plant. A 2500 ton-ayear commercial unit to produce the acrylic resin will be completed about the end of 1972. In a field dominated by epoxy and polyvinyl chloride resins, acrylics have an edge in weatherability—and Mitsui Toatsu estimates that roughly half the present applications of powder coating are for products intended for outdoor use. The problem has been to adapt acrylics to the conditions under which such finishes are applied. Mitsui Toatsu will supply the resin from which coating powders are made as well as the know-how for making acrylic powder. The initial price of the acrylic resin will be a bit more than that of epoxy resins sold for the same application. Solventless. Powder coating is a solventless approach to applying fin ishes to metals. The powder—a mix ture of resin, pigment, and curing agent—can be flame-sprayed or ap plied to a preheated surface by a fluidized bed system. Another ap plication technique is to attract the powder electrostatically to the metal item. The coating step is then fol lowed by baking. Since no solvent is used, powder coating avoids the problems of air pol lution and fire hazard that industrial use of solution-type coatings usually entails. Excess powder can be re covered and re-used, thus providing savings when products such as screens or grillwork are painted. Building products—such as guard rails, fences, steel pipe, and sashes—are among the main applications of powder coat ing at present. Another is household appliances, such as washing machines and air conditioners. Because it pro duces a slightly crinkled finish rather than a mirror finish, powder coating hasn't found its way into use by auto makers for applying automotive fin ishes. Demand for powder coating ma terials is small, but fast growing. A C&EN estimate (Dec. 22, 1969, page 44) puts annual U.S. application of coating powders by fluidized-bed tech niques at $7 to $9 million worth of coating materials. Mitsui Toatsu es
timates the growth of the total pow der coatings market at about 200% a year in the areas of both Europe and
Japan. Although epoxies, followed by PVC, are the most used resins in these sys tems, nylons 11 and 12, polyolefins, and others are used in special applica tions. Epoxies, however, are limited in outdoor uses because they tend to lose their gloss rather easily by weath ering. PVC coatings are better on that score, but a primer coat is re quired because of poor adhesion be tween a PVC coating and unprimed metal. Acrylic thermosetting resins need no primer coat, and they beat both epoxies and PVC in weather resistance. Using a standard sunshine weathermeter test (ASTM D-750-43-P), Mi tsui Toatsu reports 82% gloss reten tion by acrylic-coated samples after 1000 hours' exposure, compared to 63% for PVC coatings and only 5% for epoxies. A problem in developing acrylic coating powders, however, has been production of an acrylic copolymer that retains its desirable qualities and yet meets the needs of a powder coat ing operation—to melt when needed, for example, while avoiding premature curing. A critical point, says Mitsui Toatsu's Hiroshi Ozawa, is choice and control of both molecular weight aver age and molecular weight distribution of the resin. The result of the company's work is a copolymer consisting of a methacrylic ester, another acrylic monomer, and a functional monomer that can react with a curing agent to cross-link the copolymer chains. (Styrene is also a component for some applica tions.) The functional monomer may contain an epoxy group, Mr. Ozawa tells C&EN, which links with a polycarboxylic acid in the curing agent when the applied coating is heated. Other functional monomer-curing agent combinations have proved prac tical as well. The coatings are baked at temperatures between 160° and 240° C.—the same range used for epoxy powder coatings, and somewhat lower than the temperatures used for baking PVC coatings. Once a usable combination of mono mers was found, the other technical hurdle was to polymerize them. In the end, Mitsui Toatsu settled on a sequence of bulk and suspension poly merization steps, with intermediate ad ditions of monomer in the production sequence. JAN. 25 1971 C&EN 37
