Othmer points out, since a higher overall heat transfer coefficient can be expected. As a result, the only increase in capital cost of the plant is substantially that of the heating tube bundle of the half stage. Although the half-stage and heaterevaporator modification is relatively simple and could be incorporated in current plants or designs, the vapor reheat concept involves a redesign of the process. In this case, the flow of condensed fresh water is reversed, moving from low-temperature to hightemperature stages. Also, sea water enters the process at the hot end rather than at the cool end. Essentially, product water, which has been externally cooled, is introduced into the condensing section of the coolest stage as a spray. Vapors from flashing brine contact the spray and condense, mixing with the spray. This stream then goes to the condensing section of the next hotter stage as a spray. As hot condensate leaves the hottest stage, it goes to a heat exchanger where it preheats raw sea water. This sea water then flows to the heaterevaporator. Some evaporates and goes to the half stage, while the remainder becomes hot brine feeding to the first stage. Dr. Othmer points out that a basic advantage of vapor reheat is the very low temperature difference necessary as a driving force for heat transfer when vapors condense directly on water sprays. It allows a much closer approach temperature in all stages. The half-stage modification can also be used with vapor reheat. Advantages are the same as for the conventional process. The heat reject modification—the third of the group—can be applied to conventional MSF or to the vapor reheat version. In this modification, heat which must be withdrawn to maintain the desired heat balance in the system is withdrawn at a high temperature level in the form of a stream of preheated sea water, rather than at the lowest possible temperature by the usual coolant stream. This way, it's possible to recover or utilize the heat otherwise rejected. The stream of preheated sea water is withdrawn just before going to the prime heater. The stream bypasses the prime heater and, in an emergency, it could be diverted to waste. However, it would normally go to the flashing side of a stage at a point where its temperature is that of the stage. The sensible heat of the preheated stream is thus substantially recovered. It is given up in flash evaporation after combining with the main flow of evaporating brine. 66 C&EN SEPT. 25. 1967
Starch xanthates show rubber chemical promise
154TH
ACS NATIONAL MEETING Rubber Chemistry
Starch xanthates show promise as contenders in the multibillion pound market for rubber chemicals and reinforcing agents, according to chemists at the U.S. Department of Agriculture Northern Regional Research Laboratory, Peoria, 111. USDA's Russell A. Buchanan says that zinc starch xanthate, when masterbatched with SBR, natural, or nitrile rubber latexes, acts as a rubber reinforcer, coagulation modifier, and curing accelerator. Starch xanthate masterbatching is compatible with current industrial
masterbatching processes, and reinforcement with starch xanthate is competitive in cost, on an equal volume basis, with some carbon blacks and many nonblack fillers. Mr. Buchanan points out that even though starch and elastomers are incompatible, fine particles of starch xanthates in rubber may be prepared by masterbatching. Such dispersions are analogous to reactive resin-rubber blends in that xanthate groups are chemically reactive toward elastomer molecules during vulcanization. Thus, zinc starch xanthate and starch xanthide accelerate sulfur curing of elastomers at a rate proportional to the loading. When dispersed as colloidal particles in compounded blends, starch xanthates improve the tensile properties of a variety of elastomers. The
Incorporation of zinc starch xanthate into rubber by latex masterbatching Starch xanthate solution
Latex
Water
Antioxidant emulsion
Mixing
Dilute sulfuric acid Zinc sulfate solution
Coprecipitation
Curd
Drying
Filtration
Serum (discarded)
Additional antioxidant
Milling Masterbatch (product)
How do you make a process stroam behave?
Control it with MSA process analyzers
The MSA process instruments shown above are: (top left) the M-S-A® Model 801 Oxygen Analyzer; (bottom left) M-S-A LIRA Infrared AnalyzerModel 300; (right) M-S-A Model 550 Gas Chromatograph.
Process stream instrumentation is an MSA speciality. Our analyzers cover a wide scope of process applications . . . chromatography, infrared, oxygen and thermal conductivity analysis, total hydrocarbons, water vapor . . . and more. For example, the ethylene oxide plant shown in the simplified schematic drawing above is typical of the many process operations which utilize
MSA analyzers for precise control of stream variables. For the whole story on our capability in process instrumentation, call the instrument specialists at MSA. And before you buy any process analyzer, check MSA first. Instrument Division, Mine Safety Appliances Company, Pittsburgh, Pa. 15208. Phone (412) 421-5900.
reinforcement is comparable to some types of reinforcing and semireinforcing blacks, and superior to nonblack fillers such as barytes, precipitated whiting, zinc oxide, regenerated clay, and silica, which are used when polymer color is more important than tensile properties. In the process developed by Mr. Buchanan and his coworkers, Dr. Charles Russell, Orville E. Weislogel, and Carl E. Rist, masterbatches are prepared by a coprecipitation similar to that used for making lignin-reinforced rubber. Starch xanthate solutions are mixed with latex, and the starch and elastomer precipitated by adding zinc sulfate solution. The zinc ion simultaneously coagulates the latex and forms zinc starch xanthate to produce a curd in which the xanthate is the continuous phase. Normally, coagulation by zinc solutions is not practical because the solids which form occlude serum and are difficult to dewater and dry. Incorporation of as little as 6 parts per hundred zinc starch xanthate gives a precipitate that is easily filtered and dried. Subsequent milling reverses the starch and rubber phases, consolidating the hard crumb to a rubber mass containing fine particle dispersions of zinc starch xanthate. When compounded with standard recipes containing sulfur and accelerators, xanthate-masterbatched rubber cured faster than blends with thiazolethiuram vulcanization accelerators alone. The cure acceleration was proportional to the starch xanthate loading. These reinforced vulcanizates had tensile properties better than rubbers containing either unmodified starch or starch xanthide incorporated as a dry powder by mill mixing. However, starch xanthide, when incorporated by dry milling, accelerated vulcanization. In the USDA study, the reinforcement properties of zinc starch xanthate were determined with styrene-butadiene, natural, and nitrile rubbers. Typical results were an increase in tensile stress at 300% elongation of 70% for natural rubber and 350% for SBR. Reinforcement of nitrile and carboxylic rubbers was most effective, probably because their polar groups make them more compatible with starch. Reinforcement of nitrile rubber gave tensile strengths equal to that obtained with an equal loading of HAF black. In SBR, starch xanthate behaved differently from common reinforcing agents in that its tensile maximum occurred at a low level of loading. Up to 22 parts per hundred, starch xanthate had a reinforcing effect equal to that of an equal volume loading of SRF black. But at higher loadings, 68 C&EN SEPT. 25, 1967
there was a tensile strength decrease. This probably resulted from a particle size dependency on xanthate level, with the higher loadings causing a larger particle size. Additional USDA-sponsored programs will be aimed at obtaining higher loadings of xanthate of smaller particle size to improve reinforcing properties. Also slated are compounding studies to fully utilize the combined reinforcing and cure accelerating properties of starch xanthates.
Organic coatings resist nitrogen tetroxide 154
ACS NATIONAL MEETING Organic Coatings and Plastics TH Chemistry
Organic coatings that resist corrosion by nitrogen tetroxide, an oxidizer used in booster rockets such as the Titan II missile, have been developed by George E. Cremeans and Louis J. Nowacki of the Columbus, Ohio, laboratories of Battelle Memorial Institute, and by Donald Stang and Kenneth Karki of Martin Co., Denver, Colo. These coatings include epoxidized polybutadiene and epoxy Novalac resin cured at about 250° F. with either trimellitic anhydride (TMA) or pyromellitic dianhydride (PMDA) and resin-filled epoxidized polybutadiene formulations cured at room temperature. The coatings, developed under an Air Force contract, were designed to protect electrical circuitry in the interior of the Titan and auxiliary equipment from fumes and splash of nitrogen tetroxide. Several coating systems, including alkyd-urea resins, catalyzed phenolicepoxy systems, epoxy amines, epoxy polysulfides, butyl rubber formulations, vinyls, acrylics, and anhydride-cured epoxy systems, were screened in search for resistant coatings, Mr. Cremeans says. Screening tests consisted of several hours of exposure to 70% nitric acid and methyl ethyl ketone. Resistance to the ketone was indicative of degree of cure in many cases. Failure of coatings was determined by film softening, cracking, or loss in adhesion to the substrate. Coatings that passed initial screening were exposed to nitrogen tetroxide. This test consisted of exposing a coated printed circuit board to the vapors of liquid nitrogen tetroxide for one hour and then airing the panel for 24 hours. Electrical properties and visual inspection for loss of film or nitrogen tetroxide attack on the copper determined whether the coating failed.
Although TMA-cured epoxidized polybutadiene coatings proved resistant to nitrogen tetroxide, the curing temperature necessary to obtain the resistance was higher than requested by the Air Force. Thus another anhydride—pyromellitic dianhydride (PMDA)—was used as a curing agent for epoxidized polybutadiene with the hope of making a cross-linked film with good chemical resistance, but with curing temperatures lower than those necessary for TMA-cured coatings. The PMDA coatings also proved resistant to nitrogen tetroxide, Mr. Cremeans says, but still with a curingtemperature dependence. If curing temperature was less than 140° F., the resistance of the coating to nitrogen tetroxide was marginal, as indicated by severe chalking and a loss of electrical properties. However, a technique for making resistant coatings that would cure at room temperature was found. Since some of the coatings showed good resistance, but only with high curing temperatures, the Battelle workers tried baking only part of the coating. This was done by curing the proper ratio of epoxy and anhydride at about 250° F. and grinding the resin into a fine powder. The powder was then dispersed into additional liquid epoxyanhydride resin mixture which was then allowed to cure at room temperature. This coating showed significant improvements in resistance to nitrogen tetroxide over the same basic coating without fillers or with inert fillers. The best coating obtained using the resinous fillers was an epoxidized polybutadiene-PMDA composition that had an acid-to-epoxy ratio of 0.5 and contained 50 parts resinous filler per 100 parts uncured epoxy resin in the binder. Enough anhydride was added to this mixture to cure the liquid resin to give the 0.5 acid-to-epoxy ratio. The resin-filled, epoxidized polybutadiene formulations were the only coatings cured at room temperature that were rated nitrogen tetroxide-compatible according to fumes, splash, and electrical resistivity tests conducted at Martin. The concept of resinous fillers to improve chemical resistance of coatings in general (not only to nitrogen tetroxide but to other corrosive chemicals as well) carries over to other systems, Mr. Nowacki asserts. Various resin-filled coatings were applied to cold-rolled steel panels, aged one week, then subjected to corrosive chemicals such as nitric and sulfuric acids, and strong solvents such as methyl ethyl ketone and phenol. Although improvement varied for different coatings and with different reagents, there was generally a twofold to 10-fold improvement in chemical resistance over unfilled coat-
