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closures or engine-base areas of missiles from high temperatures of the rocket blast. This usage in cludes inside surfaces of skirts, out side surfaces of nozzles, and interior portions of the rocket chambers themselves. These rubbers can pro tect outside structural joints, too— probe holes, raceways, and joints between stages—from rocket blast or thermodynamic heat. Maybe for Nose Cones, Too Heat-flux densities in nose cones of uncontrolled ballistic missiles rise to about 1000 B.t.u. per sq. foot per
Polyurethanes N o w in Fire-Safe Foam Form Novel polyester urethane rigid foams contain chlorine-rich organic acid in alkyd resin for built-in flame retardance
AND
INTERPRETS
second. Consequently the possi bility of silicone rubber nose-cone formulations for controlled re-entry, where heat-flux densities run only about 4 % of this level, is not too farfetched. In a "more conventional" ap plication the Mercury capsule which carried Cmdr. Alan Shepard to the edge of space last M a y used R T V silicone rubber as a sealant around rivets, bolts, and joints, to protect against both the high-altitude vac u u m and the waters of the Atlantic Ocean. And every Minuteman mis sile contains literally hundreds of
pounds of silicone rubber, chiefly RTV-88 reinforced with red iron oxide to give bolt-heads and other fasteners a smooth covering, and RTV-60 to provide caulking for the skirt areas of the three stages and to insulate large areas inside all three stages. Polaris, Titan, Bomarc, and other missiles also use silicone-rubber compounds where convenient ap plication of heat-resistant material is required. But the concept of these elastomers as ablative materials is new and in triguing—another facet of the ver satility of silicones.
the 200 million board-foot annual level) will require 600 million board feet a year within the "foreseeable future" ; needs for panels and siding should rocket from today's 30 million board feet to 400 million board feet— and other building-product uses should about double—during the same period. Another burgeoning field exists in aviation where foams form "transparent" radomes and
burning foam board, the fire goes out at once without smoldering. Key to this built-in permanent fire resistance is Hooker's H E T acid— chlorendic acid (also known as hexachloroendomethylene t e t r a h y d r o phthalic acid), 1,4,5,6,7,7-hexachlorobicyclo - (2.2.1) - 5 - heptene - 2,3dicarboxylic acid, made by allowing hexachloropentadiene to react with maleic anhydride: Cl
Ο
I HE ASTRONOMICAL boom forecast for structural plastic foams over the next few years will bring forth many new and improved foam formula tions, new polymers for foams, and foams with new properties. O n e of the first properties to get widespread attention is fire resistance. And Hooker Chemical's new fire-retardant polyurethane foam displayed re cently at the Ninth National Plastics Exposition in New York's mammoth coliseum, Durez Plastics Division's Hetrofoam, is a major step in this di rection. Polystyrene foams treated or for mulated for fire retardance are already established on the market. (In most cases, this property is so important that the resistance to flame is now "manufactured i n " at no price increase; some producers are not even furnishing untreated ma terial at all anymore for structural uses.) And today fire-retardant urethanes are making their bid for the lucrative rigid-foam market in the building field. For instance, foam manufacturers estimate roof construction (now at 32 A
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S HC-C
Ck
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HC-σ
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-Cl - COOH
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ο serve as core material for sheet metal or reinforced plastic sandwich as semblies. And urethane foams want a big piece of these tremendous in creases. Generally, urethane foams are formulated with phosphated or chlo rinated plasticizers to provide a de gree of fire retardance. T h e first types of Hooker's Hetrofoam—rang ing in density from 1.9 to 6.0 pounds per cubic feet—are formulated to include an alkyd resin containing combined halogen. This reacts with a conventional polyisocyanate in the presence of a foaming system. Be sides exhibiting excellent structural properties up to 250° F., low thermal conductivity ( " K " factor 0.10 to 0.12 at 75° F.), and a high degree of adhesion to sandwich panels between which they foam, these new foams are immediately self-extinguishing. Upon removal of a flame from a
COOH Cl-
\ Η Cl
Unique among dibasic acids, it con tains 5 4 . 7 % of its weight in stable chlorine. This compound, and other similar Diels-Alder reaction products of a hexahalocyclopentadiene with an unsaturated polycarboxylic acid (or a polycarboxylic acid anhydride, halide, or ester) can be esterified with a polyhydric al cohol (such as glycerol), according to U . S. Patent 2,865,869, Canadian Patent 607,104, and other recent foreign patents. A higher propor tion of trifunctional alcohol provides greater cross linking for the alkyd component and makes a more rigid foam; difunctional alcohols give less rigid products. An aliphatic acid (generally adipic, but any of a number of sat urated or unsaturated acids will do) added to the alkyd improves the reac(Continued on page 35 A) Circle No. 27 on Readers' Service Card
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tion. And tertiary-alcohol foaming agents dehydrate and blow more effectively in the presence of a small amount of strong acid catalyst (inorganic, organic, or Lewis-type acids). Any of a number of various polyisocyanates will give good foams. Aromatic polyisocyanates are preferred because they show less toxicity and more reactivity than the aliphatics. Best semiprepolymers for H E T systems contain 2,4- or 2,6tolylene diisocyanate, or mixtures of the two isomers. Isocyanate concentrations in the range of 85 to 110 mole % produce strong rigid foams. First customers for Durez's new flame-retardant polyester urethane foams are: • Toyad Corp. (Latrobe, Pa.)— makes slab stock and molded shapes for such applications as insulation in cold-storage chambers, truck trailers, and air conditioners. • U. S. Steel's American Bridge Division (Harrisburg, Pa.)—enamelled and vinyl-clad sheet-steel panels for AmBridge modular building. These two Hetrofoam systems differ in that the AmBridge material is formulated for particularly good adhesion to steel and for high rise capacity. T h e firm fills preheated (to 165° F.) wall and door panels bycontinuous foaming; complete curing takes only 15 minutes. Toyad adds a conventional blowing agent (trichlorofluoromethane) in the semiprepolymer, lets slabs cure overnight before slicing, but post-cures its shaped forms only 20 to 30 minutes after foaming in preheated (150° to 155° F.) molds. Conventional urethane foams made fire-retardant by use of additives suffer from loss of high temperature strength or show poorer aging characteristics. Also these additives, where they are plasticizers, disappear gradually through evaporation or leaching. O n the other hand, the inflammable nontreated urethanes are degraded by water (at elevated temperatures) and char (in sections thicker than three inches) from heat built up during the exothermic foaming process. With its new Hetrofoam and with a one-shot system just introduced at the plastics show, Hooker expects to open the door even wider to the fast-growing urethane foams.
^ ^ CUTS PASTE MIXING COSTS AS MUCH AS Jested by a leading producer of solid rocket fuels and compared with paste-mixing equipment costing ten times as much, Rietz Extructors exhibited dramatic highs in production efficiency and substantial design and construction advantages.
RIETZ EXTRUCTOR AS A C O N T I N U O U S
PASTE
MIXER
Performance
Mixing efficiency as checked by radio-active tracers . . . 99.7% (This figure is now somewhat higher because of modifications introduced since the test.) Dollar per ton mixing cost 80% to 90% less than other equipment tested. (Based on initial investment and operating cost.) Design
Much easier to clean as all parts of the crevice-free interior are readily accessible. Mechanically simple with a minimum of moving parts. Practically no maintenance because of rugged all-welded construction and slow-speed, high torque operation at very low horsepower. RIETZ EXTRUCTORS a r e a v a i l a b l e in a v a r i e d r a n g e o f capacities in carbon or stainless steel, and with a selection of extrusion plates to adapt to any paste mixing requirement, and for the efficient mixing of dry powders and granular solids. OFFICES I N PRINCIPAL CITIES
Rietz DISINTEGRATORS
MANUFACTURING CO. Santa Rosa, Calif. · West Chester, Penna. PREBREAKERS · EXTRUCTORS · THERMASCREWS Circle No. 2 on Readers' Service Card VOL. 53, N O . 8
·
AUGUST 1961
35 A
