TECHNOLOGY
Du Pont introduces modified neoprene foam Isocya η ate-modified, fire-resistant material will compete with SBR and urethane foams Du Pont has developed a fire-resistant foam cushioning material that will be competing in the styrene-butadiene rubber and urethane foam markets. The Du Pont material is an isocyanatemodified neoprene. The foam is made by adding a blend of undistilled isocyanates to a frothed neoprene latex. In announcing the product at the ACS Division of Rubber Chemistry meeting in Cleveland, Du Pont's Fran cis J. Asti, a chemical engineer in the elastomer chemicals department, cited its potential applications. They in clude car, truck, bus, and aircraft seats; carpet and fabric backing; insti tutional, maritime, and truck cab mat tresses; auto and truck body seals; and as a foam adhesive to polypropylene. Because Du Pont is the only U.S. producer of neoprene, the modified neoprene is a logical extension of its product line. Although Du Pont isn't saying which "selected undistilled iso cyanate" it mixes with neoprene, tolu ene diisocyanate (TDI) would prob ably be a bookmaker's choice. Du Pont has a captive supply, and it's rel atively inexpensive to make. While retaining neoprene's out standing resistance to oil and deterio ration by aging, Du Pont says, the isocyanate-modified foam has superior wet gel strength, broader density range (especially lower), as well as im proved flame resistance. Uncured foams rapidly develop high wet gel strength which reduces linear shrink age and permits densities as low as 3 pounds per cu. ft. Conventional neo prene latex foam is difficult to prepare below 6 pounds per cu. ft. The wet gel strength also allows demolding of low-density foams from pin core molds after 15 to 30 minutes at room temper ature. The usual steam vulcanization precure is eliminated. Cured neo prene/isocyanate foams have two to three times the load-bearing capacity (per unit density) of neoprene foams. For cushioning applications using 3 to 7 pounds per cu. ft., the isocya nate content is about 20 parts per hun dred rubber (phr) of dry neoprene. This concentration gives an optimum balance between rate of gel develop ment and compression modulus per unit density, Mr. Asti explains. How ever, there is a slight sacrifice in resist ance to compression set. For higher 46 C&EN MAY 6, 1968
Continuous frothing units are not usually equipped to monitor the rate of latex flow. Thus, in the neoprene/isocyanate system equipment changes must be made, such as adding a flowmeter for the latex and an isocyanate pumping and metering unit
density applications, such as rug back ing or auto body seals, the isocyanate content is reduced below 20 phr to improve compression set resistance and to increase gel time. In fire-retardancy tests the neo prene/isocyanate foam outperformed all other cushioning samples evaluated, Mr. Asti says. The addition of anti mony trioxide combined with chlori nated paraffin further improves its fire resistance. In the radiant panel flame spread test (ASTM E162-66T), the index for neoprene/isocyanate foam at 5 pounds per cu. ft. is 150 to 200. Poly ether urethane fire-retardant foam at 1.5 pounds per cu. ft. has an index of 470 to 650, according to Du Pont's figures. With fire-retardant additives in the neoprene/isocyanate foam the index is below 25 for densi ties ranging from 4 to 20 pounds per cu. ft. Subjected to a hot bolt test (de
scribed in MIL-R-20092E), the neo prene/isocyanate foam did not fail at 500° C. Fire-retardant urethane fails this test below 400° C , and styrenebutadiene rubber fails below 300° C. Both the new Du Pont material and urethane foams fail to burn when touched to a flame in the candle test. However, styrene-butadiene rubber is consumed. Compounded neoprene/isocyanate latex may be processed in either batch or continuous frothing equipment, Du Pont says. However, in a continuous frothing operation a monitoring device is required to control the latex flow, and a separate pumping and metering unit is needed for the isocyanate (see flow diagram). This modification is needed to accurately control the neo prene latex to isocyanate ratio to pro duce consistent quality foams. The unit is typical of those used in the ure thane foam industry.
Batch processing is done in Hobart or similar mixing equipment. First, the neoprene latex is frothed for about three minutes. Then the "polyfunctionai" isocyanate and potassium silicofluoride (the gelling agent) are blended in. The foam gels for 15 to 20 minutes, and it cures and dries at 250° to 330° F. Petrolatum is used to retard the rate of foaming and pro duce finer cell structure, though it is not required for improvement in com pression set as with conventional neo prene foams. In continuous opera tions petrolatum is unnecessary. After being frothed by either con tinuous or batch methods, the foam is molded using Dunlop or Talalay tech niques which have been slightly modi fied and simplified. Dunlop process ing steps call for frothing, steam precure, demold, wash, squeeze, dry, and cure. The steam precure (vulcaniza tion), wash, and squeeze steps are eliminated. The simplification of the Dunlop process suggests a possible de crease in mill cost, Du Pont says. Also, in carpet and foam on fabric applica tions, the neoprene/isocyanate foam can be processed on a continuous molding belt or conveyors used for natural or S BR foams.
The conventional Talalay process ing steps include frothing, vacuum, freeze, carbon dioxide, heat, demold, and cure. The freeze cycle is not used for neoprene/isocyanate rubber. Hence, processing costs are reduced. The foam's rapid development of wet gel strength at room temperature obviates heating the molded foam to high temperatures (above 150° F.) be fore demolding. Foam is demolded after 30 minutes or less at room tem perature. Therefore, low-cost rein forced plastic molds may be used. Conventional aluminum and steel molds can also be used, as can pin de signs. It should be noted, Mr. Asti says, that although the wet gel strength is very high, it is not equivalent to vulcanized neoprene. So, care in han dling the molded material and certain pin mold redesigns may be required. Also, molded isocyanate-modified neo prene foams are more difficult to re lease than conventional foams. The new foam, like neoprene, re quires more intense curing conditions than other latex foams. However, because of the better heat resistance of the modified foam, it can be sub jected to higher temperatures to shorten the drying and curing time.
Du Pont's modified neoprene shows increased flame resistance Index** Neoprene/isocyanate foam with fire retardant additives 4 - 2 0 lb./cu. ft.
