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ROBERT C. WADE Metal Hydrides, Inc., Beverly, Mass.
Borohydride-Based Blowing Agent A mountain of laboratory d a t a can still not b e sufficient for predicting production results
SINCE
the discovery of the borohydrides by Schlesinger, Brown, and others in 1943, a great interest in the chemistry of these compounds has developed. For the past six years Metal Hydrides, Inc., has devoted a major part of its corporate energy and resources to the development of commercial processes for the manufacture of borohydrides and to the development of new uses for these materials. One of the potential uses for sodium borohydride and potassium borohydride is as a pneumatogen or blowing agent for elastomers. A brief look at the properties of borohydrides will help to understand this case. Sodium borohydride, NaBH,, and potassium borohydride, KBHI, are white crystalline compounds. They are soluble and stable in water under alkaline conditions. However, they can be hydrolyzed to evolve large quantities of hydrogen by lowering the p H of the solution with acidic materials, by raising the temperature of solutions, or by adding metallic ions such as cobalt, nickel, or iron which catalyze the deMBH,
+ 2 H 2 0 + MBO? + 4H2
composition. I n nonaqueous systems the borohydride may also react with the acidic hydrogen atoms in many organic compounds to evolve hydrogen in a similar manner.
potential outlets in the foamed plastic and rubber field. Scouting work in the laboratories indicated that materials such as sodium silicate, poly(viny1 acetate), polv(viny1 alcohol), epoxies, polyester and other resins could be foamed with the borohydrides. The possibility of developing a blowing agent for dry rubber and other elastomers appeared good. Success would open a large potential market in this field for the borohydrides. ,4 manufacturer‘s representative with a strong technical background in the rubber and plastic industry was engaged to assist in the development of a suitable blowing agent for dry rubber. His efforts were directed to a testing program lvith potential customers in the h-e\$ England area. At about this time, a private market research organization was engaged to conduct a survey of markets for pneumatogens. Their findings confirmed published estimates of markets for expanded rubber and other foamed products. This survey also made a number of useful recommendations for developing an acceptable product and for marketing such a product. I t also pointed out the technical and economic advantages and disadvantages of competitive products. The competitive materials are the so called “nitrogen blowing agents.” These materials decompose on heating to
release nitrogen. They are mild explosives. There are a number of such organic compounds produced commercially under various trade names. They sell for anywhere from 75 cents to 81.95 per pound. Obviously the company was not competing with sodium or ammonium bicarbonate which sell for only a few cents per pound. These materials are used extensively where the quality of sponge structure is of secondary- importance. However, in many cases combinations of a nitrogen blowing agent and a bicarbonate are used to obtain the necessary properties of the sponge. How then could sodium borohydride selling at $33 per pound or potassium borohydride selling at $17 per pound hope to compete in the market? The answer lies in the cost of the gas produced from these various sources. (See table.) At $17 per pound for potassium borohydride a competitive price can be established with most of the nitrogen blowing agents, When the prices of the borohydrides are lowered, as expected, their economic advantages increase. I t is very doubtful that competitive pricing could be maintained for the other blowing agents. Thus: the potential market and economic attractiveness of the borohydrides as pneumatogens was established. Then
Borohydrides as Pneumatogens The large amounts of hydrogen evolved in these reactions can be used to produce foams of rubber latex and vinyl plastisols (developed by the B. F. Goodrich Co., Sponge Products Division). T h e latex application has been patented but is not commercially attractive at present borohydride prices. The vinyl plastisol application is commercial and was described recently [Chem. Eng. X e w s 37, 38 (April 6, 1959)]. This application is growing at a very encouraging rate. With these technical and commercial developments well established, Metal Hydrides in 1957 looked for other
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Costs o f Gas from Pneumatogens Compounds
NaHC03
Gas Yield,
CO?
Theoretical Cu. Ft./Lb. 4.27
Nz Nz N2 N2 HP
2.87 2.0 3.1 3.9 39.4
180 125 193 240 2370
Hz
27.8
1660
Selling
Gas
Price, $
Produced
0.02 1.95 1.95 1.95 0.72 33.00 20.00 10.00 5.00 17.00 10.00 5.00
Cc./Gram 266
Cobt of Gas, O/Cu. Ft. 0.006
NO
11
C~HI(CON-CH~)~ 0 ( C sHaS0zNHNHz)z (NC0NHz)z (CHZ)SN~(NOZ)filler NaBHd NaBHa NaBH4 NaBHd KBHa KBHI KBHa
+
0.68 0.97 0.63 0.19 0.84 0.51 0.26 0.13 0.61 0.36 0.18
PITFALLS IN COMMERCIAL DEVELOPMENT
An Ideal 6lowing Agent Should Possess These Properties Gas must b e released over a definite and short range. Gas must be released a t a controllable but reasonably rapid rate. The gas should not be corrosive. The compound should disperse readily in the stock and preferably dissolve. The compound should b e cheap in application. The compound must b e stable in storage. The residue should not have an unpleasant smell. The residue should b e colorless and nonstaining. The compound and residue should b e nontoxic. The compound should not give rise to a large exotherm on decomposition. The compound should not affect the rate of cure adversely. The compound should function equally well in a closed mold.
But Secondary Characteristics M a y Assume Great Importance in Some Applications It should not b e flammable. It should not induce collapse or retraction after curing. It should b e solid, if possible, as liquids tend to induce collapse or shrinking of pores.
it was necessary to develop a suitable product that would find commercial acceptance for elastomers. The market survey enumerated twelve properties which an ideal blowing agent would possess. It had been determined previously that pure sodium borohydride or potassium borohydride could be milled into a rubber compound. The stearic acid used in the compound would decompose the borohydride to release hydrogen. This would expand the rubber prior to and during the vulcanization stage and give a sponge structure. Aside from the flammability hazard from hydrogen which can be minimized by adequate ventilation, hydrogen has a real advantage in its excellent heat transfer characteristics. This property permits faster and more uniform vulcanization of the expanded rubber. This is an important consideration in reducing unit production costs of sponge products. Hydrogen rapidly diffuses from a vulcanized sponge and is replaced by air in the cell structure, even in predominantly closed cell structures. The borohydride appeared to meet most of the requirements for an ideal blowing agent. I t was realized that the major difficulty in marketing a pure borohydride would be the very high unit price. A second technical difficulty would be to get production personnel accustomed to
handling and accurately weighing very small quantities of an expensive product. A diluent for the borohydride was obviously necessary. This would reduce the unit cost to a more palatable level for the purchasing agent and greatly simplify use in production. A suitable mixture of potassium borohydride and a clay was developed which appeared to meet all of the requirements for a good, if not ideal, pneumatogen for rubber. The combination could be used as a part for part by weight replacement for the nitrogen blowing agents. A name for the product, M H I HydriFoam was selected and copyrighted. The price range was established at $1.35 to $1.60 per pound, clearly under that of most of the nitrogen products. Laboratory tests looked promising. Market testing was undertaken in the New England area. Customer evaluations in laboratory equipment looked promising. A technical brochure was prepared based on laboratory experience. Sales started for commercial evaluation. Then a major problem developed.
Decomposition Problem
In the commercial blending and milling of rubber stocks, much higher temperature of the stocks is obtained than on laboratory mills. Laboratory mills seldom generate over 185' F. stock
temperature, while production mills generally generate about 200' F. in the stock. At these higher production temperatures, a slow decomposition of the borohydride was initiated. Because large batches of rubber are milled prior to the blowing and curing step of the smaller individual pieces, a considerable time differential exists between the production of the first piece and the last. During this time the slow decomposition of the blowing agent continued and hydrogen was lost by diffusion, The expansion of the individual pieces became noticeably less as time of storage lengthened. A good example of this is illustrated in pogo stick balls. When the pneumatogen was fresh the product was good, and the mold filled completely during the expansion and cure cycle. Alas, when made much later, the product was a complete failure. Insufficient hydrogen was left to expand the rubber sufficiently to fill the mold. There is an unhappy molder in this country who has several barrels of pogo stick balls which will never be used on pogo sticks. Yes, his lab tests were very good also. A second difficulty arose in making sponge rubber for shoe insoles. Again, laboratory tests of Hydri-Foam gave excellent results, and substantial savings were indicated in its use. I n this case, however, the difficulty arose in the difference between the laboratory and production curing cycle. These shoe insoles are placed in the rubber shoes and blown and vulcanized on an aluminum last. I n the laboratory cure cycle, the temperature is rapidly raised to the vulcanizing temperature, held for 10 to 15 minutes, then quickly cooled. Under these conditions HydriFoam worked well. In production, however, the temperature is raised slowly over a period of perhaps an hour or more in steam autoclaves. During this warmup period, decomposition of HydriFoam started, and the hydrogen diffused out of the soft, uncured stock and the mold, When the shoes were cooled and the pressure released, no cell structure was obtained. Fortunately, these deficiencies were discovered during market tests in the New England area. No advertising of Hydri-Foam has been undertaken. Hydri-Foam now has been withdrawn from further market development except in two cases where testing is going on with specialty rubbers. The problem is being examined further in these laboratories. Recently an entirely different approach has been devised for solving the problems encountered. The company is convinced that borohydrides will still find their place as pneumatogens for rubber, as the prices have now started their downward trend. VOL. 52, NO. 1
JANUARY 1960
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