Manufacture of Azodicarbonamide (1961)
U.S. Patent No. 2,988,545, granted on June 13, 1961, to Henry A. Hill, describes a significant improvement in the production of azodicarbonamide (ADC). This chemical was widely proposed as a “blowing agent”—the substance responsible for creating the tiny air bubbles in cellular (sponge) rubber and plastics.
Before Hill’s invention, azodicarbonamide suffered from poor efficiency, inconsistent particle size, and a tendency to leave behind unreacted orange-red crystals that discolored the final product. Hill discovered a way to produce ADC in microcrystalline form (roughly one micron in size) with a yield of over 98%, making it vastly more effective for industrial use.
The Chemical Process
The manufacture of azodicarbonamide is a two-step synthesis involving hydrazine and urea.
Step 1: Formation of Hydrazodicarbonamide
Urea is reacted with hydrazine sulfate and sulfuric acid to form a white intermediate slurry called hydrazodicarbonamide.
2NH_2CONH_2 + N_2H_4 * H_2SO_4 ————————————-> NH_2CONHNHCONH_2 + (NH_4)_2SO_4
Step 2: Oxidation to Azodicarbonamide
The intermediate slurry is then oxidized using chromic acid (derived from an alkali metal chromate and sulfuric acid). Hill’s innovation focused on precisely controlling the temperature during this stage.
NH_2CONHNHCONH_2 + [O] ———> NH_2CON=NCONH_2 + H_2O
The Innovation: Temperature & Concentration Control
The “old way” of producing ADC often resulted in large, orange-red crystalline aggregates that were difficult to disperse in rubber. Hill found that by using a more concentrated oxidant at lower temperatures, he could produce discrete, light-yellow microcrystals.
Key Parameters of the Hill Process:
- Initial Cooling: The reaction mixture is kept between 32°F and 85°F during the initial addition of acid.
- Partial Conversion: Conversion is allowed to reach 5% to 30% at these low temperatures.
- Final Heating: The mixture is then heated to a maximum of 150°F to complete the conversion.
- Resulting Particle Size: The particles are approximately one micron in their major dimension.
Performance in Rubber Vulcanization
To prove the superiority of his microcrystalline ADC, Hill compared it to the standard ADC of the time (referred to in the patent as the “Example 3” product).
Density Reduction in Shoe Soling Stock (Example 5):
| Property | Hill’s ADC (Ex. 1) | Standard ADC (Ex. 3) |
| Parts per 100 parts Rubber | 12.5 | 12.5 |
| Specific Gravity | 0.18 | 0.37 |
| Percent Expansion | 615% | 262% |
The data showed that Hill’s version produced more than a three-fold reduction in density. Because the particles were smaller and more uniform, they dispersed better and decomposed completely, meaning no leftover yellow/orange pigment remained to stain white or light-colored rubber products.
Industrial Significance
This patent was crucial for the mass production of lightweight, high-quality “sponge” materials.
- Shoe Soling: Allowed for the creation of lightweight, durable soles for footwear.
- Insulation: Improved the efficiency of plastic foams used in thermal and acoustic insulation.
- Cost Efficiency: Since hydrazine was an expensive raw material, increasing the yield from 84% to over 98% represented a massive cost saving for manufacturers like National Polychemicals, Inc.
Summary of Claims
The patent claims the specific process of forming an aqueous slurry of hydrazodicarbonamide, adding an alkali metal chromate, and slowly adding sulfuric acid while maintaining a temperature not exceeding 85°F for partial conversion, followed by heating to no more than 150°F for final conversion.
