Foamable Composition and Method of Foaming (1964)
U.S. Patent No. 3,141,002, granted on July 14, 1964, to Henry A. Hill, represents a major breakthrough in the production of cellular plastics and synthetic rubbers. Assigned to National Polychemicals, Inc., the invention introduced a specialized “blowing agent” capable of surviving the extreme temperatures required to process modern high-density plastics like polypropylene and nylon.
Prior to Hill’s work, manufacturers struggled to create “foamed” or “sponge” versions of these high-melting-point materials because existing chemical blowing agents would decompose too early in the manufacturing process, resulting in collapsed or uneven foams.
The Innovation: Barium Azocarbonate
The heart of this patent is the use of barium azocarbonate as a blowing agent. While other nitrogen-releasing chemicals existed, they typically activated at temperatures too low for the “new” hydrocarbon polymers of the 1950s and 60s.
1. High-Temperature Stability
Materials like Polypropylene (melting point 325–340°F) and Type 6 Nylon (melting point 390–425°F) require processing heat that would vaporize standard blowing agents instantly. Barium azocarbonate, however, remains stable until temperatures reach 475–525°F, allowing the plastic to melt and flow before the “bubbles” are released.
2. The Closed-Cell Structure
When the barium azocarbonate finally decomposes, it releases gas uniformly, creating a “closed-cell” structure. This means the bubbles are discrete and sealed, making the material:
- Odorless: No chemical after-scent.
- Non-Discolored: The final product remains white or clear rather than turning yellow.
- Stable: The chemical byproduct actually helps protect the plastic from degrading under heat and light.
Key Applications: Wire Coating and Insulation
Hill identified a critical industrial use for this foamed plastic: electrical wire insulation.
- Low Dielectric Constant: Foamed polypropylene has superior electrical properties compared to solid plastic.
- Flexibility: The cellular structure allows wires to bend more easily without cracking the insulation.
- Material Economy: Because the plastic is “puffed up” with gas, manufacturers can use significantly less raw polymer to coat the same length of wire.
Performance: Comparative Foam Densities
The patent demonstrates the effectiveness of the blowing agent across several high-performance resins by measuring their Specific Gravity (density) before and after heating.
| Material | Process Temp (°F) | Initial Density | Blown Density | Result |
| Low-Density Polyethylene | 475°F | 0.94 | 0.58 | Uniform White Foam |
| High-Density Polyethylene | 525°F | 0.96 | 0.60 | High-Strength Cell |
| Polypropylene | 525°F | 0.93 | 0.44 | 50% Weight Reduction |
The Manufacturing Process
Hill provided a precise method for creating the blowing agent and integrating it into plastics:
- Chemical Synthesis: Reacting Barium Oxide with Azodicarbonamide in water to produce a light yellow, fine-grained powder.
- Dispersion: Tumble-mixing the powder (typically 1 to 10 parts per 100) into the raw plastic resin.
- Extrusion or Molding: Heating the mixture under pressure (up to 940 p.s.i.).
- Expansion: Reducing the pressure to atmospheric, allowing the trapped gas to expand the plastic into a foam.
About the Inventor: Henry A. Hill
Dr. Henry A. Hill was a distinguished chemist and a trailblazer in the American chemical community.
- Education: He earned his Ph.D. from MIT in 1942, during an era when African Americans faced immense barriers in high-level scientific research.
- Leadership: In 1977, Dr. Hill became the first African American president of the American Chemical Society (ACS).
- Legacy: Beyond this patent, Hill was a staunch advocate for laboratory safety and professional standards. His work on “plastomers” helped bridge the gap between heavy industrial rubber and the modern age of lightweight, high-performance plastics.
Summary of Claims
The patent explicitly claims:
- A foamable composition consisting of a thermoplastic polymer and barium azocarbonate.
- A method for foaming high-melting polymers including Polyethylene, Polypropylene, Nylon, and Butyl Rubber.
- The use of activators (like stearic acid) to lower the decomposition temperature for use with heat-sensitive materials like natural rubber.
