Polyolefins Stabilized by Boric Acid Combinations (1966)
U.S. Patent No. 3,235,532, granted on February 15, 1966, to Jack R. Hopper, Eugene E. Poirot, and Raymond A. Speed (assigned to Esso Research and Engineering Company), describes a multi-component stabilizer system formulated to prevent oxidative degradation and discoloration in polyolefins such as polypropylene and polyethylene.
This invention resolved two compounding problems in early industrial plastics manufacturing: severe polymer breakdown caused by transition metal contaminants (such as catalyst residues or copper wiring in electrical insulation) and thermal discoloration that turned clear or white polymers into an off-color tan during high-temperature molding.
The Innovation: Boric Acid Synergy
Standard stabilization packages typically combined a hindered phenolic antioxidant with a sulfur-containing secondary antioxidant. While partially effective at low temperatures, these combinations failed under severe thermal stress or when exposed to metal contamination (iron, titanium, aluminum, or copper).
The inventors discovered that adding small amounts of boric acid (H3BO3)—typically 0.01% to 1.0% by weight—acts synergistically with conventional phenolic and sulfur-based systems to neutralize color degradation and dramatically extend thermal-oxidative life.
Why Boric Acid?
- Metal Deactivation: Suppresses catalytic degradation provoked by traces of residual Ziegler polymerization catalysts or direct contact with metals like copper during wire-coating applications.
- Color Suppression: Halts discoloration caused by high-heat processing (up to 550°F), keeping molded articles clear-white instead of yellow or tan.
- Thermal Synergism: Significantly boosts long-term resistance to embrittlement and oxidative decay under sustained elevated temperatures.
Key Chemical Components
The composition is a synergistic formulation where each additive targets a distinct degradation pathway:
| Component | Function |
| Alpha-Olefin Polymer | The base solid resin (e.g., polypropylene or polyethylene) with molecular weights ranging from 10,000 to 1,000,000. |
| Boric Acid (H3BO3) | The inorganic stabilizer (0.01% to 1.0% by weight) that deactivates metal-driven breakdown and eliminates processing discoloration. |
| Phenolic Inhibitor | Primary radical scavenger (e.g., Ionol / 2,6-di-t-butyl-4-methylphenol or dipinene diphenol) that traps free radicals to stop polymer chain scission. |
| Sulfur-Containing Costabilizer | Secondary antioxidant (e.g., dilaurylthiodipropionate / DLTDP or dialkyl sulfides) that decomposes hydroperoxides into non-reactive species. |
Performance: Thermal Stability and Color Retention
The patent demonstrates that while standard dual systems fail rapidly, adding boric acid yields dramatic improvements in both oxidative endurance and visual appearance.
Oxidative Life at 315°F (in pure O2):
- Polypropylene with 0.1% Ionol + 0.25% DLTDP: Failed in 4.5 hours.
- Polypropylene with 0.1% Ionol + 0.25% DLTDP + 0.3% H3BO3: Remained stable for 14.5 hours (extending performance in air at 300°F from ~1 day to over 6 days).
Color Rating Under Iron Contamination (FeCl3):
- With Ionol + DLTDP + Iron: Rated 6 (Tan/severely discolored).
- With Ionol + DLTDP + Iron + 0.10% H3BO3: Rated 2+ (Clear White).
Compounding and Processing
The inventors outlined practical industrial methods for incorporating the stabilizer suite into the base resin:
- Recovery: Isolate the polymer powder from the polymerization slurry after deactivating catalysts with methanol.
- Blending: Introduce finely divided boric acid powder (particle size between 5 and 500 microns) or an aqueous boric acid solution directly to the dry polymer particles.
- Additive Incorporation: Dissolve the phenolic inhibitor and sulfur costabilizer in a volatile solvent (such as xylene, hexane, or methanol) and spray over the polymer mix, or add them directly in dry form.
- Homogenization: Mill or extrude the compounded mixture to yield uniform pellets suitable for injection molding, high-temperature film blowing (e.g., food packaging), or fiber extrusion.
Industrial Application and Context
Developed at Esso Research and Engineering Company during the expansion of the commercial plastics industry, this stabilization method directly addressed the limitations of first- and second-generation Ziegler-Natta catalyzed polyolefins.
By neutralizing trace metallic catalyst fragments without requiring intensive polymer washing, and by enabling the use of polypropylene in direct contact with copper conductors, this formulation expanded the use of polyolefins into higher-temperature electrical insulations, extruded textiles, and food-grade packaging films.
Summary of Claims
The patent explicitly claims:
- A composition comprising a solid alpha mono-olefin polymer (2 to 8 carbon atoms) containing copper contaminants, stabilized by effective amounts (0.01% to 1.0% by weight) of a phenolic inhibitor, a sulfur-containing costabilizer, and boric acid.
- Specific combinations utilizing polyethylene or polypropylene as the base polymer.
- Finished industrial articles comprising polypropylene formulated with this ternary stabilizer system in direct mechanical contact with copper.
