The Evolution of Polymer Materials: Part 6 – From Thermosets to Thermoplastics

In the 1930s, the rubber industry had reached a century of growth, celluloid had been on the market for more than 50 years, and phenolic resins were a staple across many sectors. Up to that time, the most impactful advances in polymer science were rooted in crosslinked, thermosetting systems.

Today the landscape has shifted dramatically, with thermoplastics taking center stage. The commodity polymers—polypropylene, polyethylene, polystyrene, and PVC—make up the bulk of global consumption. However, for high‑temperature applications that rival crosslinked polymers and metals, advanced thermoplastics such as polyamides (nylons), polycarbonates, polysulfones, and PEEK prevail.

Tracing the evolution of thermoplastics is complex because laboratory breakthroughs often lagged behind market entry. Polystyrene, first identified in 1839, only entered commercial production in 1931 after overcoming challenges in controlling its exothermic polymerization. PVC, discovered in 1872, faced early commercial hurdles because its melting point exceeded its thermal degradation temperature.

In 1926, Waldo Semon of BF Goodrich solved this problem. While experimenting with dehydrohalogenating PVC in a high‑boiling solvent to bond rubber to metal, he found the solvent plasticized the PVC, reducing its softening point and enabling melt processing.

German chemist Hans von Pechmann first synthesized polyethylene in 1898 by decomposing diazomethane, a toxic, explosive gas he had discovered earlier. The hazardous nature of diazomethane precluded large‑scale production, despite polyethylene now reaching annual consumption figures over 100 million metric tons (220 billion lb).

ICI researchers Eric Fawcett and Reginald Gibson independently rediscovered polyethylene in 1933 while testing high‑pressure gas mixtures. By combining ethylene with benzaldehyde under extreme pressure, they produced a white, waxy product now recognized as low‑density polyethylene (LDPE). The process was initially inconsistent, but in 1935 chemist Michael Perrin refined the reaction, enabling reliable production that went commercial in 1939—over four decades after the first laboratory synthesis.

High‑density polyethylene (HDPE) emerged in the early 1950s with the advent of new catalysts. Phillips Petroleum’s J. Paul Hogan and Robert Banks developed a chromium‑oxide catalyst in 1951; patents followed in 1953, and the process entered commercial use in 1957, still known as the Phillips catalyst. Simultaneously, Karl Ziegler introduced a titanium halide–aluminum organometallic system, which Giulio Natta refined. These catalyst families lowered the temperature and pressure for polyethylene synthesis and produced a linear, highly crystalline polymer that was stronger, stiffer, and more heat‑resistant than its branched counterpart.

Such parallel breakthroughs often lead to patent disputes. In this instance, the legal contest over HDPE patents was resolved only in 1983 in favor of Phillips scientists. Yet Ziegler and Natta, who first published their titanium‑halide system, received the Nobel Prize in 1963—20 years earlier—recognizing their foundational contribution.

These catalyst innovations paved the way for polypropylene, the fourth commodity polymer. Fawcett and Gibson had earlier synthesized polypropylene in the mid‑1930s, but their initial product was a tacky, atactic polymer unsuitable for structural use, serving merely as an adhesive.

Polypropylene differs from polyethylene in that each repeating unit carries a sizable methyl side group alongside three hydrogens. In atactic polypropylene, these methyl groups occupy random positions, inhibiting crystallization. The new catalysts enforced a uniform stereochemistry—placing the methyl group consistently—allowing the polymer to crystallize.

By enforcing a regular stereochemistry, the new catalysts yielded crystalline polypropylene with superior strength, stiffness, and a higher melting point than HDPE. This breakthrough gave rise to two polymers that together represent over 50 % of global annual polymer output. The terminology—atactic, isotactic, and syndiotactic—was coined by Giulio Natta’s wife, Rosita Beati, a non‑chemist, to describe the stereoregular architectures now standard in polymer science.

All four commodity polymers emerged from serendipitous laboratory observations, a pattern that recurs in many modern materials. Parallel to these breakthroughs, the foundational chemistry initiated by Hyatt in the 1850s continued to evolve, leading to further significant innovations.

Although these subsequent advances did not yield the massive volumes of the four commodity polymers, they addressed critical challenges and leveraged chemistry now classified as biopolymers—aligning with contemporary sustainability goals. We’ll explore these developments in the next installment.

ABOUT THE AUTHOR: Michael Sepe is an independent materials and processing consultant based in Sedona, Ariz., with clients throughout North America, Europe, and Asia. He has more than 45 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203‑0408 • mike@thematerialanalyst.com