The 1935 material called fiber 66
On 28 February 1935, at DuPont’s Experimental Station in Wilmington, Delaware, Wallace Hume Carothers’s team obtained a polyamide from hexamethylenediamine and adipic acid. It later became nylon 66. The two sixes refer to the six carbon atoms in each feedstock, rather than an experiment number or polymer-chain length. The team selected it from many combinations within a longer history of synthetic fibers.
Individual laboratory roles mattered. In his 1996 oral history, technician Joseph Labovsky attributed production of polymer 66 to Gerard Berchet and described testing it afterward, entering results in research notebook 2286. Research organized by Carothers, synthesis by Berchet and technical spinning tests together constitute the node.
How small molecules formed long chains
Carothers studied building long chains step by step from small molecules with reactive groups at both ends. The early Linear Condensation Polymers patent distinguishes such bifunctional reactions from multifunctional ones that readily produce three-dimensional structures. It discusses removal of volatile reaction products to promote condensation. Filed in 1931 and granted in 1937, it shows that research into spinnable chains preceded selection of nylon 66.
For nylon 66, hexamethylenediamine and adipic acid can first form a salt. Heating and polycondensation produce chains of repeated amide linkages while releasing water. Feedstock proportions, extent of reaction and impurities affect whether molecules become sufficiently long and processable. The later Synthetic Fiber patent discusses viscosity stabilization: the goal was a polymer retaining suitable properties under spinning conditions.
Making a filament and improving it by drawing
Pulling hot polymer into a filament was only the first step. Earlier polyester work had shown that a cooled fiber could be drawn again, changing strength and elasticity. Early patents used flexibility, ability to knot and orientation to distinguish useful fibers from resins that could only just form threads. Selecting nylon 66 involved testing performance as well as chemical composition.
Cold drawing aligns molecules more strongly along the fiber axis. The patent granted in 1938 describes a continuous sequence: melt passes through spinneret holes, and a faster second roller stretches the formed filaments. Synthesis, spinning and drawing serve distinct purposes. Different processing conditions can give fibers of different thickness and properties even with the same composition. The patent includes spinning examples for the polymer made from hexamethylenediamine and adipic acid.
From test tubes to integrated equipment
DuPont chose fiber 66 for further development in July 1935, with industrialization still ahead. Reliable feedstock supply, uniform heating and continuous flow through small spinneret holes were required. Labovsky recalled the move from dry nylon salt to aqueous salt solutions to improve heat transfer as batch sizes increased. Equipment progressed from test tubes to autoclaves.
Melt spinning also required control of oxidation, bubbles and viscosity. Pumps and alloys used in existing fiber processes did not automatically meet the new temperatures and pressures, requiring cooperation between engineers, metallurgists and operators. Labovsky’s accounts of gas pressure, pumping, cooling and winding describe the equipment and quality control between obtaining a promising polymer and making acceptable yarn daily.
Public introduction and stockings came later
The Synthetic Fiber patent was granted on 20 September 1938. DuPont introduced nylon at a public forum in New York on 27 October. Its Seaford plant in Delaware began production in December 1939, and nationwide U.S. sales of nylon stockings followed in May 1940. Laboratory synthesis, disclosure in legal documents, plant operation and consumer sales were distinct stages.
Nylon 66 connected selection of molecular composition with forming, orientation and manufacturing conditions, enabling fibers to be designed through chemical synthesis and engineering control. The 1935 experiment was a key selection result; subsequent products emerged from years of continuing development.