Spanagel decided to perform a simple experiment. Nylon 6-6 is made from hexamethylene diamine and adipic acid. While he heated a mixture of hexamethylene diamine and adipic acid to produce nylon polymer, he held a piece of litmus paper in the vapor coming from the reaction mixture. Sure enough, the litmus paper turned blue, indicating that there was base in the vapor. That base was hexamethylene diamine.

Hexamethylene diamine was evaporating from the reaction mixture. This upset the balance of adipic acid to hexamethylene diamine in the reaction mixture. High molecular weight nylon can only be produced when the numbers of molecules of two reactants are exactly equal(1:1 stoichiometric ratio). So the loss of hexamethylene diamine led to low molecular weight nylon, which was useless.

To keep the hexamethylene diamine from evaporating, Spanagel had to make nylon polymer under high pressure, 250 pounds per square inch (psi). To make nylon at such a high pressure, he had to carry out the reaction in an autoclave. Until then, the polymerization had always been carried out in glass vessels. Glass would easily shatter at 250 psi. So Spanagel had to carry out the polymerization in a a silver-lined steel autoclave. Under the high pressure, the hexamethylene diamine stopped evaporating, and the desired molecular weight nylon polymer was easily produced.

But there were more problems to be solved. At first, the autoclave had to be silver-lined, because silver was the only known material that would not discolor the nylon polymer. This might be acceptable for a small autoclave which makes two pounds of nylon polymer, but for a large commercial-scale autoclave, something cheaper had to be found. Eventually it was found that stainless steel would work. The first two batches of nylon made in a stainless steel autoclave would be discolored, but subsequent batches would not be. Later it was found that treating a new autoclave with hydrochloric acid made "breaking-in" the equipment unnecessary.

To view a sketch of the polymerization process, made by Joe Labovsky, click here. As you can see in the picture, after polymerization, the nylon was extruded as a ribbon through the bottom of the reactor. The ribbon was then chopped into chips.

Also from Jow labovsky, here is the step-by-step process used to make nylon polymer in the 250 lb. autoclave.

Making Nylon Salt

• Add 1 molal part of hexamethylene diamine [NH₂(CH₂)₆NH₂].
• Add 1 molal part of adipic acid [HOOC(CH₂)₄COOH].
• Stir in deionized water at 70 ^oC.
• Precipitate the nylon salt from solution.

• Evaporate nylon salt solution to ~80% salt. Drop (feed) solution into autoclave.
• Add acetic acid (stabilizer) and TiO₂ (delusterant) if needed.
• Set autoclave controls to 295 ^oC and 250 psi.
• At 250 ^oC instantaneous polymerization occurs resulting in a polymer of 15,000 to 18,000 desired molecular weight.
• Automatically the H₂O is bled off to 0 psi over a 30 minute period.
• Hold autoclave at 0 psi for 30 minutes to "settle" molten polymer.
• Extrude (cast) nylon [as ribbon] over a roll sprayed with water.
• Cut nylon ribbon into chips.
• Blend batch with five other batches to obtain average uniformity (molecular weight).