Making nylon 6-6 'greener,' and without zinc

A solar reflector heats a chemical reaction to make nylon 6-6. Credit: Brian Agee
Nylon 6-6 is a type that can be used to make outdoor stadium seats, ski bindings, tire reinforcements, and other products that need strength, durability, and weather resistance. This material is produced using the environmentally-unfriendly catalyst zinc. Researchers have now developed alternative metals to make this step more sustainable. Researchers may even be able substitute for the endangered element waste iron, either in the form ferric oxide or rust.

Today, the researchers will present their findings at the American Chemical Society's fall meeting.

According to the ACS Green Chemistry Institute estimates, zinc is only 50-100 years away from becoming extinct," said Amina Aly (an undergraduate student) who will present the work at the meeting. "And currently, manufacturers use zinc as the reducing agent and catalyst for making cyclohexene from trans-1,2-dibromocyclohexane, which is the first step in the five-step synthesis of nylon 6-6."

The general term "Nylon" refers to a group of synthetic polymers called polyamides that are made up of repeating units. Nylon 6-6 and nylon 6-6 are two examples of different types of nylons. They use different building blocks, which gives them unique properties. Nylon 6-6 gets its name because it is composed of two molecules that each have six carbon atoms and are linked together to form the repeating unit.

Aly, a Ph.D. student at Augusta University, searched for a replacement for zinc by looking to other metals in the periodic table that had similar chemical properties. The metals must be more plentiful than zinc and safer to work with. As catalysts for the production of cyclohexene, the team studied cobalt and aluminum as well as iron, copper, nickel, and iron. The researchers also wanted to find greener ways to save energy and water while using less toxic chemicals. They used a solar reflective dish to replace an electric hotplate and a water-saving condensationer instead of a regular condenser. The team also substituted propylene glycol in place of the more dangerous ethylene glycol to heat transfer agent in the water saving condenser. This cools the reaction without requiring a continuous flow cold water as with regular condensers.

Aly discovered that iron was the most effective catalyst, yielding slightly less than zinc. She says that the sun is stronger than any hotplate and can also synthesise faster. The synthesis was done outside using a solar reflector, which took only 30 minutes as opposed to 3-4 hours in the laboratory with a hot plate. Researchers also discovered that increasing the amount of reflux heating the reaction and using a condenser continuously cooling the vapors to make them liquid again from 15 to 30 minutes significantly increased the yield. Agee says that because we are using the radiant energy from the sun, we don't waste electricity by heating more. Researchers believe their method could be scaled up to industrial nylon 6-6 manufacturing.

Iron is a common metal. Aly and Agee would like to catalyze the reaction using rust, a more sustainable iron waste product. Agee states, "If ferricoxid purchased from a chemical firm works for the reaction," Agee said. "Because I'm a green chemist, there is no better source of a catalyst than something that you can find anywhere."

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More information: Nylon 6-6 synthesis is underway.