Make nylon 6-6 “greener” and zinc-free

Outdoor stadium seats, ski bindings, tire reinforcements, and other products that require strength, durability, and weather resistance are all made with a type of nylon called nylon 6-6. However, the production of this material requires an environmentally unfriendly process, the first step of which uses the endangered element zinc as a catalyst. Now, researchers have developed “greener” methods for this step that use alternative metals. They might even be able to substitute waste iron in the form of rust or ferric oxide for the endangered element.

The researchers will present their findings today at the American Chemical Society (ACS) fall meeting.

“According to ACS Green Chemistry Institute estimates, zinc is only 50 to 100 years from its extinction,” said Amina Aly, an undergraduate student who presented the work at the meeting. “And currently, manufacturers are using zinc as a reducing agent and catalyst to make cyclohexene from trans-1,2-dibromocyclohexane, which is the first step in the five-step synthesis of nylon 6-6.”

“Nylon” is a general term for a family of synthetic polymers, called polyamides, which are made up of repeating units. Different types of nylons, such as nylon 6 and nylon 6-6, use different building blocks and therefore have unique properties. Nylon 6-6 is so named because it is made up of two molecules, each having six carbon atoms, which are linked together as a repeating unit.

To find a substitute for zinc, Aly, who works in Brian Agee, Ph.D.’s lab at the University of Augusta, looked for other metals that were found nearby in the periodic table and had properties similar chemicals. Other criteria were that the metals had to be more abundant than zinc and safe to work with. The team chose to study cobalt, aluminum, iron, copper and nickel as possible catalysts in the production of cyclohexene. In addition, the researchers wanted to identify greener methods that save energy and water, while using less aggressive chemicals. So, they incorporated a reflective solar dish instead of an electric hotplate and a water-saving condenser instead of a regular condenser. In addition, the team replaced the propylene glycol with the more dangerous ethylene glycol as a heat transfer agent in the water-saving condenser, which cools the reaction without the need for a continuous flow. of cold water like ordinary condensers.

Aly found that iron was the best catalyst tested so far, with slightly lower yields than zinc. “We also found that solar power is really the way to go for this synthesis, because the sun is much stronger than any hotplate you are going to find, and much faster,” she says. Carrying out the synthesis outdoors with a solar reflector only required 30 minutes, compared to 3 to 4 hours in the laboratory with a hot plate. The researchers also found that the increase in reflux time –– heating the reaction for a specific time and using a condenser to continuously cool the vapors produced to convert them back to liquid form –– from 15 minutes to 30 minutes increased dramatically. the yield . “Since we use the radiant energy of the sun, we do not waste electricity on additional heating,” notes Agee. The researchers say their methods could easily be extended to industrial nylon 6-6 manufacturing.

Although iron is an abundant metal, Aly and Agee want to try and catalyze the reaction with an even more environmentally friendly scrap iron that can be found anywhere the metal is left outside to get wet: the ferric oxide or rust. “If the ferric oxide purchased from a chemical company works for the reaction, I seriously consider going to my parents and scraping some rust from their barn to try,” Agee said. “…

More information:
Zinc Alternative Metals Investigation for the First Stage of Nylon 6-6 Synthesis Using Green and Conventional Techniques, ACS Fall 2021.