In the USA more than 13 billion plastic bottles are disposed of each year. Most plastic beverage bottles are made from PET. After being emptied and hopefully tossed in the waste bin the beverage bottle may end up being recycled. While plastics are recyclable, the resulting materials are limited to “second generation reuse” only, such as fleece clothes, carpet and other ‘second-life’ products. This means that at the final end of the life-cycle the materials made from recycled plastic bottles are ultimately disposed in landfills. Although there is a tendency to re-use recycled PET in new beverage bottles, such as the PlantBottle of Coca-Cola, only a relatively small percentage is added to the virgin resins used. In the United States, up to 63 lbs of plastic packaging per capita is discarded each year, instead of being repeatedly recycled.
This might come to an end.
In a paper published in the American Chemical Society journal “Macromolecules”, scientists from IBM and Stanford University detail discoveries that could lead to the development of new types of biodegradable, biocompatible plastics. The breakthrough also could lead to a new recycling process that has the potential to significantly increase the ability to recycle and reuse common PET and plant-based plastics in the future.
In general, mechanical, rather than chemical, recycling is used for PET because ….it’s too expensive to break the polymer down into its chemical parts, according to the National Association for PET Container Resources. There are two existing methods for accomplishing the chemical reaction, but they are very energy-intensive and have been abandoned because of the cost. Even with the use of existing catalysts to help the recycling reactions along, these processes must be done at high temperatures and under great pressure, and take a long time.
The researchers describe in their paper several new developed catalysts, one of which can be used to chemically recycle PET in a short time at 75 ºC. PET is made from two feedstocks, one of them an organic acid, the other ethylene glycol, which is relatively inexpensive. The catalyst works in an ethylene glycol solution. When shredded water bottles are placed in the solution, the catalyst causes the organic acid in the plastic to react with the ethylene glycol in solution to make PET that is of the same quality of which the bottle was made originally.
A major focus of the research has been on ring-opening polymerization, a strategy dominated by metal oxide or metal hydroxide catalysts. The research shows that organic catalysts both exhibit activities that rival the most active metal-based catalysts, and provides access to polymer architectures that are difficult to access by conventional approaches. This discovery and new approach using organic catalysts could lead to well-defined, biodegradable molecules made from renewable resources in an environmentally responsible way.
The paper also describes recycling or degradation strategies that would enable a “closed-loop” life cycle for materials that meet the needs of the marketplace while helping to minimize the environmental footprint left for future generations.
The PET-recycling catalyst, a type of molecule called a carbene, was inspired by vitamin B1, says Stanford chemistry professor Robert Waymouth. The Stanford and IBM researchers guessed that a similar organic small molecule might be good at catalyzing reactions that string esters together to make long polymers.
The IBM researchers will now collaborate with the King Abdulaziz City for Science and Technology in Saudi Arabia to test the chemical recycling of PET on a larger scale. In initial tests, they will focus on breaking down the polymer into its constituents. However, the company has also had good results using its organic catalysts to de-polymerize PET to make specialized materials such as feedstocks for high-strength plastics that are more valuable but are expensive to make using other pathways.
“You start with trash, and build it back up into higher value materials,” says Robert Allen, senior manager of advanced materials chemistry at IBM Almaden.
If the new catalysts have “even a modicum of success, it would be big news,” says Dennis Sabourin, executive director of the National Association for PET Container Resources.