The High Cost of Plastic Convenience
Since their creation, plastics have been an indispensable ingredient in consumer lifestyles. They have found their way into various household and commercial products, such as water bottles, food containers, packaging materials, or disposable utensils. Plastic materials are convenient and inexpensive, but their disposal poses an environmental dilemma (Fig. 1). Although plastics only accounted for 12% of total municipal solid waste generation in the United States in 2008, they steadily increased since the 1960s and now constitute the greatest amount of discarded material with a low rate of biodegradation . Furthermore, of the 30 million tons of plastic that ended up in municipal solid waste centers in 2008, over 75% was discarded into landfills . The problem with plastic material in landfills, besides the space they occupy, is that they contribute a barrage of toxic chemicals to the fluids that drain and percolate through the landfill (known as leachate). Toxic chemicals that are derived from plastics (for example, phthalates) have been found in ground water due to leachate infiltration, posing a great concern to human and wildlife health .
Dealing with Different Plastics
A plastic is made up of individual molecules called monomers, which are linked together to form long chains called polymers. Each polymer has unique chemical properties, physical properties, and functions . Consumer plastics are largely made from six different polymer resins, which are indicated by a number etched onto the surface. The numbers or resin codes are numbered from 1 to 7. Fig. 2 outlines the different polymer resins, their resin codes, main properties, general applications, and potential recycled products. The chemical composition and function of each resin controls where the resin can be recycled and the recycling rate . According to the EPA, plastic with resin code 7 (mixed plastic or other less-commonly used polymers) accounts for 22% of total plastic waste, but is the least recycled with a rate of 6%. This could be attributed to the difficulty of separating mixed plastic during the recycling process. On the other hand, PET, or resin code 1, only accounts for 12% of the total plastic waste, but has a recycling rate of about 20% . Because of its widespread use in drinking bottles, PET is very identifiable and easy to sort.
All resins are held in separate holding tanks and then make their way to two other tanks for solvent removal and the release of vapors . The polymer resins can then be cut into pellets and shipped to processing plants to be made into new products. A comparison between the Rensselaer technology (Fig. 3) and physical sorting demonstrates that the selective dissolution process involves more technology and is more complex than current reclamation technology. However, this technique has been shown to produce recycled plastics that can economically compete with virgin plastics, thereby providing an incentive to boost recycling rates . Furthermore, it can better accommodate the mixture of polymers in resin code 7. The Rensselaer technology is a good short-term solution that can help reduce the amount of plastic that ends up in landfills, but other engineering technology is needed for long-term use.
Another way to create BPs is the production of polyesters by the bacterial fermentation of sugars and lipids extracted from plants . There are five types of polyesters that can be isolated from the fermentation process—PHA, pullulan, and xanthan are removed directly, while PLA and TPA are extracted from the lactic acid and aspartic acid produced in fermentation. These biodegradable polymers can be modified with synthetic or natural polymers like starch and cellulose to make other products like shampoo bottles, packaging, fibers, trash bags, and cutlery . BPs provide a long-term solution for replacing hazardous non-degradable plastics in landfills that potentially release toxic compounds and adversely affect human and wildlife health, but they are still under debate for wide use.
Development for the Future
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