Recycling Plastics: New Recycling Technology and Biodegradable Polymer Development

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 [1]. Furthermore, of the 30 million tons of plastic that ended up in municipal solid waste centers in 2008, over 75% was discarded into landfills [1]. 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 [2].

Although most of the discarded plastic finds its way into landfills, about 16% is combusted to produce energy [2]. Since plastic is derived from petroleum products, its energy yield is almost as high as fuel oil; plastic yields 19,900 BTU/lb whereas oil yields 20,900 BTU/lb [3]. However, the incineration of plastics produces air pollution by releasing chemicals such as hydrogen chloride, dioxin and fine particulate matter [2]. Another way of dealing with plastics is recycling, but this only accounts for about 7% of the plastics encountered in municipal solid waste centers [1]. Because plastics pose a threat to human and wildlife health when disposed of in landfills or through incineration, engineers are working to increase recycling rates by improving current recycling technologies while also developing biodegradable polymers for future use.

To better understand the difference in recycling rates of various polymers, it must first been seen that different resins are not compatible with others because varying molecular structures can repel one another [5]. Polymer mixtures can also “lead to the deterioration of the mechanical performance of plastic products,” if they are not engineered properly [5]. The molecules that make up various resins have different sizes and chemical composition. For example, PVC (resin code 3) contains chloride, while HDPE and LDPE contain only carbon and hydrogen. Plastics also have additives to make the flame retardant or flexible or resistant to UV damage. All of these factors make it nearly impossible to obtain a homogenous plastic mixture with uniform mechanical properties. When resins are separated, the range of recycled products created is much larger and the processing is cheaper and less complex. This is why plastics that are generally easier to separate, like PET and HDPE, have higher recycling rates. It is important that the sorting process be well regulated to insure the integrity and overall performance of a recycled plastic product.

Although sorting is the step in the current recycling process that has the largest impact on product integrity, it is also very difficult to perform well, which explains why only 7% of plastics end up being recycled [2]. Sorting can be done by mechanical or physical methods [5]. Sorting by mechanical methods require the plastic to first be reduced in size and then washed to remove contaminants [5]. According to the Association of Postconsumer Plastic Recyclers, mechanical technologies involve one or more techniques that exploit the different chemical compositions or properties of each resin. For example, one of the processes called ‘float-and sink’, which sorts plastics by density, sends the washed and chipped plastic into tubs of water and separates the pieces that float or sink [5]. However, some plastics, like PET and PVC, have similar densities so an additional mechanical procedure is needed for complete separation [5]. One possible technique uses x-ray fluorescence to detect the chlorine atoms in a sample, allowing others with similar numbers of chlorine atoms to be combined and then separated from the mixture [6]. Another technique is infrared sorting, which requires the plastic fragments to be exposed to infrared light. Exposure causes the plastic fragments to emit light of a specific wavelength unique to their chemical composition, allowing for separation of the different components [5].

These mechanical sorting methods don’t always work. Coloring, adhesives, residues and additives applied when the plastics were first processed can interfere with the mechanical sorting process [5]. The alternative is physical sorting, which can sometimes be more economical than mechanical methods [2]. In this case, the plastic products move past human operators on a conveyor belt and are separated based on shape and color. The disadvantage of this procedure is that it is very labor intensive and subject to human error, which can compromise the integrity of the recycled plastic products [2]. Because the sorting process is one of the main reasons for low plastic recycling rates, new recycling technologies are being developed that improve or eliminate this step.

By taking advantage of the physical and chemical properties of polymers, new processes that eliminate the sorting step can be implemented. One such technology, developed at the Rensselaer Polytechnic Institute, relies on a process known as selective dissolution, a more complex reclamation step compared to the one used in the current recycling process. A detailed sketch of the selective dissolution process can be seen in Fig. 3. In general this process starts with comingled plastics being shredded, washed, and dried, then placed in a dissolver where a xylene solvent is added. At a temperature of 15˚C, polystyrene (resin code 5) dissolves and drains through a filter from the dissolver to be placed in a holding tank. Next, more xylene is added and the temperature is increased to dissolve the next polymer resin. The process is repeated until only PET and PVC are left in the mix. These comingled plastics are then moved to a smaller dissolver for better mixing and the process is repeated with a xylene-cyclohezanone​ solvent and a higher temperature [2].

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 [2]. 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 [2]. 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.

Engineering new materials like biodegradable polymers can be a long-term solution for eliminating plastics from landfills. Biodegradable polymers, as the name suggests, are meant to degrade upon disposal with the help of microorganisms [7].

Biodegradable polymers, or BPs, can be made from a number of materials such as starch, cellulose, and polyesters [7]. Fig. 4 shows a diagram depicting the life cycle of biodegradable polymers. Starch and cellulose, called biopolymers because they are produced by plants, are extracted and blended with synthetic polymers to produce biodegradable polymers. By varying the amount of starch, cellulose, and synthetic polymers in the mixture, different plastic properties can be achieved. Once processed, the BPs can be used for many applications such as packing foam, toothbrush handles, adhesive tape backing, films, and trays [7]. Upon reaching the municipal waste center they can be sent to bio-waste collection or compost sites where they can be properly degraded by microorganisms to form carbon dioxide, water, biomass, and humic matter, all of which serve as nutrients for plant life. With the addition of sunlight, plants can grow and produce another batch of biodegradable polymers, repeating the cycle [7].

Another way to create BPs is the production of polyesters by the bacterial fermentation of sugars and lipids extracted from plants [7]. 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 [7]. 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.

Plastics are too important to be eliminated from consumer products, but their disposal is an environmental problem that cannot be overlooked. Current recycling rates are very low, because of the difficulty in separating the different resins. New plastic recycling techniques eliminate the need for a sorting step by exploiting the chemical properties of each resin. However, this process still requires a lot of energy and the use of other chemical solvents. Therefore, a long-term solution seeks to replace current plastics with biodegradable polymers that can be composted with other organic waste. New material of this type would stop dangerous leaching of landfill plastic compounds into drinking water and the release of toxins and carbon dioxide through combustion. Through process development and materials engineering, scientists and engineers are tackling the challenge of polymer disposal. With increased consumer awareness, corporate sponsorship, and government incentives, these technologies have the potential to create more sustainable lifestyles and a healthier planet.