Challenges and Opportunities in Plastic Recycling

Plastics are inexpensive, durable and easily moldable materials with wide-ranging applications. For this reason, plastic production volumes have reached significant levels over the past 60 years. The expansion of plastic applications has led to various environmental problems. Of petroleum and gas-derived products produced globally, 4% is spent on non-recyclable plastic raw materials and 3-4% is spent on the energy required for their production. Plastic raw materials are mostly used in single-use packaging products or products with a service life not exceeding one year. Although polymers are durable and complete their service lives, they can remain undegraded in nature and cause environmental pollution. Recycling will be one of the most effective methods to eliminate environmental problems and constitutes the most dynamic sector of today's plastics industry. Recycling reduces the amount of waste that must be disposed of, carbon dioxide emissions, and petroleum consumption. In this study, we will focus on important recycling methods such as downgauging or reuse of products, use of alternative biodegradable materials, and fuel and energy recovery compared to other waste reduction strategies.1 Plastics have been recycled geographically in different quantities since 1970 depending on raw material type and application. In some countries, packaging material recycling has shown rapid growth in recent years. Advancing technology and plastic collection and sorting systems are creating new opportunities for recyclable plastics to be reprocessed.

1. Overview of Waste Management

Within European Union countries, priority is given to waste management issues such as waste collection and energy generation through incineration to control urban solid waste quantities. Today, approximately 520 kg of residential solid waste per capita per year falls in Europe. This figure is expected to rise to 680 kg per capita per year by 2020. Generally, plastic waste is recovered from landfills or dumpsites. Plastic packaging is discarded along with other organic waste due to its light, fragile and flexible nature. The amount of material entering the waste management system can be reduced to lower levels through plastic downgauging. (For example: replacing heavy packaging formats with lighter ones.) Plastic waste quantities can be reduced by designing products suitable for reuse, repairable or recyclable.

1.1 Waste Incineration and Energy Recovery

Waste incineration reduces landfill demand for plastic waste. However, the release of hazardous gases into the atmosphere during the combustion process raises concerns. PVC and halogenated additives are found mixed in plastic waste. These products cause the release of substances such as dioxins, polychlorinated biphenyls and furans into the environment. Due to the environmental pollution risk, plastic incineration is a less common practice than landfill and mechanical recycling applications. Japan and some European countries (such as Denmark and Sweden) are among countries with comprehensive infrastructure for incinerating residential solid waste containing plastic.

1.2 Reduction

By reducing the amount of packaging used per product, waste quantity can also be reduced. Economically, most producers use the minimum necessary material for a given application. (Table 1) This principle can also lead to excessive packaging use in terms of aesthetics, accessibility and marketing. Existing processing and production investments may result in over-packaging of some products.

1.3 Reuse of Plastic Packaging

In many European countries, take-back and refill schemes exist. Along with glass bottles, PET bottles are included in these systems. For local businesses, this is a more appropriate method than large-scale packaging waste reduction strategies. Recycled plastics have wide applications ranging from electronic equipment to vehicle components and regain added value. Additionally, in the transport industry, containers and pallets are seen to be made from reusable materials. In Australia, reusable plastic shopping bags are used instead of some single-use plastic bags that are difficult to include in recycling processes, in Ireland plastic bags used in accordance with laws are collected for return, and in China and Bangladesh examples, the use of light plastic bags has been banned.

1.4 Plastic Recycling

Recycling terminology is grouped into four categories: primary recycling (obtaining products with equivalent properties through mechanical reprocessing), secondary recycling (obtaining products with inferior properties through mechanical reprocessing), tertiary recycling (recycling of chemical components), quaternary recycling (energy recovery). Primary recycling is generally referred to as closed-loop recycling, secondary recycling is referred to as recycling based on the same system but waste collection. Tertiary recycling is expressed as chemical or raw material recycling obtained through depolymerization of polymer chemical components. Quaternary recycling is energy recovery from waste. Biodegradable plastics can be used as compost and are defined as organic or biological recycling, and also serve as examples for quaternary recycling. Closed-loop recycling is the highest value-added recycling for thermoplastics. However, different polymers and materials such as metal, paper, ink and adhesive are used in plastic packaging, which complicates the recycling process. Closed-loop recycling is the most practical method in terms of effectively separating polymer components from the waste source and stabilizing them against degradation during reprocessing. For ideal plastic waste recovery applications, the range of polymers used should not be wide. For example; if all PET bottles are made from similar PET material they can be recycled and used as polyester fiber, but HDPE bottles produced by the blow molding method are not suitable for these cases. As a result, PET bottles and HDPE milk bottles found in some countries among post-consumer plastic waste can be effectively mechanically recycled.

2. Plastic Recycling Systems

Plastic materials are recycled in various ways. The ease of recycling varies depending on the polymer type, packaging design and product shape. For example, plastic containers made from a single polymer type have more economical and simpler recycling compared to multilayer and multi-component packaging. All thermoplastics including PET, PE and PP are materials with high mechanical recycling potential. For example, unsaturated polyester or epoxy resins cannot be mechanically recycled but can be used as filler material by reducing their size or converting them to powder form. These thermoset plastics cannot be remelted and reshaped because they permanently contain cross-links. Recycling of cross-linked rubber is based on shredding car tires and mixing them into other products. As waste tire levels are expected to increase, the European Union has banned the accumulation of waste tires in landfills. The biggest challenge in plastic waste recycling stems from the incompatibility of different plastic types with each other. The inability of contents to mix at the molecular level and the different processing conditions of plastics cause incompatibility. For example, a small amount of PVC present in PET waste causes degradation of PET during recycling. Since PET is melted and recycled at higher temperatures, hydrochloric acid gas released from PVC at these temperatures causes degradation. PET present in PVC waste remains as solid lumps during recycling and reduces the value of the recycled material. For this reason, it is not suitable to mix unprocessed polymer into technically recovered plastic as it reduces some quality properties (color, transparency or mechanical). In contrast, in multilayer/multi-component polymers, contamination can occur during recycling due to different processing characteristics. Recycling of plastic waste consists of basic stages such as collection, sorting, cleaning, size reduction and separation of incompatible polymers.

2.1 Collection

Collection of plastic waste is carried out either through return by individuals after use or based on separate collection at source. The desired levels have not been reached due to insufficient public awareness of take-back and lack of necessary enforcement of deposit schemes. For this reason, in recent periods, emphasis has been placed on separate collection at source of recyclable plastics. For these programs to be more economically beneficial, recyclable materials (paper/cardboard, glass, aluminum, steel and plastic) must be separated (Figure 2). With source separation programs, 30-40% of produced plastic bottles are recycled. Considering these factors, if daily waste is separated, higher value-added recycling rates can be achieved. Plastics have different thermal properties from each other and therefore need to be separated and recycled individually. Plastic waste must be collected separately using globally accepted recycling codes. Table 6 provides recycling codes, recycling quantities and application areas of commonly used plastics.

2.2 Sorting

Separation of mixed recyclable materials is done manually or with automatic systems (Figure 3). The automatic pre-sorting process is used to separate plastic waste from glass, metal and paper waste. Generally, it is an appropriate method for detecting and separating clean PET and uncolored HDPE bottles from waste. Automatic sorting of containers is widely used by recycling companies. These systems generally consist of using near-infrared spectroscopy (Fourier-transform Near-infrared) to detect polymer type and optical color identification camera systems to separate colored materials (Figure 4). With the FT-IR method, many commercial plastics (such as PET, PP, PS, ABS, PE, PPS, PET, PC, PMP, PBT, PA, PETG, Nylon and PMMA) can be separated in a very short time and at low cost.   Optical sorters are used to separate transparent, light blue, dark blue, green and other colored PET bottles from each other (Figure 5). Systems with multiple detectors are preferred to maximize sorting performance. Another separation technology is systems containing X-ray detectors. These systems are used for separating PVC containers. These containers are easily identified because they contain 59% chlorine by weight. Recycling factories do not actively collect flexible packaging materials because they lack sufficient equipment to separate them from other waste. Many plastic recycling facilities use rotating screens and density-based air separation systems for sorting flexible materials such as bags or film packaging. Different technologies in this field such as ballistic separation, advanced hydrocyclones and air classification are used for flexible packaging separation.

2.3 Size Reduction and Washing

Hard plastics are generally made into flakes and cleaned to remove food residue and materials such as adhesive. Thanks to next-generation washing technologies, one ton of material can be washed with 2-3 m3 of water. Some innovative technologies perform dry cleaning with friction force on surfaces to remove organic materials and dirty surfaces.

2.4 Advanced Separation Methods

After size reduction, different separation techniques can be applied. Density-based wet methods and surface property-based separation methods are the basic methods used for plastic separation.

2.4.1 Density-Based Wet Method

This method is frequently used to separate polymer mixtures based on their density. Plastics have a wide density range between 1 g/cm3 and 1.5 g/cm3. In density-based separation, the general process is: if the density difference is sufficient, less dense material floats and denser material sinks. Density-based separation is performed in 3 different ways: flotation-sinking, hydrocyclone and centrifuge methods.

Flotation-Sinking Method

It is used to separate simple density differences. A large flotation-sinking tank and water are used. For example; HDPE bottles with a density of 1 g/cm3 and PET bottles with a density of 1.34 g/cm3 can be separated by this method (Figure 6).

Hydrocyclone Method

This method is an economical and effective option for separating plastic mixtures. It also takes advantage of density differences. The shape of the particles is quite important in the hydrocyclone separation method. If only water is used as the separation liquid, drying costs are exceeded. Water velocity is important. The higher the speed, the more efficient the separation. Hydrocyclones have two outlets and one inlet. The outlet at the top is the fine material outlet pipe (vortex) and the outlet at the bottom is the coarse material outlet pipe (apex) (Figure 7).

Centrifuge Method

This method is a technique that separates the most complex densities and is highly efficient (Figure 8). However, the disadvantage of this method is that the equipment and separation costs are high. Nevertheless, only small particles are preferred. For the centrifuge method, particle shape is not important and it can be used to separate hard plastics.  

2.4.2 Wet Separation Method Based on Surface Properties Froth Flotation

It is an effective and efficient separation method. Froth flotation is used to separate two plastics with small density differences but different surface properties (hydrophilic-hydrophobic) (Figure 9). The steps of this method are: 1. First, the polymer is placed in an appropriately sized mill. 2. Then, in the mixing tank, the polymer, water and some chemicals are made into a slurry. 3. Finally, froth flotation is carried out by adding air or gas to the flotation machine. Water-repellent particles stick to air bubbles while water-loving particles remain in the cell and are discharged with the cell. For example, PET (1.33-1.37 g/cm3) and PVC (1.33-1.37 g/cm3) polymers have very similar density values. Since the water contact angles of both polymers are approximately 20-85 degrees, the hydrophobic properties of both polymers are also similar. Excessively basic sodium hydroxide solution changes the hydrophobic properties of the PET surface while having little effect on PVC. Thus, separation of these two polymers is possible.2

Dry Separation Method

It is a method of separating plastics using the difference in electrostatic charge. This method depends on the triboelectrification of dissimilar plastics (Figure 10). For example, two different polymers can be separated by means of an electric field by creating a charge difference from sanding their surfaces simultaneously to remove roughness. This method is used while most plastics are separated.

2.4.3 Advances in Recycling

Over the past ten years, innovations in the recycling field have increased as investment in sorting detectors. Special identification and decision-making software have been produced to enable more accurate and efficient sorting. For example, FT-NIR detectors operate with minimal error for up to 8,000 hours. Another developing application is the use of recycled polymers in closed systems in place of original polymers. For example, in the United Kingdom since 2005, 50% to 70% recycled PET (rPET) has been made into A/B/A sheet form and used in food packaging materials. The (A) layer is made from original raw material because it contacts food, the (B) layer can be used in the middle section of the sheet because it does not contact food. Recycled PETs are classified as super clean and therefore have extensive application in the food industry. rPET is less transparent than original PET and is used mixed with original PET at rates of 30-50%. Leading European countries such as Germany, Austria, Norway, Italy and Spain collect plastic waste bottles along with plastic materials such as pots and cans, form them into film and bales and perform recycling operations. These wastes, which are not just bottles, are classified with developing technology and then their recovery is possible through washing processes. In summary, recycling is an important strategy for managing plastic waste that has reached the end of its life. It is extremely important in preventing the damage of plastic waste to the environment and contributing to the economy. Recycling methods continue to be applied today, but problems in the collection of plastic waste continue due to technological, economic and social reasons. For post-consumer plastic packaging to be recycled on a wider scale, they need to be separated from other plastic waste and residential organic waste. To increase recycling rates and use recycled plastic instead of original plastic, polymer producers need to conduct more work on this issue. Assoc. Prof. M. Atilla Taşdelen - Yalova University Faculty of Engineering Department of Polymer Engineering Serhat Oran - Doctoral Student Yalova University Faculty of Engineering Department of Polymer Engineering References *"J. Hopewell, R. Dvorak and E. Kosior, Plastics recycling: challenges and opportunities,Philosophical Transactions of the Royal Society B (2009) 364, 2115–2126" 1S. Oran, M. Babacan, Plastik geri dönüşümünde son teknolojiler, Yalova Üniversitesi,Polimer Mühendisliği Bitirme Tezi 2014. 2J. Drelich,T. Payne, J. H. Kim, and J. D. Miller, Selective Froth Flotation of PVC From PVC/PET Mixtures for the Plastics Recycling Industry, Polymer Engineering and Science, 1998, Vol. 38, No. 9