Perspective

Recycling of the #5 Polymer

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Science  10 Aug 2012:
Vol. 337, Issue 6095, pp. 700-702
DOI: 10.1126/science.1221806

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Abstract

Polypropylene (PP) is a widely used plastic with consumer applications ranging from food packaging to automotive parts, including car battery casings. To differentiate it from other recyclable plastics, it is designated as #5. Here, the factors contributing to PP recycling rates are briefly reviewed. Considerations include collection and separation efficiency, processing chemistry, and market dynamics for the products derived from recyclates.

In 1970, recycling of plastics from the municipal solid waste (MSW) stream was virtually nonexistent. By 1994, the recycling rate of different types of plastics found in the United States MSW (containers and packaging, durable goods, and nondurable goods) increased to 4.7% (1), and it reached 8% in 2010 (2). Similar growth can also be found in the plastics recycling rates of many Western European countries—and also in Japan since the introduction of government legislation in 1995. The low levels of what is known as “mechanical” recycling (the term excludes incineration with or without energy recovery or conversion into chemical feedstock) reflect the complexity of discarded and used materials and the corresponding issues with their effective separation and cleaning. However, the recycling rate for some individual plastics is much higher. Sorting of commodity thermoplastics used in packaging (e.g., containers and films) is facilitated by using a number identification code from 1 to 7. This is a voluntary system introduced by the Society of Plastics Industry in 1988 with the goal of helping consumers to determine whether certain types of plastics are collected for recycling in their area (2). The system is also in use in countries other than the United States (e.g., Canada and Switzerland). In North America, successful recycling operations mostly deal with #1 and #2 packaging plastics [polyethylene terephthalate (PET) and high-density polyethylene (HDPE), respectively], for which large-scale collection infrastructures exist and commercial applications for the recyclates have existed for some time. For example, in 2010, 28% of HDPE bottles and 29% of PET bottles and jars were recycled (2).

Polypropylene Recycling Overview

Polypropylene (PP), designated as #5, is among the commodity thermoplastics found in the MSW. Its recycling is technically possible and has been practiced for certain single-industrial products and industrial scrap for more than three decades. Recycling of PP bottles, although increased in 2010 to 35.4 million pounds, is still lower than either PET or HDPE (3). In addition to the need for adequate collection means, the challenge for postconsumer packaging is in separating it from other plastics—including its own many variations—once it arrives at the waste station and beyond. For PP, recyclability is also related to a certain extent to its structural instability during melt reprocessing. PP may be found in other mixed waste streams, such as wire and cable coverings, discarded electronics, automotive shredder residue, and carpets (4). Here, I refer to recycling of PP items in general and not only to the ones that may appear in the MSW.

Polypropylene has several attributes (low density, thermal and chemical stability, stiffness, and low cost) that make it quite appealing for use in food packaging, including microwave-safe containers. These same properties have made PP an excellent choice in a variety of automotive and other industrial contexts. Specifically, it is used in the manufacture of carpets, ropes, automotive parts, car batteries, containers, crates, piping, furniture, consumer electronics, bottle tops, living hinges, laboratory equipment, storage boxes, buckets, medicine packaging, and even banknotes (Fig. 1). Recycled PP products may be the same as the original (e.g., auto parts, pallets, and storage bins) or repurposed to applications with less stringent performance requirements (e.g., bike racks, brooms, ice scrapers, buckets, and consumer items) (5). For example, PP banknotes, when withdrawn from circulation, are granulated and then recycled in household and industrial products such as wheelbarrows, compost bins, and plumbing fittings (6). So far, recycling of food containers to food-contact-grade packaging has not been possible given the stringent quality checks that recycled plastics need to meet under government standards; substantial efforts, however, are still in progress in the United Kingdom (7).

Fig. 1

An array of polypropylene products.

PHOTO COLLAGE BY L. BLIZARD; THINKSTOCK; ISTOCKPHOTO; SHUTTERSTOCK; WIKIMEDIA COMMONS

Recycling of industrial scrap from extrusion or injection molding has been a common practice over the years. Recycled PP products may include high-value or lower-value items, as discussed earlier. In general, demand and markets for post-consumer recyclates appear to be steadily growing. Driving forces include legislated minimum recycled–content mandates, procurement policies, expanded waste collection networks, and improvements in recycling technology.

Restabilization of PP

In general, recycled plastics are less costly than the original materials, unless separation and cleaning (beneficiation) costs were excessive; many recycled plastic items are considered to be useful only for low-value applications. It is common for quality and appearance to degrade with each reprocessing cycle. This is particularly true for polypropylene items, which, due to the presence of tertiary carbon atoms in the polymer backbone, are susceptible to pronounced thermo-oxidative degradation during melt processing and/or during use, as well as photo-oxidative degradation during use.

Plastics usually contain stabilizers that protect the polymer from such degradation. These stabilizers are usually consumed during melt processing and/or leach out during exposure of plastic parts and need to be replaced (through a process termed restabilization) to enhance the processing stability and increase the service life of the recyclate. The molecular weight and/or molecular-weight distribution of the initial polymer can change as a result of chain scission (typical for polypropylene) or cross-linking reactions (typical for polyethylene). Both these types of reactions may lead to irreversible changes in mechanical and rheological properties (8).

In the case of virgin PP, melt processing is only possible in the presence of combinations of a processing stabilizer or a blend of processing stabilizers with different, synergistic modes of action. Typical compounds for this purpose include a combination of sterically hindered phenols with phosphite or phosphonite substituents and an acid acceptor. Restabilization can substantially minimize further melt viscosity reduction that would correspond to an increase of the melt flow rate (MFR) as a result of molecular-weight reduction. Types of additives and their associated concentrations for restabilization can be screened by performing multiple extrusions, which would simulate repeated processing, or long-term exposure of the recyclate to elevated temperatures. Depending on the extent of the polymer degradation during use, different types and levels of restabilization will be required that may increase the overall materials cost of the recyclate; it has been shown, however, that restabilization is the key for successfully recycling PP in a variety of items such as crates and films (8).

Packaging

A few prominent food and beverage companies in the United States are moving on their own to recapture their packaging after its use by their customers. There is still little demand from recyclers for used PP cups, and many communities and municipalities in the United States lack the infrastructure—and the capital—to collect #5 containers and reprocess them. Companies that manufacture the packaging are logical candidates to be part of the recycling scenario, as experience has shown in Europe, Canada, and elsewhere where such responsibility was imposed on packaged goods companies.

Through the initiative of a yogurt manufacturer (9), a deal was struck with a major supermarket to place collection bins in its stores spread throughout most U.S. states. Yogurt, one of the most widely sold dairy products, is packaged in PP. Customers return their #5 containers (not only yogurt), which are then taken to a plant for cleaning, size reduction, and further processing to consumer items such as toothbrushes, razors, and cutlery (10). This successful program is still based on the initiative of individual companies; however, it remains to be seen whether legislation that would mandate extended producer responsibility will be introduced in different U.S. states.

Car Batteries

A successful collection infrastructure for plastic waste from car batteries also exists in the United States. About 95% of used automotive batteries are recycled for their lead, sulfuric acid, and polypropylene content. The polypropylene in the casings from end-of-life batteries represents a centralized and relatively homogeneous source of polymer and finds many markets, including automotive products and the next generation of batteries. The cost of recycling car batteries is partly covered by advanced disposal fees and/or take-back fees. This is acceptable to ensure that the lead is properly managed, but it also helps to subsidize the cost of the PP recycling. The PP casing makes up about 7% of the total battery and may be obtained in quantities sufficient to warrant the operation of a plastics recycling plant. Typical operations involve crushing and separation of materials into a heavy fraction (lead, metals, and ebonite) and a light fraction (PP and impurities). Upgrading of the light fraction—through grinding, sedimentation, drying, and cyclone separation—brings the PP regrind’s purity to about 99.5% (11). The material is then modified with additives by standard melt-mixing operations to produce compounds suitable for injection molding.

Mixed Streams

PP may be present at different concentrations in a variety of postindustrial or postconsumer waste streams. Pressures on landfill capacity, as well as impending legislation or regulations, have induced the plastics industry to pay much greater attention to the recycling of heterogeneous plastic waste. Standard sorting and cleaning beneficiation techniques may be used to increase the purity of the polymer. In some cases, at low concentrations of impurities in the waste streams, reactive chemical modification can be carried out, assuming that the contaminants will not affect the course of the reaction (12). The goal here is to modify the PP for new applications. For example, modification with maleic anhydride will enhance adhesion through the introduction of polar groups; peroxide reaction with or without coagents will modify the PP rheological characteristics. It has been reported recently that a high-performance grade of black PP derived from end-of-life vehicles can be potentially used in new automotive-related products (13). Furthermore, molding compounds from multicomponent recycled PP carpets have been evaluated (14).

In addition to separation and cleaning techniques, physical and chemical techniques have been developed to improve the quality of mixed plastics. For example, in carpets that may contain PP fibers and PP backing in combination with other fibers such as polyamides, near-infrared (NIR) spectroscopic methods, among others, can identify the types of fibers present and contribute to the advanced sorting of the face fiber. In the case of waste streams containing polymers with different structures such as polyamides, polyesters, EPDM (ethylene propylene diene monomer), or other commodity polymers, useful compounds can be prepared by melt mixing with suitable reactive or nonreactive compatibilizers (12). As an example, attempts have been made to compatibilize mixtures of polyamide and PP fibers found in abandoned fishnets and ropes, respectively, collected from the sea (15).

Outlook

Recycling rates of PP found in U.S. municipal waste streams will increase as consumers become aware that items from this #5 plastic can be easily identified and separated, similarly to those made of PET and HDPE, and successful sorting technologies are already in place. Legislation mandating extended producer responsibility may contribute to an acceleration of the recycling rates; similarly, successes in the ongoing efforts to create recyclates intended to be used for food-contact applications will increase the volume of recyclable PP. The issue of reduced properties as a result of the inherent thermo-oxidative instability of PP upon reprocessing may be addressed through the addition of appropriate restabilization packages and additives, including fillers and reinforcements.

Recycling rates of PP by melt reprocessing will likely continue to be dictated by the availability of the recyclable resin, the cost of the virgin resin, markets for the recyclates, and the overall economics of the recycling process, because technological issues related to processability appear to be easily addressed.

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