PerspectivePOLYMER CHEMISTRY

Designed to degrade

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Science  17 Nov 2017:
Vol. 358, Issue 6365, pp. 872-873
DOI: 10.1126/science.aap8115

Around 50 years ago, interest arose in making plastics that can degrade in the environment (1). Since then, a stream of research efforts has chased the dream of environmentally friendly materials that disappear without leaving behind fragments or harmful products. Such environmentally degradable plastics are, however, difficult to produce in practice. Durability is one of the requirements for plastic in most technical applications, whereas degradability is necessary for recycling in nature. Although advances are being made in developing degradable materials with suitable properties for particular applications, it is crucial that they are seen as part of a range of approaches and that degradation will always require particular conditions that depend on the specific material and its chemical and physical structure and composition.

Challenging Conditions

In biomedical applications, degradable polymers, including polyglycolide, polylactide, polycaprolactone, and polytrimethylene carbonate, have been used successfully for decades. The materials can be kept under safe conditions such as low temperature and nitrogen until used, after which they start to hydrolyze. Through designing the polymer's molecular architecture, it is possible to tune the degradation rate and even degradation products (2). The human body is a relatively controlled environment with known temperature and degradation conditions, making it possible to create these materials with optimum degradation properties.

By comparison, natural environments have much wider diversity in variables such as humidity, microorganisms, oxygen, sunlight, and temperature, making it extremely difficult if not impossible to control and ensure the complete degradation of even potentially degradable plastic materials. Controlled conditions can be created in commercial composting plants, allowing plastics classified as compostable to be successfully degraded through the combined action of heat, moisture, and microorganisms. In a compost, degradation of a material to acceptable degradation products can be sufficient; this is, however, different from complete mineralization. One of the major problems connected to the development, use, and disposal of degradable materials is that none are degraded in all natural environments; a specific environment is needed, and the conditions required depend on the type of plastic. Claims of degradability or environmental degradability should thus always be connected to a specific environment. This is of course quite difficult, if we consider environmentally degradable plastics as an answer to the problem of plastic litter.

For example, plastic debris in marine environments is now widely recognized as an enormous environmental problem, requiring critical improvements in plastic handling and waste management (3). However, degradable plastics might not be the solution, because the conditions in seawater are not ideal for rapid degradation. Even plastics such as starch-based plastic carrier bags that rapidly degrade in compost might not readily degrade in seawater or even soil, because the favorable degradation process achieved in compost is not always achieved in the sea and in other natural environments (4). The degradation rate and the degradation products are also highly dependent on local characteristics such as the presence of light, oxygen, bacteria, and the temperature. This can cause serious problems; both conventional and degradable bags can rapidly alter marine assemblages and the ecosystem services they provide (4). A further complication arises from the fact that although we want the material to rapidly degrade in natural environments, it should not degrade during its shelf and service life.

Biobased Does Not Mean Degradable

Today there are huge research efforts worldwide to use biomass as a feedstock for production of polymer materials (5, 6). In all cases, it is crucial to remember that degradability is connected to the chemical and physical structure and composition of the material and its interactions with the surrounding environment and not to the origin of the raw material (7). Even if a natural starting material is degradable, this does not mean that the material after chemical modification is still degradable. In fact, all modifications or additives will potentially influence the degradability, the type of degradation products formed, and the ultimate environmental fate of the product.

Corn is sowed under a biodegradable film in a field in France.

PHOTO: FORGET PATRICK/SAGAPHOTO.COM/ALAMY STOCK PHOTO

Experimental Approaches

Different approaches to achieve degradable or environmentally degradable plastics have been proposed (8). Some early approaches tried to promote the environmental degradation of cheap plastics, such as polyolefins used for packaging and mulch film. However, polyolefins consist of a carbon chain with covalent carbon-carbon bonds, which no natural enzyme can cut directly. The first step in degradation is oxidation, but usually the materials contain a stabilizer to avoid oxidation. Making the materials more sensitive to oxidation by, for example, sunlight also makes them less stable during use. Materials with pro-oxidants have been commercialized and marketed as oxo-degradable plastics or pro-oxidant additive containing (PAC) plastics. It is much easier to start the oxidation and fragmentation of a material than it is to control the oxidation rate and degradation products and to ensure and convincingly prove that the material will be completely mineralized or degraded to acceptable degradation products (9). As a result, many countries have come to the conclusion that the use of these materials should be avoided or forbidden. In 2014, members of the European Parliament proposed a ban on PACs in the European Union. The ban was blocked, but a 2016 European report summarizes the impact of PACs on the environment and calls for suspending their use (10). A new legislative proposal is also expected.

So far, the most widely accepted “environmentally degradable” polymer materials are based on aliphatic polyesters or starch with hydrolyzable and/or biodegradable chemical bonds—the same type of materials that have been successfully used in biomedical and bioresorbable applications. Polylactide (modified to reduce brittleness and facilitate processing) is the commercially most used aliphatic polyester for packaging and other commodity applications (11). Use of this material will likely continue to grow because its price and properties make it a possible replacement for several current packaging materials. Polylactide disposables and compost bags can be degraded in commercial or household composting facilities, but they will not rapidly degrade in all natural environments. However, commercial polylactides may contain additives or be chemically modified, improving the properties during processing and use but at the same time also decreasing or even preventing degradation during composting or as litter.

To further expand the library of degradable materials, as well as their quality, controllability, and application range, scientists need to learn from biological systems, which often combine weak or switchable linkages with hierarchical structures. The hierarchical structure of wood is a perfect example that still remains incompletely understood. In addition, development of thermoplastics, biocomposites, and even thermosets from lignocellulosic raw materials is expected to result in a library of new materials from compostable to inert structures based either on modified cellulose, hemicellulose, and lignin or on the monomers and nanoproducts derived from them (12).

In the scientific literature on degradable or environmentally degradable plastics, many claims of degradability have been made without proper scientific proof. Only reporting weight loss is not a proof of degradation. There are many reasons for weight loss, such as extraction of additives or loss of parts or fragments of the polymer. Also, potential degradation products could be persistent in nature and must be identified. Using the correct terminology for describing the type of degradation as well as correct test methods for the intended aim is important to avoid misunderstandings and incorrect claims. A degradable polymer is a polymer that undergoes chain scission, resulting in a decrease of molar mass. A biodegradable polymer is susceptible to degradation by biological activity with degradation accompanied by a lowering of its molar mass (13).

Well-Defined Aims

Degradable plastics have important roles in applications such as biomedicine, agricultural mulching films (see the photo), and compostable waste bags, where degradability is part of the function. However, for other applications, effective plastic waste collection followed by material or energy recycling is more desirable (14). One concern with degradable plastics is the serious negative influence they may have on the quality of recycled plastic products. If not separated, their large-scale production could lead to substantial problems for plastic recycling. They could also, in the worst case, lead to more littering if consumers assume that it is safe to throw them into the environment.

Whether degradable polymers are the most environmentally friendly solution depends on the application, as well as the whole life cycle, from raw materials to production processes and end-of-life management, which must be evaluated on a case-by-case basis (15). Even materials designed for environmental degradation will take time to degrade under nonideal environmental conditions. Throwing plastics, whether traditional or degradable, into the environment cannot be a solution from either an environmental or an economical point of view.

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