Bioplastics: Biobased Plastics As Renewable And/Or Biodegradable Alternatives to Petroplastics

BIOPLASTICS: BIOBASED PLASTICS AS RENEWABLE AND/OR BIODEGRADABLE ALTERNATIVES TO PETROPLASTICS 1. Introduction ‘‘Plastics’’ were introduced approximately 100 years ago, and today are one of the most used and most versatile materials. Yet society is fundamentally ambivalent toward plastics, due to their environmental implications, so interest in bioplastics has sparked. According to the petrochemical market information provider ICIS, ‘‘The emergence of bio-feedstocks and bio-based commodity polymers production, in tandem with increasing oil prices, rising consumer consciousness and improving economics, has ushered in a new and exciting era of bioplastics commercialization. However, factors such as economic viability, product quality and scale of operation will still play important roles in determining a bioplastic’s place on the commercialization spectrum’’ (1). The annual production of synthetic polymers (‘‘plastics’’), most of which are derived from petrochemicals, exceeds 300 million tons (2), having replaced traditional materials such as wood, stone, horn, ceramics, glass, leather, steel, concrete, and others. They are multitalented, durable, cost effective, easy to process, impervious to water, and have enabled applications that were not possible before the materials’ availability. Plastics, which consist of polymers and additives, are defined by their set of properties such as hardness, density, thermal insulation, electrical isolation, and primarily their resistance to heat, organic solvents, oxidation, and microorganisms. There are hundreds of different plastics; even within one type, various grades exist (eg, low viscosity polypropylene (PP) for injection molding, high viscosity PP for extrusion, and mineral-filled grades). Applications for polymeric materials are virtually endless; they are used as construction and building material, for packaging, appliances, toys, and furniture, in cars, as colloids in paints, and in medical applications, to name but a few. Plastics can be shaped into films, fibers, tubes, plates, and objects such as bottles or boxes. They are sometimes the best available technology. Many plastic products are intended for a short-term use, and others have long-term applications (eg, plastic pipes, which are designed for lifetimes in excess of 100 yr). On the other hand, there is a growing debate about crude oil depletion and price volatility, and environmental concerns with plastics are becoming more serious. Approximately half of all synthetic polymers end up in short-lived products, which are partly thermally recycled (burnt), but to some extent end up on landfills or, worse, in the oceans, where large plastic objects are washed ashore, sink or float (eg, the ‘‘North Pacific Garbage Patch,’’ which has continental dimensions), and get fragmented to ‘‘microplastics’’ (particles between a few mm and <5 mm) that harm and kill various organisms, finally ending up on our plates. It is estimated that globally some 900 billion plastic bags (shopping bags, waste bags, etc) are produced each year, with a typical average useful life of only a few minutes and a significant fraction of them ending up as litter in the environment (3), having wasted energy, spoiling the scene, and seriously harming wildlife. 1 Kirk-Othmer Encyclopedia of Chemical Technology. Copyright # 2015 John Wiley & Sons, Inc. All rights reserved. DOI: 10.1002/0471238961.koe00006 2 BIOPLASTICS: BIOBASED PLASTICS AS RENEWABLE It is estimated that since the 1950s, approximately 1 billion tons of plastics have been discarded and some of that material might persist for centuries or even significantly longer, as it is demonstrated by the persistence of natural materials such as amber (4). One of the biggest advantages of plastics, their durability, is likewise one of their biggest problems: The rate of degradation (biodegradation) does not match their intended service life, and buildup in the environment occurs. Recycling of waste plastics, in principle, a meaningful approach, can follow different routes: 1. Reuse of the product (eg, a bag). 2. Material recycling (collection, sorting, and reprocessing). 3. Feedstock recycling (depolymerization to capture the monomers). 4. Thermal recycling (use of the energy content in waste incineration, steel works, or cement kilns). Recycling plastics is not always feasible, and it can have a negative eco- balance due to the efforts for collecting, sorting, and processing them. In most cases, they need to be washed, and waste grinding and processing are energy consuming. The recycling rate of plastics differs from country to country; there are also differences in the plastics concerned. In the United States, the recycling rate for polyethylene terephthalate (PET) packaging (bottles) was 31.2% in 2013 (5). PET has the highest value of commodity plastics and is used mainly for drinking bottles; hence, efforts are made to collect it. Recycled plastics go through different processing steps such as sorting and melt filtration. They can often only be used in lower grade products, typically not with direct food contact or high performance applications. A ‘‘usage cascade’’ can be created, ending in thermal recycling (combustion: incineration or pyrolysis). To summarize, the extensive use of plastics has become a problem in many aspects. Therefore, growing interest in ‘‘bioplastics’’ is observed (for reuse and recycling of bioplastics, an unsolved issue, see Reference 6 and Section 9). The term ‘‘bioplastics’’ stands for ‘‘biobased polymers.’’ According to IUPAC, a bioplastic is derived from ‘‘biomass or . monomers derived from the biomass and which, at some stage in its processing into finished products, can be shaped by flow’’ (7). In the area of bioplastics, several terms are used vaguely, ambiguously, or wrongly. Hence, some important definitions are provided as follows (see also Reference 7). Plastics (plastic materials) in general are a huge range of organic solids that are malleable (pliable, moldable). Malleability is a material’s ability to deform under compressive stress. Plastics usually consist of organic polymers with high molecular weight and other substances (fillers, colors, and additives). They are typically synthetically produced. The term ‘‘natural plastics’’ is sometimes used in the industry for unfilled and uncolored plastics, as opposed to compounds. BIOPLASTICS: BIOBASED PLASTICS AS RENEWABLE 3 Often, the expression bioplastics is used to make a distinction from polymers derived from fossil resources (monomers). The term is, to some extent, misleading, 4 BIOPLASTICS: BIOBASED PLASTICS AS RENEWABLE as the prefix ‘‘bio’’ suggests that any polymer derived from biomass is environment-friendly. Biobased polymers are neither necessarily biocompatible nor biodegradable. According to industry association European Bioplastics, bioplastics are ‘‘polymers that are biobased, biodegradable, or both’’ (8). So the industry has adopted a rather large definition. An alternative expression could be ‘‘technical biopolymers.’’ In case polymers are obtained from agro-resources such as polysaccharides (eg, starch) (9), one can talk about ‘‘agro-polymers.’’ ‘‘Biomaterials’’ denote materials that are exploited in contact with living tissues, organisms, or microorganisms. Hence, ‘‘polymeric biomaterials’’ are used in applications such as medicine (catheters, bone cements, and contact lenses) (10). Many of them are conventionally produced polymers. Implantable biomaterials are PET, PP, PEEK (polyetheretherketone), UHMWPE (ultrahigh molecular weight polyethylene), and PTFE (polytetrafluoroethylene) (11,12), on the one hand, and (bio-)resorbable polymersPGA (polyglycolide), PLA (polylac- tide), PCL (polycaprolactone), and PGS (poly(glycerol sebacate)), on the other hand (12,13). Generally, a polymer is a substance composed of macromolecules. A macromolecule is a very large molecule commonly made by polymerization of smaller subunits. In biochemistry, the term is applied to the main biopolymers such as nucleic acids (eg, DNA), proteins, and carbohydrates (natural polymers), plus other large, nonpolymeric molecules such as lipids and polyphenols. Natural polymers (‘‘biopolymers’’) can be organic or inorganic (14), the latter having a skeleton devoid of carbon (15). Examples for the former include cellulose, starch, latex, and chitin; examples for the latter include polyphosphazenes, polysilicates, polysiloxanes, polysilanes, polysilazanes, polygermanes, and polysulfides. In between, one can find so-called hybrid polymers, ie, polymers containing inorganic and organic components such as polydimethylsiloxane (silicone rubber: --[O--Si (CH3)2]n - - ). Synthetic polymers (artificial polymers) are man-made polymers. They are built from monomers by polymerization, polycondensation, or polyaddition. Most synthetic polymers have significantly simpler and more random (stochastic) structures than natural ones. They show a molecular mass distribution, which does not exist in biopolymers (polydispersity vs monodispersity). They are substances that are not produced by nature (xenobiotics). Due to their high molecular weight, they are not mobile. From a practical processing point of view, synthetic polymers can be classified into the four main categories: thermoplastics (thermo- softening plastics), thermosets (duromers), elastomers, and synthetic fibers. The most common synthetic polymers are • polyethylene (PE: PE-HD and PE-LD, with HD being high density and LD being low density); • polypropylene; • acrylonitrile–butadiene–styrene

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