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These are some common bioplastics, with varying degrees of biodegradability:
Starch is cheap, abundant, and renewable and makes up about half of the bioplastics market. It is so simple it can be made at home, and if you ever made paper-mâché as a child, chances are you have already made some. Flexibilizers and plasticizers, such as sorbitol and glycerine, can also be added for a resulting bioplastic called thermo-plastic starch. Starch-based bioplastics can be blended to produce starch/polylactic acid, which has better water-shedding and mechanical properties. These blends are also compostable, but others, such as the starch/polyoleﬁn blends, are not, although they do have a lower carbon footprint than petroleum-based plastic.
Cellulose is the original natural plastic and still the most abundant organic polymer on Earth. It’s the primary structural ingredient of the cell walls of green plants, many forms of algae, and the oomycetes (water molds). Some species of bacteria secrete it to form bioﬁlms. The cellulose content of cotton ﬁber is 90 percent, wood 40-50 percent, and hemp approximately 57 percent. Cellulosic ﬁbers added to starches can improve mechanical properties, permeability to gas, and water resistance due to being less hydrophilic (water loving) than starch.
Wheat gluten, soy protein, and casein (from milk) show promising properties as biodegradable polymers. Soy proteins have been used in plastic production for over one hundred years but lost out to fossil-based plastics due to their water sensitivity and relatively high cost. A new frontier lies in blends of soy with biodegradable polyesters to boost water resistance and lower cost.
Polylactic acid (PLA) is a transparent plastic produced from corn or dextrose. It is superﬁcially similar to polystyrene but degrades to non-toxic products. Unfortunately, it is inferior to polystyrene in impact strength, heat and cold tolerance, and blocking air permeability com-pared to other blends in commercial use.
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Poly-3-hydroxybutyrate (PHB) is a polyester produced by certain bacteria processing glucose, cornstarch, or wastewater. It’s similar to polypropylene. Brazil is taking it to large-scale production using the waste by-products of growing sugar. It can be processed into a trans-parent ﬁlm with a melting point higher than 130 degrees C and is 100 percent biodegradable without residue.
Polyhydroxyalkanoates (PHA) are produced in nature by bacterial fermentation of sugar or lipids. They are produced by the bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to make materials with widely different properties. PHA is more ductile and less elastic than other plastics and is also biodegradable. PHA is widely used in the medical industry.
Polyamide 11 (PA 11) is known under the trade name Rilsan and comes from castor oil. Because of its toughness, ﬂexibility, and chemical and permeation resistance, it can be found in automotive fuel lines, pneumatic air brake tubing, electrical cable anti-termite sheathing, ﬂexible oil and gas pipes, control ﬂuid umbilicals, sports shoes, and catheters. It is not biodegradable. A similar plastic is Polyamide 410 (PA 410), derived 70 percent from castor oil, using the trade name EcoPaXX and sold in several different blends.
Bio-derived polyethylene is chemically and physically identical to traditional polyethylene but produced by fermentation of sugarcane, beets, or corn. It does not biodegrade but can be recycled. The Brazilian chemicals group Braskem claims that, using its method of producing sugarcane and reﬁning polymer, it captures (removes from the atmosphere) 2.15 tons of CO2 per ton of Green Polyethylene it produces. That is a big deal.
Polyhydroxyurethane (PHU) is bio-based and isocyanate-free polyurethane. The substitution of natural-oil-based polyols was the first route developed. The second strategy is blending polyamines to poly-cyclic carbonates without the use of harmful isocyanate. The chemical transformation of epoxidized vegetable oils or glycerine-carbonate-based intermediates provided that path. New families of bio-based PHUs are capable of recycling and reprocessing.
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Polyurethanes, polyesters, epoxy resins, and other types of polymers have been developed with comparable properties to petroplastics through the development of oleﬁn metathesis from vegetable oils and microalgae. Algae has been called “the ultimate winner” in the bio-feedstock race because it absorbs water, sunlight, and CO2 from the air, the organisms grow rapidly, and their production does not disrupt food markets. Bio-oil production rate per unit of cultivation area is ﬁfteen times higher than with other biomass sources.
Surplus soybean oil, reinforced with lignin from wood, is being developed by the United Soybean Board into resin monomers for sheet and bulk molding compounds and resin transfer molding, including a dashboard panel for John Deere tractors. Ground-up corncobs are being developed into a bioﬁller for a vinyl window and door manufacturer. Non-edible cellulose and cashew nutshell oil (cardanol) have been chemically bonded with other additives to create a 70 percent plant-based thermoplastic material said to have multiple times the strength and heat resistance of PLA and the equivalent water resistance.
Keratin resin made from poultry feathers, combined with a poly-oleﬁn, is being molded into biodegradable ﬂowerpots for nurseries, making use of at least some of the three billion pounds of chicken feathers created by US poultry processors per year (roughly 80 percent is currently sent to landﬁlls).
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Resins are also being experimentally produced from carbon dioxide itself and synthesized into a polypropylene carbonate resin (PPC). The latest is a 44 percent CO2-based Greenpol PPC using a proprietary catalyst and a continuous polymerization process. Its makers see potential uses in packaging materials, supplanting petroplastic polyoleﬁns. Because the UN’s Nobel Prize-winning science advisory, the Intergovernmental Panel on Climate Change (IPCC), has reported that carbon dioxide removal will be essential if catastrophic global warming is to be avoided in this century, much of our future energy will likely derive from biomass energy with carbon capture and storage (BECCS). Greenpol or PPC resins like it could supply the missing CCS part of that equation at a proﬁt, rather than attempting to pump CO2 liquids into deep reservoirs or to the ocean ﬂoor.
In addition to bioplastics, there are many more novel or experimental polymers being studied or test-marketed. One is high-performance nanoporous nanoﬁbers from biodegradable clays, which form nanofiber webs. Early researchers report that these nanopolymers form physically and chemically responsive functional groups that “tend to strong self-assembly.” When stressed, the increased loading of partner polymer nanocomposites speeds self-adaptation, which improves the morphology and thermal behaviors of these webs, making them potentially useful in nanomedicine, nanomicrobiology, and water ﬁltration.
Self-assembling nanoﬁber webs sounds like the future has just arrived.
More from Transforming Plastic:
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Reprinted with permission from Transforming Plastic: From Pollution to Evolution by Albert Bates and published by GroundSwell Books, 2019.