Getting the Chemistry Right: Analyzing the Benefits and Challenges of Bioplastics

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What chemical reaction developed in the lab has had the greatest impact on the world? I think I can make a case for polymerization, the process by which small molecules join together to form long chains. I mean specifically the polymerization that produces plastics, a group of materials that can be shaped when soft and then hardened to hold that shape.

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How has plastic changed our world? Our food and drinks are packaged in plastic; our medicines are coated in it; our personal care products come in plastic containers; our cars, planes and hospitals cannot function without them; and our homes are filled with a range of plastic items.

Unfortunately, our landfills, beaches, and oceans are also filled with plastic. And the micro- and nanoplastics that are created when these materials decompose end up in our food, water, and ultimately our bodies. In addition, most plastics are made from raw materials derived from petroleum, a non-renewable resource, and their production is an energy-intensive process that releases greenhouse gases.

Of course, recycling, reusing and reducing non-essential uses of plastic are desirable goals. But we also need to discover ways to source raw materials from renewable sources and develop plastics that break down into harmless substances in the environment. This is where bioplastics come into play.

“Bio” means life, so bioplastics are plastics that are at least partially made from raw materials derived from living organisms or are biodegradable. Biodegradable means that they can be broken down by bacteria, fungi or microorganisms into simple compounds such as carbon dioxide and water, which can be reabsorbed by the environment, ideally without causing pollution.

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Bioplastics are classified by their origin or biodegradability. Some can be derived from renewable resources and are not biodegradable, some can be produced from petroleum and are biodegradable, and some can be derived from renewable resources and are biodegradable. The latter is the most desirable and also the most difficult to achieve. None of the most commonly used plastics—polyethylene, polyethylene terephthalate, polyvinyl chloride, polystyrene, and polycarbonate—fall into the bioplastic category. At this point, only about one percent of all plastics produced are bioplastics—hardly a significant amount to consider.

Microbes can not only break down plastic, they can also make it. Bacteria and fungi are like tiny chemical factories that produce special proteins called enzymes that catalyze chemical reactions, such as converting starch to glucose and then glucose to lactic acid, a process first described in 1857 by French chemist Louis Pasteur. Adding lactic acid-producing bacteria to cornstarch produces lactic acid, which can then be polymerized into polylactic acid (PLA) using a metal catalyst. PLA is the most widely used bioplastic, suitable for disposable cutlery and compost bags. It is biodegradable, but only in an industrial composting facility. It does not biodegrade in household compost. It eventually biodegrades in the anaerobic conditions of a landfill, but the byproduct is methane, a potent greenhouse gas.

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The second most commonly used bioplastic is PHA, which stands for polyhydroxyalkanoate. PHA is actually a family of polyesters that are naturally produced by certain bacteria. For example, the soil bacterium Cupriavidus necator can produce different PHAs depending on the nutrient it is fed, which can be methane, starch, or fats from plants or animals.

The great advantage of PHAs is that they biodegrade in the natural environment, even in the ocean. This makes them suitable for single-use products such as food packaging, straws and cutlery. Their disadvantage is cost: PHAs are about 10 times more expensive than PLA.

Some clever chemistry has led to another biodegradable plastic that, while not as readily biodegradable as PHA, has physical properties similar to those of conventional plastics. It’s known by the tongue-twisting name polybutylene adipate terephthalate, or PBAT.

Scientists looked at the molecular structure of polyethylene terephthalate, a plastic used to make drinks bottles that are often heartlessly thrown away. Because they don’t biodegrade, they pollute the environment. Microbes can’t seem to get at the bonds that hold these molecules together because their access is blocked by the terephthalate components of the polymer. But replacing some of the terephthalate with a smaller adipate and increasing the distance between these components by replacing the two-carbon ethylene linkage with a four-carbon butylene linkage allows microbes to break down the molecule. But—and there’s always a “but”—all the ingredients still come from petroleum! Nevertheless, PBAT qualifies as a bioplastic because it’s biodegradable.

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Bioplastics would be particularly welcome in diapers, given that about 250 million are thrown away every day, most of which end up in landfills. The introduction of acrylic acid-based superabsorbent polymers in the 1980s reduced the incidence of diaper rash by 50 percent, but these polymers are not biodegradable and are made from acrylic acid derived from petroleum. Scientists at Procter & Gamble, the maker of Pampers diapers, have developed a catalyst that can convert lactic acid to acrylic acid, which is then polymerized. Because lactic acid is fermented from plant material, this version of the superabsorbent polymer would be a bioplastic, even though it is not biodegradable. But it would open the door to terms like “naturally derived” or “biotechnology-based,” which are very much in the market.

Similarly, Coca-Cola went overboard with its “plant-based bottle.” Like other beverage bottles, it’s made from polyethylene terephthalate, except the ingredients are made from corn or sugar cane. The finished product isn’t biodegradable, though. It can be recycled.

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Another thing to consider is that even bioplastics that are entirely plant-based and biodegradable are not without their environmental concerns. The crops that provide the necessary raw materials require pesticides and fertilizers, chemicals that have their own problems. Then there’s the concern that the land to grow these crops could be better used, for example, in agriculture to produce food for the world’s growing population.

Another issue is that even if bioplastics can be derived from renewable sources, there is still a lot of chemistry involved in their transformation into the final product. Polymerization catalysts, plasticizers, antioxidants, antistatic agents, lubricants, dyes, flame retardants and degradation products are just as likely to be present in bioplastics as in conventional ones. A study that examined the toxicological profile of extracts from samples of the two types of plastic found no differences.

There is no doubt that there is some interesting chemistry involved in the quest for better bioplastics, but these materials will not solve the world’s plastic problem.

Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3:00 to 4:00 p.m.

[email protected]

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