Are compostable bags actually compostable?
Biodegradable Plastic and Its Impact
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For plastics derived from renewable resources, see Bioplastic. For plastics that biodegrade in the human body, see Biodegradable polymer.
Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually microbes, into water, carbon dioxide, and biomass. Biodegradable plastics are commonly produced with renewable raw materials, micro-organisms, petrochemicals, or combinations of all three.
While the words "bioplastic" and "biodegradable plastic" are similar, they are not synonymous. Not all bioplastics (plastics derived partly or entirely from biomass) are biodegradable, and some biodegradable plastics are fully petroleum-based. As more companies are keen to be seen as having "Green" credentials, solutions such as using bioplastics are being investigated and implemented more. The definition of bioplastics is still up for debate. The phrase is frequently used to refer to a wide range of diverse goods that may be biobased, biodegradable, or both. This could imply that polymers made from oil can be branded as "bioplastics" even if they have no biological components at all. However, there are many skeptics who believe that bioplastics will not solve problems as others expect.
History of Biodegradable Plastics
Polyhydroxyalkanoate (PHA) was first observed in bacteria in 1888 by Martinus Beijerinck. In 1926, French microbiologist Maurice Lemoigne chemically identified the polymer after extracting it from Bacillus megaterium. It wasn't until the early 1960s that the groundwork for scaled production was laid. Several patents for the production and isolation of PHB, the simplest PHA, were administered to W.R. Grace & Co. (USA), but due to low yields, tainted product, and high extraction costs, the operation was dissolved. The halting of oil exports by OPEC in 1973 spurred more companies to invest in the biosynthesis of sustainable plastics. Imperial Chemical Industries (ICI UK) successfully produced PHB at a yield of 70% using the strain Alcaligenes latus. The PHA produced then was a scl-PHA.
In 1983, ICI received venture capital funding and founded Marlborough Biopolymers to manufacture the first broad-application biodegradable plastic, PHBV, named Biopol. Though it showed promise, Biopol was still too costly to produce to disrupt the market. In 1996, Monsanto discovered a method to produce one of the two polymers in plants and acquired Biopol from Zeneca, anticipating cheaper production. The early 2000s saw a steep increase in oil prices, motivating the plastics industry to seek alternatives to petroleum-based products. Since then, many alternatives have emerged, produced chemically or by bacteria, plants, seaweed, and plant waste, also influenced by geopolitical factors.
Application of Biodegradable Plastics
Biodegradable plastics are commonly used for disposable items, such as packaging, cutlery, and food service containers.
In principle, biodegradable plastics could replace many applications for conventional plastics. However, this entails a number of challenges.
- Many biodegradable plastics are designed to degrade in industrial composting systems. However, this requires a well-managed waste system to ensure that this actually happens. If products made from these plastics are discarded into conventional waste streams such as landfill, or find their way into the open environment such as rivers and oceans, potential environmental benefits are not realized and evidence indicates that this can actually worsen, rather than reduce, the problem of plastic pollution.
- Plastic items labeled as 'biodegradable', but that only break down into smaller pieces like microplastics, or into smaller units that are not biodegradable, are not an improvement over conventional plastic.
- A 2009 study found that the use of biodegradable plastics was financially viable only in the context of specific regulations limiting the usage of conventional plastics. For example, biodegradable plastic bags have been compulsory in Italy since 2011 with the introduction of a specific law.
Types of Biodegradable Plastics
Bio-based Plastics
Biologically synthesized plastics (also called bioplastics or biobased plastics) are produced from natural origins such as plants, animals, or micro-organisms.
Polyhydroxyalkanoates (PHAs)
Polyhydroxyalkanoates are a class of biodegradable plastic naturally produced by various microorganisms. Specific types of PHAs include poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and polyhydroxyhexanoate (PHH). The biosynthesis of PHA is usually driven by depriving organisms of certain nutrients and supplying an excess of carbon sources. PHA granules are then recovered by rupturing the microorganisms.
Polylactic Acid (PLA)
Polylactic acid is a thermoplastic aliphatic polyester synthesized from renewable biomass, typically from fermented plant starch such as corn, cassava, sugarcane, or sugar beet pulp. In 2010, PLA had the second-highest consumption volume of any bioplastic worldwide. PLA is compostable, but non-biodegradable according to American and European standards because it does not biodegrade outside of artificial composting conditions.
Starch Blends
Starch blends are thermoplastic polymers produced by blending starch with plasticizers. Starch polymers on their own are brittle at room temperature, so plasticizers are added to enhance their properties. While all starches are biodegradable, not all plasticizers are. The biodegradability of the plasticizer determines the biodegradability of the starch blend.
Cellulose-based Plastics
Cellulose bioplastics are mainly the cellulose esters (including cellulose acetate and nitrocellulose) and their derivatives. Cellulose can become thermoplastic when extensively modified. An example is cellulose acetate, which is expensive and rarely used for packaging.
Lignin-based Polymer Composites
Lignin-based polymer composites are bio-renewable natural aromatic polymers with biodegradable properties. Lignin is a byproduct of polysaccharide extraction from plant material through the production of paper, ethanol, and more. It is high in abundance, with reports showing that 50 million tons are created by chemical pulp industries each year. Lignin's low weight and environmentally friendly qualities make it a useful alternative, neutral to CO2 release during the biodegradation process.
Petroleum-based Plastics
Petroleum-based plastics are derived from petrochemicals obtained from fossil crude oil, coal, or natural gas. While widely used types such as polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), and polystyrene (PS) are not biodegradable, some petroleum-based plastics are.
Polyglycolic Acid (PGA)
Polyglycolic acid is a thermoplastic polymer and an aliphatic polyester often used in medical applications. It can degrade into its nontoxic monomer, glycolic acid, through hydrolysis, especially with the help of esterases. In the body, glycolic acid can enter the tricarboxylic acid cycle, after which it is excreted as water and carbon dioxide.
Polybutylene Succinate (PBS)
Polybutylene succinate is a thermoplastic polymer resin comparable to propylene, used in packaging films for food and cosmetics. In the agricultural field, PBS serves as a biodegradable mulching film. Several species of microorganisms have been shown to consume PBS in various forms.
Polycaprolactone (PCL)
Polycaprolactone has gained prominence as an implantable biomaterial due to its biodegradable properties. It has been shown that various microbes can degrade PCL.
Considerations and Challenges
Factors Affecting Biodegradation
One of the design challenges is that biodegradability is a "system property." Whether a particular plastic item will biodegrade depends not only on its intrinsic properties but also on environmental conditions. The rate at which plastic biodegrades depends on temperature, specific microorganisms present, humidity, and other factors.
Environmental Concerns
Though the terms "compostable," "bioplastics," and "oxo-degradable plastics" are often used interchangeably with "biodegradable plastics," they are not synonymous. Mismanagement and improper labeling can lead to confusion, contamination of waste streams, and increased pollution. Reliable companies should provide clear and accurate labeling on how to properly dispose of their biodegradable products.
Sorry, Compostable Plastic Isn't Really ... - Chatelaine Magazine
Unless compostable plastic is labelled specifically as “home compostable,” it cannot go in a traditional backyard compost, because backyard composts do not meet the specific conditions required for the plastic to break down (such as the minimum temperature or moisture levels, which can vary by manufacturer). “There are some plastics that have been developed to be composted in smaller facilities, like a backyard, but those are less commercialized right now. The dominant [compostable] would only break down in an industrial facility,” Li says.
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