Chloe Zhang
Biodegradable plastics are plastics that can be broken down by microorganisms into water, carbon dioxide, methane, and biomass under specified conditions. The term biodegradable on product labelling can be misleading, as it may be interpreted as implying that the product will degrade under any environmental conditions. In reality, biodegradable plastics are tested under controlled conditions in a lab, and their ability to break down in nature is uncertain. Biodegradable plastics are derived from renewable raw materials, such as biomass, and can be produced chemically or by using bacteria, plants, seaweed, and plant waste. They are used in a variety of applications, including food packaging, disposable containers, and agricultural products. While biodegradable plastics offer a promising approach to reducing plastic waste, they are not a perfect solution, and proper waste management is crucial to ensure their effectiveness in addressing the plastic pollution crisis.
| Characteristics | Values |
|---|---|
| Definition | Biodegradable plastic is defined by its ability to break down completely into substances found in nature, and in a reasonable timeframe. |
| Biodegradation Process | Biodegradable plastics are degraded by microorganisms into water, carbon dioxide (or methane) and biomass under specified conditions. |
| Composting | Composting of biodegradable plastics requires strict control of environmental factors, including temperature, moisture, pressure, oxygen levels, UV exposure, and nutrient concentration. These conditions can only be recreated in industrial composting plants. |
| Bioplastics | Bioplastics are derived from renewable raw materials or petrochemicals with biodegradable additives. Examples include PHA (polyhydroxyalkanoate), PHB (polyhydroxybutyrate), and PLA (poly(lactide)). |
| Production Methods | Biodegradable plastics can be produced chemically or using bacteria, plants, seaweed, plant waste, and cyanobacteria (blue-green algae). |
| Applications | Biodegradable plastics are commonly used for disposable items such as packaging, cutlery, food service containers, grocery bags, and agricultural products. They can be foamed, extruded, or injection-moulded. |
| Environmental Impact | Biodegradable plastics can help reduce waste and have a lower environmental impact than traditional plastics, but they do not solve the plastic pollution crisis on their own. Misconceptions about biodegradability can lead to increased littering. |
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What You'll Learn
Polyhydroxyalkanoate (PHA)
The synthesis of PHA is promoted by unbalanced growth during the fermentation and accumulation of PHA granules as part of a survival mechanism of the microbes. The biosynthesis of PHA is usually caused by deficiency conditions, such as the lack of macro elements like phosphorus, nitrogen, trace elements, or lack of oxygen, and the excess supply of carbon sources. The yield of PHA obtained from the intracellular granule inclusions can be as high as 80% of the organism's dry weight.
The simplest and most common form of PHA is the fermentative production of poly-beta-hydroxybutyrate, which consists of 1000 to 30000 hydroxy fatty acid monomers. In industrial production, the polyester is extracted and purified from bacteria by optimising the conditions of microbial fermentation of sugar, glucose, or vegetable oil. PHA can be synthesized from over 150 molecular precursors, and its mechanical properties and biocompatibility can be changed by blending, modifying the surface, or combining it with other polymers, enzymes, and inorganic materials.
The mechanical behaviour of PHA depends on the length of the pendant groups and the distance between ester linkages. PHAs with short pendant groups exhibit stiff and brittle behaviour, while those with longer pendant groups are ductile. The introduction of other units in the PHA chain (besides 3HB) significantly affects its mechanical behaviour.
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Biologically-based polymers
Biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, classified according to the monomers used and the structure of the biopolymer formed: polynucleotides, polypeptides, and polysaccharides. Polynucleotides, such as RNA and DNA, are long polymers of nucleotides. Polypeptides, including proteins and shorter polymers of amino acids, encompass examples like collagen, actin, and fibrin. Polysaccharides, or sugar polymers, are linear or branched chains of sugar carbohydrates; starch, cellulose, and alginate fall into this category.
Other examples of biopolymers include natural rubbers, suberin, lignin, cutin, cutan, melanin, and polyhydroxyalkanoates (PHAs). PHAs, first observed in bacteria in 1888, are a family of biodegradable polyesters produced by bacterial fermentation. Biopolymers offer good biocompatibility and degradability, presenting a more environmentally friendly alternative to synthetic materials.
The global interest in bio-based polymers is rising due to the increasing demand for non-petroleum-based polymers. Companies like Ikea and Lego have expressed interest in biodegradable packaging alternatives, and niche markets already exist for bio-based and biodegradable plastics in the food and healthcare industries.
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Compostable plastics
There are several types of biodegradable plastics on the market today, including Polyhydroxyalkanoate (PHA), which was first observed in bacteria in 1888. PHA is made by microorganisms that produce plastic from organic materials. Companies can harvest the microbe-made PHA, which has a chemical structure similar to that of traditional plastics. Because it is biodegradable and does not harm living tissue, PHA is often used for medical applications and single-use food packaging.
Another type of biodegradable plastic is poly(lactide) (PLA), which can be combined with starch to make it easier for bacteria to break down. Plasticizers such as glycerol, sorbitol, and triethyl citrate are added to starch-PLA compounds to prevent brittleness. PLA is fully biodegradable when composted in a large-scale operation with temperatures of 60°C and above.
Biodegradable plastics are commonly used for disposable items such as packaging, cutlery, and food service containers. They can be foamed into packing materials, extruded, and injection-moulded in modified conventional machines. Different types of fillers can be used, such as wood flour, lime, clay, or waste paper, and the fillers can be coloured and used in various granulation sizes to change the material's external appearance.
While compostable and biodegradable plastics can play a helpful role in reducing waste, they are not necessarily better for the environment. They are often made from plants, which require agricultural processes that can have negative impacts on the environment. Additionally, the term "biodegradable" on product labels can be misleading, as it may be misinterpreted to imply that the product will degrade under any environmental conditions. In reality, biodegradable plastics may not break down as intended if they are not managed properly once they become waste.
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Microorganisms
Biodegradable plastics are plastics degraded by microorganisms into water, carbon dioxide (or methane) and biomass under specified conditions. The biodegradation process can take place in both aerobic and anaerobic environments.
The feasibility of plastic degradation by microorganisms has attracted a lot of attention due to the ecological problems caused by plastic and microplastic pollution. Existing research and data show a large number of microorganisms involved in plastic biodegradation, including bacteria, fungi, microalgae, and gut microbes of insects.
Several bacterial enzymes have been identified as playing a role in the degradation of different polymers. For example, PET hydrolase is involved in the degradation of PET, while PCL-cutinase is involved in the degradation of PCL. Additionally, pretreatments to improve the efficiency of biodegradation have been discussed, which can cause a significant reduction in toxic plastic pollution.
Some biodegradable plastics are fully petroleum-based, while others are derived from renewable raw materials such as plants, seaweed, plant waste, and bacteria. For example, polyhydroxyalkanoate (PHA) was first observed in bacteria in 1888 and later chemically identified by French microbiologist Maurice Lemoigne. Another biodegradable plastic, PHBV, was produced using the bacterial strain Alcaligenes latus.
While biodegradable plastics offer an ideal solution for many single- or short-term use applications, there are also challenges to their implementation. Many biodegradable plastics are designed to degrade in industrial composting systems, which require well-managed waste systems. Additionally, the specific mechanisms of microbial degradation of plastics have not been fully explored yet, and the efficiency of plastic degradation by microorganisms is relatively slow, hindering their practical application in the industry.
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Cyanobacteria
Synechocystis, Spirulina, Anabaena, and Nostoc muscorum are cyanobacteria that can serve as bio-factories for the production of biofuel and bioplastic. These cyanobacteria can produce biopolymers like PHB and PHAs, which are both affordable and sustainable. Additionally, several cyanobacteria can produce succinate and lactate, commodity chemicals used to generate bioplastics. Genetic manipulation has successfully increased the productivity of these chemicals, offering a promising approach for sustainable bioplastic production.
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Frequently asked questions
Biodegradable plastic is a type of polymer that can be degraded by microorganisms into small molecules such as H2O, CO2, and CH4.
Biodegradable plastics are often made from bio-based sources like seaweed, sugar beets, or other plants instead of fossil fuels. They can also be made from organic materials using microorganisms such as bacteria or algae.
PHA, or polyhydroxyalkanoate, is a type of biodegradable plastic made by microorganisms that produce plastic from organic materials. It has a chemical structure similar to that of traditional plastics.
Biodegradable plastics can help reduce waste and have a lower environmental impact than traditional plastics. They can also be used to create medical devices that will not harm living tissue.
Biodegradable plastics are commonly used for disposable items such as packaging, cutlery, food service containers, and single-use food packaging. They can also be used for agricultural applications such as mulch films and plant pots.
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