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December 23, 2020

Biodegradable plastics; Enrichment of global sustainability

Plastic is a material that has changed the face of the world since its discovery. The invention of bakelite, the first fully synthetic plastic, in 1907 was really the beginning of new evolution. The demand for plastic that took over the world is rising day by day. Even though we applaud for plastics, we know that it is also depicted as a huge potential threat to the planet because the microorganisms are not capable of decomposing this wonderful human creation. The influence of plastic is so deeply ingrained in our lives that we cannot even imagine a world without plastic, even in our distant dreams. So people began to find more ways like 4R’s (Reduce, Reuse, Recycle, Recovery) to bring down the adverse effects. Also, there emerges the concept of biodegradable plastic as an eco-friendly solution to ensure global sustainability. The invention of biodegradable plastics or bio-plastics is a milestone in the plastic industry. Biodegradable plastics are plastic materials that can be degraded by microorganisms under the right conditions into water, CO2, and compost. Biodegradable plastics are primarily produced from renewable biological sources like corn starch, potato starch, straw, sugarcane, cassava, cotton, vegetable fats and oils, and cellulose from softwood trees, etc. Plastics produced from fossil fuels with added biodegradable additives also come under the category of biodegradable plastics. These bio-plastics are better for the environment and a retort to control plastic litter.

Classification of biodegradable plastics

Biodegradable plastics are not merely one type of plastic; it is a collection of various sorts of plastics made from different raw materials. Their properties and applications vary based on the nature of the materials. Biodegradable plastics are mainly categorized into two broad groups,
namely bio-based plastics and petrochemical-based biodegradable plastics.

Bio-based Plastics

Bio-based plastics are plastic materials that are made entirely or partially from components derived from renewable biomass resources such as vegetable oil or corn starch or sugarcane or potato etc instead of crude oil. These are also called first generation bio-based plastics. Plastic can also be produced using food waste or wood or sawdust or feedstock waste etc as raw material, which is known as second generation bio-based plastics. Third generation bio-plastics made from raw materials extracted from algae are also under the developmental phase.

Types of Bio-based plastics

The bio-based plastics synthesized from renewable organic materials can be sorted into different types based on the raw materials used. Each type of plastic has specific properties and applications. Some of the common types are listed below:

Polyhydroxyalkanoates (PHAs)

Polyhydroxyalkanoates are well-known biodegradable plastics synthesized from numerous micro-organisms present in nature like Cupriadavidus necator or Ralstonia eutroph. PHAs are generally thermoplastics with physical properties that are comparable to conventional plastics.
Bio-based feedstocks are used as the raw material for the fermentative synthesis. There is a wide variety of PHAs that demonstrate unique attributes and specific applications. Certain types of PHAs include Poly (3-hydroxybutyrate) (PHB), Poly-4-hydroxybutyrate (P4HB), Poly (3-
hydroxybutyrate-co-4-hydroxybutyrate) (P (3HB-co-4HB)), Poly (3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV), polyhydroxyhexanoate (PHH). Each of the PHAs offers an extensive range of properties and the structure of the polymer can be tailored to various industrial applications. PHAs are widely used to make carrier bags, disposable gloves, foam packaging, phone cases, plant containers, absorbable mesh, furniture, etc.

Polylactides (PLA)

Polylactides, or PLA, or Polylactic acid is one of the most commonly used biodegradable plastic in the world. Lactic acid and lactide monomers are the two main building blocks of PLA. These monomers formed from the fermentation of any carbohydrate source derived from renewable resources like corn starch, cassava starch, or sugarcane. The biggest concern to PLA is that it only decomposes under specific industrial composting conditions. Several kinds of polylactic acid are available, comprising of Poly-L-lactic Acid (Regular PLLA), Racemic PLLA, PDLA (Poly-D-lactic Acid), and PDLLA (Poly-DL-lactic Acid). Higher mechanical strength, good barrier properties, low toxic level, increased aesthetics, and chemical resistance are counted as some of its major merits, and PLA is widely used to make food packaging bags, shrink wraps, 3D printable filaments, medical implants, sports apparel, etc.

Cellulose-based plastics

Cellulose is one of the most copious organic substances on earth and is an excellent raw material to produce biodegradable plastics. Cellulose esters derived from the pulp of softwood trees through various procedures are used in the production of cellulose plastics. Cellulose-
based plastics are power-packed with a wide variety of properties like greater tensile strength, high transverse rupture strength, and good thermal stability. The principal uses of cellulose plastics are in the making of films, eyeglass frames, protective cover sheets of books, table tennis balls, etc.

Starch-based plastics

Like cellulose, starch is also one of the most abundant organic materials on earth and is obtained naturally from corn, wheat, peas, potato, rice, cassava, etc. The pure form of starch does not possess thermoplastic properties. The blending of native starch with plasticizer under controlled conditions aids in infusing thermoplastic properties and is known as the thermoplastic starch. They are the most commonly used bio-plastic accounts for more than 50 percent of the global bio-plastic market. Starch (carbohydrate polymer) can also be incorporated with different synthetic and natural polymers. Starch-based plastics have high tensile strength and good heat tolerance compared to other bio-based plastics. They can be
used in a vast array of applications like the manufacturing of bottles, cups, cutlery, phone cases, and kitchen bags.

Lignin-based plastic

Lignin is a biopolymer abundantly found in the cell walls of plants that strengthen their structure. This prominent plant component has often been discarding as a useless by-product from the paper manufacturing process. But now, it becomes a rising raw material in the production of bio-plastic. Lignin is an ideal unit for the production of plastic composite materials owing to its remarkable properties such as reinforcing potential, low weight, biodegradable nature, and carbon neutrality. The incorporation of lignin to polylactide matrix increases the overall impact strength and thermal stability of the composites. The lignin plastics with high lignin content show similar properties to conventional polystyrene and acrylic.

Petrochemicals-based biodegradable plastics

The fossil-based plastics can be made biodegradable by inducing specific degradable additives that cause fast decomposition in the presence of light and oxygen. These types of plastics often confuse us a lot with different names like degradable or oxo-biodegradable or photo-biodegradable etc. These are nothing but the conventional plastics supplemented with certain
pro-degradant additives during manufacturing to enhance biodegradation. Oxo-biodegradable plastics are plastics that decompose faster than traditional plastics in the presence of molecular oxygen in the atmosphere. The additives with the oxo-degradable property are normally incorporated in petrochemical-based plastics like polyethylene (PE) and polypropylene (PP). Photo-biodegradable plastics are plastic materials that can degrade in the presence of sunlight and oxygen. These are conventional fossil-based plastics that either contain chemical bonds that can be weakened by sunlight or contain degradable additives that enhance degradation in
the presence of light.

Advantages of Biodegradable plastics

Biodegradable plastics are so popular because they have many incredible advantages over traditional plastics. Some of the significant advantages are listed below:

  • Biodegradable plastics assists in reducing the carbon footprint in products
  • Help to save more non-renewable sources as most of the biodegradable plastics are produced from renewable sources
  • Lesser energy consumption during the manufacturing and recycling process
  • Eco-friendly alternatives that aids in the significant reduction of environmental pollution
  • It is noteworthy that greenhouse gases emit much less than conventional plastics.
  • Minimize the amount of waste production as it decomposes easily and reduce the need for landfill area
  • Biodegradable plastics can be recycled easily through the organic process

Disadvantages of Biodegradable plastics

Biodegradable plastics also have certain disadvantages. The major demerits include:

  • Difficult to degrade in landfills along with other waste and may produce methane as they breakdown
  • Generally more expensive than non-degradable plastics because the production cost is high
  • Proper disposal of bio-plastics is necessary; in case it mixes with other recycled plastics, the whole batch can be contaminated and become useless
  • Bio-plastics have higher eutrophication potential compared to conventional plastics that can cause consequential effects like algae blooms.

The major challenge faced by the bio-plastics industry is the cost of production; they are way more expensive than conventional plastics. There are also a lot of uncertainties arising about the technologies, processes, recycling, composting, etc. The availability of raw materials on an
industrial scale is also a challenge, and the cultivation of food crops for plastic manufacturing creates controversies. Despite the challenges, the biodegradable plastic industry is steadily flourishing. Sustainable plastics made from renewable resources have the potential to attenuate some of the serious environmental pollutions caused by conventional plastics. Even
though bio-plastics wouldn’t end all the pollution problems associated with traditional plastics, it is an inspiring start to protecting our environment. The invention of biodegradable plastics marks the beginning of a journey towards global sustainability.