Synthesised by living organisms

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Definition of biopolymer:

Biopolymers are naturally occurring polymers which are synthesised by living organisms. This synthesis can occur either internally within an organism's structure, or externally in appropriate conditions. The term biopolymer also encompasses those polymers which are produced by the physical or chemical manipulation of production environments. However, depending on the term's origination, it does not strictly include those polymers produced by biological manipulating. For this reason, it is best to classify such polymers as ‘partially synthetic biopolymers'. Through the chemical and physical manipulation of production environments, a large variety of biopolymers have been synthesised. Each of these newly developed biopolymers are available with unique and beneficial properties, along with the ability to biodegrade and provide a renewable source of plastic like material.

Reasons why biopolymers may become increasingly important in society:

Petroleum derived plastics have formed an integral relationship with modern society, providing a cheap, convenient and durable method for developing numerous consumer goods and other products. The negative impacts associated with plastic favour the use of biopolymers. Such impacts, along with other factors, involve:

  • An overuse of non-biodegradable plastics. These plastics are produced at a rate of over 100million tonnes per year- consumption patterns which have lead to serious problems concerning environmental pollution, waste management and danger to animals.
  • An uncertainty about the future resources of the petrochemical industry. This industry may become obsolete or produce goods too expensive for mainstream consumption.
  • The initiatives of recycling have failed to make any great progress over the previous decades.
  • The recent success of biocompatible plastics which have revolutionised the medical industry.
  • The ability to use industrial waste (such as food waste) as a substrate for biopolymer production. This has the added benefit of improving waste usage and reducing other forms of pollution and treatment.
  • Burning of electronic waste (e-waste) to recover the precious metals contained in chips and circuits. With the continued exponential growth of the electronics industry, the illegal practice of secretly burning e-waste releases many toxic gases, especially if coated in PVC.

For these reasons, much interest has arisen in the design and development of biodegradable, renewable, practical and economically viable biopolymers to replace the synthetic plastics consumed today. The new age of renewable energy and waste management have resulted in great emphasis on the future of biopolymers and the relative efficiency of their production.

Selected Biopolymer:

PHB is a biopolymer belonging to a group of biopolymers called polyhydroxyalkanoates (PHA). It is also classified as a polyester due to containing an ester functional group. PHB is synthesised by the polymerisation of (R)-3-hydroxybutynl-CoA.

PHB is produced by bacteria as the result of physiological stress. During this process PHB acts as an energy storage molecule to be used later when other energy sources are depleted. The most common form of PHB is poly-3-hydroxybutyrate,(as shown in the above diagram), however discussed below are the generalised notes for all PHB isomers.

PHB was first discovered in 1925 by Maurice Lemoigne who concluded that bacteria could produce polyesters. However Maurice's discovery was not officially recognised as PHB until its rediscovery in 1957. This stimulated much interest in the future of biopolymers, an interest which has reignited in recent years due to the environmental debate and uncertain future of the petrochemical industry. Using traditional production methods, up to 80% of the dry weight of the bacteria can be composed of PHB.

Properties of the biopolymer

  • 100% biodegradable in both aerobic and anaerobic environments
  • Biocompatible - the polymer can be naturally incorporated into and decomposed by the human body
  • Thermoplastic
  • Piezoelectric - produces an electric potential when compressed 8
  • Low thermal stability 9
  • Ultra violet resistance 10
  • High melting point 175˚C8
  • Low resistance towards acids and bases 10
  • Transparent and lustrous
  • High crystallinity - structural arrangement 8
  • Stiff 8
  • More dense then water 10
  • Brittle - depends on the level of crystallinity 8
  • Does not have chain branching - it is isotactic (uniform structure) and therefore flows well during processing8
  • Is not soluble in water - hydrophobic 8
  • Has a low permeable level (penetration) for oxygen, water and carbon dioxide8

Uses or potential uses of the biopolymer

Relationship between uses and properties

PHB could become the new material for use in bottles, bags, wrapping, nappies and other disposables where biodegradability is a concern

Due to the biodegradability of PHB in both aerobic and anaerobic environments (both in the presence or lack of oxygen) there is a great incentive for the potential replacement of the polymers derived from petrochemicals. PHB is also hydrophobic, has low permeability by oxygen, water and CO2, has UV resistance, high melting point, and is isotactic - properties which make PHB a suitable replacement for many plastic products.

PHB can be used as a medical tool. These include surgical implants, treads and coatings.

In medical applications, PHB is biocompatible with the blood and tissues of humans and other mammals. The normal metabolism of humans produces the monomer of PHB, (R)-3-hydroxybutynl-CoA,and thus does not reject the polymer's use as a medical tool. Surgical implants and threads all reabsorb into the body.

In the pharmaceutical industry, PHB can coat capsules and provide slow or controlled drug release.

The property of biocompatibility allows this process to occur. Also, a low permeability for O2 and H2O allows it to be released slowly.

The electronics industry currently burns the plastic (usually PVC) coatings around chips and circuits to retain precious metals. PHB could provide an alternative which prevents toxic gases being released by this practice.

Due to the property of biodegradability, special treatment facilities could be established to extract the precious metals in an environmentally friendly manner.

Potential benefits of the biopolymer to society and the environment

Environmental impacts:

  • Recycling plastic as an alternative to environmental pollution and landfill usually requires more energy compared to creating new plastic. A biopolymer with the ability to biodegrade, such as PHB, removes the need to consider the less energy efficient recycling method.
  • Burning waste plastic to harness energy is an option towards to landfill issue, but this releases toxic gases and increases carbon dioxide concentration in the atmosphere. Conversely, biological polymers form part of a natural cycle whereby carbon dioxide and water are used during photosynthesis and released during natural decomposition.
  • The ability to synthesise PHB from a wide variety of carbon rich sources means that a secondary use or market can be found for some waste products. Using substrates such as industrial food waste and molasses from sugar processing reduces the need for the treatment and disposal of such wastes.
  • The complete changeover to PHB from normal petroleum derived plastics would reduce landfill volumes by approximately 20%, given this is the percentage composition of plastics in our rubbish. This would reduce overall volumes of pollution.

Societal impacts:

  • PHB and other biopolymers have revolutionised the medical industry. PHB is biocompatible with human blood and tissues, and readily reabsorbs into the body objects such as implants and threading. The biopolymer can also be used as a material for slow releasing drugs. Improvements in this field are inevitable.
  • Petrol derived plastics can be carcinogenic. Examples include those containing benzene and vinyl chloride. PHB is a safer material for use in containers and drink bottles where this is an inconclusive concern.
  • Reducing the volume of landfill by 20% has the social benefit of increasing overall domestic, commercial and industrial land use. It also reduces the public ‘eyesore' the landfill creates.
  • Production of PHB using food substrates can have negative societal impacts. A higher demand for substrates which form the staple diet of developing countries may reduce the ability of these countries to purchase this food. Such a consequence would worsen the food shortages of these developing countries.

HSC Chemistry

Assessment Task 1: Biopolymers

Current problems with the biopolymer

HSC Chemistry

Applying PHB as a substitute material for petroleum derived synthetic plastics would cost substantially more and offer no real performance advantages other than its biodegradability. In the production of PHB four major factors influence overall cost:

  • the price of the substrate
  • the effective yield achieved from that substrate
  • the price of other input factors
  • tedious production procedures such as the need for a pure culture of alcaligenes eutrophus

The cost of harvesting the PHB directly from alcaligenes eutrophus costs approximately $8/kg. This is substantially more than the $1/kg production cost for most oil based plastics. These high costs are reflected in the relative costs of different substrates. The cost of the petrochemical substrate for polypropylene is US$0.185/kg of polypropylene . This is a large variation compared to the prices of different PHB substrates given in the following table:

Substrate effectiveness based on substrate costs and yield of PHB

Substrate

Price of substrate (US$/kg)

Yield (kg/kg of substrate)

Substrate Cost (US$/kg of PHB)

Glucose

0.493

0.38

1.350

Sucrose

0.295

0.40

0.720

Methanol

0.180

0.430

0.420

Acetic Acid

0.595

0.380

1.560

Ethanol

0.502

0.500

1.000

Molasses

0.220

0.420

0.520

Cheese whey

0.071

0.330

0.220

Corn Starch

0.220

0.185

0.580

Hemicellulose

0.069

0.200

0.340

In addition to the economical restraints of PHB, various mechanical issues are also apparent:

  • PHB is stiff and brittle compared to polyethylene and polypropylene. This has hindered its wide acceptance as a practical replacement for these materials. Brittleness is directly related to the degree of crystallinity in the material.
  • At room temperature, over time, secondary crystallisation occurs and the material becomes more brittle.
  • The polymer chains degrade during processing
  • The effect of the mass production of PHB on the environment has not been thoroughly investigated. While the material is biodegradable and renewable, major environmental consequences not yet identified may exist.

Properties/production processes which need further research

The main directions of improvement and research into modifying PHB and/or its production process can be classified into two categories:

HSC Chemistry

1. Methods which involve the physical or chemical manipulation of production environments:

  • Adding lubricants and plasticisers to prevent degrading of chains during processing.
  • Researching new bacteria which naturally produce plasticisers along with the biopolymer to address the issue of brittleness. Such progress would directly reduce the production costs as the plasticisers otherwise added are expensive.
  • Suppression of the secondary crystallisation that occurs over time
  • Making products that are programmed degradable - a biopolymer that allows you to control when and how it degrades. This will insure that the biopolymer remains practical while still in use.
  • Investigating the influence of additives on PHB degrading and level of brittleness
  • Increasing the productivity of processing techniques such as:
  • Extrusion: the process in which blends are mixed to create a uniform product
  • Injection moulding: the process of injecting the molten polymer into a mould to solidify
  • Investigating which solvents used in the extraction process are most productive and efficient
  • Distinguishing methods which decrease the production time. Time means money, and the time taken by the bacteria to produce PHB is an economical factor hindering its commercial use.

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