Green Polymers – A Case Study (HSC SSCE Chemistry): Revision Notes

Green Polymers – A Case Study

Introduction: The need for green polymers

Most synthetic polymers today come from crude oil, which is a non-renewable resource. Experts predict that the world's oil reserves could be depleted by the middle or end of this century. This creates two major problems for the plastics industry:

1. Resource depletion: Oil supplies are running out
2. Plastic pollution: Poor recycling practices and throwaway consumer habits lead to environmental damage

There are two main viewpoints about the future of plastics:

View 1 - Develop alternatives now: Some scientists argue that we should develop alternative raw materials for plastics immediately. They suggest using ethanol as a source of ethylene, which is already used as a fuel additive and could provide the key monomer needed for many common polymers.

View 2 - Oil will remain viable: Others believe that as oil becomes scarcer and more expensive for fuel, the plastics industry will be less affected by price rises. This is because raw material costs are a smaller proportion of the final product cost for plastics. These people think oil will eventually become the exclusive domain of the plastics industry and last for many more decades.

Regardless of which view is correct, it makes sense for the plastics industry to develop alternative sources for its core monomers, particularly ethylene and propene.

Cellulose as a chemical feedstock

What is cellulose?

Cellulose is a naturally occurring condensation polymer called a polysaccharide. It consists of 3000 or more glucose monomer units linked together. Cellulose is a major component of plant material, making up approximately 33% of all vegetable matter from sources like cereal crops. In comparison, starches and sugars are only minor components of plants.

Each glucose unit in cellulose contains six carbon atoms. This makes it potentially useful as a building block for petrochemicals that typically contain 2-4 carbon atoms, such as:

• Ethylene (2 carbon atoms)
• Propylene (3 carbon atoms)
• Butane (4 carbon atoms, used for synthetic rubber)

Converting cellulose to useful chemicals

The diagram above shows how cellulose can be converted to ethylene and eventually to various polymers. The process works as follows:

1. Cellulose (from straw, sugar cane waste, or wood) is broken down into glucose
2. Glucose is fermented with yeast to produce ethanol
3. Ethanol is dehydrated to form ethylene
4. Ethylene can be used to make monomers (like vinyl chloride and styrene)
5. These monomers form polymers (like polyethylene, PVC, and polystyrene)

Starch can also follow a similar pathway through glucose to ethanol and ethylene.

Methods for breaking down cellulose

Breaking cellulose into glucose is very difficult because the long, near-linear chains of cellulose hydrogen bond to each other, forming compact fibres. This makes it hard for chemicals to reach the glucose-glucose links. However, there are two main processes:

Method 1: Digestion by cellulase enzymes

Cellulase enzymes are found in bacteria in the stomachs of herbivores and in certain fungi. The industrial process works as follows:

1. Finely grind cellulose-containing materials (grain husks, stalks, bagasse, old newspapers)
2. Treat with sodium hydroxide solution or hot water to swell and open up the cellulose fibres
3. Digest with cellulase enzymes to produce a glucose solution

Method 2: Digestion with strong acid

This method involves:

1. Heat a suspension of cellulose-containing materials (including wood chips) with moderately concentrated sulfuric acid solution
2. The acid breaks cellulose into glucose
3. Filter off insoluble matter (especially lignin from wood pulp)
4. Remove impurities
5. Neutralise the acid to produce a glucose solution

Energy and cost considerations

While converting cellulose to ethanol and ethylene does work, there are important limitations:

• Making ethanol from cellulose is considerably more expensive than making it from starch or sugars, despite cellulose being cheaper as a starting material
• However, scientists are working hard to develop more efficient methods

Alternative sources

Algae show promise as another alternative source:

• Can be grown in controlled, large-scale environments
• Produce oils that can be converted to hydrocarbons and ethanol
• Grow very quickly and can be harvested within a week
• This is faster than traditional plant crops

The chemical industry can also reduce raw material needs through improved recycling practices.

Biopolymers from nature

What are biopolymers?

Biopolymers are polymers that are made totally or in large part by living organisms.

Originally, this term referred only to biologically synthesised polymers such as:

• Cellulose
• Starch
• Proteins
• Nucleic acids

Now the definition includes:

• Chemically modified versions of natural polymers
• Polymers produced by manipulating biological organisms

Historical context

Partially synthetic biopolymers based on cellulose have been used commercially for nearly a century. The first plastic was cellulose nitrate, patented in 1869 as celluloid. This was a synthetically modified form of cellulose.

Examples of renewable polymers

The table below shows various renewable polymers, their source materials, and common applications:

Polymer	Crop or renewable material	Common uses
Starch-based polymers	Wheat, potatoes, corn	Compost bags, foam packaging fill, some food packaging
Polylactic acid (PLA)	Wheat, potatoes, corn	Food packaging, medical applications
Cellulose	Farmed wood	Cereal bar wrappers
Polyhydroxyalkanoates (PHA)	Plant-derived sugars and oils	Medical devices such as orthopaedic pins
Polyesters	Corn	Textiles
Polyurethane	Soy or castor oil	Car seats
Polyethylene	Sugar cane	Resins

Biodegradable synthetic plastics

The biodegradability problem

One major issue with synthetic polymers compared to natural polymers is that they are not biodegradable. This means plastics discarded into the environment or placed in rubbish dumps are not decomposed naturally by bacteria or fungi.

Solutions: biodegradable plastics

Synthetic biodegradable plastic materials have been developed and are used in some packaging applications. For example, photodegradable plastics are sometimes used for six-pack can carriers.

However, there are still potential problems:

• A photodegradable plastic buried in landfill won't degrade without light exposure
• When biodegradable plastics decompose, they may release undesirable substances like mineral fillers, metal salts, or other potentially dangerous additives into the environment

Graft copolymers

One innovative approach involves alternating biopolymer sections with synthetic sections in the same polymer molecule. This is called a graft copolymer.

When disposed of:

1. Microbes attack the biopolymer sections
2. This breaks the polymer chain into many smaller units
3. Biological processes convert the remaining fragments into shorter hydrocarbon chains

The image above shows a close-up view of biodegradable plastic under a scanning electron microscope. The orange spheres are starch granules embedded in the plastic. When the plastic is disposed of:

1. Starch granules absorb water and expand
2. The plastic breaks up
3. This increases the surface area in contact with soil bacteria
4. Bacteria digest the plastic more effectively

Environmental triggers

Researchers are investigating ways to add an environmental trigger to increase biodegradability:

• A catalyst is added to the starch molecule
• When the rubbish bag enters the environment, microbes release the catalyst as they consume the starch
• This catalyst makes the bag decompose even more quickly

The development of biopolymers and biodegradable plastics demonstrates how chemists have designed materials in response to environmental and economic issues.

Investigation 16.3: Biopolymers

Aim: To research a biopolymer and link its structure to its properties and uses.

Tasks:

1. Choose a biopolymer to research
2. Identify and draw:
   • The monomer(s) that make up the biopolymer (structural formulae)
   • At least one unit of the biopolymer (structural formula)
3. Analyse the biopolymer:
   • Identify its properties
   • Describe at least one use and relate it to the properties
   • Relate the properties to the structure
   • Determine if it can replace existing petrochemical polymers or has a completely new use
4. Present your findings as a video for the class
5. Watch and critique at least two other class videos

Remember!