Materials and Recycling

What are polymers

Polymers are large molecules made up of many repeating units.

• Can be linear molecules or branched molecules

• Can have cross-linking by covalent bonds between chains or secondary interactions between chains

 

What are addition polymers

Addison polymers are formed when unsaturated molecules add together under certain conditions of temperature and pressure with the help of initiators. This occurs without the elimination of any atoms.

What are condensation polymers

These are formed from monomers with two functional groups that undergo a condensation reaction. A small molecule (usually water) is eliminated for each condensation.

There are two main types of condensation polymers:

·       Polyesters

·       Polyamides

What are polyesters

Polyesters have ester links holding their chain together. Ester links form from hydroxyl and carboxyl functional groups combining.

They are formed from either hydroxycarboxylic acids reacting with each other or dicarboxylic acids reacting with diols.

What are polyamides

Polyamides have amide links holding their chain together. These are the result of reaction between an amine and a carboxyl functional group.

They are formed from either diamine and dicarboxylic acid monomers or by amino acid monomers. 

How can polymers be engineered for a particular use

The properties of synthetic polymers are extremely diverse. They can be “engineered” so that their properties match a particular use. This is done by:

• Controlling certain features of the structure such as the extent of branching and cross-linking along with polarity of side groups.

• Adding substances to enhance properties.

 

This gives synthetic polymers a distinct advantage over natural materials.

Advantages of synthetic polymers

Polymer materials are cheaper and less dense than glass Now used to make: •       Soft drink bottles •       Reading glasses •       Contact lenses

Do not corrode •       Now used to make car components rather than metal

More durable than cotton and wool •       Now used in production of textiles

Relatively cheap to produce

Polymer can be created for a specific use and properties can be altered accordingly •       Additives can be included to further modify properties

Disadvantages of synthetic polymers

Disadvantages

Degradation via sunlight •       Restricted to use at low temperatures

Disposal via combustion produces toxic fumes (e.g. HCN)

Litter problems (not biodegradable) and difficult to recycle (additives difficult to separate)

Raw material for most organic polymers is crude oil (non-renewable)

Toxic additives can leach from polymers

Describe Thermosetting plastics

Thermosetting plastics have sufficient cross-linking of their chains to prevent molecular motion on heating. They do not soften or change shape when heated. They simply char or decompose.

Describe Thermoplastics

Thermoplastics are polymers that soften when heated and return to their original condition when cooled. There is little or no cross-linking between the polymer chains. They are suited to recycling.

What are cross links

Covalent bonds that form between polymer chains are called cross-links.

what do The properties of a polymer depend on:

     Monomers

Degree of cross-linking between chains

How are crosslinks formed in polymers

Cross-links are either formed during polymerisation or they may be introduced by reacting a polymer with a cross-linking agent.

How does cross linking influence the properties of polymers

The greater the extent of cross-linking, the greater the hardness, rigidity, brittleness and durability of the polymer.

What is a resin

If there is extensive cross-linking, a 3D network structure is established and the material is termed a resin.

What are polar polymers with hydrogen bonding

H bonds form between chains that have O-H, N-H and C=O groups present in side groups of their structure. Polyamides have these groups and, as such, they are more rigid and elastic. Nylon is an example of a polymer with hydrogen bonding and is shown below:

Dispersion forces in polymers

These increase as the length of the chain increases and so for large polymer chains, dispersion forces are more significant than small molecules.

properties of non polar polymers

These are soft, flexible and are non-elastic (they do not regain their shape when stretched).

They have low melting points and can be softened easily and consequently reshaped which makes them good for recycling.

eg. polyethene and polypropene

How can polymers be produced

Polymers can be made from fossil resources or from renewable materials. The depletion of fossil fuel reserves, along with the need for more environmentally sustainable production, has led to greater research into and application of synthetic biopolymers as an alternative.

What are synthetic bio-polymers

Synthetic biopolymers are made from renewable plant material. Polysaccharides, proteins and triglycerides can all be used as raw materials in the production of biopolymers.

Advantages of Polymer Production from Fossil Resources

Polymer Production from Fossil Resources

Petroleum and natural gas are relatively abundant feedstock

Petroleum and natural gas are easily extracted and processed into petrochemicals used as raw materials

Well-established industries exist for a multitude of polymer products derived from petroleum and associated petrochemicals

A wide variety of polymers with varying polymer properties can be derived from petrochemicals

Disadvantages of Polymer Production from Fossil Resources

Petroleum and natural gas reserves are being depleted at an accelerated rate. Their rate of depletion is much faster than their geological rate of formation

Petroleum and natural gas are non-renewable feedstock

Advantages of Polymer Production from Renewable Materials

Carbohydrates, proteins and oils derived from biomass are relatively abundant as feedstock

Raw materials are easily extracted and processed from biomass

The sources of biomass can often be a by-product or waste product of other industries such as farming of crops

Biomass is a renewable feedstock

Disadvantages of Polymer Production from Renewable Materials

A limited range of polymers derived from biomass are currently possible

Use of biomass for polymer production may compete directly with land cropped for food production

What are degradable polymers

Degradable polymers are designed to maintain structural integrity and functionality during use and then break down over time in the environment after disposal. The polymers have the ability to degrade due to the chemical reactions (oxidation and hydrolysis) that can take place in the environment, as well as reactions with microorganisms. Sunlight can also break down some polymers.
Example of degradable polymers :Natural polymers (carbohydrates, proteins, nucleic acids) and synthetic polymers derived from petroleum and biomass

What are Biodegradable polymers

Biodegradable polymers are polymers which degrade due to enzyme-assisted reactions of microorganisms such as bacteria, fungi and algae.

Many biodegradable polymers contain ester and amide groups. These bonds can readily undergo hydrolysis reactions to break down the polymer. Polylactic acid (PLA) and polyglycolic acid (PGA) are examples of biodegradable polymers.

What are photodegradable polymers

Photodegradable polymers become brittle and break down when exposed to sunlight. The carbonyl groups present in the polymer absorb radiation and the polymer breaks down. Light-sensitive additives can also be incorporated to catalyse the reaction.

limitations of degradable polymers

In landfill, degradable synthetic polymers are exposed to limited oxygen, water and sunlight. This means that photodegradable polymers will not degrade and biodegradable polymers (which require oxygen and water) will degrade at much slower rates, persisting in the environment for longer.

limitations of photodegradable polymers

The breakdown of photodegradable polymers also has its concerns, small particles of polymer are produced which persist in the environment with unknown consequences.