Unit 2: The World of Carbon

(e) Polymers

Early plastics and fibres

Ethene is a starting material of major importance in the petrochemical industry especially for the manufacture of plastics.

Cracking

Ethene can be formed by cracking the ethane from the gas fraction or the naphtha fraction from oil.

Propene can be formed by cracking the propane from the gas fraction or the naphtha fraction from oil.

Cracking produces shorter chain molecules for use as fuels and also alkenes as by products.

Cracking gives a saturated and an unsaturated molecule. This makes the total numbers of C atoms and H atoms equal before and after the reaction. The reaction always produces a mixture of products which must be separated by fractional distillation. The mixture results from the fact that there is no weak spot or functional group at which the molecule will break in a predictable way.


The products of cracking are of crucial importance to the manufacture of many other chemicals, plastics and solvents.

These feedstocks are cracked at very high temperature (above 1000° C) over an alumina catalyst in the presence of steam. Other products are produced at the same time, eg. methane, propene, hydrogen

Condensation polymers

Condensation polymers are made from monomers with two functional groups per molecule.

The repeating unit or the structure of a condensation polymer can be drawn given the monomer structures and vice-versa.

Polyesters

Polyesters are examples of condensation polymers.

Polyesters are manufactured for use as textile fibres and resins.

Polyesters used for textile fibres have a linear structure.

If an organic acid with two carboxylic groups (a diacid) and an alcohol with two hydroxyl groups (a diol) react a polyester is formed


Esters like this are used in the textile industry eg. terylene

Terylene is made from

If the dicarboxylic acid has unsaturated chains in it further cross linking can take place to give a three dimensional structure - a cured polyester resin.

GRP - glass reinforced polyester is manufactured like this. This is used for boat hulls and motor cycle helmets.

Another method of producing cross linking is to use a triol instead of a diol - three links instead of two take place. These are used for bonders in paint.

 

Nylon

Amines

An amine can be identified from the functional group.

Functional group -NH2 (amino) NB this is for primary amines

Name

Structure

methyl amine

CH3-NH2

ethyl amine

CH3CH2-NH2

propyl amine

CH3CH2CH2-NH2

butyl amine

CH3CH2CH2CH2-NH2


Properties of ethyl amine (can be contrasted with ammonia)

1.

Completely miscible with water

2.

pH > 7 weakly alkaline

CH3NH2 + H2O CH3NH3+ + OH-

3.

Reacts with copper sulphate to give a deep blue complex, similar to ammonia

 
Polyamides are examples of condensation polymers.

The amide link is formed by the reaction of an amine group with a carboxyl group.


An example of a polyamide is nylon which is a very important engineering plastic. An engineering plastic is a plastic which can be used in place of a metal for engineering applications. Nylon can be used for machine parts.

The strength of nylon is related to the hydrogen bonding between polymer chains. Nylon is also useful because of its chemical unreactivity in many environments.

As an engineering material nylon is far superior to poly(ethene) or poly(propene). The polymer chains in nylon need only be about half as long as high density poly(ethene) chains to show the same strength. The strong intermolecular forces which act between nylon chains are the source of this increased strength.

 

Synthesis Gas

Synthesis gas is a very important feedstock

Synthesis gas can be obtained by steam reforming of methane from natural gas or by steam reforming of coal.

Methanol, a feedstock for methanal, is made industrially from synthesis gas, a mixture of carbon monoxide and hydrogen.

CO + 2H2 ® CH3OH

Low pressure and a catalyst based on copper is used. Only about 5% conversion takes place. However unused reactants are recycled.

 

Methanal and Thermosetting Plastics

Methanal is an important feedstock in the manufacture of thermosetting plastics.

Methanal is used in the synthesis of bakelite

Phenol is one of the monomer units used to make the condensation polymer Bakelite (used in electrical devices)



 

The problem with bakelite is that it is always brown due to the phenol reacting with air. If the phenol is replaced with urea, a polymer known as urea formaldehyde resin is produced. It is white and can have its colour altered with pigments.

 

Recent developments

Kevlar

Kevlar (like nylon) is an example of a polyamide; it is an aromatic polyamide.

 

The molecule is flat because of the aromatic groups.

The uses of Kevlar are related to its strength ; this is related to the packing together of sheets of molecules held together by hydrogen bonds.

Hydrogen bonds are formed between N – H groups and C = O groups on adjacent molecules.

Kevlar is used to replace the steel in the cords of car tyres. Kevlar ropes have 20 times the strength of steel ropes of the same weight. A stiffer form of Kevlar is used in aircraft wings, where its strength combined with its low density is important. It is ideal for making bullet-proof vests and jackets for fencers are made from composites of Kevlar.

The use of Kevlar as a lining in aircraft holds to protect an aircraft from an explosion is presently being investigated.

Leading racing motorcyclists wear protective suits containing Kevlar since the abrasion resistance of the polymer is better than leather.

 

Poly(ethenol)

 
It would be expected to be formed from the monomer ethenol.

Ethenol is highly unstable

Instead the monomer used is ethenyl ethanoate

 

 
The poly (ethenyl ethanoate) is reacted with methanol.

 

Poly(ethenol) is a plastic which readily dissolves in water.

The uses poly(ethenol) are related to the solubility, this in turn, is related to the intermolecular forces

If soiled laundry from a hospital is mishandled, there is a risk of infection. This risk can be avoided by making the laundry bags out of a dissolving plastic. The dirty linen is safely contained until the bag is placed in the wash - then the bag dissolves and the washing is let out. The dissolving plastic which is used is poly(ethenol). It is a new synthetic polymer, made from another polymer, poly(ethenyl ethanoate). This process is known as ester exchange.

The percentage of acid groups which have been removed in the production process influences the strengths of the intermolecular forces upon which the solubility depends .

The extent of reaction can be controlled by adjusting either the temperature or the reaction time. The plastic's solubility depends on the percentage of acid groups which have been removed. Different solubilities give the plastic different uses.

% of OH groups

solubility in water

100 - 99

insoluble

99 - 97

soluble in hot water

96 - 90

soluble in warm water

below 90

soluble in cold water


Dissolving polymers have another medical use - as sutures or stitches in surgery. Once the surgeon has chosen to use soluble thread, a decision is made about how quickly it should dissolve and the appropriate poly(ethenol) is selected.

 

Poly (ethyne)

 

Poly (ethyne) can be treated with iodine to make a polymer which conducts electricity.

The conductivity depends on delocalised electrons along the polymer chain. This is similar to graphite, benzene or buckminsterfullerene chemistry.

Poly (ethyne) is used to make the membrane for high-performance loudspeakers.

 

Poly(vinyl carbazole)

The monomer unit is vinyl carbazole

It forms an addition polymer

Poly(vinyl carbazole) is a polymer which exhibits photoconductivity and is used in photocopiers.

At the heart of the photocopying process is a photoreceptor surface which is sensitive to light. In many machines, the photoreceptor is in the form of a metal drum coated with a very thin layer (about 10-5 cm thick) of a poly (vinyl carbazole).

A remarkable property of poly(vinyl carbazole) is that it exhibits photoconductivity - it conducts electricity much better when light shines on it than when it is in the dark.

 

Biopol

Biopol is an example of a biodegradable polymer.

Biopol is the trade name used by Zeneca for a family of polyesters, mainly poly(hydroxybutaneoate), or PHB. It is made by bacteria in a fermentation process.

 

 
In practice a modified form of PHB is synthesised where some of the hydrogen atoms on the central carbon of a repeating unit are replaced by ethyl groups.

It is a natural polyester made by certain bacteria and used by them as a source of energy. Micro-organisms found in the soil, and in streams and the sea, are able to break the polymer down. PHB degradation in the environment is usually complete within 9 months. But there is a price to pay - PHB is about 15 times more expensive than poly(ethene).

 

 
Photodegradable polymers

The structure of low density polythene (LDPE) can be modified during manufacture to produce a photodegradable polymer. A photodegradable polymer is broken down by sun light. The functional groups which trap light are introduced to the polymer and this causes fission of bonds in the neighbourhood of the groups.

Carbonyl groups (C=O) absorb radiation in the wavelength range 270 - 360 nm (about 1015Hz frequency). This corresponds to light in the near ultraviolet region of the spectrum. These groups can be incorporated into polymer chains to act as energy trappers. The trapped energy causes fission of bonds in the neighbourhood of the carbonyl group, and the polymer chain breaks down into short fragments which can then biodegrade.