Coláiste Lorcáin Leaving Certificate Engineering Polymers History of Polymers A plastic material is one that is easily shaped and takes on a permanent

Coláiste Lorcáin Leaving Certificate Engineering

PolymersHistory of Polymers

A plastic material is one that is easily shaped and takes on a permanent set.

The term polymer refers to a material which has been made by some polymerisation process.

Additives and polymers combined yield plastics.

The history of plastics is difficult to pin down. Man has been using polymers such as shellac, horn, gutta, percha, bitumen, lacquer and amber for centuries.


PloymersHistory of polymers

It is not surprising that research was undertaken to develop and improve on nature, thus synthetic polymers were developed.

With an increase in understanding in the structure of these materials there was rapid development in the technology of polymers. Their historical development generally follows five stages:


PolymersHistory of Polymers

1- from earliest time to 1900: the realisation of a new material group,

2- 1900 -1930 The birth of a plastic technology,

3 - 1930 -1950 Plastics as substitute materials

4 - 1950 1970 The “Plastics” age

5 - 1970 - now Engineering Plastics.


PolymersStructure of polymers

New polymers are being discovered and developed all the time. Many are developed for particular property combinations which are required for specific tasks.

Until 1920 people thought polymers were just thick jellies or liquids like bitumen. Nobody really understood them fully. In 1920 Staudinger showed that polymers were composed of very large molecules,- thus the term polymer was developed.


Polymersstructures

Polymers consist of long chains, which are composed of simple structural units (mers) strung together.

mermer

mermer

mer mer

mer mer mer

“poly” = many

Mers strung together to form polymers


There are lots of different types of mers that can join together to form polymers. Mers are simple organic molecules. These are special molecules that will link together such as double link molecules. These are like linking arms together to form a human chain. However, if two people link together with one hand each and don’t use the other hand they can only form pairs, such molecules can’t form mers.

Polymersstructures


Polymersstructures

1One-handed Closed molecule

Two-handed Linear molecule

Three or morehandles Cross-linked

polymers


Polymersstructures Types of chains

1-Linear chains

Thesse are formed when chains are built up without any deviation. They are generally formed when the mers used have only two reactive sites.

2-Branched chains

Deviation can occur when the chain is being formed and chain branching can result.

3- Cross-linking

The branches can join up to give cross-links between adjacent chains.


Polymersstructures

Level 1 The types of mers Level 2 Combining mers together Level 3 The way mers are joined

together Level 4 The degree of order

in polymers Level 5 Polymer mixtures


Polymersstructures Thermoset and Thermplastics

The basic structure of polymers is that the mers are strung together in chains. Analogies like paper clip chains and spaghetti are very useful.

There is a strong covalent bond along the length of the chain. The way the chains relate to each other determines the type of structure and, hence properties. If the bonds between the chains are primary then a rigid 3D network is constructed and a Thermoplastic polymer will result.


Polymersstructures Thernmoset and Thermplastics

If the bonds between the chains are weak secondary bonds, then these will easily be disrupted by heat. Such polymers are termed Thermoplastics.

The difference is explained by describing how butter can be melted in a frying pan and when allowed to cool, will harden again. Whereas if an egg is broken and fried in the pan until hard, it cannot be resoftened.


Polymers [Polymerisation] Methods of sticking Mers together

The process of sticking mers together is called polymerisation. The two ways of doing this are;

1- Additional polymerisation, this is the most important of the two because most important polymers are made this way and,

2- Condensation polymerisation. This method tends to yield rigid plastics.


Polymers Addition Polymerisation The basic idea of sticking mers together like

this is similar to a zip. The mers are added onto the end of a growing chain. There are three basic parts to the process:

Initiation - Polyethylene C2H4, has a double bond between the carbon atoms. One of these bonds is stronger, while the other is weak. Each bond contains two electrons. The weak bond is attacked by a radical.


Polymers Addition Polymerisation

This radical grabs an electron and leaves one electron behind.

Propagation -When other ethylene molecules come near the ethylene radical they too are attacked and a mer is attached. The combined molecule is a growing chain radical. The process continues by adding other ethylene molecules in a similar fashion.


Polymers Addition Polymerisation

Termination - Possible stops to the growing chains could simply be running out of mers or radicals.

R-C-C + -C-C R-C-C-C-C-

H H

H H

H H

H H

H H H H

H H H H

Addition Polymerisation

Radical

Ethylene polyethylene


Polymers Condensation Polymerisation

This process depends on splicing rather than zipping. The starting molecules are spliced or joined together with small molecules, like water, being condensed out of the reaction at the end of polymerisation. The remaining structure is a comples 3D structure which is rigid and has a significant cross-linking feature.


Polymers Condensation Polymerisation

OH

H

C = O

H

OH H

C

H+H2O

OH

.

Phenol

Formaldehyde


Polymers Types of Polymers

Thermoplastics:These are thermosoftening polymers and make up

about 85 - 90% of the bulk of polymers. They include ;

High Density Polyethylene --HDPE

Low Density Polyethylene --LDPE

Poly Vinyl Chloride --PVC

Polystyrene -- PS

Polypropylene -- PP



Thermosetting:

The important ones in this group include;

Phenolics - This is a group of polymers based on phenol formaldehyde (PF) the original developed by bakeland. It is a cheap, brittle, muddy-brown (0r black or blue) polymer.

Urea-formaldehyde - A white polymer that can be easily coloured. Uses include electric plugs, sockets, and other fittings.



Melamine-formaldehyde. This is harder, more scratch resistant and clearer. It can be found in ‘melaware’ and formica table tops

Polyesters - This is the resin used in fibreglass production.

Epoxies - Tis polymer is used in adhesives.

Polyurethanes - Are hard to classify as they can exist as hard glassy polymer or as a soft flexible sponge.


Polymers Additives for Polymers

Just as metals are alloyed to improve their properties, substances are added to polymers to improve theirs. A typical example

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Coláiste Lorcáin Leaving Certificate Engineering Polymers History of Polymers A plastic material is one that is easily shaped and takes on a permanent