Polymers
History 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.
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Ploymers
History 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:
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Polymers
History 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.
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Polymers
Structure 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.
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Polymers
structures
Polymers consist of long chains, which are
composed of simple structural units (mers) strung
together.
mer
mer
mer
“poly” = many
mer
mer
mer
mer
mer
Mers strung together to form polymers
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mer
Polymers
structures
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.
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Polymers
structures
1
Closed molecule
One-handed
Two-handed
Three or more
handles
Linear molecule
Cross-linked
polymers
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structures
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.
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Polymers
structures
Level 1
Level 2
Level 3
Level 4
Level 5
The types of mers
Combining mers together
The way mers are joined
together
The degree of order
in polymers
Polymer mixtures
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Polymers
structures 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.
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Polymers
structures 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.
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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.
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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.
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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.
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Addition Polymerisation
Termination - Possible stops to the growing
chains could simply be running out of mers
or radicals.
Radical
H H
HHHH
HH
R-C-C + -C-C
HH
R-C-C-C-C-
HH
HHHH
Ethylene
polyethylene
Addition Polymerisation
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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.
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Polymers
Condensation Polymerisation
H
.
Formaldehyde
OH
C=O
H
Phenol
OH H
OH
C
H
+H2O
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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
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Polymers
Types of Polymers
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.
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Polymers
Types of Polymers
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.
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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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