1

Key to polymer acronyms used in this presentation:

ABS poly(acrylonitrile-butadiene-styrene)

EP poly(ethylene-propylene)

EVA poly(ethylene-vinyl acetate)

PAN poly(acrylonitrile)

PBD poly(butadiene)

PE poly(ethylene)

PEEK poly(ether ether ketone)

PEK poly(ether ketone)

PET poly(ethylene terephthalate)

PM poly(methylene)

PMMA poly(methyl methacrylate)

PP poly(propylene)

PS poly(styrene)

PTFE poly(tetra fluoro ethylene)

PVC poly(vinyl-chloride)

PVDC poly(vinylidene chloride)

PVF poly(vinyl fluoride)

PVDF poly(vinylidene fluoride)

SAN poly(styrene-acrylonitrile)

SBR poly(styrene-butadiene)

poly THF poly(tetrahydrofuran)

VAE poly(vinyl acetate-ethylene)

2

Polymers are all pervasive in our world today. There is hardly an implement,

item of clothing, household furnishing, wall covering, mode of transport which

does not use polymers in one form or another.

The need for communication, control and power supply within the military field

has led to high performance wire insulation which is now used on cars and

civilian land transport. The wiring in trams and train carriages is now measured

in kilometres; the need to reduce weight, even on ships, has led to new products

- lighter, smaller, more robust (e.g 40 tons weight is saved on the wire insulation

alone in a frigate, mostly above the waterline).

Cars: high under-bonnet temperatures calls for airframe style wiring. ABS

brake sensor wiring require protection from heat, abrasion and solvents.

Probably the most important items, the tyres, are created using 4 or 5 different

polymers, all chosen for specific properties.

In all these environments, a predominant requirement now is to reduce fire and

toxicity hazards, whilst retaining the high performance characteristics which

allow the production of efficient systems and elegant design.

Specific properties are called for in each environment which often results in a

multiplicity of polymers being used - even the cheapest ball point pen uses up to

three or four different polymers.

The Alaska pipeline - 800 miles long (half of which is above ground), 48”

diameter, crosses 3 mountain ranges, 800 rivers and streams - is insulated with a

polymer coating. See: www.alyeska-pipe.com.

2

Creating an aircraft wiring loom

3

Under your feet and in your street – 1,200 pair telephone cables.

4

How not to do it!

5

6

A few details:

shellac - secretion of lac insects in SE Asia

amber - fossilised pine resin found in the Baltic

natural rubber - cis 1:4 polyisoprenefrom hevea brasiliensis tree

gutta percha - trans 1:4 polyisoprene from palaquium trees (SE Asia)

good dielectric - used for early undersea cables

other trans 1:4 polyisoprenes:

balata from mimosups balata - Caribbean - golf balls;

chicle from sapota achras - used for chewing gum

Reference: Plastics Materials, fifth ed. Brydson J. A. Butterworths 1989

7

Polymers consist of long chains of small repeat units (monomers). They can be

classified in a variety of ways: i) by source: natural products synthetic modified natural

ii) by properties e.g.:

elastic mod % elongation

elastomers (rubbers) 105 - 106 500 - 1000

plastics 107 - 108 100 - 200

Elastomers are extensible / deformable and revert very rapidly at room

temperature to their original dimensions.

Plastics deform but do not revert to their original dimensions on removal of the

applied stress.

Thermoplastics are capable of inelastic deformation at elevated temperatures,

i.e they can be re-processed indefinitely. Some elastomers exhibit thermoplastic

behaviour, but most thermoplastic polymers are not elastomers.

iii) by polymerisation mechanism (see slide below)

addition

condensation

polyaddition

iv) by microstructure (molecular structure & configuration)

semi-crystalline vs amorphous

random vs block copolymers

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9

Addition and condensation are the two main polymerisation routes, and are

explained on the subsequent set of slides:

10

Polymerisation proceeds by the addition of subsequent monomer units with the

opening of double bonds - usually C=C, but not necessarily so. There are no

by-products formed in this process.

The three steps in polymerisation are detailed on the next slides.

11

The initiation step requires the formation of a radical a chemical moiety with

an unpaired valence electron (radicals are VERY chemically active), or ionic

species which attaches to a monomer unit, opening the double bond and

creating an active moiety which attacks a further monomer unit, leading to:

12

the propagation step.

The example chosen here is ethylene, and the original (radical formation)

process required high pressures and temperatures ( typically 20,000psi at

250degC ). Very spectacular when the process went awry! Traces of oxygen

are necessary to initiate the process. Other initiators used have been: peroxides,

hydroperoxides and azo compounds. This process gives highly branched

polymers of low density, low melting point (for PE) and low crystallinity.

Modern processes use a catalyst and aliphatic hydrocarbon solvent, with

pressures close to ambient and temperatures of about 60degC an ionic

mechanism of initiation is produced. This yields linear polyethylenes of higher

density, higher melting point and higher crystallinity than the high pressure

process example above.

13

Termination the various mechanisms result in a variety of differing main chain

lengths –

reaction with initiator radical: Init-(CH2-)n-CH2• + • Init Init-(CH2-)n-CH2-Init

mutual combination of growing chains:

Init-(CH2-)n-CH2• + •CH2-(CH2)m-Init Init-(CH2-)n-CH2-CH2-(CH2)m-Init

disproportionation:

Init-(CH2-)n-CH2• + CH3-(CH2)m-Init Init-(CH2-)n-CH3 + CH2=CH2(CH2)m-1-Init

Chain transfer agents may be used to limit the molecular weight reached. An

example is in the production of SBR latex, where a mercaptan is used as the

transfer agent.

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A list (not exhaustive!) of common polymer types with the backbone bonds:

bond typical polymers

C-C olefines: polyethylene, polypropylene, polyalkenes, polybutadiene,

polyisoprene, polystyrene, polyparaphenylenes.

vinyls: polyvinylchloride, polyvinylidene chloride, polyvinylidene

fluoride, polytetrafluoroethylene, polyacrylonitrile,

polymethylmethacrylate, polyvinylacetate,

polyvinylalkylacrylate, polyvinylether

C-O aromatic polyesters, polyethers, polyacetals, polycarbonates

C-N polyamides, aramids, polyimides, polyurethanes.

C-S polysulphides, polyphenylene sulphide, polysulphones.

polyether sulphones (both O and S in the chain)

Si-O silicones (note backbone is inorganic, with organic side groups)

These are mainly homopolymers - i.e. only one monomer is used. However

copolymers may be formed from both within each bond type and across bond

types, e.g Tefzel - poly(ethylene co- tetrafluoro ethylene) and

Ultem - poly(ether-imide)

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16

Result of termination reactions:

range of molecular weights

range of chain branching

range of end groups

i.e. a Complex microstructure is produced.

This slide shows the actual molecular weight distribution of a polyethylene as

determined by Gel Permeation Chromatography (red trace). Note the peak mol

wt = 120,000; but that there are species present as high as 1,000,000 to

2,000,000 and as low as 700. Also presented are oil - blue trace - (effectively

oligomeric polyethylene) and a graphical representation of ethylene. Note the

mol wt axis is logarithmic. (note that in PE, a mol wt of 1,000,000 is equivalent

to approx. 36,000 ethylene units)

Note that ultra high molecular weight polyethylene (UHMWPE) has mol wts up

to 1 - 2E7. These are VERY difficult to process.

Note also that chain branching and co-monomer distribution can vary with

molecular weight, i.e. there is a third axis representing molecular composition

passing into the plane of the graph above.

A production problem was found to relate to specific incoming batches of base

polymer. All the testing showed that there were no detectable differences

between the batches, both of which met the normal acceptance tests.

GPC, however, showed some batches to have a high mol wt “tail”. The

variability of the test method meant that multiple tests on each sample were

required, with the sets of data being averaged.

The averaged data of each batch were then compared (see above) where it is

obvious that two samples clearly have a high end “tail”. These two samples

correlated with “good” material, the rest with “bad”.

Faced with this data the suppliers admitted to having two reactors, and the two

sets of data correlated with these.

It should be noted that the presence of the high mol wt material did not explain

the difference in products, but probably reflects compositional differences

which are related to differing mol wts.

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18

Chain branch length has a direct impact on density, crystallinity (chain packing)

and melt rheology (chain entanglement).

The linearity of LLDPE results from the different manufacturing processes of

LLDPE and LDPE. In general, LLDPE is produced at lower temperatures and

pressures by copolymerization of ethylene and such higher alpha-olefins as

butene, hexene, or octene. The copolymerization process produces a LLDPE

polymer that has a narrower molecular weight distribution than conventional

LDPE and in combination with the linear structure, significantly different

rheological properties.

LLDPE has penetrated almost all traditional markets for polyethylene; it is used

for plastic bags and sheets (where it allows using lower thickness than

comparable LDPE), plastic wrap, stretch wrap, pouches, toys, covers, lids,

pipes, buckets and containers, covering of cables, and mainly flexible tubing

19

Although main chain unsaturation can be used to crosslink polymers, there are a

variety of routes to creating a three dimensional network.

All such processes rarely lead to a completely crosslinked material, some

polymer chains remaining isolated.

Cross linking only occurs in the amorphous regions

A commonly encountered crosslinked product is a vehicle tyre.

Polymer: styrene-butadiene rubber

Filler: carbon black (often several types)

Process oil: highly aromatic oil used as lubricant.

Antioxidants & anti-ozonants

Cross-linking agents: sulphur; zinc oxide; accelerators

The cross-links are sulphur bridges created by the sulphur / accelerator system.

20

As the degree of crosslinking increases, the modulus of the material increases,

ultimate elongation decreases; solvent resistance increases.

21

The linear backbones of many polymers provide a tendency for alignment of the

chains, creating crystallites within an amorphous matrix. These semi-

crystalline polymers have reasonably sharp melting points (by DSC) and exhibit

X-ray diffraction patterns, albeit with broad diffraction peaks due to internal

strain and disorder. DSC, XRD FTIR and NMR can all be used to measure the

levels of crystallinity present (although the techniques may give differing values

since they are measuring different parameters.)

The presence of these crystallites can be used to create heat recoverable

plastics. The material is fabricated to its final dimensions and then crosslinked

(by peroxide or radiation). The crosslinking occurs in the amorphous regions. If

the material is now heated above its melting point, expanded and cooled in the

expanded state, crystallisation occurs to “hold out” the material in its expanded

form. On re-heating above the melting point, the crosslinks pull the material

back into its original form - on cooling the crystallites reform. A prime example

of such a product is heat shrinkable tubing.

DSC: Differential Scanning calorimetry

XRD: X-ray Diffraction

FTIR: Fourier Transform Infra Red

NMR: Nuclear Magnetic Resonance

21

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Substitution of one or more of the H atoms of ethylene gives other polymers,

with markedly different properties.

Use of a diene, (molecule with conjugated double bond) leads to polymers with

unsaturation along the backbone. This can readily be used to crosslink the

polymer.

PE: films, bags, insulation

PP: textiles, containers, polymeric banknotes

EVA: hot melt adhesives, foamed products

EEA: higher heat abrasion and heat resistance

PVC: unplasticised: window frames. Plasticised: electrical wiring

PAN: fibres, basis for carbon fibres

Poly-vinyl alcohol: adhesive

PS: packaging, containers………………………

PVDF: wire insulation, piezoelectric applications (microphones etc)

PTFE: water repellent coatings, non-stick ware, etc……

See: Plastics Materials, J A Brydson, Butterworths 1988

23

A summary of the main polymer conformations.

In the case of graft copolymers, the grafted chain need not be long - for example

CCl4 can be grafted onto natural rubber in the latex form simply using a

peroxide catalyst. The resultant graft polymer is flame retarded.

A more typical graft copolymer is ABS. Styrene and acrylonitrile are

copolymerised in the presence of polybutadiene latex. The SAN copolymer not

only grafts, but forms the continuous phase with “balls” of PBD embedded in it

(about 700 - 1000Å diameter). These balls act as stress relieving centres and

improve the impact resistance dramatically.

24

So far we have mainly presented a homopolymer - polyethylene. In reality

small amounts of co-monomers (e.g. propylene) are usually added to control

mol wt, chain branching, unsaturation - the result is a strictly called a

copolymer, although at low levels of co-monomer the term homopolymer

remains valid

25

.

Shrinking down using a gas torch.

26

A sectioned view of a heat shrinkable joint (the bar through the middle is for

presentation purposes only).

To protect a very large number of connections, a sheet of heat recoverable

polymer is wrapped around the joints and closed with a stainless steel “rail”.

The sheet is torched down, shrinking the polymer around the joints.

Note the (red) hot melt adhesive at the far end of the closure, which ensures a

hermetic seal.

27

Schematic of a polymer extruder.

28

Change the extruder head and tubing can be made.

29

Take extruded tubing (thick wall) of a semi crystalline polymer.

30

Cross link the tubing, in this case by electron beam

The crosllinking occurs in the amorphous regions only.

31

Heat the extruded, crosslinked tubing and expand with compressed air, passing

it through a sizing die.

Cool the tubing, which results in crystallisation of the polymer., holding the

tube in the expanded form.

32

On heating the expanded form, the crystals melt allowing the crosslinks to pull

the tubing back to its unexpanded form – a “memory effect”.

33

Note the heat recovered high voltage connection to the left of the picture (red),

with the polymeric “sheds”. These increase the voltage leakage path to avoid

arc tracking.

The grey units are surge arrestors, also with polymeric shedding.

34

Solder sleeves; heat recoverable tubing with solder rings at either end,

permitting the soldered connection of two wires and simultaneous shrink down

to protect the connection.

35

Heat shrinkable moulded “boots” which, with a deposited metal inner coating,

are used to connect and protect electrical plugs, creating a continuous electrical

shielding of cable and plug.

36

37

Second of the polymerisation processes.

This usually involves the reaction of two dissimilar monomers each with

reactive end groups. The end groups react with the elimination of a small

molecule such as water or hydrochloric acid.

A few examples are presented in the next slides:

38

An example of condensation polymerisation is the production of polyamides

(Nylon). Here the reactive bifunctional monomers are an amine and acid (or

acid chloride), where HCl is eliminated in the reaction. To show how simple

the reaction is, refer to the next slide:

39

The reactant solutions are placed in a beaker, with minimal mixing, and the

polymer which forms at the interface is drawn off over a pulley. This can be

continued until the reactants are exhausted.

A simple example of interfacial polymerisation.

Note: CCl4 cannot now be used, but tetrachloroethylene is a reasonable but not

perfect substitute.

40

Here we see that by the elimination of a molecule of water between the ethylene

glycol and terephthalic acid, an adduct is formed which then reacts with further

acid and glycol (or adduct) to produce a linear copolymer - polyethylene

terephthalate (PET). In practice, the dimethyl ester of terephthalic acid is used,

with methanol being eliminated in a trans-esterification reaction.

A by product of the reaction, which is often detectable by GPC is the cyclic

trimer

41

To control the physical properties of aromatic polyesters, the glycol and simple

acid can be replaced with complex pre-cursors to give a block copolymer. The

properties of the polymer can be tailored by the choice of hard block and soft

block chemistry, together with the relative proportions of each.

Combines resilience, and heat and chemical resistance with strength and

durability.

Can flex in multiple directions, cycle after cycle, long after rubber would break.

A prime example is the Constant Velocity Joint (CVJ) boot, an automotive

component that is subject to an average of 150,000 miles of pounding, and a

wide range of temperatures.

42

43

To obtain polymers with desired specific characteristics, two approaches are

possible:

blends - simply a mixture of two or more dissimilar but compatible polymers.

Dissimilar polymers can be blended using compatibilisers.

or:

copolymers - polymerisation of two monomers to give random or block

products. In the first, the distribution of each monomer is truly random along

the chain. In the second case, blocks of monomer A are interspersed with

monomer B.

44

Originally, UHMWPE was claimed to have yield strength and stiffness slightly

worse than linear PE of “conventional” mol wt. However, fibres hot drawn

from the melt under tension (which orients the polymer) and then braided into

rope (Plasma rope), gives a material as strong as steel but density < 1.

45

Power & data cables are wrapped round rope as Plasma rope paid out from ship

(simpler than umbilical and doesn’t crush fibre optics). Therefore full strength

can be used to lift salvage. At that depth, steel cable would be too heavy.

30/12/1915 Eastern Mediterranean - SS Persia, carrying £30million of Au &

Ag (today’s prices), sunk by U Boat U38 & left upright on seabed at

2800metres. In 2004 recovery using new cable was attempted. After cutting

through 5 decks, no gold was found, but amethysts (earliest known synthetics –

these days more valuable than real ones!).

New Scientist 16 dec 2006 pp32 et seq

46

Additives are used to overcome some of the fundamental drawbacks of

polymers.

Polymers are susceptible to degradation, hence antioxidants and stabilisers are

often needed.

They can be difficult to process, so lubricants are added.

They may need flexibilising - thus PVC requires 30% of plasticiser added to it

to render it flexible.

Fillers may be added to improve hardness, abrasion resistance or even to

cheapen the product.

Dyes or pigments are required to colour the product.

Flame retardants (hydrated fillers or halogen/antimony systems) reduce

flammability and / or smoke emissions.

After glow supressants reduce the smouldering after the flame has gone out.

High levels of conductive fillers may be added to yield a conductive or semi-

conductive product. Note that a recent innovation is a UV-curing acrylate-

styrene-acrylate adhesive filled with conductive particles and designed to

replace solder (especially for organic light emitting diodes, etc)

ref New Scientist vol 179 issue 2407 - 09 Aug 2003 p23

Given that all or some of these additives are required, together with the matrix

of polymers that we have briefly looked at, it can be seen that “a piece of

polymeric material” might consist of up to 20 discrete components, present at

levels from fractions of a percent to 50 or 60%.

46

Heat “tracing” of process pipes.

47

Domestic examples.

48

With a conductively filled crosslinked semi crystalline polymer with embedded

parallel wires, applying a suitable voltage results in current flow causing a rise

in temperature.

As the melting point of the polymer is approached, the melting of the crystalline

regions results in the conductive paths being disrupted. The consequential

increase in resistance limits the current flow, shutting off the resistive heating.

The cable therefore reaches a temperature which depends on the choice of

polymer used and which cannot be exceeded, avoiding the inevitable hot spots

created by overwrapping simple resistive heater. The system needs no

thermocouple and controller and can be cut to any length.

49

50

An inversion of the heat tracing idea. In this case the current flow is through a

short length of compound. As the specific temperature of the device is reached,

the resistance rises exponentially, switching off the current.

On cooling, the current will begin flowing again – a self resetting fuse.

Typically used with lithium batteries to avoid potential explosive decomposition

if a short occurs. Also used to limit the force available on car window closures

to ensure no injuries occur if a body part is trapped.

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52

53

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Polyhydroxyalkanoates or PHAs are linear polyesters produced by bacterial

fermentation of sugars or lipids. More than 100 different monomers can be

combined to give materials with extremely different properties; either

thermoplastic or elastomeric materials, with melting points ranging from 40 to

180 °C. The most common type of PHA is PHB (poly-beta-hydroxybutyrate).

PHB has properties similar to those of PP, however it is stiffer and more brittle.

A PHB copolymer, PHBV, (polyhydroxybutyrate-valerate) is less stiff and

tougher, and it is used as packaging material.

There are potential applications for PHA produced by micro-organisms under

unbalanced growth conditions within the medical and pharmaceutical

industries, primarily due to their biodegradability. (wikipedia)

55

Degradation at low levels leads to:

Discoloration (often by formation of conjugated species). At these levels,

although the colour formation can be quite marked visually, there is often no

evidence of unsaturation in, for example, the Infra Red spectrum of the

polymer.

Loss of properties (chain scission).

As the level of degradation increases: next slide

56

See next slide

57

Chain Scission:

only 1% of bonds need to rupture to cause major changes in physical properties.

58

Depolymerisation:

The catastrophic fire on the Isle of Man in 1973 was due to the use of PMMA

panels as ceiling lights. The release of monomer which caught fire gave rise to

an extensive and massive fire.

59

Elimination:

In the case of PVC, not only is dense smoke released, but the loss of HCl

produces unsaturation which lead to the formation of aromatics and

subsequently “pre-graphitic” char. The degree of ordering in the char can be

followed by X-ray diffraction.

Char may be useful in presenting an intumescent barrier, excluding oxygen and

creating a thermal barrier thus limiting flammability. It may also create

conditions where, in the presence of an electric current, arc tracking occurs, due

to electrical conductivity of the char. Several instances have been reported of

cable bundles exploding within airframes where chafing of the wires has

resulted in a small surface discharge between two conductors, leading to an

even bigger explosion of the mass of cables, puncturing the outside skin of the

aircraft.

60

Polymers are < stable than model cpds due to:

• structural abnormalities

(head to head, unsaturation, chain branching)

• defects introduced in processing

• presence of foreign atoms (catalyst residues etc)

• chain processes - decomposition can activate closest

neighbours

Ref: Cullis C.F. & Hirschler M.M. The Combustion of Organic Polymers - Clarendon 1981

61

The spiders web:

some species of spider generate seven different filaments, the most common being:

• structural web: high strength (greater than steel)

• capture web: 400% elongation to absorb the impact of flying prey

Each fibre (approx 1m in diameter) is a complex structure with microstructure extending from

nanometres to microns. A central oriented core is overlaid with spiral nanofibrils which are held

together with a reformable adhesive. The latter is key to the physical properties.

The oldest web has just been discovered encased in amber and is said to be 130 million years

old.

Result:

A recycable material (some spiders eat their web before dawn and spin a new one - a process

thought to conserve moisture) which is stronger than Kevlar and is produced at neutral pH, room

temperature and at phenomenal speed.

We have a lot to learn, but recent advances using ionic liquids to spin silk has resulted in tailored

products. And now spider genes have been inserted into silkworms to mass produce spider silk.

At present only 10% has been achieved, but the silk is stronger, softer and more durable than

ordinary silk. It is hoped to reach 50% in due course.

Ref: New Scientist vol 162 issue 2183-24 April 1999 page 38 et seq

vol 134 issue 1827-27 June 1992 page18

vol 179 issue 2407-09Aug 2003 page 24

vol 194 issue 2607-09 June 2007 page45

Chemistry World vol 2 no. 11 Sept 2005 page 23

The Times 10 Dec 2007