Engineering Materials vol 2 Part 10 pot

Proper design with polymers requires agood understanding of their properties and where they come from.. As with metals and ceramics, there is a bewilderingly large number of polymers and

C Polymers and composites

And the new polymers are as exciting as the new composites By crystallising, or bycross-linking, or by orienting the chains, new polymers are being made which are asstiff as aluminium; they will quickly find their way into production The new process-ing methods can impart resistance to heat as well as to mechanical deformation, open-ing up new ranges of application for polymers which have already penetrated heavilyinto a market which used to be dominated by metals No designer can afford toneglect the opportunities now offered by polymers and composites.

But it is a mistake to imagine that metal components can simply be replaced bycomponents of these newer materials without rethinking the design Polymers are lessstiff, less strong and less tough than most metals, so the new component requirescareful redesign Composites, it is true, are stiff and strong But they are often veryanisotropic, and because they are bound by polymers, their properties can changeradically with a small change in temperature Proper design with polymers requires agood understanding of their properties and where they come from That is the func-tion of the next four chapters

In this chapter we introduce the main engineering polymers They form the basis

of a number of major industries, among them paints, rubbers, plastics, syntheticfibres and paper As with metals and ceramics, there is a bewilderingly large number

of polymers and the number increases every year So we shall select a number of

“generic” polymers which typify their class; others can be understood in terms ofthese The classes of interest to us here are:

(a) Thermoplastics such as polyethylene, which soften on heating.

(b) Thermosets or resins such as epoxy which harden when two components (a resin

and a hardener) are heated together

(c) Elastomers or rubbers.

(d) Natural polymers such as cellulose, lignin and protein, which provide the

mechan-ical basis of most plant and animal life

Although their properties differ widely, all polymers are made up of long moleculeswith a covalently bonded backbone of carbon atoms These long molecules are bondedtogether by weak Van der Waals and hydrogen (“secondary”) bonds, or by these pluscovalent cross-links The melting point of the weak bonds is low, not far from roomtemperature So we use these materials at a high fraction of the melting point of theweak bonds (though not of the much stronger covalent backbone) Not surprisingly,

they show some of the features of a material near its melting point: they creep, and the

elastic deflection which appears on loading increases with time This is just one ant way in which polymers differ from metals and ceramics, and it necessitates adifferent design approach (Chapter 27)

import-Most polymers are made from oil; the technology needed to make them from coal isstill poorly developed But one should not assume that dependence on oil makes thepolymer industry specially vulnerable to oil price or availability The value-addedwhen polymers are made from crude oil is large At 1998 prices, one tonne of oil isabout $150; 1 tonne of polyethylene is about $800 So doubling the price of oil does notdouble the price of the polymer And the energy content of metals is large too: that ofaluminium is nearly twice as great as that of most polymers So polymers are no moresensitive to energy prices than are most other commodities, and they are likely to bewith us for a very long time to come

The generic polymers

Thermoplastics

Polyethylene is the commonest of the thermoplastics They are often described as

linear polymers, that is the chains are not cross-linked (though they may branch

occa-sionally) That is why they soften if the polymer is heated: the secondary bonds whichbind the molecules to each other melt so that it flows like a viscous liquid, allowing it

to be formed The molecules in linear polymers have a range of molecular weights,and they pack together in a variety of configurations Some, like polystyrene, areamorphous; others, like polyethylene, are partly crystalline This range of molecularweights and packing geometries means that thermoplastics do not have a sharp melting

point Instead, their viscosity falls over a range of temperature, like that of an ganic glass.

inor-Thermoplastics are made by adding together (“polymerising”) sub-units (“monomers”)

to form long chains Many of them are made of the unit

HCH

HCR

repeated many times The radical R may simply be hydrogen (as in polyethylene), or

—CH3 (polypropylene) or —Cl (polyvinylchloride) A few, like nylon, are more plicated The generic thermoplastics are listed in Table 21.1 The fibre and film-formingpolymers polyacrylonitrile (ACN) and polyethylene teraphthalate (PET, Terylene,Dacron, Mylar) are also thermoplastics

com-Thermosets or resins

Epoxy, familiar as an adhesive and as the matrix of fibre-glass, is a thermoset

(Table 21.2) Thermosets are made by mixing two components (a resin and a hardener)

which react and harden, either at room temperature or on heating The resultingpolymer is usually heavily cross-linked, so thermosets are sometimes described as

network polymers The cross-links form during the polymerisation of the liquid resin

and hardener, so the structure is almost always amorphous On reheating, the tional secondary bonds melt, and the modulus of the polymer drops; but the cross-links prevent true melting or viscous flow so the polymer cannot be hot-worked (itturns into a rubber) Further heating just causes it to decompose

addi-The generic thermosets are the epoxies and the polyesters (both widely used asmatrix materials for fibre-reinforced polymers) and the formaldehyde-based plastics(widely used for moulding and hard surfacing) Other formaldehyde plastics, which nowreplace bakelite, are ureaformaldehyde (used for electrical fittings) and melamine-formaldehyde (used for tableware)

Elastomers

Elastomers or rubbers are almost-linear polymers with occasional cross-links in which,

at room temperature, the secondary bonds have already melted The cross-links vide the “memory” of the material so that it returns to its original shape on unloading.The common rubbers are all based on the single structure

pro-CH

CR

A B C

D E F

HCH

HC

with the position R occupied by H, CH or Cl They are listed in Table 21.3

Natural polymers

The rubber polyisoprene is a natural polymer So, too, are cellulose and lignin, themain components of wood and straw, and so are proteins like wool or silk We usecellulose in vast quantities as paper and (by treating it with nitric acid) we makecelluloid and cellophane out of it But the vast surplus of lignin left from wood process-ing, or available in straw, cannot be processed to give a useful polymer If it could, it

COOCH 3

Polyethylene, PE Tubing, film, bottles, cups, electrical insulation,

packaging.

Table 21.1 Generic thermoplastics

A B C

D E F

H C

D E F

H C

A B C

D E F

F C

F n

Partly crystalline.

Polystyrene, PS Cheap moulded objects Toughened with butadiene to

make high-impact polystyrene (HIPS) Foamed with

CO 2 to make common packaging.

A B C

D E F

H C

Amorphous.

Polyvinylchloride, PVC Architectural uses (window frames, etc.) Plasticised to

make artificial leather, hoses, clothing.

A B C

D E F

H C

D E F

H C

Amorphous.

Partly crystalline when drawn.

H C

CH 3

H C

C 6 H 5

H C Cl

CH 3

C

C 6 H 11 NO

Elastomer Composition Uses

Table 21.3Generic elastomers (rubbers)

Amorphous except at high strains.

Amorphous except at high strains.

Polychloroprene Neoprene An oil-resistant rubber used for seals.

Amorphous except at high strains.

A B C

D E F

H C

A B C

D E F

H C

C C C H

H

H H

A B C

D E F

H C

C C C Cl

H

H H

Expensive.

Table 21.2 Generic thermosets or resins

A B C

D E F

Cheaper than epoxy.

A B C

D E F

D E F

) n

Cellulose Framework of all plant life, as the main structural

component in cell walls.

Table 21.4 Generic natural polymers

Amorphous.

Lignin The other main component in cell walls of all plant life Protein

Crystalline ( C 6 H 9 O 6

Gelatin, wool, silk.

A B C

D E F

n

NH C C

H O R

R is a radical.

Partly crystalline.

Table 21.5 Properties of polymers

($US) tonne −1 ) (Mg m −3 ) modulus strength

(20°C 100 s) (MPa) (GPa)

would form the base for a vast new industry The natural polymers are not as ated as you might expect They are listed in Table 21.4.

complic-Material data

Data for the properties of the generic polymers are shown in Table 21.5 But you have

to be particularly careful in selecting and using data for the properties of polymers.Specifications for metals and alloys are defined fairly tightly; two pieces of Type 316Lstainless steel from two different manufacturers will not differ much Not so withpolymers: polyethylene made by one manufacturer may be very different frompolyethylene made by another It is partly because all polymers contain a spectrum ofmolecular lengths; slight changes in processing change this spectrum But it is alsobecause details of the polymerisation change the extent of molecular branching andthe degree of crystallinity in the final product; and the properties can be further changed

by mechanical processing (which can, in varying degrees, align the molecules) and byproprietary additives For all these reasons, data from compilations (like Table 21.5), or

data books, are at best approximate For accurate data you must use the manufacturers’

data sheets, or conduct your own tests

Fracture Glass Softening Specific heat Thermal Thermal toughness temperature expansion (J kg −1 K −1 ) conductivity coefficient

There are other ways in which polymer data differ from those for metals or ics Polymers are held together by two sorts of bonds: strong covalent bonds whichform the long chain backbone, and weak secondary bonds which stick the long chains

ceram-together At the glass temperature Tg, which is always near room temperature, thesecondary bonds melt, leaving only the covalent bonds The moduli of polymers re-

flect this Below Tg most polymers have a modulus of around 3 GPa (If the polymer is

drawn to fibres or sheet, the molecules are aligned by the drawing process, and the

modulus in the draw-direction can be larger.) But even if Troom is below Tg, Troom will

still be a large fraction of Tg Under load, the secondary bonds creep, and the modulusfalls* The table lists moduli for a loading time of 100 s at room temperature (20°C);for loading times of 1000 hours, the modulus can fall to one-third of that for the short

(100 s) test And above Tg, the secondary bonds melt completely: linear polymersbecome very viscous liquids, and cross-linked polymers become rubbers Then themodulus can fall dramatically, from 3 GPa to 3 MPa or less

You can see that design with polymers involves considerations which may differfrom those for design with metals or ceramics And there are other differences One ofthe most important is that the yield or tensile strength of a polymer is a large fraction

of its modulus; typically, σy= E/20 This means that design based on general yield

(plastic design) gives large elastic deflections, much larger than in metals and ceramics.The excessive “give” of a poorly designed polymer component is a common experi-ence, although it is often an advantage to have deflections without damage – as inpolyethylene bottles, tough plastic luggage, or car bumpers

The nearness of Tg to room temperature has other consequences Near Tg most

polymers are fairly tough, but KIC can drop steeply as the temperature is reduced.(The early use of polymers for shelving in refrigerators resulted in frequent fractures

at +4°C These were not anticipated because the polymer was ductile and tough atroom temperature.)

The specific heats of polymers are large – typically 5 times more than those of metalswhen measured per kg When measured per m3, however, they are about the samebecause of the large differences in density The coefficients of thermal expansion ofpolymers are enormous, 10 to 100 times larger than those of metals This can lead toproblems of thermal stress when polymers and metals are joined And the thermalconductivities are small, 100 to 1000 times smaller than those of metals This makespolymers attractive for thermal insulation, particularly when foamed

In summary, then, design with polymers requires special attention to time-dependenteffects, large elastic deformation and the effects of temperature, even close to room tem-perature Room temperature data for the generic polymers are presented in Table 21.5

As emphasised already, they are approximate, suitable only for the first step of thedesign project For the next step you should consult books (see Further reading), andwhen the choice has narrowed to one or a few candidates, data for them should besought from manufacturers’ data sheets, or from your own tests Many polymerscontain additives – plasticisers, fillers, colourants – which change the mechanical prop-erties Manufacturers will identify the polymers they sell, but will rarely disclose their

* Remember that the modulus E = σ/ε ε will increase during creep at constant σ This will give a lower

apparent value of E Long tests give large creep strains and even lower apparent moduli.

additives So it is essential, in making a final choice of material, that both the polymer

and its source are identified and data for that polymer, from that source, are used in the

design calculations

Further reading

F W Billmeyer, Textbook of Polymer Science, 3rd edition, Wiley Interscience, 1984.

J A Brydson, Plastics Materials, 6th edition, Butterworth-Heinemann, 1996.

C Hall, Polymer Materials, Macmillan, 1981.

International Saechtling, Plastics Handbook, Hanser, 1983.

R M Ogorkiewicz (ed.), Thermoplastics: Properties and Design, Wiley, 1974.

R M Ogorkiewicz, Engineering Design Guide No 17: The Engineering Properties of Plastics,

Oxford University Press, 1977.

Problems

21.1 What are the four main generic classes of polymers? For each generic class:(a) give one example of a specific component made from that class;

(b) indicate why that class was selected for the component

21.2 How do the unique characteristics of polymers influence the way in which thesematerials are used?

The simpler polymers (like polyethylene, PMMA and polystyrene) are linear: thechains, if straightened out, would look like a piece of string These are the thermoplastics:

if heated, the strings slither past each other and the polymer softens and melts And, atleast in principle, these polymers can be drawn in such a way that the flow orients thestrings, converting the amorphous tangle into sheet or fibre in which the molecules aremore or less aligned Then the properties are much changed: if you pull on the fibre(for example) you now stretch the molecular strings instead of merely unravellingthem, and the stiffness and strength you measure are much larger than before.The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks: each chain is cross-linked in many places to other chains,

so that, if stretched out, the array would look like a piece of Belgian lace, somehowwoven in three dimensions These are the thermosets: if heated, the structure softensbut it does not melt; the cross-links prevent viscous flow Thermosets are usually abit stiffer than amorphous thermoplastics because of the cross-links, but they cannoteasily be crystallised or oriented, so there is less scope for changing their properties byprocessing

In this chapter we review, briefly, the essential features of polymer structures Theyare more complicated than those of metal crystals, and there is no formal framework(like that of crystallography) in which to describe them exactly But a looser, lessprecise description is possible, and is of enormous value in understanding the propert-ies that polymers exhibit

Molecular length and degree of polymerisation

Ethylene, C2H4, is a molecule We can represent it as shown in Fig 22.1(a), where thesquare box is a carbon atom, and the small circles are hydrogen Polymerisation breaks