Polymers/Composites

Introduction

• Almost all biological systems are built of polymers

• perform mechanical functions (like wood, bone, cartilage, leather) but also

• contain and regulate chemical reactions (leaf, veins, cells)

• Natural polymers

• high-performance glass, carbon, or Kevlar-fibre reinforced polymers (GFRP,CFRP, KFRP)

• enjoy a faster growth rate (over 10% per year) than almost any other branch of materials production

• Industry developed new materials are stiff, strong and light.• Though expensive, they are finding increasing use in

aerospace,• transport and sporting goods; in other fields like hiking

equipment, medical goods and even apparently insignificant things like spectacle frames: world-wide, at least 1,000,000,000 people wear spectacles.

• the new polymers are as exciting as the new composites

• By crystallising, or by cross-linking, or by orienting the chains, new polymers are being made which are as stiff as aluminium

• Composites, it is true, are stiff and strong. But they are often very anisotropic, and because they are bound by polymers, their properties can change radically with a small change in temperature

The interest to us are

• Thermoplastics such as polyethylene, which soften on heating.

• Thermosets or resins such as epoxy which harden when two components (a resin and a hardener) are heated together.

• Elastomers or rubbers.• Natural polymers such as cellulose, lignin and

protein, which provide the mechanical basis of most plant and animal life.

• all polymers are made up of long molecules with a covalently bonded backbone of carbon atoms. These long molecules are bonded together by weak Van der Waals and hydrogen (“secondary”) bonds, or by these plus covalent cross-links

• Most polymers are made from oil; the technology needed to make them from coal is still poorly developed

• At 1998 prices, one tonne of oil is about $150; 1 tonne of polyethylene is about $800.

• So doubling the price of oil does not double the price of the polymer

• the energy content of metals is large too: that of aluminium is nearly twice as great as that of most polymers.

The generic polymers

• Thermoplastics• Polyethylene is the commonest of the

thermoplastics- called linear polymers, that is the chains are not cross-linked

• branch occasionally-soften if the polymer is heated

• Thermoplastics are made by adding together (“polymerising”) sub-units(“monomers”) to form long chains

R may simply be hydrogen (as in polyethylene), or CH3 (polypropylene) or Cl polyvinylchloride)

The fibre and film-forming polymers polyacrylonitrile (ACN) and polyethylene teraphthalate (PET, Terylene, Dacron, Mylar) are also thermoplastics.

• Thermosets or resins• Epoxy, familiar as an adhesive and as the matrix

of fibre-glass, is a thermoset• Thermosets are made by mixing two components

(a resin and a hardener) which react and harden, either at room temperature or on heating.

• The resulting polymer is usually heavily cross-linked, so thermosets are sometimes described as network polymers.

• Structure is amorphous• On reheating, the additional 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 (it turns into a rubber).

• Further heating just causes it to decompose

• The generic thermosets are the epoxies and the polyesters (both widely used as matrix materials for fibre-reinforced polymers) and

• the formaldehyde-based plastics (widely used for moulding and hard surfacing).

• Other formaldehyde plastics, which now replace 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 provide the “memory” of the material-returns

Generic thermosets/resins

Natural polymers• The rubber polyisoprene is a natural polymer. So, too, are cellulose and lignin, the main components of wood and straw, and so are proteins like wool or silk.• We use cellulose in vast quantities as paper and (by treating it with nitric acid) we make celluloid and cellophane out of it. But the vast surplus of ligninleft from wood processing, or available in straw, cannot be processed to give a useful polymer.

Generic elastomers (rubbers)

Generic natural polymers

Properties

Flanged connector in an internally pressurised piping system.

• where the square box is a carbon atom, and the small circles are hydrogen. Polymerisation breaks the double bond, activating the ethylene monomer (Fig. 22.1b), and allowing it to link to others, forming a long chain or macromolecule

• (Fig. 22.1c). The ends of the chain are a problem: they either link to other macromolecules, or end with a terminator (such as an OH group), shown as a round blob.

• If only two or three molecules link, we have created a polymer.

(a) The ethylene molecule or monomer; (b) the monomer in the activated state, ready to polymerise with others; (c)–(f ) the ethylene polymer (“polyethylene”); the chain length is limited by the addition of terminators like ---OH. The DP is the number of monomer units in the chain.

(a) Linear polymers are made of chains with a spectrum of lengths, or DPs. The probability of a given DP is P(DP); (b) and (c) the strength, the softening temperature and many other properties depend on the average DP.

• The chain must be longer-at least 500 monomers long. They are called high polymers (to distinguish them from the short ones) and, obviously, their length, or total molecular

• weight, is an important feature of their structure. It is usual to speak of the degree of polymerisation or DP: the number of monomer units in a molecule.

• Commercial polymers have a DP in the range 103 to 105.