Elastomers

XXXX

2.7 Polymeric structures .1 Thermoplastics

2.7.2 Elastomers

The development of a relatively small number of crosslinking chains between linear molecules can pro- duce an elastomeric material which, according to an ASTM definition, can be stretched repeatedly at room temperature to at least twice its original length and which will, upon sudden release of the stress, return forcibly to its approximate original length. As shown in Figure 2.24, the constituent molecules are in a coiled and kinked condition when unstressed; during elastic strain, they rapidly uncoil. Segments of the structure are locally mobile but the crosslinks tend to prevent any gross relative movement of adjoining molecules, i.e. viscous deformation. However, under certain con- ditions it is possible for elastomers, like most poly- mers, to behave in a viscoelastic manner when stressed and to exhibit both viscous (time-dependent) and elas- tic (instantaneous) strain characteristics. These two effects can be broadly attributed, respectively, to the l In the late 1940s this copolymer was chosen to provide the superior surface texture and durability required for the first long-play microgroove gramophone records. This

33 ~ r.p.m, system, which quickly superseded 78 r.p.m.

shellac records, has been replaced by compact discs made from polycarbonate thermoplastics of very high purity.

Figure 2.24 Unstrained elastomeric structure showing entanglement, branching points, crosslinks, loops and free ends (after Young, 1991).

relative movement and the uncoiling and/or unravel- ling of molecular segments.

Elastomers include natural polymers, such as poly- isoprene and polybutadiene in natural rubbers, and synthetic polymers, such as polychloroprene (Neo- prene), styrene-butadiene rubber (SBR) and silicone rubbers. The structural repeat units of some impor- tant elastomers are shown in Table 2.7. In the orig- inal vulcanization process, which was discovered by C. Goodyear in 1839 after much experimentation, iso- prene was heated with a small amount of sulphur to a temperature of 140~ causing primary bonds or crosslinks to form between adjacent chain molecules.

Individual crosslinks take the form C - ( S ) , , - C , where n is equal to or greater than unity. Monosulphide links (n = 1) are preferred because they are less likely to break than longer links. They are also less likely to allow slow deformation under stress (creep). Examples of the potentially-reactive double bonds that open up and act as a branching points for crosslinking are shown in Table 2.7. Nowadays, the term vulcaniza- tion is applied to any crosslinking or curing process which improves elasticity and strength; it does not necessarily involve the use of sulphur. Hard rubber (Ebonite) contains 3 0 - 5 0 % sulphur and is accordingly heavily crosslinked and no longer elastomeric. Its long- established use for electrical storage battery cases is now being challenged by polypropylene (PP).

The majority of polymers exhibit a structural change known as the glass transition point, Tg; this tem- perature value is specific to each polymer and is of great practical and scientific significance. (Its impli- cations will be discussed more fully in Chapter 11.) In general terms, it marks a transition from hard, stiff and brittle behaviour (comparable to that of an inor- ganic glass) to soft, rubbery behaviour as the tempera- ture increases. The previously-given ASTM definition described mechanical behaviour at room temperature;

it follows that the elastomeric condition refers to tem- peratures well above Tg. Table 2.7 shows that typi- cal values for most elastomers lie in the range - 5 0 ~ to -80~ When an elastomeric structure is heated through Tg, the segments between the linkage or branching points are able to vibrate more vigorously.

36 Modern Physical Metallurgy and Materials Engineering T a b l e 2.7 R e p e a t u n i t s o f t y p i c a l e l a s t o m e r s

Polyisoprene

Polychloroprene

Polybutadiene

S t y r e n e - b u t a d i e n e rubber (SBR)

H I C

I

-(-

H

-(-

H

I I

C C

I I

C6H.~ H

Silicone rubber - ~ O

CH 3 H

I I

C C

C!

I C

C "~ " C

I I

H H

cH, ,

_Si

I CH3

I C

I H

I C I H

I C

I H

H_):.

^I

C I H

r,

-72oC

-50oc

-85oc

-50~

-120~

A simple linear equation expresses the temperature- dependence of an elastomer's response to shear stress:

tx = N k T

where /z is the shear modulus, N is the number of segments per unit volume of structure (between successive points of crosslinking), k is the Boltzmann constant and T is absolute temperature. Segments are typically about 100 repeat units long. Clearly, for a given polymer, the stiffness under shear conditions is directly proportional to the absolute temperature. As temperature increases, deflection under load becomes less. This rather unusual feature has raised engineering problems in suspension systems.

At higher temperatures, well above Tg, the poly- meric structure is likely to deform slowly under applied stress (creep) and ultimately to break down into smaller chemical entities, or degrade. As indicated in Figure 2.24, unstressed elastomers are disordered and non-crystalline. Interestingly, stressing to produce a high elastic strain, say 200% or more, will induce a significant amount of crystallinity. Stress aligns the chains and produces regions in which repeat units form ordered patterns. (This effect is readily demon- strated by projecting a monochromatic beam of X-rays through relaxed and stretched membranes of an elas- tomer and comparing the diffraction patterns formed upon photographic film.)

2.7.3 T h e r m o s e t s

In the third and remaining category of polymers, known generally as thermosets or network polymei~s, the degree of crosslinking is highly developed. As a result, these structures contain many branching points.

They are rigid and strong, being infinitely braced in three dimensions by numerous chain segments of relatively short length. Unlike thermoplastics, molec- ular mobility is virtually absent and Tg is accord- ingly high, usually being above 50~ Thermosets are therefore regarded generally as being hard and brittle ('glassy') materials. Examples of thermoset resins in common use include phenol-formaldehyde (P-F resin;

Bakelite), epoxy resins (structural adhesives; Araldite), urea formaldehyde (U-F resin; Beetle) and polyester resins. A resin is a partially-polymerized substance which requires further treatment.

The utilization of thermosets typically involves two stages of chemical reaction. In the first stage, a liq- uid or solid prepolymer form or precursor is produced which is physically suitable for casting or moulding.

Resins are well-known examples of this intermedi- ate state. Their structures consist mostly of linear molecules and they are potentially reactive, having a specifiable shelf-life. In the second stage, extensive crosslinking is promoted by heating, pressure applica- tion or addition of a hardening agent, depending upon

the type of polymer. This stage is commonly referred to as curing. The resultant random network possesses the desired stiffness and strength. When heated, this structure does not exhibit viscoelastic flow and, being both chemically and physically stable, remains unal- tered and hard until the decomposition temperature is reached. The formation of a thermoset may thus be regarded as an irreversible process. With pheno- lic resins, final crosslinking is induced by heating, as implied by the term thermoset. The latter term is also applied, in a looser sense, to polymers in which final crosslinking occurs as the result of adding a hard- ener; for example, in epoxy resin adhesives and in the polyester resin-based matrix of glass-reinforced polymer (GRP; Fibreglass). in these substances, an increase in the ratio of hardener to resin tends to increase the Tg value and the modulus.

Many thermoset structures develop by condensa- tion polymerization. This process is quite different in

Atomic arrangements in materials 37 chemical character to the process of addition polymer- ization by which linear molecules grow in an endwise manner in thermoplastics. Although it is one of the oldest synthetic polymers, having appeared as Bake- lite in the early 1900s, phenolformaldehyde retains its industrial importance. It is widely used for injection- mouldings in the automotive and electrical industries, for surface coatings and as a binder for moulding sands in metal foundries. It is therefore appropriate to use phenolformaldehyde as an illustrative example of the condensation reaction, focusing upon the novolac resin route. In the first stage (Figure 2.25a), phenol and formaldehyde groups react to form an addition compound (methylol derivative). Figure 2.25b shows these derivatives joining with phenol groups in a con- densation reaction. Methylene bridges (CH2) begin to form between adjacent phenol groups and molecules of water are released. The two reactions shown dia- grammatically in Figures 2.25a and 2.25b produce a

(a) Methylolation reaction

OH OH

H.. c

+H/, =O

phenol formaldehyde derivative

OH

(b) Condensation reaction

OH OH OH

H2 +

(c) Cured structure

"---- CH2

OH OH OH OH

CHz 2 CH2

I

Figure 2.25 Interaction of phenol and formaldehyde to form a thermoset structure.

+ H20

38 Modem Physical Metallurgy and Materials Engineering relatively unreactive novolac resin. (Control of the ini- tial phenol/formaldehyde ratio above unity ensures that a deficiency of formaldehyde will inhibit crosslinking during this first stage.) After drying, grinding and the addition of fillers and colourants, the partly-condensed resin is treated with a catalysed curing agent which acts as a source of formaldehyde. Network formation then proceeds during hot-moulding at a temperature of 200-300~ Each phenol group is said to be tri- functional because it can contribute three links to the three-dimensional random network (Figure 2.25c).

Another type of phenolic resin, the resole, is pro- duced by using an initial phenol/formaldehyde ratio of less than unity and a different catalyst. Because of the excess of formaldehyde groups, it is then possible to form the network structure by heating without the addition of a curing agent.