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MATERIALS INFORMATION SERVICE The Materials Information Service helps those interested in improving their knowledge of engineering materials and highlights the national network of materials expertise. This Profile is one of a series produced by the Materials Information Service. For advice relating to your particular materials problem, you can contact the MIS at: The Materials Information Service The Institute of Materials, Minerals and Mining Danum House, South Parade Doncaster DN1 2DY Tel:
01302 320 486
Fax: 01302 380 900 MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining
CORROSION John Hickman, Trowbridge College
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Introduction Elasticity is a property possessed by many solid materials, but the class of materials known as rubbers or elastomers possess a different property called high elasticity. This differs from conventional Hookean elasticity in that it exhibits much higher values of elongation at break, up to 1000% or more. This huge value for elongation at break means that many of the other properties measured in the accepted manner for metals show interesting variations. Tensile strength, for example, although still calculated in the usual way using the initial cross-sectional area, will give values much lower than the true stress in the material because the actual cross-section at break will be approximately one-tenth of the original value. Another important point is that, over the elongation at break range, the slope of the line varies substantially, giving typically an S-shaped curve, so that the normal engineering term of modulus becomes meaningless. Rather confusingly a description of the relative stiffness of the rubbery materials, frequently used, is also called ’modulus’. This is simply a measure of the load at a given elongation, e.g. ’modulus at 300% ’ is simply a stress value for the material at an elongation of 300% (i.e. at four times the original length). Although 300% is a commonly used value, a range of extensions is used to give ’modulus’ values relevant to different applications. Throughout this Profile modulus refers to this rubber property.
High Elasticity or Rubbery Elasticity Elastomers are materials of a very high molecular weight, generally composed of one or more monomers polymerised or co-polymerised together to form a polymer (or copolymer). The polymer consists of a very long chain of monomer molecules chemically bonded together to form a single molecule with a molecular weight of several million. This large molecule, which will consist of several tens of thousands of the small (monomer) units bound together, has a very large length-to-diameter ratio (often more than 10,000 -1) and does not exist as a straight, rod-like structure, but in a form known as a random coil. In a raw uncompounded polymer, each random coil will be entangled with many neighbours making flow difficult. The mass behaves like a liquid but with a viscosity some five million times that of water, but will eventually flow under the effects of stress and temperature.
Flow under stress is a severe limitation to the industrial utilisation of plain elastomers. It can be overcome by crosslinking. In this process the random coils are tied, or cross linked, to others at intervals randomly along their length. This is achieved by reaction with added chemicals assisted by heating to 150200°C. The resultant mass still exhibits the characteristic high elongation and modulus behaviour because lengths of random coil between crosslinks can still straighten under load. The extent of elasticity will depend upon the length of random coil between crosslinks. A heavily Closslinked polymer will exhibit lower extensibility and higher stiffness than a lightly crosslinked example. A simple indentation hardness test is used to characterise blends for industrial applications.
Range of Properties Rubber materials in use are required to have a wide range of properties, from very soft to very hard. These are generally obtained by the compounder, using a base polymer or polymer blend to give essential properties such as strength, ageing and environmental resistance, then modifying hardness and modulus properties by additives such as fillers to achieve a suitable compound. Compounds containing only a vulcanising (crosslinking) system are generally known as gum compounds and generally have a Specific Gravity of close to 1.0. Their modulus is affected by both the crosslink density and the ambient temperature. Crosslink density has to be kept between limits to ensure that ’rubbery’ properties are retained. The modulus shows an almost linear increase as the crosslink density increases but is mainly manipulated for engineering purposes by use of relatively large quantities of solid particulate materials called fillers. These range from cheap, inert materials such as washed clays and Whitings (calcium carbonates) to active, property enhancing materials, by far the most common being Carbon Black — this is why engineering rubber compounds tend to be black in colour. By using a range of type, volume fractions, and particle sizes of black - perhaps in conjunction with other fillers - the shear modulus (G) of the compound can be varied over the approximate range 03 - 2.5 Mnm-2.
Hardness Rubber hardness is measured, like metals, by an indentation method. The normal UK method is to use International Rubber Hardness Degrees (IRHD). Values range from about 30 for a soft gum rubber to about 85 for a highly filled material, there is an almost linear relationship between Youngs Modulus and IRHD.
Physical Properties The physical properties of a rubber compound have a complex dependence on the crosslink system, crosslink density, and the type and quantity of filler, but in general can be represented as in Figure 1. As the article deforms under a steadily increasing tensile stress its behaviour is not linear, the curve obtained is a characteristic S curve, see Figure 2. However, the material behaves almost linearly in shear or compression deformation. Figures 3 and 4.
Fig.1: How properties vary with Filler Loading or Crosslink Density
Fig.2: Typical tensile behaviour of an Elastomer
Fig.3: Shear Curves
Fig. 4: Compression Curves
The early part of the Tensile curve is linear and the gradient of this is taken as a measure of the Youngs Modulus of the material (E). The Shear modulus, which can also be measured independently, can be shown to be one-third of this value for a simple gum rubber network. The Shear modulus (G) ranges from about 0.3 MNm2 upwards. For filled materials G can increase to about 2.5 Mnm2, but E values then increase to 4 to 5G. The Bulk (compressive) Modulus has the relatively high value of 1000 -1300 Mnm2 and elastomers can be regarded, like most liquids, as effectively incompressible in a closed system. If the sample is relaxed before failure occurs the return stress-strain curve follows closely the outward path, i.e. almost all the energy absorbed by the sample is
returned on relaxation. This low level of energy loss (low hysteresis) in a cycle of deformation can be particularly useful. Rubber compounds can be used to isolate vibrating bodies (usually engines of all types), and the energy loss per cycle of vibration, although appearing in the rubber as heat, can be controlled by compound selection so that the bearing does not overheat. One unique property of high quality rubber compounds is that high stresses can be sustained over a wide range of elongation, thus the area underneath the stress-train plot is high, and a considerable amount of energy can be absorbed before the sample fails. All crosslinked polymers under stress suffer from stress relaxation or creep as the article undergoes small internal changes such as breaking (and possibly reforming) of individual crosslinks suffering the highest stress levels. This is inconvenient but is fortunately quite predictable (linear against log time) and properly formulated Natural Rubber compounds can have very low creep rates (of the order of 2% per decade of time).
Types of Rubbery Polymers NR Natural Rubber This is the original rubber and in many ways is an ideal polymer for dynamic or static engineering applications. It has excellent dynamic properties, with a low hysteresis loss, and good low temperature properties, it can be bonded well to metal parts, has high resistance to tear and abrasion and it is relatively easy to process. It also has excellent low temperature properties (Tg approx. minus 7QPC) see later for a definition.
Unfortunately it has a relatively high reactivity with its environment, with oxygen and particularly ozone. Ozone causes surface cracking that can rapidly penetrate the component when even a low threshold value of tensile stress is applied. However, in components of fairly large cross sectional area, whilst there may be extensive surface reaction, depending upon the external stress pattern, actual penetration of the oxygen and ozone can be low and the inside is protected by the degraded exterior. Articles in shear or compression remain unaffected provided that the surface itself does not enter a tension mode, (this can be ensured by design). One hundred year old seals from Victorian water and drainage systems demonstrate this very effectively as the seals still function. In tension, ozone cracking can propagate quite rapidly through an otherwise satisfactory sample. This means that the lives and performance of thin and thick items made of the same material in the same environment can be very different. Attack by contact with oils is usually restricted to a thin surface layer due to slow diffusion rates. Lighter solvents will attack the rubber more rapidly, actual rates
depend on the type of solvent and the type of rubber. Both oils and solvents will cause a loss of physical strength — thin articles being the worst affected. The major advantage of Natural Rubber, which makes it dominant in many engineering applications, is its dynamic performance. It has a low level of damping, and its properties remain fairly constant over the range l-200Hz, and show only slight increase to l000Hz. Its combined dynamic properties generally out-perform any synthetic rubbers or combinations available to date. Despite proliferation of general and special purpose synthetics. Natural Rubber still holds a significant market share between 30 and 40%.
Synthetic Rubbers Although Natural Rubber, with the benefit of modern compounding, is very satisfactory for many applications, it is also a strategically important material, a natural crop only produced in tropical countries and has relatively poor ageing properties. Therefore synthetic materials have been developed to replace Natural Rubber in a wide range of applications. There is now a wide range of synthetics available able to cope with high and low temperatures, contact with fluids of various types (including at high pressures), and aggressive or corrosive environments The main Synthetic Rubbers are: SBR Styrene Butadiene Rubber A general purpose rubber, which, when compounded with carbon black, behaves similarly to NR (Tg is higher at about minus 55°C). BR Butadiene Rubber A non-polar rubber like NR and SBR, with a very low Tg (approximately minus 80PC). Very high resilience (very low loss) rubber used in ’superballs’, but also much used in combination with NR and SBR in long-life rubber tyre treads. Difficult to process unless blended with another elastomer. CR Chloroprene Rubber A polar polymer with improved resistance to attack by non-polar oils and solvents. It has high toughness, good fire resistance, good weatherability, and is easily bonded to metals. NBR (Acrylo) Nitrite Butadiene Rubber Variation of the Acrylonitrile (ACN) content from 18-50% controls polarity arid other properties. High resistance to non-polar oils and fuels (eg used in seals, fuel lines, hydraulic pipes) but high Tg. Improved versions of this much used polymer are becoming available.
IIR Iso Blrtylene Isoprene (Butyl) Rubber This material has a low Tg but has very little ’bounce’. It has excellent ageing properties and has a very low permeability to gases, so it is much used as a tubeless tyre liner, as well as for reservoir linings and other membranes. Chemically modified forms are frequently used. EPDM Ethylene Propylene Rubber (also EPR). This is a much used non-polar rubber in applications that require good ageing properties, such as in heater and radiator hose, car door water and draught seals. The structure of the polymer can be altered to give a fairly wide range of properties and uses. Other more expensive varieties are generally designed to increase the working temperature range, especially at the high end, and usually contain chemical elements such as fluorine to increase the stability of the carbon backbone. Silicone rubber is unique in not having a carbon backbone, being -Si-O-Si-O-, and this extends the useful temperature range noticeably. It has a Tg as low as minus 127°C depending on type, and can be used in service at temperatures of 200°C or more for several years. Further modification with fluorine will give even better performance. Several other special purpose rubbers are available, including polyurethanes. Chloroprene rubber, an early synthetic rubber, has been used in many outdoor applications due to its superior weathering properties and oil resistance. It performs well compared with Natural Rubber in many ways but can suffer from long-term stiffening (change in properties) and its low temperature performance is not as good as Natural Rubber.
Properties and Applications It is a paradox that elastomers whose unique property is high elasticity are rarely used in tension. Elastomeric products are used in areas such as vibration isolation, ridge bearings, suspension bearings, earthquake protection, flexible jointings, seals, electrical insulators, energy absorption, hoses, belts, tyres, and many other areas. Virtually all industrial applications utilise rubbers in shear, compression or a combination of these whilst taking advantage of other properties. Typical applications are bridge bearings, suspension bearings for rail vehicles and anti-vibration mountings. To be useful most rubber components have to be properly fixed in the appropriate location. Therefore most articles comprise an appropriate rubber shape permanently bonded to suitable rigid mounts, usually steel, for bolting to the structures concerned. Good quality natural rubber compounds can be strongly bonded to properly prepared steel surfaces by standardised one or two coat bonding systems obtained from
various trade suppliers. Excellent, reliable bonds can be obtained on a production line basis provided proper attention is given to cleanliness and procedures. Non-bonded components can be used, relying on the high coefficient of friction of rubber to keep the component in position, but are generally much less satisfactory than bonded components which have more consistent deformation characteristics and can be located much more positively. Also, bonded components can tolerate the load going through a zero or even negative force during unusual situations such as a resonant vibration.
Bridge and Suspension Bearings A typical bridge bearing, which generally is a solid square or cylindrical piece of rubber, is sandwiched between steel plates as in Figure 5a. Substantial height changes can be avoided when intermediate metal plates are inserted, with bonding into the structure. These contribute to non-linear behaviour by steadily incurring shear and surface tensile forces within the block which inhibit the large sideways bulge, Figure 5b. Figure 6 further shows that the insertion of steel plates into a bearing can radically increase the load bearing capacity. Note that inserted plates are completely encapsulated by the rubber to prevent delamination by rusting, bond failure, or water ingress. This type of bearing can accommodate vertical deflection (compression), horizontal deflection due to ’spread’ or lateral movement (shear) in any direction and tilt (rotation) due to flexure. All this is achieved with one solid, maintenancefree bearing involving no moving parts: typical sizes would be up to 250mm square, arid 100 - 150mm thick: with a design life in excess of 50 years, thus replacing complex conventional bearings which require routine maintenance and inspection. An example of a bearing designed for a range of stiffnesses in different directions is shown in Figure 7. This type can be used, for example, as suspension units for rail cars. The main weight of the car is carried on the top face and down into the ’V area, putting the rubber into combined shear and compression, with different load bearing capabilities vertically (up and down the page), in the direction of travel (left to right or right to left across the page) and along the axle (perpendicular to the page).
Vibration Isolation All moving or vibrating masses, such as a car engine, have a resonant frequency at which any applied deflection is greatly amplified, see Figure 8. The bearing design is devised so that the natural frequency of the assembly is some one third of the applied frequency. At this level any applied deflection is suppressed, see Figure 8.
This is fine in steady state service but of course the mass has to pass through the resonant frequency during starting and stopping. It is therefore essential that the mass be damped to minimise movement, for example the engine compartment is not a large space and excessive movement could damage both engine and bodyshell! Steel coiled springs with separate mechanical dampers would be adequate but relatively complex, whereas solid rubber mounts of appropriate design are simpler and superior, using the natural damping qualities (hysteresis) of the elastomer to achieve the performance.
Fig. 5(a) and (b): The difference in compression and bulging of plain and plated bearings under the same load
Fig.6: Increasing the number of steel plates increases load bearing capacity
Fig.7: Bearing design for a range of stiffness
Fig.8:
Energy Absorption A major application for rubbery materials is in energy absorption, for example absorbing the large amounts of energy from the ’way’ of a large ship coming in to dock is important to avoid damage to both ship and dockside. Simple dock fenders are often a thick wall tube, wall thickness around 75mm and overall diameter
300mm, giving a collapsing distance of about 150mm before a steep rise in transmitted load becomes apparent. For a typical fender, a load of about 12 tonnes/metre would be required to compress the fender flat. Larger fenders with a range of geometries can provide a collapsing distance of about I metre. Lighter construction pneumatic fenders, built similarly to a tyre, are also available and give very good load deflection characteristics. Other uses for rubber in energy absorption include transportation of delicate or expensive machinery, which can be mounted on collapsible supports for transport. The supports are designed to buckle to extend the load/deflection plot for extra energy absorption, they can also damp the harmonic frequency and suppress excessive movement.
Glass Transition Temperature (Tg) Elastomers have limited performance generally if a wide range of temperatures is considered. Low temperatures increase stiffness, and at a characteristic temperature called the Glass Transition temperature (Tg) the material loses all rubbery properties and becomes ’glassy’, with characteristically brittle properties. The Tg for NR is about minus 72°C, lower than most synthetics, but applications involving arctic environments need careful testing. Many elastomers become progressively leathery’ as they cool towards Tg, their use in this state is not recommended.
Crystallisation Under certain conditions some elastomers can ’crystallise’. This is not usually desirable as it interferes with rubbery properties and occurs at temperatures well above Tg. Crystallisation involves segments of a random coil coming into close proximity to a neighbouring coil allowing areas of high organisation (crystallites) to form within the bulk. It is not possible particularly with crosslinked polymers, for the whole mass to be crystalline and a ’percentage crystallinity’ figure is used to describe the condition, which varies with polymer structure, level of orientation, and temperature. The stiffening (increased modulus) effect becomes evident as ambient temperature reduces, and can manifest itself over long periods of time: the effect, however, is completely reversible if temperature increases. Vulcanised Natural Rubber also suffers from crystallisation at high levels of extension when polymer chains are in sufficiently close alignment to enable associations to form, this is useful in resistance to cutting and tearing. However, normal usage of NR in applications in compression or shear will not produce this type of crystallinity.
Chloroprene rubbers have a higher tendency to crystallise and can show stiffening (modulus increase) with ageing in service. The type of crosslinking system used can have marked effects on long term performance, and even NR compounds can show stiffening on exposure to temperatures below 10°C, which is commonly experienced in many parts of the world.
Higher Temperature Effects High temperatures generally increase the rate of ageing (degradation of properties with time) due to the environment. Temperatures of the order of 120°C, typical of automotive under-the-bonnet temperatures, are quite severe for many common elastomers and careful selection of compounds is necessary for satisfactory performance.
Where to get advice The following organisations are all expert in rubbers and elastomers. RAPRA Technology Ltd Shrewsbury Shropshire SY44NR Tel: 01939 250383 Fax: 01939 251118 Malaysian Rubber Producers’ Research Association, (MRPRA) Tun Abdul Razak Laboratory Brickendonbury Hertford SG13 8NL Tel: 01992 584966 Fax: 01992 554837 Centre for Polymer Studies Trowbridge College College Road Trowbridge, Wiltshire BA14 OES Tel: 01225 766241 Fax: 01225 777148 The major manufacturers of general rubber and rubber goods have much expertise in design and manufacture of rubber articles for engineering purposes and are usually willing to provide advice.
Sources of Further Information There are many good texts on Rubber in engineering, the following are just a selection. A very useful, concise publication is: Engineering Design with Natural Rubber Malaysian Rubber Producers Research Association. More specialised texts include: Theory and Practice of Engineering with Rubber, A R Payne & P K Freakley, Applied Science Publishers, 1978. ISBN 0-85334-772-7. Rubber Spring Design, E F Gobel, Newnes Butterworth, 1974. ISBN 0-408-70557-4. Engineering with Rubber, A N Gent, Hanser, 1992. ISBN 3-446-17010-3. Natural Rubber Science and Technology A D Roberts, Oxford University Press, 1988. ISBN 0-408 70557-4.
Acknowledgements Some diagrams are reproduced with permission of the Malaysian Rubber Producers Research Association.
