Laboratory testing of rubber durability

Laboratory testing of rubber durability

PolymerTesting1 (1980) 167-189 LABORATORY TESTING OF RUBBER DURABILITY P. M. LEWIS Malaysian Rubber Producers' Research Association, Tun Abdul R...

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PolymerTesting1 (1980) 167-189

LABORATORY

TESTING

OF RUBBER

DURABILITY

P. M. LEWIS

Malaysian Rubber Producers' Research Association, Tun Abdul Razak Laboratory, Brickendonbury, Hertford SG13 8NL, UK

SUMMARY

One aim of the ever-increasing standardization of test methods for assessing the resistance of rubber to various types of degradation and other potential failure processes is to give a more reliable laboratory estimate of service performance. If this aim is to succeed, those developing, and using, such tests should be aware of the limitations of laboratory testing, and be acquainted with the factors that can affect the properties being determined. In his search for a quick estimate the user must recognise that an increase in test severity can also change the nature of the processes involved, and that a test conducted at a single set o]: conditions cannot be expected to provide the relevant data for a product which may be exposed to a widely varying environment. He also needs to take account of differences in size between the test piece and the intended product. The paper is illustrated by a discussion of the factors influencing resistance to ozone cracking, fatigue in tension, creep and stress relaxation, and liquids.

1.

INTRODUCTION

Since its formation in 1948, ISO TC 45--the technical committee of the International Organization for Standardization (ISO) responsible for rubber and rubber products--has published about 170 international standards. 1 Most of these are test methods and analytical procedures; only about twenty are specifications for finished products, although the proportion is gradually increasing. There are at least two reasons for this difference. First, it must be recognized that universally acceptable specifications cannot be established as 167 Polymer Testing 0142-9418/80/0001-0167502-25 © Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

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v.M. LEWIS

long as methods of test continue to vary from country to country. Secondly, a committee that drafts a product specification, or a material specification for a product, usually has a more difficult task than one responsible for test methods. It has not only to decide which tests to use, but also needs to choose suitable test conditions; to decide whether to conduct tests on the finished product, on test pieces taken from the product, or on test pieces prepared in the laboratory; to devise suitable sampling procedures; and to specify property limits or grades which are acceptable to all the parties concerned. A further complication is that a specification can mean different things to different people. Some users may regard it essentially as a quality control schedule to ensure one batch has the same properties as its predecessor. Others will expect to see requirements which once met ensure satisfactory service performance. This paper is not a guide to specification design; that subject is a complex one and the needs of the particular application would have to be considered. However, it is concerned with an aspect that is often a cause of argument among those preparing, or using, specifications--namely the use of laboratory tests to assess the durability of materials. As rubber increasingly finds its way into new engineering applications, the paper is also a plea for the adoption of tests which will provide the engineer with data that enables him to design a component for optimum performance by taking into account the limitations of the materials he uses. It relates to natural rubber, but many of the conclusions made will apply more generally.

2.

TESTING FOR DURABILITY

So far ISO TC 45 has issued in the region of forty international standards for the physical testing of solid vulcanized rubber, and those relating to degradation and other aspects of durability are shown in Fig. 1. In time this list will be increased by other test methods approved by the thirty or so member bodies belonging to the technical committee. For example, the working group concerned with degradation testing is currently examining methods for the assessment of resistance to abrasion, ozone cracking under dynamic strain conditions, outdoor weathering, accelerated light ageing, fatigue in tension, heat build-up and fatigue in flexometers, corrosion of metals in contact with rubber components, and stress relaxation at ageing temperatures. The objective is a straightforward one: to provide the rubber industry with a series of internationally recognized test methods covering all major aspects of durability or serviceability. Whether or not each of these standards will actually fulfil a need will depend on the extent to which industry has confidence in the method or methods specified; there are already a number of standards that have fallen into disuse. Part of the problem is that standardization necessarily

ISO R812 Impact brittleness ISO 1432 Gehman test (torsion modulus)

I

Glass transition effects

I

I

I

I

Compiession

I

I S O 815 Compression set

I

I

I I

i

ISO 3865 Staining of lightcoloured surfaces

I

LIGHT

ISO 132 De Mattia flex cracking ISO 133 De Mattia cut --growth

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ISO 1431 Accelerated static ozone exposure

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OZONE

CYCLIC DEFORMATION

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Tensionl

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I S O 1817 Resistance to liquids

ISO 2285 Tension set

I

I

LIQUIDS

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STATIC DEFORMATION

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ISO 3384 Stress relaxation

ISO 188 Accelerated ageing and heat resistance (air oven and oxygen bomb methods)

High temperatures

OXYGEN

I

MECHANICAL ENVIRONMENT Fig. 1. Approved international standard (ISO) test methods for rubber durability.

[

Set

ISO 3387 Hardness change

Cryitallizati°n

ISO 1653 Compression set ISO 2921 Temperature retraction

temperatures

I Low

TEMPERATURE

I

CLIMATIC AND MATERIAL ENVIRONMENT

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ffl ©

0 > ,-] 0

r" >

170

P.M. LEWIS

involves some restriction of choice, for example of test conditions, and this often means there is going to be a restriction in the field of application. It must also be admitted that in several standards there are deficiencies which can be traced back to a lack of understanding of the various factors that can influence the property being determined. Misuse of tests is another danger. Standards like those listed in Fig. 1 are basically test procedures, and as such normally consist of clauses on apparatus, test piece preparation, test procedure and expression of results. They are not text books. Thus the user is expected to have sufficient background knowledge to enable him to choose the procedures which best suit his purpose and to interpret the results he obtains. Unfortunately this is not always found to be the case. The features just mentioned will now be discussed in terms of four specific properties. Two of them--resistance to ozone cracking and resistance to liquids--are the subjects of already established international standards, and requirements for them are frequently stipulated in product and material specifications. The others--tension fatigue and resistance to creep and stress relaxation--are relatively new items as far as international standardization is concerned, but are among those properties of concern to design engineers.

3.

RESISTANCE TO OZONE CRACKING

3.1. Background Although seldom present in more than a few parts per hundred million parts (pphm) of air, ozone is regarded by many people as the greatest environmental threat to natural rubber and other elastomers having main-chain unsaturation, a view often reinforced by the rapidity with which some rubbers fail under accelerated test conditions. It is important to remember, however, that ozone cracking is a surface phenomenon which only occurs in stretched rubbers. Many products are bulky and are largely used in compression. In these, ozone attack is very much slower than it is in pure tension and confined to any regions of tensile strain at free surfaces 2 (Fig. 2). Even those strains can be eliminated at the design stage. In surfaces held at constant tensile strain the severity of ozone cracking depends on the magnitude of the extension. Regardless of the ambient ozone concentration no cracks will appear until a characteristic threshold strain is exceeded. 3 In the absence of protective agents this strain normally lies in the region of 1-7% for most unsaturated rubbers, the exact figure depending on vulcanizate stiffness and surface quality. Just above the threshold value cracks are initiated at the largest of the stress-raising flaws in the surface, and since these are relatively small in number the rate of crack growth is at a maximum. Consequently the strain region just above the threshold strain is the most

LABORATORY TESTING OF RUBBER DURABILITY

171

Fig. 2. Disc of a transparent natural rubber vulcanizate (44 mm diameter) after exposure at 10% compression for 14 h at l0 s pphm ozone, equivalent to at least 50 years atmospheric exposure in the UK. 2 The thickness of the thin dark ring of ozone-degraded rubber is about l ram.

severe one for ozone attack. As the extension is further increased, cracks develop from smaller flaws and so the crack density increases, but the rate at which individual cracks grow decreases. 4 Protective agents are added to rubber to reduce the rate of crack growth or, more usefully, to raise the threshold strain above the maximum extension found in the product. Chemical antiozonants of the p-phenylenediamine type and waxes accomplish the latter by diffusing to the rubber surface to form protective barriers, antiozonants by reaction with ozone 4 (and possibly with ozonized rubber) and waxes by blooming from supersaturated solution. 5 These barriers cannot withstand continuous flexing, so that protection against cracking under dynamic strain conditions relies mainly on a reduced rate of crack growth, for which p-phenylenediamines are also effective. 3.2. Test methods Anyone responsible for standardizing, and making use of, accelerated ozone tests is confronted with three major tasks. The first is to replace the traditional, subjective methods of assessing ozone resistance by more quantitative methods. One suitable for static strain conditions is the determination of threshold strain, and special test pieces have been devised for this purpose. They include the annulus developed in the U K by

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P.M. L~WlS

Amsden, 6 and a trapezoidal-shaped test piece used in Czechoslovakia. 7 Both these test pieces provide a strain gradient, thus eliminating the need for a large number of test pieces at different strains. [The second task is to improve test reproducibility, especially between laboratories. In this respect there is urgent need for the adoption of a single, internationally acceptable method of measuring ozone concentration. Several methods are currently in use and can give different results. 8 The third task is to choose test conditions which correlate with service. A reasonable estimate of the service performance of unprotected rubbers can be made from short-term tests conducted at the same deformation in the laboratory at high ozone concentrations, provided the level of ozone in the service environment is known. This follows because the threshold strain of an unprotected rubber is independent of ozone concentration, while the rate of crack growth for many rubbers is proportional to ozone concentration and varies only slightly with temperature. 3 Predicting the performance of protected rubbers is more difficult since account must be made of the factors influencing the behaviour of waxes and antiozonants. A reliable, quantitative estimate of life will not be possible until more is known of the mechanisms responsible for protection. Therefore, the first priority must be to devise a test which will at least rank rubbers in the same order as in service. This is obviously best accomplished by matching the test conditions as closely as possible to those found in service even if it means extending the duration of the test. As is now well known, temperature is the most important climatic variable influencing the behaviour of waxes because of the considerable effect it has on solubility and on the rate of diffusion to the rubber surface. 5'9 It follows that a common cause of poor correlation is the use of a test temperature which is not typical of service (Table 1). This problem is aggravated by the recommendation in many national standards that tests be conducted at either 30 °C or 40 °C. These two temperatures are not suitable for assessing the performance of wax-protected rubbers which are to be exposed in cool environments, and especially so when the wax bloom is allowed t o form during a preconditioning period in the stretched state at normal laboratory temperatures. As is usually the case in ozone testing the ability of a wax to provide protection at low temperatures depends critically on the rate at which it can diffuse to the surface, and as a recent paper has warned the wax component involved in this protection may not be the same as that which blooms out at higher temperatures. 1° The same paper recommends that any preconditioning required should be carried out at the temperature of exposure. The inability of laboratory tests to estimate the service behaviour of rubbers containing chemical antiozonants more often than not originates from the use of an unrealistically high ozone concentration or too high a test extension. The

9 20

27 1 1 2

0 3

Jail

25 pphm ozone and 17 °C~

pass

EXPOSURE FOR W A X - P R O T E ~

10 19

26 2

pass

0 3

1 2

Jail

25 pphm ozone and 30 ° C b

10 4

12 2

pass

0 18

15 2

fail

50 pphm ozone and 50 °C b

N u m b e r o[ passes or [allures at 20% strain alter 6 days at

a Test pieces exposed i m m e d i a t e l y after stretching. b Test pieces p r e c o n d i t i o n e d in stretched state for 48 h at 20 °C before exposure.

10 passes 22 failures

S u m m e r exposure

27 passes 4 failures

Winter exposure

N u m b e r o[ passes or [allures after 14 weeks outdoor exposure at 2 0% strain

TABLE 1 EFFEC~ OF TEST TF3dPERATL1RE ON CORRELATION BETWEEN ACCELERATED OZONE TESTS AND O ~ R NATURAL RUBBER

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r-

>

"~

z 0 0

0

>

0

174

P.M. LEWIS

% 500

A 100 50

B 10 5

1

1

0

2

antiozonant

3

concentration, parts phr

Fig. 3. Effect of ozone concentration on the activity of dioctyl-p-phenylenediamine antiozonant in natural rubber. The activity is expressed as the ratio between the time to first crack of a protected vulcanizate (Tp) and the time to first crack of an uprotected vulcanizate (Tc) the test being conducted at 20% strain. A = Atmospheric exposure in UK; B = laboratory exposure at 25 pphm ozone and 30°C. a b i l i t y o f p - p h e n y l e n e d i a m i n e s to r a i s e t h r e s h o l d s t r a i n is s t r o n g l y d e p e n d e n t o n o z o n e c o n c e n t r a t i o n . T h e p r o t e c t i o n c o n f e r r e d b y a g i v e n a m o u n t of a n t i o z o n a n t is h i g h at all o z o n e c o n c e n t r a t i o n s u p to a c r i t i c a l v a l u e , a b o v e w h i c h it falls r a p i d l y t o a l e v e l s i m i l a r to t h a t f o r a n u n p r o t e c t e d r u b b e r . 4 E v e n tests c o n d u c t e d at 25 p p h m o z o n e , t h e l o w e s t c o n c e n t r a t i o n p e r m i t t e d in m o s t TABLE 2 OZONE CONCENTRATIONS AT DIFFERENT LOCATIONS

Location

Outdoors Indoors, closed room Indoors, draughty room Motor car, under bonnet Motor car, under seat

Average ozone concentration a (pph m)

1-2 0.04-0.1 0.1-0.3 0.1-0.7 0.01-0.04

Estimated from the cracking produced in test pieces of an unprotected natural rubber vulcanizate over a period of at least 1 month. The measurements were taken in Hertfordshire.

175

L A B O R A T O R Y T E S T I N G OF R U B B E R D U R A B I L I T Y

standard m e t h o d s , can seriously underestimate the protection often f o u n d outdoors. Figure 3 relates to l a b o r a t o r y roof exposure at an average o z o n e c o n c e n t r a t i o n of not m o r e than 2 p p h m ozone. Table 2 shows that this c o n c e n t r a t i o n is itself significantly higher than those m e a s u r e d at locations where r u b b e r p r o d u c t s are frequently employed.

4.

RESISTANCE TO TENSION FATIGUE

4.1. Background F o r the purposes of this paper fatigue is defined as the failure f r o m crack growth initiated at small flaws or cuts in the rubber, and fatigue life is defined as the n u m b e r of cycles for a test piece to break. Figure 4 shows h o w life varies with strain and how the d e p e n d e n c e varies a m o n g different rubbers. Figure 4(a) is for test pieces which are relaxed to zero fatigue life, cycles

fatigue life,cycles

10/

107

(a)

(b) NR

10'

lo'

5

10

4

lO'

10

NR

i

\

\ \.

10'

3

10 I

0

I

I

100 maximum strain,~o

I

200

i

I

300

I

0

i

I

100

i

200

minimum strain,~o

Fig. 4. Dependence of fatigue life on test strain. 11 (a) Test pieces returned to zero minimum strain. (b) Non-relaxing fatigue: maximum strain 250%; minimum strain varied as shown. NR = natural rubber; CR=chloroprene rubber; IIR=butyl rubber; SBR=styrene-butadiene rubber; NBR= acrylonitrile-butadiene rubber; BR= butadiene rubber.

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P.M. LEWIS

strain for part of each cycle. Life increases rapidly as the maximum extension is reduced and were it not for the initiation of cracks by ozone would be almost indefinite below what has become known as the mechanical fatigue limit. Ix For the vulcanizate shown this limit lies in the range of 60-90% strain. Failure above the fatigue limit is caused mainly by mechanical, or more generally, mechanico-oxidative crack growth since the process is faster in air than it is in vacuo. Oxygen can also lower the fatigue limit. The oxidative contribution can be reduced by adding antiflex cracking agents, among which pphenylenediamines are the most effective, and becomes less significant as the extension is increased. Under non-relaxing conditions (Fig. 4b) the fatigue life of natural rubber is enhanced considerably, an important effect because many components function in this way. The improvement is attributed to the ability natural rubber has to undergo strain-induced crystallization because the effect is very much less evident in non-crystallizing rubbers such as styrene-butadiene copolymer (SBR). Cracks develop at stress-raising flaws present in the rubber, and the rate of growth will depend on the stress concentration at the tip of the crack. This stress cannot be determined directly but can be assessed using a fracture mechanics approach, which enables crack growth behaviour to be expressed independently of test piece shape or the type of deformation. II'x2 It also provides a quantitative basis for relating laboratory measurements to service performance. In this the rate of growth is treated as a function of tearing energy, T, which is defined as the elastic strain energy available per unit area of growth and given by T = 2KWc

(1)

where K is a slowly varying function of strain, W is the strain energy density in the bulk of the test piece (equivalent to the area under the stress-strain curve) and c is the crack length. A critical tearing energy value, To, must be exceeded before mechanical crack growth can occur and obviously this corresponds to the mechanical fatigue limit just described. Just above To, the rate of growth per cycle shows an approximately linear relation with tearing energy and can be expressed by dc dn

~=

A(T-

To)+ rz

(2)

where the first term on the right-hand side represents the mechanico-oxidative growth, A being a constant. Crack growth due to ozone attack is represented by rz, which is proportional to the ambient ozone concentration. At moderate and high tearing energies crack growth shows a stronger dependence and the rate can be expressed by a power-law relationship of the

LABORATORY TESTING OF RUBBER DURABILITY

177

form

dc dn

--

= BT

~

(3)

where B and /3 are constants for a given rubber. By a process of integration eqns. (1) and (3) can be used to derive the following equation for the fatigue life, N, at moderate to high maximum extensions, the rubber being returned to zero strain during each cycle. N=

1 (/3 -- 1)B(2KW)¢~ C~o--1

(4)

where Co is the effective length of naturally occurring flaws. Equation (4) predicts that a plot of N against 2 K W on logarithmic paper should give a straight line. That is, in fact, the case and the values for/3 of 2 and 4 respectively for natural rubber and SBR are similar to those calculated from crack growth measurements on test pieces containing inserted cuts. Equation (4) also suggests that Co is 0.025 mm for natural rubber and 0.05 mm for SBR, values which are consistent with the size of moulding or cutter imperfections and particulate impurities in the rubbers. 11'1a 4.2. Test methods Our current understanding of fatigue has been summarized in some detail to show how essential it is to take account of basic studies when designing laboratory tests. Fatigue is a complex phenomenon and the strong strain dependence illustrated in Fig. 4 indicates why so many traditional laboratory tests have failed to provide an accurate forecast of service performance. Tests conducted under arbitrary conditions, including the internationally standardized De Mattia flex cracking and cut growth methods (see Fig. 1), can give very misleading data, and are not always amenable to quantitative analysis. The basic studies indicate that a more meaningful test procedure would be one in which test pieces are cycled to a clearly defined extension, and in which the extension can be varied to suit the needs of particular applications. Suitable test equipment, using dumb-beU or ring test pieces, is now available for the purpose, and gives quantitative results which do not depend on operator interpretation.13'14 If service life is to be estimated from laboratory measurements there is need to determine the relation between tearing energy, crack size and overall forces or deformations, and this can present a major difficulty for complex shapes in which stresses vary over a small area. Two techniques found suitable for this task have been numerical computation by finite element analysis 15 and the estimation of tearing energy from measurements of the amount by which a crack opens under stress. 16 The second of these has enabled the fracture

178

e.M. LEWIS crack growth per revolution,10-6mm 1.5

1.0

0.5

O0

I

i

I

5

10

15

crack

a 20

25i

length, mm

Fig. 5. Observed and predicted groove cracking behaviourJ 2'16 The experimental results are for service tests (Q) and rig tests (A) on Avon 'Supreme' size 9.00-20 cross-ply truck tyres. The line is a theoretical curve for a groove strain cycle of 0-9%.

mechanics approach to be successfully applied to the hitherto ill-understood phenomenon of tyre groove cracking. 16 Figure 5 shows the close agreement found between service and tyre rig tests and crack growth tests determined under appropriate conditions in the laboratory. The study has also indicated that tread rubbers are best screened by conducting normal fatigue tests at extensions near the mechanical fatigue limit. It is important to interpret fatigue data correctly. Most tests are conducted at constant extension, but in practice it may be necessary to compare results on the basis of constant stress or constant strain energy, andffor rubbers varying in stiffness that could mean a change in ranking order. Allowance should also be made for any set developed during the test--with certain rubbers this can be appreciable and result in deceptively long lives. A feature of fatigue testing is the often very large variation in life found among test pieces taken from the same sample. Some of the scatter may be traced back to testing errors and sample preparation, but however much care is exercised--as indeed it always should be during mixing, moulding, diestamping and testing--the inherent variability of fatigue life will remain very much greater than that of tensile strength and other properties (Table 3). Natural rubber is reasonably reproducible in this respect--the overall variation in life for replicate tests is commonly two-fold or less and often obeys a Gaussian distribution. In general, the rubbers giving the greatest scatter in lives

179

LABORATORY TESTING OF RUBBER DURABILITY TABLE 3 VARIABILITY IN FATIGUE LIFE

Fatigue life of ring test pieces cycled between 0-100% strain, at 5Hz and 23 °C. (Kilocycles to

Natural r u b b e r Results for individual test pieces taken from same sheet Median result Ratio of highest to lowest result

165.1, 132.3 126.1, 116.9 114-3, 110-6 121.5

break) SBR/polybutadiene blend 393.7, 150.1 125.9, 116.7 103.7. 40.3 121.3

1.5

9.8

are those which are incapable of strain-induced crystallization, since their rate of crack growth is more strongly dependent on tearing energy and hence on flaw size (high/3 values; eqn. (3)). The fatigue life of a carefully prepared SBR vulcanizate, for example, can be subject to a ten-fold variation or more and can show a skew-type distribution. This can have important practical consequences, so that it is always advisable to report some measure of scatter as well as any central or averaged value. 17 5.

RESISTANCE TO CREEP AND STRESS RELAXATION

5.1. Background When rubber is held for any length of time in the deformed state it relaxes, and this leads to one of two changes. One is creep, the increase in strain under constant load, and the other is stress relaxation, the decay in stress under constant strain. Figure 6 shows how these related phenomena depend on time and indicates the existence of two distinctive processes. 2 First, there is a physical (primary) relaxation associated with the realignment of polymer molecules and filler particles as the deformed rubber slowly approaches its equilibrium state. It exhibits a near-linear dependence on logarithmic time, and thus is conveniently expressed in terms of per cent creep or stress relaxation per decade of time A study of creep has shown that this dependence holds for test pieces cycled during testing as well as for statically held ones, although dynamic creep proceeds more rapidly than static creep, is The rate of physical relaxation is also dependent on the previous strain history of the rubber, 19 temperature fluctuations during the course of measurement ~9 and humidity2°--factors that should be taken into account when designing laboratory tests to estimate service behaviour. Secondly, there is chemical (secondary) relaxation, which is caused by the scission of stress-bearing bonds in the rubber network. The main degradative

180

P.M. LEWIS creep,~ 12 10 8 6 4 2 process

01

I

10

|

102

I

103

time, minutes

Fig. 6. Dependence of creep on time. The curve is for a conventional sulphur-natural rubber vulcanizate tested at 70°C. influence at play is oxidation, but anaerobic ageing can become operative at elevated temperatures leading to the breakdown of any thermally unstable crosslinks. Chemical relaxation is distinguished from physical relaxation in that it can show an essentially linear dependence on time and has a stronger temperature dependence. At normal ambient temperatures physical relaxation dictates behaviour during the early stages and some time may elapse before the chemical process becomes evident. It may take several months for the rates of the two processes to coincide even for thin test pieces and a much longer time will be required for thick products because of the diffusion control of oxidation. 5.2. Test methods Creep is a property which many engineers have regarded as a potential weakness of rubber in load-bearing applications. This concern has focused attention on the need to collect service data and to develop tests that give an accurate prediction of performance. As part of its own studies M R P R A has monitored the creep of several natural rubber building mounts from the time of their installation. One result has been a remarkably close agreement between the creep measured over almost 10 years and the creep predicted from short-term laboratory tests on thin test pieces of the same rubber. 2a The prediction shown in Fig. 7 was constructed on the basis that the chemical component of creep would be

181

LABORATORY TESTING OF RUBBER DURABILITY creep, mm

1.0

0.8

0.6

0.4

0.2

o;

I

i

I

I

2

4

6

8

time,

years

Fig. 7. Observed and predicted creep of natural rubber building mountings. 21 The points are averages of readings taken from five mountings, corrected for temperature fluctuations and contraction of the concrete support to which the reference plates were attached to eliminate all effects other than creep in the rubber. The best line through these points is well within the laboratory estimates shown by the broken lines.

superimposed on the physical component, the total creep between 1 day and t days being calculated from the equation Bt % creep -- A log t + - 365

(5)

where A is the physical creep rate and B is the chemical creep rate in per cent per year. One outcome of this work has been the inclusion of a creep test in a BSI Draft for Development on anti-vibration building mountings. 22 Manufacturers and users are also begining to recognize that stress relaxation in compression is the property of rubber most likely to determine the longterm performance of seating applications. 23'24 To this end, a stress relaxation requirement is now included in the BSI material specification for sealing rings used in gas pipelines. 25 The intention is that the requirement will be extended to rings for water mains and drainage once suitable limits are established. An international standard test method for stress relaxation in compression (ISO 3384) has only just been approved and there is still not one for creep in solid vulcanized r u b b e r - - t o some extent this delay can be regarded as an indication of the unpopularity of relaxation tests among users. Many believe

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LEWIS

that such tests are not suitable for specification or quality control purposes, since they may require the purchase of relatively expensive equipment. There has also been an assumption, now discredited, that much the same information could be obtained from conventional compression set tests, which are quick and comparatively straightforward. Compression set is a measure of recovery after release of the stress, and a rubber having low set characteristics would only become desirable in a pipe sealing ring where the pipeline shifted to such an extent that the configuration of the sealing ring was seriously altered. U n d e r normal circumstances the life of the seal depends primarily on its ability to maintain a sufficient stress to resist the fluid pressure--a requirement that need not exclude the use of a rubber having high compression set. The processes involved are distinct. A n y crosslinks formed during compression will hinder recovery upon release and increase the permanent set, but they need not significantly affect continuous stress relaxation which depends on the crosslinks inserted before compression. Conversely, the breakdown of the

stress relaxation rate at 30°C, ~oper decade

(80) (50)

.(0> I

1o compression

2'o set

3'o

i

4o

at 70°C,

Fig. 8. Comparisonof stress relaxation measured at 30°C and compression set measured at 70 °C. The results are for two types of natural rubber vulcanizate varyingin filler level. 0, Heat resistant vulcanizate (soluble EV system); A, conventional sulphur vulcanizate. The figures given at the points refer to the amounts of SRF black present (in parts phr). The stress relaxation rate was measured at 100oYostrain and 30% relative humidity. Compression set was measured after 1 day at 25% compression and after 60-min recovery.

L A B O R A T O R Y TESTING OF R U B B E R D U R A B I L I T Y

183

original network of crosslinks is reflected in an increase in stress relaxation but may have little effect on set. Compression set tests are frequently carried out at elevated temperatures and, as Fig. 8 illustrates, the results should not be used to predict service performance at ambient temperatures. At 70 °C the magnitude of the set in sulphur-vulcanized natural rubber is largely determined by the thermal stability of the crosslink network and can be relatively insensitive to increasing filler levels. In contrast, the physical stress relaxation which predominates at normal temperatures is less affected by the type of vulcanizing system used than by the type and amount of filler present. 26 Compression set tests at the service temperature are more relevant, but it should be emphasized that the results of short-term laboratory tests (for example 24h) can show a poor correlation even with the results of long-term set tests. 27 This is due in part to the inadequacy of the standard 30-min recovery time for test pieces which have been compressed for long periods. 2s

6.

RESISTANCE T O LIQUIDS

6.1. Background The absorption of liquids by rubber is a diffusion-controlled process and therefore one whose effect depends both on the time of immersion and the thickness of the rubber. Initially all the liquid absorbed is confined to a surface layer of rubber in which it reaches its equilibrium swelling value. The thickness of the swollen layer then increases until the whole mass of rubber becomes swollen. This can take some considerable time in bulky products because the amount of liquid absorbed increases in proportion to the square root of time, at least up to 50% of total equilibrium swelling. Since the boundary between the swollen and unswollen rubber is normally quite sharp, it is reasonable to assume that all the absorbed liquid is contained in a layer of swollen rubber of uniform concentration. In this way it is possible to define a penetration rate, P, which is the rate of movement of the boundary between swollen and unswollen rubber. 29 P can be calculated from the equation P =

M,

L - - --=

CoAx/t x/t

(6)

where M, is the mass of liquid absorbed by a surface area A in time t; Co is the concentration of liquid in the surface of the rubber; and L is the depth of the swollen layer. P is related to the diffusion coefficient of the liquid in the rubber by the

184

P.M.

LEWIS

equation D =¢rP 2 4

(7)

6.2. Test methods The theory of swelling outlined above has two messages for laboratory testing. The first is that measurements made on thin test pieces (as they usually are to facilitate the determination of properties after swelling) can fail to distinguish between the two parameters which characterize the absorption of liquids--(a) the value of the equilibrium swelling, and (b) the rate at which the swollen layer increases. The performance of many thick products will be governed by (b) rather than (a). It may not matter how high the equilibrium swelling value is, provided the swollen layer remains small in relation to the cross-section of the product. The second message relates to the choice of test liquid. Use of the actual service liquid is always advisable, but when this is not possible and a standard liquid has to be specified the following observations should be recognized. They are that the equilibrium swelling value is largely dependent upon the chemical nature of the liquid, whereas the rate of penetration is related to the viscosity of liquid, decreasing as the viscosity is raised (Fig. 9). time to penetrate 5mm 10 years 4 years

I year

4 months

1 month

1 week

oils

4 days f

0.1

I

i

1

10 liquid

Fig. 9.

a

10 2

I

103

i

10 4

viscosity, m N s / m 2

Dependence upon liquid viscosity of tthe time taken by a liquid to penetrate a given distance into natural rubber. (Based on Ref. 29.)

L A B O R A T O R Y T E S T I N G OF RUBBER D U R A B I L I T Y

7.

185

DISCUSSION

7.1. Design of laboratory tests What general lessons can be learned from the preceding sections? Surely we begin by recognizing three basic differences that exist between laboratory testing and service performance. The first is that most laboratory tests of the type listed in Fig. 1 are designed to measure material properties under controlled, and generally limited, conditions using test pieces that may be considerably different in size from the intended product. Some emphasis is placed on the ease and speed of measurement and on reproducibility. Can such tests be expected to assess the suitability of materials for a product whose life may be affected by its size and shape, the mode and magnitude of any stresses and strains, and the working environment, including perhaps a widely varying climate? The second is that an increase in severity of testing--such as the use of a higher-than-normal ozone concentration or the use in fatigue testing of a higher-than-normal test extension--not only has the desired effect of shortening the duration of the test, but can also change the nature of the processes involved. In some cases the result can be a reversal in ranking order among materials being screened. The third is one of interpretation. The use in laboratory testing of severe test conditions and small test pieces can shift emphasis towards failure, and away from durability. It should be remembered that rapid failure of the test piece does not necessarily mean the product will fail prematurely, or even that the property concerned will play a significant role in determining the life of the product. Given these limitations there are none the less practical lessons for improved laboratory tests, and four are summarized as follows: 1. Laboratory results cannot be expected to correlate with service if the test conditions used are unrelated to those encountered in service. It follows that a test conducted at a single set of conditions is unlikely to assess the performance of a material in a product which is subjected to a widely varying environment. In this case examination over a range of test conditions will become necessary. An example is the use of at least two test temperatures representative of the service range when the ozone resistance of wax-protected rubber is being assessed. 2. Before carrying out the test and when interpreting the results, due account must be made of the differences in size and thickness between the test piece used and the product. In this respect many of the comments made earlier for

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swelling by liquids also apply to oxidation, which is another diffusioncontrolled process, especially at elevated temperatures. 3° 3. It should not be assumed that one test will give the same information as another; for example, the determination of compression set where stress relaxation or creep would be more appropriate. 4. Unless it is inherently resistant to degradation, a rubber depends largely upon protective agents for its stabilization and due consideration should be given to the factors which could affect the behaviour of these additives in service. Antiozonants, for example, are reactive materials and can be consumed not only by ozone, but also by oxidation. Levels may also be depleted by volatilization or extraction. On finding that laboratory tests fail to eliminate unsatisfactory materials, many users are tempted to increase severity by raising the ozone concentration, whereas a more promising approach would be to pre-age test pieces before exposure to more realistic test conditions. 7.2. Use of tests in specifications 'Specifications may be an unfortunate necessity, but very often they are drawn up in such a way that the rubber manufacturer does better to ignore them and provide components giving good service than to observe them faithfully and to supply components giving inferior service, resulting perhaps in loss in business. '31 One of the criticisms made by people who hold this view is that many product specifications are written in terms of tests which, though relatively easy to carry out, have little in common with actual service requirements. This can result from a failure to distinguish between what are really quality control tests for materials already in use and tests that are intended to assess performance of the material in the finished product. In theory the test conditions employed for quality control purposes are not critical provided the test is quick, reproducible and capable of detecting a faulty batch. There is no reason, for example, why a test at a high ozone concentration should not be used to check that one batch has the same level of antiozonant protection as its predecessor. Nor is there any objection to the use of a high temperature compression set test to ensure that each batch has been properly vulcanized. Inasmuch as undercure is deleterious to stress relaxation or creep as well as to set, there can be a correlation with service behaviour provided the same composition, or one very similar to it, is always used. As long as the composition of the rubber remains the same a quality control specification can become the material specification for the product. Some of the properties specified may not have a direct relevance to service, tensile strength

LABORATORY TESTING OF RUBBER DURABILITY

187

and elongation at break being common examples in this category, but the correlation, such as it is, may be based on years of experience. A problem arises when the same specification is later amended to allow the use of other rubbers. These may have very different characteristics and display different property-property relationships compared to the original choice although they are still found suitable for the product. More often than not the rubbers which suffer most from this type of specification are those which are less inherently resistant to degradation. The result is that they either are excluded from use or require unnecessarily high levels of protection for a product where the particular property or type of degradation may be of only minor importance. In a number of specifications these difficulties have partially been resolved by the insertion of separate physical property limits for the various rubbers permitted for use in the product. The weakness of this approach remains, however, that any empirical relationships used to establish the limits may not hold for all mixes in view o~ the considerable effect compounding can have on the properties of any one rubber. An alternative, and obviously more desirable, approach is to draft specifications wholly in terms of functional properties. It would mean, for example, the acceptance of a stress relaxation test in any specification for sealing applications, or the inclusion of creep tests in specifications for rubber springs. Such a requirement would be common to all rubbers, although as a safeguard it may be necessary to restrict the rubbers allowed to those which have established themselves in the application. A major stumbling block for this type of specification would appear to be a shortage of suitable data from which reliable limits could be drawn. Any survey of the literature will show that the quality of rubbers is normally characterized in terms of relatively few properties. Foremost are tensile strength, elongation at break, stress at a relatively high elongation (usually 300%), hardness, compression set at an elevated temperature and the retention of stress-strain properties after a period of accelerated ageing. Little information will be found for stress relaxation, creep, long-term changes in stiffness and fatigue life measured over a wide range of stresses and strains--the properties of rubber with which design engineers are often most concerned. The hope must be that once approved standard methods are available this type of information will be more forthcoming.

8.

CONCLUSIONS

Of the 150 or so technical committees within ISO, only twt>--those for chemicals and plastics materials--have managed to produce more international standards than ISO TC 45. This reflects the versatility of rubber as an industrial material, but above all is a tribute to the firm commitment of the

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v.M. LEWIS

r u b b e r i n d u s t r y towards s t a n d a r d i z a t i o n . Test m e t h o d s , despite all their shortc o m i n g s a n d abuses, are r e c o g n i z e d as essential tools in o r d e r to define in m e a s u r a b l e terms those p r o p e r t i e s r e q u i r e d to give satisfactory service perform a n c e . W i t h o u t t h e m specifications could n o t exist. E v e n so s t a n d a r d i z a t i o n should n e v e r be t a k e n to the p o i n t where the quality of r u b b e r is j u d g e d solely o n the basis of its p e r f o r m a n c e in the l a b o r a t o r y . It is n o t too difficult, for e x a m p l e , to devise tests which show that n a t u r a l rubi~er does n o t have as a m a t e r i a l the i n h e r e n t resistance to heat, o x i d a t i o n , o z o n e a n d oils that several synthetic r u b b e r s have. B u t n o r is it difficult to find n a t u r a l r u b b e r e n g i n e e r i n g products, such as b u i l d i n g m o u n t s a n d bridge bearings, which have s h o w n n o m o r e t h a n negligible surface d e g r a d a t i o n after 1 0 - 2 0 years of c o n t i n u o u s service a n d which are e x p e c t e d to last for periods up to 100 years a n d e v e n b e y o n d . W h i c h is to be b e l i e v e d ?

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16.

17. 18. 19.

20. 21

22. 23. 24.

25.

ISO Catalogue, 1979. Geneva, International Organization for Standardization. DERHAM,C. J., SOUTHERN,E. and THOMAS,A. G. (1970). NR Technology, 1 (2), 7. BRADEN,M. and GENT, A. N. (1960). J. Appl. Polym. Sci., 3 (7), 90; ibid, 3 (7), 100. LAKE,G. J. (1970). Rubber Chem. Technol., 43 (5), 1230. LEWIS,P. M. (1972) NR Technology, 3 (1), 1. AMSDEN,C. S. (1965). Trans. Inst. Rubber Ind., 13 (3), T91. DLAB,J. (1976). Intern. Polymer Sci. Technol., 3 (1), T/3. BROWN,R. P. and HUGHES,R. C. (1973). R A P R A Members Bulletin, 1, 113. BRISTOW,G. M., LAKE, G. J. and LEWIS, P. M. (1976). The Weathering of Plastics and Rubbers. Paper E9. London, PlasticsCand Rubber Institute. JOWETr,F. Paper presented to American Chemical Society Rubber Division Spring meeting, Atlanta, Georgia, March 1979. LAKE, G. J. and LINDLEY,P. B. (1964). Rubber Journal, 146 (10), 24; ibid, 146 (10), 30. LAKE,G. J. (1972). Rubber Chem. Technol., 45 (1), 309. 'Fatigue to Failure Tester,' Monsanto Ltd, Swindon, England. 'The WaUace-MRPRA Fatigue Tester for Rubber,' Cat. Ref. F13, H. W. Wallace and Co. Ltd, Croydon, England. LINDLEY, P. B. (1972). Jr. Strain Analysis, 7 (2), 132. CLAPSON,D. E. and LAKE, G. J. (1970). Rubber Journal, 152 (12), 36. BS 5324, 1976. Guide to application of statistics to rubber testing. British Standards Institution. DERHAM,C. J. and THOMAS, A. G. (1977). Rubber Chem. Technol., 50 (2), 397. DERHAM, C. J. (1973). J. Materials Sci., 8, 1023. DERHAM,C. J. (1972). Proc. Intern. Rubber Conf., Brighton, England, Paper F1. Institution of the Rubber Industry. DERHAM, C. J. and WALLER, R. A. (1975). Consulting Engineer, 39 (7), 49. DD 47, 1975. Vibration isolation of structures by elastomeric mountings. Technical Review 53, 1970. Solid rubber seals and joints. Rubber and Plastics Research Association of Great Britain. DERHAM, C. J. (1975). NR Technology, 6 (3), 41. BS 2494, 1976. Specification for materials for elastomeric joint rings for pipework and pipelines.

LABORATORY TESTING OF RUBBER DURABILITY 26. 27. 28. 29. 30. 31.

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GREGORY, M. J. (1977). NR Technology, 8 (1), l. MORRELL, S. H. and WATSON, W. F. (1976). R A P R A Members Journal, 4, 59. ANON. (1973). NR Technology, 4 (1), 7. SOUTHERN, E. (1967). Use of Rubber in Engineering, Chapter 4 (Ed. P. W. Allen, P. B. Lindley and A. R. Payne). London, Applied Science Publishers. KNIGHT, G. T. and LIM, H. S. (1976). Proc. Intern. Rubber ConiC, Kuala Lumpur, 1975, Vol. 5.57. Rubber Research Institute of Malaysia. HmST A. J. (1967). Use of Rubber in Engineering. Chapter 8 (Ed. P. W. Allcm P. B. Lindley and A. R. Payne). London, Applied Science Publishers