- Email: [email protected]
Reinforcement of rubber with carbon black
Reinforcement of rubber with carbon black A. R. PA YNE* and R. E. WHITTAKER*
One of the most important composite materials commercially available is rubber containing a colloidal filler such as carbon black in quantities of up to 20 to 30% by volume of the rubber. Rubber by itself is unsuitable for a large number of important applications (eg tyres) and in these it must be manufactured into a composite material before it is of commercial use
In contrast with the ordered and rigid crystalline arrangement of atoms in metals, elastomers are composed of a tangled mass of kinked, twisted and intertwined chain-like molecules. The basic element of an elastomer is the monomer -- the link in the chain which determines the chemical character of the material and its resistance to oil, ozone, solvents and chemicals. Thousands of monomers link end-to-end to form a polymer, or molecular chain, free to bend or to rotate at most points where the chain links join. The tangled chains are free to slide past each other except where they are attached together here and there by cross-links. These cross-links tie the molecular chains together and limit the amount of stretch and increase the elasticity. Cross-links are formed by chemical reaction of the chain with certain materials (curatives and accelerators) mixed into the mass. Heating increases the rate and extent of cross-linking or curing and this process is termed vulcanisation. Addition of inert materials, dispersed in the elastomeric mass prior to curing, can increase strength and hardness or resistance to deformation, usually with a reduction in elasticity and resilience. Size and shape of the particles and degree of dispersion are important factors; carbon black, for example, is useful in increasing strength and hardness for most elastomers. When an elastomer is stretched, the * Shoe and Allied Trades Research Association, Kettering, Northants, England
tangled mass of irregular, kinked and cross-linked molecular chains tend to straighten out, sliding and slipping past each other. When the external load is removed, the molecules tend to return to their original coiled-up and kinked condition. The amount and character of the added compounding materials have a strong influence on the elasticity. The term 'reinforcement' as applied to natural and synthetic rubbers, though one of the most familiar in the rubber technologist's vocabulary, is difficult to define; in fact, it does not necessarily mean the same thing to different people. This is because the properties of rubber vulcanisates are modified to different degrees by different reinforcing agents, and one's view of what constitutes high reinforcement is coloured by the particular kind of product in which one is interested. Thus certain rubber compounds (eg for engineering components) to which high stiffness is imparted by the incorporation of a suitable filler are sometimes said to be 'reinforced' because of the stiffening action of the filler, irrespective of its influence on other properties such as tensile or tear strength. On the other hand, most rubber technologists would probably agree that stiffness in itself is quite inadequate as a criterion of reinforcement. It should be accompanied by enhanced tensile strength (though the increase may be small in the case of natural rubber which, without fillers, has high tensile strength) and, in particular, by a high resistance to tearing and abrasion.
COMPOSITES June 1970
203
300
A similar plot is shown in Fig 2 for the gum and filled vulcanisates of SBR. At room temperature, in contrast to the NR vulcanisates, the addition of 30phr HAF has improved the breaking stress of the gum SBR by at least an order of magnitude. However, at higher temperatures the improvement is less marked. The results for other amorphous polymers are similar to those for SB R. Similar results 1 to those shown for tensile strength are found when work done or energy density to break is plotted against temperature.
0 NR GUM • NR +30 HAF
..~o I \ \ 1
•
E~
o ~'
o~ ~
•
2oo
;n b
°\o\ o
~oo
\o I
O
40
8(3 Tcmpero t ure, [°C]
o
\. \
I
t20
t60
FIG 1 Variation o f tensile stress at break with temperature for natural rubber (NR ) gum vulcanisate and NR with 30 phr HA F carbon black (phr = parts per hundred)
The mechanism by which carbon black improves the physical properties of rubber such as tensile strength, modulus, tear, abrasion resistance and stiffness is very complicated and has been the subject of numerous studies. This paper will briefly review the reinforcement of rubber by carbon black with respect to tensile properties, dynamic properties, wear resistance and electrical properties, but other important technological aspects of the effect of fillers on heat conduction, tear resistance and the rheological properties of extrusion and flow have had unfortunately to be omitted from this brief review.
Stress s o f t e n i n g All rubbers, vulcanised or unvulcanised, filled or unfilled, require a greater stress to produce a given elongation in the first extension than during subsequent extensions. This is illustrated by the stress-strain curves shown in Fig 3. If at point A on the first extension curve OA, the specimen is retracted to zero stress, then the second extension curve to a higher strain is represented by curve OBAC After retraction, the third extension goes to a still higher strain and is represented by the curve ODE. Clearly, the first and subsequent extension curves are markedly different and this phenomenon is referred to as stress softening. One of the first references to stress-softening was by Bouasse and Carriere 3 in 1903 and many investigations into the phenomenon have been reported since including a large study by Mullins, 4, s from where the term, 'Mullins' Effect' has arisen to describe the phenomenon in filled rubber vulcanisates. Several recent theories have been proposed to explain stress-softening behaviour in filled rubber vulcanisates. Dannenberg, 6 Boonstra 7 and co-workers 8,9 envisage a
150 O SBR GUM • SBR','30 HAF
TENSIL E PROPERTIES
Tensile strength The nominal tensile stress at break, (oB), of a natural rubber gum vulcanisate falls with increasing temperature of test (Fig 1). At about 80°C, the stress at break for this particular vulcanisate falls dramatically before showing a less sensitive temperature dependence. There is good reason to believe that the dramatic decrease in strength at 80°C arises from the inability of the vulcanisate to crystallise on stretching above this temperature. ~'2 It is effected by the type and degree of crosslinking. The results for the fdled N R are also shown in Fig 1. Between room temperature and 80°C, the addition of 30 phr HAF carbon black Idler improves the stress at break of the gum rubber by less than 20%. However, at the higher temperatures the presence of f'filer increases oj by approximately 200%.
204
COMPOSITES June 1970
'E u
IOO
b
"e'~-o--o..o_e_o_o..o_8 4~0
8i0 Tempcrotur¢, [o C ]
I;~
~ ~0
FIG 2 Variation o f tensile stress at break with temperature for styrene-butadiene (SBR ) gum vulcanisate and SBR with 30 phr HA F carbon black
E/
150
extension such as a monosulphide or carbon-carbon crosslink by swelling the stress-softened rubber in a suitable solvent and extracting off the solvent in vacuo. The recovery of stress-softening in some vulcanising systems indicates that no basic breakdown process has occurred and hence is attributed to quasi-irreversible re-arrangement of molecular networks due to localised non-affine deformation. This results from short chains reaching the limit of their extensibility leading to a relative displacement of the network junctions from their initial random state. The incomplete recovery in the case of vulcanisates cured with labile cross-links such as a polysulphide is attributed to the breakage of crosslinks and their reformation in the extended state to create a secondary network.~ 7-2 o
c/ Eu IOC
A
°/
tD 5(:
Strain at break
0
I
2
3
Strain
FIG 3 Stress-strain curves demonstrating the phenomenon o f stress-softening
slippage of the rubber network chains over the surface of the carbon black particles whilst Kraus et al 1o consider the effect to be the result of several mechanisms, thixotropy involving transient carbon black structures, rupture of network chains connecting filler particles and disruption of the 'permanent structure' of the carbon black. Harwood and Payne, however, in a series of recent papers. ~1-~ s claim that most of the stress-softening occurs in the gum phase of filler loaded vulcanisates leaving little to be explained in terms of breakage or slippage of rubber to filler bonds. The various theories will not be considered in great depth here as the whole phenomenon of stress-softening was the subject of a fairly comprehensive review recently. 1,16 A typical set of stress-strain cycles are shown in Fig 4 for NR gum and NR containing 60phr HAF carbon black. When the curves for the gum and filled rubbers are taken up to the same stress level, it is shown that the amount of stress-softening between the first and third extension cycles (curves 1 and 5) is remarkably similar. This figure shows that stress-softening effects in t'dled vulcanisates are not just a consequence of the filler in the rubber, but are due to the polymer phase of the composite. Similar types of curves are obtained in a non-crystallising rubber such as butadieneacrylonitrile (NBR) and the phenomenon has also been demonstrated in a whole range of other non-crystallising materials.l, 16 Stress softening can be recovered 13 in unfilled vulcanisates cured to produce crosslinks which do not break on
T. L. Smith 2~ and others =2 over recent years have developed the concept of a 'failure envelope' which is a unique curve peculiar to each polymer relating the stress at break (o8) or real stress at break oB(1 + c~) to the strain at break (eB) over a wide range of temperature and rate of strain. It was found by Harwood and Payne 1'23,24 that a quantitative relationship between the failure parameters could be obtained by using work done (or energy density) to break (UB) instead of stress at break. Fig 5a shows the energy input to break plotted against the strain at break for SBR containing increasing amounts of HAF carbon black. The energy input to break is multiplied by the ratio between the chosen reference temperature (294°K) and the experimental temperature to allow fi~r the temperature dependence of rubber-like elasticity as predicted by the kinetic theory. 2s It is found that as the temperature is increased, the failure points for the gum rubber move clockwise around an envelope passing through a maximum strain (en(max)) at a temperature of about 0 cC. The results for the filled rubbers lie along lines parallel to the gum rubber•
20C
NR+bOphr BLACK additional extension
NR GUM ] additional extcnsi~/
,4
150
5 ~'toc
I
I 2~j
6
6
~sc i
i
Stro,n
FIG 4 Stress-strain curves for gum NR and NR containing 60 phr HA F carbon black, showing similar stresssoftening behaviour (phr = parts per hundred)
C O M P O S I T E S June 1970
205
Hysteresis at break
IOC "-d
m
O; ?
r, I'O
i
O'1 OI
//7/ i
o SBR ® SBR • SBR • SBR
GUM 30HAF 60HAF 80HAF
i
I0 I0 Strain at break (t B)
/
/
/
i
I0 xE; B
FIG 5 Variation of reduced energy input to break with (a) strain at break, (b) strain at break corrected by hydrodynamic factor for SBR with O, 30, 60 and 80 phr HA F carbon black (phr = parts per hundred}
Mullins and Tobin 2 6 have shown that Young's modulus of carbon black filled rubber vulcanisates can be expressed in terms of the modulus of the unfilled rubber by use of an expression from hydrodynamic theory developed by Guth and Gold 2 7 relating the viscosity of a liquid containing hard spherical particles to that of the liquid alone X = 1 + 2'5c + 14.1c 2
1
where c is the volume concentration of filler. It was found by applying this factor X to the strain axis in Fig 5a the gum and filler loaded results coincided as shown in Fig 5b for strains up to en(max). The slope of the graph on the log scales was found to be 2 and hence the relationship between energy input and strain at break can be expressed as
2~-) UB = A (XeB) 2
2
where A is a constant. This type of relationship has also been found to hold both in other amorphous polymers such as NBR and also in crystalline polymers such as natural rubber. 2 Mullins and Tobin 26 have suggested that when the factor X is applied to filled vulcanisates, it takes account of both the disturbance of the strain distribution and the absence of deformation in that fraction of material composed of filler. It is shown in Fig 5b that the results at low temperatures do not coincide and this indicates that the hydrodynamic correction factor X is too large when the moduli of the filler and rubber approach parity.28 The bulk strains at break, therefore, of the filled and unfilled rubbers are related by a factor X under conditions of constant energy input. It must be remembered that when this type of agreement is obtained, the temperature of the filler loaded rubber is always higher than that of the unfilled rubber. Recent work has shown that it is possible to unify failure results from filled and unfilled rubbers of different cross-link densities 1, 2 by including in Equation 2 a crosslinking parameter from rubber elasticity theory.
206
COMPOSITES June 1970
When a viscoelastic material such as rubber is deformed by the application of a force, some of the mechanical work required is stored in the material and the remainder is dissipated as heat. This energy dissipation or hysteresis results from several different mechanisms which are dependent on the type of material and the experimental conditions. The inclusion of carbon black filler greatly modifies the hysteresial behaviour of the polymer. A measure of hysteresis in a material can be obtained from the area of the loop between the extension and retraction curves in a single tensile stress-strain cycle. The major sources of hysteresis in filled rubbers can be listed as follows 1 Viscoelasticity of the polymer; this becomes increasingly important as the temperature of test is lowered or the rate of straining is increased 2 9-31 2 Crystallisation of the polymer; this occurs in some rubbers (eg natural rubber) when they are stretched and the molecules align 3 ] 3 Changes in network configurations; this effect, known as stress-softening or Mullins effect, results in apparent softening of the rubber by a previous stre t ching 1-16 4 Changes in network structure; this results from the breakage of weak crosslinks or chains such as a polysulphide crosslink; the crosslinks may reform in the stretched state to produce the permanent set 17-2 0 5 Breakdown of carbon black or filler aggregates; this feature of filled rubbers occurs principally at low strains 3 2 - 3 6 The last three processes are discussed in other sections. Processes (1) and (2) are well documented and will not be discussed further in this review. It has been found recently that the strength of a rubber vulcanisate is not dependent on any individual contribution to the energy dissipation, but is dependent on the combined effect of all the processes which are applicable to the particular rubber and experimental conditions used. 1,2 3 , 3 4 The energy input to break (Us) is related to the hysteresis at break (HB) for amorphous polymers by the relationship
2_~__01/3Un = KH 2/3
3
where K is a constant. This is demonstrated in Fig 6 where log Un(294/T) is plotted against log Ha(294/T) for SBR containing increasing amounts of HAF carbon black. Equation 3 can be rewritten in terms of hysteresis ratio (h~) where
hB =HB/UB to yield
~)
UB =K3h~
IOC 0 SBR Gum t, SBR ÷30 Hal • SBR Haf 8
~o ~ o/-40°C
*60
o/.,~
• S BR + 8 0 Hof
O.OI
1
I lid
O,l
I I0 k
I
J"
K)O --
FIG 6 Variation o f reduced energy input to break with reduced hysteresis at break for SBR containing O, 30, 60 and 80 phr HA F carbon black (phr = parts per hundred)
conditions of rapid repeated deformations which are relatively small compared with their ultimate breaking strengths. Although ultimate values and fatigue tests are important as criteria of quality, they are of little or no assistance to the engineer, designer or technologist for anticipating the response of his products to the forces experienced in service. In order to take account of this problem a collection of technical and scientific methods, data and principles referred to as dynamic properties has been built up in recent years. There are several reviews and text books 2 9,37-42 dealing with the subject of dynamic properties in general, but there is little work done on the dynamic properties of rubber containing reinforcing fillers especially using amplitude of shear oscillation as a parameter. This is particularly surprising in view of the importance of being able to obtain data in the strain ranges applicable to service conditions in view of the fact that the normal tests such as tensile tests can provide completely misleading data about dynamic characteristics.3 3,3 6
Variation of dynamic modufi with strain The maximum value of Ua(294/T), therefore, occurs when all the work done in stretching the rubber is dissipated (hB = 1) and is, therefore, equal to K 3 . This is thought to occur at the glass transition temperature of the polymer. 23 The results for the filled rubbers are displaced from the gum rubber, the displacement increasing with increasing concentration of carbon black and it was found that by dividing the hysteresis by the factor X used in the previous section, the results 23'24 for the gum and filler-loaded rubbers coincided. Some disagreement is noticed at higher energies (low temperatures) and this is presumably due again to the hydrodynamic correction being too large when the moduli of the rubber and filler particle approach parity. The relationship therefore for gum and filler-loaded rubbers can be expressed as
(~/1/3
Ua
=
K
t--~--)2/3
5
Values of energy density and hysteresis measured before failure have been shown not to obey any of the above equations 23,=4 and hence equations have been found to represent an excellent description of failure between a lower limit at approximately 0'15 joules cm-3 when the material is essentially elastic to a higher limit about 60joulescm -3 for S B R when the material is totally hysteresial. Similar results have been shown to hold in a number of other amorphous polymers 23,24 and also to a limited extent 1'2 in natural rubber which crystallises on extension.
D YNAMIC PROPER TIES Some of the large volume uses of filled rubbers such as in tyres, belting and engine mounts employ the material under
The application of a low amplitude sinusoidal oscillation on a rubber containing filler results in the ensuing strain lagging behind the stress. This is because of the viscoelastic nature of rubber. The complex shear modulus (G*) of rubber can be expressed in terms of an elastic in-phase shear modulus (G 1) and a viscous out-of-phase shear modulus (G ~ 1) which for low amplitude sinusoidal strain are related by the following two equations G* = G 1 + iG 11
Tan 6 = G t
l/G1
6
7
where 6 is the phase angle between the sinusoidal stress and strain and tan 6 is a measure of the hysteresis exhibited by a rubber at low strains. The variation of these dynamic parameters for rubbers containing carbon black filler with strains up to about 10% in shear has been the subject of numerous papers by Payne.32-50 The in-phase shear modulus decreases with small sinusoidal strain oscillations as shown in Fig 7, for butyl rubber containing increasing concentrations of HAF carbon black. The effect increases with increasing concentration of the carbon black and is hence associated with properties of the filler particle. The in-phase shear modulus has been found to have a limiting value both at low strains (G ~o) and at high strains (G 1=) when there is no further change of modulus with increasing strain. G ~o for normal black loadings in the general purpose rubbers attains values up to some 2 orders of magnitude greater than the in-phase shear modulus of a gum vulcanisate whereas GI= for the same black loaded rubber is less than 1 order of magnitude greater than that of the gum vulcanisate. The difference between the two limits (G~0 G~=) which represents the change of dynamic modulus with strain is believed to be due to the structure breakdown of carbon black aggregates. The quantity G~= being the dynamic shear modulus when
COMPOSITES June 1970
207
I0 x
xv
X
,° i
23-2 x
'Ev 7
g ~"
6
OOOOI
,g
20-2
~
O'OI O'1 Double strain amplitude
I'O
FIG 9 Variation o f phase angle with double strain ampfitude for butyl rubber containing various concentrations up to 38"6% volume HA F carbon black
3
a
(3'OOI
~x
2 I •
0 0.0001
"",,x
0
i
i
O-OOI
0.01 011 Double strain amplitude
I'0
FIG 7 Variation o f in-phase shear modulus with double strain amplitude for butyl rubber containing various concentrations up to 23.2% volume HA F carbon black
Butyl 28.~¥o vol HAF
?
x
Effects of temperature
0
23"2J
o i
d
16.8 0
311 o 0001
o
all the carbon black structure has been broken down. This is higher than the gum modulus due to hydrodynamic effects plus other smaller contributions. The variation of out-of-phase shear modulus (G") with strain is shown in Fig 8 for a typical series of vulcanisates with increasing ffdler concentration up to 28-8% volume in butyl rubber. It is seen that the out-of-phase modulus passes through a maximum value in a region of strain where G 1 changes most rapidly. Figure 9 shows the variation of the phase angle 6 (phase angle between stress and strain) with the double strain amplitude. The results shown in Fig 9 indicate that (a)at very low strains, the phase angles of all vulcanisates are low and constant and although the phase angle increases with increasing carbon black content, it is small compared with the changes that occur at higher strains, (b) at strains above the yield point strain, the phase angle increases with increasing strain and passes through a maximum. With high concentrations of carbon black, the values of phase angle become very large in the order of 30 ° .
~i 9~J'°--°--c" ~ i ~ O0 &l Double strain amplitude
"
1.0
The effect of temperature on the structure breakdown curve is shown in Fig 10 for a natural rubber compound containing 32% by volume of HAF carbon black. An increase in temperature from 20°C to 90°C has a large effect on G I o and decreases its value by about 50%. On the other hand, however, at the end of the strain range GI= is virtually independent of temperature over the temperature range considered.
Effect of frequency FIG 8 Variation o f out-of-phase shear modulus with double strain amplitude for butyl rubber containing various concentrations up to 28"8% volume H A F carbon black
208
COMPOSITES June 1970
Little work has been carried out on the effect of frequency on the structure breakdown curve, this is presumably due to the difficulty of obtaining a wide enough frequency
range and a wide strain range together in one apparatus. Work by Warnaka 43 indicates that the frequency does not substantially alter the shape of the breakdown curve and this is confirmed in some results quoted by Payne 36 which are shown in Fig 11. The lack of a marked frequency effect is quite surprising and to date unaccounted for, unless the carbon black structure itself is a frequency independent structure, ie a purely elastic structure.
--•.
4 0 vol loading HAF black in butyl
25 'E u
~2o ?
o ×
15
Effect of type of carbon black E
Values of (]1 o have been found to be dependent markedly on the type of black used. Carbon black differs both in area of the surface and in nature and can approximately be divided into two main classes (a)channel blacks and (b) furnace and thermal blacks. Within these classes differing surface areas confer differing properties, but differences in the surface constitution also exist between the classes. To indicate how this affects dynamic properties, a series of compounds were prepared with a range of carbon blacks of differing particle size and type. The shear modulus of these compounds was measured at very low strains and the values obtained (G 1 o) are shown in Fig 12 plotted against the published figures for surface area which were determined by nitrogen adsorption measurements. 44 The plot shows that all the results for furnace blacks lie on a single curve together with lampblack. The thermal blacks MT and FT have the lowest values of all. Channel blacks lie well below the furnace blacks on another curve. Acetylene black is distinct in behaviour from all the other blacks as is well known from many of its properties. 4 s-47
\\\
z
s ~p~
g.
L
I
0"01 Ot Double strain amplitude
OOCI
I0
FIG 11 Variation of in-phase shear modulus with double strain amplitude as a function or frequency o f test for butyl rubber containing 40% volume HA F carbon black
IO jmSCF
B ~v
Effect o f dispersion of carbon black
.ss ~
SAF..jb~,<
6
8~
Boonstra and Medalia48 studied the effect of mixing times on the dispersion of the carbon black for an SBE rubber
containing IS A F carbon black. They showed that increasing the time of mixing improves the dispersion of the carbon
..~-
¢
~ L B , _. " - ' " HAg zw-(t -F ~INT FEF m..a~...-HMF
20
a
40
ISAF
.-~7~
~c~
. ' " ' " C C II~.S s:~'~¢'#! I"1"0 a l ss
419
//
.~-d~c
b..-l~pC "'"EPC
i
i
60
80
Nitrogen SA
1(50 1~) lhK) I(~D t1~ Z:~
Surface a r m [m2/g]
FIG 12 Variation of dynamic shear modulus (Glo) with surface area o f various carbon blacks determined by nitrogen adsorption o
of increasing temperature
Effect
>.
4O JO
.~x I O -
g o
Natural rubber HAF
E
o 0 u
0-001
~
n
001 0"1 Double strain amplitude
I'O
FIG 10 Variation o f in-phase shear modulus with double strain amplitude as a function of temperature for natural rubber containing 32% volume HA F carbon black
black as measured by optical and electron microscopy and counting techniques. Payne 49 using the same compounds as Boonstra and Medalia and with time of mixing varying from 1 to 16min showed how the dynamic modulus of the materials decreased with increasing mixing times. The graph is shown in Fig 13. The excellent colour photographs published by Boonstra and Medalia 4 s indicate extremely well how dispersion is improved with increasing mixing times and Payne concluded from his measurements on dynamic properties that the structure breakdown effect (Glo GI~) is solely attributed to the dispersion of the carbon black. He also
COMPOSITES June 1970
200)
and water, s 7 carbon black in oil s 8 and dendritic crystals of phenyl-beta-naphthylamine (PB N) in rubber.l' 59 Rheological studies 60-62 of clay-water systems show the existence of three dimensional or 'pack-of-cards' structures by the clay. It is presumably this three dimensional structure that is broken down by straining the samples. Similarly, the effect in carbon-black filled vulcanisates is attributed to the breakdown and reformation of the three dimensional or aggregated carbon black structure during the imposed oscillation. This has been confirmed in a recent paper by Payne and Whittaker 63 which demonstrates that the breakdown and reformation of the carbon black structure can be described by a domain theory similar to that found in magnetism.
6"0
~X~x
x
~'E 5.0 v >..
'0 x4-O
E 3.0
H-2 stage _2.© i c
A-ISmm B-2Omm C - 2 5 m,n
I0
D-3Qm,n
E F - 4 - 8 m,n G-16"O re,r,
OI 0'001
I
001 Double
i
OI s t r a i n amphtud¢
i
P0
FIG 13 Variation of in-phase shear modulus with double strain amplitude for SBR containing 70phr ISA F carbon blacks for various times of mixing from 1"5 to 16min
found that the increased mixing times reduced the out-ofphase (viscous) shear modulus and the phase angle at high strains. In parallel with the reduction in G 1o - G l ~ there is an enhancement of the ultimate tensile properties. This is accompanied as has been shown by Boonstra and Medalia with improvements in permanent set (reduced) heat build up (reduced), Mooney viscosity (reduced), DC resistivity (increased), abrasion loss (reduced), cut growth (improved), hardness (reduced) but with tear resistance and specific gravity remaining substantially the same. The correlation of the structure breakdown curve with dispersion of the filler within the mix has also been confirmed by Payne in conductivity measurements, s o This relationship is very important practically as other methods of measuring dispersion such as optical and electron microscopy, conductivity and counting techniques are difficult to use. The measurement of GIo-GI~ has subsequently been used in technological development work as a reliable index of dispersion in rubber filler composites.S 1,s 2
EL ECTRICA L PROPER TIES Rubber is usually regarded as an insulating material and has therefore been used for many electrical insulation purposes. It is made conductive 64'6s by the inclusion of adequate amounts of carbon black in the compounding formulation. Certain types of carbon black produce good values of conductivity at loadings even as low as 20 parts per weight per 100 parts of rubber. The actual mechanism of conductivity is still the subject of a considerable amount of discussion but both the particle size and the chain structure of the type of black used have an important effect. The
\
\
\
\
\ I
E u
I I
O
I
>
,,\
t_
\
J
\
\
\
\
Other effects A number of papers have considered the effect of special processing or compounding on the structure breakdown curve such as attrition of the carbon black, s 3-s 4 mixing technique and initial plasticity of the rubber, sl chemical promoters s2 and also change in the crosslinking of the vulcanisate.S s Similar breakdown curves have been shown in a number of other two phase systems such as clay in both rubber s 6
2|0
COMPOSITES June 1970
)1
I-0
I
I-2
I
I
1.4 I-6 Log weight Ofo carbon black
I
I'~
I
2-0
FIG 14 Variation of resistivity with carbon black loading for series of commercial carbon blacks in natural rubber 1. Spheron 9 2. Pelletex 3. Philblack A 4. Philblack 0 5. Philblack E (After Studebaker 6 6)
particles of most carbon blacks range in diameter from 10-400 millimicrons. The values of resistivity which are usually obtained 6s lie between 1 ohm/cm and i01 s ohm/cm. In the range 1 to 10 ~ ohm/cm, carbon black is the only conductive agent which has been used since it is the only material combining high conductivity with good mechanical properties and ease of manufacture. Very often a distinction is made between conductive and anti-static rubbers. The latter usually have a resistance above 105 ohms, but the resistivity giving this limit of resistance varies with the product. Therefore, there is really no point in distinguishing between these two types of rubber. The lower resistance limit is, however, required to prevent shock and fire risks resulting from any accidental contact with mains electricity. The important engineering applications of conductive or anti-static rubbers is in situations where the dissipation of dangerous, harmful or even merely annoying charge is required. The major use of non-insulating rubbers appears to have been developed in the 1930's when the surgical world became alarmed about the frequency of explosions in operating theatres. The elimination of static charges to reduce explosion hazards is obtained by using conducting rubbers, indeed a number of hospital products are specifically 'anti-static', ie they have a lower limit of resistance as well as an upper one. 6 s The conductivity of rubber depends on the amount, type and structure of the carbon black and on the mixing procedures used. Acetylene black was the forerunner of a wide range of carbon blacks. The types of black now used are mainly furnace blacks or conductive channel blacks. Fig 14 shows how the electrical resistivity in ohm/cm varies with the amount of carbon black included in the vulcanisate? 6 The curves are sigmoidal and change from an initial value of about 101 s ohm/cm (the value of the rubber without any carbon black) to an asymptotic value of between 1 to 1 000 ohm/cm at high loadings. The plasticity and the degree of breakdown of the rubber chains before the incorporation of carbon black affects the conductivity. The remilling of the mixed stocks, the effect of any heat treatment following the remilling, the incorporation of processing or dispersion aids also have appreciable effects on the resistivity of the product. Most rubber units in use are subjected to small amplitudes of oscillation or distortions and it is found that these small strains affect the conductivity in a similar manner to that described for the change in dynamic modulus with strain in a previous section.
5AF
14(2
12(:
I0£ c
Bo
///
0
< 40
2£ 0
0
m
I
I
I
I
I
I
I
I0
20
30
40
50
60
70
80
Loading, [ports
by wt per cent rubber]
FIG 15 Variation of abrasion resistance with filler content (parts by weight per cent rubber) for a range of different types of carbon black. (After Leigh-Dugmore 6 8)
interplay between these three factors, it is useful to consider them separately as is done later in this section. When the best reinforcement is required such as in tyre treads or conveyor belting, one of the following grades of black is generally used: S A F , I S A F , H A F , MPC, HPC o r EPC. The general trend of abrasion resistance (DunlopLambourn constant power test) with increased loading of these carbons 68 is shown in Fig 15. At any loading it decreases in the order in which they have been named above. The most important result of adding black to rubber is to improve the abrasion resistance by factors of five or six, or more. Of course, there is a limiting amount of filler that can be added as abrasion properties tend to fall away rapidly with carbon black contents above 50 to 60 parts by weight mixed with a hundred parts by weight of rubber.
Influence of strength Abrasion involves tearing, cut growth and ultimate rupture, ('7 and the paper has already discussed how ultimate tensile rupture is altered by the use of particulate fillers. Improvements in abrasion are paralleled by improvements in these three failure processes in particular tensile strength.
WEAR RESISTA NCE The inclusion of carbon black in the rubber formulation for a tyre-tread compound is absolutely necessary if reasonable wear resistance is to be obtained. The wear resistance of rubber is a complex quantity, but it is clear that three properties, those of stiffness, strength and hysteresis are very important, 67 and although there is considerable
Influence of stiffness When rubber is abraded without change of direction, sets of parallel ridges are often found on the surface of the samples at right angles to the direction of motion; these have been called abrasion patterns. Two relevant characteristics of
COMPOSITES
June
1970
2 | |
to normal driving), gives a ratio of abrasions proportional to the reciprocal value of the stiffness ratios. Thus the increase in stiffness produced by fillers in a rubber leads once again to a reduction in abrasion losses.
wheel
Influence of hysteresis
c
--~
$
o, crab ,k
FIG 16 Reaction of a rolling wheel against a constant side force S (infinite coefficient of friction): B, C and D show the direction of travel. (From A. Schallamach, Rev. Gen. Caoutchouc, 36, 1480 (1959) Fig 1)
abrasion patterns are that their intensity increases with, firstly, increasing coarseness of the track and, secondly, with decreasing stiffness of the compound as the softer the compound the easier it is for the part of the abraded rubber surface to be gripped and elongated in the direction of abrasion. A third, and possibly the most important characteristic is that they increase the rate of abrasion. 67,09 The inclusion of carbon black in a compound prevents abrasion patterns being formed to the extent they form in unfilled rubbers. With a normal tyre-tread compound (containing about 5 0 - 6 0 parts per hundred of rubber of reinforcing black) a pattern only develops after the surface of rubber has been considerably softened by repeated prestressings. This softening, however, affects only the surface layer, and the main stiffness of the rubber is still contributed by the hardly-deformed inner bulk of the tread.7 o The amount of energy which is available for the abrasion of a slipping wheel depends on the degree of slip, and the stiffness and resilience of the compound. The wear A per unit distance of travel is given by: A = 27pF 2/ka2
COMPOSITES June 1970
,120
p
,so
6p
p
,c
Sliding abrasionx r~silience
~
E
,,,Io
'p
~p
FI[~
BO ~
-90
8
where 7 is the proportionality constant between abrasion and energy dissipation and is termed the abradability, p is the resilience of the wheel, F is the reaction force in the area of contact, k is a stiffness constant defined by tangential stress per unit length of contact, and a is the length of area of contact. 7~,72 The importance of this equation is that it draws attention to the parts which stiffness and hysteresis have to play in wear processes. The abrasion ranking of two samples differing in dynamic stiffness and compared at equal force (a condition relevant
2]2
The influence of hysteresis in Equation 8 on the abrasion process arises from the fact that road wear is largely due to the sliding of the rubber over the road surface in the rear of the area of contact. This sliding occurs during the recovery of the deformation of the tyre surface imposed by contact with the road surface. With more hysteresial rubbers the recovery is less complete, and thus the amount of sliding and wear is proportionately less. 73 Fig 16 is an illustration of the reaction of a rolling wheel against a constant side force (crab walk), and Fig 17 illustrates well how the theoretically-predicted and previously-unsuspected importance of hysteresis in the wear process of slipping wheels can make sense of the experimental results when the product of sliding abrasion and resilience is plotted against crab walk abrasion. Resilience is a measure of the hysteresis of a rubber and is small when the hysteresis is high 72 Earlier in this paper, it was shown that the hysteresis of a vulcanisate increases with carbon-black content. It also increases with decreasing particle size. 7~ Provided other quantities determining the abradability of a rubber are not adversely affected, this greater hysteresis will result in less wear. It has, in fact, been found that the increased wear resistance of tyre treads containing furnace blacks of finer particle size is due not to a change in abradability but due to an increase in hysteresis. There are practical limits to how much hysteresis can be compounded into a vulcanisate, as too high a concentration
IIA
.6 ~ -50
I.0
FIG 17 Variation of abrasion in crab walk at 15° slip angle with the product of sliding abrasion and resilience. (From A. Schallamach, Wear, 3, 21 (1960), Fig 16)
of filler reduces tensile strength, 73 and leads to a large temperature rise in the rubber under flexing conditions which can cause premature failure. Improvement in wear rating by increasing hysteresis has, however, to be paid for by increased fuel consumption.
l0
Kraus, G., Childers, C. W. and Rollman, K. W., J Appl Poly Sci 10, 229, (1966)
11 Harwood, J. A. C., Mullins, L. and Payne, A. R . , J A p p l Poly Sci 9, 3011 (1965) 12 Harwood, J. A. C. and Payne, A. R . , J A p p l P o l y Sci, 10, 315 (1966)
CONCLUSION
This paper has been, of necessity, a review of only a few of the many relevant topics concerning the complex composite material formed from rubber and filler. It has not been possible to discuss in great depth the subtle interplay of surface properties of the black and the effect of particle size and of the tendency of fillers to agglomerate on all the physical properties discussed in this chapter. Other properties such as tear resistance and the rheological properties of extrusion and flow have been completely omitted. It is obvious that the excellent enhancement of rubber properties brought about by compounding rubber with fillers is little understood, and there is still much basic research needed before we can even begin to understand how it performs its task so admirably. Recent work on 1' 24 analysis of tensile stress strain curves has indicated that there exists a shell of immobilised rubber around the filler particles. This work is in agreement with the findings of several other studies ~4-78 and it is thought that this immobilised layer gives rise to the perturbed relaxation behaviour of the rubber matrix which has been discussed quantitatively in the work of Radok and co-workers 79. The perturbed relaxation behaviour of the matrix then causes the enhancement of tensile strength, and abrasion resistance.
REFERENCES
l Harwood, J. A. C., Payne, A. R. and Whittaker, R. E., Paper presented to IRI Conference 'Advances in Polymer Blends and Reinforcement', Loughborough (1969) 2 Harwood, J. A. C., Payne, A. R. and Whittaker, R. E., A waiting publication in J Appl Poly Sei
13 Harwood, J. A. C. and Payne, A. R., J ApplPoly Sci, i0, 1203 (1966) 14 Harwood, J. A. C. and Payne, A. R., Trans IRL 42, T14 (1966) 15 Harwood, J. A. C. and Payne, A. R., J Appl Poly Sci, I1, 1825 (1967) 16 Harwood, J. A. C., Mullins, L. and Payne, A. R., J of IRI, 1, 17 (1967) 17 Brucksch, W. F., Rubb Chem Tech, 35, 453 (1962) 18 Brucksch, W. F., Rubb Chem Tech, 36, 975 (1963) 19 Bateman, L., et al, Chapter 19 in 'Chemistry and Physics of Rubber-like Substances' Edited by L. Bateman, Maelaren, London (1963) 20 Brown, H. P., Rubb Chem Tech, 36, 931 (1963) 21 Smith, T. L.,JAppIPhys, 35, 27 (1964) 22 Kelley, F. N.,ApplPoly Symp, No 1,229 (1965) 23 Harwood, J. A. C. and Payne, A. R., J Appl Poly Sci, 12, 889 (1968) 24 Harwood, J. A. C., Payne, A. R. and Smith, J. F., Kaut u Gummi Kunst, 22, 548 (1969) 25 Ferry, J. D., Fitzgerald, E. R., Grandine, L. D. and Williams, M. L., Ind Eng Chem, 44, 703 {1952) 26 Mullins, L. and Tobin, N. R., J Appl Poly Sci, 9, 2993 (1965) 27 Guth, E. and Gold, O., Phys Rev, 53, 322 (1938)
3 Bouasse, H. and Carrier6, Z., Annales de la Faculte des Sciences de Toulouse, 5, 257 (1903)
28 Landel, R. F., Trans Soc Rheol, 2, 53, (1958)
4 Mullins, L . , J R u b b Res, 16, 275 [1947)
29 Ferry, J. D., 'Viscoelastic Properties of Polymers', Wiley (1961)
5 Mullins, L., J Phys and Colloid Chem, 54, 239, (1950) 6 Dannenberg, E. M., Trans IRL 42, T26 (1966)
30 Grosch, K. A. and Mullins, L., Rev Gen Caouteh, 39, 1 781 (1962)
7 Boonstra, B. B., Chapter 16 in 'Reinforcement of Elastomers', Edited by G. Kraus, Interseience (1965)
31 Harwood, J. A. C. and Schallamach, A., J Appl Poly Sci, 11, 1835 (1967}
8 Dannenberg, E. M. and Brennan, J. J., Paper No 22, ACS Div Rubb Chem, Philadelphia (1965)
32 Payne, A. R . , J ApplPoly Sei, 6, 57 (1962) 33 Payne, A. R . , J A p p l P o l y Sci, 7, 873 (1963)
9 Brennan, J. J., Jermyn, T. E. and Perdagio, M. F., Paper No 36, ACS Div Rubb Chem, Detroit (1964)
34 Payne, A. R . , J Appl Poly Sci, 8, 2661 (1964)
COMPOSITES June 1970
213
35 Payne, A. R., Trans IRL 40, T135 (1964)
57 Payne, A. R. and Whittaker, R. E., Rheol Acta, 9, 91 (1970)
36 Payne, A. R., Chapter 3 in 'Reinforcement of Elastomers', Edited by G. Kraus, Interscience (1965)
58 Payne, A. R., J Colloid Sci, 19, 744 (1964)
37 German, S. D., Rubb Chem Tech, 30, 1202 (1967)
59 Payne, A. R . , J A p p l P o l y Sci 11,383 (1967}
38 Bueche, F., 'Physical Properties of Polymers,' Wiley (1962)
60 Astbury, N. F., Proc Royal Soc, A258, 27 (1960) 61 Moore, F.,J Sci lnstr, 40, 228 (1963)
39 Tobolsky, A. V., 'Properties and Structure of Polymers', Wiley (1960) 40 Scott, J. R. and Payne, A. R., 'Engineering Design with Rubber', Maclaren (1960) 41 Davey, A. B. and Payne, A. R., 'Rubber in Engineering Practice,' Maclaren (1965) 42 Lord, P. and Wetton, R. E., 'Progress in Non-Destructive Testing', Volume 3, Heywood (1961) 43 Warnaka, G. E., ASME Rubber and Plastics Div, New York, Preprint 62-WA:323 (November, 1962)
62 Astbury, N. F. and Moore, F., Mat Res Std, 5, 178 (1965) 63 Payne, A. R. and Whittaker, R. E., Awaiting publication in Trans Farad Soc 64 Payne, A. R. and Whittaker, R. E., Chapter 4 in Design Engineering Series 'Rubbers' Edited by L. Mernagh, Morgan-Gram pian (1969) 65 Norman, R. H., 'Conductive Rubbers, its production, application and test methods', Maclaren (1957) 66 Studebaker, M. L., India Rubb World, 129, 485 (1954)
44 Gessler, A. M., Rubb Age, 88, 658 (1961) 67 Schallamach, A., Proc Phys Soc, 67B, 883 (1954) 45
Studebaker, M. L., Rubb Chem Tech, 30, 1400 (1957)
46 Leigh-Dugmore, C. H., 'Microscopy of Rubber', Heifer (1961) 47 Parkinson, D., 'Reinforcement of Rubber', Lakeman (1957)
68 Leigh-Dugmore, C. H. in 'The Applied Science of Rubber' Edited by W. J. S. Naunton, Edward ArnoM, p 475 (1961) 69 Schallamach, A., Trans IRL 28, 256 (1952) 70 Schallamach, A., Wear, 1,384 (1958)
48 Boonstra, B. B. and Medalia, A. I., Rubb Age, 92, 892 (1963) 49 Payne, A. R.,J Appl Poly Sei, 9, 2273 (1965) 50 Payne, A. R . , J A p p l P o l y Sci, 9, 1073 (1965) 51 Barker, L. R., Payne, A. R. and Smith, J. F., J o f l R L 1,206 (1967) 52 Payne, A. R., Swift, P. M. and Wheelans, M. A., J of RRIM, 22, 275 (1969) 53 Gessler, A. M.,Rubb Age, 87, 64 (1960) 54 Gessler, A. M. and Payne, A. R., J Appl Poly Sci, 7, 1815 (1963)
71 Schallamach, A., Rubb Chem Tech, 33, 857 (1960) 72 Schallamach, A., 'The Chemistry and Physics of Rubber-like Substances', Edited by L. Bateman, Maclaren (1964) 73 Studebaker, M. L., Rubb Chem Tech, 30, 1400 (1957) 74 Hess, W. M., private communication 75 Westlinning, H., Kaut U Gummi Kunst, 15, 475 (1962) 76 Schoon, T. G. F. and Alder, K., Kaut U Gummi Kunst, 19, 414 (1966) 77 Grosch, K. A . , J Appl Poly Sci, 12, 915 (1968)
55 Payne, A. R., Smith, J. F. and Whittaker, R. E., to be published
78 Smith, P. P. A.,Rheol Acta, 5, 277 (1966)
56 Payne, A. R. and Whittaker, R. E., Rheol Acta, 9, 97 (1970)
79 Radok, J. R. M. and Tai, C. L . , J A p p l P o l y Sci, 6, 518 (1962)
2]4
COMPOSITES June 1970
One of the most important composite materials commercially available is rubber containing a colloidal filler such as carbon black in quantities of up to 20 to 30% by volume of the rubber. Rubber by itself is unsuitable for a large number of important applications (eg tyres) and in these it must be manufactured into a composite material before it is of commercial use
In contrast with the ordered and rigid crystalline arrangement of atoms in metals, elastomers are composed of a tangled mass of kinked, twisted and intertwined chain-like molecules. The basic element of an elastomer is the monomer -- the link in the chain which determines the chemical character of the material and its resistance to oil, ozone, solvents and chemicals. Thousands of monomers link end-to-end to form a polymer, or molecular chain, free to bend or to rotate at most points where the chain links join. The tangled chains are free to slide past each other except where they are attached together here and there by cross-links. These cross-links tie the molecular chains together and limit the amount of stretch and increase the elasticity. Cross-links are formed by chemical reaction of the chain with certain materials (curatives and accelerators) mixed into the mass. Heating increases the rate and extent of cross-linking or curing and this process is termed vulcanisation. Addition of inert materials, dispersed in the elastomeric mass prior to curing, can increase strength and hardness or resistance to deformation, usually with a reduction in elasticity and resilience. Size and shape of the particles and degree of dispersion are important factors; carbon black, for example, is useful in increasing strength and hardness for most elastomers. When an elastomer is stretched, the * Shoe and Allied Trades Research Association, Kettering, Northants, England
tangled mass of irregular, kinked and cross-linked molecular chains tend to straighten out, sliding and slipping past each other. When the external load is removed, the molecules tend to return to their original coiled-up and kinked condition. The amount and character of the added compounding materials have a strong influence on the elasticity. The term 'reinforcement' as applied to natural and synthetic rubbers, though one of the most familiar in the rubber technologist's vocabulary, is difficult to define; in fact, it does not necessarily mean the same thing to different people. This is because the properties of rubber vulcanisates are modified to different degrees by different reinforcing agents, and one's view of what constitutes high reinforcement is coloured by the particular kind of product in which one is interested. Thus certain rubber compounds (eg for engineering components) to which high stiffness is imparted by the incorporation of a suitable filler are sometimes said to be 'reinforced' because of the stiffening action of the filler, irrespective of its influence on other properties such as tensile or tear strength. On the other hand, most rubber technologists would probably agree that stiffness in itself is quite inadequate as a criterion of reinforcement. It should be accompanied by enhanced tensile strength (though the increase may be small in the case of natural rubber which, without fillers, has high tensile strength) and, in particular, by a high resistance to tearing and abrasion.
COMPOSITES June 1970
203
300
A similar plot is shown in Fig 2 for the gum and filled vulcanisates of SBR. At room temperature, in contrast to the NR vulcanisates, the addition of 30phr HAF has improved the breaking stress of the gum SBR by at least an order of magnitude. However, at higher temperatures the improvement is less marked. The results for other amorphous polymers are similar to those for SB R. Similar results 1 to those shown for tensile strength are found when work done or energy density to break is plotted against temperature.
0 NR GUM • NR +30 HAF
..~o I \ \ 1
•
E~
o ~'
o~ ~
•
2oo
;n b
°\o\ o
~oo
\o I
O
40
8(3 Tcmpero t ure, [°C]
o
\. \
I
t20
t60
FIG 1 Variation o f tensile stress at break with temperature for natural rubber (NR ) gum vulcanisate and NR with 30 phr HA F carbon black (phr = parts per hundred)
The mechanism by which carbon black improves the physical properties of rubber such as tensile strength, modulus, tear, abrasion resistance and stiffness is very complicated and has been the subject of numerous studies. This paper will briefly review the reinforcement of rubber by carbon black with respect to tensile properties, dynamic properties, wear resistance and electrical properties, but other important technological aspects of the effect of fillers on heat conduction, tear resistance and the rheological properties of extrusion and flow have had unfortunately to be omitted from this brief review.
Stress s o f t e n i n g All rubbers, vulcanised or unvulcanised, filled or unfilled, require a greater stress to produce a given elongation in the first extension than during subsequent extensions. This is illustrated by the stress-strain curves shown in Fig 3. If at point A on the first extension curve OA, the specimen is retracted to zero stress, then the second extension curve to a higher strain is represented by curve OBAC After retraction, the third extension goes to a still higher strain and is represented by the curve ODE. Clearly, the first and subsequent extension curves are markedly different and this phenomenon is referred to as stress softening. One of the first references to stress-softening was by Bouasse and Carriere 3 in 1903 and many investigations into the phenomenon have been reported since including a large study by Mullins, 4, s from where the term, 'Mullins' Effect' has arisen to describe the phenomenon in filled rubber vulcanisates. Several recent theories have been proposed to explain stress-softening behaviour in filled rubber vulcanisates. Dannenberg, 6 Boonstra 7 and co-workers 8,9 envisage a
150 O SBR GUM • SBR','30 HAF
TENSIL E PROPERTIES
Tensile strength The nominal tensile stress at break, (oB), of a natural rubber gum vulcanisate falls with increasing temperature of test (Fig 1). At about 80°C, the stress at break for this particular vulcanisate falls dramatically before showing a less sensitive temperature dependence. There is good reason to believe that the dramatic decrease in strength at 80°C arises from the inability of the vulcanisate to crystallise on stretching above this temperature. ~'2 It is effected by the type and degree of crosslinking. The results for the fdled N R are also shown in Fig 1. Between room temperature and 80°C, the addition of 30 phr HAF carbon black Idler improves the stress at break of the gum rubber by less than 20%. However, at the higher temperatures the presence of f'filer increases oj by approximately 200%.
204
COMPOSITES June 1970
'E u
IOO
b
"e'~-o--o..o_e_o_o..o_8 4~0
8i0 Tempcrotur¢, [o C ]
I;~
~ ~0
FIG 2 Variation o f tensile stress at break with temperature for styrene-butadiene (SBR ) gum vulcanisate and SBR with 30 phr HA F carbon black
E/
150
extension such as a monosulphide or carbon-carbon crosslink by swelling the stress-softened rubber in a suitable solvent and extracting off the solvent in vacuo. The recovery of stress-softening in some vulcanising systems indicates that no basic breakdown process has occurred and hence is attributed to quasi-irreversible re-arrangement of molecular networks due to localised non-affine deformation. This results from short chains reaching the limit of their extensibility leading to a relative displacement of the network junctions from their initial random state. The incomplete recovery in the case of vulcanisates cured with labile cross-links such as a polysulphide is attributed to the breakage of crosslinks and their reformation in the extended state to create a secondary network.~ 7-2 o
c/ Eu IOC
A
°/
tD 5(:
Strain at break
0
I
2
3
Strain
FIG 3 Stress-strain curves demonstrating the phenomenon o f stress-softening
slippage of the rubber network chains over the surface of the carbon black particles whilst Kraus et al 1o consider the effect to be the result of several mechanisms, thixotropy involving transient carbon black structures, rupture of network chains connecting filler particles and disruption of the 'permanent structure' of the carbon black. Harwood and Payne, however, in a series of recent papers. ~1-~ s claim that most of the stress-softening occurs in the gum phase of filler loaded vulcanisates leaving little to be explained in terms of breakage or slippage of rubber to filler bonds. The various theories will not be considered in great depth here as the whole phenomenon of stress-softening was the subject of a fairly comprehensive review recently. 1,16 A typical set of stress-strain cycles are shown in Fig 4 for NR gum and NR containing 60phr HAF carbon black. When the curves for the gum and filled rubbers are taken up to the same stress level, it is shown that the amount of stress-softening between the first and third extension cycles (curves 1 and 5) is remarkably similar. This figure shows that stress-softening effects in t'dled vulcanisates are not just a consequence of the filler in the rubber, but are due to the polymer phase of the composite. Similar types of curves are obtained in a non-crystallising rubber such as butadieneacrylonitrile (NBR) and the phenomenon has also been demonstrated in a whole range of other non-crystallising materials.l, 16 Stress softening can be recovered 13 in unfilled vulcanisates cured to produce crosslinks which do not break on
T. L. Smith 2~ and others =2 over recent years have developed the concept of a 'failure envelope' which is a unique curve peculiar to each polymer relating the stress at break (o8) or real stress at break oB(1 + c~) to the strain at break (eB) over a wide range of temperature and rate of strain. It was found by Harwood and Payne 1'23,24 that a quantitative relationship between the failure parameters could be obtained by using work done (or energy density) to break (UB) instead of stress at break. Fig 5a shows the energy input to break plotted against the strain at break for SBR containing increasing amounts of HAF carbon black. The energy input to break is multiplied by the ratio between the chosen reference temperature (294°K) and the experimental temperature to allow fi~r the temperature dependence of rubber-like elasticity as predicted by the kinetic theory. 2s It is found that as the temperature is increased, the failure points for the gum rubber move clockwise around an envelope passing through a maximum strain (en(max)) at a temperature of about 0 cC. The results for the filled rubbers lie along lines parallel to the gum rubber•
20C
NR+bOphr BLACK additional extension
NR GUM ] additional extcnsi~/
,4
150
5 ~'toc
I
I 2~j
6
6
~sc i
i
Stro,n
FIG 4 Stress-strain curves for gum NR and NR containing 60 phr HA F carbon black, showing similar stresssoftening behaviour (phr = parts per hundred)
C O M P O S I T E S June 1970
205
Hysteresis at break
IOC "-d
m
O; ?
r, I'O
i
O'1 OI
//7/ i
o SBR ® SBR • SBR • SBR
GUM 30HAF 60HAF 80HAF
i
I0 I0 Strain at break (t B)
/
/
/
i
I0 xE; B
FIG 5 Variation of reduced energy input to break with (a) strain at break, (b) strain at break corrected by hydrodynamic factor for SBR with O, 30, 60 and 80 phr HA F carbon black (phr = parts per hundred}
Mullins and Tobin 2 6 have shown that Young's modulus of carbon black filled rubber vulcanisates can be expressed in terms of the modulus of the unfilled rubber by use of an expression from hydrodynamic theory developed by Guth and Gold 2 7 relating the viscosity of a liquid containing hard spherical particles to that of the liquid alone X = 1 + 2'5c + 14.1c 2
1
where c is the volume concentration of filler. It was found by applying this factor X to the strain axis in Fig 5a the gum and filler loaded results coincided as shown in Fig 5b for strains up to en(max). The slope of the graph on the log scales was found to be 2 and hence the relationship between energy input and strain at break can be expressed as
2~-) UB = A (XeB) 2
2
where A is a constant. This type of relationship has also been found to hold both in other amorphous polymers such as NBR and also in crystalline polymers such as natural rubber. 2 Mullins and Tobin 26 have suggested that when the factor X is applied to filled vulcanisates, it takes account of both the disturbance of the strain distribution and the absence of deformation in that fraction of material composed of filler. It is shown in Fig 5b that the results at low temperatures do not coincide and this indicates that the hydrodynamic correction factor X is too large when the moduli of the filler and rubber approach parity.28 The bulk strains at break, therefore, of the filled and unfilled rubbers are related by a factor X under conditions of constant energy input. It must be remembered that when this type of agreement is obtained, the temperature of the filler loaded rubber is always higher than that of the unfilled rubber. Recent work has shown that it is possible to unify failure results from filled and unfilled rubbers of different cross-link densities 1, 2 by including in Equation 2 a crosslinking parameter from rubber elasticity theory.
206
COMPOSITES June 1970
When a viscoelastic material such as rubber is deformed by the application of a force, some of the mechanical work required is stored in the material and the remainder is dissipated as heat. This energy dissipation or hysteresis results from several different mechanisms which are dependent on the type of material and the experimental conditions. The inclusion of carbon black filler greatly modifies the hysteresial behaviour of the polymer. A measure of hysteresis in a material can be obtained from the area of the loop between the extension and retraction curves in a single tensile stress-strain cycle. The major sources of hysteresis in filled rubbers can be listed as follows 1 Viscoelasticity of the polymer; this becomes increasingly important as the temperature of test is lowered or the rate of straining is increased 2 9-31 2 Crystallisation of the polymer; this occurs in some rubbers (eg natural rubber) when they are stretched and the molecules align 3 ] 3 Changes in network configurations; this effect, known as stress-softening or Mullins effect, results in apparent softening of the rubber by a previous stre t ching 1-16 4 Changes in network structure; this results from the breakage of weak crosslinks or chains such as a polysulphide crosslink; the crosslinks may reform in the stretched state to produce the permanent set 17-2 0 5 Breakdown of carbon black or filler aggregates; this feature of filled rubbers occurs principally at low strains 3 2 - 3 6 The last three processes are discussed in other sections. Processes (1) and (2) are well documented and will not be discussed further in this review. It has been found recently that the strength of a rubber vulcanisate is not dependent on any individual contribution to the energy dissipation, but is dependent on the combined effect of all the processes which are applicable to the particular rubber and experimental conditions used. 1,2 3 , 3 4 The energy input to break (Us) is related to the hysteresis at break (HB) for amorphous polymers by the relationship
2_~__01/3Un = KH 2/3
3
where K is a constant. This is demonstrated in Fig 6 where log Un(294/T) is plotted against log Ha(294/T) for SBR containing increasing amounts of HAF carbon black. Equation 3 can be rewritten in terms of hysteresis ratio (h~) where
hB =HB/UB to yield
~)
UB =K3h~
IOC 0 SBR Gum t, SBR ÷30 Hal • SBR Haf 8
~o ~ o/-40°C
*60
o/.,~
• S BR + 8 0 Hof
O.OI
1
I lid
O,l
I I0 k
I
J"
K)O --
FIG 6 Variation o f reduced energy input to break with reduced hysteresis at break for SBR containing O, 30, 60 and 80 phr HA F carbon black (phr = parts per hundred)
conditions of rapid repeated deformations which are relatively small compared with their ultimate breaking strengths. Although ultimate values and fatigue tests are important as criteria of quality, they are of little or no assistance to the engineer, designer or technologist for anticipating the response of his products to the forces experienced in service. In order to take account of this problem a collection of technical and scientific methods, data and principles referred to as dynamic properties has been built up in recent years. There are several reviews and text books 2 9,37-42 dealing with the subject of dynamic properties in general, but there is little work done on the dynamic properties of rubber containing reinforcing fillers especially using amplitude of shear oscillation as a parameter. This is particularly surprising in view of the importance of being able to obtain data in the strain ranges applicable to service conditions in view of the fact that the normal tests such as tensile tests can provide completely misleading data about dynamic characteristics.3 3,3 6
Variation of dynamic modufi with strain The maximum value of Ua(294/T), therefore, occurs when all the work done in stretching the rubber is dissipated (hB = 1) and is, therefore, equal to K 3 . This is thought to occur at the glass transition temperature of the polymer. 23 The results for the filled rubbers are displaced from the gum rubber, the displacement increasing with increasing concentration of carbon black and it was found that by dividing the hysteresis by the factor X used in the previous section, the results 23'24 for the gum and filler-loaded rubbers coincided. Some disagreement is noticed at higher energies (low temperatures) and this is presumably due again to the hydrodynamic correction being too large when the moduli of the rubber and filler particle approach parity. The relationship therefore for gum and filler-loaded rubbers can be expressed as
(~/1/3
Ua
=
K
t--~--)2/3
5
Values of energy density and hysteresis measured before failure have been shown not to obey any of the above equations 23,=4 and hence equations have been found to represent an excellent description of failure between a lower limit at approximately 0'15 joules cm-3 when the material is essentially elastic to a higher limit about 60joulescm -3 for S B R when the material is totally hysteresial. Similar results have been shown to hold in a number of other amorphous polymers 23,24 and also to a limited extent 1'2 in natural rubber which crystallises on extension.
D YNAMIC PROPER TIES Some of the large volume uses of filled rubbers such as in tyres, belting and engine mounts employ the material under
The application of a low amplitude sinusoidal oscillation on a rubber containing filler results in the ensuing strain lagging behind the stress. This is because of the viscoelastic nature of rubber. The complex shear modulus (G*) of rubber can be expressed in terms of an elastic in-phase shear modulus (G 1) and a viscous out-of-phase shear modulus (G ~ 1) which for low amplitude sinusoidal strain are related by the following two equations G* = G 1 + iG 11
Tan 6 = G t
l/G1
6
7
where 6 is the phase angle between the sinusoidal stress and strain and tan 6 is a measure of the hysteresis exhibited by a rubber at low strains. The variation of these dynamic parameters for rubbers containing carbon black filler with strains up to about 10% in shear has been the subject of numerous papers by Payne.32-50 The in-phase shear modulus decreases with small sinusoidal strain oscillations as shown in Fig 7, for butyl rubber containing increasing concentrations of HAF carbon black. The effect increases with increasing concentration of the carbon black and is hence associated with properties of the filler particle. The in-phase shear modulus has been found to have a limiting value both at low strains (G ~o) and at high strains (G 1=) when there is no further change of modulus with increasing strain. G ~o for normal black loadings in the general purpose rubbers attains values up to some 2 orders of magnitude greater than the in-phase shear modulus of a gum vulcanisate whereas GI= for the same black loaded rubber is less than 1 order of magnitude greater than that of the gum vulcanisate. The difference between the two limits (G~0 G~=) which represents the change of dynamic modulus with strain is believed to be due to the structure breakdown of carbon black aggregates. The quantity G~= being the dynamic shear modulus when
COMPOSITES June 1970
207
I0 x
xv
X
,° i
23-2 x
'Ev 7
g ~"
6
OOOOI
,g
20-2
~
O'OI O'1 Double strain amplitude
I'O
FIG 9 Variation o f phase angle with double strain ampfitude for butyl rubber containing various concentrations up to 38"6% volume HA F carbon black
3
a
(3'OOI
~x
2 I •
0 0.0001
"",,x
0
i
i
O-OOI
0.01 011 Double strain amplitude
I'0
FIG 7 Variation o f in-phase shear modulus with double strain amplitude for butyl rubber containing various concentrations up to 23.2% volume HA F carbon black
Butyl 28.~¥o vol HAF
?
x
Effects of temperature
0
23"2J
o i
d
16.8 0
311 o 0001
o
all the carbon black structure has been broken down. This is higher than the gum modulus due to hydrodynamic effects plus other smaller contributions. The variation of out-of-phase shear modulus (G") with strain is shown in Fig 8 for a typical series of vulcanisates with increasing ffdler concentration up to 28-8% volume in butyl rubber. It is seen that the out-of-phase modulus passes through a maximum value in a region of strain where G 1 changes most rapidly. Figure 9 shows the variation of the phase angle 6 (phase angle between stress and strain) with the double strain amplitude. The results shown in Fig 9 indicate that (a)at very low strains, the phase angles of all vulcanisates are low and constant and although the phase angle increases with increasing carbon black content, it is small compared with the changes that occur at higher strains, (b) at strains above the yield point strain, the phase angle increases with increasing strain and passes through a maximum. With high concentrations of carbon black, the values of phase angle become very large in the order of 30 ° .
~i 9~J'°--°--c" ~ i ~ O0 &l Double strain amplitude
"
1.0
The effect of temperature on the structure breakdown curve is shown in Fig 10 for a natural rubber compound containing 32% by volume of HAF carbon black. An increase in temperature from 20°C to 90°C has a large effect on G I o and decreases its value by about 50%. On the other hand, however, at the end of the strain range GI= is virtually independent of temperature over the temperature range considered.
Effect of frequency FIG 8 Variation o f out-of-phase shear modulus with double strain amplitude for butyl rubber containing various concentrations up to 28"8% volume H A F carbon black
208
COMPOSITES June 1970
Little work has been carried out on the effect of frequency on the structure breakdown curve, this is presumably due to the difficulty of obtaining a wide enough frequency
range and a wide strain range together in one apparatus. Work by Warnaka 43 indicates that the frequency does not substantially alter the shape of the breakdown curve and this is confirmed in some results quoted by Payne 36 which are shown in Fig 11. The lack of a marked frequency effect is quite surprising and to date unaccounted for, unless the carbon black structure itself is a frequency independent structure, ie a purely elastic structure.
--•.
4 0 vol loading HAF black in butyl
25 'E u
~2o ?
o ×
15
Effect of type of carbon black E
Values of (]1 o have been found to be dependent markedly on the type of black used. Carbon black differs both in area of the surface and in nature and can approximately be divided into two main classes (a)channel blacks and (b) furnace and thermal blacks. Within these classes differing surface areas confer differing properties, but differences in the surface constitution also exist between the classes. To indicate how this affects dynamic properties, a series of compounds were prepared with a range of carbon blacks of differing particle size and type. The shear modulus of these compounds was measured at very low strains and the values obtained (G 1 o) are shown in Fig 12 plotted against the published figures for surface area which were determined by nitrogen adsorption measurements. 44 The plot shows that all the results for furnace blacks lie on a single curve together with lampblack. The thermal blacks MT and FT have the lowest values of all. Channel blacks lie well below the furnace blacks on another curve. Acetylene black is distinct in behaviour from all the other blacks as is well known from many of its properties. 4 s-47
\\\
z
s ~p~
g.
L
I
0"01 Ot Double strain amplitude
OOCI
I0
FIG 11 Variation of in-phase shear modulus with double strain amplitude as a function or frequency o f test for butyl rubber containing 40% volume HA F carbon black
IO jmSCF
B ~v
Effect o f dispersion of carbon black
.ss ~
SAF..jb~,<
6
8~
Boonstra and Medalia48 studied the effect of mixing times on the dispersion of the carbon black for an SBE rubber
containing IS A F carbon black. They showed that increasing the time of mixing improves the dispersion of the carbon
..~-
¢
~ L B , _. " - ' " HAg zw-(t -F ~INT FEF m..a~...-HMF
20
a
40
ISAF
.-~7~
~c~
. ' " ' " C C II~.S s:~'~¢'#! I"1"0 a l ss
419
//
.~-d~c
b..-l~pC "'"EPC
i
i
60
80
Nitrogen SA
1(50 1~) lhK) I(~D t1~ Z:~
Surface a r m [m2/g]
FIG 12 Variation of dynamic shear modulus (Glo) with surface area o f various carbon blacks determined by nitrogen adsorption o
of increasing temperature
Effect
>.
4O JO
.~x I O -
g o
Natural rubber HAF
E
o 0 u
0-001
~
n
001 0"1 Double strain amplitude
I'O
FIG 10 Variation o f in-phase shear modulus with double strain amplitude as a function of temperature for natural rubber containing 32% volume HA F carbon black
black as measured by optical and electron microscopy and counting techniques. Payne 49 using the same compounds as Boonstra and Medalia and with time of mixing varying from 1 to 16min showed how the dynamic modulus of the materials decreased with increasing mixing times. The graph is shown in Fig 13. The excellent colour photographs published by Boonstra and Medalia 4 s indicate extremely well how dispersion is improved with increasing mixing times and Payne concluded from his measurements on dynamic properties that the structure breakdown effect (Glo GI~) is solely attributed to the dispersion of the carbon black. He also
COMPOSITES June 1970
200)
and water, s 7 carbon black in oil s 8 and dendritic crystals of phenyl-beta-naphthylamine (PB N) in rubber.l' 59 Rheological studies 60-62 of clay-water systems show the existence of three dimensional or 'pack-of-cards' structures by the clay. It is presumably this three dimensional structure that is broken down by straining the samples. Similarly, the effect in carbon-black filled vulcanisates is attributed to the breakdown and reformation of the three dimensional or aggregated carbon black structure during the imposed oscillation. This has been confirmed in a recent paper by Payne and Whittaker 63 which demonstrates that the breakdown and reformation of the carbon black structure can be described by a domain theory similar to that found in magnetism.
6"0
~X~x
x
~'E 5.0 v >..
'0 x4-O
E 3.0
H-2 stage _2.© i c
A-ISmm B-2Omm C - 2 5 m,n
I0
D-3Qm,n
E F - 4 - 8 m,n G-16"O re,r,
OI 0'001
I
001 Double
i
OI s t r a i n amphtud¢
i
P0
FIG 13 Variation of in-phase shear modulus with double strain amplitude for SBR containing 70phr ISA F carbon blacks for various times of mixing from 1"5 to 16min
found that the increased mixing times reduced the out-ofphase (viscous) shear modulus and the phase angle at high strains. In parallel with the reduction in G 1o - G l ~ there is an enhancement of the ultimate tensile properties. This is accompanied as has been shown by Boonstra and Medalia with improvements in permanent set (reduced) heat build up (reduced), Mooney viscosity (reduced), DC resistivity (increased), abrasion loss (reduced), cut growth (improved), hardness (reduced) but with tear resistance and specific gravity remaining substantially the same. The correlation of the structure breakdown curve with dispersion of the filler within the mix has also been confirmed by Payne in conductivity measurements, s o This relationship is very important practically as other methods of measuring dispersion such as optical and electron microscopy, conductivity and counting techniques are difficult to use. The measurement of GIo-GI~ has subsequently been used in technological development work as a reliable index of dispersion in rubber filler composites.S 1,s 2
EL ECTRICA L PROPER TIES Rubber is usually regarded as an insulating material and has therefore been used for many electrical insulation purposes. It is made conductive 64'6s by the inclusion of adequate amounts of carbon black in the compounding formulation. Certain types of carbon black produce good values of conductivity at loadings even as low as 20 parts per weight per 100 parts of rubber. The actual mechanism of conductivity is still the subject of a considerable amount of discussion but both the particle size and the chain structure of the type of black used have an important effect. The
\
\
\
\
\ I
E u
I I
O
I
>
,,\
t_
\
J
\
\
\
\
Other effects A number of papers have considered the effect of special processing or compounding on the structure breakdown curve such as attrition of the carbon black, s 3-s 4 mixing technique and initial plasticity of the rubber, sl chemical promoters s2 and also change in the crosslinking of the vulcanisate.S s Similar breakdown curves have been shown in a number of other two phase systems such as clay in both rubber s 6
2|0
COMPOSITES June 1970
)1
I-0
I
I-2
I
I
1.4 I-6 Log weight Ofo carbon black
I
I'~
I
2-0
FIG 14 Variation of resistivity with carbon black loading for series of commercial carbon blacks in natural rubber 1. Spheron 9 2. Pelletex 3. Philblack A 4. Philblack 0 5. Philblack E (After Studebaker 6 6)
particles of most carbon blacks range in diameter from 10-400 millimicrons. The values of resistivity which are usually obtained 6s lie between 1 ohm/cm and i01 s ohm/cm. In the range 1 to 10 ~ ohm/cm, carbon black is the only conductive agent which has been used since it is the only material combining high conductivity with good mechanical properties and ease of manufacture. Very often a distinction is made between conductive and anti-static rubbers. The latter usually have a resistance above 105 ohms, but the resistivity giving this limit of resistance varies with the product. Therefore, there is really no point in distinguishing between these two types of rubber. The lower resistance limit is, however, required to prevent shock and fire risks resulting from any accidental contact with mains electricity. The important engineering applications of conductive or anti-static rubbers is in situations where the dissipation of dangerous, harmful or even merely annoying charge is required. The major use of non-insulating rubbers appears to have been developed in the 1930's when the surgical world became alarmed about the frequency of explosions in operating theatres. The elimination of static charges to reduce explosion hazards is obtained by using conducting rubbers, indeed a number of hospital products are specifically 'anti-static', ie they have a lower limit of resistance as well as an upper one. 6 s The conductivity of rubber depends on the amount, type and structure of the carbon black and on the mixing procedures used. Acetylene black was the forerunner of a wide range of carbon blacks. The types of black now used are mainly furnace blacks or conductive channel blacks. Fig 14 shows how the electrical resistivity in ohm/cm varies with the amount of carbon black included in the vulcanisate? 6 The curves are sigmoidal and change from an initial value of about 101 s ohm/cm (the value of the rubber without any carbon black) to an asymptotic value of between 1 to 1 000 ohm/cm at high loadings. The plasticity and the degree of breakdown of the rubber chains before the incorporation of carbon black affects the conductivity. The remilling of the mixed stocks, the effect of any heat treatment following the remilling, the incorporation of processing or dispersion aids also have appreciable effects on the resistivity of the product. Most rubber units in use are subjected to small amplitudes of oscillation or distortions and it is found that these small strains affect the conductivity in a similar manner to that described for the change in dynamic modulus with strain in a previous section.
5AF
14(2
12(:
I0£ c
Bo
///
0
< 40
2£ 0
0
m
I
I
I
I
I
I
I
I0
20
30
40
50
60
70
80
Loading, [ports
by wt per cent rubber]
FIG 15 Variation of abrasion resistance with filler content (parts by weight per cent rubber) for a range of different types of carbon black. (After Leigh-Dugmore 6 8)
interplay between these three factors, it is useful to consider them separately as is done later in this section. When the best reinforcement is required such as in tyre treads or conveyor belting, one of the following grades of black is generally used: S A F , I S A F , H A F , MPC, HPC o r EPC. The general trend of abrasion resistance (DunlopLambourn constant power test) with increased loading of these carbons 68 is shown in Fig 15. At any loading it decreases in the order in which they have been named above. The most important result of adding black to rubber is to improve the abrasion resistance by factors of five or six, or more. Of course, there is a limiting amount of filler that can be added as abrasion properties tend to fall away rapidly with carbon black contents above 50 to 60 parts by weight mixed with a hundred parts by weight of rubber.
Influence of strength Abrasion involves tearing, cut growth and ultimate rupture, ('7 and the paper has already discussed how ultimate tensile rupture is altered by the use of particulate fillers. Improvements in abrasion are paralleled by improvements in these three failure processes in particular tensile strength.
WEAR RESISTA NCE The inclusion of carbon black in the rubber formulation for a tyre-tread compound is absolutely necessary if reasonable wear resistance is to be obtained. The wear resistance of rubber is a complex quantity, but it is clear that three properties, those of stiffness, strength and hysteresis are very important, 67 and although there is considerable
Influence of stiffness When rubber is abraded without change of direction, sets of parallel ridges are often found on the surface of the samples at right angles to the direction of motion; these have been called abrasion patterns. Two relevant characteristics of
COMPOSITES
June
1970
2 | |
to normal driving), gives a ratio of abrasions proportional to the reciprocal value of the stiffness ratios. Thus the increase in stiffness produced by fillers in a rubber leads once again to a reduction in abrasion losses.
wheel
Influence of hysteresis
c
--~
$
o, crab ,k
FIG 16 Reaction of a rolling wheel against a constant side force S (infinite coefficient of friction): B, C and D show the direction of travel. (From A. Schallamach, Rev. Gen. Caoutchouc, 36, 1480 (1959) Fig 1)
abrasion patterns are that their intensity increases with, firstly, increasing coarseness of the track and, secondly, with decreasing stiffness of the compound as the softer the compound the easier it is for the part of the abraded rubber surface to be gripped and elongated in the direction of abrasion. A third, and possibly the most important characteristic is that they increase the rate of abrasion. 67,09 The inclusion of carbon black in a compound prevents abrasion patterns being formed to the extent they form in unfilled rubbers. With a normal tyre-tread compound (containing about 5 0 - 6 0 parts per hundred of rubber of reinforcing black) a pattern only develops after the surface of rubber has been considerably softened by repeated prestressings. This softening, however, affects only the surface layer, and the main stiffness of the rubber is still contributed by the hardly-deformed inner bulk of the tread.7 o The amount of energy which is available for the abrasion of a slipping wheel depends on the degree of slip, and the stiffness and resilience of the compound. The wear A per unit distance of travel is given by: A = 27pF 2/ka2
COMPOSITES June 1970
,120
p
,so
6p
p
,c
Sliding abrasionx r~silience
~
E
,,,Io
'p
~p
FI[~
BO ~
-90
8
where 7 is the proportionality constant between abrasion and energy dissipation and is termed the abradability, p is the resilience of the wheel, F is the reaction force in the area of contact, k is a stiffness constant defined by tangential stress per unit length of contact, and a is the length of area of contact. 7~,72 The importance of this equation is that it draws attention to the parts which stiffness and hysteresis have to play in wear processes. The abrasion ranking of two samples differing in dynamic stiffness and compared at equal force (a condition relevant
2]2
The influence of hysteresis in Equation 8 on the abrasion process arises from the fact that road wear is largely due to the sliding of the rubber over the road surface in the rear of the area of contact. This sliding occurs during the recovery of the deformation of the tyre surface imposed by contact with the road surface. With more hysteresial rubbers the recovery is less complete, and thus the amount of sliding and wear is proportionately less. 73 Fig 16 is an illustration of the reaction of a rolling wheel against a constant side force (crab walk), and Fig 17 illustrates well how the theoretically-predicted and previously-unsuspected importance of hysteresis in the wear process of slipping wheels can make sense of the experimental results when the product of sliding abrasion and resilience is plotted against crab walk abrasion. Resilience is a measure of the hysteresis of a rubber and is small when the hysteresis is high 72 Earlier in this paper, it was shown that the hysteresis of a vulcanisate increases with carbon-black content. It also increases with decreasing particle size. 7~ Provided other quantities determining the abradability of a rubber are not adversely affected, this greater hysteresis will result in less wear. It has, in fact, been found that the increased wear resistance of tyre treads containing furnace blacks of finer particle size is due not to a change in abradability but due to an increase in hysteresis. There are practical limits to how much hysteresis can be compounded into a vulcanisate, as too high a concentration
IIA
.6 ~ -50
I.0
FIG 17 Variation of abrasion in crab walk at 15° slip angle with the product of sliding abrasion and resilience. (From A. Schallamach, Wear, 3, 21 (1960), Fig 16)
of filler reduces tensile strength, 73 and leads to a large temperature rise in the rubber under flexing conditions which can cause premature failure. Improvement in wear rating by increasing hysteresis has, however, to be paid for by increased fuel consumption.
l0
Kraus, G., Childers, C. W. and Rollman, K. W., J Appl Poly Sci 10, 229, (1966)
11 Harwood, J. A. C., Mullins, L. and Payne, A. R . , J A p p l Poly Sci 9, 3011 (1965) 12 Harwood, J. A. C. and Payne, A. R . , J A p p l P o l y Sci, 10, 315 (1966)
CONCLUSION
This paper has been, of necessity, a review of only a few of the many relevant topics concerning the complex composite material formed from rubber and filler. It has not been possible to discuss in great depth the subtle interplay of surface properties of the black and the effect of particle size and of the tendency of fillers to agglomerate on all the physical properties discussed in this chapter. Other properties such as tear resistance and the rheological properties of extrusion and flow have been completely omitted. It is obvious that the excellent enhancement of rubber properties brought about by compounding rubber with fillers is little understood, and there is still much basic research needed before we can even begin to understand how it performs its task so admirably. Recent work on 1' 24 analysis of tensile stress strain curves has indicated that there exists a shell of immobilised rubber around the filler particles. This work is in agreement with the findings of several other studies ~4-78 and it is thought that this immobilised layer gives rise to the perturbed relaxation behaviour of the rubber matrix which has been discussed quantitatively in the work of Radok and co-workers 79. The perturbed relaxation behaviour of the matrix then causes the enhancement of tensile strength, and abrasion resistance.
REFERENCES
l Harwood, J. A. C., Payne, A. R. and Whittaker, R. E., Paper presented to IRI Conference 'Advances in Polymer Blends and Reinforcement', Loughborough (1969) 2 Harwood, J. A. C., Payne, A. R. and Whittaker, R. E., A waiting publication in J Appl Poly Sei
13 Harwood, J. A. C. and Payne, A. R., J ApplPoly Sci, i0, 1203 (1966) 14 Harwood, J. A. C. and Payne, A. R., Trans IRL 42, T14 (1966) 15 Harwood, J. A. C. and Payne, A. R., J Appl Poly Sci, I1, 1825 (1967) 16 Harwood, J. A. C., Mullins, L. and Payne, A. R., J of IRI, 1, 17 (1967) 17 Brucksch, W. F., Rubb Chem Tech, 35, 453 (1962) 18 Brucksch, W. F., Rubb Chem Tech, 36, 975 (1963) 19 Bateman, L., et al, Chapter 19 in 'Chemistry and Physics of Rubber-like Substances' Edited by L. Bateman, Maelaren, London (1963) 20 Brown, H. P., Rubb Chem Tech, 36, 931 (1963) 21 Smith, T. L.,JAppIPhys, 35, 27 (1964) 22 Kelley, F. N.,ApplPoly Symp, No 1,229 (1965) 23 Harwood, J. A. C. and Payne, A. R., J Appl Poly Sci, 12, 889 (1968) 24 Harwood, J. A. C., Payne, A. R. and Smith, J. F., Kaut u Gummi Kunst, 22, 548 (1969) 25 Ferry, J. D., Fitzgerald, E. R., Grandine, L. D. and Williams, M. L., Ind Eng Chem, 44, 703 {1952) 26 Mullins, L. and Tobin, N. R., J Appl Poly Sci, 9, 2993 (1965) 27 Guth, E. and Gold, O., Phys Rev, 53, 322 (1938)
3 Bouasse, H. and Carrier6, Z., Annales de la Faculte des Sciences de Toulouse, 5, 257 (1903)
28 Landel, R. F., Trans Soc Rheol, 2, 53, (1958)
4 Mullins, L . , J R u b b Res, 16, 275 [1947)
29 Ferry, J. D., 'Viscoelastic Properties of Polymers', Wiley (1961)
5 Mullins, L., J Phys and Colloid Chem, 54, 239, (1950) 6 Dannenberg, E. M., Trans IRL 42, T26 (1966)
30 Grosch, K. A. and Mullins, L., Rev Gen Caouteh, 39, 1 781 (1962)
7 Boonstra, B. B., Chapter 16 in 'Reinforcement of Elastomers', Edited by G. Kraus, Interseience (1965)
31 Harwood, J. A. C. and Schallamach, A., J Appl Poly Sci, 11, 1835 (1967}
8 Dannenberg, E. M. and Brennan, J. J., Paper No 22, ACS Div Rubb Chem, Philadelphia (1965)
32 Payne, A. R . , J ApplPoly Sei, 6, 57 (1962) 33 Payne, A. R . , J A p p l P o l y Sci, 7, 873 (1963)
9 Brennan, J. J., Jermyn, T. E. and Perdagio, M. F., Paper No 36, ACS Div Rubb Chem, Detroit (1964)
34 Payne, A. R . , J Appl Poly Sci, 8, 2661 (1964)
COMPOSITES June 1970
213
35 Payne, A. R., Trans IRL 40, T135 (1964)
57 Payne, A. R. and Whittaker, R. E., Rheol Acta, 9, 91 (1970)
36 Payne, A. R., Chapter 3 in 'Reinforcement of Elastomers', Edited by G. Kraus, Interscience (1965)
58 Payne, A. R., J Colloid Sci, 19, 744 (1964)
37 German, S. D., Rubb Chem Tech, 30, 1202 (1967)
59 Payne, A. R . , J A p p l P o l y Sci 11,383 (1967}
38 Bueche, F., 'Physical Properties of Polymers,' Wiley (1962)
60 Astbury, N. F., Proc Royal Soc, A258, 27 (1960) 61 Moore, F.,J Sci lnstr, 40, 228 (1963)
39 Tobolsky, A. V., 'Properties and Structure of Polymers', Wiley (1960) 40 Scott, J. R. and Payne, A. R., 'Engineering Design with Rubber', Maclaren (1960) 41 Davey, A. B. and Payne, A. R., 'Rubber in Engineering Practice,' Maclaren (1965) 42 Lord, P. and Wetton, R. E., 'Progress in Non-Destructive Testing', Volume 3, Heywood (1961) 43 Warnaka, G. E., ASME Rubber and Plastics Div, New York, Preprint 62-WA:323 (November, 1962)
62 Astbury, N. F. and Moore, F., Mat Res Std, 5, 178 (1965) 63 Payne, A. R. and Whittaker, R. E., Awaiting publication in Trans Farad Soc 64 Payne, A. R. and Whittaker, R. E., Chapter 4 in Design Engineering Series 'Rubbers' Edited by L. Mernagh, Morgan-Gram pian (1969) 65 Norman, R. H., 'Conductive Rubbers, its production, application and test methods', Maclaren (1957) 66 Studebaker, M. L., India Rubb World, 129, 485 (1954)
44 Gessler, A. M., Rubb Age, 88, 658 (1961) 67 Schallamach, A., Proc Phys Soc, 67B, 883 (1954) 45
Studebaker, M. L., Rubb Chem Tech, 30, 1400 (1957)
46 Leigh-Dugmore, C. H., 'Microscopy of Rubber', Heifer (1961) 47 Parkinson, D., 'Reinforcement of Rubber', Lakeman (1957)
68 Leigh-Dugmore, C. H. in 'The Applied Science of Rubber' Edited by W. J. S. Naunton, Edward ArnoM, p 475 (1961) 69 Schallamach, A., Trans IRL 28, 256 (1952) 70 Schallamach, A., Wear, 1,384 (1958)
48 Boonstra, B. B. and Medalia, A. I., Rubb Age, 92, 892 (1963) 49 Payne, A. R.,J Appl Poly Sei, 9, 2273 (1965) 50 Payne, A. R . , J A p p l P o l y Sci, 9, 1073 (1965) 51 Barker, L. R., Payne, A. R. and Smith, J. F., J o f l R L 1,206 (1967) 52 Payne, A. R., Swift, P. M. and Wheelans, M. A., J of RRIM, 22, 275 (1969) 53 Gessler, A. M.,Rubb Age, 87, 64 (1960) 54 Gessler, A. M. and Payne, A. R., J Appl Poly Sci, 7, 1815 (1963)
71 Schallamach, A., Rubb Chem Tech, 33, 857 (1960) 72 Schallamach, A., 'The Chemistry and Physics of Rubber-like Substances', Edited by L. Bateman, Maclaren (1964) 73 Studebaker, M. L., Rubb Chem Tech, 30, 1400 (1957) 74 Hess, W. M., private communication 75 Westlinning, H., Kaut U Gummi Kunst, 15, 475 (1962) 76 Schoon, T. G. F. and Alder, K., Kaut U Gummi Kunst, 19, 414 (1966) 77 Grosch, K. A . , J Appl Poly Sci, 12, 915 (1968)
55 Payne, A. R., Smith, J. F. and Whittaker, R. E., to be published
78 Smith, P. P. A.,Rheol Acta, 5, 277 (1966)
56 Payne, A. R. and Whittaker, R. E., Rheol Acta, 9, 97 (1970)
79 Radok, J. R. M. and Tai, C. L . , J A p p l P o l y Sci, 6, 518 (1962)
2]4
COMPOSITES June 1970