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Cure meters and their problems
PolymerTesting 1 (1980) 247-257
CURE
METERS
AND
THEIR
PROBLEMS
R. H. NORMAN
Rubber And Plastics Research Association o[ Great Britain, Shawbury, Shrewsbury, Salop S Y 4 4NR, UK
S UMMAR Y
The rate of cure of a rubber mix needs to be known for development and for quality control. This paper discusses the meaning of 'rate of cure', the methods of calculating it from cure-meter measurements and some of the problems involved, in particular the rate of attainment of the required temperature in the test piece.
1.
INTRODUCTION
In the introduction of a new rubber mix into production, one of the first questions is 'How long will it take t o ' c u r e ? ' In the manufacture of repeated batches of a compound a quality control test is required to ensure that the material will cure adequately under the scheduled cure conditions. For these purposes a number of instruments have been developed and this paper outlines the various types and discusses methods of interpretation of the results and some problems associated with the measurements, with specific reference to the Monsanto R h e o m e t e r and the Wallace-Shawbury Curometer.
2.
WHAT IS MEANT BY 'RATE OF CURE'?
If vulcanisation involved only a single type of reaction which proceeded at a constant rate at a given temperature, this question would be simpler to answer and probably everyone would give the same answer. However, curing is a collection of various reactions which take place, some sequentially and some 247 Polymer Testing 0142-9418/80/0001-0247/$02.25 © Applied Science Publishers Ltd, England, 1980 Printed in Northern Ireland
248
R.H. NORMAN
concurrently, each of which starts rapidly and then gradually reduces in rate as the reactants are used up. This collection of reactions generally results in a change of 'stiffness' which starts slowly and then, after a certain scorch period, proceeds rapidly, followed by a gradual deceleration until a constant stiffness is reached or a further reaction (reversion), which reduces the stiffness, sets in. With some compounds, a 'marching modulus cure' occurs and the stiffness continues to rise slowly for a very long time (longer than we are prepared to continue a measurement). I have chosen to illustrate the changes taking place in terms of stiffness, but changes in many other properties are also taking place. What, logically, should we take as an indication of the state of cure required of the product? Ultimately this will depend upon the use of the product. The required state of cure is that which gives the required collection of physical and chemical properties for satisfactory product performance and life. D o we need a door stop to have a high tensile strength, low creep and low permanent set? No; it must have medium hardness, not too high a resilience and adequate resistance to the environment, none of which demands a very high degree of cure. A moderate degree of cure is probably adequate. Ask the same question about a bridge bearing and the answer will be very different. The required state of cure is that which imparts to the product all those properties which are important in service and nothing more. The ultimate question is therefore 'Given that the compound and the product design are right, what are the most economic curing conditions which will result in a product which will have all the physical and chemical properties which are appropriate to the application?' Up to the middle of the century, it was usual to assess optimum cure conditions by curing flabs for various times and carrying out tensile tests and hardness measurements. However, with these and many other (sometimes more relevant) tests, the between-test-piece variability of the results was such that a large error in assessing cure time could easily occur, 1 and there was considered to be a need for a method of test which could be made continuously on a single test piece of rubber while it was being cured. The chosen measurement was of modulus at the curing temperature. Since that time, at least six different 'cure meters' have been developed to make such measure-, merits and two are in common use. These machines measure a property (stiffness) which is approximately proportional to a shear modulus, and the end point is considered as the time in which the change (from the stiffness of the uncured material to the stiffness of the cured material) is, say, 90 or 95% complete. The technologist will decide upon the appropriate figure, according to anticipated service requirements, on the basis of his background experience or from other test results. A figure of 90% is generally used, but 95% may be more appropriate if low set is important.
CURE METERS AND THEIR PROBLEMS
249
Since the hot modulus is approximately proportional to the effective crosslink density, the term '90 (or 95)% crosslink cure' is often used. In the case of two materials having different scorch times but the same time measured from the beginning of the heating cycle to the 90% crosslink cure time it is obvious that the cure reaction is proceeding faster in the latter stages for the material with the longer scorch time. It has, however, in the past been adequate to ignore this fact in carrying out estimates of cure cycles and to take the reciprocal of the total time to, say, 90% crosslink cure as the cure rate. 3.
HOW DO WE MEASURE STIFFNESS DURING CURING?
I use the term 'stiffness' rather than modulus because (a) the geometry of cure meters generally does not permit the precise calculation of a modulus and (b) viscous, in addition to elastic, responses are involved. We do not need to know an absolute stiffness. Any measured parameter (e.g. pen movement) proportional to stiffness is adequate. Basically the various cure meters consist of heated cavities or pairs of plattens in (or between) which the test piece can be strained approximately sinusoidally, the deformation amplitude being held approximately constant and the force amplitude measured or vice versa. The instruments can be classified in three ways. The first classification is by the type of oscillation: 1. 2.
linear, rotary.
The second is by which variable is held substantially constant: (a) (b)
force (or torque) amplitude, deformation amplitude.
The third method of classification is between instruments with and without paddles (or rotors): (i) instruments with a paddle (or rotor), (ii) rotorless (and paddleless) instruments. In the latter, the two halves of the heating cavity move relative to each other to strain the test piece. Using these classifications three of the instruments may be classified as Wallace-Shawbury Curometer Monsanto Rheometer Elastograph 2
l/a/i 2/b/i 2/b/ii
The assessment of the 90% cure time from the Rheometer trace (Fig. 1) is relatively simple since the trace height is proportional to torque and therefore very nearly proportional to stiffness. Thus the 90% crosslink cure time (tgo) is
250
R . H . NOR/vlAN
I I
O" 0 I--
I I
09 (M"I /
i M.
j. o Usual curve , I
I
,
I
Time Fig. 1.
----- Reversion . . . . Marching modulu
)
Rheometer trace.
found where the curve crosses the M90 line, M90 being found from M90-
ML = 0 " 9 ( M H - ML)
It may be noted that the lowest point of the curve ML is taken as representing the initial stiffness. The start of the curve is always higher due to the effect of the heating time delay. Analysis of the Curometer curve is slightly more complex, since the curve (Fig. 2) represents the amplitude of motion for a nearly constant force amplitude. The percentage crosslinking is here given I by 100 ( 1 - z(t)), where 1~ z (t) = a ~
1
aao
1 a,
1
a0
where at is the trace height at time t. The difference between the methods of calculation for the two instruments tends to give the impression that two completely different measurements are being made, but this is not so. The apparent difference arises from the fact that
251
CURE M E T E R S A N D T H E I R P R O B L E M S
Usuat curve Reversion Marching modulus
u~ >,
R
~
---
Time
a00
.~ Fig. 2.
Curometer trace.
in one case the force is being measured at constant deformation amplitude and in the other the reverse, and that z = 1 for the uncured material and zero at full cure. Thus the 90% crosslink cure corresponds to z = 0.1.
4.
P R O B L E M S IN T H E USE O F C U R E M E T E R S
T h e cure meters in c o m m o n use are excellent for quality control purposes although at high t e m p e r a t u r e s (very short times) they may b e c o m e rather insensitive. However, for the purpose of cure cycle design, they have four problems: (a)
(b) (c) (d)
T h e heating of the test piece is not instantaneous and even when a steady state is reached, the test piece may not be at a uniform temperature. Occasionally the r u b b e r may slip over the paddle or rotor or in the cavity. S o m e materials develop porosity during test. No satisfactory way has been developed to determine cure rate on a material which exhibits a marching modulus.
252
R.H. NORMAN
The first problem is significant at all temperatures but can result in especially misleading results for times of the order of a minute (see Section 4.1) and the magnitude of the error is a function of instrument design. The second (Section 4.2) and third (Section 4.3) problems are only occasionally encountered and may be minimised by instrument design. The fourth problem (Section 4.4) is fundamental to the type of measurement and cannot be ameliorated by instrument design.
4.1. Rate of attainment of temperature All instruments have a heated cavity whose temperature alters little as the result of insertion of a test piece. Most instruments have a paddle or rotor which is initially cold, or cools during insertion of the test piece, and is then heated by conduction of heat through the sample from the cavity. Heat is lost continuously by conduction and radiation from the paddle or rotor during the test, with the result that the paddle or rotor is always cooler than the cavity. In the Mark VI Curometer the paddle is somewhat heat-insulated from the rest of the instrument. In the Rheometer, the stem of the rotor is in contact with bearings which are warm, though presumably not at platten temperature. Even if the paddle or rotor is heated (or absent as in rotorless instruments), there is still a time lag before the sample is at a uniform temperature. An early Vulkameter used a paddle heated to the platten temperature, but it used a thick test piece which required several minutes to heat through. Consider an ideal R h e o m e t e r with a rotor temperature equal to that of the cavity. The test piece is 2.2 mm thick between the edge of the rotor and the flat of the cavity. The centre of this thick portion would still take about 20 s to heat to within 5 °C of a platten temperature of 170 °C. A corresponding calculation for the thin (0.5 mm) web of rubber between the paddle and platten of a C u r o m e t e r gives about I s. With a rotorless cure meter, the total heat-up time is only that to heat the rubber. If one assumes that the stiffness of the test piece is controlled substantially by the very thin layer at the edges (see also Section 4.3), then the heating time for, say, a 0-05 mm clearance is measured in fractions of a second. If, however, as is implied in a paper by Gibttfert2 the torque in the Elastograph is controlled primarily by shear in the bulk of the test piece (4.3 mm maximum thickness) the heating time is about I min. The errors due to slow and incomplete temperature uniformity have been assessed for the Curometer and for the R h e o m e t e r with a micro-die. For comparison the degree of cure has been assessed by swelling measurements on 0-5 mm thick test pieces which have been cured substantially isothermally in a rapidly operating press. The degree of cure in the isothermal case has been taken as the reciprocal of the equilibrium swollen modulus. T h e difference between this measure of the
253
CURE METERS AND THEIR PROBLEMS TABLE 1 90% CURETIMES (S) MEASUREDIN THREE WAYS
Temperature (°C)
Isothermal
Curometer
Rheometer
120 140 160 180 200
3120 870 280 7'2 17
4800 1100 320 105 47
7800 1800 460 195 97
degree of crosslinking and that given by the Florey Rehner equation is small and cannot alter the general picture given by the results, which are given in Table 1. It will be seen that the Rheometer can give very much longer 90% cure times than the Curometer and also that the Curometer gives longer times than the isothermal results. The large discrepancies at lower temperatures are surprising and must represent a considerable difference between the true average temperature and the measured temperature even after very long times. The very large differences at the high temperatures are not unexpected. It was thought that part of the difference between the isothermal results and the cure meter results might lie in the difference between the assessment of cure time from hot modulus and that from reciprocal swelling. Samples were therefore cured for various times in the Curometer and then quenched and the degree of cure assessed by reciprocal swelling. With samples thus cured under identical conditions the 90% cure time assessed by swelling was 40% greater than that from the hot modulus results. Thus the results in Table 1 underestimate the errors due to poor heat transfer. It is obvious that there are large errors in measuring cure times, due to poor temperature equilibration both for long and short cure times. 4.2. Slippage This has been observed particularly with nitrile rubbers. Occasionally Curometer traces show the wine glass effect (Fig. 3) where a catastrophic slippage has occurred after the material is nearly cured. A less marked case of slippage may show itself as a change in slope of the plot of z on a logarithmic scale against time (Fig. 4). Slippage has also been observed by cutting open Curometer pellets, when it is manifest by the elliptical shape of the rubber which has moulded through round holes in the paddle. It is often thought that this can be overcome by enclosing the sample in a cavity and applying pressure by closing the cavity. The pressure may however be inadequate initially and shrinkage during cure may decrease it. Some rotorless cure meters sometimes attempt to overcome this problem 2 by
254
R.H. N O ~ n N
Time
Fig. 3.
Wine glass effect observed on an early Curometer. The MK VI gives only the envelope of the half trace.
Common curve
Slipping
0.4 Z 0.2
0.1
0.06 I
I
I
Time
Fig. 4.
)
z-Plots.
255
CURE METERS AND THEIR PROBLEMS
continuous pressure application, but I have no information on the efficacy of this. 4.3. Porosity T h e presence of air or m o r e particularly water in a r u b b e r may cause the r u b b e r to b e c o m e extremely porous when heated. It has been reported f r o m time to time that the C u r o m e t e r produced curious and variable curves when used on such a mixing. Normally a simple check on the a p p e a r a n c e of the test piece on its removal f r o m the machine will indicate whether one has a porosity problem. T o assess whether porosity really could be a problem, a clay-loaded natural r u b b e r was tested in a C u r o m e t e r under two conditions: (a) at atmospheric pressure and (b) in an autoclave at 0-55 M P a air pressure. T h e results are shown in Fig. 5. These cannot be c o m p a r e d with normal C u r o m e t e r traces because the sample was inserted at r o o m t e m p e r a t u r e to avoid 'blowing' before the pressure was applied. T h e t e m p e r a t u r e was raised to 140 °C as rapidly as
Am t osphepcri ci) .c ==
r.n 0.55
MPa
Time
Fig. 5.
Effect of porosity.
256
R.H. NORMAN
Fig. 6. Porous samples.
the platten heaters would permit. Admittedly the material chosen was extreme, as the samples from the atmospheric pressure test show (Fig. 6), but this does indicate that porosity can be a very serious problem. With most mixes, however, no porosity of a test piece is evident. Closed-cavity type instruments probably remove the problem of porosity in all but extreme cases, while rotorless systems probably still suffer from it. 4.4. Marching modulus The interpretation of cure traces is sometimes complicated by a 'marching modulus' where, for example, the Rheometer trace height continues to increase slowly over a period much greater than that in which the majority of the change takes place, and the Curometer trace becomes progressively narrower in a corresponding way. In this case the value of a~ or Mn becomes meaningless unless interpreted as the value measured after a fixed arbitrary time. Technologists at RAPRA have generally adopted 1 h for the arbitrary time. It is tempting to use the crossing of the estimated asymptote and the line through the steepest part of a Rheometer trace, but this can give a value of tgo at which other physical properties are not well developed. 5.
CONCLUSIONS
While existing cure meters are adequate for quality control purposes, they present several problems when they are used for other than comparative measurements.
CURE METERS AND THEIR PROBLEMS REFERENCES 1.
2.
HICKMAN, J. A., NORMAN, R. H. and PAYNE, A. R. (1965). Rubber World, 152 (2), 76. G~I'H-P-RT, O. (1976). Kaut. u Gummi, 29 (5), 261; 29 (6), 341.
257
CURE
METERS
AND
THEIR
PROBLEMS
R. H. NORMAN
Rubber And Plastics Research Association o[ Great Britain, Shawbury, Shrewsbury, Salop S Y 4 4NR, UK
S UMMAR Y
The rate of cure of a rubber mix needs to be known for development and for quality control. This paper discusses the meaning of 'rate of cure', the methods of calculating it from cure-meter measurements and some of the problems involved, in particular the rate of attainment of the required temperature in the test piece.
1.
INTRODUCTION
In the introduction of a new rubber mix into production, one of the first questions is 'How long will it take t o ' c u r e ? ' In the manufacture of repeated batches of a compound a quality control test is required to ensure that the material will cure adequately under the scheduled cure conditions. For these purposes a number of instruments have been developed and this paper outlines the various types and discusses methods of interpretation of the results and some problems associated with the measurements, with specific reference to the Monsanto R h e o m e t e r and the Wallace-Shawbury Curometer.
2.
WHAT IS MEANT BY 'RATE OF CURE'?
If vulcanisation involved only a single type of reaction which proceeded at a constant rate at a given temperature, this question would be simpler to answer and probably everyone would give the same answer. However, curing is a collection of various reactions which take place, some sequentially and some 247 Polymer Testing 0142-9418/80/0001-0247/$02.25 © Applied Science Publishers Ltd, England, 1980 Printed in Northern Ireland
248
R.H. NORMAN
concurrently, each of which starts rapidly and then gradually reduces in rate as the reactants are used up. This collection of reactions generally results in a change of 'stiffness' which starts slowly and then, after a certain scorch period, proceeds rapidly, followed by a gradual deceleration until a constant stiffness is reached or a further reaction (reversion), which reduces the stiffness, sets in. With some compounds, a 'marching modulus cure' occurs and the stiffness continues to rise slowly for a very long time (longer than we are prepared to continue a measurement). I have chosen to illustrate the changes taking place in terms of stiffness, but changes in many other properties are also taking place. What, logically, should we take as an indication of the state of cure required of the product? Ultimately this will depend upon the use of the product. The required state of cure is that which gives the required collection of physical and chemical properties for satisfactory product performance and life. D o we need a door stop to have a high tensile strength, low creep and low permanent set? No; it must have medium hardness, not too high a resilience and adequate resistance to the environment, none of which demands a very high degree of cure. A moderate degree of cure is probably adequate. Ask the same question about a bridge bearing and the answer will be very different. The required state of cure is that which imparts to the product all those properties which are important in service and nothing more. The ultimate question is therefore 'Given that the compound and the product design are right, what are the most economic curing conditions which will result in a product which will have all the physical and chemical properties which are appropriate to the application?' Up to the middle of the century, it was usual to assess optimum cure conditions by curing flabs for various times and carrying out tensile tests and hardness measurements. However, with these and many other (sometimes more relevant) tests, the between-test-piece variability of the results was such that a large error in assessing cure time could easily occur, 1 and there was considered to be a need for a method of test which could be made continuously on a single test piece of rubber while it was being cured. The chosen measurement was of modulus at the curing temperature. Since that time, at least six different 'cure meters' have been developed to make such measure-, merits and two are in common use. These machines measure a property (stiffness) which is approximately proportional to a shear modulus, and the end point is considered as the time in which the change (from the stiffness of the uncured material to the stiffness of the cured material) is, say, 90 or 95% complete. The technologist will decide upon the appropriate figure, according to anticipated service requirements, on the basis of his background experience or from other test results. A figure of 90% is generally used, but 95% may be more appropriate if low set is important.
CURE METERS AND THEIR PROBLEMS
249
Since the hot modulus is approximately proportional to the effective crosslink density, the term '90 (or 95)% crosslink cure' is often used. In the case of two materials having different scorch times but the same time measured from the beginning of the heating cycle to the 90% crosslink cure time it is obvious that the cure reaction is proceeding faster in the latter stages for the material with the longer scorch time. It has, however, in the past been adequate to ignore this fact in carrying out estimates of cure cycles and to take the reciprocal of the total time to, say, 90% crosslink cure as the cure rate. 3.
HOW DO WE MEASURE STIFFNESS DURING CURING?
I use the term 'stiffness' rather than modulus because (a) the geometry of cure meters generally does not permit the precise calculation of a modulus and (b) viscous, in addition to elastic, responses are involved. We do not need to know an absolute stiffness. Any measured parameter (e.g. pen movement) proportional to stiffness is adequate. Basically the various cure meters consist of heated cavities or pairs of plattens in (or between) which the test piece can be strained approximately sinusoidally, the deformation amplitude being held approximately constant and the force amplitude measured or vice versa. The instruments can be classified in three ways. The first classification is by the type of oscillation: 1. 2.
linear, rotary.
The second is by which variable is held substantially constant: (a) (b)
force (or torque) amplitude, deformation amplitude.
The third method of classification is between instruments with and without paddles (or rotors): (i) instruments with a paddle (or rotor), (ii) rotorless (and paddleless) instruments. In the latter, the two halves of the heating cavity move relative to each other to strain the test piece. Using these classifications three of the instruments may be classified as Wallace-Shawbury Curometer Monsanto Rheometer Elastograph 2
l/a/i 2/b/i 2/b/ii
The assessment of the 90% cure time from the Rheometer trace (Fig. 1) is relatively simple since the trace height is proportional to torque and therefore very nearly proportional to stiffness. Thus the 90% crosslink cure time (tgo) is
250
R . H . NOR/vlAN
I I
O" 0 I--
I I
09 (M"I /
i M.
j. o Usual curve , I
I
,
I
Time Fig. 1.
----- Reversion . . . . Marching modulu
)
Rheometer trace.
found where the curve crosses the M90 line, M90 being found from M90-
ML = 0 " 9 ( M H - ML)
It may be noted that the lowest point of the curve ML is taken as representing the initial stiffness. The start of the curve is always higher due to the effect of the heating time delay. Analysis of the Curometer curve is slightly more complex, since the curve (Fig. 2) represents the amplitude of motion for a nearly constant force amplitude. The percentage crosslinking is here given I by 100 ( 1 - z(t)), where 1~ z (t) = a ~
1
aao
1 a,
1
a0
where at is the trace height at time t. The difference between the methods of calculation for the two instruments tends to give the impression that two completely different measurements are being made, but this is not so. The apparent difference arises from the fact that
251
CURE M E T E R S A N D T H E I R P R O B L E M S
Usuat curve Reversion Marching modulus
u~ >,
R
~
---
Time
a00
.~ Fig. 2.
Curometer trace.
in one case the force is being measured at constant deformation amplitude and in the other the reverse, and that z = 1 for the uncured material and zero at full cure. Thus the 90% crosslink cure corresponds to z = 0.1.
4.
P R O B L E M S IN T H E USE O F C U R E M E T E R S
T h e cure meters in c o m m o n use are excellent for quality control purposes although at high t e m p e r a t u r e s (very short times) they may b e c o m e rather insensitive. However, for the purpose of cure cycle design, they have four problems: (a)
(b) (c) (d)
T h e heating of the test piece is not instantaneous and even when a steady state is reached, the test piece may not be at a uniform temperature. Occasionally the r u b b e r may slip over the paddle or rotor or in the cavity. S o m e materials develop porosity during test. No satisfactory way has been developed to determine cure rate on a material which exhibits a marching modulus.
252
R.H. NORMAN
The first problem is significant at all temperatures but can result in especially misleading results for times of the order of a minute (see Section 4.1) and the magnitude of the error is a function of instrument design. The second (Section 4.2) and third (Section 4.3) problems are only occasionally encountered and may be minimised by instrument design. The fourth problem (Section 4.4) is fundamental to the type of measurement and cannot be ameliorated by instrument design.
4.1. Rate of attainment of temperature All instruments have a heated cavity whose temperature alters little as the result of insertion of a test piece. Most instruments have a paddle or rotor which is initially cold, or cools during insertion of the test piece, and is then heated by conduction of heat through the sample from the cavity. Heat is lost continuously by conduction and radiation from the paddle or rotor during the test, with the result that the paddle or rotor is always cooler than the cavity. In the Mark VI Curometer the paddle is somewhat heat-insulated from the rest of the instrument. In the Rheometer, the stem of the rotor is in contact with bearings which are warm, though presumably not at platten temperature. Even if the paddle or rotor is heated (or absent as in rotorless instruments), there is still a time lag before the sample is at a uniform temperature. An early Vulkameter used a paddle heated to the platten temperature, but it used a thick test piece which required several minutes to heat through. Consider an ideal R h e o m e t e r with a rotor temperature equal to that of the cavity. The test piece is 2.2 mm thick between the edge of the rotor and the flat of the cavity. The centre of this thick portion would still take about 20 s to heat to within 5 °C of a platten temperature of 170 °C. A corresponding calculation for the thin (0.5 mm) web of rubber between the paddle and platten of a C u r o m e t e r gives about I s. With a rotorless cure meter, the total heat-up time is only that to heat the rubber. If one assumes that the stiffness of the test piece is controlled substantially by the very thin layer at the edges (see also Section 4.3), then the heating time for, say, a 0-05 mm clearance is measured in fractions of a second. If, however, as is implied in a paper by Gibttfert2 the torque in the Elastograph is controlled primarily by shear in the bulk of the test piece (4.3 mm maximum thickness) the heating time is about I min. The errors due to slow and incomplete temperature uniformity have been assessed for the Curometer and for the R h e o m e t e r with a micro-die. For comparison the degree of cure has been assessed by swelling measurements on 0-5 mm thick test pieces which have been cured substantially isothermally in a rapidly operating press. The degree of cure in the isothermal case has been taken as the reciprocal of the equilibrium swollen modulus. T h e difference between this measure of the
253
CURE METERS AND THEIR PROBLEMS TABLE 1 90% CURETIMES (S) MEASUREDIN THREE WAYS
Temperature (°C)
Isothermal
Curometer
Rheometer
120 140 160 180 200
3120 870 280 7'2 17
4800 1100 320 105 47
7800 1800 460 195 97
degree of crosslinking and that given by the Florey Rehner equation is small and cannot alter the general picture given by the results, which are given in Table 1. It will be seen that the Rheometer can give very much longer 90% cure times than the Curometer and also that the Curometer gives longer times than the isothermal results. The large discrepancies at lower temperatures are surprising and must represent a considerable difference between the true average temperature and the measured temperature even after very long times. The very large differences at the high temperatures are not unexpected. It was thought that part of the difference between the isothermal results and the cure meter results might lie in the difference between the assessment of cure time from hot modulus and that from reciprocal swelling. Samples were therefore cured for various times in the Curometer and then quenched and the degree of cure assessed by reciprocal swelling. With samples thus cured under identical conditions the 90% cure time assessed by swelling was 40% greater than that from the hot modulus results. Thus the results in Table 1 underestimate the errors due to poor heat transfer. It is obvious that there are large errors in measuring cure times, due to poor temperature equilibration both for long and short cure times. 4.2. Slippage This has been observed particularly with nitrile rubbers. Occasionally Curometer traces show the wine glass effect (Fig. 3) where a catastrophic slippage has occurred after the material is nearly cured. A less marked case of slippage may show itself as a change in slope of the plot of z on a logarithmic scale against time (Fig. 4). Slippage has also been observed by cutting open Curometer pellets, when it is manifest by the elliptical shape of the rubber which has moulded through round holes in the paddle. It is often thought that this can be overcome by enclosing the sample in a cavity and applying pressure by closing the cavity. The pressure may however be inadequate initially and shrinkage during cure may decrease it. Some rotorless cure meters sometimes attempt to overcome this problem 2 by
254
R.H. N O ~ n N
Time
Fig. 3.
Wine glass effect observed on an early Curometer. The MK VI gives only the envelope of the half trace.
Common curve
Slipping
0.4 Z 0.2
0.1
0.06 I
I
I
Time
Fig. 4.
)
z-Plots.
255
CURE METERS AND THEIR PROBLEMS
continuous pressure application, but I have no information on the efficacy of this. 4.3. Porosity T h e presence of air or m o r e particularly water in a r u b b e r may cause the r u b b e r to b e c o m e extremely porous when heated. It has been reported f r o m time to time that the C u r o m e t e r produced curious and variable curves when used on such a mixing. Normally a simple check on the a p p e a r a n c e of the test piece on its removal f r o m the machine will indicate whether one has a porosity problem. T o assess whether porosity really could be a problem, a clay-loaded natural r u b b e r was tested in a C u r o m e t e r under two conditions: (a) at atmospheric pressure and (b) in an autoclave at 0-55 M P a air pressure. T h e results are shown in Fig. 5. These cannot be c o m p a r e d with normal C u r o m e t e r traces because the sample was inserted at r o o m t e m p e r a t u r e to avoid 'blowing' before the pressure was applied. T h e t e m p e r a t u r e was raised to 140 °C as rapidly as
Am t osphepcri ci) .c ==
r.n 0.55
MPa
Time
Fig. 5.
Effect of porosity.
256
R.H. NORMAN
Fig. 6. Porous samples.
the platten heaters would permit. Admittedly the material chosen was extreme, as the samples from the atmospheric pressure test show (Fig. 6), but this does indicate that porosity can be a very serious problem. With most mixes, however, no porosity of a test piece is evident. Closed-cavity type instruments probably remove the problem of porosity in all but extreme cases, while rotorless systems probably still suffer from it. 4.4. Marching modulus The interpretation of cure traces is sometimes complicated by a 'marching modulus' where, for example, the Rheometer trace height continues to increase slowly over a period much greater than that in which the majority of the change takes place, and the Curometer trace becomes progressively narrower in a corresponding way. In this case the value of a~ or Mn becomes meaningless unless interpreted as the value measured after a fixed arbitrary time. Technologists at RAPRA have generally adopted 1 h for the arbitrary time. It is tempting to use the crossing of the estimated asymptote and the line through the steepest part of a Rheometer trace, but this can give a value of tgo at which other physical properties are not well developed. 5.
CONCLUSIONS
While existing cure meters are adequate for quality control purposes, they present several problems when they are used for other than comparative measurements.
CURE METERS AND THEIR PROBLEMS REFERENCES 1.
2.
HICKMAN, J. A., NORMAN, R. H. and PAYNE, A. R. (1965). Rubber World, 152 (2), 76. G~I'H-P-RT, O. (1976). Kaut. u Gummi, 29 (5), 261; 29 (6), 341.
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