Applications for stress relaxation from the RPA in characterization and quality control.

Uncured and cured rubber is neither completely viscous or elastic in nature, but rather viscoelastic in its properties. Understanding this viscoelastic profile is very important in predicting processing behavior as well as the nature of the cured physical properties imparted to the final rubber product. A very fast and simple way to quantify a rubber's viscoelasticity is to perform a stress relaxation experiment. The Maxwell Model shown in figure 1 illustrates this principle very well with a spring and dashpot in series (ref. 1). A sudden applied extensional deformation results in a characteristic stress relaxation curve as shown. A log-log plot of the resulting curve is often linear. Determining the steepness of the slope of this log-log stress relaxation curve is a rapid method for quantifying the viscoelasticity. When stress relaxation is fast, then there is a higher viscous quality relative to the elastic quality. When the stress relaxation is slow, then elasticity is the dominant quality.

[Figure 1 ILLUSTRATION OMITTED]

With new enhancements to the software for the RPA 2000 analyzer, this instrument can now perform a stress relaxation subtest in addition to its strain, frequency, temperature and time sweep subtest capabilities. The strain pulse is applied by torsional shear to the rubber specimen through movement of the lower die in a sealed pressurized cavity (ref. 2). The stress relaxation is measured by the torque transducer which is attached to the upper die. Figure 2 shows some of the stress relaxation characteristics which can be calculated with this new software. These characteristics are:

* Peak torque - the highest torque recorded resulting from the deformational pulse. This property was reported to correlate to compound viscosity (ref. 3). However, experimental work reported in this article suggests that the log-log regression line intercept from the initial portion of the stress relaxation curve (the first zone) may correlate better with Mooney viscosity.

* Zone - this is the portion of a stress relaxation curve predefined by the operator by time or stress. A log-log regression is performed over the data in the zone.

* Slope - this is the slope of the regression line through the zone boundaries preprogrammed in a log-log plot of torque vs. time. Slopes can be calculated for up to three preprogrammed zones of the stress relaxation curve. The zones are defined by either time or stress.

* Regression line intercept - this is the y intercept of the log-log regression line. The intercept corresponds to the predicted stress at one second. An intercept is calculated for one to three zones. This can relate to such traditional properties as Mooney viscosity for raw rubber.

* Integrated area - this is the area under the stress relaxation curve between predesignated zone boundaries. Integrated areas for up to three zones under the stress relaxation curve can be reported. These zones can overlap.

* Time to a given % drop - this test parameter indicates how much time was required to achieve a given % drop from the peak torque. The user can define this parameter. For example, time to 35% drop, 50% drop and 80% drop can be selected. Under certain circumstances, this property can relate to molecular weight distribution of a raw rubber (ref. 4).

* % drop at a given time - this test parameter indicates the % drop from the initial peak torque at a given time. For example, the % drop at 1. 10 and 100 seconds can be selected.

The following test conditions are set when configuring the RPA for stress relaxation testing:

* Preheat time - the time period from die closure until the application of strain. Usually 60 seconds is sufficient for sample warmup and conditioning. The minimum time is 10 seconds.

* Applied strain - the amplitude of the sudden shear deformation applied to the sample. Earlier work suggested that a five degree arc (70% strain) gave good repeatability when testing raw polymers on the RPA (ref. 5).

[Figure 2 ILLUSTRATION OMITTED]

Stress relaxation decay time - the time period starting at the end of the applied deformational pulse to the end of the SR test. Earlier work also suggested that a 120 second time period for measuring stress relaxation decay on uncured rubber is usually sufficient (ref. 6). The SR time period should be longer for cured rubber.

* Torque threshold - the torque level to which the stress relaxation decay must reach before the test automatically terminates before the end of the decay time. Usually a value of 0.05 lb.-in. torque works well. Torque values below this level approach the noise region when testing uncured compounds or raw polymers.

Raw rubber testing

Figure 3 shows the log-log master curve for the relaxation modulus vs. time for an amorphous polymer such as rubber in thermodynamic equilibrium (ref. 7). Stage 1 (conformational relaxation or unkinking) and stage 2 (intermolecular slippage involving a limited chain length) are not measurable in the following RPA experiments because the relaxation times are orders of magnitude too small to be measurable in a normal manner. In the first plateau at stage 1, the rubber acts like a solid for a very short time period. Here the rubber's submolecular units cannot rearrange quickly enough to dissipate the strain energy. With time the rubber enters stage 2 where it relaxes at the molecular unit level until it reaches a "time barrier" called the chain entanglement plateau zone (stage 3). This plateau results because more time is required for the rubber's entanglement network to start to disentangle. In stage 4 enough time has passed for the rubber molecular chains to start to disentangle with the longer chains requiring a longer time to disentangle. As more and more chains relax, the concentration of unrelaxed chains becomes less and less which promotes a further drop in modulus in what is called the terminal zone. Only at stages 3 and 4 (which involves the relaxation of groups of long chain segments), can this phenomenon be measured by the RPA.

[Figure 3 ILLUSTRATION OMITTED]

In general the stress relaxation curves for stages 3 and 4 are different for rubber samples which differ only in average molecular weight as shown in figure 4. As can be seen, the chain entanglement plateaus are longer for rubbers with higher average MW. However, figure 5 shows how the molecular weight distribution (MWD) can affect the slope of stage 4 (the terminal zone) for a given rubber. Figure 6 shows the effect of higher polymer linearity (less branching) which is very similar to the effects from increased MWD (refs. 8-10).

A variety of different raw synthetic and natural rubber samples were tested with the RPA stress relaxation test in order to determine the optimal test conditions.

[Figure 4 to 6 ILLUSTRATION OMITTED]

Characterization and quality control of raw rubber

A polymer conference in Belgium in 1991 concluded that raw rubber cannot be totally characterized in relation to processability by Mooney viscosity as described in ASTM D1646. While Mooney viscosity can correlate to average molecular weight, there are many other polymer characteristics which relate to rheological behavior, but which cannot be measured by Mooney viscosity alone. Some of these polymer variations include the following (ref. 11):

* molecular weight distribution;

* chain branching;

* monomer ratios for copolymers (such as percent bound styrene for SBR or percent bound ACN for NBR);

* oil and/or carbon black content in extended polymers (masterbatches);

* microstructural differences in the polymers (such as percent vinyl, cis or bans structures);

* soap emulsions;

* chemical modification of solution SBRs.

The test methods presently available to measure these polymer characteristics are very laborious and/or time consuming and do not always lend themselves well to a quality control environment. Examples of these methods include such procedures as gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Kjeldahl Nitrogen Determination. None of the test methods mentioned above can detect quality variations for all of the polymer characteristics listed above. Through frequency, strain and temperature sweep subtests the RPA has demonstrated its ability to detect variations in many or all of the polymer characteristics listed above. Now with the added capability of the stress relaxation subtest, even greater analysis and quality control of these polymer variations can be achieved. Also, the stress relaxation subtest can be used alone as a fast and effective quality control test for raw polymers.

Experimental work with raw rubbers

A total of 27 grades of raw synthetic and natural rubber were selected for this study. The composition of this selection consisted of 5 cis-BRs, 2 clear SBRs. 1 carbon black-SBR masterbatch. 3 oil-extended SBR masterbatches. 7 NBRs, 4 CRs and 2 Nits. These elastomers were then tested on the MV 2000 Mooney viscometer at ML 1+4 (in accordance with ASTM D1646) for Mooney viscosity and Mooney stress relaxation. These elastomers were also tested by the RPA stress relaxation with a one minute pre-heat, a 5 degree arc applied strain (70% strain), and a two minute stress relaxation time. Figure 7 shows that the correlation between RPA stress relaxation peak torque and Mooney viscosity is only fair with a correlation coefficient of r = 0.88. However, the regression line intercept for the first zone gave a significantly better correlation (r = 0.93). Therefore the RPA S.R. intercept gave a good correlation to Mooney viscosity but requires much less test time to run than a traditional Mooney ML 1+4. Also, a very good correlation was achieved in earlier work (ref. 12) in relating RPA stress relaxation to Mooney stress relaxation testing.

[Figure 7 ILLUSTRATION OMITTED]

Styrene butadiene rubber

There are many examples where two SBRs will nominally be the same by IISRP nomenclature and will have the same Mooney viscosity; however they will have quite different viscoelastic properties. An example of this can be seen from a comparison of two sources of SBR 1006. The Mooney viscosity tests showed these two sources of SBR 1006 to have essentially the same Mooney value (ML 1+4 of 48.1 vs. 48.2). However, figure 8 shows different RPA stress relaxation decays as seen from a log-log plot. This difference is particularly noticeable when comparing the results in the latter part of the SR curve (zone 3). The SBR 1006 from source A has a steeper slope and requires less time to drop 80% of peak torque than the SBR 1006 from source B. This is shown in table 1. These differences are detected later in the stress decay curve which may indicate these differences are due to differences at the high molecular weight fraction. Greater differences occurring later in the stress relaxation curve are equivalent to greater differences being observed at the lower portion of a frequency sweep. This can be seen from the tan [Delta] responses from RPA frequency sweeps.

Table 1

RPA stress relaxation   SBR 1006   SBR 1006
characteristics         source A   source B

Zone 1 slope             -.408       -.370
(0.6 to 2 sec.)

Zone 2 slope
(2 to 20 sec.)           -.523       -.468

Zone 1 slope
(20 to 100 sec.)         -.504       -.461

Time to 80% drop         2.89        3.77

[Figure 8 ILLUSTRATION OMITTED]

Figures 9 and 10 show viscoelastic differences between different sources of SBR 1502 and 1778. These are additional examples of polymers which have close to the same Mooney viscosity values yet have different viscoelastic profiles as seen from RPA stress relaxation.

[Figure 9 & 10 ILLUSTRATION OMITTED]

Polybutadiene rubber

A study was made using RPA stress relaxation in testing two different cis 1,4 polybutadiene polymers. Both of these cis-BRs have a final Mooney viscosity of 35 MU (ML 1+4 @ 125 [degrees] C). BR #1 has more branching and a narrower molecular weight distribution than BR #2. These differences affect such processing properties as carbon black incorporation times. Typically BR #1 has the [M.sub.W]/[M.sub.N] = 2.4 while BR #2 has [M.sub.W]/[M.sub.N] = 2.7. Figure 11 compares the stress relaxation curves for each polymer as log-log plots. The overall log-log slope for BR #1 is flatter than for BR #2. This is because BR #1 has significantly more branching. The slopes for each of these curves were measured in three different zones. The slope for zone one is measured between 0.6 seconds and 2.0 seconds. The slope for zone 2 is measured between 2.0 seconds and 20 seconds; while the slope for zone 3 is measured between 20 and 100 seconds. This is illustrated in figure 11. The slopes are calculated from a linear regression line. The figure compares the log-log slopes in each of the three zones for both polymers. While BR #1 has a much steeper slope in zone 1 and a slightly steeper slope in zone 2, it also imparts a flatter slope in zone 3 when compared to BR #2. The zone 3 represents the "tail" of the stress relaxation curve and gives good information regarding differences on the high side of the molecular weight distribution. By partitioning the stress relaxation curve into three zones and observing changes in slope from one zone to the next, one can obtain additional information regarding the differences in molecular weight distribution and branching.

[Figure 11 ILLUSTRATION OMITTED]

Acrylonitrile butadiene rubber

There is no standard classification system for acrylonitrile butadiene rubber (NBR) which has been accepted by any consensus organization. However, some NBR producers use a classification system based on percent bound acrylonitrile (% ACN), Mooney viscosity and whether the polymer is "hot" or "cold" emulsion polymerized. By using such a scheme to classify NBR, one finds there are over nine NBR producers worldwide who manufacture polymers which can meet what we have arbitrarily called category A (33% ACN, Mooney viscosity 30, cold polymerized) and eight NBR producers who manufacture to a category B specification (33% ACN, Mooney viscosity 80, cold polymerized). These two classification categories contain by far the largest population of commercially offered NBRS. (In total there are over 101 "categories" established based on this classification scheme with % ACN ranging from IS to 53 and Mooney viscosities from 30 to 140 M.U. - a total of 84 categories for "cold" and 17 categories for "hot" polymerization.)

Two NBRs from category A and four NBRs from category B were tested by the RPA stress relaxation test. These NBRs all possessed about the same Mooney viscosity within each category. Figures 12, 13a and 13b show the stress relaxation differences seen among these NBRs which are all within the same category and should be nominally the same. However, it is not uncommon for NBRs with the same % ACN and Mooney viscosity to still process quite differently.

[Figure 12, 13a and 13b ILLUSTRATION OMITTED]

Figure 14 shows the RPA stress relaxation response for two nitrile rubbers which have about the same Mooney viscosity but have significantly different bound ACN and molecular weight. As can be seen, even though the Mooney viscosities are very close, the stress relaxation profiles are quite different and the NBR with the higher % bound ACN (and lower butadiene content) drops at a faster rate.

[Figure 14 ILLUSTRATION OMITTED]

Polychloroprene

Figure 15 compares the stress relaxation curves for two different sources of a crystallization resistant grade of polychloroprene, both of which have nearly the same Mooney viscosity. Even with the match in Mooney viscosity, these polymers possess differences in viscoelastic properties.

[Figure 15 ILLUSTRATION OMITTED]

Silicone rubber

Two samples of low mw silicone rubber of different molecular weights were tested on the RPA for stress relaxation. Table 2 compares the results from the stress relaxation testing. It is interesting to note that repeatable differences could be seen after only three seconds of S.R testing.

Table 2- comparison of two polysilicones of different MW

RPA stress relaxation       Polysilicone,      Polysilicone,
characteristics           source A, test 1   source A, test 2

Zone 1 slope                 -0.607             -0.608
(0.6 to 2 sec.)

Zone 2 slope                 -0.750             -0.730
(2 to 20 sec)

Zone 3 slope                 -0.747             -0.748
(20 to 100 sec.)

Integrated area, Zone 1       0.69               0.688
(In.-lb.)

Integrated area, Zone 2       2.35               2.34
(In -lb.)

Integrated area, Zone 3       3.04               3.02
(In -lb.)

Time to 50% drop (sec.)      0.1206            0.1206
Time to 80% drop (sec.)      0.5004            0.4935

RPA stress relaxation       Polysilicone,      Polysilicone,
characteristics           source A, test 1   source A, test 2
RPA stress relaxation
characteristics

Zone 1 slope                -0.603              -0.598
(0.6 to 2 sec.)

Zone 2 slope                -0.843              -0.823
(2 to 20 sec)

Zone 3 slope                -0.800              -0.793
(20 to 100 sec.)

Integrated area, Zone 1      0.726               0.738
(In.-lb.)

Integrated area, Zone 2       2.22                2.29
(In -lb.)

Integrated area, Zone 3       2.95                3.03
(In -lb.)

Time to 50% drop (sec.)      0.1228             0.1187
Time to 80% drop (sec.)      0.5030             0.5224

Effects of temperature on the stress relaxation curve

Many of the RPA stress relaxation polymer comparisons were made at 100 [degrees] C. However, changing the temperature at which the stress relaxation experiment is performed has a great effect on the stress relaxation decay curve.

The stress relaxation curve of raw polymers is directly affected by temperature. Time-temperature superposition predicts that changing the temperature produces a shift in the stress relaxation curve. This is similar to the relationship between the temperature of a test and the frequency of a test in dynamic measurements. In stress relaxation, higher temperatures produce lower torque/modulus levels and faster relaxations. Figure 16a illustrates the effect of temperature on a typical EPDM elastomer. This figure shows the log-log plot of stress relaxation curves taken at four different temperatures using different samples from the same homogeneous EPDM polymer. Note the similarity of the curve shapes if the curves are shifted along the x-axis. Figure 16b shows the same plot as figure 16a except that the curves are all shifted into a "master curve" at 80 [Degrees] C. In this example, the actual shift factor is log(2) for each 20 [Degrees] C increase in temperature. The "master curve" in figure 16b predicts the curve shape at up to 1,000 seconds at 80 [Degrees] C even though all of the relaxations were done at 120 seconds. This is one of the advantages when testing at different temperatures. We can trade temperature for time to get the same information. Please note the faster relaxations which occur at the higher temperatures.

[Figure 16a to 16b ILLUSTRATION OMITTED]

The relationship between a stress relaxation decay and a frequency sweep

There is a theoretical relation between the RPA stress relaxation decay curve and the RPA frequency sweep. This involves the mathematical conversion of G(t) to G'([Omega]) and G"([Omega]).

The linear viscoelastic region is defined as the strain range where the modulus of a material is independent of the strain. In this region, elastomers have some unique properties. One of these properties is a theoretical exact relationship between the dynamic shear moduli G'([Omega]) and G'([Omega]) and the stress relaxation modulus G(t). However, the equations which relate these moduli are often difficult to use. They contain integrations of complex functions going from - [Infinity] to + [Infinity]. As a result of these difficulties, a variety of approximation methods were developed. These approximation methods are not empirical but are based on the original integration methods. The approximation method used in the RPA 2000 to convert G(t) to G'([Omega]) and G"([Omega]) was developed by Yagii and Maekawa. The equations are (ref. 13):

(1) G'([Omega])=[G(t)+ 1.08 {G(1.59t)-G(2.50t)} = 0.159 {G(0.25t)-

G(0.398t)}]| t=1/[Omega]

(2) G"([Omega]) = [2.70 {G(0.631t)-G(t)} + 0.794 {G(0.100t)-

G(0.158t)}]| t=1/[Omega]

The equations use the relationship t = 1/[Omega] where t is the time in seconds during the stress relaxation test and [Omega] is the oscillation frequency of the frequency sweep in radians/second. The RPA 2000 stress relaxation subtest generates G(t) at discrete points in time. The values for G(t) at any point in time required in the above equations are estimated from these discrete points. The final result is an estimate of the frequency sweep from the stress relaxation curve.

This approximation should be used with caution since it is made under the assumption that the material under test is in the linear viscoelastic region. The usable RPA frequency range for the conversion is limited by the time during which useful data are collected. The highest usable frequency for this method is typically

[Omega] = 1/t = 1/0.05 sec = 20 r/sec [nearly equal to] 191 cpm [nearly equal to] 3.2 Hz

while the lowest useful frequency is

[Omega] = 1/t = 1/100 sec = 0.010 r/sec [nearly equal to] 0.1 cpm [nearly equal to] 0.0016 Hz

The RPA frequency conversion will generate values outside of this range. Values outside of this range should be used with caution.

The RPA 2000 analyzer can verify the conversion equation for a particular material by running an actual RPA frequency sweep subtest after an RPA stress relaxation subtest. Figures 17a to 17d show plots of G measured dynamically versus the G([Omega]) calculated from a stress relaxation curve using the above method. The theory states that both values of G' and G" should be identical.

[Figure 17a to 17d ILLUSTRATION OMITTED]

A comparison of the predicted G'([Omega]) from an RPA S.R. test vs. the actual RPA dynamic modulus from an RPA frequency sweep was made for five different rubber samples. These included two SBR elastomers, two EPDM elastomers and an uncured SBR compound based on SBR 1606, (52 parts of N330 carbon black and 10 parts aromatic oil) with a sulfenamide cure system. Each sample was tested twice at four temperatures (80, 100, 120 and 140 [degrees] C) for a total of eight results per sample.

Figure 17a plots the actual RPA dynamic G' at 10 cycles per minute (cpm) versus G'([Omega]) at 10 cpm calculated from the stress relaxation curve. The data in figure 17a show that there is a linear relationship between the dynamic G' and the stress relaxation G'([Omega]) for all of the raw polymers tested. Figure 17a also has the equivalence line where both values of G' would be identical. G'([Omega]) is either comparable to or less than the G' from actual dynamic measurements. The results from all of the raw polymers followed a single line (a best fit line) which is close to the equivalent line. The SBR compound, however, deviates from this equivalent line much more than the polymers. This is probably because of the high carbon black loading. Also the linear relationship observed for the raw polymer samples was not affected by the test temperature.

Figure 17b plots the dynamic G' at 500 cpm versus G'([Omega]) at 500 cpm calculated from the stress relaxation curve. The data in figure 17b show that there is also a linear relationship at 500 cpm between the dynamic G' and the stress relaxation G'([Omega]) for all of the raw polymers tested (excluding the SBR compound). However, the deviation of the data from the equivalence line is greater at the higher frequency. The results at 500 cpm are approaching the limits of stress relaxation test accuracy because they are based on data points very early in the stress relaxation test in the milk second range.

Figure 17C shows a graph comparable to figure 17a except that loss modulus G" at 10 cpm is plotted. Results for both G' and G" at 10 cpm were similar.

Figure 17d shows a graph comparable to figure 17b except that the loss modulus G" at 500 cpm is plotted. Results show an even greater deviation from the equivalence line for G"([Omega]) compared to the G'([Omega]) result. However, these stress relaxation data are extrapolated and are beyond the actual measurement capability of the RPA.

A stress relaxation curve contains molecular weight distribution information. The initial part of the curve has information concerning the low molecular weight components of the test sample. The latter part of the curve has information concerning the high molecular weight components. The RPA is more adept at measuring the stress relaxation curve in the latter or high molecular weight area. Therefore, the RPA may discern low molecular weight differences better with an RPA frequency sweep and a high molecular weight difference better with an RPA stress relaxation test.

Mixed stocks

The manner in which a rubber stock is mixed has a large effect on its viscoelastic characteristics and on its performance in downstream processing. During the mixing process, breakdown of elastomers and incorporation and dispersion of compound ingredients, such as carbon black, all have a profound effect on the processability characteristics of the stock. An experiment was developed to determine the best RPA stress relaxation test conditions to measure state-of-mix from a laboratory internal mixer. A generic SBR truck tread formulation was selected and is shown in table 3. Laboratory simulation of factory mixing techniques was applied to a BR internal mixer to achieve different states-of-mix. The Monsanto power integrator was used to record the total amount of work in kilowatt hours at dump for each batch. Because only uncured properties were to be studied, no curatives were included in the formulation.

Table 3 - generic truck tread

Ingredients(*)

SBR 1500             100.00
N220 carbon black     50.00
Aromatic oil           8.00
TMQ antioxidant        2.00
6 PPD antiozonant      2.00
Stearic acid            1.5
Zinc oxide              5.0
Total                 168.5

(*) No curatives were added since only variations in the quality of mix were studied.

Energy at dump as recorded by a power integrator is a more effective method of controlling the rubber mixing process than traditional time and temperature techniques. This assumes the addition sequence and time-temperature profiles remain relatively unchanged (refs. 14 and 15).

Predicting quality of mix with RPA stress relaxation

Figure 18 shows three stress relaxation curves for the three different states of mix for the generic truck tread stock shown earlier in table 3. This comparison shows that as a greater amount of work history is applied, the following changes occur with the stress relaxation characteristics: Peak torque decreases; slope per zone decreases (the slope decreases as it becomes steeper because it is a negative (inverse) slope); regression line intercept decreases; integrated area per zone decreases; time to a given % drop decreases; and % drop at a given time increases.

[Figure 18 ILLUSTRATION OMITTED]

Table 4 gives the RPA stress relaxation test results for the three different levels of mix. Comparative testing of the generic truck compound at 10 different qualities of mix was performed. Statistically, the parameter which gave the best prediction of the quality of mix (as indicated by energy at dump) was integrated area under the stress relaxation curve. A correlation was found for integrated area under the SR curve for zone 2 vs. energy at dump. Integrated area under the stress relaxation curve can be used as a rapid and effective measure of the stock mixing quality.

Table 4 - comparison of rubber stocks at three different
qualities of mix

RPA stress relaxation        Stock at 0.105       Stock at 0.200
characteristics                kwh @ dump           kwh @ dump

Peak torque (lb.-in.)           71.08                 57.64
Zone 1 slope (0.6 to 2         -0.311                -0.346
sec.)
Zone 2 slope (2 to 20          -0.331                -0.350
sec.)
Zone 3 slope (20 to 100        -0.388                -0.394
sec.)
Integrated area, zone 1         33.75                24.61
(in.-lb. sec.)
Integrated area, zone 2        226.71                157.33
(in.-lb. sec.)
Integrated area, zone 3        529.50                360.57
(in -lb. sec.)
Time to 50% drop (sec.)        0.3387                0.2799
Time tn 80% drop (sec.)        6.1575                4.0956

RPA stress relaxation           Stock at 0.480
characteristics                  kwh @ dump

Peak torque (lb.-in.)              53.3
Zone 1 slope (0.6 to 2           -0.365
sec.)
Zone 2 slope (2 to 20            -0.376
sec.)
Zone 3 slope (20 to 100          -0.428
sec.)
Integrated area, zone 1           21.79
(in.-lb. sec.)
Integrated area, zone 2          133.16
(in.-lb. sec.)
Integrated area, zone 3          289.34
(in -lb. sec.)
Time to 50% drop (sec.)          0.2664
Time tn 80% drop (sec.)          3.3448

Effects of carbon black loading on uncured stress relaxation characteristics

Figure 19 shows three stress relaxation curves for the three different loadings of N330 for the recipe given in table 5. This comparison shows that with a higher loading of carbon black, the changes with the stress relaxation characteristics all increase with the exception of % drop at a given time which decreases.

Table 5 - comparison of rubber stocks with three
different loadings of N330 carbon black

Ingredients                 PHR

SBR 1500                    100
Aromatic oil                 20
Zinc oxide                    3
Stearic acid                1.5
TMQ antioxidant             1.0
N330 carbon black           variable

Total                       125.5 + blk. loading

[Figure 19 ILLUSTRATION OMITTED]

Table 6 gives the RPA stress relaxation test results for the SBR compounds with three different loadings of N330 carbon black. Comparative testing of the SBR compound at six different loadings of N330 was performed. Statistically, the parameters which gave the best predictive correlation with carbon black loading were initial peak torque and integrated area under the stress relaxation curve. These two stress relaxation parameters are very sensitive to changes in carbon black concentration. Changes resulting from changes in carbon black concentration are shown on a log scale. Linear correlations are from semi log plots.

Table 6 - comparison of rubber stocks at three
different loadings of N330 carbon black

RPA stress relaxation      30 phr       70 phr          130 phr
characteristics
Peak torque (lb.-in.)       21.27        44.37          283.46
Zone 1 slope (0.6 to 2     -0.447       -0.326          -0.122
sec.)
Zone 2 slope (2 to 20      -0.447       -0.294          -0.116
sec.)
Zone 3 slope (20 to 100    -0.435       -0.291          -0.141
sec.)
Integrated area, zone 1      4.03         9.79             137
(in.-lb. sec.)
Integrated area, zone 2      20.5         68.2           1,390
(in -lb. sec.)
Integrated area, zone 3        39          182           4,900
(in.-lb. sec.)
Time to 50% drop (sec.)      .047         .041            .096
Time to 80% drop (sec.)     0.496         0.59              -

Effects of oil loading on uncured stress relaxation characteristics

Figure 20 shows three stress relaxation curves for the three different loadings of aromatic oil for the recipe given in table 7. This comparison shows that a higher loading of aromatic oil will decrease the stress relaxation characteristics with the exception of % drop at given time which increases.

Table 7 - comparison of rubber stocks with three
different loadings of aromatic oil

Ingredients                 PHR

SBR 1500                    100
N330 carbon black            70
Zinc oxide                    3
Stearic acid                1.5
TMQ antioxidant             1.0
Aromatic oil                variable
Total                       175.5 + Aro. oil loading

[Figure 20 ILLUSTRATION OMITTED]

Table 8 gives the RPA stress relaxation test results for the SBR compounds with three different loadings of aromatic oil. Comparative testing of the SBR compound at six different loadings of aromatic oil was performed. Statistically, the parameters which gave the best predictive correlation with oil loading were initial peak torque and integrated area under the stress relaxation curve. Just as we saw with the carbon black experiment, these two stress relaxation parameters are very sensitive to changes in oil concentration. Changes resulting from changes in oil concentration are shown on a log scale. Linear correlations are from semi log plots.

Table 8 - comparison of rubber stocks at three
different loadings of aromatic oil

RPA stress relaxation           0 phr     20 phr      80 phr
characteristics

Peak torque (lb.-in.)           88.8      46.72       12.24
Zone 1 slope (0.6 to 2        -0.220     -0.308       -0.426
sec.)
Zone 2 slope (2 to 20         -0.214     -0.265       -0.421
sec.)
Zone 3 slope (20 to 100       -0.286     -0.268       -0.390
sec.)
Integrated area, zone 1        29.5       10.8         1.95
(in.-lb. sec.)
Integrated area, zone 2         247        79          10.8
(in.-lb. sec.)
Integrated area, zone 3         698       220          23.4
(in.-lb. sec.)
Time to 50% drop (sec.)        0.059     0.042        0.033
Time to 80% drop (sec.)         2.69      0.65        0.33

Conclusions

The RPA stress relaxation subtest is an effective method for quickly characterizing and controlling the viscoelastic quality of raw elastomers. The RPA stress relaxation subtest can quickly provide information concerning variations in raw polymer average molecular weight, molecular weight distribution and/or branching. The RPA stress relaxation subtest can correlate to Mooney viscosity but requires significantly less time to perform. The RPA stress relaxation subtest is also a fast and effective method for controlling the mixing quality of rubber compounds. The presence of carbon black and possibly other compounding ingredients in a mixed rubber compound can have an effect on the Yagii and Maekawa model for predicting the storage and loss moduli for a frequency sweep from stress relaxation data.

References

(1.) John D. Ferry, "Viscoelastic properties of polymers, " Third Edition, John Wiley & Sons, 1980, p. 15.

(2.) H. Pawlowski, J. Dick, "Viscoelastic characterization of rubber with a new dynamic mechanical tester," Rubber World, June, 1992.

(3.) R. I. Barker, G. L Hanna, E. R. Rodger, "Processability: The key to process control," paper no. 70, 110th meeting of the Rubber Division, ACS at San Francisco, CA, Oct. 1976.

(4.) Op. Cit. Barker, Hanna and Rodger.

(5.) H. Pawlowski, J. Dick, "Measurement of the viscoelastic properties of elastomers with a new dynamic mechanical rheological tester, " Philadelphia ACS Rubber Division meeting, Spring, 1995, p. 13.

(6.) Ibid.

(7.) Shiro Matsuoka, Relaxation phenomena in polymers, Hanser, NY, 1992, p. 144.

(8.) W.J. McGrory, W.H. Tuminello, J. of Rheology, Vol. 34 (1990), p. 867

(9.) D.H. Kerner, Proceedings of the physics society, Vol. B69 (1956), p. 808.

(10.) New proposed ASTM standard titled "Standard practice for stress relaxation testing of raw rubber, unvulcanized rubber compounds and thermoplastic elastomers," in ASTM Subcommittee D11.12, 1995.

(11.) H. Pawlowski, J.S. Dick, "A new dynamic mechanical tester designed for testing rubber," paper no. 70, presented at the Spring, 1992 meeting of the Rubber Division ACS, Louisville, pp. 8-9.

(12.) H. Pawlowski, J. Dick, "Measurement of the viscoelastic properties of elastomers with a new dynamic mechanical rheological tester," presented at the Philadelphia Rubber Division, ACS, meeting on May 2-5, 1995 (paper no. 50).

(13.) K. Yagii and E. Maekawa, Nippon Gomukyokaishi, 40, 46 (1967).

(14.) "Power integrator, a more precise and efficient method for control of batch-to-batch rubber processing and property uniformity, " Monsanto Technical Bulletin, IE3, p. 1.

(15.) John S. Dick, H.A. Pawlowski, "Applications for the rubber process analyzer," Rubber & Plastics News, April 26 and May 10, 1993.